Perovskite solar cells - An overview of critical issues

Progress in Quantum Electronics 53 (2017) 1–37

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Perovskite solar cells - An overview of critical issues A.B. Djuri
sic a, *, F.Z. Liu a, H.W. Tam a, M.K. Wong a, A. Ng b, C. Surya b, W. Chen a, c, Z.B. He c a b c

Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Department of Materials Science and Engineering, Southern University of Science and Technology (SUSTech), China

A R T I C L E I N F O

A B S T R A C T

Keywords: Perovskite Photovoltaics

Perovskite solar cell research has been attracting increasing attention in recent years. In this review paper, we will provide an overview of the recent developments in terms of material composition, deposition techniques, and the device architecture (the choice of charge transport layers and electrodes). Then, we will critically discuss some of the major problems, namely device stability, hysteresis, environmental implications due to the presence of a toxic metal (lead), and difficulties in fabrication of large area and/or flexible devices. In addition, we will also discuss tandem cells and modules, as well as the application of perovskites in other devices and the integration of perovskite solar cells with other devices. Finally, we discuss future outlook and important issues which need to be addressed for the wide scale applications of these devices. Lifetime and stability are identified as the key issue to be addressed for wide scale applications, and the majority of environmental impact is due to the use of organic solvents or other components in the device, not the lead-containing perovskite absorber. The standardisation of the testing conditions and more studies involving outdoor testing are needed for convincing demonstrations of good stability as opposed to dark storage testing. Another key issue is upscaling and reproducibility of the film preparation, which can be problematic due to high sensitivity of the perovskite film to the processing conditions. To overcome these obstacles multilaboratory collaborative efforts would be highly desirable.

1. Introduction Perovskite solar cells (PSCs) have become a very hot research topic in photovoltaics community. Since the initial reports on solid state perovskite solar cells with efficiency of
10% in 2012 [1,2], there has been a rapid increase in the number of publications in this area as well as rapid increase in the reported efficiencies. Certified record efficiency according to NREL now exceeds 22%, as illustrated in Fig. 1. It can be observed that based on the record certified efficiency, the PSCs (record efficiency of 22.1%) already outperform some of the well established technologies, such as amorphous Si (13.6%), thin film and multicrystalline Si (21.2% and 21.3%, respectively) and they are comparable to CdTe (22.1%) and CIGS (22.3%) cells. Despite rapid progress in the perovskite solar cell efficiency, there have been concerns about issues which could affect the measurement accuracy and/or practical applications of these devices, namely the hysteresis, stability, scaling up (large area devices), and possible environmental effects related to the use of lead-based active material. A number of review papers have been published on the topic of perovskite materials and devices (over 70 just in 2016). Our

* Corresponding author. E-mail address: [email protected] (A.B. Djuri
sic). http://dx.doi.org/10.1016/j.pquantelec.2017.05.002 Available online 10 June 2017 0079-6727/© 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. Efficiency chart of different solar cell technologies. This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO, USA.

objective is to provide an overview of the materials and device architectures, followed by a critical discussion of issues of concern. To distinguish this review from other numerous reviews on this topic, we aim to provide objective critical discussions as well as practical information on the best practices for device characterization. For example, perovskite films and devices are extremely sensitive to the perovskite deposition conditions. Furthermore, some of the reports on high performance devices prepared in ambient environment may not be applicable to areas with higher humidity (for example, indoor laboratory humidity in Hong Kong during summer months is typically around 60% with several dehumidifiers operating in the lab). Also, small details related to film drying procedure can significantly affect the morphology and the presence of pinholes. In addition, it is possible to obtain excellent stability results for devices stored in the dark, which is not very relevant for practical applications since the device degradation rapidly accelerates upon exposure to solar illumination. Some of the problems in published studies (small device sizes, non-standard measurement conditions) are similar to the problems experienced in organic photovoltaics (OPV), but they are further compounded by the hysteresis effect which is dependent on the measurement (scan) details. Considering the fact that there are already established good practices for characterization of organic photovoltaics (OPV) [3–6], it is straightforward to extend those to PSCs (provided that hysteresis issue, absent in OPV, is taken into account [3]). For example, ISOS protocols developed for studying the stability of OPV [4], are readily applicable to the perovskite solar cells [7]. Typical cautions related to reporting of the research results namely clearly reporting device area, avoiding too small device areas, and providing information about reproducibility and/or variation [5] are applicable to both OPV and PSCs, while for PSCs it is also necessary to provide additional information on efficiency measurement (both forward and reverse bias I-V curves, as well as clearly state scan details). In addition, the measurement of tandem cells has also been discussed recently [8]. OPV research is applicable not only to the efficiency and stability measurement protocols but also in principle applies to other characterisation procedures, due to the similarity of properties of organic and organometallic halide perovskite materials. Nevertheless, care needs to be taken when adopting experimental procedures suitable for organic materials due to the fact that perovskites commonly exhibit significantly higher sensitivity to moisture. For example, ultramicrotomy for preparation of cross sections applicable to OPV would likely not work for perovskite solar cells since it requires floating the section on water [9]. Similarly, while cooling the device in liquid nitrogen to prepare cross-section for SEM observation would typically result in a clean and sharp break, in a humid environment this can cause significant water condensation which might damage the sensitive perovskite layer. The adoption of relevant already established standard procedures would enable even more rapid progress of the field, once standardized practices are followed so that the results from different literature reports could be compared. It would also be highly beneficial for the further progress of the field if there would be large inter-laboratory collaborative efforts similar to the work which has been done for OPV [10–14]. These and similar issues will be discussed, providing the newcomers to the field not only with a summary what has been achieved so far but also with practical details they need to pay attention to in developing and characterizing high performance devices. In particular, we will provide a comprehensive discussion of the common problems of the perovskite solar cells, rather than just focus on progress achieved with various proposed methods for improving the film quality and/or device architecture. 2

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2. Main motivator of perovskite photovoltaics research - high efficiency Organometallic halide perovskite photovoltaics is a fast developing field. Since the reports of solid state devices with efficiencies close to 10% in 2012 [1,2] the field has been developing rapidly with a growing number of publications on halide perovskite photovoltaics. ISI Web of Science search with keywords ”perovskite” and ”halide” and ”solar or photovoltaic” results in 32 hits in 2013, 338 in 2014, 927 in 2015, and over 1200 in 2016. The first devices had the same structure as solid-state dye sensitised solar cells, with FTO substrate, TiO2 mesoscopic layer as electron transport layer, tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9-spirobifluorene (spiroOMeTAD) as a hole transport layer, with a methyl ammonium (MA) lead iodide CH3NH3PbI3 (MAPI) as the absorber instead of an organic dye [1]. Cells with similar efficiency could be obtained with TiO2 and alumina mesoporous layers, and using MAPI and CH3NH3PbI2 Cl perovskite materials [1,2]. Unlike dye-sensitised solar cells, the devices had significantly higher efficiency in solid-state architecture compared to the liquid electrolyte, and in addition the stability was considerably improved compared to the liquid cells with perovskite sensitiser. Consequently, there was significant interest in the investigation of perovskite materials for photovoltaics applications and the rapid progress of the efficiency, which now exceeds 22%. Although efficiencies of ~ 18% can be commonly achieved in devices with inverted architectures and organic charge transport layers, high efficiency devices with PCE  19% are commonly based on an oxide electron transport layer, such as titania or tin oxide. The improvements in the efficiency have mainly been achieved via optimising the perovskite composition and deposition method, with some modifications to the device architecture. The majority of the reported research works address various modifications targeting the improvement in the efficiency and/or improvement in our understanding of how these devices work and how we can further improve the efficiency, reduce hysteresis, and improve the stability. We will briefly describe the reports on the most efficient devices. Best achieved efficiency of 21.6% (average of 20.2%) was obtained for perovskite solar cells with mixed cation (Cs, Rb, MA and formamidinium (FA)) [15]. High efficiency of 21.1% was also obtained for a mixed cation combination of Cs, MA and FA [16]. Mixed anion (MA,FA) and mixed halide (I,Br) by a single step solution processing resulted in devices with power conversion efficiency (PCE) of 20.8% for optimised composition, which included small excess of PbI2 [17]. On the other hand, high efficiency (exceeding 20%) was also reported in devices without residual PbI2 prepared by intermolecular exchange [18]. Other approaches were also used to control the crystallisation and quality of the perovskite layer and prepare high performance devices. For example, polymer template nucleation was also reported to result in high efficiency mixed perovskite (MA, FA, I, Br) solar cells, with PCE over 21% [19]. Similarly, the addition of methyl ammonium formate resulted in improved crystallinity of mixed perovskite and consequently high efficiency of 19.5% [20]. This also demonstrates that other oxides, such as SnO2, can result in high efficiency PSCs [20,21]. Another strategy employed to obtain high performance devices was interface engineering, with PCE of 19.3% obtained by a one-step solution method perovskite deposition from the solutions of MAI and PbCl2 with annealing under controlled humidity [22]. Solvent engineering, with the use of N,N-dimethylformamide (DMF) and N,N,-dimethylsulfoxide (DMSO) mixed solvents, and washing with diethyl ether during spin-coating was also found to result in efficient devices, with the best obtained efficiency of 19.7% [23]. Replacement of a TiO2 electron transport layer with vacuum-deposited C60 in a conventional architecture resulted in hysteresis-free devices with an efficiency of 19.1% [24]. High efficiency (18.4% average, 21.7% best efficiency) was also reported for graded band gap solar cells with a more complex device architecture, consisting of MASnI3 and MAPbI3xBrx layers separated by a monolayer h-BN, sandwiched between GaN and graphene aerogel/spiroOMeTAD [25]. In addition, high efficiency devices include not only small devices, but also larger cells. A perovskite solar cell with area of 1 cm2 and maximum efficiency of 20.5% and certified efficiency of 19.6% was reported [26]. The crystallisation of the perovskite was improved by vacuum-flash assisted technique, and the perovskite composition was also mixed cation (FA,MA) and mixed halide (I,Br) [26]. It should be noted that this vacuum-flash assisted solution process, abbreviated as VASP in the paper, is different from vapor-assisted solution process [27], also commonly labeled as VASP. Certified high PCE of 18.21% was also reported for an inverted perovskite with a gradient distribution of [6,6]-phenylC61 butyric acid methyl ester (PCBM) for an aperture area of 1.022 cm2 [28]. Finally, the route towards high efficiency devices should include the optimisation of the perovskite layer thickness and the use of antireflection coatings [29]. In addition, the obtained efficiency depends on the FTO sheet resistance [30]. Thus, if aiming for very high efficiencies it is necessary to use high quality FTO substrates and possibly consider antireflection coatings to minimise reflection losses in the devices. Optical optimization of the device is also desirable to achieve high efficiencies [31]. In the applications of the proposed approaches, i.e. composition optimisation, solvent engineering, vacuum-flash assisted process etc., it is best to focus on the desired outcome, and that is the quality of the perovskite layer. Due to high sensitivity of the perovskite formation on the processing conditions, it is possible that the same procedures applied in different laboratories with small differences in processing details, different properties of oxide layer, and/or different sources of precursor chemicals may cause considerable scattering of the obtained results. For example, different lot numbers of PbI2 from Sigma Aldrich Co. can result in differences in PbI2 solubility in DMF and consequently considerably different efficiencies of the PSCs. In addition, differences in solvent grade (HPLC vs. anhydrous) as well as solvent supplier can result in significant differences in PbI2 film morphology. Furthermore, in high humidity environments it is essential that all the solvents are stored in the glovebox for reproducible results since over time the solvents miscible with water such as isopropanol will change their properties if stored in ambient. These and other issues can significantly affect the reproducibility in the same laboratory, as well as across laboratories. To illustrate this further, let us consider the reported results for the FTO/TiO2/mesoporous (mp)-TiO2/MAPI/spiro-OMeTAD/Au PSCs prepared by a two-step method (depositing PbI2 first, followed by spin-coating MA or dipping in MA solution). Efficiencies close to 14% have been obtained for this device architecture [32–34]. Efficiency as high as 15.76% was obtained for cells prepared in ambient air at humidity of 50% [35]. The efficiency was found to be dependent on PbI2 solution concentration and substrate pre-heating temperature [35]. Spiro-OMeTAD was doped with commonly used dopants lithium-bis(trifluoromethane)sulfonamide (LiTFSI) and 3

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Fig. 2. Schematic illustration of a) 2D and b) 3D perovskite structures.

4-tert-butylpyridine (TBP) [35]. Similar efficiencies of 14.3% [32] and 14.1% [33] were obtained for devices with different dipping times (20 min [32] and 60 s [33]). The dopants used for spiro-OMeTAD were LiTFSI and tris(2-(1H-pyrazol-1-yl)pyridine)cobalt(II) di [bis (trifluoromethane) sulfonimide] (FK102 dopant) [32,33]. It should be noted that FK102 dopant is available as TFSI and PF6 (tris(2(1H-pyrazol-1-yl)pyridine) cobalt(II) di[hexafluorophosphate]) salt, as well as in Co(II) or Co(III) versions. In some cases in the literature, type of FK102 dopant is not specified. Other reported efficiencies for this device architecture, with only small details in the deposition procedure as well as spiro-OMeTAD dopant varying from one report to another, include 13.9% [34], 12.9% [36],11.0–12.8% (depending on TiO2 compact layer) [37], 10.2% for conventional two-step process with dipping into MAI precursor solution and 12.5% for spray-assisted deposition [38], 8.8–11.9% depending on the dipping time in MAI solution, with further improvement to 13.47% by optimising mesoporous titania layer [39], 4.9%–9.7% [40] and 2.55–5.75% (with Ag electrode) [41] again dependent on the dipping time. This is not a phenomenon isolated to the devices with metal oxide layers, where preparation details of the metal oxide are expected to have a significant effect on the device performance. Let us consider the inverted planar perovskite device with the architecture indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate (PEDOT:PSS)/MAPI/PCBM/metal electrode. Efficiency of 18.1% was reported for a device with Au electrode and dense, high quality pinhole free MAPI prepared by a one step deposition [42]. Other reported efficiencies for this device architecture include 14.1% for a MAPI prepared by two-step deposition and LiF/Al electrode [43], 9.6% for a device with Ag electrode and a MAPI layer prepared by a one-step method [44], and 9.74% for a MAPI layer prepared by a one-step method with a solution additive (3.63% without the additive) and Al electrode [45]. Obviously, the device performance is significantly affected by the properties of the perovskite layer, and the perovskite layer is significantly affected by the details of the deposition procedure. There is a clear need for a more fundamental study of the effect of grain sizes, native defects, surface properties and energy level alignments at the interfaces on PSC performance. For a recent review of surfaces and interfaces in PSCs, see Ref. [46]. However, despite cell performance sensitivity to the perovskite layer quality, working devices can still be obtained even with noncontinuous films due to perovskite film de-wetting [47]. Devices appeared semi-transparent and exhibited efficiencies in the range ~ 3.5–6.5%, depending on the transmittance (with 10 nm Au top electrode) [47]. However, high efficiency devices require high quality perovskite films.

3. Perovskite solar cell device structure A PSC typically consist of a transparent electrode, usually conductive oxide, charge transport layers, such as electron transport layer (ETL) and hole transport layer (HTL), an absorber layer which is organometallic halide perovskite sandwiched between the charge transport layers, and a counter electrode which is typically metal. Different interlayers can also be present. We will briefly discuss materials used for each of these layers, and provide references for detailed overview of specific aspects. For example, a recent review on the effect of ETL and HTL on efficiency and stability has been reported [48]. Two architectures are common, conventional one with perovskite layer on top of the ETL followed by HTL and hole collection electrode and the inverted structure, with perovskite on top of the HTL, followed by ETL and electron collection electrode. It should be noted that this is an opposite convention compared to organic solar cells, where inverted architecture implies an HTL and hole collecting electrode on top of the active layer. Inverted organic solar cells have been found to have improved stability mainly due to higher stability of the high work function electrode such as Au compared to low work function metals such as Al or Mg:Ag alloy. It is yet not clear what are the differences, if any, 4

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in the stability of perovskite solar cells in conventional and inverted architectures. However, it should be noted that there are different factors (reactivity with halogen elements as well as reactivity with oxygen) which affect the stability of perovskite solar cells. For a review of inverted perovskite solar cells, see Ref. [49]. In addition to conventional and inverted architecture, PSCs can also be classified into planar devices where all the layers are thin films and mesoscopic and other nano structured morphologies, where one or more layers, typically metal oxides, have nano structured and/or porous morphology. In this section, we will briefly discuss commonly used perovskite materials in PSCs as well as their deposition methods, followed by the discussion of ETL and HTL materials, interlayers and electrodes. 3.1. Organometallic halide perovskite materials In this review, we will mainly concentrate on perovskite solar cell devices, while perovskite material properties will be briefly discussed with the exception of stability issues. For a detailed overview of material properties of organometallic halide perovskites, see Refs. [50–53]. These include detailed reviews of structure and optical and electronic properties [50]. Among properties important for device applications, it should be noted that they typically have large carrier diffusion lengths and mobilities, as well as composition tunable band gaps. These properties make them particularly suitable for applications in optoelectronic devices, in particular solar cells. Perovskites commonly used in PSCs are the so-called 3D perovskites with a common crystal structure ABX3, where A is a monovalent cation (organic or Csþ), B is a divalent metal which is typically Pb, and X is a halogen, typically I, Br, or Cl [50,51]. The two most commonly used perovskite materials are MAPI and FA lead iodide (CH(NH2)2)PbI3 (FAPI), as well as various mixed cation and mixed anion combinations of these materials. The band gap and consequently colour of the perovskite film/solar cell can be varied by changing the composition [54]. It should be noted, however, that wider band gap perovskites will exhibit lower efficiency due to lower overlap with the solar spectrum. Nevertheless, the colour change property can be of interest for building integrated photovoltaics. However, it can alternatively be obtained by using porous photonic crystal scaffolds instead of perovskite composition engineering [55]. In addition to 3D perovskites, other perovskite structures such as layered 2D perovskites are also possible [50–52]. This is illustrated in Fig. 2. Synthesis of various 2D perovskites with the formula (R-(CH2)nNH3)2PbX4, where R is an organic group and X is halogen, has been reported [56]. The materials exhibited improved stability and high photoluminescence efficiency, but no device performances were reported [56]. 2D layered perovskites are generally considered to have advantages for applications in light-emitting devices [57], since 2D layered perovskites are analogous to multilayer quantum wells, with alternating sheets of organic and inorganic layers with different band gaps and conductivities [58]. Lower dimensional perovskites are particularly versatile since they allow incorporation of larger organic molecules with tailored functions [52,59]. However, perovskites with different dimensionality compared to common 3D perovskites are rarely applied in photovoltaics since they typically yield low efficiency, likely due to their higher band gap, higher exciton binding energy and poor conductivity in certain crystallographic directions [60]. Nevertheless, a promising result with 12.52% efficiency with no hysteresis and good stability in ambient under illumination has been reported recently for Ruddlesden-Popper perovskite n-butylammonium (BA)2(MA)n1PbnI3nþ1 for n ¼ 3 and n ¼ 4 [61]. Thus, there is still potential for the use of layered perovskites in solar cells. Considerable effort has been made to prepare novel perovskite materials beyond common MA and FA based lead halides. There have been several reports on lead-free perovskite materials, but in all cases the achieved efficiencies were significantly lower compared to records obtained for organic lead halide materials. Among the highest reported efficiencies for a lead free PSC was the cell with CsSnI3 quantum rods as an active material, with efficiency as high as 12.96% [62]. These devices also exhibited better stability compared to the commonly used MAPI devices [62], despite the fact that Sn2þ is not intrinsically stable compared to Sn4þ. Sn and Ge are unstable in their 2þ oxidation state, and tend to get readily oxidised into a more stable 4þ oxidation state upon exposure to ambient (oxygen, moisture) [57]. Generally a number of divalent metal cations which can adopt octahedral anion coordination could be a candidate for the formation of organometallic halide perovskite, such as Cu2þ, Ni2þ, Co2þ, Fe2þ, Mn2þ, Cr2þ, Pd2þ, Cd2þ, Ge2þ, Sn2þ, Pb2þ, Eu2þ, Yb2þ etc. [58]. However, the formation of 3D perovskite is dependent on the ionic radii of the organic and metal cations A (rA) and B (rB) and halide anion X (rX) [50]. The Goldschmidt tolerance factor t needs to be in the range 0:8  t  1:0 for a perovskite to form, where t is defined as [50,52]:

rA þ rX t ¼ pffiffiffi 2ðrB þ rX Þ

(1)

Octahedral factor μ defined as μ ¼ rB =rX needs to be in the range from 0.44 to 0.9 for a perovskite to form [50]. If both empirical tolerance factors are in the correct range, it does not necessarily mean that the perovskite will form or that the perovskite structure is the most stable one for a given compound [50]. A calculation of tolerance factors for over 2500 different cation and anion combinations has been reported recently [63]. The calculations suggest that there are over 600 undiscovered perovskites [63]. Unfortunately, some of the promising metal cations include Cd and Hg, which would be of no improvement over Pb in terms of toxicity. Also, it remains to be seen how many of these could possibly be synthesized. Successful synthesis of a perovskite would require extensive exploration of a very wide range of experimental conditions for each material combination. A number of parameters to be considered in the perovskite synthesis make exploration of novel lead-free materials difficult and time-intensive, and in many cases may not be feasible realistically. Some possible methods, such as droplet based microfluidic platform parametric space mapping approach recently proposed for the synthesis of CsPbX3 (X ¼ Cl, Br, I) nano crystals [64], may allow faster exploration of possible combinations but it is still very time and labor intensive process to examine alternative materials. Theoretical calculations may possibly offer the guidelines on narrowing down possible material combinations since narrow band 5

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Fig. 3. Illustration of different perovskite deposition methods a) solution-based one step method b) co-evaporation c) solution-based two-step method d) sequential evaporation e) vapor-assisted solution process (VASP).

gaps are more desirable for photovoltaic applications. Relationships between organic cation size and the band gap is complex and dependent on metal cation, so that different variations are obtained for Pb and Sn iodides for example [60]. Nevertheless, it is possible to perform calculations of electronic structures of various possible perovskites, and this information would be helpful in identifying on which materials should synthesis efforts be concentrated. Due to difficulties associated with sensitivity of perovskite formation to the experimental conditions and large number of experimental variables, both theoretical calculations to narrow down material choices and experimental procedures enabling faster exploration of the parameter space are of significant interest for the development of novel materials. 6

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Fig. 4. SEM images of PbI2 (left) and perovskite films (right) prepared with different volume of covering vessel a) 20 mL beaker b) 50 mL beaker c) Petri dish 100 mL volume d) 500 mL beaker e) 1000 mL beaker f) no cover. Reprinted with permission from Ref. [97]. 7

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3.2. Deposition of perovskite layer The deposition methods for the perovskite layer can be broadly classified into two groups, more commonly used solution deposition methods and less commonly used vapour deposition methods [27]. For solution deposition, common methods include one step deposition where both precursors are mixed in the same solution, and sequential deposition where one precursor is deposited first, followed by the other (either by immersion into precursor solution or spin-coating the other precursor) [27], as illustrated in Fig. 3. Vapor deposition methods include co-deposition, sequential deposition, and vapor-assisted solution process (VASP) [27], as illustrated in Fig. 3. Vapor-deposition methods have an advantage of being highly controllable, but the solution processing has an advantage of possibility of low cost manufacturing on large area substrates. Nevertheless, while one of the often mentioned disadvantages of vacuum deposition is the need for vacuum equipment and hence higher cost, it should be noted that if solution processing needs to be done in an inert atmosphere there would be a significant equipment cost increase for up-scaled production. Vacuum deposition can be performed as co-deposition [31,65] or sequential deposition [66,67]. In the case of co-deposition, independent thickness/rate quartz sensors and multichannel thickness monitor or co-deposition controller is needed. In the case of sequential deposition, deposited layers can be annealed after deposition [66] or deposited at elevated temperature [67]. Closely related to sequential vapor deposition are the vapor assisted solution process (VASP), where metal halide film is deposited by a solution process while the organic halide precursor is evaporated [68–71] and chemical vapor deposition (CVD) method, where metal halide film can be thermally evaporated, but the organic precursor is evaporated in a tube furnace, either in nitrogen flow or in air [72,73] or both precursors are loaded in a tube furnace [74]. Solution processing of the perovskite solar cells can be performed as a one-step [75,76] or two-step deposition method [18,33,77–79]. Sequential deposition or two-step method was initially proposed for better control over the perovskite morphology compared to a one-step method [33]. However, efficient PSCs have been reported for both one-step and two-step depositions. Similar efficiencies for the two types of deposition methods have been reported by different groups, and in some cases similar efficiencies were demonstrated in the same report [80]. The deposition of the perovskite layer is typically very sensitive to the deposition conditions. It has been pointed out that the interaction between precursors and solvents used is very complex [81]. Attempts have been made to improve understanding of the nucleation kinetics, intermediates, transformation pathways and structure-property relationships of perovskite films [82]. Nevertheless, considerable work is still needed to fully understand the influence of different factors in the deposition process on the resulting perovskite film properties. Consequently, a number of methods has been proposed to improve the quality of the perovskite layer. For example, ionic liquids have been introduced to control the crystallisation of the perovskite and achieve large grains and consequently high PSC efficiency of 19.5% (18.8% average of 7 devices) [20]. In general, different solution additives have also been proposed to improve the perovskite layer quality and photovoltaic performance, with varying degrees of improvement and final efficiency [19,45,80,83–89]. In some cases, inclusion of an additive enabled elimination of the annealing step, although achieved efficiency was not impressive [85]. Perovskite film deposition from solution process is notoriously sensitive to very small details of the experimental procedure and there is a huge variability in material properties from one report to another [53]. Thus, it is unfortunately common that procedures that lead to improvements in performance in one laboratory fail to give improved performance in a different laboratory. This is mainly due to the fact that the solvent used and its evaporation and consequent transformation of the coated film into solid perovskite were found to play a critical role in the quality of PSC layer and consequently the obtained efficiency. For example, it was found that the solvent used affected crystalline orientation and the grain size of the perovskite films [90]. Solvent engineering with different solvent combinations has been proposed as a method for improving the perovskite layer deposition [23,75,91–94]. Different combinations of solvents were proposed for solvent engineering, such as DMF and DMSO with diethyl ether dripping/rinsing [23], gamma-butyrolactone (GBL) and DMSO with toluene dripping/rinsing [75], diethyl ether dripping during spin-coating of organic halide precursor in isopropyl alcohol onto a metal iodide thin film prepared from DMF/DMSO [93], etc. The principle of the method is to form a well-defined intermediate phase (DMSO-PbI2-MAI adduct) which can be controllably transformed into high quality, pinhole free perovskite film [23]. Solvent engineering is also useful for perovskite single crystal growth [95] and organic charge transport layer deposition [96]. The importance of solvent evaporation rate in the formation of perovskite film is really critical. Both solvent engineering, some of the solution additives, as well as modifications of drying method are trying to address this point and improve the control over the film formation and the reproducibility of film properties. It has been shown that the small steps in the film preparation affecting the evaporation rate of the solvent were found to significantly affect the device performance. For example, covering the lead iodide film during drying was found to result in the formation of pinholes in the perovskite film, as illustrated in Fig. 4 [97]. This has significant implications on the reproducibility of the reported work in other laboratories if the process is sensitive to such small details whether the sample is covered by a Petri dish during drying or not. In addition to solvent engineering and solution processing additives, various other treatments and process modifications have also been proposed to obtain a high quality perovskite layer, such as the use of different precursors [98], the treatment of transparent polymer electrode with dimethylsulphoxide [99], surface passivation with an amine-polymer which functions as a surfactant and crystallization promoter [100], the introduction of additives in CVD growth [101], the control of the crystallization by the choice of hole transport material or underlayer [78,102], etc. Furthermore, processing (including annealing as post-treatment) in ambient and/or controlled humidity was proposed to prepare improved quality perovskite films and/or high performance PSCs [22,35,68,80,103–111]. It was reported that exposure to humidity leads to self-healing of the perovskite lattice due to partial solvation of MA [103] and large grain size of the perovskite film [105,107]. It 8

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was noted however that the best results were obtained at ambient humidities below 30% and that high humidity worsens the substrate coverage [103,107]. Nevertheless, there are reports that relative humidity of 40% results in optimum device performance [108]. Thus, the optimum humidity likely depends on the details of processing conditions as well as the composition of the perovskite material. For example, it was reported that high performance CH3NH3PbI3x(SCN)x based PSCs could be prepared at ambient humidities exceeding 70% [109]. Despite high humidity, best efficiency exceeding 15% was achieved [109]. However, it was also reported that the efficiency of blade-coated devices fabricated in ambient was strongly humidity dependent [112], with films prepared at 15–25% RH, 40–50% RH, and 60–70% RH having efficiencies of 10.44%, 3.63%, and 0.35%, respectively. However, it should be noted that despite initial efficiency increase the humidity exposure may have some concerns for the long term stability in particular under illumination. Degradation was observed for devices processed in ambient after 150 days of storage in glove box, and PCE decreased from over 12% to 9.91% [104]. In addition, while improved stability was obtained for CH3NH3PbI3x(SCN)x compared to MAPI devices and 87% of initial efficiency was obtained after over 500 h of storage of unencapsulated devices in ambient air at 70% RH [109], there is no guarantee that the cells would be stable under illumination. While improved film morphology and large grain sizes may result in smaller moisture diffusion, any residual moisture would result in a rapid degradation of perovskite under illumination. In addition to commonly used solution-based deposition (one- or two-step) and vapour deposition, other deposition methods have also been reported. For example, a three-step solution process was proposed as an improvement over a two-step deposition, but all the devices were low efficiency and thus it is likely unnecessary for high performance devices [41]. Similarly, gas-blowing during spincoating was found to affect the performance, but devices with and especially without gas blowing had low efficiency [113]. Nevertheless, high efficiency devices (17% best efficiency) have also been reported for gas-blowing during spin-coating, which was attributed to improved film morphology by changing the kinetics of grain nucleation and growth [114]. While this technique is not necessary for obtaining films consisting of densely packed grains, it can be potentially beneficial for improving the film morphology. In addition, a vapor-assisted chemical bath deposition method was proposed to prepare PSCs on large and/or curved substrates [115]. Even though compact films and full conversion of PbS to MAPI was demonstrated, obtained efficiencies were not high (4.68%) and the proposed reaction releases H2S gas [115]. Nevertheless, further modification and/or optimisation of the method may yield improved results. Modifications of common deposition methods of MAI precursor have also been proposed. For example, spray-assisted deposition in a two step method (for MAI solution) was also reported [38]. There have also been promising alternative deposition methods which resulted in devices with good performance. For example, direct contact and intercalation method was also proposed for preparation of the perovskite films, which involved bringing spin-coated PbI2 films in direct contact with methyl ammonium iodide powder, followed by annealing in a closed container and washing away the excess methyl ammonium iodide powder with isopropanol [116]. This method could result in a large grain size of the perovskite film for  optimised reaction temperature, with the largest grain sizes obtained at 200 C [117]. Furthermore, vacuum flash-assisted film formation method, where solvent is rapidly removed from a spin-coated perovskite solution, was reported to enable the formation of large area uniform films with high crystallinity and consequently excellent PCE (20.5%) [26]. Large grain sizes (mm size grains) were also reported for other deposition methods, such as hot-casting technique using solvents with a high boiling point [118]. The obtained cell efficiencies were close to 18% [118]. Somewhat related method for improving the perovskite film quality is gas pump drying method, reported for devices prepared in ambient air on flexible substrates [119]. In addition, spray deposition under ambient conditions was also reported [105]. Solvent assisted gel-printing and micropatterning of the perovskite films was also demonstrated [120]. Finally, different annealing processes have also been proposed to improve the device performance, such as two-step annealing [121], vacuum-assisted annealing [111,122], etc. Exposure to moisture at room temperature instead of annealing was also proposed to induce crystallisation of perovskite [110]. In addition, solvent annealing of PSCs was also demonstrated to result in high quality perovskite films and/or high performance devices [123–125]. Finally, laser crystallization using an infrared laser was proposed for the PSC fabrication [126]. 3.3. Electron transport layers Electron transport layer (ETL) in PSCs can be organic or inorganic. Deposition methods of an oxide ETL obviously affect its properties, and consequently solar cell performance as well as degree of hysteresis observed. Progress in electron transport layers in PSCs has been recently reviewed [127]. In addition, an overview of deposition methods and different device structures with various oxide layers has been recently reported [27]. The ETLs can be organic or inorganic. Inorganic charge transport layers are commonly n-type oxides, although other materials have also been reported. Devices with oxide ETLs can either be planar, or nano structured with messcopic oxide layer (which can also be an insulating material such as alumina [2]). Mesoscopic devices were proposed first, following the architecture of solid state dye-sensitised solar cells [1]. Planar devices represent a simpler alternative to mesoscopic devices and are more likely to be compatible with flexible substrates since the deposition temperature (depending on the material) can be lower. In addition to commonly used planar and mesoscopic metal oxide layers, nano structured arrays could also be used as an ETL in perovskite solar cells [128]. Use of nano structured morphologies other than mesoscopic nano particle layers has been less common compared to both planar and mesoscopic devices. The main attraction of nano structured architecture is improved charge collection. Nanorods commonly result in higher short-circuit current densities due to improved charge collection [129,130]. However, nanorod devices may require very careful optimisation in order to prevent deterioration of FF while keeping the advantage of increased Jsc , i.e. there should be a perovskite overlayer preventing direct contact and increased recombination between ETL and HTL [130]. In addition to nanorods, other nano structured architectures have also been reported, such as SnO2/TiO2 nano wires [131], TiO2 nanobowl arrays [132], Au-nanoparticle embedded TiO2 nanofibers [133], TiO2 nano wires and nano sheets [134], flower-like array [135] etc. 9

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In terms of material choice, TiO2 remains the most popular inorganic ETL and it has resulted in the highest reported efficiencies [127]. It has several drawbacks, namely the need for higher deposition temperature in particular if mesoporous layer is used, its photocatalytic activity which can contribute to the sensitiser degradation, and relatively inefficient electron extraction which can lead to significant hysteresis. As a commonly used ETL, it has been extensively studied. Efforts have been made to optimise the TiO2 ETL and thus the effects of deposition methods and titania morphology have been studied [134,136,137]. The performance of the PSC cells is dependent on the thickness and deposition method for the titania blocking layer [30]. It was proposed that the optimal thickness is 20 nm, and the layer should be deposited by methods allowing conformal coating, such as ALD or sputtering [30]. It should be noted however that the optimal thickness may be dependent on the deposition method. Furthermore, solution processing of the ETL is nevertheless of significant interest due to potential for low cost, large area devices. In addition to studying the effect of layer morphology, thickness and deposition method, different surface modifications have been employed to improve the efficiency and/or reduce hysteresis in TiO2-based PSCs [138–140]. Despite high efficiencies of cells with titania ETL and the development of methods for its low temperature deposition, such as low temperature atomic layer deposition (ALD) [141,142] or low temperature solution growth [135], there has been considerable interest in the development of an alternative ETL which could be processed at lower temperatures. Due to its similar band gap and high electron conductivity and ease of deposition at low temperatures, ZnO has been an obvious candidate. It can be readily deposited using low temperature, solution based, fast deposition techniques [143]. However, the efficiency and/or stability of devices using ZnO ETL is generally inferior compared to those on TiO2 [7,144,145]. The perovskite films deposited on ZnO could not withstand the same temperature as those deposited on TiO2 [7,144]. The thermal instability of the perovskite deposited on ZnO is attributed to the protontransfer induced decomposition of methyl ammonium caused by the basic nature of ZnO surface, which is further compounded by the residual surface adsorbates on the ZnO surface [146]. This problem could be addressed by the deposition of interfacial layers, which can improve both the efficiency and the stability [147]. Processing of ZnO layer can also affect the PSC performance, and it was reported that ageing of ZnO in ambient results in improved performance [148]. However, the best achieved efficiencies of devices with ZnO ETL are typically below 16% [147], and their stability is typically inferior compared to devices with titania ETL. Thus, despite advantages in low temperature deposition of ZnO, it is likely that this material does not necessarily represent a better candidate for ETL compared to TiO2. In addition to TiO2 and ZnO, in recent years the use of SnO2 as ETL has been gaining popularity [20,21,149–155]. High efficiency (over 20%) and stability was recently reported in devices with SnO2 [21]. Optimization of SnO2 deposition was shown to lead to increased efficiency and reduced hysteresis in planar devices [21], which makes this material a promising alternative to TiO2. In recent years, there have also been other reports of high efficiency devices, also having low hysteresis, based on SnO2 [149,155]. Efficiencies exceeding 18% were obtained with ALD-deposited SnO2 layer [20,149], as well as plasma-enhanced ALD [150]. In addition, Li doping could be used to improve the properties of SnO2 and achieve efficiency exceeding 18% [156]. High certified efficiency of 19.9% ± 0.6% was also reported in a device with SnO2 nano particle ETL processed at a low temperature (150 C) [155]. This indicates that SnO2 represents a highly promising alternative to titania as an ETL in PSCs. Its additional advantage is that unlike TiO2 and ZnO, it is not an efficient photocatalyst, and thus it is not expected to accelerate the perovskite degradation under illumination. Finally, in addition to commonly used oxides such as TiO2, ZnO, and SnO2, other n-type inorganic semiconductors have also been proposed as ETLs in PSCs. These include CdS [157], In2O3 [158], Zn2SnO4 [159], WO3/TiO2 [160], etc. However, these materials typically resulted in inferior efficiencies compared to SnO2. Another possible solution for making PSCs processable at low temperature is to use an organic ETL. The most commonly used organic ETLs in perovskite solar cells are fullerene derivatives, such as PCBM [127]. It has an advantage of low temperature processing and devices with PCBM typically exhibit less hysteresis. Other organic molecules have also been used as ETLs in PSCs, although less commonly compared to PCBM. Nevertheless, there have been several organic ETL alternatives to PCBM proposed. For example, efficiency of 17.1% has been achieved in a PSC with a coronene based ETL [161]. Even higher efficiency, 17.66% was obtained for aminosubstituted perylene diimide derivative [162]. Other organic ETLs used include C60 [24,163] as well as fullerene derivatives different from PCBM [124], poly(9,9-diocylfluorene-co-benzothiazole) (F8BT) [164], F16 CuPc [165], and an amino-functionalized copolymer [166]. In addition to purely organic or inorganic ETLs, bilayer fullerene/oxide ETLs have also been reported. For example, a combination ETL consisting of PCBM and ZnO was reported [167]. While this combination exhibited better performance compared to individual PCBM and ZnO layers in that report, the reported efficiencies were much lower than those reported by others for the same ETLs and thus there is no guarantee that combining the two layers could result in an improvement in efficiencies in already optimised devices with individual PCBM or ZnO ETLs. On the other hand, PCBM was demonstrated to passivate defects in In2O3 and improve cell efficiency [158]. Thus, combination of a metal oxide and PCBM could possibly perform better than a metal oxide (different from optimised titania or tin oxide) alone, but the obtained efficiency was lower compared to fully optimised PCBM based cells. However, high efficiency of 17.7% was obtained in the case of PCBM/Zn2SnO4 bilayer [159]. Improvements in performance were also reported for SnO2 bilayers with PCBM or C60 [152,168]. Although an improvement in the efficiency and reduction in hysteresis was obtained, which was attributed to the PCBM passivation of traps in SnO2, the highest obtained efficiency of 19.12% still lags behind the best obtained result for SnO2 alone [21]. Thus, high efficiencies could be obtained by optimisation of the oxide deposition, or by using bilayers. From the point of view of simplicity of the deposition procedure, a single layer ETL would be preferred. However, further studies are needed to examine whether bilayer ETLs would offer advantages in terms of stability. Finally, it should be noted that devices without an ETL have also been proposed, as a possible alternative towards simplified device production by roll-to-roll processing on flexible substrates [169] or recyclable FTO/glass substrates [170]. An efficiency of 12.70% was achieved for a device with a FA-based perovskite layer directly deposited on ITO [169]. In addition, metal-insulator-semiconductor structures were also proposed as an ETL-free alternative for flexible devices [171]. Although the simplicity of the device and 10

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compatibility with roll-to-roll processing are an advantage for ETL-free devices, the lower efficiencies are a significant drawback. Among the alternatives to commonly used TiO2 and PCBM ETLs, at present SnO2 appears to be the most promising candidate due to lower processing temperature compatible with flexible substrates, low hysteresis, good stability and high efficiency devices, with the best reported PCE value to date exceeding 20% [21]. 3.4. Hole transport layers Similar to ETLs, HTLs can be organic or inorganic. A number of materials can be used for this purpose, and small molecule materials for HTL have been recently reviewed [172]. The most commonly used HTLs are organic materials, Spiro-OMeTAD and PEDOT:PSS. Efforts have been made to improve the performance of these two commonly used materials. For example, it was shown that the addition of imidazole to PEDOT:PSS alters its pH, and improves its electronic properties as well as the quality of the perovskite layer, resulting in higher efficiency and higher stability [173]. It should be noted, however, that this refers to stability tests involving unencapsulated cells stored in the dark in ambient with 20% RH [173]. In the case of Spiro-OMeTAD, main efforts in performance improvements target the replacement of common dopants such as TBP which degrades the perovskite and LiTFSI which absorbs moisture [174]. Various replacement dopants have been used, such as copper salts [175], Co salts such as FK 102 dopants [32,33], Pd nano sheets [176], as well as other organic dopants [17,177]. Another commonly used HTL material in high efficiency devices is polytriarylamine (PTAA) [18]. While PEDOT:PSS remains the most commonly used HTL in devices with organic charge transport layers and Spiro-OMeTAD is the most commonly used in devices with oxide ETL, other organic materials have been proposed for the use in PSC [151,174,178–188], as well as carbon nanotube/polymer composites [189] and reduced graphene oxide (rGO) [190] and nitrogen doped GO nano ribbons [191]. Common reasoning for the use of other organic materials includes the fact that PEDOT:PSS is acidic and hygroscopic, while additives to spiro-OMeTAD may contribute to reduced PSC stability. Nevertheless, despite a large number of materials reported to date, none of them truly stands out as an obvious candidate for replacement of the commonly used materials in high performance devices. While the majority of the reported HTL have been organic materials, inorganic HTLs have also been used and they are of significant interest for possible stability improvements. It should be noted however that the use of inorganic HTLs can result in potential difficulties due to the layer deposition process. In a conventional architecture with oxide HTL on top of perovskite, it is necessary to consider deposition temperature limitations, issues associated with using oxidants or water (for example in ALD precursors), and solvents for solution processed materials. However, these problems could be avoided in inverted device architecture with oxide HTL deposited directly on the substrate. For example, perovskite solar cells with Cu:NiO HTL prepared by solution processing have been reported, with the obtained PCE of
15% [192]. NiO, with or without dopants, is a common choice for a p-type metal oxide [192–198]. Surface modifications of NiO have also been reported [198]. Depending on the deposition method, NiO can also be compatible with the use of flexible substrates [195,196]. Other inorganic material choices include CuSCN [199], CuI [200], Cu2ZnSnS4 [201], Cu2O [202], FeS2 (octadecylamine-capped) [203], etc. Similarly to devices without ETL, devices without HTL have been reported [77,204–213], which also included TiO2/perovskite (iodide)/perovskite(bromide) device [205]. As expected, lower efficiencies have been obtained. For devices consisting of compact and mesoporous titania, MAPI and a gold electrode, the best efficiency of 10.85% was obtained in fully optimised devices (7.7% on average) [204]. Nevertheless, higher efficiencies can still be achieved. In a device with mixed cation (5-ammoniumvaleric acid (5-AVA) and MA) lead iodide, titania, mesoporous zirconia and carbon counter electrode a certified PCE of 12.8% was reported, and the devices also exhibited good stability [210]. Also, efficiency as high as 14.58% was reported for a device consisting of compact and mesoporous titania, MAPI and carbon electrode with an optimised deposition process for MAPI films involving solvent engineering [213]. 3.5. Interfacial and/or barrier layers The use of interfacial layers to improve stability has been reported in different device architectures. In addition, they can serve the function of adjusting the energy level alignment at interfaces and thus improving the charge collection and/or reducing recombination, and possibly improving the film quality for films deposited on top of the interfacial layer (which can include modifications to address the wettability issues of subsequently deposited layers [214]). Basic physical properties of the interfaces in PSCs have been reviewed recently [215]. A number of different materials has been used as interfacial and/or barrier layers, including both inorganic and organic materials. The use of molecular materials as interfacial layers has been recently reviewed [216]. This type of interfacial layers are particularly suitable for the purpose of adjusting the work function and energy level alignment at the electrode interface. For example, triazine-based molecule as an electrode interlayer was demonstrated to improve the performance of a variety of devices, including PSCs [217]. Interfacial layers can also be used to improve other electrode properties in addition to work function. Ultrathin Ag electrodes (8 nm) with low resistance of 6 Ω/sq were obtained using thiol-functionalized self-assembled monolayers [218]. The best obtained efficiency (using zirconium oxide antireflective coating) was close to 15%, comparable to devices with indium tin oxide (ITO) electrode [218]. Interlayers can also serve the purpose of protecting the top metal contacts. Chromium oxide/chromium interlayer was proposed for this purpose [99]. Inorganic interfacial layers have been used in devices with different architecture mainly to improve the stability. Among inorganic materials, the most commonly used material is alumina, while there have been reports on other materials, such as montmorillonite for preventing the degradation of perovskite by TBP dopant commonly added to Spiro-OMeTAD [219]. Devices with ZnO ETL are typically less stable compared to those with TiO2 ETL. However, it has been shown that the insertion of alumina interfacial layer at ZnO/perovskite interface can improve the efficiency and stability of ZnO-based solar cells [147]. In addition, it has been shown that insertion of a thin ALD-grown alumina between the HTL and Ag electrode significantly improved stability of perovskite solar cells [220]. Effective 11

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alumina buffer layers could also be prepared from colloidal dispersions of alumina nano particles [221]. Similarly, an improvement in the stability was obtained by inserting alumina layer between the perovskite and HTL, with alumina prepared from a precursor solution [222,223]. In addition to beneficial effects of alumina interfacial layers on the stability, improvements have also been demonstrated with Sb2S3 interfacial layer inserted between titania and the perovskite [224]. In addition, graphene-based materials can be used as interlayers in PSCs, as well as charge transport materials and/or electrodes. Detailed review on the use of graphene-based materials in PSCs has been published recently [225]. 3.6. Electrodes Bottom electrode of a PSC is typically transparent conductive oxide, either fluorine doped tin oxide (FTO) for devices with TiO2 ETL processed at high temperature, or ITO for devices processed at lower temperatures. In addition, metal oxide free electrodes have been explored for flexible devices. These include graphene [226], as well as metal/graphene based electrodes, such as silver nanowire/ graphene oxide (GO) flake composite electrode [227]. Similar examples of metal-oxide free electrodes for flexible devices will be discussed in more detail in Subsection 4.4. The top electrode is commonly metal, with the exception of devices prepared on nontransparent substrates such as metal foils and/or tandem devices. Thus, transparent top electrodes will be discussed in more detail in the Subsection 5.1, while here we will mainly discuss the top metal electrodes for standard device configurations. It is well known that commonly used metal electrodes such as Ag, Al, and Au can react with the perovskite material [193,228]. However, it was reported that devices with Cu electrodes exhibited stable operation (98% of initial efficiency) after over 800 h storage in ambient without encapsulation [228]. On the other hand, Al electrode even in the presence of MoOx interlayer rapidly degrades (damage visible by naked eye) after a single measurement of performance under simulated solar illumination in ambient at
60% RH [7]. In addition to metal electrodes, carbon electrodes have also been proposed for PSCs. They have the advantage of being stable, and they are low cost and readily processable by inexpensive methods. However, in a number of cases efficiencies of cells with carbon electrodes are below 10% [207–209,229–231], with rare exceptions [91,210,212,232,233]. However, efficiency as high as 16.1% could be obtained with printable low temperature processed carbon electrode in fully optimised devices which include copper phthalocyanine nano rod HTL [212]. Higher efficiencies could also be obtained in devices using titania/zirconia layers [233] as well as by optimisation of the deposition process of perovskite layer by solvent engineering [91]. Although a number of reports involve cells with lower efficiency, higher efficiencies are possible. Therefore, the improved stability upon ambient exposure and an obvious advantage of simple fabrication at low temperature such as doctor-blading [207,209] still make carbon-based electrodes and attractive option for PSCs, in particular for upscaling to large area, printable devices on flexible substrates. However, while the technique is suitable for large area devices, the higher resistivity of a carbon electrode compared to a metal electrode would have even larger effect in large area devices. One possible solution to improve the efficiency of carbon based electrodes would be to use carbon composites or add a metal grid, using Cu to avoid detrimental effects on stability. 4. Main problems of perovskite solar cells We will discuss several of the significant problems of perovskite solar cells, namely stability, hysteresis, environmental concerns, and scaling up to large area devices. Among these issues, the most detailed discussion will be devoted to the issue of stability of PSCs as the most significant concern for their practical application. 4.1. Stability Stability has been identified as one of the significant problems of perovskite materials and consequently perovskite solar cells [234]. In discussing the stability of the perovskite materials and devices, it is necessary to consider the effects of temperature, illumination and ambient (oxygen, moisture) exposure. There have been several reviews of this important issue [57,235–241]. In comparison of stability of PSCs compared to organic photovoltaics and dye sensitized solar cells, it was found that PSCs likely have inferior stability but also that more data are needed, especially under standardized testing conditions in particular under illumination [239]. We will first discuss the degradation mechanisms in PSCs, followed by brief summary of approaches to mitigate the degradation, and finally summarise the reported achievements in stability under illumination and/or outdoors. While there are a number of reports on the stability of PSCs with storage in the dark, often without encapsulation, these cannot be considered representative estimate of the device lifetime under realistic conditions due to significant effect of illumination on accelerating the degradation of PSCs. Thus, we will provide an overview of general strategies for improving stability, and also discuss encapsulation details and reported stability tests under illumination and/or outdoor conditions. While accelerated aging tests under constant illumination would likely underestimate the device lifetime, they would still represent a more realistic estimate compared to dark storage. 4.1.1. Degradation mechanisms The degradation of the device can occur due to the degradation of the active layer, degradation of charge transport layers, or the degradation of electrodes. We will discuss the active layer first. Let us first consider the thermal stability of the perovskite films. It was shown that the elevated temperature accelerates decomposition of MAPI in vacuum [235]. Degradation can also occur due to exposure to X-rays or electron beam [235]. It was proposed that MAPI films are intrinsically unstable and that they decompose at temperatures   exceeding 85 C even in inert atmosphere [242] or 100 C [243]. In another work, irreversible changes are found to occur at temperatures  above 50 C [244]. Nevertheless, there have been accelerated ageing reports of perovskite solar cells. This indicates the possibility that 12

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the film thermal stability may be dependent on the film preparation technique and film properties, and conclusions derived from cells with moderate performance (efficiencies below 12%) may not necessarily apply to cells with good performance due to different crystallinity of the films, presence of defects etc. The other factor which contributes to degradation is the exposure to ambient air. It is generally considered that oxygen exposure alone does not result in the decomposition of the perovskite film in the absence of moisture [97]. Exposure to oxygen can also have beneficial effect on the spiro-OMeTAD HTL [79,245]. However, it was proposed that the perovskite film on mesoporous alumina can degrade via reaction induced by superoxide ions generated by electron transfer from photoexcited perovskite to molecular oxygen [246]. This effect was found to be reduced when titania was used instead of alumina, due to reduced generation of superoxide ions [246]. Thus, the effect of oxygen cannot be entirely excluded (and it is likely dependent on the properties of ETL used), but in any case it is expected that it will be less significant compared to effects of moisture. It has been demonstrated that the device degradation is humidity dependent and very rapid at high humidity [247,248]. The degradation mechanism of MAPI upon exposure to moisture in absence of illumination involves the formation of hydrate form, which can be reversible [235,247,249,250]. However, continued exposure to moisture and/or exposure to illumination leads to the irreversible degradation to PbI2 [235]. In addition, the water infiltration into perovskite is very fast and it occurs readily at time scale of seconds and at low humidity (10% RH) where the formation of hydrates is not yet detectable [251]. Prolonged exposure to humidity and/or exposure to very high humidity levels leads to irreversible degradation of the perovskite. Volatile degradation products are considered to be NH3 and HI [235], although there has been some disagreement in the literature concerning actual degradation reactions. It was proposed that MAPI degrades into PbI2 and MAI in the presence of water, followed by decomposition of MAI into CH3NH2 and HI [237]. HI could then either react with oxygen and produce iodine and water, or photochemically decompose in the presence of UV illumination into H2 and I2 [223,237]. Thus, commonly proposed degradation of MAPI involved decomposition to CH3NH2 and HI [97,223,237]. However, it was shown recently that thermal decomposition resulted in release of CH3I and NH3 gases using a mass spectrometer connected to thermal gravimetric and differential thermal analysis system [252]. Based on XPS analysis of the perovskite degradation, it was proposed that MAPI decomposed to lead carbonate, lead hydroxide, and lead oxide [253]. Lead iodide PbIx 2þx was proposed to be a transient structure, and lead carbonate, lead hydroxide and lead oxide to be amorphous due to the absence of corresponding peaks in the XRD patterns [253]. While XRD would indeed detect only crystalline phases present, it should be noted that exposure to vacuum and/or

Fig. 5. Average power conversion efficiency of devices with (S1) and without (S3) excess PbI2 in ambient air at 60% RH as a function of time a) for alternating  simulated solar illumination 10 h/dark 14 h cycles at room temperature, and b) at 85 C and under constant simulated solar illumination. Reprinted with permission from Ref. [97]. 13

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Fig. 6. Stability of the devices in an ambient environment without encapsulation. Device performances of ITO/PEDOT:PSS/perovskite/PCBM/Al (black) and ITO/  NiOx/perovskite/ZnO/Al (red) structures as a function of storage time in an ambient environment (30–50% humidity, T ¼ 25 C). a, Normalized PCE. b, Normalized VOC. c, Normalized JSC. d, Normalized FF. Reprinted with permission from Macmillan Publishers Ltd: Nature Nanotechnology [193], copyright 2016. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

X-rays could result in loss of volatile components and/or additional degradation and requires cautious interpretation of the obtained results. Thus, exact nature of chemical reactions during perovskite degradation under realistic operating conditions is still unresolved, since that involves illumination, as well as trace ambient gases exposure and increased temperature. In situ TEM study of degradation of PSCs has demonstrated that perovskite degrades at modest temperatures of 50–60oC under illumination [254]. It was also proposed that oxygen in ambient air likely reacts with methyl ammonium [254]. Some damage due to high vacuum and electron beam was served, but the authors argued that this damage was not significant since no further damage occurred for 30 days in high vacuum after initial formation of Pb particles [254]. However, in several works, no significant degradation of the perovskite film upon exposure to oxygen has been observed, but there have also been reports that the degradation rate of solar cells was dependent on the oxygen concentration [235]. Thus, it should be noted that the device performance is dependent on the degradation of all the layers in the device, not only the perovskite. Furthermore, in mixed halide devices effect of phase segregation on the device stability should be considered [255–258]. 4.1.2. Strategies for improving stability There are several strategies that can be pursued to improve the PSC stability. It should be noted, however, that in a number of these strategies the films or devices were not exposed to multiple stressors (humidity, UV illumination, elevated temperature) at the same time. Thus, it is difficult to extrapolate how significant would those improvements be under more rigorous testing conditions compared to commonly reported storage in ambient without illumination. One strategy is to alter the chemical structure of the perovskite, since MAPI in general has relatively poor stability. For example, it has been shown that MAPbBr3 is more stable compared to MAPI [235]. Stability could also be modified by doping the perovskite, for example with MABr [259]. On the other hand, it was reported that while MAPBr3 was stable, MAPI and mixed halide (iodide and bromide) were not [260]. Accelerated degradation in the mixed halide was attributed to larger defect density [260]. Thus, this also needs to be taken into account and likely composition and morphology both need to be optimised for stable films and devices. Mixed cation perovskites may also result in improved stability [15,210]. In addition, the replacement of an organic ligand such as MA with an alkali metal such as Cs or Rb [57] or Csþ incorporation [261] are also expected to improve stability, although the band gap in case of Cs is too wide for effective photovoltaic applications. In addition, improved stability was reported for CH3NH3PbI3x(SCN)x compared to MAPI [109]. Furthermore, layered perovskites typically exhibit better stability but lower efficiency compared to 3D perovskites [61,262]. The second strategy is to optimise the deposition and properties of the perovskite film. The crystallinity and coverage were found to affect the stability of perovskite films and devices [263]. Stability of the perovskite devices with storage time in air was also dependent on the perovskite deposition process, with blade-coated films more stable compared to spin-coated ones [263]. This is likely due to the 14

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fact that the film morphology (grain size, defects in grain boundaries) affects the diffusion of oxygen and moisture. It was also demonstrated that the stability of perovskite films is grain size dependent, with films with larger grain sizes demonstrating better stability [264]. Therefore, various strategies in film deposition can be used to achieve the desired results. For example, solvent engineering was shown to result in improved stability of the perovskite films at elevated temperature and low humidity (20–30%) stored in the dark [93]. Also, improved moisture resistance for storage of unencapsulated devices in air was reported for perovskite films prepared with phosphonic acid ammonium additive, which was proposed to crosslink neighbouring grains in the perovskite film [89]. Counterintuitively, processing in ambient or controlled humidity environment [22,265], as well as the addition of water-soluble additives such as polyvinyl alcohol (PVA) [266] or even water itself [267], have been proposed to improve the efficiency and stability of PSCs. It was proposed that the addition of PVA improves the quality of perovskite film and consequently enhances the efficiency as well as the stability upon storage without encapsulation in a high humidity environment (90% RH) for 30 days [266]. Likely in all these reports improved film quality obtained result in improved stability due to slower diffusion of moisture into the film. It should be noted however that the storage was in the dark, and it is unclear whether the observed stability improvement would hold under illumination. In general, illumination significantly accelerates the degradation of perovskite films and PSCs, and thus the dark storage stability tests likely overestimate the stability of the devices. In addition to grain size, crystallinity and coverage other factors may also affect the stability, such as the presence of residual PbI2. It has been demonstrated that the presence of residual PbI2 can contribute to the degradation of the perovskite under illumination even in inert atmosphere [97]. This was attributed to the intrinsic instability of PbI2 under illumination [97]. Despite inferior stability of unencapsulated perovskite film with PbI2 residue compared to PbI2 free-film, the device stability was actually improved, as shown in Fig. 5 [97]. This indicates that the entire device degradation is a combination of a number of different processes and that it may not necessarily follow the same trends as the degradation of the perovskite film alone [97]. Negative effects of PbI2 residue on the PSC stability have also been confirmed in another study with perovskite layer prepared by a different method [88]. The improved stability in the absence of PbI2 was attributed to the lack of voids and/or pinholes and thus lower moisture penetration [88]. This again illustrates the importance of the film morphology on its stability. In addition to the modifications of the active layer, another strategy involves altering the device architecture by changing the charge transport layers, inserting interface/barrier layers, as well as changing the electrodes. Buffer layers have been shown to significantly improve the stability of perovskite solar cells [221], and we have already discussed various buffer/interface layers. Alumina is one example of a barrier layer [223]. In addition to the use of additional layers, common method of modification of device architecture is the replacement of the charge transport layers. This can improve the device stability in two ways, one is via improved stability of the charge transport layer itself, while the other is increased stability of the active layer due to the reduction of moisture diffusion or diffusion of other components (dopants for example) into the active layer. It was found that the HTL significantly affects the stability of a PSC [250]. For example, the replacement of both spiro-OMeTAD as an HTL with copper phthalocyanine nano rods and Au as an electrode with carbon electrode resulted in considerable improvement in the device stability [212]. It should be noted however that the devices were relatively small (cell area was 0.12–0.16 cm2 and aperture area was
0.09 cm2) [212], and that for larger cells electrode conductivity may be an issue. In general, replacing the commonly used spiro-OMeTAD with other organic materials, including hydrophobic polymers, has been proposed to improve the stability of the perovskite solar cells [178–180]. It should also be noted that dopants used in spiro-OMeTAD can affect the stability, as previously discussed. In addition to spiro-OMeTAD, common HTL used in PSCs, in particular those processed at low temperature, is PEDOT:PSS. However, this polymer is hygroscopic, which has been identified as a contributing factor to the degradation of OPVs [10,11]. In general, replacement of PEDOT:PSS with another organic material typically leads to improved stability [182]. Devices with hydrophobic charge transport layers or electrodes which would slow down water ingress are also expected to show improved stability. An examples of this approach is a carbon nanotube/polymer composite as a hole transport layer, which also improved thermal stability [189]. The choice of ETL can also affect the stability. It was stated that mesoporous devices exhibit better stability compared to the planar ones [240]. The thickness of the mesoporous layer was also found to affect the stability of the devices [268]. The degradation of the

Fig. 7. Photo of a perovskite solar cell incorporating oxide layer deposited by atomic layer deposition without encapsulation in a direct contact with a droplet of water. Reprinted with permission from Ref. [141]. 15

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Fig. 8. Power conversion efficiency of the best performing TiO2-based device with optimized encapsulation condition for outdoor testing over 1000 h. These data represent continuation of the outdoor testing experiment reported in Ref. [7].

perovskite film is faster on TiO2 compared to glass in presence of illumination [97]. It was also reported that the replacement of TiO2 with alumina in mesoporous layer considerably improved the PSC stability under illumination [269]. Decreased stability of devices with TiO2 was attributed to UV light induced desorption of oxygen adsorbed at surface defects [269]. While the titania devices exhibited rapid degradation within several hours of illumination without UV filter, devices with alumina mesoporous layer exhibited stable performance for 1000 h after initial degradation [269]. The efficiency in these devices exhibited a significant drop in the first 200 h down to 6% but then it stabilised at that value [269]. It should be noted, however, that the devices were prepared in air and then placed into glove box and encapsulated using a very simple encapsulation method, just a simple two part epoxy and a cover glass [269]. On the other hand, it was also reported that the presence of mesoporous titania layer contributes to improved environmental stability compared to alumina [246,270]. Nevertheless, it can be worthwhile to explore possible replacement of the TiO2 with a different charge transport material. One of the most common alternatives in ZnO. However, devices with ZnO ETL generally exhibit inferior stability compared to TiO2 [7,144]. There have been very few reports on stable performance of ZnO-based PSCs, typically involving devices not subjected to light soaking and relatively low efficiencies. For example, it was reported that efficiency of ZnO-nanorod based cells decreased only slightly from 5% to 4.35% over 500 h of ambient storage [145]. It was reported that the cells with titania nano rods exhibited significantly improved shelf-life compared to devices with nano particle layers and compact layers, which was attributed to improved phase stability of CH3NH3PbI3xClx perovskite infiltrated into devices with nano rod scaffolds [129]. Good stability was also reported for devices with aluminum doped zinc oxide (AZO) under ISOS-D-1 testing protocol [271]. It should be noted, however, that both ZnO and TiO2 are wide bandgap semiconductors which are known to be efficient photocatalysts. Thus, a likely candidate material for stable devices would be a wide bandgap n-type oxide with good charge transport properties and poor photocatalytic efficiency and low desorption of adsorbed species from the surface under illumination. Possible candidates include indium oxide and tin oxide. It has been shown that PSCs with SnO2 ETL exhibit good stability after ageing under simulated solar illumination for 60 h, showing a drop in efficiency to
17% from 20.4%, and after 10 h in the dark the PCE recovered to a value of 20.7% [21]. Further improvements in the performance can also be expected if all-inorganic charge transport layers are used. The use of inorganic charge extraction layers, so that the device had the architecture FTO/NiMgLiO/MAPI/PCBM/Ti(Nb)Ox/Ag, was associated with a significant improvement in stability [194]. The devices with an area of
1 cm2 exhibited the efficiency of 16.2% [194]. After storage in the dark for 1000 h, cells maintained the 97% of the initial efficiency, while in the case of illumination this was about 90% [194]. It should be noted however that the light exposure included a UV cutoff filter (420 nm), and thus the stability under realistic solar illumination is likely lower. Excellent stability during storage for two months in an ambient environment (30–50% RH, no encapsulation) was also reported for a device with the structure ITO/NiOx/perovskite/ZnO/Al, while the device with PEDOT:PSS and PCBM exhibited rapid degradation [193], as shown in the Fig. 6. The inferior stability of the devices with organic charge transport layers was attributed to the degradation of PCBM, damage to the metal electrode, and instability of PEDOT:PSS [193]. It was also shown that allinorganic PSCs could be fabricated in ambient, and operate without encapsulation and no degradation for 2640 h at 90–95% ambient humidity as well as exhibit exceptional thermal stability [272]. The cells consisted of TiO2 (compact and mesoporous), CsPbI3, and carbon and exhibited efficiency of 6.7% [272]. Finally, the choice of electrode can also affect the stability. Commonly used metal electrodes such as Al, Ag and Au readily react with iodine contained in volatile products of perovskite degradation or the mobile iodine ions which diffuse within the device [235]. Even the use of silver paint to make a contact to the encapsulated devices can result in inferior contacts [7]. Thus, possible strategy for improving stability also involves optimising the electrode material. It was reported that Cu metal electrodes result in stable performance of perovskite solar cells stored in ambient without encapsulation. Excellent stability ( > 2000 h in ambient conditions) of large 70 cm2 perovskite modules was attributed to hydrophobic carbon electrodes [273]. Another alternative is carbon-based electrode. Good stability has been reported for devices with carbon electrodes [210,229]. However, due to low conductivity of carbon compared to metal electrodes, carbon electrodes are not suitable for large area devices and modules. One possible alternative is a combination of carbon 16

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and Cu, either in the form of nano composite, or a carbon coated metal mesh. 4.1.3. Encapsulation The last part of the device which has significant effect on the lifetime is device encapsulation. A large number of reports on device stability of PSCs involve devices without encapsulation. When comparing the stability of devices without encapsulation, the ambient humidity must be taken into account. In OPVs [10,11] as well as PSCs [248] it has been demonstrated that ambient humidity significantly affects the degradation rate of devices. While good stability could be achieved in a low humidity ambient without encapsulation, for a realistic estimate of device lifetime under operating conditions which involve a range of temperatures and relative humidities, encapsulation is essential. It can be achieved by depositing protective layers and/or sealing the device. The deposition of protective layers is desirable for flexible device encapsulation, and it can also be combined with sealing the devices on rigid substrates to enhance the lifetime. Protective layers can be deposited using different techniques. ALD in particular is a promising technique for conformal coating of very thin oxide layers. However, the use of ALD for depositing layers on top of perovskite films could be challenging due to substrate temperatures required, as well as the use of oxidants (water, ozone, hydrogen peroxide) [141]. Lowering the deposition temperature or using the acetic acid based ALD deposition could result in a reduced damage to the perovskite layer [141]. It has been demonstrated that with an optimised ALD deposition of an oxide layer, perovskite solar cells with efficiency of 8.8% could even survive direct contact with liquid water without encapsulation [141], as shown in the Fig. 7. Stable device operation was reported throughout 3 min measurement despite contact with liquid water, and rinsing with liquid water for 10 s also did not cause performance degradation [141]. Different approaches have been reported to encapsulation of the devices, Some encapsulation methods reported in the literature are very simple, and may involve just covering the active area with a cover glass and sealing the edge. Different encapsulation materials have been reported as well. For example, polydimethylsiloxane (PDMS) was proposed for encapsulation of the perovskite solar cells, and the PDMS-packaged devices exhibited improved stability compared to unpackaged devices with carbon electrodes [232]. However, caution is needed in extrapolating conclusions from stability studies involving ambient storage rather than more harsh testing conditions which would be closer to realistic outdoor performance. However, comparisons of the sealing procedures and materials used have been scarce. Nevertheless, it was shown that the encapsulation method significantly affects the device performance under stability tests [7,248]. For some epoxy materials, encapsulated devices may exhibit lower performance after encapsulation [7,248]. It has been demonstrated that UV-curable epoxies may be preferable for encapsulating PSCs compared to thermally curable ones, likely due to PSC damage by solvent outgassing from the epoxy [7]. Presence of a desiccant sheet was also found to be essential for improved stability [7,248], and additional protection could be provided by coating the devices with SiO2 film before encapsulation [7]. Furthermore, it was demonstrated that edge-sealing with an UV epoxy had an advantage compared to oversealing (entire device covered with epoxy) during  stability testing at 100 C [230]. However, this may not necessarily apply to other types of epoxies [7]. In addition, encapsulated devices with both contacts made via conductive oxide outside packaged area sealed on the side with UV epoxy can exhibit good stability under thermal and other types of stress [7,230]. Overall, further work is needed in optimizing the encapsulation procedure for PSCs. 4.1.4. Testing protocols Final issue concerning the stability is the testing protocol. Since it is well known that the perovskite materials and devices degrade faster under light exposure, it is essential that testing protocols involve illumination and that they are standardized. Light induced degradation of perovskite films and/solar cells was observed even in an inert (nitrogen) atmosphere [97,274]. The recovery of the cell performance was observed upon annealing in the dark [274]. The degradation was attributed to ion generation and ion migration upon illumination [274]. The combination of exposure to light and humidity accelerates the degradation [97]. However, studies of PSC degradation under illumination, either by accelerated ageing under laboratory light soaking or by outdoor exposure, have been scarce [7,97,221,269,275]. Other accelerated ageing tests may include elevated temperature and/or ageing under bias [163,275]. This is because accelerated degradation in the presence of moisture occurs not only upon illumination but also upon exposure to heat and electric fields [57]. Application of an electric field can also cause degradation in the presence of a solvent vapour, such as DMF vapour inside the glove box [57]. The fact that increased temperature can contribute to the degradation is well known. The effects of heat on the device stability can be evaluated by thermal cycling tests [276–278]. ISOS-T and ISOS-LT protocols provide standardised testing conditions for thermal cycling without and with illumination, respectively [3,4]. For perovskite devices, thermal cycling is commonly performed without illumination. It should be noted, however, that the simulated solar illumination without UV filters combined with high ambient humidity and elevated temperature is the most severe accelerated ageing test that could be applied. Even without elevated temperature, illumination in a humid environment represents a harsh test for PSCs, considerably more so than adding other types of stress (bias, temperature) without illumination. It is very common to use non-standard testing conditions, with selective application of stressors, such as the most commonly used dark storage in ambient (humidity) or illumination in inert environment, with or without elevated temperature or bias. For example, good stability for 250 h after the initial burn in and initial efficiency of 21.1% and final efficiency of about 18% were reported for PSCs under constant illumination maintained at the maximum power point (MPP) in nitrogen [16]. The rationale behind such testing is that it is supposed to simulate realistic operational conditions of an encapsulated device. However, since encapsulation is typically imperfect compared to an inert environment in a glove box, such measurements are expected to overestimate the stability. It should also be noted that for flexible devices mechanical stability testing over a number of bending cycles is also of interest [279,280]. To obtain a realistic estimate on the performance of the devices, it would be highly desirable to conduct more outdoor tests in different climates. Outdoor tests have been even more scarce compared to accelerated ageing tests under light soaking [7,275,281]. It should be noted, however, that for OPVs lifetimes of samples tested outdoors (ISOS-O-1 and ISOS-O-2) are between the values obtained 17

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from the accelerated ageing (ISOS-L-2, ISOS-L-3, and ISOS-D-3) and dark tests under moderate conditions (ISOS-D-1 and ISOS-D-2) [282,283]. In other words, accelerated ageing tests typically underestimate the lifetime which would be obtained outdoors. Furthermore, the estimated lifetime in indoor light soaking is also dependent on the spectrum of illumination source, with low UV component resulting in similar lifetime estimates for dark condition and illumination with low UV component [283]. Considering the fact that perovskites are more sensitive to humidity and illumination compared to OPVs, it is expected that the difference in obtained lifetime estimates under mild and severe testing conditions would be even more pronounced. In addition, lifetime estimates for devices subject to illumination from LED sources without significant UV component [187,284] would likely be overestimated. Thus, it is essential to obtain more data on the perovskite solar cells under more rigorous testing conditions, including outdoor tests. Thus, additional outdoor testing studies are urgently needed, as well as improvements in the encapsulation methods. To date, there have been three reports on outdoor testing of PSCs with different architecture under different testing conditions (different climates). Planar MAPI cells with TiO2 charge transport layer with optimal encapsulation exhibited stable performance after initial burn in (efficiency drop from
15% to
11% during the first day) for over 400 h outdoors during Hong Kong summer (including high ambient temperature and humidity as well as heavy rain, and the best performing cell even survived one typhoon) [7]. Considering the fact that this device architecture is expected to have inferior stability, obtained result is quite promising. The normalised PCE of the best performing device during extended outdoor testing according to ISOS-O1 protocol (beyond hours reported in Ref. [7].) is shown in Fig. 8. Stable outdoor performance was also reported for a device consisting of FTO glass, TiO2, (5-AVA)xMA1xPbI3, mesoporous zirconia and carbon counter electrode for one week of outdoor testing in Jeddah, Saudi Arabia in September 2014 [275]. This device architecture also exhibited stable performance with small PCE decrease for over 1000 h of continuous light soaking [210]. Good outdoor stability (tests conducted during winter in Barcelona, Spain according to ISOS-O-2 protocol) was reported for devices with architecture glass/FTO/compact TiO2/mesoporous TiO2/FAPbI3(0.85)MAPBr3(0.15)/spiro-OMeTAD/Au [281]. The device efficiency retained 80% of the initial value (11%) at 846 h, and 60% of the initial value at the end of the test at 1080 h [281]. While the outdoor testing reports to date are promising, additional tests are needed. Finally, stability tests on PSC modules have also been reported. The encapsulated  modules with spiro-OMeTAD and P3HT charge transport layers were tested in ambient under light soaking and at 40 C, and it was found that spiro-OMeTAD resulted in better stability [285]. The performance decreased in the first 72 h, followed by stable performance to 330 h [285]. 4.2. Hysteresis and measurement standards Hysteresis has been commonly reported in PSCs, with planar devices typically exhibiting more pronounced hysteresis compared to mesoscopic devices, and devices with organic charge transport layers typically exhibiting lower hysteresis compared to those with metal oxide charge transport layers [286,287]. Hysteresis is generally less common in inverted PSCs with organic charge transport layers [42,49], and it could also be reduced by using mixed halide (bromide iodide) perovskite active layers [288]. It should be noted that due to a large variation of hysteresis reports in the literature, there can be exceptions to the claims above. Proposed reasons for variations of reported hysteresis in the literature include factors affecting ion diffusion, such as material stoichiometry which affects defect concentrations and self-healing ability, and material morphology which affects grain boundaries and interfaces [289]. Possible origins of hysteresis phenomenon have been discussed in numerous research papers, as well as reviews of perovskite solar cells [57]. Proposed reasons include ferroelectric polarisation of the perovskite, capacitive effects, ionic motion within the perovskite material, and bias dependent trap filling at the interfaces [46,57]. Phenomena related to hysteresis, such as ferroelectricity [290,291], ion migration [288,292], and charge build-up [293] have been extensively discussed. While it is clear that the perovskite materials are ferroelectrics which can be poled, recent results cast doubt on the ferroelectricity as a possible explanation of hysteresis [57]. Ionic motion under bias is expected to occur in the perovskite materials, and it likely contributes to hysteresis [57]. Hysteresis effect is exacerbated by non-selective contacts and charge accumulation at interfaces [57]. While it was proposed that bias dependent trap filling could not explain observed reversible poling in the perovskites [57], it likely affects the charge extraction and thus the magnitude of hysteresis. Despite the uncertainties in the details of mechanisms responsible for the hysteresis, general consensus is that mobile ionic species contribute to the inefficient charge collection [286]. In devices with mesoporous titania with large interfacial area, as well as devices using PCBM which infiltrates perovskite grain boundaries and passivates defects, electron transfer can occur more efficiently, thus resulting in reduced hysteresis [286]. Another possible reason for reduced or no hysteresis in devices with PCBM is stabilisation of ion migration due to formation of fullerene-halide radical [287]. Regardless of the mechanism details, efficient charge extraction appears to be the key in reducing the hysteresis, possibly due to reduced interfacial charge accumulation and consequent significant capacitance [287]. It was proposed that the hysteresis can be minimised by achieving high grain sizes of the perovskite films and passivation and interface engineering to reduce interfacial charge trapping [46]. While the question of the origin of hysteresis has not been fully settled in all the details, it is obvious that some practical guidelines on the measurement of perovskite solar cells need to be established to deal with this phenomenon. It is obviously necessary to report the data measured under both forward and reverse scanning direction. In addition, scan rate can also affect the obtained results, as well as holding the cell at a certain bias before measuring [57]. Possible approach to deal with this problem is to measure PCE over time at the MPP until the steady state is achieved [57]. Stabilized performance at the MPP has been proposed as a reliable measurement protocol, and it was advised to be included even for cells which appear to be hysteresis-free [294]. 4.3. Environmental implications - the presence of lead Environmental concerns are a well recognised issue for perovskite solar cells [295]. Perovskite solar cells share the same concern as 18

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Fig. 9. Summary of research directions necessary to fuel PSC technological translation via slot die R2R coating. (A) Material engineering can address intrinsic ion diffusivity and degradation of CH3NH3PbI3, (B) optimization of perovskite growth under R2R conditions is necessary to reach performance requirements, and (C) device engineering is required to extend device life and expand the material toolbox for PSC design. Reprinted with permission from Ref. [297] with modification. Copyright (2016) American Chemical Society.

CdTe solar cells, namely the presence of a toxic heavy metal. However, unlike CdTe which is very chemically stable, organolead halide perovskites are not stable and upon ambient exposure they can degrade into products that are readily leached into the environment [295]. Thus, ideally perovskite solar cells should be subject to even more stringent safety standards compared to CdTe cells and any commercial products should have clear plans for end-of-life disposal and/or recycling [295]. Comprehensive life cycle assessments (LCA) to evaluate the environmental impact are needed, since hazards exist in all stages, such as raw material extraction, precursor synthesis, fabrication, use, and decommissioning [295]. In fabrication stage, not only heavy metals but also toxic organic solvents which are miscible with water exacerbate the risks [295]. While the use stage has been discussed in several studies, the possibility of toxic fumes emission in case of a fire has not been evaluated [295]. Attempts have been made to estimate the effect of exposing Pb-containing perovskite film to water, simulating rain falling onto the cell with damaged encapsulation [296]. It was estimated that even in a 19

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complete failure of a module with one meter square area resulting in a dissolution of perovskite layer the impact would not be catastrophic due to relatively small amount of lead contained (under 1 g), although the care should be taken to avoid possible situation where lead would concentrate in a groundwater reservoir rather than spread out in the environment [296]. Furthermore, device encapsulation could limit the Pb leakage during the cell operation, and appropriate end-of-life disposal could limit further environmental effects of Pb [297]. Nevertheless, the presence of lead has been raised numerous times as a significant concern in the possible wide range application of PSCs. To address the concerns about lead, considerable efforts have been made to develop lead-free perovskite materials. In addition to tin-based perovskites which have been mostly commonly studied, a number of other materials has been reported in recent years. These include Cs3Bi2A9 and MA3Bi2A9 [298,299], CsGeI3, MAGeI3, and FAGeI3 [300], MA2 CuClxBr4x [301], Cs2BiAgCl6 [302], A3Sb2I9 (A ¼ Cs, Rb) [303], (N-methylpyrrolidinium)3 Sb2Br9 [304], etc. Unfortunately, in many cases the achieved PCE values for novel leadfree and tin-free perovskites have been below 1% [298–301, 303]. Tin-based perovskites generally result in higher efficiencies compared to novel perovskite materials based on other elements, but their efficiency is still bellow that of lead-based perovskites, commonly well below 10% [305–308]. For FASnI3, PCE of 6.22% was reported [306], while PCE of 5.73% was reported for devices using MASnI3xBrx [307]. In the case of MASnI3, reported efficiencies cover a wide range from 3.15% [309] to over 6% [308], which is likely due to the differences in the processing of perovskite layer and the device architecture. Thus, obviously improvements in Sn-based device efficiency are possible with careful optimisation of the deposition of the perovskite layer and the device architecture. Nevertheless, instability of the valence state Sn2þ remains a concern, as well as the fact that due to lower efficiency and chemistry of tin halides, tin-based devices may not be as environmentally benign as initially assumed [310]. While Sn is generally considered less toxic than Pb, there have been reports questioning this view. For example, it was found that Sn would result in greater toxicity to zebrafish, due to the fact SnI2 results in reduced pH [310]. Thus, even though Sn metal itself did not result in toxicity, unlike lead, the toxicity was observed due to acidification and this type of effects also needs to be considered [310]. It was also pointed out that due to the lower efficiency of Sn-based cells, larger areas would be needed to produce the same output as Pb-based cells [311]. Similarly, the choice of electrodes should also be considered when examining environmental impact. It is possible that in perovskites similar to organic photovoltaics carbon electrodes would be more environmentally favourable, although they would result in lower efficiency due to nonoptimal area usage in modules [312]. Carbon also has an advantage of being a stable electrode for perovskite materials, different from Ag. However, it is also necessary to consider the increase of area needed to produce the same current and consequently possible larger environmental impact of Pb if carbon electrodes are chosen to be used. As already mentioned, the environmental impact of a technology is best assessed via LCA studies. Life-cycle assessments of PSCs have been reported [311,313–316]. It was found that while manufacturing environmental impact of PSCs was lower compared to single crystal Si, the impact per unit of electricity was higher due to lower lifetime of PSCs [313]. The devices considered used FTO electrodes, SnO2 ETL, MAPI as active perovskite layer, while two alternative configurations were considered HTL-free architecture with carbon electrode, and CuSCN as HTL with MoOx/Al as the electrode [313]. The need to avoid precious metals such as Au and Ag has already been pointed out by previous LCA analysis [314]. Solution-based and vapour-based deposition methods were considered [313]. It was found that HTL-free cells had the lowest environmental impact except in marine eutrophication, but for all kinds of perovskite solar cells considered the main limiting factor in outcompeting the existing technologies was the lifetime of the devices [313]. Energy payback time (EPBT) was found to be 1–1.5 years, while global warming potential (GWP) was found to be 100–150 g CO2 (per KWh) equivalence [313]. However, the lifetime would need to exceed 30 years for efficiency of 15% for PSCs to be competitive with other technologies [313]. Vacuum deposited PSCs were found to have lower marine eutrophication impact compared to solution-processed and HTL-free devices, which is mainly due the use of toxic solvents such as DMF in solution processing [313]. In addition, the use of CuSCN contributed to ecotoxicity in devices with this HTL material [313], but this problem could easily be eliminated by different material choices. One significant observation from LCA was that energy requirements were not significantly lower compared to commercial technologies, but there was a significant uncertainty in exact specifications of for commercial scale fabrication [313]. In addition, the impact from the lead in the absorber layer was found to be negligible [313]. Low environmental impact of the absorber layer is in agreement with the reports from other studies [311,314,316]. While in some cases perovskite layer was found to have a significant environmental impact, only 1.14% of its human toxicity potential is due to lead, with the major impact due to the use of toxic solvents [315]. One of the major reasons for low environmental impact of Pb is its low content [315], and there is also a possibility that it could be further reduced while retaining a respectable efficiency of 14% by using a mixture of Pb and Sn [317]. The LCA studies also generally agree on the short energy payback time, although there is some spread among reported EPBT values [313,314]. LCA studies also agree that a issue of concern in considering environmental impact of perovskite solar cells is the solvents used in device preparation [313,314]. The use of nonhazardous solvents instead of toxic ones would obviously be highly desirable. It has been demonstrated that the use of nonhazardous solvents can result in comparable device efficiency (15.1% for optimised solvent combination) to commonly used toxic solvents (16.7%) [318]. Furthermore, the method is scalable so that 4 cm2 modules with efficiency of 11.9% could be prepared by blade coating method [318]. Thus, lead-based perovskite appears to be only technologically viable choice at present based on the results of LCA studies in absence of a technological breakthrough. However, care needs to be taken with the disposal of the devices. It was proposed that perovskite solar cells could be recycled by using polar aprotic solvents to dissolve the perovskite layer [319]. It was demonstrated that reuse of mesoporous titania coated FTO substrate resulted in constant PCE for 10 recycling cycles [319]. In addition, ETL-free devices with efficiencies of
10% have been reported on recycled FTO/glass substrates [170]. In addition to end-of-use recycling, it was proposed that it is possible to mitigate environmental effects of PSCs by using materials sourced from recycled car batteries [320]. It was found that the devices prepared from material from recycled batteries and from high purity precursors from Sigma-Aldrich exhibited comparable performance [320]. However, it should be noted that the devices exhibited efficiency below 10%. Whether this conclusion would hold in 20

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Fig. 10. Schematic diagram of tandem cell structures.

high performance PSCs is uncertain, since often claims derived for devices with non-optimal performance do not necessarily hold for high performance devices. For example, if the device efficiency is limited by other factors which affect the performance to a greater degree than source material purity, no difference in performance would be observed. After such factors are eliminated in high performance devices, it may occur that the source material purity becomes a significant factor affecting the efficiency. This does not necessarily mean that using recycled lead sources is not an option, but it is possible that extensive purification may be required for high performance devices.

4.4. Large area and flexible devices Progress in low temperature PSCs, flexible devices and scaling up the cell fabrication has been reviewed recently [234,297,321]. Motivation for the development of large area and/or flexible devices is primarily an economic one, to lower the production cost of PSCs. 21

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In addition, flexible devices are of interest for various niche applications where flexibility and light weight properties are desirable, such as wearable electronics [322,323] or unmanned aerial vehicles [99]. While lots of progress has been made, further improvements in this area are still needed. A challenge towards upscaling of the PSC is uniformity of the perovskite layer, contact resistance of the electrode, and suitability of the manufacturing technique for large area devices. In the case of flexible substrates, which are suitable for roll-to-roll printing, additional challenge is to ensure that all the device layers can be prepared by low temperature, printable processes. In terms of the uniformity of the perovskite layer, that can be a significant concern in particular for commonly used solution based methods such as spin-coating. While various approaches have been proposed to improve the film quality as discussed in the Subsection 3.2 and while there have been reports on efficient large area devices, this remains a concern. In addition, it has been pointed out that low efficiency of precursor solution utilisation is a concern for spin-coating, in particular for Pb-precursor solutions [324]. To address this issue, different device architectures and fabrication procedure details have been developed for large area devices prepared by printing (inkjet printing, screen printing) and/or doctor-blading [112,263,297,325–327]. Another proposed method for large area film fabrication is direct contact intercalation method [328], while other methods such as electrodeposition and spray coating are also possible [297]. While each method has its advantages and disadvantages, roll-to-roll process is probably the best candidate due to high throughput and effective material usage [297]. However, considerable efforts need to be made to optimise this process for fabrication of efficient PSCs due to the fact that perovskite films and devices have tremendous sensitivity to the small details of fabrication procedure which affects the perovskite formation, as illustrated in Fig. 9. In addition to the perovskite film uniformity, the electrode conductivity is also a concern for large area devices. One possible solution is to use metal grids, similar to large area cells of other photovoltaics. It has been demonstrated that with using aluminium grid lines to increase electrode conductivity PSCs with the area 25 cm2 and efficiency 6.8% have been obtained [329]. While preparing high performance large area PSCs is a challenge even on rigid substrates, it is even more difficult to achieve this goal on flexible substrates. Flexible devices are of considerable interest in order to achieve low cost and large area manufacturing. We will discuss some general concerns in achieving efficient flexible devices. For a recent review of achievements in flexible PSCs, see Ref. [330]. For device architectures involving organic charge transport layers, this can be achieved in a more straightforward manner compared to devices using TiO2 electron transport layer. In the latter case, similar problems compared to dye-sensitised solar cells (DSSCs) are encountered, namely that the device performance is usually better for higher processing temperature of titania layer. Thus, efforts have been made to develop low temperature processing for titania. Atomic layer deposition (ALD) for the compact layer and UVirradiation for the mesoporous layer were proposed [331]. The efficiency of the devices on ITO/PET was lower compared to glass/FTO, (9.8% on FTO/glass compared to 7.1% on ITO/PET), but nevertheless the authors demonstrated that ALD deposition can deposit compact layers at low temperatures, while UV irradiation could substitute annealing in preparing a mesoporous oxide layer [331]. UVassisted solution process was also reported for synthesis of compact TiO2 layers [332]. This resulted in efficiencies of 19.57% and 16.01% for PSCs with UV processed Nb doped TiO2 on rigid and flexible substrates [332]. Also, the efficiencies of 15.0% and 11.2% were reported for PSCs with photonic-cured TiO2 on ITO/glass and ITO/PET, respectively [333]. In addition, SnO2 is a very promising for flexible devices, since efficient solar cells were reported for low deposition temperatures compatible with flexible substrates [21,149]. Although low temperature deposition was reported for TiO2 as well [331], commonly efficiencies below 18% with very few exceptions are obtained. ZnO is another possible candidate, and although flexible ZnO-based PSCs have been demonstrated [334], but as already discussed its performance is inferior to both TiO2 and SnO2 especially in terms of stability. Flexible devices have also been reported with Zn2SnO4 ETL [335]. Different flexible substrates for perovskite solar cells have been reported to date, including ultra thin willow glass, polyethylene terephtalate (PET), polyethylene naphtalate (PEN), polyethersulfone (PES), and metallic foils [279,336–344]. It should be noted that metallic foils such as Ti [342] and ultra thin willow glass [336] are expected to have an advantage in terms of stability over devices on ITO/PEN and ITO/PET, due to slower ingress of water. Water vapor transmission rate (WVTR) of glass is as low as 1014 g/m2/day, while for PET it is 101 g/m2/day [240]. ITO on plastic substrates also has a disadvantage of increased resistance with bending. This issue could be addressed with metal meshes embedded into a plastic substrate, with or without conductive film coating. For example, a possible choice for a transparent electrode suitable for flexible substrates is ITO coated embedded metal nanowire mesh electrode [345]. Device efficiencies with Ag and Cu nanowire mesh electrodes were 14.15% and 12.95%, respectively [345]. Multilayer electrodes combining conductive oxides and metal nano wires are also possible [346]. Other types of electrodes include dielectric/metal dielectric films [347], thin metal films and high conductivity PEDOT:PSS [344], etc. Furthermore, for conductive oxide electrodes it was also demonstrated that nano structured plastic substrates performed significantly better compared to flat substrates after mechanical bending [348]. Thus, it is possible to overcome the challenge of maintaining the device performance with repeated bending. However, care needs to be taken concerning the water diffusion rate through the substrate to ensure that the devices are stable. Possible approaches include different choices of polymeric substrates, as well as the deposition of hydrophobic and/or barrier layers. It should also be noted that the use of metal foils or willow glass also has drawbacks. Willow glass is less flexible compared to other flexible substrate choices, while the use of metal foils requires transparent top electrodes which can be challenging.

5. Beyond simple single photovoltaic cells In this section, we will discuss more complex device structures. For practical applications, modules rather than individual solar cells are of interest. Hence, we will discuss what has been achieved and what difficulties need to be addressed in the development of efficient PSC modules. Then, we will discuss the challenges facing tandem cells as a possible avenue towards high efficiency devices, and finally we will consider issues related to other integrated devices. 22

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5.1. Tandem cells Tandem cells are an attractive option for achieving high efficiency. Theoretical simulations on tandem cells involving PSCs have been conducted [349–352]. It was proposed that project efficiency for optimized perovskite/Si tandem cell could achieve or exceed 30% [350–352], while for PSC/c-Ge and CIGS tandem cells the efficiencies of
25% were predicted [349,353]. For CIGS/perovskite tandem this included the use of hydrogenated indium oxide instead of commonly used ITO for optimised device structure [353]. High efficiencies exceeding 30% could be achieved for both two-terminal and four-terminal configurations with Si/perovskite tandem cells [352,354,355], although experimentally achieved values are commonly well below the theoretical estimate. Two-terminal monolithic cells and four-terminal cells, which are essentially two stacked solar cells connected in series are illustrated in the Fig. 10. Four terminal devices is typically easier to realize compared to monolithic two terminal tandem cells, where the second cell is grown directly on top of the first one. While transparent top electrode can be a shared challenge for both four terminal and monolithic tandem cells with nontransparent substrates, monolithic tandem cells have additional difficulty of preparing a high quality tunnel junction/recombination layer. Practical challenges in the realization of transparent conductive electrodes in a monolithic two terminal structure can result in a significant reduction of actual achieved efficiency over an estimate what would be the efficiency in case of connecting individual cells in series with taking into account shadowing effect [356], which is essentially a four terminal device situation. In situations where the substrate is transparent, i.e. polymer solar cells, it would be possible to have non-transparent metal electrodes which would simplify the fabrication of the tandem cell. However, this would not fully eliminate the challenges since the polymers would be susceptible to damage from solution processing of the perovskite part of the device. General considerations for tandem cells is that the current matching should be achieved for cells connected in series, and that any layers fabricated on top would not damage the layers underneath. A significant advantage of the mechanically stacked tandem is that there are fewer underlying layers that could potentially be damaged since two cells are fabricated independently. It should be noted that an additional advantage of mechanically stacked tandems is that the current matching can be achieved by changing the areas of the devices while both top and bottom cells can be independently optimised [357]. In the case of inorganic bottom cells, damage from solution processing and temperatures needed for preparing the perovskite is typically not a concern. Tunneling junction and the top cells could thus be deposited without significant problems. On the other hand, the deposition of transparent electrode on top of the perovskite solar cell is a significant concern, due to sensitivity of the perovskite and/ or organic charge transport materials to solvents and elevated temperature. Thus, the deposition of a transparent conductive oxide such  as ITO would be problematic since typically temperatures exceeding 200 C are required for high conductivity and high transparency films. Sputtering the ITO on top of spiro-MeOTAD can result in very poor cell performance even if the sputter process is optimised to minimise damage [355]. Nevertheless, successful sputtering of ITO contact on top of MoO3 layer was demonstrated by some groups [358,359]. Thus, this is likely a difficult but not impossible achievement, and may require careful optimisation of the process and it may possibly depend on the geometry of the sputtering chamber. For example, it was shown that suppression of secondary electron bombardment during sputtering of ITO improves the performance of organic light emitting diodes (OLEDs) [360]. It was shown that the presence of a WO3 metal oxide buffer layer could not entirely prevent damage induced by sputtering [361]. Nevertheless, efficient OLEDs with ITO sputtered top electrode have been reported [362], although MoOx was used instead of WO3 buffer layer. It is possible, especially considering that some successful PSC devices have been achieved [358,359], that procedures developed to minimise the damage in OLEDs during the ITO deposition [363–365] could be adapted to PSCs, but it will likely require careful process optimisation. It was proposed that the transparent electrode of an efficient cell should have a resistance below 10 Ω/sq and have high transmittance in the relevant regions determined by the absorption properties of top and bottom cells [357]. It should be noted, however, that this requirement is significantly relaxed if a metal grid is used [366]. Possible choices for transparent electrodes include metal nanowire electrodes (mechanical transfer of the electrode eliminated the possibility of solvent damage although damage due to high pressure can still occur [357]) or multilayer stacks involving thin metal layers and dielectric overlayer to optimise transmission [367]. It was also proposed that MAPb(I1xBrx)3 would be a more suitable material for a tandem cell since the bandgap could be adjusted between 1.6 eV for pure iodide and 2.25 for pure bromide [357]. In the case of MAPbI2Br, a close to ideal bandgap of 1e.76 eV could be obtained [357]. Various four terminal and/or mechanically stacked tandem cells have been reported to date [357]. It was demonstrated that mechanically stacked tandem cell consisting of bottom Si or CIGS cell and a top perovskite could result in an improved performance compared to individual single cells [357]. Obtained efficiencies of semitransparent perovskite solar cell (with laminated Ag NW electrode and antireflection coatings on both glass and top electrode) was 12.7%, while 11.4% and 17.0% were the efficiencies of low grade Si cell and CIGS cell, respectively [357]. In a tandem configuration, modest improvement to 18.6% was obtained for CIGS and a significant improvement to 17.0% was obtained for low-grade Si cell [357]. In general, more dramatic improvements in tandem devices are reported for devices with lower efficiency single cells. For example, 13.4% efficiency tandem cell was reported (6.2% perovskite top and 7.2% Si bottom cell), while the efficiency of a perovskite reference device was 11.6% [355]. A four terminal tandem cell with efficiency of 15.5% was demonstrated for CIGS cell (12.4% as a single cell without shadowing) with MoOx/Au,Ag/MoOx top electrode. Even higher efficiency of 19.5% was obtained for MoOx/AZO electrode and an improved performance CIGS cell (18.4% as a single cell without shadowing) [368]. Graphene (with Au grid) was another proposed choice for a transparent electrode in four terminal tandem cells [369]. It should be noted, however, that the graphene electrode sheet resistance was rather high at 350 Ω/sq, which is not suitable for large area devices. In this particular case, while the tandem efficiency of 13.2% was higher than that of single perovskite and shaded a-Si:H/c-Si cells (6.2% and 7.0%, respectively), it was lower than that of a Si cell without shading (18.5%) [369]. Nevertheless, perovskkite/Si tandem devices are very promising for achieving high efficiencies. For example, a high efficiency of 23.0% [370] was obtained for a perovskite/Si tandem cell. In these devices, very thin Cu (1 nm)/Au (7 nm) electrode was used in a perovskite cell, while Si cell contained antireflection coating on front side and MgF2 reflector at back side [370]. 23

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It should be noted that this metal electrode is exceptionally thin, and thus it likely requires exceptional quality of perovskite and charge transport layers for good performance devices. To date, the highest efficiency reported for a four terminal device was for a perovskite/Si tandem cell, with In-doped TiO2 electron transport layer, which achieved a steady-state efficiency of 24.5% [371]. As already discussed, monolithic tandem cells are more challenging to fabricate. Nevertheless, there have been reports on monolithic tandem cells consisting of perovskite and range of other materials. For example, a monolithic tandem cell consisting of a kesterite Cu2ZnSn(S,Se)4 and MAPI cell was reported [356]. The cell structure was glass/Mo/Cu2ZnSn(S,Se)4/CdS/ITO/PEDOT:PSS/MAPI/ PCBM/Al [356]. The obtained efficiency was quite low, 4.6%, which was attributed to the inferior properties of the top Al contact (low transmittance and high resistivity for thin Al contact) [356]. This was much lower than the efficiency of individual cells (with perovskite cells with thick Al contact), which was 11.6% for CZTSSe and 12.3% for perovskite [356]. This illustrates critical importance of electrode properties in monolithic tandem devices. The main difficulty for this material combination is that Mo layer is necessary for growing high quality CZTSSe, so that the tandem devices need to be illuminated through the top electrode. The deposition of ITO as a top electrode to the perovskite solar cells would be difficult due to the need for higher substrate temperature to achieve low resistivity and high transmission. Solving the problem of low temperature processed transparent electrodes with high conductivity is non trivial, and it remains one of the big challenges for tandem devices involving non-transparent substrates (for example those with CZTSSe, crystalline Si, etc.). For similar reasons, the reported efficiency in a monolithic tandem perovskite/CIGS cell was 10.9%, lower than the predicted efficiency based on a shadowed CIGS cell and reference perovskite device and a perfectly transparent top electrode, which was 15.9% [372]. In this case as well the transparent top electrode remains the main challenge, since Mo film was needed for growing CIGS layer [372]. Nevertheless, some high efficiency monolithic devices have been reported. For example, Si/perovskite monolithic tandem cell with ITO tunneling/recombination layer and a top MoO3/ITO/LiF contact was reported to have an efficiency of 18.1% [358]. It was proposed that the efficiency could be increased further by optimising the optical losses in the device [358]. Very impressive performance for a tandem cell was obtained for monolithic Si/perovskite tandem cells with efficiencies as high as 21.2% for the device with area 0.17 cm2 and 19.2% for a device with area 1.22 cm2 [359]. IZO was used as a recombination layer, while the top contact was MoOx/ ITO/hydrogenated indium oxide stack [359]. The use of low temperature processes for the perovskite device was emphasised to avoid damage to the bottom Si cell containing a Si layers [359]. The highest efficiency to date reported for a monolithic tandem cell is a perovskite/Si device with the efficiency of 23.6% [373]. Majority of the reported PSC tandem devices was with inorganic solar cells, while the works on organic devices have been scarce. However, it has been demonstrated that polymers can be incorporated into a perovskite solar cell in a single device to increase

Fig. 11. Fabrication processes of an organometallic halide perovskite-based solar module. Reprinted from Refs. [387,388] with permission. 24

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absorption at longer wavelengths and thus increase the efficiency [374]. The use of bulk heterojunction as an ETL to increase photoresponse at longer wavelengths has also been demonstrated [375]. However, it should be noted that this has been achieved for relatively low efficiency PSCs, and it may not hold for fully optimised high efficiency devices. In a report on tandem polymer-perovskite device with poly [(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)/doped MoO3/MoO3 as tunnelling/recombination layer [376]. However, in this case as well low efficiencies below 10% are obtained for the tandem as well as single individual cells [376]. In the case of a PEDOT:PSS based tunneling/recombination layer, a tandem efficiency of 10.2% was reported [377]. Obviously, there are significant difficulties in the preparation of high efficiency tandem devices for organic/perovskite tandem cells, mainly due to processing difficulties for all-solution processed devices, and insufficient optimisation of individual cell performance (photocurrent matching) and the need for improved tunneling/recombination layers. Nevertheless, this is still an attractive material combination despite practical difficulties due to the potential for low cost devices. Another possibility for tandem devices are perovskite-perovskite tandem devices [378]. A tandem device involving one MAPI cell and a MAPbBr3 cell with organic charge transport layers by simple sandwiching of organic cells without the top electrode followed by drying [378]. The advantage of this approach is that both electrodes are transparent oxide and the fabrication method is very simple. The disadvantage is that the obtained efficiency of a tandem cell is lower than that of an individual MAPI cell with a top metal contact (14.2% or 10.4% depending on hole transport polymer for the bromide cell, compared to 18.0% for PEDOT:PSS/MAPI/PCBM/Au cell) [378]. One straightforward improvement for this type of devices would be to deposit a metal film on ITO/glass for the back cell in a tandem configuration, which would likely improve the performance due to increased optical path through the perovskite layer. In another report on perovskite-perovskite tandem (with two MAPI layers which is obviously non-optimal in terms of absorption) which was fully solution-processed and utilised a multilayer tunneling/recombination layer, even lower efficiency was obtained, 7.0% and 5.2%, depending on the direction of illumination [379]. On the other hand, high efficiency (15.3% average, 18.1% best for reverse scan) was obtained for a perovskite (MAPI)- perovskite Cs0.15FA0.85PbI0.3Br0.7 tandem cell [380]. Although higher than previous reports, this performance was still below that of a single-junction MAPI cell (17.4% average, 19.1% best for reverse scan) [380]. Obviously, there is lots of room for improvement in PSC tandem cells, in particular those involving perovskite-perovskite and organic-perovskite combinations. In general, monolithic cells with very few exceptions need significant improvements in performance. In addition, careful measurements with taking care to avoid various possible artefacts [8] is needed for good characterisation of monolithic tandem devices. 5.2. Modules PSC modules with different areas and achieved efficiencies have been reported. For a module consisting of 5 cells with active area 3.36 cm2 connected in series, an efficiency of 5.1% was obtained [285]. Also, PSC modules with different ETL and an area of 10.8 cm2 were reported, with efficiencies ranging from 4.9% to 7.8% [129]. Optimization of perovskite deposition on titania nano rod could result in improved module efficiency of 8.1% for a module with 9.6 cm2 area [130]. Efficiency of 13.6% was reported for 4 cm2 planar PSC module [381]. The use of fluorine doped RGO for charge extraction and improved crystallisation of the perovskite layer was demonstrated to result in 10 cm2 modules with efficiencies of 10.0% and 8.1% on glass and flexible substrates, respectively [382]. In addition, large area modules of 100 cm2 were also reported, with an efficiency of 4.3%, while the efficiency for a small module with the area of
10 cm2 was 10.4% [383]. Nevertheless, similar efficiency numbers of over 10% were reported even for large modules with areas 31 cm2 and 70 cm2 [273]. In addition, modules with area 10.08 cm2 and efficiency 13.0% were also reported [384]. Finally, a high efficiency module (14.3%) was reported for inverted device with organic charge transport layers and an area of 25.2 cm2 [385] Efficient module fabrication appears to be mainly dependent on the quality of the device layers, in particular the perovskite layer, the conductivity of the electrodes, as well as the completeness of the removal of different materials in the patterning process. Thus, the challenges in module fabrication involve not only fabrication of large area perovskite solar cells, but also the patterning of the module and interconnections between the cells. Connection of individual cells in parallel and in series was attempted, and it was found that the connection in series was more favourable due to large current losses in parallel connection [386]. However, it should be noted that these cells were relatively low efficiency and therefore the conclusion may not hold for improved fabrication procedures resulting in high efficiency devices. Lasers have been used for patterning the FTO [285,383] or PbI2 layer and hole transport layer [383], while masks could be used for depositing patterned TiO2 by spray pyrolysis or screen-printing and metal electrode by evaporation [285,383]. Both FTO and titania can be patterned using a laser [129,130]. For large cells and modules, solvent could be used to remove perovskite and hole transport material from contact areas [285,331], but it is expected that this process would be less efficient compared to laser ablation. The type of laser can also affect the ablation results, and in general the completeness and cleanliness of removal of a layer can have significant effect on the fill factor of the patterned devices [384,387]. Nd:YVO4 laser results in better efficiency compared to CO2 laser [384]. Laser patterning techniques for PSC applications have been reviewed recently [387]. Laser patterning and PSC module fabrication processes are illustrated in Fig. 11 [387,388]. 5.3. Other devices In this section, we will provide a brief overview of PSCs with added functionalities, as well as integration of PSCs with other devices as well as other types of perovskite devices. There are fairly simple methods to add functionality to perovskite solar cells, some of which can also improve the efficiency and/or the stability of the devices. For example, phosphor coating of the PSCs results in UV protection of the devices as well as increased efficiency due to down-conversion of the UV light [389]. Photocurable fluoropolymers could also be used for this purpose [390]. Antireflection and self-cleaning nano cone structures have also been demonstrated for perovskite devices [336]. Other PSC structures with added functionalities include fiber-shaped solar cells for potential applications in flexible and wearable 25

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electronics [322,323]. In addition, PSCs with ZnO microwire and perovskite film on flexible substrate whose performance could be improved utilising piezo-phototronic effect upon application of strain [391]. Furthermore, closely related to solar cell applications is the use of perovskite materials in photodetectors [392–400]. This includes flexible devices [392–394], as well as devices sensitive to short wavelengths, either using MAPbCl3 [397] or novel 2D perovskites [398,401]. In addition, 3D arrays of 1024-pixel image sensors made of perovskite nanowires have been reported [402]. Image sensors have also been demonstrated for 1D aligned perovskite nanowire arrays [403]. Furthermore, flexible phototransistors based on MAPI/graphene channels have been reported as well [404–406]. Pure perovskite phototransistors with am bipolar transport have also been reported [407], as well as perovskite based field effect transistors (FETs) [408] and light-emitting FETs [409]. In addition to solar cells with added functionalities and photodetectors, the most common proposed use of perovskites has been in light emitting applications. For example, the use of perovskites in polymer/perovskite amplifying waveguides on Si/SiO2 for integration with semiconductor photonics was reported [410]. In general, perovskite materials and devices are highly promising as light emitters. A number of reports on perovskite-based light emitting diodes as well as stimulated emission from perovskite materials has been made in recent years. Due to a large number of reports on perovskite light emission (including quantum dots and nano structures) and their applications in light emitting diodes, this topic is beyond the scope of our review. Several recent reviews have covered the topic of perovskite applications in light emitting devices in detail [286,411–413]. Finally, a number of other devices have been demonstrated or proposed for perovskite materials, and the integration of different features has also been demonstrated. It has been demonstrated that PSCs could charge a lithium ion battery (LIB) [414]. This demonstrates the possibility of integrating PSCs with LIB storage. In addition, perovskite photovoltachromic super capacitors have also been reported [415]. In this device, a PSC for energy harvesting with a transparent electrode is integrated with electrochromic super capacitor which provides colour tunability and energy storage [415]. Thin film electrochemical capacitors based on organometallic halide perovskites which exhibited stable capacitance for over 10000 cycles have also been reported [416]. In addition, the potential of organometallic halide perovskites for thermoelectric applications has been investigated theoretically [417]. It was found that n-type perovskites would be promising thermoelectric materials, comparable to p-type Bi2Te3 in terms of figure of merit ZT [417]. It was also proposed, based on theoretical calculations, that the PSCs are a good choice for hybrid photovoltaic-thermoelectric systems [418]. Furthermore, it was demonstrated that organometallic halide perovskites, in particular MAPI, have high X-ray absorption cross-section and fast photoresponse [419]. The sensitivity and responsively of MAPI devices was found to be comparable to the established X-ray detectors, such as those based on amorphous Se [419]. Gamma photons detection using perovskite single crystals was also demonstrated [420]. Perovskite materials (both 3D and 2D perovskites) were also found to be suitable candidates for integrated optical cooler devices since they exhibited laser cooling by 23 K for MAPI and 58.7 K for a 2D perovskite [421]. In addition, flexible nonvolatile memory based on MAPI has also been demonstrated [422], as well as resistive switches, memristors and synaptic devices [423–428]. Other possible used include humidity sensors [429] and photoelectrocatalysis [430,431]. All these diverse possible applications demonstrate great potential of this class of materials. 6. Conclusion and future outlook There has been lots of progress in all aspects of organometallic halide perovskites research since the initial reports on efficient solar cells in 2012. Nevertheless, the majority of research efforts have been concentrating on the improvement of efficiency, which has now exceeded 22%. Progress has been made in manufacturing larger area cells as well as modules, which is of interest for commercialisation of the technology. However, based on life cycle assessments, the key issue appears to be the lifetime of the devices. Substantial improvements are needed in the stability of PSCs to make them commercially competitive. To achieve these improvements, issues which require attention is the perovskite material composition and film quality, the choice of charge transport layers (inorganic layers, such as SnO2 for ETL and NiO for HTL are likely to yield superior stability), electrodes (Cu or Cu/C are likely the best candidates) and barrier layers and encapsulation strategies. For comparisons of results in different literature reports, standardisation of the testing conditions is necessary and it is expected to be very helpful in further development of PSCs. The encapsulation requirements are likely to be similar to OLEDs, but different from OLEDs UV curing rather than thermal curing and minimal outgassing of the epoxy are needed for good performance. Stability issue is likely considerably more significant problem than the presence of Pb in the active layer for wide deployment of the devices. Another significant issue in addition to stability is upscaling and the reproducibility of the film preparation technique from one laboratory to another. Very high sensitivity of the perovskite film to processing conditions results in considerable variation in the film properties with small changes in the experimental procedure. This complicates the issue of reproducibility since often small details such as substrate size and not only device size (substrate size affects the uniformity of spin coated film), drying details etc. are not reported. Techniques with controllable solvent removal such as vacuum-flash assisted solution process may be promising to improve reproducibility. Nevertheless, a number of experimental groups from different countries have obtained high performing cells. This indicates that in spite of the difficulties in reproducibly preparing high quality perovskites this is a problem which can be solved by careful optimisation of material composition and the deposition conditions. In future development, it would be advisable to focus more effort on scalable deposition techniques different from spin-coating, as well as make attempts to eliminate the use of toxic organic solvents. Multilaboratory collaborative efforts are also expected to be helpful for further progress and addressing concerns regarding reproducibility and stability. Finally, in addition to solar cells, light emitting devices appear to be a highly promising application of perovskite materials, in addition to memories as well as a variety of other devices. There is rapidly increasing interest in the use of perovskites for light emission, although the focus on material choices is different from solar cells. In this area, the optimisation of device architectures as well as new 26

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material synthesis are needed to improve the efficiency, in particular at shorter emission wavelengths. The use of inorganic lead halide perovskites as well as less moisture sensitive 2D organometallic halide perovskites in light emitting devices may result in less severe stability problems compared to photovoltaics, but it should be noted that these devices are expected to operate under bias and thus they face different stability challenges. Acknowledgments Financial support from the ECF project 35/2015, the Strategic Research Theme and University Development Fund of University of Hong Kong is acknowledged. References [1] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Gr€atzel, N.-G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591. [2] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643–647. [3] R. Roesch, T. Faber, E. von Hauff, T.M. Brown, M. Lira-Cantu, H. Hoppe, Procedures and practices for evaluating thin-film solar cell stability, Adv. Energy Mater. 5 (2015) 1501407. [4] M.O. Reese, S.A. Gevorgyan, M. Jørgensen, E. Bundgaard, S.R. Kurtz, D.S. Ginley, D.C. Olson, M.T. Lloyd, P. Moryillo, E.A. Katz, A. Elschner, O. Haillant, T.R. Currier, V. Shrotriya, M. Hermenau, M. Riede, K.R. Kirov, G. Trimmel, T. Rath, O. Ingan€as, F. Zhang, M. Andersson, K. Tvingstedt, M. Lira-Cantu, D. Laird, C. McGuiness, S.J. Gowrisanker, M. Pannone, M. Xiao, J. Hauch, R. Steim, D.M. DeLongchamp, R. R€ osch, H. Hoppe, N. Espinosa, A. Urbina, G. Yaman-Uzunoglu, J.-B. Bonekamp, A.J.J.M. van Breemen, C. Girotto, E. Voroshazi, F.C. Krebs, Consensus stability testing protocols for organic photovoltaic materials and devices, Sol. Energy Mater. Sol. Cells 95 (2011) 1253–1267. [5] L.J. Rozanski, C.T.G. Smith, K.K. Gandhi, M.J. Beliatis, G.D.M.R. Dabera, K.D.G.I. Jayawardena, A.A.D.T. Adikaari, M.J. Kearney, S.R.P. Silva, A critical look at organic photovoltaic fabrication methodology: defining performance enhancement parameters relative to active area, Sol. Energy Mater. Sol. Cells 130 (2014) 513–520. [6] D. Gao, D.S. Seferos, Size-dependent behavior of polymer solar cells measured under partial illumination, Sol. Energy Mater. Sol. Cells 95 (2011) 3516–3519. [7] Q. Dong, F.Z. Liu, M.K. Wong, H.W. Tam, A.B. Djuri
sic, A. Ng, C. Surya, W.K. Chan, A.M.C. Ng, Encapsulation of perovskite solar cells for high humidity conditions, ChemSusChem 9 (2016) 2597–2603. [8] F.T. Si, O. Isabella, M. Zeman, Artifact interpretation of spectral response measurements on two-terminal multijunction solar cells, Adv. Energy Mater. (2016) 1601930. [9] M. Corazza, S.B. Simonsen, H. Gnaegi, K.T.S. Thyden, F.C. Krebs, S.A. Gevorgyan, Comparison of ultramicrotomy and focused-ion-beam for the preparation of TEM and STEM cross section of organic solar cells, Appl. Surf. Sci. 389 (2016) 462–468. [10] G. Teran-Escobar, D.M. Tanenbaum, E. Voroshazi, M. Hermenau, K. Norrman, M.T. Lloyd, Y. Galagan, B. Zimmermann, M. H€ osel, H.F. Dam, M. Jørgensen, S. Gevorgyan, S. Kudret, W. Maes, L. Lutsen, D. Vanderzande, U. Würfel, R. Andriessen, R. R€ osch, H. Hoppe, A. Rivaton, G.Y. Uzuno glu, D. Germack, B. Andreasen, M.V. Madsen, E. Bundgaard, F.C. Krebs, M. Lira-Cantu, On the stability of a variety of organic photovoltaic devices by IPCE and in situ IPCE analyses - the ISOS-3 inter-laboratory collaboration, Phys. Chem. Chem. Phys. 14 (2012) 11824–11845. [11] D.M. Tanenbaum, M. Hermenau, E. Voroshazi, M.T. Lloyd, Y. Galagan, B. Zimmermann, M. H€ osel, H.F. Dam, M. Jørgensen, S.A. Gevorgyan, S. Kudret, W. Maes, L. Lutsen, D. Vanderzande, U. Würfel, R. Andriessen, R. R€ osch, H. Hoppe, G. Teran-Escobar, M. Lira-Cantu, A. Rivaton, G.Y. Uzuno glu, D. Germack, B. Andreasen, M.V. Madsen, K. Norrman, F.C. Krebs, The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices, RSC Adv. 2 (2012) 882–893. [12] S.A. Gevorgyan, M. Corazza, M.V. Madsen, G. Bardizza, A. Pozza, H. Müllejans, J.C. Blakesley, G.F.A. Dibb, F.A. Castro, J.F. Trigo, C.M. Guillen, J.R. Herrero, P. Morvillo, M.G. Maglione, C. Minarini, F. Roca, S. Cros, C. Seraine, C.H. Law, P.S. Tuladhar, J.R. Durrant, F.C. Krebs, Interlaboratory indoor ageing of roll-toroll and spin coated organic photovoltaic devices: testing the ISOS tests, Polym. Degrad. Stab. 109 (2014) 162–170. [13] S.A. Gevorgyan, M.V. Madsen, H.F. Dam, M. Jørgensen, C.J. Fell, K.E. Anderson, B.C. Duck, A. Mescheloff, E.A. Katz, A. Elschner, R. Roesch, H. Hoppe, M. Hermenau, M. Riede, F.C. Krebs, Interlaboratory outdoor stability studies of flexible roll-to-roll coated organic photovoltaic modules: stability over 10,000 h, Sol. Energy Mater. Sol. Cells 116 (2013) 187–196. [14] R. R€ osch, D.M. Tanenbaum, M. J, M. Seeland, M. B€arenklau, M. Hermenau, E. Voroshazi, M.T. Lloyd, Y. Galagan, B. Zimmermann, U. Würfel, M. H€ osel, H.F. Dam, S.A. Gevorgyan, S. Kudret, W. Maes, L. Lutsen, D. Vanderzande, R. Andriessen, G. Teran-Escobar, M. Lira-Cantu, A. Rivaton, G.Y. Uzuno glu, D. Germack, B. Andreasen, M.V. Madsen, K. Norrman, H. Hoppe, F.C. Krebs, Investigation of the degradation mechanisms of a variety of organic photovoltaic devices by combination of imaging techniques —the ISOS-3 inter-laboratory collaboration, Energy Environ. Sci. 5 (2012) 6521–6540. [15] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, J.-P. Correa-Baena, W.R. Tress, A. Abate, A. Hagfeldt, M. Gr€atzel, Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science 354 (2016) 206–209. [16] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gr€atzel, Cesiumcontaining triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989–1997. [17] D.Q. Bi, W. Tress, M.I. Dar, P. Gao, J.S. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P.C. Baena, J.-D. Decoppet, S.M. Zakeeruddin, M.K. Nazeeruddin, M. Gr€ atzel, A. Hagfeldt, Efficient luminescent solar cells based on tailored mixed-cation perovskites, Sci. Adv. 2 (2016), e1501170. [18] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S.C. Ryu, J.W. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [19] D.Q. Bi, C.Y. Yi, J.S. Luo, J.-D. Decoppet, F. Zhang, S.M. Zakeeruddin, X. Li, A. Hagfeldt, M. Gr€atzel, Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%, Nat. Energy 1 (2016) 16142. [20] J.-Y. Seo, T. Matsui, J.S. Luo, J.-P. Correa-Baena, F. Giordano, M. Saliba, K. Schenk, A. Ummadisingu, K. Domanski, M. Hadadian, A. Hagfeldt, S.M. Zakeeruddin, U. Steiner, M. Gr€ atzel, A. Abate, Ionic liquid control crystal growth to enhance planar perovskite solar cells efficiency, Adv. Energy Mater. 6 (2016) 1600767. [21] E.H. Anaraki, A. Kermanpur, L. Steier, K. Domanski, T. Matsui, W. Tress, M. Saliba, A. Abate, M. Gr€atzel, A. Hagfeldt, J.-P. Correa-Baena, Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide, Energy Environ. Sci. 9 (2016) 3128–3134. [22] H.P. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z.R. Hong, J.B. You, Y.S. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (2014) 542–546. [23] N.Y. Ahn, D.-Y. Son, I.-H. Jang, S.M. Kang, M.S. Choi, N.-G. Park, Highly reproducible perovskite solar cells with average efficiency of 18.3% and best efficiency of 19.7% fabricated via Lewis base adduct of lead(II) iodide, J. Am. Chem. Soc. 137 (2015) 8696–8699. [24] H.T. Yoon, S.M. Kang, J.-K. Lee, M.S. Choi, Hysteresis-free low-temperature-processed planar perovskite solar cells with 19.1% efficiency, Energy Environ. Sci. 9 (2016) 2262–2266. [25] O. Ergen, S.M. Gilbert, T. Pham, S.J. Turner, M.T.Z. Tan, M.A. Worsley, A. Zettl, Graded bandgap perovskite solar cells, Nat. Mater. 16 (2017) 522–525 in print. [26] X. Li, D.Q. Bi, C.Y. Yi, J.-D. Decoppet, J.S. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Gr€atzel, A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells, Science 353 (2016) 58–62. [27] S.W. Shi, Y.F. Li, X.Y. Li, H.Q. Wang, Advancements in all-solid-state hybrid solar cells based on organometal halide perovskites, Mater. Horiz. 2 (2015) 378–405. 27

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[28] Y.Z. Wu, X.D. Yang, W. Chen, Y.F. Yue, M.L. Cai, F.X. Xie, E.B. Bi, A. Islam, L. Han, Perovskite solar cells with 18.21% efficiency and area over 1 cm2 fabricated by heterojunction engineering, Nat. Energy 1 (2016) 16148. [29] J.-P. Correa-Baena, M. Anaya, G. Lozano, W. Tress, K. Domanski, M. Saliba, T. Matsui, T.J. Jacobsson, M.E. Calvo, A. Abate, M. Gr€atzel, H. Miguez, A. Hagfeldt, Unbroken perovskite: interplay of morphology, electro-optical properties, and ionic movement, Adv. Mater. 28 (2016) 5031–5037. [30] H.M. Yates, M. Afzaal, A. Walter, J.L. Hodgkinson, S.-J. Moon, D. Sacchetto, M. Br€auninger, B. Niesen, S. Nicolay, M. McCarthy, M.E. Pemble, I.M. Povey, C. Ballif, Progression towards high efficiency perovskite solar cells via optimisation of the front electrode and blocking layer, J. Mater. Chem. C 4 (2016) 11269–11277. [31] Q.Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photonics 9 (2014) 106–112. [32] N. Yantara, D. Sabba, F. Yanan, J.M. Kadro, T. Moehl, P.P. Boix, S. Mhaisalkar, M. Gr€atzel, C. Gr€atzel, Loading of mesoporous titania films by CH3NH3PbI3 perovskite, single step vs. sequential deposition, Chem. Commun. 51 (2015) 4603–4606. [33] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gr€atzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316–319. [34] D.Q. Bi, A.M. El-Zohry, A. Hagfeldt, G. Boschloo, Unraveling the effect of PbI2 concentration on charge recombination kinetics in perovskite solar cells, ACS Photonics 2 (2015) 589–594. [35] H.-S. Ko, J.-W. Lee, N.-G. Park, 15.76% efficiency perovskite solar cells prepared under high relative humidity: importance of PbI2 morphology in two-step deposition of CH3NH3PbI3, J. Mater. Chem. A 3 (2015) 8808–8815. [36] H.Q. Li, S.B. Li, Y.F. Wang, H. Sarvari, P. Zhang, M.J. Wang, Z. Chen, A modified sequential deposition method for fabrication of perovskite solar cells, Sol. Energy 126 (2016) 243–251. [37] S.J. Hong, A.R. Han, E.C. Lee, K.-W. Ko, J.-H. Park, H.-J. Song, M.-H. Han, C.-H. Han, A facile and low-cost fabrication of TiO2 compact layer for efficient perovskite solar cells, Curr. Appl. Phys. 15 (2015) 574–579. [38] F.M. Li, C.X. Bao, H. Gao, W.D. Zhu, T. Yu, J. Yang, G. Fu, X.X. Zhou, Z.G. Zou, A facile spray-assisted fabrication of homogenous flat CH3NH3PbI3 films for high performance mesostructure perovskite solar cells, Mater. Lett. 157 (2015) 38–41. [39] Y. Luo, F.L. Meng, E.F. Zhao, Y.-Z. Zheng, Y. Zhou, X. Tao, Fine control of perovskite-layered morphology and composition via sequential deposition crystallization process towards improved perovskite solar cells, J. Power Sources 311 (2016) 130–136. [40] D.H. Cao, C.C. Stoumpos, C.D. Malliakas, M.J. Katz, O.K. Farha, J.T. Hupp, M.G. Kanatzidis, Remnant PbI2, an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells? Apl. Mater. 2 (2014) 091101. [41] Y.X. Zhao, K. Zhu, Three-step sequential solution deposition of PbI2-free CH3NH3PbI3 perovskite, J. Mater. Chem. A 3 (2015) 9086–9091. [42] J.H. Heo, H.J. Han, D.S. Kim, T.K. Ahn, S.H. Im, Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency, Energy Environ. Sci. 8 (2015) 1602–1608. [43] J.W. Seo, S.M. Park, Y.C. Kim, N.J. Jeon, J.H. Noh, S.C. Yoon, S.I. Seok, Benefits of very thin PCBM and LiF layers for solution-processed p-i-n perovskite solar cells, Energy Environ. Sci. 7 (2014) 2642–2646. [44] W. Chen, Y.D. Zhu, Y.Z. Yu, L.M. Xu, G.N. Zhang, Z.B. He, Low cost and solution processed interfacial layer based on poly(2-ethyl-2-oxazoline) nanodots for inverted perovskite solar cells, Chem. Mater. 28 (2016) 4879–4883. [45] Y.-J. Jeon, S.H. Lee, R.R. Kang, J.-E. Kim, J.-S. Yeo, S.-H. Lee, S.-S. Kim, J.-M. Yun, D.-Y. Kim, Planar heterojunction perovskite solar cells with superior reproducibility, Sci. Rep. 4 (2014) 6953. [46] L.K. Ono, Y.B. Qi, Surface and interface aspects of organometal halide perovskite materials and solar cells, J. Phys. Chem. Lett. 7 (2016) 4764–4794. [47] G.E. Eperon, V.M. Burlakov, A. Goriely, H.J. Snaith, Neutral color semitransparent microstructured perovskite solar cells, ACS Nano 8 (2014) 591–598. [48] S.S. Mali, C.K. Hong, P-i-n/n-i-p type planar hybrid structure of highly efficient perovskite solar cells towards improved air stability: synthetic strategies and the role of p-type hole transport layer (HTL) and n-type electron transport layer (ETL) metal oxides, Nanoscale 8 (2016) 10528–10540. [49] T.H. Liu, K. Chen, Q. Hu, R. Zhu, Q.H. Gong, Inverted perovskite solar cells: progresses and perspectives, Adv. Energy Mater. 6 (2016) 1600457. [50] J.S. Manser, J.A. Christians, P.V. Kamat, Intriguing optoelectronic properties of metal halide perovskites, Chem. Rev. 116 (2016) 12956–13008. [51] C.C. Stoumpos, M.G. Kanatzidis, Halide perovskites: poor man's high-performance semiconductors, Adv. Mater 28 (2016) 5778–5793. [52] B. Saparov, D.B. Mitzi, Organic-inorganic perovskites: structural versatility for functional materials design, Chem. Rev. 116 (2016) 4558–4596. [53] J. Berry, T. Buonassisi, D.A. Egger, G. Hodes, L. Kronik, Y.-L. Loo, I. Lubomirsky, S.R. Marder, Y. Mastai, J.S. Miller, D.B. Mitzi, Y. Paz, A.M. Rappe, I. Riess, B. Rybtchinski, O. Stafsudd, V. Stevanovic, M.F. Toney, D. Zitoun, A. Kahn, D. Ginley, D. Cahen, Hybrid organic-inorganic perovskites (HOIPs): opportunities and challenges, Adv. Mater. 27 (2015) 5102–5112. [54] D. Cui, Z. Yang, D. Yang, X.D. Ren, Y.C. Liu, Q.B. Wei, H.B. Fan, J.H. Zeng, S.Z.F. Liu, Color-tuned perovskite films prepared for efficient solar cell applications, J. Phys. Chem. C 120 (2016) 42–47. [55] W. Zhang, M. Anaya, G. Lozano, M.E. Calvo, M.B. Johnston, H. Míguez, H.J. Snaith, Highly efficient perovskite solar cells with tunable structural color, Nano Lett. 15 (2015) 1698–1702. [56] S.J. Zhang, G. Lanty, J.-S. Lauret, E. Deleporte, P. Audebert, L. Galmiche, Synthesis and optical properties of novel organic-inorganic hybrid nanolayer structure semiconductors, Acta Mater. 57 (2009) 3301–3309. [57] T. Leijtens, G.E. Eperon, N.K. Noel, S.N. Habisreutinger, A. Petrozza, H.J. Snaith, Stability of metal halide perovskite solar cells, Adv. Energy Mater. 5 (2015) 1500963. [58] D.B. Mitzi, K. Chondroudis, C.R. Kagan, Organic-inorganic electronics, IBM J. Res. Dev. 45 (2001) 29–45. [59] D.B. Mitzi, K. Chondroudis, C.R. Kagan, Design, structure, and optical properties of organic-inorganic perovskites containing an oligothiophene chromophore, Inorg. Chem. 38 (1999) 6246–6256. [60] P.P. Boix, S. Agarwala, T.M. Koh, N. Mathews, S.G. Mhaisalkar, Perovskite solar cells: beyond methylammonium lead iodide, J. Phys. Chem. Lett. 6 (2015) 898–907. [61] H.H. Tsai, W.Y. Nie, J.-C. Blancon, C.C.S. Toumpos, R. Asadpour, B. Harutyunyan, A.J. Neukirch, R. Verduzco, J.J. Crochet, S. Tretiak, L. Pedesseau, J. Even, M.A. Alam, G. Gupta, J. Lou, P.M. Ajayan, M.J. Bedzyk, M.G. Kanatzidis, A.D. Mohite, High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells, Nature 536 (2016) 312–316. [62] L.-J. Chen, C.-R. Lee, Y.-J. Chuang, Z.-H. Wu, C.Y. Chen, Synthesis and optical properties of lead-free cesium tin halide perovskite quantum rods with highperformance solar cell application, J. Phys. Chem. Lett. 7 (2016) 5028–5035. [63] G. Kieslich, S. Sun, A.K. Cheetham, An extended tolerance factor approach for organic-inorganic perovskites, Chem. Sci. 6 (2015) 3430–3433. [64] I. Lignos, S. Stavrakis, G. Nedelcu, L. Protesescu, A.J. deMello, M.V. Kovalenko, Synthesis of cesium lead halide perovskite nanocrystals in a droplet-based microfluidic platform: fast parametric space mapping, Nano Lett. 16 (2016) 1869–1877. [65] M.Z. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395–398. [66] A. Ng, Z.W. Ren, Q. Shen, S.H. Cheung, H.C. Gokkaya, G.X. Bai, J.C. Wang, L.J. Yang, S.K. So, A.B. Djuri
sic, W.W.-F. Leung, J.H. Hao, W.K. Chan, C. Surya, Efficiency enhancement by defect engineering in perovskite photovoltaic cells prepared using evaporated PbI2/CH3NH3 I multilayers, J. Mater. Chem. A 3 (2015) 9223–9231. [67] C.-W. Chen, H.-W. Kang, S.-Y. Hsiao, P.-F. Yang, K.-M. Chiang, H.-W. Lin, Efficient and uniform planar-type perovskite solar cells by simple sequential vacuum deposition, Adv. Mater. 26 (2014) 6647–6652. [68] J. Yin, H. Qu, J. Cao, H.L. Tai, J. Li, N.F. Zheng, Vapor-assisted crystallization control toward high performance perovskite photovoltaics with over 18% efficiency in the ambient atmosphere, J. Mater. Chem. A 4 (2016) 13203–13210. [69] Q. Chen, H.P. Zhou, T.-B. Song, S. Luo, Z. Hong, H.-S. Duan, L. Dou, Y.S. Liu, Y. Yang, Controllable self-induced passivation of hybrid lead iodide perovskites toward high performance solar cells, Nano Lett. 14 (2014) 4158–4163. [70] Q. Chen, H.P. Zhou, Z.R. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y.S. Liu, G. Li, Y. Yang, Planar heterojunction perovskite solar cells via vapor-assisted solution process, J. Am. Chem. Soc. 136 (2014) 622–625.

28

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[71] C. Ying, C.W. Shi, N. Wu, J.C. Zhang, M. Wang, A two-layer structured PbI2 thin film for efficient planar perovskite solar cells, Nanoscale 7 (2015) 12092–12095. [72] M.R. Leyden, L.K. Ono, S.R. Raga, Y. Kato, S.H. Wang, Y.B. Qi, High performance perovskite solar cells by hybrid chemical vapor deposition, J. Mater. Chem. A 2 (2014) 18742–18745. [73] S. Yuan, Z.W. Qiu, H.L. Zhang, X.F. Qiu, C.M. Gao, H.B. Gong, S.K. Yang, J.H. Yu, B.Q. Cao, Growth temperature-dependent performance of planar CH3NH3PbI3  solar cells fabricated by a two-step subliming vapor method below 120 C, RSC Adv. 6 (2016) 47459–47467. [74] M.M. Tavakoli, L.L. Gu, Y. Gao, C. Reckmeier, J. He, A.L. Rogach, Y. Yao, Z.Y. Fan, Fabrication of efficient planar perovskite solar cells using a one-step chemical vapor deposition method, Sci. Rep. 5 (2015) 14083. [75] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S.C. Ryu, S.I. Seok, Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897–903. [76] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476480. [77] B.-E. Cohen, S. Gamliel, L. Etgara, Parameters influencing the deposition of methylammonium lead halide iodide in hole conductor free perovskite-based solar cells, Apl. Mater. 2 (2014) 081502. [78] C. Bi, Q. Wang, Y.C. Shao, Y.B. Yuan, Z.G. Xiao, J.S. Huang, Non-wetting surface-driven high-aspect-ratio crystalline grain growth for efficient hybrid perovskite solar cells, Nat. Commun. 6 (2015) 7747. [79] Z.W. Ren, A. Ng, Q. Shen, H.C. Gokkaya, J.C. Wang, L.J. Yang, W.K. Yiu, G.X. Bai, A.B. Djuri
sic, W.W.-F. Leung, J.H. Hao, W.K. Chan, C. Surya, Thermal assisted oxygen annealing for high efficiency planar CH3NH3PbI3 perovskite solar cells, Sci. Rep. 4 (2014) 6752. [80] G. Li, T.Y. Zhang, Y.X. Zhao, Hydrochloric acid accelerated formation of planar CHCH3NH3PbI3 perovskite with high humidity tolerance, J. Mater. Chem. A 3 (2015) 19674–19678. [81] R. Munir, A.D. Sheikh, M. Abdelsamie, H.L. Hu, L.Y. Yu, K. Zhao, T. Kim, O.E. Tall, R.P. Li, D.-M. Smilgies, A. Amassian, Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases, Adv. Mater. (2017) 29, 1604113. [82] S.T. Williams, C.-C. Chueh, A.K.Y. Jen, Navigating organo-lead halide perovskite phase space via nucleation kinetics toward a deeper understanding of perovskite phase transformations and structure-property relationships, Small 11 (2015) 3088–3096. [83] Y.Y. Wang, J.M. Luo, R.M. Nie, X.Y. Deng, Planar perovskite solar cells using CH3NH3PbI3 films: a simple process suitable for large-scale production, Energy Technol. 4 (2016) 473–478. [84] Q.B. Wei, D. Yang, Z. Yang, X.D. Ren, Y.C. Liu, J.S. Feng, X.J. Zhu, S.Z.F. Liu, Effective solvent-additive enhanced crystallization and coverage of absorber layers for high efficiency formamidinium perovskite solar cells, RSC Adv. 6 (2016) 56807–56811. [85] Y.N. Chen, Y.X. Zhao, Z.Q. Liang, Non-thermal annealing fabrication of efficient planar perovskite solar cells with inclusion of NH4Cl, Chem. Mater. 27 (2015) 1448–1451. [86] Y.L. Guo, W. Sato, K. Shoyama, E. Nakamura, Sulfamic acid-catalyzed lead perovskite formation for solar cell fabrication on glass or plastic substrates, J. Am. Chem. Soc. 138 (2016) 5410–5416. [87] S. Bag, M.F. Durstock, Large perovskite grain growth in low-temperature solution-processed planar p-i-n solar cells by sodium addition, ACS Appl. Mater. Interfaces 8 (2016) 5053–5057. [88] H. Zhang, J. Mao, H.X. He, D. Zhang, H.L. Zhu, F.X. Xie, K.S. Wong, M. Gr€atzel, W.C.H. Choy, A smooth CH3NH3PbI3 film via a new approach for forming the PbI2 nanostructure together with strategically high CH3NH3I concentration for high efficient planar-heterojunction solar cells, Adv. Energy Mater. 5 (2015) 1501354. [89] X. Li, M.I. Dar, C.Y. Yi, J.S. Luo, M. Tschumi, S.M. Zakeeruddin, M.K. Nazeeruddin, H.W. Han, M. Gr€atzel, Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid omega-ammonium chlorides, Nat. Chem. 7 (2015) 703–711. [90] C.-M. Tsai, G.-W. Wu, S. Narra, H.-M. Chang, N. Mohanta, H.-P. Wu, C.-L. Wang, E.W.-G. Diau, Control of preferred orientation with slow crystallization for carbon-based mesoscopic perovskite solar cells attaining efficiency 15%, J. Mater. Chem. A 5 (2017) 739–747. [91] H.N. Chen, Z.H. Wei, H.X. He, X.L. Zheng, K.S. Wong, S.H. Yang, Solvent engineering boosts the efficiency of paintable carbon-based perovskite solar cells to beyond 14%, Adv. Energy Mater. 6 (2016) 1502087. [92] J.H. Kim, T.H. Hwang, S.H. Lee, B.H. Lee, J.W. Kim, G.S. Jang, S.H. Nam, B.W. Park, Solvent and intermediate phase as boosters for the perovskite transformation and solar cell performance, Sci. Rep. 6 (2016) 25648. [93] W.Z. Li, J.D. Fan, J.W. Li, G.D. Niu, Y.H. Mai, L.D. Wang, High performance of perovskite solar cells via catalytic treatment in two-step process: the case of solvent engineering, ACS Appl. Mater. Interfaces 8 (2016) 30107–30115. [94] P.F. Luo, W. Xia, S.W. Zhou, L. Sun, J.G. Cheng, C.X. Xu, Y.W. Lu, Solvent engineering for ambient-air-processed, phase-stable CsPbI3 in perovskite solar cells, J. Phys. Chem. Lett. 7 (2016) 3603–3608. [95] Z.L. Ku, N.H. Tiep, B. Wu, T.C. Sum, D. Fichou, H.J. Fan, Solvent engineering for fast growth of centimetric high-quality CH3NH3PbI3 perovskite single crystals, New J. Chem. 40 (2016) 7261–7264. [96] Z.H. Liu, E.-C. Lee, Solvent engineering of the electron transport layer using 1,8-diiodooctane for improving the performance of perovskite solar cells, Org. Electron 24 (2015) 101–105. [97] F.Z. Liu, Q. Dong, M.K. Wong, A.B. Djuri
sic, A. Ng, Z.W. Ren, Q. Shen, C. Surya, W.K. Chan, J. Wang, A.M.C. Ng, C.Z. Liao, H.K. Li, K.M. Shih, C.R. Wei, H.M. Su, J.F. Dai, Is excess PbI2 beneficial for perovskite solar cell performance? Adv. Energy Mater 6 (2016) 1502206. [98] F. Kamal Aldibaja, L. Badia, E. Mas-Marza, R.S. Sanchez, E.M. Barea, I. Mora-Sero, Effect of different lead precursors on perovskite solar cell performance and stability, J. Mater. Chem. A 3 (2015) 9194–9200. [99] M. Kaltenbrunner, G. Adam, E.D. Glowacki, M. Drack, R. Schw€ oediauer, L. Leonat, D.H. Apaydin, H. Groiss, M.C. Scharber, M.S. White, N.S. Sariciftci, S. Bauer, Flexible high power-per-weight perovskite solar cells with chromium oxide-metal contacts for improved stability in air, Nat. Mater. 14 (2015) 1032–1039. [100] N. Tripathi, Y. Shirai, M. Yanagida, A. Karen, K. Miyano, Novel surface passivation technique for low-temperature solution processed perovskite PV cells, ACS Appl. Mater. Interfaces 8 (2016) 4644–4650. [101] Z.F. Liu, P.F. Luo, W. Xia, S.W. Zhou, J.G. Cheng, L. Sun, C.X. Xu, Y.W. Lu, Acceleration effect of chlorine in the gas-phase growth process of CH3NH3PbI3(Cl) films for efficient perovskite solar cells, J. Mater. Chem. C 4 (2016) 6336–6344. [102] Z.-K. Wang, X. Gong, M. Li, Y. Hu, J.-M. Wang, H. Ma, L.-S. Liao, Induced crystallization of perovskites by a perylene underlayer for high-performance solar cells, ACS Nano 10 (2016) 5479–5489. [103] G.E. Eperon, S.N. Habisreutinger, T. Leijtens, B.J. Bruijnaers, J.J. van Franeker, D.W. deQuilettes, S. Pathak, R.J. Sutton, G. Grancini, D.S. Ginger, R.A.J. Janssen, A. Petrozza, H.J. Snaith, The importance of moisture in hybrid lead halide perovskite thin film fabrication, ACS Nano 9 (2015) 9380–9393. [104] X.C. Bao, Q.Q. Zhu, M. Qiu, A.L. Yang, Y.J. Wang, D.Q. Zhu, J.Y. Wang, R.Q. Yang, High-performance inverted planar perovskite solar cells without a hole transport layer via a solution process under ambient conditions, J. Mater. Chem. A 3 (2015) 19294–19298. [105] Z.R. Liang, S.H. Zhang, X.Q. Xu, N. Wang, J.X. Wang, X. Wang, Z.N. Bi, G. Xu, N.Y. Yuan, J.N. Ding, A large grain size perovskite thin film with a dense structure for planar heterojunction solar cells via spray deposition under ambient conditions, RSC Adv. 5 (2015) 60562–60569. [106] H. Gao, C.X. Ban, F.M. Li, T. Yu, J. Yang, W.D. Zhu, X.X. Zhou, G. Fu, Z.G. Zou, Nucleation and crystal growth of organic-inorganic lead halide perovskites under different relative humidity, ACS Appl. Mater. Interfaces 7 (2015) 9110–9117. [107] M.H. Lv, X. Dong, X. Fang, B.C. Lin, S. Zhang, X.Q. Xu, J.N. Ding, N.Y. Yuan, Improved photovoltaic performance in perovskite solar cells based on CH3NH3PbI3 films fabricated under controlled relative humidity, RSC Adv. 5 (2015) 93957–93963. [108] B.J. Jeong, S.M. Cho, S.H. Cho, J.H. Lee, I. Hwang, S.K. Hwang, J.H. Cho, T.-W. Lee, C.M. Park, Humidity controlled crystallization of thin CH3NH3PbI3 films for high performance perovskite solar cell, Phys. Status Solidi RRL 10 (2016) 381–387. [109] Q.D. Tai, P. You, H.Q. Sang, Z.K. Liu, C.L. Hu, H.L.W. Chan, F. Yan, Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity, Nat. Commun. 7 (2016) 11105. [110] A. Dubey, N. Kantack, N. Adhikari, K.M. Reza, S. Venkatesan, M. Kumar, D. Khatiwada, S. Darling, Q.Q. Qiao, Room temperature, air crystallized perovskite film for high performance solar cells, J. Mater. Chem. A 4 (2016) 10231–10240.

29

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[111] G. Wang, D. Liu, J. Xiang, D.C. Zhou, K. Alameh, B.F. Ding, Q.L. Song, Efficient perovskite solar cell fabricated in ambient air using one-step spin-coating, RSC Adv. 6 (2016) 43299–43303. [112] Z.B. Yang, C.-C. Chueh, F. Zuo, J.H. Kim, P.-W. Liang, A.K.-Y. Jen, High-performance fully printable perovskite solar cells via blade-coating technique under the ambient condition, Adv. Energy Mater 5 (2015) 1500328. [113] T. Gotanda, S. Mori, A. Matsui, H. Oooka, Effects of gas blowing condition on formation of perovskite layer on organic scaffolds, Chem. Lett. 45 (2016) 822–824. [114] F.Z. Huang, Y. Dkhissi, W.C. Huang, M.D. Xiao, I. Benesperi, S. Rubanov, Y. Zhu, X.F. Lin, L.C. Jiang, Y.C. Zhou, A. Gray-Weale, J. Etheridge, C.R. McNeill, R.A. Caruso, U. Bach, L. Spiccia, Y.-B. Cheng, Gas-assisted preparation of lead iodide perovskite films consisting of a monolayer of single crystalline grains for high efficiency planar solar cells, Nano Energy 10 (2014) 10–18. [115] P.F. Luo, S.W. Zhou, Z.F. Liu, W. Xia, L. Sun, J.G. Cheng, C.X. Xu, Y.W. Lu, A novel transformation route from PbS to CH3NH3PbI3 for fabricating curved and large-area perovskite films, Chem. Commun. 52 (2016) 11203–11206. [116] Z. Yang, B. Cai, B. Zhou, T.T. Yao, W. Yu, S.Z.F. Liu, W.-H. Zhang, C. Li, An up-scalable approach to CH3NH3PbI3 compact films for high-performance perovskite solar cells, Nano Energy 15 (2015) 670–678. [117] X.D. Ren, Z. Yang, D. Yang, X. Zhang, D. Cui, Y.C. Liu, Q.B. Wei, H.B. Fan, S.Z.F. Liu, Modulating crystal grain size and optoelectronic properties of perovskite films for solar cells by reaction temperature, Nanoscale 8 (2016) 3816–3822. [118] W.Y. Nie, H.H. Tsai, R. Asadpour, J.-C. Blancon, A.J. Neukirch, G. Gupta, J.J. Crochet, M. Chhowalla, S. Tretiak, M.A. Alam, H.-L. Wang, A.D. Mohite, Highefficiency solution-processed perovskite solar cells with millimeter-scale grains, Science 347 (2015) 522–525. [119] L.-L. Gao, L.-S. Liang, X.-X. Song, B. Ding, G.-J. Yang, B. Fan, C.-X. Li, C.-J. Li, Preparation of flexible perovskite solar cells by a gas pump drying method on a plastic substrate, J. Mater. Chem. A 4 (2016) 3704–3710. [120] B.J. Jeong, I. Hwang, S.H. Cho, E.H. Kim, S.Y. Cha, J.S. Lee, H.S. Kang, S.M. Cho, H.Y. Choi, C.M. Park, Solvent-assisted gel printing for micropatterning thin organic-inorganic hybrid perovskite films, ACS Nano 10 (2016) 9026–9035. [121] H.-L. Hsu, C.-P. Chen, J.-Y. Chang, Y.-Y. Yu, Y.-K. Shen, Two-step thermal annealing improves the morphology of spin-coated films for highly efficient perovskite hybrid photovoltaics, Nanoscale 6 (2014) 10281–10288. [122] F.X. Xie, D. Zhang, H.M. Su, X.G. Ren, K.S. Wong, M. Gr€atzel, W.C.H. Choy, Vacuum-assisted thermal annealing of CH3NH3PbI3 for highly stable and efficient perovskite solar cells, ACS Nano 9 (2015) 639–646. [123] X. Sun, C.F. Zhang, J.J. Chang, H.F. Yang, H. Xi, G. Lu, D.Z. Chen, Z.H. Lin, X.L. Lu, J.C. Zhang, Y. Hao, Mixed-solvent-vapor annealing of perovskite for photovoltaic device efficiency enhancement, Nano Energy 28 (2016) 417–425. [124] C.-G. Wu, C.-H. Chiang, S.H. Chang, A perovskite cell with a record-high Voc of 1.61 V based on solvent annealed CH3NH3PbBr3/ICBA active layer, Nanoscale 8 (2016) 4077–4085. [125] B.H. Wang, K.Y. Wong, S.F. Yang, T. Chen, Crystallinity and defect state engineering in organo-lead halide perovskite for high-efficiency solar cells, J. Mater. Chem. A 4 (2016) 3806–3812. [126] T.W. Jeon, H.M. Jin, S.H. Lee, J.M. Lee, H.I. Park, M.K. Kim, K.J. Lee, B.H. Shin, S.O. Kim, Laser crystallization of organic-inorganic hybrid perovskite solar cells, ACS Nano 10 (2016) 7907–7914. [127] G. Yang, H. Tao, P.L. Qin, W.J. Ke, G.J. Fang, Recent progress in electron transport layers for efficient perovskite solar cells, J. Mater. Chem. A 4 (2016) 3970–3990. [128] Z.J. Wang, D.W. Cao, R. Xu, S.C. Qu, Z.G. Wang, Y. Lei, Realizing ordered arrays of nanostructures: a versatile platform for converting and storing energy efficiently, Nano Energy 19 (2016) 328–362. [129] A. Fakharuddin, F. Di Giacomo, A.L. Palma, F. Matteocci, I. Ahmed, S. Razza, A. D'Epifanio, S. Licoccia, J. Ismail, A. Di Carlo, T.M. Brown, R. Jose, Vertical TiO2 nanorods as a medium for stable and high-efficiency perovskite solar modules, ACS Nano 9 (2015) 8420–8429. [130] A. Fakharuddin, A.L. Palma, F. Di Giacomo, S. Casaluci, F. Matteocci, Q. Wali, M. Rauf, A. Di Carlo, T.M. Brown, R. Jose, Solid state perovskite solar modules by vacuum-vapor assisted sequential deposition on Nd:YVO4 laser patterned rutile TiO2 nanorods, Nanotechnol 26 (2015) 494002. [131] G.S. Han, H.S. Chung, D.H. Kim, B.J. Kim, J.-W. Lee, N.-G. Park, I.S. Cho, J.-K. Lee, S. Lee, H.S. Jung, Epitaxial 1D electron transport layers for high-performance perovskite solar cells, Nanoscale 7 (2015) 15284–15290. [132] X.L. Zheng, Z.H. Wei, H.N. Chen, Q.P. Zhang, H.X. He, S. Xiao, Z.Y. Fan, K.S. Wong, S.H. Yang, Designing nanobowl arrays of mesoporous TiO2 as an alternative electron transporting layer for carbon cathode-based perovskite solar cells, Nanoscale 8 (2016) 6393–6402. [133] S.S. Mali, C.S. Shim, H.J. Kim, P.S. Patil, C.K. Hong, In situ processed gold nanoparticle-embedded TiO2 nanofibers enabling plasmonic perovskite solar cells to exceed 14% conversion efficiency, Nanoscale 8 (2016) 2664–2677. [134] W.-Q. Wu, F. Z. Huang, D. H. Chen, Y.-B. Cheng, R. A. Caruso, Solvent-mediated dimension tuning of semiconducting oxide nanostructures as efficient charge extraction thin films for perovskite solar cells with efficiency exceeding 16%, Adv. Energy Mater. 6. [135] X. Chen, L.J. Tang, S. Yang, Y. Hou, H.G. Yang, A low-temperature processed flower-like TiO2 array as an electron transport layer for high-performance perovskite solar cells, J. Mater. Chem. A 4 (2016) 6521–6526. [136] J.M. Choi, S.K. Song, M.T. H€ orantner, H.J. Snaith, T.H. Park, Well-defined nanostructured, single crystalline TiO2 electron transport layer for efficient planar perovskite solar cells, ACS Nano 10 (2016) 6029–6036. [137] Y.Y. Du, H.K. Cai, H.B. Wen, Y.X. Wu, L.K. Huang, J. Ni, J. Li, J.J. Zhang, Novel combination of efficient perovskite solar cells with low temperature processed compact TiO2 layer via anodic oxidation, ACS Appl. Mater. Interfaces 8 (2016) 12836–12842. [138] D. Yang, X. Zhou, R.X. Yang, Z. Yang, W. Yu, X.L. Wang, C. Li, S.Z.F. Liu, R.P.H. Chang, Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells, Energy Environ. Sci. 9 (2016) 3071–3078. [139] S.F. Shaikh, H.-C. Kwon, W.S. Yang, H.W. Hwang, H.S. Lee, E.S. Lee, S.I. Ma, J.H. Moon, La2 O3 interface modification of mesoporous TiO2 nanostructures enabling highly efficient perovskite solar cells, J. Mater. Chem. A 4 (2016) 15478–15485. [140] L.F. Zhu, Y.Z. Xu, J.J. Shi, H.Y. Zhang, X. Xu, Y.H. Zhao, Y.H. Luo, Q.B. Meng, D.M. Li, Efficient perovskite solar cells via simple interfacial modification toward a mesoporous TiO2 electron transportation layer, RSC Adv. 6 (2016) 82282–82288. [141] I.S. Kim, D.H. Cao, D.B. Buchholz, J.D. Emery, O.K. Farha, J.T. Hupp, M.G. Kanatzidis, A.B.F. Martinson, Liquid water- and heat-resistant hybrid perovskite photovoltaics via an inverted ALD Oxide electron extraction layer design, Nano Lett. 16 (2016) 7786–7790. [142] I.S. Kim, R.T. Haasch, D.H. Cao, O.K. Farha, J.T. Hupp, M.G. Kanatzidis, A.B.F. Martinson, Amorphous TiO2 compact layers via ALD for planar halide perovskite photovoltaics, ACS Appl. Mater. Interfaces 8 (2016) 24310–24314. [143] J. Zhang, E. Jose Ju arez-Perez, I. Mora-Ser o, B. Viana, T. Pauporte, Fast and low temperature growth of electron transport layers for efficient perovskite solar cells, J. Mater. Chem. A 3 (2015) 4909–4915. [144] Y. Dkhissi, S. Meyer, D. Chen, H.C. Weerasinghe, L. Spiccia, Y.-B. Cheng, R.A. Caruso, Stability comparison of perovskite solar cells based on zinc oxide and titania on polymer substrates, ChemSusChem 9 (2016) 687–695. [145] D.Q. Bi, G. Boschloo, S. Schwarzmüller, L. Yang, E.M.J. Johansson, A. Hagfeldt, Efficient and stable CH3NH3PbI3-sensitized ZnO nanorod array solid-state solar cells, Nanoscale 5 (2013) 11686–11691. [146] J. Yang, B.D. Siempelkamp, E. Mosconi, F. De Angelis, T.L. Kelly, Origin of the thermal instability in CH3NH3PbI3 thin films deposited on ZnO, Chem. Mater. 27 (2015) 4229–4236. [147] H.N. Si, Q.L. Liao, Z. Zhang, Y. Li, X. Yang, G.J. Zhang, Z. Kang, Y. Zhang, An innovative design of perovskite solar cells with Al2O3 inserting at ZnO/perovskite interface for improving the performance and stability, Nano Energy 22 (2016) 223–231. [148] Y. Guo, X. Li, L.L. Kang, X. He, Z.Q. Ren, J.D. Wu, J.Y. Qi, Improvement of stability of ZnO/CHCH3NH3PbI3 bilayer by aging step for preparing high-performance perovskite solar cells under ambient conditions, RSC Adv. 6 (2016) 62522–62528. [149] J.P.C. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, F. Giordano, T.J. Jacobsson, A.R.S. Kandada, S.M. Zakeeruddin, A. Petrozza, A. Abate, M.K. Nazeeruddin, M. Gr€atzel, A. Hagfeldt, Highly efficient planar perovskite solar cells through band alignment engineering, Energy Environ. Sci. 8 (2015) 2928–2934.

30

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[150] C.L. Wang, D.W. Zhao, C.R. Grice, W.Q. Liao, Y. Yu, A. Cimaroli, N. Shrestha, P.J. Roland, J. Chen, Z.H. Yu, P. Liu, N. Cheng, R.J. Ellingson, X.Z. Zhao, Y.F. Yan, Low-temperature plasma-enhanced atomic layer deposition of tin oxide electron selective layers for highly efficient planar perovskite solar cells, J. Mater. Chem. A 4 (2016) 12080–12087. [151] M.W. Park, J.-S. Park, I.K. Han, J.Y. Oh, High-performance flexible and air-stable perovskite solar cells with a large active area based on poly(3-hexylthiophene) nanofibrils, J. Mater. Chem. A 4 (2016) 11307–11316. [152] Z.L. Zhu, Y. Bai, X. Liu, C.-C. Chueh, S. Yang, A.K.Y. Jen, Enhanced efficiency and stability of inverted perovskite solar cells using highly crystalline SnO2 nanocrystals as the robust electron-transporting layer, Adv. Mater 28 (2016) 6478–6484. [153] W.J. Ke, G.J. Fang, Q. Liu, L.B. Xiong, P.L. Qin, H. Tao, J. Wang, H.W. Lei, B.R. Li, J.W. Wan, G. Yang, Y.F. Yan, Low-temperature solution-processed tin oxide as an alternative electron transporting layer for efficient perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 6730–6733. [154] H.-S. Rao, B.-X. Chen, W.-G. Li, Y.-F. Xu, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, Improving the extraction of photogenerated electrons with SnO2 nanocolloids for efficient planar perovskite solar cells, Adv. Funct. Mater 25 (2015) 7200–7207. [155] Q. Jiang, L.Q. Zhang, H.L. Wang, X.L. Yang, J.H. Meng, H. Liu, Z.G. Yin, J.L. Wu, X.W. Zhang, J.B. You, Enhanced electron extraction using SnO2 for highefficiency planar-structure HC(NH)22PbI3-based perovskite solar cells, Nat. Energy 2 (2016) 16177. [156] M.W. Park, J.-Y. Kim, H.J. Son, C.-H. Lee, S.S. Jang, M.J. Ko, Low-temperature solution-processed Li-doped SnO2 as an effective electron transporting layer for high-performance flexible and wearable perovskite solar cells, Nano Energy 26 (2016) 208–215. [157] W.A. Dunlap-Shohl, R. Younts, B. Gautam, K. Gundogdu, D.B. Mitzi, Effects of Cd Diffusion and doping in high-performance perovskite solar cells using CdS as electron transport layer, J. Phys. Chem. C 120 (2016) 16437–16445. [158] M.C. Qin, J.J. Ma, W.J. Ke, P.L. Qin, H.W. Lei, H. Tao, X.L. Zheng, L.B. Xiong, Q. Liu, Z.L. Chen, J.Z. Lu, G. Yang, G.J. Fang, Perovskite solar cells based on lowtemperature processed indium oxide electron selective layers, ACS Appl. Mater. Interfaces 8 (2016) 8460–8466. [159] X. Liu, C.-C. Chueh, Z.L. Zhu, S.B. Jo, Y. Sun, A.K.Y. Jen, Highly crystalline Zn2SnO4 nanoparticles as efficient electron-transporting layers toward stable inverted and flexible conventional perovskite solar cells, J. Mater. Chem. A 4 (2016) 15294–15301. [160] K. Mahmood, B.S. Swain, A.R. Kirmani, A. Amassian, Highly efficient perovskite solar cells based on a nanostructured WO3/TiO2 core-shell electron transporting material, J. Mater. Chem. A 3 (2015) 9051–9057. [161] Z.L. Zhu, J.-Q. Xu, C.-C. Chueh, H.B. Liu, Z.A. Li, X.S. Li, H.Z. Chen, A.K.-Y. Jen, A low-temperature, solution-processable organic electron-transporting layer based on planar coronene for high-performance conventional perovskite solar cells, Adv. Mater. 28 (2016) 10786–10793. [162] H. Zhang, L.W. Xue, J.B. Han, Y.Q. Fu, Y. Shen, Z.G. Zhang, Y.F. Li, M.K. Wang, New generation perovskite solar cells with solution-processed amino-substituted perylene diimide derivative as electron-transport layer, J. Mater. Chem. A 4 (2016) 8724–8733. [163] K. Wojciechowski, T. Leijtens, S. Siprova, C. Schlueter, M.T. H€ orantner, J.T.-W. Wang, C.-Z. Li, A.K.Y. Jen, T.-L. Lee, H.J. Snaith, C60 as an efficient n -type compact layer in perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 2399–2405. [164] L.X. Zhao, X.Y. Wang, X.D. Li, W.J. Zhang, X.H. Liu, Y.J. Zhu, H.-Q. Wang, J.F. Fang, Improving performance and reducing hysteresis in perovskite solar cells by using F8BT as electron transporting layer, Sol. Energy Mater. Sol. Cells 157 (2016) 79–84. [165] F.M. Jin, C.Y. Liu, F.H. Hou, Q.G. Song, Z.S. Su, B. Chu, P.F. Cheng, H.F. Zhao, W.L. Li, Hexadecafluorophthalocyaninatocopper as an electron conductor for highefficiency fullerene-free planar perovskite solar cells, Sol. Energy Mater. Sol. Cells 157 (2016) 510–516. [166] C. Sun, Z.H. Wu, H.-L. Yip, H. Zhang, X.-F. Jiang, Q.F. Xue, Z.C. Hu, Z.H. Hu, Y. Shen, M.K. Wang, F. Huang, Y. Cao, Amino-functionalized conjugated polymer as an efficient electron transport layer for high-performance planar-heterojunction perovskite solar cells, Adv. Energy Mater. 6 (2016) 1501534. [167] W.M. Qiu, M. Buffi ere, G. Brammertz, U.W. Paetzold, L. Froyen, P. Heremans, D. Cheyns, High efficiency perovskite solar cells using a PCBM/ZnO double electron transport layer and a short air-aging step, Org. Electron 26 (2015) 30–35. [168] W.J. Ke, D.W. Zhao, C.X. Xiao, C.L. Wang, A.J. Cimaroli, C.R. Grice, M.J. Yang, Z. Li, C.-S. Jiang, M. Al-Jassim, K. Zhu, M.G. Kanatzidis, G.J. Fang, Y.F. Yan, Cooperative tin oxide fullerene electron selective layers for high-performance planar perovskite solar cells, J. Mater. Chem. A 4 (2016) 14276–14283. [169] X.B. Xu, Q. Chen, Z.R. Hong, H.P. Zhou, Z.H. Liu, W.-H. Chang, P.Y. Sun, H.J. Chen, N. De Marco, M.K. Wang, Y. Yang, Working mechanism for flexible perovskite solar cells with simplified architecture, Nano Lett. 15 (2015) 6514–6520. [170] L.K. Huang, Z.Y. Hu, J. Xu, X.X. Sun, Y.Y. Du, J. Ni, H.K. Cai, J. Li, J.J. Zhang, Efficient electron-transport layer-free planar perovskite solar cells via recycling the FTO/glass substrates from degraded devices, Sol. Energy Mater. Sol. Cells 152 (2016) 118–124. [171] J. Wei, H. Li, Y.C. Zhao, W.K. Zhou, R. Fu, H.Y. Pan, Q. Zhao, Flexible perovskite solar cells based on the metal-insulator-semiconductor structure, Chem. Commun. 52 (2016) 10791–10794. [172] C.H. Teh, R. Daik, E.L. Lim, C.C. Yap, M.A. Ibrahim, N.A. Ludin, K. Sopian, M.A.M. Teridi, A review of organic small molecule-based hole-transporting materials for meso-structured organic-inorganic perovskite solar cells, J. Mater. Chem. A 4 (2016) 15788–15822. [173] Q. Wang, C.-C. Chueh, M. Eslamian, A.K.-Y. Jen, Modulation of PEDOT: PSS pH for efficient inverted perovskite solar cells with reduced potential loss and enhanced stability, ACS Appl. Mater. Interfaces 8 (2016) 32068–32076. [174] G.-W. Kim, G.H. Kang, J.S. Kim, G.-Y. Lee, H.I. Kim, L.M. Pyeon, J.C. Lee, T.H. Park, Dopant-free polymeric hole transport materials for highly efficient and stable perovskite solar cells, Energy Environ. Sci. 9 (2016) 2326–2333. [175] M. Li, Z.-K. Wang, Y.-G. Yang, Y. Hu, S.-L. Feng, J.-M. Wang, X.-Y. Gao, L.-S. Liao, Copper salts doped spiro-OMeTAD for high-performance perovskite solar cells, Adv. Energy Mater 6 (2016) 1601156. [176] J. Cao, S.G. Mo, X.J. Jing, J. Yin, J. Li, N.F. Zheng, Trace surface-clean palladium nanosheets as a conductivity enhancer in hole-transporting layers to improve the overall performances of perovskite solar cells, Nanoscale 8 (2016) 3274–3277. [177] W.H. Nguyen, C.D. Bailie, E.L. Unger, M.D. McGehee, Enhancing the hole-conductivity of spiro-OMeTAD without oxygen or lithium salts by using spiro(TFSI)2 in perovskite and dye-sensitized solar cells, J. Am. Chem. Soc. 136 (2014) 10996–11001. [178] Y.S. Kwon, J. Lim, H.-J. Yun, Y.-H. Kim, T. Park, A diketopyrrolopyrrole-containing hole transporting conjugated polymer for use in efficient stable organicinorganic hybrid solar cells based on a perovskite, Energy Environ. Sci. 7 (2014) 1454–1460. [179] H.J. Choi, S.J. Park, S.H. Paek, P. Ekanayake, M.K. Nazeeruddin, J.J. Ko, Efficient star-shaped hole transporting materials with diphenylethenyl side arms for an efficient perovskite solar cell, J. Mater. Chem. A 2 (2014) 19136–19140. [180] F.H. Hou, F.M. Jin, B. Chu, Z.S. Su, Y. Gao, H.F. Zhao, P.F. Cheng, J.X. Tang, W.L. Li, Hydrophobic hole-transporting layer induced porous PbI2 film for stable and efficient perovskite solar cells in 50% humidity, Sol. Energy Mater. Sol. Cells 157 (2016) 989–995. [181] Y.M. Xiao, G.Y. Han, Y.Z. Chang, H.H. Zhou, M.Y. Li, Y.P. Li, An all-solid-state perovskite-sensitized solar cell based on the dual function polyaniline as the sensitizer and p-type hole-transporting material, J. Power Sources 267 (2014) 1–8. [182] H.S. Choi, C.-K. Mai, H.-B. Kim, J.K. Jeong, S.Y. Song, G.C. Bazan, J.Y. Kim, A.J. Heeger, Conjugated polyelectrolyte hole transport layer for inverted-type perovskite solar cells, Nat. Commun. 6 (2015) 7348. [183] J. Liu, Y.Z. Wu, C.J. Qin, X.D. Yang, T. Yasuda, A. Islam, K. Zhang, W.Q. Peng, W. Chen, L.Y. Han, A dopant-free hole-transporting material for efficient and stable perovskite solar cells, Energy Environ. Sci. 7 (2014) 2963–2967. [184] J.W. Jo, M.-S. Seo, M.W. Park, J.-Y. Kim, J.S. Park, I.K. Han, H.J. Ahn, J.W. Jung, B.-H. Sohn, M.J. Ko, H.J. Son, Improving performance and stability of flexible planar-heterojunction perovskite solar cells using polymeric hole-transport material, Adv. Funct. Mater 26 (2016) 4464–4471. [185] C.V. Kumar, G. Sfyri, D. Raptis, E. Stathatos, P. Lianos, Perovskite solar cell with low cost Cu-phthalocyanine as hole transporting material, RSC Adv. 5 (2015) 3786–3791. [186] J.Y. Xiao, J.J. Shi, H.B. Liu, Y.Z. Xu, S.T. Lv, Y.H. Luo, D.M. Li, Q.B. Meng, Y.L. Li, Efficient CH3NH3PbI3 perovskite solar cells based on graphdiyne (GD)-modified P3HT hole-transporting material, Adv. Energy Mater. 5 (2015) 1401943. [187] H.-W. Chen, T.-Y. Huang, T.-H. Chang, Y. Sanehira, C.-W. Kung, C.-W. Chu, M. Ikegami, T. Miyasaka, K.-C. Ho, Efficiency enhancement of hybrid perovskite solar cells with MEH-PPV hole-transporting layers, Sci. Rep. 6 (2016) 34319. [188] Z.W. Wang, Q.Q. Dong, Y.J. Xia, H. Yu, K.C. Zhang, X.D. Liu, X. Guo, Y. Zhou, M.J. Zhang, B. Song, Copolymers based on thiazolothiazole-dithienosilole as holetransporting materials for high efficient perovskite solar cells, Org. Electron 33 (2016) 142–149.

31

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[189] S.N. Habisreutinger, T. Leijtens, G.E. Eperon, S.D. Stranks, R.J. Nicholas, H.J. Snaith, Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells, Nano Lett. 14 (2014) 5561–5568. [190] J.-S. Yeo, R.R. Kang, S.H. Lee, Y.-J. Jeon, N.S. Myoung, C.-L. Lee, D.-Y. Kim, J.-M. Yun, Y.-H. Seo, S.-S. Kim, S.-I. Na, Highly efficient and stable planar perovskite solar cells with reduced graphene oxide nanosheets as electrode interlayer, Nano Energy 12 (2015) 96–104. [191] J.M. Kim, M.A.M. Teridi, A.R.B.M. Yusoff, J. Jang, Stable and null current hysteresis perovskite solar cells based nitrogen doped graphene oxide nanoribbons hole transport layer, Sci. Rep. 6 (2016) 27773. [192] J.H. Kim, P.-W. Liang, S.T. Williams, N. Cho, C.-C. Chueh, M.S. Glaz, D.S. Ginger, A.K.Y. Jen, High-performance and environmentally stable planar heterojunction perovskite solar cells based on a solution-processed copper-doped nickel oxide hole-transporting layer, Adv. Mater. 27 (2015) 695–701. [193] J.B. You, L. Meng, T.-B. Song, T.-F. Guo, Y.M. Yang, W.-H. Chang, Z.R. Hong, H.J. Chen, H.P. Zhou, Q. Chen, Y.S. Liu, N. De Marco, Y. Yang, Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nanotechnol. 11 (2016) 75–81. [194] W. Chen, Y.Z. Wu, Y.F. Yue, J. Liu, W.J. Zhang, X.D. Yang, H. Chen, E.B. Bi, I. Ashraful, M. Gr€atzel, L.Y. Han, Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers, Science 350 (2015) 944–948. [195] X.T. Yin, P. Chen, M.D. Que, Y.L. Xing, W.X. Que, C.M. Niu, J.Y. Shao, Highly efficient flexible perovskite solar cells using solution-derived NiOx hole contacts, ACS Nano 10 (2016) 3630–3636. [196] H. Zhang, J.Q. Cheng, F. Lin, H.X. He, J. Mao, K.S. Wong, A.K.-Y. Jen, W.C.H. Choy, Pinhole-free and surface-nanostructured NiOx film by room-temperature solution process for high-performance flexible perovskite solar cells with good stability and reproducibility, ACS Nano 10 (2016) 1503–1511. [197] X.B. Xu, Z.H. Liu, Z.X. Zuo, M. Zhang, Z.X. Zhao, Y. Shen, H.P. Zhou, Q. Chen, Y. Yang, M.K. Wang, Hole selective NiO contact for efficient perovskite solar cells with carbon electrode, Nano Lett. 15 (2015) 2402–2408. [198] Y. Bai, H.N. Chen, S. Xiao, Q.F. Xue, T. Zhang, Z.L. Zhu, Q. Li, C. Hu, Y. Yang, Z.C. Hu, F. Huang, K.S. Wong, H.-L. Yip, S.H. Yang, Effects of a molecular monolayer modification of NiO nanocrystal layer surfaces on perovskite crystallization and interface contact toward faster hole extraction and higher photovoltaic performance, Adv. Funct. Mater. 26 (2016) 2950–2958. [199] P. Qin, S. Tanaka, S. Ito, N. Tetreault, K. Manabe, H. Nishino, M.K. Nazeeruddin, M. Gr€atzel, Inorganic hole conductor-based lead halide perovskite solar cells with 12.4% conversion efficiency, Nat. Commun. 5 (2014) 3834. [200] J.A. Christians, R.C.M. Fung, P.V. Kamat, An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper iodide, J. Am. Chem. Soc. 136 (2014) 758–764. [201] Q.L. Wu, C. Xue, Y. Li, P.C. Zhou, W.F. Liu, J. Zhu, S.Y. Dai, C.F. Zhu, S.F. Yang, Kesterite Cu2 ZnSnS4 as a low-cost inorganic hole-transporting material for highefficiency perovskite solar cells, ACS Appl. Mater. Interfaces 7 (2015) 28466–28473. [202] W.L. Yu, F. Li, H. Wang, E. Alarousu, Y. Chen, B. Lin, L.F. Wang, M.N. Hedhili, Y.Y. Li, K.W. Wu, X.B. Wang, O.F. Mohammed, T. Wu, Ultrathin Cu2O as an efficient inorganic hole transporting material for perovskite solar cells, Nanoscale 8 (2016) 6173–6179. [203] B.K. Koo, H.S. Jung, M.W. Park, J.-Y. Kim, H.J. Son, J.H. Cho, M.J. Ko, Pyrite-based bi-functional layer for long-term stability and high-performance of organolead halide perovskite solar cells, Adv. Funct. Mater. 26 (2016) 5400–5407. [204] S. Aharon, S. Gamliel, B. El Cohen, L. Etgar, Depletion region effect of highly efficient hole conductor free CH3NH3PbI3 perovskite solar cells, Phys. Chem. Chem. Phys. 16 (2014) 10512–10518. [205] S. Aharon, B. El Cohen, L. Etgar, Hybrid lead halide iodide and lead halide bromide in efficient hole conductor free perovskite solar cell, J. Phys. Chem. C 118 (2014) 17160–17165. [206] S. Aharon, A. Dymshits, A. Rotem, L. Etgar, Temperature dependence of hole conductor free formamidinium lead iodide perovskite based solar cells, J. Mater. Chem. A 3 (2015) 9171–9178. [207] H.W. Zhou, Y.T. Shi, Q.S. Dong, H. Zhang, Y.J. Xing, K. Wang, Y. Du, T.L. Ma, Hole-conductor-free, metal-electrode-free TiO2/CH3NH3PbI3 heterojunction solar cells based on a low-temperature carbon electrode, J. Phys. Chem. Lett. 5 (2014) 3241–3246. [208] Z.Y. Liu, T.L. Shi, Z.R. Tang, B. Sun, G.L. Liao, Using a low-temperature carbon electrode for preparing hole-conductor-free perovskite heterojunction solar cells under high relative humidity, Nanoscale 8 (2016) 7017–7023. [209] F.G. Zhang, X.C. Yang, H.X. Wang, M. Cheng, J.H. Zhao, L.C. Sun, Structure engineering of hole-conductor free perovskite-based solar cells with lowtemperature-processed commercial carbon paste as cathode, ACS Appl. Mater. Interfaces 6 (2014) 16140–16146. [210] A.Y. Mei, X. Li, L.F. Liu, Z.L. Ku, T.F. Liu, Y.G. Rong, M. Xu, M. Hu, J.Z. Chen, Y. Yang, M. Gr€atzel, H.W. Han, A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295–298. [211] P. Liu, Z.H. Yu, N. Cheng, C.L. Wang, Y.N. Gong, S.H. Bai, X.-Z. Zhao, Low-cost and efficient hole-transport-material-free perovskite solar cells employing controllable electron-transport layer based on P25 nanoparticles, Electrochim. Acta 213 (2016) 83–88. [212] F.G. Zhang, X.C. Yang, M. Cheng, W.H. Wang, L.C. Sun, Boosting the efficiency and the stability of low cost perovskite solar cells by using CuPc nanorods as hole transport material and carbon as counter electrode, Nano Energy 20 (2016) 108–116. [213] X.W. Chang, W.P. Li, H.N. Chen, L.Q. Zhu, H.C. Liu, H.F. Geng, S.S. Xiang, J.M. Liu, X.L. Zheng, Y.L. Yang, S.H. Yang, Colloidal precursor-induced growth of ultraeven CH3NH3PbI3 for high-performance paintable carbon-based perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016) 30184–30192. [214] C. Liu, K. Wang, P.C. Du, T.Y. Meng, X.F. Yu, S.Z.D. Cheng, X. Gong, High performance planar heterojunction perovskite solar cells with fullerene derivatives as the electron transport layer, ACS Appl. Mater. Interfaces 7 (2015) 1153–1159. [215] J.J. Shi, X. Xu, D.M. Li, Q.B. Meng, Interfaces in perovskite solar cells, Small 11 (2015) 2472–2486. [216] W. Yin, L.J. Pan, T.B. Yang, Y.Y. Liang, Recent advances in interface engineering for planar heterojunction perovskite solar cells, Molecules 21 (2016) 837. [217] N. Chakravarthi, K. Gunasekar, W.S. Cho, D.X. Long, Y.-H. Kim, C.E. Song, J.-C. Lee, A. Facchetti, M.K. Song, Y.-Y. Noh, S.-H. Jin, A simple structured and efficient triazine-based molecule as an interfacial layer for high performance organic electronics, Energy Environ. Sci. 9 (2016) 2595–2602. [218] C.-Y. Chang, Y.-C. Chang, W.-K. Huang, W.-C. Liao, H. Wang, C. Yeh, B.-C. Tsai, Y.-C. Huang, C.-S. Tsao, Achieving high efficiency and improved stability in largearea ITO-free perovskite solar cells with thiol-functionalized self-assembled monolayers, J. Mater. Chem. A 4 (2016) 7903–7913. [219] W.Z. Li, H.P. Dong, L.D. Wang, N. Li, X.D. Guo, J.W. Li, Y. Qiu, Montmorillonite as bifunctional buffer layer material for hybrid perovskite solar cells with protection from corrosion and retarding recombination, J. Mater. Chem. A 2 (2014) 13587–13592. [220] X. Dong, X. Fang, M.H. Lv, B.C. Lin, S. Zhang, J.N. Ding, N.Y. Yuan, Improvement of the humidity stability of organic-inorganic perovskite solar cells using ultrathin Al2O3 layers prepared by atomic layer deposition, J. Mater. Chem. A 3 (2015) 5360–5367. [221] S. Guarnera, A. Abate, W. Zhang, J.M. Foster, G. Richardson, A. Petrozza, H.J. Snaith, Improving the long-term stability of perovskite solar cells with a porous Al2O3 buffer layer, J. Phys. Chem. Lett. 6 (2015) 432–437. [222] W.Z. Li, J.L. Li, L.D. Wang, G.D. Niu, R. Gao, Y. Qiu, Post modification of perovskite sensitized solar cells by aluminum oxide for enhanced performance, J. Mater. Chem. A 1 (2013) 11735–11740. [223] G.D. Niu, W.Z. Li, F.Q. Meng, L.D. Wang, H.P. Dong, Y. Qiu, Study on the stability of CH3NH3PbI3 films and the effect of post-modification by aluminum oxide in all-solid-state hybrid solar cells, J. Mater. Chem. A 2 (2014) 705–710. [224] S. Ito, S. Tanaka, K. Manabe, H. Nishino, Effects of surface blocking layer of Sb2S3 on nanocrystalline TiO2 for CH3NH3PbI3 perovskite solar cells, J. Phys. Chem. C 118 (2014) 16995–17000. [225] M. Acik, S.B. Darling, Graphene in perovskite solar cells: device design, characterization and implementation, J. Mater. Chem. A 4 (2016) 6185–6235. [226] Z.K. Liu, P. You, C. Xie, G.Q. Tang, F. Yan, Ultrathin and flexible perovskite solar cells with graphene transparent electrodes, Nano Energy 28 (2016) 151–157. [227] H.F. Lu, J.S. Sun, H. Zhang, S.M. Lu, W.C.H. Choy, Room-temperature solution-processed and metal oxide-free nano-composite for the flexible transparent bottom electrode of perovskite solar cells, Nanoscale 8 (2016) 5946–5953. [228] J.J. Zhao, X.P. Zheng, Y.H. Deng, T. Li, Y.C. Shao, A. Gruverman, J. Shield, J.S. Huang, Is Cu a stable electrode material in hybrid perovskite solar cells for a 30year lifetime? Energy Environ. Sci. 9 (2016) 3650–3656. [229] Z.L. Ku, Y.G. Rong, M. Xu, T.F. Liu, H.W. Han, Full printable processed mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells with carbon counter electrode, Sci. Rep. 3 (2013) 3132.

32

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37 

[230] A.K. Baranwal, S. Kanaya, T.A.N. Peiris, G. Mizuta, T. Nishina, H. Kanda, T. Miyasaka, H. Segawa, S. Ito, 100 C thermal stability of printable perovskite solar cells using porous carbon counter electrodes, ChemSusChem 9 (2016) 2604–2608. [231] B.X. Wang, T.F. Liu, Y.B. Zhou, X. Chen, X.B. Yuan, Y.Y. Yang, W.P. Liu, J.M. Wang, H.W. Han, Y.W. Tang, Hole-conductor-free perovskite solar cells with carbon counter electrodes based on ZnO nanorod arrays, Phys. Chem. Chem. Phys. 18 (2016) 27078–27082. [232] Z.Y. Liu, B. Sun, T.L. Shi, Z.R. Tang, G.L. Liao, Enhanced photovoltaic performance and stability of carbon counter electrode based perovskite solar cells encapsulated by PDMS, J. Mater. Chem. A 4 (2016) 10700–10709. [233] X.X. Jiang, Y.L. Xiong, A.Y. Mei, Y.G. Rong, Y. Hu, L. Hong, Y.X. Jin, Q.J. Liu, H.W. Han, Efficient compact-layer-free, hole-conductor-free, fully printable mesoscopic perovskite solar cell, J. Phys. Chem. Lett. 7 (2016) 4142–4146. [234] M.D. Ye, X.D. Hong, F.Y. Zhang, X.Y. Liu, Recent advancements in perovskite solar cells: flexibility, stability and large scale, J. Mater. Chem. A 4 (2016) 6755–6771. [235] H.-S. Kim, J.-Y. Seo, N.-G. Park, Material, Device Stability, In perovskite solar cells, ChemSusChem 9 (2016) 2528–2540. [236] N.-G. Park, M. Gr€ atzel, T. Miyasaka, K. Zhu, K. Emery, Towards stable and commercially available perovskite solar cells, Nat. Energy 1 (2016) 16152. [237] G.D. Niu, X.D. Guo, L.D. Wang, Review of recent progress in chemical stability of perovskite solar cells, J. Mater. Chem. A 3 (2015) 8970–8980. [238] T.A. Berhe, W.-N. Su, C.-H. Chen, C.-J. Pan, J.-H. Cheng, H.-M. Chen, M.-C. Tsai, L.-Y. Chen, A.A. Dubale, B.-J. Hwang, Organometal halide perovskite solar cells: degradation and stability, Energy Environ. Sci. 9 (2016) 323–356. [239] A.B. Djuri
sic, F.Z. Liu, A.M.C. Ng, Q. Dong, M.K. Wong, A. Ng, C. Surya, Stability issues of the next generation solar cells, Phys. Status Solidi RRL 10 (2016) 281–299. [240] S. Castro-Hermosa, S.K. Yadav, L. Vesce, A. Guidobaldi, A. Reale, A. Di Carlo, T.M. Brown, Stability issues pertaining large area perovskite and dye-sensitized solar cells and modules, J. Phys. D. Appl. Phys. 50 (2016) 033001. [241] T.T. Xu, L.X. Chen, Z.H. Guo, T.L. Ma, Strategic improvement of the long-term stability of perovskite materials and perovskite solar cells, Phys. Chem. Chem. Phys. 18 (2016) 27026–27050. [242] B. Conings, J. Drijkoningen, N. Gauquelin, A. Babayigit, J. D'Haen, L. D'Olieslaeger, A. Ethirajan, J. Verbeeck, J. Manca, E. Mosconi, F. De Angelis, H.-G. Boyen, Intrinsic thermal instability of methylammonium lead trihalide perovskite, Adv. Energy Mater. 5 (2015) 1500477. [243] B. Philippe, B.-W. Park, R. Lindblad, J. Oscarsson, S. Ahmadi, E.M.J. Johansson, H. Rensmo, Chemical and electronic structure characterization of lead halide perovskites and stability behavior under different exposures - a photoelectron spectroscopy investigation, Chem. Mater. 27 (2015) 1720–1731. [244] T.J. Jacobsson, W. Tress, J.-P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (2016) 11382–11393. [245] A.D. Sheikh, A. Bera, M.A. Hague, R.B. Rakhi, S. Del Gobbo, H.N. Alshareef, T. Wu, Atmospheric effects on the photovoltaic performance of hybrid perovskite solar cells, Sol. Energy Mater. Sol. Cells 137 (2015) 6–14. [246] N. Aristidou, I. Sanchez-Molina, T. Chotchuangchutchaval, M. Brown, L. Martinez, T. Rath, S.A. Haque, The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactive layers, Angew. Chem. Int. Ed. 54 (2015) 8208–8212. [247] J.A. Christians, P.A. Miranda Herrera, P.V. Kamat, Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air, J. Am. Chem. Soc. 137 (2015) 1530–1538. [248] Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J.M. Pringle, U. Bach, L. Spiccia, Y.-B. Cheng, Degradation observations of encapsulated planar CH3NH3PbI3 perovskite solar cells at high temperatures and humidity, J. Mater. Chem. A 3 (2015) 8139–8147. [249] A.M.A. Leguy, Y.H. Hu, M. Campoy-Quiles, M. Isabel Alonso, O.J. Weber, P. Azarhoosh, M. van Schilfgaarde, M.T. Weller, T. Bein, J. Nelson, P. Docampo, P.R.F. Barnes, Reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells, Chem. Mater 27 (2015) 3397–3407. [250] J. Yang, B.D. Siempelkamp, D. Liu, T.L. Kelly, Investigation of CH3NH3PbI3 degradation rates and mechanisms in controlled humidity environments using in situ techniques, ACS Nano 9 (2015) 1955–1963. [251] C. Müller, T. Glaser, M. Plogmeyer, M. Sendner, S. D€ oring, A.A. Bakulin, C. Brzuska, R. Scheer, M.S. Pshenichnikov, W. Kowalsky, A. Pucci, R. Lovrin
cic, Water infiltration in methylammonium lead iodide perovskite: fast and inconspicuous, Chem. Mater. 27 (2015) 7835–7841. [252] E.J. Juarez-Perez, Z. Hawash, S.R. Raga, L.K. Ono, Y.B. Qi, Thermal degradation of CHCH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermogravimetry-mass spectrometry analysis, Energy Environ. Sci. 9 (2016) 3406–3410. [253] W.X. Huang, J.S. Manser, P.V. Kamat, S. Ptasinska, Evolution of chemical composition, morphology, and photovoltaic efficiency of CH3NH3PbI3 perovskite under ambient conditions, Chem. Mater 28 (2016) 303–311. [254] B. Yang, O. Dyck, W. Ming, M.-H. Du, S. Das, C.M. Rouleau, G. Duscher, D.B. Geohegan, K. Xiao, Observation of nanoscale morphological and structural degradation in perovskite solar cells by in situ TEM, ACS Appl. Mater. Interfaces 8 (2016) 32333–32340. [255] B.A. Rosales, L. Men, S.D. Cady, M.P. Hanrahan, A.J. Rossini, J. Vela, Persistent dopants and phase segregation in organolead mixed-halide perovskites, Chem. Mater. 28 (2016) 6848–6859. [256] O. Hentz, Z. Zhao, S. Gradecak, Impacts of ion Ssegregation on local optical properties in mixed halide perovskite films, Nano Lett. 16 (2016) 1485–1490. [257] W. Rehman, D.P. McMeekin, J.B. Patel, R.L. Milot, M.B. Johnston, H.J. Snaith, L.M. Herz, Photovoltaic mixed-cation lead mixed-halide perovskites: links between crystallinity, photo-stability and electronic properties, Energy Environ. Sci. 10 (2017) 361–369. [258] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613–617. [259] J. Xi, Z.X. Wu, K. Xi, H. Dong, B. Xia, T. Lei, F. Yuan, W. Wu, B. Jiao, X. Hou, Initiating crystal growth kinetics of α-HC(NH)22PbI3 for flexible solar cells with longterm stability, Nano Energy 26 (2016) 438–445. [260] R.K. Misra, L. Ciammaruchi, S. Aharon, D. Mogilyansky, L. Etgar, I. Visoly-Fisher, E.A. Katz, Effect of halide composition on the photochemical stability of perovskite photovoltaic materials, ChemSusChem 9 (2016) 2572–2577. [261] R.G. Niemann, L. Gouda, J.G. Hu, S. Tirosh, R. Gottesman, P.J. Cameron, A. Zaban, Csþ incorporation into CH3NH3PbI3 perovskite: substitution limit and stability enhancement, J. Mater. Chem. A 4 (2016) 17819–17827. [262] I.C. Smith, E.T. Hoke, D. Solis-Ibarra, M.D. McGehee, H.I. Karunadasa, A layered hybrid perovskite solar-cell absorber with enhanced moisture stability, Angew. Chem. Int. Ed. 53 (2014) 11232–11235. [263] J.H. Kim, S.T. Williams, N.C. Cho, C.-C. Chueh, A.K.-Y. Jen, Enhanced environmental stability of planar heterojunction perovskite solar cells based on bladecoating, Adv. Energy Mater. 5 (2015) 1401229. [264] C.-H. Chiang, C.-G. Wu, Film grain-size related long-term stability of inverted perovskite solar cells, ChemSusChem 9 (2016) 2666–2672. [265] S. Casaluci, L. Cin a, A. Pockett, P.S. Kubiak, R.G. Niemann, A. Reale, A. Di Carlo, P.J. Cameron, A simple approach for the fabrication of perovskite solar cells in air, J. Power Sources 297 (2015) 504–510. [266] Y. Sun, Y.H. Wu, X. Fang, L.J. Xu, Z.J. Ma, Y.T. Lu, W.-H. Zhang, Q. Yu, N.Y. Yuan, J.N. Ding, Long-term stability of organic-inorganic hybrid perovskite solar cells with high efficiency under high humidity conditions, J. Mater. Chem. A 5 (2017) 1374–1379. [267] X. Gong, M. Li, X.-B. Shi, H. Ma, Z.-K. Wang, L.-S. Liao, Controllable perovskite crystallization by water additive for high-performance solar cells, Adv. Funct. Mater. 25 (2015) 6671–6678. [268] J. Yin, J. Cao, X. He, S.F. Yuan, S.B. Sun, J. Li, N.F. Zheng, L.W. Lin, Improved stability of perovskite solar cells in ambient air by controlling the mesoporous layer, J. Mater. Chem. A 3 (2015) 16860–16866. [269] T. Leijtens, G.E. Eperon, S. Pathak, A. Abate, M.M. Lee, H.J. Snaith, Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells, Nat. Commun. 4 (2013) 2885. [270] F.T.F. O'Mahony, Y.H. Lee, C. Jellett, S. Dmitrov, D.T.J. Bryant, J.R. Durrant, B.C. O'Regan, M. Gr€atzel, M.K. Nazeeruddin, S.A. Haque, Improved environmental stability of organic lead trihalide perovskite-based photoactive-layers in the presence of mesoporous TiO2, J. Mater. Chem. A 3 (2015) 7219–7223. [271] A. Savva, I. Burgues-Ceballos, S.A. Choulis, Improved performance and reliability of p-i-n perovskite solar cells via doped metal oxides, Adv. Energy Mater. 6 (2016) 1600285.

33

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[272] J. Liang, C.X. Wang, Y.R. Wang, Z.R. Xu, Z.P. Lu, Y. Ma, H.F. Zhu, Y. Hu, C.C. Xiao, X. Yi, G.Y. Zhu, H.L. Lv, L.B. Ma, T. Chen, Z.X. Tie, Z. Jin, J. Liu, All-inorganic perovskite solar cells, J. Am. Chem. Soc. 138 (2016) 15829–15832. [273] A. Priyadarshi, L.J. Haur, P. Murray, D. Fu, S. Kulkarni, G. Xing, T.C. Sum, N. Mathews, S.G. Mhaisalkar, A large area (70 cm2) monolithic perovskite solar module with a high efficiency and stability, Energy Environ. Sci. 9 (2016) 3687–3692. [274] P.H. Joshi, L. Zhang, I.M. Hossain, H.A. Abbas, R. Kottokkaran, S.P. Nehra, M. Dhaka, M. Noack, V.L. Dalal, The physics of photon induced degradation of perovskite solar cells, AIP Adv. 6 (2016) 115114. [275] X. Li, M. Tschumi, H. Han, S.S. Babkair, R.A. Alzubaydi, A.A. Ansari, S.S. Habib, M.K. Nazeeruddin, S.M. Zakeeruddin, M. Gr€atzel, Outdoor performance and stability under elevated temperatures and long-term light soaking of triple-layer mesoporous perovskite photovoltaics, Energy Technol. 3 (2015) 551–555. [276] Y.G. Rong, L.F. Liu, A.Y. Mei, X. Li, H.W. Han, Beyond efficiency: the challenge of stability in mesoscopic perovskite solar cells, Adv. Energy Mater. 5 (2015) 1501066. [277] G. Divitini, S. Cacovich, F. Matteocci, L. Cina, A. Di Carlo, C. Ducati, In situ observation of heat-induced degradation of perovskite solar cells, Nat. Energy 1 (2017) 15012. [278] J.C. Kim, J.S. Yun, X.M. Wen, A.M. Soufiani, C.F.J. Lau, B. Wilkinson, J. Seidel, M.A. Green, S.J. Huang, A.W.Y. Ho-Baillie, Nucleation and Growth Control of HC(NH)22PbI3 for planar perovskite solar cell, J. Phys. Chem. C 120 (2016) 11262–11267. [279] C. Rold an-Carmona, O. Malinkiewicz, A. Soriano, G. Minguez Espallargas, A. Garcia, P. Reinecke, T. Kroyer, M.I. Dar, M.K. Nazeeruddin, H.J. Bolink, Flexible high efficiency perovskite solar cells, Energy Environ. Sci. 7 (2014) 994–997. [280] Y.W. Li, L. Meng, Y.M. Yang, G.Y. Xu, Z.R. Hong, Q. Chen, J.B. You, G. Li, Y. Yang, Y.F. Li, High-efficiency robust perovskite solar cells on ultrathin flexible substrates, Nat. Commun. 7 (2016) 10214. [281] Y. Reyna, M. Salado, S. Kazim, A. Perez-Tomas, S. Ahmad, M. Lira-Cantu, Performance and stability of mixed FAPbI3(0.85)MAPbBr3(0.15) halide perovskite solar cells under outdoor conditions and the effect of low light irradiation, Nano Energy 30 (2016) 570–579. [282] M. Corazza, F.C. Krebs, S.A. Gevorgyan, Lifetime of organic photovoltaics: linking outdoor and indoor tests, Sol. Energy Mater. Sol. Cells 143 (2015) 467–472. [283] S.A. Gevorgyan, M.V. Madsen, B. Roth, M. Corazza, M. H€ osel, R.R. Søndergaard, M. Jørgensen, F.C. Krebs, Lifetime of organic photovoltaics: status and predictions, Adv. Energy Mater. 6 (2016) 1501208. [284] C.H. Law, L. Miseikis, S. Dimitrov, P. Shakya-Tuladhar, X.E. Li, P.R.F. Barnes, J. Durrant, B.C. O'Regan, Performance and stability of lead perovskite/TiO2, polymer/PCBM, and dye sensitized solar cells at light intensities up to 70 suns, Adv. Mater. 26 (2014) 6268–6273. [285] F. Matteocci, S. Razza, F. Di Giacomo, S. Casaluci, G. Mincuzzi, T.M. Brown, A. D'Epifanio, S. Licoccia, A. Di Carlo, Solid-state solar modules based on mesoscopic organometal halide perovskite: a route towards the up-scaling process, Phys. Chem. Chem. Phys. 16 (2014) 3918–3923. [286] S.D. Stranks, H.J. Snaith, Metal-halide perovskites for photovoltaic and light-emitting devices, Nat. Nanotechnol. 10 (2015) 391–402. [287] L. Meng, J.B. You, T.-F. Guo, Y. Yang, Recent advances in the inverted planar structure of perovskite solar cells, Acc. Chem. Res. 49 (2016) 155–165. [288] T. Zhang, H.N. Chen, Y. Bai, S. Xiao, L. Zhu, C. Hu, Q.Z. Xue, S.H. Yang, Understanding the relationship between ion migration and the anomalous hysteresis in high-efficiency perovskite solar cells: a fresh perspective from halide substitution, Nano Energy 26 (2016) 620–630. [289] J.M. Frost, A. Walsh, What is moving in hybrid halide perovskite solar cells? Acc. Chem. Res. 49 (2016) 528–535. [290] Z. Fan, J.X. Xiao, K. Sun, L. Chen, Y.T. Hu, J.Y. Ouyang, K.P. Ong, K.Y. Zeng, J. Wang, Ferroelectricity of CH3NH3PbI3 perovskite, J. Phys. Chem. Lett. 6 (2015) 1155–1161. [291] M. Coll, A. Gomez, E. Mas-Marza, O. Almora, G. Garcia-Belmonte, M. Campoy-Quiles, J. Bisquert, Polarization switching and light-enhanced piezoelectricity in lead halide perovskites, J. Phys. Chem. Lett. 6 (2015) 1408–1413. [292] K. Miyano, M. Yanagida, N. Tripathi, Y. Shirai, Hysteresis, stability, and ion migration in lead halide perovskite photovoltaics, J. Phys. Chem. Lett. 7 (2016) 2240–2245. [293] W. Tress, N. Marinova, T. Moehl, S.M. Zakeeruddin, M.K. Nazeeruddin, M. Gr€atzel, Understanding the rate-dependent J-V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field, Energy Environ. Sci. 8 (2015) 995–1004. [294] E. Zimmermann, K.K. Wong, M. Müller, H. Hu, P. Ehrenreich, M. Kohlst€adt, U. Würfel, S. Mastroianni, G. Mathiazhagan, A. Hinsch, T.P. Gujar, M. Thelakkat, T. Pfadler, L. Schmidt-Mende, Characterization of perovskite solar cells: towards a reliable measurement protocol, Apl. Mater. 4 (2016) 091901. [295] A. Babayigit, A. Ethirajan, M. Muller, B. Conings, Toxicity of organometal halide perovskite solar cells, Nat. Mater. 15 (2016) 247–251. [296] B. Hailegnaw, S. Kirmayer, E. Edri, G. Hodes, D. Cahen, Rain on methylammonium lead iodide based perovskites: possible environmental effects of perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 1543–1547. [297] S.T. Williams, A. Rajagopal, C.-C. Chueh, A.K.-Y. Jen, Current challenges and prospective research for upscaling hybrid perovskite photovoltaics, J. Phys. Chem. Lett. 7 (2016) 811–819. [298] B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E.M.J. Johansson, Bismuth based hybrid perovskites A3Bi2I9 (A: methylammonium or cesium) for solar cell application, Adv. Mater. 27 (2015) 6806–6813. [299] T. Singh, A. Kulkarni, M. Ikegami, T. Miyasaka, Effect of electron transporting layer on bismuth-based lead-free perovskite (CH3NH3)3Bi2I9 for photovoltaic applications, ACS Appl. Mater. Interfaces 8 (2016) 14542–14547. [300] T. Krishnamoorthy, H. Ding, C. Yan, W.L. Leong, T. Baikie, Z.Y. Zhang, M. Sherburne, S.Z. Li, M. Asta, N. Mathews, S.G. Mhaisalkar, Lead-free germanium iodide perovskite materials for photovoltaic applications, J. Mater. Chem. A 3 (2015) 23829–23832. [301] D. Cortecchia, H.A. Dewi, J. Yin, A. Bruno, S. Chen, T. Baikie, P.P. Boix, M. Gr€atzel, S. Mhaisalkar, C. Soci, N. Mathews, Lead-free MA2CuClxBr4x, hybrid perovskites, Inorg. Chem. 55 (2016) 1044–1052. [302] G. Volonakis, M.R. Filip, A.A. Haghighirad, N. Sakai, B. Wenger, H.J. Snaith, F. Giustino, Lead-free halide double perovskites via heterovalent substitution of noble metals, J. Phys. Chem. Lett. 7 (2016) 1254–1259. [303] P.C. Harikesh, H.K. Mulmudi, B. Ghosh, T.W. Goh, Y.T. Teng, K. Thirumal, M. Lockrey, K. Weber, T.M. Koh, S.Z. Li, S. Mhaisalkar, N. Mathews, Rb as an alternative cation for templating inorganic lead-free perovskites for solution processed photovoltaics, Chem. Mater. 28 (2016) 7496–7504. [304] Z.H. Sun, A. Zeb, S.J. Liu, C.M. Ji, T. Khan, L.N. Li, M.C. Hong, J.H. Luo, Exploring a lead-free semiconducting hybrid ferroelectric with a zero-dimensional perovskite-like structure, Angew. Chem. Int. Ed. 55 (2016) 11854–11858. [305] S.J. Lee, S.S. Shin, Y.C. Kim, D.S. Kim, T.K. Ahn, J.H. Noh, J.W. Seo, S.I. Seok, Fabrication of efficient formamidinium tin iodide perovskite solar cells through SnF2-pyrazine complex, J. Am. Chem. Soc. 138 (2016) 3974–3977. [306] W.Q. Liao, D.W. Zhao, Y. Yu, C.R. Grice, C.L. Wang, A.J. Cimaroli, P. Schulz, W.W. Meng, K. Zhu, R.-G. Xiong, Y.F. Yan, Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%, Adv. Mater. 28 (2016) 9333–9340. [307] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solid-state organic-inorganic halide perovskite solar cells, Nat. Photonics 8 (2014) 489–494. [308] N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A.-A. Haghighirad, A. Sadhanala, G.E. Eperon, S.K. Pathak, M.B. Johnston, A. Petrozza, L.M. Herz, H.J. Snaith, Lead-free organic-inorganic tin halide perovskites for photovoltaic applications, Energy Environ. Sci. 7 (2014) 3061–3068. [309] F. Hao, C.C. Stoumpos, P. Guo, N.J. Zhou, T.J. Marks, R.P.H. Chang, M.G. Kanatzidis, Solvent-mediated crystallization of CH3NH3PbI3 films for heterojunction depleted perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 11445–11452. [310] A. Babayigit, D.D. Thanh, A. Ethirajan, J. Manca, M. Muller, H.-G. Boyen, B. Conings, Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism Danio rerio, Sci. Rep. 6 (2016) 18721. [311] L. Serrano-Lujan, N. Espinosa, T.T. Larsen-Olsen, J. Abad, A. Urbina, F.C. Krebs, Tin- and lead-based perovskite solar cells under scrutiny: an environmental perspective, Adv. Energy Mater. 5 (2015) 1501119. [312] N. Espinosa, A. Laurent, G.A.D.R. Benatto, M. Hosel, F.C. Krebs, Which electrode materials to select for more environmentally friendly organic photovoltaics? Adv. Eng. Mater. 18 (2016) 490–495. [313] I. Celik, Z.N. Song, A.J. Cimaroli, Y.F. Yan, M.J. Heben, D. Apul, Life cycle assessment (LCA) of perovskite PV cells projected from lab to fab, Sol. Energy Mater. Sol. Cells 156 (2016) 157–169.

34

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[314] J. Gong, S.B. Darling, F.Q. You, Perovskite photovoltaics: life-cycle assessment of energy and environmental impacts, Energy Environ. Sci. 8 (2015) 1953–1968. [315] J.Y. Zhang, X.F. Gao, Y.L. Deng, B.B. Li, C. Yuan, Life cycle assessment of titania perovskite solar cell technology for sustainable design and manufacturing, ChemSusChem 8 (2015) 3882–3891. [316] N. Espinosa, L. Serrano-Lujan, A. Urbina, F.C. Krebs, Solution and vapour deposited lead perovskite solar cells: ecotoxicity from a life cycle assessment perspective, Sol. Energy Mater. Sol. Cells 137 (2015) 303–310. [317] C. Liu, J.D. Fan, H.L. Li, C.L. Zhang, Y.H. Mai, Highly efficient perovskite solar cells with substantial reduction of lead content, Sci. Rep. 6 (2016) 35705. [318] K.L. Gardner, J.G. Tait, T. Merckx, W. Qiu, U.W. Paetzold, L. Kootstra, M. Jaysankar, R. Gehlhaar, D. Cheyns, P. Heremans, J. Poortmans, Nonhazardous solvent systems for processing perovskite photovoltaics, Adv. Energy Mater. 6 (2016) 1600386. [319] B.J. Kim, D.H. Kim, S.L. Kwon, S.Y. Park, Z. Li, K. Zhu, H.S. Jung, Selective dissolution of halide perovskites as a step towards recycling solar cells, Nat. Commun. 7 (2016) 11735. [320] P.-Y. Chen, J.F. Qi, M.T. Klug, X.N. Dang, P.T. Hammond, A.M. Belcher, Environmentally responsible fabrication of efficient perovskite solar cells from recycled car batteries, Energy Environ. Sci. 7 (2014) 3659–3665. [321] C.-Y. Chu, C.-Y. Chang, W.-F. Su, Low-temperature solution processable n-i-p perovskite solar cell, Jpn. J. Appl. Phys. 55 (2016), 04EA01. [322] L.B. Qiu, S.S. He, J.H. Yang, F. Jin, J. Deng, H. Sun, X.L. Cheng, G.Z. Guan, X.M. Sun, H.B. Zhao, H.S. Peng, An all-solid-state fiber-type solar cell achieving 9.49% efficiency, J. Mater. Chem. A 4 (2016) 10105–10109. [323] L. Qiu, S. He, J. Yang, J. Deng, H. Peng, Fiber-shaped perovskite solar cells with high power conversion efficiency, Small 12 (2016) 2419–2424. [324] Y.X. Zhao, K. Zhu, Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655–689. [325] H.C. Back, J.H. Kim, G.J. Kim, T.K. Kim, H.K. Kang, J.M. Kong, S.H. Lee, K.H. Lee, Interfacial modification of hole transport layers for efficient large-area perovskite solar cells achieved via blade-coating, Sol. Energy Mater. Sol. Cells 144 (2016) 309–315. [326] Y.H. Deng, E. Peng, Y.C. Shao, Z.G. Xiao, Q.F. Dong, J.S. Huang, Scalable fabrication of efficient organolead trihalide perovskite solar cells with doctor-bladed active layers, Energy Environ. Sci. 8 (2015) 1544–1550. [327] Q. Hu, H. Wu, J. Sun, D.H. Yan, Y.L. Gao, J.L. Yang, Large-area perovskite nanowire arrays fabricated by large-scale roll-to-roll micro-gravure printing and doctor blading, Nanoscale 8 (2016) 5350–5357. [328] Z. Yang, B. Cai, B. Zhou, T.T. Yao, W. Yu, S.Z.F. Liu, W.-H. Zhang, C. Li, An up-scalable approach to CH3NH3PbI3 compact films for high-performance perovskite solar cells, Nano Energy 15 (2015) 670–678. [329] M. Hambsch, Q.Q. Lin, A. Armin, P.L. Burn, P. Meredith, Efficient, monolithic large area organohalide perovskite solar cells, J. Mater. Chem. A 4 (2016) 13830–13836. [330] H. Halim, Y. Guo, Flexible organic-inorganic hybrid perovskite solar cells, Sci. China Mater. 59 (2016) 495–506. [331] F. Di Giacomo, V. Zardetto, A. D'Epifanio, S. Pescetelli, F. Matteocci, S. Razza, A. Di Carlo, S. Licoccia, W.M.M. Kessels, M. Creatore, T.M. Brown, Flexible perovskite photovoltaic modules and solar cells based on atomic layer deposited compact layers and UV-irradiated TiO2 scaffolds on plastic substrates, Adv. Energy Mater. 5 (2015) 1401808. [332] I.Y. Jeong, H.S. Jung, M.W. Park, J.S. Park, H.J. Son, J. Joo, J.W. Lee, M.J. Ko, A tailored TiO2 electron selective layer for high-performance flexible perovskite solar cells via low temperature UV process, Nano Energy 28 (2016) 380–389. [333] S. Das, G. Gu, P.C. Joshi, B. Yang, T. Aytug, C.M. Rouleau, D.B. Geohegan, K. Xiao, Low thermal budget, photonic-cured compact TiO2 layers for high-efficiency perovskite solar cells, J. Mater. Chem. A 4 (2016) 9685–9690. [334] A. Dymshits, L. Iagher, L. Etgar, Parameters influencing the growth of ZnO nanowires as efficient low temperature flexible perovskite-based solar cells, Materials 9 (2016) 60. [335] S.S. Shin, W.S. Yang, E.J. Yeom, S.J. Lee, N.J. Jeon, Y.-C. Joo, I.J. Park, J.H. Noh, S.I. Seok, Tailoring of electron-collecting oxide nanoparticulate layer for flexible perovskite solar cells, J. Phys. Chem. Lett. 7 (2016) 1845–1851. [336] M.M. Tavakoli, K.-H. Tsui, Q.P. Zhang, J. He, Y. Yao, D.D. Li, Z.Y. Fan, Highly efficient flexible perovskite solar cells with antireflection and self-cleaning nanostructures, ACS Nano 9 (2015) 10287–10295. [337] M.H. Kumar, N. Yantara, S. Dharani, M. Graetzel, S. Mhaisalkar, P.P. Boix, N. Mathews, Flexible, low-temperature, solution processed ZnO-based perovskite solid state solar cells, Chem. Commun. 49 (2013) 11089–11091. [338] X.Y. Wang, Z. Li, W.J. Xu, S.A. Kulkarni, S.K. Batabyal, S. Zhang, A.Y. Cao, L.H. Wong, TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode, Nano Energy 11 (2015) 728–735. [339] J. You, Z. Hong, Y.M. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou, Y. Yang, Low-temperature solution-processed perovskite solar cells with high efficiency and flexibility, ACS Nano 8 (2014) 1674–1680. [340] B.J. Kim, D.H. Kim, Y.-Y. Lee, H.-W. Shin, G.S. Han, J.S. Hong, K. Mahmood, T.K. Ahn, Y.-C. Joo, K.S. Hong, N.-G. Park, S. Lee, H.S. Jung, Highly efficient and bending durable perovskite solar cells: toward a wearable power source, Energy Environ. Sci. 8 (2015) 916–921. [341] P. Docampo, J.M. Ball, M. Darwich, G.E. Eperon, H.J. Snaith, Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates, Nat. Commun. 4 (2013) 2761. [342] J. Troughton, D. Bryant, K. Wojciechowski, M.J. Carnie, H. Snaith, D.A. Worsley, T.M. Watson, Highly efficient, flexible, indium-free perovskite solar cells employing metallic substrates, J. Mater. Chem. A 3 (2015) 9141–9145. [343] H.S. Jung, N.-G. Park, Perovskite solar cells: from materials to devices, Small 11 (2015) 10–25. [344] F. Qin, J.H. Tong, R. Ge, B.W. Luo, F.Y. Jiang, T.F. Liu, Y.Y. Jiang, Z.Y. Xu, L. Mao, W. Meng, S.X. Xiong, Z.F. Li, L.Q. Li, Y.H. Zhou, Indium tin oxide (ITO)-free, top-illuminated, flexible perovskite solar cells, J. Mater. Chem. A 4 (2016) 14017–14024. [345] H.-G. Im, S. Jeong, J. Jin, J. Lee, D.-Y. Youn, W.-T. Koo, S.-B. Kang, H.-J. Kim, J. Jang, D. Lee, H.-K. Kim, I.-D. Kim, J.-Y. Lee, B.-S. Bae, Hybrid crystalline-ITO/ metal nanowire mesh transparent electrodes and their application for highly flexible perovskite solar cells, NPG Asia Mater 8 (2016) e282. [346] A. Kim, H.S. Lee, H.-C. Kwon, H.S. Jung, N.-G. Park, S.H. Jeong, J.H. Moon, Fully solution-processed transparent electrodes based on silver nanowire composites for perovskite solar cells, Nanoscale 8 (2016) 6308–6316. [347] X.-L. Ou, M. Xu, J. Feng, H.-B. Sun, Flexible and efficient ITO-free semitransparent perovskite solar cells, Sol. Energy Mater. Sol. Cells 157 (2016) 660–665. [348] M.M. Tavakoli, Q.F. Lin, S.-F. Leung, G.C. Lui, H. Lu, L. Li, B. Xiang, Z.Y. Fan, Efficient, flexible and mechanically robust perovskite solar cells on inverted nanocone plastic substrates, Nanoscale 8 (2016) 4276–4283. [349] W. Zi, X.D. Ren, X.P. Ren, Q.B. Wei, F. Gao, S.Z.F. Liu, Perovskite/germanium tandem: a potential high efficiency thin film solar cell design, Opt. Commun. 380 (2016) 1–5. [350] S. Foster, S. John, Light-trapping design for thin-film silicon-perovskite tandem solar cells, J. Appl. Phys. 120 (2016) 103103. [351] R. Asadpour, R.V.K. Chavali, M.R. Khan, M.A. Alam, Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient (ηT*~ 33%) solar cell, Appl. Phys. Lett. 106 (2015) 243902. [352] M. Filipi
c, P. L€ oper, B. Niesen, S. De Wolf, J. Kr
c, C. Ballif, M. Topi
c, CH3NH3PbI3 perovskite/silicon tandem solar cells: characterization based optical simulations, Opt. Express 23 (2015) A263–A278. [353] G. Yin, A. Steigert, P. Manley, R. Klenk, M. Schmid, Enhanced absorption in tandem solar cells by applying hydrogenated In2O3 as electrode, Appl. Phys. Lett. 107 (2015) 211901. [354] D. Shi, Y. Zeng, W. Shen, Perovskite/c-Si tandem solar cell with inverted nanopyramids: realizing high efficiency by controllable light trapping, Sci. Rep. 5 (2015) 16504. [355] P. L€ oper, S.-J. Moon, S.M. de Nicolas, B. Niesen, M. Ledinsky, S. Nicolay, J. Bailat, J.-H. Yum, S. De Wolf, C. Ballif, Organic-inorganic halide perovskite/ crystalline silicon four-terminal tandem solar cells, Phys. Chem. Chem. Phys. 17 (2015) 1619–1629. [356] T. Todorov, T. Gershon, O. Gunawan, C. Sturdevant, S. Guha, Perovskite-kesterite monolithic tandem solar cells with high open-circuit voltage, Appl. Phys. Lett. 105 (2014) 173902.

35

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[357] C.D. Bailie, M.G. Christoforo, J.P. Mailoa, A.R. Bowring, E.L. Unger, W.H. Nguyen, J. Burschka, N. Pellet, J.Z. Lee, M. Gr€atzel, R. Noufi, T. Buonassisi, A. Salleo, M.D. McGehee, Semi-transparent perovskite solar cells for tandems with silicon and CIGS, Energy Environ. Sci. 8 (2015) 956–963. [358] S. Albrecht, M. Saliba, J.P.C. Baena, F. Lang, L. Kegelmann, M. Mews, L. Steier, A. Abate, J. Rappich, L. Korte, R. Schlatmann, M.K. Nazeeruddin, A. Hagfeldt, M. Gr€ atzel, B. Rech, Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature, Energy Environ. Sci. 9 (2016) 81–88. [359] J. Werner, C.-H. Weng, A. Walter, L. Fesquet, J.P. Seif, S. De Wolf, B. Niesen, C. Ballif, Efficient monolithic perovskite/silicon tandem solar cell with cell area > 1 cm2, J. Phys. Chem. Lett. 7 (2016) 161–166. [360] D. Hamaguchi, S.-I. Kobayashi, T. Uchida, Y. Sawada, H. Lei, Y. Hoshi, Improvement in luminance efficiency of organic light emitting diodes by suppression of secondary electron bombardment of substrate during sputter deposition of top electrode films, Jpn. J. Appl. Phys. 55 (2016) 106501. [361] J.H. Kim, Y.J. Lee, Y.S. Jang, J.N. Jang, D.H. Kim, B.C. Song, D.H. Lee, S.N. Kwon, M.P. Hong, The effect of Ar plasma bombardment upon physical property of tungsten oxide thin film in inverted top-emitting organic light-emitting diodes, Org. Electron 12 (2011) 285–290. [362] H. Schmidt, H. Flügge, T. Winkler, T. Bülow, T. Riedl, W. Kowalsky, Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode, Appl. Phys. Lett. 94 (2009) 243302. [363] H.-K. Kim, D.-G. Kim, K.-S. Lee, M.-S. Huh, S.H. Jeong, K.I. Kim, T.-Y. Seong, Plasma damage-free sputtering of indium tin oxide cathode layers for top-emitting organic light-emitting diodes, Appl. Phys. Lett. 86 (2005) 183503. [364] H. Lin, J.S. Yu, S.L. Lou, J. Wang, Y.D. Jiang, Low temperature DC sputtering deposition on indium-tin oxide film and its application to inverted top-emitting organic light-emitting diodes, J. Mater. Sci. Technol. 24 (2008) 179–182. [365] Y.J. Lee, J.H. Kim, J.N. Jang, I.H. Yang, S.N. Kwon, M.P. Hong, D.C. Kim, K.S. Oh, S.J. Yoo, B.J. Lee, W.G. Jang, Development of inverted OLED with top ITO anode by plasma damage-free sputtering, Thin Solid Films 517 (2009) 4019–4022. [366] D.A. Jacobs, K.R. Catchpole, F.J. Beck, T.P. White, A re-evaluation of transparent conductor requirements for thin-film solar cells, J. Mater. Chem. A 4 (2016) 4490–4496. [367] Y.M. Yang, Q. Chen, Y.-T. Hsieh, T.-B. Song, N. De Marco, H.P. Zhou, Y. Yang, Multilayer transparent top electrode for solution processed perovskite/Cu(In,Ga) (Se,S)2 four terminal tandem solar cells, ACS Nano 9 (2015) 7714–7721. [368] L. Kranz, A. Abate, T. Feurer, F. Fu, E. Avancini, J. L€ ockinger, P. Reinhard, S.M. Zakeeruddin, M. Gr€atzel, S. Buecheler, A.N. Tiwari, High-efficiency polycrystalline thin film tandem solar cells, J. Phys. Chem. Lett. 6 (2015) 2676–2681. [369] F. Lang, M.A. Gluba, S. Albrecht, J. Rappich, L. Korte, B. Rech, N.H. Nickel, Perovskite solar cells with large-area CVD-graphene for tandem solar cells, J. Phys. Chem. Lett. 6 (2015) 2745–2750. [370] B. Chen, Y. Bai, Z.S. Yu, T. Li, X.P. Zheng, Q.F. Dong, L. Shen, M. Boccard, A. Gruverman, Z. Holman, J. Huang, Efficient semitransparent perovskite solar cells for 23.0%-efficiency perovskite/silicon four-terminal tandem cells, Adv. Energy Mater. 6 (2016) 1601128. [371] J. Peng, T. Duong, X.Z. Zhou, H.P. Shen, Y.L. Wu, H.K. Mulmudi, Y.M. Wan, D.Y. Zhong, J.T. Li, T. Tsuzuki, K.J. Weber, K.R. Catchpole, T.P. White, Stability issues pertaining large area perovskite and dye-sensitized solar cells and modules, Adv. Energy Mater. 7 (2017) 1601768. [372] T. Todorov, T. Gershon, O. Gunawan, Y.S. Lee, C. Sturdevant, L.-Y. Chang, S. Guha, Monolithic perovskite-CIGS tandem solar cells via in situ band gap engineering, Adv. Energy Mater. 5 (2015) 1500799. [373] K.A. Bush, A.F. Palmstrom, Z.J. Yu, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin, R.L.Z. Hoye, C.D. Bailie, T. Leijtens, I.M. Peters, M.C. Minichetti, N. Rolston, R. Prasanna, S. Sofia, D. Harwood, W. Ma, F. Moghadam, H.J. Snaith, T. Buonassisi, Z.C. Holman, S.F. Bent, M.D. McGehee, Stability issues pertaining large area perovskite and dye-sensitized solar cells and modules, Nat. Energy 2 (2017) 17009. [374] L. Ye, B.H. Fan, S.Q. Zhang, S.S. Li, B. Yang, Y.P. Qin, H. Zhang, J.H. Hou, Perovskite-polymer hybrid solar cells with near-infrared external quantum efficiency over 40%, Sci. China Mater. 58 (2015) 953–960. [375] C.T. Zuo, L.M. Ding, Bulk heterojunctions push the photoresponse of perovskite solar cells to 970 nm, J. Mater. Chem. A 3 (2015) 9063–9066. [376] J. Liu, S.M. Lu, L. Zhu, X.C. Li, W.C.H. Choy, Perovskite-organic hybrid tandem solar cells using a nanostructured perovskite layer as the light window and a PFN/ doped-MoO3/MoO3 multilayer as the interconnecting layer, Nanoscale 8 (2016) 3638–3646. [377] C.-C. Chen, S.-H. Bae, W.-H. Chang, Z.R. Hong, G. Li, Q. Chen, H.P. Zhou, Y. Yang, Perovskite/polymer monolithic hybrid tandem solar cells utilizing a lowtemperature, full solution process, Mater. Horiz. 2 (2015) 203–211. [378] J.H. Heo, S.H. Im, CH3NH3PbBr3-CH3NH3PbI3 perovskite-perovskite tandem solar cells with exceeding 2.2 V open circuit voltage, Adv. Mater. 28 (2016) 5121–5125. [379] F.Y. Jiang, T.F. Liu, B.W. Luo, J.H. Tong, F. Qin, S.X. Xiong, Z.F. Li, Y.H. Zhou, A two-terminal perovskite/perovskite tandem solar cell, J. Mater. Chem. A 4 (2016) 1208–1213. [380] D. Forg acs, L. Gil-Escrig, D. Perez-Del-Rey, C. Momblona, J. Werner, B. Niesen, C. Ballif, M. Sessolo, H.J. Bolink, Efficient monolithic perovskite/perovskite tandem solar cells, Adv. Energy Mater. 7 (2017) 1602121. [381] W. Qiu, T. Merckx, M. Jaysankar, C.M. de la Huerta, L. Rakocevic, W. Zhang, U.W. Paetzold, R. Gehlhaar, L. Froyen, J. Poortmans, D. Cheyns, H.J. Snaith, P. Heremans, Pinhole-free perovskite films for efficient solar modules, Energy Environ. Sci. 9 (2016) 484–489. [382] J.-S. Yeo, C.-H. Lee, D.W. Jang, S.H. Lee, S.M. Jo, H.-I. Joh, D.-Y. Kim, Reduced graphene oxide-assisted crystallization of perovskite via solution-process for efficient and stable planar solar cells with module-scales, Nano Energy 30 (2016) 667–676. [383] S. Razza, F. Di Giacomo, F. Matteocci, L. Cina, A.L. Palma, S. Casaluci, P. Cameron, A. D'Epifanio, S. Licoccia, A. Reale, T.M. Brown, A. Di Carlo, Perovskite solar cells and large area modules (100 cm2) based on an air flow-assisted PbI2 blade coating deposition process, J. Power Sources 277 (2015) 286–291. [384] F. Matteocci, L. Cin a, F. Di Giacomo, S. Razza, A.L. Palma, A. Guidobaldi, A. D'Epifanio, S. Licoccia, T.M. Brown, A. Reale, A. Di Carlo, High efficiency photovoltaic module based on mesoscopic organometal halide perovskite, Prog. Photovoltaics 24 (2016) 436–445. [385] C.-H. Chiang, J.-W. Lin, C.-G. Wu, One-step fabrication of a mixed-halide perovskite film for a high-efficiency inverted solar cell and module, J. Mater. Chem. A 4 (2016) 13525–13533. [386] G. Sfyri, C.V. Kumar, D. Raptis, V. Dracopoulos, P. Lianos, Study of perovskite solar cells synthesized under ambient conditions and of the performance of small cell modules, Sol. Energy Mater. Sol. Cells 134 (2015) 60–63. [387] G. Mincuzzi, A.L. Palma, A. Di Carlo, T.M. Brown, Laser processing in the manufacture of dye-sensitized and perovskite solar cell technologies, ChemElectroChem 3 (2016) 9–30. [388] F. Matteocci, S. Razza, F. Di Giacomo, S. Casaluci, G. Mincuzzi, T.M. Brown, A. D'Epifanio, S. Licoccia, A. Di Carlo, Solid-state solar modules based on mesoscopic organometal halide perovskite: a route towards the up-scaling process, Phys. Chem. Chem. Phys. 16 (2014) 3918–3923. [389] J. Cui, P.F. Li, Z.F. Chen, K. Cao, D. Li, J.B. Han, Y. Shen, M.Y. Peng, Y.Q. Fu, M.K. Wang, Phosphor coated NiO-based planar inverted organometallic halide perovskite solar cells with enhanced efficiency and stability, Appl. Phys. Lett. 109 (2016) 171103. [390] F. Bella, G. Griffini, J.-P. Correa-Baena, G. Saracco, M. Gr€atzel, A. Hagfeldt, S. Turri, C. Gerbaldi, Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers, Science 354 (2016) 203–206. [391] G.F. Hu, W.X. Guo, R.M. Yu, X.N. Yang, R.R. Zhou, C.F. Pan, Z.L. Wang, Enhanced performances of flexible ZnO/perovskite solar cells by piezo-phototronic effect, Nano Energy 23 (2016) 27–33. [392] C.X. Bao, W.D. Zhu, J. Yang, F.M. Li, S. Gu, Y.R.Q. Wang, T. Yu, J. Zhu, Y. Zhou, Z.G. Zou, Highly flexible self-powered organolead trihalide perovskite photodetectors with gold nanowire networks as transparent electrodes, ACS Appl. Mater. Interfaces 8 (2016) 23868–23875. [393] P.C. Zhu, S. Gu, X.P. Shen, N. Xu, Y.L. Tan, S.D. Zhuang, Y. Deng, Z.D. Lu, Z.L. Wang, J. Zhu, Direct conversion of perovskite thin films into nanowires with kinetic control for flexible optoelectronic devices, Nano Lett. 16 (2016) 871–876. [394] H. Lu, W. Tian, F.R. Cao, Y.L. Ma, B.K. Gu, L. Li, A self-powered and stable all-perovskite photodetector-solar cell nanosystem, Adv. Funct. Mater. 26 (2016) 1296–1302. [395] M.I. Saidaminov, M.A. Haque, M. Savoie, A.L. Abdelhady, N. Cho, I. Dursun, U. Buttner, E. Alarousu, T. Wu, O.M. Bakr, Perovskite photodetectors operating in both narrowband and broadband regimes, Adv. Mater. 28 (2016) 8144–8149.

36

A.B. Djuri
sic et al.

Progress in Quantum Electronics 53 (2017) 1–37

[396] J.P. Lu, A. Carvalho, H.W. Liu, S.X.D. Lim, A.H. Castro Neto, C.H. Sow, Hybrid bilayer WSe2-CH3NH3PbI3 organolead halide perovskite as a high-performance photodetector, Angew. Chem. Int. Ed. 55 (2016) 11945–11949. [397] V. Adinolfi, O. Ouellette, M.I. Saidaminov, G. Walters, A.L. Abdelhady, O.M. Bakr, E.H. Sargent, Fast and sensitive solution-processed visible-blind perovskite UV photodetectors, Adv. Mater. 28 (2016) 7264–7268. [398] J.C. Zhou, Y.L. Chu, J. Huang, Photodetectors based on two-dimensional layer-structured hybrid lead iodide perovskite semiconductors, ACS Appl. Mater. Interfaces 8 (2016) 25660–25666. [399] D.-H. Kang, S.R. Pae, J.W. Shim, G. Yoo, J.H. Jeon, J.W. Leem, J.S. Yu, S.J. Lee, B.H. Shin, J.-H. Park, An ultrahigh-performance photodetector based on a perovskite-transition-metal-dichalcogenide hybrid structure, Adv. Mater. 28 (2016) 7799–7806. [400] H.L. Zhu, J.Q. Cheng, D. Zhang, C.J. Liang, C.J. Reckmeier, H. Huang, A.L. Rogach, W.C.H. Choy, Room-temperature solution-processed NiOx:PbIx nanocomposite structures for realizing high-performance perovskite photodetectors, ACS Nano 10 (2016) 6808–6815. [401] Z.H. Nie, J. Yin, H.W. Zhou, N. Chai, B.L. Chen, Y.T. Zhang, K.G. Qu, G.D. Shen, H.Y. Ma, Y.C. Li, J.S. Zhao, X.X. Zhang, Layered and Pb-free organic-inorganic perovskite materials for ultraviolet photoresponse: (010)-oriented (CH3NH3)2MnCl4 thin film, ACS Appl. Mater. Interfaces 8 (2016) 28187–28193. [402] L.L. Gu, M.M. Tavakoli, D.Q. Zhang, Q.P. Zhang, A. Waleed, Y.Q. Xiao, K.-H. Tsui, Y.J. Lin, L. Liao, J.N. Wang, Z.Y. Fan, 3D arrays of 1024-pixel image sensors based on lead halide perovskite nanowires, Adv. Mater. 28 (2016) 9713–9721. [403] W. Deng, X.J. Zhang, L.M. Huang, X.Z. Xu, L. Wang, J.C. Wang, Q.X. Shang, S.-T. Lee, J.S. Jie, Aligned single-crystalline perovskite microwire arrays for highperformance flexible image sensors with long-term stability, Adv. Mater. 28 (2016) 2201–2208. [404] V.Q. Dang, G.-S. Han, T.Q. Trung, L.T. Duy, Y.-U. Jin, B.-U. Hwang, H.-S. Jung, N.-E. Lee, Methylammonium lead iodide perovskite-graphene hybrid channels in flexible broadband phototransistors, Carbon 105 (2016) 353–361. [405] M. Spina, M. Lehmann, B. Nafradi, L. Bernard, E. Bonvin, R. Gaal, A. Magrez, L. Forro, E. Horvath, Microengineered CH3NH3PbI3 nanowire/graphene phototransistor for low-intensity light detection at room temperature, Small 11 (2015) 4824–4828. [406] Y.S. Wang, Y.P. Zhang, Y. Lu, W.D. Xu, H.R. Mu, C.Y. Chen, H. Qiao, J.C. Song, S.J. Li, B.Q. Sun, Y.-B. Cheng, Q.L. Bao, Hybrid graphene-perovskite phototransistors with ultrahigh responsivity and gain, Adv. Opt. Mater. 3 (2015) 1389–1396. [407] F. Li, C. Ma, H. Wang, W.J. Hu, W.L. Yu, A.D. Sheikh, T. Wu, Ambipolar solution-processed hybrid perovskite phototransistors, Nat. Commun. 6 (2015) 8238. [408] Y.X. Wu, J. Li, J. Xu, Y.Y. Du, L.K. Huang, J. Ni, H.K. Cai, J.J. Zhang, Organic-inorganic hybrid CH3NH3PbI3 perovskite materials as channels in thin-film fieldeffect transistors, RSC Adv. 6 (2016) 16243–16249. [409] X.Y. Chin, D. Cortecchia, J. Yin, A. Bruno, C. Soci, Lead iodide perovskite light-emitting field-effect transistor, Nat. Commun. 6 (2015) 7383. [410] I. Su arez, E.J. Juarez-Perez, J. Bisquert, I. Mora-Ser o, J.P. Martínez-Pastor, Polymer/perovskite amplifying waveguides for active hybrid silicon photonics, Adv. Mater. 27 (2015) 6157–6162. [411] P. Docampo, T. Bein, A long-term view on perovskite optoelectronics, Acc. Chem. Res. 49 (2016) 339–346. [412] B.R. Sutherland, E.H. Sargent, Perovskite photonic sources, Nat. Photonics 10 (2016) 295–302. [413] S.A. Veldhuis, P.P. Boix, N. Yantara, M. Li, T.C. Sum, N. Mathews, S.G. Mhaisalkar, Perovskite materials for light-emitting diodes and lasers, Adv. Mater. 28 (2016) 6804–6834. [414] J.T. Xu, Y.H. Chen, L.M. Dai, Efficiently photo-charging lithium-ion battery by perovskite solar cell, Nat. Commun. 6 (2015) 8103. [415] F.C. Zhou, Z.W. Ren, Y.D. Zhao, X.P. Shen, A.W. Wang, Y.Y. Li, C. Surya, Y. Chai, Perovskite photovoltachromic supercapacitor with all-transparent electrodes, ACS Nano 10 (2016) 5900–5908. [416] S. Zhou, L.K. Li, H. Yu, J.Z. Chen, C.-P. Wong, N. Zhao, Thin film electrochemical capacitors based on organolead triiodide perovskite, Adv. Electron. Mater. 2 (2016) 1600114. [417] C.H. Lee, J.S. Hong, A. Stroppa, M.-H. Whangbo, J.H. Shim, Organic-inorganic hybrid perovskites ABI3(A ¼ CH3NH3, NH2CHNH2; B ¼ Sn, Pb) as potential thermoelectric materials: a density functional evaluationt, RSC Adv. 5 (2015) 78701–78707. [418] J. Zhang, Y. Xuan, L. Yang, A novel choice for the photovoltaic-thermoelectric hybrid system: the perovskite solar cell, Int. J. Energy Res. 40 (2016) 1400–1409. [419] S. Yakunin, M. Sytnyk, D. Kriegner, S. Shrestha, M. Richter, G.J. Matt, H. Azimi, C.J. Brabec, J. Stangl, M.V. Kovalenko, W. Heiss, Detection of X-ray photons by solution-processed lead halide perovskites, Nat. Photonics 9 (2015) 444–449. [420] S. Yakunin, D.N. Dirin, Y. Shynkarenko, V. Morad, I. Cherniukh, O. Nazarenko, D. Kreil, T. Nauser, K.M. V, Detection of gamma photons using solution-grown single crystals of hybrid lead halide perovskites, Nat. Photonics 10 (2016) 585589. [421] S.-T. Ha, C. Shen, J. Zhang, Q.H. Xiong, Laser cooling of organic-inorganic lead halide perovskites, Nat. Photonics 10 (2016) 115–121. [422] C.W. Gu, J.-S. Lee, Flexible hybrid organic-inorganic perovskite memory, ACS Nano 10 (2016) 5413–5418. [423] Z.G. Xiao, J.S. Huang, Energy-efficient hybrid perovskite memristors and synaptic devices, Adv. Electron. Mater. 2 (2016) 1600100. [424] J.H. Choi, S.H. Park, J.H. Lee, K.T. Hong, D.-H. Kim, C.W. Moon, G.D. Park, J.M. Suh, J.Y. Hwang, S.Y. Kim, H.S. Jung, N.-G. Park, S.W. Han, K.T. Nam, H.W. Jang, Organolead halide perovskites for low operating voltage multilevel resistive switching, Adv. Mater. 28 (2016) 6562–6567. [425] E.J. Yoo, M.Q. Lyu, J.-H. Yun, C.J. Kang, Y.J. Choi, L.Z. Wang, Resistive switching behavior in organic-inorganic hybrid CH3NH3PbI3x Clx perovskite for resistive random access memory devices, Adv. Mater. 27 (2015) 6170–6175. [426] C. Muthu, S. Agarwal, A. Vijayan, P. Hazra, K.B. Jinesh, V.C. Nair, Hybrid perovskite nanoparticles for high-performance resistive random access memory devices: control of operational parameters through chloride doping, Adv. Mater. Interfaces 3 (2016) 1600092. [427] W.T. Xu, H.C. Cho, Y.-H. Kim, Y.-T. Kim, C. Wolf, C.-G. Park, T.-W. Lee, Organometal halide perovskite artificial synapses, Adv. Mater. 28 (2016) 5916–5922. [428] K. Yan, M. Peng, X. Yu, X. Cai, S. Chen, H.W. Hu, B.X. Chen, X. Gao, B. Dong, D.C. Zou, High-performance perovskite memristor based on methyl ammonium lead halides, J. Mater. Chem. C 4 (2016) 1375–1381. [429] W. Xu, F.M. Li, Z.X. Cai, Y.R. Wang, F. Luo, X. Chen, An ultrasensitive and reversible fluorescence sensor of humidity using perovskite CH3NH3PbBr3, J. Mater. Chem. C 4 (2016) 9651–9655. [430] W.G. Ma, J.F. Han, W. Yu, D. Yang, H. Wang, X. Zong, C. Li, Integrating perovskite photovoltaics and noble-metal-free catalysts toward efficient solar energy conversion and H2S splitting, ACS Catal. 6 (2016) 6198–6206. [431] M.T. Hoang, N.D. Pham, J.H. Han, J.M. Gardner, I. Oh, Integrated photoelectrolysis of water implemented on organic metal halide perovskite photoelectrode, ACS Appl. Mater. Interfaces 8 (2016) 11904–11909.

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