Passivation in perovskite solar cells: A review

Materials Today Energy xxx (2018) 1e20

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Passivation in perovskite solar cells: A review Pengjun Zhao, Byeong Jo Kim, Hyun Suk Jung* School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, 440-746, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 November 2017 Received in revised form 18 December 2017 Accepted 9 January 2018 Available online xxx

Photovoltaic device based on inorganiceorganic hybrid perovskite structured materials have been one of the brightest spotlights in the energy-conversion research field in recent years. However, due to their inherent properties and the architecture of the fabricated device, many defects trap states or carrier transport barriers are present at the interfaces between each functional layer and at the grain boundaries of the perovskite. These defects cause undesirable phenomena such as hysteresis and instability in the perovskite solar cells, which has slowed their commercialization. To address these issues, intensive research effort has been devoted recently to the development of passivation materials and approaches that can reduce the amount of interface and surface defect states in perovskite solar cells. Here, we have reviewed the state of the research progress in the development of passivation of different interfaces in the perovskite solar cell, including the interface (a) between transparent conductive oxide and electron transport material; (b) between the electron transport material and perovskite; (c) between the perovskite grains (grain boundaries); (d) between the perovskite and hole transport layer; (e) between the hole transport layer and electrode, and (f) between the electrode material and atmospheric environment. We also look into the prospects and challenges in the passivation of hybrid perovskite solar cells. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Passivation Perovskite solar cell Interface Defect states

1. Introduction In the five years since the first fabrication of the all solid-state perovskite solar cell (PSC) by Nam Gyu Park et al. in 2012 [1], the photoelectric conversion efficiencies (PCEs) of PSCs have experienced an explosive growth. Certified PCEs of 22.7% and 19.7% for small area PSCs with areas of 0.095 cm2 and 1.0 cm2, respectively [2], and of 12.1% for large area of 36.1 cm2 [3] have been achieved. This remarkable achievement has resulted in PSCs being considered the most promising class of third generation photovoltaic devices to replace the currently widely used silicon solar cells. In order to commercialize PSCs, three major barriers remain to be overcome [4,5]: (a) The environmental toxicity caused by the use of Pb in PSCs, (b) their unsatisfactory stability against temperature, humidity and light exposure, and (c) the dependence of their differential JeV curves on the scan directions (i.e., from open circuit voltage to short circuit current or vice versa), which is the so-called “hysteresis” phenomenon [6]. The searching for Lead-free perovskite with high photovoltaic performance still has a long way to go. Theoretical investigation has

* Corresponding author. E-mail address: [email protected] (H.S. Jung).

implied that a promising perovskite absorber should exhibit high electronic dimensionality, a criterion that presently fulfilled only by Pb-based three dimensioned (3D) structured perovskite. Some reported double perovskites, such as the Ag- and Bi-based halide double perovskites are structurally 3D but electronically 0 dimensioned (0D), making it quite difficult to find promising candidates to replace the current Pb-based perovskite [7]. Fortunately, research into eliminating hysteresis and improving stability has resulted in many substantial achievements recently. PSCs with low or even no hysteresis [8e13], and with outstanding stability towards temperature [14], humidity [15e19], and light exposure [20e25] have been successfully fabricated. Interface passivation is one of the most commonly used and efficient strategies to improve the photovoltaic performance of PSCs. According to International Union of Pure and Applied Chemistry (IUPAC), passivation, in physical chemistry and engineering, refers to a material becoming “passive,” that is, less affected or corroded by the environment in which it will be used. Passivation involves the application of an outer layer of a shielding material as a microcoating, created by chemical reaction with the base material [26]. The transition process from the “active state” to the “passive state” by the formation of a passivating film [27]. For perovskite solar cells, passivation generally refers to either chemical passivation,

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which reduces the defects trap states in order to optimize the charge transfer between various interfaces [9,28e31], or physical passivation, which isolates certain functional layers from the external environment to avoid degradation of the device. Typical PSC devices contain six main interface, including (a) the interface between the transparent conductive oxide and electron transport layer (ETL); (b) the interface between the electron transport material and perovskite; (c) the interface between the perovskite grains (grain boundaries); (d) the interface between the perovskite and hole transport layer (HTL); (e) the interface between the hole transport layer and electrode, and (f) the interface between the electrode material and atmospheric environment [32]. In this review, we summarize the research advances of the past several years, and focus on interface passivation in perovskite solar cells, organized according to the interface classifications listed above. A brief prospective on the challenges and opportunities in passivation technology for enhancing the performance and stability of perovskites solar cells is also provided. It should be noted that in addition to perovskite solar cells, passivation strategies have also been applied to perovskite nanocrystals/quantum dots [33e35] and light emitting devices [36e39]; however, these will not be discussed here. 2. Passivation at the interface between ETL and perovskite 2.1. Passivation of TiO2 surface The most popularly used electron transfer material in perovskite solar cells is titanium dioxide (TiO2), with a planar or mesoporous structure. Due to its inherent properties, the surface trap states are highly abundant, which limits the photovoltaic performance of the resulting perovskite solar cells. In addition, TiO2 is an outstanding photocatalyst under UV light, however UV light can decompose organic groups, and thus attenuate the photovoltaic performance of perovskite solar cells under sunlight. Numerous strategies have been attempted to resolve this problem and many passivators have been utilized to eliminate the surface trap state of TiO2. Common methods to passivate the surface trap states of TiO2 are to deposit another TiO2 layer to coat the original surface, for example, TiCl4 chemical bath deposition (CBD) [40], or ultrathin TiO2 film deposited by atomic layer deposition (ALD) [41], or the deposition of an insulator layer (Al2O3, Y2O3, MgO [42e47]). TiO2 itself can also act as a passivation layer on ZnO electron transfer material, to slow down the charge recombination rate in the ZnO layer [48]. While the above approaches can reduce the surface defects of TiO2, the use of TiO2 as the electron transport layer in perovskite solar cells still has an apparent disadvantage. In TiO2 based perovskite solar cells, the JeV curves usually exhibit a large discrepancy between the reverse and forward scan directions (Fig. 1 (a)) [9], partly due to the non-equilibrium injection rate of electrons and holes at the two electrodes. This hysteresis phenomenon causes uncertainty as to true efficiency of the cells. To solve this issue, lithium (Liþ) doped TiO2 was successfully developed to improve the electronic properties of the TiO2 mesoporous layer, by reducing electronic trap states. The monovalent Liþ causes a partial reduction of Ti4þ to Ti3þ within the TiO2 lattice and passivation of the electronic defect states that act as nonradiative recombination centers [49]. Perovskite solar cells with enhanced efficiency as well as reduced hysteresis have been obtained using Liþ doped TiO2 [14,30,50] Additionally, high electron mobility materials, such as selfassembled fullerene derivatives [8,28,51,52], pyridine [53], carboxyl groups [54], or other semiconductor shell layers [55] have been developed to passivate the TiO2 surface. These materials create a physical barrier between TiO2 and the perovskite layer and

decrease the TiO2 trap states, thus enhancing the efficiencies of the perovskite solar cells as well as improving their light-stability [28]. One example of effective solution using this strategy was inserting [6]-phenyl-C61-butyric acid methyl ester (PCBM) or another fullerene/graphene derivative [28,56e64] as a thin layer between TiO2 and the perovskite (Fig. 1 (b)) or as individual electron transport materials [63,65]. Because of the high carrier mobility of PCBM, hysteresis can be significantly reduced (Fig. 1 (c)). More importantly, the constant power output at the maximum power point (MPP) becomes comparable to the performance obtained from the JeV curve (Fig. 1 (d)) [9]. This is a key parameter that should be noted when using PCBM for the passivation of TiO2 surfaces. Because PCBM absorbs part of the incident illumination, slightly reducing the photo-generated current, an ultra-thin PCBM coating should be used [13]. More than the conductive interlayer, insulating polymers can form interface chemical interactions between the thin insulating layer and the perovskite films, resulting in significant improvement of the device stability while high PCE can still be maintained [66]. Sargent et al. [67] developed a contact-passivation method using a chlorine-capped TiO2 colloidal nanocrystal film that mitigated interfacial recombination and improved interface binding in planar structured perovskite solar cells. Certified efficiencies of 20.1% and 19.5% were achieved for active areas of 0.049 and 1.1 cm2, respectively. These excellent photovoltaic performances resulted from the reduction in trap-like localized anti-site defects between PbeI bonds by replacing them with PbeCl bonds (see Fig. 2 (a and b)). After contact doping, the hysteresis almost disappeared in Fig. 2 (c), and both the transient photocurrent decay and transient photovoltage decay lifetimes were longer in the TiO2eCl samples than that of in the TiO2 sample (Fig. 2 (d and e)). This analysis indicates that the strong binding at the TiO2eCl/ perovskite interface suppressed the interfacial recombination, which accounts for the superior stability of planar PSCs based on TiO2eCl (Fig. 2 (feh)). 2.2. Interface passivation by self-assembled monolayer Self-assembled monolayers (SAMs) of organic molecules are molecular assemblies that form spontaneously on surfaces by adsorption and are organized into ralatively large ordered domains [68]. SAMs can be formed on semiconductors or on other dielectric substrates, and have been used in a variety of technological applications [69e72]. SAMs containing different functional groups have been utilized to passivate the interface between the electron transfer and perovskite layer [73e76]. Zuo et al. studied four SAMs (BA-SAMs, PA-SAMs, CBA-SAMs, ABA-SAMs, and C3-SAMs, see Fig. 3 (a)) on SnO2 compact planar layers [74]. The chemical groups of the different SAMs exhibited two different interactions with the SnO2 and perovskite layers: van der Waals interactions with benzoic acid (BA) and dipolar interactions with the 4pyridinecarboxylic acid (PA), 3-aminopropanoic acid (C3) [75], 4aminobenzoic acid (ABA), or 4-cyanobenzoic acid (CBA). As seen in Fig. 3 (b), the work functions of the SAMs showed a negative correlation with the efficiencies of the corresponding perovskite solar cells, which is the opposite of what would be expected from the energy level alignment theory. The photoluminescence (PL) quenching exhibited the same tendency as the efficiency, as shown in Fig. 3 (c) and (d). These results indicate that the interfacial optoelectronic properties were mainly governed by chemical interactions, rather than the energy level alignment. The PA-SAM passivated cell exhibited the highest efficiency of 18.8%. To explain this, the schematic diagram in Fig. 3 (f) shows the carrier dynamics of the transfer stages and the mechanism of enhanced photovoltaic performance in the PA-SAM devices. Firstly, in order to generate a

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Fig. 1. (a) A typical hysteresis JeV loop for a perovskite solar cell. (b) Electronic band structure of the PCBM passivated device (c) JeV curve showing reduced hysteresis after PCBM interposition and (d) a static PCE scan with the voltage held at the MPP voltage. Open squares represent the device without PCBM, while closed circles indicate the device with a PCBM layer. Reprinted with permission from Ref. [9]. Copyright 2015 AIP Publishing LLC.

photocurrent, the photo-generated carriers must be transferred from the perovskite to the electrode. There are two trap-stateinduced barriers that reduce the charge injection efficiency: PL quenching via trap states, and charge recombination via trap states. In most cases, the terminal groups of SAM contain nitrogen atoms, which tend to form hydrogen-bonding interactions (NeH/I) with the methylamine groups of the perovskite crystal lattice. This can improve their miscibility with the perovskite substrate, further enhancing the crystallization of perovskite and reducing surface trap states [73,75]. Therefore, modification with PA-SAM suppressed the surface trap states, as evidenced by the enhanced TPV decay time in Fig. 3 (e). Meanwhile, due to the reduced work function, carrier transfer between the perovskite and SnO2 became more efficient [74,77]. Investigations of the surface chemistry combined with timeresolved photoluminescence spectroscopy have indicated that charge recombination centers in hybrid metal-halide perovskites are almost exclusively localized on the surfaces of the crystals, rather than in the bulk [34]. Thus, passivation of these surface defects could be the most efficient method to prolong charge carrier lifetimes and further improve solar cell performance.

3. Passivation in perovskite grain boundaries 3.1. Grain boundary self-passivation by PbI2 Controlling charge carrier trapping, which introduces competitive recombination channels, is an extremely important issue in the development of high-performance solar cells. As a kind of polycrystalline thin film, it is necessary for perovskite thin film to have a low density of charge carrier traps, at both grain boundaries and at interfaces with electron or hole extraction layers [78]. Supasai et al. first reported the passivation effect of a PbI2 layer on perovskites in 2013 [79]. The evidence for this effect was then investigated, by Wang et al. using femtosecond time-resolved transient absorption spectroscopy technology (fs-TA) [80]. The injection rates were found to be slowed in the presence of a greater mount of PbI2, and carrier recombination lifetimes were also lengthened upon passivation [81,82]. Moreover, in a typical neiep structured perovskite solar cell, the formation of the PbI2 passivation layer is highly related to the TiO2 architecture. Mesoporous TiO2 is more likely to induce the formation of PbI2 than compact TiO2, which leads to the passivation of perovskite grain boundaries

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Fig. 3. (a) Schematic diagram of the SAM between SnO2 and the perovskite film. (b) Work functions and efficiencies of perovskite and organic solar cells with different SAMs. (c) Steady state and (d) transient PL spectra of the perovskite film on SnO2 with different SAMs. (e) Transient photo-voltage of perovskite solar cells with different SAMs. (f) Schematic diagram of the charge dynamics at the perovskite/SnO2 interface in perovskite solar cells: ① Photoluminescence process, ② PL quenching via trap states, ③ charge transfer process, ④ charge recombination via trap states, ⑤ power generation. Reprinted with permission from Ref. [74]. Copyright 2017 American Chemical Society.

(Fig. 4 and Table 1). A systematical study indicated that the present of suitable amount of PbI2 species in the CH3NH3PbI3 film led to improved carrier behavior, possibly due to reduced recombination at the grain boundaries (GBs) and the TiO2/perovskite interface.

Before passivation with PbI2, the trap density of TiO2 was 1019 cm3. This value decreased to 1016 cm3 after PbI2 passivation, suggesting that the TiO2 surface traps can be passivated by IePbeI bonding [83]. Additionally, the properties of GBs of the perovskite film were

Fig. 2. (a) Trap-like localized antisite defects form near the valence band edge of the PbI2-terminated TiO2/perovskite interface (left). Shallow and delocalized PbeCl anti-site defects are seen for the PbCl2-terminated interface (right). (b) Device structure and cross-sectional scanning electron microscopy (SEM) image of planar PSC. (c) JeV curves of PSCs containing TiO2 and TiO2eCl as ESLs measured at reverse and forward scans. (d) Normalized transient photocurrent decay and (e) normalized transient photovoltage decay of solar cells containing TiO2 and TiO2eCl as ESLs. (f) JeV curves of the best-performing small-area (0.049 cm2) CsMAFA PSC measured using reverse and forward scans. (g) Dark storage stability of non-encapsulated PSCs containing TiO2 and TiO2eCl. The unsealed cells were kept in a dry cabinet (<30% relative humidity) in the dark and measured regularly under nitrogen. (h) JeV curves of PSC (CsMAFA) at various stages: fresh, immediately after 500 h of MPP operation, and after recovery overnight in the dark. Reprinted with permission from Ref. [67]. Copyright 2017 American Association for the Advancement of Science.

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Fig. 4. Transient band edge bleaching kinetics (symbols) and their fits (lines) for AeD perovskite architectures (inset) at the probe wavelengths l noted in Table 1. Reprinted with permission from Ref. [80]. Copyright 2016 American Chemical Society.

Table 1 Kinetic fitting parameters for the perovskite architectures. Architecture

lprobe/nm

t1/ps

t2/ps

perovskite/FTO perov/comp/FTO perov/meso/FTO perov/meso/comp/FTO

748 750 728 722

2.2 ± 0.2 (55%) 1.7 ± 0.1 (51%) 9.1 ± 0.8 (52%) 13.5 ± 1.0 (52%)

39.0 ± 2.8 (45%) 39.9 ± 2.5 (49%) 89.6 ± 12.5 (48%) 150.0 ± 17.6 (48%)

Reprinted from Ref. [80]. Copyright 2016 American Chemical Society.

altered by passivation [81]. Several possible passivation mechanisms were proposed (Fig. 5) [81,84] based on the fact that the interface between PbI2 and perovskite in the film shows a type I band edge alignment. The schematic peien structure is shown at the bottom left of Fig. 5 (a). The perovskite/TiO2 interface (I) is shown at the top right of Fig. 5 (a); the recombination of the electrons from TiO2 and holes from the perovskite is reduced by the introduction of PbI2. The perovskite/HTM interface (II) described at the bottom right of Fig. 5 (a); the presence of PbI2 changes the grain-to-grain boundary-bending from downward to upward, which helps to reduce recombination between the electrons from the perovskite and the holes from the HTM. Some other reports in the literature have asserted that energy level alignment is the key factor for the passivation effect of PbI2. Due to band edge matching between TiO2, PbI2, and the perovskite, PbI2 is able to passivate the TiO2 interface and further decrease hole recombination (Fig. 5 (b)). Moreover, PbI2 can facilitate electron injection into TiO2. At the perovskite/hole transport layer (HTL) interface, PbI2 can act as an electron blocking layer, facilitating hole injection and thereby decreasing recombination (Fig. 5 (c)). In contrast, if the PbI2 layers are too thick, or if the energy matching is unsuitable, they may instead insulate individual grains and block charge transfer (Fig. 5 (d and e)). PbI2 may also influence the chemical composition at the grain boundaries. The composition of the interior of the perovskite grains could be ralatively insensitive toward small changes in overall stoichiometry. The grain boundaries and the regions between the grains would thus by necessity be highly sensitive to the overall composition (Fig. 5 (feh)). This could affect defect states, dangling bonds, conductivity, doping, and ion migration of the perovskite absorber layer. Some results indicate that recombination is faster within grain boundaries that are deficient in PbI2. Thus, PbI2 acts as a passivation layer between the grains [84].

When PbI2 is used as a passivator, it is usually found at the grain boundaries of perovskite grains. The grain boundaries usually appear relatively brighter contrast than the grains nearby [81,85,86] in scanning electron microscopy (SEM) images, possibly because of their low conductivity results in the accumulated charge accumulated. There are three methods to introduce a PbI2 passivation layer at perovskite grain boundaries. The first one is self-induced formation of PbI2 from the controlled degradation of pristine perovskite thin films by thermal [79,81,86,87] or water vapor treatment [88e90]; the second is the preparation of a nonstoichiometric perovskite precursor solution with excess of PbI2 (usually 3%e10% molar ratio relative to the perovskite) [84,85,91e93]; and the final one is the incomplete reaction of PbI2 through a two-step solution or vapor reaction method [92e94]. Many previous articles have reported that an excess of PbI2 can have beneficial effects on the photovoltaic performance of a perovskite solar cell, including suppressed charge recombination, increased fluorescence emission lifetime [92] and increased polaron binding energy [95] in perovskite thin films, thus enhancing the open circuit voltage (as high as 1.15 V for CH3NH3PbI3) [91], and reducing hysteresis between the forward and reverse scans [6,37,96,97]. Because PbI2 is a commonly observed byproduct during the aging and degradation processes of perovskite solar cells, identifying and quantifying PbI2 formation is necessary to improving the performance and stability of perovskite solar cells. Nazeeruddin et al. reported the influence of non-stoichiometric PbI2: CH3NH3I ratios in the precursor solution on the passivation effect of PbI2 [98]. In stoichiometric of PbI2: CH3NH3I was tuned from 1:1 to 1.2:1 in regular neiep mesoporous TiO2 structured perovskite solar cells (Fig. 6 (a)). As indicated in the JeV curves in Fig. 6 (b), the photovoltaic performance of the cells was improved at all tested non-stoichiometric precursor ratios, and the optimized PbI2 excess ratio was 10%. The X-ray diffraction (XRD) and SEM images in Fig. 6 (c) and (d) suggest that the presence of unreacted PbI2 improved the crystallinity as well as the grain sizes of the perovskite film, thus enhancing the electron transfer capacity from perovskite to the TiO2 layer. By using confocal based PL/time resolved photoluminescence (TRPL) spectroscopy and microscopy, Chen et al. [35] observed the spatial distribution of PbI2 between perovskite boundaries. Their results showed that the perovskites in PbI2-rich grains exhibited a longer lifetime than that of PbI2-poor grains, due to the suppression of defect trapping. In addition to the commonly used fluorescence spectrum technique [92,93,95,99], light-modulated scanning tunneling microscopy (LMSTM) enables spatially resolved mapping of the photoinduced interfacial band bending of valence and conduction bands, and of the photo-generated electron and hole carriers at the hetero-interfaces of perovskite crystal grains. Shih et al. explored the interfacial electronic structures of individual perovskite grains, and directly observed enhanced charge separation and reduced back recombination when interfacial PbI2 passivation layers were applied to the perovskite crystal grains [100]. They also concluded that the thickness of the PbI2 passivation layers should be less than 20 nm, in order to maintain high photo-induced charge separation and transfer efficiency at the hetero-interfaces between the CH3NH3PbI3 perovskite crystals and the PbI2 passivation layers. However, the influence of excess PbI2 in perovskite thin films is still disputed. Some researchers believe that unreacted PbI2þ acts as a “double-edged sword” for the enhancement of the performance of perovskite solar cells [84,85,101]. Liu et al. [85] reported that due to their inferior film morphology (i.e., smaller grain size and the presence of pinholes), perovskite films without PbI2 have lower

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Fig. 5. (a) A diagram of the possible mechanism of the PbI2 passivation effect, and the effects of PbI2 on the perovskite film related to possible energy alignments suggested in the literature and an artistic illustration of different grain boundary types as a function of overall stoichiometry. (b) Schematic of PbI2 as a passivating layer on the back contact. (c) Schematic of PbI2 as a passivation layer next to the hole-selective layer. (d) Schematic of PbI2 as an electron blocking layer next to the back contact. (e) Schematic of PbI2 as a charge carrier barrier between perovskite grains. (f) Grain boundary with a large surplus of PbI2. (g) Grain boundary with a small deficiency of organic species. (h) Grain boundary with a large surplus of organic species. Reprinted with permission from Refs. [81,84]. Copyright 2016 American Chemical Society.

Fig. 6. (a) Diagram showing the device architecture of a neiep mesoporous TiO2 structured perovskite solar cell, where CH3NH3PbI3: X PbI2 represents the active film with different stoichiometric ratios of the precursors. (b) Current densityevoltage (JeV) curves obtained for devices containing a 0, 5, 10, 15 or 20% molar excess of PbI2 in the perovskite layer measured under AM1.5G solar irradiation of 100 mW cm2 and with scan-rate fixed at 10 mV s1. (c) XRD patterns of the different perovskite films, with the composition of each indicated in the legend. The inset shows a magnified graph in the 2q range of 14.1e14.31 corresponding to the characteristic peak (110) of the perovskite structure as well as the FWHM values. (d) SEM pictures corresponding to the surface analysis of the films. Reprinted with permission from Ref. [98]. Copyright 2015 Royal Society of Chemistry.

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Fig. 8. (a) Pristine perovskite solution (left) and the formulated perovskiteePCBM hybrid solution. (b) A schematic of in situ passivation of the halide-induced deep trap. (c) Ultravioletevisible absorption spectroscopy of the hybrid solution shows the interaction between PCBM and perovskite ions. The inset of (c) shows the details of such interaction (d) The wavefunction overlap shows the hybridization between PCBM and defective surface, enabling the electron/hole transfer for absorbance and passivation. (e) DFT calculation of density of states (DOS) shows that deep trap state (black) induced by PbeI anti-site defect is reduced and becomes much shallower (red) upon the adsorption of PCBM on defective halide. (f) The instantaneous JeV curve of the control device (perovskite film) with high hysteresis. The black point indicates the ‘maximum-power output point’ (MPP). (g) The JeV scan of a hybrid device shows very low hysteresis and low current loss. The inset of figure (g) shows the external quantum efficiency (EQE) of a hybrid device. The inset figure of (f) shows the thickness of the active layer. Reprinted with permission from Ref. [37]. Copyright 2015 Nature publication group.

Fig. 7. (a) Photographs of the coated CH3NH3PbI3 films prepared from a non-stoichiometric precursor solution with an excess of CH3NH3I. (b) Plots of Jsc, Voc, FF and PCE as a function of x in (1þx) CH3NH3I:PbI2. (ced) c-AFM images for the MAPbI3 perovskite films with x ¼ 0 (c) and 0.06 (d) obtained at a bias voltage of 2 V in the dark, where the perovskite films were sandwiched between metal electrodes. Insets show the corresponding topographies. (e) JeV curves of perovskite solar cell employing MAPbI3 for x ¼ 0.06 (f) EQE spectrum and the integration of the value of Jsc based on the EQD data. (g) Reverse and forward scanned JeV curves for 50 cells. (h) Histogram of the PCEs for the reverse and forward scanned data for 50 cells. Solid lines represent statistical data. Reprinted with permission from Ref. [102]. Copyright 2016 Nature publication group.

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Fig. 9. KPFM measurements were performed on a CH3NH3Pb(I1exBrx)3/TiO2/FTO/glass devices. (a, d) Topography images and (b, e) surface potential images of the topography of two devices. The GB regions can be easily distinguished in the reverse topography images. In the x ¼ 0.12 sample, a positive surface potential is observed at the GB, and a negative potential is exhibited on the surface of the intra-grains (IGs). (c, f) One-dimensional potentials and topography line profiles near the GBs in the perovskite thin films. The region of the line profiles is marked in (a) and (d). The potential value at the GBs is ~280 mV, and that at the IGs is 150 mV. (g) Structure of mesoporous TiO2 with a mixed halide perovskite absorber. (h) The schematic band diagram near the GBs in Br containing MAPb(I0.88Br0.12)3 thin films. The electronehole carrier separates the mesoporous TiO2 layer from the perovskite layer. The charged GBs (potential value at GBs ~300 mV) have a high local built-in potential, which improves the carrier separation. The electrons are attracted to the TiO2 layer, and the holes move to the HTM layer. Reprinted with permission from Ref. [122]. Copyright 2014 American Chemical Society.

efficiency. On the other hand, although perovskite devices with excess PbI2 can exhibit high initial efficiency (15.1 ± 0.9%), a small amount of excess PbI2 has detrimental effects on the perovskite film stability. The presence of unreacted PbI2 results in intrinsic instability of the film under illumination, leading to degradation of the film even under an inert atmosphere, as well as causing faster degradation upon exposure to illumination and humidity. In addition, by optimizing the spin-coating process, the PbI2-free perovskite films can achieve efficiency comparable to that of with excess PbI2 (14.2 ± 1.3%), and significantly improve film stability. However, the photostability of perovskite film appears to represent only a minor contribution to overall device degradation, and encapsulated devices with excess PbI2 can still exhibit good stability.

Du et al. also found that the presence of a residual PbI2 layer had undesirable effects on the performance of planar PSC [101], the residual PbI2 layer not only greatly impeded carrier extraction and transport, but also accelerated the degradation of the CH3NH3PbI3 film. The perovskite film with an excess of PbI2 decomposed rapidly at high temperature (100  C). 3.2. Grain boundary self-passivation by CH3NH3I (MAI) Interestingly, in perovskite materials with either an excess of PbI2 or a deficiency of PbI2, the grain boundaries can be passivated by unreacted PbI2 or MAI [84]. Son et al. [102] discovered that excess MAI could self-assemble at perovskite grain boundaries, and

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Fig. 10. (a) A diagram of the structure of a device with PCBM passivation layer. (b) Photocurrent upon turning on and turning off the incident light for the devices without PCBM layer (yellow) and with PCBM layer (blue). (c) Schematic of the blue-shift of the PL peaks due to the passivation effect. (d) The PL spectra of samples after thermal annealing PCBM layer with 532 nm green laser as excitation source from the air side (dark blue), from the ITO side (pink), and samples without a PCBM layer from the air side (orange), and from the ITO side (sky blue). Reprinted with permission from Ref. [97]. Copyright 2014 Nature publication group.

that is significantly increased the efficiency and decreased the hysteresis of the cells. The excess MAI is expected to form at the perovskite grain boundaries because it cannot be accommodated in the perovskite lattice. The optical images in Fig. 7 (a and b) clearly show that the films developed a slightly turbid appearance for 0.02 < x < 0.1, and that the turbidity increased with increasing MAI content. In addition, the c-AFM images (Fig. 7 (c) and (d)) suggest that the conductivity of the perovskite thin film improved after MAI passivation. The grain boundaries in Fig. 7 (d) appear brighter than those of the pristine film (Fig. 7 (c)), indicating that MAI might be considered as charge transporting channels. The content of excess MAI was optimized, and the most efficient perovskite can be obtained was for x ¼ 0.06, (overall PCE ¼ 20.4%, reverse scan: Jsc ¼ 23.72 mA cm2, Voc ¼ 1.117 V, FF ¼ 0.779, and PCE ¼ 20.6% forward scan: J sc ¼ 23.67 mA cm2, Voc ¼ 1.110 V, FF ¼ 0.768 and a PCE ¼ 20.2%), and with slight hysteresis (Fig. 7 (eeh)). Yabing Qi et al. [103] further clarified the role of excess MAI at the interface between perovskite and spiro-MeOTAD hole-transport layer in PSCs. By controlling the thickness (0e32 nm) of excess MAI at the interface, it was found that the interfacial energy-level tuning was induced by the dissociated species, rather than the MAI layer. Because MAI can be transformed from a solid to vapor at a relatively low temperature (around 150e200  C) [23,104e106], MAI vapor post-treatment is an efficient method to passivate defect sites on the perovskite grain surface [107,108]. Unlike unreacted PbI2, MAI can passivate not only the grain boundaries but also the bulk defects in MAPbI3 films, as the MAI vapor can diffuse into the

perovskite lattice. While excess PbI2 usually lowers the device stability, the presence of MA-rich species in the perovskite introduces an anti-degradation reaction in the presence of moisture. Excess-MA has been proven to not only result in surface passivation through coordination to lead(II), but also through reaction with H2O to produce the derivative perovskite MAPbI2(OH), which lead to high stability at high humidity (65%) [109]. 3.3. Grain boundary passivation by organic molecules In addition to self-passivation by PbI2 and MAI, the passivation of perovskite grain boundaries can be achieved by the addition of organic molecules to the perovskite precursor solution. A common and valid additive is PCBM. As shown in Fig. 8 (a), the resulting perovskiteePCBM hybrid solution became brown. PCBM is believed to passivate the PbI3 anti-site defects during the formation of perovskite grains (Fig. 8 (bed)) [37]. Theoretical calculations (DFT) also indicated that through the incorporation of PCBM near such PbeI anti-site defects, the deep trap state (black peak in Fig. 8 (e)) induced by PbeI anti-site defect is reduced and becomes much shallower (red peak) upon the adsorption of PCBM on the defective halide [37]. The elimination of the hysteresis index after passivation is clearly observable. (Fig. 8 (f and g)). PCBM has also been used to passivate the perovskite thin film surface, which will be discussed later. The use of a grain boundary passivator seems to affect the crystallization kinetics of perovskite grains. According to a report by Fang et al., when different concentrations of graphene quantum dots (GQDs) are incorporated

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Fig. 11. (a) Device configuration of a planar heterojunction (PHJ) PVSC and the chemical structures of the n-type fullerene derivatives, IC60BA, PC61BM, and C60 used for the study. (b) The energy diagram of each layer of the devices. (c) Steady-state PL spectra and (d) JeV characteristics of the perovskite in the presence of the studied fullerene quenchers. Reprinted with permission from Ref. [136]. Copyright 2015 John Wiley and Sons.

Table 2 Electrical characteristics and photovoltaic performance of the studied PVSCs using different fullerene-based ETLs. Employed Fullerene

VOC [V]

Jsc [mA cm2]

FF

PCE [%]

Mobility [cm2 V s1]

Conductivity [S cm1]

IC60BA PC61BM C60

0.95 0.89 0.92

11.27 18.85 21.07

0.75 0.80 0.80

8.06 13.37 15.44

6.9  103 6.1  102 1.6

6.5  105 3.2  104 2.4  103

Reprinted from Ref. [136]. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

into perovskite films, the grain size enlarges with increasing GQD concentration (from about 100 nm for pristine perovskite to 250 nm for 10% GQD), which could be explained as result of the merging of adjacent crystals [110]. Some organic additives with functionalized hydrophobic groups displayed superior passivation effects compared to the PCBM molecule. An instance is that the 2-(6-bromo-1,3-dioxo-1H-benzo [de]isoquinolin-2(3H)-yl)ethan-1-ammonium iodide (2-NAM) cation, which is recently reported by Qi et al. [111]. Due to the strong Lewis acid and base interaction between C]O group and Pb2þ in perovskite lattice, can effectively increase crystalline, the 2NAM molecule can effectively increase crystalline grain size and reduce charge carrier recombination in perovskite film, and the

efficiency as high as 20.0% was achieved with 2-NAM additives. Meanwhile, the hydrophobic groups, hydrophobic self-assembled 2-NAM molecules at the grain boundaries behaves as molecular locks to isolate the moisture, which contributes to improve the moisture stability of PSCs (85% RH level and dark environment). 3.4. Grain boundary passivation by Br/Cl Several previous reports have demonstrated that the efficiency and stability of perovskite (MAPbI3) solar cells can be enhanced by substituting various proportions of bromide or chloride ions for the iodide in CH3NH3PbI3 with [112e117]. The effects of bromide substitution include tuning of the band gap (from 1.5 to 2.2 eV)

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Fig. 12. (a) Schematic view of the halogen bond interaction between the iodopentafluorobenzene (IPFB, halogen bond donor) and a generic halogen anion (X ¼ I, Br, Cl, halogen bond acceptor) with sp3-hybridized valence electrons. (b) Illustration of the electrostatic interaction between the undercoordinated halide (X) on the perovskite surface and the hole injected in the Spiro-OMeTAD. (c) Schematic view of the interaction between Al2O3, perovskite, and IPFB (d) Illustration of the electrostatic screening of the halide (X) via halogen bond complexation of IPFB on the perovskite surface. Reprinted with permission from Ref. [137]. Copyright 2014 American Chemical Society.

[115,117,118], optimization crystallization rate [119], and increased crystal grain [112,120,121], among others. To understand the physical mechanism of the electronic effects of the Br in perovskite solar cells, Jung et al. [122] investigated the grain boundaries of the perovskite film via Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM). They observed that there is a significant potential barrier bending at the grain boundaries and induced passivation. As shown in Fig. 9 (aef), the positively charged GBs in the Br-substituted perovskite thin film induced a higher local built-in potential at the grain boundaries, which can prompt downward energy band bending. Thus, as shown in Fig. 9 (g) and (h), the charged grain boundaries induced movement of the electrons to the TiO2 side, while the holes were attracted to the perovskite and hole transport layer side. Thus, the electron-hole carrier efficiently separated and suppressed the recombination of charges between the n-type TiO2 layer and the ptype HTM layer. From the above, Br substitution would be expected to play a beneficial role in the passivation of the perovskite thin-film solar cells, through improving their electrical characteristics [122]. In addition to Br, Cl, O, Cuþ, I and thiourea has also been observed to spontaneously segregate into GBs, passivate the defect levels and deactivate the trap states, leading to increased carrier transport in perovskite thin films, via a passivation mechanism similar to what discussed above [123e126]. 3.5. Other passivation additives It is also worth noting that some inorganic and organic monovalent halide additives, such as formamidinium bromide (FABr)

[50], trimethlsulfonium iodide [127], cesium iodide (CsI) [12], iodomethane (CH3I) [128], guanidinium chloride (GuCl) [129,130], alkali metal halides [131] and even quasi-2-dimensional perovskite, e.g. phenylethylammonium iodide (PEAI) [132], and n-butylammonium iodide (BAI) [133] have been reported to modify the perovskite/hole (electron) transporting material interfaces, and to enhance the resulting photovoltaic performance or stability. However, because these halide materials can enter into the crystalline lattice of the perovskite, this kind of modification should be classified into composition engineering, rather than interface passivation. The enhanced properties are mainly caused by the reduction in the number of iodide vacancies [11]. An interesting and mysterious phenomenon observed by Huang et al. [134] is the diffusion of sodium ions (Naþ) from the soda lime glass substrate (ITO glass), which contributed to the defect (grain boundary) passivation. This effect led to an enhancement in the efficiency of a peien planar structure device from 18.8 to 20.2% after 24 h of storage in nitrogen atmosphere. 4. Perovskite film surface passivation by organic molecule 4.1. Surface passivation by PCBM and its derivative The surface of deposited perovskite thin films contains a high density of charge traps, which might be the origin of the notorious photocurrent hysteresis in perovskite solar cells. As with TiO2 surfaces, fullerene derivatives can also be used in reverse structured peien structured perovskite solar cells. The fullerene layers deposit on the perovskite layers, eliminating photocurrent hysteresis and improving the device performance. The surface passivation effect of

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Fig. 13. (a) Chemical structures of aniline (A) benzylamine (BA), and phenethylamine (PA). (b) Schematic illustration of amine treatment of perovskite films through a spin-coating method, followed by an annealing process. (c) Images of unmodified FAPbI3, A-FAPbI3, BA-FAPbI3, and PA-FAPbI3 films for different durations (fresh, 3 days, and 4 months) of exposure to 50 ± 5 RH% air. (d) Stabilized photocurrent and power output as a function of time for the champion BA-FAPbI3 device at a bias of 0.91 V. (e) Moisture stability of unmodified FAPbI3 (black line) and BA-FAPbI3 (red and blue lines) devices to air exposure (50 ± 5 RH%). “Half cells” means that only the BA-FAPbI3 films on the TiO2/FTO substrates were exposed to air and that the spiro-MeOTAD and Au layers were deposited onto the films before the JeV measurement. Reprinted with permission from Ref. [149]. Copyright 2016 John Wiley and Sons.

fullerene derivatives was first reported by Huang [97] in 2014. As shown in Fig. 10 (a), an ultra-thin PCBM layer was coated on the perovskite surface, followed by heat treatment, during which the PCBM diffused into the grain boundaries as well as the surface defects of the perovskite thin film. The effective mitigation of defect states through this method is apparent from the significant increase in the photocurrent response speed (Fig. 10 (b)) and the decrease of the trap density of states (tDOS) (Fig. 10 (c)). The PL results in Fig. 10 (d) indicate that PCBM can passivate the trap states close to the top surface and/or along the grain boundaries. The PCBM passivation leads to improved electronic properties for the perovskite films, including reduced interface charge recombination, longer charge carrier lifetime, and greater mobility, which contributed to the enhancement of device performance [97]. Moreover, the intrinsic fullerene (C60) layer has also been reported to be an effective passivator for the surface traps of perovskite film [135]. As discussed above, different fullerene derivatives can be utilized as passivators. A question is that if there is any difference on

the passivation effects between diverse ones? Jen et al. compared the diversity of the passivation performance between three different fullerenes: IC60BA, PC61BM, and C60 (Fig. 11 (a)). The passivation effects were indirectly observed from the quenching of the PL intensities. The quenching effect of the fullerene derivatives clearly followed the order: C60 > PC61BM > IC60BA (Fig. 11 (b)). As shown in Table 2, the PCEs of the fullerene-derived PVSCs (Fig. 11 (c and d)) also clearly showed a positive correlation with the electron mobility of the fullerene materials, illustrating that high-mobility fullerenes can effectively promote charge dissociation/transport, which in turn leads to a greater passivation effect [136]. 4.2. Surface passivation by Lewis bases Snaith et al. developed a surface passivation method for organicinorganic halide perovskite solar cells by introducing the Lewis bases thiophene, pyridine, and iodopentafluorobenzene (IPFB) (Fig. 12 (a)) via supramolecular halogen bonding [137,138]. After treatment with the Lewis bases, the PCE of the tested devices

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increased from 13.0% to over 17%. The author explained the mechanism responsible for this enhancements as follows: without treatment, under-coordinated halide anions act as hole traps, leading to faster recombination and resulting in a disadvantageous charge density profile within the hole transporter and perovskite film, inhibiting fast and efficient charge extraction under working conditions (Fig. 12 (b)). The Lewis bases can bind to and screens the electrostatic charge from the under-coordinated halide ions, resulting in a significant decrease in the rate of nonradiative recombination [138] in perovskite films, overcoming the above defects (Fig. 12 (c) and (d)). Although PCBM has shown to have an excellent passivation effect to the surfaces as well as the grain boundaries of perovskite materials, due to its low-lying lowest unoccupied molecular orbital level (LOMO), the open-circuit voltage of PCBM based inverted structured perovskite solar cells are still much lower (usually below 1.0 V) [61,139e141] than those of regular neiep cells. Xue et al. introduced C60(CH2)(Ind), a fullerene derivative with a shallow LOMO (3.66 eV Vs 3.8 eV for PCBM), to replace PCBM in the inverted perovskite solar cells [142]. C60(CH2)(Ind) has better energy level matching with the conduction band of the perovskite layer than PCBM, and shows better electron extraction capability, the C60(CH2)(Ind) film exhibited more efficient PL quenching than PCBM and surface trap passivation effect (transient PL decay lifetime increase from 20.1 to 32.6 ns). This resulted in the Voc, FF and power conversion efficiency of the PVSCs increasing from 1.05 V, 0.74 and 16.2% to 1.13 V, 0.80 and 18.1% when the PCBM was replaced by C60(CH2)(Ind).

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4.3. Surface passivation by organic materials with hydrophobic groups Some other organic materials that contain hydrophobic groups, such as polystyrene (PS) [143], poly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA) [144e146], Teflon [15,143], poly(4-vinylpyridine) (PVP) [19,147], polyvinylidenetrifluoroethylene copolymer (PVDF-TrFE) [143], and even ionic liquids [148], polymers, polystyrene (PS), Teflon, and polyvinylidene-trifluoroethylene copolymer (PVDF-TrFE) can cover the surface and diffuse into the grain boundaries of polycrystalline perovskite thin film. These have been used as protective polymer film on perovskite thin film/arrays, and not only passivate surface defects but also block atmospheric moisture, allowing perovskite solar cells to sustain over 80% of their initial performance after 30 days of storage in high moisture (50%) conditions [15]. Amine functionalized molecules containing cyclobenzene combine a hydrophobic benzene ring and a p conjugated structure, which favors charging transport, with an amino group that can graft the molecule to the PbeI framework through coordination with the Pb ions or hydrogen bonding, making these molecules favorable candidates for passivating the perovskite film surface from moisture [149]. Fig. 13 depicts research by Wang et al. [149], in which three different amine functionalized molecules were coated on the surface of a perovskite thin film surface using a post-deposition process (Fig. 13 (a and b)). When exposed to high humidity (50%), the unmodified FAPbI3 films turned yellow after 3 days. In contrast, the FAPbI3 films modified with the amine-functionalized molecules

Fig. 14. PL enhancements of large (a) and small (b) perovskite crystals in ambient air and in argon. Phase I: Enhancement due to consumption of the oxygen dissolved in the material (identical in air and in argon). Phase II: Further PL enhancement that requires oxygen diffusion from the surface to the bulk (does not occur in argon). (c) Normalized PL kinetics measured in the as-prepared micrometer-sized crystals and scratched area (<100 nm grains) of the sample in argon. (d) Accumulated image of a large perovskite crystal under continuous excitation of 0.2 W/cm2. (e) Super-resolution image showing the localized clusters less than 100 nm in size responsible for light emission (emitting sites). (f) PL intensity (counts per pixel) transient measured locally at the squares marked in panel (d). Reprinted with permission from Ref. [150]. Copyright 2015 American Chemical Society.

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Fig. 15. Diagram summarizing the passivation media for perovskite solar cells.

showed much higher stability against humidity with the BA-FAPbI3 thin film maintaining a pure a-FAPbI3 phase even after four months (Fig. 13 (c)). However, although modification with aniline, benzylamine, and phenethylamine was tested, only the benzylaminemodified film exhibited enhanced solar cell efficiency perovskite films stability for more than four months in moist air (Fig. 13 (d and e)). That was because, according to DFT calculations, the steric arrangement of the benzene rings was quite sensitive to the group by which they were grafted to the perovskite lattice. 4.4. Surface passivation by oxygen An interesting passivation medium is the oxygen in air. Tian et al. discovered the synergistic passivation effect of oxygen and illumination on the perovskite single crystal defect states, using PL microscopy and super-resolution optical imaging [150]. As can be observed from Fig. 14 (a) and (b), both large and small crystals exhibited the PL intensities with increasing as irradiation time in air atmosphere, while the intensities of the samples kept in an Ar atmosphere were almost constant. In addition, the PL lifetime of small particles kept in the Ar (in Fig. 14 (c)) was much longer than that of a large micrometer-sized crystal in Ar. This evidence implied that the oxygen in air acts as a reactant to passivate the traps of small crystals quickly and efficiently. When the PL signals of isolated micrometer-sized crystals were investigated at high spatial

resolution, the characteristic PL intensity enhancement time was found to vary greatly depending on the measurement position within one crystal. In general, the blinking sites on the surface in Fig. 14 (d) and (e) showed much higher PL brightness per unit area in comparison with the surrounding areas with stable PL (Fig. 14 (f)). The same author also investigated the mechanism by which light-induced curing in the presence of oxygen causes the perovskite photoluminescence enhancement. One proposed hypotheses was that the trap sites responsible for non-radiative charge recombination can be de-activated by a photochemical reaction involving oxygen, and that the reaction zone is spatially limited by the penetration depth of the excitation and diffusion length of the charge carriers [36]. Other reports have also demonstrated that the oxygen can reduce deep defect states in perovskite materials, not only under illumination but also by thermal treatment in air [105,151]. In addition to producing oxygen passivation, oxygen can also act as a p-type dopant in MAPbI3 [123]. Similar to the “double-edged sword” effect of PbI2 passivation at perovskite grain boundaries [84], oxygen diffusion can also induce rapid degradation of perovskite thin film. Using a combination of ab initio simulation and experimental characterizations, including isothermal gravimetric analysis (IGA), photoluminescence, and secondary ion mass spectroscopy (SIMS), Aristidou et al.

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determined the mechanistic of the oxygen and light-induced degradation of perovskite solar cells. Fast oxygen diffusion into CH3NH3PbI3 films is accompanied by the formation of superoxide (O 2 ) species, which accelerate the decomposition of perovskite materials. Fortunately, thin-film passivation with iodide salts can mitigate the photo-induced formation of superoxide species from O2, and thus enhance film and device stability [127]. This strategy has been successfully applied to fabricate the most efficient perovskite solar cells (22.1%) [2].

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passivate that highly oxygen-sensitive bivalent tin cation (Sn2þ). Overall, we believe that the passivation method will continue to play an important role in the development of the emerging technology of perovskite solar cells. Conflict of interest The authors declare no conflict of interests. Acknowledgements

4.5. Functionalized silica nanoparticles The addition of 3-aminopropyl (3-oxobutanoic acid) functionalized silica nanoparticles (f-SiO2) to the precursor solution of perovskites, results in surface passivation effect whereby the f-SiO2 nanoparticles hinder recombination inside a pinhole void, where the c-TiO2 layer and the Spiro-OMeTAD would otherwise come into contact [152].

The authors thank the financial support from the National Research Foundation (NRF) of Korea grant funded by the Korea government (No. 2017R1A2B3010927), Basic Science Research Program through the National Research Foundation of Korea (NRF2014R1A4A1008474) and Creative Materials Discovery Program (2016M3D1A1027664). References

5. Passivation of the top electrode The most commonly utilized metal electrodes are Ag and Au electrodes; however, both of these electrodes can diffuse through the HTM layer and even the perovskite layer, causing the performance degradation in the perovskite solar cells [153]. Sanehira et al. compared the stability of perovskite solar cells with different electrode configurations: Au, Ag, MoOx/Au, MoOx/Ag, and MoOx/Al [154]. Devices with MoOx/Al electrodes were more stable than devices with more conventional, Au and Ag electrodes. The spontaneously formed ultra-thin aluminum oxide layer is believed to be responsible for the increased stability of MoOx/Al, and the introduction of Al oxide then slows the iodization of the back contact. There have been only a few reports of the passivation of the top metal electrode (Au/Ag). In one, an ultra-thin Ni surface layer was applied to the Au electrode of a CH3NH3PbI3-based photoanode solar-to-fuel system. The Ni layer functioned as both a physical passivation barrier and a hole-transfer catalyst. Enhanced photocurrent density and substantially better water stability were achieved using the Ni layer [155,156]. Finally, encapsulation processes, such as sealing the perovskite solar cells with polyimide or UVcured polymers [157], are able to effectively isolate the active layer in perovskite solar cells from the O2/moisture. 6. Summary and prospective In summary, we reviewed the passivation mediums used for perovskite solar cells (Fig. 15). Currently, surface/interface passivation has been developed as a universal method to improve the photovoltaic performance and stability of perovskite solar cells. Two inherent disadvantages of perovskite solar cells, hysteresis and instability, can be partly compensated through passivation. Reported passivators include a wide variety of materials ranging from inorganic to organic molecules or even polymers, and form insulators to semiconductors, from nonstoichiometric reactant to second phase compounds, indicating new passivating materials and technical approaches remain to be explored. To guide such an exploration, a fundamental understanding of the types and densities defects that form in perovskite crystals and their influence on electronic transport properties as well as the photovoltaic performance in perovskite crystals should be lucubrated. Surface and interface passivation might be a promising method to surpass the present record PCE of 22.7%. Passivation strategies are also likely to provide new opportunities to develop PSCs with a stable high performance based on less-toxic PbeSn mixed perovskite or even totally Pb-free, completely Sn-based PSCs, by providing a means to

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Please cite this article in press as: P. Zhao, et al., Passivation in perovskite solar cells: A review, Materials Today Energy (2018), https://doi.org/ 10.1016/j.mtener.2018.01.004

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