Advances in stability of perovskite solar cells

Organic Electronics 78 (2020) 105590

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Review

Advances in stability of perovskite solar cells Qamar Wali a, *, Faiza Jan Iftikhar a, Muhammad Ejaz Khan a, Abid Ullah b, Yaseen Iqbal b, Rajan Jose c a

NUTECH School of Applied Sciences & Humanities and Computer Engineering Department, National University of Technology, Islamabad, 42000, Pakistan Materials Research Laboratory, Department of Physics, University of Peshawar, Peshawar, 25120, Pakistan c Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Keywords: Renewable energy Future technology Portable devices Emerging technology Perovskite solar cells

Perovskite Solar Cells (PSCs) with efficiency greater than 25% have shown promising prospects for future green technology. However, exposure to moisture, along with thermal and photo instability are critical issues limiting commercialization of the PSC devices. Indeed, perovskite-provoked instability of PSCs together with decompo­ sition of hole transport layer (HTL) and electron transport layer (ETL) contribute to overall degradation process and hence affecting the performance of the device. Herein, we discuss instability of PSCs in various operating conditions such as UV light, humidity, environmental ingredients and temperature. Furthermore, we report the recent progress towards improvement in long-term stability of PSCs and those efforts include but not limited to introducing new HTLs, engineering of perovskite materials, interfacial modification, electrodes and novel device configurations and behavior of the device under encapsulation and un-encapsulation conditions. Moreover, we also discuss the researcher’s efforts to improve the optical, electrical and chemical properties of different layer of PSCs. Additionally, to address the future research directions such as the need to improve the intrinsic stability of the perovskite absorber layer, design architecture of the device, and search for new durable materials are also proposed.

1. Introduction

for a duration of 1000 h. PSCs are on their way towards this milestone and are being constantly investigated. The sources of degradation are manifold e.g., degradation of PSCs occurs not only because of the poor stability of the perovskite layers e.g., methylammonium (MA) molecule is thermally unstable, but it also accelerates due to the instability caused by the employed hole transport media (HTM) or electron transport media (ETM) of the device as shown in Fig. 1a. For example, when HTM en­ counters moisture or water, it degrades and thereby deteriorates the performance of the device. Perovskite materials are sensitive to ultra­ violet light, water and moisture, and thermal stress [4–6] as depicted in Fig. 1b. Before focusing on the instability and nature of degradation of PSCs, it is important to introduce the basic structure of these materials. PSCs are relatively new photovoltaic devices based on light absorbing mate­ rials ABX3 of crystal structure shown schematically in Fig. 1c. Where ABX3 represents a combination of organic inorganic metal halide perovskite material where A is a large monovalent organic cation mostly þ composed of CH3NHþ 3 (MA ), B is an inorganic smaller divalent cation

Although perovskite solar cells (PSCs) technology has an excellent commercialization potential, it is still in the early stages as there are many concerns that remain to be addressed before its entry into the market. The major issues include; unaffordable high cost as most of the common electrodes in PSCs are made of gold, perovskite materials deteriorate in the presence of moisture, and need strong protective encapsulation adding to the cost as well as weight of the cell. Another major issue is the toxicity of Pb, endangering the environment. The development of solution processable methods of PSCs, and its decent performance with power conversion efficiency (PCE) of 25.2% compa­ rable to the market leader-silicon solar cells at a half price makes them promising cells for future technology [1]. Despite these outstanding potentials, PSCs exhibit instability under normal operative conditions thereby restricting their commercialization [2,3]. Less than 10% drop in solar cells performance and a warranty of 20–25 years are pre-requisites for such market products. This is accomplished by the standard accel­ erated aging test for a device to exhibit less than 10% drop in efficiency

* Corresponding author. E-mail address: [email protected] (Q. Wali). https://doi.org/10.1016/j.orgel.2019.105590 Received 15 February 2019; Received in revised form 3 December 2019; Accepted 9 December 2019 Available online 14 December 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.

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added functionality such as flexibility, transparency, aesthetics, and less stringent operating conditions. While the first and second generation solar cells offer the first three (i, ii, and iii) the third-generation solution processable solar cells offer the last and the third one. However, the early members of this group, i.e., dye-sensitized and bulk heterojunction polymer solar cells, failed to a great extent in exhibiting the first three criteria. Emergence of hybrid organic inorganic PSCs boosted the morale of the third-generation solar cells in relation to its efficiency like the first two generations with a cost lower than the two. The compelling features offered by the perovskite solar cells are light absorbing materials that generates charge carriers with long diffusion lengths, extremely high absorption co-efficients, and tunable band gaps. An overview of publi­ cations and efficiency values in PSCs is depicted in Fig. 2. The data is taken from the Scopus on 10.14.2019 using the keyword “Perovskite solar cells”. The most commonly used perovskite MAPbI3 poses an issue due to the relative volatility of the methylammonium iodide (MAI), which evolves out of the film during the heating process which expedites further in the presence of the moisture and air [5,10,11]. Furthermore, the device based on MAPbI3 perovskite degrades tremendously when exposed to light in ambient atmosphere owing to the chemical reaction between oxygen and MAþ cations [12]. The effect of various factors on the stability of PSCs are summarized in Fig. 3. Devices based on for­ mamidinium (FA) employing FAPbI3 having narrower band gap show better thermal stability than MAPbI3 counterpart [13,14]. However, FAPbI3 is structurally not stable (i.e., in a metastable state) due to its large size. It tends not to form the black perovskite phase and is trans­ formed to a yellow phase at room temperature. There are a few A cations in ABX3 perovskite which support photoactive black phase. The most reliable empirical approach is the Goldschmidt tolerance factor (t), which lies between 0.8 and 1.0 for photoactive black phase perovskite [15]. Only few cations such as Cs, MA, and FA fall in the range of photoactive black perovskite, while other cations are too large (imida­ zolium, ethylamine, and guanidinium) or too small (Na, K, Rb) for consideration. For example, the t for FAPbI3 (t ~ 1) and CsPbI3 (t ~ 0.8) are at the edge and have a distorted lattice resulting in the presence of a yellow phase at room temperature. While MAPbI3 (t ~ 0.9) does not have a yellow phase. This structural instability could be improved by

Fig. 1. (a) device architecture of PSC, and (b) PSC exposed to air (i.e., moisture and oxygen), light illumination, and an electric field during operation, leading to initial degradation of the perovskite layer and generation of I2 (indicated by paired purple spheres without a tail) as a product. Simultaneously, the perov­ skite layer is self-exposed to I2 vapor (indicated by paired purple spheres with tails) because of the high vapor pressure of I2 [7], Copyright Nature 2019 c) Perovskite crystal structure ABX3: Corner sharing PbX6, (d) the central site occupied by methyl ammonium CH3NHþ 3 , A cation which surrounded by 12 nearest anions [8,9]. Figures adapted from ref. 7–9.

such as Pb2þ or Sn2þ and X is a monovalent anion halide such as I , Cl , Br etc. The building blocks of the organic inorganic halide perovskites are the Pb-halide octahedra (PbX6) that are corner shared and form a 3D structure as shown in Fig. 1c. The voids in the inorganic network are filled by the organic ‘A’ cation mostly CH3NHþ 3 in order to balance the charge. The CH3NHþ 3 cation occupying the central A site is surrounded by 12 nearest neighbor ‘X’ anions (Fig. 1d). The four important requirements for an adaptable solar cell tech­ nology are; (i) high efficiency, (ii) high stability, (iii) low cost, and (iv)

Fig. 2. An overview of publications and efficiency values in PSCs. Data is taken from Scopus in October 2019 using keywords “perovskite solar cells”. 2

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decomposition reactions (1–5): n [(CH3NH3)þPbI3] þ H2O → (CH3NH3)þn-1(CH3NH2) PbI3(H3O)þ þ

(1)

þ

(CH3NH3) n-1(CH3NH2)PbI3(H3O) → CH3NH2(aq) þ HI(aq) þ PbI2(S) þ n1[(CH3NH3)þ PbI3 þ H2O (2) n[(CH3NH3)þ PbI3] þ nH2O → nCH3NH2(aq) þ nHI(aq) þ PbI2(S)þ nH2O (3) 4HI(aq) þ O2 →2I2 þ 2H2O

(4)

2HI þ UV → H2 þ I2

(5)

These reactions reveal that as a water molecule extracts proton (Hþ) from the perovskite crystal, the color of the perovskite layer changes to yellow which is the experimentally confirmed color of PbI2. The HI decomposition occurs in two ways; (i) the redox reaction in the presence of oxygen, and (ii) under UV radiation. It should be noted that the moisture behaves like a catalyst required for irreversible decomposition of the perovskite layer [3]. Burschka et al. [17], suggested that the perovskite layer is very sensitive to moisture, and therefore, the device preparation must be carried out in a controlled atmosphere with relative humidity (RH) < 1% in order to ensure high efficiency. The authors have previously reported that decomposition of perovskite starts at a RH of 55% which can be confirmed from the change of color from dark brown to yellow [10]. According to Walsh et al. in the degradation of perovskite, water molecules act as Lewis base which chemically combine with the perovskite and accept one Hþ from the ammonium leading to its degradation via intermediate steps. The degradation of intermediates yields CH3NH2, HI, and finally PbI2, where the HI and solid PbI2 are soluble in water. The decomposition of PbI2 leads to further toxicity while CH3NH2 is a polar organic compound; therefore, it is highly vol­ atile and soluble in water [16,18,19]. More recently Choi et al. [20], explained detailed degradation pathway of hybrid halide perovskites (MAPbBr3) at atomic-scale in humid conditions under dark and vacuum environment (Fig. 4). They reported the evaporation of water vapors and volatile CH3NH2 gas together with formation of large-sized PbBr2 clus­ ters at the perovskite surface during decomposition process in humid condition. þ CH3NHþ 3 PbI3 is more sensitive to humidity than CH3NH3 PbBr3 and þ þ CH3NH3 PbCl3, but CH3NH3 PbI3 offers high absorption coefficient than the latter. In order to maintain the high absorption coefficient of iodine (I) based perovskites and less sensitivity of bromine (Br) and chlorine (Cl) based perovskite toward moisture. Seok et al. [10] demonstrated stable device performance by employing mixed (CH3NH3)þ(PbI3-xBrx

Fig. 3. A schematic of perovskite solar cells’ degradation by various factors [8]. Reproduced from ref 8.

adding a fraction of MA; however, MA is chemically unstable and therefore causes instability in the device. To cover different aspects of the subject, this review has been divided into four sections. Section 1 gives a brief overview of the stability issues of perovskite solar cells, the related technology, and crystal structure. Various causes of instability arising due to a range of factors such as photon dose (UV), temperature, air and moisture are discussed in Sec­ tion 2 (Fig. 3). Section 3, gives a comprehensive description of the progress made in addressing the instability issues in PSCs using different approaches. For example, introducing new HTMs, engineering of perovskite materials, interfacial modification, electrodes as well as novel device configurations and device encapsulation. Finally, the cur­ rent status of research, the application potential, and commercialization prospects of this technology have been highlighted in the conclusion of this review. Power conversion efficiencies of organic–inorganic lead halide based PSCs have dramatically arisen in the last few years reaching a certified value of 25.2% in 2019 ahead of any of the photovoltaic technologies, yet the research for efficient PSCs has not stopped here. Nevertheless reaching a certified high value of PCE does not mask the issue pertaining to stability of PSCs. Hence, instability issues with different degrees of sensitivity towards light, moisture and temperature as well as the ma­ terial used plague their development for commercial purposes. It is argued that the MAPbI3 may not be a suitable material for futuristic solar cell applications due to generation of volatile I2 leading to chain reaction and degradation and hence it is ever so more necessary to look for other robust materials that can resist physical and chemical change and overcome stability issues [7]. This review will help the esoteric reader to develop an in-depth understanding of the problem at hand with relation to stability and find real time solutions for better and efficient use of solar energy. 2. Causes of instability in perovskite solar cells 2.1. Water and moisture Exposure to water/moisture is considered to be one of the most dominant factors that degrades and hence, destabilizes PSCs. Degrada­ tion of the perovskite layer is a chemical process in which the moisture acts as a catalyst. The relevant literature has been reviewed by Niu at al [16]. In brief, a perovskite material decomposes in the presence of moisture because of its high sensitivity towards water and tends to hy­ drolyze due to its polar nature. It is reported that PbI2, CH3NH2I and HI species are formed when CH3NH3PbI3 is exposed to moisture. The decomposition of CH3NHþ 3 PbI3 initiates when water molecules interact with the proton of the perovskite layer, as described by the following

Fig. 4. Degradation pathway of MAPbBr3 single crystal in humid conditions. Variation of perovskite surface are shown after each degradation step [20]. Adopted from ref. 20 with permission. 3

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perovskite than that of the CH3NHþ 3 PbI3. Interestingly, the device fabricated using the (CH3NH3)þ(PbI3-xBrx) sensitizer demonstrated sta­ ble performance for one day when RH increased beyond 55% at values of x ¼ 0.2, and 0.29, in comparison to the x ¼ 0 and 0.06 compositions. The reported excellent performance may be due to the presence of Br because it is smaller in size and possess more negative charges in com­ parison to I. Therefore, the binding force or attraction between the organic and in-organic constituents is stronger, and the suitable atomic ratio of the ions play an important role in improving the stability of the device. Fig. 5 compares the device efficiency made of (CH3NH3) þ (PbI1-xBrx) materials (x ¼ 0.2, 0.29) where the device showed stable performance when the RH was in the range of 35% and 55%. On the other hand, device stability decreased when RH increased to >55%, obviously due to the decomposition of the (CH3NH3) þ(PbI1-xBrx) layer [3].

[23]. Furthermore, conductive atomic force microscopy (C-AFM) method was also used to confirm the structural change. A careful inspection of the area where the grain appears in the topography image indicates dark region thereby not contributing to the electric current. Upon increasing temperature, the conductivity of the film decreases due to the presence of metal halide (PbI2) which is poor light absorber because of large band gap i.e., requires high energy to promote electrons from the valence band to the conduction band. As mentioned above, PbI2 is a bad conductor; therefore, its presence causes a decrease in the electric cur­ rent which it is a clear indication of device internal structure decom­ position. It is clear from above discussion that the decomposition of the layer starts at ~85 � C and the rate of the degradation processes depends on the surrounding atmosphere [3,24]. The possible temperature-induced degradation reaction can be written as follows: CH3NH3PbI3 → PbI2 þ CH3NH2↑þHI↑

2.2. Thermal stress

(9)

Philippe et al. [25] employed photoelectric spectroscopy (PES) approach and determined thermal instability in terms of I/Pb and N/Pb ratios. During heating and cooling of the sample, the X-rays beam was kept off to avoid its effect. Furthermore, the sample was heated in a moisture-free environment to investigate the effect of temperature on sample decomposition. Initially the film was inspected at room tem­ perature and then heated up to 100 � C for 20 min and finally the tem­ perature was raised to 200 � C. Fig. 7 shows that as the temperature was gradually increased, both the I/Pb and N/Pb ratios first declined linearly to 2.0 and then 0 which confirmed the conversion of the sample into byproduct PbI2. Additionally, the elevated temperature not only effect pure CH3NH3PbI3 but also mixed CH3NH3PbI3_xClx layer [25,26]. Katz et al. [26] reported that the decomposition induced due to light energy can be thermally accelerated i.e., the combined effect of light and heat energy coming from concentrated sun light increases degradation of the device. Recently, Leong et al. [27] studied the performance of m-TiO2 based PSCs in temperature range 80–360K to investigate the relationship be­ tween temperature and device performance. Initially, the efficiency shows increment for T < 330K and then decreased significantly as the temperature increased above 330 K up to 360 K. Upon further increase in heating time as well as temperature, the device started to gradually decompose due to thermal instability of the MA cations in the perovskite structure [28]. On the basis of temperature dependence, the open circuit voltage (VOC) can be described in terms of two regimes. At low tem­ peratures (T ˂ 250 K), the VOC remains in the range of 1.0 V–1.1 V whereas in the high temperature regime i.e. 250–360 K, VOC decreases linearly with increasing temperature [27,29].

The commonly employed perovskite are highly sensitive to heat where successive phase transitions occur from low temperature (T < 162.2 K) distorted orthorhombic state to the medium temperature (162.2–327.4 K) tetragonal phase, and then, the ideal cubic phase at higher temperatures >327.4 K. Upon further increase in temperature, the perovskite decomposes to volatile CH3NH2 and HI compounds [21, 22]. The thermal degradation of perovskite structure could be seen from its optical absorption, electrical and chemical properties of the layers. Coning et al. [23] reported that the phase transition of CH3NH3PbI3 occurs at 85 � C, indicating thermal instability of the perovskite material. Thermal stability tests have been performed under pure dry O2, pure dry N2 and ambient atmosphere with RH ~50%, and at temperatures up to 85 � C for 24hrs, in dark, respectively. When the samples were heated in the presence of N2, scanning electron microscopy (SEM) revealed a variation in the microstructure of the samples in the form of spots with bright contrast (with some of these slightly elongated) on the top of the perovskite layer, indicating the beginning of decomposition (Fig. 6a–b). When the samples were heated solely in the presence of O2, elongated tiny features were observed on the top of the perovskite layer (Fig. 6c). In case of heating the same samples in ambient atmosphere, the struc­ tural transitions were more visible. The determination of exact morphology of the new features was difficult; however, the elongated grainy features on the top of the perovskite layer increased in number as well as size (Fig. 6d) demonstrating the dominant induced degradation

2.3. Light and oxygen induced degradation In most of the solar cells (dye-sensitized and PSCs), ultraviolet (UV) cut off filters are often employed to avoid instability. The instability of PSCs caused by UV is attributed to the changes occurring in compact -TiO2 (c-TiO2) or mesoporous -TiO2 (mp-TiO2). The UV effect is domi­ nant when the devices are sealed and encapsulated properly from oxy­ gen ingression and moisture. Photo-generated holes created in TiO2 upon exposure to UV light react with the oxygen adsorbed at oxygen surface vacancies thereby act as deep traps, leading to recombination. Recent reports have been shown that planar encapsulated c-TiO2 based PSCs still encounter degradation issues under UV light; therefore, for long term operation of PSCs, stable ETM should be a pre-requisite [30, 31]. Leijtens et al. [31] reported that decomposition of the TiO2 ETM of the device occurs under full radiation spectrum with a simulated in­ tensity (I) ¼ 100 mW cm 2 at 40 � C. According to this study, unsealed, sealed and sealed along with UV filter-added devices were illuminated with UV light in an inert atmosphere and their efficiencies were

Fig. 5. The efficiency decay curve of the (CH3NH3) þ(PbI1-xBrx) based device (in the presence of Br). Note in the red lines when RH increases beyond 55% the efficiency drops rapidly [10]. Reproduced from ref.10 with permission. 4

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Fig. 6. SEM images of the samples that decomposed into the byproduct PbI2 when subjected to 24 h heat treatments at 85 � C [23]. Figures taken from ref. 23 with permission.

compared. Interestingly, the sealed device was reported to exhibit inferior stability relative to the unsealed device, and the efficiency of sealed device with UV filter was reported to exhibit superior perfor­ mance (Fig. 8). Transient absorption spectroscopy (TAS) results of the TiO2 based device revealed that the charge collection efficiency of encapsulated cells deteriorated after exposure to UV light. This effect was more severe in the case of the encapsulated PSCs as compared to the un-encapsulated ones. The rapid drop in efficiency may be due to the injected electrons into TiO2 getting trapped in the deep lying vacancies [31]. The decomposition mechanism is closely related to the surface chemistry of TiO2 as shown in Fig. 9. Due to crystal defects, many va­ cancies are present in the structure of TiO2, particularly on the surface where molecular oxygen (O2) is adsorbed from the surrounding. The vacancies are positively charged which adsorb O2 molecule and transfer electrons from TiO2-x and form the Ti (IV)þO2 complex. When the device is exposed to UV light, electron-hole pairs generate in the TiO2 layer as shown in Fig. 9a–b. The holes in the valence band recombine with electrons which release O2 as shown in Fig. 9c–d. Consequently, free electrons and empty vacancies are generated which lead to the donation of inner shell electrons. The energy of these electron is ~1eV smaller than the conduction band electrons in the TiO2 crystal [32]. The remaining excess free electrons in the conduction band recombine with the holes in the p-doped transfer layer. The rate of such recombination significantly affects the performance of the device [33]. Ito et al. reported that degradation mechanism of PSCs with device

architecture (FTO/TiO2/CH3NH3PbI3/CuSCN/Au) rapidly increased under light irradiation without Sb2S3 blocking layer between TiO2 and perovskite. It seems that the decomposition occurs at the interface be­ tween the CH3NH3)þ PbI3 and TiO2 layers under UV light (Fig. 10a). The decomposition reaction consists of the following reaction steps (6–8): (6)

2I ↔ I2 þ 2e at TiO2 and CH3NH3) PbI3 interface, þ

þ 3CH3NHþ 3 ↔ 3CH3NH2↑ þ 3H

(7)

I þ I2 þ 3Hþ þ 2e ↔ 3HI↑

(8)

When UV light shines on PSCs, TiO2 gets excited and has the ability to extract electrons from the halide ion (I ) thereby destroying the crystal structure of perovskite layer, leading to the formation of byproduct I2 (reaction 6). In reaction 7, the equilibrium basically moves to the right side because the layers are no more integrating. In reaction 8, the extracted electrons return back from TiO2 to the perovskite layer starting reaction with I and Hþ and upon completion give rise to vol­ atile HI and continuous release of Hþ as a byproduct. With the evapo­ ration of CH3NH2 and continuous release of Hþ, the reaction (7) moves to forward direction to attain equilibrium [33]. Devices with and without Sb2S3 blocking layer (Fig. 10a–b), revealed that inclusion of Sb2S3 between TiO2 and perovskite layer significantly decreased the UV-induced degradation. For an unsealed device in air and without the Sb2S3 blocking layer, the efficiency dropped to almost 0% after 12 h. On 5

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Fig. 9. Schematic of the degradation of the TiO2 layer which adsorbs O2 from the surrounding in the interstitial defects. When UV light falls on this layer, electron-hole pairs are produced in the TiO2, the vacancies in the conduction band accept electrons from the O2 molecule and detach the oxygen molecule, giving rise to deep electron sites as shown in (a) and (b) The free electrons in the conduction band recombine with HTL as shown in (c) and (d) [31]. Figures reproduced from ref.31 with permission.

Fig. 7. Red circles represent I/Pb and black squares denote the N/Pb atoms ratios plotted at different temperatures employed in this study. The values 3 and 1 respectively, represent the undistorted perovskite, while the o and 2 show almost complete decomposition of the perovskite layer and conversion into PbI2. reproduced from Ref. [25]. Reprinted from ref. 25 with permission.

Fig. 10. (a) Degradation of the structure upon exposure to light of the device without interface layer between the TiO2 and pervoskite and (b) PSCs with Sb2S3 layer at the interface preventing the direct interaction between TiO2 and the pervoskite layer; therefore, decreasing the UV-induced degradation. The inclusion of this layer improves the stability of the layer. Adopted from Ref. [33]. Reproduced from ref.33. Fig. 8. Normalized PCE curves for PSCs in which the TiO2 was used as ETL, showing the efficiency of the sealed (red open squares), unsealed (open blue squares) and UV filter 435 nm (black closed squares) devices. Note the rapid drop in the efficiency of sealed device in comparison to the other two [31]. Figure taken from ref.31.

photoactive layers was significantly affected upon switching the oxide substrate. Moreover, the photoactive layers of mp-Al2O3/CH3NH3PbI3 rapidly decomposed into CH3NH2, PbI2, and I2. In contrast, the surface decomposition was reduced when the mp-Al2O3 was replaced with an mp-TiO2 as electron transport layer. The increased degradation in mp-Al2O3/CH3NH3PbI3 occurred due to large amount of light-induced superoxide O2 generation compared to mp-TiO2/MAPbI3. The process of transfer of the photoexcited electrons that generated higher super­ oxide the Al2O3/MAPbI3 compared to the TiO2/MAPbI3 is depicted in Fig. 11a, which was also evident from their fluorescence intensity plots as shown in Fig. 11b. In the Al2O3/MAPbI3, the photoexcited electrons remained on the perovskite surface and reacted with oxygen to form a superoxide. Contrarily, while the perovskite was prepared on TiO2,

the other hand, the device with Sb2S3 layer retained almost 65% of its initial efficiency after illumination at the same conditions. Fourier transformation infra-red (FTIR) spectroscopy results demonstrated that Sb2S3 layer decelerated the decomposition of perovskite crystalline structure [24]. In addition to TiO2, its other wide band gap ETM counterparts such as Al2O3 were also considered to determine their stability under the light [34]. The light-induced degradation of CH3NH3PbI3 perovskite-based 6

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Fig. 12. Schematic of the, (a) diffusion of the oxygen molecule into the perovskite layer and its interaction with the, (b) light falling on the perovskite layer producing electrons and holes, (c) the absorption of the O2 molecule by the excited electron absorbs forming superoxide, and (d) the reaction of the layer with oxide, thereby, decomposing the device [36]. Figure taken from ref. 36.

desorption of O2 at the TiO2 cathode. The formation of deep trap state Ti (3) (3d1) occurs due to the transfer of one electron to Ti (4) (3d0) of TiO2 from perovskite layer. The unsaturated coordination of Ti (3) occurs due to oxygen vacancies residing in the sub-band gaps, which acts like a deep trap state. The vacancies attract molecular oxygen from atmosphere and form α (Ti (4)þO2 ) [38,39], and if electrons are present in the sub-band gap, β(Ti(IV)þO2 ) is formed. These modified states do not affect the per­ formance of a device when exposed to light and the depletion layer forms due to O2 when positive potential applies at the TiO2 surface. The layer decomposes upon continuous exposure to high temperature. In this process O2 is liberated and forms α complex superoxide which converts to Ti (3) when UV light falls on the TiO2, and results in the generation of electron-hole pairs. The hole in the valence band gets trapped with O 2 in the Ti (4)þO2 as shown in Fig. 13b. The corresponding free electrons in the conduction band regenerate deep trap states and finally recom­ bine with holes in the HTL (Fig. 13c). The adsorbed O2 may broaden the depletion region, causing the energy band to bend upwards into the conduction band which increases the band gap, and reduces interfacial charge carriers’ recombination. In addition, O2 desorption upon light exposure vanishes the upward bending of the band, and hence, results in the disappearance of the barrier, thereby, flattening the conduction band and lowering the Fermi-level (Fig. 13d). O2 creates defects in the TiO2 and it is one of the major reasons to cause device degradation during operation [40]. Similarly, it has been reported that the top Ag electrode turns yellow when exposed to air showing the formation of AgI layer. This layer forms due to the migration of I2 species from the perovskite layer to the Ag electrode through pin holes in the hole transport layer [41].

Fig. 11. (a) Schematic models showing the electron transfer process of the photoexcited electrons to oxygen in the MAPbI3 perovskite layers that result in formation of superoxide. The generation of higher number of superoxide in Al2O3/MAPbI3 can be observed compared to TiO2/MAPbI3. (b) Normalized fluorescence intensity. The ratio IF(t)/IF(t0) indicates the amount of superoxide generated showing higher concentration of O2 in Al2O3 compared to TiO2 substrate [34]. Figures are adopted from ref 0.34.

those photoexcited electrons could also injected into mp-TiO2, resulting in reduced O2 generation. Therefore, TiO2 as ETM was shown as more suitable choice to design stable photovoltaic devices. Generally, MAPbI3 perovskite layer changes color from dark brownish to yellow when exposed to air showing perovskite degrada­ tion. It is reported that the performance of PSCs significantly de­ teriorates when tested in dry air and in N2 atmosphere and needs a proper protection for proper working [35]. Oxygen molecules diffuse into the perovskite layer and trap into the iodine vacancies. Upon exposure to light, the perovskite film produces electron-hole pairs in it as shown in Fig. 12(a–b). The electron transfers to O2, produces highly reactive super oxide species (O2 ). Such O2 attacks the sample and ex­ tracts Hþ from the photo-excited perovskite layer leading to the for­ mation of PbI2, water, CH3NH2 and I2 and hence, decompose the active perovskite layer as shown in Fig. 12(c–d) [36]. Depending on the nature of cathode (n-type), interstitial defects and oxygen vacancies adversely affect the performance of fast working de­ vices, for example, as a result of non-stoichiometric composition of TiO2x. Two under-coordinated Ti (3) ions give rise to one oxygen vacancy (Vo) where the bridging oxygen atoms are removed from lattice due to thermal energy. These under-coordinated Ti (3) and Vo act like a reac­ tive center and adsorb O2 molecules which form a complex compound (Ti4þO2 ) via the reaction Ti3þ þ O2 →Ti4þO2 [37]. Formation of O2 thermodynamically is the most probable outcome through different re­ actions if excess electrons are available in the sub-band or conduction band (CB) i.e. Ti4þ þ eCB þ O2 → Ti4þO2 . Fig. 13 shows the formation of deep trap states and depletion regions due to the adsorption and

2.4. Electric field induced decomposition Electric field activates ion migration in the perovskite layer which constitutes the dominant degradation mechanism. The resulting move­ ment of ions is one of the possible ways to change the band structure and alter the barrier height for hole and electron diffusion which affects device performance. The major reason for non-steady current through perovskite layer and hysteresis in current-voltage (I–V) measurement are due to the mobile ions [42]. Furthermore, the organic-inorganic perovskites are ionic-electronic conductors i.e., conduct electrons and holes at the same time due to its low activation energy (Ea) for halide, methyl and Pb ions, respectively [43]. In the absence of an electric field, 2þ a significant amount of I , CH3NHþ ions are present at room 3 and Pb temperature which diffuse randomly into vacancies. Employing the first principle method, Azpiroz et al. investigated the mobility of ions in the perovskite layer and showed the diffusion path of the model vacancies 7

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Fig. 13. Schematic of, (a) O2 adsorption at the interstitial spaces, forming the complex compound, (b) production of electrons and holes upon excitation of TiO2 when exposed to UV light, (c) release of O2 molecule as a result of reaction of O2 with holes in VB, and (d) the effect of complex oxide on the depletion area. Reproduced from Ref. [3]. Figures reproduced from ref. 3.

and migrated ions along the sample as shown in Fig. 14 [44]. Vacancies and interstitial defects are represented by doted red circles and red spheres, respectively, dashed lines indicate vacancies, and solid lines represent ions migration. Fig. 14a represents the generation of iodine vacancy (VI) which mi­ grates from equatorial position towards an axial site. It is to be noted that the VI at the equatorial or axial site have almost the same energy <0.01eV whereas for bromine vacancy (VBr), the axial site has higher energy (0.07 eV) than that of the equatorial site. The inorganic barrier is

responsible for jumping between its near methyl ammonium vacancies (VMA) that lie in ab plane as shown in Fig. 14b. Furthermore, the VMA migrate through the Pb4I4 framework structure with Ea ~0.46 eV. Fig. 14c shows the plane moment of the Pb vacancy (VPb) where VPb vacancies do not travel via square symmetry made of the four Pb and four I atoms rather they migrate along the diagonal of the square ge­ ometry. In Fig. 14d, the iodide defect is located between two axial or interstitial I atoms at almost the same distances i.e. 3.87 Å and 3.96 Å, respectively [44,45]. Eames et al. showed the lowest Ea for ion migration i.e. ~0.58 eV for I , ~0.84 eV for MAþ, and ~2.31 eV for Pbþ2 where the highest energy barrier for Pbþ2 suggested the immobility of Pb lat­ tice. The superior ion mobility of I and MAþ due to low Ea is responsible for hysteresis [46]. As mentioned above, in the absence of an electric field, the mobile ions move randomly in the perovskite layer. During the operation of PSCs, the photo-generated field compels the mobile ions to move to HTL and ETL sides. As the migrated ions reach the electrodes, they get stable due to strong electrostatic interaction between the mobile ions and contacts i.e. HTL/ETL. The migrated charges induce their own electric field in opposite direction to the photo-generated field (Vph) and decrease the efficiency of the device. The biasing i.e. external electric field can further increase movement of the defects in the given direction, thereby, decreasing the time required for accumulation of ions at the given contacts. The time and field are variables in the sense that if the applied field is weak, the ions will take longer to accumulate while a high electric field will perform the same task more quickly [47]. The impact of the most mobile charges (i.e. I and MAþ ions) on the performance of PSCs was investigated under normal working condition which generate voltage VPh in the direction from ETL (e.g. TiO2) towards HTL (e.g. Au-coated Spiro-MeO-TAD). In such environment, Vph will attract I towards ETL and MAþ will migrate towards HTL. As a result, the I and MAþ migrated ions pile up on the ETL and HTL, respectively. These ions generate their own electric field (Ein) in opposite direction of the photo-generated field where the Ein opposes charge collection to the selective contacts and reduces the PCE of the device [48]. Biasing of the device can be performed in two ways, (i) forward bias and (ii) reverse bias. Under reverse bias, the electric field exists due to the contacts which are at different potentials. The I and MAþ ions migrate towards the contacts which oppose the electric field; therefore, the internal

Fig. 14. Diffusion routes for the (a) VI (iodide ion vacancy), (b) VMA (methyl ammonium ion vacancy), (c) VPb (lead ion vacancy), and (d) Ii (iodide inter­ stitial) for I migration initial and final configuration. The vacancies are repre­ sented by dashed red circles; the interstitial defects are denoted by red spheres. The dotted lines indicate the rout followed by the vacancy whereas the solid lines stand for the migration of the ions. Reproduce from Ref. [44]. 8

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electric field is screened by the mobile ions and poor accumulation of charges. On the other hand, in forward bias, the field hinders the ion migration, thus, the performance of the device is not perturbed [49]. Furthermore, TiO2 has the ability to strongly extract electrons from the perovskite layer at TiO2/CH3NH3PbI3 interface. The perovskite þ consists of CH3NHþ 3 , Pb2 and I ions where TiO2 extracts electrons from migrated I ions forming I2 which leads to the degradation of the perovskite layer. The extracted electrons when moved back to the film, form HI as a byproduct [50]. The possible perovskite degradation path is given by following reactions (10–12):

All commercial solar modules degrade with 0.5% per year of the rated power output amounting 40 years life time for 80% of its perfor­ mance [54]. Nevertheless, this rate of degradation expedites after 20 years [54]. Furthermore, Wang et al. fitted post “burn in” section of the PCE to a straight line and extrapolated the curve back to zero time in order to obtain t ¼ 0 efficiency. The life time to 80% (t80) from the t ¼ 0 efficiency was 656 h and the stabilized power output (SPO) was 583 h for the doped C60 and FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 device which was 30 times longer than the t80 for the device based on MAPbIxCl3-x. Fig. 15b compares the sealed (using a glass coverslip and hot melt polymer foil) devices based on doped FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 and control FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 and MAPbIxCl3-x. The t80 life time was re­ ported to be 3423 h and 2958 h for the SPO and JV measured for the encapsulated FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 with 1 wt% C60 and neat C60. It was reported that the sealed device comprised of neat C60 and FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 degraded further and slightly faster in the long decay versus the doped C60 [53]. Saliba et al. [55] incorporated rubidium (Rbþ) cations into the photoactive perovskite structure using multiple A-site cation formula­ tions and achieved a stabilized PCE ~21.6%. Fig. 16a shows the JV curve of the best device based on RbCsMAFA cell which reached a sta­ bilized power output of 21.6% with JSC ~22.8 mA cm 2, FF ~81%, and VOC ~1180 mV. The highest VOC ~1240 mV was achieved confirmed by observing the inset that tracks VOC over time (Fig. 16b). This is one of the highest reported VOC for any PV material so far as the loss in potential which is the difference between the band gap and the obtained VOC is only ~0.39 V revealing very small non-radiation recombination losses [56]. Even silicon cannot reach this limit owing to its indirect band gap and Auger recombination (loss in potential of 0.4 V in the most efficient devices). Such a high VOC is intriguing since it limits the device from reaching the optimum performance as JSC and FF have already reached their maximum values. Theoretically, in a defect-free pure material with only radiative recombination, the loss in potential can be small e.g. 0.23 V for materials with 1.0 eV band gap and 0.3 V for materials with 2 eV band gap. Fig. 16c displays the external electroluminescence quantum effi­ ciency (EQEEL) which quantifies the non-radiative recombination. In addition, the EQEEL reaching 3.8% makes the perovskite suitable for light emitting diodes emitting light in the near-infrared/red spectral range [57–59]. As spiro-OMeTAD HTL is permeable to metal contact diffusion to the perovskite, it causes irreversible degradation at elevated temperatures. This effect can be mitigated either via employing buffer layers or by avoiding the use of metal electrodes [60–62]. The device based on RbCsFAMA was reported to retain 95% of its initial perfor­ mance over 500 h under constant illumination at 85 � C (Fig. 16d). As it is seen that exploration of alternative cations such as Csþ and Rbþ and their application in hybrid (FAPbI3)0.85 (MAPbBr3)0.15 yielded PCE ~20%. Recently a very high and stable PSC was conceived based on a novel cation guanidinium (CH6Nþ 3 , Gua) that showed improved ther­ mal and environmental stability versus the state of the art MAPbI3 [63] as shows in Fig. 17. Gua having ionic radius of 278 p.m. which is slightly above the upper limit of tolerance factor (t ~1.03) forming low dimensional perovskite upon mixing with PbI2 yield MA1-xGuaxPbI3 with composition (0 < x < 0.25). Saliba et al. [51] showed that the inclusion of inorganic cesium to the mixed MA and FA system results in a triple cation system with notable thermal stability due to its higher less phase impurity as well as less vulnerability to the processing condition. Fig. 18a shows J-V charac­ teristic curves of the best stabilized device based on triple cations Csx (MA0.17FA0.83) (100-x)Pb(I0⋅83Br0.17)3 termed as CsxM where ‘M’ stands for the mixed perovskite phase and ‘x’ denotes the “Cs” content. The device based on Cs5M yielded η >21% with a maximum output power point tracking reaching up to 21.1% under constant illumination shown in the inset, in good agreement with the J-V scan data. Results of the stability test performed for the best performing device based on Cs5M and compared with Cs0M (i.e. no Cs) in N2 atmosphere at

(10)

2I → I2 þ 2e þ 3CH3NHþ 3 → 3CH3NH2(g) þ 3H þ

I þ I2 þ 3H þ 2e → 3HI(g)

(11) (12)

3. Developments in PSCs stability 3.1. Effect of chemical structure of perovskite The inclusion of Cs into FAPbI3 system improved the structural sta­ bility, particularly its transition from black to yellow phases in humid environment [51,52]. In another report, FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 exhibited improved stability in comparison to MAPb(I0⋅6Br0.4)3 [14]. Wang et al. [53] doped 4-(1,3-dimethyl-2,3-dihy­ dro-1H-benzimidazol-2-yl)-N,N-diphenylaniline (N-DPBI) in n-type C60 ETL for n-i-p planar PSCs and reported enhancement in device efficiency as well as long term stability. They employed ‘mixed cations’ and ‘mixed halide’ perovskite composition FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 ((where FA ¼ HC(NH2)2) and studied the stability under fully simulated solar spec­ trum at ambient environment without encapsulation. The unencapsu­ lated devices based on FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 and n-doped C60 ETL layer maintained 80% of their “post burn in” efficiency after 650 h while the same efficiency was maintained for 3400 h for the sealed devices. The long term operational stability of the unsealed doped FA0.83Cs0⋅17Pb (I0⋅6Br0.4)3 PSCs was comparable with the FA0.83Cs0⋅17Pb(I0⋅6Br0.4)3 and MAPbIxCl3-x, respectively under humid conditions (55% humidity in air) (Fig. 15a). It can be seen that the device based on mixed FA/Cs cations showed much improved and stronger resistance to aging while other devices showed fast degradation. Efficiency of the device based on MAPbIxCl3-x dropped to 0% over the first 50 h.

Fig. 15. (a–b) Long term stability comparison of FA0.83Cs0⋅17Pb(I0⋅6Br0.4) PSCs with neat C60 and 1 wt% N-DPBI doped C60 ETL along with the MAPbIxCl3-x, respectively. Adopted with permission from Ref. [53]. 9

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Fig. 16. (a) JV characteristic curves of PSCs recorded at a scan rate of 10 mVs 1 along with the stabilized efficiency in the inset, (b) JV curve of the highest VOC ~1240 mV, where the inset shows the VOC for over 120 s, (c) EQE electroluminescence (EL) vs voltage. The inset on the LHS shows EL spectrum and on the RHS shows device photographs, and (d) thermal stability test of the prepared PSCs for ~ 500 h at 85 � C [55]. Adopted from ref. 55.

Fig. 17. (a) Schematic illustration of the device architecture based on MA1-xGuaxPbI3 perovskite light absorber, (b) the simulated crystalline structure of the same, and (c) Stability test of the novel MA1-xGuaxPbI3 perovskite material under continuous light illumination compared with the most employed MAPbI3 perovskite. Reproduced from Ref. [63].

Fig. 18. J-V curves of the best performing device based on Cs5M at voltage scan rate of 10 mV/s (the inset shows the maximum power output tracking point for 60 s depicting stabilized η ~21.1%), (b) stability of the Cs5M and Cs0M devices were compared for 250 h in N2 atmosphere at room temperature under constant light illumination [51]. Reprinted from ref. 51.

room temperature under constant illumination are shown in Fig. 18b. It is evident from Fig. 18b that the Cs5M dropped from ~20% to ~18% during the first few hours whereas it showed stable performance for the rest of the time (for 250 h). On the contrary, the device based on Cs0M showed much less stable performance over the same period. Duong et al. [64] improved structural stability of the perovskite

material by merely substituting Cs with rubidium (Rb) despite its smaller size (Rb ¼ 1.51 Å) than that of the former (1.18 Å). The incor­ poration of Rb as dopant significantly enhanced the device performance as well as thermal and photo-stability. It has been reported previously that an excess of PbI2 alleviates the non-radiative recombination as well as ionic defect migration, thereby, enhancing the performance of the 10

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device [65–69]. The JV curve of the champion device with 15% excess PbI2 along with 5% doping of RbI is shown in Fig. 19a. The highest PCE was 18.80% for reverse scan direction whereas 17.03% for the forward scan direction along with the other PV parameters depicted in the inset of Fig. 19a. The EQE presented in inset shows a plateau over the whole visible spectrum and a sharp edge near the band gap of the material. Fig. 19b shows the steady state efficiency of 18.42% and JSC ~20.27 mA cm 2 for about 400 s at a maximum power point with the device photograph shown in the inset. The stability of the prepared un-encapsulated devices was tested under ambient environment with relative humidity of 40–50% and continuous illumination of white light from solar simulator without a UV filter. Fig. 19c compares the stability for about 10 h of un-doped (0% RbI) and 5% RbI doped devices where they degrade significantly; however, the latter exhibits relatively better stability. The combination of light dose and humidity both considerably degrades and speeds up the degradation as the devices were not encapsulated. On the other hand, the scenario completely changed when the same devices were kept in the N2 environment as shown in Fig. 19d. As can be seen in the mentioned figure, the un-doped (0% RbI) device encountered a sudden drop of about 30% in PCE in the first 10 h; however, it maintained stable per­ formance for the rest of the 100 h. In stark contrast, the device based on 5% RbI showed a mild drop of about 10% in the first few hours: how­ ever, restored the original performance for about 100 h.

Fig. 20. (a) Thermal stability test at MPP tracking of Au and SWCNT-contacted devices from low to high temperatures (20–60 � C), b) device’s temperature was varied from 20 to 60 � C after 14 h into the experiment. MPP tracking of an Au device at 20 � C is shown for 90 h [70]. Adopted from ref. 70.

3.2. Alternative hole transport layers Aitola et al. [70] employed new HTM-hole contact based on single walled carbon nanotubes (SWCNT) which showed superior stability at an elevated temperature 60 � C. PSC with standard configuration with Au contact based on glass-FTO/c-TiO2/mp-TiO2/perovskite/Spiro-OMe TAD/Au was compared with glass-FTO/c-TiO2/mp-TiO2/perovskite/ SWCNT-Spiro-OMeTAD where the perovskite layer comprises Cs5 (MA0.17FA0.83)95Pb(I0⋅83Br0.17)3. The devices based on Au contact and SWCNT were tested for stability at 20 � C and 60 � C, as shown in Fig. 20a–b. The test was performed under white light emitting diode illumination with intensity comparable to 1 Sun condition in N2 atmo­ sphere for 140 h at maximum power point tracking. Initially, the PSCs were kept at 20 � C for 14 h. It can be seen that the Au contacted device

showed pronounced changes within this period. After increasing the temperature to 60 � C, the Au based device degraded dramatically and showed an exponential decay. In stark contrast, the device based on SWCNT contact showed stable performance with minute degradation. €tzel group fabricated a stabilized PSC employing CuSCN Recently Gra as a HTL with PCE beyond 20% [71]. They employed a fast solvent approach that generates a compact and highly conformal CuSCN layer to facilitate robust charge carrier extraction and collection. The fabricated PSCs employing pure CuSCN showed high thermal stability under a prolonged heating treatment; however, its operational stability was inferior. The cause of the operational instability was mainly the potential-induced degradation at the interface of CuSCN/Au layer. This issue was resolved by incorporation of a thin layer of reduced graphene oxide (rGO) between the CuSCN and Au layer that retained more than 95% operational stability. The JV curve of the PSCs based on Spiro-OMeTAD and CuSCN as HTL are shown in Fig. 21a-b, where the former yielded PCE ~20.8% (JSC ~23.35 mA cm 2, VOC ¼ 1137 mV and FF ~77.5%) while for the latter, the PCE was ~20.4% (JSC ~23.24 mA cm 2, VOC ¼ 1112 mV and FF ~78.2%). The hysteresis index values reveal that the scan direction has a negligible effect on the CuCSN based devices while it was discernible for their Spiro-OMeTAD based counterparts. A slightly lower VOC for CuCSN device was associated with the dominant recombination in the working device which can be seen from the ideality factor, and was reported to be ~1.50 for CuSCN and 1.46 for Spiro-OMeTAD as shown in the inset (Fig. 21 a-b). Fig. 21c compares the statistical analysis of PV parameters derived from the JV curves of the 20 individual independent devices; the average PCE for CuSCN and Spiro-OMeTAD devices were 19.22 � 0.84% and 19.6 � 0.77%, respectively. The stability test of the device based on CuSCN was performed at 85 � C in ambient environment in the dark. It can be seen that the CuSCN device under full Sun illumination at the maximum power point lost its >50% PCE in merely 24 h (red line in Figure). However, the inclusion of rGO thin layer between the CuSCN and Au resulted in an amazing operational stability under full Sun illumination at 60 � C – the exact reason for this stability is however, not clear. The resultant PSCs maintained more than 95% of its initial per­ formance for 1000 h as can be seen from the green line in Fig. 21d. In addition to organic-inorganic hybrid perovskite based HTL, a

Fig. 19. (a) JV curve of the best device with respect to scan direction with EQE in the inset, (b) steady state PCE and current monitoring at VMPP ~ 0.9 V, photographs of the same device shown in the inset, (c) stability of the 0% RbI and 5% doped RbI un-encapsulated devices under ambient condition, and (d) stability of a similar device in the presence of N2 environment [64]. Adopted from ref 64. 11

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Organic Electronics 78 (2020) 105590

Fig. 22. (A) Dark storage stability of the fabricated unencapsulated PSCs based on TiO2 and TiO2–Cl, (B) continuous maximum power point tracking for 500 h of the device based on CsMAFA and TiO2–Cl, and (C) J-V curve of the same device [74]. Adapted with permission from ref. 74.

Fig. 21. (a–b) JV curve of the PSCs based on Spiro-OMeTAD and CuSCN HTLs, the inset illustrates that VOC is a function of illumination intensity with an ideality factor of 1.50 and 1.46, (c) Data extracted from JV curve of the 20 independent PSCs based on the Spiro-OMeTAD and CuSCN HTLs with an illu­ mination aperture of 0.16 cm2, and (d) Stability test for the pure CuSCN and with rGO over a time of 1000 h. Adapted from Ref. [71].

after MPP operation which demonstrated that the performance drop during MPP operation may be a consequence of defect generation in the perovskite layer. Fig. 22C compares the fresh, 500 h MPP, and selfrecovered PSC with a PCE ~19.8% (97% of the initial value) following dark storage. The instability of PSCs may come from several factors, including the perovskite materials itself, ETL, HTL and the interfaces in the device configuration. Particularly when m-TiO2 is employed as ETL in a PSC, it significantly affects the device stability under UV irradiation [75,76]. Various strategies proposed to address this issue include, replacement of TiO2, insertion of a thin layer between the perovskite and TiO2 layer [33], doping of TiO2 or employing UV filters [77]. To avoid additional processing, search for new materials could be the best alternative for TiO2 based PSCs. Towards this end, BaSnO3 (BSO) is an n-type semi-conductor perovskite oxide having wide band gap of 3.2 eV com­ parable to the TiO2. In particular, La-doped BSO (LBSO) possesses a high electrical mobility of 300 cm2 V 1s 1 [78]. In addition, the reported poor UV photocatalytic activities due to its small dipole moment, attri­ butes to the cubic perovskite structure without octahedral tilting [79, 80]. However, the LBSO film requires very high crystallization tem­ perature (>1000 � C) to form the perovskite phase which puts stringent condition on the selection of substrates i.e., cannot be used for flexible as well as FTO glass [81]. Shin et al. [82] fabricated PSCs based on m-TiO2 and La-doped BaSnO3 (LBSO) electron extracting layers and compared their perfor­ mance. They employed MAPbI3 as a light absorber due to its suitable conduction band edge with respect to the LBSO ETL. A very high PCE (~21.2%) was achieved using LBSO as ETL for the MAPbI3 system. Fig. 23a shows the cross-sectional SEM view of the fabricated device based on LBSO ETL (a uniform layer of thickness 120 nm) with MAPbI3 as a perovskite material and PTAA as HTL. Fig. 23b compared the J-V characteristics curve of the PSCs fabricated using LBSO ETL with PCE ~21.2%, and the control TiO2 ETL based with PCE ~19.6%. These de­ vices were tested for photostability under the air mass (AM) 1.5 illu­ mination of xenon including UV radiations as shown schematically in Fig. 23c. The tests were performed for unencapsulated PSCs based on LBSO and TiO2 based ETLs with device architectures FTO/LBSO/MAPbI3/ PTAA/Au and FTO/TiO2/MAPbI3/PTAA/Au in a nitrogen filled cham­ ber at 25 � C (Fig. 23d). It can be seen that unlike its TiO2 ETL based counterpart, the LBSO based cells showed greater resistance against photodegradation, and there was an abrupt decrease in PCE upon initial

continuous quest for alternates of hole transport layer materials drove the researchers attention to explore their inorganic substitutes and therefore, they found various promising candidates such as Cu2O, CuOx, CuSCN, MoO3 (MoOx), and NiOx etc. for their applications in photo­ voltaics. Recently, Kung and Pitchaiya et al. [72,73] reviewed the progress in development of these suitable inorganic candidates of HTL. Those HTL materials became popular due to their low cost and intrinsic chemical stability at ambient conditions. The stability of inorganic HTL materials at different operational conditions including humidity, light, and heat was also discussed in Refs. [72,73]. 3.3. Alternative electron transport layers Tan et al. [74] fabricated planar PSCs employing chlorine-capped TiO2 colloidal nanocrystal as ETL processed at low temperature (150 � C), compatible with the flexible substrates. The inclusion of interfacial Cl atoms on the TiO2 nanocrystals suppress deep trap states at the perovskite interface, thereby, considerably reducing the interfacial recombination at the TiO2/perovskite interface contact. The fabricated planar PSCs showed no hysteresis and yielded a PCE ~20.1%. The de­ vice based TiO2–Cl ETL shows an exceptional long-term stability as compared to that of its TiO2 counterpart (Fig. 22A). Device based on TiO2–Cl ETL with MAFA perovskite retained 95% of its initial perfor­ mance stored in the dark for 60 days while those for only TiO2 retained 38% of the initial value. On the other hand, the device fabricated using TiO2–Cl ETL and CsMAFA with an initial PCE ~21% exhibited excep­ tional shelf stability of PCE ~96% over 2000 h (90 days) (Fig. 22A). Any solar cell must show high stability under operation at its maximum power point (MPP) condition. The PSC based on TiO2–Cl ETL and CsMAFA perovskite has been reported to retain 90% of its original PCE for 5000 h, as directly determined from the MPP tracking. More­ over, the device was shone by continuous operation under 1 full Sun condition. A 520 nm UV cutoff filter was used, (Fig. 22B). It can be seen from Fig. 22B that there was a slight increase in PCE in the first tens of hours of MPP operation, which may have been caused by the lightinduced defect healing in the perovskite active layer. In addition, the relevant XRD data showed that there was no perovskite decomposition 12

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with 40% relative humidity in room light showing only 2.5% degrada­ tion in η indicating strong intermolecular bonds with perovskite layer in the solid state. The device based on NDI-PM exhibited superior thermal stability as compared to devices based on PCBM. In this vein, Naph­ thalene diimide (NDI) based semiconducting polymer such as NDI-thiophene and NDI-selenophene copolymers [90–92] have been used in organic solar cells as electron acceptors and have demonstrated excellent performance for the device. Thus synthesis of NDI-based selenophene polymer as efficient ETLs in PSCs are reported in Ref. [93]. The ETL were synthesized with outstanding stability and crystallinity and well matched HOMO and LUMOs thus reducing barrier for electron transfer and resulting in enhanced electron mobility and durability in ambient air with device configuration as ITO/­ NiOx/MAPbCl0.2I2.8/ETL/Ag using it as TiO2 free to make it solution processable at low temperatures [93]. Additionally, several nano­ composites have been used as ETL to boost the performance of the PSC however the FF could not exceed 76% [94–97]. Thus to improve the FF, a compact nanocrystal of SnO2 modified NDI-graphene placed on top of a perovskite layer is reported [98]. This modification results in an increased hydrophobicity and van der Waal interactions with the Perovskite layer with an η value of 20.2% and FF of 82% due to its enhanced charge transport properties. Hence PSC with device configu­ ration of ETL/FA0.83MA0.17PbI2⋅63Br0.37/spiro-OMe-TAD/Au with ETL as modified graphene with NDI, SnO2 or SnO2/NDI-Graphene showed a boost in η of 9.8%, 15.2% and 20.2% respectively. The encapsulated device demonstrated enhanced stability for 300 h at room temperature and 30% humidity in dark. Sara Oh et al. have discussed polymer based poly ([N,N0 -bis(2-octyldodecyl)-1,4,5,8-naphthalene bis (dicarboximide)-2,6-diyl]-alt-5,5’-(2,20 -bithiophene)) PNDI-2T acting as the electron acceptor combined with electron donor molecule as BDT2TR which has shown excellent properties with a η of 4.43%. PNDT-2T maintained photovoltaic efficiency for 15 h at 150 � C exhib­ iting 93% of the initial value of η i.e., 4.85%. The respective device showed a slight decrease in photovoltaic properties of Jsc and FF when annealed for 15 h. Thermal stability of device was confirmed by deter­ mining the value of charge carrier mobility. It was found that the hole and electron charge mobility for PNDI-2T using device configuration ITO/PEDOT:PSS/BDT2TR:Acceptor/Au and ITO/ZnO/BDT2TR:Accept­ or/Ca/Al was the same as for pristine PNDI-2T film [99].

Fig. 23. (a) Cross sectional SEM view of the prepared PSCs (scale bare 500 nm), (b) J-V curves of the devices based on LBSO and TiO2, see the stabilized PCEs at a maximum power point for the same in the inset, (c) Schematic illustration of the relevant photostability, (d) photostability of the unencapsu­ lated devices based on LBSO and TiO2 using xenon lamp including UV–Vis, and (e) photostability of the encapsulated devices based on LBSO and TiO2 [82]. Reproduced with permission from ref. 82.

illumination for the former device. It is understood that PSCs containing organic HTLs such as Spiro-OMeTAD and PTAA can cause degradation of the device (particularly for long time i.e., 1000 h) due to, for example, morphological deformation, metal diffusion, and other movable addi­ tives. To avoid the influence of HTM on the photostability, the devices were properly encapsulated using two-sided glass and employing NiO HTL. The device architecture was glass/FTO/n-type oxide/MAPbI3/ NiO/FTO/glass, by laminating two half cells (glass/FTO/n-type oxide/ MAPbI3) and (MAPbI3/NiO/FTO/glass). It was reported that the LBSO cells retained 93.3% of initial performance after 1000 h, while TiO2 based device completely degraded in merely 500 h (Fig. 23e). Different ETL materials such as fullerenes C60, its derivatives [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) and as well as 2, 9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) have been employed in inverted PSC devices to improve their performance and stability, however their η is compromised when used alone. The η can be enhanced by using multiple ETLs [83–85]. Nevertheless employing multiple ETLs have its own drawbacks as introducing more layers is complex and necessitates a high vacuum thermal evaporation method which makes it expensive [86] in addition to the cost and unstable na­ ture of fullerenes themselves [87,88]. Hence it become imperative to search for materials that are cost effective, are oxygen and light stable, can be using as a single ETL, follow simple fabrication processes and are solution processable for their practical applications [83]. Inverted PSCs with fullerene based phenyl-C61-butyric acid methyl ester PCBM have been used as ETL due to better photovoltaic performance, however its poor solubility as compared to spiro-OMeTAD as the HTL, morpholog­ ical changes with time and formation of pin holes on the perovskite layer during spin coated synthesis requires it to be replaced with a more stable ETL [89]. Hence n-type naphthalene diimide (NDI) as the ETL seems a robust solution because of its ease of synthesis, solution processibility and good solubility in solvents of organic nature. Thus Heo et al. [89] report (N,N0 -Bis(phenylmethyl)naphthalene-1,4,5,8-tetracarboxylic dii­ mide) NDI-PM as ETL with inverted PSC configuration given as: glass/ITO/PEDOT:PSS/perovskite (MAPbI3 or FAPbI3-xBrx)/ETM (NDI-PM or PCBM)/Al. Hence using NDI-PM the η value engaging MAPbI3 or FAPbI3-xBrx is 18.4% and 19.6% respectively. The unencap­ sulated devices was subjected to heat treatment at 90 � C for 100 min

3.4. Electron transport layer free The development of another very stable PSCs/module using an innovative approach by employing multijunction comprising 2D/3D perovskite material with a prolong stability of >10,000 h has been re­ ported with no loss in the PCE [100]. Such an innovative approach of 2D/3D multijunction interface utilized the improved stability of a 2D perovskite with the panchromatic absorption and outstanding charge transport of the 3D perovskite. They used protonated salt of amino-valeric acid iodide (HOOC(CH2)4NH3I) referred to as AVAI from now onwards, mixed with PbI2 which resulted in a low dimensional (HOOC(CH2)4NH3)2PbI4 structure. The schematic device architectures are shown in Fig. 24a for HTM and Au based as well as HTM-free printable devices. The JV curve of an optimized 3% AVAI composition in 2D/3D PSCs is shown in Fig. 24b with a decent PCE ~14.6% along with the photographs and statistics of the prepared device shown in the inset. A careful inspection of Fig. 24b shows that the device based on 3% AVAI composition in 2D/3D perovskite exhibits enhanced stable per­ formance in comparison to the pure 3D perovskite (PCE ~13%) with the same device architecture as illustrated in Fig. 24d. The 2D/3D perov­ skite based device stored in argon atmosphere and under constant illu­ mination maintained up to 60% PCE for 300 h, hence, is much stable than the device based on pure 3D perovskite counterpart. In the same study, the authors also developed HTM-free PSCs with a small area (0.64 cm2) and module with large area 10 � 10 cm2. Fig. 24 ef shows the J-V curves of the HTM-free PSCs and module along with the 13

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Organic Electronics 78 (2020) 105590

accumulation that may change the device performance, or light-induced trap formation. One thing is obvious that due to the lack of HTM in the device, the fill factor (FF) deteriorates from 75% (in case of HTM spiroOMeTAD) to 58% for the HTM-free device. Best performing devices in terms of stability and PV are listed in Table 1. To further explore stability of PSCs under practical conditions, perovskite materials alone [26] and complete solar cell devices [101] were simultaneously exposed to light and damped heat. It was observed that degradation is strongly dependent on the composition of halide perovskite at light and elevated temperature, e.g. while exposed to sunlight at 45–55 � C sample temperature, MAPbI3 went through sig­ nificant decomposition whereas its counterpart MAPbBr3 exhibited no degradation, as depicted in Fig. 25. Similarly, light and elevated tem­ perature were simultaneously applied on PSCs with device configuration ITO/cp-TiO2/ms-TiO2/perovskite/PTAA/Au and substantial degrada­ tion was found. Their stability was further improved by introducing transparent contact and robust perovskite composition, where PSCs revealed over 90% of the initial efficiency after passing 160 h for continuously operating the device in nitrogen environment at 85 � C together with illumination [101]. In addition to the conventional perovskite solar, in past few years, researchers have also considered inverted organic–inorganic hybrid perovskite solar cells (by reversing position of ETM and HTM) that have been shown as a promising approach to design PSCs with high efficiency (>20%) and stability in different operating conditions [103–105]. However, those are out of scope in the current review context and we will discuss it in detail in our future work.

Fig. 24. (a) Device architecture of HTM-free and mesoporous standard cell, (b) JV curve using 2D/3D perovskite with 3% HOOC(CH2)4NH3I (image of the prepared device and statistics are depicted in the inset), (c) comparison of stability of the device based on Spiro-OMeTAD/Au using standard 3D and 2D/ 3D mixed perovskite along with the JV data of the champion device in the inset, (d) JV curve using 2D/3D mixed perovskite with 3% AVAI in HTM-free PVSC and device photograph and statistics in the inset, (e) JV of the 10 � 10 cm2 module of the same device (photograph and statistics of the device in the inset), and (f) module’s stability along with JV data in the inset [100]. Reproduced with permission from ref.100.

statistics and photographs in the inset. The best PSCs and module yiel­ ded PCE 1–2.71% and ~11.2% for HTM free architecture, respectively. The prepared module was tested at cycling temperature of up to 90 � C under simulated AM 1.5 G solar illumination at 1000 Wm-2 in ambient conditions. The devices were reported to show extraordinary long-term stable performance for more than 10,000 h and an amazing response at high temperatures. It could be seen during the stability test that there was an increase in the performance in the first 500 h which may be due to the associated effects. For example, light or field-induced ion move­ ment along with the structural rearrangement, and interfacial charge

4. Conclusion and outlook Production cost and serious environmental concerns for Si/poly­ crystalline and Se based thin film solar cells, although having achieved high photovoltaic efficiency, limit their use for industrial scale appli­ cations [106]. Dye sensitized solar cells seemed an answer to problems encountered with previous solar cells, yet came with their own baggage of problems such as requirement of thick absorbing layer and bleaching

Table 1 Device configuration, their PV parameters and long-term stability of perovskite solar cells. Degradation Factor

Test conditions

Device configuration

PCE

Loss in PCE

Stability time

Ref

Humidity

Unsealed devices and stored in dark with relative humidity <30%. Exposed to air at an ambient humidity ~55%.

FTO/TiO2–Cl/MAPbI3/SpiroOMeTAD/Au FTO/doped C60/mixed perovskite/ spiro-OMeTAD/Au GlassITO/PEDOT:PSS/MAPbI3/ PCBM/EFGnPs-F/Al FTO/LBSO/MAPbI3/PTAA/Au FTO/TiO2/MAPbI3/PTAA/Au

21%

4%

[74]

17.6%

20%

2000 hours 650 h

[53]

14.3%

~10%

30 days

[102]

21.2% 19.6%

<10% ~50%

120 h 120 h

[82]

FTO/LBSO/MAPbI3/NiO/Au

–

6.7%

FTO/TiO2/MAPbI3/NiO/Au

–

100%

GlassFTO/c-TiO2/mp-TiO2/mixed perovskite/Spiro-OMeTAD-SWCNT FTO/c-TiO2/m-TiO2/perovskite/ Spiro-OMeTAD/Au FTO/c-TiO2/m- TiO2/perovskite/ ZrO2/Carbon FTO/c-TiO2/m-TiO2/Cs5M/HTL/Au FTO/c-TiO2/m-TiO2/Cs0M/HTL/Au ITO/cp-TiO2/msTiO2/perovskite/ PTAA/Au

15.0%

1000 hours 1000 hours 580 h

[70]

300 h,

[100]

12, 000 h

[100]

250 h 250 h 160 h

[101]

Exposed to air at an ambient humidity 50% Illuminations

Unencapsulated devices in a nitrogen-filled chamber with a constant device temperature 25 � C and illuminated with UV radiation Encapsulated devices tested in open air by including UV radiation. N2 atmosphere, LED white light equivalent to 1 Sun, and at 60 C The temperature of cells was set to 45 � C in a sealed cell holder with a glass cover The cell was sealed under an ambient environment and kept at temperature 55 � C (1 full Sun) Nitrogen environment at room temperature under constant illumination Nitrogen environment at an elevated temperature and illumination

�

Temperature

Environment Illuminations þ elevated Temperature a b c d

With 3%AVAI. 3D perovskite. Module. Cell. 14

20% a

14.6% 15.95%b 11.9%d 10.10%c 21.2% – 20.6

40% >50% 0 0 <10% 80% 10%

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Organic Electronics 78 (2020) 105590

interface and encapsulation of the device is of great importance to ensure practicability of such SC devices and enhance the device effi­ ciency. Degradation mechanism and effect of continuous illumination has been studied in Ref. [24]. It is also of interest to sought HTL mate­ rials other than Spiro-OMeTAD to reduce the overall cost of the perov­ skite based cell thus making commercialization of such a devices a possibility. Methods to deposit a film of large area perovskite and a need to understand the mechanism behind the conversion efficiency is an area where research efforts can be deployed. The explanation of the mech­ anism by theoretical model provides ease in developing simple and efficient materials before handling and conducting experiments in real times in wet labs. Other issues pertain to unifying the conditions that lead to stability of the device and hence standard measurements and test should be developed such as temperature, humidity, water and oxygen content and time and intensity of light irradiated for achieving maximum device stability. Solar cells’ stability is pre-requisite for proper working and stable performance, especially, when in contact with different kinds of envi­ ronment. Consequently, the materials used in a device must be able to retain its stability in a given atmosphere. The unprecedented perfor­ mance of perovskite solar cells (PSCs) with efficiency ~25.2%, is a promising development for the much needed improvement in the present-day technologies. For instance, PSCs are compatible with cheap, roll to roll processing and relatively more flexible in the choice of ma­ terials. However, the fast degradation of the organometallic lead halides-based perovskites upon continuous exposure to light, moisture and elevated temperatures, impede their commercialization which de­ mands urgent redressal. One of the major causes of PSCs’ instability is the instability of the perovskite layer upon exposure to moisture. Additionally, thermal, and photo-instabilities are also considered crit­ ical issues to be resolved before their commercialization. As a high de­ vice efficiency is crucial for any industry, the stability is equally important. Recent reports show that the stability of PSCs has been significantly improved at indoor; however, it must go beyond this level to cross the barrier for their commercialization. To achieve the required stability, future research must be focused upon improvements in the intrinsic stability of the perovskite absorber layer, optimization of de­ vice architecture, and identification and engineering of durable mate­ rials for encapsulation.

Fig. 25. The decomposition behavior of MAPbI3 and MAPbBr3 perovskites under sunlight and elevated temperature condition [101]. Adopted with permission from ref. 101.

of light by the organic dyes [107]. Further research in this direction to improve properties to develop an all solid dye material lead to the development of PSCs that could achieve relatively high efficiency accompanied with low production cost. The advancement of novel perovskite SCs proved promising for its scientific value and have so far acquired breakthrough power conversion effi­ ciencies [1,24]. However device stability is still an area where attention needs to be focused on. Improving optical properties by tuning the band gap is crucial to high performance of the device affecting the geometry and phase formation in the cell [108]. Stability of perovskite SCs with relation to electronic properties can be improved by replacing at site A the organic cation methylammonium MA and formamidinium FA in MAPI with an inorganic ion such Cs as the best counterpart to the organic cations exhibiting PCE of 10.77% [109] which compensates for phase separation and large bandgap [110]. Thus an all inorganic com­ pound that improves device performance can be the best alternative to organic cation based SCs, however this has proven to be challenging yet promising. This, if achieved will contribute toward maximum efficiency of the device and will be remarkable feat towards stability of perovskite based SCs. Yet another direction could be development and under­ standing of organic inorganic hybrid halide perovskites based solar cells such as trimethylsulfonium lead triiodide as low dimensional form of halides that might lead to outstanding efficiencies and stabilities with remarkable quantum confinement and optoelectronic properties [111]. The toxicity of Pb thwarts any effort to commercialize and promote perovskite based SCs. Thus since, Pb being toxic can be replaced at the B site in the perovskite cell with low or non-toxic metals such as Sn and Ge belonging to elements in the same periodic group that improves the optoelectronic properties for sustainable and stable perovskite based SCs. However, still bandgap need to be tuned for efficient absorption of light as well as hindering oxidation of tin and controlling its self-doping before it can outperform Pb based SC devices [112,113]. Additionally, stability of perovskite SC based on organic lead halide is effected by light, humidity, temperature and UV which also affects the packaging of the cell at a later stage and hence a very fast degradation is evident under ambient conditions [24]. Thus developing composition that confers maximum device stability with relation to the light absorbing layer, modification of ETL/HTL and their optimization, engineered

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