Stability of all-inorganic perovskite solar cells

Journal Pre-proof Stability of all-inorganic perovskite solar cells Nabonswende Aida Nadege Ouedraogo, Yichuan Chen, Yue Yue Xiao, Qi Meng, Chang Bao Han, Hui Yan, Yongzhe Zhang PII:

S2211-2855(19)30956-5

DOI:

https://doi.org/10.1016/j.nanoen.2019.104249

Reference:

NANOEN 104249

To appear in:

Nano Energy

Received Date: 15 August 2019 Revised Date:

30 October 2019

Accepted Date: 30 October 2019

Please cite this article as: N.A.N. Ouedraogo, Y. Chen, Y.Y. Xiao, Q. Meng, C.B. Han, H. Yan, Y. Zhang, Stability of all-inorganic perovskite solar cells, Nano Energy, https://doi.org/10.1016/ j.nanoen.2019.104249. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Graphical abstract

Stability of all-inorganic perovskite solar cells Nabonswende Aida Nadege Ouedraogoa,b, Yichuan Chena, c, Yue Yue Xiaoa,d, Qi Menga, Chang Bao Hana*, Hui Yana, Yongzhe Zhanga,b* a

College of Material Sciences and Engineering, Beijing University of Technology, Beijing, 100124, China b The Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology, Beijing, 100124, China c School of Mechanical and Electrical Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, Jiangxi, China d College of Material Sciences and Engineering, Hebei University of Science and Technology, Hebei, 050080, China Corresponding Authors *email: [email protected] (Yongzhe Zhang), [email protected] (Chang Bao Han)

Highlights •

The most relevant studies of all-inorganic perovskite solar cells are reviewed. An overview of materials, preparation methods, as well as the materials and device stability are provided.

•

The effects of different engineering/modifications methods on the phase stability of CsPbI3 are reviewed.

•

The influence of the different layers on the device efficiency and stability are pointed out.

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Abstract Organometal lead halides perovskites are promising solar cells material due to their outstanding properties such as tuneable bandgap, impressive tolerance to defects, long exciton diffusion length, high carrier mobility and absorption coefficient. Up to now, the organometal lead halides based solar cells (PSCs) have demonstrated impressive power conversion efficiency reaching 25.2% (not stabilised). However, their operating life-times are limited due to degradation of the organic components under some environmental conditions. Therefore, researchers have focused their interest on the all inorganic perovskite; especially on the caesium lead triiodide perovskite (CsPbI3) which exhibits a better compositional and chemical stability. Nevertheless, the phase instability of the black phase of this material constitutes its main limitation for its use in the solar cell devices production. This review aims to present the most impactful research giving insights on the factors that may cause the instability of all-inorganic lead halide perovskite materials, as well as the instability of the whole device. In addition to deposition methods, the composition, structure and optical properties of inorganic perovskite materials have also been presented. Furthermore, this review highlights the different strategies used in order to improve the phase stability of caesium lead halide perovskite material through either engineering on the material structure or the fabrication method.

Graphical abstract

Keywords: Perovskite solar cells, All-inorganic, Stability, Phase degradation, Caesium lead triiodide perovskite (CsPbI3)

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1

Introduction

The depletion of fossil fuels and global warming resulting from the use of these traditional energy sources reveals the urge to focus on alternative energies. As a renewable and clean energy, photovoltaics are nowadays receiving greater attention, making them one of the most promising sustainable and clean energy. Since the development the modern photovoltaic in 1954 by Bell laboratories, many types of solar cells have been fabricated [1]. Solar cell technologies are typically named based on their light-absorbing material. Therefore in the past decades, huge effort has been focused on new photovoltaic materials development in order to produce high quality solar cells with a low levelized cost of electricity. According to the materials used for the fabrication of solar cells, they can be classified into 3 main groups (Fig. 1): the organic, inorganic and organic-inorganic hybrid solar cells [2]. While the single-junction solar cells have the maximum limit of the PCE about 33.7% which is defined by Shockley-Queisser limit [3], silicon-based solar cells, especially single crystalline (sc-Si) have a high power conversion efficiency (PCE) up to 26.1% and a lifetime over 20 years [4]. Multijunction solar cells with efficiency around 46% are currently the most widely used and mature technology among all photovoltaic devices. However, all these solar cells with high efficiency have high production costs. Thus, some new concepts of solar cells, such Dye-Sensitized Cells (DSC), organic cells, inorganic cells (CZTSSe), quantum dot cells and perovskite cells have emerged in these last ten years. Among these new solar cells, the perovskite solar cells are the most cost-effective photovoltaic technology because of its low production costs (simple preparation process) and lightweight. The term perovskite comes from a mineral first discovered in the Russia Ural Mountains of by Gustav Rose in 1839, and afterwards renamed by a Russian mineralogist called L. A. Perovski (1792-1856) [5]. This organic-inorganic hybrid mineral has a general formula ABX3 with A=MA+, FA+; B=Pb2+, Sn2+; X = Cl-, Br-, I-and exhibits exceptional optoelectronic properties such as remarkable tolerance to defects, high carrier mobility and absorption coefficient, tuneable bandgap and, long exciton diffusion length [6-10]. All these outstanding properties make it suitable to be used as active material in solar devices. Miyasaka et al. [11] have been the first in 2009 to use the perovskite material as absorber layer in the fabrication of solar cell, with a PCE of 3.81%. Since that, pretty much improvement has been done in this field, leading to reach a PCE of 25.2 % nowadays [4]. Nevertheless, despite their

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great power achieved, they still go through some compositional and structural degradation mechanisms resulting from thermal and chemical instabilities, phase transformations, exposure to visible and UV light, moisture as well as oxygen and most importantly sealing issues [12-17]. These problems are mainly due to the presence of organic components. Therefore, in order to overcome these environmental instability issues, inorganic cations such as Cs+, Rb+ or Sn+ have been proposed to be used to form an all inorganic perovskites ((CsPbX3 (X=halide)). This cation substitution aims to improve the chemical and thermal stability compared to organic-inorganic hybrid solar cells.

Fig. 1. Photovoltaic solar cells classification based on materials [2]

The common inorganic perovskite solar cells used currently are mainly CsPbI3, CsPbI2Br CsPbIBr2, and CsPbBr3. The CsPbI3 material with a narrow band gap (Eg=1.73 eV), is good candidate for solar energy harvesting. The caesium lead halide perovskite solar cells, exhibits the best efficiency among the inorganic ones as presented in Fig. 2 [18]. However, its black phase (α-CsPbI3) suffers from notorious instability at room temperature generating a quick degradation to a non-perovskite yellow phase (δ-CsPbI3) [19-24] which constitute a major issue in the cell preparation. Incorporating some bromide ions into Cs-perovskites in replacement of iodide ions to form CsPbBr3, could be a good alternative for the phase stability improvement, however, its large band gap (Eg=2.25 eV) limits the light harvesting reducing then the cells efficiency [25, 26]. Hence, many efforts have been stressed on the development of all-inorganic PSCs, with the aim to improve its phase stability which constitutes its main limitation.

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This review focuses on the stability of perovskite materials and devices especially phase instability of inorganic perovskite material used as perovskite solar cells. More specifically, it will be about reviewing in section 2, the composition and physical properties of inorganic perovskite materials, followed by its synthetizing methods. In section 3, we will bring a deep understanding on the causes of the material phase instability and review the methods used to overcome this issue. The last section 4 will mainly discuss the stability of the whole device and the parameters to take into account to improve it.

Fig. 2. Performance of CsPbIxBr3-x solar cells. (a) Evolution of CsPbI3, CsPbI2Br, and CsPbIBr2 perovskite solar cell energy conversion efficiencies. (b) Demonstrated efficiencies of state-of-the-art organic and reported inorganic perovskite solar cells as a percentage of Shockley-Queisser (SQ) limits [18].

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Inorganic perovskite materials

2.1 Composition, structure and optical properties of inorganic perovskite materials In this section, we will discuss the 3D inorganic perovskite materials (Fig. 3). The different structures of perovskites are dependent to the Goldschmidt tolerance factor value (t): + = √2( + )

5

with rA, rB and rX the radii of the A, B and X atom respectively [27-29]. In the range of 0.8≤ t ≤1.0, octahedral structure is formed as the result of the smallness of the A cation radius.

Fig. 3. 3D structure of perovskite material

Based on the spatial configurations, lead halide octahedrons can form four (04) different types of perovskites structures (Fig. 4): three-dimensional (3D), two-dimensional (2D), one-dimensional (1D) and zero-dimensional (0D) [30]. The majority of the inorganic perovskite compounds undergoes therefore the same process, leading to the experimentally observed orthorhombic (Pnma) structures of CsPbCl3 and CsSnI3 [31].

Fig. 4. Structures of 3D, 2D, corrugated 2D, 1D, and 0D perovskites [30]

Analysis involved in the crystallography of halide perovskites was first carried out by Moller in 1958 [32]. This study revealed that the high-temperature crystal 6

structures of the CsPbX3 series were determined to be the quintessential cubic perovskite structure. General caesium based inorganic perovskite CsBX3 is isostructural to perovskite CaTiO3 and related oxides, and its structure consists of cuboctahedral cavity at the centre of the corner-sharing lead halide octahedral network [33]. For instance, the crystal structure of 3D inorganic perovskite CsPbX3 is similar to CH3NH3PbX3 one; however its physical properties are completely different [34]. Different structure means different phase for inorganic perovskite materials. Phase transitions to lower symmetry perovskite phases are ineluctably noted at lower temperatures. The case of CsPbCl3 which gradually transits from a tetragonal, orthorhombic and monoclinic phase respectively at 320K, 316 K, and 310 K is a good example [35]. Herein in Table 1 are some other examples of known Inorganic Halide Perovskite phase transition and corresponding temperatures. Table 1: Some inorganic Halide Perovskite phase transition and corresponding temperatures [35]. Cubic (PmĀm) Inorganic

Tetragonal (P4/mbm)

(K)

Orthorhombic

Monoclinic

(Pmma) (K)

(P21/n) (K)

perovskite CsSnCl3

293

<293

CsSnBr3

292, 300

270

100

CsSnI3

500, 446

380, 373

300

CsPbF3

186

CsPbCl3

320

315

310

CsPbBr3

403

361

<361

CsPbI3

634

<292

<310

298

At room temperature, CsPbX3 crystals are highly coloured. For example, CsPbCl3 crystal is pale yellow, CsPbBr3 orange and CsPbI3 black while crystals of the type Cs4PbX6 (X= Cl, Br, or I) are colourless. By heating the CsPbI3 perovskite crystal at temperature above 310 °C, it experiences a phase change and the colour shift from yellow to black [36]. This black colour can also be obtained by melting the caesium iodide and the lead iodide together in the appropriate stoichiometric ratio. The coloured perovskite CsPbX3 crystals are photoconductive and CsPbCl3, CsPbBr3, and CsPbI3 have their maximum spectra sensitivity respectively in the violet, blue to green and red region [21, 32].

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The different phase colours have different optical properties, and induce at the same time some specific band gaps. Regarding the optical properties of Caesium lead halide perovskite, several studies have revealed its interesting light absorber properties [34, 37-43] due to their relatively high absorption coefficient (Fig. 5). In recent years, CsPbBr3, CsPbI2Br, CsPbIBr2, and CsPbI3 are the most studied inorganic perovskite. Among them, CsPbIBr2 and CsPbBr3 have large band gaps (~2.05 eV and 2.3 eV) [26, 44-46] compared to CsPbI3 which has relatively narrow band gap (~1.73 eV) when in its cubic phase (α-CsPbI3) [19, 21, 22]. Chen et al. (2019) [34] pointed out that In-doped CsPbI3 exhibits an enhanced optical absorption in visible light region, which may have great potential for optoelectronic devices. In the same vein, Singh et al. (2019) [38] investigates the optical and dielectric properties of CsPbI3 inorganic lead iodide perovskite thin film. They conveyed that CsPbX3 inorganic perovskites are excellent for the different light harvesting due to their tuneable band gap throughout the entire visible region. Additionally, they are the pioneers who have measured the refractive index, extinction coefficients, and dielectric functions at different incident angles in visible range. The refractive index of CsPbI3 was found to be 2.46 at 435 nm. This low value implies there is little light loses to reflection at the front of the active layer, which suggests that Cs based lead halide solar cells may be excellent antireflection coating for tandem solar cells. Furthermore, the microstructure of the cubic phase Pm-3 m of CsPbI3 investigated by X-ray diffraction, revealed sharp absorption edge and PL emission near infrared region with direct band gap of 1.67 eV with high colour purity of red emission [38].

Fig. 5. (a) Absorbance spectra and (b) photoluminescence spectra for mixed halide CsPb (IxBr1-x)3 films with varying iodide concentration “x” [19]

A recent study by Huang et al. [43] reported that CsPb (Cl/Br)3 nanocrystals can be used as blue light and ultraviolet blocking material. Indeed, they revealed that physical and photochemical properties of perovskite nanocrystals are preserved 8

during the fabricating process and by modifying the Cl/Br ratio; the light blocking range is adjustable. The materials exhibit high transparency above the blue light range (> 95%), which is beneficial to many applications such as photodectors or photo sensors; to list a few.

2.2 Preparation Methods The all-inorganic perovskite preparation mechanisms are similar to those of organic-inorganic perovskite. The only difference subsists in the use of precursor materials. The conditions of perovskite layer deposition are highly important and the intense complexity of the relation between precursors and solvents has been pointed out. Efforts have been made to enhance the awareness of transformation pathways, nucleation kinetics, intermediates and structure-property relationships of perovskite films [47, 48]. However, huge effort is still needed to entirely figure out the repercussion of different factors in the deposition process on the emanating perovskite film properties. There are two main methods (Fig. 6) typically used in perovskite material deposition that are the solution processing and the vacuum deposition. In addition, the combination of solution and the vacuum deposition has also been used. Here, solution process is most used in the synthesis of perovskite-like compounds in Cs-Pb-Br system, CsPbBr3, CsPb2Br5 and Cs4PbBr6 [49].

Fig. 6. Schemes of the diverse perovskite deposition methods [50].

Solution processing methods have the advantage to be fast and easy to process. Moreover, they are low-cost and compatible with roll-to-roll device production. One step and two steps spin-coating are the most commonly used deposition. In one step 9

deposition, both precursors are blended in the same solution with solvent like γ-butyrolactone (GBL), N, N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). For instance, in one step method, CsPbI3 precursor solution is obtained by combining PbI2 and CsI in a mixture solvent of N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The perovskite film is then achieved by spin-coating the formed solution on the substrate films, followed by immediate annealing at temperature above 320 °C to form the black phase. This easy to perform method has however poor reproducibility of the solar cell performance and involve the formation of disparate perovskite crystal size. To solve this problem, You et al. [36] developed the solvent-controlled growth (SCG) method, in which, after the spin-coating of the perovskite precursor, the film is dried for 0 to 50 min before annealing at 350 °C for 10 min in nitrogen glove box (Fig. 7).

Fig. 7. (a) Illustration of solvent-controlled growth (SCG) for CsPbI3 deposition. (b,c)-SEM of CsPbI3-precursor films without and with solvent-controlled growth (SCG) respectively. (d,e) SEM images of annealed CsPbI3 perovskite precursor films without and with SCG, respectively [36].

As for the two-step spin-coating or inter-diffusion method, it consists of preparing the active inorganic perovskite material in two different phases. For instance, in a classic two-step spin-coating for the preparation of CH3NH3PbI3 film, CH3NH3PbI layer is spread by spin-coating or soaking on an early dried PbI2 deposited on the substrate [50]. Liu et al. [51] used this method to produce CsPbBr3. They first spin coated the PbBr2 solution which was a mixture of PbBr2 precursor with DMF solvent. After annealing this first layer for few minutes, the CsBr dissolved in methanol was spin coated to form the resulted inorganic perovskite CsPbBr3. This 10

method was basically developed to improve the control of the perovskite crystals formation and growth. Nevertheless during the process, some impurities such as CsPb2Br5 and Cs4PbBr6 can be produced, leading to a decrease of the device performance. Moreover, pretty much time is required to completely convert PbI2 into perovskite [52, 53]. Therefore the annealing time and temperatures are important for the uniformity and repeatability of the perovskite preparation. In comparison to the one-step spin-coating, PbI2 layer covers more uniformly the substrate. The separation of the evaporation of solvent and the growth of perovskite in two-step deposition method, allows a better control of the perovskite film morphologies. Nonetheless, in the preparation process of CsPbI2Br, the huge differences between PbI2 and CsBr solution solubility contribute to make the two-step process inappropriate for CsPbI2Br inorganic perovskite film [54]. In addition to one step and two steps spin-coating as solution processing methods described previously, various other treatments and process modifications have also been proposed to obtain a high quality perovskite layer, such as the use of anti-solvent. In this vein, Dong et al. [55], developed an anti-solvent assisted multi-step deposition method (Fig. 8) is proposed to achieve controllable preparation of high-quality CsPbI2Br inorganic perovskite film. In this method, PbI2 (DMSO) intermediate phase film is first prepared by anti-solvents nano-sized voids treatment. Here, they used toxic chlorobenzene (CB), green ethyl acetate (EA), and ethanol (EtOH). Multi-step spin-coating of CsBr methanol solution is therefore applied to the pre-formed PbI2 (DMSO) layer, seconded by thermal annealing to generate complete crystallization of CsPbI2Br inorganic perovskite [55].

Fig. 8. Anti-solvent assisted multi-step deposition method for CsPbI2Br inorganic perovskite films fabrication [55]

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Vacuum deposition methods consist in the deposition of two kinds of precursors, typically organic and metal halides (mainly lead halides), in vacuum chamber. Generally, it includes the co-deposition where two precursors are deposited simultaneously and the sequential deposition where the precursors can either be annealed at high temperature or annealed after deposition. The precursors evaporation rates establish the composition of the obtained perovskite [53]. Liu et al. [56] first reported this method for a planar MAPbI3-xClx PSCs by co-evaporating PbCl2 and MAI in a vacuum disposition. Similarly, Chen et al. [57] used the same all vacuum-deposition method for the fabrication of an inorganic caesium lead halide PSCs (CsPbI2Br). In their process, the caesium halide and the lead halide precursors were developed by co-sublimation in high vacuum. This method has the advantage of being highly controllable and taking place in a neat environment, producing then a smooth and crystalline layer without pinholes. However it is a high cost technic due to the use of vacuum chamber and may be inappropriate for widely application. Chemical vapor deposition method in which both inorganic precursors can also be thermally evaporated in a tube furnace [46, 58] and, vapor assisted solution process (VASP) which consists first in a solution deposition followed by the evaporation of the both inorganic precursors, are akin to sequential vapor deposition [45, 59]. The Table 2 summarizes the different inorganic perovskite deposition methods with their stability and best efficiency documented.

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Table 2: Comparison of different inorganic perovskite deposition methods with their stability and best efficiency documented Deposition methods

Vacuum deposition process

Solution processing

Benefits Reduce contamination from solvents. Allows control over the film thickness for better film uniformity Low fabrication temperature

One step spin-coating

Easy and fast process

Two step spin-coating

Better control over the film morphology

Anti-solvent method

Controllable preparation of high-quality crystals

Active material

Inconvenient

High cost due to vacuum requirement Low efficiency

Poor morphology Low crystallinity Thin thickness Long-time process Thickness limitation on the films

Toxicity of most anti-solvent used

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PCE (%)

Stability

11.8%

Device showed a promising stabilized PCE of 11.5%,

[57]

CsPbI2Br

13.71%

The devices retain 93% of the initial efficiency after 370 hr under 100 mw cm2 continuous white light illumination

[60]

CsPbBr3

9.72 %

87% PCE over 130 days at 90% RH and 25 °C

[54]

10.21%

Long-term stability with no obvious efficiency degradation under ambient atmosphere at 15–30% RH at room temperature for 44 days.

[55]

CsPbI2Br

CsPbI2Br

Ref.

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Stability of the all-inorganic lead halide perovskite materials

3.1 Photochemical and thermal stability The crystal structure of inorganic metal halide perovskite material has a general chemical formula ABX3 (Fig. 3) where A represents monovalent inorganic cation (Cs+), B divalent cations (ever Pb2+ or Sn2+) and C halogen anion (I-, Br-or Cl-) [61]. These inorganic perovskite materials are good alternative to chemical decomposition and air-stability problems of the organic-inorganic hybrid perovskite due to their better stability in ambient environment. Indeed, caesium lead halide materials show lower hygroscopicity compared to methylammonium (MA) widely used in the organic PSCs fabrication [62-65]. Moreover, organic-inorganic hybrid perovskite solar cells are well known to undergo severe decomposition under light, thus resulting mainly from the decomposition of PbI2. Its photochemical decomposition pathways are well addressed in Scheme 1, where the decomposition of MAPbI3 results in the formation of intermediate PbI2, metal lead (Pb0), molecular iodine (I2), and some other volatile products issuing from MAI [66]. Scheme 1: MAPbI3 Photochemical Degradation Pathway [66].

Considering the two most used inorganic metal halide perovskite material for solar cells devices (CsPbI3 and CsPbBr3), Harma et al. [67] have established since 1992 their compositional stability up to their melting points which are generally above 460 °C. Under illumination or submitted to elevated temperature, all-inorganic perovskites do not engender volatile decomposition products, remaining then essentially stable to different stress effects [68]. In 2017, Akbulatov et al. [66] has undertaken some research about the intrinsic thermal and photochemical stability of CsPbBr3 and CsPbI3 perovskite compared to the hybrid MAPbBr3. It can be inferred from their work, that the hybrid perovskite films are very sensitive toward the elevated temperatures and are subject to an easy thermal decomposition, compared to the 14

inorganic perovskites which are pretty much more stable. Indeed, EDX analysis conducted on CsPbI3 revealed that the film does not lose iodine under continuous illumination for 900 h in oxygen and moisture free-environment (Fig. 9 a). Regarding the photochemical stability of the CsPbBr3, they found that under the same conditions, the material does not exhibit any evident degradation and its composition remains unchanged, contrary to MAPbBr3 perovskite (Fig. 9 b,c). In addition, CsPbBr3 has shown minor changes in the optical spectra after thermal annealing, which were attributed to the evolution of film morphology. Moreover, its electrical properties such as charge transport properties and photovoltaic characteristics have been improved due to the heat-induced crystallization and specific “fusion” at the grain boundaries. More recently, Sanchez et al. [69] revealed that un-encapsulated pulsed-infrared-light-annealed CsPb1.8IBr1.2, CsPb1.5IBr1.5, and CsPb1.2IBr1.8 cells have shown great photo stability with less than 10% loss of efficiency after 1,000 h of continuous illumination under an AM 1.5 spectrum in an inert atmosphere. Therefore, it is obvious that under operating temperatures on earth, the structural stability of inorganic lead halide perovskites materials remains the most crucial issue to overcome.

Fig. 9. (a) EDX analysis revealing the evolution of the iodine-to-lead atomic ratio in the CsPbI3 films when exposed to light. (b) Comparison of photochemical degradation of the hybrid MAPbBr3 and all-inorganic CsPbBr3 perovskite material (c) Relative intrinsic photochemical stability of different perovskite materials as follows from the evolution of their absorption spectra under illumination [66].

In the same vein, Liang et al [39] also demonstrated that the inorganic perovskite solar cells had a better thermal stability compared to the hybrid ones. Their work was about to 15

fabricate CsPbBr3/carbon based all-inorganic PSCs and to examine them in contrast to MAPbI3/carbon PSCs. They achieved a PCE as high as 6.7% for the inorganic PSCs, with the photovoltaic parameters presented in Fig. 10 (a,b). In addition they noticed that CsPbBr3/carbon based PSCs were not only more resistant in ambient conditions, but they were also even more stable when subjected to relative severe temperatures, compared to hybrid devices. In fact, without any encapsulation, the all-inorganic PSCs present no performance degradation in humid air (90-95% RH, 25 °C) for over 3 months (2640 h) (Fig. 10 c), as well as when heated at 100 °C in very humid ambient environment (90-95% RH, 25 °C) without encapsulation (Fig. 10 d). More importantly, they highlighted the high thermal stability of their CsPbBr3/carbon based solar cells, by exposing them to temperature circles between -22 °C and 100 °C. As a result, the devices showed no dramatic PCE decay even when stored without any encapsulation and in an environment of 90-95% RH. These results thus confirm the strong thermal stability of inorganic perovskite materials.

Fig. 10: (a) J-V plot of CsPbBr3/carbon based all-inorganic PSCs. (b) Statistical histogram of the PCEs of 40 individual CsPbBr3/carbon based all-inorganic PSCs. (c) Normalized PCEs of CsPbBr3/carbon based all-inorganic PSCs, MAPbI3/carbon and MAPbI3/spiro-MeOTAD based hybrid PSCs as a function of storage time. (d) Normalized PCEs of CsPbBr3/carbon based all-inorganic PSCs and MAPbI3/carbon

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based hybrid PSCs as a function of time. (e) Normalized PCEs of CsPbBr3/carbon based all-inorganic PSCs as a function of storage time during temperature circles (between -22 °C and 100 °C) [39].

Later in 2018, li et al [70] developed a surface passivation engineering for preparing long-term stable cubic phase CsPbI3 films via a reproducible solution-chemistry process with the assistance of PVP. They investigated the moisture (Fig. 11 a) and thermal stability of inorganic cubic perovskite (CsPbI3) PSCs in comparison to typical organic–inorganic hybrid perovskite (MAPbI3) under different conditions. Overall, their results demonstrated that the CsPbI3 inorganic perovskite possesses more outstanding thermal stability. Indeed, from (Fig. 11 b) which presents the device efficiency variation as a function of temperature from 20 to 100 °C, it can be observed that with the increase of temperature, the inorganic cubic perovskite exhibits more outstanding thermal stability (90% efficiency retention at 80 °C), compared to the typical organic–inorganic hybrid perovskite (MAPbI3). Even stored at high temperature (60 °C) under normal sunlight exposure in N2, the cubic CsPbI3 based PSC shows a slight efficiency lost (only 10%), compared to MAPbI3 based solar cell with 70% efficiency loss which confirm its impressive superior thermal stability (Fig. 11 c). Nevertheless, it should be noted that above 100 °C, the both CsPbI3 and MAPbI3 devices present meaningful efficiency decrease, which might result from the failure of the organic hole transport material [70].

Fig. 11: Moisture and thermal stability investigation of perovskite solar cells based cubic CsPbI3. (a) Efficiency evolution of the devices exposed in ambient air under relative humidity of 45-55% without any sealing. The measurements were carried every 50 h during 500 h. (b) Efficiency variation as a function of temperature from 20 to 100 °C. The PCEs were measured under nitrogen atmosphere after an equilibration time of 30 min at each temperature setting. (c) Efficiency evolution of the cells in a nitrogen atmosphere at 60 °C during 500 h [70].

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3.2 Phase instability of CsPbI3 perovskite and methods of stabilization Despite their relative advantage, inorganic perovskite materials also have some limitations in terms of phase stability, especially the CsPbI3 perovskite which has been firstly synthesised in 1893 by Wells et al. [71]. In fact, based on Sharma et al. [67] work, the Cs+ cation of the CsPbI3 is not strong enough to form a stable perovskite structure at room temperature, which leads to a need of high temperature (above 310 °C) to transform the light yellow phase (γ-CsPbI3) into a dark brown or black phase (α-CsPbI3) [21, 23, 24, 65, 72-74]. The biggest challenge in the use of CsPbI3 as active layer in perovskite solar cell is thus, the stabilisation of the black phase at room temperature [75, 76]. Numerous attempts have been made in order to overcome this phase instability issue; among them the most commonly used are (1) solvent-additives engineering, (2) alloying-element doping, and (3) 2D nanocrystal engineering. 3.2.1

Solvent-additives engineering

Preliminary work on the inorganic caesium lead iodide perovskite phase transition at room temperature was undertaken by Eperon et al. in 2015 [21]. They established that the ambient conditions affect the stability of the black phase. Indeed, a temperature of above 335 °C was necessary to form the black phase which alters to yellow orthorhombic phase when exposed to the air [21]. In their work, they use a small amount of hydroiodic acid (HI) as additive to the precursor CsPbI3 solution (1:1 CsI:PbI mixed in DMF), to produce a stabilised black phase at room temperature with an low efficiency of 2.9% . The role of the HI is to induce microstrain in the crystal lattice leading to the stabilization of α-CsPbI3 at room temperature. The all process has been carried out in inert atmosphere at only 100 °C, which is a relatively low compared to the temperature of above 310 °C initially needed to form the black phase. In Fig. 12 are presented the absorbance spectra of black and yellow phases, as well as their structural configurations (γ-orthorhombic at room temperature, and α-cubic at T>310 °C or 100 °C with HI).

Fig. 12. (a) Absorbance spectra of black and yellow phases of CsPbI3 thin films; (b) Diagrammatic structure of CsPbI3 phases [21]

18

Similarly, Luo et al. in 2016 [22] has developed sequential solvent engineering including the addition of hydroiodic acid (HI) and subsequent isopropanol (IPA) treatment for the fabrication of stable CsPbI3 PSCs in fully ambient environment with humidity below 30%. According to their work, after optimizing the Cs4PbI6 precursor solution with 66 µL/mL of HI at low temperature, a hot IPA solution was mixed with, and the layer was annealed at 100 °C for 5 minutes in the air leading consequently, to the formation of yellow−brown Cs4PbI6 films. Afterward, the film quickly changed into dark brown α-CsPbI3 and stayed stable for 72 h (Fig. 13). The role of the hydroiodic acid (HI) was to stabilise and produce uniform film. However, depending on the quantity introduced in the solution, it can either facilitate the dissolution of the CsI (Eq.1) or suppress the solubility of PbI2 and form the complex compound PbI2·DMF films (Eq.2). The Eq. (3), (4), and (5) bellow present the different reactions occurring after IPA treatment [22]. CsI + PbI2 → α-CsPbI3 → δ-CsPbI3

1)

4CsI + PbI2 → Cs4PbI6

2)

Cs4PbI6 → α-CsPbI3 +3CsI

3)

3CsI + 3PbI2 → 3α-CsPbI3

4)

Cs4PbI6 + 3PbI2 → 4α-CsPbI3

5)

19

Fig. 13. (a) Photographs of S2 samples freshly coated and exposed for 24 and 72 h and (b) their corresponding XRD patterns; (c) experimental and simulated XRD patterns of the Cs4PbI6 phase [22]

More recently, it has been found that the use of small amount of H2O as additive in the CsPbI3 precursor could thermodynamically stabilise the orthorhombic γ-CsPbI3 thin films in its crystal structure through proton transfer reaction [77]. Indeed, Zhao et al. in their work have demonstrated using both theoretical calculations (calculated Gibbs free energy) and experiments, the effect of H2O additive on the film morphology and stability. Thus, using Rietveld refinement in a XRD test helped to confirm the crystal structure of the γ-CsPbI3 polymorph as shown in (Fig. 14 a). Moreover, by using the density functional theory (DFT) to calculate Gibbs free energy of the bulk δ-CsPbI3, they underlined the fact that for a surface area greater than ~8600 m2/mol, γ-CsPbI3 gradually shows a smaller Gibbs free energy than δ-CsPbI3 (Fig. 14 b), which implies a comparably stronger phase stability.

Fig. 14. (a) Rietveld refinement of the x = 0.02 film XRD pattern, revealing pure orthorhombic γ-CsPbI3 with the Pbnm space group. (b) Calculated Gibbs free energy of g-CsPbI3 and a-CsPbI3 [77]

In the same trend of using additives as solution to black phase instability at room temperature, the treatment with long-chain ammonium has been found to be a good alternative. 20

For instance, Li et al. [78] have reported that the incorporation of phenylethylammonium (PEA+) into the CsPbI3 perovskite considerably improves the film phase stability. Indeed, an optimised CsxPEA1-xPbI3 perovskite film exhibits a stable black α-phase up to 250 °C in air with a power conversion efficiency of 5.7%. Similarly, Zhang et al. [79] discovered that the introduction of small amount of ethylenediamine cations (EDA2+) in CsPbI3, not only facilitates the formation of α-CsPbI3 films at lower temperature (100 °C), but also helps its phase stabilisation even more than 150 hours in ambient conditions. Liao et al. [80] used bulky ammonium (butyl ammonium) cation, to form stable 2D caesium lead iodide perovskite BA2CsPb2I7; (BA=CH3(CH2)3NH3). The resulted 3D CsPbI3 exhibits very good stability under the pressure of heat (85 °C for 72 h) and humidity (30% RH). Later, Wang et al. [81] used a small amount of sulfobetaine zwitterions (~1.5 wt %) to stabilize the α-CsPbI3 film at room temperature via a single-step film deposition process. The zwitterions were found to impede the crystallization of CsPbI3 perovskite films via electrostatic interaction with the ions and colloids in the CsPbI3 precursor solution. In Fig. 15 is presented the CsPbI3 α-phase stabilization mechanism by the use of Zwitterion.

Fig. 15. Mechanism of CsPbI3 α-Phase Stabilization by Zwitterion: (a-f) Schematic representation of CsPbI3 crystal formation from precursor solution without (a-c) or with (d-f) the zwitterion [81].

The studies presented thus far provide evidence that the incorporation of additives in the perovskite production process not only helps to the crystallisation of the material, but also stabilises it in its 3D crystal phase. 3.2.2

Alloying/element doping engineering

The cubic phase (black phase) of CsPbI3 can also be stabilised at room temperature by partially alloying or doping its B-site with smaller radius metal cations [82], in order to increase the t value. According to the previous reports, the common metal cation used are Mn2+, Ge2+,

21

Sn2+, Sb3+, Bi3+, Eu3+ [83]. Hu et al. [20] have been the first to attempt the B-site doping using a compositional engineering approach via incorporation of Bi3+ in CsPbI3. The Bi-incorporated CsPb1-xBixI3 (0 ≤ x < 0.1) has been obtained by using the one step deposition method and replacing the Pb2+ (1.19 Å) with a smaller cation Bi3+ (1.03 Å), which leads to an increase of t value from 0.81 (α-CsPbI3) to 0.84 (α-CsPb1−xBixI3). When used as active layer in the solar cells, it is found that with an optimal doping of 4 mol % Bi, (CsPb0.96Bi0.04I3), the cell exhibit PCE of 13.21% remain 68% stable for 168 h under ambient conditions. Furthermore, based on the Fig. 16, it can be inferred that the stabilisation mechanism of the α-CsPbI3 phase, ever by using the HI or Bi3+ are much alike. The use of Br alloying in the CsPbI3 to form CsPbI2Br has been found to exhibit a good ambient stability and a suitable bandgap (1.92 eV) for tandem solar cells [84]. But in the path for a better stability, recently, Xiang et al. [60] used Eu3+ as dopant in the fabrication of CsPbI2Br perovskite solar cell. This experience resulted in the stabilization of the black photoactive phase of the material and a considerable improvement of its photovoltaic performance has been observed. Indeed, the cell showed a stability of 93% after 370 h of 100 mW cm-2 continuous white light illumination. Akkerman et al. [85] have demonstrated in 2017 that the use of Mn2+ as dopant could improve the stability of the CsPbI3 perovskite films as wells as its nanocrystals. However, Mn2+ cation (0.70Å) having a relative smaller radius compared to the Pb2+ (1.19Å), it is important to underline that only small amount of Mn2+ can be used to allow the phase separation in the CsPbI3 crystal. Sn2+ can be used to tune the t value of CsPbI3 since the Pb2+ and Sn2+ have relatively close radii value, (1.19Å and 1.18Å respectively). However due to the rapid oxidation of Sn2+ in ambient air, there is a need to combine it with another alloy. In this trend, Br-has been used by Liang et al. [40] to form CsPb1-xSnI3-xBrx which exhibits a better phase stabilisation. In view of all that has been mentioned so far, one may suppose that alloying or doping the CsPbI3 perovskite layer with small cation, constitute good alternatives to the phase instability problems. However, more studies still need to be undertaken in order to resolve definitively this matter. Co-doping methods could be an interesting pave to investigate deeper.

22

Fig. 16. (a)Possible mechanisms for stabilization of α-CsPbI3 by adding HI or Bi3+ in the precursor solution [20] (b)Crystal structure of Eu-doped CsPbI2Br (c) Stability and PCE comparison of Eu-doped and Eu-non doped CsPbI2Br [60].

3.2.3

2D nanocrystal engineering

The improved stability of CsPbI3 perovskite by using CsPbI3 nanocrystals is mostly due to the strong micro-strain or great surface energy of nanocrystals [86]. However, the size dependent phase diagrams imply that the stability of CsPbI3 cubic phase and its nanocrystal size are inversely proportional. In fact, the α-phase shows higher stability with the decrease of the nanocrystal size. This hypothesis is supported by Protesescu et al. [86], who in work have reported that the cubic 4-15 nm CsPbI3 nanocrystals shows a relative stability when kept at ambient temperature for one month, whereas the bigger size 100-200 nm nanocrystals, briskly degenerate into yellow δ-phase. A broader perspective has been adopted by Dutta et al. [87] who argue that the reaction temperature matters in CsPbI3 nanocrystals phase stability. They reported in their study that a thermally and colloidal stable CsPbI3 nanocrystals can be obtained via high temperature reactions. In fact, an increase of 100 °C compared to the standard perovskite synthesis temperature (160 °C) allowed the alkylammonium ions to passivate the surface strongly and to prevent the nanocrystals from phase degradation. The obtained nanocrystals were found to be stable either on long annealing at high temperature or in film under ambient atmosphere. 23

Recently, Mir and co-workers [88] proposed to use Mn as dopant to improve the stability of CsPbI3 nanocrystals (NCs) black phase. From their work, it has been found that Mn-doped CsPbI3 NCs exhibits better black phase stability from few days to nearly a month, compared to the undoped NCs. Indeed, the Mn dopant has improved both colloidal and phase stability of black CsPbI3 NCs though surface passivation. These results have been achieved in ambient air conditions by using post-synthesis Mn doping method. It consists of sparing the CsPbI3 NCs prepared into two fractions; one containing 4.8% Mn to constitute the Mn-doped CsPbI3 nanocrystals and the other undoped fraction used as witness solution (Fig. 17a). The experimental parameters of this method allow us to better understand the role of Mn in the structural stability of CsPbI3 NCs. In Fig. 17b, it can be observed that the Mn doping stops the transformation of the perovskite black phases (both the α-and γ-phases) into the yellow δ-phase when under ambient conditions. Collectively, these studies outline the critical role of understanding the fundamental chemistry of these nanocrystals, which could help the scientific community to improve the stability of nanocrystals based inorganic perovskite solar cells.

Fig. 17. (a) Post treatment synthesis method, (b) Schematic illustration of the effect of Mn-dopant on CsPbI3 NCs black phases [88].

4

Stability of inorganic perovskite solar cell device

The stability of inorganic perovskite solar cells is known to be better compared to the hybrid solar cells. Indeed, with or without encapsulation, some researchers have pointed out the good stability of inorganic solar cells during some specific period. For instance in 2017, Zhou et al. [89] have compared CsPbI2Br and MAPbI3 PSCs stability at various stages: fresh and right after corresponding MPP (Maximum Power Point) operation. It results from their work, that CsPbI2Br exhibits important long-term stability under continuous light illumination at MPP 24

tracking during 1500 hours, in comparison to its MA counterpart; without any encapsulation (Fig. 18 a). In the same trend, in 2018, You and co-workers [36] developed a high performance inorganic perovskite device with a good photo stability under more than 500 h without efficiency drop (Fig. 18 b,c). More recently, Akin’s group [90] developed PSCs based on perovskite quantum dots which showed a better operational stability under continuous light irradiation over 400 h and still without encapsulation. Indeed, the device employing CsPbBrxI3-x demonstrated a good shelf-stability against to humidity under ambient conditions (R.H.≥40%) (Fig. 18 d,e). In order to further improve the stability of the device, different methods such as encapsulation, optimizations of HTL and ETL, as well as the passivation of interface layer have been developed.

Fig. 18 (a) Continuous maximum point tracking for over 1500 hours of the unsealed CsPbI2Br and MAPbI3 solar cell in nitrogen glovebox (25 °C) under constant AM1.5G illumination with 420 nm UV filter [89]. (b) Photo stability of the CsPbI3 solar cells under continuous one-sun illumination (100mWcm-2) with UV cut filter (420 nm) in nitrogen glove box (temperature: approximately 25 °C) [36]. (c) J-V curve of the devices under different continuous light-soaking time [36]. (d)Aging test for devices with the control (black line) and QDs-10 modified (red line) perovskite films in ambient air (at room temperature and 40-50% humidity) without any encapsulation for 30 days [90]. (e) The devices were maintained at the maximum power point (MPP) under one sun and N2 atmosphere over 400 hours [90]

25

4.1 Effect of perovskite materials and its fabrication method Enhancing the stability of the perovskite material layer also contribute to improve the whole device performance and stability. Therefore, some optimization methods of perovskite layer have been developed in order to higher these parameters. In this following section, we will present some of these methods such as the introduction of perovskite nano crystals or quantum, surface passivation or improving fabrication process. Through the past years, numerous studies have been conducted into the preparation of CsPbI3 perovskite solar cells. In 2015, Eperon et al. [21] was the first to fabricate a working black phase caesium lead iodide (CsPbI3) solar cells, by incorporating slight amount of hydroiodic acid (HI) to form smaller grains with a distorted structure that could stabilize the cubic phase at room temperature. The resulted highest PCE was therefore only of 2.9%. In 2016, Luo et al. [22] improved the PCE to 4.13% by using a sequential solvent engineering process (HI and IPA) and for the first time a low-temperature phase-transition route from new intermediate Cs4PbI6 to stable α-CsPbI3. In the same year, a slight improve has been made by Ripolles et al., [74] increasing the PCE to 4.68%. In 2017, by using the vacuum vapor deposition, Yonezawa et al. [91] and Hutter et al. [92] increased the PCE to 5.71% and 8.80%, respectively. In the same vein of increasing the PCE of caesium lead iodide solar cells, Chen et al. [57] with a stoichiometrically balanced precursor ratios, reached 9.40%, while Frolova et al. [76] obtained 10.5% by using a thermal co-evaporation of CsI and PbI2 precursors. Although the PCE of the cells has been increased in the aforementioned studies, their stability still remains very poor; which implies that it is highly crucial to boost the initial PCE while maintaining the cells stable. Based on the size-dependent phase diagrams, it can be deduced that when the nanocrystals decrease in size, the α-cubic phase gradually grows up in stability [93, 94]. For instance, in 2016, Swarnkar et al. [95] produced α-CsPbI3 quantum dots (QDs) PSCs with a relative high PCE of 10.77%. More interestingly, these cells exhibited a phase-stability for months in ambient air. One year later, Sanehira et al. [96] reached a PCE of 13.43% using lead halide perovskite quantum dot (QD) films, with tuned surface chemistry based on A-site cation halide salt (AX) treatments. Furthermore, Wang et Al. fabricated CsPbI3 PSCs using hydroiodic acid (HI) and phenylethylammonium iodide (PEAI) as additives (Fig. 19). The HI was used to generate the formation of the intermediate distorted black phase, hydrogen lead iodide (HPbI3+x), whilst the PEAI offers nucleation for optimized crystallization. All this process resulted in the production of high PCE of 15.07 % with a relative good stability (92% of the device initial efficiency remains) when stored in ambient air for 2 months [97]. In 2018, You et al. [36] developed an innovative deposition method for the production of efficient and stable inorganic solar cells. Indeed, thanks to the solvent controlled growth method, a high PCE of 15.7% and a certified PCE of 14.67% have been obtained, and more than 95% of the initial PCE remains after 500 h in continuous light soaking without any encapsulation. More recently, Wang’s group [98] discovered that an effective PTABr post-treatment on the α-CsPbI3 thin films could enhance its phase stability through simultaneous gradient Br doping and organic PTA cation surface passivation. The resulted PTABr treated CsPbI3 based

26

PSCs, exhibited a high efficiency up to 17.06%, and retained 91% of its initial PCE after stored in a N2 glovebox with 500 h of continuous white light LED illumination.

Fig. 19. Mechanism of HI/PEAI-induced phase stability [97]

Although the CsPbBr3 exhibits the best stability against moisture, oxygen and heat among the Cs-based inorganic perovskites [99], its use in solar cells devices fabrication remains limited due its wide band gap (2.3eV). The classic solution deposition methods are more challenging when operated on caesium precursors for the synthesis of CsPbBr3, due to their weak solubility in organic solvents [100, 101]. Nevertheless, Miao et al. [101] fabricated a series of air-stable CsPb1−x BixBr3 (0 ≤ x < 1) inorganic halide perovskite single crystals via a modified anti-solvent vapour-assisted crystallization method (Fig. 20). The optical absorption measurements showed that the Bi-doped CsPb0.975Bi0.025Br3 and CsPb0.9Bi 0.1Br3 crystals have large absorption covering the entire visible spectrum, making them promising materials for photovoltaic applications. However, very few scientists have focused their research based on the CsPbBr3 perovskite solar cells. Kulbak et al. in 2015 [65] was the first to use CsPbBr3 a two-step deposition method, as active material in PSCs with an obtained PCE of 5.95%. In 2016, Hoffman’s group [102] developed a controlled film growth method through layer-by-layer CsPbBr3 quantum dot deposition to fabricate perovskite solar cells. Multiples aspects among them, the final film thickness, the spin-coating rate and the annealing temperature have been taken into account in order to increase the cells efficiency. Then, PCE ranging from 1.35 to 2.25% have been obtained corresponding to around 6-24 deposition cycles. In 2016, Chang and co-workers [103] using a radical optimization of the CsPbBr3 fabrication process, reached a PCE of 5% with TiO2 (c-TiO2)/m-TiO2/CsPbBr3/carbon structure. Still, the improvement of CsPbBr3 PCE is inhibited by the low phase-purity and poor morphology of the CsPbBr3 film prepared by two-step solution-processing methods [54]. Therefore in order to control the crystallization dynamics, some scientists have undertaken some research. In this trend, Teng et al. [99] after discovering the rapid decomposition process of the CsPbBr3 into CsBr/methanol solution have split the face-down liquid space restricted deposition to limit the decomposition. The resulted CsPbBr3 cells reached a PCE of 5.86% planar structure. Duan et al. [54] created a multi-step spinning method to fabricate high-purity CsPbBr3 films and achieved a PCE of 7.54% without any further modification. Chang et al. [41] fabricated a carbon-based PSCs (C-PSCs) in ambient atmosphere obtaining a PCE of about 5.0% and only 11.7% loss in PCE was observed after 250 h storage at 80 °C. On this note, Liu’s group [26] developed a carbon-based planar CsPbBr3 perovskite solar through a traditional two-step deposition method, and obtained a PCE of 8.12%. 27

Moreover, the un-encapsulated device showed good humidity and thermal stability with no decline in efficiency when stored in ambient air at room temperature (25 °C) for over 1000 h and 60 °C for one month, respectively. More recently, Frolova and co-workers [104] have developed an efficient and stable all-inorganic perovskite solar cells based on nonstoichiometric CsxPbI2Brx (x > 1) alloys. The best devices based on Cs1.2PbI Br1.2 exhibits a PCE of around 10% with advanced thermal and improved photochemical stabilities in both thin film samples and photovoltaic cells.

Fig. 20. Single crystals of CsPbBr3 and CsPb1-x BixBr3 prepared by a modified anti solvent vapour-assisted crystallization method [101].

4.2 Effect of the structure and the electron transport material (ETM) Generally, 5 types of PSCs device structures can be found; among them the normal planar, inverted planar, mesoscopic or mesoporous, triple mesoscopic and, tandem structure (Fig. 21) [105]. Fakharuddin et al. [106] in their study in 2015, compared for the first time the long term performance stability of the planar and scaffold devices for approximately 1300 h. From this study, it can be inferred that the performance stability is related to crystallinity and chemical stability of the scaffold layer. Moreover, the cells with mesoporous TiO2 structure are more stable in comparison to those with planar structure. Indeed, based on the absorption and XRD studies, it is found that the performance in the planar devices downed slide from an initial 12.1% PCE to <1% PCE in ~300 h while the CH3NH3PbI3-xClx attachment is slightly at the surface of TiO2 which result in an increase of surface degradation. Therefore, the planar device structure may not be suitable for long term performance devices. Contrarily in the mesoporous structure, the hole transport layer above acts like a barrier which inhibits the physical contact with the moisture allowing then a better protection and high power conversion efficiency of the device [107, 108]. A few studies have further confirmed this hypothesis, but more need to be done in order to clearly estate the role of the device structure on the device stability, especially in inorganic perovskite solar cells [109-113]. Many types of ETLs such as SnO2, TiO2, C60 and ZnO2 are used in inorganic perovskite solar cells. However, the commonly used are TiO2 and SnO2. Nowadays, SnO2 is gaining stronger interest due to its deeper conduction band and higher electron mobility compared with traditional TiO2, which could enhance the charge transfer from perovskite to electron transport layers, and reduce charge accumulation at the interface [113]. In the study conducted by Zhou’s group, it has been demonstrated that the PSCs based on SnO2 ETL with MoO3 interfacial layer significantly exhibits an enhanced performance, thermal stability, light-soaking stability and long-term stability [114]. 28

The Table 3 summarizes the evolution of different inorganic lead halide perovskite devices with a focus on their efficiency and stability depending on the nature of the perovskite, the device configuration and the fabrication method.

29

Table 3: Summary of reported studies of lead halide inorganic perovskite solar cells CsPbI3, CsPbI2Br, CsPbBr3 and CsPbIBr2

Deposition method

PCE (%)

Stability conditions

PCE under stabilisa tion process (%)

2.9

Un-encapsulated at room temperature

1.7

2015

[21]

4.13

Un-encapsulated in air (RH<30%)

1.88

2016

[22]

10.77

Un-encapsulated in ambient air

7.9

2016

[95]

-

2017

[115]

-

2017

[96]

-

2017

[79]

8.2

2017

[76]

-

2017

[57]

8.98

2017

[20]

Active material

Device configuration

CsPbI3

FTO/TiO2/P/Spiro-OMeTAD/Au

CsPbI3

FTO/TiO2/P/Spiro-OMeTAD/Ag

CsPbI3QDs

FTO/TiO2/P/Spiro-OMeTAD/MoOx/Al

CsPbI3

ITO/PEDOT: PSS/P/PCBM/Al

CsPbI3QDs

FTO/TiO2/P/Spiro-OMeTAD/MoOx/Al

CsPbI3·0.025EDAPbI4

FTO/TiO2/P/Spiro-OMeTAD/Ag

CsPbI3

FTO/TiO2/P/P3HT/Al

CsPbI3

ITO/Ca/C60/P/TAPC/TAPC: MoO3/Ag

Vacuum deposition

9.4

CsPb0.96Bi0.04I3

FTO/c-TiO2/P/CuI/Au

Solvent Engineering

13.21

Solution processing Low-tempera ture solution method Multi-step spin coating One-step spin coating Multi-steps spin coating One-step spin coating Thermal co-evaporati on

30

6.5 13.43 11.8 10.5

Un-encapsulated room temperature Un-encapsulated in N2-filled glovebox Un-encapsulated at room temperature Un-encapsulated in ambient air UV-epoxy encapsulated in dark environment at room temperature (≈25 °C) Un-encapsulated for168 h under ambient conditions

Year

Ref

Solution processing with PTABr doping One-step spin coating Solvent-contr olled growth

PTABr-CsPbI3

FTO/c-TiO2/P/Spiro-OMeTAD/Ag

CsPbI3(γ-phase)

FTO/TiO2/P/P3HT/Au

CsPbI3

ITO/SnO2/P/Spiro-OMeTAD/Au

γ-CsPbI3

N-GQD/ FTO/TiO2/P/PTAA/Au/

CsPbI2Br

FTO/TiO2/P/Spiro-OMeTAD/Ag

CsPbI2Br

ITO/PEDOT: PSS/P/PCBM/BCP/Al

CsPbI2Br

ITO/Ca/C60/P/TAPC/TAPC: MoO3/Ag

Vacuum deposition

11.8

CsPbI2Br

FTO/TiO2 /CsPbBrI2 QDs/ PTAA/Au

Solution processing

14.81

CsPbI2Br

FTO/TiO2/P/Spiro-OMeTAD /Au

Solution processing

14.78

CsPbI2Br

FTO/TiO2/P/CsPbI3QDs/PTAA/Au

Solution processing

14.45

CsPbI2Br

ITO/TiO2/P/P3HT/Au

CsPbI2Br

FTO/TiO2/P/PTAA/Au

-

17.06

11.3 15.71 16.02

Two-step spin coating Solution processing

QD Solution processing Solution processing

31

9.84 6.8

12.02 12.39

Un-encapsulated in N2 glovebox (500 h of continuous white light LED illumination) Un-encapsulated in ambient atmosphere Encapsulated by the UV-epoxy in nitrogen glove box 10 days under 50% RH in N2 atmosphere Un-encapsulated in air at room temperature UV-epoxy encapsulated in dark environment at room temperature (≈25°C) Un-encapsulated in ambient environment (RH 25~35% at 25°C) Un-encapsulated in air with RH~20% Un-encapsulated in dry nitrogen and oxygen atmosphere for 3 weeks Un-encapsulated for 960 h in a dry glovebox Un-encapsulated and stored in dark at 25 °C with RH 25-35%

16.3

2018

[98]

9.7

2018

[77]

14.67

2018

[36]

15.70

2019

[116]

5.6

2016

[19]

6.5

2016

[117]

11.5

2017

[57]

14.55

2018

[118]

14.67

2018

[119]

14.41

2018

[120]

10.8

2018

[121]

No detectab

2018

[122]

CsPbI2Br

FTO/TiO2/P/PTAA/Au

Solution processing

12.39

CsPbI2Br

ITO/PEDOT/P/C60BCP/Ag

Vacuum-assi st method

8.67

CsPbI2Br

ITO/SnO2/P/PTAA/MoO3/Al

Hot casting process

13.8

CsPbI2Br

ITO/ TiO2/P /PTAA /Au

Solution processing

14.86

Cs1.2PbI 2Br1.2

ITO/TiO2/P/P3HT/Au

CsPbI2.94Cl0.06

ITO/PTAA/P/PCBM/C60/BCP/Al

CsPbIBr2

FTO/c-TiO2/P/Spiro-OMeTAD/Au

CsPbBr3

FTO/TiO2/P/C

CsPbBr3

FTO/TiO2/Graphene Ds/P/C

Thermal Co-evaporati on One-step spin coating Gas-assisted method Solution processing Multistep solution-proc essing

32

Un-encapsulated and stored in dark at 25 °C with RH 25-35% Un-encapsulated for and stored 10 min RH of 30% ±4% in open air 100 ᵒC in a N2-filled glove box with H2O and O2 levels below 1 ppm Un-encapsulated under continuous 1 sun light soaking at 85 °C for 1000 h.

le decay No detectab le decay

2018

[122]

-

2018

[123]

-

2019

[124]

12.78-1 3.37

2019

[125]

~10

Un-encapsulated at 70 °C in inert atmosphere for 300h

~8

2019

[104]

11.4

Un-encapsulated in air for over 30 days

9.69

2017

[81]

8.02

-

6.7

2017

[126]

6.3

Un-encapsulated in humid air (90-95% RH, 25 °C)

-

2016

[39]

9.72

Un-encapsulated in RH 90% 25 °C for 130 days

8.46

2018

[54]

Fig. 21. Different types of PSCs device configurations [105]

4.3 Effect of hole transport material (HTM) It has been reported that a good HTM should have four main characteristics: 1) high hole mobility; 2) an optimal HOMO energy level; 3) good solubility and film forming properties; and 4) low cost [17]. A wide range of HTLs can be used in inorganic PSCs, ranging from organic types such as 2,2’,7,7’-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene (spiro-OMeTAD), poly[bis(4-phenyl)(2,4,6-trimethylphenyl) amine] (PTAA), poly(3,4-ethylenedioxythiophene) (PEDOT) and poly (3-hexylthiophene-2,5-diyl) (P3HT); to inorganic ones such as phthalocyanine (CuPc) and Nickel oxide (NiO) [127]. Overall, the most common used HTLs in inorganic PSCs are spiro-OMeTAD and PTAA. Frist introduced in 2012 and replacing the liquid electrolytes, the organic hole-transport material spiro-OMeTAD, contributed to improve the perovskite solar devices stability, and contributed to reach a PCE of 9.7% [128]. Even if Spiro-OMeTAD is the most commonly used HTM in PSCs, it still has some drawbacks linked to its huge synthesis costs and its thermal and moisture instability [129]. In fact, in order to improve the conductivity of spiro-OMeTAD, the addition of some additives like bis(trifluoromethane) sulfonimidelithiumsalt (Li-TFSI) and 4-tert-butylpyridine (TBP) is required, which increases the synthesis costs. Moreover, these additives have breakdown effects on the perovskite layer, contributing to increase the device instability [130, 131]. A great way to encounter these limitations regarding the use of spiro-OMeTAD as HTL is to introduce an interfacial layer between the HTL and the electrodes. In this trend, Zhou et al. [114] proceeded to interface engineering at low temperature of all-inorganic CsPbI2Br perovskite solar cells, with a resulted PCE of 14.05%. More importantly, the introduction of MoO3 showed excellent thermal stability and long-term stability of the device without any encapsulation. 33

Considering these reasons aforementioned, it is imperative to focus research on new type of hole transporting materials which could potentially be more thermally and moisture stable. For this purpose, investigating on inorganic materials as HTLs could be a good alternative since, they exhibit good ambient stability and relative low synthesis costs [132, 133]. Liu et al. [134] first used the copper phthalocyanine (CuPc) as hole transporting layer (HTL) in CsPbBr3 PSC and got a PCE of 6.32% with outstanding durability and a promising thermal stability. In 2019, Liu and co-workers [26] used CuPc as HTL in CsPbBr3 fabrication which resulted in a high ambient stable device. Indeed, they obtained a PCE of 6.9% and the devices exhibit no decline in PCE after being stored in ambient air with a relative humidity of ~ 40% at room temperature (25 °C) for over 1000 h and 60 °C for 720 h, respectively. This exceptional stability could be imputed to the robust CsPbBr3 itself and the highly stable CuPc. Quantum dots (QD) have also been found to not only improve the device performance, but also its stability. For instance, Tang and co-workers [54] used graphene based-QDs as interfacial modification layer or HTL for CsPbBr3 PSCs and obtained a stability of 87 % PCE over 130 days or 95 % over 40 days under 90 % relative humidity (RH) at 25 °C, compared to the initial efficiency. Polymeric HTMs, like poly (3-hexylthiophene-2, 5-diyl) (P3HT) or poly (triarylamine) (PTAA) are also interesting in terms of good device stability. Zhao et al. [72] and Frolova et al. [76] used P3HT as HTL resulting in pretty good stability in ambient atmosphere for months, whereas Wang’s group [97] obtained 15% PCE with high stability. Without any encapsulation the device showed negligible efficiency loss after 300 h of light soaking and kept 92% of its initial efficiency after being stored for 2 months under ambient conditions. Overall, there seems to be some evidence to indicate that the use of HTL could protect the perovskite from atmospheric aggressions even if it depends on its nature. Moreover, it is nowhere mentioned the impact of HTL on the common phase instability of CsPbI3, which is the biggest issue in the perovskite solar cells stability. Since the most commonly used HTL (Spiro-OMeTAD) presents a low hydrophobicity constituting a limiting parameter of the overall device performance, development of new adequate materials is needed.

4.4 Effect of hysteresis Frist reported in 2014 by Dualeh et al. [135], hysteresis effect results from the irreproducibility of the current-voltage curves based on the scan direction and rate, which lead to a modification of photovoltaic parameters [136, 137]. It is stated from many literatures, that hysteresis effects are mainly caused by the 1) ferroelectric polarization effect, 2) charge trapping at the interface, 3) ion migration, 4) grain size and grain boundaries [138-143]. Among above mentioned causes, the ion migration within the crystal structure remains the most impactful on the device stability. Indeed, from Tress et al. [144], Bag et al. [145] and Leijtens et al. [146] work, it resulted that the process of ion migration in hybrid perovskite solar cells, engender the degradation of the device under working conditions. This view is supported by Zhang et al. [147] who indicates that the stability of the cells could be improved by decreasing or stabilizing the mobile halide ion species. Eperon and co-workers in 2015 [21] studied the hysteresis effects in CsPbI3 devices. From their research, they concluded that current–voltage hysteresis is present in CsPbI3 devices 34

and then, this phenomenon is not unique to the hybrid materials with a dipolar organic molecule. Indeed, since CsPbI3 does not have any polar space group, it cannot therefore be ferroelectric, either via dipole alignment or by classical lattice distortion. This implies that the J-V hysteresis noticed in CsPbI3, and possibly in other perovskite solar cells, is not induced by the ferroelectric nature of the material, contrarily to what is claimed in the previous reports [148, 149]. However according to them, the ion motion within the perovskite, could likely constitute, the main alternative to ferroelectricity proposed as a cause for the hysteretic effect. Moreover, they established that the hysteresis takes place on the timescale of seconds as the hysteresis is most severe in J-V curves scanned at slowest rate whereas the faster scans keep the device in the good state during scanning. Then one can conclude that the current-voltage measurement depends on the electron scanning direction and scanning speed. The hysteresis seriously affects not only the cells stability, but also their application and commercialized development. Therefore, for the future development of the perovskite solar cell, it urges to conduct effective research in this field in order to weaken or remove completely the hysteresis effect. Altogether, this will lead in the production of high stable perovskite solar cells.

5

Conclusion

As observed in this review, inorganic perovskite solar cells were found to be a good alternative to the stability issue of hybrid PSCs, notably to the moisture instability induced by the material high hygroscopicity. However, the most commonly used inorganic lead halide perovskite CsPbI3 is found to exhibit a severe phase instability issues under ambient atmosphere. Plentiful efforts have been made in order to overcome these phase instability problems including, solvent-additives engineering, alloying or element doping, and 2D nanocrystal engineering, where doping method revealed to be the most effective one. Furthermore, it has been established that for a better stability of the whole devices, some other parameters have to be considered, like the fabrication method including encapsulation, the type of ETL & HTM and the hysteresis effect. In the light of aforementioned, there is a chance to overcome the inorganic perovskite materials ambient instability issue. However, more research has to be undertaken in order to potentially achieve better results in efficiency and stability field, compared to the organic-inorganic hybrid perovskite solar cells. Conflict of interest form The authors of this manuscript have no conflict of interest. Acknowledgments This work was financially supported by National Key Research and Development Program of China (Grant No. 2016YFB0700700 and 2017YFA0206600), National Natural Science Foundation of China (NSFC) under Grant Nos. 61575010, 61574009, 11574014 and 51671006, the Beijing Municipal Natural Science Foundation (BNSF) under Grant No. 4162016, the Beijing Municipal Science and Technology Commission (BSTC) under Grant Nos. 35

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