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Fabrication and Stability of All-Inorganic Perovskite Solar Cells

Huirong Peng , Molang Cai , Shuang Ma , Xiaoqiang Shi , Xuepeng Liu , Songyuan Dai

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (1) : 136 -150.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (1) : 136-150 DOI: 10.7536/PC200652
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Fabrication and Stability of All-Inorganic Perovskite Solar Cells
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Abstract

The all-inorganic perovskite solar cells(PSCs) have attracted much attention because of their good thermal stability, high carrier mobility and excellent compatibility with tandem devices. With the in-depth study of all-inorganic PSCs and continuous optimization of the fabrication process, the power conversion efficiency of all-inorganic PSCs have exceeded 19%. However, the phase stability of all-inorganic perovskite materials is relatively poor, therefore, the preparation and long-term application of all-inorganic PSCs in the air environment still faces great challenges. By analyzing the phase transition mechanism of all-inorganic perovskite, many researchers have proposed various methods including additive engineering, interface engineering and the development of all-inorganic perovskite quantum dot solar cells to improve their long-term stability. This review summarizes the research progress of all-inorganic PSCs in recent years from the aspects of all-inorganic perovskite materials and structure of solar cells, the fabrication method of the active layer and its phase stability.

Contents:

1 Introduction

2 All-inorganic perovskite solar cells

2.1 Crystal structure of all-inorganic perovskite materials

2.2 Architecture of all-inorganic perovskite solar cells

3 Fabrication methods of all-inorganic perovskite films

3.1 Solution processing technique

3.2 Vacuum processing technique

4 Research progress on the phase stability of all-inorganic perovskite

4.1 Mechanism of phase instability

4.2 Strategy for improving phase stability

5 Conclusion and outlook

Graphical abstract

Keywords

perovskite solar cells / all-inorganic perovskite / fabrication method / phase stability / additive

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Huirong Peng, Molang Cai, Shuang Ma, Xiaoqiang Shi, Xuepeng Liu, Songyuan Dai. Fabrication and Stability of All-Inorganic Perovskite Solar Cells. Progress in Chemistry, 2021, 33(1): 136-150 DOI:10.7536/PC200652

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1 Introduction

In recent years, perovskite materials, which can be prepared in solution, have been widely used in the fields of high-efficiency solar cells, light-emitting diodes, photodetectors and lasers, and have received unprecedented attention and in-depth research[1,2]. Among them, perovskite solar cells have been developed rapidly and steadily, and are considered to be one of the most promising photovoltaic cells[3 ⇓~5]. In just a few years, the photoelectric conversion efficiency of perovskite solar cells has increased from 3.8% to 25.2%[6 ⇓ ⇓ ⇓ ⇓~11]. However, for organic-inorganic hybrid perovskite materials, high temperature or long time ultraviolet irradiation will lead to the volatilization of organic components, which will affect the long-term stability of the cell and hinder the development of perovskite solar cells. Relatively speaking, all inorganic perovskite materials (CsPbX3(X=I, Br, Cl or mixed halogen) have more excellent component stability. The excellent composition stability and photoelectric properties of CsPbX3 materials make it a promising photovoltaic material. In particular, the all-inorganic CsPbX3 quantum dot material has the advantages of low defect state density, high fluorescence quantum yield, long diffusion length and the like, and is widely applied to preparing all-inorganic perovskite quantum dot solar cells[12]. In recent years, both all-inorganic CsPbX3 bulk materials and CsPbX3 quantum dot materials have been well studied and developed in the field of perovskite solar cells.
In 2015, Eperon et al. First prepared an all-inorganic CsPbI3 cell with a FTO/TiO2/CsPbI3/Spiro-OMeTAD/Au cell structure and a photoelectric conversion efficiency of 2.9%. This historic breakthrough has become a turning point in the development of all-inorganic CsPbX3 perovskite solar cells[13]. The results show that the all-inorganic CsPbX3 perovskite solar cell has better thermal stability than the organic-inorganic hybrid perovskite solar cell, and the carrier mobility is similar to that of the reported hybrid solar cell, while it has better photoelectric performance[14][15,16]. In particular, the CsPbI3 cell has a suitable bandgap (~ 1.73 eV), which is beneficial for it to be used as a top cell to form a tandem cell with a silicon cell or other narrow bandgap cells[17 ⇓~19]. With these unique properties, all-inorganic perovskite solar cells have been favored by scientists in recent years, showing an unprecedented momentum of development. In just a few years, the photoelectric conversion efficiency of all-inorganic perovskite solar cells has increased from 2.9% to 19.03%[20,21].
Although all-inorganic CsPbX3 perovskite solar cells have shown many excellent performances, their long-term development is also facing many challenges, and the phase stability of all-inorganic CsPbX3 is the first one. When the relative humidity of the environment is more than 20%, the black phase of the all-inorganic CsPbX3 will rapidly degrade into a yellow phase which has little ability to absorb sunlight, resulting in a sharp decline in its photoelectric conversion efficiency[22,23]. Exploring the reasons for the poor phase stability of all-inorganic perovskite solar cells and how to improve its phase stability has become a research hotspot in this field. Partial substitution of elements, additive engineering, and obtaining all-inorganic perovskite solar cells based on quantum dots have proven to be effective ways to improve the long-term stability of the cells. In particular, the thermal stability of all-inorganic quantum dot perovskite solar cells is significantly better than that of organic-inorganic hybrid perovskite solar cells, and the long-term stability of all-inorganic quantum dot perovskite-type solar cells is comparable to that of organic-inorganic hybrid perovskite-type solar cells, which has attracted wide attention and research. In this review, the crystal configuration and cell structure of all-inorganic CsPbX3 perovskite materials are introduced, and the preparation methods of all-inorganic perovskite light absorption layers are sorted out. At the same time, this review summarizes the phase instability mechanism of all-inorganic perovskite solar cells, and summarizes the stability research progress of all-inorganic perovskite solar cells from the aspects of element doping, additive engineering, interface engineering and quantum dot solar cells. Finally, the development prospects and research directions of all-inorganic perovskite solar cells in the future are prospected.

2 All-inorganic perovskite solar cell

2.1 Crystal structure of all-inorganic perovskite

CsPbX3(X=I-, Br-, Cl-, or mixed halogens) have four crystal configurations. The crystal configuration of the (T>360℃),CsPbX3 at high temperature is shown in fig. 1A, and the Pb2+ and the X- form a three-dimensional structure of an angle-sharing PbX6 octahedron.The Cs+ occupies the central position of the eight octahedra connected by the X-, and the crystal structure of the CsPbX3 is characterized by high symmetry, which is called cubic phase (α-, Pm3m)[24]. In addition to the cubic phase, the CsPbX3 may also exist in three other crystal forms, namely the tetragonal phase (β-, P4/mbm), the orthorhombic black phase (γ-, Pbnm), and the orthorhombic yellow phase (δ-, Pnma). The crystal structures of these phases are shown in figs. 1b-d, respectively[25 ⇓ ⇓~28]. At present, the crystal structure of the light absorption layer material in most CsPbI3 perovskite solar cells is alpha phase, and the cells based on α-CsPbI3 have also obtained high photoelectric conversion efficiency[15,29,30]. However, pure α-CsPbI3 needs to be obtained at high temperature, which increases the difficulty and cost of the preparation of CsPbI3 cells, so the preparation process of perovskite solar cells based on β-CsPbI3 and γ-CsPbI3 has been gradually developed[31 ⇓~33].

2.2 All-inorganic perovskite solar cell structures

The CsPbX3 perovskite solar cell is generally composed of conductive glass, an electron transport layer (ETL), a CsPbX3 perovskite light absorption layer (active layer), a hole transport layer (HTL) and an electrode layer. There are two kinds of conductive glass in common use, which are fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO). Examples of host materials for the electron transport layer include titanium oxide (TiO2), tin oxide (SnO2), zinc oxide (ZnO), and phenyl-3-methyl butyrate (PCBM)[34]. The perovskite light absorption layer is a CsPbX3 thin film capable of absorbing visible light. 2,2 ', 7,7' -tetrakis [N, N-bis (4-methoxyphenyl) amino] -9,9 '-spirobifluorene (Spiro-OMeTAD), poly (3,4-ethylenedioxythiophene): poly (styrenesulfonic acid) (PEDOT: PSS), poly [bis (4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), nickel oxide (NiOx) and the like are host materials commonly used for preparing the hole transport layer[30,35 ⇓ ~37]. The electrode layer is mainly gold electrode or silver electrode, and some researchers choose aluminum or carbon as the electrode of CsPbX3 perovskite solar cells[22,38]. The layer-by-layer structure of all-inorganic CsPbX3 perovskite solar cells requires that the energy level matching principle must be met between the layers of the cells[39]. Only when the energy levels of the materials in each layer of the battery match, can the charge be effectively transmitted between the layers and the battery work properly. Under the condition of meeting the energy level matching of the materials in each layer, the optimal selection is made by considering the factors such as battery performance and preparation cost.
When sunlight passes through the transparent conductive glass and irradiates the light absorption layer of the CsPbX3, photons with energy larger than the forbidden band width of the CsPbX3 are captured, valence electrons are excited to jump to a conduction band, and holes with positive charges are left at the original position to generate photo-generated excitons, namely electron-hole pairs. Aft being separate, that electrons and the holes are respectively transmitted to the photoanode and the count electrode through the electron transmission layer and the hole transmission layer. The electrons and holes are collected by the corresponding electrodes and generate current in the external circuit[40].

3 Method for preparing all-inorganic perovskite film

As the core part of all-inorganic perovskite solar cells, the preparation method of active layer directly affects the morphology and coverage of the film, and then affects the photoelectric performance of the cell[41]. At present, there are two main ways to prepare CsPbX3 thin films in the laboratory, which are solution method and vacuum deposition method. Among them, the solution method involves simple instruments, reduces the requirements for experimental conditions, and consumes less time in the preparation process, which is favored by scientific researchers. Although the vacuum deposition method requires high experimental conditions and complex operation, which greatly increases the preparation cost, the vacuum deposition method can accurately control the thickness of the film and is expected to achieve large-scale production of all-inorganic perovskite films. Moreover, the grains obtained by vacuum deposition method are large and dense, which can effectively improve the quality of the film and then improve the photoelectric performance of the cell, so the process of preparing CsPbX3 film by vacuum deposition method has been continuously developed and improved[42].

3.1 Solution Method

The most common way of solution method is to use a high-speed spin coater to coat the precursor solution on the conductive substrate, evaporate part of the solvent and roughly control the thickness of the film through high-speed rotation, and finally anneal to obtain the perovskite film with visible light absorption ability. According to different deposition steps, the solution method can be divided into one-step deposition method and two-step deposition method.

3.1.1 One-step deposition method

The one-step deposition method firstly needs to prepare a precursor solution of a CsPbX3, and the precursor solution is usually obtained by fully dissolving PbX2 and CsX with a certain stoichiometric ratio into a polar organic solvent. DMF and DMSO are the main polar solvents used in the preparation of CsPbX3 perovskite thin films. After the precursor solution is fully dissolved, the precursor solution is directly spin-coated on a clean conductive substrate, and the CsPbX3 perovskite thin film capable of absorbing visible light is obtained after annealing at a proper temperature. Although the one-step deposition method is easy to operate, the spin-coating process, ambient atmosphere and annealing process involved in the one-step deposition method will affect the quality of perovskite thin films and reduce the performance of solar cells. Therefore, many researchers have been committed to studying how to optimize the one-step deposition process to improve the quality of CsPbX3 films.
Eperon et al. prepared the thin film of CsPbI3 by one-step deposition method for the first time. By adding CsI and PbI2 with equal molar ratio in DMF solution, the precursor solution of CsPbI3 was spin-coated on the conductive substrate by one-step method, and the black thin film of CsPbI3 was obtained by annealing at 335 ℃[13]. However, the high temperature over 300 ℃ will inevitably increase the cost of the preparation of the battery. It is found that the addition of a trace amount of hydriodic acid solution to the precursor solution can not only improve the solubility of the perovskite precursor solution and induce the formation of a smooth and dense film, but also reduce the phase transition temperature of the CsPbI3 from 335 ℃ to 100 ℃. The grain size of the CsPbI3 annealed at 100 ℃ with the addition of HI was significantly reduced to about 100 nm, compared with the grain size of the film annealed at high temperature without HI, which reached the micron level (Figure 2a). It is another method to prepare high quality CsPbX3 thin films at low temperature by using the HPbI3 pre-synthesized by HI, PbI2 and DMF instead of PbI2 to prepare precursor solution. The thin film prepared by Wang et al. With the precursor solution composed of HPb I 3 + x (X = 0.1 ~ 0.2), CsI and PbBr2 can undergo phase transition after annealing at 100 ℃, and the ultraviolet absorption spectrum of the thin film is consistent with the characteristics of the α-CsPbI2Br (Figure 2b)[43]. However, the thin films prepared from the precursor solution composed of PbI2, CsI and PbBr2 are difficult to phase change after annealing at the same temperature.It shows the characteristics of δ-CsPbI2Br, which indicates that CsPbX3 thin films can be prepared at low temperature by using HPb I 3 + x instead of PbI2. Xiang et al. Used HPbI3 instead of PbI2 to prepare α-CsPbI3 films with high stability by one-step deposition in dry air environment (temperature: 20 ~ 30 ℃, humidity: 10% ~ 20%)[44]. Due to the lattice strain, the band gap of the α-CsPbI3 decreases from 1.72 eV to 1.68 eV, which significantly improves the absorption and utilization of visible light. Finally, the photoelectric conversion efficiency of the cell with FTO/TiO2/CsPbI3/Carbon (carbon electrode) is as high as 9.5%, which is the highest efficiency of the CsPbI3 cell without hole transport layer reported at that time.
It is found that the solubility of CsBr and PbBr2 in DMF is low, which leads to the concentration of CsPbX3 precursor solution can not be increased, and the thickness of the thin film is thin, which is easy to cause battery leakage and short circuit. DMSO can promote the transport and diffusion of materials, but excessive DMSO will increase the roughness of the CsPbX3 film, affect the contact between layers, cause a lot of charge loss, and reduce the performance of the battery[45]. The optimization of the ratio of DMF to DMSO has also become a research topic for the preparation of high-quality CsPbX3 films by one-step deposition. By optimizing the ratio of DMF and DMSO and improving the one-step deposition process, Zai et al. Carefully regulated the crystallization process of CsPbI2Br to improve the transport and diffusion of carriers[46]. Fig. 2C shows the mechanism of DMSO regulating the crystallization kinetics of the CsPbI2Br. DMSO molecules are inserted between adjacent planes of the PbX2, so that the PbX2 is vertically symmetrically distributed along the central axis, and CsI is inserted to form a CsI-PbX2-DMSO intermediate. The residual DMSO can promote the transport and diffusion of substances and improve the quality of the film. The DMSO is gradually evaporated during the annealing process, so that a high-quality pure CsPbI2Br film is obtained. Similarly, Zhang et al. Also reported the optimization of the ratio of DMF and DMSO in the precursor solution to obtain high-quality all-inorganic perovskite thin films[47]. Yin et al. Dissolved PbI2 and PbBr2 in DMSO in advance, and obtained the adduct of PbI2(DMSO) and PbBr2(DMSO) by filtration and vacuum drying.Then the adduct is dissolved in a mixed solution of DMF and DMSO(VDMF∶VDMSO=4∶1) to obtain a precursor solution of the CsPbI2Br to prepare the thin film[48]. The fluorescence lifetime test of the film (Fig. 2D, e) shows that the carrier lifetime of the film prepared by using the adduct is long, which indicates that the defect state density of the film is low. In addition, the highest efficiency (14.88%) of gadolinium-based perovskite solar cells was achieved based on this high-quality thin film.
On the basis of one-step deposition method, the method of using anti-solvent to obtain high-quality perovskite thin films has also been proposed[35]. Chen et al. Tried to use isopropanol and toluene as antisolvents and assisted by gradient annealing to obtain high purity and high quality CsPbI2Br films[49]. As shown in fig. 3A, the anti-solvent can wash away the residual DMSO during the spin coating process, eliminate the nucleation aggregation phenomenon and the solvent edge effect, and effectively "freeze" the precursor film in the early stage of gradient annealing, thus forming larger perovskite grains. Finally, the thin film with large size and dense grains was obtained under the combined effect of anti-solvent and gradient annealing, and the cell prepared in this way also showed the best photoelectric conversion efficiency of 16. 07%, and maintained the initial efficiency of more than 90% after working in the air environment for more than 1000 H. In order to obtain CsPbX3 thin films with visible light absorption ability, annealing process is essential. However, if the film is annealed at high temperature immediately after spin-coating, the rapid evaporation of polar solvent and the accelerated crystallization of perovskite easily lead to the formation of low-quality film with a large number of holes. In order to delay the evaporation process of organic solvents, Wang et al. Reported a simple solvent controlled growth (SCG) method to solve the problem of rapid evaporation of organic solvents[16]. The method proposes placing the spin-coated CsPbI3 film in a dry nitrogen glove box for more than a few tens of minutes before annealing (fig. 3B). The experimental results show that the thin film after SCG treatment is continuous and has no pores, and the grain size is about 5 μm.The highest photoelectric conversion efficiency and conversion efficiency of the solar cell based on this high-quality thin film are 15. 7% and 14. 67%, respectively.
In a word, the one-step deposition method for preparing all-inorganic CsPbX3 thin films is a mainstream method for preparing all-inorganic perovskite thin films, especially CsPbI3 thin films, because of its simple process, short preparation time and low requirements for production equipment, and has been widely studied and applied, and the preparation process is constantly being optimized.

3.1.2 Two-step deposition method

The process of the two-step deposition method comprises depositing a PbX2 layer, then reacting CsX with the PbX2 by dip coating or spin coating to form CsPbX3 perovskite, and finally annealing to obtain a thin film with visible light absorption capability. Generally speaking, the two-step deposition method is more complex than the one-step deposition method and the reaction process is not controllable, so it is mostly used to deposit bromine-rich CsPbBr3 and CsPbIBr2 films. This is because when PbBr2 and CsBr are dissolved in polar solvents at the same time, their solubility will be greatly reduced, and they need to be deposited separately by a two-step method[50 ⇓ ⇓~53]. Teng et al. describe a method of immersing a FTO/c-TiO2/PbBr2 substrate face down in a CsBr solution to obtain a high quality CsPbBr3 film (fig. 4A)[54]. Compared with the traditional two-step method of substrate up soaking, the substrate down method avoids the reverse process of PbBr2+CsBr→CsPbBr3 reaction, and has high reproducibility. Based on this method, the open circuit voltage of the cell is as high as 1. 34 V, and the photoelectric conversion efficiency reaches the highest record efficiency of the CsPbBr3 cell at that time. On the basis of the traditional two-step deposition method, Duan et al optimized the number of times of the second spin coating of CsBr and studied the reaction process of multiple deposition of CsBr[55]. The results show that when the spin coating number of CsBr is less than or equal to 3 times, the reaction is CsPb2Br5, when the spin coating number of CsBr is 4 times, the CsPb2Br5 reacts with CsBr to form CsPbBr3, and when the spin-coating number is more than or equal to 5 times, Cs4PbBr6 is obtained. Therefore, only when the number of spin-coating times is 4, the stoichiometric CsPbBr3 thin film can be obtained, the thin film is uniform and dense, the grain size is appropriate, and the obtained solar cell has excellent performance, and the photoelectric conversion efficiency is 9.72%. Duan et al. also reported a method for preparing CsPbBr3 thin films by spray assisted deposition of CsBr[56]. The spray-assisted deposition method can not only simplify the film preparation process, but also help to obtain large-area all-inorganic perovskite solar cells. By optimizing the times of spray deposition of CsBr, a cell with an effective area of 1 cm2 was obtained, which has a certain guiding significance for the preparation of large area all-inorganic perovskite solar cells.
The two-step deposition process is relatively complex, and the reaction process is uncontrollable. In most cases, it is used to deposit bromine-rich all-inorganic perovskite thin films, and its preparation process needs further study and optimization.

3.2 Vacuum deposition

The advantage of the vacuum deposition method is that it can control the film thickness more accurately and has high repeatability, which is conducive to the production of large area perovskite films. The process flow is shown in Figure 4B. According to the evaporation source and evaporation sequence, vacuum deposition can be divided into double-source co-evaporation, sequential evaporation and alternate evaporation[57][58][59,60]. Frolova et al. Used CsI and PbI2 as evaporation sources, obtained high-quality CsPbI3 films with high purity, smoothness and good crystallinity after thermal evaporation and annealing under vacuum conditions[14]. Shahiduzzaman et al. Developed a way of depositing PbI2/CsI layer by layer to prepare CsPbI3 films[59]. Compare with that traditional double-source co-evaporation mode, the layer-by-layer deposition mode is simpler, and in addition, the numb of deposition layers is continuously increased in the layer-by-layer deposition of the PbI2/CsI, so that the precise intercalation control of the CsPbI3 film can be realized. Through the deposition of different layers of PbI2/CsI and repeated experimental verification, the most accurate intercalation control and the highest repeatability can be achieved by four layer-by-layer deposition of PbI2/CsI.
Although the vacuum deposition method can accurately control the thickness of the film, easily obtain dense films with large grains, and has high repeatability, it is still not widely used due to the limitations of experimental conditions and preparation costs.Further research and optimization are needed to reduce production costs and improve the quality of the film.

4 Research Progress on Phase Stability of All-inorganic Perovskite

4.1 Phase instability mechanism

Although the all-inorganic perovskite solar cells have excellent photoelectric performance, the black phase of the CsPbX3 can be rapidly degraded into the yellow phase (delta phase) which has almost no absorption ability to visible light in the environment with relative humidity of more than 20%, resulting in a sharp decline in the photoelectric conversion efficiency of the CsPbX3 perovskite solar cells, which seriously limits its further development. In order to improve the performance of Er-perovskite solar cells, the phase stability of Er-perovskite solar cells needs to be improved urgently. In order to improve the phase stability of CsPbX3 efficiently and effectively, the cause of phase instability of CsPbX3 must be clarified first.
First, the phase instability of the CsPbX3 is closely related to the value of its tolerance factor (denoted by t). In 1926, Goldschmidt first proposed the concept of tolerance factor to describe the relationship between the structural stability of materials and the size of ions[61]. The value of the tolerance factor is calculated according to the formula: t = r A + r B 2 ( r B + r X ), where the rA represents the size of the A-site ions, here the size of the Cs+, the rB represents the size of B-site ions,Here, the size of the Pb2+ is referred to, and the rX indicates the size of the X-site ion, which here refers to the size of a halogen ion (I-, Br-, Cl-) or a mixed halogen ion. In general, the t value of the cubic perovskite material is in a suitable range of 0.9 < t < 1, but the tolerance factor value of the CsPbX3 is difficult to reach this range, so that the phase stability of the CsPbX3 perovskite material is poor[62]. Secondly, the four phases of the CsPbX3 are temperature-dependent and can only exist stably at a specific temperature. Taking the CsPbI3 as an example, the process of phase change of each phase of the CsPbI3 with the change of temperature is shown in fig. 5A. When the δ-CsPbI3 is heated to 645 K, the δ-CsPbI3 is transformed into a α-CsPbI3. When cooled to 510 K, the α-CsPbI3 transforms into a tetragonal β-CsPbI3, and when further cooled, the β-CsPbI3 transforms into a black ——γ-CsPbI3 with an orthorhombic structure[63]. Only the δ-CsPbX3 can exist stably at room temperature, which is one of the reasons for the poor stability of the black phase of CsPbX3 perovskite at room temperature. Again, moisture is an important factor in the rapid degradation of the CsPbX3 film (fig. 5B). During the preparation of CsPbX3 perovskite solar cells, defects are unavoidable. These defects not only affect the efficiency of CsPbX3 solar cells, but also provide an invasion path for water molecules to accelerate the degradation of CsPbX3. Finally, the study shows that the contact angle between the CsPbX3 film and water is usually less than 50 °, which tends to be a hydrophilic film. In the humid environment, it is very easy to be eroded by water, and the structure is inclined, which leads to the rapid degradation of the CsPbX3 film[48,64].
Once the causes of the poor phase stability of the CsPbX3 are identified, it is easy to formulate strategies to improve the phase stability. Firstly, the stability of the CsPbX3 can be effectively improved by adjusting the tolerance factor. Since the tolerance factor of the CsPbX3 is small, the phase stability of the CsPbX3 can be effectively improved by finding a suitable method to increase the tolerance factor. According to the calculation formula of the tolerance factor value, the tolerance factor value of the CsPbX3 can be improved by increasing the size of A-site ions or reducing the size of X-site ions or adjusting the size of B-site ions. Because the Cs+ is already the larger ion in inorganic ions, it is difficult to increase the size of A-site ions, so adjusting the size of X-site and B-site ions has become the mainstream trend. Secondly, although it has been confirmed that solar cells based on cubic phase CsPbX3 can achieve the best photoelectric conversion efficiency, the cubic phase needs to be obtained at high temperature and can only exist stably at high temperature.The other two black phases require relatively low temperatures, so developing the preparation process of CsPbX3 all-inorganic perovskite batteries based on β phase or γ phase is also an effective way to improve the phase stability[66 ⇓ ⇓~69]. Thirdly, using appropriate methods to passivate the defects of the CsPbX3 can not only reduce the density of defect States and improve the photoelectric performance of the cell, but also reduce the attack sites of water and delay the degradation rate of water on the all-inorganic perovskite, thereby improving the phase stability. Compared with the CsPbX3 with large grains, the surface energy of the CsPbX3 quantum dots is greatly reduced, and the phase stability can be obviously improved. In addition, the use of materials containing hydrophobic ions or groups to improve the hydrophobicity of the perovskite film can also play a role in isolating water and delaying the degradation of the film. In addition to the above methods, increasing the phase transition barrier and reducing the surface energy can also effectively improve the phase stability of the CsPbX3.

4.2 Phase stability improvement strategy

With the development of all-inorganic CsPbX3 perovskite solar cells and the further understanding of the phase instability mechanism of perovskite materials, researchers have proposed many constructive schemes to improve the stability of all-inorganic CsPbX3 perovskite solar cells. The following is a summary of the work done in recent years to improve the stability of all-inorganic CsPbX3 perovskite solar cells from five aspects: element substitution, additive engineering, interface engineering, quantum dot cells and other ways.

4.2.1 Element Substitution

In the related work of improving the stability of CsPbX3, partial element substitution is usually used to adjust the tolerance factor or introduce ions into the lattice of CsPbX3 to improve the phase stability. Partial element substitution can change the size of A-site, B-site or X-site ions by introducing ions of appropriate size, increase the tolerance factor value or change the lattice constant to improve the stability of CsPbX3 perovskite structure[70,71]. Sutton et al. Prepared a full range of perovskite thin films from CsPbBr3 to CsPbI3 by a two-step method[72]. The results show that the mixed halide perovskite thin films have the best structural stability in the air environment. Finally, a CsPbI2Br cell with a suitable band gap (1.92 eV) was selected, which can operate in air at room temperature and has a photoelectric conversion efficiency of nearly 10%. In the whole series of CsPbX3 perovskite solar cells, the CsPbI3 has the most appropriate band gap (~ 1.73 eV), a wide range of solar absorption, and the highest theoretical photoelectric conversion efficiency. The phase stability of the all-inorganic perovskite can be significantly improved by partially or completely replacing the I- with the Br-. The CsPbBr3 has a wide band gap (~ 2.3 eV), and its ability to absorb and utilize sunlight is low, which leads to its poor photoelectric conversion efficiency. In contrast, cells based on iodine-bromine mixture (CsPbIxBr3-x), especially CsPbI2Br cells, which can meet the requirements of optimized photoelectric conversion efficiency and achieve better phase stability, have been widely studied[37,73 ⇓ ~75]. In addition to the partial substitution of X-site ions to form CsPbIxBr3-x based mixed halide perovskite batteries, the partial substitution of B-site ions (Pb2+) is another way to improve phase stability[76]. Hu et al. Added a certain stoichiometric ratio of bismuth ions (Bi3+) to the precursor solution of CsPbI3 to partially replace Pb2+.The value of the tolerance factor is increased from 0.8 in the α-CsPbI3 to 0.84 in the BixI3 of the alpha-CsP b 1 - x[77]. Moreover, the grain size of the CsP b 1 - x BixI3 film is also significantly reduced, which is also conducive to the improvement of phase stability. After optimizing the concentration of Bi3+, the cell doped with 4%mol Bi3+ achieved the best photoelectric conversion efficiency of 13. 21%. Without any packaging protection, the initial efficiency of more than 68% can still be maintained when the battery is placed in a humid environment for 168 H (Figure 6a). Guo et al introduced niobium ion (Nb5+) to replace part of Pb2+ in the CsPbI2Br to stabilize the α-CsPbI2Br[78]. The results show that the effective ion size of the Pb2+ and the Nb5+ becomes 1.168168 Å when the 4%Nb5+ is doped, and the tolerance factor is calculated to be 0.815. In addition, the lattice strain induced by the Nb5+ can also effectively improve the phase stability of the α-CsPbI2Br. Xiang et al. Successfully introduced europium (Eu) into the CsPbI2Br. The results of solid-state nuclear magnetic resonance and high-angle annular dark-field transmission electron microscopy showed that Eu entered the crystal lattice of the CsPbI2Br[79]. The incorporation of Eu into the crystal lattice of the CsPbI2Br suppresses the nonradiative recombination and prolongs the fluorescence lifetime of the CsPb1-xEuxI2Br film. In addition, the ion size of Eu is smaller, which is helpful to improve the tolerance factor and reduce the lattice distortion, so as to improve the stability of the battery. Stability tests of CsPb0.95Eu0.05I2Br films and cells under different aging conditions show that CsPb0.95Eu0.05I2Br based cells are more stable than CsPbI2Br based cells regardless of environmental conditions (Figure 6 B). Imilarly,Tin ions (Sn2+), calcium ions (Ca2+), manganese ions (Mn2+), germanium ions (Ge2+),Strontium ions (Sr2+) have also been used to improve the phase stability of TiO2 perovskite batteries[75,80 ⇓ ⇓ ~83]. In addition to the partial substitution of B-site cations, the partial substitution of Cs+ can also improve the phase stability of CsPbX3. Nam et al. Demonstrated that the partial replacement of Cs+ by potassium ions (K+) helps to tighten the volume of the PbX6 octahedron.O that the Cs+ can firmly support the corner-shared PbX6 octahedron, thereby remarkably improving the phase stability of the Cs0.925K0.075PbI2Br based battery[84]. Guo et al. Also proposed to partially replace Cs+ with rubidium ions (Rb+) to improve the long-term stability of CsPbI2Br cells[85]. Similarly, the partial replacement of Cs+ with Na+ to improve the stability of all-inorganic perovskites has also been reported[38]. Table 1 summarizes the stability performance of the cells or films obtained by the above element substitution, and it is not difficult to see that element substitution is an effective way to improve the stability of the CsPbX3.
Although partial element substitution can effectively improve the phase stability of CsPbX3, it is not easy to introduce ions of appropriate size at the atomic level. In addition, strictly speaking, the components formed by the introduced ions are not controllable, and it is very likely that other types of defects will be produced, resulting in an increase in the concentration of defect States and a negative impact on the efficiency of the cell. Further, the uncontrollable ionic composition also greatly reduces the repeatability in the battery manufacturing process. Therefore, the way of element substitution needs to be further optimized to obtain a more mature process to achieve accurate and reliable partial element substitution to improve the phase stability of the CsPbX3.

4.2.2 Additive Engineering

Compared with element substitution, which needs to introduce ions with appropriate size from the atomic level and the uncontrollability of ion components reduces the repeatability of battery preparation, additive engineering only needs to add appropriate additives to the precursor solution of CsPbX3 and mix them fully, which greatly reduces the difficulty of experiment[86]. In addition, additive engineering can introduce macromolecular or long-chain compounds to improve the hydrophobicity of CsPbX3 films, isolate water and delay the degradation of perovskite materials. Zhang et al. Added a small amount of ethylenediamine cation (EDA2+) to the precursor solution of the CsPbI3, without affecting the carrier transport.The amino groups in the EDA2+ cross-link with the CsPbI3 perovskite crystals, making the α-CsPbI3 less susceptible to degradation to δ-CsPbI3 (Fig. 6C)[87]. Stability tests showed that the CsPbI3·xEDAPbI4 film retained its alpha phase after heating at 100 ° C for one week, showing excellent thermal stability (Fig. 6d). In addition, the cells based on the CsPbI3·0.025EDAPbI4 can still maintain a photoelectric conversion efficiency of more than 10% (initial efficiency of 11.86%) when stored in a dry environment for one month without any packaging. Li et al. Prepared all-inorganic perovskite solar cells by adding a proper amount of polyvinylpyrrolidone (PVP) to the mixed polar solvent of DMF and DMSO, and then dissolving CsI and PbI2 with a certain stoichiometric ratio to obtain a CsPbI3 precursor solution[88]. The mechanism by which PVP improves the phase stability of the CsPbI3 is shown in fig. 6e. PVP is first coordinated to the surface of the CsPbI3 by oxygen atoms, and then the long chains and negatively charged amido groups of PVP attract cations of the CsPbI3 and further promote the formation of cubic CsPbI3. With the growth of the PVP-CsPbI3 crystal, the interaction between the amido group and the Cs+ exposed on the surface is enhanced, which leads to the enhancement of the electron cloud of the Cs+ and greatly reduces the surface energy of the CsPbI3, thus improving its phase stability. The stability evaluation of the PVP-treated CsPbI3 cell shows that the PVP-treated solar cell can achieve the humidity stability which is comparable to that of the organic-inorganic hybrid perovskite solar cell and the thermal stability which is obviously superior to that of the organic-inorganic hybrid perovskite solar cell. Wang et al. Reported a method of stabilizing the α-CsPbI3 by adding a small amount of zwitterions to the CsPbI3 precursor solution. The zwitterions improve the stability of the battery by inducing the formation of extremely small CsPbI3 grains. The mechanism is shown in Figure 6f[30]. In the precursor solution, the zwitterions can react with the Cs+ and the I-. During the annealing process, the zwitterions move to the surface and grain boundaries of the CsPbI3 film to restrict the further growth of the grains while the CsPbI3 grains grow, and small CsPbI3 grains with an average grain size of about 30 nm are obtained. The reduction of the grain size is beneficial to reducing the surface energy of the material, thereby improving the stability of the material[89,90]. Hu et al. Added ascorbic acid (AA) to the precursor solution of the CsPbI3 to improve the phase stability of the film, and the mechanism was similar to that proposed by Wang et al[36]. Liu et al. Developed a simple soft template controlled growth (STCG) method to prepare pore-free and highly stable CsPbI3 films[91]. Adamantane methylamine (ADMA) is introduced as a soft template agent to be tightly adsorbed on the surface of the CsPbI3 crystal and rapidly wrap the CsPbI3 crystal grains, and the crystal growth space is squeezed through a strong space effect, so that the nucleation rate is maximally improved at the initial stage of film formation, and the foundation for obtaining the high-coverage film is laid. During the annealing process, ADMA gradually releases the CsPbI3 crystal to precisely adjust the growth rate of the CsPbI3 in the high temperature stage. Finally, pinhole-free CsPbI3 films with micron-scale grain size were obtained, and the cells based on these high-quality films showed ultra-high photoelectric conversion efficiency and outstanding light stability. It has been reported that appropriate additives can induce the formation of two-dimensional or quasi-two-dimensional perovskites to improve the stability of materials[92]. Jiang et al. Dissolved cesium acetate (CsAc) and HPbX3 in DMSO to prepare CsPbIxB r 3 - x perovskite precursor solution, which overcame the problem of low solubility of Cs component and obtained perovskite thin films with appropriate thickness[93]. On this basis, phenylethylamine halide (PEAX) was added to the precursor solution with improved solubility to induce the formation of quasi-two-dimensional PEA2C s n - 1 Pbn X 3 n + 1 perovskite structure. The defect state density of the quasi-two-dimensional perovskite thin film is obviously reduced, and the phase transition is effectively inhibited. Finally, the cell based on the PEA2C s n - 1 Pbn(I2/3Br1/3 ) 3 n + 1 showed the best photoelectric conversion efficiency of 12.4% and significantly improved stability. Wu et al. Tried to improve the phase stability by adding three different organic terminal groups as additives to the CsPbI3 precursor solution[94]. The results show that the surface defects of the films are passivated by the organic terminal groups, and the steric hindrance provided by the alkyl chain can effectively inhibit the lattice distortion. Finally, the photovoltaic conversion efficiency of the cell with the additive reached the highest efficiency (13. 2%) recorded in the trans-structure cell, and the initial efficiency of 85% was still maintained after 30 days of storage in a humidity environment of < 10% RH.
Additive engineering is a simple way to improve the stability of europium perovskite batteries, and has a wide range of additives to choose from, so it is popular with researchers[95]. Additive engineering has been widely studied, and great breakthroughs have been made in improving the stability of thulium perovskite solar cells.

4.2.3 Interface Engineering

The layer-by-layer structure of CsPbX3 perovskite solar cells leads to unavoidable interface problems. Poor layer-to-layer interfacial contact not only leads to severe charge recombination, but also provides a path for water to enter the perovskite film and accelerate the degradation process[40]. Therefore, the work of interface modification is particularly important from the point of view of improving the performance of CsPbX3 cells and the long-term stability of batteries[96,97]. Wang et al. Post-treated CsPbI3 films with phenyltrimethylammonium bromide (PTABr)[15]. As shown in fig. 7A, the bromine component can enter the CsPbI3 film during the post-treatment process, and the concentration of the bromine component is depth-dependent, so that the visible light absorption range of the film is not narrowed, but the grain regrowth can be induced, and the stability of the film is improved. In addition, the organic cation PTA+ can passivate the surface defects and improve the water resistance of the film. Finally, the humidity stability and thermal stability of the PTABr-based post-treated film, as well as the photostability of the cell, showed a significantly higher level of stability than the untreated CsPbI3 (fig. 7B). Wang et al. attempted to post-treat the CsPbI3 film with phenethylammonium iodide (PEAI), the mechanism of which is shown in fig. 7c[29]. PEA+ and I- can passivate Cs+ and I- respectively.And forming an organic functional layer over the CsPbI3 to stabilize the α-CsPbI3,The cells based on PEAI post-treatment showed excellent stability (fig. 7d). Liu et al. attempted to post-treat the CsPbI2Br film by dissolving a small amount of PEAX (X = Cl, Br, I) in isopropanol[98]. The results show that the smaller size of the Cl- enters the CsPbI2Br lattice during the post-treatment process, resulting in a slight lattice contraction of the perovskite, so that the Cs+ can support the PbX6 octahedron more firmly. In addition, the hydrophobic aromatic group is PEA+ sealed on the surface of the CsPbI2Br to prevent water from entering the perovskite thin film, thereby improving the phase stability of the CsPbI2Br. As shown in fig. 7e, the combined effect of the PEA+ and the Cl- resulted in the PEACl post-treated cell exhibiting significantly better humidity stability than the PEAI and PEABr post-treated cells. Zhao et al. Reported a method of using a high-mobility conjugated polymer DPP-DTT as an interface modification layer of a CsPbI2Br to maintain the photovoltaic performance of a CsPbI2Br perovskite solar cell in a humidity or thermal stress environment[99]. Thiophene, carbonyl and pyrrole groups in DPP-DTT act as Lewis bases to coordinate with unbonded Pb2+ in the CsPbI2Br, passivating the defects at the surface and grain boundaries of the films. In addition, the alkyl chain of DPP-DTT acts as a water barrier above the perovskite to prevent water from entering the film, which is essential to enhance the moisture resistance of the CsPbI2Br film under external environmental conditions. The combination of DPP-DTT passivation of surface and grain boundary defects and improved hydrophobicity significantly improved the long-term stability of the CsPbI2Br in high humidity environments (Figure 7f).
Interface engineering can not only improve the charge transport between the interfaces of perovskite solar cells, reduce the recombination loss, but also improve the stability of the cells, which is an effective way to improve the photoelectric performance and long-term stability of the cells[100,101].

4.2.4 Quantum dot solar cell

CsPbX3 quantum dots (QDs) have the advantages of tunable size, high photoluminescence and quantum yield[102]. Most importantly, when the CsPbX3 is in the form of quantum dots, its surface energy is significantly reduced relative to other large-grained CsPbX3, so that significantly improved phase stability can also be obtained[103]. It is reported that CsPbX3 quantum dot perovskite solar cells have excellent thermal stability as well as long-term stability comparable to organic-inorganic hybrid perovskite solar cells[12]. Therefore, perovskite solar cells based on CsPbX3 quantum dots have been widely studied.
Swarnkar et al. Successfully prepared a CsPbI3 quantum dot solar cell with a photoelectric conversion efficiency of more than 10%. The cell structure and cross section are shown in Figure 8A[89]. The results of XRD and UV absorption showed that the CsPbI3 QDs could be stored in humid environment for 60 days without significant degradation. When quantum dot materials are used to prepare solar cells, the defects in the active layer of quantum dots affect the conversion efficiency of the cells as the recombination centers of carriers. Therefore, if these defects are eliminated in the process of synthesizing quantum dots, the photoelectric performance of the cell can be further improved. Liu et al. Obtained a precursor solution with high reactivity and stability by using heated trioctylphosphine (TOP) to dissolve the PbI2[104]. The CsPbI3 quantum dots (TOP-QDs) synthesized by the precursor solution have ultra-high luminosity and strong chemical stability. Fluorescence quantum yield measurements (fig. 8 B) showed that all sizes of the TOP-QDs exhibited quantum yields close to 100%, demonstrating that the non-radiative defects in the TOP-QDs were almost completely eliminated. In addition, the initial fluorescence quantum yield of 85% of the TOP-QDs was maintained after 30 days of storage in a humid environment (Fig. 8 C). Whether in terms of ultra-high fluorescence quantum yield or strong chemical stability, the method proposed by Liu et al laid the foundation for the preparation of quantum dot solar cells with high photoelectric performance and stability. Although the band gap of the CsPbI3 is narrower than that of the CsPbBr3, due to the quantum confinement effect, the band gap of the CsPbI3 will be widened to 1.85 ~ 1.97 eV when it exists as a nanostructure[103,105]. Therefore, it is of great significance to find a material with a wider spectral response range to improve the performance of CsPbI3 quantum dot solar cells. Liu et al. Synthesized CsSnxPb1-xI3 quantum dot materials by combining CsPbI3 and CsSnI3[12]. Studies have shown that CsSnxPb1-xI3 quantum dots show excellent stability whether stored in solution or directly exposed to air. In addition, both UV absorption and fluorescence measurements show that the absorption edge of the CsSnxPb1-xI3 QDs is located at 850 nm, while the absorption edge of the pure CsPbI3 QDs is located at about 700 nm (Figure 8 d). CsSnxPb1-xI3 quantum dots not only have a wider spectral response range, but also have better phase stability than CsPbI3 quantum dots, which opens up a new possibility for the preparation of perovskite quantum dot solar cells with high photoelectric conversion efficiency and excellent stability. Sanehira et al. Proposed a method of dropping a halide salt solution on the surface of quantum dots to modify the surface of the quantum dots to obtain a quantum dot solar cell with superior performance[106]. In addition, Ling et al. Also reported a method of post-treating CsPbI3 quantum dots with a series of inorganic salts to achieve surface passivation and obtain high-efficiency quantum dot solar cells[107]. Cesium acetate (CsAc) -treated CsPbI3 quantum dot (QD) cells achieved the best photoelectric conversion efficiency of 14. 1%. Moreover, the efficiency profile of CsAc-based quantum dot cells shows higher efficiency and narrower efficiency distribution range, which indicates that CsAc-based quantum dot cells have high repeatability.
The all-inorganic CsPbX3 quantum dot material has the advantages of adjustable size, high fluorescence quantum yield and extremely strong chemical stability, and a cell prepared based on the CsPbX3 quantum dot also shows superior photoelectric performance and outstanding humidity stability, so that a new possibility is provided for improving the stability of the all-inorganic perovskite solar cell.

4.2.5 Other means

In addition to the above mentioned element substitution, additive engineering, interface engineering and preparation of quantum dot perovskite batteries, passivation of defects to reduce moisture attack sites, development of CsPbX3 batteries based on beta and gamma phases, introduction of hydrophobic ions or groups into thin films, and improvement of phase transition barriers can also effectively improve the phase stability of all-inorganic perovskite.
Zeng et al. Prepared stable and efficient CsPbI2Br cells by using lead acetate (Pb(Ac)2) to passivate the defects at the grain boundary of the film in situ[108]. The mechanism of improving battery performance is that the addition of Pb(Ac)2 makes the CsPbI2Br crystallize rapidly, resulting in smaller grains and lower surface energy. In addition, the strong chemical bond between the Ac- and the CsPbI2Br also helps to stabilize the α-CsPbI2Br. It is a common method to increase the phase transition barrier of CsPbX3 to inhibit its degradation by adding hydrogen peroxide to the precursor solution or replacing PbI2 with HPbI3[44,109]. Xiang et al. Found that α-CsPbI3 thin films could be obtained at 200 ℃ by using HPbI3 instead of PbI2 to prepare perovskite precursor solution[44]. As shown in fig. 9a, the film obtained in this way can be placed in dry air for many days and still maintain the cubic phase, which is a significant improvement in stability compared to the film obtained by PbI2. In addition to improving the stability of the α-CsPbX3 in the above manner, the β-CsPbX3 and the γ-CsPbX3 have relatively low requirements on temperature conditions,It has better stability than α-CsPbX3, so more and more research is focusing on the development of batteries based on β-CsPbX3 and γ-CsPbX3. Wang et al. prepared a highly stable β-CsPbI3 film with a wider absorption range for sunlight (Figure 9b), and used choline iodide (CHI) as an interface modification material to obtain a highly efficient and stable all-inorganic perovskite solar cell based on β-wCsPbI3[31]. Zhao et al. Added a small amount of water to the CsPbI3 precursor solution, and the film obtained after annealing showed the characteristics of γ-CsPbI3 (Fig. 9c)[32]. It is found that the H2O molecules can finely adjust the proton transfer equilibrium in the precursor solution, thus controlling the formation of thermodynamically favorable phases by adjusting the size of perovskite crystals. Both theoretical and experimental results show that the surface energy of the γ-CsPbI3 is much lower than that of the δ-CsPbI3, although both the γ-CsPbI3 and the δ-CsPbI3 are orthorhombic. As a result, the γ-CsPbI3 based cell exhibited significantly enhanced reliability, with no significant efficiency loss for months in humid environments and for hours under continuous operating conditions.
In a word, after the phase instability mechanism of all-inorganic CsPbX3 perovskite solar cells is clarified, the phase stability can be improved pertinently. With the joint efforts of numerous researchers, the improvement of device performance and phase stability of all-inorganic perovskite solar cells has been fruitful[110].

5 Conclusion and prospect

The all-inorganic CsPbX3 perovskite solar cell has excellent thermodynamic stability. In particular, the CsPbI3 cell has good spectral matching performance with narrow bandgap cells such as silicon cells, and can be used for preparing tandem solar cells, which is conducive to further opening up the market for commercial applications. There is no doubt that CsPbX3 solar cells will have unlimited development potential and broad application prospects for a long time in the future. Photoelectric conversion efficiency, long-term stability and manufacturing cost are important evaluation indexes of solar cells. However. Although the highest photoelectric conversion efficiency of the reported all-inorganic CsPbX3 cell is more than 19%, there is still a certain gap with the highest efficiency of 25.2% of the organic-inorganic hybrid perovskite cell, and there is still room for further improvement. In addition, there are few reports on large area all-inorganic perovskite batteries. The following is a summary of the existing problems and research prospects of all-inorganic europium perovskite solar cells:
(1) At present, the reported work mainly focuses on the optimization of the performance of thulium perovskite solar cells, but the mechanism is lack of in-depth and systematic research. The exciton separation mechanism of all-inorganic perovskites is still controversial, so it is necessary to establish a complete theoretical system to provide a solid theoretical basis for improving the performance of CsPbX3. (2) The band gap of all inorganic CsPbX3 perovskite is more than 1.7 eV, and it can only absorb visible light in the range of 300 ~ 730 nm. In order to improve the absorption and utilization of visible light in all-inorganic perovskite solar cells, appropriate methods should be further studied to broaden the absorption spectrum of all-inorganic perovskite solar cells. (3) The phase transition temperature of all-inorganic CsPbX3 is high. Although some methods have been proposed to achieve low-temperature preparation, the repeatability and film quality can not be guaranteed. We should continue to optimize the low-temperature preparation process of all-inorganic perovskite thin films to achieve the preparation of high-quality perovskite thin films with high stability at low temperatures. (4) At present, there are few reports on the packaging process of CsPbX3 batteries. It is very important to develop the packaging process of batteries while optimizing the efficiency and stability of batteries, and to extend the service life of batteries without negative impact on the performance of batteries, so as to promote the commercial application of CsPbX3 perovskite batteries. (5) At present, the working environment and working time of batteries are different when evaluating the stability of batteries in the reported literature, and there is a problem that the standard of stability evaluation is not uniform. The characterization of stability is different from the characterization of photoelectric conversion efficiency, which has a unified standard instrument and calculation formula, and can only reflect the relationship between efficiency attenuation and time under certain conditions. However, during the test process, the environment in which the battery is located may change, and the continuous cycling of the battery will also cause the attenuation of efficiency. Therefore, it is necessary to formulate a unified stability evaluation standard in the stability test process of all-inorganic perovskite solar cells.

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Funding

National Key R&D Program of China(2018YFB1500101)

the 111 Project(B16016)

the National Natural Science Foundation of China(51702096)

the National Natural Science Foundation of China(U1705256)

the National Natural Science Foundation of China(51572080)

the National Natural Science Foundation of China(61904053)

and the Fundamental Research Funds for the Central Universities(2019MS026)

and the Fundamental Research Funds for the Central Universities(2019MS027)

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