Inorganic and lead-free CsBi3I10 thin-film solar cell prepared by single-source thermal evaporation

All inorganic lead-free halide perovskites have attracted much attention due to their non-toxic and suitable bandgap. In this paper, we first prepared all inorganic lead-free perovskite CsBi3I10 thin-films by single-source thermal evaporation deposition. The results show that CsBi3I10 thin films prepared by single-source thermal evaporation have layered structure, high purity hexagonal phase and high crystallinity, which are consistent with the theoretical calculation results. The surface of the thin film was compact and uniform, and had high homology with the crystal structure of the evaporation source material. After annealing, the crystallinity of the film was further improved. The photovoltaic conversion efficiency of perovskite solar cells based on the CsBi3I10 thin film is as high as 0.84%. These results indicate that the proposed single-source thermal evaporation method has the potential to prepare high efficiency inorganic lead-free perovskite solar cells.


Introduction
In recent years, organic-inorganic perovskite materials have attracted great attention in the photovoltaic field all over the world due to its simple production process, low cost and high photoelectric performance. These excellent properties make it have a huge application market in photodetectors, optical converters, light-emitting diodes and solar cells [1][2][3][4]. In particular, perovskite solar cells are developing rapidly. At present, the highest photovoltaic conversion efficiency (PCE) of perovskite solar cells has reached 25.5% [5]. This efficiency is comparable to that of Si, CdTe and CIGS solar cells already commercially available [6]. However, the organicinorganic perovskite solar cells have poor stability, lead-based perovskite materials are highly toxic and pollute the environment, which seriously hinder the commercialization of perovskite solar cells [7,8]. Therefore, it is necessary to find other metals to replace lead in order to solve the problem of environmental pollution. At this time, non-toxic inorganic non-lead perovskite materials have attracted the attention of researchers and related exploration has been carried out [9,10]. It is of great significance to replace Pb 2? with all inorganic non-lead perovskite materials that are narrower, more chemically stable and more environmentally friendly, such as Sn 2? /Ge 2? base perovskite, Bi 3? /Sb 3? base double perovskite and halide perovskite added with S 2-. However, Sn 2? is easily oxidized to Sn 4? , leading to Sn-based perovskite instability in the air, which greatly limits its development as a light absorbing layer [11][12][13][14]. CsGeI 3 has a stable diamond crystal structure, but the maximum photoelectric conversion efficiency of CsGeI 3 solar cells is only 0.11%, and its manufacturing cost is high. Bismuth-based ternary iodide is used as perovskite solar cell absorbing materials [15]. The low power conversion efficiency of these materials limits the practical application of bismuth-based solar cells. Bi-based total inorganic perovskite A a B b X x (A = Cs, Ag, Cu; B = Bi, Sb; X = I, Br; X = a ? 3b), such as Cs 3 Sb 2 I 9 , Cs 3 Bi 2 I 9 , CsBi 3 I 10 , CuBiI 4 , and Ag 3 BiI 6 become new choices of absorption layers for solar cells [16,17]. Its stability can be improved by replacing MA ? or FA ? with Cs ? . In the bismuth-based perovskite material CsBi 3 I 10 , Bi 3? has the same potential as lead cation to obtain high efficiency 6S 2 6P 0 electron configuration to form the perovskite conduction band and valence band, and CsBi 3 I 10 band gap width of about 1.77 eV is suitable as the perovskite solar cell absorption layer [18]. However, the highest photoelectric conversion efficiency of Cs 3 Bi 2 I 9 -based perovskite solar cells prepared by traditional spin-coat method is only 3.2%. Therefore, the preparation of high-quality CsBi 3 I 10 thin films by traditional solution method has become a new challenge [19,20].
At present, there are few studies on CsBi 3 I 10 thinfilm solar cells. CsBi 3 I 10 thin films are usually prepared by the solution method. CsBi 3 I 10 thin films are obtained by heat treatment after spin-coating the precursor solution of CsI and BiI 3 [18]. However, the solution method film formation environment is difficult to ensure the reproducibility of the process. And it is difficult to prepare large continuous and homogeneous films, which makes it difficult to commercialize. In addition, the dual-source thermal evaporation method requires precise control of the evaporation parameters of both organic and inorganic materials, which makes it difficult to effectively control the formation of thin films and tends to cause films to deviate from the stoichiometric ratio [21]. Therefore, the development of large area, high quality, non-toxic and high stability perovskite film is an important topic for future application, especially for the prospect of non-lead inorganic perovskite film. As far as we know, single-source thermal evaporation has been very mature in the preparation of high quality CH 3 NH 3 PbI 3 thin films [22]. However, there have been few reports of this approach in CsBi 3 I 10 thin-film solar cells.
In this work, the first principle density functional theory (DFT) was used to calculate the crystal structure and electronic structure of CsBi 3 I 10 . Inorganic lead-free perovskite CsBi 3 I 10 thin-films were prepared by single-source thermal evaporation method. The CsBi 3 I 10 powder was directly heated and then deposited on the substrate to form the CsBi 3 I 10 film. Meanwhile, the microstructure, composition, energy band structure, optical and electrical properties of the CsBi 3 I 10 film were analyzed. The photoelectric conversion efficiency of CsBi 3 I 10 thin-film solar cells based on single-source thermal evaporation is 0.84%, which provides a new method for preparing inorganic lead-free perovskite.  Fig. 1. Then, the prepared CsBi 3 I 10 crystal was ground into powder and used as film evaporation material. Figure 1 shows the preparation process of CsBi 3 I 10 thin-film, including crystal preparation, single-source thermal evaporation and annealing. TiO 2 dense layer and mesoporous layer were prepared before deposition of thin-film CsBi 3 I 10 . Fluorine doped tin oxide (FTO) (sheet resistance \ 15 X, South China Xiangcheng Tech., China) conductive glass was used in the experiment. First, the FTO glass is cleaned with isopropanol, acetone, desolvent and ethanol, and treated with UV/O 3 before use. According to the steps described in the references [23], The TiO 2 dense layer solution was spin-coated on FTO and then sintered at 450°C for 30 min to form a TiO 2 dense layer. The TiO 2 paste (30NR-D, Dyesol, Queanbeyan, Australia) diluted with ethanol (mass ratio 1:6) was used as the mesoporous layer solution, and then it was spincoated on the dense TiO 2 layer and sintered at 500°C for 30 min to form the mesoporous structure of TiO 2 . The sample (glass/FTO/c-TiO 2 / m-TiO 2 ) was transferred to the vacuum chamber, and 300 mg of CsBi 3 I 10 powder was put into the evaporation boat with the distance between the sample and the evaporation source at 25 cm and the sample speed at 40 rpm. When the vacuum level reached 6 9 10 -4 Pa, the evaporation power supply was turned on, and the working current of the evaporation boat was rapidly increased from 0 to 140 A rapidly until the powder was completely evaporated to form a 250 nm thick CsBi 3 I 10 film. Slowly adjust the evaporation current to 0A, turn off the molecular and mechanical pumps in turn, and finally fill the vacuum chamber with nitrogen, remove the thin-film sample and put it into the glove box.

Characterization
The crystal orientation and structure of the films were determined by X-ray diffraction (XRD, Ultima IV) at 40 kV and 40 mA under Cu K a radiation (k = 0.15406 nm). Field emission scanning electron microscopy (FE-SEM, Zeiss, Supra 55) was used to analyze the surface and cross-sectional morphology of the thin-films and the fabricated device. Energy dispersive X-ray spectroscopy combined with scanning electron microscopy (EDX: Bruker Quantax 200) was used to determine its composition. The valence band of CsBi 3 I 10 thin-films was measured by UV electron spectroscopy (UPS) using a 21.2 eV monochromatic black source and VG Scienta R4000 analyzer. Spectrophotometer (lambda 950) was used to evaluate the light absorption characteristics. Thermogravimetric analysis (TGA) was carried out in

Results and discussion
The crystal structure of CsBi 3 I 10 thin films has been predicted in previous papers [24], from rhombic structure in BiI 3 to hexagonal structure in Cs 3 Bi 2 I 9 . Figure 2a shows the crystal and electronic structures of CsBi 3 I 10 thin films calculated by first principles DFT, indicating that CsBi 3 I 10 is a layered crystal structure. We calculated all the XRD spectra, and determined the crystal structure of CsBi 3 I 10 powder, as-deposited CsBi 3 I 10 thin-film and annealed thin film by X-ray diffraction (XRD). The results are shown in Fig. 2b. It is found that the calculated XRD patterns are basically consistent with the experimental XRD patterns of as-deposited CsBi 3 I 10 thinfilm and annealed thin film. From the XRD patterns, it can be seen that the most significant peaks of CsBi 3 I 10 films are (003), (006) and (300), which can correspond to the calculated XRD peak positions. This result further indicates that CsBi 3 I 10 has a layered crystal structure. After annealing, the characteristic peaks (003) and (006) of CsBi 3 I 10 films are more pronounced, which indicates that the annealed CsBi 3 I 10 films have higher phase purity and better crystallinity, and (003) enhanced preferred orientation can be obtained. Figure 2c shows the weight change of CsBi 3 I 10 powder with temperature in nitrogen atmosphere. It can be seen from the figure that the weight of the sample decreases with the increase of temperature. For CsBi 3 I 10 powder, when the temperature was higher than 300°C, the weight of CsBi 3 I 10 begins to lose, which indicates that the CsBi 3 I 10 powder is thermally unstable and begins to decompose when the temperature is higher than 300°C . Figure 3 shows the SEM images of as-deposited CsBi 3 I 10 thin-film and annealed thin-film. Before annealing, the CsBi 3 I 10 film was dense and uniform, and the surface of TiO 2 was completely covered and compact. After annealing, the CsBi 3 I 10 thin-films were compact, the grain size increases, and the layered structure can be seen, which is consistent with the observation of XRD. The reason of pinhole formation in annealed films is not clear, but it may be related to the growth process of thin films. The composition of CsBi 3 I 10 film is an important factor affecting its structure, electrical and optical properties. Table 1 shows the composition of as-deposited CsBi 3 I 10 thin-film and annealed thin-film measured by EDS. The Cs/Bi/I ratios of CsBi 3 I 10 powder, asdeposited thin-film and annealed thin-film were calculated as shown in Table 1. The Cs/Bi/I ratio of powder and annealed film is very close to the stoichiometric ratio of CsBi 3 I 10 thin-film, which also indicates the formation of pure phase CsBi 3 I 10 film.
The optical properties of CsBi 3 I 10 films are strongly affected by the microstructure. Figure 4 shows the UV-visible spectra of annealed films and as-deposited films. As shown in Fig. 4a the prepared CsBi 3 I 10 films have low reflectivity, and obvious interference phenomenon occurs at the wavelength of 670 nm to 1400 nm. The reflection intensity of the annealed film is slightly lower than that of the asdeposited film, and the lower reflection can be attributed to the larger grain size and the possible presence of pinholes in the annealed film. The transmission spectrum of annealed films is lower than that of as-deposited films, which is mainly due to the existence of grain boundaries in annealed films to reduce light scattering. Therefore, the absorption spectra calculated from reflectance and transmittance are shown in Fig. 4c, indicating that the CsBi 3 I 10 thinfilms prepared by single-source thermal evaporation have good absorption, and most of the absorption peaks begin at the wavelength of * 600 nm. Finally, according to absorption spectrum calculation, the band gap width of as-deposited film and annealed film was 1.79 eV and 1.78 eV, respectively, which were close to the theoretical value of 1.77 eV reported in the literature [18,23].
The Fermi energy (E f ) and valence band energy (E v ) of CsBi 3 I 10 thin-films were measured by ultraviolet photoelectron spectroscopy (UPS). Figure 5a, b depict the determined valence band levels. The energy of monochromatic light source (He I light) was 21.22 eV and the secondary cut-off edge was 16.33 eV. We can find that E f was 4.89 eV, according to the equation E f = 21.22 eV (He I)-E cutoff [25]. The E v value was determined by linear extrapolation (E v -E f ) in the low binding-energy region, calculated E v was 6.43 eV. Figure 5c shows a sharp absorption edge, indicating a high light absorption coefficient in the visible light range. The relationship between band gap and optical absorption was in accordance with the Tauc relation, and the formula is ahm = C(hm -E g ) 1/2 , where a is the absorption coefficient, h is the Planck constan, C is the constant, and m is the photon frequency [18,26]. The calculated energy band gap (E g ) of CsBi 3 I 10 film is 1.83 eV, which is close to the previous band gap (E g ) of 1.79 eV calculated from the absorption spectrum. The conduction band energy (E c ) is estimated according to E v ? E g , and the band diagram as shown in Fig. 5d. The CsBi 3 I 10 thin-films deposited by singlesource thermal evaporation may be an N-type semiconductors, because E f is higher than the median value of band gap.
In order to evaluate the potential application of CsBi 3 I 10 thin-films prepared by single-source thermal evaporation method in solar cells, we prepared all inorganic perovskite solar cells with the device structure of glass/FTO/c-TiO 2 /m-TiO 2 / CsBi 3 I 10 / Spiro-OMeTAD/Ag, as shown in Fig. 6a. Figure 6b shows the SEM cross-section images of solar cell based on annealed CsBi 3 I 10 thin-film. As shown in Fig. 6b the cross section of the cell based on annealed CsBi 3 I 10 thin-film was more compact and smooth  without pinholes. TiO 2 and Spiro-OMeTAD were used as electron transport materials and hole transport materials (ETM and HTM), respectively. The annealed CsBi 3 I 10 film has compact and uniform cross-section, which is mainly due to the infiltration of some CsBi 3 I 10 films into the mesoporous TiO 2 layer after annealing and the further growth of the grains. Figure 6c depicts the energy level arrangement of the solar cell. The small gap between the CBs of TiO 2 and CsBi 3 I 10 may affect the smooth transition of photogenerated electrons, resulting in the low performance of CsBi 3 I 10 solar cells [27][28][29]. Figure 6d, e show the J -V curves of as-deposited and annealed CsBi 3 I 10 thin-film solar cells. Table 2 shows the CsBi 3 I 10 solar cells parameters (J sc , V oc , FF, PCE). Tables 3 and 4 show the photovoltaic parameters of forward and reverse scans of as-deposited and annealed CsBi 3 I 10 solar cells, respectively. It can be found that as-deposited CsBi 3 I 10 thin-film solar cell exhibit J sc of 1.07 mA/cm 2 , V OC of 0.47 V, FF of 47.36%, leading to a low PCE of 0.24% (Forward). CsBi 3 I 10 thin-film after annealing, the PCE of CsBi 3 I 10 thin-film solar cell was significantly improved, J sc was significantly increased to 4.86 mA/cm 2 , and the final device efficiency reaches 0.84% (Forward). However, CsBi 3 I 10 thin-film after annealing, V oc decreases to 0.38 V, which may be due to the poor morphology of the HTM on the rough surface of the annealed film, which may reduce the maximum voltage of the solar cell, this phenomenon also leads to hysteresis effect. Figure 6f shows the statistics power conversion efficiency of solar cell based on as-deposited and annealed CsBi 3 I 10 thin-film. As shown in Fig. 6f the solar cell based on annealed CsBi 3 I 10 thin-film, although V oc drops to 0.38 V, the solar cell efficiency was significantly improved. Although the photoelectric conversion efficiency of CsBi 3 I 10 thin-film solar cell prepared by single-source thermal evaporation is still lower than that of ordinary Pb-based perovskite solar cells. However, the inorganic Pb-free CsBi 3 I 10 thin films prepared by single-source thermal evaporation have a potential application prospect in the field of photovoltaic.

Conclusions
In summary, we report a facile, efficient and reproducible new method for the fabrication of all inorganic Pb-free perovskite CsBi 3 I 10 thin film prepared by single-source thermal evaporation. Firstly, we prepared CsBi 3 I 10 crystal powder with layered structure. Secondly, CsBi 3 I 10 thin-films were prepared by single-source thermal evaporation. CsBi 3 I 10 films had high purity hexagonal phase and crystallinity, which were consistent with the theoretical calculation results. CsBi 3 I 10 thin-film was uniform,   smooth and non-porous. The surface of TiO 2 was completely covered and the chemical composition was similar to that of precursor powder. In the visible light range, CsBi 3 I 10 thin-film had high optical absorption coefficient, and the band gap of the film is 1.83 eV, which is close to the theoretical value. The PCE of CsBi 3 I 10 perovskite thin-film solar cells prepared by single-source thermal evaporation is 0.84%, which has good performance and reproducibility.
The results show that the inorganic Pb-free CsBi 3 I 10 thin film prepared by single-source thermal evaporation method have potential applications in photovoltaic field.