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The quality of perovskite films plays a crucial role in solar cell, which can affect the stability and power conversion efficiency (PCE). As one of inorganic perovskites with excellent stability, CsPbBr3 perovskite is usually prepared by multi-step method due to the large difference in solubility between its precursor salts (PbBr2 and CsBr). The main reason is that the formation mechanism of CsPbBr3 film is not thoroughly studied. The incomplete reaction of PbBr2 and emergence of Cs4PbBr6 when the CsBr is excessive become problems that need to be solved urgently. In this paper, the phase transition of films during spin coating is observed in detail. In the process of film formation, the CsBr diffuses into the predeposited PbBr2 film to complete the reaction. The short reaction time results in insufficient reactions inside the film but overreaction on the surface of film. The CsPb2Br5 and Cs4PbBr6 appear with CsPbBr3 perovskite, and the film formed by repetitively annealing blocks the diffusion of CsBr. Methanol has an etching effect on the perovskite film which can eliminate the blocking effect. By extending the reaction time of CsBr solution on the film surface, the PbBr2 in the bottom layer is fully reacted, and after being annealed, the perovskite film will recrystallize to form a compact film. With the reaction time controlled appropriately, the CsPb2Br5 in the film can be effectively reduced and Cs4PbBr6 will not appear. The film grain size increases, grain boundary decreases, and the recombination is effectively inhibited, which ensures the improvement of the photoelectric performance of the solar cell. Under the condition of spin-coating four times and reaction time of 30 s, the solar cell has 6.30% PCE, Voc = 1.28 V, Jsc = 8.40 mA/cm2, FF = 0.59 . Comparing with the solar cells with no extended reaction time, the PCE improves more than 18%. This work will provide an important insight into the growth mechanism of perovskite film toward high crystallinity and less defects.
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Keywords:
- inorganic perovskite /
- CsPbBr3 /
- multi-step spin-coating method /
- formation mechanism /
- reaction time
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图 1 (a) 多步旋涂法示意图; (b) 不同旋涂次数钙钛矿薄膜照片
Figure 1. (a) Schematic of multi-step spinning method; (b) photographs of perovskite films with different times of spin coating.
图 2 不同旋涂次数CsPbBr3钙钛矿薄膜的(a) XRD图、(b) 紫外吸收光谱图、(c) Tauc图和 (d) PL光谱
Figure 2. (a) XRD patterns, (b) UV-vis absorption spectra, (c) Tauc plots of (αhν)2 vs. the photo energy, and (d) PL spectra of CsPbBr3 perovskite films with different spin-coating times.
图 3 不同旋涂次数制备的CsPbBr3钙钛矿薄膜SEM图 (a) 旋涂3次; (b) 旋涂4次; (c) 旋涂5次; (d) 旋涂6次; (e) 旋涂7次; (f) 旋涂4次时薄膜的截面
Figure 3. The SEM images CsPbBr3 perovskite films with different spin coating times: (a) 3 times; (b) 4 times; (c) 5 times; (d) 6 times; (e) 7 times; (f) cross-section image of the film with 4 times.
图 4 CsPbBr3钙钛矿薄膜的形成机理图
Figure 4. Formation mechanism of CsPbBr3 in multi-step spin-coating.
图 5 (a) 不同反应时间下CsPbBr3钙钛矿薄膜照片; (b)—(h) 未退火的薄膜SEM图; (i)—(o) 退火后的薄膜SEM图. 标尺1 μm
Figure 5. (a) Images of as-prepared films with varied CsBr solution reaction time; (b)–(h) SEM images of unannealed films; (i)–(o) SEM images of annealed films. All films spin-coating four times. Scale bar: 1 μm.
图 6 不同反应时间下CsPbBr3钙钛矿薄膜的(a) XRD图、(b) 紫外吸收光谱、(c) Tauc图和 (d) PL光谱
Figure 6. (a) XRD patterns, (b) UV-vis absorption spectra, (c) Tauc plots of (αhν)2 vs. the photo energy, and (d) steady-state PL of the cesium lead bromide films deposited on FTO substrates with varied CsBr solution reaction time.
图 7 (a) 不同反应时间下的CsPbBr3钙钛矿薄膜器件J-V曲线; (b) CsPbBr3电池Nyquist 图, 插图为等效电路图及相关参数; (c) 暗态下结构为FTO/TiO2/CsPbBr3/PC61BM/Ag的器件的J-V曲线
Figure 7. (a) J-V curves of CsPbBr3 perovskite solar cell based on different reaction time; (b) Nyquist plots of CsPbBr3 PSCs under 1 sun illumination, the inset provides the equivalent circuit and relevant parameter; (c) J-V curves of the device with an architecture of FTO/TiO2/CsPbBr3/PC61BM/Ag under dark conditions.
表 1 不同反应时间下CsPbBr3钙钛矿薄膜的电池器件J-V参数
Table 1. J-V parameters of CsPbBr3 perovskite for solar cell with different reaction time.
反应时间/s Jsc/(mA·cm–2) Voc/V FF PCE/% 0 8.78 1.17 0.52 5.32 10 8.51 1.22 0.56 5.86 20 7.95 1.20 0.58 5.55 30 8.40 1.28 0.59 6.30 40 7.94 1.03 0.52 4.22 50 6.28 1.08 0.59 4.02 60 4.19 1.03 0.48 2.09 
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[1] National Renewable Energy Laboratory. Best Research-Cell Efficiencies https://www.nrel.gov/pv/cell-efficiency.html [2022-01-24]
[2] Min H, Lee D Y, Kim J, et al. 2021 Nature 598 444
Google Scholar
[3] Colsmann A, RöhmA H 2020 Energy Technol. 8 2000912
Google Scholar
[4] Abdulrahim S M, Ahmad Z, Bhadra J, Al-Thani N J 2020 Molecules. 25 5794
Google Scholar
[5] Wei J W, Huang F R, Wang S N, et al. 2018 Mater. Res. Bull. 106 35
Google Scholar
[6] Yu S S, Liu H L, Wang S R, Zhu H W, Dong X F, Li X G 2021 Chem. Eng. J. 403 125724
Google Scholar
[7] Zhu C, Yang Y, Lin F, Luo Y, Ma S, Zhu L, Guo X 2021 Rare Met. 40 2402
Google Scholar
[8] Yang Y, Chen T, Pan D, Gao J, Zhu C, Lin F, Zhou C, Tai Q, Xiao S, Yuan Y, Dai Q, Han Y, Xie H, Guo X 2020 Nano Energy 67 104246
Google Scholar
[9] Cheng N, Li W, Zhang M, Wu H, Sun S, Zhao Z, Xiao Z, Sun Z, Zi W, Fang L 2019 Curr. Appl. Phys. 19 25
Google Scholar
[10] Hu Y, Bai F, Liu X, Ji Q, Miao X, Qiu T, Zhang S 2017 ACS Energy Lett. 2 2219
Google Scholar
[11] Zhang J, Bai D, Jin Z, Bian H, Wang K, Sun J, Wang Q, Liu S 2018 Adv. Energy Mater. 8 1703246
Google Scholar
[12] Bai D, Bian H, Jin Z, Wang H, Meng L, Wang Q, Liu S 2018 Nano Energy 52 408
Google Scholar
[13] Lin F, Yang Y, Zhu C, Chen T, Ma S, Luo Y, Zhu L, Guo X 2020 Acta Phys. Chim. Sin. 37 2005007
Google Scholar
[14] Kulbak M, Cahen D, Hodes G 2015 J. Phys. Chem. Lett. 6 2452
Google Scholar
[15] Duan J, Zhao Y, He B, Tang Q 2018 Angew. Chem. 57 3787
Google Scholar
[16] Liu X, Tan X, Liu Z, Ye H, Sun B, Shi T, Tang Z, Liao G 2019 Nano Energy 56 184
Google Scholar
[17] Teng P, Han X, Li J, Xu Y, Kang L, Wang Y, Yang Y, Yu T 2018 ACS Appl. Mater. Interfaces 10 9541
Google Scholar
[18] Lan H, Xiao H, Zhao J, Chen X, Fan P, Liang G 2021 Mater. Sci. Semicond. Process. 132 105869
Google Scholar
[19] Lei J, Gao F, Wang H, Li J, Jiang J, Wu X, Gao R, Yang Z, Liu S 2018 Sol. Energy Mater. Sol. Cells 187 1
Google Scholar
[20] Wang H, Wu Y, Ma M, Dong S, Li Q, Du J, Zhang H, Xu Q 2019 ACS Appl. Energy Mater. 2 2305
Google Scholar
[21] Yang X, Li M, Jiang J, Ma L, Tang W, Xu C, Cai H L, Zhang F M, Wu X S 2021 J. Phys. D 54 154001
Google Scholar
[22] Li H, Tong G, Chen T, Zhu H, Li G, Chang Y, Wang L, Jiang Y 2018 J. Mater. Chem. A 6 14255
Google Scholar
[23] Saidaminov M I, Almutlaq J, Sarmah S, Dursun I, Zhumekenov A A, Begum R, Pan J, Cho N, Mohammed O F Bakr O M 2016 ACS Energy Lett. 1 840
Google Scholar
[24] Ryu J, Yoon S, Lee S, Lee D, Parida B, Kwak H W, Kang D W 2021 Electrochim. Acta 368 137539
Google Scholar
[25] Zhang X, Jin Z, Zhang J, Bai D, Bian H, Wang K, Sun J, Wang Q, Liu S F 2018 ACS Appl. Mater. Interfaces. 10 7145
Google Scholar
[26] Jiang Y, Juarez-Perez E J, Ge Q, Wang S, Leyden M R, Ono L K, Raga S R, Hu J, Qi Y 2016 Mater. Horiz. 3 548
Google Scholar
[27] Ding Y, He B, Zhu J, Zhang W, Su G, Duan J, Zhao Y, Chen H, Tang Q 2019 ACS Sustainable Chem. Eng. 7 19286
Google Scholar
[28] Zhou F, Liu H, Wang X, Shen W 2017 Adv. Funct. Mater. 27 1606156
Google Scholar
[29] Li H, Guo L, Li C N, Wang C, Wang G, Wen S, Wu J, Dong W, Li Z J, Ruan S 2019 ACS Sustainable Chem. Eng. 7 8579
Google Scholar
[30] Saidaminov M I, Haque M A, Almutlaq J, Sarmah S, Miao X H, Begum R, Zhumekenov A A, Dursun I, Cho N, Murali B 2017 Adv. Opt. Mater. 5 1600704
Google Scholar
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