Abstract:

Chemical components of perovskite layers play a key role in improving the efficiency and stability of perovskite solar cells. Pure inorganic perovskites exhibit good thermal and light stabilities; however, the smaller radius of Cs⁺ leads to a poor perovskite phase stability. In this case, the Cs-rich (CH(NH₂)₂)_xCs_1−xPbI₃ ((CH(NH₂)₂⁺=FA⁺) perovskite seems more promising because it simultaneously offers the above-mentioned properties, while not forming an unstable perovskite phase. Thus far, the synthesis of Cs-rich FA_xCs_1−xPbI₃ perovskite has been realized by introducing excess formamidinium iodide (FAI) as an additive. However, FAI sublimates at a high temperature and excessive FAI sublimation necessitates even greater temperatures. Therefore, it is difficult to precisely control the ratio of the sublimated FAI from the perovskite film. Herein, the precise synthesis of Cs-rich FA_xCs_1−xPbI₃ perovskites at relatively low sublimation temperatures using amine additives, such as methylammonium iodide (MAI), dimethylamine iodide (DMAI), ethylamine iodide (EAI), ammonium iodide (NH₄I), and formamidine acetate (FAAC), was studied. The reaction temperature was reduced when utilizing these additives. Moreover, the window period for the preparation has been widened, which is particularly important for the preparation of pure phase Cs-rich FA_xCs_1−xPbI₃ perovskite films for large devices. In the experiment, perovskite FA_0.15Cs_0.85PbI₃ was selected because of its good stability. The reaction process of the additive that assisted perovskite preparation was studied. Firstly, 0.85 mmol of MAI, DMAI, EAI, FAAC, and NH₄I each were added to 1 mmol of FA_0.15Cs_0.85PbI₃ solution. Then, the precursor solution was spin-coated and thermally annealed. The FA_0.15Cs_0.85PbI₃ films were formed by sublimation of the additives during thermal annealing. The influence of different additives on the film formation process was traced using X-ray diffraction (XRD) measurements and UV-visible absorbance spectra (UV-Vis abs). The results showed that MAI and DMAI could be used as additives in the preparation of FA_0.15Cs_0.85PbI₃ films. The strong intermolecular interaction between these additives and PbI₂ could benefit the formation of Cs₄PbI₆ and prevent the formation of δ-CsPbI₃. Cs⁺ is easier to migrate in Cs₄PbI₆ than in δ-CsPbI₃, which provides a necessary condition for the ion exchange reaction. Simultaneously, the mild sublimation temperature of the additives ensured that the films maintain their perovskite phase. Finally, pure phase Cs-rich FA_xCs_1−xPbI₃ perovskites were prepared using this method at a relatively lower temperature of 200 ℃. The XRD and UV-Vis absorption results confirmed the precise synthesis of FA_0.15Cs_0.85PbI₃. The FA_0.15Cs_0.85 PbI₃ solar cells synthesized with MAI and DMAI achieved the maximum power conversion efficiencies of 15.6% and 15.1%, respectively.

Key words: Inorganic halide perovskite, Formamidine, Doping, Additive, Solar cell