All-inorganic halide perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their heat-sensitive hybrid organic-inorganic counterparts. However, controlling the crystallinity and morphology of all-inorganic perovskites is a significant challenge. Furthermore, tuning the composition of the perovskites between CsPbI₃ and CsPbBr₃, there is a trade-off between high phase stability and low bandgap energy, both of which are desirable for solar cell applications. CsPbI₂Br with a bandgap energy of about 1.9 eV and a Goldschmidt tolerance factor of 0.84 shows the highest potential for developing thermally stable perovskite solar cells (PSCs) among all-inorganic compositions. A typically used method for CsPbI₂Br film crystallization is the so-called thermal gradient (TG) annealing, which consists of several annealing steps at different temperatures ranging from 50 to 160°C. As this procedure is rather time- and resource-intensive, it is crucial to establish a facile one-step synthesis method for high-quality all-inorganic perovskite films and further advance the field.

Recently, Malekshahi Byranvand et al. [1] introduced such a simple method, called spin-force (SF) method, to precisely control the formation pathway, i.e. nucleation and growth processes, achieving a homogenous, pinhole-free perovskite film with large grain sizes. Here, we use in situ absorbance measurements to investigate the dynamic film formation during spin coating and annealing to understand and optimize the evolving film properties. Comparing films prepared by the TG and the SF method as well as by varying the process parameters of the latter method, we investigate the formation pathways of the perovskite. Based on the in situ absorbance data, we propose that the amount of solvent retained in the film after spin coating crucially influences the film evolution and eventual quality. By optimizing the duration of the spin coating step in the SF method, we accordingly improve the film formation process and validate it in complete solar cell devices. The fabricated n-i-p PSCs using the optimized synthesis conditions showed a high V_OC of 1.27 V and a power conversion efficiency of 15.7%. Furthermore, CsPbI₂Br perovskite films with excellent thermal stability at 300°C for 1 h were demonstrated.

[1] M. Malekshahi Byranvand et al., Submitted to Joule, 2021.