Initiating crystal growth kinetics of α-HC(NH₂)₂PbI₃ for flexible solar cells with long-term stability

Flexible and stable energy conversion devices are core components for next generation flexible electronics. Perovskite solar cells using formamidinium lead iodide (FAPbI₃) exhibit superior power conversion efficiency (PCE), but the strenuous process of n-i-p configuration and the instability of α-FAPbI₃ become inevitable obstacles for practical application in flexible electronics. Here, we for the first time report the particular crystal growth process of α-FAPbI₃ grain with a controlled size for the development of flexible perovskite solar cells. Through morphology, element and thermostability measurements, the crystalline size and film homogeneity are found to mainly depending on decent thermal energy and doping density of methylammonium bromine. Additionally, stable "pseudocubic" lattice with larger crystalline size is established owing to the varied tolerance factor. We demonstrated that the flexible solar cells exhibiting an average PCE of 12.41±0.52% (the optimal 13.03%), which slightly decreased to 10.98±1.00% (the optimal 12.0%) over one month at 35% humidity without encapsulation, based on the modified α-FAPbI₃ for most efficient devices. This work favors the understanding of the crystal growth kinetics of α-FAPbI₃ for flexible substrate, and paves the way for potential application of flexible electronics.