Unveiling the GeI₂-Assisted Oriented Growth of Perovskite Crystallite for High-Performance Flexible Sn Perovskite Solar Cells

Tin perovskites are emerging as promising alternatives to their lead-based counterparts for high-performance and flexible perovskite solar cells. However, their rapid crystallization often leads to inadequate film quality and poor device performance. In this study, the role of GeI₂ as an additive is investigated for controlling the nucleation and crystallization processes of formamidinium tin triiodide (FASnI₃). The findings reveal the preferential formation of a Ge-rich layer at the bottom of the perovskite film upon the introduction of GeI₂. It is proposed that the initial formation of the Ge complex acts as a crystallization regulator, promoting oriented growth of subsequent FASnI₃ crystals and enhancing overall crystallinity. Through the incorporation of an optimal amount of GeI₂, flexible Sn perovskite solar cells with an efficiency of 10.8% were achieved. Furthermore, it was observed that the GeI₂ additive ensures a remarkable shelf-life for the devices, with the rigid cells retaining 91% of their initial performance after more than 13800 h of storage in an N₂ gas environment. This study elucidates the mechanistic role of GeI₂ in regulating the nucleation and crystallization process of tin perovskites, providing valuable insights into the significance of additive engineering for the development of high-performance flexible tin perovskite solar cells.