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.

Development of tin (Sn)-based perovskite solar cells (PSCs) largely lags behind that of lead counterparts due to fast crystallization process of Sn perovskite and numerous defects in both bulk and surface of Sn perovskite films. Herein, this work reports a facile strategy of introducing 4-fluorobenzylammonium iodide (FBZAI) as additives into Sn perovskite precursor to synergistically modulate the roles of benzylamine and fluorine in Sn-based PSCs. Incorporation of FBZAI can increase crystallinity, passivate defects, and inhibit the oxidation of Sn²⁺, leading to suppression of nonradiative recombination and enhancement of charge transport and collection in devices. As a result, the best-performing Sn-based PSC with the FBZAI additive achieves the maximum PCE of 13.85% with the enhanced fill factor of 77.8% and open-circuit voltage of 0.778 V. Our unencapsulated device exhibits good stability by maintaining 95% of its initial PCE after 160 days of storage.