  Tin perovskites are emerging as promising alternatives to their lead-based
counterparts for high-performance and flexible perovskite solar cells (PSCs).
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
formamidium 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 PSCs 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 13,800 hours 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 PSCs.
