3D interaction of zwitterions for highly stable and efficient inorganic CsPbI${}_3$ solar cells

All-inorganic cesium lead iodide (CsPbI${}_3$) perovskites, which replace volatile and hygroscopic organic components with stable inorganic cesium cations, have promising photoelectronic properties for potential application in solar cells. However, highly stable and efficient CsPbI${}_3$-based perovskite solar cells are rarely reported because the optically active black phases of CsPbI${}_3$ tend to change into a photo-inactive yellow $\delta$-phase. Herein, a highly stable CsPbI${}_3$ film that is formed by introducing a small quantity of zwitterions with different dimensions to the perovskite precursor solution is reported. The zwitterions effectively inhibit the formation of the yellow $\delta$-phase during perovskite crystallization and promote the development of a stable black $\alpha$-phase. In addition, a systematic analysis reveals strong interaction between 3D zwitterions and perovskites in both the solution and film states, which leads to a dense and pinhole-free CsPbI${}_3$ film with suppressed trap states. The resulting perovskite solar cells with 3D zwitterions achieve a significantly improved power conversion efficiency of 18.4% with high reproducibility. The devices without encapsulation retain 98% of the initial efficiency after 25 days at 25 °C and relative humidity of 25% ± 5%. Importantly, the 3D zwitterion-based devices demonstrate excellent phase stability when subjected to thermal aging at 100 °C. (© 2021 Wiley‐VCH GmbH)