Acidity Control of Interface for Improving Stability of All‐Perovskite Tandem Solar Cells

Abstract Developing all‐perovskite tandem solar cells has been proved to be an effective approach to boost the efficiency beyond the Shockley–Queisser limit. However, the Sn‐based narrow‐bandgap (NBG) perovskite solar cells (PSCs) suffer from the relatively low photostability, which limits their further application in all‐perovskite tandem solar cells. In this work, the instability of NBG PSCs is found to come from the commonly used acidic hole transporting material PEDOT:PSS, which reacts with the indispensable basic additive SnF₂ in the perovskite layer. By acidity control of PEDOT:PSS via aqueous ammonia, the NBG PSCs yield an efficiency of 22.0% with much improved photostability, which can maintain 91.3% of the initial value after 800 h illumination under AM 1.5G. As an application, the corresponding all‐perovskite tandem cells exhibit a stabilized efficiency of 25.3% with 92% remaining after 560 h illumination. This work reveals an origin of instability of NBG PSCs and provides an effective approach to enhance the device stability, which can promote the development of all‐perovskite tandem solar cells.