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

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.