Reconstruction of the Buried Interface of Triple‐Halide Wide‐Bandgap Perovskite for All‐Perovskite Tandems

All-perovskite tandem solar cells (TSCs) paired by wide-bandgap (WBG) perovskites with narrow-bandgap perovskites holds the potential to overcome the Shockley-Queisser limitation. However, the severe phase segregation and non-radiative recombination of WBG perovskite put on a shadow for their power conversion efficiency and stability. Here, an interfacial engineering strategy is introduced into the triple-halide WBG perovskite. Potassium trifluoromethanesulfonate (TfOK) is utilized to reconstruct the buried interface of the triple-halide WBG perovskite. The distribution of (chlorine) Cl^- changes from perovskite bulk toward the buried interface due to the TfOK addition. Therefore, a wider bandgap perovskite thin layer is formed at buried layer, which can form a graded heterojunction with bulk WBG perovskite to improve carrier separation and transfer. Meanwhile, the (potassium) K⁺ of TfOK diffuses into WBG perovskite bulk to suppress halide phase segregation. Consequently, the 1.78 eV WBG PSCs deliver an impressive power conversion efficiency of 20.47% and an extremely high fill factor over 85%. Furthermore, the resultant two-terminal all-perovskite TSCs achieves a champion efficiency of 28.30%. This strategy provides a unique avenue to improve performance and photostability of WBG PSCs, a new function of Cl^- in triple-halide is illustrated.