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

Abstract 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.