Multifunctional Ammonium Sulfide Enables Highest Efficiency Lead–Tin Perovskite Solar Cells

The commonly used hole transport layer (HTL) Poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) still has some shortcomings that will limit the development of Pb–Sn perovskite solar cells (PSCs). In this work, multifunctional ammonium sulfide (AS) is attempted to modify PEDOT:PSS/perovskite interface. AS has multiple effects on different functional layers and the device as a whole: a) optimize the perovskite crystallization by the formation of PbS nucleation sites; b) neutralize the acidity of PEDOT:PSS by reaction with PSS; c) promote carrier transport at HTL/perovskite interface by tuning the energy level of PEDOT:PSS. As a result, a power conversion efficiency of 24.23% is obtained in the reverse scanning direction and 23.84% in the steady‐state output power test for the single‐junction Pb–Sn PSCs, and 27.48% for all‐perovskite tandem solar cells. These results show that AS modification can be an effective way to boost the development of Pb–Sn PSCs. Weakly basic ammonium sulfide ((NH4)2S, AS) is employed to modify PEDOT:PSS/Pb‐Sn perovskite interface. AS can react with both PbI2 and PEDOT:PSS to regulate perovskite crystallization, optimize PEDOT:PSS performance, and promote interface carrier transport. Consequently, single‐junction Pb–Sn and all‐perovskite tandem solar cells obtained power conversion efficiencies of 24.23% and 27.48%, respectively.