Multi‐Functional Spirobifluorene Phosphonate Based Exciplex Interface Enables Voc Reaching 95% of Theoretical Limit for Perovskite Solar Cells

Abstract Metal halide perovskite solar cells (PSCs) show significant advancements in power conversion efficiency (PCE). However, the open‐circuit voltage ( V OC ) of PSCs is limited by interfacial factors such as defect‐induced recombination, energy band mismatch, and non‐intimate interface contact. Here, an exciplex interface is first developed based on the strategically designed and synthesized two spirobifluorene phosphonate molecules to mitigate V OC loss in PSCs. The exciplex interface constructed by the intimate contact between the multi‐functional molecules and hole transport layer takes the roles to promote the hole extraction by donor–acceptor interaction, passivate coordination‐unsaturated Pb 2+ defects by equipped phosphonate groups, and optimize the energy level alignment. As a result, a record V OC of 1.26 V with a perovskite bandgap of 1.61 eV is achieved, representing over 95% of theoretical limit. This advancement leads to an increase in PCE from 21.29% to 24.12% and improved stability. The exciplex interface paves the way for addressing the long‐standing challenge of V OC loss and promotes the wider application of PSCs.