Copper (II) Chloride Mediated Interfacial Permeation for High‐Efficient and Stable Wide‐Bandgap Perovskite Solar Cells

ABSTRACT Wide‐bandgap (WBG) perovskite solar cells (PSCs) are crucial for high‐efficiency tandem photovoltaics. However, their performance is severely limited by lattice disorder at the buried interface, which leads to the accumulation of deep‐level defects and interfacial morphological inhomogeneities, thereby inducing pronounced non‐radiative recombination and halide segregation. Here, we report a copper (II) chloride (CuCl 2 ) mediated interfacial permeation strategy that achieves the redox mediation and halide coordination, thereby stabilizing the interfacial chemical state, suppressing the formation of deep‐level defects. The strategy effectively suppresses lattice disorder at the buried interface, while simultaneously constructing an efficient hole‐transfer structure, thus enhancing interfacial phase stability and charge‐transport properties. Consequently, our modified 1.79 eV WBG PSCs exhibit minimized non‐radiative recombination losses and superior charge transport, achieving state‐of‐the‐art performance with an open circuit voltage ( V OC ) of 1.35 V, a fill factor of 84.29%, and power conversion efficiency (PCE) of 21.24%, one of the highest reported so far. The device can maintain 91% of its original efficiency after 1400 h of maximum power point tracking.