Pure 2D Perovskite Formation by Interfacial Engineering Yields a High Open‐Circuit Voltage beyond 1.28 V for 1.77‐eV Wide‐Bandgap Perovskite Solar Cells

Abstract Surface post‐treatment using ammonium halides effectively reduces large open‐circuit voltage ( V OC ) losses in bromine‐rich wide‐bandgap (WBG) perovskite solar cells (PSCs). However, the underlying mechanism still remains unclear and the device efficiency lags largely behind. Here, a facile strategy of precisely tailoring the phase purity of 2D perovskites on top of 3D WBG perovskite and realizing high device efficiency is reported. The transient absorption spectra, cross‐sectional confocal photoluminescence mapping, and cross‐sectional Kelvin probe force microscopy are combined to demonstrate optimal defect passivation effect and surface electric‐field of pure n = 1 2D perovskites formed atop 3D WBG perovskites via low‐temperature annealing. As a result, the inverted champion device with 1.77‐eV perovskite absorber achieves a high V OC of 1.284 V and a power conversion efficiency (PCE) of 17.72%, delivering the smallest V OC deficit of 0.486 V among WBG PSCs with a bandgap higher than 1.75 eV. This enables one to achieve a four‐terminal all‐perovskite tandem solar cell with a PCE exceeding 25% by combining with a 1.25‐eV low‐bandgap PSC.