Grain‐Boundary Passivation by Cation Reaction Minimizes VOC-deficit of 1.8 eV Perovskites for Efficient All-Perovskite Tandem Solar Cells

Abstract

The anticipated high open-circuit voltage (V_OC) of Br-rich wide-bandgap (WBG) perovskite solar cells (PSCs) look bright to enhance the performance of all-perovskite tandem solar cells (APTSCs). However, the inherent photoinduced halide segregation and high V_OC loss of WBG PSCs severely restrict their development. Herein, a cation reaction between the mixed formamidinium hydrochloride (FACl) and methylammonium hydrochloride (MACl) is proposed to address the issues and the produced N-methylformamidinium (MFA⁺) chloride efficiently reacts with the residual PbI₂ to clean grain boundaries (GBs) and passivate the defects, thereby significantly reducing non-radiative recombination losses at the interfaces. Ultimately, a 1.80 eV single-junction WBG device achieves a champion efficiency of 19.52%, with a high V_OC of 1.366 V, exceeding 90% of the radiative limit. Meanwhile, the hysteresis of the target device (0.67%) is almost negligible compared to the control device (3.33%). By integrating with 1.23 eV narrow-bandgap PSC, an APTSC with an efficiency of 27.25% is prepared with long-term operational stability exceeding 500 h (~80% initial efficiency).