Strain Regulation of Mixed‐Halide Perovskites Enables High‐Performance Wide‐Bandgap Photovoltaics

Wide‐bandgap mixed‐halogen perovskite materials are widely used as top cells in tandem solar cells. However, serious open‐circuit voltage (Voc) loss restricts the power conversion efficiency (PCE) of wide‐bandgap perovskite solar cells (PSCs). Herein, it is shown that the resulting methylammonium vacancies induce lattice distortion in methylammonium chloride‐assisted perovskite film, resulting in an inhomogeneous halogen distribution and low Voc. Thus, a lattice strain regulation strategy is reported to fabricate high‐performance wide‐bandgap PSCs. Rubidium (Rb) cations are introduced to fill the A‐site vacancy caused by the methylammonium volatilization, which alleviates shrinkage strain of the perovskite crystal. The reduced lattice distortion and increased halide ion migration barrier result in a homogeneous mixed‐halide perovskite film. Due to improved carrier transport and suppressed nonradiative recombination, the Rb‐treated wide‐bandgap PSC (1.68 eV) achieves an excellent PCE of 21.72%, accompanied by a high Voc of 1.22 V. The resulting device maintains more than 90% of its initial PCE after 1500 h under 1‐sun illumination conditions. The Rb+ fills the A‐site vacancy caused by the MA volatilization, relieving shrinkage strain of perovskite crystal. The reduced lattice distortion increases the halide migration barrier, resulting a homogeneous mixed‐halide perovskite phase, and suppressing nonradiative recombination. As a result, the Rb‐treated wide‐bandgap perovskite solar cell exhibits an improved power conversion efficiency and stability.