Lead Derivative‐Based Precursor Engineering Enables Halogen‐Uniform Perovskite Solar Cells with Enhanced Stability and Mechanical Tolerance

Perovskite solar cells (PSCs) with supreme opto-electrical properties and solution-processability have attracted tremendous interest. To realize state-of-the-art efficiencies in PSCs, delicate control of bandgap (E _g) is required, which generally involves using mixed halogens. This, however, can result in unfavorable phase segregation to negatively influence on the target efficiency and long-term stability. Herein, a viable precursor method is demonstrated for preparing halide-uniform perovskites based on lead derivatives of nPbI₂:1PbXA. It is found that nPbI₂:1PbXA enables tuning the bonding preference and strength between PbI₂ and PbBr₂ in the precursor, leading to generating stable -I-Br-I-Br- fragments, which eventually minimizes halide segregation in the perovskite. The precursor approach have been applied to a series of wide-bandgap mixed halide perovskites, achieving boosted efficiencies of 21.3% and 20.3% in CsPbI_2.8Br_0.2 (bandgap of 1.74 eV) and Cs_0.2FA_0.8I_1.9Br_1.1 (bandgap of 1.77 eV) based solar cells. Interestingly, the connection between the modified halide homogeneity and mechanical tolerance is found: the better the uniformity in the halide distribution, the higher the mechanical resistance of the perovskite to compressive or bending forces. The solar cells with modified halogen uniformity exhibit impressive long-term stability, with the retention of >90% of the initial efficiencies after 1500 h of continuous illumination under maximum power point tracking.