Heteroepitaxy with PbSe Nanocrystals Enables Highly Stable Wide‐Bandgap Perovskite Solar Cells

Abstract The perovskite‐based tandem solar cell is one of the promising technological pathways to achieve high efficiency. However, the mixed‐halide perovskite top cell is prone to phase segregation under continuous illumination, which leads to rapid degradation of the tandem device's overall power output. To tackle this challenge, ligand‐free lead selenide (PbSe) nanocrystals are introduced into the precursor solution to promote the heteroepitaxial growth of mixed‐halide perovskite. The incorporation of PbSe results in a high‐quality perovskite film with excellent uniformity and low defect density, effectively suppressing halide phase segregation. This improved perovskite thin film enables the fabrication of wide‐bandgap (1.68 eV) perovskite Cs 0.05 (FA 0.77 MA 0.23 ) 0.95 Pb(I 0.77 Br 0.23 ) 3 p ‐i‐ n devices, achieving a power conversion efficiency (PCE) of 22.87% and a fill factor (FF) of 84.79%. After 1000 h of maximum power point (MPP) tracking under 1‐sun continuous illumination, the perovskite solar cells retain 88% of their initial efficiency. Additionally, by mechanically stacking the semi‐transparent perovskite on copper indium gallium selenide (CIGS) solar cells, the 4‐terminal tandem cell has demonstrated a PCE of 28.24%.