Copper‐Poor Copper Sulfide Enables High‐Efficiency and Stable Perovskite Solar Cells via Interface Modification

Abstract The copper content in copper sulfide significantly influences the material properties, potentially affecting interfacial non‐radiative recombination in perovskite solar cells. Here, a simple ball‐milling strategy is used to obtain Cu‐poor (cupric sulfide, CuS) and Cu‐rich (cuprous sulfide, Cu 2 S) nanoparticles (in which CuS and Cu 2 S are the dominant phases respectively, containing a mixture of CuO/Cu(OH) 2 species) from commercial cupric‐ and cuprous‐sulfide powders respectively. The resulting Cu‐poor nanoparticles exhibit a well‐crystallized hexagonal structure, suppressing the formation of harmful CuO/Cu(OH) 2 byproducts. Additionally, the Cu‐poor nanoparticles uniformly cover the perovskite, effectively addressing residual PbI 2 at grain boundaries and constructing a hole‐transport channel at the interface between perovskite and spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N, N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene]. In contrast, the Cu‐rich nanoparticles exhibit a non‐crystalline structure with excessive CuO/Cu(OH) 2 byproducts. Their aggregation and non‐uniform morphology on the perovskite surface likely result from the charge‐controlled interactions from the big bond polarity in cupric sulfide. Moreover, the presence of excessive CuO/Cu(OH) 2 byproducts increases hydrophilic sites, weakening interfacial defect passivation and compromising device performance. Consequently, perovskite solar cells using copper‐rich CuS interface passivation layers achieved a champion power conversion efficiency (PCE) of over 25% and maintained ≈87% of the initial PCE after 2,300 h of storage in air, outperforming copper‐rich Cu 2 S‐based devices.