Synergistic Sulfide–Alkali Surface Reconstruction Engineering Enables High‐Efficiency Cadmium‐Free Kesterite Thin Film Solar Cells

Sulfide kesterite Cu 2 ZnSnS 4 (CZTS) solar cells offer a sustainable route toward thin film photovoltaics, yet their efficiency remains limited by severe open‐circuit voltage loss associated with defective absorber surfaces and unfavorable heterojunction energetics. Here, a K 2 S post‐deposition treatment is reported, which targets the crystallized CZTS surface and simultaneously regulates surface chemistry and interfacial band alignment. Unlike conventional alkali treatments, K 2 S provides a dual‐function chemical environment in which alkaline surface cleaning removes detrimental secondary phases, while K + and S 2− ions participate in defect passivation and surface electronic reconstruction at the CZTS/ZTO interface. This process reduces surface potential fluctuations and alleviates Fermi‐level pinning, leading to a reduced electron‐transport barrier at the CZTS/ZTO junction. These synergistic effects suppress trap‐assisted recombination and facilitate carrier extraction. As a result, the cadmium‐free CZTS device achieves a champion efficiency of 12.10%, with a large‐area (1.02 cm 2 ) device delivering 10.27%. This work highlights sulfide alkali post‐treatment as an effective strategy for interface engineering in pure‐sulfide kesterite photovoltaics.