Self‐Sacrificing Polymer Interlayer Suppress Buried‐Interface Se Depletion for High‐Efficiency Cu ₂ ZnSn(S, Se) ₄ Photovoltaics

Suppressing the formation of secondary phases during selenization has long been recognized as essential for achieving high‐efficiency Cu 2 ZnSn(S, Se) 4 (CZTSSe) photovoltaics. However, Se depletion arising from the kinetic imbalance between Se supply and consumption at the buried interface significantly increases the propensity of secondary phase formation. Here, we introduce a self‐sacrificing polymethyl methacrylate (PMMA) interlayer to mitigate Se depletion, thereby avoiding the formation of secondary phases and improving device performance. During selenization, the PMMA layer inhibits the competitive reaction of the Mo substrate for Se, alleviates local Se deficiency, and promotes a direct one‐step phase conversion into CZTSSe without generating secondary phases. As selenization proceeds, the PMMA layer gradually decomposes and disappears, leaving no additional barrier to carrier transport across the buried interface. We provide direct visualization of the composition and spatial distribution of secondary phases formed under Se‐depletion conditions at the buried interface, including ZnS, Zn x Sn 1‐x S, SnSe, and Cu 2 SnS(Se) x . Transient photocarrier dynamics analysis verifies enhanced carrier extraction at the buried interface upon introduction of the PMMA buried layer. Incorporating a PMMA buried layer enhances the device efficiency from 12.81% to 13.91%. This work provides new strategies for controllable phase transformation at buried interfaces in CZTSSe and related chalcogenide photovoltaic systems.