Defect engineering via Ag and Na Co-doping in wide-bandgap CIGS: from interfacial suppression to bulk enhancement

The efficiency of wide-bandgap (WBG) CIGS solar cells is restricted by bulk quality degradation and severe heterojunction interface recombination, resulting in high V_oc loss and suboptimal fill factor (FF). Although Ag doping has proven effective in enhancing WBG performance, its synergistic mechanisms with monovalent Na dopants remain poorly understood. This study systematically investigates the cooperative effects of Ag-Na co-doping. The key findings reveal that both dopants reduce the Fermi level and enhance the built-in potential, albeit through distinct pathways: Na increases carrier concentration but concurrently raises interface defect density. In contrast, Ag improves lattice ordering and reduces point defects through isoelectronic substitution at Cu sites to suppress interfacial defects and reduce recombination losses. Although Ag-Na co-doping improves both V_oc and FF, excessive Na incorporation introduces oversized conduction band offset (CBO) barriers, degrading device performance. To address this concern, we propose an Na-depletion strategy (30% lower than the standard Ag-alloyed CIGS Na baseline) under Ag doping, achieving a balanced optimization of bulk carrier transport and interfacial band alignment. The optimized device demonstrates a V_oc of 801.4 mV, J_sc of 27.3 mA cm^-2, FF of 71.3%, and a champion PCE of 15.6%, providing a viable pathway for high-performance WBG CIGS development. Notably, the resulting bandgap energy (1.36 eV) closely aligns with the Shockley-Queisser model optimum.