Defect Engineering via Ag and Na Co-doping in Wide-Bandgap CIGS: From Interfacial Suppression to Bulk Enhancement

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