Improvement of wide-bandgap Cu(In,Ga)Se2 solar cells by Cd-free ZnSnO buffer layers deposited via atomic layer deposition

Wide-bandgap chalcopyrite Cu(In,Ga)Se2 (CIGS) thin-film solar cells are promising candidates for tandem photovoltaic applications due to their high optical absorption and tunable bandgap. However, achieving both a high open-circuit voltage (VOC) and high fill factor (FF) in wide-gap devices remains a significant challenge, primarily due to substantial VOC losses originating from the bulk absorber quality relative to the bandgap, while interface properties with conventional CdS buffer layers also play an important role. In this study, a Cd-free Zn1−xSnxO (ZTO) buffer layer is applied to wide-gap CIGS solar cells, and its interfacial and device properties are systematically investigated. Temperature-dependent J–V analysis suggested a near-flat to spike-type conduction-band alignment at the ZTO/CIGS interface, which effectively suppressed interface recombination and enabled a high VOC. However, a conduction-band well-like structure within the CIGS-absorber layer initially limited short-circuit current density (JSC) and FF. Both JSC and FF were significantly improved using compositional tuning of the absorber to optimize this well-like structure. A maximum conversion efficiency of 13.5% was achieved (VOC: 806 mV, JSC: 24.9 mA cm−2, and FF: 67.3%), surpassing the performance of the simultaneously fabricated CdS reference cell. These results demonstrate the potential of ZTO as a Cd-free buffer layer for high-efficiency wide-gap CIGS solar cells.