Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

Earth‐abundant Cu 2 BaSnS 4 (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 10 4 cm −1 , and a p‐type conductivity, suitable as a top‐cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open‐circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine‐doped SnO 2 (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum‐doped ZnO (front contact).