Bulk and interface engineering of 1.7 eV-bandgap chalcogenide solar cells enabling record efficiency

Wide-bandgap chalcogenide photovoltaics offer strong potential for tandem solar cells and solar-driven hydrogen generation via water splitting, yet their performance remains limited by persistent interfacial and bulk defects. Here, we demonstrate enhanced efficiency in 1.7-electron volt CuGaSe₂ thin-film solar cells through aluminum (Al) alloying and rubidium (Rb) incorporation. The Al- and Rb-modified CuGaSe₂ absorber exhibits fundamentally distinct interfacial chemistry, structural properties, and metastable defect behavior compared with conventional narrow-bandgap Cu (In,Ga)Se₂ solar cells. Moreover, we demonstrate that introducing a steep Al concentration gradient to engineer a back-surface electric field is a highly effective means of boosting device performance, even at an Al content of approximately 1 atomic % or less. By integrating these strategies, we achieve a higher open-circuit voltage without compromising photovoltaic efficiency, establishing a performance benchmark for wide-bandgap (1.65 to 1.75 electron volts) chalcogenides. These findings highlight the unique characteristics of wide-bandgap chalcopyrites and suggest a promising pathway toward next-generation, high-efficiency photovoltaic technologies.