Achieving over 860 mV Open‐Circuit Voltage in Low‐Ag Wide‐Bandgap Cu(In,Ga)Se2 Solar Cells Through Ion Diffusion and Band Structure Optimization

Abstract Wide‐bandgap Cu(In, Ga)Se 2 (CIGS) solar cells offer excellent thermal stability and significant potential for tandem applications. However, conventional CdS buffer layers exhibit poor band alignment with wide‐bandgap absorbers, resulting in severe interface recombination losses. Similarly, employing Zn(O,S) alone as a buffer layer leads to insufficient surface inversion, limiting its effectiveness. Here, a tailored interface engineering strategy is introduced using a 30 nm Zn(O,S)/15 nm CdS bilayer structure. The high conduction band value of Zn(O,S) establishes an optimal spike contact with the wide‐bandgap absorber, effectively suppressing interface recombination. Simultaneously, Cd ions diffuse from the modified thin CdS film to the Zn(O,S) layer, which sequentially enhances surface inversion and facilitates efficient carrier extraction in synergy with Zn ions. Additionally, Zn(O,S) offers superior absorber surface cleaning effects, ensuring a more uniform phase and potential distribution. Collectively, these improvements contribute to reduced interface recombination losses and lower reverse saturation current, leading to a power conversion efficiency (PCE) of 15.35% and an open‐circuit voltage ( V oc ) of 866 mV in a 1.46 eV wide‐bandgap CIGS device, which is the highest V oc in wide bandgap CIGS solar cells with low Ag content [Ag]/([Ag]+[Cu]) (AAC). This work provides critical insights into the development of high‐efficiency wide‐bandgap CIGS solar cells.