Wide-Bandgap Cu(In, Ga)S₂ Solar Cell: Mitigation of Composition Segregation in High Ga Films for Better Efficiency

Cu(In, Ga)S₂ demonstrates potential as a top cell material for tandem solar cells. However, achieving high efficiencies has been impeded by open-circuit voltage (V_OC) deficits arising from In-rich and Ga-rich composition segregation in the absorber layer. This study presents a significant improvement in the optoelectronic quality of Cu(In, Ga)S₂ films through the mitigation of composition segregation in three-stage co-evaporated films. By elevating the substrate temperature during the first stage, the intermixing of In and Ga is promoted, leading to reduced Cu(In, Ga)S₂ composition segregation. Furthermore, the optimization of Cu-excess during the second stage minimizes non-radiative voltage loss. These combined strategies yield quasi-Fermi level splitting exceeding 1 eV and a record V_OC of 981 mV in Cu(In, Ga)S₂ devices. Consequently, a champion device achieves an in-house power conversion efficiency (PCE) of 16.1% (active area) and a certified PCE of 14.8%, highlighting the potential of Cu(In, Ga)S₂ as a stable and efficient top-cell device for tandem photovoltaics.