Wide‐Bandgap Cu(In, Ga)S 2 Solar Cell: Mitigation of Composition Segregation in High Ga Films for Better Efficiency

Cu(In, Ga)S 2 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)S 2 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)S 2 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)S 2 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)S 2 as a stable and efficient top‐cell device for tandem photovoltaics.