Influence of Varying Cu Content on Growth and Performance of Ga-Graded Cu(In,Ga)Se2 Solar Cells

Cu(In,Ga)Se₂ thin-film solar cells with Ga-graded absorber layers and a [Cu]/([In] + [Ga]) ratio varying between 0.5 and 1.0 were prepared by coevaporation and investigated. Except for the sample with a final [Cu]/([In] + [Ga]) ratio of 1.0, the samples were Cu-poor at all times during the evaporation. The variation in copper was found to influence the material properties in several ways: 1) Changing the Cu content had a strong impact on In and Ga interdiffusion, resulting in decreased Ga gradients in samples with large Cu deficiency; 2) the Cu-poor Cu(In,Ga)₃Se₅ phase was detected in absorbers with [Cu]/([In] + [Ga]) ratios of 0.65 and below; and 3) the grain size changed significantly with the Cu variation. We observe a trend of reduced solar cell efficiencies for [Cu]/([In] + [Ga]) ratios of 0.65 and below, with an efficiency of 13.4% for the sample with a [Cu]/([In] + [Ga]) ratio of only 0.5, i.e., far from stoichiometry. We tentatively attribute the efficiency loss to a high concentration of point defects caused by the Cu deficiency.