Effects of silver-doping on properties of Cu(In,Ga)Se2 films prepared by CuInGa precursors

Abstract The AgCuInGa alloy precursors with different Ag concentrations are fabricated by sputtering an Ag target and a CuInGa target. The precursors are selenized in the H2Se-containing atmosphere to prepare (Ag,Cu)(In,Ga)Se2 (ACIGS) absorbers. The beneficial effects of Ag doping are demonstrated and their mechanism is explained. It is found that Ag doping significantly improves the films crystallinity. This is believed to be due to the lower melting point of chalcopyrite phase obtained by the Ag doping. This leads to a higher migration ability of the atoms that in turn promotes grain boundary migration and improves the film crystallinity. The Ga enrichment at the interface between the absorber and the back electrode is also alleviated during the selenization annealing. It is found that Ag doping within a specific range can passivate the band tail and improve the quality of the films. Therefore, carrier recombination is reduced and carrier transport is improved. The negative effects of excessive Ag are also demonstrated and their origin is revealed. Because the atomic size of Ag is different from that of Cu, for the Ag/(Ag + Cu) ratio (AAC) ≥ 0.030, lattice distortion is aggravated, and significant micro-strain appears. The atomic radius of Ag is close to those of In and Ga, so that the continued increase in AAC will give rise to the AgIn or AgGa defects. Both the structural and compositional defects degrade the quality of the absorbers and the device performance. An excellent absorber can be obtained at AAC of 0.015.