Bismuth-ion doped Cu(In,Ga)Se2 thin films: Preparation, microstructures, and electrical properties

The influence of doping with bismuth ions in the properties of synthesized Cu(In,Ga)Se2 films was investigated via the solution coating process followed by a selenization step. The incorporation of bismuth ions in Cu(In,Ga)Se2 significantly increased the size of the grains in the obtained films. The intermediate compound of bismuth copper oxide was thought to react with selenium vapor to form copper bismuth selenide, which acted as a flux agent at elevated temperatures. The low-temperature photoluminescence spectra of bismuth-containing Cu(In,Ga)Se2 films revealed attenuated intensity of the peak that was attributed to the donor-acceptor pair transition following the incorporation of bismuth. The donor-acceptor transition is caused by the selenium deficiency. Doping with bismuth ions reduced the amounts of the selenium deficiencies in the films. The incorporation of bismuth ions increased the open-circuit voltage and the fill factor of solar cells by increasing the carrier concentration and reducing the resistivity. The efficiency of solar cells increased from 4.37 to 6.29% as bismuth-ion doping f was increased from 0 to 1.0 mol%, revealing that the properties of Cu(In,Ga)Se2 absorber layers were effectively improved via the addition of bismuth ions.