CuInSe₂ (CIS) thin film solar cells by direct coating and selenization of solution precursors

CuInSe₂ (CIS) absorber layer was formed by a direct nonvacuum coating and a subsequent selenization of precursor solutions of Cu(NO ₃)₂ and InCl₃ dissolved in methanol. The viscosity of precursor solutions was adjusted by adding ethyl-cellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu²⁺ ions in the starting solution were reduced to Cu⁺, resulting in precursor films consisting of CuCl crystals and amorphous In compound embedded in EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In₂Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absorber film and the devices exhibited reproducible conversion efficiency as high as about 2%.