Cu(In,Sn)Se2 thin films for absorption of energy from infrared radiation

In this study, we sought to lower the bandgap of thin film solar cells by replacing the Ga used in the absorber layer of Cu(In,Ga)Se2 with Sn (bandgap of 0.07 eV) to form Cu(In,Sn)Se2. The proposed scheme was shown to reduce the bandgap of the absorber layer from 1.0 eV to 0.88 eV. Sn films of various thicknesses were deposited using precursors of Sn-In-Cu metal in order to study the effects of Sn/(In + Sn) ratio (SIR) on the structure of the material and photoelectrical characteristics of the Cu(In,Sn)Se2 absorber layer. Experiment results revealed that a higher SIR following selenization increased the grain size and surface roughness of the absorber layer. It increased the quantity of secondary phases of SnSe2 and Cu2SnSe3 and improved the distribution of Cu and In in the absorber layer. A higher SIR was also shown to increase electron mobility while decreasing carrier concentration and conductivity. When SIR≧0.25, the replacement of In3+ with Sn4+in the Cu+ vacancies decreased the electron strength of In. We speculate that an increase in SIR caused a relative increase in the quantity of Sn2+ compared to Sn4+, thereby increasing the electron strength of Sn and switching the absorber layer from a p-type to an n-type semiconductor.