Wide bandgap Cu(Ga,In)Se2/(Zn,Cd)S heterojunctions

The preparation of thin-film heterojunctions for solar cells consisting of vacuum evaporated Cu(Ga,In)Se₂ absorber and (Zn,Cd)S window layers is discussed. Efficiencies between 7.8% at 30% Ga and 6.7% at 70% Ga are reported. The open-circuit voltage increases according to the bandgap. Reasonable properties are obtained even for large deviations from stoichiometry to the Ga,In-rich side of the phase diagram in comparison to CuInSe₂. Cells with high Ga content deteriorate during air annealing and improve with reducing treatments. Measurements of capacitance, jV-characteristic, and quantum efficiency indicate that the space-charge region is shifted into the window by annealing in air. Since the electrical field in the absorber decreases, fewer photogenerated carriers are collected.