Dark currents in Cu2S–CdS single crystal heterojunctions. Consequences for photocells

An investigation is made of the dark-current conduction processes in Cu2S–CdS single crystal heterojunctions. The reverse currents are multistep tunneling currents as in thin film Cu2S–CdS photocells, but the number of required steps is smaller. For the forward currents, thermal currents over the barrier are dominant above 300 K, and tunneling currents appear only at lower temperatures. It follows that crystallographic defects enhance the tunneling currents, and that high efficiency solar photocells have to be made with recrystallized CdS layers. A band profile is given, and a photovoltaic mechanism is proposed for the Cu2S–CdS heterojunctions.