Photocapacitance Effects at a Cu2S–CdS Heterojunction

Abstract

The influence of trapped charge on the photovoltaic properties of an efficient Cu 2S–CdS single‐crystal heterojunction has been studied by a photocapacitance technique. For the non‐heat‐treated cell, a persistent increment in capacitance as high as 70% of the dark capacitance remained after illumination of the junction by band‐gap light at 100°K. The additional capacitance is due to holes trapped in deep levels in the CdS depletion region near the interface. The trapped holes enhanced the 100°K photocurrent spectrum uniformly by a factor of 3 before heat treatment. After a 200°C heat treatment in air, the maximum trapped charge at 100°K enhanced the photocurrent by two orders of magnitude. The results are interpreted in terms of the tunneling through a conduction band spike of electrons photoexcited in the Cu 2S. The width of the spike, and thus the tunneling probability, is controlled by the magnitude of the trapped charge. The spike height is estimated to be in the range 13–60 meV.