Photocapacitance Effects at a Cu2S–CdS Heterojunction

Abstract

The influence of trapped charge on the photovoltaic properties of an efficient Cu2S–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 Cu2S. 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.

Interface effects in Cu2S-CdS photovoltaic cells formed by the dry barrier process

“…The thresholds for infrared quenching (figure 3) show that the binding energies of the ground and excited hole states of the copper acceptor are at 1.1 and 0.35 eV above the valence band respectively. This is in excellent agreement with the conclusions of Grimmeiss et a1 (1981), and in reasonable agreement with the results of Lindquist and Bube (1972).…”

“…However, when the experiment was repeated at 85 K the threshold had shifted to above 1.1 eV. A similar effect was reported by Lindquist and Bube (1972). The quenching at 85 K is attributed to the excitation of an electron from the valence band to the ground state of the Cu2acceptor.…”

In situ preparation and interface characterization of TiO2/Cu2S heterointerface

“…1,2 Cu 2 S thin films as one of the most promising absorbing materials have been widely investigated as represented in the CdS-Cu 2 S system during recent decades. [3][4][5][6] The stability of this system has been proven to be a fatal problem due to the diffusion of Cu into the CdS layer, which resulted in a degradation of photovoltaic performance. However deposition of Cu 2 S onto TiO 2 surface as an alternative substrate would be expected to produce a stable device without such interface diffusion.…”

Copper diffusion and photovoltaic mechanisms at Cu-CdS contact

“…Figure 9 shows the C-2 ( V ) curves under white light illumination for a Cu-CdS thin film sample after one year storage at 20 "C. When the diodes are illuminated, with white light or intrinsic radiation, an increase of the capacitance is observed related to the narrowing of the depletion layer in the CdS by the trapped holes (Lindquist and Bube 1972). The variation of the C-2(V) curve is characterised by a modification of the slope for all samples, and a slow parallel displacement for the diodes kept at ambient temperature.…”