ELSEVIER Solar Energy Materials and Solar Cells 41/42 (1996) 295-305 so~ rm~w k mtl .solar cel~ A photoacoustic study on the effect of Se content on defect levels in CulnSe 2 single crystals A. Zegadi a, M.V. Yakushev b, E. Ahmed c, R.D. Pilkington b, A.E. Hill b, R.D. Tomlinson b a Institut d'Electronique, Centre Universitaire M ea KhMer Biskra, B.P. 145, Biskra 07000, Algeria b Electronic and Electrical Engineering Department, Salford University, Salford, UK e Physics Department, BZ University, Multan, Pakistan Abstract The objective of this paper is to investigate the impact of changes in the selenium content of CulnSe 2 single crystals on their optical properties in the subgap region of the infrared spectrum. A high resolution near-infrared photoacoustic spectrometer of the gas-microphone type is used for room temperature analysis of non-radiative defect states in as-grown n- and p-type CIS crystals. Samples with an excess and a deficiency of Se (5% off the stoichiometric composition) were grown from the melt by the vertical Bridgman technique. The absorption coefficient has been derived from photoacoustic spectra in order to establish activation energies for several defect-related energy levels. These results are compared with similar data obtained from near-stoichiometric single crystals which were annealed under maximum and minimum selenium vapour pressures. 1. Introduction In recent years, CulnSe 2 (CIS) and related compounds have been the subject of a vigorous development and study programme stimulated by their potential for use in photovoltaic devices. Although thin-film solar cells employing CulnSe 2 based absorber layers have achieved efficiencies in excess of 16% [1], these devices have yet to reach their peak efficiency limit. This is due in part to a fundamental lack of understanding of the complex intrinsic defect structure in the compound [2]. An efficient solar cell requires good collection of photoexcited carriers, and to this effect control of the defect state populations is essential. Numerous experimental investigations have shown that the electrical properties of the compound are dominated by various types of electrically active intrinsic defects associated with 0927-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-0248(95)00102-6