The Optical Properties of the Interface Layer in CdS-CdTe Heterojunctions
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The Optical Properties of the Interface Layer in CdS-CdTe Heterojunctions Sergiu A. Vatavu Faculty of Physics, Moldova State University, 60 A.Mateevici str., Chisinau, MD 2009, Republic of Moldova ABSTRACT The photosensitivity and electro-reflection spectra of CdS/CdTe heterojunctions annealed up to one hour in presence of CdCl2 have been studied. The lifetime of charge carriers, generated in the interface layer, has been determined. The SnO2/CdS/CdTe/Ni thin films solar cells, deposited by close-spaced sublimation (CSS) technique are photosensitive in the 1.5-2.4 eV spectral region. The spectral distribution of photocurrent is not essentially modified by the increase of the annealing time. A slight decrease of the photocurrent in the 1.5-1.7 eV energy range is observed and the photocurrent slightly increases in the 1.7-2.4 eV region. The recombination of the nonequilibrium charge carriers in the interface layer takes place via recombination levels having a lifetime value, which decreases from 46 µs down to 30 µs along with photon energy increase from 1.55 eV to 2.4 eV respectively (T=293K). The correlation between the light penetration depth in component layers, estimated from the absorption spectral distribution and the photon energies was established. INTRODUCTION Cadmium telluride (CdTe) is an ideal material for photovoltaic solar energy conversion [1]. CdTe solar cells efficiency already reached 16.5% [2] and theoretical efficiency was estimated to 29%[3]. nCdS/pCdTe heterojunctions are currently studied for being used for solar energy conversion. The generation of nonequilibrium charge carriers takes place in a thin CdTe layer at the junction interface. The thickness of the photo-active layer is determined by the value of the absorption coefficient, which can reach 104 cm-1 [4]. At the interface of CdS/CdTe heterojunction CdSxTe1-x solid solutions with different composition x is formed [5, 6]. The different values of the lattice parameter for CdTe and CdS [7] result in formation of a high concentration of defects, which act as traps with a high value of lifetime. The presence of these defects diminishes the charge carriers flux to be separated by the heterojunction and the efficiency of the solar cell. The spectral distribution of photosensitivity of solar cells based on CdS/CdTe heterojunctions deposited by CSS and annealed in presence of CdCl2 is limited by the absorption band edge of CdS and CdTe layers respectively [8]. In this energy range, only generation and recombination processes in the interface layer CdSxTe1-x, which has a composition close to CdTe, determine the photosensitivity. The annealing in presence of CdCl2 leads to a homogenization of the thin films and to a modification of the interface layer structure. As a result, changing the technological parameters can modify the photogeneration properties of the structure. The results of investigation of the optoelectronic properties of the component layers and of the heterojunction in ensemble as a function of thermal annealing in CdCl2 are
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