Photostimulated Changes of Electrical Characteristics of Ag/CdTe Thin Film Structures
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Photostimulated Changes of Electrical Characteristics of Ag/CdTe Thin Film Structures T.D.Dzhafarov1,2, M. Caliskan1 1 Department of Physics, Yildiz Technical University, Davutpasa, 34210 Istanbul, Turkey 2 Institute of Physics, Azerbaijan National Academy of Sciences, Javid str,Az-1143 Baku, Azerbaijan ABSTRACT Electrical, optical and structural properties of Ag/CdTe structures exposed to thermal (in dark) and photoannealing (under illumination) have been studied. The effective diffusion coefficie nt of Ag in CdTe films have been estimated from resistance versus duration of annealing curves. In the range of 280-420o C the effective coefficient of thermal diffusion (Dt ) and photodiffusion (Dph ) are described as Dt = 1.9x105 exp (-1.60/kT) and Dph =8.7x103 exp(-1.36/kT). The acceleration of Ag diffusion under illumination was tentatively attributed to photoionization of Ag increasing the interstitial flux of silver. Ag/CdTe structures exposed to annealing were characterized by X-ray diffraction (XRD), I-V, C-V, conductivity-temperature and optical transmission measurements. In XRD patterns of annealed Ag/CdTe structures, besides the intensive (111) peak of cubic CdTe, the weak peaks of Ag2 Te phase are also present. The temperature dependence of conductivity of annealed Ag/CdTe structures showed the energy levels 0.13 eV. INTRODUCTION The CdTe thin film is promising base material for a solar cell owing to its nearly optimum band gap and high absorption coefficient. Elements of the group 1 (Cu and Ag) are an everpresent residual impurity in CdTe. Moreover they are used as contact material for fabrication of CdTe/CdS solar cells and metal/CdTe interfaces play an important role in cells. Cu and Ag are known as substitution acceptor impurities in Cd positions (CuCd or AgCd ). In addition, they are reported to be incorporated also as interstitial donors and the diffusion coefficient of interstitial ions is known to be very high [1,2]. Besides, Cu and Ag is able to form complexes with VCd and VTe vacancies. These impurities are considered to be principal component responsible for the degradation of CdTe/CdS cells due to their ability to diffuse through the CdTe film to the cell junction. In the photoluminescence spectra of Ag-doped CdTe peaks at 1.491 eV and 1.590 eV are attributed to a donor-acceptor pair between cadmium- vacancy (VCd ) and Ag in interstitial position (Agi) [3]. A photoemission study of Ag/CdTe interfaces prepared by evaporation of Ag on the roomtemperature, surface of CdTe showed a significant concentration of dissociated Te in reacted form with the overlayer Ag metal [4]. According to Wolf et al. [5] the diffusion profiles of Ag and Cu introduced by implantation of radiotracers Ag111 and Cu67 in bulk CdTe directly reflect the distributio n of intrinsic defects, such as VCd and Cd i . Influence of illumination on diffusion in Cu/CdTe thin film structures and photostimulated degradation of characteristics of CdTe(Cu)/CdS cells was reported in [6]. According to our knowledge, photostimulated dif
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