Impedance Spectroscopy and Hall Measurements on CdTe Thin Polycrystalline Films
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Impedance Spectroscopy and Hall Measurements on CdTe Thin Polycrystalline Films A.S. Gilmore, V. Kaydanov, T.R. Ohno, D. Grecu* and D. Rose* Colorado School of Mines, Dept. of Physics, Golden, CO 80401, U.S.A. *First Solar, LLC, 12900 Eckel Junction Road, Perrysburg, OH 43551, U.S.A. ABSTRACT Measurements of film impedance in a wide frequency range (impedance spectroscopy, IS) were used to evaluate the grain boundary (GB) and the bulk material (B) resistance. Undoped films demonstrated very low carrier concentration and negligible GB resistance, both due to depletion of the bulk material by the GB deep states. Doping the film with Cu led to a significant increase in the GB resistance, in addition to the expected increase in carrier concentration. Illumination of doped films reduced GB resistance by orders of magnitude. This effect was used for the analysis of Hall concentration and Hall mobility data, aimed at exclusion of GB influence on measured parameters, and was used to determine the carrier concentration and mobility in the grain bulk. Hall studies on undoped films under illumination were also used to estimate the lifetime of the photogenerated carriers. INTRODUCTION Polycrystalline cadmium telluride thin film solar cells demonstrate high efficiency (record value of about 16% ). Recently this material has progressed to the commercialization stage with manufacturers such as BP Solar and First Solar, L. L. C., proposing the production of modules. Faster progress in this area is deterred due to a lack of knowledge and understanding of fundamental electronic properties of the materials behind cell performance, especially those of the absorber CdTe layer. Electronic properties of grain boundaries (GB) are of special interest in polycrystalline materials. CdTe films in the cells demonstrate a fine-granular structure (grain size of ~1 µm) and high density of deep GB electronic states indicated by both studies of CdTe bicrystals [1] and especially of polycrystalline films (up to 1013 cm-2 [2]). These deep states may act as effective recombination centers. Another important effect of GB states is trapping of the majority carriers (holes). The resulting GB potential barrier for holes provides high resistance for grain-to-grain transport. Recent studies showed that the barrier height in CdTe films extracted from a cell can be as high as 0.8 eV [3]. Besides the straightforward impact of GB on the CdTe electronic properties, hence the cell performance, their influence should also be taken into account when characterizing properties of grain bulk, in particular carrier concentration and mobility derived from Hall measurements [4]. In this paper, combined studies of impedance spectroscopy (IS) and Hall effect under varying illumination in thin CdTe polycrystalline films are presented. The IS (measurements of the frequency-dependent impedance) allows one to separate and estimate individual contributions of GB and bulk (B) material to the film in-plane resistance, see, e.g. [2, 3]. Illumination of films can significantly reduc
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