Influence of Deposition Parameters on the Morphology of CdTe Films and on the Performance of CdTe Solar Cells
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1012-Y12-22
Influence of Deposition Parameters on the Morphology of CdTe Films and on the Performance of CdTe Solar Cells Mathias H‰drich, Sebastian Mack, Heinrich Metzner, Udo Reislˆhner, and Wolfgang Witthuhn Institut f¸r Festkˆrperphysik, Friedrich-Schiller-Universit‰t Jena, Max-Wien-Platz 1, Jena, 07743, Germany ABSTRACT For industrial CdTe solar cell production, thin, dense and pinhole-free films have to be deposited at high rates. Using a close-to-industrial close space sublimation (CSS) process, we deposited CdTe films at different temperatures and rates. The morphology of these films was analyzed by scanning electron microscopy (SEM) and x-ray diffraction (XRD). The measurements show the size, size distribution and orientation of the grains to strongly depend on deposition temperature and rate. A suitable combination of these parameters yields large grains of about 3 microns diameter even for deposition temperatures below 750 K and at high deposition rates of 3 microns per minute. The interplay of CdTe layer morphology and solar cell performance is elucidated employing a TCO-CdS-CdTe-Mo film sequence for complete solar cells. INTRODUCTION CdTe with a direct band gap of 1.45 eV is a very suitable absorber material for thin film solar cells. It grows p-type and highly stoichiometric forming a heterojunction with n-type CdS. High AM1.5 conversion efficiencies of 16% for small area laboratory cells have been obtained using the close space sublimation (CSS) technique [1]. As the efficiency of industrial modules with large areas is still less than 10%, improving CdTe thin film technology is essential. We established a new baseline for CSS-CdS-CdTe solar cells. Using this baseline, we produced solar cells and investigated structural and electrical properties [2,3]. The morphology of polycrystalline CdTe films strongly depends on several deposition parameters among which the deposition rate and especially the growth temperature are of major importance. The impact of these parameters on film growth has been investigated by several authors [4-6]. A generalized structure zone model for film growth was suggested by Thornton [7]. Luschitz et al. [4] identified regimes of CdTe film growth resembling these structure zones. These regimes were found to have an impact on the size as well as on the orientation of the crystal grains. The discussed investigations on CdTe films however refer to the as-grown state though an additional post-deposition treatment is necessary to improve the performance of CdTe solar cells. Therefore we investigated the influence of the activation step on CdTe layers deposited at different substrate temperatures. To this end, we used SEM measurements to study the CdTe film surface and to measure the grain size as well as the grain size distribution. Complementary XRD measurements were used to study texture effects. We tried different deposition rates in order to obtain a fast film growth. Complete solar cells were produced using the differently deposited CdTe films.
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