On Industrial Production of Thin Film CdTe PV Modules
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1012-Y05-01
On Industrial Production of Thin Film CdTe PV Modules Dieter Bonnet SOLARPACT e.V., Breslauer Ring 9a, Friedrichsdorf, D-61381, Germany ABSTRACT Due to its basic physical and chemical properties CdTe has become a favored base material for thin film solar cells using robust, high-throughput manufacturing procedures. The technology shows significant potential for attaining cost levels of 105 cm-1 so that the absorber layer needs only be a few µm thick to absorb >90% of photons at energies above 1.45 eV. Current densities of up to 27 mA cm-2 and open-circuit voltages of 880 mV, leading to AM 1.5 efficiencies of 18.5%, can be expected for cells made from CdTe [4]. CdTe has an extremely simple phase-diagram, which indicates that only the compound CdTe is stable at temperatures of 500∞C or more. It also sublimes congruently, which allows to vaporize it from open crucibles filled with CdTe granulate and have it condense onto closely spaced substrates by CSS. At temperatures above 500∞C CdTe under such conditions grows p-doped to approximately 1014 cm-3. Such a doping-level - although somewhat low - can be tolerated in thin film solar cells. The CSS process - as indicated in Fig. 2 - can be applied for in-line deposition of both semiconductors CdS and CdTe successively. In view of low required vacuum conditions (mbar-region) exceptionally low-cost equipment can be built.
Substrate, 500∞C, moving CdTe granulate
Carbon-crucible, 700∞C
Figure 2. Schematic illustration of close space sublimation . Activation As light-generated electrons (minority carriers in p-CdTe) have to traverse the CdTe-CdS junction, conventional wisdom would expect significant recombination at the abrupt interface, which should have abundant recombination centers. CdS and CdTe in thermal equilibrium can form mixed compounds CdSxTe1-x only for limited ranges of x (0
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