Cadmium Telluride Solar Cells on Molybdenum Substrates
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Cadmium Telluride Solar Cells on Molybdenum Substrates Ilvydas Matulionis, Sijin Han, Jennifer A. Drayton, Kent J. Price, and Alvin D. Compaan Dept. of Physics and Astronomy, The University of Toledo, Toledo, OH, 43606 [email protected] ABSTRACT We report on the development of Mo/CdTe/CdS/indium-tin-oxide, thin-film solar cells grown by radio-frequency magnetron sputtering. This is an inverted configuration compared to the conventional glass/tin-oxide/CdS/CdTe/metal cells. Molybdenum was chosen as a substrate because its thermal expansion coefficient and the work function are close to those of CdTe. We have achieved AM1.5 conversion efficiencies of 7.8 percent on 0.05 cm2 area devices. Our best cells had a nitrogen-doped ZnTe layer between the molybdenum and the CdTe for a somewhat improved back contact. However, we observe a significant rollover in the IV curve in forward current that indicates a back-diode effect. This implies the need for improvement of the electronic properties of the molybdenum - CdTe and possibly CdS - ITO interfaces. INTRODUCTION Conventional cadmium telluride solar cells are fabricated in the superstrate configuration, glass/SnO2/CdS/CdTe/contact. These glass superstrate cells have achieved superior performance with efficiencies exceeding 16% on borosilicate glass[1]. For terrestrial power applications, ordinary soda-lime glass provides a convenient and inexpensive growth substrate. In addition, it can be heat-strengthened; it facilitates laser scribing for monolithic cell integration; and with a back lamination it facilitates fabrication into frameless modules[2]. However, the glass superstrate has two important limitations. Its large mass per unit area, in a 2-3 mm sheet, precludes consideration for space applications, and from a more fundamental viewpoint, the cell structure buries the electrically active junction under the 2-10 µm thick CdTe absorber layer and contact. In the present work we have used magnetron sputtering [3] to fabricate substrate structures on Mo sheet substrates. Several aspects are similar to recent work of Singh, et al.[4], although we have used magnetron sputtering for the deposition of the CdTe, CdS, and ITO layers. Furthermore, we find that the use of a nitrogen-doped ZnTe interfacial layer between the Mo and the CdTe yields somewhat improved performance. The fact that this inverted structure (for CdS/CdTe cells) makes the CdS and ITO emitter the final deposited layers appears to lead to some qualitatively different S/Te interdiffusion behavior at the CdTe/CdS junction. CELL FABRICATION The Mo/CdTe/CdS/indium tin oxide (ITO) cells and Mo/ZnTe/CdTe/CdS/ITO cells were fabricated on Mo sheet substrates 100 µm thick. Cadmium telluride has a relatively high work function of 5.7 eV [5] and this, together with a thermal expansion coefficient close to that of CdTe, was some consideration in the choice of Mo for a substrate material. The polycrystalline CdTe, CdS, ITO, and ZnTe films were grown using planar magnetron radio-frequency (RF) sputtering. CdTe is natu
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