Electron Reflector Strategy for Thin CdTe Solar Cells
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1210-Q03-01
Electron Reflector Strategy for Thin CdTe Solar Cells Kuo-Jui Hsiao and James R. Sites Department of Physics, Colorado State University, Fort Collins, CO 80523, USA
ABSTRACT Incorporation of an electron reflector is a proposed strategy to improve the open-circuit voltage of CdTe thin-film solar cells. An electron reflector is basically a conduction-band barrier at the back surface, which can reduce the recombination resulting from the electron flow to the back surface. It should be particularly beneficial for cells with thicknesses below two microns when the CdTe absorber layer is fully depleted at its typical carrier density, because back-surface recombination is a primary limitation to the performance of fully depleted cells. Cells with thickness below two microns can also benefit from optical reflection at the back interface. One-dimension numerical simulation is used to investigate the electron reflector strategy and optical back reflection for thin CdTe cells. Theoretically, about a 200-mV increase in voltage and 3% in efficiency are achievable for a thin CdTe solar cell with 2x1014-cm-3 hole density, 1-ns lifetime, and a 0.2-eV electron reflector barrier. Furthermore, with the electron reflector, good CdTe cell performance at thicknesses as thin as 0.4 μm should be possible.
INTRODUCTION The CdTe solar cell is one of the most promising photovoltaic devices. The CdTe band gap is a good match to solar spectrum. In addition, CdTe has a large optical absorption coefficient, so that two microns of CdTe already can absorb more than 99% of incident light at 600 nm. CdTe solar cells can be fabricated with a variety of deposition techniques, because the electronic properties and the structure are generally optimized by a post-deposition treatment. Economic fabrication processes are already utilized in large-scale manufacturing [1,2]. Thinning solar cells without compromising their performance is one approach to low-cost PV devices due to less fabrication time and less material needed for thinner cells. The University of Toledo has successfully fabricated a 0.3-μm CdTe solar cell with 6.8 % efficiency and a 0.5μm cell with 9.7 % [3]. The thin CdTe solar cell with its typical carrier density (1013-2x1014cm-3) is usually fully depleted. However, the back-surface recombination limits the performance of fully depleted cells. Moreover, incomplete optical absorption with a thin absorber layer causes current loss. The two strategies discussed here to minimize the loss due to back-surface recombination and incomplete absorption are the electron reflector and optical back reflection. An electron reflector, which is a conduction-band barrier of height φe at the back surface, can reduce the recombination resulting from the forward electron flow to the back surface, especially at forward bias (see fig. 1). It can be fabricated by adding a layer of material with expanded band gap, such as the alloy CdZnTe. It should be particularly beneficial for CdTe thicknesses below two microns when the CdTe is fully depleted at it
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