Temperature-Dependent Electroluminescence from CdTe/CdS Solar Cells
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Temperature-Dependent Electroluminescence from CdTe/CdS solar cells K. J. Price1,2, A. Vasko, L. Gorrell and A. D. Compaan Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606 1 summer faculty scholar from Morehead State University, Morehead, KY 40351 2 email address: [email protected] ABSTRACT Electroluminescence (EL) from polycrystalline CdTe/CdS solar cells was studied over the temperature range – 30 C to 25 C. We are able to observe above-background EL at forward current densities as low as 3 mA/cm2 , allowing us to explore the EL behavior at current-voltage regimes within the normal operating parameters of the device. The EL spectrum is very similar to the photoluminescence (PL) spectrum, and is independent of applied voltage. We show that the EL most likely originates from injected electron-hole recombination at the CdTe/CdS junction. The total EL intensity is found to vary as a power-law function of current, EL ~ Ib , where I is the forward current density and b is a constant. The value of b varies from sample to sample and decreases with increasing temperature. EL intensity typically is much more sensitive to device deterioration with light soak stress than is cell efficiency.
INTRODUCTION Thin film CdTe/CdS solar cells show great promise for low-cost photovoltaic applications, having already achieved conversion efficiencies of over 16 % (Ref 1). Much progress has been made in the past decade in achieving a fundamental understanding of CdTe/CdS device operation, but further advances are necessary to make continued progress in conversion efficiency. In a-Si photovoltaic devices, an important characterization tool has been electroluminescence (EL) (see, for example, the review by Han et al2 and the references therein). However, in CdTe/CdS devices, few EL studies have been performed. Potter et al first reported the existence of EL at temperatures of approximately 10 K in CdTe/CdS solar cells3. We first reported EL measurements in Ref. 4. Recently, Feldman et al5 presented room-temperature spatially resolved EL data from cells (stressed and unstressed) operating at forward current densities of over 200 mA/cm2 and more recently6 similar studies with currents as low as 33 mA/cm2. In this work, we report EL at room temperature in CdTe/CdS solar cells operating at forward current densities from 3 mA/cm2 to 20 mA/cm2. These conditions are comparable to those experienced by a typical device. Comparison with photoluminescence spectra from the same samples suggests the EL emission originates from the CdTe/CdS junction region. The total EL intensity is found to vary as a power-law function of current, EL ~ Ib , where I is the forward current density. The exact value of b varies with temperature and from sample to sample. We also present preliminary results of the effect of stressing on EL. EXPERIMENTAL DETAILS Samples were standard glass/SnO2:F/CdS/CdTe/Cu-Au photovoltaic devices. CdS (0.12 microns) and CdTe (2.3 microns) were deposited by rf-sputtering. Back contacts were
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