Open-Circuit Voltage Decay in CdTe/CdS Solar Cells
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Open-Circuit Voltage Decay in CdTe/CdS Solar Cells Kent Price1, Kevin Cooper2, and Chris Lacy Department of Physical Sciences, Morehead State University Morehead KY 40351, USA 1 [email protected] 2 Lewis County High School Vanceburg KY 41179, USA ABSTRACT Open-Circuit Voltage Decay (OCVD) is a common technique used to characterize numerous semiconductor devices. However, to the knowledge of the authors, the technique has not previously been applied to CdTe-based solar cells. We use a simple setup consisting of a function generator, rectifying diode, and digital oscilloscope to measure the dark open circuit voltage decay as a function of time across a CdTe solar cell. We find the decay to be described by the equation v(t) = v0 + A1exp (–t/τ1) + A2exp (–t/τ2) where v is the voltage, t is time, τ1 and τ2 are characteristic decay times, and A1, A2 and v0 are constants. The two characteristic decay times are on the order of 10 µs and 500 µs. The relative contribution of the two decay times depends on the magnitude of the initial applied voltage pulse. We will describe preliminary results on the correlation between the OCVD and solar cell performance, including the effects of light-soaking on OCVD behavior. 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. Open Circuit Voltage Decay (OCVD) is a technique that has been used extensively to characterize silicon and GaAs solar cells. However, to the knowledge of the authors, the technique has not previously been applied to CdTe-based solar cells. We report results of OCVD study of CdTe/CdS solar cells showing the decay to be described by the equation v(t) = v0 + A exp (–t/τ1) + B exp (–t/τ2) where v is the voltage, t is time, τ1 and τ2 are characteristic decay times, and A, B and v0 are constants. The two characteristic decay times are approximately 10 µs and 500 µs. The decay times depend on the magnitude of the initial applied voltage pulse and the CdTe fabrication method. EXPERIMENTAL DETAILS Samples used in this study were provided by the University of Toledo. Samples were standard ITO/CdS/CdTe/Cu/Au devices. The semiconductor layers were prepared by RFmagnetron sputtering on commercially available ITO-coated glass, and the metal contacts were applied by evaporation. The OCVD setup consisted of a solar cell in series with a rectifying diode and a square-wave generator, as shown in figure 1. Square waves with fall time of less than 2 ns, peak-to-peak amplitude between 100 mV and 1000 mV, and frequency of 50 Hz were
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CdTe solar cell Rectifying diode
Oscilloscope
Function generator
Figure 1. Diagram of OCVD setup.
applied to the solar cell using the function generator. The DC offset of the square wave was set so that the
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