Evaluation of Four Imaging Techniques for the Electrical Characterization of Solar Cells
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Evaluation of Four Imaging Techniques for the Electrical Characterization of Solar Cells Gregory M. Berman1,3, Nathan J. Call2,3, Richard K. Ahrenkiel2,3, and Steven W. Johnston3 1
Department of Electrical Engineering, University of Colorado, Boulder, CO 80309, U.S.A. Department of Materials Science, Colorado School of Mines, Golden, CO 80401, U.S.A. 3 National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. 2
ABSTRACT We evaluate four techniques that image minority carrier lifetime, carrier diffusion length, and shunting in solar cells. The techniques include photoluminescence imaging, carrier density imaging, electroluminescence imaging, and dark lock-in thermography shunt detection. We compare these techniques to current industry standards and show how they can yield similar results with higher resolution and in less time. INTRODUCTION In this paper, we address four imaging techniques that measure minority carrier lifetime, diffusion length, and the location of shunts in a solar cell, and we compare our techniques to methods currently used in industry. Minority-carrier-lifetime and diffusion-length measurements are critical for characterizing the quality of solar materials to predict the efficiency of a device made from them [1-2]. Shunts can drastically lower a solar cell’s efficiency and are introduced in the processing of a wafer into a working solar cell [3]. Minority carrier lifetime is the average time it takes for a free carrier generated in a material to recombine. We measure it with photoluminescence imaging (PLI) and carrier-density imaging (CDI). PLI probes the radiative recombination of an optically excited sample, which is proportional to the sample’s carrier density [4-6]. CDI measures the transmission of infrared (IR) light through an optically excited sample, which is also proportional to carrier density [7-8]. We compare these two carrier-lifetime techniques to each other and to microwave-reflection photoconductive decay on a Semilab tool, an industry standard that relates a material’s conductivity to its carrier density. Carrier diffusion length is the average distance a free carrier travels in a material before it recombines. We measure it with electroluminescence imagining (ELI). ELI probes the radiative recombination of a finished solar cell that is electrically driven in forward bias [9-10]. We compare ELI images with light-beam-induced current (LBIC) maps [11], an industry standard that measures the optically induced current in a solar cell. Shunts are defects created during solar cell processing that leak current. We use dark lock-in thermography (DLIT) to detect them. While shunts can be detected by measuring a cell’s I-V (current-voltage) curve, no information about a shunt’s cause or location is identified. In DLIT imaging, the solar cell is put under reverse bias, and the shunts are detected by a thermal signature, generated by current leaking across the p-n junction [12]. We demonstrate shunt detection and identify the defect with a scanning electron microscope.
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