Study of the Electronic Properties of Matched Na-Containing and Reduced-Na CuInGaSe 2 Samples Using Junction Capacitance
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Study of the Electronic Properties of Matched Na-Containing and Reduced-Na CuInGaSe2 Samples Using Junction Capacitance Methods Peter T Erslev1, Adam Halverson1, William Shafarman2, and J. David Cohen1 1 Physics Department, University of Oregon, Eugene, OR, 97405 2 Institute of Energy Conversion, Newark, DE, 19716 ABSTRACT Junction capacitance methods were used to examine a matched pair of Cu(In1-xGax)Se2 (CIGS) thin film solar cells, one with Na incorporated into the absorber and the other with a diffusion barrier to inhibit the Na incorporation from the soda-lime glass. Typical cells showed a 50% increase in efficiency with the addition of Na. Forward biased admittance spectroscopy revealed a large defect density located near the CdS/CIGS heterojunction in the reduced Na samples not present in the higher Na samples. This may be responsible for the lower Voc, contributing to the loss in efficiency when Na is not added. Drive-level capacitance profiles revealed free carrier densities of 3×1014 cm-3 and 1.1×1014 cm-3 for the higher and reduced Na samples, respectively. Transient photocapacitance spectra indicated a slight improvement in absorber properties with the addition of Na, but not enough to account for the large loss in efficiency. INTRODUCTION Thin film photovoltaic devices have the potential to provide an inexpensive renewable source of electrical power. Polycrystalline Cu(In1-xGax)Se2 (CIGS) materials is one of the most promising, with the highest efficiency (η = 19.4 %) reported for a thin film solar cell [1]. The addition of Na to the CIGS absorber layer is a commonly followed procedure, boosting the efficiency by up to 50% primarily through a sizeable increase in the open-circuit voltage (Voc) and the fill factor (FF). Although the positive role of Na is well known, there is an ongoing debate as to the exact mechanism of the beneficial effect of Na. Most of the debate centers around where in the cell the Na has an effect on the performance of the device. Possible sites are grain boundaries, in the bulk of the grains, or the CdS/CIGS heterojunction. Recently published experimental results have been contradictory, with one group finding no evidence of Na at the grain boundaries[2] and another group concluding that the Na is only found in significant amounts at the grain boundaries[3]. Another hypothesis is that the Na acts only during the growth of the sample to organize and passivate point defects[4]; however, this is disputed by similar benefits obtained through diffusion of Na into the sample in a postdeposition treatment[5]. In this paper we present results from junction capacitance measurements performed on a pair of co-deposited CIGS sample devices, one with Na and the other with a diffusion barrier to inhibit the movement of Na from the soda-lime glass substrate into the absorber layer. These have been investigated with junction capacitance methods including admittance spectroscopy, drive-level capacitance profiling (DLCP) and transient photocapacitance (TPC) spectroscopy. Secondary Io
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