Investigation of the Trapping Mechanism for Transient Current-Voltage Behavior In CIGSS-Based Solar Cells
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Investigation of the Trapping Mechanism for Transient Current-Voltage Behavior In CIGSS-Based Solar Cells Pamela K. Johnson1, James R. Sites1and Dale E. Tarrant2 1 Colorado State University, Fort Collins, CO 80523, U.S.A. 2 Siemens Solar Industries, Camarillo, CA 93012, U.S.A. ABSTRACT Some thin-film CIS photovoltaic devices exhibit reversible transient behavior in their electrical properties induced by modestly elevated (70 - 100 oC) temperatures. This paper evaluates changes due to light exposure, thermal exposure, and applied bias in cells fabricated by Siemens Solar Industries (SSI). When a constant bias was maintained across cells subjected to elevated temperatures in the dark, and subsequent moderatetemperature light exposure, there was little reversible transient behavior. When the bias was cycled between zero and open-circuit voltage (VOC), independent of illumination, the fill factor (FF) decreased for zero bias and increased at VOC. Hence, it is the bias rather than photon absorption that drives the transient current-voltage behavior in these cells. Investigations of the relationship between trapping mechanisms and transient behavior using the frequency and temperature dependence of capacitance showed clear cyclic behavior in the trap-response frequency. Trap density profiles were found to be relatively independent of measurement temperature, and the total trap density varied only slightly with the bias cycle. INTRODUCTION As with many thin-film CIS photovoltaic devices, CIGSS absorbers made at Siemens Solar Industries (SSI) exhibit transient changes in electrical properties. The transient behavior exhibited generally improves performance for normal operating conditions [1]. Nonetheless, the changes are a complicating factor when trying to measure the photovoltaic parameters of devices or modules and for interpretation of accelerated environmental tests that include exposures to temperatures above normal operating conditions [2]. The U.S. National CIS R&D Team [3] is studying these effects with the goal of eliminating or minimizing them and improving overall device performance based on improved knowledge regarding the details of junction formation. While transient behavior has been observed for some time, the mechanism has not been understood. Hence, in this paper we attempt to identify what drives the transient current-voltage behavior in these cells and investigate potential correlations between the trap density profiles and the observed device properties during elevated-temperature cycles. EXPERIMENTAL The details of transient behavior are dependent on the temperature-illumination-bias history of the cell. Conditions studied include thermal exposure in the dark (dark heat soaking) and exposure to light at moderate temperatures (light soaking.) All results reported in this paper are for 20-hour soaking cycles in an air atmosphere, though longer H5.12.1
times are sometimes needed for complete reversibility. During dark heat soaking for the standard cycle, the cells were held at 85 oC and zero bias, VOC,
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