Striving for a standard protocol for preconditioning or stabilization of polycrystalline thin film photovoltaic modules

Abstract

Polycrystalline photovoltaic (PV) modules containing cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS) thin film materials can exhibit substantial transient or metastable current-voltage (I-V) characteristics depending on prior exposure history. Transient I-V phenomena confound the accurate determination of module performance, their reliability, and their measured temperature coefficients, which can introduce error in energy ratings models or servicelifetime predictions. Indeed, for either of these two technologies, a unique performance metric may be illusory without first specifying recent exposure or state—even at standard test conditions. The current standard preconditioning procedure for thin-film PV modules was designed for amorphous silicon (a-Si), and is likely inadequate for CdTe and CIGS. For a-Si, the Staebler-Wronski effect is known to result from defects, created via breaking of weak silicon bonds or light-activated trapping at the device junction, occurring rapidly upon light-exposure. For CdTe and CIGS devices, there is less agreement on the causes of metastable behavior. The data suggests that either deep-trapping of charge carriers, or the migration and/or electronic activation of copper may be responsible. Because these are quite disparate mechanisms, we suspect that there may be a more practical preconditioning procedure that can be employed prior to accurate performance testing for CdTe and CIGS modules. We devise a test plan to examine and compare the effects of light soaking versus forward-biased dark exposure at elevated temperatures, as parallel strategies to determine a feasible standard protocol for preconditioning and stabilizing these polycrystalline PV technologies, and report on the results of our tests.