The correlation of capacitance-voltage hysteresis measurements with performance during accelerated lifetime testing of polycrystalline thin film solar cells

Cell performance, or efficiency, η% is the most used, if not most important metric for evaluating different solar cell technologies. Efficiency in turn is determined directly by the product of open-circuit voltage, V_oc, short-circuit current density, J_sc, and fill-factor, FF. From numerous studies involving the measurement of η% as a function of stress, it has been determined that η% can be most correlated with changes in FF. In order to better study changes in FF, and thus, to better understand what causes cells to degrade, we have developed a new and appropriate failure analysis technique which can be readily incorporated within the framework of existing accelerated lifetime testing (ALT). This non-destructive, two-terminal technique is relatively easy to perform on both polycrystalline thin film solar cells as well as finished series-connected thin film modules and is believed to be benign as a failure analysis tool. In this technique, capacitance data is collected using a bi-directional voltage scan in order to capture commonly observed hysteretic or “transient” effects in polycrystalline thin film cells and modules.