Direct analysis of the current density vs. voltage curves of a CdTe module during outdoor exposure

Abstract

Current density–voltage (JV) curves of a CdTe photovoltaic module measured during 2.5years of outdoor operation are analyzed using a phenomenological four-parameter equation. This approach is shown to be more accurate than fitting the same data set to the conventional five parameter diode equation. The four extracted parameters are the short-circuit current density Jsc, open-circuit voltage Voc, and the differential resistances Rsc and Roc, i.e., the reciprocal slopes of the JV curves at short circuit and open circuit, respectively. The dependencies of all four parameters on module temperature and irradiation intensities are analyzed in terms of physically motivated models. The coefficients derived from these models are then used to transform the photovoltaic parameters to reference conditions and to investigate the degradation of the module. The models for Rsc and Roc involve voltage-dependent carrier collection. This feature appears prominent already in the initial JV curves, but becomes even more dominant with increasing exposure time. Voltage-dependent carrier collection also explains that the diode ideality factor determined from Voc vs. Jsc curves is different from that determined using Roc vs. 1/Jsc plots. The degradation of the module is essentially reflected in fill factor losses caused by a decrease of Rsc and an increase of Roc. Additionally, the Voc vs. log(Jsc) curves of the module behavior at low irradiation unveil an increasing influence of a double-exponential diode behavior.