Current density-voltage (JV) curves of a CdTe photovoltaic module measured during 2.5 years 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 J(sc), open-circuit voltage V-oc, and the differential resistances R-sc and R-oc, 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 R-sc and R-oc 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 V-oc vs. J(sc) curves is different from that determined using R-oc vs. 1/J(sc) plots. The degradation of the module is essentially reflected in fill factor losses caused by a decrease of R-sc and an increase of R-oc. Additionally, the V-oc vs. log(J(sc)) curves of the module behavior at low irradiation unveil an increasing influence of a double-exponential diode behavior. (C) 2014 Elsevier Ltd. All rights reserved.