The stability of organic photovoltaic (OPV) devices in ambient conditions has been a serious issue which needs to be addressed and resolved timely. In order to probe the degradation mechanism in a donor-acceptor polymer PDPP-TNT: PC71BM bulk heterojunction basedOPVdevices, we have studied current density-voltage ( J-V ) behavior and impedance spectroscopy of fresh and aged devices. The current-voltage characteristic of optimized fresh devices exhibit a short circuit current density ( Jsc) of 8.9 mA cm2, open circuit voltage (Voc) of 0.79 V, fill factor (FF) of 54.6\%, and power conversion efficiency (PCE) of 3.8\%. For aged devices, Jsc,Voc, FF, and PCE were reduced to 57.3\%, 89.8\%, 44.3\% and 23.7\% of its initial value, respectively. The impedance spectra measured under illumination for these devices were successfully fitted using a CPE-based circuit model. For aged devices, the low-frequency response in impedance spectra suggests an accumulation of the photogenerated charge carriers at the interfaces which leads to a significant lowering in fill factor. Such degradation in device performance is attributed to the incorporation of oxygen and water molecules in devices. An increase in the recombination resistance indicates a deterioration of free charge carrier generation and conduction in devices. © 2016 IOP Publishing Ltd.