Analysis of degradation mechanisms in donor–acceptor copolymer based organic photovoltaic devices using impedance spectroscopy
Materials Research Express
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 block copolymer PDPP-TNT:PC71BM bulk heterojunction based OPV devices, we have studied current-voltage (J-V) behaviour and impedance spectroscopy (IS) of fresh and aged devices. The current-voltage characteristic of optimized fresh devices exhibit a short circuit current (Jsc) of 8.9
... ent (Jsc) of 8.9 mA/cm 2 , 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 was reduced to 42 %, 10 %, 56% and 76% of its initial value, respectively. The impedance spectra measured under illumination for these devices was successfully fitted using a CPE based circuit model. For aged devices, the low frequency response in impedance spectra suggests an accumulation of the photo-generated charge carriers at the interfaces which impedes the charge extraction at the electrodes and 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 associated with the bulk heterojunction active layer indicates a deterioration of free charge carrier generation and conduction in devices. Such observations from impedance measurement appear as an electrical signature of the probable physical mechanisms responsible for degradation of OPV devices.