Predicting thermal stability of organic solar cells through an easy and fast capacitance measurement

Degradation of organic photovoltaic (OPV) devices is currently a topic under intense research as it is one of the main limitations towards the commercialization of this technology. Morphological changes at both active layer and interfaces with the outer contacts are believed to determine main key issues to be overcome. In-line techniques are essential to rule out any effect arising during sample fabrication. Unfortunately, the number of physical techniques able to provide morphological information on complete and operational devices is certainly limited. In this work, we study the thermal degradation of bulk heterojunction (BHJ) solar cells composed by different donor polymers with techniques developed to provide in-situ information on operational devices. Capacitance measurement as a function of temperature monitors the electrical integrity of the active layer and provides the threshold temperature (T<inf>MAX</inf>) at which the whole device becomes thermally unstable. We found a direct correlation between the threshold temperature T<inf>MAX</inf>, obtained by capacitanceerature measurements on complete OPV devices, and the power conversion efficiency decay measured at 85°C. Devices tend to be thermally stable when the temperature of the thermal stress is below T<inf>MAX</inf>, while above T<inf>MAX</inf> evident changes in the active layer or at the active layer/electrode interface are also detected by confocal fluorescence microscopy. The capacitance method gives precious guidelines to predict the thermal stability of BHJ solar cells using an accelerated and easy test.