Morphological, Chemical, and Electronic Changes of the Conjugated Polymer PTB7 with Thermal Annealing

There is considerable interest in improving the performance of organic optoelectronic devices through processing techniques. Here, we study the effect of high-temperature annealing on the properties of the semiconducting polymer PTB7 and PTB7:fullerene blends, of interest as efficient organic photovoltaic (OPV) devices. Annealing to moderate temperature improves the PTB7 morphology and optoelectronic properties. High-temperature annealing also improves morphology but results in poorer optoelectronic properties. This is a result of side chain cleavage that creates by-products that act as trap states, increasing electronic disorder and decreasing mobility. We further observe changes to the PTB7 chemical structure after thermal cleavage that are similar to those following solar irradiation. This implies that side chain cleavage is an important mechanism in device photodegradation, which is a major “burn-in” loss mechanism in OPV. These results lend insight into side chain cleavage as a method of improving optoelectronic properties and suggest strategies for improvement in device photostability.