Probing the bulk heterojunction morphology in thermally annealed active layers for polymer solar cells

A combination of fast scanning chip calorimetry and X-ray ptychography is explored to study the effects of thermal annealing on the active layer of bulk heterojunction organic photovoltaics. The well-known P3HT/PC61BM 1: 1 system is investigated as a test case. By using a custom chip calorimetry setup, it is possible to give a thermal treatment at 127 degrees C (400 K) to P3HT/PC61BM 1: 1 thin layers, using a heating and cooling rate of 30000 K s (-1), after which the resulting morphology is investigated with X-ray ptychography. Applying only heating and cooling, without isothermal annealing, yields a featureless morphology. This corresponds well with thermal data which indicate a mixed amorphous phase only. For increasing isothermal annealing times, a well-defined morphology appears with increasing domain size, corresponding to the formation of an endothermal melting trajectory. This melting trajectory is expected to consist of both eutectic melting and melting of coarsened crystals. In contrast to chip calorimetry results, large domain sizes are obtained for heating and cooling without isothermal annealing at a conventional rate of 20 K min(-1). This initial morphology then develops further with increased isothermal annealing. The combination of chip calorimetry and ptychography allows separating the effects of each single thermal step on morphology development.