Thermally induced morphological changes in PAT/PCBM bulk heterojunctions studied by time-resolved small- and wide-angle X-ray scattering

In polymer bulk heterojunctions (BHJs), the thermally induced phase separation between electron-donating and -accepting molecules is considered as a key process in determining the overall nano-morphology of the BHJ, which can limit the performance of the fully fabricated BHJ device. Although it is often understood that the spinodal decomposition or the constitutional solidification (crystallization) of polymer is the main mechanism for phase separation, there are still questions remained to be answered because the crystallization rate can be comparable to the kinetics of the spinodal decomposition when the BHJ is made of semicrystalline polymer and it is difficult to separate those mechanisms to establish fundamental principles. This study uses the in-situ small- and wide-angle X-ray scattering to collect the information on the crystallization of polymer in BHJs under thermal annealing and its phase separation at the same time. The data showed the faster diffusion of PCBM compared to the crystallization of the polymer in the BHJs and, in fact, the crystallization was strongly limited by the diffusivity of PCBM in the polymer.