The molecular regioregularity induced morphological evolution of polymer blend thin films

Abstract

The polymer regioregularity has a dramatic effect on device performance of organic photovoltaics in terms of material crystallinity and phase separated morphology. In this study, we investigated the molecular regioregularity induced morphological evolution of P3HT and PF12TBT blend thin films processed by solvents with different solubility and boiling point. After cast by good solvents chloroform (CF) and o-dichlorobenzene (oDCB), the regioregular (RR) and regiorandom (RA)-P3HT/PF12TBT blend films showed the similar phase separated morphology, namely fast solidification induced unsharp morphology for CF blend films and long-range demixing via spinodal decomposition within extending drying process for oDCB blend films, while the domain size for RA blend films were larger than those of RR blend films which could be attributed to easily disentanglement and diffusion of RA-P3HT molecules facilitated by smaller aggregation size in RA blend solutions. In addition, the morphology of RR blend films exhibited micro change while the RA blend films developed into long-range phase separation after thermal annealing for CF blend films which also confirmed the molecular diffusion induced morphological evolution. For marginal solvents p-xylene (XY), the RR P3HT showed obvious molecular aggregation in XY solution due to the intensive solvent–polymer interaction (Δδ) and high regioregularity and therefore the value of aggregation size (Ra) for RR-P3HT is as three times as that of RA-P3HT. After cast from XY solution, the RA blend films displayed droplet and bicontinuous morphology based on various blend ratios via spinodal decomposition. Meanwhile, the nanofiber morphology could be observed when the weight fraction of RR-P3HT beyond 10% in RR blend films, which resulting from the continuous growth of P3HT aggregations formed anterior of the phase separation in the film solidification process.