No abstract

The large-scale manufacture of organic electronics devices becomes more feasible if the molecular orientation and morphology of the semiconductor can be controlled. Here, we report on a previously unidentified crystal shape of terraced nanoscale "ribbons" in thin films of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT). The ribbons form after a pBTTT film is heated above its highest temperature phase transition. In contrast to the wide terrace crystal shape previously reported, terraced ribbons have lateral widths of approximately 60 nm and lengths greater than 10 microm, with a common orientation between adjacent ribbons. Further, we report a simple and scalable flow coating process that can control the ribbon orientation without requiring special substrates or external fields. The degree of molecular orientation is small after coating but increases dramatically after the terraced ribbons are formed, indicating that an oriented minority templates the whole film structure. The large extent of orientation obtained in these polythiophene crystallites provides potential opportunities to exploit anisotropic electrical properties and to obtain detailed information about the structure of organic semiconductor thin films.

The carrier mobility of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) semiconductors can be substantially enhanced after heating through a thermotropic mesophase transition, which causes a significant improvement in thin film structural order. By directly measuring film structure throughout a heating and cooling cycle, we identify the molecular origin of this mesophase transition as the melting of interdigitated linear alkane side chains, in this case quaterdecyl. The morphology and phase behavior throughout the thermal cycle are controlled by the changing conformation of the side chains. Surprisingly, the melting of the side chains allows increases in the backbone order, π−π stacking, and carrier mobility. Upon cooling, the side chains recrystallize to preserve the excellent mesophase order and enhanced electrical performance.