In-Plane Liquid Crystalline Texture of High-Performance Thienothiophene Copolymer Thin Films

Poly(2,5-Bis(3-alkylthiophen-2-yl) thieno[3,2-b] thiophenes (pBTTTs) are a new class of solution-processable polymer semiconductors with high charge carrier mobilities that rival amorphous silicon. This exceptional performance is thought to originate in the microstructure of pBTTT films, which exhibit high crystallinity and a surface topography of wide terraces. However, the true lateral grain size has not been determined, despite the critical impact grain boundaries can have on the charge transport of polymer semiconductors. Here a strategy for determining the lateral grain structure of pBTTT using dark-field transmission electron microscopy (DF-TEM) and subsequent image analysis is presented. For the first time, it is revealed that the in-plain pBTTT crystal orientation varies smoothly across a length scale significantly less than one micrometer (e. g., with only small angles between adjacent diffracting regions). The pBTTT polymers thus exhibit an in-plane liquid crystalline texture. This microstructure is different from what has been reported for small molecule semiconductors or polymer semiconductors such as poly (3-hexyl thiophene) (P3HT). Even though films processed differently exhibit different apparent domain sizes, they exhibit similar charge carrier hopping activation energies because they possess similar low densities of abrupt grain boundaries.