| Future development of organic semiconductors requires understanding of the primary chemical structure and processing conditions and how they relate to the thin film microstructure and the final organic field effect transistor (OFET) performance. Establishing these relationships is a necessary step towards controlling thin film crystallization processes to realize greater carrier mobility and more facile processing methods. Probing the thin film crystal structures of complex organic molecules is challenging because they often form unit cells with complex spatial and orientational arrangements of their chemical moieties. Developing complete structure models for more complex molecules requires multiple, complementary measurements.;Using a unique characterization approach, the thin film crystal structure of small molecule organic semiconductor 6,7,15,16-tetrakis(alkylthio)-quinoxalino[2',3',9,10]-phenanthro[4,5-abc]phenazine (TQPP-[SR]4) was determined using a combination of polarized photon absorption spectroscopies (X-ray, vis, and infrared), X-ray diffraction, and scanning probe techniques. The combined techniques allowed the determination of crystal order and local molecular orientation within thin films. The TQPP-[SR]4 core orientation changes dramatically depending on the length of its side chains. Longer side chains enforce a core orientation with greater potential for pi-overlap in the source-drain plane of OFETs. Surprisingly, TQPP-SC8H17 exhibits room temperature polymorphism, where the core packing motif changes after thermal treatment. The thin film microstructure and core packing styles appear to directly correlate with variations in OFET performance; packing styles with improved pi-overlap and larger lateral domain size exhibit greater field effect carrier mobility. |
