Microstructure foundations of high carrier mobility in polymers.

The microstructure of organic semiconductor films can impact charge carrier mobility because it defines the persistence and quality of $\backslash $pi bond overlap in the source-drain plane. Important aspects of microstructure include the intermolecular packing arrangement within crystals, the surface-relative crystal orientation, and the overall crystal size and connectivity. We combine complementary microstructure measurements including polarized absorption spectroscopies, scanning probe techniques, and X-ray diffraction to investigate the microstructure details of polymer semiconductors for organic thin film transistors (OTFTs). Here, we demonstrate this approach by solving the packing arrangement of a polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTTs), with hole mobility of (0.2 to 0.6) cm2/Vs. NEXAFS combined with FTIR spectroscopy reveals nearly all-trans side chains that strongly tilt. With the XRD lamellar spacing, we show that vertically adjacent layers interdigitate. A general consideration of side chain configuration reveals a striking signature packing motif that sets high performance polymers such as pBTTT apart from the lower performance poly(3-alkylthiophenes).