ABSTRACT:
The microstructure of organic semiconductor films strongly affect charge carrier mobility
because it defines the persistence and quality of pi-orbital 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 (IR, vis, and X ray), scanning probe techniques, and X-ray diffraction to
determine the microstructure 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. Near edge X-ray absorption fine structure (NEXAFS)
spectroscopy indicates a conjugated plane tilt that strongly impacts electron and hole
bandwidths. NEXAFS combined with Fourier transform infrared spectroscopy (FTIR) reveals
nearly all-trans side chains that strongly tilt; reconciled with the XRD lamellar spacing it proves
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). A simple model provides a synthetic
design rule describing the side chain graft density necessary to achieve this signature packing.

Unless otherwise noted, all seminars are held in the Molecular Science and Engineering Building in Room G011. Refreshments are served at 3:30 PM. The seminar starts promptly at 4:00 PM.