Microstructure foundations of high carrier mobility in polymers

Abstract

The microstructure of organic semiconductor films can impact charge carrier mobility because it defines the persistence and quality of overlap in the source-drain plane. Important aspects of microstructure include the intermolecular packing arrangement within crystals, the substrate-relative crystal orientation, and the overall crystal size and connectivity. We combine complementary microstructure measurements including polarized absorption spectroscopies (infrared, visible, and X-ray), scanning probe techniques, and X-ray diffraction (XRD) to investigate the microstructure details of polymer semiconductors for organic thin film transistors (OTFTs). The investigations establish correlations between primary chemical structure, processing, film microstructure, and carrier mobility. These fundamental relationships yield practical guidelines for synthesis and processing. 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. XRD of pBTTT films indicates lamellar packing, and atomic force microscopy (AFM) reveals terraces several microns wide. Spectral ellipsometry (SE) indicates that polymer long axes are confined to the film plane. The XRD, AFM, and SE evidence supports a narrow orientation distribution, which permits tilt angle assignment from linearly polarized spectroscopies. 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. Similar analyses explain the influences of dielectric chemistry, dielectric roughness, and thermal history, and the mechanisms by which each impacts semiconductor microstructure and OTFT performance.