High carrier mobility polythiophene thin films: Structure determination by experiment and theory

The conjugated planes of poly(2,5-bis(3-alkythiophene-2yl) thieno[3,2-b] thio-phenes)(pBTTT) substantially tilt within their crystalline lamellae were studied using density functional theory (DFT). First-principles total energy calculations of the pBTTT backbone showed that tilting of the conjugated plane was energetically favorable. Conjugated-plane orientation was measured using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. AFM microscopy image and SE dielectric function for pBTTT films with tetradecyl side chains (pBTTT-C₁₄) indicated a single type comprehensive crystalinity. The DFT calculations indicate that tilt of the conjugated plane is common feature in alkyl-functionalized thiophene polymers. The experimental verification of conjugated-plane tilt, made possible by the highly crystal pBTTT-C₁₄, supports the validity of the calculations and indicates that πstacking slip is an important structural feature for polymer semiconductors.