Structure and Charge Transport Anisotropy of Polythieno[3,4-b]-Thiophene-co-Benzodithiophene (PTB7) Oriented by High-Temperature Rubbing

Structure determination in high performance polymer semiconductors such as polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) is essential to establish proper structure-property correlations. A combination of high-temperature rubbing and isothermal crystallization leads to oriented and crystalline films of face-on oriented PTB7 crystals. Electron diffraction indicates that crystallinity is marginal in the rubbed films but develops upon postrubbing annealing at T >= 250 degrees C. The best oriented and crystalline PTB7 films have a dichroic ratio of 12 for UV-vis absorption. The hole mobilities are improved by a factor of six along the rubbing direction over nonaligned films (mu(//) = 5.8 x 10(-3) cm(2) V-1 s(-1) vs mu(perpendicular to) = 3.1 x 10(-4) cm(2) V-1 s(-1)). Structural analysis yields three possible models that share similar structural features, however; namely (i) PTB7 chains form a layered packing such as poly(2,5-bis(3-dodecyl-2-yl)thieno[3,2-b]thiophene) with pi-stacked backbones alternating with layers of strongly interdigitated alkyl side chains, (ii) the PTB7 chains adopt a nonplanar zigzag conformation, and (iii) PTB7 chains show a mixed stacking of thieno[3,4-b]thiophene and benzodithiophene blocks. Overall, these results highlight the key role played in polymer semiconductor crystals by the steric constraints due to the branched alkyl side chains interdigitation on the pi-stacking of conjugated backbones.