We report on a strong field-dependent mobility in organic field-effect transistors fabricated by using poly(2,5-bis(3-tetradecylthiophene-2-yl) thieno[3,2-b]thiophene) (pBTTT-C14) as the active polymer layer. Charge transport and mobilities in devices annealed in the mesophase show a more pronounced dependence on channel length as compared with as-cast devices. Analysis reveals that the contact effects in both sets of devices are negligible from room temperature down to ≈100 K. We show that this field dependence is consistent with a Poole-Frenkel model of mobility. Finally, the nonlinear transport data for short channel devices are modeled consistently in the Poole-Frenkel framework over a broad temperature range. © 2007 American Institute of Physics.