Using a gradient combinatorial approach, the authors report the effects of temperature on the microstructure and hole mobility of poly(2,5-bis(3- dodecylthiophen-2yl)thieno[3,2-b]thiophene) thin films for application in organic field-effect transistors. The gradient heating revealed a detailed dependence on thermal history. Optimal heat treatment achieved mobilities as high as 0.3 cm2 V-1 s-1. Mobility enhancement coincides with an increase in crystal domain size and orientation, all of which occur abruptly at a temperature closely corresponding to a bulk liquid crystal phase transition. © 2007 American Institute of Physics.