The effects of the roughness of the gate dielectric on the performance of thin film transistors (TFTs) fabricated with poly(2,5-bis(3-hexadecylthiophen-2- yl)thieno[3,2- b] thiophene) (PBTTT-C16) were examined. The field effect mobility of coplanar TFTs made with PBTTT-C16 and plasma-enhanced chemical vapor deposited Si O2 Si Nx dielectrics decreased nearly exponentially with surface roughness. Films of PBTTT-C16 have similar crystalline structure on smooth and rough surfaces, but the domain size decreases with increasing roughness. Surface roughness was found to have less of an impact on field effect mobility than the chemical treatment of the dielectric. © 2007 American Institute of Physics.