The transport properties of high-performance thin-film transistors (TFT) made with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The roomtemperature field-effect mobility of the devices varied between 0.004 cm 2 /V s and 0.1 cm 2 /V s and was controlled through thermal processing of the material, which modified the structural order. The transport properties of TFTs were studied as a function of temperature. The field-effect mobility is thermally activated in all films at T<200 K and the activation energy depends on the charge density in the channel. The experimental data is compared to theoretical models for transport, and we argue that a model based on the existence of a mobility edge and an exponential distribution of traps provides the best interpretation of the data. The differences in room-temperature mobility are attributed to different widths of the shallow localized state distribution at the edge of the valence band due to structural disorder in the film. The free carrier mobility of the mobile states in the ordered regions of the film is the same in all structural modifications and is estimated to be between 1 and 4 cm 2 /V s.2 1-IntroductionThe carrier mobility of polymer semiconductors has improved tremendously over the past few years. Field-effect mobility as high as 0.1 cm 2 /V s has recently been measured in regio-regular poly(thiophenes). [1][2][3] Transport characteristics are strongly dependent on the degree of order of the polymer semiconductor at the dielectric interface. Because the structure of the polymer depends on the processing conditions, it is not uncommon to find in the literature widely differing mobility values obtained for nominally the same polymer. In particular, the energy of the dielectric surface prior to the deposition of the polymer, 4-7 the solvent evaporation rate, 2 the molecular weight of the polymer 8 and thermal post-processing of the film 9 all influence the carrier mobility.There is no general consensus on the mechanism of charge transport in these amorphous or poly-crystalline materials. A complete model of the electrical properties should include a description of the energy distribution of the carriers and how the conduction varies as a function of energy. Disorder-induced localized states are clearly important for the transport, and the essential question is the relation between atomic structure, electronic structure and the transport. Generally, charge transport in disordered materials is described either as hopping between localized states, or trapping and release from localized states into a higher energy mobile state. 10 The degree of structural order may change the mechanism even within the same class of polymer.Because the electronic structure of semiconducting polymer films is not known experimentally, a simplified model has to be assumed. The model proposed by Bässler 11 assumes that the energy distribution of the carriers is Gaussian due to the random disorder in the material. The standard deviation of the Gaussian (typicall...