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We present a study performed on organic thin film transistors based on poly(3-hexylthiophene) with molecular weights ranging from 20 to 80 kDa as active material. Besides having a strong influence on the absolute value of the mobility, we show that the molecular weight also drastically affects the mobility functional dependence on the gate voltage and on the longitudinal electric field. While the medium range of molecular weight (37–53 kDa) yields a high (about 10−2 cm2/V s) and practically constant mobility, the low and high ranges yield a lower mobility, which in addition shows a strong dependence on both the charge density and the electric field. By means of a detailed analysis of experimental transfer characteristics of transistors, this behavior is traced back to the broadness of the density of states, which turns out to be higher for low mobility polymers. Finally, consequences on transistor modeling due to the simultaneous dependence of the mobility on charge density and electric field are discussed.

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