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The effects of molecular packing on the field-effect mobility and external quantum efficiency of polymer light-emitting transistors based on films composed of a donor–acceptor polymer, poly(9,9-dioctylfluorene- al t-benzothiadiazole) (F8BT), are investigated. The molecular packing structures of F8BT films are modified by annealing at different temperatures, such that large grains in a mesophase are formed. In the case of F8BT films in the mesophase, highly crystalline three-dimensional configurations with adjacent chains result in effective electron transport. The electron mobility, μ e , in a device in which the F8BT film is in the mesophase is approximately ten times greater than that for F8BT films annealed at other temperatures, and is estimated to be approximately 3.4 × 10 −3 cm 2 V −1 s −1 . This same device exhibits the highest hole and electron mobilities along with the highest maximum external quantum efficiency (approximately 1.4%) due to improved electron transport and carrier balance. [ABSTRACT FROM AUTHOR]

Abstract: The effects of film morphology on ambipolar transport in solution-processed top-gate-type organic field-effect transistors (OFETs) utilizing poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) films were investigated. All the F8T2 OFETs without toluene-vapor treatment showed ambipolar characteristics. In contrast, for toluene-vapor-treated F8T2 films at below 250°C, which were observed to have a small fibrillar structure and a large number of grain boundaries, electron conduction decreased remarkably. Structure ordering in F8T2 films has a strong influence on ambipolar charge transport in OFETs. With an increase in the annealing temperature, the electron transport characteristics were improved. Improving the interface geometry of the F8T2 film, which is related with the decrease of grain boundaries, led to an increase in mobility, and the appearance of ambipolar characteristics. These results suggest that the appearance of electron conduction results not only from the increase in grain size but also from improvements in the connections between grain boundaries in F8T2. Ambipolar F8T2 OFETs exhibited yellow EL emissions, hole and electron field-effect mobilities of approximately 10−3 cm2 V−1 s−1, and maximum external quantum efficiency of approximately 0.2%. [Copyright &y& Elsevier]