Nature and extent of solution aggregation determines the performance of polymer thin-film transistors

Full text: Semiconducting polymers are interesting materials that are being developed for a wide range of applications including polymer solar cells, polymer field-effect transistors and polymer light emitting diodes. The thin film microstructure of semiconducting polymers critically influences the performance of such devices. Since semiconducting polymers have to be processed from solution, the way in which polymer chains behave in solution can influence the resulting thin film microstructure. In this presentation it will be shown that the way in which chains of the well-known n-type polymer P(NDI2OD-T2) aggregate in solution strongly affects the resulting transistor performance. The nature of aggregation is varied by using P(NDI2OD-T2) chains of different molar mass and by varying the solvent quality. In general, poor solvents are found to be better for transistor performance, with the strong aggregation of P(NDI2OD-T2) chains in poor solvents promoting elongated rod-like aggregates. These large, rod-like aggregates facilitate the correlated packing of polymer backbones in thin film leading to larger orientational correlation lengths and higher charge transport mobilities. Aggregation is not always found to be beneficial, with the weak aggregation of longer P(NDI2OD-T2) chains in tolerably good solvents frustrating the correlated ordering of backbones compared to shorter chains that maintain a more open conformation. To assess thin-film microstructure a range of techniques including synchrotron based scattering and spectroscopy techniques are used. (author)