Chris McNeill1 Masrur Nahid1 Eliot Gann3 Lars Thomsen2 Kamendra Sharma4

1, Monash University, Clayton, Victoria, Australia
3, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
2, Australian Synchrotron, Clayton, Victoria, Australia
4, Indian Institute of Technology, Bombay, Mumbai, , India

The effect of solvent quality on the microstructure and organic field-effect transistor (OFET) performance of thin films of the high mobility naphthalene-diimide-thiophene copolymer P(NDI2OD-T2) is presented. A strong correlation between OFET mobility and solvent quality is observed with average electron mobility increasing from 0.21 cm2/Vs for samples spin-coated from tolerably-good solvents to 0.56 cm2/Vs for samples spin-coated from poor solvents, with a maximum electron mobility of 1.5 cm2/Vs observed for transistors processed from toluene. The variation in transistor performance with solvent quality is linked to the nature and extent of the solution aggregation of P(NDI2OD-T2) chains. Small angle X-ray scattering measurements reveal elongated rod-like aggregates up to 300 nm in length in solutions prepared using poor solvents, in contrast to more coil-like chains with radius of gyration of ~ 10 to 15 nm for solutions based on good to tolerably-poor solvents. Thin films produced from solvents of decreasing quality show an increase in the extent of correlated ordering of backbones and the degree edge-on orientation of polymer chains at the air/film interface. This work establishes the important link between solution-phase chain aggregation behavior, thin-film microstructure and transistor performance in the P(DNI2OD-T2) system.