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.