Processing of a P(NDI2OD-T2) film without an intermediate wetting layer during spin-coating to improve charge carrier mobility

Abstract

The thin film morphology of a conjugated polymer is one of the important factors in determining its electrical performance, which is controlled by thermodynamic and kinetic factors. However, attaining a clear solution processing window for film preparation remains challenging due to the intrinsic complex properties of polymers and solvents. Herein, we proposed a strategy to obtain a uniform fibrous morphology by direct formation of conjugated polymer films on a substrate without an intermediate wetting layer via controlling solvent evaporation. We chose a poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)] solution as the model system. The intermediate wetting layer vanishes during the spin-coating deposition when the solvent evaporation speed is greater than 130–140 μm s⁻¹ achieved by increasing the substrate temperature. At this point, in the time-resolved in situ photoluminescence (PL) spectra of P(NDI2OD-T2), an intermediate transition state of the 0–1 peak appears at 756 nm in addition to the 0–0 peak at 834 nm. This indicates that the polymer chains are rearranged to form homogeneous fibers without detrimental aggregation. Compared to the film formed with an intermediate wetting layer, some large aggregates can be observed. Thus, the electron mobility reaches 0.33 ± 0.02 cm² V⁻¹ s⁻¹ for the film without the intermediate wetting layer, which is 3 times higher than that of the film formed with the wetting layer. The relationship between the deposition regimes, microstructure and charge carrier mobility was also revealed in other solvents, which can serve as generalized guidelines for conjugated polymer film deposition.