Control of roughness at interfaces and the impact on charge mobility in all-polymer field-effect transistors

The interfacial roughness at a buried dielectric-semiconductor interface in an all-polymer field-effect transistor (FET) is characterised by specular neutron reflectivity. Using a mixed solvent (toluene cyclohexane) to deposit poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) directly on top of poly(methylmethacrylate) (PMMA), we are able to fabricate bilayer FET architectures with systematically controlled roughness at the buried polymer-polymer interface over a range of approximately 1 to 3 nm, simply by changing the solvent ratio. This system has the advantage that these two solvents are completely miscible and the mixture can dissolve F8T2, but the solvent quality of the mixture with respect to PMMA can be continuously varied by changing the ratio of toluene to cyclohexane from 100% toluene (good solvent for PMMA) to 100% cyclohexane (non-solvent for PMMA). By also fabricating F8T2 FETs from the same mixed solvents, but with silicon oxide (SiO2) as the dielectric layer, we are able to compare the effects of solvent quality and interface roughness on field-effect mobility. We find that the solvent ratio strongly affects mobility in F8T2-SiO2 FETs. In contrast charge mobility in F8T2-PMMA FETs shows relatively little dependence on solvent ratio, indicating that, to a first approximation, the effects of solvent quality and interfacial roughness cancel one another out in this system.