Influence of the semiconductor thickness on the charge carrier mobility in P3HT organic field-effect transistors in top-gate architecture on flexible substrates

In this study, we report on the dependence of the charge carrier mobility μ on the organic semiconductor thickness in organic field-effect transistors (OFETs) fabricated in top-gate architecture on flexible and transparent plastic foils as substrates with regio-regular poly-(3-hexylthiophene) (rr-P3HT) as organic semiconductor and poly(methyl methacrylate) (PMMA) as gate insulator. The gate-voltage dependence of the charge carrier mobility is taken into account by analyzing the mobility according to the Vissenberg and Matters model for charge transport in disordered systems. The transistors exhibit high mobility of almost 0.01 cm 2 /(Vs) and long-term stability. After a strong mobility increase by two orders of magnitude with P3HT thickness for very thin films of several nanometer, saturation was observed around 50 nm thickness. It was observed that the exponent of the mobility’s gate-voltage dependence increases from 1.2 to 3.2 when decreasing the semiconductor thickness. A larger exponent is related to a wider distribution of localized states in the tail of an exponential density of states and thus to higher disorder of the system. This finding explains the low mobility in these very thin films and is confirmed by phase contrast AFM measurements, which demonstrate the low degree of order in thin P3HT films.