Charge density increase in submonolayer organic field-effect transistors

Interface confinement plays a central role in charge carrier accumulation and transport along the channel of organic field-effect transistors. Understanding the relevant interfacial interactions that affect the energy landscape experienced by carriers in the channel is of fundamental interest. Here we investigate charge transport in the submonolayer regime of pentacene transistors in which confinement arises due to the finite size of the interconnected semiconducting islands. In situ real-time electrical characterization is used to monitor the formation and evolution of the accumulation layer at the very early stages of growth. The morphology of the confining interfaces is controlled by growth conditions and pentacene coverage. Charge transport occurs when percolation pathways connecting source and drain electrodes are formed at a critical coverage. The displacement current across the oxide/semiconductor interface is observed starting from the onset of percolation (0.69 monolayer coverage). The analysis of the characteristics shows that already the submonolayer film fully screens the gate field and accumulates higher charge carrier density as compared to the monolayer film. We propose an electrostatic model to correlate the charge density to the characteristic length scale of the submonolayer film and the thickness of the dielectric layer. This explains charge mobility and threshold voltage of thin-film transistors in the submonolayer regime.