Relation between injection barrier and contact resistance in top-contact organic thin-film transistors

We theoretically investigate the carrier injection into top-contact bottom-gate organic thin film transistors. By means of a two-dimensional drift–diffusion model, we explicitly consider thermionic and tunneling injection in combination with subsequent carrier transport into the device. Based on numerical simulations with this model, we determine the contact resistance as a function of the nominal hole injection barrier height and temperature. Depending on the barrier height or the operating temperature, we find three distinct injection regimes. Our work reveals that in all three regimes self-regulating processes exist due to which the influx of current is adjusted according to the needs of the channel at the given point of operation.We explain why the transmission/transfer line method (TLM) for the determination of the contact resistance, RcRc, quantitatively fails for non-quasi-ohmic injection. Self-regulation links the contact resistance to the channel resistance and the contact resistance becomes dependent on the channel length. For larger channel lengths, RcRc is underestimated by TLM; the method yields overestimated values for small channel devices.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Direct relation of the contact resistance to the injection barrier for an ideal device geometry using a 2D drift–diffusion based model. ► Contact resistance strongly related to channel resistance due to shared current. ► Contact resistance strongly dependent on channel length. ► Systematic increase of injection barrier height reveals three distinct injection regimes: quasi-ohmic, barrier regulated thermionic, tunneling. ► Temperature analogously controls injection regime.