Gate-bias assisted charge injection in organic field-effect transistors

The charge injection barriers in organic field-effect transistors (OFETs) seem to be far less critical as compared to organic light-emitting diodes (OLEDs). Counter intuitively, we show that the origin is image-force lowering of the barrier due to the gate bias at the source contact, although the corresponding gate field is perpendicular to the channel current. In coplanar OFETs, injection barriers up to 1 eV can be surmounted by increasing the gate bias, enabling extraction of bulk transport parameters in this regime. For staggered transistors, however, the injection is gate-assisted only until the gate bias is screened by the accumulation channel opposite to the source contact. The gate-assisted injection is supported by two-dimensional numerical charge transport simulations that reproduce the gate-bias dependence of the contact resistance and the typical S-shaped output curves as observed for OFETs with high injection barriers.

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► Charge injection barriers in OFETs are less critical than in OLEDs.
► Image-force lowering of the injection barrier by gate-bias-induced electric field.
► Typical S-shaped output curves of OFETs with high injection barriers are reproduced.
► Supported by two-dimensional numerical charge transport simulations.
► Analysis of the influence of different OFET geometries on the barrier lowering.