Contact resistance in ambipolar organic field-effect transistors measured by confocal photoluminescence electro-modulation microscopy.

While it is theoretically expected that all organic semiconductors support ambipolar charge transport, most organic transistors either transport holes or electrons effectively. Single-layer ambipolar organic field-effect transistors enable the investigation of the different mechanisms in hole and electron transport in a single device since the device architecture provides a controllable planar pn-junction within the transistor channel. However, a direct comparison of the injection barriers and of the channel conductivities between electrons and holes within the same device cannot be measured by standard electrical characterizations. This article introduces a novel approach for determining threshold gate voltages for the onset of the ambipolar regime from the position of the pn-junction observed by photoluminescence electro-modulation (PLEM) microscopy. Indeed, the threshold gate voltage in ambipolar bias regime considers a vanishing channel length thus correlating to the contact resistance. PLEM microscopy is a valuable tool to directly compare the contact and channel resistances for both carrier types in the same device. The reported results demonstrate that designing the metal/organic-semiconductor interfaces by aligning the bulk metal Fermi levels to the HOMO or LUMO levels of the organic semiconductors is a too simplistic approach for optimizing the charge injection process in organic field-effect devices.