Fermi-Level Pinning in Organic Thin-Film Transistors

Organic thin-film transistors (TFTs) are of interest for flexible electronics applications, such as flat-panel displays and sensors. An important performance parameter is the transit frequency, which is the highest frequency at which the TFTs are able to amplify electrical signals. The transit frequency is determined by several parameters, including the intrinsic channel mobility, the channel length, the gate-to-contact overlaps, and the contact resistance. It can be shown that in organic TFTs that have an intrinsic channel mobility of at least 5 cm²/Vs and critical dimensions of a few microns, the transit frequency is limited almost entirely by the contact resistance. This implies a significant incentive for reducing the contact resistance to improve the dynamic TFT performance.<br/><br/>The contact resistance is determined mainly by the energy-level alignment at the interface between the semiconductor and the source/drain contacts, and (in the case of TFTs fabricated in the inverted coplanar device architecture) by the thin-film morphology of the semiconductor on the contact surfaces [Adv. Mater. <b>32</b>, 2104075, 2022]. Functionalizing the contact surfaces with a chemisorbed monolayer (e.g., a thiol) prior to the organic-semiconductor deposition can be helpful in improving the semiconductor morphology and in adjusting the Fermi level of the contacts by introducing an interface dipole. In principle, this dipole lowers the Schottky barrier between the Fermi level of the contact and the transport level of the semiconductor, thereby reducing the contact resistance. For TFTs fabricated on flexible polyethylene naphthalate (PEN) substrates using the small-molecule semiconductor diphenyl-dinaphthothienothiophene (DPh-DNTT) and gold source/drain contacts functionalized with a monolayer of pentafluorobenzenethiol (PFBT), we have measured an average contact resistance of less than 100 Ωcm (and an intrinsic channel mobility of 9 cm²/Vs, a subthreshold swing of 65 mV/dec, a threshold voltage of -1 V, and an on-off current ratio above 10⁸).<br/><br/>Perfect energy alignment at the interface between metal contacts and an organic semiconductor is, however, often prevented by a pinning of the Fermi level to the energy of localized electronic states in the semiconductor. Measurements have shown that any work-function changes induced by functionalizing the contacts with a thiol are eliminated once the organic semiconductor is deposited. Further significant reductions of the contact resistance will thus require that the Fermi level is depinned to allow energy-level alignment.<br/>A promising approach to depinning the Fermi level is to introduce an interlayer between the metal and the semiconductor in order to decouple the metal from the localized states in the semiconductor. To investigate the effectiveness of this approach in organic TFTs, we fabricated DPh-DNTT TFTs using a wide range of interlayer materials. The interlayers and the organic semiconductor were deposited by thermal sublimation in vacuum. The interlayer thickness was limited to a few nanometers to balance the beneficial effect of charge decoupling and the detrimental effect of the tunneling barrier.<br/><br/>In the literature, it has been suggested that in the case of p-channel TFTs, the interlayer needs to have a large ionization energy [Adv. Electron. Mater. <b>6</b>, 1901352, 2020]. However, we have found that a high ionization energy alone is insufficient and that other factors, including the semiconductor morphology, the interlayer thickness and possibly electronic interactions such as doping must be considered. For some interlayer materials, we have observed a substantial reduction in contact resistance compared to TFTs without any interlayer (contact resistance as small as 15 Ωcm in TFTs fabricated using a 2-nm-thick tetratetracontane interlayer), but a clear correlation to the ionization energy of the interlayer remains elusive.