Many future technologies involve devices fabricated on mobile, wearable, and bio-degradable substrates that are highly flexible, such as plastics or paper. Whenever electronic components are needed for applications in such devices, inorganic semiconductors can typically not be used, since their high processing temperature is incompatible with mechanically flexible substrates. Organic semiconductors, however, are compatible with these substrates and can be used to fabricate organic thin-film transistors (TFTs). The performance of organic TFTs is limited mainly by their contact resistance, which should be minimized in order to improve their performance.
In this thesis, the contact resistance is investigated for organic p-channel TFTs based on the organic semiconductor DNTT and its derivative DPh-DNTT. The TFTs with channel lengths as short as 0.9 µm are fabricated in the bottom-gate, bottom-contact architecture and feature a thin hybrid gate dielectric that allows for operation at voltages below 3 V. The contact resistance of the organic TFTs is determined by the transmission line method (TLM). It allows for the comparison of the effectiveness of multiple approaches to modify the charge-injection behavior from the metal source and drain contacts into the organic semiconductor.
The impact of several environmental parameters during the fabrication of otherwise identical organic TFTs on the contact resistance was examined. Statistical analysis
showed that none of the environmental parameters had a strong correlation, and the observed variability of the contact resistance is mostly stochastic. Functionalizing the
gold contacts with thiol molecules changes the work function of the contact, and therefore the energy-level alignment at the metal-semiconductor interface. Pentafluorobenzenethiol (PFBT) resulted in the most reproducible low contact resistance for DPh-DNTT TFTs. It is found, however, that the Fermi level of the contacts is pinned to in-gap states in the semiconductor, hindering the reduction of the Schottky barrier. To reduce the electronic interaction and to lift the Fermi-level pinning (FLP), a decoupling interlayer has been introduced between the metal and the semiconductor. Out of 11 materials tested as an interlayer, TFTs with a tetratetracontane (C44H90) interlayer have shown the greatest reduction in contact resistance compared to TFTs without an interlayer - suggesting that the FLP is at least partially lifted. Furthermore, geometric factors impacting the contact resistance and its extraction have been investigated, especially the effect that fringe currents have on the determination of the contact resistance. Employing contacts with a triangular layout - instead of a rectangular one - enhances the electric field at the apex of the contact, which is found to reduce the contact resistance.
The DPh-DNTT TFTs that use PFBT-treated gold contacts showed contact resistances as low as 30 Ohm-cm and intrinsic channel mobilities as high as 9.2 cm²/Vs. The expected
values for TFTs on more than 100 substrates over the course of three years were 110 Ohm-cm and 6.7 cm²/Vs, respectively. Incorporating a tetratetracontane interlayer between the contacts and DPh-DNTT yielded TFTs with about 10% reduced contact resistances, as low as 11 ± 5 Ohm-cm. This value is on par the smallest contact resistance of 12 Ohm-cm reported to date for organic TFTs.