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Zachary Lamport1 Katrina Barth1 Iain McCulloch2 3 John Anthony4 Oana Jurchescu1

1, Wake Forest University, Winston Salem, North Carolina, United States
2, Imperial College London, London, , United Kingdom
3, King Abdullah University of Science and Technology, Thuwal, , Saudi Arabia
4, University of Kentucky, Lexington, Kentucky, United States

The performance of organic thin-film transistors (OTFTs) depends on a variety of factors including the type of semiconductor and processing conditions, the dielectric, the device structure, and the choice of electrodes. In particular, the quality of the source and drain contacts is of the utmost importance to facilitate efficient charge injection and minimize contact resistance. Here, we focus on the study of contact effects in OTFTs and their relation to processing conditions. We fabricate bottom-contact, top-gate OTFTs with pentafluorobenzenethiol (PFBT)-treated Au/Ti contacts, the organic semiconductor 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene (diF-TES ADT) and Cytop gate dielectric. We deposit the source and drain electrodes using thermal evaporation and vary the deposition rate from 0.1 Å/s to 2 Å/s, while maintaining the geometry of the device unmodified. We find that the field-effect mobility is dependent on the details of the electrode fabrication, with a maximum value being obtained for a rate of 0.5 Å/s, µmax = 19 cm2/Vs, and gradually decreasing to a value of µ = 8 cm2/Vs, at a rate of 0.1 Å/s, and µ = 4 cm2/Vs at 2 Å/s, a value that is consistent with earlier reports obtained under similar conditions.1 By examining the dependence of mobility on gate-source voltage, we observe an abrupt increase, followed by a plateau at the reported values, confirming that the reported mobility values are not simply an artefact of the measurement due to gated contacts. We evaluate the contact resistance through gated-TLM measurements, and find that it follows the same trend as the mobility vs. deposition rate, with the lowest value aggressively lowered to 0.5 kΩcm for 0.5 Å/s deposition rate. We discuss the factors that contribute to lowering of the contact resistance, which, in turn, gives improved device characteristics.
To determine if this drastic improvement in electrical characteristics through modifying the electrode processing is material-dependent, we further performed a similar study using another well-known semiconductor, in this case the copolymer indacenodithiophene-co-benzothiadiazole (IDTBT). Using the same device structure, we find a maximum mobility of 12 cm2/Vs at a deposition rate of 0.5 Å/s, which is approximately 4 times greater than the highest value reported in the literature for this material,2 and results from a low contact resistance of 0.2 kΩcm. The vastly improved performance exhibited by these devices indicate that the deposition rate of source and drain electrodes is a salient parameter that must be accounted for in device design and is effective for both small-molecule and polymer semiconductors.
1 P. J. Diemer, et al., Appl. Phys. Lett., 2015, 107, 103303.
2 X. Zhang, et al., Nat. Commun., 2013, 4, 1–9.

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