Ionic self-assembled monolayer for low contact resistance in inkjet-printed coplanar structure organic thin-film transistors

• To reduce the contact resistance, a new surface treatment concept is proposed.
• Ionic SAM treated electrodes show better charge injection than the electrodes with conventional treatment.
• Ionic SAM containing ionic species effectivey increase the charge density in near the interface so may make the contact barrier thinning.

To reduce the contact resistance in inkjet-printed organic thin-film transistors (OTFTs), the use of a newly synthesized ionic self-assembled monolayer (SAM) consisting of an anchoring group, a linker group, and an ionic functional group, is investigated. According to the gated transmission line method (TLM) measurements of a series of OTFT devices, where one type has no charge injection layer, another type having a pentafluorobenzenethiol (PFBT) injection layer, and a third type containing a (6-mercaptohexyl)trimethylammonium bromide (MTAB) ionic SAM, the latter exhibits the lowest contact resistance value of ∼3.1 K Ω cm. The OTFTs without charge injection layer and with the PFBT SAM have relatively higher contact resistance values of ∼6.4 K Ω cm and ∼5.0 K Ω cm, respectively. The reduced contact resistance in the OTFTs with ionic SAMs is attributed to the large charge carrier density induced by the ionic SAM, which allows sufficient tunneling-assisted injection of the carriers from the metal electrode to the polymer semiconductor. These results suggest that the use of appropriate ionic SAM injection layer is an effective way to reduce the contact resistance, hence improving the charge transport characteristics of inkjet-printed OTFTs.

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