Publication date: 28th August 2024
Organic electrochemical transistors (OECTs) are being widely studied due to their numerous applications such as organic bioelectronics, neuromorphic systems, sensors, etc. OECTs offer the advantages of transporting both electrons and ions due to the organic mixed ionic-electronic conductors (OMIECs) that allow ions to penetrate the channel throughout its volume. However, the design rules to fully optimize the OECTs are still unclear. It is possible to “tune” the performance, that is, the transconductance (gm), of OECTs according to the Bernard model for variations in channel length, thickness and width. On the other hand, the effects due to the morphology/structure of the channel are always present, but their influence is not yet clear. The effect of gate-voltage-dependent resistance, known as contact resistance (RC), is critical to performance. This parasitic RC, obtained by transmission-line method (TLM), is present at the interface between the source/drain electrodes and the channel. Here, the results of the doping effect of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) on RC, more specifically in the source-drain/channel interfaces, are analyzed. Unlike organic field-effect transistors (OFETs) in which molecular contact doping (a dopant layer is deposited between the channel and the metallic contacts) or OECTs using source/drain-electrode surface modification, LiTFSi is used in ultra-low quantities. This ultra-low LiTFSi content is defined by the ratio between the number of LiTFSi molecules and the number of olygo(ethylene-glycol) (OEG). By introducing LiTFSi into the bulk of the channel, the RC is enhanced by several orders or magnitude. Our results showed that an ultra-low presence of LiTFSi improves (reduces) RC by ~ 1-2 orders of magnitude. RC improvement has been observed in p-type (p(g2T-T)) and n-type (p(gNDI-gT2)) OECTs and can be reduced to very low values of approximately 0.002 Ω∙cm during operation. Both polymers present OEG chains that facilitate the transport of Li+ ions through the bulk of the channel and could explain the RC improvement. On the other hand, in some devices, the performance (transconductance) increases significantly up to 3-4 times due to low RC values.
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