Doped Organic Electrochemical Transistors—Effect on Channel/Electrodes Interfaces (Contact Resistance)

Organic electrochemical transistors (OECTs) are being widely studied due to their numerous applications such as organic bioelectronics, neuromorphic systems, sensors, etc. OECTs transport benefits of 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, this is the transconductance (g_m), of OECTs according to the Bernard model for variations in channel length, thickness and width, but effects due channel morphology/structure is not considered. The effect of gate-voltage-dependent resistance, better known as contact resistance (R_C), is critical to performance. This parasitic R_C can be obtained using the transmission-line method (TLM) and is present in the interface between the source/drain electrodes and the channel. Here results of the doping effect of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) on R_C, more specifically in the source-drain/channel interfaces, are analyzed. Unlike organic field-effect transistors (OFETs) in which through molecular contact doping (a dopant layer is deposited between the channel and the metallic contacts) or OECTs using source/drain-electrode surface modification , we use LiTFSi 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 this LiTFSi in the bulk of the channel, the R_C is enhanced in several orders. Our results showed that an ultra-low presence of LiTFSi improves (reduces) the R_C by ~ 1-2 orders of magnitude. This R_C improvement has been observed in p-type (p(g2T-T)) and n-type (p(gNDI-gT2)) polymers and can be reduced to very low values of approximately 0.002 Ω cm. Both polymers present OEG chains that facilitate the transport of Li⁺ ions through the bulk of the channel and could lead to the explanation for this R_C improvement. On the other hand, in some devices the performance (transconductance) increases significantly up to 3 times due this low R_C values.