Impact of Charge Carrier Injection/Extraction Performances in Low-Dimension PEDOT:PSS Organic Electrochemical Transistors

Organic electrochemical transistors (OECT) are gaining momentum in future applications of biosensors and bioelectronics. Nonetheless, contact (or series) resistances (R_S/D) remain underexplored, even though physical processes between the source/drain electrodes and organic mixed ionic-electron conductors (OMIECs) drive a substantial part of their performances. To address this shortcoming, in this study, low-dimension OECTs featuring 2 µm-long poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) channel are explored. Normalized contact resistances (R_S/D⋅W) values as low as 1.4 W cm are obtained. It is observed that channel PEDOT:PSS thickness is not detrimental to R_S/D but is impacting the cut-off frequency. A figure-of-merit (h) expressing the charge-carrier injection (or extraction, respectively) efficiency shows that planar depletion-mode OECTs are not contact-limited up to L = 30 µm channel length. Finally, an unprecedented approach that highlight the importance of optimizing the micro-fabrication technologies is shown, by decreasing the contact overlap length, according to OMIECs physicochemical contact properties. Indeed, a transfer-length method coupled to a current-crowding model allow to fully understand the behavior of low-dimension PEDOT:PSS OECTs and next, to optimize its circuits design. This is paving the way toward the development of OECTs-based integrated circuits with faster switching speed, broadening further their scopes and future use as advanced bioelectronics platforms.