How the series resistance influence the transconductance of the OECT planar structure

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The organic electrochemical transistor (OECT) plays an important role in modern bioelectronics. Its use in study of living cells physiology keeps this device very attractive and at the forefront of various organic bioelectronic devices [1]. In a case of some bioelectronic applications, namely in sensing, the high gain of the OECT is needed. The key parameter which describes high gain in OECT is transconductance. Since the transconductance describes the modulation of the gate voltage to the drain current [2,3], a strong dependence on channel parameters is known [3,4]. To develop the OECT with high transconductance, an optimal electrical properties of the semiconductor and the length and width of the channel must be found. However, for the sample with high ratio of width and thickness to channel length (Wd/L), the dependence of transconductance on channel parameters was shown to be nonlinear [5]. Kaphle et al. shown that the contact resistance plays the significant role in a case of highly doped organic semiconductors [6]. This means that the extracted OECT parameters are burdened by contact resistance error. To compensate for this effect the corrected values of transconductance, i.e. intrinsic transconductance, can be calculated [7]. To prepare high transconductance OECT devices, the effect of the thickness of organic semiconductor and electrode system and the effect of busbars on the OECT transconductance were studied in this contribution. The electrode systems with variable thickness were prepared by chemical vapor deposition and the semiconductor films with variable thickness were prepared by spin-coating. As a semiconductor film, the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was used. By varying the geometric parameters of the electrode system and the channel, the OECT serial resistance was decreased and the transconductance value 63 mS was achieved. By lowering the serial resistance, the peak transconductance was increased and the difference between the peak transconductance and intrinsic transconductance was lowered, see Figure. In the Figure the lines represents calculated theoretical dependences for given contact resistance based on Chou model7. As can be seen from the Figure 1, our measured data for different electrodes thickness well represents the model. Due to additionally lowered serial resistance with the help of silver-plated electrodes, the transconductance was further increased. We shoved that the OECTs can be limited by their series resistance. With proper analysis based on the voltage drop division on resistors in series it is possible to (i) design an arrangement that will not be burdened by this effect when analyzing new materials, and (ii) optimize the OECT layout to obtain the maximum transconductance and thus the sensitivity of the OECT sensor.

OECT, PEDOT:PSS, series resistance, transconductance

MARKOVÁ, A.; VALA, M.; STŘÍTESKÝ, S.; WEITER, M.

17. 9. 2020

Masaryk University Press

Brno

978-80-210-9655-4

XX. Workshop of Biophysical Chemists and Electrochemists Book of abstracts

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