First in situ THz measurements on organic electrochemical transistors: New perspectives from the microscale
Organic electrochemical transistors (OECTs) are highly sensitive sensors used in increasingly challenging biologic applications such as wearable textiles with integrated biosensors and in vivo recording of brain activity.[1,2] They can be described as an ionic circuit embedded with an electronic circuit. Upon a voltage bias, ions penetrate and modifiy the doping level of the organic channel thereby changing its conductivity at both macro- and micro-scales.
One of today’s challenge to further develop this promising technology lies in the fundamental understanding of the interplay between doping level and conductivity.  Previous investigations have been addressing this question via macroscopic approaches which are inherently sensible to devices geometries and morphological features. [3-4] In this work, we propose an innovative in situ bottom-up approach that correlates the channel doping level (neutral, polaron and bipolaron populations) with its microscopic conductivity (hundred of nanometers).
We use UV-vis-NIR absorption spectroelectrochemistry combined with a multivariance curve resolution (MCR) analysis  to extract neutral, polaron and bipolaron populations. The microscopic conductivity is probed via in situ THz spectroscopy , which represents the first measurement of this kind on OECTs. Overall, this new versatile approach provides two promising perspectives:
Results on the archetypal PEDOT:PSS-based OECTs will be discussed along with the promising perspectives of this new approach on the development of OECTs in general.
 I. Gualandi, M. Marzocchi, A. Achilli et al., Sci Rep, 2016, 6, 33637.
 D. Khodagholy, T. Doublet, P. Quilichini et al., Nat Commun, 2013, 4, 1575.
 R. Colucci, H. F. de Paula Barbosa, F. Günther et al., Flexible and Printed Electronics, 2020, 5, 013001
 J. Rivnay, S. Inal, A. Salleo et al., Nat Rev Mater, 2018, 3, 17086
 A dę Juan, J. Jaumot, R. Tauler, Anal. Methodes, 2014, 6, 4964-4976
 T. Unuma, N. Yamada, A. Nakamura et al., Appl Phys Lett, 2013, 103, 053303