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. [3] 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 [4] to extract neutral, polaron and bipolaron populations. The microscopic conductivity is probed via in situ THz spectroscopy [5], which represents the first measurement of this kind on OECTs. Overall, this new versatile approach provides two promising perspectives: 

1. Disentangle underlying mechanisms limiting long-range charge transport such as device geometries and morphological features
2. Accessing fondamental properties to push further microscale modelling

 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. 

[1] I. Gualandi, M. Marzocchi, A. Achilli et al., Sci Rep, 2016, 6, 33637.

[2] D. Khodagholy, T. Doublet, P. Quilichini et al., Nat Commun, 2013,  4, 1575.

[3] R. Colucci, H. F. de Paula Barbosa, F. Günther et al., Flexible and Printed Electronics, 2020, 5, 013001

[4] J. Rivnay, S. Inal, A. Salleo et al., Nat Rev Mater, 2018, 3, 17086

[5] A dę Juan, J. Jaumot, R. Tauler, Anal. Methodes, 2014, 6, 4964-4976

[6] T. Unuma, N. Yamada, A. Nakamura et al., Appl Phys Lett, 2013,  103, 053303