Mixed Ionic-Electronic Transport for PEDOT:PSS-Based Zero-Gated Organic Electrochemical Transistors Using Impedance Spectroscopy and Micro-Raman Imaging

Abstract

Recent studies are being focused on establishing a link between structural and mixed ionic-electronic transport properties based on the characterization of organic mixed ionic-electronic conductors (OMIECs) under operation conditions in organic electrochemical transistor (OECT) devices. Here, we report the simultaneous access to microstructural, chemical, and physical information using impedance spectroscopy and micro-Raman imaging techniques to address the mixed ionic-electronic transport mechanisms in PEDOT:PSS-based OMIECs working as OECT channel materials. We focused on the out-of-plane and in-plane mixed ionic-electronic transport mechanisms and complement our experimental studies with computational simulations using molecular dynamics (MD) and density functional theory (DFT) methodologies. Our studies revealed that the out-of-plane transport is based on ionic injection and diffusion mainly associated with the formation of nanopore channels in PEDOT:PSS exhibiting sulfonic groups toward the aqueous media. Thus, we also evidenced that the dedoping is not only due to the direct interaction of positive ions with PEDOT:PSS bulk region but also indirectly by the rearrangements of sulfonic groups from PSS toward the aqueous media. The in-plane transport in the OECT channel was properly described as a mixed ionic-electronic transport, with not only ionic transport of sodium (Na+) but also hydronium (H3O+) species mainly through nanopores and PEDOT:PSS bulk regions and with electronic transport mainly through the PEDOT:PSS bulk region. Although it is shown that the electronic transport monotonically declines with increasing ionic concentration, the Na+ diluted regime (1 mM) exhibits weak selectivity compared to H3O+ species, and the Na+ concentrated regime (100 mM) induces extreme porosity in PEDOT:PSS yielding to the destruction of the device. These results introduce some particular features for PEDOT:PSS-based OECT channel materials that need to be considered not only for ionic concentration and selectivity but also other related stability issues.

Exploring Mixed Ionic–Electronic-Conducting PVA/PEDOT:PSS Hydrogels as Channel Materials for Organic Electrochemical Transistors

“…Polymers 2024, 16, x FOR PEER REVIEW 11 of 14 gated mode (Vg = 0) condition, the presence of the chloride anion is more or less passive, but the potassium cation is more active by directly interacting with thiophene rings of PEDOT but also with the sulfonate group of PSS, as also evidenced for sodium cations using the same level of theory in our recent report [56]. Finally, in the OFF mode (Vg > 0) condition, the potassium cation is freer to interact with the thiophene ring of PEDOT and at the same time to block the doping ability of the sulfonate group of PSS, leading to an increment in PEDOT:PSS de-doping.…”

“…In addition to this, we can find peaks at 976 and 820 cm −1 associated with the C-C and C-S stretching mode for the PEDOT thiophene ring, respectively [55]. [56,59]. Two additional peaks at 1509 and 1567 cm −1 are assigned to C=C asymmetrical stretching modes [60,61].…”

“…It is important to note that the Raman modes observed for the pure PVA reference material exhibit no drastic modifications for PVA hydrogel samples not containing PEDOT:PSS (i.e., PP-0). [56,59]. Two additional peaks at 1509 and 1567 cm −1 are assigned to C=C asymmetrical stretching modes [60,61].…”

“…The peaks near 1718 cm −1 are probably associated with the C=O stretching modes from partially oxidized PVA but also coexist with those associated with acetal and hemiacetal groups which are products of the crosslinking between PVA and glutaraldehyde [ 43 , 54 ]. Moreover, we can also see evidence of signals corresponding to the structure of PEDOT:PSS such as the characteristic peak at 1647 cm −1 attributed to the C=C asymmetric stretching mode for the PEDOT thiophene structure [ 55 , 56 ]. However, it is important to note that these peaks also coexist with those associated with O-H bending modes of PVA.…”

“…However, it is important to note that these peaks also coexist with those associated with O-H bending modes of PVA. Interestingly, there is a peak at 1525 cm −1 which is only present for samples with PEDOT:PSS that can be attributed to the C=C symmetric stretching mode for the PEDOT thiophene structure [ 56 ]. The presence of broad peaks at 1090–1030 and 830–840 cm −1 can be associated with the C-O and C-C stretching of PVA, respectively, and the peaks at 1250–1200 cm −1 can be related to the C-O-C stretching modes of acetal and hemiacetal groups [ 57 ].…”