Microscopic Understanding of the Granular Structure and the Swelling of PEDOT:PSS

Abstract

The conjugated polymer poly­(3,4-ethylenedioxythiophene) polymerized and stabilized in the presence of polystyrene­sulfonate (best known as PEDOT:PSS) is a working horse of organic electronics and bioelectronics and one of the most important conductive polymers. While its morphology is complex and depends on the details in synthesis and post-treatment, its distinctive and common feature is a two-phase granular structure attributed to PEDOT- and PSS-rich regions. Yet, there is still no well-established consensus concerning the precise nature of PEDOT- and PSS-rich regions as well as their chemical composition and structure. In this study we perform coarse-grained MARTINI molecular dynamics simulations of PEDOT:PSS focusing on understanding its two-phase morphology as well as water intake and ion exchange. We demonstrate that PEDOT:PSS is an essentially three-component system consisting of positively charged PEDOT chains, PSS chains with mostly deprotonated sulfonate groups, and protonated PSS chains. PEDOT-rich regions are predominantly composed of PEDOT and deprotonated PSS chains, whereas PSS-rich regions are composed of protonated PSS chains. Our calculations unravel how PEDOT- and PSS-rich regions are formed from the solution phase during the drying process. We show that when the dry polymer film is immersed in water, its swells by nearly 60%, and we demonstrate that the origin of swelling is related to deprotonation of the sulfonate groups in the PSS-rich regions. It is mostly PSS-rich regions that swell while the PEDOT-rich regions remain rather unchanged. We demonstrate that swelling of the film is rather insignificant during reduction/oxidation within the cyclic voltammetry (CV) conditions. We show that during CV experiments each counterion brings on overage ≈4 water molecules into the polymer region. Our simulations of swelling, CV experiments, and π–π stacking formation in PEDOT and PSS match well the experimental results. Our theoretical studies unravel the most important morphological aspects of one of the most important polymers for organic electronics, providing the essential insight needed for the material and device design and improvements.

The Martini Model in Materials Science

“…Simulations of neat P3HT [ 192 ] have also been performed, while more organic semiconductor mixtures have been tested in the context of organic thermoelectric devices [ 185,196 ] and organic mixed ion–electron conductors (which will be described as part of the next section). [ 194,195,199–201 ]…”

“…Martini has also been used to investigate [ 194,195,199–201,204 ] organic mixed ion–electron conductors, which are soft (semi‐)conductors—often polymers—that readily solvate and transport ionic species. [ 205,206 ] Applications of such systems include organic electrochemical transistors for biological interfacing and neuromorphic devices, among others.…”

“…[ 195 ] The same authors, combining PEDOT with the available [ 209 ] PSS model, went on to investigate PEDOT:PSS morphologies in detail. [ 199,201 ] The simulations allowed the effect of pH on the morphology of in silico solution‐processed PEDOT:PSS thin films to be studied. Changes in pH were found to greatly affect the morphology ( Figure A), and in turn the distribution of the 5–15 weight % of water content in the polymer film, which is of critical importance for ion diffusion.…”

Degradable endovascular neural interface for minimally invasive neural recording and stimulation

“…On the other hand, the increase of the CSC might be explained by the swelling of PEDOT:PSS. [28][29][30] Furthermore, this swelling would cause electrolyte's diffusion within the PEDOT:PSS network and its penetration into those areas covered by the encapsulation layer, increasing the effective GSA of the electrode. The electrolyte leakage is detectable at slow scanning rates (50 mV s -1 ) but not at high scanning rates since the pore resistance represents an obstacle limiting the access to the area under the encapsulation.…”

Top–Down Coarse-Grained Framework for Characterizing Mixed Conducting Polymers

“…At present, only one CG-MD model of OMIECs has been published specific to the mixed conducting polymer PEDOT. [27][28][29][30][31] Thus, we have also developed a generic CG-MD force-field for OMIECs that can be used to systematically interrogate the various designable OMIEC components and their effect on morphology and transport processes. As an initial benchmark demonstration of this framework, we have investigated effects of oxidation and hydration levels on the morphology and transport processes of a generic p-type conjugated homopolymer with glycolated sidechains in the presence of an aqueous electrolyte.…”