Synthesis and polymerization of polyelectrolyte-based conductive inks: the protagonism of the coadjutant oxidizing salt†

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has gained significant popularity as a material in both academic and industrial fields, especially due to the its vast application in electronic-based devices. However, despite its popularity and prominence in organic electronics, there is still a lack of scientific references regarding the synthesis and production of PEDOT:PSS inks. Here, we aim to discuss and elucidate the relationship between the conducting properties of PEDOT:PSS inks and the oxidizing agent concentration used during the oxidative polymerization of EDOT in the presence of PSS. To achieve this, we first studied the effect of addition of an oxidant to PSS solutions via dynamic light scattering (DLS) to characterize whether a higher oxidant concentration impacts the chain conformation of PSS in solution. Furthermore, we conducted UV-Vis spectroscopy measurements at various stages during the synthesis and purification of PEDOT:PSS to assess the formation of polarons and bipolarons. Finally, we evaluated the properties of deposited PEDOT:PSS films. Our findings demonstrate that the addition of an oxidant to PSS solutions leads to polyelectrolyte chain shrinkage, due to the screening of electrostatic interactions between the negatively charged sulfonate groups. Consequently, the likelihood of effective coupling between the PEDOT⁺ and PSS⁻ counterparts is reduced due to the presence oxidant ions. This hypothesis was confirmed since inks polymerized with a higher oxidant concentration underwent depletion from bipolarons to polarons during the purification process as counterions originating from the oxidizing agent were removed.