The ability of organic electrochemical transistors (OECTs) to conduct both holes and ions has made them interesting candidates for biosensing, neural interfacing, and targeted drug delivery applications. Researchers in this relatively new field have used the well-known PEDOT:PSS (conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)), which has been used for decades in optoelectronics applications and as an anti-static coating. The excellent hole conducting properties of PEDOT:PSS have been extensively investigated for electrodes in optoelectronic devices. The hydrated polymer also conducts ions fairly well, along with holes, but until now the mechanism was poorly understood.

In a recent article in Nature Communications, Jonathan Rivnay and colleagues at the Ecole Nationale Supérieure des Mines in Gardanne, France, describe a unique method of decoupling electronic and ionic charges, and relating their transport to the polymer's microstructure. "I view this as one of the first cases where we've been able to perform a structure–processing–property study on organic mixed conductors," Rivnay says. "We were able to understand what it is about the microstructure that allows for both good electronic and ionic transport."

The researchers faced two main challenges in this work: the low contrast and relatively disordered film microstructure posed difficulty in distinguishing the PEDOT:PSS dispersed phase from the PSS matrix in traditional electron or x-ray methods, and the use of standard conductance measurements could not distinguish between ion and hole transport. They solved the first by combining synchrotron radiation and resonant soft x-ray scattering (rSoXS) to determine the microstructure of the film. For the second, they adapted a one-dimensional "moving front" experiment by using an optical microscope to monitor changes in visible light transmission through the electrochromic polymer thin film as K+ ions were injected into and drifted through the film. When an ion penetrates the bulk of material and dopes or dedopes the polymer backbone, it changes color, Rivnay says. This phenomenon enables researchers to visualize how quickly ions penetrate into the material by observing the moving front between colored and uncolored parts of the film, thereby quantifying ionic transport rates. The mobility of the ions could thus be decoupled from the mobility of the holes.

To change the microstructure and morphology of the PEDOT:PSS films, Rivnay's team added ethylene glycol (EG) to the system as a dispersion additive in amounts varying from 0 to 50 vol%, which modified the film structure at both the molecular and mesoscale. With no EG added, small PEDOT:PSS gel particles were dispersed in a PSS matrix, with relatively wide channels of impure PSS between the gel islands, through which K+ ions moved; hole transport occurs through PEDOT chains in the PEDOT:PSS islands. Addition of EG caused larger gel islands to form and increased the PEDOT aggregates therein. The swelling narrowed the pathways for K+ ion transport while increasing the hole transport through the larger islands. The researchers found that 5 vol% EG resulted in the optimal transport of ions and holes, thus yielding the highest performing OECTs. However, while interesting, this finding was not the main goal of the work.

"The main takeaway from this research is not that we found optimal conditions for mixed-conduction in the PEDOT:PSS system," Rivnay says. "It's that we now have tools that allow us to understand why it is that certain conditions give us good mixed conduction or not." Using these methods, they plan to work with polymer chemists to develop novel polymers with improved hole and ion conduction for bioelectronics. Rivnay envisions using these devices as active recording sites for neural interfacing, active scaffolds for tissue engineering, and for drug delivery.

"This is the first in-depth study connecting the morphology with the ionic and electronic conduction in a conducting polymer," says Xavier Crispin, an expert in organic electronics at Linköping University in Sweden. "It shows a systematic and elegant methodology to study those complex interrelated phenomena. It is becoming obvious that the performance of any type of electrochemical device could be optimized following the same strategy."

Read the article in Nature Communications.