🔬 materials science

Morphology Engineering of Mixed Ionic Electronic Conductors through Aqueous Phase Separation

This paper demonstrates that engineering the internal morphology of PEDOT:PSS:PEI films via pH-induced aqueous phase separation creates a porous network that overcomes the gain-speed trade-off in organic electrochemical transistors, enabling high transconductance and fast response times at ultralow voltages even in thick channels.

Original authors: Siqi Wang, Maria Restrepo, John Linkhorst, Matthias Wessling

Published 2026-08-10

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CC BY 4.0

Original authors: Siqi Wang, Maria Restrepo, John Linkhorst, Matthias Wessling

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where your body's own electrical signals could talk directly to a computer. This isn't science fiction; it's the goal of a field called bioelectronics. To make this happen, scientists use special materials called "mixed ionic–electronic conductors." Think of these materials as busy two-lane highways. One lane carries electrons (the electricity that powers our gadgets), and the other lane carries ions (charged particles floating in water, like the salt in your tears or blood). For a device to work well, it needs to let both lanes flow smoothly at the same time.

The star player in this story is a material called PEDOT:PSS. It's like a super-conductive plastic that loves water. Scientists use it to build Organic Electrochemical Transistors (OECTs), which are tiny switches that can amplify weak biological signals, like a heartbeat or a nerve impulse, into a strong electrical signal a computer can read. However, there's a tricky problem: if you make the material too thick to hold more signal, the ions get stuck trying to travel through the solid plastic, slowing everything down. It's like trying to run a marathon through a crowded room; the more people (ions) you have, the slower you move. This creates a frustrating trade-off: you can have a fast device or a powerful one, but usually not both.

This paper by Siqi Wang and colleagues tackles that exact problem. They asked: "What if we could build a highway that isn't a solid block, but a sponge?" Instead of making the material a dense, solid wall, they used a clever trick involving water and pH levels to create a film full of tiny, connected holes. They mixed their conductive plastic with another ingredient called PEI and dropped it into an acidic bath. This caused the mixture to separate into a porous, sponge-like structure. The result? A thick film (over 100 micrometers thick) that ions can zip through easily because they have a network of tunnels to travel in, while the electricity still flows smoothly through the solid parts.

The team found that by tweaking the recipe—specifically using twice as much PEI as another component called PSS—they created a "super-sponge" that kept its shape even after heating. This specific mix allowed the device to achieve a massive signal boost (a transconductance of 30 mS) and react incredibly fast (in just 13 milliseconds), all while using a tiny amount of voltage (0.05 V). The authors suggest that this works because the sponge-like structure lets the electrolyte soak deep into the material, turning the whole volume of the film into an active switch, rather than just the surface.

Crucially, the paper argues that simply making a material porous isn't enough; the holes must be connected, and the electric pathways must remain unbroken. They showed that a different mix (1:1 ratio) created holes that collapsed when heated, ruining the performance, while their 1:2 mix stayed open and connected. This proves that the shape of the inside of the material is just as important as what the material is made of. By mastering this internal architecture, the researchers suggest we can finally build thick, powerful, and fast bio-electronic devices without the usual speed limits, opening the door for better sensors and brain-computer interfaces that work with the gentle, wet environment of the human body.

Technical Summary: Morphology Engineering of Mixed Ionic–Electronic Conductors through Aqueous Phase Separation

Problem Statement
The performance of organic electrochemical transistors (OECTs) is fundamentally constrained by a trade-off between transconductance (gain) and switching speed. While increasing the channel thickness ( $d$ ) enhances the electrochemically active volume and thus the transconductance ( $g_m$ ), it simultaneously prolongs the distance ions must diffuse through the polymer. In dense, solid-state films, this restricts electrochemical modulation to the interface near the electrolyte, slowing the device response. Conventional fabrication methods like spin coating offer limited control over the internal transport architecture of thick channels, making it difficult to achieve high gain and fast response simultaneously in films exceeding typical sub-micron thicknesses.

Methodology
The authors propose using aqueous phase separation (APS), a technique adapted from membrane science, to engineer the internal morphology of mixed ionic–electronic conductors. The study focuses on films composed of PEDOT:PSS (the conductive polymer), PSS (polyelectrolyte), and PEI (branched polyethyleneimine).

Fabrication: A homogeneous precursor solution containing PEDOT:PSS, PSS, and PEI (with 25 wt% DMSO as a co-solvent/dopant) is cast and immersed in an acidic coagulation bath.
Mechanism: The pH shift triggers the protonation of PEI amine groups, inducing complexation with PSS sulfonate groups. This drives phase separation and precipitation, forming a porous PEDOT:PSS:PEI matrix.
Variables: The study systematically varies the PSS:PEI molar ratio (1:1 vs. 1:2), the coagulation bath composition (acetate buffer vs. H $_2$ SO $_4$ at varying concentrations), and post-processing treatments (annealing at 140°C and DMSO content).
Characterization: Films are analyzed via Scanning Electron Microscopy (SEM) for morphology, four-point probe/two-probe methods for electronic conductivity, and tensile testing for mechanical properties. OECT performance is evaluated through transfer curves, transconductance ( $g_m$ ), and response time ( $\tau$ ) measurements in 0.1 M NaCl electrolyte.

Key Contributions

Morphology as a Design Parameter: The work establishes internal morphology—specifically pore connectivity and continuity—as a critical design parameter for OECTs, complementing molecular structure and device geometry.
Scalable Water-Based Strategy: It demonstrates a scalable, water-based fabrication route (APS) to create freestanding, porous mixed-conductor films without relying on specialized freeze-drying or hydrogel templating.
Decoupling Gain and Speed: The study provides a pathway to mitigate the gain–speed trade-off by creating interconnected pore networks that reduce effective ionic diffusion lengths while maintaining electronic percolation.

Results

Morphological Control: The PSS:PEI ratio and coagulation conditions dictate the pore structure. Films with a 1:2 PSS:PEI ratio coagulated in 1 M H $_2$ SO $_4$ retained a stable, interconnected macroporous network after annealing and rehydration. In contrast, 1:1 films lost their porous structure upon annealing, likely due to incomplete charge compensation and structural collapse.
Electronic Conductivity: DMSO treatment and annealing significantly enhanced electronic conductivity (up to ~15 mS/cm) by improving PEDOT chain ordering and interchain coupling. However, conductivity alone did not explain performance differences; 1:1 and 1:2 films exhibited comparable conductivities but vastly different OECT behaviors.
OECT Performance: The optimized 1:2 PSS:PEI device achieved a maximum transconductance of 30 mS and a response time of 13 ms at an ultralow gate voltage of 0.05 V. Crucially, these metrics were achieved despite channel thicknesses of 120–200 µm, which are orders of magnitude thicker than conventional OECT channels.
Mechanism Validation: The superior performance of the 1:2 film is attributed to its interconnected pore network, which facilitates rapid electrolyte penetration (reducing ionic transport distance) and increases the electrochemically addressable volume, while the DMSO/annealing treatment ensures a continuous electronic pathway for charge transport.

Significance and Claims
The paper claims that this work establishes a general framework for developing high-performance OECTs by shifting the design paradigm from purely surface-limited modulation toward spatially distributed mixed ionic–electronic transport.

The authors assert that the porous architecture generated via pH-induced APS allows for:

Volumetric Modulation: Electrolyte access throughout the bulk of the thick film, rather than just at the surface.
Mitigated Trade-offs: The combination of a large electrochemically active volume (high gain) with shortened local ion-diffusion pathways (fast speed).
Design Flexibility: A method to tune morphology, mechanical compliance, and electronic properties independently through processing parameters (stoichiometry, pH, annealing).

The study concludes that while the results support the transition to volumetric modulation, definitive quantification of the degree of volumetric mixing (e.g., via volumetric capacitance or impedance spectroscopy) remains a target for future work. The approach is positioned as a scalable alternative to existing porous-channel strategies, relevant for soft bioelectronics, ion sensing, and neuromorphic materials.

Technical Summary: Morphology Engineering of Mixed Ionic–Electronic Conductors through Aqueous Phase Separation

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