Conductivity Engineering of PEDOT:PSS for High-Performance Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have gained significant attention as enabling components for bioelectronic systems due to their low operating voltages, large signal amplification, and compatibility with aqueous and mechanically soft environments. The conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is most commonly employed as the OECT channel material because it can be processed from solution and remains electrochemically stable in water. Despite these advantages, PEDOT:PSS in its pristine form suffers from limited electrical conductivity, an excess of electrically inactive PSS, and structural changes upon electrolyte exposure. These factors collectively degrade charge transport, reduce device gain, and compromise operational stability during OECT operation. In this work, we investigate how different conductivity-engineering approaches applied to PEDOT:PSS influence OECT performance, with the goal of establishing clear correlations between processing, polymer structure, and device characteristics. Four distinct modification strategies are examined and directly compared: (i) secondary doping using high-boiling-point polar solvents, (ii) post-deposition solvent treatments, (iii) surfactant-assisted film formation combined with thermal optimization, and (iv) polymer blending with mechanically flexible or insulating additives. Each approach alters the PEDOT:PSS microstructure through different mechanisms, enabling a systematic assessment of their effectiveness under identical device fabrication and testing conditions. Secondary doping with polar solvents such as dimethyl sulfoxide (DMSO) and ethylene glycol (EG) enhances conductivity by weakening electrostatic interactions between positively charged PEDOT backbones and negatively charged PSS chains. This interaction screening facilitates phase segregation and promotes a more ordered arrangement of PEDOT domains, leading to improved carrier mobility. We find that the method of dopant incorporation plays a critical role: while solvent addition during solution preparation or vapor exposure after deposition improves conductivity, post-deposition immersion of PEDOT:PSS films in DMSO produces the most substantial enhancement. This treatment effectively removes excess PSS and increases structural order, resulting in significantly higher conductivity in the dry state. Under practical OECT operating conditions, PEDOT:PSS channels are in direct contact with an electrolyte and undergo swelling due to ion uptake, which reduces conductivity relative to dry films. Nevertheless, devices based on DMSO-treated channels maintain superior effective conductivity in the hydrated state and exhibit enhanced hole mobility. As a result, these OECTs demonstrate increased drain currents, higher transconductance, and improved switching behavior compared to devices fabricated from untreated PEDOT:PSS. Surfactant-assisted processing offers an additional pathway to control film morphology and electrical performance. The incorporation of a non-ionic surfactant such as Tween-80 improves wetting behavior and enables the formation of uniform, defect-free PEDOT:PSS films. However, the presence of surfactants constrains the thermal processing window, requiring careful optimization of annealing conditions to induce beneficial polymer rearrangement without damaging the film. Our results indicate that conductivity improvements arising from this approach are primarily associated with changes in polymer packing rather than simple removal of residual solvent or water. Polymer blending further expands the tunability of PEDOT:PSS films, particularly for applications requiring enhanced mechanical compliance. Blending with polyethylene glycol (PEG) reveals a strong dependence on molecular weight: low-molecular-weight PEG (approximately 200–400) significantly enhances conductivity, whereas higher-molecular-weight PEG provides minimal electrical benefit. Similarly, incorporating flexible polymers such as poly(vinyl alcohol) (PVA) improves mechanical robustness but can reduce conductivity unless combined with secondary doping. The addition of EG to PEDOT:PSS–PVA blends restores electrical performance while preserving mechanical durability. Across all investigated strategies, a unifying design guideline emerges: high-performance OECTs require continuous PEDOT-rich pathways, minimized PSS content at critical interfaces, and processing routes that maintain conductivity under electrolyte-swollen conditions. By applying these principles, we demonstrate PEDOT:PSS-based OECTs with enhanced transconductance, stable aqueous operation, and improved overall device performance. These findings provide practical guidance for tailoring PEDOT:PSS channels for biosensing, neural interfaces, and wearable bioelectronic devices.