![]() ![]() Here, we leverage the electrochromic response of CPs to study their mixed ionic-electronic transport properties using operando optical microscopy. Hence, measurements that differentiate between ionic and electronic transport are needed to improve the fundamental understanding of mixed conducting materials. The lack of mechanistic understanding in turn holds back the rational design of new mixed conducting materials with optimized properties 14. Furthermore, it is not possible to probe the transport of each carrier type independently with standard electrical and electrochemical techniques. However, achieving a mechanistic picture of mixed transport is challenging due to the dynamic interactions among ions, holes and/or electrons and the material microstructure. Like other mixed ionic-electronic conductors including lead-halide perovskites 12 and battery electrodes 13, it is critical to understand the interaction between and transport properties of ionic and electronic charges in OMIECs. While newly developed materials have been optimized for electronic transport, little is known about their ionic transport properties. The homopolymer p(g1T2-g5T2) is an intrinsic semiconductor with a polythiophene backbone and hydrophilic ethylene glycol sidechains to enable ion transport, where the distribution of sidechains between adjacent bithiophene units is optimized to maximize performance ( ♜ * of 500 F cm −1 V −1 s −1) 11. This approach has led to the high performing polymer p(g1T2-g5T2) (Fig. While PEDOT:PSS is the standard benchmark for OMIEC performance ( ♜ * of roughly 50 F cm −1 V −1 s −1), there has been a recent trend in developing higher performance materials by modifying traditional semiconducting polymer backbones via addition of hydrophilic sidechains 10. 1a), which is a blend of the semiconducting polymer PEDOT, which gives the composite its electronic properties, and an ion conducting polymer, PSS, which also acts as a dopant. The most widely used OMIEC is PEDOT:PSS (Fig. However, this quantity does not capture critical device properties such as speed of (de)doping, which is presumed to primarily depend on the ionic transport properties 9 since ionic masses greatly exceed those of electronic carriers. The magnitude of amplification results primarily from the electronic properties of the material, namely the hole and/or electron mobility, µ, and the amount of charge modulation or (de)doping for a given change in potential (called volumetric capacitance, C *), where the product ♜ * is the typical materials figure of merit 7. The performance of OMIECs for use in organic electrochemical transistors (OECTs) is benchmarked by their ability to amplify small voltage signals. At the core of OMIEC device operation is the conversion of ionic currents from an external electrolyte to modulations in the electronic carrier density of a conjugated polymer (CP) 8. ![]() These devices benefit from a combination of excellent electronic properties (hole mobilities of 1 to 10 cm 2 V −1 s −1) and high ionic conductivities (up to 10 −2 S cm −1) 7. Organic mixed ionic-electronic conductors (OMIECs) have emerged for applications in bioelectronics where their properties can be exploited for sensing 1, neural recording 2, complementary logic 3, energy harvesting 4 and storage 5, and memory 6. We show that the timescale of hole-limited doping can be controlled by the degree of microstructural heterogeneity, enabling the design of conjugated polymers with improved electrochemical performance. ![]() Using operando optical microscopy, we reveal that electrochemical doping speeds in a state-of-the-art polythiophene can be limited by poor hole transport at low doping levels, leading to substantially slower switching speeds than expected. Here, we show that this basic assumption does not hold for conjugated polymer electrodes. In semiconducting electrodes, electrochemical doping is assumed to be limited by motion of ions due to their large mass compared to electrons and/or holes. While the mixed conductors enabling these technologies are widely used, the dynamic relationship between ionic and electronic transport is generally poorly understood, hindering the rational design of new materials. Simultaneous transport and coupling of ionic and electronic charges is fundamental to electrochemical devices used in energy storage and conversion, neuromorphic computing and bioelectronics. ![]()
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