Conductive polymer electrodes have exceptional promise for next-generation bioelectronics and energy conversion devices due to inherent mechanical flexibility, printability, biocompatibility, and low cost. Conductive polymers uniquely exhibit hybrid electronic–ionic transport properties that enable novel electrochemical device architectures, an advantage over inorganic counterparts. Yet critical structure–property relationships to control the potential-dependent rates of charge transfer at polymer/electrolyte interfaces remain poorly understood. Herein, we evaluate the kinetics of charge transfer between electrodeposited poly-(3-hexylthiophene) films and a model redox-active molecule, ferrocenedimethanol. We show that the kinetics directly follow the potential-dependent occupancy of electronic states in the polymer. The rate increases then decreases with potential (both normal and inverted kinetic regimes), a phenomenon distinct from inorganic semiconductors. This insight can be invoked to design polymer electrodes with kinetic selectivity toward redox active species and help guide synthetic approaches for the design of alternative device architectures and approaches.

Controlling interfacial electron-transfer rates is fundamental to maximizing device efficiencies in electrochemical technologies including redoxflow batteries, chemical sensors, bioelectronics, and photo-electrochemical devices. Conductive polymer electrodes offer the possibility to control redox properties through synthesis and processing, if critical structure−property relationships governing charge transfer are understood. In this work, we show that the rate and symmetry of electron transfer at conductive polymer electrodes are directly connected to the microstructure and the density of states (DOS) using the model system of poly(3-hexylthiophene) (P3HT) and ferrocene/ ferrocenium (Fc/Fc + ), as predicted by the Marcus−Gerischer model. Experimentally, crystalline P3HT exhibits a sufficient overlap between the polymer DOS and the DOS of both Fc and Fc + , resulting in a reversible electron transfer. Conversely, the DOS of amorphous electrodeposited P3HT does not overlap with that of Fc + , inhibiting reduction (i.e., kinetic selectivity for oxidation). This proofof-concept work offers a paradigm to predict and control the kinetics at the polymer/liquid interface for applications from biology to energy.

A pathway to improve the light-harvesting efficiency of dye-sensitized solar cells based on porous electrodeposited ZnO films is presented. Cosensitization with the indoline dyes D149 and D131 and the squaraine dye SQ2 leads to panchromatic light harvesting and photovoltaic activity. When coadsorbates are employed to prevent dye aggregation on the ZnO surface, an increase in the short-circuit photocurrent and overall efficiency compared to reference cells with D149, or D149 and D131 is attained. The overall performance of cosensitized cells containing SQ2, however, is limited by low open-circuit voltages and fill factors. By use of electrochemical impedance spectroscopy, current–voltage characterization in the dark and under illumination, and charge extraction measurements, recombination reactions and the distribution of trap states in the ZnO films are investigated, and the origins of the limited open-circuit voltages and fill factors in cells with SQ2 are revealed.

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