Probing Polarons in Mixed Conductors via the Photoinduced Stark Effect

Conjugated polymers are promising candidates for high performance bioelectronics due in part to their ability to conduct ions and charge carriers simultaneously. However, the fundamentals of mixed ion and charge transport, including their complex dependence on polymer structural dynamics occurring <i>in operando</i>, remain unclear due to limited characterization methods. Using ultrafast transient absorption spectroscopy, we demonstrate a new approach to probing the nature and local environment of polarons in a mixed conducting, electrochromic polymer, 3,4-propylenedioxythiophene-<i>co</i>-3,4-ethylenedioxythiophene (ProDOT-<i>co</i>-EDOT).[1] Selectively photoexciting hole polarons via the P1 and P2 polaronic transitions causes spatial separation of the hole from its counterion during thermalization. The resulting electric field, which induces a Stark shift of the bandgap transition of nearby polymer chains, creates signals in the transient spectrum that reflect the electronic structure and environment of the charge carrier. As a first step, we show how this optical spectroscopy method, which effectively senses polarons in both amorphous and crystalline domains, can be implemented in spectroelectrochemical measurements to elucidate mixed conduction mechanisms.<br/> <br/>[1] Umar, AR., Dorris, A.L., Kotadiya, N.M., Giebink, N.C., Collier, G.S., and Grieco, C.: Probing Polaron Environment in a Doped Polymer via the Photoinduced Stark Effect, J. Phys. Chem. C. <b>127</b>, 9498-9508 (2023)