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Organic mixed ionic–electronic conducting polymers remain at the forefront of materials development for bioelectronic device applications. During electrochemical operation, structural dynamics and variations in electrostatic interactions in the polymer occur, which affect dual transport of the ions and electronic charge carriers. Such effects remain unclear due to a lack of in situ spectroscopic methods capable of capturing these dynamics, which hinders the rational design of higher-performance polymers. Herein, we present the first in situ transient absorption spectroelectrochemical measurement of a conducting polymer in the near-infrared, where photoexcited charge carrier dynamics are used to directly probe their nanoscale environment and trapping behavior in working electrodes. In this method, voltage is applied to charge or discharge the polymer, and the picosecond relaxation dynamics of directly photoexcited charge carriers are spectroscopically monitored to relate their location within the heterogeneous polymer nanostructure to their transport behavior. Applying this technique to working PEDOT:PSS electrodes, we investigated the impacts of voltage-induced changes in polymer chain packing and ion–carrier interactions on charge trapping. At lower voltages, carriers initially form within J-aggregated PEDOT chains that are deeply trapped due to strong electrostatic coupling to PSS− counterions. At higher voltages, the PEDOT lamellae expand and charge–ion pairs enter the PEDOT-rich domains, where trapping is decreased and carriers delocalize among the more tightly stacked, H-aggregated PEDOT chains. Further, this in situ spectroscopic method can also be more broadly applied to study electrochemical dynamics in accumulation-mode and n-type polymer electrodes and electrochemical transistors. 

Organic mixed ionic-electronic conducting polymers continue to emerge as promising next-generation materials for electrochemical applications ranging from bioelectronics to energy storage. However, we lack a clear understanding of how electrostatic and nanostructural changes in the polymer, which evolve during electrochemical device operation, influence charge and ion conductivity. In this work, we applied ultrafast near-infrared pump-probe spectroscopy, which is sensitive to the local nanostructure and electronic environment of charge carriers (polarons), to an electrochemically doped poly(3-hexylthiophene-2,5-diyl) [P3HT] model polymer system. The impact of electrolyte infiltration on carrier trapping was tested by comparing its photoexcited polaron dynamics to those measured in a chemically doped P3HT film lacking electrolyte and containing the same polaron mole fraction (~15%) and ClO4 − counterions. The transient absorption spectra revealed the presence of Coulombically free and trapped (ion-bound) polarons in both the electrochemically and chemically doped P3HT films, with a greater relative number of free polarons in the latter. However, the ion-bound polarons were less deeply trapped on average in the electrochemically doped film, suggesting that charge trapping was suppressed due to electrostatic screening by the electrolyte. This study highlights how fundamental knowledge gained from past chemically doped polymer studies cannot directly be applied to understand conductivity in mixed conducting polymers, encouraging future spectroscopic studies of charge trapping behavior in their electrochemically active states. 

Charge carrier mobility is a key factor underlying the performance of conjugated polymers as conductive materials for flexible and lightweight electronics. Chemical doping is typically used to improve polymer conductivity by increasing the carrier density. However, doping consequently induces both morphological and electrostatic changes within the polymer that impact charge mobility, the extent to which remains unclear. Using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer films with tuned morphology and controlled ion-carrier distance, we investigated the influence of nanoscale chain ordering on the device-scale mobility of its chemically-induced carriers. Grazing-incidence X-ray diffraction measurements revealed that chemically doping the films resulted in a similar lamellar d-spacing of ∼18.5 Å, despite differences in chain ordering within their nanocrystalline domains. Transient absorption (TA) spectroscopy was used to examine the relaxation of hole polarons excited with 0.62 eV (2000 nm) light to study their trapping behavior, and the results were compared with field-effect mobility measurements. Despite a 4-fold difference in hole mobility, the average relaxation times of the mobile and trapped polarons were identically ∼0.1 ps and 17 ps, respectively, between the two films. The TA results only showed qualitative differences in the ratio of mobile to trapped polarons, indicating that ordered nanocrystalline domains facilitate the formation of free polarons, which enhance the hole mobility. The results from this study suggest that TA spectroscopy can be used as an electrode-free method of assessing the local mobility of doping-induced charge carriers, and that nanoscale chain ordering – and not just mesoscale structure or ion-carrier distance – is essential to control for improving the device-scale mobility of polarons. 

Charge carrier mobility is a key factor underlying the performance of conjugated polymers as conductive materials for flexible and lightweight electronics. Chemical doping is typically used to improve polymer conductivity by increasing the carrier density. However, doping consequently induces both morphological and electrostatic changes within the polymer that impact charge mobility, the extent to which remains unclear. Using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer films with tuned morphology and controlled ion-carrier distance, we investigated the influence of nanoscale chain ordering on the device-scale mobility of its chemically-induced carriers. Grazing-incidence x-ray diffraction measurements revealed that chemically doping the films resulted in a similar lamellar d-spacing of ~18.5 Å, despite differences in chain ordering within their nanocrystalline domains. Transient absorption (TA) spectroscopy was used to examine the relaxation of hole polarons excited with 0.62 eV (2000 nm) light to study their trapping behavior, and the results were compared with field-effect mobility measurements. Despite a 4-fold difference in hole mobility, the average relaxation times of the mobile and trapped polarons were identically ~0.1 ps and 17 ps, respectively, between the two films. The TA results only showed qualitative differences in the ratio of mobile to trapped polarons, indicating that ordered nanocrystalline domains facilitate the formation of free polarons, which enhance the hole mobility. The results from this study suggest that TA spectroscopy can be used as an electrode-free method of assessing the local mobility of doping-induced charge carriers, and that nanoscale chain ordering – and not just mesoscale structure or ion-carrier distance – is essential to control for improving the device-scale mobility of polarons.