Effect of Local Chain Ordering on Macroscopic Charge Mobility in Chemically Doped P3HT

Abstract

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.