Quantifying the localization of charges generated upon molecular doping of conjugated polymers

Abstract

Quantifying whether the charges created by molecular doping of organic semiconductors reside in localized versus mobile states is important to understanding doping mechanisms of organic semiconductors. Nevertheless, such quantification remains challenging, as it typically requires Hall effect measurements or specialized equipment. Here, we quantify the mobile and localized charge concentrations in molecularly doped conjugated polymers using spectroelectrochemical calibration, which we cross-validate by applying X-ray photoemission spectroscopy and Hall effect measurements and by fitting to established transport models. As a test case, we study doping with 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ) in two different polymers: poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and (poly(bithiophene-thienothiophene) derivative modified with a triethylene glycol side chain (Pg2T-TT). In PBTTT, we find that at most ∼10% of the carriers become mobile once carrier concentrations reach ∼3 × 1020 cm–3 (∼0.4 molar ratio of F4TCNQ:thiophene). In contrast, when using Pg2T-TT, we find that the mobile fraction exceeds 50% at carrier concentrations of ∼3 × 1020 cm–3 (∼0.1 of the F4TCNQ molar ratio). Not only are these results consistent with Hall effect measurements, but they agree with an analysis of the semilocalized transport (SLoT) model. We fit the SLoT model and extract the depth of potential well in combination with Fermi-level measurements via scanning Kelvin probe microscopy. These fits further indicate that Pg2T-TT reaches delocalized transport conditions while transport in PBTTT remains localized. This work provides a straightforward platform for quantifying charge localization in molecularly doped conjugated polymers by providing explicit parameters governing the charge transport, including carrier concentration, mobility, and the fraction of mobile carriers.