Quantifying the Delocalized and Localized Charges Upon Molecular Doping of Conjugated Polymers

Doping of conjugated polymers is widely studied for both fundamental and practical applications. Tailoring the electrical properties of conjugated polymers via molecular doping is well established. However, quantifying number of charges per dopant, and the ratio of delocalized charges to localized charges remains a challenge. Herein, we demonstrate a practical platform to quantify the delocalized and localized charges separately upon molecular doping using a combination of spectroelectrochemistry, and Hall measurements. First, we quantify the generated charges from doping the conjugated polymer poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) using 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as a dopant. Using spectroelectrochemistry, we probe the generated charges, which include delocalized and localized charges and validate our quantification by using X-ray photoelectron spectroscopy (XPS). Second, we quantify delocalized charges selectively by using Hall measurement, as Hall effect is based on free mobile charges. By plotting the generated charges, and mobile charges as a function of doping level, we find that most (> 99%) of the generated charges upon doping does not contribute to charge transport at the lower doping regime (dopant molar ratio per thiophene unit, X_F4TCNQ < 0.05). We further quantify the generated charges and delocalized charges upon different polymer-dopant system (poly-(bithiophene-thienothiophene) with a triethylene glycol sidechain, Pg2T-TT:F4TCNQ). We find that the fraction of mobile charges is far higher (10×) in doped Pg2T-TT than in doped PBTTT at lower doping levels (X_F4TCNQ: ~0.05). Our results quantitatively reveal that polar sidechain can substantially increase the mobile fraction in generated charges upon doping, which we hypothesize may be due to the reduced Coulombic interaction between ionized dopants and generated charges due to the polar sidechains. This work will provide simple but solid model to study charge transport model in molecular doping of conjugated polymers.