Chain Conformational Ordering and Aggregation in Precursor Solution Affects the Charge Mobility of p-Doped P3HT Films

The charge carrier density and chain aggregation state of the conjugated polymer film are regarded unanimously as two factors affecting its electric transport properties. While chemical doping enhances the carrier concentration in polymer semiconductors, the dopant molecules also function as extraneous impurities, which hinder the aggregation ordering and crystallization of the polymer chains. Thus, the dopant level and the aggregation ordering should be compromised especially for organic field effect transistors (OFET) application. In this work, the processing-structure-performance relationships in poly(3-hexylthiophene) (P3HT) doped with low levels of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is investigated. A microfluidic and ultrasonic field-assisted solution processing strategy (FU) is employed to enhance the chain conformation order in the solution. By virtue of the UV-vis absorption, X-ray scattering, and X-ray absorption spectroscopy, the FU-processed P3HT solution with 1 wt.% doping level demonstrated the highest conformation order in terms of large chain conjugation length and optimal aggregate packing, which further dictating the highest mobility of the corresponding F4TCNQ-doped P3HT film. The chain conjugation length is found to correlate more directly with OFET performance than the macroscopic crystallinity of the film and thus is proposed to be more intrinsic for understanding the electronic transport physics of conjugated polymers.