Regulating Competitive Doping in Solution-Mixed Conjugated Polymers for Dramatically Improving Thermoelectric Properties

Abstract

Largely soluble tris(pentafluorophenyl)borane (BCF) has recently emerged as a promising molecular dopant for preparing highly conductive organic thermoelectrics (TEs) using a one-step solution-mixing method. However, the unique doping mechanisms that include both undesirable Lewis acid doping with BCF and effective Bronsted acid doping with BCF-water complexes limit its widespread applications. Herein, the feasibility of modulating the two doping mechanisms by utilizing the competitive Lewis acid-base interactions of BCF with H2O or Lewis basic groups in conjugated polymers is demonstrated. The polymer without strong Lewis basic groups undergoes Bronsted acid doping, which efficiently forms delocalized free charge carriers and leads to superior TE power factors and figures of merit of 49.6 mu W m(-1) K-2 and 0.061, respectively. However, the polymer with strong Lewis basic groups undergoes both doping mechanisms competitively. BCF-polymer Lewis complexes not only generate free charge carriers inefficiently, but also hinder possible Bronsted acid doping and localize charge carriers, significantly lowering the TE properties. Nevertheless, the reduced TE properties can be dramatically improved by thermally annealing the predoped polymer films because Lewis acid doping can be substantially replaced by Bronsted acid doping owing to the different thermal stabilities between the BCF-polymer and BCF-water complexes.