Dependence of charge carrier transport on molecular relaxations in glassy poly(3-hexylthiophene-2,5-diyl) (P3HT)

Abstract

A positive effect of molecular relaxations and the movement of the molecular backbone and of chain segments on charge carrier transport processes in organic semiconductors, has not previously been reported. In this work, charge carrier transport mechanisms in amorphous glassy poly(3-hexylthiophene-2,5-diyl) (P3HT) are determined unambiguously, both above and below the glass transition temperature, T_α. This is the result of measurements of temperature, electric field, and ac frequency dependence of charge carrier mobility and density in organic field effect transistors, based on regioregular P3HT of three different molecular weights. Relaxations play an important role in dc conductivity. At temperatures above T_α, under constant source-drain voltage, the hole-like polaronic carriers move with, and hop between, mobile polymer backbones. Below T_α, they hop between mobile chain segments of essentially immobile neighboring molecules. Under source-drain voltages at frequencies of 100 Hz and higher, charge carriers move on polymer backbones. This motion does not contribute to dc conductivity. These insights into the dependence of charge carrier transport properties upon the molecular properties should contribute to advances in stable organic photovoltaics, organic thermoelectrics, and thermally degradable electronics. The approach taken here should also be useful for clarifying conduction mechanisms in other polymeric semiconductors.