Soft Coulomb Gap Limits the Performance of Organic Thermoelectrics

TL;DR

This study reveals that the soft Coulomb gap in doped organic semiconductors limits thermoelectric performance, and suggests strategies like high dielectric materials to improve power factors by shifting the optimal doping point.

Contribution

It combines experimental data and kinetic Monte Carlo simulations to establish a universal transport framework involving Efros-Shklovskii VRH in organic thermoelectrics.

Findings

Optimal power factor occurs at the transition to Efros-Shklovskii VRH.

Soft Coulomb gap constrains the Seebeck coefficient and conductivity correlation.

High dielectric materials can potentially enhance thermoelectric performance.

Abstract

Although consensus exists that the thermoelectric properties of doped organic semiconductors result from a complex interplay between a large number of mutually dependent factors, there is no consensus on which of these are dominant, or even on how to best describe the charge and energy transport. This holds particularly in the intermediate doping regime where the optimal performance is typically observed at the roll-off in the Seebeck coefficient - conductivity (S-{\sigma} correlation, fundamentally limiting the rational advancement of organic thermoelectric materials. Here, we combine experiments on a board set of conjugated polymers with kinetic Monte Carlo simulations across varying doping levels to uncover a general transport framework. We demonstrate that the optimal thermoelectric power factor (PF_max) consistently occurs at the transition between conventional variable-range hopping (VRH) and VRH in a density of states in which a soft Coulomb gap forms at the Fermi level, as described by Efros and Shklovskii (ES-VRH). This suggests the use of high dielectric constant materials or the promotion of charge delocalization as an avenue to shift the roll-off of the S-{\sigma}} curve, which constrains PF_max, to higher doping levels and accordingly higher PF.