Tuning charge transport dynamics via clustering of doping in organic semiconductor thin films

Connor J Boyle,Ljiljana Korugic-Karasz,Peijian Wang,Lawrence A Renna,Michael D Barnes,Meenakshi Upadhyaya,Zlatan Aksamija,Seung Pyo Jeong,Nicholas Hight-Huf,Michael Lu-Díaz,D Venkataraman

doi:10.1038/s41467-019-10567-5

Connor J Boyle, Ljiljana Korugic-Karasz + Show 9 more

Open Access

https://doi.org/10.1038/s41467-019-10567-5

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Abstract

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. Conventional wisdom posits that the polymer alone dictates thermoelectric efficiency. Herein, we show that doping — in particular, clustering of dopants within conjugated polymer films — has a profound and predictable influence on their thermoelectric properties. We correlate Seebeck coefficient and electrical conductivity of iodine-doped poly(3-hexylthiophene) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] films with Kelvin probe force microscopy to highlight the role of the spatial distribution of dopants in determining overall charge transport. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang–Snyder transport parameter. These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications.

Highlights

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient
We show that the spatial distribution of dopants in the conjugated polymer has a profound impact on the shape of the α vs. σ curve and that clustering of dopants in the polymer modifies the shape of the density of states (DOS) and alters the trend of α vs. σ curve
Modulation doping using electrochemical field-effect transistors has been used to control the doping density of thermoelectric materials[37,38]

Summary

Introduction

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang–Snyder transport parameter These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications. Thermoelectrics (TEs) are solid-state devices that offer reliable and environmentally friendly scavenging of waste heat into electricity Their modest efficiency and relatively high cost have hampered their widespread adoption. Organic thermoelectrics (OTEs), on the other hand, are lightweight, flexible, low-cost, and solution-processable, but progress in raising their conversion efficiency has been sporadic and severely hampered by the complexity of their thin film structure and a lack of systematic guidelines for materials discovery and improvement. This implies that precise control of the amount of doping is critical for obtaining the maximum power factor, α2σ, and further progress will rely on altering the shape of the α vs. σ curve

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