Enhanced Electrical Conductivity and Seebeck Coefficient in PEDOT:PSS via a Two-Step Ionic liquid and NaBH4 Treatment for Organic Thermoelectrics.

Jonathan Atoyo,James Mcgettrick,Matthew J Carnie,Matthew R Burton

doi:10.3390/polym12030559

Abstract

A two-step approach of improving the thermoelectric properties of Poly(3,4-ethylenedioxythiophene)poly(4-styrenesulfonate) (PEDOT:PSS) via the addition of the ionic liquid, 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM:TFSI) and subsequent reduction with NaBH4 is presented. The addition of 2.5 v/v% of EMIM:TFSI to PEDOT:PSS increases the electrical conductivity from 3 S·cm−1 to 1439 S·cm−1 at 40 °C. An additional post treatment using the reducing agent, NaBH4, increases the Seebeck coefficient of the film from 11 µV·K−1 to 30 µV·K−1 at 40 °C. The combined treatment gives an overall improvement in power factor increase from 0.04 µW·m−1·K−2 to 33 µW·m−1·K−2 below 140 °C. Raman and XPS measurements show that the increase in PEDOT:PSS conductivity is due to PSS separation from PEDOT and a conformational change of the PEDOT chains from the benzoid to quinoid molecular orientation. The improved Seebeck coefficient is due to a reduction of charge carriers which is evidenced from the UV–VIS depicting the emergence of polarons.

Highlights

Organic thermoelectric generators (OTEGs) are solid state devices that can convert heat energy into electrical energy [1,2]
A thin film PEDOT: PSS ionic liquid composite utilizing the ionic liquid Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM):TFSI was synthesized with superior thermoelectric properties to Pristine PEDOT:PSS
The ionic liquid significantly improved the electrical conductivity from 3 S·cm−1 to 1439 S·cm−1 for the 2.5% EMIM:TFSI film thereby leading to a PF of 28 μW·m−1 ·K−2 at 40 ◦ C

Summary

Introduction

Organic thermoelectric generators (OTEGs) are solid state devices that can convert heat energy into electrical energy [1,2]. Has been widely studied as an OTEG material, which is in part due to its low thermal conductivity. 0.1–0.3 W·m−1 ·K−1 [7,8] and tuneable doping states from chemical treatment [9]. Properties such as, water solubility, processability, ease of printing and device fabrication, as well as miscibility with polar solvents have made PEDOT:PSS a common starter material in OTEG research [10,11]. Due to the pristine PEDOT:PSS exhibiting low electrical conductivity, σ (≈ 2 S·cm−1 ) and Seebeck coefficient,

Methods

Results

Conclusion