Enhanced Electrical Conductivity and Seebeck Coefficient in PEDOT:PSS via a Two-Step Ionic liquid and NaBH(4) Treatment for Organic Thermoelectrics

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 NaBH(4) 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, NaBH(4), 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.