LAUSR

DOI: 10.1002/aelm.202001190 materials with high zT values are available, solid-state waste heat recovery schemes become practically attractive for increasing system energy efficiencies.[1] Organic conductors and semiconductors are compelling systems for near-ambient thermoelectric applications because of their intrinsically low thermal conductivity, elemental ubiquity, and scalable, low-temperature processing, rendering organic electronics commercially viable despite reduced efficiencies relative to their inorganic counterparts.[2] The physical robustness of polymer materials expands their markets to flexible applications and curved surfaces for which the cost of custom design of rigid thermoelectric devices would exceed their value. Additionally, conductive polymer-based materials exhibit weaker coupling between S, κ, and σ than inorganic materials, mainly due to two factors: 1) the relatively low carrier concentrations and mobilities result in a weak correlation between the thermal and electrical conductivities; and 2) the non-band-like nature of the density of states leads to nontraditional relationships between the Seebeck coefficient and electrical conductivity.[2,3] These relaxed interdependencies bound a vast design space for synthesis, processing, and doping of organic materials to increase their overall figure of merit. This optimization process is focused on improving the electrical transport properties of organic materials, which have historically limited their performance in thermoelectric energy conversion. To increase the thermoelectric viability of conductive polymers, research efforts have focused on increasing the electrical conductivity and Seebeck coefficient of polymer material systems, such as poly(3,4-ethylenedioxythiophene):poly(styrenes ulfonate) (PEDOT:PSS). PEDOT:PSS consists of a conjugated hydrophobic polymer (PEDOT) within an insulating hydrophilic matrix (PSS), which dopes PEDOT to increase the conductivity and helps to disperse any excess PEDOT in water. However, the insulating PSS that remains in deposited films does not contribute to electrical conduction and prevents the PEDOT phase from ordering with a consequently higher conductivity. The electrical conductivity of this formulation can be improved through preand postdeposition treatments, involving various acids (camphorsulfonic acid,[4] dichloroacetic acid,[5] H2SO4, and p-toluenesulfonic acid[8]), solvents (dimethyl sulfoxide,[9] ethylene glycol (EG), diethylene glycol, methanol (MeOH),[10] and formamide[11]), and surfactants.[12,13] These methods have produced electrical conductivities of nearly 5000 S cm−1 and simultaneous enhancements of the Seebeck coefficient through Polymer-based materials hold great potential for use in thermoelectric applications but are limited by their poor electrical properties. Through a combination of solution-shearing deposition and directionally applied solvent treatments, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films with metallic-like conductivities can be obtained with high power factors in excess of 800 μW m−1 K−2. X-ray scattering and absorption data indicate that structural alignment of PEDOT chains and larger-sized domains are responsible for the enhanced electrical conductivity. It is expected that further enhancements to the power factor can be obtained through device geometry and postdeposition solvent shearing optimization.