Interface Engineering in Hybrid Organic-Inorganic Composites for Flexible and High-Performance Thermoelectrics

Abstract

Thermoelectric devices are typically rigid and their performance and reliability could be compromised by thermal stress. There is also a growing need to develop flexible thermoelectric devices for integration with wearable electronics. By using inherently soft polymers, hybrid organic/inorganic composites are considered a promising candidate for flexible thermoelectrics. In addition, the composites also provide an opportunity for enhanced thermoelectric performance by taking the advantages of low thermal conductivity and high Seebeck coefficient from the organic and inorganic components, respectively. Experimentally, however, enhanced thermoelectric performance in hybrid composites has not been observed, primarily due to inefficient carrier transport across the organic/inorganic interfaces. In particular, if the composites are energetically heterogeneous, their thermoelectric performance will be dictated by the more conductive component and there is no contribution from the less conductive component, preventing higher power factor in the composites . Here we demonstrate a strategy for interface engineering to improve the charge carrier transport from a conducting polymer, i.e., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which has an electrical conductivity greater than 1000 S/cm [1-3], to nanostructures of a prototypical inorganic thermoelectric material, i.e., Bi0.5Sb1.5Te3 (BST), which is the state-of-the-art thermoelectric materials at room temperature (with thermoelectric figure of merit ZT~1) [4]. PEDOT:PSS consists of hydrophobic PEDOT in core and hydrophilic PSS in shell, where PSS is an insulator but it is required to dope EDOT as an oxidizing agent during the polymerization process of EDOT. Therefore, conduction in PEDOT:PSS is mediated by phonon-assisted hopping that requires an activation energy due to the delocalized carriers within the backbones of PEDOT. The key to enhance this hopping process in a BST-PEDOT composite is to reduce the hopping distance for transport of PEDOT-to-PEDOT and PEDOT-to-BST. This could be achieved in a uniform dispersion of BST nanostructures in PEDOT, in which the discrepancy in the activation energy is minimized. To demonstrate this concept, we fabricated BST particles by using a spark erosion method, which involves repetitive spark discharges to produce fine powders [5], and then mixed the particles in PEDOT:PSS (CLEVIOS PH1000) using a non-ionic surfactant and controlling its micelle concentration (concentration ranging from 0.5 wt % to 10 wt%). We then characterized thermoelectric properties and morphologies of the hybrid films fabricated under various conditions. We demonstrated homogeneous dispersion of BST in the PEDOT matrix and consequently efficient interfacial charge transport by optimizing the surfactant. Our results show that hybrid BST/polymer composites are promising for flexible and high-performance thermoelectrics. The strategy developed in this work may also be applicable to other heterogeneous energy conversion architectures consisting of organic and inorganic materials.