In this paper, we review recent advances in the development of flexible thermoelectric materials and devices for wearable human body-heat energy harvesting applications.

We present detailed theoretical predictions on the enhancement of the thermoelectric figure of merit by minority carrier blocking with heterostructure barriers in bulk narrow-band-gap semiconductors. Bipolar carrier transport, which is often significant in a narrow-band-gap material, is detrimental to the thermoelectric energy conversion efficiency as it suppresses the Seebeck coefficient and increases the thermal conductivity. When the minority carriers are selectively prevented from participating in conduction while the transport of majority carriers is relatively unaffected by one-sided heterobarriers, the thermoelectric figure of merit can be drastically enhanced. Thermoelectric transport properties such as Seebeck coefficient, electrical conductivity, and electronic thermal conductivity including the bipolar term are calculated with and without the barriers based on the near-equilibrium Boltzmann transport equations under the relaxation time approximation to investigate the effects of minority carrier barriers on the thermoelectric figure of merit. For this, we provide details of carrier transport modeling and fitting results of experimental data for three important material systems, $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$-based alloys, $\mathrm{M}{\mathrm{g}}_2\mathrm{S}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{n}}_x$, and $\mathrm{S}{\mathrm{i}}_{1-x}\mathrm{G}{\mathrm{e}}_x$, that represent, respectively, near-room-temperature (300 K–500 K), midtemperature (600 K–900 K), and high-temperature ($>1000\mathrm{K}$) applications. Theoretical maximum enhancement of thermoelectric figure of merit that can be achieved by minority carrier blocking is quantified and discussed for each of these semiconductors.

Hybrid thermoelectric (TE) nanocomposites containing conducting polymers and nanocarbon materials have been extensively studied in recent years due to their unique advantages over single-phase organic/inorganic TE materials. Nanocarbon materials have been developed as conductive nanofillers to improve the electrical conductivity of the polymer matrix, and to create a strong π–π interfacial interaction with the matrix to enhance the TE performance. However, previous designs of the hybrid TE nanocomposites tend to cause aggregation of nanocarbon materials, which is detrimental to the TE performance. Also, they are limited in their fabrication to thin film technologies with submicron thicknesses, which prevents these composites from being used in practical TE devices. Herein, we present the synthesis and thermoelectric properties of free-standing, three-dimensional graphene (3DG)–polyaniline (PANI) composites with greater than 100 μm thicknesses for high performance flexible p-type thermoelectrics. Our 3DG matrix has been synthesized by Chemical Vapor Deposition (CVD) with particulate nickel catalysts, and used as a scaffold for the polymer composites. This material provides an excellent electrical conductivity and a reasonable Seebeck coefficient along with very good mechanical integrity preserved when bending, thus making it a promising candidate for flexible TE. PANI polymer was electrochemically grown on the 3DG scaffold as a filler to further tune the TE properties. The proposed 3DG–PANI composites showed a maximum power factor of 81.9 μW m⁻¹ K⁻² with a PANI loading of 80 wt% and highly reproducible TE performance after repeated mechanical bending tests. This novel material provides a different strategy for simple and scalable fabrication of flexible thermoelectrics with high performance TE energy harvesting and improved mechanical properties.