High Figure-of-Merit Telluride-Based Flexible Thermoelectric Films through Interfacial Modification via Millisecond Photonic-Curing for Fully Printed Thermoelectric Generators

The thermoelectric generator (TEG) shows great promise for energy harvesting and waste heat recovery applications. Cost barriers for this technology could be overcome by using printing technologies. However, the development of thermoelectric (TE) materials that combine printability, high-efficiency, and mechanical flexibility is a serious challenge. Here, flexible (SbBi)₂ (TeSe)₃ -based screen-printed TE films exhibiting record-high figure of merits (ZT) and power factors are reported. A high power factor of 24 µW cm^-1  K^-2 (ZT_max  ≈ 1.45) for a p-type film and a power factor of 10.5 µW cm^-1  K^-2 (ZT_max  ≈ 0.75) for an n-type film are achieved. The TE inks, comprised of p-Bi_0.5 Sb_1.5 Te₃ (BST)/n-Bi₂ Te_2.7 Se_0.3 (BT) and a Cu-Se-based inorganic binder (IB), are prepared by a one-pot synthesis process. The TE inks are printed on different substrates and sintered using photonic-curing leading to the formation of a highly conducting β-Cu_{2- δ} Se phase that connects "microsolders," the grains resulting in high-performance. Folded TEGs (f-TEGs) are fabricated using the materials. A half-millimeter thick f-TEG exhibits an open-circuit voltage (V_OC ) of 203 mV with a maximum power density (p_max ) of 5.1 W m^-2 at ∆T = 68 K. This result signifies that a few millimeters thick f-TEG could power Internet-of-Things (IoTs) devices converting low-grade heat to electricity.

Photonic Curing Enables Ultrarapid Processing of Highly Conducting β-Cu2-δSe Printed Thermoelectric Films in Less Than 10 ms

It has been a challenge to obtain high electrical conductivity in inorganic printed thermoelectric (TE) films due to their high interfacial resistance. In this work, we report a facile synthesis process of Cu-Se-based printable ink for screen printing. A highly conducting TE β-Cu_2-δSe phase forms in the screen-printed Cu-Se-based film through ≤10 ms sintering using photonic-curing technology, minimizing the interfacial resistance. This enables overcoming the major challenges associated with printed thermoelectrics: (a) to obtain the desired phase, (b) to attain high electrical conductivity, and (c) to obtain flexibility. Furthermore, the photonic-curing process reduces the synthesis time of the TE β-Cu_2-δSe film from several days to a few milliseconds. The sintered film exhibits a remarkably high electrical conductivity of ∼3710 S cm^-1 with a TE power factor of ∼100 μW m^-1 K^-2. The fast processing and high conductivity of the film could also be potentially useful for different printed electronics applications.

Recent Progress in Multiphase Thermoelectric Materials

Thermoelectric materials, which directly convert thermal energy to electricity and vice versa, are considered a viable source of renewable energy. However, the enhancement of conversion efficiency in these materials is very challenging. Recently, multiphase thermoelectric materials have presented themselves as the most promising materials to achieve higher thermoelectric efficiencies than single-phase compounds. These materials provide higher degrees of freedom to design new compounds and adopt new approaches to enhance the electronic transport properties of thermoelectric materials. Here, we have summarised the current developments in multiphase thermoelectric materials, exploiting the beneficial effects of secondary phases, and reviewed the principal mechanisms explaining the enhanced conversion efficiency in these materials. This includes energy filtering, modulation doping, phonon scattering, and magnetic effects. This work assists researchers to design new high-performance thermoelectric materials by providing common concepts.