Advances in thermoelectric materials research: Looking back and moving forward

BACKGROUND Heat and electricity are two forms of energy that are at opposite ends of a spectrum. Heat is ubiquitous, but with low quality, whereas electricity is versatile, but its production is demanding. Thermoelectrics offers a simple and environmentally friendly solution for direct heat-to-electricity conversion. A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity. Electricity also can drive a TE device to work as a solid-state heat pump for distributed spot-size refrigeration. TE devices are free of moving parts and feasible for miniaturization, run quietly, and do not emit greenhouse gasses. The full potential of TE devices may be unleashed by working in tandem with other energy-conversion technologies. Thermoelectrics found niche applications in the 20th century, especially where efficiency was of a lower priority than energy availability and reliability. Broader (beyond niche) application of thermoelectrics in the 21st century requires developing higher-performance materials. The figure of merit, ZT, is the primary measure of material performance. Enhancing the ZT requires optimizing the adversely interdependent electrical resistivity, Seebeck coefficient, and thermal conductivity, as a group. On the microscopic level, high material performance stems from a delicate concert among trade-offs between phase stability and instability, structural order and disorder, bond covalency and ionicity, band convergence and splitting, itinerant and localized electronic states, and carrier mobility and effective mass. ADVANCES Innovative transport mechanisms are the fountain of youth of TE materials research. In the past two decades, many potentially paradigm-changing mechanisms were identified, e.g., resonant levels, modulation doping, band convergence, classical and quantum size effects, anharmonicity, the Rashba effect, the spin Seebeck effect, and topological states. These mechanisms embody the current states of understanding and manipulating the interplay among the charge, lattice, orbital, and spin degrees of freedom in TE materials. Many strategies were successfully implemented in a wide range of materials, e.g., V2VI3 compounds, VVI compounds, filled skutterudites and clathrates, half-Heusler alloys, diamond-like structured compounds, Zintl phases, oxides and mixed-anion oxides, silicides, transition metal chalcogenides, and organic materials. In addition, advanced material synthesis and processing techniques, for example, melt spinning, self-sustaining heating synthesis, and field-assisted sintering, helped reach a much broader phase space where traditional metallurgy and melt-growth recipes fell short. Given the ubiquity of heat and the modular aspects of TE devices, these advances ensure that thermoelectrics plays an important role as part of a solutions package to address our global energy needs. OUTLOOK The emerging roles of spin and orbital states, new breakthroughs in multiscale defect engineering, and controlled anharmonicity may hold the key to developing next generation TE materials. To accelerate exploring the broad phase space of higher multinary compounds, we need a synergy of theory, machine learning, three-dimensional printing, and fast experimental characterizations. We expect this synergy to help refine current materials selection and make TE materials research more data driven. We also expect increasing efforts to develop high-performance materials out of nontoxic and earth-abundant elements. The desire to move away from Freon and other refrigerant-based cooling should shift TE materials research from power generation to solid-state refrigeration. International round-robin measurements to cross-check the high ZT values of emerging materials will help identify those that hold the most promise. We hope the renewable energy landscape will be reshaped if the recent trend of progress continues into the foreseeable future.