Electronic Band Structure Dominates Below-Room-Temperature Thermoelectrics

Thermoelectric materials operating below room temperature (BRT, <300 K) offer a compelling pathway to solid-state refrigeration without moving parts or environmentally harmful refrigerants, while also enabling efficient harvesting of low-grade heat. Despite this potential, progress in BRT thermoelectrics has remained limited since the discovery of CsBi 4 Te 6 in 2000. At low temperatures, charge transport is confined within a narrow energy window on the order of ∼ k B T, which renders thermoelectric performance highly sensitive to fine electronic-structure features. Under these conditions, subtle variations in band curvature, valley degeneracy, and Fermi surface topology are nonlinearly amplified, making electronic structure engineering—rather than further reduction of lattice thermal conductivity—the dominant lever for improving performance. This perspective examines recent progress in representative BRT systems, including Mg 3 (Bi, Sb) 2, TlCu 3 Te 2, and two-dimensional SnQ (Q = S, Se) single crystal materials. These materials exemplify distinct yet convergent electronic strategies—valley degeneracy, unconventional Fermi surface geometry, and multiband synglisis—that maximize the power factor within the constrained transport window. We highlight how these developments collectively establish an electrical-first design paradigm for BRT thermoelectrics, in which band structure engineering governs both performance breakthroughs and remaining limitations. Finally, we outline key challenges, including achieving balanced n- and p-type performance, improving structural robustness and scalability, and reducing interfacial losses in device architectures, and discuss future opportunities for advancing thermoelectric cooling and low-temperature energy conversion.