The Remarkable Role of Quadruple-Band Synglisis in High-Thermoelectric Performance Tin Sulfide Crystal
Thermoelectric energy conversion is governed by the dimensionless figure of merit ZT = S²σT/κtot, yet decoupling Seebeck coefficient, electrical conductivity, and thermal conductivity remains a persistent challenge. Commercial Bi₂Te₃-based alloys are constrained by tellurium scarcity and high cost, motivating the search for earth-abundant alternatives such as SnSe and SnS. SnS, an analog of SnSe, offers higher elemental abundance and intrinsic low thermal conductivity, but its wide bandgap (Eg ≈ 46 kBT) and poor electrical transport in polycrystalline form have historically limited ZT. Single-crystal p-type SnS exhibits high in-plane carrier mobility and a complex valence band structure with multiple valence band maxima (VBM) separated by small energy offsets (ΔE). Prior work achieved triple-band convergence and triple-band synglisis, but further enhancement requires activating additional valence bands. This study reports the activation of four valence bands and the realization of quadruple-band synglisis in p-type SnS crystals via SnS₂ introduction, which promotes simultaneous convergence of energy and momentum across four valence bands. The resulting band manipulation substantially improves thermoelectric efficiency, yielding high power factor and ZT. Notably, despite an exceptionally wide bandgap of 46 kBT—far exceeding the conventional 6–10 kBT range for thermoelectric cooling—the engineered SnS demonstrates viability as a refrigeration material. These findings establish a pathway for low-cost, earth-abundant, environmentally friendly thermoelectric materials that challenge the dominance of tellurium-based systems.