Surface Oxide Removal for Polycrystalline SnSe Reveals Near-Single-Crystal Thermoelectric Performance

Tin selenide (SnSe) has emerged as a surprising new material with exceptional thermal transport and charge transport properties such as ultralow thermal conductivity, which give it a record-high thermoelectric figure of merit (ZT) of similar to 2.5-2.7 at around 800 K. These properties, however, have been only observable in well-prepared and properly handled single-crystal samples. Polycrystalline SnSe samples have markedly inferior properties paradoxically with higher apparent thermal conductivity and much lower ZT values than single crystals. The high thermal conductivity in polycrystalline samples has been attributed to surface tin oxides. Based on this hypothesis, we have employed an oxide-removing strategy that involves a chemical reduction process at 613 K under a 4% H-2/Ar atmosphere. This leads to an exceptionally low lattice thermal conductivity of similar to 0.11 W m(-1)K(-1) in polycrystalline hole-doped SnSe alloyed with 5% lead selenide, even lower than that of single crystals, and boosts the ZT to similar to 2.5 at 773 K.