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 ∼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 ∼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 ∼2.5 at 773 K. Well prepared and properly handled single-crystal SnSe samples exhibit exceptional thermoelectric performance with a record-high thermoelectric figure of merit (ZT) of ∼2.5–2.7 due to the ultralow thermal conductivity. Achieving comparable thermoelectric performance to single crystals in polycrystalline SnSe samples has been the widely sought-after goal. These properties, however, have never been achievable in the polycrystalline samples. Particularly, serious debates have arisen on markedly higher apparent thermal conductivity for the latter. Here, we show that the underperformance of polycrystalline SnSe is mainly due to the presence of tin oxides in the sample. Our H 2 reduction process removes the majority of tin oxide films that ubiquitously cover SnSe grains and eventually unveils the ultralow thermal conductivity intrinsic in this material. It also simultaneously enhances the hole mobility, electrical conductivity, and Seebeck coefficient, leading to an extraordinarily high ZT of ∼2.5 at 773 K. Nearly intrinsic charge and thermal transport properties of polycrystalline SnSe materials are unveiled for the first time. We confirm that tin oxide layers on the surface of SnSe powder are the origin of paradoxically higher apparent thermal conductivity in polycrystalline samples over single crystal. Our surface oxide removing strategy on polycrystalline SnSe materials reveals even lower thermal conductivity than single-crystal samples and simultaneously enhances the hole mobility, electrical conductivity, and Seebeck coefficient, leading to an extraordinarily high thermoelectric figure of merit (ZT) of ∼2.5.
- polycrystalline SnSe
- surface oxide
- ultralow thermal conductivity
- waste heat
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