Unidentified major p-type source in SnSe: Multivacancies

Tin selenide (SnSe) is considered a robust candidate for thermoelectric applications due to its very high thermoelectric figure of merit, ZT, with values of 2.6 in p-type and 2.8 in n-type single crystals. Sn has been replaced with various lower group dopants to achieve successful p-type doping in SnSe with high ZT values. A known, facile, and powerful alternative way to introduce a hole carrier is to use a natural single Sn vacancy, VSn. Through transport and scanning tunneling microscopy studies, we discovered that VSn are dominant in high-quality (slow cooling rate) SnSe single crystals, while multiple vacancies, Vmulti, are dominant in low-quality (high cooling rate) single crystals. Surprisingly, both VSn and Vmulti help to increase the power factors of SnSe, whereas samples with dominant VSn have superior thermoelectric properties in SnSe single crystals. Additionally, the observation that Vmulti are good p-type sources observed in relatively low-quality single crystals is useful in thermoelectric applications because polycrystalline SnSe can be used due to its mechanical strength; this substance is usually fabricated at very high cooling speeds.