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, V_Sn. Through transport and scanning tunneling microscopy studies, we discovered that V_Sn are dominant in high-quality (slow cooling rate) SnSe single crystals, while multiple vacancies, V_multi, are dominant in low-quality (high cooling rate) single crystals. Surprisingly, both V_Sn and V_multi help to increase the power factors of SnSe, whereas samples with dominant V_Sn have superior thermoelectric properties in SnSe single crystals. Additionally, the observation that V_multi 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.