Thermoelectric conversion elements made from p-type and n-type semiconductors. Image: Tokyo Tech.
Thermoelectric conversion elements made from p-type and n-type semiconductors. Image: Tokyo Tech.

Energy consumption in developed countries is rather wasteful. Nearly two-thirds of total energy consumption is typically lost to the environment as 'waste heat', which ends up contributing to global warming. Hence the growing interest in finding ways to make use of this waste heat.

One option is to take advantage of what is known as 'thermoelectric conversion', in which an electric voltage is produced from the temperature difference across a semiconductor. Thermoelectric devices contain n-type and p-type semiconductors with two types of charge carrier – negative electrons for n-type and positive holes for p-type. These n-type and p-type semiconductors are connected in series to produce a large thermoelectric voltage, making it necessary to develop both p-type and n-type semiconductors with high thermoelectric conversion efficiencies.

One particular semiconductor material that scientists have recently turned their attention to is tin monoselenide (SnSe), which reportedly exhibits the world's highest ZT value, a measure of thermoelectric conversion performance. But SnSe cannot easily be turned into an efficient n-type or p-type semiconductor.

Doping SnSe with alkali ions can improve its p-type thermoelectric performance, but the alkali ions are volatile and diffusive, and are not suitable for high-temperature applications. Adding bismuth and iodine, on the other hand, can turn SnSe into an n-type semiconductor, but results in low electron concentrations.

Now, in a paper in Advanced Functional Materials, a team of scientists from Tokyo Institute of Technology in Japan, led by Takayoshi Katase, reports that when SnSe is doped with antimony (Sb), denoted as (Sn1-xSbx)Se, it exhibits a peculiar switching of conduction type. The team found that at low doping concentrations (Sn1-xSbx)Se started out with p-type conduction but switched to n-type with increased doping, before switching back to p-type for high doping concentrations.

Detailed analyses and calculations revealed an interesting charge type switching mechanism, which, the team found, has to do with the distribution of Sb substitution sites between Sn and Se. They attributed this behavior to a switching of the major Sb substitution site from Se (SbSe) to Sn (SbSn) with increased doping.

At very low Sb concentrations, the p-type conduction occurs solely due to holes supplied by the Sn vacancy. But as doping increases, SbSn starts to donate electrons, while SbSe forms an 'impurity band' that allows conduction through it, resulting in the observed n-type behavior. However, as doping levels increase further, the Fermi level approaches the mid-gap level, located between the SbSe impurity band and the conduction band minimum, resulting in the p-type conduction.

"Now that we understand the mechanism at play in the polarity switching of Sb-doped SnSe, we can hope to optimize the bulk synthesis process to further improve its thermoelectric performance and, in turn, realize high-performance thermoelectric conversion devices with it," says Katase.

What's more, the researchers also expect that this doping-site-switching-based polarity control will become more versatile in the future, allowing it to be applied to other semiconductor materials whose carrier types are difficult to control.

This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.