The best commercial materials are alloys of Bi 2 Te 3 with Bi 2 Se 3 (n-type) and with Sb 2 Te 3
(p-type). The alloys are used because phonon thermal conductivity can be reduced significantly
with only a small reduction in the electronic power factor (S^2 σ). Bi 2 Te 3 -based alloys have a
maximum ZT around 1 near room temperature. Thus these materials are used for refrigerators.
Thermoelectric refrigerators based on these materials have a COP ~ 1 in the same operational
temperature range of compressor-based household refrigerators, which have a COP ~3–4. Thus,
current commercial materials are not competitive. However, thermoelectric refrigerators have
advantages for small refrigeration applications and have found niche markets in a variety of
applications, such as picnic coolers, automobile car seats (luxury models), medical equipment,
and laser-diode temperature control. Bi 2 Te 3 -based materials are also used in some power
generation applications; however, the module efficiency is limited to 5%. The U.S. National
Aeronautics and Space Agency used SiGe alloys (and PbTe-based alloys) to make radioisotope-
powered thermoelectric power generators operating in the temperature range of 300–900°C (and
300–600°C for PbTe-based alloys), with a system conversion efficiency ~6–7%. These materials
all have a maximum ZT less than but close to 1. Commercial PbTe-based power generation
systems using fossil fuel have a fuel-to-electricity efficiency of ~2.5%. The lower efficiency is
due to heat loss carried by combustion gas.
The commercial materials discussed above, with a maximum ZT ~ 1, were mostly discovered in
the 1950s. Little progress was made in the subsequent years. In the 1990s, the possibility of
improving the thermoelectric figure of merit based on electron band-gap engineering and phonon
engineering in nanostructures was investigated. These ideas have led to resurgence in
thermoelectric research and significant progress in improving ZT, particularly based on
nanostructured materials (Chen 2003). Venkatasubramanian et al. (2001) reported that
Bi 2 Te 3 /Sb 2 Te 3 -based p-type superlattices have a room-temperature ZT of 2.4. Harman et al.
(2002) reported that PbTe/PbTeSe superlattices with nanodots formed by strain have a room-
temperature ZT of 1.6 and a ZT ~ 3.5 around 300°C. Hsu et al. (2004) reported bulk
nanostructures of AgPb 2 SbTe2+m. with a ZT of 2.2 at 527°C. These results suggest that
thermoelectric materials can have major impacts in energy conversion technology. Meanwhile,
several research projects aiming at improving device efficiency based on more mature materials
are underway. The Jet Propulsion Laboratory reported a segmented thermoelectric unicouple
with an efficiency of ~14% with the hot side at 975K and cold side at 300K.
Current commercial thermoelectric modules based on Bi2Te3 are at ~$4.60/W because the
market is very small. With current efficiency, it has been projected that the cost can reach
$0.74/W if the annual consumption is more than 2 million modules. If the efficiency can be
improved, significant cost reduction is possible. It was projected that $0.3/W (electrical power)
could be realized by using nanostructured materials. Assuming a 35% energy conversion
efficiency of thermoelectric devices and a concentrator cost of $0.24/W (solar input power), the
cost of solar-based thermoelectric power generator systems has the potential to reach the
$1–1.5/W (electric) range.
The enabler for the cost/performance improvement is cost-effective materials with high ZT
values. Nanostructured materials have broken the ZT ~ 1 barrier, and it seems that ZT ~ 4 is
reachable. A major effort should be aimed at mass-producible nanostructures with high ZT and
further understanding of the scientific underpinnings of high-ZT values. Other important factors
are material reliability, system design, and thermal management.