Chemistry Reference
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d
n
3
r
4
n
g
|
7
Figure 6.5
(a) Distortion of density of states with resonant level. (b) Figure of merit
for 2 wt% Tl-PbTe. (c) Pisarenko plot of Tl-PbTe. Solid black line is
calculated; solid black dots are experimental data.
Reprinted with permission from Ref. 26. Copyright
r
2008 American
Association for the Advancement of Science.
26
.
Thallium produces resonant levels inside the valence band of PbTe. Special
impurities, e.g., thallium in PbTe, can create resonant states inside the va-
lence or conduction bands in the host material. Then, these resonant states
can distort the electronic density of states to achieve a large differential
density of states [dD(E)/dE] at the Fermi level. If the Fermi level is located on
the rapidly rising side of the distorted density of states, according to eqn (6.9)
we can increase the Seebeck coecient without significantly decreasing the
electrical conductivity.
26
Figure 6.5 (b) and (c) show the figure of merit and
Pisarenko plot for this material, respectively. Doping PbTe with 2 wt% Tl
yielded a ZT of
1.5 at 773 K.
26
The Pisarenko plot shows the relationship
between the carrier concentration and the Seebeck coecient. As mentioned
above, the Seebeck coecient generally decreases with increasing carrier
concentration (Figure 6.2). However, in Tl-PbTe, the Seebeck coecient does
not decrease as the carrier concentration increases, but instead stays high at
B
B
150 mVK
1
even at very high carrier concentrations because of the
resonant level.
In 2011, a ZT of
1.8 was reported for Na-doped PbTe
1-x
Se
x
.
36
In PbTe
systems, there are two valence bands, the L band and S band. In the alloys
with Se, PbTe
1-x
Se
x
, when the temperature increases, the band maximum of
B
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