Environmental Engineering Reference
In-Depth Information
Figure 8.3 Two separate photon absorptions
are illustrated in schematic diagram of
intermediate band solar cell, as realized using
coupled quantum dots to form a
mini-band.
The idea is that the two smaller gaps allow
utilization of the part of the fixed solar spectrum
below E
C
E
V
¼E
G
. In this device, the usual
absorption for hc/l
l
E
G
also occurs, at least in
principle. The upper curve shown in Figure 8.1
assumes that carriers in the intermediate band
have high mobility [102].
would be small, in the range of meV, as was discussed in Chapter 3 in relation to
hydrogenic impurities. A similar situationmight occur with chemical impurities that
form deeper levels within the gap. In the work cited, the authors suggest the
intermediate band could be formed by a regular array of quantum dots (QDs),
three-dimensionally localizing electron states.
The small overlap between adjacent dots is assumed to form the intermediate band
(referred to as mini-band) in Figure 8.3.
A sketch of an intermediate band quantum dot solar cell device is given in
Figure 8.4. It is assumed that the metallic intermediate band results from half
filling with electrons the quantum dot wells, as indicated. In this structure, the
intermediate band is isolated from the external contacts by means of heavily doped p-
and n-type emitter layers.
Experimental evidence of photocurrent from subbandgap light in an intermediate
band solar cell structure has recently been demonstrated [104].
The authors [104], however, state that their devices do not exhibit the main features
expected for the intermediate band solar cell. The results so far obtained, thus, give
little indication that a practical intermediate band solar cell device is near. Since it is
dif
cult to fabricate a complicated structure as asked for in Figures 8.3
-
8.5, the
failure of the experiment may re
ect the dif
cult fabrication rather than a failure of
the essential intermediate band cell concept.
8.2
Impact Ionization and Carrier Multiplication
The Li-drifted germanium detector (http://en.wikipedia.org/wiki/Germanium_
detector#Germanium_detector.) is an elegant commercially available solid-state
