Complete Report - University of New South Wales

The phonon dispersions show that if the superlattice structure is engineered correctly, the
mini-gaps can in principle be arranged such that they prevent the decay of optical phonons by
interrupting the same mechanism. However the density of states (DOS) in Figure 4.5.25 and
Figure 4.5.26 indicate that complete gaps in the DOS for all directions in reciprocal space
only occur for the QD superlattice. Hence this mechanism of slowing carrier cooling should
only be present in QD structures. Experimental evidence for this in the literature is being
assessed and effects on effi ciencies calculated. However, to achieve such effects in practice,
the structure would need to be fi nely tuned.
A further effect under investigation is that of the QD interface with the matrix, on phonon
dispersion. Figure 4.5.27 shows that with a soft interface the mini-gaps group into doublets
with a larger gap in the centre. This would improve the ease of fi ne tuning the QD superlattice
such as to block the optical phonon decay. Further work on modelling such interfaces is
underway.
Figure 4.5.27: Dispersion curve of
acoustic phonons for 1nm3 QDs in a
superlattice with 1nm spacing between
QDs – incorporating soft interface modes
between QDs and matrix.
Finally, some progress has been made
towards predicting the effi ciencies that a
Hot Carrier cell might achieve using the
non-ideal properties of existing materials
[4.5.21]. Preliminary computations show
that, at the illumination level used for the
experiments in [4.5.17, 4.5.18] (equivalent to about 2500 suns), effi ciencies of 50% could be
achievable. This is in spite of thermal losses in state of the art materials such as the multiple-
QW structures discussed elsewhere [4.5.17, 4.5.18], even though these have a non optimal
band gap of 1.5 eV. This result compares to a calculated 54% effi ciency at these concentrations
without any thermalisation losses at all [4.5.13]. Hence, at these concentrations the loss to
thermalisation for existing materials is not large. However, these fi gures assume perfect
energy selection at the contacts. Therefore, as operation at lower concentrations would be
preferable and because of the diffi culties in the fabrication of optimal contacts, it is desirable
to search for further reduction in carrier cooling and/or a reduction in the threshold injection
levels at which this occurs. Phononic band gap engineering in superlattices seems a potential
route to achieving this.
4.5.7 Supporting project areas
There are three projects associated with Third Generation Strand work but not directly
concerned with either of Si nanostructures, Up-converters or Hot Carrier cells. These are
continuing work on limiting effi ciency computer programs, coupling of light with surface
plasmons and quantum antennae.
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