Complete Report - University of New South Wales

absorption
emission
absorption, emission [au]
Figure 4.5.20:
Absorption/emission
from three different PbSe
quantum dot samples. The
emission of the quantum
dots coincide with the
absorption of Er 3+ doped
phosphors.
1200 1300 1400 1500 1600 1700
wavelength [nm]
4.5.6 Hot Carrier Cell
The concept underlying the hot carrier solar cell is to slow the rate of photoexcited carrier
cooling, caused by phonon interaction in the lattice, to allow time for the carriers to be
collected whilst they are still at elevated energies (“hot”) and thus allow higher voltages to be
achieved from the cell [4.5.13]. This approach thus tackles the major PV loss mechanism of
thermalisation of carriers (loss (2) in Figure 4.5.1 and strategy (b) discussed above in Section
4.5.1). In addition to an absorber material that slows the rate of carrier relaxation, a hot
carrier cell must allow extraction of carriers from the device through contacts which accept
only a very narrow range of energies (selective energy contacts).
Within the Third Generation Strand, the parallel project, funded by Toyota, on thermoelectric
and energy conversion devices has strong parallels with the Hot Carrier cells project. They
both require the selective energy contacts discussed below but in addition the Hot Carrier
cell requires an absorber in which carrier thermalisation is very signifi cantly slowed from the
picosecond timescale of normal semiconductors, to closer to the nanosecond timescale of
carrier recombination.
Experimental work in the Third Generation Strand is currently focussed on selective energy
contacts using resonant tunnelling structures based on a similar Si quantum dot approach
to that discussed in Section 4.5.2. Theoretical work is directed at investigating mechanisms
for slowing carrier cooling by modifying phononic band structures in nanostructure
superlattices.
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