Hot Carrier Cells
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 “hot” thus enhancing
the voltage of a cell. Thus it tackles the major PV loss mechanism
of thermalisation of carriers. To be effective such carriers must
be collected over a very small energy range with selective energy
contacts so as to prevent cold carriers in the contacts cooling
the hot carriers to be extracted. In thermodynamic terms the carriers
are thus collected with a very small increase in entropy. Ideally
this collection would be isoentropic using mono-energetic contacts.
Schematic of an ideal hot carrier cell
The absorber has a hot carrier distribution at temp
TH . Carriers cool isoentropically in the mono-energetic
contacts to TA . The difference of the Fermi levels
of these two contacts is manifested as a difference in chemical
potential of the carriers at each contact and hence an external
voltage, V.
Band diagram of an ideal hot carrier cell
The challenges for such a hot carrier cell fall into
two categories. The first concerns absorption under such difficult
constraints and retardation of the thermalisation mechanisms relative
to radiative recombination rates. Techniques to achieve this are
at present mainly theoretical, although slowing of carrier cooling
rates has been observed in superlattices (see Hot
Carrier absorbers), where radiative recombination rates are
also enhanced. The second category concerns contacts that are able
to collect carriers from such hot carrier distributions with no
entropy generation. [P. Würfel, “Solar Energy Conversion
with Hot Electrons from Impact Ionisation”, Sol. Energy Mats.
and Sol. Cells. 46, 43 (1997)]
The Centre's research on hot carrier cells has focused
on the two following areas:
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