The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalize to the band
edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 65%
under 1 sun and 86% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but
modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the
required slowing of carrier cooling. A number of materials and structures are being investigated with these properties and
test structures are being fabricated. Initial measurements indicate slowed carrier cooling in III-Vs with large phonon
band gaps and in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the
way for their development to fully functioning high efficiency solar cells.
The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalize to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 65% under 1 sun and 86% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. A number of materials and structures are being investigated with these properties and test structures are being fabricated. Initial measurements indicate slowed carrier cooling in III-Vs with large phonon band gaps and in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.
The Hot Carrier solar cell is a third generation photovoltaic concept which has the potential to achieve high efficiencies,
exceeding the Shockley-Queisser limit for a conventional p-n junction solar cell. The theoretical efficiencies achievable
for the Hot Carrier solar cell is 65% for non-concentrated solar radiation and 85% for maximally concentrated light,
very close to the limits of an infinite tandem solar cell. The approach of the Hot Carrier solar cell is to extract carriers
generated before thermalisation to the bandgap edge occurs when their excess energy is lost to the environment as heat.
To achieve this, the rate of carrier cooling in the absorber must be slowed down sufficiently enough to allow carriers to
be collected while they are hot. This work investigates using hafnium nitride as such an absorber to restrict mechanisms
of carrier cooling. Hafnium nitride’s phononic properties, where a large ‘phononic band gap’ exist can reduce the carrier
cooling rate by means of a phonon bottleneck such that optical phonons cannot decay into acoustic phonons by means of
the Klemens’ mechanism. Optical phonon-electron scattering can maintain a hot electron population while acoustic
phonons are irrecoverable and lost as heat. The electronic and phononic properties of hafnium nitride are evaluated for
their suitability to be used in a Hot Carrier solar cell absorber. Recent work on the fabrication of hafnium nitride at
UNSW is presented.
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