Hot Carrier Solar Cell
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Transcript of Hot Carrier Solar Cell
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Hot Carrier Solar Cell
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Basic concept
Absorption of all solar photons with energies greater than the absorber
threshold energy.
Collect carriers before they thermalize with lattice.
Requires:
Energy selective contacts (ESCs)
Slowing of carrier cooling
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Band diagram of an ideal hot carrier solar cell.
The absorber has a hot carrier distribution attemp 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.
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Energy selective contacts (ESCs)
Carriers must be collected over narrow energy range:
- in conduction band for e- and valence band for h+ ;
- prevents loss of energy to cold carriers in contacts;
- minimises increase in entropy;
Replenishment of extracted energy by carrier-carrier scattering in hotcarrier population.
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These are collected at the optimal energy and hence electrochemicalpotential of the appropriate contact.
Those carriers at the
energy of the ESC:
The lower band edge of the contact prevents are verse flow of coldcarriers into the hot absorber which would otherwise decrease the
average energy and hence electrochemical potential of collectedcarriers.
A distribution of carrier
energies below the ESCenergy:
This is moresubtle-collection of these carriers would indeedincrease current (as occurs in a thermoelectric cell) but the
excess energy of each carrier above the ESC energy would belost in thermalisation in the external contact. A more efficient
hot carrier cell is achieved if these carriers are reflected
back into the absorber by an upper energy cut-off in thecontact. A further requirement is that carriers in the absorber
with energy higher and lower than the ESC are then able tore-normalise their energy through carriercarrier scatteringso as to re-populate the ESC level.
A distribution of
carrier energies above
the ESC energy:
The operation of the ESCs is best understood by considering the hot
carrier distribution in the absorber as being comprised of three
components:
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Layers deposited by RF-sputtering
On annealing at 1100C- The Si rich
layer (middle layer) crystallises to SiQD in a matrix of SiO2
selected
Energy
Double barrier resonant
tunnelling in a single layer
QD structure used for SEC
QD not QW required to give
Total energy selection
Sample Structure
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I-V
IV data at 300 K for the double-barrier resonant tunnelling structure, showing
NDR (indicative of resonanttunnelling) for two different devices.
NDR at 300K- indicates SEC
To improve quality of peak
need smaller area.
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Conductive AFM
the current profile across two
spots in left figure indicated by the
circle.
CAFM on a double oxide resonant tunnelling structure:
current map obtained at 10V (white dots
at up to 800 pA)
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Optically assisted I-V
Light I-V gives larger current- carriers excited to SEC level
Tentative evidence for resonance at ~ 3.5V for dark and ~ 2.1V under
illumination- collection of hot carriers at lower bias with optical assistance.
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Thermal Characterisation
Thermoelectric voltage
generation- evidence of
tunnelling through QD structure
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Slowing of carrier cooling
Decay via O LA + LA (only)
TO
LO
LA & TA quasi elastic
Electrons carry most energy
Cool predominantly via
small wave vector optical phonon
emission - timescale of ps
inelastic energy relaxation
Hot Optical phonon population
phonon bottleneck effect
Slows further carrier cooling
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Phononic band gaps in
bulk materials
Ratios of phononic band gap energy values for various binary compounds,
based on simple elemental mass calculations: energy gap between
acoustic and optical modes normalised to acoustic frequency; energy
dispersion of optical modes. The dashed line indicates the max. acoustic
phonon energy to which the other data are normalised.
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Phonon energy as a function of phonon momentum and Density of States (DOS)
for (a) hexagonal InN measured and calculated data from [21] and (b) dataappropriate for cubic InN extracted from (a). [The lower symmetry in the
for hexagonal as compared to for cubic, gives the extra degree of folding in
the former.] The data in (b) matches well the calculated data for InN in Fig.4. Also
shown are the optical phonon decay mechanisms (seetext). [NB. The phonon
decays shown in Fig.5 are schematic. The actual phonon interactions conserve
energy and momentum in detail, but are difficult to illustrate.]
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Conclusions
Hot carrier cells require two difficult implementations: slowing of the rate of
carrier cooling and collection of carriers through ESCs of a narrow width.
Improvement in quality factor of the resonance observed is being tackled by
improving the quality of materials in the structure.
Carrier cooling by emission of optical phonons which then decay into LA phonons
can in principle be reduced by blocking the latter decay which operates via the
Klemens anharmonicity mechanism.
InN seems a good candidate for a possible hot carrier absorber material because
of its wide gap, larger than the acoustic phonon energy and its narrow band gap
for absorption. However, the important elements of these properties seem also
possible in carefully tailored QD superlattices, such that coherent Bragg reflection
of phonons allows gaps in the dispersion to open up.
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