Hot Carrier Solar Cell

7/30/2019 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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Hot Carrier Solar Cell