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Ab-initio model of Si:Ge by VASP: (a) 1D phonon DOS
for Si:Ge MQW. 3D DOS for QD arrays, Si dots of (b) 1.3 nm and (c) 2.1 nm. Insets show detail over 60-64 meV: indicating very small mini-gaps in 3D.
3D force constant modelling of hypothetical QD supe rlattice
Hot Carrier Solar Cells – structures for slowed carr ier coolingG. Conibeer, M.A. Green, D. König, S. Shrestha, S. Huang , P. Aliberti, L. Treiber, R. Patterson, B. Puthen Vee ttil, A. Hsieh
– ARC Photovoltaics Centre of Excellence, UNSW, Sydney
A.Luque, A. Marti, P.G. Linares, E. Cánovas, E. Ant olín, D. Fuertes Marrón, C. Tablero, E. Hernández- Instituto de Energía Solar-Universidad Politécnica de Madrid
J-F. Guillemoles, L. Huang, A. Lebris, S. Laribi, P . Olsson- IRDEP, joint EDF-CNRS Institute for Photovoltaic R&D , Paris
T.W. Schmidt, R.G.C.R. Clady, M.J.T. Tayebjee- School of Chemistry, University of Sydney
ConclusionsConclusionsConclusionsConclusions
Concept of the Hot Carrier cell [1,2]Concept of the Hot Carrier cell [1,2]Concept of the Hot Carrier cell [1,2]Concept of the Hot Carrier cell [1,2]
•Absorption of all solar photons with energies greater than the absorber threshold energy. •Collect carriers before they thermalise. Requires:Requires:Requires:Requires:- Selective energy contacts -- Slowing of carrier cooling -
Selective energy contacts
Hot carrier absorber HolecontactElectroncontact
Hot Optical phonon population“phonon bottleneck effect”
Slows further carrier cooling
Decay via O → LA + LA (only)
Electrons carry most energy
Cool predominantly via small wave vector optical phonon
emission - timescale of psinelastic – energy relaxation
LO
TO
LA & TA – quasi elastic
Hot Carrier coolingHot Carrier cell requirements
• InP has large phonon band gap - should block Klemens decay• GaAs has no phonon gap – but similar Eg
Eg
GaAs InP
Eg
E
k
TA
LA
LOTO
63meV
O →→→→ LA + LA (Klemens) – principal mode in semiconductors [3]
LO
TO
LALA
Large phonon gap in bulk materials → M >>m- Klemens blocked
LA
TO
TA
LA
XL Γ
LA
LA
TO
QD nanstructure - mini-gaps- folded phonon modesBlock Klemens?- need specific periodic
superstructureBlock Ridley?
•Peak intensities (black arrows) shift closer to the band gap (white lines) with timeInP stays further above the band gap for longer times than GaAs
• Slower cooling in InP due to suppression of Klemens’ decay
Low temperature QEadapted for the UV
Deposition ofDeposition ofDeposition ofDeposition of nanocrystalnanocrystalnanocrystalnanocrystal superstructuressuperstructuressuperstructuressuperstructures
Four characterisation techniques for HCSC: • Low temperature current-voltage • Low temperature quantum efficiency (QE) • Photoreflectance for band diagram analysis• Time resolved PL – measure carrier cooling
Low temperature I-Vhigh I and very low I
Time resolved PL spectra for bulk GaAs and InP 730 nm excitation – carrier density - 8.5x10-19 cm-3
• Carriers cool by emission of phonons - restricting Optical to Acoustic phonon decay can slow cooling • Binary semiconductors can have large band gaps between O and A modes,
e.g. InP: phonon gap large enough to block Klemens decay – from TRPL• Ridley mode allowed but this has lower energy loss • Cubic materials restrict Ridley loss through narrow optical dispersion
• Folding of Brillouin zone in QD nanostructures gives gaps in phonon DOS • These can prevent Klemens decay if tuned correctly • Modelling in 1D and 3D – Group IV and hypothetical superlattices • Complete gaps in reciprocal space can block Klemens decay• Langmuir Blodgett deposition of ordered arrays of NP superstructures
Ma
Tacoustic
12=ω
+=Mma
Tand
ma
Toptical
11212ω
Simple force constant model derived from the equationof motion gives phonon frequency, ω, as:
Acoustic max. and optical min. & max. phonon energies[‘M’ (heavy), ‘m’ (light) atomic masses; T/a = force constant]
Optical to acoustic phonon decay
BlockingBlockingBlockingBlockingOptical Phonon decayOptical Phonon decayOptical Phonon decayOptical Phonon decay
O → TO + LA – Ridley [4] smaller energy loss
Cubic << Ridley
Time resolved PL Time resolved PL Time resolved PL Time resolved PL ---- resultsresultsresultsresults
.
Maximum phonon energy, ωoptical
ωoptical / 2aligned with mini-gap
(a) 1D phonon DOS Si:Ge MQW
(b) 3D DOS
(c) 3D DOS
Very narrow mini-gaps – bigger for 1.3nm cf. 2.1nm QDs
(111)(110)diamond superlattice (100)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
BiB
BS
b
InN
SnO
BA
s
GaN
AlS
b
InP
BP
SiC
AlN
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Eoptical (M
ax -M
in)
Eacoustic
Eop
tical
−E
acou
stic
Eac
oust
ic
Phonon miniPhonon miniPhonon miniPhonon mini----gaps ingaps ingaps ingaps in nanostructuresnanostructuresnanostructuresnanostructures
3D phonon dispersions for a diamond QD superlattice, with 1nm diamond QDs. Mass ratio MMatrix:MQD = 1:7. There are complete gap in reciprocal space – sufficient to block Klemens’ decay, indicated by the arrow at ωoptical / 2
PhononicPhononicPhononicPhononic band gaps in bulk materialsband gaps in bulk materialsband gaps in bulk materialsband gaps in bulk materials
References: References: References: References: 1. P. Würfel, SOLMAT. 46 (1997) 43-47. 2. M.A. Green, Third Generation Photovoltaics (2003). 3. P. Klemens, Phys Rev 148 (1966) 845; 4. J.W. Pomeroy et al., AAPL. 86 (2005) 223501.
Characterisation for Hot Carrier Solar CellsCharacterisation for Hot Carrier Solar CellsCharacterisation for Hot Carrier Solar CellsCharacterisation for Hot Carrier Solar Cells
substrate
Langmuir-Blodgett deposition of amonolayer of encapsulated NPs
InP/CdS core-shell NPs: 1. Synthesis 2. Characterization3. Test of phonon engineering concepts (Planned)
Langmuir-Blodgett deposition to fabricate ordered arrays of nanoparticles (NP)• Initial work on Si & Au NPs of uniform size• Functionalise to give close packing• Langmuir-Blodgett deposition in layers• Characterise periodicity• Later work on NPs with MQD>>mmatrix
IES-UPM
Raman spectra of InP/CdS NP’s
(crossed polarization)Displaying LO & TO modes
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