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 superlattice Hot Carrier Solar Cells – structures for slowed carrier cooling G. Conibeer, M.A. Green, D. König, S. Shrestha, S. Huang, P. Aliberti, L. Treiber, R. Patterson, B. Puthen Veettil, A. Hsieh – ARC Photovoltaics Centre of Excellence, UNSW, Sydney A.Luque, A. Marti, P.G. Linares, E. Cánovas, E. Antolín, 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 Conclusions Conclusions Conclusions Conclusions 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 Hole contact Electron contact 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 ps inelastic – energy relaxation LO TO LA & TA – quasi elastic Hot Carrier cooling Hot Carrier cell requirements • InP has large phonon band gap - should block Klemens decay • GaAs has no phonon gap – but similar E g E g GaAs InP E g E k TA LA LO TO 63meV O → LA + LA (Klemens) – principal mode in semiconductors [3] LO TO LA LA Large phonon gap in bulk materials → M >>m - Klemens blocked LA TO TA LA X L Γ LA LA TO QD nanstructure - mini-gaps - folded phonon modes Block Klemens? - need specific periodic superstructure Block Ridley? •Peak intensities (black arrows) shift closer to the band gap (white lines) with time InP stays further above the band gap for longer times than GaAs • Slower cooling in InP due to suppression of Klemens’ decay Low temperature QE adapted for the UV Deposition of Deposition of Deposition of Deposition of nanocrystal nanocrystal nanocrystal nanocrystal superstructures superstructures superstructures superstructures 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-V high 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 M a T acoustic 1 2 = ϖ + = M m a T and m a T optical 1 1 2 1 2 ϖ Simple force constant model derived from the equation of 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 Blocking Blocking Blocking Blocking Optical Phonon decay Optical Phonon decay Optical Phonon decay Optical Phonon decay O → TO + LA – Ridley [4] smaller energy loss Cubic << Ridley Time resolved PL Time resolved PL Time resolved PL Time resolved PL - results results results results Maximum phonon energy, ω optical ω optical / 2 aligned 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 BSb InN SnO BAs GaN AlSb InP BP SiC AlN 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% E optical (Max - Min) E acoustic E optical -E acoustic E acoustic Phonon mini Phonon mini Phonon mini Phonon mini-gaps in gaps in gaps in gaps in nanostructures nanostructures nanostructures nanostructures 3D phonon dispersions for a diamond QD superlattice, with 1nm diamond QDs. Mass ratio M Matrix :M QD = 1:7. There are complete gap in reciprocal space – sufficient to block Klemens’ decay, indicated by the arrow at ω optical / 2 Phononic Phononic Phononic Phononic band gaps in bulk materials band gaps in bulk materials band gaps in bulk materials band 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 Cells Characterisation for Hot Carrier Solar Cells Characterisation for Hot Carrier Solar Cells Characterisation for Hot Carrier Solar Cells substrate Langmuir-Blodgett deposition of a monolayer of encapsulated NPs InP/CdS core-shell NPs: 1. Synthesis 2. Characterization 3. 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 M QD >>m matrix IES-UPM Raman spectra of InP/CdS NP’s (crossed polarization) Displaying LO & TO modes