GCEP Symposium 5 October 2011 HOT CARRIER SOLAR CELLS

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1 GCEP Symposium 5 October 2011 HOT CARRIER SOLAR CELLS Gavin Conibeer - Photovoltaics Centre of Excellence, UNSW Robert Patterson, Pasquale Aliberti, Shujuan Huang, Yukiko Kamakawa, Hongze Xia, Dirk König, Binesh Puthen-Veettil, Santosh Shrestha, Martin Green - PV CoE, University of New South Wales Jean-François Guillemoles, Arthur LeBris, Par Olsson, Sana Laribi IRDEP: EDF/CNRS/ENSCP, Paris Raphael Clady, Murad Tayebjee, Tim Schmidt, Nicholas Ekins-Daukes University of Sydney Antonio Luque, Antonio Marti, Pablo Linares, Enrique Canovas, IES-UPM

2 E f f i c i e n c y, % Photovoltaics: Three Generations U S $ / W U S $ / W Energy 1. Sub bandgap losses 2. Lattice thermalisation U S $ / W T h e r m o d y n a m i c l i m i t III 2 U S $ / W P r e s e n t l i m i t qv II I U S $ / W C o s t, U S $ / m 2 [Green, 3rd Gen PV (S-Verlag) 2003]

3 Outline Motivation what are Hot Carrier solar cells? Hot Carrier cooling Carrier cooling by emission of optical phonons Restricting Optical to Acoustic phonon decay Quantum Dot materials MQWs Evidence for slowed cooling Mechanisms - superlattices Large phonon gap materials Analogues of InN Complete structure QW of phononic material Selective contacts Conclusions

4 Hot Carrier cell Extract hot carriers before they can thermalise: Need to slow carrier cooling Collect carriers over narrow range of energies High voltage + high current = high η e - energy selective contact E s E Ross & Nozik, JAP, 53 (1982) 3813 Würfel, SOLMAT, 46 (1997) 43 Green, 3rd Gen PV (S-Verlag) 2003 Würfel, PIP, 13 (2005) 277 Takeda, JAP, 105 (2009) Alibert, JAP, 108 (2010) Paterson, SOLMAT, 94 (2010) 1931 Conibeer, SOLMAT, 93 (2009) 713 Theoretical η 65% - 1 sun 85% - max concn. E f(n) µ qv E f small E g h + energy selective contact E f(p) E s T A Hot carrier distribution T H T A

5 Resonant Tunneling Transport Energy Selective Contact Si QD Energy Resonant Transport Filter I Dielectric matrix E f E C E f V

6 Ig(A) I-V Resonant of the MOS structure with Tunneling a single Transport layer of Si QD (Size 4 mm 2 ) Two different sites on the wafer Gate voltage (V) Illuminated I-V at 100K shows resonance I-V at 300K shows

7 Optical phonons emitted Hot Carrier cooling 6. Extraction of Hot electron 4. Thermalisation 5. Re-absorption of Optical phonon 1. High energy photon 2. High energy electron Hot Optical phonon population phonon bottleneck effect 3. Emission of Optical phonon Alternative 5. Optical phonon decays into two acoustic phonons Decay of Optical phonons to Acoustic is critical Slows further carrier cooling LO 2LA Klemens (1965)]

8 Discrete Quantum Dots Not good for absorption of wide photon energy range Prevent phonon emission if > E LO DOS Core shell QD - radiatively efficient

9 Multiple Quantum Wells DOS Multiple quantum well (MQW) Continuous DOS - absorbs wide photon range

10 Characteristic relaxation time (ps) Evidence of slowed cooling in MQW Multiple QW show slowed carrier cooling Injection level Bulk GaAs 1x10 19 cm -3 5x10 18 cm -3 2x10 18 cm Carrier temperature, shifted from room temp data (T-300K) GaAs MQW Guillemoles (2005) Re-calc. from (Rosenwaks, Phys Rev B, 48 (1993) 14675) Similar data in: Westland (1988); Snow (1989); Recently confirmed for strain balanced MQW, Hirst, Ekins-Daukes, PVSC, Seattle 2011

11 Reduced hot carrier diffusion in MQW - phonon bottleneck Bulk hot carrier diffusion enhanced MQW hot carrier diffusion supressed Superlattice mini-bands MQW discrete wells

12 Phonon energies (density of states) Allowed phonon energies Element e.g. Si Compound e.g. InN mev 60 Optical phonons (standing waves) 30 E Acoustic phonons (heat in the lattice) DOS 0 Some evidence for slowed carrier cooling in InN: Chen & Cartwright, APL, 83 (2003) 4984 And for longer phonon lifetimes in GaN, AlSb, InP all of which have large phonon gaps - due to large difference in element masses

13 Characteristic relaxation time (ps) Slowed cooling with phononic band gap InN also shows slowed cooling; Chen & Cartwright, APL 83 ( 03) 4984 Multiple QW show slowed carrier cooling Injection level Bulk GaAs 1x10 19 cm -3 5x10 18 cm -3 2x10 18 cm -3 InP InN: 5x10 18 (indirect comparison) AlSb GaAs MQW Guillemoles (2005) Re-calc. from (Rosenwaks, Phys Rev B, 48 (1993) 14675) Similar data in: Westland (1988); Snow (1989); recently confirmed for strain balanced MQW, Ekins-Daukes, Hirst. Carrier temperature, shifted from room temp data (T-300K)

14 BiN InN SnO GaN AlSb InP SiC AlN BN AlP Phononic band-gaps for various binary compounds % E optical E acoustic E acoustic % 60% 40% 20% Eoptical (Max - Min) E acoustic 0.0 Gap < E acoustic 0% E g = 0.7eV E g same as GaAs

15 Time resolved photoluminescence, InP & GaAs GaAs InP excitation 730nm E g excitation 730nm E g Clady PIP 2011 InN:)O Hot carrier temperature vs. time InP cools slower InN than GaAs low quality O impurity

16 Phononic analogues of InN IIA IIIA IB IIB IIIB IVB VB VIB Be B C N O Mg Al Si P S Ca Sc Cu Zn Ga Ge As Se Sr Y Ag Cd In Sn Sb Te Ba La Au Hg Tl Pb Bi Po Er

17 Phononic analogues of InN IIA IIIA IB IIB IIIB IVB VB VIB Be B C N O Mg Al Si P S Ca Sc Cu Zn Ga Ge As Se Sr Y Ag Cd In Sn Sb Te Ba La Au Hg Tl Pb Bi Po Er

18 Phononic analogues of InN IIA IIIA IB IIB IIIB IVB VB VIB Be B C N O Mg Al Si P S Ca Sc Cu Zn Ga Ge As Se Sr Y Ag Cd In Sn Sb Te Ba La Au Hg Tl Pb Bi Po Er

19 Phononic analogues of InN IIA IIIA IB IIB IIIB IVB VB VIB Be B C N O Mg Al Si P S Ca Sc Cu Zn Ga Ge As Se Sr Y Ag Cd In Sn Sb Te Ba La Au Hg Tl Pb Bi Po Er

20 Phononic analogues of InN IIA IIIA IB IIB IIIB IVB VB VIB Be B C N O Mg Al Si P S Ca Sc Cu Zn Ga Ge As Se Sr Y Ag Cd In Sn Sb Te Ba La Au Hg Tl Pb Bi Po Er

21 Electron selective contact Phononic band gap IIIA N, IV-IV Si oxides, organics thin film Electron selective contact Combined superlattice / phononic gap Selective contact, electrons Thin barrier for superlattice InN InGaN InN Hole selective contact Wide well phononic band gap Selective or normal contact for holes Superlattice + Phononic gap wells small E normal contact

22 Towards a complete cell Fabrication of slowed cooling absorber Transport and Renormalisation of carrier energies Energy Selective Contacts Colloidal QD array

23 Conclusions Hot Carrier solar cell Slowed Carrier cooling Carrier cooling primarily: electron Optical phonons LO 2LA decay Klemens Discrete Quantum Dots prevent phonon emission but narrow absorption bandwidth Multiple Quantum Wells Slow carrier cooling, reduced diffusion phonon bottleneck Large phonon band gaps with large mass difference InN phonon bottleneck Analogues of InN group IV compounds/alloys Superlattice with phononic band gap wells or a QD array Further work on fabricating and measuring these materials