Woods Energy Seminar, 28 May Third Generation Photovoltaics

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1 Woods Energy Seminar, 28 May 2008 Third Generation Photovoltaics Gavin Conibeer Deputy Director ARC Photovoltaics Centre of Excellence University of New South Wales Photovoltaics Centre of Excellence supported by the Australian Research Council, the Global Climate and Energy Project and Toyota CRDL

2 Meeting the IPCC target of 60% reduction in GHG emission by 2050 Transforming the global energy mix: The exemplary path until 2050/ 2100 appointed for a term of four years by the federal cabinet (Bundeskabinett)

3 The business case for early action 60% reduction in GHG emission by 2050

4 Outline The importance of Photovoltaics Three generations of Photovoltaics The main losses in photovoltaic cells Third Generation approaches Silicon nanostructure tandem cells Band gap engineering quantum confinement Fabrication of materials / devices Hot Carrier cells Contacts energy filtering Hot Carrier cooling energy loss to phonons Summary

5 Booming Photovoltaics MWp Market growth at 35%/yr for last 10 years, 60%+ in 2007 Approx 1 million jobs in PV by 2020 Approx 1 million jobs in RE by 2010 USA Europe Japan Rest of World Total Global PV market US$6.5 billion in 2006 $16.4 billion in 2012 Driven by rebates/tariffs: Japan, Germany Now other Euro. Countries and S Australia USA: Power purchase agreements Japan: market is stable with reducing rebates

6 Annual capacity increase Solar Heating 15 New Capacity, GW Wind Nuclear Google s Mountain View campus Photovoltaics Sources: Photon International, WNA, WWEA, IEA

7 Learning curves Photovoltaics st Generation Photovoltaics 2003 US$/kW Wind turbines 1982 Gas turbines (USA) (~20%) bulk-si (~10%) 2003 US$/kW Thin-film PV 2002 (~20%) 2nd Generation 3rd Generation bulk-si (~10%) Cumulative GW installed Cumulative GW installed. more potential for learning. lower cost at smaller volumes

8 Photovoltaics: Three Generations 100 US$0.10/W US$0.20/W US$0.50/W 80 Thermodynamic limit Efficiency,% II III a-si tandem mc-si I c-si US$1.00/W concentration Present limit US$3.50/W III-V tandem thin film Cost, US$/m2

9 Efficiency Loss Mechanisms 1. Sub bandgap losses Energy 2 2. Lattice thermalisation Two major losses 50% Also: 3. Junction loss 4. Contact loss 5. Recombination qv 1 Limiting efficiencies 1 sun Max concn. Single p-n junction: 31% 40.8% Multiple threshold: 68.2% 86.8%

10 Third generation options J VC CB J h e - E 2,e E f VB E rela x J l intermediate level 100% E h E l E g One photon h + e - e - h + E 0,e E 0,h E 2,h Multiple electrons - circulators 74% 68% 65% 58% 54% 49% 44% 39% 31% tandem (n ) hot carrier tandem (n = 6) thermal, thermopv, thermionics tandem (n = 3) impurity PV & band, up-converters impact ionisation tandem (n = 2) down-converters single cell 0%

11 Silicon based Tandem Cell Sunlight Decreasing band gap Free choice or Si cell AM1.5G Efficiency % 42.5% 33% 47.5% 50.5% 45% Free choice Si bottomcell Intrinsic radiative and Auger losses included Number of cells

12 Silicon based Tandem Cell Decreasing band gap Engineer a wider band gap Si QDs Solar Cell 1 Solar Cell 2 Solar Cell 3 2nm QD, E g =1.7eV Thin film Si cell E g = 1.1eV Si QDs SiO 2 barriers defect or tunnel junction SRO SiO 2 Anneal 1100 C Si precipitation Substrate Substrate x x SiO x SiO2 + 1 Si 2 2

13 Si nanostructure tandem cell Alternate matrices SiO 2 Si 3 N 4 SiC 3.2 ev 0.5 ev 1.9 ev c-si 1.1 ev c-si 1.1 ev c-si 1.1 ev PL energy [ev] Si QDs in oxide/nitride Y. Kanemitsu et al H. Takagi et al S. Takeoka et al T. Y. Kim et al T. W. Kim et al Oxide (UNSW) Nitride (UNSW) 0.9 ev 2.3 ev Si QDs in SiC 4.7 ev Dot diameter [nm] 508 cm -1 Si nanocrystal Si (111) Annealed at 1100 o C Intensity (a.u.) Nanocrystalline SiC 1100 o C 1000 o C 800 o C Intensity (a.u.) β-sic (111) Si (220) Si (311) a-si As deposited Raman shift (cm -1 ) Theta (deg.)

14 Gaussian modelling of Si QD in SiC Alternate QDs Tin QDs in SiO 2 Ge QDs in SiO 2

15 Various material combinations Quantum Dot / Matrix combinations and current status of investigations Increasing conductivity Decreasing processing temperature SiO 2 Si 3 N 4 SiC Si SPOED SPOED SPOD Ge SP - - Sn SPO PO - S = Simulation (ab-initio modelling - DFT) P = Physical (electron microscopy, X-ray difraction) O = Optical (photoluminescence, absorptance) E = Electronic (conductivity, conductivity with Temp.) D = Devices (Diodes, Cells)

16 Si substrate - problem Can t be sure absorption is not in Si Hence transparent substrate or pseudo-substrate SiC substrate Light Si QD SiO 2 SiC pseudo-substrate Light Si QD SiO 2 SiO 2 Homojunction front back contact Light p-si QD n-si QD n-sic wafer sputtered SiC quartz sputtered SiC quartz emitter absorber Barrier 3mm Voc = 93 mvsi substrate N + - Si NC:SiC (100nm, (200 Sb-doping) P-Si NC:SiC (600 (200nm, B B-doping) SiN (70 nm) Open circuit Voltage = 83mV

17 Hot Carrier cell Ross & Nozik, JAP, 53 (1982) 3813 Extract hot carriers before they can thermalise: Würfel, Need SOLMAT, to slow carrier 46 cooling (1997) Collect carriers over narrow range of energies Green, 3rd Gen PV (S-Verlag) 2003 Würfel, PIP, 13 (2005) 277 Ross & Nozik, 1982 Würfel, 1995 Green, 2003 Würfel, 2005 Takeda et al, SOLMAT, 08 e - energy selective contact E s δe E f(n) Δµ A = qv E f small E g h + energy selective contact E s E f(p) T A Hot carrier distribution T H T A

18 Resonant Tunneling Transport Energy Selective Contact Si QD Energy Resonant Transport Filter 0.04 Ig(A) Two different sites on the wafer Dielectric matrix 0.01 I Gate voltage (V) NDR at 300K - Repeatable E f E C Ef V

19 Hot Carrier cooling Energy Optical phonons emitted Electrons carry most energy Cool predominantly via small wave vector optical phonon emission - timescale of ps inelastic energy relaxation Decay of Optical phonons to Acoustic is critical Hot Optical phonon population phonon bottleneck effect Slows further carrier cooling

20 Optical phonon decay

21 Optical phonon decay O LA + LA (Anharmonicity or Klemens mechanism)

22 Allowed phonon energies Element e.g. Si Compound e.g. InN mev E Phonon energies (density of states) Optical phonons (standing waves) Acoustic phonons (heat in the lattice) Nō 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

23 Phononic gaps in nanostructures mev Nanostructure 40 Phonon energies (density of states) 20 0 Linear force constant model: l = 4a 1 + 4a 2 mass ratio = 2; force constant ratio = 5

24 Phonon propagation in nanostructure Acoustic phonon reflected from zone edges standing wave

25 Towards a complete cell Fabrication of slowed cooling absorber Transport and Renormalisation of carrier energies Energy Selective Contacts

26 Summary Relevance and growth of Photovoltaics Three PV Generations Main energy losses Third Generation approaches Si nanostructure tandem cells Band gap eng. Range of QD materials Early devices Hot Carrier cells Energy filter contacts Phonon bottleneck Nanostructures - QD based cell Third generation multi-energy level devices tend to involve QD nanostructures enable tailoring of material properties

27 Third Generation Strand (2008) Research Staff: Martin Green, Richard Corkish, Gavin Conibeer, Dirk König, Eun-Chel Cho, Tom Puzzer, Yidan Huang, Shujuan Huang, Dengyuan Song, Santosh Shrestha, Ivan Perez-Wufl, Supriya Pillai PhD students: Chris Flynn, Jeana Hao, Sangwook Park, Lara Treiber, Yong So, Pasquale Aliberti, Yong So, Andy Hsieh, Bo Zhang, Rob Patterson, Binesh Puthen Veettil, Visiting researchers: Fei Gao, Dong-Ho Kim, Ke Ma, Veronique Gevaerts, Martin Kirkengen, Martina Schmid