Third Generation Photovoltaics: Silicon nanostructure & Hot Carrier solar cells. GCEP Symposium 2 October 2008

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1 Third Generation Photovoltaics: Silicon nanostructure & Hot Carrier solar cells GCEP Symposium 2 October 2008 Gavin Conibeer Deputy Director Photovoltaics Centre of Excellence University of New South Wales, Sydney, Australia Photovoltaics Centre of Excellence supported by the Australian Research Council

2 Outline The main losses in photovoltaic cells Third Generation approaches Silicon nanostructure tandem cells Band gap engineering quantum confinement Materials and devices Hot Carrier cells Hot Carrier cooling Interrupting energy loss to phonons Summary

3 Photovoltaics: Three Generations Wholesale parity 100 US$0.10/W US$0.20/W US$0.50/W Efficiency (%) II III I Thermodynamic limit US$1.0/W Single material limit US$3.50/W Retail parity Cost (US$/m 2 )

4 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 Single p-n junction: 31% Multiple threshold: 68.2%

5 Silicon based Tandem Cell Martin Green, Gavin Conibeer, Dirk König, Eunchel Cho, Tom Puzzer, Yidan Huang, Shujuan Huang, Dengyuan Song, Angus Gentle, Ivan Perez-Wufl, Chris Flynn, Jeana Hao, Sangwook Park, Yong So, Bo Zhang Decreasing band gap Solar Cell 1 Solar Cell 2 2nm QD, E g =1.7eV Solar Cell 3 Thin film Si cell E g = 1.1eV Si, Ge or Sn rich layer Dielectric layer Anneal 1100 C Si precipitation Substrate Substrate Si QDs SiO 2 barriers defect or tunnel junction Engineer wider band gap Si QDs Zacharias, 2000

6 Si QD characterisation XRD Si QDs in oxide d QD 4.5nm PL optical energy levels PL energy [ev] Diameter of Si QDs [nm] Integrated PL intensity (au) Deposition time [sec]

7 Range of QD materials Alternative matrices SiO 2 Si 3 N 4 SiC 3.2 ev c-si 0.5 ev 1.1 ev 0.9 ev c-si 1.9 ev 1.1 ev c-si 1.1 ev 2.3 ev 4.7 ev Conductivity (S/cm) PL energy [ev] E E Si QDs in oxide/nitride Greater σ for Si 3 N 4 but also lower E act Eσ =0.30eV 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 Eσ =0.36eV (UNSW) SiQDs in Si3N4 1.0E E-05 Eσ = 0.63eV SiQDs in SiO2 1.0E Eσ = 0.76eV 1.0E Dot 1000/T diameter (1/K)[nm] DFT model -ling

8 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)

9 Current (A) 5.00E E+00 PV devices with increasing V OC V OC = 390mV Quartz Substrate (H 2 /Ar anneal) -5.00E Volatage (V) Si Substrate with 1μm thermal SiO 2 V OC (mv) ID (A) Ilight 25 C Ilight 32.8 C Ilight 50.2 C Ilight 62.8 C Ilight 75.2 C Ilight 87.6 C 0 4.E-06 3.E-06 2.E-06 1.E-06 0.E+00-1.E-06-2.E-06-3.E Si QDs in SiC -4.E-06 Mar April May 700mV Light EIV act = 1.45eV (1 sun) Si QDs in SiO 2 I SC unexpected June July f(t) V D (V) 2008 Aug B doped Si QD P doped Si QD Light sputtered SiC quartz p-si QD n-si QD sputtered SiC Light p-si QD n-si QD 1μm thick SiO 2 Si wafer Continuing Work Quality of dielectrics Passivate QD interfaces Improved device design Increase absorption

10 Hot Carrier solar cell Started September 2008 University of New South Wales, Sydney: Gavin Conibeer, Martin Green, Dirk König, Shujuan Huang, Santosh Shrestha, Chris Flynn, Lara Treiber, Pasquale Aliberti, Andy Hsieh, Rob Patterson, Binesh Puthen Veettil, Martin Kirkengen Institute Energie Solar, Universitas Polytechnic Madrid: A. Luque, A. Marti, E. Cánovas, A. Martí, P.G. Linares, E. Antolín, D. Fuertes Marrón, C. Tablero Inst. Research Development Energie Photovoltaic / CNRS, Paris: Jean Francois Guillemoles, Lunmei Huang University of Sydney: Timothy Schmidt, Raphael Clady, Murad Tayebjee

11 Hot Carrier solar cell: Concept Extract hot carriers before they can thermalise: Need to slow carrier cooling Collect carriers over narrow range of energies Renormalisation of electron (hole) energies e - energy selective contact E s E f(n) δe Ross & Nozik, JAP, 53 (1982) 3813 Würfel, SOLMAT, 46 (1997) Green, 3rd Gen PV (S-Verlag) 2003 Würfel, PIP, 13 (2005) 277 Conibeer, TSF, 516(2008) 6948 Enrique Canovas et al: Poster session: Predicted photoreflectance signatures on QD selective contacts for hot carrier solar cells 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

12 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

13 Optical phonon decay

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

15 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

16 Phononic gaps in nanostructures Linear force constant model: mass ratio = 2; force constant ratio = 5 mev 40 Phonon energies (density of states) 20 0 Phononic band gaps modulate acoustic impedance analogy to Photonic band gap modulate refractive index

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

18 1D to 3D modelling Uniform 3D periodic QD array Coherent interference Periodic QD array probably fcc probably core shell QDs 1D modelling to 3D Lunmei Huang Long range / short range defects Andy Hsieh, Binesh PV

19 Colloidal dispersion of nanoparticles Colloidal dispersion of Si or other nanocrystals want uniform spacing and mono-disperse size Core shell nanocrystals hetero-interface Guillemoles, IRDEP, Paris Organosilanes of varying alkyl chain lengths a)trimethoxy(propyl)silane, b) Trimethoxy(octyl)silane Langmuir-Blodgett deposition of monolayers build up multiple mono-layers

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

21 Summary Principal energy losses Si nanostructure tandem cells Band gap engineering Range of QD materials Devices now up to 390mV V OC 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

22 Third Generation Strand (2008) Thank you for your attention 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, Craig Johnson, Darryl Wang, Dawei Dai Visiting researchers: Fei Gao, Dong-Ho Kim, Frank Koo, Ke Ma, Veronique Gevaerts, Martin Kirkengen, Martina Schmid