Efficency of Converting Solar Irradiance into Electrical or Chemical Free Energy

1. Efficency of Converting Solar Irradiance into Electrical or Chemical Free Energy A.J. Nozik National Renewable Energy Laboratory and Department of Chemistry, Univ. Colorado, Boulder

2. The U.S. Department of Energy’s National Renewable Energy Laboratory www.nrel.gov Golden, Colorado

3. FY02 EERE Funding at National Labs Dollars in $M FY02 Budget Authority

4. Renewable Energy Cost Trends Levelized cents/kWh in constant $20001 4030 20 10 0 100 80 60 40 20 0 PV Wind COE cents/kWh 1980 1990 2000 2010 2020 1980 1990 2000 2010 2020 70 60 50 40 30 20 100 1512 9 6 30 10 8 6 4 20 Solar thermal Biomass Geothermal COE cents/kWh 1980 1990 2000 2010 2020 1980 1990 2000 2010 2020 1980 1990 2000 2010 2020 Source: NREL Energy Analysis Office 1These graphs are reflections of historical cost trends NOT precise annual historical data. Updated: October 2002

5. Solar Spectrum and Available Photocurrent

6.  Solar Electricity ● Solar Fuels

8. National Geographic, Sept., 2004

9. Millions of Barrels per Day (Oil Equivalent) World Energy 300 200 100 0 1860 1900 1940 1980 2020 2060 2100 Source: John F. Bookout (President of Shell USA) ,“Two Centuries of Fossil Fuel Energy” International Geological Congress, Washington DC; July 10,1985. Episodes, vol 12, 257-262 (1989).

10. Conventional PV Cell e- heat loss Energy e- ηmax = 32% hν usable photo-voltage (qV) p-type n-type heat loss 1 e- - h+ pair/photon

11. Photoeffects in Semiconductor-Redox Electrolyte Junction Photoelectrochemistry (PEC) C434703 Absorption of light in depletion layer results in creation and separation of electron-hole pairs. For n-type semiconductors, holes move toward surface and electrons toward semiconductor bulk. For p-type semiconductors, reverse process occurs. Redox couples in electrolyte capture injected photogenerated carriers and reactions occur.

12. SOLAR PHOTOCHEMISTRY/PHOTOELECTROCHEMISTRY

13. Some Endergonic Fuel Generation Reactions

14. SOLAR HYDROGEN--PHOTOELECTROLYSIS

15. Outstanding Technological Issues Discovery of “Holy Grail” of Photoelectrolysis: Semiconductor with: Bandgap  1/6–2.0 eV Appropriate flatband potential Catalytic surface for O2 (or H2) evolution Long-term stability against photocorrosion Conversion efficiency > 10% Low cost and environmentally benign or p-n combination of two different semiconductors in a tandem configuration with above properties, except bandgaps can be 1 eV.

16. Electrochemical Photovoltaic Cells

17. Dye-Sensitized Nanocrystalline TiO2 Photochemical Solar Cell (Graetzel Cell) Band Diagram

18. B084717

19. Main Process Limiting Conversion Efficiency Hot e- Relaxation

21. Detailed Balance Efficiency Calculation The theoretical maximum efficiency of a solar cell is calculated using the Detailed Balance Model first introduced by Shockley and Queisser*. ASsUMPTIONS Absorption of one photon produces one electron-hole pair. Quantum Yield = 1. Only photons with hn > Eg are absorbed. Radiative recombination is the only recombination mechanism present. Hot carriers are relaxed to the band edges The quasi-Fermi level separation is constant through- out the cell.  infinite carrier mobility Eg EFn V EFp J(V) Load *Shockley and Queisser, J.Appl. Phys. 32, 510 (1961) GBB = blackbody photon flux

22. Net absorbed photon flux = solar flux + ambient flux – radiant emission flux INET ABS (ν) = ∫{IS(ν) + IA (ν) – I(ν,μ,TQ,2π)}σ(ν, μ,TQ) dν dA P= INET ABS (ν) μ μ = chemical potential produced by light ηQ = power converison efficiency ηQ = INET ABS (ν) μ/ ∫ IS(ν) hν dν For single threshold absorber, maximum efficiency = ηQ =.31

24. 3rd Generation Photon Conversion Valid Thermodynamic Approaches to Achieve Photon Conversion Efficiencies > 32% (Exceeding the Shockley-Queisser Limit) • 1. Tandem Cells (exceed S-Q limit but not new approach) • 2. Hot Carrier Conversion • Extract, collect, and utilize hot carriers • Impact ionization/exciton multiplication • Intermediate Band Solar Cell • Thermophotonic Solar Cells • Down conversion and upconversion of incident photons (M. Green and P. Wuerfel) • See: • M. Green, “Third Generation Photovoltaics”. Springer, 2003 • A. Marti and A. Luque, “Next Generaton Photovoltaics”, Inst. Of Physics Series in Optics and Optoelectronics, 2003

25. Efficiency of Hot Carrier Photoconversion Ross & Nozik, J. Appl. Phys. 53, 3813 (82)

26. Multiple Threshold Absorbers For an infinite number of tandem of tandem absorbers:

27. 2-PHOT0SYSTEM PEC CONVERSION

29. Multi-Layered/Multi-Photon Photoelectrochemical Converters (Photochemical Diode) p n H2 e- H+/H2 e- h2 h O2 h+ h1 H2O/O2 h+ Transparent ohmic contact

30. Wavelength Contours for Efficiency of Water Splitting Utilizing Two Tandem Photosystems

31. High Efficiency Multijunction Solar Cells • Want 1eV material lattice-matched to GaAs  Try GaInNAs 034016319

33. Maximum Efficiency of Tandem Solar Cells Calculated using a 6000K blackbody spectrum

34. Best Research-Cell Efficiencies Spectrolab 36 Multijunction ConcentratorsThree-junction (2-terminal, monolithic)Two-junction (2-terminal, monolithic) Crystalline Si CellsSingle crystalMulticrystalline Thin Film TechnologiesCu(In,Ga)Se2CdTeAmorphous Si:H (stabilized) Emerging PVDye cells Organic cells(various technologies) Spectrolab Japan Energy 32 NREL/ Spectrolab NREL NREL 28 UNSW UNSW 24 UNSW Spire UNSW NREL Cu(In,Ga)Se2 14x concentration UNSW Stanford Spire UNSW Georgia Tech ARCO 20 Efficiency (%) NREL Sharp Georgia Tech Westing- house Varian NREL NREL NREL 16 UniversitySo. Florida NREL No. Carolina StateUniversity NREL Euro-CIS Boeing Solarex ARCO 12 Boeing Kodak Boeing United Solar AMETEK University ofLausanne Masushita United Solar Kodak Boeing 8 Monosolar Photon Energy RCA Solarex Boeing Siemens Groningen University ofLausanne Princeton University of Maine 4 RCA RCA RCA RCA Cambridge RCA UniversityLinz UCSB Kodak RCA University Linz 0 Berkeley 1975 1980 1985 1990 1995 2000 2005

35. ~94% PV Module Production in 2003 by Technology Type * * Source: PV News, March 2004

36. Sunlight p n p n p n Solid state solar cells H2 O2 e- e- H2O H+ Dark electrolysis cell Photovoltaic Electrolysis

37. p p n n e- H2 e- h2  H+/H2 h O2 h1 2 h+ H2O/O2 h+ Ohmic contact and metal cathode Transparent ohmic contact Ohmic contact and metal anode Two-Junction Cascade PV/PEC Device for Water Splitting

38. Multi-Layered/Multi-Photon Photoelectrochemical Converters (Photochemical Diode) p n H2 e- H+/H2 e- h2 h O2 h+ h1 H2O/O2 h+ Transparent ohmic contact

39. John Turner Cell - > 11% efficient water splitting

40. Projected Need for Carbon-Free Primary Power Bottom Line: New “disruptive” energy technology is needed

41. PV Power Costs as Function of Cell Efficiency and Module Cost min BOS UltimateThermodynamic limit at 1 sun Shockley- Queisser limit From Martin Green For PV or PEC to provide the level of C-free energy required for electricity and fuel—power cost needs to be 2-3 cents/kWh ($0.40 – $0.60/W)

42. $/peak watt = (module cost/Eff ) + (BOS cost/Eff) + 0.1 where: Eff = cell conversion efficiency x 1 Kw/m2 BOS = balance of systems (support structure, installation,wiring, land, etc) $0.1 = power conditioner, AC – DC inverter Also: 1$/Wp $0.05/kWh Therefore, to achieve $0.02/kWh, need total cost of $0.40/ Wp If BOS can be reduced to $75/ m2 (currently  $250/m2), and module cost reduced to $50/ m2 (currently  $300/ m2 ), then module efficiency needs to be 41% (and cell efficiency at least 50%). Disruptive technology required.

43. World PV Cell/Module Production (MW) 800 744.1 700 600 561.8 Rest of world 500 Europe Japan 390.5 400 U.S. 300 287.7 201.3 200 154.9 125.8 100 88.6 77.6 69.4 60.1 57.9 55.4 46.5 40.2 33.6 0 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Source: PV News, March 2004

44. Two Ways to Utilize Photogenerated Hot e- for Useful Work and Increase Efficiency • Higher photovoltage via hot e- transport, transfer, and conversion • Higherphotocurrent via carrier multiplication through impact ionization (inverse Auger process)

45. e-  Hot e- Energy lost as heat Heat loss e-  e-  Thermalized e- h Eg Available Energy  h+ Liquid Redox Electrolyte p-type photoelectrode Thermalized vs Hot Electron Transfer Nozik, et. al. ,J. Applied Physics54, 6463 (1983) Nozik &Turner, Appl. Phys. Lett., 41, 101 (1982)

46. e e e e e Eg hν 2 Eg h+ h+ h+ h+ Photocurrent Multiplication by Impact Ionization h+ 1 photon yields 2 (or more) e- - h+ pairs (I.I. previously observed in bulk Si, Ge, InSb)

48. Maximum Single Bandgap Efficiency at 1 Sun Impact Ionization Impact Ionization Detailed Balance Shockley- Queisser limit A. De Vos, B. Desoete, Solar Energy Materials and Solar Cells 51 (1998) 413–424

49. Impact Ionization Processesin Bulk Semiconductors Reverse biased p-i-n junction Optically excited hot carriers Electron initiated Hole initiated I ETH>Eg F F F hn I Field hn>2Eg F I I distance I – initial states e- gain kinetic energy in a high electric field, then scatter by II generating a secondary e-h pair. F – final states

50. Queisser, et al. 1994 Impact Ionization along the (100) direction ( axis) of Si. Absorption of a photon h creates a first electron hole pair (e1/h1) at the  point. The excess energy Ex = h - Eg of the electron suffices to generate a second electron hole pair (e2/h2) while the electron e1 relaxes towards the conduction-band minimum (e’1). Conservation of energy E and momentum hk/(2) is fulfilled if the two dash-dotted arrows add vectorially to zero. QDs: Requirement for conservation of momentum is relaxed. Threshold should be lower.