Van der Waals-Zeeman Institute, University of Amsterdam

1. “Twee halen - een betalen” Si nano-photovoltaics Tom Gregorkiewicz Van der Waals-Zeeman Institute, University of Amsterdam

2. Absorption of solar energy is a natural process Preferred solutions for energy • Use processes occurring in nature • - do not produce “new” components • (nuclear waste, CFC, …) • - CO2, CO, SO2 do occur in nature but in small quantities (e.g. burning of wood) • The scale needs to be “small” (best negligible) when compared to those occurring naturally PV “shapes” this natural process in the way useful to men, using only a (very) small part Van der Waals-Zeeman Institute, University of Amsterdam

3. Solar power: 120.000 TW Calibrating the energy needs Daily food consumption: 2000 cal/day  100 W  ~ 1 kW 2 kW pp  13 TW (2010) 28 TW (2050) ~0.02% of the total is enough to power our civilization! Van der Waals-Zeeman Institute, University of Amsterdam

4. light  low/high temperature heat light  electricity Main solar energy conversion options Van der Waals-Zeeman Institute, University of Amsterdam

5. light  low/high temperature heat light  electricity light  chemical energy (solar fuels, art. photosynthesis) Main solar energy conversion options Van der Waals-Zeeman Institute, University of Amsterdam

6. Don’t worry Mr. President, solar will be economical in 5 years! I can’t believe he said that. Oil crisis of the 1970’s Jimmy Carter at SERI (now NREL) May 5, 1978 Van der Waals-Zeeman Institute, University of Amsterdam

7. “Global warming” crisis Barack Obama at Nellis AFB May 2009 Van der Waals-Zeeman Institute, University of Amsterdam

8. Van der Waals-Zeeman Institute, University of Amsterdam

9. Van der Waals-Zeeman Institute, University of Amsterdam

10. Solar electricity solutions • Indirect conversion: light-high T heat-electricity • Solar thermal energy: photons-to-phonons-to-electrons • - without energy storage • - with energy storage Van der Waals-Zeeman Institute, University of Amsterdam

11. Solar thermal power Van der Waals-Zeeman Institute, University of Amsterdam

12. Solar thermal power Van der Waals-Zeeman Institute, University of Amsterdam

13. Solar electricity solutions • Indirect conversion: light-high T heat-electricity • Solar thermal energy: photons-to-phonons-to-electrons • - without energy storage • - with energy storage • Direct conversion:light-to-electricity • Photovoltaics: photons-to-electrons • - without light concentration • - with light concentration Van der Waals-Zeeman Institute, University of Amsterdam

14. Photovoltaic cell top metal contact load active material (with asymmetry for charges) bottom metal contact mobile negative charge mobile positive charge Van der Waals-Zeeman Institute, University of Amsterdam Courtesy W. Sinke, ECN

15. Researchers at Bell Labs, N.J. (USA) 1953, firstphotovoltaic solar cells based on silicon (  5%) PV history In 1954, the U.S. News & World Report wrote : …..one day such silicon strips……“may provide more power than all the world’s coal, oil and uranium” Van der Waals-Zeeman Institute, University of Amsterdam

16. 17th March 1958: The Vanguard 1 satellitewith solar panels - 0.1 watt peak power – is put onto orbit PV history Van der Waals-Zeeman Institute, University of Amsterdam

17. Van der Waals-Zeeman Institute, University of Amsterdam

18. Polycrystalline silicon – a cheap & easy-to-make alternative Van der Waals-Zeeman Institute, University of Amsterdam

19. Van der Waals-Zeeman Institute, University of Amsterdam

20. PV application limits? Van der Waals-Zeeman Institute, University of Amsterdam

21. Source: Photon International March 2010 Van der Waals-Zeeman Institute, University of Amsterdam

22. Price development 22% price decrease for every doubling of cumulative production 1979 wafer Si silicon feedstock shortage 2007 Thin film 2009 2009 Source: EPIA, October 2009 Van der Waals-Zeeman Institute, University of Amsterdam

23. Over 90% of today’s PV modules are based on Crystalline Silicon Excellent performance  modules: ~20%  lab: up to ~25% Current status PV Van der Waals-Zeeman Institute, University of Amsterdam

24. Silicon for PV

25. Silicon and light • indirect bandgap • low emission/absorption rates • (at low energies) Van der Waals-Zeeman Institute, University of Amsterdam

26. PV conversion – basic concept recombination X gap energy light X X heat generation Van der Waals-Zeeman Institute, University of Amsterdam

27. PV conversion loses X X Van der Waals-Zeeman Institute, University of Amsterdam

28. Shockley-Queisser limit Conversion efficiency maximum for single junction PV cell with Egap=1.1 eV (≈ 31 %) Van der Waals-Zeeman Institute, University of Amsterdam

29. Optimal bandgap energy  Abundant  Mechanically strong  High mobilities possible  Si for photovoltaics Van der Waals-Zeeman Institute, University of Amsterdam

30. Manipulate band-structure Light management: waveguiding, cloaking, multiple reflection, dispersing Si nanowires Si nanocrystals Quantum cutting and pasting “Smart” solutions for Si PV TGG TGG TGG Van der Waals-Zeeman Institute, University of Amsterdam

31. Si nanocrystals

32. Nanocrystals (NCs) Bandstructure modification induced by quantum confinement Bands →quantized energy levels Relaxation of k-vector conservation for indirect bandgap Tuning optical properties SiNC 4.3 nm Silicon

33. Si Nanocrystals in SiO2 Paillard et al., Tolouse Van der Waals-Zeeman Institute, University of Amsterdam

34. Si NC photoluminescence SiNC PL CB VB Van der Waals-Zeeman Institute, University of Amsterdam

35. Si NC photoluminescence SiNC PL CB VB Van der Waals-Zeeman Institute, University of Amsterdam

36. Si NC photoluminescence SiNC Van der Waals-Zeeman Institute, University of Amsterdam

37. Si NC photoluminescence SiNC PL CB Auger VB Van der Waals-Zeeman Institute, University of Amsterdam

38. Si NC PL saturation Van der Waals-Zeeman Institute, University of Amsterdam

39. Si nanocrystals • photon convertors: • size-tunable energy • photon limiters • only one photon out Hot electrons are not used! Van der Waals-Zeeman Institute, University of Amsterdam

40. Using “hot electrons”: Cutting and emitting photons with Si-NCs

41. PL from SiNCs in SiO2 Van der Waals-Zeeman Institute, University of Amsterdam

42. λexc=323 nm f = 3.8 MHz ~2 ps PL •370 ≤ λdet ≤ 700 nm •τresolution ~25 ps MCM PMT PL from SiNCs d=4.5 nm τ1≈25 ps τ2≈100 ps ~ns ~μs Hot PL Excitonic recombination O-related PL

43. Hot PL for all the samples Van der Waals-Zeeman Institute, University of Amsterdam

44. Theoretical model Si Nanocrystal Direct Indirect 1.17 eV 3.32 eV Van der Waals-Zeeman Institute, University of Amsterdam

45. Pulsed vs. semi-cw excitation Pulsed <Nexc><1 ~2 ps cooling 1 – 10 ps ~ns 420 nm ~μs NIR <Nexc>>1 Semi-cw ~5 ns Auger 10 – 100 ps ~ns 420 nm ~μs NIR

46. hot PL s-PL hot PL s-PL ≈ 5 ≈ 1 Relative efficiency “hot” PL in Si NC 1000 stronger than in bulk Si enhanced emission and absorption in the visible W.D.A.M. de Boer et al. Nature Nanotechnology 2010 Van der Waals-Zeeman Institute, University of Amsterdam

47. Cutting photonswith Si NCs

48. Absolute QE of Si-NCs PL Solid state sample: SiO2:Si-NCs Experimental setup Colloidal sample: SiNCs in ethanol • HF chemical • etching: po-Si • suspended in • ethanol Van der Waals-Zeeman Institute, University of Amsterdam

49. Nem Nabs η = Relative quantum efficiency Q.E. for different wavelengths in visible and near UV Definition relative quantum efficiency: • η is constant up to a photon energy threshold of Ethreshold ≈ 2Egap • For larger photon energies a second excitation mechanism takes place Van der Waals-Zeeman Institute, University of Amsterdam

50. Quantum cutting with Si-NCs Eexc≥ 2Egap Space-separated quantum cutting (SSQC) Multi-exciton generation (MEG) D. Timmerman et al., Nature Photonics (2008)