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

2. Preferred solutions for energy Use processes occurring in nature -do not produce “new” components (nuclear waste, CFC, …) - CO 2, CO, SO 2 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 Absorption of solar energy is a natural process 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. Calibrating the energy needs Daily food consumption: 2000 cal/day  100 W  ~ 1 kW Solar power: 120.000 TW ~0.02% of the total is enough to power our civilization! 2 kW pp  13 TW (2010)  28 TW (2050) 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. Jimmy Carter at SERI (now NREL) May 5, 1978 Oil crisis of the 1970’s Don’t worry Mr. President, solar will be economical in 5 years! I can’t believe he said that. 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

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. load top metal contact bottom metal contact active material (with asymmetry for charges) mobile negative charge mobile positive charge Photovoltaic cell Courtesy W. Sinke, ECN

15. Researchers at Bell Labs, N.J. (USA) 1953, first photovoltaic solar cells based on silicon (   5%) 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” PV history Van der Waals-Zeeman Institute, University of Amsterdam

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

18. Polycrystalline silicon – a cheap & easy-to-make alternative

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. Thin film 1979 2009 wafer Si silicon feedstock shortage 2007 2009 22% price decrease for every doubling of cumulative production Source: EPIA, October 2009 Price development 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. indirect bandgap low emission/absorption rates (at low energies) Van der Waals-Zeeman Institute, University of Amsterdam Silicon and light

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

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

28. Shockley-Queisser limit Conversion efficiency maximum for single junction PV cell with E gap =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 Van der Waals-Zeeman Institute, University of Amsterdam TGG

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 Silicon 4.3 nm SiNC

33. Paillard et al., Tolouse Si Nanocrystals in SiO 2 Van der Waals-Zeeman Institute, University of Amsterdam

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

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

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

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

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

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

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

41. PL from SiNCs in SiO 2 Van der Waals-Zeeman Institute, University of Amsterdam

42. λ exc = 323 nm f = 3.8 MHz MCM PMT 370 ≤ λ det ≤ 700 nm τ resolution ~25 ps ~2 ps PL O-related PL Hot PL Excitonic recombination ~μs~μs ~ns τ 1 ≈ 25 ps τ 2 ≈ 100 ps PL from SiNCs d=4.5 nm

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

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

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

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

47. Cutting photons with Si NCs

48. Solid state sample: SiO 2 :Si-NCs Colloidal sample: SiNCs in ethanol HF chemical etching: po-Si suspended in ethanol Experimental setup Absolute QE of Si-NCs PL Van der Waals-Zeeman Institute, University of Amsterdam

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

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

51. SSQC with SiNCs in SiO 2 E exc >2E gap 1 in → 2 out Van der Waals-Zeeman Institute, University of Amsterdam

52. Quantum cutting with Si-NCs  QE is constant up to photon energy threshold of hν ≈ 2E g  ~100 % increase of initial value  Step-like behavior  Two types of Si-NC samples:  Si-NCs in SiO 2  po-Si in EtOh  In two different calibrated QE setups Van der Waals-Zeeman Institute, University of Amsterdam

53. Shockley-Queisser limit Conversion efficiency  up to 44%!!!

54. D. Timmerman et al., under review Nature Materials PV impact Van der Waals-Zeeman Institute, University of Amsterdam

55. XXI st century will begin “(Si) Solar Energy Age” Reaching ultimate PV cost and performance levels at sufficient sustainability critically depends on (Si) materials development Conclusion “you have seen nothing yet” Van der Waals-Zeeman Institute, University of Amsterdam

56. TGG at WZI, UvA Van der Waals-Zeeman Institute - UvA