Photo- voltaics

1. H2 production & storage Fuel cells Photo-voltaics Ljus framtid för solenergi tack vare nanoteknik ? Photo-catalysis Batteries Emission cleaning Chalmers vägar mot enhållbar värld 2009 Michael ZächChalmers tekniska högskolaInstitutionen för teknisk fysik

2. Innehåll • Några inledande tankar kring solenergi, solceller och nanoteknik • Nanopartikelplasmoner – vad är det för något ? • Nanopartikelplasmoner i solceller

3. Kol Uran Gas Olja Energikällor • Solen täcker världens årliga energibehov på en timma! Världensenergibehov(15 TW) Sol Fotosyntes Vatten Vind cont.

4. Hur mycket är en TW? • 1 TW = 1 Terawatt = 1’000 GW = 1’000 Gigawatt =1’000’000 MW = 1’000’000 Megawatt =1’000’000’000 kW = 1’000’000’000 kilowatt1’000’000’000’000 W back 1 W 1 kW 1 MW 1 GW 1 TW

5. Si Solar Cell(1st Generation PV) Source: Surek, Tom - Solar Power - Today, Tomorrow, and Forever.pdf (NREL)

6. Monocrystalline Si Solar Cells(1st Generation Photovoltaics) E • Essentially a p-n junction, with contacts to p- and n-sides • Photons with E ≥ 1.12eV generate charge carriers (e-h pairs) in silicon • Charges are separated by built-in electric field and driven through external load electrontransport p-Si n-Si holetransport I R

7. Issues with 1st Generation PV • Si has an indirect bandgap  low optical absorption Si needs to be thick to absorb most of the light (>> 100m) • e-h pairs must diffuse to the junction region • Minority carrier diffusion length (recombination rate) depends on material purity and crystallinity • Efficient devices can only be made with very pure (solar-grade) Si, which is expensive and energy-intensive in the production The most widely used technique for making single-crystal silicon is the Czochralski process, in which a seed of single-crystal silicon contacts the top of molten silicon. As the seed is slowly raised, atoms of the molten silicon solidify in the pattern of the seed and extend the single-crystal structure. Czochralski process to make single-crystal Si

8. Dye-Sensitized Solar Cells(3rd Generation PV) • Fundamental difference: light absorption occurs in a dye rather than in a semiconductor (i.e. it is separated from charge separation) • Need rather thick layer of dye  use 3-D scaffold of mesoporous titania • (Rather) cheap raw materials, and “simple” production process  cost advantages • Efficiency is smaller than for Si cells (≈ 10%) • Good price/performance ratio

9. Overview of State-of-the-Art • LabModule (%) (%) • Monocrystalline Si 24.7 14-16 • Polycrystalline Si 20.3 12-15 • Amorphous Si 12.1 6-8 • CIGS 19.9 9-11 • DSSC 11.1 3-8 • Organic 5.4 ?

10. How small is a nm? 1 µm = one millionth of a meter 1 nm = one billionth of a meter ≈ 1/50,000 thickness of a hair If we shrunk all distances 110,000,000,000 times, the sun and earth would be separated by 1m. A football field would then be 1nm. 110,000,000 km Human hair thickness ~ 50 µm

11. Early Nanotechnology The Lycurgus CupThe British Museum 4th century AD Window of the Seasons, (Jan. and Feb.) Chartres Cathedral (France)13th century (?)

12. E-field Metalsphere - - + + - - + + - - + + e- cloud Nanoparticle Plasmons • Observed when electronsin metallic nanostructuresoscillate collectivelyunder the influence ofan electric field (light) • Resonance frequency of theoscillation (= color) can be tuned by varying particle size • For particles in the size range ≈ 20-200nm, the resonance frequency falls into the visible regime Metallic nanoparticles are good at absorbing and scattering sunlight

13. Solar Spectrum and Plasmons Particle ø: Absorbance [a.u.] 700 800 500 600 900 1000 Wavelength [nm]

14. Model Si Solar Cell Polarized light Al-electrode Y 200µm p-Si X Z n-Si 2000µm Al-electrode Au nanoparticles I

15. Optical properties • Elliptical particles with two distinct NPPs (corresponding to long- and short axis) • Light polarization can be used to “switch NPPs on and off”. (Photocurrent with nanoparticles) Enhancement = (Photocurrent without nanoparticles) Hägglund, C., Zäch, M., Petersson, G., and Kasemo, B., Appl. Phys. Lett.92 (2008) 053110

16. (Photocurrent with nanoparticles) (Photocurrent without nanoparticles) Photocurrent • Photocurrent clearly polarization dependent • Clear correlation with plasmon resonance peaks • Net decreaseof photocurrentat resonance (<1) • Increase off resonance (>1) s-polarized light p-polarized light // to minor axis // to major axis Wavelength [nm] Hägglund, C., Zäch, M., Petersson, G., and Kasemo, B., Appl. Phys. Lett.92 (2008) 053110

17. Y X Z Model DSSC Polarized light 10µm Au/Ti-electrode TiO2 2000µm Glass support Dye ± V ElongatedAg or Aunanoparticle

18. P P Optical properties • Polarization-dependent microextinction measurement • Two clearly separated peaks corresponding to particle short and long axis

19. G// P P G Photoconductance with Au particles • Clear polarization dependence • Clear correlation with plasmon peak • Net increase of photoconductance with Au particles Hägglund et al, Applied Physics Letters 92 013113 (2008)

20. Plasmonic charge carrier generation in photovoltaic solar cells Electromagneticfieldinfluence on the charge carrier generation PV PV PV Far field effects Near field effects Photoemission of charge carriers Schematic taken from Carl Hägglund’s PhD thesis

21. Acknowledgements • EU, Mistra, SSF and Chalmers Foundation for financial support • Collaborators in the Chemical Physics group • Many of the students who attended the course TIF165 - “Nanotechnology for Sustainable Energy” • And you for your attention ! Tack !