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Nanotechnology and Solar Energy

Nanotechnology and Solar Energy. Solar Electricity Photovoltaics Fuel from the Sun Photosynthesis Biofuels Split Water Fuel Cells . Solar cell.

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Nanotechnology and Solar Energy

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  1. Nanotechnology and Solar Energy • Solar Electricity • Photovoltaics • Fuel from the Sun • Photosynthesis • Biofuels • Split Water • Fuel Cells

  2. Solar cell A photonfrom the Sun generates an electron-hole pair in a semiconductor. The electron is pulled to the front, the hole to the back of the solar cell, thereby creating a battery.

  3. Energy diagram of a solar cell Energy The electron and hole are pulled apart by the electric field between the p- and n-doped regions. It is critical not to lose electronsand holes on their way out. Crystalline semiconductors are good at that, but expensive.

  4. Energy Electron Photon Band Gap Hole exciton Electrons, Holes, and Excitons A photon excites an electron across the band gap of a semiconductor. That leaves a hole among the occupied levels in the valence band and an electron among the unoccupied levels in the conduction band. Electron and hole are attracted electrically and can form an exciton (similar to a hydrogen atom). In organic semiconductors it takes a significant amount of energy to break them apart into free carriers.

  5. Creation of an electric field at a pn-junction

  6. Applications of the pn-junction

  7. Many types of solar cells Phy107 Lecture 30

  8. DOE Report http://www.er.doe.gov/bes/reports/files/SEU_rpt.pdf Solar cells containing nanoparticles. Sensitized by a dye molecule similar to that on the cover. TiO2 nanoparticles act as acceptors, collecting electrons. The electrolyte acts as donor, collecting the holes.

  9. Ruthenium dye molecule for Grätzel cells • Absorbs more than 90% of the sunlight. • The dyes are made with ruthenium, an expensive 4d transition metal. • Can one replace Ru by inexpensive 3d transition metals, such as Fe ?

  10. LUMO EF h eVopen EF HOMO Contact Acceptor Dye Donor Contact + Energy level diagram of a molecular solar cell Discrete energy levels in molecules, instead of energy bands in solids.

  11. LUMO EF h eVopen EF HOMO Acceptor Dye Donor Contact + Contact Generic Molecular Solar Cell Another version: Instead of holes going uphill, electrons going downhill fill the holes.

  12. Efficiency Limits Semiconductors: 30% for a single junction (Shockley-Queisser limit) 70% for multiple junctions Molecules: 20% for a dye-sensitized solar cell, single junction Snaith, Adv. Funct. Mater. 19, 1 (2009) Track down the losses systematically and eliminate them one by one.

  13. Spectroscopy of the HOMO and LUMO (donor and acceptor orbital) Calculation for Zn-OEP • LUMO from X-ray absorption spectroscopy. • HOMOfrom photoelectron spectroscopy. Cook et al. (2009)

  14. The transition metal center of a dye molecule Cook et al., J. Chem. Phys. 131, 194701 (2009) • X-ray absorption is element-selective (here via the Fe2p core level). • Detects the oxidation state, spin state, and ligand field.

  15. What happens during a photochemical reaction? • Spin excitations in 100 picoseconds (data below) • Atomic motion in 100 femtoseconds (vibration period) • Electronic motion in 1femtosecond (Fermi velocity = nm/fs) Huse and Schoenlein (2008) Pump-probe X-ray absorption spectra of a solvated Fe complex for thelow-spin ground stateand a high-spinexcited state

  16. Nanostructured solar cells Better design: Regular array of nanorods Use nanostructured “fractal” structures to minimize the path of excitons, electrons, holes, to the nearest electrode. Avoid losses.

  17. ZnO nanorods as electrode Growth time increases from left to right. (a)-(c) side view (500 nm bar), (d)-(f) top view (100 nm bar). Baxter et al., Nanotechnology 17, S304 (2006) and Appl.Phys.Lett. 86, 053114 (2005).

  18. Nanorods coated with nanocrystals CsSe nanodots (3nm) replace the dye. Absorption spectrum tunable via the size of the dot (Lect. 9, Slides 6,7). More robust against radiation damage. Leschkies et al., Nano Letters 7, 1793 (2007).

  19. Polymer solar cells Polymer chain with a diffusing polaron (electron + distorted polymer) surrounded by fullerene molecules as acceptors. A fullerene can accept up to six electrons in its LUMO (Lect. 7a, Slide 8). Nanotubes show similar performance.

  20. Fuel from the Sun • Photosynthesis • Biofuels • Split Water • Fuel Cells How does nature convert solar energy to chemical energy? Convert plants into fuel: Make ethanol, diesel fuel from sugar, corn starch, plant oil, cellulose, algae, ... Split water into hydrogen and oxygen using sunlight. Use hydrogen as fuel. No greenhouse gases. Producing electricity directly from fuel and oxygen.

  21. Next slide How Does Nature Do it? Plants convert solar energy into chemical energy (e.g. sugar). Less than 2% of the solar energy gets converted. But the initial part of the conversion is very efficient.

  22. Next slide Light-harvesting proteins Chlorophyll

  23. The Oxygen Evolving Complex 4 Mn + 1 Ca Instead of rare Pt (5d), Rh (4d), nature uses plentiful Mn (3d), Fe (3d), Ca(3d) as catalysts.Can we do that in artificial photosynthesis ? What does it take? (3D cage?)

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