Transcript Energy02 - University of Wisconsin–Madison

Nanotechnology and Solar Energy
Solar Electricity
• Photovoltaics
Fuel from the Sun
• Photosynthesis
• Biofuels
• Split Water
• Fuel Cells
Solar cell
A photon from 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.
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 electrons and holes on their way out.
Crystalline semiconductors are good at that, but expensive.
Electrons, Holes, and Excitons
Energy
Electron
Photon
Band
Gap
Hole
exciton
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.
Creation of an electric field at a pn-junction
Applications of the pn-junction
Many types of solar cells
Wed. Apr. 11, 2006
Phy107 Lecture 30
7
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.
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 ?
Energy level diagram of
a molecular solar cell
LUMO
EF
h
eVopen
EF
HOMO
Contact Acceptor
Dye
Donor
Contact +
Discrete energy levels in molecules, instead of energy bands in solids.
Generic Molecular Solar Cell
LUMO
EF
h
eVopen
EF
HOMO
Contact Acceptor
Dye
Donor
Contact +
Another version:
Instead of holes going uphill, electrons going downhill fill the holes.
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.
Spectroscopy of the HOMO and LUMO
(donor and acceptor orbital)
Calculation
for Zn-OEP
• LUMO from X-ray absorption spectroscopy.
• HOMO from photoelectron spectroscopy.
Cook et al. (2009)
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 Fe 2p core level).
• Detects the oxidation state, spin state, and ligand field.
What happens during a photochemical reaction ?
• Spin excitations in 100 picoseconds (data below)
• Atomic motion in 100 femtoseconds (vibration period)
• Electronic motion in 1 femtosecond (Fermi velocity = nm/fs)
Huse and
Schoenlein
(2008)
Pump-probe X-ray absorption spectra of a solvated Fe complex for the
low-spin ground state and a high-spin excited state
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.
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).
Nanorods coated with nanocrystals
CsSe nanodots (3 nm) 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).
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.
Fuel from the Sun
• Photosynthesis
How does nature convert solar energy to chemical energy ?
• Biofuels
Convert plants into fuel: Make ethanol, diesel fuel from
sugar, corn starch, plant oil, cellulose, algae, ...
• Split Water
Split water into hydrogen and oxygen using sunlight.
Use hydrogen as fuel. No greenhouse gases.
• Fuel Cells
Producing electricity directly from fuel and oxygen.
How Does Nature Do
it ?
Next slide
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.
Light-harvesting proteins
Chlorophyll
Next
slide
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 ?)