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

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.

Energy

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.

PhotonElectron

Hole

Energy

Band Gap

exciton

Creation of an electric field at a pn-junction

Applications of the pn-junction

Wed. Apr. 11, 2006 Phy107 Lecture 30 7

Many types of solar cells

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

HOMO

EF

EF

eVopen

Contact Contact +Acceptor DonorDye

hν

Discrete energy levels in molecules, instead of energy bands in solids.

Generic Molecular Solar Cell

LUMO

HOMO

EF

EF

eVopen

Contact Contact +Acceptor DonorDye

hν

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)

• LUMO from X-ray absorption spectroscopy.• HOMO from photoelectron spectroscopy.

Calculationfor Zn-OEP

Cook et al. (2009)

The transition metal center of a dye molecule

• X-ray absorption is element-selective (here via the Fe 2p core level).• Detects the oxidation state, spin state, and ligand field.

Cook et al., J. Chem. Phys. 131, 194701 (2009)

What happens during a photochemical reaction ?

Pump-probe X-ray absorption spectra of a solvated Fe complex for thelow-spin ground state and a high-spin excited state

• 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)

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

• Biofuels

• Split Water

• Fuel Cells

How does nature convert solar energy to chemical energy ?

Convert plants into fuel: Make ethanol, diesel fuel fromsugar, 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.

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.

Next slide

Light-harvesting proteins

Next slide

Chlorophyll

4 Mn + 1 Ca

The Oxygen Evolving Complex

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 ?)