Solar Cells

Solar Cells

Photoelectric Effect

This effect is in direct contradiction to the laws of classical physics and without which solar cells would not exist. Here light travels in the form of photons with energy described by:

E hf

Not all photons are reflected some are absorbed or transmitted When a photon is absorbed, the energy of the photon is transferred to an electron in the crystal latticeThis is known as the photovoltaic effectSolar cells are made of semiconducting material, traditionally Silicon

Energy of the photon is transferred to the valence electrons in the n-type layer

These valence electrons escape their orbits leaving holes(electrons are majority carriers, holes are minority

carriers) This creates mobile electron-hole pairs

In the p-type layer electrons are the minority carriers and holes are the

majority carriers

When p-type and n-type layers are placed together a p-n junction is formed and current begins to flowWhen electrons from the n layer move into the p layer a depletion zone is formed This zone separates the positive and negative charges and prevents a flow between them – an electric field is created

The diode created by the electric field allows current to flow in only one direction across the junction

Connecting the sides of the cell externally will cause electrons to flow to their original p side to meet with holes

The flow of electrons is a DC current (I)

The electric field of the cell is a voltage (V)

Here power (P) is given by:

P VI

Many of these cells are connected to in order to create a solar panel

By connecting the cells in series a higher voltage is obtained

By connecting the cells in parallel a higher current is obtained

Efficiency ( )

Maximum power (Pm) in W

Irradiance of input light (E) measured in W/m2

Surface area or solar cell (Ac) in m2

Fill factor (FF) Open circuit voltage (Voc)

Short circuit current (Isc)

m

c

P

E A

m c

oc sc oc sc

P A EFF

V I V I

Solar cells

1st generation- large area single layer n-p junction

Crystalline silicon

2nd generation- multi. Layer p-n junctions

Usually silicon

3rd generation- Semiconducting device which does not rely on

p-n junctions

Monocrystalline

Quantum dot cells- Electron-confined

nanoparticles

Thin Films deposited on supporting

substrates

Dye sensitized cellsOrganic polymer cells

Polycrystalline

Ribbon silicon

Thin-Films

Less material needed to create solar cell but less energy conversion efficiencyHowever, multi-layer thin films may have higher efficiencies then silicon wafersUse flexible resin film substrates instead of glass sheet substrates

Solar Shingles

Thin film PVs can be used as shingles

Cells shown here are triple junction

Each shingle has a pair of wires coming off its back so the system can be wired inside the attic

Conductive PolymersBuilt from thin films of organic semiconductorsThe high-efficiency cells made from GaAs where used in Deep Space 1

Dye-sensitive solar cells

Absorption occurs in dye molecules Electrons are passed on to the n-type TiO2, holes are passed to an electrolyte on the other side of the dyeHeat and UV light cause cells to degrade Low-cost productionModerate efficiency (less then 10%)

Quantum Dots

Semiconductor quantum dots can slow the cooling of hot electrons

This could enhance efficiencies up to 66%

Combination of quantum dots with polymers creates cells that can absorb IR radiation

Energy Storage

Energy produced by solar cells can be stored in batteries

For domestic systems it is most effective to run the meter backward- in effect selling the electricity to the grid

When electricity is needed and the solar cells are not producing the electricity can be bought back from the network

References Dye-Sensitized Solar Cells. European Institute for Energy Research.

2005. 23 April 2006 <http://www.eifer.uni-karlsruhe.de>.Green, Martin A. Solar Cells. New Jersey:Prentice-Hall, 1982.Highlights of the 2003 NCPV and Solar Program Review Meeting.

NREL. 2003. 23 April 2006 <http://www.nrel.gov>. Lovgre, Stefan. Spray-On Solar-Power Cells Are True Breakthrough.

National Geographic News. 14 Jan. 2005. 18 April 2006 <http://news.nationalgeographic.com>.

Photodetectors. HyperPhysics. 20 April 2006 <http://hyperphysics.phy-astr.gsu.edu/hbase/ligdet.html>.

PV Cells. Specmat.com. 23 April 2006 <http://www.specmat.com>.Solar Cells. 23 April 2006

<http://www.corrosion-doctors.org/Solar/cells.html>.Solar Cell. Wikipedia. 25 April 2006

<http://en.wikipedia.org/wiki/Solar_cell>.Solar Electricity. The Electricity Forum. 23 April 2006

<http://electricityforum.com/solar-electricity.html>.U.S. Department of Energy. Roofing. 10 Dec. 2004. 23 April 2006

<http://www.eere.energy.gov>.U.S. Department of Energy. Solar Shingles. 5 Jan. 2006. 23 April

2006 <http://www.eere.energy.gov>.Wave-Particle Duality. HyperPhysics. 25 April 2006

<http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html>.