Download 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
lattice
This is known as the
photovoltaic effect
Solar 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 flow
When 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 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
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
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)
Pm
E Ac
Pm
 Ac E
FF 

Voc I sc
Voc I sc
Solar cells
1st
generation- large area
single layer n-p junction
Crystalline silicon
Monocrystalline
3rd generationSemiconducting device
which does not rely on
p-n junctions
2nd
generationmulti. Layer p-n junctions
Usually silicon
Polycrystalline
Ribbon silicon
Thin Films deposited
on supporting
substrates
Dye sensitized cells
Organic polymer cells
Quantum dot cellsElectron-confined
nanoparticles
Thin-Films
Less material needed to
create solar cell but less
energy conversion efficiency
However, multi-layer thin
films may have higher
efficiencies then silicon
wafers
Use 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 Polymers
Built from thin films of organic semiconductors
The 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 dye
Heat and UV light cause cells to degrade
Low-cost production
Moderate 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.phyastr.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.phyastr.gsu.edu/hbase/mod1.html>.