Solar cells convert sunlight into electricity by using semiconductor materials, most commonly silicon, where photons knock electrons loose allowing them to flow as an electric current; different types of solar cells have varying efficiencies with single crystalline silicon cells being the most efficient at 25%; solar cells can be connected together in panels and arrays to provide power for various applications from small electronics to entire homes.

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2. The Solar Cell
• The most common type of solar cells are Photovoltaic
Cells (PV cells)
• Converts sunlight directly into electricity
• Cells are made of a semiconductor material (eg. silicon)
• Light strikes the PV cell, and a certain portion is
absorbed
• The light energy (in the form of photons) knocks
electrons loose, allowing them to flow freely, forming a
current
• Metal contacts on the top and bottom of PV cell draws
off the current to use externally as power

3. Why Use Solar Cells?
• Low maintenance, long lasting sources of energy
• Provides cost-effective power supplies for people
remote from the main electricity grid
• Non-polluting and silent sources of electricity
• Convenient and flexible source of small amounts
of power
• Renewable and sustainable power, as a means to
reduce global warming
• In 2002, the global market for photovoltaic panels
and equipment was valued at 3.5 billion dollars

4. The Single Crystalline Silicon
Solar Cell
• Pure silicon is a poor
conductor of electricity
• “Doping” of silicon with
phosphorus and boron is
necessary to create n-type
and p-type regions
• This allows presence of
free electrons and electron-
free ‘holes’
• The p-n junction generates
an electric field that acts
as a diode, pushing
electrons to flow from the
P side to the N side

5. The Solar Cell

6. When Light Hits the Cell
• Light energy (photons) ionizes the atoms in the
silicon and the internal field produced by the
junction separates some of the positive charges
(holes) from the negative charges (electrons)
• The holes are swept into the p-layer and the
electrons are swept into the n-layer
• The charges can only recombine by passing
through an external circuit outside the material
• Power is produced since the free electrons have to
pass through the load to recombine with the
positive holes

7. Efficiency of Solar Cells
• The amount of power available from a PV device
is determined by
– Type and area of the material
– The intensity of the sunlight
– The wavelength of the sunlight
• Single crystalline solar cells  25% efficency
• Polycrystalline silicon solar cells  less than
20%
• Amorphous silicon solar cells  less than 10%
• Cells are connected in series to form a panel to
provide larger voltages and an increased current

8. Arrays and Systems
• Panels of solar cells can be linked together
to form a larger system – an array
(a) a PV panel array, ranging from two to
many hundreds of panels;
(b) a control panel, to regulate the power
from the panels;
(c) a power storage system, generally
comprising of a number of specially
designed batteries;
(d) an inverter, for converting the DC to
AC power (eg 240 V AC)
(e) backup power supplies such as diesel
startup generators (optional)
(f) framework and housing for the system
(g) trackers and sensors (optional);

9. Solar Cells are used in a wide
variety of applications
• Toys, watches, calculators
• Electric fences
• Remote lighting systems
• Water pumping
• Water treatment
• Emergency power
• Portable power supplies
• Satellites

10. Future Applications
The Flexible Solar Cell
• Looks like denim
• Can be draped over any
shape
• No rigid, silicon base
• Made of thousands of
flexible, inexpensive solar
beads between two layers of
aluminum foil
• Each bead functions as a tiny
solar cell

11. Future Applications
Organic Solar Cells
• Based on photosynthesis in plants
• Use of light-sensitive dyes
• Cost of manufacture is decreased by
60%
New Alloys
• Indium, gallium, and Nitrogen
• Converts full spectrum of sunlight from
near-infrared to far-ultraviolet

12. Future Applications
Nano Solar Cells • Tiny rods are embedded
in a semi-conducting
plastic layer sandwiched
between two electrodes
• Rods act like wires,
absorbing light to create
an electric current
Tetrapod Nanocrystals
• May double the efficiency of plastic solar cells
• Made of cadmium, tellurium

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