Why Use Solar Cells

1
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

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
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

4
The Solar Cell
5
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

6
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

7
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)
8
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

9
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

10
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

11
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