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

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

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

The Solar Cell

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

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

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);

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

Future Applications

• 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

The Flexible Solar Cell

Future Applications

• Based on photosynthesis in plants• Use of light-sensitive dyes• Cost of manufacture is decreased by 60%

Organic Solar Cells

New Alloys

• Indium, gallium, and Nitrogen

• Converts full spectrum of sunlight from near-infrared to far-ultraviolet

Future Applications

• 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

Nano Solar Cells

Tetrapod Nanocrystals

• May double the efficiency of plastic solar cells

• Made of cadmium, tellurium