Solar Energy: Conversion into Electricity

How Solar Energy is Generally Used:

What about the Direct Conversion into Electricity?

Two Choices:
• Heat Water into Steam
• Turn photons into electrons

Water Into Steam:

Photons into Electrons: PhotoVoltaic Devices

• Charge Generation --> Photoelectric Effect

• When photons stike a metal, their energy is used to liberate loosely bound electrons and therefore induce a current. This will be demonstrated in class on Wednesday.
• Efficiency of this process depends upon the material
• This is the principle behind many digital cameras, otherwise known as CCD cameras . These kinds of cameras are used in astronomy to take digital pictures . Incoming photons are converted into units of electric charge and stored at individual pixel locations. Different amounts of charge represent different intensity levels. This encoded information is a digital image.

To make use of the photoelectric effect, we need material that is a goo conductor of electricity and which can be manufactured in bulk at reasonable cost. This conditions strongly constrain the available choices. For most practical aspects, Silicon is the material of choice.

Silicon:

• is abundant on the earth and readily found in the crust. It is a direct product of fusion inside stars. It can be easily recovered from the crust and mass-produced. Computers are cheap because silicon works well for circuit boards and is an easily recoverable material from the earth's crust. The result is a world-wide enconomy centered around semi-conductor technology.
• Has four outer (valence) electrons to bond silicon atoms together in a crystal
• Under normal circumstances, there are no free electrons available in silicon to conduct electricity. All the electrons are used to bind the atoms in place to form the crystal.
• The conduction band is empty and therefore no current can be carried by the material.

Schematic structure of energy bands in Silicon:

Hence, if a silicon atom receives at least 1.11 Electron Volts from some source, a valence electron will move to the conduction band. Once an electron is in the conduction band, the material can carry a current and the material is now a conductor.

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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu