Good explanation of alternative technologies, courtesy of the U.S. Department of Energy:

• http://www1.eere.energy.gov/...
  
  
• The Photoelectric Effect Explained
• Solar Cell Materials/Structures
• Solar Energy International
• Union of Concerned Scientists
• U. S. Department of Energy - Energy Efficiency &
  Renewable Energy
• Renewable Energy Access
• National Renewable
  Energy Laboratory
• DSIRE - Renewable Energy Incentives by State
• Solar Energy Glossary
  
  
• www.homepower.com