Semiconductor Based Solar Cells
Today, most conventional silicon solar cells efficiently capture light in only the red, and a small portion of the yellow solar spectrum. The remaining solar energy (in the green and blue spectrum), that is the most energy intensive is converted to and dissipates as heat. As a result, today's technology is only realizing a small portion of the energy that could ultimately be converted. It is possible to increase the maximum obtainable efficiency by use of multi-junction solar cells, but at much increased complexity and cost.
The two main categories of solar technology are defined by the choice of semiconductor: either crystalline silicon that is in a wafer form, or thin films, which can be produced with other materials. The emerging thin film technologies are starting to make significant in-roads but crystalline technologies still dominate the market today.
Conventional Si solar cells share the following characteristics:
- The band gap of silicon and other semiconductors limits absorption to a narrow frequency range
- Rectification is limited by recombination and thermalization of charge carriers.
- Temperature degradation is a factor in device performance and material lifetime.
- Theoretical limit is ~41% based on thermodynamic considerations.
- Material and device fabrication costs can be important considerations.
Typical solar cells use a three-step process:
- The silicon absorbs the solar energy, and creates free charge carriers both positive and negative.
- The separation of positive and negative charge creates a voltage in the silicon.
- The voltage causes the charge carriers to move across the silicon, creating an electric current.
There are three key elements in a solar cell that form the basis of manufacturing technology:
- First is the semiconductor, which absorbs light and converts it into electron-hole pairs.
- Second is the semiconductor junction, which separates the electrons and holes.
- Third is the contacts on the front and back of the cell that allow the current to flow to an external circuit.