Solar Cell Characteristics
- Photovoltaic cells are built of semiconductor material that absorbs sunlight and transfers the energy of the light photon to an electron, which then can power an electrical circuit. The choices for bulk semiconductors include silicon, germanium, cadmium telluride, copper indium selenide and copper gallium selenide.
- Semiconductors absorb best when the energy in the light they absorb is just enough to transfer their electrons from a localized valence state to a conduction band in the semiconductor, where they are free to move. Each material has specific valence and conduction band energies; the difference between them is called the bandgap. The energy carried in a photon depends upon its wavelength. Each material will absorb different wavelengths best.
- Quantum efficiency (QE) is a measure of how well a semiconductor transfers energy from photons to electrons. It is given in percent, so a QE of 87 percent means that 87 out of every 100 photons is absorbed by electrons within the semiconductor. A semiconductor's QE is best when a photon has an energy just above the semiconductor's bandgap energy, so QE will vary with wavelength. A QE curve will show the absorption efficiency over the whole solar spectrum.
- Unfortunately, not every electron that absorbs energy is available to power a circuit. Some "recombine," which means they drop down to the valence band, giving up their energy and getting tied to a specific atom once again. Some crash into other atoms along the way, giving up their energy in the form of heat -- electrical resistance in the semiconductor and the electrical contacts is the culprit here.
- The maximum amount of power a solar cell can put out is determined by measuring its performance under different circuit loads. The maximum current (I) is put out when there is no load; the maximum voltage (V) builds up when there's an open circuit, that is, no connection at all. Somewhere between these two extremes is a point where the current times the voltage is a maximum. Power is the product of current and voltage, so this point is called the maximum power point.
