^Solar or photovoltaic cells
A solar cell is a pn junction diode generally made from Si or GaAs. It works on following three basic processes: generation, separation and collection:
When sun light of energy E > Eg falls near the junction (area large) they excite the electrons from VB to CB, leaving behind equal number of holes in the VB. These photo generated electron – hole pairs generated in the depletion region move in opposite directions due to the barrier electric field (electrons move towards n side and holes move towards p side of the junction) & are collected near the side of the diode, which makes the p – side a positive electrode & the n – side a negative electrode & thus a photovoltaic potential difference is developed across the junction. If external load is connected across a solar cell a photocurrent IL starts flowing through it. This current is proportional to the intensity of the illumination falling on the diode.
Solar intensity received is maximum near 1.5 eV. As, for photo excitation E > Eg, so, semiconductor with band gap energy Eg ~ 1.5 eV or lower gives better solarconversion energy & thus are ideal materials for solar cell fabrication. Since Si has Eg ~ 1.1 eV while for GaAs it is ~1.43 eV, so they have relatively higher absorption coefficient than other materials like CdS or CdSe. This is why Si and GaAs are preferred materials for solar cells. Note that sunlight is not always required for a solar cell.
Any light with photon energies greater than the band gap will do.
Solar cells are used to power electronic devices in satellites and space vehicles and also as power supply to some calculators.