photovoltaics The conversion of solar energy into electrical power using p-n junction diode. Transparent p region of a diode allows light photons of proper frequency to reach the transition region. If the photon energy (hn ) exceeds the energy gap, Eg then covalent bonds are broken producing electron- hole pairs. The junction is reverse biased, which sweeps the electrons and holes in opposite direction giving a current. The longest wavelength of electromagnetic waves capable of producing a photocurrent in pure silicon is ~ 1.1 m m, which for germanium is~ 1.7 m m. Both these wavelengths lie in the infrared region. Solar cells are similar diodes, which are employed for charging batteries.