Photovoltaic cell

Photovoltaic cell


A device that detects or measures electromagnetic radiation by generating a current or a voltage, or both, upon absorption of radiant energy. Specially designed photovoltaic cells are used for power generation, as in solar batteries or solar cells, and for sensitive detection of electromagnetic radiation in radiometry, optical communications, spectroscopy, and other applications. An important advantage of the photovoltaic cell in these particular applications is that no separate bias supply is needed—the device generates a signal (voltage or current) simply by the absorption of radiation.

Most photovoltaic cells consist of a semiconductor pn junction or Schottky barrier in which electron-hole pairs produced by absorbed radiation are separated by the internal electric field in the junction to generate a current, a voltage, or both, at the device terminals. Under open-circuit conditions (current I = 0) the terminal voltage increases with increasing light intensity, and under short-circuit conditions (voltage V = 0) the magnitude of the current increases with increasing light intensity. When the current is negative and the voltage is positive, the photovoltaic cell delivers power to the external circuit. In this case, if the source of radiation is the Sun, the photovoltaic cell is referred to as a solar battery or solar cell. When a photovoltaic cell is used as a photographic exposure meter, it produces a current proportional to the light intensity, which is indicated by a low-impedance galvanometer or microammeter. For use as sensitive detectors of infrared radiation, specially designed photovoltaic cells can be operated with either low-impedance (current) or high-impedance (voltage) amplifiers, although the lowest noise and highest sensitivity are achieved in the current or short-circuit mode. Another mode of operation of a pn junction diode as a photodetector involves the application of a reverse bias voltage to the diode. In this case, the photogenerated current is directly proportional to the incident power, and the diode is said to be operated in the photodiode mode rather than the photovoltaic mode.