HOW SOLAR CELLS WORK
The are various different types photo cells, 3 are discussed below of which 2 can be used in a solar cell.
The electrons in a metal can have energy supplied to them by radiation eg. light rays. This is known as the photo-electric effect. The energy of a light quantum, photon, is imparted to the most loosely bound electron of an atom. This energy may be sufficient to liberate the electron but not enough to eject it entirely from the metal (photo-conductive effect); alternately, it may be sufficient not only to liberate the electron but also to cause it to be ejected into the vacuum (normal photo-electric effect). The energy balance of the elementry process involved is given by Einstein's equation:
hv=A+½mev2
where hv denotes the energy of the photons, in which v is the frequency of the light radiation and h is Plank's constant (h=6.626 x 10-27 erg-seconds), A denotes the photo-electric work function (i.e. the energy required by a photon to eject an electron from a metal), me denotes the mass of the electron, and v its velocity in a vacuum.
The normal photo-electric effect is applied in the photo-electric cell. The light-sensitive photo-cathode, which isusuall installed in an evacuated glass tube, may consist of a very thin film of cesium deposited by vaporisation on to an oxidised silver base. For greater sensitivity the glass tube may be filled with an inert gas at low pressure. A battery in the external circuit serves to amplify the current by ionisation of the gas filling.
The photo-conductive effect is utilised in the photo-conductive cell. The sensitive material usually employed in this case is cadmium sulphide or cadmium selenide. These substances undergo changes in resistance in the ratio of 109:1 between the extremes of darkness and maximum exposure to light.
When the photo-conductive effect occurs at the P-N boundary of semi-conductors or at the boundary between a semiconductor and a metal (eg. corous oxide and copper), a potential difference will develop: this is known as the photo voltaic effect, and a cell of this kind is called a photo-voltaic cell. The photo-electric and photo-voltaic cells generate an electromotive force on their own account, causing a current to flow in the circuit even if no battery is included in the circuit, whereas the photo-conductive cell requires an auxiliary voltage provided by a battery. Photo-electric cells are used in "solar batteries" as sources of electric power for rockets and satellites used in space research. For this purpose silicon photo-electric cells are used; about 10% of the radiation energy which they absorb is converted into electric energy.
SOLAR POWER