A solar cell is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires firstly, a material in which the absorption of light raises an electron to a higher energy state, and secondly, the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell.
A phototransistor is a device that converts light energy into electric energy. Phototransistors are similar to photoresistors but produce both current and voltage, while photoresistors only produce current. This is because a phototransistor is made of a bipolar semiconductor and focuses the energy that is passed through it. Photons (light particles) activate phototransistors and are used in virtually all electronic devices that depend on light in some way.
A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode. A small amount of current is also produced when no light is present. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas.The common, traditional solar cell used to generate electric solar power is a large area photodiode. Many diodes designed for use specifically as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response
An optically coupled isolator is a photo emissive which is optically coupled to a photo electric device both contain in the same encapsulation the device provide electrical isolation b/w the two circuits and typically provide a band width from DC to about 100 KHz. Earlier type Contained either a near lamp or a tungsten filament lamp together with a photo conductive cell.