Solar cell

(electronics) A pn-junction device which converts the radiant energy of sunlight directly and efficiently into electrical energy.

A semiconductor electrical junction device which absorbs and converts the radiant energy of sunlight directly and efficiently into electrical energy. Solar cells may be used individually as light detectors, for example in cameras, or connected in series and parallel to obtain the required values of current and voltage for electric power generation.

Most solar cells are made from single-crystal silicon and have been very expensive for generating electricity, but have found application in space satellites and remote areas where low-cost conventional power sources have been unavailable.

The conversion of sunlight into electrical energy in a solar cell involves three major processes: absorption of the sunlight in the semiconductor material; generation and separation of free positive and negative charges to different regions of the solar cell, creating a voltage in the solar cell; and transfer of these separated charges through electrical terminals to the outside application in the form of electric current.

When light is absorbed in the semiconductor, a negatively charged electron and positively charged hole are created. The heart of the solar cell is the electrical junction which separates these electrons and holes from one another after they are created by the light. An electrical junction may be formed by the contact of: a metal to a semiconductor (this junction is called a Schottky barrier); a liquid to a semiconductor to form a photo-electrochemical cell; or two semiconductor regions (called a pn junction).

The fundamental principles of the electrical junction can be illustrated with the silicon pn junction. Pure silicon to which a trace amount of a group V element (in the periodic table) such as phosphorus has been added is an n-type semiconductor, where electric current is carried by free electrons. Each phosphorus atom contributes one free electron, leaving behind the phosphorus atom bound to the crystal structure with a unit positive charge. Similarly, pure silicon to which a trace amount of a group III element such as boron has been added is a p-type semiconductor, where the electric current is carried by free holes. The interface between the p- and n-type silicon is called the pn junction. The fixed charges at the interface due to the bound boron and phosphorus atoms create a permanent dipole charge layer with a high electric field. When photons of light energy from the Sun produce electron-hole pairs near the junction, the built-in electric field forces the holes to the p side and the electrons to the n side. This displacement of free charges results in a voltage difference between the two regions of the crystal. When a load is connected at the terminals, an electron current flows and useful electrical power is available at the load. See also Semiconductor; Solar energy.