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Semiconductors and the Built-In Electric Field for Crystalline Silicon Photovoltaic Cells
To separate electrical charges, crystalline silicon cells must have a built-in electric field. Light shining on crystalline silicon may free electrons within the crystal lattice, but for these electrons to do useful work—such as provide electricity to a light bulb—they must be separated and directed into an electrical circuit.
To create an electric field within a crystalline silicon photovoltaic (PV) cell, two silicon semiconductor layers are sandwiched together. P-type (or positive) semiconductors have an abundance of positively charged holes, and n-type (or negative) semiconductors have an abundance of negatively charged electrons. When n- and p-type silicon layers contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.
Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet"what is called the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.
Creating P-Type and N-Type Semiconductors
In a crystalline silicon PV cell, p-type silicon must contact n-type silicon to create the built-in electrical field. The process of doping, which is used to create these materials, introduces an atom of another element into silicon crystal to alter its electrical properties. The dopant, which is the introduced element, has either three or five valence electrons—which is one less or one more than silicon’s four.
Phosphorus atoms, which have five valence electrons, are used to dope n-type silicon because phosphorus provides its fifth free electron. A phosphorus atom occupies the same place in the crystal lattice formerly occupied by the silicon atom it replaced. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free, having no bonding responsibilities. When phosphorus atoms are substituted for silicon in a crystal, many free electrons become available.
The most common method of doping is to coat a layer of silicon material with phosphorus and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. The temperature is then reduced so the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray-on process, and a technique in which phosphorus ions are precisely driven into the surface of the silicon.
But the n-type silicon cannot form an electric field by itself. It also needs p-type silicon. Boron, which has only three valence electrons, is used for doping p-type silicon. Boron is introduced during silicon processing when the silicon is purified for use in photovoltaic devices. When a boron atom takes a position in the crystal lattice formerly occupied by a silicon atom, a bond will be missing an electron. In other words, there is an extra positively charged hole.
In a PV cell, photons are absorbed in the p-layer. It is therefore important that this layer be "tuned" to the properties of incoming photons so it can absorb as many as possible and, thus, free up as many electrons as possible. The design of the p-layer must also keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. To accomplish these goals, p-layers are designed to free electrons as close to the junction as possible, so that the electric field can help send the free electrons through the conduction layer (the n-layer) and out into the electrical circuit.
By optimizing these characteristics, the PV cell’s conversion efficiency (how much light energy is converted into electrical energy) is improved.