Optics 101

Converting Light into Electricity - How Does a Photovoltaic Cell Work

Introduction to Photovoltaic Cells
Materials Used for the Construction of Photovoltaic Cells
How Silicons are Used to Produce Photovoltaic Cells
Generation of Electric Field Within Photovoltaic Cells
Antireflective Coatings in Photovoltaic Cells

Introduction to Photovoltaic Cells

Photovoltaic cells, more commonly known as solar cells, are found in applications such as calculator and satellites. First used almost exclusively in space, photovoltaic cells are used in more common applications.

In simple terms, photovoltaic cells and devices convert light energy into electrical energy. Photovoltaic cells are available in many different shapes and sizes. When individual photovoltaic cells are joined, they form photovoltaic modules.

Photovoltaic arrays are available in different shape and sizes. However, their sizes are determined by factors like the required power output and the available amount of sunlight where the photovoltaic arrays are located.

Materials Used for the Construction of Photovoltaic Cells

Special materials are used for the construction of photovoltaic cells. These materials are called semiconductors. The most commonly used semiconductor material for the construction of photovoltaic cells is silicon. Several forms of silicon are used for the construction; they are single-crystalline, multi-crystalline and amorphous. Other materials used for the construction of photovoltaic cells are polycrystalline thin films such as copper indium diselenide, cadmium telluride.

How Silicons are Used to Produce Photovoltaic Cells

When light, for example, sunlight, strikes the photovoltaic cell, a certain amount of the light is absorbed, while the rest of the light beam is reflected or passes right through the photovoltaic cell. Only the absorbed light can be used to generate electricity.

Special Chemical properties exist in silicon. This is especially when the silicon is in the crystalline form. When energy, such as light or heat is added to pure silicon, it results in a number of electrons to break free of their bonds and leave their atoms. This results in the generation of a hole in the crystalline lattice. These electrons, known as free carriers, then wander randomly around the crystalline lattice looking for another hole to fall into. These free carriers have the ability to carry an electrical current. Unfortunately, very few of the free carriers are in pure silicon.

In order for the photovoltaic cells to work, the silicon in a solar cell has been modified slightly.

Generation of Electric Field Within Photovoltaic Cells

To generate an electric field within a photovoltaic cell, two different semiconductor materials are placed in direct contact with each other. The two different semiconductor materials are commonly referred to as N-Type semiconductor and P-type semiconductor materials. The area where the N-type and P-type semiconductor materials meet is referred to as the p/n junction.

The N-type semiconductor is negatively charged and has an abundance of electrons. The P-type semiconductor is positively charged and has a large number of holes in its crystalline lattice.

It is well known that pure silicon is a poor conductor in electricity. A photovoltaic cell has silicon with impurities. When phosphorous atoms mixed in with silicon atoms will take less energy to strike loose one of the extra phosphorus electrons since the phosphorus atoms are not tied in a bond.

This results in a large number of free carriers than compared to pure silicon. The process of adding an additional element of impurities is known as doping. When the silicon is doped with phosphorus, this is known as N-type silicon. Compared to pure silicon, N-type silicon is a much better conductor of electricity.

P-type silicon is manufactured by doping the silicon with chemical elements such as boron. Boron has three electrons rather than four in its outer shell. This results in the silicon having a positive charge.

Excess electrons from the N-type side moves to the P-type side when the two semiconductor materials are placed together. Because of this movement of electrons, a buildup of positive charge can be found along the side where the N-Type silicon is located. On the other hand, a buildup of negative charge can be found along the P-type side.

The electrical field causes the electrons to move towards to the negative side, where they become available to the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive side, where they await incoming electrons. Because of the flow of electrons and holes, the two semiconductors behave like a battery.

Antireflective Coatings in Photovoltaic Cells

Unfortunately, silicon is known to be a very shiny material, hence it is very reflective. As a result, an antireflective coating is applied to the top of the photovoltaic cell since photons that are reflected cannot be used by the photovoltaic cell.

Source: AZoOptics

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