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Solar cells

Solar cells convert light energy into electrical energy through the photoelectric effect. There are many different types of solar cells, made from a large range of materials, and they all convert light using the same process. It is also called the photovoltaic effect and solar cells are often called photovoltaic cells. The two main groups of cells are bulk silicon and thin-film.

The photoelectric effect occurs when the solar cell is exposed to light above a certain frequency. Some light photons are absorbed by the electrons in the atoms, while others pass through the cell or reflect away from it. The amount of absorption depends largely on the properties and thickness of the material used to make the cell. When a light photon strikes an electron, energy is transferred to the electron, allowing it to move away from the atom nucleus. These free electrons move freely through the material and even cross over to other conductors. The spaces left behind by free electrons are called holes, and the electrons will eventually fall back into them. To move the electrons to the surface before this recombination happens, an electrostatic field is needed to draw them away.

An internal electrostatic charge is created by changing the chemical makeup of a semiconductor. The basic solar cell is made from two different semiconductor layers, a positively charged layer and a negatively charged layer. This charge difference creates the internal electrostatic field needed to move free electrons to the surface, and it lowers the energy required to produce free electrons, making the cell more efficient.

Solar cells have many thin metal wires running across their front and back surfaces. One side is for the collection of electrons and the other is for the return of electrons. Each solar cell produces a small potential difference, usually less than two volts, that is only strong enough to power a wristwatch or calculator. To achieve higher voltages, the cells need to be connected together in series, like the lead plates in a car battery. The wires from each cell are connected to busbars that run between the cells and which are connected to two terminals, negative for the front wires and positive for the back wires.

Bulk polysilicon solar cells are the most common type used today and can be easily identified by their distinctive deep blue color. They are thick compared to other cells, allowing them to absorb more light photons which gives them a higher efficiency. However, being thick and brittle is also a disadvantage because they cannot be used in the many places that require a curved panel.

Thin-film solar cells have the advantage of being flexible, but usually that comes at the cost of lower efficiency. They use other compounds, such as cadmium telluride (CdTe), copper-indium selenide (CIS), and gallium arsenide (GaAs). They have a lower efficiency because there is less chance of the photons being absorbed in a thin cell. However, their efficiency can be improved by using several layers of thin-film cells. Nanotechnology is also being used to increase the internal surface area of cells to improve their efficiency.

Solar cells can convert other sources of light into electrical energy, such as starlight and artificial light, but they work best under intense sunlight. Even moonlight, which is just reflected sunlight, is not strong enough to produce useful current due to the poor efficiency of the cells. Solar cells are a practical application of quantum mechanics and as our knowledge of atomic structures improves, along with advances in manufacturing, smaller and more efficient solar cells should appear in the near future.

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