What are solar cells in physics?

What are Solar Cells?

Solar cells are devices that work on the principle of Photovoltaic Effect. These tiny compact devices convert solar light energy directly into electricity. In Greek, Photo-means light whereas voltaic means electrical current or electricity (light-electricity). Since, a solar cell gives out direct current (DC) electricity; it could be used for various purposes such as to power DC motors and to light bulbs. They also can be used to charge rechargeable batteries which could store electricity and could be used later in the absence of sun. Once the batteries are completely charged they could be used anywhere at any time especially in places where electricity isn’t reachable or in case of emergency backup.

How does it Work?

Solar cells or Photovoltaic cells are made up of special materials called semi-conductors such as silicon, which are most commonly used. When light falls on the cell, a certain amount of the light is absorbed within the semi-conductor material. The energy absorbed from the light is transferred to the semiconductor which helps knocks electrons loose, allowing them to flow freely.

PV cells have one or more electric field that force electrons freed by light absorption to flow in a certain direction and this flow of electrons is called current. Metal contacts are placed on the top and bottom of the PV cell, which draw current off for external use, for example to power a watch or a calculator. This current, along with the cells voltage gives power (wattage) that the solar cell can produce.

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As we see from the diagram there are three important layers for energy conversion in the solar cell.
The first one is the top junction layer (made of N-type semi-conductors).
The second layer is the core of the device which is the absorber layer (P-N junction).
The last of the conversion layers is a black junction layer (made of P-type semi-conductor).

As we can also see from the diagram, that there are two additional layer in the solar cell. These are the electrical contact layers. These two layers allow electric current to flow out of and into the cell.

On the top of the cell, from where light enters the electrical contact layer is present in a grid pattern is usually made of a very good conductor such as a metal. The grid pattern are mostly widely spaced so as to allow light to enter the solar cell, but just enough so that the electrical contact layer won’t have difficult collecting current produced by the cell.

To sum up a typical solar cell which is multi-layered would consist:

• Cover Glass- provides outer protection ( made from clear glass layer)
• Anti-reflective Coating- prevents the entered light from escaping and bouncing off so that max. energy is absorbed.
• Front Contact- transmits electric current.
• N-Type Semiconductor Layer- thin layer of silicon which has been doped with phosphorous.
• P-Type Semiconductor Layer- thin layer of silicon which has been doped with boron.
• Back Contact- transmits electric current.

Characteristics of a Solar Cell

Usable voltage from solar cells depends on the type of semi-conductor material like in Silicon it approximates only up to 0.5 V. The terminal voltage is somewhat dependent on light, while current intensity increases with higher luminosity. For example, a 100 cm² Silicon cell can reach up to a max. current intensity of 2 A approx. radiated by 1000 W/m².

The output of a solar cell depends on the temperature. Higher the cell temperature, lower the output which implies to lower efficiency. The level of efficiency tells us how much amount of radiated light is converted into useable electrical energy.

Types of Solar Cells

• Monocrystalline Silicon- made up of single layer of silicon for the semi-conductor. Extremely pure and most expensive type of solar cell to produce.
• Polycrystalline Silicon- made by pouring liquid silicon into blocks which are later on sawed into plates. This sometimes degrades the silicon crystals making them less efficient. But, this type approach is easier and cheap to manufacture.
• Amorphous Thin Film Silicon- made up of layers of semi-conductors that are only a few micrometers thick.This makes it lower the material cost but also makes it less efficient than other types of silicon. Since, it’s so thin it is used in wide variety of flexible material like solar shingles or roof tiles

All of them follow the same principle, but some are more efficient than the other while some are cheaper.

Uses & Application

Scientific research & Aerospace Industry:

• Civilian aircraft,
• satellites
• experimental rovers
• telescopes

Home Appliances-

• Water heaters
• air conditioners,
• water distillers

Solar cells are also being used for electricity storage and emergency backup, especially in far off remote places or rural areas in some parts of the world and also where it’s difficult for electricity to reach like in space.