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The Basics Of A Solar Cell

What is a solar cell? How does it work and what is it used for? Find out here.

A solar cell, also known as a photovoltaic cell, converts sunlight directly into electricity by a process called the photovoltaic effect. The term "photovoltaic" comes from the Greek word (phos) that means "light", and the word "voltaic", derived from the name of the Italian physicist; Volta, after whom a unit of electro-motive force, the volt, is named. The term "photo-voltaic" has been in use in English since 1849.

The photovoltaic effect involves creating a voltage (an electric current) in a material when the material is exposed to electro-magnetic radiation. In the photovoltaic process the generated electrons are transferred from one material to another. The process of transferring electrons results in a buildup of voltage between two electrodes.

Most photovoltaic applications use sunlight as the source of elctro-magnetic radiation and that is the reason the devices using the photovoltaic effect to convert solar energy into electrical energy are known as solar cells.

solar cell basics

Solar Cell Basic Facts

So, how does a solar cell work?

This will be quite technical, but I'll try to make it understandable.

When photons contained in sunlight hit a solar panel they are absorbed by semiconducting material, most commonly crystalline silicon. Electrons (which have a negative charge) are knocked loose from their atoms. This allows the electrons to flow through the material to produce electricity. Solar cells are constructed in a special way that forces the electrons to move in a single direction. Complementary positive charges are also created (like bubbles). These are called holes and they flow in the direction opposite of the electrons in a silicon solar panel.

This flow of electrons within the photovoltaic cell creates a current, and by placing metal contacts on the top and bottom of the photovoltaic cell, we can draw that current off to use externally. For example, the current created in a solar cell can power a calculator. This current combined with the cell's voltage determines the power (or wattage) that a solar cell produces. An array (several solar cells or solar panels connected together) of solar cells converts solar energy into a usable amount of direct current (DC) electricity.

Crystalline silicon, the most common semiconductor used in solar cells, has unique chemical properties, especially in a crystallized form. An atom of silicon has 14 electrons that are contained within three different "shells". The first two shells are closest to the center of the atom and have completely full sets of electrons (8). The outer shell, furthest from the core of the atom, only has four electrons. A silicon atom always looks for a way to fill up its last shell from 4 to 8 electrons. In order to accomplish this task it must share electrons with four of its neighbor silicon atoms. The effect of this process is that every atom "holds hands" with its neighbors. In the case of crystallized silicon each atom has four hands (electrons) joined to four of its neighboring atoms electrons. This is what actually creates the crystalline structure. It is the crystalline structure that is so important to this type of photovoltaic cell.

Pure silicon does not conduct electricity very well because none of its electrons are free to move about. The electrons are all locked within the crystalline structure. The silicon used in a solar cell is adapted slightly so that it will work as a solar cell.

Solar cells use silicon that has impurities. This means there are other atoms that are present with the silicon atoms. This changes the way silicon functions. Normally, impurities are not desirable in an element or product but our solar cell wouldn't work without them. The impurities in the case of silicon for solar cells are added on purpose.

Let's look at an example of silicon mixed with phosphorous atoms. Phosphorous has five electrons in its outer shell, not four like silicon. It still adheres with neighboring silicon atoms, but the phosphorous has one extra electron that doesn't have a free electron to bind with from the silicon atom. This extra electron doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus that holds it in place. When energy (normally heat) is added to pure silicon it can cause a few of the electrons to break free from their bonds and leave their atoms. A hole is left behind in each case. These electrons then flow randomly around the crystalline lattice until it finds another hole to fall into. These types of electrons are named free carriers because they carry electrical current. Impure silicon with phosphorous atoms mixed in turns out to be a very useful combination. It takes much less energy to knock loose the "extra" phosphorous electrons because they aren't tied up in a bond. These electrons don't "hold hands" with neighboring atoms like silicon does so it is much easier for them to break free to carry current.

The process of introducing impurities into pure silicon on purpose is called doping. When phosphorous is used the silicon created is referred to as N-type ("n" for negative) because there are so many free electrons. N-type silicon is a much better conductor than pure silicon.

Only one part of a photovoltaic cell is composed of the N-type silicon. The other half is doped with boron, which has three electrons in its outer shell instead of four. Silicon doped with boron is called P-type silicon. P-type silicon ("p" for positive) has free holes. Holes are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do.

It gets interesting (trust me...) when you put N-type silicon together with P-type silicon. Every photovoltaic cell has at least one electric field. Without an electric field, a solar cell would not work. This electric field is created when the N-type and P-type silicon come in contact with each other. The free electrons in the N side that look for holes to fall into, see all the free holes on the P side, and there's a mad rush (current) to fill in the holes.

When the holes and electrons mix at the junction (connection point) between the N-type and P-type silicon neutrality is disrupted. Right at the junction the electrons and holes do mix and form a barrier. This makes it harder for electrons on the N side to cross to the P side. Eventually, an equilibrium is reached, and an electric field is created that separates the two sides.

The electric field acts as a diode, allowing (sometimes pushing) electrons to flow from the P side to the N side, but not the other way around. Electrons can easily pass to the N side but they cannot "climb up" to the P side. The electric field acts as a diode in which electrons only move in one direction.

When light (photons) hits our solar cell, its energy (heat) frees the electron-hole pairs. Each photon with enough energy will normally free exactly one electron also creating a free hole. If this happens close enough to the electric field (barrier), or if the free electron and free hole happen to wander into its range of control, the field will send the electron to the N side and the hole to the P side. This causes a disruption in the neutrality of the atoms, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage present we generate electricity or power.

Before a photovoltaic cell is useable a few more things need to occur. Silicon is a very shiny material. This means it is highly reflective. Photons that are reflected can't be used by the cell. They bounce off the cell. Because of that, an antireflective coating is applied to the top of the cell to reduce the number of photons lost due to reflection.

The last thing that must be done is to add a glass cover plate that protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and in parallel to achieve high enough levels of voltage and current to produce a useful amount of electrical power. The solar cells are housed in a sturdy frame with the glass cover and positive and negative terminals on the back side.

How are solar cells used?

Solar cells are used in a wide variety of applications. Currently photovoltaic cells are being used in all sorts of technology from solar powered backpacks and ipod chargers to water heating, lighting and of course home and business electrical needs.

With the dawn of concentrated solar energy we are beginning to see the development of larger scale solar generation fields to generate electricity directly into the electrical grids.

Space research has been the incubator for much of the solar cell technology we have today. The need to find new ways to refuel in space and power our vehicles has helped push the solar technology envelope. Solar cells are used in satellites, telescopes, space rovers and the International space station. The ISS functions completely on solar cells that collect energy from the sun. It sports over 200,000 solar cells that are mounted on four large wings.

There are many home applications that utilize energy collected from photovoltaic cells. One of the most common is lighting, another common use for solar cell technology is solar hot water heating. Then there is general electrical service to run all of your appliances, computers etc.

Apart from powering your home, photovoltaic cells also power a broad range of gadgets we have all come to depend on. Some of these include MP3 players, portable CD players, cell phones, laptops and GPS units.

Even our cars are going solar. In 2011 we will see a new car enter the market that uses a rooftop solar panel to supplement the electrical needs of the car. You can expect to see more and more solar powered products in the years to come.

Video explanation

If you want to see how a photovoltaic cell works, have a look at this video clip.

Instructions: When watching the video clips, start the videos by clicking the small arrow down to the left, not the large one in the middle of the screen. If you're using Internet Explorer and your browser and the video doesn't start, try clicking twice on the small arrow.

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