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Solar Cells: Photovoltaic (PV) Cells

Solar Engery 101

In the most simplistic terms, solar or photovoltaic cells convert one form of energy (sunlight) into another form of energy (electricity). Commonly known as solar cells, individual photovoltaic cells are electricity-producing devices made of semiconductor materials. Solar modules capture the sun's energy and convert it to electricity using these photovoltaic (PV) cells.

Solar Cells: The Building Blocks for Solar Systems

A photovoltaic or solar cell is the basic building block of a PV system. An individual PV cell is usually quite small, typically producing about 1 or 2 watts of power. To boost the power output of PV cells, they are connected together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, PV systems can be designed to be able to meet almost any electric power need, whether small or large.

Really Simple Chemistry Review

Remember your high school chemistry class? Remember that all things are made up of atoms and all atoms have electrons circling a ball of matter called the nucleus. And if you remembered all that, one thing you may not remember is that the nucleus of the atom is positively charged and the electrons are negatively charged. All atoms have an essentially neutral charge, meaning the charge on the nucleus of the atom (positive) and the charge of its electrons (negative) cancel each other out. Since most solar cells are made of silicon, we're going to show you a representation of a silicon atom and its electron representation (right). Other materials are used as well, but the fundamental process of how those cells work is the same as for silicon cells. In this article, we will be referring to silicon only.

silicon atom

Really Simple Electricity Review

Are you still with us? To give a simple illustration of electricity, let's look at an ordinary battery, like the A, AA, C or D battery that you use in digital cameras, and other small electronic devices. When you are replacing a battery in an electronic device, the top or bottom says positive (indicated by something which looks like a +) and another side says "-", which means negative. You need to put the battery in the device the right way or it won't work. Why? Electricity only flows because electrons in the battery material flows from negative to positive. If you don't put the battery in the right way, the electricity never flows into the device because the electrons are moving the wrong way. In solar cells, all electricity is created by the flow of electrons from a negatively charged material to a positively charged material. Got that?

How do Solar Cells Work?

Solar cells are made from semiconductor material, as we mentioned, typically silicon. A typcial solar cell consists of two layers of silicon, one p-type (positive type) and the other n-type (negative type), sandwiched together to form a 'pn junction. That's it in a nutshell, a solar cell is basically just two different types of silicon material pressed together and wired to draw off the current.

Positively Charged Materials (p-type) - How Are They Created?

The p-type solar layer is "doped" by adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers (in this case positive). This process creates a "hole" in the material by removing electrons in individual atoms, therefore decreasing the negative charge and making the atom a positive charge.

Negative Charged Materials (n-type) - How Are They Created?

The n-type material layer is "doped" by adding a certain type of atoms to the semiconductor in order to increase the number of electrons. This process creates "free electrons" in the material by removing electrons in individual atoms, therefore decreasing the negative charge and making the atom a positive charge. They easily "detach" from the atom when it is acted on by heat or light.

I See the Light!

When photons in sunlight hit the solar panel they are absorbed by the semi-conducting material; in other words, the energy of the absorbed light is transferred to the semiconductor. Ah-ha! The electrons in the n-type material are free to move about. Opposites attract, and they move towards the "holes" in the p-type material.

The PN Junction

Where the p-type materials and the n-type materials meet is called the PN Junction and it creates an electric field between the two types of materials. It is this electric fields which is the electrical current produced by the solar cell.

Voila! Electricity!

The current generated at the PN Junction is the basis of electricity generated by the solar cell. By placing metal contacts on the top and bottom of the PV cell, the current can be drawn off and sent to wherever the PV cell is wired. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

Limitations on Efficiencies of Solar Cells

Conventional silicon solar cells have an efficiency of about 17% and the solar panels last about 25 years. At the end of 25 years, the solar panels are still producing electricity, but less than their efficiency rating.

Many factors limit the efficiency of photovoltaic cells. Silicon is cheap, for example, but in converting light to electricity it wastes most of the energy as heat. The most efficient semiconductors in solar cells are alloys made from elements from group III of the periodic table, like aluminum, gallium, and indium, doped with elements from group V, like nitrogen and arsenic.

One way to make solar cells more efficient is to find a material that will capture energy from a large portion of the spectrum of sunlight -- from infrared to visible light to ultraviolet.

The trouble is, most photovoltaic materials absorb a relatively narrow range of light energy. The most efficient silicon solar cells capture about 25 percent of the sun's energy. Multi-junction solar cells combine several materials to capture multiple bands of photonic energy. Today's most efficient combination -- germanium, gallium arsenide and gallium indium phosphide -- boosts efficiency to 36 percent, but is relatively difficult to make and therefore expensive.

What Happens at Night, or Cloudy Days?

If a cloud passes by, the intensity of the photon flow is reduced, and the amount of current flowing from the cells and entire array diminishes proportionally. When the cloud passes away and the sunlight returns, the conversion process resumes. As the sun slowly sets in the evening, the current is reduced until finally at darkness there is no electric current flow from the solar array at night. Solar cells do not store electricity. At night, a solar electric system needs some form of energy storage, usually batteries, to draw upon instead of the solar cells. The next morning, the process begins again automatically.