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Explore Photovoltaics

History

The French scientist Alexandre Edmond Becquerel discovered around 1840 that some materials produce a current (electricity) when light shines on them. PV cells, as we know them now, were first developed in 1954 by Bell Telephone researchers and first applied to power satellites in space. Over the past three decades, cost has been decreasing continuously, while efficiencies have been increasing. PV cells are now widely used to power:

• Homes 
• the utility grid (in modest amounts)
• Satellites (the first application to use PV panels)
• Billboards 
• Highway signs 
• Remote transmitters, pumps, and other equipment
• Construction equipment lighting
• outdoor lights over doorways and along sidewalks
• calculators

Project: See how many applications you can find in your own neighborhood!

Types of PV Cells

Most PV cells are made from purified silicon, which is doped with other elements to achieve the desired photoelectric properties. There are several basic kinds of cells:

• Crystalline: Each cell is a single silicon crystal: These have pretty high efficiency (10-30%), and very long lifetimes because the crystal structure is very stable.

• Polycrystalline: Each cell is a collection of many small silicon crystals, as one can see in the photo below. These also have pretty high efficiency (10-30%), and very long lifetimes because the crystal structure is very stable.

• Thin film or amorphous silicon: These are the type most commonly found in calculators. Each cell is made from non-crystalline thin films of silicon atoms, and typically have a uniform gray appearance. These tend to have lower efficiencies (5-10%), and more limited lifetimes because the silicon atoms have some freedom to move around over time. On the other hand, thin film cells can be much cheaper and require much less energy to produce, and promising efforts are being made to extend their lifetimes.

Typical commercially available PV panels have an efficiency of about 15%, which means that they can deliver about 150 watts of power per square meter.

How they work

Solar cells are mostly made of silicon. Each silicon atom has four electrons in its outermost (valence) shell. To complete the shell and achieve the most stable configuration, the atom would like to have eight instead (this is due to the quantum mechanical properties of electron orbitals). To achieve this, each silicon atom shares each of its four electrons with four other silicon atoms. This sharing of atoms binds the atoms to each other, and these bonds are called "covalent" bonds. These covalent bonds cause the silicon atoms to form a very stable silicon crystal. Because each of these other four atoms also each share one of their electrons with the original atom, our original atom gets to use eight electrons, and so achieves the stable configuration it likes.   

Because all the valence electrons are involved in the covalent bonds, they can't move from one atom to another, and therefore a pure silicon crystal is a very bad conductor of electricity.

However, we can make the silicon crystal conduct electricity with a sneaky trick: we add a small number of phosphorous atoms to the silicon crystal. Each phosphorous atom has five electrons in its valence shell, instead of four. But only four of these electrons are needed to bond with four nearby silicon atoms, so the fifth one is left over. Because it is not involved in a bond, it is can move much more freely through the silicon.

This process of adding another element is called "doping". As we have seen, when phosphorous is the dopant, extra electrons are added. Because electrons have a negative charge, we call the doped material "n-material", where the n stands for the negative charge of the electrons. Its important to keep in mind that n-material doesn't have a net negative charge, because the nucleus of the phosphorous atoms have an extra proton as well (relative to silicon), and this balances out the extra electrons. What the n-material does have that the silicon doesn't have is charge carriers that can move, and so can conduct electricity.

Another way that charge carriers can be added to the silicon is to add an element such as boron, which has only three instead of four electrons in its valence shell. The doped silicon crystal that results will then have electron vacancies in its structure, called "holes". These holes can actually move, because nearby electrons can fill these holes, leaving behind a new hole nearby. This kind of material is called p-material, where p stands for positive, because we may think of the holes as having a positive charge. 

The electron-hole concept may seem a little tricky at first. The simplest way to think of it is simply that in the p-material, the electrons can't move unless other electrons move out of their way. A hole is simply the space created by an electron moving out of the way.

In any case, for either p or n type material, electrons can move, so that electricity can be conducted.

When the two types of material are brought together, say, with the n-material on the top, a very interesting thing happens. Some of the extra, mobile electrons in the n-material migrate over into the p-material and fill some of the holes there. This makes the upper layer of the p-material negatively charged, while the nearby n-material now lacks electrons and becomes positively charged. In the diagram below, these charges are symbolized with minus signs (for the negative charges), and plus signs (for the positive charges).

These charges create an electric field, or voltage, across the junction of the two wafers, called a p-n junction, balances (stops) further (net) migration. This electric field remains permanently "built-in".

When there is no sunlight shining on the material, there is no net movement of electrons in the material, despite the fact that there is an electric field inside the material.

When photons of light strikes the material, however, some normally non-mobile electrons in the material absorb the photons, and become mobile by virtue of their increased energy. This creates new holes too - which are just the vacancies created by the newly created mobile electrons. Because of the "built in" electric field, the new mobile electrons in the n-material cannot cross over into the p-material. In fact, if they are created near or in the junction where the electric field exists, they are pushed by the field towards the upper surface of the n-material (such an event is shown in the diagram below). If a wire is connected from the n-material to the p-material, however, they can flow through the wire, and deliver their energy to a load. 

On the other hand, the holes created in the n-material, which are positively charged, are pushed over into the p-material. In fact, what is really happening here is that an electron from the p-material, which was also made mobile by the adsorption of a photon, is pushed by the electric field across the junction and into the n-material to fill the newly created hole. This completes the circuit - we now see that there are electrons flowing all the way around the circuit, dropping the energy they acquired from photons at a load. 

The crucial step in the whole process is that just described - the pushing of mobile electrons across the p-n junction. This suggests a nice way to think of the PV process - like a tennis player making an overhead serve:

1. First, an electron absorbs a photon and become mobile. This is like the first step in a tennis players serve, when they throw the ball upwards into the air.

2. Secondly, the built in electric field pushes the electron into the n-material. This is like the tennis racket crashing into the ball, and accelerating across the net.

Here is a diagram showing the whole process:

How PV cells are packaged 

Each individual pv cell is about 1/2 inch to 4 inches in size, and can produce from 1 to 2 watts of power. To produce more power, many cells are electricity wired together into a larger, weather-tight modules, which usually have an aluminum frame. These modules can be further connected to form an array.  In the field of photovoltaics, the term array refers to the entire set of modules an installation uses, whether it is made up of one or several thousand modules.  

PV solar cells can be used singly in small applications such as calculators, or they can be bundled together into "PV solar panels", which can be used for generating arbitrary large amounts of electricity. A typical house in the sunbelt can be powered with about 200 square feet of solar panels, less than the surface area of a typical bedroom!

System Components

The basic components of a PV system are:

• PV panels
• Batteries: Typically about 12 deep-cycle lead acid batteries
• Charge controller: To regulate the charging of the batteries
• Inverter (pictured below): Converts the low voltage DC (direct current) power from the batteries into 110 volt alternating current for use by appliances .

The following diagram shows they are connected together: 

First, the Sun shines on the panels to produce electrical power. That power is routed through a charge controller to the batteries. The charge controller regulates the charging of the batteries - the voltage on the batteries needs to be increased slowly, because charging them too fast or routinely overcharging the batteries quickly degrades them. Next, the inverter converts the dc (direct current) electrical power from the batteries into ac (alternating current) electrical power at 110 volts. This can then be fed to household appliances via a wall socket.

System Costs

The costs of typical PV system range from anywhere between a few thousand for a hunting cabin system, to about ten thousand for a small home, and upwards of $35,000 for a large home (for more details on home PV system costs, see our curriculum project "Calculate the cost of Photovoltaic Systems"). Component costs break down roughly as follows:

• About $5-7 per watt for the panels, so for a typical 2 kilowatt system the panels cost about $10,000-14,000. 
• Several hundred dollars for the charge controller.
• About $1 per watt for the inverter: a typical 2 kilowatt system would therefore need a $2000 inverter
• About $100 per kilowatt-hour of energy storage: a typical 2 kilowatt system might require 20 kwh of storage (only 10 kwh in active use to extend battery life) and therefore about $2000 worth of batteries. 

Today's crystalline PV panels have a very long lifetime, at least 25 years, and possibly much longer. This is because crystalline silicon is very stable (silicon crystals can remain intact on geological time scales). The primary cause of failure is due to degradation of the transparent laminates that protect the cells from the elements, and from problems such as broken contacts. 

Today's batteries typically last 3-10 years before they need to be replaced. Fortunately, US law requires that the batteries be recycled. Many solar enthusiasts are hopeful that energy storage systems using hydrogen fuel cells will become available in coming decades to replace the need for short-lived batteries. You may want to see our materials on electrolysis and fuel cells to see how this might be accomplished.

A common myth, probably promoted by those who oppose the development of renewable energy for competitive reasons, is that PV panels take more energy to manufacture than they produce. In fact, PV panels typically pay back their energy in 2 to 5 years, depending on the available sunlight. 

Here are some Quick Facts About PV from the US Department of Energy Photovoltaics Program (http://www.nrel.gov/ncpv/):

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