How solar cells work pdf

The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in , who found that certain materials would produce small amounts of electric current when exposed to light.

In , Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics. The first photovoltaic module was built by Bell Laboratories in It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use.

In the s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the s, photovoltaic technology gained recognition as a source of power for non-space applications. The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell.

Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. A solar cell provides direct current electricity similar to batteries; however battery uses electro-chemical reaction. The generated electricity depends on various factors such as intensity of sunlight, wavelength of sunlight, type and surface area of the solar cell etc.

Ni-Cd cells are rechargeable cells used in satellites and automatic traffic lights. Let us understand solar cell working principle. Protons are positively charged and electrons are negatively charged. Holes are created when electrons move from its position. Holes contain positive charge.

Above this materials anti-reflective layer and protective glass cover is provided. First, what do we do when the sun isn't shining? The thought of living at the whim of the weatherman probably doesn't thrill most people, but three main options can ensure you still have power even if the sun isn't cooperating.

If you want to live completely off the grid, but don't trust your PV panels to supply all the electricity you'll need in a pinch, you can use a backup generator when solar supplies run low. The second stand-alone system involves energy storage in the form of batteries. Unfortunately, batteries can add a lot of cost and maintenance to a PV system, but it's currently a necessity if you want to be completely independent.

The alternative is to connect your house to the utility grid, buying power when you need it and selling it back when you produce more than you use. This way, the utility acts as a practically infinite storage system. Keep in mind though, government regulations vary depending on location and are subject to change.

Your local utility company may or may not be required to participate, and the buyback price can vary greatly. You'll also probably need special equipment to make sure the power you're looking to sell the utility company is compatible with their own. Safety is an issue as well.

The utility has to make sure that if there's a power outage in your neighborhood, your PV system won't continue to feed electricity into power lines that a lineman will think are dead. This is a dangerous situation called islanding , but it can be avoided with an anti-islanding inverter -- something we'll get to on the next page.

If you decide to use batteries instead, keep in mind that they'll have to be maintained, and then replaced after a certain number of years. Most solar panels tend to last about 30 years and improved longevity is certainly one research goal , but batteries just don't have that kind of useful life [source: National Renewable Energy Laboratory ]. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, nonmetallic enclosure for them.

Although several different kinds of batteries are commonly used, the one characteristic they should all have in common is that they are deep-cycle batteries. Unlike your car battery, which is a shallow-cycle battery, deep-cycle batteries can discharge more of their stored energy while still maintaining long life. Car batteries discharge a large current for a very short time -- to start your car -- and are then immediately recharged as you drive.

PV batteries generally have to discharge a smaller current for a longer period of time such as at night or during a power outage , while being charged during the day. The most commonly used deep-cycle batteries are lead-acid batteries both sealed and vented and nickel-cadmium batteries , both of which have various pros and cons. On the next page, we'll dig a little deeper into the components that'll be needed for the sun to start saving you some cash.

The use of batteries requires the installation of another component called a charge controller. Batteries last a lot longer if they aren't overcharged or drained too much. That's what a charge controller does. Once the batteries are fully charged, the charge controller doesn't let current from the PV modules continue to flow into them. Similarly, once the batteries have been drained to a certain predetermined level, controlled by measuring battery voltage, many charge controllers will not allow more current to be drained from the batteries until they have been recharged.

The use of a charge controller is essential for long battery life. The other problem besides energy storage is that the electricity generated by your solar panels, and extracted from your batteries if you choose to use them, is not in the form that's supplied by your utility or used by the electrical appliances in your house.

The electricity generated by a solar system is direct current, so you'll need an inverter to convert it into alternating current. And like we discussed on the last page, apart from switching DC to AC, some inverters are also designed to protect against islanding if your system is hooked up to the power grid.

Most large inverters will allow you to automatically control how your system works. Some PV modules, called AC modules , actually have an inverter already built into each module, eliminating the need for a large, central inverter, and simplifying wiring issues. Throw in the mounting hardware, wiring , junction boxes, grounding equipment, overcurrent protection, DC and AC disconnects and other accessories, and you have yourself a system.

You must follow electrical codes there's a section in the National Electrical Code just for PV , and it's highly recommended that a licensed electrician who has experience with PV systems do the installation. Once installed, a PV system requires very little maintenance especially if no batteries are used , and will provide electricity cleanly and quietly for 20 years or more.

We've talked a lot about how a typical PV system operates, but issues concerning cost-effectiveness which we'll get into more on the next page have spurred endless research efforts aimed at developing and fine-tuning new ways to make solar power increasingly competitive with traditional energy sources.

For example, single-crystal silicon isn't the only material used in PV cells. Second-generation solar cell technology consists of what's known as thin-film solar cells. While they also tend to sacrifice some efficiency, they're simpler and cheaper to produce -- and they become more efficient all the time.

Thin-film solar cells can be made from a variety of materials, including amorphous silicon which has no crystalline structure , gallium arsenide, copper indium diselenide and cadmium telluride. Another strategy for increasing efficiency is to use two or more layers of different materials with different band gaps.

Remember that depending on the substance, photons of varying energies are absorbed. So by stacking higher band gap material on the surface to absorb high-energy photons while allowing lower-energy photons to be absorbed by the lower band gap material beneath , much higher efficiencies can result.

Such cells, called multi-junction cells , can have more than one electric field. Concentrating photovoltaic technology is another promising field of development. Instead of simply collecting and converting a portion of whatever sunlight just happens to shine down and be converted into electricity, concentrating PV systems use the addition of optical equipment like lenses and mirrors to focus greater amounts of solar energy onto highly efficient solar cells.

Although these systems are generally pricier to manufacture, they have a number of advantages over conventional solar panel setups and encourage further research and development efforts. All these different versions of solar cell technology have companies dreaming up applications and products that run the gamut, from solar powered planes and space-based power stations to more everyday items like PV-powered curtains, clothes and laptop cases.

Not even the miniature world of nanoparticles is being left out, and researchers are even exploring the potential for organically produced solar cells. But if photovoltaics are such a wonderful source of free energy, then why doesn't the whole world run on solar power?

Some people have a flawed concept of solar energy. While it's true that sunlight is free, the electricity generated by PV systems is not. There are lots of factors involved in determining whether installing a PV system is worth the price. Near the junction of the two layers, the electrons on one side of the junction n-type layer move into the holes on the other side of the junction p-type layer. This creates an area around the junction, called the depletion zone, in which the electrons fill the holes Fig.

When all the holes are filled with electrons in the depletion zone, the p-type side of the depletion zone where holes were initially present now contains negatively charged ions, and the n-type side of the depletion zone where electrons were present now contains positively charged ions. The presence of these oppositely charged ions creates an internal electric field that prevents electrons in the n-type layer to fill holes in the p-type layer.

If this happens in the electric field, the field will move electrons to the n-type layer and holes to the p-type layer. If you connect the n-type and p-type layers with a metallic wire, the electrons will travel from the n-type layer to the p-type layer by crossing the depletion zone and then go through the external wire back of the n-type layer, creating a flow of electricity. An increasing number of everyday items are powered with the sun, including backpacks, watches, cars, and airplanes.