Photovoltaic Cells – Generating electricity

We’ve come a long way to gain an understanding of semi-conductors to see how they relate to making solar cells. A solar cell is essential a PN junction with a large surface area. The N-type material is keep thin to allow light to pass through to the PN junction.
Light travels in packets of energy called photons. The generation of electric current happens inside the depletion zone of the PN junction. The depletion region as explained previously with the diode is the area around the PN junction where the electrons from the N-type silicon, have diffused into the holes of the P-type material. When a photon of light is absorbed by one of these atoms in the N-Type silicon it will dislodge an electron, creating a free electron and a hole. The free electron and hole has sufficient energy to jump out of the depletion zone. If a wire is connected from the cathode (N-type silicon) to the anode (P-type silicon) electrons will flow through the wire. The electron is attracted to the positive charge of the P-type material and travels through the external load (meter) creating a flow of electric current. The hole created by the dislodged electron is attracted to the negative charge of N-type material and migrates to the back electrical contact. As the electron enters the P-type silicon from the back electrical contact it combines with the hole restoring the electrical neutrality.

LED’s

A Light Emitting diode (LED) is very similar to the standard diode we already looked at. LED’s are made to emit light at the PN junction. When forward-biased the excited electrons from the N-type silicon combine with the holes in the P-Type silicone emit photons of light. Typically LED’s only emit one color of light. The manufacturer can adjust the frequency of the emitted light from an LED from infrared to ultraviolet.
Using a bank of parallel LED's to generate electric power from light.

What is not commonly known is that most PN junctions are photovoltaic. While solar cells are made with a large area PN junction, a LED has only a small surface area in comparison. We can show the photovoltaic effect by wiring 10 LED’s in parallel. When exposed to sunlight, the LED’s will clearly generate electric current. See photograph. The ten LED’s will not generate as much electric power as a solar cell, but it does demonstrate the photovoltaic property of the PN junction.



source:
www.imagesco .com

Photovoltaic energy efficiency record

A photovoltaic cell that reaches record-breaking efficiency could make solar energy competitive with fossil fuels, says the company that created the cell.
Alta Devices, a start-up in Santa Clara, Calif., presented research at the 37th IEEE Photovoltaic Specialist Conference, in Seattle, this week that claims its thin-film gallium-arsenide cell can convert 27.6 percent of the sunlight striking the cell into electricity, under standardized conditions. Since the paper was submitted, the company says it has upped the efficiency to 28.2 percent. That beats the previous record of 26.4 percent for a solar cell with a single p-n junction, which was the first improvement in years over 26.1 percent. Both numbers, according to Alta, were independently confirmed by the National Renewable Energy Laboratory.
The efficiency was measured on a laboratory-made solar cell. Efficiency tends to decrease once the cells are packaged into usable modules. "We assume we will ultimately be able to achieve modules that are around 26 percent, and that’s plenty to be competitive with fossil fuels," says Christopher Norris, CEO of Alta.
The key to achieving the record was photon recycling. When the photons in sunlight are absorbed in a photovoltaic material, they kick electrons into the conduction band and leave behind holes. The electrons that pass out of the cell can be used as electricity, but many of them are lost in the semiconductor when they recombine with a hole to produce either waste heat or a new photon. By carefully growing a high-quality single crystal of gallium arsenide, the company managed to ensure that more than 99 percent of the recombinations would result in new photons. Those photons could then create a new electron-hole pair and give the electron another chance to be captured as electricity. The Alta team also improved the reflectivity of the metal contacts on the back of the solar cell, so that any photons that exited the cell would be sent back in for possible reabsorption.
The theoretical maximum conversion efficiency for a solar cell with a single junction is 33.5 percent. "We can see a path to 30 percent with our same design right now," says Norris. Adding a second junction could also increase the energy output.
The more efficient a solar cell is, the faster it pays back the cost of manufacturing and installing it. But efficiency and cost have been at odds with each other in solar cell design. Gallium arsenide is naturally better at converting light to electricity than the chief contenders, such as silicon and cadmium telluride, but it tends to be more expensive.
The most efficient materials are single-crystalline semiconductors, but those are usually pricier. Low-cost materials, such as amorphous silicon, cadmium telluride, and copper indium gallium selenide, are less efficient; CdTe cells are around 12 percent efficient. Alta solves this problem by using only a small amount of a high-quality material—a thin film of gallium arsenide about 1 micrometer thick.
"That is the whole trick. Don’t use much gallium and don’t use much arsenic," Norris says. He says an Alta module should cost about the same as a CdTe module but produce three times the energy.
The company cut down on the material cost by using a process called epitaxial liftoff, developed by Eli Yablonovitch, an engineering professor at the University of California, Berkeley, and a cofounder of Alta. Technicians start with a GaAs wafer as a seed layer and grow a thin-film photovoltaic device structure on top of that. They peel off the thin film, attach it to a metal backing, and finish processing it into a solar cell. The process leaves the original wafer, which they can reuse for the next batch of solar cells.
Alta is working on a pilot production line to produce samples of its solar cells sometime this year and expects to have early commercial shipments by late next year, Norris says. The company has raised US $72 million to develop its production process.
This article was updated on 11 August 2011.

source : http://spectrum.ieee.org

Photovoltaic : Barometer on Renewable Energy

In the field of renewable energy in Europe, the EurObserv'ER (www.eurobserv-er.org) ‘Barometer’ assists policy makers to measure the progress made by renewable energies in each Member State of the European Union.
Thematic Barometers are published in a two-months interval and present statistical data on renewables in the preceding year. One of the important features of the Barometer is that the figures are mostly more up-to-date than official statistical offices, because of the direct links to country representatives.
Besides information on realisations, the Barometer discusses the backgrounds of renewable energy policy and it reviews selected countries. Also, industry news is provided.

Link to The bilingual (French/English) EurObserv’ER Barometers are available for download under the page:
http://www.ecn.nl/barometer/