Random Ramblings of Everyday Life

What are Solar Cells?

Posted in Guides by ziptrickhead on July 16, 2010

Solar cells are devices that convert energy from light into electricity. They accomplish this through the photovoltaic effect. Solar cells are specific in that they generated electricity from captures sunlight while general photovoltaic devices convert any light into electricity.

The most commonly known solar cells are created from doping silicon into 2 different layers, a p-type (positive) and n-type (negative) semiconductor. When the p and n type silicon is placed in contact with each other, there is a diffusion of electrons from the region with a higher electron density (the n-type material) to the region with a lower electron density (the p-type material). As the electrons diffuse from the negative side they recombine with the holes in the positive side. This diffusion of electrons has a limit however because when there is a built up of charges on either side of the junction, an electric field is formed, and the junction region is effectively turned into a depletion region (like the name implies, the region is depleted of all mobile charge carriers).

If any astute readers are getting a feeling of déjà vu, that is because this process is very similar to that of a diode which was briefly described in our article Introduction to LEDs (Light-Emitting Diodes). It is this generated electric field that transforms p-n junctions into diodes. LED’s, or any diode for that matter, can not only produce light and heat, but they can also produce electricity. It is just that different diodes are built in certain ways to optimize whatever their function is. In an LED’s case, its function is to produce light with as little heat as possible. So when a forward bias voltage is applied to the LED (think positive end of a battery connected to the positive end of the LED and the negative connected to the negative), light is generated. The opposite of that would be a photodiode. Photodiode are diodes that detect when light hits the surface of the diode and produces a current, or electricity. When a photodiode is operated in photovoltaic mode (zero bias voltage), is essentially is a solar cell.

When a photodiode us run in photovoltaic mode, the lack of a bias voltage causes the flow of photocurrent (current generated by photons from sunlight) to be restricted. This results in a build up of voltage. Once there is a significant voltage built up, the diode effectively becomes forward biased by itself and dark current (electrical current that flows through a photosensitive device even when no protons are entering the device) begins to flow across the junction in the opposite direction of the photocurrent. It’s confusing, but the rule of thumb is that electrons flow in the opposite direction of current.

There is currently a lot of research being done to make solar cells cheaper and more efficient. As efficiency and production costs decrease, this means an overall decrease in the price of solar technology for the consumer.

One topic that has been researched constantly is how to reduce the costs of obtaining pure silicon. Technically, silicon is everywhere around us. The problem is that it is usually bound to other things and must be processed. The current method is to produce a reaction between carbon and silica in very high temperatures, which accounts for the high energy process required to produce silicon. This also generates a lot of carbon dioxide.

Another approach is to just use less silicon. Silicon wafers are produced and then sliced into very thin layers. This produces many slices of silicon with the same surface dimensions as the original wafer. Basically, say you have a 1 foot by 1 foot piece of silicon that is 5 inches thick. If you slice the silicon into 1 inch thick pieces, you have effectively increased the surface area of silicon by 5 times. Since this solar conversion process depends mostly on the surface area exposed to sunlight, you increase the effectiveness of the same amount of silicon.

Another process being researched is thin-film processing. This promises to use less than 1% of the raw materials compared to wafer-based solar cells. This leads to a price drop per Watt capacity. This process involves depositing thin crystalline silicon films on glass or some other material. That surface material gives the crystalline silicon structure, making a more robust package compared to the sliced silicon wafers.

The most interesting to me is infrared solar cells. The Earth continuously emits heat absorbed throughout the day from sunlight. Infrared solar cells allow that energy to be harvest to produce electricity, even when there is no sunlight available. That would answer the old question, “What happens when the Sun goes down?”

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