Photovoltaic Cells

Photovoltaic (PV) cells are made of special materials called semiconductors, such as silicon. Basically, when light strikes the cell, a certain portion of the light’s energy is absorbed within the semiconductor material. The energy knocks electrons loose, allowing them to flow freely. PV cells have one or more electric fields that act to force the electrons freed by light absorption to flow in a certain direction. N-type silicon ("n" for negative) has free electrons. P-type silicon ("p" for positive) has free holes. Holes really are just the absence of electrons. When N-type and P-type silicon come into contact, an electric field forms within the PV cell. Suddenly, the free electrons in the N side, which have been looking all over for holes to fall into, see all the free holes on the P side, and there's a mad rush to fill them in. This mad rush of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, you can use that current to power external devices.

To see how that current is actually put into practice, let's consider a solar yard light. Basically, a solar yard light generates and stores its own power during the day and then releases it at night from a battery. A typical solar yard light consists of a plastic case, a few solar cells wired together in a series, a single AA nickel-cadmium (NiCad) battery, a small controller board, a light-emitting diode (LED) light source and a photoresistor to detect darkness. The solar cells are wired directly to the battery through a diode. The NiCad battery produces about 1.2 volts and can store a maximum of approximately 700 milliamp-hours. Except on short winter days or on days that are heavily overcast, the battery can reach maximum charge during daylight hours.

Image courtesy © Lester Lefkowitz/Stone/Getty

At night, the solar cells stop producing power. The photoresistor turns on the LED. The controller board accepts power from the solar cell and battery, as well as input from the photoresistor. It has a three-transistor circuit that turns on the LED when the photoresistor indicates darkness. The LED draws about 45 milliamps with the battery producing about 0.055 watts; about half of the light a candle would produce. It's not super bright, but it is enough to mark a trail. So, what if you want to power up something bigger – a lot bigger – than a solar yard light? That's where silicon-wafer solar panels come into play. Maybe you've seen them -- they look like black-paned rectangles. While these panels do work, there is room for improvement – especially when it comes to cost. In fact, many would argue that the biggest barrier to widespread adoption of solar technology has been the high price tag. Enter the newest solar cell on the block: the thin-film photovoltaic (PV) cell.

Thin-film PV Cells
Traditional silicon-wafer solar panels require a complex, time-consuming manufacturing process that drives up the per-watt cost of electricity. Non-silicon thin-film solar cells are much easier to manufacture – some are made using a process that resembles offset printing. The presses used in semiconductor printing are easy to use and maintain. Not only that, very little raw material is wasted. This contributes to the overall efficiency of the process and drives down the cost of the electricity generated by the solar panels. Electricity from traditional solar panels costs about $3 per watt. Conventional wisdom suggests that solar will not be competitive until it can produce electricity at $1 per watt. One thin-film solar cell manufacturer, Nanosolar, claims that it can reduce the cost of making electricity from sunlight to a mere 30 cents per watt.

If thin-film solar cells achieve their full potential, it's easy to imagine a future where solar power is as ever-present as, well, sunlight. Thin-film cells could blanket rooftops or form building fronts and storefronts throughout towns and cities everywhere. And it's possible they could help power a new generation of solar cars and trucks. It's been reported that Toyota plans to top some 2010 Prius models with solar panels. Although, right now, it looks like they're going with old-school solar panels.

Image courtesy University of Michigan Solar Car Team University of Michigan solar car Continuum

Racing with the Sun The North American Solar Challenge (NASC) is a competitive race with a goal -- a solar goal. College teams from all over North America rally against each other to create and race a winning solar vehicle. Twenty-four teams participated in the 2008 NASC, traveling 2,400 miles from Plano, Texas to Calgary, Alberta, Canada. The University of Michigan and their car "Continuum" retained bragging rights again this year; it was their fifth win in nine races.