What is a p-n junction as they are used in solar cells?

We've created a blog entry dealing with the p-n junction terminology and functions.

What are the generations of solar cells?

First Generation: This term refers to the classic p-n junction photovoltaic. Typically, this is made from Silicon (multicrystalline and single crystalline) doped with other elements to make them preferentially positive (p) or negative (n) with respect to electronic charge carriers. However, in the past these devices were made from other materials like Germanium as well.

Second Generation: Thin films of photon-absorbers and layered stacks of thin films. These families of devices are working toward the purest, most efficient capture of light and conversion to electricity. They are elegant and artful, and also delicate and difficult to scale up to industrial levels of production. If the First Gen cells can be viewed as analogous to Microsoft (in that they work, but not optimally), then these varieties are surely the Macintosh-version of solar cells. The materials used in these cells are often designer semiconductor films, and can combine multiple light absorbing materials in a "stack" of films, with each absorbing a slightly different range of light wavelengths than the one below it. While they have been made in the lab and successfully applied in space technologies, they have not gained public attention in the way that Si has. They may prove to be much more efficient in solar energy conversion, but they are much more expensive than a simple silicon wafer. In addition, these films have provided a much more complex structure that steps away from a traditional p-n junction to describe charge carrier generation and separation.

Third Generation: The wild west! An open territory characterized by monsters of photovoltaic science. This can include dye-sensitized cells, polymer-fullerene cells, ETA cells (for Extremely Thin Absorber, not the controversial Basque resistance group). Most of the time these are broadly referred to them as "non"-p-n junction photovoltaics, because the old models of p-n semiconductors required a mesoscopic electric field to induce charge carrier "drift", and these devices are a bit more intricate. Some parts of this research are avoided by traditional PV materials scientists and physicists because there is a lot to be learned about the new rules of photovoltaics here that is closer to chemistry.

What are "advanced photovoltaics"?

An arbitrary definition, but we are deferring to the US government to answer this one. If you view the DOE Energy Efficiency and Renewable Energy website, advanced photovoltaics or advanced solar cells can be equated with Third Generation solar cells.

What about thin film solar cells (e.g.: CIGS, or CuInGaSe), aren't they pretty advanced?

We completely agree that multi-thin film solar cells like the CIGS system are advanced solar cell technologies. However, at the moment this wiki presents background information based on third generation solar cells. The AP wiki is not affiliated with the DOE or its subsidiary research arms, but it seemed like a good point of reference for folks on the street and science educators. See Second Generation definition for more information.

Where can I find out more information about the traditional solar cells like the ones I can buy now?

I recommend the Energy Efficiency and Renewable Energy website and Photovoltaic Fundamentals from the Florida Solar Energy Center. Both have accurate descriptions of currently available technologies.

Have a sunny day!