Q & A/Mulligan: Solar Panels

In response to my post the other day about photovoltaic cells, I received a number of responses indicating I had done an inadequate job explaining the technology. One dear friend wrote:

okay, I believe you because I trust you, but I don't totally comprehend the concept of how PV cells generate energy. It seems like scientists are trying to fool me into believing some gobbledygook they made up.

So here goes a more complete-- and hopefully better-- explanation:

The most basic idea with a solar cell is that it works by using energy from the sun, in the form of photons, to create electricity. In order to do this, engineers start off by putting two layers of "doped" silicon in contact with each other-- one of these layers is positively-charged; the other is negatively-charged.

The positively-charged layer (p-layer) has a lot of "holes"-- places where electrons could be if there were electrons available. The n-layer, on the other hand, has a lot of electrons that are extra-- you can think of them like a bunch of understudies on an acting set (they have a role, but they're really just waiting around to jump into a more stable, more important role).

When the two layers come together, the extra electrons on the n-side, close to the barrier, rush to fill the "holes" on the p-side. On the p-side, the holes migrate over to the n-side. This happens really fast, until equilibrium is reached, right at the boundary between the two sides (called the p-n junction):



Because the n-layer now has this positive charge right by the barrier, it takes too much energy for more electrons to move from the n-side to the p-side. However, electrons can move easily from the p-side to the n-side.

Of course, once the equilibrium is reached when the two layers are put together, electrons aren't going to keep moving around on their own-- they need energy to push them. Here's where photons come into play: photons are a form of energy, and when they strike the layers of silicon, they transfer that energy to electrons and dislodge them from their place. Electrons on the p-side cross the barrier to the n-side, but the electrons on the n-side can't cross to the p-side, so they get knocked around the n-side, looking for somewhere to go.



In a solar cell, some of those electrons end up in metal conducting strips (remember those thin silver lines on the PV cell on your calculator?) in the solar panel, which move the electrons out of the cell and into a wire-- generating electricity!



The solar panel is a closed-loop system, meaning that the electrons that are lost through the metal conducting strips and into the wire travel aren't lost forever-- after doing their job generating electricity, they travel back through the wire and are picked up by a conductive metal backing on the back of the solar panel, where they transfer back to the p-layer.

The whole solar panel has a number of parts-- we've mentioned the two layers of silicon, the metal conducting bands, and the metal backing. The top of the panel is also covered in an anti-reflective coating so that the panel can absorb as much light as possible, and the whole thing is covered in a layer of protective glass:



If you're with me to this point, you can see why solar panels are fairly inefficient producers of energy, even though the sun is so powerful. Only certain levels of energy from the sun knock the electrons in the silicon out of place; also, of the electrons that do start to fly around, only a few of them make it into the metal conducting strips.