This article marks the start of a series about renewable technology and their operation. The goal being a concise treatise of this information where anyone can easily pick up the basics. Today’s article will introduce solar cells. Solar cells by themselves are more in-depth than other technology like wind or geothermal, so as a result this article will be continuously updated until complete.
“You can cell me PV Junior!”
The main point to drive home is that the sunlight is converted to electricity when it hits a solar cell. Also note that when you put a bunch of these little solar cells together, you can make the PV panels you see on roofs.
How does light become electricity? If you remember your high school or college chemistry class, every atom has electrons organized into levels. The outermost level, or shell, have the valence electrons; these electrons have more energy than the innermost ones and are the electrons that bond with other atoms.
Wait a second! What do I mean? Look at the picture above. The shells are akin to the orbiting rings around the nucleus (the center with Cu). The dots are electrons. The outermost ring’s electrons are called “valence electrons” which have more energy than electrons closer to the center.
Now say if we wanted to get salt (NaCl). A sodium atom uses its spare electron to bond with the 7 valence electrons on the chlorine atom. Why do they bond? Because they want their outermost shell complete because it makes the atom chemically stable. Here, the sodium atom loses it spare electron and thus is complete while similarly the chlorine atom is complete with 8 electrons.
What do these valence electrons have to do with solar cells? Well, as mentioned before, these electrons have a lot of energy. When light strikes an atom of a solar cell, these electrons get even more energy, and they can become loose from the atom. Think of the above Cu atom’s nucleus as being the sun, and the electrons are the planets. When sunlight hits the valence electrons, they gain enough energy to escape the “orbit” of the atom to enter the so-called conduction band.
Only photons with more energy than the “band gap” will have any chance of loosening these electrons. The band gap is simply the difference in energy between the native valence electron and the energy it needs to reach the conduction band.
Let’s recap so far. Light hits the solar cells. Light is made up of photons. These photons give energy to the valence electrons to escape to the conduction band.
Now, when these electrons leave the outer shell, think in your mind that they leave a “reserved seat” where that electron should be. It’s a lot like an empty hole. In fact, when photons hit a solar cell, not only is an electron energized, but a positively charged hole is left in its place. The purpose behind this positive vs. negative charge will become apparent later in the article.
The flash animation on this page shows how this electron-hole pair interacts to create electricity. The most common PV atom is Silicon. You can see a part of the animation labeled ‘p’ and the other ‘n’. These two together make a p-n junction. Now what’s this? A For Silicon, a ‘p’ type simply means the overall charge of the atom is positive usually done artificially and is called “doping” i.e. add positive charge to a neutral atom. If steroids help you make the analogy that’s fine. Likewise, the ‘n’ type is jacked out on negative charge.
In a ‘p’ type where positive charge dominates, the electron is called the “minority carrier” while the hole is the “majority carrier”. The opposite applies in ‘n’ type materials. Simply think p = positive (hole) and n = negative (electrons).
In the animation in the link, the light (photons) hits the n-type semiconductor (Si). This makes the electron the majority carrier, and the hole the minority carrier. Where the n-type meets the p-type (p-n junction aka. depletion layer), the minority carrier (hole) is swept across to the p-type side. On the other hand, the majority carrier (electron) is repelled by an electric field at the junction.
However, the electron with its negative charge really wants to combine with a positive charge i.e. the hole that went to the p-type side. Opposites attract! How does the electron get there? Simple. We put a wire connected the n-type to the p-type side. The electron (electricity) travels through the wire and recombines on the other side with the hole. Note that there is the possibility that the electron and hole recombine before even running through the wire, dropping the efficiency of the solar cell.
You put light in and you get light out. How bout that?
While traveling through this wire, if we put a light bulb or other load across it, the light will turn on. Thus, we have successfully used a solar cell to create electricity to power a light bulb. The beauty of this process is that since the electron and hole recombine, they can be recreated for the life of the PV panel whenever light hits the panel and producing more electricity in the same way. This whole process of generating a carrier combination, separating them, doing work, and recombining is aptly called “photovoltaic action”.
I realize that solar cells necessarily involve topics like physics and may be confusing. I hope I made it as clear as possible while still introducing the technical lingo. As I mentioned before, this series will be continuously updated to provide a complete yet understandable background on solar cells.
Stay tuned for Part 2 of this series which will delve into performance measurements of solar cells.