#2: The Mystery of Solar Panels…

Hello everybody!

After my last article, which you can access by clicking here, I realize that I should probably have started with something a bit tamer (instead of jumping straight into photoelectric theory). Now, I’m not going to take that article down, since it’s actually some really interesting stuff. However, for the next few articles, I’m going to try to say a bit more…simple.

Now, don’t be alarmed by this. I’m still gonna explore some really cool stuff on here, stuff that most other energy blogs won’t be talking about. However, it won’t be as intensive as what we did last time since I realized many of you don’t need to go that deep into the weeds. Also, on my own solar energy discovery journey, I think it’s important for me to get a really in-depth look at some of these concepts, in order to understand them as well.

There are also a few of you who will look at this and think it’s too simple, and that’s totally fine (I prefer it actually). If you have anything to contribute, to add, or to build upon, please make a response article, I’m excited to read them. Anyway, I’m not gonna waste too much of your time: today, I’ll be exploring a simple, yet awesome topic. Specifically, we’ll be looking at how solar cells actually work. Enjoy!

Have you ever asked yourself how a solar panel actually works?

Like, it’s kind of weird right? This sheet of metal and glass that you stick up on your roof, and somehow, it creates electricity, even enough to power a home?

Now, in most cases, to find out how something works you crack it open. Like, if you want to know how a computer works, you can open it up, and see each component. After some guidance, you’ll be able to understand why each piece goes where it does, and how it contributes to the full system.

That doesn’t exactly work for a solar panel. If you crack it open (and believe me, people have tried), all you’ll end up with is some metal, and some glass, along with your no-longer-functional panel. Yeah, it sucks but DIY cracking-open-stuff isn’t always the most effective method to understand how things work.

So, for this one, we’re going to need to trust what others have written on the topic, as well as using some good ol’ common sense. But first, if you haven’t read my first article on Einstein’s photoelectric effect, you should. That’ll give you some background information that will help you understand what’s about to come a little bit better.

Okay, so without further ado, let’s get started. A solar panel is essentially composed of three layers. These are the p-type, the n-type, and the depletion zone. Let’s go through each one and try to understand how they work, and why they’re important. But first, a diagram to show you where they’re localized inside of the solar panel.

Got it? Okay, let’s go.

P-Type

Before I explain this one, I should probably point out that a solar cell is made of a semiconducting material. In most cases, a solar cell will be made from silicon. A semiconductor is pretty much a material which is in between an insulator and a conductor–in other words, it conducts electricity well, but not that well.

So, the word used is ‘p-type’, and as it turns out, it’s short for ‘positive-type’. Just from the name alone, we can, therefore, deduce that the p-type material is somehow made positive. How, you ask? Well, quite simply, by doping the silicon with another material.

Now, what’s doing? It’s pretty much taking another material and replacing some of the silicon atoms with atoms of that other materials. In order to make the material more positive than normal, we’re going to want to have fewer electrons than regular silicon. In this case, we should, therefore, use an element which contains three valence electrons, rather than four (which is what silicon contains).

Looking at the periodic table, we clearly have a number of choices to pick from, but one of the most popular elements to use for doping is actually boron. So, we add a little bit of it into our pure silicon semiconductor, and that results in a sheet which is more positive than neutral. Got it? Good. Let’s quickly move one from this.

N-Type

Now, if you know how the p-type works, you know how the n-type works. N-type is short for ‘negative-type’, which indicates that we’re trying to dope the silicon, but the other way around this time.

Instead of doping the silicon sheet with an element containing fewer electrons, we’re going to dope it with an element that contains an excess of electrons. That way, the n-type material will be more negative than a typical sheet of silicon. In most cases, phosphorus is used for doping in the n-type material.

Pretty simple, right? This sheet of n-type silicon, is located at the top of the solar panel, is not actually where electrons get ejected from, however. That is called the depletion zone. What’s that? Let’s find out

Depletion Zone:

As you saw in the diagram above, the depletion is actually sandwiched between the n-type and p-type materials. For the purposes of this simplified explanation, we can refer to this material as being neutral. In reality, it is ever so slightly positive, for a number of reasons regarding the real-world workings of these materials.

Essentially, as we can see in the picture above, the depletion zone is pretty much just a regular layer of silicon. It’s called the depletion zone because…well, we’ll see that soon.

Anyway, to recap, we’ve got three layers. The top layer is slightly negative due to doping, the bottom layer is slightly positive due to doping, and the middle layer is pretty much neutral. We also know that we’ve got an excess of electrons in our top layer, as well as an excess of ‘positive holes’ in our bottom layer. So, the natural question is…how does this setup end up transforming light into electricity?

Well, it’s a simple process. First, you need to understand the photoelectric effect, which was incidentally discovered by Albert Einstein in the early 20th Century. Actually, his Nobel Prize was actually awarded for this discovery, not the formulation of the theory of Relativity. Anyway, if you haven’t seen my full post on the photoelectric effect, you should check it out before continuing.

If you’ve already learned about this, but it’s been a while, I’ve provided a little refresher below (it’s actually some really interesting stuff). If you read my article already, or you have a solid grasp of it, you can just skip the section below.

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Crash Course: The Photoelectric Effect

Okay, so Einstein was one smart cookie. Not only was he able to disprove Newtonian mechanics in certain situations, but he also managed to discover the photoelectric effect to prove the particle nature of light.

So, how does this work? Well, Einstein set up an experiment like the one shown in the image below, in order to see how light interacts with electrons.

Simple enough. It’s just a regular circuit, but instead of being completed, we eventually reach a part where two metal plates are placed parallel to one another, with nothing but air between them (like a pseudo-capacitor).

So, when the battery is turned on, the electrons will start flowing, but eventually gather on one of the plates, unable to move any further. Next, the light comes in.

When light is shone onto the plate with electrons, the light serves to excite them and allow them to start moving across the gap. The electrons can only fully make it across if the energy provided by the light is high enough. Through this experiment, Einstein made a number of key observations, proving the particle nature of light.

1. Given that the intensity of light is proportional to the amplitude of a wave, as given by the formula

1. that means that increasing the intensity of the light would also increase the amplitude, and hence increase the energy. This would mean that average kinetic energy of emitted electrons should increase as intensity is increased. However, this didn’t happen. Instead, the increasing intensity increased the number of electrons emitted, which supported the particle model.
2. According to the wave model, as the light hits the metal sheet, there should be a brief time lag, before the electrons were emitted, as it takes some time for the wave to provide energy to the electrons. However, no time lag was observed, which indicated that it was more likely that energy was distributed in small unitary packets (i.e. photons).
3. When light strikes the sheet, electrons should also be emitted at every frequency of light, though the time lag may vary. However, it was observed that no electrons were omitted at lower frequencies (i.e. lower energies), indicating again that the particle model was correct.

This really was a huge step forward for the particle model developed by De Broglie. Through this, Einstein developed the following equation:

It relates the work function of the metal, o which is the energy required to liberate an electron from the sheet, the maximum energy of the electrons, Ek, and the energy of the light hf. And, what was so shocking about all of this was the fact that there was also other evidence supporting the wave theory. But, we’ll save that for another time.

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Okay, so now that we understand that, we know that photons can serve to excite electrons in a metal. Well, as it turns out, the very same process occurs in a semiconductor material, such as silicon. See where I’m going with this?

When photons strike the panel’s surface, we get a kinda-sorta-version of the photoelectric effect that takes place. Well, more like an exact version of the photoelectric effect. When a photon strikes an electron, it naturally gains energy. After, all a photon is simply a packet of electromagnetic energy, as given by the equation below.

So, what happens at this point. Well, now that the electron has gained a particular amount of energy, which is dependent on the wavelength and velocity of light, it somehow needs to release that energy. It’s not just going to remain energized for no reason, right?

How does the electron lose this energy? It quite simply bumps up to the next energy level. Okay, let’s tap the brakes for a moment here. When we consider a semiconductor like silicon, we need to know that the electrons in the material primarily exist in two energy levels: the valence band and the conduction band.

When no light is hitting the panel, all electrons are located in this valence band. If you’ve ever taken a chemistry class, you’ll recognize this term. Valence simply refers to the outer shell of the atom–the outer electron shell, to be exact.

We saw this concept above, too. Silicon has four valence electrons (four electrons in the outer shell), whereas the materials used to dope it will typically have one more or one fewer electrons in that shell. In physics, instead of talking about shells, we talk about a band–a combination of all the electron shells in all the atoms (since they should be at around the same energy level).

When an electron is excited, it makes a jump from this low-energy valence band, to a higher-energy conduction band. The conduction band is key for a panel to produce electricity. The work conduction means that electrons can flow freely in this band. This contrasts with the valence band, where the electrons were fixed ‘in place’ [okay, for all you chemists out there, yes the electrons are typically always moving around within the shell, and can’t move between the valence shells of other atoms, but I digress].

The energy difference is known as band-gap and depends on the photovoltaic material used. For silicon semiconductors, it can range between 1.1 electron volts and 1.7 electron volts. For crystalline silicon, which is the material used in the vast majority of solar cells, the band-gap is approximately 1.1 eV.

What does this mean? Well, it means that the energy contained in the photon must be greater than or equal to the energy gap between the conduction and valence bands, in order for electricity to actually be produced. Otherwise, the electron does not gain enough energy to jump up to that higher energy level.

So, that’s all good. But, what exactly happens when an electron actually is excited? To find out, let’s take a look at the GIF below.

From the above, we can clearly see that the electrons are not excited from the n-type or p-type materials. Rather, they are all excited inside the central depletion zone. Actually, that’s why that region is called the depletion zone–it is gradually depleted of electrons.

Anyway, as the electron is excited, what happens next? To put it simply, the electron being excited creates an electron-hole pair. In other words, the electron is excited, and there is a corresponding hole where it used to be.

The electron ends up flowing to the n-type material, to join its fellow excess of electrons, whereas the positive ‘hole’ does the same but in the p-type material. Why is this important? Well, because now, the electrons and the holes are separated, but they want to recombine with one another. However, since they can’t simply cross the depletion zone again, they need to find some kind of alternative path.

This is where electricity finally comes into play. If we use a wire to connect the p-type material to the n-type material, as shown in the diagram above, the electrons will quickly begin to flow through that wire. This creates a current, which can be used to provide power to any given item.

The electron does this because it desperately wants to recombine with the positive ‘hole’. Through this alternate path, it is able to do that and therefore provides current to a large array of tools and devices. Simple enough!

Thanks for reading! I hope this was quite informative. I tried to keep certain parts of it simple since I don’t want to overwhelm any of my readers with this stuff. However, there are definitely elaborations that can be made. Why is band-gap dependent on the material? How do we manipulate these solar cells to work differently in a range of natural environments? Questions like these will hopefully be answered in future articles.

I hope this provides a good basis as to the workings of solar cells, I will continue to develop this article the more I learn. However, all materials will be linked strictly to basic solar cell functionality. More specific topics will be dedicated to their own articles. Anyway, I hope you enjoyed! Until next time…see ya!