# How do solar cells work?

This question already had some answers in English, but I think I can add some.When I did my PhD research on the workings of solar cells (especially cells made of silicon), I noticed that there was actually quite a bit of discussion in the field about what is going on right now. Don’t get me wrong: We have all the mathematical equations to describe how solar cells work and everyone is in that area once with each other. Unfortunately, these equations are strongly non-linear (or: Difficult) partial (even worse) differential equations.When you try to design a solar cell, there are many buttons that you can rotate. That’s why it’s good to have a kind of intuitive model about solar cells that lets you elaborate your ideas before trying to figure out a cell design. These intuitive models are very useful when you exchange ideas with other scientists, because you can’t count everything that comes into your mind every time.

The heuristic model I attach the most value to is that of Peter W眉rfel that he sets out in his book Physics of Solar Cells.It is fairly new and in the time of my promotion it was slowly gaining popularity. This is the model I want to explain below, because it is so simple and elegant.

First of all, there are a number of background issues that we need to understand.The basis of (almost) every solar cell is an absorption material such as silicon.When light falls on this material, the light particles (photons) are absorbed and converted into two load-bearing particles: a negatively charged electron and a positively charged electron beam (usually called a hole).Together, these two particles are also called an electron-hole pair. Technically, the hole is not really a particle, but in practice it is much more convenient to treat it as such. Both particles can move more or less independently from each other freely by the silicon. If you leave them alone for a long time, the electrons and holes will come back to each other and disappear when they collide against each other (where the pair is converted into a photon, heat, or both), but this recombination process is Slower than you might think.In high quality Silicon This takes several milliseconds, which on the scale of atoms and electrons is truly an eternity.

In short, yes, the absorption material of a solar cell can be seen as a type of sealed box in which molecules of 2 different gases fly around.The only difference is that you always have as many electrons as holes because they come in pairs and disappear again. When your light shines on the box you generate more and more particles and the pressure in the box increases. If you leave the box with rest again, the particles disappear gradually.

So now we have a box in which electrons and holes bounce around, but that doesn’t give us any power.If your power wires would connect to a slab of silicon and light appears on it, then the electrons and the holes both enter the wires and nothing happens. What you want is that the electrons all go into one thread and the holes the other. How do you get it? The answer is simple: you put filters for the wires that only pass one type of particle. In chemistry and biology, such filters are called semipermeable membranes and that is what we need.

So, how do you make semipermeable membranes for electrons and holes?Now we can make use of the fact that the two load carriers live in different energy tires.I’m not going to explain this in detail, but what it comes down to is that we can change the properties of silicon independently for electrons and holes. In this case, it is especially important that we can do this for the resistance of the material: it is e.g. Possible to give a piece of silicon a lot of resistance for holes, but at the same time it is very good to guide electrons. In this way we can make selective contacts .Try to imagine that you have a material that led electrons just as well as copper, but for holes a resistor has as big as glass. In silicon you can get this done by adding impurities to the material in a process called dosing .However, this is not the only way: You can also choose to put materials with specific properties on the silicon to make selective contacts in this way. In that case, we are talking about a heterojunction.

In a sense we are now working to give the electrons and holes an alternative way to recombinate.As I said before, recombination in silicon is a slow process and thus try to entice the electrons and holes to go through the contact wires so that they can “recombinate” (although that is actually not really the right term unless we leave a LED Fires) in the device what we want to power. The next picture from my dissertation summarizes it all schematically:

[Math蟽 _ {N, p} [/math: Electron/gat conductivity
[mathJ_ {n, p} [/math: Electron/hole flow
[math纬 [/math: Photon

And that’s it more or less.

Anyone who is a little familiar with solar cell physics may now notice that I have not talked about electric fields and I have not accidentally forgotten them.

Many textbooks on semiconductor physics begin by telling that a solar cell is a semiconductor device with a PN transition that generates an electric field. If there is light on the cell, load carriers are created (as I described earlier) which are then pumped around the electric field. Now it is certainly true that there is an electric field in PN transitions and that electric fields exert forces on charged particles, but if you stop your story there (and that is exactly what is usually done in the popular scientific explanation of solar cells ), you are missing an important part of the explanation.

Most textbooks then continue with a treatise on drift and diffusion currents (which in itself is also a weird picture, because it is a bit odd to apply the superposition principle to currents rather than forces, but that aside…) With which I do not want to bother my readers.Instead, let’s look at the story with the electric fields for a moment, because something is not correct: it does not strike physics anywhere. Electric fields are, what physicists call, conservative.This means that if you allow an electron to pass through an electric field and then put it back on its original spot that there is no net loss or gain in energy. It’s like when you’re going to take a bike ride: no matter how you go, Gravity won’t give you a free ride. When you go down a hill, you will also have to stop on the way back. Gravity is Conservative.Please do not calculate gravitation on its political preferences.

The same applies to electric fields.When a solar cell propel some apparatus, electrons go around and around (because a circuit must be closed) and it always goes through the cell. In the device the electrons lose energy to keep it going. This is similar to cycling with slack tires: It takes a lot of energy to get ahead at all.

So the problem is that if you say that the electric field in the solar cell is powering the power, then you do not do anything like claiming that you can go downhill cycling on slack tires without pedalling.In other words: You have created a perpetuum mobile ; Something that is totally impossible.

The electric field in a solar cell is, which is so nicely called in English, a red herring; Something that distracts attention from what is really important. Of course, the field has something to do with the operation of a solar cell, but certainly does not give any explanation.It is even possible to design a solar cell in which electric fields are barely present (as W眉rfel has demonstrated). If you really want to understand a solar cell, it is much more important to think about the pressure of the gas of electron-hole pairs in the absorption material and the selective resistances of the contacts.

Now that I’m working on explaining solar cells on the basis of bike rides (how crooked the equation might be), I can just as well finish it completely.If you are proposing a solar cell as a bike ride, it’s like having a steady wind in the back and blowing it so hard you don’t have to pedal. Wind is simply air that is pumped around by the sent. Now it turns out that you have stepped up while a big hurricane is underway:

Because a hurricane is a large vertebra, you could basically blow around and return to where you started.

Wind is not a conservative force field and can only be maintained by external energy coming from the sun.