Theory/How Solar Works 2: Bandgaps
In Lesson 1 of this "Photovoltaics for Dummies" class, we looked at how light consists of photons (little bundles of light energy) and how the annual energy we receive from the Sun dwarfs all the energy we use in a year. In this lesson, we will show you how solar cells convert light energy into electricity. There's a couple of things you should know before we start; and they're all basic high school physics (did you pay good attention there??). We will start out with the molecular structure of the most common solar cells and how they are 'tweeked' in order to be able to generate electricity from photons.
Molecular structure of Silicon solar cells
The most common solar cells around are silicon solar cells. Silicon is an atom that can share four electrons (aka valence electrons) with other atoms. As you might remember from physics class, atoms consist of a core (with protons and neutrons in it) and an 'electron cloud' where electrons fly around in orbits. In crystalline silicon solar cells, these silicon atoms are arranged so that each one of them connects with four other silicon atoms around it (so each valence electron is shared with another silicon atom). The material is so pure, that only 0.0001% of the atoms is not silicon. The silicon is then 'tweeked' by introducing impurities on the upper and lower side of the cell (also called 'doping'). The impurities are atoms that are put in the structure of Silicon who have a different number of valence electrons. In the picture below, you see how the top of the cell is doped with boron (3 valence electrons) and the bottom has phosphorus (5 valence electrons).
Because the boron atom has only three valence electrons, it misses an electron to bond with one of the silicon atoms next to it. This side of the cell is therefore called the positive side or P-type semiconductor. Similarly on the other side, the 4-valence silicon is doped with 5-valence phosphorus atoms. This side therefore has free electrons, and is called N-type semiconductor.
The magic happens when P-type and N-type semiconductors are brought in connection. The free electrons and open spots of electrons (called electron-holes) jump across the junction in an instant. They do this because their concentration there is much lower. It's similar to how an oil spill spreads by diffusion. The region in the middle where the material is not doped is called the depleted region. Because one side of the depleted region becomes negative (electron rich) and the other side positive (electron poor), an electric field is formed across the depleted region. So if an electron would find itself in the depleted region, it would be pulled towards the positive side. It would be almost impossible for electrons to travel from the negative side to the positive side.
Generation of electron-hole pairs
So how can we generate an electron (electricity) from an incoming photon? This happens when a photon goes into the cell and bounces into a valence electron that was part of the silicon molecular structure. Since valence electrons are in the outer orbit of the electron cloud, only a little bit of energy is needed to make it loose and allow it to travel freely. By leaving the organized structure of the silicon, the electron leaves a 'hole' behind. This is called the generation of an electron-hole pair. Once the electron is free, the electric field pulls it towards the P-side and the hole is pulled towards the N-side. All the free electrons forced to one side of the cell form the current. And the current combined with the voltage of the electric field gives Power!
Not all the photons can create an electron-hole pair. Only the photons that have enough energy to knock an electron out of its place can do that. As we've seen in lesson 1, photons carry a tiny amount of energy (in electronvolts or: eV). The amount of energy that is required the knock an electron off a Silicon atom (Si) is 1.1eV. This is called the 'Bandgap'. Every type of solar cell has its own bandgap. You can see that in the figure below.
Only photons with an energy higher than the bandgap energy, can knock off electrons and generate electricity. However, if a photon has 1.7 eV and falls onto a 1.1 eV cell, the excess energy (0.6 eV) will be lost in the form of heat. So there's a trade-off there: if you set the bandgap too high, you don't generate a lot of electrons (current) because few photons have so much energy. However, a bandgap too low will generate a lot of electrons, but most of the energy is lost in the form of heat. In the graph, you can see the result of this trade-off, as there is a peak in the theoretical possible efficiency.
Here's two Youtube videos that explain it in a more visual way.