It’s obvious to point out that generating renewable energy is hugely important, and one way of doing that is to make electricity using solar cells. Solar cells turn the energy carried by light into an electrical current, which can directly power a device, be connected into the grid, or be stored in a battery for future use. Understanding how solar cells work depends on the principles of quantum mechanics that I’ve already written about on this blog, so the middle of this long, dark, northern winter is the perfect time to think about it and dream of all that sunlight!
It’s possible to understand how a solar works by looking at band structure. I’ve written about band structure before, so feel free to read that post for more details. To briefly recap, quantum particles can only exist in certain allowed states and the band structure is essentially a map of these states in a crystal material. Electrons fill up all these states starting from the lowest energy, but there are usually more possible states than there are electrons to fill them, meaning that some of the higher energy states are not filled. The energy of the last filled state or first empty state is called the Fermi level.
Absorption of light
At the fundamental level, light is also made up of quantum particles, called photons. The amount of energy that is carried by a photon is directly related to the wavelength (or colour) of the light. When a photon hits an object it can be absorbed, but the energy that it carries cannot be created or destroyed, so it must be transferred into the material.
It just so happens that the amount of energy that is carried by a photon of visible light is in the same range as the energy spacing between the quantum-mechanical states in lots of crystals. This is why most materials are opaque: they are good at absorbing photons.
The energy absorbed by the material is accounted for by an electron changing its state and gaining energy in the process. I’ve tried to show this in the sketch above. A photon (green wavy line) plus an electron below the Fermi level (black circles) becomes an electron above the Fermi level. There is also a space left below the Fermi level from which the absorbing electron came (the hollow circle). This space is called a hole.
How to capture electrons
A solar cell converts light into electrical current by capturing the excited electron and hole. But this process can be quite difficult to engineer because naturally electrons (and everything else) will tend to rearrange so that they lower their energy. For the electron, this is most obviously done by “falling” back down into the state that it left, while emitting another photon to make sure that energy is conserved. So, the solar cell must have a way of driving the electron and hole apart from each other so that they can be captured before this recombination happens.
One way that can be done is to make a structure that has different band structure in different places, shown in the sketch below. There, a solar cell device is shown on the top, and the band structure of three different regions is below. The left-hand end of the solar cell is made so that it is “p-type” meaning that it has an excess of positively charged holes. Another way of saying this is that the Fermi level is in the valence band. The right-hand end is “n-type” meaning that it has extra negative electrons, or the Fermi level is in the conduction band.
The electron-hole pairs that are formed in either the p-type or n-type regions will recombine very quickly, but those that are made in the zone in between (called a “p-n junction”, highlighted by the dashed box) might not. Another way in which the excited electron can lose energy is by moving into an unoccupied state in the n-type region (shown by the blue arrow). Simultaneously, electrons in the valence band of the p-type region can lose energy by filling in the hole in the junction region. This process is equivalent to the hole moving to the left, towards the p-type region (red arrow).
This moving charge is exactly the current that the solar cell is designed to generate, and it can be collected by attaching wires (or “contacts”) at the ends (brown areas).
There are a few things that can be optimised to make solar cells more efficient. The obvious thing is to use materials which absorb a lot of photons, so finding a material that has energy transitions at lots of different energies (corresponding to a lot of different photon wave lengths) is very important. Then, the recombination time can be increased so that the electrons and holes have more time to move apart from each other. Lastly, using a material that has a good electrical conductivity will allow the electrons to move faster and so can get more separation from the holes within the recombination time. This is a massive industry, and even small gains in efficiency can be worth a lot of money!
As a little coda, there is another electronic device which can be understood from this kind of thinking. Instead of absorbing light and creating current, an LED does the opposite: It uses current flowing through it to emit light.
Electrons moving through the p-n junction have to lose energy to get from the n-type region to the p-type region and they can do this by emitting photons – the inverse of the absorption process. By changing the energy spacing between the levels that the electron has to move between, the colour of the emitted light can be changed. Of course, there is a bit of detail which I am leaving out here, but it’s kinda neat that an LED is like a solar cell running in reverse!