How do photovoltaic cells work and why do they require a crystalline structure and what is a crystalline structure?
In: Chemistry
A crystalline structure is when the atoms of a material are organized in an orderly, repeating fashion. Imagine a grid pattern where each atom sits at an intersection of the lines. In reality the shapes are three-dimensional and there are more patterns than simply a grid. In the case of photovoltaic cells, these crystalline structures are called semiconductors.
The photovoltaic effect occurs when light knocks electrons out of their orbits around atomic nuclei and those electrons can make their way from where they “detach” to the negative end of the semiconductor. The crystalline structure is key here. If you were to knock electrons out of their orbits in empty space (or any “messy” atomic structure), they could just go in any direction. The crystalline structure essentially provides a “track” for electrons to ride on so that they only go toward the negative electrode.
There a lot of intricacies I’ve neglected, but I think it’s pretty good. Anyone feel free to chime in if you think my explanation is lacking.
A crystalline structure is just a repeating pattern of how the atoms are arranged in space. This structure has an interesting effect on the fraction of electrons which are able to move freely through the material. Basically, the crystal structure limits the wavelength, direction, and energy that an electronic can move through. Only certain combinations are allowed. It’s kind of like a highway where you can only drive one way in a given lane, and you can’t drive off the road.
So let’s stay with the highway example. Let’s say it’s bumper to bumper traffic going south, no one is moving. And let’s say theres a service road running the same direction, there are no cars, it’s totally empty. The effect of an incoming photon of light is to kick a car that’s not moving in the south bound lane over to the service road. Now this car can move freely. What about the empty spot where that car was? It can get filled by the car behind, and then the one behind it, and so forth. The space moves in the opposite direction. In solar cells, the cars are the electrons and are negatively charged. The empty spot is a missing electrons, we call it a ‘hole’ and it is thought of as positively charged. They are moving in opposite directions, so you have a net movement of charge. That’s electric current.
So what determines which way the traffic is going? That is set by the impurities we add to the crystal, but you can think of them as maybe like putting a steep incline in the road. The cars want to roll down hill and the ‘hole’ will go uphill. We don’t have to add any energy to make the cars want to roll down hill, we just need to break up the traffic jam a little.
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