#1: How Einstein Inadvertently Invented Solar Energy…
Welcome back to TheClimateProject! It’s a pleasure to have you on here!
The topic being explored today is solar energy. It’s a really great, promising technology, and one day, it could even end up replacing fossil fuels for good! In fact, many countries have already started doing just that! However, a lot of people don’t know how it actually works, so today, I’ll be learning, and you’ll be learning right along with me!
Now, a quick warning: this article will be heavily focused on mathematics and physics since solar cells are very much an engineering topic. But, before you click off, don’t worry, I promise I’ll make it as colloquial, chill and fun as I possibly can. So, let’s dive right in.
A weird, but cool-looking representation of the photoelectric effect [Source]
When most people think of solar energy, they probably imagine a bunch of sheets of black glass laying over the top of the building, which, by some weird, mysterious process turn sunlight into electricity. And, to be honest, that’s not too far off.
However, as one might expect, solar panels aren’t really a natural thing. After all, you aren’t going to go for a hike and just stumble across a rock you can plug your phone into, or use to power an LED light. So, how do engineers transform our natural materials into solar energy? Well, what would you think if I told you that Einstein had actually discovered it… by accident! How? Well, to understand, we need to dive into Einstein’s photoelectric effect.
The Photoelectric Effect
So, everybody knows who Einstein was. A really smart German guy, at the start of the 20th century, who happened to have a passion for physics. Most notably, Einstein is known for his work on the General Theory of Relativity, published in 1915, and the Special Theory of Relativity, published in 1905.
But what did Einstein know about solar cells? Well, as it turns out Einstein’s work is really what allowed us to develop photovoltaics in the first place. Why? You see, in the early 20th century, there was a lot of heavy debate about what light actually was. Was it a particle? Was it a wave? Was it both?
Many scientists at the time tried to answer this question. All had their own theories on this, which you can read up yourself here. In this article, we’re going to focus on Einstein’s idea. So what exactly did Einstein think?
Well, quite simply, Einstein believed, that unlike sound or water, light was not actually a wave, but a particle. After all, the Michelson-Morley experiment had proved that the light didn’t travel through a medium. So, why couldn’t this be the case?
Truth be told, a lot of scientists initially rejected Einstein’s claims. After all, Young’s double slit experiment had shown that light diffracted, like a wave, when it was shone through both slits. So, Einstein, as all scientists do, though he would prove his hypothesis through a little experiment: the photoelectric experiment.
Quite simply, he set up a circuit like the one pictured below.
As you can see, there are a number of interesting things about the setup. Although there is a battery, an ammeter, and some wire, the circuit is wide open, and there are two metal plates facing one another. Why did Einstein do this? Well, as we know from basic circuit physics, electrons flow from the negative side of the batter to the positive side, through the wire.
In the case of Einstein’s setup, they would begin to flow but then stop on the metal plate, requiring energy they didn’t have in order to get across to the other side. However, since light contains energy, shining a light onto the electrons on the plate should excite them enough so that they can ‘jump’ over to the other plate. You can test this out yourselves using this simulation from PhET Boulder.
So, what happened when Einstein tested this? Well quite simply, he proved the dual wave-particle nature of light. How? Well, according to a few observations below:
- Given that the intensity of light is proportional to the amplitude of a wave, 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.
- 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).
- 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 equations:
It relates the work function of the metal, Φ 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.
So, now that we’ve gotten through the nitty-gritty physics of all of this, how does it relate to solar panels. How could Einstein’s photoelectric effect, carried out with a multicolor lamp and a battery in a dark lab, help solve our energy problems? Well, to find out, I invite you to check out our next section.
The Photovoltaic Effect
As it turns out, the process that Einstein detailed in his paper on the photoelectric effect is the very same process that allows solar cells to work. While I don’t want to dive into a full, detailed explanation of photovoltaics, as there is one currently in the works for its one article, I’ll give you the basics.
So, when a solar panel creates electricity, it’s using up power from the sun’s light rays. How, well, the solar cell is pretty much just a bunch of electrons on a metal sheet, freely floating, just as they were on Einstein’s sheet (this is an oversimplification, but you get the gist).
Well, in this case, however, they’re actually found in something called the ‘valence band’ meaning that in their current state, as valence electrons of the atoms making up the metal (usually a semiconductor such as crystalline silicon or cadmium telluride). How do we get them out of the valence band? The answer: the photoelectric effect.
Every time light strikes the surface of the solar panel, a tiny packet of energy, called a photon, will hit an electron. If the frequency of the light is high enough, that electron will get dislodged and will be able to bump up to the conduction band of the solar cell.
This process allows it to seamlessly flow through the circuit (connected to the solar cell), and provide electricity, completely free of charge, independent of any natural resources, and plentiful as well as renewable. So, perhaps my title was a little misleading, but without Einstein’s work, it’s safe to assume we wouldn’t have been able to invent solar cell technology…or at least we would’ve just waited until someone else came up with the photoelectric effect…
Hope you enjoyed the article! If you’d like to know more about the photoelectric effect and its role in solar energy, we’ll soon be releasing a separate episode on how solar cells work on a more precise, physical level. But until then…see ya!
