It’s 2015, and the U.S. Senate has just voted that global climate change “is real and not a hoax”. Two thousand and fifteen! AD! All this talk about climate change in recent years has shifted the world’s attention to alternative sources of energy and the technologies that can help bring humankind closer to being green. One of the most prominent technologies in the works are solar cells, these nifty things that convert solar radiation into usable electric power. Pretty cool, huh?
Okay, so everyone knows what solar cells are. They’re everywhere – on calculators, satellites, flashing road signs, buildings, the roof of the White House – even my watch runs on light. For something that we see all the time, we never really think twice about what kind of magic happens inside them.
As it turns out, asking that question was fundamental to the birth of one of physic’s most exciting fields – quantum mechanics.
Here, we’ll take a look at the development history of solar panels, how they work, and the underlying properties of light-matter interaction that make them so damn expensive.
The History
Solar panels have a quaint little beginning in Paris, France. In 1839 at the age of 19, a young physicist named Edmond Becquerel created the first solar cell in his father’s laboratory and observed “the production of an electric current when two plates of platinum or gold diving in an acid, neutral, or alkaline solution are exposed in an uneven way to solar radiation.” For his discovery of what is now called the “photovoltaic effect” – the generation of electric current from materials exposed to light – the photovoltaic effect is also called the “Becquerel Effect”. But no one ever calls it that. Poor Edmond.
The next few generations of solar panels were highly experimental, and only about 1% efficient. The first modern practical solar cells were invented by scientists working at Bell Laboratories in April of 1954. They became very popular for satellites because of their high power to weight ratio, but because space agencies were willing to pay top dollar for the best quality solar panels, development of low-cost solar panels did not flourish until very recently. Hikes in production have caused the cost of solar panels to drop drastically in the last few decades and now go for less than $0.36 per Watt, which is a totally intuitive unit of measure.
Nowadays, efficiency has reached 18-20% on average, with the newest state-of-the-art stuff getting to 25.6% efficiency. The world record: 46% from a solar cell in pristine lab conditions, developed by a French-German collaboration. It’s so nice to see them playing well together.
The How
Okay okay, so they should be called Becquerel Panels, but we both know they wouldn’t sell nearly as well. It WAS Edmond Becquerel that first observed the photovoltaic effect, but the reason that electrons are released from materials when exposed to light is due to the photoelectric effect explained by Albert Einstein in 1905.
Before we jump into that, let’s take a look at how solar panels are put together. Buckle down!
Solar panels are usually made of silicon, a semiconductor. As the same suggests, semiconductors are semi-electrically conductive – they have an electrical conductivity somewhere between a conductor and an insulator. These materials are the cornerstone of all modern electronic devices, and silicon chips show up in just about everything. By itself however, silicon lacks enough conductivity. The answer, as always: doping.
A silicon atom has 4 electrons in its outermost shell, called valence electrons. These electrons are shared with other atoms nearby, bonding atoms to another and creating a crystal structure.
Doping a silicon crystal involves adding another element to the crystal structure that allows the semiconductor to easily give up or accept electrons. With the right kind of impurity atom, silicon can become an n-type or p-type semiconductor. N-type (“negative”) semiconductors give up electrons easily, and p-type (you guessed it, “positive”) semiconductors have an “electron hole” that easily accepst spare electrons. Introducing antimony (Sb), which has 5 valence electrons into the silicon structure provides one extra electron to the picture – this makes our n-type semiconductor. On the other hand, adding boron (B), which has only 3 valence electrons, makes for a structure that lacks an electron – our p-type semiconductor.
Now, we have a material that wants to get rid of electrons, and one that wants to take in electrons. Get where I’m going with this? When we put these two materials together, we create something that allows for the flow of electrons, i.e. an electrical current. Free electrons from the n-type semiconductors rush to fill electron “holes” in the p-type semiconductors. Eventually, we reach a balance between the two, and an electric field holds our semiconductor state stable. This electric field also allows electrons to flow from the p side to the n side – but not the other way around. (Any device that only allows electricity to flow in one direction is known as a diode.)
So now we have this arrangement that’s going to allow for the flow of electricity….how does light factor into this?
That my friends, is our 0.36 dollars per Watt question. The answer?
The Physics
QUANTUM PHYSICS. (A phrase which is best said with a fake Australian accent.)
The photoelectric effect had been observed since 1887 by various scientists including Hertz and Hallwachs, Lenard, and Thompson (who elected to call electrons “corpuscles”, which is maybe the ickiest name for them) – who discovered that light shining on a metal surface would cause it to expel electrons. Naturally, the intensity of light has an effect on the number of electrons being ejected – more light, more electrons. What was strange though, is that the kinetic energy of the electrons was not effected by intensity. Through experimentation, it was found that the frequency of the light affects the kinetic energy of the ejected electrons. Weird. This relationship can be characterized by the equation:
,where 
This basically means that it takes a certain amount of energy – the work function – to free an electron from the metal. If light carrying less than that amount of energy reaches the metal, the energy is absorbed and re-emitted. If it is exactly equal, the electron is released with no kinetic energy. If it is more energetic than the work function, then the electron is released with additional energy – its kinetic energy.
As it turns out, light of a higher frequency carries more energy than light of lower frequency. For example, red light is not as energetic as orange light, which is less energetic than yellow light, which is in turn less energetic than x-rays. Whaaaat? Yes – light, x-rays, gamma rays, even microwaves and radio waves – they’re all waves that make up the electromagnetic spectrum, or EM spectrum.
Energies of electromagnetic waves are given by the equation:
,where 
We could rewrite our previous equation as:
Building on the work of many others before him, Albert Einstein came to theorize that light was not a continuous wave, but instead made up of little packets of energy – we call them photons now. This little idea that elementary particles exhibit properties of waves and of particles called “wave-particle duality” helped to lay the foundation for quantum mechanics. Quantum mechanics is basically the study of really small particles, where classical mechanics fails to properly describe the behavior of electrons, photons, etc. It gets its name from the idea that many physical quantities (energy, charge, electron spin, and all kinds of complicated stuff) are defined only in integer values or quanta, and are not continuous.
You can eat half of a cake, but in quantum mechanics, you can only eat whole cakes.
For his contribution, Albert was awarded the Smarty-Pants-Medal in 1921. Other people call it the Nobel Prize in Physics.
Now we segue smoothly back to solar panels…
When sunlight hits the solar panel, it excites what we call an “electron-hole pair”, the separation of an electron from its place in the p-type semiconductor. Due to the electric field (blah blah blah diode), the electron travels to the n-type semiconductor and leaves the electron hole in the p-type structure. A nearby electron can fall into this hole, which leaves another electron hole for yet another electron to fall into. This goes on like a bucket brigade until an electron from the n-type side is available to move down through a connected conductor. On its way down, that electron might power something, or get stored in a battery for later use.
This is a simplified version of what happens, but it gives you an idea of how the photoelectric and photovoltaic effects are related.
With these solar panels, we can produce a pretty substantial amount of power. The largest solar farm in the world, the Topaz Solar Farm in California, was completed in November 2014, and can produce 550 MW – on par with a fossil-fuel powered plant. Photovoltaic power plants have a pretty bright future, and we’re sure to see more of them in years to come.
The Physics Behind 
