Modern solar panel technology is pretty damned awesome. I say this from personal experience, since my roof is pretty much maxed out. I may even move the inverter into the lounge to replace the television as my visual entertainment of choice.

Most people don’t view solar panels as a source of entertainment, though. They want power, and the big thing that everyone talks about when it comes to power is the panels’ efficiency: how many photons that hit them liberate electrons. The usual answer is… not many.

There is a fundamental limitation, called detailed balance, that helps limit the efficiency. Essentially, absorption of a photon and emission of a photon are the same thing (you just reverse the direction of time). So, if something is good at absorbing photons, it’s also good at emitting photons. When your solar panel absorbs a lot of photons, there are lots of excited electrons around, and many of them will lose their energy by emitting other photons. In the end, where these two processes balance out helps set the maximum possible efficiency of a standard solar cell.

Now, you shouldn’t worry about that too much when it comes to the panels used outside of the lab, because other factors ensure that they are nowhere near that ideal limit. In the lab, it’s a different story; there are experimental solar cells that get close to the limit. So research is now turning to ways to beat detailed balance. It turns out that you can do this by using conservation laws to prevent electrons from radiating the energy they just absorbed. Getting it to work is a bit delicate, however, and even understanding why it works is difficult. So what follows may be a bit confusing.

Hiding in the dark

When I described the process of turning light into electrical energy above, the limit is given by the balance between absorption of a photon and emission of a photon. What if an electron was unable to emit a photon?

Let’s look at an example of that. A molecule, which is basically a cloud of electrons, is sitting around minding its own business, when a photon collides with it. The photon is absorbed by the molecule and, as a result, one of the electrons gets excited and moves to a state with higher energy. But, the electron can’t choose just any state, because the photon has also given it some angular momentum. So, the new state must match both the change in energy and the change in angular momentum. If nothing else was going on, the electron could then emit a photon with the right energy and angular momentum, allowing it to return from whence it came.

However, usually an electron has more than one choice in how it loses energy. So, it can also lose a small amount of energy and angular momentum to enter a state with less energy, but still more than it had originally. That leaves the electron stuck. It still has energy to lose, but the only way to return to its lowest energy state is to emit a photon with no angular momentum, which is impossible….More Here