hyper-efficient solar cells that aren’t actually efficient! or, giving good science a bad title

It’s ironic (or perhaps siliconic since this is about solar cells) to find a takedown article (which is otherwise OK) that says things like “relaxed momentum conservation” and explain it like this:

These strange little beasties are tiny bits of semiconductor material — tiny enough that lots of strict physical laws (like conservation of momentum) get to relax in some ways, making all sorts of fantastic things possible.

The phrase show up in a few places on the net, but the context there is not that the law is relaxed, but that the conditions are. Silicon is an indirect-bandgap material, meaning that the bands on either side of the bandgap do not line up if you lot energy vs. momentum, as you can see here. This requires a “phonon assist” for an excitation — you need the right vibration, with the right momentum at the time the photon arrives in order to promote the election. In other words, it’s harder to meet the conditions of momentum conservation.

But that’s in a crystal. So what I suspect is that in a quantum dot, the conditions are easier to meet, because it looks more like a direct bandgap material. Not that the law itself is more relaxed.