Solar panels with multiple stacked cells are currently breaking records. Remarkably, a team of researchers from Eindhoven University of Technology and TNO at Holst Center have now managed to make photodiodes—based on a similar technology—with a photoelectron yield of more than 200%. You would think that efficiencies of more than 100% are only possible using alchemy and other Harry Potter–like wizardry. But it can be done. The answer lies in the magical world of quantum efficiency and stacked solar cells.

René Janssen, professor at the Eindhoven University of Technology and co-author of a new Science Advances paper, explains. "I know, this sounds incredible. But, we're not talking about normal energy efficiency here. What counts in the world of photodiodes is quantum efficiency. Instead of the total amount of solar energy, it counts the number of photons that the diode converts into electrons.

"I always compare it to the days when we still had guilders and lira. If a tourist from the Netherlands received only 100 lira for their 100 guilders during their holiday in Italy, they might have felt a bit shortchanged. But because in quantum terms, every guilder counts as one lira, they still achieved an efficiency of 100%. This also holds for photodiodes: the better the diode is able to detect weak light signals, the higher its efficiency."

Dark current

Photodiodes are light-sensitive semiconductor devices that produce a current when they absorb photons from a light source. They are used as sensors in a variety of applications, including medical purposes, wearable monitoring, light communication, surveillance systems, and machine vision. In all these domains, high sensitivity is key.