Researchers at Aalto University have developed a photovoltaic device having an external quantum efficiency of 132%. This impossible feat was accomplished using nanostructured black silicon and could represent a breakthrough for solar cells and other photodetectors.

If a hypothetical photovoltaic device has an external quantum efficiency of 100%, that means that each photon of light that strikes it generates an electron, which passes through the circuit in the form of electricity.

This new device is the first to achieve not only 100% efficiency, it surpasses it. At 132%, that means an average of 1.32 electrons is obtained for each photon. It was made using black silicon as the active material, with nanostructures in the form of cones and columns, absorbing UV light.

Obviously, you can’t have 0.32 electrons, but in other words, you have a 32% chance of generating two electrons from a single photon. On the surface this may seem impossible, after all, physics dictates that energy cannot be created out of nothing. So where do these extra electrons come from?

It all depends on how photovoltaic materials work in general. When an incoming photon of light hits the active material, usually silicon, it pulls an electron from one of its atoms. But under certain circumstances, a high-energy photon could strike two electrons, without violating any physical law.

It goes without saying that harnessing this phenomenon could go a long way in improving solar cell design. In many photovoltaic materials, efficiency is lost in several ways.

But the Aalto team claim to have largely removed the barriers. Black silicon absorbs many more photons than other materials, and the cone and column nanostructures reduce recombination of electrons on the material’s surface.

Together, this progress a device with an external quantum efficiency of 130%. The team even had these results independently verified by the German National Metrology Institute, Physikalisch-Technische Bundesanstalt (PTB).

The researchers say this record efficiency could improve the performance of virtually any photodetector, including solar cells and other light sensors, and that the new detectors are already being manufactured for commercial use.

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