A team from Aalto University has developed a photovoltaic device that has an external quantum efficiency of more than 100%. It was produced using black silicon as the active material, with nanostructures shaped like columns and cones that absorb UV light. This significant breakthrough paves the way for improving the efficiencies of solar cells and other photodetectors.
For a hypothetical photovoltaic device to have an external quantum efficiency of 100%, it would mean that each photon of light that strikes it produces one electron, which exits via the circuit and is harvested as electricity. Impressively, this new device is the first to not only reach 100% efficiency but surpass it at 132%. An efficiency of 132% means that one incoming photon generates about 1.3 electrons.
Prof. Hele Savin, the Electron Physics research group leader at Aalto University, said:
“When we saw the results, we could hardly believe our eyes. Straight away, we wanted to verify the results by independent measurements.”
The independent measurements were conducted by the German National Metrology Institute, Physikalisch-Technische Bundesanstalt (PTB), which is well-known for providing the most precise and consistent measurement services in Europe.
Since you can’t have 0.32 of an electron, where do these extra electrons come from? The researchers found out that the root of the exceptionally high external quantum efficiency lies in how photovoltaic materials work in general. For example, when a photon of incoming light hits the active material, generally silicone, it bumps an electron out from one of its atoms. But in some situations, two electrons could get knocked out of one high-energy photon.
Tapping into this phenomenon could help improve the design of solar cells. In several photovoltaic materials, efficiencies are lost in many ways, including electrons rejoining with the “hole” they left in the atom before they get collected by the circuit or photons being reflected away from the device.
The Aalto team’s device solves these issues with black silicon, which absorbs the most photons out of any known material. Electron recombinations are reduced at the surface of the material due to the column and cone nanostructures. Overall, these advances made it possible to produce a device with 130% external quantum efficiency.
Such a record efficiency could improve the performance of photodetectors, including light sensors and solar cells. The new record-breaking detectors are currently being manufactured for commercial use.
On July 28, 2020, the research was accepted for publication in the journal Physical Review Letters.
