Black silicon photodetector breaks the limit of 100% efficiency

Researchers from Aalto University have developed a black photodetector for silicon that has reached above 130% efficiency. Thus, for the first time, a photovoltaic device has exceeded the limit of 100%, which was previously considered as the theoretical maximum for external quantum efficiency.

“When we saw the results, we could hardly believe our eyes. Right away we wanted to check the results through independent measurements,” says prof. Hele Savin, Head of the Electron Physics Research Group at Aalto University.

The independent measurements were carried out by the German National Metrology Institute, Physikalisch-Technische Bundesanstalt (PTB), which is known for providing the most accurate and reliable measurement services in Europe.

Head of the PTB Laboratory for Detector Radiometry, Dr. Lutz Werner comments, “After seeing the results, I immediately realized that this is a major breakthrough – and at the same time a very welcome advance for our metrologists who dreamed of higher sensitivity.”

The secret behind the breakthrough: Unique nanostructures

The external quantum efficiency of a device is 100% when one incoming photon generates one electron to the external circuit. 130% efficiency means that one incoming photon generates about 1.3 electrons.

The researchers found that the origin of the extraordinarily high external quantum efficiency lies in the charge-carrier multiplication process in silicon nanostructures that is triggered by high-energy photons. The phenomenon has not been previously observed in real devices, because the presence of electrical and optical loss has reduced the number of electrons collected.

“We can collect all multiple charge carriers without the need for separate external biasing, because our nanostructured device is free from recombination and reflection loss,” explains prof. Savin.

In practice, the record efficiency means that the performance of any device using light detection can be drastically improved. Light detection is already widely used in our daily life, for example in cars, mobile phones, smartwatches and medical devices.

“Our detectors are currently attracting a lot of attention, especially in biotechnology and industrial process monitoring,” says Dr. Mikko Juntunen, CEO of Aalto University spin-off company, Elfys Inc. They produce all the record detectors for commercial use.

The results that lead to the efficiency of the record have been accepted for publication in Physical review letters in an article entitled “Ultra-violet photodiodes of black-silicon achieve external quantum efficiency above 130%.”