The efficiency was so high that at first the researchers had a hard time believing the result. Now Aalto University spin-off company ElFys Inc. already supplies the record detectors for several industry sectors.
Aalto University researchers have developed a black silicon photodetector that has reached above 130% efficiency. Thus, for the first time, a single photovoltaic device has exceeded the 100% external quantum efficiency limit at UV. This result opens new avenues for improving efficiencies beyond the famous Shockley-Queisser limit.
“When we saw the results, we could hardly believe our eyes. Straight away we wanted to verify the results by 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 to provide the most accurate and reliable measurement services in Europe.
Head of the PTB Laboratory of Detector Radiometry, Dr. Lutz Werner comments, “After seeing the results, I instantly realized that this is a significant breakthrough — and at the same time, a much-welcomed step forward for us metrologists dreaming of higher sensitivities.”
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 approximately 1.3 electrons.
The researchers found out that the origin of the exceptionally high external quantum efficiency lies in the charge-carrier multiplication process inside silicon nanostructures that is triggered by high-energy photons. The phenomenon has not been observed earlier in actual devices since the presence of electrical and optical losses has reduced the number of collected electrons.
“We can collect all multiplicated charge carriers without a need for separate external biasing as our nanostructured device is free of recombination and reflection losses,” Prof. Savin explains.
In practice, the record efficiency means that the performance of any device that is utilizing light detection can be drastically improved. Light detection is already used widely in our everyday life, for example, in cars, mobile phones, smartwatches, and medical devices.
“Our detectors are gaining a lot of attraction at the moment, especially in biotechnology and industrial process monitoring,” says Dr. Mikko Juntunen, CEO of Aalto University spin-off company, Elfys Inc. They are already manufacturing the record detectors for commercial use.
The results leading to the record efficiency have been accepted for publication in Physical Review Letters with the title “Black-silicon ultraviolet photodiodes achieve external quantum efficiency above 130%.”
Reference: “Black-Silicon Ultraviolet Photodiodes Achieve External Quantum Efficiency above 130%” by M. Garin, J. Heinonen, L. Werner, T. P. Pasanen, V. Vähänissi, A. Haarahiltunen, M. A. Juntunen and H. Savin, 8 September 2020, Physical Review Letters.
However, in my humble opinion the problem with percentages and statistics is that they are based on Assumptions and most likely outputs, when the resources and costs which define the effciency are incorrectly estimated.
Going back to basics.
Efficiency is often measured as the ratio of useful output to total input, which can be expressed with the mathematical formula r=P/C, where P is the amount of useful output (“product”) produced per the amount C (“cost”) of resources consumed. When efficiency exceeds 100% i simply think the expected amount of useful output was an underestimate.
I have always beleived that the laws of physics are superior to use of statistics and measures of efficiency and effectiveness, which sometimes obfuscates rather than reveals.