20 Feb 2023
Eindhoven and TNO team use similar approach to solar panels based on multiple stacked cells.Eindhoven University of Technology (“TU/e”) has come up with a photodiode that has record sensitivity.
Solar panels with multiple stacked cells are regularly breaking output and conversion efficiency records. Now, a team of researchers from TU/e and TNO’s Holst Center, both in the Netherlands, have managed to develop photodiodes – based on a similar technology – with a photoelectron yield of more than 200 percent.
How is this possible? The answer lies in the “magical world of quantum efficiency and stacked solar cells,” according to the research group. The achievement is described in Science Advances.
René Janssen, professor at TU/e and co-author of the Science Advances paper, explained, “I know, this sound 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 short-changed. But because in quantum terms, every guilder counts as one lira, they still achieved an efficiency of 100 per cent. This also holds for photodiodes: the better the better the diode is able to detect weak light signals, the higher its efficiency.”
For a photodiode to work correctly, it has to meet two conditions. Firstly, it should minimize the current that is generated in the absence of light, the so-called dark current – the less dark current, the more sensitive the diode. Secondly, it should be able to distinguish the level of background noise from the relevant infrared light. Unfortunately, these two conditions do not usually occur together.
In 2019, Riccardo Ollearo, one of Janssen’s PhD students and lead author of the paper, set about solving this conundrum. In his research he joined forces with the photodetector team working at Holst Centre, a research institute specialized in wireless and printed sensor technologies, Ollearo built a so-called tandem diode, a device that combines both perovskite and organic PV cells.
Combining these two layers – a technique also increasingly used in state-of-the-art solar cells – he was able to optimize both conditions, reaching an efficiency of 70 per cent.
Ollearo commented, “I decided to see if I could increase the efficiency even further with the help of green light. I knew from earlier research that Illuminating solar cells with additional light can modify their quantum efficiency, and in some cases enhance it. To my surprise, this worked even better than expected in improving the photodiode sensitivity. We were able to increase the efficiency for near-infrared light to over 200 per cent.”
“We think that the additional green light leads to a build-up of electrons in the perovskite layer. This acts as a reservoir of charges that is released when infrared photons are absorbed in the organic layer,” said Ollearo. “In other words, every infrared photon that gets through and is converted in an electron, gets company from a bonus electron, leading to an efficiency of 200 per cent or more. Think of it as getting two lira for your guilder, instead of one.”
(L) How tandem photodiode monitors heart and respiration rates; (R) Detected heart and respiration signals.
Ollearo tested the photodiode, which is suitable for use in flexible devices in the lab. “We wanted to see whether the device could pick up subtle signals, such as the heart or respiration rate of a human being in an environment with realistic background light,” he said.
Ollearo added, “Holding the device at 130 cm from a finger, the researchers were able to detect minute changes in the amount of infrared light that was reflected back into the diode.
“These changes turn out to be a correct indication of changes in the blood pressure in a person’s veins, which in turn indicate heart rate. When pointing the device at the person’s chest, they were able to measure the respiration rate from light movements in the thorax.”
Considering possible future developments, Janssen said, “We want to see if we can further improve the device, for instance by making it quicker. We also want to explore whether we can clinically test the device, for instance in collaboration with the FORSEE project.”
The FORSEE project, led by TU/e researcher Sveta Zinger and in collaboration with the Catharina Hospital in Eindhoven, is developing an intelligent camera that can observe a patient’s heart and respiration rates.
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