Dutch Team Develops 200% Efficient Photodiode

Recently, a group of researchers from Eindhoven University of Technology ("TU/e") and the Holst Center, a subsidiary of TNO in the Netherlands, have developed a photodiode with record-breaking sensitivity and a photoelectron output rate of more than 200% using green light and a double-layer battery design. The results have been published in the journal Science Advances.
In recent years, solar panels with multiple stacked cells have frequently set records for output and conversion efficiency. Riccardo Ollearo, a doctoral researcher at the Eindhoven University of Technology in the Netherlands, points out that the 200 percent photoelectron output mentioned above does not refer to normal energy efficiency. In the world of photodiodes, it is the quantum efficiency that counts. Instead of counting the total amount of solar energy, it counts the number of photons that the diode converts into electrons.
For a photodiode to work properly, two conditions must be met: first, it should minimize the amount of current generated in the absence of light, known as dark current - and the smaller the dark current, the more sensitive the diode. Second, it should be able to discriminate the level of background noise from the associated infrared light. Unfortunately, these two things don't usually happen at the same time.
The Holst Center is a research institute specializing in wireless and printed sensor technologies. In the above collaboration, Riccardo Ollearo worked with the Holst Center's photodetector team to construct a tandem diode.
This tandem diode device combines chalcogenide and organic photovoltaic cells to optimize the performance of the solar cell, which reaches an efficiency of 70%. Later, they used green light as an aid, and eventually increased the efficiency of near-infrared light to more than 200 percent.
The researchers analyzed that this is mainly because the extra green light leads to a buildup of electrons in the chalcogenide layer. This acts as a reservoir of charge that is released when an infrared photon is absorbed by the organic layer - in other words, for every infrared photon that passes through and is converted into an electron, it receives an accompanying extra electron, resulting in an efficiency of 200 percent or more.
The team tested the above tandem photodiode device and showed that it is suitable for flexible devices in the lab, capable of capturing subtle signals, such as the human heart or breathing rate, in environments with realistic background light. The researchers placed the device 130 centimeters away from their finger and were actually able to detect small changes in the amount of infrared light reflected back to the diode.
The researchers want to see if the device can be further improved and explore whether the device can be clinically tested, for example by making it faster. It is also known that the "FORSEE" project led by TU/e researcher Sveta Zinger is working with Catharina Hospital in Eindhoven to develop a smart camera that can observe a patient's heart and respiratory rates.