Researchers from Eindhoven University of Technology and TNO at Holst Centre have developed a sensor that converts light into an electrical signal at an astonishing 200% efficiency – a seemingly impossible figure that was achieved through the exceptional nature of quantum physics.
SA schematic of the photodiode architecture
The team of scientists sees its invention potentially used in technology that monitors a person's vital signs (including heartbeat or respiration rate) from afar, without the need for anything to be inserted or even attached to the body.
Photodiode efficiency is typically measured as the number of available light particles it can convert into electrical signals. Here, the scientists are talking about something closely related, but a bit more specific: photoelectron yield, or the number of electrons generated by photons hitting the sensor.
The photoelectron yield of a photodiode is determined by its quantum efficiency – the essential capability of a material to produce charge-carrying particles at a fundamental level, rather than the amount of electrical power produced.
"This sounds incredible, but, we're not talking about normal energy efficiency here," says chemical engineer Rene Janssen, from the Eindhoven University of Technology in the Netherlands. "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."
As a starting point, the team worked on a device that combined two types of solar panel cells, perovskite and organic, achieving 70% quantum efficiency. To push this figure higher, additional green light was introduced. The sensor was also optimized to improve its ability to filter different types of light, and respond to no light at all. This pushed the quantum efficiency of the photodiode past 200%, although at this stage it's not clear exactly why that boost is happening.
The key might be the way photodiodes produce a current. Photons excite electrons in the photodiode material, causing them to migrate and create a build-up of charge. The researchers hypothesize that the green light might release electrons on one layer, which are converted into current only when photons strike a different layer.
"We think that the additional green light leads to a build-up of electrons in the perovskite layer," says chemical engineer Riccardo Ollearo, from the Eindhoven University of Technology. "This acts as a reservoir of charges that is released when infrared photons are absorbed in the organic layer". "In other words, every infrared photon that gets through and is converted into an electron, gets company from a bonus electron, leading to an efficiency of 200% or more."
A more efficient photodiode is also a more sensitive photodiode – one that's better able to observe very small changes in light from greater distances. This brings us back to measuring beating hearts and respiration levels.
Using their super-thin photodiode, much thinner than a sheet of paper, the researchers measured small changes in infrared light reflected back from a finger from a distance of 130 centimeters (51.2 inches). This was shown to match blood pressure and heart rate, much as a smartwatch sensor does, but operating from across a table.
With a similar set up, the team measured respiration rates from slight chest movements. There's potential here for all sorts of monitoring and medical purposes, if the technology can be successfully developed from the lab stage.
The team stated that the device has low dark currents (<10−6 mA cm−2), linear dynamic range >150 dB, and operational stability over time (>8 hours). With a narrowband quantum efficiency that can exceed 200% at 850 nm and intrinsic filtering of other wavelengths to limit optical noise, the device exhibits higher tolerance to background light than optically filtered silicon-based sensors.
"We want to see if we can further improve the device, for instance by making it quicker," says Janssen. "We also want to explore whether we can clinically test the device."
