Scientists have created a sensor that converts light into an electrical signal with a staggering 200 percent efficiency—a seemingly impossible number achieved thanks to the marvel of quantum physics. The device, known as a photodiode, is so sensitive that the team behind this innovation says the device could potentially be used in technology that monitors a person’s vital signs (including heart rate or respiratory rate) without or even wearing anything. body.

The efficiency of a photodiode is usually measured by the number of light particles present that it can convert into electrical signals. Here scientists are talking about something closely related but a little more specific: the photoelectron output, or the number of electrons produced by photons striking the sensor. The photoelectron output of a photodiode is determined by its quantum efficiency—a material’s ability to produce fundamentally charged particles, not by the amount of electricity produced.

“It sounds incredible, but we’re not talking about normal energy efficiency,” says chemical engineer René Janssen of the Eindhoven University of Technology in the Netherlands.

“Quantum efficiency is important in the photodiode world. It counts the number of photons that the diode converts into electrons, rather than the total amount of solar energy.”

As a starting point, the team worked on a device that combines two types of solar panels, perovskite and organic. The researchers achieved 70 percent quantum efficiency by arranging the cells so that light passing through one layer is absorbed by the other. An additional green light has been given to increase this figure. The sensor has also been optimized to improve its ability to filter out different types of light and respond to no light. This put the photodiode’s quantum efficiency up to 200 percent, but it’s not clear at this stage exactly why this increase was caused.

The key may be how photodiodes generate current. Photons excite electrons in the photodiode material, causing them to migrate and build up a charge. The researchers hypothesize that green light can release electrons in one layer and that these only become current when photons hit another layer. “We believe that the additional green light leads to electron deposition in the perovskite layer,” says chemical engineer Riccardo. Ollearo from Eindhoven University of Technology. “This acts as a charge reservoir that is released when infrared photons are absorbed in the organic layer.”

“In other words, every infrared photon that passes through and is converted into an electron gains the company of an extra electron, resulting in an efficiency of 200 percent or more.”

A more efficient photodiode is also a more sensitive photodiode—a photodiode that can better observe very small changes in light from greater distances. This brings us back to measuring heart rate and respiration. Using their ultrathin photodiode, which is a hundred times thinner than a sheet of newspaper, the researchers measured tiny changes in infrared light reflected from a finger at a distance of 130 centimeters (51.2 inches). It’s been shown to match blood pressure and heart rate, just as a smartwatch sensor does, but work from the other side of the table.

With a similar setup, the team measured breathing rate with gentle movements of the chest. If the technology can be developed successfully from the laboratory stage, there is potential here for all kinds of monitoring and medical purposes.

“We want to see if we can improve the device further, for example, to make it faster,” says Janssen. “We also want to explore whether we can clinically test the device.”