Light sensor can reach 200% quantum efficiency

Riccardo Ollearo demonstrates the photodiode’s ability to sense the signal from his finger. From there, the screen shows his heart rate. (Image Credit: Bart van Overbeeke)

The Eindhoven University of Technology researchers developed photodiodes that achieve a photoelectron yield of over 200% through quantum physics and stacked solar cells. The team says this sensor technology could have applications for monitoring vital signs without attaching or implanting devices into the body.

“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,” René Janssen, professor at TU/e and co-author of a new Science Advances paper, says.” 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 shortchanged. But because, in quantum terms, every guilder counts as one lira, they still achieved an efficiency of 100 percent. This also holds for photodiodes: the better the diode is able to detect weak light signals, the higher its efficiency.”

The photodiode can achieve over 200% quantum efficiency. (Image Credit: Bart van Overbeeke)

The team started with a device featuring a combination of perovskite and organic solar panel cells that achieved 70% quantum efficiency.  “Impressive, but not enough,” Riccardo Ollearo, one of Janssen’s Ph.D. students and lead author of the paper. “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 percent!”

However, the team still isn’t sure how this works. So they theorized that adding green light excites the electrons, causing them to accumulate in the perovskite layer. According to the researchers, the green light could be emitting electrons on one layer that converts into a current when photons hit the other layer. In addition, optimizing the sensor allowed it to filter various lights while responding to the absence of light. This resulted in the device achieving a quantum efficiency of over 200%. 

“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”, says Ollearo. “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 percent or more. Think of it as getting two lira for your guilder, instead of one!”

The left shows how the diode can measure a person’s heart and respiration rate. Meanwhile, the right image shows the heart and respiration signals from 51.2 inches away. (Image Credit: Eindhoven University of Technology)

The team also used their ultra-thin photodiode to measure small infrared light changes reflected back into the diode from a finger 51.2 inches away. This indicated blood pressure changes in the veins and performed similarly to a smartwatch sensor. Aiming the device at the individual’s chest allowed the team to measure the respiration rate from light movements in the thorax. Potential applications for this device include medical and monitoring. But first, the team wants to make the device perform faster and clinically test it.

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