Low-noise 150dB photodiode can detect heartbeat remotely

Eindhoven University of Technology has made a photodiode with such low noise (<10^-6mA/cm² dark current) and wide dynamic range (<150dB), that it can optically detect a heartbeat at a distance of 1.3m.

TU Eindhoven researcher Riccardo Ollearo having the pulse in his finger measured remotely by thin-film photodiode

Through an unexpected photo-multiplier effect, it can achieve a photo-electron yield above 200% at 850nm.

“I know, this sound incredible,” said project researcher Professor René Janssen, “but, we’re not talking about normal energy efficiency here. What counts in the world of photodiodes is quantum efficiency, the number of photons that the diode converts into electrons.”

This is not a silicon photodiode, but a thin-film tandem structure with a perovskite photo-active layer facing the incoming light, and a blend of organic donor and acceptor semiconductors (a bulk heterojunction) behind it.

The structure is designed to be optically self-filtering, with the front diode absorbing wavelength shorter than ~650nm, blocking everything but near-infra red from reaching the rear narrow bandgap bulk heterojunction.

To prevent the front perovskite cell from contributing to the photo-current, between the two photo-sensitive layers is an optically-inactive electrically-active layer (made from ‘PFN-Br’) which selectively blocks electrons generated in the perovskite film while passing holes from the narrow-band organic bulk heterojunction – making the cell only sensitive to the longer wavelengths.

Overall the structure has an external quantum efficiency (EQE) peaking at 70% at 850nm (full width at half max <100 nm).

However, EQE, as far as the near infra-red is concerned, rockets to 220% if the cell is also illuminated with green light (60mW/cm² at 540nm).

While the mechanism for this gain is not proven, the team thinks it is due to green illumination causing electrons to gather in the perovskite film, that are then gated through the PFN-Br barrier when near-infra-red-generated-holes on the organic bulk junction side temporarily lower the barrier’s energy.

“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,” said Ollearo (photo above).

Remote detection of heartbeat and respiration rate requires the human subject to be illuminated with near-infra-red light (left). It will work with as little as the changes in reflectivity as blood pressure varies in a single finger.

Eindhoven University of Technology worked with Netherlands research organisation TNO at the Holst Centre.

Their findings are published as ‘Vitality surveillance at distance using thin-film tandem-like narrowband near-infrared 2 photodiodes with light-enhanced responsivity‘ in Science Advances.

This clearly-written paper can be read without payment, and includes extensive descriptions of the device and the non-intrusive medical experiments.