Boosting Photodiode Efficiency to 220%

Towards more efficient power for devices

Boosting Photodiode Efficiency to 220%,   By R. Colin Johnson

Commissioned by CACM Staff, March 16, 2023

Eindhoven University of Technology’s super-sensitive photodiode will be used in sensors for medical monitors, wearables, light communication, health surveillance systems, and machine vision.

Credit: Eindhoven University of Technology

The physics of thermodynamics dictates that a photodiode's "energy efficiency" must be less than 100%, defined as "the ratio between the useful output and input of an energy conversion process." However, in sensor applications, it is the quantum efficiency that is important—the ratio between the outputs (electrons) and inputs (photons), and as such is not limited to 100%.

Until now, the highest quantum efficiency of a photodiode used as a sensor, according to its designers at Eindhoven University of Technology (EUT, Netherlands), was 70%. However, the EUT researchers say their newest invention is a tandem-like photodiode (similar to tandem-layer solar cells) that have a hitherto impossibly high quantum efficiency of 220%—since they output 22 electrons for every 10 photons input.

"The tandem-like architecture developed by this group successfully solves several key challenges in the field of near-infrared detection," said photodiode expert professor Fei Huang at the South China University of Technology, who was not involved in the project. "EUT demonstrated devices with a simple but reliable configuration, achieving excellent narrowband detection, very promising operational stability, as well as very low signal-to-noise ratio."

All photodiodes convert photons into electrons—for uses ranging from the near-infrared (NIR) pulse oximeter sensor your doctor clips on your fingertip, to the visible-light solar panels on your roof. As sensors, photodiodes are optimized for superb sensitivity in reflected NIR detection of heart rate, its variability, and blood oxygen levels, whereas photodiodes for solar cells are optimized for energy conversion efficiency from sunlight to electricity. As sensors, photodiodes are usually tuned to different frequencies, too—typically NIR for medical sensors, instead of the visible light spectrum from the Sun for solar cells.

Tandem photovoltaic cells, on the one hand, harvest photonic energy from two different bands of light coming from the Sun, then combine the resulting two separate electron streams, boosting the overall performance of the tandem solar cells to achieve 20% to 40% energy efficiency.

EUT's tandem-like photodiode, on the other hand, achieves greater than 100% quantum efficiency by separately filling a "reservoir" of extra electrons with self-generated photons from an LED shining on the tandem layer. The electrons harvested there are then "gated" into the output electron stream by the incident NIR photons, thus enhancing quantum efficiency far beyond the highest energy efficiency of any solar cell.

"The efficiency that we are talking about is the quantum efficiency—it counts the number of charges (electrons) that pass a circuit per incident photon. This is not really related to the energy efficiency," said Rene Janssen, professor and leader of the interdepartmental research group called Molecular Materials and Nanosystems at EUT. "For our photodiodes, the quantum efficiency is what counts. For a photovoltaic solar cell, it is the energy efficiency that counts. These are related, but the working principle is entirely different—we cannot claim that the effect that we demonstrate here would boost the efficiency of photovoltaic solar cells."  ... '