Photodiode Measures Heart and Respiration Rate With Zero Contact

Recently, a team of researchers from Eindhoven University of Technology and TNO at Holst Centre created photodiodes based on technology similar to solar panels with multiple stacked cells. The result: a photoelectron yield exceeding 200%. 

 

Researcher Riccardo Ollearo shows how the photodiode (right) detects his heart signal from his finger, recorded on the heart rate monitor on the screen (left). Image courtesy of Bart van Overbeeke/TU Eindhoven

 

Focusing on quantum efficiency instead of normal energy efficiency, the researchers aimed to improve the photodiode's ability to detect weak light signals, thereby increasing its efficiency. The team plans to use this technology for remote heart and respiration monitoring.

 

The Role of Photodiodes in PPG

Photodiodes are essential in many biomedical applications, including photoplethysmography (PPG), where they are used to detect changes in blood volume and flow. In this approach to measuring heart rate, a photodiode detects the amount of light absorbed or reflected by blood vessels. As blood flows through the blood vessels, the amount of light that is absorbed or reflected changes, and the photodiode detects these changes as fluctuations in the intensity of the light signal.

 

The photodiode prototype used in TU Eindhoven's experiment. Image courtesy of Bart van Overbeeke/TU Eindhoven

 

The photodiode's sensitivity determines its ability to detect these changes in light intensity. A more sensitive photodiode can detect small changes in light intensity, resulting in a more accurate and reliable measurement of blood flow and volume. Sensitivity is particularly important in biomedical applications such as PPG, where precise measurements are crucial for diagnosing and monitoring conditions such as cardiovascular diseases.

PPG remotely assesses the cardiorespiratory activity of newborns and people with skin conditions. This technique is also used when patients need to be monitored more comfortably, such as when they are sleeping or resting. With remote detection, however, the signal is weaker and has lower integrity because of optical losses and background noise due to ambient light. Background noise can add random fluctuations to the PPG signal and alter its baseline, the steady-state level of the PPG signal when there is no change in blood volume or flow.

 

Tandem Approach for High Quantum Efficiencies

An ideal photodiode should distinguish between background light and the relevant infrared light. It should also minimize the current generated in the absence of light, the so-called dark current. Minimizing the dark current maximizes the sensitivity. Both characteristics are difficult to achieve in a photodiode.

A team of researchers from the Eindhoven University of Technology and TNO at Holst Centre managed to make a photodiode featuring less than 10⁻⁶ mA cm⁻² dark currents, greater than 200% quantum efficiency, and intrinsic filtering of other wavelengths to limit optical noise. The device produces ultra-low dark currents while exhibiting higher tolerance to background light than optically filtered silicon-based sensors.

Tandem photodiodes are a type of photodiode that consist of two or more semiconductor layers stacked on top of each other. Each layer absorbs a different portion of the incident light, allowing a broader range of wavelengths to be detected. 

 

Tandem device architecture. Image courtesy of Science Advances

 

Stacked diodes increase absorption while reducing the effect of thermal noise, limiting the quantum efficiency of single-layer photodiodes. The layers are designed to have different band gap energies, which helps to distribute the thermal energy over a wide range of energy levels, reducing the likelihood of thermal noise-induced electron-hole pair generation. In addition, they typically have lower dark currents than single-layer photodiodes, leading to higher signal-to-noise ratios.

René Janssen, professor at TU/e and co-author of the study, and his Ph.D. student, Riccardo Ollearo, built a tandem photodiode combining perovskite and organic PV cells. Using these two layers, they reached quantum efficiencies of about 70%, and with additional green light, they increased the efficiency of near-infrared light to over 200%.

 

The Key to the Efficiency Boost: Green Light?

The researchers are not sure of the mechanisms behind such high sensitivity. Nevertheless, they have a theory that can explain this effect. They suggest that additional green light might lead to a build-up of high-energy electrons in the perovskite layer that acts as a reservoir of charges released when low-energy infrared photons are absorbed in the organic layer. Ollearo suggests that every infrared photon that breaks through this layer is converted into an electron and receives a bonus electron, leading to 200%+ efficiency.

The team tested their device by holding it 130 cm from a finger and detected minuscule changes in the infrared light reflected by the diode. This indicated correct blood pressure and heart rate. They were also able to measure respiration rate from light movements in the thorax.

 

Vital signals while testing the device. Image courtesy of TU Eindhoven

 

The team further plans to speed up the device and run clinical trials on it. Moreover, they plan to collaborate with the FORESEE project, led by TU Eindhoven researchers, to develop an intelligent camera to observe a patient's heart and respiration rates.