Mechanically deformed photodetectors (PDs) are important wearable health monitoring systems components. For next-generation wearable devices, experts hope to add features such as fast response time, high detectivity, and ultrathin form factor. Aside from this, point-of-care operations will also benefit from self-powered operation without using bulky power-supply units.
The Challenge in Quantum Dot Photosensor
Quantum dots are ultra-small semiconductor particles that measure only several nanometers. They possess optical and electrical properties that are superior to conventional semiconductors. This allows them to conduct faster electron and electron-hole separation, making them ideal for photosensor technologies.
However, most quantum dot photosensors used in research studies are thick and contain toxic heavy metals such as lead sulfide. They are also unsuitable for wearable technology since they are designed at a micrometer scale. The same struggles are observed in conventional silicon-based photosensors since they are usually too heavy and rigid for long-term wear. Sometimes, they also struggle to capture biometric signals accurately because of their inability to maintain close skin contact.
Advanced Photosensor
In the study "Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring," researchers from Daegu Gyeongbuk Institute of Science and Technology (DGIST) have achieved a breakthrough in optoelectronics. Led by Professor Ji-Woong Yang from the Department of Energy Science and Engineering, they developed the most advanced eco-friendly quantum dot photosensor in the world.
This project also involves collaboration with Ulsan National Institute of Science and Technology under Professor Moon-kee Choi and Seoul National University under Professor Dae-hyeong Kim. This year's Nobel Prize in Chemistry honored three scientists for their pioneering breakthrough in quantum dots, which serve as a building block of nanoscience.
The scientists revolutionized quantum dots by defying the general assumptions about their inferior performance. They did this by enhancing the electrical properties of copper-indium-selenide (Cu-In-Se) quantum dots by controlling their size and composition to free them from heavy metals. An innovative organic-inorganic hybrid charge transfer layer was also developed with custom properties for the quantum dots. This enabled them to introduce an eco-friendly photosensor that outperforms its toxic counterparts.
The team's quantum dot photosensor also exhibits exceptional performance with a quantum dot absorption layer of only 40 nanometers. This demonstrates remarkable light-detection capabilities without using an external power source, harnessing the photovoltaic effect for stable light signal measurement.
This technology is further enhanced by creating a wearable pulse sensor that combines the photosensor with a light source on a flexible polymer substrate. It ensures the operation is stable even under critical curvature and during physical activities such as walking and running. Such innovation is particularly relevant today since the COVID-19 pandemic and the aging population increase the need for healthcare monitoring devices that can be comfortably worn for extended periods.
Professor Yang highlighted their success in creating a high-performance, eco-friendly quantum dot photosensor using strategic structural control and layer optimization. Meanwhile, Professor Choi envisioned wider applications for this technology, such as lidar and infrared cameras. They also hope to develop next-generation wearable healthcare monitoring systems with ultra-thin, highly flexible design that is not dependent on external power sources.
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