Nanyang Technological University, Singapore (NTU Singapore) researchers have developed a new type of light sensor that is both flexible and highly sensitive in order to enable the development of wearable devices with advanced ultraviolet (UV) detection functions.
While UV rays are invisible to the human eye, they are present in our environment and can cause health problems such as skin cancer and premature skin aging. During weather reports, the intensity of UV rays is typically reported as an index. A wearable device, such as a T-shirt or watch, that monitors actual personal UV exposure throughout the day would be a more useful and accurate guide for people attempting to avoid sun damage.
The NTU researchers reported in their study, which was featured on the front cover of the peer-reviewed journal ACS Nano, that their flexible UV light sensors were 25 times more responsive and 330 times more sensitive than existing sensors, outperforming the performance level required for optoelectronic applications of light-based electronics.
UV light sensors, also known as photodetectors, are found in a variety of systems, ranging from smartphones to biomedical imaging. Gallium nitride (GaN) has emerged as the ideal material for fabricating UV light sensors in recent decades, owing to its superior properties in emitting, regulating, transmitting, and sensing light.
To enable the development of wearable devices that possess advanced ultraviolet (UV) detection functions, scientists have created a new type of light sensor that is both flexible and highly sensitive.
Most GaN-based UV sensors on the market today, however, are built on rigid layers, limiting their use in flexible and wearable products. While other researchers have developed flexible GaN-based UV sensors, they have not yet achieved the level of performance required for cutting-edge use. Low responsivity and low sensitivity are two of their most significant challenges.
NTU’s invention overcomes the limitations of rigid UV light sensors
The NTU team overcame these constraints by fabricating their flexible UV light sensors on an 8-inch-diameter semiconductor wafer using free-standing single-crystalline layers of GaN and aluminum gallium nitride (AlGaN) arranged using membranes made of two different thin semiconductor layers (heterostructure membranes).
This type of semiconductor structure, which can be made using existing industrially compatible methods, allows the material to be bent easily, making it ideal for use in flexible sensors. Simultaneously, the chemical composition of the material changes with depth, ensuring that high performance is maintained even when subjected to strain.
The NTU flexible UV light sensors created using the novel combined AlGaN and GaN demonstrated extremely high levels of responsivity and sensitivity in lab tests. They also performed well when subjected to multiple bending and high-temperature tests.
The sensors recorded a responsivity level of between 529 – 1340 Ampere/Watt (a unit used to measure a device’s ability to transfer an optical signal to an electrical signal) under a range of external strains (compressive, flat, and tensile), which is approximately 100 times higher than existing UV sensors. This responsiveness remained stable after 100 cycles of repetitive bending, indicating its potential for use in wearables.
Innovation opens door to UV-enabled flexible wearable tech
The high performance of the team’s flexible UV light sensors, according to lead researcher NTU Assistant Professor Kim Munho from the School of Electronic and Electrical Engineering, demonstrates that large-scale lightweight and flexible electronics for use in future relevant light-based applications are feasible.
According to Asst Prof Kim, the NTU team’s achievement could lead to significant advances in UV optoelectronic devices and circuits, as product engineers can now look forward to developing UV-enabled wearable systems.
“While the performance of rigid GaN-based UV light sensors has been greatly improved with various structural innovations over the past years,” said Asst Prof Kim, “a flexible version remains in its infancy and their performance is far behind that of the rigid counterparts.”
“Our high-performance flexible UV light sensors pave the way for a variety of future wearable applications, such as personal smart health monitoring, where people can accurately measure their UV exposure levels throughout the day to reduce their risk of skin cancer.”
Skin cancer, one of the most common types of cancer worldwide, is primarily caused by excessive sun exposure. According to data compiled by the World Cancer Research Fund, it is estimated that approximately 2 in 3 people will be diagnosed with skin cancer by the time they reach the age of 70 in regions such as Australia, which has the highest rate of skin cancer in the world.
“Skin cancer can be avoided by avoiding prolonged exposure to the sun. In this context, a reliable wearable device that can track UV exposure could be a useful tool to help monitor one’s recommended exposure, especially for those who spend a lot of time outside”, According to the research team.
Associate Professor Zhao Hongping from the Department of Electrical and Computer Engineering at Ohio State University, USA, who was not involved in the research, highlighted the significance of the NTU research team’s work, saying, “This demonstration on a flexible platform opens vast opportunities not only in UV photodetectors but also other optoelectronic and electronic device applications.”
The NTU team spent two years designing, fabricating, and testing the flexible UV light sensors. Moving forward, the researchers hope to use their innovation to create eye-type UV imagers and other applications.
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