Flexible Near-Infrared Photodetectors Pave the Way for Advanced Wearable Technology

The potential of wearable biometric devices in monitoring health and enhancing personal security © MAY - stock.adobe.com

In the rapidly advancing field of wearable technology, a significant breakthrough has emerged from a collaboration between the RIKEN Center for Emergent Matter Science (CEMS) in Saitama, Japan, and the School of Engineering at The University of Tokyo. The research, spearheaded by Zehao Wang, Simin Cheng, Kenjiro Fukuda, Wenping Hu, Xiaomin Xu, and Takao Someya, focuses on the development of flexible near-infrared organic photodetectors (OPDs) designed for next-generation wearable applications (1).

The rise of Industry 4.0—the Fourth Industrial Revolution, which is the integration of cyber-physical systems, the Internet of Things (IoT), cloud computing, and cognitive computing into manufacturing)—has placed a spotlight on the potential of organic electronics in creating high-performance, flexible, and integrative devices. OPDs, specifically those designed to operate in the near-infrared (NIR) spectrum, stand out for their application potential in non-invasive biosensing and bio-imaging. These devices offer significant advantages over traditional rigid photodetectors by mitigating motion artifacts, which are a common limitation in existing systems (1). Note that the NIR-I (near-infrared I) band covers from ~700 to 900 nanometers (nm), and the NIR-II (near-infrared II) band covers wavelengths from 1000 to 1700 nm (2).

The wavelength window for NIR optoelectronics applications begins around 760 nm, with the 700-800 nm range serving as a transition zone from visible to NIR light. Enhancing molecular absorption in the NIR range, particularly beyond 1000 nm, can be achieved through intramolecular charge transfer (ICT). This method employs the electron push-pull mechanism between electron-donating and electron-withdrawing units (D-A effect) to fine-tune the absorption spectrum. The effectiveness of extending absorption into the NIR range depends significantly on the strength of these donor and acceptor units, highlighting the importance of strong ICT effects for optimal performance (1,3).

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At the core of this innovation is the ability of NIR light to penetrate deeply into human tissues with minimal scattering. This property makes flexible NIR OPDs particularly adept at precise diagnostics, providing enhanced signal quality for medical applications. The adaptable form factor of these devices further broadens their potential for use in human-environment interaction, marking a notable shift towards more responsive and intelligent wearable technologies (1).

One of the most compelling aspects of this work is the potential for flexible OPDs to transform how we interact with our environment. For instance, in health monitoring, these devices can be used to gather detailed information from deep-seated arterial vessels, improving the accuracy of monitoring and analytical techniques. The ability of NIR light to penetrate deep into tissues makes it ideal for applications such as heart rate and oxygen saturation monitoring, which require minimal invasiveness (1).

The development of narrow-bandgap organic molecules is a prerequisite for achieving efficient NIR photodetection. The research underscores the importance of sophisticated engineering strategies in pushing the boundaries of device design and functionality. Recent breakthroughs have enabled OPDs to detect wavelengths beyond 1000 nm, a significant achievement given the challenges traditional silicon-based devices face in detecting such longer wavelengths (1).

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In addition to their optoelectronic performance, the physical design of OPDs is rapidly advancing. The move towards more flexible and miniaturized devices, combined with a higher degree of integration, allows for superior spatial resolution. This development enables OPDs to fit more conformably on the complex, curved surfaces of the human body, making them attractive candidates for wearable and epidermal health monitors (1).

The integration of these devices into arrays has further enhanced their capabilities. Solution processes and printing techniques have allowed for the creation of lightweight, flexible devices that can capture more comprehensive data sets. This integration enhances both the accuracy and reliability of monitoring outcomes, positioning flexible NIR OPDs as ideal components in advanced wearable systems (1).

Despite these advancements, several challenges remain. The development of low-bandgap organic semiconductors and the transition from rigid to flexible formats require further research. Additionally, the integration of photodetectors with other electronic components, such as data processing units and wireless data transmission antennas, is essential for creating miniaturized and multifunctional systems (1,3).

The development of flexible NIR OPDs holds promise for a wide array of applications, from health monitoring and diagnostic imaging to human-environment interfaces. This research not only highlights the potential of flexible OPDs but also sets the stage for future innovations that could transform human interactions with our world.

References

(1) Wang, Z.; Cheng, S.; Fukuda, K.; Hu, W.; Xu, X.; Someya, T., 2024. Flexible Near-Infrared Organic Photodetectors for Emergent Wearable Applications. Wearable Electronics 2024, 1, 53–77. DOI: 10.1016/j.wees.2024.04.001

(2) Zhu, S.; Yung, B. C.; Chandra, S.; Niu, G.; Antaris, A. L.; Chen, X. Near-infrared-II (NIR-II) bioimaging via off-peak NIR-I fluorescence emission. Theranostics 2018, 8 (15), 4141–4151. DOI: 10.7150/thno.27995

(3) Chow, P. C; Someya, T. Organic Photodetectors for Next‐Generation Wearable Electronics. Adv. Mater. 2020, 32 (15), 1902045. DOI: 10.1002/adma.201902045