With applications in biomedical research, information technology, and a host of other advanced industrial sectors, short-wavelength infrared (SWIR) sensors have come under increasing scrutiny in recent years.
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SWIR sensors are typically rigid and brittle due to the inorganic materials used to make them. But new research in this area lays the foundation for flexible SWIR sensors suitable for the next generations of robotics and electronics, made from SWIR-absorbing organic materials.
Creating Organic Flexible SWIR Sensors
In a paper published in Flexible Electronics in 2021, authors from Kyungpook National University, Korea demonstrated a method for creating flexible SWIR sensors with organic materials.
The chemical engineering team doped a polytriarylamine material, poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (PolyTPD), with tris(pentafluorophenyl)borane (BCF) molecules. After 48 hours, the resulting nanoengineered compound could absorb almost the entire range of the SWIR wavelength of light (λ = 1000–3200 nm).
Using spectroscopic characterization techniques, the scientists discovered that an electron transfer from the PolyTPD to BCF made a new low energy level state in the compound. This gap was what caused SWIR absorption in the new material.
Next, the researchers applied the BCF-doped PolyTBD in thin films as a gate sensing layer on organic phototransistors (OPTRs). These OPTRs using the new compound detected SWIR radiation with good responsivity: approximately 538 mA W–1 (λ = 1500 nm), 541 mA W−1 (λ = 2000 nm), and 222 mA W−1 (λ = 3000 nm).
The SWIR-OPTR technology development may lead to more uses of organic materials in flexible SWIR sensors."
Lee, C., H. Kim, and Y. Kim (2021)
Improvements can still be made by optimizing the BCF-doped layers’ thicknesses, structures between the layers, the concentration of doping particles, and so on. These would yield even more responsive flexible SWIR sensors in the future, as well as routes to market.
Why Do We Need Flexible SWIR Sensors?
SWIR sensors are used in cameras designed for machine vision and imaging beyond the visible light spectrum. They can be used to “see through” surfaces that are not transparent to the human eye, helping their human and machine users to look at features like hidden moisture, fill levels, or tamper-proof security codes.
Water detection is a common application area for SWIR sensing. As water is usually transparent in visible light, machine vision systems operating in this wavelength range cannot easily identify it. In SWIR camera images, water – which absorbs in 1450 nm and 1900 nm wavelengths strongly – appears black.
This application of SWIR sensing features in many inspection and quality control processes in the automotive, food and beverage, woodworking, and textile industries. SWIR sensors check the dryness and uniformity of paint coatings in bulk, detect fill levels through opaque containers, automatically identify bruised and damaged fruit, and determine plants’ relative water content.
The ability to see through materials that are opaque in visible light also makes SWIR cameras useful for nondestructive inspection in clinical or advanced manufacturing. Syringe manufacturers use SWIR cameras to automatically check whether there is a needle in the packaging without removing or displacing the protective cap. The cap is made out of transparent plastic in infrared light, while the needle’s metal is opaque.
In research and industrial laboratories alike, SWIR cameras find many uses in spectroscopic analysis. They can help to clarify the structure of unknown substances, categorize substances’ purity, and sort plastic for recycling.
Photoluminescence imaging is applied throughout the solar cell production process to ensure engineering tolerances are met. Electroluminescence analysis is also used in the final quality assurance step in solar cell manufacturing.
Both of these techniques work best with SWIR cameras. This is because silicon emits light at a peak of around 1150 nm. NIR-enhanced CCD and CMOS cameras cannot detect wavelengths above 1100 nm, where silicon becomes transparent.
InGaAs sensors typically used for SWIR cameras also have a higher quantum efficiency (more than 70% at 1050 nm) than either of the NIR-enhanced cameras (less than 10% at 1050 nm).
This means that SWIR cameras are ideally suited for imaging via silicon to detect metallization and electrical contact defects in electronic devices.
Next Generation Electronics with Flexible SWIR Sensors
Flexible sensors like the organic SWIR-OPTR sensor with BCF-doped PolyTBD demonstrated in the new research are enabling advances in fields like robotics and electronics. Technologists have cited flexible sensors as a key driver of future progress in robotics and artificial intelligence (AI).
Soft robots will rely on flexible sensors to operate smoothly and organically. Softer robots with multi-modal sensing capabilities will better interact with structured and unstructured environments, resulting in safer cooperation with human workers.
The new research may pave the way for more developments in flexible sensors to complement the progress achieved in flexible electronics in recent years. These developments are bringing about a new generation of wearable electronics that impact people’s health and fitness.
Continue reading: Developing Graphene Based Flexible Sensors – RF Flexible Electronics.
References and Further Reading
Lee, C., H. Kim, and Y. Kim (2021). Short-wave infrared organic phototransistors with strong infrared-absorbing polytriarylamine by electron-transfer doping. Flexible Electronics. Available at: https://doi.org/10.1038/s41528-021-00105-z
Costa, J. C., et al. (2019). Flexible Sensors—From Materials to Applications. Technologies. Available at: https://www.mdpi.com/2227-7080/7/2/35
Hashagen, J. (2014) SWIR Applications and Challenges: A Primer. Photonics.com. Available at: https://www.photonics.com/Articles/SWIR_Applications_and_Challenges_A_Primer/a56646