Low-cost SWIR Imager Shows Promise for Machine Vision

A European research group has created a device for upconverting shortwave infrared (SWIR) to visible light. The team’s all-organic SWIR upconversion device consists of a SWIR-sensitive photodetector integrated with a visible light-emitting unit in the form of an OLED display. The organic upconversion device (OUC) directly converts SWIR to visible photons. It could become a low-cost alternative to the inorganic, compound-based SWIR-imaging technologies that are currently available.

SWIR light has many uses, from identifying and sorting damaged fruit, to inspecting silicon chips, to sharpening night vision. SWIR photodetection and imaging also supports airborne remote sensing and machine vision solutions.

So far, however, SWIR cameras have been based on expensive electronics, said researchers at Swiss Federal Laboratories for Materials Science and Technology (EMPA), École Polytechnique Fédérale de Lausanne (ÉPFL), ETH Zurich, and the University of Siena. The average cost for a standard SWIR camera for industrial use is around 7000 Swiss francs, or over $7500.

The researchers sought to provide a SWIR imager for consumer and low-end applications that avoided the need for expensive intermediate electronics.

The OUC that the research team produced has an organic IR photodetector that absorbs IR light, creating electrons and holes. The electrons travel to a positive electrode, and the holes travel through the transport layer of the detector into the OLED. Inside the OLED, the holes combine with the electrons, which are injected via a negative electrode. Through this recombination process, visible light is emitted. The higher the voltage that is applied between the two electrodes, the more easily the charges generated by the IR light migrate into the OLED and recombine.

Electronic signal processing is not necessary with the OUC; the incoming, invisible SWIR light is instead amplified and displayed directly on the OLED screen. The color of the emitted visible light — blue, green, yellow, or red — can be adjusted by selecting the dye in the OLED.

The researchers synthesized a series of organic dyes called squaraines to obtain dyes with selective light absorption that extended well into the SWIR wavelength range. In experiments, the organic photodetector demonstrated an external photon-to-current efficiency of over 30%. When the photodetector was combined with a fluorescent OLED, the result was an OUC with long-term stability and with peak sensitivity at 1020 nm, extending to beyond 1200 nm.

The OUC was characterized by very low dark luminance in the absence of SWIR light and a low turn-on voltage of 2 V when SWIR light was present. A low dark luminance is an important performance metric of an OUC, researchers said in their paper; under ambient light, a dark luminance level can be hardly detectable by the human eye.

“Therefore, even a small SWIR light-induced luminance results in a high image contrast that can visually be clearly differentiated against the (black) background,” the researchers said.

In general, the OUC can be fabricated from low-cost manufacturing processes on large-area flexible substrates. And it can be operated at room temperature. In ongoing synthetic work, the researchers plan to shift the dye absorption further into the SWIR range by increasing the donor strength and using different acceptors on the squaric acid core, which they have chemically modified so that the dyes can be optically tuned to absorb in the range of SWIR light.

In designing and developing the OUC, the team also worked with narrowband polymethine dyes. The researchers said that a major advantage of narrowband polymethine dyes, compared to colloidal quantum dot absorbers, is that light absorption in the visible is small. This results in upconverters with selective SWIR response.

Structure of the photodetector: The IR photodetector resembles a sandwich of several layers. IR light is absorbed in the organic photodetector, creating electrical charges that emit visible light. Photodetector light intensity: A variable voltage is applied between the two electrodes. The higher the voltage, the more easily the charges generated by the IR light migrate into the OLED and recombine there, emitting visible light. Water absorption: Water does not absorb light in the visible range, so it appears colorless. However, it does have some prominent absorption bands between 760 and 2000 nm. Courtesy of EMPA.

“Right now, we’re working with dyes that absorb at just under 1000 nm,” EMPA researcher Roland Hany said. “But we’re already working on shifting the absorption to longer wavelengths, further into the IR range. If we succeed, our sensor will be able to detect water and moisture much better than it does now.” SWIR light makes moist objects, such as fruits and other foods, appear darker, which makes impurities such as stones and metal objects shine brightly among objects with more moisture as they move along a conveyor belt.

The research was published in Science and Technology of Advanced Materials (www.doi.org/10.1080/14686996.2021.1891842).