Holst Centre has been at the forefront of developing large-area image sensors based on organic photodiodes, creating a range of prototypes for visible light and X-rays. It has now extended this technology platform into the near-infrared (NIR). Light in this wavelength domain can pass through skin but is absorbed by hemoglobin in the blood, making it ideal for detecting vein patterns.
Researchers at Holst Centre have combined the organic NIR photodiodes with an oxide thin-film transistor backplane and a focusing lens to create an NIR image sensor measuring 2.4 x 3.6 cm, large enough to image the palm of a hand or multiple fingers at a distance. It's 500-ppi resolution is state-of-the-art for biometric image sensors, enabling high-quality images of the vein pattern. In addition, the sensor achieves external quantum efficiency (EQE) of 40% at 940 nm and a dark current of around 10-6 mA / cm2.
"Together with a NIR light source, the prototype image sensor opens the door to contactless biometric security through vein pattern detection. Our thin-film technologies make for extremely thin and potentially flexible sensors that could be easily integrated into existing displays and things like mobile phones or cash machine screens, eliminating the need for separate ID and credit cards," says Daniel Tordera, Senior Scientist at Holst Centre.
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Having demonstrated the potential of large-area NIR sensors for vein detection, Holst Centre is continuing to refine the technology and push its sensitivity deeper into the NIR region. With photodiodes efficient up to 1100 nm, these latest developments could enable new applications such as eye tracking, quality control in food production, condition monitoring of pipes and non-invasive in-body medical imaging including large area oxygen saturation (SpO2) measurements, conformable optical brain scans and cuffless blood pressure monitoring.