The idea of integrating fingerprint scanners into displays is increasingly common in smartphones. These applications place a sensor under a transparent OLED array which acts as both the display and light source for the fingerprint reading. However, the LCD displays used, for example, in automotive applications, public transport systems and cash machines are more opaque and need a backlight, so are not suited to below-display fingerprint sensors. Instead, the sensor must be placed on top of the display which means it needs to be highly transparent so that the display can still be seen.
At 70% transparency in the visible region, the new prototype is the most transparent large-area optical fingerprint sensor yet. It is based on a proven combination of organic photodiodes (OPD), thin-film barrier and oxide thin-film transistors (TFTs) pioneered by Holst Centre. To achieve the high transparency, the team uses photolithography to pattern the photodiode layer within each pixel, creating microscopic islands of photosensitive material.
The transparent sensor offers high resolution and dynamic range with a low dark current, ensuring excellent sensitivity even in low light conditions. It can also be produced in large sizes for example to read palmprints or 4 fingerprints at once. This combination of size and performance enables compliance with FAP 60, the FBI's most-stringent certification category.
It also allows the sensor to work as a document scanner, for example allowing a single system to read passports and fingerprints while also providing user guidance. Moreover, the new sensor can be used in combination with the highly transparent capacitive touch panels commonly used in LCD screens, enabling simultaneous touch and biometric functionality.
"Integrating biometric sensors into displays allows the display to give people better guidance on how to use the system and exactly where to put their hand. This would make identification, for example at customs, more reliable and much faster. Our patterned pixel design ensures high transparency and is completely compatible with existing flat-panel display production processes, making integration with widely used LCD displays both technically feasible and cost effective," said Hylke Akkerman, Program Manager at Holst Centre.