In recent years, organic photodetectors (OPDs) have attracted much attention due to their broad prospects in large-area light sensing applications. For example, current smartphone manufacturers have shifted towards full screen development, and as the physical Home button disappears due to full screen, the fingerprint sensor typically placed under this button has also lost space. A simple solution is to embed a thin film fingerprint sensor in the display area.

According to a report from Maims Consulting, Professor Gerwin Gelinck and his research team at Holst Centre in Eindhoven, Netherlands, have launched a new flexible, large-area, and high-resolution fingerprint recognition sensing technology. The research results were recently published in the journal Advanced Materials Technologies.

Although ultrasonic and thermal fingerprint sensors have been developed, they both have some drawbacks. The cost of ultrasonic fingerprint sensors in large-scale display applications is high, while the imaging time of thermal induction sensors is very short. The alternative solution proposed by researchers at Holst Center is to use optical fingerprint sensors based on OPD, which allows for the manufacture of cost-effective photodetector arrays on a large area (square centimeter level) of plastic thin substrates at lower processing temperatures. In addition, the device exhibits high photosensitivity, fast response time, and spectral absorption can extend from ultraviolet to near-infrared light.

Traditional coating techniques, such as rotary coating, make it difficult to prepare large-area and uniform OPD films. Researchers have developed a cost-effective and scalable photodetector array fabrication technology using a slit coating method for active organic photodetector materials. Researchers have demonstrated its application on high-resolution fingerprint sensors, which have a lightweight appearance that allows users to safely unlock their phones or open specific applications from anywhere on the touch screen.

The fingerprint sensor mainly consists of three modules, including: backlight light source The a-IGZO thin film transistor (TFT) backplane and bulk heterojunction (BHJ) OPD front plate and thin film protective barrier layer are shown in Figure 1.



This new sensor has a thickness of less than 0.2 millimeters and does not have large prisms or moving parts. It can be embedded into objects such as mobile phones and door handles, creating an "invisible" but secure access control system. This sensor uses a 508-ppi oxide transistor as the back panel and a printed organic photodiode as the front panel, and its manufacturing process is compatible with flat panel display technology. The sensor reads fingerprints by detecting visible light reflected back from the surface of the skin, and the level of visualization precision can reach the so-called third level, such as sweat hole details on a single mastoid line, as shown in Figure 2, which meets the FBI (Federal Bureau of Investigation) identity feature recognition standards.



The sensor can also detect some light that penetrates the skin before reflection. This allows them to perceive heartbeat through changes in the capillaries of their hands, thereby verifying that fingerprints come from living individuals. In addition, by using different photodiode materials, the detection function of the sensor can be extended to other wavelengths, such as near-infrared. This technology will make new authentication modes possible, such as identifying venous images of hands, which are said to be more specific than individual fingerprint recognition.

Gerwin Gelinck, Professor of Applied Physics at Eindhoven University of Technology in the Netherlands and Chief Technology Officer of Holst Center, said, "This flexible fingerprint sensor demo prototype demonstrates the versatility and maturity of flexible organic photodetector technology.".