Blue OLED on silicon sensor detects phosphorescence
The Fraunhofer FEP has many years of expertise in the development and manufacture of highly integrated electro-optical devices based on OLED-on-silicon technology. This technology has been established for the realization of high-resolution microdisplays for AR and VR glasses and is now increasingly being developed further for optical sensor solutions.
For example, optical fingerprint sensors have already been developed by combining the display and image sensor in a so-called bi-directional OLED microdisplay. In addition to the display function, the display pixels serve as smart illumination of the finger resting on it, whose structures are detected via the embedded photodiodes.
Now the researchers went further and developed a miniaturized phosphorescence sensor. In this sensor, a marker is excited by blue, modulated OLED light and the phosphorescence response is detected directly inside the sensor chip. The marker determines the substance to be measured; a typical application is the measurement of oxygen concentration.
Why not use commercial sensors? The challenge lay in the conception of an extremely small sensor, which combines all functionalities and perspectively can be manufactured cost-effectively due to its small size. For this purpose, the OLED control and the sensor front end were integrated into the silicon chip and different arrangements of excitation and detection areas were investigated. The first stage has now resulted in a miniaturized phosphorescence sensor. This is just the size of a thumbnail and combines marker and sensor in a single component. It consists of a blue OLED integrated on the silicon chip together with a commercially available marker.
The sensor is currently designed to detect oxygen changes. With this first setup a functional verification of the device was achieved and the miniaturized sensor chip can be used for oxygen measurements in gases. In future the sensor chip should serve as a platform for future developments, e.g. the measurement of further parameters or use in other environmental conditions.
The current sensor emits blue light for the excitation of the oxygen-sensitive marker on an area of approx. 4.7 x 2.2 mm². The decay time of the emitted marker light is a parameter for the oxygen concentration of the environment. The significantly lower phosphorescence signal is captured by integrated silicon photodiodes, amplified locally in the chip and then evaluated with regard to the phase shift to the exciter signal. In perspective, the chip should be significantly reduced in size, the target being <2x2 mm² total chip size.
Due to the advantages of the innovative phosphorescence sensor - the small size, the combination of marker and sensor on one chip as well as the fast and precise evaluation of the data - the researchers see further areas of application in which the sensor concept is to be used.
The monitoring and evaluation of cell cultures in very small disposable culture vessels or in bioreactors is an interesting application example. "Single-use bioreactors" usually only offer the smallest installation space and a limited number of ports via which measurement systems can be connected. In the future, the sensor system is to be developed in the direction of multi-parameter measurement. The monitoring of liquids after the filling process in the pharmaceutical sector in blisters or for quality control of oxygen-sensitive drugs is also conceivable. The Fraunhofer FEP is very interested in project partnerships for such (further) developments.
At embedded world 2019, in Nuremberg from February 26 - 28, 2019, the Fraunhofer FEP scientists will present the miniaturized fluorescence sensor for oxygen measurement for the first time at the Fraunhofer joint booth in Hall 4, Stand No. 4-470.