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Blue OLED-on-silicon sensor detects oxygen through phosphorescence

01/30/2019 – By John Wallace, Senior Editor

Researchers at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (FEP; Dresden, Germany), who have been involved in development of organic LED (OLED)-on-silicon sensors for years, have now developed a miniaturized phosphorescence sensor that combines a marker and sensor on a very small chip surface that could be produced at low cost when further developed. Researchers will present the first prototype of the sensor, which is currently designed for oxygen ratio monitoring, at Embedded World 2019 (26-28 February, 2019; Nuremberg, Germany).

Considering oxygen sensors, there are many electrical-current-based sensors on the market able to cover large temperature ranges, but they are difficult to miniaturize and restricted to certain measurement points. Optical oxygen sensors, such as phosphorescence sensors, overcome these hurdles. They are popular alternatives due to their ease of handling and capability of being integrated into existing systems. The reliability, low susceptibility to interference, and simple maintenance of most devices quickly persuades users.

The Fraunhofer FEP has much expertise in the development and manufacture of highly integrated OLED-on-silicon electro-optical devices; this technology has become standard for high-resolution microdisplays in augmented- and virtual-reality glasses, and is now increasingly under development for optical sensors. Optical fingerprint sensors have already been created by merging the display and image sensor into a so-called bidirectional OLED microdisplay where, in addition to the display function, the display pixels serve as smart illumination of the finger on the surface, whose features are then detected by the embedded photodiodes.

The FEP researchers have taken another step and developed a miniaturized phosphorescence sensor. In this sensor, a chemical marker is excited by modulated blue OLED light and the phosphorescent response of the commercially available marker is then detected directly inside the sensor chip. The marker determines the substance to be measured; a typical application is measurement of an oxygen concentration.

Why not use commercial sensors? The challenge lays in the design of an extremely small sensor that combines all the functionalities and could be manufactured more cost-effectively in the future due to its small size. For this purpose, the OLED control and the sensor front end were integrated into the silicon chip and different configurations of the excitation and detection areas were investigated.

The sensor emits blue light over an area of 4.7 × 2.2 mm for exciting the oxygen-sensitive marker. The decay time of the light emitted by the marker after excitation is a parameter of the oxygen concentration of the environment. The significantly lower phosphorescence signal is recorded via integrated silicon photodiodes, amplified locally in the chip, and subsequently evaluated in relation to the excitation signal with regard to the phase shift. In the future, the chip will be significantly reduced in size, with the goal being less than 2 × 2 mm total chip size.

The monitoring and evaluation of cell cultures in very small disposable culture vessels and in bioreactors is an interesting application example for the technology. Single-use bioreactors usually only offer a very small installation space and a limited number of ports to which measurement systems can be connected. In the future, the sensor system will be further developed in the direction of multiparameter measurements. Monitoring liquids after the filling process in the pharmaceutical sector, in blister packs, and for quality control of oxygen-sensitive drugs is also conceivable. The Fraunhofer FEP is very interested in project partnerships for these advanced developments.

Scientists of the Fraunhofer FEP will debut the miniaturized phosphorescence sensor for oxygen measurement at Embedded World 2019 February 26 – 28 at the Fraunhofer Joint Booth in Hall 4, Booth no. 4-470.


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