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Blue OLED on silicon sensor detects phosphorescence

02 Feb 2019

Fraunhofer FEP developers to demo prototype, which is designed for oxygen ratio monitoring, at Embedded World 2019.

Researchers at the Fraunhofer Institute for Organic Electronics, Electron Beam & Plasma Technology (FEP) have long been involved in development of OLED-on-silicon sensors. Now they have developed a miniaturized phosphorescence sensor that combines a marker and sensor on very small chip surface.

They comment that this device could eventually be produced at low cost. The team will present the first prototype of the new sensor – which is currently designed for oxygen ratio monitoring – at Embedded World 2019, 26-28 February in Nuremberg, Germany.

Considering oxygen sensors, there are many current-based sensors available to cover large temperature ranges, but FEP comments that these can be difficult to miniaturize and restricted to certain measurement points:

“Optical 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,” says the Dresden, Germany-based institute.

Its blue OLED release statement says, “Highly integrated OLED-on-silicon electro-optical devices have become standard for realizing high-resolution microdisplays in Augmented- and Virtual-reality glasses, and this technology is now increasingly under development for optical sensor solutions.”

“Optical fingerprint” sensors have already been created by merging the display and image sensor into a so-called a bi-directional OLED microdisplay. 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.

Miniaturized

Now the researchers have taken another step and developed a miniaturized phosphorescence sensor. In this sensor, a chemical marker is excited by modulated blue OLED light. The phosphorescent response of the 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.

To achieve the design of an extremely small sensor that combines all the required functionalities and which could be manufactured cost-effectively, the OLED control and the sensor front-end are integrated into the silicon chip. The FEP team then investigated different configurations of the excitation and detection areas.

In the first stage, a miniaturized phosphorescence sensor was developed. This is barely the size of a thumbnail and combines the marker and sensor in a single component. It consists of a blue OLED integrated together with a commercially available marker on the silicon chip.

Dr. Karsten Fehse, project manager in FEP’s Organic Microelectronic Devices group, commented, “Currently, the sensor is designed to detect changes in oxygen level. We have achieved functional verification of the component with this first setup, and can use the miniaturized sensor chip for oxygen measurements in gaseous environments. We see the sensor chip as a platform for future developments, such as the measurement of further parameters and deployment in other environmental conditions.”

The current sensor emits blue light in 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, the goal being to achieve a total chip size of under 2 × 2 mm. “We see further areas of application in which the mini-sensor design could be used,” said Dr. Fehse. “The monitoring and evaluation of cell cultures in very small disposable culture vessels and in bioreactors is an interesting application example for the technology. Also, monitoring liquids after the filling process in the pharmaceutical sector, in blister packs, and for quality control of oxygen-sensitive drugs is also conceivable. We are very interested in project partnerships for these advanced developments.”

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