Organic photo diodes enable innovative sensor applications
Organic photo detectors (OPDs) utilize organic molecules to define the light sensitivity within a specific part of the light spectrum. An example for materials suited for OPDs are certain colour pigments. "These devices have multiple advantages over conventional inorganic components", explained Olaf R. Hild, department manager at Fraunhofer COMEDD in Dresden. Which material suits best however depends on the part of the light spectrum a customer needs for his specific application. Organic materials are only sensitive to light of a certain wavelength, they react only to light of a certain colour. This means that the scientists can control the spectral sensitivity distribution of optical sensors through the selection of the material. Materials available already cover a wide range within the spectrum of visible light. For specific applications in the ultraviolet or near infrared range, the researchers also develop compact microsensors that combine organic semiconductors with silicon technology.
The application spectrum ranges from tiny sensor elements in cameras or in bioanalytics to large-area applications in quality control – for instance, such devices can be utilized to determine damages to the paintwork in automotive volume production. Other examples are lab-on-chip applications that detect certain DNA sequences which have been marked with fluorescence markers. And yet another application example: In CCD sensors for sophisticated photo cameras, organic photodiodes can increase the sensitivity to light. "They are more sensitive because they can use a larger active area", Hild explains.
In contrast to silicon sensors, OPDs can be implemented as flexible components. To manufacture flexible devices, the structures for the photodiodes are deposited onto polymer film which in turn can be applied to curved or bulged surfaces. In this manner it is possible to create specific shapes for quality assurance applications into which the device under test can be placed. An example is testing car doors for paintwork damages: The entire door is tested quickly in one single step.
This, OPDs are particularly well suited for large-area applications where they offer a significant cost advantage over conventional technologies. It is much more expensive and lavish to coat large curved areas with silicon sensors than with OPDs. Coating such surfaces with OPDs is done in simple processes on relatively cheap materials. Developers can utilise proven manufacturing technologies such as the processes in the production of organic photovoltaics.
COMEDD will demonstrate a colour sensor consisting of four organic photodiodes with different spectral sensitivity each at the Sensor + Test trade fair in Nuremberg.
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