Printed Image Sensors Can Be Flexible, Says Plastic Logic

26 November 2013

By Richard Wilson

Earlier this year Plastic Logic announced that it had successfully developed the world’s first flexible image sensor array in collaboration with ISORG, a leader in organic photo-detectors and large area image sensors in printed electronics.

This sensor was produced by combining a flexible organic backplane, developed by Plastic Logic, with a flexible organic photo-detector (OPD) developed by ISORG, to make a flexible image sensor array.

This is the first time an organic thin-cell transistor backplane has been combined with an organic photodiode layer. The array contained 8930 pixels, over a 4cm x 4xm sensor area, with an overall stack thickness of less than 0.15mm.

It’s exciting in that the technology can easily scale to much higher pixel densities and larger areas.

Indeed, the flexible backplane technology had already been industrialised by Plastic Logic and had been shown over larger areas and for pixel densities up to 300ppi, meaning that high performance sensor arrays are possible for small or large areas.

With a few design adjustments, this same backplane technology used for displays can be used in a sensor array: instead of writing voltages onto pixel capacitors, the same pixels are used to store charge created by the photo-detector layer, and the charge is the read off, row by row, to read the image.

This is a standard way of using backplanes in sensor arrays, for example, in digital X-ray sensors, but today all such arrays use a glass backplane.

The Advantage of Using Organic Materials

There are a number of advantages of using organic materials for the photo-detector over their inorganic counterparts.

Firstly, the organic layer can have a very high absorption coefficient, meaning that only very thin layers of the material are needed (around 0.1 micron), deposited as a continuous film. The full coating surface enables photo detection on whole surface for a 100% pixel fill factor.

Secondly, the spectral range of the OPD component can be broad, meaning that imaging systems able to employ the same component type for near-IR and visible sensing, and with the addition of a suitable scintillation layer, can also be used for X-Ray sensing, for example.

In terms of manufacture, both the backplane and the OPD layer are made using printing operations at ambient temperature, potentially leading to substantial cost-efficiencies compared to the manufacturing processes involved in traditional inorganic components. It also means that substrates such as PET can be used, rather than more exotic, high temperature substrate materials.

Potential Applications

From a use-case point of view, the real benefit is the resulting product flexibility: Since both the substrate and the active layers are plastic and fundamentally flexible, the flexibility of the entire device is much higher than would be possible with inorganic transistor technologies, resulting in a range of benefits, some more obvious than others. Such thin, light sensor arrays have clear benefits for large areas, where glass equivalents are fragile, heavy and impractical. For example, portable X-Ray sensors for security applications would benefit from such technology.

Flexible sensors can be conformed around a curved surface, and made just a few tens of microns in thickness, thereby adding almost no incremental weight to a product. This has clear benefits for mobile devices, and opens up myriad design possibilities for new products.

A less obvious, but potentially ground-breaking benefit is the ability to cut the sensor array into unusual shapes—the product need not be constrained by conventional display dimensions. Sensors that are circular, triangular or any other shape are possible by cutting the array with a laser into the appropriate pre-determined shape. In addition, custom sizes for smaller order sizes without the prohibitive tooling costs normally associated with new display designs.

Other application areas include smart packaging for logistics, biomedical diagnostics and monitoring, user identification and fingerprint scanning for security and mobile commerce, 3D interactive user interfaces and non-contact touch sensing, and many others.

Organic electronic materials have now reached a performance point where they can meet or exceed that of the equivalent inorganic technology, and has already been proven in an industrial scale for the manufacture of flexible displays.

This combined with the benefits of truly flexible, durable, or micron-thin sensor array and displays will unlock a whole raft of new possibilities and change the rules for product designers, leading to products design simply not possible with inorganic technology.

Paul Cain is head of business planning at Plastic Logic.

Source: Electronics Weekly, 26.11.2013