Organic, flexible displays

Materials World magazine
1 Nov 2008
An animated cartoon projected by a 635nm laser onto a flexible plastic screen

A prototype organic upconversion display panel that is transparent and bendable has been developed by researchers from Sony’s Materials Science Laboratory and the Max Planck Institute for Polymer Research, both in Germany. This technology could be used to create digital newspapers, rollable television screens or ultra-lightweight laptops.

The technology combines a palladium-porphyrin complex sensitiser and an antracene emitter, both extruded onto a polystyrene sheet within a viscous polymeric matrix. This is then exposed to galvo-scanned laser diodes. These emit non-coherent light in all directions, creating a multilayered display with a wide field of view. The image produced offers efficiencies of up to six per cent, which is comparable to other organic displays, and a response time of 500µs (LCDs, by comparison, exhibit response times in the millisecond range).

Previous attempts to create upconversion displays were hampered by problems with inflexible inorganic upconversion materials. ‘These examples were based on rare-earth ion upconversion in glassy materials, in bulk or dispersed form,’ explains Dr Stanislav Baluschev of the Max Planck Institute. ‘In bulk they are transparent but not flexible. In polymer-matrix dispersions they are flexible, but have huge back-reflection of the excitation laser.’

Dr Tzenka Miteva, of Sony, notes that the optically pumped display in the new 2D upconversion device does not require electronics in the screen, allowing it to be more flexible. It overcomes resolution problems by replacing the organic sealant toluene used in other displays with a trimer/tetramer mixture of styrene oligomers, which efficiently seal the device while increasing its durability. When tested for 100 days, the display did not lower its efficiency. It is powerful enough to be seen in daylight using a 10-20MW continuous wave laser, but also offers low intensities, ‘meaning it is safe to observe’, notes Miteva.

The response time of the device is controlled by the concentration of sensitiser/emitters used, allowing broad intervals to be factored in. And it has an external quantum yield of up to 3.2%, meaning for every 100 infrared photons absorbed 3.2 upconverted photons are emitted – a ‘good number for displays,’ says Miteva.

The size of the display is limited ‘only by the parameters of the laser scanning system,’ says Baluschev. The team has created screens with 15x15cm images, showing pictures in blue, green, yellow and orange – it is still working on achieving red and increasing efficiency.

The group believes the technology could be easily and economically scaled up using printing or roll-to-roll processing, as the materials used are robust.

This work was published in the October issue of the New Journal of Physics.