The German unit of Sony Corp and the Max-Planck-Institute, a German research institute, prototyped a flexible translucent display made entirely of organic materials.
The display was developed based on a method called "upconversion (UC)," which makes the phosphors in the screen emit light by photoexcitation. The team reportedly solved many of the existing problems with UC.
Sony and the Max-Planck-Institute developed three types of UC displays, which respectively emit orange, green and blue lights. The displays were manufactured by adding PdPh4TBP, a palladium (Pd) complex serving as a photosensitive material, to a polycarbonate transparent sheet and then adding organic materials (rubrene, BPEA and perylene) as emitting materials for orange, green and blue.
The display scan uses a laser beam with an intensity of 10mW and a wavelength of 635nm at a high speed (12kHz) to render images. The two organizations presented a video showing the operation of the green panel.
Prior studies relating to UC displays have been made since the 1950s. UC displays are similar to CRT displays, which render images by analog scanning, but the UC method can use glass and transparent plastics for its medium. Thus, the method is attracting attention especially in the last few years because it is suitable for flexible and 3D displays, Sony and the Max-Planck-Institute said.
However, existing UC displays have a number of major problems. (1) The UC display requires expensive rare earth elements as the emitting materials. (2) The display scan needs a laser beam with a high intensity of 10kW/m2 or higher to achieve a sufficient luminance. (3) Two or more laser light beams with different wavelengths are required. (4) The size of the display is limited to several centimeter-square. (5) And the performance degrades quickly. The latest technique resolved all of those problems, according to the organizations.
The key to the success is the triplet-triplet annihilation and other schemes used for the display. In the triplet-triplet annihilation, two materials are combined for use as the photosensitive and emitting mechanisms. When the photosensitive material is excited by light (or infrared light), the generated energy is passed on to the emitting material while still in the triplet state (state where the spin of multiple electrons is not determined), and then converted into photon energy.
The triplet-triplet annihilation approach enabled the use of palladium, which is less expensive than rare earth elements. In addition, the light emission efficiency was improved to four to five orders of magnitude higher, thus making it possible to use a laser beam with an intensity of only 10-20mW/m2.
As a result of separating the photosensitive material and the emitting material from each other, images can be rendered with one laser beam with a single wavelength.
"It is possible to display color video images with a single laser beam by painting the pixels in different colors," according to the paper published by Sony and the Max-Planck-Institute.
Regarding the durability, the two organizations reported that no deterioration in performance was observed after the operation for 100 hours.
"It is highly possible to manufacture the display by printing techniques such as the roll-to-roll processing," according to the paper.
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