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By using the room temperature phosphorescence of organic materials, transparent and flexible tags with resolutions of up to 700 dpi and an activation time of 120 ± 20 ms can be achieved. Any pattern can be written into the film by selectively illuminating the film with ultraviolet light through a shadow mask. The inscribed information can then be erased again with the aid of a heat source, and the overall cycle can be repeated many times (> 40 tested).
The functional layer of these devices consists of organic biluminescent emitters doped into a poly(methyl methacrylate) (PMMA) host matrix covered with an oxygen barrier layer. By wet processing under ambient conditions, molecular oxygen is present in the emission layer, hence quenching the phosphorescence effectively. During this quenching process, highly reactive singlet oxygen is generated, which can chemically bond to the surrounding environment. Therefore, the molecular oxygen concentration decreases in the illuminated areas. The decreasing oxygen quenching is at some point outcompeted by the radiative rate of the emitter, enabling locally resolved phosphorescence in shape of the used mask. The oxygen permeability of the barrier layer is temperature dependent and increases with rising temperature. This enables an oxygen refilling of the functional layer by heating the device and thus increases the quenching rate to a value prior to the activation process. After a short cooling phase, the film can be rewritten.
Under continuous-wave (cw) illumination, any luminescence that is independent of the oxygen concentration, be it fluorescence or phosphorescence with a high radiative rate, limits the contrast between activated and oxygenated areas. Therefore, different material systems are developed and tested, showing reduced fluorescence but maintaining the oxygen quenching ability of the phosphorescence. The emitter 4,4‘-dithianthrene-1-yl-benzophenone (BP-2TA), with a phosphorescence lifetime of 30 ms and a high phosphorescence-to-fluorescence ratio of about 20, fulfills these requirements best and thus enables high-contrast tags that can also be read under cw illumination.
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