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By utilizing the room temperature phosphorescence (RTP) of organic materials, transparent and flexible optical tags with high resolutions up to 700 dpi are realized. Through masked ultraviolet (UV) illumination or laser ray writing, any phosphorescent pattern can be printed into the transparent device. With the help of infrared (IR) light, it is possible to fully erase the imprint again. This cycle is shown to be repeatable at least 40 times.
The functional layer of these devices consists of different organic biluminescent emitters doped into a polymethylmethacrylate (PMMA) host matrix covered with an oxygen barrier layer. However, due to the sample preparation in ambient conditions, molecular oxygen is still present in the emissive layer. This leads to a full quenching of the phosphorescence and the generation of excited singlet oxygen due to triplet-triplet interactions involving the oxygen triplet groundstate. The singlet oxygen has a high chemical reactivity and thus is able to form a bond with the surrounding materials. Consequently, the molecular oxygen concentration decreases in the illuminated areas. The concurrently decreasing oxygen-quenching rate is at some point outcompeted by the radiative rate of the emitter, enabling locally resolved phosphorescence. This emission resembles the intended pattern.
The oxygen permeability of the barrier layer is temperature dependent and increases with rising temperature, which can be realized using a hotplate or IR illumination. This enables an oxygen refilling of the functional layer and therefore increases the quenching rate to a value prior to the activation process, leading to the vanishing of the phosphorescent image. After a short cooling phase, new information can be printed into the device.
In continuous wave illumination, fluorescence may limit the contrast between activated areas and those still containing oxygen. Hence, different material systems showing reduced fluorescence without losing the oxygen quenching ability of the phosphorescence, are developed and tested.
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