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.
Organic room temperature phosphorescence (RTP) is used to realize rewriteable (>40 cycles), transparent and flexible optical tags with high resolution (>700 dpi). The devices contain an organic biluminescent emitter doped into polymethylmethacrylate (PMMA). They show phosphorescence, which in general is quenched by molecular oxygen. However, by illuminating with ultraviolet light (365 nm), this molecular oxygen locally vanishes at the irradiated area, enabling RTP at defined spots. Further, by illuminating with infrared light, the system can be refilled with oxygen leading to quenching of the RTP again. Therefore, any luminescent pattern can be written into and erased from the tag using light only.
Measuring the photoluminescence quantum yield (PLQY) is a method often used within numerous fields of luminescent material science. Determining its absolute value relies on counting photons and hence, it is a very sensitive technique. Therefore, systematic errors that may occur during the measurement are discussed widely. However, the statistical uncertainty within those measurements remains mainly unconsidered.
Here, we propose a new way of data analyses that exploits multiple measurements and a subsequent evaluation using the weighted mean. This leads in an efficient way to a very low statistical uncertainty. Additionally, time-dependent influences on the measurement can be identified that way.