The organic light-emitting diode (OLED) technology has become the state-of-the-art for mobile displays, is penetrating the TV display sector, and is a light-source with further application potential in the area of automotive lighting, indoor lighting, and wearable and health-monitoring electronics. Recently, J. Rafols-Ribe et al. showed that the efficiency and lifetime of OLEDs can be increased by the layer-by-layer optimization of its fabrication [J. Rafols-Ribe et al., Sci. Adv., 4 (2018)]. This was possible by heating the substrate during deposition. In this work, we investigate how each heated OLED-layer influences the overall device-performance change and compare it to post-annealing treatment by studying the optoelectrical and photoluminescent characteristics of the OLEDs. Additionally, we employ impedance spectroscopy to calculate the giant surface potential of the thin films in an attempt to learn more about the nanoscopic changes of the layer morphologies. Here, different emitter systems, i.e., phosphorescence- or thermally activated delayed fluorescence-based ones, are tested. We find a clear correlation between the surface potential of a transport layer and efficiency of the OLED. Furthermore, we discuss that the efficiency is improved because of a reduced non-radiative decay rate. We show that the in-situ heating increases the efficiency only for OLEDs containing polar layers. These findings contribute to the understanding of the in-situ heating of OLED layers and provide further levers for OLED improvement.
Nature provides many examples of surface structures with multiple functionalities. Some of those, such as light management and self-cleaning, are of interest for increasing the efficiency of optoelectronic devices, such as OLEDs, and for adding new surface functions. However, mimicking and transferring these textures to polymers over large areas often requires complex processes at high costs. Here, we demonstrate a low-cost strategy to fabricate hierarchically textured polyethylene terephthalate (PET) films by plate-to-plate hot embossing. Laser-machined stainless-steel plates with doublescaled hole-like textures were used as master for hot embossing. The larger structure with a period between 30 µm and 70 µm and depths up to 8 µm was produced by direct laser writing (DLW), whereas the smaller structure featuring a period of 3 µm at a depth up to 2 µm was fabricated by direct laser interference patterning (DLIP). The textured surfaces of stainless steel were then molded onto PET films at a pressure of 42 MPa and a temperature of 85°C using a hydraulic press. Topographical characterization was performed by confocal microscopy and scanning electron microscopy. Experiments have shown an increased static water contact angle up to 105°. Furthermore, the hierarchically microtextured foils were studied as out-coupling layers in OLEDs, showing a potential increase in device efficiency of up to 57%. The results thus indicate a good suitability of the developed surfaces for use in highly efficient OLEDs with easy-to-clean properties.
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