Vortrag (20 Min., 5 Min. Diskussion, 5 Min. Raumwechsel)
In the last decade organic light-emitting diode (OLED) technology has shown a fast progress for becoming a strong competitor in the display and lightning markets. Some of the appealing features of OLEDs are high device efficiency, wide viewing angles, high brightness contrast, low processing costs, and the possibility of fabrication on lightweight and flexible substrates. However, OLEDs still lag behind inorganic LEDs regarding overall device efficiency. One of the main hurdles that state-of-the-art OLEDs face to reach higher efficiencies is the low light out-coupling to the air, caused by total internal reflection at the air-substrate interface. A simple, yet effective, strategy to out-couple the substrate modes into useful far radiation is attaching a microlens array (MLA) on the external surface of OLEDs. This component can be produced at industrial levels by stamping a transparent polymer with a structured master using hot-embossing methods based on conventional flat presses or continuous roll-to-roll systems. However, the most used method to fabricate the master for the replication process is photolithography, which requires several time consuming steps. A single-step method for producing the MLA master with high resolution and throughput is Direct Laser Interference Patterning (DLIP).
In this work, periodic hole-like structures were patterned on metal stamps employing four-beam DLIP. Varying the DLIP process parameters, such as laser fluence and overlapping angle, textures with periods ranging from 1.2 µm to 2.0 µm and structure heights between 50 nm to 200 nm were produced. Next, the stamps were used to produce MLAs by hot-embossing polyethylene terephthalate (PET) foils. The topographical characterization, carried out by confocal microscopy, showed a faithful pattern transfer on the PET foils. Finally, MLAs were attached on the front glass surface of different OLEDs emitting at red, green and blue wavelengths. The optoelectronic characterization, consisting in electroluminescence and external quantum efficiency (EQE) measurements, showed that the MLAs effectively couples light from the substrate to the air enhancing the device efficiency by 6-13%.