External light-outcoupling structures in organic light-emitting diodes (OLEDs) present an efficient and cost-effective solution to improve optical performance of devices. External structures can be realized, for example, by applying a microtextured foil on the emitting side of the device. We employ advanced three-dimensional optical simulations to investigate the effects of OLED structures with different external textures in relation to different emitting dipole orientations and layer thicknesses of the OLED stack. We investigate light outcoupling of a red, ITO free, large-area, bottom-emitting OLED, by applying various sinusoidal and pyramid-like external microtextures in periodic (rectangular and hexagonal) and random arrangements. It is shown that there are minimal differences in outcoupling efficiency for the optimized textures for different texture feature shapes, under condition that sufficiently high aspect ratio (height / period) of the texture is applied. With optimized external texture with three-sided micropyramids, horizontally aligned dipole sources and optimized layer thicknesses of OLED stack more than 62 % outcoupling efficiency is predicted. Interestingly, the results show that the highest efficiencies for individual orientation of dipoles, can be achieved with the same shape and size of the texture, regardless of the preferential orientation of the emitting dipoles, which simplifies further research and development of external light extraction.
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.
Nanostructures play an important role in state-of-the-art photovoltaic devices. Optical and electrical characteristics of the devices can be improved with properly designed and fabricated nanotextures. In this contribution, we highlight examples of possible fabrication of nanotextures by using UV nanoimprinted lithography, their optical characterization by camera based angular resolved spectroscopy and perform computer-assisted design of nanotextures for best optical performance of heterojunction silicon and ultra-thin chalcopyrite solar cells.
We present improvements in light outcoupling for the example of red, bottom-emitting, ITO free OLEDs. As an optimization tool we use experimentally verified coupled modelling approach, where we simulate a complete OLED device, including thin-film coherent stacks as well as thick microtextured incoherent layers (substrate). We calibrate the combined model on a fabricated small sample OLED. The research of lateral limitations and limited integrating sphere opening effects show that small area effects can lead to large deviations in outcoupling efficiency with respect to the large area devices commonly used in lighting applications. On the large area devices, we focus on the optimization of the thinfilm stack cavity in the OLED by tuning the thicknesses of thin layers. We show the importance of including the complete device in the optimization process, including the thin-film stack and the thick substrate with the outcoupling textures. We show that an OLED with an optimized planar cavity and applied external positive shaped dome texture can reach up to 50.5 % light extraction efficiency according to simulations.
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