Analysis and optimization of light outcoupling in OLEDs with external hierarchical textures

Abstract

Hierarchical textures (combining 2D periodic large and small micro textures) as an external outcoupling solution for OLEDs have been researched, both experimentally and by optical simulations. For the case of a red bottom emitting OLED, different hierarchical textures were fabricated using laser-based methods and a replication step and applied to the OLED substrate, resulting in an increased light outcoupling. Laboratory-size OLED devices with applied textured foils show a smaller increase in efficiency compared to the final large area devices. The results show that the full exploitation of textured foils in laboratory-size samples is mainly limited by the lateral size of the thin film stack area and by limited light collection area of the measuring equipment. Modeling and simulations are used to further evaluate the full prospective of hierarchical textures in large area OLED devices. Optimization of hierarchical textures is done by simultaneously changing the aspect ratios of the small and large textures and a potential of 57% improvement in EQE compared to devices without applied textures is predicted by simulations. Optimized hierarchical textures show similar outcoupling efficiencies compared to optimized single textures, while on the other hand hierarchical textures require less pronounced features, lower aspect ratios, compared to single textures to achieve the same efficiencies. Hierarchical textures also help in eliminating flat parts that limit outcoupling efficiency. Finally, the limiting factors that prevent higher outcoupling are addressed. We show that the dominant factor is non-ideal reflection from the organic thin film stack due to parasitic absorption. In addition, possible ways to further increase the outcoupling from a thick substrate are indicated.

Opto-Electronics Review

“…On the other hand, a decreased or increased transmittance does not seem to have much effect on smaller angles, which is consistent with the results in Ref. 25. A much higher T for three-sided pyramids at angles around the critical angle gives an advantage over other textures in light extraction.…”

“…In the simulations, all textures are treated as part of the glass substrate, which is justified due to very similar optical properties of lacquer, textured foils, and a glass substrate. These exhibit negligible differences in refractive index, thus, as it was already shown in previous publications [22,25], can be treated as a single layer.…”

“…As investigated in Ref. 25, the light outcoupling depends essentially on the angle-dependent transmission (T) from the glass substrate to the air. For the flat glass/air interface, a pronounced dependence is observed with a high transmittance at small angles -below the critical angleand a transmittance of 0 at angles greater than the critical angle -see black line in Fig.…”

“…Recent development in the field of light extraction has produced different highly efficient solutions, both in optimizing OLED internal and external optical losses -see for example the latest reviews on light outcoupling solutions in OLEDs [1,11,12]. In particular, much research has been done to limit substrate losses by attaching microlenses [13][14][15][16], scattering films [17][18][19][20], introducing scattering particles [21], or using microtextures [22][23][24][25]. These solutions provide high improvements at low cost and, generally, do not affect the electrical properties of the sensitive organic layers.…”

Improved Light Extraction in Organic Light‐Emitting Diodes via Semiconductor Dilution

“…[3,4] The former is effectively resolved by applying light extraction films, microlens arrays, or simply by roughening the substrate. [5][6][7] The latter has proven more difficult to overcome. Internal light extraction has been addressed extensively in both modeling and experiment using various approaches to scatter out waveguided light via corrugation, [8,9] grids, [10,11] or wrinkles [12] introduced within the ITO anode and/or device stack.…”