(A) study on outcoupling enhancement of organic lightemitting diodes based on trans-scale optical analysis

Abstract

Outcoupling efficiency has been a limiting factor in achieving highly efficient organic light-emitting diodes (OLEDs). To improve the outcoupling efficiency of OLEDs, numerous groups have attempted to extract guided modes with modification of internal/external structures or the microcavity effect. For a quantitative analysis of OLEDs with geometrical outcoupling structures, we present a trans-scale optical simulation combining wave-optics representing the dipole emission in a cavity structure and geometrical optics handling incoherent light propagation. The proposed trans-scale optical approach helps to estimate precisely the enhancement of OLEDs and to find optimized conditions including geometrical structures. The validity of the proposed optical simulation is verified by comparing experimental and simulated results. After reviewing the wave-optic simulation methods, we use them to present a systematic study for balancing the efficiency and the angular color stability of top-emitting OLEDs (TEOLEDs) to solve so-called white angular dependence (WAD) issues. We carefully explore the origins of WAD problems in multi-color subpixel type TEOLEDs and provide optical optimization guidelines as a reliable scheme. With the proposed scheme, we demonstrate TEOLEDs with a low angular color deviation satisfying a reasonably low decrease in efficiency. In the second part, we focus on a feasible outcoupling scheme that can be readily used for large area devices. A method based on high refractive index substrates is explored for this purpose. As a low-cost and reliable platform for enhancing outcoupling efficiency, we utilize commercial polyethylene naphthalate (PEN) instead conventional high refractive glass to address the cost issue of available. The inherent scattering properties induced by nano-scatter fillers inside the industrial grade PEN are shown to extract substrate confined light without any additional optical components. For a quantitative analysis, we combine the wave-optic model with geometrical/statistical optic model that describes the scattering properties by a simplified model based on Henyey-Greenstein phase function controlled by an asymmetry parameter. This approach provides fast and quantitative modeling and thus opens up new possibilities of optical optimization of OLEDs containing light extraction structures based on scattering mechanism. In the last part, OLEDs on high refractive substrates with microlens arrays are explored in particular for 2nd order cavity OLEDs, which are widely adopted in industrial for high yield. The wide angular substrate confined mode (WASM) is studied as one of the factors limiting OLED efficiency in the 2nd ？cavity OLEDs structures on high-index substrates with conventional microlens arrays. The angular intensity distribution depending on the base OLED structures is explored, and optimal geometrical substrate structures are explored in consideration of the WASM factor. We also present the advantages of scattering-based scheme over the conventional MLA-only approach in the case of the OLED structures with high WASM factors.