Optimizing OLED structures for a-Si display applications via combinatorial methods and enhanced outcoupling

Active-matrix organic light-emitting displays driven by amorphous silicon (a-Si) thin-film transistors (TFT) require superior organic light-emitting device (OLED) performance in order to achieve sufficient pixel brightness and to avoid degradation of the a-Si transistors. In addition, the a-Si TFT circuitry of high-resolution full-color displays can become rather complex and therefore occupies a large amount of the pixel area, leading to a significantly reduced aperture ratio with conventional bottom-emitting OLEDs. A top-emitting device architecture is therefore advantageous. In this paper we describe general concepts for optimizing top-emitting OLEDs via combinatorial methods and for enhancing outcoupling. To optimize OLED performance, we use combinatorial device fabrication. Our system allows the simultaneous fabrication of 10 x 10 individual devices on one substrate, enabling a systematic variation of material combinations and electrodes as well as of device parameters such as layer thickness, layer sequence, and dye dopant concentrations. In the first part, we present an over-view of the capabilities of combinatorial methods for electrical and electro-optical device optimization. We show results on multi-layer OLEDs ranging from trilayer devices to phosphorescent five-layer OLEDs. The second part describes an outcoupling concept that allows the emission characteristics of top-emitting OLEDs to be tailored. We demonstrated that and analyzed how the angular intensity distribution and the spectral characteristics can be tuned. Light outcoupling can be enhanced by simply varying the optical thickness of a dielectric layer deposited on top of a semitransparent metal electrode. Using this capping-layer concept the outcoupled light intensity in forward direction was increased by a factor of 1.7 while concomittantly achieving a high color purity. The strength of the capping-layer concept is in particular that the optical and the electrical device performance can be optimized separately. Using optimized layer thicknesses and the capping concept, we achieve an efficiency of NTSC (National Television Standard Committee) white from separate red, green and blue sub-pixels exceeding 22 cd/A under realistic display-driving conditions, thus enabling the world's largest (20-inch) a-Si active-matrix OLED display. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.