Emission Characteristics of OLEDs on Planar and Non-Planar Substrates
- PDF / 326,263 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 110 Downloads / 279 Views
L7.31.1
Emission Characteristics of OLEDs on Planar and Non-Planar Substrates Marcus Scheffel1,2, Jan Birnstock1,2, Arvid Hunze1,2, Ralph Paetzold1,2, Lothar Rau3, Wolfgang Rogler1, Sven Voegele3, and Albrecht Winnacker2 1 Siemens Corporate Technology, CT MM 1, Erlangen, Germany 2 Department of Material Science VI, University of Erlangen-Nuernberg, Erlangen, Germany 3 Institute for System Theory and Display Technology, University of Stuttgart, Germany ABSTRACT We report on light extraction experiments with polyfluorene based organic light emitting diodes (OLEDs). The intensity distribution of emission within the substrate and within air is determined experimentally. For the devices investigated in this study, we find that the external quantum efficiency can be increased by a factor of 1.82, in the case of complete suppression of waveguiding within the substrate. The experimental data is in good agreement with modeling results based on dipole emission within a multilayer system. As a consequence of these findings, we modified the device topology by introducing light extraction structures between glass substrate and indium tin oxide anode. This results in redirection of photon trajectories and thus in a higher probability to extract waveguided radiation. At a luminance of 100 cd/m², the photometric efficiency for emission in forward direction is enhanced by a factor of 1.45 to 8.9 cd/A.
INTRODUCTION The device performance of organic light emitting diodes (OLEDs) has been drastically enhanced within the last years. Nevertheless, in order to compete with well established display technologies, OLEDs are still subject to further improvement. One major goal is the increase of external quantum efficiency. This is of particular importance for mobile applications where low power consumption is essential. Total internal reflection at the device interfaces leads to waveguiding within the substrate and the thin active layers and, therefore, to a significant loss of radiated optical power [1]. Hence, efficiency can be enhanced by optimizing light extraction. In the first part of this article we determine the maximum efficiency enhancement achievable by total extraction of radiation from the substrate. In the second part we investigate the effect of light enhancement structures implemented between substrate and anode layer. The structures are produced photolithographically in the shape of cones and truncated cones, respectively. The electro-optical performance of diodes with and without structures is discussed.
PART I: BASIC INVESTIGATIONS A method to determine the ratio between extracted and waveguided radiative power is sketched in Fig. 1. By placing the emissive area of an OLED within the center of curvature of a hemispherical lens most of the otherwise trapped light is coupled out. Light that is still remaining within the substrate can be detected with a photodiode attached to the substrate edge.
L7.31.2
a)
extracted photon
substrate
θ
OLED waveguided photon
b) far-field detector index matching gel
lens
θ
extract
Data Loading...
