Influence of Substrate Temperature on Multilayer Thin Film Growth, Charge Carrier Injection and Efficiency of OVPD-Processed Organic Light Emitting Diodes

摘要

Organic light emitting diodes (OLED) have great potential for future lighting applications. At present, most OLED intended for lighting are processed by vacuum thermal evaporation (VTE). Organic vapour phase deposition (OVPD) is a more elaborate process than VTE, but allows to fabricate devices with innovative stack designs as for example mixed layers with varying compositions. In order to control the OVPD process such that efficient and long-living devices can be achieved, detailed knowledge about the influence of the substrate temperature, gas flows and deposition chamber pressure on the film and interface formation is crucial. In this work, we present a study on the influence of the substrate temperature in the OVPD process on the morphology of all individual layers of a red phosphorescent OLED as well as on the electro-optical properties of the complete device. All samples have been deposited employing AIXTRON OVPD systems. The surface roughness of all layers of the investigated OLED was shown to depend strongly on the substrate temperature during the evaporation process. However, by choosing suitable substrate temperatures, the RMS roughness of all layers could be reduced to less than 1.1 nm. Furthermore, it is shown that the substrate temperature has a great influence on the I-V characteristics of the hole injection layer and hole transport layer. Based on the obtained results, an optimized substrate temperature profile could be tailored for the investigated OLED and therefore its luminous efficacy was increased significantly compared to reference OLED which were processed at a constant substrate temperature.