OVPD-processed OLED for general lighting

摘要

Due to continuous advancements of materials for organic light emitting diodes (OLED) a new field of application currently opens up for OLED technology: General lighting. A significant reduction of OLED production cost might be achieved by employing organic vapor phase deposition (OVPD). OVPD is a novel process for depositing organic thin films from the gas phase. In contrast to the well established process of vacuum thermal evaporation (VTE), OVPD allows to achieve much higher deposition rates (and consequently a higher throughput). Furthermore, OVPD allows to process complex device structures with high reproducibility. This holds especially for devices containing multiple mixed layers consisting of several different materials. The purpose of this work is the development of OVPD-processed highly efficient white emitting OLED for general lighting. Different organic light emitting materials (phosphorescent as well as fluorescent) were investigated with respect to their compatibility with the OVPD process. Therefore, monochrome OLED were processed and characterized with respect to their electro-optical properties. Comprehensive modifications of the organic layer stack were performed, in order to understand the exciton transfer mechanisms in the investigated material systems. Using the investigated phosphorescent materials in an optimized device structure, an external quantum efficiency (EQE) higher than 17% was achieved (without improved light out-coupling, i.e. with a light out-coupling efficiency of about 20%). Using the investigated fluorescent materials in an optimized device structure, an EQE of up to 7.9% was achieved. This surprisingly high efficiency (fluorescent materials typically exhibit an EQE of only 5%) can be explained by a partial conversion of excited triplet states into excited singlet states. Based on the obtained results, different approaches for white emitting OLED were tested. By (vertically) combining a blue fluorescent emitting layer with a red and green phosphorescent emitting layer in one single OLED unit (single unit OLED) a white OLED with a maximum power efficiency of 16 lm/W was obtained. However, an efficiency of about 30 lm/W could be reached by (laterally) combining three monochrome OLED units in one device (multi unit OLED). To increase the efficiency of a multi unit OLED, it was tried to improve its light out-coupling efficiency (which is typically only about 20% for devices of this type) by placing an inorganic semitransparent reflector layer at the anode side of the organic layer stack (in order to create a weak micro cavity together with the reflective cathode). For blue emitting OLED no efficiency improvement was obtained. However, the luminous efficiency of red emitting OLED was nearly doubled by means of this approach (reaching a luminous efficiency of 60 lm/W and an EQE of 21% @ 200 cd/m2). For a white emitting OLED consisting of monochrome pixels, this corresponds to an efficiency improvement of more than 30%, so that an overall efficiency of about 40 lm/W can be obtained. In summary it has been demonstrated, that highly efficient white emitting multi unit OLED can be realized by means of OVPD. A remaining challenge is the qualification of organic n-type doping by means of OVPD. (Since organic p-type doping by means of OVPD has already been demonstrated successfully, organic n-type doping is the last missing building block for a complete p-i-n diode, which would then also enable vertically stacked multi unit OLED).