In the course of the successful closure of the BMBF funded research project TOPAS2012 (end of 2012) OSRAM presented already transparent OLEDs with remarkable performance data. With a lighting area of 116 cm² a transmission value of 57% could be achieved accompanied by a very good homogeneity. The white OLED showed a luminous efficacy of 20 lm/W. Since „transparency“ for an OLED includes the lack of haze and clear view requirements through the device, this value for luminous efficacy represents a very good performance. Compared to high performance OLEDs with more than 50 lm/W transparent OLEDs cannot make use of light extraction methods such as scatter layers because this would lead to haze. The final demonstrator of the project was a concept luminaire called “Rollercoaster” featuring 30 OLEDs in an arrangement inspired by the well-known Möbius ribbon.
Compared to standard OLEDs, transparent ones need two electrodes with look-through properties. Due to the significantly lower conductivity of transparent electrodes compared to metal cathodes the scaling to larger areas is still a major challenge. Since good homogeneity is an important requirement, the voltage drop over a laterally extended electrode is a key factor. Conductivity and transmission are competing parameters which have to be compromised when it comes to reasonable device layout. Thus, the vision of a lighting window is a goal which still needs some research.
Nevertheless, the laterally extended „light out of the nothing“ is a fascinating feature which can leverage new applications. In order to develop suitable ideas it’s worth to have a deeper look in some intrinsic properties of transparent OLEDs.
Transparent OLEDs emit light into two main directions – to the front and to the back. In the course of the manufacturing process it is possible to adjust the emission ratio between 50:50 and approximately 80:20. This is done by tuning the conductivity of the electrodes which has an impact on their reflectivity. An even more pronounced emission ratio would lead to a rather low overall transmission and consequently affect the transparency significantly.
By using an asymmetric emission ratio various application options can be targeted. Imagine, an observer is looking onto the high intensity side of an illuminating transparent OLED. Then the OLED acts as a light curtain, i.e. objects placed behind it are not visible any more. If the observer is looking onto the weak intensity side, the objects behind are illuminated and can be seen. By combing these effects with dynamic dimming scenarios new presentation concepts, e.g. for vitrine showcasing can be developed.
Equivalently to standard OLEDs, also for transparent OLEDs the lighting area can be structured, e.g. as a lighting frame or something similar.