Transparent and color-tunable organic light-emitting diodes with highly balanced emission to both sides

Future lighting applications will strongly benefit from transparent luminescent devices. Here, we demonstrate transparent organic light-emitting diodes (OLEDs), which provide real-time adjustment of the emission color. Making use of the AC/DC concept, two stacked subunits can be addressed independently via an AC signal. Combining blue and yellow emission leads to the possibility to tune the emitted color between deep blue over cold white and warm white to yellow on both emission sides. For such highly complex device architectures, the thickness of each layer needs to be adjusted carefully in order to achieve balanced and efficient emission in both directions. Therefore, optical simulations are carried out to optimize the OLED. Based on these simulations, we present transparent, indium-free OLEDs that achieve a luminous efficacy of 8.7 lm/W in bottom direction and 9.7 lm/W in top direction at a brightness level of 1000 cd/m(2) for warm white emission and a peak transmission of 56%. Using an emitter combination providing red, green, and blue emission, we were able to achieve a high color-rendering index (CRI) of 84, which further expands the range of possible applications for this promising device concept. (C) 2016 Elsevier B.V. All rights reserved.