Organic light emitting diodes with horizontally oriented thermally activated delayed fluorescence emitters†
Horizontal orientation of the emission transition dipole of light-emitting molecules plays a critical role in the light outcoupling efficiency and the performance of organic light-emitting diodes. It is well established that linear and planar small molecules are generally preferred to achieve such a horizontal molecular orientation in vapor-deposited organic thin films. Here, we designed and synthesized four novel carbazole-based thermally-activated delayed fluorescence (TADF) molecules with different shapes and degrees of planarity in order to examine the influence of the emitter structure on the molecular orientation and the electroluminescence properties in TADF OLEDs. Molecular orientation of the TADF molecules in neat films and in blends was investigated using variable angle spectroscopic ellipsometry and angle dependent photoluminescence measurements, respectively. The results provide new important insights into the influence of the planarity of TADF molecules on their molecular orientation in vapor-deposited organic thin films. The photophysical and electroluminescence properties of these TADF molecules were then investigated to examine the influence of the molecular orientation on the performance of TADF OLEDs. The most efficient device was obtained using the TADF emitter showing a nearly perfect horizontal orientation of the emitting dipoles in the blend films. The maximum electroluminescence external quantum efficiency measured in this device was found to be 15.4%, which is about 1.75 times higher than the theoretical value calculated using an isotropic distribution of transition dipoles.