Optical Outcoupling in Efficient Single-Layer TADF Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) serve as the active component in high-end displays and have received a considerable amount of interest because of their potential in lighting applications. However, for a more widespread application, production costs need to be lowered and the energy efficiency and lifetime need to be improved. Over the past two to three decades, OLEDs were designed with increasing complexity, using a multilayer structure consisting of typically five layers. Recently, we have developed an efficient and stable single-layer OLED based on thermally activated delayed fluorescence (TADF) [1], stepping aside from the conventional multilayer device concept that has been commonplace in the OLED field for more than two decades.<br/>The single-layer TADF OLED has the advantage of extremely low operating voltages and high power efficiency. In addition, due to the broad recombination zone, the operational lifetime is greatly improved. However, a remaining question is how this broad emission profile affects the optical outcoupling efficiency. Here, we demonstrate that the optical outcoupling in single-layer OLEDs can be calculated with a combined optical and electrical model. The charge-recombination profile is first simulated based on the measured charge-transport properties. This recombination profile is subsequently used to calculate the optical outcoupling efficiency. It is demonstrated that an outcoupling efficiency as high as 26% can be obtained for a CzDBA single-layer OLED. The results show that single-layer devices with a broadened emission zone can achieve similar outcoupling efficiency to multilayer OLEDs with an optimized confined emission zone, establishing a route to efficient, stable, and simplified OLEDs.<br/>[1] Kotadiya, N.B., Blom, P.W.M. & Wetzelaer, GJ.A.H. Efficient and stable single-layer organic light-emitting diodes based on thermally activated delayed fluorescence. Nat. Photonics 13, 765–769 (2019). https://doi.org/10.1038/s41566-019-0488-1