An Indolocarbazole-Based Thermally Activated Delayed Fluorescence Host for Solution-Processed Phosphorescent Tandem Organic Light-Emitting Devices Exhibiting Extremely Small Efficiency Roll-Off

The study reports the development of a solution-processed phosphorescent tandem organic light-emitting device (OLED) exhibiting extremely small efficiency roll-off. The OLED comprises two light-emitting units (LEUs) connected by an interconnecting unit and employs a thermally activated delayed fluorescence host material. One of the most difficult tasks in the fabrication of OLEDs is to form a multilayer structure without dissolving the underlayer during the coating of the upper layer. The developed host materials exhibit high tolerance to methanol. The upper-layer adjacent to the light-emitting layer consists of ZnO nanoparticles, which could be dispersed in methanol by improving the preparation method. This results in the successful fabrication of a solution-processed phosphorescent tandem OLED comprising two LEUs. The maximum external quantum efficiency (EQE) of the tandem device is 22.8%, and the EQE is 21.9% even at a high luminance of 10 000 cd m(-2). The suppression of efficiency roll-off is among the best of those previously reported. Moreover, the operational stability of the tandem device is much higher compared with single-LEU devices.