Organic light-emitting diodes (OLEDs) are widely used for displays and promising for applications lighting, sensing and medicine. Thermally assisted delayed fluorescence (TADF) is an attractive way of harvesting triplets, without the need for heavy metals such as iridium. However, many TADF materials suffer severe efficiency roll-off i.e. as the light output of the device increases, the efficiency decreases. We will show how efficiency roll-off compares between phosphorescent, fluorescent and TADF OLED materials. The TADF literature suggests that efficiency roll-off should be addressed by increasing the rate of reverse intersystem crossing. We show this incomplete and propose an improved approach. Furthermore we introduce a measurement using variable repetition rate to measure key photophysical parameters to understand efficiency roll-off. Our results and analysis suggest how the serious problem of efficiency roll-off in TADF materials can be reduced.
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