Photoluminescence turn-on method to determine the kinetics of thermally activated delayed fluorescence materials

Thermally activated delayed fluorescence (TADF) is a desirable emission mechanism that can translate into highly efficient organic light-emitting diodes. Conventionally, the TADF kinetics have been extracted by analysing the transient photoluminescence (PL) of the emitters in combination with measurements of their PL quantum yield. However, measuring transient PL can be challenging, as the emission decay of a TADF emitter occurs over a large range of intensities and timescales. The huge dynamic range can make it difficult to match data from early and late processes in the decay together. Here, we introduce a novel measurement that enables rate constants to be extracted from the turn-on of the PL of TADF emitters. In our approach, the prompt and delayed PL can be measured in a single measurement where the PL intensity changes only over one order of magnitude, thereby overcoming the problem of a huge dynamic range. This approach enables a much faster measurement of the excited-state dynamics than conventional transient PL methods, whilst using less expensive equipment. Our method also allows for the reliable estimation of prompt and delayed emission contributions. It could enable high-throughput screening of the photophysics of TADF materials and so accelerate the identification of highly promising TADF emitters.