Correlation between exciton residence time and efficiency roll-off in TADF-based organic light-emitting diodes

Abstract

Tuning exciton behavior in organic semiconductors has enabled the achievement of high efficiency for organic optoelectronic devices. Thermally activated delayed fluorescence (TADF) can be a promising candidate to overcome spin multiplicity in that these can harness the excited triplet exciton (so-called ‘dark states’) with the help of reverse intersystem crossing (RISC). Unfortunately, this spin up-conversion process with thermal energy occurs within a few microsecond time-scale in general, associated with predominant efficiency roll-off, such as singlet-triplet annihilation (STA) or triplet-triplet annihilation (TTA). From the studies, the increment of triplet exciton density (NT) dependent on current density (J) aggravated efficiency roll-off at the steady-state condition. The reduction of NT at high J can be a good strategy to suppress efficiency drop. However, NT is not rapidly consumed after electrical excitation in contrast with the optical model. Here, we focus on another perspective, the exciton density based on time-dependent population rate equations. We find through numerical calculation and photo-physical experiments, that exciton residence time at the lowest excited triplet state (T1) also affects efficiency roll-off. With this, we discuss the correlation between exciton residence time and efficiency roll-off in TADF-based OLED device.