Mechanisms of TADF/Phosphorescence-Sensitized Fluorescence in OLED Emissive Layers and Their Impacts on Device Performances: A Theoretical Insight

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence or phosphorescence-sensitized fluorescence [TADF-sensitized fluorescence (TSF) or phosphorescence-sensitized fluorescence (PSF)] open the door for devices with high efficiency, better color purity, long lifetime, and low-efficiency roll-off. However, the photophysical processes and their impact on device performances in TSF and PSF systems are not clearly understood to date. In this work, the photophysical processes for the TSF and the PSF systems are in depth investigated by combining molecular dynamics simulation and quantum-chemistry calculations. In the studied TSF [PCTF/TBRb] system and the PSF [Ir(ppy)₂acac/TBRb] system, the film morphologies, electronic properties, and kinetic processes are analyzed in detail. The calculated results indicate that the [PCTF/TBRb] system and the [Ir(ppy)₂acac/TBRb] system exhibit obvious TSF and PSF features, respectively. The difference between the two sensitization systems is the utilization process of the T₁ excitons of the sensitizers. The rates of reverse intersystem crossing (k_RISC^TADF) and the triplet-singlet Förster resonance energy transfer (k_T-SFRET^Ph-G) are the decisive factors in the internal quantum efficiency (IQE) of the TSF and the PSF systems, respectively. Moreover, it can be found that the effect of k_RISC^TADF on the IQE in the TSF system is stronger than the effect of k_T-SFRET^Ph-G on the IQE in the PSF system. We hope that these results will promote an in-depth understanding of the mechanisms of the TSF and the PSF systems and their impacts on device performances, accelerating the development of highly efficient sensitized OLEDs.