Effects of exciton deconfinement on the transient photoluminescence from thermally activated delayed fluorescence host–guest systems

For thermally activated delayed fluorescence (TADF) host–guest systems used in organic light-emitting diodes, understanding of the transient photoluminescence (PL) measurements is crucial for accurate determination of the photophysical rates of the emitter. Here, we study how the PL is affected by triplet-exciton deconfinement from the guest to the host molecules. This deconfinement can complicate the analysis of the PL decay and potentially lead to a loss of efficiency. From an analytical model, we find that the transient PL intensity remains bi-exponential in the presence of exciton deconfinement for the case of fast triplet diffusion, albeit with a longer decay time of the delayed component. Deconfinement might, therefore, not always be recognizable from a single transient PL measurement. The role of deconfinement depends on the energetic disorder, the guest concentration, and the energy difference Δ E T between triplet-exciton energies on the host and guest molecules and is effectively suppressed for Δ E T > 0.2 eV. We find from analytical modeling and kinetic Monte Carlo simulations that the decay can become non-bi-exponential and even show a distinct third decay step. The shape of the decay curves depends on the characteristic times for guest–host transfer and host diffusion, relative to the prompt and delayed decay times of the TADF emitter. A comparison with available experimental data is included, finding qualitative agreement with dedicated deconfinement studies and indicating the influence of other processes for the often observed power-law decay at long time scales.