Influence of host matrix on thermally-activated delayed fluorescence: Effects on emission lifetime, photoluminescence quantum yield, and device performance

The influence of the host molecules on the photoluminescent (PL) and electroluminescent (EL) properties of organic light-emitting diode (OLED) emitters showing efficient thermally-activated delayed fluorescence (TADF) has yet to be investigated in detail. Here we demonstrate that the choice of host can cause large variations in the PL quantum yield (Φ_PL ∼15-70%) and delayed PL transient decay (τ_del ∼2-70 ms) of a spiro-acridine-based TADF guest. We show that the effect of exciplex formation on Φ_PL must be considered even at low concentrations of the TADF guest. Using the same TADF guest but changing the host layer, we are able to greatly vary the PL transient decay time from ∼4 to ∼70 ms while maintaining a high Φ_PL ∼70%, which can lead to new applications. Detailed spectral characterization during PL decay reveals a gradually increasing singlet-triplet energy gap (ΔE_ST) as the origin of these observations. The time-varying ΔE_ST is explained based on dipole interactions between the host and guest molecules. Finally, PL and electrical considerations for host selection are discussed based on the performance of OLED devices.