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

• Host dependence of the PL and EL characteristics of a TADF emitter is evaluated.
• Exciplex formation between host and guest reduces both PL and EL efficiency.
• Length of delayed emission (τdel) varies from ∼2 ms to ∼70 ms depending on host.
• Transient change in singlet–triplet energy gap (ΔEST) explains the very long TADF.
• Long τdel could be used in oxygen sensing and short τdel in OLED applications.

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 (ΦPL ∼15–70%) and delayed PL transient decay (τdel ∼2–70 ms) of a spiro-acridine-based TADF guest. We show that the effect of exciplex formation on ΦPL 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 ΦPL ∼70%, which can lead to new applications. Detailed spectral characterization during PL decay reveals a gradually increasing singlet–triplet energy gap (ΔEST) as the origin of these observations. The time-varying ΔEST 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.

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