Disclosing the Impact of Local Host-Emitter Interactions on Donor-Acceptor Type TADF Dynamics and the Significance for Emissive Layer Design in OLEDs

Abstract: Donor-Acceptor (D-A) type thermally activated delayed fluorescence (TADF) emitters which constitute the key functional units in proposed Gen3 Organic Light Emitting Diodes (OLEDs), are sensitive to the rigidity and polarity of their local environment. In particular, the torsional freedom of the D-A dihedral angle and the excited state dipole moments of the occurring charge transfer states, condition a distribution of TADF dynamics over the emitter ensemble, concealed in standard optical spectroscopy. Here we apply spectroscopy on the single molecule level to directly access individual emitter properties, and, thus, bypass the downside of ensemble averaging. By photon correlation data and locally resolved spectral information on single D-A type TADF molecules embedded in technological relevant host materials of different polarity and rigidity, we derive host-dependent characteristics and distributions in the TADF dynamics. Those can be related to the conformational freedom and the dielectric environment imposed by the specific local host rigidity and polarity, thereby pointing out new selection criteria for host-emitter combinations in OLEDs.