Interacting Emission\nSpecies among Donor and Acceptor\nMoieties in a Donor-Grafted Polymer Host/TADF-Guest System and Their\nEffects on Photoluminescence and Electroluminescence

Thermally activated delayed fluorescence (TADF)-based\nelectroluminescence\n(EL) devices adopting a host/guest strategy in their emitting layer\n(EML) are capable of realizing high efficiency. However, TADF emitters\ncomposed of donor and acceptor moieties as guests dispersed in organic\nhost materials containing a donor and/or an acceptor are subject to\ndonor–acceptor (D–A) interactions. In addition, electron\ndelocalization between neighboring emitter molecules could form different\nspecies of aggregates. Here, we investigate the effects of intermolecular\ninteracting emission species on the optoelectronic properties of sky-blue/green/red\n(sB/G/R) TADF emitters as guests using poly(biphenyl-Si/Ge) grafted\nwith various donor moieties as hosts. We found the presence of guest/guest\nexciplex (D<sub>g</sub>/A<sub>g</sub>)*, host/guest exciplexes (D<sub>h</sub>/A<sub>g</sub>)*, and aggregates through the exploration of\ninteractions between neighboring TADF guest molecules and between\nhost and TADF-guest molecules. The nonradiative <sup>3</sup>(D<sub>h</sub>/A<sub>g</sub>)* (Δ<i>E</i><sub>ST</sub> ≈\n0.5 eV) could increase the internal conversion rate (k<sub>IC</sub>) and reduce delayed luminescence, and both of them could cause a\ndecrease in PLQY. The luminescence of <sup>3</sup>(D<sub>h</sub>/A<sub>g</sub>)* may have a positive or negative effect on PLQY depending\non its triplet energy. As the singlet and triplet energies of (aggregate)*\nare lower than those of (ICT)*, energy transfer from (ICT)* to (aggregate)*\ncould occur. The low PLQY nature of (aggregate)* means that it is\nmore likely to cause quenching in device emission. The emissions from\n(D<sub>h</sub>/A<sub>g</sub>)* and (aggregate)* are found to have\nincreased full width at half-maximum and lead to lower emission color\npurity. Such intermolecular interactions should also occur in host/guest\n(TADF) systems and nondoped TADF emitter systems and thus are important\nfactors for the molecular design of the TADF emitter and/or its accompanying\nhost for high device efficiency and emission color purity.