Ethyl cellulose as a host material for thermally-activated delayed fluorescence emitters

Thermally-activated delayed fluorescence (TADF) emitters are typically introduced to the OLEDs by doping into a host matrix to prevent emission losses caused by aggregation, concentration quenching, and unwanted excimer emission. While small molecules are usually chosen for the host matrix design, polymers could offer better solution processing and lower production costs for light-emitting structures. In this study, we demonstrate that ethyl cellulose (EC), an environmentally friendly polymer, can serve as a host matrix for specific TADF molecules, enabling the preparation of high-quality films through solution processing. We found that the relative photoluminescence (PL) quantum yield of composite samples using the EC matrix is enhanced by about ten times compared to neat solid-state films of the same dyes. Our PL emission spectroscopy and fluorescence microscopy studies on two pairs of donor-acceptor-donor molecules-each pair having the same donor units but different sulfonyl or carbonyl acceptor groups-revealed distinct morphologies and complex emission bands. The emission color was determined by the interplay of emission components in the multicomponent charge-transfer band. Neat dye films showed aggregated species with red-shifted charge-transfer emission, while guest-host systems exhibited smooth morphology with brighter, more even, and blue-shifted emission. The enhanced PL emission in composite films is attributed to a model where emitter molecules are frozen in the polymer matrix, adopting a restricted twisting disorder. In contrast, the PL emission in neat dye films is weakened due to the higher molecular twisting disorder. We suggest that the interface between EC and the guest molecules facilitates their specific intermolecular packing and suppresses intramolecular twisting upon photoexcitation, leading to a dominant locally excited state and a resulting blue shift in the emission spectrum.