Thermally Activated Delayed Fluorescence in Optical Cavities

Thermally activated delayed fluorescence (TADF) has gained great attention in light-emitting applications due to its potential to achieve 100% efficiency by recycling dark triplet excitons back into bright singlet excitons using ambient thermal energy. In recent years, multi-resonant TADF (MR-TADF) emitters have emerged as great candidates for next-generation organic light-emitting diodes (OLEDs) and lasing due to the high quantum yield and narrow emission bandwidth. However, MR-TADF emitters face molecular aggregation issues due to their planar structure, limiting their use in a host matrix at low doping concentrations. In particular, aggregate formation can lead to thermalisation and the formation of excimers that feature a large Stokes shift and broader emission. Here, we show that excimer emission can be appreciably suppressed by placing a thin film of MR-TADF emitters embedded in a host PMMA matrix within an optical cavity. Strong light-matter interactions in these microcavities result in Rabi splitting larger than 200 meV, placing lowest singlet excited state, i.e., lower polariton states close to the triplet state. Under the strong coupling regime, excimer emission is significantly reduced due to the strong emission from the lower polariton states. The rate constant of reverse intersystem crossing to the lower polariton states increases up to 33% in optical cavities, resulting from a lower activation energy barrier. This work highlights that strong light-matter interactions can mitigate excimer emission of highly aggregating emitter molecules post-synthetically, paving the way towards efficient light-emitting devices even at high doping concentrations.