The Role of Intermolecular Interactions on the Performance of Organic Thermally Activated Delayed Fluorescence (TADF) Materials

In most instances, thermally activated delayed fluorescence (TADF) emitters are incorporated into a suitable host matrix at low doping concentration in order to reduce emission quenching and to improve organic light-emitting diode (OLED) efficiency. Here, a combination of molecular dynamics simulations and density functional theory calculations is performed for thin films of 1) the neat 4CzIPN TADF emitter and 2) the (guest–host) 4CzIPN:mCBP system, in order to determine how guest–guest and guest–host interactions influence the morphological, electronic, and luminescence properties of the TADF emitters. The red-shift in emission recently observed experimentally upon increasing the concentration in TADF emitters is attributed to the formation of guest–guest, i.e., dimer, intermolecular charge-transfer states. It is found that the radiative and reverse intersystem crossing rates associated with these dimer states are similar to those of monomers. Thus, the contributions from both the dimer and monomer states need to be considered to describe TADF within the emissive layer. The exciton diffusion processes are also characterized; singlet excitons are calculated to be the main contributors to the diffusion length, in contrast to recently proposed models.