Origin of High Efficiencies for Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes: Atomistic Insight into Molecular Orientation and Torsional Disorder

Both the molecular orientation and conformation of thermally activated delayed fluorescence (TADF) emitter molecules that are doped in the host matrix are crucial to determine the performance of TADF-based organic light-emitting diodes (OLEDs). However, the amorphous molecular packing prohibits observation of the structural details at the atomic accuracy by experimental techniques. Here, using atomistic molecular dynamics simulations, we have uncovered the deposition process and molecular arrangements of a representative donor-acceptor (D-A)-structured TADF emitter along with a host material on different model substrates. The simulated results indicate that despite the distinct characters of the substrates, the emitter molecules in all the films exhibit preferential horizontal orientation because of the rodlike structure; thus, the transition dipole moments (TDMs) of the lowest singlet excited state (S-1) prefer a horizontal distribution. This is beneficial to achieve a high out-coupling efficiency. In addition, the torsion angles between the D and A units of the emitter molecules show a broadened distribution around 90 degrees because of thermal fluctuation and intermolecular interaction. Importantly, such torsional disorder can induce a drastic increase of both the S-1 TDM and the spin-orbit coupling of S-1 with the lowest triplet excited state (T-1) while still keeping a small energy difference between S-1 and T-1, which would facilitate the S-1 radiative decay and the T-1 -> S-1 reverse intersystem crossing to obtain a high internal quantum efficiency. Our work provides an atomistic insight into the critical role of both molecular orientation and torsional disorder in achieving high efficiency for an OLED based on the twisted D-A-structured TADF emitter.