jp8b08169_si_001.pdf (3.22 MB)
Origin of High Efficiencies for Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes: Atomistic Insight into Molecular Orientation and Torsional Disorder
journal contribution
posted on 2018-11-07, 00:00 authored by Taiping Hu, Guangchao Han, Zeyi Tu, Ruihong Duan, Yuanping YiBoth 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 (S1) 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° because of thermal fluctuation
and intermolecular interaction. Importantly, such torsional disorder
can induce a drastic increase of both the S1 TDM and the
spin–orbit coupling of S1 with the lowest triplet
excited state (T1) while still keeping a small energy difference
between S1 and T1, which would facilitate the
S1 radiative decay and the T1 → S1 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.