Charge Recombination in Phosphorescent Organic Light-Emitting Diode Host–Guest Systems through QM/MM Simulations

Host–guest systems are
crucial for achieving high efficiency
in most organic light-emitting diode (OLED) devices. However, charge
recombination in such systems is poorly understood due to complicated
molecular environment, making the rational design of host–guest
systems difficult. In this article, we present a computational study
of a phosphorescent OLED with 2,8-bis­(triphenylsilyl)­dibenzofuran
(BTDF) as the host and fac-tris­(2-phenylpyridine)
iridium (fac-Ir­(ppy)3) as the guest, using
a combined quantum mechanics/molecular mechanics (QM/MM) scheme. A
new reaction coordinate is introduced to measure the electrostatic
interactions between the host and guest molecules. Ionization potentials
and electron affinities of the host show broader distributions as
the host–guest interaction increases. On the basis of these
distributions, we describe a molecular picture of charge recombination
on the guest and find a direct charge trapping route for this system.
Our results suggest several strategies for the design of more efficient
host and guest combinations.