Modeling molecular properties for OLED materials
To further improve the efficiency and life-time of organic light-emitting diodes (OLEDs),
ultimately the properties of underlying materials need to be tweaked at the molecular level.
In materials science, as in other fields, modeling has become a more wide-spread tool
integrated with experiments for a holistic research & development approach.
In particular, high-throughput screening computational screening may considerably reduce
experimental costs for synthesizing and testing new materials. At the molecular level there are a
few properties that are important for OLED life-time and efficiency. A simplified set up for an
OLED device is depicted in Figure 1. The charge mobility and light emission properties can bepredicted with computational chemistry, as explained below.

Figure 1: OLED device set-up and fundamental processes. Picture taken from: Y. Suzuri et al.,Phosphorescent cyclometalated complexes for efficient blue organic light-emitting diodes, Sci.
Technol. Adv. Mater. 15 (2014) 054202
Under working conditions of the OLED, positive charges (holes) from the anode recombine withnegative charges (electrons) from the cathode to form excited states: excitons. Excitons canrelax to the ground state by emitting light. Relaxation via other pathways typically generate heatand can deteriorate the device.
Electrons and holes have a quantum mechanical property called spin, and the rules of quantum
mechanics dictate that, upon recombination, 25% of these excitons are in a so-called singlet
state (spins anti-parallel) while 75% forms in the triplet state (spins parallel). In a purely organic
material, only the singlet excitons can emit light (fluorescence), again due to the laws ofquantum mechanics, thus resulting in a maximum theoretical efficiency of 25%.
In typical present-day devices, transition metal complexes are used which can emit light fromthe triplet excitons (phosphorescence), and furthermore can harvest the singlet excited states tothe triplet state before they emit (intersystem crossing. Both these processes are allowed
through the combination of quantum mechanics and relativity, which makes the spin quantum
property couple with angular momentum of the electrons orbiting the nuclei. This spin-orbit

coupling typically only manifests itself noticeably for the heavier elements (lower rows of theperiodic table) where relativity plays a larger role for the faster moving electrons.
Modeling charge mobilities for organic electronics
With computational chemistry the properties of hole and electron mobility can be studied by
calculating the propensity (electronic coupling) for the charge to move from one molecule toanother (Figure 2).

Figure 2: mobility in organic materials can be studied by calculating the hopping probability
between different dimers, as shown here for pentacene
With different orientations of neighboring molecules in an organic crystal, charges are moremobile in certain directions than in others. This anisotropicity has been correctly predicted for aset of organic semi-conductors in S.-H. Wen et al., First-Principles Investigation of Anistropic
Hole Mobilities in Organic Semiconductors J. Phys. Chem. B 113, 8813-8819 (2009).
Modeling phosphorescence for OLEDs
Phosphorescent lifetimes can be predicted with quantum chemistry codes combining spin-orbit
coupling relativistic effects with calculating excited states. Likewise, the rate of intersystemcrossing (triplet harvesting) can be estimated.
By modifying the molecule used as a phosphor in OLED devices, a different color can beobtained and in particular the radiative rate can be optimized. Molecular modeling can be used
to predict radiative lifetimes of these phosphors, as has been demonstrated by a few companies
producing OLED materials (H. Sasabe et al., High-Efficiency Blue and White OLEDs
Incorporating a Blue Iridium Carbene Complex, Adv. Mater., 22, 5003-5007 (2010); J. M.
Younker and K. D. Dobbs, Correlating Experimental Photophysical Properties of Iridium(III)Complexes to Spin-Orbit Coupled TDDFT Predictions, J. Phys. Chem. C 117, 25714-25723(2013)).
For optimal OLED performance, a high phosphorescent rate is desirable, and computational
chemistry can be used to model this without having to go through laborious experimental
procedures first. The computational approach also works for predicting phosphorescent
properties of different transition metals, not just the often-used iridium complexes (Figure 3).

Figure 3: Predicted radiative lifetimes for 23 different transition metal complexes in good
agreement with experiment, with the exception of Rh complex Rh(bpy)33+ which also relaxes via
alternative pathways (K. Mori et al. Predicting phosphorescent lifetimes and ZFS oforganometallic complexes with SOC-TDDFT, Phys. Chem. Phys. Chem.16, 14523 (2014))
Other important molecular processes in OLEDs have to do with non-radiative relaxation of theexcitons. In particular the recombination of excitons with charges (exciton-polaron coupling) andother excitons (exciton-exciton annihilation) will reduce luminescent efficiency and candeteriorate the device via heat generation leading to damage at the molecular level. Theseproperties can also be studied with more advanced computational chemistry approaches.
This entry was written by SCM's business developer Fedor Goumans, who has many years ofcomputational chemistry experience in various experimental groups. SCM is a spin-off company
from the VU University in Amsterdam, developing the Amsterdam Density Functional (ADF)
modeling suite in collaboration with many (academic) partners. ADF has unique capabilities formodeling molecular properties for organic electronics such as described here.