Charge transports in amorphous thin films with 100 nm thickness are investigated in silico by explicitly considering organic molecules. The amorphous layer of organic molecules was constructed using molecular dynamics simulations. The rate constants for charge hopping between two organic molecules, extracted from the amorphous layers, were calculated based on quantum chemical calculations. The hopping transport in amorphous layers was simulated using a Monte Carlo method. The hole mobility was calculated to be several times larger than the electron mobility, which was consistent with the experimental results. The Monte Carlo simulation also shows that diffusion transport is dominant at low applied electric fields and that contribution of drift transport increases at high electric fields. The simulation in this study enables us to reveal molecular origin of charge transport. In the presentation, we will show the results on recently-developed new thermally activated delayed fluorescence materials and the device performances.
In organic light-emitting diodes (OLEDs), device degradation is one of the crucial problems to be solved. In this study, we have investigated material degradations in FIrpic-based phosphorescent OLEDs by solution nuclear magnetic resonance (NMR) spectroscopy. NMR experiments clearly indicate that about 18% of TPhDB molecules, which is used as an electron-transporting material, are decomposed during driving the devices. The decomposition of the TPhDB molecules is considered to be related to the device degradation. This study demonstrates that solution NMR spectroscopy is a useful tool to investigate an origin of device degradation of multi-layered OLEDs in terms of decomposition of organic molecules.
The conformations and aggregated structures, such as molecular orientations, of organic molecules in organic light-emitting
diodes (OLEDs) are considered to be closely related to the performance of OLEDs. Therefore, their
clarification is of great importance in understanding device performance; however, the structures have not been fully
analyzed because the organic molecules in OLEDs are often in amorphous states, and conventional methods for structure
analysis, such as diffraction methods, do not provide sufficient information. In this study, we have attempted the
analysis of molecular orientations of OLED materials by solid-state NMR, which is useful for structure analysis even for
amorphous solids.
Tris(8-hydroxyquinoline) aluminum(III) (Alq3) is one of the most widely used materials in organic light-emitting diodes
(OLEDs), and the relationship between the structures and the luminescent wavelengths is of recent interest; yellowish-green
emissions are observed for the α- and amorphous Alq3, whereas blue emissions are found for the γ- and δ-Alq3. In
order to clarify the relationship between the structures and the emission wavelengths, we carried out solid-state nuclear
magnetic resonance (NMR) experiments on the different polymorphs of Alq3 and the amorphous state. Based on 27Al
and 13C magic angle spinning (MAS) NMR experiments, it is found that the isomeric state of the amorphous Alq3 is the
same as that of α-Alq3 and is different from those of γ- and δ-Alq3. Not only for the amorphous, but also for α-Alq3, the
local structures are found to be disordered. We also obtained clear evidence that γ-Alq3 is in the facial isomeric state. It
is suggested that δ-Alq3 is also facial. The difference between γ- and δ-Alq3 is the intermolecular packing; the effect of
intermolecular packing is found only for δ-Alq3. A further confirmation of the isomeric states of these Alq3 samples is
obtained from temperature-dependent X-ray diffraction experiments.
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