Molecular orientation and emission characteristics of Ir complexes and exciplex in organic thin films

Abstract

Organic light-emitting diodes (OLEDs) has drawn great attention in lighting and display technologies with the advantages of high color purity, large-area processability, low cost, and flexibility. Development of the materials and the device structures are still demanded because the power efficiency of OLEDs is low to replace other lighting sources. The key importance of emitting materials for OLEDs is utilizing the non-radiative triplet excitons as photons and the increase of outcoupling efficiency. The triplet excitons can be harvested as photons by employing phosphorescent or delayed fluorescent emitters. Control of the molecular orientation in organic thin films is one of the methods to enhance the outcoupling efficiency of OLEDs. Horizontal alignment of the emitting dipole moment to the substrate largely enhances the outcoupling efficiency of light in OLEDs. Ir(III) complex is one of the phosphorescent dyes for highly efficient OLEDs with high radiative quantum efficiency and various emission color spectrum. In general, Ir complexes have octahedral structures with three cyclometalating bidentate ligands and only small amount of them are doped in an organic host in the emissive layer. Therefore, their molecular orientations have been regarded as isotropic for a decade. However, the preferred horizontal orientation of the emitting dipole moment was investigated from a heteroleptic Ir complex in 2011. Optical simulation predicts over 46% of external quantum efficiency from the OLEDs using the Ir complexes with the perfect horizontal alignment of the emitting dipole moment. Now the molecular orientation of Ir complex is an important topic of OLEDs. Exciplex is a charge transfer complex formed by charge transfer between different molecules at the exciplex state. Exciplex has a very low singlet-triplet energy gap by spatial separation of the frontier orbitals, enabling delayed fluorescence with electron exchange between singlet and triplet states at the room temperature. Exciplex has been highlighted as an emitter of OLEDs because it also can harvest the non-radiative triplet state as delayed fluorescence. Many kinds of exciplexes have been reported with efficiency radiative quantum efficiency but understanding the electronic structure and the emission mechanism of the exciplex is still insufficient. We need more concentration on the dimer arrangements and the charge transfer process in a solid state blend to understand the emission mechanism of the exciplex. The optical model of OLEDs interprets dipole radiation in a microcavity structure. It predicts external quantum efficiency and electroluminescence spectrum of OLEDs very precisely. In addition, the model has been recently applied to the method determining the emitting dipole orientation (EDO) of an emissive layer. The preferred molecular orientations in organic thin films lead to optical birefringence so that the development of the optical model of OLEDs with consideration of the birefringence is required. This thesis analyzes three subjects: (1) the origin of molecular orientation in organic thin films, (2) electronic structure and emission process of exciplex in a solid state, and (3) modeling of luminescence from a birefringent layer. The 1st chapter introduces organic light-emitting materials, OLEDs, and molecular orientation in thin films. The 2nd chapter analyzes host effect on the orientation of the iridium complexes in organic amorphous film and furthers the intermolecular interactions between the Ir complexes and host molecules on organic surfaces during vacuum deposition via quantum chemical calculation and molecular dynamics simulations. Most researches have tried to control the molecular orientation by a change of the molecular structure of Ir complexes. However, this thesis investigates that the preferred orientation of the emitting dipole moment is able to vary from horizontal, isotropic, to rather vertical directions depending on host materials. The local attraction between the host-aromatic ligand of the dopant on the surface induces the horizontal EDO. Furthermore, I demonstrate molecular dynamics (MD) simulations with various host-dopant combinations to simulate the vacuum deposition processes and observe the molecular configurations. As a result, I investigate that the aromatic ligands are aligned parallel to the substrate even though the orientation of aromatic ligands is randomized. Therefore, the host-dopant aromatic-aromatic interactions whose direction is parallel to the substrate is attributed to the origin of the preferred molecular orientation of heteroleptic Ir complexes in vacuum-deposited organic thin films. Energetic analysis indicated that the dispersion interaction is the major force for the orientation of Ir complexes but the local electrostatic attraction induces further alignment if a polar host is used. The 3rd chapter analyzes the electronic structure and emission process of the exciplex in a solid state blend employing the hybridized local and charge-transfer excited state. The exciplex energy spectrum is calculated by a product of the exciplex energy and density of the molecule as functions of distance between the heterodimer. As a result, the high energy exciplex has increased locally excited state emission, indicating the increase of the overlap of the frontier orbitals, the decay rates, and the energy gap between the singlet and triplet states. Superposition of the fast-decaying high-energy exciplex and slow-decaying low-energy exciplex is attributed to a reason of the spectral red-shift of exciplex as time delays. In addition, OLEDs using an exciplex host for a thermally activated delayed fluorescent dopant are fabricated. The combined inter- and intra-molecular charge transfer system enhance the singlet-triplet electron exchange rates, thereby suppression of the efficiency roll-off at high current density by reduction of the density of the triplet excitons. The 4th chapter describes optical modeling of the luminescence from an oriented emitting dipole moment in an anisotropic microcavity and the model is expanded for optical analysis of OLEDs. The dipole radiation in an anisotropic microcavity is solved as functions of the dipole orientation and direction of the polarization using the classical dipole theory. As a result, it enables to analyze the far field radiation from an Ir complex doped in a birefringent layer. In addition, angular emission spectra and external quantum efficiency of OLEDs employing the birefringent emissive layer as are successfully analyzed.