Organic light-emitting diodes (OLEDs) are particularly promising organic semiconductor devices for applications in lighting and displays. However, OLEDs exhibit lower power efficiencies than other light sources. Recently reported external quantum efficiencies (EQEs) of 29?30% for phosphorescent OLEDs (PhOLEDs) are close to the theoretical limit for isotropically oriented iridium complexes. The preferred orientation of the transition dipole moments has not been considered for PhOLEDs because of the lack of an apparent driving force for the molecular arrangement, even though horizontally oriented transition dipoles can result in efficiencies of > 30%. The origin of preferred orientation of the emitting dipoles of Ir complexes, and the design of a horizontal orientation for the emitting dipoles of the Ir complexes has not been studied in detail. Furthermore, EQEs in excess of 30% for horizontal emitting dipoles have not previously been reported.
Recent investigations of triplet harvesting in pure organic phosphors via reverse intersystem crossing (RISC) have used charge-transfer complexes. Intramolecular charge transfer complexes reduce the energy gap between singlet and triplet states, ΔEST, resulting in more efficient triplet harvesting, as well as an internal quantum efficiency (IQE) of almost 100%, and an EQE of 30%. The excited state intermolecular charge transfer complex (or exciplex) can result in very small values of ΔEST, and highly efficient triplet harvesting via RISC. However, fluorescent OLEDs (FOLEDs) using exciplex emission still exhibit lower efficiencies than FOLEDs based on single molecule thermally activated delayed fluorescent (TADF) emitters; furthermore, there are no clear strategies for improving the efficiency of FOLEDs using exciplex emission.
This thesis concerns two research topics: (1) exciplex dynamics for efficient triplet harvesting of FOLEDs, and (2) the emitting dipole orientation of phosphors for PhOLEDs. Our analysis shows that we could achieve unprecedented efficiency for both fluorescent and PhOLEDs.
In Chapter 2, the exciplex dynamics that determine the IQE of FOLEDs are analyzed quantitatively using temperature-dependent transient photoluminescence (PL) and electroluminescence (EL) measurements. To date, most researchers have concentrated on reducing ΔEST to obtain high RISC rates, resulting in efficient triplet harvesting; however, this study shows that a high RISC rate does not necessarily lead to efficient triplet harvesting in exciplex emitters if there is a high non-radiative transition rate (knr). Not only efficient RISC, but also low non-radiative losses in both singlet and triplet exciplex emitters is required for efficient triplet harvesting. Triplet harvesting from exciplex emission can be increased by using exciplex emitters with a low knr; furthermore, suppressing knr by cooling the exciplex can result in an IQE of 100%. As a result, FOLEDs using exciplex emission were achieved that exhibited an IQE of 100% and an EQE of 25.2% at 150 K; this compares with an IQE of 48.3% and an EQE of 11.0% at room temperature, and is due to a reduction in the non-radiative transition rate.
TADF and exciplex-based FOLEDs have disadvantages compared with conventional FOLEDs in terms of the broad emission spectra and low device stability. In Chapter 3, rather than using exciplex emission, an exciplex system was used as a host for a conventional fluorescent dopant without delayed fluorescence. By exploiting the narrow emission spectra and stability of fluorescent molecules, we use exciplex triplet harvesting via the Forster energy transfer mechanism from the exciplex to the conventional fluorescent dopant. As a result, red FOLED with conventional fluorescent dopant was obtained with an unprecedented EQE of 10.6%. A fraction of radiative excitons greater than 35% was achieved using the exciplex host, which is clearly indicative of triplet harvesting in the system.
In Chapters 4?8, we discuss the emitting dipole orientation of the phosphorescent emitter for highly efficient PhOLEDs. Origin of the preferred orientation of phosphorescent emitting dipoles and relationships between the molecular structures of the phosphorescent emitter and the orientation of the emitting dipoles are discussed. OLEDs with unprecedented efficiencies are demonstrated using horizontally emitting dipoles. The orientation of the emitting dipoles of Ir complexes is influenced significantly by the ancillary ligand (Chapter 4) and the main ligand of the Ir complex (Chapter 5). Homoleptic Ir complexes exhibit almost isotropic orientation of the emitting dipoles; by contrast, heteroleptic Ir complexes (HICs) exhibit preferred orientations of emitting dipoles, despite their globular shape. The preferred orientation of the emitting dipole moments of HICs in amorphous host films results from the preferred direction of the triplet transition dipole moments of the HICs and the strong supramolecular arrangement within the co-host environment. This study shows that the C2 axis of HICs preferentially aligns normal to the substrate with some distribution and triplet transition dipole moments in the HICs direct from Ir to N-heterocycles. This results in horizontally emitting dipoles of HICs in the amorphous emitting layer. Furthermore, OLEDs with an EQE of 35.6% (red HICs) and 32.3% (green HICs) are demonstrated, with phosphorescent transition dipole moments oriented in the horizontal (in-plane) direction (Chapters 4 and 5).
Based on these observations, HICs with a high fraction of horizontally oriented dipoles (Θ) were designed and synthesized by substituting the main ligand of the HICs. This design strategy includes two strategies: the substituents induce (1) triplet transition dipole moments in HICs located normal to the C2 axis of HICs (Chapter 6), and (2) the doubly degenerated transition dipole moments are parallel to the substrate (Chapter 7). Cyclometalated ligands of HICs were systematically substituted by methyl groups to control the direction of transition dipole moments of HICs. This methyl substitution resulted in different directions of the transition dipole moments in the iridium complexes, giving molecules with larger angles between the C2 axis and the transition dipole moments (Φ) in the molecules, resulting in a larger Θ in the emitting layer. As a consequence, a bis(2-(3,5-dimethylphenyl)-4-methylpyridine) Ir (III) (2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(3′,5′,4-mppy)2tmd) with a high Θ of 80% (which is 6% higher than that of HICs without methyl substituents) was developed, and green OLEDs exhibiting a high EQE of 34.1% were demonstrated using the new emitter (Chapter 6).
In Chapter 7, various functional groups were substituted on the 4-position of the pyridine ring of the HICs to align Ir-N bonds of HICs and transition dipole moments parallel to the substrate. These HICs exhibited Θ in the range 80?86.5%, which is 1?7.5% greater than that of the reference HIC (Ir(3′,5′,4-mppy)2tmd), coming from different molecular orientation in the emitting layer. Consequently, we demonstrated unprecedented high EQE of 38% for yellow OLED and 36% for green OLED with new HICs in iridium-based PhOLEDs.
An amorphous emission layer can have orientational ordering, as with a liquid crystal; however, the orientation of molecules exhibits some distribution around an average orientation. Thus, it will be difficult to increase Θ due to the amorphous nature of typical emitting layers. In this regard, organic crystals would be the better emitters because of the orientational and positional ordering; however, organic crystals have been rarely used in OLEDs due to low PL quantum yield that results from concentration quenching, and the low device stability that results from the rough surface. In Chapter 8, an unprecedented Θ of 93% and an EQE of 39% were realized for red OLEDs using a crystalline emission layer with perfectly oriented platinum complexes. The relationship between the orientation of the emitting dipole and the crystalline properties of the undoped Pt complex thin film are discussed based on grazing incident wide angle X-ray diffraction analysis and quantum chemical calculations. The orientation of the emitting dipole of the crystalline emission layers was affected not only by the crystallinity of the layer, but also by the molecular arrangement of the crystal, which are both influenced by the symmetry and position of the CF3-substituted pyrazolate units in Pt complexes.