Blue-phosphorescent organic light-emitting diodes (OLEDs) with 34.1% external quantum efficiency (EQE) and 79.6 lm W(-1) are demonstrated using a hole-transporting layer and electron-transporting layer with low refractive index values. Using optical simulations, it is predicted that outcoupling efficiencies with EQEs > 60% can be achieved if organic layers with a refractive index of 1.5 are used for OLEDs.

Orientation of emitting dipoles is one of the most important material properties influencing the efficiency of organic light-emitting diodes (OLEDs). Recently, even globular-shaped Ir complexes, especially heteroleptic Ir complexes, have been reported to have horizontal emitting dipole orientation (EDO) to improve the external quantum efficiency (EQE) over 30%. Still the relationship between molecular structure of Ir complexes and their EDO has not been fully understood yet. Here, we report that substituents at the para-position of the pyridine in the main ligands of Ir complexes play a pivotal role inducing the orientation of heteroleptic Ir complexes. Substitution of aliphatic and aromatic functional moieties at the position leads to high horizontal emitting dipole orientation with the horizontal dipole ratio up to 86.5% to realize unprecedentedly high-efficiency yellow and green OLEDs, with EQEs of 38% and 36%, respectively. Elongated and planar substituents with high electrostatic potential enlarge the interacting surface region between Ir complex and host molecules, resulting in stacking Ir complexes parallel to the film surface.

Excited charge-transfer complexes, or exciplexes, have attracted significant attention due to their potential applications to improving the performance of organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). In solid states, exciplexes exhibit extraordinary characteristics, including broad emission spectra, multiexponential photoluminescence (PL) decay curves, and spectral red shifts as time delays in transient PL. Here, we present experimental and theoretical evidence that all of the emission characteristics of solid-state exciplexes originate from differences in their dimer configurations, which have different charge transfer rates, emission energies, singlet−triplet energy gaps, kinetic rate constants, and emitting dipole orientations. This conclusion is based on experimental observations, quantum chemical calculations, and molecular dynamics simulations. These results enabled us to develop a model of the electronic structure of an exciplex in a solid-state medium. This comprehensive model accommodates all of the characteristics of the exciplex and can be used to further our understanding of OLEDs and OPVs.

Tetradentate Pt(II) complexes are promising emitters for deep blue organic light‐emitting diodes (OLEDs) due to their emission energy and high photoluminescence efficiency. However, to obtain a pure blue color, spectral red‐shifts, and additional emission peaks at longer wavelengths, originating from strong intermolecular interactions between parallel Pt(II) complexes, must be avoided. Herein, a new class of deep‐blue emitting tetradentate Pt(II) complexes consisting of a non‐planar ligand and a bulky adamantyl group is reported. The six‐membered metallacycle structure renders the Pt(II) complex non‐planar. In addition, the bulky adamantyl groups increase intermolecular distances and decrease red‐shifts in the emission originating from strong dipole–dipole interactions. Therefore, these Pt(II) complexes exhibit little change in emission color with increasing dopant concentration. OLEDs incorporating these new Pt(II) complexes as emitters exhibit deep blue emission with a Commission International de L'Eclairage (CIE) y under 0.13 and a maximum external quantum efficiency of 22.6%, which is one of the highest observed for deep blue (CIE y < 0.15) phosphorescent OLEDs using Pt(II) complexes. These results provide a new approach for designing Pt(II) complexes for high efficiency deep blue OLEDs.

The use of exciplex hosts is attractive for high-performance phosphorescent organic light-emitting diodes (PhOLEDs) and thermally activated delayed fluorescence OLEDs, which have high external quantum efficiency, low driving voltage, and low efficiency roll-off. However, exciplex hosts for deep-blue OLEDs have not yet been reported because of the difficulties in identifying suitable molecules. Here, we report a deep-blue-emitting exciplex system with an exciplex energy of 3.0 eV. It is composed of a carbazole-based hole-transporting material (mCP) and a phosphine-oxide-based electron-transporting material (BM-A10). The blue PhOLEDs exhibited maximum external quantum efficiency of 24% with CIE coordinates of (0.15, 0.21) and longer lifetime than the single host devices.