It has been known for decades that the emitting dipole orientation (EDO) of emitting dyes influences the outcoupling efficiency of organic light‐emitting diodes (OLEDs). However, the EDO of dopants, ...especially phosphorescent dopants, has been studied less than that of neat films and polymer emitting layers (EMLs) due to the lack of an apparent driving force for aligning the dopants in amorphous host films. Recently, however, even globular‐shaped Ir complexes have been reported to have a preferred orientation in doped films and OLEDs. External quantum efficiencies (EQEs) higher than 30% have also been demonstrated using phosphorescent and thermally activated delayed fluorescent dyes (TADF) doped in EMLs. Here, recent results on the EDO of phosphorescent and TADF dyes doped in host films, and highly efficient OLEDs using these dyes are reviewed. The origin and control of the orientation of phosphors are discussed, followed by a discussion of future strategies to achieve EQEs of over 60% without a light extraction layer, from the material point of view.
Orientations of phosphors in organic light‐emitting diodes (OLEDs) have been studied considerably because the emitting dipole orientation (EDO) of OLEDs significantly influences the outcoupling efficiency of OLEDs. The orientation of phosphorescent and thermally activated delayed fluorescent dyes doped in a host film and highly efficient OLEDs with horizontal EDOs is reviewed from the material point of view.
New blue (DBA‐SAB) and deep‐blue (TDBA‐SAF) thermally activated delayed fluorescence (TADF) emitters are synthesized for blue‐emitting organic‐light emitting diodes (OLEDs) by incorporating ...spiro‐biacridine and spiro‐acridine fluorene donor units with an oxygen‐bridged boron acceptor unit, respectively. The molecules show blue and deep‐blue emission because of the deep highest occupied molecular energy levels of the donor units. Besides, both emitters exhibit narrow emission spectra with the full‐width at half maximum (FWHM) of less than 65 nm due to the rigid donor and acceptor units. In addition, the long molecular structure along the transition dipole moment direction results in a high horizontal emitting dipole ratio over 80%. By combining the effects, the OLED utilizing DBA‐SAB as the emitter exhibits a maximum external quantum efficiency (EQE) of 25.7% and 1931 Commission Internationale de l'éclairage (CIE) coordinates of (0.144, 0.212). Even a higher efficiency deep blue TADF OLED with a maximum EQE of 28.2% and CIE coordinates of (0.142, 0.090) is realized using TDBA‐SAF as the emitter.
New blue‐emitting thermally activated delayed fluorescence (TADF) emitters are designed and synthesized for blue‐emitting organic light‐emitting diodes (OLEDs) by incorporating rigid donor units with an oxygen‐bridged boron acceptor unit. As a result, a deep‐blue OLED with a maximum external quantum efficiency of 29.3% and CIE coordinates of (0.142, 0.090) is realized.
Tandem white organic light‐emitting diodes (WOLEDs) using horizontally oriented phosphorescent dyes in an exciplex‐forming co‐host are presented, along with an orange OLED. A high external quantum ...efficiency of 32% is achieved for the orange OLED at 1000 cd m−2 and the tandem WOLEDs exhibit a high maximum EQE of 54.3% (PE of 63 lm W−1).
Triplet harvesting is important for the realization of high‐efficiency fluorescent organic light‐emitting diodes (OLEDs). Triplet–triplet annihilation (TTA) is one triplet‐harvesting strategy. ...However, for blue‐emitting anthracene derivatives, the theoretical maximum radiative singlet‐exciton ratio generated from the TTA process is known to be 15% in addition to the initially generated singlets of 25%, which is insufficient for high‐efficiency fluorescent devices. In this study, nearly 25% of the radiative singlet‐exciton ratio is realized by TTA using an anthracene derivative, breaking the theoretical limit. As a result, efficient deep‐blue TTA fluorescent devices are developed, exhibiting external quantum efficiencies of 10.2% and 8.6% with Commission Internationale de l'Eclairage color coordinates of (0.134, 0.131) and (0.137, 0.076), respectively. The theoretical model provided herein explains the experimental results considering both the TTA and reverse intersystem crossing to a singlet state from higher triplet states formed by the TTA, clearly demonstrating that the radiative singlet ratio generated from TTA can reach 37.5% (total radiative singlet‐exciton ratio: 62.5%), well above 15% (total 40%), despite the molecule having S1, T2 < 2T1 < Q1 energy levels, which will lead to the development of high‐efficiency fluorescent OLEDs with external quantum efficiencies exceeding 28% if the outcoupling efficiency is 45%.
The triplet–triplet annihilation (TTA) process can recycle nonradiative triplet excitons to radiative singlet excitons and enhance the efficiency of fluorescent organic light‐emitting diodes (OLEDs). Conventionally, the theoretical limit of delayed emission ratio by the TTA process is known to be 37.5% in anthracene‐based molecules. In this work, 48% of delayed emission ratio is achieved by TTA with carefully designed blue OLEDs.
Deep‐blue emitting Iridium (Ir) complexes with horizontally oriented emitting dipoles are newly designed and synthesized through engineering of the ancillary ligand, where ...2′,6′‐difluoro‐4‐(trimethylsilyl)‐2,3′‐bipyridine (dfpysipy) is used as the main ligand. Introduction of a trimethylsilyl group at the pyridine and a nitrogen at the difluoropyrido group increases the bandgap of the emitter, resulting in deep‐blue emission. Addition of a methyl group (mpic) to a picolinate (pic) ancillary ligand or replacement of an acetate structure of pic with a perfluoromethyl‐triazole structure (fptz) increases the horizontal component of the emitting dipoles in sequence of mpic (86%) > fptz (77%) > pic (74%). The organic light‐emitting diode (OLED) using the Ir complex with the mpic ancillary ligand shows the highest external quantum efficiency (31.9%) among the reported blue OLEDs with a y‐coordinate value lower than 0.2 in the 1931 Commission Internationale de L'Eclairage (CIE) chromaticity diagram.
A deep‐blue iridium (Ir) complex with CIE coordinate y < 0.2 and horizontal emitting dipole ratio of 86% is developed by the chemical design of ancillary ligands. The phosphorescent organic light‐emitting diode (phOLED) using the Ir complex shows an external quantum efficiency of 31.9% with CIE y < 0.2, which is the highest value ever achieved in deep‐blue phOLEDs.
Organic light‐emitting diodes with external quantum efficiency of 38.8% are realized using a Pt‐based thin‐film emitting layer with photoluminescence quantum yield of 96% and transition dipole ratio ...of 93%. The emitting dipole orientation of the thin films fabricated using Pt complexes is investigated and the structural relationship between X‐ray structural analysis and the structures in thin films are discussed based on quantum chemical calculations.
Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light‐emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an ...exciplex‐forming co‐host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex‐forming co‐hosts stimulate energy transfer and charge balance in the system. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device.
Inter‐ and intramolecular charge‐transfer processes are combined using an exciplex‐forming host and a thermally activated delayed fluorescent dopant, for fabricating efficient fluorescent organic ...light‐emitting diodes along with the reduced efficiency roll‐off at high current densities. Extra conversion on the host from triplet exciplexes to singlet exciplexes followed by energy transfer to the dopant reduces population of triplet excitons on dopant molecules, thereby reducing the triplet exciton annihilations at high current densities.
Inter‐ and intramolecular charge‐transfer processes are combined using an exciplex‐forming host and a thermally activated delayed fluorescent dopant, for fabricating efficient fluorescent organic light‐emitting diodes (OLEDs). This process reduces triplet exciton quenching and results in high‐efficiency OLEDs with the maximal external quantum efficiency of 34% and power efficiency of 121 lm W−1 along with the reduced efficiency roll‐off at high current densities.
Highly efficient, yellow‐fluorescent organic light‐emitting diodes with a maximum external quantum efficiency exceeding 25.0% and extended lifetime are reported using iridium‐complex sensitizers ...doped in an exciplex host. Energy transfer processes reduce the lifetime of the exciplex and excitons on the Ir complexes and enable an excited state to exist in a conventional fluorescent emitter, thereby increasing device lifetime. The device stability depends on the location of the excited state.
Highly efficient, conventional, fluorescent organic light‐emitting diodes with a maximum external quantum efficiency exceeding 25.0% are fabricated using iridium (Ir) complex sensitizers doped in an exciplex host. Energy‐transfer processes reduce the lifetime of the exciplex and excitons in the Ir complex, and enable an excited state to be formed in a conventional fluorescent emitter, thereby increasing device lifetime.
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.
New tetradentate Pt(II) complexes composed of adamantyl spacing group are developed. Without a spacing group, the emission spectrum can be largely changed in solution state or highly doped thin film. However, the Pt(II) complexes with a spacing group retain its own blue emission even in solution state. Highly efficient organic light‐emitting diodes with external quantum efficiency of 22.6% and Commission International de L'Eclairage y of 0.122 are reported.
