Organic light emitting diode (OLED) is a new yet promising technology that is anticipated to replace the liquid crystal display technology in the very near future. The development of both the emitter ...and host materials for OLEDs is indispensable to realize high device efficiency and optimal performance. Though the presently commercialized OLED panels mostly utilize phosphorescence emitters, the all‐organic thermally activated delayed fluorescence (TADF) emitting materials have some obvious advantages. Considerable progress has been made in search of better performing TADF OLEDs in the past few years. Although major research attention has been drawn toward reporting new TADF emitters, the hosts are equally important in TADF OLEDs, as the doped films of the emitters mostly yield better results than the nondoped films. There are already some good reviews on the TADF emitters in literature. In this review article, the literature data specifically aimed at hosting TADF dopants are carefully selected and comprehensively summarized and categorized into several sub‐groups based on their structural features to draw the attention of the organic electronics research community toward developing new host materials for TADF OLEDs.
The thermally activated delayed fluorescence (TADF) emitters are promising in organic light emitting diode (OLED) display technology where host materials play an important role for the performance and stability of devices. This review presents the structure–property–performance correlation of host materials developed for TADF OLEDs. The host materials are categorized into various groups depending on the functional groups.
Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. In addition to pure ...TADF emitters achieved by the subtle manipulations of intramolecular charge transfer processes with sophisticated molecular structures, a new class of efficient TADF-based OLEDs with emitting layer formed by blending electron donor and acceptor molecules that involve intermolecular charge transfer have also been fabricated. In contrast to pure TADF materials, the exciplex-based systems can realize small ΔE ST (0–0.05 eV) much more easily since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the prospective to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. Therefore, the challenging issue of achieving small ΔE ST in organic systems could be solved. In this article, we summarize and discuss the latest and most significant developments regarding these rapidly evolving functional materials, wherein the majority of the reported exciplex forming systems are categorized into two subgroups, viz. (a) exciplex as TADF emitters and (b) those as hosts for fluorescent, phosphorescent and TADF dopants according to their structural features and applications. The working mechanisms of the direct electroluminescence from the donor/acceptor interface and the exciplex-forming systems as cohost for the realization of high efficiency OLEDs are reviewed and discussed. This article delivers a summary of the current progresses and achievements of exciplex-based researches and points out the future challenges to trigger more research endeavors to this growing field.
The future of organic light‐emitting devices (OLEDs) is drifting from electrofluorescence toward electrophosphorescence due to the feasibility of realizing 100% internal quantum efficiency. There is ...limited availability of transition metals (TMs) such as Ir, Os, and Pt, which are used for color‐tunable phosphorescent emitters, and the use of the host‐guest strategy is necessary for suppressing the detrimental triplet–triplet annihilation inherently imparted by the TM‐centered emitters. The inevitable demands of organic host materials provide organic chemists with tremendous opportunities to contribute their expertise to this technology. With suitable molecular design and judicious selection of chemical structures featured with different electronic nature, the incorporation of hole‐transporting (HT) and electron‐transporting (ET) moieties combines the advantages of both functional units into bipolar host materials, which perform balanced injection/transportation/recombination of charge carriers and consequentially lead the OLEDs to have higher performancesand low roll‐off efficiencies. This review highlights recently developed bipolar host materials with the focus on molecular design strategies and the structure–property–performance relationships of various classes of bipolar host materials, which are classified into several categories according to the structural features of their constituents (HT/ET blocks and spacers).
Tailor‐made bipolar host molecules obtained by selection of hole‐transporting (HT) and electron‐transporting (ET) moieties connected by proper linkages provide electrophosphorescence devices with balanced charge flux, allowing the realization of high performance organic light‐emitting diodes (OLEDs). An overview of the recent development of bipolar host molecules is presented with a special emphasis on the molecular design strategies and the relationships between structure, properties, and performance.
The combination of rigid acridine donor and 1,8‐naphthalimide acceptor has afforded two orange–red emitters of NAI‐DMAC and NAI‐DPAC with high rigidity in molecular structure and strongly pretwisted ...charge transfer state. Endowed with high photoluminescence quantum yields (ΦPL), distinct thermally activated delayed fluorescence (TADF) characteristics, and preferentially horizontal emitting dipole orientations, these emitters afford record‐high orange–red TADF organic light‐emitting diodes (OLEDs) with external quantum efficiencies of up to 21–29.2%, significantly surpassing all previously reported orange‐to‐red TADF OLEDs. Notably, the influence of microcavity effect is verified to support the record‐high efficiency. This finding relaxes the usually stringent material requirements for effective TADF emitters by comprising smaller radiative transition rates and less than ideal ΦPLs.
Orange–red organic light‐emitting diodes with external quantum efficiencies of up to 29.2% are achieved using excellent thermally activated delayed fluorescence (TADF) emission of 1,8‐naphthalimide‐acridine hybrid. The relatively high photoluminescence quantum yield and horizontally oriented emitting dipoles of the TADF emitters combined with the influence of microcavity effects are verified to support the record‐high efficiency.
Solid‐state white light‐emitting electrochemical cells (LECs) exhibit the following advantages: simple device structures, low operation voltage, and compatibility with inert metal electrodes. LECs ...have been studied extensively since the first demonstration of white LECs in 1997, due to their potential application in solid‐state lighting. This review provides an overview of recent developments in white LECs, specifically three major aspects thereof, namely, host–guest white LECs, nondoped white LECs, and device engineering of white LECs. Host–guest strategy is widely used in white LECs. Host materials are classified into ionic transition metal complexes, conjugated polymers, and small molecules. Nondoped white LECs are based on intra‐ or intermolecular interactions of emissive and multichromophore materials. New device engineering techniques, such as modifying carrier balance, color downconversion, optical filtering based on microcavity effect and localized surface plasmon resonance, light extraction enhancement, adjusting correlated color temperature of the output electroluminescence spectrum, tandem and/or hybrid devices combining LECs with organic light‐emitting diodes, and quantum‐dot light‐emitting diodes improve the device performance of white LECs by ways other than material‐oriented approaches. Considering the results of the reviewed studies, white LECs have a bright outlook.
Recent developments in white light‐emitting electrochemical cells (LECs) based on different functional materials including ionic transition metal complexes, conjugated polymers, and small molecules and device engineering of various new techniques, including tandem and/or hybrid devices combining LECs with organic light‐emitting diode (LEDs) and quantum dot LEDs to improve device efficiency, are reviewed.
An organic light emitting diode based on thermally activated delayed fluorescence (TADF) has been produced using a spirobifluorene derivative (Spiro-CN) having the donor-acceptor moieties as an ...emitter.
A series of twisted D–π–A type emitters based on the acridine donor unit and CN‐substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically ...probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge‐transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN‐substitution pattern but also by the subtle control of in‐acceptor conformation (twist angles). Small triplet‐to‐singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in‐acceptor conformation, yielding a series of efficient blue‐green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue‐green to yellow TADF organic light‐emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.
Efficient blue‐green to yellow thermally activated delayed fluorescence emitters capable of generating 23%–31% electroluminescence external quantum efficiencies are developed adopting the acridine donor unit and cyano (CN)‐substituted pyridine and pyrimidine acceptor units. They permit systematic probing of influences of acceptor strengths and tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring.
Extremely efficient sky‐blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated ...delayed fluorescence emitter based on the spiroacridine–triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).
Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge‐transfer dipoles to harvest nonradiative triplets into radiative ...singlets in exciplex light‐emitting diodes are reported. Magneto‐photoluminescence studies reveal that the triplet‐to‐singlet conversion in exciplexes involves an artificially generated spin‐orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge‐transfer occurs with forming electric dipoles (D+•→A−•), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔEST) in 9,9′‐diphenyl‐9H,9′H‐3,3′‐bicarbazole (BCzPh):3′,3′″,3′″″‐(1,3,5‐triazine‐2,4,6‐triyl)tris((1,1′‐biphenyl‐3‐carbonitrile)) (CN‐T2T) (ΔEST = 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔEST = 130 meV) exciplexes, the SOC generated by the intermolecular charge‐transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQEmax (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔEST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D+•→A−•) size, enhanced SOC, increased orbital polarization, and decreased ΔEST can simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion.
The cooperative relationship between spin, energy, and polarization parameters is revealed to maximize triplet‐to‐singlet conversion based on high‐efficiency exciplex organic light‐emitting diodes (OLEDs) with the EQEmax over 21%. This cooperative relationship provides a critical guideline to further advance the development of organic light‐emitting diodes.
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