Efficient Electroluminescent Devices Research Articles

Through‐Space vs Through‐bond Charge Transfer Fluorophores as Emitters for Efficient Blue Electroluminescent Devices

AbstractHerein, we report the design, synthesis, and properties of two donor‐π‐acceptor (D‐π‐A) charge transfer fluorophores pursuing an efficient deep blue emitter for organic light‐emitting diode (OLED). TCyCN and TPhCN comprise triphenylamine as a donor and benzonitrile as an acceptor connecting by either [2.2]paracyclophane (Cy) as a through‐space π‐linker or phenyl ring (Ph) as a through‐bond π‐linker, respectively. In thin film, TCyCN shows deep blue emission with moderate fluorescence efficiency, while TPhCN displays cyan blue emission with a high fluorescence efficiency. The two molecules feature hybridized local and charge‐transfer states with good thermal stability and balanced charge transport properties. They are successfully employed as non‐doped blue emissive layers in OLEDs. The TCyCN‐based device emits deep blue light peaked at 425 nm (CIE coordinates of (0.157, 0.076)) with a moderate electroluminescent (EL) performance (EQEmax=3.23 % and CEmax=2.06 cd A−1), while the TPhCN‐based device attains an excellent EL performance (EQEmax=6.24 % and CEmax=8.26 cd A−1) with cyan blue emission peaked at 490 nm. This work provides insight into the relationship between molecular design and properties of D‐π‐A emitters, offering a guideline for tailoring new organic compounds for organic optoelectronics.

Linker aggregation engineering of TADF materials to tune carrier balance for highly efficient organic LEDs with long operational lifetime

AbstractThermally activated delayed fluorescence (TADF) molecules are regarded as promising materials for realizing high‐performance organic light‐emitting diodes (OLEDs). The connecting groups between donor (D) and acceptor (A) units in D–A type TADF molecules could affect the charge transfer and luminescence performance of TADF materials in aggregated states. In this work, we design and synthesize four TADF molecules using planar and twisted linkers to connect the aza‐azulene donor (D) and triazine acceptor (A). Compared with planar linkers, the twisted ones (Az‐NP‐T and Az‐NN‐T) can enhance A–A aggregation interaction between adjacent molecules to balance hole and electron density. As a result, highly efficient and stable deep‐red top‐emission OLEDs with a high electroluminescence efficiency of 57.3% and an impressive long operational lifetime (LT95 ∼ 30,000 h, initial luminance of 1000 cd m−2) are obtained. This study provides a new strategy for designing more efficient and stable electroluminescent devices through linker aggregation engineering in donor–acceptor molecules.

A dual ligand synergism strategy for fabrication of highly luminescent FAPbBr3 nanocrystal films and efficient electroluminescent devices

We addressed the unique regulation mechanism of dual ligands on crystallization of FAPbBr3 NCs. Based on this dual ligands synergism effect, highly luminescent and uniform FAPbBr3 films and efficient electroluminescence devices were achieved.

High solid-state fluorescent 9,9-bis(4-(diphenylamino) phenyl)fluorene-pyrenes as hole-transporting-free blue emitters for efficient solution-processed electroluminescent devices

The development of light-emitting conjugated materials (LEMs) with multiple functions not only improves photo and electronic properties to attain efficient and robust device electroluminescence (EL) performance, but also enhances the film-forming ability to manufacture large-area film via solution processing technology. Herein, new multi-functional blue LEMs (PY1 and PY2) bearing either 9,9-bis(4-diphenylaminophenyl)fluorene or 7,9,9-tris(4-diphenylaminophenyl)fluorene substituted at 1,3,6,8-positions of pyrene were designed, synthesized, and characterized as solution-processable hole-transporting-free blue emitters for simple structured solution-processed organic light-emitting diodes (OLEDs). Both molecules adopted highly bulky molecular structures. They exhibited good quality thin films by spin-coating, stable blue emissions with high fluorescence emission efficiencies (64–71%), shallow HOMO levels (−5.31 to −5.38 eV), decent hole mobilities (2.31–5.05 x 10−6 cm2 V−1s−1), and high thermal stability, and effectively applied as non-doped emissive layers in the OLEDs. Especially, PY2-based OLED showed a bright blue emission with superior EL performance (brightness of 26415 cd m−2, turn-on voltage (Von) of 2.8 V, external quantum efficiency (EQEmax) of 4.19%, and luminance efficiency (LEmax) of 10.53 cd A−1). This outstanding EL performance of the non-doped solution-processed blue OLED could be ascribed to the effective molecular design, and this work presented a new strategy for constructing useful blue emitters based on pyrene.

Tuning photophysical properties of bicarbazole-based blue TADF emitters with AIE feature: Trade-off between small singlet-triplet energy splitting and high photoluminescence quantum yield

Three aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) blue light-emitting materials were successfully synthesized using bicarbazole and phenylsulfonyl/benzophenone moieties as electron donors and acceptors, respectively, via Buchwald-Hartwig reaction. Density functional theory calculations showed that the three emitters have a moderate dihedral angle between the two carbazole planes and between the donor and acceptor, resulting in a good conjugated structure and a trade-off between the small ΔEST and high photoluminescence quantum yield of the BCzDPM molecule. The cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry tests demonstrated that the three materials had high electrochemical and thermal stability. The three emitters exhibited obvious intramolecular charge transfer characteristics in solution and film. According to the fluorescence and low-temperature phosphorescence spectra, the lowest singlet and triplet excited state energy levels of BCzDPM were 2.96 eV and 2.78 eV, respectively, with a ΔEST of 0.18 eV. Using these emitters as non-doped emissive layers, efficient blue electroluminescent devices were fabricated. The maximum current efficiency and external quantum efficiency reached 15.4 cd/A and 7.05% for the BCzDPM device, respectively, and the efficiency roll-off of BCzDPM was smaller than that of PSOBCz and PBCzPM, under high current density.

Highly Efficient Inherent Linearly Polarized Electroluminescence from Small-Molecule Organic Single Crystals.

Small-molecule organic single crystals (SCs) with an inherent in-plane anisotropic nature enable direct linearly polarized light emission without the need for spatially separated polarizers and complex optical structures. However, the device performance is severely restricted by the starvation of appropriate SC emitters and the difficulty in the construction of efficient SC electroluminescence (EL) devices, leading to a low external quantum efficiency (EQE) of usually smaller than 1.5%. Here, highly efficient inherent linearly polarized light-emitting diodes (LP-LEDs) are demonstrated by exploiting 2,6-diphenylanthracene (DPA) SCs as intrinsically polarized emitters. The LP-LEDs exhibit a 2.5-fold enhanced maximum EQE of 3.38%, which approaches the theoretical limit for the DPA SC-based EL device and is the highest among organic SC-based LEDs reported thus far. More importantly, a high degree of polarization (DOP) up to 0.74 is achieved for the intrinsically polarized EL emission of the DPA SC-based LP-LEDs. By leveraging the highly efficient LP-LED, an interchip polarized optical communication system consisting of organic SCs is demonstrated for the first time. This work creates a solid foundation for the exploitation of a vast new library of small-molecule organic SCs for LP-LEDs and carries broad implications for polarized optics and relevant optoelectronic devices.

Efficient Red Light Emitting Diodes Based on a Zero‐Dimensional Organic Antimony Halide Hybrid (Adv. Mater. 9/2023)

Organic Metal Halide Hybrids Efficient electroluminescent devices can now be fabricated using solution-processed thin films based on 0D organic metal halide hybrids (OMHHs). In article number 2209417, Biwu Ma and co-workers develop a highly luminescent 0D OMHH containing organic semiconducting cations, which successfully addresses the issues of low conductivity and poor energy alignment of existing 0D OMHHs for electrically driven light-emitting diodes (LEDs).

Carboxylic‐Free Synthesis of InP Quantum Dots for Highly Efficient and Bright Electroluminescent Device

AbstractIn the synthesis of InP‐based quantum dots (QDs), the environmentally friendly carboxylic‐free phosphor precursor tris(dimethylamino)phosphine ((DMA)3P) has been frequently used to replace tris (trimethylsilyl)phosphine (TMS)3P, in order to prevent the InP cores from oxidation and avoid the dangerous etching with hydrofluoric acid. However, the InP‐based QDs synthesized with (DMA)3P still perform poorly in quantum dot light‐emitting diodes (QLEDs) due to oxidation by other carboxylic sources. In this work, based on the synthesis with (DMA)3P, ZnCl2 precursor instead of zinc carboxylate is used for shell growing, and oleylamine is used as solvent instead of oleic acid, so that there is no oxidizing agent such as carboxylic groups during the whole synthesis process. As a result, highly efficient InP/ZnSe/ZnSeS/ZnS QDs with gradient varying shells are obtained, whose full width at half‐maximum (≈41 nm) and photoluminescence quantum yield (≈96%) are narrowest and highest among the reported ones, respectively. The QLEDs based on InP QDs synthesized with (DMA)3P achieve breakthrough efficiency of 12.39%. Especially, high efficiency and high brightness are achieved at the same time. This work opens a new avenue in the synthesis of high‐quality and environmental friendly InP‐based QDs.

Dual Hybridized Local and Charge Transfer Characteristic Fluorophore as an Efficient Nondoped Emitter for Electroluminescent Device

Hybridized local and charge transfer (HLCT) is an attractive strategy for achieving efficient electroluminescence (EL) in electrofluorescent devices by the conversion of triplet‐state (T) to singlet‐state (S) excitons via a reverse intersystem crossing, giving rise to a high exciton utilization efficiency exceeding the spin statistical limit. Excellent device performance is reached using thermally evaporated emissive layers. However, for potential large‐scale commercialization, it is crucial to attain comparable device performances using low‐cost solution‐processing techniques. Herein, a new concept of dual HLCT characteristic fluorophore (BCBF) with a high solid‐state fluorescence as a nondoped emitter for a simple structured solution‐processed EL device is presented. BCBF is effectively formulated by π‐conjugation linking two HLCT fragments with a highly soluble hole‐transporting aromatic moiety. Its dual HLCT and photoluminescence (PL) properties are experimentally and theoretically probed by the solvatochromic effect and density functional theory (DFT) calculations. The molecule exhibits an intense yellow–green emission with a good solution‐processed film‐forming quality and a high solid‐state fluorescence quantum yield of 80%. BCBF is successfully utilized as a nondoped emissive layer in a solution‐processed double‐layered organic light‐emitting diode (OLED), which shows excellent EL performance (brightness of 47 580 cd m−2, current efficiency of 15.78 cd A−1, and external quantum efficiency of 7.20%).

Spectra Stable Quantum Dots Enabled by Band Engineering for Boosting Electroluminescence in Devices

The band level landscape in quantum dots is of great significance toward achieving stable and efficient electroluminescent devices. A series of quantum dots with specific emission and band structure of the intermediate layer is designed, including rich CdS (R-CdS), thick ZnSe (T-ZnSe), thin ZnSe (t-ZnSe) and ZnCdS (R-ZnCdS) intermediate alloy shell layers. These quantum dots in QLEDs show superior performance, including maximum current efficiency, external quantum efficiencies and a T50 lifetime (at 1000 cd/m2) of 47.2 cd/A, 11.2% and 504 h for R-CdS; 61.6 cd/A, 14.7% and 612 h for t-ZnSe; 70.5 cd/A, 16.8% and 924 h for T-ZnSe; and 82.0 cd/A, 19.6% and 1104 h for R-ZnCdS. Among them, the quantum dots with the ZnCdS interlayer exhibit deep electron confinement and shallow hole confinement capabilities, which facilitate the efficient injection and radiative recombination of carriers into the emitting layer. Furthermore, the optimal devices show a superior T50 lifetime of more than 1000 h. The proposed novel methodology of quantum dot band engineering is expected to start a new way for further enhancing QLED exploration.

Efficient deep-blue electroluminescent devices based on a novel β-diketone zinc complex

A novel zinc complex Zn(PhCzKt)2 with β-diketone unit as chelating group was synthesized, where PhCzKt is dehydrogenation 1-(4-(tert-butyl) phenyl)-3-(9-ethyl-9H-carbazol- 3-yl) propane-1,3-dione. Its structure was characterized by 1H NMR, MS and EA. Its UV–vis, PL, thermal stability, and electrochemistry were studied. The luminescent mechanism was also explored by calculations of DFT (density functional theory) and time-dependent DFT. Zn(PhCzKt)2-based electroluminescent devices were manufactured by using evaporation technique. The devices present efficient luminescence with maximum EQE of 1.54 %, and the CIE(x,y)1931 of the EL spectrum is（0.1558, 0.0901), displaying a deep-blue emission.

Dibenzophenazine-Based TADF Emitters as Dual Electrochromic and Electroluminescence Materials.

Previous work has reported the synthesis of donor-acceptor-donor molecules based on dibenzophenazine acceptor group, presenting thermally activated delayed fluorescent (TADF) properties and their application in the assembly of highly efficient electroluminescent devices. Herein, we focus on the characterisation of charge carrier species through UV-Vis-NIR spectroelectrochemical and potentiostatic EPR techniques, in addition to the investigation of electropolymerisation properties of some compounds depicted in this study. The promising electrochromic features of both small molecules and conjugated polymers led to the assembly and investigation of electrochromic devices, evidencing the materials' versatility, applied in such different approaches as electrochromic windows and electroluminescent devices. Furthermore, the assembled OLEDs provided high efficiencies, with small roll-off, EQEs up to 20.5 % and luminance values up to 85 000 cd/m2 .

Solution-processed green and blue quantum-dot light-emitting diodes with eliminated charge leakage

Quantum-dot light-emitting diodes (QD-LEDs) promise a new generation of efficient, low-cost, large-area and flexible electroluminescent devices. However, the inferior performance of green and blue QD-LEDs compared with their red counterpart is hindering the commercialization of QD-LEDs in display and solid-state lighting applications. Here we demonstrate green and blue QD-LEDs with ~100% conversion of the injected charge carriers into emissive excitons. The key to success is the elimination of electron leakage at the organic/inorganic interface by using hole-transport polymers with simultaneous low electron affinity and reduced energetic disorder. Our devices exhibit high external quantum efficiencies over a wide range of luminance values (peak external quantum efficiencies of 28.7% for green and 21.9% for blue) and excellent stability (extrapolated T95 lifetime is 580,000 h for green and 4,400 h for blue QD-LEDs). We expect our work to provide a general strategy for eliminating charge leakage in solution-processed LEDs featuring organic/inorganic interfaces.

Surface state-induced barrierless carrier injection in quantum dot electroluminescent devices

The past decade has witnessed remarkable progress in the device efficiency of quantum dot light-emitting diodes based on the framework of organic-inorganic hybrid device structure. The striking improvement notwithstanding, the following conundrum remains underexplored: state-of-the-art devices with seemingly unfavorable energy landscape exhibit barrierless hole injection initiated even at sub-band gap voltages. Here, we unravel that the cause of barrierless hole injection stems from the Fermi level alignment derived by the surface states. The reorganized energy landscape provides macroscopic electrostatic potential gain to promote hole injection to quantum dots. The energy level alignment surpasses the Coulombic attraction induced by a charge employed in quantum dots which adjust the local carrier injection barrier of opposite charges by a relatively small margin. Our finding elucidates how quantum dots accommodate barrierless carrier injection and paves the way to a generalized design principle for efficient electroluminescent devices employing nanocrystal emitters.

V-shaped triazine host featuring intramolecular non-covalent interaction for highly efficient white electroluminescent devices

The exploration of host materials with simultaneously excellent electrical properties and high triplet energy levels is highly essential but challenging in the fields of blue and white phosphorescence (Ph) and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, the symmetric V-shaped triazine host of DCzT in D-A-D molecular configuration featuring non-covalent intramolecular interactions (NCIs) was designed and synthesized through the replacement of phenyl core of highly twisted blue host of mCP by the acceptor unit of 2,4-dichloro-1,3,5-triazine. By means of the NCIs, DCzT demonstrates a relatively closed plane, thus boosting ordered molecular packing arrangement for the formation of carrier transport channel to acquire enhanced and balanced electrical performance at a high triplet energy level. Excitingly, highly efficient white TADF OLEDs was achieved displaying maximum luminance of 12000 cd m−2 and external quantum, current and power efficiencies of 23.2%, 66.9 cd A-1 and 70.0 lm W−1, respectively. This work highlights that the NCI affords a simple and intelligent way to regulate the electrical attributes of organic optoelectronic materials.

Twisted Phenanthro[9,10-d]imidazole Derivatives as Non-doped Emitters for Efficient Electroluminescent Devices with Ultra-Deep Blue Emission and High Exciton Utilization Efficiency.

Herein, two deep-blue emissive molecules (SAF-PI and SAF-DPI) are designed and synthesized using spiro[acridine-9,9'-fluorene] as a donor (D) substituted with 2-(3-methylphenyl)-1-phenyl-phenanthro[9,10-d]imidazole as an acceptor (A), forming twisted D-A and A-D-A structures, respectively. The photophysical studies and density functional theory (DFT) calculations reveal that both molecules exhibit hybridized local excited and charge transfer (HLCT) characteristics with deep blue emission color. They are effectively applied as non-doped emitters in OLEDs. Particularly, SAF-PI-based device achieves the high-definition television (HDTV) standard blue color emission peaked at 428 nm with CIE coordinate of (0.156, 0.053), a narrow full width at half maximum of 55 nm, a maximum external quantum efficiency (EQEmax ) of 4.57% and an exciton utilization efficiency of 65%.

(Invited) Helicate Europium Complexes for the Design of Efficient Luminescence and Electro-Luminescence Devices

Molecular structure in metal complexes enhances their electronic structure and physicochemical properties such as luminescence. It is well known that the number of coordination of a lanthanide ion is various from six to twelve. Here, we will introduce a series of helicate europium (Eu) complexes which having hexadentate ligand with chelate effect to keep the coordination of its aromatic ligand as photo-antenna to induce ff emissions of Eu.[1] The Eu complex (EuL) has a neutral ligand consisted in two bipyridine moieties bridged by ethylenediamine and two nitrate ions, and takes a cationic complex. The molecular structure succeeded to form the efficient energy donor behavior from the measurement of electro-static potential by the synchrotron single crystal X-ray analysis (SPring-8, BL01B1), because the bipyridine moiety has kept the excited state like a terpyridine moiety. The luminescence band of Eu was observed at 616 nm with relatively high luminescent quantum yields (QY) by UV excitation. The complex is also soluble in ionic liquids (IL) when the same counter anions were used in both the complex and IL such as PF6 -.[2] Luminescence and electronic absorption bands position of EuL in IL were same. Interestingly the QY of the complex drastically increased in IL because of the high viscosity. IL was also useful for the application for electrochemistry as a medium. Actually, there are several papers of Eu complexes were reported to explain the possibility of luminescence color change by the redox reaction of Eu ion[3]. If Eu ion takes divalent state, the luminescence color will be shifted at blue, and can be switch the color between red and blue by electrochemistry. In the case of EuL in IL, we adopted the solution to electronic cells and found the luminescence intensity changing at around +2.0 and -1.8 V [4]. It exhibites the reversibility with keeping the possible luminescence intensity. Thus, the stable halicate structure and cationic state of EuL act as a electrochemical active behavior in IL.Author acknowledges Professor Y. Kim, Hokkaido University for the collaboration of electrochemistry in the ionic liquids.[1] (a)M. Hasegawa, et al., New J. Chem., 2014, 38, 1225-1234; (b) M. Hasegawa, et al., ChemPlusChem, 2020, 85, 294-300; (c) M. Hasegawa, et al., Photochem. Photobiol. Sci., 2020, DOI: 10.1039/d0pp00069h, (d) M. Hasegawa & H. Ohmagari, Chem. Lett., 2020, 49, 845-854.[2] Y. Hasegawa, M. Hasegawa, et al., Molecules, 2018, 23, 55.[3] (a)K. Binnemans, Chem Rev, 2007, 107(6), 2592–2614; (b) M. Mukaigawa & H. Ohno, Solid State Ionics, 1998, 113-115, 439-442.[4] Y. Kim & M. Hasegawa, et al., 2020, DOI:10.1021/acsami.0c13765. Figure 1

Efficient green electroluminescent devices with low operation voltage and slow efficiency roll-off by utilizing hole transport material as host

We reported efficient electroluminescent (EL) devices with low operation voltage and slow efficiency roll-off based on green emitter Ir1 by utilizing hole transport material TAPC as host. Experimental results revealed our designed single light-emitting layer (EML) devices exhibit even lower operation voltage and slower efficiency roll-off compared with double-EMLs devices attributed to the reduced carriers' injection barriers, balanced carriers' distribution and well confined recombination zone. Ultimately, efficient green EL device realized the highest current efficiency (CE), power efficiency (PE), external quantum efficiency (EQE) and brightness of 71.6 cd/A, 75.0 lm/W, 22.9% and 36600 cd/m2, respectively. Excitedly, turn-on voltage was only 2.5 V. Even at a given brightness of 1000 cd/m2 (3.4 V), CE of 67.2 cd/A (EQE = 21.5%) and PE of 65.8 lm/W can still be retained. Our work offers a method to prepare efficient devices with slow efficiency roll-off, low operation voltage and simultaneously simple device structure.

A solution-processable wholly-aromatic bipolar host material for highly efficient blue electroluminescent devices

Highly efficient blue electroluminescent devices were realized by using a solution-processable wholly-aromatic bipolar host material prepared through an insulating C(sp3) bridge linking strategy.

Cyanophenyl spiro[acridine-9,9′-fluorene]s as simple structured hybridized local and charge-transfer-based ultra-deep blue emitters for highly efficient non-doped electroluminescent devices (CIEy ≤ 0.05)

Cyanophenyl substituted spiro[acridine-9,9′-fluorene]s featured with HLCT excited states yielded highly efficient non-doped OLEDs with emissions in high-definition television standard blue color, CIEy ≤ 0.05, and CEmax/EQEmax of 6.54 cd A−1/4.63%.