Efficient Blue Emission Research Articles

Color Stable Deep Blue Multi-Resonance Organic Emitters with Narrow Emission and High Efficiency.

The development of highly efficient and deep blue emitters satisfying the color specification of the commercial products has been a challenging hurdle in the organic light-emitting diodes (OLEDs). Here, deep blue OLEDs with a narrow emission spectrum with good color stability and spin-vibronic coupling assisted thermally activated delayed fluorescence are reported using a novel multi-resonance (MR) emitter built on a pure organic-based molecular platform of fused indolo[3,2,1-jk]carbazole structure. Two emitters derived from 2,5,11,14-tetrakis(1,1-dimethylethyl)indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (tBisICz) core are synthesized as the MR type thermally activated delayed fluorescence emitters realizing a very narrow emission spectrum with a full-width-at-half-maximum (FWHM) of 16nm with suppressed broadening at high doping concentration. The tBisICz core is substituted with a diphenylamine or 9-phenylcarbazole blocking group to manage the intermolecular interaction for high efficiency and narrow emission. The deep blue OLEDs achieve high external quantum efficiency (EQE) of 24.9%, small FWHM of 19nm, and deep blue color coordinate of (0.16, 0.04) with good color stability with increase in doping concentration. To the authors' knowledge, the EQE in this work is one of the highest values reported for the deep blue OLEDs that achieve the BT.2020standard.

Exploring Efficient Blue Thermally Activated Delayed Fluorescence Emitters by Constructing Intramolecular Hydrogen Bonds

AbstractEfficient blue organic luminescent materials are highly desired for full‐color displays and white lighting based on the organic light‐emitting diode (OLED) technique, but the exploration of robust blue emitters remains challenging. In this work, a series of efficient blue thermally activated delayed fluorescence materials comprised of a benzonitrile acceptor, pyridine bridge, and carbazole‐based donors is designed and synthesized. Intramolecular hydrogen bonds are formed in these molecules, which improve molecular planarity and rigidity. These molecules exhibit excellent thermal stability, high photoluminescence quantum yields, and large horizontal dipole ratios. Highly efficient non‐doped and doped OLEDs are fabricated using these molecules as emitters, providing deep‐blue to sky‐blue lights with outstanding maximum external quantum efficiencies and good operational device lifetime. These results indicate that building intramolecular hydrogen bonds can be an effective strategy for the construction of efficient and robust blue emitters for OLED applications.

Facile Synthesis of Multicolored Stacked Quantum Dot Films for Efficient White Light Emission

Quantum dot (QD)-based white light-emitting diodes (LEDs) or white QD-LEDs were successfully fabricated by stacking QD films of primary colors (i.e., red, green, and blue). An ultrathin ZnO film was deposited between each QD layer to facilitate the formation of a tricolor-stacked QD structure. Such stacked QD films effectively suppress the Förster resonance energy transfer between QDs and enable efficient blue light emission, unlike randomly mixed QD films of different colors. The photoluminescence (PL) spectra of various QD films were obtained to study the change in the corresponding color gamut with applied voltage. In addition, QD-LEDs were fabricated based on the PL behavior and electronic band diagrams of QDs of different colors. Maximum luminance and peak EQE of white QD-LED shows 5700 cd/m2 and 1.1% compared to 2200 cd/m2 and 0.3% of QD-LED with mixed QD layer. Finally, a smooth color transition and white light emission were achieved using a blue/green/red stacking sequence for the QDs, which successfully suppressed the energy transfer effect. The overall performance and brightness of the white QD-LEDs synthesized in this study can be further enhanced by developing high-performance blue QDs.

Efficient blue emission from stable silver-based metal halide single crystals

In this paper, the one-dimensional Cs2AgI3 and Cu+-doped Cs2AgI3 single crystals had been synthesized by antisolvent infiltration method. A blue-emission with peak at 473 nm and a large Stokes shift was observed in the Cs2AgI3 single crystals, which might originate from the self-trapped excitons (STEs). Experimental studies revealed that appropriate Cu+ dopant greatly enhanced the Cs2AgI3 photoluminescence quantum yield (PLQY). Theoretical calculation confirmed that the optimized Cu+ concentration could effectively modulate the electronic band structure, and the electronic states highlighted in CuI4 tetrahedron showed strong localized characteristic, which was beneficial to improve the luminescence efficiency of the blue band. In addition, this kind of metal halides exhibited strong stability in air ambient. This work supplies a feasible strategy to improve the optoelectronic performance by extrinsic chemical doping and could prompt the commercial application of lead-free metal halides.

Efficient blue CsPbBr3 perovskite nanocrystals synthesis with the assistance of zwitterionic straight chain amino acids

Metal-halide perovskite nanocrystals (NCs) are promising semiconductor materials for the application of novel nano-optoelectronic devices and light sources, which have attracted wide attention in recent years. Many additives were applied for modulating the electronic structure, as well as improving the stability or light efficiency of the perovskite NCs, particularly the zwitterionic molecules show strong and unique interaction with perovskite lattice to passivate the defects and surface state of the perovskite NCs. Here, in this work, we synthesized the blue CsPbBr3 perovskite NCs with the assist of zwitterionic straight-chain amino acids, and systematically studied the interaction between the perovskite and amino acids with varied molecular structure. We found that the polarization of the amino acid molecules dominates the ionized state of the two functional groups of -NH2 and -COOH, leading to different interaction with perovskite. Eventually, the perovskite NCs with 7-aminophenic acid show stable and efficient blue light emission at peak wavelength of 475 nm with achieved light efficiency of ∼ 90%. This work demonstrated the mechanism of the interaction between series zwitterionic amino acids and perovskite experimentally, which provides a guideline for the additive selection and design.

Intrinsically Flexible and Aging Resistant Fluorene‐Based Rod‐Coil Copolymer for Bendable Deep‐Blue PLEDs

AbstractIn the field of flexible light‐emitting display, goal‐oriented intelligent molecular design is used to control various behaviors of molecules, which provides potential for the development of flexible light‐emitting conjugated polymers (LCPs). The introduction of non‐conjugated units into polymer molecules is a key prerequisite for realizing the intrinsic flexibility, but its easy interchain slip will also lead to the formation of interchain excited states, which is detrimental to the efficiency of light‐emitting diodes. Herein, two kinds of fluorene‐based rod‐coil copolymer with stable deep blue emission characteristics is presented and with Commission Internationale de L'Eclairage (CIE) coordinates of (0.18, 0.14) and (0.15, 0.09), respectively. Surprisingly, the copolymer films show efficient blue emission even at 100% tension. Meanwhile, the rod‐coil copolymer possesses better aging resistance compared to rigid π‐conjugated counterparts. Finally, both rigid and flexible light‐emitting diodes based on rod‐coil copolymer exhibit stable deep blue emission, and the G2‐based PLED with CIE coordinates of (0.16, 0.08), which approach National Television System Committee standard blue specification. These results confirm the validity of rod‐coil copolymer design strategy in constructing inherently flexible polymers with deep blue emission, which have great application potential in flexible PLEDs.

Synthetic Control over the Surface Chemistry of Blue-Emitting Perovskite Nanocrystals for Photocatalysis

Lead halide perovskite nanocrystals (NCs) are particularly suitable for light-emitting and photocatalysis applications, where their potential can be maximized by controlling the surface composition of their organic shell. In this study, the preparation of CsPbClxBr3–x NCs at room temperature in toluene is described. Three differently structured surfactants are utilized for the synthesis, each with a specific function, namely, the solubilization of the lead precursor (n-Hept4NBr), the surface passivation with halide modification (dimethyldioctadecylammonium chloride), and the protection of the surface-active sites (octanoic acid) for photocatalysis. Under these conditions, nearly monodispersed blue-emitting nanocubes are selectively obtained in a one-pot synthesis by combining specific amounts of the perovskite precursors. As supported by thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy investigations, the organic shell of the obtained NCs is composed of electrostatically bound dimethyldioctadecylammonium ions, granting robustness to the corresponding NCs, and octanoic acid molecules, interacting with the nanoparticle surface through weaker secondary bonds. The obtained NCs exhibit a high photoluminescence quantum yield (PLQY = 72 ± 3%) notwithstanding multiexponential recombination dynamics of the excited state, resulting from the different passivation modes at the NC surface. Moreover, the NCs show a remarkable optical stability after exposure to high temperatures and to water contact due to the high surface density of the multifunctional organic ligands. The introduction of 4-tert-butylphenyl thiol promotes a charge transfer process at the NC/thiol interface formed upon removal of the labile ligands (octanoic acid) at the NC surface. In these conditions, the NCs are prone to the photoinduced conversion of the aromatic thiol into the corresponding disulfide without varying the optical properties of the perovskite photocatalyst upon the substrate conversion. Therefore, the obtained results cast light on the versatility of the surface engineering of lead halide perovskite NCs for efficient blue emission and photocatalysis with improved stability.

Sulfide, Sulfoxide, and Sulfone Substitution: Electron Structure Modulation from Para‐D‐π‐B to Para‐A‐π‐B in Multiple‐Resonance Emitters with Narrowband Emission and High Color Purity

AbstractMulti‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have been studied to address the issue of the broadband emission in organic light‐emitting diodes (OLEDs). Herein, the authors have systematically investigated the effect of electron‐donating or ‐withdrawing units in the para position of B atom on the optoelectronic emission modulation BCz‐BN MR‐TADF emitters. Due to the enhanced spin‐orbit coupling (SOC) effect, BN(p)SCH3 with electron structure of para‐D‐π‐B is synthesized by introducing a heavy S atom into the para position of B atom of BCz‐BN. By oxidizing BN(p)SCH3, BN(p)SOCH3 and BN(p)SO2CH3 with electron structure of para‐A‐π‐B have been synthesized. The quantum simulations and photophysical studies have illustrated BN(p)SCH3 can exhibit large reverse intersystem crossing rate constant (kRISC) of 6.4 × 104 s−1 due to the large SOC constants and small singlet‐triplet energy splitting (ΔEST) of 0.12 eV. BN(p)SOCH3 and BN(p)SO2CH3 with electron structure of para‐A‐π‐B displayed red‐shift emissions with smaller full‐width at half‐maximum (FWHM) values of ≈21 nm and kRISC values owing to enhanced ΔEST and the low emission contribution of the triplet excitons in contrast to those of BN(p)SCH3 with electron structure of para‐D‐π‐B. Consequently, BN(p)SCH3‐based OLEDs show highly efficient blue emission with an external quantum efficiency (EQE) of 26.2% and excellent color purity.

Hybrid Lead‐Free Metal Antimony Oxide Chlorides with Highly Efficient Blue Light Emissions

AbstractHerein, we report two hybrid zero‐dimensional (0D) lead‐free metal oxide halides of [PPip]Sb2Cl6O (1) ([PPip]=1‐(2‐Pyridyl)piperazine) and [PmPip]Sb2Cl6O (2) ([PmPip]=1‐(2‐Pyrimidyl)piperazine). Compounds 1 and 2 are able to exhibit narrow blue light emissions excited by UV light with photo‐luminescence quantum yields (PLQYs) of 25.49 % and 24.71 %, respectively. Through systematical characterizations, these blue light emissions can be attributed to the radiative transition in conjugated organic cations. Benefiting from the highly efficient intrinsic blue emissions, compound 2 can be used as blue fluorophor to fabricate liquid crystal display. This work provides a new to assembly blue emitter through by virtue of the optical active organic cations in hybrid materials.

Organic light-emitting diodes, what’s next?

Organic light-emitting diode (OLED) technology has made significant advances in performance and becoming widely used in smartphone displays. However, the stability of efficient blue emitters, such as phosphorescent and thermally activated delayed fluorescent emitters, remains a challenge. While white OLEDs have the potential to revolutionize indoor lighting, their high cost per lumen due to efficiency roll-off and manufacturing costs is a limiting factor. To overcome these challenges, researchers must explore new material systems and device architectures to reduce efficiency roll-off and develop cost-effective film preparation processes. With continued increases in stability and decreases in cost, there is significant interest in scaling up OLED lighting and exploring its applications in other fields such as biomedical devices, anti-counterfeit labels, and optical communication.

A Deep‐Blue‐Emitting Heteroatom‐Doped MR‐TADF Nonacene for High‐Performance Organic Light‐Emitting Diodes**

AbstractWe present a p‐ and n‐doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder‐type multiresonant thermally activated delayed fluorescence (MR‐TADF) emitter. This compound shows efficient narrow deep blue emission, with a λPL of 410 nm, full width at half maximum, FWHM, of 38 nm, photoluminescence quantum yield, ΦPL of 71 %, and a delayed lifetime, τd of 1.18 ms in 1.5 wt % TSPO1 thin film. The organic light‐emitting diode (OLED) using this compound as the emitter shows a comparable electroluminescence spectrum peaked at 409 nm (FWHM=37 nm) and a maximum external quantum efficiency (EQEmax) of 8.5 % at Commission Internationale de l’Éclairage (CIE) coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2 % at 3 wt % doping of the emitter within the emissive layer of the device. At this concentration, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

A Deep-Blue-Emitting Heteroatom-Doped MR-TADF Nonacene for High-Performance Organic Light-Emitting Diodes.

We present a p- and n-doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder-type multiresonant thermally activated delayed fluorescence (MR-TADF) emitter. This compound shows efficient narrow deep blue emission, with a λPL of 410 nm, full width at half maximum, FWHM, of 38 nm, photoluminescence quantum yield, ΦPL of 71 %, and a delayed lifetime, τd of 1.18 ms in 1.5 wt % TSPO1 thin film. The organic light-emitting diode (OLED) using this compound as the emitter shows a comparable electroluminescence spectrum peaked at 409 nm (FWHM=37 nm) and a maximum external quantum efficiency (EQEmax ) of 8.5 % at Commission Internationale de l'Éclairage (CIE) coordinates of (0.173, 0.055). The EQEmax values were increased to 11.2 % at 3 wt % doping of the emitter within the emissive layer of the device. At this concentration, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

Tuning the energy transfer in Ruddlesden-Popper perovskites phases through isopropylammonium addition - towards efficient blue emitters.

Here we demonstrate blue LEDs with a peak wavelength of 481 nm, with outstanding colour purity of up to 88% (CIE coordinates (0.1092, 0.1738)), an external quantum yield of 5.2% and a luminance of 8260 cd m-2. These devices are based on quasi-2D PEA2(Cs0.75MA0.25)Pb2Br7, which is cast from solutions containing isopropylammonium (iPAm). iPAm as additive assist in supressing the formation of bulk-like phases, as pointed out by both photophysical and structural characterization. Additionally, the study of the excitation dynamics demonstrates a hindering of the energy transfer to domains of lower energy that generally undermines the performance and emission characteristics of blue-emitting LEDs based on quasi-2D perovskites. The achieved narrow distribution of quantum well sizes and the hindered energy transfer result in a thin film photoluminescence quantum yield exceeding 60%. Our work demonstrates the great potential to tailor the composition and the structure of thin films based on Ruddlesden-Popper phases to boost performance of optoelectronic devices - specifically blue perovskite LEDs.

Optical gain properties of interfacial material PFN-Br and its application potentials in future electrically pumped organic lasers

In this paper, the optical gain properties of the water/alcohol soluble conjugated polyelectrolyte (Poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) (PFN-Br) and its potential applications in future electrically pumped organic lasers are revealed and systematically studied. To the best of our knowledge, no studies on the optical gain properties of PFN-Br or its prototype, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] have been reported before. These conjugated polyelectrolytes are widely used as the interlayers in organic light emitting diodes or organic solar cells. The thickness of such an interlayer is usually less than 10 nm, which is considered not sufficient for supporting light waveguiding. Therefore, the thickness of the PFN-Br layer used in this work is increased to more than 100 nm. Through careful study, the polymer is found to possess a low threshold of amplified spontaneous emission (ASE) (~11 μJ/cm<sup>2</sup>) and a small ASE cutoff thickness (<50 nm). It is an efficient blue emission (~456 nm) gain medium. The ASE peak of the PFN-Br film is red-shifted as the thickness increases from 50 to 220 nm. By utilizing the great resistance of PFN-Br against the organic solvent, such as toluene, PFN-Br/F8BT bilayer devices on quartz and PFN-Br/MEH-PPV bilayer devices on ITO glass are fabricated and characterized. In the PFN-Br/F8BT bilayer devices, it is found that the PFN-Br interlayer has very limited influence on F8BT. The ASE threshold of F8BT increases only twice, compared with that of F8BT monolayer device, when 100-nm-thick PFN-Br layer is introduced beneath the F8BT film. No significant change in optical gain or loss is observed. Most of the extra losses in F8BT due to the introduction of PFN-Br are attributed to the larger refractive index of PFN-Br than that of quartz substrate. Furthermore, in the PFN-Br/MEH-PPV bilayer devices on ITO glass, introducing PFN-Br interlayer resulting in optimal ASE performance of MEH-PPV compared with that on bare ITO surface. The ASE threshold of MEH-PPV is reduced as much as 60% (from 402 μJ/cm<sup>2</sup> to 160 μJ/cm<sup>2</sup>) while the PFN-Br layer is sandwiched between ITO and MEH-PPV. The PFN-Br layer modifies the waveguiding modes, and reduces the interaction between excitons and ITO electrodes. As a result, the ASE performance of MEH-PPV is improved. The findings of this report indicate that the PFN-Br is not only a good carrier transport material but also a highly-efficient gain medium. PFN-Br, combined with its advantages in different fields, is expected to play various roles in future organic electrically pumped lasers.

High-efficiency all fluorescence white OLEDs with high color rendering index by manipulating excitons in co-host recombination layers.

All fluorescence white organic light-emitting diodes (WOLEDs) based on thermally activated delayed fluorescence (TADF) emitters are an attractive route to realize highly efficient and high color quality white light sources. However, harvesting triplet excitons in these devices remains a formidable challenge, particularly for WOLEDs involving conventional fluorescent emitters. Herein, we report a universal design strategy based on a co-host system and a cascaded exciton transfer configuration. The co-host system furnishes a broad and charge-balanced exciton generation zone, which simultaneously endows the devices with low efficiency roll-off and good color stability. A yellow TADF layer is put forward as an intermediate sensitizer layer between the blue TADF light-emitting layer (EML) and the red fluorescence EML, which not only constructs an efficient cascaded Förster energy transfer route but also blocks the triplet exciton loss channel through Dexter energy transfer. With the proposed design strategy, three-color all fluorescence WOLEDs reach a maximum external quantum efficiency (EQE) of 22.4% with a remarkable color rendering index (CRI) of 92 and CIE coordinates of (0.37, 0.40). Detailed optical simulation confirms the high exciton utilization efficiency. Finally, by introducing an efficient blue emitter 5Cz-TRZ, a maximum EQE of 30.1% is achieved with CIE coordinates of (0.42, 0.42) and a CRI of 84 at 1000 cd m-2. These outstanding results demonstrate the great potential of all fluorescence WOLEDs in solid-state lighting and display panels.

Rational design of phenanthroimidazole derivatives with hybridized local and charge-transfer characteristics to achieve efficient blue emission in non-doped OLEDs

Two new blue phenanthroimidazole-based emitters were designed and synthesized. The molecules after introducing a benzene ring show interesting HLCT characteristics. A HLCT-OLED shows blue emission and a full width at half-maximum of 67 nm.

Sn2+doping-induced large extra vibrational energy of an excited state for efficient blue emission in Cs2SnCl6:Bi

Sn2+and Bi3+co-doped Cs2SnCl6was achieved by using SnCl2as the precursor. Sn2+increases the extra vibrational energy of the excited state through more localized excitons, and thus greatly improves the radiative transition efficiency.

Tailoring Donor‐Acceptor Emitters to Minimise Localisation Induced Quenching of Thermally Activated Delayed Fluorescence

AbstractBy inverting the common structural motif of thermally activated delayed fluorescence (TADF) materials to a rigid donor core and multiple peripheral acceptors, it has been shown possible to achieve both a reverse intersystem crossing (rISC) rate 1×107 s−1 and a unity photoluminescence quantum yield [Dos Santos et al. Adv. Sci. 5 (2018) 1700989]. However, the rISC rate in this motif is quenched by localisation of the excited state electronic structure which causes in‐equivalence between the peripheral acceptors. In this paper, we explore a series of related molecular targets which seek to reduce the effect of localisation on the rISC rates. This includes structures that contain donors exhibiting three‐fold or four‐fold symmetry and different degrees of steric hindrance around the donor‐acceptor bond with the objective of using steric hindrance to exert finer conformational control of the excited state dynamics to enhance functional properties. We demonstrate that a triazatruxene central donor is most effective for TADF owing to the energetic position of the locally excited state, while cyano‐benzonitriles are the most effective acceptors for reducing the effect of localisation on the electronic structure. These also push the emission into the blue region of the spectrum, opening the possibilities for this to be a pathway to develop efficient blue TADF emitters.

Improving excitons utilization of blue thermally activated delayed fluorescence emitter to achieve highly efficient solution-processed hybrid white organic light-emitting diodes

5-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (DBA-DI) with high triplet energy was utilized as efficient blue emission dopant to construct solution-processed thermally activated delayed fluorescence-phosphorescence (TADF-phosphorescence) hybrid white organic light-emitting diodes (WOLEDs). Meanwhile, a series of TADF-phosphorescence hybrid WOLEDs composed of bis(2-phenylquinoline) (2,2,6,6-tetramethylheptane-3,5-dionate)iridium(III) (PQ2Ir(dpm)), tris(2-(4-tolyl)-phenylpyridine)iridium (Ir(mppy)3) and DBA-DI are successfully developed by introducing the co-host system consist of poly(N-vinylcarbazole) (PVK) and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) with high color rendering index (CRI) and improving the utilization of triplet excitons. Furthermore, energy transfer processes among different emitters were investigated in detail based on absorbance spectra, photoluminescence and transient photoluminescence spectra. Ultimately, the Commission Internationale de I'Eclairage coordinates of the optimum solution-processed WOLEDs in warm-white emission region are (0.41, 0.41) and high CRI is 82. Furthermore, the same device realized outstanding efficiencies of 32.14 cd A−1, 21.00 lm W−1 and 15.1% at high brightness of 1000 cd m−2.

Shining Transparent Displays with Stable Narrow-Band Blue-Emitting Phosphor in Layered Film.

Transparent displays (TDs) rendering "levitating" images on screen have appeared as an emerging technology toward augmented/mixed reality applications. However, the traditional phosphor design and screen construction have severely limited the TD performance owing to the lack of efficient narrow-band blue emitters and stable screen structure. Herein, the novel narrow-band (full width at half maximum: 32nm) NaLi3 SiO4 :Eu2+ phosphor with a peak at 467nm as a key blue emitter is explored, and it is sandwiched in layered film as a unique screen design. The devised screen features decent transparency, high emission color purity, and good reliability, and the TD prototype renders "floating" static images and vivid animation with broad viewing angle (15°-165°) and large color gamut (97% of National Television Standards Committee). Spectroscopic and microstructural characterizations reveal the TD superior performance originates from synergistic contributions of moderate crystal field effect (εc ≈ 1.13eV; εcfs ≈ 1.60eV), weak vibronic coupling (S≈ 3; ħω≈ 285 cm-1 ), and limited thermal ionization of 5d electrons (Ea ≈ 0.43eV) for NaLi3 SiO4 :Eu2+ emission and layered architecture for screen film. These findings establish fundamental guidelines for narrow-band emitting materials design and shine light on superior TD innovative development.