Deep-blue Electroluminescence Research Articles

Narrowing the Phase Distribution of Quasi-2D Perovskites for Stable Deep-Blue Electroluminescence.

Solution‐processed quasi‐2D perovskites contain multiple quantum wells with a broad width distribution. Inhomogeneity results in the charge funneling into the smallest bandgap components, which hinders deep‐blue emission and accelerates Auger recombination. Here, a synthetic strategy applied to a range of quasi‐2D perovskite systems is reported, that significantly narrows the quantum well dispersity. It is shown that the phase distribution in the perovskite film is significantly narrowed with controlled, simultaneous evaporation of solvent and antisolvent. Modulation of film formation kinetics of quasi‐2D perovskite enables stable deep‐blue electroluminescence with a peak emission wavelength of 466 nm and a narrow linewidth of 14 nm. Light emitting diodes using the perovskite film show a maximum luminance of 280 cd m–2 at an external quantum efficiency of 0.1%. This synthetic approach will serve in producing new materials widening the color gamut of next‐generation displays.

Chrysene and triphenylene based-fluorophores as non-doped deep blue emitters for triplet-triplet annihilation organic light-emitting diodes

Herein, two new triplet-triplet annihilation (TTA) molecules, namely CCsCN and CTpCN, based on chrysene and triphenylene polyaromatic rings to develop deep-blue emissive materials for efficient non-doped TTA-OLEDs are designed, and synthesized. The polyaromatic rings are directly functionalized with benzonitrile and electron-donating 9-(m-tolyl)carbazole moieties. Their structural, physical, and photophysical properties are experimentally and theoretically studied. Both molecules display deep blue color emissions in solution and neat films with decent hole mobility and high thermal and electrochemical stabilities. They are effectively fabricated as non-doped emitters in the OLEDs, and all devices exhibit deep blue electroluminescence (EL) spectra (CIEy = 0.08) with narrow full width at half maximum of 74 nm, low turn-on voltages of 3.0 V and TTA characteristics. In particular, the CCsCN-based device achieves the best EL performance with a maximum luminance (Lmax) of 6338 cd m−2, a maximum external quantum efficiency (EQEmax) of 3.33%, and a maximum current efficiency (CEmax) of 2.64 cd A−1.

Deep-Blue OLEDs with Rec.2020 Blue Gamut Compliance and EQE Over 22% Achieved by Conformation Engineering.

To achieve high-efficiency deep-blue electroluminescence satisfying Rec.2020 standard blue gamut, two thermally activated delayed fluorescent (TADF) emitters are developed: 5-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (TDBA-PAS) and 10-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-9,9-diphenyl-9,10-dihydroacridine (TDBA-DPAC). Inheriting from their parented organoboron multi-resonance core, both emitters show very promising deep-blue emissions with relatively narrow full width at half-maximum (FWHM, ≈50nm in solution), high photoluminescence quantum yield (up to 92.3%), and short emission lifetime (≤2.49µs) with fast reverse intersystem crossing (>106 s-1 ) in doped films. More importantly, replacing the spiro-centered sp3 C atom (TDBA-DPAC) with the larger-radius sp3 Si atom (TDBA-PAS), enhanced conformational heterogeneities in bulky-group-shielded TADF molecules are observed in solution, doped film, and device. Consequently, OLEDs based on TDBA-PAS retain high maximum external quantum efficiencies ≈20% with suppressed efficiency roll-off and color index close to Rec.2020 blue gamut over a wide doping range of 10-50 wt%. This study highlights a new strategy to restrain spectral broadening and redshifting and efficiency roll-off in the design of deep-blue TADF emitters.

Highly efficient hybridized local and Charge-transfer (HLCT) Deep-blue electroluminescence with excellent molecular horizontal orientation

Efficient deep-blue emitters with nice color purity are fundamental in organic light-emitting diode (OLED) applications. Herein, we report two phenanthroimidazole (PPI) derivative deep-blue emitters (DP, SP) with hybrid local and charge-transfer (HLCT) properties of high photoluminescence quantum yield (PLQY) and nice color purity. More importantly, the horizontal dipole orientation (Θ//) of them are both superior to the isotropic emitters (66.7%), especially the Θ// of SP is 89.5%, which can greatly improve its OLED performance. As a result, non-doped OLED of SP exhibits deep-blue emission with Commission Internationale de lEclairage (CIE) coordinate of (0.158, 0.068) and maximum external quantum efficiency (EQEmax) of 11.3%, which is nearly twice that of DP and is one of the best results among the non-delayed, non-doped deep-blue fluorescent OLEDs. Furthermore, SP-based doped OLED displays an excellent performance of near ultraviolet (NUV) emission at 406 nm with CIE coordinate of (0.157, 0.046) and EQEmax of 7.5%, which is also a competitive result in the materials with comparable NUV OLED applications.

Efficient deep blue (CIE ∼0.08) fluorophore-based benzimidazole with hybridized local and charge transfer (HLCT) excited state for OLEDs

Development of efficient deep blue fluorophore with Commission Internationale de l’Eclairage (CIE)y value <0.1 is of great importance for display technology. Herein, we designed and synthesized a thermally stable deep blue emissive material 3-(2-(4′′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′:4′,1″-terphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)benzonitrile (MCNPIBI) by integrating moderate donor and acceptor, namely, phenanthroimidazole (PI) and benzimidazole with cyanophenyl (-CN) group at N1 position of the PI to tune the CT component in the excited states. The systematic theoretical and photophysical study reveals the MCNPIBI with hybridized local and charge transfer (HLCT) excited states. Time-dependent density functional theory (TD-DFT) calculation suggests that the reverse intersystem crossing (RISC) process in MCNPIBI occurs from high-lying triplet states to a singlet state. Furthermore, the synthesized deep blue emissive materials were employed as dopants in multilayer organic light emitting diode (OLED) devices, thus resulting in deep-blue electroluminescence (EL) with an emission wavelength of 447 nm and CIE coordinates of (0.15, 0.08), which are close to the standard values for blue emitters, as suggested by NTSC (0.14, 0.08). The OLED device displays a maximum current efficiency of 2.78 cd A−1, a maximum power efficiency of 1.94 lm W−1, and a maximum external quantum efficiency of 3.69%, respectively. In addition, the OLED device has a low turn in voltage of 3.8 V.

Highly efficient non-doped deep-blue OLED with NTSC CIEy and negligible efficiency roll-off based on emitter possessing hydrogen bond and hybridized excited state

The efficient deep-blue organic emitters with satisfying National Television System Committee (NTSC) standard Commission Internationale de L'Eclairage (CIE) coordinate of (0.14, 0.08) are indispensable component for full-color organic light-emitting diodes (OLEDs) display. In this work, we designed and synthesized a novel deep-blue material TPIN with donor‒π‒acceptor (D‒π‒A) structure by utilizing triphenylamine (TPA) as donor, 1,4,5-triphenyl-1H-imidazole (PIM) as acceptor and pyridyl as π-bridge. The pyridyl is expected to endow TPIN with multiple intra-/intermolecular hydrogen bands (H-bands) interactions which could inhibit molecular vibration and affect the radiative transition, aiming to obtain efficient deep-blue OLED with high color purity and negligible efficiency roll-off. In addition, the photophysical experiments and theoretical calculations were implemented to verify the unique hybridized excited state of TPIN. And X-ray crystallographic analysis of TPIN single-crystal indicates that there exist many C–H⋯N and C–H···π interactions in the stacking structure. The non-doped device based on TPIN exhibited good deep-blue electroluminescence (EL) performance with emission peak at 443 nm, corresponding to a CIE coordinate of (0.151, 0.080), and a maximum external quantum efficiency (EQEmax) of 3.71% with negligible efficiency roll-off (EQE = 3.70% @ 1000 cd m−2).

Spiro-configured dibenzosuberene compounds as deep-blue emitters for organic light-emitting diodes with a CIEy of 0.04

Deep blue electroluminescence is highly required for organic light-emitting diode (OLED) technology. We report, in this work, blue emitting fluorophores for OLED with low CIEy.

Highly Efficient Hybridized Local and Charge-Transfer (Hlct) Deep-Blue Electroluminescence with Excellent Molecular Horizontal Orientation

Highly Efficient Hybridized Local and Charge-Transfer (Hlct) Deep-Blue Electroluminescence with Excellent Molecular Horizontal Orientation

Deep-blue high-efficiency triplet–triplet annihilation organic light-emitting diodes using hydroxyl-substituted tetraphenylimidazole-functionalized anthracene fluorescent emitters

A new triplet–triplet annihilation-OLED exhibits deep-blue electroluminescence (CIEy ≈ 0.08) with a narrow full width at half maximum of 59 nm, high external quantum efficiency of 6.43%, low turn-on voltage, and singlet exciton utilization of 38%.

Aryl-Annulated [3,2-a] Carbazole-Based Deep-Blue Soluble Emitters for High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with CIEy

In this study, we demonstrated two deep-blue TADF emitters, BO-tCzPhICz and BO-tCzDICz, for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). They were synthesized by employing an organoboron acceptor and 9-(3,6-di-tert-butyl-9H-carbazol-9-yl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (tCzPhICz) and 12-(3,6-di-tert-butyl-9H-carbazol-9-yl)-15H-diindolo[2,3-b:1',2',3'-lm]carbazole (tCzDICz) as bulky aryl-annulated [3,2-a] carbazole donors, respectively. Both emitters showed sufficient solubility in organic solvents, narrow deep-blue emission, and small energy difference (ΔEST) between singlet and triplet states, which can be applied to solution-processable deep-blue TADF-OLEDs. Solution-processed OLEDs exploiting these TADF emitters displayed deep-blue electroluminescence with CIEy <0.1, and high external quantum efficiencies of 17.8 and 14.8% were observed for BO-tCzPhICz and BO-tCzDICz, respectively. The emitter bearing bulky ICz-based donating units shows highly promising potential for high-efficiency solution-processable deep-blue TADF-OLEDs.

Highly efficient deep-blue OLEDs with CIE closely approaches Rec.2020 standard based on an acridine‒naphthalene hybrid fluorophore

9,9-Dimethyl-9,10-dihydroacridine (DMAC) and naphthalene are fused to produce a chromophore 12,12-Dimethyl-7,12-dihydrobenzo[a] acridine (DMBA). Based on DMBA, two emitters, 12,12-dimethyl-7-phenyl-7,12-dihydrobenzo[a]acridine (BACH) and 3-fluoro-12,12-dimethyl-7-phenyl-7,12-dihydrobenzo[a]acridine (BACF), were designed and synthesized. Thanks to the planar structure and weak electron donating ability of DMBA, these two emitters exhibit deep-blue emissions with photoluminescence (PL) spectra peaks (λPLs) of 402–422 nm in solutions and doped films. The non-doped devices based on BACH and BACF present excellent deep-blue electroluminescence (EL) emission with spectra peaks (λELs) of 428 and 427 nm, maximum external quantum efficiencies (EQEs) of 6.09% and 5.99%, respectively. Notably, the CIE coordinates of these non-doped devices closely approach the Rec.2020 standard (0.131, 0.046).

Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence

We report an efficient deep-blue organic light-emitting diode (OLED) based on a heptazine-based thermally activated delayed fluorescent (TADF) emitter, 2,5,8-tris(diphenylamine)-tri-s-triazine (HAP-3DPA). The deep-blue-emitting compound, HAP-3DPA, was designed and synthesized by combining the relatively rigid electron-accepting heptazine core with three electron-donating diphenylamine units. Due to the rigid molecular structure and intramolecular charge transfer characteristics, HAP-3DPA in solid state presented a high photoluminescence quantum yield of 67.0% and obvious TADF nature with a short delayed fluorescent lifetime of 1.1 μs. Most importantly, an OLED incorporating HAP-3DPA exhibited deep-blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.13), a peak luminance of 10,523 cd/m−2, and a rather high external quantum efficiency of 12.5% without any light out-coupling enhancement. This finding not only reports an efficient deep-blue TADF molecule, but also presents a feasible pathway to construct high-performance deep-blue emitters and devices based on the heptazine skeleton.

Anthracene-based emitters for highly efficient deep blue organic light-emitting diodes with narrow emission spectrum

Organic light-emitting diodes (OLEDs) are expected to support the future applications in flat-panel displays and solid-state lightings owing to their advantages of flexibility, light weight, high resolution, energy conservation, etc. Achieving efficient deep blue electroluminescence with excellent color purity (narrow emission) remains a big challenge for OLEDs. Herein, two anthracene derivatives, PIAnTPh and PyIAnTPh, with a hybridized local and charge transfer (HLCT) excited state are obtained by attaching the large periphery groups of phenanthroimidazole/pyrenoimidazole and terphenyl unit to the 9,10-positions of anthracene. The sterically asymmetric strategy can efficiently suppress the unfavorable intermolecular interactions and realize deep blue emission. The introduction of neutral terphenyl unit avoids the potential reduction of fluorescence quantum yield from strong charge transfer (CT) characteristics. More importantly, the relatively weak CT state and the rigid molecular structure can significantly reduce the vibrational motion to obtain relatively narrow emission spectra. The doped devices based on PIAnTPh and PyIAnTPh show strong deep blue electroluminescence with CIE coordinates of (0.15, 0.06) and (0.15, 0.07), high color purity, and narrow full width at half maxima (FWHM) of merely 50 and 52 nm, respectively, meeting well with the EBU standard. The EQEs are as high as 8.09% and 8.44% for PIAnTPh and PyIAnTPh, which are among the state-of-the-art deep blue OLEDs.

New Quinoxaline‐Based Blue Emitters: Molecular Structures, Aggregation‐Induced Enhanced Emission Characteristics and OLED Application

Main observation and conclusionThree new deep blue light emitters, bearing quinoxaline with different substituents (biphenyl/pyrene) at the 1‐ or 2‐position, were synthesized by a Pd‐catalysed coupling reaction in high yields, and were fully characterized by 1H/13C NMR spectroscopy, single crystal X‐ray diffraction, and high‐resolution mass spectrometry (HRMS). The designed molecules exhibit good thermal stability (Tg &gt; 452 oC). Single crystal X‐ray diffraction indicated the compounds containing pyrene units exhibiting π‐π stacking, and thus this family of compounds exhibited intramolecular charge transfer (ICT) and non‐typical and aggregation‐induced enhanced emission (AIEE) characteristics, possessing blue emission both in solution and in the aggregation (solid) state. Furthermore, a selected deep blue emitter was utilized as the emitting layer for the fabrication of doped organic light‐emitting diode (OLED) devices, which afforded a deep blue electroluminescence (EL) peak at 428 nm with a narrow full width at half maximum (FWHM) of 57 nm, and the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of (0.15, 0.06).

TADF sensitization targets deep-blue

Two independent studies employing the same narrowband deep-blue emitter, ν-DABNA, but different energy transfer schemes, achieve efficient and stable deep-blue electroluminescence.

A sterically shielded design on anthracene-based emitters for efficient deep-blue organic light-emitting diodes

Two new anthracene derivatives are designed with a sterically shielded strategy for deep-blue electroluminescence applications. The naphthyl groups of the widely used blue emitter 9,10-di-(2-naphthyl)anthracene (ADN) were modified with four peripheral phenyl groups (PPGs) to enhance intermolecular spacing in solid-state thin film for suppressing electronic coupling. Influence of the PPGs decoration on photoluminescence and electroluminescence are investigated to reveal the structure-property relationship. A nondoped organic light-emitting diode with the new emitter 9-(naphthalen-2-yl)-10-(5,6,7,8-tetraphenylnaphthalen-2-yl)anthracene (M1) exhibits excellent performance with a maximum external quantum efficiency of 5.05% and deep-blue color index of (0.15, 0.09). Although the use of bulky peripheral substituents does improve device performance, it should be paid attention to the non-conductive effect induced by the larger molecular spacing in the design of deep-blue electroluminescence materials. This work provides a promising molecular design for deep-blue electroluminescence materials.

Homoleptic cyclometalated dibenzothiophene–NHC–iridium(iii) complexes for efficient blue phosphorescent organic light-emitting diodes

Dibenzothiophene (DBT) and dibenzofuran (DBF) based NHC–Ir complexes lead to high-end deep-blue electroluminescence with unprecedented CIE coordinates of [0.14, 0.19] for m-IrSMe (EQEmax; 17.1%) and [0.14, 0.14] for m-IrOMe (EQEmax; 18.2%).

Phenanthroimidazole-based bipolar carbazoles featuring cyano substituents to realize efficient deep-blue electroluminescence with an external quantum efficiency of nearly 6%

Organic emitters comprising carbazole and phenanthroimidazole with tunable optical and electronic properties due to the position and number of cyano substituents on the carbazole core are described.

Deep-blue organic light-emitting diodes based on push-pull π-extended imidazole-fluorene hybrids

Four donor-π-acceptor (D-π-A) luminophores based on π-extended imidazole-fluorene hybrid, namely PhIF-1, PyIF-1, PhIF-2 and PyIF-2, are developed. Two imidazole derivatives, phenanthreneimidazole (PhI) and pyreneimidazole (PyI), are investigated to highlight their potential applications in deep-blue electroluminescence. A mild electron pull-push system between PhI/PyI and fluorene is achieved, enabling the mixture of locally excited (LE) and charge transfer (CT) excited state character. Systematic study is also carried out to reveal the influence of n-hexyl chain at the 9-position of fluorene on the photophysical properties of these new emitters. We find that the introduction of alkyl chain can tune photophysical properties, leading to better solid-state emission in PhIF-2 and PyIF-2. All the new compounds show intense high-energy fluorescence with excellent thermal properties suitable for organic light-emitting diode (OLED) application. Non-doped OLEDs exhibiting maximum external quantum efficiency (EQE) of 3.65% and 3.75% are realized using PhIF-1 and PyIF-1, respectively. Compared with PhIF-1 and PyIF-1, alkyl chain substituted PhIF-2 and PyIF-2 exhibit better blue color purities of (0.15, 0.12) and (0.15, 0.13), respectively. In addition, better color purity and higher luminescence efficiency are acquired simultaneously in doped devices. An OLED employing 20 wt% PyIF-1 as emitting core displays a high EQE of 4.29% at 1000 cd m−2 (EQEmax = 4.60%). This work may provide a rational design for π-extended imidazole D-π-A luminophores for deep-blue electroluminescence applications.

Asymmetrically 2,7-difunctionalized carbazole-based donor-acceptor hybrids for deep blue electroluminescence applications

Asymmetrically 2,7-difunctionalized carbazoles decorated with cyano group and different conjugated chromophores have been synthesized by following Suzuki coupling reactions. Direct C–C coupling between cyanocarbazole and peripheral chromophores lead to twist structure of the molecules. As a result, it restricts the electronic communication between the chromophores and thereby confines absorption and emission maxima in shorter wavelength region. The positive solvatochromic emission spectrum of carbazole and triphenylamine substituted compounds is ascribed to synergistic effect of large structural reorganization and intramolecular charge transfer at excited state. Density functional theory (DFT) studies reveal the involvement of ICT from donor carbazole/triphenylamine to cyanocarbazole acceptor. OLED device containing a pyrene substituted dye (5 wt% in CBP host) displayed deep blue electroluminescence with the EQE of 3.0% and CIE of (0.16, 0.06). The superior device performance can be attributed to favorable energy level alignment of this dye with CBP host and complete energy transfer from the host to guest.