Sky-blue Organic Light-emitting Diodes Research Articles

Simple-structured nicotinonitrile based material for sky-blue organic light-emitting diodes

Organic light-emitting diodes (OLEDs) perform a vital function in flat-panel displays and solid-state lighting. Herein, a simple structured donor-acceptor based organic light-emitting material (OLEM), 6-(4-(di ([1,1′-biphenyl]-4-yl)amino)phenyl)nicotinonitrile (PTPA-NN), has been prepared and applied as emissive layer in OLEDs. This OLEM has a slightly twisted structure, which enhances its thermal stability. Interestingly, a device consisting of PTPA-NN OLEM demonstrated a promising performance showing a turn-on voltage of 3.8 V with a current efficiency of 9.7 cd/A, external-quantum-efficiency of 4.17% and power efficiency of 8.0 lm/W. Small molecular simple-structured nicotinonitrile based material provide an effective strategy to discover suitable OLEMs for sky-blue OLEDs.

Derivatives of 2-Pyridone Exhibiting Hot-Exciton TADF for Sky-Blue and White OLEDs.

Development of emissive materials for utilization in organic light-emitting diodes (OLEDs) remains a highly relevant research field. One of the most important aspects in the development of efficient emitters for OLEDs is the efficiency of triplet-to-singlet exciton conversion. There are many concepts proposed for the transformation of triplet excitons to singlet excitons, among which thermally activated delayed fluorescence (TADF) is the most efficient and widespread. One of the variations of the TADF concept is the hot exciton approach according to which the process of exciton relaxation into the lowest energy electronic state (internal conversion as usual) is slower than intersystem crossing between high-lying singlets and triplets. In this paper, we present the donor-acceptor materials based on 2-pyridone acceptor coupled to the different donor moieties through the phenyl linker demonstrating good performance as components of sky-blue, green-yellow, and white OLEDs. Despite relatively low photoluminescence quantum yields, the compound containing 9,9-dimethyl-9,10-dihydroacridine donor demonstrated very good efficiency in sky-blue OLED with the single emissive layer, which showed an external quantum efficiency (EQE) of 3.7%. It also forms a green-yellow-emitting exciplex with 4,4',4″-tris[phenyl(m-tolyl)amino]triphenylamine. The corresponding OLED showed an EQE of 6.9%. The white OLED combining both exciplex and single emitter layers demonstrated an EQE of 9.8% together with excellent current and power efficiencies of 16.1 cd A-1 and 6.9 lm W-1, respectively. Quantum-chemical calculations together with the analysis of photoluminescence decay curves confirm the ability of all of the studied compounds to exhibit TADF through the hot exciton pathway, but the limiting factor reducing the efficiency of OLEDs is the low photoluminescence quantum yields caused mainly by nonradiative intersystem crossing dominating over the radiative fluorescence pathway.

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Ornamenting of Blue Thermally Activated Delayed Fluorescence Emitters by Anchor Groups for the Minimization of Solid-State Solvation and Conformation Disorder Corollaries in Non-Doped and Doped Organic Light-Emitting Diodes.

Motivated to minimize the effects of solid-state solvation and conformation disorder on emission properties of donor–acceptor-type emitters, we developed five new asymmetric multiple donor–acceptor type derivatives of tert-butyl carbazole and trifluoromethyl benzene exploiting different electron-accepting anchoring groups. Using this design strategy, for a compound containing four di-tert-butyl carbazole units as donors as well as 5-methyl pyrimidine and trifluoromethyl acceptor moieties, small singlet-triplet splitting of ca. 0.03 eV, reverse intersystem crossing rate of 1 × 106 s–1, and high photoluminescence quantum yield of neat film of ca. 75% were achieved. This compound was also characterized by the high value of hole and electron mobilities of 8.9 × 10–4 and 5.8 × 10–4 cm2 V–1 s–1 at an electric field of 4.7 × 105 V/cm, showing relatively good hole/electron balance, respectively. Due to the lowest conformational disorder and solid-state solvation effects, this compound demonstrated very similar emission properties (emission colors) in non-doped and differently doped organic light-emitting diodes (OLEDs). The lowest conformational disorder was observed for the compound with the additional accepting moiety inducing steric hindrance, limiting donor–acceptor dihedral rotational freedom. It can be exploited in the multi-donor–acceptor approach, increasing the efficiency. Using an emitter exhibiting the minimized solid-state solvation and conformation disorder effects, the sky blue OLED with the emitting layer of this compound dispersed in host 1,3-bis(N-carbazolyl)benzene displayed an emission peak at 477 nm, high brightness over 39 000 cd/m2, and external quantum efficiency up to 15.9% along with a maximum current efficiency of 42.6 cd/A and a maximum power efficiency of 24.1 lm/W.

Small dose of phosphorescent dopant enabling high efficiency and bright solution-processed sky-blue organic light-emitting diodes

In this work, we demonstrate the simultaneous use of thermally activated delayed fluorescence (TADF) material and a small dose of phosphorescent material to construct TADF-phosphorescent mixed emitting system, which enables the OLED exhibiting high device performance. To achieve this, a blue TADF material, 10,10'- (4,4′-Sulfonylbis (4,1-phenylene)) bis (9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS), and a conventional blue phosphor with similar emission color to DMAC-DPS, Bis [2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III) (FIrpic), are incorporated together into 1,3-Bis(carbazol-9-yl) benzene (mCP) host to constructed the mixed emitting system. The device has achieved the maximum current efficiency of 21.1 cd/A with FIrpic concentration of 1%; meanwhile, the luminance of the device has also been improved. There merits are superior to the optimized TADF OLED and the optimized phosphorescent OLED based on 16% FIrpic in device efficiency. In view of the expensive price of phosphorescent materials, the strategy presented here will greatly reduce the material cost without the cost of device performance. Through investigations on the energy transfer process and the film morphology in the mixed emitting system, the main cause of the improvement has been unveiled. The study may provide a simple but effective strategy to achieve low-cost, high-performance solution-processed OLEDs.

Benzofurodibenzofuran as a universal chemical platform of highly efficient sky-blue thermally activated delayed fluorescence emitters and hosts

A benzofuran based chemical platform, benzofurodibenzofuran, was designed and synthesized as a core structure to develop hosts for phosphorescent organic light-emitting diodes and thermally activated delayed fluorescence (TADF) emitters. The high triplet energy and rigid molecular structure of the benzofurodibenzofuran unit enabled the universal application of it as the building unit of the sky-blue hosts and emitters. The sky-blue TADF emitters built on the benzofurodibenzofuran platform showed high external quantum efficiency, narrow emission spectrum and blue-shifted emission color compared with the conventional phenyl based emitters. Moreover, the high triplet energy hosts obtained from benzofurodibenzofuran efficiently harvested the triplet excitons of the sky-blue phosphors and TADF emitters. Therefore, the benzofurodibenzofuran can work as the basic platform of designing host and emitters for high efficiency sky-blue organic light-emitting diodes.

Molecular designs offer fast exciton conversion

Two independent studies report new organic compounds that offer record rates of reverse intersystem crossing between triplet and singlet excited states. The result is sky-blue organic light-emitting diodes with improved efficiency, stability and reduced efficiency roll-off.

Efficient and Stable Deep-Blue Fluorescent Organic Light-Emitting Diodes Employing a Sensitizer with Fast Triplet Upconversion.

Multiple donor-acceptor-type carbazole-benzonitrile derivatives that exhibit thermally activated delayed fluorescence (TADF) are the state of the art in efficiency and stability in sky-blue organic light-emitting diodes. However, such a motif still suffers from low reverse intersystem crossing rates (kRISC ) with emission peaks <470 nm. Here, a weak acceptor of cyanophenyl is adopted to replace the stronger cyano one to construct blue emitters with multiple donors and acceptors. Both linear donor-π-donor and acceptor-π-acceptor structures are observed to facilitate delocalized excited states for enhanced mixing between charge-transfer and locally excited states. Consequently, a high kRISC of 2.36 × 106 s-1 with an emission peak of 456 nm and a maximum external quantum efficiency of 22.8% is achieved. When utilizing this material to sensitize a blue multiple-resonance TADF emitter, the corresponding device simultaneously realizes a maximum external quantum efficiency of 32.5%, CIEy ≈ 0.12, a full width at half maximum of 29 nm, and a T80 (time to 80% of the initial luminance) of > 60 h at an initial luminance of 1000 cd m-2 .

Synergetic interface and morphology modification to achieve highly efficient solution-processed sky-blue organic light-emitting diodes

Abstract Solution-processed organic light-emitting diodes (OLEDs), especially blue OLEDs, generally suffer from the low efficiency. Herein, we report an efficient approach to achieve high efficiency by synergetic interface and morphology modification with a polymer, Poly(9-vinylcarbazole) (PVK). Sky-blue thermally activated delayed fluorescent (TADF) material, 10-(4-(4,6-diphenyl-1,3,5- triazin-2-yl)phenyl)-10H- spiro[acridine-9,9′-fluorene] (SpiroAC-TRZ), is used as the emitter. The incorporation of PVK thin layer at the hole injection layer/emission layer (EML) interface and a small amount of PVK inner the EML modify the carrier behaviors at the interface and improve the EML morphology. As a result, balanced carrier distribution and reduced carrier recombination are realized at the interface and inner the EML. Through these strategies, the maximum external quantum efficiency and current efficiency of the optimal OLED achieve 25.1% and 53.5 cd/A. To the best of our knowledge, the efficiencies are the highest values ever achieved by the solution-processed sky-blue TADF OLEDs.

Dual role of a pyrene derivative as a hole transport material and an emitter in blue fluorescent organic light-emitting diodes

A pyrene derivative, N1,N6-di([1,1′-biphenyl]-4-yl)-N1,N6-bis(9,9-dimethyl-9H-fluoren-2-yl)pyrene-1,6-diamine(PyFB), was investigated as a dual function organic material working as a hole transport material and a sky blue emitter at the same time. A pyrene core was modified with an aromatic amine moiety substituted with a fluorene and a biphenyl unit. The pyrene derived PyFB material showed the highest occupied molecular orbital of −5.5 eV and better hole transport properties than a common hole transport layer. The PyFB also worked as the sky blue emitter and a sky blue organic light-emitting diode developed using the PyFB emitter demonstrated a high external quantum efficiency of 8.5% and a very long lifetime of 56 h up to 95% of initial luminance at 500 cd/m2.

Aggregation-induced emission tetraphenylethene type derivatives for blue tandem organic light-emitting diodes

Tetraphenylethene (TPE) has been demonstrated to be an effective core molecule for generating excellent aggregation-induced emissions (AIE). Herein, we propose TPE moieties decorated with different biphenyl or anthracenyl fragments to avoid the crystallization of neat film and simultaneously harvest blue AIE. The two synthesized compounds, 2-(4-biphenyl)-1,1,2-triphenylethylene (2) and 2-(9-antracenyl)-1,1,2-triphenylethylene (3), possess bipolar transport capabilities, high thermal stability, and strong sky-blue AIE. Both compounds 2 and 3 were selected as emitters to investigate their electroluminescence applications. The optimized non-doped sky-blue organic light-emitting diodes (OLEDs) with 2 and 3 exhibited respective maximum efficiencies of 3.8% (8.8 cd/A) and 2.2% (4.0 cd/A). These results illustrate the effective molecular design concepts of the TPE derivatives containing biphenyl or anthracenyl fragments. Furthermore, a tandem device architecture was used to improve device efficiency. Through adopting the specific designs of the intermediate charge generation layer structure and the corresponding device architecture, respective peak efficiencies of 8.2% (19.4 cd/A) and 4.6% (9.5 cd/A) were obtained in tandem devices with 2 and 3 emitters together with slight efficiency roll-off. The superior EL performance demonstrated that the tandem structure possesses high potential for boosting the efficiency of the AIE-based OLEDs.

Secondary Acceptor Optimization for Full‐Exciton Radiation: Toward Sky‐Blue Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency of ≈30%

Efficient blue emitters are indispensable for organic light-emitting diodes (OLEDs) with respect to display and lighting applications. Because of their high-energy excited states, both radiation enhancement and non-radiation suppression should be simultaneously optimized to realize 100% exciton utilization. Here, it is shown that the excited-state characteristics of blue thermally activated delayed fluorescence emitters can be precisely controlled by a secondary acceptor having moderate electronic effects on increasing the singlet charge-transfer component and preserving the triplet locally excited-state component. In addition of planar configuration between the donor and the primary acceptor, the radiative transition improvement and non-radiative transition suppression can be simultaneously achieved for "full-exciton radiation". A molecule using diphenylphosphine oxide as the secondary acceptor exhibits ≈100% photoluminescence quantum yield on the basis of its tenfold increased singlet radiative rate constant, fivefold decreased singlet and triplet non-radiative rate constants, and ≈100% reverse intersystem crossing efficiency, which further endows ≈100% exciton utilization efficiency to its sky-blue OLEDs.

New multifunctional aggregation-induced emission fluorophores for reversible piezofluorochromic and nondoped sky-blue organic light-emitting diodes

In this study, 4,6-diphenyl-1,3,5-triazine as the rotor, was firstly introduced to the construction of AIEgens. By conjugating phenyl-9H-carbazole and 4,6-diphenyl-1,3,5-triazine to the 9 or 10 position of anthracene, three new multifunctional aggregation-induced emission (AIE) fluorophores, 3-(10-(4,6-diphenyl-1,3,5-triazin-2-yl)anthracen-9-yl)-9-phenyl-9H-carbazole (3CzAnTrz) 9-(4-(10-(4,6-diphenyl-1,3,5-triazin-2-yl)anthracen-9-yl)phenyl)-9H-carbazole (pCzAnTrz) and 9-(3-(10-(4,6-diphenyl-1,3,5-triazin-2-yl)anthracen-9-yl)phenyl)-9H-carbazole (mCzAnTrz) were designed and synthesized. The photophysical properties indicated that all three compounds not only exhibited an AIE effect but also showed a reversible piezofluorochromic (PFC) behaviour. The experimental results revealed that the spectral shifts ΔλPFC (ΔλPFC = λrepressed – λannealed) were obviously influenced by the linking position of 9-phenyl-9H-carbazole, and 3CzAnTrz showed largest spectral shifts (ΔλPFC = 66 nm), because of the relatively loose molecular packing, weak intermolecular CH-π interactions and more twisted conformation. When three fluorophores were used as emitter in the nondoped sky-blue devices, these devices showed excellent electroluminescence (EL) performances, realizing the maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) values of 13.34 cd A−1, 11.59 lm W−1, 5.44%; 11.19 cd A−1, 9.25 lm W−1, 5.54% and 8.74 cd A−1, 6.44 lm W−1, 4.71% for 3CzAnTrz, pCzAnTrz, and mCzAnTrz, respectively. And these devices exhibit negligible efficiency roll-off even at 10000 cd m−2. These results manifest that the potential advantage of multifunctional AIEgens applicable for piezofluorochromism and nondoped high-performance EL devices.

Sky Blue-Emitting Iridium(III) Complexes Bearing Nonplanar Tetradentate Chromophore and Bidentate Ancillary.

Tetradentate chelates bearing tripodal arranged terpyridine and a functional pyrazole unit (i.e., L1-H and L2-H) were employed in preparation of Ir(III) complexes [Ir(L1)Cl2] (1) and [Ir(L2)Cl2] (2); subsequent chloride-to-bipyrazolate substitution gave [Ir(L1)(bipz)] (3) and [Ir(L2)(bipz)] (4). Single-crystal X-ray structural studies on 1 and 3 showed the possession of a tetradentate chelate, whereas the remaining cis-sites are occupied by either dual chlorides or the bipz chelate, respectively. Sky blue organic light-emitting diode with peak efficiencies (10.1%, 19.8 cd·A-1, and 20.4 lm·W-1) was successfully fabricated using 3 (or 4) as dopant emitter, highlighting the potential application of this class of Ir(III) phosphor.

Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices.

Efficient sky-blue organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) display a three orders of magnitude increase in lifetime, which is superior to those of controlled phosphorescent OLEDs used in this study. The combination of electro-oxidation and photo-oxidation of the TADF emitters in their triplet excited-states is suppressed through molecule design and device engineering.

Highly Luminous Sky-Blue Organic Light-Emitting Diodes Based on the Bis[(1,2)(5,6)]indoloanthracene Emissive Layer

An in-depth analysis of the electro-optical and light-emissive properties was carried out with a new fused heteroacene, the bis[(1,2)(5,6)]indoloanthracene. The compound was synthesized in a straightforward bidirectional manner from 1,4-dibromo-2,5-diodobenzene utilizing a reaction cascade of isomerization and cyclization sequences. Bis[(1,2)(5,6)]indoloanthracene exhibits bright luminescence with high quantum yield in solution as well as in the solid state. By using this novel semiconductor as active layer, an efficient organic light-emitting diode (OLED) has been fabricated for which its emission pattern resembles at least two emitters in a single emissive region originated from the same molecule. One has its origin from the single molecule and is similar to the luminescence spectra in solution, whereas the other functions through the formation of aggregates being similar to the emission pattern found with crystals of bis[(1,2)(5,6)]indoloanthracene. As a result, bright sky-blue OLEDs with very high lum...

Highly efficient greenish-blue platinum-based phosphorescent organic light-emitting diodes on a high triplet energy platform

We have demonstrated high-efficiency greenish-blue phosphorescent organic light-emitting diodes (PHOLEDs) based on a dimesitylboryl-functionalized C^N chelate Pt(II) phosphor, Pt(m-Bptrz)(t-Bu-pytrz-Me). Using a high triplet energy platform and optimized double emissive zone device architecture results in greenish-blue PHOLEDs that exhibit an external quantum efficiency of 24.0% and a power efficiency of 55.8 lm/W. This record high performance is comparable with that of the state-of-the-art Ir-based sky-blue organic light-emitting diodes.

Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (ηEQE) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet–triplet annihilation and triplet–triplet annihilation dominate ηEQE roll-off because of the relatively long 2CzPN triplet lifetime of 273μs. This work yields a method to relax the exciton quenching process in TADF based OLEDs.

High-efficiency organic light-emitting diodes utilizing thermally activated delayed fluorescence from triazine-based donor–acceptor hybrid molecules

We have designed and synthesized a high-efficiency purely organic luminescent material, 2,4-bis{3-(9 H-carbazol-9-yl)-9 H-carbazol-9-yl}-6-phenyl-1,3,5-triazine (CC2TA) comprising the bicarbazole donor and phenyltriazine acceptor units, which is capable of emitting thermally activated delayed fluorescence. The molecular design of CC2TA allows spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, leading to an exceptionally small singlet–triplet exchange energy (ΔEST = 0.06 eV) together with a high triplet energy. Furthermore, a high external electroluminescence quantum efficiency as high as 11% ± 1% has been achieved in the sky-blue organic light-emitting diodes employing CC2TA as an emitter.

Pyridine-Incorporated Dihexylquaterthiophene: A Novel Blue Emitter for Organic Light Emitting Diodes (OLEDs)

The synthesis and characterisation of 2,5-bis(5′-hexyl-[2,2′-bithiophen]-5-yl)pyridine (Th4PY) and its use as a blue emitter in organic light emitting diodes (OLEDs) is reported. Th4PY was synthesised in high yield using a straightforward Suzuki coupling route with commercially available starting materials. As Th4PY is both soluble and has low molecular weight, blue OLEDs were fabricated using both spin-coating and vacuum deposition thin film processing techniques to study the effect of processing on device performance. OLED devices using a spin-coated layer consisting of 4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as a host matrix together with Th4PY as emitter exhibited highly efficient sky-blue emission with a low turn-on voltage of 3 V, a maximum brightness close to 15000 cd m–2 at 8 V, and a maximum luminous efficiency of 7.4 cd A–1 (6.3 lm W–1) with CIE coordinates of x = 0.212, y = 0.320. The device performance characteristics are compared using various matrices and processing techniques. The promising sky-blue OLED performance, solution processability, and ambient stability make Th4PY a promising blue emitter for application in OLEDs.