Blue-emitting Layer Research Articles

Two tridentate pyridinyl-hydrazone zinc(II) complexes as fluorophores for blue emitting layers

Two new complexes were obtained by reaction of zinc (II) acetate with 4-fluoro-N'-(pyridin-2-ylmethylene) benzohydrazide or with 4-(hexyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide ligands in pyridine. Both ligands have a pyridinyl-hydrazone moiety able to act as a mono-negative tridentate ligand toward zinc ion in 2:1 stoichiometric ratio, producing an octahedral environment. The derived complexes are poor emitters in diluted solution. Instead, they exhibit intense blue fluorescence in the solid state due to AIE (aggregation induced emission) effect. The crystalline complexes are bright yellow in natural light and blue under UV–visible light with remarkable Stokes Shifts. Their photoluminescence quantum yields are ranging 20–30%, making them very promising as fluorophore dopants for blue emissive layers. One ligand and its complex were characterized by single crystal X-ray diffraction analysis that revealed an almost planar conformation of two ligands coordinated to the metal and the presence of significant π-π stacking of molecules at about 3.4 Å.

Creation of Efficient Blue Aggregation-Induced Emission Luminogens for High-Performance Nondoped Blue OLEDs and Hybrid White OLEDs.

Organic blue luminescent materials are essential for organic light-emitting diodes (OLEDs). However, high-quality blue materials that can fulfill the requirements of OLED commercialization are much rare. Herein, two novel blue luminogens, 9-(4-(2,6-di- tert-butyl-10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)phenyl)-9 H-carbazole and 9-(4-(2,6-di- tert-butyl-10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)1,3-di(9 H-carbazol-9-yl)benzene (TPE-TADC), consisting of anthracene, tetraphenylethene, and carbazole groups are successfully prepared, and their thermal, optical, electronic, and electrochemical properties are fully investigated. They exhibit prominent aggregation-induced emission property and strong blue fluorescence at ∼455 nm in neat films. Efficient nondoped OLEDs are fabricated with these blue luminogens, providing blue electroluminescence (EL) at 451 nm (CIE x, y = 0.165, 0.141) and high EL efficiencies of 6.81 cd A-1, 6.57 lm W-1, and 5.71%. By utilizing TPE-TADC as a blue emissive layer, high-performance two-color hybrid white OLEDs are achieved, furnishing modulatable light color from pure white (CIE x, y = 0.33, 0.33) to warm white (CIE x, y = 0.44, 0.46) and excellent EL efficiencies of 56.7 cd A-1, 55.2 lm W-1, and 19.2%. More importantly, these blue and white OLEDs all display ultrahigh color and efficiency stabilities at high luminance, indicating the great potential of these blue luminogens for the application in OLED displays and white illumination.

High CRI and stable spectra white organic light-emitting diodes with double doped blue emission layers and multiple ultrathin phosphorescent emission layers by adjusting the thickness of spacer layer

The color rendering index (CRI) and Commission Internationale de L'Eclairage coordinate (CIE x,y ) are significant parameters for the white organic light-emitting diodes (WOLEDs). We fabricate a series of the phosphorescent WOLED devices, in which there are two doped phosphorescent layers (for blue emission) on both sides of the emitting layer (EML) and three ultrathin phosphorescent layers (for red, orange and green emission) separated by the spacer layers (mCP) in EML. One doped blue emission layer near the hole transport layer contributes to CRI enhancement by increasing the doping concentration (x %) of the blue phosphorescent dye because Dexter energy transfer increases from the blue emission to the red emission. When x is 20, CRI reaches to 90 at 5 V in device A3. Based on device A3, CRI can be further improved to 94 in device B1 with a thinner thickness of the spacer layer (y = 1 nm) between the green emission layer and orange emission layer in EML because Dexter energy transfer from the green emission layer to the orange and red emission layer becomes easier. Although CRI declines, the spectra become more stable by increasing the thickness of the spacer layer (y = 3 nm) in device B2 or decreasing the thickness of the green emission layer (z = 0.05 nm) in device C, in which the CIE x,y coordinate shift is only (0.0214, 0.0102) from 5 V to 8 V. • The double doped blue phosphorescent layers (EML1 and EML5) on both sides of EML can adjust CRI and spectra stability. • The EML1 near HTL is beneficial for CRI and the red emission by increasing the doping concentration of FIrpic. • The EML5 near ETL can improve spectra stability by increasing the thickness of the spacer layer (Spacer2). • The EML5 near ETL can improve CRI by decreasing the thickness of green ultrathin phosphorescence layer (EML4).

Hybrid White Organic Light Emitting Diodes Using Dual Core Blue Fluorescent Emitter.

White OLED (WOLED) devices were fabricated using blue emitting materials, TP-AA-TP, TP-AA-TPB, and TPB-AA-TPB, and yellow emitting material polyphenylenevinylene derivative (PDY 132). Three devices were fabricated with the following structure: ITO/PEDOT:PSS (50 nm)/PDY-132 (40 nm)/NPB (10 nm)/Blue EML (30 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al (200 nm). PDY-132 was used by spin coating, and then three blue fluorescent materials were used by vapor deposition to form a white light emitting device. The color coordinates of WOLED using TP-AA-TP, TP-AA-TPB and TPB-AA-TPB were (0.291, 0.317), (0.317, 0.371) and (0.345, 0.457), respectively. Device using TP-AA-TP as blue emitting material showed the highest efficiencies at 10 mA/cm², L.E of 5.58 cd/A, P.E of 2.16 lm/W and E.Q.E of 2.96%.

Naphthalimide end-capped diphenylacetylene: a versatile organic semiconductor for blue light emitting diodes and a donor or an acceptor for solar cells

A novel compound NAI-PVP-NAI was utilized as a blue emissive layer for organic light emitting diodes and as an acceptor and a donor for organic solar cells.

Semi-fluorinated p-terphenyl liquid crystals

p-Terphenyls carrying two or four fluorine atoms in the central ring were synthesized via Suzuki-Miyaura cross-coupling reaction using phenylboronic acids and fluorinated dibromobenzenes. By calorimetric and X-ray diffraction analyses it was found that the p-terphenyls carrying two fluorine atoms develop nematic and/or smectic (A and C) phases. On the contrary, the p-terphenyls substituted with four fluorine atoms are not liquid crystals. Differences found in the thermal behavior are explained in terms π – πF interactions arising from the electron withdrawing ability of the fluorine atom. Such π – πF interactions let the p-terphenyls stack in a staggered configuration that explains the stability of the smectic C phase. On the other hand, the semi-fluorinated p-terphenyls absorb and emit in the blue region of the electromagnetic spectrum; a measured large Stokes shift indicates that these compounds are nearly transparent to their own emitted light. This optical characteristic, combined with liquid crystals properties of low order, makes these new p-terphenyls potential candidates for linearly polarized blue-emitting layers in electroluminescent devices.

Highly efficient single-stack hybrid cool white OLED utilizing blue thermally activated delayed fluorescent and yellow phosphorescent emitters

Highly efficient single-stack hybrid cool white organic light-emitting diodes (OLEDs) having blue-yellow-blue multiple emitting layers (EMLs) are designed and constructed by utilizing blue thermally activated delayed fluorescent (TADF) and yellow phosphorescent emitters. The out-coupling efficiencies of yellow and blue emissions are maximized by tuning the ITO and total device thickness that satisfies both of antinode positions for yellow and blue emissions in a limited multiple EML thickness. To obtain a cool white emission, the exciton formation ratio in the blue-yellow-blue multiple EML system is controlled by manipulating the recombination zone through charge conductivity variation of host medium in the blue TADF EML. The resulting device exhibits cool white emission with very high maximum external quantum efficiency of 23.1% and CIE color coordinates of (0.324, 0.337). We anticipate that the studied approach will raise the viability of single-stack hybrid cool white OLEDs for high performance display applications.

Optimization for white organic light-emitting diode based on DCJTB as color conversion layer

In this paper, combining phosphorescence and fluorescence to form white light was realized based on DCJTB:PMMA/ITO/NPB/TCTA/FIrpic:TCTA/TPBi/Ir(ppy)3:TPBi/TPBi/Cs2CO3/Al. The effects of red fluorescence on this white light device was studied by adjusting the concentration of DCJTB. The study shows that the device with a DCJTB concentration of 0.7% in the color conversion layer (CCL) generates a peak current efficiency and power efficiency of 23.4 cd ⋅ A[Formula: see text] and 7.5 lm ⋅ W[Formula: see text], respectively. And it is closest to the equal-energy white point of (0.33, 0.33) which shows a CIE (Commission Internationale de L’Eclairage) coordinate of (0.35, 0.43) and a color rendering index (CRI) of 70 at current density of 10 mA ⋅ cm[Formula: see text]. In order to improve the efficiency, we design and fabricate both high efficient and pure white organic light-emitting diode (WOLED) by replacing the single blue emission layer (EML) with double EMLs of FIrpic:TCTA and FIrpic:TPBi. The further study shows that, when the layers of EML is three and the concentration of DCJTB at 0.7%, the device exhibits good performance specifically, at current density of 10 mA ⋅ cm[Formula: see text], the current efficiency of 28.2 cd ⋅ A[Formula: see text] (power efficiency of 10.3 lm ⋅ W[Formula: see text]), and the CIE coordinate of (0.33, 0.31) (CRI of 80.38).

High-Performance White Organic Light-Emitting Diodes with Simplified Structure Incorporating Novel Exciplex-Forming Host.

It is a challenge to engineer white organic light-emitting diodes (WOLEDs) with high efficiency, low operating voltage, good color quality, and low efficiency roll-off, simultaneously. Herein, we employ a novel exciplex to solve this problem, which mixes a bipolar host material 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) with a common electron-transporting material 4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PyMPM) to form the host for a blue emitter iridium(III)bis(4,6-(difluorophenyl)-pyridinato- N,C2') picolinate (FIrpic). The blue OLED with maximum power efficiency (PE) over 48 lm W-1 and Commission International de I'Eclairage chromaticity diagram (0.17, 0.36) was achieved. To obtain white light emission, a complementary orange emission layer is used, which consists of the bis(4-phenylth-ieno[3,2- c]pyridine)(acetylacetonate)iridium(III) (PO-01) doped into the single host of 26DCzPPy adjacent to the blue emission layer. Benefiting from the exciplex and effective utilization of the excitons by using the optimized multifunctional device structure, the WOLEDs remarkably exhibit maximum external quantum efficiency, PE, and current efficiency of 28.5%, 95.5 lm W-1, and 82.0 cd A-1, respectively. At the luminance of 100 cd m-2, it maintains the values of 27.2%, 90.2 lm W-1, and 78.4 cd A-1, respectively. Furthermore, the WOLEDs have a low threshold voltage of about 2.6 V and remain around 4.0 V at 10 000 cd m-2. These results indicate that the exciplex-forming co-host 26DCzPPy:B4PyMPM can provide an effective strategy to fabricate high-efficiency WOLEDs for potential applications.

Color-Stable White Organic Light-Emitting Diodes Utilizing a Blue-Emitting Electron-Transport Layer.

We have fabricated efficient and color-stable white organic light-emitting diodes (WOLEDs) based on a simple double-layer device structure consisting of a blue-emitting oligoquinoline electron-transport layer and a poly(N-vinyl carbazole) layer doped with phosphorescent green and red iridium emitters. Pure white color with the CIE coordinates from (0.30, 0.31) to (0.34, 0.37) in the whole operating bias voltage range and luminous efficiency higher than 7 cd/A at a brightness up to 12 000 cd/m2 was achieved. These results demonstrate a new strategy to realizing easily processable, highly efficient, and color-stable WOLEDs with a simple structure.

Graphitic C3N4 quantum dots for next-generation QLED displays

Abstract Quantum dot light-emitting diode (QLED) displays are considered a next-generation technology, but previously reported quantum dots (QDs) consisting of heavy metals are toxic and harmful. This work examined earth-abundant, metal-free, graphitic C3N4 (g-C3N4) with exceptional optical and electronic properties, excellent chemical and thermal stability, an appropriate band gap, and non-toxicity for QLED applications. The dependence of the luminescence performance on the reaction atmosphere and temperature; the transformation of the crystal and electronic structures during the reaction, including crystal defects and surface functional groups; and the luminescence mechanisms of g-C3N4 were uncovered. The highest quantum yield of 49.8% was achieved by the sample possessing the highest graphitic-to-triazine carbon ratio synthesized at 500 °C under N2 atmosphere. The disappearance of the charge-transfer band, crystal defects (traps), and non-radiative transition (due to fast relaxation) from the absorption spectra demonstrates the enhanced quantum efficiency of the g-C3N4 QDs over that of the bulk powders. A QLED prototype device employing g-C3N4 QDs as the blue-emitting layer was demonstrated.

Electroluminescence performance of the blue, white and green-red organic light emitting diodes treated by in-situ heating

White light phosphorescent organic light emitting diodes (W-PHOLED) together with the blue light (B-PHOLED) and green-red light phosphorescent OLED (GR-PHOLED) equipped with the same blue and green-red emission layer were fabricated, and remarkable in-situ heating effects on their electroluminescence (EL) performance were observed. For the B-PHOLED, the in-situ heating increased the peak power efficiency (PE) by more than 30% with the increasing heating temperature from the room temperature to 100 °C accompanying with negligible effects on the blue light color stability. However, the optimal in-situ heating temperature of 100 °C for B-PHOLED was not beneficial to the PE of the W-PHOLED, but greatly facilitated the white light color stability. Such opposite impact on the W-PHOLEDs PE and color stability might be ascribed to the effects of the in-situ heating on the organic materials inter-diffusion and excitons decaying dynamic. The GR-PHOLED revealed the similar PE dependence on the in-situ heating as those of the W-PHOLED as well as the deteriorated color stability upon in-situ heating. The mechanism responsible for those characteristic temperature dependence of the EL performance was investigated focusing on the surface morphology related carrier transportation and carriers mobility related excitons decaying.

Efficient co-host exciplex emission for white organic light-emitting diodes

In this study, white organic light-emitting diodes (WOLEDs) based on exciplexes with a simple structure were investigated. Several combinations of hole-transport material and electron-transport material (ETM) were examined to verify the formation of the exciplexes. Various combinations of these exciplexes contributed to the blue and green emitters, as well as the host within the WOLEDs. Four types of WOLEDs were fabricated using various ETMs 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 4,7-diphenyl-1,10-phenanthroline (BPhen), bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum (BAlq), and 2,6-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (26DCzppy) within the emitting layers (EMLs). In this work, which comprised a blue EML with a 1:1 wt ratio of tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and TPBi, a green EML prepared by mixing 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) and ETMs at a weight ratio of 1:1, and a red EML. In addition, the exciplex formation was optimized according to the mixing concentration of the green EML when BPhen and TPBi were used as the ETM. As a result, a green EML was obtained at 50 wt% TPBi with improved performance, such as an increased power efficiency and a low efficiency roll-off in the range of 200–1000 cd/m2 in WOLEDs. It also showed white emission for Commission Internationale de l’Éclairage coordinates of (0.45, 0.45) at 1000 cd/m2.

Development of a Highly Efficient Hybrid White Organic-Light-Emitting Diode with a Single Emission Layer by Solution Processing.

We use a mixed host, 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine blended with 20 wt % tris(4-carbazoyl-9-ylphenyl)amine, to lower the hole-injection barrier, along with the bipolar and high-photoluminescence-quantum-yield (Φp= 84%), blue thermally activated delay fluorescence (TADF) material of 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) as a blue dopant to compose the emission layer for the fabrication of a TADF blue organic-light-emitting diode (BOLED). The device is highly efficient with the following performance parameters: maximum brightness (Bmax) = 57586 cd/m2, maximum current efficiency (CEmax) = 35.3 cd/A, maximum power efficiency (PEmax) = 21.4 lm/W, maximum external quantum efficiency (EQEmax) = 14.1%, and CIE coordinates (0.18, 0.42). This device has the best performance recorded among the reported solution-processed TADF BOLEDs and has a low efficiency roll-off: at brightness values of 1000 and 5000 cd/m2, its CEs are close, being 35.1 and 30.1 cd/A, respectively. Upon further doping of the red phosphor Ir(dpm)PQ2 (emission peak λmax = 595 nm) into the blue emission layer, we obtained a TADF-phosphor hybrid white organic-light-emitting diode (T-P hybrid WOLED) with high performance: Bmax = 43594 cd/m2, CEmax = 28.8 cd/A, PEmax = 18.1 lm/W, and CIE coordinates (0.38, 0.44). This Bmax = 43594 cd/m2 is better than that of the vacuum-deposited WOLED with a blue TADF emitter, 10000 cd/m2. This is also the first report on a T-P hybrid WOLED with a solution-processed emitting layer.

Influence of the Mixed-Host Heterojunction on the Spectrum Stability in White Phosphorescent Organic Light Emitting Diodes

In order to improve spectrum stability of white phosphorescent organic light emitting diodes (WOLEDs), a series of WOLED devices with mixed host (mCP:TPBi) blue emitting layer (EML) structure are fabricated. When TPBi concentration is 3/4, the maximum value of current density, current and power efficiency can reach to 245.63 mA/cm2, 50.57 cd/A and 52.96 lm/W, respectively. Then, another series of WOLED devices with heterojunction structure are fabricated by changing the proportion of the mixed host materials in two blue EMLs. When the TPBi concentration is 2/3 and 1 in EML2 and EML3, we obtain a spectrum stability WOLED with luminance of 12190 cd/m2, current efficiency of 47.77 cd/A and power efficiency of 45.83 lm/W, respectively.

High efficiency hybrid white organic light-emitting diodes based on a simple and efficient exciton regulation emissive layer structure.

It is well-known that hybrid white organic light-emitting diodes (WOLEDs) are constructed by blue fluorophors and red-green or orange phosphors, therefore, theoretically exhibiting the advantages of long lifetime and high efficiency. However, the efficiency is far from reaching the expected values. Here, we designed a simple and efficient exciton regulation emissive layer (EML) structure to fabricate high efficiency hybrid WOLEDs. The EML sequentially comprises a yellow EML of phosphor doped hole-transporting material, a blue EML of a blend of hole- and electron-transporting materials and an exciton regulation layer of ultra-thin green phosphor inserted in electron-transporting material. It can be seen that the emissive excitons are well regulated by such a strategic EML structure. The resulting WOLEDs exhibit a maximum forward viewing external quantum efficiency and power efficiency of 18.2% and 72.9 lm W−1, respectively, and they are as high as 16.7% and 61.7 lm W−1 at 100 cd m−2, and 12.5% and 37.7 lm W−1 at 1000 cd m−2, showing the positive properties of high efficiency and low efficiency roll-off. Such outstanding performance will greatly promote the development of high performance hybrid WOLEDs.

The Improvement of Spectral Stability by Adjusting the Recombination Zone Position in White Phosphorescent Organic Light Emitting

The recombination zone (RZ) position is an important factor for the CIE coordinate and spectral stability. We attempt to fabricate a serials of white phosphorescent organic emitting diode devices with mixed host (mCP:TPBi) multiple EMLs. The RZ position and range can be changed by the concentration ratio of mixed host or guest material, the thickness of the emitting layer (EML) and the structure of EML. Through inserting a mixed host blue emission layer as compensation layer between orange EML and ETL, we obtain a spectral stable device with the maximum luminance, power efficiency and current efficiency of 29790 cd/m2, 23.89 lm/W and 35.94 cd/A, respectively. Then, a more stable device is fabricated by decreasing the thickness of the blue EML, the range of CIE coordinate change is only (0.005, 0.002) when the voltage increases from 4 V to 8 V.

Optical Design and Optimization of Highly Efficient Sunlight-like Three-Stacked Warm White Organic Light Emitting Diodes

In this paper, we report an optical structure design method with the predicted performances of highly efficient three-stacked white organic light emitting diodes (WOLEDs) for solid state lighting applications. The efficiency and color properties of stacked WOLEDs are strongly affected by optical interference inside the thick cavity length; therefore appropriate emissive layer (EML) position is determined by thorough theoretical optical simulations to prevent such optical effect. The theoretically evaluated, three-stacked hybrid WOLED with entirely separated phosphorescent red and green as well as fluorescent blue EML has displayed a CRI and power efficiency of 91 and 33.5 lm/W, respectively. Based on our assumptions, design method, and optical simulation results, the fabricated three-stacked WOLEDs showed a CRI of 93, external quantum efficiency (EQE) of 49.4%, and power efficiencies of 33.4 lm/W. These experimentally measured characteristics are fully correlated with the performances of optically simulat...

Effect of exciton blocking layers on the color-tunable properties of organic light-emitting devices

A series of voltage-controlled color-tunable organic light-emitting devices (OLEDs) were fabricated. Four charge transport materials with different characteristics serving as the exciton blocking layers (EBLs) were inserted between yellow and blue emissive layers, respectively, and the color-tunable properties of OLEDs were studied in detail. Carrier transport and electroluminescence (EL) mechanisms of the color-tunable devices were investigated by analyzing the EL spectra and device efficiencies. The results showed that a color-tunable range of blue, white, and yellow light had been realized by tuning the film thicknesses of EBLs. The device with 16nm N,N′-dicarbazolyl-3,5-benzene hole transport material as the EBL could shift the color coordinates from (0.21, 0.32) to (0.38, 0.41), which exhibited the broadest color tuning range. Meanwhile, the device based on 18nm 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene electron transport material had the opposite color-tuning sequence and slightly higher tuning voltages, comparing with other three devices. The difference of color-tunable properties in OLEDs based on various EBLs was attributed to the variation of carrier mobility and energy level of EBL materials. Moreover, to effectively harvest the excitons for color-tunable light emission, the EBL thickness should be comparable to the triplet exciton diffusion length of EBLs.

Organic light emitting devices employing non-doped structure

A kind of efficient non-doped white organic light-emitting diodes (WOLEDs) were realized by using a bright blue-emitting layer of 4,4-bis(2,2-diphenylvinyl)-1,1-biphenyl (DPVBi) combining with red emitting ultrathin layer of [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) and green emitting ultrathin layer of 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T) with different thicknesses of 0.05 nm, 0.10 nm and 0.20 nm. For comparing, a doped WOLED was also fabricated, in which C545T and DCM2 are codoped into DPVBi layer to provide blue, green and red emission for obtaining white emission. The maximum luminance and power efficiency of the doped WOLED are 5 765 cd/m2 at 16 V and 5.23 lm/W at 5 V, respectively, and its Commission Internationale de l’Eclairage (CIE) coordinate changes from (0.393 7, 0.445 3) at 5 V to (0.300 7, 0.373 8) at 12 V. When the thickness of the ultrathin C545T layer in non-doped WLEDs increases, the emission luminance increases, but all non-doped devices are in the yellow white region. The device with 0.10-nm-thick C545T has a maximum efficiency of 15.23 cd/A at 8 V and a maximum power efficiency of 6.51 lm/W at 7 V, and its maximum luminance is 10 620 cd/m2 at 16 V. CIE coordinates of non-doped WLEDs with C545T thickness of 0.05 nm, 0.10 nm and 0.20 nm are (0.447 3, 0.455 6), (0.464 0, 0.473 1) and (0.458 4, 0.470 0) at 8 V, respectively.1