Stable and efficient blue fluorescent organic light-emitting diode by blade coating with or without electron-transport layer

Multi-layer small-molecule blue fluorescent organic light-emitting diode (OLED) is fabricated by blade coating. The emission layer is based on a mixed host of 1-(7-(9,9′-bianthracen-10-yl)-9,9-dioctyl-9H-fluoren-2-yl)pyrene (PT-404) and electron-transport material 2,7-Bis(diphenylphosphoryl)-9,9′ -spirobifluorene (SPPO13), and the blue guest emitter is 4-4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethane-2,1-diyl)bis(N,N-bis(4-hexyl- Phenyl) aniline) (Blue D). A hole-transport layer of Poly-(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4-(N-(4-sec-butylphenyl)) diphenylamine) (TFB) is added on top of PEDOT: PSS anode. The electrons are blocked away from TFB by a layer of pure host emission layer of PT-404 between TFB and the mixed –host emission layer. For the device with the electron transport layer of Tris(8-hydroxyquinolinato)aluminum (Alq₃) blade-coated over the emission layer the efficiency and lifetime at initial brightness of 500 cd m⁻² are 7.5 cd A⁻¹ and 150 h for Alq₃/CsF/Al cathode. When the Alq₃/CsF/Al is replaced by simply CsF/Al over the mixed-host emission layer the efficiency and lifetime are 6.4 cd A⁻¹ and 300 h (2 times longer than that of the Alq₃/CsF/Al cathode). The lifetime depends on the electron-hole balance tuned by the mixed-host blending ratio as well as the electron injection from the cathode. This work shows good stability is possible for all-solution-processed blue OLED.     

Phosphorescent dye-doped hole transporting layer for organic light-emitting diodes

Hole transport materials are critical to the performance of organic light-emitting diodes (OLEDs). While 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) with a high triplet energy is widely used for high efficiency phosphorescent OLEDs, devices using TAPC as a hole transport layer (HTL) have a short operating lifetime due to the build-up of trapped charges at the TAPC/emitting layer (EML) interface during device operation. In this work, to solve the operating stability problem, instead of using conventional HTLs, we use a(fac-tris(2-phenylpyridine)iridium (III))(Ir(ppy)₃) doped layer as an HTL to replace the conventional HTLs. Because of the hole injecting and transporting abilities of the phosphorescent dye, holes can be directly injected into the emitting layer without an injection barrier. OLEDs based on a phosphorescent dye-doped HTL show significant improvement in operational stability without loss of efficiency.     

Phenylimidazole-based homoleptic iridium(III) compounds for blue phosphorescent organic light-emitting diodes with high efficiency and long lifetime

In order to obtain triplet emitters with high stability and efficiency, three homoleptic iridium(III) compounds — specifically, Ir(tpim)₃ (1), Ir(mtpim)₃ (2), and Ir(itpim)₃ (3), where tpim = 1-([1,1′:3′,1″-terphenyl]-2′-yl)-2-(4-fluorophenyl)-1H-imidazole, mtpim = 2-(4-fluorophenyl)-1-(5′-methyl-[1,1′:3′,1″-terphenyl]-2′-yl)-1H-imidazole, and itpim = 2-(4-fluorophenyl)-1-(5′-isopropyl-[1,1′:3′,1″-terphenyl]-2′-yl)-1H-imidazole — were prepared by one-pot reaction of the corresponding phenylimidazole ligand with an Ir(I) complex as a starting material. Compounds 1–3 emit bright sky-blue phosphorescence with λ_max = 459–463 nm and phosphorescent quantum efficiencies of 0.38–0.50. Multi-layer phosphorescent organic light-emitting diodes using compounds 1–3 as the triplet emitters and mCBP (3,3-di(9H-carbazol-9-yl)biphenyl) as the host have been fabricated. Compound 3 doped in the emissive layer demonstrate external quantum efficiency as high as 20.1% at 1000 cd/m². In addition, the device based on compound 1 as an emitter shows a stable lifetime greater than 300 h at 1000 cd/m², which is one of the best results concerning the device lifetime.     

Fluorene derivatives for highly efficient non-doped single-layer blue organic light-emitting diodes

Diphenylamino- and triazole-endcapped fluorene derivatives which show a wide energy band gap, a high fluorescence quantum yield and high stability have been synthesized and characterized. Single-layer electroluminescent devices of these fluorene derivatives exhibited efficient deep blue to greenish blue emission at low driving voltage. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm⁻² with external quantum efficiency (EQE) of 2.0% and CIE coordinates of (0.153, 0.088) in deep blue region, while the single-layer device of oligothienylfluorene PhN-OFOT-TAZ shows a maximum brightness of 7524 cd/m² and a maximum current efficiency of 2.9 cd/A with CIE coordinates of (0.20, 0.40) in greenish blue.     

Efficient solution-processed blue phosphorescent organic light-emitting diodes with halogen-free solvent to optimize the emissive layer morphology

Efficient solution-processed blue phosphorescent organic light-emitting diodes (OLEDs) featuring with halogen-free solvent processing are fabricated in this study. The organic molecule 3,6-bis(diphenylphosphoryl)-9-(4′-(diphenylphosphoryl) phenyl)-carbazole (TPCz) that possesses good solubility in halogen-free polar solvents is selected to serve as the host of blue phosphorescent iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C²]-picolinate (FIrpic) dopant. The morphology of the TPCz:FIrpic emissive layer prepared with different polar solvents including chlorobenzene (CB), n-butanol (ButA) and isopropanol (IPA) and the effect on their electroluminescent performance have been investigated in detail. It is found that the more polar halogen-free solvent IPA restrains the FIrpic aggregation and renders a more densely packed emissive layer as compared to the CB-processed counterpart, which results in the enhanced electroluminescent performance. The luminous efficiency and power efficiency of the blue phosphorescent OLEDs prepared with CB are merely 5.7 cd/A and 3.3 lm/W, respectively. When using more polar halogen-free solvent IPA, the efficiencies are enhanced to 22.3 cd/A and 15.6 lm/W, about 2.9 and 3.7-time increment, respectively. This work provides an approach to fabricate efficient solution-processed phosphorescent OLEDs with environmental-friendly solvents, which is highly required in large-scale solution-processed manufacturing.     

Orthogonally substituted aryl derivatives as bipolar hosts for blue phosphorescent organic light-emitting diodes

Four novel bipolar hosts, namely 9,9′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(9H-carbazole) (2CzPm), 9,9′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (2TCzPm), 5,5′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzPm) and 5,5′-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzTrz) were designed and synthesized with diphenylpyrimidine and diphenyltriazine as electron-transporting units and carbazole derivatives as hole-transporting motifs for the application in blue phosphorescent organic light-emitting diodes (PHOLEDs). These electron-accepting and -donating functional groups were attached to the central phenylene bridge in an ortho-substituted fashion, which led to high triplet energies (2.97–3.00 eV) and wide bandgap (3.43–3.55 eV). The effect of modulation of electron-accepting and donating groups on the photophysical properties, frontier orbital energy levels, charge carrier transport properties and device performance of these four hosts has been investigated. 2BFCzPm and 2BFCzTrz featured with large conjugation system exhibited high thermal stability as compared to 2CzPm and 2TCzPm. The bis[2-(4,6-difluorophenyl)-pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based blue PHOLEDs hosted by 2BFCzPm exhibited excellent electroluminescence performance with a peak current efficiency of 38.2 cd/A and a maximum external quantum efficiency of 19.0%, which could be ascribed to the enhanced thermal stability, high triplet energy and good bipolar charge transport properties of the host material.     

Preparation and Characterization of Blue‐Luminescent Tris(8‐hydroxyquinoline)‐aluminum (Alq3)

Using differential scanning calorimetry (DSC) measurements in combination with structural and optical characterization we have investigated the formation conditions of different phases of tris(8‐hydroxyquinoline)aluminum (Alq₃). We have identified the δ‐phase as a high‐temperature phase of Alq₃ being composed of the facial stereoisomer, and report an efficient method to obtain blue luminescent Alq₃ by a simple annealing process. This allows the preparation of large amounts of pure δ‐Alq₃ by choosing appropriate annealing conditions, which is necessary for further characterization of this blue‐luminescent phase, and offers the possibility of fabricating blue organic light‐emitting devices (OLEDs) from this material.     

meta-Linked spirobifluorene/phosphine oxide hybrids as host materials for deep blue phosphorescent organic light-emitting diodes

This study investigated the use of a novel modification in molecular design to get two new electron-transport host materials, SF3PO and BSF3PO. By linking the phosphine oxide moieties at meta-position of spirobifluorene rings, higher triplet energies could be easily achieved for these two new materials. The steric spirobifluorene structures could guarantee their good thermal stabilities. According to these properties, their applications as host materials for deep blue phosphorescent organic light-emitting diodes (PHOLEDs) were explored. As expected, the deep blue emitting devices with Ir-complex FIr6 as phosphorescent dopants and SF3PO and BSF3PO as hosts had been fabricated and showed high efficiency of 28.5 and 22.0 cd/A, respectively, which were significantly higher than that of the para-linked analogue SPPO1.     

Novel spiro-based host materials for application in blue and white phosphorescent organic light-emitting diodes

Two novel spiro-based host materials, namely 3-(9,9′-spirobi[fluoren]-6-yl)-9-phenyl-9H-carbazole (SF3Cz1) and 9-(3-(9,9′-spirobi[fluoren]-6-yl)phenyl)-9H-carbazole (SF3Cz2) were designed and synthesized. Due to the meta-linkage of spirobifluorene backbone, both SF3Cz1 and SF3Cz2 possess triplet energies over 2.70 eV, indicating they could serve as suitable hosts for blue and even white phosphorescent organic light-emitting diodes (PHOLEDs). The fabricated bis(4,6-(difluorophenyl)-pyridinato -N,C′)picolinate (FIrpic) based PHOLEDs hosted by SF3Cz1 and SF3Cz2 exhibited excellent performance with maximum external quantum efficiencies (EQEs) of 18.1% and 19.7%, respectively. Two-color warm white PHOLEDs fabricated by utilizing SF3Cz1 and SF3Cz2 as hosts also achieved high EQEs and low efficiency roll-offs. The results demonstrate that SF3Cz1 and SF3Cz2 are promising hosts for blue and white PHOLEDs.     

Light emitting field-effect transistors with vertical heterojunctions based on pentacene and tris-(8-hydroxyquinolinato) aluminum

We report a top-contact light emitting field-effect transistor based on an asymmetric vertical heterojunction using pentacene as a field-effect layer and tris-(8-hydroxyquinolinato) aluminum (Alq₃) as an electron transport and luminescent material, which is fabricated on an indium tin oxide (ITO)-coated glass substrate with poly (methyl methacrylate) (PMMA) as a gate dielectric. The Alq₃ layer underneath the drain electrode roughly occupies one half of the pentacene surface forming an asymmetric heterojunction with pentacene. A hole transport layer N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) is introduced to occupy the other half of the pentacene surface underneath the source electrode to allow vertical hole transport in the device. We have realized the electrical switching functionality of a field-effect transistor (FET) and the control of electroluminescence (EL) simultaneously under ambient atmosphere. The device exhibits typical p-channel characteristics and green emission from Alq₃ is observed adjacent to the drain electrode. A working principle of the device is discussed in detail. Furthermore, this device configuration enables high-spatial-resolution fluorescence imaging of device operation, which is a simple and powerful tool for studying organic luminescent materials.     

Highly efficient thienylquinoline-based phosphorescent iridium(III) complexes for red and white organic light-emitting diodes

Highly efficient 2-(thiophen-2-yl)quinoline-based phosphorescent iridium(III) complexes bearing 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine or picolinic acid as ancillary ligands are designed and synthetised. The variation of ancillary ligands is attempted to finely tune the photophysical properties of these complexes, especially the solution phosphorescent quantum yields (ΦPL), full width at half maximum (FWHM), etc. The picolinic acid-based complex displays the slightly red-shifted dual-peak emission compared to triazolpyridine-based one. The complexes show bright emission with broad FWHM up to 83 nm, and the emissions are in red region with the very high absolute ΦPL up to 0.76 in solution. Moreover, high-performance red and three-color-based white organic light-emitting diodes (OLEDs) with excellent color stability have been fabricated. The maximum external quantum efficiencies of red and white OLEDs can reach 16.2% and 15.1%, respectively. The maximum current efficiency and power efficiency of white OLED are as high as 35.5 cd A⁻¹ and 34.0 lm W⁻¹, respectively. Especially, the designed white OLED exhibits excellent spectral stability under wide operating voltage range, and the 1931 Commission Internationale de L'Eclairage of white OLED only changes from (0.43, 0.42) to (0.44, 0.44), the color rendering index is in a narrow range of 75–77.     

Plasma-tolerant structure for organic light-emitting diodes with aluminum cathodes fabricated by DC magnetron sputtering: Using a Li-doped electron transport layer

We fabricate aluminum cathodes that are almost free from plasma damage by DC magnetron sputtering for organic light-emitting diodes (OLEDs). While sputtering is widely known to have numerous advantages over conventional evaporation for mass production of devices, it can cause serious damage to organic layers. In this report, we fabricate devices that are free from plasma damage by introducing a 1%-Li-doped electron transport layer (ETL). The difference of external electroluminescence quantum efficiency between OLEDs with the structure ITO/α-NPD/ETL/Al (where ITO is indium tin oxide and α-NPD is N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine) with Al cathodes deposited by conventional evaporation or sputtering is 0.1%, and their driving voltage is identical. We find that the Li-doped ETL should be thicker than 40 nm. Analysis of the depth profile of the ETL by time-of-flight secondary ion mass spectrometry indicates that considerable damage from sputtering extended to a depth of approximately 30 nm, suggesting that high-energy particles penetrated about 30 nm into the ETL.     

Analysis of the effect of solvents on the performance of solution-processed organic light-emitting diodes based on Fourier-transform infrared spectroscopy

Solution-processed organic light-emitting diode (s-OLED) is one of the strongest candidates for the next-generation display technology owing to its cost-effectiveness and broad applicability. However, its commercialization is being delayed due to the relatively low operational stability and abstruse mechanism of solvents’ influence on the device characteristics. Herein, we investigate the impact of the solvents on the characteristics of s-OLEDs by segmenting the solvents from tetrahydrofuran (THF) to chloroform (CF) and comparing them with thermally evaporated OLED as a reference device. Whereas the evaporated device offered current efficiency (CE) of 42.9 cd A⁻¹, the THF- and CF-based ones showed 38.5 cd A⁻¹ and 7.4 cd A⁻¹. Besides, the CF-based device showed significantly reduced operational stability with less than 10% of that of evaporated and THF-based ones. The main reason for the deviation between the devices is interpreted to be related to the residual chemicals and degradation of the organic materials of the CF-based emissive layer (EML), which is supplemented by Fourier-transform infrared spectroscopy (FTIR) analyses. We believe that the results will provide helpful insight into developing highly efficient and stable s-OLEDs.     

Degradation pattern of contact resistance and characteristic trap energy in blue organic light-emitting diodes

Thin and lightweight organic light-emitting diodes (OLEDs) are promising candidates for next-generation rollable displays; they offer numerous advantages, such as scalable manufacturing, high color contrast ratio, flexibility, and wide viewing angle. Despite the numerous merits of OLEDs, the insufficient lifetime and stability of blue OLEDs remain unresolved, thereby necessitating a feedback strategy for lifetime extension. Herein, we propose a simple yet effective methodology to determine the contact resistance (R_CT) and characteristic trap energy (E_T) of OLEDs simultaneously in the trapped-charge-limited-conduction regime, where electroluminescence occurs primarily. To validate our approach, the extracted R_CT and E_T values are directly compared with each other by connecting a commercial resistor (R_C) to a blue OLED in series. The percent errors discovered in R_C and E_T are less than 7% and 4%, demonstrating the high feasibility and accuracy of our approach. We further employ this method to study the degradation mechanism of a blue OLED by presenting the electrical stress time- and cycle-dependent R_CT, E_T, ideality factor, and turn-on voltage, revealing different degradation patterns of the metal-to-transport layer interface and emission layer, respectively. Our results provide better insights into the electrical parameter extraction method and electrical current degradation mechanism in blue OLEDs.     

Straight-forward control of the degree of micro-cavity effects in organic light-emitting diodes based on a thin striped metal layer

We investigated the control of micro-cavity (MC) effects in organic light-emitting diodes (OLEDs) with the introduction of a striped thin metal layer between the indium tin oxide (ITO) layer and the hole transporting layer (HTL). With an enhanced MC effect obtained through the inserted metal layer, the forward emission of the OLED became stronger and the angular distribution became more forward-directed, leading to a current efficiency (CE) that was nearly 1.45 times higher than that of the reference device without the inserted metal layer. The net CE of the OLEDs with a striped metal layer was found to be determined by the area-weighted average of the CE’s of full-cavity-enhanced OLEDs and non-cavity OLEDs. It was also observed that the trade-off between resonance enhancement in efficiency and angle-dependent color stability, often found problematic in MC-based OLEDs, could be mitigated in a straight-forward manner by changing the relative portion of the metal-covered area.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Universally applicable small-molecule co-host ink formulation for inkjet printing red,green, and blue phosphorescent organic light-emitting diodes

For organic light-emitting diodes (OLEDs), inkjet printing technology is being developed as an alternative to the traditional vacuum evaporation, because of its precise patterning, high-efficient material utilization, large-area compatibility and low-cost. In this work, we report a universal ink formulation of small-molecule co-host and binary solvents for red, green and blue phosphorescent OLEDs. Moreover, the effect of hole-transporting layers on the ink spreading, film uniformity and exciton confinement ability is investigated. Furthermore, a large-area (170 mm × 170 mm) and homogeneous light-emitting film is inkjet-printed. Finally, red, green and blue OLEDs are successfully constructed using these optimized ink formulations on the solvent resistance hole-transporting layer. This work can reduce the complexity to adjust the host materials and solvents for different color inks, and could be applied in large-area and low-cost OLED displays with high resolution.     

Solution-processed organic light-emitting diodes with emission from a doublet exciton; using (2,4,6-trichlorophenyl)methyl as emitter

Neutral-π radical based open shell molecules foster new potential in light emitting diodes because of their theoretical near-equity quantum efficiencies. In this study, we report organic light emitting diodes (OLEDs) based on a novel open shell molecule (2,4,6-trichlorophenyl)methyl (TTM) as the electroluminescent layer. The singly occupied molecular orbital (SOMO) level and optical bandgap of TTM was calculated using cyclic voltammetry and UV–visible absorption respectively. Thermogravimetric analysis showed a stable molecule capable of sublimation. Two decidedly different approaches, thermal evaporation and solution processing, were employed to deposit TTM:host blend thin films for OLED device fabrication. OLED devices fabricated via thermal evaporation and solution processing yielded external quantum efficiencies of 2.78% and 0.28% and luminances of 330 cdm^-2 and 78 cdm^-2 respectively. Further, the effect of doping ratios of the host materials on OLED device performance were investigated and optimal ratios were established. We report for the first time solution processed open shell organic molecules for light emitting diode applications. Our results elucidate the potential for low-cost and high efficiency optoelectronic devices.     

Two trans-1-(9-anthryl)-2-phenylethene derivatives as blue-green emitting materials for highly bright organic light-emitting diodes application

1-(9-Anthryl)-2-phenylethene (t-APE) is a blue-green material with high fluorescence quantum yield (Ф_f 0.44). However, it is easily crystallized. Herein, Two asymmetric blue-green emitting materials based on t-APE, (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene (6) and (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-2-yl)anthracene (7) were firstly designed and synthesized. The two compounds possess high thermal stability, morphological durability, and bipolar characteristics. The non-doped blue-green organic light-emitting diodes (OLEDs) using 6 and 7 as emitting layers showed emission at 495 nm, full width at half maximum of 80 nm, maximum brightness of 13,814, 10,579 cd m⁻², maximum current efficiency of 3.62, 7.16 cd A⁻¹, and Commission Internationale de L'Eclairage (CIE) coordinate of (0.20, 0.43), respectively. Furthermore, when employing 6 and 7 as blue-green emitting layers and rubrene doped in tris-(8-hydroxyquinolinato)aluminum (Alq3) as the orange emitting layers to fabricate white OLEDs (WOLEDs), the WOLEDs exhibit a maximum brightness of 10,984, 14,652 cd m⁻², maximum current efficiency of 2.04, 2.70 cd A⁻¹, and CIE coordinate of (0.30, 0.40), (0.37, 0.47), Color Rendering Index (CRI) of 65, 60, stable EL spectra, respectively. This study demonstrates that the t-APE-type derivatives have the excellent properties for the emitting materials of OLEDs.