Iridium-based light-emitting electrochemical cells containing ionic liquids in the luminous layer

To fabricate transition metal complex-based LECs (light-emitting electrochemical cells), ([Ir(ppy)₂(5,6-dime-1,10-phenthroline)]PF₆ was synthesized and used as a luminous material and ILs (ionic liquids) were incorporated into a luminous layer, in which two types of ionic liquid were used; 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄). ILs were added to a [Ir(ppy)₂(5,6-dime-1,10-phenthroline)]PF₆ luminous layer to improve ionic conductivity and light intensity. Both ILs significantly increased the current density and luminance. Due to the small molecule of BF₄^?, turn-on time was reduced and ionic conductivity was increased. However, the device stability was sacrificed. High current efficiency of 34.5 cd/A was investigated at 7 V of BMIMPF₆-doped luminous layer. The LECs based on [Ir(ppy)₂(5,6-dime-1,10-phenthroline)]PF₆ gave yellow emission color when ILs were added into light-emitting layer, and no significant change of color has been found in this study.     

Heteroleptic tris-cyclometalated iridium(III) complexes with phenylpridine and diphenylquinoline derivative ligands

The synthesis and photophysical study of efficient phosphorescent heteroleptic tris-cyclometalated iridium(III) complexes having two different (C^N) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet-triplet (T-T) annihilation, new heteroleptic tris-cyclometalated iridium complexes, Ir(ppy)₂(dpq), Ir(ppy)₂(dpq-3-F) and Ir(ppy)₂(dpq-CF₃), are designed and prepared where ppy, dpq, dpq-3-F and dpq-CF₃ represent 2-phenylpyridine, 2,4-diphenylquinoline, 2-(3-fluorophenyl)-4-phenylquinoline, and 4-phenyl-2-(4-(trifluoromethyl)phenyl)quinoline, respectively. Ppy ligands and dpq derivatives can act as a source of energy supply. When new heteroleptic tris-cyclometalated iridium complex, Ir(ppy)₂(dpq-3-F) is placed in the lowest excited state, the excitation energy is neither quenched nor deactivated but quickly intermolecularly transferred from two ppy ligands to one luminescent dpq-3-F ligand. Such transfer can occur because the triplet energy level of Ir(ppy)₃ is higher than that of Ir(dpq-3-F)₃ and because Ir(dpq-3-F)₃ was known to have a shorter lifetime than that of Ir(ppy)₃. As a result, Ir(ppy)₂(dpq-3-F) shows strong emission band at 620 nm from dpq-3-F ligand in the end. Thus it allows more reddish luminescent color and improves the luminescence by the decrease of quenching or energy deactivation by decreasing the number of the luminescent ligand. To analyze luminescent mechanism, we calculated these complexes theoretically by using computational method.     

Single dopant white electrophosphorescent light emitting diodes using heteroleptic tris-cyclometalated Iridium(III) complexes

We report single dopant single emissive layer white organic electroluminescent (EL) device based on the heteroleptic tris-cyclometalated iridium(III) complex, Ir(dfppy)₂(pq), as the guest, where dfppy and pq are 2-(2,4-difluorophenyl) pyridine and 2-phenylquinoline, respectively, and 1,4-phenylenesis(triphenylsilane) (UGH2) as the host. The maximum luminous and power efficiencies of the device were 11.00 cd/A (J = 0.05 mA/cm²) and 5.60 lm/W (J = 0.001 mA/cm²), respectively. The CIE coordinates of the device with Ir(dfppy)₂(pq) are (0.443, 0.473) and the EL spectrum of device shows emission band at 473 and 544 nm, at the applied voltage of 12 V. The similar phosphorescent decay rate of two ligands can lead to emit luminescence in two ligands at the same time.     

Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes

The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F₂-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)₃ has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F₂-ppy)₃ was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂ were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F₂-ppy)₂ was 3.63% at J = 10 mA/cm².     

Efficient red electrophosphorescent organic light-emitting diodes based on the new sensitized heteroleptic tris-cyclometalated Ir(III) complexes

Organic light-emitting device (OLED) was fabricated using the novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, bis(2-phenylpyridine)iridium(III)[2(5′-methylphenyl)-4-diphenylquinoline] [Ir(ppy)₂(dpq-5CH₃)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq-5CH₃) ligand. Generally, the ppy ligand in heteroleptic iridium complexes plays an important role as “sensitizer” in the efficient energy transfer from the host (CBP; 4,4,N,N′-dicarbazolebiphenyl) to the luminescent ligand (dpq-5CH₃). We demonstrated that high efficiency through the “sensitizer” can be obtained, when the T₁ of the emitting ligand is close to T₁ of the sensitizing ligand. The device containing Ir(ppy)₂(dpq-5CH₃) produced red light emission of 614 nm with maximum luminescence efficiency and power efficiency of 8.29 cd/A (at 0.09 mA/cm²) and 5.79 lm/W (at 0.09 mA/cm²), respectively.     

Impedance spectroscopy measurements of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl thin films doped with tris(2-phenylpyridine) iridium

The charge carrier mobility of green phosphorescent emissive layers, tris(2-phenylpyridine) iridium [Ir(ppy)₃]-doped 4,4'-N,N'-dicarbazole-biphenyl (CBP) thin films, has been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement by IS rests on a theory for single-injection space-charge limited current. The hole mobilities of the Ir(ppy)₃-doped CBP thin films were measured to be 10^− 10–10^− 8 cm²V^− 1 s^− 1 in the 2–7 wt.% Ir(ppy)₃-doped CBP from the frequency dependence of both conductance and capacitance. These hole mobility values are much lower than those of the undoped CBP thin films (~ 10^− 3 cm²V^− 1 s^− 1) because the Ir(ppy)₃ molecules act as trapping centers in the CBP host matrix. These mobility measurements in the Ir(ppy)₃-doped CBP thin films provide insight into the hole injection process.     

High efficiency green phosphorescent organic light emitting device with (TCTA/TCTA0.5TPBi0.5/TPBi): Ir(ppy)3 emission layer

A new high efficiency green light emitting phosphorescent device with an emission layer consisting of {4,4',4'-tris(N-carbazolyl)-triphenylamine[TCTA]/TCTA_0.5TPBi_0.5/1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene[TPBi]}:tris(2-phenylpyridine)iridium(III)[Ir(ppy)₃] was fabricated and its electroluminescence characteristics were evaluated in comparison with those of devices with emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/ TPBi):Ir(ppy)₃.The device with the emission layer consisting of (TCTA/TCTA_0.5TPBi_0.5/TPBi):Ir(ppy)₃ showed a luminance of 11,000 cd/m² at an applied voltage of 8 V and maximum current efficiency of 63 cd/A under a luminance of 500 cd/m². The peak wavelength in the electroluminescent spectral and color coordinate on the Commission Internationale de I'Eclairage(CIE) chart were 513 nm and (0.31, 0.62) in this device, respectively. Under a luminance of 10000 cd/m², the current efficiency of this device was 55 cd/A, which is 1.4 and 1.1 times better than those of the devices with the emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/TPBi):Ir(ppy)₃, respectively.     

Effect of ionic liquids on the electroluminescence of yellowish-green light-emitting electrochemical cells using bis(2-(2,4-difluorophenyl)pyridine)4,7-diphenyl-1,10-phenanthroline-iridium(III) hexafluorophosphate

A cationic iridium complex [Ir(dfppy)₂(dpphen)]PF₆, where dfppy is 2-(2,4-difluorophenyl)pyridine, dpphen is 4,7-diphenyl-1,10-phenanthroline and PF₆⁻ is hexafluorophosphate, has been synthesized and its photophysical and electrochemical properties were investigated. Light-emitting electrochemical cells (LECs) based on this complex were fabricated using air stable electrodes and emits yellowish-green light (533 nm) with Commission Internationale de L’Eclairage (CIE) coordinates of (0.35, 0.59) at 4 V. Effect of two different imidazolium based ionic liquids (ILs) viz, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) and 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIMPF₆) on the active layer has been studied and the luminance and the current density of the devices were found to be enhanced with increasing ionic conductivities.     

Strong ligand field effects of blue phosphorescent mono-cyclometalated iridium(III) complexes

A series of mono-cyclometalated blue phosphorescent iridium(III) complexes with two phosphines trans to each other and two cis-ancillary ligands, such as Ir(F₂Meppy)(PPhMe₂)₂(H)(Cl), [Ir(F₂Meppy)(PPhMe₂)₂(H)(NCMe)]⁺ and Ir(F₂Meppy)(PPhMe₂)₂-(H)(CN), [F₂Meppy = 2-(2′,4′-difluorophenyl)-4-methyl-pyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. We investigate the electron-withdrawing capabilities of ancillary ligands using the DFT and TD-DFT calculations on the ground and excited states of the three complexes to gain insight into the factors responsible for the emission color change and the different luminescence efficiency. Reducing the molecular weight of phosphine ligand with PPhMe₂ leads to a strategy of the efficient deep blue organic light-emitting devices (OLED) by thermal processing instead of the solution processing. The electron-withdrawing difluoro group substituted on the phenyl ring and the cyano strong field ancillary ligand in the trans position to the carbon atom of phenyl ring increased HOMO-LUMO gap and achieved the hypsochromic shift in emission color. As a result, the maximum emission spectra of Ir(F₂Meppy)(PPhMe₂)₂(H)(Cl), [Ir(F₂Meppy)(PPhMe₂)₂(H)-(NCMe)]⁺ and Ir(F₂Meppy)(PPh-Me₂)₂ (H)(CN) were in the ranges of 446, 440, 439 nm, respectively.     

[Ba4(S2)][ZnGa4S10]: Design of an Unprecedented Infrared Nonlinear Salt-Inclusion Chalcogenide with Disulfide-Bonds

Salt-inclusion chalcogenides (SICs) have been receiving widespread attention due to their large second harmonic generation (SHG) responses and wide bandgaps, however most of them suffer from small birefringence limiting their technical application. Herein, by introducing the π-conjugated (S₂)²⁻ units in the ionic guest of salt-inclusion structure, the first disulfide-bond-containing SIC, [Ba₄(S₂)][ZnGa₄S₁₀] has been synthesized. It exhibits the widest bandgap up to 3.39 eV among polychalcogenides and strong SHG response as large as that of AgGaS₂ (AGS). Importantly, its birefringence reaches a max value of 0.053@1064 nm among AGS-like SICs, indicating it is a promising IR nonlinear optical (NLO) material. Theoretical calculations reveal that the π-conjugated (S₂)²⁻ units and covalent Ga S layers favor the enhanced birefringence and large SHG response. This work provides not only a new type of SIC for the first time, but also new lights on the design of IR NLO materials.     

Triplet to singlet transition induced low efficiency roll-off in green phosphorescent organic light-emitting diodes

Phosphorescent organic light-emitting diodes (PHOLEDs) with an emitting layer of 4,4′-N,N′-dicarbazole-biphenyl codoped with phosphor fac-tri(phenylpyridine)iridium(III) [Ir(ppy)₃] and fluorophore N,N’-dimethy-quinacridone (DMQA) are investigated. Predominant emission from DMQA due to the efficient energy transfer from Ir(ppy)₃ to DMQA is observed. Such an energy transfer results in the transition of Ir(ppy)₃ triplet to DMQA singlet, which reduces the Ir(ppy)₃ exciton lifetime and hence suppresses the triplet-triplet annihilation and triplet-polaron annihilation of Ir(ppy)₃ excitons, leading to dramatical reduction of the efficiency roll-off of the PHOLEDs. This transition of triplet to singlet strategy provides a method to improve the efficiency roll-off of the PHOLEDs.     

Synthesis and electro-optical properties of carbazole derivatives with high band gap energy

Two materials containing carbazole moieties and exhibiting a high band gap energy, 3,8-di(9H-carbazol-9-yl)-6-phenylphenanthridine (DCzP) and 3,6-di(naphthalene-2-yl)-9-phenyl-9H-carbazole (DNaC), were synthesized via CN coupling and Suzuki coupling reactions, respectively. The compound DCzP exhibited blue emission with the CIE coordinates of x = 0.165 and y = 0.136 from the OLED device, ITO(indium–tin oxide)/NPB(N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine)/DCzP/LiF/Al. The doped device, ITO/2-TNATA(4,4′,4″-Tris(2-naphtylphenyl-phenylamino) triphenyl amine)/NPB/DCzP + Ir(ppy)₃/BCP(2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline)/Alq₃(tris(8-hydroxyquinoline)aluminum/LiF/Al, showed bright yellowish-green emission with a maximum luminance of 23,000 cd/m² when the synthesized DCzP was applied as a host material for the phosphorescent green dopant. From the double layer device, ITO/DNaC/Alq₃/LiF/Al, in which DNaC was used as the hole transporting material, the yellowish-green color arising from the Alq₃ was also observed.     

Efficient red-emitting phosphorescent iridium(III) complexes of fluorinated 2,4-diphenylquinolines

Ir(β) complexes of fluorinated dpqs(dpq-3-F, dpq-4-CF₃) as a cyclometallated ligand were prepared and their photonic properties were investigated, where dpq-3-F and dpq-4-CF₃ represent 2-(3-fluoro-phenyl)-4-phenylquinoline and 4-phenyl-2-(4-trifluoromethylphenyl)quinoline, respectively. Fluorinated dpq derivatives were introduced to the iridium complexes to increase the efficiency compared to Ir(dpq)₂(acac) which was recently reported to have emission wavelength of 614 nm with quantum efficiency of 0.14. These fluorinated ligands and their Ir(III) complexes were computationally calculated by ab initio methods to support our experimental results. It was found that the Ir complex containing dpq-3-F ligands exhibits the largest emission efficiency with maximum emission peak at 593.5 nm. The result of ab initio calculation using the time-dependent density functional theory (TD-DFT) showed that the strong ³MLCT transition of the complex occurs due to the strong coupling between the 5d orbital of the Ir atom and the highest occupied molecular orbitals (HOMOs) of these ligands.     

Synthesis and photo physical study of iridium complex of new pentafluorophenyl-substituted ligands

New iridium complexes, [Ir(dpq)₂(acac), Ir(PF-dpq)₂(acac) and Ir(PF-dpq-5F)₂(acac)] (dpq = 2,4-diphenylquinoline, dpq-5F = 2-(3′-fluorophenyl)-4-phenylquinoline), PF-dpq-5F = 2-(3-fluoro-phenyl)-6-pentafluorophenyl-4-phenylquinoline and acac = acetylacetonate) have been synthesized and characterized for efficient red organic light-emitting diodes (OLEDs). In order to improve the luminescence efficiency by preventing self-quenching and to tune photoluminescence (PL) and electroluminescence (EL) spectra to a longer wavelength, dpq ligand was fluorinated by -PF and -F moieties. However, the iridium complex of PF-dpq-5F underwent a weak MLCT transition because of the weak coupling between the 5d orbital of the iridium atom and HOMO of the substituted ligand. Thus, the maximum luminous efficiencies of the device using Ir(dpq)2(acac), Ir(PF-dpq)₂(acac) and Ir(PF-dpq-5F)₂(acac) are 4.36 cd/A, 6.04 cd/A and 4.35 cd/A, respectively.     

Introduction of the benzoquinoline ancillary ligand to the iridium complexes for organic light-emitting diodes

The new iridium complexes, Ir(C^N)2(bq), (C^N = ppy, F2-ppy, 2,3-dpqx-F2 or 4-Me-2,3-dpq) were prepared and their luminescence properties were investigated, where ppy, F2-ppy, 2,3-dpqx-F2, 4-Me-2,3-dpq and bq represent 2-phenylpyridine, 2-(4',6'-difluorophenyl)-pyridine, 2,3-bis (4'-fluorophenyl)quinoxaline, 4-methyl-2,3-diphenylquinoline and 10-hydroxybenzoquinoline ligands, respectively. We expected that the relative energy levels of the main ligands (C^N) and ancillary ligand, bq, in the complexes could determine the possibility of interligand energy transfer (ILET) in the complexes and thereby luminescence properties. The main ligands, F2-ppy and 2,3-dpqx-F2, which have drastically different energy gaps between the HOMO and LUMO energy levels were chosen and their complexes were synthesized. The photoabsorption, photoluminescence and electroluminescence of the complexes were studied. Ir(ppy)2(bq), Ir(F2-ppy)2(bq) Ir(2,3-dpqx-F2)2(bq) and Ir(4-Me-2,3-dpq)2(bq) exhibited the luminescence maxima between 600-694 nm and their efficiencies were affected by the main ligands. While Ir(ppy)2(bq) and Ir(F2-ppy)2(bq) showed relatively high luminous efficiencies (> 10 cd/A), Ir(2,3-dpqx-F2)(bq) had poor luminous efficiency (0.30 cd/A). The electrochemical properties were studied to support ILET in the ppy-based iridium complexes. Their luminescence performances were compared with those of the complexes containing acetylacetonate (acac) ancillary ligand which are not allowed to have ILET.     

New complexes of europium and gadolinium with 2,4,6-trichlorophenyl acetoacetate as ligand

The synthesis, characterization and photoluminescent properties of new europium complexes with 2,4,6-trichlorophenyl acetoacetate (TCA) and 3-amino-2-carboxypyridine-N-oxide (picNO) ligands have been investigated. Results of the characterization are in agreement with the molecular formula proposed. The emission spectra at 77 K of the [Eu(TCA)₂(H₂O)₅]OH and [Eu(TCA)₂(picNO)(H₂O)₂]OH complexes, excited at 333 nm, display the typical transitions of the europium ion, ⁵D₀ → ⁷F_J (J = 0-4), indicating an efficient luminescence sensitization of this ion by the TCA ligand. The satisfactory agreement between experimental and theoretical absorption spectra of the organic part of the complexes suggests that the geometries optimized by the Sparkle model are correct. These results suggest these complexes as potential candidates as useful markers.     

Transmission electron microscopy on the microstructure of 7050 aluminium alloy in the T74 condition

The microstructure of 7050 aluminium alloy in the T74 condition has been investigated by transmission electron microscopy. It was found that the alloy contains the superlattice Al₃Zr phase, η′ phase and Al₇Cu₂Fe constituent phase. The η′ phase is proposed to have an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The orientation relationship between the matrix and η′ phase is [11−2]_m//[100]_η′; [1−]_m//[010]_η′;[−1−1−1]_m//[001]_η′. The phases on the small-angle grain boundary are found to be mainly η′ phase and Cu/Si-rich phase, whereas on the large-angle grain boundary there is only η phase.     

Observation of phosphorescence from fluorescent organic material Bebq2 using phosphorescent sensitizer

Absorption, emission and excitation spectra of bis(10-hydroxybenzo [h] quinolinato)-beryllium (Bebq₂) were studied using polystyrene film doped with 5 wt% Bebq₂, N,N-di(naphthalene-1-yl)-N,N-diphenyl-benzidene (NPB) film doped with 60 wt% Bebq₂, and neat film. The monomer and aggregate of Bebq₂ give fluorescence at 492 and 511 nm at 12 K, respectively. A strong T₁ emission with a vibronic structure was observed from Bebq₂ below 70 K by heavily doping with phosphorescent tris(2-phenylpyridine) iridium [Ir(ppy)₃]. The T₁ energy of Bebq₂ was estimated to be 2.26 eV from the onset of the 573 nm 0–0 vibronic emission band. The energy transfer mechanism from Ir(ppy)₃ to the T₁ state of Bebq₂ is discussed.     

Determination of dopant concentration in co-deposited organic thin films by using RBS and X-ray fluorescence combined techniques

Organic light emitting diodes using phosphorescent dyes (PHOLEDs) have excellent performance and an internal quantum efficiency approaching 100%. To maximize performance, PHOLED devices use a conductive organic host material with a phosphorescent guest that is sufficiently dispersed to avoid concentration quenching. One of the most widely used organic compounds, green phosphorescent fac-tris(2-phenylpyridine)iridium, or [Ir(ppy)₃], can be used to produce PHOLEDs with very high external quantum efficiency by doping host material at different nominal concentrations. In this study, a methodology to accurately establish dopant concentration in co-deposited organic layers is proposed and discussed. X-ray fluorescence (XRF) and Rutherford backscattering (RBS) analyses were performed in co-deposited organic thin films and then combined to provide an accurate methodology. [Ir(ppy)₃] was used at different concentrations in two different hosts – 2,7-bis(9-carbazolyl)-9,9-spirobifluorene (Spiro2-CBP) and copper phthalocyanine (CuPc) – to test the proposed methodology. As Cu peak is easily detected by RBS, the CuPc host was chosen for calibration purposes, allowing more accurate determination of [Ir(ppy)₃] concentration. A linear correlation between the RBS and the XRF measurement data was found allowing the drawing up of a calibration chart used to determine the [Ir(ppy)₃] mass content in co-deposited films.     

Effect of fabrication process on characteristics of phosphorescence organic light emitting diodes with methoxy-substituted starburst low-molecule as a host

The effect of dry process and wet process on the characteristics of phosphorescence organic light-emitting devices (OLEDs) employing a phosphorescent dye fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)₃) doped into a methoxy-substituted starburst low-molecule material methoxy-substituted 1,3,5-tris[4-(diphenylamino) phenyl]benzene (TDAPB) are investigated. The FT-IR and absorption spectra of TDAPB films fabricated by a dry process, and a wet process are almost same, and the PL spectra of those films are different. The carrier transport capability of TDAPB by a dry process is lower than that by a wet process. The photoluminescence intensity of Ir(ppy)₃ doped in TDAPB fabricated by a wet process is higher than that by a dry process. A maximum external current efficiency of more than 20 cd/A and luminance of more than 10,000 cd/m² were obtained. Maximum luminance of devices monotonously decreases with increasing the thickness of a dry-processed emitting layer. The main emission zone of the OLED was located in almost at the center of the emitting layer. The improvement of device performance in the OLED fabricated by a wet process was achieved due to the high efficient energy transfer from TDAPB to Ir(ppy)₃, high carrier transporting capability and the formation of homogeneous film, compared with that fabricated by a dry process.