Photoluminescence spectroscopy study on tris(8-hydroxyquinoline) aluminum film

In situ photoluminescence spectroscopy (PL) measurements of tris(8-hydroxyquinoline) aluminum (Alq₃) film were carried out. Upon deposition of Alq₃ on the glass substrate, the PL intensity changes dramatically, while the peak position of Alq₃ emission shows a sharp red-shift from 524 nm at the initial deposition of Alq₃, and tends to a saturation value of 536 nm for the film thickness range from 2 to 500 nm. This red-shift is associated with the change from the 2D to 3D exciton state with increasing Alq₃ film thickness. Temperature dependent PL spectra of Alq₃ films showed, besides the changes in the PL intensity, clearly a blue-shift of Alq₃ emission about 9 nm for the film annealing up to 150 °C, while no any shift of Alq₃ emission was observed for the film annealing below 130 °C. Both changes in PL intensity, and especially in the peak position of Alq₃ emission were attributed to crystallization (thermal) effect of Alq₃ film upon annealing.     

White organic electroluminescence from fluorescent bis (2-(2-hydroxyphenyl) benzoxazolate)zinc doped with phosphorescent material

Efficient white electroluminescence has been obtained by using an electroluminescent layer comprising of a blue fluorescent bis (2-(2-hydroxyphenyl) benzoxazolate)zinc [Zn(hpb)₂] doped with red phosphorescent bis (2-(2′-benzothienyl) pyridinato-N,C^3′)iridium(acetylacetonate) [Ir(btp)₂acac] molecules. The color coordinates of the white emission spectrum was controlled by optimizing the concentration of red dopant in the blue fluorescent emissive layer. Organic light-emitting diodes were fabricated in the configuration ITO/α-NPD/Zn(hpb)₂:0.01 wt%Ir(btp)₂acac/BCP/Alq₃/LiF/Al. The J-V-L characteristic of the device shows a turn on voltage of 5 V. The electroluminescence (EL) spectra of the device cover a wide range of visible region of the electromagnetic spectrum with three peaks around 450, 485 and 610 nm. A maximum white luminance of 3500 cd/m² with CIE coordinates of (x, y=0.34, 0.27) at 15 V has been achieved. The maximum current efficiency and power efficiency of the device was 5.2 cd/A and 1.43 lm/W respectively at 11.5 V.     

Investigation of an efficient YbF3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq₃)-based organic light-emitting diodes (OLEDs) using YbF₃ as the electron injection layer have been investigated. With an YbF₃ (3.0 nm)/Al cathode, the device with Alq₃ as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq₃-cathode interface could be well covered by YbF₃ at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq₃ and Al, and to reduce the destruction of Alq₃ by Al.     

The origination of ill-defined layer in organic spin valves

The origination of ill-defined layer in organic spin valves was investigated by using atomic force microscopy (AFM) and Rutherford backscattering (RBS) analysis. It was found that conductive bulges of LSMO film and self-grown pinholes in Alq₃ film other than Co inclusions could lead to the formation of ill-defined layer. The morphology of LSMO substrate had a strong influence on that of Alq₃ film, LSMO/Alq₃ and Alq₃/Co interfaces. Moreover, Alq₃ film with the thickness of 1-4 nm could be barriers which was explained by small active area and added insulated layer in organic magnetic tunnel junctions.     

Efficient and stable single-dopant white OLEDs based on 9,10-bis (2-naphthyl) anthracene

Efficient white organic light-emitting diodes (WOLEDs) are fabricated with a thin layer of 9,10-bis (2-naphthyl) anthracene (ADN) doped with Rubrene as the source of white emission. A device with the structure of ITO/NPB (70 nm)/ADN: 0.5% Rubrene (30 nm)/Alq₃ (50 nm)/MgAg shows a maximum current efficiency of 3.7 cd/A, with the CIE coordinates of x=0.33, y=0.43. The EL spectrum of the devices and the CIE coordinates remains almost the same when the voltage is increased from 10 to 15 V and the current efficiency remains quite stable with the current density increased from 20 to 250 mA/cm².     

Blue organic light-emitting diodes with 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as hole transport and emitting layer and the impedance spectroscopy analysis

2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) based fluorescent blue organic light-emitting diodes (OLEDs) are demonstrated. With MADN as emitting layer, experiments indicate that thick MADN (40–60 nm) is preferable for constructing efficient blue OLED. With MADN as hole-transport and emitting layer and tris(8-hydroxy-quinolinato)aluminium (Alq₃) as electron-transport layer, the OLED electroluminescent characteristics show a mixture emission of MADN and Alq₃ with Commission Internationale d'Eclairage (CIE) color coordinates of (0.25, 0.34), indicating feasible hole transporting in MADN. Using 4,7-diphenyl-1,10-phenanthroline (BPhen) replacing Alq₃ as electron-transport layer, the OLED shows deep blue emission with a maximum luminous efficiency of 4.8 cd/A and CIE color coordinates of (0.16, 0.09). The hole transport characteristics of MADN are further clarified by constructing hole-only device and performing impedance spectroscopy analysis. The results indicate that MADN shows superior hole-transport ability which is almost comparable to typical hole-transport material of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB), suggesting a promising application for constructing efficient blue OLED with integrated hole-transport layer and emitting layer.     

Improved performances of red organic light-emitting devices by co-doping a rubrene derivative and DCJTB into tris-(8-hydroxyquinoline) aluminum host

Performances of red organic light-emitting device were improved by co-doping 2-formyl-5,6,11,12-tetraphenylnaphthacene (2FRb) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)-4H-pyran (DCJTB) in tris-(8-hydroxyquinoline) aluminum (Alq₃) host as the emitting layer. The device with 1 wt% DCJTB and 2.4 w% 2FRb in Alq₃ host gave a saturated red emission with CIE chromaticity coordinates of (0.65, 0.35) and a maximum current efficiency as high as 6.45 cd/A, which are 2 and 2.4 fold larger than that of the device with 1 wt% DCJTB (3.28 cd/A) in Alq₃ host and the device with 2.4 wt% 2FRb (2.72 cd/A) in Alq₃ host at the current density of 20 mA/cm², respectively. The improvement could be attributed to the effective utilization of host energy by both energy transfer and trapping in the electroluminescence process and the depression of concentration quenching between the dopants molecules.     

New heteroleptic cyclometalated iridium(III) complexes containing 2-(2′,4′-difluorophenyl)-4-methylpyridine for organic light-emitting diode applications

Two phosphorescent iridium(III) complexes (dfpmpy)₂Ir(ppc) and (dfpmpy)₂Ir(prz) [dfpmpy=2-(2′,4′-difluorophenyl)-4-methylpyridine, ppc=pipecolinate, prz=2-pyrazine carboxylate] were synthesized from the reaction of the chloro-bridged dimeric complex [(dfpmpy)₂Ir(μ-Cl)]₂ and the ancillary ligand. Their structures and photoluminescence properties were investigated and device performances for application in organic light-emitting diodes (OLEDs) were studied. The complexes adopt a distorted, octahedral geometry around the iridium metal, exhibiting cis C-C and trans N-N arrangements. The photoluminescent (PL) properties reveal that (dfpmpy)₂Ir(ppc) emits in the blue-green region (λ_max=497 nm), whereas (dfpmpy)₂Ir(prz) shows red phosphorescence (λ_max=543 nm) in the film state (5% wt. doped in PMMA). The (dfpmpy)₂Ir(ppc)- and (dfpmpy)₂Ir(prz)-based OLEDs exhibited sky-blue and greenish-yellow electroluminescence with similar current-voltage characteristics, repectively. Maximum current efficiency of (dfpmpy)₂Ir(ppc) and (dfpmpy)₂Ir(prz) were 4.4 and 7.4 cd/A, respectively. Maximum luminance values were approximately 10,000 cd/m² for the both compounds.     

Photoactive thin films of polycaprolactam doped with europium (III) complex using phenylalanine as ligand

A photoactive complex based on europium(III) using the amino acid phenylalanine as ligand was prepared and characterized. The obtained europium(III)/phenylalanine complex presents an effective energy transfer from ligands to the rare earth center. The observed photoluminescent behavior for europium(III)/phenylalanine complex was similar to the well known europium(III)/ acetyl-β-acetonate hydrate. New photoactive polyamide thin films were prepared using polycaprolactam as host of these complexes. The structural characterizations of the films were studied through Rutherford backscattering (RBS), Fourier transform infrared (FTIR) and Raman spectroscopies. The polyamide films doped with the amino acid and acetyl-β-acetonate rare earth complexes maintain the original photoluminescent behavior, narrow emission bands corresponding to transitions ⁵D₀ → ⁷F_0-4, which indicates that this polymer is an excellent host to these complexes.     

Improved electron injection of OLEDs with a thin PBD layer at Alq3/Cs2CO3 interface

In this study, the effect of one oxadiazole derivative (PBD) using as an electron injection layer (EIL) at Alq₃/Cs₂CO₃ interface has been investigated. The present of PBD EIL was showed an interesting enhanced electron injection for OLEDs although the nominal electron injection barrier for PBD based OLEDs is much larger, because PBD owns an obvious higher intrinsic the Lowest Unoccupied Molecular Orbital level (2.3 eV) than that of Alq₃ (3.0 eV). For example, the current density of OLEDs at 8 V was increased from 54 mA/cm² to 168 mA/cm² when inserting a thin PBD layer (5 nm) at Alq₃/Cs₂CO₃ interface. Here the increased current is suggested associating with the changed electronic structure of PBD when it contacts with Cs₂CO₃.     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

The roles of zinc selenide sandwiched between organic layers and its applications in white light-emitting diodes

In this paper, the roles of zinc selenide (ZnSe) sandwiched between organic layers, i.e. organic/ZnSe/aluminum quinoline (Alq₃), have been studied by varying device structure. A broad band emission was observed from ITO/poly(N-vinylcarbazole)(PVK)(80 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al under electric fields and it combined the emissions from the bulk of PVK, ZnSe and Alq₃, however, emission from only Alq₃ was observed from trilayer device ITO/N,N^′-bis-(1-naphthyl)-N,N^′-diphenyl-1, 1^′-biphenyl-4, 4^′-diamine (NPB) (40 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al. Consequently the luminescence mechanism in the ZnSe layer is suggested to be charge carrier injection and recombination. By thermal co-evaporating Alq₃ and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), we get white light emission with a Commission Internationale de l’E clairage (C.I.E) co-ordinates of (0.32, 0.38) from device ITO/PVK(80 nm)/ZnSe(120 nm)/ Alq₃:DCJTB(0.5 wt% DCJTB)(15 nm)/Al at 15 V and the device performs stably with increasing applied voltages.     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

Red organic light-emitting devices with dotted-line doped emitting layers

High efficiency red organic light-emitting devices (OLEDs) with several dotted-line doped layers (DLDLs) were fabricated by using an ultra-high vacuum organic molecular-beam deposition system. The red OLEDs consisted of indium-tin-oxide (ITO)/N, N′-diphenyl-N, N′-bis(1-naphthyl)-(1, 1′-biphenyl)-4, 4′-diamine (α-NPD): 40 nm/tris(8-hydroxyquinoline)aluminum (Alq₃)+4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljuloldyl-9-enyl)-4H-pyran (DCJTB); 3%wt.: x nm/(Alq₃+DCJTB; 3%wt./ Alq₃)_n−1: (30−x) nm/ Alq₃: 30 nm/Mg:Ag with n of 2, 4, 6, or 8, and x=30/(2n−1). The luminance yield of the device with 8 DLDLs was 75% higher than that of the device with a common doped layer. This was attributed to more formation of the excitons formed in a wider region resulting from the existence of the DLDLs. The dominant mechanisms of the dopant emission for the devices with DLDLs were described on the basis of the sequential carrier trapping process.     

Bright electrophosphorescent devices based on sterically hindered spacer-containing Cu(I) complex

A Cu(I) complex, [Cu(Dppp)(DPEphos)]BF₄ (Dppp=2,3-diphenyl-pyrazino[2,3-f][1,10]phenanthroline, DPEphos=Bis[2-(diphenylphosphino)phenyl]ether), is synthesized and used as the dopant in bright electrophosphorescent devices with the general structure ITO/m-MTDATA (30 nm)/NPB (20 nm)/CBP: ×wt% [Cu(Dppp)(DPEphos)]BF₄ (30 nm)/Bphen (20 nm)/Alq₃ (20 nm)/LiF (0.8 nm)/ Al (200 nm). These devices exhibit a maximum brightness of 4483 cd/m² and a peak efficiency of 3.4 cd/A. Compared with previously reported similar devices based on Cu(I) complexes, the brightness of the devices presented in this article is the best. Meanwhile, 2% [Cu(Dppp)(DPEphos)]BF₄-based devices exhibit white light-emitting properties with CIE coordinates of (0.32. 0.35) at 10 V.     

Study of photophysical properties AlBaq5

Pure and Ba²⁺ doped Alq₃ complexes were synthesized by simple precipitation method at room temperature, maintaining stoichiometric ratio. These complexes were characterized by XRD, UV–vis and FT-IR and photoluminescence (PL) spectra. XRD analysis reveals the polycrystalline nature of the synthesized complexes, while UV and FTIR confirm the molecular structure and the completion of quinoline ring formation and presence of quinoline structure in the metal complex. PL spectra of Alq₃ compared with barium doped complexes exhibit highest intensity in comparison to Alq₃ phosphor, which proves that barium enhances PL emission intensity of Alq₃ phosphor. The excitation spectra of the synthesized complexes are in the range of 300–480 nm with a broad peak in the range of 429–440 nm and shoulder at 380 nm, but with varying intensity. The emission wavelength lies in the range of 501–506 nm. Among all the synthesized complexes, AlBa₂q₅ exhibits maximum emission intensity. These remarkable properties of AlBaq₅ could be considered as promising materials as optoelectronic materials as well as green light emissive materials for OLEDs, PLLCD and solid state lighting applications.     

Synthesis, crystal structure and blue electroluminescence of a new zinc complex based on 2,6-bis(benzimidazolyl)pyridine

A very stable and blue luminescent complex Zn(bbp)Cl₂ (bbp: 2,6-bis(benzimidazolyl)pyridine) was synthesized. X-ray crystal structural analysis for the complex revealed that there are intermolecular π?π interactions in the solid state. The fluorescence properties for this complex were studied. The similar devices with the structure of [ITO/CuPc(31 nm)/NPB(80 nm)/[Zn(bbp)Cl₂] (or L) (85 nm)/LiF15/Al] were constructed to investigate their electroluminescent performance. Both the complex and the ligand can be fabricated as blue-emitting materials. The complex shows emission peak at 555 nm, electroluminescent efficiency 0.017 cd A⁻¹ and turn-on voltage 7 V, compared to 470 nm, 0.036 cd A⁻¹ and 9 V for the ligand.     

Efficient white organic light-emitting devices using 4,7-diphenyl-1,10-phenanthroline as block layer

A white light-emitting device has been fabricated with a structure of ITO/m-MTDATA (45 nm)/NPB (10 nm)/DPVBi (8 nm)/DPVBi:DCJTB 0.5% (15 nm)/BPhen (x nm)/Alq₃ [(55−x) nm]/LiF (1 nm)/Al, with x=0, 4, and 7. BPhen was used as the hole-blocking layer. This results in a mixture of lights from DPVBi molecules (blue-light) and DCJTB (yellow-light) molecules, producing white light emission. The chromaticity can be readily adjusted by only varying the thickness of the BPhen layer. The CIE coordinates of the device are largely insensitive to the driving voltages. When the thickness of BPhen is 7 nm, the device exhibits peak efficiency of 6.87 cd/A (3.59 lm/W) at the applied voltage of 6 V, the maximum external quantum efficiency η_ext=2.07% corresponding to 6.18 cd/A, and the maximum brightness is 18494 cd/m² at 15 V.     

Photoluminescence of 1,3-Diphenyl-1H-pyrazolo[3,4-b]quinoline and its derivatives: Experiment and quantum chemical simulations

The optical absorption and photoluminescent spectra are studied in recently synthesized diphenyl pyrazoloquinolines (DPPQ): 1,3-diphenyl-1H-pyrazolo[3,4-b] quinoline and its 6-vinyl, 6-N,N-diphenyl, 6-methyl, 6-fluoro, 6-bromo and 6-chloro derivatives. The photoemission spectra are recorded in organic solvents of different polarity and found to be highly solvatochromic. The measured spectra are compared with the quantum chemical calculations performed by means of the semiempirical methods (AM1 or PM3) in combination with the equilibrium molecular conformation (EMC) in vacuo (T = 0 K, Γ = 0.12 eV) or MD simulations (T = 300 K). The broadening of absorption and emission bands and their red-shift with increasing of temperature may be well reproduced by MD simulations. The Stokes shift of the photoluminescent spectra is obtained by including vibrational modes into the emission equation. The quantum chemical method AM1 in combination with MD simulations gives in most cases the best agreement with the experimental data. By comparing the emission spectra of 6-N,N-diphenyl-DPPQ with other DPPQ-derivatives one concludes that the molecular fragment diphenyl-amin [(C₆H₅)₂N-] is likely subjected to strong conformational changes in solvents. The large difference between the excited- and state-dipole moments indicates on a strong electron transfer effect being common for all DPPQ derivatives.     

Spectral studies of thin films based on poly(N-vinylcarzole) and red dopant

Organic light-emitting diodes were fabricated with a structure of indium-tin-oxide (ITO)/poly(N-vinylcarzole)(PVK):4-(dicyanom-ethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/8-tris-hydroxyquinoline aluminum (Alq₃)/lithium fluoride (LiF)/Al. The energy transfer from PVK to Alq₃ then to DCJTB and the charge trapping processes were investigated by employing the photoluminescence (PL) and electroluminescence (EL) spectra. With increasing thickness of the Alq₃ layer, the PL and EL emission from PVK were decreased gradually, which indicated that the effective energy transfer occurred from PVK to Alq₃ and then from Alq₃ to DCJTB. At the same time, we found that the exciton recombination zone could be adjusted by controlling the Alq₃ layer thickness and the applied voltages. The effects of different DCJTB concentrations on the optical and electrical characteristics of the devices were investigated, and an obvious red-shift was observed with the DCJTB dopant concentrations increasing in the PL and EL spectra.