Photoluminescence and electroluminescence by gallium(III) complexes of N-salicylidene-o-aminophenol and its derivatives

N-salicylidene-o-aminophenol and two of its derivatives bearing either an electron-donating methyl group or an electron-withdrawing Br group were used as ligands for the synthesis of three Ga(III) complexes. The complexes involved the participation of one acetylacetonate and one ethanol or methanol molecule. The geometry of the dianion of the Schiff base in the complexes is planar while photoluminescence data showed that photoluminescence intensity was extensively increased upon complex formation. Complexes emitted ligand-centered luminescence by ligand-centered excitation. Substituent groups broadly modified emission maximum. These substitutions can be exploited to tune light emission by the complex. All three complexes were used for the construction of electroluminescence devices and all emitted electroluminescence. Both photoluminescence and electroluminescence emission was enhanced in the case of substituted Schiff bases.     

Synthesis and electroluminescence properties of benzothiazole derivatives

Benzothiazole-based blue fluorescent materials N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (BPPA) and N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylnaphthalen-1-amine (BPNA) were synthesized for use in organic light-emitting diodes (OLEDs). Electroluminescent device with a configuration of ITO/NPB/BPPA/BCP/Alq₃/LiF/Al showed a maximum brightness of 3760 cd/m² at 14.4 V with the CIE coordinates of (0.16, 0.16). A current efficiency of 3.01 cd/A and an external quantum efficiency of 2.37% at 20 mA/cm² were obtained from this device. Molecules derived from BPPA and BPNA with incorporated dicyanomethylidene, which is a functional group for most red fluorescent molecules, were designed, synthesized and characterized to study the red fluorescence properties of the benzothiazole derivatives.     

Photoluminescence and electroluminescence from porous silicon

The properties and origins of the red, blue and infrared photoluminescence bands of porous silicon are reviewed and discussed in the light of the models that have been proposed to explain the experimental and theoretical results. The red band is due to quantum confinement possibly supplemented by surface states; the blue band is linked to the presence of silicon dioxide; the infrared band is correlated with dangling bonds and bandgap luminescence in large crystallites. The fabrication and characterization of light-emitting devices made of porous silicon are reported and discussed with respect to critical issues such as the device stability, efficiency, modulation speed, emission wavelength, and compatibility with microelectronic processing.     

Voltage-tunable photo- and electroluminescence of porous silicon

Experimental results showing two electrically induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence (QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5 V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination taking place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.     

Photoluminescence and electroluminescence studies of polyvinyl carbazole films

Polyvinyl carbazole (PVK) film has been prepared by solution method. Its absorption, photoluminescence (PL) and electroluminescence (EL) have been studied. For PL and absorption studies, film of PVK was prepared by spreading PVK solution on clean glass plate. The dried film was taken out of the glass plate and used for absorption, thickness measurement and PL studies. The film is transparent in the visible region and absorption starts at 340 nm wavelength. The absorption peaks are obtained at 280, 250 and 220 nm, indicating that optical gap of film is 3.65 eV and molecular orbitals exist at 4.43, 4.96 and 5.64 eV. PL studies reveal that excitation by violet light gives luminescence at 430, 480 and 690 nm. For EL studies, cell is prepared by depositing PVK film on a portion of conducting glass plate and taking aluminum foil as second electrode. It is observed from the characteristics that the current varies linearly, where as EL intensity varies non-linearly with increasing voltage. Higher brightness has been observed at higher frequencies. EL spectrum shows a sharp peak at 400 nm and a broad and less intense peak at 700 nm, which are attributed to radiative decay of singlet exciton and defect centers, respectively.     

Preparation,characterization and electroluminescence studies of copper doped zinc sulfide nanocrystals

Copper doped zinc sulfide nanoparticles were prepared by chemical precipitation method. The size of the particles was varied by changing the concentration of capping agent. The XRD studies indicate that most of the samples are cubic in nature. The broadening of peaks tends to increase with increasing capping agent concentration showing decrease in particle size. The crystalline size computed using Scherrer formula is found to be in range of 3–10 nm. Absorption spectra show absorption edge in UV region. The edge was found to shift towards shorter wavelength as the capping agent concentration is increased. This indicates increased effective band gap and hence reduced particle size. The nanoparticle size has been estimated in the range 5–10 nm using effective mass approximation model. For electroluminescence (EL) study of ZnS:Cu nanocrystals, the EL cells were prepared by placing ZnS:Cu nanoparticles between SnO₂ coated conducting glass plate and aluminum foil. Alternating voltage of various frequencies was applied and EL brightness (B) at different voltages (V) was measured and reported in this paper.     

Photo- and electroluminescence of sulfide and silicate phosphors embedded in synthetic opal

The sulfide (ZnS:Mn, Zn_xCd_1−xS:Mn, Zn_xCd_1−xS:Ag) and silicate (Zn₂SiO₄:Mn) phosphors were synthesized directly inside the pores of synthetic opal by chemical bath deposition. These composites are perfect three-dimensional photonic crystals, which produce effective photo- and electroluminescence at room temperature. The emission spectra are considerably modified by the photonic crystal structure to become anisotropic in accordance with the photonic band gap angular dispersion.     

Ag纳米颗粒对富Ag二氧化硅薄膜电致发光谱的影响

利用磁控溅射和热退火在硅衬底上制备了Ag纳米颗粒镶嵌的氧化硅薄膜(SiO2∶ Ag),制作了电致发光结构ITO/SiO2∶Ag/p-Si,观测到了可见区的电致发光.发现薄膜中的Ag纳米颗粒不仅成倍地提高器件的发光强度,还明显地移动电致发光的峰位.Ag含量越高,颗粒越大,发光峰位越红移.氧化硅中的发光中心与纳米Ag间的电...     

Novel green-emitting copper(I) complexes with electron donors incorporated ligands: Synthesis, photophysical properties, and electroluminescence performances

In this paper, we explore a diimine ligand of 2-thiazol-4-yl-1H-benzoimidazole (TB) with strong electron donors in its molecule. In addition, an excess electron-donor moiety of carbazole is incorporated through an inert alkyl chain to form another diimine ligand of 1-(4-carbazolylbutyl)-2-thiazol-4-yl-1H-benzoimidazole (CTB). Their corresponding Cu(I) complexes are synthesized as well. Experimental data confirm that both the Cu(I) complexes are high-energy-emitting ones. What is more, it is found that the emitter’s photoluminescent and thermal performances can be greatly improved when active hydrogen is eliminated. Using the CTB-based Cu(I) complex as a dopant, we realize green-emitting devices with a maximum brightness of 1500 cd/m² peaking at 525 nm, and the device efficiency roll-off in these devices is largely depressed due to the shield provided by carbazole moiety.     

A novel Re(I) complex for electroluminescence application: synthesis, theoretical studies, photophysical and electroluminescent properties

In this paper, we report a novel phosphorescent Re(I) complex of Re(CO)₃(ETCP)Br, where ETCP=1-ethyl-2-naphthalen -1-yl-1H-1,3,7,8-tetraaza-cyclopenta[l]phenanthrene, including its photophysical properties, geometric/electronic structures, electrochemical and thermal properties. Experimental data suggest that Re(CO)₃(ETCP)Br is a promising yellow emitter peaking at 540--nm with short excited state lifetime of ∼0.06 μs. Re(I) center localizes in a distorted octahedral field in Re(CO)₃(ETCP)Br and the emissive state of Re(CO)₃(ETCP)Br has metal-to-ligand-charge-transfer character, leading to the room-temperature phosphorescence. Further analysis reveals that Re(CO)₃(ETCP)Br has HOMO and LUMO energy levels at −6.03 V and −3.56 V, respectively, as well as its high thermal decomposition temperature of 377 °C. Using Re(CO)₃(ETCP)Br as a dopant, an electroluminescence peaking at 565-nm is realized, with a maximum luminance of 5900 cd/m² and a maximum current efficiency of 11.3 cd/A.     

Correlation of charge trapping and electroluminescence in highly efficient Si-based light emitters

In this paper we report on recent results on charge trapping and electroluminescence (EL) from Ge rich SiO₂ layers. Thermally grown 80 nm thick SiO₂ layers were implanted with Ge ions at energies of 30–50 keV to peak concentrations of 1–6 at%. Subsequently rapid thermal annealing was performed at 1000°C for 6, 30 and 150 s under a nitrogen atmosphere in order to form luminescence centers. A combination of capacitance–voltage (CV) and current–voltage (IV) methods was used for the investigation of the trapping properties. It was found that at electric fields <8 MV/cm electron trapping dominates while at higher electric fields which are typically required for the EL operation of the devices positive charge trapping occurs. It is assumed, that the trapping sites which are responsible for the trapping of the positive charge are in strong relation to the defects causing the luminescence.     

Photoluminescence and electroluminescence investigations at Ge-rich SiO2 layers

Strong blue and violet photo (PL) and electroluminescence (EL) at room temperature was obtained from SiO₂-films grown on crystalline Si, which were either single (SI) or double implanted (DI) with Ge ions and annealed at different temperatures. The PL spectra of Ge-rich layers reach a maximum after annealing at 500–700°C for DI layers or 900–1000°C for SI layers, respectively. Both, PL and EL of 500 nm thick Ge-rich layers are easily visible by the naked eye at ambient light due to their high intensity. Based on excitation spectra we tentatively interpret the blue PL as due to the oxygen vacancy in silicon dioxide.

The EL spectrum of the Ge-implanted oxide correlates very well with the PL one and shows a linear dependence on the injected current over three orders of magnitude. For DI layers much higher injection currents than for SI layers can be achieved. An EL efficiency in the order of 10⁻⁴ for Ge⁺-implanted silicon dioxide was determined.     

Calculation of force constants and interpretation of vibrational spectra of cyclopentadiene and its deutero derivatives

The force constants, influence coefficients, frequencies, and forms of the vibrations are calculated and an interpretation is given for the vibrational spectra of cyclopentadiene and its deutero derivatives.     

Synthesis, structure and luminescence properties of zinc (II) complexes with terpyridine derivatives as ligands

Five zinc (II) complexes (1-5) with 4′-phenyl-2,2′:6′,2″-terpyridine (ptpy) derivatives as ligands have been synthesized and fully characterized. The para-position of phenyl in ptpy is substituted by the group (R), i.e. tert-butyl (t-Bu), hexyloxy (OHex), carbazole-9-yl (Cz), naphthalen-1-yl-phenyl-amine-N-yl (NPA) and diphenyl amine-N-yl (DPA), with different electron-donating ability. With increasing donor ability of the R, the emission color of the complexes in film was modulated from violet (392 nm) to reddish orange (604 nm). The photoexcited luminescence exhibits significant solvatochromism because the emission of the complexes involves the intra-ligand charge transfer (ILCT) excited state. The electrochemical investigations show that the complexes with stronger electro-donating substituent have lower oxidation potential and then higher HOMO level. The electroluminescence (EL) properties of these zinc (II) complexes were studied with the device structure of ITO/PEDOT/Zn (II) complex: PBD:PMMA/BCP/AlQ/LiF/Al. Complexes 3, 4 and 5 exhibit EL wavelength at 552, 600 and 609 nm with maximum current efficiency of 5.28, 2.83 and 2.00 cd/A, respectively.     

Two peaks observed in the electroluminescence spectra of Alq3-based OLEDs

Tris(8-hydroxyquinolato) aluminum (Alq₃)-based organic light-emitting diodes were fabricated with or without using a hole transport layer (HTL). As a conventional device, the ITO/Alq₃/Mg-Ag device yielded a green-light emission with a single peak at 525 nm in the electroluminescence (EL) spectrum. In contrast, two sub-peaks were observed in the EL spectrum of some ITO/HTL/Alq₃/Mg-Ag devices. This difference was tentatively explained by comparing EL with the photoluminescence (PL) spectrum reported in the literature.     

Development of electroluminescence cell using a Eu0.5Y0.5(TTA)3Phen organic luminescent complex

Novel red emitting organic luminescent complexes, namely Eu_0.5Ln_0.5(TTA)₃ Phen (Eu: europium, Ln: Y/Tb, Y: yttrium, Tb: terbium, TTA: thenoyl tri fluoro acetone, Phen: phenanthroline) were synthesized by solution technique, maintaining stoichiometric ratio. These complexes were characterized by various techniques such as XRD, optical absorption and photoluminescence (PL) spectra. Electroluminescence cells were designed by sandwiching Eu_0.5Ln_0.5(TTA)₃Phen between indium tin oxide (ITO) and aluminum (Al). Voltage?current characteristics and voltage?brightness characteristics of the developed electroluminescent cell were carried out. Turn on voltage of both the devices was found to be 9 V. These devices emit intense red emission at 611 nm, proving their potential applications as organic light emitting diodes and displays.     

Surface electroluminescence phenomena correlated with trapping parameters of insulating polymers

Electroluminescence (EL) phenomena are closely linked to the space charge and degradation in insulating polymers, and dominated by the luminescence and trap centers. EL emission has been promising in defining the onset of electrical aging and in the investigation of dissipation mechanisms. Generally, polymeric degradation reveals the increment of the density of luminescence and trap centers, so a fundamental study is proposed to correlate the EL emission of insulating polymers and their trapping parameters. A sensitive photon counting system is constructed to detect the weak EL. The time- and phase-resolved EL characteristics from different polymers (LDPE, PP and PTFE) are investigated with a planar electrode configuration under stepped ac voltage in vacuum. In succession, each sample is charged with exposing to multi-needle corona discharge, and then its surface potential decay is continuously recorded at a constant temperature. Based on the isothermal relaxation current theory, the energy level and density of both electron and hole trap distribution in the surface layer of each polymer is obtained. It is preliminarily concluded that EL phenomena are strongly affected by the trap properties, and for different polymers, its EL intensity is in direct contrast to its surface trap density, and this can be qualitatively explained by the trapping and detrapping sequence of charge carriers in trap centers with different energy level.     

Photo and electroluminescence properties in Zns: (Cu,La) and Zns: (Ag,La) phosphors

A method for preparation of ZnS: (Cu, La) and ZnS: (Ag, La) phosphors is described. Photo and electroluminescence of these phosphors have been studied. The voltage and frequency variation of EL brightness have also been reported. The results agree with the collision-excitation mechanism in the Schottky barrier. The emission of blue, green and yellow bands has been interpreted in terms of different electronic transitions. Simultaneous action of both the field and the 3650 Å radiation has been studied. An attempt has also been made to explain the quenching and enhancement.     

The insert of zinc oxide thin film in indium tin oxide anode for organic electroluminescence devices

Zinc oxide films were prepared by rf magnetron sputtering on glass substrates with designed ZnO target using high-purity of zinc oxide (99.99%) powder. Systematic study on dependence of target-to-substrate distance (D_ts) on structural, electrical and optical properties of the as-grown ZnO films was mainly investigated in this work. XRD showed that highly preferred ZnO crystal in the [0 0 1] direction was grown in parallel to the substrate, while the D_ts did not effect to the peak position of XRD. With decreasing D_ts, the growth rate is increased while the electrical resistivity as well as crystal size in the ZnO films was decreased. The XPS data showed that the O/Zn ratio in ZnO films was increased with increasing D_ts in the films. The as-grown ZnO films have an average transmittance of above 85% at the visible region. The optical band gap of the as-grown ZnO films was changed from 3.18 to 3.36 eV with D_ts. With decreasing D_ts, the electrical resistivity was decreased, while the growth rate was increased.A bilayer is used as an anode electrode for organic electroluminescent devices. The bilayer consists of an ultrathin ZnO layer adjacent to a hole-transporting layer and an Indium tin oxide (ITO) outerlayer. We tried to bring low the barrier between the devices as deposited ZnO films on ITO substrates. We fabricated the organic EL structure consisted of Al as a cathode, Al₂O₃ as an electro transport layer, Alq₃ as a luminously layer, TPD as a hole transport layer and ZnO (1 nm)/ITO (150 nm) as an anode. The result of this experiment was not good compared with the case of using ITO, nevertheless, at this structure we obtained the lowest turn-on voltage as the value of 19 V and the good brightness (6200 cd/m²) of the emission light from the devices. Then the quantum efficiency was to be 1.0%.     

Photoluminescence and electroluminescence of a novel green-yellow emitting material-5,6-Bis-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-pyrazine-2,3-dicarbonitrile

A new compound with intramolecular charge transfer (ICT) property—5,6-Bis-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-pyrazine-2,3-dicarbonitrile(BNPPDC) was synthesized. The new compound was strongly fluorescent in non-polar and moderately polar solvents, as well as in thin solid film. The absorption and emission maxima shifted to longer wavelength with increasing solvent polarity. The fluorescence quantum yield also increased with increasing solvent polarity from non-polar to moderately polar solvents, then decreased with further increase of solvent polarity. This indicates both “positive” and “negative” solvatokinetic effects co-existed. Using this material as hole-transporting emitter and host emitter, we fabricated two electroluminescent (EL) devices with structures of A (ITO/BNPPDC (45 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (45 nm)/Mg:Ag (200 nm) and B (ITO/N,N′-diphenyl-N,N′-bis-(3-methylphenyl) (1,1′-diphenyl)4,4′-diamine (TPD) (50 nm)/BNPPDC (20 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (45 nm)/Mg:Ag (200 nm). The devices showed green-yellow EL emission with good efficiency and high brightness. For example, the device A exhibited a high brightness of 17400 cd/m² at a driving voltage of 11 V and a very low turn-on voltage (2.9 V), as well as a maximum luminous efficiency 3.61 cd/A. The device B showed a similar performance with a high brightness of 12650 cd/m² at a driving voltage of 13 V and a maximum luminous efficiency 3.62 cd/A. In addition, the EL devices using BNPPDC as a host and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant (configuration: ITO/TPD (60 nm)/BNPPDC:DCJTB (2%) (30 nm)/TPBI (35 nm)/Mg:Ag (200 nm)) showed a good performance with a brightness of 150 cd/m² at 4.5 V, a maximum brightness of 12600 cd/m² at 11.5 V, and a maximum luminous efficiency of 3.30 cd/A.