Substituent Dependent Optical Properties of <Emphasis Type="Italic">p</Emphasis>-phenyl Substituted ethenyl-<Emphasis Type="Italic">E</Emphasis>-thiophenes

Various electron donor and acceptor substituted (NO₂, CN, Cl, H, OCH₃, NH₂) p-phenyl ethenyl-E- thiophenes (1–6) were synthesized and substituent dependent optical properties (dipole moment, transition dipole moment, oscillator strength, optical band gap, hyperpolarizability) were studied using Solvatochromism and Density functional theory. It is shown that thiophene acts as a weak electron donor in presence of an electron withdrawing p-phenyl substituent (NO₂, CN, Cl), whereas thiophene acts as a weak electron acceptor in presence of an electron donating p-phenyl substituent (OCH₃, NH₂). In comparison to ethenyl thiophene 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing capacity of p-phenyl substituent. From the excited state dipole moment calculation, it is shown that the excited state is highly dipolar for nitro and amino compounds 1 and 6, whereas compounds 2–5 show a non-polar excited state. As compared to the ethenyl thiophene 4, the first hyperpolarizability (β) increases upon substitution either with a strong electron withdrawing or strong electron donating p-phenyl substituent. A large β value is found for p-nitro phenyl ethenyl-E-thiophene and p-amino phenyl ethenyl-E- thiophene. Overall, these studies provide useful information in understanding the optical properties of phenyl and heterocyclic based ethenyl systems.     

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.     

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.     

The influence of synthesis conditions on the stability of tris(8-hydroxyquinoline) aluminum organometallic luminophore

Single-phase crystalline luminophore tris(8-hydroxyquinoline) aluminum (Alq₃) has been synthesized at Т = 483 K and a partial pressure of 8-hydroxyquinoline vapor from 0.15 to 6.12 Torr. The influence of P _8-Hq on the luminescent characteristics of crystalline Alq₃ samples has been studied. It has been found that an increase in P _8-Hq led to a shift of the photoluminescence-band maximum and to a change in the photoluminescence-decay kinetics. It has been shown that Alq₃ synthesized at Т = 483 K and P _8-Hq = 6.12 Torr had the most stable spectral-luminescent characteristics. The results obtained are discussed taking into account defect formation in crystalline Alq₃.     

Investigation of charge carrier mobility in 5,6,11,12-tetraphenylnapthacene (rubrene) and coumarin 6 doped Alq3 films

The doping effect on charge carrier mobility in tris (8-hydroxyquinolinato) aluminum (Alq₃) was studied by time-of-flight (TOF) measurements. The polar dopant, coumarin 6 (C-6) and extensive π conjugated dopant, 5,6,11,12-tetraphenylnaphthacene (rubrene) were used for this study. The co-doped of rubrene (Rb) with C-6 into Alq₃ improved the carrier mobility compared to the single doped Alq₃:C-6 film. The carrier mobility in single doped Alq₃:C-6 film did not follow the linear relationship of Poole-Frenkel (PF) model with applied electric field. The mobility was in agreement with the PF model at two different ranges of electric fields (F) separated by a critical field . The mobility in co-doped Alq₃:(Rb:C-6) film followed the linear relationship with the PF model. The energetic disorder was found as ∼0.32 eV in co-doped films. It was ∼0.55 and ∼0.27 eV before and after the critical field in Alq₃:C-6 film. The values of positional disorders in co-doped films were estimated as ∼1.8 and it was ∼2 in Alq₃:C-6 film at . The organic light emitting diode performance of the co-doped film was improved compared to single doped film. The luminescence efficiency was improved tremendously to ∼6  Cd/A in co-doped device at 45 mA/cm² current compared to Alq₃:C-6 film device of ∼1  Cd/A.     

Effect of polar dopant on energetic and positional disorders in tris(8-hydroxyquinolinato) aluminum (Alq3)

The influence of polar dopant on the charge carrier transport in amorphous tris (8-hydroxyquinolinato) aluminum (Alq₃) was studied by time-of-flight measurement. The 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was doped into Alq₃ with various concentration from 0.5 to 24 wt. %. The electron mobility was reduced by about one order by DCM doping in Alq₃. The electric-field dependence electron mobility in Alq₃:DCM films separated into two discrete regions of critical fields E_c ^1/2. The value of E_c ^1/2ranged from 360 to 405 (V/cm)^1/2 depending on the DCM concentration in Alq₃ films. The energetic disorder in Alq₃:DCM films increased from 0.01 eV to 0.09 eV with DCM doping concentration. The positional disorder in Alq₃:DCM films also increased from 0.3 to 6.5 with DCM doping concentration up to 24 wt. %. These results indicated the strong Coulombic and dipole–dipole interactions between DCM and Alq₃ molecules. The interactions between randomly located DCM molecules, Alq₃ dipoles and oriented dipoles are the major caused of positional disorder. PACS 73.50.-h; 73.61.Ph; 71.20.Rv     

The influence of exciton behavior on luminescent characteristics of organic light-emitting diodes

We used N,N′-bis-(1-naphthyl)-N,N′-1-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,4′-N,N′-dicarbazole-biphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq₃) to fabricate tri-layer electroluminescent (EL) device (device structure: ITO/NPB/CBP/Alq₃/Al). In photoluminescence (PL) spectra of this device, the emission from NPB shifted to shorter wavelength accompanying with the decrease of its emission intensity and moreover the emission intensity of Alq₃ increased relatively with the increase of reverse bias voltage. The blue-shifted emission and the decrease in emission intensity of NPB were attributed to the polarization and dissociation of NPB excitons under reverse bias voltage. The increase of emission intensity of Alq₃ benefited from the recombination of electrons (produced by the dissociation of NPB exciton) and holes (injected from the Al cathode).     

Bipolar Alq3-based complexes: Effect of hole-transporting substituent on the properties of Alq3-center

Two bipolar Alq₃-based complexes, tris{5-[(carbazole-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(CzHQ)₃) and tris{5-[(phenothiazine-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(PHQ)₃), involving an Alq₃-center and three hole-transporting substituents (carbazole or phenothiazine), were prepared and characterized. Effects of hole-transporting substituent on the properties of Alq₃-center were investigated in detail. It is found that the two complexes have improved hole-transporting performance and appropriate thermal stability (the 5%-weight-loss temperatures T_5%>260 ^oC). Photoluminescence (PL) spectra indicate that both energy transfer and electron transfer can take place simultaneously in the PL process of these complexes. Both thermodynamics and dynamics of the electron transfer were studied and corresponding parameters were calculated. Energy transfer is favorable for the PL of Alq₃-center, while electron transfer is unfavorable for the PL of Alq₃-center. These results will be useful to explore novel OLEDs material with increased efficiency.     

Mixed-ligand Al complex—a new approach for more high efficient OLEDs

The mixed-ligand Aluminum bis(8-hydroxyquinoline) acetylacetonate (Alq₂Acac) complex was presented and its performance as electroluminescent and electron transporting layer for OLED was studied. The photophysical properties of the novel complex were investigated and compared with the properties of the parent Alq₃. Highly efficient OLED based on the mixed-ligand Al complex was developed with two times higher luminescence and efficiency compared to the identical OLED based on the conventional Alq₃ The better performance of the devices make the novel Al complex a very promising material for OLEDs.     

Effect of Alkyl Side Chain Length on Relaxation Behaviors in Poly(n-alkyl Acrylates) and Poly(n-alkyl Methacrylates)

Dynamic mechanical analysis (DMA) is used to investigate the effect of alkyl side chain length on the relaxation behavior of poly(n-alkyl acrylates) (PnAA) and poly(n-alkyl methacrylates) (PnAMA) above the glass transition temperature (T_g). Master curves and shift factors (log a_T) were obtained using the time–temperature superposition (TTS) principle. The log a_T curves of PnAA and PnAMA exhibit a dynamic crossover from one Vogel–Fulcher–Tammann–Hesse (VFTH) equation to another above T_g. The corresponding temperature was designated as the dynamic crossover temperature (T_c). It is found that T_c/T_g and the apparent activation energy (E_g) increases, e whereas the fragility index (m) decreases with increasing alkyl side chain length. Further analysis shows that m ∝ T_g, E_g∝ , and E_g∝ m² for both PnAA and PnAMA.     

Bond cutting in K‐doped tris(8‐hydroxyquinoline) aluminium

A series of Al 2p, K 2p, O 1s and N 1s core‐level spectra have been used to characterize the interaction between potassium (K) and tris(8‐hydroxyquinoline) aluminium (Alq₃) molecules in the K‐doped Alq₃ layer. All core‐level spectra were tuned to be very surface sensitive in selecting various photon energies provided by the wide‐range beamline at the National Synchrotron Radiation Research Center, Taiwan. A critical K concentration (x = 2.4) exists in the K‐doped Alq₃ layer, below which the K‐doped atoms generate a strained environment near the O and N atoms within 8‐quinolinoline ligands. This creates new O 1s and N 1s components on the lower binding‐energy side. Above the critical K coverage, the K‐doped atoms attach the O atoms in the Al—O—C bonds next to the phenoxide ring and replace Al—O—C bonds by forming K—O—C bonds. An Alq₃ molecule is disassembled into Alq₂ and Kq by bond cutting and bond formation. The Alq₂ molecule can be further dissociated into Alq, or even Al, through subsequent formations of Kq.     

Bond cutting in Cs‐doped tris(8‐hydroxyquinoline) aluminium

A series of Cs 4d and Al 2p spectra associated with valence‐band and cut‐off spectra have been used to characterize the interaction between caesium and tris(8‐hydroxyquinoline) aluminium (Alq₃) molecules in a Cs‐doped Alq₃ layer. The Cs 4d and Al 2p spectra were tuned to be very surface sensitive by selecting a photon energy of 120 eV at the National Synchrotron Radiation Research Center, Taiwan. A critical Cs concentration exists, above which a new Al 2p signal appears next to the Al 2p peak of Alq₃ in the lower binding‐energy side. The Al 2p signal was analyzed and assigned as being contributed from a mixture of Alq₂, Alq and Al. Experimental data supported the observation that bond cutting of Alq₃ by the doped Cs atoms occurred at high Cs doping concentration.     

EPR study of Cu(II) dopant ions in single crystals of bis(l-asparaginato)Zn(II)

We report an electron paramagnetic resonance (EPR) study at 33.9 GHz and room temperature of oriented single crystal samples of bis(l-asparaginato)Zn(II) doped with Cu(II). The variation of the spectra with magnetic field orientation was measured in three crystal planes (a*b, bc and a*c, with a*=b×c). These spectra display two groups of four peaks arising from the hyperfine interaction with the I_Cu=3/2 nuclear spins of copper. They were assigned to Cu(II) ions in two lattice sites related by a 180° rotation around the b-crystal axis. The g and hyperfine coupling (A) tensors of the Cu(II) ions were evaluated from the single crystal data. Some indeterminacy in the assignment of the signals was avoided measuring the EPR spectrum of a powder sample. Their principal values are g₁=2.060(1), g₂=2.068(2), g₃=2.283(2), and A₁≈0.1×10⁻⁴, A₂=13×10⁻⁴ and A₃=165×10⁻⁴ cm⁻¹. The eigenvectors corresponding to g₃ and A₃ are coincident within the experimental error; the other eigenvectors are rotated 5.6° in the perpendicular plane. Considering the crystal structure of bis(l-asparaginato)Zn(II), our EPR results indicate that the Cu(II) impurities replace Zn(II) ions in the host crystal. We propose a molecular model based on the EPR data and the structural information, and analyse the results comparing the measured values with those obtained in similar systems.     

Effect of desiccant on the performance of green organic light-emitting device

A green organic light-emitting diodes (OLED) with a multilayer structure of indium-tin oxide (ITO)/copper-phthalocyanine (CuPc) (200Å)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) (600Å)/N′- diphenyl-N,N′-tris(8-hydroxyquinoline) aluminium (Alq₃) (400Å):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7- tetrahydro-1H,5H,11H-(l)benzopyropyrano(6,7,8-i, j)quinolizin-11-one (C545T) (2%)/Alq₃ (200Å)/LiF (10Å)/Al (1000Å) was prepared via vacuum thermal evaporation. To reduce the impact of water vapor and oxygen on the device, we encapsulated it with a kind of specific and efficient desiccant, called DESIPASTE, under the protection of high-purity nitrogen. By analyzing a series of optical characteristics of OLEDs, the results showed that this desiccant can improve the brightness about 500 and 250 cd/m² at same driving voltage and current density, respectively. The electroluminescent (EL) spectra were hardly affected except a very weak blue shift of broadband emission peak. It turns out that encapsulation with DESIPASTE is a simple and efficient way to improve the performance of OLED.     

Influence of the deposition temperature on the structure and performance of tris(8-hydroxyquinoline) aluminum based flexible organic light-emitting devices

Based on indium tin oxide (ITO)/N,N′diphenyl-N-N′-di(m-tdyl) benzidine (TPD)/Alq₃/Al structure, flexible OLEDs on polyethylene terephthalate (PET) substrates were fabricated by physical vapor deposition (PVD) method. Tris(8-hydroxyquinoline)aluminum (Alq₃) films were deposited at 90, 120 and 150 °C to examine the influence of the deposition temperature on the structure and performance of OLEDs. Electroluminescence (EL) spectra and current-voltage-luminance (I-V-L) characteristics of the OLEDs were examined. It was found that the device fabricated at a high temperature had a higher external efficiency and longer lifetime. Atomic force microscope (AFM) was adopted to characterize the surface morphology of ITO/TPD/Alq₃. The higher uniform morphology of the Alq₃ formed at high temperature might contribute to the performance improvement of the OLEDs.     

Relaxor behavior of (Sr,Ba,Bi)TiO3 ferroelectric ceramic

The dielectric behavior of (Sr_0.4Ba_0.6)_0.925Bi_0.05TiO₃ (SBBT) ceramic was investigated in the temperature range from 100 K to 450 K. Broad dielectric maxima, which shift to higher temperature with increasing frequency, and the value of the relaxation parameter γ=1.6-2 estimated from the linear fit of the modified Curie-Weiss law, indicated the relaxor nature of the SBBT ceramic. The dielectric relaxation which follows the Vogel-Fulcher relationship with  K, E_a=0.0392 eV, and ν_o=2.98×10¹¹ Hz, further supports such a relaxor nature. The P-E hysteresis loop at different temperatures and ‘butterfly’ shape dc bias field dependence of permittivity at T>T_m (the temperature of permittivity maximum) signifies the occurrence of nanopolar clusters, which is the typical characteristic of relaxor ferroelectrics. At 300 K and 10 kHz, the dielectric constant and loss tgδ are ∼2210 and 0.00118, respectively. The tunability (28%) and figure of merit (237) at room temperature show that the SBBT ceramic could be a promising candidate for tunable capacitor applications.     

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.     

Correlation of the piezoelectric and dielectric properties of soft ferroelectric ceramics

The dependences of the piezoelectric (g ₃₃) and electrostriction (Q ₃₃) coefficients of nonpolarized soft relaxor ferroelectric ceramics of the PbMg_1/3Nb_2/3O₃-PbTiO₃ system on electric field E ₃ are calculated and interpreted. The nonmonotonic g ₃₃(E ₃) dependence and negative values of Q ₃₃ were revealed in strong electric fields. These features are shown to be the manifestation of correlation between the piezoelectric and dielectric properties.     

利用空间电荷限制电流方法确定三(8-羟基喹啉)铝的电子迁移率特性初步研究

材料的迁移率是其关键电学特性之一.有机材料迁移率的研究对于有机电致发光器件、 有机太阳电池、有机薄膜场效应晶体管性能的提高有重要的意义. 应用简单易行的空间电荷限制电流方法,对基于三(8-羟基喹啉)铝(Alq₃) 的四种单载流子器件电流密度-电压曲线特性进行研究, 根据空间电荷限制电流模型,拟合出Alq₃材料在四种器件中的零场电子迁移率和电场依赖因子,并且给出Alq₃电子迁移率随外加偏压的变化趋势. 实验结果表明,顶电极铝蒸镀到缓冲层氟化锂(1 nm)和Alq₃ (100 nm)的表面后, 可以明显改善Alq₃的零场迁移率和电场依赖因子. 认为产生这种现象的原因是氟化锂可以使铝和Alq₃发生络合反应, 形成Li⁺¹Alq^-1粒子,形成良好的欧姆接触,使得电子的注入效率大大提高.