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.     

Transient photoluminescent response with three decay lifetimes of phosphorescent organic molecule Ir(ppy)3 doped in polycarbonate

A calculation has been done to explain the temperature dependence of photoluminescence (PL) lifetime of fac tris(2-phenylpyridine) iridium (Ir(ppy)₃) observed in polycarbonate (PC) at 8–295 K. Taking into account (1) three zero-field splitting substates in the lowest-energy triplet state and (2) one-phonon non-radiative transitions among these substates, the rate equations have been obtained for the populations of these substates. We derived the PL time response with three PL lifetimes, which increase with decreasing temperature from 300 K to 1 K. A good agreement has been obtained between the calculated PL lifetimes and the observed ones.     

Impedance spectroscopy and relaxation phenomena of (Na,K) excess Na0.5K0.5NbO3 thin films grown by chemical solution deposition

Na_0.5K_0.5NbO₃ (NKN) and 10 mol% (Na,K) excess Na_0.5K_0.5NbO₃ (NKN10) thin films on Pt/Ti/SiO₂/Si substrate were prepared by chemical solution deposition. Crystallization of NKN10 thin films was confirmed by X-ray diffraction. The (Na,K) excess Na_0.5K_0.5NbO₃ thin film shows a ferroelectric P-E hysteresis loop. Dielectric properties and impedance spectroscopy of thin films were investigated in the frequency range from 0.1 Hz to 100 kHz and the temperature range of 25 ~ 500 °C. By analyzing the complex impedance relaxation with Cole-Cole plots, we found impedance relaxations for the thin film. The contribution of electrical conduction is discussed in relation to grain, grain boundary, and interface effects.     

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.     

Comparative study of Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current-voltage and spectral response measurements

Co-evaporated Cu(In,Ga)Se₂ (CIGSe) based solar cells with Physical Vapour Deposited (PVD) Indium Sulphide (In₂S₃) as buffer layer have been studied by admittance spectroscopy and current-voltage characteristics measurements. The results have been compared to those obtained with a reference CBD-CdS/CIGSe device. In darkness, the PVD-In₂S₃ buffer layer devices exhibit higher densities of trapping defects and low values of shunt resistance. However, under illumination we have observed an important improvement of the In₂S₃/CIGSe electronic transport properties. This behavior seems to be linked to the presence of a metastable defect with activation energy of 0.3 eV.     

Growth and characterization of evaporated thin films of Y1Ba2Cu3Ox

We have prepared thin films of Y-Ba-Cu-O by coevaporation in the presence of oxygen onto room-temperature substrates, followed by postannealing in oxygen. Films have been grown on (100) SrTiO₃, (100) MgO, and LiNbO₃. The films have been characterized by electrical resistivity, X-ray diffraction, and elastic backscattering (EBS) measurements.     

Complex electrical (impedance/dielectric) properties of electroceramic thin films by impedance spectroscopy with interdigital electrodes

The complex electrical properties of isotropic, electroceramic thin films can be measured with interdigital electrodes, analyzed by impedance spectroscopy (IS). A periodic two-dimensional film/interdigital electrode (IDE) structure was simulated by finite-difference numerical method and a generalized model was developed to characterize the electrical properties of thin films. Variable frequency simulations showed that the film/IDE system can be modeled as a parallel resistor-capacitor equivalent circuit. Equations were developed to extract from the equivalent circuit's fitted resistance and capacitance, the materials properties of the thin film, both conductivity and permittivity. The electrical properties of a polydomain BaTiO₃ film grown on a MgO substrate were measured with an IDE structure by IS to demonstrate how the methodology can be readily used.     

Electric response as a function of applied voltage of Nb-doped Bi4Ti3O12 thin films

We have investigated the electrical properties of sol-gel deposited Nb-doped Bi₄Ti₃O₁₂ (NBIT) ferroelectric thin films. The obtained values of remanent polarization (2P_r) and coercive voltage (V_c) were 7 μC/cm² and 2.5 V of NBIT thin film, respectively. From complex dielectric spectra, we observed the dielectric response consisting of two regions for measuring frequency; the low frequency region may be due to diffusion charge transport caused by impurities, while the dielectric relaxation mechanism of high frequency region seems to be the modified Debye type. A model was proposed to account for the observed phenomena, which fits very well to the dielectric dispersion relation: . The occurrence of an anomaly in n, σ, τ, and ε_S − ε_∞ parameters near V_c indicates a coupling between the charge carriers and ferroelectricity.     

Influence of Se on the electron mobility in thermal evaporated Bi2(Te1−xSex)3 thin films

Thermoelectric solid solutions of Bi₂ (Te_1−xSe_x)₃ with x = 0, 0.2, 0.4, 0.6, 0.8 and 1 were grown using the Bridgman technique. Thin films of these materials of different compositions were prepared by conventional thermal evaporation of the prepared bulk materials. The temperature dependence of the electrical conductivity σ, free carriers concentration n, mobility μ_H, and seebeck coefficient S, of the as-deposited and films annealed at different temperatures, have been studied at temperature ranging from 300 to 500 K. The temperature dependence of σ revealed an intrinsic conduction mechanism above 400 K, while for temperatures less than 400 K an extrinsic conduction is dominant.The activation energy, ΔE, and the energy gap, E_g, were found to increase with increasing Se content. The variation of S with temperature revealed that the samples with different compositions x are degenerate semiconductors with n-type conduction. Both, the annealing and composition effects on Hall constant, R_H, density of electron carriers, n, Hall mobility, μ_H, and the effective mass, m^∗/m₀ are studied in the above temperature range.     

Time-resolved photoluminescence in Cu(In,Ga)Se2 thin films and solar cells

Room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on Cu(In,Ga)Se₂ (CIGS) thin films and solar cells to clarify the recombination process of the photo-generated minority carrier. Both films and solar cells exhibited PL decay curves composed of the dominant fast (0.7-2 ns) and weak slow (3-10 ns) exponential decay curves. PL lifetime of the cell is longer than that of the thin films, indicating the longer minority carrier lifetime for the hetero-structures than in thin films. The increase of PL lifetime is consistent with the enhancement of the PL intensity and the elimination of defect-related PL as a result of the solar cell formation. These results are discussed in terms of the recombination process of carriers in films and hetero-structures. The relationship between the PL lifetime of the CIGS solar cells and the cell conversion efficiency is described.     

White polymer light-emitting diodes based on poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) doped with a poly(p-phenylene vinylene) derivative

Efficient white polymer light-emitting diodes based on the polymer blend of poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) doped with poly{2-[3′,5′-bis(2?-ethylhexyloxy) benzyloxy]-1,4-phenylenevinylene}-co-poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) were fabricated. The electroluminescence (EL) spectrum is easily controlled by changing the dopant concentration. A white light emission was realized on the device with the dopant concentration of 0.194‰ and the emission light is less sensitive to the applied voltage in a wide voltage range. The maximum luminance and the maximum EL efficiency of the single-layer device were 2330 cd/m² and 0.29 cd/A, respectively. By introducing an Alq₃ layer as an electron transporting and hole blocking layer, the overall performance of the double layer device was dramatically improved, the maximum luminance and the maximum EL efficiency reached 3300 cd/m² and 2.37 cd/A, respectively.     

Sc Addition to (Bi,Pb)2Sr2Ca2Cu3Ox Freeze-Dried Ceramic

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc₂O₃ was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis, X-ray diffraction (XRD), Scanning electron microscopy (SEM), and (T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.     

Phase transition behaviors and thermal conductivity measurements of nitrogen-doped Sb2Te3 thin films

Crystallization temperature of nitrogen-doped Sb₂Te₃ (ST) thin films increased with increasing nitrogen doping concentration, which indicates that the long-term stability of the metastable amorphous state can be improved by nitrogen doping. The root-mean-square (rms) roughness values of the films showed a significant decrease with nitrogen doping. Thermal conductivity of nitrogen-doped ST thin films was measured using a transient thermoreflectance (TTR) technique. It was found that the thermal conductivity decreased with increasing nitrogen doping concentration and increased with increasing annealing temperature. Nitrogen-doped ST thin films are suitable phase-change materials for low programming power consumption applications of phase-change random access memory (PCRAM).     

Study of the growth of CuAlS2 thin films on oriented silicon (111)

Within the chalcopyrite family the sulphur based compounds CuMS₂ (M = In, Ga, Al) have attracted much interest in recent years because they show a direct wide band-gap covering from E_gap = 1.53 eV (CuInS₂) over E_gap = 2.43 eV (CuGaS₂) to E_gap = 3.49 eV (CuAlS₂). Therefore they are particularly suitable for optoelectronic as well as photovoltaic applications. The CuAlS₂ semiconductor is one of these compounds and has good luminescent properties and a wide direct gap of 3.5 eV making it suitable for the use as material for light-emitting devices in the blue region of the spectrum. To dig up fully its potential a better understanding of the fundamental properties of the CuAlS₂ film itself is essential, which could be achieved from high-quality single-crystalline materials. So, the aim of this work has been to study the growth of multilayer CuAlS₂ thin films on Si(111) substrates at a substrate temperature of 723 K. One, two and three layers with 60, 120 and 180 nm thicknesses, respectively, were deposited on Si(111) substrate. The effect of the CuAlS₂ layer numbers on the structure, morphology and optical properties of the samples was investigated. The X-ray diffraction studies revealed that all the samples are polycrystalline in nature, single CuAlS₂ phase and exhibiting chalcopyrite structure with a preferred orientation along the (112) direction. However, the sample with three CuAlS₂ layers exhibit the highly oriented (112) plane with grain sizes of 80 nm. So we show that this experimental process affects significantly the structural properties of the CuAlS₂ films. Raman spectroscopic measurements indicated five prominent peaks at 193, 205, 325, 335 and 370 cm^− 1. The possible origin of the 370 cm^− 1 peak was investigated and was found to be some local vibration in the structure. The peaks at 193-205 and 335 cm^− 1 were ascribed to A₁ and B₂ modes, respectively.     

Transparent conducting Nb-doped anatase TiO2 (TNO) thin films sputtered from various oxide targets

Transparent conducting Nb-doped anatase TiO₂ (TNO) epitaxial films were sputtered from TiO₂-, Ti₂O₃-, and Ti-based targets at various oxygen partial pressures (Po₂). Using the TiO₂- and Ti₂O₃-based targets, highly conductive films showing a resistivity (ρ) of ~ 3 × 10^− 4 Ω cm could be formed without postdeposition treatment. In the case of the TNO films formed from the Ti-based target, reductive annealing had to be carried out at a temperature of 600 °C to achieve similar resistivity values. Thus, the use of oxide targets is preferable to obtain as-grown transparent conducting TNO films. In particular, the Ti₂O₃-based target is practically advantageous, because it offers a wide range of optimal Po₂ values at which ρ values of the order of 10^− 4 Ω cm are achievable.     

Structure, morphology and optical properties of nanocrystalline yttrium oxide (Y2O3) thin films

Yttrium oxide (Y₂O₃) thin films were grown onto Si(1 0 0) substrates using reactive magnetron sputter-deposition at temperatures ranging from room temperature (RT) to 500 °C. The effect of growth temperature (T_s) on the growth behavior, microstructure and optical properties of Y₂O₃ films was investigated. The structural studies employing reflection high-energy electron diffraction RHEED indicate that the films grown at room temperature (RT) are amorphous while the films grown at T_s = 300-500 °C are nanocrystalline and crystallize in cubic structure. Grain-size (L) increases from ∼15 to 40 nm with increasing T_s. Spectroscopic ellipsometry measurements indicate that the size-effects and ultra-microstructure were significant on the optical constants and their dispersion profiles of Y₂O₃ films. A significant enhancement in the index of refraction (n) (from 2.03 to 2.25) is observed in well-defined Y₂O₃ nanocrystalline films compared to that of amorphous Y₂O₃. The observed changes in the optical constants were explained on the basis of increased packing density and crystallinity of the films with increasing T_s. The spectrophotometry analysis indicates the direct nature of the band gap (E_g) in Y₂O₃ films. E_g values vary in the range of 5.91-6.15 eV for Y₂O₃ films grown in the range of RT-500 °C, where the lower E_g values for films grown at lower temperature is attributed to incomplete oxidation and formation of chemical defects. A direct, linear relationship between microstructure and optical parameters found for Y₂O₃ films suggest that tuning optical properties for desired applications can be achieved by controlling the size and structure at the nanoscale dimensions.     

The etching characteristics of Al2O3 thin films in an inductively coupled plasma

In this study, the etching characteristics of ALD deposited Al₂O₃ thin film in a BCl₃/N₂ plasma were investigated. The experiments were performed by comparing the etch rates and the selectivity of Al₂O₃ over SiO₂ as functions of the input plasma parameters, such as the gas mixing ratio, the DC-bias voltage, the RF power, and the process pressure. The maximum etch rate was obtained at 155.8 nm/min under a 15 mTorr process pressure, 700 W of RF power, and a BCl₃ (6 sccm)/N₂ (14 sccm) plasma. The highest etch selectivity was 1.9. We used X-ray photoelectron spectroscopy (XPS) to investigate the chemical reactions on the etched surface. Auger electron spectroscopy (AES) was used for the elemental analysis of the etched surfaces.     

Energy transfer between Ir(ppy)3 molecules in neat film and concentration quenching of phosphorescence

Numerical calculations are performed for temperature dependences of phosphorescence quantum efficiency and lifetime of Ir(ppy)₃ monomer in photo-excited neat film by solving the rate equations for three zero-field splitting substates of the triplet T₁ state. In the analysis, we take into account spin–phonon relaxations among the three substates and excitation energy transfer to the excimer state by thermal activation process. Good agreement of the calculated quantum efficiency and lifetime with experimental results is obtained by considering both the excimer formation and triplet–triplet annihilation, e.g., (1) the efficiency increases with decreasing temperature from 300 K to 125 K and then decreases on going from 125 K to 5 K, and (2) the phosphorescence decay curve shows multi-exponential, where the lifetimes increase with decreasing temperature from 300 K. It is suggested that concentration quenching of monomer emission in the neat film is mainly due to the formation of triplet excimer with activation energy of 975 cm⁻¹ and partly triplet–triplet annihilation and polariton pair annihilation.     

Effect of lead on formation and dielectric tunability of (Pbx,Sr1? x )0.85Bi0.1TiO3 thin films

(Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin films with the perovskite phase structure were prepared on an ITO glass substrate by sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and an impedance analyzer were respectively used in order to characterize the phase status, morphology and dielectric properties of the thin films. The results show that during the formation process of (Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin films, the nucleus of the perovskite phase are initially formed and then congregated. These aggregated nucleus are then transformed as the perovskite-phase crystalline in the thin film. Finally, the crystalline phase grows and separates gradually to form the perfect crystalline-phase structure. The content of the perovskite phase formed in the thin film under rapid thermal process (RTP) is more than that formed under traditional heat treatment with kinetic equilibrium. This is due to the high active decomposed ions that form the perovskite phase directly when heat-treated by RTP. The formation of the perovskite phase therefore overcomes a much lower barrier under RTP than that under traditional calcinations. The structure of the perovskite phase has a close relation to the ratio of Pb/Sr in the system because of the radius difference between Pb²⁺ and Sr²⁺. The transformation temperature between the cubic and the tetragonal structures of the perovskite phase increases with increasing Pb²⁺ content because the radius of Pb²⁺ is larger than that of Sr²⁺. It appears at room temperature when the content of Pb²⁺/Sr²⁺ is about 40/60 in the thin film. Meanwhile, the tetragonality of the perovskite phase is increased when Pb²⁺ ions increase due to its high polarization. The higher tunability of the (Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin film is exhibited when the film composition is close to the transformation point between the paraelectric and ferroelectric phases. Pb²⁺ ions show a dominant factor to affect the Curie point of the system and then changing tunability. Translated from Journal of Inorganic Materials, 2006, 21(2): 466–472 [译自: 无机材料学报]     

Electrical properties of chemically prepared nonstoichiometric CuIn(S,Se)2 thin films

Polycrystalline thin films of copper indium sulphoselenide [CuIn(S,Se)₂] were deposited on glass substrate by chemical bath deposition technique. The deposition parameters such as pH, temperature and time were optimized. A set of films having different elemental compositions was prepared by varying Cu/In ratio from 1·87–12·15. The films were characterized by X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDAX). The chemical composition of the CuIn(S,Se)₂ was found to be nonstoichiometric. The d.c. conductivities of the films were studied below and near room temperature. The thermo-electric power of the films was also measured and type of semiconductivity was ascertained.