Solution-processed WOx hole injection layer for efficient fluorescent blue organic light-emitting diode

Solution-processed tungsten oxide (s-WO_x) interfacial layer for efficient hole injection in fluorescent blue organic light-emitting diode (OLED) is demonstrated. The OLED using 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as emitter shows luminous efficiency of 3.3 cd/A, power efficiency of 2.5 lm/W and external quantum efficiency of 4.6% with Commission Internationale d'Eclairage (CIE) color coordinates of (0.154, 0.102). Using MADN doped 1-4-di-[4-(N,N-diphenyl)amino]styryl-benzene as emitter, luminous efficiency of 10.8 cd/A, power efficiency of 6.4 lm/W and external quantum efficiency of 7.2% with CIE color coordinates of (0.167, 0.283) are achieved. Atomic force microscopy and X-ray photoelectron spectroscopy show that s-WO_x features superior film morphology and non-stoichiometry with slight oxygen deficiency. Current-voltage characteristics and impedance spectroscopy analysis indicate that s-WO_x behaves slightly enhanced hole injection and accordingly contributes to improved device performance in comparison with conventional vacuum thermal evaporation WO_x. Our results pave an alternative way for broadening WO_x application with solution process and advancing fluorescent blue OLEDs.     

γ-Ray-induced synthesis and electrochemical properties of a mesoporous layer-structured α-Co(OH)2 for supercapacitor applications

α-Cobalt hydroxide is synthesized through a novel radiation-induced route using a cobalt nitrate hexahydrate isopropanol solution by irradiating ⁶⁰Co γ-rays. The as-prepared product shows a layer-structured mesoporous morphology with an average size of 250 nm. Crystalline property analysis exhibits the presence α-Co(OH)₂ as a predominant phase with a tiny amount of secondary phases of β-Co(OH)₂ and CoO. X-ray photoelectron spectroscopy discloses the existence of Co as Co(OH)₂ and CoO with a stoichiometric ratio of 76.7:23.3. In addition, the supercapacitive properties of the product are investigated using cyclic voltammetry and impedance spectroscopy in 1M KOH aqueous solution, showing a maximum capacitance value of 246.7 F g⁻¹ at a scan rate of 5 mV s⁻¹.     

磁控共溅射法制备的Zn2GeO4多晶薄膜结构及其光致发光研究

用射频磁控共溅射方法在不同温度的单晶硅基片上生长薄膜,然后在800℃真空环境下对薄膜进行退火处理,成功获得了结晶状态良好的Zn₂GeO₄多晶薄膜.利用X射线衍射(XRD),X射线光电子能谱(XPS)和原子力显微镜(AFM)对薄膜进行了结构、成分和形貌分析,研究了基片温度对三者的影响. 结果显示,当基片温度升高到400℃以上时,薄膜中的Zn₂GeO₄晶粒在(220)方向上显示出了明显的择优取向. 当基片温度在500—600℃范围内,有利于GeO₂结晶相的形成. XPS显示薄膜中存在着Zn₂GeO₄,GeO₂,GeO,ZnO四种化合态. 同时,随着基片温度的升高,晶粒尺寸增大且薄膜表面趋于平整. 薄膜的光致发光在绿光带存在中心波长为530和550nm两个峰,应该归因于主体材料Zn₂GeO₄中两个不同的Ge²⁺的发光中心. 关键词： 射频磁控溅射 2GeO₄')" href="#">Zn₂GeO₄ 荧光体     

Effect of Ca interlayer thickness on electron injection of lithium quinolate/Ca/Al cathodes

Electron injection behavior of lithium quinolate (Liq)/Ca/Al cathode was investigated by using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (UPS). Interfacial energy barrier lowering of Liq/Ca/Al cathode was dependent on Ca thickness and maximum energy level shift was observed at a Ca thickness of 1 nm. Maximum current density could be obtained in Liq/Ca/Al device at a Ca thickness of 1 nm and it was well correlated with energy level shift from UPS measurement. Power efficiency of Liq/Al device could be improved by more than 70% by inserting Ca layer between Liq and Al.     

Synthesis and characterization of a solution processible deep blue light-emitting molecule composed of fluorene and pyrene units

A solution processible deep blue light-emitting molecule composed of pyrene and dialkylfluorene units, 1,6-bis(9,9′-dioctylfluorene-2-yl)pyrene (BDOFP) was synthesized and characterized. The synthesized compound was soluble in common organic solvents and the solution gave a smooth thin film after spin coating. The compound was characterized by using thermogravimetric analysis (TGA), differential calorimetry (DSC), UV–visible spectroscopy, fluorescence spectroscopy and cyclic voltammetry. The maximum UV–visible absorption and PL emission of BDOFP thin film were more red-shifted than those of BDOFP solution due to strong intermolecular interaction between flat segments. To improve color purity and film stability BDOFP was doped to a well-known charge-transporting polymer, poly(N-vinylcarbazole) (PVK). BDOFP thin film showed it maximum PL at 457 nm but the thin films of BDOFP doped PVK films showed it at 443 nm. Organic light-emitting diodes were fabricated with the simple structure of ITO/PEDOT:PSS/emitter/BmPyPB/LiF/Al configuration. BDOFP or three kinds of BDOFP:PVK blends with different ratios (10:90, 30:70, 50:50 by weight) were used as the emissive layers and [1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene] (BmPyPB) as the electron-transporting layer. All of light-emitting devices showed their electroluminescence in blue region of spectrum, especially EL using BDOFP: PVK (1:9) showed a deep-blue light emission with CIE coordinates of (0.14, 0.07). Maximum brightness, external quantum efficiency and current efficiency of the device were 500 cd/m², 0.7% and 0.44 cd/A, respectively.     

High-performance organic red-light-emitting device based on DCJTB and a new host material

An efficient red-light-emitting device using a new host material (DPF) and a red dopant (DCJTB) with a configuration of ITO/NPB (50 nm)/DCJTB:DPF (2%, 10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Mg:Ag has been fabricated and investigated. The red OLED yields a brightness of 9270 cd/m² at 10 V, a maximum current efficiency of 4.2 cd/A and a maximum power efficiency of 3.9 lm/W. Using DPF as host material, the performance is much better than that of a prototypical Alq₃-based device, which has a maximum efficiency of 1.9 cd/A and 0.6 lm/W. The performance is even comparable with red OLEDs using an assist dopant or a cohost emitter system. Results of this work indicate that DPF is a promising host material for red OLEDs with high efficiency and simple device structure.     

A terbium (III) complex with triphenylamine-functionalized ligand for organic electroluminescent device

A novel ligand, 4-diphenylamino-benzoic acid (HDPAB), and the corresponding Tb (III) complex, Tb (DPAB)₃ which can be dissolved easily in organic solvents were synthesized and characterized. Organic electroluminescent (EL) device with a structure of indium tin oxide (ITO)/poly(N-vinylcarbazole) (PVK): Tb (DPAB)₃ (50 wt%, 80 nm)/1,3,5-tris-(N-phenylbenzimidazol-2-yl)benzene (TPBI) (30 nm)/tri(8-hydroxyquinoline)aluminum (AlQ) (20 nm)/LiF (1 nm)/Al (150 nm) in which Tb (DPAB)₃ acted as an emitter were fabricated. The maximum luminance of 230 cd m⁻² at 20 V and the maximum efficiency of 0.62 cd A⁻¹ were obtained due to the introduction of hole-transporting group, representing the best result to date among Tb (III) carboxylate complexes based EL devices. These results indicate that modifications of rare earth complexes are a promising way to improve the properties of EL devices.     

Enhanced performance of light-emitting diodes based on a nanocomposite of dehydrated nanotube titanic acid and poly(vinylcarbazole) (PVK)

Performance of light-emitting diodes (LEDs) based on a nanocomposite of dehydrated nanotube titanic acid (DNTA) and poly(vinylcarbazole) (PVK) as emissive layer, i.e., ITO/PVK:DNTA(2 wt%)(100 nm)/BCP(30 nm)/Alq₃(10 nm)/LiF(1 nm)/Al [device D₂], was reported. By investigation of the luminance and electric characteristics of the device, we found that the performance of device D₂ was greatly improved. The recombination zone changed evidently due to the improvement of holes mobility. Compared with those devices without incorporating DNTA, both the maximum luminance and the electroluminescent efficiency of the device D₂ can be improved by a factor of three. Furthermore, the turn-on voltage of the device decreased dramatically.     

Structural, optical and electrical properties of ZnO/Zn2GeO4 porous-like thin film and wires

Zinc oxide/zinc germanium oxide (ZnO/Zn₂GeO₄) porous-like thin film and wires has been fabricated by simple thermal evaporation method at temperature about 1120 °C for 2.5 h. The structural and optical properties of the porous-like-thin film and wires have been investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. Metal semiconductor metal (MSM) photodetector structure was used to evaluate the electrical characteristics by using current-voltage (I-V) measurements. Room temperature photoluminescence spectrum of the sample shows one prominent ultraviolet peak at 378 nm and a shoulder at 370 nm. In addition, broad visible blue emission peak at wavelength 480 nm and green emission peak at 500 nm are also observed. Strong photoelectric properties of the MSM in the UV demonstrated that the porous-like-thin film and wires contribute to its photosensitivity and therefore making ZnO/Zn₂GeO₄ wires potential photodetector in the shorter wavelength applications.     

Spectroscopic analysis of CIGS2/CdS thin film solar cell heterojunctions on stainless steel foil

This paper presents a spectroscopic analysis of the interface between a CuIn_1−xGa_xS₂ (CIGS2) absorber and a CdS buffer layer on stainless steel foil by Auger electron spectroscopy (AES), inverse photoemission spectroscopy (IPES) and photoelectron spectroscopy (PES) such as X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). By combining these spectroscopic techniques, detailed information about the electronic and chemical properties of the CIGS2 surface and the CdS/CIGS2 interface can be obtained. The gallium concentration in CIGS2 films was found to increase continuously towards the Mo back contact. XPS analysis showed the presence of KCO₃ on the surface of CdS, deposited on etched and un-oxidized samples indicating diffusion of potassium. No potassium was observed on oxidized as well as samples having thicker CdS (50 nm) indicating the effectiveness of oxidation and chemical bath deposition (CBD) process in cleaning the sample surface effectively. In addition, investigation of the electronic level alignment at the interface has been carried out by combining PES and IPES. Conduction band offset of −0.45 (±0.15) eV and a valence band offset of −1.06 (±0.15) eV were measured. These unfavorable conditions limit efficiency of CIGS2 thin film solar cells.     

Interactions of germanium atoms with silica surfaces

GeH₄ is thermally cracked over a hot filament depositing 0.7-15 ML Ge onto 2-7 nm SiO₂/Si(1 0 0) at substrate temperatures of 300-970 K. Ge bonding changes are analyzed during annealing with X-ray photoelectron spectroscopy. Ge, GeH_x, GeO, and GeO₂ desorption is monitored through temperature programmed desorption in the temperature range 300-1000 K. Low temperature desorption features are attributed to GeO and GeH₄. No GeO₂ desorption is observed, but GeO₂ decomposition to Ge through high temperature pathways is seen above 750 K. Germanium oxidization results from Ge etching of the oxide substrate. With these results, explanations for the failure of conventional chemical vapor deposition to produce Ge nanocrystals on SiO₂ surfaces are proposed.     

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E_F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO₂)Ge). The concentration of oxide on the (GeO₂)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO₂)Ge surface to 6 nm was studied. At energies below 9 eV above E_F, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface, E_vac–E_F, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.     

Spectroscopic study of chromium-doped transparent calcium germanate glass-ceramics

The optical properties of transparent chromium-doped glass-ceramics with the chemical composition similar to that of cunyite (Cr⁴⁺:Ca₂GeO₄) laser crystal were investigated. Room and low temperature absorption, excitation, fluorescence and Raman spectroscopy, as well as lifetime measurements were used to develop a model for the optical centers in the glass and nanocrystals. Parent (as quenched) glass does not exhibit any fluorescence; after the heat treatment Cr⁴⁺ fluorescence band appears at ∼1280 nm. Optical properties of nanocrystals formed in the glass-ceramics differ from those in the bulk single crystal Ca₂GeO₄. By comparison of Ca₂GeO₄ - like glass-ceramics with the Ca₂GeO₄ bulk single crystals and Mg₂SiO₄ - glass-ceramics we concluded that the nanocrystal phase is a modified Ca₂GeO₄ structure. Broadband fluorescence and a high quantum efficiency of new glass-ceramics make the new Cr-doped glass-ceramics promising medium for fiber lasers and amplifiers.     

Synthesis of germanium encapsulated fullerene

A method has been described for encapsulating Ge in a fullerene cage using GeO₂ and a graphite composite rod as anode and graphite as cathode. Annealing in an inert atmosphere before arcing, and the amount of GeO₂ in the rod determined the yield of Ge doped metallofullerene. Solvent extraction using soxlet in inert atmosphere followed by calcination in air was used to isolate metallofullerene from the soot. The insertion of Ge inside the fullerene was proven by ultra violet–visible absorption spectroscopy, matrix assisted laser desorption ionization-time of flight mass spectroscopy, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.     

Silicon nanoparticle size-dependent open circuit voltage in an organic–inorganic hybrid solar cell

We have incorporated silicon nanoparticles (Si-nps) into organic–inorganic hybrid solar cells in place of the chalcogenide nanocrystals that are commonly employed in such devices. Poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) and phenyl-C₆₁-butyric acid methyl ester (PCBM) were employed as hole and electron transport layers, respectively. We used transmission electron microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy to fully characterize the Si-nps and relate their characteristics to the performance of the hybrid solar cells. We show that the open circuit voltage (V_OC) was largely dependent on the size and amorphous volume fraction of Si-nps. Our findings imply that the amorphous phase and small size of Si-nps produce band gap widening that increases the V_OC when coupled with PCBM as acceptor. The maximum V_OC was up to 0.634 V in a hybrid device with 5.7 nm Si-nps.     

Methoxycarbonyl-terminated hyperbranched poly(amine-ester) as templates for synthesis of silver nanoparticles

Highly stable and spherical silver nanoparticles, stabilized by methoxycarbonyl-terminated hyperbranched poly(amine-ester) (HPAE-COOCH₃), were synthesized in water with reducing AgNO₃/HPAE-COOCH₃ using two methods, viz. NaBH₄ and ultraviolet irradiation. HPAE-COOCH₃ was found to play a key role in the formation of nanoparticles. UV–visible absorption, Transmission electron microscopic (TEM), and Fourier transform infrared spectroscopy (FT-IR) had been used to study the structure and characterization of the silver nanoparticles. The absorption peaks of the silver nanoparticles appear at ~420 nm in UV–visible absorption spectra; average particle size reduced by NaBH₄ is ~30 nm, which is ~10 and ~15 nm, respectively, when ultraviolet irradiation time is 12 and 24 h. FT-IR spectra confirm that there is strong interaction between silver nanoparticles and HPAE-COOCH₃. And silver nanoparticles/HPAE-COOCH₃ aqueous solution can keep stable for more than 3 months.     

The electronic structure and photoluminescence properties of Ca2GeO4:Eu in ultraviolet and vacuum ultraviolet region

Ca₂GeO₄:Eu³⁺ phosphors were synthesized by the solid state method. The ultraviolet and vacuum ultraviolet excited photoluminescence properties were investigated in detail. It revealed that the emission of Ca₂GeO₄:Eu³⁺ comprised two parts: the red emission of Eu³⁺ and host defect emission in 330-550 nm. Ca₂GeO₄:Eu³⁺ presented intense excitation intensity at 163-200 and 466 nm, which suggested the potential applications in plasma display panels and light emitting diodes. The excitation spectra were studied to identify the photoluminescence mechanisms of Ca₂GeO₄:Eu³⁺. First principles calculation within the local density approximation of the density functional theory was applied to calculate the electronic structure and linear optical properties of Ca₂GeO₄.     

Nanomechanical properties inside the scratch grooves of soda–lime–silica glass

Zn_2?2x Mn_2x GeO₄ (x=0, 0.001, 0.01) phosphors were prepared by conventional solid state reaction technique. X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), diffuse reflection spectra, photoluminescence (PL), and cathodoluminescence (CL) spectroscopy were utilized to characterize the synthesized phosphors. The Mn²⁺-activated Zn₂GeO₄ phosphors exhibit narrow emission band at 532 nm under the excitation of ultraviolet light, which due to the ⁴T₁(⁴G)–⁶A₁(⁶S) transition of Mn²⁺ ions. Also it is observed that there exists energy transfer between the Zn₂GeO₄ host lattice and the activator (Mn²⁺). Under excitation of low-voltage electron beams, Zn₂GeO₄:Mn²⁺ shows strong green emission band dominating at 535 nm, corresponding to the ⁴T₁(⁴G)–⁶A₁(⁶S) emission of Mn²⁺ ions. The emission intensity and chromaticity coordinates of Zn₂GeO₄:Mn²⁺ as a function of accelerating voltage and the filament current were also investigated.     

Anomalous temperature dependence of photoluminescence in GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films and GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiO<Subscript>2</Subscript> nano-heterostructures

The optical properties of GeO _x film and GeO _x /SiO₂ multilayer heterostructures (with thickness of GeO _x layers down to 1 nm) were studied with the use of Raman scattering and infrared spectroscopy, ellipsometry and photoluminescence spectroscopy including temperature dependence of photoluminescence. The observed photoluminescence is related to defect (dangling bonds) in GeO _x and interface defects for the case of GeO _x /SiO₂ multilayer heterostructures. From analysis of temperature dependence of photoluminescence intensity, it was found that rate of nonradiative transitions in GeO _x film has Berthelot type, but anomalous deviations from Berthelot type temperature dependence were observed in temperature dependences of photoluminescence intensities for GeO _x /SiO₂ multilayer heterostructures.     

Ca2GeO4 ∶Eu3+ 的电子结构和紫外-真空紫外发光特性的研究

采用高温固相法合成了发光材料Ca₂GeO₄ ∶Eu³⁺ ,并详细研究了其紫外-真空紫外发光特性. 发现并解释了Eu³⁺ 离子在空气中的自还原以及在不同波长激发下的颜色转换现象. Ca₂GeO₄ ∶Eu³⁺ 在163—230和301,466 nm处具有强激发带,表明Ca₂GeO₄ ∶Eu³⁺ 关键词： 2GeO₄ ∶Eu³⁺')" href="#">Ca₂GeO₄ ∶Eu³⁺ 光致发光机理 空气中自还原 颜色转换