Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.     

Study of electroplex emission from a blend of two basic blue-emitting materials PVK and NPB

An organic light emitting diode was fabricated with the simple structure of ITO/PEDOT:PSS/PVK:NPB/LiF/Al, which used the blend of two blue-emitting materials poly(N-vinylcarbazole) (PVK) and N,N₀-bis-(1-naphthyl)-N,N₀-diphenyl-1,10-biphenyl-4,40-diamine (NPB) as the emitting layer. In EL spectra of the device, except for the emission of NPB, a new peak at 650 nm was observed too, but was not in PL spectra. So the new emission was proposed to be electroplex that occurred between PVK and NPB molecules. White emission via electroplex formation can be obtained with Commission International d’Eclairage (CIE) coordinates (0.31, 0.3 0) at 28 V. By use of electroplex formed between two blue-emitting materials, a new route to get white organic light-emitting diode (WOLED) may be obtained.     

Electrochemical synthesis and electrochromic application of a novel polymer based on carbazole

The compound containing carbazole and thiophene, named as B1 was synthesized with 4-(9H-carbazol-9-yl) phenol and 3,4-dibromo thiophene. Additionally, the electrochemical polymer of B1 was synthesized and coated onto an ITO-glass surface via electrochemical oxidative polymerization. The electrochemical synthesis of the polymer was performed both in 0.05 M LiClO₄ supporting electrolyte in AN/BF₃EtE (1:1, v/v) and an AN/LiClO₄ solvent/electrolyte solution. The compounds were characterized by FT-IR and NMR techniques. The spectroelectrochemical and electrochromic properties of this polymer were also investigated for two electrolyte solution systems. The switching ability of this polymer was measured as the percent transmittance (%T) at its point of maximum contrast. According to the electrochromic measurements, the synthesized polymer had a blue color when it was oxidized, and also when it was reduced, it had a transparent color. Additionally, redox stability measurements indicates that the polymer had a high stability and it could be used to produce new polymeric electrochromic devices and also, it was a good candidate for electrochromic devices (ECDs) applications.     

The effect of gamma irradiation and forward bias on the 76 K electroluminescence spectrum of GaP:Zn,O

Both the effects of gamma irradiation and high-current forward bias on the low temperature electroluminescence (EL) spectra of GaP:Zn,O light emitting diodes (LEDs) have been studied by measuring constant current (1 mA) 76 K EL spectra below 1000 nm. The LEDs were divided into two sets: a control set which was subjected only to forward bias, and a set exposed to gamma irradiation and forward bias. The EL spectra reveal the growth of sharp, bound exciton emission bands (Z lines) in the range 580 nm to 620 nm following extended high current bias at 300 K of irradiated LEDs and, more weakly, in control LEDs which show pre-bias evidence of these lines. The emergence of these lines is tenatively associated with the recombination-enhanced motion of a defect, possibly the Zn interstitial, which can be introduced by irradiation and is sometimes present in unirradiated LEDs. This article sponsored by the U. S. Department of Energy (DOE) under Contract No. DE-AC04-76-DP00789. A United States Department of Energy Facility.     

液晶分子取向对液晶/聚合物光栅电光特性以及激光出射特性的影响

报道了液晶分子不同取向对液晶/聚合物光栅电光特性以及激光出射的影响。通过扫描电子显微镜观察液晶/聚合物光栅的截面,成功观察到了光栅的体光栅结构。液晶分子垂直光栅矢量排列时,由于光栅形貌变差,衍射效率由液晶分子沿光栅矢量排列时的83.2%降低至72%,同时散射损耗由11.8%增加至19.1%。液晶分子垂直光栅矢量时,液晶/聚合物光栅的调谐电场由液晶分子沿光栅矢量时的13.6V/μm下降至3.1V/μm。液晶沿光栅矢量排列时,激光出射阈值更低,为利于激光出射的方向。本文工作为进一步加深对液晶/聚合物光栅以及染料掺杂分布式反馈选频的理解和认识,提供了指导和借鉴。     

The effect of charge transfer state on the open-circuit voltage of small-molecular organic photovoltaic devices: A comparison between the planar and bulk heterojunctions using electroluminescence characterization

This work studies the open-circuit voltage (V_OC) of planar and bulk heterojunction organic photovoltaic (OPV) devices by means of electroluminescent (EL) technique to resolve the charge transfer (CT) states between donor and acceptor. The OPV devices containing a small part of bulk heterojunction increases the V_OC as a result of the enhanced CT process as compared with a complete planar structure. Red shift of the CT charge transfer was observed by increasing the bulk volume, which indicates the increased degree of interaction between both molecules and excitons. By characterizing the EL spectra of OPV devices and relating them to the CT absorption, the interfacial property between the donor and acceptor is shown to be crucial for determining the V_OC in small-molecule OPV devices. Detailed analysis of the energetic loss was also used to interpret the V_OC under the effect of CT states.     

Synthesis,field emission and optical properties of ZnSe nanobelts,nanorods and nanocones by hydrothermal method

ZnSe nanostructures, such as nanobelts, nanorods and nanocones, were successfully synthesized on Zn foils via a hydrothermal method using EDTA as soft template at low temperature. EDTA played a significant role on the morphology of ZnSe nanomaterials. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) were carried out to characterize the microstructures and chemical compositions of the as-synthesized ZnSe samples. XRD patterns indicated that the as-synthesized ZnSe samples belonged to a cubic zinc blende structure. SEM observation obviously showed that the nanocones had very sharp tips compared to nanorods and nanobelts. The field emission (FE) measurement showed that the as-synthesized ZnSe nanocones had a lower turn-on field of ~1.6 V μm⁻¹ at the current density of 10 μA cm⁻². A high field enhancement factor of ~4514 was achieved for the ZnSe nanocones. The superior field emission properties were probably attributed to the sharp tips of the ZnSe nanocones. Room temperature photoluminescence (PL) spectroscopy of the ZnSe nanostructures showed a wide band emission from blue light to orange light. The as-prepared ZnSe nanomaterials have promising applications in optoelectronic devices. A possible formation mechanism of ZnSe nanobelts, nanorods and nanocones was also proposed and discussed.     

薄膜衬底电极CNT阴极制备及场发射性能研究

采用电泳沉积(EPD,electrophoretic deposition)法在不同薄膜衬底电极上制备碳纳米管(CNT,carbon nanotube)场发射阴极.采用场发射扫描电子显微镜(FESEM)对其进行表面形貌表征,结果表明,EPD可以制得CNT均匀分布的场发射阴极.场发射测试结果表明衬底电极对CNT阴极的场发...     

基于高温制备的氧化钙镁复合薄膜光电特性研究

采用电子束蒸镀工艺制备氧化钙掺杂氧化镁复合介质保护膜并深入分析了制备温度对该复合保护膜透过率及二次电子发射效率的影响。实验表明,高温制备能够使复合薄膜表面形貌更为致密,结晶粒径增加,薄膜的透过率和发光效率提高。当制备温度为300℃时,复合薄膜的形貌更为致密平整,裕度从25V增加到32V,发光效率从1.70lm/W提高到1.91lm/W,提高了12.35%。     

Cu2ZnSnS4四种晶型的电子结构、光学性质和力学性质

本文采用密度泛函理论详细计算了Cu₂ZnSnS₄四种晶型的晶体结构、电子结构、光学性质和力学性质。结果表明它们之间在光学性质和力学性质方面没有非常明显的差异,计算结果与文献报道的实验数据基本一致。根据计算结果,Cu2ZnSnS4的基本带隙是由孤立导带的带宽来决定。Cu₂ZnSnS₄的载流子有效质量非常小,尤其是闪锌矿衍生的Cu₂ZnSnS₄在导带底具有极小的电子有效质量。根据所计算的力学常数矩阵可知,四种晶型的Cu₂ZnSnS₄; 均符合Born稳定性条件,而且较高的B/G比例表明Cu2ZnSnS4的四种晶型都具有突出可塑性。     

The properties of structures based on oxidized porous silicon under the effect of illumination and a gas environment

The effect of illumination and adsorption of polar molecules on the properties of structures based on oxidized porous silicon has been investigated. The specific features of these structures are fluctuations and a change in the sign of open-circuit voltage under the influence of long-term photoexcitation, time-related instabilities of the current in media with a high content of hydrogen sulfide, long periods of photocurrent decay after switching off the illumination, and the occurrence of negative photoconductivity in structures with space-charge-limited currents. The results obtained are explained by the presence of multiply charged traps of different types in oxidized porous silicon, their charge exchange, and the tunneling of charge carriers between silicon nanocrystallites.     

新型挤压法制备硫系玻璃光纤及其性能研究

选用组分为Ge15Sb20S65和Ge25Sb5Se70的两组硫系玻璃,制备了多种尺寸的高质量硫系玻璃,分析了两组玻璃的物理、热学和光学性能;利用复合层叠挤压法制备了硫系玻璃光纤预制棒,避免了钻孔法或其他机械加工引起的预制棒界面缺陷,且光纤端面结构可随模具尺寸自由设计,挤压后光纤预制棒结构整齐,内外界面整洁光滑;利用高温聚合物保护的预制棒具有良好的机械性能,用光纤拉丝机将具有保护层的预制棒拉制成光纤;利用普通光纤抛光机进行端面抛光,光纤端面结构均匀,界面无明显缺陷。傅里叶红外光纤光谱仪(FTIR)测试光纤损耗谱表明,在波长4.8μm处光纤的最低损耗为2.63dB/m。     

ZnS:Er电子结构和光学性质的第一性原理计算

采用第一性原理平面波赝势方法,分析了ZnS及掺杂Er3+之后的电子结构,进而预测了其光学性质。结果表明,掺杂之后体系的带隙变窄,同时价带与导带之间形成一个新的中间带。随着掺杂Er3+浓度的增加,体系的绝缘性呈下降趋势,吸收光谱则发生红移现象。这一结论与前人报道的实验结果相符。     

双掺Cr,Nd∶ZW晶体的光性、热学和激光性能

生长出Ｃｒ３＋，Ｎｄ３＋：ＺｎＷＯ４（Ｃｒ，Ｎｄ：ＺＷ）单晶，尺寸为２５ｍｍ×５０ｍｍ，研究了Ｃｒ，Ｎｄ：ＺＷ晶体的光学性质、热学性质，测定了其吸收和发射截面，激发态寿命和激光性能。结果表明，双掺有效地改善了激光性能，室温下获得０．９５μｍ单脉冲激光能量输出１．６２ｍＪ，斜效率为０．７９％。     

The effect of AlN/AlGaN superlattices on crystal and optical properties of AlGaN epitaxial layers

We investigate the effect of AlN/AlGaN superlattices (SLs) on crystal and optical properties of AlGaN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for (0002)-and (1012)-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases. 280 nm deep ultraviolet light-emitting diodes (DUV-LEDs) structures are further regrown on the n-AlGaN layers. The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.     

First-principles study on the electronic, elastic and thermodynamic properties of three novel germanium nitrides

The ultrasoft pseudo-potential plane wave method combined with the quasi-harmonic approach have been used to study the electronic, elastic and thermodynamic properties of the tetragonal, monoclinic and orthorhombic Ge₃N₄. The negative formation enthalpies, the satisfactory of Born''s criteria and the linear variations of elastic constants with pressure indicate that the three polymorphs can retain their stabilities in the pressure range of 0-25 GPa. The three Ge₃N₄ are brittle solids at 0 GPa, while they behave in ductile manners in the pressure range of 5-25 GPa. t- and o-Ge₃N₄ are hard materials but anisotropic. m-Ge₃N₄ has the largest ductility among the three phases. The results reveal that m-Ge₃N₄ belongs to an indirect band gap semiconductor, while t- and o-Ge₃N₄ have direct band gaps. For the thermal properties, several interesting features can be observed above 300 K. o-Ge₃N₄ exhibits the largest heat capacity, while m-Ge₃N₄ shows the highest Debye temperature. The results predicted in this work can provide reference data for future experiments.     

Ab initio study on physical properties of wurtzite,zincblende, and rocksalt structures of zinc oxide using revised functionals

To provide a more comprehensive understanding of zinc oxide (ZnO), the structural, optical, dielectric and thermodynamic properties of wurtzite (B4), zincblende (B3) and rocksalt (B1) phases are studied based on density functional theory with various revised functionals. It is found that the equilibrium volume, bulk modulus, phase transition point and covalent degree of B3-ZnO are close to those of B4-ZnO. Similar behaviors can be observed from optical properties, dielectric constants and Born effective charges. Moreover, thermodynamic parameters including isothermal bulk modulus, thermal expansion coefficients, Grüneisen parameter, and heat capacity with temperature of B3-ZnO are also in accordance with B4-ZnO. In contrast, B1-ZnO obviously undergoes dissimilar physical properties. Thus, it can be concluded that B3 phase may be the substitute of B4 phase in case of growing on the closely cubic lattice-matched substrates.     

成核层形貌对AlGaN/GaN异质结晶体质量，表面形貌以及电学特性的影响

Nucleation layer formation is a key factor for high quality gallium nitride（GaN）growth on a sapphire substrate.We found that the growth rate substantially affected the nucleation layer morphology,thereby having a great impact on the crystal quality,surface morphology and electrical properties of AlGaN/GaN heterostructures on sapphire substrates.A nucleation layer with a low growth rate of 2.5 nm/min is larger and has better coalescence than one grown at a high growth rate of 5 nm/min.AlGaN/GaN heterostructures on a nucleation layer with low growth rate have better crystal quality,surface morphology and electrical properties.     

Effect of nucleation layer morphology on crystal quality, surface morphology and electrical properties of AlGaN/GaN heterostructures

Nucleation layer formation is a key factor for high quality gallium nitride (GaN) growth on a sapphire substrate. We found that the growth rate substantially affected the nucleation layer morphology, thereby having a great impact on the crystal quality, surface morphology and electrical properties of AIGaN/GaN heterostructures on sapphire substrates. A nucleation layer with a low growth rate of 2.5 nm/min is larger and has better coalescence than one grown at a high growth rate of 5 nm/min. AIGaN/GaN heterostructures on a nucleation layer with low growth rate have better crystal quality, surface morphology and electrical properties.     

The properties of aluminum, platinum silicide and copper based contacts for silicon high-power devices

Electrical, thermo-electrical and thermo-mechanical properties were compared for PtSi, aluminum (Al, PtSi-Al, PtSi-AlCuSi, PtSi-TiW-AlCuSi), and copper based contacts (TiW-Ni-Cu, TiW-Ni-Cu-Ni-Au, PtSi-TiW-Ni-Cu, PtSi-TiW-Ni-Cu-Ni-Au). High-power 2.5 kV/150 A P-i-N diode with both lapped and etched anode surface was used to characterize the performance of the contacts in the conditions of free floating silicon in pressed package. The devices with PtSi contacts have the lowest forward voltage drop, but do not survive the operation in pressed package under thermal cycling. The copper based contacts with PtSi layer give lower voltage drop and better thermo-mechanical ruggedness compared to that of aluminum. These features are conserved even with passivation against corrosion using Ni-Au. The contacts without the PtSi layer have high voltage drop and fail under thermal cycling. The composition of the contact layer is shown to influence the thermal behavior of device voltage drop. The crossing point current is found to decrease with increasing contact layer thickness. The lowest magnitude gives the aluminum contact.