有源矩阵OLED

介绍了OLED的工作原理、器件结构、有源驱动等,有机发光器件由ITO阳极、金属阴／极和多层有机薄膜构成,其中各有机层分别起电子、空穴传输、复合发光及缓冲作用,讨论了发光效率和器件寿命及可靠性等问题。     

Characterization of closed space vapor transport GaP epitaxial layers

The growth of homoepitaxial GaP layers using Te-doped GaP as source material has been obtained by the so-called closed space vapor transport technique. The photoluminescence study shows that these layers, when grown under optimized thermodynamical conditions, have both a large luminescence efficiency and the same optical quality as the ones obtained by liquid phase epitaxy. The variation of the luminescence properties with the conditions of growth has been investigated. Both electron paramagnetic resonance and deep level transient spectroscopy detect the presence of deep levels that are not observed in liquid phase epitaxy materials.     

基于全电介质复合纳米天线荧光传感器的研究

为了增强量子点的定向发光强度,提出了一种由硅柱二聚体和二氧化钛圆盘组成的复合纳米天线结构。利用时域有限差分方法系统研究了硅柱二聚体的轴参数、截面类型以及复合纳米天线结构对量子点定向发光增强的影响。结果表明,对于中心波长为600nm的量子点,硅柱二聚体的轴参数对量子点的发光影响不大,椭圆形截面的硅柱二聚体可以实现较大的量子点发光增强。此外,在复合纳米天线的作用下,不仅可以获得较大的量子效率增强,还可以实现量子点高度定向的发射效果,量子效率增强约6倍,定向收集效率可以达到50%。     

High-efficiency blue luminescence from MOCVD-grown ZnSe at room temperature

The cathodoluminescence properties of undoped ZnSe layers grown by MOCVD onto GaAs substrates have been studied. Intrinsic and extrinsic luminescence processes are observed at liquid nitrogen temperatures, and at room temperature luminescence is shown to result from band-to-band recombination. The external quantum efficiency for this blue luminescence at 300 K is estimated to be 0.1%, and an internal quantum efficiency as high as 30% is deduced from the result.     

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.     

自组织生长InAs／GaAs量子点的退火效应

通过研究ＧａＡｓ衬底上不同厚度ＩｎＡｓ层光致发光的退火效应，发现它和应变量子阱结构退火效应相类似，ＩｎＡｓ量子点中的应变使退火引起的互扩散加强，量子点发光峰蓝移．量子点中或其附近一旦形成位错，其中的应变得到释放，互扩散现象就不明显了，退火倾向于产生更多的位错，量子点的发光峰位置不变，但强度减弱．     

Design of Highly Efficient Light-Trapping Structures for Thin-Film Crystalline Silicon Solar Cells

We present a design optimization of a highly efficient light-trapping structure to significantly increase the efficiency of thin-film crystalline silicon solar cells. The structure consists of an antireflection (AR) coating, a silicon active layer, and a back reflector that combines a diffractive reflection grating with a distributed Bragg reflector. We have demonstrated that with careful design optimization, the presented light-trapping structure can lead to a remarkable cell-efficiency enhancement for the cells with very thin silicon active layers (typically 2.0-10.0 mum) due to the significantly enhanced absorption in the wavelength range of 800-1100 nm. On the other hand, less enhancement has been predicted for much thicker cells (i.e.,>100 mum) due to the limited absorption increase in this wavelength range. According to our simulation, the overall cell efficiency can be doubled for a 2.0-mum-thick cell with light-trapping structure. It is found that the improvement is mainly contributed by the optimized AR coating and diffraction grating with the corresponding relative improvements of 36% and 54%, respectively. The simulation results show that the absolute cell efficiency of a 2.0-mum-thick cell with the optimal light-trapping structure can be as large as 12%.     

异质外延GaN中穿透位错对材料发光效率的影响

用阴极射线致发光(CL)法、透射电子显微镜(TEM)和X射线衍射(XRD)法研究了异质外延GaN材料的发光性质与结构特性的关系.结果表明,GaN外延层中的穿透位错是材料有效的非辐射复合中心,但GaN的CL带边峰强度并不随位错密度的增加而减少.两步法生长GaN形成的马赛克结构的亚晶粒尺寸和晶粒间合并产生的位错的弯曲程度是影响材料发光效率的关键.     

Photon assisted growth of nitrogen-doped CdTe and the effects of hydrogen incorporation during growth

Nitrogen doping in CdTe epilayers grown by photo-assisted molecular beam epitaxy was demonstrated using an rf plasma source. The effect of the presence of atomic hydrogen during growth of undoped and nitrogen-doped CdTe was investigated. The layers were characterized using photoluminescence spectros-copy (PL), Hall effect, secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy, and atomic force microscopy. PL confirmed the incorporation of nitrogen as acceptors. While p-type carrier concentrations greater than 10¹⁸ cm⁻³ were easily obtained, SIMS measurements indicated that nitrogen was concentrated near the undoped-doped and epilayer-substrate interfaces which complicates interpretation of activation efficiency. Hydrogen incorporation was found to be enhanced by the presence of nitrogen. Infrared absorption measurements strongly suggested the formation of N-H complexes. Hall measurements indicated that complexes are formed which are donor-like in nature. The presence of atomic hydrogen during growth radically changed the low temperature photoluminescence in both undoped and nitrogen-doped layers. Exciton-related luminescence was quenched at low temperature. Nitrogenrelated donor-acceptor pair luminescence was also absent from the N-doped hydrogenated layers, consistent with complex formation. Copper (a cation-site acceptor) donor-acceptor pair luminescence appeared to be enhanced by hydrogenation.     

Stabilized Blue Emission from Polymer–Dielectric Nanolayer Nanocomposites

Blue‐light‐emitting polymer (polyfluorene)/dielectric nanolayer nanocomposites were prepared by the solution intercalation method and employed in an electroluminescent (EL) device. Their photoluminescence (PL) and electroluminescence characteristics demonstrates that the interruption of interchain interaction in intercalated organic/inorganic hybrid systems reduces the low‐energy emission that results from keto‐defects. By reducing the probability that the excitons initially generated on the polyfluorenes will find keto‐defects, both the color purity and the luminescence stability were improved. Furthermore, the dielectric nanolayers have an aspect ratio of about five hundred, and therefore act as efficient exciton blocking layers and barriers to oxygen diffusion, producing a dramatic increase in the device stability. A nanocomposite device with a Li:Al alloy cathode gave a quantum efficiency of 1.0 %(ph/el), which corresponds to an approximate five times enhancement compared to the neat polymer device. The nanocomposite emitting layer is considered to have a pseudo‐multiple quantum well structure.     

Effect of irradiation on the luminescence properties of low-dimensional SiGe/Si(001) heterostructures

This study is concerned with the effect of irradiation on the luminescence properties of low-dimensional Si/Ge heterostructures with different degrees of spatial localization of charge carriers. It is shown that the radiation stability of Si/Ge heterostructures is improved with increasing efficiency of localization of charge carriers in the structures. The spatial localization of charge carriers in the SiGe nanostructures decreases the probability of nonradiative recombination of charge carriers at radiation defects produced in the Si matrix. It is demonstrated that, among the structures explored in the study, the highest radiation stability of luminescence properties is inherent in the multilayered structures containing self-assembled Ge(Si) nanoislands, in which the most efficient spatial localization of charge carriers is attained. In this case, the localization is three- and two-dimensional, correspondingly, for holes in the islands and for electrons in the Si layers that separate neighboring layers containing the islands.     

Elastic strain and enhanced light emission in dry etched Si/Si1-xGex quantum dots

Quantum dots of 50 ~ 60 nm diameter fabricated from both Si/Si_1-xGe_x (x = 0.1 ~ 0.3) strained layer superlattices and a strain symmetried Si₉/Ge₆ superlattice were investigated by a combination of Raman scattering, photoluminescence, and electroluminescence spectroscopy. It was found that, in addition to an enhanced luminescence intensity of the dots by over two orders of magnitude and improved luminescence quenching temperature, all of the nanostructure dots have residual built-in elastic strains, which are of the order of ~50% of the values in corresponding pseudomorphic heterostructures. This result suggests a possible mechanism for explaining the huge enhancement of the optical efficiency in our luminescence measurements.     

Temperature and flow modulation doping of manganese in ZnS electroluminescent films by low pressure metalorganic chemical vapor deposition

A temperature and flow modulation (TFM) technique has been developed to modulate the manganese doping profile in ZnS phosphor material grown by lowpressure metalorganic chemical vapor deposition for alternating-current thin film electroluminescent devices (TFELDs). In the TFM technique, modulation of both the substrate temperature as well as the flows of metalorganic sources, diethylzinc and tricarbonyl-(methylcyclopentadienyl)-manganese (TCPMn), was used to grow a structure consisting of alternating layers of undoped ZnS at 400°C and Mn-doped ZnS where Mn being incorporated into the undoped ZnS at 550°C. X-ray results indicated that MnS_x phases were present within the ZnS host crystal matrix for the modulation doped samples, while a Mn_xZn_1-xS solid solution was present in the uniformly doped samples. The luminescence efficiency of the TFELDs could be modified by growing the phosphor with dopant (luminescent center) modulation. The TFELDs with a single modulated doping phosphor layer showed lower threshold voltages in the range 70 to 80 V with light emission in the 580 to 587 nm wavelength range. With a twofold increase in the total thickness of the undoped ZnS layer, the brightness and the luminescence efficiency, measured at the threshold voltage plus 40 V, increased by a factor of 20 and 10, respectively. The electroluminescent (EL) characteristics of the phosphors with multiple dopant layers showed higher luminescence efficiency. By using the TFM growth technique, one can engineer the luminescent center distribution in the phosphor layer to improve the EL characteristics.     

Cu2ZnSnS4 photovoltaic cell with improved efficiency fabricated by high‐temperature annealing after CdS buffer‐layer deposition

To improve the photovoltaic properties of Cu₂ZnSnS₄ (CZTS) cells, we investigated the effect of both the thickness of the deposited CdS layers and the post‐annealing temperature following CdS deposition on the photovoltaic properties of CZTS cells using a two‐layer CZTS structure. By depositing a thin CdS layer (40 nm) followed by high temperature annealing (603 K), we observed a remarkable increase in the short‐circuit current density because of the enhancement of the external quantum efficiency in the wavelength range of 400–800 nm. The best CZTS cell exhibited a conversion efficiency of 9.4% in the active area (9.1% in the designated area). In addition, we also fabricated a CZTS cell with open‐circuit voltage of 0.80 V by appropriately tuning the composition of the CZTS layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Enhancement of Crystalline Quality of Strained InAs／InP Quantum Well Structures by Rapid Thermal Annealing

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Enhancement of light extraction of green top-emitting organic light-emitting diodes with refractive index gradually changed coupling layers

By means of refractive index gradually changed coupling layers, a highly efficient green top-emitting OLED (TOLED) with enhanced light coupling efficiency and stable colors over angles has been realized. The refractive index transition of the coupling layers including the doping layer smoothes light extraction from the semitransparent cathode metal to the air, which is the reason for the enhancement of light coupling efficiency. The doping layer in the coupling layers also acts as a microparticle diffuser to eliminate the shift in EL spectra with viewing angles. A universal simulation has also been carried out, and the result suggests that the light coupling efficiency will be enhanced further if the refractive index transition of the coupling layers is continuous.     

Growth of AlGaN alloys exhibiting enhanced luminescence efficiency

Interest in developing ultraviolet emitters using the III-Nitride family of semiconductors has sparked considerable effort in fabricating AlGaN alloys that exhibit enhanced luminescence based on strong carrier localization, similar to their InGaN brethren. In this paper, we report on the growth of such alloys by plasma-assisted molecular beam epitaxy (PA-MBE) without the use of indium. This enhancement is attributed to the presence of nanoscale compositional inhomogeneities (NCIs) in these materials. The emission wavelength in these materials has been tuned between 275 nm and 340 nm by varying growth conditions. The effects of dislocations on double heterostructures (DHs) that employ an NCI AlGaN active region has been investigated, with an internal quantum efficiency as high as 32% obtained for the lowest dislocation density samples (3×10¹⁰ cm⁻²). Prototype DH-ultraviolet light emitting diodes (DH-UVLEDs) emitting at 324 nm were fabricated employing an NCI AlGaN alloy as the active region.     

Plasmonic Au nanoparticles for enhanced broadband light absorption in inverted organic photovoltaic devices by plasma assisted physical vapour deposition

We use a low vacuum plasma assisted physical vapour deposition (PAPVD) method to deposit a Au nanoparticles (NPs) thin film onto the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer in inverted poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methylester (P3HT:PCBM) organic photovoltaic (OPV) devices. The Au NPs that incorporated into the PEDOT:PSS layer and reached to the active P3HT:PCBM layer can provide significant plasmonic broadband light absorption enhancement to the active layer. An approximately 50–90% improvement in short-circuit current density and in power convention efficiency has been achieved compared with those OPV devices without the plasmonic light absorption enhancement. This technique can be adopted and easily fit into most OPV device fabrication processes without changing other layers’ processing methods, morphologies, and properties.     

Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO₂ or reduced graphene oxide–TiO₂ (RGO–TiO₂) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO₂ or RGO–TiO₂ interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO₂ and RGO–TiO₂ interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.     

Efficient 2.0–2.6 μm wavelength photoluminescence from narrow bandgap InAsP/InGaAs double heterostructures grown on InP substrates

Photoluminescence spectra and efficiency have been measured for several strained InAs_yP_1−yIn_xGa_1−xAs (0.28 < y ≤ 0.62; 0.66 ≤ x ≤ 0.83) double heterostructures grown by vapor phase epitaxy on InP substrates with graded InAsP buffer layers. Luminescence peak positions between the wavelengths of 1.99 and 2.57 (μm at a temperature of 295K are consistent with bandgap luminescence from the In_xGa_1−xAs active regions. Despite a high density of dislocations in the buffer layers, internal radiative recombination efficiencies of from 25 to 50% for the structures are found at 295K.