Fabrication of PDMS microlens array by digital maskless grayscale lithography and replica molding technique

This paper presents a facile and effective method to fabricate microlens array in polydimethylsiloxane (PDMS). The microlens array model is fabricated in photoresist via digital maskless grayscale lithography technique and the replica molding technique is used to fabricate PDMS microlens array. A convex PDMS microlens array with rectangular aperture and concave PDMS microlens array with hexagonal aperture are fabricated. The morphological characteristics of the microlens arrays are measured by microscope and 3D profiler. The results indicate that the profiles of the PDMS microlens arrays are clear and distinct. This method provides a simple and low-cost approach to prepare large area, concave or convex with arbitrary shape microlens array, which has potential application in many optoelectronic devices.     

Numerical and experimental studies of mixed-host organic light emitting diodes

Electrical characteristic and luminance of three mixed-host organic light emitting diodes (OLEDs): namely the uniformly mixed, step-wise graded and mixed, and continuously graded and mixed, were compared with the conventional hetero-junction OLED in both numerical and experimental studies. These mixed-host OLEDs were fabricated by a mixed-source thermal evaporation process, and half-cell devices were also fabricated to provide some input parameters for OLED simulations. The current efficiencies were largely influenced by their device structures and strongly agreed with the computed current balance factors. The improved mixed-host OLED performances can be discussed with aid from simulations, which include spatial distributions of electron and hole, carrier mobility, electric field profiles, the total recombination rates in the light emitting layer.     

Electroluminescence from hybrid organic–inorganic LEDs based on thermally evaporated CdS thin films

Hybrid organic–inorganic light emitting devices combine the color purity and durability of inorganic light emitting diodes (LEDs) with high efficiency, flexibility and low processing cost of organic LEDs (OLEDs). A significant challenge is to incorporate inorganic nanocrystals inside the OLED structure. In the present work, thin films of CdS were successfully incorporated inside standard OLED structure using vacuum thermal evaporation technique. For the characterization of these films, they were deposited on plain glass plates at room temperature and studied using structural (XRD and TEM), morphological (SEM and AFM) and optical (UV and PL) techniques. The films were found to be composed of nanocrystals of CdS in which the size of the crystals increased with the increase in film thickness. The hybrid organic–inorganic LEDs showed improved luminance and efficiency as compared to the organic LED without CdS layers.     

Efficiency improvement and spectral shift of an organic light-emitting device with a square-based microlens array

In this paper, we present and analyze the influences of the fill factor and the height ratio of square-based microlens arrays on the optical characteristics of an organic light-emitting device (OLED). These properties include spectral shift, CIE coordinates, viewing-angle dependence, luminous current efficiency and luminous power efficiency. Both the luminous current efficiency and luminous power efficiency of the OLED were found to increase linearly with increasing the fill factor of microlenses. It is also found that the full width at half maximum (FWHM) of the OLED spectra and CIE coordinate decreased linearly with increasing the fill factor of the microlenses. Besides, the efficiency improvement of the OLED increased with the height ratio of attached microlenses. Compared to the OLED, the luminous current efficiency and luminous power efficiency of the device can be enhanced by 42% and 47%, respectively, by attaching the microlens array having a fill factor of 90% and a height ratio of 0.57. We also observed blue shifts at different viewing angles when microlens arrays were attached to the OLED, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifted and the FWHM decreased with increasing the viewing angles due to the microcavity effect.     

Depth dependence of Néel wall pinning on amorphous CoxSi1−x films with diluted arrays of elliptical antidots

Diluted arrays of elliptical antidots have been fabricated by optical lithography, electron beam lithography and plasma etching on amorphous Co₇₄Si₂₆ magnetic films with a well-defined uniaxial anisotropy. The magnetic behavior of two identical antidot arrays but with different hole depth in comparison with film thickness has been studied by transverse magneto-optical Kerr effect. Significant differences appear in the coercivity depending on whether the magnetic film is completely perforated or not, indicating a much more effective domain wall pinning process when the depth of the holes is smaller than the magnetic film thickness.     

Organized molecular assemblies for scanning probe microscope lithography

Nanostructures down to a few ten nanometers in size have been fabricated with Langmuir–Blodgett (LB) films and self-assembled monolayers (SAMs) using scanning probe microscope lithography. The SAMs have been prepared with organosilane and bipolar amphiphiles, alkanethiol molecules as ultrathin resists on Si and Au substrates. The LB films on silicon substrates using both the polymer of thiophene derivatives and the mixture of palmitic acid and hexadecylamine were prepared and fabricated. The effect of functional groups of molecules on the atomic force microscope (AFM) anodization has been studied in the optimized process conditions. Applied voltage between the AFM tip and sample, the scanning speed and the relative humidity in the laboratory are also important factors for nanometer-scale lithography of the ultrathin films. The STM lithography with an octadecanethiol SAM on Au film in the air was carried out at the pulse mode of tip bias with respect to the suitability of STM lithography. The high structural orderness and perfect thickness of ultrathin organic molecular assemblies are the major advantages as required for nanoscale lithography.     

有机微腔绿色发光二极管

光学微腔是指尺寸在光波长量级的光学微型谐振腔。微腔结构可以使腔内物质和光场的相互作用与体材料相比发生很大变化,出现了自发辐射谱线窄化和增强等腔效应。利用这些腔效应,可以改善有机发光器件的性能。采用微腔结构,优化设计并研制了有机微腔绿色发光二极管,器件结构为Glass/DBR/ITO/NPB/Alq∶Rubrene/Alq/MgAg,获得了最大亮度40100 cd/m2、最大发光效率为6.44 cd/A、半峰全宽为28 nm的纯绿色有机微腔电致发光器件。而与之比较的无腔器件最大亮度为22580 cd/m2、最大发光效率为2.98 cd/A、半峰全宽为120 nm。相同电流密度下微腔电致发光谱的峰值发射强度是无腔器件的4.2倍。结果表明将微腔结构引入有机电致发光器件中,不但改善了发光的色纯度,而且使器件的发光效率和亮度都得到明显增强。     

Design and Fabrication of Microlens Array for Near-Field Vertical Cavity Surface Emitting Laser Parallel Optical Head

A GaP microlens for collecting laser light was developed in the tip of a near-field probe. It is important to realize a near-field optical probe head with high throughput and a small spot size. The design and fabrication results of the GaP microlens array are described. The most suitable GaP microlens with a probe was calculated as having a 10 μm radius using the two-dimensional finite difference time domain (2-D FDTD) method. The full width half maximum (FWHM) spot size variation and optical power density tolerance were calculated as 157 nm ± 5 nm and 7%, respectively. A spherical GaP microlens was fabricated with a radius of 10 μm by controlling the Cl₂/Ar gas mixture ratio. The difference between the theoretical spherical shape and the fabricated GaP microlens was evaluated as 40 nm at peak to valley. The FWHM spot size and optical throughput of the fabricated microlens were measured as 520 nm and 63%, respectively. The microlens was the same as a theoretical lens with a 10 μm radius. The micron-lens array fabrication process for a near-field optical head was demonstrated in this experiment.     

IPTO薄膜制备及其在有机光电器件中的应用

新型IPTO(Pr Ti O3掺杂In2O3)薄膜的可见光透过率及导电性可与商业化的ITO薄膜媲美。采用双源电子束设备制备了一种新型的IPTO透明导电薄膜,通过开尔文探针法测试,其功函数为5.14 e V。为验证新型IPTO透明导电阳极对有机电致发光器件性能的影响,将IPTO替代商业化ITO作为阳极制备了有机电致发光器件。基于IPTO阳极的器件的亮度最大值为85 140 cd/m2,外量子效率最大值为3.16%,分别为以ITO为阳极的器件的3倍及1.13倍。这种性能的改善是由于IPTO具有较小的表面粗糙度及较高的功函数,可以降低阳极的注入势垒,有利于电荷向有机层注入,改善了器件内的空穴及电子的注入平衡。     

聚合物自写入光波导的时域有限差分模拟

利用时域有限差分方法,对非线性光学聚合物SU-8环氧树脂在激光作用下,其折射率会升高的过程进行了数值模拟与分析,得到SU-8薄膜与激光作用一定时间之后,其内部折射率的分布。分别对表面是平面及表面带有微透镜的SU-8薄膜进行了模拟。结果表明,利用折射率升高引起的自聚焦效应,可以在SU-8薄膜内自写入波导;写入的波导以自聚焦焦点为分界点,分为锥形波导和柱形波导。写入的波导长度随着与激光作用时间的增加而增长;在与激光相同的作用时间里,表面带有微透镜的SU-8薄膜,与表面是平面的SU-8薄膜相比,锥形波导的旁瓣更收敛,写入的波导长度更长;在微透镜底面半径保持不变以及曝光时间一定时,微透镜冠高与底面半径比例为0.08时比比例为0.24时写入的波导长度要长12.2%,其中锥形波导部分要长19.2%。     

Photoetching of spherical microlenses on glasses using a femtosecond laser

We fabricated spherical microlenses on optical glasses by femtosecond laser direct writing (FLDW) in ambient air. To achieve good appearances of the microlenses, a meridian-arcs scanning method was used after a selective multilayer removal process with spiral scanning paths. A positive spherical microlens with diameter of 48 μm and height of 13.2 μm was fabricated on the surface of the glass substrate. The optical performances of the microlens were also tested. Compared to the conventional laser direct writing (LDW) technique, this work could provide an effective method for precise shape-controlled fabrication of three-dimensional (3D) microstructures with curved surfaces on difficult-to-cut materials for practical applications.     

Improvement of the outcoupling efficiency of an organic light-emitting device by attaching microstructured films

In this paper, we present and analyse the optical characteristics, such as spectral shift, CIE coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency, of an organic light-emitting device (OLED) with a commercial diffuser film or a brightness-enhancement film (BEF) attached. Compared to a planar green OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 29% and 23%, respectively. The overall luminous power efficiencies are enhanced by 28% and 7%. Compared to the planar green device, we observe blue shifts at different viewing angles when microstructured films are attached, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifts and the full width at the half maximum (FWHM) decrease with increasing viewing angles due to the microcavity effect. When the diffuser is attached, the spectral peak has a constant blue shift (6 nm) compared to that of the planar OLED. On the other hand, in the BEF case, the spectral shift depends on the viewing angle (2-12 nm blue shifts from 0 to 80°). This is due to the different operating principles (scattering and redirected light) of the diffuser and BEF. Since the transmittance spectra of both the diffuser film and the BEF are flat over the visible range, it is suitable for lighting applications by using white OLED. When attaching the films on a commercial white OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 34% and 31%, respectively. The overall luminous power efficiencies are enhanced by 42% and 8%.     

基于石墨烯/PEDOT：PSS叠层薄膜的柔性OLED器件

石墨烯具有独特的电学性能、优异的机械延展性和良好的化学稳定性,是制备高性能导电薄膜的理想材料,但是当前石墨烯的高电阻率限制了它的实际应用。本文采用喷涂方法制备了石墨烯/聚（3,4-亚乙二氧基噻吩）-聚（苯乙烯磺酸）（PEDOT：PSS）复合导电薄膜,对复合薄膜的表面形貌与光电性能进行了研究。PEDOT：PSS的引入不仅降低了石墨烯薄膜的表面电阻,同时还平滑了薄膜表面。在此基础上,成功制备了柔性黄光有机电致发光器件,器件在12 V时达到效率最大值0.9 cd/A。器件在曲率半径为10 mm时弯曲了100次后,发光亮度并无明显变化。该复合薄膜可实际应用于柔性有机电致发光显示器件。     

Microlens array diffuser for a light-emitting diode backlight system

Microlens array (MLA) diffusers for light-emitting diode (LED) backlight systems have been developed. A high fill-factor photoresist mold for the MLA was fabricated using three-dimensional diffuser lithography, and the patterns were transferred to a nickel master mold for UV-curable polymer replication. The fabricated microlens had various paraboloidal profiles, and its aspect ratio was controlled from 1.0 to 2.1. The MLA diffuser showed a batwing radiation pattern with a radiation angle of 150 degrees. The fabricated MLA diffuser may greatly enhance the color-mixing characteristics of LED backlight systems and help reduce the number of LEDs required.     

用于头盔显示液晶像源的透镜薄膜设计与制备

LED已经成为头盔液晶显示的主流背光光源,为了提升LED主视角亮度,减小背光模块的体积,对双自由曲面透镜进行了扁平化设计,形成透镜薄膜,仿真结果表明光线透过透镜薄膜后,存在中心亮度升高、光斑缩小的问题。采用反馈优化方法修改了双自由曲面透镜设计时的能量分布,并重新设计了透镜薄膜,仿真结果表明优化后的光斑尺寸从2.3 mm增加到7.1 mm。采用无掩模直写光刻技术制备了两层透镜薄膜,分别包含一个扁平化后的自由曲面。对包含和不包含透镜薄膜的背光模块进行了测试,测试结果表明:包含透镜薄膜的背光模块平均亮度比不包含的提升了18.1%,设计的透镜薄膜可有效提升LED背光亮度,减小背光模块体积。     

Research on micro-optical lenses fabrication technology

We present a detail investigation on the development of a series of gradient index (GRIN) optical glass microlens and polymer microlens and microlens arrays in our laboratory in recent years. The special glass material GRIN lenses have been fabricated mainly by using ion-exchange technology, which are applied to construct micro-optic devices and other applications. On one hand, we demonstrated the light propagation and imaging properties of GRIN lenses and the results analyzed. On the other hand, we have explored a drop-on-demand ink-jet printing method to produce microlens array using nano-scale polymer droplets involved with a uniform ultraviolet light and heat solidifying process. The experimental setup for manufacturing polymer microlens array and the performance of refractive microlens elements are also given in this paper.     

Nano-image profiles transferred by near field phase-shifting lithography precisely simulated by finite element method and fabricated

The operating frequency of the SAW filters is limited by the gap width but not the line width. The narrow gap width is required for the high operating frequency SAW filters. Therefore, in this study, high precision nano-image profiles transferred by near field phase-shifting mask (NFPSM) lithography at various exposure-energy-intensities (EEIs) are simulated by the finite element method (FEM). The transferred energy-intensity distribution (TEID) in the photoresist during the NFPSM process (at the wavelength of 248 nm) can be accurately simulated by the FEM. The TEID at the interface between the mask and the photoresist is also simulated. The fabricated pattern widths clearly match the simulation results. The study of the simulated image profiles shows that they are dependent on the EEI. The greater the EEI is, the narrower the width and the shorter the height of the image profile. The nano-linewidth of 60 nm is simulated and fabricated. The fabricated nano-imaging profile precisely fits the simulation results. Therefore, any expected nano-image profile can easily be fabricated by way of the simulation.     

Plasma treatment of ITO surfaces to improve luminescence characteristics of organic light-emitting devices with dopants

Indium-tin-oxide (ITO) surfaces were treated with O₂ and H₂ plasmas. The contact angle of water, X-ray photoelectron spectroscopy (XPS) spectra, and luminescence characteristics of organic light-emitting devices (OLEDs) fabricated on plasma-treated ITO surfaces using poly(N-vinylcarbazole) (PVK) films doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) or Nile Red were investigated.Using O₂ plasma treatment, the contact angle was reduced from 35° to 13°. The luminance of OLEDs with rubrene was significantly improved. The luminous color of an OLED inserted poly (3,4-ethylenedioxythiophene)-polystyrene sulphonic acid (PEDOT-PSS) layer between Nile Red-doped PVK film and O₂-plasma-treated ITO surface turned white as the applied voltage increased.     

Optical film with microstructures array for slim-type backlight applications

A novel optical film with optical microstructures is proposed for enhancing the light control in slim-type backlight modules. The proposed film enables a homogeneous luminance distribution to be obtained though an appropriate adjustment of the filling ratio of the optical microstructures in different regions of the backlight unit. The simulation results show that the luminance provided by a backlight incorporating a filling-ratio-optimized optical film and a pyramid-structured brightness enhancement film is approximately 1.5 times of the luminance obtained from a backlight with conventional optical films. In addition, the proposed film enables a significant reduction in the thickness of the backlight unit. Thus, the novel optical film is suitable for a slim bottom-lit backlight module.     

Improvement in crystalline quality and surface smoothness of ZnO film by multi-step deposition process

The effect of multi-step deposition process on the crystalline quality and surface smoothness of ZnO film was investigated. ZnO films were composed of multi-layers, in which each layer was deposited at different temperatures. The maximum intensity and the smallest FWHM of (0 0 2) diffraction peak in XRD spectrum were observed for the multi-layered ZnO film of which each layer was deposited at progressively higher temperatures. In addition, the smoothest film surface was also observed for the ZnO film deposited through multi-step process in which deposition temperatures gradually increase. On the other hand, the large difference between the deposition temperatures in multi-step process did not result in the significant improvement of the crystalline quality of ZnO film. The ZnO film prepared by using multi-step process had high transmittance over 70% in visible region and the optical band gap of 3.22 eV.