Hydrothermal Growth: Polymer‐Templated Hydrothermal Growth of Vertically Aligned Single‐Crystal ZnO Nanorods and Morphological Transformations Using Structural Polarity (Adv. Funct. Mater. 18/2010)

Position‐configurable, vertical, single‐crystalline ZnO nanorod arrays are fabricated via a polymer‐templated hydrothermal growth method at a low temperature of 93 °C. A sol‐gel processed dense c‐oriented ZnO seed layer film is employed to grow nanorods along the c‐axis direction [0001] regardless of any substrate crystal mismatches. Here, one‐beam laser‐interference lithography is utilized to fabricate nanoscale holes over an entire 2‐in. wafer during the preparation of the polymer template. As such, vertically aligned ZnO nanorods can be grown from the seed layer exposed at the bottom of each hole. Furthermore, morphological transformations of the ZnO nanorods into pencil‐like, needle‐like, tubular, tree‐like, and spherical shapes are obtained by controlling the growth conditions and utilizing the structural polarity of the ZnO nanorods.     

Polymer‐Templated Hydrothermal Growth of Vertically Aligned Single‐Crystal ZnO Nanorods and Morphological Transformations Using Structural Polarity

Position‐configurable, vertical, single‐crystalline ZnO nanorod arrays are fabricated via a polymer‐templated hydrothermal growth method at a low temperature of 93 °C. A sol‐gel processed dense c‐oriented ZnO seed layer film is employed to grow nanorods along the c‐axis direction [0001] regardless of any substrate crystal mismatches. Here, one‐beam laser‐interference lithography is utilized to fabricate nanoscale holes over an entire 2‐in. wafer during the preparation of the polymer template. As such, vertically aligned ZnO nanorods can be grown from the seed layer exposed at the bottom of each hole. Furthermore, morphological transformations of the ZnO nanorods into pencil‐like, needle‐like, tubular, tree‐like, and spherical shapes are obtained by controlling the growth conditions and utilizing the structural polarity of the ZnO nanorods.     

ZnO Nanorod Arrays as Electron Injection Layers for Efficient Organic Light Emitting Diodes

Nanostructured oxide arrays have received significant attention as charge injection and collection electrodes in numerous optoelectronic devices. Zinc oxide (ZnO) nanorods have received particular interest owing to the ease of fabrication using scalable, solution processes with a high degree of control of rod dimension and density. Here, vertical ZnO nanorods as electron injection layers in organic light emitting diodes are implemented for display and lighting purposes. Implementing nanorods into devices with an emissive polymer, poly(9,9‐dioctyluorene‐alt‐benzothiadiazole) (F8BT) and poly(9,9‐di‐n‐octylfluorene‐alt‐N‐(4‐butylphenyl)dipheny‐lamine) (TFB) as an electron blocking layer, brightness and efficiencies up to 8602 cd m^?2 and 1.66 cd A^?1 are achieved. Simple solution processing methodologies combined with postdeposition thermal processing are highlighted to achieve complete wetting of the nanorod arrays with the emissive polymer. The introduction of TFB to minimize charge leakage and nonradiative exciton decay results in dramatic increases to device yields and provides an insight into the operating mechanism of these devices. It is demonstrated that the detected emission originates from within the polymer layers with no evidence of ZnO band edge or defect emission. The work represents a significant development for the ongoing implementation of ZnO nanorod arrays into efficient light emitting devices.     

Enhancement of Light Extraction Through the Wave‐Guiding Effect of ZnO Sub‐microrods in InGaN Blue Light‐Emitting Diodes

The improvement of the light extraction efficiency (LEE) of a conventional InGaN blue light‐emitting diode (LED) by the incorporation of one‐dimensional ZnO sub‐microrods is reported. The LEE is improved by 31% through the wave‐guiding effect of ZnO sub‐microrods compared to LEDs without the sub‐microrods. Different types of ZnO microrods/sub‐microrods are produced using a simple non‐catalytic wet chemical growth method at a low temperature (90 °C) on an indium‐tin‐oxide (ITO) top contact layer with no seed layer. The crystal morphologies of needle‐like or flat‐top hexagonal structures, and the ZnO microrods/sub‐microrod density and size are easily modified by controlling the pH value and growth time. The wave‐guiding phenomenon within the ZnO rods is observed using confocal scanning electroluminescence microscopy and micro‐electroluminescence spectra.     

Beryllium‐Assisted p‐Type Doping for ZnO Homojunction Light‐Emitting Devices

A key step in realization of a ZnO homojunction light‐emitting diode is the effective p‐type doping in ZnO:N. In this article, a feasible route is demonstrated to enhance hole doping in ZnO:N films by the assistance of Beryllium. The newly synthesized p‐type ZnO is applied in light‐emitting devices. The corresponding p–i–n junction exhibits excellent diode characteristics, and strong near band edge ultraviolet emissions is also observed even at temperatures as high as 400 K under the injection of continuous current. The results represent a critical advance toward the development of high‐efficiency and stabilized p‐type ZnO, which is also a desirable key step for future ZnO‐based optoelectronic applications.     

An Indolocarbazole‐Based Thermally Activated Delayed Fluorescence Host for Solution‐Processed Phosphorescent Tandem Organic Light‐Emitting Devices Exhibiting Extremely Small Efficiency Roll‐Off

The study reports the development of a solution‐processed phosphorescent tandem organic light‐emitting device (OLED) exhibiting extremely small efficiency roll‐off. The OLED comprises two light‐emitting units (LEUs) connected by an interconnecting unit and employs a thermally activated delayed fluorescence host material. One of the most difficult tasks in the fabrication of OLEDs is to form a multilayer structure without dissolving the underlayer during the coating of the upper layer. The developed host materials exhibit high tolerance to methanol. The upper‐layer adjacent to the light‐emitting layer consists of ZnO nanoparticles, which could be dispersed in methanol by improving the preparation method. This results in the successful fabrication of a solution‐processed phosphorescent tandem OLED comprising two LEUs. The maximum external quantum efficiency (EQE) of the tandem device is 22.8%, and the EQE is 21.9% even at a high luminance of 10 000 cd m^?2. The suppression of efficiency roll‐off is among the best of those previously reported. Moreover, the operational stability of the tandem device is much higher compared with single‐LEU devices.     

ZnO纳米棒的低温制备及表征

以硝酸锌、氨水为原料,采用低温水浴法在不同的温度下大规模制备了团簇状ZnO纳米棒,采用扫描电子显微镜(SEM)、能谱分析(EDX)、X射线衍射(XRD)、室温光致发光(PL)等手段对ZnO纳米棒进行了表征.SEM结果表明,环境温度对ZnO的形貌和性质有很大的影响,随着温度增加,ZnO长径比越来越大,当温度为90℃时ZnO的平均直径100 nm,长度约为5 μm;EDX和XRD图谱表明,ZnO纳米棒是高纯的六角纤锌矿结构;对90℃条件下制备的ZnO进行光致发光性能测试,观察到波长位于423 nm附近有较强的蓝光发射.     

Enhancing Light Emission of Nanostructured Vertical Light‐Emitting Diodes by Minimizing Total Internal Reflection

Nanostructured vertical light‐emitting diodes (V‐LEDs) with a very dense forest of vertically aligned ZnO nanowires on the surface of N‐face n‐type GaN are reported with a dramatic improvement in light extraction efficiency (～3.0×). The structural transformation (i.e., dissociation of the surface nitrogen atoms) at the nanolevel by the UV radiation and Ozone treatments contributes significantly to the initial nucleation for the nanowires growth due to the interdiffusion of Zn into GaN, evident by the scanning photoemission microscopy (SPEM), high‐resolution transmission electron microscopy (HR‐TEM), and ultraviolet photoelectron spectroscopy (UPS) measurements. This enables the growth of densely aligned ZnO nanowires on N‐face n‐type GaN. This approach shows an extreme enhancement in light extraction efficiency (>2.8×) compared to flat V‐LEDs, in good agreement with the simulation expectations (～3.01×) obtained from 3D finite‐difference time‐domain (FDTD) tools, explained by the wave‐guiding effect. The further increase (～30%) in light extraction efficiency is also observed by optimized design of nanogeometry (i.e., MgO layer on ZnO nanorods).     

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.     

Charge‐Generating Mode Control in High‐Performance Transparent Flexible Piezoelectric Nanogenerators

In this work, we demonstrate the mode transition of charge generation between direct‐current (DC) and alternating‐current (AC) from transparent flexible (TF) piezoelectric nanogenerators (NGs), which is dependent solely on the morphology of zinc oxide (ZnO) nanorods without any use of an AC/DC converter. Tilted ZnO nanorods grown on a relatively low‐density seed layer generate DC‐type piezoelectric charges under a pushing load, whereas vertically aligned ZnO nanorods on a relatively high‐density seed layer create AC‐type charge generation. The mechanism for the geometry‐induced mode transition is proposed and characterized. We also examine the output performance of TF‐NGs which employ an indium zinc tin oxide (IZTO) film as a TF electrode. It is demonstrated that an IZTO film has improved electrical, optical, and mechanical properties, in comparison with an indium tin oxide (ITO) film. Enhanced output charge generation is observed from IZTO‐based TF‐NGs when TF‐NGs composed of only ITO electrodes are compared. This is attributed to the higher Schottky barrier and the lower series resistance of the IZTO‐based TF‐NGs. Thus, by using IZTO, we can expect TF‐NGs with superior mechanical durability and power generating performance.     

Flash‐Assisted Processing of Highly Conductive Zinc Oxide Electrodes from Water

Fabricating high‐quality transparent conductors using inexpensive and industrially viable techniques is a major challenge toward developing low cost optoelectronic devices such as solar cells, light emitting diodes, and touch panel displays. In this work, highly transparent and conductive ZnO thin films are prepared from a low‐temperature, aqueous deposition method through the careful control of the reaction chemistry. A robotic synthetic platform is used to explore the wide parameter space of a chemical bath system that uses only cheap and earth abundant chemicals for thin film deposition. As‐deposited films are found to be highly resistive, however, through exposure to several millisecond pulses of high‐intensity, broadband light, intrinsically doped ZnO films with sheet resistances as low as 40 Ω □^?1 can be readily prepared. Such values are comparable with state‐of‐the‐art‐doped transparent conducting oxides. The mild processing conditions (<150 °C) of the ZnO electrodes also enable their deposition on temperature sensitive substrates such as PET, paving the way for their use in various flexible optoelectronic devices. Proof‐of‐concept light emitting devices employing ZnO as a transparent electrode are presented.     

生长时间对氧化锌纳米杆形貌的影响

采用旋涂法在洗净的玻璃衬底上制备了醋酸锌薄膜,并进一步在空气中退火获得了氧化锌(ZnO)薄膜,X射线衍射分析显示退火后获得的ZnO薄膜具有c轴(002)择优取向生长特性.通过水热法以ZnO薄膜为种子层,生长了ZnO纳米杆阵列.研究了在相同的ZnO种子层、前驱液浓度和生长温度条件下,不同生长时间对ZnO纳米杆形貌的影响.扫描电子显微镜照片显示,随着生长时间的增加,ZnO纳米杆阵列的生长具有阶段性规律,并且在经过52h生长后得到了顶端中心被溶解的ZnO纳米管.分析认为该现象和前驱液中Zn2+离子和OH-离子的浓度变化有关,同时也和ZnO的非极性结构有关.     

ZnO纳米棒薄膜光学性能的系统研究

采用简化的种子层制备工艺在ITO基底上制备了ZnO种子层,并使用化学溶液沉积法制备了高度取向的ZnO纳米棒阵列。采用XRD和SEM对ZnO纳米棒的结构和形貌进行表征,并对样品的光学性能进行了测试。测试结果表明,所制备的ZnO纳米棒为c轴择优取向的六角纤锌矿结构,直径为66~122nm可控,且排列紧密,形貌规整。光学性能测试结果表明,吸收光谱在375nm附近表现出强烈的紫外吸收边是由于禁带边吸收引起的;反射光谱具有一定的周期振荡性,可用于薄膜厚度的估算;光致发光谱在378nm附近有很强的紫外发射峰;增大生长液浓度和高温退火可降低缺陷发光,改善结晶质量。     

Uniform 3D hydrothermally deposited zinc oxide nanorods with high haze ratio

We present low cost hydrothermally deposited uniform zinc oxide (ZnO) nanorods with high haze ratios for the a-Si thin film solar cells. The problem of low transmittance and conductivity of hydrothermally deposited ZnO nanorods was overcome by using RF magnetron sputtered aluminum doped zinc oxide (ZnO:Al ~300 nm) films as a seed layer. The length and diameters of the ZnO nanorods were controlled by varying growth times from 1 to 4 h. The length of the ZnO nanorods was varied from 1 to 1.5 µm, while the diameter was kept larger than 300 nm to obtain various aspect ratios. The uniform ZnO nanorods showed higher transmittance (~89.07%) and haze ratio in the visible wavelength region. We also observed that the large diameters (>300 nm) and average aspect ratio (3–4) of ZnO nanorods favored the light scattering in the longer wavelength region. Therefore, we proposed uniformly deposited ZnO nanorods with high haze ratio for the future low cost and large area amorphous silicon thin film solar cells.     

High‐Performance,Phosphorescent, Top‐Emitting Organic Light‐Emitting Diodes with p–i–n Homojunctions

High‐performance, green, orange, and red top‐emitting organic light‐emitting diodes (TOLEDs) with p–i–n homojunction are demonstrated. An excellent ambipolar host, 2,5‐bis(2‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (o‐CzOXD), which has good thermal and morphological stabilities, a high triplet energy level, and equally high electron and hole mobilities, is chosen as the organic host material for the homojunction devices. By electrical doping, the carrier injection and transporting characteristics are greatly improved. The optical structure is optimized in view of light emission of different colors to enhance the color purity and improve the view characte­ristics. As a result, high efficiency p–i–n homojunction TOLEDs with saturated intrinsic emission of the emitting materials and angular independence of the emission are realized. The performances of these p–i–n homojunction TOLEDs are even higher than the multi‐layer heterojunction bottom‐emitting devices using the same emitting layers.     

Solution‐Processed Extremely Efficient Multicolor Perovskite Light‐Emitting Diodes Utilizing Doped Electron Transport Layer

A specially designed n‐type semiconductor consisting of Ca‐doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high‐performance multicolor perovskite light‐emitting diodes (PeLEDs) fabricated using an all‐solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub‐bandgap turn‐on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A^?1 and 5.8% for red emission, 16 cd A^?1 and 4.2% for yellow, and 21 cd A^?1 and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W^?1 for green light‐emitting PeLED, 28 lm W^?1 for yellow, and 36 lm W^?1 for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two‐step hot‐casting technique that affords highly continuous (>95% coverage) and pinhole‐free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m^?2 are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite‐based materials a step closer to practical application in multicolor light‐emitting devices.     

籽晶层退火温度对ZnO纳米棒形貌及发光特性的影响

以不同退火温度处理后的ZnO籽晶层为基底,采用水热法生长了ZnO纳米棒阵列。对制备得到的ZnO纳米棒阵列的形貌、结构以及发光特性进行了表征,分析了籽晶层的退火温度对ZnO纳米棒阵列的形貌及发光性质的影响,发现通过调节籽晶层的退火温度,可以控制ZnO纳米棒的大小及密度,并发现在经400℃退火后的籽晶层上生长的ZnO纳米棒阵列形貌最佳,发光性能最优。     

White Light‐Emitting Diode From Sb‐Doped p‐ZnO Nanowire Arrays/n‐GaN Film

A whole interfacial transition of electrons from conduction bands of n‐type material to the acceptor levels of p‐type material makes the energy band engineering successful. It tunes intrinsic ZnO UV emission to UV‐free and warm white light‐emitting diode (W‐LED) emission with color coordinates around (0.418, 0.429) at the bias of 8–15.5 V. The W‐LED is fabricated based on antimony (Sb) doped p‐ZnO nanowire arrays/Si doped n‐GaN film heterojunction structure through one‐step chemical vapor deposition with quenching process. Element analysis shows that the doping concentration of Sb is ≈1.0%. The I–V test exhibits the formation of p‐type ZnO nanowires, and the temperature‐dependent photoluminescence measurement down to 4.65 K confirms the formation of deep levels and shallow acceptor levels after Sb‐doping. The intrinsic UV emission of ZnO at room temperature is cut off in electroluminescence emission at a bias of 4–15.5 V. The UV‐free and warm W‐LED have great potential application in green lights program, especially in eye‐protected lamp and display since television, computer, smart phone, and mobile digital equipment are widely and heavily used in modern human life, as more than 3000 h per year.     

Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes

We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a single electron injection layer for indium tin oxide cathode, which possess comparable efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a PEIE layer between light emitting polymer layer and aluminum has been shown to significantly enhance device efficiency as well. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE serves as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels.     

Polymeric Light‐Emitting Diodes Based on Poly(p‐phenylene ethynylene), Poly(triphenyldiamine), and Spiroquinoxaline

In this paper polymeric light‐emitting diodes (LEDs) based on alkoxy‐substituted poly(p‐phenylene ethynylene) EHO‐OPPE as emitter material in combination with poly(triphenyldiamine) as hole transport material are demonstrated. Different device configurations such as single‐layer devices, two‐layer devices, and blend devices were investigated. Device improvement and optimization were obtained through careful design of the device structure and composition. Furthermore, the influence of an additional electron transporting and hole blocking layer (ETHBL), spiroquinoxaline (spiro‐qux), on top of the optimized blend device was investigated using a combinatorial method, which allows the preparation of a number of devices characterized by different layer thicknesses in one deposition step. The maximum brightness of the investigated devices increased from 4 cd/m² for a device of pure EHO‐OPPE to 260 cd/m² in a device with 25 % EHO‐OPPE + 75 % poly(N,N′‐diphenylbenzidine diphenylether) (poly‐TPD) as the emitting/hole‐transporting layer and an additional electron‐transport/hole‐blocking spiro‐qux layer of 48 nm thickness.