Effect of the polymer emission on the electroluminescence characteristics of n-ZnO nanorods/p-polymer hybrid light emitting diode

Hybrid light emitting diodes (LEDs) based on zinc oxide (ZnO) nanorods and polymers (single and blended) were fabricated and characterized. The ZnO nanorods were grown by the chemical bath deposition method at 50°C. Three different LEDs, with blue emitting, orange-red emitting or their blended polymer together with ZnO nanorods, were fabricated and studied. The current–voltage characteristics show good diode behavior with an ideality factor in the range of 2.1 to 2.27 for all three devices. The electroluminescence spectrum (EL) of the blended device has an emission range from 450 nm to 750 nm, due to the intermixing of the blue emission generated by poly(9,9-dioctylfluorene) denoted as PFO with orange-red emission produced by poly(2-methoxy-5(20-ethyl-hexyloxy)-1,4-phenylenevinylene) 1,4-phenylenevinylene) symbolized as MEH PPV combined with the deep-band emission (DBE) of the ZnO nanorods, i.e. it covers the whole visible region and is manifested as white light. The CIE color coordinates showed bluish, orange-red and white emission from the PFO, MEH PPV and blended LEDs with ZnO nanorods, respectively. These results indicate that the choice of the polymer with proper concentration is critical to the emitted color in ZnO nanorods/p-organic polymer LEDs and careful design should be considered to obtain intrinsic white light sources.     

Hybrid fluorescent polymer-zinc oxide nanoparticles: Improved efficiency for luminescence conversion LED

An incorporation of few weight percentage of n-type zinc oxide (ZnO) on the surface of yellow-emitting fluorescent polymer under mild conditions was demonstrated. Here, a deep level emissive ZnO was selectively deposited on the surface of fluorescent polymer via a simple chemical deposition bath method, at relatively low temperature. The polymer-zinc oxide hybrids, consisting of uniform nanosized spherical fluorescent polymer, having mean diameter ca. ∼500-700 nm were subjected as core molecules capped with different weight ratio of ZnO on the surfaces were prepared successfully. The relative photoluminescence emission efficiency was drastically enhanced as two-fold with just 4 wt% of ZnO incorporation and also more than 10-fold improvement in 50 wt% of ZnO content with respect to pure fluorescent polymer. Bright and efficient white light-emitting devices have been fabricated with these hybrid materials, such as luminescence converter light-emitting diodes (LUCO LEDs), using commercially available GaN LED (460 nm), as a primary pumping source. A device containing 20 wt% of ZnO incorporated hybrid material (2 wt%) exhibits nearly pure white light, having Commission Internationale de I’Eclairage coordinates of (0.30, 0.36) and total luminous flux of 1.80 lm, at an operating voltage of 20 mA. The lifetime measurement data of fabricated device containing polymer-ZnO hybrid materials showed significant improvements over the pure counterpart, due to the “caging effect” of the ZnO shell, which can reduce the self-quenching of the polymer molecules in the core.     

Bandgap engineering of GaxZn1–xO nanowire arrays for wavelength‐tunable light‐emitting diodes

Wavelength‐tunable light‐emitting diodes (LEDs) of Ga_xZn_1–xO nanowire arrays are demonstrated by a simple modified chemical vapor deposition heteroepitaxial growth on p‐GaN substrate. As a gallium atom has similar electronegativity and ion radius to a zinc atom, high‐level Ga‐doped Ga_xZn_1–xO nanowire arrays have been fabricated. As the x value gradually increases from 0 to 0.66, the near‐band‐edge emission peak of Ga_xZn_1–xO nanowires shows a significant shift from 378 nm (3.28 eV) to 418 nm (2.96 eV) in room‐temperature photoluminescence (PL) measurement. Importantly, the electroluminescence (EL) emission of Ga_xZn_1–xO nanowire arrays LED continuously shifts with a wider range (∼100 nm), from the ultraviolet (382 nm) to the visible (480 nm) spectral region. The presented work demonstrates the possibility of bandgap engineering of low‐dimensional ZnO nanowires by gallium doping and the potential application for wavelength‐tunable LEDs.     

广色域钙钛矿量子点/荧光粉转换白光LED

报道了一种使用绿色CsPb（Br_0.75I_0.25）₃无机钙钛矿量子点（PeQDs）和红色K₂SiF₆：Mn⁴⁺（KSF）荧光粉作为荧光转换材料实现广色域白光LED的方法。合成了绿色CsPb（Br_0.75I_0.25）₃量子点,峰值波长为526 nm,半高宽度为27 nm,具有很好的单色性。采用蓝光LED芯片、红色KSF荧光粉和绿色CsPb（Br_0.75I_0.25）₃ PeQDs组合能够覆盖CIE 1931颜色空间中很广的色域,达到NTSC标准色域的107%。利用丝网印刷和紫外固化工艺制作了PeQDs薄膜、KSF薄膜和PeQDs-KSF混合薄膜,与蓝光LED芯片组合得到了3种不同封装形式的白光LED器件。研究了不同封装形式对器件光学特性的影响,KSF薄膜在外侧的样品光效最高,为102 lm/W,色温为7 100 K。     

Gallium and indium co‐doped ZnO thin films for white light emitting diodes

Ga and In co‐doped ZnO (GIZO) thin films together with ZnO, In‐doped ZnO (IZO), Ga‐doped ZnO (GZO), and IZO/GZO multilayer for comparison, were grown on corning glass and boron doped Si substrates by PLD. The photoluminescence spectra of GIZO showed a strong white light emission and the current–voltage characteristics showed relatively lower turn‐on voltage and larger forward current. The CIE coordinates for GIZO were observed to be (0.31, 0.33) with a correlated colour temperature of 6650 K, indicating a cool white light, and establishing a possibility of white light emitting diodes. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of UV excited white light‐emitting diodes for optimization of luminous efficiency and color rendering index

Wavelength down‐converted white‐light sources excited by near ultraviolet light‐emitting diodes require specific phosphor properties in order to generate high‐quality white light (namely, light with good color rendition and stability of color coordinate). Simulation and experimental results are discussed, with particular emphasis on the spectral distribution property of red phosphor required to realize high values of luminous efficiency and color rendition. A peak wavelength of 610 nm and an FWHM of 80 nm for the spectral power distribution were proposed, and co‐doped phosphate materials were synthesized successfully. This can contribute to a white‐light source with a luminous efficiency of 45 lm/W and color rendering index greater than 90 at a color temperature of 5600 K and an operational current of 20 mA. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Color tunable light‐emitting diodes with modified pulse‐width modulation

In this study, a color tunable light source, operated by a modified pulse width modulation method, is investigated. By utilizing this method along with anti‐parallel connected discrete light‐emitting diodes (LEDs) and two electrical terminals, a wide range of the chromaticity coordinates is attained and varied by electrical control. Using the combination of a blue LED and a phosphor‐converted yellow LED (blue LED plus yellow phosphor), the chromaticity range is varied by electrical control from pure blue to pure yellow. In addition, using the modified pulse‐width modulation method and a combination of white and red LEDs, white light with correlated color temperatures ranging from 5000 K to 2000 K is demonstrated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced Device Performances of Blue‐Emitting PLEDs Coupled with Silver‐Nanoicosahedrons

Silver‐nanoicosahedron particles (AgNIPs) are produced by chemical reduction and photochemical methods and doped into the hole transport layer (HTL) or emissive layer (EML) of blue‐emitting polymer light‐emitting diodes (PLEDs) to improve their luminous efficiency. The optimal distributed‐densities of the AgNIPs are determined from current density–voltage–luminance measurements at different doping concentrations. The AgNIP dopant doses that maximize the average luminous efficiency of the proposed PLED are 6.71 µg cm^?2 in EML (achieving 3.48 cd A^?1) and 6.88 µg cm^?2 in HTL (achieving 3.35 cd A^?1). Although the luminous efficiencies of the blue‐emitting PLEDs fabricated by both doping methods are not significantly different, the maximum plasmonic enhancement (around 30‐fold) of the blue‐emitting PLED with AgNIPs in EML is red‐shifted to the green region (≈530 nm in the electroluminescence spectrum), seriously degrading the luminescent monochromaticity of the blue‐emitting PLED. The maximum plasmonic enhancement (around 33‐fold) of blue‐emitting PLED with AgNIPs in HTL occurred at 430 nm, overlapping the localized surface‐plasmon resonance extinctions of the AgNIPs in HTL (425 nm), thus favoring the enhancement of fluorescence emission. Therefore, to enhance the large‐area emission of blue‐emitting PLEDs, the AgNIPs should be doped in the HTL rather than the EML.     

Si(110)和Si(111)衬底上制备InGaN/GaN蓝光发光二极管

分别在Si(110)和Si(111)衬底上制备了In Ga N/Ga N多量子阱结构蓝光发光二极管(LED)器件.利用高分辨X射线衍射、原子力显微镜、室温拉曼光谱和变温光致发光谱对生长的LED结构进行了结构表征.结果表明,相对于Si(111)上生长LED样品,Si(110)上生长的LED结构晶体质量较好,样品中存在较小的张应力,具有较高的内量子效率.对制备的LED芯片进行光电特性分析测试表明,两种衬底上制备的LED芯片等效串联电阻相差不大,在大电流注入下内量子效率下降较小;但是,相比于Si(111)上制备LED芯片,Si(110)上LED芯片具有较小的开启电压和更优异的发光特性.对LED器件电致发光(EL)发光峰随驱动电流的变化研究发现,由于Si(110)衬底上LED结构中阱层和垒层存在较小的应力/应变而在器件中产生较弱的量子限制斯塔克效应,致使Si(110)上LED芯片EL发光峰随驱动电流的蓝移量更小.     

掺杂聚合物蓝光发光二极管

报道了用有机染料ＴＰＢ（１，ｌ，４，４－四苯基丁二烯）分散到ＰＶＫ（聚乙烯基咔唑）中的掺杂聚合物作有源层制作的蓝光发光二极管及其发光特性。聚合物发光层用旋转涂敷的方法制备，用透明导电材料ＩＴＯ（铟锡氧化物）、金属Ａｌ作为正负电极。器件正向偏压为１３Ｖ时，可以看到蓝光发射，峰值波长为４５５ｎｍ，注入电流为５０ｍＡ／ｃｍ２时，亮度为４４ｃｄ／ｍ２。     

Effect of MgZnO barrier layer on the UV emission of n-ZnO/p-Si heterojunction diodes

ZnO-based heterojunction light emitting diodes (LEDs) with MgZnO barrier layer had been fabricated on the p-Si substrate by metal-organic chemical vapor deposition (MOCVD) technology. The current-voltage (I-V) characteristics exhibited a typical p-n diode behavior. Both ultraviolet (UV) and visible emissions could be detected in the electroluminescence (EL) measurement. The result was compared with the EL spectrum of n-ZnO/p-Si heterojunction LED without MgZnO barrier layer. An improved light extraction efficiency by about 31% was realized owing to the current-blocking effect of MgZnO layer. The result indicated that MgZnO barrier layer can prevent the electrons as expected and realize electron-hole recombination in ZnO layer effectively.     

Blue electroluminescence nanodevice prototype based on vertical ZnO nanowire/polymer film on silicon substrate

We present a polymer-complexing soft template technique to construct the ZnO-nanowire/polymer light emitting device prototype that exhibits blue electrically driven emission with a relatively low-threshold voltage at room temperature in ambient atmosphere, and the ZnO-nanowire-based LED’s emission wavelength is easily tuned by controlling the applied-excitation voltage. The nearly vertically aligned ZnO-nanowires with polymer film were used as emissive layers in the devices. The method uses polymer as binder in the LED device and dispersion medium in the luminescence layer, which stabilizes the quasi-arrays of ZnO nanowires embedding in a thin polymer film on silicon substrate and passivates the surface of ZnO nanocrystals, to prevent the quenching of luminescence. Additionally, the measurements of electrical properties showed that ZnO-nanowire/polymer film could significantly improve the conductivity of the film, which could be attributed to an increase in both Hall mobility and carrier concentration. The results indicated that the novel technique is a low-cost process for ZnO-based UV or blue light emission and reduces the requirement for achieving robust p-doping of ZnO film. It suggests that such ZnO-nanowire/polymer-based LEDs will be suitable for the electro-optical application.     

The luminescence of ZnO film grown on GaAs interlayer by PA-MOCVD

ZnO film was firstly prepared by PA-MOCVD method on the substrate pre-coated with GaAs interlayer. Hall measurement found that the GaAs interlayer had important effects on the electrical behavior of the ZnO film. It could make the ZnO film convert to p-type conductivity. The XPS results confirmed that the acceptor was arsenic. And the acceptor level was 130 meV above the ZnO valence band maximum. Low-temperature PL measurement was introduced to investigate the optical properties of both as-grown n-type and arsenic doped p-type ZnO films. Then, based on this technology, ZnO homojunction light emitting device (LED) was fabricated with arsenic doped p-type ZnO and unintentionally doped n-type ZnO on GaAs/p⁺-Si substrate. Its current-voltage (I-V) character showed a typical rectification behavior, which was different from the n-ZnO/p⁺-Si structure. The UV-visible (385-580 nm) electroluminescence was detected under relatively low current injection condition from the n-ZnO/p-ZnO/p⁺-Si LED.     

A comparative study of the electrodeposition and the aqueous chemical growth techniques for the utilization of ZnO nanorods on p-GaN for white light emitting diodes

Vertically well aligned zinc oxide nanorods (ZnO NRs) were grown on p-GaN by electrodeposition (ED) and aqueous chemical growth (ACG) techniques and the structures were employed to fabricate white light emitting diodes (LEDs). Room temperature current voltage (I–V$I–V$), photoluminescence (PL), and electroluminescence (EL) measurements were performed to investigate and compare both LEDs. In general, the I–V$I–V$ characteristics and the PL spectra of both LEDs were rather similar. Nevertheless, the EL of the ED samples showed an extra emission peak shoulder at 730 nm. Moreover, at the same injection current, the EL spectrum of the ED light emitting diode showed a small UV shift of 12 nm and its white peak was found to be broader when compared to the ACG grown LED. The broadening of the EL spectrum of the LED grown by ED is due to the introduction of more radiative deep level defects. The presented LEDs have shown excellent color rendering indexes reaching a value as high as 95. These results indicate that the ZnO nanorods grown by both techniques possess very interesting electrical and optical properties but the ED is found to be faster and more suitable for the fabrication of white LEDs.     

基于Ce:YAG单晶的白光发光二极管性能研究

以Ce:YAG单晶取代传统Ce:YAG荧光粉用于制备白光发光二极管(LED),研究了Ce:YAG单晶厚度及驱动电压的变化对其发射光谱、色坐标、亮度、光视效能和色温的影响。研究结果表明,在基于Ce:YAG单晶的白光LED中,发射光的色坐标以及蓝光与黄绿光之间的相对强度可通过对Ce:YAG单晶片厚度的改变进行调整。在恒定电压驱动下,白光LED样品的亮度、光视效能和色温均随单晶片厚度的减小而增加。当Ce:YAG单晶厚度为0.6mm时,可获得较纯的白发射光,并且其色坐标具有较高的可靠性和稳定性,基本不受驱动电压变化的影响。研究结果表明Ce:YAG单晶是一种可用于新型白光LED的理想荧光材料。     

多基色LED的照明通信光源显色性研究

可见光无线通信(visible light communication,VLC)是将LED照明技术和光通信技术相结合的一种新兴技术。针对目前LED照明通信光源显色性差、光效低且色温不可调等问题,依据多基色LED白光通信原理进行了相关研究,以Yoshi等提出的高斯分布形式作为基色LED的光谱模型,利用国际照明委员会(CIE)推荐的一般显色指数(R_a)和美国国家标准与技术研究所(National Institute of Standards and Technology,NIST)推荐的一般色质指数(Q_a)评价光源的显色性。采用遗传算法,在2 700~6 500 K色温范围内优化出单个色温以及色温可调光源满足显色性最优的光谱组合,并基于R_a和Q_a大于80的原则优化出色温可调光源光视效能(luminous efficacy of radiation,LER)最优化的光谱组合。最后根据实验结果分析了光源的显色性、光视效能和色温可调性三者之间的关系。结果表明：三基色色温可调白光LED满足显色性最优的峰值波长组合为613 nm/541 nm/464 nm,此时R_a和Q_a的最小值分别为81.2和81,可以满足一般条件下的照明通信需求;四基色色温可调白光LED满足显色性最优的峰值波长组合为620 nm/562 nm/505 nm/449 nm,此时R_a和Q_a的最小值分别为96.7和92.2。在特殊照明场所或要求较高的通信速率时,应采用四基色白光LED作为照明通信光源。仿真得到了三基色和四基色白光LED的最佳光谱组合,为宽通道可见光通信光源的设计提供了参考依据。     

Inhomogeneity of a highly efficient InGaN based blue LED studied by three‐dimensional atom probe tomography

The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). The average In mole fraction by three‐dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis. The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be non‐uniform in the InGaN active layer. In clustering or In rich regions in the range of 2–3 nm size were found, in contrast to recent reports. Our results thus indicate that In clustering is essential for high‐brightness InGaN based LEDs. We have also observed a discontinuity in the range of 50–100. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The demonstration of hybrid n-ZnO nanorod/p-polymer heterojunction light emitting diodes on glass substrates

We report a demonstration of heterojunction light emitting diode (LED) based on a hybrid n-ZnO-nanorod/p-polymer layered structure. The ZnO was grown using the aqueous chemical growth (ACG) on top of the polymer(s) which were deposited on glass. The current–voltage (I–V) behavior of the heterojunctions showed good rectifying diode characteristics. Room-temperature electroluminescence (EL) spectra of the LEDs provided a broad emission band over a wide LED color range (430–650 nm), in which both zinc and oxygen vacancy peaks are clearly detected. We present here luminescent devices based on the use of ZnO-nanorods in combination with two different blended and multi-layered p-type polymers. Electroluminescence of the first batch of devices showed that white bluish strong emission for the presently used polymers is clearly observed. We obtained a turn-on voltage of 3 V and break-down voltage equal to −6 V for PVK-TFB blended device. The corresponding values for the NPD-PFO multilayer device were 4 V and −14 V, respectively. The rectification factors were equal to 3 and 10 for the two devices, respectively. The films and devices processed were characterized by scanning electron microscopy (SEM), DEKTAK 3ST Surface Profile, Semiconductor Parameter Analyzer, photoluminescence (PL), and electroluminescence (EL).     

White light emission from blue and near ultraviolet light-emitting diodes precoated with a Sr3SiO5:Ce3+,Li+ phosphor

White-light-emitting diodes (WLEDs) were fabricated by combining a yellow Sr3SiO5:Ce3+, Li+ phosphor with a blue light-emitting diode (LED) (460 nm chip) or a near ultraviolet (n-UV) LED (405 nm chip), respectively. Color temperature (Tc) of Sr3SiO5:Ce3+, Li+-based WLEDs could be tuned from 6500 to 100,000 K (blue LED pumping) and from 4900 to 50,000 K (n-UV LED pumping) without mixing with other phosphors. The blue LED-pumped WLED showed excellent white light (luminous efficiency=31.7 lm/W, Tc=6857 K) at 20 mA. This WLED showed a stable color coordinates property against an increase of the forward current. An n-UV LED-pumped WLED also showed bright white light (25.0 lm/W, 5784 K) at 20 mA.     

白光LED用稀土荧光粉的制备和性质

在还原气氛保护下利用高温固相法合成了化学组分为(M₁,M₂)₁₀(PO₄)₆X₂(M₁=Ca,Sr,Ba;M₂=Eu,Mn;X=F,Cl,Br)的可被紫光激发的蓝光、绿光和红光荧光粉,制备了紫光LED芯片+蓝光荧光粉+YAG荧光粉的二基色白光LED;紫光LED芯片+蓝光荧光粉+红光荧光粉的二基色白光LED,以及紫光LED芯片+蓝光荧光粉+绿光荧光粉+红光荧光粉的三基色白光LED。测试了所有制备的白光LED在不同的直流电驱动下的色度坐标、相关色温和显色指数。