Review of blue perovskite light emitting diodes with optimization strategies for perovskite film and device structure

Perovskite light emitting diodes(PeLEDs)have attracted considerable research attention because of their external quantum efficiency(EQE)of>20%and have potential scope for further improvement.However,compared to red and green PeLEDs,blue PeLEDs have not been extensively investigated,which limits their commercial applications in the fields of luminance and full-color displays.In this review,blue-PeLED-related research is categorized by the composition of perovskite.The main challenges and corresponding optimization strategies for perovskite films are summarized.Next,the novel strategies for the design of device structures of blue PeLEDs are reviewed from the perspective of transport layers and interfacial layers.Accordingly,future directions for blue PeLEDs are discussed.This review can be a guideline for optimizing perovskite film and device structure of blue PeLEDs,thereby enhancing their development and application scope.

A recent advances of blue perovskite light emitting diodes for next generation displays

The halide perovskite blue light emitting diodes(PeLEDs)attracted many researchers because of its fascinating optoelectrical properties.This review introduces the recent progress of blue PeLEDs which focuses on emissive layers and interlayers.The emissive layer covers three types of perovskite structures:perovskite nanocrystals(PeNCs),2-dimensional(2D)and quasi-2D perovskites,and bulk(3D)perovskites.We will discuss about the remaining challenges of blue PeLEDs,such as limited performances,device instability issues,which should be solved for blue PeLEDs to realize next generation displays.

Pure blue and white light electroluminescence in a multilayer organic light-emitting diode using a new blue emitter

We characterized the 6,12-bis{[N-（3,4-dimethylphenyl）-N-（2,4,5-trimethylphenyl）]amino} chrysene （BmPAC）, which has been proven to be a blue fluorescent emission with high EL efficiency. The blue fluorescent device exhibits good performance with an external quantum efficiency of 5.8% and current efficiency of 8.9 cd/A, respectively. Using BmPAC, we also demonstrate a hybrid phosphorescence/fluorescence white organic light-emitting device （WOLED） with high efficiency of 36.3 cd/A. In order to improve the relative intensity of blue light, we plus a blue light-emitting layer （BEML） in front of the orange light emitting layer （YEML） to take advantage of the excess singlet excitons. With the new emitting layer of BEML/YEML/BEML, we demonstrate the fluorescence/phosphorescence/fluorescence WOLED exhibits good performance with a current efficiency of 47 cd/A and an enhanced relative intensity of blue light.

High color rendering index white organic light-emitting diode using levofloxacin as blue emitter

Levofloxacin （LOFX）, which is well-known as an antibiotic medicament, was shown to be useful as a 452-nm blue emitter for white organic light-emitting diodes （OLEDs）. In this paper, the fabricated white OLED contains a 452-nm blue emitting layer （thickness of 30 nm） with 1 wt% LOFX doped in CBP （4,4＇-bis（carbazol-9-yl）biphenyl） host and a 584-nm orange emitting layer （thickness of 10 nm） with 0.8 wt% DCJTB （4-（dicyanomethylene）-2-tert-butyl-6-（1,1,7,7- tetramethyljulolidin-4-yl-vinyl）-4H-pyran） doped in CBE which are separated by a 20-nm-thick buffer layer of TPBi （2,2＇,2＂-（benzene-1,3,5-triyl）-tri（1-phenyl-lH-benzimidazole）. A high color rendering index （CRI） of 84.5 and CIE chromaticity coordinates of （0.33, 0.32）, which is close to ideal white emission CIE （0.333, 0.333）, are obtained at a bias voltage of 14 V. Taking into account that LOFX is less expensive and the synthesis and purification technologies of LOFX are mature, these results indicate that blue fluorescence emitting LOFX is useful for applications to white OLEDs although the maximum current efficiency and luminance are not high. The present paper is expected to become a milestone to using medical drug materials for OLEDs.

Fine-tuning the thicknesses of organic layers to realize high-efficiency and long-lifetime blue organic light-emitting diodes

By using p-bis（p - N, N-diphenyl-aminostyryl）benzene doped 2-tert-butyl-9, 10-bis-β-naphthyl）-anthracene as an emitting layer, we fabricate a high-efficiency and long-lifetime blue organic light emitting diode with a maximum external quantum efficiency of 6.19% and a stable lifetime at a high initial current density of 0.0375 A/cm2. We demonstrate that the change in the thicknesses of organic layers affects the operating voltage and luminous efficiency greater than the lifetime. The lifetime being independent of thickness is beneficial in achieving high-quality full-colour display devices and white lighting sources with multi-emitters.

Efficiency droop alleviation in blue light emitting diodes using the InGaN/GaN triangular-shaped quantum well

The InGaN/GaN blue light emitting diode（LED） is numerically investigated using a triangular-shaped quantum well model,which involves analysis on its energy band,carrier concentration,overlap of electron and hole wave functions,radiative recombination rate,and internal quantum efficiency.The simulation results reveal that the InGaN/GaN blue light emitting diode with triangular quantum wells exhibits a higher radiative recombination rate than the conventional light emitting diode with rectangular quantum wells due to the enhanced overlap of electron and hole wave functions（above 90%） under the polarization field.Consequently,the efficiency droop is only 18% in the light emitting diode with triangular-shaped quantum wells,which is three times lower than that in a conventional LED.

Novel Field Emission Organic Light Emitting Diodes with Dynode

This work presents novel field emission organic light emitting diodes(FEOLEDs) with dynode,in which an organic EL light-emitting layer is used instead of an inorganic phosphor thin film in the field emission display(FED).The proposed FEOLEDs introduce field emission electrons into organic light emitting diodes(OLEDs),which exhibit a higher luminous efficiency than conventional OLED.The field emission electrons emitted from the carbon nanotubes(CNTs) cathode and to be amplified by impact the dynode in vacuum.These field emission electrons are injected into the multi-layer organic materials of OLED to increase the electron density.Additionally,the proposed FEOLED increase the luminance of OLED from 10 820 cd/m2 to 24 782 cd/m2 by raising the current density of OLED from an external electron source.The role of FEOLED is to add the quantity of electrons-holes pairs in OLED,which increase the exciton and further increase the luminous efficiency of OLED.Under the same operating current density,the FEOLED exhibits a higher luminous efficiency than that of OLED.

Carrier Dynamics Determined by Carrier-Phonon Coupling in InGaN/GaN Multiple Quantum Well Blue Light Emitting Diodes

Phonon sidebands in the electrolumiescence(EL) spectra of InGaN/GaN multiple quantum well blue light emitting diodes are investigated. S-shaped injection current dependence of the energy spacing(ES) between the zero-phonon and first-order phonon-assisted luminescence lines is observed in a temperature range of 100–150 K.The S-shape is suppressed with increasing temperature from 100 to 150 K, and vanishes at temperature above200 K. The S-shaped injection dependence of ES at low temperatures could be explained by the three stages of carrier dynamics related to localization states:(i) carrier relaxation from shallow into deep localization states,(ii) band filling of shallow and deep localization states, and(iii) carrier overflow from deep to shallow localization states and to higher energy states. The three stages show strong temperature dependence. It is proposed that the fast change of the carrier lifetime with temperature is responsible for the suppression of S-shaped feature.The proposed mechanisms reveal carrier recombination dynamics in the EL of InGaN/GaN MQWs at various injection current densities and temperatures.

White organic light-emitting diodes based on a combined electromer and monomer emission in doubly-doped polymers

We report on white organic light-emitting diodes （WOLEDs） based on polyvinylcarbazole （PVK） doped with 1,1-bis（（di-4-tolylamino）phenyl）cyclohexane （TAPC） and perylene, and investigate the luminescence mechanism of the devices. The chromaticity of light emission can be tuned by adjusting the concentration of the dopants. White light with the Commission Internationale de L＇Eclairage （CIE） coordinates of （0.33, 0.34） is achieved by mixing the yellow electromer emission of TAPC and the blue monomer emission of perylene from the device ITO/PVK： TAPC： perylene （100：9：1 in wt.） （100 nm）/tris-（8-hydroxyquinoline aluminum （Alq3） （10 nm）/A1. The device exhibits a maximal luminance of 3727 cd/m2 and a current efficiency of 2 cd/A.

Observation of Blue-Light Emission Band from Eu-Doped Ta₂O₅Thin Films Prepared Using Co-Sputtering

In this paper, we report on the first observation of blue-light emission bands from europium-doped tantalum pentoxide (Ta2O5:Eu) thin films prepared using a simple co-sputtering method. We prepared four specimens from one as-deposited sample, and we subsequently annealed them at 700&degC, 800&degC, 900&degC, or 1000&degC for 20 min. Four remarkable photoluminescence (PL) peaks at wavelengths of 600, 620, 650, and 700 nm due to the 5D0→7F1, 5D0→7F2, 5D0→7F3, and 5D0→7F4 transitions of Eu3+ were observed from all the specimens, and blue PL peaks around a wavelength of 450 nm were also observed from the specimens annealed at 800&degC, 900&degC, and 1000&degC. The blue PL peaks seem to be originated from the 4f65d1→4f7 transition of Eu2+. Both Eu3+ and Eu2+ ions seem to exist in our Ta2O5:Eu co-sputtered thin films annealed at temperatures from 800&degC to 1000&degC. Such Ta2O5:Eu co-sputtered thin films seem to be used as multi-functional coating films having both anti-reflection and down-conversion effects for realizing high-efficiency silicon solar cells.

Two-color light-emitting diodes with polarization-sensitive high extraction efficiency based on graphene

In the present study, graphene photonic crystals are employed to enhance the light extraction efficiency（LEE） of two-color, red and blue, light-emitting diode（LED）. The transmission characteristics of one-dimensional（1D） Fibonacci graphene photonic crystal LED（FGPC-LED） are investigated by using the transfer matrix method and the scaling study is presented. We analyzed the influence of period, thickness, and permittivity in the structure to enhance the LEE. The transmission spectrum of 1D FGPC has been optimized in detail. In addition, the effects of the angle of incidence and the state of polarization are investigated. As the main result, we found the optimum values of relevant parameters to enhance the extraction of red and blue light from an LED as well as provide perfect omnidirectional and high peak transmission filters for the TE and TM modes.

Developments and challenges ahead in blue perovskite light-emitting devices

Recently,perovskite light-emitting diodes(Pe LEDs)have developed rapidly in the green,red,and nearinfrared light emissions,owing to the unique optoelectronic characteristics of halide perovskites,such as high carrier mobility,narrow emission linewidths,high photoluminescence quantum yield,as well as bandgap tunability.However,the efficiency improvement in blue(especially deep-blue)Pe LEDs is still inferior to other analogs,which severely restricts the Pe LED applications.Here,we systematically summarize the substantial progress in the performance of blue Pe LEDs based on different blue perovskite candidates,and recent advances from three aspects(i.e.,the sky-blue,pure-blue,and deep-blue light emissions).Then,we point out several challenges existing in deep-blue Pe LEDs,such as the effect of Cl-ions incorporation,spectral instability,ion migration,and the difficulty of charge injection,and highlight the strategies to improve device efficiency,to motivate further research and development of blue Pe LEDs.

Blue-Red Tuning Emission of ZnO: Europium Quantum Dots with Different Excitation Wavelengths

Optoelectronic applications require the development of new fluorescent and efficient luminescent materials, free of toxicity, low in cost, and easy to produce. In this way the synthesis of zinc-oxide (ZnO) quantum dots (QDs) has recently received special attention due to their good optical, electrical and chemical properties with low production costs and blue light emission. In this work ZnO QDs were successfully doped with europium in order to obtain a tunable emission luminescence from blue emission of ZnO to red emission of europium as a function of wavelength excitation. Results show an efficient blue to red tuning when the excitation wavelength was changed from 317 nm to 395 nm, respectively. This opens the possibility of having new optical devices to produce different color emission using the same material.

NaYSiO_(4)∶Ce^(3+)蓝色荧光粉的发光性质及其在白光发光二极管上的应用

采用高温固相法合成了NaYSiO_(4)∶xCe^(3+)(0.01≤x≤0.05)系列蓝色荧光粉。NaYSiO_(4)∶xCe^(3+)荧光粉在250~360 nm之间的宽带吸收能与紫外LED芯片很好地匹配。NaYSiO_(4)∶xCe^(3+)荧光粉中存在多个Ce^(3+)离子荧光中心,且在紫外光激发下表现出峰值波长位于414 nm附近的宽带蓝光发射。NaYSiO_(4)∶0.02Ce^(3+)荧光粉在300~350 nm紫外光激发下量子效率在25%以上。NaYSiO_(4)∶0.02Ce^(3+)荧光粉表现出优良的化学稳定性,在水中浸泡14 d后荧光强度和量子效率几乎不变。将NaYSiO_(4)∶0.02Ce^(3+)蓝色荧光粉、商用(Sr,Ba)_2SiO_(4)∶Eu^(2+)绿色荧光粉和商用(Ca,Sr)AlSiN_(3)∶Eu^(2+)红色荧光粉涂覆于310 nm紫外LED芯片上制备得到了显色指数高达95的LED器件。当驱动电流从50 mA逐渐增大到300 mA时,制备的LED器件表现出稳定的暖白光发射,其色坐标几乎不变。上述结果说明,本研究报道的NaYSiO_(4)∶0.02Ce^(3+)蓝色荧光粉在紫外LED芯片驱动的白光发光二极管照明上有着潜在应用价值。

含氟膦氧衍生物修饰的纯蓝光钙钛矿发光二极管

实现高效蓝色电致发光是钙钛矿发光二极管(PeLEDs)商业应用的关键挑战,特别是波长为465~475 nm的纯蓝光。蓝光PeLEDs的低效率主要源于混合卤素策略导致的高缺陷密度和降维策略导致的载流子注入困难。本文以氯/溴混合卤素准二维钙钛矿为研究对象,探究了小分子添加剂膦氧衍生物((五氟苯基)甲基)二苯基氧化膦(PFPO)在钙钛矿前驱体溶液中的作用机理及其对钙钛矿薄膜形貌、光学和器件性能的影响。实验结果显示,PFPO修饰后的钙钛矿薄膜光致发光强度和寿命均有增加,说明PFPO具有良好的缺陷钝化能力。紫外-可见光吸收光谱证实修饰后薄膜中小n相吸收强度明显降低,说明小n相得到了显著抑制,减少了非辐射复合路径,同时有利于激子能量转移。器件结果表明,PFPO处理后的PeLEDs器件最大外量子效率从1.83%提升到3.62%,发光峰位于466 nm,对应的CIE为(0.123,0.079),与标准蓝光CIE坐标(0.131,0.046)接近。

Bi^(3+)、Eu^(3+)共掺杂La_(4)GeO_(8)荧光粉的制备及可调发光

为了开发出高效稳定的单一基质白光发射荧光材料,本工作通过高温固相法成功合成了一系列La_(4)GeO_(8)∶Bi^(3+),Eu^(3+)荧光粉样品,并通过X射线衍射、室温光谱、变温光谱等手段研究了实验样品的结构与发光性能。研究发现,Bi^(3+)离子在该结构中占据两种不同的格位(Bi^(3+)(Ⅰ)和Bi^(3+)(Ⅱ)),且在紫外光激发下呈现两个峰值分别在475 nm和620 nm的宽带发射。对于Bi^(3+)、Eu^(3+)共掺样品,由于Bi^(3+)(Ⅰ)与Eu^(3+)之间的竞争吸收、Bi^(3+)(Ⅰ)至Bi^(3+)(Ⅱ)以及Bi^(3+)(Ⅱ)至Eu^(3+)的能量传递作用,可实现蓝色至红色、橙红色至红色的可调发光。特别地,样品La_(4)GeO_(8)∶0.07Bi^(3+),0.06Eu^(3+)在313 nm光激发下可获得CIE值为(0.335,0.319)的优异白色发光。此外,该白光发射材料具有较佳的发光热稳定性,当温度升高至380 K时,发光积分强度仍然为室温的59%,表明其在白光二极管上具有潜在的应用价值。

以二氢苊骨架构建的深蓝光有机电致发光材料

合成了4种在二氢苊骨架上连接咔唑或三苯胺电子给体和氰基苯电子受体的发光材料,考察了电子给体连接方式及种类对材料光电性能的影响.目标化合物的单晶结构分析结果显示,位阻导致给/受体与二氢苊骨架不共平面,从而降低了共轭程度,消除了分子间π⁃π堆积作用,分子间依靠大量的C—H…π分子间相互作用稳定,只有3CzAB存在分子内π⁃π堆积作用.4种化合物均具有非常高的热稳定性,5%热失重温度(Td,5%)高于343℃.这些材料在PMMA薄膜中的发光均为深蓝光发射(403~432 nm),光致发光效率高(0.60~0.94),荧光寿命为5.4~8.6 ns.电致发光性能测试结果表明,以三苯胺为给体的材料PaAB的电致发光性能最佳,最大亮度可达2427 cd/m,峰值电流效率、功率效率和外量子效率分别为2.43 cd/A,1.53 lm/W和1.58%,色坐标(CIEx,y)为(0.15,0.09),为深蓝光发射.

电泵浦有机发光二极管衬底侧向辐射研究

有机发光二极管衬底侧向辐射光谱与正向辐射光谱相比,存在明显的窄化现象。研究影响器件侧向辐射光谱窄化的因素,进一步减小辐射光谱的线宽,可为电泵浦有机发光二极管激光辐射研究打下基础。本文研究了随有机发光二极管空穴传输层NPB厚度的变化,器件衬底侧向辐射光谱的半高宽、峰位以及偏振特性的变化情况。比较了有机发光二极管衬底边缘两侧蒸镀银膜与未蒸镀银膜时的衬底侧向辐射光谱。研究发现蒸镀银膜时有机发光二极管的衬底侧向辐射光谱半高宽变窄,并且当空穴传输层NPB的厚度为130 nm时,器件衬底侧向辐射光谱半高宽低至14 nm。说明器件衬底两侧存在银膜作为反射镜的情况下,衬底中侧向传播的光将受到光学谐振腔的作用。本文的研究结果为有机发光二极管辐射光谱的窄化和辐射光放大提供了一种新思路。

Zn^(2+)共掺提高Cr^(3+)掺杂钙铌镓石榴石热稳定性

近红外荧光粉转换发光二极管(pc-LEDs)具有体积小、效率高等优点,在无损检测和生物成像等领域具有广泛应用前景。本研究通过高温固相法成功合成了Ca_(3)Nb_(1.6875)Ga_(3.1875)O_(12):Cr^(3+)(CNGG:Cr^(3+))荧光粉,在460 nm蓝光激发下,可以产生650~1000 nm宽带近红外发射,峰值波长为770 nm。对材料的晶体结构和光谱数据进行了系统分析,计算了Cr^(3+)在CNGG基质中的晶体场强度等参数。通过引入Zn^(2+),显著提高了荧光粉的发光热稳定性,并分析了可能的原因。将CNGG∶Cr^(3+)荧光粉与460 nm蓝光芯片结合,封装成近红外pc-LED,并展示了其在夜视和血管成像领域的应用。

LiCl掺杂PEDOT∶PSS提高蓝光量子点发光二极管的性能

基于全溶液制备的量子点发光二极管(QLEDs)通常存在电子-空穴注入不平衡的问题,严重地限制了蓝光QLEDs器件性能的提升。本文研究了不同浓度的金属卤化物(Li Cl)掺杂对聚(乙烯二氧噻吩)∶聚苯乙烯磺酸盐(PEDOT∶PSS)薄膜的形貌、导电率、透光性以及所制备QLEDs器件性能的影响规律。实验结果表明,Li Cl的掺杂浓度(质量分数)为2%时蓝光QLED器件性能的提升效果最佳,这主要归因于Li Cl掺杂后的PEDOT∶PSS薄膜导电率和透光性的增强以及器件中空穴注入效率的提高。相较于未掺杂的PEDOT∶PSS基QLED器件,基于Li Cl掺杂的蓝光QLED器件的最大亮度、峰值电流效率、峰值功率效率和峰值外量子效率分别从5 083 cd·m^(-2)、0.91 cd·A^(-1)、0.59 lm·W^(-1)和2.31%提升到7 451 cd·m^(-2)、1.38 cd·A^(-1)、0.89 lm·W^(-1)和3.51%。结果表明,采用Li Cl掺杂PEDOT∶PSS空穴注入层是提高蓝光QLED性能的一种有效策略。