Influence of Nickel Oxide Anode Buffer Nanolayer on Blue Organic Light-emitting Diodes

SUN Yong-zhe, ZHENG Bo, LI Hai-rong,3,4*, WANG Fang, YUAN Chao-xin, CAO Zhong-tao, CHANG Fang-zhi, QIAN Kun, LEI Qi， LEI ong, XUE Xin, HAN Gen-liang， SONG Yu-zhe

(1. School of Physical Science and Technology, Lanzhou Universitys, Lanzhou 730000, China;2. Institute of Sensor Technology, Gansu Academy of Sciences, Lanzhou 730000, China; 3. Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, China; 4. Key Laboratory for Magnetism and Magnetic Materials of The Ministry of Education, Lanzhou University, Lanzhou 730000, China)*Corresponding Authors, E-mail: hrli@lzu.edu.cn; zhengboem@163.com

Influence of Nickel Oxide Anode Buffer Nanolayer on Blue Organic Light-emitting Diodes

SUN Yong-zhe1, ZHENG Bo2*, LI Hai-rong1,3,4*, WANG Fang1, YUAN Chao-xin1, CAO Zhong-tao1, CHANG Fang-zhi1, QIAN Kun1, LEI Qi1， LEI ong1, XUE Xin1, HAN Gen-liang2， SONG Yu-zhe2

(1.SchoolofPhysicalScienceandTechnology,LanzhouUniversitys,Lanzhou730000,China;2.InstituteofSensorTechnology,GansuAcademyofSciences,Lanzhou730000,China; 3.KeyLaboratoryofSpecialFunctionMaterialsandStructureDesign,MinistryofEducation,LanzhouUniversity,Lanzhou730000,China; 4.KeyLaboratoryforMagnetismandMagneticMaterialsofTheMinistryofEducation,LanzhouUniversity,Lanzhou730000,China)*CorrespondingAuthors,E-mail:hrli@lzu.edu.cn;zhengboem@163.com

The electrical and optical properties of blue organic light-emitting diodes fabricated by utilizing nickel-oxide nano buffer layers between the anodes and hole transport layers were investigated. NiO nanolayer on ITO was prepared by the electrochemical methods. The effects of NiO nanolayer on the device performances were studied. The experimental results show that NiO buffer layer can effectively enhance the probability of hole-electron recombination due to an efficient holes injection into and charge balance in an emitting layer. The sample with Ni deposition time of 30 s has the highest luminance of 42 460 cd/m2, maximum current efficiency of 24 cd/A, and CIEx,ycoordinates of (0.212 9, 0.325 2), respectively.

blue organic light-emitting diode(BOLED); anode buffer layers; NiO nanoparticles; electrochemical method

1 Introduction

In the past few decades, organic light-emitting diodes (OLEDs) have attracted an enormous degree of attention and have already proved to be a promising, renewable, and low-cost device[1]. In particular, due to their potential application on wearable device field and flat-panel displays[2], OLEDs are one of the most suitable candidates compared to the other emitting devices. Blue OLED (BOLED) is a crucial solid-state lighting as well as component for full-color display[3]. Blue emitter as one of the three main sources of primary color is used to obtain green or red emission from the fluorescent materials[4-5]. But now, a number of technological challenges and difficulties still need to be solved such as increasing the luminous efficiency, reducing the cost, and improving the operational stability[6-8].

Recently, an enhancement of the luminous efficiency of OLEDs utilizing inorganic buffer layers between the anode and the holes transport layer (HTL) has been reported[9-11]. Among various kinds of inserted buffer layers in OLEDs, nickel-oxide (NiO) buffer layers have become particularly attractive because of their wide band gap (transparent over the visible spectral range) and high work function[12-14]. NiO is a kind of inorganic compound, compared to the traditional organic buffer layer, which has high compressive strength, high temperature resistance, corrosion resistance and other advantages[15-16]. Therefore, we combined the characteristics of organic materials and inorganic materials to enhance the efficiency of the device. Although some investigations of the electrical and the optical properties of OLEDs utilizing NiO buffer layers between the anodes and the HTLs are important for achieving high performance OLEDs[17-19], most of these previous methods are expensive and cannot be effectively aligned with typical manufacturing processes of OLEDs. The usual methods of obtaining NiO are metal organic chemical vapor deposition (MOCVD) and magnetron sputtering. Wangetal. have used the MOCVD method to prepare NiO. It is inevitable to produce a certain pollution to the environment because of the presence of organic metal[20-21]. In addition, there were also some problems such as slow growth, plasma instability and low utilization rate of target in the sputtering method to prepare NiO by Chanetal.[22]. All of these are virtually increase the cost of production and damage to the functional layer of OLEDs. However, few studies concerning electrical and optical properties of OLEDs with NiO buffer layers formedviaa simple method of the electrochemical have been reported. Electrochemical methods due to its own characteristics such as adjustable control potential, provides a convenient and feasible method for the preparation of nanoparticles with controllable size and shape[23]. Compared to other methods to obtain NiO, electrochemical method has the advantage of cheap, easy operation, mild reaction conditions, high purity of the obtained nanoparticles and less environmental pollution.

Hence, in this work, we attempted to use the method of electrochemical to introduce NiO buffer layer on OLEDs. The current density, luminance, current efficiency, electroluminescence (EL) spectra, and Internationale de L’Eclairage (CIEx,y) coordinates of BOLEDs were characterized by comparison to conventional BOLED.

2 Experiments

All the organic materials with the purity of more than 99% were purchased from Aldrich Chemical Co., Ltd.. The chamber pressure during deposition was kept at 4.5×10-4Pa with the deposition rate of 0.1-0.3 nm/s. The film thickness was detected by a quartz crystal oscillator. And the devices were encapsulated by epoxy resin in N2atmosphere. The NiO buffer layer was prepared as the following procedure. First, the deposition was carried out with an electrochemical workstation (Corrs Test CS350S type) under constant current density of -1.0 mA·cm-2for 0, 15, 30 and 60 s，respectively. Three electrodes were used in this process: the nickel plate (99.9%, mass fration) as the counter electrode and saturated calomel as the reference electrode. ITO glass was connected with the working electrode which was immersed in NiSO4solution, parallel to the ITO substrate at a distance of～2 cm. Next, Ni films were oxidized at 350 ℃ for 2 h by using a thermal annealing system. In order to compare the influence of NiO buffer layer to OLED, we prepared A, B, C and D four contrast experiments correspond to Ni deposition time of 0, 15, 30 and 60 s, respectively. Then, a 160 nm indium tin oxide (ITO) with a sheet resistance of ～10 Ω/□ was used as the transparent anode electrode, all organic layers were sequentially deposited onto the surface of the ITO substrate. As hole transport layer (HTL),N,N-diphenyl-N,N-bis (1-naphthyl)-(1,1-biphenyl)-4,4-diamine (α-NPB) film was thermally deposited on top of NiO film. Next,4,4-bis [4-(di-ptolylamino) styryl] biphenyl (DPAVBi) (as the fluorescent materials) was doped in the host 2-methyl-9,10-di(2-naphthyl)-anthracene (MADN) (as light-emitting layer,EML) and subsequently the EML was evaporated onto the NPB film. After that,as hole blocking layer (HBL),2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-line (BCP) film was consecutively deposited on the surface of EML and then the tris(8-hydroxy quinoline)-aluminium (Alq3) film as electron transport layer (ETL) was formed by thermal evaporation in a same method. Finally,an Al cathode was deposited at a rate of 1 nm/s for complete BOLED with active emitting area of 3 mm×3 mm. The device configurations are ITO/NPB/MADN∶DPAVBi/BCP/Alq3/Al and ITO/NiO/NPB/MADN∶DPAVBi/BCP/Alq3/Al which were shown in Fig.1.

The as-synthesized NiO nanoparticles were measured by X-ray diffraction (XRD,Rigaku D/max-2400 with Cu Kα radiation). The morphology of the samples was examined using scanning electron microscopy (SEM,Hitachi S-4800). The current density and luminanceversusthe voltage of the devices were characterized by a Kiethley 2410 programmable current-voltage digital source unit. The CIE coordinates and the electroluminescence (EL) intensities were measured by Photo Research PR-650. All the measurements were carried out in air at room temperature.

Fig.1 Device structures of fabricated BOLEDs

3 Results and Discussion

Fig.2(a) shows XRD diffraction patterns of Ni film prepared on ITO substrate. (200) diffraction peak indicates that a good Ni film forms on ITOviathe electrochemical method. Fig.2(b) shows the presence of NiO diffraction peaks from (001),(111) and (021),and none of Ni diffraction peak is observed,which means only the two substances on glass substrate after annealing at 350 ℃ for 4 h in dry air. Fig.3 is a cross section scanning electron microscopy (SEM) photo of NiO nano thin film. The results show that the surface of ITO is composed of many small nanoparticles. A layered structure can be seen from the local enlarged photograph of NiO as shown in Fig.3(b). The NiO particles are mainly spherical,forming NiO powder with similar flower structure. The mean diameter is about 0.7 μm and the petal thickness is about 100 nm.

Fig.2 XRD patterns of Ni nanoparticles deposited on ITO substrates (a) and NiO nanoparticles obtained after oxidation (b)

Fig.3 Scanning electron microscopy (SEM) photo of NiO nano thin film

Fig.4 (a) Current density-voltage (J-V) and (b) luminance-voltage (L-V) curves of BOLEDs

The current efficiency is plottedvs. the current density of BOLEDs with different deposition time of NiO, as shown in Fig.5. The maximum current efficiency of the devices A, B, C and D is 21, 22, 24 and 23 cd/A, respectively. The current efficiency of the device B, C, D is significantly higher than the device A, indicating that the current efficiency of the device is improved obviously after the adding of NiO buffer layer. This phenomenon can be explained in terms of the energy band. The work function of ITO is 4.7 eV and the highest occupied molecular orbital (HOMO) energy level of NPB is 5.5 eV, which means the holes from ITO to NPB injection need to cross the barrier of 0.8 eV, as shown in Fig.6(a). Nevertheless, the holes from ITO to NiO injection should cross the barrier down to 0.7 eV after the joining of NiO buffer layer, as shown in Fig.6(b). At the same time, the holes from NiO buffer layer migrating to NPB layer only need to cross the barrier of 0.1 eV, which would greatly increase the number of electron-hole pairs. As a result, more carriers combining near the anode eventually lead to the improvement of efficiency. In addition, the efficiency of device C is higher than device B, D, which further demonstrates that the increase of current density can improve the efficiency of the device.

Fig.5 Current efficiency-current density

Fig.6 Energy band gap schematics of device A (a) and device B, C, D (b), respectively.

Fig.7(a) shows EL spectra of BOLEDs with Ni deposition time of 0, 15, 30 and 60 s as A, B, C and D, respectively. The EL spectra were normalized to compare the relative change of blue and green emission peaks. It is found that the spectral intensity of device B, C, D is slightly higher than device A, where the spectral intensity of device C is the strongest. A strong blue emission of each device appeared around 473 nm due to a phosphorescent MADN host emitter.

Another shoulder peak appears in 505 nm. This change of the EL spectra may be caused by a planar optical microcavity effect. The appearance of the intensity of the shoulder peak in the line-spectral shape is through the light interference and reflection in the multilayer structure[24]. Fig.7(b) describes some changes at the CIEx,ycoordinates of BOLEDs. The CIE coordinates of device A, B, C and D are (0.201 5, 0.313 9), (0.212 9, 0.325 2), (0.212 9, 0.325 2) and (0.220 9,0.350 1),respectively. The color changed slightly from blue to blue-green with the increasing of Ni deposition time.

Fig.7 EL spectra (a) and CIE chromaticity coordinates (b) of BOLEDs

4 Conclusion

Insummary, BOLEDs with NiO nano materials as anode buffer layer were fabricated. The introduction of NiO nanolayer not only greatly enhanced the holes injection performance in OLEDs, but also had significantly improved device efficiency. In addition, a new electrochemical method for the preparation of Ni nanolayer was studied. The BOLEDs with Ni deposition time of 30 s has the highest luminance of 42 460 cd/m2, maximum current efficiency of 24 cd/A, and CIEx,ycoordinates of (0.212 9, 0.325 2), respectively. These results indicate that OLEDs fabricated with NiO nano buffer layer hold promise for the potential applications in the field of highly-efficient flat-panel displays.

[1] YAO J H, ZHEN C G, LOH K P,etal.. Novel iridium complexes as high-efficiency yellow and red phosphorescent light emitters for organic light-emitting diodes [J].Tetrahedron, 2008, 64(48):10814-10820.

[2] BURROWS P E, GU G, BULOVIC V,etal.. Achieving full-color organic light-emitting devices for lightweight, flat-panel displays [J].IEEETrans.ElectronDev., 1997, 44(8):1188-1203.

[3] JEFFREY P, TUKARAM K. Fluorescent-based tandem white OLEDs designed for display and solid-state-lighting applications [J].J.Soc.Inf.Dis., 2009, 17(10):861-868.

[4] CHEN S M, KWOK H S. Full color organic electroluminescent display with shared blue light-emitting layer for reducing one fine metal shadow mask [J].Org.Electron., 2012, 13(1):31-35.

[5] CHEN S J, ZHAO X Y, WANG Y R,etal.. White light emission with red-green-blue lasing action in a disordered system of nanoparticles [J].Appl.Phys.Lett., 2012, 101(12):123508-1-4.

[6] KIM W J, LEE Y H, KIM T Y,etal.. Dependence of efficiency improvement and operating-voltage reduction of OLEDs on thickness variation in the PTFE hole-injection layer [J].J.KoreanPhys.Soc., 2007, 51(3):1007.

[7] KANNO H, HAMADA Y, TAKAHASHI H. Development of OLED with high stability and luminance efficiency by co-doping methods for full color displays [J].IEEEJ.Sel.Top.Quant.Electron., 2004, 10(1):30-36.

[8] FURUKAWA T, NAKANOTANI H, INOUE M,etal.. Dual enhancement of electroluminescence efficiency and operational stability by rapid upconversion of triplet excitons in OLEDs [J].Sci.Rep., 2015, 5:8429.

[9] IM H C, CHOO D C, KIM T W,etal.. Highly efficient organic light-emitting diodes fabricated utilizing nickel-oxide buffer layers between the anodes and the hole transport layers [J].ThinSolidFilms, 2007, 515(12):5099-5102.

[10] YAMAMORI A, ADACHI C, KOYAMA T,etal.. Doped organic light emitting diodes having a 650-nm-thick hole transport layer [J].Appl.Phys.Lett., 1998, 72(17):2147-2149.

[11] GROZEA D, TURAK A, YUAN Y,etal.. Enhanced thermal stability in organic light-emitting diodes through nanocomposite buffer layers at the anode/organic interface [J].J.Appl.Phys., 2007, 101(3):033522-1-6.

[12] WANG Y L, NIU Q L, HU C D,etal.. Ultrathin nickel oxide film as a hole buffer layer to enhance the optoelectronic performance of a polymer light-emitting diode [J].Opt.Lett., 2011, 36(8):1521-1523.

[13] STEIRER K X, CHESIN J P, WIDJONARKO N E,etal.. Solution deposited NiO thin-films as hole transport layers in organic photovoltaics [J].Org.Electron., 2010, 11(8):1414-1418.

[14] MEYBODI S M, HOSSEINI S A, REZAEE M,etal.. Synthesis of wide band gap nanocrystalline NiO powderviaa sonochemical method [J].Ultrason.Sonochem., 2012, 19(4):841-845.

[15] JUNG K, SEO H, KIM Y,etal.. Temperature dependence of high- and low-resistance bistable states in polycrystalline NiO films [J].Appl.Phys.Lett., 2007, 90(5):052104-1-3.

[16] OKUYAMA M, KAWAKAMI M, ITO K. Corrosion resistance of iron coated by plasma spray ceramic coatings to molten fluoride [J].HighTemper.Corros.Adv.Mater.Protect.Coat., 1992, 78(2):275-283.

[17] MA H, YIP H L, HUANG F,etal.. Interface engineering for organic electronics [J].Adv.Funct.Mater., 2010, 20(9):1371-1388.

[18] ZHANG D D, FENG J, LIU Y F,etal.. Enhanced hole injection in organic light-emitting devices by using Fe3O4as an anodic buffer layer [J].Appl.Phys.Lett., 2009, 94(22):223306-1-3.

[19] HAN S C, SHIN W S, SEO M,etal.. Improving performance of organic solar cells using amorphous tungsten oxides as an interfacial buffer layer on transparent anodes [J].Org.Electron., 2009, 10(5):791-797.

[20] WANG H, WU G G, CAI X P,etal.. Effect of growth temperature on structure and optical characters of NiO films fabricated by PA-MOCVD [J].Vacuum, 2012, 86(12):2044-2047.

[21] MIN K C, KIM M, YOU Y H,etal.. NiO thin films by MOCVD of Ni(dmamb)2, and their resistance switching phenomena [J].Surf.Coat.Technol., 2007, 201(22-23):9252-9255.

[22] CHAN I M, HONG F C. Improved performance of the single-layer and double-layer organic light emitting diodes by nickel oxide coated indium tin oxide anode [J].ThinSolidFilms, 2004, 450(2):304-311.

[23] KIM G P, NAM I, PARK S,etal.. Preparationviaan electrochemical method of graphene films coated on both sides with NiO nanoparticles for use as high-performance lithium ion anodes [J].Nanotechnology, 2013, 24(47):475402.

[24] BENISTY H, DE NEVE H, WEISBUCH C. Impact of planar microcavity effects on light extraction—Part I: basic concepts and analytical trends [J].IEEEJ.Quant.Electron., 1998, 34(9):1612-1631.

孙永哲(1991-)，男，河南郑州人，硕士研究生，2013年于兰州大学获得学士学位，主要从事有机半导体器件的研究。

E-mail: 15117116001@163.com李海蓉(1971-),女,甘肃临洮人,博士，教授，2004 年于兰州大学获得博士学位,主要从事半导体光电子学方面的研究。

E-mail: hrli@lzu.edu.cn郑礴(1972-)，男，河北保定人，总工程师，1994年于哈尔滨科技大学获得学士学位，主要从事嵌入式技术及仪器仪表方面的研究。

E-mail: zhengboem@163.com

2016-11-01；

2016-11-30

甘肃省自然科学基金(1606RJZA026)； 兰州市城关区科技计划(2016cgkj280)资助项目 Supported by Natural Science Foundation of Gansu Province(1606RJZA026); Science and Technology Planning of Chengguan District of Lanzhou(2016cgkj280)

纳米结构氧化镍缓冲层对蓝色有机发光二极管性能的影响

孙永哲1， 郑 礴2*， 李海蓉1，3,4*， 王 芳1， 员朝鑫1， 曹中涛1，常芳芝1， 钱 坤1， 雷 琦1， 雷 栋1， 薛 鑫1， 韩根亮2， 宋玉哲2

(1. 兰州大学物理科学与技术学院 微电子研究所， 甘肃 兰州 730000； 2. 甘肃省科学院 传感技术研究所， 甘肃 兰州 730000； 3. 兰州大学 特殊功能材料与结构设计教育部重点实验室， 甘肃 兰州 730000；4. 兰州大学 磁学与磁性材料教育部重点实验室， 甘肃 兰州 730000)

在阳极和空穴传输层分别引入氧化镍纳米结构缓冲层，制备了蓝色有机发光二极管，对二极管的电学和光学特性进行了测试分析，研究了采用电化学方法制备的氧化镍纳米结构对器件的影响。结果表明，纳米结构氧化镍缓冲层能够有效地提高空穴-电子对的产生和复合效率，它的引入带来了高效的空穴注入及发光层中的载流子平衡，能有效提高有机发光二极管的电致发光特性。氧化镍缓冲层沉积时间为30 s的器件具有最高的亮度和电流效率，分别为42 460 cd/m2和24 cd/A，该器件的CIEx,y色坐标为(0.212 9,0.325 2)。

蓝色有机发光二极管； 阳极缓冲层； 氧化镍纳米结构； 电化学法

1000-7032(2017)04-0492-07

TN383+.1 Document code： A

10.3788/fgxb20173804.0492