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.

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孫永哲(1991-)，男，河南鄭州人，碩士研究生，2013年于蘭州大學(xué)獲得學(xué)士學(xué)位，主要從事有機(jī)半導(dǎo)體器件的研究。

E-mail: 15117116001@163.com李海蓉(1971-),女,甘肅臨洮人,博士，教授，2004 年于蘭州大學(xué)獲得博士學(xué)位,主要從事半導(dǎo)體光電子學(xué)方面的研究。

E-mail: hrli@lzu.edu.cn鄭礴(1972-)，男，河北保定人，總工程師，1994年于哈爾濱科技大學(xué)獲得學(xué)士學(xué)位，主要從事嵌入式技術(shù)及儀器儀表方面的研究。

E-mail: zhengboem@163.com

2016-11-01；

2016-11-30

甘肅省自然科學(xué)基金(1606RJZA026)； 蘭州市城關(guān)區(qū)科技計劃(2016cgkj280)資助項目 Supported by Natural Science Foundation of Gansu Province(1606RJZA026); Science and Technology Planning of Chengguan District of Lanzhou(2016cgkj280)

納米結(jié)構(gòu)氧化鎳緩沖層對藍(lán)色有機(jī)發(fā)光二極管性能的影響

孫永哲1， 鄭 礴2*， 李海蓉1，3,4*， 王 芳1， 員朝鑫1， 曹中濤1，常芳芝1， 錢 坤1， 雷 琦1， 雷 棟1， 薛 鑫1， 韓根亮2， 宋玉哲2

(1. 蘭州大學(xué)物理科學(xué)與技術(shù)學(xué)院 微電子研究所， 甘肅 蘭州 730000； 2. 甘肅省科學(xué)院 傳感技術(shù)研究所， 甘肅 蘭州 730000； 3. 蘭州大學(xué) 特殊功能材料與結(jié)構(gòu)設(shè)計教育部重點實驗室， 甘肅 蘭州 730000；4. 蘭州大學(xué) 磁學(xué)與磁性材料教育部重點實驗室， 甘肅 蘭州 730000)

在陽極和空穴傳輸層分別引入氧化鎳納米結(jié)構(gòu)緩沖層，制備了藍(lán)色有機(jī)發(fā)光二極管，對二極管的電學(xué)和光學(xué)特性進(jìn)行了測試分析，研究了采用電化學(xué)方法制備的氧化鎳納米結(jié)構(gòu)對器件的影響。結(jié)果表明，納米結(jié)構(gòu)氧化鎳緩沖層能夠有效地提高空穴-電子對的產(chǎn)生和復(fù)合效率，它的引入帶來了高效的空穴注入及發(fā)光層中的載流子平衡，能有效提高有機(jī)發(fā)光二極管的電致發(fā)光特性。氧化鎳緩沖層沉積時間為30 s的器件具有最高的亮度和電流效率，分別為42 460 cd/m2和24 cd/A，該器件的CIEx,y色坐標(biāo)為(0.212 9,0.325 2)。

藍(lán)色有機(jī)發(fā)光二極管； 陽極緩沖層； 氧化鎳納米結(jié)構(gòu)； 電化學(xué)法

1000-7032(2017)04-0492-07

TN383+.1 Document code： A

10.3788/fgxb20173804.0492