Efficiency Enhancement Utilizing A Hybrid Anode Buffer Layer in Organic Light-emitting Devices

XIAO Jing, DENG Zhen-bo

(1.CollegeofPhysicsandElectronicEngineering,TaishanUniversity,Tai’an271021,China;2.InstituteofOpoelectronicTechnology,BeijingJiaotongUniversity,Beijing100044,China)

Efficiency Enhancement Utilizing A Hybrid Anode Buffer Layer in Organic Light-emitting Devices

XIAO Jing1*, DENG Zhen-bo2

(1.CollegeofPhysicsandElectronicEngineering,TaishanUniversity,Tai’an271021,China;2.InstituteofOpoelectronicTechnology,BeijingJiaotongUniversity,Beijing100044,China)

We report the incorporation of lithium fluoride doped 4,7-diphenyl-1,10-phenanthroline (Bphen∶LiF), Al, and molybdenum trioxide (MoO3) which is utilized to form the charge injection buffer layer in single-unit organic light-emitting devices (OLEDs). This hybrid buffer layer at the anode/organic interface was found to be very effective, which increased accumulation of holes at the NPB-buffer interface to improve the balance of the carrier injection. Both the maximum current efficiency and maximum power efficiency of the device were improved by 1.3 times. The results strongly indicate that carrier injection ability and balance shows a key significance in device performance.

hybrid buffer layer; balance of the carrier injection; carrier injection ability

1 Introduction

Organic light-emitting devices (OLEDs) possess tremendous potentials in the development of solid-state lighting and flat-panel displays due to their low-voltage operation, wide viewing angle, and mechanical flexibility[1-7]. Although the performance of OLEDs is influenced by the properties of their constituent organic materials, the interfaces the electrodes form with the carrier transport layers are also critical.

As we all know, light is emitted after the opposite charges are injected from electrodes into organic layers in OLEDs. The injection efficiency of charges is an important parameter in such devices. Therefore much work has been devoted to improving electrode interface and to understanding their underlying mechanisms[8-12]. For example, various electrode interfacial treatments, such as O2plasma and UV-ozone, cause significant changes on the work function of the electrode to improve device performance[13-15]. A significant enhancement of device performance was achieved by using a thin insulator layer at the interface between the electrodes and the electroluminescent (EL) layers, such as MgO, SiO2, Al2O3, LiF and CuPc[16-20]. The insulator layer, generally referred to as the buffer layer, operates by changing the effective barrier for charge injection, thereby improving the EL efficiency. Buffer layers have been widely adopted to modulate hole and electron injection in OLEDs[11-23].

In this study, we employed lithium fluoride (LiF)-doped 4,7-diphenyl-1,10-phenanthroline (Bphen∶LiF)/Al/molybdenum trioxide (MoO3) as anode buffer layer in single-unit OLEDs. For comparison, the device using a common cathode buffer layer LiF has also been fabricated. We compared the EL properties of OLEDs based on the hybrid buffer layer with the reference device based on LiF as cathode buffer layer. The hybrid buffer layer was useful to improve the current efficiency and the power efficiency in the single-unit OLEDs, and it is ascribed to the efficient charge separation in the hybrid buffer layer and the resultant improvement of charge balance for light emission[23，25].

2 Experiments

For fabricating the devices, the patterned indium tin oxide (ITO, 6 Ω/□) substrates (Lum. Tech.), organic (Nichem. Fine Tech.) and inorganic materials (Alfa Aesar), were commercially purchased. After the routine cleaning and ultraviolet (UV) ozone treatment, the ITO substrates were introduced into a high-vacuum deposition chamber (Trovato MFG, base pressure about 1×10-4Pa) with multiple thermal evaporation sources, where film thickness was monitoredinsituwith a calibrated quartz crystal microbalance. All the organic and inorganic layers were evaporated on the substrates in sequence by vacuum deposition. The EL spectra and the current density-voltage-luminance (J-V-L) characteristics of the corresponding devices were measured simultaneously with a Photo Research PR-655 spectrometer and a computer controlled programmable Keithley model 2400 power source. All measurements were carried out at room temperature under ambient conditions after the devices have been encapsulated in a glove box.

3 Results and Discussion

To obtain a high-quality light emission, device A with hybrid buffer layer has been performed. This kind of buffer layer structure is adopted to improve the device performance. The layer structures of the single-unit OLEDs with the hybrid buffer of Bphen∶LiF/Al/MoO3are summarized in Tab.1, where EL-Unit and HBL-Unit refer to electroluminescence and hybrid buffer layer units, respectively. The optimal doping mass fraction of LiF in Bphen is 6%. The fluorescent EL-Unit is composed of NPB/Alq3. The reference device (device B) for comparison was also fabricated with the configuration of ITO/NPB/Alq3/LiF/Al.

Tab.1 Layer structures of OLEDs A and B, where EL-Unit and BL-Unit refer to electroluminescence and hybrid buffer layer units, respectively

DevicesorunitLayerstructuresAITO/BL-Unit/EL-Unit/LiF(0.5nm)/Al(100nm)BITO/EL-Unit/LiF(0.5nm)/Al(100nm)El-UnitNPB(75nm)/Alq3(75nm)BL-UnitBphen∶LiF(5nm;6%)/Al(1nm)/MoO3(5nm)

Fig.1 represents the device characteristics of current density (J)vs. operating voltage, where the operating voltage of device A is markedly increased compared with device B. It implies that HBL-Unit as anode buffer layer could increase the accumulation of holes at the NPB-buffer interface and restrict the injection current improvement.

Fig.2 portrays the comparison in luminous efficiency of two devices. Compared with reference device (device B), device A with anode buffer layer

Fig.1 Current density-voltage (J-V) curves of single-unit devices A and B

of HBL-Unit shows higher luminous efficiency. The results demonstrate that HBL-Unit can act as an effective anode buffer layer. Under certain electric field, the holes mobility of NPB is higher than the electronic transfer rate of Alq3by two orders of magnitude. Hole and electron injection are unbalance. HBL-Unit can improve the carriers into balance and make effective recombination of electrons and holes. As demonstrated in Fig.2, the maximum luminous efficiency of device A is up to 4.684 cd/A, which is

about 1.3 times higher than that of device B. Inserting HBL-Unit to form the anode buffer layer, the interface resistance increased and the current decreased, thus improved the balance of the carrier injection and the device efficiency. For device B, the hole and electron injection are imbalance, which make its efficiency lower. In addition, the use of HBL-Unit can modify the ITO surface well and hinder the spread of indium to organic layers effectively. So the HBL-Unit holes injection layer is benefit

Fig.2 Luminous efficiency-current density (η-J) curves of devices A and B

Tab.2 Device parameters at current density of 10 mA/cm2and maximum current and power efficiencies of devices A and B

DevicenameVoltage/VCurrentefficiency/(cd·A-1)Powerefficiency/(lm·W-1)Maximumcurrentefficiency/(cd·A-1)Maximumpowerefficiency/(lm·W-1)A6.144.052.0704.6842.45B4.842.861.8553.5581.92

to the device efficiency.

The device parameters at given current density, the maximum luminous and power efficiencies of two devices are summarized in Tab.2. Device A using Bphen∶LiF/Al/MoO3as anode buffer layer, has higher current and power efficiencies than reference device B based on LiF cathode buffer layer. This efficiency enhancement might arise from the improvement of the charge balance factor (γ)[23-25], which can be expressed as

(1)

with

(2)

ThelayerstructureofdeviceAissimilartoreferencedeviceB,butithastheHBL-Unitasanode

Fig.3DevicesstructuresofdevicesAandB,whereinjectionandextractioncurrentdensityforholes(Jh) and electrons (Je) of two devices are schematically shown.

4 Conclusion

In summary, highly efficient single-unit OLEDs using Bphen∶LiF/Al/ MoO3as an effective buffer layer were fabricated. Results show that HBL-Unit of Bphen∶LiF/Al/MoO3can as anode buffer layer. The results indicated that device A with HBL-Unit as anode buffer layer has better carriers balance. Comparing with reference device, the maximum luminous efficiency of device A was 4.684 cd/A, increased by 1.3 times.

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肖静(1979-)女，山东泰安人，博士，副教授，2007年于北京交通大学获得博士学位，主要从事有机电致发光方面的研究。

E-mail: xiaojingzx@163.com

2016-08-21；

2017-01-19

国家自然科学基金(61574098，61204051)； 2015年度山东省教育厅优秀中青年骨干教师国际合作培养项目资助 Supported by National Natural Science Foundation of China (61574098，61204051)； International Cooperation Program for Excellent Lecturers of 2015 of Shandong Provincial Education Department

基于一种新型杂化阳极修饰层的高效有机电致发光器件

肖 静1*， 邓振波2

(1. 泰山学院 物理与电子工程学院， 山东 泰安 271021； 2. 北京交通大学 光电子技术研究所， 北京 100044)

设计了基于Bphen∶LiF、Al和MoO3的杂化电荷注入层，并将其应用于有机电致发光器件中。实验研究表明，这种杂化层作为阳极修饰层是非常有效的，它可以增加器件中载流子注入的平衡性，提高器件的性能。相对参考器件，基于杂化阳极修饰层的电致发光器件的最大电流效率和最大功率效率均提高1.3倍左右。我们对器件性能及其提高的机理进行了分析。

杂化修饰层； 载流子注入的平衡性； 载流子注入能力

1000-7032(2017)05-0601-05

TN383+.1; TN873.3 xDocument code： A

10.3788/fgxb20173805.0601

*CorrespondingAuthor,E-mail:xiaojingzx@163.com