J Shanghai Univ (Engl Ed), 2011, 15(4): 252–255 Digital Object Identifier(DOI): 10.1007/s11741-011-0732-3
High efficient and high color rendering index white organic light-emitting diodes
) , SHANG Yu-zhu ( Û) , WEI Bin (ï Ê)
ZHANG Min-yan ( YU Jian-ning (
1
2
2
)1 , XU Hong (
DZ) ,
¨ Yan-fang ( )1 , LU
1
1. School of Materials Science and Engineering, Shanghai University, Shanghai 200072, P. R. China 2. Key Laboratory of Advanced Display and System Application (Shanghai University), Ministry of Education, Shanghai 200072, P. R. China ©Shanghai University and Springer-Verlag Berlin Heidelberg 2011
Abstract We have fabricated high-efficient white organic light-emitting diodes (WOLEDs) using two types of electron transport materials with different electron mobility. The effect of the electron mobility on the device performance is discussed. In addition, to generate the desired white emission and high color rendering index, we perform the structure design of OLED, in which the functions of co-host of blue and green dopants on chromatic-stability are investigated. Experimental results find that the maximum color rendering index reaches as high as 91 at the voltage of 8 V. Keywords white organic light emitting diodes (WOLEDs), charge balance, energy transfer, color rendering index
Introduction White organic light-emitting diodes (WOLEDs) are under the intense investigation for the application of solid-state lighting and full color display nowadays[1]. To date, some approaches to fabricating one WOLED have been developed, including a single emissive layer structure doped with different fluorescent materials[2−3] , and a stacked multi-emissive layer structure in which each layer emits different light colors to generate combined white light[4−9] or electrophosphorescent WOLED[10] , as well as a tandem cell structure. In this paper, we focus on the development of highefficiency and high color rendering index WOLED with stacked multi-emissive layer structure. Two channels of light emitting in emissive layers are discussed: (i) the energy transfers from excitons generated by host to guest, and (ii) hole or electron can directly be captured by the guest, and then the excitons are formed and the dye materials emit. However, it is still a challenge to fabricate highperformance OLEDs, for the reasons: (i) imbalanced charge injection, which largely depends on the mobility of the minority carrier, namely electron in the OLED, and (ii) energy transfers, happening in the adjacent recombination site and transferred from the high emis-
sion energy of light emitting excitons to low emission energy of a dye material. For example, compared with the hole-mobility of N, N’-bis-(3-naphthyl)-N, N’-biphenyl-(1, 1’-biphenyl)-4, 4’-diarnine (NPB), the electron-mobility of 5.1×10−4 cm2 /(V·s)[11] , 4,7-diphenyl-1, 10-phenanthroline (Bphen), is 2.4×10−4 cm2 /(V·s)[12] . It leads to the undesired leakage current and reduces the power efficiency. Furthermore, it is important for a white emission device to hold a high color rendering index (CRI).
1 Experimental and discussion 1.1
Effect of charge balance on efficient WOLEDs
high-
It is crucial to choose electron transport material in that the imbalanced carrier injection is largely caused by the slower transport of electron. To improve the injection of electron and control the recombination site, we have fabricated two white devices A and B with the following structures: ITO/hole injection layer (HIL, 45 nm)/NPB(3 nm)/TCTA:RD (2 nm, 4%)/TCTA(6 nm)/EPH31:BD(5 nm, 5%)/ EPH31 (2 nm)/EPH31:YD (10 nm, 10%)/EPH31 (2 nm)/ETL(25 nm)/LiF(0.3 nm)/AL (100 nm), in which devices A and B used Bphen and 1007 as the
Received Apr.15, 2011; Revised June 8, 2011 Project supported by the Development Foundation for Electronic and Information Industry (2010), the Science and Technology Commission of Shanghai Municipality (Grant No.10DZ1140502), and the Mechatronics Engineering Innovation Group Project from Shanghai Education Commission Corresponding author WEI Bin, Ph D, Prof, E-mail:
[email protected]
J Shanghai Univ (Engl Ed), 2011, 15(4): 252–255
electron transport material respectively. In these structures, the following chemicals are used respectively. NPB as the hole transport layer (HTL); 4,4 ,4 -tris(N -carbazolyl)-triphenylamine (TCTA) doped with RD as red emitting material layer (EML); EPH31 doped with BD as blue EML; EPH31 doped with YD as yellow EML; TCTA and EPH31 as the space layer, for the purpose of confining the excitons, especially the excitons produced by blue dopant. The energy-level diagram of devices used in this study was schematically represented in Fig.1.
Fig.1
253
As is shown in Fig.3(a), two emission peaks, centered at 468 nm and 564 nm are observed in device A, while another emission of 596 nm is observed in device B. This phenomenon is consistent with the explanation that the electron mobility of 1007 is higher than that of Bphen, and more electrons are able to reach the interface of red EML/HTL. Thus, increasing number of excitons are formed in the region of red EML, and the intensity of red light is enhanced. However, Fig.1 shows that, for a given voltage, the current density of device B is larger than that of device A. This may be ascribed to the fact that the injected electrons not only contribute to the red light emission, but also induce increasing leakage current and reduce power efficiency in device B. The lower efficiency of device B in Fig.3(b) confirms that 1 007 exhibits higher electron mobility.
Energy-level diagram of the devices
The J-V-L characteristics of devices A and B are shown in Fig.2. It is observed that the turn-on voltage of both devices A and B is almost the same (2.2 V), owning to the lower energy barrier and good mobility of carrier. However, it should be noted that the luminance of both devices is similar to a low-voltage region (
