CHEM. RES. CHINESE UNIVERSITIES 2009, 25(4), 590 594 Bright White Organic Light-emitting Device Based on 1,2,3,4,5,6-Hexakis(9,9-diethyl-9H-fluoren-2-yl)benzene MA Tao 1, YU Jun-sheng 1*, LOU Shuang-ling 1, TANG Xiao-qing 1, JIANG Ya-dong 1 and ZHANG Qing 2* 1. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China 2. Department of Polymer Science, School of Chemistry and Chemical Technology, Shanghai Jiaotong University, Shanghai 200240, P. R. China Abstract Organic light-emitting devices(oleds) with the structure of indium-tin-oxide(ito)/n,n -bis-(1- naphthyl)-n,n -diphenyl-(1,1 -biphenyl)-4,4 -diamine(npb)/2,9-dimenthyl-4,7-diphenyl-1,10-phenanthroline(bcp)/ tris(8-hydroxyquinoline)aluminum(alq 3 )/Mg:Ag or that of ITO/NPB/1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2- yl)benzene(hkethflyph)/alq 3 /Mg:Ag were studied. White light emission was achieved with the two devices when the thicknesses of BCP and HKEthFLYPh were 1.5 nm(device B) and 5 nm (device II), respectively. The obvious difference was that the EL spectrum of device II was not sensitive to the thickness of HKEthFLYPh compared to that of BCP layer. Moreover, the maximum luminance of device II was about 1000 cd/m 2 higher than that of device B at a forward bias of 15 V, and it exhibited a maximum power efficiency of 1.0 lm/w at 5.5 V, which is nearly twice that of device B. The performance of device II using a novel star-shaped hexafluorenylbenzene organic material was improved compared with that of BCP. Keywords Organic light-emitting diode; Star-shaped hexafluorenylbenzene; White light; BCP; Energy transfer Article ID 1005-9040(2009)-04-590-05 1 Introduction Organic optoelectronic devices such as light-emitting devices [1,2], thin-film transistors [3], and solar cells [4], are actively investigated in anticipation of the fascinating characteristics of organic materials such as lightweight, flexibility, and excellent processing capability. Especially, organic lightemitting devices(oleds) precede other organic devices and have already been put into practical use. They have the advantages of short response time, wide angle of vision, simple fabrication process, low turn-on voltage, high luminescence efficiency, and low power dissipation [5 8]. Owing to the superior achieved performance, OLEDs are potential candidates not only for next generation displays, but also for solid state lighting application, which requires high efficiency and low operating voltage. Novel functional materials and techniques should be developed for the fabrication of OLEDs on a large scale. Fluorene-based molecular materials and polymers have become an important class of materials for OLED applications owing to their high photoluminescent quantum yield and good charge transport properties [9,10]. However, rigid-rod polyfluorene has a tendency toward a nematic type of packing arrangement and is inherently prone to chain aggregation in condensed state. Chain aggregation tends to degrade the device performance as crystalline formation destroys film homogeneity and crystalline boundaries so as to increase the resistance of the device, eventually leading to an electric short. The morphological instability of organic electroluminescence(el) materials is one of the major problems for device application. Star-shaped molecules have been proven to overcome this problem [11 13]. The performance of device consisting of star-shaped material is significantly improved owing to its tendency to form stable amorphous films with a relatively high glass transition temperature(t g ) and improved thermal stability [14,15]. EL from OLEDs is generated by a recombination between holes and electrons, which are injected from an anode and a cathode at forward bias. However, hole *Corresponding author. E-mail: jsyu@uestc.edu.cn; qz14@sjtu.edu.cn Received January 21, 2008; accepted April 30, 2008. Supported by the National Natural Science Foundation of China(Nos.60425101 and 20674049), Program for New Century Excellent Talents in Universities of China(Nos.060812) and Young Talent Project of University of Electronic Science and Technology of China(Nos.060206).
No.4 MA Tao et al. 591 mobility in the device is 2 to 3 orders of magnitude higher than electron mobility in conventional OLEDs [16]. Consequently, in order to balance the mobility of holes and electrons, a hole-blocking layer(hbl) is usually inserted between hole transporting layer(htl) and electron transporting layer (ETL). Besides conventional hole-blocking materials of BCP and BAlq, several novel materials especially with star-shaped architecture have been synthesized as hole blocker [17,18], e.g., a new class of hole-blocking trisubstituted benzene molecular materials were developed by Shirota [19], which are 1,3,5-tri(4-biphenylyl)- benzene(tbb), 1,3,5-tris(4-fluorobiphenyl-4'-yl)benzene(F-TBB), 1,3,5-tris(9,9-dimethylfluoren-2-yl)benzene(TFB), and 1,3,5-tris[4-(9,9-dimethylfluoren-2- yl)phenyl]benzene(tfpb). However, despite a large volume of previous study, most of the reported molecular structures are with three arms from the center core. In this study, a bright WOLED was fabricated from a novel star-shaped hexafluorenylbenzene with six arms as an HBL, and the EL properties of WOLED were investigated. 2 Experimental A novel star-shaped hexafluorenylbenzene 1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2-yl) benzene(hkethflyph) with six arms star-shaped fluorene structure was synthesized in our group [20 22]. The organic materials 2,9-dimenthyl-4,7-diphenyl- 1,10-phenanthroline(BCP), N,N -bis-(1-naphthyl)-n, N -diphenyl-(1,1 -biphenyl)-4,4 -diamine(npb) and tris(8-hydroxyquinoline) aluminum(alq 3 ) were all purchased from Sigma-Aldrich Co. To compare the characteristics of OLEDs with BCP and HKEthFLYPh, two separate experiments were conducted. For the first kind of OLEDs, the devices of ITO/NPB(40 nm)/bcp(x nm)/alq 3 (50 nm)/mg:ag consisted of BCP film with different thicknesses, where, A, B, and C stand for the devices with 1, 1.5 and 2 nm thick BCP layers, respectively. For the second kind of OLEDs, devices with a structure of ITO/NPB(40 nm)/hkethflyph(y nm)/alq 3 (50 nm)/mg:ag were studied, where, y varied from 2, 5, to 8 nm corresponding to devices I, II, and III, respectively. The ITO-coated glasses used in this study have a sheet resistance of 10 Ω/. For the preparation of OLEDs, the ITO glasses were cut into square plates. Prior to the organic films being grown on ITO glasses, ITO glasses were ultrasonically cleaned with detergent water, acetone, ethanol, and deionized water for 10 minutes at each step, and the glass was blown dry by nitrogen gas. The pre-cleaning procedure was used to remove organic contamination and particles from the ITO surface. Afterwards, the glasses underwent an oxygen plasma treatment for ca. 5 min in order to enhance the working function of ITO and thus improve its hole injecting capacity [23,24]. All the organic materials were deposited by conventional thermal evaporation in a vacuum circumstance at 4 10 4 Pa. The deposition rate and thickness of the deposited layers by evaporation were monitored by a quartz-crystal oscillator. The deposition rates were about 0.1 0.2 nm/s in thickness. After the organic materials grew, the samples were transferred to another chamber, an alloy of Mg and Ag with a ratio of 10:1 was deposited as cathode using a shadow mask, and the film thickness was 200 nm. The active emissive area of both devices is 0.5 cm 0.5 cm. Luminance-voltage(L-V) and current density-voltage(j-v) characteristics were measured with a Keithley 4200 semiconductor characterization system. The absorption spectrum was investigated with a SHIMADZU UV-1700 spectrophotometer. Commissions Internationale de L Eclairage(CIE) coordination, Photoluminescence(PL) and EL spectra of these devices were measured on an OPT-2000 spectrophotometer. All measurements were carried out at room temperature under ambient atmosphere. Fig.1 shows the molecular structures of the used Fig.1 Molecular structures of used materials and device architectures in this study
592 CHEM. RES. CHINESE UNIVERSITIES Vol.25 materials and the device architecture. 3 Results and Discussion For the first kind of device with BCP as HBL, Fig.2 shows the normalized EL spectra of OLED. The EL spectra of the devices can be tuned by adjusting the film thickness of BCP layer, which is used as a hole blocking layer to change the recombination region of the devices. It was reported that the hole mobility of NPB was 1.6 10 5 cm 2 /Vs [25] and the electron mobi- lity of Alq 3 was 1.4 10 6 cm 2 /Vs [26]. When the film thickness of BCP layer is as thin as 1 nm, it almost could not effectively block holes, and such a device suffers hole leakage to the cathode while electrons are more and less consumed before reaching HTL; thus, the luminescence of device A is only in the green light region, and the spectral peak at about 530 nm is associated with the emission of Alq 3 layer. In device B, the film thickness of BCP layer reaches 1.5 nm; owing to the partial hole blocking of BCP layer, the luminance is not only in the green region, both NPB and Alq 3 layers are the luminescence areas. Increasing the BCP layer is favorable to the accumulation of holes at the interface of HBL and HTL, which increases the electric field at the HTL again, thereby increasing the probability of direct exciton formation at NPB layer. It is seen that the peaks of EL spectra of device B locate at 435 and 530 nm, respectively, and corresponding to this, the Commissions Internationale De L Eclairage (CIE) chromaticity coordinates are (0.27, 0.33), which belong to the white light region. In device C, most of the holes are blocked by BCP at the NPB/BCP interface, resulting in lesser holes at the BCP/Alq 3 interface. It can be seen from Fig.2 that the emission mainly takes place at the NPB with a thicker BCP layer. Compared to the light emission of NPB, there is only a tiny shoulder of light emission originated from Alq 3. Fig.3 shows the normalized EL spectra of the devices with different HKEthFLYPh film thicknesses. It can be seen that light emission intensity from NPB is proportional to that of Alq 3 with the enhanced film thickness of HKEthFLYPh, which is similar to that of BCP in the first kind of devices. The only difference is that the EL spectrum is not as sensitive to the thickness of HKEthFLYPh as that of BCP layer. With the film thickness of HKEthFLYPh varying from 2 to 8 nm, EL of NPB and that of Alq 3 both keep considerable intensity at all times. The relatively small change is attributed to the fact that star-shaped materials have very good thermal and morphological stabilities [27]. As seen in Fig.4, the CIE coordinates of the three devices are all within the white region and move slightly as the thickness of HKEthFLYPh layer changes. The CIE coordinates of device II are (0.29, 0.34) when the thickness of HKEthFLYPh is 5 nm, which are nearer to (0.33, 0.33) than those of the other devices. Thus, we can conclude that HKEthFLYPh layer plays an identical role as BCP layer, which efficiently prevents holes from reaching the green emission layer and accelerates electrons injecting into the blue emission layer. Fig.3 EL spectra of the devices with a structure of ITO/NPB/ HKEthFLYPh/Alq 3 /Mg:Ag Fig.2 EL spectra of the devices with a structure of ITO/NPB/BCP/Alq 3 /Mg:Ag Fig.4 The CIE coordinate of device ITO/NPB/ HKEthFLYPh/Alq 3 /Mg:Ag The diversification of EL spectra of two kinds of devices is mainly originated from the variation of charge carrier recombination region, and this
No.4 MA Tao et al. 593 difference is considered to be largely owing to the impact of electric field on charge carrier tunneling effect with the variation of film thickness. According to Fowler-Nordheim tunneling model [28], the relationship between the tunneling probability of charge carriers(rate T ) and the electric field can be expressed as follows: 2 ( ) 1 2 Eg RateT E exp π 2mEg h qe (1) where, E=V/d is the electric field at a bias voltage of V, q is the electronic charge, m is the electronic mass, and d and E g represent the film thickness of organic layers and the bandgap energy between the highest occupied molecular orbital(homo) and the lowest unoccupied molecular orbital(lumo), respectively. From Eq.(1), we can predict that Rate T varies with electric field, which is related to the film thickness of organic layers(at identical bias voltage). In these two kinds of devices, with the increase of HBL film thickness, more holes are accumulated at the interface of HBL/NPB, and the electric field between the interface of HBL/NPB and cathode is enhanced, which directly results in the augment of Rate T. Consequently, it will eventually lead to diverse recombination regions in the devices. In these two kinds of devices, the CIE coordinates of devices B and II are the nearest to those of pure white light(0.33, 0.33), and therefore, the characteristics comparison of the two devices has been focused on. Luminance variation as a factor of applied voltage for devices B and II is shown in Fig.5. At a forward bias of 15 V, the maximum luminance of device II reaches 8523 cd/m 2, which is about 1000 cd/m 2 higher than that of device B. The turn-on voltages (defined as the bias required to attain a measurable luminance of 1 cd/m 2 ) of the two devices are 4 and 4.5 V, respectively. The inset of Fig.5 shows the power efficiency-voltage characteristic curve of the two devices; device II exhibits a maximum power efficiency of 1.0 lm/w at 5.5 V, which is nearly twice that of device B. Obviously, the performance of device II, such as maximum luminance or power efficency, is higher than that of device B, which is potentially useful for solid state lighting application. The significant improvement was caused by high luminescent quantum yield and good charge transport properties of fluorene based compounds [29,30]. Moreover, in the second kind of OLEDs, energy transfer may occur from HKEthFLYPh to NPB and Alq 3. Fig.6 shows the UV-Vis absorption and PL spectra of NPB, Alq 3, and KEthFLYPh. It can be seen that there is a large overlap between HKEthFLYPh emission spectrum and NPB absorption spectrum, and a considerable overlap between HKEthFLYPh emission spectrum and Alq 3 absorption spectrum. As well known, energy transfer of both Förster-type and Dexter-type requires the sufficient overlap of donor material PL spectrum with acceptor material absorption emission. Therefore, energy transfer from HKEthFLYPh to NPB and Alq 3 can be expected. Fig.5 Luminance-voltage characteristic curves of device B and device II Inset shows their power efficiency-voltage characteristic curves. Fig.6 Absorption and PL spectra of HKEthFLYPh and NPB 4 Conclusions In summary, we have fabricated a high performance white OLED with an easy processed structure from a novel star-shaped hexafluorenylbenzene. The device with a structure of ITO/NPB(40 nm)/ HKEthFLYPh(5 nm)/alq 3 (50 nm)/mg:ag(200 nm) was processed by thermal evaporation. The device shows a maximum luminance of 8523 cd/m 2 at 15 V, and a power efficiency of 1.0 lm/w at 5.5 V. A significant improvement of the characteristics can be attributed to a high luminescent quantum yield and good charge transport properties of HKEthFLYPh, which make most holes confined within the emission layer
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