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Efficient Deep Blue Organic Light-Emitting Diodes Based on Wide Band Gap 4-Hydroxy-8-Methyl1.5-Naphthyridine Aluminum Chelate as Emitting and Electron Transporting Layer Chih-Chien Lee, Chih-Hsien Yuan, Shun-Wei Liu, and Yih-Shiun Shih
Abstract—We have developed a high efficiency and deep blue organic light-emitting diodes (OLEDs) incorporating a 4-hydroxy-8-methyl-1,5-naphthyridine aluminum chelate (AlmND3 ) as the emitter and electron transporting layer (ETL). The blue AlmND3 emitter, with an electron withdrawing group added to the well-known green fluorophore tris(8-hydroxyquinolinato)aluminum (Alq3 ), exhibited ambipolar charge transport as well as high electron and hole mobilities on the order of 10 5 cm2 V s, as deduced time-of-flight measurements. The magnitude of the electron mobility was 10 times greater than that of the widely used Alq3 ETL, resulting in efficient charge balance in the AlmND3 device. Based on a simple configuration of double heterojunction device, a blue device with the maximum external quantum efficiency of 1.58% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.08) was achieved at a brightness of 200 cd/m2 . This study has revealed the fundamental nature of charge transport in hydroxynaphthyridine metal chelate and shed a new light on the design of high performance blue OLEDs. Index Terms—AlmND3 , Alq3 , deep blue organic light-emitting diodes (OLEDs), EML (emitter material layer), ETL (electron transporting layer), time-of-flight.
I. INTRODUCTION
O
RGANIC light-emitting diodes (OLEDs) have attracted much attention in various display applications since the first report on a bilayer OLED by Tang et al. [1]. The configuration of a typical single heterojunction device consists of a hole-transporting layer (HTL) and an electron-transporting layer (ETL) sandwiched between a low work function cathode and a transparent indium tin oxide (ITO) anode. The carrier mobilities of the two layers, which may play a critical role in determining the recombination region of the device, depend sensitively on the chemical structure of the organic material and the device structure [2]–[4]. Tris(8-hydroxyquinolinzto)aluminum (Alq ) has found widespread as a green emitter, a common ETL, as well as a host material for saturated green and red fluorescent Manuscript received October 26, 2010; revised March 21, 2011; accepted March 25, 2011. Date of current version July 06, 2011. This work was supported in part by Academia Sinica and the National Science Council of Taiwan under Grant NSC 100-3113-E-001-001. C.-C. Lee and C.-H. Yuan are with the Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10617, Taiwan (e-mail:
[email protected]). S.-W. Liu is with the Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan (e-mail:
[email protected]). Y.-S. Shih is with Lunghwa University of Science and Technology, Taipei 33306, Taiwan. Digital Object Identifier 10.1109/JDT.2011.2136319
dopants and still remains one of the most widely used [5]. In the previous works, the electron mobility of Alq has been estabto cm V s [6]. This lished to be on the order of value is lower than that those of common HTL materials. For example, the hole mobility of -naphthylphenylbiphenyl amine (NPB) is on the order of 10 cm V s [6]. It is commonly known that the electrical properties of the NPB/Alq bilayer device structure are dominated by the lower electron mobility of Alq rather than the hole mobility of NPB [7]. Therefore, quite a number of structurally modified Alq derivatives with high electron and hole mobilities have been developed over the last two decades to cater to the requirements for full color display applications [8]. In our recent work, a series of Group III metal chelates were synthesized and characterized for deep-blue OLED applications [8]. These chelating ligands and their metal chelates can easily be prepared with improved synthetic methods, suggesting that Alq derivatives exhibiting deep-blue fluorescence in non-doped devices can be realized. On the other hand, to achieve efficient device performance, the active emitting material should satisfy several requirements: 1) thermally stable and morphological properties in the solid state; 2) appropriate charge mobilities for the transport of both carriers; 3) high photoluminance (PL) quantum efficiency; 4) appropriate energy levels for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO); and 5) straightforward purification with the vacuum deposition process. In order to further enhance the luminous efficiency and device reliability, it is therefore necessary to verify the electron and hole transport capabilities of these metal chelates via vacuum fabrication processes. In this paper, we report a time-of-flight (TOF) study of the hole and electron drift mobilities of a deep-blue 4-hydroxy-8-methyl-1,5-naphthyridine aluminum chelate AlmND . We also examined the origin of the carrier transport properties using TOF transient profiles based on the Poole-Frenkel (PF) model. In addition, the resultant simple nondoped device exhibited a high electroluminescence (EL) quantum efficiency of 1.58% at 200 cd/m with AlmND as the deep blue emitter and transporting layer. II. EXPERIMENTAL AlmND has been used as the emitting and transporting layer in deep-blue OLED, owing to the fact that its optical C is larger than those bandgap ( 3.4 eV) and Tg
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Fig. 1. The chemical structure of NPB, mCP, and AlmND and energy alignment of HOMO and LUMO in double heterojunction OLED.
of Alq . In the study of the blue device performances of EL devices, the device configuration was ITO/NPB (40 nm)/mCP nm AlmND (50- nm)/LiF (1 nm)/Al (100 nm), where -dicarbazolyl-3,5-bebzebe as the buffer mCP represents layer for optimization the device efficiency. In order to obtain the optimal thickness of mCP, various layers with thickness of 0 (Device I), 2 (Device II), 5 (Device III), and 10 nm (Device IV) were inserted into our blue device. All organic materials were synthesized in-house and purified by temperature-gradient sublimation in this work. The chemical structures of these materials and energy alignments of the HOMO and LUMO in the bilayer OLED are shown in Fig. 1. A DC current/voltage source meter (Model 2400; Keithley) was used to measure the characterisluminance-current density-voltage tics, while the brightness was monitored with Si photodiodes calibrated by a spectrophotometer (PR650; PhotoResearch). The PL and EL spectra were recorded with a fluorescence spectrometer (Hitachi F-4500). External quantum efficiency (EQE) was assumed a Lambertian emission to calculate the EQE values. For the TOF measurements, the samples were fabricated with a configuration of glass/ITO (100 nm)/organic layer 1 m Ag (20 nm). During the film deposition of the devices, the pressure of the chamber was maintained below 4 10 Torr to minimize defects and the evaporation rates of the organic materials were controlled with the respective quartz-crystal monitors. The thickness of the organic layer was measured by a surface profiler (Sloan Dektak 3). For absorption measurement, the frequency-doubled output of a 5-ns pulsed dye laser at 340 nm was chosen to maximize the optical absorption of the AlmND layer, with the laser beam directed through the samples from the semitransparent Ag side to create photoexcited carriers. The excitation power density W/cm to avoid the space-charge effect and was set at the instrumental response time was kept much shorter than the transit time. The transient photocurrent was recorded by measuring the voltage across a 50 resistor with a 1 GHz digital oscilloscope (LC574 AM, LeCroy) while the transit time, , was obtained from the intersection of two clear straight lines in the double-logarithmic plot of the transient photocurrent , where profile. The carrier mobility is given by is the thickness of the organic layer and is the applied
Fig. 2. (a) Hole and (b) electron photocurrent traces the AlmND film at an electric field of 5.6 MV/cm. The inset in each plot is a double logarithmic plot from which the carrier transit time (t ) is determined.
electric field. The detection of the transient photocurrent signal was similar to the one described in [9]. These devices were completed with an encapsulation in a glove box (O and H O concentration below 0.1 ppm). III. RESULTS AND DISCUSSION Fig. 2 shows the hole and electron TOF transient profiles of MV/cm. AlmND under an electric field strength of The transient photocurrent of holes and electrons exhibited highly dispersive behavior and the transit time could not be determined from the double linear plot of photocurrent versus time characteristics. The dispersive character of the TOF trace indicated that a large spread of the local site-energy existed in the charge conduction pathway of the organic thin film [10]. As shown in Fig. 2, more impurities remaining in the material may degrade a packet of carriers propagating through the localized states of AlmND . Similar observations have been reported by Malliaras [11], who demonstrated that the nondispersive electron transport of Alq indicated the absence of intrinsic traps in well-purified films. Exposure of the Alq to the atmosphere introduced traps, which gave rise to dispersive transport [12]. This result implies that the presence of deep traps in the AlmND thin film could be removed with thorough purification. From the inserts of Fig. 2(a) and (b), in the double logarithmic representation, the carrier transit time, , was determined from the intersection of the two slopes in the transient photocurrent profile. We obtained a high electron cm V s and a hole mobility of nearly mobility of cm V s at an electric field of 5.6 MV/cm. In particular, the hole and electron mobilities were comparable
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TABLE I CARRIER MOBILITIES, ZERO-FIELD MOBILITIES ( ), AND PF SLOPE ( ) OF AlmND ; Alq , AND Bphen AT THE APPLIED ELECTRIC FIELD OF 0.64 MV/CM
Fig. 3. EL spectra of three different thicknesses of NPB/AlmND structures.
to those of the best performing currently dominant metal chelates-based materials [13]. According to the PF model for Coulombic trap controlled transport, the relationship between the carrier mobility and elec, where is the tric field is given by mobility at zero electric field and is the PF constant [13]. Such a relationship could be attributed to the effects of energetic and positional disorders on the hopping conduction in amorphous molecular solids [14]. This model is used to analyze the plot for logarithmic mobilities (hole and electron) versus AlmND , Alq , and BPhen (4,7-diphenyl-1,10-phenanthroline) and the fitting parameters are listed in Table I. The high hole and electron mobilities in AlmND , even large than those of common metal chelate Alq ETL, clearly indicated ambipolar carrier transport [13]. The higher hole and electron mobilities of AlmND may be attributed to naphthyrine derivatives (mND) which enhance the stacking in metal chelate. In fact, it can be seen that the electron mobility of AlmND approaches that of the highly efficient hole-blocking layer of BPhen [15], [16]. One would thus expect that the ETL with high electron mobility to reduce the driving voltage of the OLED and hence increase the power efficiency of the device [5]. The experimental details of efficient blue double heterojunction OLED with AlmND as the emitting and transporting layer will be described later. As previously reported, we found that the exciplex emission occurred at the interface of the NPB and AlmND due to the large energy difference between the two materials [8]. Therefore, fine-tuning the energy levels between HTL/EML or EML/ETL is crucial to minimize the exciplex emission [17]. Here, we focus on the thickness dependence of NPB/AlmND relative to the exciplex emission. Fig. 3 shows the EL spectra of three NPB/AlmND bilayer device structures. It is clear that exciplex formation depended strongly on the number of holes or electrons at the NPB/AlmND interface. As shown in Fig. 3, it has been found that the EL intensity at around 430–450 nm, i.e. the AlmND emission peak, roughly doubled as the NPB thickness increased from 20 to 60 nm. It is known that the balance of holes and electrons accumulated at the organic/organic interface is effective in reducing the exciplex emission as evident in the ITO/NPB (60 nm)/AlmND (30 nm)/LiF/Al device. Therefore, by improving the charge balance of holes and electrons at the NPB/AlmND interface, the problem of the exciplex associated with AlmND can be largely alleviated.
Fig. 4. (a) Quantum efficiency and (b) luminance vs. current density of mCP buffer layer with 0 ( ), 2 ( ), 5 ( ), and 10 ( ) nm in AlmND double heterojucntion OLED.
4
5
}
In order to obtain a high performance deep blue device, we have further eliminated the exciplex emission by inserting a buffer layer of mCP between the NPB and AlmND . To the best of our knowledge, we believe that the mCP buffer layer can be used in an AlmND double heterojunction OLED because its HOMO/LUMO energy levels are matched with NPB and AlmND at the interface for hole or electron transport. This indicates that the mCP buffer layer is very effective for eliminating of the exciplex formation in AlmND OLED as compared with 4,4 -di(9H-carbazol-9-yl)biphenyl (CBP) [8]. Detailed performances of the blue double heterojunction OLEDs are summarized in Fig. 4 and Table II.
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TABLE II EL CHARACTERISTICS OF AlmND DOUBLE HETEROJUNCTION OLED CONTAINING MCP WITH DIFFERENT LAYER THICKNESS. ALL DEVICES USED THE CONSTANT CURRENT DENSITY OF 20 mA/cm
Fig. 5. EL and PL spectra of nondoped AlmND OLEDs containing mCP with thickness of 0 ( ), 2 ( ), 5 ( ), and 10 ( ) nm. Notice that the devices were driven under the constant current density of 20 mA/cm .
4
5
}
Thus, the devices with the appropriate mCP thicknesses (about 5 10 nm) exhibited a notable improvement in the EL performances such as the turning-on voltage, current efficiency, and color purity (see Table II) due to the elimination of the exciplex emission at the NPB/AlmND interface. Unlike most of the currently known blue analogues of Alq or other deep blue materials, AlmND exhibited very deep blue fluorescence, wide-band gap, and high carrier mobilities. Many studies have reported structurally modified ETLs which exhibited an transition for achieving increase in the energy of the large electron affinity, thereby resulting in superior electron transport behavior [20]. However, the AlmND emitter can be easily prepared with improved synthetic methods, making the material readily available in bulk quantities and facilitating their usage in blue OLEDs. IV. CONCLUSION
Compared to the device with the buffer layer of mCP (see Devices II–IV), the blue device without the buffer layer of mCP (Device I) exhibited a spectral single blue-green exciplex emission (with EL around 480–540 nm), as shown in Fig. 5. The exciplex emission originated at the interface of two molecules by charge transfer from the electron-donor molecule to electron-acceptor molecule [18], [19]. This inference seemingly indicates that the exciplex emission originated from the interface of the NPB and AlmND . Therefore, the suitable HOMO level of mCP is effective in preventing the exciplex emission in our device structure as evident from the EL spectra of these devices. It should be note that the PL spectrum of AlmND thin film peaked at around 430–440 nm (see Fig. 5). This might be important to verify that the observed EL of Devices III and IV indeed corresponds to AlmND emission, and is not due to emission from other materials in our case. In addition, having solved the problem of exciplex EL and demonstrated the EL emission and electron transport properties of AlmND , we arrived at the optimal buffer thickness of 10 nm mCP for AlmND devices. Based on such a double heterojunction device, an AlmND OLED with deep blue EL emission of (0.16, 0.08), satisfactory EQE reaching 1.48% at 20 mA/cm , and maximum brightness of 3730 cd/m (see Device IV) was achieved. Interestingly, the EQE value was much higher than that of the Alq emitter with a similar structure under same driving current density [1]. This is attributed to the high PL quantum efficiency of the AlmND emitter at 43%. On the other hand, we noted that the EQE increased monotonically with the increase in the mCP thickness (see Table II).
We have determined the ambipolar charge transport and high carrier mobilities of AlmND by using the TOF technique. The transient photocurrent exhibited a highly dispersive shape. The hole and electron mobilities of AlmND were on the order of cm V s at an electric field in excesses of 0.64 MV/cm. The nondoped double heterojunction device employing an ambipolar AlmND emitting/transporting layer successfully achieved deep blue EL with good blue CIE of (0.16, 0.08), external quantum efficiency of 1.58%, and maximum brightness of 3730 cd/m with a 10 nm mCP buffer layer. ACKNOWLEDGMENT The authors give special thanks to H.-H. Wu, Syskey Technology Corporation (Taiwan), for the assistance in the design of fabrication system. REFERENCES [1] C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes,” Appl. Phys. Lett., vol. 51, pp. 913–915, 1987. [2] V.-E. Choong, S. Shi, J. Curless, C.-L. Shieh, H. C. Lee, F. So, J. Shen, and J. Yang, “Organic light-emitting diodes with a bipolar transport layer,” Appl. Phys. Lett., vol. 75, pp. 172–174, 1999. [3] J.-H. Lee, C.-I. Wu, S.-W. Liu, C.-A. Huang, and Y. Chang, “Mixed host organic light-emitting devices with low driving voltage and long lifetime,” Appl. Phys. Lett., vol. 86, pp. 103506–103503, 2005. [4] C.-C. Lee, M.-Y. Chang, P.-T. Huang, Y. C. Chen, Y. Chang, and S.-W. Liu, “Electrical and optical simulation of organic light-emitting devices with fluorescent dopant in the emitting layer,” J. Appl. Phys., vol. 101, pp. 114501–114511, 2007. [5] L. S. Hung and C. H. Chen, “Recent progress of molecular organic electroluminescent materials and devices,” Mater. Sci. Eng. R, vol. 39, pp. 143–222, 2002.
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Chih-Chien Lee was born in Taoyuan, Taiwan, on 1961. He received the Ph.D. degree in physics from Stevens Institute of Technology, Hoboken, NJ, in 1998. Following his Ph.D. degree, , he joined National Taiwan University of Science and Technology, Taipei, Taiwan, as Assistant Professor of Department of Electronic Engineering. In recent years, he has started research on the developing a simulation model for organic light-emitting diodes.
Chih-Hsien Yuan was born in Taoyuan, Taiwan, in 1978. He received the M.S. degree in Electro-Optical Engineering from National Taiwan University of Science and Technology in 2010. He is currently a research assistant at the Institute of Chemistry, Academia Sinica (Taiwan). His current research interests include the solution processes of full color organic light-emitting diodes.
Shun-Wei Liu was born in Taoyuan, Taiwan, in 1978. He received the Ph.D. degree from the Graduate Institute of Photonics and Optoelectronics, National Taiwan University in 2010. After his Ph. D., he moved to the Institute of Chemistry, Academia Sinica (Taiwan), and served as a Special Research Staff. His current research interests involve the device physics and fabrication of organic optoelectronics, including organic light-emitting diodes, organic solar cells, and organic field-effect transistors.
Yih-Shiun Shih was born in Changhua in 1956. He received the Ph.D. degree in material science engineering from Tatung University in 2000. His current research interests include the material property of diamond-like carbon (DLC) film.
