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Enhancement in brightness and efficiency of organic electroluminescent device using novel N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline as hole injecting and transporting material
Synthetic Metals 146 (2004) 85–89
Enhancement in brightness and efficiency of organic electroluminescent device using novel N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline as hole injecting and transporting material Di Liua , Chang-Gua Zhenb , Xue-Song Wanga,∗∗ , De-Chun Zoub,∗∗ , Bao-Wen Zhanga,∗ , Yi Caoa a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, PR China b College of Chemistry, Peking University, Beijing 100871, PR China Received 18 December 2003; received in revised form 8 May 2004; accepted 13 June 2004 Available online 7 August 2004
Abstract A novel hole injecting and transporting material, N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA), was prepared by one-step reaction for use in organic electroluminescent (EL) devices. Organic EL device with configuration of ITO/DECMA (60 nm)/NPB (10 nm)/Alq3 (60 nm)/Mg:Ag/Ag turned on at 3.7 V and displayed a brightness of 1150 cd m−2 and a current efficiency of 5.5 cd A−1 at the current density of 20 mA cm−2 , while a reference device with configuration of ITO/NPB (70 nm)/Alq3 (60 nm)/Mg:Ag/Ag turned on at 3.9 V and exhibited a brightness of 500 cd m−2 and a current efficiency of 2.4 cd A−1 at the same current density. The lower turn on voltage and higher brightness and efficiency obtained in DECMA-containing device can be ascribed to the improved hole injection at anode/organic interface due to the low ionization potential of DECMA and the better balanced hole-electron recombination in emitting layer due to the moderate hole drift mobility of DECMA. © 2004 Elsevier B.V. All rights reserved. Keywords: Organic light-emitting devices; N,N-di(9-ethylcarbazyl-3)-3-methylaniline; Hole injecting and transporting
1. Introduction Organic light-emitting devices (OLEDs) have attracted considerable attention due to their potential applications in full-color flat-panel displays. Since the initial work on small molecules and polymer OLEDs by Tang and Vanslyke [1] and Friend and coworkers [2], much research work has focused on the improvement of efficiency and durability and other performance of OLEDs. Using multilayer structures [1], doped emitter system [3], low work-function cathodes for electron injection [1,3,4], and high work-function transparent indium–tin-oxide (ITO) bottom anode for hole injection, ∗
Corresponding author. Tel.: +86 10 64888103; fax: +86 10 64879375. Co-corresponding authors. E-mail address: [email protected] (B.-W. Zhang).
∗∗
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.06.022
remarkable improvements in device performance have been achieved. However, to enhance the efficiency, to prolong the durability and to lower the driving voltage of OLEDs continue to be the key challenges to meet the requirements of practical application in flat panel display technology. It is well proved [5] that the formation of cationic tris(8-hydroxyquinolinato)aluminium (Alq3 )+ in the emitting layer of the prototype 4,4 -bis[N-(1-naphthyl)-Nphenylamino]biphenyl (NPB)/Alq3 device, the result of the hole drift mobility of NPB is a factor of three orders higher than the electron transport mobility of Alq3 , is responsible for the low efficiency and intrinsic degradation. Therefore, it is essential to balance the hole and electron injected into the emitting layer to improve the OLEDs efficiency and reduce the intrinsic degradation. In order to balance the holes and electrons in OLEDs, there are currently two general
86
D. Liu et al. / Synthetic Metals 146 (2004) 85–89
methods in terms of hole injection [6,7] and transport [8], besides to improve the electron injection at cathode/organic interface [9] and to increase the electron transport mobility of the electron transporting materials [10]. One is to hold back hole injection to a certain extent by inserting hole buffer layer such as SiO2 [6] and Teflon [7] between ITO anode and the hole transporting layer (HTL). The other is to reduce properly the hole drift mobility of the HTL by doping hole trapping material such as rubrene [8]. However, the former method usually increases the driving voltage of the OLEDs due to the insulating properties of the hole buffer layer, and the later one inevitably complicates the device fabrication. Therefore, it is necessary to combine the improvement of the hole injection and the reduction of the hole drift mobility in the HTL to achieve low driving voltage and high efficiency and durability of OLEDs. 4,4 ,4 -Tris(3methylphenylphenylamino)triphenylamine (m-MTDATA) seems, so far, the only candidate of such hole injecting and transporting materials in OLEDs [11]. The usage of it can reduce the turn-on voltage and increase the brightness and efficiency due to its low ionization potential (Ip = 5.1 eV) and moderate hole drift mobility. However, it has a quite low glass transition temperature (Tg = 75 ◦ C), which limits its utilization in devices operated under high temperature. It is generally accepted that a morphologically stable amorphous organic layer, especially HTL, will lead to a longer lasting OLEDs [12]. In addition, the charge transporting materials in OLEDs should be transparent to the visible light so as to improve light output from the devices. Therefore, a morphologically stable amorphous HTL having a high Tg , a low Ip , a high transparency to visible light, and moderate hole drift mobility to match the present electron transporting material is desirable for realizing highly efficient and stable OLEDs. In this paper, we report that a novel carbazole-based triarylamine, N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA), functions as an excellent hole injecting and transporting material for OLEDs. DECMA is well characterized by low Ip , high Tg , high transparency to visible light, moderate hole drift mobility, and facile synthesis. The organic electroluminescent (EL) device consisting of DECMA as hole injecting and transporting layer, a thin layer of NPB as a second hole transporting layer and Alq3 as emitting layer exhibits extremely high efficiency of 5.63 cd A−1 and 2.6 lm W−1 . The power efficiency is 2.5 times of the traditional NPB/Alq3 device fabricated under the same conditions and also higher than the value of 2.3 lm W−1 reported for the similar device using m-MTDATA as hole injecting and transporting material [11].
1H
NMR spectrum was recorded on a Bruker DPX-400 spectrometer (400 MHz). The Infra-red (IR) spectrum was measured as KBr pellets on a BIO-RAD FTS-165 FT-IR spectrometer. Mass spectrum (MS) was measured on a Finnigan GC-MS 4021 C spectrophotometer. Elemental analysis (EA) was carried out on a Carlo Erba 1106 by the Flash EA 1112 method. The photoluminescence and absorption spectra of DECMA in films were recorded with a Perkin–Elmer LS50 fluorescence spectrometer and a Perkin–Elmer Lambda 2S UV–vis spectrophotometer, respectively. The differential scanning calorimetry (DSC) measurement and thermogravimetric analysis (TGA) were performed on a TA Instruments DSC 2910 and a TA Instruments TGA 2050, respectively. 2.2. Preparation of DECMA The molecular structure of DECMA is shown in Scheme 1. It can be synthesized by one-step palladium-catalyzed Narylation [13–15] of 3-methylaniline along with the following procedure. N,N-Di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA): A three-necked flask charged with 9-ethyl-3-bromocarbazole (3.025 g, 11 mmol), m-toluidine (0.535 g, 5 mmol), Pd(AcO)2 (74 mg, 0.33 mmol), 1,1 -Bis(diphenylphosphino) ferrocene (DPPF) (220 mg, 0.396 mmol), t-BuONa (1.58 g, 16.5 mmol) and dry toluene (30 mL) was heated to 80 ◦ C and stirred under nitrogen for 5 h. The solvent was evaporated under vacuum and the dark residue was isolated by column chromatography on silica gel using a hexane/ethyl acetate mixture (20:1) as eluent to yield 2.27 g (92%) of the title compound as a white powder. 1 H NMR (CDCl3 , δ): 1.35 (t, 6H), 2.2 (s, 3H), 4.4 (q, 4H), 6.61 (s, 1H), 6.64 (s, 2H), 7.05 (t, 1H), 7.12 (t, 2H), 7.3 (d, 2H), 7.44 (t, 2H), 7.6 (m, 4H), 7.97 (d, 2H), 8.04 (s, 1H), 8.06 (s, 1H); FT-IR (KBr pellets, cm−1 ): 3047.5, 2960, 1593, 1475, 1316, 1209, 1132, 753; MS: m/z 493 ([M]+ ). Anal. Calc. for C35 H31 N3 : C 85.19%, H 6.29%, N 8.52%. Found: C 85.529%, H 6.36%, N 8.532%. 2.3. Fabrication and testing of OLEDs A three-layer OLED was fabricated by vacuum deposition with a configuration of ITO (30 cm−2 )/DECMA (60 nm)/NPB (10 nm)/Alq3 (60 nm)/Mg:Ag/Ag, in which DECMA was used as a hole injecting and transporting layer,
2. Experimental section 2.1. Materials and instruments All the reagents and solvents used for the synthesis of DECMA were purchased from Aldrich and Across companies and used without further purification.
Scheme 1. Molecular structure and synthetic route of DECMA.
D. Liu et al. / Synthetic Metals 146 (2004) 85–89
NPB acts as a second HTL and Alq3 as electron transporting emitter layer. Before being loaded into a deposition chamber, the ITO-coated glass substrate was cleaned with detergents and deionized water, dried in an oven at 120 ◦ C for 2 h, and treated with UV-ozone for 25 min. The organic materials were deposited at a rate of 0.1–0.3 nm s−1 sequentially on the ITO substrate in the chamber with a base pressure lower than 1 × 10−6 mbar. Onto the Alq3 layer a 250 nm thick Mg:Ag (mass ratio of 9:1) alloy was deposited at a rate of 0.6 nm s−1 as the cathode. Finally a 50 nm thick layer of Ag was deposited to protect the alloy cathode from oxidation in air. For comparison, a standard double-layer NPB (70 nm)/Alq3 (60 nm) device was also prepared and tested under identical conditions. The EL spectra and voltage-current densityluminance characteristics were measured with an Advantest R6145 dc voltage current source, Keithley Multimeter 2000 and Minolta Camera LS-110 Luminescence-meter under ambient conditions. The emission area of the devices is 0.04 cm2 as determined by the overlap area of the anode and the cathode.
3. Results and discussion 3.1. Thermal properties of DECMA Fig. 1 shows the DSC thermograms of DECMA at a heating rate of 10 ◦ C min−1 . When a polycrystalline sample of DECMA was heated, an endothermic peak due to melting was observed at 236 ◦ C. When the isotropic liquid sample was cooled down quickly and heated again, an amorphous glass of DECMA changed to a super-cooled liquid state at 106 ◦ C and finally melted at 236 ◦ C. In the temperature range from Tg to melting point, no noticeable crystallization took place. The glass transition temperature (Tg ) of DECMA is 106 ◦ C and higher by 30 ◦ C than that of the representative hole injecting and transporting material m-MTDATA (75 ◦ C), indicating a higher morphological stability of DECMA. The thermal property of DECMA was further confirmed by TGA measurement. No decomposition of DECMA was detected before and during the evaporation at 417 ◦ C.
Fig. 1. DSC thermograms of DECMA at a heating rate of 10 ◦ C min−1 .
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Fig. 2. Absorption and fluorescence spectra of DECMA film.
3.2. Optical properties of DECMA Fig. 2 shows the UV–vis absorption and fluorescence spectra of the vacuum-evaporated film of DECMA on quartz substrate. The absorption spectrum of DECMA exhibits the characteristic absorption peaks of carbazole below 350 nm, with very weak absorption over 400 nm, indicating DECMA is highly transparent to visible light. This merit of organic material is highly beneficial for the efficient light output from the OLEDs. The fluorescence spectrum of DECMA film shows a sharp and strong emission with peak at 447 nm. The fullwidth at half-maximum (FWHM) is only 48 nm, implying a good color purity when DECMA is used as blue emitting material in OLEDs. The Ip and electron affinity (Ea ) of solid-state DECMA were determined as 4.94 and 2.19 eV, respectively, by ultraviolet photoelectron spectroscopy along with the optical absorption spectra of the DECMA film. This extra low Ip value indicates that DECMA can be used as hole injecting material in organic EL devices. The energy levels of DECMA and NPB and Alq3 used to fabricate the OLEDs in the present study are shown in Fig. 3. Based on the low Ip character and the hole transporting nature of the triarylamine structure of DECMA, organic Alq3 -emitting EL device utilizing DECMA as hole injecting and transporting layer was deliberately fabricated and investigated.
Fig. 3. Energy level of DECMA and other materials used to fabricate OLEDs in the present study.
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Fig. 4. Schematic diagram of EL device configurations. (A) ITO/DECMA (60 nm)/NPB (10 nm)/Alq3 (70 nm)/Mg:Ag/Ag. (B) ITO/NPB (70 nm)/Alq3 (70 nm)/Mg:Ag/Ag.
3.3. EL performances of DECMA-based OLEDs The configuration of the DECMA-based devices in the present study is shown as A in Fig. 4. DECMA was used as hole injecting and transporting layer and Alq3 as electron transporting emitter layer. A thin layer of NPB was inserted between DECMA and Alq3 layers to overcome the high energy barrier for hole injection from the former to the later. To evaluate the hole injecting and transporting ability of DECMA, a prototype NPB/Alq3 reference device B was fabricated for comparison. The total thickness of the DECMA and NPB layers in device A was controlled identical with that of the single NPB layer in device B to avoid the possible driving voltage difference caused by organic thickness variation [10]. To ensure that no parameter differences other than the introduction of DECMA layer would influence the experimental results, all the organic and metallic depositions for both the devices were done in the same batch. Both devices A and B emitted bright green light with the same spectra as the photoluminescence of Alq3 , which confirms that the hole and electron recombine in the Alq3 layer. Fig. 5 shows the voltage-current density–luminance characteristics for both the devices. Device A can turn on at 3.7 V, while device B turns on at 3.9 V. The decrease in turn-on voltage is mainly because DECMA layer lowers the hole injection barriers at ITO/DECMA and DECMA/NPB interfaces and consequently facilitates the hole injection at these interfaces, as shown in the energy level scheme (Fig. 3). At any given voltage, the brightness of device A is enhanced compared to device B. High maximum brightness of 21,500 cd m−2 at the voltage of 15 V and 16,500 cd m−2 at 16 V were obtained for device A and device B, respectively. Shown in Fig. 6 is the current efficiency versus current density curves for the two devices. At a given constant current density of 20 mA cm−2 , DECMA-containing device A displayed a brightness of 1150 cd m−2 and an current efficiency of 5.5 cd A−1 , a factor of over two higher than 500 cd m−2 and 2.4 cd A−1 for the reference device B. The maximum current efficiency for device A is 5.62 cd A−1 at 50 mA cm−2 , which is much higher than 3.5 cd A−1 for device B. It should be noted that the current efficiency almost remain constant for both the devices even when the current density is increased up to 450 mA cm−2 , which is really an important merit highly beneficial for applications requiring high exci-
Fig. 5. Voltage–current density–luminance characteristics of the EL devices studied here.
tation density such as in passive dot matrix devices. The high thermal and morphological stability of DECMA should be responsible for the stable EL efficiency. A high power efficiency of 2.6 lm W−1 at a luminescence of 200 cd m−2 was achieved for device A. This power efficiency is a factor of two and a half higher than that of the reference device B (1.04 lm W−1 ), also higher than the value of 2.3 lm W−1 reported by Shirota for the similar device using m-MTDATA as the main hole injecting and transporting material and TPD as the second HTL [11]. The enhancement in brightness and efficiencies in DECMA-containing device should be attributed to the better balanced hole and electron recombination in the emitting layer due to the moderate hole drift mobility of DECMA. It has been well established that the positive charge surplus causes low EL efficiency and intrinsic degradation of OLEDs
Fig. 6. Current efficiency vs. current density curves of the devices.
D. Liu et al. / Synthetic Metals 146 (2004) 85–89
due to the hole drift mobility of NPB is a factor of three orders higher than the electron transport mobility of Alq3 in NPB/Alq3 device [5]. The better balance in hole and electron in OLEDs is essential to improve the efficiency and durability. The hole drift mobility of DECMA is determined by transient EL methodology [16] in the present study. It is independent of the electric field and is calculated as 2 × 10−4 cm2 V−1 s−1 under electric field intensities ranging from 6 × 105 to 1 × 106 V cm−1 , which is a factor of one order lower than that of NPB. Therefore, the hole drift mobility in DECMA layer matches in value much better with the electron transport mobility of Alq3 than that of NPB. Accordingly the hole and electron injected into the emitting layer is better balanced in device A. In addition, the merits including the high Tg , good film formation property and high transparency to visible light of DECMA should also contribute to the high efficiency and stability of device A especially at high current density.
4. Conclusion A novel compound N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA) was synthesized by facile method with excellent yield for use as hole injecting and transporting materials in OLEDs. The usage of DECMA as hole injecting and transporting layer in OLED reduced turn-on voltage and enhanced brightness and efficiencies. Especially the power efficiency was 2.5 times increased compared with the reference device without DECMA layer. The improved hole injection at interfaces is responsible for the reduction of driving voltage due to the low Ip value of DECMA. The enhancement in brightness and efficiencies resulted from the better balanced hole and electron injected into the emitting layer due to the moderate hole drift mobility of DECMA. The merits of DECMA including low Ip , high Tg , high transparency to visible light, the moderate hole drift mobility and easy synthesis combine to promise that DECMA is an excellent hole injecting and transporting material in organic EL de-
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vices and other organic optoelectronic devices. The present study provides a practical strategy by developing novel hole transporting materials with moderate hole drift mobility to improve the efficiency and durability of OLEDs.
Acknowledgments This project was financially supported by the Ministry of Science and Technology of China (G2000028204) and the National Natural Science Foundation of China (nos. 20272065, 90101013, 50125310).
References [1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913. [2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Mackay, K. Marks, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [3] C.W. Tang, S.A. Vanslyke, C.H. Chen, J. Appl. Phys. 85 (1989) 3610. [4] I.D. Parker, J. Appl. Phys. 75 (1994) 1656. [5] H. Aziz, Z.D. Popovic, N.X. Hu, A.M. Hor, G. Xu, Science 283 (1999) 1900. [6] Z.B. Deng, X.M. Ding, S.T. Lee, W.A. Gambling, Appl. Phys. Lett. 74 (1999) 2227. [7] Y. Gao, L. Wang, D. Zhang, L. Duan, G. Dong, Y. Qiu, Appl. Phys. Lett. 82 (2003) 155. [8] H. Aziz, Z. Popovic, Appl. Phys. Lett. 80 (2002) 2180. [9] L.S. Liang, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. [10] D.G. Ma, G. Wang, Y.F. Hu, Y.G. Zhang, L.X. Wang, X.B. Jing, F.S. Wang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 82 (2003) 1296. [11] Y. Shirota, Y. Kuwabara, H. Inada, Appl. Phys. Lett. 65 (1994) 807. [12] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 56 (1990) 799. [13] J.F. Hartwig, Angew. Chem. Int. Ed. 37 (1998) 2046. [14] M.S. Driver, J.F. Hartwig, J. Am. Chem. Soc. 118 (1996) 7217. [15] K.R.J. Thomas, J.T. Lin, Y.T. Tao, C.W. Ko, J. Am. Chem. Soc. 123 (2001) 9404. [16] T.C. Wong, J. Kovac, C.S. Lee, L.S. Hung, S.T. Lee, Chem. Phys. Lett. 334 (2001) 61.
Enhancement in brightness and efficiency of organic electroluminescent device using novel N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline as hole injecting and transporting material Di Liua , Chang-Gua Zhenb , Xue-Song Wanga,∗∗ , De-Chun Zoub,∗∗ , Bao-Wen Zhanga,∗ , Yi Caoa a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, PR China b College of Chemistry, Peking University, Beijing 100871, PR China Received 18 December 2003; received in revised form 8 May 2004; accepted 13 June 2004 Available online 7 August 2004
Abstract A novel hole injecting and transporting material, N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA), was prepared by one-step reaction for use in organic electroluminescent (EL) devices. Organic EL device with configuration of ITO/DECMA (60 nm)/NPB (10 nm)/Alq3 (60 nm)/Mg:Ag/Ag turned on at 3.7 V and displayed a brightness of 1150 cd m−2 and a current efficiency of 5.5 cd A−1 at the current density of 20 mA cm−2 , while a reference device with configuration of ITO/NPB (70 nm)/Alq3 (60 nm)/Mg:Ag/Ag turned on at 3.9 V and exhibited a brightness of 500 cd m−2 and a current efficiency of 2.4 cd A−1 at the same current density. The lower turn on voltage and higher brightness and efficiency obtained in DECMA-containing device can be ascribed to the improved hole injection at anode/organic interface due to the low ionization potential of DECMA and the better balanced hole-electron recombination in emitting layer due to the moderate hole drift mobility of DECMA. © 2004 Elsevier B.V. All rights reserved. Keywords: Organic light-emitting devices; N,N-di(9-ethylcarbazyl-3)-3-methylaniline; Hole injecting and transporting
1. Introduction Organic light-emitting devices (OLEDs) have attracted considerable attention due to their potential applications in full-color flat-panel displays. Since the initial work on small molecules and polymer OLEDs by Tang and Vanslyke [1] and Friend and coworkers [2], much research work has focused on the improvement of efficiency and durability and other performance of OLEDs. Using multilayer structures [1], doped emitter system [3], low work-function cathodes for electron injection [1,3,4], and high work-function transparent indium–tin-oxide (ITO) bottom anode for hole injection, ∗
Corresponding author. Tel.: +86 10 64888103; fax: +86 10 64879375. Co-corresponding authors. E-mail address: [email protected] (B.-W. Zhang).
∗∗
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.06.022
remarkable improvements in device performance have been achieved. However, to enhance the efficiency, to prolong the durability and to lower the driving voltage of OLEDs continue to be the key challenges to meet the requirements of practical application in flat panel display technology. It is well proved [5] that the formation of cationic tris(8-hydroxyquinolinato)aluminium (Alq3 )+ in the emitting layer of the prototype 4,4 -bis[N-(1-naphthyl)-Nphenylamino]biphenyl (NPB)/Alq3 device, the result of the hole drift mobility of NPB is a factor of three orders higher than the electron transport mobility of Alq3 , is responsible for the low efficiency and intrinsic degradation. Therefore, it is essential to balance the hole and electron injected into the emitting layer to improve the OLEDs efficiency and reduce the intrinsic degradation. In order to balance the holes and electrons in OLEDs, there are currently two general
86
D. Liu et al. / Synthetic Metals 146 (2004) 85–89
methods in terms of hole injection [6,7] and transport [8], besides to improve the electron injection at cathode/organic interface [9] and to increase the electron transport mobility of the electron transporting materials [10]. One is to hold back hole injection to a certain extent by inserting hole buffer layer such as SiO2 [6] and Teflon [7] between ITO anode and the hole transporting layer (HTL). The other is to reduce properly the hole drift mobility of the HTL by doping hole trapping material such as rubrene [8]. However, the former method usually increases the driving voltage of the OLEDs due to the insulating properties of the hole buffer layer, and the later one inevitably complicates the device fabrication. Therefore, it is necessary to combine the improvement of the hole injection and the reduction of the hole drift mobility in the HTL to achieve low driving voltage and high efficiency and durability of OLEDs. 4,4 ,4 -Tris(3methylphenylphenylamino)triphenylamine (m-MTDATA) seems, so far, the only candidate of such hole injecting and transporting materials in OLEDs [11]. The usage of it can reduce the turn-on voltage and increase the brightness and efficiency due to its low ionization potential (Ip = 5.1 eV) and moderate hole drift mobility. However, it has a quite low glass transition temperature (Tg = 75 ◦ C), which limits its utilization in devices operated under high temperature. It is generally accepted that a morphologically stable amorphous organic layer, especially HTL, will lead to a longer lasting OLEDs [12]. In addition, the charge transporting materials in OLEDs should be transparent to the visible light so as to improve light output from the devices. Therefore, a morphologically stable amorphous HTL having a high Tg , a low Ip , a high transparency to visible light, and moderate hole drift mobility to match the present electron transporting material is desirable for realizing highly efficient and stable OLEDs. In this paper, we report that a novel carbazole-based triarylamine, N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA), functions as an excellent hole injecting and transporting material for OLEDs. DECMA is well characterized by low Ip , high Tg , high transparency to visible light, moderate hole drift mobility, and facile synthesis. The organic electroluminescent (EL) device consisting of DECMA as hole injecting and transporting layer, a thin layer of NPB as a second hole transporting layer and Alq3 as emitting layer exhibits extremely high efficiency of 5.63 cd A−1 and 2.6 lm W−1 . The power efficiency is 2.5 times of the traditional NPB/Alq3 device fabricated under the same conditions and also higher than the value of 2.3 lm W−1 reported for the similar device using m-MTDATA as hole injecting and transporting material [11].
1H
NMR spectrum was recorded on a Bruker DPX-400 spectrometer (400 MHz). The Infra-red (IR) spectrum was measured as KBr pellets on a BIO-RAD FTS-165 FT-IR spectrometer. Mass spectrum (MS) was measured on a Finnigan GC-MS 4021 C spectrophotometer. Elemental analysis (EA) was carried out on a Carlo Erba 1106 by the Flash EA 1112 method. The photoluminescence and absorption spectra of DECMA in films were recorded with a Perkin–Elmer LS50 fluorescence spectrometer and a Perkin–Elmer Lambda 2S UV–vis spectrophotometer, respectively. The differential scanning calorimetry (DSC) measurement and thermogravimetric analysis (TGA) were performed on a TA Instruments DSC 2910 and a TA Instruments TGA 2050, respectively. 2.2. Preparation of DECMA The molecular structure of DECMA is shown in Scheme 1. It can be synthesized by one-step palladium-catalyzed Narylation [13–15] of 3-methylaniline along with the following procedure. N,N-Di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA): A three-necked flask charged with 9-ethyl-3-bromocarbazole (3.025 g, 11 mmol), m-toluidine (0.535 g, 5 mmol), Pd(AcO)2 (74 mg, 0.33 mmol), 1,1 -Bis(diphenylphosphino) ferrocene (DPPF) (220 mg, 0.396 mmol), t-BuONa (1.58 g, 16.5 mmol) and dry toluene (30 mL) was heated to 80 ◦ C and stirred under nitrogen for 5 h. The solvent was evaporated under vacuum and the dark residue was isolated by column chromatography on silica gel using a hexane/ethyl acetate mixture (20:1) as eluent to yield 2.27 g (92%) of the title compound as a white powder. 1 H NMR (CDCl3 , δ): 1.35 (t, 6H), 2.2 (s, 3H), 4.4 (q, 4H), 6.61 (s, 1H), 6.64 (s, 2H), 7.05 (t, 1H), 7.12 (t, 2H), 7.3 (d, 2H), 7.44 (t, 2H), 7.6 (m, 4H), 7.97 (d, 2H), 8.04 (s, 1H), 8.06 (s, 1H); FT-IR (KBr pellets, cm−1 ): 3047.5, 2960, 1593, 1475, 1316, 1209, 1132, 753; MS: m/z 493 ([M]+ ). Anal. Calc. for C35 H31 N3 : C 85.19%, H 6.29%, N 8.52%. Found: C 85.529%, H 6.36%, N 8.532%. 2.3. Fabrication and testing of OLEDs A three-layer OLED was fabricated by vacuum deposition with a configuration of ITO (30 cm−2 )/DECMA (60 nm)/NPB (10 nm)/Alq3 (60 nm)/Mg:Ag/Ag, in which DECMA was used as a hole injecting and transporting layer,
2. Experimental section 2.1. Materials and instruments All the reagents and solvents used for the synthesis of DECMA were purchased from Aldrich and Across companies and used without further purification.
Scheme 1. Molecular structure and synthetic route of DECMA.
D. Liu et al. / Synthetic Metals 146 (2004) 85–89
NPB acts as a second HTL and Alq3 as electron transporting emitter layer. Before being loaded into a deposition chamber, the ITO-coated glass substrate was cleaned with detergents and deionized water, dried in an oven at 120 ◦ C for 2 h, and treated with UV-ozone for 25 min. The organic materials were deposited at a rate of 0.1–0.3 nm s−1 sequentially on the ITO substrate in the chamber with a base pressure lower than 1 × 10−6 mbar. Onto the Alq3 layer a 250 nm thick Mg:Ag (mass ratio of 9:1) alloy was deposited at a rate of 0.6 nm s−1 as the cathode. Finally a 50 nm thick layer of Ag was deposited to protect the alloy cathode from oxidation in air. For comparison, a standard double-layer NPB (70 nm)/Alq3 (60 nm) device was also prepared and tested under identical conditions. The EL spectra and voltage-current densityluminance characteristics were measured with an Advantest R6145 dc voltage current source, Keithley Multimeter 2000 and Minolta Camera LS-110 Luminescence-meter under ambient conditions. The emission area of the devices is 0.04 cm2 as determined by the overlap area of the anode and the cathode.
3. Results and discussion 3.1. Thermal properties of DECMA Fig. 1 shows the DSC thermograms of DECMA at a heating rate of 10 ◦ C min−1 . When a polycrystalline sample of DECMA was heated, an endothermic peak due to melting was observed at 236 ◦ C. When the isotropic liquid sample was cooled down quickly and heated again, an amorphous glass of DECMA changed to a super-cooled liquid state at 106 ◦ C and finally melted at 236 ◦ C. In the temperature range from Tg to melting point, no noticeable crystallization took place. The glass transition temperature (Tg ) of DECMA is 106 ◦ C and higher by 30 ◦ C than that of the representative hole injecting and transporting material m-MTDATA (75 ◦ C), indicating a higher morphological stability of DECMA. The thermal property of DECMA was further confirmed by TGA measurement. No decomposition of DECMA was detected before and during the evaporation at 417 ◦ C.
Fig. 1. DSC thermograms of DECMA at a heating rate of 10 ◦ C min−1 .
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Fig. 2. Absorption and fluorescence spectra of DECMA film.
3.2. Optical properties of DECMA Fig. 2 shows the UV–vis absorption and fluorescence spectra of the vacuum-evaporated film of DECMA on quartz substrate. The absorption spectrum of DECMA exhibits the characteristic absorption peaks of carbazole below 350 nm, with very weak absorption over 400 nm, indicating DECMA is highly transparent to visible light. This merit of organic material is highly beneficial for the efficient light output from the OLEDs. The fluorescence spectrum of DECMA film shows a sharp and strong emission with peak at 447 nm. The fullwidth at half-maximum (FWHM) is only 48 nm, implying a good color purity when DECMA is used as blue emitting material in OLEDs. The Ip and electron affinity (Ea ) of solid-state DECMA were determined as 4.94 and 2.19 eV, respectively, by ultraviolet photoelectron spectroscopy along with the optical absorption spectra of the DECMA film. This extra low Ip value indicates that DECMA can be used as hole injecting material in organic EL devices. The energy levels of DECMA and NPB and Alq3 used to fabricate the OLEDs in the present study are shown in Fig. 3. Based on the low Ip character and the hole transporting nature of the triarylamine structure of DECMA, organic Alq3 -emitting EL device utilizing DECMA as hole injecting and transporting layer was deliberately fabricated and investigated.
Fig. 3. Energy level of DECMA and other materials used to fabricate OLEDs in the present study.
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Fig. 4. Schematic diagram of EL device configurations. (A) ITO/DECMA (60 nm)/NPB (10 nm)/Alq3 (70 nm)/Mg:Ag/Ag. (B) ITO/NPB (70 nm)/Alq3 (70 nm)/Mg:Ag/Ag.
3.3. EL performances of DECMA-based OLEDs The configuration of the DECMA-based devices in the present study is shown as A in Fig. 4. DECMA was used as hole injecting and transporting layer and Alq3 as electron transporting emitter layer. A thin layer of NPB was inserted between DECMA and Alq3 layers to overcome the high energy barrier for hole injection from the former to the later. To evaluate the hole injecting and transporting ability of DECMA, a prototype NPB/Alq3 reference device B was fabricated for comparison. The total thickness of the DECMA and NPB layers in device A was controlled identical with that of the single NPB layer in device B to avoid the possible driving voltage difference caused by organic thickness variation [10]. To ensure that no parameter differences other than the introduction of DECMA layer would influence the experimental results, all the organic and metallic depositions for both the devices were done in the same batch. Both devices A and B emitted bright green light with the same spectra as the photoluminescence of Alq3 , which confirms that the hole and electron recombine in the Alq3 layer. Fig. 5 shows the voltage-current density–luminance characteristics for both the devices. Device A can turn on at 3.7 V, while device B turns on at 3.9 V. The decrease in turn-on voltage is mainly because DECMA layer lowers the hole injection barriers at ITO/DECMA and DECMA/NPB interfaces and consequently facilitates the hole injection at these interfaces, as shown in the energy level scheme (Fig. 3). At any given voltage, the brightness of device A is enhanced compared to device B. High maximum brightness of 21,500 cd m−2 at the voltage of 15 V and 16,500 cd m−2 at 16 V were obtained for device A and device B, respectively. Shown in Fig. 6 is the current efficiency versus current density curves for the two devices. At a given constant current density of 20 mA cm−2 , DECMA-containing device A displayed a brightness of 1150 cd m−2 and an current efficiency of 5.5 cd A−1 , a factor of over two higher than 500 cd m−2 and 2.4 cd A−1 for the reference device B. The maximum current efficiency for device A is 5.62 cd A−1 at 50 mA cm−2 , which is much higher than 3.5 cd A−1 for device B. It should be noted that the current efficiency almost remain constant for both the devices even when the current density is increased up to 450 mA cm−2 , which is really an important merit highly beneficial for applications requiring high exci-
Fig. 5. Voltage–current density–luminance characteristics of the EL devices studied here.
tation density such as in passive dot matrix devices. The high thermal and morphological stability of DECMA should be responsible for the stable EL efficiency. A high power efficiency of 2.6 lm W−1 at a luminescence of 200 cd m−2 was achieved for device A. This power efficiency is a factor of two and a half higher than that of the reference device B (1.04 lm W−1 ), also higher than the value of 2.3 lm W−1 reported by Shirota for the similar device using m-MTDATA as the main hole injecting and transporting material and TPD as the second HTL [11]. The enhancement in brightness and efficiencies in DECMA-containing device should be attributed to the better balanced hole and electron recombination in the emitting layer due to the moderate hole drift mobility of DECMA. It has been well established that the positive charge surplus causes low EL efficiency and intrinsic degradation of OLEDs
Fig. 6. Current efficiency vs. current density curves of the devices.
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due to the hole drift mobility of NPB is a factor of three orders higher than the electron transport mobility of Alq3 in NPB/Alq3 device [5]. The better balance in hole and electron in OLEDs is essential to improve the efficiency and durability. The hole drift mobility of DECMA is determined by transient EL methodology [16] in the present study. It is independent of the electric field and is calculated as 2 × 10−4 cm2 V−1 s−1 under electric field intensities ranging from 6 × 105 to 1 × 106 V cm−1 , which is a factor of one order lower than that of NPB. Therefore, the hole drift mobility in DECMA layer matches in value much better with the electron transport mobility of Alq3 than that of NPB. Accordingly the hole and electron injected into the emitting layer is better balanced in device A. In addition, the merits including the high Tg , good film formation property and high transparency to visible light of DECMA should also contribute to the high efficiency and stability of device A especially at high current density.
4. Conclusion A novel compound N,N-di(9-ethylcarbaz-3-yl)-3-methylaniline (DECMA) was synthesized by facile method with excellent yield for use as hole injecting and transporting materials in OLEDs. The usage of DECMA as hole injecting and transporting layer in OLED reduced turn-on voltage and enhanced brightness and efficiencies. Especially the power efficiency was 2.5 times increased compared with the reference device without DECMA layer. The improved hole injection at interfaces is responsible for the reduction of driving voltage due to the low Ip value of DECMA. The enhancement in brightness and efficiencies resulted from the better balanced hole and electron injected into the emitting layer due to the moderate hole drift mobility of DECMA. The merits of DECMA including low Ip , high Tg , high transparency to visible light, the moderate hole drift mobility and easy synthesis combine to promise that DECMA is an excellent hole injecting and transporting material in organic EL de-
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vices and other organic optoelectronic devices. The present study provides a practical strategy by developing novel hole transporting materials with moderate hole drift mobility to improve the efficiency and durability of OLEDs.
Acknowledgments This project was financially supported by the Ministry of Science and Technology of China (G2000028204) and the National Natural Science Foundation of China (nos. 20272065, 90101013, 50125310).
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