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Hole transport materials with high glass transition temperatures for highly stable organic light-emitting diodes
Thin Solid Films 520 (2012) 7157–7163
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Hole transport materials with high glass transition temperatures for highly stable organic light-emitting diodes Jeonghun Kwak a, Yi-Yeol Lyu b, Seunguk Noh c, Hyunkoo Lee c, Myeongjin Park c, Bonggoo Choi e, Kookheon Char d,⁎, Changhee Lee c,⁎ a
Department of Electronic Engineering, Dong-A University, Busan 604-714, Republic of Korea Unitech Co., Ltd., 398-6, Moknae-dong, Danwon-gu, Ansan-city, Gyeonggi-do, 425-100, Republic of Korea School of Electrical and Computer Engineering, Inter-university Semiconductor Research Center (ISRC), Seoul National University, Seoul 151-744, Republic of Korea d School of Chemical and Biological Engineering, Intelligent Hybrids Research Center, Seoul National University, Seoul 151-744, Republic of Korea e Department of Chemical and Biomolecular Engineering, Electronic Material Lab, Yonsei University, Seoul 120-749, Republic of Korea b c
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Article history: Received 8 February 2012 Received in revised form 31 July 2012 Accepted 31 July 2012 Available online 10 August 2012 Keywords: Organic light-emitting diode Hole transport material High glass transition temperature High stability
a b s t r a c t Two hole transport materials with high glass transition temperatures (Tg ~200 °C) have been synthesized by replacing the phenyl groups of 4,4′-bis[N-(1-naphthyl-1)-N′-phenyl-amino]-biphenyl (α-NPD) with the bulkier phenanthrene (N,N′-di(naphthalene-1-yl)-N,N′-di(phenanthrene-9-yl)biphenyl-4,4′-diamine, NPhenD) or anthracene (N, N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)biphenyl-4,4′-diamine, NAD). The organic light-emitting diodes (OLEDs) using these hole transport materials exhibited stable operation at high temperatures up to 420 K, improved device lifetimes, and reduced operating voltage changes compared to the conventional hole transport materials owing to their high Tg. Although NAD has quite small bandgap as a hole transport material, superior thermal properties of NPhenD and NAD suggest that they can be promising materials for highly stable and high temperature-durable OLEDs and other organic optoelectronic devices. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have shown remarkable progress in their luminous efficiency, driving voltage, and device stability since the first efficient OLED had been demonstrated [1]. Due to their high potential for realizing the flat panel displays and lightings, the interests in organic functional materials based on π-conjugated molecules as well as in electroluminescent (EL) devices have attracted attentions for a few decades [2–9]. At this time, small-sized display panels are widely used for mobile instruments on a mass production basis. However, it still has been widely agreed that achieving much more enhanced durability is one of the most significant issues for the spread of practical OLED applications. The exact degradation mechanisms in the OLED devices are not fully comprehended yet, but there are several studies indicating the morphological changes of amorphous organic layers, especially the hole transport layer (HTL) [3–5]. In particular, Joule heating during device operation increases the device temperature, which causes the recrystallization of organic molecules with low glass transition temperature (Tg) [6–8]. Therefore, it is important to develop organic functional materials possessing high glass transition temperature for high thermal and morphologic stability, which can lead to produce the high-durability OLEDs. ⁎ Corresponding authors at: School of Electrical Engineering and Computer Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea. Tel.: +82 2 880 9093; fax: +82 2 877 6668. E-mail addresses: [email protected] (K. Char), [email protected] (C. Lee). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.130
Since prototypical HTL materials such as N,N′-(3-methyl-phenyl)1,1′-biphenyl-4,4′-diamine (TPD) and 4,4′-bis[N-(1-naphthyl-1)-N′phenyl-amino]-biphenyl (α-NPD) have relatively low Tg of 65 °C and 95 °C, respectively [9], the longevity of devices using those materials were hindered considerably. Thus, various methods have been reported to improve the stability of HTLs for a longer device lifetime: utilizing high Tg HTL materials [9–11], or doping the HTL with other organic or inorganic materials [4,12,13]. For instance, the derivatives of TPD such as 4,4′-N,N′-dicarbazolylbiphenyl (CBP) [5] or N,N′-di(biphenyl-4-yl)-N,N′diphenyl-[1,1′-biphenyl]-4,4′-diamine (p-BPD) [14] have been developed because using materials having higher Tg can contribute to increase the stability and operational lifetime of OLEDs. However, the highest occupied molecular orbital (HOMO) energy levels of them are too low to inject hole carriers from the typical anodes (e.g., indium-tin-oxide (ITO)), and also their thermal properties were not good enough to adopt them for the commercialized fabrication. Here, we report the synthesis of two amorphous hole transport materials containing a biphenyl backbone with naphthyl and bulkier phenanthrene or anthracene groups attached to the nitrogen atoms, i.e., N,N′-di(naphthalene-1-yl)-N,N′-di(phenanthrene-9-yl)biphenyl4,4′-diamine(NPhenD) or N,N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)biphenyl-4,4′-diamine (NAD), respectively, and their application to OLEDs in simple bilayer device structure. These materials have high Tg (195 °C for NPhenD and 206 °C for NAD) and moderate hole mobility (2.2 × 10−4 cm2/Vs for NPhenD). We believe that these high thermal properties are attributed to the bulkier and more rigid
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anthracenyl and phenanthrenyl groups replacing the phenyl groups in α-NPD. Compared with the standard device using α-NPD as a HTL, the devices with NPhenD and NAD exhibited better efficiency, improved lifetime and small operating voltage changes over time, and stable operation even at high temperature up to 420 K. We verified the synthesis, analyzed the optical, thermal properties of them, and characterized the device performances. 2. Experimental details
at 180 °C for 36 h. After evaporation of the solvent in vacuum, the residue was washed with water several times, dried, re-dissolved in hot chloroform, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography (silica-gel, hexanes/ chloroform (7:3)) to yield the title compound as a pale yellow solid. (2.26 g, 42%) 1H-nuclear magnetic resonance (1H-NMR) (300 MHz, THF-d8): δ 8.14 (d, 2H, J=1.5 Hz), 7.84 (d, 2H, J=9.0 Hz), 7.32–7.49 (m, 16H), and 7.14 (d, 4H, J=22.5 Hz). 13C-NMR (75 MHz, THF-d8): δ 145.0, 140.9, 136.0, 133.8, 129.0, 128.8, 127.6, 126.8, 126.6, 125.7, 123.3, 122.6, 118.6, and 115.5.
2.1. Synthesis of NPhenD and NAD NPhenD and NAD were synthesized according to the procedures in Scheme 1. Unless stated otherwise, all reagents were used as received from commercial sources. The solvents were dried using standard procedures. All reactions were performed under a purified nitrogen atmosphere using the standard Schlenk technique. 2.1.1. N,N′-di(naphthalene-1-yl)-biphenyl-4,4′-diamine (DBD) 4,4′-diiodobiphenyl (5.00 g, 12.31 mmol), 1-naphthylamine (3.61 g, 25.21 mmol), copper (0.13 g, 2.05 mmol), and potassium carbonate (3.74 g, 27.06 mmol) were charged in a two-necked flask equipped with a reflux condenser and maintained under N2 atmosphere. o‐ Dichlorobenzene (150 ml) was added and the mixture was heated
2.1.2. N,N′-di(naphthalene-1-yl)-N,N′-di(phenanthren-9-yl)-biphenyl4,4′-diamine (NPhenD) N,N′-di(naphthalene-1-yl)-biphenyl-4,4′-diamine(2.50 g, 5.73 mmol), 9-iodophenanthrene (5.23 g, 17.20 mmol), sodium tert-butoxide (1.66 g, 17.27 mmol), tris(dibenzylideneacetone) dipalladium (0) (265 mg, 0.29 mmol), and tri-tert-butylphophine (115 mg, 0.57 mmol) were allowed to react according to a procedure for compound NAD. Yield: 65% (2.94 g). Matrix-assisted laser desorption/ionization time-offlight mass spectrometry (MALDI-TOF) (m/z): Calcd. 788.32. Found: 788.24. Liquid chromatography/mass spectrometry-ion trap-time-offlight (LCMS-IT-TOF) [M+H]+: Calcd. 789.3270. Found: 789.3272. Anal. Calcd. for C60H40N2: C, 91.34; H, 5.11; and N, 3.55%. Found: C, 91.35; H, 5.09; and N, 3.54%.
Scheme 1. Synthetic route to the NPhenD and NAD.
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2.1.3. N,N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)-biphenyl4,4′-diamine (NAD) N,N′-di(naphthalene-1-yl)-biphenyl-4,4′-diamine (2.50 g, 5.73 mmol), 9-bromoanthracene (4.42 g, 17.19 mmol), sodium tertbutoxide (1.66 g, 17.27 mmol), tris(dibenzylideneacetone) dipalladium (0) (265 mg, 0.29 mmol), and tri-tert-butylphophine (115 mg, 0.57 mmol) were charged in a two-necked flask equipped with a reflux condenser and maintained under N2 atmosphere. Toluene (100 ml) was added and the mixture was heated at 110 °C for 24 h. The residue was filtered, and washed with chloroform several times, dried, and concentrated under reduced pressure. The crude product was purified by repeated temperature gradient vacuum sublimation to yield the title compound as a yellow solid. (2.76 g, 61%) MALDI-TOF (m/z): Calcd. 788.32. Found: 788.32. LCMS-IT-TOF [M + H] +: Calcd. 789.3270. Found: 789.3263. Anal. Calcd. for C60H40N2: C, 91.34; H, 5.11; and N, 3.55%. Found: C, 91.33; H, 5.08; and N, 3.55%. 2.2. Measurement The 1H and 13C NMR spectra were recorded at 25 °C on a Bruker DPX-300 spectrometer. Mass spectra were recorded on a Voyager-DE™ STR Biospectrometry Workstation MALDI-TOF MS spectrometer and Shimadzu LCMS-IT-TOF. Elemental analyses were recorded on a CE Instrument EA 1110 Elemental Analyzer. Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC 2020 instrument with a heating rate of 10 °C min−1 and a cooling rate of 20 °C min−1. Thermogravimetric analyses (TGA) were conducted on a TA Instruments TGA 2050 unit under a heating rate of 10 °C min−1 and a nitrogen flow rate of 90 ml min−1. UV–visible spectra were measured with an HP 8453 spectrophotometer. Photoluminescence (PL) spectra were obtained with the F-4500 fluorescence spectrometer. The HOMO energy levels were measured with the Riken Keiki AC-2 (750H) measurements, and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the optical absorption edge. 2.3. Hole mobility measurements The hole mobilities of α-NPD, NPhenD, and NAD films were measured with a time-of-flight photoconductivity (TOF-PC) technique as described in our previous work [15]. About 1-μm-thick films of α-NPD, NPhenD, and NAD sandwiched between ITO and Al electrodes were photoexcited by a N2 pulse laser (337 nm, PTI GL-3300), and the transient photocurrent (PC) was measured with a 500 MHz oscilloscope (Tektronix TDS 5054B). Single carrier (hole-only) devices of α-NPD, NPhenD, and NAD were also fabricated as follows: as a hole injection layer, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spin-coated on the top of the ITO substrate at a rate of 4000 rpm, and then dried in a vacuum oven at 120 °C for more than 30 min. After that, 100 nm of organic materials and 100 nm of Al were deposited sequentially by thermal evaporation. The current–voltage (I–V) characteristics of single carrier devices were measured with a Keithley 236 sourcemeasure unit.
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and 4–6 Å/s for Al electrodes. Fabricated device was mounted onto the cryostat in order to measure the electrical and optical characteristics with changing the substrate temperature. The current–voltage– luminance (I–V–L) characteristics and the EL spectra were measured using a Keithley 236 source-measure unit, a Konica-Minolta CS-1000A, and a Keithley 2000 multimeter equipped with a calibrated Si photodiode and a photomultiplier tube (PMT) through an ARC 275 monochromator. For the lifetime tests, each device was capped with a glass lid using UVcurable epoxy sealant in an Ar-filled glove box. 3. Result and discussion 3.1. Synthesis and characterization Scheme 1 shows the chemical structures and their synthetic procedures. NAD and NPhenD were prepared by palladium-catalyzed C–N coupling reaction of DBD with 9-bromoanthracene and 9iodophenanthrene, respectively, which was obtained from 4,4′dibromobiphenyl and 1-naphthylamine through copper-catalyzed Ullmann condensations. The structures of the synthesized compounds were characterized by using nuclear magnetic resonance (NMR), mass spectroscopy, and elemental analysis. These materials have biphenyl backbone similar to TPD, α-NPD, CBP, and p-BPD, but in order to realize higher Tg, we applied the naphthyl groups to the nitrogen atom of each amine groups of one material and bulkier aromatic compounds to the other. Thus two amorphous hole transport materials possess high Tg values of 195 °C (NPhenD) and 206 °C (NAD). 3.2. Optical properties Fig. 1 shows the optical absorption and photoluminescence (PL) spectra of the NPhenD and NAD in solid films formed by vacuum deposition on a quartz plate. The absorption spectra of the NPhenD and NAD reveal the maximum absorption peaks at 375 and 382 nm, while the PL spectra exhibit the maximum emission peaks at 441 and 524 nm, respectively. The PL peak of NAD is a little broader and red-shifted about 80 nm compared with that of NPhenD because the symmetric anthracenyl group in NAD has more ordered molecular packing due to its longer conjugation length than the asymmetric phenanthrenyl group in NPhenD [16]. The energy bandgaps of NPhenD and NAD were 2.94 eV and 2.50 eV, respectively, calculated from the threshold of absorption spectra. The HOMO energy levels, 5.52 eV and 5.44 eV, were measured by a photoelectron spectrometer, and the LUMO energy levels were calculated as 2.58 eV and 2.94 eV from the bandgaps, respectively. These results are consistent with the recent report that the LUMO energy level was determined by anthracene and phenanthrene moieties while the HOMO energy level was controlled by the triarylamine
2.4. Fabrication and characterization of the OLEDs The ITO substrates were cleaned ultrasonically in organic solvents (isopropyl alcohol, acetone, and methanol), rinsed in deionized water, followed by UV-ozone (UVO-42) treatment for 5 min. After that, 4,4′, 4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (mMTDATA) as a hole injection layer, a hole transporting material (α-NPD, NPhenD, or NAD, respectively), tris(8-hydroxyquinolinato) aluminum (Alq3) as an emitting and electron transport layer, LiF, and Al were successively deposited on the substrates under a base pressure of 4×10−4 Pa. The deposition rate was about 1–2 Å/s for organic materials,
Fig. 1. Normalized UV–vis. absorption and fluorescence spectra of NPhenD (solid lines) and NAD (dashed lines) in films.
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core [16]. Consequently, NAD with anthracene has smaller bandgap and more red-shifted PL than those of NPhenD with phenanthrene. 3.3. Thermal properties Fig. 2 shows the thermal properties of NPhenD and NAD, investigated with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA indicates that NPhenD and NAD exhibit decomposition temperatures (Td) (corresponding to 5% weight loss) as high as 488 and 473 °C, respectively. The melting temperatures (Tm) were detected for NPhenD and NAD at 385 and 393 °C, respectively, by DSC examination in the first heating scan. The glass transition temperatures (Tg) were observed for NPhenD and NAD as 195 and 206 °C, respectively, in the second, the third, and the forth heating scan. Both NPhenD and NAD show superior thermal stability from TGA and DSC measurements. In particular, they show impressively high Tg which is one of the highest values among those of reported hole transport materials. This high thermal stability is obtained by applying bulkier and more rigid anthracenyl and phenanthrenyl groups in α-NPD structure instead of phenyl group. 3.4. Mobilities of hole injection and transport materials To compare the hole conducting properties of the materials as the HTL, we measured the I–V characteristics of the single carrier devices of α-NPD, NPhenD, and NAD with the structure of ITO/PEDOT:PSS (40 nm)/HTL (100 nm)/Al (100 nm). As can be seen in Fig. 3(a), the current density of NPhenD is smaller than that of α-NPD at a given bias voltage, while the current density of NAD is similar with that of α-NPD at the same condition. It means that NAD possesses slightly higher hole mobility compared to NPhenD when we ignore the injection barrier between HTLs by using PEDOT:PSS buffer layer [17].
In order to characterize the hole transport properties precisely, we also measured the hole mobilities of α-NPD, NPhenD, and NAD films (about 1-μm-thick) with the TOF-PC technique. From the transit time (τ), defined as the edge of the plateau where the photocurrent starts to decrease steeply as holes discharge at the opposite Al electrode, the hole mobility (μ) was calculated by using the relation μ = d 2/Vτ, where d is the sample thickness and V is the bias voltage. Fig. 3(b) describes their hole mobilities as a function of the electric field, and the inset plots the transient PC waveforms of NPhenD measured under an electric field of 0.2 MV/cm at room temperature. The hole mobilities of α-NPD, and NPhenD at 0.2 MV/cm were found to be 5.7 × 10 −4 and 2.2 × 10 −4 cm 2/Vs, respectively, which are similar to the previously reported results [18,19]. The PC waveform of NPhenD showed a characteristic plateau of the non-dispersive hole transport, while the PC signal of NAD was highly dispersive so that we could not determine the carrier transit time as shown in the inset of Fig. 3(b). From the I–V characteristics in Fig. 3(a), the hole mobility of NAD can be estimated to be similar with that of α-NPD. 3.5. Electroluminescence properties In order to examine the performances of synthesized materials as the HTL, we adopted a simple OLED structure, which is ITO/m-MTDATA (15 nm)/HTL (60 nm)/Alq3 (70 nm)/LiF (0.5 nm)/Al (100 nm). Fig. 4 depicts the schematic energy band diagrams of devices using m-MTDATA as the HIL, α-NPD, NPhenD, or NAD as the HTL, and Alq3 as the lightemitting and electron transport layer. The HOMO energy level of NPhenD is slightly lower by 0.1 eV compared to those of α-NPD and NAD. The I–V–L characteristics of the devices with various HTLs are shown in Fig. 5(a) and (b). They showed higher current densities in sequence of NAD, α-NPD, and NPhenD at a given voltage, which also supports the prior supposition that the hole mobility of NAD
Fig. 2. TGA thermograms of (a) NPhenD and (b) NAD, and DSC thermograms of (c) NPhenD and (d) NAD. Both materials exhibit high thermal properties in terms of decomposition, melting, and glass transition temperatures.
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Fig. 3. (a) The I–V characteristics of hole-only devices (ITO/PEDOT:PSS/HTL (100 nm)/Al) with α-NPD (circle), NPhenD (triangle), and NAD (square). (b) The electric field dependence of the hole mobilities of α-NPD (circle) and NPhenD (triangle) films with a thickness of 1 μm, measured by TOF-PC, and the inset plots TOF-PC waveforms of NPhenD and NAD films under an electric field of 0.2 MV/cm at room temperature.
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nearby NAD layer due to the smaller band gap of NAD compared with Alq3. The external quantum efficiency of the device with NPhenD is comparable with that with α-NPD, but the device with NAD exhibits reduced efficiency because the recombination occurs in NAD layer (see Fig. 5(d)). Since the properties of low LUMO energy level and small bandgap of NAD are demerits for being used as the HTL, a thin electron blocking layer might be required to utilize NAD as the HTL. Even though the device efficiencies of OLEDs using NPhenD or NAD as the HTL were not noticeable, we can expect enhanced stability during the device operation since their glass transition temperatures (~ 200 °C) are exceptionally high. In order to investigate the effect of Joule heating on the materials, we fabricated 50-nm-films of α-NPD, NPhenD, and NAD on the substrate and then examined the changes of film morphology by using a high resolution optical microscope before and after the annealing them at 400 K for an hour. As shown in Fig. 6, NPhenD and NAD films have no noticeable change in their morphology, while α-NPD film is deformed to hemispherical islands during the heating. This phenomenon, which is usually observed in the materials with low Tg [20], exerts a bad effect on the lifetime of OLED devices. Fig. 7 plots the operational stability of the OLED devices with the HTL of α-NPD, NPhenD, or NAD under a constant current bias for the initial luminance of 1000 cd/m 2 at room temperature. The stability of NPhenD and NAD devices is significantly improved by 3–8 folds compared with α-NPD device, attributed to their higher Tg than that of α-NPD (95 °C). It is also noticeable that both the device lifetime and the voltage change of NAD device are much better than those of NPhenD device although two materials possess similar high Tg values. We believe that there may be more fundamental origin responsible for the OLED degradation in addition to morphological changes. As Aziz et al. reported [21], the cationic species of Alq3 (Alq3+) formed by excess holes injected from the HTL into the Alq3 layer could be one of intrinsic degradation mechanisms of the OLED based on Alq3. Thus, in spite of the poor efficiency, the NAD device can exhibit higher stability since the exciton recombination occurs predominantly in the NAD layer rather than the Alq3 layer, which leads to prevent from the Alq3+ formation. Similarly, the Alq3+ formation can be reduced in the
may be higher than that of NPhenD. The EL spectra of the devices with α-NPD and NPhenD are identical, originating from Alq3 layer, but the EL spectrum of NAD device shows a peak at about 560 nm, which is 40 nm away from the emission peak of Alq3 (see Fig. 5 (c)). As can be seen in the schematic energy band diagram in Fig. 4, the electron injection barrier between Alq3 and NAD (LUMO offset ~ 0.16 eV) is lower than the hole injection barrier between them (HOMO offset ~ 0.36 eV). Therefore, it is considered that the electrons and holes recombine mainly in the NAD layer rather than Alq3 layer, and also there is a possibility of energy transfer from the Alq3 layer to the
Fig. 4. Schematic energy band diagrams of the OLED devices using α-NPD, NPhenD, or NAD as the HTL.
Fig. 5. Device characteristics by using α-NPD (circle), NPhenD (triangle), or NAD (square) as the HTL in terms of (a) current density–voltage, (b) luminance–voltage, (c) EL spectra, and (d) external quantum efficiency–current density.
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Fig. 7. (a) The ratio of the relative luminance to the initial luminance and (b) voltage changes as a function of operating time of the OLED devices adopting various HTL materials biasing the constant current for the initial luminance of 1000 cd/m2. Fig. 6. Comparison of surface morphology between films as-deposited and annealed at 400 K for α-NPD, NPhenD, and NAD films.
NPhenD and NAD with enhanced thermal and electrical stability can be one of the promising materials in OLEDs or other organic optoelectronic devices. device with NPhenD compared with the device with α-NPD since the hole mobility of NPhenD is slightly lower than that of α-NPD even though it is still orders of magnitude higher than the electron mobility of Alq3. Therefore, we consider that much higher device stability by using NPhenD and NAD than using α-NPD is attributed to the effective prevention of the Alq3+ formation in this device structure as well as the high Tg of two materials. We also examined the device performances and durability at an elevated temperature. Fig. 8 plots the external quantum efficiency of three devices at 100 mA/cm 2 according to the operating temperature. The device with α-NPD showed unstable performances from 340 K, and failed to emit light above 360 K. On the other hand, both devices with high-Tg HTLs exhibited stable operation up to 420 K maintaining their efficiency. Consequently, these results clearly show that improved device stability at high temperature as well as the operational device lifetime can be achieved by using these hole transport materials possessing high Tg.
Acknowledgment This work was supported by the Industrial strategic technology development program (10041556) funded by the Ministry of Knowledge Economy (MKE), and by the Creative Research Initiative Program for “Intelligent Hybrids Research Center” (2010-0018290) and Basic Science Research Program (2011-0022716) of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea.
4. Conclusion We synthesized and characterized two hole transport materials possessing high Tg based on biphenyl backbone with bulkier and more rigid phenanthrenyl or anthracenyl group. Both materials, NPhenD and NAD, showed similar hole mobilities compared with α-NPD, and good electrical stabilities. We also demonstrated simple OLED devices adopting these materials as the HTL possessing superior performances in terms of lifetime, operating voltage variation, and high temperature operation up to 420 K. The result implies that the OLED operational stability is highly affected by the thermal stability of the HTL. We believe that
Fig. 8. External quantum efficiencies of each device by changing the operating temperature at the same current density of 100 mA/cm2.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Hole transport materials with high glass transition temperatures for highly stable organic light-emitting diodes Jeonghun Kwak a, Yi-Yeol Lyu b, Seunguk Noh c, Hyunkoo Lee c, Myeongjin Park c, Bonggoo Choi e, Kookheon Char d,⁎, Changhee Lee c,⁎ a
Department of Electronic Engineering, Dong-A University, Busan 604-714, Republic of Korea Unitech Co., Ltd., 398-6, Moknae-dong, Danwon-gu, Ansan-city, Gyeonggi-do, 425-100, Republic of Korea School of Electrical and Computer Engineering, Inter-university Semiconductor Research Center (ISRC), Seoul National University, Seoul 151-744, Republic of Korea d School of Chemical and Biological Engineering, Intelligent Hybrids Research Center, Seoul National University, Seoul 151-744, Republic of Korea e Department of Chemical and Biomolecular Engineering, Electronic Material Lab, Yonsei University, Seoul 120-749, Republic of Korea b c
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Article history: Received 8 February 2012 Received in revised form 31 July 2012 Accepted 31 July 2012 Available online 10 August 2012 Keywords: Organic light-emitting diode Hole transport material High glass transition temperature High stability
a b s t r a c t Two hole transport materials with high glass transition temperatures (Tg ~200 °C) have been synthesized by replacing the phenyl groups of 4,4′-bis[N-(1-naphthyl-1)-N′-phenyl-amino]-biphenyl (α-NPD) with the bulkier phenanthrene (N,N′-di(naphthalene-1-yl)-N,N′-di(phenanthrene-9-yl)biphenyl-4,4′-diamine, NPhenD) or anthracene (N, N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)biphenyl-4,4′-diamine, NAD). The organic light-emitting diodes (OLEDs) using these hole transport materials exhibited stable operation at high temperatures up to 420 K, improved device lifetimes, and reduced operating voltage changes compared to the conventional hole transport materials owing to their high Tg. Although NAD has quite small bandgap as a hole transport material, superior thermal properties of NPhenD and NAD suggest that they can be promising materials for highly stable and high temperature-durable OLEDs and other organic optoelectronic devices. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have shown remarkable progress in their luminous efficiency, driving voltage, and device stability since the first efficient OLED had been demonstrated [1]. Due to their high potential for realizing the flat panel displays and lightings, the interests in organic functional materials based on π-conjugated molecules as well as in electroluminescent (EL) devices have attracted attentions for a few decades [2–9]. At this time, small-sized display panels are widely used for mobile instruments on a mass production basis. However, it still has been widely agreed that achieving much more enhanced durability is one of the most significant issues for the spread of practical OLED applications. The exact degradation mechanisms in the OLED devices are not fully comprehended yet, but there are several studies indicating the morphological changes of amorphous organic layers, especially the hole transport layer (HTL) [3–5]. In particular, Joule heating during device operation increases the device temperature, which causes the recrystallization of organic molecules with low glass transition temperature (Tg) [6–8]. Therefore, it is important to develop organic functional materials possessing high glass transition temperature for high thermal and morphologic stability, which can lead to produce the high-durability OLEDs. ⁎ Corresponding authors at: School of Electrical Engineering and Computer Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea. Tel.: +82 2 880 9093; fax: +82 2 877 6668. E-mail addresses: [email protected] (K. Char), [email protected] (C. Lee). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.130
Since prototypical HTL materials such as N,N′-(3-methyl-phenyl)1,1′-biphenyl-4,4′-diamine (TPD) and 4,4′-bis[N-(1-naphthyl-1)-N′phenyl-amino]-biphenyl (α-NPD) have relatively low Tg of 65 °C and 95 °C, respectively [9], the longevity of devices using those materials were hindered considerably. Thus, various methods have been reported to improve the stability of HTLs for a longer device lifetime: utilizing high Tg HTL materials [9–11], or doping the HTL with other organic or inorganic materials [4,12,13]. For instance, the derivatives of TPD such as 4,4′-N,N′-dicarbazolylbiphenyl (CBP) [5] or N,N′-di(biphenyl-4-yl)-N,N′diphenyl-[1,1′-biphenyl]-4,4′-diamine (p-BPD) [14] have been developed because using materials having higher Tg can contribute to increase the stability and operational lifetime of OLEDs. However, the highest occupied molecular orbital (HOMO) energy levels of them are too low to inject hole carriers from the typical anodes (e.g., indium-tin-oxide (ITO)), and also their thermal properties were not good enough to adopt them for the commercialized fabrication. Here, we report the synthesis of two amorphous hole transport materials containing a biphenyl backbone with naphthyl and bulkier phenanthrene or anthracene groups attached to the nitrogen atoms, i.e., N,N′-di(naphthalene-1-yl)-N,N′-di(phenanthrene-9-yl)biphenyl4,4′-diamine(NPhenD) or N,N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)biphenyl-4,4′-diamine (NAD), respectively, and their application to OLEDs in simple bilayer device structure. These materials have high Tg (195 °C for NPhenD and 206 °C for NAD) and moderate hole mobility (2.2 × 10−4 cm2/Vs for NPhenD). We believe that these high thermal properties are attributed to the bulkier and more rigid
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anthracenyl and phenanthrenyl groups replacing the phenyl groups in α-NPD. Compared with the standard device using α-NPD as a HTL, the devices with NPhenD and NAD exhibited better efficiency, improved lifetime and small operating voltage changes over time, and stable operation even at high temperature up to 420 K. We verified the synthesis, analyzed the optical, thermal properties of them, and characterized the device performances. 2. Experimental details
at 180 °C for 36 h. After evaporation of the solvent in vacuum, the residue was washed with water several times, dried, re-dissolved in hot chloroform, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography (silica-gel, hexanes/ chloroform (7:3)) to yield the title compound as a pale yellow solid. (2.26 g, 42%) 1H-nuclear magnetic resonance (1H-NMR) (300 MHz, THF-d8): δ 8.14 (d, 2H, J=1.5 Hz), 7.84 (d, 2H, J=9.0 Hz), 7.32–7.49 (m, 16H), and 7.14 (d, 4H, J=22.5 Hz). 13C-NMR (75 MHz, THF-d8): δ 145.0, 140.9, 136.0, 133.8, 129.0, 128.8, 127.6, 126.8, 126.6, 125.7, 123.3, 122.6, 118.6, and 115.5.
2.1. Synthesis of NPhenD and NAD NPhenD and NAD were synthesized according to the procedures in Scheme 1. Unless stated otherwise, all reagents were used as received from commercial sources. The solvents were dried using standard procedures. All reactions were performed under a purified nitrogen atmosphere using the standard Schlenk technique. 2.1.1. N,N′-di(naphthalene-1-yl)-biphenyl-4,4′-diamine (DBD) 4,4′-diiodobiphenyl (5.00 g, 12.31 mmol), 1-naphthylamine (3.61 g, 25.21 mmol), copper (0.13 g, 2.05 mmol), and potassium carbonate (3.74 g, 27.06 mmol) were charged in a two-necked flask equipped with a reflux condenser and maintained under N2 atmosphere. o‐ Dichlorobenzene (150 ml) was added and the mixture was heated
2.1.2. N,N′-di(naphthalene-1-yl)-N,N′-di(phenanthren-9-yl)-biphenyl4,4′-diamine (NPhenD) N,N′-di(naphthalene-1-yl)-biphenyl-4,4′-diamine(2.50 g, 5.73 mmol), 9-iodophenanthrene (5.23 g, 17.20 mmol), sodium tert-butoxide (1.66 g, 17.27 mmol), tris(dibenzylideneacetone) dipalladium (0) (265 mg, 0.29 mmol), and tri-tert-butylphophine (115 mg, 0.57 mmol) were allowed to react according to a procedure for compound NAD. Yield: 65% (2.94 g). Matrix-assisted laser desorption/ionization time-offlight mass spectrometry (MALDI-TOF) (m/z): Calcd. 788.32. Found: 788.24. Liquid chromatography/mass spectrometry-ion trap-time-offlight (LCMS-IT-TOF) [M+H]+: Calcd. 789.3270. Found: 789.3272. Anal. Calcd. for C60H40N2: C, 91.34; H, 5.11; and N, 3.55%. Found: C, 91.35; H, 5.09; and N, 3.54%.
Scheme 1. Synthetic route to the NPhenD and NAD.
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2.1.3. N,N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)-biphenyl4,4′-diamine (NAD) N,N′-di(naphthalene-1-yl)-biphenyl-4,4′-diamine (2.50 g, 5.73 mmol), 9-bromoanthracene (4.42 g, 17.19 mmol), sodium tertbutoxide (1.66 g, 17.27 mmol), tris(dibenzylideneacetone) dipalladium (0) (265 mg, 0.29 mmol), and tri-tert-butylphophine (115 mg, 0.57 mmol) were charged in a two-necked flask equipped with a reflux condenser and maintained under N2 atmosphere. Toluene (100 ml) was added and the mixture was heated at 110 °C for 24 h. The residue was filtered, and washed with chloroform several times, dried, and concentrated under reduced pressure. The crude product was purified by repeated temperature gradient vacuum sublimation to yield the title compound as a yellow solid. (2.76 g, 61%) MALDI-TOF (m/z): Calcd. 788.32. Found: 788.32. LCMS-IT-TOF [M + H] +: Calcd. 789.3270. Found: 789.3263. Anal. Calcd. for C60H40N2: C, 91.34; H, 5.11; and N, 3.55%. Found: C, 91.33; H, 5.08; and N, 3.55%. 2.2. Measurement The 1H and 13C NMR spectra were recorded at 25 °C on a Bruker DPX-300 spectrometer. Mass spectra were recorded on a Voyager-DE™ STR Biospectrometry Workstation MALDI-TOF MS spectrometer and Shimadzu LCMS-IT-TOF. Elemental analyses were recorded on a CE Instrument EA 1110 Elemental Analyzer. Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC 2020 instrument with a heating rate of 10 °C min−1 and a cooling rate of 20 °C min−1. Thermogravimetric analyses (TGA) were conducted on a TA Instruments TGA 2050 unit under a heating rate of 10 °C min−1 and a nitrogen flow rate of 90 ml min−1. UV–visible spectra were measured with an HP 8453 spectrophotometer. Photoluminescence (PL) spectra were obtained with the F-4500 fluorescence spectrometer. The HOMO energy levels were measured with the Riken Keiki AC-2 (750H) measurements, and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the optical absorption edge. 2.3. Hole mobility measurements The hole mobilities of α-NPD, NPhenD, and NAD films were measured with a time-of-flight photoconductivity (TOF-PC) technique as described in our previous work [15]. About 1-μm-thick films of α-NPD, NPhenD, and NAD sandwiched between ITO and Al electrodes were photoexcited by a N2 pulse laser (337 nm, PTI GL-3300), and the transient photocurrent (PC) was measured with a 500 MHz oscilloscope (Tektronix TDS 5054B). Single carrier (hole-only) devices of α-NPD, NPhenD, and NAD were also fabricated as follows: as a hole injection layer, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spin-coated on the top of the ITO substrate at a rate of 4000 rpm, and then dried in a vacuum oven at 120 °C for more than 30 min. After that, 100 nm of organic materials and 100 nm of Al were deposited sequentially by thermal evaporation. The current–voltage (I–V) characteristics of single carrier devices were measured with a Keithley 236 sourcemeasure unit.
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and 4–6 Å/s for Al electrodes. Fabricated device was mounted onto the cryostat in order to measure the electrical and optical characteristics with changing the substrate temperature. The current–voltage– luminance (I–V–L) characteristics and the EL spectra were measured using a Keithley 236 source-measure unit, a Konica-Minolta CS-1000A, and a Keithley 2000 multimeter equipped with a calibrated Si photodiode and a photomultiplier tube (PMT) through an ARC 275 monochromator. For the lifetime tests, each device was capped with a glass lid using UVcurable epoxy sealant in an Ar-filled glove box. 3. Result and discussion 3.1. Synthesis and characterization Scheme 1 shows the chemical structures and their synthetic procedures. NAD and NPhenD were prepared by palladium-catalyzed C–N coupling reaction of DBD with 9-bromoanthracene and 9iodophenanthrene, respectively, which was obtained from 4,4′dibromobiphenyl and 1-naphthylamine through copper-catalyzed Ullmann condensations. The structures of the synthesized compounds were characterized by using nuclear magnetic resonance (NMR), mass spectroscopy, and elemental analysis. These materials have biphenyl backbone similar to TPD, α-NPD, CBP, and p-BPD, but in order to realize higher Tg, we applied the naphthyl groups to the nitrogen atom of each amine groups of one material and bulkier aromatic compounds to the other. Thus two amorphous hole transport materials possess high Tg values of 195 °C (NPhenD) and 206 °C (NAD). 3.2. Optical properties Fig. 1 shows the optical absorption and photoluminescence (PL) spectra of the NPhenD and NAD in solid films formed by vacuum deposition on a quartz plate. The absorption spectra of the NPhenD and NAD reveal the maximum absorption peaks at 375 and 382 nm, while the PL spectra exhibit the maximum emission peaks at 441 and 524 nm, respectively. The PL peak of NAD is a little broader and red-shifted about 80 nm compared with that of NPhenD because the symmetric anthracenyl group in NAD has more ordered molecular packing due to its longer conjugation length than the asymmetric phenanthrenyl group in NPhenD [16]. The energy bandgaps of NPhenD and NAD were 2.94 eV and 2.50 eV, respectively, calculated from the threshold of absorption spectra. The HOMO energy levels, 5.52 eV and 5.44 eV, were measured by a photoelectron spectrometer, and the LUMO energy levels were calculated as 2.58 eV and 2.94 eV from the bandgaps, respectively. These results are consistent with the recent report that the LUMO energy level was determined by anthracene and phenanthrene moieties while the HOMO energy level was controlled by the triarylamine
2.4. Fabrication and characterization of the OLEDs The ITO substrates were cleaned ultrasonically in organic solvents (isopropyl alcohol, acetone, and methanol), rinsed in deionized water, followed by UV-ozone (UVO-42) treatment for 5 min. After that, 4,4′, 4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (mMTDATA) as a hole injection layer, a hole transporting material (α-NPD, NPhenD, or NAD, respectively), tris(8-hydroxyquinolinato) aluminum (Alq3) as an emitting and electron transport layer, LiF, and Al were successively deposited on the substrates under a base pressure of 4×10−4 Pa. The deposition rate was about 1–2 Å/s for organic materials,
Fig. 1. Normalized UV–vis. absorption and fluorescence spectra of NPhenD (solid lines) and NAD (dashed lines) in films.
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core [16]. Consequently, NAD with anthracene has smaller bandgap and more red-shifted PL than those of NPhenD with phenanthrene. 3.3. Thermal properties Fig. 2 shows the thermal properties of NPhenD and NAD, investigated with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA indicates that NPhenD and NAD exhibit decomposition temperatures (Td) (corresponding to 5% weight loss) as high as 488 and 473 °C, respectively. The melting temperatures (Tm) were detected for NPhenD and NAD at 385 and 393 °C, respectively, by DSC examination in the first heating scan. The glass transition temperatures (Tg) were observed for NPhenD and NAD as 195 and 206 °C, respectively, in the second, the third, and the forth heating scan. Both NPhenD and NAD show superior thermal stability from TGA and DSC measurements. In particular, they show impressively high Tg which is one of the highest values among those of reported hole transport materials. This high thermal stability is obtained by applying bulkier and more rigid anthracenyl and phenanthrenyl groups in α-NPD structure instead of phenyl group. 3.4. Mobilities of hole injection and transport materials To compare the hole conducting properties of the materials as the HTL, we measured the I–V characteristics of the single carrier devices of α-NPD, NPhenD, and NAD with the structure of ITO/PEDOT:PSS (40 nm)/HTL (100 nm)/Al (100 nm). As can be seen in Fig. 3(a), the current density of NPhenD is smaller than that of α-NPD at a given bias voltage, while the current density of NAD is similar with that of α-NPD at the same condition. It means that NAD possesses slightly higher hole mobility compared to NPhenD when we ignore the injection barrier between HTLs by using PEDOT:PSS buffer layer [17].
In order to characterize the hole transport properties precisely, we also measured the hole mobilities of α-NPD, NPhenD, and NAD films (about 1-μm-thick) with the TOF-PC technique. From the transit time (τ), defined as the edge of the plateau where the photocurrent starts to decrease steeply as holes discharge at the opposite Al electrode, the hole mobility (μ) was calculated by using the relation μ = d 2/Vτ, where d is the sample thickness and V is the bias voltage. Fig. 3(b) describes their hole mobilities as a function of the electric field, and the inset plots the transient PC waveforms of NPhenD measured under an electric field of 0.2 MV/cm at room temperature. The hole mobilities of α-NPD, and NPhenD at 0.2 MV/cm were found to be 5.7 × 10 −4 and 2.2 × 10 −4 cm 2/Vs, respectively, which are similar to the previously reported results [18,19]. The PC waveform of NPhenD showed a characteristic plateau of the non-dispersive hole transport, while the PC signal of NAD was highly dispersive so that we could not determine the carrier transit time as shown in the inset of Fig. 3(b). From the I–V characteristics in Fig. 3(a), the hole mobility of NAD can be estimated to be similar with that of α-NPD. 3.5. Electroluminescence properties In order to examine the performances of synthesized materials as the HTL, we adopted a simple OLED structure, which is ITO/m-MTDATA (15 nm)/HTL (60 nm)/Alq3 (70 nm)/LiF (0.5 nm)/Al (100 nm). Fig. 4 depicts the schematic energy band diagrams of devices using m-MTDATA as the HIL, α-NPD, NPhenD, or NAD as the HTL, and Alq3 as the lightemitting and electron transport layer. The HOMO energy level of NPhenD is slightly lower by 0.1 eV compared to those of α-NPD and NAD. The I–V–L characteristics of the devices with various HTLs are shown in Fig. 5(a) and (b). They showed higher current densities in sequence of NAD, α-NPD, and NPhenD at a given voltage, which also supports the prior supposition that the hole mobility of NAD
Fig. 2. TGA thermograms of (a) NPhenD and (b) NAD, and DSC thermograms of (c) NPhenD and (d) NAD. Both materials exhibit high thermal properties in terms of decomposition, melting, and glass transition temperatures.
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Fig. 3. (a) The I–V characteristics of hole-only devices (ITO/PEDOT:PSS/HTL (100 nm)/Al) with α-NPD (circle), NPhenD (triangle), and NAD (square). (b) The electric field dependence of the hole mobilities of α-NPD (circle) and NPhenD (triangle) films with a thickness of 1 μm, measured by TOF-PC, and the inset plots TOF-PC waveforms of NPhenD and NAD films under an electric field of 0.2 MV/cm at room temperature.
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nearby NAD layer due to the smaller band gap of NAD compared with Alq3. The external quantum efficiency of the device with NPhenD is comparable with that with α-NPD, but the device with NAD exhibits reduced efficiency because the recombination occurs in NAD layer (see Fig. 5(d)). Since the properties of low LUMO energy level and small bandgap of NAD are demerits for being used as the HTL, a thin electron blocking layer might be required to utilize NAD as the HTL. Even though the device efficiencies of OLEDs using NPhenD or NAD as the HTL were not noticeable, we can expect enhanced stability during the device operation since their glass transition temperatures (~ 200 °C) are exceptionally high. In order to investigate the effect of Joule heating on the materials, we fabricated 50-nm-films of α-NPD, NPhenD, and NAD on the substrate and then examined the changes of film morphology by using a high resolution optical microscope before and after the annealing them at 400 K for an hour. As shown in Fig. 6, NPhenD and NAD films have no noticeable change in their morphology, while α-NPD film is deformed to hemispherical islands during the heating. This phenomenon, which is usually observed in the materials with low Tg [20], exerts a bad effect on the lifetime of OLED devices. Fig. 7 plots the operational stability of the OLED devices with the HTL of α-NPD, NPhenD, or NAD under a constant current bias for the initial luminance of 1000 cd/m 2 at room temperature. The stability of NPhenD and NAD devices is significantly improved by 3–8 folds compared with α-NPD device, attributed to their higher Tg than that of α-NPD (95 °C). It is also noticeable that both the device lifetime and the voltage change of NAD device are much better than those of NPhenD device although two materials possess similar high Tg values. We believe that there may be more fundamental origin responsible for the OLED degradation in addition to morphological changes. As Aziz et al. reported [21], the cationic species of Alq3 (Alq3+) formed by excess holes injected from the HTL into the Alq3 layer could be one of intrinsic degradation mechanisms of the OLED based on Alq3. Thus, in spite of the poor efficiency, the NAD device can exhibit higher stability since the exciton recombination occurs predominantly in the NAD layer rather than the Alq3 layer, which leads to prevent from the Alq3+ formation. Similarly, the Alq3+ formation can be reduced in the
may be higher than that of NPhenD. The EL spectra of the devices with α-NPD and NPhenD are identical, originating from Alq3 layer, but the EL spectrum of NAD device shows a peak at about 560 nm, which is 40 nm away from the emission peak of Alq3 (see Fig. 5 (c)). As can be seen in the schematic energy band diagram in Fig. 4, the electron injection barrier between Alq3 and NAD (LUMO offset ~ 0.16 eV) is lower than the hole injection barrier between them (HOMO offset ~ 0.36 eV). Therefore, it is considered that the electrons and holes recombine mainly in the NAD layer rather than Alq3 layer, and also there is a possibility of energy transfer from the Alq3 layer to the
Fig. 4. Schematic energy band diagrams of the OLED devices using α-NPD, NPhenD, or NAD as the HTL.
Fig. 5. Device characteristics by using α-NPD (circle), NPhenD (triangle), or NAD (square) as the HTL in terms of (a) current density–voltage, (b) luminance–voltage, (c) EL spectra, and (d) external quantum efficiency–current density.
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Fig. 7. (a) The ratio of the relative luminance to the initial luminance and (b) voltage changes as a function of operating time of the OLED devices adopting various HTL materials biasing the constant current for the initial luminance of 1000 cd/m2. Fig. 6. Comparison of surface morphology between films as-deposited and annealed at 400 K for α-NPD, NPhenD, and NAD films.
NPhenD and NAD with enhanced thermal and electrical stability can be one of the promising materials in OLEDs or other organic optoelectronic devices. device with NPhenD compared with the device with α-NPD since the hole mobility of NPhenD is slightly lower than that of α-NPD even though it is still orders of magnitude higher than the electron mobility of Alq3. Therefore, we consider that much higher device stability by using NPhenD and NAD than using α-NPD is attributed to the effective prevention of the Alq3+ formation in this device structure as well as the high Tg of two materials. We also examined the device performances and durability at an elevated temperature. Fig. 8 plots the external quantum efficiency of three devices at 100 mA/cm 2 according to the operating temperature. The device with α-NPD showed unstable performances from 340 K, and failed to emit light above 360 K. On the other hand, both devices with high-Tg HTLs exhibited stable operation up to 420 K maintaining their efficiency. Consequently, these results clearly show that improved device stability at high temperature as well as the operational device lifetime can be achieved by using these hole transport materials possessing high Tg.
Acknowledgment This work was supported by the Industrial strategic technology development program (10041556) funded by the Ministry of Knowledge Economy (MKE), and by the Creative Research Initiative Program for “Intelligent Hybrids Research Center” (2010-0018290) and Basic Science Research Program (2011-0022716) of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea.
4. Conclusion We synthesized and characterized two hole transport materials possessing high Tg based on biphenyl backbone with bulkier and more rigid phenanthrenyl or anthracenyl group. Both materials, NPhenD and NAD, showed similar hole mobilities compared with α-NPD, and good electrical stabilities. We also demonstrated simple OLED devices adopting these materials as the HTL possessing superior performances in terms of lifetime, operating voltage variation, and high temperature operation up to 420 K. The result implies that the OLED operational stability is highly affected by the thermal stability of the HTL. We believe that
Fig. 8. External quantum efficiencies of each device by changing the operating temperature at the same current density of 100 mA/cm2.
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