dimethylsilane as Electron-Transport Material for Deep Blue

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Bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane as Electron-Transport Material for Deep Blue Phosphorescent OLEDs Soonnam Kwon, Kyung-Ryang Wee, Ae-Li Kim, and Sang Ook Kang* Department of Advanced Material Chemistry, Korea University, Sejong, Chungnam 339-700, South Korea

ABSTRACT Bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane (SiTAZ) was designed and synthesized as an electron-transporting material for deep blue phosphorescent organic light-emitting devices (PHOLEDs). Introducing a Si atom between two 3,4,5-triphenyl-1,2,4-triazole molecules, a high triplet energy of 2.84 eV, high glass transition temperature of 115 °C, and high electron mobility of 6.2 10-4 cm2 V-1 s-1 were achieved. By employing SiTAZ as a hole-blocking and electron-transporting material of iridium(III)[bis(4,6-difluorophenyl)pyridinatoN,C20 ]tetrakis(1-pyrazolyl)borate (FIr6)-based deep blue phosphorescent OLEDs, a maximum external quantum efficiency (EQE) of 15.5%, an EQE of 13.8% at high luminance of 1000 cd m-2, and deep blue color coordinates of (0.16, 0.22) were achieved. The reduced efficiency roll-off at high luminance was attributed to the high triplet energy of the SiTAZ. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

(ETL). Despite the successful development of FIrpic-based PHOLED, there is urgent need for the development of efficient deeper blue light-emitting devices. However, a new ETL material with higher ET is needed to employ deeper emitters, such as iridium(III)[bis(4,6-difluorophenyl)pyridinato-N,C20 ]tetrakis(1-pyrazolyl)borate (FIr6) (ET =2.80 eV).16-18 In this report, a silicon atom was introduced between two 3,4,5-triphenyl-1,2,4-triazole molecules to develop an ETL material with high thermal stability, wide energy gap, and high electron mobility, as shown in Scheme 1. The Si atom has advantages, such as conjugation blocking for a wide band gap,19 tetrahedral geometry for morphology control, and high thermal stability.20,21 Initially, N0 -benzoyl-4-bromobenzohydrazide was prepared in quantitative yield from 4-bromobenzohydrazide and benzoyl chloride. 3-(4-Bromophenyl)-4, 5-diphenyl-1,2,4-triazole was synthesized by reacting N0 benzoyl-4-bromobenzohydrazide and aniline using a literature protocol.22 Finally, bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3yl)phenyl)dimethylsilane, SiTAZ, was achieved by a direct substitution reaction between 2 equiv of 3-(4-lithiumphenyl)-4,5-diphenyl-1,2,4-triazole and dichlorodimethylsilane. The details of this synthesis are given in the Supporting Information (SI). SiTAZ was isolated by flash column chromatography in 32% yield and purified further by train sublimation in 67% yield. The composition of SiTAZ was confirmed based on elemental and high-resolution mass

S

ince the pioneering report of Tang et al., organic lightemitting devices (OLEDs) have attracted enormous interest for their high efficiency, pure color reproduction, fast response, and wide viewing angle.1 In particular, efforts to achieve 100% internal quantum efficiency of phosphorescent OLEDs (PHOLEDs) have led to considerable advances in materials chemistry and device physics.2,3 As a result, highly efficient PHOLEDs with external quantum efficiency (EQE) as high as 29 and 15% for green4 and pure red5 light-emitting devices, respectively, have been reported. In the case of blue color-emitting devices, there have been two different approaches. The color purity has been the main target of research for applications for flat panel displays, where the efficiency is not high.6-8 On the other hand, efficiency maximization has been the aim of the research but at the expense of color purity.9,10 In the later case, lighting would be the main application of the devices. The best reported result of iridium(III)[bis(4,6-difluorophenyl)pyridinato-N,C20 ]picolinate (FIrpic)-based PHOLED exhibited a maximum EQE and Commission Internationale de L'Eclairage (CIE) coordinates of 26.2% and (0.16, 0.38),11 respectively, which reveals enormous enhancement compared to the first report of the FIrpic-based PHOLED with an EQE of 7.5%.12 Evolutionary enhancement is due mainly to the efficient confinement of triplet energy into FIrpic. In that report, all of the materials surrounding FIrpic were chosen carefully to meet the requirement that the triplet energy (ET) should be greater than that of FIrpic (ET = 2.77 eV).13 They employed 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane (TAPC) (ET = 2.87 eV)14 as a hole-transport layer (HTL) and TmPyPB (ET = 2.75 eV)15 as an electron-transport layer

r 2009 American Chemical Society

Received Date: November 8, 2009 Accepted Date: November 28, 2009 Published on Web Date: December 02, 2009

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DOI: 10.1021/jz900238h |J. Phys. Chem. Lett. 2010, 1, 295–299

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Scheme 1. Synthetic Route and Structure of SiTAZ

analyses, and the structure was authenticated based on 1H and 13C NMR spectroscopy. The HOMO energy level was estimated to be -6.45 eV from the cyclic voltammetry data. The LUMO energy level (-2.52 eV) was calculated from the HOMO energy level and gap energy, which was defined as the intersection of the absorption and fluorescence spectra of SiTAZ, as shown in Figure 1a. The thermal properties of SiTAZ were estimated from differential scanning calorimetry (DSC). The compound was melted at 294 °C during the first heating cycle. The glass transition temperature (Tg) was observed at 115 °C in the second and third heating cycles. The crystallization temperature (Tc) was observed at 185 °C in the second and third heating cycles. The triplet energy level (ET) of SiTAZ was determined from the phosphorescent spectrum at 77 K, as shown in Figure 1a. The spectrum shows the first phosphorescence peak at 436 nm, corresponding to a triplet energy of 2.84 eV, which is higher than the triplet energy of FIr6 (ET =2.80 eV). The ET of FIr6 was defined as a high-energy cutoff of the PL spectrum, which is shown in Figure 1a. As a result, it was expected that SiTAZ would effectively confine the triplet energy into FIr6. Figure 1b shows the transient photoluminescence decay of 10 wt % FIr6-doped SiTAZ and 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) films deposited on fused silica substrates at a wavelength of 460 nm and room temperature. A monoexponential decay curve was observed for the FIr6-doped SiTAZ film with a decay lifetime of 3.05 μs. On the other hand, for the FIr6-doped TAZ film, the decay curve could be fitted with two exponential components with a short and long lifetime of 0.63 and 7.3 μs, respectively, which shows obvious backward triplet energy transfer from FIr6 to TAZ.14 In addition to the triplet energy, the electron mobility is also an important physical quantity for achieving high efficiency because the electron and hole balance is an elementary factor governing the efficiency of the device.11 The electron mobility of a 1 μm thick SiTAZ film was determined using a time-of-flight method (TOF), which was estimated to be 6.2 10-4 cm2 V-1 s-1 under an electric field of 2 105 V cm-1. Considering the physical properties reported above, SiTAZ is believed to be one of the most competitive ETL materials ever reported, except for the energy level of LUMO. As stated above, the LUMO level was determined to be 2.52 eV, which is high relative to the energy levels of the LUMO of TAZ (2.7 eV).


Figure 1. (a) UV/vis absorption, fluorescence, and phosphorescence spectra of SiTAZ. (b) Transient phosphorescence decay curves of 5 wt % FIr6-doped SiTAZ and TAZ films.

The high LUMO level is expected to cause a relatively high energy barrier for electron injection. A series of PHOLEDs were fabricated using two different ETL materials to examine the influence of high LUMO level and confirm the utility of SiTAZ as an ETL. The PHOLEDs had the following device structure: ITO/NPB (30 nm)/TAPC (15 nm)/Me2Si(CBP)2 (5 nm)/Me2Si(CBP)2-FIr6 (30 nm, 15%)/ETL (40 nm)/LiF (0.5 nm)/Al (100 nm). Two different ETL materials were used in three different OLEDs, where the ETL layers for devices A, B, and C were TAZ, SiTAZ, and SiTAZ (35 nm)/TAZ (5 nm), respectively. Adetailed description of the device structure and host material (Me2Si(CBP)2) is given in the SI and a separate report.23 Figure 2a shows the current density-voltage and luminance-voltage characteristics. As anticipated from the electron mobility results, the device using SiTAZ as an ETL (devices B and C) showed a much higher current density and luminance at a high voltage (>9 V) than device A. However, device A showed a slightly higher current than device B at a lower voltage. The energy levels shown in Figure 2a suggest that the electron injection barrier of device B is higher than that of device A, resulting in a slightly higher driving voltage (0.4 V difference in turn-on voltage). In the case of device C, the energy barrier was reduced by a thin

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Figure 2. Device characteristics of the PHOLEDs using SiTAZ and TAZ as an ETL. (a) Current density-voltage curves. Inset: luminance-voltage curves. (b) EQE-current density curves. Inset: EQE-luminance curves.

Figure 3. (a) EL spectra of devices A and B with different voltages. Right upper inset: magnified spectra of the second vibration peaks. Left lower inset: magnified spectra of the short wavelength region. (b) Change of color coordinates as a function of voltage (or current density) of devices A and B.

layer of TAZ inserted between the SiTAZ and cathode, resulting in enhanced current. Devices A, B, and C exhibited driving voltages of 5.5, 5.8, and 5.4 V at 100 Cd m-2 and 7.6, 7.4, and 6.9 V at 1000 Cd m-2, respectively. Device B showed a lower driving voltage than device A at a practical luminance of 1000 Cd m-2 for active matrix OLED (AMOLED) applications. As a result, despite its high LUMO level, SiTAZ is believed to be a competitive ETL in view of the driving voltage. Furthermore, the external quantum efficiency of device B was much higher than that of device A, as shown in Figure 2b. The maximum EQEs of devices A, B, and C were 12.5, 15.5, and 16.0%, respectively. Moreover, the EQEs of devices B and C at high luminance were even higher than that of device A. The EQEs of devices B and C at 1000 Cd m-2 were as much as 89.0 and 90.0% of the maximum EQE, whereas the EQE of device A at 1000 Cd m-2 was only 65.0% of the maximum EQE. The enhanced maximum EQE and reduced efficiency roll-off of device B could be explained by two factors, (i) SiTAZ supplied more electrons to the EML due to its enhanced electron mobility, which resulted in enhanced electron and hole balance and (ii) SiTAZ (ET ∼ 2.84 eV) effectively confined

the triplet energies into the EML layer due to its high triplet energy. For the second factor, the reduced efficiency roll-off of devices B and C was attributed to the high triplet energy of SiTAZ. Figure 3b shows a change of color coordinates as a function of voltage (or current density) of devices A and B. In both devices, the CIE x and y values decreased with increasing voltage (or current density) until the voltage reached 8-9 Vor the current density reached approximately 12 mA cm-1. The change in CIE coordinates was attributed to the change in the relative distance between the cathode and emission zone.23 The same trends were also observed in Figure 3a, where the intensity of the second vibration peaks with a wavelength of approximately 485 nm decreased with increasing induced voltage to 8-9 V. At a low voltage, the average emission zone was supposed to be away from the EML/ETL interface. As the voltage or current density was increased, the average emission zone was believed to be moved near the interface of the EML/ETL. This resulted in a decrease in distance between the cathode and emission zone, where the effect of interference by reflected light from the cathode would be altered.24 The probability of quenching the triplet energy of FIr6 by the ETL molecules would be enhanced as the emission zone


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approaches the ETL, resulting in new emission. Evidence of the above suppositions is provided in the left inset of Figure 3a, which shows the magnified EL spectra in the range of 380-440 nm. In the case of the device using TAZ as an ETL, a new EL spectrum appeared at a bias voltage of 8 V, which could not be observed at a bias voltage of 5 V. On the other hand, in the device using SiTAZ as the ETL, no new spectrum appeared as the bias voltage was increased. The new spectrum of the device A was attributed to TAZ emission. Therefore, the triplet energy of FIr6 would be quenched by the ETL as the bias voltage or current density is increased, which results in serious efficiency roll-off. However, in the case of SiTAZ, the triplet energy of FIr6 was not quenched by SiTAZ, despite the approach of the emission zone to the interface of the EML/ETL due to its high triplet energy, resulting in lower efficiency roll-off. In conclusion, a new ETL material, SiTAZ, was synthesized by inserting a Si atom between two 3,4,5-triphenyl-1,2,4triazole molecules. The compound exhibited a high ET, high Tg, and high electron mobility of 2.84 eV, 115 °C, and 6.2 10-4 cm2 V-1 s-1, respectively. High efficiency (15.5%), low efficiency roll-off at high luminance (13.8% at 1000 Cd m-2), and deep blue color coordinates (0.16, 0.22) were achieved for FIr6-based PHOLED using SiTAZ as an ETL.

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SUPPORTING INFORMATION AVAILABLE Experimental details and characterization data, cyclic voltammetry, DSC, electron mobility of SiTAZ, and detailed description of devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. Tel: þ82-41-8601334. Fax: þ82-41-867-5396. E-mail: [email protected].

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ACKNOWLEDGMENT This work was supported by the National

Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2009-0083181) and the Strategic Technology Development Project from the Ministry of Knowledge Economy of Korea and LG Chem., Ltd. We also acknowledge support from Korea Basic Science Institute for HR-ESI-MS, LTQ-FT, and 300 NMR.

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dimethylsilane as Electron-Transport Material for Deep Blue

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