Highly efficient, deep-blue phosphorescent organic light emitting diodes

Highly efficient, deep-blue phosphorescent organic light emitting diodes

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Unformatted text preview: APPLIED PHYSICS LETTERS 93, 133312 2008 Highly efficient, deep-blue phosphorescent organic light emitting diodes with a double-emitting layer structure H. Fukagawa,1,a K. Watanabe,2 T. Tsuzuki,1 and S. Tokito1 1 2 NHK Science and Technical Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan Tokyo University of Science, 1-3 Kagurazaka, Tokyo 162-8610, Japan Received 9 July 2008; accepted 11 September 2008; published online 3 October 2008 We have demonstrated a highly efficient, deep-blue organic light-emitting diode OLED using a host material with a high triplet energy. The OLED device that we have prepared utilizes a phosphorescent guest material, iridium III bis 4 , 6 ,-difluorophenylpyridinato tetrakis 1pyrazolyl borate, exhibits a peak quantum efficiency of about 15.7%. We employed a double-emitting layer DEL structure that distributes the carrier recombination region within the device. In this DEL structure, the emission mechanism is such that the energy transfers from the host material in one emitting layer, and the other emitting layer provides for direct charge trapping in the guest material. This DEL structure proved to be quite useful in achieving the reported device characteristics. © 2008 American Institute of Physics. DOI: 10.1063/1.2996572 Because of their relatively high emission efficiencies in comparison with conventional fluorescent organic lightemitting diodes OLEDs , it is widely felt that OLEDs using phosphorescent dyes will be applied to future full-color displays.1,2 However, the realization of highly efficient blue phosphorescent OLEDs has been difficult because blue light is generated from wide-gap excited states. The blue phosphorescent OLED was reported by Adachi et al., whereby the guest phosphorescent dye utilized was iridium III bis 4,6-di-fluoropheny -pyridinato-N , C2, picolinate FIrpic .3 The quantum efficiency of this device was limited to 5.7% due to poor confinement of its triplet energy. Here, the triplet energy of the host material was lower than that of FIrpic. Subsequently, Tokito et al. succeeded to improve the OLED device efficiency by using a high triplet energy host to confine the triplet energy.4 More recently, a FIrpic-based device, reported by Tanaka et al., demonstrated an efficiency of almost 20%, corresponding to a 100% internal quantum efficiency.5 However, deeper blue phosphorescent emission is desired for high quality full-color displays. Holmes et al. reported a deep-blue phosphorescent OLED using iridium III bis 4 , 6 difluorophenylpyridinato tetrakis 1-pyrazolyl borate FIr6 as the guest material.6 The blue light emission using FIr6 exhibited CIE coordinates of x = 0.16, y = 0.26 , which is significantly better than that of FIrpic. The Holmes group prepared their device using p-bis triphenylsilyly benzene UGH2 as the host material, and reported a peak quantum efficiency of about 12%, which is indeed lower than for the FIrpic-based device mentioned above. Electrophosphorescence can be generated by two mechanisms, one is energy transfer from a fluorescent host to a phosphorescent guest and the other is direct charge recombination within the guest. The same group demonstrated emission from FIr6 that was generated via direct charge trapping followed by exciton formation by the guest in a UGH2 host. Several challenges make it difficult to generate a highly efficiency, deep-blue phosphorescent OLED by host-guest energy transfer. Primary among these is that few existing host materials that a Electronic mail: fukagawa.h-fe@nhk.or.jp. combine both carrier-transport ability and the high triplet energy needed to confine triplets within the guest. Some carbazole derivatives can satisfy these requirements.4,5,7–10 However, the use of carbazole derivatives as a host for deep-blue phosphorescent OLEDs presents its own difficulties. While many carbazole derivatives act as hole-transport materials, few hole-blocking materials have yet been reported that satisfy the following requirements: i strong hole-blocking ability, ii high triplet energy, and iii electron-transport capability. In summary, a host material with good holeblocking properties, as well as a suitable device structure is required to realize highly efficient, deep-blue OLED devices. In this letter, we would like to report on our success in achieving a high external quantum efficiency of over 15%, obtained using a host material with a high triplet energy and a suitable hole-blocking material. We employed a double emitting layer DEL structure in an FIr6-based OLED device, whereby phosphorescence resulted both from hostguest energy transfer emitting layer I and direct charge recombination within the guest material emitting layer II . For emitting layer I, the host material used was 2,2-bis 4carbazolyl-9-ylphenyl adamantane Ad-Cz , which is a carbazole derivative. We used UGH2 as a host material in emitting layer II, which has strong hole-blocking properties, weak electron-transport capability, and high triplet energy of about 3 eV.11 The UGH2 layer not only acts to block holes but also emits light in the DEL device. Here, the Ad-Cz and UGH2 layers are doped with FIr6. We have found that the DEL device structure is quite effective in obtaining highly efficient, deep-blue OLED devices since DELs distribute the carrier recombination region within the device. Two types of FIr6-based OLED devices were developed, one having a single emitting layer SEL and the other having a DEL, each produced on a glass substrate coated with a 200-nm-thick indium tin oxide layer having a sheet resistance of 10 / sq. Prior to fabrication of the organic layers, the substrate was cleaned with ultrapurified water and organic solvents, and treated with a UV-ozone ambient. To reduce the possibility of electrical shorts within the device, poly 3,4-ethylenedioxythiophene /poly styrene sulfonic acid PEDOT:PSS , diluted in water, was spun onto the sub© 2008 American Institute of Physics 0003-6951/2008/93 13 /133312/3/$23.00 93, 133312-1 133312-2 Fukagawa et al. 0 Appl. Phys. Lett. 93, 133312 2008 10 N -1 N 10 10 -2 Ad-Cz External Quantum Efficiency (%) PL Intensity (a. u.) 15 10 : DEL-type device : SEL-type device 5 10 0 -3 10 Time (µs) 20 30 010-2 -1 10100 101 Current Density (mA/cm2) 102 FIG. 1. Transient photoluminescent decay for a Ad-Cz film with 3 wt % FIr6, and the molecular structure of Ad-Cz. FIG. 2. External quantum efficiency vs. current density for two types of FIr6-based OLED devices. One is a SEL device and the other is a DEL device. strate to form a 40-nm-thick PEDOT:PSS layer after being baked for 1 h at 180 ° C. The other organic layers were sequentially deposited onto the substrate without breaking vacuum at a pressure of about 10−5 Pa. First, a 40-nm-thick 4 , 4 -bis N- 1-naphthyl -N-phenyl-amino biphenyl -NPD was deposited as the hole-transporting layer onto the PEDOT:PSS. Next, a 35-nm-thick emissive layer consisting of a Ad-Cz and FIr6 was formed by codeposition. For the SEL-type device, a 5-nm-thick UGH2 layer was formed as the electron-transporting and hole-blocking layer. For the DEL-type device, FIr6 was codeposited with UGH2 to form a 5-nm-thick emitting layer II. Then, a 50-nm-thick 1,3,5-tris N-phenylbenzimidazol-2-yl benzene TPBI was deposited as the electron-transporting layer. Finally, a 0.5-nm-thick LiF layer and 100-nm-thick Al layer were deposited as the cathode. The devices were encapsulated using a UV-epoxy resin and a glass cover within a nitrogen atmosphere, after cathode formation. The electroluminescent EL spectra and luminance were measured with a spectroradiometer Minolta CS-1000 . A digital source meter Keithley 2400 and a desktop computer used to operate the devices were connected. We assumed that emission from the OLED device was isotropic, such that the luminance was Lambertian, and calculated the external quantum efficiency from the luminance, current density, and EL spectra. Figure 1 shows the molecular structure of Ad-Cz. The triplet energy of Ad-Cz was estimated from the highest energy peak of phosphorescent spectra of Ad-Cz film at 8 K, and was found to be 2.88 eV. Thus, the triplet energy of Ad-Cz is higher than that of FIr6 2.7 eV , which is expected due the effective confinement of triplets in FIr6. This was confirmed by measuring transient photoluminescent decay of a 3% FIr6-doped Ad-Cz thin film at a wavelength of 460 nm and at room temperature, as shown in Fig. 1. A monoexponential decay curve was observed, indicating that the triplet energy transfer from FIr6 to Ad-Cz is completely suppressed and the energy is confined in FIr6.4 We measured the holetransport capability of Ad-Cz using a time-of-flight method and found that the hole mobility is about 1 10−5 cm2 / V s. Conversely, the electron-transport property was too low to measure. The highest occupied molecular orbital HOMO level of Ad-Cz, which plays a key role in hole transporting, is estimated to be 5.8 eV from the spectroscopic measurement of photoemission in air AC-3, Rikenkeiki and is lower than that of guest molecule FIr6 6.1 eV . Based on the results of the carrier-transport ability and the HOMO level position, it is likely that the carrier recombination occurs at Ad-Cz/UGH2 interface and the excited state of Ad-Cz is generated in the SEL-type device. Therefore, the phosphorescence can be expected to be obtained by energy transfer from the host to guest. The external quantum efficiencies of the two phosphorescent OLED devices, one being SEL-type and the other DEL-type, are shown as a function of current density in Fig. 2. The optimal guest concentration of FIr6 in the SELtype device was 3 wt %. In the DEL-type device the best combination of concentrations for each of two emitting layers was 3 wt %. The maximum efficiency of the SEL-type device was about 10%. However, the maximum external quantum efficiency of the DEL-type device was 15.7% at 0.1 mA / cm2, which is relatively high compared to previously reported values for FIr6-based devices.6,11 The maximum power efficiency of the DEL-type device was found to be 14.3 lm / W at 0.01 mA / cm2. The improved efficiency in the DEL-type device can be explained by the distribution of the triplet-exciton generation region within the two emitting layers. Figure 3 shows the EL spectra of both SEL and DEL devices. The increased spectra level in the short-wavelength region is also shown in the figure approximately 20 times . Weak emission was observed at around 380 nm in the SEL-type device. The fluorescence spectrum of Ad-Cz is also shown in Fig. 3, which agrees well with the increased spectra level of the SEL-type : DEL-type device : SEL-type device Intensity (a. u.) fluorescence spectrum of Ad-Cz x 20 350 450 550 Wavelength (nm) FIG. 3. EL spectra for two types of FIr6-based OLED devices. The fluorescent spectrum of Ad-Cz is also shown. 133312-3 Fukagawa et al. Appl. Phys. Lett. 93, 133312 2008 2.4 eV 2.6 eV 2.8 eV 2.8 eV α-NPD Ad-Cz UGH2 TPBI 5.5 eV FIr6 5.8 eV 6.1 eV 7.2 eV 6.3 eV FIG. 4. Structure of DEL OLED device and the assumed energy diagram. device by adapting the scale size. This emitted light from the Ad-Cz host indicates that the guest concentration is insufficient for excited states of the host material. In fact, the photoluminescence PL efficiencies PL of FIr6-doped Ad-Cz film were measured using an absolute PL quantum yield measurement system C9920, Hamamatsu , and found that PL of 3 wt % film 67% is lower than that of 20 wt % film 78% . For comparison, we prepared several SEL-type devices, where the guest concentrations are 7, 14, and 20 wt %. The fluorescence from Ad-Cz in the SEL-type device disappeared with increasing the guest concentration. The disappearance of fluorescence from Ad-Cz with increasing guest concentration suggests that the guest concentration is insufficient in the 3 wt % device and the emission mechanism of SEL-type device is energy transfer from the host to guest. However, the efficiency got worse with increasing the guest concentration. In this SEL-type device, the high density of Ad-Cz excitons is formed at Ad-Cz/UGH2 interface. The increase in the guest concentration in such a high-exciton density region may cause not only the disappearance of fluorescence but also the triplet-triplet annihilation and/or the exciton annihilation by the interaction between exciton and the charge carrier. In contrast, the DEL-type device exhibits no fluorescence from Ad-Cz, as shown in Fig. 3. This indicates that almost all excitons in the device are emitted phosphorescently from FIr6. As a result, the quantum efficiency was improved in comparison with that of the SEL-type device, as shown in Fig. 2. Here, we discuss the mechanism of the improvement of emission efficiency by using the energy diagram of the DEL device shown in Fig. 4, where the HOMO level and the lowest unoccupied molecular orbital LUMO level for each material are indicated. The HOMO-LUMO gaps are based on reported values except for Ad-Cz.6 In the DEL device, a portion of the accumulated holes at the Ad-Cz/UGH2 interface is directly injected into the FIr6 residing in the UGH2 layer. Next, carrier recombination occurred between the in- jected holes and electrons transported within the UGH2 layer, such that excited triplet states of FIr6 were generated. Direct hole injection into the FIr6 residing in the UGH2 layer is possible since the HOMO energy difference between Ad-Cz and FIr6 is much smaller than the HOMO energy difference between Ad-Cz and UGH2, as illustrated in Fig. 4.6 In the Ad-Cz hole-transport layer, the excited states of the host were generated at the Ad-Cz/UGH2 interface and phosphorescence occurred through host-guest energy transfer. However, in the UGH2 electron-transport layer, carrier recombination occurred directly with guest molecules, resulting in phosphorescence. By employing two emitting layers with different emission mechanisms in the OLED device, carrier recombination region is distributed over the two emitting layers. The distribution of the carrier recombination region efficiently provides for excitons that are attributed phosphorescent from FIr6. In summary, we succeeded in demonstrating of a highly efficient, deep-blue OLED by using DEL structure, whereby the emission mechanism in each emitting layer is quite different. By distributing the carrier recombination region, fluorescence from the host, any possible exciton quenching mechanisms are excluded. The DEL structure, which consists of hole-transporting host material and material which has both high electron-transport and strong hole-blocking properties, is useful in achieving highly efficient, deep-blue phosphorescent OLEDs since almost all host materials with high triplet energy are hole transporting. Optimization of the DEL-type devices in terms of film thickness and guest concentration would provide for deep-blue OLED devices that are even more efficient. M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R. Forrest, Nature London 395, 151 1998 . 2 V. Cleave, G. Yahioglu, P. L. Barny, R. H. Friend, and N. Tessler, Adv. Mater. 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