Fluorinated Carbazole Derivatives as Wide-Energy-Gap Host Material

Sep 5, 2012 - Corporate Research & Development center, Toshiba Corp. ... sized as new building blocks of wide-energy-gap host material for ...
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Fluorinated Carbazole Derivatives as Wide-Energy-Gap Host Material for Blue Phosphorescent Organic Light-Emitting Diodes Yukitami Mizuno,*,† Isao Takasu,† Shuichi Uchikoga,† Shintaro Enomoto,† Tomoaki Sawabe,† Akio Amano,† Atsushi Wada,† Tomoko Sugizaki,† Jiro Yoshida,† Tomio Ono,† and Chihaya Adachi‡ †

Corporate Research & Development center, Toshiba Corp. 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki, 212-8582, Japan Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan

‡

S Supporting Information *

ABSTRACT: 2,7-Difluo-carbazole and 2,4,5,7-tetrafluoro-carbazole were synthesized as new building blocks of wide-energy-gap host material for phosphorescent organic light-emitting diodes (PHOLEDs). These fluorinated positions in the carbazole ring were determined on the basis of density functional theory calculation results. Spectroscopic analyses supported the hypothesis that poly(Nvinyl-2,7-difluoro-carbazole) (2,7-F-PVK) with the fluorinated pendant group possessed a wide energy gap, leading to the exciton energy confinement on the blue phosphorescent dopant as well as nonsubstituted poly(N-vinyl-carbazole) (PVK). 2,7-F-PVK was used in solution-processed blue PHOLED to achieve 27 cd/A at 760 cd/m2, which is 1.8 times higher than that of nonsubstituted PVK. We assumed that the replacement of nonsubstituted PVK with 2,7-F-PVK improved the charge balance in the emission layer, while keeping the exciton confinement effect. The fluorination of the carbazole ring is a useful molecular design strategy for wide-energy-gap host material.

1. INTRODUCTION The luminous efficiency of organic light-emitting diodes (OLEDs) is an important performance value for their application to flat-panel displays or solid-state lighting sources. The efficiency has been greatly improved by the development of phosphorescent OLEDs (PHOLEDs), which can theoretically achieve 100% internal quantum efficiency.1 In order to achieve the theoretical efficiency, the triplet-exciton energy should be confined on the phosphorescent dopant without backward energy transfer to the host material.2 The confinement can be realized by the employment of host material possessing a higher triplet energy (T1) level than the dopant. In blue PHOLEDs, the T1 level of the host material should be very high, because the T1 level of the blue dopant is rather high (>2.6 eV). However, suitable molecular building blocks for such an ideal host material are limited to carbazole or the oxadiazole ring. The T1 level of material has been reported to correlate with the singlet energy (S1) level.3 One of the ways to develop host materials possessing high S1 level is to widen the energy-level gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Thus, the HOMO−LUMO energy gap should be widened by tuning the HOMO and LUMO energy levels of building blocks for the host material, leading to the sufficiently high T1 level. The general tuning method of the HOMO and LUMO energy levels is substituent introduction into the molecular skeleton. However, the introduction into the aforementioned building blocks such as carbazole possessing the π-conjugated system usually enhances the electron delocalization effect, resulting in a narrow gap between the © 2012 American Chemical Society

HOMO and LUMO energy levels. Therefore, with regard to the use of the conventional building blocks of the host material, widening the energy gap through substituent effects is an important research topic. The purpose of our research is the development of fluorinated building blocks for suitable host material possessing high T1 level in blue PHOLEDs. The fluorine substituent is known to shift the HOMO and LUMO energy levels by its electron-withdrawing effect. Furthermore, the fluorination has also been reported to widen or narrow the HOMO−LUMO energy gaps of materials.4 For example, Alq3 shows blue or red shift of the emission spectra depending on the fluorinated positions in the ligand moiety. In this article, we applied the fluorination effect to carbazole as a building block for the host material. In order to widen the HOMO−LUMO energy gap of the carbazole, we determined the appropriate fluorinated position in the carbazole ring on the basis of density functional theory calculation results. After that, the effect was confirmed by the spectroscopic analyses of the synthesized carbazole having fluorine substituents. Finally, the polymers consisting of nonsubstituted or fluorinated carbazole were compared in terms of the luminous efficiency of their solution-processed blue and white PHOLEDs. To the best of our knowledge, this is the first example of applying the energy-gap-widening effect of the fluorination to the host material in PHOLEDs. Received: March 31, 2012 Revised: August 28, 2012 Published: September 5, 2012 20681

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Scheme 1. Synthesis of Fluorinated Cz and PVKa

Reagents and conditions: (i) Pd(OAc)2, Cs2CO3, 100 °C, 3 h, air, toluene, Tf = trifluoromethane sulfonyl group; (ii) Pd(OAc)2, 100 °C, 3 h, air, acetic acid; (iii) 2-chloroethyl p-toluenesulfonate, potassium hydroxide (KOH), r.t., 5 h, air, dimethyl sulfoxide; (iv) KOH, reflux, 2 h, air, ipropanol/ethanol; (v) AIBN, 60 °C, 5 h, N2, tetrahydrofuran (THF). a

2. EXPERIMENTAL PROCEDURES 2.1. Materials. All the organic materials used in this study were obtained commercially and used as received. 2.2. Synthesis of the New Host Materials. Each fluorinated carbazole (4) was synthesized by coupling 3-fluoro or 3,5-difluorophenyl trifluoromethanesulfonate (1)5 and 3fluoro or 3,5-difluoroaniline (2) in the presence of a palladium catalyst.6 The yield of 2,7-difluoro-carbazole (2,7-F-Cz) was 20%. 1H NMR (270 MHz, CDCl3): δ (ppm) 8.10 (br, 1H), 7.94−7.89 (dd, J = 8.6 Hz, 5.3 Hz, 2H), 7.12−7.07 (dd, J = 9.6 Hz, 2.3 Hz, 2H), 7.01−6.94(d, J = 9.6 Hz, 8.6 Hz, 2.3 Hz, 2H). MS (EI) m/z: 203 (M+). The yield of 2,4,5,7-tetrafluoro-carbazole (2,4,5,7-F-Cz) was 30%. 1H NMR (270 MHz, CDCl3): δ (ppm) 8.34 (br, 1H), 6.94−6.90 (m, 2H), 6.80−6.72 (m, 2H). MS (EI) m/z: 239 (M +). After vinylation of these fluorinated carbazoles under the basic condition, these vinyl carbazoles were polymerized with azobisisobutyronitrile (AIBN) (Scheme 1).7 The yield of poly(N-vinyl-2,7-difluoro-carbazole) (2,7-F-PVK) was 70%. 1H NMR (270 MHz, CDCl3): δ (ppm) 7.71−6.09 (br, 6H), 4.69−0.82 (br, 3H). 2.3. Device Fabrication. The 50 nm-thick poly(3,4ethylenedioxy-thiophene)/poly(styrene-sulfonate) (PEDOT/ PSS) layer was spin-coated onto a clean indium−tin-oxide (ITO) coated glass substrate. In the blue PHOLED, the emission layer (85 nm thick), spin-coated on top of the PEDOT/PSS layer, contained the host material (95 wt % or 65 wt %), 1,3-bis[4-tert-butylphenyl]-1,3,4-oxadiazolyl]-phenylene (OXD-7) (0 wt % or 30 wt %) as the electron-transporting material and iridium(III) bis[2-(4,6-difluorophenyl)pyridinatoN,C2]picolinate (FIrpic) (5 wt %) as the blue phosphorescent dopant. In the white PHOLED, the blue emission layer contains 0.2 wt % yellow dopant, iridium(III) bis (2-(9,9dihexyl-fluorenyl)-1-pyridine)-acetylacetonate). The 1 nm-thick CsF layer and 150 nm-thick Al layer were deposited as the cathode to complete the device. Devices were encapsulated in a nitrogen atmosphere after deposition, using a glass cover sealed to the substrate with a UV-curable epoxy. 2.4. Measurement. 1H NMR spectra were recorded on a JEOL GSX-270 MHz spectrometer. Absorption and photoluminescence spectra of the solutions and films were recorded using a UV−vis spectrophotometer (UV-2500(PC) SGLP from

Shimadzu) and spectrofluorometer (Fluoromax-4 from Horiba Jobin Yvon), respectively. Gel permeation chromatography (GPC) measurements were conducted on a Shimadzu SCL10AVP equipped with a Shimadzu RID-10A refractive index detector and Polymer Laboratories PLgel Mixed-D columns. Polystyrene standards were used as the molecular weight references, and THF was used as the eluent. The luminous efficiencies of devices were measured using a Hamamatsu Photonics C9920-12 system. The HOMO energies of organic thin films were measured using a photoelectron spectrometer (Riken-Keiki, AC-3). A Keithley 2400 electrometer was used to measure the current−voltage (J-V) characteristics of devices. Phosphorescence of the host material and transient decay characteristics of the mixture of host material and FIrpic were measured under vacuum using a streak camera system (C4334, Hamamatsu Photonics Co.). A nitrogen gas laser with an excitation wavelength of 337 nm was used. 2.5. Molecular Orbital Calculation. Quantum chemical calculations of the carbazole derivatives were carried out using density functional theory (DFT) as implemented in the Gaussian 03 software package.8 Molecular structures of the ground electronic state were optimized using the Becke threeparameter hybrid exchange-correlation functional (known as B3LYP) with the 6-31G* basis set.

3. RESULTS AND DISCUSSION 3.1. Molecular Design. To determine the appropriate fluorinated position in the carbazole ring, we undertook molecular orbital calculations to estimate the HOMO and LUMO energy levels. Figure 1 shows the calculated values of the HOMO−LUMO energy gaps (ΔHOMO−LUMO gap) of fluorinated carbazoles plotted with reference to those of nonsubstituted carbazole. In monofluorinated carbazoles, the ΔHOMO−LUMO gap was widened by the fluorination at the 2-position (0.12 eV) or 4-position (0.06 eV) and was narrowed by the fluorination at the 1-position (0.01 eV) or 3-position (0.18 eV). The 2-position and 4-position seem to be appropriate for fluorination to widen the HOMO−LUMO energy gap of carbazole. Next, we designed difluorinated carbazoles, which were fluorinated at symmetrical positions of the carbazole ring. The calculated ΔHOMO−LUMO gap of 2,7-F-Cz was 0.03 eV wider than that of 2-monofluorocarbazole, which possessed the widest energy gap of 20682

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difference (0.30 eV) of the HOMO energy level between 2,7-FCz and nonsubstituted Cz was bigger than the difference (0.15 eV) of the LUMO energy level between them. This means that the fluorination at the 2- and 7-positions can notably lower the HOMO energy level by 0.15 eV in comparison to the LUMO energy level. As a result, the HOMO−LUMO energy gap of 2,7-F-Cz was wider than that of nonsubstituted Cz. Similarly, the HOMO energy level downward shift (0.24 eV) of 2,4,5,7-FCz was bigger than the LUMO energy level downward shift (0.05 eV). In contrast, the difference (0.11 eV) of the HOMO energy level between 3,6-F-Cz and nonsubstituted Cz was smaller than the difference (0.44 eV) of the LUMO energy level between them. This means that the fluorination at the 3and 6-positions can notably lower the LUMO energy level by 0.33 eV in comparison to the HOMO energy level. As a result, the HOMO−LUMO energy gap of 3,6-F-Cz was narrower than that of nonsubstituted Cz. In terms of the molecular orbital shape of nonsubstituted Cz, there were no molecular orbitals at the 2- and 7-positions of the HOMO energy level and at the 3and 6-positions of the LUMO energy level. These calculation results indicate that the HOMO and LUMO energy levels tend to be notably lowered by the fluorination at these positions having no molecular orbital.4a According to this tendency, the HOMO energy level of carbazole may be lowered over the HOMO-1 energy level by fluorination at the 2- and 7-positions. This is an explanation of why the orbital shape of the HOMO energy level of 2,7-F-Cz is similar to that of the HOMO-1 energy level of nonsubstituted Cz. 3.2. Properties of Fluorinated Carbazoles. 2,7-F-Cz and 2,4,5,7-F-Cz, which were expected to possess wider HOMO− LUMO energy gaps than nonsubstituted Cz by molecular orbital calculation methods, were synthesized. Figure 3 shows the absorption spectra of nonsubstituted and fluorinated Cz in THF solution. In general, the HOMO−LUMO energy gap can be estimated from the edge of the wavelength of each absorption spectrum. The edge of the wavelength was blueshifted in the order of nonsubstituted Cz (345 nm), 2,7-F-Cz (334 nm), and 2,4,5,7-F-Cz (321 nm). The HOMO−LUMO

Figure 1. Calculated values of the HOMO−LUMO energy gap (ΔHOMO−LUMO gap) of fluorinated carbazoles plotted with reference to that of nonsubstituted carbazole. Numbers in parentheses, (*,*), indicate the fluorinated position of the carbazole ring. The hyphen indicates the nonsubstitution in the carbazole ring. For example, (2,-), 2-monofluoro-carbazole; (2,7), 2,7-difluoro-carbazole; and (2,4,5,7), 2,4,5,7-tetrafluoro-carbazole.

monofluorinated carbazoles. Furthermore, we designed 2,4,5,7F-Cz having four fluorine substituents at the symmetrical position of carbazole ring. The calculated ΔHOMO−LUMO gap was 0.19 eV wider than that of 2,7-F-Cz, which possessed the widest energy gap of difluorinated carbazoles. These results suggest that the HOMO−LUMO energy gap of carbazole can be widened by fluorination at 2-, 4-, 5-, and/or 7-positions and by increasing fluorine substituents. As noted above, we assume that the widening of the HOMO−LUMO energy gap leads to the development of new host material possessing the high T1 level. Thus, fluorination at the appropriate position in carbazole is a useful molecular design strategy for a suitable building block development for the host material. Figure 2 shows the calculated molecular orbital shapes and energy levels of nonsubstituted and fluorinated carbazoles. The

Figure 2. Plots of calculated molecular orbitals at LUMO (red top lines), HOMO (red bottom lines), and HOMO-1 (black lines) energy levels of 3,6-difluoro-carbazole (3,6-F-Cz), nonsubstituted Cz, 2,7-F-Cz, and 2,4,5,7-F-Cz. 20683

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indicate that S1 levels of these fluorinated carbazole are higher than that of nonsubstituted carbazole. Because of the correlation between S1 and T1 levels,3 these fluorinated carbazoles have the potential to possess high T1 levels. Therefore, modified host materials, which are replaced with these fluorinated carbazole, are expected to possess higher T1 levels than the conventional host material composed of nonsubstituted carbazole. 3.3. Polymerization. We tried to use the fluorinated carbazoles in solution-processable-polymer host material because the coating method has the potential to reduce OLED fabrication cost. Fluorinated carbazoles were used as pendant groups of poly(N-vinyl-carbazole) (PVK) for the polymer host material. Table 1 shows the results of radical Table 1. Characteristics of Polymerized Compounds

Figure 3. Absorption spectra of nonsubstituted Cz (black line), 2,7-FCz (blue line), and 2,4,5,7-F-Cz (red line) in THF solution. HOMO− LUMO energy gaps of these materials were estimated from the edge of wavelength of each absorption spectrum. Note that the peak intensities of each spectrum are normalized at main peak intensities for comparison.

nonsubstituted PVK 2,7-F-PVK 2,4,5,7-F-PVK

energy gaps of nonsubstituted Cz, 2,7-F-Cz and 2,4,5,7-F-Cz were estimated to be 3.59, 3.71, and 3.86 eV, respectively, from these edges. Therefore, the gaps of 2,7-F-Cz and 2,4,5,7-F-Cz were 0.12 and 0.27 eV wider, respectively, than that of nonsubstituted Cz. These measurement data on the widening gap were consistent with the molecular orbital calculation results of 2,7-F-Cz (0.15 eV) and 2,4,5,7-F-Cz (0.34 eV). The HOMO energy levels of these carbazole films are shown in Supporting Information. Downward shift behaviors of the experimental HOMO energy level were consistent with that of the calculation results in Figure 2. Figure 4 shows the photoluminescence spectra of nonsubstituted and fluorinated carbazole in THF solution. The

number of fluorinesa

Mnb

Mwb

Mw/ Mn

solubilityc

0

22400

4840 0

2.16

+

2 4

22300 890

4690 0 905

2.10 1.02

+ −

Number of fluorine substituents in each carbazole unit. bNumberaverage molecular weight (Mn) and weight-average molecular weight (Mw) by GPC measurement in THF with polystyrene standards. c Solubility in chlorobenzene as a solvent for the formation of the emission layer in OLEDs. +, soluble; −, insoluble. a

polymerization experiments on the vinyl carbazole monomer. 2,7-F-PVK, which was composed of 2,7-F-Cz, was dissolved in chlorobenzene as the solvent for the solution process. However, the synthesized material from the vinyl monomer of 2,4,5,7-FCz could hardly dissolve in the solvent. This means that the solubility may be reduced by increasing the fluorine substituents in the carbazole ring. By the gel permeation chromatography (GPC) analysis, it was found that 2,7-F-PVK had roughly the same average molecular weight as nonsubstituted PVK. However, low average molecular weight materials, which corresponded to the trimer or tetramer, were detected from the slightly dissolved solution of the synthesized material from the vinyl monomer of 2,4,5,7-F-Cz. 2,7-F-PVK is a new desirable solution-processable host material. 3.4. Properties of Fluorinated PVK. Figure 5 shows the absorption spectra of nonsubstituted PVK and 2,7-F-PVK in their film states. The edges of the wavelength of 2,7-F-PVK (339 nm) was 17 nm shorter than that of nonsubstituted PVK (356 nm). Estimated from the edges, the HOMO−LUMO energy gap of 2,7-F-PVK (3.65 eV) was 0.17 eV wider than that of nonsubstituted PVK (3.48 eV). As described in the absorption spectra of carbazole, 2,7-F-Cz as a pendant group of 2,7-F-PVK possessed a 0.12 eV wider HOMO−LUMO energy gap than that of nonsubstituted Cz. These results mean that the fluorinated polymer host material inherits the energygap-widening effect of the fluorination of carbazole, resulting in it possessing a wider energy gap than that of the nonsubstituted polymer host material. Next, HOMO energy levels of these PVK were estimated by experiments with photoelectron spectroscopy. The HOMO energy level of 2,7-F-PVK (−6.2 eV) was 0.2 eV lower than that of nonsubstituted PVK (−6.0 eV). The aforementioned results of the DFT calculation showed that 2,7-F-Cz possessed a 0.30 eV lower HOMO energy level than that of nonsubstituted Cz. These experimental results seem to be consistent with calculation

Figure 4. Photoluminescence spectra of nonsubstituted Cz (black line), 2,7-F-Cz (red line), and 2,4,5,7-F-Cz (blue line) excited at 290 nm in THF solution. Note that the peak intensities of each spectrum are normalized at the highest peak intensity of each spectrum for comparison.

peak wavelengths of 2,7-F-Cz (329 nm) and 2,4,5,7-F-Cz (315 nm) were shorter than that of nonsubstituted Cz (342 nm). These peak wavelengths of 2,7-F-Cz and 2,4,5,7-F-Cz were 13 and 27 nm blue-shifted in comparison with nonsubstituted Cz. These observed blue-shifts in photoluminescence spectra 20684

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nonsubstituted PVK.9 This conclusion was confirmed by measuring the phosphorescence decays of 5 wt % FIrpic in 2,7-F-PVK or nonsubstituted PVK thin film. As shown in Figure 6b, these phosphorescence transient decays of FIrpic were monoexponential. The transient data in the FIrpic/2,7-FPVK and FIrpic/nonsubstituted PVK systems could be fitted by a monoexponential function, and phosphorescent lifetimes were 1.2 and 1.1 μs, respectively. These lifetimes were roughly the same as those reported in the literature about the confinement effect on FIrpic. These results indicate that exciton energy can be effectively confined in FIrpic without backward energy transfer to these host materials. 3.5. Device Performance. To evaluate the effect of fluorinated PVK on the device's luminous efficiency, the blue PHOLEDs were fabricated with the device configuration of ITO/PEDOT/PSS/emission layer/CsF/Al. The emission layer was composed of 5 wt % FIrpic as phosphorescent dopant, 30 wt % OXD-7 as electron transport material, and 65 wt % nonsubstituted PVK or 2,7-F-PVK as host material.10 Figure 7a

Figure 5. Absorption spectra of PVK (black line) and 2,7-F-PVK (red line) in their film states on the quartz substrates. Inset: photoluminescence spectra of PVK (black line) and 2,7-F-PVK (red line) in the same film states. Note that the peak intensities of each spectrum are normalized for comparison.

results of fluorinated carbazole. Calculated by subtracting the HOMO−LUMO energy gap from the HOMO energy level, the LUMO energy level of 2,7-F-PVK (−2.6 eV) was 0.1 eV lower than that of nonsubstituted PVK (−2.5 eV). The photoluminescence spectra are shown in the inset of Figure 5. Nonsubstituted PVK and 2,7-F-PVK had main peaks at 405 and 389 nm, respectively. The peak of 2,7-F-PVK was shifted 16 nm toward the short-wavelength side from that of nonsubstituted PVK. As mentioned in the photoluminescence spectra of carbazole, the peak wavelength of 2,7-F-Cz was also 13 nm shorter than that of nonsubstituted Cz. These results mean that 2,7-F-PVK possesses high S1 level even if nonsubstituted carbazole is replaced with fluorinated carbazole as a pendant group. 2,7-F-PVK was expected to possess the same or higher T1 level than that of nonsubstituted PVK. Since the phosphorescence spectrum generally depends on the T1 level, phosphorescence spectra of nonsubstituted PVK and 2,7F-PVK thin film were measured at 5 K as shown in Figure 6a. The peak wavelength of the phosphorescence spectrum of 2,7F-PVK was around 480 nm, which was almost the same as that of nonsubstituted PVK. From the foregoing spectroscopic analysis, the 2,7-F-PVK also has a high T1 level to confine the exciton energy on blue phosphorescent dopant as well as

Figure 7. (a) Luminous efficiency and (b) current−voltage (J-V) characteristics of blue PHOLED composed of 2,7-F-PVK (red line) or nonsubstituted PVK (black line).

depicts the luminous efficiency and luminous intensity characteristics of blue PHOLEDs. In the PHOLED composed of nonsubstituted PVK, the luminous efficiency of 15 cd/A (430 cd/m2) was obtained at 2.8 mA/cm2. In contrast, the luminous efficiency of 27 cd/A (760 cd/m2) was achieved at the same current density in the PHOLED composed of 2,7-FPVK. The luminous efficiency of the PHOLED composed of 2,7-F-PVK was 1.8 times higher than that of the PHOLED composed of nonsubstituted PVK. At such high luminous intensity, the luminous efficiency is one of the highest reported data in the literature for solution-processed blue polymer PHOLEDs. Furthermore, yellow phosphorescent material was added in the emission layer with a 25:1 ratio of FIrpic and the yellow dopant. In this case, the fabricated device emitted white light. In the white PHOLED, the luminous efficiency at 1000 cd/m2 of the PHOLED composed of 2,7-F-PVK was 24 cd/A, which was 1.4 times more efficient than the PHOLED composed of nonsubstituted PVK, 18 cd/A (Figure 8). There are the following three possible reasons for the luminous current efficiency improvement by replacement of host material: (1) effective exciton energy confinement on emission dopant, (2) efficient transfer of generated exciton energy in the host material to emission dopant, and (3) better balance between hole and electron carrier in the emission layer. As mentioned above, both 2,7-F-PVK and nonsubstituted PVK can confine the triplet exciton energy on FIrpic. Therefore, reason 1 is unrelated to the luminous efficiency improvement. In regard to reason 2, the properties of the exciton energy

Figure 6. (a) Phosphorescence spectra of nonsubstituted PVK (black line) and 2,7-F-PVK (red line) in their film states on the Si substrate at 5 K. These spectra were measured after 1 ms from excitation by 337 nm wavelength. (b) Phosphorescence decays of 5 wt % FIrpic-doped nonsubstituted PVK (black line) and 2,7-F-PVK (red line) films. The excitation wavelength was 337 nm, and the detection wavelength was 400−600 nm. Note that the peak intensities of each spectrum are normalized for comparison. 20685

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current in the PHOLED composed of 2,7-F-PVK may be lower in comparison with that composed of nonsubstituted PVK. However, regardless of the LUMO energy level of these PVK, the electron carrier is likely to be injected into OXD-7 as electron transport material. Therefore, the electron current is expected to be almost the same value in these PHOLEDs. On the basis of these speculations, the hole current in the PHOLED composed of 2,7-F-PVK seems to be lower than that of nonsubstituted PVK owing to the possibility of the high hole injection barrier and/or efficient hole trap of FIrpic. The current−voltage (J-V) characteristics of these devices are shown in Figure 7b. The driving voltage of PHOLED composed of 2,7-F-PVK was higher than that of PHOLED composed of nonsubstituted PVK. As expected, the current density of PHOLED composed of 2,7-F-PVK was lower than that of PHOLED composed of nonsubstituted PVK at the same voltage. We assume that the excess hole current is reduced by employment of 2,7-F-PVK instead of nonsubstituted PVK, leading to the improvement of the balance between the hole and electron carrier in the emission layer. Hence, the better balance (reason 3) seems to be the dominant factor accounting for the improvement of the luminous efficiency. These results suggest that the 2,7-F-Cz is a suitable building block for modifying conventional host materials while keeping a high T1 level for the exciton confinement effect.

Figure 8. Luminous efficiency of white PHOLED composed of 2,7-FPVK (red line) or nonsubstituted PVK (black line). In the emission layer, the weight concentration ratio of FIrpic and the yellow dopant is 25:1.

transfer from these PVK to FIrpic are likely to be different from each other. According to the Förster−Dexter energy transfer theory,11 the rate of the energy transfer depends upon the extent of the spectral overlap between the donor emission spectrum and acceptor absorption spectrum. The spectral overlap between the photoluminescence of 2,7-F-PVK as the donor and the absorption of FIrpic as acceptor was more extensive than that in the case of the system composed of nonsubstituted PVK and FIrpic because of the blue-shifted photoluminescence spectrum of 2,7-F-PVK. In order to evaluate the difference of the energy transfer, the photoluminescence quantum yield (PLQY) was measured by excitation of these PVK at 295 nm. The measured films were composed of 95 wt % host and 5 wt % dopant. Contrary to the above expectation, PLQY of the film composed of 2,7-F-PVK (51%) was almost the same as that of the film composed of nonsubstituted PVK (54%). These results mean that the improvement of the device's luminous efficiency is unlikely to be caused by the rate of energy transfer (reason 2). In order to consider reason 3, energy level diagrams of FIrpic, OXD-7, these PVK, and adjacent layer materials are illustrated in Figure 9.12 In the charge injection process from PEDOT/PSS (−5.3

4. CONCLUSIONS We have designed and synthesized fluorinated carbazoles as suitable building blocks for wide-energy-gap host materials, leading to sufficiently high T1 level of the host material to confine the exciton energy on blue phosphorescent dopants. DFT calculation results showed that the fluorination at 2-, 4-, 5-, and 7-positions lowered the HOMO energy level and also showed that the HOMO−LUMO energy gap was widened by increasing the fluorine substituents at these positions. As expected from the calculation, synthesized 2,7-F-Cz and 2,4,5,7F-Cz had HOMO−LUMO energy gaps 0.12 and 0.27 eV wider than those of nonsubstituted carbazole, respectively. The synthesized 2,7-F-PVK, which was the solution-processable host material with 2,7-F-Cz as a pendant group, possessed a HOMO energy level 0.2 eV lower and a HOMO−LUMO energy gap 0.17 eV wider than those of nonsubstituted PVK. The wider HOMO−LUMO energy gap and the phosphorescence spectrum suggested that 2,7-F-PVK possessed the same high T1 level as nonsubstituted PVK. The phosphorescence decay of the FIrpic-doped film showed that 2,7-F-PVK could confine the exciton energy on the dopant. The blue PHOLED using 2,7-F-PVK showed 1.8 times higher luminous efficiency, 27 cd/A at 760 cd/m2, than that of the blue PHOLED using nonsubstituted PVK. Similar improvement was also observed in the white PHOLED composed of blue and yellow phosphorescent dopants. The hole injection barrier from adjacent charge-transport layer to 2,7-F-PVK as the host material seemed to be increased owing to the lower HOMO energy level of 2,7-F-PVK than that of nonsubstituted PVK, giving rise to a reduction of the hole current in these devices. We assume that these high luminous efficiencies are achieved by the charge balance improvement due to hole current reduction while keeping the exciton confinement effect by using wide-energygap host material. To the best of our knowledge, this is the first example of host material applying the energy-gap-widening effect by fluorination. Further study on optimization of device

Figure 9. Energy level diagrams of PHOLED composed of (a) 2,7-FPVK or (b) nonsubstituted PVK as host material.

eV) into the host material, the hole injection barrier into 2,7-FPVK (−6.2 eV) is 0.2 eV higher than that into nonsubstituted PVK (−6.0 eV). Thus, the injection into 2,7-F-PVK seems to be harder than that into nonsubstituted PVK. Even if the hole carriers are injected into the host material, FIrpic should efficiently trap hole carriers in the emission layer composed of 2,7-F-PVK owing to the relatively wider HOMO energy gap between FIrpic and 2,7-F-PVK (0.4 eV). Hence, the hole 20686

dx.doi.org/10.1021/jp303085h | J. Phys. Chem. C 2012, 116, 20681−20687

The Journal of Physical Chemistry C

Article

structure and materials is underway, and the results will be reported in due course.

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ASSOCIATED CONTENT

* Supporting Information S

HOMO energy level data of 2,7-F-Cz, 2,4,5,7-F-Cz, and nonsubstituted Cz. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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