ARTICLE pubs.acs.org/JPCC
Effects of Functional Groups in Unsymmetrical Distyrylbiphenyl on the Performances of Blue Organic Light Emitting Diodes Kyoung Soon Choi, Hyunjong Jo, Kwangyong Park,* and Soo Young Kim* School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul, 156-756, Korea
Bon Hyeong Koo, Kihyon Hong, and Jong-Lam Lee* Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk, 790-784, Korea
bS Supporting Information ABSTRACT: The effects of derivatives in unsymmetrical distyrylbiphenyl compounds on the performance of blue organic light emitting diodes (OLEDs) were investigated. Organic compounds based on unsymmetrical distyrylbiphenyl were synthesized with respect to the numbers and locations of methyl and tert-butyl groups. The main peak wavelength of photoluminescence (PL) spectra in a solid state and the difference in the PL main peak between the solid and solution states decreased as the number of tert-butyl groups in unsymmetrical distyrylbiphenyl decreased. Furthermore, discernible diffraction peaks and roll-like grain were not observed as the number of tertbutyl groups increased, indicating that the tert-butyl group prohibits the formation of a planar structure and thus reduces chromophoric ππ interaction and fluorescence quenching. The luminance value of OLEDs used as an active layer in the tert-butyltert-butyl functional group was as high as 1860 cd/m2 at a current density of 150 mA/cm2 due to the reduction in fluorescence quenching. Therefore, the tert-butyl group is effective at prohibiting fluorescence quenching in distyrylbiphenyl compounds.
1. INTRODUCTION Organic light emitting diodes (OLEDs) have received considerable attention as potential candidates for applications in next-generation full-color flat-panel displays due to their low driving voltage, low power consumption, high contrast, low cost, and fast response time.1,2 Studies on blue OLEDs are important for fabrication of full-color flat-panel displays and white OLEDs. Many researchers have tried to synthesize ultimate, nondoped host materials for blue OLEDs utilizing the derivatives of bistriphenylenyl,3 diarylfluorene/spirobifluorene,4,5 fluorene,6 pyrene,7 fluoranthene,8 and anthracene/diarylanthracene.911 However, blue OLEDs still have low efficiency, poor color purity, and short lifetimes compared to those of red or green OLEDs due to the poor quality of organic materials, wide band gap, and limited research.1214 Distyrylarylene derivatives are generally adopted as one of the blue fluorescent materials.15 However, stilbene components in distyrylarylene derivatives have trans to cis photoisomerization profiles and close intermolecular chromophoric ππ stacking, inducing emission quenching or shifting both in solution and in the solid state due to excimer formation.16 Furthermore, it is believed that distyrylarylene derivatives with symmetric structures crystallize better than does distyrylarylene with an unsymmetrical structure, inducing a short lifetime.17 Therefore, it is expected that r 2011 American Chemical Society
use of an unsymmetrical distyrylbiphenyl-based active layer with a different functional group as the active layer will cause the OLEDs to be brighter and have longer lifetimes by prohibiting intermolecular chromophoric ππ stacking and an unsymmetrical structure. In the present work, we investigated the change in performance in blue OLEDs as a function of the functional group in an unsymmetrical distyrylbiphenyl-based active layer. Compounds with four different types of functional groups were synthesized, including hydrogenmethyl, methylhydrogen, tert-butylmethyl, and tertbutyltert-butyl groups. The optical properties of these materials were characterized with UVvisible and photoluminescence (PL) spectroscopies. X-ray diffraction (XRD) and atomic force microscope (AFM) were adopted to check the degree of crystallinity of compounds. After these different compounds were used as active layers in OLEDs, the current densityvoltage, luminancecurrent density, and luminous efficiencycurrent density characteristics of each device were measured. Based on these measurements, the effects of derivatives of unsymmetrical distyrylbiphenyl-based compounds on the performance of blue OLEDs are discussed. Received: March 3, 2011 Revised: April 8, 2011 Published: April 25, 2011 9767
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Scheme 1. Synthesis Methods of Asymmetric Distyrylarylene Compounds
2. EXPERIMENTAL SECTION 2.1. Synthesis. All reactions were carried out under an inert Ar atmosphere. Tetrahydrofuran (THF) was distilled from sodiumbenzophenone ketyl. 4-Bromobenzophenone, 4-formylphenylboronic acid, diethyl benzylphosphonate (1a), diethyl 4-methylbenzylphosphonate (1b), and tetrakis(triphenylphosphine) palladium(0) were purchased and used as received. Diethyl 4-tertbutylbenzylphosphonate (1c) was prepared via the appropriate reactions in the laboratory. The overall synthetic pathway for the unsymmetrical distyrylbiphenyls 4 is described in Scheme 1. Bromotriphenylethenes 2 were prepared through the HornerWadsworthEmmons reactions18,19 of 4-bromobenzophenone with diethyl benzylphosphonates 1 using potassium tert-butoxide as a base. Although the crude products 2 were obtained as 60:40 mixtures of (Z)- and (E)-geometric isomers that were not separable through common chromatographic methods, a careful recrystallizing process in 2-propanol facilitated the selective isolation of (Z)-isomers in 5053% isolated yields.20,21 4-Triphenylethylene benzaldehydes 3 were prepared via palladium-catalyzed Suzuki cross-coupling reactions2224 of 2 with 4-formylbenzeneboronic acid using Na2CO3 in dimethylformamide (DMF) and were isolated using column chromatography to show 6785% isolated yields. Additional HornerWadsworthEmmons reactions of 3 with 1 produced the desired unsymmetrical distyrylbiphenyls 4, which were purified via simple column chromatography to produce 6781% isolated yields. According to the attached functional groups in unsymmetrical distyrylbiphenyls, four kinds of compounds were synthesized: (Z)-4-(2-p-tolylvinyl)-40 -(1,2-diphenylvinyl)biphenyl (U-DSB_HM), (Z)-4-styryl-40 -[1-phenyl-2-(p-tolyl)vinyl] biphenyl (U-DSB_MH), (Z)-4-(2-p-tolylvinyl)-4 0 -[1-phenyl-2-(4-tert-butylphenyl)vinyl]biphenyl (U-DSB_tBM), and (Z)-4[2-(4-tert-butylphenyl)vinyl]-40 -[1-phenyl-2-(4-tert-butylphenyl) vinyl]biphenyl (U-DSB_tBtB). Detailed experimental procedures are described in the Supporting Information. 1 H NMR (300 and 500 MHz) and 13C NMR (75 and 125 MHz) were performed in CDCl3, and tetramethylsilane (TMS)
was used as an internal standard using a Gemini 2000 and Bruker Avance-500. Chemical shifts are reported in δ units (ppm) by assigning TMS resonance in the 1H spectrum as 0.00 ppm and CDCl3 resonance in the 13C spectrum as 77.2 ppm. Column chromatography was performed on silica gel 60 and 70230 mesh. Analytical thin-layer chromatography (TLC) was performed using Merck Kieselgel 60 F254 precoated plates (0.25 mm) with a fluorescent indicator and was visualized with UV light (254 and 365 nm). Melting points were obtained using a Barnstead 9100 and are uncorrected. HRMS were obtained using a Micromass Autospec. Ultravioletvisible (UVvis) spectra were recorded with a Hitachi U-3300. Band gaps were obtained using a Hitachi AC-2. 2.2. Fabrication of OLEDs. A glass coating with ITO (150 nm thick, ∼20 Ω/sq) was used as the starting substrate. The samples were cleaned in sequence with acetone, isopropyl alcohol, and deionized water and then dried with high purity nitrogen gas. The samples were then treated with O2 plasma for 1 min at a power of 150 W in order to optimize the workfunction of ITO. After the samples were loaded into a thermal evaporator, 40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl (RNPD, 70 nm), active layer (40 nm), tris(8-hydroxyquinoline) aluminum (Alq3, 20 nm), LiF (1 nm), and Al (100 nm) layers were deposited in sequence as the hole transport layer, active layer, electron transport layer, electron injection layer, and cathode, respectively. Four types of active layer, U-DSB_HM (sample HM), U-DSB_MH (sample MH), U-DSB_tBM (sample tBM), and U-DSB_tBtB (sample tBtB), were used to investigate the effect of the functional group on the performance of an OLED. Figure 1 shows the basic structure of an OLED, in which the active area was 3 3 mm2. The current densityvoltage, luminance current density, luminous efficiencycurrent density characteristics, and CIE chromaticity coordinates of the devices were measured. The inset of Figure 1 shows the emitting OLED images of each sample after applying 100 mA/cm2. All measurements were performed in a glovebox under N2 ambient without encapsulation. 9768
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Figure 1. Structures of the blue OLED devices. U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB were used as active layers in sample HM, sample MH, sample tBM, and sample tBtB, respectively. The inset of Figure 1 shows the emitting OLED images of each sample after application of 100 mA/cm2.
Figure 3. Luminance efficiencycurrent density characteristics of the four types of devices.
Figure 2. (a) The current densityvoltage and (b) luminancecurrent density characteristics of the four types of devices. All measurements were performed in a glovebox under N2 ambient without encapsulation.
3. RESULTS AND DISCUSSION Figure 2(a) shows the current densityvoltage characteristics of the four types of devices. The operation voltages at a current density of 150 mA/cm2 were 10.4 V for sample HM, 11.8 V for sample MH, 8.5 V for sample tBM, and 11.4 V for sample tBtB. Figure 2(b) shows the luminancecurrent density curves of the materials. The luminance values and CIE coordinates at a current density of 150 mA/cm2 were 1160 cd/m2 and (0.18, 0.17) for sample HM, 1100 cd/m2 and (0.19, 0.20) for sample MH, 2400 cd/m2
and (0.26, 0.51) for sample tBM, 1860 cd/m2 and (0.19, 0.16) for sample tBtB. The CIE coordinates of all samples except for sample tBM were less than (0.20, 0.20), indicating that the thickness of the active layer was adequate for emission of blue light. However, the CIE coordinates in the case of sample tBM suggested that the active layer was too thin to emit blue light. Therefore, it seems that some of the anode holes traverse the active layer without recombination and recombine with electrons from the cathode in the Alq3 region, producing a mixed blue-green color. Figure 3 shows luminance efficiencycurrent density curves where the luminance efficiency values at a current density of 150 mA/cm2 were 0.77 cd/A for sample HM, 0.73 cd/A for sample MH, 1.6 cd/A for sample tBM, and 1.24 cd/A for sample tBtB. The luminance efficiency of sample tBM was much higher than those of the other samples because the emission in that sample is due to both the U-DSB_tBM and Alq3 layers. Figure 4(a) shows the UVvisible spectra of the synthesized compounds. The maximum absorption wavelengths of U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB in chloroform appeared at 341 nm, 320 nm, 332 nm, and 331 nm, respectively. The adsorption edges were determined by extrapolating 9769
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obtained from the first reflection (2θ = 23.7°) is 3.77 Å. The XRD pattern of U-DSB_tBtB shows no discernible diffraction peaks, suggesting that the films are amorphous and the molecules are randomly oriented. The images of AFM are shown in the inset of Figure 5. It is shown that the film of U-DSB_tBM consists of roll-like grains in good connectivity. However, the grain was not shown in the film of U-DSB_tBtB, indicating the amorphous characteristics. The values of root-mean-square were measured to be 3.31 and 1.88 nm for U-DSB_tBM and U-DSB_tBtB, respectively. Based on these experimental observations, the effect of the functional group in the unsymmetrical distyrylbiphenyl-based active layer on the performance of blue OLEDs can be explained as follows. Table 1 shows that a red shift in the PL spectra occurred as the number of tert-butyl groups in unsymmetrical distyrylbiphenyl based compounds decreased. Red-shifted emissions are reported to be caused by the formation of aggregates, and this phenomenon usually takes place due to the planar structures of organic materials.25 The main peak differences (Δλ) between PL in solid states and PL in solution were calculated to be 42 nm, 43 nm, 33 nm, and 21 nm for U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB, respectively, indicating that Δλ decreased as the number of tert-butyl groups in an unsymmetrical distyrylbiphenyl-based organic compound increased. It is reported that the ππ interactions, which induce fluorescence quenching, become less frequent as Δλ decreases.26 The planar structure usually induces increased ππ
two solid lines from the background and straight onset in the spectra. Adsorption edges of U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB were 384 nm, 376 nm, 376 nm, and 377 nm, respectively. This result indicates that the optical band gaps of U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB were 3.23 eV, 3.30 eV, 3.30 eV, and 3.29 eV, respectively. Photoelectron spectroscopy data showed that the highest occupied molecular orbital (HOMO) level of each compound was 5.8 eV (data not shown). From these data, the lowest unoccupied molecular orbital (LUMO) levels were calculated to be 2.57 eV, 2.50 eV, 2.50 eV, and 2.51 eV for U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB, respectively. Figure 4(b) shows the normalized PL spectra of the compounds in the solid state. The main peak positions of the PL spectra were measured to be 464 nm, 466 nm, 454 nm, and 446 nm for U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB_tBtB, respectively. This result indicates that a red shift occurred as the number of tert-butyl groups decreased. Table 1 summarizes the main peak positions (λmax) of UV absorption, PL spectra in solution, PL spectra in the solid state, Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, and the calculated optical band gaps. Substitution of tert-butyl in U-DSB derivatives exerts a dramatic influence on molecular ordering. The orientation of the vapor-deposited films was investigated by XRD. Figure 5 shows the XRD patterns of U-DSB_tBM and U-DSB_tBtB deposited on SiO2. For U-DSB_tBM, the peaks in the XRD patterns indicate the existing crystalline microstructure in the films. As for polycrystalline films, such structure is known to be a favorable structure in terms of achieving high mobility. The d-spacing
Figure 5. XRD patterns of U-DSB_tBM and U-DSB_tBtB deposited on SiO2. The inset of Figure 5 shows AFM images of the thin film of U-DSB_tBM and U-DSB_tBtB deposited on SiO2.
Figure 4. (a) UVvis and (b) PL spectra of U-DSB_HM, U-DSB_MH, U-DSB_tBM, and U-DSB-tBtB.
Table 1. The Main Peak Positions of UV absorption, PL Spectra in a Solution State, PL Spectra in a Solid State, and Optical Band Gap compound
a
UV (nm) (soln, λmax)
PLa (nm) (soln, λmax)
PLb (nm) (λmax)
CIE (x,y)b
band gap (eV)
U-DSB_HM
341
422
464
(0.137, 0.057)
3.23
U-DSB_MH
320
423
466
(0.131, 0.071)
3.30
U-DSB_tBM U-DSB_tBtB
332 331
421 425
454 445
(0.143, 0.083) (0.144, 0.047)
3.30 3.29
Obtained as a solution in CHL solution. b Obtained as a pure solid. 9770
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The Journal of Physical Chemistry C interactions. From the main peaks of PL in the solid state, XRD data, AFM images, and the Δλ data, the tert-butyl functional group seems to prohibit the occurrence of the planar structure, reducing the ππ interactions. However, it is reported that increasing tert-butyl substitution reduces both electron and hole mobility primarily due to a reduction in wave function overlap.27 Excluding sample tBM is thought to be reasonable in the comparison of the device properties because recombination takes place not only in the U-DSB_tBM region but also in the Alq3 region. The luminance value at 150 mA/cm2 in sample tBtB was measured to be as high as 1860 cd/m2. It is reported that ππ interactions induce fluorescence quenching and thus reduce the luminance value.16 Although substitution of tert-butyl functional group reduces both electron and hole mobility, the tert-butyl functional group in sample tBtB hinders U-DSB_tBtB from ππ interaction, producing the highest luminance value in a high current.
4. CONCLUSIONS We investigated the effects of derivatives in unsymmetrical distyrylbiphenyl-based compounds on the performance of blue OLEDs. Four compounds were synthesized according to the numbers and locations of methyl and tert-butyl groups. Red shifts in the PL spectra and decreases in Δλ occurred as the number of tert-butyl groups in the organic compounds decreased. According to XRD data and AFM images, crystallinity of unsymmetrical distyrylbiphenyl-based compounds decreased as the attachment of tert-butyl groups. These results indicated that tert-butyl groups prohibit the formation of a planar structure, reducing chromophoric ππ interaction and fluorescence quenching. The luminance value at 150 mA/cm2 in sample tBtB was the highest at 1860 cd/m2, which is evidence that the tert-butyl group prohibits the reduction of fluorescence quenching. Therefore, it is believed that the tert-butyl group is very efficient at prohibiting fluorescence quenching in distyrylbiphenyl-based compounds. ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed experimental procedures. 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: K.P.,
[email protected]; S.Y.K.,
[email protected]. kr; J.-L.L.,
[email protected].
’ ACKNOWLEDGMENT This research was supported in part by the Seoul R&BD Program (ST10004M093171), in part by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2010-0011590) and in part by WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the ministry of Education, Science and Technology (Project No. R31-2008-000-10059-0). 9771
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