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Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode
Dyes and Pigments 101 (2014) 221e228
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Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode Nam-Jin Lee b,1, Joon Ho Jeon a,1, Insik In b, Ji-Hoon Lee b, *, Min Chul Suh a, ** a
Department of Information Display and Advanced Display Research Center, Kyung Hee University, Dongdaemoon-Gu, Seoul 130-701, Republic of Korea Department of Polymer Science & Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju-Si, Chungbuk 380-702, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2013 Received in revised form 23 August 2013 Accepted 30 September 2013 Available online 12 October 2013
We report series of triphenylene derivatives with good electronic properties for the use as host materials for green phosphorescent organic light emitting diodes (PHOLEDs). We applied highly planar triphenylene moieties which could provide good electron transport ability and the carbazole or dibenzothiophene moieties which could provide good hole transport ability to obtain a bipolar host materials for green PHOLEDs. From this approach we achieved relatively good current efficiency up to 64 cd/A and external quantum efficiency up to 20.3%, respectively. This was the much more improved value (by w22.3%) compare to that obtained from the 4,40 -N,N0 -dicarbazolebiphenyl system as a reference. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Charge balance Host materials Green phosphorescent OLED Triphenylene Carbazole Dibenzothiophene
1. Introduction Phosphorescent organic light-emitting diodes (PHOLEDs) have attracted intense interest because of their merit of high quantum efficiency as compared to conventional fluorescent OLEDs through utilizing both singlet and triplet excitons for emission [1e4]. In fact, red phosphorescent materials have already been applied in the main-display of commercial mobile phones since 2007. Recently, green emission is almost approaching to the theoretical limitation of efficiency (>20% of external quantum efficiency, EQE) by utilizing various dopants such as fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3] as the phosphorescent emitter, and the commercialization of which has been successfully started [5e9]. However, compared with the great achievements in red phosphorescent materials, green component has many obstacles against commercialization including its relatively short lifetime issue. To overcome such issue, the mixed host system is widely utilized [10e12]. However, many research groups are concentrating on preparation of the host materials which possess bipolar characteristics because the minute change of mixing ratio of hole transport type and electron transport * Corresponding author. Tel.: þ82 43 841 5427; fax: þ82 43 841 5420. ** Corresponding author. Tel.: þ82 2 961 0694; fax: þ82 2 968 6924. E-mail addresses: [email protected] (J.-H. Lee), [email protected] (M.C. Suh). 1 Both authors were equally contributed as a first author to this work. 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.09.046
type host materials could change the resultant device characteristics. For this purpose, many achievements have been reported in this field in recent years [2,13,14]. Especially, the host materials with hole transport type functional groups such as carbazoles or triarylamine moieties combined with electron transport type functional groups such as heterocyclic moieties such as pyridine, triazine, benzimidazole showed great progress in the device performances [15,16]. Aryl silanes [17,18] and phosphine oxides [19e21] have also been utilized to obtain bipolar properties as electron transport type functional groups [22]. As a part of such an effort, the efficiency greater than 20% (EQE) was frequently reported from the green PHOLEDs with such kinds of bipolar host materials [23,24]. In this study, we report three different types of new host materials for green PHOLEDs having triplet energy greater than 2.5 eV with highly planar triphenylene moieties to improve their electron transport ability. The resultant PHOLEDs containing such moieties showed moderate EQE up to 20.3%. 2. Results and discussion 2.1. Materials We prepared three host materials to realize a highly efficient green PHOLED. We selected triphenylene moiety as a basic
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functionality because they are renowned as discotic liquid crystalline molecules for their potential in one-dimensional charge transporting properties. Conductivity along the columns in columnar mesophases has been reported to be several orders of magnitude greater than that in the perpendicular direction. The triphenylene and its derivatives have been known as hole transporting materials although it could also be used as an electron transporting functionality [25] Their photoinduced charge-carrier mobilities ranged from 105 cm2 V1 s1 in the isotropic phase [26] to 102 cm2 V1 s1 in highly ordered discotic mesophases [27] Meanwhile, we changed the numbers of triphenylene moieties which are connected to the central core unit (1,3,5-trisubstituted benzene) which also contains a hole transporting functional groups such as carbazole and/or dibenzothiophene units. In principle, carbazole containing host materials exhibit high triplet energies if they have no extended p-conjugation length [28,29]. In other words, the most common approach to increase a triplet
energy level as a green host might be the disruption of p-conjugation just as in the case of 9,90 -(1,3-Phenylene)bis-9H-carbazole which has two carbazole units connected meta position of a single benzene ring. Scheme 1 shows the schematic diagram of synthetic route to prepare such compounds. Fig. 1 shows the geometries of core parts of those three resultant host materials obtained from the simulation at a DNP/GGA(PBE) level of theory using Dmol3 module (Material Studio 6.1, Accelrys software). The triphenylene moieties were rather twisted to the core benzene unit which could arouse a broken conjugation as we expected and we could utilize those materials as a green host materials (T1 > 2.6 eV). Besides, the molecular orbital distribution of 9,90 -(5-(triphenylen-2-yl)-1,3phenylene)bis(9H-carbazole) (TP-mCP), 9-(3,5-di(triphenylen-2yl)phenyl)-9H-carbazole (DTP-mCP), and 4-(3-(triphenylen-2-yl) phenyl)dibenzo[b,d]thiophene (TP-PDBTh) are also shown in Fig. 1. As we expected from the molecular structure, the electrons in highest occupied molecular orbital (HOMO) was localized on the
Scheme 1. Synthesis of green host materials.
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223
Fig. 1. Geometries of three host materials having twisted core structures.
carbazole and/or dibenzothiophene units, while the electrons in lowest unoccupied molecular orbital (LUMO) was dispersed over the triphenylene and benzene core units. In other words, the hole carriers injected from the hole transport layer (HTL) could move through the carbazole and/or dibenzothiophene units, while the electrons injected from the electron transport layer (ETL) could be transported through the triphenylene and/or central benzene moieties. Fig. 2(a) shows the ultravioletevisible (UVeVis) absorption and photoluminescence spectra of new host materials in chloroform (CHCl3). The band gap (Eg) of TP-PDBTh was slightly larger than those of TP-mCP and DTP-mCP (TP-PDBTh: 3.63 eV, TP-mCP: 3.57 eV, DTP-mCP: 3.49 eV). All three compounds showed vibronic fine structures in their emission spectra, especially at the low temperature (77 K) as shown in Fig. 2(b). The triplet energies (T1) of new host materials which were determined by the first phosphorescence peak from the shorter wavelength region were 2.63, 2.63, 2.64 eV for TP-mCP, DTP-mCP, and TP-PDBTh, respectively, as marked in the spectra as shown in Fig. 2(b). The representative results are summarized in Table 1. The energy levels of HOMO and LUMO were collected from cyclic voltammetry and band edges of UVevisible absorption spectra and they were also summarized in Table 1. Fig. 3 shows the thermal stability behavior of the materials synthesized in this study. All of the compounds having triphenylene moieties showed outstanding thermal stability up to 430 C. The decomposition temperature (Td) of TP-mCP, DTP-mCP, and TPPDBTh were 436, 488, and 445 C, respectively. Very interestingly, the glass transition temperatures (Tg) of TP-mCP and DTP-mCP with carbazole moieties were higher (159 and 179 C) than that of TPPDBTh with dibenzothiophene moiety (108 C). Besides, the melting temperature (Tm) and crystallization temperature (Tc) of the materials were also summarized in Table 1.
2.2. Device characteristics Bipolar character of host materials with proper triplet energy level is very important for the remarkable enhancement of efficiency upon doping. To investigate the bipolar characteristics, we prepared the hole only devices (HODs) as well as electron only devices (EODs) [30e32] of 4,4-N,N0 -Dicarbazole-1,10 -biphenyl (CBP) and newly synthesized host materials by using molybdenum oxide (MoO3) and lithium quinolate (LiQ) as charge carrier injection layers from indium tin oxide (ITO) and aluminum (Al) as follows: HOD A: ITO/MoO3 (0.75 nm)/CBP (200 nm)/MoO3 (10 nm)/Al (100 nm) HOD B: ITO/MoO3 (0.75 nm)/TP-mCP(200 nm)/MoO3 (10 nm)/Al (100 nm) HOD C: ITO/MoO3 (0.75 nm)/DTP-mCP (200 nm)/MoO3 (10 nm)/ Al (100 nm) HOD D: ITO/MoO3 (0.75 nm)/TP-PDBTh(200 nm)/MoO3 (10 nm)/Al (100 nm) EOD A: Al (50 nm)/LiQ (1.5 nm)/CBP(200 nm)/LiQ (1.5 nm)/Al (100 nm) EOD B: Al (50 nm)/LiQ (1.5 nm)/TP-mCP (200 nm)/LiQ (1.5 nm)/ Al (100 nm) EOD C: Al (50 nm)/LiQ (1.5 nm)/DTP-mCP(200 nm)/LiQ (1.5 nm)/ Al (100 nm) EOD D: Al (50 nm)/LiQ (1.5 nm)/TP-PDBTh (200 nm)/LiQ (1.5 nm)/Al (100 nm) Fig. 4(a) and (b) shows the energy band diagram of HODs and EODs of host materials. For the effective ohmic injection of hole carriers into the HOMO of new host materials, we utilized 0.75 nm of MoO3 as a charge injection layer [30]. Meanwhile, we used LiQ/Al to inject the electrons into the LUMO of new host materials
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Fig. 3. TGA of newly synthesized host materials.
Fig. 2. (a) UVevisible absorption and photoluminescence spectra (at 298 K), (b) photoluminescence spectra (at 77 K) of three host materials in 2-methyl tetrahydrofuran.
although newly synthesized materials showed more or less higher lying LUMOs because it has been reported as an effective cathode toward tris(8-hydroxyquinolinato)aluminum [33]. Fig. 5(a) and (b) shows the hole and electron current characteristics obtained from the HODs and EODs prepared with newly synthesized host materials (HOD A w D; EOD A w D). The hole current density was significantly reduced in the order of HOD C (DTP-mCP) > HOD A (CBP) > HOD D (TP-PDBTh) > HOD B (TP-mCP) while the electron current density was slightly reduced in the order of EOD D (TP-PDBTh) > EOD A (CBP) > EOD C (DTP-mCP) > EOD B (TP-mCP). To estimate the charge balance if we use those materials as an emission layer, we calculate the relative charge density by dividing the hole current density by electron current density [e.g. J(hole)/J(electron) where J: current density, mA/cm2] as shown in Fig. 6. From this calculation, we could expect that TP-mCP could give the most desirable device performance because the relative charge density value was very close to unity. In other words, we
could expect that TP-mCP might show best efficiency value. Then the efficiency could be decreased in order of increasing the relative charge density value which may deviated more from the unit value (1) (e.g. TP-PDBTh > CBP > DTP-mCP). To verify the final characteristics as a full devices with new synthetic host materials, we choose well-known standard reference device structure with CBP because it has similar T1 energy value (w2.6 eV) to those obtained from new synthetic host materials prepared in this study [14,34]. Fig. 7 shows the perspective images of the OLED devices fabricated in this study. We used ITO as an anode, N,N0 -Bis(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) as a hole transport layer, 4,40 ,400 -tris(carbazol-9-yl)-triphenylamine (TCTA) as an electron blocking layer, CBP and new materials synthesized in this study as host materials for emission layer (EML) and Ir(ppy)3 as a dopant material for EML, 4,7diphenyl-1,10-phenanthroline (Bphen) as an electron transport layer as well as a hole blocking layer (HBL), lithium fluoride (LiF) as an electron injection layer (EIL), Al as a cathode. The exact device configuration used in this work was as follows (See also Fig. 7): Device A: ITO/NPB (30 nm)/TCTA (10 nm)/CBP: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Device B: ITO/NPB (30 nm)/TCTA (10 nm)/TP-mCP: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Device C: ITO/NPB (30 nm)/TCTA (10 nm)/DTP-mCP: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Device D: ITO/NPB (30 nm)/TCTA (10 nm)/TP-PDBTh: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Fig. 8(a) shows the current densityevoltage (JeV) and luminanceevoltage (LeV) characteristics of fabricated green devices and the representative results are summarized in Table 2. At a given constant voltage of 5.0 V, current density values of 1.5, 2.1, 5.5 and 2.7 mA/cm2 were observed in the fabricated Devices A, B, C, and D, respectively. The reason that the DTP-mCP showed the
Table 1 Summary of physical properties of new host materials. Compound
labs [nm]
lpl. [nm] Solution
Film
TP-mCP DTP-mCP TP-PDBTh
245, 273, 293 273 246, 271
375 377 369
391 391 385
T1 (eV) (Exp)
T1 (eV) (Cal)
HOMO [eV] (UPS)
HOMO [eV] (CV)
LUMO [eV]
Eg [eV]
Tg [ C]
Tm[ C]
Tc [ C]
Td [ C]
2.63 2.63 2.64
2.81 2.92 2.97
5.91 5.93 5.95
5.68 5.98 5.84
2.11 2.39 2.21
3.57 3.59 3.63
159 179 108
273 354
267
436 488 445
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225
Fig. 4. Energy band diagrams of used materials in this work.
greatest current density value could be due to the columnar packing of triphenylene moieties with the neighboring molecules which also makes the distances between carbazole moieties very short. This is well consistent with the result of HOD characteristics that the hole current level in the HOD C was the greatest as shown
in Fig. 5(a). On the other hand, the electron current level of EOD C with DTP-mCP was about the same as those of other devices such as EOD A (with CBP) and D (with TP-PDBTh), which means that the triphenylene moiety could be acted as a hole transporter although we designed it as an electron transporting functionality. The driving voltages to reach 1000 cd/m2 were 5.2, 4.8, 4.5, and 4.8 V for the Devices A, B, C, and D, respectively (Table 2). The turn-on voltages of 3.0, 3.0, 3.0, and 3.0 V were observed for the Devices A, B, C, and D, respectively. The current and power efficiency characteristics of fabricated devices are shown in Fig. 8(b) and also summarized in Table 2. At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 46 cd/A and 28 lm/W for the Device A, 64 cd/A and 42 lm/W for the Device B, 48 cd/A and 34 lm/W for the Device C, 65 cd/A and 40 lm/W for the Device D, respectively. These efficiency data correspond to 16.6, 20.3, 15.6, and 18.7% external quantum efficiencies of Devices A, B, C and D, respectively. The maximum current and power efficiencies were 50 cd/A and 28 lm/W for the Device A, 64 cd/A and 49 lm/W for the Device B, 52 cd/A and 51 lm/W for the Device C, 66 cd/A and 40 lm/W for the Device D, respectively. These results are slightly different with our expectation from the calculation of relative current density aforementioned. However, those are well consistent with those of EQE. Thus, TP-mCP and TP-PDBTh have emerged as good candidates for excellent host materials for green PHOLEDs. The normalized EL spectra at a brightness of 1000 cd/m2 of fabricated green devices are shown in Fig. 9. The EL spectra have clear Ir(ppy)3 emission with peak wavelength at 512 nm, 515 nm, 518 nm, 514 nm for the Devices A, B, C, and D, respectively. 3. Conclusion
Fig. 5. (a) Hole current characteristics of HOD A w D and (b) electron current characteristics of EOD A w D.
In conclusion, we prepared new green PHOLEDs with newly synthesized host materials which have bipolar characteristics. The host materials with more triphenylene moieties showed higher hole carrier characteristics although we expected those moiety could have high electron transporting ability. Materials with relative charge density close to unity showed fairly good current efficiency as well as EQE values when it was fabricated as green PHOLEDs. Finally, we found that the TP-mCP and TP-PDBTh could be emerged as good candidates for excellent host materials for green PHOLEDs which could give high EQE up to 20.3% and 18.7%, respectively.
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Fig. 6. Relative charge density in host materials.
4. Experimental 4.1. Instruments 1
H NMR and 13C NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer and chemical shifts were referenced to chloroform (7.26 ppm). UVeVis spectra were recorded with a HEWLETT PACKARD 8453 UVeVis spectrophotometer. Photoluminescence Spectrometer was used PERKIN ELMER LS 50B model. The electrospray ionization (ESI) source was coupled to a hybrid quadrupole orthogonal time-of-flight (Q-TOF) mass spectrometer (SYNAPT G2, Waters, MS Technologies, Manchester, U.K.) was used to mass spectra acquirement in positive- and negative-ion mode. A capillary and cone voltage of 3.0 kV and 30 V, and capillary temperature of 120 C were used for both polarities, respectively. The desolvation source conditions were as follows; for the desolvation gas 800 L/h was used and the desolvation temperature was kept at 600 C. Data acquisition took place over the mass range of m/z 50 to m/z 1200 for MS modes. The sample was introduced into the ESI source at a constant flow rate of 20 ml/min by using an external syringe pump (HARVARD 11Plus, Holliston, MA, USA). Also, another mass measurement was performed using
Fig. 8. (a) JeVeL characteristics of fabricated blue devices. (b) Luminance vs current efficiency and power efficiency characteristics of fabricated blue PHOLEDs.
an UltrafleXtreme MALDI time-of-flight (TOF) mass spectrometer equipped with a pulsed smartbeam II (355 nm Nd:YAG laser, repetition rate 1 kHz) in reflector mode (Bruker Daltonics, Billerica, MA) Thermal properties were measured by differential scanning calorimetry (DSC, TA instruments) under nitrogen atmosphere. We also utilized thermogravimetric analysis (TGA) using TGA-1000 (SINCO) for investigation of thermal stability of the materials. Cyclic voltammetry (CV) [35,36] were measured by a Autolab/PGSTAT 2 model at room temperature in a solution of tetra-nbuthylammonium hexafluorophosphate (0.1 N n-Bu4NPF6) in acetonitrile under nitrogen gas protection at a scan rate of 100 or 50 mV/s. The working electrode was platinum (Pt) disk type and reference electrode was Ag/0.1 M AgNO3. The HOMO energies were also measured with Ultraviolet Photoelectron Spectrometer (UPS) in a Hitachi High Tech AC-2 surface analysis system [37]. 4.2. Synthesis of new host materials
Fig. 7. Schematic diagram of device architecture of green devices fabricated in this study.
4.2.1. Synthesis of 4,4,5,5-tetramethyl-2-(triphenylen-2-yl)-1,3,2dioxaborolane (1) 2-bromotriphenylene (10 g, 32.6 mmol), bis(pinacolato) diborate (9.9 g, 39.1 mmol), [1,10-bis(diphenylphosphino)ferrocene] dichloropalladium(II) (0.5 g, 0.7 mmol), and potassium acetate (16.2 g, 162.8 mmol) were dissolved in anhydrous 1,4-dioxane (180 ml) and refluxed under nitrogen for 24 h. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane-tetrahydrofuran (5:1)
N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228 Table 2 Device characteristics of phosphorescent green OLEDs.
Turn-on voltage (1 cd/m2) Operating voltage (1000 cd/m2) Efficiency (1000 cd/m2) Efficiency (Max) CIE (x,y) (1000 cd/m2) E.Q.E. (1000 cd/m2)
Device A
Device B
Device C
Device D
3V
3V
3V
3V
5.2 V
4.8 V
4.5 V
4.8 V
46 cd/A 28 lm/W 50 cd/A 28 lm/W 0.265, 0.631
64 cd/A 42 lm/W 64 cd/A 49 lm/W 0.279, 0.635
48 cd/A 34 lm/W 52 cd/A 51 lm/W 0.292, 0.634
65 cd/A 40 lm/W 66 cd/A 40 lm/W 0.273, 0.631
16.6%
20.3%
15.6%
18.7%
eluent to afford (1) a white solid, yield 80%. 1H NMR (400 MHz, CDCl3) 9.15 (s, 1H), 8.84e8.82 (m, 1H), 8.71e8.64 (m, 4H), 8.07e8.05 (d, J ¼ 8.0 Hz, 1H), 7.70e7.66 (m, 4H), 1.43 (s, 12H) ppm. 4.2.2. Synthesis of 9,90 -(5-bromo-1,3-phenylene)bis(9H-carbazole) (2) 9H-carbazole (40 g, 240 mmol), 1,3,5-tribromobenzene (39.2 g, 120 mmol), copper(I) iodide (6.83 g, 36 mmol), potassium phosphate (203 g, 96 mmol) and trans-1,2-diaminocyclohexane (5.8 ml, 48 mmol) in 200 ml of anhydrous toluene was stirred at 120 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane eluent to afford (2) a white solid, yield 34%. 1H NMR (400 MHz, CDCl3) 8.16e8.14 (d, J ¼ 7.6 Hz, 4H), 7.87 (s, 2H), 7.8 (s, 1H), 7.57e7.54 (d, J ¼ 8.4 Hz, 4H), 7.49e7.45 (t, J ¼ 7.2 Hz, 4H), 7.36e7.32 (t, J ¼ 7.6 Hz, 4H) ppm. 4.2.3. Synthesis of 9-(3,5-dibromophenyl)-9H-carbazole (3) 9H-carbazole (40 g, 240 mmol), 1,3,5-tribromobenzene (98 g, 300 mmol), copper(I) iodide (6.83 g, 36 mmol), potassium phosphate (203 g, 96 mmol) and trans-1,2-diaminocyclohexane (5.8 ml, 48 mmol) in 300 ml of anhydrous toluene was stirred at 120 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried
Fig. 9. The normalized EL spectra at a brightness of 1000 cd/m2 of fabricated green devices.
227
over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane eluent to afford (3) a white solid, yield 27%. 1H NMR (400 MHz, CDCl3) 8.14e8.12 (d, J ¼ 7.6 Hz, 2H), 7.78 (s, 1H), 7.45 (s, 2H), 7.47e7.41 (m, 4H), 7.34e7.3 (t, J ¼ 8 Hz, 2H) ppm. 4.2.4. Synthesis of 9,90 -(5-(triphenylen-2-yl)-1,3-phenylene)bis(9Hcarbazole) (TP-mCP) 4,4,5,5-tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane (0.65 g, 1.84 mmol), 9,90 -(5-bromo-1,3-phenylene)bis(9H-carbazole) (0.91 g, 1.87 mmol), tetrakis(triphenylphosphine) palladium(0) (0.06 g, 0.06 mmol), and 1 M Na2CO3 (7.3 ml) in 40 ml of Toluene was stirred at 130 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexanetetrahydrofuran (5:1). The final product was obtained as a white powder after purification by vacuum sublimation at a synthetic yield of 38%. 1H NMR (400 MHz, CDCl3) 8.97 (s, 1H), 8.79e8.77 (d, J ¼ 8.8 Hz, 1H), 8.71e8.67 (m, 4H), 8.22e8.20 (d, J ¼ 7.6 Hz, 4H), 8.16 (s, 2H), 8.02e8.00 (d, J ¼ 8.8 Hz, 1H), 7.88 (s, 1H), 7.72e7.62 (m, 8H), 7.52e7.48 (t, J ¼ 8.4 Hz, 4H), 7.37e7.33 (t, J ¼ 6.8 Hz, 4H) ppm; 13C NMR (CDCl3, 400 MHz) 142.5, 138.6, 137.8, 135.5, 128.2, 128, 127.9, 127.7, 127.3, 127.2, 127, 126.2, 125.5, 125.3, 124.2, 123.8, 122.5, 122.2, 122, 121.6, 121.4, 121.3, 121.2, 119.8, 118.5, 118.4 ppm. ESI-MS [M þ H]þ: m/z calcd. 635.2487; found 635.2460. 4.2.5. Synthesis of 9-(3,5-di(triphenylen-2-yl)phenyl)-9H-carbazole (DTP-mCP) 9-(3,5-dibromophenyl)-9H-carbazole (1.8 g, 4.49 mmol), 4,4,5,5tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane (3.34 g, 9.42 mmol), tetrakis(triphenylphosphine) palladium(0) (0.26 g, 0.22 mmol), and 1 M Na2CO3 (19.1 ml) in 140 ml of Toluene was stirred at 130 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduce pressure. The crude product was purified by silica gel column chromatography using n-hexane-tetrahydrofuran (5:1). The final product was obtained as a white powder after purification by vacuum sublimation at a synthetic yield of 74%. 1H NMR (400 MHz, CDCl3) 9.02 (s, 2H), 8.83e8.81 (d, J ¼ 8.8 Hz, 2H), 8.78e8.77 (d, J ¼ 7.2 Hz, 2H), 8.74e8.69 (m, 6H), 8.34 (s, 1H), 8.25e8.23 (d, J ¼ 7.6 Hz, 2H), 8.10e8.08 (d, J ¼ 5.6 Hz, 4H), 7.72e7.65 (m, 10H), 7.52e7.48 (t, J ¼ 7.2 Hz, 2H), 7.38-7.34 (t, J ¼ 7.6 Hz, 2H) ppm. MALDITOF [M þ H]þ: 695.329. 4.2.6. Synthesis of 4-(3-(triphenylen-2-yl)phenyl)dibenzo[b,d] thiophene (TP-PDBTh) 1-bromo-3-iodobenzene (2.0 g, 7.07 mmol), 4,4,5,5-tetramethyl2-(triphenylen-2-yl)-1,3,2-dioxaborolane (2.5 g, 7.07 mmol), tetrakis(triphenylphosphine)palladium(0) (0.25 g, 0.21 mmol), and 1 M Na2CO3 (35.4 ml) in 100 ml of toluene was stirred at 90 C for 48 h under nitrogen atmosphere. To the reaction mixture was added a dibenzo[b,d]thiophene-4-ylboronic acid (1.61 g, 7.07 mmol) and then stirred at 120 C for 24 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane-tetrahydrofuran (5:1). The final product was obtained as a white powder after purification by
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vacuum sublimation at a synthetic yield of 46%. 1H NMR (400 MHz, CDCl3) 8.96 (s, 1H), 8.79e8.75 (m, 2H), 8.71e8.67 (m, 3H), 8.24e8.21 (m, 3H), 8.01e8.00 (d, J ¼ 8.8 Hz,1H), 7.91e7.86 (m, 2H), 7.82e7.80 (d, J ¼ 7.6 Hz, 1H), 7.70e7.68 (m, 5H), 7.63e7.62 (d, J ¼ 4 Hz, 2H), 7.52e 7.49 (m, 2H) ppm. ESI-MS [M þ H]þ: m/z 487.1539. 4.3. Fabrication of PHOLEDs 4.3.1. Materials NPB as a hole injection layer, TCTA as an HTL, Ir(ppy)3 as a green dopant, Bphen as an HBL as well as an ETL, LiF, and LiQ as an EIL and Al as a cathode were purchased from commercial suppliers and were used without purification. 4.3.2. Device fabrication To fabricate OLED devices, clean glass substrates pre-coated with an 150-nm-thick ITO layer with a sheet resistance of w12 U/ sq were used. Line patterns of ITO were formed on glass by photolithography process. The ITO glass was cleaned by sonification in an isopropylalcohol and acetone, rinsed in deionized water, and finally irradiated in a UV-ozone chamber. All organic materials were deposited by the vacuum evaporation technique under a pressure of w1 107 Torr. The deposition rate of organic layers was about 0.5 A/s. Deposition rates of LiF and Al were 0.1 A/s and 3 A/s, respectively. 4.3.3. Measurements The JeV and LeV data of OLEDs were measured by Keithley 2635A and Minolta CS-100A, respectively. The OLED area was 4 mm2 for all the samples studied in this work. Electroluminescence spectra and CIE coordinate were obtained using a Minolta CS2000A spectroradiometer. Acknowledgments Prof. M. C. Suh was supported by the National Research Foundation of Korea (NRF) (NRF-2011-00006847). Prof. J.-H. Lee was supported by a grant (Catholic Univ.) from the Fundamental R&D program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. High resolution mass spectroscopy and MALDI-TOF was carried out at Korea Basic Science Institute (Mass Spectrometry Research Team). References [1] Kawamura Y, Goushi K, Brooks J, Brown JJ, Sasabe H, Adachi C. 100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films. Appl Phys Lett 2005;86:071104. [2] Sasabe H, Pu YJ, Nakayama K, Kido J. m-Terphenyl-modified carbazole host material for highly efficient blue and green PHOLEDS. Chem Commun 2009: 6655e7. [3] Kim SY, Jang JS, Lee JY. High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer. Appl Phys Lett 2007;90:223505. [4] Krummacher BC, Choong VE, Mathai MK, Choulis SA, Franky. Highly efficient white organic light-emitting diode. Appl Phys Lett 2006;88:113506. [5] King KA, Spellane PJ, Watts RJ. Excited-state properties of a triply orthometalated iridium(III) complex. Am Chem Soc 1985;107:1431. [6] Dedeian K, Djurovich PI, Garces FO, Carlson G, Watts RJ. A new synthetic route to the preparation of a series of strong photoreducing agents: fac tris-orthometalated complexes of iridium(III) with substituted 2-phenylpyridines. Inorg Chem 1991;30:1685. [7] Colombo MG, Brunold TC, Riedener T, Gudel HU, Fortsch M, Burgi HB. Facial tris cyclometalated Rh3þ and Ir3þ complexes: their synthesis, structure, and optical spectroscopic properties. Inorg Chem 1944;33:545. [8] Tokito S, Iijima T, Suzuri Y, Kita H, Tsuzuki T, Sato F. Confinement of triplet energy on phosphorescent molecules for highly-efficient organic bluelightemitting devices. Appl Phys Lett 2003;83:569e71. [9] Watanabe S, Ide N, Kido J. High-efficiency green phosphorescent organic lightemitting devices with chemically doped layers. Jpn J Appl Phys 2007;46:1186.
[10] Lee JH, Wu CI, Liu SW, Huang CA, Chang Y. Mixed host organic light-emitting devices with low driving voltage and long lifetime. Appl Phys Lett 2005;86:103506. [11] Tsai CY, Jou JH. Long-lifetime, high-efficiency white organic light-emitting diodes with mixed host composing double emission layers. Appl Phys Lett 2006;89:243521. [12] Chopra N, Swensen JS, Polikarpov E, Cosimbescu L, Franky. High efficiency and low roll-off blue phosphorescent organic light-emitting devices using mixed host architecture. Appl Phys Lett 2010;97:033304. [13] Hung WY, Chi LC, Chen WJ, Chen YM, Chou SH, Wong KT. A new benzimidazole/carbazole hybrid bipolar material for highly efficient deep-blue electrofluorescence, yellowegreen electrophosphorescence, and two-color-based white OLEDs. J Mater Chem 2010;20:10113e9. [14] Tao Y, Wang Q, Yang C, Wang Q, Zhang Z, Zou T, et al. A simple carbazole/ oxadiazole hybrid molecule: an excellent bipolar host for green and red phosphorescent OLEDs. Angew Chem 2008;120:8224e7. [15] Wong KT, Chen YM, Lin YT, Su HC, Wu CC. Nonconjugated hybrid of carbazole and fluorene: a novel host material for highly efficient green and red phosphorescent OLEDs. Org Lett 2005;7(24):5361e4. [16] Ding J, Gao Jia, Cheng Y, Xie Z, Wang L, Ma D, et al. Highly efficient greenemitting phosphorescent iridium dendrimers based on carbazole dendrons. Adv Funct Mater 2006;16:575e81. [17] Holmes RJ, D’Andrade BW, Forrest SR, Ren X, Li J, Thompson ME. Efficient, deep-blue organic electrophosphorescence by guest charge trapping. Appl Phys Lett 2003;83:3818e20. [18] Ren X, Li J, Holmes RJ, Djurovich PI, Forrest SR, Thompson ME. Ultrahigh energy gap hosts in deep blue organic electrophosphorescent devices. Chemitry Mater 2004;16:4743e7. [19] Lee J, Lee JI, Lee JY, Chu HY. Improved performance of blue phosphorescent organic light-emitting diodes with a mixed host system. Appl Phys Lett 2009;95:253304e13. [20] Chou HH, Cheng CH. A highly efficient universal bipolar host for blue, green, and red phosphorescent OLEDs. Adv Mater 2010;22:2468e71. [21] Jeon SO, Yook KS, Joo CW, Lee JY. High-efficiency deep-blue-phosphorescent organic light-emitting diodes using a phosphine oxide and a phosphine sulfide high-triplet-energy host material with bipolar charge-transport properties. Adv Mater 2010;22:1872e6. [22] Hsu FM, Chien CH, Shih PI, Shu CF. Phosphine-oxide-containing bipolar host material for blue electrophosphorescent devices. Chemitry Mater 2009;21: 1017e22. [23] Chen HF, Yang SJ, Tsai ZH, Hung WY, Wang TC, Wong KT. 1,3,5-Triazine derivatives as new electron transport etype host materials for highly efficient green phosphorescent OLEDs. J Mater Chem 2009;19:8112e8. [24] Jeon WS, Park TJ, Kim SY, Pode R, Jang J. Low roll-off efficiency green phosphorescent organic light-emitting devices with simple double emissive layer structure. Appl Phys Lett 2008;93:063303. [25] Togashi K, Nomura S, Yokoyama N, Yasuda T, Adachi C. Low driving voltage characteristics of triphenylene derivatives as electron transport materials in organic light-emitting diodes. J Mater Chem 2012;22:20689e95. [26] Adam D, Schuhmacher P, Simmerer J, Maubling L, Paulus W, Siemensmeyer K, et al. Photoconductivity in the columnar phases of a glassy discotic twin. Adv Mater 1995;7:276. [27] Simmerer J, Glusen B, Paulus W, Kettner A, Schuhmacher P, Adam D, et al. Transient photoconductivity in a discotic hexagonal plastic crystal. Adv Mater 1996;8:815. [28] Schrögel P, Tomkevi ciene_ A, Strohriegl P, Hoffmann ST, Köhler A, Lennartz C. A series of CBP-derivatives as host materials for blue phosphorescent organic light-emitting diodes. J Mater Chem 2011;21:2266e73. [29] Gong S, He X, Chen Y, Jiang Z, Zhong C, Ma D, et al. Simple CBP isomers with high triplet energies for highly efficient blue electrophosphorescence. J Mater Chem 2012;22:2894e9. [30] Matsushima T, Kinoshita Y, Murata H. Formation of ohmic hole injection by inserting an ultrathin layer of molybdenum trioxide between indium tin oxide and organic holetransporting layers. Appl Phys Lett 2007;91:253504. [31] Matsushima T, Murata H. Observation of space-charge-limited current due to charge generation at interface of molybdenum dioxide and organic layer. Appl Phys Lett 2009;95:203306. [32] Matsushima T, Jin GH, Kanai Y, Yokota T, Kitada S, Kishi T, et al. Interfacial charge transfer and charge generation in organic electronic devices. Org Electron 2011;12:520e8. [33] Liu Z, Salata OV, Male N. Improved electron injection in organic LED with lithium quinolate/aluminium cathode. Synth Met 2002;128:211e4. [34] Jeon WS, Choi JW, Park JS, Yu JH, Suh MC, Kwon JH. The ideal doping concentration in phosphorescent organic light emitting devices. Jpn J Appl Phys 2011;50:061603. [35] Tonzola CJ, Alam MM, Kaminsky W, Jenekhe SA. New n-type organic semiconductors: synthesis, single crystal structures, cyclic voltammetry, photophysics, electron transport, and electroluminescence of a series of diphenylanthrazolines. J Am Chem Soc 2003;125(44):13548e58. [36] Shafiee A, Salleh MM, Yahaya H. Determination of HOMO and LUMO of [6,6]phenyl C61-butyric acid 3-ethylthiophene ester and poly (3-octyl-thiophene2, 5-diyl) through voltametry characterization. Sains Malaysiana 2011;40(2): 173e6. [37] Hwang JH, Kim EG, Liu J, Brédas JL, Duggal A, Kahn A. Photoelectron spectroscopics of the electronic band structure of polyfluorene and fluorenearylamine copolymers at interfaces. J Phys Chem C 2007;111(3):1378e84.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode Nam-Jin Lee b,1, Joon Ho Jeon a,1, Insik In b, Ji-Hoon Lee b, *, Min Chul Suh a, ** a
Department of Information Display and Advanced Display Research Center, Kyung Hee University, Dongdaemoon-Gu, Seoul 130-701, Republic of Korea Department of Polymer Science & Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju-Si, Chungbuk 380-702, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2013 Received in revised form 23 August 2013 Accepted 30 September 2013 Available online 12 October 2013
We report series of triphenylene derivatives with good electronic properties for the use as host materials for green phosphorescent organic light emitting diodes (PHOLEDs). We applied highly planar triphenylene moieties which could provide good electron transport ability and the carbazole or dibenzothiophene moieties which could provide good hole transport ability to obtain a bipolar host materials for green PHOLEDs. From this approach we achieved relatively good current efficiency up to 64 cd/A and external quantum efficiency up to 20.3%, respectively. This was the much more improved value (by w22.3%) compare to that obtained from the 4,40 -N,N0 -dicarbazolebiphenyl system as a reference. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Charge balance Host materials Green phosphorescent OLED Triphenylene Carbazole Dibenzothiophene
1. Introduction Phosphorescent organic light-emitting diodes (PHOLEDs) have attracted intense interest because of their merit of high quantum efficiency as compared to conventional fluorescent OLEDs through utilizing both singlet and triplet excitons for emission [1e4]. In fact, red phosphorescent materials have already been applied in the main-display of commercial mobile phones since 2007. Recently, green emission is almost approaching to the theoretical limitation of efficiency (>20% of external quantum efficiency, EQE) by utilizing various dopants such as fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3] as the phosphorescent emitter, and the commercialization of which has been successfully started [5e9]. However, compared with the great achievements in red phosphorescent materials, green component has many obstacles against commercialization including its relatively short lifetime issue. To overcome such issue, the mixed host system is widely utilized [10e12]. However, many research groups are concentrating on preparation of the host materials which possess bipolar characteristics because the minute change of mixing ratio of hole transport type and electron transport * Corresponding author. Tel.: þ82 43 841 5427; fax: þ82 43 841 5420. ** Corresponding author. Tel.: þ82 2 961 0694; fax: þ82 2 968 6924. E-mail addresses: [email protected] (J.-H. Lee), [email protected] (M.C. Suh). 1 Both authors were equally contributed as a first author to this work. 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.09.046
type host materials could change the resultant device characteristics. For this purpose, many achievements have been reported in this field in recent years [2,13,14]. Especially, the host materials with hole transport type functional groups such as carbazoles or triarylamine moieties combined with electron transport type functional groups such as heterocyclic moieties such as pyridine, triazine, benzimidazole showed great progress in the device performances [15,16]. Aryl silanes [17,18] and phosphine oxides [19e21] have also been utilized to obtain bipolar properties as electron transport type functional groups [22]. As a part of such an effort, the efficiency greater than 20% (EQE) was frequently reported from the green PHOLEDs with such kinds of bipolar host materials [23,24]. In this study, we report three different types of new host materials for green PHOLEDs having triplet energy greater than 2.5 eV with highly planar triphenylene moieties to improve their electron transport ability. The resultant PHOLEDs containing such moieties showed moderate EQE up to 20.3%. 2. Results and discussion 2.1. Materials We prepared three host materials to realize a highly efficient green PHOLED. We selected triphenylene moiety as a basic
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functionality because they are renowned as discotic liquid crystalline molecules for their potential in one-dimensional charge transporting properties. Conductivity along the columns in columnar mesophases has been reported to be several orders of magnitude greater than that in the perpendicular direction. The triphenylene and its derivatives have been known as hole transporting materials although it could also be used as an electron transporting functionality [25] Their photoinduced charge-carrier mobilities ranged from 105 cm2 V1 s1 in the isotropic phase [26] to 102 cm2 V1 s1 in highly ordered discotic mesophases [27] Meanwhile, we changed the numbers of triphenylene moieties which are connected to the central core unit (1,3,5-trisubstituted benzene) which also contains a hole transporting functional groups such as carbazole and/or dibenzothiophene units. In principle, carbazole containing host materials exhibit high triplet energies if they have no extended p-conjugation length [28,29]. In other words, the most common approach to increase a triplet
energy level as a green host might be the disruption of p-conjugation just as in the case of 9,90 -(1,3-Phenylene)bis-9H-carbazole which has two carbazole units connected meta position of a single benzene ring. Scheme 1 shows the schematic diagram of synthetic route to prepare such compounds. Fig. 1 shows the geometries of core parts of those three resultant host materials obtained from the simulation at a DNP/GGA(PBE) level of theory using Dmol3 module (Material Studio 6.1, Accelrys software). The triphenylene moieties were rather twisted to the core benzene unit which could arouse a broken conjugation as we expected and we could utilize those materials as a green host materials (T1 > 2.6 eV). Besides, the molecular orbital distribution of 9,90 -(5-(triphenylen-2-yl)-1,3phenylene)bis(9H-carbazole) (TP-mCP), 9-(3,5-di(triphenylen-2yl)phenyl)-9H-carbazole (DTP-mCP), and 4-(3-(triphenylen-2-yl) phenyl)dibenzo[b,d]thiophene (TP-PDBTh) are also shown in Fig. 1. As we expected from the molecular structure, the electrons in highest occupied molecular orbital (HOMO) was localized on the
Scheme 1. Synthesis of green host materials.
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223
Fig. 1. Geometries of three host materials having twisted core structures.
carbazole and/or dibenzothiophene units, while the electrons in lowest unoccupied molecular orbital (LUMO) was dispersed over the triphenylene and benzene core units. In other words, the hole carriers injected from the hole transport layer (HTL) could move through the carbazole and/or dibenzothiophene units, while the electrons injected from the electron transport layer (ETL) could be transported through the triphenylene and/or central benzene moieties. Fig. 2(a) shows the ultravioletevisible (UVeVis) absorption and photoluminescence spectra of new host materials in chloroform (CHCl3). The band gap (Eg) of TP-PDBTh was slightly larger than those of TP-mCP and DTP-mCP (TP-PDBTh: 3.63 eV, TP-mCP: 3.57 eV, DTP-mCP: 3.49 eV). All three compounds showed vibronic fine structures in their emission spectra, especially at the low temperature (77 K) as shown in Fig. 2(b). The triplet energies (T1) of new host materials which were determined by the first phosphorescence peak from the shorter wavelength region were 2.63, 2.63, 2.64 eV for TP-mCP, DTP-mCP, and TP-PDBTh, respectively, as marked in the spectra as shown in Fig. 2(b). The representative results are summarized in Table 1. The energy levels of HOMO and LUMO were collected from cyclic voltammetry and band edges of UVevisible absorption spectra and they were also summarized in Table 1. Fig. 3 shows the thermal stability behavior of the materials synthesized in this study. All of the compounds having triphenylene moieties showed outstanding thermal stability up to 430 C. The decomposition temperature (Td) of TP-mCP, DTP-mCP, and TPPDBTh were 436, 488, and 445 C, respectively. Very interestingly, the glass transition temperatures (Tg) of TP-mCP and DTP-mCP with carbazole moieties were higher (159 and 179 C) than that of TPPDBTh with dibenzothiophene moiety (108 C). Besides, the melting temperature (Tm) and crystallization temperature (Tc) of the materials were also summarized in Table 1.
2.2. Device characteristics Bipolar character of host materials with proper triplet energy level is very important for the remarkable enhancement of efficiency upon doping. To investigate the bipolar characteristics, we prepared the hole only devices (HODs) as well as electron only devices (EODs) [30e32] of 4,4-N,N0 -Dicarbazole-1,10 -biphenyl (CBP) and newly synthesized host materials by using molybdenum oxide (MoO3) and lithium quinolate (LiQ) as charge carrier injection layers from indium tin oxide (ITO) and aluminum (Al) as follows: HOD A: ITO/MoO3 (0.75 nm)/CBP (200 nm)/MoO3 (10 nm)/Al (100 nm) HOD B: ITO/MoO3 (0.75 nm)/TP-mCP(200 nm)/MoO3 (10 nm)/Al (100 nm) HOD C: ITO/MoO3 (0.75 nm)/DTP-mCP (200 nm)/MoO3 (10 nm)/ Al (100 nm) HOD D: ITO/MoO3 (0.75 nm)/TP-PDBTh(200 nm)/MoO3 (10 nm)/Al (100 nm) EOD A: Al (50 nm)/LiQ (1.5 nm)/CBP(200 nm)/LiQ (1.5 nm)/Al (100 nm) EOD B: Al (50 nm)/LiQ (1.5 nm)/TP-mCP (200 nm)/LiQ (1.5 nm)/ Al (100 nm) EOD C: Al (50 nm)/LiQ (1.5 nm)/DTP-mCP(200 nm)/LiQ (1.5 nm)/ Al (100 nm) EOD D: Al (50 nm)/LiQ (1.5 nm)/TP-PDBTh (200 nm)/LiQ (1.5 nm)/Al (100 nm) Fig. 4(a) and (b) shows the energy band diagram of HODs and EODs of host materials. For the effective ohmic injection of hole carriers into the HOMO of new host materials, we utilized 0.75 nm of MoO3 as a charge injection layer [30]. Meanwhile, we used LiQ/Al to inject the electrons into the LUMO of new host materials
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Fig. 3. TGA of newly synthesized host materials.
Fig. 2. (a) UVevisible absorption and photoluminescence spectra (at 298 K), (b) photoluminescence spectra (at 77 K) of three host materials in 2-methyl tetrahydrofuran.
although newly synthesized materials showed more or less higher lying LUMOs because it has been reported as an effective cathode toward tris(8-hydroxyquinolinato)aluminum [33]. Fig. 5(a) and (b) shows the hole and electron current characteristics obtained from the HODs and EODs prepared with newly synthesized host materials (HOD A w D; EOD A w D). The hole current density was significantly reduced in the order of HOD C (DTP-mCP) > HOD A (CBP) > HOD D (TP-PDBTh) > HOD B (TP-mCP) while the electron current density was slightly reduced in the order of EOD D (TP-PDBTh) > EOD A (CBP) > EOD C (DTP-mCP) > EOD B (TP-mCP). To estimate the charge balance if we use those materials as an emission layer, we calculate the relative charge density by dividing the hole current density by electron current density [e.g. J(hole)/J(electron) where J: current density, mA/cm2] as shown in Fig. 6. From this calculation, we could expect that TP-mCP could give the most desirable device performance because the relative charge density value was very close to unity. In other words, we
could expect that TP-mCP might show best efficiency value. Then the efficiency could be decreased in order of increasing the relative charge density value which may deviated more from the unit value (1) (e.g. TP-PDBTh > CBP > DTP-mCP). To verify the final characteristics as a full devices with new synthetic host materials, we choose well-known standard reference device structure with CBP because it has similar T1 energy value (w2.6 eV) to those obtained from new synthetic host materials prepared in this study [14,34]. Fig. 7 shows the perspective images of the OLED devices fabricated in this study. We used ITO as an anode, N,N0 -Bis(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) as a hole transport layer, 4,40 ,400 -tris(carbazol-9-yl)-triphenylamine (TCTA) as an electron blocking layer, CBP and new materials synthesized in this study as host materials for emission layer (EML) and Ir(ppy)3 as a dopant material for EML, 4,7diphenyl-1,10-phenanthroline (Bphen) as an electron transport layer as well as a hole blocking layer (HBL), lithium fluoride (LiF) as an electron injection layer (EIL), Al as a cathode. The exact device configuration used in this work was as follows (See also Fig. 7): Device A: ITO/NPB (30 nm)/TCTA (10 nm)/CBP: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Device B: ITO/NPB (30 nm)/TCTA (10 nm)/TP-mCP: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Device C: ITO/NPB (30 nm)/TCTA (10 nm)/DTP-mCP: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Device D: ITO/NPB (30 nm)/TCTA (10 nm)/TP-PDBTh: Ir(ppy)3 (8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm) Fig. 8(a) shows the current densityevoltage (JeV) and luminanceevoltage (LeV) characteristics of fabricated green devices and the representative results are summarized in Table 2. At a given constant voltage of 5.0 V, current density values of 1.5, 2.1, 5.5 and 2.7 mA/cm2 were observed in the fabricated Devices A, B, C, and D, respectively. The reason that the DTP-mCP showed the
Table 1 Summary of physical properties of new host materials. Compound
labs [nm]
lpl. [nm] Solution
Film
TP-mCP DTP-mCP TP-PDBTh
245, 273, 293 273 246, 271
375 377 369
391 391 385
T1 (eV) (Exp)
T1 (eV) (Cal)
HOMO [eV] (UPS)
HOMO [eV] (CV)
LUMO [eV]
Eg [eV]
Tg [ C]
Tm[ C]
Tc [ C]
Td [ C]
2.63 2.63 2.64
2.81 2.92 2.97
5.91 5.93 5.95
5.68 5.98 5.84
2.11 2.39 2.21
3.57 3.59 3.63
159 179 108
273 354
267
436 488 445
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225
Fig. 4. Energy band diagrams of used materials in this work.
greatest current density value could be due to the columnar packing of triphenylene moieties with the neighboring molecules which also makes the distances between carbazole moieties very short. This is well consistent with the result of HOD characteristics that the hole current level in the HOD C was the greatest as shown
in Fig. 5(a). On the other hand, the electron current level of EOD C with DTP-mCP was about the same as those of other devices such as EOD A (with CBP) and D (with TP-PDBTh), which means that the triphenylene moiety could be acted as a hole transporter although we designed it as an electron transporting functionality. The driving voltages to reach 1000 cd/m2 were 5.2, 4.8, 4.5, and 4.8 V for the Devices A, B, C, and D, respectively (Table 2). The turn-on voltages of 3.0, 3.0, 3.0, and 3.0 V were observed for the Devices A, B, C, and D, respectively. The current and power efficiency characteristics of fabricated devices are shown in Fig. 8(b) and also summarized in Table 2. At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 46 cd/A and 28 lm/W for the Device A, 64 cd/A and 42 lm/W for the Device B, 48 cd/A and 34 lm/W for the Device C, 65 cd/A and 40 lm/W for the Device D, respectively. These efficiency data correspond to 16.6, 20.3, 15.6, and 18.7% external quantum efficiencies of Devices A, B, C and D, respectively. The maximum current and power efficiencies were 50 cd/A and 28 lm/W for the Device A, 64 cd/A and 49 lm/W for the Device B, 52 cd/A and 51 lm/W for the Device C, 66 cd/A and 40 lm/W for the Device D, respectively. These results are slightly different with our expectation from the calculation of relative current density aforementioned. However, those are well consistent with those of EQE. Thus, TP-mCP and TP-PDBTh have emerged as good candidates for excellent host materials for green PHOLEDs. The normalized EL spectra at a brightness of 1000 cd/m2 of fabricated green devices are shown in Fig. 9. The EL spectra have clear Ir(ppy)3 emission with peak wavelength at 512 nm, 515 nm, 518 nm, 514 nm for the Devices A, B, C, and D, respectively. 3. Conclusion
Fig. 5. (a) Hole current characteristics of HOD A w D and (b) electron current characteristics of EOD A w D.
In conclusion, we prepared new green PHOLEDs with newly synthesized host materials which have bipolar characteristics. The host materials with more triphenylene moieties showed higher hole carrier characteristics although we expected those moiety could have high electron transporting ability. Materials with relative charge density close to unity showed fairly good current efficiency as well as EQE values when it was fabricated as green PHOLEDs. Finally, we found that the TP-mCP and TP-PDBTh could be emerged as good candidates for excellent host materials for green PHOLEDs which could give high EQE up to 20.3% and 18.7%, respectively.
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Fig. 6. Relative charge density in host materials.
4. Experimental 4.1. Instruments 1
H NMR and 13C NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer and chemical shifts were referenced to chloroform (7.26 ppm). UVeVis spectra were recorded with a HEWLETT PACKARD 8453 UVeVis spectrophotometer. Photoluminescence Spectrometer was used PERKIN ELMER LS 50B model. The electrospray ionization (ESI) source was coupled to a hybrid quadrupole orthogonal time-of-flight (Q-TOF) mass spectrometer (SYNAPT G2, Waters, MS Technologies, Manchester, U.K.) was used to mass spectra acquirement in positive- and negative-ion mode. A capillary and cone voltage of 3.0 kV and 30 V, and capillary temperature of 120 C were used for both polarities, respectively. The desolvation source conditions were as follows; for the desolvation gas 800 L/h was used and the desolvation temperature was kept at 600 C. Data acquisition took place over the mass range of m/z 50 to m/z 1200 for MS modes. The sample was introduced into the ESI source at a constant flow rate of 20 ml/min by using an external syringe pump (HARVARD 11Plus, Holliston, MA, USA). Also, another mass measurement was performed using
Fig. 8. (a) JeVeL characteristics of fabricated blue devices. (b) Luminance vs current efficiency and power efficiency characteristics of fabricated blue PHOLEDs.
an UltrafleXtreme MALDI time-of-flight (TOF) mass spectrometer equipped with a pulsed smartbeam II (355 nm Nd:YAG laser, repetition rate 1 kHz) in reflector mode (Bruker Daltonics, Billerica, MA) Thermal properties were measured by differential scanning calorimetry (DSC, TA instruments) under nitrogen atmosphere. We also utilized thermogravimetric analysis (TGA) using TGA-1000 (SINCO) for investigation of thermal stability of the materials. Cyclic voltammetry (CV) [35,36] were measured by a Autolab/PGSTAT 2 model at room temperature in a solution of tetra-nbuthylammonium hexafluorophosphate (0.1 N n-Bu4NPF6) in acetonitrile under nitrogen gas protection at a scan rate of 100 or 50 mV/s. The working electrode was platinum (Pt) disk type and reference electrode was Ag/0.1 M AgNO3. The HOMO energies were also measured with Ultraviolet Photoelectron Spectrometer (UPS) in a Hitachi High Tech AC-2 surface analysis system [37]. 4.2. Synthesis of new host materials
Fig. 7. Schematic diagram of device architecture of green devices fabricated in this study.
4.2.1. Synthesis of 4,4,5,5-tetramethyl-2-(triphenylen-2-yl)-1,3,2dioxaborolane (1) 2-bromotriphenylene (10 g, 32.6 mmol), bis(pinacolato) diborate (9.9 g, 39.1 mmol), [1,10-bis(diphenylphosphino)ferrocene] dichloropalladium(II) (0.5 g, 0.7 mmol), and potassium acetate (16.2 g, 162.8 mmol) were dissolved in anhydrous 1,4-dioxane (180 ml) and refluxed under nitrogen for 24 h. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane-tetrahydrofuran (5:1)
N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228 Table 2 Device characteristics of phosphorescent green OLEDs.
Turn-on voltage (1 cd/m2) Operating voltage (1000 cd/m2) Efficiency (1000 cd/m2) Efficiency (Max) CIE (x,y) (1000 cd/m2) E.Q.E. (1000 cd/m2)
Device A
Device B
Device C
Device D
3V
3V
3V
3V
5.2 V
4.8 V
4.5 V
4.8 V
46 cd/A 28 lm/W 50 cd/A 28 lm/W 0.265, 0.631
64 cd/A 42 lm/W 64 cd/A 49 lm/W 0.279, 0.635
48 cd/A 34 lm/W 52 cd/A 51 lm/W 0.292, 0.634
65 cd/A 40 lm/W 66 cd/A 40 lm/W 0.273, 0.631
16.6%
20.3%
15.6%
18.7%
eluent to afford (1) a white solid, yield 80%. 1H NMR (400 MHz, CDCl3) 9.15 (s, 1H), 8.84e8.82 (m, 1H), 8.71e8.64 (m, 4H), 8.07e8.05 (d, J ¼ 8.0 Hz, 1H), 7.70e7.66 (m, 4H), 1.43 (s, 12H) ppm. 4.2.2. Synthesis of 9,90 -(5-bromo-1,3-phenylene)bis(9H-carbazole) (2) 9H-carbazole (40 g, 240 mmol), 1,3,5-tribromobenzene (39.2 g, 120 mmol), copper(I) iodide (6.83 g, 36 mmol), potassium phosphate (203 g, 96 mmol) and trans-1,2-diaminocyclohexane (5.8 ml, 48 mmol) in 200 ml of anhydrous toluene was stirred at 120 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane eluent to afford (2) a white solid, yield 34%. 1H NMR (400 MHz, CDCl3) 8.16e8.14 (d, J ¼ 7.6 Hz, 4H), 7.87 (s, 2H), 7.8 (s, 1H), 7.57e7.54 (d, J ¼ 8.4 Hz, 4H), 7.49e7.45 (t, J ¼ 7.2 Hz, 4H), 7.36e7.32 (t, J ¼ 7.6 Hz, 4H) ppm. 4.2.3. Synthesis of 9-(3,5-dibromophenyl)-9H-carbazole (3) 9H-carbazole (40 g, 240 mmol), 1,3,5-tribromobenzene (98 g, 300 mmol), copper(I) iodide (6.83 g, 36 mmol), potassium phosphate (203 g, 96 mmol) and trans-1,2-diaminocyclohexane (5.8 ml, 48 mmol) in 300 ml of anhydrous toluene was stirred at 120 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried
Fig. 9. The normalized EL spectra at a brightness of 1000 cd/m2 of fabricated green devices.
227
over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane eluent to afford (3) a white solid, yield 27%. 1H NMR (400 MHz, CDCl3) 8.14e8.12 (d, J ¼ 7.6 Hz, 2H), 7.78 (s, 1H), 7.45 (s, 2H), 7.47e7.41 (m, 4H), 7.34e7.3 (t, J ¼ 8 Hz, 2H) ppm. 4.2.4. Synthesis of 9,90 -(5-(triphenylen-2-yl)-1,3-phenylene)bis(9Hcarbazole) (TP-mCP) 4,4,5,5-tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane (0.65 g, 1.84 mmol), 9,90 -(5-bromo-1,3-phenylene)bis(9H-carbazole) (0.91 g, 1.87 mmol), tetrakis(triphenylphosphine) palladium(0) (0.06 g, 0.06 mmol), and 1 M Na2CO3 (7.3 ml) in 40 ml of Toluene was stirred at 130 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexanetetrahydrofuran (5:1). The final product was obtained as a white powder after purification by vacuum sublimation at a synthetic yield of 38%. 1H NMR (400 MHz, CDCl3) 8.97 (s, 1H), 8.79e8.77 (d, J ¼ 8.8 Hz, 1H), 8.71e8.67 (m, 4H), 8.22e8.20 (d, J ¼ 7.6 Hz, 4H), 8.16 (s, 2H), 8.02e8.00 (d, J ¼ 8.8 Hz, 1H), 7.88 (s, 1H), 7.72e7.62 (m, 8H), 7.52e7.48 (t, J ¼ 8.4 Hz, 4H), 7.37e7.33 (t, J ¼ 6.8 Hz, 4H) ppm; 13C NMR (CDCl3, 400 MHz) 142.5, 138.6, 137.8, 135.5, 128.2, 128, 127.9, 127.7, 127.3, 127.2, 127, 126.2, 125.5, 125.3, 124.2, 123.8, 122.5, 122.2, 122, 121.6, 121.4, 121.3, 121.2, 119.8, 118.5, 118.4 ppm. ESI-MS [M þ H]þ: m/z calcd. 635.2487; found 635.2460. 4.2.5. Synthesis of 9-(3,5-di(triphenylen-2-yl)phenyl)-9H-carbazole (DTP-mCP) 9-(3,5-dibromophenyl)-9H-carbazole (1.8 g, 4.49 mmol), 4,4,5,5tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane (3.34 g, 9.42 mmol), tetrakis(triphenylphosphine) palladium(0) (0.26 g, 0.22 mmol), and 1 M Na2CO3 (19.1 ml) in 140 ml of Toluene was stirred at 130 C for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduce pressure. The crude product was purified by silica gel column chromatography using n-hexane-tetrahydrofuran (5:1). The final product was obtained as a white powder after purification by vacuum sublimation at a synthetic yield of 74%. 1H NMR (400 MHz, CDCl3) 9.02 (s, 2H), 8.83e8.81 (d, J ¼ 8.8 Hz, 2H), 8.78e8.77 (d, J ¼ 7.2 Hz, 2H), 8.74e8.69 (m, 6H), 8.34 (s, 1H), 8.25e8.23 (d, J ¼ 7.6 Hz, 2H), 8.10e8.08 (d, J ¼ 5.6 Hz, 4H), 7.72e7.65 (m, 10H), 7.52e7.48 (t, J ¼ 7.2 Hz, 2H), 7.38-7.34 (t, J ¼ 7.6 Hz, 2H) ppm. MALDITOF [M þ H]þ: 695.329. 4.2.6. Synthesis of 4-(3-(triphenylen-2-yl)phenyl)dibenzo[b,d] thiophene (TP-PDBTh) 1-bromo-3-iodobenzene (2.0 g, 7.07 mmol), 4,4,5,5-tetramethyl2-(triphenylen-2-yl)-1,3,2-dioxaborolane (2.5 g, 7.07 mmol), tetrakis(triphenylphosphine)palladium(0) (0.25 g, 0.21 mmol), and 1 M Na2CO3 (35.4 ml) in 100 ml of toluene was stirred at 90 C for 48 h under nitrogen atmosphere. To the reaction mixture was added a dibenzo[b,d]thiophene-4-ylboronic acid (1.61 g, 7.07 mmol) and then stirred at 120 C for 24 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed three times with distilled water and extracted with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane-tetrahydrofuran (5:1). The final product was obtained as a white powder after purification by
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vacuum sublimation at a synthetic yield of 46%. 1H NMR (400 MHz, CDCl3) 8.96 (s, 1H), 8.79e8.75 (m, 2H), 8.71e8.67 (m, 3H), 8.24e8.21 (m, 3H), 8.01e8.00 (d, J ¼ 8.8 Hz,1H), 7.91e7.86 (m, 2H), 7.82e7.80 (d, J ¼ 7.6 Hz, 1H), 7.70e7.68 (m, 5H), 7.63e7.62 (d, J ¼ 4 Hz, 2H), 7.52e 7.49 (m, 2H) ppm. ESI-MS [M þ H]þ: m/z 487.1539. 4.3. Fabrication of PHOLEDs 4.3.1. Materials NPB as a hole injection layer, TCTA as an HTL, Ir(ppy)3 as a green dopant, Bphen as an HBL as well as an ETL, LiF, and LiQ as an EIL and Al as a cathode were purchased from commercial suppliers and were used without purification. 4.3.2. Device fabrication To fabricate OLED devices, clean glass substrates pre-coated with an 150-nm-thick ITO layer with a sheet resistance of w12 U/ sq were used. Line patterns of ITO were formed on glass by photolithography process. The ITO glass was cleaned by sonification in an isopropylalcohol and acetone, rinsed in deionized water, and finally irradiated in a UV-ozone chamber. All organic materials were deposited by the vacuum evaporation technique under a pressure of w1 107 Torr. The deposition rate of organic layers was about 0.5 A/s. Deposition rates of LiF and Al were 0.1 A/s and 3 A/s, respectively. 4.3.3. Measurements The JeV and LeV data of OLEDs were measured by Keithley 2635A and Minolta CS-100A, respectively. The OLED area was 4 mm2 for all the samples studied in this work. Electroluminescence spectra and CIE coordinate were obtained using a Minolta CS2000A spectroradiometer. Acknowledgments Prof. M. C. Suh was supported by the National Research Foundation of Korea (NRF) (NRF-2011-00006847). Prof. J.-H. Lee was supported by a grant (Catholic Univ.) from the Fundamental R&D program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. High resolution mass spectroscopy and MALDI-TOF was carried out at Korea Basic Science Institute (Mass Spectrometry Research Team). References [1] Kawamura Y, Goushi K, Brooks J, Brown JJ, Sasabe H, Adachi C. 100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films. Appl Phys Lett 2005;86:071104. [2] Sasabe H, Pu YJ, Nakayama K, Kido J. m-Terphenyl-modified carbazole host material for highly efficient blue and green PHOLEDS. Chem Commun 2009: 6655e7. [3] Kim SY, Jang JS, Lee JY. High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer. Appl Phys Lett 2007;90:223505. [4] Krummacher BC, Choong VE, Mathai MK, Choulis SA, Franky. Highly efficient white organic light-emitting diode. Appl Phys Lett 2006;88:113506. [5] King KA, Spellane PJ, Watts RJ. Excited-state properties of a triply orthometalated iridium(III) complex. Am Chem Soc 1985;107:1431. [6] Dedeian K, Djurovich PI, Garces FO, Carlson G, Watts RJ. A new synthetic route to the preparation of a series of strong photoreducing agents: fac tris-orthometalated complexes of iridium(III) with substituted 2-phenylpyridines. Inorg Chem 1991;30:1685. [7] Colombo MG, Brunold TC, Riedener T, Gudel HU, Fortsch M, Burgi HB. Facial tris cyclometalated Rh3þ and Ir3þ complexes: their synthesis, structure, and optical spectroscopic properties. Inorg Chem 1944;33:545. [8] Tokito S, Iijima T, Suzuri Y, Kita H, Tsuzuki T, Sato F. Confinement of triplet energy on phosphorescent molecules for highly-efficient organic bluelightemitting devices. Appl Phys Lett 2003;83:569e71. [9] Watanabe S, Ide N, Kido J. High-efficiency green phosphorescent organic lightemitting devices with chemically doped layers. Jpn J Appl Phys 2007;46:1186.
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