Research Article www.acsami.org
Synthesis of Soluble Host Materials for Highly Efficient Red Phosphorescent Organic Light-Emitting Diodes Min Chul Suh,*,† So-Ra Park,† Ye Ram Cho,† Dong Heon Shin,† Pil-Gu Kang,† Dong A Ahn,† Hyung Suk Kim,† and Chul-Bae Kim‡ †
Organic Electronic Materials Laboratory, Department of Information Display, Kyung Hee University, Dongdaemoon-Gu, Seoul 02447, Republic of Korea ‡ Central Research Institute, KISCO, 69, Yangcheon-ro 75-gil, Gangseo-gu, Seoul 07537, Republic of Korea S Supporting Information *
ABSTRACT: New soluble host materials with benzocarbazole and triphenyltriazine moieties, 11-[3-(4,6-diphenyl[1,3,5]triazin-2-yl)-phenyl]-11H-benzo[a]carbazole and 11[3′-(4,6-diphenyl-[1,3,5]triazin-2-yl)-biphenyl-4-yl]-11Hbenzo[a]carbazole, were synthesized for highly efficient red phosphorescent organic light-emitting diodes (PHOLED). Hole-transporting benzocarbazole moiety and electron transporting triphenyltriazine moiety, which are severely twisted each other enhance the solubility of those materials in common organic solvent. The improved solubility from this molecular design could be due to a reduced π−π stacking interaction, which gives a very uniform film morphology after spin coating of those materials. As a result, we obtained highly efficient soluble PHOLEDs combined with an evaporated blue common layer structure. The resultant red PHOLED exhibited the maximum current efficiency as well as external quantum efficiency values up to 23.7 cd/A and 19.0%. KEYWORDS: organic light-emitting diodes, cross-linked HTL, solution processed red PHOLED, blue common layer, relative current density
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INTRODUCTION Active-matrix organic light-emitting diodes (AMOLEDs) have attracted continuous attention as a next generation flat panel display device followed by active-matrix liquid-crystal displays (AMLCDs) because they are thinner, lighter, more vivid, and more power efficient than AMLCDs. 1−4 Nevertheless, AMOLEDs are facing with a difficulty to realize a mass production of high resolution as well as large area displays. In addition, a difficulty of reducing the manufacturing costs of AMOLEDs is also very important hurdle to make it as a dominant display mode for wider applications. Thus, many research groups have been developing next generation process to overcome those problematic issues by utilizing a solution processes, such as spin coating, inkjet printing, or nozzle printing instead of vacuum thermal evaporation process.5−7 Unfortunately, those processes aforementioned have lots of critical limitations (e.g., short lifetime) which make them hard to be applied to TV application. Nevertheless, most of the panel makers have started to develop printing technology because the performances of solution processed red and green emitters have almost met the commercial requirements for TV applications even though it is very hard to obtain blue emitters, yet. However, we could realize a full color AMOLED TV sets having competetive stability by utilizing a hybrid structure with thermally evaporated blue common layer (BCL) on top of solution processed red and green layers. © 2016 American Chemical Society
In principle, there have been two main synthetic approaches to obtain highly efficient emitters for solution processed OLED. The first approach is to use a polymeric emitters such as polyfluorene, poly(p-phenylenevinylene), etc., with flexible spacers as a side group, while the other approach is to utilize a small molecular emitter containing charge transport units and alkyl moieties to improve the solubility in common organic solvents. Recently, many research groups are more focusing on the development of small molecular emitting materials because the polymeric species often cause problems related to the poor reproducibility of their properties in each batch because it is too difficult to obtain an exactly same molecular weight of such polymers on each occasion. The clogging of printhead nozzle is also a serious problem when we use the polymer as an emitting material with high viscosity. In contrast, we can reproduce the same molecules with exactly same properties if we use the small molecular species although the poor solubility of small molecular species which were already qualified in the mass production with thermal evaporation process may be a representative obstacle for mass production for soluble process. To overcome those problems, the materials with short alkyl chains or short alkoxy chains have been developed by many Received: March 17, 2016 Accepted: June 14, 2016 Published: June 14, 2016 18256
DOI: 10.1021/acsami.6b03281 ACS Appl. Mater. Interfaces 2016, 8, 18256−18265
Research Article
ACS Applied Materials & Interfaces
obtained by ultraviolet−visible (UV−vis) spectrophotometer (JASCO V-570). Photoluminescence (PL) specta were collected by PerkinElmer photoluminescence spectrophotometer (LS 55 model). The surface morphology such as surface roughness was investigated by atomic force microscope (AFM, XE-100, Park system). The differential scanning calorimetry (DSC) measurements were performed on PerkinElmer model−DSC 4000. Low and high resolution mass spectra were recorded using a LC-mass (Shimadzu LCMS-2020). HOMO (highest occupied molecular orbital) was measured with a photoelectron spectrometer (AC2, Riken Keiki). Carbon, hydrogen, and nitrogen analysis was conducted on an elemental analyzer (Flash1112/Flash2000, CE Instrument, Italy). General Procedure for Synthesis of Soluble Host Materials. 2-(3-Bromophenyl)-4,6-diphenyl-1,3,5-triazine (1). To a solution of 3-bromobenzaldehyde (10.0 g, 54.4 mmol) and benzamidine hydrochloride (16.97 g, 108.8 mmol) in DMSO (54 mL) was added Cs2CO3 (35.45 g, 108.8 mmol), and the reaction mixture was stirred at 90 °C for 12 h. The reaction mixture was cooled to room temperature, and then pure into water (54 mL) and extracted with dichloromethane (108 mL). The organic layers were separated, dried with anhydrous MgSO4, and concentrated under reduced pressure. The crude product was purified by recrystallization from acetone to give (1) as a white powder (11.54 g, 55%): 1H NMR (500 MHz, CDCl3) δ 7.37 (t, J = 8.0 Hz, 1H), 7.51−7.59 (m, 6H), 7.67 (d, J = 8.0 Hz, 1H), 8.61 (d, J = 8.0 Hz, 1H), 8.68 (d, J = 7.1 Hz, 2H), 8.80 (s, 1H) ppm; 13C NMR (125 MHz, CDCl3) 122.8, 127.4, 128.6, 128.9, 130.0, 131.7, 32.6, 135.2, 135.8, 138.2, 170.2, 171.6 ppm; HRMS (EI, 70 eV) calcd for C21H14BrN3 (M+) 387.0371, found 387.0432; Anal. Calcd for C21H14BrN3 C 64.96, H 3.63, N 10.82; found C 64.91, H 3.61, N 10.85. 11-[3-(4,6-Diphenyl-[1,3,5]triazin-2-yl)-phenyl]-11H-benzo[a]carbazole (TRZ-PBC). To a solution of 1 (3.88 g, 10.0 mmol), 11Hbenzo[a]carbazole (2.28 g, 10.5 mmol), sodium tert-butoxide (1.92 g, 20.0 mmol), and bis(dibenzylideneacetone)palladium(0) (0.17 g, 0.3 mmol) in dry toluene (80 mL) was added 1.0 M in toluene tri-tertbutylphosphine (0.3 mL), and the resulting mixture was stirred at refluxing temperature for 48 h under a nitrogen atmosphere. After it was cooled to room temperature, ethyl acetate (80 mL) and water (40 mL) was added to the reaction mixture. The organic layer was filtered through a bed of silica gel. The combined filtrate was evaporated under reduced pressure. The crude product was purified by recrystallization from toluene to give 11-[3-(4,6-diphenyl-[1,3,5]triazin-2-yl)-phenyl]11H-benzo[a]carbazole (TRZ-PBC) (3.83g, 73%) as a white powder: 1 H NMR (500 MHz, CDCl3) δ 7.18 (t, J = 6.9 Hz, 1H), 7.25 (d, J = 6.9 Hz, 1H), 7.36−7.40 (m, 3H), 7.49−7.56 (m, 5H), 7.56−7.58 (m, 2H), 7.72−7.76 (m, 2H), 7.85 (t, J = 8.0 Hz, 1H), 7.99 (d, J = 8.0 Hz, 1H), 8.23 (d, J = 6.8 Hz, 1H), 8.26 (d, J = 8.8 Hz, 1H), 8.71−8.72 (m, 4H), 9.00 (t, J = 1.8 Hz, 1H), 9.05 (dd, J = 8.8, 0.9 Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3) 110.4, 119.1, 119.6, 120.4, 121.4, 122.0, 122.2, 123.5, 124.8, 125.1, 125.2, 128.6, 129.0, 129.2, 129.3, 129.4, 130.5, 132.7, 133.1, 133.5, 135.5, 135.8, 138.6, 140.6, 142.0, 170.7, 171.9 ppm; HRMS (EI, 70 eV) calcd for C37H24N4, 524.2001; found, 524.2048; Anal. Calcd for C37H24N4 C 84.71, H 4.61, N 10.68; found C 84.69, H 4.60, N 10.65. 2-(4′-Chloro[1,1′-biphenyl]-3-yl)-4,6-diphenyl-1,3,5-triazine (2). To a solution of 2 (3.88 g, 10.0 mmol), 1-chlorophenylboronic acid (1.64 g, 10.5 mmol) and tetrakis(triphenylphosphine) palladium (0.35 g, 0.3 mmol) in tetrahydrofuran (40 mL) was added 2 M aq Na2CO3 (10 mL), under nitrogen atmosphere, and the reaction mixture was stirred at reflux temperature for 5 h. The reaction mixture was cooled to room temperature, and then pure into water (40 mL) and extracted with dichloromethane (40 mL). The organic layers were separated, dried with anhydrous MgSO4, and concentrated under reduced pressure. The crude product was purified by recrystallization from toluene to give 1 (3.49 g, 83%) as a white powder. 1H NMR (400 MHz, CDCl3) δ 7.47 (d, J = 8.1 Hz, 2H), 7.55−7.66 (m, 9H), 7.77 (d, J = 7.6 Hz, 1H), 8.76−8.77 (m, 5H), 8.93 (s, 1H) ppm; 13C NMR (100 MHz, CDCl3) 127.7, 128.4, 128.8, 128.9, 129.2, 129.3, 129.4, 131.2, 132.8, 134.0, 136.4, 137.2, 139.5, 140.7, 171.7, 172.0 ppm; HRMS (EI, 70 eV) calcd for C27H18ClN3, 419.9120; found, 419.9144;
research groups because those chains normally help to improve the solubility of such molecules by increasing the free volumes inside such molecules.8 However, the introduction of such alkyl chains generally deteriorates the long-term stability, so-called, lifetime although the chain length of alkyl group is very short. Thus, there are many other methodologies to enhance the solubility of the organic light emitting matters. For example, molecular design to give asymmetric structure, star-shaped configuration, and meta-linked molecular structure also improves the solubility of the small molecular organic species. Besides, the proper device engineering is very important to maximize device characteristics. A representative way to achieve enhanced performances of OLED devices is to realize a good charge balance by using various types of layers such as hole injection layer (HIL), hole transport layers (HTL), electron blocking layer (EBL), emitting layers (EMLs), hole blocking layer (HBL), electron transport layer (ETL), electron injection layer (EIL), etc.9−15 However, it is really hard to obtain the highly efficient and stable solution processed devices with perfect charge balance because a multilayer stacking of the functional layers aforementioned is too difficult. The biggest hurdle is a lack of the materials with orthogonal solubility, which typically causes a mixing of the polymeric or small molecular interfaces between functional layers during consecutive soluble processes.16−19 Hence, in most cases, hydrophilic HIL [e.g., PEDOT [a poly(3,4-ethylene dioxythiophene)) doped with poly(styrene p-sulfonate) (PSS), polyaniline (PA) doped with PSS, etc.] and hydrophobic cross-linkable HTL with orthogonal solubility are utilized for solution processed OLED fabrication.20−22 Then, hydrophobic EML is successively spin-casted or printed on the cross-linked HTL. After all of those solution processes, ETL, EIL, and cathode is continuously deposited by thermal evaporation process. From this approach, we can obtain comparable device performances for red and green phosphorescent OLEDs (PHOLEDs). However, there are still lots of difficulties to achieve reliable and practical performances for the blue OLEDs. To overcome such an issue, “advanced hybrid device structure” has been suggested by Matsumoto et al., which utilizes BCL on the patterned red and green EMLs.23−25 In other words, the hybrid process (e.g., solution + evaporation) architecture to obtain highly efficient solution processed phosphorescent OLEDs was proposed as follows: anode/HIL (solution process, polymers)/ HTL (solution process, polymers or small molecules with cross-link units)/red or green EML (solution process, polymers or small molecules)/bipolar exciton blocking layer (B-EBL, vacuum deposition, small molecules)/blue EML (vacuum deposition, small molecules)/ETL (vacuum deposition, small molecules)/cathode. Hereat, the B-EBL plays very important role to suppress the triplet exciton migration toward blue EML which could result in side emission originated from the triplet− triplet annihilation inside blue EML as established in the previous reports.26−28 In this Research Article, we report highly efficient solutionprocessed red PHOLEDs by using new bipolar host materials with relatively good solubility without any alkyl group. We applied hybrid structure with BCL to improve the device performances as explained before.
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EXPERIMENTAL SECTION
Instruments. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 500 MHz NMR spectrometer and chemical shifts were referenced to chloroform-d (7.26 ppm). The absorption spectra were 18257
DOI: 10.1021/acsami.6b03281 ACS Appl. Mater. Interfaces 2016, 8, 18256−18265
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthetic Methodology for (a) TRZ-PBC and (b) TRZ-BBCa
a
TRZ-PBC and TRZ-BBC are utilized as host materials for highly efficient red PHOLEDs. (TPBI) as an ETL, lithium fluoride (LiF) as the materials for an EIL, and aluminum (Al) as a cathode were also purchased from the commercial suppliers and were used without purification. Device Fabrication. To fabricate solution processed OLED devices, a 150 nm thick patterned indium−tin oxide (ITO) glasses with an open emission area of 4 mm2 were used. The ITO glasses were cleaned in acetone and isopropyl alcohol with sonication process and rinsed in deionized water. Then, ITO glass substrates were treated in UV− ozone to eliminate all the organic impurity remained during previous fabrication processes. PEDOT:PSS was spin-coated on ITO glass in ambient condition and annealed at 120 °C for 15 min in nitrogen atmosphere. Subsequently, HTL material dissolved in chlorobenzene by 0.5 wt % was spin-coated and cross-linked by standard process. In case of EML materials, TRZ-PBC and RD were dissolved in toluene to give 1 and 0.5 wt % solutions while TRZ-BBC were dissolved in chlorobenzene to give 1 wt % solution. A red EML solution obtained by mixing of host and dopant solutions was spin coated and dried at 100 °C for 10 min inside glovebox. Then, B-EBL, blue EML, and TPBI were thermally deposited by the rate of 0.5 Å/s under typical vacuum condition (
