Highly Efficient New Hole Injection Materials for OLEDs Based on

Mar 3, 2011 - LCD R&D Center, Samsung Electronics Co. ... New hole injection layer (HIL) materials for organic light-emitting diodes (OLEDs) based on ...
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Highly Efficient New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine and Phenoxazine Derivatives Youngil Park,† Beomjin Kim,† Changjun Lee,† Aeran Hyun,† Sanghee Jang,‡ Ji-Hoon Lee,§ Yeong-Soon Gal,|| Tae Hyung Kim,^ Kyoung-Soo Kim,^ and Jongwook Park†,* †

Department of Chemistry, Catholic University of Korea, Bucheon, 420-743, Korea LCD R&D Center, Samsung Electronics Co. Ltd., Youngin, 446-711, Korea § Department of Polymer Science and Engineering, Chungju National University, Chungju, 380-702, Korea Polymer Chemistry Laboratory, College of Engineering, Kyungil University, Gyeongsan 712-701, Gyeongsangbuk-Do, Korea ^ Doosan R&D Center, Doosan Electronics Co. Ltd., Yongin 449-795, Korea

)

‡

bS Supporting Information ABSTRACT:

New hole injection layer (HIL) materials for organic light-emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of the synthesized materials were examined by UV-vis and photoluminescence spectroscopy, and by cyclic voltammetry. 10,100 -bis(4-tert-butylphenyl)-N7,N70 -di(naphthalen-1-yl)-N7,N70 -diphenyl-10H,100 H3,30 -biphenoxazine-7,70 -diamine (1-PNA-BPBPOX) showed glass transition temperatures (Tg) of 161 C, which was higher than that (110 C) of Tris(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2-TNATA), a commercial HIL material. The HOMO levels of the synthesized materials were 4.9-4.8 eV, indicating a good match between the HOMO of indium tin oxide (ITO) (4.8 eV) and the HOMO of N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) (5.4 eV), a common hole transfer layer (HTL) material. Because the synthesized materials showed minimal absorption at wavelengths shorter than 450 nm, they have good potential for use as effective HIL materials. The synthesized materials were used as the HIL in OLED devices, yielding power efficiencies of 2.8 lm/W (1-PNA-BPBPOX) and 2.1 lm/W (2-TNATA). These results indicate that 1-PNA-BPBPOX yields a higher power efficiency, by a factor of 33%, than the 2-TNATA commercial HIL material. Also, 1-PNA-BPBPOX exhibited a longer device lifetime than 2-TNATA.

’ INTRODUCTION Organic light emitting diodes (OLEDs) are optoelectronic devices based on the photoluminescent properties of π-conjugated organic materials, and are useful for lighting applications and next-generation flat panel displays.1-5 Multilayer devices that use small molecules can be produced through deposition methods to achieve high energy conversion efficiencies. The layers in a multilayered system comprising small molecules include hole injection/transport layers (HIL/HTL), emitting layers (EML), and electron injection/transport layers (EIL/ ETL). Therefore, development of materials with characteristics r 2011 American Chemical Society

that are optimized for each layer is an important objective for OLED development. Good HIL materials used in OLEDs have the following common properties: (1) the highest occupied molecular orbital (HOMO) level should fall between the HOMO levels of the ITO and HTL materials; (2) absorption of red, blue, and green (RGB) emission from the EML should be minimized; (3) high glass Received: September 13, 2010 Revised: January 24, 2011 Published: March 03, 2011 4843

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Scheme 1. Chemical Structures of New Phenothiazine and Phenoxazine Materials and 2-TNATA

transition temperatures (Tg) confer better device endurance with respect to stability and degradation by Joule heat generation during device driving.6-8 Recently, starburst amine 4,40 ,400 -Tris(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2TNATA), 4,40 ,400 -Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA), or copper phthalocyanine (CuPc), acting as the HIL, were inserted between the HTL and the transparent anode to improve device durability.7 Unfortunately, CuPc slightly absorbed blue and red light, and hence is unsuitable for use in full color displays. m-MTDATA had the disadvantage of a low Tg. Crystallization and melting of amorphous organic materials caused by Joule heat generation are common causes of device degradation. In principle, all organic layers that form electroluminescence (EL) devices should have a Tg that is as high as possible. The individual layer with the lowest Tg limits the thermal stability of the OLED.6,9 Therefore, in this work, new alternative hole injection materials based on dimeric phenothiazine and phenoxazine derivatives were synthesized in an attempt to address the above problem. Phenothiazine and phenoxazine have been mainly developed as p-type or ambipolar materials that can transport hole and/or electron in organic photovoltaic (OPV) devices and organic thinfilm transistors (OTFT) in the field of optoelectronics.7-25 To fundamentally strengthen the transfer of electrons to the basic moieties of phenothiazine and phenoxazine using hole transfer characteristics and to produce an amorphous structure with an optimal HOMO energy level, the bulky aromatic amine groups were substituted and four new HIL materials 1-PNA-BPBP, 2-PNA-BPBP, 1-PNA-BPBPOX, and 1-PNA-BPBPOX were synthesized as shown in Scheme 1.

The electro-optical and thermal properties of the synthesized compounds, based on dimeric phenothiazine and phenoxazine, were compared by measuring the UV-visible (UV-vis) and photoluminescence (PL) spectra, cyclic voltammetry (CV), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) analysis. Furthermore, the HIL properties of phenothiazine and phenoxazine derivatives incorporated into an EL device were compared with those of the commercial HIL material 2-TNATA as well as OLED device lifetime.

’ EXPERIMENTAL SECTION General Method. 1H NMR spectra were recorded using Bruker Advance 300 and 500 instruments, fast atom bombardment (FAB) mass spectra were recorded using a JEOL, JMSAX505WA, HP5890 series II. The optical absorption spectra were obtained using an HP 8453 UV-vis-NIR spectrometer. A Perkin-Elmer luminescence spectrometer LS50 (Xenon flash tube) was used for photoluminescence (PL) and EL) spectroscopy. The melting temperatures (Tm), Tg, crystallization temperatures (Tc), and degradation temperatures (Td) of the compounds were measured by DSC under a nitrogen atmosphere using a DSC2910 (TA Instruments), and TGA was performed using an SDP-TGA2960 (TA Instruments). The redox potentials of the compounds were determined by CV using an AUTOLAB/PG-STAT128N model system with a scan rate of 100 mV/s. The working electrode was formed by depositing the synthesized materials onto ITO substrates, Ag/ AgNO3 in acetonitrile (AN) as reference electrode, Pt wire as counter electrode, and 0.1 M tetrabutylammonium perchlorate 4844

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Scheme 2. Synthetic Routes of New Hole Injection Materials

(TBAP) in acetonitrile as the electrolyte. All potentials are quoted vs the ferrocene/ferrocenium (Fc/Fcþ) couple as the internal standard. The band gap was estimated from the tailing edges of UV-vis spectra. From the oxidation potential, HOMO energy level was derived by assuming that the energy level of ferrocene/ferrocenium (Fc/Fcþ) is 4.8 eV. Finally, their LUMO energy level was derived from HOMO energy level and the band gap. Atomic force microscopy (AFM) imaging was performed in air using a PicoScan system (Molecular Imaging) equipped with a 5 6 5 mm scanner. Magnetic-ac (Mac) mode (a noncontact mode) was used for all of the AFM images. OLED devices were fabricated with the following structure: ITO/2-TNATA or synthesized material (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm), where 2-TNATA and the synthesized material formed the HILs, N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) formed the HTL, 8-hydroxyquinoline aluminum (Alq3) formed the ETL and EL, lithium fluoride (LiF) formed the EIL, ITO formed the anode, and Al formed the cathode. The organic layer was vacuum-deposited by thermal evaporation at a vacuum base pressure of 10-6 Torr and a rate of deposition of 1 Å/s to give an emitting area of 4 mm2; the Al layer was continuously deposited under the same vacuum conditions. The current-voltage (I-V) characteristics of the fabricated OLED devices were obtained using a Keithley 2400 electrometer. Light intensity was measured using a Minolta CS-1000A. The operational stability of the devices was measured with encapsulation in glovebox. The device lifetime was recorded by using Polaronix OLED Lifetime Test System of McScience company.

General Syntheses of New Materials. The materials were synthesized by a Suzuki aryl-aryl coupling reaction in the presence of a Pd catalyst. A typical synthetic procedure was as follows: To 7,70 -dibromo-10,100 -bis-(4-tert-butyl-phenyl)10H,100 H-[3,30 ]biphenothiazinyl (0.5 g, 0.6 mmol) and phenyl-4-yl-naphthalen-1-yl-amine (0.41 g, 1.4 mmol) in a 250 mL round-bottomed flask under a nitrogen atmosphere were added Pd2(dba)3 (0.04 g, 0.04 mmol), sodium tert-butoxide (0.40 g, 4.1 mmol), and toluene. The temperature was increased to 110 C. Stirring was continued at this temperature, and the reaction was monitored by TLC. When the reaction was complete, the product was extracted with water and toluene. The organic extract was dried over MgSO4, filtered, and the solvent was removed in vacuo. The resulting crude mixture was passed through a short column of silica using THF as the eluent, and the product was recrystallized from THF to obtain a yellow solid. Synthesis of 10,100 -Bis(4-tert-butylphenyl)-N7,N70 -di(naphthalen1-yl)-N7,N70 -diphenyl-10H,100 H-3,30 -biphenothiazine-7,70 -diamine (1-PNA-BPBP). The yield was 63%. 1H NMR(500 MHz, THF-d8): δ (ppm) 1.37(s, 18H), 6.04-6.05(d, 2H), 6.17-6.18(d, 2H), 6.49-6.51(d, 2H), 6.79-6.85(m, 8H), 6.94-6.96(d, 2H), 7.097.10(m, 6H), 7.24-7.25(d, 2H), 7.29-7.32(m, 6H), 7.41-7.44(m, 4H), 7.63-7.64(d, 4H), 7.73-7.75(d, 2H), 7.85-7.87(d, 2H), 7.90-7.92(d, 2H). 13C NMR(300 MHz, THF-d8): δ (ppm) 152.2, 149.9, 144.6, 144.5, 140.6, 139.7, 136.6, 135.0, 132.4, 131.2, 129.8, 129.3, 128.5, 127.7, 127.2, 127.1, 126.9, 125.3, 125.1, 124.9, 122.6, 122.1, 121.8,121.6, 121,2, 117.6, 117.0, 114.2, 35.5, 31.8. FT-IR (KBr pellet, cm-1): 3037, 2959, 1593, 1508, 1491, 1461, 1392, 1301, 1267, 4845

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The Journal of Physical Chemistry C 1235, 803, 775, 694. HRMS: m/z 1094.4406 (Mþ, calcd 1094.4416). Anal. Calcd for C76H62N4S2: C, 83.33; H, 5.70; N, 5.11; S, 5.85. Found: C, 83.36; H, 5.71; N, 5.09; S; 5.88. Synthesis of 10,100 -Bis(4-tert-butylphenyl)-N7,N70 -di(naphthalen2-yl)-N7,N70 -diphenyl-10H,100 H-3,30 -biphenothiazine-7,70 -diamine (2-PNA-BPBP). The yield was 65%. 1H NMR(500 MHz, THFd8): δ (ppm) 1.39(s, 18H), 6.14-6.16(d, 2H), 6.18-6.20(d, 2H), 6.60-6.62(d, 2H), 6.82(d, 2H), 6.95-7.00(m, 4H), 7.037.05(d, 4H), 7.15(s, 2H), 7.19-7.22(m, 6H), 7.27(t, 2H), 7.327.36(m, 8H), 7.56-7.58(d, 2H), 7.66-7.69(m, 8H), 13C NMR(300 MHz, THF-d8): δ (ppm) 152.4, 148.9, 146.5, 146.1, 144.6, 143.8, 14.5, 139.6, 135.8, 135.1, 131.3, 131.2, 13.0.1, 129.7, 128.6, 128.4, 127.8, 127.0, 125.4, 125.1, 124.5, 123.5, 12.3, 120.2, 117.6, 117.1, 35.6, 31.8. FT-IR (KBr pellet, cm-1): 3055, 2959, 1629, 1592, 1507, 1491, 1461, 1365, 1301, 1267, 1240, 810, 746, 697. HRMS: m/z 1094.4435(Mþ, calcd 1094.4416). Anal. Calcd for C76H62N4S2: C, 83.33; H, 5.70; N, 5.11; S, 5.85. Found: C, 83.28; H, 5.79; N, 5.04; S; 5.82. Synthesis of 10,100 -Bis(4-tert-butylphenyl)-N7,N70 -di(naphthalen1-yl)-N7,N70 -diphenyl-10H,100 H-3,30 -biphenoxazine-7,70 -diamine (1-PNA-BPBPOX). The yield was 37%. 1H NMR(500 MHz, THFd8): δ (ppm) 1.36(s, 18H), 5.77-5.79(d, 2H), 5.89-5.90(d, 2H), 6.29-6.31(d, 2H), 6.44(s, 2H), 6.68(d, 2H), 6.70(d, 2H), 6.80(t, 2H), 6.84-6.86(d, 4H), 7.08-7.11(t, 4H), 7.25-7.27(d, 6H), 7.33(t, 2H), 7.41-7.44(m, 4H), 7.62-7.64(d, 4H), 7.737.75(d, 2H), 7.85-7.87(d, 2H), 7.92-7.94(d, 2H) 13C NMR(300 MHz, THF-d8): δ (ppm) 152.5, 150.0, 145.4, 144.8, 144.6, 143.5, 137.6, 136.6, 134.3, 134.1, 132.4, 131.0, 130.8, 129.8, 129.3, 128.8, 127.6, 127.1, 126.9, 125.2, 121.7, 121.5, 118.8, 114.6, 114.3, 113.5, 111.9, 35.6, 31.8. FT-IR (KBr pellet, cm-1): 3040, 2958, 1594, 1483, 1392, 1321, 1266, 1242, 798, 771, 693. HRMS: m/z 1062.4874(Mþ, calcd 1062.4873). Anal. Calcd for C76H62N4O2: C, 85.84; H, 5.88; N, 5.27; S, 3.01. Found: C, 85.82; H, 5.83; N, 5.21; S; 3.09. Synthesis of 10,100 -Bis(4-tert-butylphenyl)-N7,N70 -di(naphthalen2-yl)-N7,N70 -diphenyl-10H,100 H-3,30 -biphenoxazine-7,70 -diamine (2-PNA-BPBPOX). The yield was 44%. 1H NMR (500 MHz, THF-d8) δ (ppm): 1.37(s, 18H), 5.87-5.88(d, 2H), 5.915.93(d, 2H), 6.37-6.39(d, 2H), 6.49(s, 2H), 6.72-6.74(d, 2H), 6.77(s, 2H), 6.93-6.96(t, 2H), 7.05-7.07(d, 4H), 7.217.22(m, 6H), 7.30-7.32(m, 8H), 7.36(s, 2H), 7.57-7.58(d, 2H), 7.65-7.71(m, 8H), 13C NMR(300 MHz, THF-d8): δ (ppm) 152.6, 148.9, 146.5, 145.6, 144.9, 142.7, 137.6, 135.8, 134.3, 134.2, 131.8, 131.1, 131.0, 130.0, 129.6, 128.9, 128.3, 127.8, 127.0, 125.0, 124.5, 123.4, 121.6, 121.1, 120.1, 114.8, 114.4, 114.0, 113.6, 35.6, 31.8. FT-IR (KBr pellet, cm-1): 3053, 2959, 1628, 1593, 1485, 1386, 1338, 1270, 801, 745, 694. HRMS: m/z 1062.4886(Mþ, calcd 1062.4873). Anal. Calcd for C76H62N4O2: C, 85.84; H, 5.88; N, 5.27; S, 3.01. Found: C, 85.80; H, 5.86; N, 5.26; S; 3.07.

’ RESULTS AND DISCUSSION The new materials 1-PNA-BPBP, 2-PNA-BPBP, 1-PNABPBPOX, and 2-PNA-BPBPOX, which are based on the dimeric phenothiazine and phenoxazine moieties, were synthesized by C-C and C-N coupling reactions using a palladium catalyst, as shown in Scheme 2. The synthesized material was purified on a silica column and recrystallized to yield a pure solid material. The chemical structure of the synthetic material was confirmed by NMR, mass analyses.

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Table 1. Thermal Properties of the Synthesized Materialsa Tm (C)

Td (C)

1-PNA-BPBP

335

455

2-PNA-BPBP

330

447

259

489

compounds

1-PNA-BPBPOX 2-PNA-BPBPOX

Tg (C)

161

481

a

Tg: glass transition temperature, Tc: crystallization temperature, Tm: melting-point temperature, Td: decomposition temperature (5% weight loss).

Figure 1. UV-vis absorption spectra (a) in benzene solution state (b) in film state; 1-PNA-BPBP (9), 2-PNA-BPBP (0), 1-PNA-BPBPOX (b), and 2-PNA-BPBPOX (O).

DSC measurements yielded a Tg of 161 C for 1-PNABPBPOX as shown in Table 1 and Figure S2 of the Supporting Information. The value was higher than that (110 C) of 2-TNATA, a commercial HIL material. Devices using the synthesized materials are expected to yield longer lifetimes under Joule heating due to a Tg that is higher than that of 2-TNATA.9 Figures 1, S1 of the Supporting Information, and Table 2 show the summarized results of the UV-vis, PL spectra, and CV for the synthesized materials in the film state. The UV-vis absorption maxima for the synthesized materials in the solution state were 300, 315, 371, and 382 nm for 1-PNA-BPBP, 2-PNA-BPBP, 1-PNA-BPBPOX, and 2-PNA-BPBPOX, respectively, and the maxima of the PL spectrum were 473, 473, 492, and 447 nm. 4846

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Table 2. Optical Properties of the Synthesized Materials film on glass

solutiona compounds

Eoxonset

HOMOb

LUMOc

band gapd

477

0.12

4.92

2.04

2.88

475

0.15

4.95

2.01

2.94

374

488

0.06

4.86

2.04

2.82

381

465

0.09

4.89

2.07

2.82

UVmax (nm)

PLmax (nm)

UVmax (nm)

1-PNA-BPBP

300

473

300

2-PNA-BPBP

315

473

309

1-PNA-BPBPOX

371

492

2-PNA-BPBPOX

382

447

PLmax (nm)

Benzene solution (1  10-5 M). b From the oxidation potential, HOMO energy level was derived by assuming that the energy level of ferrocene/ ferrocenium (Fc/Fcþ) is 4.8 eV. c LUMO energy level was determined from the HOMO level and the optical band gap. d The optical band gap was derived from the absorption edge of the thin film. a

Figure 2. Cyclic voltammetry 0.1 M TBAP in acetonitrile; 1-PNABPBP (9), 2-PNA-BPBP (0), 1-PNA-BPBPOX (b), and 2-PNABPBPOX (O).

The UV-vis absorption maxima were 300, 309, 374, and 381 nm for 1-PNA-BPBP, 2-PNA-BPBP, 1-PNA-BPBPOX, and 2-PNA-BPBPOX, respectively, in the film state. The UV-vis spectra and the absorption edges of these materials displayed virtually no changes on going from the solution to the film state. Additionally, because the synthesized materials in the film state showed almost no absorption at wavelengths longer than 450 nm, the synthesized materials are appropriate HIL materials due to their transparency properties. The maxima of the PL spectrum for 1-PNA-BPBP, 2-PNABPBP, 1-PNA-BPBPOX, and 2-PNA-BPBPOX were 477, 475, 488, and 465 nm in the film state (see Figure S1 of the Supporting Information). According to the maximum values, when the sulfur atom is changed into oxygen atom in 1-naphtyl pendant group such as 1-PNA-BPBP and 1-PNA-BPBPOX, PL was red-shifted 11 nm. However, in 2-naphtyl pendant group such as 2-PNABPBP and 2-PNA-BPBPOX, there was blue-shifted 10 nm. The reason for this is not clearly explained. However, it might be due to the different link positions of 1- and 2- as well as different sizse and inductive effects from different atoms.26,27 CV was performed over the range -1.25 to 1.50 V, and the four compounds displayed normal reversible oxidation and reduction waves as shown in Figure 2. The area corresponding to oxidation in the CV curve was larger than that for the reduction step, suggesting that these compounds favored the cationic state, consistent with a p-type material.

Figure 3. J-V-L characteristics for the EL devices: ITO/2-TNATA or synthesized materials (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm); 1-PNA-BPBP (0, 9), 2-PNA-BPBP (O, b), 1-PNA-BPBPOX (4, 2), 2-PNA-BPBPOX (), (), and 2-TNATA (3, 1).

On the basis of these results, the HOMO and LUMO energy levels were calculated from the CV and optical band gap measurements (see Table 2).28,29 From the oxidation potential, the HOMO energy level was derived by assuming that the energy level of ferrocene/ferrocenium (Fc/Fcþ) is 4.8 eV. The LUMO energy level was determined from the HOMO level and the optical band gap. The optical band gap was derived by analyzing the absorption edge with a plot of (hν) vs (Rhν)2, where R, h, and ν are absorbance, Plank’s constant, and the frequency of light. The HOMO levels for the synthesized phenothiazine and phenoxazine materials were 4.86-4.95 eV, intermediate between the HOMO of ITO (4.8 eV) and that of NPB (5.4 eV), the HTL material. Thus, the HOMO levels of the new materials were well-matched with the required HOMO level of HIL. OLED devices were fabricated using the synthesized materials as the HIL, and the EL characteristics were examined using a device configuration of ITO/2-TNATA or the synthesized material (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF (1 nm)/ Al (200 nm). The results are summarized in Table 2. As shown in the J-V-L graph of Figure 3 and Table 3, the operating voltages of the EL devices decreased in the following order: phenothiazine derivatives > 2-TNATA > phenoxazine derivatives. 1-PNA-BPBP and 2-PNA-BPBP exhibited similar luminance-voltage results with that of 2-TNATA. In particular, the 1-PNA-BPBPOX device showed a lower operating voltage 4847

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Table 3. EL Performances of Synthesized Material: ITO/ 2-TNATA or Synthesized Material (60 nm)/NPB (15 nm)/ Alq3 (70 nm)/LiF (1 nm)/Al (200 nm) at 10 mA/cm2 operating compounds

luminance

voltage (V) efficiency (cd/A) power efficiency (lm/W)

1-PNA-BPBP

7.2

4.7

2.2

2-PNA-BPBP

7.4

4.6

2.2

1-PNA-BPBPOX

5.7

4.7

2.8

2-PNA-BPBPOX 2-TNATA

5.8 6.3

4.4 4.1

2.7 2.1

Figure 5. Operational lifetime of the device: ITO/2-TNATA (dashed line) or 1-PNA-BPBPOX (line) (60 nm)/NPB (15 nm)/Alq3 (70 nm)/ LiF (1 nm)/Al (200 nm) at 5000 cd/m2.

Figure 4. I-V characteristics of hole only device: ITO/2-TNATA or synthesized materials (100 nm)/Au (200 nm); 1-PNA-BPBP (9), 2-PNA-BPBP (b), 1-PNA-BPBPOX (2), 2-PNA-BPBPOX ((), and 2-TNATA (1).

of 5.7 V than that of the 6.3 V needed for the 2-TNATA device at 10 mA/cm2. The power efficiencies with respect to the operating voltage of the phenoxazine materials were improved to be as high as 2.8 and 2.7 lm/W for 1-PNA-BPBPOX and 2-PNA-BPBPOX, respectively. In particular, 1-PNA-BPBPOX showed a power efficiency that exceeded that of 2-TNATA, which is a commercial HIL material with high performance, by a factor of 33%. No marked relation was found between the HOMO level and operating voltage in the series of compounds studied because of their very similar HOMO values. The superior property of 1-PNA-BPBPOX is not clearly understood, but it might be due to the different hole transporting properties of these HILs. That is, the hole transporting property is probably a crucial determinant of the operating voltage and power efficiency, especially when the HOMO level of the HIL is situated in an appropriate position between the energy levels of ITO and NPB as a hole transporting material. In order to explain the result, hole-only devices [ITO/2TNATA or synthesized materials (100 nm)/Au (200 nm)] were fabricated. It is very difficult to inject and transport the electron into the device, because the work function of gold (Au) is as large as 5.1 eV and there is a big gap between the LUMO level of organic layers and the metal Fermi level. Thus, the device current is considered to be driven mainly by hole carriers, which are injected from the ITO to the organic layer. As shown in Figure 4, the phenoxazine derivatives exhibited much lower turn-on voltages and left-shifted J-V curves compared to 2-TNATA

and the phenothiazine derivatives. These J-V characteristics of hole-only devices were well matched with the J-V-L results of normal OLED devices (phenoxazine derivatives > 2-TNATA > phenothiazine derivatives). In this additional result, the current comes from only the holes injected from ITO, and it means that phenoxazine derivatives are superior to 2-TNATA and phenothiazine derivatives in terms of hole transport. On the basis of the high Tg and high power efficiency of phenoxazine derivatives, these materials are expected to have much longer lifetime in OLED devices. In order to test the device lifetimes compared with a newly synthesized HILs and 2-TNATA as a reference, we fabricated both nondoped and doped emitting OLED devices. Figure 5 showed the lifetime data of nondoped green emitting devices using each 1-PNA-BPBPOX or commercial 2-TNATA as an HIL: ITO/HIL (60 nm)/NPB (15 nm)/ Alq3 (70 nm)/LiF (1 nm)/Al (200 nm). In Figure 5, the lifetime of 1-PNA-BPBPOX (99 h) was about three times higher than that of 2-TNATA (30 h) at 5000 cd/m2. Also, the C545T-doped green emitting device was prepared as ITO/HIL (100 nm)/NPB (15 nm)/Alq3 þ 2% C545T (30 nm)/Alq3 (25 nm)/LiF (1 nm)/Al (200 nm). In this case, 2-TNATA had a 220-h lifetime and 1-PNA-BPBPOX showed a 290-h lifetime at 5000 cd/m2. 1-PNA-BPBPOX, which has the best efficiency of OLED device in this series of compounds and high Tg exhibited highly improved OLED lifetime than 2-TNATA. This result can be explained by much lower operating voltage and smaller current density than that of 1-PNA-BPBPOX under the same high brightness of 5000 cd/m2: it is attributed to a better hole injection property of 1-PNA-BPBPOX than that of 2-TNATA. Also we believe that the thermal properties of the films used in OLEDs are strongly associated with device lifetimes because of Joule heating during device operation. Therefore, the changes in the morphology of 2-TNATA and the synthesized materials that result from heating and storage were determined by depositing them onto ITO glass in vacuum and monitoring the surfaces of the films with atomic force microscopy (AFM). To investigate the morphological stabilities of 1-PNA-BPBPOX and 2-TNATA, we prepared thin films (60 nm) of these materials by vapor deposition on ITO glass substrates, and determined their surface morphologies before and after annealing at 110 C for 3 days 4848

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Figure 6. AFM images of 1-PNA-BPBPOX and 2-TNATA: (a) immediately following deposition, (b) following storage at room temperature under nitrogen for 3 days, and (c) following storage at 110 C under nitrogen for 3 days.

under a nitrogen atmosphere by using AFM. Figure 6 shows the surface immediately after vacuum deposition (Figure 6a), after storage for 3 days at room temperature under N2 (Figure 6b), and after storage for 3 days at 110 C under nitrogen (Figure 6c). Immediately after deposition, the rms roughnesses of 1-PNABPBPOX and 2-TNATA were 2.4 and 1.1 nm, respectively. The stability of the deposited films was assessed by exposing the film to N2 and room temperature for 3 days, at which time the rms was not changed relative to the rms immediately after deposition (Figure 6b). The 2-TNATA film had an rms value of 43.5 nm after 3 days at 110 C under nitrogen (Figure 6c), which was about 40 times higher than that immediately after vapor deposition. For 1-PNA-BPBPOX, almost no damage to the film was evident (rms = 1.2 nm). Consequently, devices using 1-PNA-BPBPOX as the HIL would be expected to have a longer lifetime than those using 2-TNATA.

’ CONCLUSIONS In this work, new hole injection materials for OLEDs based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of the synthesized materials were examined by UV-vis and PL spectral and CV measurements. The HOMO levels of the synthesized materials were 4.9-4.8 eV for the phenothiazine and phenoxazine moieties, respectively, which match well with the HOMO levels of ITO (4.8 eV) and NPB (5.4 eV), a commercial HTL material. Judging from the fact that the synthesized materials showed minimal absorption at wavelengths shorter than 450 nm, the synthesized materials may be effective HIL materials. OLED devices fabricated using the synthesized materials as the HIL yielded a power efficiency of 2.8 lm/W for 1-PNABPBPOX, which was higher than that of 2-TNATA, 2.1 lm/W. These results indicated that 1-PNA-BPBPOX demonstrated an

excellent power efficiency, which provided an improvement of nearly 33% over 2-TNATA, a commercial HIL material. Furthermore, 1-PNA-BPBPOX, which has the best efficiency of OLED device and high Tg, exhibited much longer OLED lifetime than 2-TNATA.

’ ASSOCIATED CONTENT

bS

Supporting Information. 1H, 13C NMR data for 1-, 2-PNA-BPBP and 1-, 2-PNA-BPBPOX. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ82-11-9759-8485. Fax: þ82-2-2164-4764. E-mail: hahapark@ catholic.ac.kr.

’ ACKNOWLEDGMENT This research 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. This work was supported by a Midcareer Researcher Program through an NRF grant funded by the MEST (No. 2009008019920100000422).This work was supported by the Technology Innovation Program (New Growth of Technology 2009-2011) funded by the Ministry of Knowledge Economy (MKE, Korea). ’ REFERENCES (1) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Sun, Y. R.; Giebink, N.; Kanno, C.; Ma, B.; Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908. 4849

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