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Triplet bipolar host materials for solution processed organic light-emitting devices
Organic Electronics 11 (2010) 1624–1630
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Triplet bipolar host materials for solution processed organic light-emitting devices Hyoung-Yun Oh a,b, Chandramouli Kulshreshtha c, Jang Hyuk Kwon c,*, Seonghoon Lee a,** a
Molecular Electronics and NanoStructures Lab, School of Chemistry, NS60, Seoul National University, Shillim-dong, San 56-1, Seoul 151-747, Republic of Korea LG Display R&D Center, 1007 Deongeun-ri, Wollong-myeon, Paju-si, Gyeonggi-do 413-811, Republic of Korea c Department of Information Display, Kyung Hee University, Dongdaemoon-ku, Seoul 130-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 1 March 2010 Received in revised form 5 June 2010 Accepted 7 July 2010 Available online 10 August 2010 Keywords: OLED Soluble Red Host Phosphorescent
a b s t r a c t Two solution-processible new triplet bipolar host molecules, 4,40 -bis[N-(1-naphthyl)-N(3pyridinylamino]biphenyl (NPyB) and 4,40 -bis [N-(3-quinolinyl)-N-phenylamino]biphenyl (QuPB) were synthesized and characterized for phosphorescent organic light-emitting diodes (PHOLEDs). They exhibit good triplet energies as 2.3–2.4 eV for red PHOLEDs and show good amorphous film characteristics with high solubility, allowing a solution processing. The NPyB and QuPB hosts were evaluated for soluble red PHOLEDs with ITO/PEDOT:PSS/Ir complex:host/TPBI/LiF/Al structure. A maximum light output of 14,440 cd/m2, driving voltage of 5.9 V, external quantum efficiency of 14.2%, and maximum current/ power efficiencies of 21.0 cd/A and 11.3 lm/W have been achieved by NPyB host and bis(2-phenylquinoline(acetylacetonate)iridium (Ir(phq)2acac) red dopant. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have become more mature in recent decades and their applications now include a variety of materials with superior properties for low power consumption displays, lightings and printed electronics industry [1,2]. Thermal vacuum deposition process has been in use for commercial OLED products, owing to high purity and good device performance as the advantages. Thermal vacuum deposition allows complicated device structure with improved performance, utilizing a fine shadow mask but its disadvantages include high cost, high resolution, and scalability of the method to very large mother glass formats. In contrast, solution processing such as spin coating or ink-jet printing overcomes these disadvantages for low-cost by reducing material consumption and equipment cost compared to incumbent vapor deposi-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.H. Kwon), [email protected] (S. Lee).
tion processes [3]. Despite the poor solubility and crystallization issues of polymeric and small molecule materials, efforts have been aggrandized to improve the device performance of solution processed OLEDs. Over the years, attempts have been limited to polymeric and/or small molecule solution-processed phosphorescent materials [4,5]. Use of high-efficiency phosphorescent OLED (PHOLED) technology is one of the most efficient strategy employed to achieve 100% internal quantum efficiency [6–8]. In this regard, iridium containing transition–metal complexes as dopant materials are the most promising candidates [9]. For instance, we have demonstrated promising and efficient red PHOLEDs using small molecule mixed host system [10]. It is important that charge transporting host should have higher triplet excited state energy (T1) than emitter guest due to T1 energy confinement in the guest. As a consequence, polymer materials are not suitable generally as they have low T1 energy and high band gap due to considerably high exchange energy, 1.0–1.5 eV values, between the singlet and triplet states [11]. To resolve this problem in solution processing PHOLEDs, small molecules
1566-1199/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.07.005
H.-Y. Oh et al. / Organic Electronics 11 (2010) 1624–1630
were considered ideal for host–guest relationship but their crystallization issues and non-uniform film thickness have aggravated as additional problems. Generally, most of the host materials either transport holes or electrons in a device but unbalanced hole and electron currents makes the recombination zone narrower, thereby shifting it either close to hole transport layer (HTL) or electron transport layer (ETL). Thus narrower charge recombination zone in PHOLEDs are prone to triplet–triplet exciton quenching due to local high density of triplet excitons and also long diffusion of the triplet excitons [12,13]. Therefore, bipolar small molecule hosts can meet the requirements of balancing the charge density for making high performance simple device structure. For improving the charge-transferring capability of bipolar molecules, it is desirable to localize the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) level at their respective hole and electron transporting moieties. To the best of our knowledge very few devices based on solution process bipolar small molecules have been reported so far [14–16]. Typically, N,N0 -diphenyl-N,N0 -bis(1-naphthyl)(1,10 -biphenyl)-4,40 diamine (NPB) has bipolar characteristics in OLEDs with both hole and electron transporting property. It has 5.4 eV HOMO and 2.3 eV LUMO values and a low triplet energy (2.3 eV) [17]. For practical use of the host in PHOLEDs, its LUMO energy is too high compared with general host materials even though its electron mobility is good as 9 104 cm2/vs [18]. In solution process devices, it forms non-uniform surface with high roughness due to crystallization of its molecules upon thermal treatment [19]. Therefore, NPB was reported as a hole-type host in the mixed host system [10]. In order to reduce the electron-injection barrier and enhancing the electron mobility of NPB molecule, we report, NPB modified two novel bipolar molecules, 4,40 -bis[N-(1-naphthyl)-N-(3-pyridinylamino]biphenyl (NPyB) and 4,40 -bis [N-(3-quinolinyl)-Nphenylamino]biphenyl (QuPB) with their synthetic method and material characteristics. We also employed NPyB and QuPB host materials for fabricating high-efficiency electrophosphorescent solution processed red OLEDs. The multilayered electrophosphorescent OLEDs fabricated from 1,2-dichlorobenzene solution containing spin-coated NPyB and QuPB hosts blended with a dopant, bis(2-phenylquinoline(acetylacetonate)iridium (Ir(phq)2acac) emitted red phosphorescent light. The bipolar NPyB and QuBP hosts show good solubility and stability in 1,2-dicholobenzene solvent, uniform film growth characteristics without any crystallization issues, and good electron and hole-transporting properties. A luminance of 14,440 cd/m2 at 8.4 V and luminous efficiency of 21.0 cd/A and 11.3 lm/W at 5.9 V are demonstrated in the NPyB host device.
2. Experimental details 2.1. Material synthesis The compounds NPyB and QuPB were synthesized according to the procedure shown in Fig. 1. All reagents and solvents were purchased from Aldrich chemical Co.
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and Fluka. Other reagents in the scheme were used as from commercial sources. 2.1.1. 4,40 -Bis[N-(1-naphthyl)-N-(3-pyridinylamino]biphenyl (NPyB) To a solid mixture of 1-naphthyl-3-pyridinylamine (3.0 g, 13.6 mmol) and 4,40 -dibromobiphenyl (2.0 g, 6.5 mmol), a solution of 50 ml toluene was added. It was degassed by bubbling nitrogen for 30 min. Additionally, 0.2 mmol of tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3], 0.4 mmol of tri-tert-butylphosphine, and 20 mmol of sodium-tert-butoxide was added to the mixture. The mixture was vigorously refluxed under nitrogen for 24 h. When the reaction mixture was cooled down to room temperature, 30 ml of water was added to remove some salts. After separating the water, the solution was dried by MgSO4, and then concentrated under reduced pressure. The mixture was purified by column chromatography on silica gel eluting with a 1:2 mixture of ethylacetate and hexane. By removing the eluting solvent, light yellow powder was obtained (2.5 g, 65%). Tm 241 °C, Tg 93 °C, dH (500 MHz; CDCl3; Me4Si) 8.38 (2H, d, J 2.6), 8.16 (2H, d, J 4.5), 7.90 (4H, J 8.5), 7.49 (4H, m), 7.37 (8H, m), 7.28 (2H, d, J 8.7), 7.11 (6H, m). Elemental Analysis Calc. for C42H30N4: C, 85.40; H, 5.12; N, 9.48%. Found: C, 85.54; H, 5.04; N, 9.47%. M/S (m/z, M+). Found: 591.2543, Calc.: 591.2549. 2.1.2. 4,40 -Bis [N-(3-quinolinyl)-N-phenylamino]biphenyl (QuPB) The synthetic procedure of QuPB is same except using 3-quinolinyl-phenylamine (3.0 g, 13.6 mmol) instead of 1-naphthyl-3-pyridinylamine. Yellow powder was obtained (2.3 g, 60%). Tm 186 °C, Tg 94 °C. dH (500 MHz; CDCl3; Me4Si) 8.79 (2H, d, J 2.5), 8.03 (2H, d, J 8.3), 7.70 (2H, d, J 2.2), 7.59 (4H, m), 7.52 (4H, d, J 8.6), 7.46 (2H, t, J 7.7), 7.32 (4H, t, J 7.9), 7.20 (8H, d, J 8.4), 7.13 (2H, t, J 7.3). Elemental Analysis Calc. for C42H30N4: C, 85.40; H, 5.12; N, 9.48%. Found: C, 85.49; H, 5.11; N, 9.45%. M/S: (m/z, M+). Found: 591.2551, Calc.: 591.2549. 2.2. OLED fabrication and measurements OLED devices were fabricated on the indium–tinoxide (ITO) glass having an emission area of 2 mm 2 mm with a sheet resistance of 12 X/square. A line pattern of ITO was formed by the photolithography process. Before coating organic layers, the patterned ITO glasses were cleaned by sonification in isopropyl alcohol (IPA) and acetone, and then rinsed in deionized water; finally it was irradiated in a UV-ozone chamber. A 45 nm poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spincoated on the ITO substrates and then dried using a hot plate at 120 °C for 10 min to remove the solvent. A 40 nm thick emissive host system i.e. NPyB and QuPB doped with Ir(phq)2acac in 1,2-dichlorobenzene was later spin-coated on the PEDOT:PSS layer. The spin-coated emissive layer was then baked on a hot plate at 120 °C for 10 min. The spin coating and baking process were carried out in a glove box with nitrogen ambient. Subsequently, 30 nm TPBI was
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Fig. 1. Synthesis procedure of NPyB and QuPB.
deposited in an organic chamber using vacuum thermal evaporation at a base pressure of 107 Torr, while LiF (1 nm) and Al (100 nm) were deposited in an adjacent metal chamber without breaking the system vacuum. Current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the fabricated OLEDs were measured using a Keithley SMU 2635 and Konika Minolta CS100A. Also, the electroluminescence (EL) spectra and CIE color coordinates were obtained using a Konika Minolta CS-1000A spectroradiometer. For optical measurements, UV–Visible absorption spectra of NPyB in dichloromethane solution were recorded with a Shimadzu 3100 UV–VIS–NIR spectrophotometer. In case of photoluminescence (PL) measurement, emission spectrum of NPyB and QuPB in dichloromethane solution, and deposited thin films on glass substrate were measured with a Jasco FP-6500 spectroflurometer. Low temperature emission spectra of NPyB and QuPB solutions were also recorded using liquid nitrogen at 77 K with Jasco FP-6500 spectroflurometer. For investigation of smoothness and homogenous surface of emissive layer, atomic force microscopy (AFM) was done. Cyclic voltammetry (CV) was performed using EC epsilon electrochemical analysis equipment. Platinum wire and ITO film on glass were used as counter and working electrodes, respectively. Tetrabutylammonium perchlorate (Bu4NClO4) is used as a supporting electrolyte. Ag wire in 0.1 M AgNO3 was used as a reference electrode. By means of internal ferrocenium/ferrocene (Fe+/Fe) standard, the potential values were converted to the saturated calomel electrode (SCE) scale. The position of LUMO of each material is estimated from the optical band gap determined from the absorption onset of the spectra. The films that had been coated with the material were made on ITO glass through the solution drop coating and then dried in a vacuum oven at 80 °C. Solid-state electrochemistry was performed at a scan rate of 150 mV/s and start potential voltage have supplied from 0.5 eV for reducing potential damage. 3. Results and discussion The chemical structure and synthetic route of 4,40 bis[N-(1-naphthyl)-N-(3-pyridinylamino]biphenyl (NPyB)
and 4,40 -bis [N-(3-quinolinyl)-N-phenylamino]biphenyl (QuPB) are shown in Fig. 1. The molecular structures of NPyB and QuPB were confirmed by 1H NMR, mass spectrometry, and elemental analysis. Thermal properties of NPyB and QuPB were investigated by means of differential scanning calorimetry (DSC). A glass-transition temperature (Tg) was obtained from the second heating scan of the glassy samples cooled after the first heating up to melting temperature (Tm). The heating rate was 10 °C/min. The Tm’s of NPyB and QuPB appeared at 241 and 186 °C, respectively. The crystallization temperature (Tc) of NPyB was observed about 180 °C by the second heating but no Tc of QuPB was found, which indicates that QuPB has more glass-like property. Considering the melting point of NPB about 280 °C, intermolecular dipole interaction is reduced in the order of NPB, NPyB, and QuPB. However, the glasstransition temperatures of both the molecules are around 93–95 °C which is about same as that of commercialized NPB (95 °C) [20]. NPyB and QuPB can readily dissolve in common organic solvents, such as 1,2-dichloromethane, 1,2-dichlorobenzene, toluene which enables the preparation of uniform thin film by solution processing such as spin coating. AFM studies of NPyB and QuPB host materials with spin coating in 1,2-dichlorobenzene solution were carried out to investigate the surface morphology of thin films. As shown in Fig. 2, root-mean-square-roughness (rms) values are 1.90 Å and 1.89 Å for NPyB and QuPB. These results shows that both the host films were smooth and homogeneous without any crystallization issues by implementing the host compatibility with dopant material used in fabricating solution-processible devices. The bipolar host materials, NPyB and QuPB having pyridine and quinoline units as electron transport moieties lead to bipolar charge transport property for both holes and electrons. The photophysical properties of NPyB and QuPB host materials were measured using UV–visible absorption and photoluminescence spectra as depicted in Fig. 3 and Table 1. The absorption and emission spectra of both the host materials were carried out in dilute dichloromethane solution. The absorption spectrum of NPyB has one absorption band from 305 to 360 nm which could be from p–p* transitions of the conjugated p-electron system.
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Fig. 2. AFM image of host (a) NPyB and (b) QuPB.
Fig. 3. UV spectra in dichloromethane (a) NPyB, (b) QuPB, and PL spectra in dichloromethane, (c) NPyB solution, (d) NPyB thin film, (e) NPyB low temperature (77 K), (f) QuPB solution, (g) QuPB thin film and (h) QuPB low temperature (77 K).
QuPB exhibits four vibronic absorption bands centered at 284, 320, 345 and 390 nm, respectively. The PL emission maximas of NPyB and QuPB in solution were observed around 442 and 510 nm, respectively. The PL emission spectra of NPyB and QuPB thin films were slightly blue shifted by 6 and 36 nm, respectively. The low temperature phosphorescent spectrum of the NPyB host, measured in 2methyltetrahydrofuran (THF) solution at 77 K, revealed the shifting of main peak at 405 nm while a phosphorescent sub-band of two peaks with peak positions, 520 and 558 nm could also be noticed. Similarly, QuPB at 77 K shows the main peak at 446 nm and phosphorescent subbands with peak positions at 527 and 572 nm. From the phosphorescent spectra, triplet energies of 2.4 and 2.3 eV were calculated from 520 and 527 nm first peaks of NPyB
and QuPB materials. It confirms that their triplet energies are in good agreement in transferring excitons from the bipolar hosts to Ir(phq)2acac dopant. In order to make appropriate comparisons of triplet energies, energy transfer experiments from the red dopant to NPyB and QuPB hosts were carried out in 1,2-dichlorobenzene solution. When NPyB was added to Ir(phq)2acac solution at different concentrations, the quantum efficiency did not change. However, addition of QuPB has decreased the quantum efficiency, significantly. These results indicate that NPyB triplet energy is higher than the red dopant while QuPB has a triplet energy similar to Ir(phq)2acac dopant. In order to obtain more accurate results, energy transfer rate of QuPB was calculated using Stern–Volmer plots and it came out 3 109 s1M1, which is similar to diffusion rate of
Table 1 Physical properties of NPyB and QuPB.
a b
Compounds
kabsa (nm)
kPLb (nm) sol./film/77 K
Tm, Tg (°C)
HOMO/LUMOcalc De (eV)
HOMO/LUMOexp De (eV)
T1calc/exp (eV)
NPyB QuPB NPB
340 284, 320, 345, 390 350
442/436/405,520 510/476/445,527 467/438/418,528
241, 93 186, 94 277, 95
4.4, 2.2, 2.0 4.4, 2.3, 2.3 4.2, 2.0, 2.2
5.5, 2.4, 3.1 5.5, 2.7, 2.8 5.4, 2.3, 3.1
2.0/2.4 1.8/2.3 2.1/2.3
Absorption maximum. PL maxima, Tm – melting temperature, Tg – glass-transition temperature.
4.5 5.3 8751 (8.6 V) 7.2, 4.5 7.2, 4.2 0.60, 0.38 598
NPB:TPBI (7%) QuPB (7%) QuPB (5%)
4.9 5.5 7714 (8.0 V) 7.8, 4.5 7.6, 3.3 0.60, 0.38 598 5.1 6.3 7888 (8.2 V) 6.9, 3.3 6.7, 3.3 0.60, 0.38 598
QuPB (3%) NPyB (3%)
4.3 6.1 14,190 (9.2 V) 12.8, 5.8 11.6, 5.9 0.59, 0.39 596 Turn-on voltage (at 1 cd/m2) Operating voltage (at 1000 cd/m2) Luminance (max.) at voltage Maximum efficiency (cd/A , lm/W) Efficiency (cd/A, lm/W at 1000 cd/m2) CIE (x, y) at 1000 cd/m2) Emission peak (nm)
NPyB (2%) NPyB (1%)
5.1 5.9 12,820 (8.4 V) 17.9, 9.21 17.6, 9.1 0.57, 0.39 593
Doping ratios of Ir(phq)2acac
Table 2 A summary of device performance as a function of doping concentration in NPyB and QuPB hosts.
5.4 6.3 19,250 (9.2 V) 11.9, 5.8 11.9, 5.8 0.61, 0.38 600
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5.3 5.9 14,400 (8.8 V) 21.0, 11.3 20.6, 10.8 0.59, 0.39 595
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organic solvents. However, NPyB does not reach to such value. This proves that NPyB has higher triplet energy than Ir(phq)2acac and can be used with red dopant effectively. In order to investigate the electrochemical properties of NPyB and QuPB, cyclic voltammetry experiments were performed to know HOMO levels. Table 1 summarizes the calculated and experimentally obtained energy levels values. The host materials showed reversible oxidation and reduction waves, indicating that these materials are quite relevant for bipolar transport in OLEDs. The HOMO energy levels for NPyB and QuPB were estimated to be 5.5 eV by cyclic voltammogram, which is considerably close to 5.4 eV HOMO of hole-transporting material, NPB [21]. The LUMO energy levels of both the hosts are at 2.4 and 2.7 eV which can be deduced from calculation of band gaps. The triplet energy values of host materials and NPB were estimated by molecular modeling with density functional theory (DFT) using DMOL3 program (version 4.2) [22–24]. The energy levels of phosphorescent dopant Ir(phq)2acac were reported to have approximately 2.2 eV triplet energy, 5.3 eV HOMO, and 3.1 eV LUMO energies [25], which allow to receive triplet energy from both the hosts. Our host materials are expected to be good agreement in confining triplet excitons on the guest molecule effectively. A series of solution processed devices were fabricated by using a host/guest blend system and evaluated in terms of their J–V and L–V output characteristics. Devices A and B were fabricated using NPyB:Ir(phq)2acac (2 wt.%), and QuPB:Ir(phq)2acac (5 wt.%), respectively. These devices were fabricated on ITO with a multilayered structure of ITO/ PEDOT:PSS (45 nm)/host:dopant (35 nm)/TPBI (30 nm)/ LiF(1 nm)/Al(100 nm). For comparison, NPB:TPBI (1:1) type of mixed host device with 7 wt.% of Ir(phq)2acac concentration in similar experimental conditions was also fabricated [10] and its data is outlined in Table 2. Here PEDOT:PSS improves the hole injection as its HOMO level is 5.2 eV which is close to HOMO of NPyB and QuPB hosts. The operating conditions and characteristics of these devices with varying concentration ranges are compared and summarized in Table 2. Among all the prepared devices, devices A and B demonstrate their best performances with a dopant concentration of 2 and 5 wt.%. Fig. 4 shows the current density–voltage (J–V) and luminance–voltage (L–V) characteristics of all three devices. Device A exhibited turn-on voltage of 5.3 V which is similar to hole transport NPB:TPBI mixed host doped device while lower turn-on voltage of 4.9 V was obtained in device B. It is interesting to note that NPyB type of device has similar current density compared to NPB:TPBI mixed host system. This clearly indicates that NPyB host has very good hole and electron transporting ability. Devices A and B have lower driving voltages compared to NPB:TPBI based device. It was observed that driving voltages to reach 1000 cd/m2 were found to be 5.9 and 5.5 V for devices A and B, lower than 6.3 V for NPB:TPBI based device. Fig. 4 also shows L–V characteristics, exhibiting maximum brightness 14,440 cd/m2 at 8.8 V for device A, and 7714 cd/m2 at 8.0 V for device B, respectively. However, NPB:TPBI type of device exhibited slightly better maximum brightness compared to device A but overall performance of device A
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Fig. 4. Current–voltage and luminance–voltage of devices A, B and NPB:TPBI. Inset figure shows EL spectra of devices A, B and NPB:TPBI.
Fig. 5. Current and power efficiency characteristics of devices A, B and NPB:TPBI.
was found to be promising and convincing. This also signifies that NPyB host has better hole and electron transporting properties than NPB:TPBI type devices. Inset of Fig. 4 also shows the EL spectra of devices A and B exhibiting emissions in the red regions. It indicates complete energy transfer from the host materials to the guest due to confinement of triplet excitons at Ir(phq)2acac dopant molecule. The EL spectral peaks and CIExy coordinates of all devices are summarized in Table 2. At 1000 cd/m2, the EL spectral peaks and CIExy coordinates of devices A and B were 595 nm/(0.59, 0.39) and 598 nm/(0.60, 0.38). The EL spectrum of each device does not change significantly with the applied voltage. It is manifested that phosphorescence photoluminescence quantum efficiency at high doping concentration tends to reduced inevitably due to self quenching or triplet–triplet annihilation by dopant molecules. In both the devices, efficiency decreases at high concentration of the dopant molecules. The current and power efficiencies of devices A and B are depicted in Fig. 5 and Table 2. At a given constant lumi-
Fig. 6. I–V characteristics of the hole-only devices (a) NPB:TPBI, (b) NPyB, (c) QuPB, and electron-only devices (d) NPB:TPBI, (e) NPyB and (f) QuPB.
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nance of 1000 cd/m2, the current and power efficiencies of these devices were 20.6 cd/A and 10.8 lm/W for device A, and 7.6 cd/A and 3.3 lm/W for device B. Furthermore, the maximum current and power efficiencies obtained were 21.0 cd/A and 11.3 lm/W for device A, and 7.8 cd/A and 4.5 lm/W for device B. It clearly reveals that external quantum efficiency (EQE) of device A was 14.2%, which is nearly 40% higher than NPB:TPBI based device exhibiting EQE of about 8.6%. In case of device B, 5.2% of external quantum efficiency was achieved. To examine the bipolar charge transport ability of NPyB and QuPB host materials, hole-only and electron-only devices were fabricated. For comparison, NPB:TPBI mixed host device as a reference was also fabricated. Fig. 6 shows the J–V characteristics of hole-only and electron-only devices. In case of hole-only device, the structure of ITO/ PEDOT:PSS(45 nm)/host(35 nm)/sublimated NPB(60 nm)/ Al(100 nm) was adopted in which a large barrier of NPB was used to block the electron-injection from Al [26]. The results clearly demonstrate that NPyB host has better hole conduction than NPB:TPBI type of hole-only device while QuPB shows the lowest hole current than that of NPB:TPBI reference. To evaluate the electron transporting performance of host materials, we adopted the device structure of glass/Al(100 nm)/PEDOT:PSS(45 nm)/host(50 nm)/ LiF(1 nm)/Al(100 nm). It can be seen that electron transport property of NPyB and QuPB hosts are better than that of NPB:TPBI host system. This suggests that both NpyB and QuPB act as efficient bipolar materials because of comparable hole and electron mobility than that of reference. The above results confirm that NPyB and QuPB hosts have good bipolar charge transport properties. 4. Conclusions We have synthesized and characterized two new bipolar host materials, NPyB and QuPB, containing both electron and hole-type of moieties. Their phosphorescence and electrochemical studies confirmed that both the hosts are capable of electron and hole-transport, suitable for harvesting red electrophosphorescence. The bipolar transport properties of NPyB and QuPB hosts were investigated by hole-only and electron-only devices. The multilayered solution-processed phosphorescent devices utilizing NPyB host was found superior in red electroluminescence performance. In particularly, this device exhibited a maximum brightness of 14,440 cd/m2, with promising current and power efficiencies of 21.0 cd/A and 11.3 lm/W, and is considered an efficient bipolar host material for applications in low-cost electroluminescence devices. Consequently, this highly efficient promising red-emitting PHOLED have a possible space for improving its performance for further
development of new generation OLEDs. Thus, the present investigation determined that new NPyB host material yielded a promising performance in terms of efficiency and color chromaticity for practical applications. Acknowledgments This work was supported by a grant from the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korea government Ministry of Knowledge Economy (No. 2009-0074667). This work was also financially supported by the Grant (C1AAA001-2009-0093282) of MEST and the Seoul R&BD Program (10543). References [1] N. Koch, Chem. Phys. Chem. 8 (2007) 1438. [2] M.A. Wolak, J. Delacamp, C.A. Landis, P.A. Lane, J. Anthony, Z. Kafafi, Adv. Funct. Mater. 16 (2006) 1943. [3] D.H. Kim, D.H. Choi, J.J. Park, S.T. Lee, J.H. Kwon, Chem. Lett. 37 (2005) 1150. [4] M. Ooe, S. Naka, H. Okada, H. Onnagawa, Jpn. Appl. Phys. Part 1 91 (2007) 093512. [5] H. Kim, Y. Byun, R.R. Das, B.-K. Choi, P.-S. Ahn, Appl. Phys. Lett. 91 (2007) 093512. [6] J.H. Kwon, T.J. Park, W.S. Jeon, J.J. Park, Proc. SPIE 6828 (2007) 682802. [7] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [8] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [9] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adach, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304. [10] K.H. Kim, J.Y. Lee, T.J. Park, W.S. Jeon, G.P. Kennedy, J.H. Kwon, Synthetic Met. 160 (2010) 631. [11] A.P. Monkman, H.D. Burrows, L.J. Hartwell, L.E. Horsburgh, I. Hamblett, S. Navaratnam, Phys. Rev. Lett. 86 (2001) 1358. [12] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328. [13] M.A. Baldo, D.F. O’Brien, M.E. Thompson, S.R. Forrest, Phys. Rev. B 60 (1999) 14422. [14] S. Tao, L. Lu, J. Yu, Y. Zhou, C.-S. Lee, S.-T. Lee, X. Zhang, O. Kwon, Chem. Mater. 21 (2009) 1284. [15] T. Youtian, Q. Wang, C. Yang, K. Zhang, Q. Wang, T. Zou, J. Qin, M. Dongge, J. Mater. Chem. 18 (2008) 4091. [16] Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akike, H. Miyamoto, T. Kajita, M. Kakimoto, Chem. Mater. 20 (2008) 2532. [17] W.S. Jeon, T.J. Park, Y.S. Kim, R. Pode, J. Jang, J.H. Kwon, Appl. Phys. Lett. 93 (2008) 063303. [18] S.C. Tse, K.C. Kwok, S.K. So, Appl. Phys. Lett. 89 (2006) 262102. [19] J.P. Park, T.J. Park, W.S. Jeon, R. Pode, J. Jang, J.H. Kwon, E.-S. Yu, M.-Y. Chae, Org. Elect. 10 (2009) 189. [20] S.A. VanSlyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160. [21] T.J. Park, W.S. Jeon, J.J. Park, Y.S. Kim, K.Y. Lee, J. Jang, J.H. Kwon, R. Pode, Appl. Phys. Lett. 92 (2008) 113308. [22] B. Delley, J. Chem. Phys. 92 (1990) 508. [23] B. Delley, J. Phys. Chem. 100 (1996) 6107. [24] B. Delley, J. Chem. Phys. 113 (2000) 7756. [25] M.C. Suh, M.-H. Kim, S.T. Lee, H.D. Kim, H.K. Chung, Appl. Phys. Lett. 86 (2005) 133505. [26] C.-H. Kuo, K.-C. Peng, L.-C. Kuo, K.-H. Yang, J.H. Lee, M.K. Leung, K.H. Hsieh, Chem. Mater. 18 (2006) 4121.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Triplet bipolar host materials for solution processed organic light-emitting devices Hyoung-Yun Oh a,b, Chandramouli Kulshreshtha c, Jang Hyuk Kwon c,*, Seonghoon Lee a,** a
Molecular Electronics and NanoStructures Lab, School of Chemistry, NS60, Seoul National University, Shillim-dong, San 56-1, Seoul 151-747, Republic of Korea LG Display R&D Center, 1007 Deongeun-ri, Wollong-myeon, Paju-si, Gyeonggi-do 413-811, Republic of Korea c Department of Information Display, Kyung Hee University, Dongdaemoon-ku, Seoul 130-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 1 March 2010 Received in revised form 5 June 2010 Accepted 7 July 2010 Available online 10 August 2010 Keywords: OLED Soluble Red Host Phosphorescent
a b s t r a c t Two solution-processible new triplet bipolar host molecules, 4,40 -bis[N-(1-naphthyl)-N(3pyridinylamino]biphenyl (NPyB) and 4,40 -bis [N-(3-quinolinyl)-N-phenylamino]biphenyl (QuPB) were synthesized and characterized for phosphorescent organic light-emitting diodes (PHOLEDs). They exhibit good triplet energies as 2.3–2.4 eV for red PHOLEDs and show good amorphous film characteristics with high solubility, allowing a solution processing. The NPyB and QuPB hosts were evaluated for soluble red PHOLEDs with ITO/PEDOT:PSS/Ir complex:host/TPBI/LiF/Al structure. A maximum light output of 14,440 cd/m2, driving voltage of 5.9 V, external quantum efficiency of 14.2%, and maximum current/ power efficiencies of 21.0 cd/A and 11.3 lm/W have been achieved by NPyB host and bis(2-phenylquinoline(acetylacetonate)iridium (Ir(phq)2acac) red dopant. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have become more mature in recent decades and their applications now include a variety of materials with superior properties for low power consumption displays, lightings and printed electronics industry [1,2]. Thermal vacuum deposition process has been in use for commercial OLED products, owing to high purity and good device performance as the advantages. Thermal vacuum deposition allows complicated device structure with improved performance, utilizing a fine shadow mask but its disadvantages include high cost, high resolution, and scalability of the method to very large mother glass formats. In contrast, solution processing such as spin coating or ink-jet printing overcomes these disadvantages for low-cost by reducing material consumption and equipment cost compared to incumbent vapor deposi-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.H. Kwon), [email protected] (S. Lee).
tion processes [3]. Despite the poor solubility and crystallization issues of polymeric and small molecule materials, efforts have been aggrandized to improve the device performance of solution processed OLEDs. Over the years, attempts have been limited to polymeric and/or small molecule solution-processed phosphorescent materials [4,5]. Use of high-efficiency phosphorescent OLED (PHOLED) technology is one of the most efficient strategy employed to achieve 100% internal quantum efficiency [6–8]. In this regard, iridium containing transition–metal complexes as dopant materials are the most promising candidates [9]. For instance, we have demonstrated promising and efficient red PHOLEDs using small molecule mixed host system [10]. It is important that charge transporting host should have higher triplet excited state energy (T1) than emitter guest due to T1 energy confinement in the guest. As a consequence, polymer materials are not suitable generally as they have low T1 energy and high band gap due to considerably high exchange energy, 1.0–1.5 eV values, between the singlet and triplet states [11]. To resolve this problem in solution processing PHOLEDs, small molecules
1566-1199/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.07.005
H.-Y. Oh et al. / Organic Electronics 11 (2010) 1624–1630
were considered ideal for host–guest relationship but their crystallization issues and non-uniform film thickness have aggravated as additional problems. Generally, most of the host materials either transport holes or electrons in a device but unbalanced hole and electron currents makes the recombination zone narrower, thereby shifting it either close to hole transport layer (HTL) or electron transport layer (ETL). Thus narrower charge recombination zone in PHOLEDs are prone to triplet–triplet exciton quenching due to local high density of triplet excitons and also long diffusion of the triplet excitons [12,13]. Therefore, bipolar small molecule hosts can meet the requirements of balancing the charge density for making high performance simple device structure. For improving the charge-transferring capability of bipolar molecules, it is desirable to localize the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) level at their respective hole and electron transporting moieties. To the best of our knowledge very few devices based on solution process bipolar small molecules have been reported so far [14–16]. Typically, N,N0 -diphenyl-N,N0 -bis(1-naphthyl)(1,10 -biphenyl)-4,40 diamine (NPB) has bipolar characteristics in OLEDs with both hole and electron transporting property. It has 5.4 eV HOMO and 2.3 eV LUMO values and a low triplet energy (2.3 eV) [17]. For practical use of the host in PHOLEDs, its LUMO energy is too high compared with general host materials even though its electron mobility is good as 9 104 cm2/vs [18]. In solution process devices, it forms non-uniform surface with high roughness due to crystallization of its molecules upon thermal treatment [19]. Therefore, NPB was reported as a hole-type host in the mixed host system [10]. In order to reduce the electron-injection barrier and enhancing the electron mobility of NPB molecule, we report, NPB modified two novel bipolar molecules, 4,40 -bis[N-(1-naphthyl)-N-(3-pyridinylamino]biphenyl (NPyB) and 4,40 -bis [N-(3-quinolinyl)-Nphenylamino]biphenyl (QuPB) with their synthetic method and material characteristics. We also employed NPyB and QuPB host materials for fabricating high-efficiency electrophosphorescent solution processed red OLEDs. The multilayered electrophosphorescent OLEDs fabricated from 1,2-dichlorobenzene solution containing spin-coated NPyB and QuPB hosts blended with a dopant, bis(2-phenylquinoline(acetylacetonate)iridium (Ir(phq)2acac) emitted red phosphorescent light. The bipolar NPyB and QuBP hosts show good solubility and stability in 1,2-dicholobenzene solvent, uniform film growth characteristics without any crystallization issues, and good electron and hole-transporting properties. A luminance of 14,440 cd/m2 at 8.4 V and luminous efficiency of 21.0 cd/A and 11.3 lm/W at 5.9 V are demonstrated in the NPyB host device.
2. Experimental details 2.1. Material synthesis The compounds NPyB and QuPB were synthesized according to the procedure shown in Fig. 1. All reagents and solvents were purchased from Aldrich chemical Co.
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and Fluka. Other reagents in the scheme were used as from commercial sources. 2.1.1. 4,40 -Bis[N-(1-naphthyl)-N-(3-pyridinylamino]biphenyl (NPyB) To a solid mixture of 1-naphthyl-3-pyridinylamine (3.0 g, 13.6 mmol) and 4,40 -dibromobiphenyl (2.0 g, 6.5 mmol), a solution of 50 ml toluene was added. It was degassed by bubbling nitrogen for 30 min. Additionally, 0.2 mmol of tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3], 0.4 mmol of tri-tert-butylphosphine, and 20 mmol of sodium-tert-butoxide was added to the mixture. The mixture was vigorously refluxed under nitrogen for 24 h. When the reaction mixture was cooled down to room temperature, 30 ml of water was added to remove some salts. After separating the water, the solution was dried by MgSO4, and then concentrated under reduced pressure. The mixture was purified by column chromatography on silica gel eluting with a 1:2 mixture of ethylacetate and hexane. By removing the eluting solvent, light yellow powder was obtained (2.5 g, 65%). Tm 241 °C, Tg 93 °C, dH (500 MHz; CDCl3; Me4Si) 8.38 (2H, d, J 2.6), 8.16 (2H, d, J 4.5), 7.90 (4H, J 8.5), 7.49 (4H, m), 7.37 (8H, m), 7.28 (2H, d, J 8.7), 7.11 (6H, m). Elemental Analysis Calc. for C42H30N4: C, 85.40; H, 5.12; N, 9.48%. Found: C, 85.54; H, 5.04; N, 9.47%. M/S (m/z, M+). Found: 591.2543, Calc.: 591.2549. 2.1.2. 4,40 -Bis [N-(3-quinolinyl)-N-phenylamino]biphenyl (QuPB) The synthetic procedure of QuPB is same except using 3-quinolinyl-phenylamine (3.0 g, 13.6 mmol) instead of 1-naphthyl-3-pyridinylamine. Yellow powder was obtained (2.3 g, 60%). Tm 186 °C, Tg 94 °C. dH (500 MHz; CDCl3; Me4Si) 8.79 (2H, d, J 2.5), 8.03 (2H, d, J 8.3), 7.70 (2H, d, J 2.2), 7.59 (4H, m), 7.52 (4H, d, J 8.6), 7.46 (2H, t, J 7.7), 7.32 (4H, t, J 7.9), 7.20 (8H, d, J 8.4), 7.13 (2H, t, J 7.3). Elemental Analysis Calc. for C42H30N4: C, 85.40; H, 5.12; N, 9.48%. Found: C, 85.49; H, 5.11; N, 9.45%. M/S: (m/z, M+). Found: 591.2551, Calc.: 591.2549. 2.2. OLED fabrication and measurements OLED devices were fabricated on the indium–tinoxide (ITO) glass having an emission area of 2 mm 2 mm with a sheet resistance of 12 X/square. A line pattern of ITO was formed by the photolithography process. Before coating organic layers, the patterned ITO glasses were cleaned by sonification in isopropyl alcohol (IPA) and acetone, and then rinsed in deionized water; finally it was irradiated in a UV-ozone chamber. A 45 nm poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spincoated on the ITO substrates and then dried using a hot plate at 120 °C for 10 min to remove the solvent. A 40 nm thick emissive host system i.e. NPyB and QuPB doped with Ir(phq)2acac in 1,2-dichlorobenzene was later spin-coated on the PEDOT:PSS layer. The spin-coated emissive layer was then baked on a hot plate at 120 °C for 10 min. The spin coating and baking process were carried out in a glove box with nitrogen ambient. Subsequently, 30 nm TPBI was
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Fig. 1. Synthesis procedure of NPyB and QuPB.
deposited in an organic chamber using vacuum thermal evaporation at a base pressure of 107 Torr, while LiF (1 nm) and Al (100 nm) were deposited in an adjacent metal chamber without breaking the system vacuum. Current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the fabricated OLEDs were measured using a Keithley SMU 2635 and Konika Minolta CS100A. Also, the electroluminescence (EL) spectra and CIE color coordinates were obtained using a Konika Minolta CS-1000A spectroradiometer. For optical measurements, UV–Visible absorption spectra of NPyB in dichloromethane solution were recorded with a Shimadzu 3100 UV–VIS–NIR spectrophotometer. In case of photoluminescence (PL) measurement, emission spectrum of NPyB and QuPB in dichloromethane solution, and deposited thin films on glass substrate were measured with a Jasco FP-6500 spectroflurometer. Low temperature emission spectra of NPyB and QuPB solutions were also recorded using liquid nitrogen at 77 K with Jasco FP-6500 spectroflurometer. For investigation of smoothness and homogenous surface of emissive layer, atomic force microscopy (AFM) was done. Cyclic voltammetry (CV) was performed using EC epsilon electrochemical analysis equipment. Platinum wire and ITO film on glass were used as counter and working electrodes, respectively. Tetrabutylammonium perchlorate (Bu4NClO4) is used as a supporting electrolyte. Ag wire in 0.1 M AgNO3 was used as a reference electrode. By means of internal ferrocenium/ferrocene (Fe+/Fe) standard, the potential values were converted to the saturated calomel electrode (SCE) scale. The position of LUMO of each material is estimated from the optical band gap determined from the absorption onset of the spectra. The films that had been coated with the material were made on ITO glass through the solution drop coating and then dried in a vacuum oven at 80 °C. Solid-state electrochemistry was performed at a scan rate of 150 mV/s and start potential voltage have supplied from 0.5 eV for reducing potential damage. 3. Results and discussion The chemical structure and synthetic route of 4,40 bis[N-(1-naphthyl)-N-(3-pyridinylamino]biphenyl (NPyB)
and 4,40 -bis [N-(3-quinolinyl)-N-phenylamino]biphenyl (QuPB) are shown in Fig. 1. The molecular structures of NPyB and QuPB were confirmed by 1H NMR, mass spectrometry, and elemental analysis. Thermal properties of NPyB and QuPB were investigated by means of differential scanning calorimetry (DSC). A glass-transition temperature (Tg) was obtained from the second heating scan of the glassy samples cooled after the first heating up to melting temperature (Tm). The heating rate was 10 °C/min. The Tm’s of NPyB and QuPB appeared at 241 and 186 °C, respectively. The crystallization temperature (Tc) of NPyB was observed about 180 °C by the second heating but no Tc of QuPB was found, which indicates that QuPB has more glass-like property. Considering the melting point of NPB about 280 °C, intermolecular dipole interaction is reduced in the order of NPB, NPyB, and QuPB. However, the glasstransition temperatures of both the molecules are around 93–95 °C which is about same as that of commercialized NPB (95 °C) [20]. NPyB and QuPB can readily dissolve in common organic solvents, such as 1,2-dichloromethane, 1,2-dichlorobenzene, toluene which enables the preparation of uniform thin film by solution processing such as spin coating. AFM studies of NPyB and QuPB host materials with spin coating in 1,2-dichlorobenzene solution were carried out to investigate the surface morphology of thin films. As shown in Fig. 2, root-mean-square-roughness (rms) values are 1.90 Å and 1.89 Å for NPyB and QuPB. These results shows that both the host films were smooth and homogeneous without any crystallization issues by implementing the host compatibility with dopant material used in fabricating solution-processible devices. The bipolar host materials, NPyB and QuPB having pyridine and quinoline units as electron transport moieties lead to bipolar charge transport property for both holes and electrons. The photophysical properties of NPyB and QuPB host materials were measured using UV–visible absorption and photoluminescence spectra as depicted in Fig. 3 and Table 1. The absorption and emission spectra of both the host materials were carried out in dilute dichloromethane solution. The absorption spectrum of NPyB has one absorption band from 305 to 360 nm which could be from p–p* transitions of the conjugated p-electron system.
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Fig. 2. AFM image of host (a) NPyB and (b) QuPB.
Fig. 3. UV spectra in dichloromethane (a) NPyB, (b) QuPB, and PL spectra in dichloromethane, (c) NPyB solution, (d) NPyB thin film, (e) NPyB low temperature (77 K), (f) QuPB solution, (g) QuPB thin film and (h) QuPB low temperature (77 K).
QuPB exhibits four vibronic absorption bands centered at 284, 320, 345 and 390 nm, respectively. The PL emission maximas of NPyB and QuPB in solution were observed around 442 and 510 nm, respectively. The PL emission spectra of NPyB and QuPB thin films were slightly blue shifted by 6 and 36 nm, respectively. The low temperature phosphorescent spectrum of the NPyB host, measured in 2methyltetrahydrofuran (THF) solution at 77 K, revealed the shifting of main peak at 405 nm while a phosphorescent sub-band of two peaks with peak positions, 520 and 558 nm could also be noticed. Similarly, QuPB at 77 K shows the main peak at 446 nm and phosphorescent subbands with peak positions at 527 and 572 nm. From the phosphorescent spectra, triplet energies of 2.4 and 2.3 eV were calculated from 520 and 527 nm first peaks of NPyB
and QuPB materials. It confirms that their triplet energies are in good agreement in transferring excitons from the bipolar hosts to Ir(phq)2acac dopant. In order to make appropriate comparisons of triplet energies, energy transfer experiments from the red dopant to NPyB and QuPB hosts were carried out in 1,2-dichlorobenzene solution. When NPyB was added to Ir(phq)2acac solution at different concentrations, the quantum efficiency did not change. However, addition of QuPB has decreased the quantum efficiency, significantly. These results indicate that NPyB triplet energy is higher than the red dopant while QuPB has a triplet energy similar to Ir(phq)2acac dopant. In order to obtain more accurate results, energy transfer rate of QuPB was calculated using Stern–Volmer plots and it came out 3 109 s1M1, which is similar to diffusion rate of
Table 1 Physical properties of NPyB and QuPB.
a b
Compounds
kabsa (nm)
kPLb (nm) sol./film/77 K
Tm, Tg (°C)
HOMO/LUMOcalc De (eV)
HOMO/LUMOexp De (eV)
T1calc/exp (eV)
NPyB QuPB NPB
340 284, 320, 345, 390 350
442/436/405,520 510/476/445,527 467/438/418,528
241, 93 186, 94 277, 95
4.4, 2.2, 2.0 4.4, 2.3, 2.3 4.2, 2.0, 2.2
5.5, 2.4, 3.1 5.5, 2.7, 2.8 5.4, 2.3, 3.1
2.0/2.4 1.8/2.3 2.1/2.3
Absorption maximum. PL maxima, Tm – melting temperature, Tg – glass-transition temperature.
4.5 5.3 8751 (8.6 V) 7.2, 4.5 7.2, 4.2 0.60, 0.38 598
NPB:TPBI (7%) QuPB (7%) QuPB (5%)
4.9 5.5 7714 (8.0 V) 7.8, 4.5 7.6, 3.3 0.60, 0.38 598 5.1 6.3 7888 (8.2 V) 6.9, 3.3 6.7, 3.3 0.60, 0.38 598
QuPB (3%) NPyB (3%)
4.3 6.1 14,190 (9.2 V) 12.8, 5.8 11.6, 5.9 0.59, 0.39 596 Turn-on voltage (at 1 cd/m2) Operating voltage (at 1000 cd/m2) Luminance (max.) at voltage Maximum efficiency (cd/A , lm/W) Efficiency (cd/A, lm/W at 1000 cd/m2) CIE (x, y) at 1000 cd/m2) Emission peak (nm)
NPyB (2%) NPyB (1%)
5.1 5.9 12,820 (8.4 V) 17.9, 9.21 17.6, 9.1 0.57, 0.39 593
Doping ratios of Ir(phq)2acac
Table 2 A summary of device performance as a function of doping concentration in NPyB and QuPB hosts.
5.4 6.3 19,250 (9.2 V) 11.9, 5.8 11.9, 5.8 0.61, 0.38 600
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5.3 5.9 14,400 (8.8 V) 21.0, 11.3 20.6, 10.8 0.59, 0.39 595
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organic solvents. However, NPyB does not reach to such value. This proves that NPyB has higher triplet energy than Ir(phq)2acac and can be used with red dopant effectively. In order to investigate the electrochemical properties of NPyB and QuPB, cyclic voltammetry experiments were performed to know HOMO levels. Table 1 summarizes the calculated and experimentally obtained energy levels values. The host materials showed reversible oxidation and reduction waves, indicating that these materials are quite relevant for bipolar transport in OLEDs. The HOMO energy levels for NPyB and QuPB were estimated to be 5.5 eV by cyclic voltammogram, which is considerably close to 5.4 eV HOMO of hole-transporting material, NPB [21]. The LUMO energy levels of both the hosts are at 2.4 and 2.7 eV which can be deduced from calculation of band gaps. The triplet energy values of host materials and NPB were estimated by molecular modeling with density functional theory (DFT) using DMOL3 program (version 4.2) [22–24]. The energy levels of phosphorescent dopant Ir(phq)2acac were reported to have approximately 2.2 eV triplet energy, 5.3 eV HOMO, and 3.1 eV LUMO energies [25], which allow to receive triplet energy from both the hosts. Our host materials are expected to be good agreement in confining triplet excitons on the guest molecule effectively. A series of solution processed devices were fabricated by using a host/guest blend system and evaluated in terms of their J–V and L–V output characteristics. Devices A and B were fabricated using NPyB:Ir(phq)2acac (2 wt.%), and QuPB:Ir(phq)2acac (5 wt.%), respectively. These devices were fabricated on ITO with a multilayered structure of ITO/ PEDOT:PSS (45 nm)/host:dopant (35 nm)/TPBI (30 nm)/ LiF(1 nm)/Al(100 nm). For comparison, NPB:TPBI (1:1) type of mixed host device with 7 wt.% of Ir(phq)2acac concentration in similar experimental conditions was also fabricated [10] and its data is outlined in Table 2. Here PEDOT:PSS improves the hole injection as its HOMO level is 5.2 eV which is close to HOMO of NPyB and QuPB hosts. The operating conditions and characteristics of these devices with varying concentration ranges are compared and summarized in Table 2. Among all the prepared devices, devices A and B demonstrate their best performances with a dopant concentration of 2 and 5 wt.%. Fig. 4 shows the current density–voltage (J–V) and luminance–voltage (L–V) characteristics of all three devices. Device A exhibited turn-on voltage of 5.3 V which is similar to hole transport NPB:TPBI mixed host doped device while lower turn-on voltage of 4.9 V was obtained in device B. It is interesting to note that NPyB type of device has similar current density compared to NPB:TPBI mixed host system. This clearly indicates that NPyB host has very good hole and electron transporting ability. Devices A and B have lower driving voltages compared to NPB:TPBI based device. It was observed that driving voltages to reach 1000 cd/m2 were found to be 5.9 and 5.5 V for devices A and B, lower than 6.3 V for NPB:TPBI based device. Fig. 4 also shows L–V characteristics, exhibiting maximum brightness 14,440 cd/m2 at 8.8 V for device A, and 7714 cd/m2 at 8.0 V for device B, respectively. However, NPB:TPBI type of device exhibited slightly better maximum brightness compared to device A but overall performance of device A
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Fig. 4. Current–voltage and luminance–voltage of devices A, B and NPB:TPBI. Inset figure shows EL spectra of devices A, B and NPB:TPBI.
Fig. 5. Current and power efficiency characteristics of devices A, B and NPB:TPBI.
was found to be promising and convincing. This also signifies that NPyB host has better hole and electron transporting properties than NPB:TPBI type devices. Inset of Fig. 4 also shows the EL spectra of devices A and B exhibiting emissions in the red regions. It indicates complete energy transfer from the host materials to the guest due to confinement of triplet excitons at Ir(phq)2acac dopant molecule. The EL spectral peaks and CIExy coordinates of all devices are summarized in Table 2. At 1000 cd/m2, the EL spectral peaks and CIExy coordinates of devices A and B were 595 nm/(0.59, 0.39) and 598 nm/(0.60, 0.38). The EL spectrum of each device does not change significantly with the applied voltage. It is manifested that phosphorescence photoluminescence quantum efficiency at high doping concentration tends to reduced inevitably due to self quenching or triplet–triplet annihilation by dopant molecules. In both the devices, efficiency decreases at high concentration of the dopant molecules. The current and power efficiencies of devices A and B are depicted in Fig. 5 and Table 2. At a given constant lumi-
Fig. 6. I–V characteristics of the hole-only devices (a) NPB:TPBI, (b) NPyB, (c) QuPB, and electron-only devices (d) NPB:TPBI, (e) NPyB and (f) QuPB.
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nance of 1000 cd/m2, the current and power efficiencies of these devices were 20.6 cd/A and 10.8 lm/W for device A, and 7.6 cd/A and 3.3 lm/W for device B. Furthermore, the maximum current and power efficiencies obtained were 21.0 cd/A and 11.3 lm/W for device A, and 7.8 cd/A and 4.5 lm/W for device B. It clearly reveals that external quantum efficiency (EQE) of device A was 14.2%, which is nearly 40% higher than NPB:TPBI based device exhibiting EQE of about 8.6%. In case of device B, 5.2% of external quantum efficiency was achieved. To examine the bipolar charge transport ability of NPyB and QuPB host materials, hole-only and electron-only devices were fabricated. For comparison, NPB:TPBI mixed host device as a reference was also fabricated. Fig. 6 shows the J–V characteristics of hole-only and electron-only devices. In case of hole-only device, the structure of ITO/ PEDOT:PSS(45 nm)/host(35 nm)/sublimated NPB(60 nm)/ Al(100 nm) was adopted in which a large barrier of NPB was used to block the electron-injection from Al [26]. The results clearly demonstrate that NPyB host has better hole conduction than NPB:TPBI type of hole-only device while QuPB shows the lowest hole current than that of NPB:TPBI reference. To evaluate the electron transporting performance of host materials, we adopted the device structure of glass/Al(100 nm)/PEDOT:PSS(45 nm)/host(50 nm)/ LiF(1 nm)/Al(100 nm). It can be seen that electron transport property of NPyB and QuPB hosts are better than that of NPB:TPBI host system. This suggests that both NpyB and QuPB act as efficient bipolar materials because of comparable hole and electron mobility than that of reference. The above results confirm that NPyB and QuPB hosts have good bipolar charge transport properties. 4. Conclusions We have synthesized and characterized two new bipolar host materials, NPyB and QuPB, containing both electron and hole-type of moieties. Their phosphorescence and electrochemical studies confirmed that both the hosts are capable of electron and hole-transport, suitable for harvesting red electrophosphorescence. The bipolar transport properties of NPyB and QuPB hosts were investigated by hole-only and electron-only devices. The multilayered solution-processed phosphorescent devices utilizing NPyB host was found superior in red electroluminescence performance. In particularly, this device exhibited a maximum brightness of 14,440 cd/m2, with promising current and power efficiencies of 21.0 cd/A and 11.3 lm/W, and is considered an efficient bipolar host material for applications in low-cost electroluminescence devices. Consequently, this highly efficient promising red-emitting PHOLED have a possible space for improving its performance for further
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