- Email: [email protected]
A new tricarbazole phosphine oxide bipolar host for efficient single-layer blue PhOLED
Organic Electronics 12 (2011) 2025–2032
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
A new tricarbazole phosphine oxide bipolar host for efficient single-layer blue PhOLED Hsin-Hua Chang a,⇑, Wan-Shan Tsai b, Chien-Ping Chang a,c, Nien-Po Chen c, Ken-Tsung Wong b,⇑, Wen-Yi Hung d, Shou-Wei Chen d a
Department of Electro-Optical Engineering, Vanung University, Taoyuan 320, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan c Department of Electro-Optical Engineering, Yuanze University, Taoyuan 320, Taiwan d Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan b
a r t i c l e
i n f o
Article history: Received 22 July 2011 Received in revised form 29 August 2011 Accepted 29 August 2011 Available online 15 September 2011 Keywords: Electrophosphorescence Bipolar host material Phosphine oxide Carbazole
a b s t r a c t A novel tricarbazole phosphine oxide (POCz3) with high triplet energy and promising physical properties serves as a bipolar host material of blue-emitting phosphor (FIrpic) to realize highly efficient single-layer blue PhOLED which achieves a maximum external quantum efficiency up to 9% (gp = 10.4 lm/W, gc = 21.3 cd/A) and shows low efficiency roll-off effect. POCz3 was also utilized in a three-layer device with a double confinement effect exhibiting maximum luminance 60098 cd/m2 and maximum EQE (power efficiency) of 14.5% (31.3 lm/W). Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Recent progresses on phosphorescence organic lightemitting diodes (PhOLEDs) receive considerable research attentions owing to their capability of harvesting both singlet and triplet excitons to realize internal quantum efficiencies up to 100% [1]. In order to efficiently extract the electro-generated emissive excitons in typical PhOLEDs device, various functional layers were required for their individual role-play such as charge carrier injection, transport, and blocking as well as the confinement of exciton diffusion. Such sophisticated device configurations inevitably increase the manufacture complexity and the production cost. Therefore, it is highly desirable to fabricate PhOLEDs with simplified device structures. Particularly potential
⇑ Corresponding authors. Tel.: +886 3 4515811x73607; fax: +886 3 4515811 334 (H.-H. Chang), tel.: +886 2 33661665; fax: +886 2 33661667 (K.-T. Wong). E-mail addresses: [email protected] (H.-H. Chang), ken [email protected] (K.-T. Wong). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.030
goal is the single layer device, which can perform comparable efficiency as compared to their multilayer counterparts. There are several important factors required for efficient single layer PhOLEDs: (1) host material with suitable energy levels match to cathode/anode for easy charge injection, (2) host material with balanced electron and hole mobilities for large charge flux and efficient charge recombination, (3) high triplet energy relative to emissive phosphors for exciton confinement within the emissive layer. More recently, reports of adopting direct charge injection and transport onto the triplet dopants dispersed in an appropriate host matrix pave the way to realize efficient single-layer PhOLEDs. The host materials incorporated in these single-layer devices include mixed host materials [2–4], bipolar host materials [5], and electron-transporting host material [6]. The judicious choice of hosts with appropriate energy levels combining with hole-/electron -transporting characters in mixed systems and/or the single molecular bipolar host play the crucial role for the success of single layer device. Among these elegant works, efficient green and red single-layer PhOLEDs have been
2026
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
successfully demonstrated [5]. However, single layer blue PhOLEDs [2,5a,6] reported as far were not satisfactory yet, giving challenges for material scientists. The most popular blue emitter FIrpic has revealed its bipolar charge carrier transport behavior by using the time-of-flight (TOF) transient photocurrent technique [7]. Accordingly, highly efficient single layer blue PhOLED should be feasible if one uses FIrpic dispersed into a tailor-made high triplet energy host with suitable energy levels for direct carriers injection. In this paper, we report a novel high triplet energy host material: 3,30 ,30 0 -phosphoryl tris(9-phenyl-9Hcarbazole) (POCz3) for efficient single-layer blue PhOLEDs. The device: ITO/PEDOT:PSS/POCz3 (3 nm)/POCz3:FIrpic 20 wt.% (80 nm)/LiF/Al gives a maximum external quantum efficiency (gext) of 9%, power efficiency (gp) of 10.4 lm/W, and current efficiency (gc) of 21.3 cd/A. To the best of our knowledge, this work represents an unprecedented example of realizing a single-layer blue PhOLED with remarkable device characteristics.
2. Results and discussion 2.1. Materials The most successful host materials used for FIrpic are carbazole derivatives [8]. Without extended p-conjugation from the carbazole core, carbazole-based host materials usually exhibit high triplet energies. Thus, the triplet excitons can be efficiently confined within the emitting layer. However, typical FIrpic-based blue PhOLEDs incorporated with carbazole-based hosts require multilayer structures for facilitating the hole/electron injections into the emitting layer. In order to enhance the electron injection capability of carbazole, structural modification by introducing electron deficient moiety onto the carbazole core is necessary. Recently, triarylphosphine oxides have been reported to perform facile electron injection and transport properties [9–11]. The combination of the structural features of carbazole and phosphine oxide had led to promising bipolar hosts for blue PhOLEDs with excellent device characteristics [12]. The HOMO level of carbazole compound can be modulated by introducing appropriate substitutions at the C3 and/or C6 positions of carbazole [13–14], giving the opportunity of developing potential host materials for single layer device. Scheme 1 depicts
the structure of the new bipolar host material POCz3, which was synthesized in one step with reasonable yield from a known precursor. The 3-bromo-9-phenylcarbazole reacted with magnesium turning in THF to give a Grignard solution, which was then quenched with POCl3 to give the desired product POCz3 in 55% yield. The structural identity of POCz3 was confirmed by satisfactory 1 H, 13C and 31P NMR spectra, high-resolution mass spectrometry (HRMS). 2.2. Physical properties characterizations POCz3 exhibits a high thermal tolerance [5% weight loss occurred at 395 °C, determined by thermogravimetric analysis (TGA)] and a distinct glass transition temperature (Tg) of 163 °C analyzed by differential scanning calorimetry (DSC). Therefore, POCz3 can form homogeneous and stable amorphous films upon thermal evaporation, which is a crucial criterion for OLED application. The photophysical property of POCz3 as compared to that of parent compound 9-phenyl carbazole is shown in Fig. 1. Apparently, the introduction of phosphine oxide onto the carbazole ring only brings out limited perturbation on the photophysical property of carbazole. In solution, POCz3 exhibits a strong absorption band range from 260 to 300 nm that is attributed to the typical p–p⁄ electronic transition of carbazole, whereas the weak absorption bands (300–350 nm) are ascribed to the n–p⁄ transitions of carbazole. The emission spectrum of POCz3 is almost superimposable to that of 9-phenyl carbazole. The phosphorescent spectrum of POCz3 was obtained at 77 K (in EtOH). Again, there is no significant difference on the triplet energy upon introducing the electron deficient phosphine oxide to the carbazole as compared to that of 9-phenyl carbazole. The difference between the fluorescence (351 nm) and phosphorescence (408 nm) spectra corresponds to an exchange energy of ca. 0.67 eV between the singlet and triplet of POCz3. The triplet energy (ET) of 3.03 eV (defined as the 0–0 transition in phosphorescent spectrum) is sufficiently high for POCz3 to confine the triplet excitons of the blue phosphorescence emitter FIrpic (ET = 2.7 eV) within the emitting layer. Fig. 2 displays the cyclic voltammograms of POCz3 and 9-phenyl carbazole in CH2Cl2 using a glassy carbon electrode with 0.1 M of nBu4NPF6 as electrolyte. The oxidation of POCz3 showed typical electrochemical behavior of C3
Scheme 1. The structure and synthesis of bipolar host POCz3.
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2027
Fig. 1. A comparison of UV–vis absorption, photoluminescence (PL) (CH2Cl2), and phosphorescence (EtOH, 77 K) spectra of POCz3 and its parent counterpart N-phenyl carbazole.
Fig. 2. The cyclic voltammogram of POCz3 as compared to that of parent compound 9-phenylcarbazole.
and C6 unprotected carbazole derivatives, which usually performed a lower reduction peak originated from the more conjugated dimeric species generated from the coupling of carbazole cationic radicals. It is noteworthy that the oxidation onset of POCz3 is evidently shifted from 1.28 V of 9-phenyl carbazole to 1.44 V. Thus, the highest occupied molecular orbital (HOMO) energy of POCz3 is strongly affected by introducing the electron-withdrawing phosphine oxide. The electron deficient substituent is also beneficial to the reduction behavior of POCz3, in which one quasi-reversible reduction potential, onset potential at 2.25 V, was observed in dimethylformamide (DMF) using a glassy carbon electrode with 0.1 M of nBu4NClO4 as electrolyte. For practical evaluation, the HOMO of POCz3 were measured using UV photoemission spectroscopy (AC-2) to be 5.5 eV, and the lowest unoccupied molecular orbital (LUMO) energy was deduced to be 1.8 eV by adding the energy gap of POCz3 to the HOMO energy. To further understand the charge-carrier transport properties of POCz3, we used the time-of-flight (TOF) technique
[15] to evaluate the carrier mobilities. Fig. 3(a) displays representative TOF transient for electrons of POCz3. The transit time, tT, can be evaluated from the intersection point of two asymptotes in the double-logarithmic representation of the TOF transient. It is then used to determine the carrier mobilities, according to the equation l = d2/VtT, where d is the sample thickness and V is the applied voltage. Fig. 3(b) reveals that the field dependence of the electron mobility follows the nearly universal Poole–Frenkel relationship, with values in the range from 1.4 106 to 6.4 106 cm2/Vs for fields varying from 4 105 to 6.8 105 V/cm. In contrast, the TOF transient for holes exhibited weak photocurrent signals, which could not be used to determine the hole mobility. From the chemical structure of POCz3 (Scheme 1), the carbazole unit is a good hole transport unit and is effective in improving the hole transport property of the organic material. Hence, we also prepared hole- and electron-only single-carrier devices, to evaluate the bipolar transport behavior. We used high-LUMO 4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, lh of 103 cm2/Vs order) [16] to limit the electron carrier in hole-only devices and low-HOMO 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, le of 107 cm2/Vs order) [17] to limit the hole carrier in electron-only devices. As shown in Fig. 4, POCz3 shows bipolar transport property. However, the current density of electron-only device is larger than that of hole-only device at the same voltage, indicating the superior capability of POCz3 to transport electron other than hole-transport. This result is in agreement with the observed TOF mobility that only electron mobility can be resolved. 2.3. Device characteristics The energy levels diagram of single-layer blue PhOLEDs with host molecule POCz3 and blue emitter FIrpic is depicted in Fig. 5. It is obvious that the energy level alignment allows energetically favorable electron injection from cathode directly into FIrpic (energy barrier 0.1 eV) instead of POCz3 which gives an energy barrier 1.2 eV.
2028
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
Fig. 3. Typical transient photocurrent signals for POCZ3 (2.8 lm thick): (a) electron at E = 5.4 105 V/cm. Insets are the double logarithmic plots of (a). (b) Electron mobility plotted with respect to E1/2 at ambient temperature.
In contrast, the HOMO energy level of POCz3 allows hole injection from anode and transporting by way of host to reach to FIrpic. Based on the promising physical properties of POCz3, a single layer blue PhOLEDs with POCz3 as host for FIrpic was fabricated with the device configuration as ITO/PEDOT:PSS/POCz3:FIrpic/LiF/Al. It is inevitable that the recombination zone should be as away as possible from both electrodes for avoiding exciton quenching. Thus, the thickness of the single layer device needs to be carefully optimized. We used the typical doping concentration 10 wt.% of FIrpic for devices configured as ITO/PEDOT:PSS/POCz3:FIrpic (x nm)/LiF/Al, where the thicknesses x were varied as 70 nm, 80 nm, and 90 nm, respectively. The J–V and efficiency characteristics of these devices are shown in Fig. 6. It is clear that the devices have higher driving voltages as the thickness of POCz3:FIrpic layer increases. Among these three devices, the single-layer device with thickness of 80 nm gave the optimal maximum external quantum efficiency (EQE) of 7.1% at brightness of 100 cd/m2, and
showed slow efficiency roll-off as the EQE keeps at 6.4% at the brightness of 1000 cd/m2. As a consequence, we utilized 80 nm as the optimal thickness of the emitting layer in the following experiments. It should be noted that the hole-injection material PEDOT:PSS may act as a strong quencher of blue phosphorescence, rendering low device efficiency. In order to suppressing the possible triplet exciton diffusion, a neat thin film of POCz3 was introduced between the PEDOT:PSS and emitting layer comprised of POCz3 and FIrpic. Thus, we fabricated two devices with device configuration of ITO/PEDOT:PSS/POCz3 (x nm)/ POCz3:FIrpic 10.0 wt.% (80 nm)/LiF/Al, where the thickness x of POCz3 is 3 and 5 nm, respectively. Fig. 7 shows the J–V and external quantum efficiency-brightness characteristics of these two devices as compared to those of the parent device. Fig. 7(b) shows the EL spectra of the devices with and without introducing a thin film of POCz3 as exciton confinement layer. As shown, all devices have almost identical EL spectra which gave blue light originating from the FIrpic, indicating that the triplet excitons of FIrpic are
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2029
Fig. 4. Current density–voltage (I–V) characteristics of carrier-only device: hole-only device (ITO/NPB (10 nm)/POCz3 (30 nm)/NPB (10 nm) /LiF/Al and electron-only device: (ITO/BCP (10 nm)/POCz3 (30 nm)/BCP (10 nm)/LiF/Al.
Fig. 5. Relative energy levels of a single-layer PhOLED device with blue emitter FIrpic and host POCz3.
Fig. 6. Current–voltage and external quantum efficiency characteristic of POCz3:FIrpic based single layer blue PHOLEDs as a function of device thickness.
efficiently confined within the POCz3:FIrpic layer. The device with POCz3 as the interfacial layer (5 nm) achieves higher efficiency of 7.7% as compared to that of the parent
Fig. 7. (a) Current–voltage and external quantum efficiency–brightness characteristic of the exciton confined devices, (b) EL spectra of the devices with different thickness of exciton confinement layer.
device due to the exciton confinement effect. However, due to the low hole transport ability of POCz3, the driving voltages are higher with the thickness of interfacial POCz3 layer increases. As a consequence, a 3 nm neat thin film of POCz3 was utilized as the exciton confinement layer in the following experiments. In the case of direct injection and transport on the dopant, the increase of the doping concentration may lead to enhanced probability of carrier injection and transport channels, resulting in higher device efficiency [6]. In order to improve device efficiency, four devices of varied concentrations of FIrpic were fabricated with a configuration of ITO/ PEDOT:PSS/POCz3(3 nm)/POCz3: FIrpic x wt.% (80 nm)/LiF/ Al, where the doping concentration x was varied as 10, 15, 20, and 25 wt.%, respectively, in order to determine the optimal dopant concentration. J–V–L curve and efficiency characteristics of these devices are shown in Fig. 8. The driving voltage decreases as the concentration of dopant concentration increases from 10 to 20 wt.%. There was limited difference on driving voltage between the doping concentration of 20 and 25 wt.%. This result indicates that device with higher FIrpic concentration imparts better carrier injection and more transport channels. Fig. 8(b) shows that maximum external quantum efficiency (power efficiency) enhanced
2030
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
Fig. 8. (a) Current–voltage–brightness characteristic, (b) external quantum efficiencies and power efficiencies of single layer blue PhOLEDs.
from 7% (6.7 lm/W) to 9% (10.4 lm/W), when the FIrpic concentration increases from 10 to 20 wt.%. At higher doping concentration, more carriers can be directly injected and transported on FIrpic, leading to better device efficiency.
However, at dopant concentration reaches to 25 wt.%, the efficiency decreased dramatically due to the triplet–triplet annihilation and concentration quenching effects. With the optimized dopant concentration of 20 wt.%, the device
Fig. 9. Current–voltage–brightness and efficiency characteristic of POCz3 hosted three-layer blue PhOLEDs.
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
achieves the best performance, yielding maximum external quantum efficiency 9%, power efficiency 10.4 lm/W, and current efficiency 21.3 cd/A. The efficiencies of this optimized device show low roll-off effect with comparably high gext (8.1%), gp (5.6 lm/W), and gc (19.6 cd/A) when the brightness was increased to 1000 cd/m2. For evaluate the potential usefulness of POCz3 as a suitable host in multilayer OLEDs, a three-layer device using hole transport material 1,1-bis[4-[N,N-di(4-tolyl)-amino] phenyl]cyclohexane (TAPC) with high hole mobility (1 103 cm2/Vs) and electron transport material 1,3,5-tri(mpyrid-3-yl-phenyl)benzene (TmPyPB) with high electron mobility (1 103 cm2/Vs) were fabricated. Considering the triplet exciton confinement within the FIrpic (ET = 2.65 eV), a thin neat film of POCz3 (ET = 3.03 eV) was introduced between TmPyPB (ET = 2.78 eV) and POCz3:FIrpic. After optimizing charge balance by tuning the thickness of TAPC and TmPyPB, the J–V–L and efficiency characteristics of a three-layer device was shown in Fig. 9 with device configuration: ITO/TAPC (40 nm)/POCz3:FIrPic 8.0 wt.% (25 nm)/POCz3 (5 nm)/TmPyPB (50 nm)/LiF (0.8 nm)/Al. This device exhibits maximum luminance 60098 cd/m2 and maximum external quantum efficiency and power efficiency of 14.5% and 31.3 lm/W, respectively. This device shows slow efficiency roll-off as the gext retains at 12.1% as the brightness is 1000 cd/m2. 3. Conclusion In summary, an efficient one-step synthesis gives a new bipolar host material POCz3 with high triplet energy and good thermal/morphological stabilities useful for efficient single-layer and tri-layer blue PhOLED devices. The introduction of electron-withdrawing phosphine oxide renders POCz3 to give a shallow HOMO energy suitable for injecting hole from anode and transporting through to reach to the bipolar blue phosphorescence dopant FIrpic, and recombining with the electrons direct-trap from the cathode. This strategy was evidenced by the higher device efficiency as doping concentration of FIrpic increased due to enhanced probability of carrier injection and transporting channels. With the optimized dopant concentration of 20 wt.%, and incorporates a thin POCz3 film as exciton diffusion blocking layer between PEDOT:PSS and the emitting layer comprised of POCz3:FIrpic, the single-layer blue PhOLED achieves maximum external quantum efficiency of 9%, power efficiency of 10.4 lm/W, and current efficiency of 21.3 cd/A which are comparable to the common multilayer PhOLEDs devices. A three-layer device with a double confinement effect exhibits maximum luminance 60098 cd/m2 and maximum EQE (power efficiency) of 14.5% (31.3 lm/W). We believe that these results can trigger the molecular design of new bipolar host materials for more cost-effective PhOLEDs. 4. Synthesis of POCz3 Mg (243 mg, 10 mmol) in a 50 mL double-necked round-bottomed flask, equipped with a reflux condenser and an addition funnel, under an atmosphere of argon,
2031
few anhydrous THF was added in the flask. 3-bromo-9phenylcarbazole (3.22 g, 10 mmol) and anhydrous THF (10 mL) in the addition funnel were added dropwise to the flask and the reaction mixture was refluxed until Mg is disappeared. Return to room temperature, POCl3 (0.28 mL, 3 mmol) was added in the Grignard reagent. The resulting pale-yellow solution was stirred for 8 h. The mixture was quenched with saturated aqueous solution of NH4Cl (10 mL). The aqueous layer was extracted with dichloromethane (3 20 mL), and the combined organic layers were washed with water (2 20 mL), dried (MgSO4), and concentrated under reduced pressure. The residue was purified through column chromatography (SiO2; EtOAc/hexane 1:1) to yield POCz3 as a white yellow solid (4.3 g, 55%) 1H NMR (400 MHz, CDCl3) d 8.67 (d, J = 12 Hz, 3H), 8.09 (d, J = 7.6 Hz, 3H), 7.76 (td, J = 10, 1.6 Hz, 3H), 7.62–7.55 (m, 12H), 7.50–7.41 (m, 12H), 7.27 (td, J = 7.2, 2.4 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 142. 5, 141.2, 136.9, 129.8, 129.7, 129.6, 129.4, 127.8, 127.0, 126.5, 125.3, 125.2, 124.7, 123.6, 123.3, 123.1, 122.9, 120.7, 120.5, 110.0, 109.8, 109.6; 31P NMR (161 MHz, CDCl3) d 33.9; HRMS(M/z, FAB+) Calcd. For C54 H36N3OP 774.2596, found 774.2674.
References [1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151–154. [2] L.D. Hou, L. Duan, J. Qiao, W. Li, D.Q. Zhang, Y. Qi, Appl. Phys. Lett. 92 (2008) 263301–263301-3. [3] C.W. Joo, S.O. Jeon, K.S. Yook, J.Y. Lee, Org. Electron. 11 (2010) 36–40. [4] W.S. Jeon, T.J. Park, K.H. Kim, R. Pode, J. Jang, J.H. Kwon, Org. Electron. 11 (2010) 179–183. [5] (a) M.-Y. Lai, C.-H. Chen, W.-S. Huang, J.T. Lin, T.-H. Ke, L.-Y. Chen, M.-H. Tsai, C.-C. Wu, Angew. Chem. Int. Ed. 47 (2008) 581–585; (b) W.-Y. Hung, T.-C. Tsai, S.-Y. Ku, L.-C. Chi, K.-T. Wong, Phys. Chem. Chem. Phys. 10 (2008) 5822–5825; (c) W.-Y. Hung, T.-C. Wang, H.-C. Chiu, H.-F. Chen, K.-T. Wong, Phys. Chem. Chem. Phys. 12 (2010) 10685–10687; (d) C.-H. Chen, W.-S. Huang, M.-Y. Lai, W.-C. Tsao, J.T. Lin, Y.-H. Wu, T.-H. Ke, L.-Y. Chen, C.-C. Wu, Adv. Funct. Mater. 19 (2009) 2661– 2670; (e) X. Qiao, Y. Tao, Q. Wang, D. Ma, C. Yang, L. Wang, J. Qin, F. Wang, J. Appl. Phys. 108 (2010) 034508–034508-8; (f) Y. Tao, C. Yang, J. Qin, Chem. Soc. Rev. 40 (2011) 2943–2970; (g) S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 22 (2010) 5370–5373; (h) Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin, D. Ma, Angew. Chem., Int. Ed. 47 (2008) 8104–8107. [6] Z. Liu, M.G. Helander, Z. Wang, Z. Lu, Org. Electron. 10 (2009) 1146–1151. [7] N. Matsusue, Y. Suzuki, H. Naito, Jpn. J. Appl. Phys. 44 (2005) 3691–3694. [8] M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang, Y.-L. Liao, K.-T. Wong, C.–I. Wu, Adv. Mater. 18 (2006) 1216–1220. [9] F.-M. Hsu, C.-H. Chien, P.-I. Shih, C.-F. Shu, Chem. Mater. 21 (2009) 1017–1022. [10] A.B. Padmaperuma, L.S. Sapochak, Paul E. Burrows, Chem. Mater.18 (2006) 2389–2396. [11] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E. Thompson, Appl. Phys. Lett. 88 (2006) 183503–183503-3. [12] (a) H.-H. Chou, C.-H. Cheng, Adv. Mater. 22 (2010) 2468; (b) S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Adv. Funct. Mater. 19 (2009) 3644–3649; (c) S.O. Jeon, S.E. Jang, H.S. Son, J.Y. Lee, Adv. Mater. 23 (2011) 1436– 1441; (d) J. Ding, Q. Wang, L. Zhao, D. Ma, L. Wang, X. Jing, F. Wang, J. Mater. Chem. 20 (2010) 8126–8133; (e) S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Adv. Mater. 22 (2010) 1872–1876; (f) F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh, K.-W.
2032
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032 Wang, P.-T. Chou, Adv. Funct. Mater. 19 (2009) 2834–2843; (g) C.-H. Chien, F.-M. Hsu, C.-F. Shu, Y. Chi, Org. Electron. 10 (2009) 871–876; (h) P.A. Vecchi, A.B. Padmaperuma, H. Qiao, L.S. Sapochak, P.E. Burrows, Org. Lett. 8 (2006) 4211–4214; (i) X. Cai, A.B. Padmaperuma, L.S. Sapochak, P.A. Vecchi, P.E. Burrows, Appl. Phys. Lett. 92 (2008) 083308–082208-3; (j) E. Polikarpov, J.S. Swensen, N. Chopra, F. So, A.B. Padmaperuma, Appl. Phys. Lett. 94 (2009) 223304–223304-3.
[13] P. Marsal, I. Avilov, D.A. da Silva Filho, J.L. Gredas, D. Beljonne, Chem. Phys. Lett. 392 (2004) 521–528. [14] K. Brunner, A. Van Dijken, H. Brrner, J.J.A.M. Bastiaansen, N.M.M. Kiggen, B.M.W. Langeveld, J. Am. Chem. Soc. 126 (2004) 6035–6042. [15] P.M. Borsenberger, D.S. Weiss, Org. Photorecep. Imag. Sys., Marcel Dekker, New York, 1993. [16] S.C. Tse, K.C. Kwok, S.K. So, Appl. Phys. Lett. 89 (2006) 262102. [17] Z.-Y. Xie, T.-C. Wong, L.-S. Hung, S.-T. Lee, Appl. Phys. Lett. 80 (2002) 1477–1479.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
A new tricarbazole phosphine oxide bipolar host for efficient single-layer blue PhOLED Hsin-Hua Chang a,⇑, Wan-Shan Tsai b, Chien-Ping Chang a,c, Nien-Po Chen c, Ken-Tsung Wong b,⇑, Wen-Yi Hung d, Shou-Wei Chen d a
Department of Electro-Optical Engineering, Vanung University, Taoyuan 320, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan c Department of Electro-Optical Engineering, Yuanze University, Taoyuan 320, Taiwan d Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan b
a r t i c l e
i n f o
Article history: Received 22 July 2011 Received in revised form 29 August 2011 Accepted 29 August 2011 Available online 15 September 2011 Keywords: Electrophosphorescence Bipolar host material Phosphine oxide Carbazole
a b s t r a c t A novel tricarbazole phosphine oxide (POCz3) with high triplet energy and promising physical properties serves as a bipolar host material of blue-emitting phosphor (FIrpic) to realize highly efficient single-layer blue PhOLED which achieves a maximum external quantum efficiency up to 9% (gp = 10.4 lm/W, gc = 21.3 cd/A) and shows low efficiency roll-off effect. POCz3 was also utilized in a three-layer device with a double confinement effect exhibiting maximum luminance 60098 cd/m2 and maximum EQE (power efficiency) of 14.5% (31.3 lm/W). Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Recent progresses on phosphorescence organic lightemitting diodes (PhOLEDs) receive considerable research attentions owing to their capability of harvesting both singlet and triplet excitons to realize internal quantum efficiencies up to 100% [1]. In order to efficiently extract the electro-generated emissive excitons in typical PhOLEDs device, various functional layers were required for their individual role-play such as charge carrier injection, transport, and blocking as well as the confinement of exciton diffusion. Such sophisticated device configurations inevitably increase the manufacture complexity and the production cost. Therefore, it is highly desirable to fabricate PhOLEDs with simplified device structures. Particularly potential
⇑ Corresponding authors. Tel.: +886 3 4515811x73607; fax: +886 3 4515811 334 (H.-H. Chang), tel.: +886 2 33661665; fax: +886 2 33661667 (K.-T. Wong). E-mail addresses: [email protected] (H.-H. Chang), ken [email protected] (K.-T. Wong). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.030
goal is the single layer device, which can perform comparable efficiency as compared to their multilayer counterparts. There are several important factors required for efficient single layer PhOLEDs: (1) host material with suitable energy levels match to cathode/anode for easy charge injection, (2) host material with balanced electron and hole mobilities for large charge flux and efficient charge recombination, (3) high triplet energy relative to emissive phosphors for exciton confinement within the emissive layer. More recently, reports of adopting direct charge injection and transport onto the triplet dopants dispersed in an appropriate host matrix pave the way to realize efficient single-layer PhOLEDs. The host materials incorporated in these single-layer devices include mixed host materials [2–4], bipolar host materials [5], and electron-transporting host material [6]. The judicious choice of hosts with appropriate energy levels combining with hole-/electron -transporting characters in mixed systems and/or the single molecular bipolar host play the crucial role for the success of single layer device. Among these elegant works, efficient green and red single-layer PhOLEDs have been
2026
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
successfully demonstrated [5]. However, single layer blue PhOLEDs [2,5a,6] reported as far were not satisfactory yet, giving challenges for material scientists. The most popular blue emitter FIrpic has revealed its bipolar charge carrier transport behavior by using the time-of-flight (TOF) transient photocurrent technique [7]. Accordingly, highly efficient single layer blue PhOLED should be feasible if one uses FIrpic dispersed into a tailor-made high triplet energy host with suitable energy levels for direct carriers injection. In this paper, we report a novel high triplet energy host material: 3,30 ,30 0 -phosphoryl tris(9-phenyl-9Hcarbazole) (POCz3) for efficient single-layer blue PhOLEDs. The device: ITO/PEDOT:PSS/POCz3 (3 nm)/POCz3:FIrpic 20 wt.% (80 nm)/LiF/Al gives a maximum external quantum efficiency (gext) of 9%, power efficiency (gp) of 10.4 lm/W, and current efficiency (gc) of 21.3 cd/A. To the best of our knowledge, this work represents an unprecedented example of realizing a single-layer blue PhOLED with remarkable device characteristics.
2. Results and discussion 2.1. Materials The most successful host materials used for FIrpic are carbazole derivatives [8]. Without extended p-conjugation from the carbazole core, carbazole-based host materials usually exhibit high triplet energies. Thus, the triplet excitons can be efficiently confined within the emitting layer. However, typical FIrpic-based blue PhOLEDs incorporated with carbazole-based hosts require multilayer structures for facilitating the hole/electron injections into the emitting layer. In order to enhance the electron injection capability of carbazole, structural modification by introducing electron deficient moiety onto the carbazole core is necessary. Recently, triarylphosphine oxides have been reported to perform facile electron injection and transport properties [9–11]. The combination of the structural features of carbazole and phosphine oxide had led to promising bipolar hosts for blue PhOLEDs with excellent device characteristics [12]. The HOMO level of carbazole compound can be modulated by introducing appropriate substitutions at the C3 and/or C6 positions of carbazole [13–14], giving the opportunity of developing potential host materials for single layer device. Scheme 1 depicts
the structure of the new bipolar host material POCz3, which was synthesized in one step with reasonable yield from a known precursor. The 3-bromo-9-phenylcarbazole reacted with magnesium turning in THF to give a Grignard solution, which was then quenched with POCl3 to give the desired product POCz3 in 55% yield. The structural identity of POCz3 was confirmed by satisfactory 1 H, 13C and 31P NMR spectra, high-resolution mass spectrometry (HRMS). 2.2. Physical properties characterizations POCz3 exhibits a high thermal tolerance [5% weight loss occurred at 395 °C, determined by thermogravimetric analysis (TGA)] and a distinct glass transition temperature (Tg) of 163 °C analyzed by differential scanning calorimetry (DSC). Therefore, POCz3 can form homogeneous and stable amorphous films upon thermal evaporation, which is a crucial criterion for OLED application. The photophysical property of POCz3 as compared to that of parent compound 9-phenyl carbazole is shown in Fig. 1. Apparently, the introduction of phosphine oxide onto the carbazole ring only brings out limited perturbation on the photophysical property of carbazole. In solution, POCz3 exhibits a strong absorption band range from 260 to 300 nm that is attributed to the typical p–p⁄ electronic transition of carbazole, whereas the weak absorption bands (300–350 nm) are ascribed to the n–p⁄ transitions of carbazole. The emission spectrum of POCz3 is almost superimposable to that of 9-phenyl carbazole. The phosphorescent spectrum of POCz3 was obtained at 77 K (in EtOH). Again, there is no significant difference on the triplet energy upon introducing the electron deficient phosphine oxide to the carbazole as compared to that of 9-phenyl carbazole. The difference between the fluorescence (351 nm) and phosphorescence (408 nm) spectra corresponds to an exchange energy of ca. 0.67 eV between the singlet and triplet of POCz3. The triplet energy (ET) of 3.03 eV (defined as the 0–0 transition in phosphorescent spectrum) is sufficiently high for POCz3 to confine the triplet excitons of the blue phosphorescence emitter FIrpic (ET = 2.7 eV) within the emitting layer. Fig. 2 displays the cyclic voltammograms of POCz3 and 9-phenyl carbazole in CH2Cl2 using a glassy carbon electrode with 0.1 M of nBu4NPF6 as electrolyte. The oxidation of POCz3 showed typical electrochemical behavior of C3
Scheme 1. The structure and synthesis of bipolar host POCz3.
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2027
Fig. 1. A comparison of UV–vis absorption, photoluminescence (PL) (CH2Cl2), and phosphorescence (EtOH, 77 K) spectra of POCz3 and its parent counterpart N-phenyl carbazole.
Fig. 2. The cyclic voltammogram of POCz3 as compared to that of parent compound 9-phenylcarbazole.
and C6 unprotected carbazole derivatives, which usually performed a lower reduction peak originated from the more conjugated dimeric species generated from the coupling of carbazole cationic radicals. It is noteworthy that the oxidation onset of POCz3 is evidently shifted from 1.28 V of 9-phenyl carbazole to 1.44 V. Thus, the highest occupied molecular orbital (HOMO) energy of POCz3 is strongly affected by introducing the electron-withdrawing phosphine oxide. The electron deficient substituent is also beneficial to the reduction behavior of POCz3, in which one quasi-reversible reduction potential, onset potential at 2.25 V, was observed in dimethylformamide (DMF) using a glassy carbon electrode with 0.1 M of nBu4NClO4 as electrolyte. For practical evaluation, the HOMO of POCz3 were measured using UV photoemission spectroscopy (AC-2) to be 5.5 eV, and the lowest unoccupied molecular orbital (LUMO) energy was deduced to be 1.8 eV by adding the energy gap of POCz3 to the HOMO energy. To further understand the charge-carrier transport properties of POCz3, we used the time-of-flight (TOF) technique
[15] to evaluate the carrier mobilities. Fig. 3(a) displays representative TOF transient for electrons of POCz3. The transit time, tT, can be evaluated from the intersection point of two asymptotes in the double-logarithmic representation of the TOF transient. It is then used to determine the carrier mobilities, according to the equation l = d2/VtT, where d is the sample thickness and V is the applied voltage. Fig. 3(b) reveals that the field dependence of the electron mobility follows the nearly universal Poole–Frenkel relationship, with values in the range from 1.4 106 to 6.4 106 cm2/Vs for fields varying from 4 105 to 6.8 105 V/cm. In contrast, the TOF transient for holes exhibited weak photocurrent signals, which could not be used to determine the hole mobility. From the chemical structure of POCz3 (Scheme 1), the carbazole unit is a good hole transport unit and is effective in improving the hole transport property of the organic material. Hence, we also prepared hole- and electron-only single-carrier devices, to evaluate the bipolar transport behavior. We used high-LUMO 4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, lh of 103 cm2/Vs order) [16] to limit the electron carrier in hole-only devices and low-HOMO 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, le of 107 cm2/Vs order) [17] to limit the hole carrier in electron-only devices. As shown in Fig. 4, POCz3 shows bipolar transport property. However, the current density of electron-only device is larger than that of hole-only device at the same voltage, indicating the superior capability of POCz3 to transport electron other than hole-transport. This result is in agreement with the observed TOF mobility that only electron mobility can be resolved. 2.3. Device characteristics The energy levels diagram of single-layer blue PhOLEDs with host molecule POCz3 and blue emitter FIrpic is depicted in Fig. 5. It is obvious that the energy level alignment allows energetically favorable electron injection from cathode directly into FIrpic (energy barrier 0.1 eV) instead of POCz3 which gives an energy barrier 1.2 eV.
2028
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
Fig. 3. Typical transient photocurrent signals for POCZ3 (2.8 lm thick): (a) electron at E = 5.4 105 V/cm. Insets are the double logarithmic plots of (a). (b) Electron mobility plotted with respect to E1/2 at ambient temperature.
In contrast, the HOMO energy level of POCz3 allows hole injection from anode and transporting by way of host to reach to FIrpic. Based on the promising physical properties of POCz3, a single layer blue PhOLEDs with POCz3 as host for FIrpic was fabricated with the device configuration as ITO/PEDOT:PSS/POCz3:FIrpic/LiF/Al. It is inevitable that the recombination zone should be as away as possible from both electrodes for avoiding exciton quenching. Thus, the thickness of the single layer device needs to be carefully optimized. We used the typical doping concentration 10 wt.% of FIrpic for devices configured as ITO/PEDOT:PSS/POCz3:FIrpic (x nm)/LiF/Al, where the thicknesses x were varied as 70 nm, 80 nm, and 90 nm, respectively. The J–V and efficiency characteristics of these devices are shown in Fig. 6. It is clear that the devices have higher driving voltages as the thickness of POCz3:FIrpic layer increases. Among these three devices, the single-layer device with thickness of 80 nm gave the optimal maximum external quantum efficiency (EQE) of 7.1% at brightness of 100 cd/m2, and
showed slow efficiency roll-off as the EQE keeps at 6.4% at the brightness of 1000 cd/m2. As a consequence, we utilized 80 nm as the optimal thickness of the emitting layer in the following experiments. It should be noted that the hole-injection material PEDOT:PSS may act as a strong quencher of blue phosphorescence, rendering low device efficiency. In order to suppressing the possible triplet exciton diffusion, a neat thin film of POCz3 was introduced between the PEDOT:PSS and emitting layer comprised of POCz3 and FIrpic. Thus, we fabricated two devices with device configuration of ITO/PEDOT:PSS/POCz3 (x nm)/ POCz3:FIrpic 10.0 wt.% (80 nm)/LiF/Al, where the thickness x of POCz3 is 3 and 5 nm, respectively. Fig. 7 shows the J–V and external quantum efficiency-brightness characteristics of these two devices as compared to those of the parent device. Fig. 7(b) shows the EL spectra of the devices with and without introducing a thin film of POCz3 as exciton confinement layer. As shown, all devices have almost identical EL spectra which gave blue light originating from the FIrpic, indicating that the triplet excitons of FIrpic are
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2029
Fig. 4. Current density–voltage (I–V) characteristics of carrier-only device: hole-only device (ITO/NPB (10 nm)/POCz3 (30 nm)/NPB (10 nm) /LiF/Al and electron-only device: (ITO/BCP (10 nm)/POCz3 (30 nm)/BCP (10 nm)/LiF/Al.
Fig. 5. Relative energy levels of a single-layer PhOLED device with blue emitter FIrpic and host POCz3.
Fig. 6. Current–voltage and external quantum efficiency characteristic of POCz3:FIrpic based single layer blue PHOLEDs as a function of device thickness.
efficiently confined within the POCz3:FIrpic layer. The device with POCz3 as the interfacial layer (5 nm) achieves higher efficiency of 7.7% as compared to that of the parent
Fig. 7. (a) Current–voltage and external quantum efficiency–brightness characteristic of the exciton confined devices, (b) EL spectra of the devices with different thickness of exciton confinement layer.
device due to the exciton confinement effect. However, due to the low hole transport ability of POCz3, the driving voltages are higher with the thickness of interfacial POCz3 layer increases. As a consequence, a 3 nm neat thin film of POCz3 was utilized as the exciton confinement layer in the following experiments. In the case of direct injection and transport on the dopant, the increase of the doping concentration may lead to enhanced probability of carrier injection and transport channels, resulting in higher device efficiency [6]. In order to improve device efficiency, four devices of varied concentrations of FIrpic were fabricated with a configuration of ITO/ PEDOT:PSS/POCz3(3 nm)/POCz3: FIrpic x wt.% (80 nm)/LiF/ Al, where the doping concentration x was varied as 10, 15, 20, and 25 wt.%, respectively, in order to determine the optimal dopant concentration. J–V–L curve and efficiency characteristics of these devices are shown in Fig. 8. The driving voltage decreases as the concentration of dopant concentration increases from 10 to 20 wt.%. There was limited difference on driving voltage between the doping concentration of 20 and 25 wt.%. This result indicates that device with higher FIrpic concentration imparts better carrier injection and more transport channels. Fig. 8(b) shows that maximum external quantum efficiency (power efficiency) enhanced
2030
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
Fig. 8. (a) Current–voltage–brightness characteristic, (b) external quantum efficiencies and power efficiencies of single layer blue PhOLEDs.
from 7% (6.7 lm/W) to 9% (10.4 lm/W), when the FIrpic concentration increases from 10 to 20 wt.%. At higher doping concentration, more carriers can be directly injected and transported on FIrpic, leading to better device efficiency.
However, at dopant concentration reaches to 25 wt.%, the efficiency decreased dramatically due to the triplet–triplet annihilation and concentration quenching effects. With the optimized dopant concentration of 20 wt.%, the device
Fig. 9. Current–voltage–brightness and efficiency characteristic of POCz3 hosted three-layer blue PhOLEDs.
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
achieves the best performance, yielding maximum external quantum efficiency 9%, power efficiency 10.4 lm/W, and current efficiency 21.3 cd/A. The efficiencies of this optimized device show low roll-off effect with comparably high gext (8.1%), gp (5.6 lm/W), and gc (19.6 cd/A) when the brightness was increased to 1000 cd/m2. For evaluate the potential usefulness of POCz3 as a suitable host in multilayer OLEDs, a three-layer device using hole transport material 1,1-bis[4-[N,N-di(4-tolyl)-amino] phenyl]cyclohexane (TAPC) with high hole mobility (1 103 cm2/Vs) and electron transport material 1,3,5-tri(mpyrid-3-yl-phenyl)benzene (TmPyPB) with high electron mobility (1 103 cm2/Vs) were fabricated. Considering the triplet exciton confinement within the FIrpic (ET = 2.65 eV), a thin neat film of POCz3 (ET = 3.03 eV) was introduced between TmPyPB (ET = 2.78 eV) and POCz3:FIrpic. After optimizing charge balance by tuning the thickness of TAPC and TmPyPB, the J–V–L and efficiency characteristics of a three-layer device was shown in Fig. 9 with device configuration: ITO/TAPC (40 nm)/POCz3:FIrPic 8.0 wt.% (25 nm)/POCz3 (5 nm)/TmPyPB (50 nm)/LiF (0.8 nm)/Al. This device exhibits maximum luminance 60098 cd/m2 and maximum external quantum efficiency and power efficiency of 14.5% and 31.3 lm/W, respectively. This device shows slow efficiency roll-off as the gext retains at 12.1% as the brightness is 1000 cd/m2. 3. Conclusion In summary, an efficient one-step synthesis gives a new bipolar host material POCz3 with high triplet energy and good thermal/morphological stabilities useful for efficient single-layer and tri-layer blue PhOLED devices. The introduction of electron-withdrawing phosphine oxide renders POCz3 to give a shallow HOMO energy suitable for injecting hole from anode and transporting through to reach to the bipolar blue phosphorescence dopant FIrpic, and recombining with the electrons direct-trap from the cathode. This strategy was evidenced by the higher device efficiency as doping concentration of FIrpic increased due to enhanced probability of carrier injection and transporting channels. With the optimized dopant concentration of 20 wt.%, and incorporates a thin POCz3 film as exciton diffusion blocking layer between PEDOT:PSS and the emitting layer comprised of POCz3:FIrpic, the single-layer blue PhOLED achieves maximum external quantum efficiency of 9%, power efficiency of 10.4 lm/W, and current efficiency of 21.3 cd/A which are comparable to the common multilayer PhOLEDs devices. A three-layer device with a double confinement effect exhibits maximum luminance 60098 cd/m2 and maximum EQE (power efficiency) of 14.5% (31.3 lm/W). We believe that these results can trigger the molecular design of new bipolar host materials for more cost-effective PhOLEDs. 4. Synthesis of POCz3 Mg (243 mg, 10 mmol) in a 50 mL double-necked round-bottomed flask, equipped with a reflux condenser and an addition funnel, under an atmosphere of argon,
2031
few anhydrous THF was added in the flask. 3-bromo-9phenylcarbazole (3.22 g, 10 mmol) and anhydrous THF (10 mL) in the addition funnel were added dropwise to the flask and the reaction mixture was refluxed until Mg is disappeared. Return to room temperature, POCl3 (0.28 mL, 3 mmol) was added in the Grignard reagent. The resulting pale-yellow solution was stirred for 8 h. The mixture was quenched with saturated aqueous solution of NH4Cl (10 mL). The aqueous layer was extracted with dichloromethane (3 20 mL), and the combined organic layers were washed with water (2 20 mL), dried (MgSO4), and concentrated under reduced pressure. The residue was purified through column chromatography (SiO2; EtOAc/hexane 1:1) to yield POCz3 as a white yellow solid (4.3 g, 55%) 1H NMR (400 MHz, CDCl3) d 8.67 (d, J = 12 Hz, 3H), 8.09 (d, J = 7.6 Hz, 3H), 7.76 (td, J = 10, 1.6 Hz, 3H), 7.62–7.55 (m, 12H), 7.50–7.41 (m, 12H), 7.27 (td, J = 7.2, 2.4 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 142. 5, 141.2, 136.9, 129.8, 129.7, 129.6, 129.4, 127.8, 127.0, 126.5, 125.3, 125.2, 124.7, 123.6, 123.3, 123.1, 122.9, 120.7, 120.5, 110.0, 109.8, 109.6; 31P NMR (161 MHz, CDCl3) d 33.9; HRMS(M/z, FAB+) Calcd. For C54 H36N3OP 774.2596, found 774.2674.
References [1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151–154. [2] L.D. Hou, L. Duan, J. Qiao, W. Li, D.Q. Zhang, Y. Qi, Appl. Phys. Lett. 92 (2008) 263301–263301-3. [3] C.W. Joo, S.O. Jeon, K.S. Yook, J.Y. Lee, Org. Electron. 11 (2010) 36–40. [4] W.S. Jeon, T.J. Park, K.H. Kim, R. Pode, J. Jang, J.H. Kwon, Org. Electron. 11 (2010) 179–183. [5] (a) M.-Y. Lai, C.-H. Chen, W.-S. Huang, J.T. Lin, T.-H. Ke, L.-Y. Chen, M.-H. Tsai, C.-C. Wu, Angew. Chem. Int. Ed. 47 (2008) 581–585; (b) W.-Y. Hung, T.-C. Tsai, S.-Y. Ku, L.-C. Chi, K.-T. Wong, Phys. Chem. Chem. Phys. 10 (2008) 5822–5825; (c) W.-Y. Hung, T.-C. Wang, H.-C. Chiu, H.-F. Chen, K.-T. Wong, Phys. Chem. Chem. Phys. 12 (2010) 10685–10687; (d) C.-H. Chen, W.-S. Huang, M.-Y. Lai, W.-C. Tsao, J.T. Lin, Y.-H. Wu, T.-H. Ke, L.-Y. Chen, C.-C. Wu, Adv. Funct. Mater. 19 (2009) 2661– 2670; (e) X. Qiao, Y. Tao, Q. Wang, D. Ma, C. Yang, L. Wang, J. Qin, F. Wang, J. Appl. Phys. 108 (2010) 034508–034508-8; (f) Y. Tao, C. Yang, J. Qin, Chem. Soc. Rev. 40 (2011) 2943–2970; (g) S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 22 (2010) 5370–5373; (h) Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin, D. Ma, Angew. Chem., Int. Ed. 47 (2008) 8104–8107. [6] Z. Liu, M.G. Helander, Z. Wang, Z. Lu, Org. Electron. 10 (2009) 1146–1151. [7] N. Matsusue, Y. Suzuki, H. Naito, Jpn. J. Appl. Phys. 44 (2005) 3691–3694. [8] M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang, Y.-L. Liao, K.-T. Wong, C.–I. Wu, Adv. Mater. 18 (2006) 1216–1220. [9] F.-M. Hsu, C.-H. Chien, P.-I. Shih, C.-F. Shu, Chem. Mater. 21 (2009) 1017–1022. [10] A.B. Padmaperuma, L.S. Sapochak, Paul E. Burrows, Chem. Mater.18 (2006) 2389–2396. [11] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E. Thompson, Appl. Phys. Lett. 88 (2006) 183503–183503-3. [12] (a) H.-H. Chou, C.-H. Cheng, Adv. Mater. 22 (2010) 2468; (b) S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Adv. Funct. Mater. 19 (2009) 3644–3649; (c) S.O. Jeon, S.E. Jang, H.S. Son, J.Y. Lee, Adv. Mater. 23 (2011) 1436– 1441; (d) J. Ding, Q. Wang, L. Zhao, D. Ma, L. Wang, X. Jing, F. Wang, J. Mater. Chem. 20 (2010) 8126–8133; (e) S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Adv. Mater. 22 (2010) 1872–1876; (f) F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh, K.-W.
2032
H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032 Wang, P.-T. Chou, Adv. Funct. Mater. 19 (2009) 2834–2843; (g) C.-H. Chien, F.-M. Hsu, C.-F. Shu, Y. Chi, Org. Electron. 10 (2009) 871–876; (h) P.A. Vecchi, A.B. Padmaperuma, H. Qiao, L.S. Sapochak, P.E. Burrows, Org. Lett. 8 (2006) 4211–4214; (i) X. Cai, A.B. Padmaperuma, L.S. Sapochak, P.A. Vecchi, P.E. Burrows, Appl. Phys. Lett. 92 (2008) 083308–082208-3; (j) E. Polikarpov, J.S. Swensen, N. Chopra, F. So, A.B. Padmaperuma, Appl. Phys. Lett. 94 (2009) 223304–223304-3.
[13] P. Marsal, I. Avilov, D.A. da Silva Filho, J.L. Gredas, D. Beljonne, Chem. Phys. Lett. 392 (2004) 521–528. [14] K. Brunner, A. Van Dijken, H. Brrner, J.J.A.M. Bastiaansen, N.M.M. Kiggen, B.M.W. Langeveld, J. Am. Chem. Soc. 126 (2004) 6035–6042. [15] P.M. Borsenberger, D.S. Weiss, Org. Photorecep. Imag. Sys., Marcel Dekker, New York, 1993. [16] S.C. Tse, K.C. Kwok, S.K. So, Appl. Phys. Lett. 89 (2006) 262102. [17] Z.-Y. Xie, T.-C. Wong, L.-S. Hung, S.-T. Lee, Appl. Phys. Lett. 80 (2002) 1477–1479.