High-efficiency blue-green electrophosphorescent light-emitting devices using a bis-sulfone as host in the emitting layer

Organic Electronics 12 (2011) 1314–1318

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High-efficiency blue-green electrophosphorescent light-emitting devices using a bis-sulfone as host in the emitting layer Sung-Jin Kim a,1, Julie Leroy b, Carlos Zuniga b, Yadong Zhang b, Lingyun Zhu b, John S. Sears b, Stephen Barlow b, Jean-Luc Brédas b, Seth R. Marder b, Bernard Kippelen a,⇑ a b

Center for Organic Photonics and Electronics (COPE), School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA Center for Organic Photonics and Electronics (COPE), School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA

a r t i c l e

i n f o

Article history: Received 11 February 2011 Received in revised form 15 April 2011 Accepted 26 April 2011 Available online 10 May 2011 Keywords: Organic light emitting diodes Phosphorescence

a b s t r a c t A bis-sulfone small molecule, 4,40 -bis(phenylsulfonyl)biphenyl has been evaluated as a 0 host for the phosphor iridium(III) bis(2-(4,6-difluorophenyl)pyridinato-N,C2 )picolinate in blue-green organic light-emitting devices. In addition, a poly(norbornene)-based polymer functionalized with a 3,6-bis(carbazol-9-yl)carbazole moiety as a side group is utilized as a solution-processible hole-transport layer; comparison is made to the widely-used holetransport polymer poly(N-vinylcarbazole) (PVK). At 100 cd/m2, the highest efficiency device incorporating the new polymer achieved an external quantum efficiency (EQE) of 6.9% (12.9 cd/A) with a turn-on voltage of 4.3 V. In comparison, the highest efficiency device using PVK achieved an EQE of 6.4% (11.3 cd/A) with a turn-on voltage of 6.0 V. Ó 2011 Elsevier B.V. All rights reserved.

In recent years, significant progress has been made towards the development of organic-based semiconductors for display and lighting applications. These materials have the potential to lead to cost-effective, ultrathin, lightweight, and large-area organic light-emitting diodes (OLEDs) [1–7]. In particular, bright blue emission from OLEDs is of importance for the development of satisfactory full-color flat-panel displays and of white lighting in combination with complementary yellow light emission [8– 11]. In comparison with red or green light-emitting devices, it is typically more challenging to obtain high-efficiency blue phosphorescent OLEDs because host materials must be developed that have triplet energies that are higher than those of the phosphorescent dopant in order to effectively capture and confine triplet excitons on the phosphor. Recently, phosphine oxide derivatives, such as 4,40 bis(diphenylphosphoryl)biphenyl (PO1, Fig. 2), have been ⇑ Corresponding author. E-mail address: [email protected] (B. Kippelen). Present address: College of Electrical and Computer Engineering, Chungbuk National University, Cheongju 361-763, Republic of Korea. 1

1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.04.015

identified as effective host materials for blue-green-emitting phosphors such as iridium(III) bis(2-(4,6-difluoro0 phenyl)pyridinato-N,C2 )picolinate (FIrpic) [12,13]. The phosphoryl substituents of PO1 are believed to exert a predominantly inductive electron-withdrawing effect on the biphenyl core [12], thus largely retaining its high triplet energy while lowering the lowest unoccupied molecular orbital energy and thereby facilitating electron injection. Since sulfonyl substituents have similar, albeit somewhat more potent, electron-withdrawing character and may provide additional flexibility in terms of synthesis, we were interested in investigating the efficacy of sulfones as host materials. Precedence for this approach can be found in the work of Hsu et al. who reported highly efficient red phosphorescent OLEDs with a triarylamine-functionalized bis-sulfone as a host material [14]. In this work, we report high-efficiency blue-green electrophosphorescent OLEDs in which the vacuum-deposited emissive layer consists of 4,40 -bis(phenylsulfonyl)biphenyl (SO1) [15] doped with FIrpic and in which a spin-coated carbazole-based polymer is used as hole-transport layer. Density functional theory (DFT) computations [16] at the B3LYP/6-31G⁄⁄ level [17–19] indicate the similarity of

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PO1 and SO1. The computed electronic properties of PO1 and SO1 are consistent with the available experimental data (discussed below). The vertical triplet energies evaluated at the TDDFT level (Table 1) are identical, 3.14 eV; they are significantly higher than that of FIrpic (2.70 eV at the same level of theory) and only 0.17 eV lower than that of the unsubstituted biphenyl core. The adiabatic triplet energies computed at the DSCF level (computed as the difference in the SCF energies at the optimized S0 and T1 geometries) are nearly identical, being 2.87 and 2.88 eV for PO1 and SO1, respectively. The dominant excitation giving rise to the triplet state is localized on the biphenyl moiety and corresponds in both cases to a HOMO-to-LUMO transition. The very similar nature of the HOMO and LUMO orbitals of PO1 and SO1 is evident from the isosurface plots displayed in Fig. 1. The stronger, predominantly inductive, electron-withdrawing character of the sulfonyl moiety relative to that of the phosphoryl substituent can be seen from the trends in the computed HOMO/LUMO energies and electron affinities (EAs). In general, the values for PO1 are calculated to lie between those of SO1 and biphenyl. Thus, the computational results are consistent with a largely inductive effect for the sulfonyl substitution and predict that the high triplet energy of PO1 should be maintained in SO1.

Compound SO1 is reversibly reduced at 2.06 V vs. FeCp2+/0 in 0.1 Bu4NPF6/N,N-dimethylformamide, i.e. somewhat more readily than PO1 under the same conditions (2.4 V) [12], consistent with the trend in computed EAs. Its absorption and fluorescence maxima in dichloromethane (277 and 331 nm, respectively) are very similar to those reported for PO1 in the same solvent [12]. The adiabatic triplet energy for SO1 is estimated to be 2.72 eV from the phosphorescence spectrum acquired in a 2-methyltetrahydrofuran glass at 77 K. As in the case of the TDDFT values, the value for SO1 is indistinguishable within experimental uncertainty from the value for PO1 (2.72 eV) [12] and is higher than the value of 2.65 eV reported for FIrpic [20], suggesting that SO1 might also be an effective host for this phosphor. As expected, the experimental adiabatic triplet energies (based on the highest energy emission at the relaxed triplet geometry) are smaller than the computed vertical triplet energies (calculated at the singlet geometry), while the computed adiabatic triplet energies (DSCF level) provide energies that are only slightly higher than those measured from the phosphorescence spectra. Small-molecule derivatives of the 3,6-bis(carbazol-9yl)carbazole (‘‘tricarbazole’’) moiety have been used as hole-transport components for OLEDs [21,22]. In order to simplify the fabrication process of our OLEDs we were

Table 1 Computed HOMO and LUMO energies, adiabatic ionization potentials and electron affinities, vertical TDDFT transition energies from the ground state to the lowest singlet (S0-S1⁄) and triplet (S0-T1⁄) excited states, and the DSCF adiabatic triplet energies (S0  T1).a

PO1 SO1 Biphenyl a

HOMO

LUMO

IP

EA

S0  S1⁄

S0  T1⁄

S0  T1

6.42 6.85 6.06

1.51 2.02 0.69

7.40 7.96 7.61

0.65 1.04 0.70

4.47 4.39 5.02

3.14 3.14 3.31

2.87 2.88 3.04

All values are in eV and computed at the DFT B3LYP/6-31G⁄⁄ level. EAs are defined as the negative of the free-energy change for the process M + e ? M.

Fig. 1. Illustration of the HOMO and LUMO wave functions computed at the DFT B3LYP/6-31G⁄⁄ level for PO1 (left) and SO1 (right).

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Fig. 2. Chemical structures of the materials used in this study.

Fig. 3. Energy level diagram based on the estimated solid-state IPs and EAs of the materials discussed in this work.

interested in using a solution-processible tricarbazolebased material as a hole-transport layer (HTL), since this would reduce the number of layers requiring vacuum deposition. As earlier work on poly(norbornene) polymers functionalized with charge-transport side groups has shown that the backbone of the polymer does not adversely affect the energy levels of the moiety [23,24], polymer P-TCz, a poly(norbornene) with pendant tricarbazole moieties, was synthesized (Mw = 49,000; PDI = 1.9 according to GPC in chloroform vs. polystyrene standards) [25]. For comparison to devices incorporating P-TCz, analogous devices were also fabricated using the widely used hole-

transport polymer, poly(N-vinylcarbazole) (PVK, Mw = 81,800; PDI = 1.68) as a solution-processible HTL with good hole-transport properties, as reported, for example, by Kido et al. [26]. Devices were fabricated on glass substrates precoated with indium tin oxide (ITO) with a sheet resistance of 20 X/h (Colorado Concept Coatings, L.L.C.). The substrates were first cleaned in an ultrasonic bath using a dilute solution of Triton-X (Aldrich) in deionized (DI) water (20 min) followed by successive ultrasonication in DI water, acetone, and finally ethanol (20 min each). Washed ITO substrates were then dried in a vacuum oven at 70 °C under

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Fig. 4. J–V characteristics for devices using PVK (device I) and polymer P-TCz (device II) as a HTL. (Inset) Log scale J–V plot.

10

1000

Device I: 6 wt.% Device I:10 wt.% Device II: 6 wt.% Device II:10 wt.%

6 4

2

Luminance (cd/m )

100

8

2 10

EL (a. u.)

1000

0 10

100

8 6

400

600

800

External Quantum Efficiency (%)

vacuum (1  102 Torr) for 1 h. Films of PVK and polymer P-TCz with a thickness of 35 nm were spin-coated from toluene onto air-plasma treated ITO coated substrates in a nitrogen inert atmosphere. The substrates were then loaded into a Kurt J. Lesker Spectros vacuum system without being exposed to atmosphere. For all subsequent organic layers, materials were first purified using gradient zone sublimation, and were then thermally evaporated at a pressure below 1  107 Torr. The OLED devices have the following architecture: ITO/spin-coated carbazolebased polymer (35 nm) – either PVK (device I) or polymer P-TCz (device II) – as a HTL/compound SO1 layer doped with 6 or 10 wt% FIrpic (20 nm) as an emitting layer/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (40 nm) as a hole-blocking layer. The cathode consisted of a thin layer of lithium fluoride (LiF) (with an effective thickness of 2.4 nm based on the deposition rate and time) followed by a 200-nm thick layer of aluminum (Al). The thickness of the different layers was selected based on our experience with the optimization of electrophosphorescent devices with comparable geometry. The chemical structures of the materials used are shown in Fig. 2 and the estimated ionization potentials (IPs) and electron affinities (EAs) [27] of the materials used are shown in Fig. 3. Luminance–current density–voltage (L–J–V) characteristics of the devices were measured using a Keithley 2400 source meter for current–voltage measurements. The devices were tested inside a nitrogen-filled glovebox with O2 and H2O levels <20 and <1 ppm, respectively. The current density–voltage (J–V) characteristics of the two device architectures, with PVK (device I) or polymer PTCz (device II) as a HTL, are shown in Fig. 4. It is noted that the operation voltage of devices of type I are greater than that for devices of type II; for instance, the operation voltage at 2 mA/cm2 is 9.3 V for device I with 6 wt% doping of the FIrpic phosphor and 9.7 V for device I with 10 wt% doping, whereas for device II the operation voltage is 7.1 V at 6 wt% doping and 6.4 V at 10 wt%. At a luminance of 100 cd/m2, device II shows maximum external quantum efficiencies (EQE) of 6.9% and a current efficiency of

4

Wavelength (nm)

2 10 -2

0

2 4 6 Applied Voltage (V)

8

0 10

Fig. 5. Luminance and external quantum efficiency as a function of applied voltage for devices. (Inset) EL spectrum of device II (6 wt%).

12.9 cd/A, while device I shows efficiencies of 6.4% and 11.3 cd/A, respectively. The luminance–voltage (L–V) and EQE curves of the OLED devices are compared in Fig. 5 and show that the luminances for SO1-based blue-green OLEDs incorporating polymer P-TCz are higher than for those incorporating PVK. Furthermore, the turn-on voltages (defined as the voltage required to obtain a brightness of 10 cd/m2) of the P-TCz-based devices II (4.4 and 4.3 V for devices with 6 and 10 wt% FIrpic, respectively) are clearly lower than those of the PVK-based devices I (6.4 and 6.0 V for 6 and 10 wt%). The EQE, current efficiencies, and electroluminescence (EL) spectra demonstrate that compound SO1 is a good host material for FIrpic in electrophosphorescent OLEDs. Indeed, the performance of the current hybrid (mixed solution/vacuum-deposited) devices approaches that of fully vacuum-deposited devices in which its phosphine-oxide analog, PO1, was used as a host for FIrpic (EQE up to 7.8%) [28]. Carrier injection and transport efficiency may be crucial issues affecting the charge balance and quantum efficiency of OLEDs. The differences between devices of types I and II may be attributable either to different hole mobility values and/or injection efficiencies in the two polymers. Although the electrochemically estimated ionization potential of PTCz is a little lower than that of PVK, this apparent difference should be treated with caution due to the irreversible oxidation of the latter material. Moreover, the increased chemical stability of charge carriers in P-TCz relative to PVK, which is reflected by the more reversible electrochemistry of the former, may play a role. The hole mobility of P-TCz will be investigated as part of our continuing study of this polymer.

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In summary, we have demonstrated that a bis-sulfone host (SO1) can act as a high-triplet-energy host suitable for the fabrication of highly efficient blue-green phosphorescent OLEDs. We believe that the synthetic flexibility of sulfone chemistry opens the possibility for a range of useful organic electronic materials based upon bis-sulfones. Devices employing the new tricarbazole-functionalized polymer P-TCz as a HTL showed that the current density was increased by 20 times and that the operation voltage was also significantly lowered as compared to that of PVK devices. Acknowledgments This material is based upon work supported by Solvay SA and in part by the STC Program of the National Science Foundation under Agreement No. DMR-0120967. We are grateful to Jean-Pierre Catinat and Véronique Mathieu for measuring the adiabatic triplet energy of SO1. References [1] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, Nearly 100% internal phosphorescence efficiency in an organic light-emitting device, J. Appl. Phys. 90 (2001) 5048. [2] V.V. Bulovic, B. Khalfin, G. Gu, P.E. Burrows, D.Z. Garbuzov, S.R. Forrest, Weak microcavity effects in organic light-emitting devices, Phys. Rev. B 58 (1998) 3730. [3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Macay, R.H. Friend, P.L. Burns, A.B. Holmes, Light-emitting-diodes based on conjugated polymers, Nature (London) 347 (1990) 539. [4] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, A. Stocking, Organic electroluminescent devices, Science 273 (1996) 884. [5] X. Zhou, M. Pfeiffer, J. Blochowitz, A. Werner, A. Nollau, T. Fritz, K. Leo, Very-low-operating-voltage organic light-emitting diodes using a pdoped amorphous hole injection layer, Appl. Phys. Lett. 78 (2001) 410. [6] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A.D. Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Electroluminescence in conjugated polymers, Nature (London) 397 (1999) 121–128. [7] C.W. Tang, S.A. VanSlyke, C.H. Chen, Electroluminescence of doped organic thin films, J. Appl. Phys. 65 (1989) 3610–3616. [8] S.E. Shaheen, G.E. Jabbour, M.M. Morrell, Y. Kawabe, B. Kippelen, N. Peyghambarian, M.-F. Nabor, R. Schlaf, E.A. Mash, N.R. Armstrong, Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode, J. Appl. Phys. 84 (1998) 2324–2327. [9] F. Li, W. Jia, S. Wang, Y. Zhao, Z.-H. Lu, Blue organic light-emitting diodes based on Mes2B [p-4,40 -biphenyl-NPh(1-naphthyl)], J. Appl. Phys. 103 (2008) 034509. [10] M.-F. Lin, L. Wang, W.-K. Wong, K.-W. Cheah, H.-L. Tam, M.-T. Lee, C.H. Chen, Highly efficient and stable sky blue organic light-emitting devices, Appl. Phys. Lett. 89 (2006) 121913. [11] H. Fukagawa, K. Watanabe, T. Tsuzuki, S. Tokito, Highly efficient, deep-blue phosphorescent organic light emitting diodes with a double-emitting layer structure, Appl. Phys. Lett. 93 (2008) 133312. [12] L.S. Sapochak, A.B. Padmaperuma, P.A. Vecchi, H. Qiao, P.E. Burrows, Design strategies for achieving high triplet energy electron transporting host materials for blue electrophosphorescence, Proc. SPIE 6333 (2006) 63330F. [13] E. Polikarpov, J.S. Swensen, N. Chopra, F. So, A.B. Padmaperuma, An ambipolar phosphine oxide-based host for high power efficiency blue phosphorescent organic light emitting devices, Appl. Phys. Lett. 94 (2009) 223304.

[14] F.-M. Hsu, C.-H. Chien, Y.-J. Hsieh, C.-H. Wu, C.-F. Shu, S.-W. Liub, C.T. Chen, Highly efficient red electrophosphorescent device incorporating a bipolar triphenylamine/bisphenylsulfonylsubstituted fluorene hybrid as the host, J. Mater. Chem. 19 (2009) 8002. [15] R. Adams, W. Reifschneider, M.D. Nair, The preparation of aryl thioethers by reaction of aromatic halogen compounds with cuprous thiophenolate or cuprous thiobutylate, Croat. Chem. Acta 29 (1957) 277. [16] M.J. Frisch, G.W. Trucks, H.B. Schlegel, GAUSSIAN 09, Gaussian Inc., Pittsburgh, PA, 2009. [17] W.J. Hehre, R. Ditchfie, J.A. Pople, Self-consistent molecular-orbital methods: 12. Further extensions of Gaussian-type basis sets for use in molecular-orbital studies of organic-molecules, J. Chem. Phys. 56 (1972) 2257–2261. [18] C.T. Lee, W.T. Yang, R.G. Parr, Development of the Colle–Salvetti correlation-energy formula into a functional of the electron-density, Phys. Rev. B 37 (1988) 785–789. [19] A.D. Becke, Density-functional thermochemistry: 3. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648–5652. [20] R.J. Holmes, S.R. Forrest, Y.-J. Tung, R.C. Kwong, J.J. Brown, S. Garon, M.E. Thompson, Blue organic electrophosphorescence using exothermic host–guest energy transfer, Appl. Phys. Lett. 82 (2003) 2422. [21] Q. Zhang, Y.F. Hu, Y.X. Cheng, G.P. Su, D.G. Ma, L.X. Wang, X.B. Jing, F.S. Wang, Carbazole-based hole-transporting materials for electroluminescent devices, Synth. Met. 137 (2003) 1111. [22] M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu, A. Matoliukstyte, J. Simokaitiene, S. Grigalevicius, J.V. Grazulevicius, C.-P. Hsu, 3-(9-Carbazolyl)carbazoles and 3,6-di(9carbazolyl)carbazoles as effective host materials for efficient blue organic electrophosphorescence, Adv. Mater. 19 (2007) 862. [23] J.-Y. Cho, B. Domercq, S. Barlow, K.Y. Suponitsky, J. Li, T.V. Timofeeva, S.C. Jones, L.E. Hayden, A. Kimyonok, C.R. South, M. Weck, B. Kippelen, S.R. Marder, Synthesis and characterization of polymerizable phosphorescent platinum(II) complexes for solution-processible organic light-emitting diodes, Organometallics 26 (2007) 4816. [24] A. Kimyonok, B. Domercq, A. Haldi, J.-Y. Cho, J.R. Carlise, X.-Y. Wang, L.E. Hayden, S.C. Jones, S. Barlow, S.R. Marder, B. Kippelen, M. Weck, Norbornene-based copolymers with iridium complexes and bis(carbazolyl)fluorene groups in their side-chains and their use in light-emitting diodes, Chem. Mater. 19 (2007) 5602. [25] Y. Zhang, S.R. Marder, C. Zuniga, S. Barlow, B. Kippelen, A. Haldi, B. Domercq, M. Weck, A. Kimyonok, Carbazole group-containing ROMP-prepared norbornene derivative polymer hole transport and/or electron blocking materials and/or host materials for luminescent layers, PCT Int., WO2009080799 (2009). [26] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Bright blue electroluminescence from poly(N-vinylcarbazole), Appl. Phys. Lett. 63 (1993) 2627. [27] The EA of SO1 was estimated from the electrochemical reduction potential vs. ferrocene according to EA ¼ eEred 1=2 þ 4:8 eV and the IP from the EA estimate and the absorption onset, as previously described for PO1 in Ref. [12]. The IP of P-TCz was estimated from the oxidation potential of the corresponding monomer vs. ferrocene according to IP ¼ eEox 1=2 þ 4:8 eV and its EA from the IP estimate and the absorption onset. Accordingly the IP of SO1 and the EA of P-TCz are likely to be under- and overestimated, respectively, due to the neglect of exciton binding energy. The values for FIrpic and BCP were taken from V.I. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B.W. D’Andrade, S.R. Forrest, New charge-carrier blocking materials for high efficiency OLEDs, Org. Electron. 4 (2003) 77 and A. Kahn, N. Koch, W. Gao, Electronic structure and electrical properties of interfaces between metals and p-conjugated molecular films, J. Polym. Sci. B 41 (2003) 2529, respectively. [28] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E. Thompson, Ultraviolet electroluminescence and blue-green phosphorescence using an organic diphosphine oxide charge transporting layer, Appl. Phys. Lett. 88 (2006) 183503.