Letter www.acsami.org
Enhanced Luminance of Electrochemical Cells with a Rationally Designed Ionic Iridium Complex and an Ionic Additive Kristin J. Suhr,† Lyndon D. Bastatas,‡ Yulong Shen,‡ Lauren A. Mitchell,† Bradley J. Holliday,† and Jason D. Slinker*,‡ †
Department of Chemistry, The University of Texas at Austin, 105 E. 24th Street, Stop A5300, Austin, Texas 78712-1224, United States ‡ Department of Physics, The University of Texas at Dallas, 800 West Campbell Road, PHY 36, Richardson, Texas 75080-3021, United States S Supporting Information *
ABSTRACT: Light-emitting electrochemical cells (LEECs) offer the potential for high efficiency operation from an inexpensive device. However, long turn-on times and low luminance under steady-state operation are longstanding LEEC issues. Here, we present a single-layer LEEC with a custom-designed iridium(III) complex and a lithium salt additive for enhanced device performance. These devices display reduced response times, modest lifetimes, and peak luminances as high as 5500 cd/m2, 80% higher than a comparable device from an unoptimized complex and 50% higher than the salt-free device. Improved device efficiency suggests that salt addition balances space charge effects at the interfaces. Extrapolation suggests favorable half-lives of 120 ± 10 h at 1000 cd/m2 and 3800 ± 400 h at 100 cd/m2. Overall, complex design and device engineering produce competitive LEECs from simple, single-layer architectures. KEYWORDS: light-emitting electrochemical cells, ionic iridium complex, ionic transition metal complex, ionic additives, organic light-emitting diodes, luminance
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Another longstanding issue with small molecule LEEC devices is achieving high luminance for sustained driving periods. To date, LEEC devices have achieved lifetimes on the order of 1000 h, but these devices have generally been limited to luminances of hundreds of candelas per meter squared (cd/ m2).9,10 Such luminance levels, while sufficient for display applications, do not meet the 3000 cd/m2 lighting target established by the U.S. Department of Energy for OLED emitters.19 Furthermore, long lifetimes are often achieved at the expense of response time, with turn-on times ranging from hours to days under constant driving conditions.9,10 Increased luminance can be achieved in many LEEC devices through high voltage driving, but this generally leads to rapid degradation of the emitter.20,21 Likewise, fast response times can be achieved with high voltages, with the same drawback; or with pulsed driving, which complicates practical implementation of iTMCs in simple applications.12,20,21 Recently, we demonstrated a strategy for dramatically improved luminance and response of iTMC devices by using ionic additives.22 This low-cost approach was implemented in a single-layer LEEC device with [Ir(ppy)2(bpy)][PF6], which is an archetypal, but unoptimized emitter. A series of salt additives
rganic light-emitting diodes (OLEDs) are currently a viable technology for display applications.1−6 OLEDs have numerous advantages over alternative technologies, including high efficiencies, tunable emission color, and compatibility with glass and flexible plastic substrates. However, in general, these high efficiency OLEDs rely on complex fabrication methods that can significantly increase the cost. The most common method, vacuum deposition, is used to add multiple semiconducting layers, tailored specifically for electron or hole injection, transport, or recombination for emission.1 Achieving high performance OLEDs in a cost-effective device format would unlock the full potential for display and lighting applications and result in considerable energy savings. Light-emitting electrochemical cells (LEECs) consist of a single, mixed conducting layer between two electrodes. LEECs are a promising approach to decrease fabrication costs, as they are solution processable and function efficiently as single-layer devices with air-stable electrodes.7−10 Among these, LEECs using ionic transition metal complexes (iTMCs) are particularly attractive as efficient and stable materials.9,10 Studies have shown iTMCs can produce long-lasting, efficient devices, and their unique operational mechanism, involving mobile ions, facilitates their implementation in passive lighting applications.11−14 Still, some of the main challenges for LEECs involve slow turn-on times and the need for efficient, stable blue LEECs to achieve white light emission.15−18 © XXXX American Chemical Society
Received: February 12, 2016 Accepted: March 29, 2016
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DOI: 10.1021/acsami.6b01816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
lighting, along with improved efficiency metrics. This performance enhancement is discussed in view of the underlying device physics of mixed conductor devices. The phosphorescent, cationic, heteroleptic iridium(III) complex, [Ir(DiPhPy)2(bpy)][PF6], was synthesized in three steps following modified literature procedures.23−25 The composition and purity were confirmed by 1H and 13C{1H} NMR spectroscopy, electrospray ionization mass spectrometry, and elemental combustion analysis, with the measured values showing excellent agreement with the expected theoretical values. Details of the iTMC synthesis and analyses are described in the Supporting Information. Yellow crystals suitable for X-ray diffraction were obtained by slow vapor diffusion of diethyl ether into a concentrated solution of [Ir(DiPhPy)2(bpy)][PF6] in DCM (Table S1). As expected, the crystal structure in Figure 1b shows a distorted octahedral geometry around the iridium metal center with the two pyridine rings of the cyclometalating ligands oriented trans to each other and the two phenyl rings in a cis orientation. The bond lengths and angles, shown in Table S2, are in agreement with literature values for similar complexes. Steady-state and time-resolved emission spectra were recorded with the complex in solution (DCM), in the solidstate (crystals crushed into powder), as a drop-cast film (evaporated from DCM), and as a frozen solution (2-MeTHF, 77 K). The emission spectra showed a broad, featureless peak at 598 nm in solution, characteristic of metal-to-ligand chargetransfer (MLCT) emission (Figure S1). Emission spectra narrowing was observed in going from solution to the solid state, potentially due to a decreased range of vibrational energy levels, as molecular motions are more restricted in a dropcast film, solid, and frozen state.26 At 77 K, a hypsochromic shift of 82 nm is observed when compared to the solution-state emission. This is due to a strong rigidochromic effect, indicating a polar excited state, which is linked to a high degree of charge-transfer character.27 The photoluminescence (PL) quantum yield was 21% in solution and 12% in the solid state. In both cases, the complex showed short microsecond lifetimes (100 cd/m2) is reduced from over 10 s to only 5 s with addition of LiPF6, and maximum luminance is attained in under 5 min (Figure S5). In this case, the small radius of the lithium cation contributes to higher ionic mobility within the film for faster ionic redistribution to a steady-state configuration for balanced carrier injection. For a clear view of the relative carrier injection upon salt addition, we plotted external quantum efficiency (EQE) versus time for [Ir(DiPhPy)2(bpy)][PF6] with and without LiPF6, Figure 3 (top graph). In the absence of salt, the device achieves 3.45% EQE, that is, photons emitted from the device per electron injected (ph/el). Imbalanced carrier injection and nonradiative decay pathways of excitons can limit the EQE. The addition of LiPF6 improves this value to 4.72%. Increasing the spacing between emitters can improve the EQE by limiting quenching modes, such as excimer formation, but considering
⎛ L ⎞ AF T2 = T1⎜ 1 ⎟ ⎝ L2 ⎠
(1)
where T2 is the extrapolated lifetime at a maximum luminance of L2, T1 is the measured lifetime at a maximum luminance of L1, and AF is a dimensionless exponential acceleration factor generally taken to be 1.5−1.6. Assuming AF is 1.5, for an L2 of 1000 cd/m2, a lifetime of 120 ± 10 h is estimated. Going further, for AF of 1.5 and L2 of 100 cd/m2, a lifetime of 3800 ± 400 h is obtained. Thus, it appears that the [Ir(DiPhPy)2(bpy)][PF6] LEEC device luminances and lifetimes are competitive with the best iTMC devices reported to date.9,10 It should be noted that a recent report of a similar phenyl substituted complex led to devices with exceptional extrapolated half-lives of over 2500 h at lower luminance levels under pulsed driving.30 Thus, it appears that phenyl substituted complexes have a high potential for long-lasting iTMC-LEECs. Overall, the combination of a rational phosphorescent, cationic iridium complex design with a lithium salt additive brings about gains in the luminance and efficiencies of the iTMC LEECs. Luminance values surpassing the US Department of Energy benchmark are attained with response times of minutes under simple, steady state operation. Gains in device metrics are rationalized in terms of balanced carrier injection after increased redistribution of ionic space charges. Modest lifetimes are achieved, but extrapolated lifetimes at lower luminances show that this [Ir(DiPhPy)2(bpy)][PF6]/0.3% LiPF6 combination is competitive with the best iTMC devices reported. Clearly, both rational synthesis of novel complexes and control of space charge effects with ionic additives are key factors in the optimization of iTMC LEEC devices.
Figure 3. (Top) External quantum efficiency (EQE) vs time and (bottom) current efficiency vs time of [Ir(DiPhPy)2(bpy)][PF6] LEEC devices under constant current driving with and without (Pristine) 0.3% LiPF6. C
DOI: 10.1021/acsami.6b01816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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(9) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard, S.; Abruna, H. D.; Malliaras, G. G. Electroluminescent Devices from Ionic Transition Metal Complexes. J. Mater. Chem. 2007, 17, 2976− 2988. (10) Costa, R. D.; Ortí, E.; Bolink, H. J. Recent Advances in LightEmitting Electrochemical Cells. Pure Appl. Chem. 2011, 83, 2115− 2128. (11) Slinker, J. D.; Rivnay, J.; DeFranco, J. A.; Bernards, D. A.; Gorodetsky, A. A.; Parker, S. T.; Cox, M. P.; Rohl, R.; Malliaras, G. G.; Flores-Torres, S.; Abruna, H. D. Direct 120 V, 60 Hz Operation of an Organic Light Emitting Device. J. Appl. Phys. 2006, 99, 074502. (12) Tordera, D.; Meier, S.; Lenes, M.; Costa, R. D.; Ortí, E.; Sarfert, W.; Bolink, H. J. Simple, Fast, Bright, and Stable Light Sources. Adv. Mater. 2012, 24, 897−900. (13) Bolink, H. J.; Coronado, E.; Costa, R. D.; Ortí, E.; Sessolo, M.; Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.; Constable, E. C. Long-Living Light-Emitting Electrochemical Cells - Control through Supramolecular Interactions. Adv. Mater. 2008, 20, 3910− 3913. (14) Su, H.-C.; Wu, C.-C.; Fang, F.-C.; Wong, K.-T. Efficient SolidState Host-Guest Light-Emitting Electrochemical Cells Based on Cationic Transition Metal Complexes. Appl. Phys. Lett. 2006, 89, 261118. (15) Meier, S. B.; Tordera, D.; Pertegás, A.; Roldán-Carmona, C.; Ortí, E.; Bolink, H. J. Light-Emitting Electrochemical Cells: Recent Progress and Future Prospects. Mater. Today 2014, 17, 217−223. (16) Costa, R. D.; Ortí, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Luminescent Ionic Transition-Metal Complexes for Light-Emitting Electrochemical Cells. Angew. Chem., Int. Ed. 2012, 51, 8178−8211. (17) Hu, T.; He, L.; Duan, L.; Qiu, Y. Solid-state Light-Emitting Electrochemical Cells Based on Ionic Iridium(III) Complexes. J. Mater. Chem. 2012, 22, 4206−4215. (18) Dumur, F.; Bertin, D.; Gigmes, D. Iridium (III) Complexes as Promising Emitters for Solid-State Light-Emitting Electrochemical Cells (LECs). Int. J. Nanotechnol. 2012, 9, 377−395. (19) Brodrick, J. R. Solid-State Lighting Research and Development: Multi-Year Program Plan; U. S. Department of Energy: Washington, D.C., 2012. (20) Handy, E. S.; Pal, A. J.; Rubner, M. F. Solid-State Light-Emitting Devices Based on the Tris-Chelated Ruthenium(II) Complex. 2. Tris(Bipyridyl)Ruthenium(II) as a High-Brightness Emitter. J. Am. Chem. Soc. 1999, 121, 3525−3528. (21) Buda, M.; Kalyuzhny, G.; Bard, A. J. Thin-Film Solid-State Electroluminescent Devices Based on Tris(2,2′-Bipyridine)Ruthenium(II) Complexes. J. Am. Chem. Soc. 2002, 124, 6090−6098. (22) Shen, Y.; Kuddes, D. D.; Naquin, C. A.; Hesterberg, T. W.; Kusmierz, C.; Holliday, B. J.; Slinker, J. D. Improving Light-Emitting Electrochemical Cells with Ionic Additives. Appl. Phys. Lett. 2013, 102, 203305. (23) Edkins, R. M.; Wriglesworth, A.; Fucke, K.; Bettington, S. L.; Beeby, A. The Synthesis and Photophysics of Tris-Heteroleptic Cyclometalated Iridium Complexes. Dalton Trans. 2011, 40, 9672− 9678. (24) Lowry, M. S.; Hudson, W. R.; Pascal, R. A.; Bernhard, S. Accelerated Luminophore Discovery through Combinatorial Synthesis. J. Am. Chem. Soc. 2004, 126, 14129−14135. (25) Hesterberg, T. W.; Yang, X.; Holliday, B. J. Polymerizable Cationic Iridium(III) Complexes Exhibiting Color Tunable Light Emission and Their Corresponding Conducting Metallopolymers. Polyhedron 2010, 29, 110−115. (26) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular Photochemistry: An Introduction; Stiefel, J., Ed.; University Science Books: Sausalito, CA, 2009. (27) Lees, A. J. The Luminescence Rigidochromic Effect Exhibited by Organometallic Complexes: Rationale and Applications. Comments Inorg. Chem. 1995, 17, 319−346.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01816. Details of synthesis, iTMC analyses, and device fabrication; crystal structure determination, UV−vis absorption spectrum, cyclic voltammogram, photoluminescence spectra, photophysical properties, electroluminescence spectra, LEEC device properties, and plot of device turn-on time (PDF) crystallographic information file for [(2,2'-bipyridine)-bis(2,4-diphenylpyridine)-iridium(III)] hexafluorophosphate (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
J.D.S. acknowledges support from UT Dallas startup funds. B.J.H. gratefully acknowledges the Welch Foundation (F-1631) and the National Science Foundation (CHE-0847763) for financial support of this research. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank David Taylor, Gary Frazier, and Timothy Mulone for their roles in assembling the OLED test station for this work.
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ABBREVIATIONS OLED, organic light-emitting diode LEEC, light-emitting electrochemical cell iTMC, ionic transition metal complex DiPhPy, 2,4-diphenylpyridine bpy, 2,2′-bipyridine EQE, external quantum efficiency
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REFERENCES
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DOI: 10.1021/acsami.6b01816 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
