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White Light-Emitting Electrochemical Cells based on the Langmuir-Blodgett Technique Jesús Miguel Fernández-Hernández, Luisa De Cola, Henk J Bolink, Miguel ClementeLeon, Eugenio Coronado, Alicia Forment-Aliaga, Angel López-Muñoz, and Diego Repetto Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503144v • Publication Date (Web): 27 Oct 2014 Downloaded from http://pubs.acs.org on October 30, 2014
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White Light-Emitting Electrochemical Cells based on the Langmuir-Blodgett Technique Jesús M. Fernández-Hernández,a,c Luisa De Cola,a,d Henk J. Bolink,b* Miguel ClementeLeón,b* Eugenio Coronado,b Alicia Forment-Aliaga,b Angel López-Muñoz,b Diego Repettob,e [a] Dr. J. M. Fernández-Hernández, Prof. Luisa De Cola Physicalisches Institut and Center for Nanotechnology (CeNTech), Westfälische WilhelmsUniversität Münster, Heisenbergstrasse 11, 48149 Münster, Germany [b]
Dr. H. J. Bolink, Dr. M. Clemente-León, Prof. E. Coronado, Dr. A. Forment-Aliaga, A. López-Muñoz, Dr. D. Repetto
Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedrático José Beltrán 2, 46980 Paterna, Spain Tel: (+34) 96 3544415 Fax: (+34) 96 354 3273 E-mail:
[email protected] and
[email protected] [c]
Present address: Departamento de Química Inorgánica, Facultad de Química, Universidad de Murcia, Apdo. 4021, 30071 Murcia, Spain. 1 ACS Paragon Plus Environment
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[d]
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Present address: Université de Strasbourg – Institut de Science et d'Ingénierie
Supramoléculaires (ISIS), 8 Rue Gaspard Monge, 67083 Strasbourg, France. [e]
Present address: Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy.
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)
Abstract
Light-emitting electrochemical cells (LECs) showing a white emission have been prepared with
Langmuir-Blodgett
(LB)
films
of
the
metallosurfactant
Bis[2-(2,4-
difluorophenyl)pyridine][2-(1-hexadecyl-1H-1,2,3-triazol-4-yl)pyridine]iridium(III) chloride (1), which work with an air-stable Al electrode. They were prepared by depositing a LB film of 1 on top of a layer of Poly(N,Nʹ′-diphenyl-N,Nʹ′-bis(4-hexylphenyl)-[1,1ʹ′biphenyl]-4,4ʹ′-diamine (pTPD) spin-coated on indium tin oxide (ITO). The white color of the electroluminescence of the device contrasts with the blue color of the photoluminescence of 1 in solution and within the LB films. Furthermore, the crystal structure of 1 is reported together with the preparation and characterization of the Langmuir monolayers (π−A compression isotherms and Brewster Angle Microscopy (BAM)) and LB films of 1 (IR, UV-Vis and emission spectroscopy, X-ray photoelectron spectroscopy (XPS), Specular X-ray reflectivity (SXR) and Atomic Force microscopy (AFM)).
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Keywords: Organic light-emitting-diodes (OLEDs), Light-emitting electrochemical cell (LECs), Iridium complexes, Langmuir-Blodgett, monolayer.
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Introduction Electroluminescent molecular devices are becoming a serious alternative to their inorganic analogues as their efficiencies and stabilities have improved dramatically over the last years.1 The most efficient and stable organic light-emitting devices (OLEDs) are based on a multistack architecture of low-molecular weight components. In the last few years the preparation of efficient white OLEDs has generated significant interest for solid-state lighting applications, due to potential savings in both costs and energy use.2,3 Many white light emission devices are obtained by sequential evaporation of the active materials under high vacuum conditions, which uses the materials in an inefficient manner and requires high investments.4,5 In the search of simple and stable processing and low cost devices it is desirable to use large area compatible solution based processes. Light-emitting electrochemical cell (LECs) represent an interesting alternative.6-8 In its simplest form LECs consist of a single active layer composed of an ionic transition-metal complex that provide the ionic transport in the device.9-12 LECs have a much simpler architecture and operate with air-stable electrodes.13-15 This allows for their preparation using solution-based techniques, which makes them suitable for low-cost and large-area applications. Cationic heteroleptic iridium(III) complexes with the structure [Ir(C-N)2(N-N)]PF6, where C-N is an anionic cyclometallating ligand (e.g., 2-phenylpyridine, ppy) and N-N is a neutral diimine ancillary ligand (e.g., 2,2´-bipyridine, bpy) represent the most promising candidates for long-living, stable LECs. This is as a consequence of the excited state properties of the Ir(III) complexes, which typically exhibit long-lived excited states and high emission quantum yields.16 One possible strategy to improve the blue-emission of the devices is the
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replacement of the pyridine ring of the C-N ligand with a 1,2,3-triazole as it promotes a blue-shift in emission.17 The Langmuir-Blodgett (LB) technique is a well-established method toward the controllable fabrication of well-organized multilayered films with a very precise control of the thickness and composition that is not available with other techniques.18 This technique has been applied to create ultrathin films, which can be used as chemical sensors, modified electrodes or molecular electronic devices.18-20 There are many examples of the use of this technique to prepare emissive,21-26 hole-injection27,28 or insulating layers for OLEDs.29,30 Recently, our group have prepared electroluminescent devices formed by LB films containing alternating layers of Iridium(III) and Ruthenium(II) complexes that show dual emission by simple mixing of the two emitters in a single LB film, and by preparing two stacked configurations, in which a LB layer of the ruthenium complexes is deposited on top of a LB layer of the iridium complexes and the inverse situation. The color of the electroluminescence can be tuned by changing the thickness of each LB layer. However, a white emission was not achieved due to shift of the emission of the iridium complex from blue in solution to the green within the LB film.31 In this work, we have tried to improve this result by preparing LB films of a blueemitting amphiphilic iridium complex containing a 1,2,3-triazole. The crystal structure of this complex is reported. Langmuir monolayers and LB films of this complex have been prepared and characterized. Finally, electroluminescent devices have been prepared using these LB films. As in our previous work, the color of the electroluminescence of the complex in the device is modified with respect to photoluminescence in solution and in the LB film. Interestingly, this results in the onset of a new emission that coexists with the blue 5 ACS Paragon Plus Environment
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emission, which is preserved, leading to a white emission. Furthermore, the device presents a clear LEC behavior and works with an air-stable Al electrode.
Cl-
+
N NN
F
N
F
*
N
N
Ir F
N
* n
N
F
1
pTPD
Chart 1. Molecular structure of 1 and pTPD. Experimental section Materials.
Poly(N,Nʹ′-diphenyl-N,Nʹ′-bis(4-hexylphenyl)-[1,1ʹ′-biphenyl]-4,4ʹ′-diamine
(pTPD) was obtained from American Dye Source and used without further purification. Synthesis and characterization of Bis[2-(2,4-difluorophenyl)pyridine][(3-hexadecyl-1,2,3triazole)pyridine]iridium(III) chloride (1) have been reported elsewhere.32 Single crystals of 1 were grown by diffusing hexane into a chloroform solution of the compound.
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Single crystal X-ray diffraction. A single crystal of 1 was mounted on glass fibers using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream for data collection. All reflection data were collected at 120 K on a Xcalibur, Sapphire3, Gemini diffractometer equipped with a graphite-monochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). The CrysAlisPro program, Oxford Diffraction Ltd., was used for unit cell determinations and data reduction. Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. Crystal structure was solved by direct methods with the SIR97 program,33 and refined against all F2 values with the SHELXL-2013 program,34 using the WinGX graphical user interface.35 All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. Data collection and refinement statistics are collected in Table 1. CCDC-1017983 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Preparation of the Langmuir monolayers and LB Films. A 1 mM solution of 1, in CHCl3 was used as spreading solution. An appropriate amount of this solution was carefully spread onto a 10-3 M KPF6 aqueous subphase. After spreading, the solvent was allowed to evaporate for 10 minutes prior to compression. The monolayer was compressed up to a surface pressure of 17 mN m-1 for transfer. The LB films were assembled to the substrate by the vertical lifting method, i.e., immersion and withdrawal of the substrate through the interface covered with the charged complex monolayer. An odd number of monolayers was obtained for hydrophilic substrates such as quartz due the lack of transfer
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during the first immersion of the substrate.18 The dipping speed of the substrates was 1 cm min-1. Isotherms were obtained with a NIMA trough (type 702BAM) equipped with a Wilhelmy plate and maintained at 22°C. A KSV3000 trough was used to prepare the LB films. Millipore water with a resistivity higher than 18 MΩ cm was used in all the experiments. A EP3-BAM from NFT was used for the Brewster Angle Microscopy experiments. Characterization of the LB films. Infrared (IR) spectra were recorded on a FTIR 320 Nicolet spectrometer with a resolution of 2 cm-1. UV-vis spectra were recorded on a Shimadzu UV-2401PC spectrometer. Emission spectra were measured on a Hamamatsu, model C9920-01. X-ray photoelectron spectroscopy (XPS, K-ALPHA, Thermo Scientific) was used to analyze the surface of a LB film of 21 monolayers deposited on quartz. All spectra were collected using Al-K_ radiation (1486.6 eV), monochromatized by a twin crystal monochromator, yielding a focused X-ray spot (elliptical in shape with a major axis lenght of 400µm) at 3 mA × 12 kV. The alpha hemispherical analyser was operated in the constant energy mode with survey scan pass energies of 200 eV to measure the whole energy band and 50 eV in a narrow scan to selectively measure the particular elements. XPS depth profiles were obtained by sputtering the specimen with a 1 keV Ar+ ion beam. XPS data were analyzed with Avantage software. A smart background function was used to approximate the experimental backgrounds and surface elemental composition were calculated from background-subtracted peak areas. Charge compensation was achieved with the system flood gun that provides low energy electrons and low energy argon ions from a single source. Specular X-ray reflectivity (SXR) measurements were performed on a Empyrean PANalytical diffractometer, using CuKα radiation (λ = 1.54177 Å). A
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commercial Atomic Force Microscope (AFM) (Multimode SPM by Veeco) was employed for surface sample characterization. AFM was operated in tapping mode. Sharp silicon probes were purchased from Nanosensors (PPP-NCH-W; force constant: 10-130 Ν/m; resonance frequency: 204-497 kΗz). Scan rate was adjusted during the scanning of each image (usually below 1 Hz, 512 samples/line). AFM images were processed by using WSxM.36 Preparation of the devices. Devices were prepared by spin coating a thin layer (80 nm) of the pTPD from a chlorobenzene solution on the ITO covered glass substrates. Before spin coating, the solutions were filtered over a 0.20 μm PTFE filter. The spin-coated films were annealed at 150° C for 10 minutes. Afterwards, the LB films were transferred on the ITO/pTPD substrate. Then, the thin films were dried and transferred into a high vacuum chamber integrated in an inert atmosphere glovebox ( 2σ(I)
0.0391
wR2(F2),[b] all data
0.0848
S(F2),[c] all data
0.907
α
[a]
R1(F) = Σ||Fo|–|Fc||/Σ|Fo|; [b]wR2(F2) = [Σw(Fo2–Fc2)2/ΣwFo4]½; [c]S(F2) = [Σw(Fo2–Fc2)2/(n + r – p)]½
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