Charge Transport and Sensitized 1.5 μm Electroluminescence

Article pubs.acs.org/JPCC

Charge Transport and Sensitized 1.5 μm Electroluminescence Properties of Full Solution-Processed NIR-OLED based on Novel Er(III) Fluorinated β‑Diketonate Ternary Complex Pablo Martín-Ramos,† Carmen Coya,*,‡ Á ngel L. Á lvarez,‡ Manuela Ramos Silva,§ Carlos Zaldo,∥ José A. Paixaõ ,§ Pedro Chamorro-Posada,† and Jesús Martín-Gil⊥ †

Departamento de Teoría de la Señal y Comunicaciones e Ingeniería Telemática, Universidad de Valladolid, ETSI Telecomunicación, Paseo Belén 15, Campus Miguel Delibes, 47011 Valladolid, Spain ‡ Departamento de Tecnología Electrónica, Universidad Rey Juan Carlos, Escuela Superior de Ciencias Experimentales y Tecnología (ESCET), C/Tulipán s/n, Móstoles, 28933 Madrid, Spain § CEMDRX, Physics Department, Coimbra University, Rua Larga, P-3004-516 Coimbra, Portugal ∥ Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, C/Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain ⊥ Advanced Materials Laboratory, ETSIIAA, Universidad de Valladolid, Avenida de Madrid 44, 34004 Palencia, Spain S Supporting Information *

ABSTRACT: Solution-processed near-infrared organic light-emitting diodes (NIROLEDs) with structure glass/indium−tin oxide/poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate)/Er-complex/Ca/Al based on a novel Er(III) complex, [Er(tfnb)3(bipy)] (Htfnb = 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione and bipy = 2,2′-bipyridine) have been manufactured and their properties have been studied. A complete quenching of the organic ligand visible emission is shown, and only the sensitized 1.5 μm electroluminesce from Er(III) results. From the electrical characteristic we present the mobility dependence on applied voltage using a numerical model, comparing it to poly(9,9-dioctylfluorene), a commercial semiconducting polymer with optical properties close to those of the molecular ligands. The synthesis of the novel complex together with a detailed analysis of its structure elucidated by XRD, 1H NMR, Raman, and Fourier-transform infrared spectroscopies is presented. A wide-ranging characterization of its photophysical properties in terms of absorption and steady and transient photoluminescence is used to investigate the energy-transfer process from the organic ligand to the central Er(III) ion. properties of these radiations are well-established.2 Other high-added-value applications could be in clothing, night vision equipment, irregular surfaces signalization, greenhouse lighting, or environmental heating. For these purposes, lanthanide complexes that emit in the NIR region are among the most promising approaches. Despite the fact that direct optical excitation of the lanthanide ion is a low-efficiency process due to its weak optical absorption, it is known that the optical population of its emitting levels can be achieved by employing organic ligands as chromophores, with strong absorption in the UV spectral region. These ligands can sensitize the central lanthanide ion by intramolecular energy transfer, a process known as “antenna effect”.3,4 For example, βdiketones containing aromatic groups are known to provide efficient energy transfer to lanthanide ions5,6 because they have

1. INTRODUCTION In recent past years, the development of organic light-emitting diode (OLED) technology has acquired great maturity, moving rapidly from basic research to new generation devices, which are already in the market.1 Much attention is focused on solution-processed methods due to their compatibility with inexpensive manufacturing techniques for large area operation, such as inkjet printing or roll-to-roll processing. In this stage there is a significant need for new organic semiconducting materials that combine the ability of being processed by costeffective methods in a stable manner with efficient charge transport and light emission. Thus, the versatility presented by organic semiconductor design, together with their flexiblesubstrate compatibility, allows us to consider applications with high-added-value, which would not be affordable by conventional methods. For example, wet-processed organic semiconductors emitting in the near-infrared (NIR) region, 900− 1600 nm, deposited on flexible substrates could be used for Health Care applications, provided that the therapeutic © 2013 American Chemical Society

Received: March 3, 2013 Revised: April 11, 2013 Published: April 18, 2013 10020

dx.doi.org/10.1021/jp402174s | J. Phys. Chem. C 2013, 117, 10020−10030

The Journal of Physical Chemistry C

Article

2. EXPERIMENTAL SECTION X-ray Crystallographic Analysis. Prior to structural characterization, powder diffractograms were obtained using an ENRAF-NONIUS FR590 powder diffractometer equipped with an INEL120 detector (Debye−Scherrer geometry). For the determination of crystal structure by X-ray diffraction, a crystal of [Er(tfnb)3(bipy)] was glued to a glass fiber and mounted on a BruKer APEX II diffractometer. Diffraction data were collected at room temperature 293(2) K using graphite monochromated MoKα (λ = 0.71073 Å) radiation. Absorption corrections were made using SADABS.20 The structure was solved by direct methods using SHELXS-9721 and refined anisotropically (non-H atoms) by full-matrix least-squares on F2 using the SHELXL-97 program.21 PLATON22 was used to analyze the structure and Figure plotting. All CF3 groups show signs of disorder with large ellipsoids. One of the groups could be refined over two positions, with F atoms refined isotropically, with 60/40% occupation. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). Any request to the CCDC for this material should quote the full literature citation and the reference number CCDC 919674. Physical Measurements. The C, H, and N elemental analyses were conducted using a Perkin-Elmer CHN 2400 apparatus. Differential scanning calorimetry (DSC) data were obtained on a DSC TA Instruments model Q100 v.9.0 with a heating rate of 10 °C/min under a N2 atmosphere. The optical absorption infrared spectrum was recorded with a Thermo Nicolet 380 FT-IR spectrometer using products dispersed in KBr pellets. The Raman spectrum was recorded with an FTRaman Bruker FRA106 apparatus by using a NIR (Nd:YAG, 1064.1 nm) laser to excite the sample. The RMN spectrum was register from deuterated chloroform solution (CDCl3) using a 400 MHz RMN spectrometer from Varian model Mercury 400 (9.4 T) at 400.123 MHz. Cyclic voltammetry was performed at 50 mV/s using a Biologic VMP3 multichannel potentiostat with silver wire working and reference electrodes and platinum mesh counter electrodes. A 10−3 M solution of the complex in chloroform was prepared by adding 0.5 M of tetrabutylammonium hexafluorophosphate as supporting electrolyte. Spectroscopic Measurements. Optical absorption and PL spectra of the materials in solution or powder have been measured at room temperature. The 200−800 nm range absorption spectra were recorded with a Cary 4000 Varian spectrophotometer in methanol-diluted solutions (10−5 and 10−3 M). The absorbance in the 900−3500 nm NIR range was recorded using a Varian 660-IR FT-IR spectrometer in 10 wt % KBr pellet. The NIR PL emission at 1.5 μm was excited at the ligand absorption, λexc = 337 nm (N2 laser), or at the Er:4I11/2 absorption level, λexc = 980 nm (MOPO laser system). The emitted light was dispersed by a Spex model 340E (f = 34 cm) spectrometer and detected with a Peltier-cooled NIR Hamamatsu photomultiplier and a lock-in amplifier. Lifetimes were measured by using a Tektronix model TDS 520 (500 MHz) oscilloscope. The infrared EL spectrum from the diode was analyzed using a SPEX 1702/04 (f = 1m) monochromator and detected with a 77 K cooled Ge detector connected to a Stanford Research system SR530 lock-in amplifier using 50% duty cycle electrical excitation waveform from a TTi40 MHz arbitrary waveform generator and a TREK-601C amplifier.

a strong absorption within a large wavelength range and consequently have been targeted to sensitize the lanthanide luminescence. Another beneficial effect of the ligands is that they can prevent water from binding to the coordination Ln(III) sphere, which has been shown to quench the luminescence very efficiently.7,8 Furthermore, the replacement of C−H bonds with lower-energy C−F oscillators in the β-diketone ligand decreases the nonradiative losses of the lanthanides and therefore enhances their emission intensity.9,10 In the late 1990s, Hasegawa et al. described the improved NIR luminescence of several deuterated and fluorinated Ln3+ β-diketonate complexes in solution.10,11 Furthermore, it is known that fluorination does not significantly modify the energy of the triplet levels of the ligands, so the resonant transfer to the lanthanide ion is not affected.12 Among lanthanides, Er(III) ion is particularly interesting because its 1.5 μm emission matches the spectral region used in long-distance optical fiber telecommunications. The observation of NIR photoluminescence (PL)13 emission and electroluminescence (EL) from evaporated erbium-8-quinolinolate (ErQ3) complexes triggered investigations in the field of lanthanide NIR-OLEDs,14 but few works have been reported to date.15,16 Then, adequate film-forming properties by costeffective methods, good thermal stability of the Er-complexes, together with the capability of NIR emission by electrical excitation (EL) is especially interesting. This motivated us to focus on a series of optically active complexes having the general formula [Er(β-diketonate)3(N,N-donor)],17 a wide family of materials intended for their use in the development of large area optoelectronic applications. We present the synthesis, structural data and physical properties of a novel Er(III) complex using 4,4,4-trifluoro-1(2-naphthyl)-1,3-butanedione (Htfnb or Hnta) as the fluorinated β-diketonate primary sensitizer, together with 2,2′bipyridine (bipy) as the N,N-donor molecule acting as the synergistic ligand, with a resulting structure [Er(tfnb)3(bipy)]. The N,N-donor ligand not only completes the coordination sphere of the Er(III) but also causes a higher intensity of the sensitized luminescence because its rigid planar structure allows a better energy transfer.18,19 This Er complex exhibits very good film-forming properties upon solution in methanol, which allows us to study the device characteristics of a simple and solution-processed structure (glass/indium−tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS)/active layer/Ca/Al) OLED. The NIR EL device emission at 1.5 μm (Er:4I13/2→4I15/2 transition) is observed as a result of a complete charge transfer from the organic ligands to the Er(III) ion, with no residual visible emission from the ligands. To investigate the energy transfer from the organic ligands to the central Er(III) ion in the NIR EL process, we have performed a detailed study of its photophysical properties in terms of steady and transient PL, at different excitation wavelengths. The results help to prove the antenna effect and to propose a mechanism for the indirect excitation process of Er(III). Besides, the device current density−voltage (J−V) response is studied using a numerical model including field-dependent carrier mobility under a single carrier approach that allows to evaluate the carrier mobility dependence on the applied voltage in this novel complex and to compare it with that of a commercial conjugated polymer with optical properties close to those of the molecular ligands. 10021

dx.doi.org/10.1021/jp402174s | J. Phys. Chem. C 2013, 117, 10020−10030

The Journal of Physical Chemistry C

Article

Table 1. Crystal Data and Structure Refinement of Complex [Er(tfnb)3(bipy)] empirical formula formula weight temperature wavelength crystal system space group a b c β volume Z density (calculated) absorption coefficient F(000) crystal size θ range for data collection index ranges reflections collected independent reflections completeness to 2θ = 51° refinement method data/restrains/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole

C52H32ErF9N2O6 1119.06 293(2) K 0.71073 Å monoclinic C2/c 22.301(2) Å 22.919(3)Å 20.220(2)Å 107.657(2)° 9847.6(18) Å3 8 1.510 g cm−3 1.788 mm−1 4440 0.28 × 0.14 × 0.12 mm3 1.49−25.70° −19 < h < 27; −24 < k < 27; −24 < l < 17 25752 9328 99.6% full matrix LS on F2 3885/0/629 0.917 R = 0.0612; wR = 0.1000 R = 0.1841; wR = 0.1353 −0.611/0.641

Figure 1. Chemical structure (left) and structural diagram (right) of the [Er(tfnb)3(bipy)] complex, showing the antisquare prismatic conformation of the first coordination shell of the Er(III) ion. H atoms were omitted for clarity.

Device Fabrication and Evaluation. The structure of the device is ITO/PEDOT:PSS/[Er(tfnb)3(bipy)]/Ca/Al. A prepatterned ITO glass plate with four circular diodes (1 and 1.5 mm radii) was extensively cleaned, using chemical and UV− ozone methods, just before the deposition of the organic layers. PEDOT:PSS (CLEVIOS P VP AI 4083) was deposited at 2000 rpm by spin-coating and then cured on a hot plate at 140 °C for 15 min. Methanol precursor solution (4 wt %) was kept in an ultrasonic bath for 45 min and filtered through a 0.2 μm polytetrafluoroethylene (PTFE) syringe filter prior to being spin-coated. The active layer was then deposited by spin coating (1500 rpm) and cured on a hot plate at 90 °C for 10 min plus 120 °C for 10 min to achieve complete solvent removal. The thickness of the layer was measured using an Alpha Step D120 profilometer (KLA-Tenkor Instruments), obtaining 70 nm for the PEDOT:PSS layer and 97 nm for the active layer. The Ca/Al cathode was thermally evaporated in an atmosphere of 8 × 10−6 Torr on top of the organic layer surface, and the device was finally encapsulated using a glass

cover attached by a bead of thermally cured epoxy adhesive [EPO-TEK(730)]. All of the process was carried out in an inert atmosphere glovebox (