Exploiting Photo- and Electroluminescence Properties of FIrpic

Article pubs.acs.org/IC

Exploiting Photo- and Electroluminescence Properties of FIrpic Organic Crystals Antonio Maggiore,† Marco Pugliese,†,▲ Francesca Di Maria,†,‡ Gianluca Accorsi,*,§ Massimo Gazzano,‡ Eduardo Fabiano,∥,△ Vittorianna Tasco,§ Marco Esposito,† Massimo Cuscunà,§ Laura Blasi,§ Agostina Capodilupo,§ Giuseppe Ciccarella,⊥,§ Giuseppe Gigli,†,§ and Vincenzo Maiorano§ †

Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Campus Universitario, via Monteroni, 73100 Lecce, Italy ‡ CNR-ISOF, Istituto per la Sintesi e la Fotoreattività, Consiglio Nazionale delle Ricerche, 40129 Bologna, Italy § CNR NANOTEC-Institute of Nanotechnology c/o Campus Ecotekne, University of Salento, Via Monteroni, 73100 Lecce, Italy ∥ Istituto Nanoscienze-CNR, Euromediterranean Center for Nanomaterial Modelling and Technology (ECMT), Via per Arnesano, 73100 Lecce, Italy ⊥ Department of Biological and Environmental Sciences and Technologies, University of Salento & Udr INSTM of Lecce △ Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia, Via Barsanti, I-73010 Arnesano, Italy ▲ Echolight srl, R&D Dpt, campus Ecotekne, via per monteroni, 73100 Lecce, Italy S Supporting Information *

ABSTRACT: In this work, we investigate the optical and structural properties of the well-known triplet emitter bis(4′,6′-difluorophenylpyridinato)-iridium(III) picolinate (FIrpic), showing that its ability to pack in two different ordered crystal structures promotes attractive photophysical properties that are useful for solid-state lighting applications. This approach allows the detrimental effects of the nonradiative pathways on the luminescence performance in highly concentrated organic active materials to be weakened. The remarkable electro-optical behavior of sky-blue phosphorescent organic light-emitting diodes incorporating crystal domains of FIrpic, dispersed into an appropriate matrix as an active layer, has also been reported as well as the X-ray diffraction, nuclear magnetic resonance, electro-ionization mass spectrometry, and scanning electron microscopy analyses of the crystalline samples. We consider this result as a crucial starting point for further research aimed at the use of a crystal triplet emitter in optoelectronic devices to overcome the long-standing issue of luminescence selfquenching.

1. INTRODUCTION Phosphorescent materials, incorporating heavy metal complexes, are intensively investigated because of their potential high efficiency in solid-state lighting, particularly in phosphorescent organic light-emitting diodes (PHOLEDs).1 On the other hand, although their maximal theoretical internal quantum electroluminescence (EL) efficiency is 100%, as compared to 25% for fluorescent materials,2 triplet emitterbased devices rarely maintain their performance because of some detrimental mechanisms (triplet−triplet annihilation, triplet−polaron quenching, and exciton dissociation).3 Important results in this realm have been obtained for both red and green-emitting devices,4 but the fabrication of blue phosphorescent OLEDs with stable efficiency at high luminance is still a challenge.5,6 For instance, FIrpic is considered an excellent dopant and some optimized device structures7−11 have been proposed to ensure better charge carrier balance inside the active layer and to optimize the confinement of triplet excitons to reduce the nonradiative decay rates. However, an efficiency © XXXX American Chemical Society

roll-off at a high current density is still observed, preventing its practical use (required brightness around 5000−10000 cd/m2). Most of the followed approaches in optoelectronic applications include the use of amorphous films for several reasons linked to higher efficiencies and ease of fabrication.12 However, the crystallinity of fluorescent molecules has recently been used to enhance their overall phosphorescence efficiency.13 In fact, for almost all classes of triplet emitters (including octahedral iridium complexes), with an increase in the dopant concentration within an opportune matrix as requested by solid-state lighting, the output emission is strongly suppressed by self-quenching processes.3 In this work, we observe the possibility of attenuating such a drawback by a proper structural organization of the molecules into the film forcing the FIrpic to pack in two different crystal forms. The photoluminescence data clearly indicate the Received: March 21, 2016

A

DOI: 10.1021/acs.inorgchem.6b00701 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a) XRD patterns of FIrpic I obtained from diffusion of hexane into a toluene solution (1), FIrpic II obtained from a toluene/ dichloromethane solution (2), a sublimated 100% film (3), and a TCTA:50 wt % FIrpic form (4).( b) Confocal laser scanning microscope image and structure of FIrpic I crystals. (c) CLSM image and structure of FIrpic II crystals. (d) View of the FIrpic II structure along the c-axis that highlights the presence of channels.

calculated from the previously reported FIrpic I crystal structure, even though the preparation does not contain methanol.20 The XRD pattern is not significantly modified by the absence of methanol molecules and the FIrpic molecules pack in the same lattice (see Figure S2). Crystals obtained by slow evaporation of a toluene/ dichloromethane (1:1) solution grew as thin yellow needles (Figure 1c) and showed an XRD profile that does not belong to the known structures, with main peaks at 7.1°, 10.5°, and 12.8° [Figure 1a(2)]. The structure, labeled FIrpic II, was determined, and the details are reported in Table 1. FIrpic II shows toluene and dichloromethane as cocrystallized solvents (Figure 1c). The Ir(III) complex shows a slightly deformed octahedral coordination geometry around the iridium center very similar to that observed in the FIrpic I structure with almost identical metal−ligand distances. It is also observed that the structure of FIrpic II reveals the presence of channels parallel to the c-axis with a diameter of ∼2.5 Å (see Figure 1d). In both structures, FIrpic I and FIrpic II, the presence of an extensive network of π−π stacking interactions between the phenylpyridine moieties of FIrpic molecules as well as some edge-to-face intramolecular interactions is evident. The ability of such interactions to promote aggregations has been also investigated by density functional theory calculations on FIrpic dimers, showing that FIrpic molecules presenting a mix of π−π stacking and edge-to-face interactions (Figure S6) display binding energies of up to 20 kcal/mol. Figure 1a also reports the X-ray diffraction pattern of a sublimated 100 wt % FIrpic neat film [line 3 (for characterizations, see Figures S1 and Table S1, FIrpic H NMR and mass analysis)] and a TC:50 wt % FIrpic film (line 4), obtained by finely controlling the deposition rate and temperature (0.02 Å/

formation of additional levels (crystal exciton states signaled, among others, by a spectral red-shift relative to monomers)14 that strongly enhance the optical performance of the devices at high dopant concentrations. On the basis of recent studies in the crystal field,15 we introduce a crystalline FIrpic approach on a basic blue-emitting OLED structure, leading to the weakening of detrimental effects (responsible for the decrease in the efficiency at high brightness, typically corresponding to a high current density) 9,16−19 in PHOLEDs, particularly blue-emitting ones.1−6

2. RESULTS AND DISCUSSION: PHOTOPHYSICS AND MORPHOLOGY From a photophysical point of view, interpretation of the crystal excited states generally starts from a description of the dimer extended to an array of identical molecules with a specified orientation and separation. In this representation, the excitation energy initially localized on a single molecule is instantaneously shared with the crystal network, so that a series of new and quasi-degenerate levels (i.e., a band) is produced. This so-called crystal exciton band constitutes a proper starting point for the interpretation of the optical properties of the crystal.14 Two crystal structures involving FIrpic have been also reported: FIrpic crystals17 and FIrpic methanol cocrystals20 (hereafter named FIrpic I). We prepared crystalline materials by two methods producing different morphologies. Crystals obtained by diffusion of hexane vapors into a toluene solution were green rhombic prisms (Figure 1b), with an X-ray diffraction (XRD) pattern {main peaks at 2θ = 10.5°, 11.5°, 13.5°, and 20.4° [Figure 1a(1)]} that corresponds to the one B


Article

Inorganic Chemistry

s and 28 °C, respectively). These are typical patterns of a semicrystalline material because two bell-shaped “bumps” appear on the background line at 2θ around 11° and 23° together with sharp and well-resolved reflections that correspond to the position of the main peaks of FIrpic I (Figure 1a, line 1). This confirms the formation upon sublimation of crystalline domains constituted predominantly by FIrpic I, although it is not possible to exclude the presence of the other polymorphs. The photophysical properties of the different crystals have been also investigated in detail. Photoexcitation (PE) spectra of the two samples (FIrpic I and FIrpic II) have been obtained and are depicted in Figure 2a, while the corresponding lifetime decay values are listed in Table 2. Figure 2b shows a remarkable difference between the normalized PL spectra of the two different crystalline forms. For comparison, the profile of the isolated FIrpic (0.1 wt % in PMMA) is also reported. When one moves from FIrpic− PMMA to FIrpic II and FIrpic I, the emission progressively redshifts (25 nm for FIrpic II and 120 nm for FIrpic I) and additional spectral shoulders appear. Accordingly, from the PE spectra (Figure 2a), the intensity of the low-lying bands (λ > 450 nm) greatly increases for FIrpic I and FIrpic II, with respect to that of FIrpic−PMMA, suggesting the presence of different emitting states between the amorphous and crystal species. The analysis of the lifetime decays (Table 2) shows that while FIrpic−PMMA follows a monoexponential decay, for

Table 1. Crystal Data and Details of Measurements of FIrpic II formula formula weight crystal dimensions (mm3) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z δ (g/mL) μ (mm−1) λ (Å) temp (K) no. of reflections (measured/ independent) Rint R wR (all data)

C28H16IrN3O2F4· C7H8·1/2CH2Cl2 829.24 0.18 × 0.05 × 0.02 monoclinic I2/a 15.965(5) 13.971(5) 27.521(5) 90.0 91.383(5) 90.0 6137(3) 8 1.81 4.502 0.71073 293(2) 15290/7062 0.0811 0.0834 0.1839

Figure 2. Photophysical characterization of FIrpic crystals. (a) Normalized excitation (λem0.1% = 550 nm; λem FIrpic I = 600 nm; λem FIrpic II = 500 nm). (b) Normalized photoluminescence of FIrpic at 0.1 wt % in PMMA, FIrpic I, and FIrpic II. (c) PL emission of a 100 wt % FIrpic film (black line) and its deconvolution into four peaks (green lines) (performed with Origin). C


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Inorganic Chemistry

proximity of the characteristic peaks of both crystal polymorphs FIrpic I and FIrpic II (Figure S9), further demonstrating the presence of a crystalline domain in the sublimated 100 wt % neat film, already provided by X-ray diffraction measurements (Figure 1a). Furthermore, with the aim of evaluating the effect of crystallinity on the electroluminescence, FIrpic thin films that co-evaporated at different weight concentrations inside a suitable 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) matrix with an appropriate triplet energy (>2.62 eV for the FIrpic dopant) have also been created and their optical properties evaluated. The UV−vis electronic absorption spectra of the films, including that of isolated FIrpic (0.1 wt % in inert matrix PMMA), have been obtained and are reported in Figure 3b. The UV region (up to ∼350 nm) is dominated by intense absorption bands assigned to ligand-centered (LC) 1(π−π*) transitions, while the weaker and broader bands at lower energies (∼350−440 nm) are of charge transfer (CT) character and are related to electronic transitions occurring from the metal center to the cyclometalated ligands (MLCT).20 To be noted that, in the ∼250−370 nm range the TCTA absorbs also, thus the profiles reported in Figure 3a result by both contributions, from host and doping materials, according to the relative concentration ratio.

Table 2. Lifetime Decays of FIrpic I, FIrpic II, and 0.1 wt % FIrpic in PMMA at Room Temperature sample

λdet (nm)

τ1 (ns)

τ2 (ns)

0.1 wt % in PMMA

470 500 550 470 500 550 600 470 500 530 570

1612 1580 1580 11 24 90 300 36 120 135 175

a a a 75 100 715 1000 124 420 500 770

FIrpic I

FIrpic II

a

Not detected.

FIrpic I and FIrpic II an additional decay has been found, confirming the formation of new emitting states. It should also be noted that the crystal-emitting states (FIrpic I and II) are intrinsically shorter-lived than those of the isolated molecules in the PMMA film (FIrpic−PMMA). Figure 2c shows the multi-Gaussian curve fitting of sublimated 100 wt % FIrpic neat film emission spectra. It is remarkable to note that these peaks are localized in the

Figure 3. Photophysical characterization of evaporated FIrpic films at different doping concentrations. (a) Normalized excitation (λem = 550 nm) spectra. (b) Electronic absorption spectra. (c) Normalized photoluminescence. The inset shows lifetime decays (λexc = 405 nm). D


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Inorganic Chemistry

Figure 4a shows the FIrpic electroluminescence spectrum at different weight concentrations (from 15 to 80 wt %). For FIrpic concentrations in the TCTA matrix of 15 wt %, the efficiency decreases, reaching a relative minimum for concentrations around 30 wt %, and then increases to the absolute maximum of 20 cd/A at a concentration of 50 wt %. Similarly, EQE shows the same trend, with a maximum of ∼8.5% at 50 wt % (Figure 4d). It is also possible to note that the roll-off efficiency at concentrations of 40 wt % is similar to that obtained at a concentration of 15 wt % and lower than that at 30 and 50 wt %. We attribute this behavior to the presence of new excited crystal states (signaled by the biexponential decay in Table 3) that could lead to a reduction in the frequency of annihilation phenomena, because of the higher carrier mobility and recombination yield provided by crystals.15 Although the overall efficiencies are relatively low compared to that of the state-of-the-art for FIrpic-based OLEDs,22 our results can be considered a starting point for the study of OLEDs with crystalline domains within the emissive layer, with the aim of increasing efficiencies and improving the performance at high current densities.

The excitation spectra recorded at 550 nm (Figure 3a), in agreement with the absorption profiles, show an even clearer spectral trend in which the lowering of UV bands intensity is accompanied by an increase of the low-lying ones that is strictly correlated to the increasing absorption intensity of the FIrpic, while that of the host (absorbing in the UV region only) decreases. The photoluminescence spectra of the different concentration films have been also obtained and reported in Figure 3c, showing a significant change upon increasing the amount of FIrpic. They go from monomer-like shape (0.1 wt %), finely matching with that obtained in dilute solution (Figure S8), up to a red-shifted profile (pure dye) that in addition shows the formation of a new band centered at ∼550 nm. Given the presence of crystalline domains in the sublimated films, it could be ascribed to the interaction of adjacent phenylpyridine pyridyl rings in the crystal structure that significantly modify the excited-state properties of the FIrpic complex, leading to the formation of additional levels from crystal exciton states, reported above.14 The luminescence decays have been measured for all the samples and are listed in Table 3. From the experimental data, Table 3. Photophysical Properties of a FIrpic Doped Thin Film at Different Concentrations at Room Temperature

a

doped FIrpic (wt %)

λmax (nm)

ΦPL (%)

τ1 (ns)

τ2 (ns)

5 10 15 20 25 30 40 50 70 100

497 499 499 496 501 499 498 503 507 522

55 71.5 71 62 60 56 42 35 20 14

1290 1264 1215 1124 995 844 657 450 240 66

a a a a a a 1050 1200 1400 890

4. CONCLUSION

Not detected.

Despite the fact that the emission output of iridium octahedral complex aggregates is quenched by nonradiative deactivation, in this work we demonstrated the ability of this material to pack efficiently in two different ordered crystal structures (mainly through π−π interactions of the ligands), leading to a weakening of such detrimental effects. In particular, we observed, at high dopant concentrations, the formation of new low-lying bands with specific photophysical features different from those of the corresponding monomers, which can be attributed to crystal exciton states. To integrate our results into an optoelectronic application, we used the crystalline FIrpic in a basic blue-emitting OLED structure, observing that crystalline domains within the emitting layer allow the efficiency to increase at high dopant concentrations and maintain a high efficiency at a high operating luminance: the current efficiency drops from 16.7 cd/A (at 6.000 cd/m2) to 14.6 cd/A for 26.000 cd/m2 of luminance, for a dopant concentration of ∼40 wt %. Although the trend in OLED research is to decrease the doping amount of noble metal phosphors to reduce the cost of fabrication of devices and to avoid efficiency roll-off at high brightness, we think that a deep understanding of transport and recombination phenomena occurring in the molecular crystal could represent a new strategy for improving the optical output of crystal-based OLEDs.

the following observations were made. (i) The contribution of the monomer to the total emission decreases with an increase in the dye concentration in the films. This is evident in view of both the change in the spectral profile and the shortening of the lifetime (from ∼1.0 to 0.07 μs). (ii) The increase in the new emission component on the red region can be reasonably attributed to the radiative deactivation from a new state as signaled by the biexponential decays (from a 50 wt % doped film and beyond) that could be assigned to the crystal exciton levels. From a spectral point of view, these results are in line with those obtained via electroluminescence (as shown in the next section) and reveal that they could be useful for enhancing the color output in terms of both energy and intensity.

3. FABRICATION AND CHARACTERIZATION OF OLEDS The effects of crystal structures on electroluminescent FIrpicbased p-i-n21 OLEDs (sky-blue PHOLED) have also been investigated. The device structure is represented in Figure 4a (inset). The active layer consists of TCTA doped with a variable concentration (from 15 to 80 wt %) of FIrpic and a total thickness of 20 nm. E


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Inorganic Chemistry

Figure 4. Electro-optical characterization of sky-blue OLEDs at different FIrpic weight concentrations. (a) Normalized electroluminescence spectra. The inset shows the proposed energy level diagram of the sky-blue OLED. (b) Current efficiency vs current density curves. (c) Current density and luminance as a function of applied voltage. (d) Maximal EQE of OLEDs at different FIrpic concentrations. Institute of Standards and Technology (NIST) calibrated using a standard lamp and is directly connected by an RS232 cable to a Keithley 2420 current−voltage source meter.

5. EXPERIMENTAL SECTION X-ray diffraction patterns were obtained with Cu Kα radiation in reflection mode by means of an X’Pert PANalytical diffractometer equipped with a fast X’Celerator detector, with a 0.05° step and 80 s/ step. SIR-9223 was used for structure solution and SHELX9724 for the refinement based on F2. The CCDC deposition contains the crystallographic data for this structure. The data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html. Emission spectra were recorded with an Edinburgh FLS980 spectrometer equipped with a Peltier-cooled Hamamatsu R928 photomultiplier tube (185−850 nm). An Edinburgh Xe900 450 W xenon arc lamp was used as the exciting light source. Confocal microscopy images have been acquired at room temperature by using a confocal microscope (FV-1000, Olympus). Emission lifetimes in the nanosecond to microsecond range were determined with the single-photon counting technique by means of the same Edinburgh FLS980 spectrometer using a laser diode as the excitation source (1 MHz, λexc = 407 nm, 200 ps time resolution after deconvolution) and the PMT mentioned above as the detector. Analysis of the luminescence decay versus time profiles was accomplished with the Decay Analysis Software provided by the manufacturer. The OLEDs have been fully fabricated by high-vacuum thermal evaporation in a Kurt J. Lesker multiple-chamber system with a base pressure of ∼10−8 mbar. The electrical−optical characteristics of the devices were measured under vacuum with an Optronics OL770 spectrometer, coupled, through an optical fiber, to the OL610 telescope unit for the luminance measurements, with an experimental uncertainty of approximately ±10%. The whole system is National

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00701. Additional figures (Figures S1−S11) and one additional table (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +39 0832 298227. Author Contributions

A.M., M.P., and F.D.M. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Italian Projects PON R&C 20072013 “MAAT” Molecular NAnotechnology for HeAlth and EnvironmenT (PON02 00563 3316357) and EFOR-Energia da F


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Inorganic Chemistry FOnti Rinnovabili (Iniziativa CNR per il Mezzogiorno L. 191/ 2009 art. 2 comma 44).

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Exploiting Photo- and Electroluminescence Properties of FIrpic

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