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Design of heteroleptic Ir complexes with horizontal emitting dipoles for highly efficient organic light-emitting diodes with an EQE of 38% Kwon-Hyeon Kim, Eun Soo Ahn, Jin-Suk Huh, Yun-Hi Kim, and Jang-Joo Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03428 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Chemistry of Materials

Design of heteroleptic Ir complexes with horizontal emitting dipoles for highly efficient organic light-emitting diodes with an EQE of 38% Kwon-Hyeon Kim1‡, Eun Soo Ahn2‡, Jin-Suk Huh1, Yun-Hi Kim2*, Jang-Joo Kim1* 1

Research Institute of Advance Materials and Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, South Korea 2

Department of Chemistry and Engineering Research Institute (ERI), Gyeongsang National University, Jinju 66-701, South Korea

ABSTRACT: Orientation of emitting dipoles is one of the most important material properties influencing the efficiency of organic light-emitting diodes. Recently, even globular shaped Ir complexes, especially heteroleptic Ir complexes, have been reported to have horizontal emitting dipole orientation (EDO) to improve the external quantum efficiency (EQE) over 30%. Still the relationship between molecular structure of Ir complexes and their EDO has not been fully understood yet. Here, we report that substituents at the para-position of the pyridine in the main ligands of Ir complexes plays pivotal role inducing the orientation of heteroleptic Ir complexes. Substitution of aliphatic and aromatic functional moieties at the position leads to high horizontal emitting dipole orientation with the horizontal dipole ratio up to 86.5% to realize unprecedentedly high-efficiency yellow and green OLEDs, with EQEs of 38% and 36%, respectively. Elongated and planar substituents with high electrostatic potential enlarge the interacting surface region between Ir complex and host molecules, resulting in stacking Ir complexes parallel to film surface.

■ Introduction Horizontally oriented emitting dipoles in organic light-emitting diodes (OLEDs) lead to reduced losses from surface plasmon modes and waveguide modes, which increases the outcoupling efficiency of OLEDs. 1-10 The theoretically predicted maximum achievable EQE is 45–55% if emitters are horizontally oriented and have 100% photoluminescence quantum yield (PLQY), which is considerably higher than the EQE of 25–30% for randomly oriented emitting dipoles.10,11 This prediction is supported by the demonstration of an EQE of 39% using a crystal OLED with an undoped Pt complex where the emitting dipoles were horizontally oriented.12 Horizontal orientation of the emitting dipoles has been reported for spin-coated polymer films and vacuum-evaporated neat films for rod- or disk-like molecules.1-5 Recently, the emitting dipoles of heteroleptic, and some homoleptic, Ir-based phosphorescent dyes were reported to exhibit a preferential orientation in doped films.7-23 There have been attempts to increase the fraction of horizontal emitting dipoles (Θ) by modifying the main and ancillary ligands of heteroleptic Ir complexes (HICs)13,14, increasing Θ to 74–80%, and achieving EQEs of > 30%.10-21 It transpires that Θ is strongly dependent on the host molecules, indicating that intermolecular interactions are important in the orientation of the emitting dipoles.15,16 Models have been proposed to explain why some Ir complexes have a preferred orientation;15,22,23 Kim et al. reported that horizontal emitting dipoles of HICs originated from preferred di-

rection of the triplet transition dipole moment along Ir-N direction in HICs and the strong interaction between HICs and host molecules.15 Jurow et al. reported inherent asymmetry at the surface of the deposited film promotes HIC alignment in amorphous film.23 Still the relationship between molecular structure of Ir complexes and their EDO has not been fully understood yet. There are two factors that should be considered to understand the preferred orientation of Ir complexes: the symmetry of the transition dipole moment (TDM) in a specific molecule, and the orientation of that dopant molecule in a film of host molecules.14,15,23 The degeneracy of the TDMs and their relative orientations in the molecule needs to be taken into account in the molecular symmetry. High values of Θ are expected for heteroleptic Ir complexes if the emitting dipole vectors are perpendicular to the C2 axes in the molecules, and the doubly degenerated TDMs are parallel to the substrate. The influence of the orientation of the transition dipoles in a molecule with respect to the C2 axis on Θ (i.e. the average dipole orientation in films) was investigated by slight modifications to the main phenyl pyridine (ppy) ligands; i.e. by substituting methyl group(s) at different positions of ppy with the same ancillary ligand in the same host to minimize variations in the intermolecular interactions (or the molecular axis against the substrate) in the film.13 The larger the angle (Φ) between the C2 axis and the emitting dipole vector, the larger the value of Θ. A value of Θ = 80% was obtained from an Ir complex where the angle between the C2 axis and the emitting dipole vector

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Figure 1. (a) Chemical structures and (b) optimized geometries along with triplet transition dipole moments of Ir(3’,5’,4-mppy)2tmd, Ir(dmppy-CF3)2tmd, Ir(dmppy-pro)2tmd, and Ir(dmppy-ph)2tmd from three sublevels are shown as bars based on the center of mass. The transition dipole moments of the T x, Ty, Tz sublevels correspond to the blue, green, and red bars, respectively. Scaled TDMs were shown in inset of Figure 1(b). was almost 90; this may be the limit that is achievable using this approach.13,15. This limitation may result from variations in the alignment of the dopants in a doped amorphous film. This can be represented as the average and the standard deviation of the angle between the TDMs of the molecules and the film surface. Therefore, it would be valuable to design molecules such that their TDMs are aligned parallel to the surface. Here, we show that orienting Ir complexes with the TDMs parallel to the substrate is possible if they are elongated by substituting aliphatic or aromatic functional moieties at the para-position of the pyridine in the main ligands. Elongated and planar substituents with high electrostatic potential enlarge the interacting surface region between Ir complex and host molecules, resulting in stacking Ir complexes parallel to the film surface. We demonstrate values of Θ up to 86.5%, obtaining unprecedentedly high-efficiency yellow and green OLEDs, with EQEs of 38% and 36%, respectively.

■ Results and discussion Figure 1 shows the chemical structures of Ir complexes used in this work, together with optimized geometries and the triplet TDMs of the Ir complexes. The 4-position of pyridine of bis(2-(3,5-dimethylphenyl)-4-methylpyridine) Ir (III) (2,2,6,6tetramethylheptane-3,5-diketonate) [Ir (3’,5’,4-mppy)2tmd]13 was substituted with functional groups such as a trifluoromethyl (CF3) group, which is strongly electron withdrawing, as well as with a propyl group, which is elongated, and with a phenyl group, which is planar and charge conjugated, creating the following compounds: bis(2-(3,5-dimethylphenyl)-4-(trifluoromethyl)pyridine) Ir (III) (2,2,6,6-tetramethylheptane3,5-diketonate) [Ir(dmppy-CF3)2tmd], bis(2-(3,5-dime-

thylphenyl)-4-propylpyridine) Ir (III) (2,2,6,6-tetramethylheptane-3,5-diketonate) [Ir(dmppy-pro)2tmd] and bis(2-(3,5-dimethylphenyl)-4-phenylpyridine) Ir (III) (2,2,6,6tetramethylheptane-3,5-diketonate) [Ir(dmppy-ph)2tmd]. Geometry optimizations and calculations of the frontier orbitals of the T1 states were implemented using the density functional theory (DFT) package Gaussian 09,24 where the triplet energies and TDMs of the molecules were found using time-dependent calculations, and where spin-orbit coupling was included using the quantum chemistry package Jaguar25,26 (part of the Schrödinger Materials Science Suite27). Details of calculations are given in the experimental section. Ir(dmppypro)2tmd and Ir(dmppy-ph)2tmd have relatively high aspect ratios of 1.81 and 1.99, respectively (c.f. 1.34 for Ir(3’,5’,4mppy)2tmd and Ir(dmppy-CF3)2tmd). The dihedral angle between the pyridine and phenyl groups at the 4-position of the pyridine of Ir(dmppy-ph)2tmd was 23°. The small dihedral angle enables conjugation between the phenyl and pyridine groups, resulting in sufficiently large dipole length in the horizontal direction. The isosurfaces of electrostatic potential (ESP) of HICs shows that hydrogens of methyl, propyl, phenyl substituents on para-position of pyridine exhibit strong electro-deficient nature (+8~20 kcal/mol), in contrast CF3 substituents on para-position of pyridine exhibit strong electro-rich nature (~15 kcal/mol) (Figure S1). These electro-deficient (or rich) regions of HICs would strongly interact with electro-rich (or deficient) regions of host molecules as well as induced dipole of host molecules, resulting in preferred molecular orientation of HICs. The triplet TDMs of Tx, Ty and Tz sublevels of the HICs are shown by the bars in Figure 1b. The triplet TDMs of all the Ir complexes were aligned almost vertically with the C2 axis (see


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Table 1. Photophysical properties and DFT results of Ir complexes Φ

Aspect ratio

(˚)

MLCT b [%]

LCb [%]

79

1.34

88.2

32.6

19.2

75

80

1.34

88.1

32.7

45.7

Ir(dmppypro)2tmd

98

83.5

1.81

88.2

32.3

19.7

Ir(dmppyph)2tmd

98

86.5

1.99

89.1

35.2

47

Ir complex

PLQYa [%]

Ir(3’,5’,4mppy)2tmd

97

Ir(dmppyCF3)2tmd

Θ [%]

aMeasured

using 50-nm-thick TCTA:B3PYMPM films doped with 4 wt% HICs. bContribution of the frontier orbitals of HICs to the triplet transitions from HOMO to LUMO level

Tables 1 and S1 for further details). The TDMs and oscillator strengths of the T1 transition of Ir(dmppy-CF3)2tmd and Ir(dmppy-ph)2tmd were significantly larger than those of Ir(dmppy-pro)2tmd and Ir(3’5’,4-mppy)2tmd. This is attributed to conjugation between the pyridine and phenyl substituents of Ir(dmppy-ph)2tmd, the increased dipole length, and the phenyl and CF3 substituents at the 4-position of pyridine pull the large amount of electrons at the lowest unoccupied molecular orbital (LUMO) (see Figure S2). Thus, the contributions of the ligand-centred (LC) orbitals of Ir(dmppyCF3)2tmd and Ir(dmppy-ph)2tmd were almost twice that of Ir(3’,5’,4-mppy)2tmd, resulting in increased oscillator strengths for Ir(dmppy-CF3)2tmd and Ir(dmppy-ph)2tmd. By contrast, the LUMO and highest occupied molecular orbital (HOMO) of Ir(3’,5’,4-mppy)2tmd and Ir(dmppy-pro)2tmd exhibited similar electron distributions (see Figure S2); therefore the metal-to-ligand charge transfer (MLCT) and LC of the two emitters were similar (Table 1). The molecules were synthesized via the reaction of 2,2,6,6tetramethyl-3,5-heptanedione as an ancillary ligand with corresponding μ-chloro-bridged dimer, which was obtained from bis(2-(3,5-dimethylphenyl)-4-methylpyridine), 2-(3,5-dimethylphenyl)-4-phenylpyridine, 2-(3,5-dimethylphenyl)-4-(trifluoromethyl)pyridine, and 2-(3,5-dimethylphenyl)-4propylpyridine, respectively. Synthesis and characterization details are in the Supporting Information. All of the Ir complexes exhibited good thermal stability (Figure S3). Figure 2a shows photoluminescence (PL) and absorption spectra of HICs in solution. The absorption at 300–400 nm can be assigned to a spin-allowed singlet MLCT (1MLCT) transition, and the weak absorption at 430–500 nm can be assigned to a spin-forbidden triplet LC (3LC) and/or triplet MLCT (3MLCT) transitions. The peak wavelengths of the PL spectra were 587 nm for Ir(dmppy-CF3)2tmd, 532 nm for Ir(3’5’,4mppy)2tmd and Ir(dmppy-pro)2tmd, 579 nm for Ir(dmppyph)2tmd. The HOMO levels of the compounds were measured using cyclic voltammetry, and the LUMO levels were calculated from the HOMO levels and the optical bandgaps; these data are listed in Table S2. The PLQYs of 4 wt% HICs doped in a 4,4’,4”-tris(N-carbazolyl)-triphenylamine (TCTA): bis4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM)

Figure 2. (a) Absorption (close scatter) and PL spectra (open scatter) of Ir complexes in choloroform. (b) Angle-dependent PL intensities of the p-polarized light from the 30 nm thick films composed of TCTA:B3PYMPM: 4 wt% Ir complexes measured at 530 nm for Ir(3’,5’,4-mppy)2tmd and Ir(dmppypro)2tmd, 570 nm for Ir(dmppy-ph)2tmd, and 590 nm for Ir(dmppy-CF3)2tmd, respectively. co-host film were 97 ± 2%, 75 ± 2%, 98 ± 2%, and 98 ± 2%, for Ir(3’,5’,4-mppy)2tmd, Ir(dmppy-CF3)2tmd, Ir(dmppy-pro)2tmd and Ir(dmppy-ph)2tmd, respectively. The orientation of the emitting dipoles of the Ir complexes doped at 4 wt% into 30-nm-thick TCTA:B3PYMPM films was determined based on angle-dependent PL spectra (Figure 2b)6,10 using a classical dipole model considering the birefringence of the film.28 Figure S4 shows the angle-dependent ppolarized PL intensities at various wavelengths. The measured angle-dependent PL spectra were well fitted by Θ values of 79% for Ir(3’,5’,4-mppy)2tmd, 80% for Ir(dmppy-CF3)2tmd, 83.5% for Ir(dmppy-pro)2tmd, and 86.5% for Ir(dmppy-ph)2tmd. Note that larger values of Θ correspond to a greater degree of parallel orientation of TDMs of the molecules in the films, rather than orientation of the TDM against the C2 axis in the molecules; this is because the TDMs are almost 90 with respect to the C2 axis for all molecules. Parallel orientation of IrN bonds with the substrate becomes more effective as the aspect ratio and the surface area of the substituents increases, which corresponds to substitution of an elongated group (propyl and phenyl). This is because elongated linear substituents



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Scheme 1. Schematic of molecular orientation and intermolecular interaction of Ir complexes on host film under vacuum deposition (propyl group) interacted with host film as face-to-line interaction, elongated planar substituents (phenyl group) interacted with host film as face-to-face interaction, but methyl and CF3 group interacted with host film as face-to-point interaction (Scheme 1). Thus face-to-face interaction (phenyl substituents) becomes more effective to align Ir-N bonds and TDMs of HICs parallel to the substrate and to reduce degree of freedom for molecular orientation of HICs than face-topoint interaction. Similar Θ values of Ir(3’,5’,4-mppy)2tmd and Ir(dmppy-CF3)2tmd shows that interaction area significantly influences the molecular orientation of HICs. We fabricated OLEDs using these dyes to investigate the influence of the preferred horizontal orientation of the emitting

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dipoles on the efficiency of the devices. The device structure was as follows: indium tin oxide (ITO) (70 nm)/ 1,1-bis((di-4tolylamino)phenyl)cyclohexane (TAPC) (75 nm)/TCTA (10 nm)/TCTA:B3PYMPM: 4 wt% green dyes (30 nm)/B3PYMPM (45 or 55 nm) / LiF (0.7 nm)/Al (100 nm). TCTA was used as the hole-transport material, with B3PYMPM used as the electron-transport material. These two compounds were also used as the exciplex-forming co-hosts of the EML, where the molar ratio was 1:1.29 The thickness of the B3PYMPM layer was 45 nm for Ir(3’,5’,4-mppy)2tmd and Ir(dmppy-pro)2tmd, compared to 55 nm for Ir(dmppy-ph)2tmd and Ir(dmppy-CF3)2tmd. The device results of Ir(3’,5’,4-mppy)2tmd are from ref. 13. Figure 3a shows the current density–voltage–luminance (J-V-L) characteristics of these OLEDs. The OLEDs all exhibited turnon voltages of 2.4 V. The electroluminescence (EL) spectra of the devices were almost identical to those of the PL spectra of the dyes (Figure 3b). The power efficiencies (PEs) and current efficiencies (CEs) of the devices are shown in Figure 3c, and the EQEs are shown in Figure 3d. The EQE and PE were calibrated using angle-dependent emission intensity profiles (see the inset of Figure 3a). The maximum EQEs were 34.1%, 25.5%, 36.0% and 38.1% for the devices doped with Ir(3’,5’,4mppy)2tmd, Ir(dmppy-CF3)2tmd, Ir(dmppy-pro)2tmd, and Ir(dmppy-ph)2tmd, respectively; the corresponding PEs were 123, 51, 139 and 108 lm W-1 at 1,000 cd m-2, and the maximum respective CEs of the dyes were 120, 54, 126 and 108 cd A-1. The EQE of 36.0% for the green OLED that was doped with Ir(dmppy-pro)2tmd, and the EQE of 38.1% for the yellow OLED that was doped with Ir(dmppy-ph)2tmd, represent the highest efficiencies yet reported for Ir-based phosphorescent OLEDs with no outcoupling structures. This is attributed to

Figure 3. (a) Current densityvoltageluminance characteristics (b) EL spectra, (c) current efficiency and power efficiency as a function of the luminance of OLEDs, and (d) EQEs as a function of the current density with different Ir complexes as dopants. Inset of (a) shows the angular distributions of the EL intensities of the OLEDs with the dashed line of the Lambertian distribution.


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the large values of Θ and PLQY. To the best of our knowledge, the highest previously reported EQEs are 34.1% for a green OLED and 27.2% for a yellow OLED.13,30 The experimentally measured EQEs reported here are in good agreement with the calculated maximum achievable EQEs of 27.2%, 36.4%, and 38.2% for Ir(dmppy-CF3)2tmd, Ir(dmppy-pro)2tmd, and Ir(dmppy-ph)2tmd, OLEDs respectively (these calculations were based on measured values of Θ and PLQY and assumed no electrical losses.)

■ Conclusion In conclusion, we have shown that HICs can be aligned with the Ir-N bonds and TDMs parallel to the surface of doped films if the substituents have an elongated and planar structure with a high aspect ratio, and exhibit intermolecular interactions with face-to-face or face-to-line between the HICs and the host molecules. Ir-based complexes with unprecedentedly large horizontal emitting dipoles were formed in doped films by substituting functional groups at the 4-position of pyridine in the ppy main ligands, allowing it to realise high EQEs of 38.1% for a yellow OLED and 36.0% for a green OLED. Substituents at the 4-position of the pyridine ring significantly affect the molecular orientation of HICs. Although this approach was developed using HICs, it may also be applied to homoleptic Ir complexes.

■ Experimental section Materials. All starting materials were purchased from SigmaAldrich (St. Louis, MO, USA) and TCI (Tokyo, Japan). Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) was purchased from Umicore (Brussels, Belgium). All reagents were used as-received and without further purification. All solvents were further purified prior to use. The hole- and electron-transport materials (> 99%) were purchased from Nichem Fine Technology Co., Ltd (Hsinchu County, Taiwan). Al and LiF were purchased from Meterion (Fremont, CA, USA). Material characterization. 1H nuclear magnetic resonance (NMR) spectra were recorded using Bruker Avance 300-MHz and DRX 500-MHz spectrometers (Bruker, Billerica, MA, USA). The chemical shifts are reported in ppm units with tetramethylsilane as the internal standard. Thermogravimetric analysis (TGA) was performed in a nitrogen atmosphere using a TA Instruments 2050 thermogravimetric analyser (New Castle, DE, USA). Samples were heated at 10°C/min from 50ºC to 800°C. Differential scanning calorimetry (DSC) was carried in a nitrogen atmosphere using a TA Instruments 2100 thermal analyser. Samples were heated at 10°C/min from 30ºC to 350°C. Mass spectra were measured using a Jeol JMS-700 (Tokyo, Japan). Ultraviolet–visible (UV-vis) absorption spectra were measured using a Perkin-Elmer LAMBDA-900 spectrophotometer (Waltham, MA, USA) and a Perkin-Elmer LS-50B luminescence spectrophotometer. Cyclic voltammograms of the materials were recorded using an epsilon E3, at room temperature, in a 0.1-M solution of tetrabutylammonium perchlorate (Bu4NClO4) in acetonitrile under nitrogen gas at a scan rate of

50 mV/s. A Pt wire was used as the counter electrode and Ag/AgNO3 as the reference electrode. DFT calculations. Optimized geometries and HOMO and LUMO frontier orbitals were calculated using Gaussian 09.24 Geometry optimisation of the triplet states was performed using the B3LYP exchange–correlation functional, where the ‘double-ξ’ quality LANL2DZ basis was used for the Ir atom, and the 6-31G+(d) basis set was used for all other atoms. A pseudopotential (LANL2DZ) was used to describe the inner core electrons of the Ir atom. The triplet transition properties of the Ir complexes were investigated using quantum chemistry calculations, implemented via the Jaguar package25,26 that is included in Schrödinger Materials Science Suite27. Geometry optimisation of triplet states was performed using the B3LYP exchange–correlation functional, with the LACVP** basis set for all atoms. The triplet energies and triplet TDMs of the Ir complexes were determined using time-dependent DFT (TDDFT) calculations, where the spin-orbit Hamiltonian was described using the Tamm–Dancoff approximation. The B3LYP exchange–correlation functional was used with the DYALL2ZVCP_ZORA-J-PT-GEN basis set for all atoms, and the spinorbit zeroth order regular approximation (ZORA) was used to calculate relativistic effects. Analysis of emitting dipole orientation. The orientation of the dipoles of Ir complex was determined based on angle-dependent PL using a classical dipole model.6,10 This model allows for separate calculation of the p- and s-polarised optical emissions from horizontally and vertically oriented dipoles in a non-absorbing anisotropic medium. The experimental setup has been reported previously.6,10 Briefly, it consisted of a motorised rotation stage, a half cylinder lens with a sample holder, a dichroic mirror to filter the excitation beam, a polariser to select the polarisation of the emitted light, and a fibre optic spectrometer (Maya2000; Ocean Optics, Oxford, UK). An HeCd continuous-wave laser (325 nm; Melles Griot, Rochester, NY, USA) was used as the excitation source. The emitting films were thermally deposited on a 1-mm-thick fused silica substrate and encapsulated under an N2 atmosphere prior to use. P-polarised light was used to analyse the orientation of the dipoles in the films. Device fabrication and characterization. OLEDs were fabricated on cleaned glass substrates that were pre-patterned with 70-nm-thick ITO, which were formed using thermal evaporation at a pressure of 5 × 10−7 Torr without breaking the vacuum. Prior to deposition of the organic layers, the ITO substrates were pre-cleaned using isopropyl alcohol and acetone, then exposed to ultraviolet–ozone for 10 min. Each layer was deposited at a rate of 1 Å /s, and the total deposition rate of the co-deposited layers was 1 Å /s. Current density, luminance, and EL spectra were measured using a programmable source meter (Keithley 2400; Keithley Instruments, Cleveland, OH, USA) and a spectrophotometer (Spectrascan PR650; Photo Research Inc., Chatsworth, CA, USA), which measures power per steradian per unit wavelength per unit area (in units of W/(nm·sr·m2)). Luminance was calculated based on the measured optical power considering the CIE luminosity function. The angular distribution of the EL was measured using a programmable source meter (Keithley 2400), a goniometer, and a fibre optic spectrometer (Ocean Optics S2000). The EQE and



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PE were calculated based on the J-V-L characteristics, EL spectra and angular distributions of the EL intensity.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website Calculated triplet transition properties of HICs; photophysical and electronic properties of HICs; Isosurfaces of electrostatic potential of HICs; Frontier molecular orbitals of HICs; TGA and DSC thermograms of HICs; Synthesis and characterization details of compounds

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions ‡These authors contributed equally.

Funding Sources This work was supported by the the Midcareer Researcher Programs (2014R1A2A1A01002030 and 2015R1A2A1A10055620) through a National Research Foundation (NRF) grant funded by the Ministry of Science, Information and Communications Technology (ICT) and Future Planning (MSIP).

Notes The authors declare no competing financial interest.

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