Highly Efficient Red and White Organic Light-Emitting Diodes with

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Highly Efficient Red and White Organic Light-Emitting Diodes with External Quantum Efficiency Beyond 20% by Employing Pyridylimidazole-Based Metallophosphors Yanqin Miao, Peng Tao, Kexiang Wang, Hongxin Li, Bo Zhao, Long Gao, Hua Wang, Bingshe Xu, and Qiang Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10300 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Applied Materials & Interfaces

Highly Efficient Red and White Organic Light-Emitting Diodes with External Quantum Efficiency Beyond 20% by Employing Pyridylimidazole-Based Metallophosphors

Yanqin Miao,a,†,* Peng Tao,a,b,† Kexiang Wang,a Hongxin Li,b Bo Zhao,a Long Gao,a Hua Wang,a,* Bingshe Xu,a and Qiang Zhaob,*

a

Research Center of Advanced Materials Science and Technology and MOE Key Laboratory

of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, P. R. China. E-mail: [email protected]; [email protected]

b

Key Laboratory for Organic Electronics and Information Displays and Institute of Advanced

Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications (NUPT), Nanjing, 210023, P. R. China. E-mail: [email protected]

ABSTRACT: Two highly efficient red neutral iridium(III) complexes, Ir1 and Ir2, were rationally designed and synthesized by selecting two pyridylimidazole derivatives as the ancillary ligands. Both Ir1 and Ir2 show nearly the same photoluminescence emission with the maximum peak at 595 nm (shoulder band at about 638 nm), and achieve high solution quantum yields up to 0.47 for Ir1 and 0.57 for Ir2, respectively. Employing Ir1 and Ir2 as emitters, the fabricated red organic light-emitting diodes (OLEDs) show outstanding performance with the maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 20.98 %, 33.04 cd/A, and 33.08 lm/W for the Ir1-based device and 22.15 %, 36.89 cd/A, and 35.85 lm/W for the Ir2-based device. Furthermore, using Ir2 as 1

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red emitter, a tri-chromatic hybrid white OLED, showing good warm white emission with low correlated colour temperature of < 2200 K under the voltage of 4-6 V, was fabricated successfully. The white device also realizes excellent device efficiencies with the maximum EQE, CE, and PE reaching 22.74%, 44.77 cd/A, and 46.89 lm/W, respectively. Such high EL performance for red and white OLEDs indicates that Ir1 and Ir2 as efficient red phosphors have great potential for future OLED displays and lightings applications.

KEYWORDS: external quantum efficiency, imidazole derivatives, organic light-emitting

diodes, iridium(III) complexes, phosphorescence

1. INTRODUCTION The explorations of high-performance organic light-emitting diodes (OLEDs) are always keeping on going owing to their promising utilizations for the future solid-state lighting and display.1-4 Phosphorescent OLEDs, which could remarkably improve the efficiency of devices by harvesting both singlet and triplet excitons, have attracted extensive attentions.5-7 In addition to the device structure, phosphorescent dopants have also equally importance in determining the efficiency of OLEDs.8,9 The cyclometalated neutral iridium(III) complexes are widely regarded as the promising candidates for the electroluminescence (EL) because of their high luminescent efficiency, tunable emission energy, and excellent chemical stability among various phosphorescent transition-metal complexes.10-16 For realization of high-quality white OLEDs, especially in the warm WOLEDs, the red emission is the essential chromaticity component.17-20 Thus, the explorations of highly efficient neutral red iridium(III) phosphors are particularly important and necessary. Up to date, many efforts have been made in the design and synthesis of these complexes.17-20 The 1-phenylisoquinoline (Hpiq), 2-phenylquinoline (Hpq) and their derivatives have been successfully incorporated into iridium(III) complexes as the cyclometalated ligands for the red 2


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phosphors.21-27

The

majority

of

the

reported

neutral

red

phosphors

based

on

1-phenylisoquinoline or 2-phenylquinoline are the homoleptic complexes (e.g., Ir(piq)3) or β-diketonate-based complexes (e.g., (piq)2Ir(acac) or (pq)2Ir(acac)).26,28,29 The introduction of the alkyl group (such as methyl group) into the cyclometalated ligand was also commonly used strategy for the finely tuning of the photophysical properties of these complexes. For example, Shu et al. reported two quinoline-based red iridium(III) complexes with solution quantum yields (ФPL) of 0.11 and 0.14 by incorporating the phenyl and fluorenyl moieties, the external quantum efficiency (EQE) of red device is 8.16%.[19] Kim et al. developed a group of 2-phenylquinoline-based red iridium(III) complexes with β-diketonates by introducing methyl groups into both phenyl ring and quinoline ring, realizing the very high EQE of red device up to 24.6%.26 However, these complexes still often suffer from the relatively low solution quantum yields (less than 0.15). For the phenylisoquinoline-based red phosphors, Liu et al. reported

fluorine-modified

phenylisoquinoline-based

red

phosphors

with

emission

wavelengths at 595, 600 nm, and the quantum yields of these phosphors are 0.15 and 0.33, the EQE of corresponding device is 8.67%.[20] The well-known (piq)2Ir(acac) also shows relatively low quantum yield of 0.20.28 Recently, thiophenquinolone was empolyed as the cyclometalated ligand for the design of highly efficient red phosphors. Cheng et al. developed a thiophene-based iridium(III) complex with acetylacetone ligand.30 This thiophene-based complex exhibited much higher ФPL up to 0.55, and the red electroluminescent device using 4,4'-bis(9H-carbazol-9-yl)biphenyl as host material showed a EQE as high as 15.1%. The alteration of ancillary ligands is another important method to tune the photophysical properties of phosphors. Very recently, we reported two thiophenquinolone-based red iridium(III) complexes by incorporating triazolpyridine or picolinic acid as ancillary ligands, and the EQE for best red device reached 17.6%.11,12 Pyridylimidazole is an important ligand commonly used for the design of cationic iridium(III) complexes owing to the N-H or N-Calkyl moiety.31,32 For the pyridylimidazole with the N-H moiety, the proton of the N-H bond could 3



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be easily removed from the cationic complex in the presence of weak base to form its corresponding neutral complex suitable for the high-performance OLEDs. So the pyridylimidazole with the N-H moiety should be the potential ancillary ligand for the design of neutral iridium(III) complexes, especially for highly efficient red phosphors. In this work, we select two pyridylimidazole derivatives with various degrees of conjugation, 2-(pyridin-2-yl)-1H-benzo[d]imidazole (HPbi) and 2-(1H-imidazol-2-yl)pyridine (HPi) (The chemical structures of HPbi and HPi are shown in Scheme S1 in Supporting Information), serving as the ancillary ligands to investigate the effects of pyridylimidazole on

thiophenquinolone-based

iridium(III)

complexes.

Correspondingly,

two

novel

high-efficiency neutral red iridium(III) complexes, Ir1 and Ir2, were rational designed and synthesized. These two phosphors show nearly the same photoluminescence (PL) emission with the maximum peak at 595 nm (shoulder band at about 638 nm), and also own a very broad full width at half maximum (FWHM) as high as 84 nm. And it is found that the variation of ancillary ligands could tune the ФPL from 0.47 (Ir1) to 0.57 (Ir2). The red OLEDs based on Ir1 and Ir2 as emitters all achieve excellent device efficiencies with the maximum EQE, current efficiency (CE), and power efficiency (PE) of 20.98 %, 33.04 cd/A, and 33.08 lm/W for the Ir1-based device and 22.15 %, 36.89 cd/A, and 35.85 lm/W for the Ir2-based device, which are comparable or even superior to the best data among the reported platinum(II) and iridium(III)-based red OLEDs. Furthermore, employing Ir2 as red emitter, a tri-chromatic hybrid white OLED was developed. The white device realizes good warm white emission with low correlated color temperature (CCT) of < 2200 K under the voltage of 4-6 V. And the white device also exhibits excellent device efficiencies with the maximum EQE, CE, and PE reaching 22.74%, 44.77 cd/A, and 46.89 lm/W, respectively. To the best of our knowledge, the EQE is the highest record so far for hybrid white OLEDs with low CCT below 2500 K.

4


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2. RESULTS AND DISCUSSION 2.1 Synthesis and Characterization The chemical structures of the novel iridium(III) complexes (Ir1 and Ir2) and the synthetic routes

are

shown

in

Scheme

1.

The

cyclometalated

ligand

4-methyl-2-(thiophen-2-yl)quinolone was refluxed with IrCl3·3H2O in the mixture solvents of water and 2-ethoxyethanol to give the cyclometalated iridium(III) µ-chloro-bridged dimers according to the method proposed by M. Nonoyama.33 Then, the target complexes could be obtained in excellent yields by stirring the pyridylimidazole (HPbi and HPi) and the iridium(III) dimers together with the fine powders of potassium carbonate in dichloromethane at 25 oC developed previously by us,10 indicating that the proton of the N-H bond could be removed easily in the presence of weak base from the cationic complex formed in the process of complexation reaction to give its corresponding neutral complex suitable for high-performance OLEDs.

Scheme 1. Chemical structures and the synthetic routes of Ir1 and Ir2. The chemical structures were fully confirmed by the 1H NMR and mass spectra as shown in Supporting Information (Figure S1, S2, and S14-17). Figure S1 and S2 display the enlarged view of 1H NMR of Ir1 and Ir2. The enlarged spectra show much complicated resonance signals due to the asymmetrical ancillary ligand of the complexes, and most of them are well-resolved. Notably, by the analysis of the coupling constants (J) and chemical shifts (δ) as summaried in Figure S1 and S2, a couple of signals in the relatively high field of aromatic region should be corrsponding to the protons adjacent to the C-Ir bonds of the

5



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complexes. Furthermore, the 1H NMR also confirmed that these complexes show excellent purities suitable for the device fabrication (see Figure S16 and S17). Thermogravimetric analysis was conducted to investigate the thermal stability of these phosphors, and the thermogravimetric curves are shown in Figure S3. From Figure S3, it can be seen that the temperatures of 5% loss in weight of these phosphors are all exceeding 300 o

C (383 oC for Ir1 and 331 oC for Ir2), supporting that the phosphors exhibit enough thermal

stability in the fabrication of OLEDs. 2.2 Photophysical Properties The PL spectra and UV-visible absorption were carried out at room temperature to explore the photophysical properties of the phosphors in details. Figure 1a shows the PL spectra and UV-Vis absorption of Ir1 and Ir2 in CH2Cl2. Table S1 summarized the absorption data. Both phosphors show the intense absorption bands in the range of 200-400 nm attributed to 1π→π* transition of cyclometalated ligand,26 and exhibit nearly the same absorption peaks (227, 289, 344 nm for Ir1 and 227, 289, 339 nm for Ir2) because of the same cyclometalated ligands in these complexes. The relatively weak absorption bands in the region of 400-550 nm are probably attributed to the metal-to-ligand charge-transfer transitions (MLCT).26 Despite the fact that the ancillary HPbi and HPi ligands possess quite different conjugated degree, the two complexes show nearly the same absorption peaks, indicating that the absorptions are almost independent on the conjugated degree of pyridylimidazole. However, from the normalized absorption spectra of Ir1 and Ir2 shown in Figure S4, the intensities of absorption in the range of 250-550 nm of Ir2 is much stronger than that of Ir1, suggesting that the different conjugated degree of pyridylimidazole could give rise to the various intensities of absorption of complexes. This increment in absorption intensity, especially in lower energy band (400-550 nm), may then further influence the performance of electroluminescence (EL) through different efficiency of energy transfer from the host (energy donor) to the dopant (energy acceptor).6 6


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Figure 1. UV-visible absorption (a), PL spectra (b), phosphorescent decays (c) of Ir1 and Ir2 in degassed CH2Cl2 at 298 K; PL spectra (d) of Ir1 and Ir2 in 2-MeTHF glass matrix at 77 K. Inset: luminescence photographs in CH2Cl2. The emission properties of the phosphors were also affected by the variations in ancillary ligand. Both Ir1 and Ir2 phosphors exhibit broad, dual-peak emissions. The bright red emission in wavelength of 595 nm with shoulder emission at ∼639 nm is found in the PL spectra of two phosphors (Figure 1b). From the normalized PL spectra, we also note a slight intensity difference in the emission shoulder, which should be the result of the different pyridylimidazole ligands. In addition, the ancillary ligands with various conjugated degrees have evident effects on the aspects of the emission lifetime (τ) and the absolute ФPL. The Ir1 based on HPbi ligand shows 2.26 µs in emission lifetime with the ФPL of 0.47 in CH2Cl2 (Figure 1c and Table 1). The relatively smaller conjugated ligand Pi-based complex Ir2 exhibits more longer emission lifetime of 2.88 µs and higher ФPL up to 0.57 in degassed CH2Cl2 (Figure 1b and Table 1). This increments in emission lifetimes and ФPL for Ir2 7



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should be attributed to the decrement of the conjugated degree in ancillary ligands by increasing the radiative decay rate.34 Here the high ФPL of 0.57 for Ir2 is superior to those of most of the red phosphors, for instance, ФPL = 0.55 for (tmq)2Ir(acac),30 0.43 for (MTQ)2Ir(PIC),11

0.21

for

Ir(MePQ)2(acac),10 0.20

for

(piq)2Ir(acac),28

0.19

for

Ir(2-tpq)2(DPAP),35 and 0.13 for (pq)2Ir(acac).24 In addition, both Ir1 and Ir2 possess quite broad FWHMs up to 84 nm as listed in Table 1, which will play an important role in the realization of excellent color quality white devices. These results suggest that the pyridylimidazole with various conjugated degrees plays significant roles in determining the excited-state properties of thiophenquinolone-based iridium(III) phosphors. The triplet energies (T1) of the novel phosphors were estimated by the low-temperature spectra (at 77 K) in 2-methyltetrahydrofuran (2-MeTHF) as shown in Figure 1d and Table S1. The phosphors show evident blue-shift of the emission and exhibit fine structures of vibronic bands (∼12 nm blue-shift for first vibronic sub-band, ∼5 nm blue-shift for second vibronic sub-band). T1 levels were calculated from the highest-energy vibronic sub-band according to the low-temperature spectra.11 Two phosphors exhibits almost the same T1 (2.13 eV) as summarized in Table 1, which agree with the fact that the emission spectra are almost the same at room temperature. In order to further understand the effect of the pyridylimidazole ligands on the emission properties of iridium(III) phosphors, the theoretical calculations for two red phosphorescent iridium(III) phosphors have been carried out using the Gaussian 09 package.[36] We calculated the electron cloud distributions and energy levels of the complexes at ground state, singlet state and triplet state, as shown in Figure 2 and Figure S5-S10. The calculated energies and oscillator strengths for the lowest-energy singlet and triplet transitions are listed in Table S2. We can conclude that the triplet transitions for both complexes are from HOMO to LUMO (73.14% for Ir1 and 76.35% for Ir2). From Figure 2, for both complexes, the HOMOs mainly distributed in the thiophene moieties and iridium atom, while the LUMOs mainly located in 8


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quinoline moieties, which is consistent with the previous report.[11] The degrees of conjugation of pyridylimidazole ligands indeed have little influences on the excited states of thiophenquinolone-based iridium(III) complexes. The theoretical calculations also agree well with the experimental results discussed above.

Figure 2. The energy levels and electron cloud distributions of LUMOs, HOMOs of Ir1 and Ir2 at T1 state. 2.3 Electrochemical Properties The electrochemical properties of the phosphors were further explored by cyclic voltammetry in CH2Cl2. Both iridium(III) complexes exhibit two oxidation waves as shown in Figure S11a. Each complex has one less positively irreversible oxidation wave and the other more positively reversible oxidation wave. In order to rule out the interferences from the solvent, we also carried out the blank measurement under the same conditions. As shown in Figure S11b, there are no any signals in the blank measurement. Thus, the control experiment indicated that the first less positively irreversible oxidation waves indeed originated from the 9



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complexes. We attribute the first oxidation wave to the Ir centered oxidation of complexes,11 and the oxidation potential of Ir1 and Ir2 is 0.35 and 0.34 V, respectively (Table 1). The oxidation potential of Pbi-based complex with larger conjugated degree shows a slightly positive shift of ~0.01 V compared with the Pi-based complex, indicating that the variation in the electronic properties at the metal center of iridium(III) complexes could also be finely tuned by the degree of conjugations in ancillary ligands. Table 1. Luminescence and electrochemical data for iridium(III) complexes.

Complex Ir1 Ir2

λem [nm] 595, 639 (sh) 595, 638 (sh)

Emissiona) FWHM τ [µs] [nm] 84 2.26 81 2.88

ΦPL

Eonsetox b) [eV]

Eg c) [eV]

T1 d) [eV]

HOMO/ LUMOc) [eV]

0.47 0.57

0.35 0.34

2.08 2.08

2.13 2.13

-5.15/-3.07 -5.14/-3.06

a)

At a concentration of 1.0 × 10-5 mol/L in degassed CH2Cl2, λex = 365 nm; b)In CH2Cl2; c)HOMO (eV) =

-e(Eonsetox + 4.8), Eg = 1240/λ, LUMO (eV) = Eg + HOMO; d)The triplet energy (T1) was estimated from the highest-energy vibronic sub-band of the phosphorescence spectra at a concentration of 1.0 × 10-5 mol/L in 2-MeTHF glass matrix at 77 K, λex = 365 nm.

2.4 Electroluminescence Performance 2.4.1 Red OLEDs based on Ir1 and Ir2 From the above discussion, phosphors Ir1 and Ir2 all exhibit highly efficient red emission and broad PL emission, which are beneficial for the development of superior efficiency and high color quality white OLEDs. To evaluate the EL performances of Ir1 and Ir2, a suitable device structure is of great importance. The doping OLEDs are commonly used to achieve high performance due to the reduction of excitons quenching.37 For the doping OLEDs, an ideal host is the prerequisite for obtaining high device efficiency.38 The previous work reported by Jeon et al. confirmed that in phosphorescent OLEDs, the employment of strong fluorescent host materials is helpful to achieve high device performance because of efficient Förster

energy

transfer

from

host

materials

to

doped

emitters.39

Herein,

bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2) was selected as such ideal host because of the following reasons: i) Bepp2 shows strong fluorescence emission with the high ФPL of 10


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0.80; ii) there exists the big spectral overlaps between the PL emission spectrum of Bepp2 and the absorption spectrum of Ir1 and Ir2 (Figure S5); iii) the triplet energy level (T1 = 2.6 eV)40 for Bepp2 is higher than these of Ir1

and Ir2 (T1 = 2.13 eV). These ensure the efficient

energy transfer from the Bepp2 host to Ir1 and Ir2 guests. Next, two red OLEDs were fabricated, and the device structure is ITO/ MoO3 (3 nm)/ TCTA (40 nm)/ Bepp2: 3wt% Ir1 or Ir2 (20 nm)/ TPBi (50 nm)/ LiF (1 nm)/ Al (100 nm), here, the device comprising Ir1 is denoted as device R1, and the device comprising Ir2 is denoted as device R2. The structure diagrams for two red devices are shown in Figure 3a, where 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA) served as the hole transport layer (HTL), Bepp2 doped with Ir1 or Ir2 served as the light-emitting layer (EML), 1,3,5-tris(phenyl-2-benzimidazolyl)benzene (TPBi) served as the electron transport layer (ETL), LiF, MoO3, and Al served as the electron injection layer (EIL), the hole injection layer (HIL), and the cathode, respectively. The energy level diagrams of Ir1 and Ir2-based red OLEDs are shown in Figure 3b. Obviously, the high energy barriers of 0.3 eV and 0.6 eV are observed at HTL/EML and EML/ETL interfaces, which are cuased by a higher LUMO level (-2.3 eV) of TCTA and a deeper HOMO level (-6.2 eV) of TPBi. Thus, the carrier recombination zone in these two devices can be well confined in the EML. In addition, the higher triplet energy levels of TCTA (2.8 eV)41 and TPBi (2.7 eV)42 than Ir1 (2.13 eV) and Ir2 (2.13 eV) will also limit well the triplet excitons in the Bepp2 EML. These will be beneficial for boosting device performance.

11


(b)

LiF (1 nm)/Al (100 nm) TPBi (50 nm) Bepp2: Ir1 3wt% (20 nm)

R1

Bepp2: Ir2 3wt% (20 nm)

R2

TCTA (40 nm) MoO3 (3 nm) ITO (180 nm) Glass (1.1 mm)

-2

R1 R2

LUMO LiF/Al

-3 TCTA

-4 ITO

Bepp2

(a)

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Bepp2

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Energy levels (HOMO and LUMO) (eV)


TPBi

-5 -6 -7

MoO3

EML Ir1 Ir2

HOMO

Figure 3. (a) The device structure diagram of the red devices R1 and R2; (b) The energy level diagram of the red devices R1 and R2. Figure 4a displays the normalized EL spectra and CIE coordinates of the red devices R1 and R2 at 5 V, and the inset in Figure 4a is a photograph for the device R2 at 6 V. Obviously, both of two devices R1 and R2 realize red emission, and the CIE coordinates are (0.624, 0.371) for device R1 and (0.620, 0.377) for the device R2 at a voltage of 5 V (see Figure S12a). And two red devices also exhibit similar EL spectra with the same emission peak at 600 nm and a shoulder peak at about 641 nm. These are consistent to the PL spectra of Ir1 and Ir2, suggesting that red emission in two devices exactly originated from Ir1 and Ir2. However, compared with the PL spectra of Ir1 and Ir2, a 3 nm red-shift for device R1 and a 5 nm red-shift for the device R2 are found in the EL spectra of two red devices, which are attributed to the intermolecular dipole-dipole interactions caused by the short distance between the dopant molecules in solid film.43,44 Here, two red devices also show broad FWHMs of emission of 78 nm for the device R1 and 76 nm for the device R2, which is very beneficial for developing high color quality white OLEDs. Figure

4b,

4c,

and 4d show

the

current density-voltage-luminance (J-V-L),

CE-lumiance-PE (CE-L-PE) and EQE-luminance (EQE-L) curves of the devices R1 and R2, and the corresponding EL data are listed in Table 2. From Figure 4b, two red devices show the same turn-on voltage of 3.0 V, and realize high maximum luminance of 28880 cd/m2 for the device R1 and 27510 cd/m2 for the device R2. From Figure 4c and Figure 4d, two red 12


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devices also all achieve excellent device efficiencies. The maximum EQE, CE, and PE of 20.98 %, 33.04 cd/A, 33.08 lm/W for the device R1 and 22.15 %, 36.89 cd/A, 35.85 lm/W for the device R2 are obtained. It is found that compared with the device R1, the device R2 shows slightly higher device efficiencies for EQE, CE, and PE, which can be ascribed to i) a higher ΦPL for Ir2 (0.57) than Ir1 (0.47); ii) a more efficient energy transfer from Bepp2 to Ir2 due to a bigger spectral overlap between the PL spectrum of Bepp2 and absorption spectrum of Ir2. It is worth noting that the EQEs for the devices R1 (20.98%) and R2 (22.15%) all exceed a theoretical value of 20 %, indicating a complete excitons harvesting, which is attributed to i) the efficient red emitters with ФPL for Ir1 (0.47) and Ir2 (0.57) ; ii) an ideal host-guest system by employing a narrow band-gap and strong fluorescent host material for two red devices; iii) a suitable device structure with a strictly limited carrier recombination zone by broad band-gap HTL and ETL for Ir1 and Ir2. In addition, the EQE and CE roll-offs for the two red devices are negligible. Under a luminance of 1000 cd/m2, both red devices still possess high EQE and CE values, 18.97% and 29.87 cd/A for the device R1, and 19.96% and 33.24 cd/A for the device R2, indicating a well-balanced charge-transport property in these devices.45 Based on the above analysis, such high performance for the Ir1 and Ir2-based red devices are comparable or even superior to the best data among the reported platinum(II) and iridium(III)-based red OLEDs.11,26,41,43,44,46 For example, Ir(pq)2(acac) is a typical red phosphors commonly employed to develop high performance red and white OLEDs. And the OLEDs based on Ir(pq)2(acac) show a red emission peak at about 600 nm, which is well coincident to the EL spectra of the devices R1 and R2. However, the efficiencise for these Ir(pq)2(acac)-based OLEDs are modest, showing forward-viewing EQEs below 20%.47 In contrast, the superior EL performance for the Ir1 and Ir2-based red devices confirms that Ir1 and Ir2 can act as efficient red phosphors for developing high-performance monochrome and white OLEDs.

13



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Figure 4. The normalized EL spectra (a), J-V-L (b), CE-L-PE (c) and EQE-L (d) characteristics curves for the red devices R1 and R2. Inset: a photograph for the device R2 at 6V. 2.4.2 White OLED based on Ir2 In order to verify the feasibility of these excellent red phosphors applied to white OLEDs, Ir2 as a representative was selected as red emitter to develop white OLED because of its higher monochrome

device

efficiency.

By

inserting

ultrathin

(
10 nm), the fromed triplet excitons at Bepp2/TPBi and TCTA/Bepp2 interfaces can diffuse to ultrathin Ir2 and Ir(ppy)3 layers for red and green emission. A 3 nm-thick Bepp2 layer between ultrathin Ir2 and Ir(ppy)3 layers is employed to adjust the emission intensity of Ir2 emitter by controlling the energy transfer rate from Ir(ppy)3 to Ir2, for obtaining a balanced white emission. Figure 6a displays the EL spectra, luminance, CCT, CIE coordinates, and CRI of the device W(Ir2) at different voltages. Clearly, the EL spectra for the device W(Ir2) contains three main emission peaks at 448 nm, 508 nm and 596 nm corresponding to emissions of 15



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Bepp2, Ir(ppy)3 and Ir2, respectively. And the dominant emission intensity is located at red band in EL spectra, which makes the device W(Ir2) possesses a low CCT of 2148-2131 K under the voltage of 4-6 V. The white device W(Ir2) also exhibits extremely high color stability. As the voltage increases from 4-6 V, corresponding to luminance increases from 335 cd/m2 to 5684 cd/m2, the CIE coordinates reveal slight change from (0.525, 0.435) to (0.522, 0.427). Clearly, the CIE coordinates values for the white device W(Ir2) are close to the CIE values of (0.52, 0.42) for candle white light,45 and lie close to the Planckian locus, as shown in Figure S12b. Besides, the device W(Ir2) also shows a relatively high CRI of 77, very close to the threshold value of 80 for white OLEDs in practical lighting. These high performance confirms that the device W(Ir2) realizes excellent warm white emission.49

Figure 6. (a) The normalized EL spectra (a), J-V-L (b), CE-L-PE (c) and EQE-L (d) characteristics curves for the white device W(Ir2). Inset: a photograph for the white device W(Ir2) at 6 V.

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Figure 6b, 6c, and 6d show the J-V-L, CE-L-PE and EQE-L curves of the device W(Ir2), and the related EL data are also listed in Table 2. From Figure 6b, the device W(Ir2) exhibits a low turn-on voltage of 3.0 V, and realizes a high maximum luminance of 21450 cd/m2. As shown in Figure 6c and 6d, the device W(Ir2) also exhibits excellent device efficiencies, the maximum EQE, CE, and PE reaching 22.74%, 44.77 cd/A, and 46.89 lm/W, respectively. Herein, the EQE value of 22.74% also obviously exceeds the theoretical value of 20 %, indicating a complete excitons harvesting, which is attributed to i) a highly efficient red emitter Ir2, ii) effective management and utilization of singlet and triplet excitons via a novel white device structure. To the best of our knowledge, the EQE for the white device based on Ir2 is the highest record so far among all the hybrid warm white OLEDs with low CCT below 2500 K.50-52 Actually, even though employing inferior Ir1(ФPL = 0.47) red emitter and using the same device structure as the case of the white device W(Ir2), the fabricated Ir1 based white OLED (device W(Ir1)) also realizes good warm with a low CCT of 1946-2058 K under the voltage of 4-6V (see Figure S13a). And similar to the white device W(Ir2), the white device W(Ir1) shows a low turn-on voltage of 3.0 V and a high maximum luminance of 23020 cd/m2 (see Figure S13b). Owing to a lower ФPL (0.47) for Ir1, the device efficiencies for the white device W(Ir1) are slightly lower than these of the white device W(Ir2). Even so, the maximum CE, PE, and EQE for device W(Ir1) are still up to 41.21 cd/A, 43.15 lm/W, and 20.21 %, respectively, where the EQE value of 20.21% also exceeds the theoretical value of 20 % (see Figure S13c and S13d). These primary results indicate that these two excellent red phosphors have promising applications for the future OLED display and solid-state lighting.

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Table 2. Summary of the performances of the red devices R1and R2, and white devices W(Ir1) and W(Ir2).

a

Device

Von [V]a

R1 R2 W(Ir1) W(Ir2)

3.0 3.0 3.0 3.0

Maximum Luminance [cd/m2] 28880 27510 23020 21450

EQE [%] 20.98 22.15 20.21 22.74

CE [cd/A] 33.04 36.89 41.21 44.77

PE [lm/W] 33.08 35.85 43.15 46.89

CIE (x,y) @5V

CCT (K) @ 4 V-6 V

CRI @ 4/5/6V

(0.624, 0.371) (0.620, 0.377) (0.543, 0.425) (0.530, 0.427)

----1946−2058 2148−2131

----73/70/71 77/75/77

Voltage at luminance of > 1 cd/m2.

3. CONCLUSIONS In summary, by selecting two pyridylimidazole derivatives as the ancillary ligands, two highly efficient red neutral iridium(III) complexes (Ir1 and Ir2), showing nearly the same red PL emission with the maximum peak at 595 nm (shoulder band at about 638 nm) were rationally designed and synthesized. Ir1 and Ir2 phosphors also achieved high solution quantum yields up to 0.47 for Ir1 and 0.57 for Ir2, respectively. The red devices using Ir1 and Ir2 as emitters all obtained the excellent device efficiencies with the maximum EQE, CE, and PE of 20.98 %, 33.04 cd/A, and 33.08 lm/W for the Ir1-based device and 22.15 %, 36.89 cd/A, and 35.85 lm/W for the Ir2-based device, which are comparable or even superior to the best data among the reported platinum(II) and iridium(III)-based red OLEDs. A tri-chromatic hybrid white OLED based on Ir2 as red emitter realizes good warm white emission with low CCT of < 2200 K, showing high color stability with CIE coordinates slightly change from (0.525, 0.435) to (0.522, 0.427) under the voltage of 4-6 V. And the white device also achieved excellent device efficiencies with the maximum EQE, CE, and PE reaching 22.74%, 44.77 cd/A, and 46.89 lm/W, respectively. To the best of our knowledge, the EQE value is the highest record so far for hybrid white OLEDs with low CCT below 2500 K. Such high EL performance for red and white OLEDs indicates that Ir1 and Ir2 as efficient red

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phosphorescent emitters have great potential for future OLED solid-state lighting and display applications.

4. EXPERIMENTAL SECTION 4.1. Experimental Information The detailed description about materials, theoretical calculations, measurements (NMR spectra, mass spectra, UV-vis absorption spectra, steady-state emission, excited-state lifetime, quantum yields, and cyclic voltammetry) can be found in Section S1 in Supporting Information. And the detailed process about OLEDs fabrication and testing can be found in Section S2 in Supporting Information. 4.2. Synthesis of Iridium(III) Complexes The procedures for the synthesis of iridium(III) complexes are according to our reported methods previously.10 According to the method proposed by M. Nonoyama,33 cyclometalated iridium(III) µ-chloro-bridged dimers were obtained by refluxing IrCl3·3H2O with 2.2 equiv 4-methyl-2-(thiophen-2-yl)quinolone in a 1:3 mixtures of H2O and 2-ethoxyethanol under N2 for 20 hours. To the solution of CH2Cl2 (30 mL) the mixture of ancillary ligand HPbi or HPi (0.5 mmol), chloro-bridged dimer (0.2 mmol) and K2CO3 powder (2 mmol) were added, then the mixtures was stirred at 25 °C under N2 for 12 hours. The reaction mixtures were filtered to remove salts after completing the reaction. Then the solvent was removed by rotary evaporator. The crude product was purified to obtain the target complexes by column chromatography (CH2Cl2 as eluent). (MTQ)2Ir(Pbi): Red powder (60% yield). 1H NMR (400 MHz, CDCl3, δ): 8.10 (d, J = 7.60 Hz, 1H), 7.89 (t, J = 7.20 Hz, 1H), 7.82-7.71 (m, 5H), 7.68 (s, 1H), 7.53 (t, J = 4.40 Hz, 2H), 7.39 (t, J = 7.60 Hz, 2H), 7.30 (d, J = 7.20 Hz, 1H), 7.21 (t, J = 8.80 Hz, 2H), 6.96-6.88 (m, 2H), 6.70 (t, J = 7.60 Hz, 1H), 6.50 (t, J = 8.00 Hz, 1H), 6.24 (d, J = 4.80 Hz, 1H), 6.20 (d, J 19



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= 4.80 Hz, 1H), 6.04 (d, J = 8.40 Hz, 1H), 2.81 (s, 3H), 2.74 (s, 3H). MALDI-TOF-MS (m/z): calcd for C40H28IrN5S2, 835.03; found, 835.19. (MTQ)2Ir(Pi): Red powder (42% yield). 1H NMR (400 MHz, CDCl3, δ): 8.24 (s, 1H), 7.80 (d, J = 5.20 Hz, 1H), 7.75 (d, J = 8.40 Hz, 2H), 7.63 (t, J = 7.20 Hz, 1H), 7.61 (s, 1H), 7.57 (d, J = 8.80 Hz, 1H), 7.54 (s, 1H), 7.33-7.24 (m, 4H), 7.22 (d, J = 4.80 Hz, 1H), 7.20 (s, 1H), 7.09 (ddd, J = 1.20 Hz, J = 6.80 Hz, J = 8.40 Hz, 1H), 7.04 (t, J = 6.40 Hz, 1H), 6.91 (ddd, J = 1.20 Hz, J = 6.80 Hz, J = 8.80 Hz, 1H), 6.53 (s, 1H), 6.46 (t, J = 4.80 Hz, 1H), 6.26 (d, J = 4.80 Hz, 1H), 2.82 (s, 3H), 2.79 (s, 3H). MALDI-TOF-MS (m/z): calcd for C36H26IrN5S2, 784.97; found, 785.22.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Descriptions of general experimental methods and detailed OLEDs fabrication and testing process, chemical structures for derivatives, absorbance spectra, PL spectra, 1H NMR spectra, MS spectra, thermogravimetry, and cyclic voltammograms curves of Ir1 and Ir2, theoretical calculations of energy levels for Ir1 and Ir2, CIE coordinates for red and white devices, device performance for device W(Ir1).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Yanqin Miao). *E-mail: [email protected] (Hua Wang). *E-mail: [email protected] (Qiang Zhao). Author Contributions † Y. Q. Miao and P. Tao contributed equally to this work. 20


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Funding This work was supported by K. C. Wong Education Foundation-Hong Kong Baptist University (HKBU) Fellowship Programme for Mainland Visiting Scholars 2017-2018, National Natural Science Foundation of China (61705156), Key Innovative Research Team in Science and Technology in Shanxi Province (201513002-10), Natural Science Foundation of Shanxi Province (201601D011031, 201601D021018), National Program for Support of Top-Notch Young Professionals, Scientific and Technological Innovation Teams of Colleges and Universities in Jiangsu Province (TJ215006), Priority Academic Program Development of Jiangsu Higher Education Institutions (YX03001).

Notes The authors declare no competing financial interest.

REFERENCES (1) Yang, X.; Zhou, G.; Wong, W.-Y. Functionalization of Phosphorescent Emitters and Their Host Materials by Main-Group Elements for Phosphorescent Organic Light-Emitting Devices. Chem. Soc. Rev. 2015, 44, 8484−8575. (2) Ho, C.-L.; Wong, W.-Y. Charge and Energy Transfers in Functional Metallophosphors and Metallopolyynes. Coord. Chem. Rev. 2013, 257, 1614−1649. (3) Ying, L.; Ho, C.-L.; Wu, H.-B.; Cao, Y.; Wong, W.-Y. White Polymer Light-Emitting Devices for Solid-State Lighting: Materials, Devices, and Recent Progress. Adv. Mater. 2014, 26, 2459−2473. (4) Miao, Y.-Q.; Gao, Z.-X.; Wang, H.; Jia, H.-S.; Liu, X.-G.; Xu, B.-S.; Wei, B. Extremely High Chromatic-Stability White Organic Light-Emitting Device with Symmetrical Cascade Emissive Layer. Org. Electron. 2015, 23, 199−207.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5) W.-Y. Wong, C.-L. Ho, Functional Metallophosphors for Effective Charge Carrier Injection/Transport: New Robust OLED Materials with Emerging Applications. J. Mater. Chem. 2009, 19, 4457−4482. (6) Wu, S.-F.; Li, S.-H.; Wang, Y.-K.; Huang, C.-C.; Sun, Q.; Liang, J.-J.; Liao, L.-S.; Fung, M.-K. White Organic LED with a Luminous Efficacy Exceeding 100 lm W−1 without Light Out-Coupling Enhancement Techniques. Adv. Funct. Mater. 2017, DOI: 10.1002/adfm.201701314. (7) Sarada, G.; Cho, W.; Maheshwaran, A.; Sree, V.-G.; Park, H.-Y.; Gal, Y.-S.; Song, M.; Jin, S.-H. Deep-Blue Phosphorescent Ir(III) Complexes with Light-Harvesting Functional Moieties for Efficient Blue and White PhOLEDs in Solution-Process. Adv. Funct. Mater. 2017, DOI: 10.1002/adfm.201701002. (8) Wong, W.-Y.; Ho, C.-L. Heavy Metal Organometallic Electrophosphors Derived from Multi-Component Chromophores. Coord. Chem. Rev. 2009, 253, 1709−1758. (9) Lee, S.; Kim, K.-H.; Limbach, D.; Park, Y.-S.; Kim, J.-J. Low Roll-Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co-Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co-Host. Adv. Funct. Mater. 2013, 23, 4105−4110. (10) Tao, P.; Li, W.-L.; Zhang, J.; Guo, S.; Zhao, Q.; Wang, H.; Wei, B.; Liu, S.-J.; Zhou, X.-H.; Yu, Q.; Xu, B.-S.; Huang, W. Facile Synthesis of Highly Efficient Lepidine-Based Phosphorescent Iridium(III) Complexes for Yellow and White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 881−894. (11) Tao, P.; Miao, Y.-Q.; Zhang, Y.-B.; Wang, K.-X.; Li, H.-X.; Li, L.; Li, X.-L.; Yang, T.-T.; Zhao, Q.; Wang, H.; Liu, S.-J.; Zhou, X.-H.; Xu, B.-S.; Huang, W. Highly Efficient Thienylquinoline-Based Phosphorescent Iridium(III) Complexes for Red and White Organic Light-Emitting Diodes. Org. Electron. 2017, 45, 293−301.

22


Page 22 of 29

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Tao, P.; Miao, Y.-Q.; Wang, K.-X.; Li, H.-X.; Zhao, Q.; Wang, H.; Li, J.; Xu, B.-S.; Huang, W. Novel Phosphorescent Neutral Iridium(III) Complex with the Steric Hindrance for Highly Efficient Red Organic Light-Emitting Diodes. Tetrahedron Lett. 2017, 58, 3598–3601. (13) Lin, J.; Wang, Y.; Gnanasekaran, P.; Chiang, Y.-C.; Yang, C.-C.; Chang, C.-H.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Chi, Y.; Liu, S.-W. Unprecedented Homoleptic Bis-Tridentate Iridium(III) Phosphors: Facile, Scaled-Up Production, and Superior Chemical Stability. Adv. Funct. Mater. 2017, DOI: 10.1002/adfm.201702856. (14) Gao, Z.-Q.; Mi, B.-X.; Tam, H.-L.; Cheah, K.-W.; Chen, C.-H.; Wong, M.-Shing.; Lee, S.-T.; Lee, C.-S. High Efficiency and Small Roll-Off Electrophosphorescence from a New Iridium Complex with Well-Matched Energy Levels. Adv. Mater. 2008, 20, 774– 778. (15) Im, Y.; Byun, S.-Y.; Kim, J.-H.; Lee, D.-R.; Oh, C.-S.; Yook, K.-S.; Lee, J.-Y. Recent Progress in High-Efficiency Blue-Light-Emitting Materials for Organic Light-Emitting Diodes. Adv. Funct. Mater. 2017, DOI: 10.1002/adfm. 201603007. (16) Tao, P.; Zhang, Y.-B.; Wang, J.; Wei, L.-W.; Li, H.-X.; Li, X.-L.; Zhao, Q.; Zhang, X.-W.; Liu, S.-J.; Wang, H.; Huang, W. Highly Efficient Blue Phosphorescent Iridium(III) Complexes with Various Ancillary Ligands for Partially Solution-Processed Organic Light-Emitting Diodes. J. Mater. Chem. C 2017, DOI: 10.1039/c7tc02496g. (17) Ho, C.-L.; Hua, L.; Wong, W.-Y. Red to Near-Infrared Organometallic Phosphorescent Dyes for OLED Applications. J. Organomet. Chem. 2014, 751, 261−285. (18) Fan, C.; Yang, C.-L. Yellow/Orange Emissive Heavy-Metal Complexes as Phosphors in Monochromatic and White Organic Light-Emitting Devices. Chem. Soc.Rev. 2014, 43, 6439−6469. (19) Wu, F.-I.; Su, H.-J.; Shu, C.-F.; Luo, L.; Diau, W.-G.; Cheng, C.-H.; Duan, J.-P.; Lee, G.-H. Tuning the emission and morphology of cyclometalated iridium complexes and 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

their applications to organic light-emitting diodes. J. Mater. Chem. C 2005, 15, 1035−1042. (20) Su, Y.-J.; Huang, H.-L.; Li C.-L.; Chien, C.-H.; Tao, Y.-T.; Chou, P.-T.; Datta, S.; Liu, R.-S. Highly Efficient Red Electrophosphorescent Devices Based on Iridium Isoquinoline Complexes: Remarkable External Quantum Efficiency Over a Wide Range of Current. Adv. Mater. 2003, 15, 884−888. (21) Ding, J.-Q.; Gao, J.; Fu, Q.; Cheng, Y.-X.; Ma, D.-G.; Wang, L.-X. Highly Efficient Phosphorescent Bis-Cyclometalated Iridium Complexes Based on Quinoline Ligands. Synth. Met. 2005, 155, 539−548. (22) Wu, F.-I.; Su, H.-J.; Shu, C.-F.; Luo, L.-Y.; Diau, W.-G.; Cheng, C.-H.; Duan, J.-P.; Lee, G.-H. Tuning the Emission and Morphology of Cyclometalated Iridium Complexes and Their Applications to Organic Light-Emitting Diodes. J. Mater. Chem. 2005, 15, 1035−1042. (23) Thomas, K.-R.-J.; Velusamy, M.; Lin, J.-T.; Chien, C.-H.; Tao, Y.-T.; Wen, Y.-S.; Hu, Y.-H.; Chou, P.-T. Efficient Red-Emitting Cyclometalated Iridium(III) Complexes Containing Lepidine-Based Ligands. Inorg. Chem. 2005, 44, 5677−5685. (24) Zhao, Q.; Jiang, C.-Y.; Shi, M.; Li, F.-Y.; Yi, T.; Cao, Y.; Huang, C.-H. Synthesis and Photophysical, Electrochemical, and Electrophosphorescent Properties of a Series of Iridium(III) Complexes Based on Quinoline Derivatives and Different β-Diketonate Ligands. Organometallics 2006, 25, 3631−3638. (25) Ho, C.-L.; Wong, W.-Y.; Gao, Z.-Q.; Chen, C.-H.; Cheah, K.-W.; Yao, B.; Xie, Z.-Y.; Wang, Q.; Ma, D.-G.; Wang, L.-X.; Yu, X.-M.; Kwok, H.-S.; Lin, Z.-Y. Red-Light-Emitting Iridium Complexes with Hole-Transporting 9-Arylcarbazole Moieties for Electrophosphorescence Efficiency/Color Purity Trade-off Optimization. Adv. Funct. Mater. 2008, 18, 319−331.

24


Page 24 of 29

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Kim, D.-H.; Cho, N.-S.; Oh, H.-Y.; Yang, J.-H.; Jeon, W.-S.; Park, J.-S.; Suh, M.-C.; Kwon, J.-H. Highly Efficient Red Phosphorescent Dopants in Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 2721−2726. (27) Sarada, G.; Yoon, J.; Cho, W.; Cho, M.; Cho, D.-W.; Kang, S.-O.; Nam, Y.; Lee, J.-Y.; Jin, S.-H. Substituent Position Engineering of Diphenylquinoline-Based Ir(III) Complexes for Efficient Orange and White PhOLEDs with High Color Stability/Low Efficiency Roll-Off Using a Solution-Processed Emission Layer. J. Mater. Chem. C 2016, 4, 113−120. (28) Su, Y.-J.; Huang, H.-L.; Li, C.-L.; Chien, C.-H.; Tao, Y.-T.; Chou, P.-T.; Datta, S.; Liu, R.-S. Highly Efficient Red Electrophosphorescent Devices Based on Iridium Isoquinoline Complexes: Remarkable External Quantum Efficiency Over a Wide Range of Current. Adv. Mater. 2003, 15, 884−888. (29) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. Homoleptic Cyclometalated Iridium Complexes with Highly Efficient Red Phosphorescence and Application to Organic Light-Emitting Diode. J. Am. Chem. Soc. 2003, 125, 12971−12979. (30) Fan, C.-H.; Sun, P.-P.; Su, T.-H.; Cheng, C.-H. Host and Dopant Materials for Idealized Deep-Red Organic Electrophosphorescence Devices. Adv. Mater. 2011, 23, 2981−2985. (31) Sun, H.-B.; Liu, S.-J.; Lin, W.-P.; Zhang, K.-Y.; Lv, W.; Huang, X.; Huo, F.-W.; Yang, H.-R.; Jenkins, G.; Zhao, Q.; Huang, W. Smart Responsive Phosphorescent Materials for Data Recording and Security Protection. Nat. Commun. 2014, 5, 3601−3609. (32) Gong, D.-P.; Gao, T.-B.; Cao, D.-K.; Michael, D.-W. Cyclometalated Ir(III) Complexes Containing Quinoline–Benzimidazole-Based N^N Ancillary Ligands: Structural and Luminescence

Modulation

by

Varying

the

Substituent

Groups

or

the

Protonation/Deprotonation State of Imidazole Units. Dalton Trans. 2017, 46, 275−286. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(33) Nonoyama, M. Benzo[h]quinolin-10-yl-N Iridium(III) Complexes. Bull. Chem. Soc. Jpn. 1974, 47, 767−768. (34) Zhao, Q.; Liu, S.-J.; Shi, M.; Wang, C.-M.; Yu, M.-X.; Li, L.; Li, F.-Y.; Yi, T.; Huang, C.-H. Series of New Cationic Iridium(III) Complexes with Tunable Emission Wavelength and Excited State Properties: Structures, Theoretical Calculations, and Photophysical and Electrochemical Properties. Inorg. Chem. 2006, 45, 6152−6160. (35) Shen, L.; Chen, Z.; Zhao, Q.; Li, F.-Y.; Yi, T.; Cao, Y. C.-H. Huang, High Efficient Polymer Light-Emitting Diode Based on a New Iridium(III) Complex with Pyrazolone Ligand. Inorg. Chem. Commun. 2006, 9, 620−623. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian09, Revision B.01; Gaussian Inc., Wallingford, CT, 2010. (37) Zhao, Y.-B.; Chen, J.-S.; Ma, D.-G. Ultrathin Nondoped Emissive Layers for Efficient and Simple Monochrome and White Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2013, 5, 965−971.

26


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(38) Liu, B.-Q.; Luo, D.-X.; Gao, D.-Y.; Wang, X.; Xu, M.; Zou, J.-H.; Ning, H.-L.; Wang, L.; Peng, J.-B.; Cao, Y. An Ideal Host-Guest System to Accomplish High-Performance Greenish Yellow and Hybrid White Organic Light-Emitting Diodes. Org. Electron. 2015, 27, 29−34. (39) Jeon, W.-S.; Park, T.-J.; Kim, S.-Y.; Pode, R.; Jang, J.; Kwona, J.-H. Ideal Host and Guest System in Phosphorescent OLEDs. Org. Electron. 2009, 10, 240-246. (40) Zhao, F.-C.; Zhang, Z.-Q.; Liu, Y.-P.; Dai, Y.-F.; Chen, J.-S.; Ma, D.-G. A Hybrid White Organic Light-Emitting Diode with Stable Color and Reduced Efficiency Roll-Off by Using a Bipolar Charge Carrier Switch. Org. Electron. 2012, 13, 1049−1055. (41) Xiao, L.-X.; Chen, Z.-J.; Qu, B.; Luo, J.-X.; Kong, S.; Gong, Q.-H.; Kido, J. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926−952. (42) Seino, Y.; Inomata, S.; Sasabe, H.; Pu, Y.-J.; Kido, J. High-Performance Green OLEDs Using Thermally Activated Delayed Fluorescence with a Power Efficiency of over 100 lm W−1. Adv. Mater. 2016, 28, 2638−2643. (43) Divayana, Y.; Sun, X.-W. An Efficient Bis(2-phenylquinoline)(acetylacetonate) Iridium(III)-Based Red Organic Light-Emitting Diode with Alternating Guest:Host Emitting Layers. Org. Electron. 2009, 10, 320−325. (44) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Theoretical Maximum Quantum Efficiency in Red Phosphorescent Organic Light-Emitting Diodes at a Low Doping Concentration Using a Spirobenzofluorene Type Triplet Host Material. Org. Electron 2010, 11, 881−886. (45) Xu, H.-X.; Sun, P.; Wang, K.-X.; Li, J.; Wang, F.; Miao, Y.-Q.; Wang, H.; Xu, B.-S.; Wong, W.-Y. Bipolar Hosts and Non-Doped Deep-Blue Emitters (CIEy = 0.04) Based on Phenylcarbazole and 2-(2-Phenyl-2H-1,2,4-Triazol-3-yl)Pyridine Groups. J. Mater. Chem. C, 2017, 5, 4455−4462. 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(46) Fukagawa, H.; Shimizu, T.; Hanashima, H.; Osada, Y.; Suzuki, M.; Fujikake, H. Highly Efficient and Stable Red Phosphorescent Organic Light-Emitting Diodes Using Platinum Complexes. Adv. Mater. 2012, 24, 5099−5103. (47) Peng, T.; Liu, H.-X.; Liu, Y.; Fan, Y.; Gao, H.-Z.; Wang, Y.; Hou, Z.-M. Very High-Efficiency Red-Electroluminescence Devices Based on an Amidinate-Ligated Phosphorescent Iridium Complex. J. Mater. Chem. 2009, 19, 8072−8074. (48) Zheng, C.-J.; Wang, J.; Ye, J.; Lo, M.-F.; Liu, X.-K.; Fung, M.-K.; Zhang, X.-H.; Lee, C.-S.

Novel Efficient Blue Fluorophors with Small Singlet-Triplet Splitting: Hosts for

Highly Efficient Fluorescence and Phosphorescence Hybrid WOLEDs with Simplified Structure. Adv. Mater. 2013, 25, 2205−2211. (49) Jou, J.-H.; Hsieh, C.-Y.; Tseng, J.-R.; Peng, S.-H.; Jou, Y.-C.; Hong, J.-H.; Shen, S.-M.; Tang, M.-C.; Chen, P.-C.; Lin, C.-H. Candle Light-Style Organic Light-Emitting Diodes. Adv. Funct. Mater. 2013, 23, 2750−2757. (50) Miao, Y.-Q.; Wang, K.-X.; Zhao, B.; Gao, L.; Tao, P.; Liu, X.-G.; Hao, Y.-Y.; Wang, H.; Xu, B.-S.; Zhu, F.-R.

High-Efficiency/CRI/Color Stability Warm White Organic

Light-Emitting Diodes by Incorporating Ultrathin Phosphorescence Layers in a Blue Fluorescence Layer. Nanophotonics, 2017, DOI: 10.1515/nanoph-2017-0021. (51) Sun, N.; Wang, Q.; Zhao, Y.-B.; Chen, Y.-H.; Yang, D.-Z.; Zhao, F.-C.; Chen, J.-S.; Ma, D.-G.

High-Performance Hybrid White Organic Light-Emitting Devices without

Interlayer between Fluorescent and Phosphorescent Emissive Regions. Adv. Mater. 2014, 26, 1617−1621. (52) Liu, B.-Q.; Tao, H.; Wang, L.; Gao, D.-Y.; Liu, W.-C.; Zou, J.-H.; Xu, M.; Ning, H.-L.; Peng, J.-B.; Cao, Y.

High-Performance Doping-Free Hybrid White Organic

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