High performance solution-processed green phosphorescent organic light-emitting diodes with high current efficiency and long-term stability

Article abstract of DOI:10.1039/c9tc03559a

In this study, we design and synthesize a new host and two new highly efficient green-emitting heteroleptic Ir(iii) complexes. These new materials are based on an amide-bridged, trifluromethyl-substituted, phenylpyridine skeleton with a longer alkyl chain as the main ligand, and on a phosphine oxide containing symmetrical dipyridinylphosphinate and asymmetrical phenyl(pyridin-2-yl)phosphinate as ancillary ligands. Their thermal, photophysical, electrochemical, and electroluminescent (EL) properties are fully investigated. The solution-processed green devices were fabricated using bis[5-ethylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](dipyridinylphosphinate)iridium(iii) as dopant, and (4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)di-o-tolylphosphine oxide (m-CBPPO1) and TPBi as hosts. The optimized devices containing a symmetrical-type ancillary ligand show excellent EL performance with a maximum current efficiency (CE_max) of 68.72 cd A^-1 and a maximum external quantum efficiency (EQE_max) of 20.82% without compromising the color purity. This is one of the best reported CE_max values with high EQE for solution-processed phosphorescent organic light-emitting diodes (PHOLEDs). To the best of our knowledge, this is the first report on green solution-processed PHOLEDs with EQE over 20% by using phosphine oxide functionalized symmetrical type ancillary ligand. Furthermore, these devices with symmetrical Ir(iii) complexes show better device stability than that of asymmetrical Ir(iii) complexes, which is attributed to the formation of undesirable isomers in asymmetrical complexes.

Full text of DOI:10.1039/c9tc03559a

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Page 1 of 38
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DOI: 10.1039/C9TC03559A
High performance solution-processed green phosphorescent organic light-emitting diodes
with high current efficiency and long-term stability
Athithan Maheshwaran,a Vijaya Gopalan Sree,a Ho-Yeol Park,a Woosum Cho,a Hyein Kim,a Raja
Kumaresan,a Youngkwang Kim,b Jae Chol Lee,b Myungkwan Song,c* and Sung-Ho Jin,a*
A. Maheshwaran, Dr. V. G. Sree, H. Kim, H.-Y. Park, Dr. W. Cho, H. Kim, R. Kumaresan, Prof.
S.-H. Jin
aDepartment of Chemistry Education, Graduate Department of Chemical Materials, Institute for
Plastic Information and Energy Materials, Pusan National University, Busandaehakro 63-2, Busan
46241, Republic of Korea
E-mail: shjin@pusan.ac.kr
Y. Kim, Dr. J. C. Lee
bLG Display E6 Block LG Science Park 30, Magokjungang 10-ro, Gangseo-gu, Seoul 07796,
Republic of Korea
Dr. M. Song
cAdvanced Functional Thin Films Department Surface Technology Division Korea Institute of
Materials Science (KIMS) 797 Changwondaero, Changwon, Gyeongnam 51508, Republic of
Korea E-mail: smk1017@kims.re.kr

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Abstract
In this study, we design and synthesize a new host and two new highly efficient green-emitting
hetroleptic Ir(III) complexes. These new materials are based on an amide-bridged, trifluromethyl-
substituted, phenylpyridine skeleton with a longer alkyl chain as the main ligand, and on a
phosphine oxide containing symmetrical dipyridinylphosphinate and asymmetrical
phenyl(pyridin-2-yl)phosphineate as ancillary ligands. Their thermal, photophysical,
electrochemical, and electroluminescent (EL) properties are fully investigated. The solution-
processed green devices were fabricated using bis[5-ethylhexyl-8-trifluoromethyl-5H-
benzo(c)(1,5)naphthyridin-6-one](dipyridinylphosphiate)iridium(III) as dopant, and (4'-(9H-carb
azol-9-yl)-[1,1'-biphenyl]-4-yl)di-o-tolylphosphine oxide (m-CBPPO1) and TPBi as hosts. The
optimized devices containing a symmetrical-type ancillary ligand show excellent EL performance
with a maximum current efficiency (CEmax) of 68.72 cd/A and a maximum external quantum
efficiency (EQEmax) of 20.82% without compromising the color purity. This is one of the best
reported CEmax values with high EQE for solution-processed phosphorescent organic light-
emitting diodes (PHOLEDs). To the best of our knowledge, this is the first report on green solution-
processed PHOLEDs with EQE over 20% by using phosphine oxide functionalized symmetrical
type ancillary ligand. Furthermore, these devices with symmetrical Ir(III) complexes show better
device stability than that of asymmetrical Ir(III) complexes, which is attributed to the formation of
undesirable isomers in asymmetrical complexes.

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Introduction
Phosphorescent organic light-emitting diodes (PHOLEDs) have become very popular and
are preferred for displays and energy saving solid-state lighting applications in the past decade.1-5
Although green, red and blue triplet emitters have reached commercialization, there is still much
room for improvement in efficiency, color purity and stability.6 In addition, a highly efficient green
triplet emitter for PHOLEDs is crucial to generate an efficient white light7-8. Using
electroluminescence (EL) efficient green triplet emitters via a wet process attained the maximum
benefit in case of commercial products and cost-effective large-area production.9-13 The well-
known and highly efficient commercial green triplet emitter, fac-tris(2-phenylpyridine)iridium
[Ir(ppy)3], is commonly used in AMOLED displays. However, the color purity of Ir(ppy)3 remains
low for commercial display applications.10,13,14 Furthermore, several research groups have been
trying to improve the color purity and stability of green emitting Ir(III) complexes by introducing
bulky groups on the 2-phenylpyrdine (ppy) ligands with ancillary ligands such as acetylacetonate,
picolinate, triazolates, tetrazolates, or quinolinolates for solution-processed PHOLEDs.15-17
Moreover, only a few studies and literature reports have examined phosphine oxide containing
ancillary ligands to improve the device performance of green triplet emitter in wet-processed
PHOLED.12 In addition to an efficient green emitter, an appropriate host matching the needs of
the emitter is also essential to realize high efficiency.18,19 Sufficient generation and efficient the
transfer of excitons from host to guest are necessary for achieving high efficiency. Thus, both
successful host and guest materials need to be developed.
Hence, in this communication, we report a novel approach to improve the efficiency and
color purity by introducing, firstly, symmetrical dipyridinylphosphinate (sym) and asymmetrical
phenyl(pyridin-2-yl)phosphinate (asym) as ancillary ligands, secondly, an amide-bridged,

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trifluromethyl (-CF3)-substituted, phenylpyridine skeleton with a longer alkyl chain as the main
ligand, and, finally, a guest with a new donor-pi-acceptor (D−π–A)- type (4'-(9H-carb azol-9-yl)-
[1,1'-biphenyl]-4-yl)di-o-tolylphosphine oxide (m-CBPPO1) as host by combining the hole
transporting carbazole unit, biphenyl and electron-transporting di-o-tolylphosphine oxide as D, π
and A, respectively. The bulky –CF3 group on the ppy main ligand is likely to reduce self-
quenching, which could enhance the device performance of solution-processed PHOLEDs.
Furthermore, the presence of the high polar phosphoryl (P=O) bond in the ancillary ligand may
improve the electron mobility and effectively reduce triplet–triplet annihilation (TTA) and triplet-
polaron quenching (TPQ) effects.12,19 In addition, the presence of a longer alkyl chain in the metal
complexes may reduce the intermolecular interaction and improve solubility towards common
organic solvents, thereby producing a uniform surface morphology. The solution-processed green
devices were fabricated using bis[5-ethylhexyl-8-trifluoromet hyl-5H-benzo(c)(1,5)naphthyridin-
6-one](dipyridinylphosphinate)iridium(III) (sym-Ir1) and bis[5-ethylhexyl-8-trifluoromethyl-5H-
benzo(c)(1,5)naphthyridin-6-one](phenyl(pyridin-2-yl)phosphinate)iridium(III) (asym-Ir2) as
dopants, and TCTA ((4,4’,4’’-tris(N-carbazolyl)triphe nylamine) and TPBi (1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene) as hosts. The optimized devices show excellent EL performance with
a maximum current efficiency (CEmax) of 60.08 cd/A, a maximum external quantum efficiency
(EQEmax) of 17.23%, and a notable CIE of (0.30, 0.62) when using sym-Ir1, compared to 47.86
cd/A, 13.62%, and (0.29, 0.62) with asym-Ir1, respectively. The phase alternation by line (PAL)
color coordinate system for standard green CIE (x, y) is (0.29, 0.60), which is highly pursued
despite this PAL standard being hard to achieve with high efficiencies.20 The obtained CIE(x, y)
color coordinates of sym-Ir1 and asym-Ir2 are very close to those suggested by the PAL standard
for a green emitter. The results suggest that both Ir(III) complexes show high color purity with

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high EQE, which is a prerequisite for high performance displays and lightning applications.
Furthermore, our synthesized new host, m-CBPPO1, exhibits a high triplet energy (ET) of 2.68 eV,
which renders it a potential host for green and red phosphorescent dopants. Thus, the new host
material m-CBPPO1 was incorporated into the emissive layer (EML) as host for two green
dopants, sym-Ir1 and asym-Ir2, and achieved superior EL performance, with more enhanced EQE
and CE, to that of TCTA host. We achieved a CEmax of 68.72 cd/A and an EQEmax of 20.82%,
without compromising the color purity CIE (0.30, 0.62) in solution-processed PHOLEDs for the
first time by using m-CBPPO1 as the new host material. The balanced charge transportation and
high ET that are favored for efficient energy transfer from host to dopant ensure high EQE and
high color purity. The integration of P=O groups successfully increased the electron mobility,
resulting in the balanced charge injection and transportation. Introduction of such new ligands
dramatically improves the photoluminance quantum yield and improved EL performance by
reducing the formation of additional isomers for sym-complex. Additionally, we achieved a
highest EQE and CE for green emitting solution-process PHOLEDs with high color purity. As of
now, there have been only few reports on the phosphine oxide functionalized green Ir(III)
complexes for solution-processed PHOLEDs and their EQE has been surpassed successfully with
our new design concept by suppressing the formation of undesired isomers. To the best of our
knowledge, this is the highest reported EQE with high color purity in solution-processed
PHOLEDs by using phosphine oxide containing symmetrical ancillary ligands (Table S1,
Supporting Information). In addition, we also examined the stability of the Ir(III) complexes and
found that sym-Ir1 showed better device stability. This difference in device stability might be due
to the formation of undesirable isomers of the asymmetrical Ir(III) complex, asym-Ir2. The above
results clearly reveal that the symmetrical ancillary ligand-based Ir(III) complex shows better EL

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performance than that of the asymmetrical ancillary ligand-containing Ir(III) complex. We
therefore conclude that the Ir(III) complexes with dipyridinylphosphinate ancillary ligands could
be useful in improving both the performance and color purity of solution-processed PHOLEDs.
Experimental
General information
1H NMR spectra were recorded on a Varian Mercury Plus 300 MHz spectrometer in CDCl3 using
tetramethylsilane as an internal reference. The chemical shifts were reported in ppm relative to the
singlet of CDCl3 at 7.26 ppm for the 1H NMR. UV-visible and the emission spectra were recorded
with a JASCO V-570 and Hitachi F-4500 fluorescence spectrophotometers at room temperature.
Transient PL measurements were carried out using compact fluorescence lifetime spectrometer
C11367 at room temperature. Thermal analyses were carried out on a Mettler Toledo TGA/SDTA
851e analyzer under N2 atmosphere at a heating rate of 10 °C min-1. Cyclic voltammetry (CV)
studies were carried out with a CHI 600C potentiostat (CH Instruments) at a scan rate of 100 mV
s-1 in anhydrous dichloromethane (CH2Cl2) solvent with 0.1 M TBAClO4 as supporting electrolyte.
A platinum wire was used as the counter electrode and an Ag/AgCl electrode was used as the
reference electrode. The potentials were referenced to the ferrocene/ferrocenium redox couple
(Fc/Fc+)
Synthesis of sodium dipyridinylphosphinate (sym). A solution of n-BuLi (6.40 mL, 16.00
mmol) in diethyl ether (Et2O, 15 mL) was cooled to -78 °C and 2-bromopyridine (2.52 g, 15.94
mmol) in Et2O (6 mL) at -78 °C was added quickly and the resulting dark red solution was stirred
at -78 oC for 4 h. A solution of phosphorus trichloride (0.74 g, 5.40 mmol) in Et2O (10 mL) was
added slowly over 1 h at -78 °C the solution stirred for 2 h at the same temperature, and the reaction
mixture was slowly warmed to RT. The tan-colored mixture was extracted with sulfuric acid

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(H2SO4) (20 mL, 2N) and the extract made alkaline with saturated sodium hydroxide (NaOH)
solution. The solid product was collected and recrystallized from acetone/n-hexane (1:1, v/v) to
get 0.59 g of pure product with 41.09% yield. Then the product (0.59 g, 2.23 mmol) was dissolved
in dimethylsulfoxide (DMSO) (4 mL)/H2O (4 mL) and H2O2 (30%, 0.3 g, 2.67 mmol) was added.
The mixture was stirred at RT for 1 h. The solid of sodium hydroxide (NaOH) (0.27 g, 6.69 mmol)
was added and the mixture was stirred at RT for 12 h. Then the solvent was evaporated off and the
residue was extracted with methanol (MeOH) to give the compound sym as a white solid (0.72 g).
(yield: 70%). 1H NMR (300 MHz, D2O, δ): 8.40 (d, J = 4.8 Hz, 2H), 7.91 (t, J = 6.7 Hz, 2H), 7.89
– 7.82 (m, 2H), 7.43 – 7.34 (m, 2H).
Synthesis of sodium phenyl(pyridin-2-yl)phosphinate (asym). A solution of n-BuLi (6.40 mL,
16.00 mmol) in (Et2O, 15 mL) was cooled to -78 oC and 2-bromopyridine (2.52 g, 15.94 mmol) in
Et2O (6 mL) at -78 °C was added quickly and the resulting dark red solution was stirred at -78 °C
for 4 h. A solution of dichlorophenylphosphine (1.44 g, 8.04 mmol) in Et2O (10 mL) was added
slowly and the solution was stirred at -78 °C for 2 h and the reaction mixture was slowly warmed
to RT. After reaction completion, the tan-colored mixture was extracted with H2SO4 (20 mL, 2N)
and the extract made alkaline with saturated NaOH solution. The solid product was collected and
recrystallized from acetone/n-hexane (1:1, v/v) to get 0.84 g of pure product with 39.59% yield.
Then the product (0.84 g, 3.17 mmol) was dissolved in DMSO (5 mL)/H2O (5 mL) and H2O2
(30%, 0.43 g, 3.80 mmol) was added. After the mixture was stirred at RT for 1 h, solid NaOH
(0.38 g, 9.52 mmol) was added, the mixture was stirred at RT for 12 h, the solvent was evaporated
off, and the residue was extracted with MeOH to give the compound asym as a white solid (0.72
g). (yield: 60%). 1H NMR (300 MHz, D2O, δ): 8.38 (d, J = 4.8 Hz, 1H), 7.87 – 7.71 (m, 2H), 7.66
– 7.52 (m, 2H), 7.43 – 7.26 (m, 4H).

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Synthesis of iridium(III) dimer complex. Ligand (1.5 g, 53.91 mmol) and IrCl3·3H2O (0.76 g,
134.77 mmol) were added to a mixture of 2-ethoxyethanol and water (40 mL, 3:1 v/v). The reaction
mixture was stirred at 140 °C for 20 h under nitrogen (N2) atmosphere and a yellow precipitate
was obtained and cooled to RT. The precipitate was filtered and washed with water (60 mL) and
MeOH (10 mL) and subsequently dried under vacuum to afford a yellow solid compound.
Synthesis of bis[5-ethylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](dipyr
idinylphosphinate)iridium(III) (sym-Ir1) complex. Dimer (1 g, 0.51 mmol) and sym (0.30 g,
1.27 mmol) were dissolved in 2-ethoxyethanol (30 mL). After being degassed, the reaction was
maintained at 140 °C for 24 h under N2 atmosphere, the solvent was removed and the crude
compound purified by column chromatography with MeOH/EtOAc (6:4, v/v) as eluent to afford
sym-Ir1 as a yellow solid. Further purification by high vacuum train sublimation afforded pure
product (yield: 50%). 1H NMR (300 MHz, CDCl3, δ): 9.51 (s, 1H), 8.66 (s, 1H), 8.32 (d, 1H), 8.16
(s, 1H), 8.04 (d, 1H), 7.77 (m, 6H), 7.68- 7.52 (m, 2H), 7.42 (d, 2H), 7.16 (s, 1H), 6.69 (d, 1H),
13
6.46 (d, 1H), 4.35 (s, 4H), 1.98 (s, 2H), 1.36 (d, 16H), 1.01-0.84 (m, 12H). C NMR (75 MHz,
CDCl3, δ): 161.5, 150.6, 149.1, 149.8, 148.1, 147.2, 145.7, 144.2, 144.8, 140.5, 137.3, 134.2,
132.8, 130.8, 129.6, 127.5, 127.1, 127.6, 126.4, 125.3, 125.8, 124.5, 124.1, 123.5, 123.3, 123.0,
119.8, 118.1, 46.6, 46.4, 46.3, 38.0, 37.9, 28.8, 28.7, 23.9, 23.0, 14.1, 14.1, 14.0, 13.9, 10.9, 10.8,
10.8, 10.7, 10.5. FAB+ MS (m/z): [M + H]+ calcd. for C52H52F6IrN6O4P, 1162.33; found, 1163.34.
Synthesis of bis[5-ethylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](phen
yl(pyridin-2-yl)phosphinate)iridium(III) (asym-Ir2) complex. Dimer (1.5 g, 0.76 mmol) and
asym (0.46 g, 1.91 mmol) were dissolved in 2-ethoxyethanol (30 mL). After being degassed, the
reaction was maintained at 140 °C for 24 h under N2 atmosphere, the solvent was removed and the
crude compound purified by column chromatography with EtOAc/MeOH (6:4, v/v) as eluent to

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afford asym-Ir2 as a yellow solid. (yield: 30%) 1H NMR (300 MHz, CDCl3, δ): 9.62 (d, 1H), 8.21
– 8.06 (m, 2H), 7.94 – 7.66 (m, 6H), 7.66 – 7.39 (m, 2H), 7.33 (t, 3H), 7.19 (dd, 3H), 6.74 – 6.42
(m, 2H), 4.34 (t, 4H), 1.98 (s, 2H), 1.42 (dd, 8H), 1.24 (s, 8H), 0.96 – 0.85 (m, 12H). 13C NMR
(75 MHz, CDCl3, δ): 161.6, 161.4, 150.2, 149.7, 149.6, 147.9, 147.3, 147.3, 146.7, 144.9, 141.7,
140.5, 137.7, 137.6, 134.9, 134.8, 134.5, 134.2, 132.2, 132.1, 131.7, 131.5, 131.3, 131.1, 130.9,
130.7, 130.1, 129.9, 128.3, 128.1, 127.9, 127.3, 126.3, 124.9, 123.8, 123.5, 123.2, 118.9, 118.2,
46.6, 38.1, 38.0, 37.9, 37.8, 29.6, 28.8, 28.7, 14.1, 14.0, 14.0, 13.9, 10.9, 10.8, 10.7, 10.7. FAB+
MS (m/z): [M + H]+ calcd. for C53H53F6IrN5O4P, 1161.34; found, 1162.34.
Synthesis of 9-(4-bromophenyl)-9H-carbazole (1). 9H-Carbazole (0.3 g, 1.79 mmol), K2CO3
(0.74 g, 5.38 mmol), Cu (0.34 g, 5.38 mmol), and 1-bromo-4-iodobenzene (0.55 g, 1.97 mmol)
were mixed in dimethylformamide (10 mL) under N2 atmosphere. The reaction mixture was heated
to 130 °C for 48 h. After reaction completion, the reaction mixture was cooled to RT, extracted
with methylene chloride (MC), washed with brine solution, and dried over anhydrous Na2SO4. The
solvent was distilled off and the solid residue was purified by column chromatography on silica
gel using EtOAc/n-hexane (1:9, v/v) as eluent to afford 1 as a white solid. (yield: 70%). 1H NMR
(300 MHz, CDCl3, δ): 7.20–7.37 (m, 6H), 7.44 (d, 2H), 7.69 (d, 2H), 8.07 (d, 2H).
Synthesis of 9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole(2). n-
BuLi (5.21 mL, 13.03 mmol) was added to the solution of 1 (2.8 g, 8.69 mmol) at -78 °C in dry
tetrahydrofuran. After the mixture was stirred for 2 h at the same temperature, it was slowly
warmed to RT, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.42 g, 13.03 mmol) was
added and the mixture was stirred at RT for 48 h. After reaction completion, the reaction mixture
was quenched by the addition of water (50 mL). Finally, the organic layer was extracted with MC,
washed with brine solution and dried over anhydrous Na2SO4. The solvent was distilled off, and

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the solid residue was purified by column chromatography on silica gel using EtOAc/n-hexane (1:9,
v/v) as eluent to afford 2 as a white solid. (yield: 50%). 1H NMR (300 MHz, CDCl3, δ): 8.19 (d,
2H), 8.09 (d, 2H), 7.67 (d, 2H), 7.48 (d, 2H), 7.45-7.38 (m, 2H), 7.29 (t, 2H), 1.42 (d, 12H).
Synthesis of (4-bromophenyl)di-o-tolylphosphine oxide (3). 1-Bromo-4-iodobenzene (1 g, 3.53
mmol), di-o-tolylphosphine oxide21 (0.81 g, 3.53 mmol), Pd(PPh3)4 (0.20 g, 0.17 mmol), and
triethylamine (0.35 g, 3.53 mmol) were dissolved in toluene (30 mL) under N2 for 10 min and the
reaction mixture was stirred at 110 °C for 12 h. After reaction completion, the mixture was cooled
to RT and water (50 mL) was added. The resulting mixture was extracted with MC (100 mL) and
the organic phase was dried over anhydrous Na2SO4. The solvent was evaporated off, and the solid
residue was purified by column chromatography on silica gel using EtOAc/n-hexane (5:5, v/v) to
afford 3 as a white color solid. (yield: 75%). 1H NMR (300 MHz, CDCl3, δ): 7.63-7.60 (m, 2H),
7.49-7.43 (m, 4H), 7.31 (m, 2H), 7.14-7.11 (t, 2H), 7.02-6.97 (m, 2H), 2.50 (s, 6H).
Synthesis of (4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)di-o-tolylphosphine oxide (m-CBPP
O1). (4-Bromophenyl)di-o-tolylphosphine oxide 3 (1.5 g, 3.89 mmol) and 9-(4-(4,4,5,5-tetrame
thyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazol 2 (1.5 g, 4.28 mmol) were mixed in a degassed
toluene, ethanol, and water (3:1:1) system and heated to 80 °C for 12 h. After reaction completion,
the organic layer was extracted with MC, washed with brine solution and dried over anhydrous
Na2SO4. The solvent was distilled off, and the solid residue was purified by column
chromatography on silica gel using EtOAc/n-hexane (4:6, v/v) as eluent to afford m-CBPPO1 as
a white solid. Further purification by high vacuum train sublimation afforded pure product (yield:
95%). 1H NMR (300 MHz, CDCl3, δ ): 8.14 (s, 2H), 7.76 (dd, 8H), 7.46 (s, 6H), 7.32 (s, 4H), 7.12
13
(d, 4H), 2.56 (s, 6H). C NMR (75 MHz, CDCl3, δ): 143.4, 143.4, 143.3, 141.6, 139.8, 138.7,
133.0,133.9, 133.8, 132.3, 132.1, 132.0, 132.9, 131.4, 131.0, 130.0, 129.6, 127.4, 127.2, 127.0,

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126.0, 125.4, 125.3, 123.4, 120.36, 120.1, 110.7, 22.9, 22.8.
Device fabrication and measurement
As the hole injection layer (HIL), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate
(PEDOT:PSS) was spin coated at 3000 RPM for 30 s onto the UV-ozone-treated (20 min) indium
tin oxide-coated glass substrate, followed by annealing in air for 15 min at 150 oC. The EML was
then spin coated onto the HILs and annealed at 110 oC for 20 min in N2 atmosphere. The electron-
transporting layer, LiF and Al were sequentially deposited over the EML layer in a vacuum at a
pressure of 5 x 10-6 Torr. All devices were measured under ambient condition at RT. The
concentration of the hosts and dopants in solvents were uniformly 1 wt%. The thickness of each
layer was measured by an alpha-step IQ surface profiler (KLA Tencor, San Jose, CA). Different
devices with different hosts and dopants were fabricated with different RPM with varying time.
So, the measured thickness of EML of different optimized devices were approximately 50, 45, 40
and 40 nm for devices A, B, C and D, respectively. The current density and voltage were controlled
with a measurement unit (Keithley, model 236) power source. The luminance, CE, EQE, CIE
coordinates (x, y), current density characteristics, and EL spectra were measured by a spectra scan
CS-1000 photometer (Minolta).
Results and discussion
Synthesis and characterization
Our group has recently reported phosphine oxide (Ph2P=O) containing symmetrical
ancillary ligands as one of the most promising candidates to achieve highly stable and efficient
Ir(III) complexes and PHOLEDs. Here, in order to develop highly efficient solution-processed
green PHOLEDs and investigate the effect of symmetrical ancillary ligands on the device

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performance, two Ir(III) complexes, sym-Ir1 and asym-Ir2, were designed and successfully
synthesized according to the route shown in Scheme 1. The main ligand was synthesized by our
previously reported synthetic procedure.12 Most research groups have tried to introduce
asymmetrical ancillary ligands containing electron-transporting groups (nitrogen-containing
heterocycles, P=O, oxadiazole, etc.) into the Ir(III) metal complexes to improve the PHOLED
device performance, whereas symmetrical ancillary ligands with an electron-transporting material
in PHOLEDs still remain unexplored.22,23
Based on the above considerations, we developed two novel green emitting Ir(III)
complexes with P=O containing sym and asym ancillary ligands. Furthermore, the new green host
material m-CBPPO1 was also designed and synthesized by Suzuki-coupling reaction, as shown in
Scheme 2, and the intermediates synthesis is shown in Scheme S1 of the supporting information.
In addition, m-CBPPO1 and sym-Ir1 were sublimed via high vacuum train sublimation to achieve
high purity. Fig. S1 (supporting information) shows the molecular structures of sym-Ir1, asym-Ir2
1
and m-CBPPO1 were confirmed by H, 13C NMR and mass spectroscopy. The detailed
photophysical, electrochemical and thermal properties of sym-Ir1, asym-Ir2 and m-CBPPO1 are
summarized in Table 1. Furthermore, Fig. S2 (supporting information) shows the purity of the
synthesized Ir(III) complexes was analyzed by high performance liquid chromatography. Sym-Ir1
shows higher purity level without any shoulder peak, whereas asym-Ir2 shows both a shoulder
peak and a main peak. We clearly attributed this to the formation of an undesirable isomer in asym-
Ir2 due to the asymmetrical structural arrangement of the ancillary ligand. Purifying the mixture
of an undesirable isomer in phosphorescent material synthesis remains a very challenging issue in
PHOLED fabrication. This challenge is further complicated by high vacuum train sublimation,
which is the most prevalent purification method for these materials and which suffers the major

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drawback of potential thermal isomerization occurring in situ during sublimation. 24,25 Moreover,
this purification technique often results in an undesirable isomeric mixture that degrades the
PHOLED device performance and lifetime. The results show that the asymmetrical structural
arrangement of ancillary ligands is not suitable for PHOLED device fabrication. Furthermore, in
order to control the undesirable isomer formation, ligands with a symmetrical structural
arrangement are highly preferred for PHOLED devices with improved device performance and
longer lifetime.
Photophysical and electrochemical properties
Fig. 1a and 1b shows the UV−vis absorption spectra and photoluminescence (PL) of sym-
Ir1 and asym-Ir2 in both dichloromethane (CH2Cl2) solution and neat film. The strong absorption
bands below 340ꢀnm in the ultraviolet region were assigned to ligand-centered (LC) π–π*
transitions in the Ir(III) complexes. The weak absorption bands in the low energy region from 400-
450 nm were assigned to an admixture of metal-to-ligand charge transfer 1MLCT and 3MLCT due
to the strong spin-orbit coupling. Both emitters show bright green PL emission in CH2Cl2 solution
at room temperature (RT) with a main emission peak of 509 nm and a weak shoulder peak at 540
nm. The low-temperature spectra of both Ir(III) complexes at 77K exhibit a well-resolved
vibronically structured line shape, which reveals the mixing between the 3MLCT and 3π–π* levels
3
and, further, that the PL emission of both Ir(III) complexes mainly arises from the LC π→π*
3
excited state, and not from MLCT or ligand-to-ligand charge transfer (3LLCT), which always
shows featureless emission spectra. In neat films, sym-Ir1 and asym-Ir2 show broad and
structureless emission and were red shifted compared to the solution, which clearly attributes the
broad emission bands in neat films to aggregation effects.26-28 Fig. 1c shows the UV-vis absorption,
fluorescence and phosphorescence spectra of m-CBPPO1 in 2-methyltetrahydrofuran (2-MeTHF)

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solution at RT and 77 K. The absorption peak at 292 nm and the absorption band from 318 nm to
337 nm were assigned to π-π* transition of the carbazole unit.29 The fluorescence spectra of m-
CBPPO1 exhibited a featureless emission peak maximum of 396 nm at RT. The ET was calculated
from the first emission peak of the low-temperature phosphorescence spectrum.
The ET measured for sym-Ir1, asym-Ir2 and m-CBPPO1 were 2.47, 2.48 and 2.68 eV, respectively.
Fig. 2 shows molecular orbital distributions obtained from the density functional theory (DFT)
calculation for sym-Ir1, asy-Ir2 and m-CBPPO1. The observed dihedral angle between the two
phenyl groups was 36.54o. No orbital overlap was observed for deep-blue host m-CBPPO, whereas
an orbital overlap over the phenyl was observed in m-CBPPO1.29 This is due to the presence of
methyl in the biphenyl, which induced the twisted conformation of the deep-blue host, while its
absence prompted the orbital overlap. The highest occupied molecular orbital (HOMO) and
HOMO-1 of sym-Ir1 and asym-Ir2 are mainly distributed over the Ir atom and main ligand,
whereas the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are mostly localized on
the main ligand and the ancillary ligand. The HOMO of m-CBPPO1 is prevailingly localized on
the phenylcarbazole unit, whereas HOMO-1 is mainly localized on the carbazole unit. LUMO is
mainly distributed over the phosphine oxide unit and the center of the biphenyl system, whereas
LUMO+1 is localized only on the phosphine oxide unit. Fig. S3 (supporting information) shows
the calculated electrostatic potential surfaces of sym-Ir1, asym-Ir2 and m-CBPPO1 molecules,
along with the visualization of the ET states of Ir(III) complexes. Both Ir(III) complexes show a
highly electronegative region in the molecules due to the presence of P=O. The PL quantum yields
(PLQYs) of sym-Ir1 and asym-Ir2 were measured in degassed CH2Cl2 at RT and the well-known
green emitter fac-Ir(ppy)3 was used as a reference. Sym-Ir1 showed higher quantum yield of 78%,
compared to 58% for asym-Ir2. The obtained PLQYs were much higher than that of the standard

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green emitter fac-Ir(ppy)3 (40%).1,30 Furthermore, the study results reveal that the introduction of
a symmetrical ancillary ligand into the Ir(III) metal complex not only improves the device
performance but also increases the PLQY.
The triplet lifetime (τ) was measured in deaerated CH2Cl2 (1 x 10-4 M), and is shown in Fig.
S4 (supporting information). The measured τ values were 0.36 and 0.37 μs for sym-Ir1 and asym-
Ir2, respectively. By assuming that the emitting state of the Ir(III) complex represents the unit
efficiency, the radiative (kr) and non-radiative (knr) rate constants can be calculated using the
relationships, kr = ΦPL/τ and ΦPL = kr/(kr + knr), as given in Table 2. The calculated kr and knr values
were 2.17 x 10-6 and 6.1 x 10-7 for sym-Ir1 complex, and 1.57 x 10-6 and 1.14 x 10-6 for asym-Ir2,
respectively. Sym-Ir1 exhibited higher kr and lower knr than asym-Ir2. The possible contributing
factors for sym-Ir1 are its high emission quantum yield and high kr for achieving high EQE.
The electrochemical behaviors of sym-Ir1, asym-Ir2 and m-CBPPO1 were investigated by
cyclic voltammetry (CV; Fig. S5, Supporting Information). The two Ir(III) complexes show similar
oxidation and reduction behaviors, indicating that they possess frontier molecular orbitals with
similar energy levels. The oxidation potential was calculated by the onset peak at (0.76 eV) with
respect to Fc/Fc+, which was attributed to the metal-centered IrIII/IrIV oxidation couple. Therefore,
as estimated from the optical Eg, HOMO and LUMO were -5.0 and -2.5 eV, respectively.
To further analyze the charge transporting properties of m-CBPPO1, we also calculated the
reorganization energies (Marcus theory) for m-CBPPO1, which is associated with the transport
properties of the charge carriers. From Fig. 3, the reorganization energies for m-CBPPO1 were
calculated as 0.255 eV for hole hopping and 0.418 eV for electron hopping. The lower
reorganization energy for hole hopping as compared to that of electron hopping suggests that it

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might have relatively capability for hole transporting than for electron hopping. The calculated
ratio of rate constants for m-CBPPO1 was much higher than unity, showing a possible charge
imbalance. The contributions from HOMO→LUMO transition to both the lowest singlet transition
(S0→S1) and the lowest triplet transition (S0→T1) were large (91–97.2%), and all the S0→T1
transitions for these complexes were assigned to an MLCT character mixed with some contribution
from LLCT and ππ* features (Table S2, Supporting Information). Figure S6 shows the absorption
coefficient spectra for sym-Ir1, asym-Ir2 and m-CBPPO1 and the corresponding values are
tabulated in Table S3 of the supporting information 30.
Thermal properties
In general, the thermal stability of triplet emitters is crucial for developing highly efficient
PHOLED devices. The thermal properties of sym-Ir1, asym-Ir2 and m-CBPPO1 were investigated
under nitrogen (N2) atmosphere using thermogravimetric analysis and differential scanning
calorimetry (DSC) (Fig. S7 and S8, Supporting Information). The decomposition temperature (Td)
of sym-Ir1 is much higher than that of asym-Ir2, and this difference might be due to the formation
of undesirable isomers in asym-Ir2; the relevant data are summarized in Table 1. The DSC analysis
of sym-Ir1 and asym-Ir2 shows high glass transition temperature (Tg) in the range of 110-180°C,
which supports good film morphological stability at RT. Moreover, sym-Ir1 complex clearly has
better thermal stability than that of asym-Ir2. In addition, the Td of m-CBPPO1 was 375 °C with a
5% loss of weight. The thermal stability of m-CBPPO1 is increased compared to that of our
previous reported bipolar host m-CBPPO, which was attributed to the ortho- substitution of two
methyl groups into the phosphine oxide moiety.29 Furthermore, the high Tg of the host material is
most useful for improving the operational stability of PHOLEDs. Notably, we observed the high
Tg value of m-CBPPO1 during DSC measurement and the no exothermic peak related to

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crystallization was observed at temperatures up to 250 °C. These observations indicate that the
enhanced thermal stability of the amorphous glass states of m-CBPPO1 may arise from more
crowded arrangement of two alkyl groups around the P=O atom, and prevents the easy
intermolecular packing and hence ready crystallization which could improve the carrier transport
property in EML. Furthermore, m-CBPPO1 shows Tg >100 °C, which was very high compared to
that of commonly used high ET hosts in PHOLEDs device.31,32
Carrier Transport and Surface Analysis
To study the charge carrier transport property of m-CBPPO1, we fabricated single carrier
devices and their current density-voltage (J-V) curve is shown in Fig. 4. From the previously
reported host m-CBPPO, we observed a bipolar transporting nature from m-CBPPO.29 But the
single carrier device results showed that the hole mobility was higher than the electron mobility in
m-CBPPO1, as was observed from the calculated hole and electron carrier mobilities. The hole
and electron mobilities of m-CBPPO1 were calculated to be 1.08 x 10-5 and 5.75 x 10-6 cm2/Vs,
respectively. From these carrier mobilities, we assume that the relatively reduced twist angle of
the biphenyl in m-CBPPO1 led to the imbalance in the carrier transport of the molecule due to
partial overlap of molecular orbitals (HOMO and LUMO), whereas the orbitals were completely
separated in bipolar m-CBPPO with 87.6° twist.
Atomic force microscopy (AFM) was used to measure the morphology of the EML films
and Fig. S8 (supporting information) shows the 3D images of mixed m-CBPPO1:TPBi and
TCTA:TPBi host films containing the symmetrical and asymmetrical Ir(III) complexes. The AFM
test results reveal that the sym-Ir1 complex-based films show lower surface roughness with both
mixed hosts. The root mean square (RMS) surface roughness values of the spin-coated films with
m-CBPPO1:TPBi and TCTA:TPBi as mixed hosts were 326 and 658 nm for sym-Ir1 and 393 and

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698 nm for asym-Ir2, respectively. In addition, films with m-CBPPO1 in the host matrix exhibited
a comparatively better surface morphology compared to that of films with TCTA as host.
Homogeneous and smooth morphology without any noticeable aggregation was observed for all
four films. This negligible aggregation may lower the TTA effect, and result in better roll-off.
Remarkable emission redshifts of these Ir(III) complexes were not observed, which was further
confirmed from the absence of aggregation.
Electroluminescent Properties of Solution-Processed PHOLEDs
Further, to understand the EL properties of the Ir(III) complexes, we fabricated devices A
(sym-Ir1) and B (asym-Ir2) with TCTA and TPBi as hosts to compare the performance of the two
Ir(III) complexes and the device energy level diagram is shown in Fig. 5. TCTA and TPBi were
chosen because of their different carrier transporting properties, which could help achieve a
balanced transport of carriers and in turn improve device lifetime and prevent charge accumulation
in EML.33 TCTA and TPBi were mixed in a 1:1 ratio doped with 12 wt% of guest Ir(III) complexes.
Fig. 6 shows the current density-voltage-luminance (J-V-L) characteristic of devices A and B and
their key parameters are summarized in Table 3. Devices A and B showed a turn-on voltage (Von)
of 5.3 and 5.7 V, respectively. From Fig. 6b, the CEmax and PEmax of device A were 55.91 cd/A
and 47.92 cd/A at 30 mA/cm2 and 18.56 lm/W, compared to 47.79 cd/A and 31.07 cd/A at 30
mA/cm2 and 16.68 lm/W for device B, respectively. Device A achieved a maximum brightness of
33145 cd/m2, while device B showed 51421 cd/m2. Devices A and B achieved an EQEmax of 17.75
and 13.63%, respectively.
In addition, to understand the PHOLED performance of the newly designed host, m-
CBPPO1, we fabricated devices with m-CBPPO1 in place of TCTA. Devices C (sym-Ir1) and D

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(asym-Ir2) were fabricated with m-CBPPO1 and TPBi (7:3) as a mixed host and their performance
as PHOLEDs was evaluated. Fig. 7 shows the J-V-L characteristic and the efficiency curves of
devices C and D versus luminance. Device C exhibited a higher efficiency than that of device D:
CEmax, PEmax and EQEmax of 68.72 cd/A, 29.44 lm/W and 20.82% for C, compared to 54.74 cd/A,
18.26 lm/W and 15.62% for D, respectively.
From Fig. 7c, the ELmax spectra of sym-Ir1 and asym-Ir2 complexes were measured at 515
and 514 nm. The EL spectra were similar to the observed PL spectra of the Ir(III) complexes in
neat film. CIE coordinates of all four PHOLEDs (A, B, C, and D) were recorded at 30 mA cm−2
and represent standard green color at (0.306, 0.627), which is very close to the suggested PAL
standard for a green emitter. The devices with sym-Ir1 complex (A and C) showed better
performance than asym-Ir2 complex devices (B and D), which further demonstrates the advantages
of the new host and dopant system. The higher efficiency of the devices with sym-Ir1 complex
was attributed to the distinctly higher PLQY. Further, the higher kr and lower knr calculated using
τ and the shorter lifetime of sym-Ir1 complex may account for the superior performance.
However, roll-off in PHOLEDs usually occurs due to the possible quenching process: TTA
and TPQ.34 To further analyze this issue, a TTA model was used to fit the J-EQE curves using the
equation
퐽0
1 + 8퐽퐽
η
=
― 1
(
)
η0
4퐽
0
where η and
refer to EQE with and without TTA, respectively, and is the critical current
퐽0
η0
density at η= /2. EQE curves fitted for the complexes were in agreement with the TTA model
η0
(Fig. S10, supporting information), indicating the TTA process is responsible for the efficient roll-

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off at high current densities. When defined as the current density at η= /2, 0 for devices with
η0
퐽
sym-Ir1 was 75.9 mA cm-2. We also studied the reproducibility of the results by fabricating devices
with the optimized structures (device C and D). The results show that the materials have notable
repeatability (Fig. S11 a and b). Finally, we also studied the long-term stability of the encapsulated
devices (C and D) measured at regular intervals. Fig. 8 shows the time versus EQE and luminance
of these devices and their stability was measured. The stability was measured at J value of 10 mA
cm-2 for both devices and, the EQE and brightness values are shown in Table S4 (supporting
information). The results reveal that sym-Ir1 complex has better stability and longer device
lifetime than sym-Ir2 complex. Device D exhibited a faster rate of efficiency decline and retained
~50% of initial EQE, compared to ~67% for device C. This difference in device stability might be
due to the formation of undesirable isomers in asym-Ir2 complex and may further explain the rapid
degradation observed in device D.
Conclusion
In this work, we have introduced a new host material, (4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-
yl)di-o-tolylphosphine oxide (m-CBPPO1), and two new green dopants, bis[5-ethylhexyl-8-
trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](dipyridinylphosphinate)iridium(III) (sym-
Ir1),
and
bis[5-ethylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](phenyl(
pyridin-2-yl)phosphineate)iridium(III) (asym-Ir2) for solution-processable green PHOLEDs. The
carbazole/di-o-tolylphosphine oxide hybrid compound of m-CBPPO1 showed a high ET of 2.68
eV. Therefore, solution-processed green PHOLEDs were fabricated and we achieved a CEmax of
68.72 cd/A and an EQEmax of 20.82% without compromising color purity CIE (0.30, 0.62) in
solution-processed PHOLEDs for the first time by using our new host material m-CBPPO1. This
is the highest reported EQE for solution-processed PHOLEDs using phosphine oxide

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functionalized symmetrical type ancillary ligand. Our results have demonstrated the potential for
this newly developed host and dopant system to be effectively applied for highly efficient solution-
processed PHOLEDs. From the device lifetime studies, devices with sym-Ir1 complex showed
comparatively better stability, which was attributed to the formation of undesirable isomers in
asym-Ir2. These results clearly reveal that the symmetrical ancillary ligand-based Ir(III) complex
shows better overall EL performance than the asymmetrical ancillary ligand-based Ir(III) complex.
Thus, we summarize that Ir(III) complexes with dipyridinylphosphinate ancillary ligands could be
useful in improving both the performance and high color purity of solution-processed PHOLEDs.
Acknowledgements
A. Maheshwaran and V. G. Sree contributed equally to this work. This work was supported by a
grant fund from the National Research Foundation (NRF) (2018R1A5A1025594) by the Ministry
of Science, ICT of Korea.
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N
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DOI: 10.1039/C9TC03559A
O PONa
N
N
N
O P O
N
N
Ir
2
O
CF3
sym-Ir1
Cl
Cl
N
N
2-ethoxyethanol
135 °C, 24 h
N
N
Ir
Ir
O
O
CF3
O PONa
CF3
2
2
N
Dimer
N
Ir
N
N
O P O
N
N
Ir
+
O P O
O
O
N
CF3
CF3
2
2
asym-Ir2
undesirable isomer
Scheme 1. Synthesis of Ir(III) complexes.
O
N
B
O
2
P
O
N
P
Br
K2CO3,Pd(PPh3)4
Toluene/H2O/EtOH
80° C, 12 h.
O
3
m-CBPPO1
Scheme 2. Synthesis of green-host m-CBPPO1.

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UV-solution
1.0
0.8
0.6
0.4
0.2
0.0
PL-solution
(a)
PL-neat film
2-MeTHF matrix at 77K
Sym
300
400
500
600
700
Wavelength (nm)
UV-solution
1.0
0.8
0.6
0.4
0.2
0.0
PL-solution
(b)
PL-neat film
2-MeTHF matrix at 77K
Asym
300
400
500
600
700
Wavelength (nm)
UV-Vis
Flu at RT
Flu at 77K
Phos at 77K
Film state
1.0
0.8
0.6
0.4
0.2
0.0
(c)
300
400
500
600
700
Wavelength (nm)
Fig. 1. UV-vis and PL spectra of (a) sym-Ir1, (b) asym-Ir2 and (c) m-CBPPO1 in solution and
in film state.

Page 27 of 38
Journal of Materials Chemistry C
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DOI: 10.1039/C9TC03559A
Fig. 2. Contours and contributions of the frontier molecular orbitals of the studied molecules.

Journal of Materials Chemistry C
Page 28 of 38
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DOI: 10.1039/C9TC03559A
Fig. 3. m-CBPPO1 energetics for the reorganization energies for hole and electron transfer
calculated by DFT method.

Page 29 of 38
Journal of Materials Chemistry C
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DOI: 10.1039/C9TC03559A
150
120
90
60
30
0
(a)
Hole
0.00
0.05
0.10
0.15
0.20
Electric filed (109V/m)
60
40
20
0
(b)
Electron
0.00
0.05
0.10
0.15
0.20
Electric filed (109V/m)