New bipolar host materials for high performance of phosphorescent green organic light-emitting diodes

Article abstract of DOI:10.1039/c5ra04593b

We report a series of four bipolar host materials with indenocarbozole and biphenyl pyrimidine moieties for efficient phosphorescent green organic light-emitting diodes (OLEDs). All four host materials possess excellent electro-optical properties as well as good thermal and morphological stability due to their high glass transition temperature of 129°C and decomposition temperature of 363-401°C. The new materials also show good triplet energy of 2.76 eV and excellent charge carrier transport properties. The fabricated OLED device with 11-[3-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-dimethyl-11,12-dihydro-11-aza-indeno[2,1-a]fluorene (mDPPICz1) as green host shows maximum current and power efficiency as high as 75.8 cd A^-1 and 87.1 lm W^-1 at 1000 cd m^-2, and an almost ideal external quantum efficiency of 21.7%.

Full text of DOI:10.1039/c5ra04593b

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New bipolar host materials for high performance of
phosphorescent green organic light-emitting
diodes†
Cite this: RSC Adv., 2015, 5, 31282
Gyeong Heon Kim,a Raju Lampande,a Ji Hoon Kong,a Jung Min Lee,a
a
Jang Hyuk Kwon, Jae Kyun Leeb and Jung Hwan Parkc
*
We report a series of four bipolar host materials with indenocarbozole and biphenyl pyrimidine moieties for
efficient phosphorescent green organic light-emitting diodes (OLEDs). All four host materials possess
excellent electro-optical properties as well as good thermal and morphological stability due to their high
glass transition temperature of 129 ꢀC and decomposition temperature of 363–401 ꢀC. The new
materials also show good triplet energy of 2.76 eV and excellent charge carrier transport properties. The
fabricated OLED device with 11-[3-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-dimethyl-11,12-dihydro-
11-aza-indeno[2,1-a]fluorene (mDPPICz1) as green host shows maximum current and power efficiency
as high as 75.8 cd Aꢁ1 and 87.1 lm Wꢁ1 at 1000 cd mꢁ2, and an almost ideal external quantum efficiency
of 21.7%.
Received 17th March 2015
Accepted 20th March 2015
DOI: 10.1039/c5ra04593b
www.rsc.org/advances
triplet annihilation but also decreases the driving voltage and
enhances the operational lifetime.
Introduction
Recently, several bipolar host materials for phosphorescent
OLEDs have been well reported for providing a charge balance
characteristics by incorporating hole and electron transporting
moieties in the host molecules.12 The reported good charge
transport moieties for phosphorescent green host materials are
arylamine,13–15 carbazole16–18 group as hole-transporting units,
and uorine,13,20,21 oxadiazole,14 phosphine oxide,15,19,22 benz-
imidazole,16 phenanthroimidazole,17 triazine,18 pyrimidine,23,24
sulfone25,26 as electron-transporting units. D. Ma et al. devel-
oped a bipolar host materials with triphenylamine and oxadi-
azole as hole and electron transporting moieties, which exhibits
a maximum EQE of 22.7% and good charge transport charac-
teristics for phosphorescent green device.14 Furthermore,
carbazole and benzimidazole moieties were used for improving
the bipolar characteristics of green host material and demon-
strated a maximum EQE, current efficiency, and power effi-
ciency of 20.1%, 70.4 cd Aꢁ1, and 63.2 lm Wꢁ1, respectively.16
Likewise, phenanthroimidazole and carbazole were utilized as
the electron and hole transporting units for bipolar host
To achieve theoretical 100% internal quantum efficiency in
organic light-emitting diodes (OLEDs), the external quantum
efficiency (EQE) should be up to 20–25%.1–5 Over the past several
years, numerous classes of heavy-metal complex based phos-
phorescent materials have been reported that efficiently utilize
triplet energy to enhance the EQE of OLED devices.6–10 In order
to produce efficient emission from phosphorescent materials,
an ideal host-guest system is essential, which can transfer
triplet energy from host to guest molecules. Generally, in many
host-guest systems charge trapping emission is caused by large
energy gaps between host and guest energy levels. In such
systems, exciton self-quenching and triplet–triplet annihilation
problems are unavoidable using high doping concentrations.
To reduce charge trapping, the energy gap of HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied
molecular orbital) between host and dopant molecules should
be less than 0.2–0.3 eV.11 In addition, charge balance is also one
of the crucial parameters to enhance the efficiency of OLED
devices. Better charge balance in the emissive layer not only
broadens the charge recombination zone and reduces triplet–
materials, which, displayed
a very high glass transition
temperature (Tg) of 113–243 ꢀC and decomposition temperature
(Td) of 395–417 ꢀC. They also showed a maximum EQE of
21.0%.17 Recently, we reported two new host materials based on
the indenocarbazole unit, and they exhibit an excellent OLED
device performances with good EQE value as compare to the
widely used 4,40-di(N-carbazolyl)biphenyl (CBP).27 Based on
these results we have developed series of host materials using
indenocarbazole moiety to further enhance the OLED perfor-
mances in terms of efficiency and lifetime. There are various
aDepartment of Information Display, Kyung Hee University, Dongdaemoon-gu, Seoul,
130-701, Republic of Korea. E-mail: jhkwon@khu.ac.kr; Fax: +82-2-961-9154; Tel:
+82-2-961-0948
bCenter for Neuro-Medicine, Korea Institute of Science & Technology, Hwarangro 4-gil,
Seongbuk-gu, Seoul 136-791, Republic of Korea
cDuksan Neolux Co., Ltd., 21-32, Ssukgol-gil, Ipjang-myeon, Seobuk-gu, Cheonan-si,
Chungcheongnam-do 331-821, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra04593b
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other good host materials are available for phosphorescent 12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (2) and
organic electroluminescent devices; however, for the develop- 11,11-dimethyl-5,11-dihydroindeno[1,2-b]carbazole (3), respec-
ment of OLEDs synthesis of new and efficient organic host tively. (yield: 12,12-dimethyl-11,12-dihydroindeno[2,1-a]carba-
materials are still needed.
zole 42.5 g, 11,11-dimethyl-5,11-dihydroindeno[1,2-b]carbazole
In this article, we demonstrate four new bipolar host materials, 37.5 g, 73%). 1H NMR (CDCl3, 300 MHz) d (ppm) (12,12-
11-[4-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-dimethyl-11,12- dimethyl-11,12-dihydroindeno[2,1-a]carbazole) 8.14–8.10 (m,
dihydro-11-aza-indeno[2,1-a]uorene (pDPPICz1), 6-[4-(2,6- 3H), 7.85–7.83 (m, 1H), 7.71–7.69 (d, J ¼ 8.0 Hz, 1H), 7.55–7.50
diphenyl-pyrimidin-4-yl)-phenyl]-12,12-dimethyl-6,12-dihydro- (m, 2H), 7.48–7.29 (m, 4H), 1.73 (s, 6H) (11,11-dimethyl-5,11-
6-aza-indeno[1,2-b]uorene (pDPPICz2), 11-[3-(2,6-diphenyl- dihydroindeno[1,2-b]carbazole) 8.11–8.06 (m, 2H), 7.75–7.70
pyrimidin-4-yl)-phenyl]-12,12-dimethyl-11,12-dihydro-11-aza- (m, 1H), 7.67 (s, 1H), 7.49–7.44 (m, 1H), 7.40–7.32 (m, 5H), 7.26–
indeno[2,1-a]uorene (mDPPICz1) and 6-[3-(2,6-diphenyl-pyr- 7.22 (m, 1H), 1.60 (s, 6H). 13C NMR (CDCl3, 500 MHz) d (ppm)
imidin-4-yl)-phenyl]-12,12-dimethyl-6,12-dihydro-6-aza-indeno 153.3, 140.3, 140.1, 137.4, 135.3, 134.2, 127.2, 126.8, 125.7,
[1,2-b]uorene (mDPPICz2) by incorporating new inden- 123.9, 123.5, 122.2, 120.3, 120.0, 119.8, 117.4, 112.3, 110.7, 46.7,
ocarbazole as the hole-transporting and biphenyl pyrimidine as 25.7.
the electron-transporting moiety for green phosphorescent
OLEDs. Herein, the indenocarbazole and biphenyl moieties are Synthesis of 11-(4-bromophenyl)-12,12-dimethyl-11,12-
expected to provide high hole and electron transporting prop- dihydroindeno[2,1-a]carbazole (4)
erty and better charge carrier stability due to longer conjugation
length. We investigated the thermal, photo-physical and elec-
A mixture of 12,12-dimethyl-11,12-dihydroindeno[2,1-a]carba-
zole (2, 30 g, 105.87 mmol), 1-bromo-4-iodobenzene (89.85 g,
trochemical properties of newly synthesized compounds. As
317.61 mmol), copper powder (7.40 g, 116.46 mmol), potassium
expected the fabricated OLED devices with new host materials
carbonate (14.63 g, 105.87 mmol) and 18-crown-6 (2.80 g, 10.59
and Ir(ppy)3 as triplet dopant show a reasonably good perfor-
mmol) was reuxed for 24 hours in 2 L round bottom ask at
nitrogen atmosphere. Aer nal reaction, the residue was
mance with high EQE values and longer operational stability.
divided by column chromatography (eluent ¼ CH2Cl2/hexane,
1 : 3). Finally, the residue was recrystallized using CH2Cl2/
Experimental section
Synthesis of 9,9-dimethyl-2-(2-nitrophenyl)-9H-uorene (1)
hexane. (yield: 31.5 g, 68%). 1H NMR (CDCl3, 300 MHz) d (ppm)
8.19–8.17 (d, J ¼ 8.1 Hz, 1H), 8.13–8.10 (d, J ¼ 7.2 Hz, 1H), 7.96–
2-(9,9-Dimethyl-9H-uorene-2-yl)-4,4,5,5-tetramethyl-1,3,2-di-
7.93 (d, J ¼ 8.4 Hz, 1H), 7.80–7.73 (m, 3H), 7.43–7.40 (d, J ¼ 8.4
oxaborolane (150 g, 468.41 mmol), 1-bromo-2-nitrobenzene
(89.89 g, 444.99 mmol), tetrakis(triphenylphosphine) palla-
Hz, 1H), 7.37–7.25 (m, 6H), 6.81–6.78 (d, J ¼ 7.2 Hz, 1H), 1.19 (s,
6H). 13C NMR (CDCl3, 500 MHz) d (ppm) 154.4, 144.44, 144.42,
dium(0) (27.07 g, 23.42 mmol) and potassium carbonate (2.2 M,
640 mL, 1405.24 mmol) with tetrahydrofuran (2200 mL) and
water (500 mL) solvents were reuxed for 4 hours in 5 L round
bottom ask. The solution was extracted with dichloromethane,
and dried over anhydrous MgSO4. Aer removal of solvent
under reduced pressure, nal residue was puried by column
chromatography (eluent ¼ CH2Cl2/hexane, 1 : 3) to get the pale
yellow solid of 9,9-dimethyl-2-(2-nitrophenyl)-9H-uorene
(yield: 147.7 g, 84%). 1H NMR (CDCl3, 300 MHz) d (ppm) 7.88–
7.85 (m, 1H), 7.79–7.74 (m, 2H), 7.66–7.61 (m, 1H), 7.55–7.42
(m, 3H), 7.39–7.30 (m, 4H), 7.37–7.25 (m, 6H), 1.51 (s, 6H).
139.5, 139.3, 138.9, 133.2, 133.0, 132.9, 127.0, 126.9, 125.9,
125.2, 123.8, 122.0, 120.4, 119.7, 119.5, 119.4, 113.2, 110.7,
110.6, 48.0, 26.5.
Synthesis of 5-(4-bromophenyl)-11,11-dimethyl-5,11-
dihydroindeno[1,2-b]carbazole (5)
A mixture of 11,11-dimethyl-5,11-dihydroindeno[1,2-b]carba-
zole (3, 3 g, 17.64 mmol), 1-bromo-4-iodobenzene (15 g, 52.92
mmol), copper powder (1.23 g, 19.40 mmol), potassium
carbonate (2.44 g, 17.64 mmol) and 18-crown-6 (0.46 g, 1.76
mmol) was reuxed for 24 hours in 500 mL round bottom ask
at nitrogen atmosphere. Aer reaction, the residue was divided
by column chromatography (eluent ¼ CH2Cl2/hexane, 1 : 3) and
recrystallized using CH2Cl2/hexane to get white solid. (yields:
Synthesis of 12,12-dimethyl-11,12-dihydroindeno[2,1-a]-
carbazole and 11,11-dimethyl-5,11-dihydroindeno[1,2-b]-
carbazole (2 and 3)
1
3.0 g, 65%). H NMR (CDCl3, 300 MHz) d (ppm) 8.17–8.14 (m,
2H), 8.13–8.10 (d, J ¼ 8.6 Hz, 1H), 7.79–7.76 (d, J ¼ 8.8 Hz, 1H),
7.74–7.71 (m, 1H), 7.66–7.65 (m, 1H), 7.53–7.46 (m, 2H), 7.40–
7.25 (d, 6H), 1.62 (s, 6H). 13C NMR (CDCl3, 500 MHz) d (ppm)
154.4, 146.7, 141.2, 140.7, 140.6, 139.2, 138.3, 137.0, 127.3,
127.0, 125.8, 123.7, 123.4, 123.3, 122.7, 120.2, 120.1, 119.9,
114.1, 109.5, 100.8, 100.7, 46.3, 28.0.
A mixture of 9,9-dimethyl-2-(2-nitrophenyl)-9H-uorene (56 g,
459.78 mmol) and triphenylphosphine (301.49 g, 1149.44
mmol) was dried in vacuum and lled with nitrogen gas in 5 L
round bottom ask. Later, 1,2-dichlorobenzene (2200 mL) was
added to dissolve the mixture and then reuxed for 12 hours in
nitrogen atmosphere. Aer removal of solvent using distilla-
tion, the residue was divided by column chromatography
(eluent ¼ CH2Cl2/hexane, 1 : 7) to 12,12-dimethyl-11,12-dihy-
Synthesis of 11-(3-bromophenyl)-12,12-dimethyl-11,12-
dihydroindeno[2,1-a]carbazole (6)
droindeno[2,1-a]carbazole
and
11,11-dimethyl-5,11-dihy-
droindeno[1,2-b]carbazole, respectively. Finally, two residues A mixture of 12,12-dimethyl-11,12-dihydroindeno[2,1-a]carba-
were recrystallized with CH2Cl2/hexane to get white solid of zole (2, 12 g, 42.35 mmol), 1,3-dibromobenzene (36.32 g, 127.04
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mmol), copper powder (2.96 g, 46.58 mmol), potassium
carbonate (5.85 g, 42.35 mmol) and 18-crown-6 (1.12 g, 4.23
mmol) was reuxed for 24 hours in 500 mL round bottom ask
at nitrogen atmosphere. Aer reaction, the solution was puri-
ed by column chromatography (eluent ¼ CH2Cl2/hexane, 1 : 4)
and recrystallized using CH2Cl2/hexane to get white solid.
(yields: 11.13 g, 60%). 1H NMR (CDCl3, 300 MHz) d (ppm) 8.20–
8.17 (d, J ¼ 7.9 Hz, 1H), 8.14–8.11 (m, 1H), 7.81–7.74 (m, 3H),
7.69–7.68 (m, 1H), 7.54–7.46 (m, 2H), 7.38–7.25 (m, 5H), 6.86–
6.83 (m, 1H), 1.20 (s, 4H), 1.17 (s, 2H). 13C NMR (CDCl3, 500
MHz) d (ppm) 154.3, 144.3, 142.7, 139.5, 139.2, 135.7, 134.4,
132.4, 130.6, 129.8, 126.9, 126.8, 125.8, 125.1, 123.8, 122.7,
121.9, 120.4, 119.6, 119.4, 113.3, 110.6, 47.9, 26.3.
Synthesis of 11,11-dimethyl-5-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-5,11-dihydroindeno[1,2-b]carbazole (9)
mixture of 5-(4-bromophenyl)-11,11-dimethyl-5,11-dihy-
A
droindeno[1,2-b]carbazole (5, 2.5 g, 5.70 mmol), 4,4,40,40,5,5,50,50-
octamethyl-2,20-bi(1,3,2-dioxaborolane) (1.59 g, 6.27 mmol),
PdCl2(dppf) (0.14 g, 0.17 mmol) and potassium acetate (1.678 g,
17.10 mmol) was dried and lled with nitrogen in 250 mL round
bottom ask. DMF (30 mL) was added in the mixture and reuxed
for 2 hours in nitrogen atmosphere. The nal reacted mixture was
extracted using ethyl acetate and dried over anhydrous MgSO4. The
dried solution was ltered. And the solution was puried by
column chromatography (eluent ¼ ethyl acetate/hexane, 1 : 4) and
recrystallized using CH2Cl2/hexane to get white solid. (yields: 1.7 g,
62%). 1H NMR (CDCl3, 300 MHz) d (ppm) 8.18–8.10 (m, 4H), 7.74–
7.65 (m, 4H), 7.49–7.46 (m, 1H), 7.43–7.41 (m, 1H), 7.40–7.36 (m,
1H), 7.35–7.27 (m, 3H), 1.62 (s, 6H), 1.43 (s, 12H). 13C NMR (CDCl3,
500 MHz) d (ppm) 154.4, 146.5, 141.2, 140.7, 140.6, 139.4, 138.2,
136.5, 127.2, 126.9, 126.3, 125.7, 123.7, 123.4, 122.7, 120.0, 119.9,
119.8, 113.9, 109.8, 101.0, 84.1, 46.3, 28.0, 24.9.
Synthesis of 5-(3-bromophenyl)-11,11-dimethyl-5,11-
dihydroindeno[1,2-b]carbazole (7)
A mixture of 11,11-dimethyl-5,11-dihydroindeno[1,2-b]carba-
zole (3, 8.50 g, 30.00 mmol), 1,3-dibromobenzene (25.73 g, 89.99
mmol), copper powder (2.10 g, 33.00 mmol), potassium
carbonate (4.15 g, 30.00 mmol) and 18-crown-6 (0.79 g, 3.00
mmol) was reuxed for 24 hours in 250 mL round bottom ask
at nitrogen atmosphere. Aer reaction, the nal mixture was
divided by column chromatography (eluent ¼ CH2Cl2/hexane,
1 : 4) and recrystallized using CH2Cl2/hexane to get white solid.
Synthesis of 12,12-dimethyl-11-(3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-11,12-dihydroindeno[2,1-a]
carbazole (10)
A mixture of 11-(3-bromophenyl)-12,12-dimethyl-11,12-dihy-
1
(yields: 8.80 g, 67%) H NMR (CDCl3, 300 MHz) d (ppm) 8.19–
droindeno[2,1-a]carbazole
(6,
9.8
g,
22.36
mmol),
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (6.25 g,
24.60 mmol), PdCl2(dppf) (0.55 g, 0.67 mmol) and potassium
acetate (6.58 g, 67.08 mmol) was dried and lled with nitrogen
in 250 mL round bottom ask. DMF (100 mL) was added in the
mixture and reuxed for 3 hours in nitrogen atmosphere. Aer
reaction, the mixture was extracted using ethyl acetate and
dried over MgSO4. The dried solution was ltered. And the
solution was puried by column chromatography (eluent ¼
ethyl acetate/hexane, 1 : 4) and recrystallized using CH2Cl2/
8.16 (m, 2H), 7.83–7.82 (m, 1H), 7.78–7.75 (m, 1H), 7.71–7.70
(m, 1H), 7.68–7.65 (m, 1H), 7.62–7.58 (m, 1H), 7.56–7.53 (d, J ¼
7.7 Hz, 1H), 7.51–7.48 (m, 1H), 7.43–7.41 (m, 2H), 7.36–7.30 (m,
3H), 1.64 (s, 6H). 13C NMR (CDCl3, 500 MHz) d (ppm) 154.3,
146.7, 141.2, 140.6, 139.3, 138.3, 131.2, 130.6, 130.3, 127.3,
126.9, 125.9, 125.8, 125.7, 123.6, 123.3, 123.2, 122.6, 120.6,
120.0, 119.9, 114.0, 109.5, 100.7, 46.3, 27.9.
Synthesis of 12,12-dimethyl-11-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-11,12-dihydroindeno[2,1-a]
carbazole (8)
1
hexane to get white solid. (yields: 6.4 g, 59%) H NMR (CDCl3,
300 MHz) d (ppm) 8.20–8.17 (d, J ¼ 7.9 Hz, 1H), 8.12–8.09 (m,
1H), 8.03–8.00 (m, 1H), 7.89–7.89 (m, 1H), 7.79–7.75 (m, 2H),
7.71–7.67 (m, 1H), 7.65–7.60 (m, 1H), 7.35–7.29 (m, 2H), 7.28–
7.26 (m, 2H), 7.25–7.22 (m, 1H), 6.83–6.80 (m, 1H), 1.32 (s, 7H),
1.30 (s, 5H), 1.15 (s, 3H), 1.07 (s, 3H). 13C NMR (CDCl3, 500 MHz)
d (ppm) 154.6, 144.7, 140.8, 139.9, 139.4, 139.3, 137.6, 135.9,
135.4, 133.7, 128.8, 126.8, 126.7, 125.6, 125.1, 123.7, 121.9,
120.0, 119.5, 119.3, 119.3, 112.8, 110.9, 84.1, 48.0, 26.3, 26.0,
25.0, 24.6.
A
mixture of 11-(4-bromophenyl)-12,12-dimethyl-11,12-dihy-
droindeno[2,1-a]carbazole (4, 20.0 g, 45.62 mmol),
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (12.74 g,
50.19 mmol), PdCl2(dppf) (1.12 g, 1.37 mmol) and potassium
acetate (13.43 g, 136.87 mmol) was dried and lled with nitrogen
gas in 1 L round bottom ask. DMF (230 mL) was added to dissolve
the mixture and then reuxed for 3 hours in nitrogen atmosphere.
Aer reaction, the solution was extracted with ethyl acetate and
dried over anhydrous MgSO4. The dried solution was ltered. And
nally the solution was puried with column chromatography
(eluent ¼ ethyl acetate/hexane, 1 : 3) and recrystallized using
Synthesis of 11,11-dimethyl-5-(3-(4,4,5,5-tetramethyl-1,3,2
dioxaborolan-2-yl)phenyl)-5,11-dihydroindeno[1,2-b]carbazole
(11)
1
CH2Cl2/hexane to get white solid. (yield: 13.7 g, 62%). H NMR
(CDCl3, 300 MHz) d (ppm) 8.21–8.19 (d, J ¼ 7.9 Hz, 1H), 8.14–8.11
A
mixture of 5-(3-bromophenyl)-11,11-dimethyl-5,11-dihy-
(7, 8.0 g, 18.25 mmol),
(m, 1H), 8.08–8.05 (d, J ¼ 8.3 Hz, 2H), 7.81–7.76 (m, 2H), 7.57–7.54 droindeno[1,2-b]carbazole
(d, J ¼ 8.2 Hz, 1H), 7.37–7.25 (m, 5H), 6.80–6.78 (m, 1H), 1.45 (s, 4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (5.10 g,
12H), 1.18 (s, 6H). 13C NMR (CDCl3, 500 MHz) d (ppm) 154.6, 144.6, 20.07 mmol), Pd(dppf)Cl2 (0.45 g, 0.55 mmol) and potassium
144.0, 139.6, 139.3, 135.94, 135.91, 130.6, 126.8, 126.7, 125.7, 125.1, acetate (5.37 g, 54.75 mmol) was dried by vacuum and lled with
123.7, 121.9, 120.1, 119.5, 119.3, 119.3, 112.9, 110.8, 84.2, 48.0, nitrogen in 250 mL round bottom ask. DMF (90 mL) was added
26.3, 24.9.
to dissolve the mixture and reuxed for 3 hours in nitrogen
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atmosphere. The reacted solution was extracted by ethyl acetate 131.0, 129.2, 128.7, 127.5, 127.5, 127.2, 126.0, 124.0, 123.7,
and dried over using MgSO4. The solution was ltered. The 122.9, 120.5, 120.3, 120.2, 114.3, 110.5, 110.0, 101.2, 46.5, 28.2.
ltered solution was puried by column chromatography Found: C, 87.5; H, 5.2; N, 7.1. Calc. for C43H31N3: C, 87.6; H, 5.3;
(eluent ¼ ethyl acetate/hexane, 1 : 4) and recrystallized using N, 7.1%.
1
CH2Cl2/hexane to get white solid (yields: 5.4 g, 61%). H NMR
(CDCl3, 300 MHz) d (ppm) 8.18–8.15 (m, 2H), 8.05–8.04 (m, 1H),
7.98–7.97 (m, 1H), 7.96–7.95 (m, 1H), 7.72–7.69 (m, 3H), 7.67–
7.64 (m, 1H), 7.49–7.46 (m, 1H), 7.40–7.34 (m, 2H), 7.32–7.26
(m, 3H), 1.63 (s, 6H), 1.37 (s, 12H). 13C NMR (CDCl3, 500 MHz) d
(ppm) 154.4, 146.2, 141.8, 141.2, 139.5, 138.0, 137.4, 133.9,
133.8, 130.3, 129.4, 127.1, 126.9, 125.5, 123.4, 123.2, 122.6,
119.9, 119.9, 119.7, 113.9, 109.7, 101.0, 84.1, 46.2, 28.1, 24.9.
Synthesis of 11-[3-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-
dimethyl-11,12-dihydro-11-aza-indeno[2,1-a]uorine
(mDPPICz1)
A
mixture of 12,12-dimethyl-11-(3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-11,12-dihydroindeno[2,1-a]carbazole
(10, 5.5 g, 11.33 mmol), 4-chloro-2,6-diphenylpyrimidine (3.02
g, 11.33 mmol), tetrakis(triphenylphosphine)palladium(0) (0.39
g, 0.34 mmol) and potassium carbonate (1.5 M, 22 mL, 33.99
mmol) was added in THF (60 mL) solvent in 250 mL round
bottom ask, and then reuxed for 12 hours. Aer reaction, the
Synthesis of 11-[4-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-
dimethyl-11,12-dihydro-11-aza-indeno[2,1-a]uorine
(pDPPICz1)
A
mixture of 12,12-dimethyl-11-(4-(4,4,5,5-tetramethyl-1,3,2- solution was worked up and dried over using MgSO4. Finally,
dioxaborolan-2-yl)phenyl)-11,12-dihydroindeno[2,1-a]carbazole
the ltered residue was puried by column chromatography
(5.0 g, 10.30 mmol), 4-chloro-2,6-diphenylpyrimidine (8, 2.75 g, (eluent ¼ CH2Cl2/hexane, 1 : 4) and recrystallized using CH2Cl2/
10.30 mmol), tetrakis(triphenylphosphine)palladium(0) (0.36 g, hexane (yields: 4.0 g, 60%). 1H NMR (CDCl3, 300 MHz) d (ppm)
0.31 mmol) and potassium carbonate (1.5 M, 20 mL, 30.90 8.71–8.68 (m, 2H), 8.62–8.59 (m, 1H), 8.49 (s, 1H), 8.27–8.24 (m,
mmol) was dissolved in THF (60 mL) solvent in 250 mL round 3H), 8.19–8.16 (m, 1H), 8.03 (s, 1H), 7.84–7.81 (m, 3H), 7.75–7.73
bottom ask, and then reuxed for 12 hours. Aer reaction, the (m, 1H), 7.53–7.51 (m, 6H), 7.35–7.26 (m, 5H), 6.91–6.89 (m,
solution was worked up and dried over using MgSO4. Finally, 1H), 1.28 (s, 3H), 1.21 (s, 3H); 13C NMR (CDCl3, 500 MHz) d
the ltered residue was puried by column chromatography (ppm) 165.4, 164.9, 163.5, 154.7, 144.6, 142.1, 139.8, 139.7,
(eluent ¼ CH2Cl2/hexane, 1 : 4) and recrystallized using CH2Cl2/ 139.5, 139.4, 138.0, 137.4, 136.0, 133.6, 131.1, 131.0, 130.4,
hexane to get white solid (yields: 3.52 g, 58%). 1H NMR (CDCl3, 130.3, 129.1, 128.7, 128.3, 127.5, 127.1, 127.0, 126.1, 125.3,
300 MHz) d (ppm) 8.84–8.80 (m, 2H), 8.61–8.59 (m, 2H), 8.39– 124.0, 122.2, 120.5, 119.9, 119.6, 113.3, 111.0, 110.4, 48.3, 26.6,
8.36 (m, 2H), 8.24–8.22 (m, 1H), 8.18 (s, 1H), 8.16–8.09 (m, 1H), 26.6. Found: C, 87.5; H, 5.2; N, 7.3. Calc. for C43H31N3: C, 87.6;
7.83–7.73 (m, 4H), 7.64–7.57 (m, 6H), 7.40–7.26 (m, 5H), 6.91– H, 5.3; N, 7.1%.
6.81 (m, 1H), 1.27 (s, 6H); 13C NMR (CDCl3, 500 MHz) d (ppm)
165.4, 165.0, 163.6, 154.7, 144.7, 143.9, 139.8, 139.7, 139.5,
138.6, 138.2, 137.6, 136.1, 132.0, 131.2, 131.0, 129.2, 128.7,
128.6, 127.0, 126.1, 125.4, 124.1, 122.2, 120.5, 119.8, 119.6,
119.6, 113.4, 111.0, 110.6, 48.3, 26.7. Found: C, 87.8; H, 5.3; N,
7.2. Calc. for C43H31N3: C, 87.6; H, 5.3; N, 7.1%.
Synthesis of 6-[3-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-
dimethyl-6,12-dihydro-6-aza-indeno[1,2-b]uorene
(mDPPICz2)
A
mixture of 11,11-dimethyl-5-(3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-5,11-dihydroindeno[1,2-b]carbazole
(11, 4.0 g, 8.24 mmol), 4-chloro-2,6-diphenylpyrimidine (2.20 g,
8.24 mmol), tetrakis(triphenylphosphine)palladium(0) (0.29 g,
0.25 mmol) and potassium carbonate (1.3 M, 19 mL, 24.72
Synthesis of 6-[4-(2,6-diphenyl-pyrimidin-4-yl)-phenyl]-12,12-
dimethyl-6,12-dihydro-6-aza-indeno[1,2-b]uorine
(pDPPICz2)
A
mixture of 11,11-dimethyl-5-(4-(4,4,5,5-tetramethyl-1,3,2- mmol) was added in THF (60 mL) solvent in 250 mL round
dioxaborolan-2-yl)phenyl)-5,11-dihydroindeno[1,2-b]carbazole bottom ask, and then reuxed for 8 hours. Aer reaction, the
(9, 5.0 g, 10.30 mmol), 4-chloro-2,6-diphenylpyrimidine (2.75 g, solution was worked up and dried over using MgSO4. Finally,
10.30 mmol), tetrakis(triphenylphosphine)palladium(0) (0.36 g, the ltered residue was puried by column chromatography
0.31 mmol) and potassium carbonate (1.5 M, 20 mL, 30.90 (eluent ¼ CH2Cl2/hexane, 1 : 4) and recrystallized using CH2Cl2/
mmol) was added in THF (60 mL) solvent in 250 mL round hexane (yields: 3.1 g, 64%). 1H NMR (CDCl3, 300 MHz) d (ppm)
bottom ask, and then reuxed for 6 hours. Aer reaction, the 8.72–8.70 (m, 2H), 8.54–8.54 (m, 1H), 8.39–8.37 (m, 1H), 8.27–
solution was worked up and dried over using MgSO4. Finally, 8.26 (m, 2H), 8.19–8.17 (m, 2H), 8.02 (s, 1H), 7.83–7.76 (m, 3H),
the ltered residue was puried by column chromatography 7.72–7.70 (m, 1H), 7.52–7.44 (m, 8H), 7.41–7.38 (m, 1H), 7.32–
(eluent ¼ CH2Cl2/hexane, 1 : 4) and recrystallized using CH2Cl2/ 7.25 (m, 3H), 1.62 (s, 6H); 13C NMR (CDCl3, 500 MHz) d (ppm)
hexane (yields: 3.76 g, 65%) 1H NMR (CDCl3, 300 MHz) d (ppm) 165.3, 164.9, 164.1, 154.6, 146.9, 141.8, 141.2, 140.1, 139.6,
8.62–8.30 (m, 2H), 8.62–8.57 (m, 2H), 8.40–8.37 (m, 2H), 8.23– 139.0, 138.6, 138.1, 137.5, 131.1, 131.0, 130.8, 129.7, 129.2,
8.21 (d, 2H), 8.15 (s, 1H), 7.89–7.85 (m, 3H), 7.80–7.77 (m, 1H), 128.7, 127.5, 127.5, 127.2, 126.6, 126.5, 126.0, 123.9, 123.6,
7.66–7.44 (m, 9H), 7.44–7.34 (m, 3H), 1.67 (s, 6H); 13C NMR 122.9, 120.3, 120.3, 120.2, 114.3, 110.7, 109.9, 101.2, 46.6, 28.3.
(CDCl3, 500 MHz) d (ppm) 165.2, 164.9, 164.1, 154.6, 147.0, Found: C, 87.5; H, 5.3; N, 7.3. Calc. for C43H31N3: C, 87.6; H, 5.3;
141.4, 140.8, 140.5, 139.6, 138.5, 138.3, 137.6, 136.6, 131.1, N, 7.1%.
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HOMO and LUMO orbitals of pDPPICz1 and mDPPICz1 are well
separated but pDPPICz2 and mDPPICz2 show small coupling
between both orbitals. In particular, the HOMO orbitals are
mainly situated over the indenocarbazole moiety, whereas the
LUMO orbitals are localized on the biphenyl pyrimidine moiety.
Such well separated HOMO and LUMO orbitals are needed for
the efficient charge transport and high triplet energy.
Material characterization
1H and 13C NMR spectra were recorded using Bruker Avance 400
NMR spectrometer. The differential scanning calorimetry (DSC)
was performed using a TA Instruments DSC Q2000. UV-vis and
photoluminescence (PL) spectra were measured using SCINCO
S-4100 spectrometer and JASCO FP6500 spectrometer, respec-
tively. All reagents and solvents were purchased from Aldrich
Chemical Co. and Fluka and used as received. Cyclic voltam-
metry (CV) was performed using EC epsilon electrochemical
analysis equipment. Platinum wire and indium tin oxide (ITO)
lm on glass were used as counter and working electrodes
respectively. Tetrabutylammonium perchlorate (Bu4NClO4) was
used as a supporting electrolyte. Similarly, Ag wire in 0.1 M
AgNO3 was used as a reference electrode. Using an internal
ferrocene/ferrocenium (Fc/Fc+) standard, the potential values
were converted to the saturated calomel electrode (SCE) scale.
The optical band gap was determined from the tailing edge of
absorption spectra in solution state. The HOMO level was
obtained from the oxidation potential of CV characteristics. The
LUMO level of each material was estimated from the optical
band gap and HOMO level.
The triplet energy of the molecules are calculated from the
difference between the total energy of the molecule from singlet
ground state (S0) to triplet lowest excited state (T1). The calcu-
lated triplet energy (T1) values from the molecular simulations
of pDPPICz1, mDPPICz1 are 2.76 eV and pDPPICz2, mDPPICz2
are 2.74 eV, respectively. The relative physical properties of the
host materials are summarized in Table 1. The experimental
triplet energy of newly synthesized materials is approximately
0.14–0.16 eV lower than the calculated values, but it shows the
same tendency in theoretical and experimental data. The high
triplet energy of 2.76 eV is assigned to the proper decoupling of
HOMO and LUMO orbitals. Hence, these values are suitable to
use as host materials for phosphorescent green OLED devices.
The calculated HOMO energy values of pDPPICz1, pDPPICz2,
mDPPICz1 and mDPPICz2 are 5.26, 5.35, 5.23 and 5.29 eV,
respectively, which is slightly deeper than the experimental values.
On the other hand, the calculated LUMO values of pDPPICz1,
pDPPICz2, mDPPICz1 and mDPPICz2 are 1.92, 1.79, 1.88 and 1.81
eV, respectively, which is quite higher than the experimental
Fabrication and characterization of devices
ITO (sheet resistance of 10 U/,) covered glass substrates were
sequentially cleaned ultrasonically in acetone, isopropyl
alcohol, and rinsed with deionized water followed by UV–ozone
treatment for 10 min. All organic layers and metal cathode were
deposited by thermal vacuum deposition technique under a
vacuum pressure of ꢂ1 ꢃ 10ꢁ7 Torr without breaking vacuum
pressure. The deposition rate of 0.5 A sꢁ1 was maintained for all
˚
organic layꢁer1s. Deposition rates of LiF and Al were maintained
at 0.1 A s , 5 A sꢁ1, respectively. Finally all devices were
encapsulated using glass cover and UV curable resin inside the
nitrogen lled glove box. Current density–voltage–luminance
(J–V–L) characteristics of fabricated OLED devices were
measured by using a Keithley 2635A Source Meter Unit (SMU)
and Konica Minolta CS-100A. Electroluminescence (EL) spectra
and Commission Internationale de l'Eclairage (CIE) 1931 color
coordinate were obtained using a Konica Minolta CS-2000
spectroradiometer.
˚
˚
Results and discussion
Theoretical calculations
The detail molecule design concept is discussed in our previous
report.27 Herein phenyl group is inserted between inden-
ocarbazole and biphenyl pyrimidine moieties, which are con-
nected to their para- and meta-position by Suzuki-coupling,
respectively (See Scheme 1). In order to understand the structural
characteristic and geometry optimization of host materials,
molecular simulations were performed using density functional
theory (DFT) calculation with B3LYP (beck three parameter
hybrid functional28–30 and Lee–Yang–Parr correlation func-
tional31) and 6-31G(d) basis set implemented in Gaussian 09.32
Fig. 1 represents the molecular orbital distributions of materials
Scheme 1 Schematic route of pDPPICz1, pDPPICz2, mDPICz1 and
in the HOMO and LUMO states. It is clearly observed that the mDPPICz2.
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Fig.
1 Spatial distribution of the HOMO and LUMO from DFT
calculation.
values. However, theoretical HOMO and LUMO values of all four
materials show contrast tendency as compare to the experimental
data even aer the multiple runs of simulation, hence exact
reason behind such tendency is unclear. The calculated band-gap Fig. 2 CV analysis of pDPPICz1, pDPPICz2, mDPPICz1 and mDPPICz2
for oxidation potential.
values of pDPPICz1 and mDPPICz1 are 3.34 and 3.35 eV, and
showing a good agreement with the experimental values. Though,
this values are lower than the pDPPICz2 (3.56 eV) and mDPPICz2
summarized in Table 1. Due to relatively shorter conjugation
length of pDPPICz1 and mDPPICz1 with the inner dimethyl
group in indenocarbazole moiety, their energy band-gap is
larger than the pDPPICz2 and mDPPICz2 with the outer
dimethyl group in indenocarbazole moiety.
(3.48 eV). The expected band gap of pDPPICz2 and mDPPICz2
could be lower than the other two materials due to their low
conjugation length. Additionally, pDPPICz2 and mDPPICz2
consist of outer dimethyl group with indenocarbazole unit, which
shows a smaller torsional angle between hole and electron
transport moieties due to low steric hindrance results in longer
conjugation length and low band gap. According to our calcula-
tions, series of newly synthesized materials have proper T1 and
appropriate HOMO, LUMO values for good phosphorescent host.
The UV-vis absorption and PL measurements of all four host
materials were performed at room temperature in CH2Cl2 solution.
In order to conrm the emission properties of host materials, PL
measurements were performed at low temperature (77 K, see
Fig. 3). The UV-visible absorption and PL characteristics of host
compounds at room temperature are shown in Fig. 4. The
absorption peaks are observed near 319–328 nm, which can be
attributed to the p–p* transition from indenocarbazole to
biphenyl pyrimidine. The absorption peak of pDPPICz2 and
mDPPICz2 is obtained at relatively longer wavelength as compare
to the pDPPICz1 and mDPPICz1. On the other hand, the
maximum PL peaks of pDPPICz1, pDPPICz2, mDPPICz1 and
mDPPICz2 in CH2Cl2 solution state (1 ꢃ 10ꢁ4 M) at room
temperature are observed at 449, 445, 454 and 451 nm, respec-
tively. Although the absorption spectra of mDPPICz1 show the
shortest wavelength as compare to other host materials, the PL
spectra shows a red-shi result. This big Stokes shi in case of
mDPPICz1 molecule could be due to the more geometrical
changes as compare to other molecules from ground state to the
excited state.
Electrical and photo-physical properties
In order to nd the electrochemical properties of host mate-
rials, CV measurements were performed in acetonitrile solution
using Bu4NClO4 10ꢁ4 M. The HOMO values of host materials
were investigated from the onset of oxidation potential and the
energy level of Fc/Fc+ (i.e., 4.80 eV). The CV characteristics of the
host materials are shown in Fig. 2. The measured HOMO level of
pDPPICz1, pDPPICz2, mDPPICz1 and mDPPICz2 are 5.95, 5.76,
5.76 and 5.87 eV, respectively. Similarly, the experimental
LUMO energy levels of the host materials are 3.34, 3.10, 3.37 and
3.30 eV, respectively, which attained from the HOMO and
optical band gap values. The optical band gap values are
obtained from the tailing edge of the UV-vis absorption spec-
trum. The relevant electrochemical and optical data are
Table 1 Physical properties of pDPPICz1, pDPPICz2, mDPPICz1 and mDPPICz2
Tg [ꢀC] Tm [ꢀC] Td0.5/5.0b [ꢀC] labs [nm] lPL [nm] Eg (cal.e) [eV] HOMOf (cal.g) [eV] LUMOh (cal.g) [eV] T1 (cal.g) [eV]
a
a
c
c
d
c,i
Host
pDPPICz1 NAj
pDPPICz2 NAj
mDPPICz1 NAj
mDPPICz2 129
261
304
271
233
358/401
315/367
325/373
327/363
319
328
319
326
449
445
454
451
3.34 (3.34)
3.10 (3.56)
3.37 (3.35)
3.30 (3.48)
5.95 (5.26)
5.76 (5.35)
5.76 (5.23)
5.87 (5.29)
2.61 (1.92)
2.66 (1.79)
2.39 (1.88)
2.57 (1.81)
2.62 (2.76)
2.58 (2.74)
2.62 (2.76)
2.58 (2.74)
a
b
c
d
e
Measured from DSC. Measured from TGA. Measured in 2-methyl THF. Measured by band edge of absorption spectra. Values from the
f
g
calculated HOMO and LUMO. Determined from the onset of the oxidation potentials. Values from DFT calculation B3LYP/6-31G(d).
h
j
Calculated from the HOMO and measured Eg. i Determined from peak value of low temperature PL spectra (77 K). Not detected.
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Fig. 3 Phosphorescence photoluminescence spectra in 2-methyl
tetrahydrofuran solution (1 ꢃ 10ꢁ4 M) at 77 K.
Fig. 5 J–V characteristics of HODs and EODs.
EOD:ITO/1,3,5-tris[(3-pyridyl)phen-3-yl]benzene (TmPyPB, 80
nm)/Host (30 nm)/TmPyPB (40 nm)/LiF (1.5 nm)/Al (100 nm).
The TAPC and TmPyPB are incorporated to improve the hole
and electron transport property as well as to block the injection
of electrons and holes from the cathode and anode, respec-
tively. The current density versus voltage (J–V) characteristics of
HOD and EOD are shown in Fig. 5. All the four materials show
higher hole current density and low electron current density.
Among them mDPPICz1 shows a better hole and electron
current density as compare to the other three host materials. It
may be due to the better molecular packing. Interestingly, the
mDPPICz2 shows higher hole current density at x voltage (15
V) but their electron current density is lower than the
mDPPICz1, which could affect the charge balance property at
the emissive layer. Furthermore, pDPPICz1 and pDPPICz2
display identical charge carrier density at 15 V.
Thermal analysis
The ideal morphological and thermal stability of organic
materials are vital for the decent operation and the better life-
time of OLED devices. Thus, thermal and morphological
stabilities of new host materials were examined by using TGA
and DSC. The Td of bipolar host materials (corresponding to 5%
loss) are obtained in the range of 363–401 ꢀC, which exhibits a
good thermal stability. The Tg of mDPPICz2 was observed to be
129 ꢀC, which could be high enough to provide a morphological
stability. Whereas, the noticeable Tg of other host materials was
not observed. The Tg value of mDPPICz2 is higher than the
commercially available CBP (62 ꢀC).33 We believe that these
thermal properties are very well suited for the stable operation
of the host materials.
Charge transport properties of host materials
Electroluminescence properties
To conrm the bipolar characteristics of new host materials,
hole only device (HOD) and electron only device (EOD) were
fabricated. The congurations of charge transport devices are as
follows: HOD:ITO/di-[4-(N,N-ditolylamino)phenyl]-cyclohexane
(TAPC, 80 nm)/Host (30 nm)/TAPC (40 nm)/Al (100 nm) and
To evaluate the capability of series of indenocarbazole based
materials as a host for green phosphorescent OLEDs, four
separate devices were fabricated with the following congura-
tion: ITO/TAPC (75 nm)/Host: 5 wt% Ir(ppy)3 (15 nm)/TmPyPB
(40 nm)/LiF (1 nm)/Al (100 nm). The energy level diagram of
Fig. 4 UV-vis absorption and PL spectra.
31288 | RSC Adv., 2015, 5, 31282–31291
Fig. 6 Energy level diagram of fabricated green OLEDs.
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Fig. 7 Performances of bipolar host pDPPICz1, pDPPICz2, mDPPICz1 and mDPPICz2 devices. (a) J–V and L–V characteristics (b) power effi-
ciency-luminance characteristic (c) EQE-luminance characteristics (d) EL spectra.
the fabricated green OLED device structure is shown in Fig. 6. on voltages of all four devices are in the range of 2.5–3.2 V at 1 cd
Device A, B, C, and D are assigned to the pDPPICz1, pDPPICz2, mꢁ2. In particular, the Device C with mDPPICz2 shows a low
mDPPICz1 and mDPPICz2, respectively. In these devices, ITO turn on voltage of 2.5 V among all the studied host materials.
and Al are used as anode and cathode, respectively. The TAPC is Similarly, it also shows a low driving voltage of 3.8 V as compare
used as a hole transport layer. However, the TmPyPB layer fol- to the other host materials at 1000 cd mꢁ2. We believe that, such
lowed by thin LiF layer act as electron transport and injection low turn on and driving voltage values are due to the excellent
layer, respectively. The triplet energies (T1) of both hole and bipolar and charge balance characteristics of the host materials.
electron transport materials are high enough to block the triplet
exciton quenching from the emissive layer.34,35 Ir(ppy)3 (5%) was mDPPICz2 show a maximum current and power efficiency of 71.8,
incorporated as a green dopant in these host materials.
68.9, 75.8 and 69.3 cd Aꢁ1 and 74.8, 71.1, 87.1 and 51.7 lm Wꢁ1
The devices with pDPPICz1, pDPPICz2, mDPPICz1 and
The J–V and L–V characteristics of the fabricated phospho- (see Fig. 7(b)), respectively. In addition the Device C shows the
rescent green OLED devices are compared in Fig. 7(a). The turn- maximum EQE as high as 21.7%, whereas the Device A, B and D
Table 2 EL properties of pDPPICz1, pDPPICz2, mDPPICz1 and mDPPICz2 based phosphorescent green devices
Efficiencyb (max.)
Max. luminance
Host
Turn-on voltagea [V]
Driving voltageb [V]
[cd mꢁ2
]
CEc [cd Aꢁ1
]
PEd [lm Wꢁ1
]
EQE [%]
CIE 1931 (x, y)b
pDPPICz1
pDPPICz2
mDPPICz1
mDPPICz2
2.7
2.7
2.5
3.2
4.0
4.3
3.8
4.4
68 090
56 460
56 440
41 420
63.6 (71.8)
57.8 (68.9)
64.3 (75.8)
68.9 (69.3)
50.0 (74.8)
43.3 (71.1)
53.2 (87.1)
49.2 (51.7)
18.3 (20.6)
16.6 (19.5)
18.5 (21.7)
19.8 (19.9)
(0.29, 0.63)
(0.30, 0.63)
(0.30, 0.62)
(0.30, 0.62)
a
b
c
Measured at 1 cd mꢁ2
.
measured at 1000 cd mꢁ2
.
Current efficiency. d Power efficiency.
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show an EQE of 20.6, 19.5, and 19.9%, respectively (see
Fig. 7(c)). The EQE values of OLED device with our host mate-
9 H. Xia, C. Zhang, S. Qiu, P. Lu, J. Zhang and Y. Ma, Appl. Phys.
Lett., 2004, 84, 290.
rials are better than the previously reported indenouorene 10 Y. Chi and P. T. Chou, Chem. Soc. Rev., 2007, 36, 1421.
(EQE: 15.8%)36 and dihydroindenouorene (EQE: 14.8%)21 11 W. S. Jeon, T. J. Park, S. Y. Kim, R. Pode, J. Jang and
based host materials. The EL spectra of Device A–D are almost
J. H. Kwon, Org. Electron., 2009, 10, 240.
identical and all the devices emit green light at the emission of 12 K. S. Yook and J. Y. Lee, Adv. Mater., 2012, 24, 3169.
Ir(ppy)3 (see Fig. 7(d)). The detail performances of fabricated 13 H. F. Chen, T. C. Wang, W. Y. Hung, H. C. Chiu, C. Yun and
phosphorescent green OLEDs are summarized in Table 2.
Additionally, the operational stability of fabricated OLEDs 14 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma,
with new indenocarbazole based host materials was evaluated. Adv. Funct. Mater., 2011, 21, 1168.
The pDPPICZ1 and mDPPICz1 based devices show longer life- 15 X. K. Liu, C. J. Zheng, M. F. Lo, J. Xiao, C. S. Lee, M. K. Fung
time (LT75) of 15 and 19 hours at initial brightness value of and X. H. Zhang, Chem. Commun., 2014, 50, 2027.
1000 cd mꢁ2 as compared to the widely used CBP host device. In 16 Y. M. Chen, W. Y. Hung, H. W. You, A. Chaskar, H. C. Ting,
K. T. Wong, J. Mater. Chem., 2012, 22, 9658.
particular, mDPPICz1 based device shows 27% longer lifetime
than the pDPPICZ1. The operational lifetime of all the devices
H. F. Chen, K. T. Wong and Y. H. Liu, J. Mater. Chem., 2011,
21, 14971.
was measured until decrease in the luminance value to 75%. 17 H. Huang, Y. Wang, S. Zhuang, X. Yang, L. Wang and
The operational stability data of OLEDs are shown in Fig. S1.†
C. Yang, J. Phys. Chem. C, 2012, 116, 19458.
18 X. K. Liu, C. J. Zheng, J. Xiao, J. Ye, C. L. Liu, S. D. Wang,
W. M. Zhao and X. H. Zhang, Phys. Chem. Chem. Phys.,
2012, 14, 14255.
Conclusion
19 J. Zhao, G. H. Xie, C. R. Yin, L. H. Xie, C. M. Han, R. F. Chen,
H. Xu, M. D. Yi, Z. P. Deng, S. F. Chen, Y. Zhao, S. Y. Liu and
W. Huang, Chem. Mater., 2011, 23, 5331.
20 S. Thiery, D. Tondelier, C. Declairieux, G. Seo, B. Geffroy,
O. Jeannin, J. Rault-Berthelot, R. M'etivier and C. Poriel, J.
Mater. Chem. C, 2014, 2, 4156.
21 M. Romain, S. Thiery, A. Shirinskaya, C. Declairieux,
D. Tondelier, B. Geffroy, O. Jeannin, J. Rault-Berthelot,
R. M'etivier and C. Poriel, Angew. Chem., Int. Ed., 2015, 54,
1176.
22 L. Ding, S. C. Dong, Z. Q. Jiang, H. Chen and L. S. Liao, Adv.
Funct. Mater., 2015, 25, 645.
In this paper, we have designed and synthesized a series of new
bipolar green host materials, with indenocarbazole as hole and
biphenyl pyrimidine as electron-transporting unit. These
materials show ideal electro-optical and thermal properties for
phosphorescent green OLED devices. These host materials also
show proper charge balancing due to their excellent charge
carrier transport properties. Fabricated phosphorescence green
OLED devices show excellent maximum EQE of 21.7% as well as
current efficiency, and power efficiency of 75.8 cd Aꢁ1, and
87.1 lm Wꢁ1, respectively.
Acknowledgements
23 S. Thiery, C. Declairieux, D. Tondelier, G. Seo, B. Geffroy,
O. Jeannin, R. M'etivier, J. Rault-Berthelot and C. Poriel,
Tetrahedron, 2014, 70, 6337.
24 S. J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274.
25 S. O. Jeon, T. Earmme and S. A. Jenekhe, J. Mater. Chem. C,
2014, 2, 10129.
26 M. Romain, D. Tondelier, B. Geffroy, A. Shirinskaya,
O. Jeannin, J. Rault-Berthelota and C. Poriel, Chem.
Commun., 2015, 51, 1313.
27 G. H. Kim, R. Lampande, M. J. Park, H. W. Bae, J. H. Kong,
J. H. Kwon, J. H. Park, Y. W. Park and C. E. Song, J. Phys.
Chem. C, 2014, 118, 28757.
This work was supported by the Human Resources Develop-
ment program (no. 20134010200490) of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) grant
funded by the Korea government Ministry of Trade, Industry
and Energy.
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