Phenanthro[9,10-d]imidazole based new host materials for efficient red phosphorescent OLEDs

Article abstract of DOI:10.1016/j.dyepig.2016.11.003

We report on the synthesis and characterization of a new series of bipolar phenanthro[9,10-d]imidazole based compounds. The derivatives are highly thermally stable materials as demonstrated by thermogravimetric analysis. Electron photoemission spectra of thin layers of the materials show ionization potentials in the range of 5.1–5.4?eV. Spectroscopic properties of the phenanthro[9,10-d]imidazole based derivatives were determined by absorption and photoluminescence measurements. All the developed materials were used as hosts in red phosphorescent organic light-emitting diodes and their properties were compared with commercially available host materials. Results indicate that a device with 2-[4-(N,N-diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole exhibited superior performance with peak efficiency of 15.9% (21.5?cd/A and 29.9?l?m/W) and very low turn on voltage of 2.8?V. Efficiency of the device is about 35–67% higher than those of devices containing commercial host materials.

Full text of DOI:10.1016/j.dyepig.2016.11.003

Accepted Manuscript
Phenanthro[9,10-d]imidazole based new host materials for efficient red
phosphorescent OLEDs
Daiva Tavgeniene, Gintare Krucaite, Urte Baranauskyte, Jian-Zhi Wu, Hong-Yu Su,
Chih-Wei Huang, Chih-Hao Chang, Saulius Grigalevicius
PII:
S0143-7208(16)30674-X
10.1016/j.dyepig.2016.11.003
DYPI 5571
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 September 2016
Revised Date: 2 November 2016
Accepted Date: 3 November 2016
Please cite this article as: Tavgeniene D, Krucaite G, Baranauskyte U, Wu J-Z, Su H-Y, Huang C-W,
Chang C-H, Grigalevicius S, Phenanthro[9,10-d]imidazole based new host materials for efficient red
phosphorescent OLEDs, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.11.003.
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20
16
12
8
Host for red OLED
4
0
10-1
100
101
102
103
104
105
Luminance (cd/m2)

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Phenanthro[9,10-d]imidazole based New Host Materials for Efficient Red
Phosphorescent OLEDs
Daiva Tavgeniene1, Gintare Krucaite1, Urte Baranauskyte1, Jian-Zhi Wu2, Hong-Yu Su2, Chih-Wei
Huang2, Chih-Hao Chang2,*, Saulius Grigalevicius1,*
1Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu
plentas 19, LT50254, Kaunas, Lithuania
2Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan 32003
* Corresponding authors:
chc@saturn.yzu.edu.tw (C.H. Chang), saulius.grigalevicius@ktu.lt (S. Grigalevicius)
-------------------------------------------------------------------------------------------------------------------
Abstract
We report on the synthesis and characterization of a new series of bipolar phenanthro[9,10-
d]imidazole based compounds. The derivatives are highly thermally stable materials as
demonstrated by thermogravimetric analysis. Electron photoemission spectra of thin layers of the
materials show ionization potentials in the range of 5.1-5.4 eV. Spectroscopic properties of the
phenanthro[9,10-d]imidazole based derivatives were determined by absorption and
photoluminescence measurements. All the developed materials were used as hosts in red
phosphorescent organic light-emitting diodes and their properties were compared with
commercially available host materials. Results indicate that a device with 2-[4-(N,N-
diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole exhibited superior performance with
peak efficiency of 15.9% ( 21.5 cd/A and 29.9 lm/W) and very low turn on voltage of 2.8 V.
Efficiency of the device is about 35-67% higher than those of devices containing commercial host
materials.

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Keywords: Phenanthro[9,10-d]imidazole, host material, ionization potential, phosphorescent,
organic light-emitting diode.
Introduction
Applications of organic light-emitting devices (OLED) based displays are expanding rapidly due
to their superior performance and flexibility as compared with liquid crystal displays.
Phosphorescent OLEDs (PhOLEDs) have particularly attracted researchers because of their higher
efficiencies compared with those of fluorescent OLED devices [1]. In order to achieve the high
efficiency, the triplet energy should be confined on the phosphorescent dopant without endothermic
energy transfer to the host material. Multiple-layer configurations have been widely developed to
enhance efficiency of blue and green phosphorescent PhOLEDs. Red phosphorescent materials with
a lower triplet energy gap enable the simplification of the device architecture; however the lower
gap of the phosphors usually induces serious carrier trapping, leading to higher operation voltages
4
5
[2]. There are only several reports describing rather efficient red PhOLEDs [3, , ]. Accordingly, it
is important to develop new host materials for realizing the red devices with high performance.
Carbazole and triphenylamine fragments possess several commendable properties, including
7
8
9
10 11
chemical stability, convenient modification and large triplet energy etc. [6, , , ,
,
]
Consequently, many successful host or hole transport materials adopted carbazole or
tryphenylamine-containing designs. Our group synthesized the carbazole based material, 3,6-di(9-
carbazolyl)-9-(2-ethylhexyl) carbazole (TCz1), which possesses structurally rigid moieties and a
nonplanar molecular configuration, resulting in a morphologically-stable molecule with a wide
triplet energy gap [12]. Sky blue PhOLEDs with a TCz1 host were demonstrated with efficiencies of
up to 15 %, 31 cd/A, and 28 lm/W. We have also observed recently that carbazole-based bipolar
14
derivatives are effective host materials for PhOLED devices [13, ]. A green dry-processed device
showed a current efficiency of 61 cd/A and a power efficiency of 62.8 lm/W. In this paper, we

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describe a series of new phenanthro[9,10-d]imidazole based derivatives for bipolar emitting host.
The red PhOLEDs with 2-[4-(N,N-diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole
(H2) as host material exhibited superior performance with peak efficiency of 15.9% ( 21.5 cd/A and
29.9 lm/W) and very low turn-on voltage of 2.8 V.
Experimental
Materials
3,6-Di(9-carbazolyl)-9-(2-ethylhexyl) carbazole (TCz1) was prepared by the procedure
described in literature [12].
9H-carbazole (1), triphenylamine (4), 1-bromohexane, acetone, potassium hydroxide
(KOH), tetrabutylammonium hydrogensulfate (TBAHS), phosphorus oxychloride (POCl3),
dimethylformamide (DMF), chloroform, dichloromethane, aniline, acetic acid, ammonium acetate
and 9,10-phenanthrenequinone were purchased from Aldrich and used as received.
9-Hexylcarbazole (2) was prepared by procedure described in the literature [15]. 3-Formyl-9-
hexylcarbazole (3), 4-(diphenylamino)benzaldehyde (5) and 4,4'-diformyltriphenylamine (6) were
obtained by a Vilsmeier reaction [16] using earlier described procedures [17, 18].
2-(9-Hexylcarbazolyl-3-yl)-1-phenylphenanthro[9,10-d]imidazole
(H1).
3-Formyl-9-
hexylcarbazole (3) (1.0 g, 3.58 mmol), 9,10-phenanthrenequinone (0.82 g, 3.94 mmol), ammonium
acetate (1.11 g, 14.40 mmol) and aniline (1.67 g, 17.93 mmol) of were stirred in acetic acid (15 mL)
0
at 120 C under nitrogen for 2 h. After thin layer chromatography (TLC) control the reaction
mixture was cooled down to the room temperature and poured into ice water. The resulting residue

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that had been formed was filtered off, washed with water and dissolved in chloroform. The solution
was dried by Na2SO4 and the solvent was removed by evaporation. The product was purified by
column chromatography with silica gel using chloroform/hexane (vol. ratio 2:3) as an eluent.
Yellow crystals, 62% (1.2 g), mp 202 ºC. IR data (KBr): 2925, 1714, 1599, 1495, 1454, 1417, 1381,
1
1353, 1238, 1142, 889, 816, 767, 748, 721 cm-1. H NMR δH (CDCl3, 500 MHz): 8.96 (d, 1H, J =
8.0 Hz, Ar), 8.78 (d, 1H, J = 8.4 Hz, Ar), 8.72 (d, 1H, J = 8.0 Hz, Ar), 8.25 (d, 1H, J = 1.2 Hz,
Ar), 7.19 (d, 1H, J = 8.0 Hz, Ar), 7.82–7.72 (m, 2H, Ar), 7.73–7.55 (m, 6H, Ar), 7.56–7.17 (m, 7H,
Ar), 4.25 (t, 2H, J = 7.2 Hz, NCH2), 1.90-1.79 (m, 2H, NCH2CH2), 1.39-1.27 (m, 6H,
2×NCH2CH2CH2CH2CH2), 0.86 (t, 3H, J = 6.8 Hz, CH3). 13C NMR δC (CDCl3, 125 MHz): 151.74,
140.82, 138.69, 130.26, 129.32, 127.52, 127.45, 126.38, 126.05, 124.16, 123.10, 122.03, 120.90,
120.49, 119.34, 108.94, 108.56, 43.22, 31.55, 28.93, 26.94, 22.55, 14.02. HRMS found: [M+H]+
544.7236; molecular formula C39H33N3 requires [M+H]+ 544.7267. Elemental analysis for C39H33N3
% Calc.: C 86.15, H 6.12, N 7.73; % Found: C 86.11, H 6.14, N 7.71.
2-[4-(N,N-diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole
(H2).
4-
(Diphenylamino)benzaldehyde (5) (1.0 g, 3.66 mmol), 9,10-phenanthrenequinone (0.83 g, 3.99
mmol), ammonium acetate (1.13 g, 14.66 mmol) and aniline (1.70 g, 18.25 mmol) of were stirred in
0
acetic acid (20 ml) at 120 C under nitrogen for 2 h. After TLC control the reaction mixture was
cooled down to the room temperature and poured into ice water. The resulting residue that had been
formed was filtered off, washed with water and dissolved in chloroform. The solution was dried by
Na2SO4 and the solvent was removed by evaporation. The product was purified by column
chromatography with silica gel using chloroform/hexane (vol. ratio 2:3) as an eluent. Yellow
crystals, 64% (1.3 g), mp 285 ºC. IR data (KBr): 1588, 1516, 1485, 1468, 1453, 1424, 1383, 1330,
1281, 1195, 1076, 1037, 966, 840, 747, 735, 720 cm-1. 1H NMR δH (CDCl3, 500 MHz): 8.87 (d, 1H,
J = 8.0 Hz, Ar), 8.76 (d, 1H, J = 8.4 Hz, Ar), 8.69 (d, 1H, J = 8.4 Hz, Ar), 7.78-7.69 (m, 1H, Ar),
13
7.68-7.40 (m, 9H, Ar), 7.30-7.21 (m, 5H, Ar), 7.16-7.00 (m, 7H, Ar), 6.97-6.89 (m, 2H, Ar). C

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NMR δC (CDCl3, 125 MHz): 150.63, 148.56, 147.15, 138.83, 130.23, 130.23, 129.93, 129.38,
129.22, 129.18, 128.29, 127.90, 127.34, 126.30, 125.69, 125.13, 124.84, 124.13, 123.60, 123.09,
122.94, 121.81, 120.78. HRMS found: [M+H]+ 538.6741; molecular formula C39H27N3 requires
[M+H]+ 538.6752. Elemental analysis for C39H27N3 % Calc.: C 87.12, H 5.06, N 7.82; % Found: C
87.05, H 5.09, N 7.79.
Bis[4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl]-N-phenylamine
(H3).
4,4'-
diformyltriphenylamine (6) (1g, 3.32 mmol), 9,10-phenanthrenequinone (1.52 g, 7.30 mmol),
ammonium acetate (2.05 g, 26.6 mmol) and aniline (3.09 g, 33.18 mmol) were stirred in acetic acid
(20 mL) at 120 0C under nitrogen for 3 h. After TLC control the reaction mixture was cooled down
to the room temperature and poured into ice water. The residue that had been formed was filtered
off, washed with water and dissolved in chloroform. The solution was dried by Na2SO4 and the
solvent was removed by evaporation. The product was purified by column chromatography with
silica gel using chloroform/hexane (vol. ratio 1:1) as an eluent. Yellow crystals, 44% (1.2 g), mp
361 ºC. IR data (KBr): 1738, 1599, 1536, 1514, 1494, 1469, 1453, 1382, 1344, 1320, 1286, 1265,
1
1183, 1073, 1046, 957, 833, 759, 740, 723 cm-1. H NMR δH (CDCl3, 500 MHz): 8.87 (d, 2H, J =
7.6 Hz, Ar), 8.77 (d, 2H, J = 8.0 Hz, Ar), 8.70 (d, 2H, J = 8.0 Hz, Ar), 7.78-7.71 (m, 2H, Ar), 7.69-
7.60 (m, 7H, Ar), 7.58-7.44 (m, 9H, Ar), 7.31-7.23 (m, 6H, Ar), 7.19-7.12 (m, 2H, Ar), 7.11-7.04
13
(m, 3H, Ar), 6.99-6.93 (m, 4H, Ar). ). C NMR δC (CDCl3, 125 MHz): 149.49, 146.69, 145.57,
137.84, 136.40, 129.23, 128.94, 128.72, 128.46, 128.21, 128.11, 127.26, 127.00, 126.28, 125.28,
124.61, 123.80, 123.10, 122.07, 121.82, 119.77. HRMS found: [M+H]+ 831.0147; molecular
formula C60H39N5 requires [M+H]+ 831.0136. Elemental analysis for C60H39N5 % Calc.: C 87.05,
H 4.74, N 8.44; % Found: C 87.01, H 4.77, N 8.41.
Thermal and photophysical characterization

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Differential scanning calorimetry (DSC) measurements were carried out using a Bruker Reflex II
thermos system. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409. The
TGA and DSC curves were recorded in a nitrogen atmosphere at a heating rate of 10o C/min.
The absorption spectra were performed by using a UV-VIS Spectrophotometer (Shimadzu, UV-
1650PC). The fluorescent and phosphorescent spectra of materials in CH2Cl2 were measured by
using a Fluorolog III photoluminescence spectrometer (Horiba Jovin Yvon).
The electron photoemission method for measurement of ionization potentials (Ip) of the solid state
layers of the studied compounds was exploited in air [19]. The vacuum deposited layers of the
compounds onto commercial indium tin oxide (ITO) coated glass substrates were utilized as the
samples for the electron photoemission measurements. The deep-UV deuterium light source ASBN-
D130-CM, CM110 1/8m monochromator, and 6517B Keithley electrometer were used in the
experimental setup, which was similar as previously described [20].
Device fabrication
Indium tin oxide (ITO)-coated glass and OLED materials were purchased from Nichem and
Lumtec. The latter were subjected to temperature-gradient sublimation in a high vacuum before use.
After a routine cleaning procedure, the ITO substrate was pretreated with UV-Ozone for 5 minutes.
The organic and metal layers were deposited onto ITO-coated glass by thermal evaporation in a
vacuum chamber with a base pressure of < 10-6 Torr. Device fabrication was completed in a single
cycle without breaking the vacuum. The deposition rates of the organic materials and aluminium
were respectively kept at around 0.1 nm/s and 0.5 nm/s. The active area was defined by the shadow
mask (2 × 2 mm2). Current density-voltage-luminance characterization was measured using a
Keithley 238 current source-measure unit and a Keithley 6485 picoammeter equipped with a
calibrated Si-photodiode. The electroluminescent spectra were recorded using an Ocean Optics
spectrometer.

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Results and Discussion
Synthesis and thermal properties of the materials.
The synthesis of 1-phenylphenanthro[9,10-d]imidazole based host materials (H1-H3) was carried
out by a multi-step synthetic route as shown in Scheme 1. 9-Hexylcarbazole (2) was firstly
synthesized by reaction of commercially available carbazole (1) with an excess 1-bromohexane
under basic conditions in acetone. 3-Formyl-9-hexylcarbazole (3) was then prepared from the
compound 2 by a Vilsmeier formylation procedure. 4-(Diphenylamino)benzaldehyde (5) and 4,4'-
diformyltriphenylamine (6) were obtained from triphenylamine (4) by a Vilsmeier formylation
procedure in chlorofrom or dichloromethane, respectively. The objective materials, i.e. 2-(9-
hexylcarbazolyl-3-yl)-1-phenylphenanthro[9,10-d]imidazole
diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole
(H1),
(H2)
2-[4-(N,N-
and
bis[4-(1-
phenylphenanthro[9,10-d]imidazol-2-yl)phenyl]-N-phenylamine (H3) were prepared by the
reactions of the aldehydes 3, 5 or 6 with excess of 9,10-phenanthrenequinone, aniline and
ammonium acetate in acetic acid. The synthesized derivatives were all identified by mass
1
spectrometry and H NMR spectroscopy. The data were found to be in good agreement with the
proposed structures. The materials were soluble in common organic solvents. Transparent thin films
of these materials could be prepared by spin coating from solutions or by vacuum evaporation.

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O
N
N
O
O
N
N
1-Bromohexane,
KOH, K2CO3
N
N
POCl3, DMF
NH2
H
acetone
O
dichloromethane
1
CH3COOH,
H3C
ONH4
2
3
H1
N
N
N
O
O
N
H2
5
O
NH2
N
O
CH3COOH,
H3C
ONH4
4
c
h
l
o
r
o
N
f
o
N
r
m
N
N
N
N
O
O
6
H3
Scheme 1. Synthetic pathway of the materials H1-H3
The behaviour under heating of the objective materials H1- H3 was studied by DSC and TGA
under a nitrogen atmosphere. The data are summarized in Table 1 and presented in Figures 1 and 2.
It was established that all of the objective compounds demonstrate high thermal stability. The mass
o
o
o
loss of 5 % (TID) were at 386 C for H1, at 414 C for H2 and at 448 C for H3, as confirmed by
TGA with a heating rate of 10o C/min. It could be observed that triphenylamine-based derivatives
(H2 and H3) demonstrate slightly higher thermal stability than that of 9-alkylcarbazole containing
material H1.
All the compounds H1- H3 were obtained as crystalline materials after synthesis as confirmed by
DSC, however some of the materials could be easily converted to amorphous materials by cooling
the melted samples. The derivative H2 demonstrated very strong tendency to crystallization. DSC
thermo-grams of H2 are shown in Figure 1. When the crystalline sample was heated during the

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o
experiment, an endothermic peak due to melting (Tm) was observed at 285 C. When the melt
sample was cooled down, its crystallization (Tcr) was observed at 223 oC to form the same crystals,
which were obtained by crystallization from solution.
Tcr=223 oC
cooling
2nd heating
1st heating
Tm=285 oC
Tm=285 oC
50
100
150
200
250
300
Temperature (oC)
Figure 1. DSC curves of compound H2. Heating rate: 10 oC/min.
Compounds H1 and H3 demonstrated different behaviour during the DSC experiments. Thermo-
grams of compound H3 are shown in Figure 2 as an example. When a crystalline sample of H3 was
o
o
heated, melting was observed at 318 C followed by fast crystallization at 321 C to form new
o
crystalline modification, which melted at 361 C. When the molten sample was cooled down, it
formed an amorphous material with high glass transition temperature (Tg) of 184 oC. The compound
H1 demonstrated an analogous behaviour during the DSC test. When its crystalline sample was
o
heated during the experiment, melting was observed at 152 C followed by recrystallization at 166
o
oC. The new crystalline modification melted then at 204 C. When the melt sample was cooled
down, it formed an amorphous material with glass transition temperature of 52 oC.

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cooling
Tg=184 oC
2nd heating
Tcr=321 oC
1st heating
Tm=318 oC
Tm=361 oC
0
100
200
300
Temperature (o C)
Figure 2. DSC curves of compound H3. Heating rate: 10 oC/min.
Photoemission, absorption and photoluminescence spectra
.
(b)
(a)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Absorption
Fluorescence
H1 (5.4 eV)
H2 (5.1 eV)
H3 (5.3 eV)
H1
H2
H3
H1
H2
H3
Phosphorescence
H1
H2
H3
250
350
450
550
650
5.0 5.2 5.4 5.6 5.8
6.0 6.2 6.4
Wavelength (nm)
Photon energy (eV)
Figure 3 (a) electron photoemission spectra of thin layers of materials H1, H2, and H3; (b)
absorption spectra, fluorescence spectra, and phosphorescence spectra of the compounds in
dichloromethane.

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Figure 3(b) shows the UV-vis absorption spectra and the photoluminescence (PL) spectra of
the fabricated materials dissolved in dichloromethane recorded at 300K and 77 K. The PL spectra
measured at 77K aimed to characterize the triplet energy of the compounds. In the UV-vis
absorption spectra, each material exhibits a peak at around 260 nm, which is related to the
absorption bands of the imidazole moiety in the three compounds [21]. Notably, absorption peaks
appear at around 360 nm in H2 and H3 whereas no peak appears for H1, indicating that the
absorption bands should originate from the triphenylamine moiety [22]. On the other hand, H1
exhibits respectively a highest-energy fluorescence and phosphorescence peaks at around 404 nm
and 504 nm. Comparing the molecular structure of H1, an extension of conjugation length design
was introduced in the H2 and H3 molecules. The fluorescence (and phosphorescence) peaks of H2
and H3 were respectively red-shifted to 423 nm (514 nm) and 426 nm (517 nm). Furthermore, both
H2 and H3 possess similar spectral profiles demonstrating that the additional imidazole moiety in
H3 doesn't significantly affect its photophysical properties. The triplet energy gap of each material
was higher than those found in orange or red phosphors, thus enabling their use as host materials in
red PhOLEDs applications. The corresponding photophysical and thermal properties are
summarized in Table 1.
Table 1. Photophysical and thermal properties of compounds H1, H2 and H3.
λabs a (nm)
265, 279, 332, 367
258, 359
λfluo b (nm) λphos c (nm) Eg d (eV) ET e (eV)
Ip f (eV)
5.4
Tgg ( )
Tdh ( )
386
404
423
426
504
514
517
3.22
3.08
2.97
2.46
2.41
2.40
H1
H2
H3
5.1
414
263, 283, 376
5.3
184
448
a λabs: absorption maxima in CH2Cl2. b λfluo: emission maxima of fluorescence in CH2Cl2. c λphos
:
d
e
emission maxima of phosphorescence in CH2Cl2 at 77 K. Calculated from the absorption onset.
Estimated from the peak of phosphorescence spectrum. f Estimated from the electron photoemission
spectra. g Determined by DSC. h Analysed using TGA (5% weight loss).

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Red phosphorescent OLEDs with different host materials
Considering the triplet energy gaps of compounds H1, H2 and H3 (i.e. 2.65 eV, 2.54 eV and 2.46
eV), we attempted to fabricate red phosphorescent OLEDs with a simplified trilayer architecture
and investigate their EL characteristics. Generally speaking, a host material should possess a triplet
energy gap higher than that of the guest to ensure sufficient energy transfer as well as exciton
confinement [23]. Furthermore, the emission of the host material and the absorption spectrum of the
guest should have an adequate spectral overlap for iso-energetic transitions. In this study, a highly
efficient red phosphor, bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) iridium (III),
[Ir(MDQ)2acac], was chosen as the emitter because of its high quantum yield and adequate energy
bandgap [24]. On the other hand, to evaluate the performance of the fabricated materials, three
commercially available host materials suitable for red-emitting guests were used for comparison,
including 2,6-dicarbazolo-1,5-pyridine (PYD-2Cz), 4,4',4"-tris(carbazol-9-yl)triphenylamine
26 27
(TCTA), and 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) [25, , ]. Figure 4 shows the absorption
spectrum of Ir(MDQ)2acac and the PL spectra of the tested host materials, exhibiting different
levels of spectral overlapping. Furthermore, di-[4-(N,N-ditolylamino)-phenyl]cyclohexane (TAPC)
and 1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB) were respectively selected as the hole-
transport and electron-transport layers due to their excellent carrier transport capabilities and
adequate energy gaps [28, 29]. The tested device with a tri-layer architecture was configured as
ITO/TAPC (40 nm)/ host doped with 2 wt.% of Ir(MDQ)2acac (30 nm)/ BmPyPhB (40 nm)/ LiF
(0.8 nm)/ Al (150 nm), where the LiF and aluminium were respectively used as the electron
injection layer and the cathode. The host materials for devices A~E are as follows: PYD-2Cz,
TCTA, CBP, H1, H2, and H3. Figure 5(a) shows the chemical structure of the organic molecules
used while Figure 5(b) exhibits the energy level diagrams of the tested red PhOLEDs.

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0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ir(MDQ)2acac
PL spectra
PYD-2Cz
TCTA
CBP
H1
H2
H3
300
350
400
450
500
550
600
Wavelength (nm)
Figure 4. Absorption spectrum of Ir(MDQ)2acac and PL spectra of the host materials used in this
study.

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Dopant
Host
(b)
1.8
2.02
2.18
2.2
2.33
2.62
2.7
2.9
2.93
Cathode
Anode
5.1
5.13
5.3
5.4
5.5
5.7
5.8
6.0
6.67
ET
2.93
2.7
2.56 2.46 2.41 2.40 2.20
HTL
EML
ETL
40 nm
30 nm
40 nm
Device
A
B
C
D
E
F
Figure 5. (a) The chemical structure of the used organic materials and (b) energy level diagram of
the tested red PhOLEDs.

ACCEPTED MANUSCRIPT
(b)
(a)
0.9
CIE1931
NTSC
white
PYD-2Cz
TCTA
CBP
H1
H2
H3
@1000 cd/m2
PYD-2Cz
TCTA
CBP
1.0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.8
0.6
0.4
0.2
0.0
H1
H2
H3
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
400
500
600
700
800
Wavelength (nm)
x
(d)
(c)
16
102
12
8
100
-2
10
PYD-2Cz
TCTA
CBP
H1
H2
H3
PYD-2Cz
TCTA
CBP
H1
H2
H3
-4
10
4
-6
10
0
0
200
400
600
0
2
4
6
8
10 12 14 16
2
CurrentDensity(mA/cm)
Voltage(V)
(f)
(e)
18
16
14
12
10
8
24
20
16
12
8
PYD-2Cz
TCTA
CBP
H1
H2
H3
PYD-2Cz
TCTA
CBP
H1
H2
H3
6
4
4
2
0
10
0
10
-1
-1
100
101
102
1032
104
105
100
101
102
1032
104
105
Luminance(cd/m)
Luminance(cd/m)
Fig. 6 (a) Normalized EL spectra at a luminance of 103 cd/m2, (b) CIE coordinates, (c) current
density-voltage (J-V) characteristics, (d) luminance-current density (L-J) characteristics, (e) external
quantum efficiency versus luminance, and (f) luminance efficiency versus luminance for Devices A
(TCTA), B (PYD-2Cz), C (CBP), D (H1), E (H2) and F (H3).
Figure 6 depicts the electroluminescence (EL) characteristics of the PhOLEDs, while the
corresponding numeric data are summarized in Table 2. The normalized EL spectra of all devices

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recorded at a luminance of 103 cd/m2 are shown in Fig. 6(a). All devices showed pure
Ir(MDQ)2acac phosphorescence, except for device A, which showed an additional TCTA residual
emission, indicating the lack of sufficient doping concentration of the emitter in the TCTA host.
Overall, the emission peaks could be clearly divided into two types. The emission peaks for devices
with TCTA, PYD-2Cz, and CBP hosts were located at around 600 nm, while devices with
fabricated host materials exhibited red-shifted peaks (~611 nm) as well as wider spectral profiles.
As indicated, the full width at half maximum (FWHM) of devices using commercially available
hosts is about 73 nm, while the devices with compounds H1, H2, and H3 possessed rather wider
FWHM values (i.e. 82~91 nm). Considering the doping concentration of Ir(MDQ)2acac, thus
molecule stacking could be excluded as the cause of the red-shifted spectra. Instead, the saturated
red emissions in devices D~F resulted from the environmental polarity of solid films, making these
fabricated compounds more appropriate for use as hosts in red-PhOLEDs with a saturated red
emission, thus reducing the difficulty of developing materials for use as red emitters. The CIE
coordinates of both the devices E and F were recorded at (0.62, 0.38).
The current-voltage (J-V) curves of the tested devices are shown in Fig. 6(b). Comparing the
order of the current density recorded at 6V, the sequence of the devices is as follows: TCTA> H3~
CBP> H2~ PYD-2Cz> H1. Not only does this result in different energy barriers between
HTL/EML and EML/ETL, but also the carrier mobility of the host materials influences device
current density. Based on the experimental results, the much lower current density obtained in
device D might result from the 9-hexylcarbazole structure hindering the carrier transport [30].
Compounds H2 and H3 both possess similar energy levels. Device E with the H2 host gave a
higher current density than device D (H1), indicating that replacing the 9-hexylcarbazolyl fragment
with the triphenylamine moiety in H2 improves the carrier transport. Furthermore, based on the
molecular structure of compound H2, compound H3 has an additional imidazole moiety, giving it
an even higher carrier transport capability. Since the energy bandgaps of H2 and H3 are much

ACCEPTED MANUSCRIPT
lower than those of the other tested hosts, the respective turn on voltages of H2 and H3 are as low
as 2.8 V and 2.6 V. Moreover, the maximum luminance of device F reached 15000 cd/m2, which
exceeded that of device C with a bipolar CBP host (~13500 cd/m2).
From the efficiency curves shown in Figures 6(e) and (f), the inferior efficiency of device D
was attributed to the poor carrier transport ability of H1. A similar efficiency was obtained in
device A with the TCTA host, but for the opposite reason. Higher hole mobility of the TCTA host
resulted in the accumulation of redundant holes in the EML, leading to carrier imbalance. On the
other hand, the performance of devices E and F responded very favourably. Devices E and F
exhibited EL efficiencies peak of up to 15.9% (21.5 cd/A and 29.9 lm/W) and 12.4% (18.3 cd/A
and 23.9 lm/W), respectively. These results indicate that the gradually improved carrier transport
capabilities for H2 and H3 could result in devices easily achieving carrier balance in a simplified
tri-layer architecture. In addition, although the spectral overlap between Ir(MDQ)2acac and H2 (or
H3) is rather low as compared to that of the other hosts, there was no need to raise the doping
concentration to achieve effective exothermic energy transfer. In contrast, device C with the
commonly used bipolar CBP host only produced a peak external quantum efficiency (EQE) of
10.4%, which is much lower than that of devices E or F. Furthermore, the respective EQEs of
devices E and F recorded at 100 (and 1000) cd/m2 remained as high as 14.8% (12.7%) and 12.4%
(12.1%). Closer inspection of the roll-off behaviour of the tested devices showed the estimated
values of external quantum efficiency declined by half at current densities (J1/2) of 9.2, 19.4, 73.6,
36.8, 37.6 and 112.7 mA/cm2, respectively, for devices A, B, C, D, E and F, respectively [31]. The
mitigated efficiency roll-off behaviour observed in device F implies the existence of a diluted
exciton concentration in space without serious carrier accumulations at the HTL/EML and
33
EML/ETL interfaces. [32, ] The adequate energy levels of H3 enable abundant carrier injection
into the EML and thus enlarge the exciton formation zone, simultaneously decreasing the triplet-
triplet annihilation and polaron quenching.

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Table 2. EL characteristics of tested devices with different host materials.
Device
A
TCTA
5.3
B
PYD-2Cz
8.5
C
D
H1
E
F
Host
CBP
10.4
9.8
H2
H3
External
Quantum
[a]
[b]
5.5
15.9
14.8
12.7
21.5
21.2
18.1
29.9
12.3
7.0
12.4
12.4
12.1
18.3
18.2
17.8
23.9
13.2
9.0
3.6
6.6
4.7
Efficiency (%) [c]
Luminescence [a]
1.9
4.7
8.6
3.2
9.2
15.0
11.7
8.4
18.9
17.9
15.7
18.6
11.5
7.7
8.5
Efficiency
(cd/A)
[b]
[c]
[a]
[b]
[c]
[d]
6.4
7.3
3.3
4.9
Power
9.6
12.8
6.0
6.5
Efficiency
(lm/W)
Von (V)
5.1
2.4
1.8
3.2
1.1
3.0
4.1
3.4
4.8
2.8
2.6
J1/2 (mA/cm2)
Max. Luminance
(cd/m2) [Voltage]
λmax. (nm)
9.2
19.4
6879
[13.6]
599
73.6
13451
[11.2]
600
36.8
8914
[19.8]
609
37.6
10319
[15.0]
611
112.7
15207
[11.8]
613
2056
[8.6]
598
CIE1931
[b] (0.59, 0.40) (0.59, 0.41) (0.59, 0.41) (0.60, 0.39) (0.62, 0.38) (0.62, 0.38)
coordinates [c] (0.58, 0.40) (0.59, 0.41) (0.59, 0.41) (0.60, 0.40) (0.62, 0.38) (0.62, 0.38)
[a] Maximum efficiency; [b] recorded at 102 cd/m2; [c] measured at 103 cd/m2; [d] turn-on voltage
measured at 1 cd/m2.

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In summary, the synthesis and full characterization of three newly developed bipolar
phenanthro[9,10-d]imidazole based derivatives were reported. All the developed compounds and
three commercially available derivatives were used as host materials in bis(2-
methyldibenzo[f,h]quinoxaline) (acetylacetonate) iridium (III) based phosphorescent red organic
light emitting diodes. The results indicated that some devices with the newly-developed materials
exhibited superior performances to devices using commercial host materials. For comparison,
devices with commercial 4,4',4"-tris(carbazol-9-yl)triphenylamine and novel 2-[4-(N,N-
diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole as the host showed peak efficiencies
of 5.3% (9.2 cd/A and 9.6 lm/W) and 15.9% ( 21.5 cd/A and 29.9 lm/W), respectively. Efficiency
of device containing new host material is about 67% higher than those of device containing the
commercial host materials.
Acknowledgements
Prof. C.-H. Chang gratefully acknowledge financial support from the Ministry of Science and
Technology of Taiwan. (NSC 102-2221-E-155-080-MY3). The OLED materials were develop in
the frame of project funded by a grant No. APP-1/2016 from the Research Council of Lithuania. Dr.
D. Volyniuk is acknowledged for measurements of ionization potentials.
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Bipolar phenanthro[9,10-d]imidazole -based compounds have been synthesized and characterized.
The compounds are efficient host materials for phosphorescent OLEDs.
A red device with efficiency of 15.9% ( 21.5 cd/A and 29.9 lm/W) was demonstrated.