Structure-property relationship in high triplet energy host materials with a phenylcarbazole core and diphenylphosphine oxide substituent

Article abstract of DOI:10.1016/j.orgel.2011.03.015

A series of high triplet energy host materials with a carbazole core and a diphenylphosphine oxide substituent were synthesized and the effect of the substitution position on the photophysical properties and device performances of the host material was investigated. The substitution position of the diphenylphosphine oxide on the phenyl ring was changed and the substitution at ortho position of the phenyl ring induced the intramolecular charge transfer complex formation. The intramolecular charge transfer complex formation in the ortho substituted compound improved the current density. A maximum quantum efficiency of 20.4% was obtained in the phenylcarbazole host material with the phosphine oxide at para position of the phenyl group and the efficiency was degraded in the ortho substituted host.

Full text of DOI:10.1016/j.orgel.2011.03.015

Organic Electronics 12 (2011) 1025–1032
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Structure–property relationship in high triplet energy host materials with
a phenylcarbazole core and diphenylphosphine oxide substituent
⇑
Hyo Suk Son, Jun Yeob Lee
Department of Polymer Science and Engineering, Dankook University, Suji-gu, Jukjeon-dong, Yongin, Gyeonggi 448-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of high triplet energy host materials with a carbazole core and a diphenylphos-
phine oxide substituent were synthesized and the effect of the substitution position on
the photophysical properties and device performances of the host material was investi-
gated. The substitution position of the diphenylphosphine oxide on the phenyl ring was
changed and the substitution at ortho position of the phenyl ring induced the intramolec-
ular charge transfer complex formation. The intramolecular charge transfer complex for-
mation in the ortho substituted compound improved the current density. A maximum
quantum efficiency of 20.4% was obtained in the phenylcarbazole host material with the
phosphine oxide at para position of the phenyl group and the efficiency was degraded in
the ortho substituted host.
Received 10 September 2010
Received in revised form 14 March 2011
Accepted 14 March 2011
Available online 13 April 2011
Keywords:
Diphenylphosphine oxide
Phenylcarbazole
Deep blue phosphorescent organic
light-emitting diodes
Substitution position
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
the deep highest occupied molecular orbital (HOMO) level
of the silane based host material blocked the hole injec-
The development of deep blue phosphorescent organic
light-emitting diodes (PHOLEDs) is very important to im-
prove the power consumption of the organic light-emitting
diode (OLED) display due to the low quantum efficiency of
the current fluorescent OLED. The quantum efficiency can
be quadrupled using the PHOLED instead of the fluorescent
OLED and the power consumption of the OLED display can
be significantly reduced [1].
There are several ways to improve the quantum effi-
ciency of the deep blue PHOLED. One of the most efficient
approaches is to develop high triplet energy host materi-
als for balanced hole and electron injection in the emit-
ting layer [2–24]. Carbazole based host materials have
been widely used as the high triplet energy host material,
but the strong hole transport property of the carbazole
type host material limited the quantum efficiency of the
blue PHOLED [2,3]. Silane type host materials were also
applied as the host material in the blue PHOLED, but
tion from a hole transport layer to the emitting layer
[4,5]. Additionally, the poor thermal stability of the silane
based host material destabilized the morphology of the
evaporated host film. A silane modified carbazole host
was also synthesized as the high triplet energy host mate-
rial and it showed good electrochemical stability [6,7].
However, the host material had strong hole transport
property and it was difficult to balance holes and elec-
trons in the emitting layer. Phosphine oxide type host
materials have been developed as electron transport type
host materials and they were effective to improve the
quantum efficiency of the blue PHOLED in spite of strong
electron transport character of the phosphine oxide type
host [8–19]. The charge transport property of the phos-
phine oxide type host material could be improved by
combining the phosphine oxide group with the phenylc-
arbazole group [10,11]. The phenylcarbazole based host
material modified with two diphenylphosphine oxide
groups was effective as the host material for the deep
blue PHOLED and a high quantum efficiency over 18%
was reported in the deep blue PHOLED due to balanced
⇑
Corresponding author. Tel./fax: +82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.03.015

1026
H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
charge injection from the charge transport layer to the
emitting layer. However, more systematic study about
the relationship between the chemical structure of the
phenylcarbazole based phosphine oxide host material
and physical properties of the host material is required
to develop better high triplet host materials for deep blue
PHOLEDs.
In this work, the effect of the substitution position of
the phosphine oxide on the physical properties of the host
material and device performances of the deep blue
PHOLED were investigated. The diphenylphosphine oxide
was attached to the ortho, meta and para position of the
phenyl group in the phenylcarbazole unit. It was demon-
strated that the para substitution is better than ortho or
meta substitution in terms of quantum efficiency. A high
quantum efficiency of 20.4% was demonstrated from the
deep blue PHOLED with the phenylcarbazole based host
substituted at para position of the phenyl group.
2.3. Synthesis of 9-(3-bromophenyl)-9H-carbazole (3)
A mixtureof 9H-carbazole (4.0 g, 23.92 mmol), 1-bromo-
3-iodobenzene (8.08 g, 28.70 mmol), Cu-powder (3.04 g,
47.84 mmol), potassium carbonate (13.20 g, 95.68 mmol)
and dibenzo-18-crown-6 (0.63 g, 2.39 mmol) in dimethyl-
formamide (DMF, 40 ml) was purged with argon and heated
at reflux for 7 h. The mixture was quenched with H2O and
extracted with dichloromethane. The organic fractions were
dried with magnesium sulfate, filtered, and the solvent was
removed under reduced pressure. The residue was purified
by column chromatography on silica gel using hexane to
provided 5.67 g of title compound as white solid (m.p. 69–
70 °C; 71% yield).
1H NMR (200 MHz, CDCl3): (ppm) 8.15–8.12 (d,
J = 6.0 Hz, 1H), 7.74–7.68 (m, 2H), 7.58–7.41 (m, 7H),
7.34–7.30 (m, 2H).
2.4. Synthesis of 9-(2-bromophenyl)-9H-carbazole (5)
2. Experimental
A mixture of 9H-carbazole (3.0 g, 17.94 mmol), 1-bro-
mo-2-iodobenzene (10.15 g, 35.88 mmol), copper iodide
(2.39 g, 12.55 mmol) and potassium carbonate (4.95 g,
35.88 mmol) in xylene (40 ml) was purged with argon
and heated at reflux for 48 h. The mixture was quenched
with H2O and extracted with dichloromethane. The organic
fractions were dried with magnesium sulfate, filtered, and
the solvent was removed under reduced pressure. The res-
idue was purified by column chromatography on silica gel
using hexane to provide 2.5 g of title compound as white
solid (m.p. 93–94 °C; 56% yield).
2.1. Materials and measurements
9H-carbazole, n-butyllithium, chlorodiphenylphos-
phine, N-bromosuccinimide, and 4-bromobenzene (Aldrich
Chem. Co.) and hydrogen peroxide (Duksan Sci. Co.) were
used without further purification. Tetrahydrofuran (THF)
was distilled over sodium and calcium hydride. Detailed.
The 1H and 13C nuclear magnetic resonance (NMR)
spectra were obtained using a Varian 200 (200 MHz) spec-
trometer. The photoluminescence (PL) spectra were
recorded on a fluorescence spectrophotometer (HITACHI,
F-7000) and the ultraviolet–visible (UV–vis) spectra were
obtained using UV–vis spectrophotometer (Shimadzu,
UV-2501PC). Samples were dissolved in THF at a concen-
tration of 1.0 ꢀ 10ꢁ4 M. Hexane and methanol were also
used as the solvents for the PL measurement to confirm
solvatochromic effect. Low temperature PL spectra were
obtained at 77 K under liquid nitrogen. The differential
scanning calorimetry (DSC) measurements were carried
out using a Mettler DSC 822 at a heating rate of 10 °C/
min under nitrogen atmosphere. The mass spectrometry
(MS) was performed using a JEOL, JMS-AX505WA spec-
trometer in FAB (fast atom bombardment) mode. Cyclic
voltametry (CV) measurement was carried out in acetoni-
trile solution with tetrabutylammonium perchlorate at
0.1 M concentration. Organic materials were coated on in-
dium tin oxide substrate and were immersed in electrolyte
for analysis. Scan rate was 0.1 mV/s. Ferrocene was used as
the reference material for the CV analysis. Elemental anal-
ysis of the materials was carried out using EA1110 (CE
instrument). High performance liquid chromatography
(HPLC) analysis of the synthesized materials was carried
out using HPLC from Youngrin Instrument. A mixed eluent
of acetonitrile:methanol (90:10) was used for the analysis.
1H NMR (200 MHz, CDCl3): (ppm) 8.17–8.13 (d, J = 8.0
Hz, 1H), 7.90–7.68 (m, 1H), 7.57–7.29 (m, 8H), 7.08–7.02
(t, J = 6.0 Hz, 1H).
2.5. Synthesis of 3-bromo-9-(3-bromophenyl)-9H-carbazole
Compound 3 (2.0 g, 6.20 mmol) was dissolved in dimeth-
ylformamide (DMF, 20 ml) under argon atmosphere. The
reaction vessel was immersed in a ice bath until the temper-
ature was about ꢁ20 °C and then n-bromosuccinimide (NBS,
1.32 g, 7.44 mmol) in DMF (5 ml) was added dropwise
slowly into and stirred for 12 h. The mixture was allowed
to gradually warm to room temperature overnight and then
quenched with H2O followed by extraction with dichloro-
methane. The organic fractions were dried with magnesium
sulfate, filtered, and the solvent was removedunder reduced
pressure. The residue was purified by column chromatogra-
phy on silica gel using hexane to provided 1.8 g of title com-
pound as white solid (m.p. 187 °C; 80% yield).
1H NMR (200 MHz, CDCl3): (ppm) 8.45 (s, 1H), 8.25–
8.08 (t, J = 17.0 Hz, 3H), 7.86–7.66 (m, 3H), 7.55–7.31
(m, 5H).
2.6. Synthesis of 3-(diphenylphosphoryl)-9-(3-(diphenylphos-
phoryl)phenyl)-9H-carbazole (PPO25)
2.2. Synthesis
3-Bromo-9-(3-bromophenyl)-9H-carbazole
(4.73 g,
Detailed synthesis of the 3-(diphenylphosphoryl)-9-(4-
(diphenylphosphoryl)phenyl)-9-carbazole (PPO21) was
described in previous work [10].
11.79 mmol) was dissolved in tetrahydrofuran (THF,
50 ml) under argon atmosphere. The reaction vessel was
immersed in a dry ice/acetone bath until the temperature

H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
1027
was about ꢁ78 °C. To this solution 11.32 ml (28.30 mmol)
of 2.5 M n-BuLi solution in hexane was added dropwise
slowly and stirred for 3 h. Chlorodiphenylphosphine
(5.22 ml, 28.30 mmol) was then added to the solution
and stirred for 3 h in a dry ice/acetone bath. The mixture
was allowed to gradually warm to room temperature over-
night and then quenched with methanol, extracted with
dichloromethane. The organic fractions were dried with
magnesium sulfate, filtered, and the solvent was removed
under reduced pressure. The residue was purified by col-
umn chromatography on silica gel using hexane/dichloro-
methane (10:1) to provided 2.2 g of title compound as
white solid (32% yield).
The white solid was dissolved in dichloromethane
(40 ml) and hydrogen peroxide (8 ml) was stirred over-
night at room temperature. The organic layer was sepa-
rated and washed with dichloromethane and water. The
extract was evaporated to dryness affording a white solid,
which was further purified by column chromatography to
yield 2.2 g of chemically pure PPO25 (purity 99.0% by
HPLC).
of 2.5 M n-BuLi solution in hexane was added dropwise
slowly and stirred for 3 h. Chlorodiphenylphosphine
(7.18 ml, 38.89 mmol) was then added to the solution
and stirred for 3 h in a dry ice/acetone bath. The mixture
was allowed to gradually warm to room temperature over-
night and then quenched with methanol followed by
extraction with dichloromethane. The organic fractions
were dried with magnesium sulfate, filtered, and the sol-
vent was removed under reduced pressure. The residue
was purified by column chromatography on silica gel using
hexane/dichloromethane (10:1) to provided 2.6 g of title
compound as white solid (33% yield).
The white solid was dissolved in dichloromethane
(40 ml) and hydrogen peroxide (8 ml) was stirred over-
night at room temperature. The organic layer was sepa-
rated and washed with dichloromethane and water. The
extract was evaporated to dryness affording a white solid,
which was further purified by column chromatography to
yield 2.8 g of chemically pure PPO26 (purity 99.1% by
HPLC).
Tg: 110 °C, 1H NMR (200 MHz, CDCl3): (ppm) 8.33–8.27
(d, J = 12.0 Hz, 1H), 7.96–7.64 (m, 7H), 7.58–7.43 (m, 6H),
7.39–7.29 (m, 8H), 7.18–7.09 (t, J = 9.0 Hz, 3H), 7.01–6.94
(m, 6H) 13C NMR (200 MHz, CDCl3): (ppm) 143.7, 142.6,
139.6, 136.4, 135.6, 134.6, 133.6, 132.9, 132.5, 132.1,
131.8, 130.4, 129.9, 129.2, 128.9, 128.1, 127.9, 127.5,
126.8, 123.0, 122.7, 121.2, 120.8, 120.5, 120.2, 119.6,
111.8, 110.8 MS (FAB) m/z 644 [(M + H)+] Anal. Calcd. for
Tg: 110 °C, 1H NMR (200 MHz, CDCl3): (ppm) 8.55–8.49
(d, J = 12.0 Hz, 1H), 8.08–8.05 (d, J = 6.0 Hz, 1H), 7.88–7.69
(m, 12H), 7.61–7.50 (m, 14H), 7.37–7.30 (t, J = 7.0 Hz, 3H)
13C NMR (200 MHz, CDCl3): (ppm) 142.2, 140.8, 137.5,
137.1, 136.3, 134.3, 134.0, 132.6, 132.0, 131.4, 130.8,
130.9, 129.2, 129.0, 128.9, 128.2, 128.0, 127.9, 127.4,
126.1, 124.9, 123.9, 123.6, 123.0, 121.9, 121.4, 120.8,
120.2, 110.3, 109.2 MS (FAB) m/z 644 [(M + H)+] Anal.
Calcd. for C42H31NO2P2: C, 78.37; H, 4.85; N, 2.18; O,
4.97. Found: C, 78.12; H, 4.81; N, 2.00; O, 5.04.
C42H31NO2P2: C, 78.37; H, 4.85; N, 2.18; O, 4.97. Found:
C, 78.21: H, 4.84; N, 2.18; O, 4.99.
2.9. Device fabrication
2.7. Synthesis of 3-bromo-9-(2-bromophenyl)-9H-carbazole
A
basic device configuration of indium tin
Compound 5 (5.0 g, 15.51 mmol) was dissolved in
dimethylformamide (DMF, 30 ml) under argon atmo-
sphere. The reaction vessel was immersed in a ice bath
until the temperature was about ꢁ20 °C and then n-bromo-
succinimide (NBS, 3.0 g, 17.07 mmol) in DMF (15 ml) was
added dropwise slowly into and stirred for 12 h. The mix-
ture was allowed to gradually warm to room temperature
overnight and then quenched with H2O followed by extrac-
tion with dichloromethane. The organic fractions were
dried with magnesium sulfate, filtered, and the solvent
was removed under reduced pressure. The residue was
purified by column chromatography on silica gel using hex-
ane to provided 5.54 g of title compound as white solid
(m.p. 190–194 °C; 89% yield).
oxide(ITO,
150 nm)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-
m-tolyl-amino)-phenyl]-biphenyl-4,40-diamine (DNTPD,
60 nm)/N,N0-di(1-naphthyl)-N,N0-diphenylbenzidine (NPB,
5 nm)/N,N0-dicarbazolyl-3,5-benzene (mCP, 10 nm)/PPO21
or PPO25 or PPO26: tris((3,5-difluoro-4-cyanophenyl)pyri-
dine) iridium (FCNIr) (30 nm, 15%)/diphenylphosphine
oxide-4-(triphenylsilyl)phenyl (TSPO1, 15 nm)/LiF (1 nm)/
Al (200 nm) was used for the device fabrication. Hole only
and electron only devices had device configurations of ITO/
DNTPD (60 nm)/NPB (5 nm)/mCP (10 nm)/host (30 nm)/Au
and ITO/TSPO1 (10 nm)/host (30 nm)/TSPO1 (25 nm)/LiF
(1 nm)/Al (100 nm), respectively. The evaporation rate of
the organic materials was 0.1 nm/s and the deposition rate
of the dopant was controlled according to the doping con-
centration of the dopant. Al was deposited at a deposition
rate of 0.5 nm/s, while LiF deposition rate was 0.01 nm/s.
The devices were encapsulated with a CaO getter and a glass
rid after the device fabrication. The device performances of
thebluePHOLEDsweremeasuredwithKeithley2400source
measurement unit and CS1000 spectroradiometer.
1H NMR (200 MHz, CDCl3): (ppm) 8.26–8.22 (d,
J = 8.0 Hz, 1H), 8.12–8.08 (d, J = 8.0 Hz, 1H), 7.88–7.68 (m,
2H), 7.55–7.30 (m, 5H), 7.07–7.01 (t, J = 6.0 Hz, 1H), 6.96–
6.99 (t, J = 3 Hz, 1H).
2.8. Synthesis of 3-(diphenylphosphoryl)-9-(2-(diphenylphos-
phoryl)phenyl)-9H-carbazole (PPO26)
3-Bromo-9-(2-bromophenyl)-9H-carbazole
(5.20 g,
3. Results and discussion
12.96 mmol) was dissolved in tetrahydrofuran (THF,
50 ml) under argon atmosphere. The reaction vessel was
immersed in a dry ice/acetone bath until the temperature
was about ꢁ78 °C. To this solution 15.55 ml (38.89 mmol)
Three host materials based on a phenylcarbazole core
and a diphenylphosphine oxide substituent were synthe-
sized. Three host materials had two diphenylphosphine

1028
H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
oxide groups. One diphenylphosphine oxide group was at-
tached to the phenyl unit of the phenylcarbazole, while the
other diphenylphosphine oxide group was attached to the
3 position of the carbazole unit. The position of the diphen-
ylphosphine oxide group attached to the phenyl unit was
changed to study the effect of substitution position on
the physical properties of the host material. All materials
were synthesized by the reaction of the chlorodiphenyl-
phosphine with brominated N-phenylcarbazole moiety fol-
lowed by oxidation with hydrogen peroxide. Synthetic
scheme of all materials is shown in Scheme 1. All materials
were purified by column chromatography and a high pur-
ity level over 99% was obtained in all materials.
The energy level of the host material is critical to device
performances of the PHOLED and is closely related with
the molecular orbital distribution. Therefore, density func-
tional theory (DFT) calculation of the host materials was
carried out using a suite of Gaussian 03 program. The non-
local density functional of Becke’s 3-parameters employing
Lee–Yang-Parr functional (B3LYP) with 6–31G⁄ basis sets
was used for the calculation [25]. Fig. 1 shows the HOMO
and the lowest unoccupied molecular orbital (LUMO) orbi-
tal distribution of host materials synthesized in this work.
The HOMO was mostly distributed over the carbazole unit
of the phenylcarbazole core, while the LUMO was dis-
persed over the phenyl unit of the phenylcarbazole core
and diphenylphosphine oxide in all host materials [15].
The carbazole unit is an electron donating group and the
HOMO was localized in the carbazole group. The diphenyl-
phosphine oxide is a strong electron withdrawing group
and the LUMO was dispersed over the phenyl and diphen-
ylphosphine oxide groups. There was little difference of the
molecular orbital distribution between the three materials.
The calculated HOMO levels of the PPO21, PPO25 and
HOMO
LUMO
PPO21
1.12 eV
5.56 eV
PPO25
PPO26
5.66eV
5.74eV
1.17eV
1.07eV
Fig. 1. Molecular simulation results of PPO21, PPO25 and PPO26. HOMO
and LUMO orbital distribution is shown in this figure.
PPO26 were 5.56, 5.66 and 5.74 eV, while the LUMO levels
were 1.12, 1.17 and 1.07 eV. Although the molecular orbi-
tal distribution of the three host materials was similar, the
HOMO level was increased in the PPO26 with the diphen-
ylphosphine oxide at ortho position. This indicates the
O
Br
P
P
N
Br
Cl
P
I
Br
NBS
DMF
N
N
N
H O / MC
Cu-powder
DMF
n-BuLi / THF
2
2
Br
P
P
O
PPO21
O
I
Cl
P
Br
P
Br
P
H
N
NBS
DMF
Br
N
N
N
N
H O / MC
n-BuLi / THF
2
2
Cu-powder
DMF
Br
P
P
O
PPO25
I
Cl
P
Br
O
P
Br
Br
P
P
NBS
DMF
N
N
N
N
Cu-powder
DMF
H O / MC
2
2
n-BuLi / THF
Br
P
O
PPO26
Scheme 1. Synthetic scheme of PPO21, PPO25 and PPO26.

H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
1029
reduction of the electron density in the carbazole group of
the PPO26.
Photophysical properties of the host materials were
analyzed with ultraviolet–visible (UV–vis) and photolumi-
nescence (PL) measurements. UV–vis and PL spectra of the
host materials are shown in Fig. 2. UV–vis absorption of the
three host materials was similar and there was little shift
of the UV–vis absorption peak. The main absorption of
PPO21
PPO25
PPO26
FCNIr
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
the three host materials is
pꢁp transition of the phenylcar-
bazole core at 338 nm, resulting in similar absorption spec-
tra. However, the PL spectra were different depending on
the substitution position of the diphenylphosphine oxide.
In particular, the PL spectrum of the PPO26 with the
diphenylphosphine oxide at ortho position of the phenyl
group was red-shifted and broadened compared with that
of other host materials. The peak maximum of the PPO26
was 375 nm compared with 361 nm of the PPO25. The
electron is partially transferred from the carbazole unit
to the phosphine oxide unit, resulting in intramolecular
charge transfer complex formation. The bandgap is re-
duced by the charge transfer complex formation, and the
PL emission is red-shifted and broadened.
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 3. Solid PL spectra of PPO21, PPO25 and PPO26. UV–vis spectrum of
the FCNIr dopant was also added.
with charge transfer complex formation [26]. Fig. 4 shows
the solution PL spectra of PPO25 and PPO26 at different
solvents. Hexane, THF and methanol (MeOH) were used
as the solvents. In the case of PPO25 with little intramolec-
ular charge transfer complex formation, the PL emission
was shifted only by 3–5 nm in highly polar MeOH solvent.
However, the PL emission of PPO26 was red-shifted by
more than 40 nm in highly polar solvent, proving the intra-
molecular charge transfer complex formation. Therefore,
the solid PL emission of the PPO26 at long wavelength is
originated from intramolecular charge transfer complex
formation. The PPO26 is surrounded by higly polar PPO26
molecule and red-shifted PL emission of PPO26 by intra-
molecular charge transfer complex formation is observed.
The HOMO levels of three host materials were mea-
sured from the oxidation potential of the cyclic voltametry
and the bandgap from UV–vis spectra. Energy levels of the
host materials are summarized in Table 1. There was little
difference of the HOMO and LUMO levels between the host
materials. The HOMO was dispersed over the phenylcar-
bazole core in all host materials and similar HOMO levels
were obtained. The LUMO was also distributed over the
phenyl group substituted with diphenylphosphine oxide
in all materials, resulting in similar LUMO levels. This can
be further understood by analyzing the HOMO and LUMO
distribution of 3-(diphenylphosphoryl)-9-phenyl-9H-car-
bazole (PPO1) without the diphenylphosphine oxide unit
attached to the phenyl unit of the 9-phenylcarbazole
[11]. The HOMO (6.16 eV) and LUMO (2.60 eV) of the
PPO1 were localized in the carbazole unit with little dis-
persion over the phenyl unit. This indicates that the HOMO
and LUMO of PPO1 are not greatly affected by substitution
of electron withdrawing or donating moiety to the phenyl
unit (less than 0.1 eV). Therefore, the substitution of the
diphenylphosphine oxide group to the phenyl unit did
not significantly change the HOMO and LUMO levels of
host materials. In our previous work, the substitution of
the diphenylphosphine oxide at the carbazole unit change
the energy level by about 0.3 eV [11]. Therefore, the substi-
tution of the diphenylphosphine oxide at the phenyl unit
showed less significant effect on the energy level than
the substitution of the diphenylphosphine oxide at the car-
bazole unit. Although the calculated HOMO level from
Solid PL spectra of the evaporated host materials are
shown in Fig. 3. The solid PL spectra of the host materials
were red-shifted and the full width at half maximum was
also increased due to intermolecular interaction in solid
state. In particular, the PPO26 with a phosphine oxide at
ortho position showed two PL emission peaks at 367 and
450 nm. The PL emission peak at 367 nm is assigned to
the PL emission of the phenylcarbazole core, while the PL
emission at 450 nm is assigned to the emission of the
intramolecular charge transfer complex. The intramolecu-
lar charge transfer emission was strong in the solid state
by the solvent effect of polar PPO26 molecules. The UV–
vis absorption spectrum of the dopant material is also
shown in Fig. 3. The PL emission of host materials was well
overlapped with the UV–vis absorption of the FCNIr dop-
ant, indicating efficient energy transfer from the host to
dopant in solid state.
The intramolecular charge complex formation was con-
firmed from solution PL of host material using various sol-
vents with different polarity. It is well known that strong
solvatochromic effect is observed in the organic material
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
PPO21
PPO25
PPO26
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. UV–vis and solution PL spectra of PPO21, PPO25 and PPO26.

1030
H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
(b)
(a)
Hexane
THF
MeOH
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Hexane
THF
MeOH
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 4. Solution PL spectra of PPO25 (a) and PPO26 and (b) according to the polarity of solvent.
Table 1
Physical properties of PPO21, PPO25 and PPO26.
Optical analysis
Electrical analysis
LUMO (eV)
UV absorption (nm)
PL solution (nm)
PL solid (nm)
HOMO (eV)
Bandgap (eV)
ET (eV)
PPO21
PPO25
PPO26
337
338
335
362
364
375
366
367
367
450
6.25
6.25
6.25
2.68
2.70
2.67
3.57
3.55
3.58
3.01
2.99
3.01
molecular simulation was shifted in the three host materi-
als, experimental HOMO levels were the same in the three
organic materials.
Deep blue PHOLEDs were fabricated using three host
materials doped with deep blue emitting FCNIr dopant.
Fig.
5 shows the current density–voltage–luminance
The triplet energy of the host materials was measured
from low temperature PL emission peak and the triplet en-
ergy was 2.99–3.01 eV. All host materials showed a high
triplet energy about 3.00 eV irrespective of the substitution
position of the diphenylphosphine oxide. The triplet en-
ergy of the phenylcarbazole core was 3.01 eV and the
diphenylphosphine oxide did not change the triplet energy
of the phenylcarbazole core. As reported earlier, the
diphenylphosphine oxide unit does not extend the conju-
gation of the core structure and did not reduce the triplet
energy of the core [8]. Therefore, all host materials synthe-
sized in this work can be effective as host materials for
deep blue phosphorescent device.
curves of the deep blue PHOLEDs doped with FCNIr at a
doping concentration of 20%. The current density was high
in the PPO26 with the diphenylphosphine oxide at ortho
position, while it was low in the PPO21 with the diphenyl-
phosphine oxide at para position. The current density gen-
erally depends on the hole and electron density in the
device. Therefore, hole and electron current density of
three host materials was compared.
Current density–voltage curves of hole only and elec-
tron only devices of three host materials are shown in
Fig. 6. The PPO26 showed the highest hole current density
and lowest electron current density. The hole and electron
current density depends on the energy barrier for charge
(a)
10000
100
1
(b)
1000
10
0.1
0.001
PPO21
PPO21
PPO25
PPO26
PPO25
0.00001
PPO26
0.0000001
0
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
Fig. 5. Current density–voltage (a) and luminance–voltage and (b) curves of the deep blue PHOLEDs with the PPO21, PPO25 and PPO26.

H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
1031
0.5
120
100
80
60
40
20
0
(b)
(a)
PPO21
PPO25
PPO26
PPO21
0.4
PPO25
PPO26
0.3
0.2
0.1
0
0
2
4
6
8
10
0
1
2
3
4
5
Electric field (V/nm)
Electric field (V/nm)
Fig. 6. Current density–electric field curves of the hole only (a) and electron only and (b) devices with PPO21, PPO25 and PPO26.
the low electron current density. The high hole and elec-
tron current density of the PPO25 compared with PPO21
may be due to small molecular size of the PPO25 for good
hole and electron hopping. The HOMO and LUMO of the
host materials were localized in the molecular structure
and large molecule size hinders the charge transport
through hopping.
Considering the hole and electron current density of the
device, the high current density of the PPO26 device is due
to high hole current density [28]. Both hole and electron
current density of the PPO21 was lower than that of the
PPO25, leading to low current density in the PPO21 device.
The luminance followed the same relationship as the cur-
rent density.
25
20
15
10
5
PPO21
PPO25
PPO26
0
0.1
1
10
100
2
1000
10000
Luminance (cd/m )
The quantum efficiency–luminance curves of the deep
blue PHOLEDs are shown in Fig. 7. The PPO21 showed
the highest maximum quantum efficiency of 20.4%, while
the PPO25 and PPO26 showed the same maximum quan-
tum efficiency of 17.1%. The maximum quantum efficiency
of the PPO21 device is one of the best quantum efficiency
value reported in the literature. Comparing the quantum
efficiency at 1000 cd/m2, the PPO21 and PPO25 showed
better quantum efficiency than PPO26. The quantum effi-
ciency is generally affected by the charge balance in the
emitting layer. As shown in the hole only and electron only
device data, the PPO26 showed the highest hole current
density and lowest electron current density. Therefore,
the charge balance is worse than other host materials,
which decreased the quantum efficiency of the PPO26 de-
vice. PPO25 was better than PPO21 in terms of efficiency
roll-off. Balanced charge injection and balanced hole/elec-
tron density at low and high voltage improved the quan-
tum efficiency at high luminance. Device data of the host
materials are summarized in Table 2.
Fig. 7. Quantum efficiency–luminance curves of the deep blue PHOLEDs
with PPO21, PPO25 and PPO26 hosts.
injection and charge transport properties. Considering the
HOMO level for hole injection, there should be no differ-
ence of the hole current density. Therefore, the increased
hole current density of the PPO26 is due to improved hole
transport properties of the PPO26. The origin for the im-
proved hole transport properties of the PPO26 can be
found in the intramolecular charge transfer complex for-
mation in the PPO26. The carbazole unit donates electrons
to the partially positive P atom in the phosphine oxide unit.
This can give rise to weak p type doping effect in the car-
bazole unit, enhancing the hole transport properties of
the PPO26. Similar increase of the hole current density by
the weak p-type doping effect was reported in other work
[27]. On the contrary, the electron donating effect of the
carbazole to the P atom in the phosphine oxide has nega-
tive effect on the electron transport properties, leading to
Table 2
Device performances of PPO21, PPO25 and PPO26 blue devices.
Turn on
voltage (V)
Driving
Maximum quantum
efficiency (%)
Quantum
Maximum power
efficiency (lm/W)
Power efficiency
(lm/W)a
Color
voltage (V)a
efficiency (%)a
coordinatea
PPO21 3.5
PPO25 3.5
PPO26 3.5
8.2
7.6
7.2
20.4 1.7
17.1 1.3
17.1 1.2
11.9 1.1
12.3 1.4
10.4 1.3
26.1 2.4
21.2 1.9
20.6 2.1
6.6 0.9
7.3 1.0
6.4 0.8
(0.14, 0.19)
(0.15, 0.19)
(0.15, 0.18)
a
Data were measured at 1000 cd/m2.

1032
H.S. Son, J.Y. Lee / Organic Electronics 12 (2011) 1025–1032
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400
500
600
700
800
Wavelength (nm)
Fig. 8. Electroluminescence spectra of the deep blue PHOLEDs with the
PPO21, PPO25 and PPO26 hosts.
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complex formation, while meta or para substitution did
not significantly change the physical properties of the core
structure. The ortho substitution enhanced the current
density through intramolecular charge transfer, while the
meta and para substitution enhanced the quantum effi-
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