Correlation of the substitution position of diphenylphosphine oxide on phenylcarbazole and device performances of blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c0jm03427d

Two diphenylphosphine oxide modified phenylcarbazole host materials with substituents at 2,7 (PPO27) and 3,6 (PPO36) positions were synthesized and their device performances were correlated with the chemical structure of the host materials. Low driving voltage was obtained in the PPO27 host with two diphenylphosphine oxides at 2,7 positions due to reduced band gap. The PPO27 showed better performances in sky blue phosphorescent organic light-emitting diodes, while the PPO36 showed better performances in deep blue phosphorescent organic light-emitting diodes. A high quantum efficiency of 23.9% and power efficiency of 37.5 lm W^-1 were achieved in the blue device with PPO27 host. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0jm03427d

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PAPER
Correlation of the substitution position of diphenylphosphine oxide on
phenylcarbazole and device performances of blue phosphorescent organic
light-emitting diodes†
*
Hyo Suk Son, Chang Woo Seo and Jun Yeob Lee
Received 12th October 2010, Accepted 7th February 2011
DOI: 10.1039/c0jm03427d
Two diphenylphosphine oxide modified phenylcarbazole host materials with substituents at 2,7
(PPO27) and 3,6 (PPO36) positions were synthesized and their device performances were correlated
with the chemical structure of the host materials. Low driving voltage was obtained in the PPO27 host
with two diphenylphosphine oxides at 2,7 positions due to reduced band gap. The PPO27 showed
better performances in sky blue phosphorescent organic light-emitting diodes, while the PPO36 showed
better performances in deep blue phosphorescent organic light-emitting diodes. A high quantum
efficiency of 23.9% and power efficiency of 37.5 lm Wꢀ1 were achieved in the blue device with PPO27
host.
in sky blue or deep blue PHOLEDs.19–23 In particular, the
Introduction
phenylcarbazole compounds with two diphenylphosphine oxide
groups showed high quantum efficiency in deep blue PHO-
LEDs.20
The development of blue phosphorescent organic light-emitting
diodes (PHOLEDs) is important because the power consump-
tion of full color organic light emitting diodes is dominated by
the quantum efficiency of the blue device.1 Therefore, there have
been many studies to improve the device performances of blue
PHOLEDs by synthesizing new host and dopant materials.2–16 In
particular, many host materials were synthesized and their device
performances were investigated.8–30
The structure–property relationship of phenylcarbazole based
compounds was theoretically studied in detail.24 However, there
was no experimental study about the effect of diphenylphosphine
oxide position on the physical properties and device perfor-
mances of phenylcarbazole compounds. Therefore, a more
systematic study about the effect of substitution position of the
diphenylphosphine oxide is required.
One of the best host materials to fabricate high efficiency blue
PHOLEDs is a phosphine oxide type host material.13–25 In
general, a diphenylphosphine oxide unit was attached to the
electron transport or hole transport type core. Fluorene,13,14
spirobifluorene,15,16 dibenzothiophene,17 dibenzofuran18 and
phenylcarbazole19–29 were representative core structures modified
with the diphenylphosphine oxide. The diphenylphosphine oxide
improved the electron transport properties of the host materials
while keeping the triplet energy of the core structure. Therefore,
bipolar type host materials could be developed by combining the
hole transport type phenylcarbazole core with the diphenyl-
phosphine oxide.19–29 Several phenylcarbazole based phosphine
oxide host materials were synthesized and their device perfor-
mances were investigated. Phenylcarbazole compounds modified
with one diphenylphosphine oxide or two diphenylphosphine
oxide groups were reported and showed high quantum efficiency
In this work, a phenylcarbazole type host material with two
diphenylphosphine oxide groups at 2,7 positions of carbazole
(PPO27) was synthesized and it was compared with a phenyl-
carbazole host modified with two diphenylphosphine oxide units
at 3,6 positions (PPO36). It was demonstrated that the PPO27
reduced the driving voltage and improved the quantum efficiency
of blue PHOLEDs due to the reduced highest occupied mole-
cular orbital (HOMO)–lowest unoccupied molecular orbital
(LUMO) gap and charge balance. A high quantum efficiency of
23.9% and power efficiency of 37.5 lm Wꢀ1 were reported using
the PPO27.
Experimental section
Synthesis
Department of Polymer Science and Engineering, Dankook University,
126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Republic of
Korea. E-mail: leej17@dankook.ac.kr; Fax: +82 31 8005 3585; Tel: +82
31 8005 3585
The synthetic route of the PPO27 is shown in Scheme 1. The 2,7-
dibromo-9H-carbazole was synthesized according to the method
reported earlier.31 The description of PPO36 is presented in our
previous work.19
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0jm03427d
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128.93, 128.15, 127.96, 127.47, 126.50, 125.14, 123.97, 123.78,
123.00, 122.81, 121.64, 121.35, 120.48, 120.19, 115.52, 114.36,
114.16. Anal. calcd. for C42H31NO2P2: C, 78.37; H, 4.85; N, 2.18.
Found: C, 78.39; H, 4.89; N, 2.12.
Device preparation and measurements
A basic device configuration of indium tin oxide (ITO)/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) (20 nm)/N,N0-dicarbazolyl-3,5-
benzene (mCP) (10 nm)/emitting layer/diphenylphosphine oxide-
4-(triphenylsilyl)phenyl (TSPO1, 25 nm)/LiF/Al was used to
evaluate the device performances of the blue PHOLEDs with the
PPO27 and PPO36 host materials. Four different emitting layers
were used to test the PPO27 and PPO36 host materials. Two blue
emitting layers with PPO27 : iridium(III)bis(4,6-(di-fluo-
Scheme 1 Synthetic scheme of PPO27.
Synthesis of 2,7-dibromo-9-phenyl-9H-carbazole (1)
A mixture of iodobenzene (2.1 g, 10.2 mmol), K2CO3 (1.42 g,
10.3 mmol) and ethylenediamine (0.17 ml, 2.5 mmol) in 15 ml of
1,4-dioxane was refluxed for 12 h after adding 2,7-dibromo-
carbazole in 10 ml of 1,4-dioxane drop wise. The mixture was
quenched with H2O and extracted with dichloromethane. The
organic fractions were dried over MgSO4, filtered, and the
solvent was removed under reduced pressure. The residue was
purified by column chromatography on silica gel using hexane as
the eluent to yield 1 (1.27 g, 60%) as a white solid (mp 182–
183 ꢁC).
rophenyl)-pyridinato-N,C0)picolinate
(FIrpic)
and
PPO36 : FIrpic were tested as the emitting layers in sky blue
PHOLEDs, while two blue emitting layers with PPO27 : t-
ris((3,5-difluoro-4-cyanophenyl)pyridine)iridium (FCNIr) and
PPO36 : FCNIr were evaluated as the emitting layers in deep
blue PHOLEDs. The thickness of the emitting layers was 30 nm
and the doping concentration was 10%. Electron only devices
with a device structure of ITO/TSPO1 (5 nm)/PPO27 or PPO36
(30 nm)/TSPO1 (25 nm)/LiF/Al and hole only devices with
a device structure of ITO/DNTPD (60 nm)/NPB (20 nm)/mCP
(10 nm)/PPO27 or PPO36 (30 nm)/DNTPD (5 nm)/Al were
prepared to compare electron and hole transport properties of
PPO27 and PPO36. All organic materials except dopant
1H NMR (200 MHz, CDCl3): (ppm) 7.97–7.93 (d, 3H), 7.68–
7.48 (m, 5H), 7.42–7.38 (d, 3H).
Synthesis of 2,7-bis(diphenylphosphino)-9-phenyl-9H-carbazole
(2)
Compound 1 (0.5 g, 1.2 mmol) was dissolved in 12 ml of tetra-
hydrofuran under argon atmosphere. The reaction vessel was
immersed in a dry ice/acetone bath until the temperature went
down to ꢀ78 ꢁC. A 2.5 M n-BuLi solution (1.19 ml, 2.9 mmol) in
hexane was added to the solution drop wise slowly and stirred for
3 h. Chlorodiphenylphosphine (0.55 ml, 2.9 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 tempera-
ture overnight and then quenched with methanol. The mixture
was extracted with dichloromethane. The organic fractions were
dried over MgSO4, filtered, and the solvent was removed under
reduced pressure. The residue was purified by column chroma-
tography on silica gel using hexane/dichloromethane (9 : 1) as
the eluent to yield 2 (0.36 g, 50%) as a white solid. It was not
purified and was used as the crude product for oxidation.
ꢀ1
˚
materials were deposited at 1 A s evaporation rate. LiF was
ꢀ1
˚
deposited at a rate of 0.1 A s and Al was evaporated at
a deposition rate of 5 A sꢀ1. Devices were encapsulated with CaO
˚
getter and were sealed with a glass rid after device fabrication.
Energy levels of PPO27 were measured with cyclic voltam-
metry and current density–voltage–luminance characteristics of
the PHOLEDs were measured with a Keithley 2400 source
measurement unit and CS1000 spectroradiometer. Detailed
characterization method is described in the ESI†.
Results and discussion
We reported phenylcarbazole host materials modified with
diphenylphosphine oxide in our previous work.19,20 The aim of
our previous work was to synthesize high triplet energy host
materials with improved charge balance. The diphenylphosphine
oxide unit was attached to the carbazole or phenyl unit of the
phenylcarbazole. The substitution of the diphenylphosphine
oxide group on the phenyl group was effective to improve the
device performances of blue PHOLEDs. In addition to the
substitution position of the diphenylphosphine oxide at
the phenyl or carbazole units, the substitution position of the
diphenylphosphine oxide on the carbazole may also affect the
physical properties of host materials and device performances of
blue PHOLEDs. Therefore, two different host materials with two
diphenylphosphine oxide groups at 2,7 and 3,6 positions of
carbazole were synthesized.
Synthesis of 2,7-bis(diphenylphosphoryl)-9-phenyl-9H-carbazole
(PPO27)
Compound 2 was dissolved in a mixed solution of 5 ml
dichloromethane and 1 ml of hydrogen peroxide and the solution
was stirred overnight at room temperature. The mixture was
extracted with dichloromethane after the reaction. The organic
fractions were dried with MgSO4, filtered, and the solvent was
removed under reduced pressure. A white power (0.4 g) was
ꢁ
obtained after drying. Purity: 99.4% (HPLC) mp 310–311 C.
1H NMR (200 MHz, CDCl3): (ppm) 8.22–8.18 (d, 10H), 8.03–
7.96 (d, 10H), 7.71–7.61 (t, 6H), 7.53–7.40 (m, 5H). MS (FAB)
m/z 644 [(M + H)+] 13C-NMR (200 MHz, CDCl3): d 141.46,
141.16, 136.02, 133.69, 132.52, 131.65, 130.77, 129.61, 129.22,
Two phosphine oxide modified phenylcarbazole compounds,
PPO27 and PPO36, were synthesized by the phosphorylation of
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the corresponding dibrominated phenylcarbazole intermediates.
The 2,7-dibromo-9-phenylcarbazole intermediate was synthe-
sized from 2,7-dibromo-9H-carbazole which was reported in
other work31,32 and the synthesis of PPO36 was reported in our
previous work.19 Various synthetic methods were reviewed in
previous works and we followed the synthetic procedure pre-
sented in the work.32 Two materials were synthesized at a high
purity over 99% after purification by column chromatography.
The substitution of the diphenylphosphine oxide at 2,7 and 3,6
positions has different effects on the HOMO/LUMO levels of the
host materials because the molecular orbital distribution is
different depending on the substitution position.24 HOMO and
LUMO distribution of PPO27 and PPO36 is shown in Fig. S1,
(ESI†). The substitution at 2 and 7 positions does not have any
great effect on the HOMO level, while the substitution at 3 and 6
positions affects the HOMO level of phenylcarbazole. The
substitution of diphenylphosphine oxide at 2,7 positions may
greatly change the LUMO level, while the substitution at 3,6
positions may have little effect on the LUMO level. Considering
the HOMO and LUMO distributions of the phenylcarbazole, the
HOMO is lowered in PPO36, while the LUMO is lowered in
PPO27 by the substitution of electron withdrawing diphenyl-
phosphine oxide group. As expected from the molecular orbital
distribution, the simulated HOMO level of PPO36 (5.73 eV) was
deeper than that of the PPO27 (5.64 eV), while the LUMO level
of the PPO27 (1.36 eV) was deeper than that of the PPO36 (1.07
eV). Therefore, the HOMO–LUMO energy gap of the PPO27
(4.27 eV) was reduced compared with that of the PPO36 (4.66
eV). This result is consistent with the calculated data reported by
Bredas’s group.24
Fig. 1 UV-Vis, solution PL, solid PL and low temperature PL spectra of
PPO27 and PPO36.
PPO27 and PPO36 can also be calculated from the emission
peak of low temperature PL spectra and it was 2.81 eV and 3.01
eV, respectively. The reduced triplet energy of the PPO27 is due
to the reduction of the HOMO–LUMO gap. Although the triplet
energy of the PPO27 was lowered by substitution at 2,7 posi-
tions, the triplet energy of the PPO27 was higher than those of
blue emitting dopants such as FCNIr and FIrpic. The triplet
energies of the FCNIr and FIrpic were 2.75 eV and 2.65 eV,
respectively.
UV-Vis absorption of blue dopants and PL emission of blue
host materials were plotted in Fig. 2 to confirm the energy
transfer from host materials to dopant materials. The FCNIr
showed absorption below 430 nm, which was exactly overlapped
with the singlet and triplet emissions of PPO36. In the case of
PPO27, the singlet emission was overlapped with the absorption
of FCNIr, but the triplet emission was little overlapped due to
decreased triplet energy. The FIrpic showed strong absorption
below 460 nm, which was well overlapped with the emission of
PPO27 and PPO36. Therefore, it can be expected that energy
transfer from PPO27 and PPO36 hosts to FIrpic is effective, but
the energy transfer from PPO27 to FCNIr may not be so
effective as that from PPO36 to FCNIr.
HOMO and LUMO levels of the PPO27 and PPO36 were
measured from cyclic voltammetry (CV) and absorption edge of
the ultraviolet-visible (UV-Vis) spectrum. CV data of PPO27
and PPO36 are shown in Fig. S2, (ESI†). The HOMO level of the
PPO27 was 6.25 eV compared with 6.31 eV of PPO36. The
HOMO level was shifted by 0.06 eV by the substitution of
the diphenylphosphine oxide at 3,6 positions. The LUMO level
was calculated from the HOMO level and optical band gap from
UV-Vis absorption spectra and the LUMO levels of PPO27 and
PPO36 were 3.00 eV and 2.77 eV respectively. The LUMO level
of the PPO27 was shifted by 0.23 eV by the electron withdrawing
phosphine oxide group. As shown in the molecular orbital
distribution, the LUMO was dispersed over 2,7 positions of the
phenylcarbazole. Therefore, the substitution of the strong elec-
tron withdrawing diphenylphosphine oxide at 2,7 positions
shifted the LUMO level significantly. In addition, the LUMO
was extended to the diphenylphosphine oxide group due to
extended conjugation through 2,7 positions. The shift of the
LUMO level also led to the decrease of the HOMO–LUMO gap
from 3.54 eV of PPO36 to 3.25 eV of PPO27.
The PPO27 and PPO36 showed different HOMO/LUMO
levels and molecular orbital distributions, which may affect the
hole and electron transport properties of PPO27 and PPO36.
Therefore, hole only and electron only devices of PPO27 and
PPO36 were fabricated to compare the hole and electron density
in the PPO27 and PPO36. Hole current density–voltage and
electron current density–voltage curves of PPO27 and PPO36
Photophysical properties of the PPO27 and PPO36 were
compared and they are shown in Fig. 1. UV-Vis, photo-
luminescence (PL) and low temperature PL spectra of host
materials were compared. The UV-Vis spectrum was red-shifted
in the PPO27 compared with that of the PPO36 due to the
reduced HOMO–LUMO gap. The PL spectra were also shifted
by 26 nm due to the reduced HOMO–LUMO gap of PPO27.
The maximum peak of PL emission of PPO27 was 392 nm
compared with 366 nm of PPO36. The triplet energy of the
Fig. 2 UV-Vis absorption spectra of FCNIr and FIrpic dopants
compared with PL emission spectra of PPO27 and PPO36.
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Fig. 4 Current density–voltage–luminance curves of PPO27 and PPO36
Fig. 3 Current density–voltage curves of hole only and electron only
devices of PPO27 and PPO36.
blue devices doped with FIrpic and FCNIr.
PHOLEDs. The current density of the PPO27 blue PHOLED
was much higher that of PPO36 blue PHOLED over all voltage
ranges investigated. The high current density of the PPO27
device is due to high hole and electron current densities as
explained in Fig. 3. In the case of the FIrpic devices, the turn-on
voltage of the PPO27 device was 3.0 V compared with 3.5 V of
PPO36 device and the driving voltage at 1000 cd mꢀ2 was lowered
by 2.5 V. The driving voltage at 1000 cd mꢀ2 was 8.1 V in the
PPO36 device, while it was 5.6 V in the PPO27 device.
are shown in Fig. 3. Both the hole current density and electron
current density were high in the PPO27 device compared with
those of the PPO36 device. This indicates that the PPO27 is
better than PPO36 in terms of hole and electron injection. The
hole and electron current densities are determined by the energy
barrier for charge injection and charge transport properties.
Comparing the HOMO levels, the HOMO level of PPO27 (6.25
eV) is more suitable for hole injection than that of PPO36 (6.31
eV). Therefore, the PPO27 can effectively inject holes from
a hole transport layer. In addition, the HOMO orbital distribu-
tion of the PPO27 is more suitable for hole transport. As shown
in HOMO distribution of phenylcarbazole, the substitution of
diphenylphosphine oxide at 3,6 positions disturbs the hole
transport due to the electron withdrawing character of the
diphenylphosphine oxide. However, the HOMO of phenyl-
carbazole is maintained constant irrespective of the diphenyl-
phosphine oxide substitution at 2,7 positions. Therefore, the
PPO27 is better than PPO36 for hole injection and transport.
Hole and electron mobility of the PPO27 and PPO36 was
measured to confirm the charge transport properties of the host
materials. The hole mobilities of PPO27 and PPO36 were 8 ꢂ
10ꢀ5 cm2 Vꢀ1 sꢀ1 and 2 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1, which confirm that
PPO27 is better than PPO36 in terms of hole mobility.
The quantum efficiency–luminance and power efficiency–
luminance curves of PPO27 and PPO36 devices are shown in
Fig. 5. In the case of FIrpic devices, the PPO27 device showed
much higher quantum efficiency and power efficiency than
PPO36 device. The PPO27 improved both hole and electron
current densities, and further improved charge balance in the
emitting layer. The PPO27 had a high triplet energy of 2.81 eV
for efficient energy transfer to FIrpic and balanced charge
injection through 2,7 substitution of diphenylphosphine oxide
Similarly, the electron density of the PPO27 was higher than
that of the PPO36. Although the LUMO level of PPO27 was
lower than that of PPO36 by 0.20 eV, there was no energy barrier
for electron injection from TSPO1 to PPO27 and PPO36.
Therefore, the high electron current density of the PPO27 device
is due to improved electron transport properties of PPO27,
which are related with the LUMO distribution of the PPO27.
The LUMO of PPO27 is dispersed by the electron withdrawing
diphenylphosphine oxide, while the LUMO of PPO36 is not
extensively dispersed by the diphenylphosphine oxide because
the node of the LUMO was at 3,6 positions. Therefore, PPO27
can be better than PPO36 in terms of electron transport. The
electron mobility of the PPO27 was 7 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1, which
was higher than that of PPO36 with a electron mobility of 2 ꢂ
10ꢀ6 cm2 Vꢀ1 sꢀ1. Therefore, the hole and electron current
densities of PPO27 were higher than that of PPO36.
As the PPO27 and PPO36 had high triplet energy for energy
transfer to blue phosphorescent dopants, PPO27 and PPO36
based blue PHOLEDs doped with FIrpic and FCNIr were
fabricated. The doping concentration of the blue phosphorescent
dopant was optimized at 15%. Fig. 4 shows current density–
voltage–luminance curves of the PPO27 and PPO36 blue
Fig. 5 Quantum efficiency–luminance (a) and power efficiency–lumi-
nance (b) curves of PPO27 and PPO36 blue devices doped with FIrpic
and FCNIr.
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Table 1 Device performances of blue PHOLEDs fabricated in this work
Quantum efficiency
(%)
Power efficiency/lm
Wꢀ1
Emitting layer
Max.
1000 cd mꢀ2
Max.
1000 cd mꢀ2
PPO27 : FIrpic
PPO27 : FCNIr
PPO36 : FIrpic
PPO36 : FCNIr
23.9
16.2
16.4
19.8
20.3
13.5
14.1
14.9
37.5
15.1
23.2
19.9
23.0
8.7
10.5
7.0
Fig. 6 PL spectra of PPO27 and PPO36 films doped with FIrpic and
groups. The maximum quantum efficiency of the PPO27 device
was 23.9% and the quantum efficiency at 1000 cd mꢀ2 was 20.3%,
while the maximum quantum efficiency and quantum efficiency
at 1000 cd mꢀ2 of PPO36 device were 16.4% and 14.1%, respec-
tively. The quantum efficiency was improved by more than 50%
by substituting the diphenylphosphine oxide at 2,7 positions
instead of 3,6 positions. In addition to the quantum efficiency,
the power efficiency was also significantly improved. The
maximum power efficiency and power efficiency at 1000 cd mꢀ2
of the PPO27 device were 37.5 lm Wꢀ1 and 23.0 lm Wꢀ1, while the
maximum power efficiency and power efficiency at 1000 cd mꢀ2
FCNIr. The films were excited at 400 nm.
higher than that of the FCNIr (2.75 eV) by only 0.06 eV, which
may induce reverse energy transfer from FCNIr to PPO27. The
reverse energy transfer degrades the quantum efficiency of the
device because of triplet exciton quenching by the host material.
The triplet energy of the PPO36 (3.01 eV) is high enough and no
reverse energy transfer can be induced.
To confirm the reverse energy transfer in the PPO27 : FCNIr
emitting layer, PL measurement of the PPO27 : FCNIr and
PPO36 : FCNIr films was carried out. PL spectra of
PPO27 : FIrpic and PPO36 : FIrpic were also obtained for
comparison. Fig. 6 shows the PL spectra of the four different
films. Excitation wavelength of the film was set as 400 nm, which
excites only dopant materials without any excitation of the host
materials. There was little difference of the PL spectra in the
FIrpic doped PPO27 and PPO36. As there was no energy
transfer from FIrpic to host materials, identical FIrpic spectra
were observed in both films. However, the PL spectra of the
FCNIr doped PPO27 and PPO36 films were different. They
showed same emission spectra at long wavelength, but the
emission between 430 nm and 500 nm were suppressed in FCNIr
doped PPO27 film. Assuming that there is no energy transfer
from FCNIr dopant to the PPO27 or PPO36 host, there should
be no difference of PL spectra. Therefore, the reduction of
relative PL intensity in the PPO27 : FCNIr film indicates the
energy transfer from FCNIr to the PPO27. In particular, the
vibrational peaks at short wavelength were reduced due to high
energy of the short wavelength emission. Therefore, it can be
proved that the low quantum efficiency of the FCNIr doped
PPO27 device is due to reverse energy transfer from FCNIr to
PPO27. Similar PL spectra were observed when the FCNIr
doped film was excited at 330 nm light source. PPO27 and
PPO36 are excited at 330 nm and the PL spectra show the light
emission by energy transfer from host to dopant materials. The
same PL spectra were obtained irrespective of the excitation
of the PPO36 device were 23.2 lm Wꢀ1 and 10.4 lm Wꢀ1
,
respectively. In particular, the power efficiency at 1000 cd mꢀ2
was more than doubled by using the PPO27 instead of PPO36.
The power efficiency is generally determined by the driving
voltage and quantum efficiency. Therefore, the high power effi-
ciency of the PPO27 device is due to lowered driving voltage and
improved quantum efficiency. The driving voltage was lowered
by 2.5 V and the quantum efficiency was enhanced by 60% in the
PPO27 device. This quantum efficiency value was high although
it was lower than that of the best efficiency values in sky blue
PHOLEDs.33,34 The best quantum efficiency of sky blue PHO-
LEDs was 26%. The device performances of the PPO27 and
PPO36 blue PHOLEDs are summarized in Table 1. In addition,
the quantum efficiency of the PPO27 : FIrpic device was
compared with other data reported in FIrpic based blue PHO-
LEDs with phenylcarbazole and diphenylphosphine oxide
groups (Table 2).
In the case of the FCNIr devices, the PPO36 showed better
quantum efficiency than PPO27 although the power efficiency
was high in PPO27 device owing to a low driving voltage. The
high quantum efficiency of the PPO36 device compared with that
of PPO27 device may be related with the energy transfer from
host to dopant material. In the FIrpic devices, the triplet energy
of the host materials was higher that that of FIrpic (2.65 eV),
resulting in efficient energy transfer from host materials to
dopant. However, the triplet energy of the PPO27 (2.81 eV) was
Table 2 Device performances of blue PHOLEDs with phenylcarbazole based host material doped with FIrpic
Host material
Quantum efficiency (%)
References
3,6-Bis(diphenylphosphoryl)-9-ethylcarbazole
N-(4-Diphenylphosphoryl phenyl)carbazole (MPO12)
4,40,400-Tri(N-carbazolyl)triphenylphosphine oxide
2,7-Bis(diphenylphosphine oxide)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene
2,7-Bis(diphenylphosphoryl)-9-phenyl-9H-carbazole
ꢃ5%
23
25
26
27
9.1%
ꢃ16.0%
14.8%
23.9%
This work
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PPO27. Fig. 8 shows the transient EL spectra of PPO27 and
PPO36 solid films doped with FIrpic and FCNIr. The mono-
exponential decay curve was observed and the excited state life-
time of the FIrpic doped solid film was 1.21 ms and 1.29 ms for
PPO27 and PPO36, respectively. Similar excited state lifetime for
phosphorescence emission was observed. There was little effect
of the host on the lifetime of the FIrpic device. The
PPO36 : FCNIr film also exhibited mono-exponential decay
curve and the excited state lifetime was 1.72 ms, which was a little
longer than that of the PPO36 : FIrpic. In the case of
PPO27 : FCNIr film, weak double exponential decay behavior
was observed in the transient PL decay curve. The lifetime for
the first decay and the second decay was 1.19 ms and 7.18 ms. The
double exponential decay of the transient PL indicates the
reverse energy transfer process in the PPO27 : FCNIr.
Fig. 7 Electroluminescence spectra of PPO27 and PPO36 blue devices
doped with FIrpic and FCNIr.
wavelength and this is also due to reverse energy transfer from
FCNIr to PPO27. Energy transfer from PPO27 to FCNIr is
induced at 330 nm excitation wavelength, but the reverse energy
transfer follows, leading to the reduction of the emission inten-
sity at short wavelength.
Conclusions
The substitution of diphenylphosphine oxide at 2,7 positions of
carbazole unit improved hole and electron injection into the
emitting layer and reduced the driving voltage of blue PHO-
LEDs. In particular, the PPO27 with substituents at 2,7 posi-
tions greatly improved the power efficiency of FIrpic doped sky
blue PHOLEDs due to high quantum efficiency and low driving
voltage. A high quantum efficiency of 23.9% and power efficiency
of 37.5 lm Wꢀ1 were achieved in the blue PHOLED with FIrpic
doped PPO27 host. However, the substitution at 3,6 positions
(PPO36) was better than 2,7 substitution in the FCNIr doped
deep blue PHOLEDs because of reverse energy transfer in the
PPO27 device.
Electroluminescence (EL) spectra of the PPO27 and PPO36
devices are shown in Fig. 7. Both PPO27 and PPO36 showed
strong emission peaks originated from the FIrpic and FCNIr
dopant materials. However, the EL spectra were a little different
from PL spectra and first vibrational peak was intensified in the
PPO27. The strong intensity of the vibration peak in PPO27
device is due to the optical effect caused by the recombination
zone shift. In the FCNIr device case, the reduction of the short
wavelength emission was also observed due to reverse energy
transfer. Full width at half maximum (FWHM) of the blue
PHOLEDs was between 52 nm and 54 nm except for
PPO27 : FCNIr. This indicates that there was no reverse energy
transfer in the PPO27 : FIrpic, PPO36 : FIrpic and
PPO36 : FCNIr devices. However, the FWHM of the
PPO27 : FCNIr was 61 nm, which was higher than that of other
devices. This is due to the reverse energy transfer causing the
reduction of the deep blue emission.
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