Carboline modified dibenzofuran as a high triplet host material for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2014.10.009

A carboline modified dibenzofuran compound, 2,8-bis(pyrido[2,3-b]indol-9-yl)dibenzofuran (DBFCb), was synthesized and evaluated as the host material for blue phosphorescent organic light-emitting diodes by doping iridium (III) bis((3,5-difluorophenyl)pyridine) picolinate (FIrpic) as a blue triplet emitter. The DBFCb host showed balanced charge density in the emitting layer and a triplet energy of 2.88 eV by carboline moiety. The FIrpic doped DBFCb devices realized high external quantum efficiency of 21.3% at a low doping concentration of 3%.

Full text of DOI:10.1016/j.dyepig.2014.10.009

Accepted Manuscript
Carboline modified dibenzofuran as a high triplet host material for blue
phosphorescent organic light-emitting diodes
Yirang Im, Jun Yeob Lee
PII:
S0143-7208(14)00406-9
10.1016/j.dyepig.2014.10.009
DYPI 4560
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 11 September 2014
Revised Date: 10 October 2014
Accepted Date: 11 October 2014
Please cite this article as: Im Y, Lee JY, Carboline modified dibenzofuran as a high triplet host material
for blue phosphorescent organic light-emitting diodes, Dyes and Pigments (2014), doi: 10.1016/
j.dyepig.2014.10.009.
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Carboline modified dibenzofuran as a high triplet host material for
blue phosphorescent organic light-emitting diodes
Yirang Im, Jun Yeob Leea)
Department of Polymer Science and Engineering, Dankook University
Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do, 448-701, Korea
Abstract
A carboline modified dibenzofuran compound, 2,8-bis(pyrido[2,3-b]indol-9-
yl)dibenzofuran (DBFCb), was synthesized and evaluated as the host material for blue
phosphorescent organic light-emitting diodes by doping iridium (III) bis((3,5-
difluorophenyl)pyridine) picolinate (FIrpic) as a blue triplet emitter. The DBFCb host
showed balanced charge density in the emitting layer and a triplet energy of 2.88 eV by
carboline moiety. The FIrpic doped DBFCb devices realized high external quantum
efficiency of 21.3% at a low doping concentration of 3%.
Key words : dibenzofuran, carboline, charge balance, blue device, high quantum
efficiency
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a) To whom correspondence should be addressed
E-mail : leej17@dankook.ac.kr
Tel : 82-31-8005-3585 Fax:82-31-8005-3585
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Introduction
There have been a lot of researches about developing high triplet energy host materials
to enhance the quantum efficiency of blue phosphorescent organic light-emitting diodes
(PHOLEDs). Generally, carbazole derivatives have been extensively studied as the high
triplet energy host materials, but relatively poor electron transport properties of the
carbazole derivatives limited the device performances of the blue PHOLEDs[1-7].
Therefore, several moieties have been developed as alternatives of the carbazole moiety
and one of the most useful unit structures was carboline[8-15]. The carboline moiety is
composed of conjugated pyridine and indole sub-units and has better electron transport
properties than carbazole due to electron deficiency of the pyridine sub-unit. The good
electron transport properties of carboline balanced holes and electrons, contributing to
the high quantum efficiency of the blue PHOLEDs. There have been several papers
reporting high external quantum efficiency above 20% in blue PHOLEDs using host
materials derived from carboline[10-15]. However, the molecular structure of the
carboline compounds was not diversified in spite of high quantum efficiency of the
carboline derivatives. Typically, the carboline moiety was combined with the carbazole
moiety for both hole and electron transport properties[12,13] although a carboline
compound with two carboline moieties linked via a biphenyl core was reported[15].
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In this work, a new carboline derivative, 2,8-bis(pyrido[2,3-b]indol-9-yl)dibenzofuran
(DBFCb), was developed as a host material for blue PHOLEDs. The synthesis and
device application of DBFCb as the host material of blue PHOLEDs were investigated
and high external quantum efficiency 21.3% at a low doping concentration of 3% in
blue PHOLEDs was demonstrated.
Experimental Section
General information
Dibenzofuran (TCI Chem. Co.) was used without further purification. Periodic acid,
copper iodide (CuI), trans-1,2-diaminocyclohexane (Aldrich Chem. Co.), sodium
thiosulfate, sulfuric acid, acetic acid, 1,4-dioxane (Duksan Sci. Co.), iodine (Samchun
Sci. Co.), 9H-pyrido[2,3-b]indole (α-carboline) (P&H tech Co.), and potassium
phosphate (K3PO4) (Daejung Sci. Co.) were also used as received. Synthesized material
was analyzed according to the method reported in the literature[10].
Synthesis
Synthetic scheme of the DBFCb is described in Scheme 1.
2, 8-Diiododibenzofuran (1)
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Dibenzofuran (7.00 g, 41.62 mmol) and periodic acid (11.38 g, 49.94 mmol) were
dissolved in acetic acid (300 ml). And then, iodine (9.34 g, 73.6 mmol) was added to the
solution. The reaction mixture was stirred at 60ꢀ for 30 min followed by addition of
distilled water (60 ml) and sulfuric acid (0.6 ml). The solution was refluxed for 38 h,
cooled to room temperature and poured into distilled water. The mixture was filtered
and diluted with ethyl acetate and washed with distilled water and sodium thiosulfate.
The organic layer was dried over anhydrous MgSO4 and evaporated in vacuo. The crude
product was washed with hexane after drying in vacuum. The synthesized 2, 8-
diiododibenzofuran was used in the next reaction without further purification. A white
powder was obtained as a product (7.70 g, yield : 44%)
2, 8-Bis(pyrido[2,3-b]indol-9-yl)dibenzofuran (DBFCb) (2)
2, 8-Diiododibenzofuran (1.15 g, 2.74 mmol), α-carboline (1.02 g, 6.03 mmol), K3PO4
(2.33 g, 10.96 mmol) and CuI (0.53 g, 2.74 mmol) were dissolved in 1,4-dioxane (60
ml) under a nitrogen atmosphere. The reaction mixture was stirred for 30 min and trans-
1,2-diaminocyclohexane (0.34 ml, 2.74 mmol) was added to the solution followed by
reflux overnight. After cooling to room temperature, the mixture was filtered and
diluted with methylene chloride and washed with distilled water. The organic layer was
dried over anhydrous MgSO4 and evaporated in vacuo. The product was purified by
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column chromatography using methylene chloride/n-hexane as an eluent. A white
powder was obtained by vacuum train sublimation. (0.30 g, yield : 42%).
1H NMR (400 MHz, CDCl3): δ 8.48 (d, 1H, J = 6.40 Hz), 8.40 (d, 1H, J = 8.80 Hz),
8.20 (d, 1H, J = 1.60 Hz), 8.14 (d, 1H, J = 7.60 Hz), 7.86 (d, 1H, J = 8.80 Hz), 7.76 (d,
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1H, J = 11.20 Hz), 7.49-7.47 (m, 2H), 7.36-7.33 (m, 1H), 7.26-7.23 (m, 1H) C NMR
(100 MHz, CDCl3): δ 156.14, 152.49, 146.71, 140.79, 131.64, 128.52, 127.57, 127.22,
125.43, 121.13, 120.94, 120.90, 120.70, 116.46, 116.26, 113.16, 110.45, MS (FAB) m/z
501 [(M+H)+]. Elemental Analysis (calculated for C34H20N4O) : C, 81.58; H, 4.03; N,
11.19; O, 3.20. Found : C, 81.57; H, 4.05; N, 11.18; O, 3.19.
Device fabrication and measurements
Blue PHOLEDs were fabricated by growing organic materials on poly(3,4-
ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS, 60 nm) coated indium tin
oxide (ITO, 120 nm) substrate by vacuum thermal evaporation. 4,4'-
Cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC, 20 nm) was a hole
transport layer and N,N'-dicarbazolyl-3,5-benzene (mCP, 10 nm) was used as a hole
transport type exciton blocking layer. Iridium (III) bis((3,5-difluorophenyl)pyridine)
picolinate (FIrpic) doped DBFCb (25 nm) was a blue emitting layer and
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diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 35 nm) was deposited as an
electron transport type exciton blocking layer. LiF(1 nm)/Al(200 nm) double layer was
evaporated as a cathode for the blue PHOLEDs. Single carrier devices had device
structures of ITO (120 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/DBFCb
(25 nm)/TAPC (5 nm)/Al (200 nm) (hole only device) and ITO (120 nm)/Ca (5
nm)/DBFCb (25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm) (electron only device).
Encapsulation of the blue PHOLEDs and single carrier devices was carried out to
protect the devices from moisture and oxygen using a glass cover. A desiccant was
inserted inside the device and the glass cover was sealed with an epoxy adhesive by
ultraviolet curing. Electrical performances of the devices were characterized using
Keithley 2400 source measurement unit and luminance performances were measured
using CS 2000 spectroradiometer.
Results and discussion
The carboline based host material, DBFCb, has a weak electron transport type
dibenzofuran core and two carboline moieties. Compared with other carboline
compounds reported earlier[10-15], the dibenzofuran core can allow more electron
transport character to the host material and can produce electron transport type host
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materials. The DBFCb host was simply synthesized by the reaction of 2,8-
diiododibenzofuran with α-carboline using CuI. Synthetic yield of the product after
column chromatography and vacuum train sublimation was 42%. Purity of the final
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sublimed compound was 99.9% and the final compound was confirmed by 1H and C
NMR, mass and elemental analysis.
The highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of DBFCb were studied by calculating electron distribution
of optimized geometrical structure. B3LYP 6-31G* basis set of Gaussian 09 program
was used for the molecular simulation. Figure 1 represents the electron distribution of
DBFCb. The HOMO was widespread over the entire molecule with more HOMO
dispersion on the carboline and the LUMO was centered on the dibenzofuran moiety.
Relatively low electron density of the aromatic units of dibenzofuran by electron
withdrawing oxygen allowed the localization of LUMO on the dibenzofuran moiety and
dominant HOMO distribution on the carboline moiety. As the carboline and
dibenzofuran moieties are weak hole transport and electron transport units, respectively,
DBFCb may show bipolar charge transport properties.
Ionization potential (IP) and electron affinity (EA) of DBFCb were analyzed by
measuring the oxidation and reduction potentials. Oxidation and reduction curves
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obtained from cyclic voltammetry (CV) measurement were plotted against voltage in
Figure 2. The oxidation and reduction potentials of DBFCb were 1.30 V and -2.13 V
from the onset of oxidation and reduction curves, which were -6.17 eV and -2.74 eV as
IP and EA using ferrocene as the standard material. The IP of DBFCb was stabilized by
the HOMO dispersion on the carboline moiety. The pyridine unit of the carboline
moiety decreased electron density and deepened the HOMO level of DBFCb compared
to carbazole derived molecules. The EA was also stabilized by the LUMO distribution
on the dibenzofuran. The HOMO and LUMO of DBFCb can be estimated to be -6.17
eV and -2.74 eV, respectively, from the IP and EA.
Photophysical parameters of DBFCb were extracted using UV-vis absorption and PL
emission analysis. Figure 3 shows UV-vis absorption, solution PL and low temperature
PL spectra of DBFCb. Solution and low temperature PL measurements were carried out
in tetrahydrofuran solvent at a concentration of 1.0 × 10-5 M. UV-vis absorption
assigned to π−π* and n-π* absorption of DBFCb was observed below 360 nm and
solution PL peak position was 379 nm. First phosphorescent peak position was 427 nm
and the triplet energy of DBFCb was 2.90 eV from the low temperature PL
measurement at 77 K. The high triplet energy of DBFCb is due to the distortion of the
o
carboline moiety from the dibenzofuran plane by 53.86 as presented in Figure 4. The
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distortion of the carboline moiety limited p-orbital overlap between dibenzofuran and
carboline, which lead to the high triplet energy.
Thermal transition temperatures of DBFCb were measured using differential
scanning calorimeter (DSC) to elucidate thermal stability of DBFCb. Figure 5 shows a
DSC themogram of DBFCb. Glass transition temperature (Tg) is an important parameter
for the thermal stability and the Tg of DBFCb was 132 oC. As the Tg of DBFCb was well
o
above 100 C, the DBFCb host can be considered as a thermally stable host material.
The Tg of DBFCb was higher than that of the carboline host material with a biphenyl
core[15]. Thermal decomposition temperature (Td) of DBFCb was investigated by
thermogravimetric analysis (TGA). TGA curve of DBFCb is displayed in Figure 6. The
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Td at 5 % weight loss was 378 C, which supports the thermal stability of DBFCb as a
host material.
The high triplet energy of DBFCb allowed us to grow blue PHOLEDs by doping
common FIrpic as a blue triplet emitter because the triplet energy of FIrpic (2.65 eV) is
lower than that of DBFCb (2.90 eV). FIrpic was doped at doping concentrations of 3%
and 5%. Current density-voltage-luminance plots of the DBFCb:FIrpic devices at 3%
and 5% doping concentrations are presented in Figure 7. The current density was
sharply increased at a turn-on voltage of 3.0 V. Low energy barrier for hole injection
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(0.07 eV) and electron injection (0.22 eV) lowered the turn-on voltage of the DBFCb
device by facilitating charge injection from the charge transport layers to the emitting
layer as shown in the energy level diagram of the device in Figure 8. Driving voltage at
1,000 cd/m2 was 5.7 V and 6.7 V at 3% and 5% doping concentrations, respectively.
The high driving voltage of the DBFCb device at 5% doping concentration is originated
by the low recombination efficiency and low current density compared to the DBFCb
device at 3% doping concentration.
External quantum efficiency of the DBFCb device was presented against luminance
in Figure 9. The external quantum efficiency was optimized at 3% doping concentration
by concentration quenching effect at 5% doping concentration and the maximum
quantum efficiency of the DBFCb device was 21.3%. The high quantum efficiency
above 20% was maintained up to a luminance of 3,500 cd/m2. Several factors can
explain the origins of the high external quantum efficiency of the DBFCb devices. One
key factor is the suppression of triplet exciton quenching of FIrpic by the DBFCb host
due to high triplet energy of DBFCb (2.90 eV). Another factor is complete energy
transfer from DBFCb to FIrpic as can be ensured by the electroluminescence (EL)
spectra in Figure 10. Pure emission of FIrpic without DBFCb emission supports the
complete energy transfer in the DBFCb device. Although DBFCb was synthesized as an
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electron transport type host as shown in current density-voltage curves of single carrier
devices (Figure 11), the triplet exciton harvesting and good energy transfer increased
the quantum efficiency. Additionally, electron trapping effect by FIrpic in the emitting
layer contributed to the high quantum efficiency through balancing holes and electrons
in the emitting layer[16]. The quantum efficiency of the DBFCb devices was higher
than that of mCP device over all luminance range and efficiency roll-off was also
reduced.
Conclusions
In summary, a carboline and dibenzofuran derived material, DBFCb, was synthesized
as an electron transport type host material for FIrpic triplet emitter. The high triplet
energy of 2.90 eV and good energy transfer characteristics of DBFCb contributed to
high external quantum efficiency and the maximum external quantum efficiency of
DBFCb reached 21.3%. Therefore, the DBFCb host can be diversely applied as the host
material for blue PHOLEDs.
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List of figures
Scheme 1. Synthetic scheme of DBFCb.
Figure 1. HOMO and LUMO distribution of DBFCb
Figure 2. Oxidation and reduction curves of DBFCb. Oxidation and reduction were
measured separately.
Figure 3. UV-vis absorption, solution PL and low temperature PL spectra of DBFCb in
tetrahydrofuran solvent.
Figure 4. Geometrical structure of DBFCb.
Figure 5. DSC curve of DBFCb scanned at a heating rate of 10 oC/min under nitrogen.
Figure 6. TGA curve of DBFCb scanned at a heating rate of 10 oC/min under nitrogen.
Figure 7. Current density-voltage-luminance plots of DBFCb devices doped with FIrpic
at doping concentrations of 3% and 5%.
Figure 8. Energy level diagram of the blue phosphorescent organic light emitting diodes
with DBFCb host material.
Figure 9. External quantum efficiency-luminance plots of DBFCb devices doped with
FIrpic at doping concentrations of 3% and 5%. Device data of mCP device was also
added for comparison.
Figure 10. Electroluminescence spectra of DBFCb devices doped with FIrpic at doping
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concentrations of 3% and 5%.
Figure 11. Current density-voltage curves of the hole only and electron only devices of
DBFCb.
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Scheme 1
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HOMO
LUMO
Figure 1
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Figure 4
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endothermic
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2.10
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