Synthesis and electro-optical properties of adamantane-based host and hole-transporting material for thermal stable blue phosphorescent OLEDs

Article abstract of DOI:10.1166/jnn.2017.14749

New host/hole-transporting material, namely 9,9-{5-[(3r,5r,7r)-adamantan-1-yl]-1,3-phenylene} bis(9H-carbazole) (ad-mCP) and N⁴-(3-((3r,5r,7r-adamantan-1-yl)phenyl)-N⁴-(3-((2R,5S,6as)-hexahydro-2,5-methanopentalen-3a(1H-yl)phenyl)-N⁴N⁴-diphenyl-[1,1-biphenyl]-4,4-diamine (ad-TPD), were designed and synthesized by introducing rigid adamantane unit into 1,3-Bis(N-carbazolyl) benzene and N⁴N⁴N⁴,N⁴-tetraphenyl-[1,1-biphenyl]-4,4-diamine via a Suzuki coupling reaction. Thermal, photophysical, and electrochemical properties of ad-mCP and ad-TPD were investigated. Using adamantane unit leads to enhance thermal stability. Phosphorescent organic light-emitting diodes constructed using ad-mCP emitting layer and ad-TPD hole transporting layer produce bright blue emissions with a high maximum current efficiency of 7.0 cd/A.

Full text of DOI:10.1166/jnn.2017.14749

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 7292–7296, 2017
Copyright © 2017 American Scientific Publishers
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Printed in the United States of America
Synthesis and Electro-Optical Properties of Adamantane-
Based Host and Hole-Transporting Material for Thermal
Stable Blue Phosphorescent OLEDs
Sungkoo Lee∗ and Min Hye Seo
IT Convergence Materials Group, Korea Institute of Industrial Technology, Chungnam-do, 31056, Korea
New host/hole-transporting material, namely 9,9ꢀ-{5-[(3r,5r,7rꢀ-adamantan-1-yl]-1,3-phenylene}
ꢀ
bis(9H-carbazole) (ad-mCP) and N4-(3-((3r,5r,7rꢀ-adamantan-1-yl)phenyl)-N4 -(3-((2R,5S,6as)-
ꢀ
hexahydro-2,5-methanopentalen-3a(1Hꢀ-yl)phenyl)-N4ꢁN4 -diphenyl-[1,1ꢀ-biphenyl]-4,4ꢀ-diamine (ad-TPD),
were designed and synthesized by introducing rigid adamantane unit into 1,3-Bis(N-carbazolyl)
ꢀ
ꢀ
benzene and N4ꢁN4ꢁN4 ,N4 -tetraphenyl-[1,1ꢀ-biphenyl]-4,4ꢀ-diamine via a Suzuki coupling reaction.
Thermal, photophysical, and electrochemical properties of ad-mCP and ad-TPD were investigated.
Using adamantane unit leads to enhance thermal stability. Phosphorescent organic light-emitting
diodes constructed using ad-mCP emitting layer and ad-TPD hole transporting layer produce bright
blue emissions with a high maximum current efficiency of 7.0 cd/A.
Keywords: PhOLED, Host Material, Hole Transporting Material, Thermostability, Adamantane.
1. INTRODUCTION
derivatives, etc.9–11 In particular, adamantane has the rigid
alicyclic structure composed of three cyclohexane rings in
chair conformation, which enhances thermal stability and
increases the glass transition temperature.12ꢀ13 Therefore,
adamantane is the candidate for incorporation into OLED
materials without sacrificing their electrical and optical
properties. In many OLED materials, mCP (1,3-Bis(N -
carbazolyl)benzene) and TPD (N ,N ꢀ-Bis(3-methylphenyl)-
N ,N ꢀ-diphenylbenzidine) is a very good host and hole
transporting material with excellent EL efficiencies and
low turn-on voltages but has rather low glass transition
temperature (Tg) of 60 ꢁC (mCP) and 65 ꢁC (TPD). There-
fore, there are needs for improving the thermal stability
of mCP and TPD for forming amorphous glass film and
stable operating of devices.
In this paper, we introduce adamantane unit into mCP
and TPD structure because it is bulky and rigid, and thus
expected to improve the thermal stability of OLED mate-
rials. The synthesis and thermal and optical properties
of mCP and TPD with adamantane unit are investigated.
Finally, the EL device was fabricated using ad-mCP and
ad-TPD as an EML and HTL material. The relationship
between molecular structure and the electroluminescence
properties of OLED device was systematically investigated
herein.
Organic light-emitting diodes (OLEDs) has attracted the
attention and efforts of a substantial number of researchers,
due to their superior properties such as high efficiency,
low driving voltage, lightweight, and large area light
source.1–4 Recently, application of OLEDs in the field of
an automotive display is rapidly increased. But in order
to be actually used in the electronics such as an automo-
tive display, the thermal stability and the operating life-
time of OLEDs are crucial factors. If OLED devices are
heated above the glass transition temperature of one of
the organic materials in the device, the expansion of the
materials is observed at its Tg, leading to significant dis-
ruption of the multilayer structure.5–7 Previous research
indicates that OLEDs based on a high glass transition
temperature amorphous thin film are less vulnerable to
heat damage, and hence, are more stable in use.8 There-
fore, studies on the design and synthesis of OLED mate-
rials have been continually focused on finding materials
with high thermal and thin film morphological stabili-
ties. To realize high thermostability of OLEDs, various
efforts have been proposed, including the use of star-
burst amorphous molecular glass, spiro linkage, isoindole
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2017.14749

Lee and Seo
Synthesis and Electro-Optical Properties of Adamantane-Based Host and Hole-Transporting Material
1
2. EXPERIMENTAL DETAILS
2.1. Materials and Synthesis
For the synthesis for ad-mCP and ad-TPD, all reagents
and chemicals were purchased from commercial sources
(Aldrich, TCI) and used without further purification unless
stated otherwise.
ad-mCP as a white power product (yield: 60%). H NMR
(CDCl3ꢁ: ꢂ (ppm) 8.26 (d, 4H), 8.20 (S, 2H), 8.0 (s, 1H),
7.71 (d, 4H), 7.60 (t, 4H), 7.40 (t, 4H), 1.30–1.50 (m, 9H),
1.0–1.10 (m, 6H).
2.1.3. N4,N4ꢀ-diphenyl-N4,N4ꢀ-bis[3-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl]-[1,1ꢀ-biphenyl]-
4,4ꢀ-diamine (2)
The same procedure for compound 1 was applied to give
powder (60%). H NMR (CDCl3ꢁ: ꢂ (ppm) 7.60 (t, 4H),
2.1.1. 9,9ꢀ-[5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-1,3-phenylene]bis(9H-carbazole) (1)
1
1,3,5-tribromobenzene (5 g, 16 mmol), carbazole (8.8 g,
52 mmol), cuprous iodide (0.3 g, 1.6 mmol), potassium
phosphate (13.8 g, 65 mmol) and 1,4-dioxane (50 mL)
were combined in a 250 mL round bottom flask. The trans-
1,2-cyclohexane diamine (1.9 mL, 16 mmol) was added to
the reaction mixture. The resultant solution was refluxed
under the atmosphere of nitrogen gas for 19 hr. The
reacted solution was cooled the room temperature. Methy-
lene chloride and water were added to the solution and
the organic layer was separated. The organic layer washed
with water and dried with anhydrous sodium sulfate. After
the organic solvent was removed by distillation under
reduced pressure until the amount of the organic solvent
decreased to about one fifth of the original amount, the
formed crystals were separated by filtration and washed
with ethyl acetate. The obtained residue was purified with
column chromatography. The purified 9,9ꢀ-(5-bromo-1,3-
phenylene)bis(9H-carbazole) (1.6 g) was obtained.
7.32–7.50 (m, 12H), 7.25–7.40 (m, 10H), 1.6 (s, 24H).
2.1.4. N4-(3-((3r,5r,7r)-adamantan-1-yl)phenyl)-N4ꢀ-
(3-((2R,5S,6as)-hexahydro-2,5-methanopentalen-
ꢀ
3a(1H)-yl)phenyl)-N4,N 4 -diphenyl-[1,1ꢀ-
biphenyl]-4,4ꢀ-diamine(ad-TPD)
The same procedure for compound ad-mCP was applied
1
to give white powder (55 %). H NMR (CDCl3ꢁ: ꢂ (ppm)
8.5 (d, 4H), 7.7 (d, 6H), 7.43–7.6 (m, 6H), 7.3–7.38
(m, 8H), 1.3–1.53 (m, 22H), 1.25–1.1 (m, 8H).
2.2. Measurement
The 1H NMR spectra were recorded on a Bruker
300 MHz spectrometer. UV/Vis and fluorescence spectra
were collected on a Shimadzu UV/Vis spectrometer and
a PerkinElmer spectro-fluorometer, respectively. Decom-
position temperature (Tdꢁ and glass transition tempera-
ture (Tgꢁ of the synthesized compounds were determined
by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) on a NETZSCH thermogravi-
metric analyzer and PerkinElmer Differential Scanning
Calorimeter. Cyclic Voltammetry (CV) was carried out on
a RersaSTAT3 electrochemistry workstation. Film thick-
ness was measured using a TENCOR surface profiler.
9,9ꢀ-(5-bromo-1,3-phenylene)bis(9H-carbazole) (6 g,
12.3 mmol), Bis(pinacolato)diboron (4.7 g, 18.4 mmol),
potassium acetate (3.6 g, 37 mmol), and Pd(PPh3ꢁ4
(0.4 g, 0.3 mmol) were dissolved in distilled 1,4-dioxane.
The reaction mixture was refluxed for 12 hours under
nitrogen and then allowed to cool to room temperature.
After the mixture washed with ethyl acetate, the solvent
was removed under vacuum. The crude material was
purified by column chromatograph on silica gel (ethyl
acetate:hexane = 1:3 as eluent) to give the title compound
2.3. Fabrication of OLED Device
To investigate electroluminescent properties, we fabri-
cated two types of devices, with the device configura-
tions: device I: ITO/PEDOT:PSS (30 nm)/NPB (30 nm)/
mCP:FIr6 (9 wt%, 30 nm)/TPBi (10 nm)/Alq3 (30 nm)/
LiF (1 nm)/Al; device II: ITO/PEDOT:PSS (30 nm)/NPB
(30 nm)/ad-mCP:FIr6 (9 wt%, 30 nm)/TPBi (10 nm)/Alq3
(30 nm)/LiF (1 nm)/Al; device III: ITO/PEDOT:PSS
(30 nm)/ad-TPD (30 nm)/mCP:FIr6 (9 wt%, 30 nm)/TPBi
(10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al; device IV: ITO/
PEDOT:PSS (30 nm)/ad-TPD (30 nm)/ad-mCP:FIr6
(9 wt%, 30 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/
Al. Bis(naphthalen-1-yl)-N ,N ꢀ-bis(phenyl)-benzidine (NPB)
was used as the hole transporting material. 2,2ꢀ,2ꢀꢀ-
(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi)
and tris(8-hydroxyquinoline)aluminum (Alq3ꢁ used as
the hole blocking material and the electron transporting
1
1 (yield: 70%). H NMR (CDCl3ꢁ: ꢂ (ppm) 8.20 (d, 6H),
7.60–7.40 (m, 9H), 7.35–7.26 (m, 4H), 1.30–1.45 (m, 12H).
2.1.2. 9,9ꢀ-{5-[(3r,5r,7r)-adamantan-1-yl]-1,3-
Phenylene} bis(9H-carbazole) (ad-mCP)
Reactant 1 (1.5 g, 2.8 mmol), 1-bromoadamantane (1.1 g,
4.2 mmol), and palladium (0.16 g, 0.14 mmol) were dis-
solved in distilled THF. The reaction mixture was then
stirred for 1 hour at room temperature. Potassium carbon-
ate was added to the reaction mixture. The reaction mix-
ture was refluxed for 12 hours under nitrogen and then
allowed to cool to room temperature. After the reaction
was finished, the mixture was washed three times with dis-
tilled water and extracted with ethyl acetate. The organic
layer was separated, dried over anhydrous magnesium sul-
fate, and the solvent was removed under vacuum. The
crude material was purified by column chromatography on
silica gel (ethyl acetate:hexane = 1:3 as eluent) to give the
material.
Bis(2,4-difluorophenylpyridinato)-tetrakis(1-
pyrazolyl) borate iridium(III) (FIr6) as a blue dopant. The
ITO-coated glass substrates were cleaned by ultrasonic
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Synthesis and Electro-Optical Properties of Adamantane-Based Host and Hole-Transporting Material
Lee and Seo
Br
60
Br
+
N
N
50
40
30
20
10
0
N
H
Br
Br
Br
O
O
B
Bis(pinacolato)diboron
ph(pph3)4, KOAc
N
N
N
N
0
50
100
150
200
250
300
(1)
Temperature (°C)
Br
110
O
O
100
90
80
70
60
50
40
30
20
10
0
B
N
N
N
N
ad-mCP
(a) ad-mCP
O
B
O
Br
Br
N
O B
O
100
200
300
400
500
600
700
Temperature (°C)
N
N
N
Figure 1. DSC and TGA curve of ad-mCP.
(2)
9,9ꢀ-[5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-
phenylene]bis(9H-carbazole) (1) and 1-bromoadamantane
with palladium catalyst led to ad-mCP. The same
O
O
B
B
O
O
Br
+
N
N
N
N
ꢀ
ꢀ
procedure for N 4,N 4 -diphenyl-N 4,N 4 -bis[3-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-[1,1ꢀ-biphenyl]-
4,4ꢀ-diamine (2) was applied to give ad-TPD. The chemical
ad-TPD
(b) ad-TPD
1
structure was confirmed by H NMR.
Scheme 1. Synthetic procedure for (a) ad-mCP and (b) ad-NPD.
The thermal properties of the adamantane-linked mate-
rials were investigated by DSC and TGA. Figures 1 and
2 show DSC and TGA curves of ad-mCP and ad-TPD,
respectively. As shown in Figures 1 and 2, no obvious
crystallization peak is observed during the scanning from
treatment in deionized water and acetone. PEDOT:PSS
layer, acting as a hole injection layer, was spin-coated
onto the ITO glass and dried at 120 C for 20 min. The
ꢁ
organic layers were thermally deposited at a base pressure
of 10−7 torr and then lithium fluoride (LiF) and Aluminum
(Al) were deposited successively.
The current–voltage (I–V ) and current density–
luminance (J–L) data of OLEDs were collected by
Keithley 236 source measurement unit. The area for mea-
surement of OLED emission was 4 mm2 for all the
samples studied in this work. Electroluminescence (EL)
spectra and CIE coordinate were measured by using a PR-
650 SpectraScan Spectrophotometer.
ꢁ
room temperature to 300 C, implying that ad-mCP and
ad-TPD show the prominent stability of these materials,
which can be attributed to the presence of the adaman-
tane unit. TGA curves show that ad-mCP and ad-TPD
haveꢁhigh thermostability with 5 wt% loss temperature of
420 C. These results indicate that the thermal stabilities
of the ad-mCP and ad-TPD are high enough for them to
be used in OLEDs.14ꢀ15
Figure 3 shows the absorption and photoluminescence
spectrum of ad-mCP and ad-TPD in chloroform solu-
tion. The two absorption peaks of ad-mCP at 310 nm
and 330 nm, respectively, appeared similar to that of
mCP. The PL spectrum of ad-mCP solution shows a
blue emission peak at 355 nm. We can see that the
3. RESULTS AND DISCUSSION
Scheme 1 illustrated the synthetic route for ad-mCP
and ad-TPD. The Suzuki cross-coupling reaction of
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Lee and Seo
Synthesis and Electro-Optical Properties of Adamantane-Based Host and Hole-Transporting Material
50
1st heating
1st cooling
2nd heating
40
30
20
10
0
0
50
100
150
200
250
300
Temperature (°C)
100
80
60
40
20
Figure 4. Energy level diagram and schematic structure of the blue
OLED.
0
100
200
300
400
500
600
700
2.5 eV by subtraction of the optical band gap from the
HOMO energy level. In the same way, absorption onset of
UV/Vis spectrum and HOMO energy level of ad-TPD was
3.1 eV and 5.4 eV, respectively. The energy diagram for
the devices is shown in Figure 4.
Temperature (°C)
Figure 2. DSC and TGA curve of ad-TPD.
PL spectrum of ad-mCP is overlapped with the absorp-
tion of FIr6 in a range of 350∼400 nm. Therefore, the
efficient energy transfer from ad-mCP to FIr6 can be
expected. The HOMO-LUMO band gap of ad-mCP was
determined from the absorption spectrum of ad-mCP film
to be 3.65 eV. HOMO level of ad-mCP was measured from
cyclic voltammetry (CV). The HOMO energy level of ad-
mCP was estimated from the oxidation onset potential was
−6.15 eV. The LUMO energy level was calculated to be
To investigate the electroluminescence properties and
the current–voltage–luminance characteristics of ad-mCP
and ad-TPD, we fabricated the device using ad-mCP and
ad-TPD as host and hole transporting material. The four
devices all showed the typical blue emission from FIr6
but no emission from either the FIr6 or any exciplex
was observed. This indicates that ad-mCP only plays the
role of a host material. The voltage–current and current
density–current efficiency of the devices are also shown
in Figure 5. The devices using ad-mCP as host material
exhibited the lowest turn-on voltage of about 4.8 V and
5.1 V (device II and IV), which is lower than that of
device I and III fabricated by mCP (about 5.7 V). The
devices using ad-mCP rapidly declined at same device
thickness than those using mCP when the current density
increased. We cannot now have a clear explanation for the
low turn-on voltage and rapid decline of the current effi-
ciency, which the reason is probably higher charge trans-
porting property of ad-mCP. The turn-on voltage of device
III using ad-TPD as the hole transporting material is sim-
ilar to that of device I using NPB. It indicated that ability
of hole transporting of ad-TPD was similar to that of NPB.
In the current efficiency, the device IV showed the best
performance with a maximum current efficiency of 7 cd/A.
On the other hand, device I to III showed 5.1 cd/A, 5.5
cd/A, and 5.5 cd/A, respectively. It indicated that ad-mCP
2
1
0
10
absorption of ad-mCP
absorption of ad-TPD
PL of ad-mCP
PL of ad-TPD
5
0
550
250
300
350
400
450
500
wavelength (nm)
Figure 3. UV/vis absorption and photoluminescence spectrum of ad-
mCP and ad-TPD in solution.
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Synthesis and Electro-Optical Properties of Adamantane-Based Host and Hole-Transporting Material
Lee and Seo
20
for applications as host and hole transporting materials in
OLEDs and related optoelectronic devices.
(a)
device I
device II
device III
device IV
15
10
5
4. CONCLUSION
We have synthesized adamantane derivatives that have
high thermal stability. OLEDs constructed using ad-mCP
as a host material produced blue emissions. PhOLEDs
made using ad-mCP and ad-TPD as host and hole
transporting layer revealed the best performance with a
maximum current efficiency of 7 cd/A, which are com-
parable to the efficiencies of the control OLED based on
mCP and NPB. This approach could prove useful in future
PHOLED applications in the automotive display. Further
modification and optimization of the structure and device
are in progress.
0
0
5
10
15
Voltage (V)
8
6
4
2
0
(b)
device I
device II
device III
device IV
References and Notes
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hence, are more stable in use. The present results suggest
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Received: 31 August 2016. Accepted: 9 March 2017.
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