A solution-processable triphenylamine-fluorene host for exciplex based white phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c4tc01870b

A novel triphenylamine-fluorene oligomer with macro-spirocyclic structure was designed and prepared as a host for exciplex based white phosphorescent organic light-emitting diodes (white PhOLEDs), in which only iridium(iii)bis(4,6-(difluorophenyl)pyridina

Full text of DOI:10.1039/c4tc01870b

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Materials Chemistry C
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A solution-processable triphenylamine-fluorene
host for exciplex based white phosphorescent
organic light-emitting diodes†
Cite this: J. Mater. Chem. C, 2014, 2,
9754
a
a
a
a
b
*
*
Zhaokang Fan, Nengquan Li, Yiwu Quan, Qingmin Chen, Shanghui Ye,
Quli Fan, Wei Huang and Hui Xuc
b
b
*
A novel triphenylamine-fluorene oligomer with macro-spirocyclic structure was designed and prepared as
a host for exciplex based white phosphorescent organic light-emitting diodes (white PhOLEDs), in which
only iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) was employed as the dopant.
The device exhibited a comparatively high performance with a maximum luminance and current
efficiency of 14 213 cd mꢀ2 and 22.6 cd Aꢀ1, respectively.
Received 21st August 2014
Accepted 22nd September 2014
DOI: 10.1039/c4tc01870b
www.rsc.org/MaterialsC
oxide,3e,9 pyridine-based phenyl-phosphine oxide10 and tetraar-
ylsilane.11 Most of the above reported PhOLEDs were fabricated
1. Introduction
through vacuum deposition.
Phosphorescent organic light-emitting diodes (PhOLEDs), in
which the emissive region consists of an organometallic dopant
in a wide-band gap host material, are currently attracting
intensive attention because they can theoretically achieve 100%
internal quantum efficiency.1 To date, triphenylamine (TPA)
derivatives,2 as well as carbazole derivatives,3 are most widely
studied for host materials because of their wide-band energy,
high triplet energy level, and good charge mobility. However,
the twisted TPA molecules lack rigidity, which is disadvanta-
geous in terms of thermal and morphological stabilities as host
materials in PhOLEDs.4a Thus far, considerable effort has been
made to improve lm morphological stability without sacri-
cing triplet energy of the TPA host. For example, to develop a
rigid triphenylamine structure, Shu's group5 described a facile
synthesis of a uorene/triarylamine hybrid, tris[4-(9-phenyl-
uoren-9-yl)phenyl]amine. Ma's group2a,4 designed a series of
fully or partially bridged triphenylamine derivatives, with the
bulky spacer rigid uorene and triphenylsilyl units rendering
the triphenylamine derivatives to be highly thermally durable
and morphologically stable. Furthermore, to balance charge
ux and improve the power efficiency of devices,6 the bulky
spacer of TPA in host materials can be electron-transport
moieties such as oxadiazole,7 triazole,8 phenylphosphine
On the other hand, solution-processed OLEDs are consid-
ered critical to the realization of low-cost and large-area
displays.12 For solution-processed blue and white PhOLEDs, the
challenge still lies in the design of a solution-processed host
material with high triplet energy (ET > 2.70 eV) and a high-lying
HOMO level.1b In order to achieve such a high triplet energy, the
extent of conjugation in the host molecules must be conned,
which in turn would usually impose constraints in molecular
size. However, for the host to form morphologically stable and
uniform amorphous lms in the solution process, it usually
requires the molecules to be bulky and steric enough to extend
the device's operational lifetime. As a result, the optimization of
one physical parameter oen adversely affects the other. Apart
from polymers12c,13 and some small molecules,12d,14 oligomers
have recently been approved to be a new class of host materials
for solution-processed PhOLEDs with high efficiency3a,f,12e,15 due
to their numerous special characteristics such as well-dened
conjugation lengths and molecular structures, high triplet
energy, ease of purication, characterization, and solution
processing.
In our previous work,16 we demonstrated a series of mono-
disperse multi-H shaped oligomers using a simple Friedel–
Cras reaction for blue electroluminescence diodes. In the
produced oligomers, the triphenylamine (or carbazole) unit was
connected to the sp3-hybridized carbon atom such that the
conjugation length and triplet energy of each individual
building block essentially remained unperturbed. Herein, we
further designed and synthesized another novel triphenyl-
amine-uorene oligomer (TPAF)3 (shown in Fig. 1) as the host
for PhOLEDs. We expected that the extended uorene periph-
eries and the hollow space structure of (TPAF)3 would improve
the host's compatibility with the dopant, resulting in a good
aDepartment of Polymer Science & Engineering, State Key Laboratory of Coordination
Chemistry, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, China. E-mail: quanyiwu@nju.edu.cn
bKey Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of
Adv. Mater (IAM), Nanjing University of Posts and Telecommunications, Nanjing
210046, China. E-mail: iamshye@njupt.edu.cn; wei-huang@njupt.edu.cn
cKey Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,
Heilongjiang University, Harbin, 150080, China
† Electronic supplementary information (ESI) available: 1H NMR. See DOI:
10.1039/c4tc01870b
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Scheme 1 Synthesis of the macrospirocyclic oligomer (TPAF)3.
Fig. 1 Structure of the host (TPAF)3.
lm forming ability, which would lead to highly efficient
phosphorescent devices. Finally, we fabricated PhOLEDs using
(TPAF)3 as the host. Due to an exciplex formation of this new
host material with TPBI or CBP, the device, in which only FIrpic
was employed as the dopant, achieved white light-emission. The
device's maximum current efficiency and luminance are 22.6 cd
Aꢀ1 and 14 213 cd mꢀ2, respectively. Moreover, at the practical
luminance of 100 and 1000 cd mꢀ2, the current efficiencies are
10.4 and 9.1 cd Aꢀ1. These values are close to those of white
PhOLEDs with a typical structure.3f,17 To the best of our
knowledge, although blue, green, orange and red PhOLEDs
based on exciplex formation have been reported recently,18 the
highly efficient exciplex based white PhOLEDs have not been
reported yet.
Fig. 2 1H NMR spectra of TPAF-OH and (TPAF)3.
2. Results and discussion
2.1. Synthesis
The synthetic procedure used to prepare oligomer (TPAF)3 is
shown in Scheme 1. First, N-(4-bromo-phenyl)-N-(4-methyl-
phenyl) aniline was reacted with n-BuLi in tetrahydrofuran at
ꢀ78 C, followed by the addition of 2,7-di-tert-butyluorenone
Fig. 3 13C NMR spectra of TPAF-OH and (TPAF)3.
ꢁ
7 disappears in the spectrum of (TPAF)3, and peaks (7.18–7.24
and 7.26–7.31 ppm) attributable to proton 6 and 4 also move to
lower eld and incorporate into a new double peak. In the 13C
NMR spectra shown in Fig. 3, the characteristic peak of the C-9
of the central uorene units moves from 84.3 to 64.2 ppm aer
self-condensation due to the disappearance of OH group. The
product was also analyzed by MALDI-TOF, and only a single
peak at 1600.4 m/z (shown in Fig. 4) can be found in MALDI-TOF
to result in the compound, TPAF–OH. Then, the oligomer was
prepared by self-condensation of TPAF–OH in mesitylene at 100
ꢁC for 5 h in the presence of triuoromethanesulfonic acid. This
polycondensation reaction follows the aromatic electrophilic
substitution mechanism of Friedel–Cras reaction.2a,5,16,19 The
reaction may also potentially give a series of linear structure
oligomers. Therefore, to optimize the production of the desired
cyclic oligomer, a very low monomer concentration (1%) has
been used, which favors the formation of the smaller cyclic
oligomer. The nal Friedel–Cras reaction afforded a high yield
of 80% for (TPAF)3 aer column chromatography purication.
The structure of (TPAF)3 was conrmed by 1H NMR, 13C NMR
and MS. In the 1H NMR spectra of TPAF–OH (see ESI, Fig. S1†),
the peaks at 2.31 ppm (singlet) and 1.32 ppm (singlet) are
obtained due to the resonance of the CH3 group on the tri-
phenylamine unit (proton 8) and the t-butyl group protons of
the uorene unit (proton 9), respectively. The peaks (Fig. 2) at
7.18–7.24 ppm (multiplet) and 6.96 ppm (triplet) can be
assigned to proton 6 and proton 7 (the proton at the para
position of nitrogen). Other characteristic peaks (such as
protons 1, 2, 3, 4 and 5) are also assigned. Aer the self-
condensation reaction, the characteristic peak of proton Fig. 4 MALDI-TOF mass spectroscopy of (TPAF)3.
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corresponding to the macrocyclic trimer (TPAF)3. All these roughness of 0.42–0.46 nm. This demonstrates that (TPAF)3
results demonstrate the formation of the C–C bond between the featuring macrospirocyclic structure has a good lm-forming
p-carbon of the triphenylamine unit and the C-9 of the central property.
uorene. The desired compound could be obtained in high
purity and agrees well with the structure of the anticipated
macrocyclic trimer.
2.3. Photophysical properties
The photophysical properties of (TPAF)3 are shown in Fig. 7(a)
with the key parameters outlined in Table 1. Two strong
absorption bands at ca. 283 and 315 nm arise mainly from the
absorptions of the uorene moieties and the TPA core, respec-
tively. There is a negligible shi in the absorption and photo-
luminescence spectra for (TPAF)3 when it is transferred from
the solution to the solid state, indicating the absence of strong
interchain interactions in the solid thin lm, i.e., molecules in
solid state were restrained effectively by the bulky uorene
peripheries. In order to measure accurately the triplet energy of
the oligomer, we performed its time-resolved phosphorescence
spectrum at 77 K with a delay of 300 ms such as to get rid of the
interference of uorescence. The 0–0 transition band at 440 nm
for (TPAF)3 was used to calculate the triplet energy, and it was
found to be 2.79 eV, which is adequately high to act as the host
of the blue triplet emitter of FIrpic (ET ¼ 2.65 eV).
2.2. Thermal and morphological properties
The thermal and morphological properties of (TPAF)3 were
characterized by differential scanning calorimetry (DSC),
thermal gravimetric analysis (TGA), and atomic force micros-
copy (AFM) with results shown in Fig. 5 and 6. The DSC curves
reveal no noticeable melting temperature and glass transition
temperature below 300 ꢁC, indicating that the oligomer is a
stable amorphous structure below 300 ꢁC. TGA analysis indi-
cates that (TPAF)3 is thermally stable with 5% weight loss at
ꢁ
temperatures over 500 C. Since excellent lm-forming proper-
ties of light emitting materials are vital to high device perfor-
mance, the surface morphologies of the spin-coated lms of
(TPAF)3 were measured by atomic force microscopy (AFM). As
shown in Fig. 6, the AFM images of solution-processed (TPAF)3
lms doped with FIrpic annealed at various temperatures (100,
120, 140, 160 ꢁC) reveal smooth and homogeneous lm
morphologies with small values of root-mean-square (RMS)
2.4. Electrochemical characterization
The electrochemical behavior of (TPAF)3 was examined by cyclic
voltammetry measurements (shown in Fig. 7(b)). (TPAF)3
exhibits a reversible oxidation process even under successive
multiple potential scans. In contrast, the oxidation process of
the pristine TPA is irreversible due to the electrochemical
dimerization of TPA through the active para positions.20 The
electrochemical result also proves that (TPAF)3 is a macrocylic
trimer rather than a linear structural trimer, as the latter
contains a uorenol moiety and an active proton at para-phenyl
position in TPA. The macrocylic structure efficiently blocks the
electrochemically active sites of TPA and imparts extraordi-
narily high electrochemical stability. The value of the onset
oxidation potential is 0.67 V, corresponding to an EHOMO of
ꢀ5.07 eV (relative to vacuum energy level). The photophysical
data (absorption onset energy) together with electrochemical
data (oxidation potential) allow us to estimate the LUMO energy
level to be ꢀ1.60 eV for (TPAF)3.
Fig. 5 TGA and DSC curves of (TPAF)3 in nitrogen at a heating rate of
20 ꢁC minꢀ1
.
Fig. 6 AFM topographic images (5 ꢂ 5 mm): (a) height map; (b) phase Fig. 7 (a) Absorption and photoluminescence spectra in CH2Cl2
diagram of the solution-processed (TPAF)3 films doped with 15 wt% solution and coated on the quartz glass slide, phosphorescence
FIrpic at annealing temperatures of 100, 120, 140, 160 ꢁC for 20 min spectrum in CH2Cl2 glass matrix at 77 K; (b) cyclic voltammetric curve
under a nitrogen atmosphere.
(10 successive scans) for the p-doping process of (TPAF)3.
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Table 1 Physical and electrochemical properties of (TPAF)3
Oligomer
labs [nm]
lem [nm]
PHc [nm]
440
EHOMO [eV]
ELUMOd [eV]
ꢀ1.60
ET [eV]
(TPAF)3
283, 315a, 288, 319b
380a, 380b
ꢀ5.07
2.79
a
b
c
d
In CH2Cl2 (10ꢀ6 mol Lꢀ1). In lm. Phosphorescence spectrum of oligomer in CH2Cl2 at 77 K. Calculated from the photophysical data
(absorption onset energy) together with electrochemical data (oxidation potential).
Fig. 9 Photoluminescence spectrum of (TPAF)3, (TPAF)3 and TPBI
(CBP) mixed film: excitation wavelength 420 nm, emission wavelength
575 nm.
Fig. 8 (a) Current density–voltage–luminance and (b) current effi-
ciency–luminance characteristic of the PhOLEDs with the configura- ETL to improve the EL spectra, we obtained a relatively pure
tion of ITO/PEDOT : PSS/(TPAF)3 : OXD-7 : FIrPic/CBP (0, 7, 10, 20, 35
nm)/TPBI(35 nm)/Ca/Ag; (c) EL spectrum of the devices; (d) CIE
coordinates of the device, (0.24, 0.32) at 6V, (0.27, 0.31) at 8V, (0.28,
0.30) at 9V.
white light emission with CIE coordinates of (0.27, 0.32). It is a
meaningful phenomenon that the device with only FIrpic
utilized as the dopant achieved white light emission, which has
not been reported before. Fig. 8 also shows J–V–L and current
efficiency–luminance characteristics of the PhOLEDs with
various CBP layer thicknesses. From Fig. 8, it can be seen that
with an insertion of CBP layer, the turn-on voltage has a distinct
2.5. Electroluminescence properties
To evaluate the utility of the oligomer (TPAF)3 as a host material
in PhOLEDs, devices with structure of ITO/PEDOT : PSS
(25 nm)/(TPAF)3 : OXD-7 : FIrpic (1 : 0.3 : 0.15) (40 nm)/CBP
(0–35 nm)/TPBI (35 nm)/Ca (10 nm)/Ag were fabricated, in
which PEDOT:PSS was used as the hole-transporting material;
OXD-7 was used to improve the charge balance; CBP and TPBI
were used as hole/exciton-blocking and electron-transporting
materials, respectively; and Ca was imported as the electron-
injecting layer. Fig. 8 shows J–V–L and current efficiency–lumi-
nance characteristics of the PhOLEDs with or without CBP layer.
The electroluminescence spectra of the PhOLEDs without CBP
layer have a maximum peak at 472 nm with a shoulder peak at
500 nm, which is the characteristic emission of the FIrpic triplet
emitter.21 Interestingly, we observed an extra peak around
575 nm in electroluminescence spectra. From the photo-
luminescence spectrum (shown in Fig. 9), we demonstrate that
this emission comes from the formation of exciplex at the
interface of (TPAF)3 and CBP or TPBI, as photoluminescence
spectrum of (TPAF)3 and TPBI (CBP) mixed lm shows a peak at
575 nm, which has not been found in the photoluminescence
spectrum of (TPAF)3 lm. These phenomena nally endow the
devices with the prospect of achieving white light emission.
Furthermore, with an inserted layer of CBP between EML and
drop from 8.1 V (0 nm) to 4.3–5.6 V (7–35 nm). The device with
15 nm CBP layer shows the highest performance with a
maximum luminance, current efficiency and turn-on voltage of
14 213 cd mꢀ2, 22.6 cd Aꢀ1 and 5.4 V, respectively. At the
luminance of 100 cd mꢀ2, the current efficiency reaches 10.4 cd
Aꢀ1, which slightly rolls off to 9.1 cd Aꢀ1 at 1000 cd mꢀ2. The
external quantum efficiencies are 5.3% and 4.4% at 100 and
1000 cd Aꢀ1, respectively. Fig. 8(d) shows the CIE coordinates of
the device at various voltages. There is only a slight change in
the CIE coordinates as the operating voltage increases, which
demonstrates the comparative stability of the white device.
3. Conclusions
In summary, a novel triphenylamine-uorene oligomer with
macrospirocyclic structure has been designed and prepared as a
host material for solution-processing based PhOLEDs, which
has excellent thermal, electrochemical stability and high triplet
energy. Due to the formation of exciplex, the devices based on
(TPAF)3 achieved white light-emission with a new structure in
which only FIrpic was employed as the dopant. The device
showed a comparatively high performance with a turn-on
voltage, maximum luminance and current efficiency of 5.4 V,
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14 213 cd mꢀ2 and 22.6 cd Aꢀ1, respectively. Moreover, at the
practical luminance of 100 and 1000 cd mꢀ2, the current effi-
ciencies are 10.4 and 9.1 cd Aꢀ1, respectively. Here, we
successfully present a new strategy for the device structure
design of the efficient solution-processed white PhOLEDs.
4.2. Materials and measurement
Tetrahydrofuran was dried with Na and distilled, prior to use.
2,7-Di-tert-butyluorene was purchased from Suzhou Bailitai
Technology Co., Ltd China. N-(4-Bromo-phenyl)-N-(4-methyl
phenyl) aniline was prepared according to the procedure given
in the literature.22
1H NMR and 13C NMR spectra of the materials were recorded
on a 500 MHz Bruker spectrometer in CDCl3, and tetrame-
thylsilane was used as an internal standard. Differential scan-
ning calorimetric (DSC) measurements were performed on a TA
Instruments DSC 2920, and thermogravimetric analysis (TGA)
data were obtained using PerkinElmer Thermal Analysis. Both
measurements were performed in a N2 atmosphere at a heating
rate of 20 ꢁC minꢀ1. Positive-ion MALDI-TOF/TOF mass spec-
trometry was performed using an Autoex II TOF/TOF time-of-
ight mass spectrometer (Bruker Daltonics, Germany). UV-vis
absorption spectra were obtained using a Shimadzu UV3600
spectrophotometer. Fluorescence measurements were carried
out on a FS-LS55 uorescence spectrophotometer. Atomic force
microscopy (AFM) measurements were carried out on Bruker
Nano Surfaces. Cyclic voltammetry measurement (CV) was
carried on a CHI600 electrochemical analyzer (Chenhua,
Shanghai, China) at room temperature in a conventional three-
electrode conguration consisting of a glassy carbon working
electrode, a platinum wire auxiliary electrode, and an Ag/AgCl
standard electrode as the reference electrode. The sweep speed
4. Experimental section
4.1. Synthesis
4.1.1 2,7-Di-tert-butyluorenone. To a solution of 2,7-di-
tert-butyluorene (4 g, 14.36 mmol) in 30 mL acetic anhydride,
chromium trioxide (4.3 g, 21.5 mmol) was slowly added for 9
hours, and the reaction mixture was stirred for 15 hours at room
temperature. Aer adding 2% hydrochloric acid and stirring for
30 minutes, the suspension was ltered off and washed with
cold water. The product was puried by column chromatog-
raphy on silica gel to get the product as a light yellow solid. 1H
NMR (500 MHz, CDCl3, d): 7.72 (s, 2H), 7.52 (d, J ¼ 10 Hz, 2H),
7.43 (d, J ¼ 10 Hz, 2H), 1.37 (s, 18H). 13C NMR (126 MHz, CDCl3,
d): 194.84, 152.27, 141.90, 134.63, 131.45, 121.50, 119.79, 35.00,
31.21. EIMS (m/z (%)): calcd 292.18, found 292.2 (100).
4.1.2 TPAF–OH. Into the solution of N-(4-bromo-phenyl)-N-
(4-methyl-phenyl) aniline (1.76 g, 5.2 mmol) in tetrahydrofuran,
n-BuLi (2.5 M in n-hexane, 2.04 ml, 5.1 mmol) was slowly added
at ꢀ78 ꢁC under a N2 atmosphere, and the reaction mixture was
stirred for 1 h before the addition of 2,7-di-tert-dibutyl-
uorenone (1.53 g, 5.2 mmol) in anhydrous tetrahydrofuran at
ꢀ78 ꢁC. The mixture was then warmed to room temperature and
stirred overnight, followed by quenching with 20 mL of water.
The aqueous layer was extracted with ethyl ether three times
before drying with anhydrous MgSO4. The crude product was
collected by evaporating off the solvent and was then puried by
column chromatography on silica gel to afford TPAF–OH as a
white solid (2.24 g, yield 78.4%). 1H NMR (500 MHz, CDCl3, d):
7.57 (d, J ¼ 10 Hz, 2H), 7.46 (s, 2H), 7.42 (d, J ¼ 10 Hz, 2H), 7.30
(d, J ¼ 10 Hz, 2H), 7.19–7.25 (m, 2H), 7.05–7.09 (m, 4H), 6.96–
7.03 (m, 5H), 2.32 (s, 3H), 1.35 (s,18H). 13C NMR (126 MHz,
CDCl3, d): 151.39, 150.41, 148.04, 147.05, 145.26, 137.34, 137.04,
132.77, 129.96, 129.15, 126.47, 126.08, 125.11, 123.83, 122.96,
122.34, 121.78, 119.44, 83.86, 35.10, 31.61, 20.91. MS (Auto-
exTOF/TOF): calcd 551.3, found 550.4.
was 0.1 V sꢀ1
.
4.3. Device fabrication and characterization
ITO-coated glass substrates were rinsed in deionized water, and
then ultrasonicated sequentially in acetone and ethanol.
Immediately prior to device fabrication, the ITO substrate was
treated in a UV-ozone oven for 20 min. Then, PEDOT:PSS was
ꢁ
spin-coated onto the ITO substrate and dried at 120 C for 30
min. Next, the emissive layer materials were dissolved in chlo-
robenzene and spin-coated on the PEDOT:PSS treated
substrate. The resulting lm was about 40 nm thick, as
measured by a Dektak surface prolometer. Finally, 1,3,5-tris(N-
phenylbenzimidazol-2-yl)-benzene (35 nm) was vacuum-depos-
ited as a hole-blocking layer, and Ca (10 nm) and Ag (100 nm)
were deposited as the cathode. EL spectra, device luminance,
and current voltage characteristics were recorded using a
combination of a Photo-Research PR-655 SpectraScan and a
Keithley 2636A Sourcemeter coupled with Si-potodiodes cali-
brated with PR655. The devices were tested in an inert box with
no protective encapsulation.
4.1.3 (TPAF)3. To a solution of TPAF–OH (1.15 g, 2.08
mmol) in 1,3,5-trimethylbenzene (115 mL), triuoromethane-
sulfonic acid (11 mL) ꢁwas added. The reaction mixture was
stirred for 5 h at 100 C under an atmosphere of N2 and was
then dried over anhydrous MgSO4. The crude mixture was
concentrated by evaporating off the solvent and puried by
column chromatography on silica gel to afford (TPAF)3 as a
Acknowledgements
1
white solid (0.91 g, yield 80%). H NMR (500 MHz, CDCl3, d):
7.65 (d, J ¼ 10 Hz, 6H), 7.48 (s, 6H), 7.39 (d, J ¼ 5 Hz, 6H), 7.14 The authors acknowledge the nancial support from the
(d, J ¼ 5 Hz, 12H), 7.01 (d, J ¼ 5 Hz, 12H), 6.89 (d, J ¼ 5 Hz, 6H), National Natural Science Foundation of China (21174061,
6.84 (d, J ¼ 5 Hz, 6H), 2.19 (s, 9H), 1.33 (s, 54H). 13C NMR (126 61106017), the National Key Basic Research Program of China
MHz, CDCl3, d): 151.93, 150.39, 146.30, 145.01, 140.12, 137.58, (973 Program, 2014CB648300), the Nature Science Foundation
132.51, 129.65, 129.03, 124.99, 124.29, 123.21, 122.97, 119.27, of Jiangsu Province (BK20131375, BM2012010), and the
64.71, 35.02, 31.65, 20.75. MS (AutoexTOF/TOF): calcd 1601.2, Program for Changjiang Scholars and Innovative Research
found 1600.4.
Team in University.
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