Solution-processed oxadiazole-based electron-transporting layer for white organic light-emitting diodes

Article abstract of DOI:10.1039/c5ra04599a

A novel alcohol-soluble electron-transporting small-molecule material comprising oxadiazole and arylphosphine oxide moieties, ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide) (OXDPPO), has been synthesized and characterized. Its

Full text of DOI:10.1039/c5ra04599a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Solution-processed oxadiazole-based electron-
transporting layer for white organic light-emitting
diodes†
Cite this: RSC Adv., 2015, 5, 36568
*
*
Ronghua Liu, Xinjun Xu, Jinghong Peng, Chuang Yao, Jinshan Wang and Lidong Li
A
novel alcohol-soluble electron-transporting small-molecule material comprising oxadiazole and
arylphosphine oxide moieties, ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide)
(OXDPPO), has been synthesized and characterized. Its single crystal structure, together with the
photophysical, electrochemical and thermal properties, has been investigated. This material not only
possesses a wide bandgap with a low HOMO level but also exhibits a strong p–p stacking with a
distance of 3.35 A˚. Moreover, this compound shows excellent thermal stability with a high glass transition
temperature of 104 ꢀC and a decomposition temperature of 384 ꢀC. The unique solubility in 2-propanol
makes it a good candidate for fabricating fully solution-processed multilayer organic light-emitting
diodes (OLEDs). Efficient solution-processed white OLEDs have been fabricated with this compound as
an electron-transporting layer (ETL). It was found that this ETL can greatly balance the electrons and
holes in devices with the high work-function metal cathode (Al) and an increase in luminous efficiency
of ꢁ70-fold can be achieved. The maximum luminous efficiency of devices with an ETL/Al configuration
is even higher than that of devices using a Ca cathode.
Received 16th March 2015
Accepted 16th April 2015
DOI: 10.1039/c5ra04599a
www.rsc.org/advances
exible substrates, and easy processability over large areas by
spin-coating, inkjet printing, or roll-to-roll coating.5,9–12
Introduction
In the past years, solution-processed hole-transporting
materials, emissive materials and host materials have been
developed.13–18 Usually, charge/exciton management plays an
important role in improving the performances of OLEDs.19–21
For the purpose of facilitating electron injection, low work
function metal cathodes such as Ba and Ca can establish ohmic
contacts with the emissive layer to achieve high-efficiency
devices. Nevertheless, these low work function metals are
sensitive to both oxygen and moisture, resulting in reducing of
device lifetime. Alternatively, the use of environment stable
metals (such as Al or Ag) oen shows poor device performances
due to a large electron injection barrier existing between the
cathode and the emitting layer.22 It is very necessary to intro-
duce an ETL to reduce the electron injection barrier from the
high work function metal cathode and to enhance the electron
transport ability in devices. However, most of ETLs are depos-
ited by vacuum thermal evaporation, which leads to a costly and
complicated fabrication process.23–29 Among the reported
electron-transporting materials (ETMs), oxadiazole (OXD) is
recognized as a good electron accepting component for
building compounds with high electron mobility.30–35 In addi-
tion, OXD derivatives exhibit wide band gaps because it restricts
extensions of p-conjugation beyond the ring even if the mole-
cule is co-planar.36,37 Such a merit is signicantly important in
phosphorescent devices where the wide bandgap can avoid
undesired triplet energy transfer from emitters to ETM and is
Organic light-emitting diodes (OLEDs) have attracted much
attention owing to their promising applications in full-color
display panels, exible displays, and solid-state lighting sour-
ces.1–6 OLEDs can be fabricated either through a vacuum evap-
oration method or through a solution-processed one. In order to
improve the device efficiency through balancing the charge
injection and transport, OLEDs usually adopt a multilayer
structure which involves using of a hole-transporting (or
injecting) layer (HTL), an emitting layer (EML) and an electron-
transporting (or injecting) layer (ETL) in device fabrication.7,8
However, fabrication of multilayer devices through the vacuum
evaporation process is complicated, and time and energy
consuming compared with the solution process. Although
highly efficient OLEDs have been produced via vacuum evapo-
ration techniques, fully solution-processed devices are more
desirable for large-size at-panel displays and solid-state
lighting due to their many unique advantages such as low-
cost and facile manufacturing process, compatibility with
School of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing 100083, P. R. China. E-mail: xuxj@mater.ustb.edu.cn; lidong@
mater.ustb.edu.cn; Fax: +86-10-82375712
† Electronic supplementary information (ESI) available: Cyclic voltammogram,
HOMO and LUMO orbital distributions, AFM, and EL spectrum of device with
ETL using Al as cathode and control devices using Al or Ca as the cathode.
CCDC 1044786. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5ra04599a
36568 | RSC Adv., 2015, 5, 36568–36574
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
also benecial for exciton blocking.38 Accordingly, OXDs are EA 1112 element analyzer. The molecular weights of
promising ETMs for OLEDs. Unfortunately, when used in compounds 1 and 2 were determined on an APEXII FT-ICR
devices, all the reported OXD-based ETMs were either deposited analyzer (Bruker Daltonics Inc.) using electrospray ionization
by vacuum evaporation (which is time and energy consuming) mass spectrometry (ESI-MS) and electron bombardment ioni-
or blended into the EML (in which a continuous electron- zation mass spectroscopy (EI-MS), respectively. The mass
transporting channel can hardly be formed).
spectra of 3 and OXDPPO were determined on a Bruker Dal-
Although there is of great interest for fabricating highly tonics BIFLEX III MALDI-TOF analyzer using the matrix-assisted
efficient multilayer OLEDs through sequential solution- laser desorption ionization time-of-ight (MALDI) mode. Cyclic
processing, the existing problem is that the former layer voltammograms (CV) were recorded on a conventional three-
would be dissolved during deposition of a latter layer. One electrode cell using a Pt button working electrode, a platinum
general strategy towards overcoming this problem is to utilize wire counter electrode and a Ag/AgCl reference electrode on an
the orthogonal solvents.39 Therefore, developing alcohol/water electrochemical workstation (CHI860D, CH instruments) in
(nonsolvent for most organic semiconductors) soluble ETMs acetonitrile solution containing 0.1 M tetrabutylammonium
will provide a good solution to this problem.
hexauorophosphate (Bu4NPF6) as the supporting electrolyte at
In this paper, we report the synthesis, characterization, and room temperature under an argon atmosphere. The redox
investigation of the structural, photophysical, electrochemical potentials were obtained with ferrocene/ferrocenium (Fc/Fc+) as
and thermal properties of a new alcohol-soluble OXD-based an internal standard in the redox system. Thermogravimetric
ETM, namely ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene)) analysis (TGA) was performed under a nitrogen atmosphere on
bis(diphenylphosphine oxide) (OXDPPO). Moreover, we have a TA Instruments SDT-Q600. The differential scanning calo-
fabricated white OLEDs with OXDPPO as an ETL and Al as a rimetry (DSC) analysis was obtained on a PerkinElmer Instru-
cathode. The device performance with OXDPPO as an ETL using ments DIAMOND DSC under a N2 ow. A heat-cool-heat method
high work-function metal (Al) as the cathode was signicantly was used with an initial heating rate of 20 C minꢂ1, a cooliꢂn1g
ꢀ
ꢂ1
ꢀ
ꢀ
improved in comparison with the device without the ETL, owing rate of 40 C min , and a nal heating rate of 20 C min
.
to enhancement in electron injection from the cathode with the Single crystal X-ray diffraction analysis was recorded on a
help of OXDPPO. Furthermore, the maximum luminous effi- Rigaku Saturn 724+ under a nitrogen atmosphere. The dimen-
ciency of device with Al cathode using OXDPPO as an ETL was sions of a single crystal measured were 0.45 ꢃ 0.44 ꢃ 0.38 mm3.
even comparable to that of the device with a Ca cathode. These The atomic force microscopy (AFM) images were obtained from
results demonstrate that OXDPPO is a promising solution- a Veeco DI Dimension V atomic force microscope operating in
processable ETM for high-performance OLEDs.
the tapping mode.
Experimental
Synthesis
Materials
The synthesis of OXDPPO was outlined in Scheme 1. The
intermediate 1 and 2 were obtained by using the general
procedure as reported in the literature.40 In the third step, the
intermediate 3 was synthesized by a low-temperature reaction.
Finally, the desired product was obtained with a high yield
(92%). OXDPPO was readily soluble and stable in common
organic solvents such as dichloromethane, methanol and 2-
Poly(N-vinyl carbazole) (PVK) and poly(3,4-ethylenedioxy-
thiophene):poly(styrenesulfonate) (PEDOT:PSS) (CLEVIOS PVP
4083) used in our experiments were purchased from Sigma
Aldrich and H. C. Starck Clevios GmbH, respectively, and were
used without further purication. 1,3-Bis[(4-tert-butylphenyl)-
1,3,4-oxadiazolyl]phenylene (OXD-7) was purchased from Nichem
Fine Technology Co. Ltd. Iridium(III)bis(4,6-diuorophenylpyr-
idinato-N,C20)picolinate (FIrpic) and bis(2-phenylbenzothiazolato-
N,C20)(acetylacetonate)iridium(III) (Ir(bt)2(acac)) were purchased
from Xi'an Polymer Light Technology Corp. Other reagents
were purchased from J&K Chemical and were used without
further purication. Unless otherwise stated, all the solvents
were purchased from Beijing Chemical Works and were used
as received. Tetrahydrofuran (THF) was distilled from
Na/diphenylketone.
1
propanol. All the compounds were characterized by H NMR,
13C NMR and MS. Moreover, the single-crystal determination on
OXDPPO conrmed the proposed structure.
Synthesis of 4-bromo-N0-(4-bromobenzoyl)benzohydrazide
(1). Hydrazine monohydrate (2.89 g, 57.7 mmol) was added
Characterization
1H NMR and 13C NMR spectra were carried out using a 400 and
101 MHz AC Bruker spectrometer, respectively. Ultraviolet-
visible (UV-Vis) absorption and photoluminescence (PL) emis-
sion spectra were measured by a Hitachi U-3900H spectropho-
tometer and a Hitachi F-7000 uorescence spectrophotometer,
respectively. Element analysis (EA) was conducted on a FLASH Scheme 1 Synthetic route for OXDPPO.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 36568–36574 | 36569

View Article Online
RSC Advances
Paper
dropwise to a mixture of triethylamine (6.68 g, 66.0 mmol) and 126.65. MS (MALDI-TOF) m/z: M+ calculated for C38H28N2P2O3
4-bromobenzoyl chloride (14.49 g, 66.0 mmol) in 150 mL of 622.59; found: 623.2 [(M + H)+]. EA found: C, 71.83; H, 4.53; N,
chloroform in an ice-water bath. The resulting mixture was 4.42%. C38H28N2O3P2 requires C, 73.31; H, 4.53; N, 4.50%.
stirred for 4 h at room temperature. The solvent was removed
and the solid was washed with petroleum ether and then a large
OLEDs fabrication and measurements
amount of deionized water twice to afford 1 (9.89 g, 75%) as a
1
The OLEDs was fabricated with structure of indium tin oxide
(ITO)/PEDOT:PSS (40 nm)/PVK : OXD-7 : FIrpic : Ir(bt)2(acac)
(80 nm)/OXDPPO (20 nm)/Al (80 nm). For comparison, we also
fabricated devices with structures of ITO/PEDOT:PSS (40 nm)/
PVK : OXD-7 : FIrpic : Ir(bt)2(acac) (80 nm)/Al (80 nm) and ITO/
PEDOT:PSS (40 nm)/PVK : OXD-7 : FIrpic : Ir(bt)2(acac) (80 nm)/
Ca (8 nm)/Al (80 nm). The pre-cleaning and hydrophilic treat-
ment of ITO glass substrates (10 U per square) was carried out
white solid. H NMR (400 MHz, DMSO): d(ppm) 10.64 (s, 2H),
7.85 (d, J ¼ 8.1 Hz, 4H), 7.74 (d, J ¼ 7.6 Hz, 4H). 13C NMR (101
MHz, DMSO): d (ppm) 165.43, 132.11, 132.03, 130.02, 126.21.
MS (ESI) m/z: M+ calculated for C14H10Br2N2O2 398.05; found:
421.1 [(M + Na)+].
Synthesis of 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (2). A
reaction mixture of 1 (8.50 g, 21.4 mmol) in 150 mL of SOCl2 was
heated to reux under nitrogen atmosphere for 5–6 h. The SOCl2
according to the reported procedures.41 For all the devices,
was evaporated and the residue was extracted with dichloro-
PEDOT:PSS was rstly spin-coated onto the ITO glass substrates
methane (3 ꢃ 60 mL) and water (50 mL). The organic layer was
and annealed at 120 ꢀC for 20 min in an oven to form a
dried over anhydrous sodium sulfate and the solvent was
40 nm thick lm. Following this, a chlorobenzene solution
removed. The crude product was puried by column chroma-
containing the mixture of PVK : OXD-7 : FIrpic : Ir(bt)2(acac)
tography on silica gel using petroleum ether : dichloromethane
(10 : 4 : 1 : 0.02 in m mꢂ1) with a total concentration of 13.6 mg
1
(5 : 1 in v/v) as eluent, yielding a white solid (5.97 g, 73%). H
mLꢂ1 was spin-coated onto the PEDOT:PSS layer and then
NMR (400 MHz, CDCl3): d (ppm) 7.98 (d, J ¼ 8.5 Hz, 4H), 7.67 (d,
baked at 100 ꢀC for 20 min in a nitrogen lled glove box (H2O <
J ¼ 8.5 Hz, 4H). 13C NMR (101 MHz, CDCl3): d (ppm) 164.07,
132.51, 128.36, 126.64, 122.65. MS (EI) m/z: M+ calculated for
0.1 ppm, O2 < 0.1 ppm) to form a 80 nm-thick EML. OXDPPO
dissolved in 2-propanol with a concentration of 10 mg mLꢂ1 was
spin-coated onto the EML and then baked at 80 ꢀC for 20 min in
a nitrogen lled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm) to form
a 20 nm thick ETL. Finally, Ca (8 nm) or Al (80 nm) were
deposited onto the ETL or EML as the cathode by thermal
evaporation under a vacuum of 3 ꢃ 10ꢂ6 torr. Film thickness
was measured by an Ambios Technology XP-2 prolometer. The
current density–luminance–voltage (J–L–V), luminous effi-
ciency–current density (h–J) were measured using a Keithley
2612 B source-measurement unit and a silicon photodiode
calibrated by a Spectrascan PR655 photometer. Electrolumi-
nescent (EL) spectra of devices with the OXDPPO/Al and Ca/Al
congurations were recorded on a Maya 2000Pro spectropho-
tometer (Ocean Optics). EL spectrum of device using the bare Al
cathode was measured by a Hitachi F-7000 uorescence spec-
trophotometer. Commission Internationale de l'Eclairage (CIE)
coordinates were calculated from the EL spectra.
C14H8Br2N2O 380.03; found: 380 (M+).
Synthesis of 2,5-bis(4-(diphenylphosphino)phenyl)-1,3,4-
oxadiazole (3). A solution of 2,5-bis(4-bromophenyl)-1,3,4-
oxadiazole (5.00 g, 13.2 mmol) in 130 mL dry THF was cooled
to ꢂ78 ꢀC under argon, and n-BuLi (1.6 M n-hexane solution,
19.8 mL, 31.68 mmol) was added dropwise. The reaction solu-
tion was stirred for
1
h
at ꢂ78 ꢀC. Then, chlor-
odiphenylphosphine (5.7 mL, 31.68 mmol) was also added
dropwise and the reaction was kept for additional 2 h at ꢂ78 ꢀC.
Aer that, the reaction solution was allowed to warm to room
temperature overnight. Aer removing of THF, the crude
product was extracted with CH2Cl2 (3 ꢃ 60 mL) and H2O (50 mL).
The organic layer was dried over anhydrous sodium sulfate and
the solvent was removed. The crude oil was puried by column
chromatography on silica gel using petrol ether : dichloro-
methane (3 : 1 in v/v) as eluent to afford 3 (4.52 g, 58%). 1H NMR
(400 MHz, CDCl3): d (ppm) 8.05 (d, J ¼ 7.9 Hz, 4H), 7.41 (t, J ¼ 7.3
Hz, 4H), 7.35 (d, J ¼ 8.6 Hz, 20H). 13C NMR (101 MHz, CDCl3): d
(ppm) 164.39, 136.14, 136.03, 134.07, 133.87, 129.23, 128.79,
128.72, 126.72, 126.66. MS (MALDI-TOF) m/z: M+ calculated for
C38H28N2OP2 590.59; found: 591.2 [(M + H)+].
Results and discussion
X-ray crystal structure
Synthesis of OXDPPO. The excess H2O2 (10 mL) was added to A single crystal of OXDPPO grown via slow evaporation of the
a solution of 3 (1.50 g, 2.54 mmol) in 130 mL CH2Cl2 and the CH2Cl2/acetone solution was determined by XRD with Mo Ka
mixture was stirred for 24 h at room temperature. The reacted radiation.† Crystallographic data at 173.15 K for OXDPPO:
ꢀ
solution was extracted with CH2Cl2 and H2O. Then, the organic triclinic, space group P1, Z ¼ 2, unit cell dimensions with a ¼
ꢀ
˚
˚
ꢀ
˚
layer was dried over anhydrous sodium sulfate and the solvent 8.6042(19) A, b ¼ 12.291(3) A, c ¼ 16.260(4) A, a ¼ 110.945(4) ,
3
ꢀ
˚
was removed. The crude product was puried by column chro- b ¼ 91.262(2) , g ¼ 94.507(3) , volume for 1598.7(7) A , F(000) ¼
matography on silica gel using petrol ether : methanol (20 : 1 in 658. A total of 20 966 reections were collected in the range of
v/v), yielding a white solid (1.45 g, 92%). 1H NMR (400 MHz, 1.343ꢀ # q # 27.502ꢀ, (hkl range indices: ꢂ11 # h # 11, ꢂ15 # k
CDCl3): d (ppm) 8.21 (d, J ¼ 6.7 Hz, 4H), 7.84 (dd, J ¼ 11.2, 8.3 Hz, # 15, ꢂ21 # l # 21), 7308 independent reections.
8H), 7.67 (dd, J ¼ 12.0, 7.5 Hz, 8H), 7.55 (d, J ¼ 7.1 Hz, 4H), 7.48
The ORTEP diagram of the single-crystal structure of
(dd, J ¼ 9.9, 4.7 Hz, 8H). 13C NMR (101 MHz, CDCl3): d(ppm) OXDPPO is shown in Fig. 1a. The central 2,5-diphenyl-1,3,4-
164.32, 137.36, 136.35, 132.94, 132.84, 132.38, 132.36, 132.19, oxadiazole core is almost planar with very small dihedral
132.12, 132.02, 131.15, 128.81, 128.68, 126.94, 126.82, 126.67, angles between benzene and oxadiazole rings (0.18ꢀ and 3.9ꢀ).
36570 | RSC Adv., 2015, 5, 36568–36574
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 2 UV-vis absorption (room temperature), PL emission (room
temperature), and phosphorescence (77 K) spectra of OXDPPO in
methanol solution, in thin film, and in CH2Cl2, respectively.
(ꢁ2.6 eV) which is a commonly used blue phosphorescent
emitter in white OLEDs.
Fig.
1 (a) Thermal ellipsoid plot (ORTEP) diagram and atomic
numbering for OXDPPO; (b) packing diagram of OXDPPO molecules.
Electrochemical properties and theoretical calculations
˚
A
The P]O bond lengths [P(1)–O(1)
¼
1.4847(12)
and
The electrochemical behavior of OXDPPO was investigated by
cyclic voltammetry method in acetonitrile solution under argon
atmosphere using Bu4NPF6 as the electrolyte at a potential scan
rate of 10 mV sꢂ1 (see Fig. S1, ESI†). Its lowest unoccupied
molecular orbital (LUMO) energy was calculated to be ꢂ2.83 eV
from the onset reduction potential, assuming the absolute
energy level of Fc/Fc+ redox couple to be 4.80 eV below
vacuum energy level. Its highest occupied molecular orbital
(HOMO) energy was estimated to be ꢂ6.42 eV according to its
LUMO energy and its optical bandgap. The deep HOMO energy
level of OXDPPO indicates that it could block holes and conne
excitons within the emissive layer. To investigate the structure–
property relationship of OXDPPO at the molecular level, the
geometry optimization and electronic properties of this
compound was carried out by Gaussian 09 D.01 program.44 The
geometry was optimized by density functional theory (DFT) at
˚
˚
P(2)–O(3) ¼ 1.4840(12) A] and average P–C bond lengths [1.804 A
˚
and 1.805 A for P(1) and P(2), respectively] are similar about
each phosphorus atom and comparable to that reported for
Ph3P]O.42 At one end, the bridging 2,5-diphenyl-
1,3,4-oxadiazole plane approximately bisects the Ph–P–Ph
bond angle and the oxygen atom is approximately in-plane with
the 2,5-diphenyl-1,3,4-oxadiazole perimeter (O–P–C–C torsion
angle of 13.65ꢀ), similar to what has been observed previously
for 9,10-bis(diphenylphosphine oxide) anthracene.43 Fig. 1b
shows the packing motif of OXDPPO molecules. A strong p–p
˚
stacking (d ¼ 3.35 A) among 2,5-diphenyl-1,3,4-oxadiazole
moieties can be found in the crystal, which will be benecial
to the electron transport through hopping mechanism.
Photophysical properties
Fig. 2 shows the UV-vis absorption, PL emission, and low- B3LYP/6-31G(d) level, and no imaginary frequency for the
temperature phosphorescent spectra of OXDPPO in methanol optimized structure was found. The electronic properties
solution, in thin lm, and in CH2Cl2, respectively. The absorp- (HOMO and LUMO distributions) of OXDPPO (see Fig. S2, ESI†)
tion spectrum shows two peaks in the dilute methanol solution were calculated at sHCTHhyb/6-311++G(d,p) level, and the
located at 205 and 296 nm. The absorption peak at 296 nm calculated HOMO and LUMO levels were ꢂ6.59 eV and ꢂ2.63
could be attributed to the p–p* transitions of the 2,5-diphenyl- eV, respectively. The spatial distribution of front molecular
1,3,4-oxadiazole core. Two absorption peaks were also observed orbital was visualized using GaussView. The HOMO and LUMO
in the thin lm, similar to those in the solution except for a levels are all distributed on the oxadiazole core, indicating that
slight red shi (6 nm) of the p–p* transition band. This the 2,5-diphenyl-1,3,4-oxadiazole moiety mostly determines the
phenomenon is due to the increased electron delocalization in HOMO/LUMO levels of the OXDPPO. What's more, there is no
the solid state. The energy gap of OXDPPO was 3.59 eV derived distribution on the diphenylphosphine oxide moieties, consis-
from the UV-vis absorption edge. Upon UV excitation, the PL tent with their weak conjugation degree. Usually, since elec-
spectrum has an emission peak at 361 nm in methanol solution trons transport through LUMOs, OXDPPO can thus be expected
and 370 nm in lm. The triplet energy level ET of OXDPPO was to have an electron transport ability due to the dominant
2.63 eV deduced from the phosphorescent spectrum in CH2Cl2 distribution of LUMO on the 2,5-diphenyl-1,3,4-oxadiazole core
at 77 K. This ET value is larger than that of the FIrpic dye and the close p–p stacking among these moieties.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 36568–36574 | 36571

View Article Online
RSC Advances
Paper
Thermal properties
The thermal properties of OXDPPO shown in Fig. 3 were investi-
gated by TGA and DSC. TGA curve revealed a high decomposition
ꢀ
temperature (Td, 5% weight loss) at 384 C, conrming the high
thermal stability of OXDPPO. The DSC curve revealed a well-
dened glass transition temperature (Tg) of 104 ꢀC, which is
much higher than other commonly used ETMs such as 2-(4-
biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) (60 ꢀC)7
and 4,7-diphenyl-1,10-phenanthroline (BPhen) (62 ꢀC).45 Both the
high decomposition temperature and the high glass transition
temperature indicate that OXDPPO can form thermally durable
and morphologically stable amorphous thin lm, which is
important for fabricating high-quality OLEDs. In addition, the
surface morphology of solution-deposited OXDPPO thin lm was
investigated by AFM (Fig. S3, ESI†). It demonstrates a smooth
surface with a root-mean-square (RMS) roughness of 0.25 nm.
Fig. 4 Electron mobility at different thicknesses of OXDPPO thin films
as a function of the square root of the electric field.
Space-charge-limited current (SCLC) carrier mobility
In order to investigate the electron-transporting property of
OXDPPO, we have applied SCLC measurement to evaluate its
electron mobility.46,47 The electron-only devices have been fabri-
cated with the structure of ITO/Al (45 nm)/OXDPPO (x)/Ca (8 nm)/
Al (80 nm), where x is 120, 160, and 190 nm, respectively. The 8
nm Ca was used to decrease the electron injection barrier between
the OXDPPO layer and the Al electrode, ensuring the ohmic
contact required by SCLC method. The 45 nm Al on ITO was
utilized to block hole injection from ITO electrode. Fig. 4 shows
the electron mobility at different thicknesses of the OXDPPO thin
lm as a function of the square root of the electric eld. The
electron mobility increases with both elevating the electric eld
and thickening the OXDPPO lm in the measured range. The
electron mobility increasing with the lm thickness can be
explained by the fact that, the contribution of interfacial effect to
the effective electron mobility of OXDPPO becomes weaker and
bulk properties are predominant in a thicker lm, resulting in
higher electron mobility.46,47 When the thicknesses of OXDPPO
thin lms are 120, 160 and 190 nm, the electron mobilities are
found to be 2.7 ꢃ 10ꢂ5, 7.0 ꢃ 10ꢂ5 and 1.1 ꢃ 10ꢂ4 cm2 Vꢂ1 sꢂ1
,
respectively, under an electric eld of 3 ꢃ 105 V cmꢂ1
.
Characterization of white organic light-emitting diodes
(WOLEDs)
WOLED with OXDPPO as the ETL to improve the device's effi-
ciency has been fabricated, where the cathode is Al. To compare
the device performances, another two WOLEDs without ETLs
using either Al or Ca as the cathodes were also fabricated as the
control devices. The device conguration and the chemical
structures of the relevant materials are shown in Fig. 5. The
luminance–voltage–current density (L–V–J) characteristics of
the three WOLEDs are shown in Fig. 6a and b. The luminous
efficiency–current density (h–J) characteristics of the three
WOLEDs are demonstrated in Fig. 6c. Obviously, the device with
the bare Al cathode (/Al) exhibited a very poor performance due
to the poor electron injection from high work-function Al (ꢁ4.3
eV) to the emitting layer (PVK : OXD-7 : FIrpic : Ir(bt)2acac). So
Fig. 3 TGA traces of OXDPPO recorded at a heating rate of 10 ꢀC
minꢂ1. The inset shows DSC curve of OXDPPO at a heating rate of Fig.
5
Device configuration and chemical structures of relevant
20 ꢀC minꢂ1
.
materials.
36572 | RSC Adv., 2015, 5, 36568–36574
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
OXDPPO/Al structure can be a good replacement to the Ca
cathode, which avoids the using of unstable low-work function
metal as electrodes in OLEDs. The EL spectrum of device with
the OXDPPO/Al conguration exhibited emission bands in the
sky-blue and the orange-red regions (see Fig. S4 (a), ESI†). Thus,
white emission with the corresponding CIE coordinates of
(0.30, 0.43) can be obtained. We also obtained the EL spectra of
control devices using Al or Ca as the cathode, and the corre-
sponding CIE coordinates were (0.20, 0.37) and (0.29, 0.41),
respectively (see Fig. S4 (b) and (c), ESI†). The CIE coordinates of
device with the OXDPPO/Al conguration were better than the
control devices. This can be ascribed to the carrier injection and
transport being more balanced when OXDPPO was introduced
into the device as ETL.
Fig. 6 (a) J–L–V characteristics of devices with the OXDPPO/Al and
Ca/Al configurations; (b) J–L–V characteristics of device using the
bare Al cathode; (c) h–J curves of devices with ETL using Al as the
cathode and without ETL using either Ca or Al as the cathode. The
filled symbols stand for current density and empty ones for brightness
in (a) and (b).
Conclusions
In conclusion, a novel alcohol-soluble electron-transporting
material OXDPPO based on oxadiazole and arylphosphine
oxide moieties was synthesized and conrmed by various
spectroscopic studies. OXDPPO exhibited a high decomposition
temperature and a high glass transition temperature (104 ꢀC)
revealed by the TGA and DSC studies. It shows an excellent lm-
forming ability with RMS roughness of 0.25 nm. All-solution-
processed WOLEDs with OXDPPO as ETLs using air stable Al
cathode had an outstanding performance with a maximum
luminous efficiency of 19.65 cd Aꢂ1 which is nearly 70-fold
higher than the device using bare Al cathode and is comparable
to the device with Ca cathode. Furthermore, the driving voltage
of the device using Al cathode can be lowered aer introduction
of OXDPPO as the ETL. In other words, OXDPPO can effectively
improve the electron injection and balancing electrons and
holes in devices, thus enhancing the OLEDs' performance.
These results suggest that OXDPPO would be a good candidate
for the application in multilayer EL devices with high efficiency
through a solution process.
only a very low luminance (Lmax ¼ 151 cd mꢂ2), a high turn-on
voltage (Vturn-on ¼ 11.21 V) and a low luminous efficiency (hmax
¼ 0.28 cd Aꢂ1) can be obtained (see Fig. 6b). By using the low
work-function metal Ca (2.8 eV) as the cathode (/Ca/Al), the
device showed much improved performance with hmax of 18.47
cd Aꢂ1. When OXDPPO was introduced into the device with Al
cathode (/OXDPPO/Al), signicantly improved performance can
be achieved with a maximum luminous efficiency of 19.65 cd
Aꢂ1, which is nearly 70-fold larger than that of the bare Al
cathode device and is even higher than that of the bare Ca
cathode device. This indicates the important role of OXDPPO in
balancing electrons and holes in OLEDs. The lower current
density of the OXDPPO based device (Fig. 6a) can be attributed
to the thicker organic layer thickness than the Ca/Al device.
Besides the luminous efficiency, device with the OXDPPO/Al
structure showed the lowest turn-on voltage of 5.10 V. Such a
turn-on voltage is much lower than that of the device with the
bare Al cathode and even a little lower than the device with the
Ca/Al cathode (see Table 1), indicating that the electron injec-
tion were signicantly facilitated by using OXDPPO as the ETL.
When the luminance reached up to 1000 cd mꢂ2, the device
with the OXDPPO/Al conguration still showed a high luminous
efficiency of 18.96 cd Aꢂ1. As a result, the device with an
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (51273020, 51373022) and Research Fund
for the Doctoral Program of Higher Education of China
(20130006110007). We acknowledge National Supercomputing
Center in Shenzhen for providing the computational resources
and Gaussian 09 (D.01).
Notes and references
Table 1 Performances of WOLEDs with and without the ETL
a
b
c
d
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2 J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332.
3 A. C. Grimsdale, K. Leok Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897.
ETL and cathode
structure
Von
[V]
Lmax
hmax
[cd Aꢂ1
h1000
[cd Aꢂ1
]
[cd mꢂ2
]
]
/Ca/Al
/OXDPPO/Al
/Al
5.70
5.10
11.21
29 907
20 954
151
18.47
19.65
0.28
16.48
18.96
—
4 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
¨
B. Lussem and K. Leo, Nature, 2009, 459, 234.
a
Turn-on voltage which is dened as the voltage at a brightness of 1 cd
5 K. T. Kamtekar, A. P. Monkman and M. R. Bryce, Adv. Mater.,
2010, 22, 572.
b
c
mꢂ2
.
Maximum luminance.
Maximum luminous efficiency.
Luminous efficiency at a brightness of 1000 cd mꢂ2
.
d
6 H. Sasabe and J. Kido, Chem. Mater., 2011, 23, 621.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 36568–36574 | 36573

View Article Online
RSC Advances
Paper
7 A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, 32 Y.-T. Chang, J.-K. Chang, Y.-T. Lee, P.-S. Wang, J.-L. Wu,
Chem. Mater., 2004, 16, 4556.
C.-C. Hsu, I.-W. Wu, W.-H. Tseng, T.-W. Pi, C.-T. Chen and
8 G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94.
C.-I. Wu, ACS Appl. Mater. Interfaces, 2013, 5, 10614.
9 B. W. D0Andrade and S. R. Forrest, Adv. Mater., 2004, 16, 33 C. Wang, G.-Y. Jung, A. S. Batsanov, M. R. Bryce and
1585.
10 A. Misra, P. Kumar, M. N. Kamalasanan and S. Chandra, 34 C. Liu, P. Zhao and W. Huang, Cent. Eur. J. Chem., 2007, 5,
Semicond. Sci. Technol., 2006, 21, R35. 303.
11 H. Wu, L. Ying, W. Yang and Y. Cao, Chem. Soc. Rev., 2009, 35 X. Wang, S. Guan, H. Xu, X. Su, X. Zhu and C. Li, J. Polym.
38, 3391. Sci., Part A: Polym. Chem., 2010, 48, 1406.
12 M. C. Gather, A. Kohnen and K. Meerholz, Adv. Mater., 2011, 36 P. Strohriegl and J. V. Grazulevicius, Adv. Mater., 2002, 14,
23, 233. 1439.
13 X. Gong, S. Wang, D. Moses, G. C. Bazan and A. J. Heeger, 37 M. Ichikawa, T. Kawaguchi, K. Kobayashi, T. Miki,
M. C. Petty, J. Mater. Chem., 2002, 12, 173.
¨
Adv. Mater., 2005, 17, 2053.
14 B. Ma, B. J. Kim, D. A. Poulsen, S. J. Pastine and
K. Furukawa, T. Koyama and Y. Taniguchi, J. Mater. Chem.,
2006, 16, 221.
´
J. M. J. Frechet, Adv. Funct. Mater., 2009, 19, 1024.
38 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Appl. Phys. Lett.,
2009, 94, 013301.
39 T. V. Pho, P. Zalar, A. Garcia, T.-Q. Nguyen and F. Wudl,
Chem. Commun., 2010, 46, 8210.
40 X. Zhang, K. Sun, Y. Liu, M. Xiong, P. Xia, Z. Li and Z. Cao,
Chin. J. Chem., 2010, 28, 1034.
15 J. J. Park, T. J. Park, W. S. Jeon, R. Pode, J. Jang, J. H. Kwon,
E.-S. Yu and M.-Y. Chae, Org. Electron., 2009, 10, 189.
16 S.-C. Lo, R. N. Bera, R. E. Harding, P. L. Burn and
I. D. W. Samuel, Adv. Funct. Mater., 2008, 18, 3080.
17 S.-C. Lo, R. E. Harding, C. P. Shipley, S. G. Stevenson,
P. L. Burn and I. D. W. Samuel, J. Am. Chem. Soc., 2009, 41 C. Yao, X. Xu, J. Wang, L. Shi and L. Li, ACS Appl. Mater.
131, 16681. Interfaces, 2013, 5, 1100.
18 W.-Y. Lai, J. W. Levell, A. C. Jackson, S.-C. Lo, P. V. Bernhardt, 42 K. A. Al-Farhan, J. Crystallogr. Spectrosc. Res., 1992, 22, 687.
I. D. W. Samuel and P. L. Burn, Macromolecules, 2010, 43, 43 Z. Fei, N. Kocher, C. J. Mohrschladt, H. Ihmels and D. Stalke,
6986.
Angew. Chem., Int. Ed., 2003, 42, 783.
19 Q. Wang and D. Ma, Chem. Soc. Rev., 2010, 39, 2387.
20 Q. Wang, I. W. H. Oswald, M. R. Perez, H. Jia, A. A. Shahub,
Q. Qiao, B. E. Gnade and M. A. Omary, Adv. Funct. Mater.,
2014, 24, 4746.
21 Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang and F. Wang,
Adv. Mater., 2009, 21, 2397.
22 G. E. Lim, Y. E. Ha, M. Y. Jo, J. Park, Y.-C. Kang and J. H. Kim,
ACS Appl. Mater. Interfaces, 2013, 5, 6508.
23 Y. Q. Li, M. K. Fung, Z. Xie, S.-T. Lee, L.-S. Hung and J. Shi,
Adv. Mater., 2002, 14, 1317.
24 C.-C. Wu, Y.-T. Lin, K.-T. Wong, R.-T. Chen and Y.-Y. Chien,
Adv. Mater., 2004, 16, 61.
25 S. O. Jeon, T. Earmme and S. A. Jenekhe, J. Mater. Chem. C,
2014, 2, 10129.
26 S.-J. Yoo, H.-J. Yun, I. Kang, K. Thangaraju, S.-K. Kwon and
Y.-H. Kim, J. Mater. Chem. C, 2013, 1, 2217.
27 J. Zhuang, W. Su, W. Wu, W. Li, Q. Shen and Q. Zhou,
Tetrahedron, 2013, 69, 9038.
28 A. B. Padmaperuma, L. S. Sapochak and P. E. Burrows, Chem.
Mater., 2006, 18, 2389.
29 S.-J. Su, T. Chiba, T. Takeda and J. Kido, Adv. Mater., 2008,
20, 2125.
44 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr,
J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith,
R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega,
J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken,
C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, R. L. Martin, K. Morokuma,
V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman,
J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09,
Revision D.01, Gaussian, Inc., Wallingford CT, 2013.
45 B. W. D'Andrade, S. R. Forrest and A. B. Chwang, Appl. Phys.
Lett., 2003, 83, 3858.
46 T.-Y. Chu and O.-K. Song, Appl. Phys. Lett., 2007, 90, 203512.
€
30 J. Bettenhausen, P. Strohriegl, W. Brutting, H. Tokuhisa and 47 M. A. Khan, W. Xu, H. Khizar ul, Y. Bai, X. Y. Jiang,
T. Tsutsui, J. Appl. Phys., 1997, 82, 4957.
31 D. Yokoyama, A. Sakaguchi, M. Suzuki and C. Adachi, Appl.
Phys. Lett., 2009, 95, 243303.
Z. L. Zhang and W. Q. Zhu, J. Appl. Phys., 2008, 103, 014509.
36574 | RSC Adv., 2015, 5, 36568–36574
This journal is © The Royal Society of Chemistry 2015