Alcohol-Soluble Electron-Transport Materials for Fully Solution-Processed Green PhOLEDs

Article abstract of DOI:10.1002/asia.201800320

Two alcohol-soluble electron-transport materials (ETMs), diphenyl(4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)phosphine oxide (pPBIPO) and (3,5-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)diphenylphosphine oxide (mBPBIPO), have been synthesized. The phys

Full text of DOI:10.1002/asia.201800320

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Alcohol-soluble Electron Transport Materials for Fully Solution-
processed Green PhOLEDs
Authors: Fudong Chen, Shirong Wang, Yin Xiao, Feng Peng, Nonglin
Zhou, Lei Ying, and Xianggao Li
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201800320
Link to VoR: http://dx.doi.org/10.1002/asia.201800320
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

10.1002/asia.201800320
Chemistry - An Asian Journal
Alcohol-soluble Electron Transport Materials for Fully
Solution-processed Green PhOLEDs
Fudong Chen, [a,b] Shirong Wang, [a,b] Yin Xiao, [a,b] Feng Peng, [c] Nonglin Zhou[a,b],Lei
Ying*[c] and Xianggao Li*[a,b]
[a] Tianjin University, School of Chemical Engineering and Technology, Tianjin China
[b] Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
[c] Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials
and Devices, South China University of Technology, Guangzhou 510640, P. R. China.
*Corresponding author. Email address: lixianggao@tju.edu.cn (Xianggao Li); msleiying@scut.edu.cn (Lei
Ying)
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Abstract: Two alcohol-soluble electron transport materials (ETMs), diphenyl(4-(1-phenyl-1H-benzo[d]imidazol-2-
yl)phenyl)phosphine oxide (pPBIPO) and (3,5-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)diphenylphosphine oxide
(mBPBIPO), are synthesized. Physical properties of the two ETMs are investigated, which show high electron transport
mobilities of 1.67 × 10−4 푐푚2푉−1푠−1 and 2.15 × 10−4 푐푚2푉−1푠−1, high glass transition temperatures of 81 °C and
110 °C and deep LUMO energy levels of -2.87 eV and -2.82 eV, respectively. The solubility of PBIPO in n-butyl alcohol
is more than 20 mg mL-1, which can meet the requirement of the fully solution-processed organic light emitting diodes
(OLEDs). Basted on it, fully solution-processed green phosphorescent OLEDs using alcohol-soluble PBIPO as ETLs
are fabricated, which exhibit high efficiencies (current efficiency, power efficiency, external quantum efficiency) of 38.43
cd A-1, 26.64 lm W-1 and 10.87%. Compared with devices without PBIPO as ETMs, performance of these devices is
significantly increased, which indicates the excellent electron transport properties of PBIPO.
Introduction
In the recent years, organic light emitting diodes (OLEDs) have attracted intense attention owing to their
potential application in display panels, solid-state lighting sources and flexible displays [1]. Although highly
efficient OLEDs are still mainly produced by evaporation methods, solution-processed method is more suitable
for flexible devices and large-size flat-panel displays [2]. However, the most serious problem of solution-
processed method is that the lower layer will be redissolved and affected by the solvent used of the upper layer,
leading to poor performance of these devices[2b]. One approach to overcome this problem is to utilize orthogonal
solvents[3] in the spin-coating process, which need water soluble hole transport materials (HTMs), oil soluble
emitting layer (EML) and alcohol soluble electron transport materials (ETMs). The first two materials above such
[4]
as PEDOT: PSS
and Ir complex[5] have been developed, so alcohol soluble ETMs is urgent needed for
solution-processed OLEDs.
[4c, 6]
Solution-processed OLEDs have been fabricated using alcohol soluble polyelectrolytes as ETMs
.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
However, it has been demonstrated that the ionic side group attachment to the polymer side chains may lead to
the unanticipated electrochemical doping effect [7]. The synthesis and purification of small molecule alcohol
soluble ETMs can be more convenient compared to the polyelectrolytes, which is more suitable for solution-
processed OLEDs [2]. In order to improve the alcohol solubility of the small molecule ETM, it is necessary to
modify the ETM with strong electron-withdrawing groups, such as the diphenyl-phosphine oxide (PO) group [3a,
8]. PO group has merits as follows: 1) PO group is widely used as electron-withdrawing unit to achieve high
electron transport mobility, producing excellent ETMs [2b, 9]. 2) According to the theory of similar dissolve mutually,
the introduction of the polar phosphine oxide moiety can afford higher solubility in polar solvents, such as
methanol, 2-propanol and n-butyl alcohol.
Using alcohol soluble ETMs modified with PO to fabricate fully solution-processed OLEDs has been
reported. For example, in 2014, Wei Jiang et al. designed the novel alcohol soluble ETM, tris(4-
(diphenylphosphoryl)phenyl)benzene, for fully0 solution-processed white phosphorescent OLED (WPhOLED),
exhibiting a high current efficiency (CE) of 32.6 cd A-1 [2b]. In 2015, Ronghua Liu et al. designed the alcohol
soluble ETM, ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide), and fully solution-
processed WPhOLED were fabricated with a high CEmax of 18.96 cd A-1 [2a]. In 2016, Xinxin Ban et al. designed
1,3,5-tris(diphenylphosphoryl)benzene (TPO) with high triplet energy level (T1) and fully solution processed
WPhOLED with CEmax of 38.5 cd A-1 and power efficiency (PE) of 22.5 lm W-1 were fabricated utilizing TPO as
exciplex-type host materials [4a]
.
With good electron transport properties and deep LUMO levels, benzimidazole units are widely used as
ETMs in OLEDs.[10] In particular, 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) has been used as ETM
for OLEDs. Using TPBI as ETMs in phosphorescent OLED (PhOLEDs), external quantum efficiency (EQE) of
10.4 % has been obtained at much lower voltages compared to those without TPBI.[11] Additionally, TPBI exhibits
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
good hole-block properties owing to its high ionization potential.[12] Inspired by the excellent properties of PO
and benzimidazole, connecting PO with benzimidazole may result in high-performing ETMs with high electron
mobility and high triplet energy level.
Herein, alcohol soluble ETMs, diphenyl(4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)phosphine oxide
(pPBIPO) and (3,5-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)diphenylphosphine oxide (mBPBIPO) were
synthesized by attaching the PO group to the benzimidazole core. Their physical properties were investigated
and fully solution-processed green PhOLEDs were fabricated. Efficiencies (CE, PE, EQE) of 38.43 cd A-1, 26.64
lm W-1, 10.87% were obtained utilizing mBPBIPO as ETL. Compared with devices without PBIPO as ETMs,
efficiencies of these PhOLEDs were significantly increased by 1-2 folds and turn on voltages (Von) were reduced
after introducing PBIPO into devices, indicating the excellent electron transport properties of PBIPO.
Result and Discussion
Synthesis Routes
The synthesis procedure of diphenyl(4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)phosphine oxide
(pPBIPO) and (3,5-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)diphenylphosphine oxide (mBPBIPO) was
illustrated in Scheme 1. Firstly, amide was synthesized by chloride according to the literature [13]. Then
benzimidazole was synthesized by a cyclization reaction [13-14]. Finally, PBIPO was synthesized by a coupling
reaction [14]. All compounds were characterized by 1HNMR and HRMS (See ESI S1-S3).
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Scheme 1. Molecular structure and synthetic routes of pPBIPO and mBPBIPO.
Photophysical properties
Figure 1a shows the UV-vis absorption and PL emission of the two compounds in CH2Cl2. The two
compounds exhibit very similar absorption and emission spectra profiles. The absorption spectra of pPBIPO
and mBPBIPO show peaks at 232 nm, 301 nm and 233 nm, 308 nm in CH2Cl2, respectively. The maximum
absorption (휆푎푏푠) at 301 nm and 308 nm may be attributed to the benzimidazole-centered n-π* transition[15]. The
energy gap (Eg) values of the two materials are 3.54 eV and 3.59 eV, respectively, which are calculated from
the onset of the UV-vis absorption spectra. The maximum PL emission peaks of pPBIPO and mBPBIPO are
376 nm and 371 nm, respectively. The slight blue-shifted (5 nm) in the PL spectrum of mBPBIPO compared with
that of pPBIPO may be attributed to the weak molecule interaction of mBPBIPO due to its larger steric hindrance
and severed molecule distortion[15-16]
.
In addition, as shown in Figure 1b, the T1 levels of pPBIPO and mBPBIPO have been determined by the
highest energy vibronic subband of the phosphorescence spectra at 77 K, which is 2.63 eV and 2.67 eV,
respectively. The high triplet energy levels of pPBIPO and mBPBIPO can efficiently confine the emitting
materials triplet excitons in the EML and improve charge recombination in the devices [2b, 15]
.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Figure 1. (a) UV-Vis absorption and PL spectra of pPBIPO and mBPBIPO. (b) Phosphorescence spectra of pPBIPO and mBPBIPO
measured at 77K.
Electrochemical properties
Energy levels matching of semiconductors used in OLEDs are crucial to making high performance devices.
Cyclic voltammetry (CV) of the two compounds, pPBIPO and mBPBIPO, was performed using Ag/AgNO3 as the
reference electrode to investigate their energy levels (See ESI Figure S4). Anhydrous tetrabutylammonium
hexafluorophosphate (Bu4NPF6) was used as the supporting electrolyte in nitrogen-saturated CH2Cl2 solvent.
The LUMO energy levels of pPBIPO and mBPBIPO were calculated to be -2.87 eV and -2.82 eV from the onset
reduction potential using Fc/Fc+ as an internal standard.[17] HOMO levels were nearly -6.41 eV for both
compounds according to the LUMO levels and Eg obtained from the UV-vis absorption spectra.[18] The nearly
HOMO and LUMO levels may be attributed to the similar molecule structure.
Thermal properties and morphology
The thermal properties of the PBIPO were investigated through thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) in a nitrogen atmosphere (Figure 2a-2b). The glass transition
temperatures (Tg) of pPBIPO and mBPBIPO are 81 °C and 110 °C and the decomposition temperatures (Td) of
pPBIPO and mBPBIPO are 371 °C and 355 °C, respectively. The lower Td of mBPBIPO compared with pPBIPO
may be attributed to the weak molecule rigidity due to its large steric hindrance and severed molecule
distortion[19]. The excellent thermal stabilities are quite significant for organic optoelectronic materials under spin-
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
coating process.[20]
The power X-ray diffraction (XRD) spectra were measured to research the crystallization property and
stability of the spin-coating PBIPO film (Figure 2c-2d). No matter before annealing or after annealing, the films
show featureless peaks compared with glass substrate, implying that pPBIPO and mBPBIPO could form
amorphous film by spin-coating process.[21] What’s more, atomic force microscopy (AFM) was used to
investigate the surface morphology of PBIPO spin-coating films (See ESI Figure S6). The root-mean-square
(RMS) values of pPBIPO and mBPBIPO spin-coating films were 2.399 nm and 2.229 nm, respectively,
demonstrating the formation of homogeneously films.
Figure 2. (a) TGA and DSC (Inset) thermograms of pPBIPO. (b) TGA and DSC (Inset) thermograms of mBPBIPO. (c) X-ray diffraction
spectra of glass substrate and pPBIPO spin-coating film on glass substrate. (d) X-ray diffraction spectra of glass substrate and
mBPBIPO spin-coating film on glass substrate.
Electron transport properties
Electron transport mobility (휇e) is one of the most important parameter for electron transport materials.
Space-charge-limited current (SCLC) measurement was applied to evaluate 휇e of PBIPO [22]. The electron-only
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
devices were designed and fabricated with the construction of ITO/LiF (1 nm)/TPBI (10 nm)/PBIPO (120 nm)/LiF
(1 nm)/Al (120 nm), in which the LiF/Al layer served as the electron-injecting layer to ensure the ohmic contact
required for SCLC method, while the LiF/TPBI layer near ITO were used as hole-block layer to ensure the single
carrier device. According to the Mott-Gurney law:
9
퐸2
퐿
퐽 = 휇0휀0휀푟
8
exp(훾√퐸)
where J is the measured current density, 휇0 is zero-field carrier mobility, 휀0 is the vacuum permittivity (휀0
=
8,85 × 10−14 퐶푉−1푐푚−1), 휀푟 is the relative dielectric constant, E is the electron field, L is the thickness of the
organic layer, 훾 is the field-activation factor. Accordingly, the 휇0 of pPBIPO and mBPBIPO can be estimated
as 1.67 × 10−4 푐푚2푉−1푠−1 and 2.15 × 10−4 푐푚2푉−1푠−1, respectively (See Figure 3 or ESI Figure S5). The
zero-field carrier mobility of mBPBIPO is higher than that of pPBIPO because mBPBIPO molecule have two
benzimidazole moiety, providing two sites for electron transportation. The high 휇0 of pPBIPO and mBPBIPO
can match the hole transport mobility of HTM, which is beneficial to the balance of hole-electron carrier of
devices.[23]
Figure 3. Dependence of electron mobilities of pPBIPO and mBPBIPO in electric field.
Alcohol Solubility
The alcohol solubility of ETM is of vital significance for the preparation of fully solution-processed OLEDs
by orthogonal solvent methods. The solubility of PBIPO in different alcohol was investigated and the detailed
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
dates are listed in Table 1. It is obvious that PBIPO has excellent methanol solubility, which is more than 500
mg mL-1, and at the same time, its solubility in 2-propanol and n-butyl alcohol meets the requirement of fully
solution-processed OLEDs. Considering the boiling point, viscosity and other parameters of the above three
alcohol solvents, n-butyl alcohol solution of PBIPO was selected to fabricate the fully solution-processed OLEDs.
Table 1. Alcohol solubility of pPBIPO and mBPBIPO.
Compounds
pPBIPO
methanol (mg mL-1)
2-propanol (mg mL-1)
n-butyl alcohol (mg mL-1)
>500
>500
>20
>20
>20
>60
mBPBIPO
Electroluminescence properties
Emitting layer with good oil solubility and bad alcohol solubility is needed for fully solution-processed OLEDs.
Herein, host 2: IrG1=95:5 were selected as an emitting layer (EML), in which host 2 and IrG1 was the host and
guest green material respectively. The energy levels and chemical structures of materials used in devices were
shown in Figure 4-5. First of all, we fabricated Device A as a control with the construction of ITO/PEDOT:PSS(40
nm)/host2:IrG1=95:5(65 nm)/CsF(0.8 nm)/Al(100 nm), in which ITO/PEDOT:PSS served as anode,
host2:IrG1=95:5 served as the green phosphorescence EML, CsF/Al served as cathode and no ETL was utilized.
Due to the lack of ETL, which results in the high energy level barriers and the unbalance of hole and electron,
the two devices exhibited high Von of 4.8 V and low efficiency (CEmax, PEmax, EQEmax) of 16.80 cd A-1, 6.67 lm
W-1 and 4.73%. Then, in order to make sure that the n-butyl alcohol used in ETL preparation process would not
affect the emitting layer prepared before, Device B was fabricated based on Device A with the construction of
ITO/PEDOT:PSS(40 nm)/host2:Ir-G1=95:5(65 nm)/n-butyl alcohol(40 μL)/CsF(0.8 nm)/Al(100 nm) by washing
the emitting layer with 40 μL n-butyl alcohol. Efficiency (CEmax, PEmax, EQEmax) of Device B only showed small
drop of 15.24 cd A-1, 5.99 lm W-1 and 4.34% compared with that of Device A, which indicated that the emitting
layer was almost unaffected by n-butyl alcohol used in ETL preparation process. EL spectra, current density-
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
voltage-luminance (J-V-L) curves, current efficiency-luminance-power efficiency (CE-L-PE), external quantum
efficiency-luminance (EQE-L) curves were shown in Figure 6 and the detailed device parameters were
summarized in Table 2.
Based on it, Device C and D, with pPBIPO (Device C) and mBPBIPO (Device D) as ETLs, were fabricated
with the construction as follows: ITO/PEDOT:PSS(40 nm)/host2:IrG1=95:5(65 nm)/PBIPO(15 nm)/CsF(0.8
nm)/Al(100 nm). EL spectra, J-V-L curves, CE-L-PE, EQE-L curves were shown in Figure 6 and the device
parameters were listed in Table 2. After introducing PBIPO into devices, the hole and electron were much
balanced and the exciton was confined within the EML due to the high 휇푒 and high T1 level of PBIPO. Much
better device performance were obtained. The maximum luminance (Lmax) as high as 21441 cd m-2 and 39132
cd m-2 of Device C and D were observed while that of Device A was just 15521 cd m-2. And Von of Device C and
D was reduced to 4.0 V compared with that of 4.8 V for Device A. What’s more, pPBIPO (or mBPBIPO) endowed
their Device C (or D) with efficiencies (CE, PE, EQE) up to 25.50 cd A-1 (25.00 cd A-1), 11.37 lm W-1 (11.95 lm
W-1) and 7.20% (7.08%). Compared with Device A without PBIPO as ETL, Lmax of Device C and D were
increased by 38% and 152%, CEmax of Device C and D were increased by 52%.
In order to reduce the Von and improve the device efficiency, PVK was introduced to further reduce the
energy level barriers and block the triplet exciton in the ETM. Device E and F, with pPBIPO (Device E) and
mBPBIPO (Device F) as ETLs, were fabricated with the construction of ITO/PEDOT:PSS(40 nm)/PVK(20
nm)/host2: IrG1=95:5(65 nm)/PBIPO(15 nm)/CsF(0.8 nm)/Al(100 nm). ELspectra, J-V-Lcurves, CE-L-PE, EQE-
L curves were shown in Figure 6 and the device parameters were listed in Table 2. Obviously, efficiencies (CEmax
,
PEmax, EQEmax) of Device E and F were increased highly. For Device F, high efficiencies (CEmax, PEmax, EQEmax
)
of 38.43 cd A-1, 26.64 lm W-1, 10.87% and low Von of 2.9 V were observed. Compared with Device C and D,
efficiencies (CEmax, PEmax, EQEmax) of Device E and F were highly raised by 26%, 43%, 26% and 54%, 123%,
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
54%, respectively. Because the photoluminescence quantum efficiency (PLQE) of the ETM prepared on the
PVK layer (0.640) was higher than that observed on PEDOT:PSS (0.366), the PVK layer worked as an efficient
exciton-block layer. The much higher device efficiency obtained after PVK was introduced arose from the much
efficient hole injection and triplet exciton block effect.[24] However, the Lmax of Device E and F were reduced,
which can be attributed to the much lower current density due to the much higher resistance after introducing
the PVK layer [25]. In addition, one phenomenon should be noted, which is that the performance of devices used
mBPBIPO as ETLs is much better than that of pPBIPO after introducing PVK into devices as HTL. This
phenomenon can be ascribed to the much higher 휇푒 of mBPBIPO than pPBIPO, which can be more matchable
with the hole transport mobility of PVK, leading to much better carrier balance of devices.
Generally, although fully solution-processed has the advantage of low-cost and material saving, devices
fabricated by vacuum evaporation would exhibit much better performance due to the homogeneity of the thin
film. Herein, Device G utilizing TPBI, whose molecular structure is similar with mBPBIPO, as ETM was fabricated
with the construction of ITO/PEDOT:PSS(40 nm)/PVK(20nm)/host2: IrG1=95:5(65 nm)/TPBI(15 nm)/CsF(0.8
nm)/Al(100 nm) , in which TPBI was deposited by vacuum evaporation. EL spectra, J-V-L curves, CE-L-PE,
EQE-L curves were shown in Figure 6 and the device parameters were listed in Table 2. Results indicated that
Device G exhibited poorer performance, including high turn-on voltages (4.6 V), severer rolls-off, lower efficiency
(34.75 cd A-1, 21.14 lm W-1, 8.48%) and lower luminance (8037cd m-2), compared with Device E and F, especially
Device F. These results demonstrated the much better properties of the spin-coating mBPBIPO film than
deposited TPBI film.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Figure 4．The energy level diagram of Device A-G.
Figure 5. Molecular structure of materials used in devices
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Figure 6. Green electrophosphorescence properties of Device A-G. (a) EL spectra. (b) J-V-L curves. (c) CE-L-PE curves. (d) EQE-L
curves.
Table 2.Key parameters of Device A-G.
Lmax
CEa
PEa
EQEa
(%)
Von
(V)
Devices
CIE
(cd m-2)
(cd A-1)
(lm W-1)
Device A
Device B
Device C
Device D
Device E
Device F
Device G
15521
11663
21441
39132
16436
19674
8037
16.80/13.10/16.68/14.51
6.67/6.31/6.19/4.12
5.99/5.72/5.31/3.02
4.73/3.61/4.74/4.02
4.34/3.51/4.32/3.09
7.20/5.33/7.12/6.93
7.08/5.89/6.92/6.90
9.09/7.21/9.01/8.55
10.87/10.70/10.79/10.13
8.48/7.89/7.12/4.69
4.8
5.0
4.0
4.0
2.9
2.9
4.6
(0.33,0.62)
(0.33,0.62)
(0.33,0.61)
(0.34,0,61)
(0.33,0.62)
(0.33,0.62)
(0.32,0.63)
15.24/12.41/15.22/10.93
25.50/18.84/25.30/14.32
25.00/20.74/24.71/24.22
32.18/25.43/31.96/30.33
38.43/37.73/38.34/35.19
34.75/32.30/29.18/19.35
11.37/10.61/10.43/7.79
11.95/11.70/10.32/8.34
16.21/16.01/14.00/10.32
26.64/24.59/18.23/13.62
21.14/14.135/9.55/4.45
a: order of measured efficiency values: maximum, measured at 100 cd m-2, measured at 1000 cd m-2, measured at 5000 cd m-2.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Conclusion
In summary, two alcohol-soluble electron transport materials (PBIPO) were designed and synthesized and
their physical properties were investigated. The two ETMs exhibited deep LUMO levels of -2.87 eV and -2.82
eV, high T1 of 2.63 eV and 2.67 eV, high electron transport mobility of 1.67 × 10−4 푐푚2푉−1푠−1 and 2.15 ×
10−4 푐푚2푉−1푠−1, respectively. Green PhOLEDs utilizing PBIPO as ETMs were fabricated. For devices using
mBPBIPO as ETMs, high efficiencies (CE, PE, EQE) of 38.43 cd A-1, 26.64 lm W-1, 10.87% and low Von of 2.9
V were obtained. Compared with devices without PBIPO as ETMs, efficiencies of these devices were
significantly increased, indicating that PBIPO was an excellent alcohol-soluble ETM.
Experimental Section
Materials
THF was freshly distilled from sodium/benzophenone under argon (Ar) atmosphere before use. Other reagents and
solvents used for the synthesis and measurement were purchased from commercial suppliers without further purification.
General Procedures
1H NMR spectrum were obtained on a Bruker ACF400 (400 MHz) spectrometer with tetramethylsilane as reference. The
mass spectra data were obtained on miorOTOF-QII. Ultravioletevisible absorption (UV-Vis) and photoluminescence (PL)
spectra were measured on a Thermo Evolution 300 UV-Visible spectrometer and a HitachiF-4500 fluorescence spectrometer,
respectively. The phosphorescence spectra was measured on Spectrofluorometer F7000. TGA was recorded on a TA Q500 at
a heating rate of 10 °C min-1 under nitrogen atmosphere. DSC measurement was recorded on a TA Q20 instrument operated
at a heating rate of 10 °C min-1 under N2 atmosphere. Cyclic voltammetry was performed using a CHI600E analyzer at a scan
rate of 100 mV s-1. XRD spectra was recorded on MiniFlex 600. AFM was measured using NTEGRA Spectra.
Photoluminescence quantum efficiency was measured on IS080 LabSphere integrating sphere.
OLED device fabrication and measurement
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Indium tin oxide (ITO) glass substrates with sheet resistance of 30 Ω per square were firstly pre-cleaned with isopropanol,
acetone, detergent, diluted water and isopropanol under succession for 15 min, then dried in an oven, treated with UV-ozone
for 2 min. An anode modified layer was obtained by spin-coating the PEDOT: PSS aqueous solution on ITO substrates at 3000
rpm, followed with annealing at 150 °C for 20 min. The 10 mg mL-1 concentrated PVK was spin-coated on the PEDOT: PSS
layer and PVK layer was obtained by annealing at 150 °C for 20 min. Then ITO was transferred to the glove box and 10 mg
mL-1 concentrated host2: IrG1=95:5 was spin-coated on the PVK layer. A 65nm thick emitting layer was obtained by annealing
at 120 °C for 20 min. The 10 mg mL-1 concentrated PBIPO n-butyl alcohol solution was spin-coated on the emitter layer. The
ITO substrate above was transferred into the deposition chamber. Then 0.8 nm CsF was deposited at a rate of 0.1 Å s-1 and
100 nm Al was deposited at a rate of 1 Å s-1 under the vacuum of 2×10-6 Pa. For all devices, the light-emitting areas were
determined as 0.16 cm2.The EL spectra, luminance of device, and CIE coordinates were measured with Ocean Optics
USB2000+. The current-voltage-luminance characteristics were measured by Keithley 2400 source meter and Konica Minolta
Chroma Meter CS-200 with silicon photodiode under ambient atmosphere with device encapsulation. The thicknesses of the
spin-coated films were measured by the Dektak 150 surface profiler terrace detector.
Synthesis and characterizations
The pPBIPO was synthesized according to the reported procedure[15] and synthesis routes of mBPBIPO are as follows:
(1) Synthesis of 5-bromoisophthaloyl dichloride (compound 3)
Oxalyl chloride was added dropwise to a mixture of 5-bromoisophthalic acid (1.23 g, 5.0 mmol), 20 mLCH2Cl2 and DMF
(two drops) under N2 at room temperature and then refluxing. After 12 hours, removing the solvent by vacuum distillation to
afford the yellow oil liquid.
(2) Synthesis of 2,2'-(5-bromo-1,3-phenylene)bis(1-phenyl-1H-benzo[d]imidazole) (compound 4)
Compound 3 (5-Bromoisophthaloyl dichloride) was added dropwise to a mixture of N-phenyl-1,2-o-phenylenediamine
(2.03 g, 11 mmol), trithylamine (2.5 mL) and NMP (20 mL) under Ar at room temperature and then heated to 160 °C . After 24
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
hours, it was poured into water and extracted with CH2Cl2. The solvent was removed and the crude product was recrystallized
in CH2Cl2 and methanol to afford a grey solid of compound 4.Yield:1.08 g,40.0%。1H NMR (400 MHz, DMSO) δ 7.81 (d, J =
7.4 Hz, 2H), 7.75 (s, 1H), 7.63 (dd, J = 13.0, 4.4 Hz, 6H), 7.58 – 7.52 (m, 2H), 7.42 – 7.27 (m, 8H), 7.20 (d, J = 7.4 Hz, 2H).
(3) Synthesis of mBPBIPO
A mixture of compound 4 (0.540 g, 1.0 mmol), diphenylphosphine oxide (0.202 g, 1.0 mmol), Zn (0.131 g, 2.0 mmol), 2, 2-
bipyridine (0.0310 g, 0.20 mmol), NiCl2·6H2O (0.0238 g, 0.10 mmol) and DMAC (20 mL) was stirred under N2 at 150 °C for 24
hours. After cooling to room temperature, it was poured into water and extracted with CH2Cl2. The organic layer was dried with
MgSO4 and filtered. The crude product was isolated by silica gel column chromatography using methanol /CH2Cl2 as eluent to
afford a white solid.Yield:0.300 g, 45.2%. 1H NMR (400 MHz, DMSO) δ=8.32 (s, 1H), 7.80 (d, J = 7.7 Hz, 2H), 7.67 – 7.60 (m,
3H), 7.59 (d, J = 1.1 Hz, 1H), 7.55 (s, 1H), 7.52 (dd, J = 13.7, 5.1 Hz, 7H), 7.48 – 7.42 (m, 2H), 7.31 (dd, J = 17.3, 6.6 Hz, 9H),
7.26 (d, J = 4.4 Hz, 2H), 7.23 (s, 1H), 7.18 (d, J = 7.7 Hz, 2H); 13C NMR (101 MHz, CDCl3) δ=150.63, 142.93, 137.12, 136.18,
134.38, 133.91, 133.13, 133.03, 132.90, 131.99, 131.94, 131.92, 131.89, 131.84, 131.37, 131.24, 130.94, 129.94, 128.74,
128.60, 128.47, 127.23, 123.81, 123.23, 120.18, 110.58. HRMS calcd for C44H31N4OP: 662.73; found, 663.2, (m+1), 685.2, (M
+23).
Acknowledgements
This work was supported by the National Key Research and Development Program of China
(2016YFB0401303).
Key words: alcohol-soluble • electron transport material • fully solution-processed • PhOLED
[1] (a) C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett. 1987, 51 (12), 913-915; (b) J. Kido, M. Kimura, K. Nagai, Science. 1995, 267 (5202),
1332-4; (c) B. W. D'Andrade, S. R. Forrest, Adv. Mater. 2004, 16.
[2] (a) R. Liu, X. Xu, J. Peng, C. Yao, J. Wang, L. Li, Rsc Adv. 2015, 5 (46), 36568-36574; (b) W. Jiang, H. Xu, X. Ban, G. Yuan, Y. Sun,
B. Huang, L. Duan, Y. Qiu, Org. Lett. 2014, 16 (4), 1140-3.
[3] (a) N. Aizawa, Y. J. Pu, M. Watanabe, T. Chiba, K. Ideta, N. Toyota, M. Igarashi, Y. Suzuri, H. Sasabe, J. Kido, Nature Commun.
2014, 5, 5756; (b) K. S. Yook, S. E. Jang, S. O. Jeon, J. Y. Lee, Adv. Mater. 2010, 22 (40), 4479-83.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
[4] (a) X. Ban, K. Sun, Y. Sun, B. Huang, W. Jiang, Org. Electron. 2016, 33, 9-14; (b) S. Ye, L. I. Le, M. Zhang, M. Quan, Z. Zhou, L.
Guo, Y. Wang, M. Yang, W. Y. Lai, W. Huang, J. Mater. Chem. C. 2017; (c) A. Lorente, P. Pingel, A. Miasojedovas, H. Krueger, S.
Janietz, Acs Appl Mater Interfaces. 2017, 9 (28); (d) W. Jiang, J. Tang, X. Ban, Y. Sun, L. Duan, Y. Qiu, Org. Lett. 2014, 16 (20),
5346-9.
[5] (a) H. J. Bolink, E. Coronado, S. S. Garcia, M. Sessolo, N. Evans, C. Klein, E. Baranoff, K. Kalyanasundaram, M. Graetzel, M. K.
Nazeeruddin, Chem. Commun. 2007, 31 (31), 3276-3278; (b) M. Lepeltier, F. Dumur, G. Wantz, N. Vila, I. Mbomekallé, D. Bertin, D.
Gigmes, C. R. Mayer, J. Lumin. 2013, 143 (6), 145-149; (c) Y. Fang, B. Tong, S. Hu, S. Wang, Y. Meng, J. Peng, B. Wang, Org.
Electron. 2009, 10 (4), 618-622; (d) G. Ge, G. Zhang, H. Guo, Y. Chuai, D. Zou, Inorg. Chim. Acta. 2012, 362 (7), 2231-2236; (e) R.
J. Holmes, S. R. Forrest, T. Sajoto, A. Tamayo, Appl. Phys. Lett. 2005, 87 (24), 243507-243507-3.
[6] (a) F. Huang, L. Hou, H. Wu, X. Wang, H. Shen, W. Cao, W. Yang, Y. Cao, J. Am. Chem. Soc. 2004, 126 (31), 9845-53; (b) F. Huang,
H. Wu, Y. Cao, Chem. Soc. Rev. 2010, 39 (7), 2500-21.
[7] (a) P. Strohriegl, J. V. Grazulevicius, Adv. Mater. 2002, 14 (20), 1439-1452; (b) S. Sax, N. Rugen-Penkalla, A. Neuhold, S. Schuh, E.
Zojer, E. J. W. List, K. Müllen, Adv. Mater. 2010, 22 (18), 2087-2091; (c) S. Sax, G. Mauthner, T. Piok, S. Pradhan, U. Scherf, E. J.
W. List, Org. Electron. 2007, 8 (6), 791-795.
[8] X. He, L. Chen, Y. Zhao, S. Ng, X. Wang, X. Sun, X. Matthew)Hu, Rsc Adv. 2015, 5 (20), 15399-15406.
[9] (a) S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, J. Mater. Chem. 2009, 19 (33), 5940-5944; (b) P. E. Burrows, A. B. Padmaperuma,
L. S. Sapochak, P. Djurovich, M. E. Thompson, Appl. Phys. Lett. 2006, 88 (18), 2082-9.
[10] A. P. Kulkarni, C. J. Tonzola, A. Amit Babel, S. A. Jenekhe, Chem. Mater. 2004, 16 (23), 4556-4573.
[11] T. D. Anthopoulos, J. P. J. Markham, E. B. Namdas, I. D. W. Samuel, S. C. Lo, P. L. Burn, Appl. Phys. Lett. 2003, 82 (26), 4824-
4826.
[12] Z. Gao, C. S. Lee, I. Bello, S. T. Lee, R. M. Chen, T. Y. Luh, J. Shi, C. W. Tang, Appl. Phys. Lett. 1999, 74 (6), 865-867.
[13] Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T. Kajita, M. A. Kakimoto, Adv. Funct. Mater. 2008, 18 (4), 584-590.
[14] F. A. Alasmary, A. M. Snelling, M. E. Zain, A. M. Alafeefy, A. S. Awaad, N. Karodia, Molecules. 2015, 20 (8), 15206-23.
[15] B. Wang, X. Lv, B. Pan, J. Tan, J. Jin, L. Wang, J. Mater. Chem. C. 2015, 3 (42), 11192-11201.
[16] S. J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 2008, 20 (11), 2125-2130.
[17] E. Ahmed, T. Earmme, S. A. Jenekhe, Adv. Funct. Mater. 2011, 21 (20), 3889–3899.
[18] N. Zhou, S. Wang, Y. Xiao, X. Li, Chem – Asian J. 2017.
[19] M. Cekaviciute, J. Simokaitiene, D. Volyniuk, G. Sini, J. V. Grazulevicius, Dyes Pigm. 2017, 140, 187-202.
[20] X. Zhao, S. Wang, J. You, Y. Zhang, X. Li, J. Mater. Chem. C. 2015, 3 (43).
[21] W. Sun, N. Zhou, Y. Xiao, S. Wang, X. Li, Chem – Asian. J. 2017, 12 (23), 3069.
[22] (a) T.-Y. Chu, O.-K. Song, Appl. Phys. Lett. 2007, 90 (20), 151; (b) M. A. Khan, W. Xu, Khizar-ul-Haq, Y. Bai, X. Y. Jiang, Z. L. Zhang,
W. Q. Zhu, Z. L. Zhang, W. Q. Zhu, J. Appl. Phys. 2008, 103 (1), 913.
[23] W. S. Jeon, O. Hyoung-Yun, J. S. Park, J. H. Kwon, Mol. Cryst. Liq. Cryst. 2011, 550 (1), 311-319.
[24] N. Aizawa, Y. J. Pu, T. Chiba, S. Kawata, H. Sasabe, J. Kido, Adv. Mater. 2014, 26 (45), 7543-6.
[25] N. Zhou, X. Shao, S. Wang, Y. Xiao, X. Li, Org. Electron. 2017, 50.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800320
Chemistry - An Asian Journal
Fully solution-processed green PhOLEDs utilizing alcohol-soluble PBIPO as ETMs.
This article is protected by copyright. All rights reserved.