Novel electron transporting materials for highly efficient fully solution-processed green PhOLEDs with low rolls-off and drive voltage

Article abstract of DOI:10.1016/j.dyepig.2018.05.007

Two novel alcohol-soluble electron transporting materials, diphenyl(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)phosphine oxide (PhOXDPO) and ((1,3-phenylenebis(1,3,4-oxadiazole-5,2-diyl))bis(4,1-phenylene))bis(diphenylphosphine oxide) (Ph2OXDPO), were synthesized and utilized in fully solution-processed green phosphorescent organic light emitting diodes (PhOLEDs). The photo-physical, electrochemical and thermal properties, and electron mobility of PhOXDPO and Ph2OXDPO had been investigated in detail. The photo-physical and electrochemical result showed that the PhOXDPO and Ph2OXDPO had lower highest occupied molecular orbitals (HOMO) level (?6.24 eV and ?6.09 eV) and high triplet level (2.79 eV and 2.66 eV). Both PhOXDPO and Ph2OXDPO achieved high electron mobility of 6.6 × 10^?4 cm² v^?1 s^?1 and 6.0 × 10^?4 cm² v^?1 s^?1. The current efficiency of the device based on PhOXDPO reached 49.5 cd A^?1, and showed only 1.5% roll-off at 1000 cd m^?2. Due to the more matchable energy gap and low HOMO level of PhOXDPO, the PhOXDPO based device presented better efficiency than the device with Ph2OXDPO as ETL. Compared with the ETL-free device, the efficiencies of the devices based on PhOXDPO and Ph2OXDPO increased up to one order of magnitude, and remained a high level (38.666 cd A^?1 and 37.834 cd A^?1, at 10000 cd m^?2). The device used TPBI as ETL made by vacuum evaporation and the device ultilized Bphen as ETL made by spin-coating showed much lower efficiency and luminance, on every voltage, than the devices based on PhOXDPO and Ph2OXDPO.

Full text of DOI:10.1016/j.dyepig.2018.05.007

Accepted Manuscript
Novel electron transporting materials for highly efficient fully solution-processed green
PhOLEDs with low rolls-off and drive voltage
Cao Wu, Zhi-Ming Zhong, Xiang-Gao Li, Yin Xiao, Feng Peng, Xue Wang, Zhen-
Qiang Huang, Shi-Rong Wang, Lei Ying
PII:
S0143-7208(18)30168-2
10.1016/j.dyepig.2018.05.007
DYPI 6736
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 January 2018
Revised Date: 11 April 2018
Accepted Date: 4 May 2018
Please cite this article as: Wu C, Zhong Z-M, Li X-G, Xiao Y, Peng F, Wang X, Huang Z-Q, Wang S-R,
Ying L, Novel electron transporting materials for highly efficient fully solution-processed green PhOLEDs
with low rolls-off and drive voltage, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.05.007.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
Graphical abstract

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Novel electron transporting materials for highly efficient fully solution-processed green
PhOLEDs with low rolls-off and drive voltage
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Cao Wu,a,b,c Zhi-Ming Zhong,d Xiang-Gao Li,a,b,c Yin Xiao,a,b,c Feng Pengd, Xue Wangd, Zhen-Qiang
Huangd, Shi-Rong Wang a,b,c,* and Lei Yingd,*
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a. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China.
b. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,
China.
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c. Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin 300072, China.
d. 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: Shi-rong Wang, wangshirong@tju.edu.cn; Lei Ying,
msleiying@scut.edu.cn.
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Keywords: high efficiency, fully solution-processed, low rolls-off, low drive voltage, high
electron mobility
Abstract:
diphenyl(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)phosphine
((1,3-phenylenebis(1,3,4-oxadiazole-5,2-diyl))bis(4,1-phenylene))bis(diphenylphosphine
Two
novel
alcohol-soluble
electron
transporting
materials,
oxide
(PhOXDPO)
and
oxide)
(Ph2OXDPO), were synthesized and utilized in fully solution-processed green phosphorescent
organic light emitting diodes (PhOLEDs). The photo-physical, electrochemical and thermal
properties, and electron mobility of PhOXDPO and Ph2OXDPO had been investigated in detail.
The photo-physical and electrochemical result showed that the PhOXDPO and Ph2OXDPO had
lower highest occupied molecular orbitals (HOMO) level ( -6.24 eV and -6.09 eV) and high triplet
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level (2.79 eV and 2.66 eV). Both PhOXDPO and Ph2OXDPO achieved high electron mobility of
6.6×10-4 cm2 v-1 s-1 and 6.0×10-4 cm2 v-1 s-1. The current efficiency of the device based on
PhOXDPO reached 49.5 cd A-1, and showed only 1.5 % roll-off at 1000 cd m-2. Due to the more
matchable energy gap and low HOMO level of PhOXDPO, the PhOXDPO based device presented
better efficiency than the device with Ph2OXDPO as ETL. Compared with the ETL-free device, the
efficiencies of the devices based on PhOXDPO and Ph2OXDPO increased up to one order of
magnitude, and remained a high level (38.666 cd A-1 and 37.834 cd A-1, at 10000 cd m-2). The
device used TPBI as ETL made by vacuum evaporation and the device ultilized Bphen as ETL made
by spin-coating showed much lower efficiency and luminance, on every voltage, than the devices
based on PhOXDPO and Ph2OXDPO.
1. Introduction
Conventional organic light emitting diodes (OLEDs) are fabricated by vacuum evaporation
techniques, with which achieved many high efficient OLEDs with stable and smooth films,
including blue1-2, green3-4, red5-6 and white7-8 OLEDs. Solution-processed technology, compared
with vacuum evaporation, were more suitable for flexible devices and large scale low-cost
manufactory. However, fully solution-processed OLEDs have their instinct weakness that the
solution deposition of an upper layer could inevitably redissolve or destroy the underlying layer,
leading to poor film quality and devices performance.
Many methods have been used to solve this problem, such as photo-9-10 or thermo-11-12
crosslinking, ink-jet printing13-14, orthogonal solvents11, 15-21 etc. The orthogonal solvent method
has been taken to fabricate fully soluble multi-layer phosphorescent organic light emitting diodes
(PhOLEDs). For the requirement of orthogonal solvents, water/alcohol-soluble electron
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transporting materials (ETMs) are needed. As is known most studies about alcohol-soluble ETMs
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in PhOLEDs are focused on polymers15,
21. However, Sax, S. demonstrates that the
uncontrollable electrochemical doping effects could be caused by the attachment of ionic groups
to the polymers’ side chains22. In Contrast with polyelectrolytes, the synthesis routes of small
molecule ETMs are usually more simplified, which makes small molecular ETMs more convenient
to purify. Furthermore, the structure of small molecular ETMs tend to be instinct, which is
beneficial for the repeatability of PhOLEDs. For the above reasons, small molecular ETMs are
more suitable for fully solution-processed devices. A few studies focused on the small molecular
ETMs23-28. The characteristics of some ETMs were not sufficiently demonstrated, which makes it
hard to choose suitable ETMs for solution-processed PhOLEDs. So more studies about novel
alcohol-soluble ETMs should be performed.
In this work, we designed and synthesized two novel alcohol-soluble ETMs,
diphenyl(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)phosphine
oxide
(PhOXDPO)
and
((1,3-phenylenebis(1,3,4-oxadiazole-5,2-diyl))bis(4,1-phenylene))bis(diphenylphosphine
oxide)
(Ph2OXDPO). The molecular structures are shown in Scheme 1. These compounds consist of
diphenyl-phosphine oxide and oxadiazole. Oxadiazole, an excellent electron accepting
component, is considered an ideal part for ETMs with high electron mobility. Diphenyl-phosphine
oxide (DPPO) is considered a desirable part for alcohol-soluble property20, 29, and many ETMs
containing P=O moiety have been investigated30-32. What’s more, the molecular design has taken
the following aspects into consideration: 1) p-conjugation of oxadiazole derivatives can be
restricted within the ring to form wide bandgap, which is effective to forbid triplet energy
transfer from emitters to ETMs so as to strengthen exciton blocking33-34; 2) the P=O bond in DPPO
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will form insulating linkages, which makes it possible to construct so-called ‘charge−exciton
separation (CES)’ strategy, endowing ETMs low highest occupied molecular orbitals (HOMO) and
high triplet level to reduce the interfacial quenching of electron transporting layers (ETLs) and
emitters35-36, then achieve high efficient and low roll-off PhOLEDs37-38. Photo-physical,
electrochemical and thermal properties, and electron mobility of PhOXDPO and Ph2OXDPO were
investigated. Utilizing PhOXDPO and Ph2OXDPO as ETMs, fully solution-processed PhOLEDs were
fabricated. Also investigated were a series of device measurements, such as electroluminescence
(EL) spectrum, luminance and efficiency.
2. Experimental Section
2.1 Materials
All materials were used as received from commercial supplier (Tianjin Heowns Biochemical
Technology Co., Ltd.) without further purification. All solvents were purchased from Tianjin
Guangfu Fine Chemical Research Institute.
2.2 General procedures
Ultraviolet-visible absorption (UV-Vis) and photo-luminance (PL) spectra were obtained on a
Thermo Evolution 300 UV-Visible spectrometer and a HitachiF-4500 fluorescence spectrometer,
respectively. The phosphorescence spectrum was operated on a Spectrofluorometer F7000.
Differential scanning calorimetry (DSC) was recorded on a TA Q20 instrument operated with
heating rate of 10 ℃min-1 from 25 ℃ to 350 ℃ in nitrogen atmosphere (N2). The glass
transition temperature (Tg) was determined from the second heating scan loop. Temperature at a
5% weight loss was used as the decomposition temperature (Td). Thermal gravity analysis (TGA)
was recorded on a METTLER TOLEDO TGA/DSC1 Thermogravimetric Analyzer heating rate of 10 ℃
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min-1 from 30 ℃ to 600 ℃ in N2. X-ray diffraction curves were recorded on a Rigaku Miniflex
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600 with 2θ range of 0°-90°, scanning at a rate of 1° min-1. H NMR and 13C NMR were recorded
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with a AVANCE III HD (400 MHz) spectrometer in chloroform-d (CDCl3), Dichloromethane-d2
(CD2Cl2) or dimethyl sulfoxide-d6 (DMSO-d6) with tetramethysilane as reference. The element
analysis was performed with a VarioMICRO CHNOS elemental analyzer. Fourier transfer infrared
spectra (FT-IR) were measured as a KBr disk on a Thermo NICOLET380 spectrometer. High
resolution mass spectrum (HRMS) was performed with a Bruker MicrOTOF-QII.
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2.3 OLED fabrication and measurements
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Two fully solution-processed multilayer green PhOLEDs A and B using PhOXDPO and
Ph2OXDPO as alcohol-soluble ETLs were fabricated with the following structure: ITO/PEDOT:PSS
(40 nm)/PVK (20 nm)/Host2:IrG = 95:5 (50 nm)/PhOXDPO or Ph2OXDPO (15 nm)/CsF/Al. An
ETL-free device C was designed as control device with the structure as follow: ITO/PEDOT:PSS (40
nm)/PVK (20 nm)/Host2:IrG = 95:5 (50 nm) /CsF/Al. The emitting layer consisted of host material
Host2 and doped with the green phosphorescent dye IrG. PEDOT:PSS served as hole injection
layer, and CsF was used as electron injection layer. PVK served as hole transporting layer.
Indium tin oxide (ITO) glass substrates with sheet resistance of 10 Ω per square were
cleaned by ultrasonic wave with isopropanol, acetone, detergent, diluted water and isopropanol,
then dried in an oven. After O2 plasma treatment for 2 min, an anode modified layer was
achieved by spin-coating the PEDOT: PSS (CLEVIOSTM P VP AI 4083) colloidal solution on the ITO
side at 3000 rpm, followed with baking at 150 ℃ for 30 min. The 10 mg mL-1 concentrated PVK
was spin-coated on the PEDOT:PSS film. A 20 nm thick emitter layer was achieved after baking at
150 ℃ for 20 min. The 10 mg mL-1 concentrated host2: IrG = 95:5 was spin-coated on the PVK
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film. A 50 nm thick emitter layer was achieved after baking at 150 ℃ for 20 min. The 10 mg mL-1
concentrated PhOXDPO or Ph2OXDPO isopropanol solution was spin-coated on the emitter film.
Then above substrate was immediately transferred into the deposition chamber with a vacuum
degree of 2 × 10-6 pa. Then 1.5 nm cesium fluoride (CsF) and 100 nm aluminium (Al) were
deposited successively at a rate of ~0.1 Å s-1 and ~4 Å s-1, respectively. The thicknesses of the
spin-coated films were measured by a Dektak 150 surface profiler terrace detector. The active
area of the device is 4 × 4 mm2, determined by the cross breadth between the cathode (Al) and
the anode (ITO). The electroluminescent spectra were measured by a Ocean Optics USB2000+
photometer. Steady current-voltage-luminance characteristics were measured by a Keithley 2400
source meter and Konica Minolta Chroma Meter CS-200 with silicon photodiode under ambient
atmosphere with device encapsulation.
2.4 Synthesis and characterizations
The synthesis of PhOXDPO and Ph2OXDPO were accomplished as shown in the Scheme 1.
Tetrabutylammonium fluoride (TBAF) as catalyst, trimethylsilyl azide (TMSN3) as azide source and
4-bromobenzonitrile as reactant resulted in 5-(4-bromophenyl)-1H-tetrazole by the [3 + 2]
cycloaddition reaction.39 3 or 4 was prepared by refluxing the pyridine solution of 2 and benzoyl
chloride or isophthaloyl dichloride29. In the third step, the target compound PhOXDPO and
Ph2OXDPO were acquired by a modified reductive Coupling reaction40. The target compound was
fully characterized by NMR, HRMS, infra-red (IR) data, melting point. The detailed synthetic
procedure is described as follows.
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Scheme 1 Synthetic pathway of the electron transporting materials PhOXDPO and Ph2OXDPO
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Synthesis of 5-(4-bromophenyl)-1H-tetrazole (2): at 85 ℃ ,
a
mixture of
Tetrabutylammonium fluoride trihydrate (TABF∙3H2O) (1.31g, 5 mmol), 4-bromobenzonitrile
(1.82g, 10mmol) and trimethylsilyl azide (TMSN3) (1.73g, 15 mmol) was under vigorous stirring
conditions for 18 hours. After completion of the reaction, the mixture was allowed to cool to
room temperature, added with ethyl acetate and washed by 3×150mL hydrochloric acid. The
organic layer was isolated and the remaining aqueous phase was further extracted with ethyl
acetate. The organic phases were combined and dried using anhydrous MgSO4 to afford
5-(4-bromophenyl)-1H-tetrazole. Yield 97.2%. melting point (MP): 260-261 ℃. 1H NMR (400
MHz, DMSO-d6): δ8.19-7.89 (m, 2H), 7.83 (d, J = 8.6 Hz, 2H). 13C NMR (400 MHz, DMSO-d6): δ
132.98 (s), 129.36 (s), 125.20 (s).
Synthesis of 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (3): 2.25 g of 2 (10 mmol) and
4.22 g of benzoyl chloride (30 mmol) were mixed with 200 mL anhydrous pyridine in a 500 mL
round bottom flask. The resulting mixture was refluxed in a nitrogen atmosphere for 6 hours.
After cooled the reaction to ambient temperature, the solid product, collected by filtration, was
washed with water to afford a gray powder. Yield 98.1%. MP: 166-168 ℃. 1H NMR (400 MHz,
CDCl3): δ8.22-8.09 (m, 2H), 8.03 (d, J = 8.5 Hz, 2H), 7.71 (d, J = 8.5 Hz, 2H), 7.63-7.48 (m, 3H). 13C
NMR (400 MHz, CD2Cl2): δ 132.44 (s), 131.86 (s), 129.15 (s), 128.28 (s), 126.83 (s), 126.22 (s),
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123.84 (s), 123.01 (s), 53.99 (s), 53.72 (s), 53.45 (s), 53.18 (s), 52.91 (s).
Synthesis of 1,3-bis(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)benzene (4): 2.25 g of 2 (10
mmol) and 4.22 g of isophthaloyl dichloride (1015.10 mg, 5 mmol) were mixed with 300 mL
anhydrous pyridine in a 500 mL round bottom flask. The resulting mixture was refluxed in a
nitrogen atmosphere for 6 hours. After cooled the reaction to ambient temperature, the solid
product, collected by filtration, was washed with water to afford a gray powder. Yield 75.1%. MP:
293-294 ℃. 1H NMR (400 MHz, CDCl3): δ 8.90 (d, J = 17.1 Hz, 1H), 8.41-8.32 (m, 2H), 8.06 (t, J =
8.2 Hz, 3H), 7.81-7.68 (m, 4H). No 13C NMR data was recorded for this compound because of its
poor solubility in organic solvents such as CHCl3, CH2Cl2, THF and DMSO.
Synthesis of PhOXDPO: to a flask containing NiCl2∙6H2O (71.4mg, 0.3mmol), zinc (390mg,
6mmol), 2,2’-bipyridine (93.6mg, 0.6mmol), diphenylphosphine oxide (848.4mg, 4.2mmol) and
60mL dimethylacetamide (DMAc), 3 (903.42mg, 3mmol) was added. In nitrogen atmosphere, the
reaction mixture was then stirred using a stir bar at 110 ℃ for 24 h. After completion of the
reaction, the mixture was allowed to cool to room temperature and added with water. The
organic layer was isolated and the remaining aqueous phase was further extracted with CH2Cl2.
Then the organic phases were combined and dried using anhydrous MgSO4, and purified by silica
gel column chromatography using dichloromethane–methanol as the eluent to afford PhOXDPO.
Yield 53.5%. MP: 196-198 ℃. IR (cm-1, KBr): 3450, 1450, 1188, 838, 700, 545. 1H NMR (400 MHz,
CDCl3): δ 8.25 (dd, J = 8.4, 2.2 Hz, 1H), 8.15 (dd, J = 7.9, 1.7 Hz, 1H), 7.87 (dd, J = 11.5, 8.3 Hz, 1H),
7.74 – 7.66 (m, 2H), 7.63 – 7.47 (m, 5H). 13C NMR (400 MHz, CD2Cl2) δ 165.20 (s), 163.97 (s),
137.36 (s), 136.36 (s), 133.04 – 132.66 (m), 132.49 – 131.99 (m), 131.80 (s), 131.27 (d, J = 10.4
Hz), 129.31 (s), 128.81 (d, J = 12.2 Hz), 128.45 (d, J = 13.2 Hz), 127.25 – 126.96 (m), 126.78 (d, J =
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12.1 Hz), 123.89 (s), 54.12 (s), 53.85 (s), 53.58 (s), 53.31 (s), 53.04 (s), 0.90 (s). HRMS Found:
[M+Na]+ 445.1082; C26H19N2O2P requires [M+Na]+ 445.1076. HRMS Found: [M+H]+ 423.1263;
C26H19N2O2P requires [M+H]+ 423.1257. Elemental analysis found: C, 73.89; H, 4.55; N, 6.64; O,
7.59; C26H19N2O2P requires C, 73.93; H, 4.53; N, 6.63; O, 7.57; P, 7.33.
Synthesis of Ph2OXDPO: to a flask containing NiCl2∙6H2O (142.83mg, 0.6mmol), zinc (780mg,
12mmol), 2,2’-bipyridine (187.2mg, 1.2mmol), diphenylphosphine oxide (1.695g, 8.4mmol) and
60mL dimethylacetamide (DMAc), 4 (1.572g, 3mmol) was added. In nitrogen atmosphere, the
reaction mixture was then stirred using a stir bar at 110 ℃ for 24 h. After completion of the
reaction, the mixture was allowed to cool to room temperature and added with water. The
organic layer was isolated and the remaining aqueous phase was further extracted with CH2Cl2.
Then the organic phases were combined and dried using anhydrous MgSO4, and purified by silica
gel column chromatography using dichloromethane–methanol as the eluent to afford Ph2OXDPO.
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Yield 40.3%. MP: 266-268 ℃. IR (cm-1, KBr): 3430, 1439, 1111, 700, 560. H NMR (400 MHz,
CDCl3): δ 8.88 (s, 1H), 8.37 (d, J = 7.8 Hz, 1H), 8.28 (d, J = 7.9 Hz, 2H), 7.89 (t, J = 9.2 Hz, 2H),
7.80-7.64 (m, 4H), 7.64- 7.55 (m, 2H), 7.52 (d, J = 7.4 Hz, 4H). 13C NMR (101 MHz, CD2Cl2) δ
164.22 (d, J = 17.8 Hz), 137.49 (s), 133.11 – 132.47 (m), 132.30 (s), 131.96 (d, J = 9.9 Hz), 131.50
(s), 130.27 (s), 130.04 (s), 128.73 (d, J = 12.2 Hz), 126.84 (d, J = 12.1 Hz), 125.02 (d, J = 15.3 Hz),
54.01 (s), 53.74 (s), 53.47 (s), 53.20 (s), 52.93 (s). HRMS Found: [M+Na]+ 789.1790; C46H32N4O4P2
requires [M+Na]+ 789.1791. Elemental analysis found: C, 72.09; H, 4.22; N, 7.29; O, 8.35;
C26H19N2O2P requires C, 72.06; H, 4.21; N, 7.31; O, 8.35; P, 8.08.
3. Result and discussion
3.1 Physicochemical properties
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The Ultraviolet visible (UV−vis) absorpꢀon, photoluminescence (PL) and phosphorescent
spectra of PhOXDPO and Ph2OXDPO are elaborated in Fig. 1. For PhOXDPO and Ph2OXDPO, the
absorption peak were at around 293 nm and 296 nm respectively. For the existence of the P=O
group, the respective spectra is very similar to that of the 2,5-diphenyl-1,3,4-oxadiazole and
1,3-bis(5-phenyl-1,3,4-oxadiazol-2-yl)benzene chromophore41. This is because the P=O group cut
down the conjugation between the oxadiazole core and the phenyl group in PhOXDPO and
Ph2OXDPO. Upon UV excitation, the PL spectrum of PhOXDPO has the emission peak at 362
nm, while the emission peak of Ph2OXDPO lays at 360 nm and 345 nm. The currency-voltage
curves of PhOXDPO and Ph2OXDPO were investigated by cathodic sweeping in dichloromethane.
As is shown in Fig. S1, one irreversible reduction peak is at -2.28 V and -2.36 V respectively, while
no oxidation wave was observed. The lowest occupied molecular orbitals (LUMO) energy levels of
PhOXDPO and Ph2OXDPO were calculated to be -2.52 eV and -2.44 eV, with regard to ferrocene
(-4.8 eV). The LUMO level of the target materials could match well with the CsF/Al (work function
= -2.6 eV). By the absorption edge of the UV-vis absorption spectrum (Fig. 1a), the energy band
gap (Eg) of PhOXDPO and Ph2OXDPO could be estimated to be 3.72 eV and 3.65 eV. The HOMO
(HOMO = LUMO - Eg) value of PhOXDPO and Ph2OXDPO were -6.24 eV and -6.09 eV, which were
deep enough to block hole efficiently. Furthermore, phosphorescent spectra of PhOXDPO and
Ph2OXDPO showed the emission peak at 445 nm and 467 nm respectively, indicating that the
triplet energy level of PhOXDPO and Ph2OXDPO were 2.79 eV and 2.66 eV. Corresponding with
the HOMO level, the high triplet energy level of PhOXDPO and Ph2OXDPO also enable those
promising applications of ETLs on PhOLEDs.
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Fig. 1 (a) UV−vis absorpꢀon, (b) photoluminescence (PL) and (c) phosphorescent spectra of
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PhOXDPO and Ph2OXDPO
3.2 Thermal properties
The thermal properties were performed by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) in Fig. 2a-2b. The TGA curves of PhOXDPO and Ph2OXDPO
exhibited a decomposition temperature (Td, corresponding to 5% weight loss) of 251 ℃ and
408 ℃. Measured by DSC, the Tg value of Ph2OXDPO (151 ℃) was much higher than that of
PhOXDPO (117 ℃), which might be led by the molecular weight of target materials. Both of the
target materials had high Tg value, due to a twisted configuration of DPPO strengthening
molecular rigidity of target materials, resulting in a high Tg value42. High Tg value is quite
beneficial for organic optoelectronic materials to form stable films and enrich the choices of
solvents. The X-ray diffraction (XRD) spectra revealed the crystallization property and stability of
the spin-coated film (Fig. 2c-2d). As a result, the film showed featureless peak as compared to
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glass substrate no matter before annealing and after annealing 24 hours at 100 ℃ and 80 ℃,
implying that PhOXDPO and Ph2OXDPO could form stable film by spin-coating.43 Besides, the
Atomic Force Microscope (AFM) had been utilized to investigate the thermal stability of films for
PhOXDPO and Ph2OXDPO in Fig. S15. According to the AFM result, the root-mean-square (RMS)
roughness of PhOXDPO and Ph2OXDPO films before and after annealing (at 80 ℃ and 100 ℃)
were very similar, which indicated that the thermal stability of films for PhOXDPO and Ph2OXDPO
were good.
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Figure 2 (a)TGA and DSC (inset) curves of PhOXDPO and (b) Ph2OXDPO; (c) X-ray diffraction spectra of glass
substrate and PhOXDPO thin film on glass substrate; (d) X-ray diffraction spectra of glass substrate and
Ph2OXDPO thin film on glass substrate
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3.3 Electron-transporting property
To study the electron-transporting property of the target materials, space-charge-limited
current (SCLC) measurement was conducted to evaluate their electron mobility44. For the
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requirement of SCLC, electron-only devices had been designed and fabricated as follow: ITO/Al
(50 nm)/LiF (1 nm)/PhOXDPO or Ph2OXDPO (100 nm)/LiF (1 nm)/ Al (50 nm). The 50 nm Al on
ITO blocked holes from ITO electrode, and the LiF layer lowered down the energy gap between
the ETL and the electrode, ensuring the ohmic contact. Fig. 3 showed the electron mobility as a
function of the square root of the electric field. As is shown in Fig. 3, under the electric field of
0.3 MV/cm, PhOXDPO or Ph2OXDPO achieved electron mobility of 6.6×10-4 cm2/vs and 6.0×10-4
cm2/vs, which were higher by 2 orders of magnitude than commercial electron transporting
materials, such as Alq3 (~10-6 cm2V-1s-1, under 0.1-1 MV/cm).
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Figure 3 Electron mobility as a function of the square root of the electric field.
3.4 Electroluminescence (EL) performance
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Two fully solution-processed multilayer green PhOLEDs A and B using PhOXDPO and
Ph2OXDPO as alcohol-soluble ETL were fabricated with the following structure: ITO/PEDOT:PSS
(40 nm)/PVK (20 nm)/Host2:IrG = 95:5 (50 nm)/PhOXDPO or Ph2OXDPO (15 nm)/CsF/Al. The
configuration of devices are shown in Fig. 4a. And the energy level of all the materials in the
devices structure is exhibited in the Figure S16. An ETL-free device C was designed as the control
device with the structure as following: ITO/PEDOT:PSS (40 nm)/PVK (20 nm)/Host2:IrG = 95:5 (50
nm) /CsF/Al. The emitting layer consisted of host material Host2 and doped with the green
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phosphorescent dye IrG. PEDOT:PSS served as hole injection layer, and CsF was used as electron
injection layer. The molecular structure is shown in the Fig. 4b. As shown in Fig. 4c, the EL
spectrum of all the devices are green emission with the Commission Internationale de L’Eclairage
(CIE) coordinates of (0.32, 0.62). No emissions of host material and ETMs were observed. Besides,
the alcohol resistance of the emitting layer were studied. As illustrated by Fig. 4d, UV-vis
absorption spectra of emitter film on ITO substrate before and after isopropanol rince showed
little difference. And the maximum intensity after rince was only 6.7% weaker then the film
before rince. It can be concluded that the alcohol resistance of the emitting layer was well
enough for the upper ETLs.
The luminance–voltage–current density (L–V–J) curves of the three green OLEDs are
exhibited in Fig. 4d. Compared with the ETL-free control device, the luminance of devices A and B,
on every voltage, increased, as the same as current density. It demonstrated that the inducing of
PhOXDPO and Ph2OXDPO balanced electrons and holes in the devices, as well as confined the
excitons within the emitter to achieve higher current density and luminance. The
electroluminescence properties of all devices are summarized in table 1. The maximum
luminance of devices A and B rose up to 32633.0 cd m-2 and 41685.0 cd m-2, while the maximum
luminance of C was 9675.9 cd m-2. The existence of PhOXDPO and Ph2OXDPO also lowered the
operating voltage of OLEDs. Especially, when the luminance reaches 1000 cd m-2, the operating
voltage of experimental devices were only 6.3V and 5.9V, which were lower enough for
commercial uses. The low operating voltage could be devoted to the low HOMO and high
electron mobility of PhOXDPO and Ph2OXDPO.
As is shown in Fig. 4e, three sorts of efficiencies (current efficiency (CE), power efficiency (PE)
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and external quantum efficiency (EQE)) were measured and calculated. The maximum efficiencies
of C were moderate (18.8 cd/A, 9.7 lm/W, 5.2%). PhOXDPO (or Ph2OXDPO) endowed their
devices A (or B) with efficiencies up to 49.5 cd A (or 46.14 cd/A), 38.58 lm W (or 34.75lm/W) and
13.76% (or 13.03%), which are 2 to 3 fold those of C. Devices A and B exhibit much higher
efficiency than device C. When the luminance reached 1000 cd m-2, the efficiencies of devices A
and B are 3 to 4 times that of device C. Particularly, device A achieved maximum current
efficiency 49.5 cd/A, which slightly rolls off to 48.8 cd/A (1.5%) at 1000 cd m-2. This performance
was better than most of the reported green fully solution-processed phosphorescence OLED. At
the luminance of 10000 cd m-2, the CE of A and B were about 7 times that of C. The investigation
of PE and EQE could also draw the same conclusion. Hence, in comparison with the ETL-free
counterpart, the devices based on PhOXDPO and Ph2OXDPO, at the same luminance, could show
much higher efficiency and lower rolls-off.
Based on conventional ETMs, the holes transported much faster than electrons, localizing
the exciton recombination at the interface of the emitting layers (EMLs) and ETLs. However,
PhOXDPO and Ph2OXDPO had high electron mobility, deep HOMO level and high triplet level,
which made it possible to confine exciton within the EMLs, and then put forward to the reduction
of quenching the EML/ETL interface. In this sense, the electron mobility and HOMO level of ETMs
were beneficial for the efficiency and low roll-off for fully solution-processed green PhOLEDs.
With these properties, the current efficiencies of device A and B, at the luminance of 10000 cd
m-2, could remain current efficiencies of 38.666 cd A-1 and 37.834 cd A-1.
For further comparison, two devices
D and E based on commercial ETM
1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI) and 4,7-Diphenyl-1,10-phenanthroline
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(Bphen) were fabricated with the following structure: ITO/PEDOT:PSS (40 nm)/PVK (20
nm)/Host2:IrG = 95:5 (50 nm)/TPBI or Bphen (15 nm)/CsF/Al. The TPBI layer was made by
vacuum evaporation, while the layer of Bphen was fabricated by spin-coating, using isopropanol
as solvent. The luminance–voltage–current density (L–V–J) curves and the current efficiency (CE)
curve were shown in the Fig. 4e-4f. As is shown in the Fig. 4e, the luminance of device D and E,
on every voltage, is smaller than the Device A and B. As for the efficiency, Fig. 4f shows that the
efficiency of device A and B are always greater than the counterpart D and E. The detail
properties are shown in the Table 1. Especially, the OLEDs made by vacuum evaporation were
always show higher efficiency comparing to the devices made by solution process. So many
works on solution-processed PhOLEDs used ETLs by vacuum evaporation.45-49 However, PhOXDPO
and Ph2OXDPO showed much better efficiency than TPBI in the same devices. So it can be
concluded that PhOXDPO and Ph2OXDPO were promising ETLs in the fully solution-processed
PhOLEDs.
While the luminance and current density of device B were higher than those of device A,
device A still appeared higher efficiency than device B, which could be illustrated as follows: 1) in
contrast with Ph2OXDPO, the LUMO level of PhOXDPO had a minor energy gap with the work
function of CsF/Al; 2) PhOXDPO had a lower HOMO level and higher triplet level than Ph2OXDPO.
These two merits enabled the ETL to block holes and transport electrons more efficiently.
Table 1 the electroluminescence properties of devices
Devices Maximum Luminance
(cd m-2)
Drive Voltage (V)
CE (cd A-1)
PE (lm W-1)
EQE(%)
[c]
[a]
[b]
[c]
[b]
[c]
[d]
[b]
[c]
[d]
[b]
[d]
A
32633
3.0
6.3
8.8 48.765 38.666 49.503 24.709 13.804 38.580 13.554 10.747 13.759
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B
C
D
E
41685
9676
7841
3104
2.9
4.0
4.2
3.7
5.9
8.5
9.5
9.4
8.0 45.694 37.843 46.144 24.750 14.861 34.752 12.904 10.687 13.031
12.7 17.391 5.410 18.817 6.5041 1.349 9.7279 4.943 1.507 5.242
-
-
18.037
4.304
-
-
20.637 5.975
12.857 1.438
-
-
8.846 4.350
6.346 1.104
-
-
4.965
3.298
324
[a] measured at 1 cd/m2; [b] measured at 1000 cd/m2; [c] measured at 10000 cd/m2 ;[d] Maximum efficiencies.
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Fig. 4 (a) The configuration of PhOLEDs and (b) the chemical structures of employed materials; (c) the EL
spectra of A, B and C; (d) the UV-vis spectra of UV-vis absorption spectra of emitter film on ITO substrate before
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and after isopropanol rince; (e) the luminance–voltage–current density (L–V–J) characteristics of A, B, C, D and E;
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(f) Efficiency curves (CE, PE and EQE curves) of A, B, C, D and E versus luminance.
4. Conclusion
In summary, two novel alcohol-soluble electron transporting materials, PhOXDPO and
Ph2OXDPO, were designed and synthesized. The photo-physical, electrochemical and thermal
properties, and electron mobility of PhOXDPO and Ph2OXDPO were investigated. Interestingly,
both of PhOXDPO and Ph2OXDPO exhibit low HOMO level (-6.24 eV and -6.09 eV), high electron
mobility (6.60×10-4 cm2/vs and 6.02×10-4 cm2/vs) and triplet level (2.79 eV and 2.66 eV), which
enabled them to confine exciton within the EMLs to achieve devices with high efficiency. With
PhOXDPO and Ph2OXDPO as ETLs, green PhOLEDs were fabricated. Compared to the ETL-free
control device, the efficiencies of A and B increased significantly and exhibited a much better
roll-off performance. Particularly, the CE value of device A reached 49.5 cd/A, and only exhibited
a 1.48% roll-off at 1000 cd m-2. At the luminance of 10000 cd m-2, comparing with the device C,
the efficiencies of the devices A and B increased by one order of magnitude, and still maintained
a high level of 38.666 cd A-1 and 37.834 cd A-1. In comparison with device D (used TPBI as ETL
made by vacuum evaporation) and device E (used Bphen as ETL made by spin-coating), the
devices based on PhOXDPO and Ph2OXDPO showed much better efficiency and luminance on
every voltage. Due to the more matchable energy gap, lower HOMO level and higher triplet level
of PhOXDPO, the device A presented better efficiency than the device B. This work offers two
promising ETMs to develop highly efficient fully solution-processed green PhOLEDs with low
rolls-off and drive voltage.
Acknowledgements
The authors acknowledge the financial support from the National Key R&D Program of China
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(2016YFB0401303), and the organic functional material service platform of technical innovation,
Tianjin Technology R&D Program (14TXGCCX00017).
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Highlights:
1, two novel alcohol-soluble electron transporting materials had been synthesized and utilized on
green PhOLEDs.
2, the current efficiency of the device based on PhOXDPO reached 49.5 cd A-1, and showed only
1.5 % roll-off at 1000 cd m-2.
3, in comparison with the ETL-free device, the efficiencies of the devices based on PhOXDPO and
Ph2OXDPO increased up to one order of magnitude, and remained a high level (38.666 cd A-1 and
37.834 cd A-1, at 10000 cd m-2). The device used TPBI as ETL made by vacuum evaporation and
the device ultilized Bphen as ETL made by spin-coating showed much lower efficiency and
luminance, on every voltage, than the devices based on PhOXDPO and Ph2OXDPO.
4, The electron mobility of PhOXDPO and Ph2OXDPO achieved 6.6×10-4 cm2 v-1 s-1 and 6.0×10-4
cm2 v-1 s-1.

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The name, affiliation and e-mail address of all co-authors are listed as below:
Mr. Cao Wu
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin
300354, China
b. Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), Tianjin 300072, China.
E-mail: wucao@tju.edu.cn
Mr. Zhi-Ming Zhong
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.
E-mail: name.zzm@gmail.com
Prof. Xiang-Gao Li
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin
300354, China
b. Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), Tianjin 300072, China.
E-mail: lixianggao@tju.edu.cn
Prof. Yin Xiao
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin
300354, China
b. Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), Tianjin 300072, China.
E-mail: xiaoyin@tju.edu.cn
Mr. Feng Peng
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.
E-mail: pengfeng2012@163.com
Miss. Xue Wang
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.
E-mail: 854069165@qq.com
Mr. Zhen-Qiang Huang
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.
E-mail: 2795514138@qq.com
Prof. Shi-Rong Wang corresponding author
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin
300354, China
b. Collaborative Innovation Center of Chemical Science and Engineering

ACCEPTED MANUSCRIPT
(Tianjin), Tianjin 300072, China.
E-mail: wangshirong@tju.edu.cn
Prof. Lei Ying corresponding author
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.
E-mail: msleiying@scut.edu.cn

ACCEPTED MANUSCRIPT
Prof. Jun-qiao Ding: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail:
junqiaod@ciac.ac.cn
Prof. Lian Duan: Key Lab of Organic Optoelectronics & Molecular Engineering of the Ministry of
Education, Department of Chemistry, Tsinghua University, Beijing, P.R. China
duanl@mail.tsinghua.edu.cn
Prof. Wai-Yeung Wong: Institute of Molecular Functional Materials and Department of Chemistry,
Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong P.R. China; HKBU
Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen,
518057, P.R. China; Department of Electrical and Electronic Engineering, Southern University of
Science and Technology, Shenzhen, 518055, P.R. China; Department of Applied Biology and
Chemical Technology, The Hong Kong Polytechnic University, Hong Hom, Hong Kong P.R. China
wai-yeung.wong@polyu.edu.hk