Tuning electrical properties of phenanthroimidazole derivatives to construct multifunctional deep-blue electroluminescent materials

Article abstract of DOI:10.1039/c7tc04972b

In this work, we introduce an n-type group, TPPO (triphenyl phosphine), to the N1-position of violet-blue fluorophore phenanthroimidazole (PI) and successfully develop one deep-blue (TPAPOPPI) and two violet-blue emitters (3-CzPOPPI and CzBPOPPI) for OLEDs (organic light-emitting diodes). With a highly twisted linkage, the TPPO group shows negligible influences on their photophysical properties of the new materials and the materials inherit highly efficient deep-blue and violet-blue emission of the PI unit and its C2-connected arylamine skeletons. Meanwhile, the TPPO group can open a new channel to transport the electron. The electron injection and transport abilities of the developed emitters are enhanced. Non-doped devices using the 3-CzPOPPI and the CzBPOPPI emitters exhibit an EQE_max (external quantum efficiency) of 5.08% and 4.42% with CIE (Commission Internationale de l'èclarage) coordinates of (0.156, 0.061) and (0.157, 0.071), respectively. Similar efficiencies and even deeper blue emissions (CIE_y = 0.050 for 3-CzPOPPI and 0.044 for CzBPOPPI) were observed in OLEDs with these emitters doped with 4,4′-bis(N-carbazolyl)-1,1′-biphenyl. TPAPOPPI is demonstrated to be a multifunctional deep-blue emitter and presents impressive performances when serving as non-doped (EQE_max = 6.69%, CIE: (0.152, 0.095)), and doped (EQE_max = 6.61%, CIE: (0.154, 0.068)) as well as a high-performance host for yellow phosphorescent OLED. By doping a yellow phosphorescent dye (PO-01) into a TPAPOPPI host, a white OLED with a high EQE of 20.3% and a low driving voltage of 3.2 V (at 1000 cd m^-2) is achieved.

Full text of DOI:10.1039/c7tc04972b

View Article Online
View Journal
Journal of
M at erials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Z. ZHU, S. Ni, W.
Chen, M. Chen, J. Zhu, Y. Yuan, Q. Tong, F. WONG and C. Lee, J. Mater. Chem. C, 2018, DOI:
1
0.1039/C7TC04972B.
Volume
4
Number
1
7
January 2016 Pages 1–224
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
Tuning electrical properties of phenanthroimidazole derivatives to construct
multifunctional deep-blue electroluminescent materials
a,b
c
a
d
d
e
d*
ZeꢀLin Zhu, ShaoꢀFei Ni, WenꢀCheng Chen, Miao Chen, JieꢀJi Zhu, Yi Yuan, QingꢀXiao Tong,
a
a,b*
FuꢀLung Wong, and ChunꢀSing Lee
a
Center of SuperꢀDiamond and Advanced Films (COSDAF) & Department of Chemistry, City
University of Hong Kong, Hong Kong SAR, PR China. Eꢀmail: apcslee@cityu.edu.hk
b
City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hiꢀ
Tech Industrial Park, Nanshan District, Shenzhen 518000, China
c
Department of Chemistry, South University of Science and Technology of China, Shenzhen,
5
18055, P.R. China
d
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered
Structural Materials of Guangdong Province, Shantou University, 243 University Road, Shantou,
Guangdong, 515063, PR China. Eꢀmail: qxtong@stu.edu.cn
e
Jiangsu Key Laboratory for CarbonꢀBased Functional Materials & Devices, Institute of Functional
Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and
Technology, Soochow University, Suzhou, 215123, PR China.
Abstract: In this work, we introduce a nꢀtype group, TPPO (triphenyl phosphine), to the N1ꢀposition of
violetꢀblue fluorophore phenanthroimidazole (PI) and successfully develop one deepꢀblue (TPAPOPPI)
and two violetꢀblue emitters (3-CzPOPPI and CzBPOPPI) for OLEDs (organic lightꢀemitting diodes).
With highly twisted linkage, the TPPO group shows negligible influences on their photophysical
properties of the new materials and the materials inherit high efficient deepꢀblue and violetꢀblue emission
of the PI unit and its C2ꢀconnected arylamine skeletons. Meanwhile, TPPO group can open a new channel
to transport electron. The electron injection and transport abilities of the developed emitters are enhanced.
Nonꢀdoped devices using the 3-CzPOPPI and the CzBPOPPI emitters exhibit EQEmax (external quantum
efficiency) of 5.08% and 4.42% with CIE (Commission Internationale de l'Èclarage) coordinates of
(
0.156, 0.061) and (0.157, 0.071), respectively. Similar efficiencies and even deeper blue emissions (CIEy
0.050 for 3-CzPOPPI and 0.044 for CzBPOPPI) were observed in OLEDs with these emitters doped
=
in 4,4’ꢀbis(Nꢀcarbazolyl)ꢀ1,1’ꢀbiphenyl. TPAPOPPI is demonstrated to be a multifunctional deepꢀblue
emitter and presents impressive performances when serving as nonꢀdoped (EQEmax = 6.69%, CIE: (0.152,
0
.095)), doped (EQEmax = 6.61%, CIE: (0.154, 0.068)) as well as a highꢀperformance host for yellow
phosphorescent OLED. By doping a yellow phosphorescent dye (POꢀ01) into a TPAPOPPI host, a white
ꢀ2
OLED with a high EQE of 20.3% and a low driving voltage of 3.2 V (at 1,000 cd m ).

Journal of Materials Chemistry C
Page 2 of 19
View Article Online
DOI: 10.1039/C7TC04972B
Keyword: triphenyl phosphine, electron transport ability, deepꢀblue electroluminescence, multifunctional.
Introduction
In the past few decays, the advancement of electroluminescent material has greatly accelerated the
1–5
development and commercialization of OLEDs (organic lightꢀemitting diodes).
Especially, the
5
6
emergence of phosphorescent (Ph) and thermally activated delayed fluorescent (TADF) materials enable
an OLED to harvest all excitons for photon generation and achieve high EQE (external quantum
2
,3,7,8
efficiency) for both display and lighting applications.
EQEs of stateꢀofꢀtheꢀart skyꢀblue and green
In terms of the red counterparts, an EQE of
9–12
PhOLED and TADF OLED have already surpassed 30%.
2
4% was reported for PhOLED and a rapid progress from 8% to 14.5% was observed in a very short
6
,13,14
period for TADF OLED.
Moreover, with optimized device structures, green and red OLED
15
possessing practical lifetimes were also realized. The mature of these longer wavelength emitters makes
it more urgent to develop high performance deepꢀblue emitters which can lower power consumption and
1,3,15
widen color gamut of fullꢀcolor displays and enhance color rendering index of lighting.
Whereas, the
nonradiative dꢀd transition of the core metal atoms and relatively low triplet energy level of common
16–18
organic luminophores limited the development of deepꢀblue phosphorescent and TADF emitters.
Besides, the popular host DPEPO for deepꢀblue phosphorescent and TADF emitters was reported to be
19
unstable and not suitable for practical application. Host materials with high triplet energy (> 3 eV) and
20
good electrical properties are still being explored.
Restricted by the spin statistics rule, OLEDs using fluorescent emitters can only achieve EQEs
around 5%. Fortunately, with progresses of device fabricating technique, this demerit can be effectively
2
1
22
23
24
remedied by using exciplex host, TADF host, TADF assistant dopant or phosphorescent dopants .
On the other hand, many deep blue fluorescent materials often have advantages in color purity, low
1
efficiency rollꢀoff at high luminance, as well as multifunctional (e.g as host). They are thus considered as
important complimentary materials to the highꢀefficiency phosphorescent and TADF emitters. To serve as
a multifunctional material, the emitter should simultaneously have a deepꢀblue emission, high
photoluminescent quantum yield (PLQY) in solid state, proper triplet energy (E ) for sensitizing the
T
ꢀ2
dopant and excellent electrical properties to reduce device turnꢀon voltage (voltage at 1 cd m , V ) and
on
driving voltages for improving power efficiency (PE) and to maintain good performance at high
brightness. However, it is difficult to develop organic materials with simultaneously deepꢀblue emission
25,26
and good carrier injection and transporting properties.
It is because the intrinsically wide energy gaps
of deepꢀblue emitters aggravate carrier injection barriers. A strong electron pullꢀpush molecular system
has been suggested to be a possible solution, but this strategy often sacrifices the color purity of

Page 3 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
27,28
emitter.
In many cases, to retain a deepꢀblue emission in solid state, deepꢀblue fluorophores often
contain bulky groups to suppress intermolecular interaction and avoid excessive redꢀshifts in solid
28,29
state.
However, this strategy will impede carrier transport, especially for electron, resulting in higher
3
0,31
V and lower PE in OLED.
on
TPPO (triphenyl phosphine) is a nꢀtype group possessing excellent electron injection and
32–34
transport abilities and is a common building block for electron transport materials and host materials.
However, TPPO is rarely used in the design of emitters. This is because of its strong electron withdrawing
ability may undermine the color purity of deepꢀblue emitters and the conjugation break in the P atom of
35
TPPO group will lower the molecules’ fluorescent yield. In our molecular design (Scheme 1), the TPPO
unit is attached to the N1 position of the phenanthroimidazole (PI) plane to form a nearly orthogonal
linkage. In this way, we are aiming at using the highly twisted linkage to weaken conjugation between the
two units effectively to minimize (ICT). The emission color is mainly dominated by the PI units and its
C2ꢀconnected (No. 2 carbon atom in PI units, Scheme S2) arylamine skeleton. Moreover, in solid state,
the bulky TPPO unit can prevent close packing of emitting center and avoid emission quenching.
Meanwhile, the electron transporting properties of the TPPO may also enhance electrical performance of
its constituting emitters. This idea is confirmed by analyzing their photophysical properties and electrical
properties. In OLED using 3-CzPOPPI and CzBPOPPI as nonꢀdoped emitters, highly efficient violetꢀ
blue emission with respectively EQE of 5.08 and 4.47% and CIE coordinates of (0.156, 0.061) and (0.157,
0
.071) were obtained. When these materials are used as dopant emitters, similar EQE were achieved with
deeper blue emission (CIE = 0.050 for 3-CzPOPPI and 0.044 for CzBPOPPI). These performances are
y
31,36–42
comparable to those of stateꢀofꢀtheꢀart violetꢀblue emitters.
OLED using another molecule,
TPAPOPPI, as nonꢀdoped emitter achieves an EQE of 6.69% with CIE coordinates of (0.152, 0.095).
Besides, TPAPOPPI can also serve as a host for yellow phosphor POꢀ01(iridium(III) bis(4ꢀ
phenylthieno[3,2ꢀc]pyridinatoꢀN,C 2’)acetylacetonate) and the yellowꢀemitting device achieve a decent
EQE of 18.9%. A F/P (fluorescence and phosphorescence hybrid) white OLED with TPAPOPPI and
POꢀ01 as emitters is shown to have a high EQE ≈ 20% and shows very low operating voltages of 2.6, 3.2
ꢀ2
and 4.6 V at 1, 1,000 and 10,000 cd m .

Journal of Materials Chemistry C
Page 4 of 19
View Article Online
DOI: 10.1039/C7TC04972B
Scheme 1 Chemical structure of the new compounds.
Experimental section
General information
1
H NMR spectra were recorded on a Bruker AVANCE III 400 spectrometer and mass spectra are
measured with a PE SCFIEX APIꢀMS spectrometer. A TA instrument TGAQ50 and a PerkinꢀElmer DSC
7
thermal analyzer were used respectively to measure decomposition temperature (5% weight loss, Td)
ꢀ1
and glass transition temperature (T ). The tests were carried out at a heating rate of 10 °C minute and in
g
nitrogen atmosphere. UVꢀvis absorption and photoluminescence (PL) spectra were obtained with a
Shimadzuꢀ1700 UVꢀvis scanning spectrometer and a Horiba Fluoromaxꢀ4 Spectrofluorometer,
TM
respectively. The absolute PLQY was measured with a Labsphere
integrating sphere using a
TM
monochromatized Xe lamp (Newport ) as an exciting source (at 360 nm). Timeꢀdependent PL was
recorded on an Edinburgh Instruments FLS980 spectrophotometer. Cyclic voltammetry (CV) was carried
out in nitrogenꢀpurged CH Cl (positive scan) and DMF (Dimethylformamide, negative scan) solutions at
2
2
room temperature using a CH1660 voltammetric analyzer. Tetrabutylammonium hexafluorophosphate
0.1 M) was used as the supporting electrolyte. The measurements were carried out at a scan rate of 100
(
ꢀ1
mV s with a conventional threeꢀelectrode configuration consisting of a glassy carbon working electrode,
a platinum plate auxiliary electrode, and a Ag/AgCl reference electrode calibrated with
+
ferrocene/ferrocenium (Fc/Fc : formal potential ꢀ4.8 eV). Molecular geometrical properties were
optimized at the B3LYP/6ꢀ31g(d, p) level using the Gaussian 09 program package. The excited states
analysis is then carried out at the TDꢀPBE0/6ꢀ31g(d, p) level.
Before device fabrication, preꢀcleaned ITOꢀcoated glass substrates with a sheet resistance of 15 ꢁ
ꢀ1
square were subjected to UVꢀozone treatment for 20 min. All organic films were deposited at the rate of

Page 5 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
ꢀ1
−6
0
.5~1.0 Å s by thermal evaporation in a deposition chamber with a base vacuum of 5×10 Torr. JꢀV
(
current densityꢀvoltage) characteristics were recorded with a Keithley 2400 Sourcemeter.
Electroluminescence spectra and CIE color coordinates were measured with a Spectrascan PR650
photometer.
Synthesis of compounds
All chemicals and solvents are received from commercial sources and used without further
purification. Column chromatography was carried out using 400 mesh silica gel. Scheme S1 in the ESI†
presents the synthetic routes of the new compounds. Three steps of reactions are involved in the synthesis
process. First, a Suzukiꢀcoupling reaction and a “oneꢀpot” reaction proceeded to form the corresponding
arylamine substituted N1ꢀ(4ꢀbromophenyl)ꢀphenanthroimidazolyl intermediates. After that, a nickel
43
catalyst CꢀP coupling reaction was applied to obtain the target products. The products were isolated with
1
moderate yields and their molecular structures were confirmed by H NMR, mass spectrometry (see ESI†).
Results and discussion
Thermal properties
Thermal properties of the new compounds were analyzed with TGA and DSC under N protection, as
2
shown in Fig. 1 and Table 1. All of them exhibit high T s of over 480 °C and T s of them are equal or
d
g
over 140 °C, which demonstrates that they are stable enough for thermal evaporation and their solid film
should have good morphologic stabilities. It is found that the two carbazole derivatives (3-CzPOPPI and
CzBPOPPI) show higher T than that of the triphenylamine derivative (TPAPOPPI). This should stem
g
from the more rigid skeleton of carbazole than triphenylamine.
Table 1. Key physical data of the new compounds.
a
b
b
c
c/d
e/d/f
Compd
T
d
(°C)
T
g
(°C)
E
g
(eV)
HOMO (eV)
LUMO (eV)
λ
abs (nm)
λ
PL (nm)
PLQY
3
-CzPOPPI 485
154
165
3.18
3.19
ꢀ5.47
ꢀ5.54
ꢀ2.36
ꢀ2.51
339
342
420/442
420/434
0.93/0.85/0.89
0.91/0.80/0.80
CzBPOPPI
506
TPAPOPPI 488
140
3.02
ꢀ5.31
ꢀ2.46
367
443/457
0.86/0.88/0.93
a
b
c
d
Estimated from absorption onset. Measured by cyclic voltammetry. Measured in THF (tetrahydrofuran) solution. Measured in
e
ꢀ6
f
neat film. Measured in hexane solution (10 M). Measured in doped film (10 wt% in CBP).

Journal of Materials Chemistry C
Page 6 of 19
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 1. a) TGA and b) DSC curves of the new compounds.
Photophysical properties
Fig. 2 gives UVꢀVis absorption and PL spectra of the new molecules (in THF solution) and key data are
also listed in Table 1. The lowest absorption band for carbazole derivatives were located around 340 nm
and a shoulder around 360 nm, which are ascribed to the πꢀπ* transition of the PI unit and the C2ꢀ
37
substituted carbazole moiety to PI core. In the triphenylamine derivative, the absorption band around
60 nm dominates. This variation is originated from the stronger electron donation character of
3
triphenylamine than carbazole. From their absorption onset, the optical bandgaps of the compounds were
estimated to be 3.18, 3.19, and 3.02 eV for 3-CzPOPPI, CzBPOPPI and TPAPOPPI, respectively. The
PL emission peaks of the TPPO derivatives in THF solution locate at 420, 420, and 443 nm for
CzBPOPPI, 3-CzPOPPI, and TPAPOPPI, respectively, ranging from violetꢀblue to deepꢀblue emission
(
Fig. 2b and Table 1). Emission spectral profiles of the carbazole derivatives present fine vibration

Page 7 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
features, while the triphenylamine derivative show structureless spectra. Comparing 3-CzPOPPI and
31
TPAPOPPI with their analogues (with N1 position substituted by tertꢀbutyl benzene) TPIBCz and
44
TPAꢀBPI (λ
= 417 and 442 nm, respectively, Scheme S3), PL spectra of these new compounds
PL,max
show rather small red shifts, which imply that N1 substitution of TPPO has little influence (charge
transfer) on the molecular radiative center (the PI unit and C2ꢀsubstituents). Solvatochromic experiments
give similar results. From nonpolar nꢀhexane to high polar acetonitrile solution (Fig. 3), carbazole
derivatives present very small red shifts (10 and 9 nm for 3-CzPOPPI and CzBPOPPI) in emission
peaks and remain fine structures in their spectral profiles. TPAPOPPI shows obvious bathochromic
31,44
effect (63 nm, Table S1) and structureless PL profiles. In our previous reports , TPIBCz and TPAꢀBPI
show similar solvatochromic effect in the same solutions (shifts 9 and 55 nm respectively) with these
TPPO substituted compounds, further evidencing our N1ꢀsubstitution strategy has negligible influence on
the emission core of the molecules. Deposited on quartz substrates, PL spectra of the emitters peak at 442,
4
34, and 457 nm for 3-CzPOPPI, CzBPOPPI, and TPAPOPPI, respectively. The relatively larger red
shifts (22 nm) from solution to solid state of 3-CzPOPPI probably stems from its more planar molecular
structure. High PLQYs of over 0.8 were observed in thin films of all compounds (Table 1), indicating
they are promising candidates of deepꢀblue OLED emitters.

Journal of Materials Chemistry C
Page 8 of 19
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 2. (a) Absorption and (b) PL spectra in THF solution.

Page 9 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 3. PL spectra of the new compounds in different solvents.
Theoretical calculation
To better understand the structureꢀproperty relationships of the new compounds, ground state energyꢀ
optimized molecular structures and frontier orbitals were calculated with the Gaussian 09 program
package on B3LYP/6ꢀ31g(d, p) level. As shown in Fig. 4, TPPO units have a nearly orthogonal dihedral
(
over 70°) angel with the PI units in all these compounds, which is consistent with our design. The
HOMO of 3-CzPOPPI distributes over the PI unit and C2ꢀsubstituent while their LUMO centers on
around the N1ꢀconnected benzene ring. This separate distribution indicates a bipolar nature of 3-
CzPOPPI. The HOMOs of CzBPOPPI and TPAPOPPI basically locate in the same segments but with
emphasis on respectively the PI unit and the triphenylamine group. The LUMOs of these two molecules
can spread from the N1ꢀconnected benzene ring to the C2ꢀconnected biphenyl group. This maybe because
of the slightly smaller twist angel between TPPO unit and PI plane. Comparing the frontier orbital
distribution of TPAPOPPI and TPAꢀBPI (see ref. 44), introducing TPPO units into the molecule will not
change the HOMO property and can widen the LUMO distribution over the N1ꢀconnected benzene ring.

Journal of Materials Chemistry C
Page 10 of 19
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 4. Optimized molecular geometries and distributions of the molecular frontier orbitals (The blue,
orange, red, grey and white ball represents the nitrogen, phosphor, oxygen, carbon and hydrogen atom,
respectively.)
Electrochemical properties
Using cyclic voltammetry, electrochemical properties of the three compounds were measured in CH Cl2
2
and DMF solution for oxidation and reduction scans respectively (Fig. 5). The HOMO levels of them are
determined to be ꢀ5.47, ꢀ5.54, and ꢀ5.31 eV for 3-CzPOPPI, CzBPOPPI and TPAPOPPI, respectively.
Comparing with TPAꢀBPI (HOMO: ꢀ5.31 eV), the very similar positive scan curves and the onset
positions imply the donor part (triphenylamine) decided the HOMO level. This is further proved by the
obvious different of oxidation curves and onset positions of carbazole derivatives. The slight shallower
HOMO of 3-CzPOPPI than CzBPOPPI can be attributed to the slightly longer conjugated length of 3-
CzPOPPI (Fig. 4). From the negative scan curves, LUMO levels of these molecules are deduced to be ꢀ
2
.36, ꢀ2.51 and ꢀ2.46 eV for 3-CzPOPPI, CzBPOPPI and TPAPOPPI, respectively. CzBPOPPI and
TPAPOPPI show different scan curves and deeper LUMO levels than others, which may be because of
their relative low twisting angles at N1ꢀposition and wider spread LUMOs. Comparing TPAPOPPI to
TPAꢀBPI (LUMO: ꢀ2.39 eV), substitution of TPPO group deepens the LUMO level of the molecule by
0
.07 eV, which is beneficial to electron injection in device.

Page 11 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 5. Cyclic voltammetry measurements of the new compounds and TPAꢀBPI.
Electrical properties
To evaluate electrical properties of CzBPOPPI, 3-CzPOPPI, and TPAPOPPI, holeꢀonly and electronꢀ
only devices were fabricated with the configuration of respectively ITO/NPB (10 nm)/one of the new
compounds (70 nm)/NPB (10 nm)/Al (120 nm) and ITO/1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)ꢀ
benzene (TPBi) (10 nm)/ one of the new compounds (70 nm)/TPBi (10 nm)/LiF (1 nm)/Al (120 nm). JꢀV
curves of the device are shown in Fig. 6. All the three materials show bipolar properties and their hole
transport abilities are better than that of electron. This should be originated from the good hole
transporting abilities of the PI unit and its C2ꢀsubstituted arylamine groups. Interestingly, CzBPOPPI
shows poorest performance in both hole and electron transports. This may because of its largely twisting
molecule structure (Fig. 4) which hinder effective molecular interaction and charge hopping. TPAPOPPI
performs well in both transporting positive and negative carriers and we suppose the good hole
transporting properties of the triphenylamine group and the relatively low LUMO level are the main
reasons. Comparing the JꢀV curve of TPAPOPPI and 3-CzPOPPI with their TPPOꢀfree counterparts
(
TPAꢀBPI and TPIBCz), TPPO substituted derivatives show inferior hole conductivity but superior
electron conductivity. This means substitution of TPPO group can enhance electron mobility of the
material and endows the molecule with a betterꢀbalanced bipolar transport ability.

Journal of Materials Chemistry C
Page 12 of 19
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 6. Current densityꢀvoltage characteristics of the electronꢀ and the holeꢀonly devices.
Electroluminescent properties
The three materials were first use as nonꢀdoped emitters in OLED, with a device structure of ITO/NPB
(
(
65 nm)/TCTA (4,4′,4″ꢀtris(carbazolꢀ9ꢀyl)ꢀtriphenylamine, 5 nm)/one of the new materials (25 nm)/TPBi
30 nm)/LiF (1 nm)/Al (120 nm). Here, NPB and TPBi are the hole and the electron transport layers
respectively. TCTA is the exciton blocking layer. As the E (~ 3.0 eV) of NPB is relatively low and
g
excitons may leak into it from the emitting layer. Inserting a TCTA layer, with its shallow LUMO (ꢀ2.20
eV) and larger E (~ 3.4 eV), electrons and excitons can be effectively blocked within the emitting
g
30,31,37
layer.
The device performances are presented in Fig. 7 and key data are summarized in Table 2. 3-
CzPOPPI and CzBPOPPI show efficient violetꢀblue emission with CIE coordinates of (0.156, 0.061)
and (0.157, 0.071), respectively. Decent performances were observed with high EQE of 4.44% and 4.13%
ꢀ2
at a practical luminance of 1,000 cd m . Higher performance in the 3-CzPOPPI based device is
attributed to its better photophysical and electrical properties. In the TPAPOPPIꢀbased device, a low Von
of 2.8 V and a high EQE of 6.69% with CIE coordinates of (0.152, 0.095) were achieved. The maximum
ꢀ1
ꢀ1
CE (current efficiency) and PE are determined to be 6.09 cd A and 5.94 lm W , respectively. At 1,000
ꢀ2
ꢀ1
cd m , high efficiency is maintained with an EQE of 6.10% and a CE of 5.27 cd A . This performance is
31,36–42
comparable to those of the stateꢀofꢀtheꢀart nonꢀdoped deepꢀblue OLED (CIE < 0.10).
As shown in
y
Fig. S4, the natural transition orbitals (NTOs) calculations of ꢀ → ꢀ transition (“hole” to “particle”)
ꢁ
ꢂ
shows a clear feature of hybrid local and chargeꢀtransfer excited state and in the LippertꢀMataga model,
the material present a LEꢀlike (locally excited) excited state dipole movement in low polarity solvents and
45,46
a CT character in high polarity solvents (Fig. S5 -S7 and Table S1).
As only nanosecond scale signal
is detected in the PL decay of TPAPOPPI and there is relatively large energy splitting of its first singlet

Page 13 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
and triplet excited state (0.51 eV, Fig. S3), this high EQE should be ascribed to the contribution of extra
47,48
singlet exciton generated from “hot” CT channels of hybrid locally and change transfer excited state.
Specifically, reverse intersystem crossing may occur between S and T state as they both have CT (or
1
5
HLCT) character and laid very close in energy (Fig. S4 -S5).
Fig. 7 a) Current densityꢀvoltageꢀluminescence characteristic curves, b) plots of current efficiencyꢀ
luminanceꢀpower efficiency, c) luminanceꢀEQE plots and d) EL spectra at 4 V of 3-CzPOPPI-,
CzBPOPPI- and TPAPOPPIꢀbased nonꢀdoped OLEDs.
Table 2. EL performances of the fabricated devices.
g
ꢀ1
g
ꢀ1
g
h
Emitter
V
on (V)
2.9
2.9
2.8
3.5
3.5
3.4
3.0
2.5
λ
EL (nm)
436
CE (cd A )
PE (lm W )
2.73, 2.39, 2.28
2.64, 2.32, 1.47
6.37, 5.72, 3.68
1.73, 1.15, 0.72
1.58, 0.90, 0.54
3.89, 2.86, 1.69
3.25, 1.89, 1.06
60.0, 60.0, 53.4
EQE (%)
CIE
a
a
3
-CzPOPPI
2.71, 2.35, 1.41
2.94, 2.88, 2.47
6.09,5.83, 5.27
1.95, 1.83, 1.61
1.76, 1.58, 1.36
4.33, 4.10, 3.59
3.66, 3.60, 3.19
57.3, 57.3, 54.1
5.08, 5.00, 4.44
4.47, 4.45, 4.13
6.67, 6.53, 6.10
4.93, 4.71, 4.15
5.62, 5.38, 4.82
6.61, 6.45, 5.62
3.57, 3.52, 3.12
18.9, 18.9, 17.8
0.156, 0.061
0.157, 0.071
0.152, 0.095
0.158, 0.050
0.157, 0.044
0.154, 0.068
0.154, 0.117
0.500, 0.494
a
CzPOBPPI
440
TPAPOPPI
444
b
1
0 wt% 3-CzPOPPI
432
b
1
0 wt% CzPOBPPI
432
b
1
0 wt% TPAPOPPI
432
c
TPAPOPPI
448
d
5
wt% PO-01
564

Journal of Materials Chemistry C
Page 14 of 19
View Article Online
DOI: 10.1039/C7TC04972B
e
f
W1
2.6
2.9
/
/
57.8, 57.1, 51.2
36.1, 35.8, 31.5
64.8, 59.3, 50.2
30.5, 26.1, 14.1
20.3, 19.2, 17.2
12.9, 12.8, 11.4
0.477, 0.468
W2
0.475, 0.455
a
b
Device structure: ITO/NPB (65 nm)/TCTA (5 nm)/EML (25 nm)/TPBi (30 nm)/LiF (1 nm)/Al (120 nm). Device structure:
c
ITO/MoO (3 nm)/NPB (60 nm)/TCTA (10 nm)/EML(30 nm)/TmPyPB (25 nm)/LiF (1 nm)/Al (120 nm). Device structure:
ITO/MoO (3 nm)/ TPAPOPPI (120 nm)/LiF (1 nm)/Al (120 nm). Device structure: ITO/NPB (65 nm)/TCTA (5 nm)/
TPAPOPPI: POꢀ01 (5 wt%, 25 nm)/TPBi (30 nm)/LiF (1 nm)/Al (120 nm), ITO/NPB (65 nm)/TCTA (5 nm)/ TPAPOPPI (10
nm)/ TPAPOPPI: POꢀ01 (5 wt%, 15 nm)/TPBi (30 nm)/LiF (1 nm)/Al (120 nm) Device structure: ITO/MoO (3 nm)/
TPAPOPPI (70 nm)/TPAPOPPI: POꢀ01 (5 wt%, 50 nm)/LiF (1 nm)/Al (120 nm). Device efficiency at maximum 100 and
1
3
d
3
e
f
3
g
ꢀ
2
h
ꢀ2
,000 cd m , respectively. Measured at 1,000 cd m .
Similar performance but better color purity was obtained when doping the materials in CBP (4,4’ꢀ
bis(Nꢀcarbazolyl)ꢀ1,1’ꢀbiphenyl). With a device configuration of ITO/MoO (3 nm)/NPB (60 nm)/TCTA
3
(
10 nm)/CBP: one of new materials (10 wt% 30 nm)/TmPyPB (25 nm)/LiF (1 nm)/Al (120 nm), EQE up
to 4.93, 4.68 and 6.61% were achieved (Fig. 8, Fig. S8 and Table 2) with CIE coordinates of (0.158,
.050), (0.157, 0.044) and (0.154, 0.068) for 3-CzPOPPI, CzBPOPPI and TPAPOPPI respectively.
0
This consistent efficiency trend with that of the nonꢀdoped devices indicates that close packing quenching
of emitters is prevented by the bulky TPPO group and current induced quenching in device is reduced as
an electron transport channel was established out of the emission core. The improved color purity is a
reasonable result as the interactions of emitters are more effectively eliminated upon dispersion in the
host and the nonꢀpolar environment provided by the CBP host may further shift the emission spectra
toward the shorter wavelength side.

Page 15 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
Fig. 8 a) Plots of current efficiencyꢀluminanceꢀEQE, b) EL spectra at 4 V of 3-CzPOPPI-, CzBPOPPI-
and TPAPOPPIꢀbased doped OLEDs.
Motivated by its excellent photophysical and electrical properties and device performances,
TPAPOPPI was further investigated as single layer OLED emitter and host for yellow phosphor POꢀ01.
As shown in Fig. 9a and Table 2, the single layer deepꢀblue OLED (ITO/MoO (3 nm)/ TPAPOPPI (120
3
nm)/LiF (1 nm)/Al (120 nm)) exhibits an impressive performance with EQE of 3.57% (maximum), 3.52%
ꢀ2
ꢀ2
(
at 100 cd m ) and 3.12% (at 1,000 cd m ). Low V = 3 V and good color index of (0.154, 0.117) were
on
49–53
observed, which is comparable to reported most efficient single layer deepꢀblue OLED.
When used as
host for POꢀ01 (ITO/NPB (65 nm)/TCTA (5 nm)/ TPAPOPPI: POꢀ01 (5 wt%, 25 nm)/TPBi (30 nm)/LiF

Journal of Materials Chemistry C
Page 16 of 19
View Article Online
DOI: 10.1039/C7TC04972B
(
1 nm)/Al (120 nm)), extremely low V of 2.5 V and EQE as high as 18.9% were achieved (Fig. 9b and
on
Table 2), which demonstrates TPAPOPPI is an excellent host for POꢀ01.
Fig. 9 a) Plots of current efficiencyꢀluminanceꢀEQE of (a) singleꢀlayer deepꢀblue OLED and (b) yellow
PhOLED (insets are EL spectra at 4 V). c) Plots of current efficiencyꢀluminanceꢀEQE of white OLEDs.
Finally, binary white OLEDs were fabricated using TPAPOPPI as blue fluorophore and POꢀ01
as yellow phosphor. In a device (W1) with the structure of ITO/NPB (65 nm)/TCTA (5 nm)/ TPAPOPPI
(
10 nm)/ TPAPOPPI: POꢀ01 (5 wt%, 15 nm)/TPBi (30 nm)/LiF (1 nm)/Al (120 nm), warm white
emission (CIE: (0.477, 0.468)) and V of as low as 2.6 V were observed. As shown in Fig. 9c, the device
on
ꢀ1
ꢀ1
exhibits a maximum CE, PE and EQE of 57.8 cd A and 64.8 lm W and 20.3%, respectively.
ꢀ2
Importantly, the luminances of 100, 1,000 and 10,000 cd m are achieved under bias of only 2.9, 3.2 and
4
.6 V respectively. To demonstrate that TPAPOPPI has also good bipolar transporting properties, device
W2 was fabricated with the structure of ITO/MoO (3 nm)/ TPAPOPPI (70 nm)/TPAPOPPI: POꢀ01 (5
3
wt%, 50 nm)/LiF (1 nm)/Al (120 nm). As shown in Fig. 9c and Table 2, EQEmax, CEmax and PEmax of
ꢀ1
ꢀ1
1
2.9%, 36.1 cd A and 30.5 lm W were achieved. Driving by a bias of 7 V, a practical luminance of
ꢀ2
1
,000 cd m (EQE of 11.4%) can be achieved in device W2. These performances comparable to the best
5
0,54–60
interlayerꢀfree white OLED
deepꢀblue emitter.
and demonstrate the good potential of TPAPOPPI as a multifunctional
Conclusion
In this work, we synthesized three deepꢀblue emitters 3-CzPOPPI, CzBPOPPI and TPAPOPPI. As
demonstrated by the spectroscopic experiments, CV measurements and single carrier device evaluations,
the introduction of TPPO group with highly twisted linkage has negligible influences on the
photophysical properties of molecules but open a new channel to facilitate electron injection and transport.
Endowed by their excellent photophysical and electrical properties, highly efficient deepꢀblue and violetꢀ
blue OLED based on them were fabricated. Nonꢀdoped devices with 3-CzPOPPI and CzBPOPPI as
emitter exhibit EQEmax of 5.08 and 4.42% with CIE coordinates of (0.156, 0.061) and (0.157, 0.071).

Page 17 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
Doped devices based on them achieve similar EQE but deeper blue emission (CIE of 0.050 for 3-
y
CzPOPPI and 0.044 for CzBPOPPI). OLEDs using TPAPOPPI shows an EQEmax of 6.69% with CIE
coordinates of (0.152, 0.095) for nonꢀdoped device and an EQE of 6.61% with CIE coordinates of (0.154,
0
.068) in doped device, which is comparable to stateꢀofꢀtheꢀart deepꢀblue emitters. Besides, a binary
white OLED with TPAPOPPI and POꢀ01 as emitters presents high EQE of 20.3% and shows a low
ꢀ2
operation voltage of 3.2 V at practical luminance of 1,000 cd m . TPAPOPPI is demonstrated to be a
promising multifunctional deepꢀblue emitter with good bipolar transporting properties.
Acknowledgement
Prof. Lee would like to acknowledge the financial support from the National Key R&D Program of China
(
Project No.: 2016YFB0401002). Prof. Tong acknowledges the support from the Natural Science
Foundation of China (Project No. 51673113) and the National Basic Research Program of China (973
Program No. 2013CB834803).
Notes and references
1
2
X. Yang, X. Xu and G. Zhou, J. Mater. Chem. C, 2015, 3, 913–944.
Y. Im, S. Y. Byun, J. H. Kim, D. R. Lee, C. S. Oh, K. S. Yook and J. Y. Lee, Adv. Funct.
Mater., DOI:10.1002/adfm.201603007.
3
4
5
M. Zhu and C. Yang, Chem. Soc. Rev., 2013, 42, 4963–4976.
F. So, J. Kido and P. Burrows, Mrs Bull., 2008, 33, 663–669.
M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R.
Forrest, Nature, 1998, 395, 151–154.
6
7
H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234–241.
Y. Tao, K. Yuan, T. Chen, P. Xu, H. Li, R. Chen, C. Zheng, L. Zhang and W. Huang, Adv.
Mater., 2014, 26, 7931–7958.
8
9
F. So, Organic electronics: materials, processing, devices and applications, CRC press, 2009.
S.ꢀY. Kim, W.ꢀI. Jeong, C. Mayr, Y.ꢀS. Park, K.ꢀH. Kim, J.ꢀH. Lee, C.ꢀK. Moon, W. Brütting
and J.ꢀJ. Kim, Adv. Funct. Mater., 2013, 23, 3896–3900.
1
1
0K. Udagawa, H. Sasabe, C. Cai and J. Kido, Adv. Mater., 2014, 26, 5062–5066.
1D. R. Lee, B. S. Kim, C. W. Lee, Y. Im, K. S. Yook, S.ꢀH. Hwang and J. Y. Lee, ACS Appl.
Mater. Interfaces, 2015, 7, 9625–9629.
1
1
1
1
2P. Rajamalli, N. Senthilkumar, P.ꢀY. Huang, C.ꢀC. RenꢀWu, H.ꢀW. Lin and C.ꢀH. Cheng, J.
Am. Chem. Soc., 2017, 139, 10948–10951.
3K. T. Ly, R.ꢀW. ChenꢀCheng, H.ꢀW. Lin, Y.ꢀJ. Shiau, S.ꢀH. Liu, P.ꢀT. Chou, C.ꢀS. Tsao, Y.ꢀC.
Huang and Y. Chi, Nat. Photonics, 2017, 11, 63–68.
4C. Li, R. Duan, B. Liang, G. Han, S. Wang, K. Ye, Y. Liu, Y. Yi and Y. Wang, Angew. Chem.
Int. Ed Engl., 2017, 56, 11525–11529.
5J.ꢀH. Jou, S. Kumar, A. Agrawal, T.ꢀH. Li and S. Sahoo, J. Mater. Chem. C, 2015, 3, 2974–
3
002.
1
1
6P.ꢀT. Chou, Y. Chi, M.ꢀW. Chung and C.ꢀC. Lin, Coord. Chem. Rev., 2011, 255, 2653–2665.
7Y. Chi, T.ꢀK. Chang, P. Ganesan and P. Rajakannu, Coord. Chem. Rev., 2017, 346, 91–100.

Journal of Materials Chemistry C
Page 18 of 19
View Article Online
DOI: 10.1039/C7TC04972B
1
1
8M. Y. Wong and E. ZysmanꢀColman, Adv. Mater., DOI:10.1002/adma.201605444.
9S.ꢀG. Ihn, N. Lee, S. O. Jeon, M. Sim, H. Kang, Y. Jung, D. H. Huh, Y. M. Son, S. Y. Lee, M.
Numata, H. Miyazaki, R. GómezꢀBombarelli, J. AguileraꢀIparraguirre, T. Hirzel, A. Aspuruꢀ
Guzik, S. Kim and S. Lee, Adv. Sci., DOI:10.1002/advs.201600502.
2
0K. S. Yook and J. Y. Lee, Chem. Rec., 2016, 16, 159–172.
2
1X.ꢀK. Liu, Z. Chen, J. Qing, W.ꢀJ. Zhang, B. Wu, H. L. Tam, F. Zhu, X.ꢀH. Zhang and C.ꢀS.
Lee, Adv. Mater., 2015, 27, 7079–7085.
2
2
2W. Song, I. Lee and J. Y. Lee, Adv. Mater., 2015, 27, 4358–4363.
3H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H. Tanaka, Y.
Sagara, T. Yasuda and C. Adachi, Nat. Commun., 2014, 5, 4016.
2
2
2
2
4J. Kido, M. Kimura and K. Nagai, Science, 1995, 1332–1332.
5W.ꢀC. Chen, Q.ꢀX. Tong and C.ꢀS. Lee, Sci. Adv. Mater., 2015, 7, 2193–2205.
6W.ꢀC. Chen, C.ꢀS. Lee and Q.ꢀX. Tong, J. Mater. Chem. C, 2015, 3, 10957–10963.
7M. Chen, Y. Yuan, J. Zheng, W.ꢀC. Chen, L.ꢀJ. Shi, Z.ꢀL. Zhu, F. Lu, Q.ꢀX. Tong, Q.ꢀD.
Yang and J. Ye, Adv. Opt. Mater., 2015, 3, 1215–1219.
2
2
3
3
3
8W.ꢀC. Chen, Y. Yuan, G.ꢀF. Wu, H.ꢀX. Wei, L. Tang, Q.ꢀX. Tong, F.ꢀL. Wong and C.ꢀS. Lee,
Adv. Opt. Mater., 2014, 2, 626–631.
9T. Qin, W. Wiedemair, S. Nau, R. Trattnig, S. Sax, S. Winkler, A. Vollmer, N. Koch, M.
Baumgarten, E. J. W. List and K. Müllen, J. Am. Chem. Soc., 2011, 133, 1301–1303.
0W.ꢀC. Chen, Y. Yuan, G.ꢀF. Wu, H.ꢀX. Wei, J. Ye, M. Chen, F. Lu, Q.ꢀX. Tong, F.ꢀL. Wong
and C.ꢀS. Lee, Org. Electron., 2015, 17, 159–166.
1W.ꢀC. Chen, Y. Yuan, S.ꢀF. Ni, Q.ꢀX. Tong, F.ꢀL. Wong and C.ꢀS. Lee, Chem. Sci., 2017, 8,
3
599–3608.
2J. E. Anthony, A. Facchetti, M. Heeney, S. R. Marder and X. Zhan, Adv. Mater., 2010, 22,
876–3892.
3
3
3
3
3
3K. S. Yook and J. Y. Lee, Adv. Mater., 2012, 24, 3169–3190.
4S. O. Jeon and J. Y. Lee, J. Mater. Chem., 2012, 22, 4233–4243.
5D. Joly, P.ꢀA. Bouit and M. Hissler, J. Mater. Chem. C, 2016, 4, 3686–3698.
6B. Liu, Z.ꢀW. Yu, D. He, Z.ꢀL. Zhu, J. Zheng, Y.ꢀD. Yu, W.ꢀF. Xie, Q.ꢀX. Tong and C.ꢀS. Lee,
J. Mater. Chem. C, 2017, 5, 5402–5410.
3
7W.ꢀC. Chen, Y. Yuan, Y. Xiong, A. L. Rogach, Q.ꢀX. Tong and C.ꢀS. Lee, ACS Appl. Mater.
Interfaces, 2017, 9, 26268–26278.
3
3
8X. Tang, T. Shan, Q. Bai, H. Ma, X. He and P. Lu, Chem. – Asian J., 2017, 12, 552–560.
9H. Xu, P. Sun, K. Wang, J. Li, F. Wang, Y. Miao, H. Wang, B. Xu and W.ꢀY. Wong, J. Mater.
Chem. C, 2017, 5, 4455–4462.
4
4
0X. Du, G. Li, J. Zhao, S. Tao, C. Zheng, H. Lin, Q. Tong and X. Zhang, Adv. Opt. Mater.
1I. Kondrasenko, Z.ꢀH. Tsai, K. Chung, Y.ꢀT. Chen, Y. Y. Ershova, A. DoménechꢀCarbó, W.ꢀ
Y. Hung, P.ꢀT. Chou, A. J. Karttunen and I. O. Koshevoy, ACS Appl. Mater. Interfaces, 2016,
8
, 10968–10976.
2A. Obolda, Q. Peng, C. He, T. Zhang, J. Ren, H. Ma, Z. Shuai and F. Li, Adv. Mater., 2016,
8, 4740–4746.
3B. Pan, H. Huang, X. Yang, J. Jin, S. Zhuang, G. Mu and L. Wang, J. Mater. Chem. C, 2014,
, 7428–7435.
4
4
4
2
2
4Y. Zhang, S.ꢀL. Lai, Q.ꢀX. Tong, M.ꢀF. Lo, T.ꢀW. Ng, M.ꢀY. Chan, Z.ꢀC. Wen, J. He, K.ꢀS.
Jeff, X.ꢀL. Tang, W.ꢀM. Liu, C.ꢀC. Ko, P.ꢀF. Wang and C.ꢀS. Lee, Chem. Mater., 2012, 24,
6
1–70.

Page 19 of 19
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC04972B
4
5S. Zhang, L. Yao, Q. Peng, W. Li, Y. Pan, R. Xiao, Y. Gao, C. Gu, Z. Wang, P. Lu, F. Li, S.
Su, B. Yang and Y. Ma, Adv. Funct. Mater., 2015, 25, 1755–1762.
4
6P. Suppan, Chem. Phys. Lett., 1983, 94, 272–275.
4
7S. Zhang, W. Li, L. Yao, Y. Pan, F. Shen, R. Xiao, B. Yang and Y. Ma, Chem. Commun.,
2
013, 49, 11302–11304.
4
4
5
5
5
8W. Li, Y. Pan, R. Xiao, Q. Peng, S. Zhang, D. Ma, F. Li, F. Shen, Y. Wang, B. Yang and Y.
Ma, Adv. Funct. Mater., 2014, 24, 1609–1614.
9T. Izumi, Y. Tian, K. Ikemoto, A. Yoshii, T. Koretsune, R. Arita, H. Kita, H. Taka, S. Sato
and H. Isobe, Chem. – Asian J., 2017, 12, 730–733.
0X.ꢀL. Li, X. Ouyang, D. Chen, X. Cai, M. Liu, Z. Ge, Y. Cao and S.ꢀJ. Su, Nanotechnology,
2
016, 27, 124001.
1X.ꢀL. Li, M. Liu, Y. Li, X. Cai, D. Chen, K. Liu, Y. Cao and S.ꢀJ. Su, Chem. Commun., 2016,
2, 14454–14457.
5
2W.ꢀC. Chen, Y. Yuan, S.ꢀF. Ni, Z.ꢀL. Zhu, J. Zhang, Z.ꢀQ. Jiang, L.ꢀS. Liao, F.ꢀL. Wong and
C.ꢀS. Lee, ACS Appl. Mater. Interfaces, 2017, 9, 7331–7338.
5
5
3T.ꢀH. Huang, J. T. Lin, L.ꢀY. Chen, Y.ꢀT. Lin and C.ꢀC. Wu, Adv. Mater., 2006, 18, 602–606.
4X. Ouyang, X.ꢀL. Li, L. Ai, D. Mi, Z. Ge and S.ꢀJ. Su, ACS Appl. Mater. Interfaces, 2015, 7,
7
869–7877.
5B. Liu, X.ꢀL. Li, H. Tao, J. Zou, M. Xu, L. Wang, J. Peng and Y. Cao, J. Mater. Chem. C,
017, 5, 7668–7683.
6Z. Wu, J. Luo, N. Sun, L. Zhu, H. Sun, L. Yu, D. Yang, X. Qiao, J. Chen, C. Yang and D. Ma,
Adv. Funct. Mater., 2016, 26, 3306–3313.
5
5
5
5
2
7N. Sun, Q. Wang, Y. Zhao, Y. Chen, D. Yang, F. Zhao, J. Chen and D. Ma, Adv. Mater., 2014,
2
6, 1617–1621.
8Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing and F. Wang, Adv. Funct. Mater., 2009,
9, 84–95.
1
5
6
9F. Zhao and D. Ma, Mater. Chem. Front., 2017, 1, 1933–1950.
0J. Ye, C.ꢀJ. Zheng, X.ꢀM. Ou, X.ꢀH. Zhang, M.ꢀK. Fung and C.ꢀS. Lee, Adv. Mater., 2012, 24,
3
410–3414.