Novel spiro[fluorene-9,9′-xanthene]-based hole transport layers for red and green PHOLED devices with high efficiency and low efficiency roll-off

Article abstract of DOI:10.1039/d0tc04676k

TwoN-phenyl-1-naphthylamine (PNA)/spiro[fluorene-9,9′-xanthene] (SFX) hybrid materials (DPNA-SFX and DOPNA-SFX) were successfully obtained by incorporating two PNA groups with a bridging phenyl ring to an SFX center. Both materials possess similar photoph

Full text of DOI:10.1039/d0tc04676k

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
0
Novel spiro[fluorene-9,9 -xanthene]-based hole
transport layers for red and green PHOLED
devices with high efficiency and low efficiency
roll-off†
Cite this: J. Mater. Chem. C, 2021,
, 3247
9
a
a
a
a
b
b
Tao Wang, Ming Shi, Daqi Fang, Junpeng He, Meng Zhang, Siwei Zhang,
b
a
Guodan Wei * and Hong Meng
*
0
Two N-phenyl-1-naphthylamine (PNA)/spiro[fluorene-9,9 -xanthene] (SFX) hybrid materials (DPNA-SFX
and DOPNA-SFX) were successfully obtained by incorporating two PNA groups with a bridging phenyl
ring to an SFX center. Both materials possess similar photophysical properties and an outstanding glass-
g
transition temperature (T ) beyond 150 1C surpassing that of most typical spirobifluorene and SFX
materials. Both green and red phosphorescent organic light-emitting diodes (PHOLEDs) based on two
materials achieved remarkable results. The DPNA-based green devices exhibited decent performance
Received 1st October 2020,
Accepted 25th January 2021
with the maximum current efficiency (CEmax), power efficiency (PEmax) and external efficiency (EQEmax)
ꢀ1
ꢀ1
of 89.8 cd A , 94.2 lm W and 24.7%, respectively. More importantly, the pure red PHOLEDs with
ꢀ
1
DPNA-SFX as the hole-transporting material achieved the CEmax, PEmax and EQEmax of 41.1 cd A
,
DOI: 10.1039/d0tc04676k
ꢀ1
4
6.4 lm W and 34.7%, respectively, which is comparable to those of the best red PHOLEDs. And the
ꢀ2
rsc.li/materials-c
EQE at 1000 cd m maintained 96% of its peak value, demonstrating an extremely low efficiency roll-off.
the current–voltage characteristic is injection-limited, and it
requires balancing charge injection to improve the current
Introduction
Organic light-emitting diodes (OLEDs) have attracted extensive performance of OLED devices. Useful and feasible HTL plays a
research interest both in academia and industry due to their critical role in improving electroluminescence efficiency since it
superior merits such as wider viewing angle, lower energy directly relates the turn-on voltage of OLED, balancing hole
4,7–11
consumption, light weight, excellent colour purity and mechanical transport and light-extraction efficiency.
flexibility. Within the past few years, OLEDs have been steadily In general, hole transport materials (HTMs) need to possess
well-developed and rapidly expanded in the mainstream display strong thermal stability with high glass transition temperature
and solid-state lighting technology for consumer electronics. (T ), high hole mobility, morphologically stable thin films and
g
OLEDs are typically composed of multi-layered thin films an appropriate HOMO level to ensure a low energy barrier for
sandwiched between two respective anode and cathode. Each hole injection and a suitable LUMO level to block electron
organic layer is constructed for its own function, such as carrier injection from the emissive layer. Traditionally, commonly
0
0
injection layer (CIL), hole transporting layer (HTL), electron used HTLs are N,N -di(1-naphthyl)-N,N -diphenylbenzidine
0
0
transporting layer (ETL), and emissive layer (EML), specifically (NPB), 4,4 -bis[N-(p-tolyl)-N-phenylamino]biphenyl (TPD), N,N -
designed for effective injection, the transport of the charge carriers di(biphenyl-4-yl)-N,N -diphenyl-[1,1 -biphenyl]-4,4 -diamine (p-BPD)
0
0
0
1–6
and electron–hole balance in the emissive region.
Therefore, and 1-bis[4-[N,N-di(4-tolyl)amino]phenyl]-cyclohexane (TAPC), due
OLED efficiency is predominantly determined by charge balance, to their high hole mobility, suitable HOMO/LUMO energy levels
12–16
the radiative decay of excitons and light outcoupling. Due to the and ease of sublimation.
It is quite apparent that triarylamine
relatively low intrinsic carrier concentrations of the transport layer, (TPA) and its derivatives with a nitrogen atom connected with
aromatic rings are excellent building blocks because of their good
17–24
electron donating properties.
In addition, HTMs are expected to
a
School of Advanced Materials, Peking University Shenzhen Graduate School,
Peking University, Shenzhen, 518055, P. R. China
have good thermal stability and morphological durability, which are
conducive to OLED fabrication and operation. Introducing a bulky
and rigid moiety into a molecule is a common strategy to increase
b
Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua Shenzhen International
Graduate School, Tsinghua University, Shenzhen, 518055, P. R. China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
24,25
thermal stability.
The utilization of a spirocyclic structure
†
d0tc04676k
endows HTMs with an intrinsic nonplanar three-dimensional
This journal is The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 3247ꢀ3256 | 3247

View Article Online
Paper
Journal of Materials Chemistry C
architecture, which results in reduced intermolecular interactions,
restricted crystallization tendency and increased molecule rigidity
2
6–30
along with higher Tg.
0
Spiro[fluorene-9,9 -xanthene] (SFX) is a famous spiro-type
platform for one-pot synthesis to design new organic semicon-
31–38
ductors for optoelectronic devices.
For example, Zhang et al.
synthesized and characterized several dicarbazole-substituted
SFX-based molecules as host materials for blue and green
3
9
PHOLEDs. Two similar SFX derivatives (SFX-Cr1 and SFX-Cr2)
with tetra-carbazole substitution were reported by Wang and
co-workers. SFX-Cr2 possesses a respectable T of 130 1C and a
g
ꢀ
3
2
ꢀ1 ꢀ1
high hole mobility (B10
cm
V
s ). Green PHOLEDs
with SFX-Cr2 as the HTL showed a high EQE of 16.3%.
Several compounds with carbazole, diphenylamine, N,N -di(4-
methoxyphenyl)amino, and phenothiazine motifs have been
demonstrated as compromising HTMs for high-efficiency PSCs
by replacing the corresponding moiety in commercialized spiro-
40
0
4
1,42
MeOTAD to the SFX core.
Recently, the relationship between
the thermal and redox properties of the HTMs and different
modified positions of TPA groups as well as their further influ-
ence on PSC efficiency were also analyzed. Therefore, these
reports have established solid progress in developing high-quality
HTMs based on the SFX core for high-performance OLEDs.
43
Scheme 1 Synthetic routes of DPNA-SFX and DOPNA-SFX.
intermediates and the final products were synthesized using
However, poor thermal stability renders their low T
leading to the detriment of device efficiency and lifetime.
g
, thus
40
common methods published in the literature. All of the
4
4–47
It
1
13
compounds were confirmed by H-NMR, C-NMR and high-
resolution mass spectrometry (HR-MS).
is essential to further develop new types of HTMs to improve the
OLED performance. In this work, we have designed two new hole
To determine the thermal properties and morphological
stabilities of the synthesized HTMs, differential scanning
calorimetry (DSC) and thermogravimetric analysis (T A) were
0
1
3
transporting materials, spiro[fluorene-9,9 -xanthene]-2,7-di-(N ,N -
1
3
di-1-naphthalenyl-N ,N -diphenyl-1,3-benzenediamine) (DPNA-SFX)
and spiro[fluorene-9,9 -xanthene]-2 ,7 -di-(N ,N -di-1-naphthalenyl-
N ,N -diphenyl-1,3-benzenediamine) (DOPNA-SFX), applying the
1
3
g
0
0
0
measured under a nitrogen atmosphere, as depicted in Fig. S5
1
3
(
see the ESI†). The decomposition temperature (T
transition temperature (T ) values are summarized in Table 1.
The T values were found to be 543 1C and 535 1C for DPNA-SFX
d
) and glass
same SFX core with bridge phenyl rings at different positions
g
(
fluorene or xanthene core), which are meta-substituted by two
electron-donating arylamine groups, N-phenyl-1-naphthylamine
PNA). It was found that both HTMs show relatively high T beyond
d
and DOPNA-SFX, respectively, indicating the excellent thermal
stability of both compounds due to the rigidity of the spiro
molecular structure. According to the DSC curves, a clear glass
(
g
150 1C, indicating dramatically enhanced thermal stability
compared to NPB. The hole mobilities of DPNA-SFX and DOPNA-
transition phenomenon can be observed with the T values of
ꢀ
5
2
ꢀ1 ꢀ1
ꢀ5
2
ꢀ1 ꢀ1
g
SFX are 7.6 ꢁ 10 cm V
s
and 6.8 ꢁ 10 cm V
s ,
DPNA-SFX and DOPNA-SFX up to 151 1C and 153 1C, respectively,
suggesting it is beneficial to form continuous thin films
compared with the typical hole transporting material NPB with
respectively, both of which are superior to those of NPB. All the
multilayer green and red PHOLEDs exhibit superb efficiency data
with a low turn-on voltage when adopting new HTLs. The
DPNA-based green OLED device represents the greatest green
g
a T of 95 1C. Moreover, the PL spectra of thin films of three
HTMs under different temperatures have been measured (see
Fig. S6, ESI†). The PL spectra of DPNA-SFX and DOPNA-SFX
remain consistent even under 300 1C; however, the PL intensity
relative to that under room temperature of NPB has been largely
decreased, which indicates the better thermal stability of our
HTMs than NPB.
ꢀ1
ꢀ1
device with CE, PE and EQE peaks of 89.8 cd A , 94.2 lm W
and 24.7%. More significantly, the DPNA-based red device R1
displays outstanding performance with the CE, PE and EQE peaks
ꢀ
1
ꢀ1
of 41.1 cd A , 46.4 lm W and 34.7%, which is comparable to the
world record of red PHOLEDs. And the EQE of the device R1 at
1
ꢀ2
000 cd m maintained 96% of its highest value, which shows an
extremely low efficiency roll-off in pure red PHOLED devices.
Electrochemical properties and theoretical calculations
To investigate the electrochemical behaviour of DPNA-SFX and
DOPNA-SFX, cyclic voltammetry (CV) measurements were con-
ducted with Ag/AgCl as the reference electrode. The CV curves
are displayed in Fig. S7 (see the ESI†) and the corresponding
Results and discussion
Synthesis and thermal properties
All the chemical structures and the synthetic processes of data are summarized in Table 1, indicating the same CV profile
the target compounds are shown in Scheme 1. The main for DPNA-SFX and DOPNA-SFX with similar oxidation couples
3248 | J. Mater. Chem. C, 2021, 9, 3247ꢀ3256
This journal is The Royal Society of Chemistry 2021

View Article Online
Journal of Materials Chemistry C
Paper
Table 1 The thermal, photophysical and electrochemical properties of DPNA-SFX and DOPNA-SFX
HOMO/LUMO (eV)
a
d
b
c
abs
c
d
e
f
g
e
h
Compound
T
/T
g
l
l
em,s /lem,f
Exp.
Cal.
E
(eV)
E
T
(eV)
DPNA-SFX
DOPNA-SFX
543/151
535/153
308
297
447/429
435/420
ꢀ5.23/ꢀ1.98
ꢀ5.23/ꢀ2.03
ꢀ4.87/ꢀ1.37
ꢀ4.85/ꢀ1.45
3.25
3.20
2.45
2.44
a
b
c
d
Decomposition temperature (5% weight loss). Glass transition temperature. Measured in degassed dichloromethane. Measured in thin
e
solid films on quartz. HOMO was calculated by the formula: HOMO = E1/2,Fc ꢀ EOX ꢀ 4.8; LUMO was calculated by the sum of HOMO and optical
f
g
band gap.h Calculated by the density functional theory level (B3LYP/6-311G**). Estimated from the absorption edge in degassed dichloro-
methane. Estimated from the phosphorescence spectra in degassed dichloromethane at 77 K.
and no reduction process. Both DPNA-SFX and DOPNA-SFX show
good stability of electrochemical oxidation. During the anodic
scan in the DCM solution, two HTMs exhibit four quasi-reversible,
four electron oxidation processes, which can be assigned to the
oxidation of four 1-(N-phenylamino)naphthalene units. The
HOMO energy levels are determined from the onset of the
oxidation curve using the empirical formula, which is at the same
value of ꢀ5.23 eV for both materials, deeper than the HOMO level
of NPB (ꢀ5.12 eV), benefiting the promotion of hole injection. On
the other hand, the LUMO energy level can be calculated using the
HOMO energy level and band gap (E
g
), which are thus ꢀ1.98 eV
and ꢀ2.03 eV, respectively. To have a better understanding of the
electronic structures of DPNA-SFX and DOPNA-SFX, density func-
tional theory (DFT) calculations were conducted to simulate the
frontier molecular orbital (FMO) levels and spatial distributions
(Fig. 1(a)). The DFT results show that the HOMOs of both
molecules are mainly localized at the two PNA moieties and the
bridge phenyl ring, and the calculated HOMOs show no big
difference which is in agreement with the experimental results.
However, the LUMO of DPNA-SFX is distributed in the fluorene
part of the SFX core and the bridge phenyl ring, while that of
DOPNA-SFX is just located at the PNA moiety due to the break-
down of conjugation extension by the xanthene moiety.
Fig. 1 (a) HOMO and LUMO distributions of DPNA-SFX and DOPNA-SFX;
b and c) UV-Vis absorption (in DCM solution), PL spectra (in DCM solution
Photophysical properties
(
To investigate the photophysical behaviours of the new HTMs, and in film at room temperature, in DCM solution at 77 K) of (b) DPNA-SFX;
UV-Vis absorption spectra and photoluminescence (PL) spectra and (c) DOPNA-SFX.
in dilute dichloromethane (DCM) solution and thin film, as
well as phosphorescence spectra in DCM at 77 K were recorded
and 435 nm, respectively. The maximum wavelength of
respectively, and are shown in Fig. 1(b) and (c), and the
DPNA-SFX exhibits a 14 nm red-shift relative to DOPNA-SFX,
summarized data are presented in Table 1. For both DPNA-
which could be due to the extended conjugation of DPNA-SFX.
The fluorescence spectra of two HTMs reveal a solvatochromic
SFX and DOPNA-SFX, the strongest absorption bands around
300 nm are attributed to localized transitions associated with
effect (see Fig. S8, ESI†), which indicates that DPNA-SFX and
DOPNA-SFX are slightly sensitive to solvent polarity. To measure
the triplet energy of the two HTMs, low temperature phosphor-
escence spectra (77 K, in DCM solution) were collected under a
nitrogen atmosphere (shown in Fig. 1(b) and (c)), corresponding
the PNA core. In the UV-Vis spectra of DPNA-SFX, a shoulder
absorption peak was observed at 320 nm due to the p–p*
transition from the PNA moiety to the fluorene moiety. The
longer wavelength band at 350 nm with weaker absorption of
DOPNA-SFX could be assigned to the p–p* transition from the
PNA moiety to the xanthene moiety, revealing the formation of
the intramolecular charge transfer (ICT) state made by some
degree of weaker spiro-conjugation effect. In addition, the band
to T
1
of 2.45 eV and 2.44 eV (Table 1), respectively, which are
guarantees better
both higher than NPB (2.33 eV). Higher T
1
exciton confinement in terms of OLED device structure.
gaps (E ) are estimated from the absorption edge, which are
g
Single carrier devices
3.25 eV and 3.20 eV for DPNA-SFX and DOPNA-SFX, respectively.
Both compounds show blue emission in dichloromethane Single carrier devices of DPNA-SFX and DOPNA-SFX were fabricated
DCM) and their maximum emission peaks were at 447 nm to study the corresponding hole-transporting properties,
(
This journal is The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 3247ꢀ3256 | 3249

View Article Online
Paper
Journal of Materials Chemistry C
according to the architecture of ITO/HAT-CN (10 nm)/DPNA-SFX the hole mobilities of DPNA-SFX and DOPNA-SFX reach 2.75 ꢁ
ꢀ
3
2
ꢀ1 ꢀ1
ꢀ3
2
ꢀ1 ꢀ1
or DOPNA-SFX (100 nm)/HAT-CN (10 nm)/Al (100 nm), in which 10 cm V
s
and 2.1 ꢁ 10 cm V
s , respectively. High
HAT-CN was used as the hole injection layer and electron blocking hole-transporting mobility with good hole injection ability
layer (inserted picture in Fig. 2(a)). No electroluminescence renders DPNA-SFX and DOPNA-SFX as potential HTM candi-
phenomenon was detected during measurement implying dates for application in OLEDs.
these are singlet-only devices. The J–V curves of the hole-only
PHOLED performance
devices in Fig. 2(a) reveal that the current densities at the same
voltage follow the sequence: DPNA-SFX 4 DOPNA-SFX 4 NPB. To evaluate the practical application for phosphorescent
The difference between the curves could further indicate that, organic light-emitting diodes (PHOLEDs), green and red
according to the HOMO energy level of HAT-CN and the above devices using the new HTMs were fabricated. Control devices
0
HTMs, a better charge injection from HAT-CN could be achieved were fabricated with the commonly-used HTL of N,N -
0
in DPNA-SFX and DOPNA-SFX based devices than in the NPB- bis(naphthalen1-yl)-N,N -bis(phenyl)-benzidine (NPB). The opti-
based one.
mized PHOLED device structure is ITO/HAT-CN (5 nm)/HTM
Space charge limited current (SCLC) theory can be used to (30 nm)/Spiro-3-BFP (10 nm)/DMIC-TRZ: DMIC-CZ: emitter
evaluate the carrier mobility of semiconductors when quasi- (40 nm)/ANT-BIZ (70 nm)/Liq (2.5 nm)/Al (100 nm) (the HTM
Ohmic contacts between the metal electrode and organic layer is DPNA-SFX for device G1, R1; DOPNA-SFX for device G2, R2),
2
are well established, which gives the formula: J = 9e
0
e
r
mE /8L. with Ir(ppy)
2
(acac) ([bis(2-phenylpyridine)(acetylacetonato)
Considering the electric-field-dependent charge mobility of iridium(III)]) and Ir(dmpibq)
2
(acac) (bis[2-(3,5-dimethylphenyl)-
0
organic semiconductors according to Poole–Frenkel mechanics 5-isobutylquinoline-C2,N ](acetylacetonato)iridium(III)) as the
ꢀ
pffiffi ffiffi ꢁ
dopant for green and red emitting layers, respectively. In these
(
m ¼ m exp b E ), the expression can be written as
0
0
0
devices, HAT-CN (dipyrazino[2,3-f: 2 ,3 -h]quinoxaline-2,3,6,7,
0,11-hexacarbonitrile) and Liq (8-quinolinolato lithium) were
ꢂ
ꢃ
pffiffiffiffi
2
1
J ¼ 9e0erm E exp g E =8L
0
used as the hole-injection and electron-injection materials,
0
ꢀ
12
ꢀ1
respectively. Spiro-3-BFP (N-([1,1 -biphenyl]-2-yl)-N-(9-dimethyl-
where e
0
is the permittivity of vacuum (B8.85 ꢁ 10
F m ), e
r
0
9
H-fluoren-2-yl)-9,9 -spirobi[9H-fluoren]-4-amine) was used as
is the relative dielectric constant, which equals three for the
the electron-blocking material to balance the electrons and
holes for green and red devices. In the emitting layer, DMIC-
TRZ (5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7,7-dimethyl-5,
7-dihydroindeno[2,1-b]carbazole) and DMIC-CZ (7,7-dimethyl-5-
phenyl-2-(9-phenyl-9H-carzole-3-yl)-5,7-dihydroindeno[2,1-b]carbazole)
served as the co-host system with a molar ratio of 1 : 1 for both
green and red PHOLEDs, and the doping concentrations of
0
organic materials approximately, m is the zero-field charge
mobility, g is the Poole–Frenkel factor, and L and E are the
thickness of the organic layer and the electric field,
6
respectively.
The value of m and g was determined from the J–V curves by the
0
ꢀ
5
2
ꢀ1 ꢀ1
linearization of the equation above, which is 7.6 ꢁ 10 cm V
s ,
double helical C/Co@CNT nanocomposite with hierarchical
structures for enhancing microwave absorption for DPNA-
Ir(ppy)
2
(acac) and Ir(dmpibq)
2
(acac) were 5 wt% and 2.5 wt%,
0
ꢀ
5
2
ꢀ1 ꢀ1
ꢀ2
ꢀ1/2
ꢀ2 ꢀ1/2
1/2
respectively. ANT-BIZ (1-(4-(10-([1,1 -biphenyl]-4-yl)anthracen-9-
SFX, 6.3 ꢁ 10
cm
V
s
, 1.48 ꢁ 10
V
cm
for
ꢀ
5
2
ꢀ1 ꢀ1
1/2 yl)phenyl)-2-ethyl-1H-benzo[d]-imidazole) was employed as the
electron-transporting material. The device configuration and
energy level diagram and the corresponding molecule struc-
tures are shown in Fig. 3(a) and (b) and Fig. S9 (ESI†). The
device performance of the green and red PHOLED devices,
including current density–voltage–luminance (J–V–L) curves,
current efficiency (CE), power efficiency (PE) and external
quantum efficiency (EQE) versus luminance (L) curves, as well
as the EL spectra are shown in Fig. 3. All the key device
DOPNA-SFX and 2.8 ꢁ 10 cm V
s
, 1.38 ꢁ 10
V
cm
for NPB. It can be clearly observed that the zero-field mobilities
show the following sequence: DPNA-SFX 4 DOPNA-SFX 4
NPB, which indicates the better hole-transporting properties
of the synthesized HTMs and the potential replacement for
commercialized NPB. The hole mobility dependent on the
electric field ascribed to the materials can be calculated with
the two parameters, with the results depicted in Fig. 2(b). It is
obviously shown that the hole mobilities increased rapidly at a
5
ꢀ1
parameters are listed in Table 2.
As depicted in Fig. 3(c), both devices G1 and G2 emit green
light with roughly identical emission peaks at 521 nm and
high bias voltage. At the high electric field of 1.9 ꢁ 10 V cm
,
2
524 nm, respectively, which was ascribed to Ir(ppy) (acac) by
efficient energy transfer from co-hosts to the guest. The EL
spectra remain unchanged over a wide range of operating
voltage from 3 V to 7 V (see Fig. S10, ESI†). The corresponding
CIE coordinates are (0.32, 0.62) and (0.33, 0.62), respectively.
The two devices exhibited a comparatively low turn-on voltage
of 2.4 V simultaneously (determined from Fig. 3(d)). It is
apparently demonstrated that the device G1 has better device
ꢀ
1
performance, with the maximum CE, PE and EQE of 89.8 cd A
94.2 lm W
,
Fig. 2 (a) J–V curves of the hole-only devices and (b) comparative field
dependence mobility of DPNA-SFX and DOPNA-SFX.
ꢀ
1
and 24.7%, respectively, compared with the
3250 | J. Mater. Chem. C, 2021, 9, 3247ꢀ3256
This journal is The Royal Society of Chemistry 2021

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 3 (a) Device architecture of PHOLEDs and (b) energy diagram of multilayer PHOLEDs. Device performances of green PHOLEDs: (c) EL spectra
4.0 V), (e) J–V–L curves, (f) CE–L–PE curves, and (g) EQE–L curves.
(
Table 2 Key electroluminescent properties of devices G1–G2 and R1–R2 indicates that the turn-on voltage of R1 and R2 is as low as
b
ꢀ1
c
ꢀ1
d
2.2 V. The DPNA-SFX based device possesses a superb device
CE (cd A
)
PE (lm W
)
EQE (%)
ꢀ
1
ꢀ1
performance, with a CE of 41.1 cd A , a PE of 46.4 lm W , and
an EQE of 34.7%, which completely outperforms that of the
other one. The EQE of the DPNA-based red device is almost
a
3
4
3
4
3
4
V
on Max/10 /10
Devices (V) (cd m
Max/10 /10
(cd m
Max/10 /10
(cd m
ꢀ2
ꢀ2
ꢀ2
e
)
)
)
CIE
G1
G2
R1
R2
2.4 89.8/88/72.2 94.2/90/50.4 24.7/24.2/19.9 (0.32, 0.62)
2.4 85.6/81.4/70.5 93.2/76.5/54.5 23.6/22.4/21.4 (0.33, 0.62)
2.2 41.1/39.5/31.6 46.4/29.6/15.5 34.7/33.3/13.1 (0.66, 0.34)
40% higher than that of the NPB-based one. To the best of our
knowledge, this excellent device performance is comparable
2.2 31.4/26.5/25.4 35.6/18.5/13.8 25.1/21.2/20.3 (0.66, 0.34) with the best records of pure red PHOLEDs4
8–57
(some repre-
a
ꢀ2
b
sentative EQE values of red PHOLEDs are shown in Fig. 5). The
Turn-on voltage at 1 cd m . c Current efficiency at maximum values/
ꢀ
2
ꢀ2
d
1
1
000 cd m /10 000 cd m
.
Power efficiency at maximum values/ efficiency roll-off values of red PHOLEDs based on DPNA-SFX,
External quantum efficiency at maximum
ꢀ
2
ꢀ2
000 cd m /10 000 cd m
.
DOPNA-SFX and NPB are 4%, 16% and 30%, respectively.
To note, the device R1 also shows better efficiency roll-off
ꢀ2
ꢀ2
e
ꢀ2
values/1000 cd m /10 000 cd m
. CIE coordinates at 1000 cd m .
compared with that of R2. It retains an EQE of 33.3% even at
corresponding data of device G2 (85.6 cd A , 93.2 lm W and 1000 cd m , which is also significantly superior compared
ꢀ1
ꢀ1
ꢀ2
23.6%). This is chiefly attributed to the higher hole mobility of with those of R2 and the NPB-based device. Therefore, the
DPNA-SFX and more harmonious charge recombination inside DPNA-SFX based red device possesses an extremely high EQE
device G1. It is noteworthy that the green devices based on and low roll-off compared with DOPNA-SFX and NPB. It is
DPNA-SFX and DOPNA-SFX are both far better than the worth mentioning that we have also tested the devices using
ꢀ
1
ꢀ1
NPB-based PHOLEDs (68.5 cd A , 79.7 lm W and 18.7%) an integrating sphere in order to exclude the possibility of
shown in Fig. S12 and Table S1 (ESI†), which is presumably due anisotropic emission of the green and red emitters. Almost
to the imbalance of hole/electron injection and transport. The similar results were exhibited (shown in Fig. S17 and S18, ESI†),
efficiency roll-off values of green PHOLEDs based on DPNA-SFX, which indicates the impossibility of anisotropic emission of
DOPNA-SFX and NPB are 2%, 5% and 1%. As we can see, both green and red emitters which might cause an overestimated
green devices show a relatively low efficiency roll-off. Sure, the performance.
roll-off of the NPB-based device is the lowest one, and the EQE is
far behind that of the others.
In order to explore the possibility of high performance blue
PHOLEDs, we fabricate blue devices based on the architecture
The device performance of the red PHOLED devices based of ITO/HAT-CN (5 nm)/HTM (40 nm)/TCTA (10 nm)/TCTA:
on DPNA-SFX and DOPNA-SFX was also measured (see Fig. 4). TmPyPB:FIrpic (20 nm)/TmPyPB (30 nm)/Liq (2.5 nm)/Al
As shown in Fig. 4(a), the devices R1 and R2 present saturated (100 nm). The devices performance is shown in Fig. S13 (ESI†)
red emissions with the same maximum peak of 620 nm, with and all the key parameters are listed in Table S2 (ESI†). Three
the CIE coordinates of (0.66, 0.34). The EL spectra remain kinds of devices possess a low turn-on voltage of 2.8 V. The
unchanged over a wide range of operating voltage from 3 V to DPNA-SFX based device exhibits the best CE, PE and EQE of
ꢀ1
ꢀ1
7
V (see Fig. S11, ESI†). The J–V–L curve of the red devices 17.93 cd A , 14.87 lm W and 8.70%, respectively. Comparing
This journal is The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 3247ꢀ3256 | 3251

View Article Online
Paper
Journal of Materials Chemistry C
Fig. 4 Device performance of red PHOLEDs: (a) EL spectra (4.0 V), (b) J–V–L curves, (c) CE–L–PE curves, and (d) EQE–L curves.
Fig. 6 Normalized luminance as a function of operating time for (a) red
and (b) green PHOLEDs with different HTMs.
based red and green devices showed a longer lifetime than
NPB, especially the DPNA-SFX based ones, indicating that the
Fig. 5 EQE versus EL peaks of red PHOLEDs based on several representa- crucial role of the thermal stability of HTMs and much better
tive iridium complexes.
charge balance render a longer lifetime for PHOLEDs.
To reveal the reasons for superb PHOLED performance,
electron-only devices based on ANT-BIZ have also been fabri-
with the NPB-based device, the performance of the DPNA-SFX cated ITO/Liq (10 nm)/ANT-BIZ (100 nm)/Liq (10 nm)/Al
and DOPNA-SFX based devices is much better, demonstrating (100 nm) (see Fig. S14, ESI†). The calculated electron mobility
ꢀ
4
2
ꢀ1 ꢀ1
their potential to replace commercialized HTMs.
under zero electric field is 2.4 ꢁ 10 cm V
s , which is
Besides high efficiency, a long lifetime of PHOLEDs is also higher than the hole mobilities of the applied HTMs. Considering
essential and is affected by the charge balance. We have the sequences of hole-transporting properties of DPNA-SFX,
confirmed the better thermal stability of our HTMs compared DOPNA-SFX and NPB, the DPNA-based PHOLEDs exhibiting the
with NPB, which should contribute to the operational stability best device performance could be attributed to more efficient
of the corresponding PHOLEDs as well. The lifetime of red and charge injection and better charge transport balance, which
green PHOLEDs with the best EQE was evaluated. The lifetime guarantee good exciton confinement. It is also in accordance with
curves shown in Fig. 6 of the green and red PHOLEDs were the optimization results of the thickness of the ETL layers (Table
ꢀ
2
measured at a high initial luminance of 50 000 cd m and S3, ESI†). In addition, we also fabricate two single carrier devices
ꢀ
2
1
0 000 cd m , respectively. The DPNA-SFX and DOPNA-SFX based on our PHOLED architecture without the emitting layers.
3252 | J. Mater. Chem. C, 2021, 9, 3247ꢀ3256
This journal is The Royal Society of Chemistry 2021

View Article Online
Journal of Materials Chemistry C
Paper
The structures of the hole-only and electron-only devices are ITO/ sodium tert-butoxide (11.52 g, 120 mmol) were added into a
HAT-CN (5 nm)/HTM (30 nm)/Spiro-3-BFP (10 nm)/Al and ITO/ 250 mL round flask and dissolved in dry toluene (100 mL) by
ANT-BIZ (70 nm)/Liq (2.5 nm)/Al (100 nm) (see Fig. S15, ESI†). It is magnetic stirring. Then, dichlorobis[di-tert-butyl(4-dimethyl-
apparent that the mobility of three HTMs show the same aminophenyl)phosphino]palladium(II) (0.848 g, 1.2 mmol) was
sequence: DPNA-SFX 4 DOPNA-SFX 4 NPB. Similarly, the elec- added into the container quickly. The mixture was degassed
tron transporting properties of ANT-BIZ are better than those of under a nitrogen atmosphere for 20 min and heated to reflux
HTMs, thus DPNA-SFX possesses the best performance because of temperature. After overnight reaction, the mixture was filtrated
better charge balance.
to remove the base and catalysts, washed with dichloromethane
According to the device performance of green and red (200 mL) several times, followed by evaporation. 11.6 g of white
PHOLEDs, respectable high efficiency, low turn-on voltage solid was obtained by silica gel column chromatography (petro-
1
and minor efficiency roll-off behavior can be achieved by the leum ether/dichloromethane = 10: 1). Yield 53%. H NMR
strategy of utilizing DPNA-SFX and DOPNA-SFX as HTMs.
(400 MHz, CDCl ) d 7.87 (s, 1H), 7.85 (s, 2H), 7.83 (s, 1H), 7.72
3
(s, 1H), 7.70 (s, 1H), 7.50–7.45 (m, 2H), 7.40 (s, 1H), 7.38 (d, J =
0
1
.7 Hz, 2H), 7.36 (s, 1H), 7.24 (d, J = 0.9 Hz, 1H), 7.22 (d, J =
.0 Hz, 1H), 7.06 (d, J = 1.9 Hz, 1H), 7.05 (d, J = 1.2 Hz, 2H), 7.03
Conclusions
(
1
s, 1H), 6.90 (s, 2H), 6.88 (s, 2H), 6.87 (s, 1H), 6.85 (d, J = 1.0 Hz,
0
In summary, two new materials, spiro[fluorene-9,9 -xanthene]
0
1
3
H), 6.44 (t, J = 2.0 Hz, 1H), 6.41 (d, J = 2.0 Hz, 2H).
(
SFX) derivatives, spiro[fluorene-9,9 -xanthene]-2,7-di-(N ,N -
di-1-naphthalenyl-N ,N -diphenyl-1,3-benzenediamine) (DPNA-
SFX) and spiro[fluorene-9,9 -xanthene]-2 ,7 -di-(N ,N -di-1-
naphthalenyl-N ,N -diphenyl-1,3-benzenediamine) were designed,
synthesized, and characterized completely. The green PHOLEDs
using DPNA-SFX and DOPNA-SFX as the hole-transporting
material exhibited maximum EQE values of 24.7% and 23.6%,
respectively. Moreover, the red PHOLEDs with DPNA-SFX
+
1
3
MALDI-TOF-MS: m/z [M + H] calcd, 546.19; found, 547.51.
0
0
0
1
3
Anal. Calcd for C H ClN : C, 83.43; H, 4.97; Cl, 6.48; N, 5.12.
Found: C, 83.41; H, 4.97; Cl, 6.47; N, 5.13.
38 27
2
1
3
1
3
1
3
N ,N -di-1-naphthalenyl-N ,N -diphenyl-5-(4,4,5,5-tetramethyl-
,3,2-dioxaborolan-2-yl)-1,3-benzenediamine (DPNA-borate).
1
DPNA-Cl (9.84 g, 18 mmol) and bis(pinacolato)diboron (5.48 g,
21.6 mmol) were added to dry 1,4-dioxane (100 mL). Potassium
acetate (5.30 g, 54 mmol), tris(dibenzylideneacetone)
dipalladium (0.99 g, 1.08 mmol) and 2-dicyclohexylphosphino-
ꢀ
2
achieved the highest EQE of 34.7% and the EQE at 1000 cd m
maintained 96% of its peak value. To the best of our knowledge,
these performances are among the highest efficiencies for pure
red PHOLEDs. This research work suggests that the SFX-core
based derivatives show great potential for application as good
hole-transporting materials in PHOLED displays with high EQE
and low turn-on voltage as well as low efficiency roll-off.
0
0 0
2
,4 ,6 -triisoprophylbiphenyl (1.03 g, 2.16 mmol) were added
to the container quickly under a nitrogen atmosphere. The
mixture was heated to reflux temperature and stirred overnight.
After the reaction, the mixture was filtrated, washed with
dichloromethane (100 mL), followed by evaporation. The residue
was subjected to silica gel column chromatography (petroleum
ether/dichloromethane = 4 : 1) to obtain 10.33 g of yellow solid.
Experimental section
General information
1
Yield 90%. H NMR (300 MHz, CDCl ) d 7.84 (d, J = 8.6 Hz, 4H),
3
7.66 (d, J = 8.2 Hz, 2H), 7.47–7.40 (m, 2H), 7.39–7.27 (m, 4H), 7.22
1
13
H and C NMR spectra were acquired using a Bruker AVANCE
00 MHz NMR spectrometer. UV–Vis spectra were obtained
(d, J = 0.9 Hz, 1H), 7.20 (d, J = 0.9 Hz, 1H), 7.11 (d, J = 2.2 Hz, 2H),
5
7
.05–6.97 (m, 4H), 6.84–6.74 (m, 7H), 1.21 (s, 12H).
MALDI-TOF-MS: m/z [M + H] calcd, 638.31; found, 639.47.
+
using a PerkinElmer Lambda 750 spectrophotometer. Fluores-
cence and phosphorescence spectra at room temperature and
2 2
Anal. Calcd for C44H39BN O : C, 82.75; H, 6.16; B, 1.69; N, 4.39;
77 K were recorded using a Horiba Fluorolog-3 in chromato-
O, 5.01. Found: C, 82.71; H, 6.18; B, 1.68; N, 4.38; O, 4.99.
graphically pure DCM. Density functional theory (DFT) calcula-
tions were performed using the 6-31g(d,p) basis set with
Gaussian 09 package. Thermo-gravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were carried out using a
METTLER TOLEDO TGA/DSC1 system with the samples under
pure nitrogen at a heating rate of 10 1C min . A typical three-
electrode configuration was applied in cyclic voltammetry (CV)
3 3
measurements, with Ag/AgNO (0.1 M, CH CN) as the reference
electrode, glass carbon as the working electrode and platinum
wire as the counter electrode.
0
2
,7-dibromospiro[fluorene-9,9 -xanthene] (2,7-diBrSFX)
A 100 mL round flask was charged with 2,7-dibromofluorenone
14.58 g, 43.14 mmol), phenol (40.6 g, 431.4 mmol) and
(
methane sulfonic acid (16.93 g, 172.6 mmol). The mixture
was heated at 150 1C under a nitrogen atmosphere to carry
out the reaction for 24 h. Then, the reaction mixture in the flask
was slowly poured into water (50 mL) and extracted three times
with 200 mL of dichloromethane. After this, the extracts were
ꢀ
1
collected, dried over MgSO and evaporated; the residue was
4
subjected to silica gel column chromatography (petroleum
Synthetic details
ether/dichloromethane = 4 : 1) to obtain 15.01 g of white solid.
1
3
1
3
1
5
-chloro-N ,N -di-1-naphthalenyl-N ,N -diphenyl-1,3-benzene- Yield 71%. H NMR (500 MHz, CDCl
3
) d 7.63 (d, J = 8.1 Hz, 2H),
diamine (DPNA-Cl). 1,3-Dibromo-5-chlorobenzene (10.8 g, 7.50 (dd, J = 8.1, 1.8 Hz, 2H), 7.27 (s, 2H), 7.24 (dd, J = 4.6,
0 mmol), N-phenyl-1-naphthylamine (11.68 g, 53.2 mmol) and 1.0 Hz, 4H), 6.85–6.80 (m, 2H), 6.40–6.36 (m, 2H).
4
This journal is The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 3247ꢀ3256 | 3253

View Article Online
Paper
Journal of Materials Chemistry C
+
MALDI-TOF-MS: m/z [M + H] calcd, 487.94; found, 488.65. round flask. Then, dichlorobis[di-tert-butyl(4-dimethylamino-
Anal. Calcd for C25 O: C, 61.24; H, 2.88; Br, 32.60; O, 3.26. phenyl)phosphino]palladium(II) (0.074 g, 0.11 mmol) was
Found: C, 61.19; H, 2.88; Br, 32.62; O, 3.27.
H14Br
2
added into the container quickly. The mixture was degassed
under a nitrogen atmosphere for 20 min and heated to 110 1C.
After overnight reaction, the mixture was filtered, poured into
0
0 0 0 0
,7 -dibromospiro[fluorene-9,9 -xanthene] (2 ,7 -diBrSFX)
2
A 100 mL round flask was charged with 9H-fluoren-9-one (2.85 g, water and extracted with dichloromethane (200 mL). 3.55 g of
5.8 mmol), 4-bromophenol (16.4 g, 94.8 mmol) and methane white solid was obtained by silica gel column chromatography
1
1
sulfonic acid (6.2 g, 63.2 mmol). The mixture was heated at 150 1C (petroleum ether/dichloromethane = 4 : 1). Yield 75%. H NMR
under a nitrogen atmosphere to carry out the reaction for 24 h. (400 MHz, CDCl ) d 7.85 (d, J = 8.2 Hz, 4H), 7.78 (d, J = 8.4 Hz,
3
Then, the reaction mixture in the flask was slowly poured into 4H), 7.66 (dd, J = 16.1, 7.9 Hz, 6H), 7.44 (dd, J = 11.1, 4.0 Hz,
water (50 mL) and extracted three times with dichloromethane 4H), 7.31 (dd, J = 15.7, 7.8 Hz, 8H), 7.27 (d, J = 2.1 Hz, 1H), 7.25
(200 mL). After this, the extracts were collected, dried over MgSO4 (d, J = 1.8 Hz, 1H), 7.16 (d, J = 6.6 Hz, 4H), 7.07 (d, J = 1.8 Hz,
and evaporated; the residue was subjected to silica gel column 1H), 7.05–7.03 (m, 3H), 7.00–6.95 (m, 8H), 6.90 (t, J = 5.8 Hz,
chromatography (petroleum ether/dichloromethane = 4 : 1) to 4H), 6.80 (dd, J = 14.4, 7.4 Hz, 12H), 6.52 (t, J = 1.9 Hz, 2H), 6.34
1
13
obtain 4.88 g of white solid. Yield 63%. H NMR (400 MHz, (d, J = 2.0 Hz, 4H), 6.20 (d, J = 1.8 Hz, 2H). C NMR (400 MHz,
CDCl ) d 7.81 (d, J = 7.6 Hz, 2H), 7.42 (td, J = 7.5, 1.0 Hz, 2H), 7.30 CDCl ) d 154.07, 151.17, 149.19, 148.03, 143.23, 141.76, 139.52,
3
3
(d, J = 2.4 Hz, 1H), 7.27 (t, J = 2.0 Hz, 2H), 7.25 (d, J = 1.0 Hz, 1H), 136.11, 135.32, 131.25, 128.95, 128.44, 128.25, 127.92, 127.11,
7
.12 (dd, J = 15.1, 8.2 Hz, 4H), 6.47 (d, J = 2.4 Hz, 2H).
MALDI-TOF-MS: m/z [M + H] calcd, 487.94; found, 488.63. 121.78, 121.59, 119.82, 116.83, 113.65, 113.32, 54.49. MALDI-
126.73, 126.50, 126.36, 126.25, 126.07, 125.98, 125.39, 124.39,
+
+
Anal. Calcd for C H Br O: C, 61.24; H, 2.88; Br, 32.60; O, 3.26. TOF-MS: m/z [M + H] calcd, 1352.54; found, 1353.53. Anal.
2
5
14
2
Found: C, 61.21; H, 2.86; Br, 32.63; O, 3.28.
Calcd for C101H N O: C, 89.62; H, 5.06; N, 4.14; O, 1.18. Found:
64 4
C, 89.24; H, 5.03; N, 4.16; O, 1.16.
0
1
3
1
Spiro[fluorene-9,9 -xanthene]-2,7-di-(N ,N -di-1-naphthalenyl-N ,
3
N -diphenyl-1,3-benzenediamine) (DPNA-SFX).
Device fabrication and measurements
DPNA-borate (3.82 g, 6 mmol) and 2,7-diBrSFX (1.47 g, 3 mmol) All the materials except DPNA-SFX and DOPNA-SFX for multi-
were added into a 250 mL round flask and dissolved in dry layer PHOLED fabrication and ITO-coated glass substrates were
toluene (80 mL) by magnetic stirring. Potassium carbonate purchased from commercial sources. The ITO-coated glass sub-
(
1.24 g, 9 mmol) was dissolved in 20 mL of water and poured strates were washed with acetone, deionized water and isopro-
into the round flask. Then, dichlorobis[di-tert-butyl(4-dimethyl- panol in order and put into a UV-ozone environment for 20 min.
aminophenyl)phosphino]palladium(II) (0.064 g, 0.09 mmol) was The deposition process of all organic layers and metal electrode
ꢀ
7
added into the container quickly. The mixture was degassed was performed under vacuum (B8 ꢁ 10 Pa). The characteriza-
under a nitrogen atmosphere for 20 min and heated to 110 1C. tion of PHOLED performance including current density–voltage–
After overnight reaction, the mixture was filtered, poured into luminance ( J–V–L) curves, electroluminescence (EL) spectra and
´
water and extracted with dichloromethane (200 mL). 2.78 g of Commission Internationale de lEclairage (CIE) coordinates were
white solid was obtained by silica gel column chromatography carried out using a Keithley 2400 semiconductor characteriza-
1
(
petroleum ether/dichloromethane = 4 : 1). Yield 69%. H NMR tion system with BM-7A luminance colorimeter and PR-788
(
400 MHz, CDCl ) d 7.83 (d, J = 8.6 Hz, 8H), 7.66 (d, J = 8.2 Hz, 4H), photometer. Further measurements were also conducted using
3
7
7
.54 (d, J = 7.9 Hz, 2H), 7.42 (ddd, J = 8.2, 6.9, 1.0 Hz, 4H), 7.36– equipment with calibrated integrating spheres and with the
.27 (m, 8H), 7.17 (dddd, J = 14.9, 9.7, 7.8, 1.3 Hz, 10H), 7.05–6.98 device’s substrate edges masked. The external quantum effi-
(
m, 8H), 6.96 (d, J = 1.3 Hz, 2H), 6.90–6.77 (m, 12H), 6.65–6.60 (m, ciency of all the fabricated devices were calculated according to
2
H), 6.58 (dd, J = 3.6, 1.5 Hz, 6H), 6.25 (dd, J = 7.9, 1.2 Hz, 2H). electroluminescence performance and EL spectra.
1
3
3
C NMR (400 MHz, CDCl ) d 155.23, 151.56, 149.16, 148.05,
All the measurements except device lifetime measurements
1
1
1
1
1
8
43.25, 142.61, 141.34, 138.51, 135.27, 131.12, 128.95, 128.45, were conducted in ambient environment without further protection
28.02, 127.61, 127.04, 126.94, 126.48, 126.36, 126.25, 126.08, at room temperature. Device stability of PHOLEDs was evaluated by
25.03, 124.40, 124.26, 123.29, 121.78, 121.62, 119.96, 116.79, collecting time dependent luminance data at a constant current
+
14.17, 113.94, 54.46. MALDI-TOF-MS: m/z [M + H] calcd, density and constant initial luminance. The lifetime measurements
352.54; found, 1353.55. Anal. Calcd for
101 64 4
C H N
O: C, were carried out in a glovebox with encapsulation.
9.68; H, 5.06; N, 4.14; O, 1.18. Found: C, 89.31; H, 5.05;
N, 4.14; O, 1.17.
Conflicts of interest
0
0
0
1
3
Spiro[fluorene-9,9 -xanthene]-2 ,7 -di-(N ,N -di-1-naphthalenyl-
There are no conflicts to declare.
1
3
N ,N -diphenyl-1,3-benzenediamine) (DOPNA-SFX)
DPNA-borate (4.7 g, 7 mmol) and 2,7-diBrSFX (1.72 g, 3.5 mmol)
were added into a 250 mL round flask and dissolved in 80 mL of
Acknowledgements
dry toluene by magnetic stirring. Potassium carbonate (1.45 g, This work was financially supported by the Key-Area Research and
0.5 mmol) was dissolved in water (20 mL) and poured into the Development Program of Guangdong Province (2019B010924003),
1
3254 | J. Mater. Chem. C, 2021, 9, 3247ꢀ3256
This journal is The Royal Society of Chemistry 2021

View Article Online
Journal of Materials Chemistry C
Paper
Guangdong Basic and Applied Basic Research Foundation 22 S.-h. Ye, L. Li, M. Zhang, Z. Zhou, M.-h. Quan, L.-F. Guo,
(
0
2020B1515120030), Shenzhen Peacock Plan (KQTD2014
62714543296), Shenzhen Engineering Research Center (Shenzhen
Y. Wang, M. Yang, W.-Y. Lai and W. Huang, J. Mater. Chem.
C, 2017, 5, 11937–11946.
development and reform commission [2018]1410), and Shenzhen 23 H. Fukagawa, T. Shimizu, H. Kawano, S. Yui, T. Shinnai,
Science and Technology Research Grant (JCYJ20180302153451987).
A. Iwai, K. Tsuchiya and T. Yamamoto, J. Phys. Chem. C,
016, 120, 18748–18755.
2
2
2
4 J. Kwak, Y.-Y. Lyu, S. Noh, H. Lee, M. Park, B. Choi, K. Char
and C. Lee, Thin Solid Films, 2012, 520, 7157–7163.
5 Z. Zhong, X. Wang, T. Guo, J. Cui, L. Ying, J. Peng and
Y. Cao, Org. Electron., 2018, 53, 35–42.
References
1
2
J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332.
S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan and 26 X.-Y. Liu, X. Tang, Y. Zhao, D. Zhao, J. Fan and L.-S. Liao,
W. Huang, Adv. Mater., 2010, 22, 5227–5239.
Z. Wu and D. Ma, Mater. Sci. Eng., R, 2016, 107, 1–42.
Z. Zheng, Q. Dong, L. Gou, J.-H. Su and J. Huang, J. Mater.
Chem. C, 2014, 2, 9858–9865.
J. Mater. Chem. C, 2018, 6, 1023–1030.
3
4
27 M. Romain, C. Quinton, D. Tondelier, B. Geffroy,
O. Jeannin, J. Rault-Berthelot and C. Poriel, J. Mater. Chem.
C, 2016, 4, 1692–1703.
5
S. Zhang, Q.-L. Xu, J.-C. Xia, Y.-M. Jing, Y.-X. Zheng and 28 M. Romain, D. Tondelier, B. Geffroy, A. Shirinskaya,
J.-L. Zuo, New J. Chem., 2015, 39, 7954–7960.
T.-Y. Chu and O.-K. Song, Appl. Phys. Lett., 2007, 90.
O. Jeannin, J. Rault-Berthelot and C. Poriel, Chem. Commun.,
2015, 51, 1313–1315.
R. Braveenth, H. W. Bae, I. J. Ko, W. Qiong, Q. P. B. Nguyen, 29 X.-D. Zhu, Y.-L. Zhang, Y. Yuan, Q. Zheng, Y.-J. Yu, Y. Li,
6
7
P. G. S. Jayashantha, J. H. Kwon and K. Y. Chai, Org.
Electron., 2017, 51, 463–470.
Z.-Q. Jiang and L.-S. Liao, J. Mater. Chem. C, 2019, 7,
6714–6720.
8
9
S. Zhang, L.-S. Xue, Y.-M. Jing, X. Liu, G.-Z. Lu, X. Liang, 30 C. Poriel and J. Rault-Berthelot, J. Mater. Chem. C, 2017, 5,
H.-Y. Li, Y.-X. Zheng and J.-L. Zuo, Dyes Pigm., 2015, 118,
–8.
A. M. Thaengthong, S. Saengsuwan, S. Jungsuttiwong,
3869–3897.
1
31 L.-H. Xie, F. Liu, C. Tang, X.-Y. Hou, Y.-R. Hua, Q.-L. Fan and
W. Huang, Org. Lett., 2006, 8, 2787–2790.
T. Keawin, T. Sudyoadsuk and V. Promarak, Tetrahedron 32 H. Li, K. Fu, A. Hagfeldt, M. Gratzel, S. G. Mhaisalkar and
Lett., 2011, 52, 4749–4752.
A. C. Grimsdale, Angew. Chem., 2014, 53, 4085–4088.
0 S. Jhulki and J. N. Moorthy, J. Mater. Chem. C, 2018, 6, 33 B.-Y. Ren, D.-K. Zhong, Y.-G. Sun, X.-H. Zhao, Q.-J. Zhang,
1
1
1
1
8
280–8325.
1 J. Li, C. Ma, J. Tang, C.-S. Lee and S. Lee, Chem. Mater., 2005,
7, 615–619.
Y. Liu, M. Jurow, M.-L. Sun, Z.-S. Zhang and Y. Zhao, Org.
Electron., 2016, 36, 140–147.
34 M. Sun, R. Xu, L. Xie, Y. Wei and W. Huang, Chin. J. Chem.,
2015, 33, 815–827.
35 D. P. Tabor, V. A. Chiykowski, P. Friederich, Y. Cao,
D. J. Dvorak, C. P. Berlinguette and A. Aspuru-Guzik, Chem.
Sci., 2019, 10, 8360–8366.
1
2 Z. Li, Z. Wu, W. Fu, D. Wang, P. Liu, B. Jiao, X. Lei, G. Zhou
and Y. Hao, Electron. Mater. Lett., 2013, 9, 655–661.
3 R. Griniene, J. V. Grazulevicius, K. Y. Tseng, W. B. Wang,
J. H. Jou and S. Grigalevicius, Synth. Met., 2011, 161,
2
466–2470.
36 X.-H. Zhao, Y.-N. Xu, J.-X. Wu, G.-D. Zou, X.-W. Zhang,
L.-H. Xie, J.-F. Zhao, C.-M. Han, X. Hui and J.-J. Zhang, Dyes
Pigm., 2019, 168, 228–234.
37 L. Calio, S. Kazim, M. Gratzel and S. Ahmad, Angew. Chem.,
2016, 55, 14522–14545.
38 H.-T. Cao, C.-S. Hong, D.-Q. Ye, L.-H. Liu, L.-H. Xie,
S.-F. Chen, C. Sun, S.-S. Wang, H.-M. Zhang and
W. Huang, J. Mol. Struct., 2019, 1196, 132–138.
1
4 O. Usluer, S. Demic, D. A. M. Egbe, E. Birckner, C. Tozlu,
A. Pivrikas, A. M. Ramil and N. S. Sariciftci, Adv. Funct.
Mater., 2010, 20, 4152–4161.
1
1
1
1
5 H. Kanai, S. Ichinosawa and Y. Sato, Synth. Met., 1997, 91,
195–196.
6 J. Zhuang, W. Su, W. Li, Y. Zhou, Q. Shen and M. Zhou, Org.
Electron., 2012, 13, 2210–2219.
7 B. Liu, J. Zhao, C. Luo, F. Lu, S. Tao and Q. Tong, J. Mater. 39 H.-j. Jiang, J. Sun, K. Yuan and Q.-w. Zhang, Synth. Met.,
Chem. C, 2016, 4, 2003–2010.
2014, 197, 217–224.
8 Q. D. Liu, J. Lu, J. Ding, M. Day, Y. Tao, P. Barrios, J. Stupak, 40 X. Liang, K. Wang, R. Zhang, K. Li, X. Lu, K. Guo, H. Wang,
K. Chan, J. Li and Y. Chi, Adv. Funct. Mater., 2007, 17,
028–1036.
9 X.-Y. Liu, Y.-J. Zhang, X. Fei, Q. Ran, M.-K. Fung and J. Fan,
J. Mater. Chem. C, 2019, 7, 1370–1378.
Y. Miao, H. Xu and Z. Wang, Dyes Pigm., 2017, 139, 764–771.
41 M. Maciejczyk, A. Ivaturi and N. Robertson, J. Mater. Chem.
A, 2016, 4, 4855–4863.
42 B. Xu, D. Bi, Y. Hua, P. Liu, M. Cheng, M. Gr ¨a tzel, L. Kloo,
A. Hagfeldt and L. Sun, Energy Environ. Sci., 2016, 9, 873–877.
1
1
2
0 Q. P. Nguyen, S. J. Baek, M. J. Kim, N. Y. Shin, G. W. Kim,
D. C. Choe, J. H. Kwon and K. Y. Chai, Molecules, 2014, 19, 43 V. A. Chiykowski, Y. Cao, H. Tan, D. P. Tabor, E. H. Sargent,
1
4247–14256.
A. Aspuru-Guzik and C. P. Berlinguette, Angew. Chem., Int.
Ed., 2018, 57, 15529–15533.
2
1 T. L. Wu, S. Y. Liao, P. Y. Huang, Z. S. Hong, M. P. Huang,
C. C. Lin, M. J. Cheng and C. H. Cheng, ACS Appl. Mater. 44 Z. Jiang, T. Ye, C. Yang, D. Yang, M. Zhu, C. Zhong, J. Qin
Interfaces, 2019, 11, 19294–19300.
and D. Ma, Chem. Mater., 2011, 23, 771–777.
This journal is The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 3247ꢀ3256 | 3255

View Article Online
Paper
Journal of Materials Chemistry C
4
5 D.-H. Lee, Y.-P. Liu, K.-H. Lee, H. Chae and S. M. Cho, Org. 51 B. Jiang, C. Zhao, X. Ning, C. Zhong, D. Ma and C. Yang,
Electron., 2010, 11, 427–433. Adv. Opt. Mater., 2018, 6, 1800108.
6 H. Sasabe, E. Gonmori, T. Chiba, Y.-J. Li, D. Tanaka, S.-J. Su, 52 K. H. Kim, S. Lee, C. K. Moon, S. Y. Kim, Y. S. Park, J. H. Lee, J. Woo
4
T. Takeda, Y.-J. Pu, K.-I. Nakayama and J. Kido, Chem.
Mater., 2008, 20, 5951–5953.
7 J. Li, D. Liu, Y. Li, C.-S. Lee, H.-L. Kwong and S. Lee, Chem.
Mater., 2005, 17, 1208–1212.
Lee, J. Huh, Y. You and J. J. Kim, Nat. Commun., 2014, 5, 4769.
53 C. Y. Kuei, W. L. Tsai, B. Tong, M. Jiao, W. K. Lee, Y. Chi,
C. C. Wu, S. H. Liu, G. H. Lee and P. T. Chou, Adv. Mater.,
2016, 28, 2795–2800.
4
4
8 C.-H. Chen, L.-C. Hsu, P. Rajamalli, Y.-W. Chang, F.-I. 54 J. H. Lee, H. Shin, J. M. Kim, K. H. Kim and J. J. Kim, ACS
Wu, C.-Y. Liao, M.-J. Chiu, P.-Y. Chou, M.-J. Huang, Appl. Mater. Interfaces, 2017, 9, 3277–3281.
L.-K. Chu and C.-H. Cheng, J. Mater. Chem. C, 2014, 2, 55 X. Y. Liu, F. Liang, Y. Yuan, L. S. Cui, Z. Q. Jiang and
183–6191. L. S. Liao, Chem. Commun., 2016, 52, 8149–8151.
9 L. S. Cui, Y. Liu, X. Y. Liu, Z. Q. Jiang and L. S. Liao, ACS 56 C. H. Shih, P. Rajamalli, C. A. Wu, W. T. Hsieh and C. H. Cheng,
Appl. Mater. Interfaces, 2015, 7, 11007–11014. ACS Appl. Mater. Interfaces, 2015, 7, 10466–10474.
0 B. Jiang, X. Ning, S. Gong, N. Jiang, C. Zhong, Z.-H. Lu and 57 C.-H. Shih, P. Rajamalli, C.-A. Wu, M.-J. Chiu, L.-K. Chu and
6
4
5
C. Yang, J. Mater. Chem. C, 2017, 5, 10220–10224.
C.-H. Cheng, J. Mater. Chem. C, 2015, 3, 1491–1496.
3256 | J. Mater. Chem. C, 2021, 9, 3247ꢀ3256
This journal is The Royal Society of Chemistry 2021