Fluorene-containing polyhedral oligomericsilsesquioxanes modified hyperbranched polymer for white light-emitting diodes with ultra-high color rendering index of 96

Article abstract of DOI:10.1016/j.jssc.2021.122122

In this work, a series of hyperbranched white-emitting conjugated polymers were synthesized with polyfluorene (PF) as the branches and three-dimensional-structured spiro[3.3]heptane-2,6-dispirofluorene (SDF) as the conjugated branching point by one-pot Suzuki polycondensation, where 4,7-dithienyl-2,1,3-benzothiadiazole (DBT) as orange-light emitting unit and fluorene-containing polyhedral oligomericsilsesquioxanes (POSSs) as conjugated linking monomer were introduced into the backbones to obtain white-light emission. The influence mechanism of POSSs for hyperbranched white-emitting polymers was explored by adjusting the feeding ratios of fluorene-containing POSSs (from 1 to 20 ?mol%). The results indicated that the synthesized polymers still maintained the high thermal stabilities, and exhibited the improved amorphous film morphology and hydrophobicity, which were beneficial for obtaining optimized interface between the aqueous hole transport layer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) layer and polymer light-emitting layer in device fabrication. As a consequence, all of the fabricated devices with the hyperbranched polymers as light-emitting layers realized white light-emission, and the optimized device exhibite good electroluminescent (EL) performance with Commission Internationale de l'Eclairage (CIE) coordinates at (0.32, 0.33) and maximum color rendering index (CRI) of 96. The fluorene-containing POSSs modified hyperbranched copolymers with broad full width at half maximum (>284 ?nm) are attractive candidates for sunlight-style white polymer light-emitting diodes.

Full text of DOI:10.1016/j.jssc.2021.122122

Journal of Solid State Chemistry 298 (2021) 122122
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Fluorene-containing polyhedral oligomericsilsesquioxanes modified
hyperbranched polymer for white light-emitting diodes with ultra-high
color rendering index of 96
,
,*
Mixue Wang a, Xiaozhen Wei a, Weixuan Zhang a, Haocheng Zhao b c, Yuling Wu a
,
,
Yanqin Miao a, Hua Wang a **, Bingshe Xu a
a Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, China
b Department of Electrical Engineering, Shanxi Institute of Energy, Taiyuan 030600, China
c College of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Polymer light-emitting diodes
White light
Color rendering index
Polyhedral oligomericsilsesquioxanes
Hydrophobicity
In this work, a series of hyperbranched white-emitting conjugated polymers were synthesized with polyfluorene
(PF) as the branches and three-dimensional-structured spiro[3.3]heptane-2,6-dispirofluorene (SDF) as the con-
jugated branching point by one-pot Suzuki polycondensation, where 4,7-dithienyl-2,1,3-benzothiadiazole (DBT)
as orange-light emitting unit and fluorene-containing polyhedral oligomericsilsesquioxanes (POSSs) as conjugated
linking monomer were introduced into the backbones to obtain white-light emission. The influence mechanism of
POSSs for hyperbranched white-emitting polymers was explored by adjusting the feeding ratios of fluorene-
containing POSSs (from 1 to 20 mol%). The results indicated that the synthesized polymers still maintained
the high thermal stabilities, and exhibited the improved amorphous film morphology and hydrophobicity, which
were beneficial for obtaining optimized interface between the aqueous hole transport layer Poly(3,4-
ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) layer and polymer light-emitting layer in device
fabrication. As a consequence, all of the fabricated devices with the hyperbranched polymers as light-emitting
layers realized white light-emission, and the optimized device exhibite good electroluminescent (EL) perfor-
mance with Commission Internationale de l’Eclairage (CIE) coordinates at (0.32, 0.33) and maximum color
rendering index (CRI) of 96. The fluorene-containing POSSs modified hyperbranched copolymers with broad full
width at half maximum (>284 nm) are attractive candidates for sunlight-style white polymer light-emitting
diodes.
1. Introduction
performance of the polymers and the electroluminescence of the corre-
sponding devices [10–15].
White polymer light-emitting diodes (WPLEDs) have been widely
recognized towards their potential applications in large area full-color
and flexible displays combined with a color filter, backlights and solid
lighting sources because they are low-carbon and environmentally
friendly planar light source [1–6] to produce highly efficient saturated
white-light with high luminance and low driving voltage [7–9]. In
particular, compared with linear polymers, the three-dimensional struc-
tured hyperbranched polymers can increase steric hindrance, effectively
In our previous research, in order to improve the luminance, effi-
ciency, and stability of the hyperbranched polymer light-emitting diodes
(PLEDs),
phosphorescent
red
emitter
of
bis(1-phenyl-
isoquinoline)(acetylacetonato)iridium(III) (Ir(piq)2acac) with high
luminescent quantum yield and fluorescent blue emitter of carbazole
with excellent thermal stability had been introduced to hyperbranched
white light-emitting polymers. It was found that the introduction of the
above groups improved the luminance and efficiency of polymer devices
to some extent, but there were still some problems in the stability and
lifetime of the devices. This is because part of the PEDOT: PSS layer could
avoid the fluorescence quenching and crystallization caused by the π-π
stacking of the molecules, thereby improving the light-emitting
* Corresponding author.
** Corresponding author.
E-mail addresses: wuyuling@tyut.edu.cn (Y. Wu), wanghua001@tyut.edu.cn (H. Wang).
https://doi.org/10.1016/j.jssc.2021.122122
Received 24 January 2021; Received in revised form 1 March 2021; Accepted 6 March 2021
Available online 11 March 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
be dissolved in the hyperbranched polymer light-emitting layer to form
an uneven interface with defects in the spin-coating process when the
single light-emitting layer WPLEDs were prepared. As the current density
was continuously increased, the defect area was easily broken-down and
a lot of heat was generated at the same time, which greatly limited the
efficiency and luminance and reduced the stability and lifetime of the
device. It is considered molecular modification would be a promising
strategy to improve thermostability, antioxidative stability, solubility
and hydrophobicity of hyperbranched white light copolymer by intro-
ducing functional groups with hydrophobic properties into the side
chains of hyperbranched polymers.
ꢀC/min under nitrogen atmosphere. Differential scanning calorimetry
(DSC) measurements were performed at both heating and cooling rates of
5
The UV–visible absorption spectra were determined on a Hitachi U-3900
spectrophotometer and the PL spectra were obtained using a Horiba
FluoroMax-4 spectrophotometer at room temperature. Fluorescence
lifetime and fluorescence quantum yield of copolymers were measured
on an Edinburgh Instrument FLS980 spectrometer. Cyclic Voltammetry
of copolymers were measured on an electrochemical workstation of
Shanghai Chenhua Instrument Co. Ltd.
ꢀC/min under nitrogen atmosphere, using DSC QIOO V9.4 apparatus.
Polyhedral oligomericsilsesquioxanes (POSSs) have got considerable
interests in the organic light-emitting diode (OLED) field due to their
excellent thermal stability, oxidation resistance, mechanical toughness,
light transmittance, conductivity, solubility and low surface energy
[16–21]. They are considered as multifunctional groups with excellent
performance because they can combine the advantages of inorganic
materials with those of organic polymers [22,23]. Meanwhile, it is
possible to introduce some special groups into POSSs to modify target
materials [24–27]. Therefore, the POSSs groups were selected as the best
polymer modificators in this work.
In this work, the hyperbranched white-emitting polymers with 2,7-
fluorene branches, spiro[3.3] heptane-2,6-dispirofluorene (SDF) core
(10 mol%), and POSSs modified 2,7-fluorene and DBT as light-dimming
units were constructed. POSSs were introduced into the polymers as
dopant side-chains. The influences of POSSs on their thermal, hydro-
phobicity, photoluminescent (PL) and EL properties were investigated in
detail. It was demonstrated the three-dimensional-structured SDF
exhibited excellent morphological stability and intense fluorescence
[28], and the POSSs group showed significant improvements in
film-forming ability and hydrophobicity. As a consequence, the fabri-
cated devices with synthesized hyperbranched polymers as emission
layers (EMLs) realized good white emission with Commission Inter-
nationale de l’Eclairage (CIE) at (0.32, 0.33) and maximum color
rendering index (CRI) of 96.
2.2. Syntheses
Spiro[3.3]
heptane-2,6-di-(20,2”,70,700-tetrabromospirofluorene)
(TBrSDF) [29–31] and 4,7-bis(2-bromo-5-thienyl)-2,1,3-benzothiadia-
zole (DBrDBT) [32–34] were synthesized according to the published
literature. The synthesis of the precursor with POSSs 6 is shown in
Scheme 1, and the detailed descriptions about 1H NMR and 13C NMR of
the precursors (2–6) are displayed in Figure S1–S7 in supplementary
information.
General Procedure for the Synthesis of copolymers PFSDF10-POSS1,
PFSDF10-POSS5, PFSDF10-POSS10, PFSDF10-POSS20.
To a solution of predetermined amount of monomers (M1, M2, POSSs
and TBrSDF) intoluene (30 mL) was added an aqueous solution (15 mL)
of K2CO3 (2 M), a catalytic amount of Pd(PPh3)4 (0.10 g，0.10 mmol)
under nitrogen. Aliquat 336 (2 mL) in toluene (5 mL) was added as the
phase transfer catalyst. The mixture was vigorously stirred at 100 ꢀC for
72 h. Then DBrDBT (3.20 mL, 2 ꢁ 10ꢂ3 mol/L) was added and the
mixture was continuously stirred at 100 ꢀC for 48 h. Phenylboronic acid
(0.14 g，1.00 mmol) in toluene (10 mL) was then added to the reaction
mixture, followed by stirring at 100 ꢀC for an additional 12 h. Finally,
bromobenzene (2 mL) was added by heating for 12 h again. When
cooling to room temperature, the reaction mixture was washed with 2 M
HCl and water. The organic layer was separated, and the solution was
added dropwise to excess methanol. The precipitated polymers were
collected by filtration and dried under vacuum. The solid was Soxhlet
extracted with acetone for 48 h and then passed through a short chro-
matographic column using toluene as the eluent to afford the polymers.
The detailed descriptions about 1H NMR of the copolymers are displayed
in Figure S8 in supplementary information.
2. Experimental section
2.1. Materials and characterization
PFSDF10-POSS1：M1 (0.37 g, 0.68 mmol), M2 (0.71 g, 1.10 mmol),
TBrSDF (0.14 g, 0.20 mmol), POSSs (0.06 g, 0.02 mmol) and DBrDBT
(3.20 mL, 2 ꢁ 10ꢂ3 mol/L). Gray powder, yield: 48.5%. 1H NMR (600
MHz, CDCl3) δ(ppm)：8.08–7.40 (-ArH-), 7.01–6.75 (-ArH-), 3.44–2.92
(–CH2–), 2.66–2.53 (–CH2–), 2.20–1.78 (-C–CH2–), 1.24–1.09 (–CH2–),
1.09–0.94 (–CH3–), 0.94–0.58 (–CH2–).
PFSDF10-POSS5：M1 (0.33 g, 0.60 mmol), M2 (0.71 g, 1.10 mmol),
TBrSDF (0.14 g, 0.2 mmol), POSSs (0.27 g, 0.10 mmol) and DBrDBT
(3.20 mL, 2 ꢁ 10ꢂ3 mol/L). Deep gray powder, yield: 52.6%. 1H NMR
(600 MHz, CDCl3) δ(ppm)：7.86–7.29 (-ArH-), 6.93–6.70 (-ArH-),
3.43–2.91 (–CH2–), 2.58–2.31 (–CH2–), 2.16–1.78 (-C–CH2–), 1.24–1.18
(–CH2–),1.18–0.92 (–CH3–), 0.92–0.58 (–CH2–).
Dihydropyran, 2-bromoethanol, p-toluenesulfonic acid monohydrate,
2,7-dibromofluorene, tetrabutylammonium chloride hydrate, allyl bro-
mide, 5-bromo-1-pentene, 7-bromo-1-heptene, 3,3,3-trifluoropropyl)tri-
methoxysilane,
(3-bromopropyl)trichlorosilane,
2,7-dibromo-9,9-
dioctylfluorene (M1, 99.8%), 9,9-dioctylfluorene-2,7-bis (boronic acid
pinacol ester) (M2, 99.5%), Aliquant336, tetrakis(triphenylphosphine)
palladium, phenylboronic acid, and bromobenzene were purchased from
Energy Chemical company and Synwitech company. Tetrahydrofuran
(THF) and toluene were distilled using standard procedures. Other sol-
vents were used without further purification unless otherwise specified.
All reactions were carried out using Schlenk techniques under a dry ni-
trogen atmosphere.
1HNMR spectra were measured on a Bruker DRX 600 spectrometer,
and chemical shifts were reported in ppm using tetramethyl silane as an
internal standard and using deuterated chloroform (CDCl3), deuterated
acetone (acetone-d6) or deuterated dimethyl sulfoxide (DMSO‑d6) as
solvents. Molecular weights and polydispersities of the copolymers were
determined using gel permeation chromatography (GPC) on an HP1100
HPLC system equipped with a 410 differential refractometer, and a
refractive index (RI) detector, with polystyrenes as the standard and THF
as the eluent at a flow rate of 1.0 mL/min at 30 ꢀC. Infrared spectra (IR)
were measured on a Bruker Tensor 27 infrared spectrometer with KBr
crystal as the carrier. Atomic force microscopy (AFM) measurements
were performed on a SPA-300HV from Digital Instruments Inc. Ther-
mogravimetric analysis (TGA) of the copolymers was conducted on a
Netzsch TG 209 F3 thermogravimetric analyzer at a heating rate of 10
PFSDF10-POSS10：M1 (0.27 g, 0.50 mmol), M2 (0.71 g, 1.10
mmol), TBrSDF (0.14 g, 0.20 mmol), POSSs (0.54 g, 0.20 mmol) and
DBrDBT (3.20 mL, 2 ꢁ 10ꢂ3 mol/L). Deep gray powder, yield: 46.2%. 1H
NMR (600 MHz, CDCl3) δ(ppm)：8.03–7.31 (-ArH-), 6.93–6.80 (-ArH-),
3.45–2.72 (–CH2–), 2.61–2.31 (–CH2–), 2.18–1.77 (-C–CH2–), 1.24–1.18
(–CH2–), 1.18–0.92 (–CH3–), 0.92–0.50 (–CH2–).
PFSDF10-POSS20：M1 (0.16 g, 0.30 mmol), M2 (0.71 g, 1.10
mmol), TBrSDF (0.14 g, 0.20 mmol), POSSs (1.07 g, 0.40 mmol) and
DBrDBT (3.20 mL, 2 ꢁ 10ꢂ3 mol/L). Deep gray powder, yield: 45.3%. 1H
NMR (600 MHz, CDCl3) δ(ppm)：8.01–7.42 (-ArH-), 6.94–6.75 (-ArH-),
3.39–2.99 (–CH2–), 2.60–2.35 (–CH2–), 2.18–1.78 (-C–CH2–), 1.23–1.17
(–CH2–), 1.17–0.94 (–CH3–), 0.85–0.62 (–CH2–).
2

M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
Scheme 1. Synthetic route of precursor 6.
synthesis and structure parameters of all copolymers.
3. Results and discussion
As shown in Fig. 1a, we verified the structure of the monomers and
polymers by infrared spectroscopy, where 600-500 cmꢂ1 is the stretching
vibration peak of the C–Br bond in TBrSDF and M1. At the same time,
1352 cmꢂ1 is the stretching vibration peak of the B–O bond in M2. The
above-mentioned vibration peaks disappeared in the copolymer
PFSDF10-POSS1, indicating that the copolymer was successfully ob-
tained. As shown in Fig. 1b, the antisymmetric stretching vibration peak
at 2924-2850 cmꢂ1 and 2912 cmꢂ1 are the C–H bands of polyfluorene
alkyl chain and the spiro C–H bands of TBrSDF, which shows that SDF has
been successfully incorporated into the copolymers. The peak at 1650-
3.1. Synthesis and characterization
The fluorene-containing polyhedral oligomericsilsesquioxanes
(POSSs) monomer 6 (Scheme 1) and modified hyperbranched co-
polymers PFSDF10-POSSx with 9,9-dioctylfluorene and
6 as the
branches, SDF as the branching point and DBT as light-dimming units
were prepared by Suzuki polycondensation (Scheme 2). According to
previous research work [35], the proportion of branching point SDF was
selected at 10 mol%. At the same time, in order to obtain white light
emission, the organic light-emitting unit DBT was incorporated into the
backbone at a feed ratio of 6.4 ꢁ 10ꢂ6 mol with respect to the blue
light-emitting groups of all copolymers. After that, we adjusted the feed
ratio of the POSSs modified 9,9-dioctylfluorene so as to get the best
synergy between the cage structure and branching point SDF, named as
PFSDF10-POSS1 (1 mol%), PFSDF10-POSS5 (5 mol%), PFSDF10-POSS10
(10 mol%) and PFSDF10-POSS20 (20 mol%), respectively. The yields of
the copolymer varied from 45% to 53%. Table 1 summarizes the
1430 cmꢂ1 is the C C stretching vibration peak of benzene ring skel-
–
–
eton. Obviously, the peaks at 1132 cmꢂ1 and 1234 cmꢂ1 are the
stretching vibration peaks of the C–O bond and C–Si bond connecting
between POSSs monomer and fluorene. Then the characteristic bands of
POSSs at 1145-1074 cmꢂ1 can be assignable to the stretching vibration of
Si– O–Si bonds, which were clearly exhibited in the FTIR spectrum and
Si–O–Si stretching bands became more intensified with increasing the
POSSs ratio. Simultaneously, a strong C–F stretching band at 1300-1120
Scheme 2. Synthetic route of the copolymers.
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M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
Table 1
Polymerization results and characterization of the copolymers.
Copolymers
nM1 (mol)
nM2 (mol)
nTBrSDF (mol)
nDBrDBT (mol)
nPOSS (mol)
yield (%)
GPC
Mn
Mw
PDI
PFSDF10-POSS1
PFSDF10-POSS5
PFSDF10-POSS10
PFSDF10-POSS20
0.68
0.60
0.50
0.30
1.10
1.10
1.10
1.10
0.20
0.20
0.20
0.20
6.4 ꢁ 10ꢂ6
6.4 ꢁ 10ꢂ6
6.4 ꢁ 10ꢂ6
6.4 ꢁ 10ꢂ6
0.02
0.10
0.20
0.40
48.5
52.6
46.2
45.3
11,112
8381
7250
6543
38,517
16,500
14,495
13,992
3.46
1.97
2.00
2.13
Fig. 1. (a) FT-IR spectra of monomers and copolymers (M1:2,7-Dibromo-9,9-dioctylfluorene, M2:9,9-Dioctylfluorene-2,7-bis (boronic acid pinacol ester), TBrSDF,
POSSs, P1:PFSDF10-POSS1), (b) FT-IR spectra of copolymers.
cmꢂ1 was observed from FT-IR [19]. This proved that the POSSs were
successfully attached to the polymer chain.
synthesis of the polymers, leading to a decreased molecular weight of the
polymers.
In addition, the structure of the polymers was verified by 1H NMR. As
can be seen from Figure S8, the peak integral intensities of the proton
signals around the ether bond on the side chain (δ 3.45–3.00) compared
with the alkyl chain of fluorene (δ 1.24–1.09) of the polymers was
gradually enhanced from PFSDF10-POSS1 to PFSDF10-POSS20, which
was consistent with the increasing trend of POSSs content.
The GPC characterization results are shown in Table 1. The number-
average molecular weights (Mn) of the copolymers were determined
ranging from 6543 to 11,112 with a polydispersity index (PDI) from 1.97
to 3.46. The copolymers were readily soluble in common organic sol-
vents, such as chloroform, tetrahydrofuran, toluene, etc. The insufficient
molecular weight of PFSDF10-POSS10 and PFSDF10-POSS20 may be
attributed to the fact that the POSSs groups introduced on the 9th carbon
atom of fluorene by alkyl chains provided steric hindrance in the
3.2. DFT calculation
As shown in Fig. 2, the density functional theory (DFT) calculation
was performed on POSSs monomer, and the B3LYP/6-31G(d) method
was used to optimize the molecule configuration. The size of the Si–O–Si
skeleton of POSSs molecule with a cage-like three-dimensional structure
is 0.42 nm. At the same time, the size of POSSs monomer after the
introduction of the divergent trifluoropropyl group was significantly
increased to 0.92 nm due to the accumulation of molecular chains. The
highest occupied molecular orbital (HOMO) of monomer is mostly
localized on the alkyl chain attached to the fluorene, while the lowest
unoccupied molecular orbital (LUMO) is mainly localized on the cage
structure and its interior.
Fig. 2. Optimized structures (a) and HOMO (b) and LUMO (c) profiles of POSSs on the basis of DFT calculations at the B3LYP/6-31G(d) level.
4

M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
3.3. Thermal property
emission peak of orange-red light group DBT is located at 624 nm,
€
indicating that the effective Forster energy transfer occurs from the
The thermal properties of the copolymers were investigated using
TGA and DSC, and the key parameters were summarized in Fig. 3 and
Table 2. All copolymers exhibited good thermal stability with the onset
decomposition temperatures (Tds, measured at a 5% weight loss) from
321 to 384 ꢀC. It is ascribed that the incorporation of the POSSs group at
the side chain of fluorene maintains the high thermal stability of poly-
fluorenes [36]. However, the thermal stability of the copolymer wasn’t
further improved with the increase in the feed ratio of the POSSs, which
may be attributed to the poor degree of polymerization of the polymers.
polyfluorene fragment to the DBT group, inducing the emission of
orange-red light. PFSDF10-POSS1 and PFSDF10-POSS5 show no sig-
nificant emission peak at 620 nm, which is attributed to the low reacted
contents of DBT by one-pot Suzuki polycondensation. Moreover, the
emission peaks of copolymers exht a slight red shift in the PL spectra
compared to that in dilute solution, which may be attributed to the in-
hibition effect of SDF on aggregation of polymers was destroyed by the
large cage structure of POSSs.
At the same time, the molecule steric hindrance is increased because
the POSSs group as a substituent on the fluorene can also play a “dilu-
tion” role, avoiding the generation of excimer and ensuring the stability
of spectra in different states.
3.4. Photophysical properties
As shown in Table 2, the fluorescence quantum yields of PFSDF10-
POSSx copolymers in solution were 70%–80%, indicating that the elec-
tronic structure of polyfluorene system was not affected by the intro-
duction of the POSSs group. There was no energy loss due to the cross-
linking effect of SDF and POSSs structures, and the high quantum effi-
ciency of polyfluorene materials was maintained. In addition, the lifetime
properties of the copolymers in CDCl3 solution and the detailed de-
scriptions are displayed in Figure S9 in supplementary information.
The normalized UV–vis absorption and PL spectra of the copolymers
in dilute solution are shown in Fig. 4a. The absorption peaks of all co-
polymers are around 370 nm, which is mainly attributed to the π-π*
transition in polyfluorene skeleton. With the increase of the POSSs group,
the absorption peaks are slightly blue-shifted gradually from PFSDF10-
POSS1 at 375 nm to PFSDF10-POSS20 at 360 nm (Table 2). This may be
because the distortion of molecular chain and effective conjugate chain
length of the polyfluorene is reduced by the large cage structure of the
POSSs monomer, which causes the decreasing electron delocalization in
polyfluorene conjugated system and the increased energy required for
molecular transitions. It can be seen from the fluorescence emission
spectra that all copolymers exhibit typical emission peaks of polyfluorene
at 414 nm and 435 nm, respectively, which indicates the fact that the
electronic structure of polyfluorene is not affected by the incorporation of
POSSs group at the side chain of fluorene. The fine vibrational structures
at near 435 nm can also indicate that all copolymers are similar to the
rigid structure of polyfluorene. At the same time, the emission peaks do
not change significantly in dilute solution with the increase of POSSs
contents, which indicates the fact that the emission behavior of co-
polymers is not affected by the introduction of POSSs. The absorption and
emission bands of DBT are not observable in the spectra owing to its
comparatively low content in the copolymers.
3.5. Electrochemical properties
As shown in Fig. 5 and Table 3, the electrochemical properties of the
PFSDF10-POSSx were investigated by cyclic voltammetry (CV). The
oxidation potentials (Eox) varied slightly from 1.00 V to 1.30 V. The
HOMO levels of PFSDF10-POSSx were calculated according to the
empirical formulas EHOMO ¼ ꢂ½4:8 þðEcox ꢂEfoxÞꢃðeVÞ [37]. The LUMO
levels were deduced from the HOMO levels and the optical band gaps (Eg)
determined from the onset value of the absorption spectrum in film in the
long-wavelength direction Eg ¼ 1240=λedge. The HOMO and LUMO levels
of the copolymers are at about ꢂ5.70 eV and ꢂ2.50 eV, respectively,
which indicate that the hole and electron injection in device are much
easy through introducing the POSSs group.
In film state, the copolymers exhibit UV–vis absorption bands at
3.6. Film morphology
around 375 nm owning to the
π–π* transitions of the polyfluorene
backbones (Fig. 4b and Table 2). The difference from that in solution is
the maximum absorption band of the copolymer in film showed no
obvious blue shift with the increase of POSSs content. In PL spectra, the
emission peaks of polyfluorene are located at 421 nm and 449 nm and the
The morphology of the spin-coated films of copolymers is a critical
factor for the PLED fabrication, which was measured by atomic force
microscopy (AFM) at a tapping mode and showed in Fig. 6. All the films
show at flat and smooth surface without any pinhole defects. The root-
mean-square (RMS) roughness values of copolymers are 0.79, 0.86,
0.52, and 0.61 nm, respectively, corresponding to the variation of con-
tent for copolymers from 1 mol% to 20 mol%. The results imply that
amorphous films with good quality can be easily prepared by the syn-
ergistic influence of the introduced POSSs group and SDF branch center,
which is favorable for PLEDs fabrication.
3.7. Hydrophobic properties
In previous studies, we found that PEDOT: PSS could be dissolved and
then defects were formed at PEDOT: PSS/EML interface when the
hyperbranched polymers emitting layer was spin-coated on the water-
soluble PEDOT: PSS layer. The defects generated on PEDOT: PSS layer
are easily broken down with increasing current density, which affects the
device efficiency, brightness and stability of the hyperbranched PLEDs.
Therefore, it is urgent to improve the hydrophobicity of the hyper-
branched polymer materials, which were characterized by the water
contact angles test and the results were shown in Fig. 7. The water
contact angle of PFSDF10-POSS1 was 95ꢀ, while the maximum water
contact angle of PFSDF10-POSS20 was 114.5ꢀ. Obviously, the water
contact angle was linearly increased with the increasing components of
the POSSs groups, which demonstrated that the hydrophobicity of the
Fig. 3. TGA and DSC (insert) curves of the copolymers in nitrogen atmosphere
with a heating rate of 10 ꢀC/min and 5 ꢀC/min, respectively.
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M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
Table 2
Thermal and photophysical properties of the copolymers.
Copolymer
Td (ꢀC)
Tg (ꢀC)
Dilute solution
Solid film
λabs (nm)
PLQY
λabs (nm)
λPL (nm)
λPL (nm)
Solution (%)
Film (%)
POSS1
POSS5
POSS10
POSS20
384
321
361
380
105
104
144
145
375
370
366
360
414, 435
414, 435
414, 434
414, 434
375
375
370
375
421, 445
418, 443
421, 442, 625
421, 443, 624
75.77
76.95
71.37
70.20
7.42
5.11
5.80
13.53
Fig. 4. UV–vis absorption and PL spectra of copolymers: (a) in CHCl3 solution (10ꢂ5 M) and (b) in solid film.
3.8. Electroluminescent properties
In order to reveal the EL performance of synthesized copolymers, the
devices were fabricated with the configuration ITO/PEDOT: PSS (40
nm)/copolymer (50 nm)/TPBi (35 nm)/LiF (1 nm)/Al (150 nm). The
structure and relative-energy-level diagram of all devices were shown in
Fig. 8a and (b), respectively. ITO and Al (150 nm) are served as the anode
and cathode, respective, PEDOT: PSS (40 nm)layer is used as the hole
injection and transporting layer; PFSDF10-POSSx (50 nm) layer is used as
the light-emitting layer; TPBi (35 nm) and LiF (1 nm) are used as the
electron-transfer and electron-injection layers no other inorganic dop-
ants, respectively [38–48]. As shown in Fig. 7b, the HOMO and LUMO
levels of the copolymers are well matched to that of TPBi and PEDOT:
PSS, which effectively facilitate the injection of electron and hole carriers
into light-emitting layer in fabricated PLEDs.
The electroluminescence spectra of copolymer materials at voltages
varying from 5 to 10 V are displayed in Fig. 9. All copolymer-based de-
vices exhibit much broader EL spectra than their PL spectra, covering
most of the visible light region. This contributes to the almost sunlight-
style white light emission with the CIE coordinates located at (0.33,
0.33). The CRI of all devices is higher than 70 and the PFSDF10-POSS5
reaches the maximum CRI value of 96 at 8 V.
Fig. 5. The cyclic voltammogram curves of the copolymer films.
4. Conclusions
Table 3
Electrochemical properties of the copolymers.
In this work, a series of white polymer light-emitting materials with
Copolymers
λabs (onset) (nm)
Eg (eV)
HOMO (eV)
LUMO (eV)
9,9-dioctylfluorene,
fluorine-containing
polyhedral
oligomer-
icsilsesquioxanes (POSSs) modified 9,9-dioctylfluorene and 4,7-bis(2-
bromo-5-thienyl)-2,1,3-benzothiadiazole (DBrDBT) as the branches, the
three-dimensional structured spiro [3,3]heptane-2,6-dispirofluorene
(SDF, 10 mol %) as the core were obtained through Suzuki poly-
condensation. It was found that the interactions between chains was
effectively suppressed by the introduction of POSSs group in their PL
spectra, leading to no apparent bathochromic shifts in solid films with
respect to those in dilute solution. Besides, because of the large amount of
fluorine element introduced in the R group of POSSs, the hydrophobicity
of all materials has been dramatically improved, which enhanced the
waterproof and oxygen insulation abilities of the hyperbranched polymer
PFSDF10-POSS1
PFSDF10-POSS5
PFSDF10-POSS10
PFSDF10-POSS20
425
423
422
420
2.92
2.93
2.94
2.95
ꢂ5.70
ꢂ5.74
ꢂ5.43
ꢂ5.43
ꢂ2.79
ꢂ2.81
ꢂ2.50
ꢂ2.48
hyperbranched polymer was improved effectively. This provided the
basis of the enhancement of the waterproof and oxygen barrier ability of
the device in the next work by the incorporation of trifluoropropyl on the
peripheral groups of the POSSs group.
6

M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
Fig. 6. AFM images (5 ꢁ 5
μ
m) of the copolymer films: (a) PFSDF10-POSS1, (b) PFSDF10-POSS5, (c) PFSDF10-POSS10, (d) PFSDF10-POSS20.
Fig. 7. Water contact angles of copolymers films: (a) PFSDF10-POSS1, (b) PFSDF10-POSS5, (c) PFSDF10-POSS10, (d) PFSDF10-POSS20.
Fig. 8. Device structure (a) and energy-level (b) diagrams of OLEDs.
light-emitting devices. The optimized device with polymer light-emitting
layer achieve good EL performance with CIE coordinates at (0.32, 0.33)
and maximum CRI of 96. The results indicate that the
fluorene-containing POSSs modified hyperbranched white polymer
light-emitting materials could be the promising candidates in WPLEDs.
7

M. Wang et al.
Journal of Solid State Chemistry 298 (2021) 122122
Fig. 9. EL spectraof the copolymer devices and the corresponding CIE coordinates and CRI at voltages varying from 5 V to 10 V: (a) PFSDF10-POSS1, (b) PFSDF10-
POSS5, (c)PFSDF10-POSS10, (d) PFSDF10-POSS20.
CRediT authorship contribution statement
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