Two-Dimensional Conjugated Polymer Based on sp2-Carbon Bridged Indacenodithiophene for Efficient Polymer Solar Cells

Article abstract of DOI:10.1021/acs.macromol.7b01738

Molecular electronic structure plays a vital role in the photovoltaic performances in polymer solar cells (PSCs) due to their influences on light-harvesting, charge carrier transfer, π-π stacking, etc. Indacenodithiophene as a star unit has been well stud

Full text of DOI:10.1021/acs.macromol.7b01738

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pubs.acs.org/Macromolecules
Two-Dimensional Conjugated Polymer Based on sp2‑Carbon Bridged
Indacenodithiophene for Efficient Polymer Solar Cells
Yijing Guo,† Miao Li,† Yuanyuan Zhou,† Jinsheng Song,*,† Zhishan Bo,*,‡ and Hua Wang*,†
†Engineering Research Center for Nanomaterials, Henan University, Kaifeng 475004, China
‡Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing
100875, China
S
* Supporting Information
ABSTRACT: Molecular electronic structure plays a vital role in the
photovoltaic performances in polymer solar cells (PSCs) due to their
influences on light-harvesting, charge carrier transfer, π−π stacking, etc.
Indacenodithiophene as a star unit has been well studied in PSCs; various
structural derivation methods have been tried, but they are still not efficient in
improvement of power conversion efficiencies (PCE) due to the narrow optical
absorptions. In this contribution, a novel planar DMIDT with extended lateral
π-electron delocalization is efficiently synthesized via introduction of sp2 hybrid
carbons as the bridge atoms. Based on this novel building block, a two-
dimensional conjugated polymer PDMIDT-TPD is prepared, and the unique
structure improves the conjugation at the lateral direction, enlarges the electron
delocalization area, and greatly broadens the absorption spectrum with a full
coverage from 350 to 700 nm. Finally, a PCE of 8.26% is achieved when
blended with PC71BM, which is the highest result among the IDT-based
polymer donors. Meanwhile, PDMIDT-TPD also presents good compatibility with the non-fullerene acceptor, and a preliminary
PCE of 6.88% is obtained. In all, this work not only provides an excellent donor material but also offers a general and simple
derivation strategy for fused aromatic building blocks.
conjugated polymers for organic field effect transistors4 and
1. INTRODUCTION
PSCs applications.5−10 Various chemical modification methods
Polymer solar cells (PSCs) have attracted more and more
attention due to the relatively high efficiency, easy fabrication,
low cost roll-to-roll process, and light weight. The impressive
have been applied on IDT skeleton by altering the bridging
atoms,11,12 replacing the central benzene ring or end-capped
thiophene with larger fused aromatic rings, and/or changing the
substituted side chains to generate a variety of π-conjugation
extended ladder-type aromatic building blocks for more planar
power conversion efficiencies (PCEs) have been achieved over
12% for single junction PSCs by several groups.1−3 Up to now,
great efforts have been devoted to modulating the conjugated
conjugated polymers. The motivation is to prevent rotational
structures of polymer donors in the active layers, which have
disorder to reduce reorganization energy, facilitate π-electron
strongly advanced the development of this field in the past
decade. Generally, the open-circuit voltage (Voc), short-circuit
current density (Jsc), and fill factor (FF) are crucial parameters
to impact the final PCEs, which are closely related to the
chemical and electronic structures of the polymer donors. The
exploration of effective polymer donors with appropriate
bandgaps and frontier molecular orbital energy levels remains
a major challenge for PSCs.
delocalization along the polymer backbones, enhance the
charge carrier mobility, and reduce their bandgaps by means of
extending the conjugation length of the ladder-type donor unit.
Unfortunately, these derivation strategies can only enhance the
carrier mobility13 but cannot broaden the absorption spectrum.
The narrower absorption for IDT-based polymers will restrict
the enhancement of Jsc and thus photovoltaic performances,
resulting in PCEs around 6% for most IDT-derivative-based
Currently, construction of donor−acceptor (D−A) alternat-
ing copolymers is the most effective way for polymer donor
designing in PSCs due to its tunability at the bandgaps, energy
PSCs.12,14,15
In the meanwhile, two-dimensional (2D) conjugated
polymers possess extended conjugation at two directions and
levels, absorption spectra, etc. In order to achieve better PCEs
may provide broadened absorption spectra, better interchain
and higher mobility in solid films, planar polymer chains are
π−π overlapping, and higher charge mobility compared with
required for close packing, so the rigid and coplanar ladder-type
aromatic molecules have been utilized as D or A sections in the
backbones to enhance the coplanarity of the final polymer main
chains. Indacenodithiophene (IDT), a popular electron-
donating building block, has been widely used in p-type
Received: August 12, 2017
Revised: October 1, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.7b01738
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Scheme 1. Synthesis of DMIDT Monomer (Compound 7) and Polymers (PDMIDT-TPD and PIDT-TPD)
their one-dimensional conjugated counterparts. Benzodithio-
phene (BDT)-based conjugated polymer donors as one typical
representative of the 2D-conjugated polymers have been well
studied and made great progress in PSCs by Li16−19 and
Hou20,21 et al. Via replacing the conjugated side chains, a
variety of BDT-based 2D-conjugated polymers have been
prepared;18,22 very recently, a trialkylsilyl-substituted 2D
polymer J71 has been reported, and a PCE of 11.41% with a
Jsc of 17.32 mA cm−2 is achieved,16 demonstrating that 2D
strategy is effective at broadening the absorption spectra. In
addition, we have previously reported that planar conjugated
polymers could be obtained via the replacement of sp3-
hybridized carbon at 9-position of fluorene by sp2-hybridized
carbon, which also favors the improvement of the Jsc.23,24
Considering the fascinating properties of 2D conjugated
polymers and the weak light-harvesting abilities for IDT-based
polymers, in this article, we designed and developed a novel
methylene-modified IDT building block (DMIDT) as shown in
Scheme 1, in which two ethylene bridges were grafted onto the
conjugated IDT main core and the sp2-hybridized bridged
carbon atoms were successfully introduced to the novel
building block. Very recently, a similar idea was reported by
Li, and a PCE of 7.3% was obtained.25 In addition, four flanking
phenyl rings at lateral direction are effectively conjugated with
the IDT segment in our DMIDT unit, which may enlarge the
π-electron delocalization region and broaden the absorption
spectrum. Such structural modification could greatly reduce the
steric hindrance of the phenyl rings and generate more planar
structures, resulting in improved charge transport mobility.
Finally, this novel building block was first applied for the D−A
polymer; since supramolecular interactions have been widely
utilized in conjugated backbones as soft conformational “locks”
(e.g., S···O, F···H, F···S, etc.)26,27 to generate planar π-
conjugated structures for higher performance PSCs, thieno-
[3,4-c]pyrrole-4,6-dione (TPD) was selected as the electron-
accepting comonomer in this work in virtue of the S···O
supramolecular locks in between the ending thiophenes of
DMIDT and carbonyl groups from the TPD unit. Meanwhile, a
polymer based on IDT and TPD was prepared as the control.
Benefiting from the two perpendicular ethylene side bridge,
which are in the same plane with the conjugated backbone,
could greatly reduce the steric hindrance for π−π stacking (as
shown in Figure 1) and provide better lateral conjugation,
polymer PDMIDT-TPD presents a broader and red-shifted
absorption spectrum compared with the control polymer
(PIDT-TPD). The syntheses, thermal stability, photophysical
properties, electrochemical properties, and DFT calculations
along with the photovoltaic device performances of the two
polymers are described. Finally, a PCE of 8.26% is achieved by
PDMIDT-TPD when blended with PC71BM, which is the
champion PCE result among the IDT-based polymer donors to
the best of our knowledge. In the meantime, this polymer also
presents good compatibility with non-fullerene acceptor, and a
preliminary PCE of 6.88% is obtained when blended with ITIC,
indicating a universal utilization of this polymer.
2. RESULTS AND DISCUSSION
Synthesis. The synthetic routes of all the monomers are
shown in Scheme 1. Starting from the commercially available
compound 1, two flanking thiophene rings could be attached
via Suzuki cross-coupling in a yield of 52%. After the hydrolysis
and Friedel−Crafts reactions, the diketone precursors (5) could
be obtained as a dark-blue solid in a yield of 83%. Then, the
crucial tetrabromide intermediate (6) carrying two perpendic-
B
DOI: 10.1021/acs.macromol.7b01738
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Figure 1. Molecular structures (a) IDT and (b) DMIDT; front and side view of crystal structures for IDT (c, e) and DMIDT (d, f); carbon and
sulfur atoms are depicted with thermal ellipsoids set at 50%; crystal packing structures of IDT (g) and DMIDT (h).
ular ethylene bridges could be generated as red solids in a yield
of 80% via the Corey−Fuchs reaction in the presence of CBr4
and triethoxyphosphine. Subsequently, Suzuki cross-coupling of
6 and 4-hexylphenylboronic ester afforded the key intermediate
DMIDT in a yield of 55%. Finally, via the deprotonation and
quenching with trimethylstannanylium chloride, the monomer
(7) could be prepared as red solids in a yield of 60%.
Compound 8 is prepared according to the literature,15 and the
final polymers are prepared via the microwave-assisted Stille
polymerization in o-xylene/DMF (5:1), aiming at higher
molecular weights.
Molecular Weights and Thermal Properties. With
enough alkyl chains attached on the conjugated backbones,
both of the two polymers displayed good solubility in
chloroform (CF), chlorobenzene (CB), and 1,2-dichloroben-
zene (DCB) at room temperature. The GPC results are
summarized in Table S5, and the number-averaged molecular
weights for PIDT-TPD and PDMIDT-TPD are 67.2 and 13.9
kg/mol, respectively. Both of the two polymers are thermally
stable below 400 °C as shown in Figure S12, and the 5% weight
loss temperatures for PIDT-TPD and PDMIDT-TPD are 404
and 437 °C, respectively.
Figure 1c, the two flanking hexylphenyl side groups are
perpendicular to the IDT core along the z-direction (as shown
in Figure 1c) due to the sp3-hybridized character of the bridged
carbons, which will hinder the molecular packing of the IDT
core. However, when the sp3-hybridized carbons are replaced
by sp2-hybridized carbons in Figure 1d and the ethylene bridges
are successfully grafted onto the IDT core along the y-direction,
the steric hindrance of the two phenyl groups is greatly
reduced. From the side view as shown in Figure 1f, DMIDT
with a more planar structure is achieved, which may facilitate
the molecular packing in solid state and enhance the charge
carrier mobility. Meanwhile, their crystal packing structures are
shown in Figure 1. Interestingly, the crystal for IDT grows
along two directions, but there is only one packing style for
DMIDT, which may lead to varied charge transport properties.
Table 1. Optical and Electrochemical Properties of the
Polymers
solution
λonset
film
λonset
λmax
λmax (nm) (nm) (nm) (eV)
Egopt HOMO LUMO
polymer
(eV)
(eV)
X-ray Crystal Structures. Fortunately, the molecular
structures of IDT and DMIDT were obtained and confirmed
by single-crystal X-ray analysis as shown in Figure 1, and both
of the two molecules belong to space group triclinic P-1(2). In
PIDT-TPD
584
597
620
688
584
640
620
700
2.00
1.77
−5.59
−5.46
−3.59
−3.69
PMDIDT-
TPD
C
DOI: 10.1021/acs.macromol.7b01738
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Figure 2. Normalized optical absorption spectra of (a) PIDT-TPD and (b) PDMIDT-TPD: measured spectra in dilute CB solutions (red solid
lines), spectra on quartz substrates (black solid lines), and spectra computed within the TD-DFT/PCM approaches based on CAM-B3LYP (dash
dot lines). (c) Energy level diagrams of polymer donor, PC71BM and ITIC. (d) Cyclic voltammograms of polymer films in 0.1 M Bu4NPF6−CH3CN
solutions at a scanning rate of 100 mV/s.
Photophysical and Electrochemical Properties. The
UV−vis absorption spectra of PIDT-TPD and PDMIDT-TPD
in chlorobenzene solution and/or on quartz substrates were
measured, and very different absorption behaviors were
observed as shown in Figure 2. The solution of polymer
PIDT-TPD presented one narrow absorption band bearing two
clear peaks at 538 and 586 nm, which is ascribed as the
intramolecular charge transfer (ICT) peak, and no bath-
ochromic shift was observed even in its thin film spectrum.
Surprisingly, PDMIDT-TPD in both solution and as film
exhibited a much broad absorption ranging from 350 to 700 nm
with two bands, which is expected to enhance the Jsc of devices.
A shorter wavelength band located around 350−490 nm (band
II) may be attributed to π → π* transitions in the polymer
chains, and the longer wavelength band located around 490−
700 nm (band I) can be ascribed to the strong intramolecular
charge transfer (ICT) interactions. In thin film, a slightly red-
shifted ICT band and a sharp shoulder peak at 638 nm are
observed; the bathochromic absorption might be attributed to
the strong intermolecular interactions (interchain packing or
aggregation) induced by the extended π-conjugated structure of
DMIDT segments in the polymer backbones. In the mean-
while, the temperature-dependent absorption spectra (TD-
Abs)28 in dilute solutions were carried for both polymers, and
the results are shown in Figure S13. It is worth noting that
these two polymers have similar main chains, but different
optical bandgaps are obtained. PIDT-TPD belongs to the
category of wide bandgap conjugated polymer with an Eg of 2.0
eV (estimated from the absorption edge in the thin films),
whereas PMDIDT-TPD is classified as medium bandgap
polymer with an Eg of 1.77 eV; such broad and red-shifted
absorption character is expected for efficient PSCs with
elevated Jsc.
contributions to the UV−vis spectra. The predicted spectra are
consistent with the experimental curves as shown in Figure 2
and the selected transition contributions are listed in Table S1.
From the calculated results, it is clear that the main absorption
band of PIDT-TPD is produced by excitations of S0 → S1 and
S0 → S3, which are mainly associated with the HOMO →
LUMO and H−1 → L+1 transitions, whereas the energy
excitations and electron distributions for PDMIDT-TPD
became slightly complicated. The band I at the long wavelength
resulted from the low-lying energy excitation of S0 → S1
associated mainly with the HOMO → LUMO transition. As
for the band II at the short wavelength, it is contributed by the
combined higher energy excitations of S0 → S6, S0 → S7, and S0
→ S9 related to mixed transitions as shown in Table S1.
Meanwhile, the frontier orbitals for PDMIDT-TPD depicted in
Figure S10 demonstrated that the electron migration between
the side chains and backbones is responsible for the varied
absorption activity.
With the structural differences, these two polymers displayed
varied electrochemical properties, which were investigated by
cyclic voltammetry with a standard three-electrode electro-
chemical cell in acetonitrile solution containing 0.1 M Bu4NPF6
at room temperature under a nitrogen atmosphere with a
scanning rate of 100 mV/s. Ag/AgNO3 was used as the
reference electrode and a standard ferrocene/ferrocenium
redox system was used as the internal standard. The CV curves
of PIDT-TPD and PDMIDT-TPD are shown in Figure 2d, and
their HOMO energy levels, which were calculated from the
onset oxidation potential (Eox) of these polymers according to
the equations EHOMO = −e[(Eox − E(Fc/Fc+) + 4.8] (eV), are
−5.59 and −5.46 eV, respectively, indicating that both
polymers should be good candidates as donor materials for
high Voc. The LUMO energy levels of PIDT-TPD and
PMDIDT-TPD were calculated to be −3.59 and −3.69 eV,
which are higher than that of PC71BM, guaranteeing the
photoinduced electron efficiently transfers from the donor to
Meanwhile, quantum chemistry calculations were performed
with Gaussian 09 suite by the TD-DFT/PCM approach at the
6-31G(d,p) level to provide further insight into the transition
D
DOI: 10.1021/acs.macromol.7b01738
Macromolecules XXXX, XXX, XXX−XXX