Pyran-Bridged Indacenodithiophene as a Building Block for Constructing Efficient A–D–A-Type Nonfullerene Acceptors for Polymer Solar Cells

Article abstract of DOI:10.1002/cssc.201701917

In recent years, nonfullerene acceptors have attracted much attention, owing to their great potential for use in high-performance polymer solar cells.The ladder-type building block, pyran-bridged indacenodithiophene (PDT), was used for constructing A–D–A nonfullerene acceptors through introduction of oxygen atoms into an indacenodithiophene (IDT) unit. The synthesis of PDT is accomplished by a BBr₃-mediated tandem cyclization–deprotection reaction to construct the pyran ring. Hence, molecular acceptor PTIC was synthesized and used in a polymer solar cell device. Compared to the IDT-based acceptor, PTIC exhibits higher HOMO levels and wider optical band gap at 550–800 nm. Devices fabricated with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione)] (PBDB-T):PTIC as the active layer give a power conversion efficiency (PCE) of 7.66 %.

Full text of DOI:10.1002/cssc.201701917

Accepted Article
Title: Pyran-bridged indacenodithiophene: A novel building block
for constructing efficient A-D-A type nonfullerene acceptor for
polymer solar cells
Authors: Renqiang Yang, Shuguang Wen, Yao Wu, Yingying Wang, Yi
Li, Ling Liu, Huanxiang Jiang, and Zhitian Liu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201701917
Link to VoR: http://dx.doi.org/10.1002/cssc.201701917
A Journal of
www.chemsuschem.org

10.1002/cssc.201701917
ChemSusChem
PER
ridged indacenodithiophene: A novel building block for
cting efficient A-D-A type nonfullerene acceptor for
r solar cells
Wen,[a] Yao Wu,[b] Yingying Wang,[a] Yi Li,[a] Ling Liu,[a] Huanxiang Jiang,[a] Zhitian Liu*[b], and
ang*[a]
Wu contributed equally to this work.
cent years, nonfullerene acceptors have attracted
due to their great potential for achieving high
ymer solar cells. In this paper, a novel ladder type
pyran-bridged indacenodithiophene (PDT) has been
nstructing A-D-A nonfullerene acceptors through
progress of these acceptors for their highly tunable physical and
photoelectric properties by changing the donor unit, end-capping
acceptor unit, side chains or π-bridge units, which inspires the
development of their donor counterparts at the same time.[10]
Importantly, it’s easy for these nonfullerene acceptors to
modulate the bandgaps, which is in favor of expanding optical
absorption and enables application in semitransparent organic
solar cells. [11]
The donor unit is core structure of A-D-A type nonfullerene
acceptors, which plays a vital role in adjusting highest occupied
molecular orbital (HOMO) levels, charge mobility and
aggregations behaviors. There are many good results obtained
by modifying the donor units including IDT,[12-16] IDTT,[9,17]
IBDT,[18] DTCC,[19] BT,[20-22] et al. For example, Zhan’s group
xygen
atoms
into
the
commonly
used
ene (IDT) unit. The key step for the synthesis of
omplished by a BBr3-mediated tandem cyclization-
ction to construct the pyran-ring. In this approach, a
acceptor PTIC was successfully synthesized and
mer solar cell device. Compared to the IDT-based
exhibits higher HOMO levels and wider optical
800 nm. Polymer solar cells devices fabricated with
s active layer show a power conversion efficiency
. The introduction of oxygen atoms would be an
ch for the modification of acceptor materials for
ls.
reported IBDT as the donor unit by π-extending the IDTT core
[18]
with thiophene units.
IBDT has larger rigid and coplanar
structure and stronger electron-donating ability, which is in favor
of enhancing the absorption and charge transport. The end-
groups have significant effect on lowest unoccupied molecular
orbital (LUMO) levels and molecular packing behaviors. These
nonfullerene acceptors were usually end-capped with IC,[19,23,24]
fluorinated IC,[9,25] methylated IC,[26] thienyl-fused indanone,[17,27]
rhodamine[14,28] et al. Hou et al. reported the acceptor material of
ITCC with thienyl-fused indanone as end groups.[17] This
molecular showed higher LUMO level, closer π-π stacking
distance and higher electron mobility than that of ITIC. The side
chains are essential components of nonfullerene acceptors to
guarantee solubility, modulate aggregation behaviors and adjust
energy levels, which includes hexylphenyl,[29,30] hexylthienyl,[21]
hexylselenophenyl,[31] and alkyl side chains[32-34]. And π bridges
were usually incorporated to elongate π-conjugation length,
such as benzothiazole,[28] thiophene,[35,36] diketopyrrolopyrrole,[14]
thiazole,[37] et al. Since the nonfullerene acceptors have
excellent flexibility in structure modification and great potential to
achieve high efficiency, much more efforts can be done to
optimize the chemical structure.
n
ells (PSCs) have attracted extensive attention in
es due to the advantages of light weight, low cost
-6]
Fullerene derivatives are usually used as
ors in the blend film of bulk heterojunction (BHJ)
se of their good electron affinity and mobility.
intrinsic drawbacks limit their photovoltaic
uch as weak optical absorption, difficult structure
d high production cost.[7] As a replacement of
tives, Zhan’s group has developed a series of A-
molecules as nonfullerene acceptor with
hene (IDT) as the core structure.[8] These
ed great potential to outperform fullerene and its
power conversion efficiencies (PCEs) of organic
ces based on these nonfullerene acceptors have
[9]
Recent years’ research has witnessed rapid
In order to improve the performance of PSC, it’s an important
strategy to expand the optical absorption range, which can be
realized by lowering LUMO level or enhancing HOMO level.
However, low LUMO level of acceptor would result in reduced
open circuit voltage (VOC) of device. So appropriately enhancing
the HOMO level is a reasonable approach to expand optical
absorption range. Herein, we have designed and synthesized a
novel nonfullerene acceptor of PTIC, which has a pyran-bridged
indacenodithiophene (PDT) as donor unit and is end-capped by
1, 1-dicyanomethylene-3-indanone groups. The incorporation of
oxygen atoms is in favor of increasing HOMO level and reducing
the band gap. As a result, this material shows a narrow band
gap of 1.55 eV and broad absorption throughout visible and near
[a]
[b]
Dr. S. Wen, Y. Wang, Y. Li, L. Liu, H. Jiang, Prof. R. Yang
CAS Key Laboratory of Bio-based Materials
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese
Academy of Sciences, Qingdao 266101, China
E-mail: yangrq@qibebt.ac.cn
Y. Wu, Prof. Z. Liu
School of Materials Science & Engineering, Wuhan Institute of
Technology, Wuhan 430205, China
E-mail: able.ztliu@wit.edu.cn
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201701917
ChemSusChem
FULL PAPER
frared region (500-800 nm). Crystal structure of PDT indicates
at an ordered molecular packing is formed, which is in favor of
harge transport in the film. PSC devices were fabricated
mploying PTIC as acceptor and PBDB-T as donor material.
ower conversion efficiency of 7.66% was achieved with VOC of
84 V, JSC of 14.2 mA cm-2 and FF of 0.64. This research
dicates that the introduction of oxygen group to IDT unit is an
mportant strategy for the modification of acceptor materials.
chemical structure can be definitely determined by X-ray
structure analysis (Figure 1). Importantly, it is shown that the
central backbone is nearly planar and parallel to each other.
Four lateral 4-hexylphenyl groups are not symmetrically
distributed to the two sides of backbone, but two of them are
perpendicular to the backbone and two are nearly in the
backbone plane. So the molecules form a very ordered close
slip-stacked packing in crystals, which is reported to prevent the
excimer formation and reduce geminate recombination.[38]
According to this result, the molecule of PTIC is deduced to form
a more ordered packing structure than the acceptors derived
from IDT.
Results and Discussion
cheme 1. Synthetic route of small molecular acceptor PTIC.
Figure 2. (a) TGA curve of PTIC; (b) DSSC thermogram of PTIC upon first and
second heating and cooling cycles.
aterial synthesis
Thermal properties
he synthesis of PTIC is started from Suzuki coupling reaction
f bromide 1 and borated 2 to produce compound 3. Then the
de chains of 4-hexylphenyl group were introduced to give
The thermal properties of PTIC were investigated using
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). As depicted in Figure 2, the decomposition
temperatures (Td) with 5% weight loss were observed at 334 °C.
PTIC exhibits good thermal stability which meets the
temperature requirement for practical photovoltaic application.
The DSC scan of neat PTIC from 50 to 270 °C with 10 °C/min
was exhibited in Figure 2b. A sharp melting endothermic
transition around 220 °C can be observed, which indicates that
PTIC is highly crystalline due to its structural planarity as
deduced from above X-ray structure analysis. Upon the cooling
cycle after melting, no recrystallization peak is observed,
suggesting the material appears to become kinetically trapped in
the amorphous phase.[39] On the second cycle of heating, no
peak can be found, signifying a feature of amorphous material.
In the first heating and cooling scan, the crystallinity of PTIC is
broken, so no melting transition is observed in the second
heating scan.
ompound 4. With
uccessfully obtained
4
in hand, compound
5
(PDT) was
tandem
by BBr3 initiated
yclization/deprotection reaction in high yield. After two aldehyde
roups were introduced, the terminal group of 2-(3-oxo-2, 3-
hydroinden-1-ylidene)-malononitrile were reacted with the
dehyde 6 to afforded the final product PTIC. The detailed
ynthetic procedures can be found in Supporting Information.
Optical and electronic properties
As shown in Figure 3, PTIC showed absorption ranging from
550 nm to 800 nm. Li et al. have reported the acceptor of IDT-IC
based on IDT core structure that showed absorption spectrum
range from ca. 520 nm to 720 nm.[24] Compared to IDT-IC, PTIC
had broader and redshifted absorption. Besides, the molar
extinction coefficient of PTIC was as high as 8.8×104 M-1 cm-1.
The broad and near-infrared absorption with high extinction
coefficient is beneficial for photocurrent generation in active
layer. Compared to the absorption of PTIC in chloroform solution,
the maximum absorption peak in solid state red-shifted about 67
nm, which means strong aggregation in PTIC solid film.
Interestingly, the onset of the absorption in blend film blue-
shifted 35 nm compared to the absorption in pure PTIC film,
gure 1. (a) Chemical structure of PDT core; (b) geometric structure of PDT;
) stacking of PDT in one crystal cell.
ngle crystal is an important approach for the investigation of
olecular packing in the blend film of PSCs. However, there’s
o report about the crystal structure of acceptors such as ITIC,
ecause the core structure of IDTT is segregated each other by
e lateral four 4-hexylphenyl groups. We have fortunately
btained the single crystal of key intermediate PDT, so the
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10.1002/cssc.201701917
ChemSusChem
FULL PAPER
hich suggests that the packing of PTIC in the blend film might
e disrupted by donor material.
DFT calculation
The molecular geometry and the frontier molecular orbitals of
PTIC were simulated by density functional theory calculations
with Gaussian 09 software at the B3LYP/6-31G(d,p) theory level.
Long hexyl chains were replaced by methyl groups to facilitate
the calculation. As shown in Figure 4, the backbone was nearly
planar and the phenyl rings in side chains were almost
perpendicular to the backbone. The coplanarity of backbone
facilitates π-π stacking in solid state, while the steric hindrance
between the side chains and the main backbone prevents
severe aggregation. Compared to IDT core which is perfectly
planar,[13,24] PDT unit showed a dihedral angle of 12.6o between
the phenyl ring and thiophene ring as a result of introducing
oxygen atoms. Besides, the thiophene ring in the PDT core and
the IC group have a dihedral angle of 12.6o. The HOMO orbital
delocalizes in the PDT core, while the LUMO orbital delocalizes
in the whole backbone. The LUMO and HOMO levels were
calculated to be -3.42 eV and -5.58 eV, giving an energy gap of
2.16 eV.
gure 3. (a) Chemical structures of PTIC and PBDB-T; (b) absorption spectra
PTIC in CHCl3 solution, pristine film, and blend film of PTIC:PBDB-T (1:1);
) cyclic voltammetry curve of PTIC.
able 1. Optical and electrochemical properties of PTIC.
opt
ε
-1
λ
λ
λ
onset
HOMO
[eV]
LUMO
[eV]
E
max
sol
film
olecule
PTIC
-1
g
[nm]
[nm]
[nm]
[M cm ]
[eV]
4
676,
734
667
800
1.55
-5.67
-3.85
8.8×10
he energy levels of PTIC is determined by cyclic voltammetry
CV) with films on the gold electrode in an acetonitrile solution of
1 mol L-1 Bu4NPF6 using ferrocene (-4.8 eV) as standard
eference (Table 1). The redox potential of the Fc/Fc+ internal
eference is 0.43 V vs. SCE. The LUMO level is determined by
= -e(ERed + 4.8 - E1/2,(Fc/Fc+)) and the HOMO level is
LUMO
etermined by EHOMO = -e(Eox + 4.8 - E1/2,(Fc/Fc+)). The onset
eduction potential of PTIC is -0.52 V. Hence, the LUMO level is
3.85 eV. Accordingly, the HOMO level was determined to be -
67 eV. The ∆HOMO and ∆LUMO level of PBDB-T:PTIC are
rger than 0.3 eV,[40] which can guarantee the exciton
ssociation between donor and acceptor materials.
Figure 5. AFM height images (a, b) and TEM images (c, d) of PBDB-
T:PTIC (1:1) blend film without (a, c) and with (b, d) thermal annealing at
160 oC.
Morphology
The morphology of active blend was illustrated by AFM and TEM
images. As the shown in AFM images (Figure 5), the blend film
with and without thermal annealing showed similar and smooth
morphology with RMS values of 2.3 and 1.7 nm respectively,
uniformly covering the entire scan area. There was no obvious
phase separation, suggesting good miscibility between PTIC
and PBDB-T. In contrast, the TEM image of the blend film with
thermal annealing revealed the crystalline grains which were not
observed in blend film without thermal annealing. In TEM
images, the bright regions are polymer-rich domains and the
dark regions are acceptor-rich domains. Apparently, the polymer
donor exhibited enhanced crystallinity in blend film processed
with thermal annealing. The crystalline domains facilitated
charge transport and were beneficial for obtaining high charge
mobilities. Besides, the pure domains reduced geminate
recombination. In addition, the crystalline domains were still in
gure 4. (a, b) The optimized geometry of PTIC; (c, d) Molecular orbitals of
TIC simulated with Gaussian B3LYP/6-31G(d, p).
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10.1002/cssc.201701917
ChemSusChem
FULL PAPER
anometer scale around 20-30 nm, which would not hamper
harge separation.
The external quantum efficiency (EQE) curves of the best
devices based on PTIC are shown in Figure 6c. The devices
exhibit a broad photo-response from 300 to 800 nm with a
highest response of 65%, indicating the donor and acceptor
made complementary contribution to photocurrent generation.
To investigate charge recombination behaviors in the active
layer of PSC devices, JSC under different light-intensity (Plight
)
was measured (Figure 6d). The dependence of JSC on light
intensity can be described by the formula, JSC ∝ Plightα. Power-
law exponent α is consistent with the extent of bimolecular
recombination. The value of α in annealed PBDB-T:PTIC blend
was 0.97, which indicates that there is weak bimolecular
recombination in the blend film.
To investigate the charge transport properties of PTIC, electron
and hole mobility were measured by space-charge-limited
current (SCLC) method with hole-only device configuration of
ITO/PEDPOT:PSS/PBDB-T:PTIC/Au and electron-only device
configuration of ITO/ZnO/PBDB-T:PTIC/Ca/Al (Figure S3). The
electron and hole mobility of the blend without annealing were
determined to be 1.03×10-5 and 1.55×10-5 cm2 V-1 s-1,
respectively. By thermal annealing, the electron and hole
mobility of the blend significantly increased to 4.88×10-5 and
9.29×10-5 cm2 V-1 s-1, which was 4.7 times and 6 times of that
without thermal annealing, respectively. Higher improvement in
hole mobility than that of electron mobility was in accordance
with higher crystallinity of polymer donors in blend observed by
TEM. The thermal annealing is in favor of improving crystallinity
and molecular packing in the blend film, which is beneficial for
improving charge mobility and reducing charge recombination.
As a result, the JSC and FF are both increased with thermal
annealing.
gure 6. (a) J-V curves for the device based on PBDB-T:PTIC; (b)
hotoluminescence (PL) spectra of PTIC and blend film of PBDB-T:PTIC (1:1)
xcited at 650 nm; (c) EQE spectra for PSCs based on PBDB-T:PTIC active
yer processed by thermal annealing (TA); (d) dependence of JSC on light
tensity.
hotovoltaic properties
he photovoltaic properties of PTIC were investigated by
bricating conventional solar cell devices with architecture of
TO/PEDOT:PSS/PBDB-T:PTIC/Ca/Al. The detailed process of
evices optimization was shown in Table S3 (Supporting
formation). The highest PCE was achieved with
a
onor/acceptor ratio of 1:1 from CB solution under thermal
o
nnealing at 160 C, as shown in Figure 6a and Table 2. The
Conclusions
olar cells based on the optimized PBDB-T:PTIC active layer
xhibited a maximum PCE of 7.66% with VOC of 0.84 V, JSC of
4.2 mA cm-2, and FF of 0.64. Compared to the devices without
nnealing which exhibited a maximum PCE of 5.89% with VOC of
88 V, JSC of 12.1 mA cm-2, and FF of 0.55, the annealed one
howed much higher JSC and FF, indicating lower charge
ecombination. The photoluminescence quenching spectra of
eat PTIC and blend film were examined to confirm the exciton
ssociation in the blends, as shown in Figure 6b. By blending
BDB-T into PTIC, an effective PL quenching (~92%) could be
bserved, indicating efficient exciton dissociation at the donor-
cceptor interface.
We designed and synthesized a novel ladder type backbone
(PDT) for constructing an A-D-A type nonfullerene acceptor for
PSCs. PDT has a nearly planar structure and it exhibits slip
stacking in crystals. Compared with the commonly used IDT
core, PDT exhibited stronger electron-donating properties. With
PDT as the core structure,
a new acceptor PTIC was
synthesized and it shows broadened and red-shifted absorption
compared to the corresponding IDT-based molecules. PSCs
devices based on PBDB-T: PTIC exhibited a maximum PCE of
7.66%, with VOC of 0.84 V, JSC of 14.2 mA cm-2, and FF of 0.64.
The introduction of oxygen atoms would be an important
strategy for the design of acceptors with wide absorption.
able 2. The photovoltaic parameters of the devices based on PBDB-T:PTIC (1:1) under illumination of AM 1.5G, 100 mW cm-2
Donor:acceptor
JSC
[mA cm-2]
FF
PCE[b]
[%]
μe
μh
TA[a]
VOC [V]
[%]
[cm2 V-1 s-1]
[cm2 V-1 s-1]
No
0.88
0.84
12.1
14.2
55
64
5.89 (5.68)
7.66 (7.48)
1.033×10-5
4.888×10-5
1.55×10-5
9.29×10-5
PBDB-T:PTIC
Yes
Thermal annealed at 160 oC for 10 min; [b) Average PCE values obtained from 10 devices are shown in parentheses.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201701917
ChemSusChem
PER
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a
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A
solution
of poly(3,4-
phene):poly(styrenesulfonate) (PEDOT:PSS, 30 nm)
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min in air, then the device was transferred into glovebox,
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re measured with a Keithley 2420 source measurement
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10.1002/cssc.201701917
ChemSusChem
PER
he Table of Contents (Please choose one layout)
PER
molecule pyran-bridged
Shuguang Wen, Yao Wu, Yingying
hene
(PDT)
is
Wang, Yi Li, Ling Liu, Huanxiang Jiang,
Zhitian Liu*, Renqiang Yang*
onstruct non-fullerene
) in polymer solar cell.
ucture of PDT exhibits
ecular packing in the
troduction of oxygen
HOMO level and
bsorption. PSC device
Page No. – Page No.
Pyran-bridged indacenodithiophene:
A novel building block for
constructing efficient A-D-A type
nonfullerene acceptor for polymer
solar cells
DB-T:PTIC shows
on efficiency of 7.66%.
a
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