Donor–acceptor photovoltaic polymers based on 1,4-dithienyl-2,5-dialkoxybenzene with intramolecular noncovalent interactions

Article abstract of DOI:10.1002/pola.28959

Donor–acceptor (D–A) conjugated polymers bearing non-covalent configurationally locked backbones have a high potential to be good photovoltaic materials. Since 1,4-dithienyl-2,5-dialkoxybenzene (TBT) is a typical moiety possessing intramolecular S…O interactions and thus a restricted planar configuration, it was used in this work as an electron-donating unit to combine with the following electron-accepting units: 3-fluorothieno[3,4-b]thiophene (TFT), thieno-[3,4-c]pyrrole-4,6-dione (TPD), and diketopyrrolopyrrole (DPP) for the construction of such D–A conjugated polymers. Therefore, the so-designed three polymers, PTBTTFT, PTBTTPD, and PTBTDPP, were synthesized and investigated on their basic optoelectronic properties in detail. Moreover, using [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as acceptor material, polymer solar cells (PSCs) were fabricated for studying photovoltaic performances of these polymers. It was found that the optimized PTBTTPD cell gave the best performance with a power conversion efficiency (PCE) of 4.49%, while that of PTBTTFT displayed the poorest one (PCE = 1.96%). The good photovoltaic behaviors of PTBTTPD come from its lowest-lying energy level of the highest occupied molecular orbital (HOMO) among the three polymers, and good hole mobility and favorable morphology for its PC₇₁BM-blended film. Although PTBTDPP displayed the widest absorption spectrum, the largest hole mobility, and regular chain packing structure when blended with PC₇₁BM, its unmatched HOMO energy level and disfavored blend film morphology finally limited its solar cell performance to a moderate level (PCE: 3.91%).

Full text of DOI:10.1002/pola.28959

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Donor–Acceptor Photovoltaic Polymers Based
on 1,4-Dithienyl-2,5-Dialkoxybenzene with Intramolecular
Noncovalent Interactions
Xue-Qiang Chen,1 Xiang Yao,1 Tianwen Bai,2 Jun Ling,2 Wen-Jing Xiao,1 Jiandong Wang,1
Si-Cheng Wu,1 Li-Na Liu,1 Guanghui Xie,3 Jingjing Li,3 Zhengquan Lu,3 Iris Visoly-Fisher,4
1,3
Eugene A. Katz,4 Wei-Shi Li
1Key Laboratory of Synthetic and Self-assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular
Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
2Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
3Engineering Research Center of Zhengzhou for High Performance Organic Functional Materials, Zhengzhou Institute of
Technology, 6 Yingcai Street, Huiji District, Zhengzhou 450044, China
4Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, The
Jacob Blaustein Institutes for Desert Research (BIDR), Ben-Gurion University of the Negev, Sede Boqer Campus, Midreshet
Ben-Gurion 8499000, Israel
Correspondence to: W.-S. Li (E-mail: liws@mail.sioc.ac.cn)
Received 10 November 2017; accepted 8 January 2018; published online 00 Month 2018
DOI: 10.1002/pola.28959
ABSTRACT: Donor–acceptor (D–A) conjugated polymers bearing
non-covalent configurationally locked backbones have a high
potential to be good photovoltaic materials. Since 1,4-dithienyl-
performance with a power conversion efficiency (PCE) of 4.49%,
while that of PTBTTFT displayed the poorest one (PCE 5 1.96%).
The good photovoltaic behaviors of PTBTTPD come from its
lowest-lying energy level of the highest occupied molecular orbital
(HOMO) among the three polymers, and good hole mobility and
favorable morphology for its PC71BM-blended film. Although
PTBTDPP displayed the widest absorption spectrum, the largest
hole mobility, and regular chain packing structure when blended
with PC71BM, its unmatched HOMO energy level and disfavored
2,5-dialkoxybenzene (TBT) is
a typical moiety possessing
intramolecular S. . .O interactions and thus a restricted planar
configuration, it was used in this work as an electron-donating unit
to combine with the following electron-accepting units: 3-
fluorothieno[3,4-b]thiophene (TFT), thieno-[3,4-c]pyrrole-4,6-dione
(TPD), and diketopyrrolopyrrole (DPP) for the construction of such
D–A conjugated polymers. Therefore, the so-designed three poly-
mers, PTBTTFT, PTBTTPD, and PTBTDPP, were synthesized and
investigated on their basic optoelectronic properties in detail.
Moreover, using [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM) as acceptor material, polymer solar cells (PSCs) were fab-
ricated for studying photovoltaic performances of these polymers.
It was found that the optimized PTBTTPD cell gave the best
blend film morphology finally limited its solar cell performance to
C
V
a moderate level (PCE: 3.91%). 2018 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2018, 00, 000–000
KEYWORDS: 1,4-dithienyl-2,5-dialkoxybenzene; donor–acceptor
conjugated polymers; intramolecular non-covalent interactions;
polymer solar cells
needed to be further improved for PSC commercialization.10
Therefore, the development of novel high performance poly-
mer photovoltaic materials is still highly desirable and would
be one of the top challenges in the field. Besides material
innovation, systematic investigations on their structure–
property relationships are of primary importance because it
can guide us in the design of new materials.11
INTRODUCTION Polymer solar cells (PSCs) have attracted
considerable attention for their potential to offer low-cost,
lightweight, flexible, and large-area energy conversion devi-
ces.1–4 With tremendous efforts in material innovation and
device engineering, PSCs have achieved a substantial pro-
gress in the past decades.5–7 The power conversion efficiency
(PCE) record of their single junction modules was raised
from 3% to 5% before 2005 to over 13% nowadays.8,9 How-
ever, compared with that of crystalline-silicon-based inor-
ganic solar cells, this efficiency still lags far behind and
According to light-to-electricity conversion mechanisms oper-
ating in bulk heterojunction (BHJ) PSCs, high performance
Additional Supporting Information may be found in the online version of this article.
C
V
2018 Wiley Periodicals, Inc.
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photovoltaic materials are generally required to possess the
following properties and characteristics:3,12–14 (1) a narrow
bandgap for efficient harvesting of solar photons in a wide
spectral range, (2) matched molecular frontier orbital energy
levels for providing a large open-circuit voltage (VOC) and
sufficient driving force for exciton dissociation, and (3)
strong interchain p–p stacking interactions for efficient
charge transportation. For the former two requirements,
donor–acceptor (D–A) conjugated polymers, which are com-
posed of electron-donating (D) and -accepting (A) moieties
in their backbones, have been developed in lieu of previously
used conjugated homopolymers represented by poly(3-
hexylthiophene). Due to the existence of intramolecular
charge transfer (ICT) between D and A units, such materials
generally possess a narrow bandgap.15,16 Meanwhile, the
material energy levels for both highest occupied and lowest
unoccupied molecular orbitals (HOMO and LUMO, respec-
tively) can be easily tuned by the proper choice of D and A
units and their mode of combination.17 For the third require-
ment, a planar structure of the conjugated backbone would
be preferred. To reduce torsional angles between two neigh-
boring moieties and make the backbone more planar, both
covalent and noncovalent binding strategies have been
reported.18,19 In the covalent approach, certain bridged
bonds in addition to those in the backbone are used to lock
two neighboring moieties in a planar geometry and thus
form ladder-type moieties.20–22 However, their syntheses
mainly combined with benzothiadiazole-derived acceptor
units and a number of D–A conjugated polymers were pro-
duced.28,29,31–33 Among them, a polymer named PPDT2FBT,
which was composed of 5,6-difluorobenzothiadiazole and
TBT moieties in an alternative fashion, achieved a PCE of
9.39% for its optimized PSCs.26 In the polymer, the existence
of numerous intra- and intermolecular non-covalent interac-
tions was demonstrated to endow the polymer tight inter-
chain packing and high charge mobility, affording high
photovoltaic performance. From these studies, one may learn
that TBT is an excellent electron-donating moiety with intra-
molecular non-covalent S. . .O interactions. It has high poten-
tial for the construction of high performance photovoltaic
polymers and the key is lying in the proper choice of
electron-accepting unit. However, most of the reported works
focused on benzothiadiazole-derived acceptor units. Only a
few studies used other accepting unit, including thienopyra-
zine, diketopyrrolopyrrole (DPP), and isoindigo.34,35 To fully
exploit the potential of TBT unit, this work investigated its
combination with 3-fluorothieno[3,4-b]thiophene (TFT) and
thieno-[3,4-c]pyrrole-4,6-dione (TPD), as well as DPP, for D–
A photovoltaic polymers. Owing to their unique structures,
all these newly chosen A moieties have excellent records in
the field and produced a number of high performance photo-
voltaic polymers, such as PTB7, PBDTTPD, and PDPP3T.
Thus, three polymers named PTBTTFT, PTBTTPD, and
PTBTDPP were designed and synthesized in the work
(Scheme 1). It was found that the different A combination
produced polymers having different optical, electrochemical,
and crystalline properties. Moreover, they also displayed
quite different photovoltaic performances, in which an opti-
mized PCE of 4.49% was achieved for PTBTTPD, while that
of 3.91% was found for PTBTDPP and 1.96% for PTBTTFT.
generally need
a tedious procedure. In an alternative
approach, intramolecular non-covalent interactions can be
adopted to induce conformation lock and backbone planari-
zation. For example, non-covalent S. . .O coulomb attraction
has been found between the sulfur of thienyl unit and the
oxygen of alkoxyl or carbonyl substituents on adjacent aro-
matics, and was utilized in the design of a variety of conju-
gated polymers and small molecular semiconductors.23–25
Most of the works demonstrated that such non-covalent
S. . .O interactions can improve the material performance and
achieve a PCE over 9.39% and mobility higher than 5.37 cm2
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthetic routes for these polymers are outlined in Scheme
1. First, 2,5-dibromohydroquinone (compound 1) was alkylated
with 2-ethylhexyl bromide, and then subjected to Stille coupling
with 2-(tributylstannyl)thiophene in the presence of Pd(PPh3)4
catalyst. The obtained compound 3 was further stannylated
with trimethyltin chloride, producing monomer M1 in a yield of
70%. Finally, this monomer was Stille-coupling polymerized
with 2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]thiophene-
2-carboxylate (M2), 1,3-dibromo-5-heptyl-4H-thieno[3,4-c]pyr-
role-4,6(5H)-dione (M3), or 3,6-bis(5-bromothiophen-2-yl)-2,5-
bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (M4),
affording the polymers PTBTTFT, PTBTTPD, and PTBTDPP,
respectively. It was found that all these polymers possess good
solubility in common solvents such as chloroform, chloroben-
zene, and o-dichlorobenzene at room temperature. Their
V21 s21 26,27
Therefore, from the above analysis, one may
.
conclude that D–A conjugated polymers with intramolecular
non-covalent interactions would be promising photovoltaic
materials with potential high performance.
In this work, we were endeavored to develop D–A photovol-
taic polymers based on 1,4-dithienyl-2,5-dialkoxybenzene
moiety, as symbolized with TBT. Since it contains two thienyl
units adjacent to a central phenylene unit bearing two
alkoxyl substituents on its 2,5-positions, TBT is a typical
building block that confers the occurrence of non-covalent
S. . .O interactions and has an almost planar configura-
tion.28,29 The first use of TBT as donor moiety can be dated
back to 2007, when Andersson and coworkers reported a D–
A conjugated polymer based on it and 2-thia-1,3,5,8-tetraaza-
cyclopenta[b]naphthalene, a strong electron-deficient unit.30
The polymer displayed an extremely narrow bandgap (ꢀ1.0
eV) and was demonstrated useful for near infrared photode-
tector application. However, its photovoltaic performance
was poor, only 0.38% for its PSC devices. Later, TBT was
-
number-average molecular weights (Mn) and dispersities (D)
were analyzed by gel permeation chromatography (GPC) with
monodispersed polystyrenes as standard and reported to be
10.3, 8.64, and 23.6 kDa for the Mn of PTBTTFT, PTBTTPD, and
PTBTDPP, and 2.71, 1.70, and 2.11 for their dispersity, respec-
tively (Table 1). Thermogravimetric analysis (TGA) revealed
2
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SCHEME 1 Synthesis of PTBTTFT, PTBTTPD, and PTBTDPP. [Color figure can be viewed at wileyonlinelibrary.com]
that PTBTTPD and PTBTDPP start to decompose over 350 8C
and have 5%-weight-loss temperature (Td) at 397 and 412 8C,
respectively (Table 1 and Supporting Information Fig. S9).
While, PTBTTFT starts to decompose around 250 8C although it
Td is as high as 331 8C. Since the weight loss in its first stage of
250–280 8C is only 3–4%, the decomposition may be ascribed
to loss fluoro-substituent on thieno[3,4-c]thiophene unit. In dif-
ferential scanning calorimetry (DSC), all these polymers did not
show any thermal transition in the range of 0–250 8C (Support-
ing Information Fig. S10).
Optical and Electrochemical Properties
The absorption spectra of PTBTTFT, PTBTTPD, and
PTBTDPP in dilute chlorobenzene solution and thin film
state are shown in Figure 1(a), and their data are summa-
rized in Table 1. It is clear that PTBTDPP displayed the
TABLE 1 Basic Properties of PTBTTFT, PTBTTPD, and PTBTDPP
kmax (nm)
konset Eox,onset Ered,onset HOMOc LUMOd Eg, opt
Eg, CV
e
f
Mn
Td
-
D
Polymer
(KDa)
(8C)
Solution
Film
(nm)
(V)
(V)
(eV)
(eV)
(eV)
(eV)
PTBTTFT 10.3a 2.71a 331 447, 564
PTBTTPD 8.64b 1.70b 397 502
PTBTDPP 23.6b 2.11b 412 433, 676, 739 435, 675, 743 886
459, 607
822
0.38
21.24
21.20
21.17
25.10
25.21
25.01
23.48
23.52
23.55
1.51
1.62
1.69
1.46
382, 541, 592 671
0.49
0.29
1.85
1.40
a
c
d
e
f
Measured by GPC operating with 1,2-dichlorobenzene as an eluent at
HOMO 5 –e(Eox,onset–EFc/Fc1 1 4.8 V).
LUMO 5 –e(Ered,onset–EFc/Fc1 1 4.8 V).
room temperature.
b
Measured by high temperature GPC operating with 1,2,4-trichloroben-
E
g,opt 5 1240/konset
.
zene as an eluent at 150 8C.
E
g,CV 5 ELUMO–EHOMO.
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FIGURE 1 (a) UV–vis absorption spectra of PTBTTFT, PTBTTPD, and PTBTDPP in diluted chlorobenzene solutions and film state at
room temperature. (b–d) The temperature-dependent UV–vis spectra of (b) PTBTTFT, (c) PTBTTPD, and (d) PTBTDPP in chloroben-
zene solutions. [Color figure can be viewed at wileyonlinelibrary.com.]
broadest absorption spectrum among the three polymers in
dilute chlorobenzene solution [Fig. 1(a)] in the range of
350–800 nm. Three absorption peaks were identified with
apexes at 433, 676, and 739 nm. The first peak is attributed
to p–p* transition of the conjugated backbone, while the
middle one originates from ICT transition between the TBT
and DPP units. The 739 nm peak may be due to interchain
p-stacking interactions when considering its film spectral
features as described below. Unlike PTBTDPP, PTBTTPD
showed only a broad peak in the range of 350–600 nm with
an apex at 502 nm, while PTBTTFT exhibited a similar
broad peak in the range of 350–700 nm with an apex at 447
and a shoulder at 564 nm. This observation implies that the
excitation energy for ICT transition in PTBTTPD and
PTBTTFT may be close to that for their p–p* transitions. In
film state, the polymer PTBTTFT showed absorption spec-
trum with a 43 nm-red-shifted band as compared with its
solution one. In the case of PTBTTPD polymer, it displayed
not only a ꢀ40 nm-red shifted band but also a new peak at
592 nm. All these indicate the occurrence of interchain p–p
stacking interactions in their films. However, PTBTDPP pre-
sented absorption peaks in film state with almost identical
positions to their solution ones. This implies that interchain
p–p stacking interactions take place even in dilute solution
state.
shifted ICT peaks, when the temperature was risen from 0 to
120 8C. Considering higher temperature generally activates
conjugated backbone to take a more twisted configuration
and leads to blue shift in absorption peaks, the observed
phenomena suggest the occurrence of little interchain p–p
interactions in PTBTTFT and PTBTTPD solutions. However,
in the case of PTBTDPP solution, the spectral shape changed
a lot with temperature increments. The peak around 739 nm
was observed at low temperature (0–40 8C), but disappeared
when the temperature was higher than 80 8C. This unambig-
uously proves that PTBTDPP polymer has strong p–p inter-
chain interactions even in dilute solution.
The bandgap energy (Eg) and energy levels of the HOMO
and LUMO (EHOMO and ELUMO) are important parameters for
a photovoltaic material. They can be estimated from the
absorption edges (konset), and onset oxidation (Eox,onset) and
reduction potentials (Ered,onset) measured by cyclic voltam-
metry. From their film absorption spectra as shown in
Figure 1(a), the konset of PTBTTFT, PTBTTPD, and
PTBTDPP was found to be 822, 671, and 886 nm, giving
their Eg,opt to be 1.51, 1.85, and 1.40 eV, respectively (Table
1). Meanwhile in their CV profiles (Figure 2a), apparent oxi-
dation waves with onset points of 0.38, 0.49, and 0.29 V
were identified for PTBTTFT, PTBTTPD, and PTBTDPP,
respectively. Since the redox couple of ferrocene/ferrocenium
(Fc/Fc1), having an absolute energy level of 24.8 eV with
respect to the vacuum level, displayed the oxidation onset at
0.08 V under the same conditions, the EHOMO of PTBTTFT,
PTBTTPD, and PTBTDPP can be calculated to be 25.10,
25.21, and 25.01 eV, respectively [Table 1 and Figure 2(b)].
To further confirm the interchain p–p interaction occurring
in solution, temperature-dependent UV–vis spectroscopy was
carried out on these polymer chlorobenzene solutions
[Figs. 1(b–d)]. It was found that the spectra of PTBTTFT
and PTBTTPD almost kept the same shape but with blue
4
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FIGURE 2 (a) CV curves of PTBTTFT, PTBTTPD, and PTBTDPP films deposited on glassy carbon electrodes and measured in aceto-
nitrile solutions containing 0.1 mol L21 Bu4NPF6 electrolyte at a scan rate of 50 mV s21. (b) Energy-level diagram of PTBTTFT,
PTBTTPD, and PTBTDPP. [Color figure can be viewed at wileyonlinelibrary.com.]
Similarly, the ELUMO of PTBTTFT, PTBTTPD, and PTBTDPP
was estimated from their Ered,onset (21.24, 21.10, and 21.17
V) and found to be 23.48, 23.52, and 23.55 eV, respectively.
With both EHOMO and ELUMO data, the energy bandgap can be
also derived to be 1.62, 1.69, and 1.46 eV for PTBTTFT,
PTBTTPD, and PTBTDPP, respectively. From these data, one
may see that PTBTTPD has the lowest-lying HOMO energy
level, which would benefit to obtain a large open-circuit volt-
age (VOC) in combination with fullerene acceptor for PSCs.
However, polymer PTBTDPP possesses the smallest
bandgap. This is consistent with the observations in absorp-
tion spectroscopy and would be favorable for achieving a
large short-circuit current (JSC) in PSC application.
alkyl side chains were replaced with methyl units and the
optimized backbone geometries are displayed in Figure 3.
It is clear that the TBT moieties in all three polymers
adopt a planar configuration with all dihedral angles less
than 158. More importantly, all sulfur atoms of thienyl units
are residing in the same side of alkoxyl substituents of cen-
tral benzene units. The distances between S and O are esti-
mated to be around 2.7 Å, much smaller than the sum of
their van der Waals radius (O: 1.52 Å, S: 1.8 Å).37 Although
lacks of direct experimental supports, the DFT optimized
backbone geometries prove the existence of non-covalent
S. . .O interactions in TBT moieties for all three polymers. It
is valuable to point out that PTBTTPD polymer has the
most planar backbone structure among the three polymers.
This is because the concurrence of another set of noncova-
lent of S. . .O interactions, which is existing between thienyl
sulfur and imide oxygen in TPD unit and evidenced by a
small S. . .O distance (3 Å) and almost flat configuration
between thienyl and TPD units with dihedral angles less
than <18.
Theoretical Studies
To gain insight into the fundamental structure and elec-
tronic properties of these three polymers, density functional
theory (DFT) was employed to simulate on their trimer
segments by using Gaussian 09 program with B3LYP/6–
31G(d,p) methods.36 To simplify the calculation, all of the
FIGURE 3 Optimized backbone geometries and electron wave functions of HOMO and LUMO orbitals of the trimer models of
PTBTTFT, PTBTTPD, and PTBTDPP via DFT calculation. [Color figure can be viewed at wileyonlinelibrary.com.]
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FIGURE 4 (a) J–V curves and (b) EQE spectra of the optimized PTBTTFT/PC71BM, PTBTTPD/PC71BM, and PTBTDPP/PC71BM-based
PSC devices. [Color figure can be viewed at wileyonlinelibrary.com.]
Figure 3 also displays the simulated electron wave functions
of HOMO and LUMO orbitals for these three polymer models.
For both HOMO and LUMO orbitals, electron delocalizes
through a wide range of backbone units, suggesting the good
p-electron conjugation in all these polymers. But their deloc-
alizing centers are different, suggesting certain D–A nature
for all three polymers.
energy level. Compared with PTBTTPD cell, PTBTDPP
device showed a higher JSC (11.11 vs. 8.70 mA cm22) and a
slightly larger FF (62.2% vs. 58.5%). However, the smallest
VOC (0.57 V) limited its final power conversion efficiency to
a moderate level (3.91%). For the PTBTTFT device, it dis-
played the smallest PCE (1.96%) with a VOC of 0.67 V, a JSC
of 5.83 mA cm22 and a FF of 50.1%.
Photovoltaic Performance
In their external quantum efficiency (EQE) spectra, as shown
in Figure 4(b), PTBTDPP cell was found to display the wid-
est photo-response range, from 300 to 800 nm, but with
moderate EQEs among the three optimized devices. The
PTBTTPD cell showed the largest EQE values in the region
of 400–600 nm but with the narrowest photo-response
range, while PTBTTFT cell displayed the smallest EQEs in
its photo-response range of 300–750 nm. Therefore, these
EQE spectra are well consistent with light-absorption behav-
iors of the polymers and can well explain the order of JSC. In
addition to PC71BM, [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM) was also used as acceptor material blended
with the polymers for PSCs and achieved similar photovol-
taic performances with PCEs of 4.41% for PTBTTPD-, 2.85%
for PTBTDPP-, and 1.68% for PTBTTFT-based devices (Sup-
porting Information Table S1).
To investigate the photovoltaic properties of these polymers,
PSC devices with a conventional configuration of ITO/
PEDOT:PSS/BHJ active layer/Ca/Al were fabricated. The BHJ
active layer was composed of the tested donor polymer and
[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as
acceptor material. To know the best performance of each
polymer, their fabrication processes were individually opti-
mized on polymer/PC71BM weight ratio, solvent, and addi-
tive. The experiments found that chlorobenzene as solvent
and polymer/PC71BM weight ratio of 1:1 were suitable for
fabrication of both PTBTTFT- and PTBTTPD-based cells,
while 1,2-dichlorobenzene and polymer/PC71BM weight ratio
of 1:1.5 were good for PTBTDPP cell fabrication. Moreover,
3% (v/v) 1-chloronaphthalene (CN) as an additive to the
processing solvent was found to effectively improve
PTBTTPD cell performance, while the tested additives (1,8-
diiodooctane and 1-chloronaphthalene) did not enhance the
performances of PTBTTFT and PTBTDPP cells. After optimi-
zation, the J–V curves of their best cells are displayed in Fig-
ure 4(a) and their device parameters are listed in Table 2. It
can be seen that the optimized PTBTTPD cell with a PCE of
4.49% displayed the best performance among these polymer
devices. This should result from its largest VOC (0.88 V),
which is closely associated with its lowest lying HOMO
Hole Mobility, Film Structure, and Morphology
To get insights into the observed photovoltaic performance
differences of these three polymers, their BHJ films with
PC71BM were investigated for hole mobility, chain packing
structure and as well as morphology. Charge mobility in the
BHJ active layer is an important factor that determines
PSC performance, especially for JSC and FF. In this work,
hole transporting only devices with a configuration of
TABLE 2 Device Parameters of the Optimized PTBTTFT-, PTBTTPD-, and PTBTDPP-Based PSCs as Shown in Figure 4(a)
b
Polymer/PC71BM
(w/w)
CN
Thickness
(nm)
VOC
JSC
(mA cm22
FF
PCEa
(%)
lh
Polymer
(v/v)
(V)
)
(%)
(cm2 V21 S21
)
PTBTTFT
PTBTTPD
PTBTDPP
1:1
0
85
0.67
0.88
0.57
5.83
8.70
11.11
50.1
58.5
62.2
1.96 (1.84)
4.49 (4.27)
3.91 (3.70)
9.62 3 1024
1.96 3 1023
5.25 3 1023
1:1
3%
0
80
1:1.5
150
a
b
The date in parentheses are average value.
Measured by SCLC method.
6
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the PTBTDPP PSC displayed the best JSC and FF value, while
PTBTTFT cell exhibited the poorest performances in these
two parameters.
Besides hole mobility, the BHJ film structure was investi-
gated by X-ray diffraction (XRD). As shown in Figure 5, only
PTBTDPP/PC71BM (1/1.5, w/w) BHJ film gave a set of dif-
fraction peaks at 2h of 4.9, 9.8, and 18.68. The former two
peaks can be assigned to (100) and (200) diffraction from a
lamellar structure with layer distance of 1.74 nm, while the
last broad peak may originate from (010) p–p stacking dif-
fraction. However, no obvious diffraction peaks were found
for PTBTTFT/PC71BM (1/1, w/w) and PTBTTPD/PC71BM
(1/1, w/w) BHJ films. This result illustrates that only
PTBTDPP polymer could form a regular packing structure
with strong p–p interchain interactions in its BHJ film, and
thus contribute to its good hole transport.
FIGURE 5 XRD profiles of polymer/PC71BM BHJ films. [Color
figure can be viewed at wileyonlinelibrary.com.]
Atomic force microscopy was also used to investigate the
BHJ film morphology. As shown in Figure 6, the blend films
of PTBTTFT/PC71BM (1/1, w/w) and PTBTTPD/PC71BM
(1/1, w/w) appeared to be rather flat. Their root-mean-
square roughnesses were estimated to be 0.60 and 0.72 nm,
respectively. However, the appearance of the blend film of
PTBTDPP/PC71BM (1/1.5, w/w) was very rough, with a
RMS roughness of 11.57 nm [Note the height scale in
Figure 6(c) is different from those of Figs. 6(a,b)]. Moreover,
its phase image revealed that the film was full of dot-like
domains with the size of 40–70 nm, while the phase image
of PTBTTPD blend film suggested short rod-like structures.
ITO/PEDOT:PSS/BHJ active layer/Au were fabricated for the
three polymers (Table 2 and Supporting Information Fig. S8).
Here, the compositions and fabrication methods of the BHJ
active layers were the same as for their optimized PSCs. J–V
curves were measured in the dark and fitted by a space
charge-limited current theory (Supporting Information Fig.
S11), and the hole mobilities were calculated to be 9.62 3
1024, 1.96 3 1023, and 5.25 3 1023 cm2 V21 S21 for
PTBTTFT-, PTBTTPD-, and PTBTDPP-based BHJ films,
respectively (Table 2). It is clear that PTBTDPP/PC71BM
blend film gave the largest hole mobility, while that of
PTBTTFT was the smallest one. So, it is not surprising that
FIGURE 6 (a–c) AFM height and (d–f) phase topographical images (5 mm 3 5 mm) of (a, d) PTBTTFT/PC71BM, (b, e) PTBTTPD/
PC71BM, and (c, f) PTBTDPP/PC71BM BHJ films. [Color figure can be viewed at wileyonlinelibrary.com.]
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Therefore, it is possible that the coarse morphology of the
PTBTDPP BHJ film is adds to the HOMO energy level consid-
eration in explaining the inferior performance of PTBTDPP
PSCs compared to PTBTTPD cells.
was measured by room temperature GPC using o-dichloroben-
zene as an eluent. Thermogravimetric analysis (TGA) was car-
ried out by a TGA Q500 instrument with a temperature rate
of 10 8C min21 under a nitrogen atmosphere. Differential
scanning calorimetry (DSC) was performed on a Q2000 modu-
lated DSC instrument with a heating rate of 10 8C min21 and
a cooling rate of 15 8C min21 under a nitrogen atmosphere.
UV–vis absorption spectroscopy was performed on a Hitachi
U-3310 spectrophotometer. Cyclic voltammetry (CV) measure-
ments were performed on a CHI 660C instrument with a
three-electrode cell using glassy carbon as a working elec-
trode, platinum wire as a counter electrode, and Ag/AgNO3 as
a reference electrode. Samples were prepared by casting their
chlorobenzene solutions onto glassy carbon electrodes. The
measurements were carried out in dehydrated acetonitrile sol-
utions containing 0.1 M Bu4NPF6 electrolyte at a scan rate of
50 mV s21. Atomic force microscopy (AFM) was performed on
a JPK NanoWizard AFM system in tapping mode. X-ray diffrac-
tion (XRD) was carried out on a PANalytical X’Pert Pro diffrac-
tometer with Cu Ka beam (40 kV, 40 mA) in h–2h scans
(0.026 Å step size, 50 s/step).
CONCLUSIONS
In this contribution, three D–A conjugated polymers
namely PTBTTFT, PTBTTPD, and PTBTDPP, which are com-
posed of 1,4-dithienyl-2,5-di(2-ethylhexyoxyl)-benzene (TBT)
electron-donating moiety and one of following electron-
accepting units: 3-fluorothieno[3,4-b]thiophene (TFT),
thieno-[3,4-c]pyrrole-4,6-dione (TPD), and diketopyrrolopyr-
role (DPP), are reported. Owing to its unique structure, TBT
has typical intramolecular S. . .O interactions and thus a non-
covalent conformation-locked backbone. When it was com-
bined with TPD, the produced PTBTTPD polymer has a
low-lying HOMO energy level (25.21 eV), thus affording the
largest VOC (0.88 V) for its PSCs. Moreover, its blend film
with PC71BM possessed good hole mobility and a favorable
morphology. Therefore, its optimized PSC showed a PCE of
4.49%, the best among the three polymer devices. In com-
parison, PTBTDPP displayed a wider absorption spectrum
and a larger hole mobility than PTBTTPD, thus giving its
PSC device the largest JSC and FF value. However, its
unmatched HOMO energy level and disfavored blend film
morphology limited the performance of its PSC device, which
displayed a PCE of 3.91%, slightly smaller than that of
PTBTTPD cell. In the case of PTBTTFT polymer, high lying
HOMO energy level, the smallest hole mobility, and irregular
chain-packing structure in the blend film are all disfavor to
its photovoltaic performance, thus leading to the smallest
PCE (1.96%) for its PSC device in this family.
Photovoltaic Device Fabrication and Characterization
All solar cell devices were fabricated with a structure of
ITO/PEDOT:PSS/BHJ active layer/Ca/Al. First, ITO-coated
glass substrates were cleaned by deionized water, acetone,
and isopropanol. Then the substrates were dried with pres-
surized nitrogen and exposed to UV2ozone plasma for 15
min. A thin layer (25 nm) of filtered PEDOT:PSS (Heraeus
Clevios PVP. Al 4083) was spin-casted on the top of cleaned
ITO at 4000 rpm and baked at 140 8C for 15 min. After-
wards, the substrates were transferred into a dry nitrogen
glovebox (O2 <0.1 ppm, H2O <0.1 ppm) for active layer
deposition. The active layer of PTBTTFT or PTBTTPD was
spin-coated from a chlorobenzene solution of the polymer
and PC71BM with a total weight concentration of 20 mg
mL21, while that of PTBTDPP was from an o-dichloroben-
zene solution of PTBTDPP and PC71BM with a total weight
EXPERIMENTAL
Measurements and Characterization
All reagents from commercial sources were used as received.
All reactions were carried out under argon atmosphere using
solvents dehydrated following standard methods. Compound
2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-car-
boxylate (M2) and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-
hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (M4)
were purchased from Suna Tech, Inc. 1,3-Dibromo-5-heptyl-
4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (M3) was synthesized
according to reported procedures.38,39 1H NMR (400 MHz)
and 13C NMR (100 MHz) were recorded on a Varian Mercury
spectrometer with tetramethylsilane (TMS) as an internal ref-
erence. The spectra of small molecular compounds were mea-
sured in CDCl3 at room temperature, while those of polymers
were recorded in 1,1,2,2-tetrachloroethane-d2 at 100 8C.
Matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass spectroscopy was carried out on a Shi-
madzu Biotech Axima Mass Spectrometer using dithranol as a
concentration of 25 mg mL21
. In the fabrication of
PTBTTPD/PC71BM BHJ active layer, 1-chloronaphthalene
(CN) in a volume fraction of 3% was added as additive to
solvent. Finally, the devices were finished by thermal deposi-
tion of 8 nm-thick Ca and 80 nm-thick Al cathode under a
vacuum of 1025 mbar. The thicknesses of all the films were
measured by a Veeco Dektak 150 profilometer. Current den-
sity–voltage (J–V) curves were recorded with a Keithley
2420 source meter. Photocurrent was acquired upon irradia-
tion using an AAA solar simulator (Oriel 94043A, 450 W)
with an AM 1.5G filter. Light intensity was adjusted to 100
mW cm22 using a standard NRE-certified silicon cell. Exter-
nal quantum efficiency (EQE) was detected with a 75 W Xe
lamp, an Oriel monochromator (74125), an optical chopper,
a lock-in amplifier, and a NREL-calibrated crystalline silicon
cell.
-
matrix. The average molecular weights and dispersities (D) of
PTBTTPD and PTBTDPP were estimated by high-
temperature gel permeation chromatography (GPC) operating
at 150 8C using 1,2,4-trichlorobenzene as an eluent and mono-
dispersed polystyrenes as standards, while those of PTBTTFT
Hole Mobility Measurement
Hole-transporting only devices were fabricated with a struc-
ture of ITO/PEDOT:PSS/polymer:PC71BM/Au. The mobility
8
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was determined by fitting the dark current to single carrier
SCLC, as described by the following equation:
room temperature, stirred for 1.5 h, cooled down to 278 8C
again, and then added with trimethyltin chloride (7.5 mL, 1 M
in n-hexane, 7.5 mmol) in one portion. Afterward, the reaction
mixture was allowed to slowly warm to room temperature
and stirred overnight. The reaction was terminated by addition
of cool water (20 mL), and the resulted mixture was extracted
with diethyl ether. Organic layers were combined, washed
with brine, dried over anhydrous MgSO4, and concentrated
under vacuum. The crude product was recrystallized from iso-
propanol to afford monomer M1 as yellow crystals (1.39 g,
yield 70%). 1H NMR (400 MHz, CDCl3): d (ppm): 7.643 (d,
J 5 3.2 Hz, 2H), 7.245 (s, 2H), 7.185 (d, J 5 3.2 Hz, 2H), 3.967
(d, J 5 5.2 Hz, 4H), 1.849–1.819 (br, 2H), 1.650–1.414 (m, 8H),
1.359–1.323 (m, 8H), 0.970–0.884 (m, 12H), 0.396 (s, 18H).
13C NMR (100 MHz, CDCl3): d (ppm): 149.2, 145.3, 137.7,
134.9, 126.4, 122.8, 112.6, 71.7, 39.7, 30.7, 29.2, 24.0, 23.1,
14.1, 11.2, 28.3. LR MS (MALDI) m/z: 824 (M1).
2
9
ðVa2VbiÞ
J5 E0Erlh
8
d3
where J is the current density, lh is the hole mobility, E0 and
Er are the permittivity of free space and the relative permit-
tivity of the materials, respectively, d is the thickness of the
active layer, Va is the applied voltage, and Vbi is the built-in
voltage.
Material Synthesis
Synthesis of 1,4-Dibromo-2,5-Bis((2-Ethylhexyl)Oxy)
Benzene (Compound 2)
A mixture of 2,5-dibromobenzene-1,4-diol (compound 1,
2.0 g, 7.52 mmol), K2CO3 (4.14 g, 30 mmol) was put into a
250 mL flask under the protection of argon. After addition of
100 mL DMF, the reaction mixture was stirred for 1 h. Then,
3-(bromomethyl)heptane (5.76 g, 30 mmol) were added into
the reaction mixture. After refluxing at 100 8C for 24 h, the
solvent was removed under reduced pressure. The crude
product washed with water and extracted with dichlorome-
thane. The product was separated by silica column chroma-
tography using hexane as the eluent to afford compound 2
as colorless oil liquid (3.263 g, yield 88%). 1H NMR (400
MHz, CDCl3): d (ppm): 7.084 (s, 2H), 3.831 (d, J 5 7.2 Hz,
4H), 1.788–1.711 (br, 2H), 1.571–1.398 (m, 8H), 1.342–1.268
(m, 8H), 0.962–0.892 (m, 12H).
General Procedure for Synthesis of Polymers PTBTTFT,
PTBTTPD, and PTBTDPP
To a two-necked flask was added dry toluene (12 mL),
monomer M1 (0.2 mmol) and one of following monomers:
M2, M3, or M4 (0.2 mmol). After the solution was frozen by
liquid nitrogen, Pd(PPh3)4 (12 mg) was added quickly into
the flask. The resulted mixture was then subjected to thor-
oughly degassed with three freeze–pump–thaw cycles and
filled back with Ar. After stirring at 110 8C for 36 h, the reac-
tion mixture was poured into 150 mL methanol to precipi-
tate the produced polymers. After filtration, the crude
polymer was subjected to Soxhlet extraction with methanol,
acetone, hexane, and chloroform in sequence, and finally
purified by SEC column with S-X1 Biobeads using chloroform
as an eluent. The yields for all polymers were 70–80%.
Synthesis of 2,20-(2,5-Bis((2-Ethylhexyl)Oxy)-1,4-
Phenylene)Dithiophene (Compound 3)
A mixture of compound 2 (1.65 g, 3.36 mmol), tributyl(thio-
phen-2-yl)stannane was dissolved in dry toluene under the
protection of Ar atmosphere. After the solution was frozen
by liquid nitrogen, tetrakis(triphenylphosphine) palladium(0)
(194 mg) was added quickly into the flask. The resulted mix-
ture was then subjected to thoroughly degassed with three
freeze–pump–thaw cycles and filled back with Ar. After stir-
ring at 110 8C for 8 h, the solvent was removed under
reduced pressure. The residue was added with water and
extracted with dichloromethane for three times. The organic
phases were combined, washed with brine, dried over anhy-
drous MgSO4, and concentrated under vacuum. The crude
product was finally separated by silica column chromatogra-
phy using hexane as eluent to afford compound 3 as yellow
solid (1.43 g, yield 86%). 1H NMR (400 MHz, CDCl3): d
(ppm): 7.531 (d, J 5 4.8 Hz, 2H), 7.347 (d, J 5 6.8 Hz, 2H),
7.260 (s, 2H), 7.102 (dd, J 5 6.8 Hz, J 5 4.8 Hz, 2H), 3.979 (d,
J 5 6.8 Hz, 4H), 1.882–1.799 (br, 2H), 1.629–1.460 (m, 8H),
1.344–1.312 (m, 8H), 0.969–0.899 (m, 12H).
ACKNOWLEDGMENTS
This work was financially supported by National Natural Science
Foundation of China (21474129, 21674125, and 51761145043),
International Science and Technology Cooperation Program of
China (2015DFG62680), Strategic Priority Research Program of
Chinese Academy of Sciences (XDB20020000), Science and Tech-
nology Open Cooperation Projects of Henan Province
(162106000018 and 172106000067) and Zhengzhou Institute
of Technology. IVF and EAK are grateful to Israel’s Ministry of Sci-
ence and Technology for the China-Israel Cooperative Scientific
Research Grant number 3–12387.
AUTHOR CONTRIBUTIONS
X.-Q. Chen was mainly in charge of research experiments. X.
Yao, J. Wang, and S.-C. Wu fabricated solar cells and charac-
terized device performance. W.-J. Xiao and L.-N. Liu helped in
polymer syntheses. G. Xie, J. Li, and Z. Lu helped in polymer
characterizations. I. Visoly-Fisher and E. A. Katz helped in
device characterization and data analysis. T. Bai and J. Lin
carried out DFT calculations. W.-S. Li supervised the research.
X.-Q. Chen and W.-S. Li wrote the manuscript, while I. Visoly-
Fisher and E. A. Katz proofread the manuscript.
Synthesis of ((2,5-Bis((2-Ethylhexyl)Oxy)-1,4-Phenylene)
Bis(Thiophene-5,2-Diyl))Bis(Trimethylstannane) (M1)
Compound 3 (1.25 g, 2.5 mmol) was dissolved in 30 mL dry
THF under the protection of argon. After the solution was
cooled down to 278 8C using a dry iceacetone bath, n-butyl-
lithium (3 mL, 2.5 M in n-hexane, 7.5 mmol) was added drop-
wise. Then, the reaction mixture was slowly warmed up to
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