Novel donor-acceptor polymers containing: O -fluoro- p -alkoxyphenyl-substituted benzo[1,2- b:4,5- b ′]dithiophene units for polymer solar cells with power conversion efficiency exceeding 9%

Article abstract of DOI:10.1039/c6ta03709g

In this work, a new electron-rich building block, o-fluoro-p-alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, has been used to construct donor (D)-acceptor (A) conjugated copolymers with electron-deficient units 5,6-difluoro-4,7-di(4-(2

Full text of DOI:10.1039/c6ta03709g

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Journal of Materials Chemistry A
ARTICLE
Novel Donor–Acceptor Polymer Containing o-Fluoro-p-
alkoxyphenyl-Substituted Benzo[1,2-b:4,5-b′]dithiophene Unit for
Polymer Solar Cells with Power Conversion Efficiency Exceeding
Received 00th January 20xx,
Accepted 00th January 20xx
9
%
DOI: 10.1039/x0xx00000x
a
a
a
a
a
a
Ning Wang,‡ Weichao Chen,‡ Wenfei Shen, Linrui Duan, Meng Qiu, Junyi Wang, Chunming
www.rsc.org/
b
a
a,c
Yang, * Zhengkun Du, Renqiang Yang
*
In this work, a new electron-rich building block, o-fluoro-p-alkoxyphenyl-substituted benzo[1,2-b:4,5-b’]dithiophene
BDT) unit has been used to construct donor (D)-acceptor (A) conjugated copolymers with electron-deficient units
,6-difluoro-4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (C8DTBTff) and 5,6-difluoro-4,7-di(4-hexyl-2-
(
5
thienyl)-2,1,3-benzothiadiazole (C6DTBTff), named P-o-FBDTP-C8DTBTff (P2) and P-o-FBDTP-C6DTBTff (P3),
respectively. The experimental results indicate that the incorporation of the fluorine onto the ortho-position of
alkoxyphenyl substituted BDT unit can enable its resultant polymer to efficiently tune the energy levels and improve the
mobility of the derived bulk heterojunction layer, which results in much higher power conversion efficiency (PCE) of P2
(
8.10%). Moreover, replacing the 2-ethylhexyl chains on the DTBTff unit with hexyl chains can improve the planarity
of the conjugated backbone of the polymer, which makes the P3/PC71BM blends exhibit higher carrier mobility than
P2/PC71BM. Finally, a PCE of 9.02% for the device of P3 is obtained without any additive treatment, which is the
highest value for the widely reported D-A polymers with fluorine substituted BDT as electron-donor units in single
junction polymer solar cells.
gaps, and mobility through modifying the chemical structure
9
-13
1
. Introduction
and conformation of D or A moieties . Until now, a large
amount of donor materials have been designed and
synthesized as new D–A copolymer photovoltaic materials. As
the electron-rich moiety, BDT has the structure of fusing
benzene with two thiophene moieties, which endows it a rigid
Bulk heterojunction polymer solar cells (PSCs) have attracted
much attention in the past decade, owing to their flexibility,
1
-6
low fabrication cost, and light weight . Recently, PSCs based
on conjugated polymers and fullerene derivatives have made
significant progress toward commercialization. The power
conversion efficiencies (PCEs) over 10% have been obtained
1
4
and planar conjugated structure . The central benzene core of
the BDT unit can be substituted by different groups to tune the
properties including energy level, band gap, and mobility of
7
, 8
for the single junction PSC devices
.
1
5
the polymers . For example, the conjugated substituted
groups with aromatic structure can enhance intermolecular π–
π interactions, which can be beneficial to improving the
In the active layer of PSCs, the conjugated polymers with
alternating electron-rich units (electron-donor, D) and
electron-deficient moieties (electron-acceptor, A), known as
D–A copolymers, are one of the most efficient design
strategies to obtain high-performance donor materials, since it
can offer the unique feature of tuning the energy levels, band
1
6-18
photovoltaic properties of conjugated polymers
.
Moreover, it has been proven that introduction of fluorine
atom onto the BDT unit can also efficiently influence the
photovoltaic performance of D-A polymers due to the strong
1
9
electron-withdrawing ability of the fluorine atom .
Furthermore fluorine substitution can also alter the surface
energy of the polymer and thus greatly influence the
morphology of the active layer through weak bonds such as
20
C−F···H and F···S interaction . Figure 1 shows the possible
substituted positions on alkoxyl-, thienyl-, and phenyl-
substituted BDT units. The performance of the reported
polymers with the above fluorinated BDT units is listed in Table
1. Interestingly, the positions where the fluorine atoms are
bonded may result in much difference in the performance of
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Figure 1. The possible fluorine substituted positions on BDT based donor-acceptor (D-A) polymers.
Table 1. The effect of the F substituted positions on the performance of the BDT based polymers.
polymer
PTBF3
F-positiona
V
oc (V)
J
sc (mA cm-2)
9.1
FF (%)d
39
PCEmax(%)
∆ PCEe
N/A
0.5
Ref.
[21]
3-b
3-b
p
0.76
0.75
0.97
0.96
0.76
0.82
0.74
0.87
0.94
0.89
0.90
0.78
0.93
2.7
3.2
PTBF2
11.1
42
[21]
b
PBO-p-FPO
PTFBDT-BZO
PBQ-2
PBDTPF-DTBT
PBT-2F
9.9
9.24
53
5.1
1.9
[25]
[26]
c
m
64
5.67
6.25
7.02
7.2
0.6
4-s
13.05
13.11
14.4
63
0.58
0.77
0.2
0.8
0.1
[22]
m
65
[24]
4-s
m
68
[23]
b
PBO-m-FPO
P2
14.3
64
8.0
[25]
o
m
12.48
12.67
13.58
15.2
69
8.10
8.24
8.55
8.6
This work
[26]
c
PTFBDT-BZS
PBQ-4
73
0.14
0.31
0
4-s
4-s
o
70
[22]
[23]
This work
PBT-3F
P3
72
12.97
74
9.02
b
0.4
a
The substituted position of fluorine on the BDT moiety with different side chains; The values were obtained from the devices
with optimized interfacial layer; Two fluorine atoms were introduced on the meta-positions of BDTP segment. The decimal digits in
some FF are rounded to the nearest unit. Incremental PCE values compared to the data in the previous row, the decimal digits in
c
d
e
some PCE are rounded to the nearest unit before calculation.
the polymers. For example, Yu and coworkers reported that the on the polymer and PC71BM exhibited good photovoltaic
modification of the thiophene ring (3-b position of BDT units in performance with PCE of 7.02%, open-circuit voltage (Voc) of
-
2
Figure 1) by fluorination could have a significant effect on the 0.82 V, short-circuit current density (
J
sc) of 13.11 mA cm and
physical properties of polymers , however, PCEs only reached fill factor (FF) of 65.28% . Meanwhile, Zou et. al. also
.3% and 2.8% for polymer PTBF2 and PTBF3, respectively, compared two polymers containing meta-fluoro-para-
2
1
24
3
which was ascribed to the poor miscibility of the polymers alkoxyphenyl- (m-FPO-) and para-fluoro-meta-alkoxyphenyl-
with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and (p-FPO-) substituted BDT segment, and showed quite different
2
5
the lower mobility of the polymers. Hou and coworkers photovoltaic performance of the two polymers . Most
reported the fluorination of the conjugated thienyl- side recently, Bo et al. introduced two fluorine atoms on the meta-
groups of the BDT unit, and showed synergistic effect on postions of the BDTP moiety, and reported two polymers with
molecular energy level modulation of the polymers. When the 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as
quinoxaline derivatives were employed as acceptor unit to the
electron
donor
moiety,
benzothiadiazole
or
construct D-A polymer, a high PCE of 8.55% was obtained for benzooxadiazole as the electron acceptor unit, named PTFBDT-
2
2
the polymer PBQ-4 ; while the thieno[3,4-b]thiophene (TT) BZS and PTFBDT-BZO, respectively. The photovoltaic devices
was selected, a comparative PCE value of 8.6% was achieved with PTFBDT-BZS:PC BM as the active layer showed PCE of
7
1
2
3
26
for polymer PBT-3F . Compared to thienyl- group, phenyl- 8.24% without any additive treatment . As shown in Figure 1,
group had better symmetric structure and weaker electron- the ortho-position is adjacent to the conjugated backbone,
donating ability. As shown in Figure 1, there are three reaction where fluorine can influence the molecular packing and
sites on the phenyl substituted BDT (BDTP) moiety including photovoltaic performance of the two-dimensional (2D)
2
7,28
para-, meta-, and ortho-positions. While fluorine atoms are conjugated polymers efficiently.
, however, to our best
introduced to the above positions, the electron density of the knowledge, there is still no report about the modification on
polymer backbone will be redistributed due to the strong the ortho-positions of BDTP.
electron inductive effects of fluorine, which will further affect
Based on the above considerations, we introduced fluorine
atom to the ortho-position of the BDTP unit, and prepared the
polymer P-o-FBDTP-C8DTBTff (P2) with the well-known
the energy levels of the conjugated polymers. Recently, our
group introduced fluorine at the meta-position of the
alkoxyphenyl group substituted BDT unit. The resulting
polymer showed a relatively low lying highest occupied
molecular orbital (HOMO) energy level, and the device based
electron-withdrawing
monomer
5,6-difluoro-4,7-di(4-(2-
ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (C8DTBTff).
2
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Figure 2. The design strategy and structures of copolymers P1, P2, and P3.
Meanwhile, PBDTP-DTBTff (P1) without F on phenyl group was
prepared as a reference polymer. The design strategy and 2. Results and Discussion
structures of polymers are shown in Figure 2. The inclusion of
2
.1. Synthesis and Characterization
the F atom on the ortho-position of alkoxyphenyl substituted
The synthetic routes for the monomers and the target poly-
BDT unit greatly decreased the HOMO energy level of P2. As a
result, the Voc value was increased to 0.94 V for P2. mers are shown in Scheme 1. The alkoxyphenyl or o-fluorine
Meanwhile, the inclusion of the F atom in the conjugated side alkoxyphenyl substituted groups were bound to BDT via the
chain of polymer also increased the mobility and regulated the nucleophilic and reduction reaction to afford compound 4 and
3
0
morphology of the active layer. These synergistic effects 2, respectively . It should be noted that the hydrogen atom on
increased PCE of P2-based devices to 8.10% after additive the meta-position of the o-fluorine alkoxyphenyl substituted
treatment. Furthermore, considering the steric effect of the group in compound 2 has high reactivity due to the combined
2
9
alkyl chain on the thiophene bridge of DTBTff unit , we ortho directing effects of the adjacent fluoro- and alkoxy-
3
1
replaced the 2-ethylhexyl chain in P2 with hexyl chain in the groups . Subsequently, the monomer o-FBDTP was prepared
DTBTff moiety, and prepared polymer P3, which showed more from bromated BDT derivative (compound 3) by lithiation
planarity backbone than P2. Inspiringly, the mobility of using butyl lithium and then quenching with trimethyltin
-4
2
-1 -1
P3/PC71BM film gave 1.5×10 cm V s , which was about 2.6 chloride with satisfying yield, which was obviously different
times higher than that of P2/PC71BM film. Finally, the PSC from the BDTP with one pot reaction. The electron deficient
2
0
32
device based on P3 and PC71BM showed the highest PCE of monomer C8DTBTff
and C6DTBTff
were synthesized
-2
9.02% with Voc of 0.93 V, a
J
sc of 12.97 mA cm and a FF of through Stille coupling reaction between 4,7-dibromo-5,6-
74.49% (Table 1).
difluoro-2,1,3-benzothiadiazole and 2-tributylstannyl-4-alkyl-
thiophene, followed by the bromination reaction, respectively.
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Scheme 1. Synthetic routes of the monomers and polymers.
Finally, the D–A structured polymers were obtained via Stille
Electronic energy levels of the polymers are crucial for their
application in OPVs. To evaluate the molecule energy levels of
2 3
cross-coupling polycondensation in the presence of Pd (dba)
3
3
catalyst and tri(o-tolyl)phosphine ligand with high yields . All the synthesized three polymers, their HOMO and lowest
the polymers can be dissolved in common organic solvents unoccupied molecular orbital (LUMO) were measured by both
such as chloroform, chlorobenzene, and 1,2-dichlorobenzene. cyclic voltammetry (CV) and ultraviolet photoelectron
The number-average molecular weights (Mn) of the polymers spectroscopy (UPS) methods. As shown in Figure S2a, the
were measured by gel permeation chromatography (GPC) polymer films exhibited an irreversible oxidative potential. The
(
Table S1). There was only small difference in the Mn and PDI HOMO energy levels of the polymers were estimated from the
values of the three polymers, which indicated that the onset potentials according to the equation HOMO = -[(Eoxonset
photovoltaic performance of these polymers was not mainly foc)+Eref] (eV), where Efoc is the potential of the external
-
E
+
influenced by the molecular weights. The thermal properties standard, the ferrocene/ferrocenium ion (Foc/Foc ) couple,
of the polymers were carried out by the thermogravimetric and Eref is the reference energy level of ferrocene (4.8 eV
3
analysis (TGA). As shown in Fig. S1 and Table S1, all the below the vacuum level ). As shown in Table 2, the HOMO
4
polymers exhibit good thermal stability and the decomposition energy levels of P1, P2 and P3 calculated in this method were
o
temperature (Td) at 5% weight loss is over 330 C.
5.50, -5.63 and -5.56 eV, respectively. It could be seen from
the CV results that the HOMO of P2 was lower than that of P1,
which was ascribed to the inclusion of fluorine on the ortho-
2
4
.2. Energy levels and Optical Properties
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Table 2. Optical and Electrochemical Properties of Polymers.
Solution
Film
opt
E
g
HOMO
LUMO
HOMO
LUMO
[eV] f
a
polymer
λ
max
λ
shoulder
λ
max-H
λ
max
λ
edge
[
eV]b
[eV]
c
[eV]
d
[eV]
e
[
nm]
[nm]
645
N/A
623
[nm]
545
543
542
[nm]
603
589
586
[nm]
708
683
686
P1
P2
P3
560
544
555
1.75
1.82
-5.50
-5.63
-5.56
-3.75
-3.81
-3.75
-5.03
-5.49
-5.13
-3.28
-3.67
-3.32
1.81
a
b
The data were obtained in chlorobenzene solution at high temperature. Band gap was calculated from the onset of the film
c
d
opt
e
absorption. Measured by cyclic voltammetry (CV). Calculated from the gap of E
and HOMO values obtained from CV data. Measured
g
f
opt
by ultraviolet photoelectron spectroscopy (UPS). Calculated from the gap of Eg and HOMO values obtained from UPS spectra.
segment. The calculated HOMO and LUMO energy levels of P1,
P2, P3 were quantitatively in agreement with the observed
experimental results, which confirmed that the fluorine
substituted on the ortho-position of the alkoxyphenyl side
groups would lower the HOMO energy level efficiently.
The normalized UV−visible absorpꢀon spectra of three
polymers P1, P2, and P3 in chlorobenzene solution and thin
o
films at 25 C are shown in Figure 3. The spectra of the three
polymers in the chlorobenzene solution under different
temperature are also presented in Figure S4. The absorption
data were summarized in Table 2. As shown in Figure 3, these
polymers showed quite different absorption bands in
chlorobenzene solution with major absorption peaks at 560
Figure 3. Normalized UV-vis absorption spectra of P1, P2, P3 in
o
nm for P1, 544 nm for P2, and 555 nm for P3, which were
chlorobenzene solution and in the thin solid film at 25 C.
3
7
assigned to the intra-molecular charge transfer (ICT) .
Meanwhile, a strong shoulder peak at around 645 nm for P1,
and a weak one at around 623 nm for P3 were observed,
which indicated the different aggregation ability of the two
position of the phenyl substituted BDT. Therefore, high Voc for
3
5
the devices based on P2 and P3 would be expected . As a
verification of the CV results, UPS spectra of the three
polymers were also provided in Figure S2. A similar changing
trend for the HOMO of the three polymers was observed from
the UPS results (Table 2), which indicated that the comparison
of HOMO energy levels among the polymers was reliable in
this work. To further understand the above experimental
results, the calculations of energies and distributions of the
frontier molecular orbitals of these polymers were performed
by density functional theory (DFT) using the Gaussian 09
3
8
polymers in solution . Interestingly, no shoulder peak for P2 in
solution was found, which indicated that the aggregation
ability of P2 was much weaker than those of P1 and P3. With
increasing the temperature, the intensities of the shoulder
peaks of P1 and P3 were declined gradually (Figure S4a and
S4c), because the aggregations of the polymers were
weakened step by step. Finally, the shoulder peaks were
disappeared and all the three polymers exhibited similar
absorption profiles (Table 2, and Figure S4), while the
aggregations of the polymers in solutions were all broken at
high temperature. The above results indicated that the
different absorption spectra in solution at room temperature
were mainly caused by the different aggregation ability of the
three polymers, which originated from the different
substituted groups or side chains. Compared to P1, the
inclusion of the fluorine on the ortho-position of the BDTP
moiety could enlarge the dihedral angle θ1 (Table S2) between
the conjugated side chains and backbone of the polymer.
Furthermore, the electron density of conjugated backbone was
reduced because of the electron-withdrawing ability of the
3
6
program at the B3LYP/6-31G(d,p) level in the gas phase . The
optimized molecular geometries were confirmed to be
minimum-energy conformations since there were no
imaginary frequencies by vibrational calculation at the same
level. In this work, only one repeating unit was chosen, and
alkyl chains on the alkoxyphenyl substituted group and DTBTff
unit were simplified to methyl group to reduce the calculation
time. The fluorine atom substituted on BDTP was put adjacent
to the hydrogen atom on the backbone of BDT unit (see the
conformation 2 in Figure S3a) to avoid the repulsive force of
the hydrogen atom, which would increase the calculated total
energy. Figure S3b showed the molecular geometries and
electronic wave function distribution of the HOMO and LUMO
of the D−π−A model compounds. For the three polymers, the
electronic wave function of the HOMO was distributed entirely
over the conjugated molecule, which was beneficial for
2
1
fluorine on the ortho-postion , which could further weaken
the π-orbital overlap between BDT and adjacent thiophene
1, 39-41
2
rings in DTBTff moiety
2
, and thus enlarge the dihedral
9, 42
2
5
angle θ3
. Therefore, the inclusion of fluorine on the ortho-
obtaining higher hole mobility . The electronic wave function
of the LUMO was mainly localized on the electron accepting
position of BDTP can significantly tune the conformation and
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Figure 4. (a) Optimized J–V curves of PSCs based on the polymer/PC BM blend film. (b) EQE spectra of the optimized devices.
7
1
Table 3. Optimized photovoltaic performance of polymers
a
sc (mA cm-2)
PCEmax (PCEave)b (%)
polymer
P1
Ratio /Addditive
1:1.5/2% DIO
1:1.5/2% DIO
1:1.5/No additive
V
oc (V)
J
FF (%)
49.43
69.03
74.49
0.84
0.94
0.93
11.81
12.48
12.97
4.91 (4.72)
8.10 (8.05)
9.02 (8.96)
P2
P3
a)
b)
The weight ratio of polymer to PC71BM; The average PCE is obtained from 20 devices.
aggregation of P2. On the other hand, the dihedral angle θ3 in same ratio, a Voc of 1.03 V was obtained for P2, which was 0.15 V
P3 was very small, probably due to the smaller steric hindrance higher than that of P1. Obviously, the higher Voc of P2 based device
of the linear hexyl in P3 than the branched 2-ethyl-hexyl side resulted from the deeper HOMO energy level of P2. Then, 1,8-
3
2
chain in P2 , indicating that the aggregation and planarity of diiodooctane (DIO) was used as the additive to further optimize the
P3 could be strengthened in some extent compared to P2. As morphology of the above polymer/PC71BM blends with volume
discussed above, the incorporation of fluorine on the ortho fraction of 1, 2, and 3%. Unfortunately, only a small improvement of
position of alkoxyphenyl substituted BDT unit will weaken the P1-device performance was observed. The optimized amount of
aggregation of its resultant polymer (from P1 to P2), and the DIO was 2% and the device showed a PCE of 4.91% with a Voc of
2
substitute of 2-ethyl-hexyl side chains with hexyl group on the 0.84 V, a Jsc of 11.81 mA/cm , a FF of 49.43%. However, DIO has
thiophene bridge will strengthen the aggregation of the significant effect on the P2/PC71BM blend. When 2% DIO was
−2
relevant polymer (from P2 to P3). It is clear that the above two added, a Jsc of 12.48 mA cm was obtained, and the FF improved to
effects on the aggregation is opposite. In our designed 69.03%, a higher PCE of 8.10% was realized (Table 3). It should be
polymer system, the effect of former (introducing fluorine) is noted that the Voc of 0.94 V for the optimized P2/PC71BM device
more effective than that of the later one (changing the alkyl was one of the highest values derived from the polymers with
1
6, 17
side chains). As a result, P3 is more aggregated than P2.
PBDT-DTBTff backbone
. Excitingly, without any processing
additives or post-treatments, the devices based on P3 exhibited a
2
In the solid state, the main absorption profiles become
broader, and the maximum absorption peaks moved toward
longer wavelength to 603 nm for P1, 589 nm for P2, and 586
nm for P3, respectively. These large red-shifts against the
absorption in solution meant stronger interchain π−π stacking
in the solid state. The optical band gaps were estimated as
maximum PCE of 9.02%, with a Voc of 0.93 V, a Jsc of 12.97 mA/cm ,
and a FF of 74.49%, which was the first reported single junction
device with PCE over 9% based on BDT-DTBTff structure,
meanwhile, it was also the highest one for the D-A polymers with
fluorine substituted BDT as electron-donor units in single junction
polymer solar cells. The performance of P3 based devices showed
no improvement when treated with DIO (Table S3).
1.75, 1.82, and 1.81 eV for P1, P2, and P3, respectively, from
the onsets of the UV-vis absorption in the thin solid films.
The EQE spectra of P3 gave the photo-to-current response in the
wavelength range from 330-700 nm with maximum 79% at 515 nm,
as shown in Figure 4b, illustrating the high Jsc of the P3-device
obtained. The substantial EQE together with a high fill factor of
2
.3. Polymer Solar Cell Performance
Photovoltaic properties of polymers were investigated with a
device structure of ITO/PEDOT:PSS/Polymer:PC71BM/Ca/Al under
-
2
7
4.49% for the P3 device, probably reflected higher hole mobility
the illumination of AM 1.5G at 100 mA cm . The device fabrication
conditions, such as the weight ratio of polymer to PC71BM, and the
amount of the 1,8-diiodooctane (DIO) additive were carefully
optimized. As shown in Figure S5 and Table 3, the optimal D/A
and more preferable morphology of active-layer for the P3-device
4
3
−2
compared to those of P1 and P2 . The Jsc values (11.35 mA cm for
−2
−2
P1, 12.02 mA cm for P2 and 12.37 mA cm for P3) integrated
from the EQE spectra were consistent well with the Jsc recorded
from the J−V measurements.
ratios for P1, P2 and P3 were all 1:1.5. The device of P1:PC71BM at
2
1
:1.5 weight ratio showed a Voc of 0.88 V, a Jsc of 11.27 mA/cm , a
FF of 46.96%, and correspondingly a PCE of 4.67% (Table S3). At the
2
.4. Morphological Characterization of the Active Layer
6
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Figure 5. AFM height images of the active layers of P1/PC71BM (a), P1/PC71BM with 2% DIO (b), P2/PC71BM (c), P2/PC71BM with 2% DIO
(
d), P3/PC71BM (e), and P3/PC71BM with 2% DIO (f).
Figure 6. TEM images of the active layers of P1/PC71BM (a), P1/PC71BM with 2% DIO (b), P2/PC71BM (c), P2/PC71BM with 2% DIO (d),
P3/PC71BM (e), and P3/PC71BM with 2% DIO (f).
The morphologies of the blend films of polymers and PC71BM
have been examined by tapping-mode atomic force microscopy
dichlorobenzene solution containing 2.0% DIO as additive, the RMS
data increased to 9.78 nm (Figure 5b). Meanwhile, its TEM image
showed large polymer and PC71BM aggregation domains (Figure
6b), which indicated poor phase separation in the blend film. This
might be one of the reasons for the much lower FF (49.43%) for
(
AFM) and transmission electron microscope (TEM). All the blend
films were prepared under the same conditions with the optimized
photovoltaic devices. As shown in Figure 5a, the blend film of P1
and PC71BM showed a surface root mean square (RMS) roughness
4
4
the P1 based device. Compared to P1, the only difference in
chemical structure of P2 is the introduction of the fluorine on the
(
4.03 nm). While the blend film was spin-coated from o-
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Journal Name
Figure 7. 2D GIWAXS images of the polymer:PC71 BM blend films processed under optimized device fabrication conditions, P1/PC71BM
1:1.5) with 2% DIO treatment (a); P2/PC BM (1:1.5) with 2% DIO treatment (b); P3/PC BM (1:1.5) without DIO treatment (c).
(
7
1
71
Figure 8. Linecuts of 2D GIWAXS images of the polymer:PC71BM blend films under optimized device fabrication conditions.
ortho-position of BDTP. However, a quite different morphology
was observed for the blend film of P2 and PC71BM. A very smooth
surface with a small RMS roughness (1.24 nm) was found. The TEM
image of the blend film also showed a well-ordered separation
network distribution between polymer and acceptor (Figure 6c).
While the blend films were spin-coated from o-dichlorobenzene
solution containing 2.0% DIO as additive, the RMS data was
increased to 5.13 nm. The morphology change of the active layer
after treated with DIO, which would resulted in different contact
situation between the donor and acceptor domains, might be one
of the possible reasons for the decrease of Voc value for the P2-
The above results indicated that the morphology of the active
layer could be optimized by precise modification of the chemical
structure of polymer. In our work, P1 had the strongest self-
aggregation ability, which resulted in the large phase separation of
P1/PC71BM blend film. It was also consistent with their optical
properties. After introduction fluorine on the ortho-position of
BDTP segment, the aggregation ability of P2 was weakened
drastically, which made it form a preferable blend film with
PC71BM after DIO treatment. To further decrease the steric
hindrance and improve the planarity, branched 2-ethylhexyl group
was replaced with linear hexyl group in P3. As a result, the blend
film of P3 and PC71BM showed a well ordered nanoscale phase
separation without DIO treatment. It can be seen from our results
that the morphology of the active layer can be optimized from the
following two methods. One is using post-treatment technique
such as adding additive, which can tune the morphology and phase
separation of the active layer in the process of film formation. In
this work, the performance of P2 is optimized through this way.
The other pathway is to optimize the chemical structures of the
polymers, which can affect on the conformation and aggregation
of the polymers. In this work, the molecular structure of P3 with
suitable side chain can make the active layer exhibit preferable
morphology, and finally show champion performance without
additive.
4
5-47
based devices with DIO additive treatment.
d, a fibrillar-like bicontinuous interpenetrating network were
observed. This might explain the higher FF of 69.03% for the P2
As shown in Figure
6
4
8
based devices. These results indicated that the inclusion of
fluorine on the ortho-poisition of BDTP segment in P2 could
efficiently tune the aggregation of the polymer and improve the
miscibility of the polymer with PC71BM in the blend films. Polymers
P2 and P3 shared the same conjugating backbone, the only
difference was the nonchromophoric alkyl chains in the thiophene
bridge in the BDT-DTBTff backbone. However, the influence of the
DIO on the morphology of the blend film was quite different.
Without treatment by DIO, the neat blend film of P3 with PC71BM
showed a uniform interpreting structure with a RMS data of 3.84
nm (Figure 5e and 6e), while 2% DIO was added, large P3 and
PC71BM domains appeared and the RMS data of the film increased
2
.5. Hole Mobility and Crystalline Structures
49
to 7.76 nm, which might account for the decline of FF to 57.61%.
8
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To further analyze and understand the photovoltaic treatment, which was 11.4% and 83.7% higher than those of P2
performance of the PSCs, the hole mobility was measured using and P1, respectively. Note that the PCE of 9.02% is the highest
a
vertical
diode
with
the
device
structure
of value for the reported D-A polymers with BDT-DTBTff as
ITO/PEDOT:PSS/polymer:PC71BM/Au and the J–V curves were backbone in single junction polymer solar cells. Our work
shown in Fig. S6. By taking current-voltage in the range of 0-4 V indicates that a BDT unit with fluorine on the ortho-position of
and fitting the results to a space charge limited current (SCLC) alkoxyphenyl side chains is a promising electron-donor building
model, where the SCLC is described by
J
SCLC
=
(9/8) block for highly efficient light-harvesting polymers, and there is
0
is the still room for further improvement of the device performance
2
3
ɛ
0
ɛ
µ
r h
((V )/(L )), where J stands for current density, ε
permittivity of free space, ε
the transport medium, μ is the hole mobility, V is the internal alkyl side chains on the D-A conjugated polymers.
r
is the relative dielectric constant of through rational selecting the electron-acceptor segment and
h
potential in the device and L is the thickness of the active layer,
the hole mobilities for P1, P2, and P3 are calculated to be 1.7×10-
5
cm2 V-1 s-1, 5.7×10-5 cm2 V-1 s-1, and 1.5×10-4 cm2 V-1 s-1, Acknowledgements
respectively. Among the three polymers, P3 exhibits the highest
hole mobility, which is 2.6 times and 8.8 times larger than those
of P2 and P1, respectively. The higher mobility contributes to
obtaining high Jsc and FF of the P3 based devices.
Grazing incidence wide-angle X-ray scattering (GIWAXS)
method was used to further characterize the crystalline
structure of the active layer of the PSCs. In our experiment, all
the blend films were prepared on the silicon substrates
following the same fabrication conditions as used for the
optimized photovoltaic devices. As shown in Figure 7a and
This work was supported by the National Natural Science
Foundation of China (21204097, 21274161, 51173199,
5
1573205, and 61405209), the Ministry of Science and
Technology of China (2014CB643501, 2010DFA52310), and
Qingdao Municipal Science and Technology Program (14-2-4-
2
8-jch). The authors thank beamline BL16B1 (Shanghai
Synchrotron Radiation Facility) for providing the beam time. N.
Wang and W. Chen contributed equally to this work.
Figure 8, the (100) diffraction peak in the out-of-plane pattern at Notes and references
q
z
= 2.34 nm-1 was observed for the blend film of P1 and PC71BM
with 2% DIO treatment, indicating an edge-on alignment of P1
with a lamellar distance of 2.69 nm. In addition, a weak (010)
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0
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The polymer, with o-fluoro-p-alkoxyphenyl- substituted benzo[1,2-b:4,5-b’]dithiophene units as
electron-rich segment and 5,6-difluoro-4,7-di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole as
electron-withdrawing unit, shows a PCE of 9.02%.