PDT-S-T: A new polymer with optimized molecular conformation for controlled aggregation and π-π Stacking and its application in efficient photovoltaic devices

Article abstract of DOI:10.1002/adma.201301174

The correlation among molecular conformation, the crystallinity of the morphology, propensity for π-π stacking, J- versus H-aggregation, and photovoltaic performance have been studied based on two newly designed polymers, PBDTTT-S-T and PDT-S-T. The results show that more linear backbone structure is helpful to improve photovoltaic properties of the polymer, and therefore, molecular conformation should be considered for molecular design of photovoltaic polymers. Copyright

Full text of DOI:10.1002/adma.201301174

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PDT-S-T: A New Polymer with Optimized Molecular
Conformation for Controlled Aggregation and π–π Stacking
and Its Application in Efficient Photovoltaic Devices
Yue Wu, Zhaojun Li, Wei Ma,* Ye Huang, Lijun Huo,* Xia Guo, Maojie Zhang,
Harald Ade, and Jianhui Hou*
In recent years, polymer solar cells (PSCs) have attracted much
attention due to their advantages of fabricating low-cost and
light-weight solar panels through roll-to-roll printing tech-
niques.[1,2] Until now, significant improvements in power con-
version efficiencies (PCEs) have been achieved and PCEs of
8–9% have been realized with conventional, single layer bulk
heterojunction (BHJ) solar cells.[3–11] Among various photovol-
taic materials for the applications in BHJ PSCs, conjugated
polymers based on fused aromatic units are particularly inter-
esting due to their well delocalized π-conjugation along the
backbones. In the past a few years, more and more fused aro-
matic units, like benzodithiophene (BDT),[4,12,13] thieno[3,4-b]
thiophene (TT),[8] benzodifuran,[14,15] indacenodithiophene
(IDT),[16] indacenodithienothiophene (IDTT),[17] dithienosilole
(DTS),[18] dithienogermole(DTG),[19] dithienobenzodithiophene
(DTBDT),[20] and so on, have been introduced into photovol-
taic polymers and contributed a lot to the development of
PSCs.[21–25] Besides the design of conjugated polymers with
different backbone structures, the modulation of the polymers’
molecular energy levels by changing the electron pushing or
withdrawing effects of their side groups has also attracted much
attention as molecular design motif for photovoltaic polymers.
Many excellent examples have been reporting this strategy.[4,5,26]
Molecular configuration and conformation are two impor-
tant but seldomly investigated structural motifs for molecular
design of photovoltaic polymers. The study of regio-regular
poly(3-hexylthiophene) (P3HT) provides the first and also the
most important example of optimizing molecular configuration
to improve photovoltaic properties of conjugated polymer.[27]
For example, highly crystalline structure can be formed in the
regio-regular P3HT, while the blend of regio-random P3HT is
amorphous.[28,29] The crystallization forces favorable morpholo-
gies that yield PCE of the PSC based on regio-regular P3HT and
PCBM of over 4%,[30,31] which is 2–6 times higher than that of
the PSC based on regio-random P3HT and PCBM.[32] In addi-
tion to configuration, the morphology of photovoltaic devices
might also be affected by the polymer's molecular conformation.
For instance, polymers with straight linear backbone structure
should be more likely to realize highly crystalline morphology
than polymers with branched or zigzagged liner backbones.
However, in the molecular design of photovoltaic polymers, the
correlation among molecular conformation, the crystallinity of
the morphology, propensity for π–π stacking, J- versus H-aggre-
gation, and photovoltaic performance has been seldomly studied
possibly due to the lack of model polymer systems.
The low band gap polymer system based on BDT and TT,
known as PBDTTTs, exhibited efficient photovoltaic proper-
ties.[4,26] Recently, several interesting results related to the
molecular design and the application of PBDTTTs in highly effi-
cient PSCs have been reported.[3,33–35] Interestingly, according
to the reported results, most of the PBDTTT polymers are
amorphous.[34,36] Therefore, figuring out why PBDTTTs are
amorphous and then to find a method to improve their crystal-
linity or π–π stacking through molecular structure design will
be helpful to achieve in-depth understanding of photovoltaic
polymers. In this work, we tried to use molecular conformation
as the root cause to interpret why PBDTTT-S-T (see Scheme 1)
is amorphous. A new polymer, PDT-S-T as shown in Scheme
1, which has similar molecular structure as PBDTTT-S-T but
different backbone conformation, was designed to realize more
ordered morphology in thin films. The morphological and pho-
tovoltaic results indicate that PDT-S-T exhibits more ordered
structure and better photovoltaic properties than PBDTTT-S-T,
that is, PCEs of the PSCs based on PBDTTT-S-T and PDT-S-T
are 5.93% and 7.79%, respectively. Besides of reporting a new
photovoltaic polymer with enhanced photovoltaic performance,
we correlate structural characteristics as revealed by X-ray dif-
fraction, tapping mode atomic force microscopy (AFM) and
transmission electron microscope (TEM), and photovoltaic
performance with the polymers’ molecular conformations, and
argue that molecular conformation should be considered in
photovoltaic polymer design.
Y. Wu, Z. Li, Y. Huang, Prof. L. Huo, X. Guo,
M. Zhang, Prof. J. Hou
State Key Laboratory of Polymer Physics
and Chemistry
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
E-mail: huolijun@iccas.ac.cn; hjhzlz@iccas.ac.cn
W. Ma, Prof. H. Ade
Department of Physics
North Carolina State University Raleigh
NC 27695, USA
E-mail: wma5@ncsu.edu
Y. Wu, X. Guo
Graduate University of Chinese Academy of Sciences
Beijing 100049, China
As demonstrated in Scheme 1. PBDTTT-S-T has zigzagged
linear backbones due to the conformational nature of BDT and
DOI: 10.1002/adma.201301174
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Scheme 1. Molecular structures and backbone conformations of PBDTTT-S-T and PDT-S-T. The molecular conformations of these two backbones are
obtained through the density functional theory (DFT) at the B3LYP/6-31G(d,p) level.
TT building blocks and the zigzagged backbone of PBDTTT-
S-T shows an angle of 36°, which may have negative influ-
ence on realizing crystalline morphology in solid film. On the
other hand, the bond angel of C–S–C in the sulfonyl group
is 108°, which causes strong steric hindrance to the adjacent
side groups, so that the alkyl in the sulfonyl cannot lie in the
same plane with the backbone.[34] Therefore, it should be hard
to obtain ordered packing in the solid film of PBDTTT-S-T. As
demonstrated in Scheme 1, from PBDTTT-S-T to PDT-S-T, the
replacement of BDT with DTBDT gives three potential advan-
tages: first, the backbone of PDT-S-T can be arranged with more
linear conformation; second, the distance between the adjacent
side groups is big enough to allow the alkyls on the sulfonyl
groups to keep in the same plane with the backbone; third, the
intermolecular π−π stacking of the polymer will be enhanced.
Hence, based on the DFT calculations, it can be predicted that
both inter-chain packing and inter-planar π−π stacking should
be more ordered and intermolecular π−π stacking should be
more compact for the solid film of PDT-S-T than for films of
PBDTTT-S-T.
The synthesis approaches for these two polymers are shown
in Scheme 2. The alkoxy-substituted DTBDT(1) was prepared
by the method reported in our previous work.[20] Ammonium
ceric nitrate (CAN) was used to oxidize the alkoxy-substituted
DTBDT(1) in acetonitrile and the corresponding quinone com-
pound (2) can be obtained easily with a yield in 75%. Then,
the thienyl-substituted DTBDT monomer (DTBDT-T) can
be prepared in high yields by the similar method as used in
the synthesis of thienyl-substituted BDT derivatives.[3] The
bis(trimethyltin)-BDT monomer (BDT-T) is commercial avail-
able. The two dibromo monomers (TT-S1 and TT-S2) were syn-
thesized by the same procedure.[37] The copolymers of PBDTTT-
S-T and PDT-S-T were prepared through the Stille coupling
reaction between the bis(trimethyltin) monomers (BDT-T and
DTBDT-T, respectively) and the dibromo monomers (TT-S1 and
TT-S2, respectively) as shown in Scheme 2. These two polymers
exhibit good solubility in commonly used organic solvents
such as chloroform, toluene, xylenes, and o-dichlorobenzene
(o-DCB),aswellasgoodthermalstabilitybelow350°Cunderinert
atmosphere (see Figure S1, Supporting Information). Through
Scheme 2. Synthesis procedures of PBDTTT-S-T and PDT-S-T. i) CAN, acetonitrile, ambient temperature, 1 h; ii) 2-(2-ethylhexyl)-thiophene, butyl-
lithium, THF, inert atmosphere, 50 °C, 1 h; then SnCl2, HCl/H2O, inert atmosphere, 50 °C, 1.5 h; iii) LDA, THF, inert atmosphere, –78 °C, 1 h, then
trimethyltin chloride, inert atmosphere, ambient temperature, 1 h; iv) Pd(PPh3)4, Toluene/DMF (10:1, v/v), inert atmosphere, reflux, 8 h.
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that molecular design indeed leads to strong coupling along the
polymer backbone, requiring the functional groups to stay in a
well defined plan without twisting. The reduction on J-aggre-
gate character in the thin film would indicate either twisting
due to disorder or increased chain-to-chain coupling, that is,
H-aggregation via π–π coupling.
The insert in Figure 1 shows the electrochemical cyclic vol-
tammetry (CV) plots of these two polymers. The onset p-doping
potentials (Eox) of PBDTTT-S-T and PDT-S-T are 0.59 V and
0.51 V, corresponding to the highest occupied molecular orbital
(HOMO) levels of –5.29 eV and –5.21 eV, respectively.[38]
Clearly, the HOMO level of PDT-S-T is slightly higher than
that of PBDTTT-S-T, implying that the DTBDT unit may have
stronger electron donating effect than the BDT unit. Consid-
ering that open circuit voltage (Voc) of PSCs is closely related
to the offset between the HOMO level of the electron donor
material and the lowest unoccupied molecular orbital (LUMO)
level of the electron acceptor material,[12,39] the replacement of
BDT with DTBDT should have some negative influence on Voc
in PSCs, that is, the PSC based on PDT-S-T might exhibit lower
Figure 1. UV-Vis absorption spectra of the solutions in chloroform and
the solid films of PBDTTT-S-T and PDT-S-T; Insert: Cyclic voltammogram
plots of PBDTTT-S-T and PDT-S-T films on glassy carbon working elec-
trode in 0.1 M Bu4NPF6 in CH3CN at a scan rate of 20 mV s−1
.
Voc than the PSC based on PBDTTT-S-T.
Grazing incidence X-ray diffraction (GIXD) has been widely
used to characterize the formation of ordered structures
within the thin films of these two polymers. The two-dimen-
sional GIXD profiles of these two pure polymers are shown in
Figure 2a and the out-of-plane and the in-plane GIXD profiles
extracted from the 2D profiles are provided in Figure 2b. Since
the diffraction signal near 1.6–1.9 Å−1 is due to the (010) π–π
differential scanning calorimetry (DSC) measurements, we
found that no endo- or exothermal signals can be observed in
the range of 0–280 °C from the second heating and cooling
runs (10 °C min−1 ).
The ultraviolet-visible (UV–vis) absorption spectra of
PBDTTT-S-T and PDT-S-T in diluted solutions and as solid thin
films are shown in Figure 1 and the detailed parameters are
colleted in Table 1. The solutions and the solid films of these two
polymers show almost identical absorption onset at ≈780 nm,
corresponding to an optical band gap (Egopt) of 1.59 eV. The
molar extinction coefficients of PBDTTT-S-T and PDT-S-T
−1
−1
are 4.57 × 104
M
cm−1 and 4.40 × 104
M
cm−1 respectively,
which are among the highest values in organic semiconduc-
tors. However, the main absorption peak of PDT-S-T is located
at 702 nm, which is red-shifted be ≈30 nm compared to that of
PBDTTT-S-T. Interestingly, the absorption in short wavelength
direction of PDT-S-T is also extended slightly compared to that
of PBDTTT-S-T. Overall, the broader absorption band of PDT-
S-T will be beneficial to utilizing more sunlight in PSCs, thus
leading to enhanced short circuit current density (Jsc) in device.
We also note that PDT-S-T is an exceptionally strong J-aggre-
gate in solution (on account of its strong low energy absorption
peak), which is slightly reduced on the solid film. This indicates
T a b le 1 . Optical absorption properties and molecular energy level data
of the polymers.
Polymer
Solutiona)
Film
Egopt [eV] HOMO [eV] LUMO [eV]
λ
max [nm]/
λ
onset [nm]
ε [M−1 cm−1
]
PBDTTT-S-T
PDT-S-T
676/4.57
× 104
768
778
1.61
1.59
–5.29
–5.21
–1.00
–1.08
700/4.40
× 104
Figure 2. a) GIXD 2D scattering of thin films of PBDTT-S-T and PDT-S-T.
b) Out-of plane (solid lines) and in-plane (dashed lines) sector average
profiles extracted from (a); GIXD profiles between polymers have been
shifted vertically for ease of comparation.
a)in diluted chloroform.
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stacking, and both pure films show clear diffraction signal in
out-of-plane direction but very weak signal in the in-plane direc-
tion, it can be concluded that the molecules in both two pure
films are dominantly arranged with face-on structure. In the
PBDTTT-S-T, the weak (010) π–π stacking peak is at 1.62 Å−1 ,
corresponding to a d-spacing of 3.88 Å. For the PDT-S-T film,
a strong (010) π–π stacking peak at 1.71 Å−1 can be observed,
corresponding to a d-spacing of 3.67 Å, one of the smallest π–π
stacking distances for conjugated polymers observed to date. In
the low q range, a (100) peak was observed only for PDT-S-T,
which is weak and broad and centered at q ≈ 0.32 Å−1 , corre-
sponding to a lamellar d-spacing of 19.6 Å. The observation in
GIXD measurements clearly indicates that the polymer’s mole-
cules can be arranged more ordered by the replacement of BDT
with DTBDT, which is consistent with the material design pro-
tocol as demonstrated above, that is, the straight linear confor-
mation of the backbone is more favorable to form ordered inter-
chain packing than the zigzagged conformation. Additionally,
the reduced steric hindrance between the adjacent alkyl side
groups is helpful to get more ordered and also more compact
π−π stacking.
The results of the GIXD and the UV-vis measurements
shown above can be interpreted as following. As demon-
strated in Scheme 3, since PBDTTT-S-T has zigzagged linear
backbone conformation, ordered inter-chain packing can be
formed only under the favorable mode and thus the polymer
aggregation will contribute to the (100) peak in GIXD profile.
Under the unfavorable mode, the inter-chain distance between
the adjacent main chains are varied along the backbones, so
that the inter-chain packing will be non-ordered. However,
the d-spacing between the adjacent backbones of PDT-S-T is
almost constant under the general packing mode, because
this polymer has more linear backbone conformation than
PBDTTT-S-T, and therefore, the film of PDT-S-T shows a clear
(100) peak in the out-of-plane profile of the GIXD analysis.
On the other hand, since it is harder to form ordered struc-
ture for inter-chain packing in PBDTTT-S-T than PDT-S-T, the
ordered inter-chain face-to-face π−π stacking will be definitely
weakened in PBDTTT-S-T. Therefore, the PBDTTT-S-T film
shows a much weaker (010) scattering peak than the PDT-S-T
film in GIXD profile. Although the packing modes shown in
Scheme 3 are extreme examples presumed according to the
optimized backbone structures, it provides a reasonable inter-
pretation for the different behaviors of these two polymers in
GIXD analysis. Overall, the concept that the linearity and the
planarity of backbone conformation play important roles in
the formation of ordered morphology for conjugated polymers
provides helpful guidance for molecular design of photovoltaic
polymers.
Photovoltaic properties of these two polymers were inves-
tigated by fabricating PSC devices with a structure of ITO/
PEDOT:PSS/Polymer:PC71BM/Ca/Al. o-DCB was used to pre-
pare the solutions for spin-coating of active layers and a concen-
trations of 10 mg mL−1 (calculated for polymer) was used in this
work. Initially, different donor/acceptor (D/A) ratios (polymer/
PC71BM, wt/wt), 1:1, 1:1.5 and 1:2, were scanned to find the
best weight ratio of device fabrication and the thickness of the
active layers used in this work is ≈110 nm 5 nm. The current
density−voltage (J−V) curves of the PSC devices under the illu-
mination of AM 1.5 G, 100 mW cm−2 are shown in Figure 3a
and the photovoltaic parameters are collected in Table 2. When
varying the mixture ratio of polymer/PC71BM, the values of Voc
seems to be depending on the ratio. For PBDTTT-S-T, the Voc
varying from 0.83 to 0.81 V and for PDT-S-T, the Voc varying
from 0.76 to 0.72 V. The phenomenon is quite similar to the
reported works on PIDTDTQx/PCBM and MEH-PPV/PCBM
photovoltaic systems. According to the literature reports, this
may be caused by the formation of PCBM cluster or increased
permittivity.[40,41] From Table 2 it can be seen that the optimum
D/A ratio of both copolymers were 1:1.5. Under the optimal
donor/acceptor weight ratio, the PSC based on PBDTTT-S-T
shows a PCE of 5.0% with Jsc = 11.79 mA cm−2 , Voc = 0.81 V
and FF = 0.52; the PSC based on PDT-S-T exhibited a PCE of
7.79% with Jsc = 16.63 mA cm−2 , Voc = 0.73 V and FF = 0.64.
Solvent additives, like 1,8-diiodooctane (DIO) and octane-1,8-
dithiol (DT), play an important role in optimizing photovoltaic
performance of PSCs, and according to the reported works,
when PBDTTTs were used as electron donor materials in PSCs,
the addition of DIO was always helpful to improve photovoltaic
performance of the devices.[4,8,42] Therefore, we used DIO to
attempt further improvements in device performance of these
two polymers, and different volume ratios of DIO (v/v, DIO/
o-DCB), 0.5%, 1%, 2% and 3%, were employed to find the
optimal composition of the processing solvent. The J−V curves
of the PSC devices prepared by the corresponding solvent mix-
tures are shown in Figure 3b and the photovoltaic parameters
are collected in Table 3. Similar to other PBDTTT derivatives,
photovoltaic performance of the PBDTTT-S-T devices can also
be improved after adding DIO, that is, after adding DIO, both Jsc
and FF improved but Voc dropped slightly. The champion device
(PCE = 5.93%) was obtained when 2% DIO was added. Very
interestingly, the blend of PDT-S-T and PC71BM show much
different behaviors upon the treatment of
DIO. As shown in Figure 3b and also listed in
Table 3, when the content of the DIO in the
solvent was stepwise increased from 0.5% to
3%, FF of the PDT-S-T based device dropped
gradually from 0.641 to 0.508 while Voc and Jsc
of the devices changed a little, and therefore,
the overall efficiency dropped to 5.81%. To
the best our knowledge, PDT-S-T is the first
photovoltaic polymer that exhibits efficient
photovoltaic performance above 7% PCE
without any further treatment such as use of
additives or annealing in device fabrication.
Scheme 3. The demonstration of the top views of the packing modes in PBDTTT-S-T and
PDT-S-T.
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T a b le 2 . Photovoltaic results of the PSC devices based on PBDTTT-S-
T:PC71BM and PDT-S-T:PC71BM with different D/A ratios under the illu-
mination of AM 1.5G 100 mW cm−2
.
Jsc [mA cm−2
]
Polymer
D:A
1:1
Voc [V]
0.83
0.81
0.81
0.76
0.73
0.72
FF [%]
48.57
52.12
54.92
59.65
64.13
63.48
PCE [%]
4.60
PBDTTT-S-T
11.41
1:1.5
1:2
11.79
5.00
9.91
4.41
PDT-S-T
1:1
14.36
6.51
1:1.5
1:2
16.63
7.79
14.75
6.74
from J-V measurements is within 5%, indicating that the J-V
measurements in this work are reliable.
AFM and TEM were used to investigate the influence
of the addition of DIO on the morphology of the blend
films. Figure 4 shows the TEM images of the thin films of
PBDTTT-S-T:PC71BM and PDT-S-T:PC71BM blends. For the
PDT-S-T:PC71BM blend, when the film was prepared by o-
DCB only, moderate phase separated morphology are formed
(see Figure 4a,b) and the RMS roughness of the film is
0.95 nm; after increasing the volume ratios of DIO in the pro-
cessing solvent, the blend films show stronger phase separation
and greater surface roughness (see Figure 4d,e; Figure S2, Sup-
porting Information). For example, the RMS roughness of the
film processed by adding 2% volume ratios of DIO as solvent
additive increased to 1.93 nm. According to the TEM images
(see Figure 4c,f), when DIO was used as additive, the phase
separation in the PDT-S-T/PC71BM blend increased slightly. For
the PBDTTT-S-T:PC71BM blend, when the film was prepared
by using only, phase separation in the blend is quite severe so
that big size aggregations are formed in the Figure 4i, that is
>100 nm dark domains can be observed; when 2% DIO was
used as additive to process the blend, the film gives a homo-
geneous morphology in Figure 4l. The results from AFM and
TEM measurements indicate that optimal phase separation can
be formed in the PDT-S-T/PC71BM blend by using pure as pro-
cessing solvent, and the use of DIO will induce stronger phase
separation as well as rougher surface of the film, resulting in
lower photovoltaic performance; however, the morphology of
T a b le 3 . Photovoltaic results of the PSC devices based on PBDTTT-S-
T:PC71BM (1:1.5) and PDT-S-T:PC71BM (1:1.5) processed by adding dif-
ferent amount of DIO under the illumination of AM 1.5G 100 mW cm−2
.
Figure 3. J-V curves of the PSC devices based on PBDTTT-S-T:PC71BM
and PDT-S-T:PC71BM under the illumination of AM 1.5G 100 mW cm−2
;
a) the PSC devices processed with different D/A ratios; b) the PSC devices
with D/A = 1:1.5 processed by adding different amounts of DIO additive;
c) EQE curves of the photovoltaic cells of both polymers under prepared
by the optimal conditions.
Jsc [mA cm−2
]
Polymer
DIO/o-DCB
[v/v,%]
Voc [V]
FF [%]
PCE [%]
PBDTTT-S-T
0.5
1
0.78
0.78
0.78
0.78
0.71
0.71
0.71
0.70
11.28
11.33
12.27
12.06
16.38
16.68
16.30
16.32
59.30
61.40
61.98
59.97
62.39
56.90
54.88
50.87
5.22
5.43
5.93
5.64
7.26
6.74
6.35
5.81
The external quantum efficiency (EQE) curves of the two
kinds of devices prepared by the optimal conditions are plotted
in Figure 3c. It is quite clear that EQE of the PDT-S-T based
device is higher in the whole range than that of PBDTTT-S-T
based device. According to the EQE curves, the integral current
density values of the devices based on PBDTTT-S-T and PDT-S-
T are 11.84 mA cm−2 and 15.78 mA cm−2 , respectively. The mis-
match between the integral values and the Jsc values obtained
2
3
PDT-S-T
0.5
1
2
3
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shows a higher PCE of 7.79% without using
any special treatment, suggesting that more
linear backbone would be helpful to improve
photovoltaic properties of the photovoltaic
polymer. More importantly, since the results
clearly shows that morphology and, thus,
photovoltaic properties of the conjugated
polymers correlate with their molecular con-
formation closely, the control of molecular
conformation would be as a useful tool to
modulate photovoltaic properties of conju-
gated polymers.
Supporting Information
Detailed information for material, instruments,
synthesis procedures of the polymers, and device
fabrication processes are listed in the Supporting
Information, available from the Wiley Online Library
or from the author.
Acknowledgements
This work was supported by the National High
Technology Research and Development Program
863 (2011AA050523), Chinese Academy of
Sciences (KJ2D-EW-J01), Ministry of Science and
Technology of China, NSFC (Nos. 51173189,
21104088), International S&T Cooperation
Program of China (2011DFG63460). X-ray analysis
by WM and HA was supported by the US Dept.
of Energy Office of Science, Basic Energy Science,
Division of Materials Science and Engineering
under Contract No. DE-FG02-98ER45737. X-ray
data was acquired the Advanced Light Source,
which is supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the
U.S. Department of Energy under Contract No.
DE-AC02-05CH11231.
Figure 4. Tapping mode AFM topography, phase and TEM images of a–c) PDT-S-T:PC71BM
(1:1.5 processed by o-DCB), d–f) PDT-S-T:PC71BM (1:1.5 processed by o-DCB with 2% DIO
additive), g–i) PBDTTT-S-T:PC71BM (1:1.5 processed by o-DCB) and j–l) PBDTTT-S-T:PC71BM
(1:1.5 processed by o-DCB with 2% DIO additive). The size of the AFM images is 5 μm × 5 μm.
Received: March 14, 2013
Revised: April 15, 2013
Published online:
the blend of PBDTTT-S-T:PC71BM can be optimized by adding
DIO.
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In conclusion, in order to investigate the correlation among
polymers’ molecular conformation, morphology, and pho-
tovoltaic properties, two new modeling polymers, named as
PBDTTT-S-T and PDT-S-T, were designed, synthesized, and
applied in PSCs. Since the backbone of PDT-S-T has a more
linear conformation compared to that of PBDTTT-S-T, more
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S-T. On the other hand, from PBDTTT-S-T to PDT-S-T, the dis-
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