A diketopyrrolopyrrole-based low bandgap polymer with enhanced photovoltaic performances through backbone twisting

Article abstract of DOI:10.1039/c6ta08130d

A diketopyrrolopyrrole (DPP)-based π-conjugated polymer is a very promising low band gap electron donor material for polymer solar cells. We have incorporated alkyl groups into the 4-position of the thiophene rings connected to the DPP fragment, which is proven to be beneficial for improving the open-circuit voltage (V_OC) and short-circuit current density (J_SC). Two DPP-based polymers are synthesized with benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron-donating segment. Both the polymers show good solubility, slightly wide optical band gaps and deep HOMO energy levels when incorporating the alkyl groups. Heterojunction solar cells are fabricated with polymer:PC₇₁BM as the active layer. V_OC and J_SC are simultaneously enhanced compared to the device performance of traditional DPP-BDT alternating polymers. A power conversion efficiency (PCE) of 8.11% was obtained, which indicates that rational utilization of backbone torsion is a promising strategy to improve the photovoltaic performance.

Full text of DOI:10.1039/c6ta08130d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Diketopyrrolopyrrole-based low bandgap polymer with enhanced
photovoltaic performances through backbone twisting
Received 00th January 20xx,
Accepted 00th January 20xx
Shuguang Wen,a Weichao Chen,a Meijie Fan,a,b Linrui Duan,a Meng Qiu,a Mingliang Sun,b Liangliang
Han,a* Renqiang Yanga*
DOI: 10.1039/x0xx00000x
Diketopyrrolopyrrole (DPP)-based -conjugated polymer is a very promising low band gap electron donor material for
polymer solar cell. We have incorporated alkyl groups to the 4-position of the thiophene rings connected to the DPP
fragment, which is proven to be beneficial for improving the open-circuit voltage (VOC) and short-circuit current density
(JSC). Two DPP-based polymers are synthesized with benzo[1,2-b:4,5-b']dithiophene (BDT) as electron-donating segment.
The both polymers show good solubility, slightly wide optical band gaps and deep HOMO energy levelswhen incorporating
the alkyl groups. Heterojunction solar cells are fabricated with polymer:PC71BM as active layer. VOC and JSC are
simultaneously enhanced compared to the device performance of traditional DPP-BDT alternating polymers. Power
conversion efficiency (PCE) of 8.11% was obtained, which indicates that rational utilization of backbone torsion is a
promising strategy to improve the photovoltaic performance.
www.rsc.org/
for polymer PBDTT-DPP.14 In order to avoid large alkyl group, it
would be a feasible approach to incorporate an additional alkyl
group to the thiophene ring. Although conjugated backbone twist
Introduction
In the past decades, polymer solar cells (PSCs) have attracted
considerable attention as an approach to direct utilization of sun light
due to their advantages such as mechanical flexibility, light weight,
low cost and solution processable for large area device fabrication.1-3
Novel materials4 and device fabrication methods5 have emerged
rapidly for improving the photovoltaic performance through
optimization of energy level, film morphology, charge carrier
mobility and so on. Encouraging power conversion efficiencies
(PCEs) of over 10% have been achieved in laboratory for single-
junction bulk-heterojunction (BHJ) PSCs.6-11
would be induced by this approach, it is reported that the controlled
degree of distortion can improve the PSC performance for
polythiophene.15 We have reported that the incorporation of
sterically hindered cyclic alkyl groups to DTBT-based conjugated
polymers can improve their photovoltaic performance. Interestingly,
the open-circuit voltage (VOC), short-circuit current density (JSC) and
fill factor (FF) were simultaneously increased, which demonstrated
that steric hindrance effect is an important approach for the design of
highly efficient photovoltaic materials.16 In this article, two new
DPP-based conjugated polymers with different side chains were
synthesized and the steric hindrance effect on the performance was
investigated. Two alkyl groups 2-ethylhexyl and cyclohexyl were
introduced to the 4-position of thiophene connected to DPP unit,
respectively, which have not been reported previously. Two
polymers PDPP-EH and PDPP-CH (See Scheme 1) based on DPP
and benzo[1,2-b:4,5-b']dithiophene (BDT) were synthesized by
Stille coupling polymerization. These polymers exhibit good
solubility, and meanwhile, both the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels are decreased. With the optimization of solar
cell devices, the efficiencies of 7.25% and 8.11% were obtained for
PDPP-EH and PDPP-CH, respectively, which is significantly higher
than the reported DPP-BDT alternating copolymers with naked
thiophene units.14
Low band gap (LBG) conjugated polymers exhibit bright prospect
on photovoltaic application for its wide optical absorption and great
potential for tandem solar cell. Diketopyrrolopyrrole (DPP)-based
polymers are intensively investigated as representative LBG
materials for single and double junction solar cells.12 These materials
show wide absorption range and good coplanar structure, which
would be in favor of light harvest and high carrier mobility.13 In
general, DPP-based polymers are incorporated with long side chains
on N-atoms to warrant the solution processability. And the adjacent
thiophene is not substituted at 3- and 4- positions in order to keep the
planar structure. Yang’s group has previously reported that the
incorporation of large alkyl group could induce the reduction of PCE
a. CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and
Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China. *E-
mail: hanll@qibebt.ac.cn; yangrq@qibebt.ac.cn
b. Institute of Materials Science and Engineering, Ocean University of China,
Qingdao 266100, China
Electronic Supplementary Information (ESI) available: [details of polymer synthesis,
hole mobility and device performance]. See DOI: 10.1039/x0xx00000x
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polymer PBDTT-DPP (1.44 eV),14 which may be caused by the
View Article Online
DOI: 10.1039/C6TA08130D
distortion of backbone.18
Electrochemical cyclic voltammetry (CV) measurements were
carried out to determine the HOMO and LUMO levels of conjugated
polymers.19 As shown in Fig. 1(b), the onsets of oxidation potentials
of PDPP-EH and PDPP-CH were 0.95 and 0.98 V, respectively, with
the corresponding calculated HOMO levels of -5.36 and -5.39 eV,
which are lower than that of PBDTT-DPP. The LUMO levels were
opt
Scheme 1 Synthetic route and molecular structures of DPP-
calculated from the equation: ELUMO = EHOMO + Eg and the
based copolymers.
corresponding LUMO levels of -3.83 and -3.89 eV were obtained,
which are higher than that of PC71BM and hence guarantee the
exciton dissociation.20 The energy levels are summarized in Table 1.
Results and discussion
Synthesis and characterization
(a)
In order to investigate the effect of backbone twist on the
photovoltaic performance, 2-ethylhexyl and cyclohexyl groups are
introduced to the 4-position of thiophene group of DPP-monomer
with 2-ethylhexyl and 2-butyloctyl side chains on the nitrogen atom,
respectively (Scheme 1). The detailed synthetic procedures of DPP-
based monomers and copolymers are described in Supporting
Information.17 Two polymers of PDPP-EH and PDPP-CH were
prepared by Stille coupling reaction between DPP and BDT
monomers. These polymers can be readily soluble in common
organic solvents, such as chloroform (CF), tetrahydrofuran (THF),
chlorobenzene (CB), and 1, 2-dichlorobenzene (DCB). The number-
average molecular weights (Mn) of PDPP-EH and PDPP-CH are
158.7 and 148.2 KDa, with polydispersity indexes (PDIs) of 3.25
and 2.62, respectively, as determined by gel permeation
chromatography (GPC) with THF as an eluent calibrated with
polystyrene standard.
(b)
Thermal stabilities of polymers were investigated by
thermogravimetric analysis (TGA) under nitrogen atmosphere, as
shown in Fig. S1. The decomposition temperatures of PDPP-EH and
PDPP-CH at 5% weight loss are 423 and 403 °C, indicating high
thermal stability of the polymers for application in optoelectronic
devices.
Optical and electrochemical properties
Fig. 1 (a) UV−vis absorption spectra of PDPP-EH and PDPP-CH in
chloroform solutions and as thin films. (b) Cyclic voltammograms of
the polymer films measured in Bu4NPF6 solution (0.1 M in
acetonitrile) at a scan rate of 50 mV s-1.
The UV-vis absorption spectra of two polymers in chloroform
solutions and as thin films are shown in Fig. 1(a) and the data are
summarized in Table 1. Two polymers exhibit very similar
absorption profiles due to the identical conjugated backbone. The
optical band gaps of PDPP-EH (1.53 eV) and PDPP-CH (1.50 eV)
are comparatively larger than that of reported DPP-BDT alternating
Table 1 Summary of optical and energy levels of polymers.
polymers
PDPP-EH
PDPP-CH
λsol (nm)a
691, 762
683, 763
λfilm (nm)a
700, 771
698, 770
λonset (nm)b
810
Egopt(eV)c
1.53
1.50
HOMO (eV)d
-5.36
LUMO (eV)e
-3.83
827
-5.39
-3.89
a the absorption peaks in chloroform solution and as thin films, respectively. the onset absorption of polymer as thin film. c Calculated from
b
1240/λonset.
d Determined from the onset of oxidation wave. e Calculated by the equation: ELUMO = EHOMO + Egopt (eV).
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Table 2 Dihedral angles (deg) along the conjugated backbone for the identical backbone to PDPP-EH/CH is used as the reference and the
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14
DOI: 10.1039/C6TA08130D
optimized molecular geometries obtained by DFT evaluated at the
B3LYP/6-31G(d) Level
photovoltaic properties are cited directly from literature.
Firstly, polymer/PC71BM blend ratios (w/w) of 1:1, 1:2 and 1:3 were
evaluated (Table S1) and it was found that the ratio of 1:2 (w/w)
showed the best performance for both polymers. PDPP-EH and
PDPP-CH exhibited good PCEs of 7.25% and 6.66%, respectively.
As DIO was added, the efficiency of PDPP-EH was reduced to
6.24%. Interestingly, the efficiency of PDPP-CH was increased to
8.11%, mainly due to the increase of JSC from 14.6 to 17.5 mA/cm2.
Compared to the reported polymer PBDTT-DPP, PDPP-EH and
PDPP-CH exhibit higher VOC, JSC and PCEs. The VOC is increased as
predicted, which is in accordance with their HOMO levels.23
Meanwhile, the JSC is also increased, which indicates that an
improved molecular packing and film morphology are formed upon
the introduction of steric hindered alkyl groups. However, PDPP-CH
shows a slightly lower FF compared to PDPP-EH, which indicates
more intrinsic charge traps are formed in the blend film. Structure
disorder could induce the formation of charge traps and
recombination.24 PDPP-CH is more twisted and the crystallization is
weaker than PDPP-EH, which can be concluded from XRD pattern.
Therefore, PDPP-CH exhibits a slightly lower FF value.
polymer
R
θ1 (deg)
13.30
13.69
9.48
θ2 (deg)
-39.86
-42.17
-9.90
PDPP-EH
PDPP-CH
PBDTT-DPP
ethylhexyl
cyclohexyl
H
Theoretical studies
In order to further investigate the effect of sterically hindered
alkyl group on the polymer structure, density functional theory
(DFT) calculations were performed at the B3LYP/6-31G(d) level
in Gaussian 09 program.21 For computational simplification, the
alkyl side chains on the BDT and DPP units were replaced with
methyl group and the backbone was simplified to one repeating The external quantum efficiency (EQE) curves for the best devices
unit (n = 1). The structures with group of ethylhexyl, cyclohexyl of PDPP-EH and PDPP-CH are shown in Fig. 2b. Both polymers
and H on the thiophene unit stand for PDPP-EH, PDPP-CH and exhibit broad photo-response from 400 to 800 nm. The JSC values
PBDTT-DPP, respectively.
integrated from the EQE spectra are 14.6 and 16.9 mA/cm2,
respectively, which are consistent well with JSC obtained from the J-
V measurements.
The optimized molecular geometries of the models are shown in Fig.
S2 and dihedral angels are given in Table 2. The torsion angles of θ1
are varied from 9.48o to 13.69o, indicating that thiophene and DPP
have good coplanar conformation for all three polymers.
Comparatively, torsion angle θ2 shows large difference ranging from
-9.9o for PBDTT-DPP to -42.17o for PDPP-CH with the
incorporation of alkyl group. PBDTT-DPP shows small torsion
angle with a nearly coplanar backbone structure. PDPP-EH and
PDPP-CH exhibit large torsion angle on backbone and PDPP-CH
with cyclohexyl group is the most twisted. Although the coplanar
structure is in favor of stacking effect and charge mobility, it would
also induce strong self-aggregation of polymer. Incorporation of
long side chain could improve the solubility, while this approach
would increase insulating content from the side chain and decrease
the crystallinity.22 Interestingly, the appropriate distortion in
backbone as the designed polymers would improve the solution
processibility and might increase photovoltaic performance.
(a)
(b)
Photovoltaic properties
The photovoltaic properties of polymers PDPP-EH and PDPP-CH
were investigated by fabricating the BHJ solar cells with the
conventional structure of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al.
The active layers were spin-coated from DCB solution and 1, 8-
diiodooctane (DIO) was added to optimize the blend film
morphology. All devices were characterized under a solar simulator
AM 1.5G at 100 mW/cm2. The details of the device fabrication and
characterization are given in the supporting information. The J-V
curves are shown in Fig. 2a and the photovoltaic parameters are
summarized in Table 3. The reported polymer PBDTT-DPP with
Fig. 2 (a) J-V curves of polymer solar cells based on PDPP-EH and
PDPP-CH under illumination of AM 1.5G, 100 mW cm-2. (b) EQE
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curves of the best PSCs based on the two polymers PDPP-EH and tapping-mode atomic force microscopy (AFM) and transmission
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DOI: 10.1039/C6TA08130D
PDPP-CH.
electron microscopy (TEM). In AFM images (Fig. S4), the surface
of PDPP-EH blend film obtained without DIO shows evenly
distributed small grains. As DIO added, the surface becomes much
coarser and the root-mean-square (rms) roughness increases from
1.66 to 12.2 nm, which is probably caused by the aggregation of
polymer and PC71BM in the active layer.28 As for the blend film of
PDPP-CH, the surface morphology showed mild evolution and the
rms roughness increased slightly from 1.71 nm to 3.35 nm upon the
addition of DIO. The increase of roughness may result from the
stronger crystallization and phase separation of blend film, which
would lead to a more efficient charge dissociation and transport.
Meanwhile, TEM was used to explore the bulk morphology of blend
film (Fig. 3). In TEM images, the bright regions are polymers-rich
Table 3 The photovoltaic parameters of the devices based on the two
polymers under illumination of AM 1.5G, 100 mW cm-2
polymers
VOC (V) JSC (mA/cm2)
FF
PCE (%)d
PDPP-EHa
PDPP-EHb
PDPP-CHa
PDPP-CHb
0.75
0.74
0.76
0.76
15.1
13.4
14.6
17.5
14.0
0.64 7.25 (7.09)
0.63 6.24 (6.13)
0.60 6.66 (6.48)
0.61 8.11 (8.01)
0.65 6.6
PBDTT-DPPc 0.73
aThe polymer/PC71BM blending ratio: 1/2 (w/w), solvent: DCB.
bDIO (1%, v/v) was added. Ref 14. Values in parentheses are
c
d
average data from over 10 devices.
domains and the dark regions are PCBM-rich domains.
A
bicontinuous network is formed for PDPP-EH blend film without
DIO, however, the network is disturbed and a severe aggregation is
To further investigate the relationship between molecular structure
and photovoltaic properties, hole mobility was measured via the observed as the addition of DIO. As a result, the PCE is dropped
space-charge-limited current (SCLC) method with a hole-only from 7.25% to 6.24%. Compared to PDPP-EH, blend film PDPP-CH
device configuration of ITO/PEDPOT:PSS/polymer:PC71BM /Au.25 exhibits a quite homogeneous phase separation without DIO. More
clearly interpenetrating network is observed with the addition of
DIO, which could facilitate exciton dissociation and charge
transport.29 The morphology results are fairly well consistent with
the photovoltaic performance of the two polymers.
J-V curves are shown in Fig. S3. The hole mobilities (μhole) of the
optimized polymer:PC71BM blend for PDPP-EH and PDPP-CH
were determined to be 4.1 × 10-4 cm2 V-1 s-1 and 2.2 × 10-4 cm2 V-1 s-
1, respectively, which are similar to that of PBDTT-DPP (2.9× 10-4
cm2 V-1 s-1).14 As is well known, backbone distortion could break out
the coplanarity of the polymer and the mobility would thus be
decreased. Interestingly, PDPP-EH and PDPP-CH still exhibit even
higher hole mobilities, which would probably result from the well-
formed bicontinuous interpenetrating network and/or molecular
packing in the blend films.26 Furthermore, high μhole is also
beneficial to obtain high JSC and FF values.27
The microstructures of the blending films were further characterized
by X-ray diffraction (XRD) of polymer:PC71BM films processed
with or without DIO, as shown in Fig. 4. The blend films only show
a wide (100) lamellar diffraction peak in the out-of-plane direction,
indicating the edge-on orientations are formed in both blend films.
The two polymers exhibit small lamellar distance of 14.7 and 15.0 Å
for PDPP-EH and PDPP-CH, respectively. As DIO is added, the
lamellar distance are slightly increased to 14.8 and 15.2 Å,
respectively, which indicates longer packing distance between side
chains of neighboring polymer backbones induced by addition of
DIO.26 Furthermore, the lamellar stacking exhibits quite small
distance compared to the widely reported conjugated polymers, such
as P3HT (~17 Å)30 and PTB7 (~18 Å)31, especially much smaller
than thiophene-flanked DPP-based polymers (~20 Å).12a This
compact packing is probably caused by the twisted backbone, which
has been reported to decrease the lamellar spacing.32 The coherence
length ζ of the polymer gives information about the lattice disorder,
which was calculated by Scherrer equation.33 Compared to PDPP-
EH, PDPP-CH/DIO exhibits a smaller coherence length (7.8 nm vs
10.8 nm) with
a low intensity, which indicates a weaker
crystallization formed in the blend film. The lower crystallization
induced by the disruption of molecular packing would be one reason
for lower FF value of PDPP-CH. Furthermore, the π-stacking
diffraction is not observed in the films, which is probably ascribed to
the backbone distortion of the polymers.
Fig. 3 TEM images of polymer/PC71BM blends (1:2 weight ratio
cast from o-dichlorobenzene) for PDPP-EH (a, b) and PDPP-CH (c,
d) without (a, c) and with 1% DIO (b, d).
To understand the influence of side chain on the photovoltaic
performance, the morphology of blend film was examined by
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out under nitrogen on
a
deoxygenated solution of tetra-n-
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DOI: 10.1039/C6TA08130D
butylammonium hexafluorophosphate (0.1 M) in acetonitrile with a
computer-controlled CHI660C electrochemical workstation, a Pt
working electrode, a platinum-wire auxiliary electrode, and an SCE
as the reference electrode. Potentials were referenced to the
ferrocenium/ferrocene(Fc/Fc+) couple by using ferrocene as a
standard. The HOMO levels of the polymers were determined using
the oxidation onset value. TGA measurements were performed on
o
STA-409at a heating rate of 10 C/min under N2 atmosphere. All
GPC analyses were made using THF as eluant and polystyrene
standard as reference. Topographic images of the active layers were
obtained through atomic force microscopy (AFM) in tapping mode
under ambient conditions using an Agilent 5400 instrument. Bright
field transmission electron microscopy (TEM) data were acquired
using a HITACHI H-7650 electron microscope operating at an
acceleration voltage of 100 kV. The structure of the films was
analyzed using X-ray diffraction (Bruker D8 ADVANCE). The
ground-state geometries of BDT-DTBT molecules were fully
optimized with DFT method under B3LYP/6-31G(d,p) level.
Fig. 4 X-ray diffraction patterns of polymer:PC71BM blend films.
Conclusions
In summary, two DPP-BDT alternating conjugated copolymers
PDPP-EH and PDPP-CH were designed and synthesized with alkyl
groups of 2-ethylhexyl and cyclohexyl incorporated to the 4-position
of thiophene unit, respectively. The influence of sterically hindered
groups on the structural, optical, electrochemical and photovoltaic
properties of the two polymers were fully investigated and compared
to the reported PBDTT-DPP without side chains on the thiophene
unit. Polymers PDPP-EH and PDPP-CH show slightly wider band
gaps and lower HOMO levels. Theoretical calculations reveal that
two polymers show more twisted backbone than PBDTT-DPP and
hence the coplanar structure is broken. Both polymers exhibit higher
PCEs (7.25% for PDPP-EH and 8.11% for PDPP-CH) than PBDTT-
DPP (6.6%). TEM image indicates that a bicontinuous fibril network
is obviously formed in PDPP-CH:PC71BM blend as addition of DIO,
which is beneficial for obtaining high JSC value. Our work
demonstrates that the appropriate twisted backbone is an effective
strategy to improve PSC performance.
Photovoltaic Device Fabrication and Characterization
The PSCs were fabricated with a configuration of ITO/PEDOT:PSS/
polymers:PC71BM/Ca/Al. A thin layer of PEDOT:PSS (30 nm,
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) was spin-
cast on pre-cleaned ITO-coated glass from a PEDOT:PSS aqueous
solution (Baytron P VP AI 4083 from H. C. Starck) at 4000 rpm and
dried subsequently at 150 °C for 30 min in air, then the device was
transferred into glovebox, where the active layer (100 nm) of the
blend of the polymer and PC71BM was spin coated on the
PEDOT:PSS layer. Finally, a Ca (10 nm)/Al (100 nm) metal top
electrode was thermal evaporated onto the active layer under about 2
× 10-4 Pa. The active area of the device was 0.1 cm2defined by
shadow mask. The current density-voltage (J-V) characteristics were
measured with a Keithley 2420 source measurement unit under
simulated 100 mW/cm2 (AM 1.5 G) irradiation from a Newport solar
simulator. Light intensity was calibrated with a standard silicon solar
cell. The external quantum efficiencies (EQE) of solar cells were
analyzed using a certified Newport incident photon conversion
efficiency (IPCE) measurement system.
Experimental section
Materials and reagents
All reactions and manipulations were carried out under Ar with the
use of standard inert atmosphere. Common solvents were dried by
standard procedures. Deuterated chloroform, 1, 8-diiodooctane and
o-dichlorobenzene were purchased from Sigma-Aldrich. All column
chromatography was performed with the use of silica gel 200-300
Space charge limited current measurements
Hole mobility was measured using the space charge limited current
(SCLC)
model,
using
a
device
configuration
of
mesh.
2,6-Bis(trimethyltin)-4,8-bis(5-ethylhexyl-2-thienyl)-
ITO/PEDOT:PSS/polymer/Au by taking current-voltage in the range
of 0-5 V and fitting the results to a space charge limited current for
hole-only device. In the presence of carrier traps in the active layer, a
trap-filled-limit (TFL) region exists between the ohmic and trap-free
SCLC regions. The SCLC behavior in the trap-free region can be
characterized by using the Mott-Gurney square law,
benzo[1,2-b:4,5-b']dithiophene, monomers and polymers were
synthesized according to the literature methods.17 Unless stated
otherwise, other reagents were purchased from commercial sources,
and used without further purification.
Instruments
9휀휇푉2
퐽 =
1H NMR and 13C NMR spectra were performed in a Bruker
AVANCE-III 600 MHz spectrometer using solutions in CDCl3 and
chemical shifts were recorded in ppm units with TMS as the internal
8퐿3
where ε is the static dielectric constant of the medium and μ is the
carrier mobility, V is the voltage drop across the device, and L is the
polymer thickness.
standard. UV-vis spectra were recorded on
a
Lambda25
spectrophotometer. The electrochemical measurements were carried
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9
J. D. Chen, C. H. Cui, Y. Q. Li, L. Zhou, Q. D. OVuie,wCA.rtLicile, OYn.linFe.
DOI: 10.1039/C6TA08130D
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Yan, Nat. Energy, 2016, 1, 15027.
General procedure for the synthesis of polymers
Monomer 1 (0.2 mmol), monomer 2 (0.2 mmol), Pd2(dba)3 (1.8 mg,
0.002 mmol) and P(o-tol)3 (3.6 mg, 0.012 mmol) were dissolved into
6 mL toluene in a 25 mL round bottom flask protected by argon. The
flask was purged three times with successive vacuum and argon
10 V. Vohra, K. Kawashima, T. Kakara, T. Koganezawa, I. Osaka,
K. Takimiya and H. Murata, Nat. Photonics, 2015, 9, 403.
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filling cycles. The oil bath was heated to 110 C gradually, and the
reaction mixture was stirred for 18 h at 110 oC under argon
atmosphere. After cooling, the mixture was cooled to room
temperature and precipitated in 200 mL methanol. The precipitate
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PDPP-EH, GPC: Mn= 158.7 kg mol-1, PDI = 3.25, Td = 423 oC.1H
NMR (600 MHz, CDCl3) δ (ppm): 9.11-8.80 (br, 2H), 7.80-6.92 (br,
6H), 4.03 (br, 4H), 2.87 (br, 8H), 1.80-0.79 (br, 90H).
PDPP-CH, GPC: Mn= 148.2 kg mol-1, PDI = 2.62, Td = 403 oC.1H
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Acknowledgements
This work was supported by National Natural Science Foundation of
China (21202181, 51503219, 51573205, 61405209), Department of
Science and Technology of Shandong Province (ZR2015EQ002,
2015GGX104007), and the Qingdao Institute of Bioenergy and
Bioprocess Technology Director Innovation Foundation for Young
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA08130D
Two DPP-BDT alternating conjugated polymers with twisted backbone were
synthesized and high efficiency of 8.11% was obtained in photovoltaic device.