Diketopyrrolopyrrole-based liquid crystalline conjugated donor-acceptor copolymers with reduced band gap for polymer solar cells

Article abstract of DOI:10.1002/pola.26394

A new liquid crystalline (LC) acceptor monomer 2,5-bis[4-(4′- cyanobiphenyloxy)dodecyl]-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (TDPPcbp) was synthesized by incorporating cyanobiphenyl mesogens into diketopyrrolopyrrole (DPP). The monomer was copolymerized with bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene (BDT) and N-9′-heptadecanylcarbazole (CB) donors to obtain donor-acceptor alternating copolymers poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene-alt-3,6-bis(thiophen-5-yl)-2,5-bis[4-(4′-cyanobiphenyloxy) dodecyl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione] (PBDTDPPcbp) and poly[N-9′-heptadecanyl-2,7-carbazole-alt-3,6-bis(thiophen-5-yl)-2, 5-bis[4-(4′-cyano-biphenyloxy)dodecyl]-2,5-dihydropyrrolo[3, 4-c]pyrrole-1,4-dione] (PCBTDPPcpb) with reduced band gap, respectively. The LC properties of the copolymers, the effects of main chain variation on molecular packing, optical properties, and energy levels were analyzed. Incorporating the mesogen cyanobiphenyl units not only help polymer donors to pack well through mesogen self-organization but also push the fullerene acceptor to form optimized phase separation. The bulk heterojunction photovoltaicdevicesshow enhanced performance of 1.3% for PBDTDPPcbp and 1.2% for PCBTDPPcbp after thermal annealing. The results indicate that mesogen-controlled self-organization is an efficient approach to develop well-defined morphology and to improve the device performance.

Full text of DOI:10.1002/pola.26394

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Diketopyrrolopyrrole-Based Liquid Crystalline Conjugated
Donor–Acceptor Copolymers with Reduced Band Gap
for Polymer Solar Cells
Yunli Han,1 Lie Chen,1,2 Yiwang Chen1,2
1Institute of Polymers/Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
2Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
Correspondence to: Y. Chen (E-mail: ywchen@ncu.edu.cn) or L. Chen (E-mail: chenlienc@163.com)
Received 21 July 2012; accepted 25 September 2012; published online 23 October 2012
DOI: 10.1002/pola.26394
ABSTRACT: A new liquid crystalline (LC) acceptor monomer 2,5-
bis[4-(40-cyanobiphenyloxy)dodecyl]-3,6-dithiophen-2-yl-pyrrolo[3,4-
c]pyrrole-1,4-dione (TDPPcbp) was synthesized by incorporating
cyanobiphenyl mesogens into diketopyrrolopyrrole (DPP). The
monomer was copolymerized with bis(2-ethylhexyloxy)-
benzo[1,2-b:4,5-b0] dithiophene (BDT) and N-90-heptadecanylcar-
bazole (CB) donors to obtain donor–acceptor alternating
copolymers poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]
dithiophene-alt-3,6-bis(thiophen-5-yl)-2,5-bis[4-(40-cyanobipheny-
loxy)dodecyl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione] (PBDTDPPcbp)
and poly[N-9’-heptadecanyl-2,7-carbazole-alt-3,6-bis(thiophen-5-
yl)-2,5-bis[4-(4’-cyano-biphenyloxy)dodecyl]-2,5-dihydropyrrolo[3,
4-c]pyrrole-1,4-dione] (PCBTDPPcpb) with reduced band gap,
respectively. The LC properties of the copolymers, the effects of
main chain variation on molecular packing, optical properties,
and energy levels were analyzed. Incorporating the mesogen
cyanobiphenyl units not only help polymer donors to pack well
through mesogen self-organization but also push the fullerene
acceptor to form optimized phase separation. The bulk hetero-
junction photovoltaicdevicesshow enhanced performance of 1.3%
for PBDTDPPcbp and 1.2% for PCBTDPPcbp after thermal anneal-
ing. The results indicate that mesogen-controlled self-organiza-
tion is an efficient approach to develop well-defined morphology
C
V
and to improve the device performance.
2012 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 51: 258–266, 2013
KEYWORDS: conjugated polymers; liquid crystals; liquid-crystal-
line polymers (LCP); morphology photovoltaics
INTRODUCTION In recent years, bulk heterojunction (BHJ)
polymer solar cells (PSC) have attracted considerable inter-
ests as promising candidates for a new renewable energy
source because of their low cost, potential for achieving
large area, flexible photovoltaic devices with fast roll-to-roll
production. PSCs are predicted to yield power conversion
efficiency (PCE) up to a commercial level, if a suitable low
band gap donor material is discovered. The research commu-
nity has made great progress in the field of BHJ PSC. The
PCE, a key parameter to assess the performance of PSC is
over 8%.1–6 However, the efficiency of PSC is still low com-
pared with inorganic counterparts, which hinder their
applications.
electron-deficient ‘‘acceptor’’ moieties can individually tune
the band gap and energy levels of the conjugated polymer.7
Due to the improvement of the PCE of the photovoltaic devi-
ces, D–A copolymers have been developed rapidly over past
few years. Among various D–A copolymers, the polymers
based on benzodithiophene (BDT)8,9 and 2,7-carbazole
(CB)10 ‘‘push’’ units have been regarded as the most promis-
ing semiconductors for highly efficient BHJ solar cells.
Recently, a group of D–A copolymer based on diketopyrrolo-
pyrrole (DPP)-based polymers that works well in field-effect
transistors and solar cells are being discovered,11–18 due to
their desirable properties, such as low band gap, broad optical
absorption, and high charge carrier mobility because of their
strong p–p interaction and good planarity. Organic photovol-
taic functional materials containing DPP core as donor have
demonstrated great potential and achieved attractive efficien-
cies,19 with the best hole mobility up to about 2 cm2/V/s and
extraordinary PCE over 5.5%.12–15 However, a systematic
study of DPP-based polymer donors to establish structure–
property relationships, namely the nature of electronic struc-
ture and charge transport properties of polymer chain–chain
To obtain high-performance photovoltaic polymer materials,
it is necessary to design and synthesize a new conjugated
polymer with ideal properties, such as low band gap, broad
absorption range, high hole mobility, and suitable molecular
energy levels. One potential strategy is to make appropriate
low-band gap polymers with tunable energy levels via the
donor–acceptor (D–A) approach. The intramolecular charge
transfer (ICT) between the electron-rich ‘‘donor’’ and the
Additional Supporting Information may be found in the online version of this article.
C
V
2012 Wiley Periodicals, Inc.
258
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
interaction and alignment in solid state of organic photovol-
taic (OPV) applications, has not been investigated.
des at a scan rate of 50 mV/s and a Ag/Agþ (0.1 M of
AgNO3 in acetonitrile) reference electrode in an anhydrous
and argon-saturated solution of 0.1 M of tetrabutylammo-
nium tetrafluoborate (Bu4NBF4) in acetonitrile. Under these
conditions, the oxidation potential (EOX1/2) of ferrocene was
ꢂ0.02 eV versus Ag/Agþ. The HOMO energy level of poly-
mers was determined from the oxidation onset of the second
scan from CV data. It is assumed that the redox potential of
Fc/Fcþ has an absolute energy level of ꢂ4.40 eV to vacuum.
The energy of HOMO and LUMO levels were calculated
according to (1) and (2), and the electrochemically deter-
mined band gaps were deduced from the difference between
onset potentials from oxidation and reduction of copolymers
as depicted in (3).30
To achieve the ordering and packing of organic donor mate-
rials and to overcome defect formation at the D–A interface,
high-mobility liquid crystalline (LC) polymer with crystalline
morphologies are introduced to control the phase separation
between the two components in BHJ. Liquid crystals possess
both order and mobility at molecular, supramolecular, and
macroscopic levels, as a result of self-assembled dynamic
functional soft materials.20–27 In our previous studies, we
have synthesized a new type D–A liquid-crystalline copoly-
mer, PFcbpDTBT, via copolymerization of LC electron-donor
fluorene and electron-acceptor dithienylbenzothiadiazole
(DTBT) units, revealing that spontaneous self-organization of
PFcbpDTBT can push [6,6]-phenyl-C61 butyric acid methyl
ester (PCBM) clusters to an oriented nanodispersing struc-
ture.28 This situation inspired us to design another type of
LC D–A alternating copolymer, containing electron-accepting
building block bearing LC side chain. Taking all of these
results into account, here we report the synthesis of a new
DPP monomer containing cyanobiphenyl mesogens and its
application of copolymerization with bis(2-ethylhexyloxy)
benzo[1,2-b:4,5-b0]dithiophene (BDT) or N-90-heptadecanyl-
2,7-carbazole (CB) to obtain polymers poly[4,8-bis(2-ethylhex-
yloxy)benzo[1,2-b:4,5-b0]dithiophene-alt-3,6-bis(thiophen-5-
yl)-2,5-bis[4-(40-cyanobiphenyloxy)dodecyl]-2,5-dihydropyrrolo
[3,4-c]pyrrole-1,4-dione] (PBDTDPPcbp) and PCBTDPPcbp. We
hope our work can shed light on the design of new organic
electronic materials with optimized liquid-crystal side chain
architecture.
EHOMO ¼ ꢂðEroexd þ 4:40ÞðeVÞ
(1)
(2)
(3)
ELUMO ¼ ꢂðEorendset þ 4:40ÞðeVÞ
Eg ¼ ðEroexd ꢂ E
ÞðeVÞ
red
onset
Annealing of films was conducted by heating in the setting
temperature for 10 min, followed by cooling to room tem-
ꢀ
perature at a cooling speed of 1 /min.
Device Fabrication and Characterization
The polymer PVCs were fabricated with the sandwiched
structure of glass/indium-tin oxide (ITO)/[poly(3,4-ethylene
dioxythiophene):poly(styrene sulfonate)] (PEDOT:PSS)/Poly-
mer:PCBM (1:2)/Aluminum. Prior to use, the substrates
were ultrasonicated for 30 min in acetone followed by deter-
gent and deionized water and then 2-propanol. The sub-
strates were dried under a nitrogen flow and subjected to
the treatment of UV ozone over 20 min. A filtered dispersion
of PEDOT:PSS in water (Baytron Al4083) was then spun-cast
onto clean ITO substrates at 4000 rpm for 60 s and then
EXPERIMENTAL
Measurements and Characterization
The ultraviolet (UV)–visible spectra of the copolymers were
characterized on a PerkinElmer Lambda 750 spectrophotom-
eter. Thermogravimetric analysis (TGA) measurements were
carried out with PerkinElmer TGA 7 for thermogravimetry at
a heating rate of 10ꢀC/min under nitrogen. Differential scan-
ning calorimetry (DSC) was used to study phase-transition
temperatures on a Perkin-Elmer DSC 7 differential scanning
calorimeter with a constant heating/cooling rate of 10 ꢀC/
min under a nitrogen flow. Texture observations were with
studied by polarizing optical microscopy (POM) a Nikon
E600POL polarizing optical microscope equipped with an
Instec HS 400 heating and cooling stage. The X-ray diffrac-
tion (XRD) study of the copolymers was performed on a
Bruker D8 Focus X-ray diffractometer operating at 30 kV and
20 mA with a copper target (k ¼ 1.54 Å) and at a scanning
ꢀ
baked at 120 C. The copolymers were dissolved in dichloro-
benzene to make 15 mg/mL for PBDTDPPcbp and 20 mg/
mL for PCBTDPPcbp solutions, followed by blending with
PCBM (purchased from Lumtec. Corp). The active layers
were got by spin-coating the blend solutions at 800 rpm for
30 s. Subsequently, a 0.6 nm layer of LiF followed by a 100
nm layer of Al were evaporated under vacuum (<10ꢂ6 Torr)
to form the electrodes. The thicknesses of all the films were
measured by a Dektak profiler. Annealing of some devices
was conducted by heating at different temperatures for 10
min, followed by cooling to room temperature at a cooling
ꢀ
speed of 1 /min. Current–voltage (J–V) characteristics were
recorded using Keithley 2400 Source Meter in the dark and
under 100 mW/cm2 simulated AM 1.5 G irradiation (Abet
Solar Simulator Sun 2000). All the measurements were per-
formed under ambient atmosphere at room temperature.
rate of
1
ꢀ/min. The gel permeation chromatography,
so-called size-exclusion chromatography analysis, was con-
ducted with a Knauer Smartline system equipped with a
Rheodyne injector, using polystyrenes as the standard and
CHCl3 as the eluent at a flow rate of 1.0 mL/min and 40ꢀC
through a Styragel column set, Styragel HT3 and HT4 (19
mm ꢁ 300 mm, 103 þ 104 Å) to separate molecular weight
ranging from 102 to 106. Cyclic voltammograms (CV) were
carried out in a three-electrode cell using platinum electro-
Materials
3,6-Dithiophen-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione
(TDPP), 4-cyanobiphenyloxy,2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaboralan-2-yl)-N-90-hepta-decanylcarbazole,2,6-bis(trime-
thyltin)-4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b0] dithio-
phene, 1,12-dibromododecane, tetrakis(triphenylphosphine)
palladium, PC60BM, and other materials are purchased from
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266
259

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Alfa Aesar, or Aldrich and used without further purification.
ITO glass was purchased from Delta Technologies Limited,
whereas PEDOT:PSS (Baytron PAl4083) was obtained from
Bayer Inc. The following compounds were synthesized accord-
ing to the procedure in the literature: PCBTDPPcbp29 and
PBDTDPPcbp30
distilled water. The extraction solution was rotorary-evap-
rated under reduced pressure. The solid was then collected
by vacuum filtration and washed with hot water and hot
methanol alternatively. Then the crude product was purified
by column chromatography using gradient elution (chloro-
form:hexanes ¼ 10:3). A purple solid was obtained.
(Yield: 60%) H NMR (CDCl3, 400 Hz): d (ppm) 8.66 (d, 2H),
7.69 (d, 4H), 7.64 (d, 4H), 7.53 (d, 4H), 7.24 (d, 2H), 6.96
(t, 4H), 4.01 (d, 4H), 3.97 (d, 4H), 2–0.9 (m, 40H)
1
4-(40-Cyanobiphenyloxy)dodecylbromide (1)
In a 500-ml round bottom flask, a mixture of 4-cyanobiphe-
nyloxy (3.9 g, 20 mmol), anhydrous potassium carbonate
(19.6 g, 60 mmol), 1,12-dibromododecane (16.5 g, 120
mmol) was added and stirred in 300 ml acetone at 135 ꢀC.
The reaction mixture was refluxed with stirring for 24 h.
After cooling down to the room temperature, the mixture
was filtered and washed with acetone of 300 ml. The solvent
was removed by reduced pressure, and the residue was
dried by vacuum to get the crude product. Purification was
accomplished by column chromatography on silica with
hexane:dichloromethane ¼ 1:2 to afford the product.
Poly[N-90-Heptadecanyl-2,7-carbazole-alt-3,6-
bis(thiophen-5-yl)-2,5-bis[4-(40-cyano-biphenyloxy)
dodecyl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione]
(PCBTDPPcbp)
Compound 3 (0.17745 g, 0.15 mmol), 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaboralan -2-yl)-N-90-heptadecanylcarbazole
(98.625 mg, 0.15 mmol), 3.5 mg (0.003 mmol) of K2CO3
(345 mg, 2.6 mmol) and tetrakis(triphenylphosphine)palla-
dium(0) were dissolved in 8 ml of THF and 2.5 ml of H2O.
The reaction mixture was vigorously stirred at 85 ꢀC, After
72 h, bromobenzene (2 lL, 0.015 mmol) was added into the
reaction mixture, 3 h later, phenylboronic acid (2.15 mg,
0.015 mmol) was added, and the reaction mixture was
refluxed overnight to complete the end capping reaction. The
polymer was purified by precipitation by methanol/water
(10:1), then filtered and washed on Soxhlet appartus with
methanol, acetone, hexane, and chloroform. The chloroform
fraction was condensed under reduced pressure, precipitated
in methanol/H2O (10:1, 250 ml), then filtered, and polymer
1
(Yield: 80%) H NMR (CDCl3, 400 Hz): d (ppm) 7.70 (d, 2H),
7.65 (d, 2H), 7.53 (d, 2H), 7.00 (d, 2H), 4.01 (t, 2H), 3.19
(t, 2H), 1.9–0.8 (m, 20H).
2,5-Bis[4-(40-Cyanobiphenyloxy)dodecyl]-3,6-
dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (2)
In a 250-ml round bottom flask, 3,6-dithiophen-2-yl-2,5-dihy-
dro-pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP) (1.5 g, 5 mmol)
and anhydrous K2CO3 (2.07 g, 15 mmol) were added under
argon. The mixture was degassed three times, and then 150
ml anhydrous N,N-dimethylformamide was injected into the
mixture and heated to 130 ꢀC under argon for 3 h, com-
pound 1 (8.85 g, 20 mmol) was added into the reaction solu-
tion. The reaction mixture was stirred for 24 h at 135 ꢀC.
The reaction mixture was allowed to cool down to the room
temperature, poured into 300 ml of water, and stirred for
0.5 h. The water layer was extracted by 300 ml of CHCl3
three times. The combined organic fractions were dried over
magnesium sulfate, and the solvent was removed under
reduced pressure. The solid was washed with several por-
tions of distilled water and methanol alternatively. The crude
product was purified by column chromatography using
dichloromethane as eluent, and the solvent was evaporated
under reduced pressure. A red powder is obtained.
ꢀ
was obtained after vacuum-drying at 50 C overnight.
(yield: 65 %) 1H NMR (CDCl3, 400 Hz): d (ppm) 9.13 (d,
2H), 8.21 (d, 2H), 7.87–7.4 (m, 18H), 7.0–6.93 (m, 7H), 4.62
(m, 1H), 4.21 (m, 4H), 3.94 (d, 4H), 2.36 (d, 4H), 2.0–1.0 (m,
78H), 0.8 (m, 4H).
PBDTDPPcbp
To a 100 ml flask, 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexy-
loxy)-benzo[1,2-b:4,5-b0]dithiophene (BDT) (0.1158 g, 0.15
mmol), Compound 3 (0.17745 g, 0.15 mmol), and toluene (8
ml) were added. The system was purged with Ar under vac-
uum. Then (PPh3)4Pd(0) (0.03 g, 0.03 mmol) was added.
The solution was stirred at 110 ꢀC, and refluxed for 24 h
under the protection of Ar. After the solution was cooled
down to room temperature, the solution was dropped into
acetone to precipitate the copolymer. The copolymer was
Soxhlet extracted with methanol, acetone, hexane, and
chloroform to remove oligmer, and then with chloroform to
collect the polymer.
1
(Yield: 70%) H NMR (CDCl3, 400 Hz): d (ppm) 8.94 (d, 2H),
7.70 (d, 10H) 7.52 (d, 4H), 7.30 (d, 2H), 6.99 (d, 4H), 4.07
(t, 4H), 4.00 (t, 4H), 1.7–0.8 (m, 40H).
3,6-Bis-(5-Bromo-thiophen-2-yl)-2,5-bis[4-(40-
cyanobiphenyloxy)dodecyl]-pyrrolo-[3,4-c]
pyrrole-1,4-dione (TDPPcbpBr) (3)
In a 250-ml round bottom flask, covered with aluminum foil,
compound 2 (1.022 g, 1mmol) and N-bromosuccinimide
(0.396 g, 2.2 mmol) were added, and then were dissolved
into 80 ml of CHCl3, at last, the reaction mixture was stirred
for 48 h at room temperature. After that, the reaction mix-
ture was poured into 300 ml of water, stirred for 0.5 h at
room temperature, the solution was extracted by CHCl3 three
times, and the combined organic phase was washed by
(Yield: 65.4%) 1H NMR (CDCl3, 400 Hz): 9.02 (d, 2H), 7.8–
7.35 (m, 13H), 7.06 (m, 3H), 6.93 (d, 4H), 4.06 (d, 6H), 3.78
(d, 4H), 2.5–0.8 (m, 74H).
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthesis of the copolymers PBDTDPPcbp and
PCBTDPPcbp is outlined in Scheme 1. Alternating copolymer
PBDTDPPcbp is synthesized via a Stille polycondensation
260
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 1 Synthesis of the LC monomer and D–A copolymer.
reaction, where 2,5-bis[4-(40-cyanobiphenyloxy)dodecyl]-3,6-
dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (TDPPcbp)
was used as an electron acceptor unit and 2,6-bis(trimethyltin)-
4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]-dithiophene (BDT)
was the donor unit. However, the copolymer of PCBTDPPcbp
The thermal stability of polymers is an important parameter
for optoelectronic device, which can be investigated with
TGA. Figure 1 shows that the polymers have good stability
and lose 5% weight at the temperature up to 350 ꢀC and
400 ꢀC, respectively. Cyanobiphenyl mesogenic appendages
may have well wrapped the conjugated backbones and thus
protect them from the perturbation by heat and degradative
species.28 Good thermal stability against oxygen ensures
both polymers for device fabrication process and other kinds
of applications.
was synthesized by
a Suzuki polycondensation reaction
between 2,7-(bis(4,4,5,5- tetramethyl-1,3,2-dioxaboralan-2-yl)-
N-90-heptadecanylcarbazole (CB) and TDPPcbp. Both the
copolymers were purified by Soxhlet extraction with metha-
nol, acetone, hexane, and chloroform in succession. The iden-
tity and purity of the new monomer and copolymers
have been confirmed by 1H NMR (Supporting Information
Figs. S1–S3).
The mesomorphic behavior of polymers has been studied by
DSC (see Supporting Information Fig. S4) and POM (Fig. 2).
The POM observation of monomer TDPPcbp and copolymers
reveals that a bright colorful texture of an enantiotropic
mesophase is formed when the sample is cooled and heated.
With the aid of XRD measurements, the mesophases have
been identified to a nature of two-domain order Smectic A
phase. The result is also supported by DSC analysis. For
polymer PBDTDPPcbp, DSC reveals two discrete exothermic
ꢀ
transitions at 149 and 165 C on second heating scan curves,
which corresponds to the solid–mesophase transition and
the mesophase–isotropic transition, respectively. However,
for copolymer PCBTDPPcbp, DSC reveals two discrete endo-
therm transitions at 142 ꢀC and 175 ꢀC. Thus, spontaneous
orientation of mesogens has effectively driven the self-
assembly of the copolymers, which could provide a favorable
pathway for charge transportation.
UV–vis–NIR absorption spectra (Fig. 3) give information
regarding the electronic structure of the polymers. Both
polymers show well-defined vibronic features in solution at
room temperature with kmax at 764 nm for PBDTDPPcbp
FIGURE 1 TGA curves of polymers with
10 ꢀC/min.
a heating rate of
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266
261

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 2 The mesomorphic textures observed by POM at LC state. (a) TDPPcbp, (b) PBDTDPPcbp, and (c) PCBTDPPcbp.
and 655 nm for PCBTDPPcbp. The UV–vis absorption peaks
of crystalline p-conjugated polymers films are usually batho-
chromic shift relative to the solution, which is as result of
increasing intermolecular interactions between the polymer
chain and polymer backbone in the solid state.31–34 Both
polymers show broad absorption spectra with extended
absorption edge in the solid state, and bathochromic shift
can be observed by 8 nm for PBDTDPPcbp and 28 nm for
PCBTDPPcbp, relative to those measured in solution. The
peak 300 nm corresponds to the cyanobiphenyl pendants.
The films of PBDTDPPcbp and PCBTDPPcbp also exhibit ICT
band at 772 nm and 683 nm, respectively. PBDTDPPcbp
shows a red-shifted absorption maximum in the film by 90
nm compared to PCBTDPPcbp, maybe due to the higher
degree coplanar of PBDTDPPcbp favoring a better stacking
in the solid state. This also indicates that the change of do-
nor moiety have an important effect on electron delocaliza-
tion with the conjugated system. The absorption spectra of
the annealed films are shown in Figure 4. Compared to those
of films as cast, both PBDTDPPcbp and PCBTDPPcbp
annealed films show slightly red-shifted bands and higher
absorption intensity as the annealing temperature falls
into mesophase region (150 ꢀC), suggesting the mesophase
pendant induced the stronger intermolecular interaction and
ordered packing arrangement in polymers.
Solid-state packing plays an important role in the electronic
properties of conjugated polymers.35–37 XRD has been used
FIGURE 4 UV–vis films absorption spectra of copolymers in
solid state at different treatments on
a quartz plate. (a)
PBDTDPPcbp as cast and after annealing 150 ꢀC and (b)
PCBTDPPcbp as cast and after annealing at 150 ꢀC. All the ther-
mal treatments are conducted at such condition except special
instructions.
FIGURE 3 UV–vis absorption spectra of copolymers in CF solu-
tion and in the solid state.
262
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 7 CV of polymers films coated on platinum electrode.
FIGURE 5 XRD patterns of polymer films as cast and after ther-
mal annealing at LC states (150 ꢀC for 0.5 h).
(d ¼ 1.85 nm), respectively, and 2h ¼ 13ꢀ (d ¼ 0.75 nm)
corresponds to biphenyl mesogenic unit length in vertical
orientation.
to investigate the structural changes of polymer films before
and after annealing treatments at LC states (Fig. 5). Com-
pared to the as-cast film, the annealed ones show relatively
sharper peaks in the region of 2–30ꢀ, especially annealed
from the LC state (See supporting Information Fig. S5),
which suggests that the formation of more well-ordered
crystalline nanostructure can be induced by oriented cyano-
biphenyl after LC thermal annealing.38,39 Distinct primary
diffraction features are observed at 2h of 6.37ꢀ for
PCBTDPPcbp, which corresponds to the laying distance (d ¼
1.54 nm), between the sheets of PCBTDPPcbp chains associ-
ated with the plane perpendicular to their longitudinal axes.
In contrast with PCBTDPPcbp, PBDTDPPcbp exhibit a higher
degree of solid state ordering. There are two low-angle
Bragg reflections at 2h of 3.40ꢀ and 5.29ꢀ, associated to
the layer spacing (d ¼ 2.88 nm) and molecular length
Thermal treatments also have profound effects on the blend
morphology of the active layer of the BHJ.40,41 To further
investigate the microstructure of polymer:PCBM blend films
as cast and after thermal annealing, we conducted XRD on
the PBDTDPPcbp:PCBM (1:2 wt %) and PCBTDPPcbp: PCBM
(1:2 wt %) films at different treatments, respectively. The
curves are shown in Figure 6, and the increasing crystalline
of blend films suggest that the spontaneous assembly of the
LC molecules with PCBM blend films form oriented layer
nanostructure after annealed at LC states, especially for the
PBDTDPPcbp blending.
Electrochemical CV has been widely used to investigate the
redox behavior of the polymer and to estimate its HOMO and
LUMO energy levels as well as electrochemical band gap
(Egec).42 Figure
7 displays the CV of PBDTDPPcbp and
PCBTDPPcbp films on a Pt electrode in a 0.1 mol/L Bu4NPF6-
CH3CN solution. The results of the electrochemical measure-
ments are listed in Table 1. It can be seen that there are
reversible n-doping/dedoping (reduction/reoxidation) proc-
esses in the negative and positive potential range for the poly-
mers. The onset oxidation potential (Eox) is 0.99 eV versus
Ag/AgNO3 for PBDTDPPcbp and 1.03 eV for PCBTDPPcbp, the
HOMO changes very slightly from ꢂ5.39 eV for PBDTDPPcbp
to ꢂ5.43 eV for PCBTDPPcbp. However, PBDTDPPcbp (ꢂ3.57
eV) has a low-lying LUMO than PCBTDPPcbp (ꢂ3.38 eV). This
contributes to PBDTDPPcbp with a lower band gap. Mean-
while, it also indicates that the donor moiety modification has
a distinct effect on the energy levels of the polymers. Addi-
tionally, polymer PCBTDPPcbp possesses a low-lying HOMO
energy level, probably providing an increased open-circuit
voltage than that of PBDTDPPcbp.
The photovoltaic properties of the polymers were studied
in solar cells with structures of ITO/PEDOT: PSS/
Polymer:[60]PCBM (1:2 wt %)/LiF/Al. Figure 8 shows the
FIGURE 6 XRD spectra of polymers:PCBM (1:2 wt %) films at
different treatments: as cast and thermal annealing from LC
states (150 ꢀC for 0.5 h).
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266
263

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 1 Electrochemical Properties and Key Polymer Properties of the Copolymers
Polymers
Eonset/ox (eV)
Eonset/red (eV)
HOMO (eV)
LUMO (eV)
Eg (eV)
Mn/PDI (kg/mol)
PBDTDPPcbp
PCBTDPPcbp
0.99
1.03
ꢂ0.83
ꢂ1.02
ꢂ5.39
ꢂ5.43
ꢂ3.57
ꢂ3.38
1.82
2.05
33.0/2.21
28.1/2.52
is in agreement well with measured value. Nevertheless, the
experimental Voc is lower than the theoretical value, prob-
ably due to the unfavorable ohmic contact with the electrode
and the field-driven photocurrent in BHJ devices.43 I–V
curves for PBDTDPPcbp and poly[N-9’-heptadecanyl-2,7-car-
bazole-alt-3,6-bis(thiophen-5-yl)-2,5-bis[4-(4’-cyano-biphenyloxy)
dodecyl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione] (PCBTDPPcpb)
as cast films show an ‘‘s-shaped’’ kink, resulting from the
defects of active layer.44 After thermal annealing at LC states,
the PCE of polymer:PCBM have an increase trend with a
higher FF and JSC, which may be due to the optimized mor-
phology. It is firmly believed that by optimizing the inter-
penetrating network morphology of the DPP-based devices
through different polymer/acceptor ratios or annealing treat-
ment, higher FF and PCE value would be obtained.
It is proven that the photovoltaic properties of BHJ relate
with the mixing morphology of the polymer and PCBM com-
posite film, which mainly affects the interpenetrated network
of the donor and the acceptor.45 Tapping mode atomic force
microscope (AFM) is used to observe the surface morpholog-
ical structure of the active layer and the phase images are
shown in Figure 9. AFM data are consistent with the FF and
JSC data of the corresponding devices. Both polymer:PCBM-
annealed films show uniform and distinct phase separated
structure compared to the as-cast films, which contribute to
form channels in the fullerene matrix. The phase-separation
appearance is decreasing after annealing while the height
images RMS are from 1.69 to 1.12 nm for PCBTDPPcbp
before and after thermal annealing, respectively (See
Supporting Information Fig. S6). The surface morphology is
quite smooth and homogeneous after annealing, which
would create excellent phase separation interfaces for charge
separation and provide favorable channels for electrons and
holes transportate to respective electrodes, consequently
reduce the electron-hole recombination ratio.46 That is why
the FF and JSC data of the corresponding devices after
annealing have an increased tendency. This observation is
FIGURE 8 J–V characteristics of photovoltaic cells based on
polymer:PCBM blend films (as cast and thermal annealing
from LC states at 150 ꢀC for 10 min) under AM 1.5 G illumina-
tion from a calibrated solar simulator with 100 mW/cm2. Inset
is a schematic of device architecture. PBDTDPPcbp:PCBM (1:2
wt %) and PCBTDPPcbp:PCBM(1:2 wt %).
J–V curves for solar cells under AM 1.5
G radiation
(100 mW/cm2). BHJ solar cells were fabricated using
PBDTDPPcbp:PCBM and PCBTDPPcbp:PCBM blends as active
layer, and the results are outlined in Table 2. We report here
preliminary results of thermal-annealed BHJ solar cell.
PBDTDPPcbp-based BHJ solar cells demonstrated a short-cir-
cuit current (JSC) of 4.11 mA/cm2, a fill factor (FF) of 0.45,
and an open circuit voltage (VOC) of 0.68 V, leading to a PCE
of 1.3% after annealing at mesophase. Conversely, compared
to PBDTDPPcbp, PCBTDPPcbp has a higher short-circuit cur-
rent JSC of 4.72 mA/cm2, but a lower FF of 0.32, a higher
open-circuit voltage of 0.80 V, resulting in PCE of 1.2%.
Meanwhile, the theoretical VOC of PCBTDPPcbp and
PBDTDPPcbp are 0.83 V and 0.79 V, respectively. The VOC of
PCBTDPPcbp is also higher than that of PBDTDPPcbp, which
TABLE 2 Device Performances of Polymers:PCBM (w/w 1:2) BHJ Solar Cells Before and After Annealing Using Dichlorobenzene
(under A.M. 1.5, 100 mW/cm2 irradiation)a
Device
Thickness (nm)
VOC (V)b
JSC (mA/cm2)c
FFd (%)
PCE [highest] (%)
PBDTDPPcbp:PCBM (1:2)
PBDTDPPcbp:PCBM (1:2)-annealede
150
150
150
150
0.68
0.68
0.69
0.80
c
2.90
4.11
4.28
4.72
25.7
44.8
26.0
31.9
0.4 6 0.1 [0.5]
1.2 6 0.1 [1.3]
0.7 6 0.1 [0.8]
1.1 6 0.1 [1.2]
PCBTDPPcbp:PCBM (1:2)
PCBTDPPcbp:PCBM (1:2)-annealed
aAll values represent averages from six 0.04 cm2 devices on a single
chip.
JSC is the short-circuit current.
FF is a graphic measure of the J–V curve.
The devices were annealed at LC state for 10 min.
d
e
b
VOC is the open-circuit voltage.
264
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
ance. Thus, incorporation of the mesogens into the conjugated
polymers is an effective approach to realize morphology
control and LC TDPPcbp is also a kind of promising acceptor
segment for the design of D–A type photovoltaic polymers.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Founda-
tion of China (51073076, 51273088, 51003045, and 51263016).
REFERENCES AND NOTES
1 Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Gang, L.; Ray, C.;
Yu, L. Adv. Mater. 2010, 22, E135–E138.
2 Chu, T. Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J. R.;
Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am.
Chem. Soc. 2011, 133, 4250–4253.
3 Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W.; J. Am.
Chem. Soc. 2011, 133, 4625–4631.
FIGURE 9 Tapping mode AFM phase images of the films
based on polymers:PCBM (1:2 wt%) as cast and after annealing
from LC states at 150 ꢀC for 10 min (3 lm ꢁ 3 lm). (a)
PCBTDPPcbp:PCBM, as cast, (b) PCBTDPPcbp:PCBM, after
annealing, (c) PBDTDPPcbp:PCBM, as cast, (d) PBDTDPPcbp:
PCBM, after annealing.
4 Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W.
Angew. Chem., Int. Ed. 2011, 50, 2995–2998.
5 Hou, L.; Zhang, S.; Guo, X.; Xu, F.; Li, Y.; Hou, J. Angew.
Chem., Int. Ed. 2011, 50, 9697–9702.
6 He, Z.; Zhong, C.; Huang, X.; Wong, W.; Wu, H.; Chen, L.; Su,
S.; Cao, Y.; Adv. Mater. 2011, 23, 4636–4643.
7 Roncali, J. Chem. Rev. 1997, 97, 173–205.
similar to our previous work that cyano biphenyl mesogenic
pendants induced well-ordered active layers morphology
after thermal treatment at LC states.28 Figure 9(d) shows the
surface topography of PBDTDPPcbp film after annealing
from LC states. The surface comprises of nanofibers with the
average width of approximately 30 nm. This nanoscale
microphase separation morphology originating from the
spontaneous assemblies of LC polymer is highly desirable for
BHJ active layer, which also can explain the enhanced per-
formance for PBDTDPPcbp after annealed from LC state.
8 Chen, H. Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.;
Yu, L.; Wu, L.; Li, G. Nat. Photonics 2009, 3, 649–653.
9 Liang, Y.; Feng, D.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L.
J. Am. Chem. Soc. 2009, 131, 7792–7799.
10 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Plesu, R. N.; Belleteˆte, M.; Durocher, G.; Tao, Y.; Leclerc, M.
J. Am. Chem. Soc. 2008, 130, 732–742.
11 Zhang, Y.; Chien, S. C.; Chen, K. S.; Yip, H. L.; Sun, Y.; Davies,
J. A.; Chen, F. C.; Jen, A. K. Y. Chem. Commun. 2011, 47,
11026–11028.
12 Bronstein, H.; Chen, Z. Y.; Ashraf, R. S.; Zhang, W. M.; Du,
J. P.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.;
Geerts, Y.; Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.;
Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc.
2011, 133, 3272–3275.
CONCLUSIONS
Two new type semiconducting conjugated polymers
PBDTDPPcbp and PCBTDPPcbp bearing LC side chain were
designed and synthesized. It was found that the donor-part
variation in the polymers lead to the different electronic prop-
erties, as polymer PCBTDPPcbp possesses a low-lying HOMO
energy level to provide a higher open-circuit voltage. The LC
polymers show red-shifted and enhanced absorption bands
after thermal annealing, suggesting that the incorporation of
mesogenic pendants could enhance the orientation degree of
the polymer. The more ordered lamella structures of the poly-
mers after annealing can also be confirmed from the XRD
results. Moreover, thermal-annealed PBDTDPPcbp:PCBM and
PCBTDPPcbp:PCBM blend films lead to enhanced PCE of 1.3%
and 1.2%, respectively. It is worthy to note that highly desired
active layer morphology can be obtained through thermal
annealing from the LC state. The performance of the devices
is not sufficient to reach the level for solar cells application,
but study of the relation between LC and the morphology of
the active layer blend has revealed that the LC thermal treat-
ment can enable the bulk form well-connected nanoscale
phase separation, consequently enhance the device perform-
13 Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.;
ꢀ
Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547–15549.
14 Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk,
M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am.
Chem. Soc. 2009, 131, 16616–16617.
15 Bijleveld, J. C.; Gevaerts, V. S.; Di Nuzzo, D.; Turbiez, M.;
Mathijssen, S. G. J.; de Leeuw, D. M.; Wienk, M. M.; Janssen,
R. A. J. Adv. Mater. 2010, 22, E242–E246.
16 Li, Y. N.; Sonar, P.; Singh, S. P.; Soh, M. S.; van Meurs, M.;
Tan, J. J. Am. Chem. Soc. 2011, 133, 2198–2204.
17 Sonar, P.; Singh, S. P.; Li, Y.; Soh, M. S.; Dodabalapur, A.
Adv. Mater. 2010, 22, 5409–5413.
18 Nelson, T. L.; Young, T. M.; Liu, J. Y.; Mishra, S. P.; Belot, J.
A.; Balliet, C. L.; Javier, A. E.; Kowalewski, T.; McCullough, R.
D. Adv. Mater. 2010, 22, 4617–4621.
19 van Duren, J. K. J.; Dhanabalan, A.; van Hal, P. A.; Janssen,
R. A. J. Synth. Met. 2001, 121, 1587–1588.
20 Dhanabalan, A.; van Duren, J. K. J.; van Hal, P. A.; van Dogen,
J. L. J.; Janssen, R. A. J. Adv. Funct. Mater. 2001, 11, 255–262.
21 Neugebauer, H.; Brabec, C. J.; Sariciftci, N. S.; Kiebooms,
R.; Wudl, F.; Luzzati, S. J. Chem. Phys. 1999, 110, 12108–12115.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266
265

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
22 Percec, V.; Asandei, A. D.; Hill, D. H.; Crawford, D. Macro-
molecules 1999, 32, 2597–2604.
34 Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc.
2004, 126, 3378–3379.
23 Percec, V.; Obata, M.; Rudick, J. G.; De, B. B.; Glodde, M.;
Bera, T. K.; Magonov, S. N.; Balagurusamy, V. S. K.; Heiney, P.
A. J. Polym. Sci. Part A: Polym. Chem. 2002, 40, 3509–3533.
35 van Bavel, S.; Veenstra, S.; Loos, J. Macromol. Rapid Com-
mun. 2010, 31, 1835–1845.
36 Yang, X.; Loos, J. Macromolecules 2007, 40, 1353–1362.
24 Percec, V.; Rudick, J. G.; Peterca, M.; Wagner, M.; Obata,
M.; Mitchell, C. M.; Cho, W. D.; Balagurusamy, V. S. K.; Heiney,
P. A. J. Am. Chem. Soc. 2005, 127, 15257–15264.
37 Giridharagopal, R.; Ginger, D. S. J. Phys. Chem. Lett. 2010,
1, 1160–1169.
38 Yuan, W. Z.; Tang, L.; Zhao, H.; Jin, J. K.; Sun, J. Z.; Qin, A.;
Xu, H. P.; Liu, J.; Yang, F.; Zheng, Q.; Chen, E.; Tang, B. Z.
Macromolecules 2009, 42, 52–61.
25 Rudick, J. G.; Percec, V. Acc. Chem. Res. 2008, 41,
1641–1652.
26 Percec, V.; Rudick, J. G.; peterca, M.; Heiney, P. A. J. Am.
Chem. Soc. 2008, 130, 7503–7508.
39 Olsen, B. D.; Gu, X.; Hexemer, A.; Gann, E.; Segalman, R. A.
Macromolecules 2010, 43, 6531–6534.
27 Percec, V.; Aqad, E.; Peterca, M; Rudick, J. G.; Lemon, L.;
Ronda, J. C.; De, B. B.; Heiney, P. A.; Meijer, E. W. J. Am.
Chem. Soc. 2006, 128, 16365–16372.
40 Po, R.; Maggini, M.; Camaioni, N. J. Phys. Chem. C 2010,
114, 695–706.
41 Ma, W.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv.
Funct. Mater. 2005, 15, 1617–1622.
28 Yao, K.; Chen, Y. W.; Chen, L.; Li, F.; Li, X. E.; Ren, X. Y.;
Wang, H. M.; Liu, T. X. Macromolecules 2011, 44, 2698–2706.
42 Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J.
Synth. Met. 1999, 99, 243–248.
¨
29 Zou, Y. P.; Gendron, D.; Badrou-Aıch, R.; Najari, A.; Tao, Y.;
Leclerc, M. Macromolecules 2009, 42, 2891–2894.
43 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
¨
30 Li, Z.; Zhang, Y. G.; Tsang, S. W.; Du, X. M.; Zhou, J. Y.;
Tao, Y.; Ding, J. F. J. Phys. Chem. C 2011, 115, 18002–18009.
C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.
44 Cheun, H.; Fuentes-Hernandez, C.; Zhou, Y.; Potscavage, W.
J.; Sung-Jin Kim, J.; Amir Dindar, J. S.; Kippelen B. J. Phys.
Chem. C 2010, 114, 20713–20718.
31 Akagi, K. J. Polym. Sci. Part A: Polym. Chem. 2009, 47,
2463–2485.
32 Percec, V.; Asandei, A. D.; Hill, D. H.; Crawford, D. Macro-
molecules 1999, 32, 2597–2604.
45 Chen, L. M.; Hong, Z. R.; Li, G.; Yang, Y. Adv. Mater. 2009,
21, 1434–1449.
33 Kokudo, H.; Sato, T.; Yamamoto, T. Macromolecules 2006,
39, 3959–3963.
46 Li, F.; Chen, W.; Chen, Y. W. J. Mater. Chem. 2012, 22,
6259–6266.
266
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2013, 51, 258–266