Enhancement of photovoltaic properties in supramolecular polymer networks featuring a solar cell main-chain polymer H-bonded with conjugated cross-linkers

Article abstract of DOI:10.1016/j.polymer.2012.01.026

Stille polymerization was employed to synthesize a low-band-gap (LBG) conjugated main-chain polymer PBTH consisting of bithiazole, dithieno[3,2-b:2′,3′-d]pyrroles (DTP), and pendent melamine derivatives. Novel supramolecular polymer networks PBTH/C and PBTH/F were developed by mixing proper molar amounts of polymer PBTH (containing melamine pendants) to be hydrogen-bonded (H-bonded) with complementary uracil-based conjugated cross-linkers C and F (i.e., containing two symmetrical uracil moieties connected with carbazole and fluorene units through triple bonds). The formation of multiple H-bonds between polymer PBTH and cross-linkers C or F was confirmed by FT-IR measurements. In contrast to polymer PBTH, the supramolecular design with multiple H-bonds can enhance the photovoltaic properties of polymer solar cell (PSC) devices containing H-bonded polymer networks PBTH/C and PBTH/F by tuning their light harvesting capabilities, HOMO energy levels, and crystallinities. Initially, the power conversion efficiency (PCE) values of PSC devices containing supramolecular polymer networks PBTH/C and PBTH/F as electron donors and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC ₇₀BM) as an electron acceptor (polymer:PC₇₀BM = 1:1 w/w) are found to be 0.97 and 0.68%, respectively, in contrast to 0.52% for polymer PBTH. The highest PCE value of 1.56% with a short-circuit current densities (J_sc) value of 7.16 mA/cm², a open circuit voltages (V_oc) value of 0.60 V, and a fill factor (FF) of 0.36 was further optimized in the PSC device containing a supramolecular polymer network PBTH/C as polymer:PC₇₀BM = 1:2 w/w. These results indicate that supramolecular design is an effective route towards better photovoltaic properties of V_oc, J_sc, and PCE values in polymer solar cells.

Full text of DOI:10.1016/j.polymer.2012.01.026

Polymer 53 (2012) 1219e1228
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Enhancement of photovoltaic properties in supramolecular polymer networks
featuring a solar cell main-chain polymer H-bonded with conjugated cross-linkers
,c,
Dhananjaya Patra a, Mohan Ramesh a, Duryodhan Sahu a, Harihara Padhy a, Chih-Wei Chu b
**
,
,
Kung-Hwa Wei a, Hong-Cheu Lin a
*
a Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30049, Taiwan, ROC
b Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, ROC
c Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Stille polymerization was employed to synthesize a low-band-gap (LBG) conjugated main-chain polymer
PBTH consisting of bithiazole, dithieno[3,2-b:20,30-d]pyrroles (DTP), and pendent melamine derivatives.
Novel supramolecular polymer networks PBTH/C and PBTH/F were developed by mixing proper molar
amounts of polymer PBTH (containing melamine pendants) to be hydrogen-bonded (H-bonded) with
complementary uracil-based conjugated cross-linkers C and F (i.e., containing two symmetrical uracil
moieties connected with carbazole and fluorene units through triple bonds). The formation of multiple
H-bonds between polymer PBTH and cross-linkers C or F was confirmed by FT-IR measurements. In
contrast to polymer PBTH, the supramolecular design with multiple H-bonds can enhance the photo-
voltaic properties of polymer solar cell (PSC) devices containing H-bonded polymer networks PBTH/C
and PBTH/F by tuning their light harvesting capabilities, HOMO energy levels, and crystallinities. Initially,
the power conversion efficiency (PCE) values of PSC devices containing supramolecular polymer
networks PBTH/C and PBTH/F as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester
(PC70BM) as an electron acceptor (polymer:PC70BM ¼ 1:1 w/w) are found to be 0.97 and 0.68%,
respectively, in contrast to 0.52% for polymer PBTH. The highest PCE value of 1.56% with a short-circuit
current densities (Jsc) value of 7.16 mA/cm2, a open circuit voltages (Voc) value of 0.60 V, and a fill factor
(FF) of 0.36 was further optimized in the PSC device containing a supramolecular polymer network
PBTH/C as polymer:PC70BM ¼ 1:2 w/w. These results indicate that supramolecular design is an effective
route towards better photovoltaic properties of Voc, Jsc, and PCE values in polymer solar cells.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 9 November 2011
Received in revised form
12 January 2012
Accepted 15 January 2012
Available online 2 February 2012
Keywords:
H-bonded cross-linker
Supramolecule
Polymer network
1. Introduction
molecular components, a variety of non-covalent interactions, for
instance hydrogen bonds (H-bonds) [3,4], ionic forces [5], and
Over the past decade, exploiting and tailoring novel materials
with promoted properties by utilizing supramolecular approaches
metal-coordinations [6], can produce unique properties, which are
not possessed by the individual components. Among these supra-
molecular interactions, H-bonds are ideal non-covalent interac-
tions to form self-assembled architectures due to their selectivity
and directionality [1c]. Tremendous research efforts have been
devoted towards engineering of polymeric materials as well as
donor-acceptor (D-A) bulk heterojunction (BHJ) solar cell devices
due to substantial future prospects as low-cost 3rd generation
photovoltaic technology [7]. Remarkable achievements in the field
of conjugated polymers and dramatic increase in device perfor-
mance, reaching a power conversion efficiencies (PCE) over 7% by
utilizing low-band-gap (LBG) polymers as electron donors and
fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester
(PC70BM), as electron acceptors [8].
in conjunction with
to design optoelectronic devices [1].Control of hierarchical self-
assembled behaviors of various -conjugated molecules through
p-conjugated structures is one of the key way
p
multiple hydrogen bonds can generate nano-structural materials
with long-range orders intrinsically, in which defect-free structures
are feasible to be the bottom-up approach via supramolecular
chemistry [2]. Due to self-assembly between complementary
* Corresponding author. Tel.: þ886 3 5712121; fax: þ886 3 5724727.
** Corresponding author. Research Center for Applied Sciences, Academia Sinica,
Taipei, Taiwan, ROC.
E-mail addresses: gchu@gate.sinica.edu.tw (C.-W. Chu), linhc@cc.nctu.edu.tw
(H.-C. Lin).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.01.026

1220
D. Patra et al. / Polymer 53 (2012) 1219e1228
An enhanced intermolecular
p
-stacking, long-range order, and
The unique features of this triple H-bonded approach are to tune
light harvesting capabilities, manipulation of HOMO energy levels,
and power conversion efficiencies of supramolecular polymer
networks PBTH/C and PBTH/F. Moreover, the functionalized
supramolecular polymer networks are developed to possess better
solubilities as well as processabilities in solvents and enhance
photovoltaic properties in this study.
solution processable electron-donating dithieno[3,2-b:20,30-d]
pyrroles (DTP) unit can be co-polymerized with various electron-
deficient units to generate LBG polymers and showed PCE values
up to w3% [9]. However, due to high values of the highest occupied
molecular orbital (HOMO) levels in DTP-based polymers, they
showed relatively low Voc values and thus to result in low PCE
values for organic solar cells [10]. This problem can be solved either
by using various electron-deficient units [10,11a] or introducing
supramolecular architectures to manipulate their HOMO energy
levels and thus to get higher Voc values. As an electron-deficient
unit, the bithiazole unit containing two electron-withdrawing
imine (eC¼N) could show high oxidative stabilities and minimize
the HOMO energy levels to induce the high Voc values [10,11a].
However, Li et al. have reported one D-A copolymer containing DTP
and bithiazole sandwiched between two thiophene units only
possessed a maximum PCE value of 0.06% [9b], but our later report
demonstrated that one of the analogous D-A copolymers contain-
ing DTP, thiophene, and bithiazole units could reach a maximum
PCE value of 0.69% [10b].
2. Experimental
2.1. Materials
All chemicals and solvents were reagent grades and purchased
from Aldrich, ACROS, Fluka, and Lancaster and Pd(PPh2)Cl2 and
Pd(PPh3)4 recived from Stream Chemical Co. Tetrahydrofuran (THF),
toluene, and diethyl ether were distilled over sodium/benzophe-
none. Absolute ethanol was obtained by refluxing with magnesium
ethoxide and then distilled. The other reagents were used as
received without further purification.
The design of supramolecular D-A systems was recognized as
a promising strategy via the charge-transfer processes between
donor and acceptor units [4]. Würthner, Schenning, and Meijer
et al. reported well-defined mesoscopic assemblies by using
complementary H-bonded moieties of perylene bisimide and
melamine derivatives [4,12]. Moreover, Bonifazi and Samori et al.
constructed several discrete and multi-component nano-scale
supramolecular assemblies from triple H-bonded D-A complexes
consisting of diamino-pyridine and uracil derivatives [1b,13]. In
addition, Meijer et al. reported a PCE value of 0.39% for the polymer
solar cell (PSC) device by utilizing a main-chain H-bonded polymer
containing oligo(p-phenylene vinyene) and ureido-pyrimidinone
units (as H-bonded groups) [14]. Small molecules and their poly-
mer analogues have attracted exceptional attentions in the recent
days owing to their corresponding advantages, including high
purities and (hole and electron) mobilities for small molecules, and
easy processing capabilities and low costs for polymers. Conse-
quently [15], one of the attractive approaches is to achieve
2.2. Measurements and characterization
1H and 13C NMR spectra were recorded on a Varian Unity
300 MHz spectrometer using CDCl3 solvent, “br” was noted for
broad peaks and chemical shifts were reported as
d values (ppm)
relative to an internal tetramethylsilane (TMS) standard. Elemental
analyses were performed on a HERAEUS CHN-OS RAPID elemental
analyzer. Thermogravimetric analyses were conducted with a TA
Instruments Q500 at a heating rate of 10 ꢀC/min under nitrogen. Gel
permeation chromatography (GPC) analyses were conducted on
a Waters 1515 separation module using polystyrene as a standard
and THF as an eluant. UVevisible absorption spectra were recorded
in dilute dichlorobenzene solutions and solid films (spin-coated on
glass substrates from dichlorobenzene solutions) on an HP G1103A
spectrophotometer. Cyclic voltammetry (CV) measurements were
performed at a scanning rate of 100 mV/s at room temperature
using a BAS 100 electrochemical analyzer with a standard three-
electrode electrochemical cell in 0.1 M tetrabutylammonium hex-
afluorophosphate (TBAPF6) solution in acetonitrile. During the CV
measurements, the solutions were purged with nitrogen for 30 s. In
each case, a carbon rod coated with a thin layer of polymers as the
working electrode, a platinum wire as the counter electrode, and
a silver wire as the quasi-reference electrode were used, and Ag/
AgCl (3M KCl) electrode was served as a reference electrode for all
potentials quoted herein. The redox couple of ferrocene/ferocenium
ion (Fc/Fcþ) was used as an external standard. The corresponding
highest occupied molecular orbital (HOMO) was calculated using
Eox/onset for experiment in solid film of polymer, which were per-
formed by drop-casting film with the similar thickness from THF
solutions (ca. 5 mg/ml). The LUMO level of PCBM employed was in
accordance with the literature data [17]. The onset potentials were
determined from the intersections of two tangents drawn at the
rising currents and background currents of the CV measurements.
advanced
p-conjugated materials by the combination of both small
molecular and polymeric designs through supramolecular archi-
tectures, so the H-bonded polymer networks are explored in this
report. Our previous studies focus on solar cell dyes H-bonded to
conjugated side-chain polymers to form either supramolecular
side-chain polymers by single H-bonds attached to one side of
(asymmetric) single carboxylic acid dyes [16a] or supramolecular
polymer networks by single H-bonds attached to both sides of
(symmetric) double carboxylic acid dyes [16b]. However, these
proton acceptor polymers contain pyridyl pendants to form weak
single H-bonds, and have shorter conjugation to exhibit worse solar
cell properties than the conjugated main-chain polymers. There-
fore, a conjugated main-chain polymer (with better PSC properties)
containing multiple H-bonded pendants (with stronger H-bonds to
the
p-conjugated small molecules) is applied to our molecular
design of supramolecular architectures in this study.
To the best of our knowledge, it is the first time to disclose the
supramolecular polymer networks containing multiple function-
alized melamine pendants and complementary H-bonded uracil
motifs for the PSC applications. To accomplish this goal, we choose
DTP, bithiazole, and thiophene units as the backbone of a conju-
gated main-chain polymer PBTH, in which the melamine pendants
can be H-bonded with complementary uracil-based conjugated
cross-linkers C and F (in proper molar amounts), i.e., the symmet-
rical uracils of cross-linkers C and F are linked to central carbazole
and fluorene units, respectively, through triple bonds (see Fig. 1.), to
generate supramolecular polymer networks PBTH/C and PBTH/F.
2.3. Fabrication and testing of polymer solar cells
The polymer photovoltaic cells in this study contained an active
layer of polymer PBTH, supramolecular polymer networks PBTH/C
and PBTH/F blended with PC70BM in solid films, which was sand-
wiched between a transparent indium tin oxide (ITO) anode and
a metal cathode (Ca). Prior to the device fabrication, ITO-coated
glass substrates (1.5 ꢁ 1.5 cm2) were ultrasonically cleaned in
detergent, deionized water, acetone, and isopropyl alcohol. After
routine solvent cleaning, the substrates were treated with UV
ozone for 20 min. Then, a modified ITO surface was obtained by

D. Patra et al. / Polymer 53 (2012) 1219e1228
1221
Fig. 1. Schematic illustration of conjugated main-chain polymer (PBTH) and supramolecular polymer networks (PBTH/C, and PBTH/F).
spin-coating a layer of poly(ethylene dioxythiophene):polystyr-
enesulfonate (PEDOT:PSS) (w30 nm). After thermal treating at
130 ꢀC for 1 h, the substrates were transferred to a nitrogen-filled
glove box. Subsequently, on the top of PEDOT:PSS layer, the active
layer was prepared by spin coating from blended solutions of PBTH,
PBTH/C and PBTH/F: PC70BM (with 1:1 w/w) with a spin rate ca.
1000 rpm, and the thickness of the active layer was typically ca.
90 nm. Initially, the blended solutions were prepared by dissolving
both polymers and PC70BM in chlorobenzene (10 mg/1 ml), fol-
lowed by continuous stirring for 12 h at 50 ꢀC. In the slow-growth
approach, blended polymers in solid films were kept in the liquid
phase after spin-coating by using the solvent with a high boiling
point (such as chlorobenzene) and allowed to dry the solvent
slowly. Finally, a calcium layer (30 nm) and a subsequent aluminum
layer (100 nm) were thermally evaporated through a shadow mask
at a pressure below 6 ꢁ 10ꢂ6 Torr to have the active device area of
0.12 cm2. All PSC devices were prepared and measured under
ambient conditions. The solar cell testing was done inside a glove
box under simulated AM 1.5G irradiation (100 mW/cm2) using
a Xenon lamp based solar simulator (Thermal Oriel 1000 W). The
light intensity was calibrated by a mono-silicon photodiode with
KG-5 color filter (Hamamatsu).
2.4. Synthesis of monomer M1, conjugated cross-linkers (C and F),
and conjugated main-chain polymer PBTH
2.4.1. 4,6-Dichloro-N,N-diphenyl-1,3,5-triazin-2-amine (1)
Under N2, a solution of diphenylamine (9.16 g, 54.22 mmol)
in THF (50 ml) was added dropwise to a mixture of diiso-
propylethylamine (9.5 ml, 54.22 mmol) and 2,4,6-trichloro-
1,3,5-triazine (10.0 g, 52.22 mmol) in THF(150 ml) at 20 ꢀC.
After stirring for 3 h, the solvent was removed by rotatary
evaporatior and the residue was purified by column chroma-
tography (hexane: ethyl acetate ¼ 4:1) to give a white color
solid. (12.9 g, yield 75%). 1H NMR (300 MHz, CDCl3,
d
):
7.45e7.41 (m, 4H), 7.35e7.33 (m, 2H), 7.30e7.27 (m, 4H). 13C
NMR (75 MHz, CDCl3,
127.40.
d): 170.83, 166.06, 141.81, 129.73, 127.91,
2.4.2. N2,N4-dibutyl-N6,N6-diphenyl-1,3,5-triazine-2,4,6-triamine (2)
Sodium bicarbonate (8.0 g, 84.01 mmol) was added to a solution
of compound 1 (10.0 g, 31.52 mmol) in 1,4-dioaxane (100 ml). After
that, butyl amine (9.4 ml, 94.58 mmol) was added and the resulting
mixture was refluxed overnight. The reaction mixtures was cooled
to room temperature and pour into cold water and extracted three

1222
D. Patra et al. / Polymer 53 (2012) 1219e1228
times with ethyl acetate. The combined organic fractions were
washed with brine, dried over magnesium sulphate (MgSO4), and
concentrated by rotary evaporation and purified by column chro-
matography, (hexane: ethyl acetate ¼ 7:3) to give a white solid.
2.4.6. 4,40-(9-(Tridecan-7-yl)-9H-carbazole-2,7-diyl)bis(2-
methylbut-3-yn-2-ol) (6)
To a solution of 2,7-dibromo-9-(tridecan-7-yl)-9H-carbazole (5)
(5.0 g, 9.84 mmol) in 50 ml of THF/Et3N (1/1), 3-methyl-1-butyn-3-
ol (3.85 ml, 39.37 mmol), Pd(PPh3)2Cl2 (0.13 g, 0.28 mol), PPh3
(0.1 g, 0.39 mol), and CuI (0.07 g, 0.39 mmol) were added. The
reaction mixture was degassed with nitrogen for 30 min, and
refluxed at 70 ꢀC under N2 for 12 h. The solvent was removed under
reduced pressure and resulting solid was extracted with DCM then
dried over MgSO4. The crude product was purified by column
chromatograph (hexane: ethyl acetate ¼ 4:1) to afford a white solid
(10.70 g, yield 87%). 1H NMR (300 MHz, CDCl3,
d):7.45e7.41 (m, 4H),
7.35e7.33 (m, 2H), 7.30e7.27 (m, 4H),4.84 (br, 2H), 3.34e3.11(br,
4H), 1.43e1.25 (m, 8H), 0.88 (t, J ¼ 6.2 Hz, 6H). 13C NMR (75 MHz,
CDCl3,
14.03.
d): 166.49, 144.61, 128.62, 128.28, 125.28, 40.57, 32.15, 20.25,
2.4.3. N2,N2-bis(4-bromophenyl)-N4,N6-dibutyl-1,3,5-triazine-
2,4,6-triamine (M1)
(3.9 g, 77%). 1H NMR (300 MHz, CDCl3,
d
): 7.97 (t, J ¼ 9.0, 2H), 7.65 (s,
N-Bromosuccinimide (NBS) (6.9 g, 38.40 mmol) was added
portion-wise to a solution of compound 2 (5.0 g, 12.80 mmol) in
af (50 ml) at 0 ꢀC. The reaction mixture was stirred for 6 h at the
same temperature and water was added to quench the reaction.
The organic layer was extracted three times by ethyl acetate
followed by brine and water washing and dried over anhydrous
MgSO4. The solvent was removed by rotary evaporator and the
product was further purified by column chromatography
(hexane: ethyl acetate ¼ 4:1) to yield a white solid. (5.7 g, 80%).
1H), 7.47 (s, 1H), 7.30 (m, 2H), 4.48 (m, 1H), 2.29e2.17 (m, 2H),
1.95e1.87 (m, 2H), 1.67 (s, 12H), 1.28e0.1.08 (m, 16H), 0.80 (t,
J ¼ 6.6 Hz, 6H).13C NMR (75 MHz, CDCl3,
d): 144.42, 131.05, 130.49,
124.29, 117.02,115.25, 93.91, 84.23, 81.55, 65.88, 65.78, 47.83, 33. 80,
31.81, 29.20, 27.10, 22.70, 14.20.
2.4.7. 2,7-Diethynyl-9-(tridecan-7-yl)-9H-carbazole (7)
A mixture of compound 6 (3.5 g, 6.81 mmol) and KOH
(2.70 mg, 47.68 mmol) in 60 ml of 2-propanol was heated to
reflux under N2 with a vigorous stirring for 4 h. Solvent was
removed and then the crude product was purified by column
chromatography (hexane) to afford a white solid (1.90 g, 70%). 1H
1H NMR (300 MHz, CDCl3,
d): 7.47e7.38 (m, 4H), 7.43e7.28 (m,
4H), 4.84 (br, 2H), 3.34e3.31(br, 4H), 1.43e1.25 (m, 8H), 0.88 (t,
J ¼ 6.2 Hz, 6H). 13C NMR (75 MHz, CDCl3,
d): 166.48, 166.11,
143.25, 131.81, 129.77, 118.79, 40.60, 32.09, 20.24, 14.01; EIMS
NMR (300 MHz, CDCl3,
d
): 8.01 (t, J ¼ 9.0, 2H), 7.72 (s, 1H), 7.56 (s,
[Mþ] calcd. m/z
¼
548.32, found 548.0. Anal. Calcd. for
1H), 7.35 (d, 2H), 4.48 (m, 1H), 3.15 (s, 1H), 2.30e2.17 (m, 2H),
1.97e1.87 (m, 2H), 1.25e1.04 (m, 16H), 0.80 (t, J ¼ 6.6 Hz, 6H). 13C
C23H28Br2N6: C, 50.38; H, 5.15; Br, 29.15; N, 15.33; found: C,
50.78; H, 5.48; N, 15.04.
NMR (75 MHz, CDCl3, d): 142.27, 138.43, 124.08, 123.21, 122.65,
120.82, 119.58, 115.67, 113.18, 85.27, 57.10, 33.89, 31.78, 29.25,
27.02, 22.73, 14.19.
2.4.4. 1-Hexyluracil (3)
Potassium carbonate (K2CO3) (14.80 g, 107.05 mmol) was added
to a suspension of uracil (10.0 g, 89.21 mmol) in dimethyl sulf-
oxide (DMSO) (150 ml), and stirred for 15e20 min at 45 ꢀC. 1-
Bromohexane (3.5 ml, 25 mmol) was added and the reaction
mixture was stirred for 48 h. The reaction was cooled to room
temperature and poured into cold water. The product was
extracted three times with dichloromethane (DCM), and washed
with dilute HCl, water, brine, and dried over MgSO4. The organic
layer was concentrated and poured into cold hexane with vigorous
stirring. The resulting precipitate was filtered and washed with
cold hexane to afford compound 3 (12.40 g, 70.9%) as a white solid.
2.4.8. 6,60-(9-(Tridecan-7-yl)-9H-carbazole-2,7-diyl)bis(ethyne-
2,1-diyl)bis(1-hexylpyrimidine-2,4(1H,3H)-dione) (C)
To a mixture of compound 7 (0.5 g, 1.26 mmol) in 20 ml THF/
NEt3 (1:1), compound 4 (1.22 g, 3.77 mmol), triphenylphosphine
(13 mg, 0.05), and CuI (10 mg, 0.05 mmol) were added. [Pd(PPh2)
Cl2] (36 mg, 0.05 mmol) was added under N2 and then the reaction
mixture was heated at 50 ꢀC for 36 h. The crude product was
extracted with DCM followed by brine wash and dried over MgSO4.
The resulting solutions were concentrated by rotary evaporator,
and purified by column chromatography, (hexane:ethyl acetate 7:3)
to give a light yellow solid. (0.60 g, yield 60%). 1H NMR (300 MHz,
1H NMR (300 MHz, CDCl3,
d
): 9.12 (br, 1H), 7.14 (d, J ¼ 9.0 Hz, 1H),
5.70 (d, J ¼ 6.0 Hz, 1H), 3.71(t, J ¼ 7.5 Hz, 2H), 1.74e1.65 (m, 2H),
CDCl3, d): 9.04 (br, 2H), 8.04 (m, 2H), 7.69 (s, 1H), 7.54 (s, 1H), 7.32
1.34e1.27 (m, 6H), 0.88 (t, J ¼ 6.6 Hz, 3H).13C NMR (75 MHz, CDCl3,
(m, 2H), 6.0 (s, 2H), 4.48 (m, 1H), 4.06 (t, J ¼ 6.0 Hz, 4H), 2.20 (m,
2H), 1.97 (m, 2H), 1.77 (m, 4H), 1.64 (m, 4H), 1.36e1.52 (m, 10H),
1.42e0.92 (m, 14H), 0.80 (t, J ¼ 6.9 Hz, 6H), 0.71 (t, J ¼ 6.2, 6H). 13C
d
): 165.24; 150.55; 144.80; 100.75; 48.67; 31.79; 28.26; 25.61;
22.48; 13.91.
NMR (75 MHz, CDCl3, d): 161.50, 156.79, 149.67, 137.77, 122.65,
2.4.5. 1-Hexyl-6-iodouracil (4)
121.97, 120.45, 114.39, 111.98, 105.50, 100.79, 79.45, 56.26, 45.75,
32.75, 30.50, 30.45, 29.27, 28.67, 27.96, 27.82, 25.78, 25.49, 21.57,
21.44, 12.96; EIMS [Mþ] calcd. m/z ¼ 786.06, found 787.0. Anal.
Calcd. For C49H62N4O4: C, 74.87; H, 8.08; N, 8.91; O, 8.14; found: C,
74.68; H, 7.97; N, 8.77.
At ꢂ78 ꢀC, lithium diisopropylamide (LDA) (20.4 ml of a 2.5 M
solution, 51.0 mmol) was added drop-wise to a solution of 1-
hexyluracil (2.0 g, 10.2 mmol) in THF (55 ml), and the resulting
solution was stirred under N2 for 2 h. I2 (12.9 g, 51.0 mmol) was
added and the reaction mixture was stirred for another 2 h at the
same temperature. Acetic acid (2.0 ml) was added under stirring at
room temperature overnight. The organic phase was extracted
with ethyl acetate, washed with saturated NaHCO3 (3 ꢁ 30.0 ml)
and Na2SO3 (3 ꢁ 30 ml) solutions. Finally, the product was washed
with brine (30 ml), and dried over MgSO4. The solvent was
removed by rotary evaporator and the crude product was purified
by column chromatography (hexane: ethyl acetate ¼ 5:5) to afford
2.4.9. 6,60-(9,9-Dihexyl-9H-fluorene-2,7-diyl)bis(ethyne-2,1-diyl)
bis(1-hexylpyrimidine-2,4(1H,3H)-dione) (F)
The synthesis procedure for F was followed using the same
procedure as that of C. After purification (hexane:ethyl acetate 7:3)
afforded a light yellow solid. (0.58 g, yield 58%). 1H NMR (300 MHz,
CDCl3,
d
): 9.60 (br, 2H), 7.77 (d, J ¼ 8.1 Hz, 2H), 7.53 (m, 4H), 6.04 (m,
2H) 4.01 (t, J ¼ 7.8 Hz, 4H), 2.0 (t, J ¼ 8.1 Hz, 4H), 1.82 (t, J ¼ 7.2 Hz,
4H), 1.44e1.34 (m, 12H), 1.36e1.08 (m, 12H), 1.15e1.08 (m, 12H),
0.88 (t, J ¼ 6.9 Hz, 6H), 0.77 (t, J ¼ 7.2, 6H), 0.59 (m, 4H). 13C NMR
compound 4 (2.2 g, 67%). 1H NMR (300 MHz, CDCl3,
d): 9.48 (br,
1H), 6.41 (s, 1H), 4.0 (t, J ¼ 8.1 Hz, 2H), 1.74e1.65 (m, 2H), 1.34e1.27
(m, 6H), 0.88 (t, J ¼ 6.9 Hz, 3H).13C NMR (75 MHz, CDCl3,
d
):
(75 MHz, CDCl3, d): 162.96, 151.95, 151.08, 142.41, 138.89, 131.72,
160.65; 148.19; 115.91; 113.52; 53.48; 31.36; 27.45; 26.32; 22.17;
14.18.
126.58, 121.05, 119.76, 106.89, 101.28, 81.30, 55.84, 46.94, 40.31,
31.67, 29.77, 29.02, 26.65, 23.97, 22.77, 22.75, 14.19; EIMS [Mþ]

D. Patra et al. / Polymer 53 (2012) 1219e1228
1223
calcd. m/z ¼ 771.04, found 772.0. Anal. Calcd. For C49H62N4O4: C,
structures of PBTH, M1, C and F were satisfactorily characterized by
1H NMR, MS spectroscopy, and elemental analyses. The weight
average molecular weight (Mw) of PBTH was determined by gel
permeation chromatography (GPC) with THF as the eluting solvent
and polystyrene as a standard. The Mw and polydispersity index (PDI)
of PBTH were found to be 21,100 g/mol and 1.65, respectively. The
decomposition temperatures with 5% weight loss (Td) of polymer
PBTH and supramolecular polymer networks PBTH/C and PBTH/F
were measured by thermogravimetric analysis (TGA) (see Fig. 2) and
found to be 380, 402, and 391 ꢀC, respectively, indicating supramo-
lecular polymer networks PBTH/C and PBTH/F have better thermal
stabilities than polymer PBTH. These thermal stabilities are adequate
for their applications in polymer solar cells and other optoelectronic
devices.
76.33; H, 8.10; N, 7.27; O, 8.30; found: C, 76.00; H, 7.97; N, 7.17.
2.5. Synthesis of PBTH via stille coupling polymerization
Into a 25 ml two-neck flask, 1 equiv. of dibromo monomers (M1
and M3) and 2 equiv. of 2,6-bis(tributylstannyl)-4-(tridecan-7-yl)-
4H-dithieno[3,2-b:20,30-d]pyrrole (M2) were added in anhydrous
toluene and deoxygenated with nitrogen for 30 min. The Pd(PPh3)4
(1 mol %), was transferred into the mixture in a dry environment.
The reaction mixture was stirred at 110 ꢀC for 48 h, and then an
excess amount of 2-bromothiophene was added to end-cap the
trimethylstannyl groups for 4 h. After cooling to room temperature,
the solution was added dropwise into MeOH (200 ml). The crude
polymer was collected, dissolved in hot CHCl3, filtered through
a 0.5-mm poly(tetrafluoroethylene) (PTFE) filter, and reprecipitated
3.2. IR measurements
in MeOH. The solid was washed with acetone, hexane, and CHCl3 in
a Soxhlet apparatus. The CHCl3 solution was concentrated and then
added dropwise into MeOH. Finally, the product was collected and
dried under vacuum to give polymer PBTH (140 mg, 77%) with
Mw ¼ 21,100 g/mol and polydispersity index (PDI) ¼ 1.65. 1H NMR
The existence of multiple H-bonds in the H-bonded complexes
can be confirmed by FT-IR spectroscopy. Therefore, IR spectra of p-
conjugated cross-linkers C and F, conjugated main-chain polymer
PBTH, and supramolecular polymer networks PBTH/C and PBTH/F
are shown in Fig. 3. Both IR spectra of cross-linkers C and F are
located likewise at 1702e1704 cmꢂ1 and 3145e3147 cmꢂ1 for free
C]O and NeH stretching bands, respectively [13,18]. The peak at
3280 cmꢂ1 can be assigned to stretching frequency of amine groups
(eNHe) present in PBTH. Therefore, the C]O stretching bands of
supramolecular polymer networks PBTH/C and PBTH/F were
(300 MHz, CDCl3): d 7.17 (m), 7.21e7.15 (m), 4.91 (br), 4.21 (m), 3.56
(br, 2H), 2.99 (m), 2.11e1.74 (m), 1.27e1.49 (m), 1.66e1.11 (m),
0.91e0.81 (m). Anal. Calcd For: C91H118N10S8: C, 67.95; H, 7.39; N,
8.71; S, 15.95; Found: C, 66.38; H, 7.85; N, 8.12.
2.6. Preparation of supramolecular polymer networks (PBTH/C and
PBTH/F)
considerably shifted to lower wave numbers of 1683 and 1680 cmꢂ1
,
respectively, and the eNHe stretching bands were significantly
shifted to higher wave numbers of 3310 and 3305 cmꢂ1, respectively.
Consequently, the large shifts of wave numbers in C]O and NeH
stretching bands during the formation of supramolecular polymer
networks (PBTH/C and PBTH/F) indicate the presence of multiple H-
bonds between polymer PBTH and cross-linkers (C and F) [13a,19].
Supramolecular polymer networks (PBTH/C and PBTH/F) were
prepared by dissolving 2 mol of polymer (PBTH) and 1 mol of bi-
functional conjugated cross-linker (C and F) in minimum
amounts of CHCl3 and then the solvent was evaporated under
ambient temperature.
3. Results and discussion
3.3. Optical properties
3.1. Synthesis and characterization
The UVevisible absorption spectra of a conjugated main-chain
polymer PBTH,
p-conjugated cross-linkers C and F (in dichloro-
The synthetic details of monomer M1,
p
-conjugated cross-linkers
benzene solutions and solid films), and supramolecular polymer
networks PBTH/C and PBTH/F (in solid films only) are shown in
Fig. 4, and their photophysical properties are summarized in
Table 1. As shown in Fig. 4b, the light harvesting capability of PBTH
can be directly tuned by forming multiple H-bonds with p-conju-
gated cross-linkers C and F, which could be clearly identified from
(C and F), and conjugated main-chain polymer PBTH are outlined in
Scheme 1. 4,6-Dichloro-N,N-diphenyl-1,3,5-triazin-2-amine (1) was
prepared by reacting of diphenylamine and 2,4,6-trichloro-1,3,5-
triazine, and subsequently converted to N2,N4-dibutyl-N6,N6-
diphenyl-1,3,5-triazine-2,4,6-triamine (2) and Finally brominated to
M1 by NBS at 0e5 ꢀC. 1-Hexyluracil (3) was prepared by reaction of
unsubstituted uracil with 1-bromohexane in the presence of K2CO3
and DMSO. Deprotonation of compound 3 with LDA and addition of I2
could yield 1-hexyl-6-iodouracil (4). Then, 2,7-dibromo-9-(tridecan-
7-yl)-9H-carbazole[11a] (5) was reacted with 3-methyl-1-butyn-3-ol
via the Sonogashira coupling reaction to form 4,40-(9-(tridecan-7-yl)-
9H-carbazole-2,7-diyl)bis(2-methylbut-3-yn-2-ol) (6) and further
deprotected by refluxing 2-propanol in a basic condition to obtain
2,7-diethynyl-9-(tridecan-7-yl)-9H-carbazole (7). The H-bonded
cross-linkers C and F were synthesized via palladium catalyzed
Sonogashira cross-couplingreactionsofcompound4 withcompound
7 and 2,7-diethynyl-9,9-dihexyl-9H-fluorene (8), respectively. In
addition, compounds 8 [6a], M2 [17], and M3 [11a] were prepared by
similar procedures reported earlier. In this study, a conjugated main-
chainpolymer (PBTH) consistingof DTP and M1 units as the electron-
donating moieties and bithiazole unit as the electron-accepting
moiety (sandwiched between two thiophene units) were synthe-
sized by Pd(0)-catalyzed Stille coupling polymerization in toluene at
110 ꢀC with an input molar ratio of m:n ¼ 1:1 (output molar ratio of
m:n ¼ 1.32:1 determined by NMR characterization). The chemical
their absorption spectra of supramolecular polymer networks
PBTH/C and PBTH/F in solid films. The absorption maxima of
p-
conjugated cross-linkers C and F in dichlorobenzene solutions are
379 and 369 nm, respectively, and in solid films are 419 and
409 nm, respectively. Because of the inter-chain association and
p-
p
stacking, the absorption wavelengths of -conjugated cross-
p
linkers C and F in solids films generally show red-shifts of 40 and
30 nm, respectively, in contrast to those in dilute solutions [15]. The
absorption maxima of PBTH in the dichlorobenzene solution and
solid film are found to be 523 and 535 nm, respectively, which can
be attributed to the intramolecular charge transfer (ICT) from the
donor to acceptor moeities [11]. In solid films, UVevisible spectra of
supramolecular polymer networks PBTH/C and PBTH/F are found
to be 550 and 544 nm, respectively, and red shifted about 15 and
9 nm (from PBTH), respectively, due to the enhanced p-p stacking
and efficient charge transfer between polymer PBTH and cross-
linkers (C and F) [19b,20]. New peaks of 407 and 396 nm were
formed in the visible region of PBTH/C and PBTH/F, respectively,
which are mainly attributed to the cross-linkers (C and F) and help
PBTH/C and PBTH/F to harvest sufficient amounts of lights from the

1224
D. Patra et al. / Polymer 53 (2012) 1219e1228
Scheme 1. Synthesis routes of monomer M1, C, F, and polymer PBTH. Reagents and conditions: (i) diisopropylethylamine, THF, 2,4,6-trichloro-1,3,5-triazine, 20 ꢀC, 3 h; (ii) sodium
bicarbonate, bromobutane,1,4-dioxane, reflux, overnight; (iii) NBS, 0e5 ꢀC, THF; (iv) K2CO3, DMSO, 45 ꢀC, overnight; (v) ꢂ78 ꢀC, LDA, THF, 5 h, I2; (vi) THF/Et3N (1/1), 3-methyl-1-
butyn-3-ol, Pd(PPh3)2Cl2, PPh3, CuI, 12 h; (vi) KOH, 2-propanol, 4 h; (viii) Ph3P, CuI, THF, Et3N, Pd(PPh2)Cl2, 50 ꢀC, overnight; (ix) Toluene, Pd(PPh3)4, 110 ꢀC, 48 h.
visible region and further to improve the performance of photo-
voltaic devices [21].
(HOMO) and lowest unoccupied molecular orbital (LUMO) levels,
the electrochemical properties of polymers were investigated by
cyclic voltammetry (CV) measurements on polymer films in aceto-
nitrile containing 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) at a potential scanning rate of 100 mV/s. The oxidation
and reduction cyclic voltammograms of all polymers are shown in
Fig. 5, and their electrochemical properties, such as the onset
oxidation potentials (Eooxnset), HOMO and LUMO levels of polymers,
are summarized in Table 1. It can be seen that all polymers PBTH,
PBTH/C, and PBTH/F possessed three oxidation and one reduction
As shown in Table 2, the optical band gaps (Egopt) of the polymers
in solid films determined by the cut off wavelengths of the optical
absorptions are in the range of 1.72e1.81 eV. Therefore, this result
implies that it is an efficient way to design supramolecular polymer
networks to absorb maximum photons from the visible region for
enhanced photovoltaic applications.
3.4. Electrochemical properties
irreversible peaks, and
p-conjugated cross-linkers C and F
To investigate the redox behavior of polymer (PBTH) and
supramolecular polymer networks (PBTH/C and PBTH/F) and their
electronic states, i.e., the highest occupied molecular orbital
possessed one oxidation and reduction irreversible peaks at positive
onset
and negative potentials. The E
values were found to be 0.55,
ox
0.66, 0.68, 1.26, and 1.49 V for PBTH, PBTH/C, PBTH/F, C, and F,

D. Patra et al. / Polymer 53 (2012) 1219e1228
1225
Fig. 2. TGA plots of polymer PBTH and supramolecular polymer networks (PBTH/C,
and PBTH/F) with a heating rate of 10 ꢀC/min under N2.
respectively. The HOMO energy levels were estimated by the
oxidation potentials from the reference energy level of ferrocene
according to the following equation
h
ꢀ
ꢁ
i
EHOMO
¼
ꢂ
Eox ꢂ EonsetðFC=FCþ VS: Ag=AgþÞ ꢂ 4:8 eV
Where 4.8 eV is the energy level of ferrocene below the vacuum
level and EonsetðFC=FCþ VS: Ag=AgþÞ ¼ 0.45 eV and Eox is the oxidation
potential of each polymer [22]. The HOMO levels of polymer PBTH,
supramolecular polymer networks (PBTH/C and PBTH/F), and
p-
conjugated cross-linkers (C and F) were found to
be ꢂ4.90, ꢂ5.01, ꢂ5.03, ꢂ5.61, and ꢂ5.84 eV, respectively. Obvi-
ously, the supramolecular incorporation of electron rich building
blocks, such as carbazole and fluorene units, enhanced oxidation
potentials of PBTH/C and PBTH/F and lowered the HOMO levels due
to their lower HOMO energy levels [23]. The LUMO levels of PBTH,
PBTH/C, PBTH/F, C and F were obtained from their optical band
gaps, and the HOMO energy levels were calculated by using the
following equation [24].
Fig. 4. Normalized UVevis spectra of (a) polymer PBTH and conjugated cross-linkers C
and F in dilute dichlorobenzene solutions and (b) supramolecular polymer networks
PBTH/C and PBTH/F in solid films (on glass substrates).
ELUMO ¼ EHOMO þ Egopt
respectively, and which depends on the strength of acceptors [11b].
The low HOMO levels of PBTH/C and PBTH/F from PBTH is an
advantage for attaining high Voc values in solar cell devices, because
the Voc value is directly proportional to the difference between the
HOMO level of each donor polymer and the LUMO level of the
acceptor derivative (PC70BM) [25]. On the other hand, the LUMO
energy level of the electron donor (polymer) has to be positioned
above the LUMO energy level of PC70BM at least 0.3 eV, so the exciton
binding energy of the polymer could be overcome and resulted in
efficient electron transfer from donor to acceptor.
The LUMO energy levels were found to be ꢂ3.09, ꢂ3.29,
ꢂ3.28, ꢂ3.03, and ꢂ3.18 eV for PBTH, PBTH/C, PBTH/F, C, and F
3.5. Photovoltaic properties
To investigate the photovoltaic properties of a conjugated main-
chain polymer (PBTH) and supramolecular polymer networks
(PBTH/C and PBTH/F) in PSC devices, the bulk heterojunction (BHJ)
solar cell devices were fabricated by using polymers as electron
donors and PC70BM as an electron acceptor with a device config-
uration of ITO/PEDOT:PSS(30 nm)/(copolymers):PC70BM(1:1w/w)
(w90 nm)/Ca(30 nm)/Al(100 nm). In order to obtain a better
absorption coefficient in visible region, PC70BM was chosen as
electron acceptor rather than PC61BM [25]. To achieve better
performance in PSC devices, chlorobenzene was chosen as the
solvent to obtain the blended polymer active layers with good film
Fig. 3. FT-IR spectra of polymer PBTH, supramolecular polymer networks (PBTH/C and
PBTH/F) and cross-linkers (C and F).

1226
D. Patra et al. / Polymer 53 (2012) 1219e1228
Table 1
Optical and electrochemical properties of polymer PBTH and supramolecular
polymer networks (PBTH/C and PBTH/F).
Solutiona
labs (nm)
Solid filmb
labs (nm)
Egopt
(eV)
E
EHOMO
(eV)
ELOMO
(eV)
c
d
onset
ox
(V)
Polymers
C
F
379
369
523
e
419
409
535
407, 550
396, 544
2.58
2.66
1.81
1.72
1.75
1.26
1.49
0.55
0.66
0.68
ꢂ5.61
ꢂ5.84
ꢂ4.90
ꢂ5.01
ꢂ5.03
ꢂ3.03
ꢂ3.18
ꢂ3.09
ꢂ3.29
ꢂ3.28
PBTH
PBTH/C
PBTH/F
e
a
In dichlorobenzene.
Spin coated from dilute dichlorobenzene solution on glass surface.
HOMO
½ꢂðEonset ꢂ EonsetðFC=FCþ VS: Ag=AgþÞÞ ꢂ 4:8ꢃ eV where 4.8 eV is the
b
c
¼
energylevel of ferrocene belowthevacuum level andEonsetðFC=FCþ VS: Ag=AgþÞ ¼ 0.45 eV.
ELUMO ¼ EHOMO þ Egopt
.
d
Table 2
Photovoltaic properties of polymer PBTH and supramolecular polymer networks
(PBTH/C and PBTH/F).
Weight ratio of polymer
to PC70BM
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
Fig. 6. JꢂV characteristics of PBTH, PBTH/C, and PBTH/F under illumination of AM 1.5
at 100 mW/cm2.
PBTH: PC70BM ¼ 1:1
PBTH/C: PC70BM ¼ 1:1
PBTH/C: PC70BM ¼ 1:2
PBTH/C: PC70BM ¼ 1:3
PBTH/F: PC70BM ¼ 1:1
PBTH/F: PC70BM ¼ 1:2
PBTH/F: PC70BM ¼ 1:3
0.51
0.60
0.60
0.58
0.60
0.61
0.58
2.73
4.58
7.16
4.97
3.28
3.66
3.09
0.36
0.35
0.36
0.30
0.35
0.36
0.32
0.52
0.97
1.56
0.87
0.68
0.80
0.57
whereas its supramolecular polymer networks PBTH/C and PBTH/F
showed improved Voc values (0.60 and 0.61 V, respectively) due to
of their lower HOMO levels (ꢂ5.01 and ꢂ5.03 eV, respectively) by
the supramolecular design. The PCE values of PSC devices con-
taining PBTH/C and PBTH/F with PC70BM in a weight ratio of 1:1
were found to be 0.97% and 0.68%, respectively. Hence, both
supramolecular polymer networks PBTH/C and PBTH/F showed
improved Jsc values owing to the enhanced crystallinities compared
with PBTH, which can be further verified by the powder X-ray
diffraction (XRD) patterns. As demonstrated in Fig. 7, the XRD
patterns of polymer (PBTH) and supramolecular polymer networks
(PBTH/C and PBTH/F) were analyzed. Compared with a single weak
peak in the small angle region of polymer PBTH, both sharp peaks
at the small angle region and broad peaks at the wide angle region
were observed in supramolecular polymer networks PBTH/C and
PBTH/F. Thus, the crystallinities of the supramolecular polymer
networks PBTH/C and PBTH/F were much improved due to the
presence of multiple hydrogen bonds, which enhanced the chain-
reorganization and self-assembled behavior and thus to induce
higher PCE values of H-bonded polymer network (PBTH/C and
PBTH/F) [1a,3a,22].
qualities. Fig. 6 shows the J-V curves of polymer solar cell (PSC)
devices under the condition of AM 1.5 at 100 mW/cm2, and the
open circuit voltage (Voc), short-circuit current density (Jsc), fill
factor (FF), and power conversion efficiency (PCE) values are
summarized in Table 2.
The obtained PCE value of conjugated main-chain polymer
PBTH was found to be 0.52% with Voc ¼ 0.51 V, Jsc ¼ 2.81 mA/cm2,
and FF ¼ 0.36. However, a similar alternating copolymer reported
by Li et al., which consisted of DTP and M3 units, only achieved
a lower PCE value of 0.06% and a Voc of 0.26 V. In addition, a poly-
mer similar to PBTH was reported to possess a PCE value of 0.69%
and a Voc value of 0.40 V [11c]. The lower Voc value (0.51 V) of
polymer PBTH was because of its higher HOMO level (ꢂ4.90 eV),
Fig. 5. Cyclic voltammograms of PBTH, C, F, PBTH/C, and PBTH/F in solid films at
a scan rate of 100 mV/s.
Fig. 7. X-ray diffraction patterns of polymer PBTH and supramolecular polymer
networks (PBTH/C and PBTH/F) in solid powders.

D. Patra et al. / Polymer 53 (2012) 1219e1228
1227
The surface morphology of the active layer in solar cell devices is
also a key parameter for the device performance [28]. AFM images of
blended polymers PBTH, PBTH/C, and PBTH/F mixed with PC70BM in
wt. ratios of 1:1, 1:2, and 1:2, respectively, are presented in Fig. 8 and
their root-mean-square roughness (Rrms) values were found to be
0.93, 0.40, and 0.85 nm, respectively. All the active layers of PBTH,
PBTH/C, and PBTH/F showed good film formation to have smooth
surfaces with no obvious aggregations (only nano-scale phase
separations) between polymers and PC70BM. A higher Rrms value
found at PBTH blended with PC70BM in a wt. ratio of 1:1 compared
with supramolecular polymer networks PBTH/C and PBTH/F
blended with PC70BM in a wt. ratio of 1:2. This larger Rrms value of
PBTH blended with PC70BM resulted in the decreased diffusional
escape probabilities for mobile charge carriers, and hence increased
their recombinations [29]. Therefore, less aggregations between
supramolecular networks PBTH/C and PBTH/F and PC70BM allows
for better photogenerated charges and inducing higher Jsc values via
supramolecular engineering of solar cell devices.
Interestingly, compared with polymer PBTH (with Voc ¼ 0.51 V),
the Voc value of the solar cell devices containing supramolecular
polymer networks PBTH/C and PBTH/F (as polymer:PC70BM ¼ 1:1
w/w) increased to 0.60 V. The supramolecular design resulting in an
increase of 0.09 V in Voc is a good agreement with the HOMO levels
(see Table 1) of ꢂ4.9, ꢂ5.01, and ꢂ5.03 eV for PBTH, PBTH/C, and
PBTH/F, respectively [27]. Various parameters improved in the
suramolecular design, such as more efficient light harvests, lower
HOMO energy levels, and higher crystallinities, are responsible for
the enhancement of Voc and Jsc values in PSC devices containing
supramolecular polymer networks PBTH/C and PBTH/F in contrast
to polymer PBTH. The enhanced photovoltaic properties of supra-
molecular polymer networks PBTH/C and PBTH/F indicate that the
supramolecular approach is one of an ideal design for high-
performance donor-acceptor polymer solar cells.
4. Conclusions
In summary, we have developed novel supramolecular polymer
networks PBTH/C and PBTH/F based on the formation of multiple
hydrogen bonds between conjugated main-chain polymer PBTH and
uracil-based conjugated cross-linkers C and F (in proper molar
amounts) through complementary melamine and uracil moieties.
The supramolecular formation of multiple hydrogen bonds success-
fully characterizated by IR, and UVeVis spectra measurements. The
supramolecular concept has been successfully applied to tune the
photophysical, electrochemical, and photovoltaic properties of
supramolecular polymer networks PBTH/C and PBTH/F compared
with polymer PBTH. The power conversion efficiencies (PCE) values
of PSC devices containing supramolecular polymer networks PBTH/C
and PBTH/F (as polymer:PC70BM ¼ 1:1 w/w) are found to be 0.97 and
0.68%, respectively, in contrast to 0.52% for polymer PBTH. The
optimum photovoltaic performance with the highest PCE value of
1.56% was obtained in the PSC device containing supramolecular
polymer networks PBTH/C as polymer:PC70BM ¼ 1:2 w/w. This
supramolecular design is a encouraging approach on the further
improvements of any most excellent donor-acceptor photovoltaic
polymers to be in conjunction with efficient conjugated cross-linkers
complementarily through supramolecular interactions for future
polymer solar cell applications.
Fig. 8. AFM images of blended polymers with PC70BM in wt. ratios (a) PBTH: PC70BM
(1:1), (b) PBTH/C: PC70BM (1:2), and (c) PBTH/F: PC70BM (1:2).
The photovoltaic properties of PBTH, PBTH/C, and PBTH/F were
investigated initially using a 1:1 weight ratio of polymer:PC70BM,
then different blending ratios of polymer:PC70BM were applied to
optimize their PCE values of PBTH/C and PBTH/F. As illustrated in
Table 2, the maximum PCE values of 1.56% and 0.80% were obtained
in the PSC devices containing PBTH/C and BPTH/F, respectively, with
a weight ratio of polymer:PC70BM ¼ 1:2, where the photovoltaic
properties of PBTH/C with the optimum PCE value (1.56%) possessed
Voc ¼ 0.60 V, Jsc ¼ 7.16 mA/cm2, and FF ¼ 0.36. Indeed, the weight
ratios between polymer donors and PC70BM acceptor played a key
role in the PCE values of all PSC devices [17,26]. However, by
increasing the weight ratios of PC70BM in supramolecular polymer
networks PBTH/C or PBTH/F (i.e., polymer:PC70BM ¼ 1:3 w/w), both
Jsc and PCE values were reduced (see Table 2), which might be
attributed to the reduced interfacial area of exciton dissociation and
the insufficient percolation for charge transport in blended films
[11a,17,27].
Acknowledgements
We acknowledge the financial supports from the National
Science Council of Taiwan (ROC) through NSC 99-2113-M-009-006-
MY2 and National Chiao Tung University through 97W807.

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D. Patra et al. / Polymer 53 (2012) 1219e1228
[13] (a) Piot L, Palma CA, Pallas AL, Prato M, Szekrényes Z, Kamarás K, et al. Adv
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