Synthesis and optoelectronic properties of novel benzodifuran semiconducting polymers

Article abstract of DOI:10.1002/pola.26243

Two new semiconducting polymers poly{4,8-bis(4-decylphenylethynyl)benzo[1, 2-b:4,5-b']difuran} (P1) and poly {4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5- b']difuran-alt-4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b']dithiophene} (P2) have been synthesi

Full text of DOI:10.1002/pola.26243

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Synthesis and Optoelectronic Properties of Novel Benzodifuran
Semiconducting Polymers
Prakash Sista, Peishen Huang, Samodha S. Gunathilake, Mahesh P. Bhatt,
Ruvini S. Kularatne, Mihaela C. Stefan, Michael C. Biewer
Department of Chemistry, University of Texas at Dallas, Richardson, Texas 75080
Correspondence to: M. C. Biewer (E-mail: biewerm@utdallas.edu)
Received 21 May 2012; accepted 25 June 2012; published online
DOI: 10.1002/pola.26243
ABSTRACT: Two new semiconducting polymers poly{4,8-bis(4-
decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran} (P1) and poly
{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran-alt-4,8-
of the solar cell devices indicated that both the polymers dis-
play a granular morphology with smoother films displaying
C
V
higher power conversion efficiencies. 2012 Wiley Periodicals,
bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene}
(P2)
Inc. J Polym Sci Part A: Polym Chem 000: 000–000, 2012
have been synthesized. These polymers were tested in bulk
heterojunction solar cells yielding power conversion efficien-
cies of 1.19% for P1 and 0.79% for P2. The surface morphology
KEYWORDS: atomic force microscopy (AFM); conjugated
polymers; UV-vis spectroscopy
INTRODUCTION Thiophene semiconducting polymers have
been in the forefront of organic electronics research in the
last decade.1,2 Poly(3-hexylthiophene) (P3HT) is one of the
most used semiconducting polymers in organic electronics
applications,2–4 and P3HT has predominantly found applica-
tions in organic solar cells3,4 and field-effect transistors.5
However, because of the position of the highest occupied mo-
lecular orbital (HOMO) energy level of P3HT relative to the
lowest unoccupied molecular orbital (LUMO) of the acceptor
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the open-
circuit voltage obtained in bulk heterojunction solar cells
with P3HT donor is relatively low.2 To enhance the light
absorption and to fine tune the energy levels of the poly-
mers, a variety of fused-ring building blocks have been
used.6–20 Benzo[1,2-b:4,5-b0]dithiophene (BDT) has gained
significant interest among the fused-ring thiophenes because
of its symmetrical structure and the ability to incorporate
substituents on the central benzene core.7–10,15–18,21
thiophene semiconducting polymers, the furan polymers
showed enhanced solubility.26 Although the mechanism
behind improved solubility is not well established, it may be
due to the differences in atomic radius, electronegativity, and
polarizability of oxygen versus sulfur atoms, which may
impact solvent and intermolecular interactions.26 Further-
more, furan can be manufactured from renewable, plant-
derived sources enabling the synthesis of biorenewable
materials.27
Benzo[1,2-b:4,5-b0]difuran (BDF) building block possesses
unique properties28 because furan displays weaker steric
hindrance to adjacent units due to the smaller oxygen atom
when compared with sulfur.29 Consequently, polymerization
of benzodifuran is expected to yield a more planar polymer
backbone with increased conjugation length. Thus, BDF semi-
conducting polymers with smaller bandgap and improved
optoelectronic properties could be envisioned. For example,
Decurtins and coworkers30 reported the synthesis of BDF
semiconducting polymers and their use in bulk heterojunc-
tion solar cells. The PCEs reported initially for BDF polymers
was only 0.17–0.59%.30 However, recently, a PCE of ꢁ5%
was reported for a donor–acceptor copolymer of BDF.28
BDT-based polymers have demonstrated good performance
in both bulk heterojunction solar cells17 and field-effect tran-
sistors.9,13,14,18,22–24 Currently, a power conversion efficiency
(PCE) of 8.37% has been reported by Cao and coworkers25
using BDT-based polymers as electron donors in bulk hetero-
junction solar cells. Moreover, field-effect mobility as high as
0.15–0.25 cm2 Vꢀ1 sꢀ1 have been reported for BDT semicon-
ducting polymers.14 Replacing the thiophene with furan moi-
eties could potentially lead to favorable changes in the opto-
electronic properties of the polymers. When compared with
In this article, we report the synthesis and optoelectronic
properties of two benzodifuran polymers containing decyl-
phenylethynyl substituents. This work is an extension of our
previous work on semiconducting polymers of benzodithio-
phene with decyl phenylethynyl substituents.18 Synthesis,
characterization, and photovoltaic performance of
a
Additional Supporting Information may be found in the online version of this article.
C
V
2012 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of benzodifuran monomers.
homopolymer of BDF with decyl phenylethynyl substituents
and an alternating copolymer of BDF and BDT with decyl
phenylethynyl substituents are reported.
of 4-decyl-1-ethynylbenzene was reacted with 4,8-dihydro-
benzo[1,2-b:4,5-b0]difuran-4,8-dione to generate 4,8-bis(4-
decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran (1) monomer
after subsequent aromatization with SnCl2 (Scheme 1). Lithi-
ated monomer (1) was reacted with trimethyltin chloride
and carbon tetrabromide to generate the distannylated and
dibromo derivatives, respectively, which were used in Stille
coupling polymerization to obtain poly{4,8-bis(4-decylphenyle-
thynyl)benzo[1,2-b:4,5-b0]difuran} (P1) homopolymer (Fig. 1).
RESULTS AND DISCUSSION
4,8-Bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran has
been synthesized similar to a procedure described in our
earlier publication (Scheme 1).18 Furan-3-carboxylic acid was
reacted with thionyl chloride to obtain the acid chloride,
which was further reacted with diethyl amine to generate N,
N-diethyl furan-3-carboxamide. The amide was reacted with
n-BuLi to form 4,8-dihydrobenzo[1,2-b:4,5-b0]difuran-4,8-
dione. Precursor 4-decyl-1-ethynylbenzene was synthesized
according to our published procedure.18 The acetylide anion
Stille coupling polymerization of 2,6-dibromo-4,8-bis(4-decyl-
phenylethynyl)benzo[1,2-b:4,5-b0]difuran and 2,6-(trimethyl-
tin)-4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithio-
phene yielded the alternating copolymer poly{4,8-bis(4-de-
cylphenylethynyl)benzo[1,2-b:4,5-b0]difuran-alt-4,8-bis(4-decyl-
FIGURE 1 Structures of benzodifuran and benzodithiophene polymers: poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-
b0]difuran} poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran-alt-4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-
(P1),
b0]dithiophene} (P2), and poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P3).
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TABLE 1 Molecular Weights and Optoelectronic Properties of the Synthesized Polymers
Mn (g molꢀ1
)
PDIa
kmax (CHCl3)b (nm)
kmax (Film)c (nm)
HOMOd (eV)
LUMOe (eV)
Eg (eV)
a
Polymer
P1
P2
P3
a
29,600
22,500
23,400
2.2
2.5
3.0
326, 522
350, 524
363, 520
333, 531
357, 541
366, 525
d
ꢀ5.39
ꢀ5.30
ꢀ5.10
ꢀ3.32
ꢀ3.26
ꢀ3.05
2.07
2.04
2.05
Estimated from SEC.
Estimated from the onset oxidation peak from
a
cyclic
b
c
UV–vis absorption maxima of polymer solution in chloroform.
UV–vis absorption maxima of polymer film drop-cast from chloroform
voltammogram.
e
Estimated from the onset of reduction peak of the cyclic
voltammogram.
solution.
phenylethynyl)benzo[1,2-b:4,5-b0] dithiophene} (P2) (Fig. 1).
The structure of the benzodithiophene polymer (P3) analog is
also shown in Figure 1 for comparison.
to 533 nm (in film). The absorption maxima of P2 shifted
from 348 nm (in CHCl3) to 356 nm (in film) and from
524 nm (in CHCl3) to 541 nm (in film). The relatively
small red shift of the absorption bands in film combined
with the vibronic peaks indicates the presence of rigid rod
conformation in both the solution and solid states of P1
and P2.20,33
The polymers P1 and P2 were soluble in organic solvents
like THF and CHCl3. However, it was observed that the solu-
bility of both the polymers in CHCl3 was slightly lower than
that of homopolymer poly{4,8-bis(4-decylphenylethynyl)-
benzo[1,2-b:4,5-b0]dithiophene} (P3) already published by
our group.18 The number–average molecular weights of both
P1 (29,600 g molꢀ1) and P2 (22,500 g molꢀ1) were compa-
rable with that reported for benzodithiophene homopolymer
P3 (Table 1). UV–visible absorption spectra of both the poly-
mers P1 and P2 were recorded in chloroform solution as
well as in thin films (Fig. 2 and Table 1). Both spectra dis-
played two absorption maxima at ꢁ340 nm and ꢁ530 nm.
The peak at ꢁ340 nm arises from the conjugation along the
1,4-bis(phenylethynyl)benzene side chains, whereas the peak
at ꢁ530 nm originates from the p–p* transition of the ben-
zodifuran backbone.18,31,32 A vibronic peak was observed at
558 nm in CHCl3 and at 569 nm in film in the UV–vis spectra
of P1. In contrast, the absorption peak arising from the back-
bone of P2 was broader and did not display a vibronic peak.
Red shift of both 1,4-bis(phenylethynyl)benzene absorption
and benzodifuran backbone absorption peaks can be
observed in the solid-state UV–vis spectra (Supporting Infor-
mation). The absorption maxima of P1 shifted from 327 nm
(in CHCl3) to 331 nm (in film) and from 524 nm (in CHCl3)
The HOMO and LUMO energy levels of P1 and P2 were cal-
culated from the values of onset of oxidation wave and onset
of reduction wave measured using cyclic voltammetry (Table
1 and Supporting Information). The polymers exhibit HOMO
energy levels of ꢀ5.39 eV for P1 and ꢀ5.30 for P2. The
more electronegative oxygen atom and the less polarizable
carbon–oxygen bond make the furan unit a weaker donor,
and hence, a deeper HOMO energy level was measured for
P1 when compared with the benzodithiophene homopolymer
P3 (Table 1).34 The HOMO energy levels of both these poly-
mers are lower than the air oxidation threshold indicating
good air stability. Similarly, the LUMO energy levels of both
the polymers are close to each other, ꢀ3.32 eV for P1 and
ꢀ3.26 eV for P2. The closeness of the HOMO and LUMO
energy levels of P1 and P2 is due to the similarity in their
structures and their molecular weights. The energy bandgaps
of both P1 and P2 are similar to P3.34
Bulk heterojunction solar cells were fabricated using both
polymers P1 and P2 with PCBM as the acceptor.
A
FIGURE 2 Solution and film UV–vis absorption spectra of polymers: (a) poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-
b0]difuran} (P1); (b) poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran-alt-4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-
b0]dithiophene} (P2); dotted line: film; solid line: solution (chloroform solvent).
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TABLE 2 Bulk Heterojunction Solar Cell Data for Polymers P1
and P2
further correlation between surface morphology and the so-
lar cell performance, TMAFM analysis was performed on the
solar cell devices of P1 at 1:1 and 1:2 weight ratios (Fig. 3).
The film obtained at polymer/PCBM 1:1 blend ratio had a
uniform surface morphology, whereas the film obtained at
1:2 weight ratio had a more evident phase separation. The
film obtained at 1:1 weight ratio is also smoother (RMS ¼
1.10 nm) than the film obtained at 1:2 weight ratio (RMS ¼
2.36 nm). This result is in agreement with our earlier obser-
vations that smoother blend films give higher PCEs in bulk
heterojunction solar cells.18,34 However, for polymer P2, the
films obtained at both 1:1 and 1:2 weight ratios display simi-
lar surface roughness (RMS for 1:1 film ¼ 2.54 nm, and RMS
for 1:2 film ¼ 2.83 nm). A comparison of the morphology of
P1 at 1:1 weight ratio (RMS ¼ 1.10 nm) and P2 at 1:1
weight ratio (RMS ¼ 2.54 nm) further indicates that a
smoother surface morphology yields higher performing solar
cell devices (Fig. 4).
Film
Voc JSC
Polymer [P]:[PCBM] (V)
(mA cmꢀ2
g
Thickness
)
FF
(%) (nm)
P1
P1
P2
P2
1:1
1:2
1:1
1:2
0.83 3.71
0.82 3.34
0.81 2.13
0.81 2.24
0.39 1.19 70.1
0.39 1.06 73.6
0.37 0.64 70.5
0.44 0.79 67.0
conventional solar cell device structure of indium tin oxide
(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/polymer:PCBM/Ca/Al was used for the meas-
urements. The devices were optimized at 15 mg mLꢀ1 con-
centration of the polymer/PCBM blend in chloroform solvent
and at polymer:PCBM weight ratios of 1:1 and 1:2 (Table 2).
The highest PCE for polymer P1 (1.19%) was obtained for a
device with 1:1 weight ratio of polymer and PCBM. Polymer
P1 displayed a relatively high Voc of ꢁ0.82 V for both 1:1
and 1:2 devices. Polymer P2 displayed the highest PCE of
0.79% at 1:2 weight ratio. The measured Voc of P2 was
about 0.81 V. The higher Voc measured for polymers P1 and
P2 are due to their deeper HOMO energy levels when com-
pared with polymer P3. The measured Voc for polymers P1
and P2 are in close agreement with the theoretical calcu-
lated values.35 The film thicknesses of the blends of both the
polymers were similar (ꢁ70 nm), allowing for a good com-
parison of the PCEs (Table 2). The lower PCE of P2 when
compared with P1 can be attributed to the different sizes of
oxygen and sulfur atoms in the polymeric backbone of P2
disrupting the packing of the polymer in the solid state. Both
P1 and P2 display a lower PCE when compared with the
benzodithiophene homopolymer P3.34 This can be attributed
to the lower solubility of both P1 and P2 in CHCl3 when
compared with P3.
The field-effect mobilities of polymers P1 and P2 were
measured in bottom-gate, bottom-contact organic field-effect
transistors (OFETs). IDS–VDS curve families were recorded
at different gate voltages [Fig. 5(a, c)]. The charge carrier
1/2
mobility was determined by plotting IDS
versus VGS in
the saturation regime [Fig. 5(b, d)], using the following
equation:22,36
"
#
2L
IDS
l ¼
;
2
WCi
ðVGS ꢀ VTÞ
where IDS is the source-drain current, W is the channel
width, L is the channel length, Ci is the capacitance of the sil-
icon dioxide dielectric, VGS is the gate voltage, and VT is the
threshold voltage. Polymers P1 and P2 had field-effect mobi-
lities of 1.34
ꢂ
10ꢀ4 cm2
V
ꢀ1sꢀ1 and 3.00
ꢂ
10ꢀ5
cm2
V
ꢀ1sꢀ1, which are higher than previously reported val-
ues for benzodifuran polymers.37
Tapping-mode atomic force microscopic (TMAFM) analysis of
both polymers P1 and P2 was performed on thin films
obtained by drop-casting from CHCl3 solutions on mica (Sup-
porting Information). Both polymers P1 and P2 displayed a
granular morphology (Supporting Information). To achieve
EXPERIMENTAL
Materials
All commercial chemicals were purchased from Aldrich
Chemical and were used without further purification unless
FIGURE 3 Three-dimensional TMAFM images of the active area of the solar cell devices of (a) [P1]:[PCBM] ¼ 1:1 (RMS ¼ 1.10 nm)
and (b) [P1]:[PCBM] ¼1:2 (RMS ¼ 2.36 nm).
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FIGURE 4 Three-dimensional TMAFM images of the active area of the solar cell devices of (a) [P2]:[PCBM] ¼ 1:1 (RMS ¼ 2.54 nm)
and (b) [P2]:[PCBM] ¼1:2 (RMS ¼ 2.83 nm).
otherwise noted. All reactions were conducted under puri-
fied nitrogen. Theꢃ polymerization glassware and syringes
were dried at 120 C for at least 24 h before use and cooled
under a nitrogen atmosphere. Tetrahydrofuran was dried
over sodium/benzophenone ketyl and freshly distilled before
use. 4,8-Dihydrobenzo[1,2-b:4,5-b0]dithiophene-4,8-dione was
prepared according to a previously reported procedure.38
spectra of the polymers were recorded on a VARIAN-INOVA-
500 MHz spectrometer at 30 ꢃC. 1H NMR data are reported
in parts per million as chemical shift relative to tetramethyl-
silane (TMS) as the internal standard. Spectra were recorded
in CDCl3. GC/MS was performed on an Agilent 6890-5973
GC-MS workstation. The GC column was a Hewlett-Packard
fused silica capillary column crosslinked with 5% phenyl-
methyl siloxane. Helium was the carrier gas (1 mL minꢀ1).
The following conditions were used for all GC/MS analyses:
injector and detector temperature, 250 ꢃC; initial tempera-
Structural Analysis
1H NMR spectra of the synthesized monomers were recorded
on a JEOL-Delta 270 MHz spectrometer at 25 ꢃC. 1H NMR
ture, 70 ꢃC; temperature ramp, 10 ꢃC minꢀ1
; final
FIGURE 5 Current–voltage characteristics of polymers P1 (a and b) and P2 (c and d) for bottom-gate, bottom-contact OFETs: (a
and c) output curves at different gate voltages and (b and d) transfer curves at VDS ¼ ꢀ50 V (W ¼ 475 mm, L ¼ 20 mm); polymer
P1: m ¼ 1.34 ꢂ 10ꢀ4 cm2
ꢀ10.8 V.
V
ꢀ1sꢀ1, on/off ratio ¼ 102, VT ¼ ꢀ4.9 V; polymer P2: m ¼ 3.00 ꢂ 10ꢀ5 cm2
V
ꢀ1sꢀ1, on/off ratio ¼ 101, VT
¼
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temperature, 280 ꢃC. The UV–vis spectra of polymer solu-
tions in chloroform solvent were carried out in 1-cm cuv-
ettes using an Agilent 8453 UV–vis spectrometer. Thin films
of polymer were obtained by evaporation of chloroform sol-
vent on glass microscope slides. Molecular weights of the
synthesized polymers were measured by size exclusion chro-
matography (SEC) analysis on a Viscotek VE 3580 system
equipped with ViscoGELTM columns (GMHHR-M), connected
to a refractive index detectors. GPC solvent/sample module
(GPCmax) was used with HPLC-grade THF as the eluent, and
calibration was based on polystyrene standards. Running
conditions for SEC analysis were as follows: flow rate ¼ 1.0
mL mꢃinꢀ1; injector volume ¼ 100 mL; detector temperature
3-carbonyl chloride (0.0893 mol) in 30 mL of methylene
chloride at 0 ꢃC. The reaction mixture was stirred at room
temperature for 2 h. The product was washed with water
(2ꢂ 200 mL), dried with magnesium sulfate, filtered, and
concentrated to yield brown oil, which was purified by vac-
uum distillation to yield 5.85 g of clear oil (0.035 mol, 39%).
1H NMR (CDCl3, 270 MHz) d: 1.18 (t, 6H), 3.44 (q, 4H), 6.56
(d, 1H), 7.38 (d, 1H), 7.67 (s, 1H).
Synthesis of 4,8-Dihydrobenzo[1,2-b:4,5-b0]
difuran-4,8-dione
N, N-Diethyl furan-3-carboxamide (5.85 g, 0.0350 mol) was
diluted in 50 mL of dry THF. To this solution, 14.0 mL of 2.5
M n-BuLi in hexane (0.0350 mol) was added dropwise at 0
ꢃC. The reaction mixture was warmed to room temperature
and stirred at room temperature for 4 h. The reaction mix-
ture was quenched with 300 mL of water and stirred over-
night. The brownish solid was filtered to obtain 0.854 g of
product (0.00454 mol, 26%).
ꢃ
¼ 30 C; column temperature ¼ 35 C. All the polymer sam-
ples were dissolved in THF, and the solutions were filtered
through polytetrafluoroethylene (PTFE) filters (0.45 mm)
prior to injection.
Tapping-Mode Atomic Force Microscopy
TMAFM investigation of thin-film surface morphology was
carried out using a NanoscopeIV-Multimode Veeco, equipped
with an E-type vertical engage scanner. Thin films were de-
posited by drop-casting chloroform solution of polymers on
mica substrate. The AFM images were recorded at room tem-
perature in air using silicon cantilevers with normal spring
constant of 42 N mꢀ1 and normal resonance frequency of
320 kHz. A typical value of AFM detector signal correspond-
ing to RMS cantilever oscillation amplitude was equal to 1–2
V, and the images were collected at 0.5 Hz scan frequency in
2 mm scan size. Polymer samples were prepared in chloro-
form solutions (1 mg mLꢀ1) and deposited on mica substrate
by drop-casting. TMAFM analysis was also performed on the
solar cell devices.
1H NMR (CDCl3, 270 MHz) d: 6.92 (d, 2H), 7.70 (d, 2H).
Synthesis of 4,8-Bis(4-decylphenylethynyl)
benzo[1,2-b:4,5-b0]difuran
The synthesis of 4-decyl-1-ethynylbenzene was previously
reported.18 4-Decyl-1-ethynylbenzene (2.13 g, 8.80 mmol)
ꢃ
was dissolved in 50 mL of dry THF. At 0 C, 3.5 mL of 2.5 M
n-BuLi in hexane (8.80 mmol) was added to the solution of
4-decyl-1-ethynylbenzene dropwise under a nitrogen atmos-
phere. The reaction mixture was stirred for 5 min at 0 ꢃC
followed by the addition of 4,8-dihydrobenzo[1,2-b:4,5-
b0]difuran-4,8-dione (0.820 g, 4.36 mmol) dissolved in 50
mL of THF. The reaction mixture was stirred for 30 min at
reflux. The reaction mixture was cooled to room temperature
followed by the addition of 4.42 g of tin chloride dihydrate
dissolved in 60 mL of 20% HCl solution. The reaction mix-
ture was refluxed for 30 min. The reaction mixture was
diluted with ether (150 mL), and the organic layer was
washed with water (2ꢂ 200 mL), dried with magnesium sul-
fate, filtered, and concentrated to yield a brownish solid,
which was dissolved in 20 mL of THF and precipitated in
200 mL of methanol to yield an off-white solid. The solid
was filtered to obtain 1.97 g of product (3.09 mmol, 71%).
Cyclic Voltammetry
Electrochemical grade tetrabutylammonium perchlorate
(TBAP) was used as an electrolyte without further purifica-
tion. Acetonitrile (99.9% grade) was distilled over calcium
hydride. Electrochemical experiments were performed using
a BAS CV-50W voltammetric analyzer. The electrochemical
cell was composed of a platinum electrode, a platinum wire
auxiliary electrode, and an Ag/AgCl reference electrode. Ace-
tonitrile solutions containing 0.1 M of TBAP were placed in a
cell and purged with argon. A drop of the polymer solution
in chloroform was deposited on the tip of platinum elec-
trode. The solvent was evaporated in air. The film was
immersed in the electrochemical cell containing TBAP for
measurement.
1H NMR (CDCl3, 270 MHz) d: 0.81 (t, 6H), 1.16 (m, 28H), 1.23
(m, 4H), 2.56 (t, 4H), 7.00 (d, 2H), 7.18 (d, 4H), 7.52 (d, 4H),
7.69 (d, 2H). 13C NMR (CDCl3, 270 MHz) d: 14.21, 22.78,
29.36, 29.42, 29.59, 29.68, 31.34, 31.99, 36.07, 81.50, 98.60,
99.61, 106.99, 120.13, 127.55 128.64, 131.82, 144. 13, 146.27.
Synthesis of the Monomers
Synthesis of Furan-3-carbonyl chloride
Synthesis of 2,6-Dibromo-4,8-bis(4-decylphenylethynyl)-
benzo[1,2-b:4,5-b0]difuran
Furan-3-carboxylic acid (10.00 g, 0.0893 mol) was heated at
reflux with thionyl chloride (30 mL) in 100 mL of methylene
chloride for 90 min. The solvent and the excess thionyl chlo-
ride were removed by distillation to yield a brownish solid.
The solid was dissolved in 30 mL of methylene chloride and
used for next step without purification.
4,8-Bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran (0.905
g, 1.42 mmol) was dissolved in 100 mL of dry THF and cooled
ꢃ
to ꢀ78 C. To this solution, 1.20 mL of 2.5 M n-BuLi in hexane
(3.0 mol, 2.1 equiv) was added, and the solution was stirred
for 30 min at ꢀ78 ꢃC. Carbon tetrabromide (1.007 g, 3.04
mmol, 2.14 equiv) dissolved in 20 mL of THF was added. Af-
Synthesis of N, N-Diethyl furan-3-carboxamide
Diethylamine (13.06 g, 0.179 mol) was diluted with 30 mL
of methylene chloride and was added to a solution of furan-
ꢃ
ter stirring for 5 min at ꢀ78 C, the solution was allowed to
warm to room temperature for 1 h. Solvent was removed, and
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the brown solid was dissolved in chloroform and washed with
water (2ꢂ 200 mL), dried with magnesium sulfate, and con-
centrated to obtain a brown solid. The brown solid was dis-
solved in 10 mL of THF and poured into 200 mL of methanol.
The precipitated off-white solid was filtered to obtain 0.746 g
of product (0.937 mmol, 60%).
To a three-necked round-bottomed flask, 2,6-(trimethyltin)-
4,8-bis(4-decylphenylethynyl)-benzo[1,2-b:4,5-b0]dithiophene
(0.314 g, 0.315 mmol), 2,6-dibromo-4,8-bis(4-decylphenyle-
thynyl)benzo[1,2-b:4,5-b0]difuran (0.249 g, 0.313 mmol), and
tetrakis(triphenylphosphine)palladium(0) (0.023 g, 0.020
mmol) were added under a nitrogen atmosphere. Toluene
(40 mL) and DMF (8 mL) were added to dissolve the mono-
mers. The solution was heated at reflux for 2 h, at which
time 25 mL of toluene was added to the reaction mixture.
After an additional 24 h at reflux, 0.020 g of the catalyst
(0.017 mmol) was added to the reaction mixture, and after
48 h at reflux, an additional 0.025 g of catalyst was added.
The polymerization was stopped after an additional 24 h at
reflux by precipitating the polymer in methanol. The polymer
was filtered, and the polymer was purified by Soxhlet extrac-
tions with methanol, diethyl ether, hexane, dichloromethane,
and chloroform. The polymer was obtained from the chloro-
form fraction on evaporation of the solvent as a dark red
solid (0.29 g, 70% yield).
1H NMR (CDCl3, 270 MHz) d: 0.84 (t, 6H), 1.25 (m, 28H),
4.33 (m, 4H), 2.62 (t, 4H), 7.38 (s, 2H), 7.21 (d, 4H), 7.56 (d,
4H). 13C NMR (CDCl3, 100 MHz) d: 14.21, 22.77, 29.41,
29.57, 29.67, 36.08, 36.19, 81.05, 98.41, 99.37, 108.88,
109.04, 119.75, 119.82, 128.22, 128.67, 129.30, 131.87,
144.31, 144.44, 151.87, 152.06.
Synthesis of 2,6-(Trimethyltin)-4,8-bis(4-decylphenyle-
thynyl)-benzo[1,2-b:4,5-b0]difuran
4,8-Bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran (0.908
g, 1.42 mmol) was dissolved in 100 mL of dry THF. Under a
nitrogen atmosphere at ꢀ78 ꢃC, 1.20 mL of 2.5 M n-BuLi in
hexane (3.0 mol, 2.1 equiv) was added. Reaction was stirred
ꢃ
at ꢀ78 C for 20 min followed by the addition of 3.0 mL of 1
1H NMR (CDCl3, 500 MHz) d: 0.88 (m, 12H), 1.25 (m, 36H),
1.52 (m, 28H), 2.67 (m, 8H), 7.12–7.25 (br, 16H), 7.40 (br,
1H), 7.51 (br, 1H), 7.63 (br, 1H), 7.66 (br, 1H).
M trimethyltin chloride. After the addition, the reaction was
allowed to warm to room temperature for over 1 h. The sol-
vent was evaporated, and the resultant solid was dissolved in
chloroform, washed with water (2ꢂ 200 mL), dried with mag-
nesium sulfate, filtered, and concentrated to yield a solid. The
solid was dissolved in 10 mL of THF and added to 200 mL of
methanol. The precipitated yellow solid was filtered to obtain
0.970 g of product (1.01 mmol, 71%).
Solar Cells Fabrication and Testing
Organic light emitting diode (OLED)-grade glass slides coated
with ITO were purchased from Luminescense Technology
(Taiwan). The ITO on these slides was patterned using
standard photolithography. The slides were cleaned with
deionized water, acetone, and isopropanol successively by
sonication for 20 min each and washed for 10 min in oxygen
plasma before use. Immediately following plasma treatment,
a 20 nm layer of PEDOT:PSS was spin coated onto the sub-
1H NMR (CDCl3, 400 MHz) d: 0.45 (s, 18H), 0.56 (t, 6H), 0.87
(m, 28H), 1.62 (m, 4H), 2.63 (t, 4H), 7.18 (d, 4H), 7.19 (s, 2H),
7.56 (d, 4H). 13C NMR (CDCl3, 270 MHz) d: ꢀ8.78, 14.21,
22.77, 29.35, 29.42, 29.58, 29.69, 31.98, 36.06, 82.57, 97.81,
117.82, 120.65, 127.80, 128.58, 131.77, 143.76, 155.19, 167.47.
strate (4000 minꢀ1, 1740 minꢀ1
s
ꢀ1, 90 s), followed by
annealing at 120 ꢃC for 20 min under nitrogen. The PCBM/
polymer blend was prepared in chloroform, at the required
weight ratio of polymer and PCBM, with a constant total
concentration of 15 mg mLꢀ1. This blend was then spin cast
Synthesis of Poly{4,8-bis(4-decylphenylethynyl)
benzo[1,2-b:4,5-b0]difuran}(P1)
To a three-necked round-bottomed flask, 2,6-dibromo-4,8-
bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran (0.322 g,
(2000 minꢀ1, 1740 minꢀ1 ꢀ1, 60 s) onto the PEDOT-PSS/
s
0.405
mmol),
2,6-(trimethyltin)-4,8-bis(4-decylphenyle-
substrate. A cathode consisting of calcium (10 nm) was ther-
mally evaporated at a rate of ꢁ1.0 Å sꢀ1, followed by alumi-
num (100 nm) at ꢁ2.5 Å sꢀ1 through a shadow mask to
define solar cell active areas.
thynyl)-benzo[1,2-b:4,5-b0]difuran (0.402 g, 0.417 mmol),
and tetrakis(triphenylphosphine) palladium(0) (0.023 g,
0.0199 mmol) were added under a nitrogen atmosphere. Tol-
uene (40 mL) and DMF (10 mL) were added to dissolve the
monomers. The reaction mixture was heated at reflux for 72
h, and the polymer was precipitated in methanol. The poly-
mer was filtered and was purified by Soxhlet extractions
with methanol, diethyl ether, hexane, dichloromethane, and
chloroform. The polymer was obtained from the chloroform
fraction on evaporation of the solvent. The polymer was
obtained as a dark red solid (0.360 g, 60% yield).
IV testing was carried out under a controlled N2 atmosphere
using a Keithley 236, model 9160 power source interfaced
with LabView software. The solar simulator used was a
THERMOORIEL equipped with a 300-W xenon lamp; the in-
tensity of the light was calibrated to 100 mW cmꢀ2 with a
NREL-certified Hamamatsu silicon photodiode. The active
area of the devices was 0.09 cm2.
1H NMR (CDCl3, 500 MHz) d: 0.88 (t, 6H), 1.25 (m, 12H),
1.52 (m, 20H), 2.62 (m, 4H), 6.9 (br, 8H), 7.6 (br, 2H).
Field-Effect Transistor (OFET) Fabrication and
Mobility Determination
Synthesis of Poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-
b:4,5-b0]difuran-alt-4,8-bis(4-decylphenylethynyl)-
benzo[1,2-b:4,5-b0]dithiophene} (P2)
We have previously reported the synthesis and characteriza-
tion of 2,6-(trimethyltin)-4,8-bis(4-decylphenylethynyl)-benzo
[1,2-b:4,5-b0]dithiophene.18
Field-effect mobility measurements of the synthesized poly-
mers were performed on thin-film transistors with a com-
mon bottom-gate, bottom-contact configuration. Highly
doped, n-type silicon wafers with a resistivity of 0.001–0.003
X cm were used as substrates. Silicon dioxide (SiO2) (200
nm thickness) was thermally grown at 1000 C on the silicon
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wafer. Chromium metal (5 nm) followed by 100 nm of gold
was deposited by E-beam evaporation as source-drain met-
als. The source-drain pads were formed by photolithographi-
cally patterning the metal layer. The SiO2 on the backside of
the wafer was etched with buffered oxide etchant (7:1 BOE
from JT Baker) to generate the common bottom gate. The
resulting transistors had a channel width of 475 mm and
channel length ranging from 2 to 80 mm. The measured ca-
pacitance density of the SiO2 dielectric was 17 nF cmꢀ2. Af-
ter the SiO2 on the backside was removed, the devices were
cleaned with UV–ozone for 7 min using a Technics Series 85
RIE etcher and stored under vacuum. This process removes
any residual organics on the substrate. For the sample prep-
aration of untreated devices, prior to the polymer deposition,
the substrates were cleaned with water, methanol, hexanes,
and chloroform with drying using N2 flow between different
solvents. The devices were baked at 80 ꢃC for 30 min in a
vacuum oven. The devices were allowed to cool under vac-
uum. For surface treatment with octyl trichlorosilane (OTS),
the devices were rinsed sequentially with water, methanol,
hexanes, and chloroform and placed in a glass container in a
solution of OTS of 4 ꢂ 10ꢀ3 M in dried toluene. The sealed
container was placed in a glove box at ambient temperature
for 48 h. After 48 h, the device was taken out of the glove
can be attributed to the different sizes of oxygen and sulfur
atoms in the polymeric backbone of P2 disrupting the pack-
ing of the polymer in its solid state. TMAFM analysis of the
polymers was performed for films of pristine polymer depos-
ited on mica and for blends used as active layer in the solar
cell devices. A granular morphology was observed for both
polymers P1 and P2. The TMAFM analysis of the polymer/
PCBM blends indicated that films with smoother surface
morphology displayed higher power conversion efficiencies.
ACKNOWLEDGMENTS
Mihaela C. Stefan acknowledges the financial support from the
NSF (DMR-0956116) and the Welch Foundation (AT-1740).
The authors thank Dr. Anvar Zakhidov at UT Dallas for giving
access to the solar cell measurement setup.
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ꢃ
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We have synthesized a new benzodifuran monomer 4,8-bis
(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran and poly-
merized it by Stille coupling polymerization to obtain
poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran}
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decylphenylethynyl)benzo[1,2-b:4,5-b0]difuran-alt-4,8-bis(4-decyl-
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synthesized. The polymers yielded reasonable molecular
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