Incorporation of furan into low band-gap polymers for efficient solar cells

Article abstract of DOI:10.1021/ja108115y

The design, synthesis, and characterization of the first examples of furan-containing low band-gap polymers, PDPP2FT and PDPP3F, with substantial power conversion efficiencies in organic solar cells are reported. Inserting furan moieties in the backbone of the conjugated polymers enables the use of relatively small solubilizing side chains because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents. Bulk heterojunction solar cells fabricated from furan-containing polymers and PC₇₁BM as the acceptor showed power conversion efficiencies reaching 5.0%.

Full text of DOI:10.1021/ja108115y

Published on Web 10/14/2010
Incorporation of Furan into Low Band-Gap Polymers for Efficient Solar Cells
Claire H. Woo,†,§ Pierre M. Beaujuge,†,‡ Thomas W. Holcombe,‡ Olivia P. Lee,‡ and
Jean M. J. Fre´chet*,†,‡,§
Materials Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States,
and Departments of Chemistry and Chemical Engineering, UniVersity of California, Berkeley,
California 94720-1460, United States
Received September 8, 2010; E-mail: frechet@berkeley.edu
Abstract: The design, synthesis, and characterization of the first
examples of furan-containing low band-gap polymers, PDPP2FT
and PDPP3F, with substantial power conversion efficiencies in
organic solar cells are reported. Inserting furan moieties in the
backbone of the conjugated polymers enables the use of relatively
small solubilizing side chains because of the significant contribu-
tion of the furan rings to overall polymer solubility in common
organic solvents. Bulk heterojunction solar cells fabricated from
furan-containing polymers and PC71BM as the acceptor showed
power conversion efficiencies reaching 5.0%.
Polymer bulk heterojunction (BHJ) solar cells have attracted
significant attention because of their potential for achieving large-
area, flexible photovoltaic devices through low-cost solution
deposition techniques.1,2 Much of the recent research effort has
focused on the development of low band-gap donor polymers that
have broad absorption spectra.3-5 The search for new building
blocks for semiconducting polymers continues as we gain mecha-
Figure 1. (a) Synthetic scheme and polymeric structures used in this study
(polymerization protocol: Pd2dba3, P(o-tol)3, chlorobenzene, 110 °C, 24 h).
(b) Thin-film absorption spectra and (c) cyclic voltammograms of PDPP2FT
nistic understandings and establish design rules relevant to organic
and PDPP3F.
electronic applications.6-8 For example, the ideal polymer should
(i) have sufficient energy level offsets with fullerenes for efficient
organic dyes for dye-sensitized solar cells and have shown very
charge separation while maximizing the open-circuit voltage,8,9 (ii)
similar optical and electronic properties.27,28 Furan-based hetero-
display an absorption spectrum extending across the visible
cycles have also been introduced as peripheral substituents on one
spectrum and into the near-IR, and (iii) maintain high extinction
of the highest performing small molecules for photovoltaics.29 The
coefficients over this spectral range.6 At the same time, it has
sparsity of studies examining polymer backbones containing furans
become increasingly apparent that a balance among the competing
in this field is surprising, given that furans exhibit similar energy
effects of solution processability, miscibility with the fullerene
levels and a comparable degree of aromaticity relative to their
component, and solid-state packing needs to be established.10-12
thiophene counterparts.24,30 Importantly, furan derivatives can be
Both the chemical structure of the backbone repeat units and the
synthesized from a variety of natural products; hence, they fall into
choice of the solubilizing side chains critically impact the above-
the category of renewable and sustainable synthetic resources.
mentioned criteria.13,14 For example, while the use of longer and
In this contribution, we demonstrate that furan heterocycles can
bulkier alkyl substituents improves solubility, it also increases
be advantageously incorporated into conjugated polymer backbones
without hindering their photovoltaic device performance. In addi-
tion, we show that furans can be employed to dramatically reduce
lamellar and π-stacking distances, hindering intermolecular ordering
and affecting the transport of charge carriers across the polymer
stacks.13,15,16 In this regard, strategies to reduce the length,
the amount of aliphatic side-chain material necessary to solubilize
bulkiness, and density of solubilizing side chains along the
polymer backbones that otherwise require the presence of long and
conjugated polymer backbone are well worth exploring.
bulky substituents. This concept is exemplified by the synthesis
A survey of state-of-the-art BHJ solar cells reveals that most
and characterization of two furan-containing semiconducting poly-
high-performance polymers reported so far rely on thiophene or
mers: PDPP2FT and PDPP3F (see Figure 1a). These polymers
thiophene-based heterocycles.17-23 While thiophene-based conju-
contain a diketopyrrolopyrrole (DPP) unit22,31-33 and are structur-
gated materials have attracted much attention in the area of organic
ally analogous to the low band-gap polymer PDPP3T previously
electronics, only a limited number of studies have examined furan-
reported by Janssen et al.34 Importantly, these furan-containing
containing materials potentially useful for device applications.24-26
derivatives were synthesized with 2-ethylhexyl substituents, whereas
Recently, furans have been used as an alternative to thiophenes in
PDPP3T (as initially reported34) was appended with 2-hexyldecyl
solubilizing groups.
While exploring the use of furans as alternatives to thiophenes
in low band-gap conjugated polymers involving DPP, we found
† Lawrence Berkeley National Laboratory.
‡ Department of Chemistry, University of California, Berkeley.
§ Department of Chemical Engineering, University of California, Berkeley.
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C O M M U N I C A T I O N S
that soluble high-molecular-weight PDPP2FT could be readily
obtained (Mn ) 66 kDa, see Supporting Information). The use of
2-ethylhexyl substituents was sufficient to impart PDPP2FT with
appropriate solubility in common organic solvents (e.g., tetrahy-
drofuran, chloroform, chlorobenzene) for device fabrication. In
contrast, the all-furan derivative PDPP3F, synthesized using the
same polymerization protocol (Mn ) 29 kDa, see Supporting
Information), was found to possess slightly reduced solubility in
the same organic solvents. While the improved solubility of
oligofurans over oligothiophenes has been reported,35 it appears
that the ratio of furan to thiophene in mixed oligomers also impacts
solubility.36,37 As a control experiment, we attempted to synthesize
the 2-ethylhexyl-substituted derivative of the all-thiophene PDPP3T
following the same polymerization procedure as that used for
PDPP2FT and PDPP3T. However, the polymerization yielded only
low-molecular-weight fractions that were minimally soluble in all
common organic solvents (Mn ) 2 kDa, see Supporting Informa-
tion).
Figure 2. (a) J-V curves of optimized PDPP2FT:PC71BM devices spin-
coated out of chlorobenzene (with no additive and with 9 vol % CN). (b)
External quantum efficiency spectra of optimized devices.
The onset of optical absorption of PDPP2FT in a thin film was
measured to be 880 nm (Eg ) 1.41 eV), while the λmax was observed
at 789 nm (see Figure 1b), which is comparable to the optical
properties of PDPP3T reported earlier by Janssen et al. (Eg ) 1.3
eV).34 PDPP3F also possesses similar optical properties, with Eg
) 1.35 eV and λmax at 767 nm. Figure 1c shows the cyclic
voltammograms of the two polymers. The onsets of oxidation and
reduction of PDPP2FT were observed at +0.28 and -1.34 V vs
Fc/Fc+, corresponding to HOMO and LUMO levels at -5.4 and
-3.8 eV vs vacuum. For PDPP3F, the onsets were observed at
+0.35 and -1.34 V, corresponding to HOMO and LUMO levels
at -5.5 and -3.8 eV. These values are comparable to those obtained
for the low-molecular-weight all-thiophene analogue, PDPP3T (see
Supporting Information).
Figure 3. AFM phase images of 1:3 PDPP2FT:PC71BM blend films spin-
coated (a) from chlorobenzene only and (b) from chlorobenzene + 9 vol
% CN. Inset: Height images of the same films. The data scale is 0-60 nm.
device performance improved by more than 5-fold, with much
higher Jsc and an average PCE of 4.7%. The best device was
obtained with the addition of 9% CN by volume relative to
chlorobenzene, and it achieved a Voc of 0.74 V, a Jsc of 11.2 mA
cm-2, a FF of 60%, and a PCE of 5.0%, results comparable to that
obtained by Janssen et al. with PDPP3T.34 The Jsc value calculated
from the integration of the EQE spectrum of the best device is
11.4 mA cm-2, which closely matches the Jsc value obtained from
the J-V measurement under white light illumination. Solar cells
containing a blend of PDPP3F and PC71BM were also fabricated
and achieved an average PCE of 3.8% (max 4.1%) after optimiza-
tion. Here again, the device performance was <1% without the
addition of CN. These device results strongly support the potential
of furan-based polymeric systems in organic photovoltaic devices.
Solar cells were fabricated using PDPP2FT as the electron donor
and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the
electron acceptor, with the device structure ITO/PEDOT:PSS/
polymer:PCBM/LiF/Al. The active layers were spin-coated from
chlorobenzene, and, in some cases, a small amount of the high-
boiling-point additive 1-chloronaphthalene (CN) was added to
optimize blend morphology for enhanced device performance. The
J-V curves and external quantum efficiency (EQE) spectra of
PDPP2FT:PC61BM devices are shown in the Supporting Informa-
tion. Without any postfabrication treatment, the PDPP2FT:PC61BM
device spin-coated from pure chlorobenzene afforded 3.4% power
conversion efficiency (PCE) under AM 1.5 G, 100 mW cm-2
illumination. The use of chlorobenzene containing 1 vol % CN for
spin-coating led to a slight improvement to 3.7% PCE, mostly
through an increase in the photocurrent. The best device was
obtained from a blend of PDPP2FT:PC61BM in a 1:3 weight ratio
and gave a PCE of 3.8%, with an open circuit voltage (Voc) of 0.76
V, a short-circuit current density (Jsc) of 9.0 mA cm-2, and a fill
factor (FF) of 55%. The EQE showed a sharp onset at the optical
band gap of the polymer and reached a maximum value of 33%.
For BHJ devices containing a 1:3 blend of PDPP3F:PC61BM,
chloroform was found to be a better solvent, and an average
efficiency of 3.0% was achieved (see Supporting Information).
The dramatic difference in device performance with and without
the CN additive is most likely due to the optimization of blend
morphology. Figure 3 compares the atomic force microscopy (AFM)
images of blend films of PDPP2FT:PC71BM at the optimized ratio.
The blend without additive exhibits coarse phase separation between
the polymer and PC71BM, with large micrometer-sized domains.
In contrast, the addition of CN led to much finer phase separation
between the two materials and the formation of fiber-like inter-
penetrating morphologies at the length scale of ∼20 nm, which is
close to the ideal domain size, assuming an exciton diffusion length
of 5-10 nm.38-40 The thin-film absorption of PDPP2FT also red-
shifts and displays more distinct vibronic structures when CN is
added to the solution before spin-coating (see Supporting Informa-
tion). The red-shift in absorption is indicative of increased
intermolecular ordering and planarity in the polymer backbone and
could be another reason for the improved performance of devices
fabricated with CN.
In an attempt to increase the breadth of the photoactive spectrum
and the overall photocurrent, we fabricated solar cells with the more
light-absorbing fullerene derivative PC71BM. Figure 2 shows the
J-V curves and the EQE spectra of optimized devices fabricated
from blends of PDPP2FT:PC71BM at a 1:3 weight ratio in
chlorobenzene. Interestingly, without any additive, the PC71BM
devices performed poorly, with an average PCE of only 0.86%.
However, with the addition of high-boiling CN to the blend solution,
In summary, we have shown that furans can be advantageously
used as an alternative to thiophenes and thiophene-based building
units in the design and synthesis of low band-gap conjugated
polymers with efficient solar cell performance. The polymers
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C O M M U N I C A T I O N S
Yang, Y. Macromolecules 2009, 42, 6564–6571.
examined, PDPP2FT and PDPP3F, exhibit nearly identical optical
and electronic properties and demonstrate power conversion ef-
ficiencies approaching 5% in BHJ devices with PC71BM. The
incorporation of furan into the conjugated backbone allowed for
shorter solubilizing groups to be used, compared to those required
to solubilize the all-thiophene polymer PDPP3T. In particular,
polymer solubility was found to improve substantially when a
combination of thiophene and furan heterocycles is incorporated.
The ca. 4% efficiency achieved with the all-furan low band-gap
polymer PDPP3F clearly demonstrates the potential of furans as
thiophene alternatives in the design of high-performance organic
solar cell materials, paving the path for the design and production
of organic electronic materials from sustainable synthetic resources.
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Acknowledgment. This work was supported by the Director,
Office of Science, Office of Basic Energy Sciences, Materials
Sciences and Engineering Division, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231, and by the
Center for Advanced Molecular Photovoltaics (Award No. KUS-
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