Functionalized isothianaphthene monomers that promote quinoidal character in donor-acceptor copolymers for organic photovoltaics

Article abstract of DOI:10.1021/ma300589k

A series of low band gap isothianaphthene-based (ITN) polymers with various electron-withdrawing substituents and intrinsic quinoidal character were synthesized, characterized, and tested in organic photovoltaic (OPV) devices. The three investigated ITN cores contained either ester, imide, or nitrile functionalities and were each synthesized in only four linear steps. The relative electron-withdrawing strength of the three substituents on the ITN moiety was evaluated and correlated to the optical and electronic properties of ITN-based copolymers. The ester- and imide-containing p-type polymers reached device efficiencies as high as 3% in bulk heterojunction blends with phenyl C₆₁-butyric acid methyl ester (PC₆₁BM), while the significantly electron-deficient nitrile-functionalized polymer behaved as an n-type material with an efficiency of 0.3% in bilayer devices with poly(3-(4-n-octyl)phenylthiophene) (POPT).

Full text of DOI:10.1021/ma300589k

Article
pubs.acs.org/Macromolecules
Functionalized Isothianaphthene Monomers That Promote Quinoidal
Character in Donor−Acceptor Copolymers for Organic Photovoltaics
Jessica D. Douglas,†,§ Gianmarco Griffini,†,§ Thomas W. Holcombe,† Eric P. Young,‡ Olivia P. Lee,†,§
Mark S. Chen,*,†,§ and Jean M. J. Frechet*,†,‡,§,⊥
́
‡
†Department of Chemistry and Department of Chemical Engineering, University of California, Berkeley, California 94720-1460,
United States
§Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
⊥King Abdullah University of Science and Technology, Thuwal, Saudi Arabia 23955-6900
S
* Supporting Information
ABSTRACT: A series of low band gap isothianaphthene-based (ITN) polymers with various electron-withdrawing substituents
and intrinsic quinoidal character were synthesized, characterized, and tested in organic photovoltaic (OPV) devices. The three
investigated ITN cores contained either ester, imide, or nitrile functionalities and were each synthesized in only four linear steps.
The relative electron-withdrawing strength of the three substituents on the ITN moiety was evaluated and correlated to the
optical and electronic properties of ITN-based copolymers. The ester- and imide-containing p-type polymers reached device
efficiencies as high as 3% in bulk heterojunction blends with phenyl C61-butyric acid methyl ester (PC61BM), while the
significantly electron-deficient nitrile-functionalized polymer behaved as an n-type material with an efficiency of 0.3% in bilayer
devices with poly(3-(4-n-octyl)phenylthiophene) (POPT).
Alongside substituent effects, polymers with quinoidal
character can have favorable electronic properties, including
reduced bond length alternation, significant electron delocaliza-
tion along the polymer backbone, and a narrow band gap.5 One
well-known, low band gap polymer with a chemical structure
that intrinsically stabilizes its quinoidal state is poly-
(isothianaphthene) (PITN).6 The ITN monomer is a bicyclic
compound comprised of a phenyl ring fused to the C3−C4
bond of thiophene, resulting in competing aromaticity between
the two rings. Depending on whether the thiophenyl or phenyl
portion of each monomer is aromatized, PITN is stabilized in
either its aromatic or quinoidal resonance form, respectively. It
has been calculated that the quinoidal form of PITN is 2.4 kcal/
mol lower in energy than the aromatic form, as a result of the
stronger aromatic stabilization energy of benzene versus
thiophene.7 This energy difference imparts significant quinoidal
INTRODUCTION
■
Organic photovoltaics (OPVs) have attracted considerable
attention due to their promise as a flexible and potentially low-
cost alternative to commercial silicon-based solar technologies.1
In the highest performing OPVs, the device active layer is
comprised of an interpenetrating network of a p-type polymer
and an n-type fullerene. To ensure that the polymer absorbs
low-energy light and charges are separated at the polymer−
fullerene interface, the energy level alignment of both the p-
and n-type materials must be considered.2 By designing
polymers with alternating electron-rich (donor) and electron-
poor (acceptor) backbone components, the HOMO and
LUMO of p-type polymers can be modulated to optimize the
polymer band gap, energy level offset with a fullerene acceptor,
and the open circuit voltage of OPV devices.3 Recently, many
performance breakthroughs, with device efficiencies surpassing
7%, have occurred as a result of this donor−acceptor
approach.4 In particular, the development of new acceptor
monomers with electron-withdrawing substituents has yielded
new, high performance polymer backbones.
Received: March 22, 2012
Revised: April 19, 2012
© XXXX American Chemical Society
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character to the structure of PITN, giving the polymer a
uniquely low band gap of about 1.0 eV.8
RESULTS AND DISCUSSION
■
Material Synthesis. In order to study the effect of different
PITN, and other low band gap polymers based on ITN, have
previously been synthesized by electrochemical or oxidative
polymerizations.9 Unfortunately, these polymerization methods
produce ITN homo- and copolymers that often have low
molecular weights and poor film-forming properties.10 In order
to achieve high molecular weight polymers with suitable
properties and tunable band gaps, ITN-based copolymers have
been synthesized with the donor−acceptor approach, through
palladium cross-coupling reactions.11 Synthesis of ITN-based
monomers for these copolymers follow the original route,
which begins with a substituted benzene that ultimately
undergoes a ring closure to form the thiophene portion of
isothianaphthene.9b−f,10−12 While this route appears short (3 or
4 steps), it requires functional groups and solubilizing chains to
be preinstalled on the phenyl ring, decreasing the flexibility of
the synthesis.
Herein, we report a new synthetic route toward three
functionalized ITN acceptor monomers for donor−acceptor
copolymers. In our approach, we construct the ITN core by
installing a phenyl ring onto thiophene in a four-step synthesis.
This route allows for electron-withdrawing functionality to be
added to the monomer in the penultimate step, thereby
facilitating the rapid synthesis of a variety of functionalized ITN
monomers. By combining the quinoidal character of ITN with
electron-withdrawing substituents, we obtain a new class of
acceptor monomer for low band gap copolymers that are
suitable for OPV applications (Figure 1). We further show that
ester, imide, or nitrile substituents on ITN can control the
optoelectronic properties of the resulting donor−acceptor
copolymers, including the polymer’s OPV performance as a
p- or n-type material.
electron-withdrawing substituents on acceptor monomers and
conjugated polymers, we investigated 5,6-disubstitued ITN
monomers functionalized with either methyl esters (ITN-ME),
an alkyl imide (ITN-I), or nitriles (ITN-CN) (Scheme 1). Each
of the monomers was synthesized in four linear steps and was
obtained through a common dialdehyde intermediate (2).
Thiophene-3,4-dicarbaldehyde (2) was synthesized by first
reducing thiophene-3,4-dicarboxylic acid with diisobutylalumi-
num hydride (DIBAL-H) to form diol 1, which was then
oxidized with pyridinium chlorochromate (PCC) to produce 2.
Next, through a one-pot Wittig olefination and Knoevenagel
condensation,13 dialdehyde 2 was reacted with an electron-
deficient, disubstituted olefin to yield the functionalized ITN
cores: dimethyl maleate was used for ITN-ME, N-octylmalei-
mide for ITN-OI, N-(2-ethylhexyl)maleimide for ITN-EHI,
and fumaronitrile for ITN-CN. The electron deficiency of these
ITN cores was evaluated by the NMR chemical shifts of the
two thiophene protons on 3, 5, and 7, which appeared at 7.82,
8.00, and 8.55 ppm, respectively. Because electron-withdrawing
functional groups generally deshield adjacent protons, leading
to a downfield (higher value) chemical shift, the observed trend
confirms that there is increasing electron deficiency when
replacing the diester with an imide and then with a dinitrile
functionality. To complete the comonomer synthesis, the three
ITN cores were dibrominated with N-bromosuccinimide
(NBS) to enable palladium-catalyzed polymerization. Com-
pared to previous preparations,9−11 the streamlined synthetic
routes presented here offer a new, quick, and simple pathway to
generate functionalized ITN monomers.
It is worth noting that while all three ITN cores were
successfully obtained and dibrominated, the ester-function-
alized core appeared to be the least oxidatively stable in the
series, as 3 became discolored if left at room temperature
overnight. The instability of 3 required bromination to be
performed immediately upon obtaining the functionalized core;
however, despite this precaution, only a small amount of 4 was
obtained from the bromination reaction. While this result was
disappointing, it was not unexpected, as the unfunctionalized
ITN monomer is known to oxidatively polymerize under
ambient conditions.9e,12 Interestingly, 5 and 7 can be easily
dibrominated with NBS because the electron-withdrawing
imide and nitrile functionalities provide increased oxidative
Figure 1. Strategy towards acceptor monomers.
Scheme 1. Synthetic Route to Functionalized ITN Monomers
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Figure 2. Structure of ITN-based polymers P1−P3.
a
Table 1. Physical and Electronic Properties of P1−P3 and Photovoltaic Performance of P1−P3 Based Devices
polymer properties
electronic properties
device properties
Jsc (mA cm−2
b
b
c
functional group Mn (kDa) PDI HOMO (eV) LUMO (eV) Eg (eV)
Voc (V)
)
FF
PCE (%)
d
P1
ester
24
19
40
43
35
16
2.2
2.2
4.2
4.2
4.6
2.3
−5.59
−5.66
−5.63
−5.69
−5.64
−5.79
−3.48
−3.63
−3.63
−3.64
−3.66
−4.16
1.60
1.54
1.51
1.57
1.53
1.51
0.87 (0.88)
0.71 (0.71)
0.78 (0.78)
0.76 (0.76)
0.75 (0.75)
0.39 (0.41)
6.69 (6.70)
7.70 (7.86)
7.05 (7.11)
8.46 (8.63)
3.66 (3.66)
1.33 (1.53)
0.46 (0.47)
0.53 (0.53)
0.55 (0.55)
0.47 (0.46)
0.48 (0.50)
0.45 (0.44)
2.70 (2.74)
2.88 (2.94)
3.01 (3.07)
3.01 (3.04)
1.32 (1.36)
0.24 (0.28)
e
P2a
P2b
imide
imide
imide
imide
nitrile
f
g
P2c
h
P2d
i
P3
a
b
Average device properties are reported with maximum values in parentheses. CV determined HOMO and LUMO values are reported relative to
c
d
Fc/Fc+ at −5.13 eV. Optical band gaps in thin films are based on the onsets of absorption. P1:PC61BM blend ratio of 1:2.5 in chlorobenzene (CB)
e
f
with no additives. P2a:PC61BM blend ratio of 1:1.5 in CB with 1 vol % diiodooctane. P2b:PC61BM blend ratio of 1:1.5 in CB with 5 vol %
g
h
chloronaphthalene. P2c:PC61BM blend ratio of 1:1.5 in CB with 1 vol % diiodooctane. P 2d:PC61BM blend ratio of 1:2.5 in CB with no additives.
POPT:P3 bilayer device annealed at 120 °C for 45 min.
i
stability to the ITN core, leading to a significant improvement
in the reaction yield.9f
synthesized (P2a−d). The number-average molecular weight
(Mn) and polydispersity index (PDI) of the six new ITN
copolymers were measured with size-exclusion chromatography
and calibrated with polystyrene standards. All of the polymers
reached Mn values between 16 and 43 kDa, with degrees of
polymerization above 20, and had PDIs between 2.2 and 4.6
(Table 1).
Electrochemical Properties. Toward studying the effect of
electron-withdrawing substituents on the synthesized copoly-
mers, the polymer HOMO and LUMO energy levels were
determined with cyclic voltammetry (CV) vs Fc/Fc+, where the
half-potential of ferrocene oxidation was set to −5.13 eV vs
vacuum (Figure 3). Since electron-withdrawing character is
known to lower orbital energy levels,17 it is reasonable that the
ITN-ME containing polymer, P1, has the highest energy levels
in the three-polymer series, whereas the ITN-CN containing
polymer, P3, has the lowest energy levels (P1 HOMO = −5.59
eV, LUMO = −3.48 eV; P3 HOMO = −5.79 eV, LUMO =
−4.16 eV). It is worth noting that while the HOMO energy
ITN monomers 4, 6, and 8 were Stille cross-coupled with
benzodithiophene (BDT) to yield new copolymers. BDT was
chosen as the donor monomer because it is known to have a
strong propensity to π-stack as a result of its large, planar
structure,14 and it has been used in many of the highest
performing OPV polymers.4a−c,15 For each polymer synthe-
sized, the alkyl side chains on BDT were judiciously chosen to
allow for a high level of solution processability while
minimizing excessive side chain length and bulk (Figure 2).
For P1 and P3, the relatively large 2-butyloctyl (BO)
solubilizing group was chosen for BDT because ITN-ME and
ITN-CN possess limited solubilizing power. Conversely, for P2,
smaller side chains were chosen for BDT because ITN-I
contains an aliphatic side chain that imparts solubility to the
polymer. Additionally, because the size and placement of
polymer side chains have been shown to affect OPV device
performance,15a,16 four different BDT-co-ITN-I polymers were
C
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Figure 3. Structure of PBDTTPD (left) and the energy level diagram
for PBDTTPD, P1, P2a, P3, and PCBM (right) where Eg represents
the optical band gap.
levels of the copolymers experience a slight decrease with
increased monomer electron-withdrawing strength, there is a
dramatic decrease in LUMO energy levels. This behavior
indicates that the electronics of the acceptor monomer has a
greater influence on the LUMO of the polymer than on the
HOMO.
The effect of quinoidal character on ITN-I was evaluated by
comparing the energy levels of P2a with PBDTTPD, a polymer
synthesized by our group that contains a thienopyrroledione
(TPD) acceptor monomer and a BDT donor monomer.15a,18
Both ITN-I and TPD have imide functionalites, but ITN-I has a
quinoidal isothianaphthene core, while TPD has only a
thiophene core. With identical solubilizing chains on both
P2a and PBDTTPD, the effect of quinoidal character on the
polymer energy levels can be clearly identified. The HOMO of
PBDTTPD was found to be at a lower oxidation potential
(−5.73 eV) than the HOMO of P2a (−5.66 eV), showing that
quinoidal character destabilizes the polymer HOMO. These
data corroborate calculations that predicted HOMO destabili-
zation and LUMO stabilization with increased quinoidal
character.8c,19
Optical Properties. Solution and thin-film absorption
spectra of the ITN copolymers demonstrate favorable overlap
with the visible region of the solar emission spectrum (Figure
4). In the solid state, the weakly electron-withdrawing ester-
containing polymer, P1, has an onset of absorption at about
750 nm, making it the material with the largest band gap (Eg) in
this series (1.60 eV). Compared to P1, the more electron-
deficient polymers, P2a and P3, have considerably red-shifted
onsets of absorption and narrower band gaps (1.54 and 1.51
eV, respectively). The strong quinoidal character of ITN
contributes to the narrow band gap of these polymers, as
revealed by the band gap comparison of P2a with PBDTTPD
(1.73 eV). Although the thin-film band gaps of P1, P2a, and P3
become narrower with increasing monomer electron deficiency,
the absorption profiles of the polymers in solution do not trend
as expected. Nitrile-based polymer, P3, has the smallest band
gap in thin films; however, in solution, P3 has a larger band gap
than imide-based polymer P2a. While this result is notable,
there are physical considerations in addition to electronics that
contribute to a material’s absorption spectra. For example, in
the solid state, increased polymer packing allows for greater
electron delocalization though π−π interactions, and a red-
shifted absorption may be observed.20 Conversely, in solution,
solvent effects may decrease interchain coupling. It is likely that
P3 has less interchain coupling in solution than in thin films,
giving it a larger-than-expected band gap in solution.
Figure 4. Absorption spectra of P1, P2a, and P3 in chloroform
solutions (top) and thin films (bottom).
Photovoltaic Properties. The OPV performance of bulk
heterojunction (BHJ) devices was investigated with the
following device architecture: ITO/PEDOT:PSS/poly-
mer:PC61BM/Ca/Al. Each polymer device was individually
optimized with processing parameters such as solvents, polymer
concentrations, donor−acceptor blend ratios, solvent additives,
and thermal annealing. With PC61BM, P1 and P2 have power
conversion efficiencies (PCE) between 1.3% and 3.0% (Figure
5). Ester-functionalized polymer, P1, has an average PCE of
2.7% and does not improve upon addition of solvent additives.
With diiodooctane or chloronaphthalene as solvent additives,
imide-functionalized polymers P2a, P2b, and P2c achieve
average PCEs between 2.9% and 3.0%, while the less soluble
P2d polymer reaches an average PCE of 1.3%, which does not
improve when employing solvent additives. Interestingly, P1
has the largest Voc (0.87 V) and the highest HOMO energy
level (−5.59 eV) of the polymer series. This result suggests that
device Voc is partly influenced by parameters other than the
polymer HOMO and that engineering of the HOMO energy
levels alone does not guarantee high Voc.
In contrast to P1 and P2, nitrile-substituted P3 does not
perform as a p-type material with PC61BM; instead, it functions
as an n-type material. To perform as a p-type material, the
polymer must have a sufficient excited-state energy level offset
with the n-type material; however, in the case of P3, the
polymer excited state (HOMO + Eg = −4.28 eV) is lower in
energy than the LUMO of PC61BM (−4.2 eV),2a energetically
hindering charge separation at the interface. As a result of P3’s
low energy levels, it was found that P3 performs as an n-type
material when combined with poly(3-(4-n-octyl)-
phenylthiophene) (POPT) in bilayer devices. Additionally,
thermal annealing was found to improve the device perform-
ance by 1 order of magnitude to give solar cells with a
maximum PCE of 0.28%.
D
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AUTHOR INFORMATION
Corresponding Author
*E-mail: frechet@berkeley.edu (J.M.J.F.); mschen@berkeley.
edu (M.S.C.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by the Director, Office of
Science, Office of Basic Energy Sciences, Material Sciences and
Engineering Division, of the U.S. Department of Energy under
́
Contract DE-AC02-05CH11231 and the Frechet “Various
Donors” gift fund for the support of research in new materials.
G.G. thanks Fondazione Banca del Monte di Lombardia,
T.W.H. thanks the National Science Foundation, and M.S.C.
thanks the Camille and Henry Dreyfus Postdoctoral Program in
Environmental Chemistry for fellowships. The authors also
thank Professors Robert Bergman and Peter Vollhardt for
helpful discussions.
ABBREVIATIONS
■
NMR, nuclear magnetic resonance; HOMO, highest occupied
molecular orbital; LUMO, lowest unoccupied molecular
orbital; ITO, indium tin oxide; PEDOT, poly(3,4-ethyl-
enedioxythiophene); PSS, poly(styrenesulfonate); PC61BM,
phenyl C61-butyric acid methyl ester.; Voc, open-circuit voltage;
Jsc, short-circuit current density; FF, fill factor
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dx.doi.org/10.1021/ma300589k | Macromolecules XXXX, XXX, XXX−XXX