Phosphole- and benzodithiophene-based copolymers: Synthesis and application in organic photovoltaics

Article abstract of DOI:10.1002/ejic.201301132

Two kinds of phosphole- and benzodithiophene-based copolymers bearing P^V=O or P^V=NSO₂C₈H₁₇ functions were prepared by Pd-CuI-promoted Stille coupling reactions, and their optical and electrochemical p

Full text of DOI:10.1002/ejic.201301132

SHORT COMMUNICATION
DOI:10.1002/ejic.201301132
Phosphole- and Benzodithiophene-Based Copolymers:
Synthesis and Application in Organic Photovoltaics
CLUSTER
ISSUE
Yoshihiro Matano,*[a] Hiroshi Ohkubo,[b] Tetsushi Miyata,[b]
Yusuke Watanabe,[b] Yukiko Hayashi,[b] Tomokazu Umeyama,[b,c]
and Hiroshi Imahori[b,d]
Keywords: Heterocycles / Phosphorus / Sulfur / Donor–acceptor systems / Copolymerization / Cross-coupling /
Photovoltaics
Two kinds of phosphole- and benzodithiophene-based co-
polymers bearing PV=O or PV=NSO2C8H17 functions were
prepared by Pd–CuI-promoted Stille coupling reactions, and
their optical and electrochemical properties were investi-
gated. Bulk heterojunction organic photovoltaic devices com-
prised of the new P,S-bridged copolymers and (6,6)-phenyl-
C71-butyric acid methyl ester (PC71BM) exhibited power
conversion efficiencies of up to 0.65% under AM1.5 irradia-
tion at 100 mWcm–2
.
phosphorus substituent.[4] In this regard, thiophene–
phosphole-based D–A-type copolymers, in which the
phosphole acts as an electron-accepting unit, are potential
low-bandgap sensitizers for use in OPV. To date, several
kinds of phosphole-containing π-conjugated copolymers
have been reported.[5–10] For example, Réau et al.[6] and
Baumgartner et al.[7] prepared phosphole–thiophene- and
dithieno[b,d]phosphole–carbazole-based copolymers, P1
and P2, respectively (Figure 1), and studied their electronic
and fluorescent properties in detail. To the best of our
knowledge, however, no attempt has been made to use thio-
phene–phosphole-based D–A copolymers in OPV devices.
Introduction
Recent progress in the field of bulk heterojunction or-
ganic photovoltaics (OPV) has encouraged many chemists
to construct a new class of low-bandgap conjugated copoly-
mers that absorb light at a longer wavelength.[1] In particu-
lar, donor–acceptor (D–A)-type copolymers containing
electron-rich thiophene derivatives and electron-deficient
heterocyclopentadienes (heteroles) have been frequently
used as the sensitizers, because alternating D–A subunits
intrinsically lower the optical bandgap owing to the charge-
transfer (CT) character. For instance, OPV devices fabri-
cated with benzo[1,2-b:4,5-bЈ]dithiophene-based D–A co-
polymers (e.g., PBDTTPD[2] and PBDTTT[3]) and fullerene
derivatives were reported to show high power conversion
efficiencies (PCE) of up to 7.6%. An important subject in
the molecular design of D–A copolymers is the develop-
ment of electronically tunable acceptor subunits with high
electron affinity.
Figure 1. Previous examples of thiophene–phosphole-based co-
polymers P1 and P2.[6,7]
The LUMO level of phosphole is influenced, among
other things, by stabilizing hyperconjugative interactions,
and can be lowered significantly further by variation of the
Recently, we reported the first examples of poly-
(phosphole P-oxide)[11] and poly(phosphole P-imide)s,[12]
both of which were successfully prepared by using Stille
coupling reactions of the corresponding phosphole mono-
mers. In addition, these polyphospholes were found to pos-
sess narrow HOMO–LUMO gaps and ambipolar charge-
carrier transport properties derived from the polarizable na-
ture of the phosphorus-bridged diene units. Herein, we re-
port the first examples of phosphole–benzodithiophene
(BDT)-based conjugated copolymers bearing PV=O or
PV=NSO2C8H17 functions. The optical and electrochemical
properties of the newly synthesized D–A copolymers and
their application in OPV devices have been investigated.
[a] Department of Chemistry,
Faculty of Science Niigata University,
Nishi-ku, Niigata 950-2181, Japan
E-mail: matano@chem.sc.niigata-u.ac.jp
http://www.niigata-u.ac.jp/index_e.html
[b] Department of Molecular Engineering,
Graduate School of Engineering, Kyoto University,
Nishikyo-ku, Kyoto 615-8510, Japan
[c] PRESTO, Japan Science and Technology Agency,
4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
[d] Institute for Integrated Cell-Material Sciences (WPI-iCeMS),
Kyoto University,Nishikyo-ku, Kyoto 615-8510, Japan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301132.
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ure S1 in Supporting Information). The observed values of
5.65 (3) and 5.32 eV (6) are comparable to those of
PBDTTPD (5.4–5.57 eV),[2] which suggests that the incor-
poration of the phosphole subunits is effective for lowering
the HOMO levels of the D–A copolymers. The LUMO
levels of 3 and 6 were calculated to be 3.92 and 3.63 eV,
respectively, from the observed ionization potentials and the
optical HOMO–LUMO gaps.
Results and Discussion
Scheme 1 summarizes the syntheses of phosphole–BDT-
based copolymers 3 and 6, which involve Stille coupling
reactions between (i) 2,5-bis(tributylstannyl)phosphole P-
imide (1)[12] and 2,6-dibromobenzo[1,2-b:4,5-bЈ]dithiophene
(2)[13] or (ii) 2,5-diiodophosphole P-oxide (4)[11] and 2,6-
bis(trimethylstannyl)benzo[1,2-b:4,5-bЈ]dithiophene (5).[14]
In both reactions, the addition of CuI was indispensable to
smoothly promote the cross-coupling, namely, polymeriza-
tion. Copolymers 3 and 6 were isolated as bluish-violet sol-
ids by repeated reprecipitation from CH2Cl2/MeOH and
CH2Cl2/hexane. The number-average molecular weight
(Mn) and the polydispersity index (PDI) of the polymers
were determined by gel permeation chromatography (vs.
polystyrene standards). The 31P NMR spectra of 3 and 6
in CD2Cl2 displayed broad peaks at δ = 35 and 52 ppm,
respectively.
Figure 2. Normalized UV/Vis/NIR absorption spectra of 3 (blue)
and 6 (red).
Table 1. Optical and electrochemical data for 3 and 6.
λmax [nm] λonset [nm] HOMO [eV][a] LUMO [eV][b]
3 (soln)
3 (film)
6 (soln)
6 (film)
588
590
579
596
710
715
690
735
n.d.
5.65
n.d.
5.32
n.d.
3.92
n.d.
3.63
[a] Determined by photoemission yield spectroscopy. [b] Deter-
mined from HOMO levels and optical HOMO–LUMO gaps. n.d.
= not determined.
To attain some insight into the fundamental properties
of the present A–D–A fragment, the phosphole–BDT–
phosphole derivative 9 was newly prepared (Scheme 2). 2-
Tributylstannyl-5-phenylphosphole P-imide (7), prepared
from 1-tributylstannyl-7-phenyl-1,6-heptadiyne,[11] was
treated with iodine to give 2-iodo-5-phenylphosphole P-
imide (8). A Stille coupling reaction between 8 and 5 in the
presence of CuI afforded one of the diastereomers of 9 (δP
= 37.0 ppm) as a blood-red solid after column chromatog-
raphy.
Scheme 1. Synthesis of copolymers 3 and 6.
The UV/Vis/NIR absorption spectra of 3 and 6 are
shown in Figure 2. In CH2Cl2, the absorption maximum
(λmax) and onset (λonset) of 3 are somewhat redshifted rela-
tive to those of 6, which suggests that the PV substituents
affect the electronic transition energies of the copolymer π
networks (Table 1). In the film state, however, the reverse
relationship was observed. The λmax and λonset values of 6
are redshifted relative to those of 3. This may be attribut-
able to the differences in the intermolecular π–π interaction
and configuration of the polymer chains in the solid state.
Presumably, the less bulky P=O groups in 6 reduce the spa-
tial separation of the polymer networks more than the
P=NSO2C8H17 groups in 3 (see below). The optical
HOMO–LUMO gaps of 3 and 6 determined from their
λonset values in films are 1.73 and 1.69 eV, respectively. The
ionization potentials of 3 and 6 were also determined by
photoemission yield spectroscopy of the thin films (Fig- Scheme 2. Synthesis of 9 (R = C4H9).
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Figure 3. HOMO (lower) and LUMO (upper) diagrams of (a) anti-9-m and (b) 10-m.
Compound 9 displayed absorption and emission maxima PPNBDT-based device. Presumably, the PPOBDT π net-
at 509 and 663 nm, respectively (see Figure S2 in the Sup- work is more densely packed (see above) and transports the
porting Information). The relatively large Stokes shift of charge carriers more effectively than the PPNBDT π net-
4560 cm–1 indicates the high intramolecular CT character in work. The preliminary PCE value of 0.65% as well as the
the excited state of 9. Indeed, the density functional theory other parameters (VOC and JSC) is still low compared to the
calculations of A–D–A model 9-m and D–A–D model 10- previously reported PCE values for OPV devices with BDT-
m support this interpretation; the HOMO is rather localized based D–A copolymers.[2,3] Nevertheless, the present results
on the BDT moieties, whereas the LUMO is localized on demonstrate for the first time that the phosphole subunits
the phosphole moieties (Figure 3). There are slight differ- can behave as key components in the D–A copolymers for
ences in the HOMO and LUMO energies between the two OPV.
diastereomers of 9-m (Figure S3 in the Supporting Infor-
mation), which implies that the stereochemistry at the phos-
phorus(V) centers does not significantly affect the electronic
properties of the A–D–A fragment.[15] The oxidation and
reduction potentials of 9 were determined to be +0.49 and
–1.65 V (vs. Fc/Fc+), respectively, by cyclic voltammetry.
The energy gap of 2.14 V is close to the optical HOMO–
LUMO gap (2.12 eV) determined from the λab and λem val-
ues.
With the new D–A copolymers 3 (PPNBDT) and 6
(PPOBDT) in hand, we investigated their photovoltaic
properties using bulk heterojunction OPV devices with a
device structure of indium tin oxide (ITO)/poly(ethylene di-
oxythiophene) doped with polystyrene sulfonic acid (PE- Figure 4. Current–voltage curves of the OPV devices comprised of
3 (squares) and 6 (circles).
DOT:PSS)/PPXBDT:PC71BM/Al (see Figures S4 and S5 in
the Supporting Information). The active layer was spin-
coated from chlorobenzene solutions onto the hole trans-
port layer (PEDOT:PSS). Both 3 and 6 are stable over
300 °C (Td10 = 325 °C for 3 and 335 °C for 6). The current–
Conclusion
voltage characteristics of the OPV devices under simulated
We have prepared the first examples of phosphole–BDT-
AM 1.5G solar irradiation at 100 mW cm–2 are illustrated based conjugated copolymers by using CuI-promoted Stille
in Figure 4 together with the open-circuit voltages (VOC), coupling reactions. These D–A copolymers were found to
short-circuit current densities (JSC), fill factors (FF), and absorb a wide range of visible light reaching 735 nm, which
PCE values. It should be noted that VOC, JSC, FF, and PCE is attributable to the intramolecular CT between the two
of the PPOBDT-based device are higher than those of the different heterole components. We also fabricated bulk
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measured by 2400 SourceMeter (Keithley) with OTENTO-SUN III
solar simulator (Bunkoukeiki).
heterojunction OPV devices containing the phosphole–
BDT-based copolymers and PC71BM, and evaluated their
device performances. The PCE and Jsc values were found
to differ significantly depending on the P=E functions (E =
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, DFT calculation results, and spectro-
O, NSO2C8H17). This may imply that the PV substituents scopic data are presented.
make a large impact on the charge-generation efficiency
and/or charge-carrier pathways in the blend films. To
achieve the higher PCE values, the synthesis of a new series
Acknowledgments
of phosphole-containing D–A-type materials based on the
more sophisticated molecular design is now in progress.
This work was supported by Grants-in-Aid from MEXT, Japan
(nos. 22350016 and 25288020), NEDO, and the Ogasawara founda-
tion. We thank Prof. Takashi Sagawa (Kyoto University) for the
help on the photoemission yield spectroscopy measurements. Y. M.
thanks Prof. Hideyuki Murata and Dr. Varun Vohra (JAIST) for
their valuable comments on the OPV devices.
Experimental Section
Synthesis of Polymer 3: A mixture of 1 (242 mg, 0.25 mmol), 2
(151 mg, 0.25 mmol), [Pd(PPh3)4] (5.0 mg, 0.0043 mmol), CuI
(47.4 mg, 0.25 mmol), and toluene (13 mL) was heated at 110 °C.
After 25.5 h at this temperature, iodobenzene (1.4 μL, 0.013 mmol)
was added to the mixture, which was then cooled down to room
temperature. The addition of methanol (300 mL) caused the pre-
cipitation of a polymer, which was collected by filtration. The poly-
mer was purified by repeated reprecipitation from CH2Cl2/MeOH
and CH2Cl2/hexane and by treatment with metal scavengers. Fi-
nally, 3 was isolated as a dark bluish-violet solid (143 mg, 68%).
1H NMR (400 MHz, CD2Cl2): δ = 0.65–1.22 (m), 1.36 (br. s), 1.45–
1.95 (m), 2.45 (br. s), 2.80–3.40 (m), 3.95–4.35 (m), 7.10–7.20 (m),
7.35–7.74 (m), 7.78–8.10 (m) ppm. 31P{1H} NMR (162 MHz,
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CDCl ): δ ≈ 35 ppm. IR (ATR): ν
1262 (P=N) cm–1.
˜
3
max
[3] BDTTT
= benzo[1,2-b:4,5-bЈ]dithiophene–thieno[3,4-b]thio-
Synthesis of Polymer 6: A mixture of 4 (119 mg, 0.25 mmol), 5
(198 mg, 0.25 mmol), [Pd2(dba)3] (dba = dibenzylideneacetone;
23 mg, 0.025 mmol), (2-furyl)3P (12 mg, 0.05 mmol), CuI (114 mg,
0.6 mmol), and N-methyl-2-pyrrolidone (NMP; 6.0 mL) was stirred
at room temperature. After 100 h, iodobenzene (3 μL, 0.025 mmol)
was added, and the mixture was stirred for a further 1 h. The re-
sulting mixture was poured into MeOH (300 mL) and the precipi-
tates were collected. The polymer was purified by repeated repre-
cipitation (CH2Cl2/MeOH/CH2Cl2 = 2:1) and by treatment with
metal scavengers (MPA). Finally, 6 was isolated as a dark blue solid
(103 mg, 62%). 1H NMR (400 MHz, CD2Cl2): δ = 0.93 (br. s),
1.26–1.67 (m), 1.70–2.05 (m), 2.15–3.72 (m), 2.95–3.20 (m), 4.05–
4.15 (m), 7.26–7.61 (m), 7.73–7.95 (m) ppm. 31P{1H} NMR
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a
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(7.2 mgmL–1) and PC71BM (14.4 mgmL–1) in chlorobenzene was
spin-coated at 1000 rpm for 1 min onto the ITO/PEDOT:PSS un-
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Supporting Information. Photocurrent–voltage characteristics were
measured under an argon atmosphere and simulated solar light
(100 mW cm–2, AM1.5). Photocurrent–voltage characteristics were
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Published Online: October 22, 2013
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