Near-infrared absorbing boron-dibenzopyrromethenes that serve as light-harvesting sensitizers for polymeric solar cells

Article abstract of DOI:10.1021/ol201770g

Hexylthiophene-conjugated boron-dibenzopyrromethenes with benzo[1,3,2]oxazaborinine rings, 1, that absorb near-infrared light with relatively high molecular extinction coefficients have been synthesized. The incorporation of 3-hexylthiophene-conjugated dye 1a at a blend ratio of 5 wt % into a polymeric solar cell based on a P3HT/indene-C₇₀ bisadduct (IC₇₀BA) bulk heterojunction structure improved power conversion efficiency from 3.7 to 4.3%. The present work suggests that well-defined near-infrared absorbing BODIPY analogues can potentially be used as photosensitizers in polymeric solar cells.

Full text of DOI:10.1021/ol201770g

ORGANIC
LETTERS
2011
Vol. 13, No. 17
4574–4577
Near-Infrared Absorbing
Boron-dibenzopyrromethenes
that Serve As Light-Harvesting
Sensitizers for Polymeric Solar Cells
Yuji Kubo,*,† Kazuki Watanabe,† Ryuhei Nishiyabu,† Rieko Hata,‡ Akinori Murakami,‡
Takayuki Shoda,‡ and Hitoshi Ota‡
Department of Applied Chemistry, Graduate School of Urban Environmental Sciences,
Tokyo Metropolitan University, 1-1 minami-ohsawa, Hachioji, Tokyo 192-0397, Japan,
and Mitsubishi Chemical Group Science and Technology Research Center, Inc.,
1000 Kamoshida, Aoba, Yokohama, Kanagawa, 227-8502, Japan
yujik@tmu.ac.jp
Received July 1, 2011
ABSTRACT
Hexylthiophene-conjugated boron-dibenzopyrromethenes with benzo[1,3,2]oxazaborinine rings, 1, that absorb near-infrared light with relatively
high molecular extinction coefficients have been synthesized. The incorporation of 3-hexylthiophene-conjugated dye 1a at a blend ratio of 5 wt %
into a polymeric solar cell based on a P3HT/indeneꢀC70 bisadduct (IC70BA) bulk heterojunction structure improved power conversion efficiency
from 3.7 to 4.3%. The present work suggests that well-defined near-infrared absorbing BODIPY analogues can potentially be used as
photosensitizers in polymeric solar cells.
Much attention has been devoted to the chemistry of
BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene).1
BODIPY, which can be easily modified by a synthetic
process, is considered greatly promising for a variety of
applications such as fluorescence probes, emitting materi-
als, chemosensors, and light-harvesting materials because
of its high molecular extinction coefficients of absorption,
high fluorescence quantum yield, and high photostability.
Its structure can be controlled during synthesis, enabling
the fine-tuning of its optical properties. In this context,
increased efforts to apply BODIPY to organic photovol-
taic cells have led to the synthesis of BODIPY derivatives
for applications to TiO2-based solar cells2 and bulk
heterojunction (BHJ) devices.3 BODIPY dyes have been
(2) (a) Hattori, S.; Ohkubo, K.; Urano, Y.; Sunahara, H.; Nagano, T.;
Wada, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Fukuzumi, S. J. Phys.
Chem. B 2005, 109, 15368–15375. (b) Erten-Ela, S.; Yilmaz, M. D.; Icli, B.;
Dede, Y.; Icli, S.; Akkaya, E. U. Org. Lett. 2008, 10, 3299–3302. (c)
Kumaresan, D.; Thummel, R. P.; Bura, T.; Ulrich, G.; Ziessel, R.
Chem.;Eur. J. 2009, 15, 6335–6339. (d) Lee, C. Y.; Hupp, J. T. Langmuir
2010, 26, 3760–3765. (e) Kolemen, S.; Cakmak, Y.; Erten-Ela, S.; Altay,
Y.; Brendel, J.; Thelakkat, M.; Akkaya, E. U. Org. Lett. 2010, 12, 3812–
3815. (f) Kolemen, S.; Bozdemir, O. A.; Cakmak, Y.; Barin, G.; Erten-El,
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Gratzel, M.; Akkaya, E. U. Chem. Sci. 2011, 2, 949–954.
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Roncali, J. Chem. Commun 2009, 1673–1675. (b) Rousseau, T.; Cravino,
A.; Bura, T.; Ulrich, G..; Ziessel, R.; Roncali, J. J. Mater. Chem. 2009,
† Tokyo Metropolitan University.
ꢀ
19, 2298–2300. (c) Kim, B.; Ma, B.; Donuru, V. R.; Liu, H.; Frechet,
‡ Mitsubishi Chemical Group Science and Technology Research Center, Inc.
(1) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932. (b)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184–1201. (c) Benstead, M.; Mehl, G. H.; Boyle, R. W. Tetrahedron
2011, 67, 3573–3601.
J. M. J. Chem. Commun. 2010, 46, 4148–4150. (d) Rousseau, T.; Cravino,
A.; Ripaud, E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali,
J. Chem. Commun. 2010, 46, 5082–5084.
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Lett. 2009, 11, 4508–4511.
r
10.1021/ol201770g
Published on Web 08/09/2011
2011 American Chemical Society

investigated as p-type or donor materials in conjugation
with electron-accepting fullerene derivatives to realize
solution-processed BHJ solar cells.
high demand for near-infrared absorbing dyes capable of
acting as a photosensitizer in BHJ solar cells to enhance the
light-harvesting capability. Despite this necessity, so far,
only a few near-infrared dyes have been reported, such as
diketopyrrolopyrrole9 and phthalocyanines.10
Here, we show that the incorporation of 1 at a blend ratio
of 5 wt % into a P3HT/IC70BA BHJ solar cell (IC70BA;
indene-C70 bis-adduct11) leads to increased carrier generation
in the near-infrared region, resulting in the enhancement of
the short circuit current and thus the PCE of the device. These
are the first examples of BODIPY analogues that improve the
photovoltaic performance of polymeric solar cells.
Scheme 1. Synthesis of Thiophene-conjugatedboron-dibenzo-
pyrromethenes 1
The synthetic path of 1 is shown in Scheme 1, the key
intermediate being dibromo- and dimethoxy-substituted
boron-dibenzopyrromethene difluoride 2. The synthesis of
2 starts with the reaction of commercially available 4-bro-
mo-2-hydroxyacetophenone 3 with hydrazine 4 to yield 5
in 62% yield, followed by the oxidation reaction with lead
tetraacetate to give 6, and then condensation with ammo-
nia to afford benzo-fused dipyrrin 7 in a moderate yield of
44%. Subsequently, BF2-chelation with 7 using BF3 Et2O
3
was carried out to afford 2 in 79% yield. As for the
preparation of 3-hexylthiophene-conjugated dye 1a, Suzuki
cross-coupling of 2 was employed with borylthiophene
derivative 8,12 followed by demethylation with BBr3 to
realize spontaneous cyclization to afford 1a in 53% yield.
On the other hand, 5-hexylthiophene derivative 1b, being
a regioisomer of 1a, could be prepared via Stille cross-
coupling of 2 with the stannic derivative 1013 followed by
treatment with BBr3. The assignments of these new dyes
From the viewpoint of achieving high power conversion
efficiency (PCE), near-infrared absorbing dyes are one of
the most important research candidates because approxi-
mately 50% of the radiation intensity of sunlight is in the
near-infraredregionfrom 700to2000nm.4 However, to the
best of our knowledge, few studies have focused on solar
cells containing near-infrared absorbing BODIPY dyes.2e
We have currently synthesized new types of boron-
dibenzopyrromethene dyes5 in which the formation of a
benzo[1,3,2]oxazaborinine based on intramolecular B,O-
chelation caused a remarkable bathochromic shift in the
absorption band,6 resulting in the production of near-
infrared absorbing dyes. The synthetic path that we em-
ployed enabled us to introduce several functional groups at
the 5-position of the isoindole moiety. Our continuous
efforts to determine the applicability of the dye to photo-
voltaic devices led to our discovery that hexylthiophene-
conjugated dye 1 could serve as a sensitizer in a BHJ solar
cell. The prototype BHJ system is based on a blend of
regioregular poly(3-hexylthiophene) (P3HT) as a donor
and a fullerene derivative as an acceptor, with reported
PCEs of up to 4ꢀ5%.7 Although P3HT has a high field
effect hole mobility (>10ꢀ2 cm2 Vꢀ1 sꢀ1),8 thefilmcanonly
absorb short wavelengths under 650 nm. Thus, there is a
Figure 1. (a) Absorption spectra of 1a, 1b, and 12 (5 μM) in THF
at 25 °C. (b) Chemical structures of 12 and 13.
were carried out using spectroscopic data (see Supporting
Information); for example, the resonance arising from
thiophene protons for 1a in d6-DMSO was exhibited at
7.13 (2H, d, J = 5.16 Hz) and 7.56 (2H, d, J = 5.10 Hz).
Figure 1 shows the absorption spectra of 1 in THF, where
(9) Tamayo, J. P. A. B.; Dang, X.-D.; Nguyen, T.-Q. Appl. Phys.
Lett. 2008, 93, 163306.
(5) Kubo, Y.; Minowa, Y.; Shoda, T.; Takeshita, K. Tetrahedron
Lett. 2010, 51, 1600–1602.
(10) Honda, S.; Ohkita, H.; Benten, H.; Ito, S. Chem. Commun. 2010,
46, 6596–6598.
(11) He, Y.; Zhao, G; Peng, B.; Li, Y. Adv. Funct. Mater. 2010, 20,
3383–3389.
(12) Janzen, D. E.; Burand, M. W.; Ewbank, P. C.; Pappenfus, T. M.;
ꢀ
Higuchi, H.; da Silva Filho, D. A.; Young, V. G.; Bredas, J.-L.; Mann,
K. R. J. Am. Chem. Soc. 2004, 126, 15295–15308.
(13) Tian, N.; Thiessen, A.; Schiewek, R.; Schmitz, O. J.; Hertel, D.;
(6) (a) Kim, H.; Burghart, A.; Welch, M. B.; Reibenspies, J.; Burgess,
K. Chem. Commun. 1999, 1889–1890. (b) Loudet, A.; Bandichhor, R.;
Burgess, K.; Palma, A.; McDonnell, S. O.; Hall, M. J.; O’Shea, D. F.
Org. Lett. 2008, 10, 4771–4774.
(7) (a) Ma, W.; Yang, C.; Heeger, A. J. Adv. Funct. Mater. 2005, 15,
1617–1622. (b) Li, G.; Shrotriya, V.; Yao, Y.; Huang, J.; Yang, Y.
J. Mater. Chem. 2007, 17, 3126–3140.
(8) Peet, J.; Heeger, A. J.; Bazan, G. C. Acc. Chem. Res. 2009, 42,
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Meerholz, K.; Holder, E. J. Org. Chem. 2009, 74, 2718–2725.
Org. Lett., Vol. 13, No. 17, 2011
4575

Table 1. Optical Properties and Theoretical Data of Boron-
dibenzopyrromethenes
λmax
(obs)a /
nm
λmax
(calcd)/nm
HOMOb/ LUMOb/ ε (obs)a
f
dye
eV
eV
M
ꢀ1 cmꢀ1 (calcd)
1a
733
747
748
746
622
635
ꢀ4.707
ꢀ4.616
ꢀ2.568
ꢀ2.519
135 000
0.621
0.662
1b
148 000
83 400
82 900
129
139
642
ꢀ4.445
ꢀ2.408
0.468
Figure 2. HOMO and LUMO surface plots for 1a and 1b.
a Spectra were measured in THF. b Values were calculated using
DFT.
the effect of the inserted thiophene on the absorption
property can be compared with the corresponding hexy-
loxy derivative 12.5 Dye 1a absorbed near-infrared light
with a λmax value of 733 nm and with the molecular
extinction coefficient (ε) being calculated to be 1.35 ꢁ
105 Mꢀ1cmꢀ1. When compared to 12, albeit with a hyp-
shochromic shift of 15nm, the ε value of 1awas foundtobe
1.6 times larger, which is favorable for light-harvesting.
The absorption properties of 1b were also elucidated (λmax
=
747 nm, ε = 1.48ꢁ 105 Mꢀ1cmꢀ1); the bathochromic shift,
with an enhanced molecular extinction coefficient com-
pared to 1a, can be interpreted to be caused by steric effects
between the isoindole segment and thiophene rings in the
chromophore (vide infra).
Figure 3. (a) Cyclic voltammograms of 1a, 1b, and 12 measured
in acetonitrile/o-dichlorobenzene (2:3 v/v) containing 0.1 M
TBAPF6 at room temperature. Ferrocene (Fc) was used as an
internal reference. (b) HOMO and LUMO energies of dyes 1a,
1b, 12, P3HT, and IC70BA. The HOMO and LUMO levels of
the dyes were calculated from the CV measurements in acetoni-
trile/o-dichlorobenzene (2:3 v/v); HOMO [eV] = ꢀ(Eox
ꢀ
For a better understanding of the spectral properties, the
transition energies and oscillator strength (f) were calcu-
lated using TD-DFT (time-dependent density functional
theory). The resulting data is summarized in Table 1 with
the data for the methoxy derivative 135 for comparison.
Geometry optimization was carried out using the B3LYP
hybrid function with the 6-31G (d,p) basis set. The trend of
the calculated λmax was almost consistent with that of
spectra observed in THF. Although the first absorption
band was characterized as a mixture of several configura-
tions, it can mainly be ascribed to the HOMOꢀLUMO
transition. Replacement of the methoxy group at the
5-position of isoindole with hexylthiophene led to signifi-
cant stabilization of both the HOMO and the LUMO
energy levels. In addition, the calculations indicated that
the π-systems extended by incorporating the thiophene
caused the significant increase in the oscillator strength of 1
compared to 13 from 0.468 to 0.621 for 1a and from 0.468
to 0.662 for 1b. The position of the hexyl substituent on the
thiophene ring was found to influence the HOMO energy
level more effectively than the LUMO energy level; the
HOMO energies of 1a and 1b were calculated to be ꢀ4.707
and ꢀ4.616, respectively. This alteration was due to steric
effects between thiophene and isoindole rings, as inferred
from the geometry optimization in Figure 2, which shows
that the HOMO level is more stable. These calculations
clarified the effectofthiophene-insertion ontheabsorption
property of the dibenzopyrromethene chromophore.
E
Fc/Fcþ þ 4.8); LUMO [eV] = ꢀ(Ered ꢀ EFc/Fcþ þ 4.8).
The electrochemical properties of 1 were further exam-
ined by cyclic voltammetry (CV). Figure 3a shows the
reference electrode in acetonitrile/o-dichlorobenzene (2:3
v/v) with 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6) as the supporting electrolyte at a scan rate of
50 mV sꢀ1. The measurements were calibrated using a
ferrocene value of ꢀ4.8 eV as the standard.14 The formal
potential of Fc/Fcþ was measured as 0.18 V against Ag/Agþ.
The dyes tested here showed fully reversible oxidation and
reduction behaviors. The anodic scan showed that the onset
oxidation potential for 1a or 1b occurred at 0.329 or 0.216
eV, respectively, which corresponded to an ionization
potential (HOMO level) of ꢀ4.95 eV for 1a and ꢀ4.84 eV
for 1b. Similarly, the onset reduction potential of 1a or 1b
allowed us to estimate the electron affinity (LUMO level) to
be ꢀ3.50 eV for 1a or ꢀ3.49 eV for 1b. Although both have
similar LUMO levels, the HOMO level of 1a was lower than
that of 1b, implying that the HOMO level was sensitive to
the position of the hexyl substituent on the thiophene ring of
the dye, which was consistent with the result of DFT
calculations (vide supra). In addition, hexylthiophene-con-
jugated dyes 1a and 1b showed lower HOMO and LUMO
(15) Cascio, A. J.; Lyon, J. E.; Beerbon, M. M.; Schlaf, R.; Zhu, Y.;
Jenekhe, S. A. Appl. Phys. Lett. 2006, 88, 062104.
(16) Taken into the absorption enhancement at about 600 nm of BHJ
film with 1a account (Figure 4a), the addition of 1a could also control the
nanoscale crystallinity of P3HT in the polymer cells, leading to improve-
ment of carrier mobility arising from P3HT.
(14) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler,
H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551–554.
4576
Org. Lett., Vol. 13, No. 17, 2011

Figure 4. (a) Absorption spectra of BHJ film comprising P3HT:
IC70BA (1:0.95) (black curve) and the BHJ film with 5 wt % of
1a (red curve); (b) JꢀV curves of devices with the BHJ film
(black curve) or the BHJ film with 5 wt % of 1a.
Figure 5. EQE spectra of the devices made using (a) BHJ film
comprising P3HT:IC70BA (1:0.95) with 5 wt % of 1a, (b) BHJ
film with 5 wt % of 1b, and (c) BHJ control film.
6.3 to 7.0 mA cmꢀ1 and the fill factor (FF) from 0.67 to
0.71, as compared to the 1a-free device. The increase in Jsc
with 1a is ascribable to an increase in photogenerated
carriers. Indeed, the external quantum efficiency (EQE),
as shown in Figure 5, exhibited a peak at ca. 760 nm,
indicating that 1a dispersed in the BHJ layer could lead to
efficient exciton dissociation and carrier generation. Sub-
sequently, the incorporation of 1a (5 wt %) into a P3HT/
IC70BA bulk heterojunction solar cell improved the PCE
from 3.7 to 4.3%.16 Although the corresponding hexyloxy
dye 12 never exhibited light-harvesting capability, as in-
ferred from the energy diagram in Figure 3b, 5-hexylthio-
phene-conjugated dye 1b exhibited a moderate effect as a
sensitizer; the PCE value of the P3HT/IC70BA BHJ device
with 5 wt % of 1b was 4.0%, which was moderately larger
than that of the control device (PCE = 3.7%). Note that
the EQE spectrum of the 1b-containing device exhibited a
diminished peak signal in the near-infrared region. This
low efficiency maybe attributable to low carrier genera-
tion, possibly due to the higher-lying HOMO level, which
is close to that of P3HT. Furthermore, we have to consider
the steric bulkiness of the dye system as being a major
factor in its performance as a light-harvesting sensitizer in
polymeric solar cells.17 In the case of 1a, steric hindrance
between thiophene and isoindole moieties could prevent
dye-aggregation in the film to provide an effective light-
harvesting behavior.
levels than did hexyloxy dyes 12 (HOMO = ꢀ4.78 eV;
LUMO = ꢀ3.43 eV), which supported our calculation
results (vide supra).
The absorption and electrochemical properties of dyes 1
make them promising for practical applications as light-
harvesting sensitizers in P3HT/IC70BA-based BHJ solar
cells, wherein IC70BA is employed as an acceptor because
of its higher LUMO energy level of ꢀ3.72 eV than that of
PC60BM.11 The energy diagrams of the dyes are shown in
Figure 3b along with the values of P3HT15 and IC70BA.
For dye 12, the high-lying HOMO level at ꢀ4.78 eV causes
a lack of driving force for charge-carrier generation. Onthe
other hand, the HOMO level of 1a (ꢀ4.95 eV) is estimated
to be lower than that of P3HT by 0.1 eV, enabling it to
avoid charge trapping on the dye. Furthermore, the
LUMO level of ꢀ3.50 eV may be sufficiently high for
electron injection into the IC70BA acceptor. This implies
that when dye 1a is located at the interface between P3HT
and IC70BA, the excitons generated in 1a could donate
holes to P3HT and electrons to IC70BA in the device. In
addition, the energydiagram suggests that1bcould act as a
photosensitizer in the device; however, a negligible differ-
ence between the HOMO levels of P3HT and those of 1b
may lead to a less effective charge-carrier function.
The dyes presented in our study were characterized
in BHJ solar cell devices. The device was fabricated
with the configuration ITO/PEDOT:PSS (40 nm)/P3HT
(Mw = 38000, PDI = 1.74, regioregularity =91%):IC70
BA (1:0.95) with a blend of 5 wt % dye 1a (100 nm)/Ca
(10 nm)/Al (80 nm). The active layer was spin-coated and
annealed at 150 °C for 20 min. As a control device, a
P3HT/IC70BA binary blended cell device was also fabri-
cated. Figure 4a shows the absorption spectra of the
activated layer; it was found that P3HT/IC70BA with
5 wt % of 1a showed a greater optical absorbance in the
near-infrared region than did P3HT/IC70BA binary
blended BHJ film. In particular, a new absorption band
was clearly observed at ca. 760 nm. The photovoltaic
devices were characterized under 100 mW cmꢀ2 AM
1.5G irradiation; the current-density vs voltage (JꢀV)
curves were measured using SourceMeter (Serious 2400
Keithley Instruments Inc.) and are shown in Figure 4b.
The P3HT/IC70BA/1a ternary system exhibited a signifi-
cant increase in both the short-circuit current (Jsc) from
In conclusion, we have described the first examples of
BODIPY analogues, thiophene-conjugated boron-diben-
zopyrromethenes, that can serve as light-harvesting sensi-
tizers in polymeric solar cells. We believe that the synthetic
flexibility of BODIPY and its analogues can lead to the
development of highly efficient sensitizers for photovoltaic
conversion in BHJ solar cell devices. Future work will
include not only structural optimization of BODIPY-
based sensitizers but also further investigation of the
mechanism behind its light-harvesting effect.
Supporting Information Available. Detailed experimen-
tal procedures of synthesis and characterization. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(17) Honda, S.; Nogami, T.; Ohkita, H.; Benten, H.; Ito, S. ACS
Appl. Mater. Interfaces 2009, 1, 804–810.
Org. Lett., Vol. 13, No. 17, 2011
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