Near-infrared absorbing merocyanine dyes for bulk heterojunction solar cells

Article abstract of DOI:10.1021/ol101492q

A series of near-infrared absorbing merocyanine dyes bearing the strong electron-accepting 2-oxo-5-dicyanomethylene-pyrrolidine unit was synthesized and applied in combination with PC₆₁BM and PC₇₁BM in solution-processed photoactive

Full text of DOI:10.1021/ol101492q

ORGANIC
LETTERS
2010
Vol. 12, No. 16
3666-3669
Near-Infrared Absorbing Merocyanine Dyes
for Bulk Heterojunction Solar Cells†
Hannah Bu¨rckstu¨mmer,‡ Nils M. Kronenberg,§ Klaus Meerholz,*,§ and
Frank Wu¨rthner*,‡
Institut fu¨r Organische Chemie and Ro¨ntgen Research Center for Complex Material
Systems, UniVersita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany, and Department
fu¨r Chemie, UniVersita¨t zu Ko¨ln, Luxemburger Str. 116, 50939 Ko¨ln, Germany
wuerthner@chemie.uni-wuerzburg.de; klaus.meerholz@uni-koeln.de
Received June 29, 2010
ABSTRACT
A series of near-infrared absorbing merocyanine dyes bearing the strong electron-accepting 2-oxo-5-dicyanomethylene-pyrrolidine unit was
synthesized and applied in combination with PC61BM and PC71BM in solution-processed photoactive layers of bulk heterojunction solar cells,
exhibiting a remarkable performance range with power conversion efficiencies from 0.01% to 1.00%.
Merocyanine dyes represent a traditional class of chro-
mophores with a general structural feature consisting of an
electron-donating and an electron-accepting moiety that are
connected by a polymethine chain. Their unique dipolar and
polarizability properties enable applications in nonlinear optics
and as photorefractive materials.1 The absorption wavelengths
as well as band gaps of these chromophores are easily tunable
by variation of the chain length and the donor and acceptor
groups.1 These features in combination with their high tinctorial
strengths add up to materials well-suited for implementation in
organic photovoltaics. Recently, we have shown the application
of merocyanine dyes as electron donors in solution-processed
and vapor-deposited bulk heterojunction (BHJ) organic solar
cells in combination with a fullerene as an acceptor.2 Moreover,
merocyanine dyes have been used in dye-sensitized solar cells
(also called Graetzel cells).3
In the field of solution-processed BHJ organic solar cells,
great attention has been paid particularly to the combination of
the polymeric donor poly(3-hexylthiophene) (P3HT) and the
electron-accepting fullerene derivative PC61BM, with enormous
research efforts on the optimization of structural features such
as regioregularity of the P3HT polymer and the morphology
of the active BHJ layer by post-treatment of the devices.4
More recently, in order to improve the absorption proper-
ties and harvest more photons, low-bandgap polymers have
been developed, and very high power conversion efficiencies
(PCEs) exceeding 7% have been achieved.5 Another recent
strategy toward BHJ solar cells is based on substitution of
the polymeric donor by small molecules whose monodis-
persity enables facile synthesis and convenient optimization
of the absorption and redox properties. In the past years,
† Dedicated to Prof. Karlheinz Drauz on the occasion of his 60th birthday.
‡ Universita¨t Wu¨rzburg.
§ Universita¨t zu Ko¨ln.
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3, 17. (b) Verbiest, T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons,
A. J. Mater. Chem. 1997, 7, 2175. (c) Blanchard-Desce, M.; Alain, V.;
Bedworth, P. V.; Marder, S. R.; Fort, A.; Runser, C.; Barzoukas, M.; Lebus,
S.; Wortmann, R. Chem.sEur. J. 1997, 3, 1091. (d) Marder, S. R.; Kippelen,
B.; Jen, A. K. Y.; Peyghambarian, N. Nature 1997, 388, 845.
10.1021/ol101492q  2010 American Chemical Society
Published on Web 07/27/2010

Scheme 1. Syntheses of Merocyanine Dyes 2 and 10a-c
increasingly promising results were reported with various
acceptor moiety 2-oxo-5-dicyanomethylene-pyrrolidine 5 (Scheme
1), which imparts absorption of the MC dyes in the NIR.17
Application of the present merocyanine chromophores in solution-
processed solar cells based on blends with PC71BM yielded devices
with PCEs of up to 1%.
types of dyes such as oligothiophene,6 squaraine,7 cyanine,8
BODIPY,9 diketopyrrolopyrrole (DPP),10 indigoid,11 acene,12
and dibenzo[b,def]chrysene,13 leading to PCEs up to 4.4%.10a
The photon flux of sunlight displays a maximum at ∼690 nm
(1.8 eV).14 Thus, to achieve an ideal spectral overlap of the
absorption of solar cells with the solar irradiation, materials with
absorption reaching in the near-infrared (NIR) region are required.15
Moreover, photovoltaics with NIR absorption render transparent
solar cells for sun shading and solar power window applications
possible.16 Here, we report the synthesis and characterization of
the optical, electrochemical, and photovoltaic properties of a series
of new merocyanine (MC) dyes containing the strong electron-
The reference chromophore 2 consisting of an aniline donor and
the heterocyclic acceptor that are directly connected was synthe-
sized by the alkylation of 118 with benzyl bromide in a high yield
of 93% (Scheme 1). The synthesis of more extended merocyanine
dyes 10a-c involves three major sequences: the syntheses of the
acceptor and the donor moieties and the condensation of these two
moieties into the desired chromophores. The synthesis of the
acceptor 5a starts with the reaction of enolate 3 with malononitrile
in the presence of triethylamine as a base to afford the intermediate
4 in 60% yield.19 The subsequent condensation reaction of 4 with
diethyl oxalate under basic condition results in the acceptor 5a,20
which exhibits poor solubility and thus was used in the further
reaction as obtained. The donor component 8a was built up by
deprotonation of indolenine 6 (“Fischer base”)21 followed by
condensation with phenylacetaldehyde in methanol.22 The two-
step reaction afforded 8a in a moderate yield of 32%. A higher
yield of 54% was obtained for the methyl derivative 8b by
condensation of the commercially available indolenine 7b with
phenylacetaldehyde.22 The coupling of the donor and acceptor
moieties comprises the in situ generation of component B by
chlorination and protonation of dianionic acceptor 5, followed by
addition of component 8 to B leading to elimination of 1 equiv
HCl to result in chromophores 9a-c. A high yield of 93% was
obtained in the case of 9c, whereas the dyes 9a,b resulting from
acceptor 5b20 were obtained in significantly lower yields of
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Steinmann, V.; Bu¨rckstu¨mmer, H.; Hwang, J.; Hertel, D.; Wu¨rthner, F.;
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Org. Lett., Vol. 12, No. 16, 2010
3667

17-35%. The final step involving the alkylation of the secondary
amine 9 with benzyl bromide afforded the dyes 10a-c in 79-83%
yield.
Figure 1 displays the UV-vis spectra and the cyclic
voltammograms of reference 2 and merocyanine dye 10a in
Table 1. Optical and Electrochemical Properties of 2 and 10a-c
and Photovoltaic Characteristics of BHJ Solar Cell Devices
Containing a MC Dye:PC61BM Blend
JSC
MC λmax ε (M-1 λmax EHOMO ELUMO wt % VOC (mA
PCE
a
dye (nm)a cm-1
)
(nm)b (eV)c (eV)d PCBM (V) cm-2) FF (%)
2
664
53300 640 -5.98 -4.11
70
70
75
75
75
0.55 0.05 0.31 0.01
0.71 2.28 0.28 0.46
0.74 2.00 0.29 0.43
0.64 3.33 0.31 0.66
0.66 4.83 0.31 1.00
10a 735 128600 783 -5.65 -3.96
10b 738 121800 783 -5.66 -3.98
10c 736
87400 771 -5.48 -3.80
87400 771 -5.48 -3.80
10ce 736
a UV-vis measurements in CH2Cl2. b UV-vis measurements of a thin
film of the blend. c From CV measurements (E1/2ox) in CH2Cl2 calibrated against
ferrocene/ferrocenium couple (Fc/Fc+, -5.15 eV) as internal standard. d ELUMO
) EHOMO - (hc/λmax). e Solar cell with PC71BM as acceptor.
Chromophore 2 exhibits an absorption maximum at 664 nm
with a molar extinction coefficient of 53 300 M-1 cm-1. The
rather low tinctorial strength and the broad absorption band
indicate a more polyene-like character of this dye.23 Notably,
2 shows very low-lying HOMO and LUMO levels at -5.98
and -4.11 eV, respectively. Dyes 10a and 10b with extended
π-systems differ only by the alkyl substituent (Me vs n-Bu)
at the donor unit and show a bathochromic shift of the
absorption maxima of 70 nm compared with that of 2 and
sharp cyanine-like absorption bands with high extinction
coefficients of 121 800-128 600 M-1 cm-1. Their HOMO
levels range at -5.65 eV, and their LUMOs are situated at
energies around -3.96 eV. Replacement of the cyano substitu-
ent R1 at the oxo-pyrrolidino acceptor unit by an ethyl ester
group in compound 10c results in a shift of both HOMO and
LUMO levels by 0.18 eV to higher energies compared to those
of 10a,b, but without any alteration of the absorption features.
The presented dyes were characterized in solution-processed
BHJ solar cell devices with the general structure ITO/PEDOT:
PSS (∼40 nm)/dye:PC61BM (25/75% by weight; ∼ 50 nm)/Al
(120 nm). The details for device fabrication are given in
Supporting Information. The photovoltaic characteristics of the
solar cells are presented in Table 1.
For the device of reference 2, a very low PCE of 0.01%
was observed. This low efficiency arises from the small short-
circuit photocurrent (JSC) of 50 µA cm-2, which is very likely
caused by a lack of driving force for charge-carrier separation
owing to the low LUMO level of -4.11 eV. The latter energy
level is not sufficiently high for electron injection into the
LUMO level of the PCBM acceptor (-4.08 eV, Figure 2).
The introduction of the indolenine donor group in merocyanine
dyes leads to the extension of the π-system and entailed better
matched HOMO levels for the dyes 10a-c. Moreover, these dyes
Figure 1
.
UV-vis absorption spectra (2 × 10-5 M, CH2Cl2) and
cyclic voltammograms (inset, CH2Cl2, calibrated against Fc/Fc+
couple) of 2 (blue line) and 10a (green line).
CH2Cl2. Further data along with those of dyes 10b,c are listed
in Table 1. The HOMO levels were derived from the half-
wave oxidation potentials determined by cyclic voltammetry
(CV), whereas the LUMO energies are calculated by the
equation ELUMO ) EHOMO - (hc/λmax).
All of these dyes possess one oxidation and two reduction
waves, both processes being fully reversible (Figure 1).
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3668
Org. Lett., Vol. 12, No. 16, 2010

The high symmetry of PC61BM renders forbidden low-energy
optical transitions and results in only weak absorption in the
visible spectral region. For this reason PC71BM has been applied
in BHJ solar cells as its lower degree of symmetry affords an
improved absorbance in the visible range.25
Thus, we have built devices containing a spin-coated 10c:
PC71BM (25/75% by weight) blend. Upon changing the
acceptor material, the fill factor and the VOC were not altered
but the photocurrent is increased by 45%, reaching a value
of 4.83 mA cm-2 and a PCE of 1% (Table 1). The EQE
characteristics emphasize that the enhanced absorption of
PC71BM especially in the range of 400-600 nm is the origin
of this significant improvement (Figure 4).
Figure 2. Illustration of the HOMO and LUMO energies of dyes
2, 10a, and 10c in comparison with the LUMO energy of PC61BM.
afforded solar cells with absorption in the NIR region (Figure S2
in Supporting Information), thus enabling the production of devices
that are transparent in the visible range (Figure 3).
Figure 4. EQE characteristics of devices containing blends of 10c
with PC61BM (filled squares) and PC71BM (empty squares) (each
75 wt % PCBM).
Figure 3. Picture of a transparent device containing a blend of
10c:PC61BM (25/75 wt %) lying on the seal of the Universita¨t Wu¨rzburg.
In summary, we have presented here a series of novel functional
merocyanine chromophores that feature NIR absorption for bulk
heterojunction solar cells. Upon optimizing the electronic properties,
we obtained dye 10c whose HOMO and LUMO levels are well-
suited for application in organic solar cells with PCBM fullerenes
as electron-accepting materials. Solar cells fabricated with the newly
synthesized merocyanine dyes afford appreciably high open-circuit
voltages of up to 0.74 V despite the low bandgap and maximum
power conversion efficiency of 1.0%. As a result of their absorption
in the NIR region, these merocyanine dyes are promising materials
for solar power windows or sun shading technologies as well as
for tandem cell devices.
The devices built with 10a-c exhibit appreciable open-circuit
voltage (VOC) values of 0.64-0.74 V, in particular, if we consider
their low bandgap. The observed fill factors (FF) of 0.3 are typical
for solution-processed MC-based solar cells.2 Devices containing
10a and 10b, whose chromophores solely differ by the side chains,
displayed similar JSC values of 2.28 and 2.00 mA cm-2, respec-
tively, and PCEs of 0.43-0.46%, though their LUMO energies
are still quite similar to the LUMO of PCBM, imparting a driving
force of only ∼0.1 eV for electron transfer according to the solution
data (which may, however, shift in the solid state).2a In the literature,
an offset of 0.3-0.4 eV is proposed as trade-off between sufficient
driving force and minimized energy loss upon charge injection.24
To increase the LUMO energy level, we have synthesized dye
10c, which bears a slightly weaker acceptor moiety owing to the
replacement of one cyano group by an ester group. Actually, both
frontier molecular orbitals of 10c are shifted to higher energies by
0.18 eV compared to those of dyes 10a,b (Table 1).
Acknowledgment. The authors thank the German Ministry
of Science and Education (BMBF) for funding through the OPEG
project.
Supporting Information Available: Synthetic procedures,
complete characterizations (NMR, UV-vis, CV), device
processing, absorption spectra, and JV-response of the solar
cells. This material is available free of charge via the Internet
at http://pubs.acs.org.
While the higher-lying HOMO of dye 10c leads to a lowering
of VOC of the respective solar cell (0.64 V) relative to the cells
fabricated with 10a (0.71 V), the increased LUMO confers a
stronger driving force for electron transfer from the MC to the
fullerene and, thus, enhances the JSC values by 46%, resulting
in an overall performance of 0.7% for 10c.
OL101492Q
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