Bulk heterojunction organic solar cells based on merocyanine colorants

Article abstract of DOI:10.1039/b813341g

Traditional low-molecular weight colorants that are widely applied in textile coloration, for printing purposes and nonlinear optics, now afford bulk heterojunction solar cells in combination with soluble C₆₀ fullerene derivative PCBM with powe

Full text of DOI:10.1039/b813341g

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Bulk heterojunction organic solar cells based on merocyanine colorantsw
Nils M. Kronenberg,a Manuela Deppisch,b Frank Wu
¨
rthner,*b Hans W. A. Lademann,a
Kaja Deinga and Klaus Meerholz*a
Received (in Cambridge, UK) 1st August 2008, Accepted 11th September 2008
First published as an Advance Article on the web 6th November 2008
DOI: 10.1039/b813341g
Traditional low-molecular weight colorants that are widely
applied in textile coloration, for printing purposes and nonlinear
optics, now afford bulk heterojunction solar cells in combination
with soluble C60 fullerene derivative PCBM with power conver-
sion efficiencies up to 1.7% under standard solar radiation.
MC dyes with [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) show remarkable PCEs of up to 1.7% under stan-
dard AM 1.5 solar radiation. To the best of our knowledge,
this represents the highest value reported for a BHJ solar cell
based on solution-processed small molecules.8
We have investigated here two series of MC dyes bearing
either a 2-aminothiophene or an indoline electron donor group
and various electron acceptor groups connected by a mono- or
dimethine chain (chemical structures are depicted in Fig. 1).
All these dyes exhibit intense absorption bands (e up to 1.4 Â
105 MÀ1 cmÀ1)7 at different wavelengths in the visible spectral
region (see Fig. SI-1 in supplementary informationw). Dye
MD344 appears orange, ATOP4 and IDOP301 magenta,
MD376 violet, and MD304 is deep blue. These MC dyes are
applied in the present study as p-type semiconducting compo-
nents in solution-processed BHJ solar cells in combination
with the soluble fullerene derivative PCBM as n-type
semiconductor (see Fig. 1).
Bulk heterojunction (BHJ) organic solar cells have attracted a
great deal of attention in view of their potential for the
fabrication of low-cost and flexible photovoltaic devices.1
BHJ solar cells can be deposited from solution, e.g. by a
printing process, and are thus compatible with low-cost role-
to-role fabrication technology, and they perform with a power
conversion efficiency (PCE) from solar light into electrical
energy of up to about 5%.2 The active layer of BHJ solar cells
is composed of interpenetrating networks of electron donor
and acceptor domains, which are formed during the
deposition/drying process. Currently, the favoured materials
for BHJ solar cells are composites of n-type semiconducting
fullerenes with p-type semiconducting polymers such as
poly(3-hexylthiophene) (P3HT). These composites have the
advantage of good charge carrier mobilities but suffer from
relatively large band gaps and low absorption coefficients, hence
they lack ideal absorption characteristics for solar radiation.
Merocyanine (MC) dyes are an important class of tradi-
tional colorants with widely tunable absorption properties
that are manufactured on a technical scale for mass applica-
tions, e.g. textile coloration.3 Owing to their high absorption
coefficients, polarizabilities and dipole moments, merocyanine
dyes are also a privileged class of functional dyes for nonlinear
optics, photorefractive materials, and other high technology
applications.4,5 In recent years, merocyanine dyes have also
been recognized as very promising substitutes for ruthenium
complex dyes in Graetzel-type dye-sensitized solar cells.6 As
for the above-mentioned applications, also in the latter case
the high tinctorial strength and the conveniently tunable
absorption wavelength are the most appreciable advantages
of merocyanine dyes. In the past, we developed MC dyes
including ATOP and IDOP derivatives (Fig. 1) for photo-
refractive applications.7 Here we report our recent discovery
that simple solution-processed BHJ cells based on blends of
For each individual dye, a series of BHJ devices with the
general structure ITO/PEDOT:PSS (40 nm)/MC:PCBM/Al
(100 nm) were fabricated by varying the layer thickness, for
which an optimum was found to be in the range of 50 nm, and
the MC:PCBM content. We discuss in more detail the devices
containing MC dye ATOP4, which has been the most
successful photorefractive dye in our earlier work due to its
outstanding glass formation properties.7c In Fig. 2 the open-
circuit voltage (VOC), the short-current density (JSC), the fill
factor (FF), and the PCE, the latter being the product of VOC
JSC and FF divided by the power of the incident light, are
,
plotted as a function of the PCBM content in the mixture of
a Universitat zu Koln, Department of Chemistry, Luxemburger Str.
116, D-50939 Koln, Germany. E-mail: klaus.meerholz@uni-koeln.de
¨
¨
¨
b Universitat Wurzburg, Institut fur Organische Chemie and Rontgen
¨
¨
¨
¨
Research Center for Complex Material Systems, Am Hubland
D-97074 Wurzburg, Germany.
¨
E-mail: wuerthner@chemie.uni-wuerzburg.de
w Electronic supplementary information (ESI) available: Experimental
details including synthesis, spectroscopy, device fabrication, Kelvin-
probe measurements, IV and EQE curves. See DOI: 10.1039/b813341g
Fig. 1 Top: Illustration of the energetic situation in BHJ solar
cells and the chemical structure of the fullerene derivative PCBM.
Bottom: Chemical structures of the investigated merocyanine dyes
(2-Ethex = 2-ethylhexyl).
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Fig.
3 JV-characteristics of ATOP4:PCBM (solid line),
MD304:PCBM (dashed line) (both 70 wt% PCBM) and
MD376:PCBM (dotted line) (75 wt% PCBM) solar cells measured
under AM 1.5 illumination.
lower the dyes’ HOMO energy. Although it has a slightly
lower VOC value (0.76 V), the highest PCE of 1.74% was
achieved for the blue dye MD304 due to a higher JSC value
(6.3 mA cmÀ2) enabled by a better absorption profile for solar
radiation (lmax = 649 nm). Notably, solar cells based on the
p-type semiconducting polymers P3HT and OC1C10-PPV
with comparable layer thicknesses exhibit almost the same
PCEs (o2%).10 The FF of all MC solar cells are rather
small (B0.3). This, together with the fact that optimum
devices are found for comparatively thin active layers, is
an indication of only moderate charge transport inside
the solar cells. No significant leakage currents were detected
in the dark current of our devices (see ESI, Fig. S2w). It is
also noteworthy that these MC:PCBM solar cells show
decent external quantum efficiencies (EQE) up to 50% at the
peak absorption wavelengths of the respective MC dyes
(see ESI, Fig. S3w).
The present dye series offers a unique opportunity for the
elucidation of structure–property relationships. For organic
BHJ materials, the general perception is that VOC reflects the
energy difference between the HOMO of the donor and the
LUMO of the acceptor11 (see Fig. 1). Thus, for one given
acceptor (here PCBM) a linear relationship between VOC and
the HOMO energy (EHOMO) with a slope of unity is
expected.12 In Fig. 4, VOC is plotted as a function of the
EHOMO of the MC dye as determined by cyclic voltammetry.
As can be seen, no correlation is given for the data. The latter
can be explained by the fact that electrochemical studies are
carried out for dissolved molecules in solution, while tightly
packed p–p-aggregates are formed for these dyes in the solid
state,13 thus we assume that the redox potentials of individual
molecules in solution do not properly reflect the energetic
situation in a solid material and hence in the actual device.
This leads to an alteration of the frontier orbital energies of
Fig. 2 (a) Dependence of VOC (n) and FF (’) and (b) JSC (n) and
PCE (’) on the PCBM content in ATOP4:PCBM solar cells.
the BHJ layer. Remarkably, the photovoltaic performance of
these blends exhibits little change upon variation of the
ATOP4 : PCBM ratio. Thus, VOC remains almost unchanged
and a broad maximum is observed for 60–80 wt% PCBM for
the JSC curve. The overall behaviour of the JSC-characteristics
can be attributed to two counteracting effects: On the one
hand, an increase of the MC portion in the active layer
enhances the numbers of absorbed photons since the
absorption of the MC dye is much stronger than that of
PCBM (see Fig. S1 in ESIw) and, on the other hand,
the transport of photo-generated charges through the
active layer seems to improve with the increasing PCBM
content. The significant (20%) increase of the FF (from
0.27 to 0.35; Fig. 2) with increasing PCBM content (from 50
to 90 wt%) can be taken as an indication for this assumption.
In BHJ solar cells, the FF is related to the series resis-
tance of the active layer, which reduces as the charge
mobility increases.9
A summary of all investigated cells at an optimized
MC : PCBM ratio of 70–80 wt% of PCBM is given in
Table 1 and the JV-characteristics of the three best cells
are presented in Fig. 3. The largest open-circuit voltage
VOC (0.9 V) was measured for a cell containing the violet
dye MD376. This is remarkable because the absorption
maximum of this dye is located at rather long wavelengths.
Thus, MCs can be easily tuned for absorption at longer
wavelength and high VOC values as well by simple attachment
of strongly electron-withdrawing acceptor heterocycles that
Table 1 Electronic properties of the MC dyes and best performance of the investigated MC:PCBM BHJ solar cells with Al top electrode
MC dye
EHOMOa/eV
lmaxb/nm
ELUMOc/eV
wt% PCBM
VOC/V
JSC/mA cmÀ2
FF
% PCE
MD344
MD352
MD334
ATOP4
IDOP301
MD376
MD304
À5.71
À5.58
À5.78
À5.64
À5.80
À5.80
À5.59
515
530
563
554
544
607
649
À3.31
À3.24
À3.58
À3.40
À3.52
À3.76
À3.68
80
70
75
70
70
75
70
0.57
0.63
0.49
0.73
0.77
0.90
0.76
2.8
2.9
3.1
4.0
4.0
5.3
6.3
0.28
0.27
0.29
0.32
0.29
0.32
0.36
0.44
0.49
0.43
0.91
0.87
1.54
1.74
a
b
Determined by cyclic voltammetry calibrated against the ferrocene/ferrocenium redox couple (À5.15 eV). UV-Vis measurements of a thin
c
film. ELUMO = EHOMO À (hc/lmax).
ꢀc
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We gratefully acknowledge financial support by DFG
(priority programme ‘‘Elementary Processes of Organic
Photovoltaics’’), Fonds der Chemischen Industrie, and
BASF SE.
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Chem. Commun., 2008, 6489–6491 | 6491