A structural study of DPP-based sensitizers for DSC applications

Article abstract of DOI:10.1039/c2cc35125k

Four D-π-A sensitizers comprising a thienyl-diketopyrrolopyrrole (ThDPP) bridge were synthesized and tested in iodide/triiodide liquid electrolyte DSC devices. The dye series was strategically designed to develop a structure-property relationship. The best performing sensitizer utilized a phenyl-based anchor and triphenylamine donor (η = 5.03%).

Full text of DOI:10.1039/c2cc35125k

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COMMUNICATION
A structural study of DPP-based sensitizers for DSC applicationsw
Thomas W. Holcombe,z*a Jun-Ho Yum,z*a Junghyun Yoon,ab Peng Gao,a
Magdalena Marszalek,a Davide Di Censo,a Kasparas Rakstys,a Md. K. Nazeeruddin*a and
Michael Graetzela
Received 17th July 2012, Accepted 6th September 2012
DOI: 10.1039/c2cc35125k
Four D-p-A sensitizers comprising a thienyl-diketopyrrolopyrrole
(ThDPP) bridge were synthesized and tested in iodide/triiodide
liquid electrolyte DSC devices. The dye series was strategically
designed to develop a structure–property relationship. The best
performing sensitizer utilized a phenyl-based anchor and triphenyl-
amine donor (g = 5.03%).
The standard metal-free sensitizer consists of a donor-
p-bridge-acceptor motif, where the bridge serves primarily to
extend p-conjugation while effectively relaying electron density
from the donor to the acceptor. Here, the aryl-DPP core serves
as an inherently coloured p-bridge, and ThDPP provides
exceptional spectral response compared to PhDPP; this is
primarily due to the planarity of the thiophene–DPP bond
and reduced aromatic stabilization energy for thiophene com-
pared to benzene (29 vs. 36 kcal molꢀ1 10). Further, planarity
of the p-bridge facilitates enhanced spectral response with the
D-p-A configuration.
Four sensitizers (Fig. 1) with similar absorption spectra and
varying electrochemical and structural features were synthesized
(Table 1 and Fig. 2). Two sensitizers with triphenylamine-based
donors and two with diphenylamine-based donors were syn-
thesized in combination with either a phenyl- or thienyl-based
acceptor/anchor; all sensitizers utilized cyanoacrylic acid
to bind the TiO2 surface. The triphenylamine donor is
expected to localize the radical cation away from the DPP
bridge at the expense of reduced electron donation from the
amine into the bridge and acceptor functionality, compared to
the diphenylamine. This is evidenced in the electrochemical
data.
Dye sensitized solar cells (DSCs) have become a powerful
alternative to conventional solar energy harvesting devices.
The underlying mechanism of device operation, mimicking
photosynthesis, is very flexible. Choosing the appropriate
nanostructured metal oxide, sensitizer, and redox shuttle offers
control over aesthetic properties such as transparency and
colour, as well as performance properties such as the attain-
able output voltage and current density.1 These advantages,
and promising power conversion efficiencies (PCE) of over
10%,2 make such devices ideal for integration with consumer
electronics such as wireless keyboards, remote controls, and
other battery powered devices. Herein, we describe how
diketopyrrolopyrrole-based sensitizers can be structurally tuned
in an effort to realize this goal of ubiquitous solar energy
harvesting.
DPP-based sensitizers have been reported for use in DSCs
only a handful of times,3–7 a remarkable fact considering the
wide-spread use of DPP in many other aspects of materials
technology, from car-paint pigments8 to small molecule and
polymeric organic photovoltaics.9 The seminal work on these
sensitizers was published in 2010 by Qu et al.,4 comparing
the phenyl-DPP (PhDPP) and ThDPP core. The success of
PhDPP in this work led to further optimization of that core
with various acceptors/anchors6 and donors.7 It was unclear,
however, why ThDPP did not outperform PhDPP. We recently
set out to determine the structural properties that underlie a
successful DPP-based sensitizer.
It is apparent from the electrochemical and UV-Vis absor-
bance data that these sensitizers should not be intrinsically
handicapped in an iodide/triiodide liquid electrolyte DSC with
TiO2, i.e. the type II energy level alignment is sufficient to
generate free charge at the TiO2 interface and to regenerate the
oxidized dye with iodide/triiodide.1 It is also apparent that the
triphenylamine donor broadens the sensitizer absorption
breadth, presumably a result of the extended conjugation,
and that the thienyl anchor results in a marginal decrease in
optical bandgap for DPP04 compared to DPP03. It is not
apparent from this data which dye will achieve the best PCE,
and this is the reason for developing a structure–property
relationship: understanding the role of structural properties,
layered over the energy level and absorption properties, is
critical for the advancement of sensitizer science.
a Laboratory for Photonics and Interfaces, Institution of Chemical
Sciences and Engineering, School of Basic Sciences, Swiss Federal
Institute of Technology, CH-1015 Lausanne, Switzerland.
E-mail: tom.holcombe@gmail.com, junho.yum@epfl.ch
b R & D Center DSC Team, Dongjin Semichem Co., LTD,
445-935 Hwasung, South Korea
w Electronic supplementary information (ESI) available: Synthetic,
spectroscopic, and devices details. See DOI: 10.1039/c2cc35125k
z These authors contributed equally.
The photovoltaic characteristics are summarized in Table 2.
Overall, structurally, the triphenylamine donor provided increased
performance over the diphenylamine with this bridge, i.e. DPP03
outperforms DPP01 and DPP04 outperforms DPP02. Addi-
tionally, the phenyl anchors outperformed the thienyl: DPP03,
c
10724 Chem. Commun., 2012, 48, 10724–10726
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Fig. 2 UV-Vis spectra of all dyes in THF solution.
Table 2 Output characteristics of DSC devices fabricated with DPP
sensitizers
Dye
CDCA [mM] Jsc [mA cmꢀ2
]
Voc [V] FF
PCE [%]
DPP01
0
2.5
0
2.5
0
2.5
0
5.24
8.66
2.14
5.04
8.89
11.9
3.14
5.49
0.54
0.55
0.51
0.52
0.59
0.64
0.56
0.58
0.72 2.02
0.72 3.41
DPP02
DPP03
DPP04
0.74 0.81
0.73 1.92
0.67 3.51
0.65 4.93
0.76 1.34
0.72 2.31
2.5
Devices fabricated with TiO2 thickness of 6 + 4 (20 nm and 400 nm
particles) microns, with electrolyte composition 0.6 M DMII, 0.05 M
LiI, 0.03 M I2, 0.25 M TBP, 0.05 M GuSCN in acetonitrile : valeronitrile
(85 : 15).
Fig. 1 Structures of DPP01-04.
Table 1 Electrochemical and optical properties of all dyes
b
Eg [eV]
Dye
Oxa [V]
Reda [V]
LUMOc [V vs. NHE]
CDCA was utilized as a co-adsorbent. The best dye, DPP03
yielded the highest PCE of 4.93%, which increased further to
5.03% (with an increase in Jsc), by employing an anti-reflection
film on the cell. Considering the IPCE data (Fig. 3), it is apparent
that these performance differences are rooted in the photon-
to-electron conversion process, rather than spectral breadth.
DPP01
DPP02
DPP03
DPP04
+0.77
+0.77
+0.96
+0.97
ꢀ0.80
ꢀ0.64
ꢀ0.74
ꢀ0.64
1.66
1.71
1.66
1.60
ꢀ0.89
ꢀ0.94
ꢀ0.70
ꢀ0.63
a
Measured in a 0.1 M Bu4NPF6 in DMF solution with glassy carbon
working, Pt counter and Pt quasi-reference electrodes; ferrocene was
used as an internal standard. Values are reported versus NHE according to
E0(Fc+/Fc) = 0.69 V vs. NHE.11 b Taken from the onset of absorbance.
c
Optical LUMO, i.e. the oxidation potential less the bandgap.
comprising a triphenylamine donor and phenyl anchor yielded
the highest PCE of 3.51%. DPP04 was the first DPP sensitizer
ever reported, in the work by Qu (vide supra); this current
structure–property relationship study identified two higher
performance derivatives of the ThDPP core – deriving their
success from the phenyl-based acceptor.
These four sensitizers were further optimized with and
without the co-adsorbent chenodioxycholic acid (CDCA).
Structural features can promote or inhibit sensitizer-sensitizer
interaction, thereby influencing exciton dynamics on the TiO2
surface and aggregation behaviour. The presence of CDCA is
known to suppress sensitizer aggregation,12–14 and CDCA was
necessary to achieve high performance with these sensitizers.
All the DPP-based sensitizers exhibited increased PCE when
Fig. 3 IPCE data for the four sensitizers.
c
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It is noted that all dyes on the TiO2 surface showed broadened
absorption spectra (see ESIw), which is in agreement with
IPCEs that also show increased absorbance in the long wave-
length region due to the thicker films15 and 400 nm scattering
particles.16
oxidation potential; however, there was a dramatic improve-
ment in IPCE when utilizing the phenyl-based anchor compared
to the thienyl, likely due to decreased electron back-recombination
as evidence by electron lifetime studies. A maximum efficiency
of 4.93% (5.03% with AR film) was achieved for DPP03
compared to 1.92% for DPP02.
A limited IPCE can result from several factors, such as
inadequate surface coverage on the TiO2 or an energy level
mismatch of the sensitizer excited state with TiO2; these two
simple cases do not appear to be the reason for such failure of
the ThDPP core, as these sensitizers performed optimally
under similar processing conditions but exhibited pronounced
differences in PCE. Structural aspects of a sensitizer are often
ignored, if there is not a rational link to parameters such as
absorbance, energy levels, or surface coverage. However,
recently, the ability of a phenyl ring to insulate the oxidized/
cationic sensitizer from the anionic TiO2 has recently been
suggested as the reason underlying a dramatic performance
difference between two otherwise similar sensitizers.17 Another
recent paper demonstrates this effect with a direct comparison
between phenyl- and thienyl-based anchors,18 although this
point was not directly addressed. It is possible that the DPP
core could require greater molecular-level-insulation from the
TiO2 surface to achieve higher performance: the single phenyl
ring here achieves 60% IPCE, while Qu et al.6 have observed
>90% IPCE for a biphenyl anchor and B70% for a compar-
able thiophene-5-phenyl-2-cyanoacrylic acid anchor. Electron
lifetime data measured by photovoltage and photocurrent
transients confirm that DPP03 should suffer from less back-
recombination loss than DPP04 (see ESIw). Moreover, a
presence of CDCA led to elongated electron lifetimes, which
is in agreement with higher Vocs in this IV data and previous
studies.13 The utilization of a triphenylamine-based donor and
further molecular level insulation of the DPP core from the
TiO2 surface is currently underway.
We acknowledge the European Community’s Seventh Frame-
work Programme (FP7/2007-2013) under grant agreement
no. 246124 of the SANS project for financial support and
the joint development project funded by Dongjin Semichem
Co., Ltd. (Korea). KR acknowledges financial support from
the Erasmus Mundus masters exchange program. J.-H.Y.
acknowledges the support from the Korea Foundation for
International Cooperation in Science and Technology through
the Global Research Lab.
Notes and references
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This journal is The Royal Society of Chemistry 2012