This presentation has been moved. To view this presentation, please visit http://pubs.acs.org/iapps/liveslides/pages/index.htm?mscNo=jz300652z Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Fluorine Substituents on Photovoltaic Performance J. Phys. Chem. Lett., 2012, 3 (13), pp 1830–1835 DOI: 10.1021/jz300652z We designed heteroleptic ruthenium complexes (RD12–RD15) containing fluoro-substituted benzimidazole ligands for dye-sensitized solar cells (DSSCs). These dyes were synthesized according to a typical one-pot procedure with the corresponding ancillary ligands produced in two simple steps; they were prepared into DSSC devices according to the same conditions of fabrication. The eventual devices show a systematic trend of increasing VOC and decreasing JSC with fluorine atoms of increasing number substituted on the ligand. The charge-extraction results show that upward shifts of the TiO2 potential occurred when the fluoro-substituted dyes were sensitized on TiO2 with a systematic trend of shift N719 > RD15 (with 5 F) > RD12 (with 2 F) >RD5 (no F); the intensity-modulated photovoltage spectra indicate that those fluoro substituents retard charge recombination with the electron lifetimes (τR) in the order RD15 > RD12 > RD5 > N719, consistent with the variation of VOC for the systems.

1. Design and Characterization of Heteroleptic Ruthenium Complexes
Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells:
The Effect of Fluorine Substituents on Photovoltaic Performance
Wei-Kai Huang, Hui-Ping Wu, Pi-Lun Lin, Yuan-Pern Lee, and Eric Wei-Guang Diau*
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung
University, Hsinchu 30010, Taiwan
J. Phys. Chem. Lett. 2012, 31830-1832 1

2. Synthetic Approach
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3. Absorption and Emission Spectra
RD5
Absorption Emission
-1
RD12
Absorption Coefficient /10 M cm
1.5 RD13
-1
Emission Intensity /a.u.
RD14
4
RD15
N719
1.0
0.5
0.0
300 400 500 600 700 800 900
Wavelength /nm
The absorption spectra of RD12-RD15 resemble the spectrum of the RD5 dye,
indicating that the effect of the fluoro substituents on the light-harvesting property is
insignificant. The emission spectra of RD12-RD15 display blue shifts with respect to
that of RD5. The positions of the intersection between the two spectra of each
species determine the band gap energy of the dye.
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4. Potential Level Diagram
The potential level diagram of RD5, RD12-RD15 and N719 shows that the
LUMO levels of the RD dyes become increasingly stabilized with an
increasing number of fluorine atoms, consistent with the electron-withdrawing
nature of the fluoro substituents.
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5. Photovoltaic Properties
Voltage /V
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Our photovoltaic results demonstrate the
20
-2
(a) same fluoro-substitution effect as those of
Current Density /mA cm
18
16 RD12-RD15 mentioned above (Table S2,
14
12
SI) with a systematic variation on VOC N719
10 RD5,  = 9.1 % > RD15 > RD12 > RD5 and an opposite
RD12,  = 9.6 %
8
RD15,  = 9.2 % trend on JSC RD12 ~ RD5 > RD15 ~ N719.
6
4
N719,  = 9.3 % As a result, the overall device performance
2 exhibits a trend RD12 > N719 > RD15 ~
RD5, with the best device (RD12) attaining
80 (b)
70
an efficiency 9.5 % of power conversion,
60 superior to that of N719 ( = 9.3 %) under
the same experimental conditions.
IPCE /%
50
40 RD5 (DL = 270)
30 RD12 (DL = 260) Dye loading JSC VOC FF η
Dye
20 RD15 (DL = 250) /nmol cm-2 /mA cm-2 /mV /%
10 N719 (DL = 170) RD5 270 17.150.26 73220 0.720.02 9.070.10
RD12 260 17.15±0.16 756±5 0.73±0.01 9.49±0.14
400 500 600 700 800
RD15 250 16.50±0.46 764±3 0.72±0.03 9.08±0.22
Wavelength /nm N719 170 16.57±0.16 776±5 0.72±0.01 9.30±0.12
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6. Charge extraction and IMVS
0.80
The charge-extraction (CE) results (a)
(a) indicate that the TiO2 potentials of the
0.75
devices show a systematic upward shift
VOC /V
0.70
with the order N719 > RD15 > RD12 >
N719 RD5, consistent with the variation of their
0.65 RD15 VOC. Increasing substitution with fluorine
RD12 atoms in the phenyl group hence shifts
RD5
0.60 the potential of the conduction band edge
of TiO2 in a more negative direction.
The Intensity-modulated photovoltage
0.1 (b) spectral (IMVS) results indicate a
R /s
systematic trend with the electron
lifetimes (corresponding to the degree of
charge recombination) showing the order
RD15 > RD12 > RD5 > N719, which is
0.01
3 4 5 6 7 8 consistent with the variation of VOC
18 -3 showing the same order for the fluoro-
Ne /10 cm
substituted devices.
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7. Conclusion
RD12-RD15 containing fluoro-substituted benzimidazole ligands were
designed and synthesized for dye-sensitized solar cells (DSSC). we
measured charge-extraction (CE) and intensity-modulated photovoltage
spectra (IMVS) at each VOC level. The CE and IMVS results indicate that
the increase of VOC upon fluoro-substitution is determined by two factors –
an upward shift of potential and a retardation of charge recombination.
Increasing substitution with fluorine atoms produces to a more negative
potential shift, but excess substituted fluorine atoms also result in a
decreased JSC because of the lower LUMO level and the smaller amount of
dye loading.
Acknowledgments
We thank Prof. Chien-Chon Chen for
providing the Zahner equipment for the
CE and IMVS measurements. National
Science Council of Taiwan and Ministry of
Education of Taiwan, under the ATU
program, provided support for this project.
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