A Comparative Study on Two RuII Complexes with Thiophene-Based Ancillary Ligands for High-Efficiency Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/ejic.201700468

Heteroleptic polypyridyl Ru complexes MMR-1 and MMR-2 containing 2-(methylthio)thienyl and 2-(4-methoxyphenyl)thienyl units on the antennas of ancillary ligands, respectively, were designed, synthesized, and characterized as sensitizers for dye-sensitized solar cells. The maximum absorption wavelength of MMR-1 is more red-shifted than that of MMR-2, but MMR-2 has a higher molar extinction coefficient, leading to better light harvesting. Under the same device-fabrication conditions, the photovoltaic performances of these sensitizers were evaluated while anchored on mesoporous TiO₂ and compared to that of the benchmark N719. Both MMR-1 and MMR-2 exhibited comparable or even higher solar-to-electric conversion efficiencies η with respect to N719 when employed as photosensitizers in DSSCs. Compared to MMR-1, MMR-2 exhibited better overall conversion efficiency, which was attributed to the electron-donating effect of the 4-methoxyphenyl group and the better absorptivity by harvesting higher-energy photons. Complex MMR-2 also showed higher open-circuit voltage V_OC than MMR-1, which is likely due to the extension of the antenna of the ancillary ligand by inserting a phenyl group, which leads to less dye aggregation. The photovoltaic performance of MMR-2 was better, with a short-circuit photocurrent density of 16.76 mA cm^–2, a V_OC of 0.673 V, a fill factor of 73.5 %, and an η value of 8.29 % with the addition of 0.5 m 4-tert-butylpyridine (TBP) compared to 8.18 % for N719.

Full text of DOI:10.1002/ejic.201700468

A Journal of
Accepted Article
Title: A comparative study of two novel thiophene-based ancillary
ligands of Ru(II) complexes for high efficiency dye-sensitized
solar cells
Authors: Rui Su, Mohamed R. Elmorsy, Mira Abed, Ashraful Islam,
Meghan Lord, Ahmed A. Fadda, and Ahmed El-Shafei
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700468
Link to VoR: http://dx.doi.org/10.1002/ejic.201700468

10.1002/ejic.201700468
European Journal of Inorganic Chemistry
A comparative study of two novel thiophene-based ancillary ligands of Ru(II) complexes
for high efficiency dye-sensitized solar cells
Rui Sua, Mohamed R. Elmorsya,b, Mira Abeda, Ashraful Islamc, Meghan Lorda, Ahmed A.
Faddab, Ahmed El-Shafeia*
aPolymer and Color Chemistry Program, North Carolina State University, Raleigh, NC, 27695,
United States.
bDepartment of Chemistry, Faculty of Science, Mansoura University, El-Gomhoria Street, 35516
Mansoura, Egypt.
cPhotovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba,
Ibaraki 305-0047, Japan
* Corresponding author: E-mail: Ahmed_El-Shafei@ncsu.edu
Home Page: https://textiles.ncsu.edu/blog/team/ahmed-el-shafei/
Graphical Abstract
Dye-sensitized solar cells (DSSCs) are potential alternatives to traditional solar cell for the solar
energy conversion. Ruthenium(II) polypyridyl complexes are very effective in achieving high
solar-to-electric efficiency. In this paper, two novel Ru(II) sensitizers were synthesized with
electron-donating thiophene-based ancillary ligands. Both the newly-synthesized Ru complexes
exhibited comparable or a bit higher performance as sensitizers for DSSCs in comparisons with
the benchmark N719, which is attributed to their higher molar extinction coefficient. Furthermore,
1
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
comparing the shifting of absorption spectra and the molar extinction coefficient, we found that
the latter factor is more substantial for the light-harvesting ability of the sensitizers, resulting in
increased efficiency for DSSC applications.
2
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
Keywords: Dye-sensitized solar cell; Ru(II) bipyridyl complexes; Thiophene-based ancillary
ligands.
Abstract
Two novel heteroleptic polypyridyl Ru-complexes, MMR-1 and MMR-2, containing 2-
(methylthio)thiophene and 2-(4-methoxyphenyl)thiophene units on the antenna of ancillary
ligands (AL), respectively, were designed, synthesized and characterized as sensitizers for dye-
sensitized solar cells. The maximum absorption wavelength of MMR-1 is more red-shifted than
that of MMR-2, however, MMR-2 has higher molar extinction coefficient, leading to better light
harvesting. Under the same device fabrication conditions, the photovoltaic performances of these
newly produced sensitizers were evaluated as anchored on mesoporous TiO2 and compared to the
benchmark N719. Both MMR-1 and MMR-2 exhibited comparable or even higher solar-to-
electric conversion efficiencies (η) with respect to N719 when employed as photosensitizers in
DSSCs. Compared to MMR-1, MMR-2 exhibited better overall conversion efficiency, which was
attributed to the electron donating effect of the 4-methoxyphenyl group and the better absorptivity
by harvesting higher energetic photons. MMR-2 also displayed higher VOC than MMR-1, which
is likely due to the extension of the antenna of ancillary ligand by inserting a phenyl group leading
to less dye aggregation. For MMR-2, the photovoltaic performance was better with JSC of 16.76
mA∙cm-2, VOC of 0.673 V, fill factor of 73.5% and η of 8.29% with the addition of 0.5M 4-tert-
butylpyridine (TBP) compared to 8.18% of N719.
1. Introduction
The demand for clean and renewable energy sources is ever increasing due to environmental
pollution and the threat of climate change caused by traditional energy usage. 1. By far, the most
plentiful renewable resource that could tackle the global energy crisis is offered by the utilization
of solar energy. Solar energy is so abundant that energy of incident sunlight on the earth in only
2
one hour is enough to supply the total energy requirement of all human activities in one year .
Among all the categories of light energy conversion applications, photovoltaic (PV) technology is
one of the most powerful strategies to harvest sunlight and directly transform it into affordable
electricity. Traditional silicon-based solar cells have dominated the photovoltaics market for years,
however, their high production cost and complex fabrication technology limits their further
3
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
3
development . In terms of developing photovoltaic technologies, dye-sensitized solar cells
4
(DSSCs), also known as the third generation of solar cells first reported in 1991 , have made
remarkable progress and found their entrance into the industry 5. DSSCs are potential alternatives
to traditional solar cells due to their comparatively low cost, molecular design opportunities,
superior performance under diffuse light conditions and low illumination levels, independence of
incident light angle and flexibility in color designing 6-8. With metal-free organic dyes as
sensitizers, the solar-to-power conversion efficiency has reached 13.0% with
dithienopicenocarbazole derivative 9.
a
Polypyridyl Ru(II) complexes based DSSCs have proven to be promising sensitizers due to their
unique energy transfer and photophysical properties such as strong metal-to-ligand charge transfer
(MLCT) properties 10-13, broad spectral response, multiple stable oxidation states 14-16, high
efficiency electron injection 17, 18 and long excited state lifetimes 19-22. In terms of the sensitizers,
23
N719 (Figure 1), which was initially reported by Grätzel and co-workers , serves as a popular
benchmark both in academia and in industry with an overall efficiency of over 10%. However,
several weaknesses of N719 deters the further improvement of its photovoltaic performance,
including low molar absorptivity and poor long-term stability. Since the sensitizer is a very crucial
component of DSSCs, an active and enduring area is the structural design of Ru(II) sensitizers
based on the N719 scaffold. An ideal sensitizer should have a wide absorption range in both the
visible solar spectrum and NIR region as well as a high molar absorptivity coefficient 24; moreover,
sensitizers should have favorable anchoring properties onto a semiconductor surface. There are
several strategies for efficient molecular design of Ru(II) sensitizers, including the modification
of ancillary ligands with electron rich donors, long alkyl chain substitution and heterocycle
introduction, as well as modification of the anchoring ligand and substitution of thiocyanate
moieties with other strong electron donor groups 25-29. Our group has developed many attractive
ruthenium sensitizers that exhibit superior conversion efficiency than N719 such as NSCU-10,
MH-1, MH-8 and HD-8 30-35
.
The incorporation of thiophene moieties as the antennas of ancillary ligand is one of the best
strategies for designing Ru(II) sensitizers because of the enhancement of the molar extinction
coefficient for both the π → π* and 4d → π* transitions 36-38. The high molar absorptivity provides
the dual benefits of decreasing dye loading amount and the film thickness of the semiconductor 25,
4
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
hence translucent devices. In this context, studies of the molecular engineering of promising Ru(II)
sensitizers with thiophene-derived ligands have been ongoing in our research group. Subtle
molecular modulations of AL can result in a considerable destabilization of the t2g (HOMO), and
hence better light harvesting. Two novel ruthenium sensitizers, MMR-1 and MMR-2, were
developed with the aim of increasing the molar extinction coefficient than N719 through a
combination of highly conjugated bipyridyl and thiophenyl groups. The methylthio or 4-
methoxyphenyl group is further attached on the end of antennas, respectively, to study the
influence of small donor groups on the thiophene-based ligands on photovoltaic performances.
Alkylthio group was previously reported in Ru(II) sensitizers that exceeded 10%39 efficiency. Its
donor strength is comparable to 4-methoxyphenyl group by potentially inducing a further red-shift
in the absorption of the dye. Herein we report the synthesis of heteroleptic ruthenium sensitizers
with 2-(methylthio)thiophene and 2-(4-methoxyphenyl)thiophene units and their superior
photovoltaic performances in DSSCs. The molecular structures of MMR-1 and MMR-2 are
displayed in Figure 1.
Figure 1 Molecular structures of MMR-1, MMR-2 and N719.
2. Results and Discussion
2.1. Photophysical Measurements
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
UV-Vis absorption and emission spectra of MMR-1, MMR-2 and N719 were measured in DMF
with the standard concentration (2  10−5 M). The spectra are shown in Figure 2, and the results
are summarized in Table 1.
9
8
7
6
5
4
3
2
1
0
60000
50000
40000
30000
20000
10000
0
MMR-1
MMR-1
MMR-2
MMR-2
N719
N719

280
380
480
580
Wavelength/nm
680
780
880
Figure 2 UV-Vis absorption (solid-line) and emission spectra (dashed-line) of MMR-1, MMR-2
and N719 measured in DMF (2  10-5 M).
MMR-1 and MMR-2 exhibited strong absorption bands at 307 nm and 297 nm, respectively,
which are assigned to intra-ligand π → π* transition of the ancillary ligand (the thiophene and
bipyridyl system). The band of MMR-2 around 300 nm is much more intense than that of MMR-
1, indicating that the additional benzene ring contributes more to the conjugation system of the
ancillary ligand. The absorption bands in the region between 350 and 380 nm are attributed to the
combination of ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfers
(MLCT), respectively. Low energy MLCT absorption peaks exhibited by MMR-1 and MMR-2
were found at 532 nm and 519 nm with molar extinction coefficients of 1.38 × 104 M-1cm-1 and
1.69 × 104 M-1cm-1, respectively. It was observed that MMR-1 displayed a slight red shift than
MMR-2 for both the high and low energy MLCT absorption bands. This alteration can be
attributed to the destabilization of the metal-based HOMO (t2g) and the decreased GSOP-ESOP
gap as shown in the energy level diagram of Figure 4. However, it was also noticed that MMR-2
exhibited an increase in molar extinction coefficient in both MLCT bands, resulting in the better
light harvesting and better photovoltaic performance when MMR-2 is used as a sensitizer.
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
Furthermore, the emission spectrum of MMR-1 was more red-shifted than that of MMR-2
indicating more energy losses in the excited state of the former.
Table 1 Absorption and emission properties for MMR-1, MMR-2 and N719.
Sensitizer Absorption λmax (nm)
ε (104M-1cm-1) Emission λmax (nm)
307; 377; 532 (d → π*) 3.12; 2.85; 1.38
297; 353; 519 (d → π*) 7.85; 4.26; 1.69
310; 381; 528 (d → π*) 4.61; 1.44; 1.32
776
717
746
MMR-1
MMR-2
N719
A comparison of absorption spectra of MMR-1, MMR-2 and N719 was depicted in Figure 3.
When adsorbed on TiO2, both ruthenium complexes exhibited broadening in their absorption
profiles compared to the corresponding spectra recorded in solution. The broadening of the MLCT
band is anticipated to present an ability to yield more photons across a wider range of the solar
spectrum in the fabricated devices, leading to an increase in the photocurrent. Additionally, it was
observed that the low energy MLCT peaks were slightly blue-shifted compared with those in
solution. This can be attributed to the deprotonation of carboxylic acid when anchored ontoTi4+ in
the semiconductor, resulting in an increase in the energy of the ESOP and destabilization of the
LUMO 40.
1.6
MMR-1
1.4
MMR-2
1.2
N719
1
0.8
0.6
0.4
0.2
0
300
400
500
600
700
800
900
Wavelength (nm)
Figure 3 Absorption spectra of MMR-1, MMR-2 and N719 adsorbed on nanoporous TiO2.
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
2.2. Electrochemical measurements
The ionization potentials (IP) of MMR-1, MMR-2 and N719 anchored to TiO2 film were
measured using a photoemission yield spectrometer (Riken Keiki AC-3E), and the results are
summarized in Table 2. Additionally, cyclic voltammetry (CV) measurements were also
performed as a supplementary method to calculate the oxidation onset which is equivalent to the
GSOP (ground state oxidation potential) or HOMO level of the dye. E0-0 (minimum energy
between the ground and excited states) which is the difference between HOMO and LUMO was
determined from the onset of the absorption spectra. Experimentally, the excited state oxidation
potential (ESOP) was derived from the values of E0-0 and GSOP according to Equation 1.
ESOP (eV) = GSOP (eV) + E0-0
(1)
Table 2 Excited state oxidation potential (Ru3+/2+), and the lowest electronic transitions (E0-0) for
MMR-1, MMR-2 and N719*.
Experimental (eV)
Theoretical (eV), TD-DFTd
GSOP (HOMO) E* (Ru3+/2+
)
E0-0
=
(S0 – S1)
1.94
Sensitizer
a
E0-0
EHOMO
ESOPsinglet
(IP)b
–5.50
–5.48
–5.76
(ESOP)c
–3.63
MMR-1
MMR-2
N719
1.87
1.99
1.99
–5.71
–5.71
–5.67
–3.77
–3.76
–3.51
–3.49
–3.77
1.94
2.15
*. HOMO, highest occupied molecular orbital; ESOP = E* (Ru3+/2+), excited state oxidation
potential; ESOPsinglet = EHOMO – E0-0; E0-0 = S0 – S1, the lowest vertical excitation energy, the lowest
singlet-singlet transition; GSOP = EHOMO, ground state oxidation potential.
a. E0-0 was calculated from the onset of absorption spectra (in DMF). b. IP = the experimental
ionization potential measured using a photoemission yield spectrometer (Riken Keiki AC-3E). c.
Exited-state oxidation potential, E* (Ru3+/2+), was calculated from: E* (Ru3+/2+) = IP – E0-0. d.
Theoretical EHOMO, ESOPsinglet and E0-0 were calculated at the TD-PBE1PBE/DGDZVP level of
theory for a complex molecule in DMF.
The GSOP values of MMR-1 and MMR-2 are 5.50 eV, and 5.48 eV, respectively, which
confirmed that there is enough thermodynamic driving force to regenerate the dye by replenishing

the hole via an electron donation from the I3 /I redox couple (5.2 eV). Moreover, the ESOP
values of MMR-1 and MMR-2 are 3.63 eV and 3.49 eV, respectively, which are both above
the conduction band (CB) edge of nanocrystalline TiO2 (4.2 eV), confirming that electron
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
injection from the excited state of the dyes into the CB of TiO2 is thermodynamically spontaneous.
Based on the energy level diagram of Figure 4, the experimental ESOP of MMR-1 and MMR-2
are both above that of N719, indicating that efficient electron injection from the excited state of
these dyes into the CB edge of TiO2 is more favorable than N719. Furthermore, MMR-2 has more
negative free energy of electron injection than MMR-1, indicating that the light-excited electrons
are injected more efficiently in case of MMR-2.
Figure 4 Energy level diagram and comparison between the GSOP and the ESOP of MMR-1,
MMR-2 and N719.
2.3. Photovoltaic device characterizations
The photovoltaic performance of complexes MMR-1 and MMR-2 on the nanocrystalline TiO2
2
electrode was studied under standard AM 1.5 irradiation (100 mW cm ) one sun illumination
using an electrolyte with the composition of 0.6 M dimethylpropylimidazolium iodide (DMPII),
0.05 M I2, 0.1 M LiI in acetonitrile. Figure 5 displays the photocurrent action spectra for solar
devices fabricated with MMR-1, MMR-2 and N719, where the incident photon-to-current
conversion efficiency (IPCE) values for each wavelength from 300 to 900 nm are plotted as a
function of wavelength.
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
The strong photon harvesting features of the thiophene ancillary ligand resulted in impressive
quantum efficiencies for MMR-1 and MMR-2. For MMR-2, up to 91% of quantum efficiency
was achieved in the plateau region. Due to the strong photon harvesting capability of the ancillary
ligands and the high molar extinction coefficient of the high energy MLCT band of MMR-1, the
measured IPCE didn’t show the sharp drop that N719 and MMR-2 exhibited at 370 nm. It was
also demonstrated that MMR-2 displayed superior quantum efficiency in the visible and NIR
regions than N719, despite the same sudden drop as N719 exhibited. Although the IPCE of N719
outperformed that of MMR-1 in the range 590-710 nm, the quantum efficiency of MMR-1
exceeded N719 throughout the rest of the spectrum. The outperformance of MMR-1 and MMR-
2 in their IPCEs over N719 can be attributed to the strong electron donating effect of their antennas
and the increased ESOP as discussed before.
100
90
80
70
60
50
40
30
20
10
0
MMR-1
MMR-1 0.5 TBP
MMR-2
MMR-2 0.5 TBP
N719
N719 0.5 TBP
300
400
500
600
700
800
Wavelength/nm
Figure 5 Photocurrent action spectra (IPCE) of solar devices based on MMR-1, MMR-2 and
N719.
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
The overall energy conversion efficiency, η, is derived from the photocurrent density measured at
short-circuit (JSC), the open-circuit photovoltage (VOC), the fill factor of the cell (FF), and the
intensity of the incident light (Pin) based on Equation 2.
퐽푆퐶푉푂퐶퐹퐹
휂 =
(2)
푃
푖푛
All the photovoltaic characteristics are summarized in Table 3.
Table 3 I-V characteristics of solar devices based on MMR-1, MMR-2 and N719.
2
JSC (mAcm )
17.70
 (%)
5.36
7.00
5.87
8.29
6.29
8.18
Sensitizer CDCA (mM) TBP (M)
VOC (V)
0.449
0.604
0.467
0.673
0.538
0.705
FF (%)
67.4
72.0
68.3
73.5
68.1
71.8
20
20
20
20
20
20
0
0.5
0
0.5
0
MMR-1
MMR-1
MMR-2
MMR-2
16.10
18.38
16.76
17.17
N719
N719
0.5
16.17
Conditions: sealed cells; coadsorbate, DCA 20 mM; photoelectrode, TiO2 (15 m thickness and
0.18 cm2); electrolyte, 0.6 M DMPII, 0.1 M LiI, 0.05 I 2 in HI-30; irradiated light, AM 1.5 solar
2
light (100 mW cm ). JSC, short-circuit photocurrent density; VOC, open-circuit photovoltage; FF,
fill factor; η, total power conversion efficiency.
Deoxycholic Acid (DCA) can be added to the dye solution as a coadsorber to achieve TiO2 surface
passivation, which suppresses recombination reactions (dark current). With the addition of an
optimized concentration of DCA, the cell sensitized with complex MMR-1 and MMR-2 exhibited
2
photocurrent densities (JSC) of 17.70 and 18.38 mAcm , respectively, corresponding to an overall
efficiency () of 5.36% and 5.87%, respectively. The JSC of measured dyes are higher than that of
2
N719 (17.17 mAcm ) due to the greater photon harvesting and more energetically favorable
electron injection into TiO2. The efficiency and JSC of MMR-2 are both higher than those of
MMR-1. Associated with their phtophysical properties (Figure 2), it is concluded that the
extinction coefficient is more improtant than the position of λmax in harvesting higher energetic
phtotons and converting into more photocurrent. Additionally, the increase in ESOP of MMR-2
results in more negative free energy between ESOP and the CB of TiO2. The VOC of MMR-2 is
also higher than that of MMR-1, which can be attributed to larger molecular size and less dye
sensitizer aggregation caused by the altered structure of MMR-2, converting into more charge
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
collection and a reduction in dark current and enhanced performance of DSSCs. The insertion of
a benzene ring in the antenna of the ancillary ligand results in the extension of the molecule and
consequently the suppression of dye aggregation on the TiO2 film during the dye uptake process.
Figure 6 shows the photocurrent density-voltage curve for MMR-1, MMR-2 and N719.
20.0
18.0
16.0
14.0
12.0
10.0
8.0
MMR-1
MMR-1 0.5 TBP
MMR-2
MMR-2 0.5 TBP
N719
N719 0.5 TBP
6.0
4.0
2.0
0.0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage/V
Figure 6 I-V characteristics of solar devices based on MMR-1, MMR-2 and N719.
In order to further optimize the fabrication conditions for MMR-1 and MMR-2, 4-tert-
butylpyridine (TBP) was also employed in the dye solution. TBP is well recognized for its

influence on inhibiting the recombination of injected electrons with I3 in the electrolyte. Due to
41-
the basicity of TBP, its addition causes a negative increase in the conduction band edge of TiO2
43. The aim of using TBP is to cover the TiO2 surface between dye molecules. Previously in our
group, TBP has been used to improve the performance of ruthenium dye based solar cells 32, 34, 44
.
The effect of TBP on the photovoltaic parameters of MMR-1 and MMR-2 was also presented in
Table 3 and Figure 6. Based on these results, both the open-circuit voltages and fill factors of
MMR-1 and MMR-2 were increased significantly with the addition of TBP, while the JSC was
slightly decreased. It is proposed that the recombination between injected electrons and the redox
couple in the electrolyte is suppressed when TBP develops a thick layer on top of the bare TiO2
surface. Overall, the usage of TBP favorably raised the efficiencies of MMR-1 and MMR-2 to
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
7.00% and 8.29%, respectively. It was noted that with the addition of TBP, MMR-2 exhibited
even higher efficiency than N719 (8.18%).
2.4. Electrochemical impedance spectroscopy characterization
To further probe the interfacial characteristics between the dyes and TiO2, electrochemical
impedance spectroscopy (EIS) was employed, which is a useful tool for analyzing charge transfer
and chemical capacitance at the interfaces between TiO2/dye/electrolyte and Pt/eletrolyte in
DSSCs 45-47. Impedance measures the dielectric properties of a material and interface as the
function of frequency. The EIS Nyquist and Bode plots for the solar devices fabricated with MMR-
1, MMR-2 and N719 are displayed in Figure 7 and Figure 8, respectively. In a Nyquist plot, a high
frequency semicircle can be correlated with charge transfer at the counter electrode and
intermediate frequency is related to electron transport through mesoporous TiO2 and back electron
transfer from TiO2 to electrolyte. In Figure 7, the radius of the larger semicircle shows the electron
recombination resistance at the TiO2/dye/electrolyte interface. The radius of N719 Nyquist plot is
larger than that of MMR-1, indicating that the decrease in charge recombination leads to an
increase of solar cell performance.
MMR-1
7
MMR-2
N719
6
5
4
3
2
1
0
15
20
25
30
35
40
45
Re(z)/ohm
Figure 7 EIS Nyquist plots for solar devices based on MMR-1, MMR-2 and N719.
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European Journal of Inorganic Chemistry
In Bode plots, frequency response in the range of 1-100 Hz is an indicator of the electron
recombination between electrolyte and TiO2 and is related to the electron lifetime in the CB of
TiO2. The electron lifetime eTiO2 can be calculated from the relation (τCB = 1/(2πf)), where f is the
mid-frequency peak in the Bode plots and τ is the lifetime of electrons in TiO2 35. Electron lifetime
depends on the density of charge traps, which is ultimately related to VOC. In Figure 8, the middle-
frequency peaks of the DSCs based on MMR-1 and MMR-2 were observed to be slightly shifted
to high frequency compared to N719, indicating a shorter recombination lifetime for the former
case, thus resulting in slightly lower VOC for MMR-1 (0.628 V) and MMR-2 (0.640 V) compared
to N719 (0.693 V). One likely explanation for the decreased eTiO2 of MMR-1 and MMR-2
employed solar devices can be ascribed to the bulkiness of the synthesized ruthenium complexes,
thus deterring the diffusion of the dye into the narrow pores of the mesoporous TiO2 film. This
results in imperfect monolayer formation, which consequently promotes the recombination of
conduction band electrons back to the electrolyte.
14
12
10
8
MMR-1
MMR-2
N719
(b)
6
4
2
0
-1
0
1
2
3
4
5
6
Freq (Log/hz)
Figure 8 EIS Bode plots for solar devices based on MMR-1, MMR-2 and N719.
2.5. Density Functional Theory/Time-Dependent Density Functional Theory Molecular Modeling
14
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
In order to gain insight into the geometrical electronic structure of two ruthenium complexes and
to evaluate the photophysical and photovoltaic properties, DFT calculations on MMR-1 and
MMR-2 were performed using density functional theory with the Guassian09 package 48. The
ground-state geometries were optimized first in DMF solution, using the PBE1PBE energy
functional and DGDZVP basis set. The optimized geometries were sequentially employed for the
TD-DFT energy calculation with the same functional and basis set. The ground and excited state
oxidation potentials and the lowest singlet-singlet electronic transitions (E0-0) were calculated and
collected in Table 2, which were in good agreement with the experimental data.
Figure 9 Calculated isodensity of frontier occupied molecular orbitals for MMR-1 and MMR-2.
The isodensity plots of frontier molecular orbitals of MMR-1 and MMR-2 are presented in Figure
9. The calculation illustrates that HOMO of both dyes exhibit ruthenium t2g character and a
substantial population on a combination of the thiocyanate, especially a major contribution from
the far-edge of sulfur atom, and π-bonding orbital of the thiophene-derived ancillary ligand. The
LUMOs of both sensitizers are predominately delocalized over the 4,4’-dicarboxy-2,2’-bipyridine
(dcbpy) anchoring ligands and ruthenium center, which has a similar distribution arrangement as
was previously published 33. Since the vectoral electron flow from across the energy gap is closely
15
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
related to light excitation, relative electron densities of the HOMOs and LUMOs of the two dyes
were examined. HOMO-LUMO excitation moved the electron density from the thiocyanate
moieties to the dcbpy ligands, which is likely due to stronger p-orbital overlap between the
thiophene-derived electron donor and the aromatic pyridine moiety. Furthermore, the HOMO-
LUMO transition sufficiently mediates the photoinduced electron transfer from the dye to TiO2
semiconductor, leading to an efficient charge separation.
3. Conclusions
In summary, incorporation of thiophene-based electron-donor antennas into polypyridyl Ru(II)
complexes has been developed, providing highly efficient sensitizers for DSSCs. MMR-2 has
higher molar extinction coefficients than MMR-1 for their photophysical properties, even though
the maximum absorption wavelength of MMR-2 is smaller than that of MMR-1. Comparing their
photovoltaic performances, MMR-2 exhibited both higher JSC and higher efficiency than MMR-
1, indicating that higher molar extinction coefficient is more important in light harvesting by
converting higher energetic photons into electrons. Solar cells fabricated with MMR-1 and MMR-
2 exhibited solar-to-electric conversion efficiency () of up to 7.00% and 8.29%, respectively,
compared to 8.18% for the benchmark N719. This work successfully establishes the relationship
between the molecular structures of Ru(II) complexes and their photophysical and photovoltaic
properties.
4. Experimental Section
4.1. Materials and Equipment
The solvents and chemicals were purchased from Sigma-Aldrich, Fisher Scientific, TCI-America
or Ark Pharm and used as received. Sephadex LH-20 was purchased from Fisher Scientific. The
mass spectrometry analysis was carried out on a high resolution mass spectrometer, the Thermo
Fisher Scientific Exactive Plus MS, a benchtop full-scan OrbitrapTM mass spectrometer using
heated electrospray ionization (HESI). Samples were dissolved in methanol and sonicated for 15
min. They were then diluted 1:1 with 20 mM ammonium acetate and analyzed via syringe injection
16
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
into the mass spectrometer at a flow rate of 10 L/min. The mass spectrometer was operated in
negative ion mode. Fourier transform infrared spectroscopy (FT-IR) (ATR) spectra were recorded
on a Nicolet Nexus 470 FT-IR spectrometer (Thermo Scientific, USA) and UV-visible spectra
1
were measured by using Cary 300 spectrophotometer. H NMR spectra were recorded using a
Varian 400 MHz spectrometer.
4.2. Cyclic voltammetry measurements
Cyclic voltammetry (CV) was carried out in DMF with 0.1 M [TBA][PF6] as an electrolyte at a
1
scan rate of 50 mV s . Glassy carbon was used as the working electrode (WE), Pt wire as counter
electrode and Ag/Ag+ in ACN was used as the reference electrode. Fc/Fc+ was used as internal
reference, voltage measured was converted to NHE by addition of 0.63 V. Figure 5S and 6S
(Supplementary Information) show the CV graphs of sensitizers.
4.3. TiO2 electrode preparation and device fabrication
A double-layer TiO2 photoelectrode (10 + 5) μm in thickness with a 10 μm thick nanoporous layer
49
and a 5 μm thick scattering layer (area: 0.25 cm2) were prepared using a reported method
.
Fluorine-doped tin oxide (FTO) coated glass (Nippon Sheet Glass Co., Japan) with a sheet
resistance of 8-10 ohm-2 and an optical transmission of greater than 80% in the visible range was
screen printed using anatase TiO2 colloids (particle size ∼ 13 nm) obtained from commercial
sources (Ti-Nanoxide D/SP, Solaronix). Nanocrystalline TiO2 thin films were deposited onto the
conducting glass by screen-printing, which was then sintered at 500 ° C for 1 h. The film thickness
was measured with a Surfcom 1400 A surface profiler (Tokyo Seimitsu Co. Ltd.). The electrodes
were impregnated with a 0.05 M titanium tetrachloride solution and sintered at 500 ° C. The films
50
were further treated with 0.1 M HCl (aq.) before examination . The dye solutions (2 × 10−4 M)
were prepared in 1:1:1 mixture of acetonitrile, tert-butyl alcohol and dimethyl sulfoxide (DMSO).
Deoxycholic acid was added to the dye solution as a coadsorbent at a concentration of 20 mM.
The electrodes were immersed in the dye solutions and then kept at 25 °C for 20 h to adsorb the
dye onto the TiO2 surface.
4.4. Fabrication of Dye-Sensitized Solar Cell
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
Photovoltaic and incident photon-to-current efficiency (IPCE) measurements were made on
sandwich cells, which were prepared using TiO2 coated working electrodes and platinum coated
counter electrodes, and were sealed using a 40 m Surlyn spacer through heating of the polymer
frame. The redox electrolyte consisted of a solution of 0.6 M DMPII, 0.05 M I2, 0.1 M LiI and 0.5
M TBP in acetonitrile (Solaronix, Iodolyte HI-30).
4.5. Photovoltaic Measurements
Photovoltaic measurements of sealed cells were made by illuminating the cell through the
conductive glass from the anode side with a solar simulator (WXS-155S-10) at AM 1.5
2
illuminations (light intensity: 100 mW cm ).
4.6. Incident photon to current efficiency (IPCE) conversion
IPCE measurements were made on a CEP-2000 system (Bunkoh-Keiki Co. Ltd.). IPCE at each
wavelength was calculated using Equation 1, where ISC is the short-circuit photocurrent density
2
(mA cm ) under monochromatic irradiation, q is the elementary charge,  is the wavelength of
incident radiation in nm and P0 is the incident radiative flux in W/m2 51
.
ISC
( )
IPCE λ = 1240 (
)
(3)
qλP
o
4.7. Electrochemical impedance spectroscopy (EIS)
The electrochemical impedance spectra were measured with an impedance analyzer potentiostat
(Bio-Logic SP-150) under illumination using a solar simulator (SOL3A, Oriel) equipped with a
450 W xenon lamp (91160, Oriel). EIS spectra were recorded over a frequency range of 100 mHz
to 200 kHz at room temperature. The applied bias voltage was set at the VOC of the DSCs, with AC
amplitude set at 10 mV. The electrical impedance spectra were fitted using Z-Fit software (Bio-
Logic).
4.8. Photoemission yield spectrometer (AC3) and E0-0 measurements
The experimental HOMO and E0-0 energy values for MMR-1, MMR-2 and N719 were measured
using a photoemission yield spectrometer and the experimental absorption/emission spectra point
52
of overlap, respectively, and the procedure was described in detail elsewhere . Figure 7S to 9S
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10.1002/ejic.201700468
European Journal of Inorganic Chemistry
(Supplementary Information) show the HOMO energies of MMR-1, MMR-2 and N719,
respectively.
4.9. Molecular modeling
Equilibrium molecular geometries of MMR-1 and MMR-2 were calculated using Adamo
53
hybridized 1996 pure functional of Perdew, Burke and Ernzerhof (PBE1PBE) and the full-
electron basis set density Gauss double-zeta with polarization functions (DGDZVP) 54, 55. The
geometry optimization calculations were followed by energy calculations using time-dependent
density functional theory (TD-DFT) utilizing the energy functional PBE1PBE and the basis set
DGDZVP. The solvent (DMF) effect was accounted for by using the conductor-like polarizable
continuum model (PCM), implemented in Gaussian 09, and the ground and excited state oxidation
potentials were calculated. All DFT and TD-DFT jobs were submitted remotely at North Carolina
State University’s Supercomputer Jasta via HPC linkage.
5. Acknowledgements
The authors are thankful to the Department of Textile Engineering, Chemistry, and Science
Department at NC State for the financial support.
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