Effect of Donor Groups on the Performance of Cyclometalated Ruthenium Sensitizers in Dye-Sensitized Solar Cells

Article abstract of DOI:10.1021/acs.inorgchem.7b02164

Three new tris-heteroleptic complexes of ruthenium(II) were designed by coordinating the metal center with cyclometalating, anchoring, and auxiliary ligands with different donor substituents. N-Hexylcarbazole, N-hexylphenothiazine, and N-hexyldiphenylamine donor moieties were used as substituents on the auxiliary ligands for SA633, SA634, and SA635, respectively. Complexes were characterized by ¹H and ¹³C 2D-COSY NMR techniques. These complexes provide power conversion efficiencies in the range of 7.6-8.2% when they are employed in state of the art dye-sensitized solar cells (DSCs) with cobalt electrolyte. Various electrochemical and transient techniques were used to unveil the unexpected differences in the performance of these very similar sensitizers.

Full text of DOI:10.1021/acs.inorgchem.7b02164

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Effect of Donor Groups on the Performance of Cyclometalated
Ruthenium Sensitizers in Dye-Sensitized Solar Cells
Sadig Aghazada,† Yameng Ren,‡,§ Peng Wang,‡ and Mohammad Khaja Nazeeruddin*,†
†
́
Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole Polytechnique
́ ́
Federale de Lausanne (EPFL), CH-1950 Sion, Switzerland
‡Center for Chemistry of Novel & High-Performance Materials, Department of Chemistry, Zhejiang University, 310028 Hangzhou,
People’s Republic of China
§Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022 Changchun, People’s Republic of China
S
* Supporting Information
ABSTRACT: Three new tris-heteroleptic complexes of ruthenium(II)
were designed by coordinating the metal center with cyclometalating,
anchoring, and auxiliary ligands with different donor substituents. N-
Hexylcarbazole, N-hexylphenothiazine, and N-hexyldiphenylamine donor
moieties were used as substituents on the auxiliary ligands for SA633,
1
SA634, and SA635, respectively. Complexes were characterized by H and
13C 2D-COSY NMR techniques. These complexes provide power
conversion efficiencies in the range of 7.6−8.2% when they are employed
in state of the art dye-sensitized solar cells (DSCs) with cobalt electrolyte.
Various electrochemical and transient techniques were used to unveil the
unexpected differences in the performance of these very similar sensitizers.
high efficiencies with electrolyte employing an I3 /3I− redox
−
INTRODUCTION
■
mediator.15−19 This results from (a) efficient regeneration
Rising energy consumption and global warming are pushing
through inner-sphere electron transfer from the iodide to the
humanity toward carbon-free sources of energy. In 1 h, the
photo-oxidized sensitizer, which is favored by NCS···I−
Earth receives more energy via solar irradiation than the
interaction,20−23 (b) slow recombination of electrons in the
amount people use in one year. Thus, using a fraction of the
24,25
−
conduction band and trap states with the I3 ,
and (c) the
incident solar energy may accelerate the transition away from
fossil fuels.1 Many different types of photovoltaic technologies
have been developed in the last half century, ranging from
conventional monocrystalline silicon-based solar cells to the
recent development of highly promising lead halide perovskite
solar cells.2−5 Most of these photovoltaic devices perform well
in outdoor conditions under direct sunlight irradiation.
However, for indoor applications dye-sensitized solar cells
(DSCs) are a better choice due to their high performance in
dim lightning and the possibility of integrating them into parts
of a building’s architecture.6,7
In DSCs, three spatially separated layers are individually
responsible for light absorption, electron transport, and hole
transport. Ideally, this structure should afford the opportunity
to optimize the layers separately.8 However, undesirable
processes at the interfaces of these layers limit the power
conversion efficiency (PCE), and only a few established
systems have been proven as efficient.9−13
relatively small size of the original sensitizer, which results in
efficient coverage of a titania surface.26,27 Thus, a ruthenium
system provides a high short-circuit current density JSC.
However, there is a big loss in open-circuit voltage VOC due
to the many-step regeneration of the photo-oxidized sensitizer
with iodide, which requires a driving force of ∼600 mV for
efficient dye regeneration.28−30 Despite the low VOC, PCE
values of over 11% were achieved.17,18
Alternatively, outer-sphere one-electron-redox mediators
such as Co3+/2+ and Cu2+/1+ require a lower driving force for
dye regeneration (∼200 mV).31−36 For example, electrolytes
based on [Co(bpy-pz)2]3+/2+ (bpy-pz = 6-(1H-pyrazol-1-yl)-
2,2′-bipyridine), and recently developed Cu2+/+ mediators
provide VOC values of over 1 V.36,37 However, outer-sphere
redox mediators usually suffer from a very strong recombina-
tion of charges from titania with oxidized redox species.38,39 To
reduce the recombination rate, many sensitizers based on
D−π−A and porphyrin have been functionalized with long
alkyl chains, which keep a redox mediator away from the
Due to unforeseen incompatibilities between a sensitizer and
established electrolytes, the discovery of new sensitizers with
suitable photophysical characteristics does not necessarily
translate into highly efficient devices.12,14 Ruthenium(II)
complexes with isothiocyanate-donating ligands provide record
Received: August 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
semiconductor surface, resulting in PCEs of over 14%.10,11 In
regard to the classic ruthenium sensitizers with NCS ligands,
the binding of an oxidized species of a redox couple with a
sensitizers leads to even higher recombination rates.39 To
overcome this problem, new ruthenium complexes that are
analogous to organic D−π−A sensitizers with long alkyl chains
have been designed, and PCEs of over 9% were achieved.40−42
To understand the main structural characteristics of
ruthenium sensitizers that limit the PCEs in DSCs with
cobalt-based electrolyte, we have designed three new tris-
heteroleptic cyclometalated ruthenium(II) complexes with
various aromatic moieties of different electron-releasing
power (Figure 1). The choice of donor groups was also limited
boronic ester adducts of the donor groups with 4,4′-dibromo-
2,2′-bipyridine. The tris-heteroleptic complexes were synthe-
sized in three steps. First, cyclometalation was carried out with
[Ru(C6H6)Cl2]2, and [Ru(C∧N)(CH3CN)4](PF6) was ob-
tained.49−51 This complex was reacted with a 1:1 mixture of
ancillary (bpyR2) and 4,4-dimethoxycarbonyl-2,2′-bipyridine
(bpy(CO2Me)2), which resulted in three compounds (two bis-
heteroleptic and one tris-heteroleptic).52,53 The tris-hetero-
leptic product was separated by column chromatography and
hydrolyzed to obtain the final sensitizer. It is worth noting that
neutral complexes with one deprotonated carboxylic acid group
were obtained. All of the products were characterized by means
1
of H, 13C, and 31P COSY NMR, and all the results are
provided in the Supporting Information.
The absorption spectra of the sensitizers in DCM show four
apparent bands in the visible region of the solar spectrum,
which could all be related to different metal to ligand charge
transfer (MLCT) transitions (Figure 2A). The band of the
lowest energy for all three sensitizers is broad and extends up to
750 nm. From SA633 to SA634 and SA635, there is a small
bathochromic shift of the lowest intense energy band from 572
nm to 578 and 579 nm. Moreover, the extinction coefficient of
this band changes from 21.2 × 103 L mol−1 cm−1 to 23.9 × 103
and 22.3 × 103 L mol−1 cm−1 (Table 1). Around 400 nm, the
bands for SA633 and SA634 have intensities of around 36 ×
103 L mol−1 cm−1, but SA635 has the most intense band with
an extinction coefficient of 51.1 × 103 L mol−1 cm−1. Moreover,
from SA633 to SA635 and SA634, there is a strong red shift of
this band, with no clear reason behind it. Thus, these sensitizers
almost completely cover the whole visible spectrum owing to
the broad nature of the MLCT bands, which should lead to
high light-harvesting efficiency.
Figure 1. Sensitizers introduced in this work.
to a few groups with very similar structures to level off the effect
of different dye loadings on the performance of solar cells.
Ruthenium complexes bearing different amine-based aromatic
substituents have been studied in DSCs, but these complexes
were usually based on NCS and were always investigated with
iodine-based electrolytes. Many works have investigated the
“antenna” effect of donor groups and the role of hole extraction
in solar cell performance.43−48
From the photoluminescence measurements in DCM for all
three complexes, a weak emission was detected with maximum
values occurring at 622, 630, and 625 nm for SA633, SA634,
and SA635, respectively (Figure S25 in the Supporting
Information). E0−0 values in the range of 1.78−1.80 eV were
determined from the intersection of the normalized absorption
and emission spectra.
The cyclic voltammograms of the new sensitizers were
obtained in a 0.1 M NBu4PF6 solution in DMF under an argon
atmosphere. Analogous measurements in DCM were un-
successful (Figure 2C). Ferrocene was used as an internal
standard, and its oxidation potential was located at 0.63 V vs
NHE. In the electrochemical window of DMF, all three
RESULTS AND DISCUSSION
■
Scheme 1 illustrates the synthesis of the complexes. First, the
cyclometalating ligand was synthesized in a way similar to
published methods.40,41 The ancillary ligands were synthesized
by Stille or Suzuki−Miyaura coupling of the tributyltin or
Scheme 1. Synthesis of Ruthenium Complexes
B
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. (A) Absorption spectra of SA633, SA634, and SA635 solutions in DCM. (B) Absorption spectra of sensitized mesoporous titania films (4
μm thick). (C) Cyclic voltammograms of sensitizers measured in 0.1 M DMF solution of (NBu4)(PF6). Ferrocene was used as an internal standard,
and its oxidation potential was located at 0.63 V vs NHE. The cyclic voltammograms without ferrocene are presented. (D) Energy diagram of
SA633, SA634, and SA635 sensitizers in DSCs using [Co(phen)3]3+/2+ as a redox mediator. The excited-state oxidation potentials for all three
sensitizers were obtained by extracting the E0−0 value from the ground-state oxidation potential.
contrast to SA633 and SA635, all three of the uppermost
occupied orbitals of SA634 have a significant contribution from
the phenothiazine substituents. This contribution is highest for
the HOMO-1, which is the result of the stronger donating
character of the phenothiazine moiety in comparison to that for
carbazole and diphenylamine. However, as shown in the cyclic
voltammograms, higher π-donating character of phenothiazine
moieties does not push the Ru3+/2+ redox potential toward
more negative values.
Table 1. Photophysical and Electrochemical Data for SA633,
SA634, and SA
abs
S+/S
S+/S
λmax −(3nm) (ε
E0
(V vs
A c
E0
*
(10 M−1
a
λmax
E0−0 NHE) (V − VC
(V vs
NHE)
PL
a
b
d
cm−1))
(nm) (eV)
(mV))
SA633
SA634
572 (21.2)
578 (23.9)
622
630
1.79 0.82 (69)
−0.97
−0.93
1.78 0.85 (66) and
1.0 (100)
SA635
579 (22.3)
625
1.80 0.82 (79)
−0.97
Generally, the oxidation potential of Ru3+/2+ redox for these
three sensitizers does not depend on the π-electron-releasing
power of the donor moieties, which is due to the twisting
between the donor moiety and bipyridine planes resulting in no
conjugation between them. The excited-state oxidation
potentials were determined according to eq 1 (n = 1) from
the obtained ground-state oxidation potentials and E0−0 values.
The excited-state oxidation potentials for all three dyes were
found between −0.93 and −0.97 V vs NHE. These values are
∼400 mV more negative than the conduction band edge for the
mesoporous titania used in conventional DSCs and should
provide enough driving force for efficient charge injection from
the photoexcited sensitizer into the conduction band of titania
(Figure 2D).
a
b
Measured in 10−5 DCM solution. Determined from the intersection
c
of normalized emission and absorption spectra. Were determined
from the CV measurements in 0.1 M NBu4PF6 solution in DMF under
argon atmosphere. Ferrocene was used as an internal standard and its
oxidation potential was fixed at 0.62 V vs NHE. The shift between the
d
anodic and cathodic wave maximums is shown in brackets. Excited-
state oxidation potential is determined by extracting the E0−0 value
from the ground-state oxidation potential.
sensitizers show two oxidations. The first oxidation potentials
for SA633, SA634, and SA635 were found at 0.82, 0.85, and
0.83 V vs NHE. Considering the very small dependence of the
values on the substituents on the auxiliary ligand, the first
oxidation for all three sensitizers was attributed to the Ru3+/2+
redox reaction.
+
S+/S
*
S /S
E0
= E0
− E0−0/n
(1)
The second oxidation for SA634 with the phenothiazine
substituent was observed at 1.0 V vs NHE, while for SA633 and
SA635, the second oxidation potentials were not determined
due to irreversibility. However, the anodic waves appear at
potentials higher than that for SA634 (1.22 and 1.36 V vs NHE
for SA633 and SA635). The second oxidation of the sensitizer
was attributed to the possible oxidation of donor moieties at
the ancillary ligand. Considering their more positive value, we
do not expect any efficient charge extraction from the metal
center after photo-oxidation except partially in SA634. More
electron-donating moieties should be attached to enable
efficient hole extraction toward the periphery of a ligand.46
We have conducted DFT calculations to support our
referencing of the first and second oxidation waves to
ruthenium and ligand oxidations, respectively (see the
Supporting Information). As shown in Figure S27, the
HOMO, HOMO-1, and HOMO-2 for SA633 and SA635
generally have Ru t2g orbital character (considering the ideal
octahedral coordination) with little composition on the
cyclometalated ligand. Additionally, HOMO-2 for SA635 has
little composition from the diphenylamine substituents. In
One of the main requirements for the sensitizers in DSCs is
stability toward the redox processes, and they should not
degrade when they are photo-oxidized. This implies that their
oxidation should be perfectly reversible. Irreversible photo-
oxidation is a disadvantage of NCS-ligated ruthenium
sensitizers, which undergo ligand substitution.54−56 In previous
work, we showed that cyclometalating complexes also suffer
from redox irreversibility, but this was generally attributed to
the degradation of polyaromatic moieties attached to the
ancillary ligand.41
We have conducted Randles−Sevcik analyses to analyze the
redox reversibility of new sensitizers.57 The Randles−Sevcik
equation defined at room temperature is
ip = 268715n3/2AC(Dν)1/2
(2)
(where ip is the current at the wave maximum, A is the
electrode area, C is the concentration of the component
undergoing redox, D is the diffusion constant of the studied
molecule, n is the number of electrons transferred in one redox
C
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Cyclic voltammograms of SA633, SA634, and SA635 at different scan rates in 0.1 M NBu4PF6 solution in DMF and Randles−Sevcik
analyses showing a linear change of maximum current value over square root of the scan rate.
event, and v is the scan rate). According to the equation there
should be a linear rise in the current maximum for the
reversible redox when it is plotted vs the square root of the scan
rate. For SA634, the Randles−Sevcik analysis showed perfect
reversibility for both oxidation waves, which we related to the
oxidation of the ruthenium and one phenothiazine substituent
(Figure 3). Perfect reversibility is also observed for the
oxidation of Ru in SA633. However, a definitive conclusion
cannot be obtained for the second oxidation, due to the shape
of the anodic wave and undeveloped cathodic wave, as well as
the perfect linear rise of the maximum current over the square
root of the scan rate for the anodic wave. For SA635, although
Randles−Sevcik analysis shows a linear dependence, the shape
of the voltammogram changes drastically when it is measured at
different scan rates.
To unravel the ambiguous results from the solution CV
analysis, the Randles−Sevcik analysis was conducted for the
sensitized mesoporous titania films in 1-ethyl-3-methylimida-
zolium bis(trifluoromethylsulfonyl)imide. In this experiment,
perfect reversibility was observed for both SA634 and SA635.
For SA633, the results are not completely clear (Figure S26 in
the Supporting Information). Thus, in both experiments, only
SA634 with phenothiazine substitution showed reversibility.
Additionally, in a perfectly reversible redox, the anodic and
cathodic currents should have comparable values. For these
sensitizers, the comparison of the values of anodic and cathodic
waves is difficult due to the overlap of the first and second
anodic waves. To show whether these sensitizers are stable in
the oxidized state and may quantitatively regenerate, we have
conducted spectroelectrochemical analyses in 0.1 M NBu4PF6
solution in DMF under an ambient atmosphere (Figure 4).
After each voltage step was applied for 60 s, the absorption
spectra were measured in the following 5 s. For all of the
sensitizers, increasing the potential results in reduction of the
MLCT band around 575 nm and in the formation of a new
absorption band in the IR region. In this process, the high-
energy bands also change their shapes and positions.
The spectra measured at high voltages do not show the
gradual changes and do not cross the isosbestic points formed
by the spectra that were observed when more negative voltages
are applied. This is the result of partial second oxidation, which
presumably leads to a slow degradation of the complex. In the
backward run, the MLCT bands for SA634 and SA635
recovered completely, while just ∼90% was recovered for
SA633. Moreover, the initial shapes of the spectra were also
recovered, with a red shift of ∼5−10 nm of the recovered
lowest energy MLCT band.
The destroyed isosbestic points and the small red shift of the
MLCT bands indicate that, at high voltages, the oxidation of
donor moieties on the ancillary ligands leads to structural
changes where the coordination core is saved. When the
spectroelectrochemical analyses were carried out at a less
positive voltage to avoid the second oxidation, clean isosbestic
points were obtained with a full recovery of the initial spectra at
the opposite voltage run. This further supports that the first
oxidation is fully reversible and that the second oxidation is
detrimental. Moreover, the attached instability undermines the
direction of research on the design of new ruthenium
D
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. Spectroelectrochemical analyses of SA633, SA634, and SA635 in 0.1 M solution of NBu4PF6 in DMF. No preliminary deoxygenation was
carried out. The absorption spectra were measured after applying each voltage step for 60 s. The top three graphs show the destructive effect of the
second oxidation. The bottom three graphs show perfect reversibility for the single oxidation of sensitizers.
Figure 5. (A) Current density−voltage (J−V) curves recorded under simulated AM1.5G sunlight (100 mW cm−2), The aperture area of the black
metal mask was 0.16 cm2. Antireflection film was used on the back of the counter electrode. (B) Incident photon to current conversion efficiency
(IPCE) at a set of wavelengths (λ) of incident monochromatic lights. The measurements were carried out at intervals of 10 nm for incident
monochromatic light under a constant white light bias (10 mW cm−2). Antireflection film was used on the back of the counter electrode.
complexes mostly comprising the attachment of various
substituents on one of the bipyridine ligands.58 This work
shows that substituents on derivatives of diphenylamine should
play a negative role in the long-term sensitizer performance.
This was also observed in our previous work on complexes with
substituents based on thiophene. However, only a single
oxidation is taking place in a DSC and the photooxidized dye is
in a reducing environment. Thus, this destructive pathway
should not influence the PCE in the short term.
DSCs were assembled using SA633, SA634, and SA635 as
sensitizers. The working electrode was fluorine-doped tin oxide
with a 4.5 μm layer of transparent titania and an additional 5
μm scattering layer on top. Films were sensitized in dye
solutions of similar concentration. The [Co(phen)3]3+/2+ redox
couple was used as a mediator with a 1:4 molar ratio of 3+ to
E
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 2. Photovoltaic Parameters of Four Cells Measured Under Simulated AM1.5G Sunlight (100 mW cm−2)
a
(mA cm−2
)
JSC (mA cm−2
)
VOC (mV)
FF (%)
PCE (%)
IPCE
JSC
SA633
SA634
SA635
13.70 0.05
13.91 0.04
13.06 0.04
13.68 0.04
13.89 0.03
13.03 0.03
819
845
809
2
2
2
71.5 0.2
70.0 0.1
72.1 0.1
8.0 0.1
8.2 0.1
7.6 0.2
a
JSCIPCEwas computed via wavelength integration of the product of the IPCE curve measured at the short circuit and the standard AM1.5G emission
spectrum (ASTM G173-03).
Figure 6. (A) Charge extracted from a dye-grafted titania film (QCE) as a function of open-circuit photovoltage (VOC). (B) Plots of half-lifetime
(t1/2TPD) of electrons in the conduction band and traps under the conduction band of titania vs QCE. (C) Dependence of open-circuit photovoltage
(VOC) on short-circuit photocurrent density (JSC). The solid lines are displayed as a visual aid.
Table 3. Comparison of Calculated Differences in VOC Due to Different Short-Circuit Current Densities and Electron
a
Recombination Lifetimes with the Differences in VOC from the J−V Measurements
ΔVOC(photocurrent) (mV)
ΔVOC(electron lifetime) (mV)
ΔVOC(total) (mV)
ΔVOC(from J−V) (mV)
SA633
SA634
SA635
∼2
∼2.5
∼14
∼24
∼16
∼26.5
10
37
a
Values for devices with SA633 and SA635 are referenced vs values for a device with SA635.
2+ species. Photocurrent density−photovoltage (J−V) curves
were obtained under AM1.5G irradiation of devices with an
active area of 0.16 cm2 (Figure 5A), and all of the parameters
are summarized in Table 2.
band gap does not depend on the sensitizer used (Figure 6A).
The similar distribution of trap states also indicates that the
adsorption modes of these three sensitizers are somehow
similar. Moreover, the position of the conduction band edge
(ECB) should be similar for all the devices, and thus, the change
in VOC should not be due to the variations in the ECB position.
TPD measurements of the devices with these three dyes
reveal very different electron lifetimes at similar capacitances.
Solar cells with SA635 show the poorest electron lifetime, while
for devices with SA633 and SA634, the electron lifetimes are
nearly 2 and 4 times longer than that with SA635 at the same
capacitance (Figure 6B).Thus, the difference in open-circuit
voltage could result from two components. First, the current
induced an increase in VOC. Second, the different electron
lifetimes lead to different steady-state concentrations of
electrons in trap states and thus different values of Fermi
energy.
The JSC value increases from 13.03 mA cm−2 for SA635 to
13.68 and 13.89 mA cm−2 for SA633 and SA634, respectively.
The VOC value rises in the same order from 809 to 819 and 845
mV. With small differences in the fill factor (FF), SA633,
SA634, and SA635 provide PCEs of 8.0, 8.2, and 7.6%. The
incident photon to current conversion efficiency (IPCE) results
are presented in Figure 5B. We obtained the short-circuit
currents by integrating the product of the IPCE curve with the
AM1.5G solar spectrum, and the results are in agreement with
the values obtained from the J−V measurements. In
comparison to the absorption spectra, the IPCE spectra have
a red-shifted onset to 820 nm. The IPCE spectra also indicate
that the higher IPCE values in the range of 420 and 620 nm are
responsible for the higher photocurrent of the devices with
SA634 and SA633 in comparison with that for SA635.
To understand the reasons behind the obtained photo-
currents and voltages, we have measured the charge extraction
(CE)59 and transient photovoltage decay (TPD)60 of the final
devices (Figure 6). The charge extraction measurements for all
three sensitizers show the same profile over the voltage range,
which indicates that the distribution of trap states within the
Equation 3 can be used to calculate the current-induced
buildup in VOC, which is the result of the diode equation:
J (2)
kTm
q
SC
VOC(2) − VOC(2) =
log
J (1)
SC
(3)
where k, T, q, and m are the Boltzmann constant, temperature,
electron charge, and nonideality factor, respectively. This
equation indicates that at room temperature the VOC value in
F
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the ideal case (m = 1) should increase by 59 mV when JSC is
increased by 10 times.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for M.K.N.: mdkhaja.nazeeruddin@epfl.ch.
The J−V curves were measured at different light intensities
obtained using various meshes. The linear fit results in rise of
ca. 87 mV in VOC when JSC increases 10 times for all sensitizers
(Figure 6C). From this result, we can use eq 1 to calculate the
gain in VOC for the solar cells with SA633 and SA635 in
reference to that for SA635 due to higher photocurrents with
former two sensitizers. A current-favored buildup in the
photovoltage occurred below 3 mV.
Equation 4 can be used to calculate the change in VOC due to
different electron recombination lifetimes. The calculated
change in VOC was substantial and was in good agreement
with the values obtained from J−V measurements (Table 3).
This indicates that the processes discussed above completely
explain the difference in the obtained voltages and PCEs.
ORCID
Sadig Aghazada: 0000-0002-7568-4481
Peng Wang: 0000-0002-6018-1515
Mohammad Khaja Nazeeruddin: 0000-0001-5955-4786
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge Valeriu Scutelnic for his valuable help with
DFT calculations.
REFERENCES
t1/2TPD(2)
■
kTm
q
VOC(2) − VOC(2) =
ln
(1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical
Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A.
2006, 103 (43), 15729−15735.
(2) Lee, Y.; Park, C.; Balaji, N.; Lee, Y.-J.; Dao, V. A. High-Efficiency
Silicon Solar Cells: A Review. Isr. J. Chem. 2015, 55 (10), 1050−1063.
(3) O’Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar Cell
Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353
(6346), 737−740.
(4) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J.
Molecular Optimization Enables over 13% Efficiency in Organic Solar
Cells. J. Am. Chem. Soc. 2017, 139, 7148.
t1/2TPD(1)
(4)
All three sensitizers have very similar spectral, electro-
chemical, and structural characteristics: (a) absorption spectra
with negligible variations in extinction coefficients, (b) position
of ground and excited oxidation potentials, and (a) structures
with tiny variations. Therefore, these sensitizers are expected to
provide the same PCEs in DSCs, which is not the case. By
comparison of the absorption spectra of sensitized films (Figure
2B) we may judge the relative dye loading. By dividing the
intensity of MLCT bands from the spectra of sensitized films by
the extinction coefficients of these bands in solution, we
approximate that the amounts of SA634 and SA633 loaded on
the titania are 10 and 18% more than the amount of loaded
SA635. Increased dye loading may result in better protection of
the surface and thus in greater electron lifetimes. In this case,
the dye-loading factor may explain the higher electron lifetime
in devices with SA633 than with SA635, but why SA634 would
provide an even higher electron lifetime is still an open
question.
̈
(5) Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-
P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.;
Hagfeldt, A.; Gratzel, M. Cesium-Containing Triple Cation Perovskite
̈
Solar Cells: Improved Stability, Reproducibility and High Efficiency.
Energy Environ. Sci. 2016, 9 (6), 1989−1997.
(6) Freitag, M.; Teuscher, J.; Saygili, Y.; Zhang, X.; Giordano, F.;
Liska, P.; Hua, J.; Zakeeruddin, S. M.; Moser, J.-E.; Gratzel, M.;
Hagfeldt, A. Dye-Sensitized Solar Cells for Efficient Power Generation
under Ambient Lighting. Nat. Photonics 2017, 11, 372.
̈
(7) Tingare, Y. S.; Vinh, N. S.; Chou, H.-H.; Liu, Y.-C.; Long, Y.-S.;
Wu, T.-C.; Wei, T.-C.; Yeh, C.-Y. New Acetylene-Bridged 9,10-
Conjugated Anthracene Sensitizers: Application in Outdoor and
Indoor Dye-Sensitized Solar Cells. Adv. Energy Mater. 2017, 7,
1700032.
(8) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-
Sensitized Solar Cells. Chem. Rev. 2010, 110 (11), 6595−6663.
(9) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
Gratzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)−
Based Redox Electrolyte Exceed 12% Efficiency. Science (Washington,
DC, U. S.) 2011, 334 (6056), 629.
(10) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B.
F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin,
CONCLUSION
■
Three new cyclometalated tris-heteroleptic ruthenium sensi-
tizers with different donor substituents were synthesized and
characterized. All of these sensitizers showed reversible metal
oxidation and irreversible ligand oxidation as a result of slow
degradation. DSCs employing these sensitizers together with
[Co(phen)3]3+/2+-based electrolyte provide good PCEs of up to
8.2% under AM1.5G irradiation. Among these sensitizers,
SA634 with phenothiazine substitution resulted in the best
performance, while SA635 with a diphenylamine group
provided the lowest performance. The difference in the
performance of these sensitizers in DSCs is generally attributed
to the different electron lifetimes.
̈
M. K.; Gratzel, M. Dye-Sensitized Solar Cells with 13% Efficiency
̈
Achieved through the Molecular Engineering of Porphyrin Sensitizers.
Nat. Chem. 2014, 6 (3), 242−247.
(11) Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya,
M. Highly-Efficient Dye-Sensitized Solar Cells with Collaborative
Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem.
Commun. 2015, 51 (88), 15894−15897.
ASSOCIATED CONTENT
■
(12) Furer, S. O.; Bozic-Weber, B.; Schefer, T.; Wobill, C.;
̈
S
* Supporting Information
Constable, E. C.; Housecroft, C. E.; Willgert, M. Understanding
Why Replacing I 3 − /I − by Cobalt(Ii)/(Iii) Electrolytes in
Bis(diimine)copper(I)-Based Dye-Sensitized Solar Cells Improves
Performance. J. Mater. Chem. A 2016, 4 (33), 12995−13004.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02164.
(13) Aghazada, S.; Gao, P.; Yella, A.; Moehl, T.; Teuscher, J.; Moser,
Detailed synthetic procedures, characterization of ma-
terial, computational analysis, and description of solar cell
preparation (PDF)
J. E.; Gratzel, M.; Nazeeruddin, M. K. Unraveling the Dual Character
̈
of Sulfur Atoms on Sensitizers in Dye-Sensitized Solar Cells. ACS
Appl. Mater. Interfaces 2016, 8 (40), 26827−26833.
G
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(14) Tejerina, L.; Martínez-Díaz, M. V.; Nazeeruddin, M. K.; Gratzel,
(30) Ardo, S.; Meyer, G. J. Photodriven Heterogeneous Charge
Transfer with Transition-Metal Compounds Anchored to TiO 2
Semiconductor Surfaces. Chem. Soc. Rev. 2009, 38 (1), 115−164.
(31) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo,
G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine
Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am.
Chem. Soc. 2010, 132 (46), 16714−16724.
̈
M.; Torres, T. Role of the Bulky Aryloxy Group at the Non-Peripheral
Position of Phthalocyanines for Dye Sensitized Solar Cells.
ChemPlusChem 2017, 82 (1), 132−135.
(15) Chen, C.-Y.; Wu, S.-J.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. A
Ruthenium Complex with Superhigh Light-Harvesting Capacity for
Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2006, 45 (35),
5822−5825.
(32) Feldt, S. M.; Lohse, P. W.; Kessler, F.; Nazeeruddin, M. K.;
Gratzel, M.; Boschloo, G.; Hagfeldt, A. Regeneration and Recombi-
̈
(16) Chen, C.-Y.; Wu, S.-J.; Li, J.-Y.; Wu, C.-G.; Chen, J.-G.; Ho, K.-
C. A New Route to Enhance the Light-Harvesting Capability of
Ruthenium Complexes for Dye-Sensitized Solar Cells. Adv. Mater.
2007, 19 (22), 3888−3891.
nation Kinetics in Cobalt Polypyridine Based Dye-Sensitized Solar
Cells, Explained Using Marcus Theory. Phys. Chem. Chem. Phys. 2013,
15 (19), 7087−7097.
(33) Cong, J.; Kinschel, D.; Daniel, Q.; Safdari, M.; Gabrielsson, E.;
Chen, H.; Svensson, P. H.; Sun, L.; Kloo, L. Bis(1,1-bis(2-
Pyridyl)ethane)copper(I/Ii) as an Efficient Redox Couple for Liquid
Dye-Sensitized Solar Cells. J. Mater. Chem. A 2016, 4 (38), 14550−
14554.
(34) Colombo, A.; Dragonetti, C.; Magni, M.; Roberto, D.;
Demartin, F.; Caramori, S.; Bignozzi, C. A. Efficient Copper Mediators
Based on Bulky Asymmetric Phenanthrolines for DSSCs. ACS Appl.
Mater. Interfaces 2014, 6 (16), 13945−13955.
(17) Chen, C.-Y.; Wang, M.; Li, J.-Y.; Pootrakulchote, N.; Alibabaei,
L.; Ngoc-le, C.; Decoppet, J.-D.; Tsai, J.-H.; Gratzel, C.; Wu, C.-G.;
̈
Zakeeruddin, S. M.; Gratzel, M. Highly Efficient Light-Harvesting
̈
Ruthenium Sensitizer for Thin-Film Dye-Sensitized Solar Cells. ACS
Nano 2009, 3 (10), 3103−3109.
(18) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.;
Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M.
̈
Enhance the Optical Absorptivity of Nanocrystalline TiO2 Film with
High Molar Extinction Coefficient Ruthenium Sensitizers for High
Performance Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130
(32), 10720−10728.
(35) Magni, M.; Giannuzzi, R.; Colombo, A.; Cipolla, M. P.;
Dragonetti, C.; Caramori, S.; Carli, S.; Grisorio, R.; Suranna, G. P.;
Bignozzi, C. A.; Roberto, D.; Manca, M. Tetracoordinated Bis-
Phenanthroline Copper-Complex Couple as Efficient Redox Media-
tors for Dye Solar Cells. Inorg. Chem. 2016, 55 (11), 5245−5253.
(19) Chang, T.-K.; Li, H.; Chen, K.-T.; Tsai, Y.-C.; Chi, Y.; Hsiao, T.-
Y.; Kai, J.-J. Substituent Effect of Ru(ii)-Based Sensitizers Bearing a
Terpyridine Anchor and a Pyridyl Azolate Ancillary for Dye Sensitized
Solar Cells. J. Mater. Chem. A 2015, 3 (36), 18422−18431.
(36) Saygili, Y.; Soderberg, M.; Pellet, N.; Giordano, F.; Cao, Y.;
̈
Munoz-García, A. B.; Zakeeruddin, S. M.; Vlachopoulos, N.; Pavone,
(20) Clifford, J. N.; Palomares, E.; Nazeeruddin, Md. K.; Gratzel, M.;
̈
M.; Boschloo, G.; Kavan, L.; Moser, J. E.; Gratzel, M.; Hagfeldt, A.;
̈
Durrant, J. R. Dye Dependent Regeneration Dynamics in Dye
Sensitized Nanocrystalline Solar Cells: Evidence for the Formation of
a Ruthenium Bipyridyl Cation/iodide Intermediate. J. Phys. Chem. C
2007, 111 (17), 6561−6567.
Freitag, M. Copper Bipyridyl Redox Mediators for Dye-Sensitized
Solar Cells with High Photovoltage. J. Am. Chem. Soc. 2016, 138 (45),
15087−15096.
(37) Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.;
Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J.-E.; Yi, C.;
(21) Lobello, M. G.; Fantacci, S.; De Angelis, F. Computational
Spectroscopy Characterization of the Species Involved in Dye
Oxidation and Regeneration Processes in Dye-Sensitized Solar Cells.
J. Phys. Chem. C 2011, 115, 18863−18872.
(22) Kusama, H.; Sugihara, H.; Sayama, K. Effect of Cations on the
Interactions of Ru Dye and Iodides in Dye-Sensitized Solar Cells: A
Density Functional Theory Study. J. Phys. Chem. C 2011, 115, 2544−
2552.
(23) Jeon, J.; Goddard, W. A.; Kim, H. Inner-Sphere Electron-
Transfer Single Iodide Mechanism for Dye Regeneration in Dye-
Sensitized Solar Cells. J. Am. Chem. Soc. 2013, 135 (7), 2431−2434.
(24) Green, A. N. M.; Chandler, R. E.; Haque, S. A.; Nelson, J.;
Durrant, J. R. Transient Absorption Studies and Numerical Modeling
of Iodine Photoreduction by Nanocrystalline TiO2 Films. J. Phys.
Chem. B 2005, 109 (1), 142.
(25) Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/
Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem.
Res. 2009, 42, 1819.
Nazeeruddin, M. K.; Gratzel, M. A Cobalt Complex Redox Shuttle
̈
for Dye-Sensitized Solar Cells with High Open-Circuit Potentials. Nat.
Commun. 2012, 3, 631.
(38) Xie, Y.; Baillargeon, J.; Hamann, T. W. Kinetics of Regeneration
and Recombination Reactions in Dye-Sensitized Solar Cells Employ-
ing Cobalt Redox Shuttles. J. Phys. Chem. C 2015, 119 (50), 28155−
28166.
(39) Mosconi, E.; Yum, J.-H.; Kessler, F.; Gom
́
ez García, C. J.;
Zuccaccia, C.; Cinti, A.; Nazeeruddin, M. K.; Gratzel, M.; De Angelis,
̈
F. Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells:
A Combined Computational and Experimental Study. J. Am. Chem.
Soc. 2012, 134 (47), 19438−19453.
(40) Polander, L. E.; Yella, A.; Curchod, B. F. E.; Ashari Astani, N.;
Teuscher, J.; Scopelliti, R.; Gao, P.; Mathew, S.; Moser, J.-E.;
Tavernelli, I.; Rothlisberger, U.; Gratzel, M.; Nazeeruddin, M. K.;
̈
Frey, J. Towards Compatibility between Ruthenium Sensitizers and
Cobalt Electrolytes in Dye-Sensitized Solar Cells. Angew. Chem., Int.
Ed. 2013, 52 (33), 8731−8735.
(26) Wang, Q.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Humphry-
(41) Aghazada, S.; Gao, P.; Yella, A.; Marotta, G.; Moehl, T.;
Baker, R.; Gratzel, M. Molecular Wiring of Nanocrystals: NCS-
̈
Teuscher, J.; Moser, J.-E.; De Angelis, F.; Gratzel, M.; Nazeeruddin, M.
̈
Enhanced Cross-Surface Charge Transfer in Self-Assembled Ru-
Complex Monolayer on Mesoscopic Oxide Films. J. Am. Chem. Soc.
2006, 128 (13), 4446−4452.
K. Ligand Engineering for the Efficient Dye-Sensitized Solar Cells with
Ruthenium Sensitizers and Cobalt Electrolytes. Inorg. Chem. 2016, 55
(13), 6653−6659.
(27) Voitchovsky, K.; Ashari-Astani, N.; Tavernelli, I.; Tetreault, N.;
(42) Wu, K. L.; Clifford, J. N.; Wang, S. W.; Aswani, Y.; Palomares,
E.; Lobello, M. G.; Mosconi, E.; De Angelis, F.; Ku, W. P.; Chi, Y.;
Rothlisberger, U.; Stellacci, F.; Gratzel, M.; Harms, H. A. In Situ
̈
Mapping of the Molecular Arrangement of Amphiphilic Dye
Molecules at the TiO2 Surface of Dye-Sensitized Solar Cells. ACS
Appl. Mater. Interfaces 2015, 7 (20), 10834−10842.
Nazeeruddin, M. K.; Gratzel, M. Thiocyanate-Free ruthenium(II)
̈
Sensitizers for Dye-Sensitized Solar Cells Based on the Cobalt Redox
Couple. ChemSusChem 2014, 7 (10), 2930−2938.
(28) Rowley, J. G.; Farnum, B. H.; Ardo, S.; Meyer, G. J. Iodide
Chemistry in Dye-Sensitized Solar Cells: Making and Breaking I-I
Bonds for Solar Energy Conversion. J. Phys. Chem. Lett. 2010, 1,
3132−3140.
(29) Gardner, J. M.; Giaimuccio, J. M.; Meyer, G. J. Evidence for
Iodine Atoms as Intermediates in the Dye Sensitized Formation of I−I
Bonds. J. Am. Chem. Soc. 2008, 130 (51), 17252−17253.
(43) Li, J.-Y.; Chen, C.-Y.; Chen, J.-G.; Tan, C.-J.; Lee, K.-M.; Wu, S.-
J.; Tung, Y.-L.; Tsai, H.-H.; Ho, K.-C.; Wu, C.-G. Heteroleptic
Ruthenium Antenna-Dye for High-Voltage Dye-Sensitized Solar Cells.
J. Mater. Chem. 2010, 20 (34), 7158−7164.
(44) Chen, C. Y.; Pootrakulchote, N.; Wu, S. J.; Wang, M.; Li, J. Y.;
Tsai, J. H.; Wu, C. G.; Zakeeruddin, S. M.; Gratzel, M. New
̈
Ruthenium Sensitizer with Carbazole Antennas for Efficient and Stable
H
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Thin-Film Dye-Sensitized Solar Cells. J. Phys. Chem. C 2009, 113 (48),
20752−20757.
ization Using Transient Photovoltage Measurements. J. Phys. Chem. B
2005, 109 (10), 4616−4623.
(45) Kono, T.; Masaki, N.; Nishikawa, M.; Tamura, R.; Matsuzaki,
H.; Kimura, M.; Mori, S. Interfacial Charge Transfer in Dye-Sensitized
Solar Cells Using SCN-Free Terpyridine-Coordinated Ru Complex
Dye and Co Complex Redox Couples. ACS Appl. Mater. Interfaces
2016, 8 (26), 16677−16683.
(46) Robson, K. C. D.; Koivisto, B. D.; Yella, A.; Sporinova, B.;
Nazeeruddin, M. K.; Baumgartner, T.; Gratzel, M.; Berlinguette, C. P.
̈
Design and Development of Functionalized Cyclometalated Ruthe-
nium Chromophores for Light-Harvesting Applications. Inorg. Chem.
2011, 50 (12), 5494−5508.
(47) Chen, W. C.; Kong, F. T.; Ghadari, R.; Li, Z. Q.; Guo, F. L.; Liu,
X. P.; Huang, Y.; Yu, T.; Hayat, T.; Dai, S. Y. Unravelling the
Structural-Electronic Impact of Arylamine Electron-Donating Anten-
nas on the Performances of Efficient Ruthenium Sensitizers for Dye-
Sensitized Solar Cells. J. Power Sources 2017, 346, 71−79.
(48) Chi, Y.; Wu, K.-L.; Wei, T.-C. Ruthenium and Osmium
Complexes That Bear Functional Azolate Chelates for Dye-Sensitized
Solar Cells. Chem. - Asian J. 2015, 10 (5), 1098−1115.
(49) Ryabov, A. D.; Estevez, H.; Alexandrova, L.; Pfeffer, M.;
Lagadec, R. Le. Unusual Phenomenon in the Chemistry of
Orthometalated Ruthenium (II) Complexes. Inorg. Chim. Acta 2006,
359 (3), 883−887.
(50) Ryabov, A. D.; Le Lagadec, R.; Estevez, H.; Toscano, R. A.;
Hernandez, S.; Alexandrova, L.; Kurova, V. S.; Fischer, A.; Sirlin, C.;
Pfeffer, M. Synthesis, Characterization, and Electrochemistry of
Biorelevant Photosensitive Low-Potential Orthometalated Ruthenium
Complexes. Inorg. Chem. 2005, 44 (5), 1626−1634.
(51) Bomben, P. G.; Koivisto, B. D.; Berlinguette, C. P.
II
∧
∧
Cyclometalated Ru Complexes of Type [Ru (N N) (C N)]
2
Z: Physicochemical Response to Substituents Installed on the Anionic
Ligand. Inorg. Chem. 2010, 49 (11), 4960−4971.
(52) Bomben, P. G.; Borau-Garcia, J.; Berlinguette, C. P. Three Is
Not a Crowd: Efficient Sensitization of TiO2 by a Bulky Trichromic
Trisheteroleptic Cycloruthenated Dye. Chem. Commun. 2012, 48 (45),
5599−5601.
(53) Bomben, P. G.; Gordon, T. J.; Schott, E.; Berlinguette, C. P. A
Trisheteroleptic Cyclometalated RuII Sensitizer That Enables High
Power Output in a Dye-Sensitized Solar Cell. Angew. Chem., Int. Ed.
2011, 50 (45), 10682−10685.
(54) Tuyet Nguyen, P.; Degn, R.; Nguyen, H. T.; Lund, T.
Thiocyanate Ligand Substitution Kinetics of the Solar Cell Dye Z-907
by 3-Methoxypropionitrile and 4-Tert-Butylpyridine at Elevated
Temperatures. Sol. Energy Mater. Sol. Cells 2009, 93, 1939−1945.
(55) Nguyen, P. T.; Lam, B. X. T.; Andersen, A. R.; Hansen, P. E.;
Lund, T. Photovoltaic Performance and Characteristics of Dye-
Sensitized Solar Cells Prepared with the n719 Thermal Degradation
Products [Ru(LH)2(NCS)(4- Tert-butylpyridine)][N(Bu)4] and
[Ru(LH)2(NCS)(1- methylbenzimidazole)][N(Bu)4]. Eur. J. Inorg.
Chem. 2011, 2011 (16), 2533−2539.
(56) Andersen, A. R.; Halme, J.; Lund, T.; Asghar, M. I.; Nguyen, P.
T.; Miettunen, K.; Kemppainen, E.; Albrektsen, O. Charge Transport
and Photocurrent Generation Characteristics in Dye Solar Cells
Containing Thermally Degraded N719 Dye Molecules. J. Phys. Chem.
C 2011, 115 (31), 15598−15606.
(57) Bard, A. J.; Faulkner, L. R. Electrochemical Methods:
Fundamentals and Applications; Wiley: Hoboken, NJ, 2001.
(58) Pashaei, B.; Shahroosvand, H.; Gratzel, M.; Nazeeruddin, M. K.
̈
Influence of Ancillary Ligands in Dye-Sensitized Solar Cells. Chem.
Rev. 2016, 116, 9485−9564.
(59) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K.
G. U. A Novel Charge Extraction Method for the Study of Electron
Transport and Interfacial Transfer in Dye Sensitised Nanocrystalline
Solar Cells. Electrochem. Commun. 2000, 2 (9), 658−662.
(60) O’Regan, B. C.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant,
J. The Effect of Al 2O 3 Barrier Layers in TiO 2/Dye/CuSCN
Photovoltage Cells Explored by Recombination and DOS Character-
I
DOI: 10.1021/acs.inorgchem.7b02164
Inorg. Chem. XXXX, XXX, XXX−XXX