Ultrafast interfacial charge transfer from the LUMO+1 in ruthenium(II) polypyridyl quinoxaline-sensitized solar cells

Article abstract of DOI:10.1039/c7dt03769d

This paper describes the implementation of robust and modular sensitizers containing aromatic-amphiphilic ligands to provide new insights into the relationship between the molecular structure and electron injection process governing the efficiency of dye-sensitized solar cells (DSSCs). The significance of this work lies in the combination of favorable experimental and theoretical results in a new class of Ru(ii) polypyridyl complexes with the molecular formula of [Ru(E101)(Dicnq)_x(Y)] which is named M101-M104 when X = 1 and Y = bpy, X = 1 and Y = phen, X = 2, and X = 1 and Y = 2 NCS, respectively. E101 and Dicnq ligands are 1,10-phenanthroline-5,6 heptan ammin and 6,7-dicyanodipyrido[2,2-d:2′,3′-f]quinoxaline, respectively. The good agreement between the experimental and the time-dependent density functional theory (TDDFT)-calculated absorption spectra of the M101-104 sensitizers allowed us to provide a detailed assignment of the main spectral features of the investigated dyes. M102 which contained phen as an ancillary ligand had the best photovoltaic performance which can be attributed to the higher light harvesting of M102 in the visible light region. A DSSC based on complex M102 without the E101 ligand did not show any observable power conversion efficiency (PCE), indicating the importance of the amphiphilic ligand, E101. Transient absorption studies indicated that the ratio of k_reg/k_rec (k_rec = the rate constant of the recombination of the dye and k_reg = the rate constant of regeneration in the presence of the electrolyte) for M101-104 is 1.1, 2.9, 1.3, and 1.2, clearly confirming a weak competition between dye regeneration and recombination. Therefore, because this ratio for M101, 103, and 104 is small, k_reg ≈ k_rec, the operation of the device has been limited by back electron transports, subsequently enhancing the recombination process. However, the rate of recombination is relatively normal for an efficient DSSC, while the rate of regeneration is very low. Subsequently, the PCE will be poor, confirming the role of aliphatic chains in reducing the recombination process. To obtain a deeper insight into the charge transfer process in the investigated devices, ab initio DFT molecular dynamics simulations and quantum dynamics of electronic relaxation were carried out, clearly showing that the interfacial electron transfer (IET) time scale particularly depends on the type of ancillary ligand. The IET results substantially proved that M102 has the fast lifetime of 2.3 ps and 90 fs for the LUMO and LUMO+1, respectively, indicating the experimentally higher PCE of M102 compared to the other three investigated sensitizers.

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Pleas
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s
j ua cs tt io
margins
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s
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DOI: 10.1039/C7DT03769D
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ARTICLE
Ultrafast Interfacial Charge Transfer from LUMO+1 in
Ruthenium(II) Polypyridyl Quinoxaline-Sensitized Solar
Cells
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
a
Hashem Shahroosvand *, Mortaza Eskandari
www.rsc.org/
This paper describes the implementation of robust and modular sensitizers containing aromatic-
amphiphilic ligands to provide new insights into the relationship between the molecular structure
and electron injection process governing the efficiency of dye-sensitized solar cells (DSSCs).
The significance of this work lies in the combination of favorable experimental and theoretical results in a
x
new class of Ru(II) polypyridyl complexes with the molecular formula of [Ru(E101)(Dicnq) (Y)] which is named
M101-M104 when X=1 and Y=bpy, X=1 and Y=phen, X=2 , X=1 and Y= 2 NCS, respectively. E101 and Dicnq
ligands are 1,10 phenanthroline -5,6 heptan ammin and 6,7-dicyanodipyrido[2,2-d:2´,3´-f] quinoxaline ,
respecively. The good agreement between the experimental and the time-dependent density functional
theory (TDDFT)-calculated absorption spectra of the M101-104 sensitizers allowed us to provide a detailed
assignment of the main spectral features of the investigated dyes. The M102 which contained phen as an
ancillary ligand had the best photovotaic performance which can be attributed to the higher light harvesting
of M102 in the visible light region. About the importance of amiphilic ligand (E101) , DSSC based on complex
M102 without E101 ligand was not shown any observable power conversion efficiency (PCE). Transient
ꢀ
ꢁꢂꢃꢄꢅꢆ
absorption studies indicated that the ratio of
Kreg/krec (krec=rate constant of recombination of dye and
ꢀ
ꢂꢃꢄꢇꢆ
k
reg= rate constant of regeneration in the presence of electrolyte) for M101-104 is 1.1, 2.9, 1.3, and 1.2,
clearly confirming a weak comptition between dye regeneration and recombination. Therefore , because this
ratio for M101,103 and, 104 is small, Kreg≈krec, the operation of the device has been limited by back electron
transports, subsequently enhancing the recombination process. However, the rate of recombination is
relatively normal for an efficient DSSC, while the rate of regeneration is very low. Subsequently, the PCE will
be poor, confirming the effect of aliphatic chains in reducing the recombination process To obtain a deeper
insight into the charge transfer process in the investigated devices, Ab-initio DFT molecular dynamics
simulations and quantum dynamics of electronic relaxation were carried out, clearly showing that the
interfacial electron transfer (IET) time scale particularly depends on the type of ancillary ligand. The IET
results substantially proved that M102 has the fast lifetime of 2.3 ps and 90 fs for LUMO and LUMO+1,
respectively, indicating the experimentally higher PCE of M102 compared to the other three investigated
sensitizers.
a.
Chemistry Department, University of Zanjan, Zanjan, IRAN
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Among the different mechanisms of assembling the dye onto a
metal oxide substrate surface including covalent attachment,
electrostatic interaction, hydrogen bonding, hydro-phobic
interaction, Van der Waals forces, or physical entrapment,
DSSCs almost exclusively employ covalent bonding between
1
. Introduction
Among the components of a dye-sensitized solar cell (DSSC),
the ancillary ligand represents outstanding multifunctional
2
the dyes and the TiO surface atoms in order to ensure strong
coupling, a homogeneous dye distribution, and device stability.
photophysical and photochemical abilities and is therefore In contrast, other interactions usually involve weaker bonds
crucial in determining the overall power conversion efficiency
and accordingly render adsorption quasi-reversible and
(PCE) [1]. The use of ancillary ligands having amphiphilic chains unstable [1, 20].
and/or a π-extended aromatic unit permits the inhibition of
Historically, the most frequently used anchors in DSSCs are
the charge recombination process and collection of a large carboxylic acid and cyanoacrylic acid groups [21-27]. However,
fraction of sunlight.
The most common modification in the design of a sensitizer for surface under long irradiation times [27]. Thus, it is pivotal to
dyes with COOH as the anchor unit will dissociate on TiO2
DSSC involves the appropriate choice of an ancillary ligand
through the tuning of the ground and excited-state properties
for achieving better light harvesting. In this regard, several
approaches have been successfully reported for enhancing the
light harvesting abilities of ancillary ligands [2] such as
incorporation of C^N ancillary ligand to construct a ruthenium
cyclometalated sensitizer [3-6]. Some other approaches
include: the extension of π-conjugation through the
incorporation of aromatic rings into different positions of
ancillary ligands for improving the molar extinction coefficient
increase the binding strength of the anchor unit. It is known
that metal scan form strong chelating bonds with the
heterocyclic ligand.
Because of the recent exponential growth of DSSC research,
many new anchors have also emerged and been tested. This
trend has significantly increased the choice of available
materials and facilitated the understanding of DSSCs.
Recently, pyridine [28], phosphinic acid [29], and 8-
hydroxylqui-noline [27] have been investigated as anchoring
groups. Nevertheless, the efficiencies of DSSCs based on these
acceptors still lag behind those of cyanoacrylic acid
(ε) along with the red shift of the absorption spectrum;
introducing electron donating/accepting groups in the ancillary counterparts. The bottleneck of this effort lies in either the low
ligand for effectively matching the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital absorption spectra.
electron-injection efficiency or the limited broadening of
-
3-
(LUMO) of dye with the redox potential of 3I /I and the lower
limit of the conduction band in TiO
In this manuscript, we employed the cyanide group as the
[7]; using amphiphilic long anchoring group as it had received considerable attention in
2
chains which lead to a slow recombination process by
preventing the water-induced desorption of the dye from the Tetracyanate anchors have been included in several organic
recent studies.
TiO surface [8]; and suppressing molecular aggregation on the
2
dyes and demonstrated their capacity to sensitize TiO2
surfaces [30]. However, the PCE of the devices is usually low
surface [9-16]. All these factors are presumed to improve the
overall performance of DSSC through ancillary ligand
substitutions.
(<0.25%). Apart from its coordination capacity, the
tetracyanate group is a strong electron acceptor and has
shown good performance in the presence of additional
anchors such as rhodanines and carboxylic acids [31].
The important features of these chromophores include
intramolecular charge-transfer interactions, potent redox
activities, and nonplanar molecular arrangements. The last
feature is expected to contribute to the suppression of dye
aggregation in DSSCs. In addition, the chromophores do not
possess any carboxylic acid groups [30].
As a remarkable report on the role of amphiphilic ancillary
ligands in DSSC, an interesting amphiphilic polypyridyl
ruthenium dye, Z907, in combination with a photochemically
stable fluorine polymer and poly(vinylidenefluoride-co-
hexafluoropropylene to solidify a 3-methoxypropionitrile-
based liquid electrolyte was synthesized and investigated in
order to obtain a quasi-solid-state gel electrolyte [17]. The
DSSC based on this conjugated system in full sunlight (air mass
−
2
1
.5, 100 mW cm ) showed a conversion efficiency of over 6%
This result suggests the possibility that cyanated derivatives
and resulted in extraordinarily stable cell performance both can efficiently adsorb onto TiO . It is interesting to examine if
2
under thermal stress at 80 °C and soaking with light. In
this is also true for the geminal dicyanated compounds in the
addition, after 1,000 h of light-soaking at 55 °C, the cells present research.
covered with an ultraviolet-absorbing polymer film maintained
These classes of Ru polypyridyl complexes have gained interest
term thermal and chemical stability compared to other
94% of their initial performance. After this exciting report, an due to their more favorable light harvesting abilities and long-
exponential growth was observed in studies and publications
dealing with the modification of ancillary ligands based on conventional sensitizers for the feasible large-scale
amphiphilic and aromatic moieties in DSSCs [18, 19].
We presented a new design concept for ruthenium sensitizers
commercialization of DSSCs. Therefore, the main idea is to
inspire readers to explore new avenues in the design of new
and reported a detailed study of DSSCs based on amphiphilic- sensitizers for DSSCs based on different ancillary and
substituted phenanthroline ligands for the first time. The
careful selection of both aliphatic and aromatic moieties on
ancillary ligands eventually led to the improvement of
photovoltaic performances. The selection of a suitable
anchoring group is the second approach for improving the
efficiency of DSSC.
anchoring ligand substitutions.
2
. The Experiment
2
.1 Materials and Instrument: All chemicals and solvents were
purchased from Merck and Aldrich and used without further
purification. IR spectra were recorded on a Perkin-Elmer 597
spectrometer. H-NMR spectra were recorded using a Bruker
1
2
| J. Name., 2012, 00, 1-3
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2
50 MHz spectrometer. Molecular and electronic structure 2.3 Synthesis of Ligand and Complexes:
calculations were performed with density functional theory 2.3.1. E101 and Dicnq Ligands. Two ligands of 1,10
DFT) using the Gaussian 03(G03) program package. The B3LYP phenanthroline -5,6 heptan ammin (E101), and6,7-
(
functional was carried out with the LANL2DZ basis set. All dicyanodipyrido[2,2-d:2´,3´-f] quinoxaline (Dicnq) were
geometry optimizations were performed in either C1 or C2 synthesized according to Figure 1.
symmetry with subsequent frequency analysis to show that The phendione ligand was synthesised according to the literature
the structures are at the local minima on the potential energy with a high-yield product (%85) [37]. For the synthesis of E101to
surface. In addition, our calculations were performed in the 0.21 g (1 mmol) of phendione in 50 mL of absolute ethanol, 0.15 mL
DMF solvent. The electronic orbitals were visualized using (2 mmol) of 1-heptan amine was added and stirred under reflux
2
Gauss View 3.0 software. The PLQY (PL quantum yields) were condition and N atmosphere for 1 h. The colour of the solution
2
calculated by comparison with [Ru(bpy)3] in degassed CH CN turned from yellow to violet, while the yellow precipitate appeared.
+
3
solution at room temperature as a standard (∅ꢈꢉꢊ = 0.0952) After the reaction was completed, the mixture was centrifuged and
using the following well-known equation:
ꢎ
the yellow sediments were separated. The solid products were
washed with ethanol and water three times to remove un-reacted
free ligands. For the synthesis of Dicnq, the mixture of 0.210 g
ꢏꢐꢑ/ꢒꢏꢐꢑ
ꢖ
ꢏꢐꢑ
ꢆꢗ
∅ꢋꢌꢍ = ∅ꢈꢉꢊ. ꢂ ꢒ
ꢆ. ꢂ
ꢖ
ꢓꢔꢕ
where Φ unk is the PL
ꢓꢔꢕ/ꢎꢓꢔꢕ
quantum yield of ruthenium complexes; Iunk and Istd are the
integrated areas of the corrected PL spectra of the ruthenium
complexes and standard, respectively; Aunk and Astd are the
(
1mmol) of phendione and 0.162 g (1.5 mmol of Dicnq was solved in
0 mL of absolute ethanol and subsequently refluxed under stirring
and N2 atmosphere for 1 h. After that, the volume of the solution
5
absorbances of ruthenium complexes and the standard at the was reduced to about 5 mL by vacuum evaporation. Immediately,
excitation wavelength (λexc= 405 nm); and ηunk and ηstd are the the concentrated solution was placed in the refrigerator until dark-
indices of refraction of the respective solvents (taken to be yellow crystals were formed. After the separation of solids by filter
equal to those of the neat solvents in both cases).
.2 computational studies:
paper, washing by ethanol and water was done three times. The
purity of two products was followed by thin-layer chromatography
2
(
TLC) which clearly showed only one trace of product with a high
The choice of the basis set is a crucial step for DFT and time
dependent density function theory (TD-DFT) calculations. The
number of literature available basis sets for ruthenium
complexes is success by using DFT/B3LYP method. DFT
calculations with the B3LYP functional and LanL2DZ basis set
are able to predict the structures of these complexes quite
reasonably [32]. All calculations were done with 6-31G(d,p)
basis set for the lighter elements (Cl, O, N, C and H). For the
transition metal (Ruthenium), the effective core potential basis
set LANL2DZ (Los Alamos ECP plus double-zeta) was used for
the valence electrons and the core electrons were treated with
LANL2DZ effective core potential. After geometry optimization
a (TDDFT) calculation was performed to assign the transient
absorption spectra of M101-M104 with the B3-LYP/6-31G(d,p)
-1
yield. FTIR (KBr), (υ, cm ): υ 2992, 2852,(CH-Alkyl, s), 1616 (C=N of
1
E101, s), 1616 (CH
2
of Alkyl, s). H NMR data (DMSO-d
0.20 (N-para CHs of E101, d, 2H); 9.70(N-ortho CHs of E101, d,
H);8.32(N-meta CHs of E101, t , 2H); 4.74 (1-CH, s, 1H) 1.43 (2-CH
, t, 2H) ; 1.60 (4-CH , t, 2H) ; 1.71 (5-CH , t, 2H);
,t, 2H) and 0.94 (7-CH - q, 3H).CHN Anal. Calc. for M101,
): calculated: C, 77.014;; H, 10.163; N, 12.835 %. Found: C,
7.022; H, 10. 172; N, 12.838 %, ESI-MS: m/z, 435.36, [M-H].
.4.2. Complexes M101-104 . [Ru(bpy)(Dicnq)(E101)](BF (M101).
.xH
and 0.045 g (1.1 m mol) of bpy was resolved in ethanol and refluxed
under N atmosphere in a three-necked-balloon for 1 h. After that,
6
), (δ: ppm):
1
2
2
,
t, 2H); 1.51 (3-CH
.87 (6-CH
2
2
2
1
(
7
2
2
3
28 44 4
C H N
4 2
)
To synthesize M101, the mixture of 0.06 g (1m mole) of RuCl
3
2
O
2
the precipitated solid was collected and washed with ethanol and
considered as a precursor for the next substitution. To synthesize
[
previous step was resolved in 15 mL of warm 2-propanole and
added to 0.11 g (1.1 mmol) of E101 in 15 mL of warm 2-propanole,
+
LANL2DZ level using an explicit Polarizable Continuum Model
PCM) for solvent. The model system was initially defined
according to the unrelaxed anatase nanostructure. The
semiconductor nanostructure was composed of 64 TiO units,
2
Ru(bpy)(E101)Cl ], 0.1 g (1 mmol) of the solids collected in the
(
2
2
refluxing under N atmosphere for 3 h. Afterwards, 0.084 g (1.1
with angles and bond lengths defined according to available
references [33]. The geometry of the nanostructure was
optimized with the Vienna Ab initio simulation package
mmol) of Dicnq in 5mLof 2 –propane was added to the solution and
subsequently another 3h of the same condition was continued.
4
Finally, an aqueous saturated solution of NaBF (5 mmol) was added
(VASP/VAMP), implementing DFT to treat the Hartree and
to the solution and the mixture was stirred for 1 h. To re-crystallize
the compound, the volume of solution was reduced by vacuum
evaporation. The solution was put in the refrigerator until the
formation of dark brown solids. The crystals were separated by
centrifugation and washed with 2-propanol, ethanol, and water
three times to remove any un-reacted reagents. For all four
complexes, the purification of products was also performed using
Sephadex LH20 as the column support and DMF/Methanol (2:1,
v/v) as the eluent, by which only one layer was extracted and
exchange-correlation electronic interactions. The Perdew-
Wang [34] generalized gradient approximation was
implemented for the exchange-correlation functional in
conjunction with ultrasoft Vanderbilt pseudo potentials [35] to
describe the ionic interactions. The Kohn-Sham (KS)
Hamiltonian was projected onto a plane-wave basis set and
high-efficiency iterative methods were implemented to obtain
the KS eigenstates and eigenvalues. Self-consistency was
accelerated by means of efficient charge density mixing
schemes.
-1
collected. FTIR (KBr), (υ, cm ): 2992, 2852,(CH-Alkyl, s), 2193 (C≡N
of Dicnq, s), 1616 (CH of Alkyl, s).
H NMR data (DMSO-d ), (δ: ppm):δ10.10(N-para CHs of Dicnq, d,
H); 10.06 (N-para CHs of E101, d, 2H); 9.39(N-ortho CHs of Dicnq,
The IET software was obtained from Batista Group and run
considering the conditions reported in the literature [36].
2
1
2
6
Computing the time-dependent wave function |Ψ(t)
Bi(t)|χi
, expanded in the basis set of AOs |χi
, required the
propagation of expansion coefficients Bi(t) )∑qQiqCqexp[-
i/ћ)Eqt], where Cq is the expansion coefficient of the initial
state |Ψ(0) )=∑qCq|q
〉
= ∑i
〉
〉
d, 2H ); 9.31(N-ortho CHs of E101, d, 2H);9.07 (ortho-C quadrupole-
CH of bpy ,d, 1H); 8.47 (N-para CHs of bpy, d, 2H); 8.10 (N-meta CHs
of E101, t , 2H); 8.02(N-meta CHs of Dicnq, t, 2H); 7.73(N-ortho CHs
of bpy , d, 2H); 7.35(N-meta CHs of bpy, t, 2H); 6.51(N-para CHs of
(
〉
〉
.
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bpy, t); 4.76 (1-CH, s, 1H) 1.42 (2-CH , t, 2H); 1.53 (3-CH , t, 2H); mmol) of RuCl .xH O in 30 mL of 2-propanole. The mixture was
2
2
3
2
1
.58 (4-CH
2
, t, 2H); 1.68 (5-CH
2
2
, t, 2H); 1.88 (6-CH ,t, 2H) and 0.92 refluxed and stirred under N2 atmosphere for 3 h. After that, 0.11 g
(
7-CH - q, 3H).CHN Anal. Calc. for M101, (C H B F N Ru): (1 mmol) of E101 was added to the mixture and refluxed for 2 h.
3 56 60 2 8 12
calculated: C, 56.313; H, 5.254; N, 14.592 %. Found: C, 56.323; H, Next, a saturated aqueous NaBF
.259; N, 14.595 %, ESI-MS: m/z, 1151.42, [M-H]. under stirring for 1 h. Finally, the volume of the solvent was
Ru(phen)(Dicnq)(E101)](BF (M102). The procedure of the reduced to reach a concentrated solution (5 mL) by vacuum
4
solution was added to the mixture
5
[
4 2
)
synthesis of M102 was the same as that of M101 except that the evaporation. The concentrated mixture was put into the
bpy ligand was replaced by phen (0.057 g, 1.1 mmol). FTIR (KBr), (υ, refrigerator to produce a dark-brown crystal solid. The solid
-
1
cm ): 2992, 2852,(CH-Alkyl, s), 2193 (C≡N of Dicnq, s), 1616 (CH of products were separated by centrifugation and then washed with 2-
2
1
-1
Alkyl, s). H NMR data (DMSO-d6), (δ: ppm):δ 10.08(N-para CHs of propanole and water. FTIR (KBr), (υ, cm ): 2992, 2852, (CH-Alkyl, s),
Dicnq, d, 2H), 9.39 (N-para CHs of E101, d, 2H), 9.32(N-ortho CHs of 2193 (C≡N of Dicnq, s), 1616 (CH of Alkyl, s). 1H NMR data (DMSO-
2
Dicnq, d, 2H ), 8.97 (ortho-C quadrupole- CH of phen ,d, 1H), d6), (δ: ppm): 9.96(N-para CHs of Dicnq, d, 4H), 9.39 (N-para CHs of
8
8
7
7
1
.51(N-ortho CHs of E101, d, 2H), 8.45 (N-para CHs of bpy, d, 2H), E101, d, 2H), 9.32(N-ortho CHs of Dicnq, d, 4H ),8.36(N-ortho CHs of
.10(N-meta CHs of E101, t , 2H), 8.05(N-meta CHs of Dicnq, t, 2H), E101, d, 2H), 7.95(N-meta CHs of E101, m , 2H), 7.85 (N-meta CHs of
.74(N-ortho CHs of phen , d, 2H), 7.33(N-meta CHs of phen, t, 2H), Dicnq, m, 4H),4.89 (1-CH, s, 1H) 1.42 (2-CH , t, 2H), 1.53 (3-CH , t,
2
2
.49(N-para CHs of phen, t), 4.76 (1-CH, s, 1H) 1.43 (2-CH
2
, t, 2H), 2H), 1.58(4-CH
, t, 2H), 1.89 (6- 0.92 (7-CH
- q, 3H). CHN Anal. Calc. for M102, calculated: C, 56.393; H, 4.575; N, 17.543 %;
2
, t, 2H) , 168 (5-CH
2
, t, 2H), 1.88 (6-CH
2
,t, 2H) and
.54 (3-CH
2
, t, 2H), 1.59(4-CH
2
, t, 2H) , 1.69 (5-CH
2
3 58 2 8
- q, 3H).CHN Anal. Calc. for M103, (C60H B F N16Ru):
CH
2
,t, 2H) and 0.93 (7-CH
3
54 60 2 8
(C H B F N12Ru): calculated: C, 57.205; H, 5.144; N, 14.291 %;.
Found: C, 57.208; H, 5.150; N, 14.293 %, ESI-MS: m/z, 1175.42, [M-
H].
[Ru(Dicnq)
2 4 2
(E101)](BF ) (M103). For the synthesis of M103, 0.17 g
(2.1 mmol) of Dicnq ligand was added to a solution of 0.06 g (1
Figure 1. The chemical process of the synthesis of E01 and Dicnq (3-CH
2
, t, 2H), 1.59(4-CH
2
, t, 2H) , 1.69 (5-CH2, t, 2H), 1.89 (6-CH2,t,
q, 3H).CHN. Anal. Calc. for M104,
ligands
2
H) and 0.93 (7-CH
3
-
(
C H N RuS ): calculated: C, 58.894; H, 5.594; N, 17.924 %;
46 52 12 2
Found: C, 56.323; H, 5.259; N, 14.595 %, ESI-MS: m/z, 1278.42, [M-
H].
Found: C, 58.897; H, 5.601; N, 17.927 %, ESI-MS: m/z, 937.29, [M-
H].
[Ru(Dicnq)
2 2
(E101)(NCS) ] (M104).The procedure of the synthesis of
M104 was similar to that of M103 except that 0.09 g (1.1 mmol) of
2
2
.5. Fabrication of DSSC
.5.1 Fabrication of Porous-TiO Electrodes. To prepare the DSSC
2
Dinq and 0.06 g (2.2 mmol) of KSCN were added to RuCl
g, 1 mmol).
FTIR (KBr) (υ, cm ): 2992, 2852,(CH-Alkyl, s), 2193 (C≡N of Dicnq, s),
3 2
.xH O (0.06
-1
working electrodes, the transparent conductive oxide (TCO) glass
plate used as the current collector was first cleaned in a detergent
solution using an ultrasonic bath for 15 min and was then rinsed
with water and ethanol. Afterwards, the TCO glass plates were
1
2
102 (C=N, s),1616 (CH2 of Alkyl, s), 806 (C=S, 806). H NMR data
DMSO-d ), (δ: ppm): δ 10.02(N-para CHs of Dicnq, d, 2H), 9.88 (N-
para CHs of E101, d, 2H), 9.48(N-ortho CHs of Dicnq, d, 2H),9.27(N-
ortho CHs of E101, d, 2H), 8.02(N-meta CHs of E101, t , 2H), 7.81(N- and washed with water and ethanol. A layer of TiCl was coated on
(
6
4
immersed into a 40mMaqueous TiCl solution at 70 °C for 30 min
4
meta CHs of Dicnq, t, 4H),4.90 (1-CH, s, 1H) 1.43 (2-CH
2
, t, 2H), 1.54 TCO glass plates by screen-printing and kept in a clean box for 3 min
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so that the layer could relax in order to reduce surface
irregularities. The plates were then dried for 6 min at 125 °C. After
H C
3
CH
2
H C
drying the films at 125 °C, the mesoscopic TiO
2
films used as
2
CH3
CH2
CH2
N
H C
N
N
(
a)
2
photoanodes consisted of layers of TiO
layer (6+4+3μminthickness) of 25nm TiO
2
(a 13μm thick transparent
anatase nanoparticles and
2
,2-bipyridine (bpy)
H C
CH2
2
H C
2
2
M101=[Ru(bpy)(E101)(Dicnq)](BF4)2
a 10μm thick scattering layer of 400nm anatase TiO
multi-layer films were heated to 520 °C and sintered for 30 min,
then cooled to 80 °C and immersed into the dye solution at room
temperature for 16 h.
.5.2 Preparation of Counter Pt-electrodes. To prepare the counter
electrode, the TCO sheet was washed with H O and 0.1M HCl
2
particles). The
CH2
Amphiphilic chain H C
H C
2
2
CH2
∼
N
N
N
1
,10-phenanthroline (phen)
2
N
M102=[Ru(phen)(E101)(Dicnq)](BF4)2
2
N
solution in ethanol and cleaned by ultrasound in an acetone bath
for 10 min. After removing the residual organic contaminants by
heating in air for 15 min at 400 °C, the Pt catalyst was deposited on
N
N
NC CN
Ancillary ligand
Ru
N
N
N
N
2 6
the TCO glass by coating with a drop of H PtCl solution (2 mg of Pt
in1 mL of ethanol) while repeating the heat treatment at 400 °C
for15 min.
N
N
N
Dicnq
Anchoring ligand
2
.5.3 DSSC assemblage. The dye-covered TiO
2
electrode and Pt-
N
M103=[Ru(E101)(Dicnq) ](BF )
CN
2
4 2
counter electrode were assembled into a sandwich-type cell. Two
clips were used to hold the two electrodes together at the corner of
the plates. The electrolyte was injected into the space between the
two electrodes.
M104=[Ru(E101)(Dicnq)(NCS)2]
NC
3
. Results and Discussion
This section is organized as follows: first, in Sections 3.1 and 3.2, we
outline the fundamental properties of the molecular structure and
photophysical properties of compounds, promising an efficient
sensitizer for use in DSSC. In Section 3.3, we present the results
followed by comparative experimental studies of DFT and TD DFT to
investigate the electronic structure of the excited states of
sensitizers. Results and discussions of the photovoltaic performance
of DSSC based on all four examined complexes are presented in
Section 3.4. The Charge career dynamic studies including transient
absorption microscopy and time resolved photoluminescence decay
Figure 2. (a) The molecular structures of the investigated complexes
named M101-104. The three parts of complexes including ancillary,
anchoring, and amphiphilic ligands as well as the type of ancillary
ligands are highlighted. (b) The optimized structure of the sensitizer
produced from DFT calculation.
are explained in section 3.5
.
Finally, details of the interfacial
electron transfer simulation (IETs) are discussed in Section 3.6
.
Moreover, the microanalysis including elemental analysis (CHN) and
ESI-Mass was carried out to determine the structure of complexes.
HNMR and FT-IR are very useful tools to characterize the structure
3
.1 An introduction to the reasonable aspects of the design of
1
complexes
The logic of designing the molecular structure of complexes is
rationally based on three functions: (i) incorporation of ligand E101
which contains an aliphatic carbon chain and reduces the
aggregation phenomena; (ii) the presence of an ancillary ligand
including bpy, phen, and NSC in order to enhance light harvesting
abilities; and (iii) introduction of Dicnq as an anchoring ligand with
the CN group which coordinately links into the semiconductor
surface. All three parts cooperatively contribute to enhance
electron regeneration and reduce charge recombination. Two
remarkable novelties in the design of molecules can be observed: (i)
the attachment of aliphatic carbon chains through the nitrogen
atom to phen moiety, and (ii) the attachment of the CN group to
the phen quinoqxaline moiety. In a certain sense, both anchoring
and ancillary ligands meticulously designed based on phen indicate
the versatile substitution of the phen ligand. The above
explanations are represented in Figure 2 as well as the optimization
of the molecule.
of complexes containing functional groups which show unique
spectroscopic evidence. The coordination of E101 and dicnq to
ruthenium in M101-M104 was clearly affirmed by 1HNMR spectra
via the assignment of their peaks to aliphatic (0.9-2.0 ppm) and
aromatic (7.5-10 ppm) regions, respectively [38]. Regarding the
heptan amine ligand as an important part of E 101, five methylene
2 2
groups of 2-CH to 6-CH and the terminal methyl group appeared
as distinct resonances at 1.42, 1.53, 1.58, 168, and 0.92 ppm,
respectively. In the FT-IR study, the obvious sharp signal in the
-1
wavenumber of approximately 2200 cm can be attributed to the
CN group of quinoxaline moiety, indicating the presence of Dicnq in
the structure of complexes [39]. FT-IR spectra along with the
assignment of finger-print regions are illustrated in ESI. Fig. S1.
3
.2 UV-Vis and Photoluminescence Spectroscopies
The UV-vis of complexes was experimentally investigated in the
dimethyl formamide (DMF) solution (Fig. 3) and subsequently the
nature of every excitation was confirmed by TD DFT studies. In the
UV-Vis spectra of complexes, two regions appeared: (i) the sharp
band in the region of the wavelength of 300-350 nm which can be
attributed to the intra-ligand charge transfer, ILCT, (π→π*) of
aromatic moieties, and (ii) the broad band in the region of 450-600
nm which differs by the substitution of ancillary ligand. The origin of
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the higher wavelength transition can be attributed to the metal-to- Figure 3. Left: the UV-Vis of M101-104 in the acetonitrile solution
2
+
-5
ligand charge transfer (MLCT) from the filled t2g of Ru
to the (10 M) (blue star). The calculated UV-Vis by TD DFT method (Red
,
empty orbitals of ligand s π*[40]. The zoom region was introduced circle). Right: PL spectra of M101-104 in acetonitrile at room
-5
from visible to near-infra-red regions where lightest harvesting temperature (10 M). The highlight of experimental UV-Vis spectra
would occur. The molar extinction coefficients (ε) at the maximum shows the active area of light harvesting in DSSC.
wavelength of MLCT were estimated to be about 12150, 14375,
-1
-1
8
720, and 11900 cm
M for M101-104, respectively, indicating 3.3 DFT and TD-DFT Studies
that M102 showed a higher ε than benchmark emitters such as N3 To achieve a deeper insight to the band absorption assignment, we
-1
-1
with the ε of ≈13550 cm
M in the same condition. However, it carried out conventional TD-DFT calculations (singlet-singlet states
should be kept in mind that the maximum wavelength of MLCT of only contributing to absorption) on the examined complexes. The
N3 occurs at approximately 540 nm which is significantly red shifted results confirmed the general assignment of low-energy features to
about 80 nm compared to M102, resulting in the more efficient N3 MLCT and higher energy ones to ILCT excitations (Figure 4 and Table
compared to M102 and others. According to this ordering, when a 1). The full set data of the optimized molecular structure is given in
bidentate ancillary ligand in M101-M103 was replaced by two NCS ESI.S3. As appeared in TD DFT results, the most contribution of the
ligands, the lower symmetry of the complex compromised the transition of the lowest energy of MLCT absorption band took place
degeneracy of MLCT transitions, leading to a broader absorption from HOMO to LUMO, nearly 70%. Interestingly, higher energy
band [14]. This remarkable change originated from the fact that the bands of all the studied complexes mainly originated from HOMO-1
electron density of the highest occupied molecular orbital (HOMO) and HOMO -2 to LUMO +1 and +2. The data are summarized in
is mainly located on the NCS group, resulting in the instability of Table 1. The origin of ICLT bands was largely based on lower HOMO
HOMO and, subsequently, a bathochromic shift in the MLC band (HOMO-1 and -2) to higher LUMO (LUMO +3). DFT calculation
[
42].
clearly showed that the density of electron in HOMO mainly located
The photoluminescence (PL) experiment was carried out in the on the ancillary ligand and metal center. Nevertheless, the density
acetonitrile solution with the excitation of 405 nm (Fig.3). All four of electron in LUMO was delocalized on Dicnq moiety. The DFT
complexes demonstrated a broad band centred at 612, 632, 601, calculation clearly pointed out that the isosurface of HOMOs and
and 616 nm for M101-104, respectively, indicating a MLCT which LUMOs of all complexes showed that there is no desnsity of
was extensively reported in homologue ruthenium polypyridyl electron on the aliphatic chain, E101 ligand. Considering these two
complexes [43]. The tuning of the PL wavelength of complexes also results, we concluded the major influence of the aromatic ancillary
indicated the influence of ancillary ligand substitution (ESI. Fig. S2). ligand on electron density distribution on molecules that contained
PL quantum yields (PLQY )were calculated by comparison with both aromatic and aliphatic ligands.
2
+
[
Ru(bpy)
3 3
] in degassed CH CN solution at room temperature as a It is necessary to note that, for an ideal electron transfer in a DSSC,
standard (∅ꢈꢉꢊ = 0.095), resulting in the PLQY of 0.048, 0.051, the electron density of electron in HOMO must be delocalized in the
0
.054, and 0.031 for M101-104, respectively [44].
metal and ancillary ligand (donor moiety), while the electron
density of electron in LUMO must be delocalized in the anchoring
group (acceptor moiety), confirming the best condition for electron
transfer from the LUMO ofa sensitizer to the conduction band of a
semicounductor [45]. As clearly seen in Figure 5, the above
mentioned condition for an ideal electron tansfer prevails for all
four investigated complexes, M101-104.
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Figure 4. The electronic density between the ground state and the first electronic excited state of M101 that mainly contributes to the
absorption and emission spectra as further represented by the color difference plot.
Figure 5. Orbital energy levels and isosurfaces for the HOMO and LUMO of complexes obtained from the DFT method through the
B3LYP/LANL2DZ basis set
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Table 1: The first two triplet states for complexes M101-104 calculated using the TDDFT approach
M101
M102
Energy
nm)
Osc
Strength
0.0008
0.007
0.0252
0.0012
Major contribsMinor contribs
Energy
(nm)
521.53
494.01
490.53
488.89
Osc.
Strength
0.0007
0.0177
0.0039
0.0123
Major contribsMinor contribs
(
5
4
4
4
24.24
93.21
92.27
86.61
HOMO->LUMO (69%),HOMO->L+3(14%)
H-1->LUMO (49%), HOMO->L+1 (47%)
H-1->LUMO (47%), HOMO->L+1 (51%)
H-2->LUMO (68%), H-2->L+3 (12%)
HOMO->LUMO (68%), HOMO->L+3 (14%)
H-2->LUMO (25%), H-1->LUMO (64%)
HOMO->L+1 (68%), H-1->LUMO (11%)
H-2->LUMO (64%), H-1->LUMO (23%)
M103
M104
MO
Energy (eV)
0.0014
0.0002
0.042
0.0165
Ru
MO
Energy (eV)
0.0035
0.0054
0.0042
0.0449
Ru
5
4
4
4
4
00.96
93.52
73.67
71.80
69.38
HOMO->LUMO (69%)HOMO->L+5(14%)
HOMO->L+1 (67%), HOMO->L+6 (19%)
H-2->LUMO (38%), H-1->LUMO (55%)
H-2->LUMO (54%), H-1->LUMO (36%)
HOMO->L+2 (65%), H-2->LUMO (14%)
543.01
478.71
451.52
299.69
286.35
HOMO->LUMO (70%)
HOMO-1>L+1 (70%)
H-2->LUMO (34%), H-1->LUMO (61%)
H-2->L+3 (29%), HOMO->L+4 (56%)
H-2->L+3 (50%), H-1->L+3 (26%), HOMO->L+4 (30%)
0.0033
0.0635
3
.4 Photovoltaic characterization
To determine the role of the ancillary ligand on its photovoltaic
performance, the DSSCs based on the examined complexes were
fabricated as explained in Section 2.5. It is very important to note
that our purpose was not to find a champion sensitizer to record a
high power conversion efficiency (PCE), although the establishment
of the employed strategies mentioned here certainly promises the
design of sensitizers which can show a high PCE.
As shown in Fig. 6 and Table 2, I-V, the results indicated that M102
which contains phen as the ancillary ligand has the best
performance among all four complexes, followed by M103 which
has two Dicnqs
M104 and M101 showed the lowest PCE, respectively. As one E101
and one Dicnq in all four complexes were unchanged and only the
ancillary was altered, we are able to correlate the nature of the
ancillary ligand with its photovoltaic performance. It is noteworthy
that, in a separate test, M102 without the presence of E101 in its
structure had a very weak performance, indicating the importance
of the presence of the aliphatic chain ligand, E101. These results
can be interpreted as follows: (1) In comparing with the two
ancillary ligands, the phen showed more efficiency than bpy; (2)
NCS groups would be effective only when at least one aromatic
ancillary ligand is also located in the molecular structure; (3) Dicnq
can act as both ancillary and anchoring ligand. However, because of
the presence of CN as the withdraw group, the influence of its
ancillary role is weak.
Figure 7. IPCE spectra of DSSC based on M101-104
To find the origin and the reason of the noted proposals, we first
focused on UV-Vis spectra, resulting in the light harvesting
properties of complexes. According to Figure 2, the highest
absorption intensity (molar extinction coefficient) belonged to
M102 which was also shown by the broadening of absorption band
in the MLCT region. In order to confirm the light harvesting
properties of complexes and its correlation with the type of
ancillary ligand, incident photon to current conversion (IPCE) was
carried out (Figure 7). IPCE results also confirmed the I-V results, i.e.
M102 had the highest IPCE value of about 29%, indicating the
highest light harvesting abilities (Table 2).
Table 2. I-V and IPCE values for M101-104 and N3
2
Dye
J
sc(mA/cm )
Voc(V) FF(%) η(%) IPCE(%)
M101 0.54
M102 1.91
M103 1.02
M104 0.74
0.45
0.43
0.47
0.52
0.8
39.9
56.6
65.1
66.9
0.44
0.12 19.8
0.58 29.2
0.39 9.8
0.34 16.4
4.00 54.7
N3
11.2
The FT-IR empirical method involving the identification of
vibrational frequencies surrounding the anchoring groups is widely
accepted in the scientific literature [46]. The FT-IR spectra of dyes
(
M101-104) adsorbed on TiO
of coordinate bonds between the CN group of Dicnq and the Lewis
acid sites of the TiO surface. To confirm this inference, the FT-IR
spectra of the dyes adsorbed on TiO -coated glass slides were
collected. Figure 8 illustrates the FT-IR spectra of the individual dyes
2
nanoparticles indicated the formation
2
Figure 6. I-V spectra of DSSC based on M101-104
2
(
2
M101-M104) and when it is adsorbed on TiO films. When a dye-
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based CN anchoring group, M101-104, was coordinated to TiO
ARTICLE
2
capacity to sensitize TiO
2
surfaces [48-50] via the formation of a
surface, two distinguished characteristic bands in FT-IR spectra can surface complex through the nucleophilic reaction of the cyano
-1
be recognized: one, a broad band in the region of 400-600 cm
2
group with superficial hydroxyl groups on TiO surface. Thus, the
which can be attributed to the vibration mode of Ti-O bond and, interfacial charge transfer transition was enhanced by the
later, the disappearing of the vibration of C≡N bond at the generation of a negatively charged CN adsorbate [51].
-1
wavenumber of 2200 cm , confirming the attachment of cyanide
group to the surface [47]. However, it was reported that the cyano-
based compound containing organic dyes demonstrated the
2
Figure 8. The FT-IR spectra of dyes (M101-104) and adsorbed dyes onto TiO surface. The disappearing of the cyano group in dyes
absorbed on TiO is highlighted.
2
2
Figure 9. Transient absorbance decay profiles obtained upon pulsed laser excitations on TiO films sensitized with dyes (M101-104), with
(
red circle) and without (dark square) the presence of LiI/I
2
electrolyte upon laser excitation at 500 nm. Solid lines are the fits obtained
after using the bi-exponential decay model.
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Table 3. Calculated lifetime(τ) and constant rate (k) for dyes (M101-104) with and without electrolyte. The employed parameter of bi-
exponential decay model included. Nonlinear Curve Fit, Iteration Algorithm: Levenberg Marquardt, Model: ExpDec , Equation: y = A *exp(-
x/t ) + A *exp(-x/t ) + y
No
2
1
1
2
2
0
M102
M104
M103
M101
Without
electrolyte
With
electrolyte
Without
electrolyte
With
12.359 (1.625)
10.725 (1.221)
5.961 (0.981)
13.435 (0.478)
11.852 (0.084)
0.074 (0.002)
0.084 (0.0034)
τ
4.280 (0.306)
0.080 (0.0108)
0.233, (0.016)
8.559 (1.739)
0.093 (0.021)
0.116 (0.023)
4.632 (1.89)
0.167 (0.0912)
0.215 (0.0174)
k
electrolyte
3
3
. 5 Charge career dynamic studies
. 5. 1. Transient absorption microscopy
Transient absorption microscopy was employed to investigate the
kinetics of dye regeneration (kreg) by the electrolyte and the kinetics
of the recombination (Krec) of photoinjected electrons in the
semiconductor with the oxidized dye in the absence of the
electrolyte [52]. The device operation of a DSSC directly depends on
the competition between Kreg and K rec . In the normal condition, the
kinetic of recombination is much slower than that of regeneration,
Kreg >> Krec,, resulting in the charge recombination between oxidized
dyes, and the charge-injected electrons are not an important loss
for limiting device efficiency [53]. The decay upon the induced light
absorption was monitored by laser excitation of the dye M101-
M104-sensitized TiO , in the presence and absence of electrolyte
2
(Figure 9), with the lifetime and constant rate summarized in Table
3
. For all four samples, there is a weak competition between Kreg
and Krec. However, the decay data clearly showed that the
regeneration of dye M10 by reduction by electrolyte (k= 0.233 ms)
is almost threefold (2.9) faster than by recombination with TiO
2
photoinjected electrons (k= 0.0.080). This competition between Kreg
and Krec (Kreg/Krec) for M101, M103, and M104 is about 1.1, 1.3, and
1
.2, respectively. These results indicate thatM101, M103, and M104
show Kreg ≈ Krec, confirming that the back electron transfer reaction
and recombination process limit the PCE of devices. However, in an
useful comparison, the half time of regeneration and recombination
of the champion sensitizer named N719, cis-bis(thisionato)bis(2,2-
bipyridine-4,4-dicarboxylato)ruthenium(II)
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Figure 10. Photoluminescence decay kinetics (PLDK) measured at λ max upon excitation at 408 nm. For PLDK experiments, solid lines are
the fits obtained after using the bi-exponential decay model. The inset of figures shows the emission lifetime and corresponding controlling
parameters for M101-104 complexes obtained using the bi-exponential decay model.
corroborate the photovoltaic performance and transient absorption
microscopy result discussed previously.
ditertbutylammonium salt), is about 1 μs and 1ms, confirming
the fact that the order of τreg of investigated dyes is in the range
of N719, while the τreg is much slower than N719 [54]. This
relatively faster recombination can be attributed to aliphatic
chains in the ancillary ligand (E101) which are absent in N719 .
3.6 Interfacial Electron Transfer Simulation (IETs)
In addition to the experimental tools for determining the
different attachment modes of an anchoring group of dye to a
semiconductor, theoretical efforts such as DFT calculation have
extensively been employed to study the interface phenomena in
DSSC. Among the powerful simulation software programs for
3
.5.2. Time-resolved photoluminescence decay
To probe the effect of the ancillary ligand on the charge carrier
dynamics, the time-correlated single photon counting (TCSPC)
technique was employed. Figure 10 presents the photoluminescence
decay kinetics (PLDK) measured at λ max upon excitation at 408 nm.
Solid lines are the fits obtained after using the bi-exponential decay
model with details given in the inset. Note that the extension of the
lifetime of the sensitizer significantly influenced the lowering of the
recombination process, subsequently enhancing the power
conversion efficiency. Using the bi-exponential decay model, the PL
decay results indicated that M102 showed a longer lifetime
estimating interfacial electron transfer in dye/TiO -anatase
2
surface, Ab-initio DFT molecular dynamics simulations and
quantum dynamics of electronic relaxation have widely been
used [56]. In summary, the process of sensitization in DSSC
starts via light absorbing by the dye attached onto TiO surface,
2
allowing to excite the electron from the steady state of dye to
its excited state. After that, the generated charge goes to the
2
conduction band of TiO and, therefore, currents in the external
circuit [57]. The zoomed region of IET studies conceptualized the
rate of electron transfer from the LUMO of the dye to the
conduction band of the semiconductor, identified as the
formation of a complex of an excited dye with a small size of
(
τ2=411.44±31.37ns) than the other three complexes. This extremely
long lifetime is among the best values for ruthenium polypyridyl
complexes which routinely show lifetimes in the range of 1-100 ns
[
55]. As given in the inset of Figure 10, the value of the lifetime (τ
2
) of
TiO
sufficient for optoelectronic property calculations on TiO
dye/TiO nanocomposites for DSSC applications [58]. The IET
simulations were performed using the supercell M101-104-TiO
2
2
cluster (TiO
2
)
n
(n is generally <80). This small cluster size is
dyes was estimated in the range of 350- 450 nm with the order of
M102 (411.44±31.37 ns)>M103 (399.172 ± 19.73 ns) >M104
(
2
and
2
386.47±16.17 ns) >M101 (350.41 ± 9.83 ns). Also, these results
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with lattice vectors which are given in Figure11. The efficient
S4-S6 for M101, M103 and M104). Similar to the LUMO + 1
dynamics, the electron injected from the LUMO state is
distributed to all Ti ions in the cluster in ps time scale.
electron transfer in dye/TiO
matching between the energy level of LUMO of dye and
conduction band of TiO which is precisely pointed out by the
2
surface strongly depends on the
2
The evolution of time-dependent charge distribution during
early-time relaxation dynamics after instantaneously populating
all four dyes was also studied. After fitting the survival
probability over time with the bi-exponential decay, we
calculated the lifetime of charge carriers from the sensitizer to
the semiconductor. This exponential fitting describes the
probability that the photo-excited electron remains in the
adsorbate molecule at time τ after the photo-excitation of the
system. The shorter the lifetime of an electron injection in dye-
density of state (DOS) [57]. The electronic coupling strength
indicates that the photoinduced charge transfer has effectively
occurred. DOS figures clearly depict the quantity of overlapping
of the LUMO of dye and conduction band of TiO . As shown in
2
Figure 12, all four dyes consist of a number of LUMOs which
have shown a good overlap degree with the conduction band of
TiO
2
. Therefore, we concluded that the attachment of the CN
surface is possible and electron transfer
anchor group onto TiO
2
happens subsequently. In what follows, we address the
quantities of the rate of interfacial electron transfer and the
population of LUMOs along with time.
TiO
higher PCE. The values of lifetimes of electron injection form
LUMO, LUMO+1of M101-104 to the conduction band of TiO are
2
surface, the faster the electron injection rate, resulting in a
2
summarized in Table 4. As stated in TD DFT explanation, the
MLCT of M101-104 was mainly originated from LUMO and
LUMO+1, demonstrating the fact that the fastest IET rate ∼2 ps,
is observed for the LUMO orbital of M102. The fastest IET rate
of LUMO+1 orbitals is investigated for M 104 with the τ of 70 fs.
Nevertheless, the average value of the lifetimes of LUMO
1 1 2
(τ .=2.0 ps) and LUMO+1(τ .=1.0 fs and τ .=90.0 fs)of M102 is
the shortest among all four sensitizers (Fig. 14 and ESI. Fig. S7-
S9). This evidence confirms the higher PCE of M102 than the
other three sensitizers, indicating a higher regeneration process
and a lower recombination process.
Table 4. Estimated lifetime of LUMO and LUMO+1 of M101-
M104 via the fitting of the survival probability with the bi-
exponential decay model.
Dyes
M101
M102
M103
M104
LUMO
2.3 ps
2.0 ps
2.1 ps
2.3 ps
LUMO+1
1 fs, 230 fs
1 fs, 90 fs
1fs, 75 fs
70 fs
2
Figure 11. Unrelaxed configuration of the TiO -anatase model
nanostructure sensitized with a sensitizer anchored on the (101)
surface of the crystal under vacuum conditions. Inset table: the
supercell dyes-TiO with lattice vectors (a, b, and c) optimized
2
with VASP.
Figure 13 depicts snap shots of the first 1 fs of the time-evolved
electron density after instantaneous photoexcitation of the
functionalized nanocrystal to M102 LUMO + 1 (Please see ESI.
1
2 | J. Name., 2012, 00, 1-3
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a
d
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o
o nn o
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t
ra
a
n
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j ua cs tt io
margins
n
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Figure 12. The density of states (DOS) obtained from the extended Hu¨ckel method for the M101-104-anatase model nanostructure. The
black line shows the valance band (left) and conduction band (right) of TiO . The filled color curve represents the DOS projected on the
2
basis functions of the adsorbate M101-104.
Figure 13. Snapshots of the electronic charge distribution at 1 fs after initiating the IET from M102 attached to a pristine (101) surface. Only
the local TiO structure, next to the photoexcited adsorbate, is illustrated for the detailed view of the time-dependent charge distribution.
2
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Figure 14. Computed survival probabilities corresponding to the photoexcited surface M102. Each curve represents the probability of the
electron to remain in the respective photoexcited surface complex. The estimated lifetime of LUMO-LUMO+3 of M102 is obtained from the
fitting of the survival probability with the bi-exponential decay model.
Finally, it is important to compare the results reported in this paper state, shows ultrafast IET with an injection time scale of
with the results of another sensitizer, i.e. TiO
Batista indicated that the rate of IET transfer from the LUMO orbital characterizations clearly indicate that the IET from LUMO is in the
of catechol to TiO at room temperature is τ=6 fs [59]. However, range of ps, while the lifetime of LUMO+1 happens in the fs range.
IETs on the catechol anchoring group which substituted terpy, Given the fast injection and the low energy of the transition relative
2
. In a pioneer study, approximately 12 fs [63]. Considering the above results, our IET
2
∼
II
2+
[
Mn (H
indicated a typical electron injection rate in the range of 50-100 fs. to be ideal.
60]. In an interesting report, Hirendra Nath Ghosh et al have been Now, if we go back to I-V results, it appears that M104 had the most
synthesized Ru(II)- and Os(II)-polypyridyl with pendent open voltage circuit among all other sensitizers as well as an ultra
O)
2 3
(catecholterpy)] /TiO
2
(terpy
)2,2′:6,2′′-terpyridine) to the LUMO state, injection from the M101-104 LUMO + 1 appears
[
acetylacetone (acac) functionality which Time-resolved transient fast injection electron, especially from LUMO+1., demonstrating
absorption spectroscopic studies in the femtosecond time domain M104 as a promising modified structure. Therfore, we propose that
have been carried out. electron injection process can be found to the fast IET contributes to lowering charge recombination from the
-
be fitting with two time constants, a pulse width limited < 120 fs conduction band of TiO2 to I /3I electrolyte energy level which
3
component and a slower 1.2-1.4 ps component . Results of such determines the value of Voc
.
studies reveal that the electron injection rates from the metal It is worth pointing out that the main contribution of LUMO+1 in all
complex based LUMO to the conduction band of the TiO2 are faster four dyes is exclusive to the absorption band in the range of 470-
that one would expect for analogous complex in which the 490 nm in UV-Vis spectrum (the molar extinction coefficient of UV-
chromophoric core and the anchoring moiety is separated with Vis and the IPCE value of M102 are significantly higher than those of
multiple saturated C-C linkages [61].
others).
In light of IETs investigations on the acetyl acetonate anchoring Further studies on efficient sensitizers are in progress, and we
group, the charge injection rate from LUMO of -4-acac-coumrin believe that improvements can be achieved by a thorough
molecule to TiO surface was estimated to be about 15 fs [62]. In optimization of aliphatic-aromatic composite structures with
2
what follows, the simulation of IET using the 3-(4- respect to, for example, carboxylic anchoring groups, π-extended
Nitrophenyl)pentane-2,4-dione (NPA-H)LUMO + 2 as the initial aromatic ancillary ligands of M102, and functionalized M102.
1
4 | J. Name., 2012, 00, 1-3
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contribute to the recognition of the main excited states which
effectively make the injection of electron from LUMOs to the
conduction band of TiO2.
4. Conclusion
Although a variety of sensitizers have been reported in dye-
sensitized solar cells (DSSCs) as the most promising component
to achieve high PCE, the preparation of a new sensitizer is still
required. In particular, the new generation of the architecture
of a sensitizer is based on multifunctional ancillary ligands
Acknowledgments: The authors wish to thank the University
of Zanjan for financial supports.
which guaranties
photochemical properties.
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electron regeneration rate in the LUMOs of sensitizers to the
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2
(
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This journal is © The Royal Society of Chemistry 20xx
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