The molecular engineering, synthesis and photovoltaic studies of a novel highly efficient Ru(ii) complex incorporating a bulky TPA ancillary ligand for DSSCs: Donor: versus π-spacer effects

Article abstract of DOI:10.1039/c9ra06150a

A novel Ru(ii) complex, denoted as IA-7, incorporating a bulky donor antenna, was synthesized and characterized as a promising inexpensive alternative to conventional p-n junction solar cells to study the influence of a bulky donor antenna on the light ha

Full text of DOI:10.1039/c9ra06150a

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PAPER
The molecular engineering, synthesis and
photovoltaic studies of a novel highly efficient Ru(II)
complex incorporating a bulky TPA ancillary ligand
for DSSCs: donor versus p-spacer effects†
a
Cite this: RSC Adv., 2020, 10, 610
and Ahmed El-Shafei*b
Islam M. Abdellah
A novel Ru(II) complex, denoted as IA-7, incorporating a bulky donor antenna, was synthesized and
characterized as a promising inexpensive alternative to conventional p–n junction solar cells to study the
influence of a bulky donor antenna on the light harvesting efficiency (LHE), ground and excited state
oxidation potentials and total conversion efficiency of sunlight to electricity (% h) for dye-sensitized solar
cells (DSSCs), and the device performance was compared to devices with MH-12 and MH-13 dyes. The
incorporation of the bulky donor enriched triphenylamine (TPA) antenna resulted in a considerable
increase in JSC and h values for DSSCs, where IA-7 outperformed MH-12 and MH-13 in terms of the total
conversion efficiency and achieved a power conversion efficiency (h) of 8.86% under full sunlight
ꢀ2
irradiation (100 mW cm ), compared to 8.09% for MH-12 and 8.53% for MH-13, which can be ascribed
to the high open circuit voltage (VOC) of IA-7. Molecular engineering utilizing DFT/TD-DFT was
employed to calculate the electronic properties of IA-7, including the HOMO/LUMO isosurfaces, the
lowest singlet–singlet electronic transitions (E0–0), and the ground and excited state oxidation potentials,
which were in ideal agreement with the empirical results. The electronic distribution of IA-7 indicated
that the HOMO is delocalized not only on Ru and NCS, but also on the substituted TPA, and the LUMO is
Received 7th August 2019
Accepted 9th December 2019
DOI: 10.1039/c9ra06150a
0
rsc.li/rsc-advances
delocalized over 4,4 -bipyridyl dicarboxylic acid.
11
levels and articial light sources. Moreover, the working
standards of DSSCs include the ingestion of a photon by the
1
. Introduction
One of the huge difficulties that mankind faces is exchanging sensitizer, and the change to an energized state, which injects
petroleum derivatives with sustainable power sources, while an electron into the semiconductor conduction band. The
combating the growing energy requirements and exhaustion of injected electron travels through the mesoporous semi-
energy resources. This challenge must be answered with low- conductor arrangement of particles to land at the back-collector
cost, clean and generously available crude materials. The sun anode and then through the external circuit to the cathode to
is a clean and cheap energy source, and supports life on earth. diminish the oxidized iodide and recover the sensitizer. Some
Accordingly, harvesting solar energy with photovoltaic innova- negative responses prompt a decline in the cell efficiency; for
tions gives the impression of being the main logical response to example, recombination of the injected electrons either with
12
the energy challenge. One of the fascinating advancements is the oxidized sensitizer or with the redox electrolyte. Subse-
1
DSSCs, which were investigated by O'Regan and Gr ¨a tzel. DSSCs quently, accomplishing higher efficiency devices is reliant on
have attracted all-inclusive academic and business interest the optimization and compatibility of all of the components in
because of their extraordinary performance under diffuse light the device at the same time, specically the semiconductor lm
1
3–15
conditions and low illumination levels, and their independence surface area onto which the dye can be adequately absorbed.
2–10
of the incident light angle. DSSCs have numerous advantages Ru(II)-based complexes have been shown to be viable sensitizers
compared to silicon-based solar cells, including transparency, for DSSCs in light of their uncommon metal to ligand charge
low cost, and high-power conversion efficiencies under low light transfer (MLCT) transitions, unique excited state photostability
and photophysics. The most efficient ruthenium constructs for
DSSCs were made by the Gr ¨a tzel group; for example, N719, N3
1
6
a
b
and black dyes. The exceptional light harvesting and durability
Department of Chemistry, Faculty of Science, Aswan University, Aswan, 81528, Egypt
Polymer and Color Chemistry Program, Department of TECS, North Carolina State features of these photosensitizers are based on the MLCT
University, Raleigh, 27606, USA. E-mail: Ahmed_El-Shafei@ncsu.edu
transition, through which the photoelectric charge is moved to
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra06150a
the TiO
2
at a quicker rate than electron recombination with the
1
610 | RSC Adv., 2020, 10, 610–619
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oxidized dye molecules, instead of moving through the spectrophotometer was used for FTIR spectroscopy analysis.
17–20
circuit.
Clearly, research on dye preparation for DSSC The mass spectra were acquired on a Thermo Scientic EXAC-
applications is becoming increasingly sophisticated and many TIVE (ESI-MS) spectrometer. The UV-visible and uorescence
21,22
ruthenium-based structures have been reported.
The most spectra were obtained utilizing SPECORD S600 and Horiba
favorable strategy for constructing effective sensitizers for Fluromax-4 spectrophotometers.
DSSCs is to molecularly build the sensitizer to exhibit
predominant light harvesting, while maintaining all of the
2
.2. Synthesis and structural characterization
2.2.1. 4-(Bis(4-bromophenyl)amino)benzaldehyde (1). In
a 100 ml three necked ask, 4-(N,N-diphenylamino) benzalde-
thermodynamic and kinetic requirements for electron injec-
tion, hole replenishment and charge recombination
suppression.
Herein we report the preparation of a new complex (IA-7) hyde (1 mmol, 1.366 g) was added to DMF (50 ml), under
based on a bulky triphenylamine ancillary ligand to form a nitrogen atmosphere, and the reaction mixture was cooled to
ꢁ
a highly light harvesting and effective donor, which is directly ꢀ5 C with an ice bath. While the solution was stirred in the
linked to Ru metal. The newly designed IA-7 has been synthe- dark, a dropping funnel was used for the drop-wise addition of
0
0
sized in three steps, starting with the preparation of 4-(bis(3 ,5 - N-bromo succinimide (2 mmol, 1.77 g) dissolved in DMF (15
0
ꢁ
dimethoxy-[1,1 -biphenyl]-4-yl)amino) benzaldehyde through ml), and the mixture was stirred for 2 hours at 0 C. Aer 2
ꢁ
Suzuki–Miyaura coupling, followed by Knoevenagel condensa- hours, the solution was stirred overnight at 25 C. The reaction
tion reaction with 4,4 -dimethyl-2,2 -bipyridine to form the mixture was extracted by equal amounts of water and chloro-
0
0
required ancillary ligand and nally target dye formation in form. The organic layer was separated using a funnel, dried over
a one-pot three-step reaction as shown in Scheme 2. The MgSO and evaporated under pressure to form a yellow solid,
4
synthetic routes and structure of IA-7 are depicted in Schemes 1 which was puried via silica gel chromatography (CHCl : Hex.)
3
and 2. Moreover, the photovoltaic and photophysical properties (3 : 1) and crystallized in methanol. Yield (97.41%), melting
ꢁ
ꢀ1
of IA-7 incorporating a bulky donor ancillary ligand were point (mp) 161 C. FT-IR (cm ): 2832 (C–H aldehydic), 1682
compared to the well-known photosensitizers MH-12 and MH- (C]O), 1573 (C]C aromatic), 1270 (C–N aromatic amine), 724
1
1
3 (ref. 23) incorporating p-spacer ancillary ligands to illustrate (C–Br). H NMR (600 MHz, chloroform-d, d): 9.75 (s, 1H), 7.73–
the effect of donor and p-spacer ancillary ligands on the PCE.
7.67 (m, 2H), 7.33 (m, 6H), 7.05–6.99 (m, 4H). MS: m/z
+
Fig. 1 depicts the chemical structure of IA-7 carrying a bulky (C H Br NO) found ¼ 431.942 (calcd 431.941 For [M + H] )
1
9
13
2
donor ancillary ligand, along with the MH-12 and MH-13 with an error of DM ¼ 2.23 ppm.
0 0 0
2.2.2. 4-(Bis(3 ,5 -dimethoxy-[1,1 -biphenyl]-4-yl)amino)
sensitizers carrying p-spacer ancillary ligands. The newly
synthesized molecules were characterized using different benzaldehyde (2). 4-(Bis(4-bromophenyl)amino)benzaldehyde
1
analytical methods; for example, H-NMR, FT-IR and high- (0.431 g, 1 mmol) and (3,5-dimethoxyphenyl)boronic acid
resolution mass spectroscopy analysis. Their electronic prop- (0.4 g, 2.2 mmol) were dissolved in 40 ml of THF and two drops
erties and band gap were resolved from UV-Vis absorption and of aliquat-336 solution were added. The reaction mixture was

uorescence emission studies. Furthermore, the energetics of then degassed under argon for 30 minutes and
the GSOP-ESOP thermodynamic parameters was determined tetrakis(triphenyl-phosphine)palladium(0) (0.068 g, 0.06 mmol)
from the cyclic voltammetry technique. Finally, the dyes were was added, followed by the addition of K CO solution (2 M, 5
2
3
ꢁ
used as photosensitizers towards the fabrication of DSSCs to ml), and the mixture was stirred for 10 hours at 85 C. The
evaluate their photovoltaic parameters, such as IPCE, PCE, JSC reaction was tracked by TLC until completion. The resulting
OC, and FF. Also, EIS studies were performed to evaluate the mixture was cooled and extracted with methylene chloride, and
recombination resistance, the transport resistance, the capaci- the organic layer was dried over Na SO , and ltered. Aer
tance of electron accumulation and measurements of electron removing the solvent, the compound was puried by silica gel
,
V
2
4
lifetimes in the fabricated DSSCs.
chromatography by using chloroform as an eluant to give
ꢁ
a yellow solid. Yield (61%), melting point (mp) 174 C. FT-IR
ꢀ1
(
cm ): 2954 & 2834 (C–H, alkane), 1687 (CO),1306 (C–N, aryl).
2
. Experimental
1
H NMR (600 MHz, chloroform-d, d): 9.74 (s, 1H), 7.66–7.60 (m,
H), 7.45–7.40 (m, 4H), 7.19–7.09 (m, 6H), 7.09–7.04 (m, 2H),
The starting materials, including N-bromosuccinimide, 4-(N,N- 6.52–6.50 (m, J ¼ 7.9 Hz, 4H), 3.42 (s, 12H). ESI-FTMS: m/z
2.1. Materials and methods
2
+
diphenylamino)benzaldehyde, (3,5-dimethoxyphenyl)boronic (C35
H
31NO
5
): found ¼ 546.22794 (calcd 546.2275 for [M + H] ).
0
0
0 0 0 0
acid, aliquat-336, K
2
CO
3
, Pd(PPh
3
)
4
, 4,4 -dimethyl-2,2 -dipyridyl
2.2.3. N,N -(((1Z,1 E)-[2,2 -Bipyridine]-4,4 -diylbis(ethene-
0
0
0
and trimethylsilane, were purchased from Sigma-Aldrich, Alfa 2,1-diyl))bis(4,1-phenylene))bis(N-(3 ,5 -dimethoxy-[1,1 -
0
0
0
0
Aesar and Ark Pharm companies. In addition, 2,2 -bipyridinyl- biphenyl]-4 yl)-3 ,5 -dimethoxy-[1,1 -biphenyl]-4-amine) (3).
0
4
,4 -dicarboxylic acid and the MH-12 and MH-13 photosensi- Ligand 3 was synthesized in a pressurized glass tube containing
23,24
0
0
0
0
tizers were synthesized according to the reported procedure
4,4 -dimethyl-2,2 -bipyridine (0.184 g, 1 mmol), 4-(bis(3 ,5 -
0
(see ESI†). All required solvents were ordered from Fischer dimethoxy-[1,1 -biphenyl]-4 yl)amino) benzaldehyde (2) (0.54 g,
1
Scientic. A Bruker advance 400 MHZ was used to obtain the H- 1 mmol), and 0.76 ml of chlorotrimethylsilane (6 mmol) in
NMR spectra, applying DMSO-d
dard for chemical shi calibration.
6
and TMS as an internal stan- 60 ml of anhydrous DMF. The tube was sealed well with the cap,
ꢁ
A
Bruker alpha the reaction temperature was raised to 110 C and the reaction
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ꢁ
Scheme 1 Synthetic routes for the bulky antenna ligand (3). (i) Aliquat-336, K
2
CO
3
, tetrakis(triphenylphosphine)palladium(0), H
2
O, THF, 90 C, 4-
0
0
ꢁ
boronobenzoic acid, and 5-formyl-2-thiopheneboronic acid; (ii) 4,4 -dimethyl-2,2 -dipyridyl, chlorotrimethylsilane, pressure tube, 115 C.
was allowed to run for 48 hours with continuous stirring, during with 40 ml anhydrous DMF, dichloro-(p-cymene)-ruthenium(II)
which time the color of the reaction mixture changed to dark dimer (0.3 g, 0.5 mmol) and antenna ligand 3 (1.23 g, 1 mmol).
orange. Aer 48 hours, the tube was cooled, the pressure of the The mixture was stirred using a magnetic stirrer bar under
ꢁ
0
0
tube was released, and the solvent was evaporated using a rotary argon at 90 C for 6 h. Then, 2,2 -bipyridyl-4,4 -dicarboxalic acid
evaporator leaving a dark orange crude product, which precip- was added (0.244 g, 1 mmol), the temperature was raised to
ꢁ
itated by adding 150 ml of icy water. Finally, vacuum ltration of 150 C, and the reaction was allowed to keep running for 5
the antenna ligand was performed, followed by washing with hours. Aer 5 hours, an excess of NH NCS (0.5 g) was added,
4
plenty of water and toluene. The crystals were collected and and the reaction mixture was permitted to keep running for
ꢁ
ꢁ
dried overnight at 50 C. The antenna ligand was recrystallized a further 4 h at 140 C. The reaction mixture was chilled off to
from methanol to form pure dark red crystals. Yield (79%), room temperature and DMF was removed utilizing a rotary
ꢁ
ꢀ1
1
melting point (mp) 190 C. FT-IR (cm ): 2936&2833 (C–H, evaporator. Ice was added to the ask and a black solid was
alkane), 1608 (C]N), 1293 (C–N, aryl). H NMR (600 MHz, precipitated. The solid was ltered, washed with diethyl ether,
chloroform-d, d): 9.00–8.94 (m, 2H), 8.12 (d, J ¼ 5.0 Hz, 1H), and dried. Aer drying, the dye was dissolved in methanol with
8
7
6
6
.09–8.02 (m, 3H), 7.68–7.62 (m, 2H), 7.58–7.51 (m, 3H), 7.49– the addition of a few drops of tetrabutylammonium hydroxide
.29 (m, 15H), 7.27–7.18 (m, 6H), 7.12 (t, J ¼ 2.0 Hz, 1H), 7.00– (TBAOH) until the pH of the solution became slightly basic, and
.91 (m, 2H), 6.74–6.55 (m, 6H), 6.52 (t, J ¼ 2.0 Hz, 1H), 6.38– then it was run through a column containing silica gel. The
.36 (m, 2H), 6.27 (t, J ¼ 2.0 Hz, 1H), 5.88 (t, J ¼ 2.0 Hz, 1H), 3.85 violet main band was collected and acidied using 0.1 M HCl to
(
1
s, 24H). ESI-FTMS: m/z (C82
H
70
N
4
O
8
) found ¼ 1239.522 (calcd reduce the pH to 2.0 and allowed to precipitate for 48 hours in
a refrigerator. The precipitate was then ltered, and washed well
.2.4. Synthesis of photosensitizer IA-7. Complex IA-7 was with water to bring the pH to neutral. The pure dye was then
synthesized in one pot using a three-step reaction. The reac- dried overnight and collected as dark color crystals. Yield (69%),
+
239.526 for [M + H] ) with an error of DM ¼ 0.812 ppm.
2
ꢁ
ꢀ1
tions were carried out in a 150 ml ask. The ask was charged melting point (mp) 267 C. FT-IR (cm ): 3409 (OH), 2935 &
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ꢁ
0
Scheme 2 Synthetic routes for the dye IA-7. (iii) Dichloro-(p-cymene)-ruthenium(II) dimer, anhydrous DMF, 95 C, 5 h, under argon gas; (iv) 2,2 -
bipyridinyl-4,4 -dicarboxylic acid.
0
2
834 (C–H, alkane), 2101 (NCS), 1724 (C]O), 1586 (C]N), 1278 preparation of aldehyde 2 utilizing C–C Suzuki coupling inter-
1
(C–N, aryl). H-NMR (600 MHz, DMSO-d , d): 9.10–9.03 (m, 2H), action, followed by the preparation of bulky antenna ligand 3
6
8
4
6
7
.74 (d, J ¼ 7.5 Hz, 2H), 8.65 (d, J ¼ 1.5 Hz, 2H), 7.38–7.32 (m, with the help of a Knoevenagel condensation reaction (Scheme
H), 7.36–7.15 (m, 7H), 7.08–7.00 (m, 2H), 6.96–6.87 (m, 5H), 1). The synthesized aldehyde and the antenna ligand were
.74 (dd, J ¼ 7.4, 1.5 Hz, 1H), 6.66 (t, J ¼ 1.9 Hz, 2H), 6.29 (dd, J ¼ puried and recrystallized.
.5, 1.4 Hz, 1H), 3.85 (s, 12H). ESI-FTMS: m/z (C96
H
78
N
8
O
12
S
2
Ru)
In the nal step, the target Ru(II) complex (IA-7) was obtained
+
found ¼ 1701.432 (calcd 1702.433 for [M + H] ) with an error of by following a one-pot three-step reaction protocol, wherein the
DM ¼ 1.40 ppm.
precursor antenna (3) was reacted with dichloro-(p-cymene)-
0
0
ruthenium(II) dimer, and then 2,2 -bipyridyl-4,4 -dicarboxalic
and ammonium thiocyanate to form the target complex (IA-7)
(Scheme 2). All of the new compounds and the target sensitizer
were puried and recrystallized utilizing column chromatog-
raphy techniques. The structures of the newly synthesized dyes
3
3
. Results and discussion
.1. Synthesis and characterization
The synthetic pathways of the Ru(II) complex (IA-7) are portrayed
in Schemes 1 and 2. The synthesis was started by the
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Fig. 1 Chemical structures of the IA-7, MH-12, and MH-13 complexes for DSSCs.
and their intermediates were conrmed by various spectral which corresponds to electron transition from the HOMO
techniques as shown in Fig. S1–S14 (see ESI†).
energy level to the LUMO energy level.
The UV-Vis spectra of all of the complexes are characterized
by metal–ligand internal charge transfer transitions in the
visible region from 400–700 nm. The molar extinction coeffi-
cients of these bands are in the following order: IA-7 > MH-13 >
MH-12. It is clear from the UV-Vis absorption spectra that the
peak positions of the lowest energy MLCT bands of the MH-12
and MH-13 sensitizers are slightly blue shied by 8 nm and
3
.2. Photophysical and electrochemical studies
The UV-Vis absorption (emission) spectra of IA-7 were measured
ꢀ
5
in methylene chloride (2 ꢂ 10 M) solution. The spectra are
depicted in Fig. 2 and the corresponding data are outlined in
Table 1. The obtained optical properties for the photosensi-
tizers MH-12 and MH-13 were compatible with the reported
7
nm, respectively, when compared with complex IA-7, and the
23
data. From the UV-Vis absorption spectra, all of the dyes
possess three distinctive absorption bands. The band in the
region of 310–320 nm is attributed to p–p* electronic transition
of the bipyridine ligand, while the peak corresponding to
wavelengths in the region of 400–445 nm can be ascribed to
ligand-to-ligand charge transfer (LLCT) mixed with metal-to-
ligand charge transfer (MLCT) (pd–p*), and the broad peak at
molar extinction coefficient of the new complex is higher. The
higher extinction coefficient of the IA-7 complex is attributed to
0
0
presence of the 4,4 -triphenyl amine-2,2 -bipyridine ligand,
which contains extended p-conjugation of the phenyl groups
with substituted methoxy groups and the directionality of the
excited state by perfect tuning of the ligand LUMO energy level
with the donating groups. As a result, the strong electron donor
ancillary ligand nature in IA-7 is responsible for the increase in
545–565 nm corresponding to the longest wavelength can be
credited to metal-to-ligand charge transfer (MLCT) (pd–p*),
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ꢀ
1
1
00 mV s scan rate using 0.1 M tetra-n-butyl ammonium
+
ꢀ
hexauorophosphate ((n-Bu) N (PF ) ) as a supporting elec-
4
6
trolyte in acetonitrile under an argon atmosphere. From the
voltammograms, it is observed that all of the dyes display two
characteristic energy levels; the rst is the ground state oxida-
tion potential (GSOP) energy level in the range of ꢀ5.46 to
ꢀ
5.56 eV and the second is the excited state oxidation potential
ESOP) energy level at about ꢀ3.58 to ꢀ3.71 eV. The GSOP
(
energy level was determined from the onset oxidation potential
Oxd
(E
) of the oxidation peak of the cyclic voltammogram,
Onset
utilizing eqn (1).
Oxd
GSOP ¼ ꢀ[EOnset + 4.7] eV
(1)
It is observed that IA-7 attained the lowest GSOP level value
as a result of the existence of the bulky donor incorporating
methoxy groups in the molecule, which lowered the GSOP level
Fig. 2 The normalized absorption and emission spectra of IA-7
ꢀ5
recorded in CH Cl
2 2
(2 ꢂ 10 M).
2
of the dye, taking it away from the CB of TiO , and pushed the
GSOP level far from the electrolyte potential. To study more
precisely the probability of dye injection and regeneration
molar extinction coefficient in the visible region as compared to processes, the excited state oxidation potentials (ESOPs) were
the thiophene p-spacer ligands in MH-12 and MH-13.
computed from the optical bandgap (E0–0) and GSOP values by
The emission spectra of all of the dyes display characteristic using eqn (2).
l
in the region of 790–810 nm. It is seen that the lemi of IA-7
emi
ESOP ¼ [GSOP ꢀ E0–0] eV
(2)
(
806 nm) is red shied when compared to that of MH-12 (796
nm) and MH-13 (803 nm). The perceived red shi is attributed
to the presence of a strong electron donor in the IA-7 complex.
The intersection of the absorption and uorescence spectra
generates the optical band-gap values. Their optical band gaps
increase in the following order: MH-13 (1.82 eV) > MH-12 (1.85
eV) > IA-7 (1.88 eV) and the Stokes shis are: MH-13 (249 nm),
IA-7 (245 nm), and MH-12 (243 nm). Among the tested dyes, IA-7
exhibited the highest molar extinction coefficient, which
translated into better population of the excitonic state and
photocurrent density due to the existence of the strong and
bulky electron donor ancillary ligand of TPA.
Cyclic voltammetry (CV) studies were performed for IA-7,
MH-12 and MH-13 to investigate the thermodynamics of elec-
tron movement from the excited states of the dyes to the CB of
TiO and the dye regeneration from the electrolyte redox couple
to the oxidized dyes. The voltammograms are presented in the
It was noticeable that the data obtained for the GSOP and
ESOP of the dyes (MH-12 and MH-13) were found to be the
2
3
same as previously reported. The predicted GSOP levels of IA-
(ꢀ5.46 eV), MH-12 (ꢀ5.56 eV), and MH-13 (ꢀ5.51 eV) were
7
ꢀ
ꢀ
observed to be more negative than the I /I electrolyte system
3
2
6
potential. Meanwhile, the calculated ESOP levels were
observed to be IA-7 (ꢀ3.58 eV), MH-12 (ꢀ3.71 eV) and MH-13
(
ꢀ3.69 eV), which are energetically higher than the CB
potential of TiO
2
. The photophysical and CV results are tabu-
lated in Table 1. The energetic diagrams of complexes IA-7,
ꢀ
ꢀ
MH-12 and MH-13 compared to TiO2 and the I /I3 redox
couple are outlined in Fig. 3. From the results, the dyes IA-7,
MH-12 and MH-13 are thermodynamically favorable for elec-
tron injection and dye regeneration in the fabricated devices.
The electron injection free energy was observed to be in the
order of IA-7 > MH-13 > MH-12 and this explains the superior
thermodynamic stability for the IA-7 complex compared to the
MH-12 and MH-13 complexes; this is due to the more ther-
modynamically favorable electron injection into the conduc-
2
25
ESI (see ESI, Fig. S15†). The measurements were performed at
2
a conventional three electrode conguration, comprising
a platinum disc as a passive electrode, glassy carbon (GC) as the
working electrode, and Ag/AgCl as the reference electrode. The
potentials are reported vs. ferrocene as a standard reference at
ꢁ
5
C using a Vertex electrochemical workstation with
tion band edge of TiO , which decreases charge recombination
2
in the fabricated devices.
Table 1 Comparison of the optical and electrochemical properties of the IA-7 complex in (2 ꢂ 10ꢀ5 M) methylene chloride solutiona
Stokes shi
ꢀ
1
ꢀ1
b
Dye
lmax (nm)
3
max (M cm
)
(nm)
Emax (nm)
I (nm)
E
0–0 (eV)
GSOP (eV)
ESOP (eV)
IA-7
561 (d–p*)
35 841
245
806
659
1.88
ꢀ5.46
ꢀ3.58
a
lmax: maximum absorption wavelength, 3max: molar extinction coefficient maxima, Emax: wavelength of maximum emission, I: intersection of the
b
absorption and emission spectra, and E0–0 : experimental optical band gap.
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out for IA-7 in order to probe their electronic excitations. TD-
DFT was performed on the optimized structure via the corre-
lation functional B3LYP and the DGDZVP basis set. From TD-
DFT calculations and GSOP and ESOP energy level calcula-
tions, clearly the theoretical results are in good agreement with
the experimental data, and electron injection for the IA-7
0
complex is thermodynamically favorable as it s ESOP, ꢀ3.46, is
positively shied compared to the calculated conduction band
edge of TiO
2
(ꢀ4.04 eV) as shown in Table 2.
3.4. Photovoltaic performance of DSSCs
27
DSSCs were fabricated as reported and more specics are
given in the ESI.† Furthermore, photovoltaic and incident
photon-to-current efficiency (IPCE) estimations were accom-
Fig. 3 An energy diagram of the complexes IA-7, MH-12 and MH-13 plished for solar cell devices, which were prepared using
ꢀ
ꢀ
redox couple. GSOP and ESOP are
compared to TiO
2
and the I /I
3
platinum-coated counter electrodes and TiO
anodes, which were prepared by using 0.2 mM dye solutions in
: 1 : 1 acetonitrile, tert-butanol and DMSO solvents; the co-
absorbent chenodeoxycholic acid (CDCA, 20 mM) was used in
2
-covered photo-
reported together with the HOMO–LUMO energy gap (S0–S1).
1
3
.3. Theoretical calculations
28–31
the solution, which acts as an anti-aggregation reagent.
The
The FMO isosurface delocalization in the HOMO/LUMO of the dyes were absorbed on the mesoporous TiO
2
electrode. The two
IA-7 complex was evaluated via applying density functional electrodes were sealed together aer adding redox electrolyte
theory (DFT) and time-dependent density functional theory (TD- (Solaronix, Iodolyte HI-30) consisting of a solution of 0.6 M
DFT). All DFT and TD-DFT calculations were performed through DMPII, 0.05 M I
2
, 0.1 M LiI and 0.5 M TBP in acetonitrile. The
North Carolina State University's High-Performance Computing photovoltaic parameters of the fabricated DSSCs were specied
ꢀ
2
utilizing GAUSSIAN 09 soware to calculate the molecular under an Oriel solar simulator (AM 1.5 G, 100 mW cm
)
32
geometry of the complex, electron delocalization over the attached to a Keithley 2400 digital-source meter. The photo-
HOMO/LUMO energy levels and vertical electronic excitation. current efficiency (h) is given in eqn (3) as a function of different
33
DFT estimations were executed utilizing the hybrid functional photovoltaic parameters and the incident light power (Pin).
B3LYP and LANL2DZ basis set. The resulting 3D optimized
J
SC
V
P
OC FF
h ¼
(3)
structures of the dyes with frontier molecular orbitals of the
HOMO–LUMO isosurfaces are represented in Fig. 4. The opti-
mized structure of the lowest energy conformer shows
a carbon–carbon double bond with E E-conformation. As ex-
pected, from the HOMO energy levels, the electron density is
predominantly delocalized on the bulky TPA units. However,
the LUMO energy level is shied from the electron donor to the
electron acceptor units (the bipyridyl ligand incorporating the
anchoring groups). The results from FMO revealed that dye IA-7
showed effective charge separation, which translated into
greater ICT behavior in the IA-7 complex and high photovoltaic
performance. On the other hand, TD-DFT studies were carried
in
The photovoltaic performance of the fabricated DSSCs with
IA-7, MH-12, and MH-13 is portrayed in Fig. 5a and the photo-
voltaic parameter information, such as VOC, JSC, FF and h, is
outlined in Table 3. The results indicate that dye IA-7 achieved
ꢀ
2
a maximum PCE of 8.86% (JSC ¼ 22.60 mA.cm , VOC ¼ 0.71 V,
and FF ¼ 56.19) compared to the PCE of 8.09% (JSC ¼ 20.23
ꢀ
2
mA.cm , VOC ¼ 0.65 V, and FF ¼ 56.39) for MH-12 and PCE of
ꢀ
2
8
.53% (JSC ¼ 22.97 mA.cm , VOC ¼ 0.66 V, and FF ¼ 56.41) for
MH-13. The J–V data indicate that the PCE of the IA-7 complex
achieved a higher total conversion efficiency compared to MH-
Fig. 4 Optimized geometry and FMO energy levels for the IA-7 complex.
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Table 2 Calculated HOMO–LUMO energy levels and lowest TD-DFT Table 3 Photovoltaic parameters of the fabricated DSSCs under the
a
ꢀ2
)
excitation energies for IA-7 compared to TiO
2
nanoparticles
illumination of AM-1.5 solar light (100 mW cm
Experimental Sensitizer
ꢀ2
Calculated energy (eV), TD-DFT
(eV)
(0.2 mM)
CDCA (mM)
JSC (mA cm
)
VOC (V) FF (%) h (%)
b
Dye
E
LUMO
E
HOMO GSOP ESOP
E
0–0¼(S
0
–S
1
)
E
0ꢀ0
IA-7
MH-12
MH-13
20
20
20
22.60
20.23
22.97
0.71
0.65
0.66
56.19
56.39
56.41
8.86
8.09
8.53
IA-7 ꢀ3.087 ꢀ5.34 ꢀ5.34 ꢀ3.48 1.86
1.88
3.20
TiO
2
ꢀ3.44
–S
ꢀ7.16 ꢀ7.16 ꢀ4.04 3.12
a
E0–0 ¼ (S
0
1
) ¼ the lowest vertical excitation energy; GSOP ¼ ground
state oxidation potential ¼ EHOMO; ESOP ¼ excited state oxidation
b
potential ¼ ELUMO; E0ꢀ0 ¼ experimental energy gap calculated from
in the region of 350–700 nm with a maximum value of 73%,
while MH-13 exhibits a broad band with a maximum value of
the point of intersection for the absorption and emission spectra
2 2
(CH Cl ).
69% and MH-12 shows a wide band in the region of 375–600 nm
with a maximum value of 55%. The superior IPCE of IA-7
translates into a higher PCE value of 8.86% compared to that of
MH-13 (8.53%) and MH-12 (8.09%).
12 and MH-13; this is due to the open circuit voltage (VOC) of
From the IPCE curves (Fig. 5b), it is evident that the response
of complex IA-7 is superior and extends into the near infrared
region more than that of MH-12 and MH-13. Moreover, the IPCE
rises gradually until it reaches over 73% for complex IA-7 in the
high energy region (570 nm), compared to MH-12 and MH-13,
and these data are in good agreement with the UV-Vis spectrum
data of these complexes.
MH-13 being decreased by DVOC ¼ 0.06 V as compared with
complex IA-7. The outstanding open circuit voltage (V ) of the
OC
IA-7 complex is attributed to the presence of a strong electron
rich donor (Bulky-TPA) motif as an ancillary ligand in the dye
molecule, which substantially enhanced charge injection into
the CB edge of TiO , while the MH-12 and MH-13 ligands
2
possess only p-spacers.
Furthermore, IPCE experiments were conducted using the
same fabricated devices utilizing the QEX10 spectral response
measurement system. IPCE quantitatively characterizes the
amount of current that a cell will produce when irradiated with
photons at a given wavelength in the solar spectrum and is
determined by eqn (4), where JSC is the photo-current density, q
is the elementary charge, l is the wavelength of incident irra-
3
.5. Electrochemical impedance studies (EIS)
The electrochemical impedance spectra were obtained from
a Biologic SP-150 potentiated under illumination using a solar
simulator. The EIS measurements were performed to under-
stand the transportation of electrons and interfacial charge
recombination behavior in the fabricated solar cells.
diation, and P
0
is the incident irradiation intensity.
34–36
The
ꢀ
ꢁ
Nyquist and Bode plots of the fabricated DSSCs with IA-7, MH-
2, and MH-13, respectively are provided in Fig. 6a and b. In the
Nyquist plots shown in Fig. 6a, a higher frequency of the
semicircle relates to the series ohmic resistance (R ), whereas
the high-frequency region gives the interface capacitance (CPt
and charge transport resistance (RPt) at the counter electrode/
J
SC
IPCE ðlÞ ¼ 1240
(4)
1
qlP
0
S
The IPCE curves of IA-7, MH-12 and MH-13 are depicted in
Fig. 5b. It is obvious that the IPCE of IA-7 displays a broad band
)
ꢀ2
Fig. 5 (a and b): Curves of solar cells sensitized with IA-7, MH-12, and MH-13 under 100 mW cm light intensity: (a) J–V curves and (b) IPCE
curves.
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Fig. 6 (a and b): Electrochemical impedance spectra: (a) Nyquist plots and (b) Bode plots for DSSCs sensitized with IA-7, MH-12, and MH-13.
electrolyte interface. Similarly, the low-frequency region is species and electron transfer into the TiO semiconductor lm.
2
attributed to chemical capacitance (C ) and the charge recom- Hence, the decrease in VOC for the MH-12 and MH-13 cells can
m
bination (RCT) resistance at the TiO /dye/electrolyte interface. be explained by their faster recombination relative to that of
2
The bigger the radius of the semicircle, the higher the charge complex IA-7.
ꢀ
ꢀ
recombination resistance from TiO
be.
2
to the I
However, a decrease in the radius of the semicircle was
observed in the order of MH-12 > MH-13 > IA-7. This trend
3
/I electrolyte will
37,38
4. Conclusions
2
demonstrates that the recombination at the TiO /dye/electrolyte
In summary, we have designed and developed a novel Ru(II)
photosensitizer, denoted as IA-7, based on a bulky TPA antenna
ligand for DSSC applications. The spectrophotometric, electro-
chemical, and device performance response studies of IA-7 were
interface is IA-7 > MH-13 > MH-12, and the high recombination
resistance of IA-7 compared to the other reported dyes enhances
electron injection into the mesoporous TiO nanoparticles and
2
so increases the overall efficiency of DSSCs. The observed
results are consistent with the achieved PCE values of IA-7, MH-
compared to our previously reported MH-12
& MH-13
complexes. Interestingly, the complex IA-7 achieved a maximum
12 and MH-13.
power conversion efficiency (PCE) of 8.86% (J ¼ 22.60 mA
The Bode frequency plots arise from electron transport at the
SC
ꢀ
2
cm ) compared to 8.09% for MH-12 and 8.53% for MH-13. The
2
TiO /electrolyte interface. The peaks from the MH-12 and MH-
13 complexes in the Bode phase plots in Fig. 6b are at higher
device performances of all dyes displayed an enhanced VOC
value for the complex IA-7, which can be claried based on its
frequencies than that from the IA-7 complex, indicating that the
electron lifetime of IA-7 is higher than that of the MH-12 and
MH-13 complexes. The effective lifetime (seff) of electrons
capacity to suppress charge recombination processes. The
superior PCE of IA-7 can be assigned to the high molar
absorptivity of IA-7 for the MLCT peak and the superiority in
light harvesting through the bulky electron donor antenna
ligand compared to the MH-12 & MH-13 dyes, which incorpo-
rate p-spacer antenna ligands. Conclusively, the donor strength
(5) in the structural motif of the dye is a key factor in enhancing the
total conversion efficiency in DSSCs.
injected into the CB of TiO
corresponding frequency peak (f) in the Bode phase plot, as
given in eqn (5)
2
was estimated as a function of the
.
ꢀ
ꢁ
1
s
eff
¼
2
pf
The effective lifetime seff is calculated for the IA-7, MH-12,
and MH-13 sensitized devices to be IA-7 (8.93 ms) > MH-13 (5.87
ms) > MH-12 (2.0 1 ms), respectively. On a fundamental level,
a longer electron lifetime indicates better suppression of back
reactions between the injected electrons and the electrolyte,
Conflicts of interest
There are no conicts to declare.
39,40
which ordinarily leads to improvement of the VOC
as shown
in Table 1. The increase in VOC value for the IA-7 complex could Acknowledgements
be caused by the retardation between injected electrons and
This work was supported by the Department of Textile Engi-
neering Chemistry and Science, NC State University, USA and
gratefully funded by the Ministry of Higher Education in Egypt
oxidized species in the electrolyte. Besides, these results clearly
show the great effect of donor and p-spacer antenna ligands on
the electron recombination processes between the electrolyte
(Grant No. Js# 3681).
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2
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