Efficiency enhancement of ruthenium-based DSSCs employing A-π-D-π-A organic Co-sensitizers

Article abstract of DOI:10.1039/d0ra03916k

A new bipyridyl Ru(ii) sensitizer incorporating triphenylamine and the 3,4-ethylenedioxythiophene (EDOT) ancillary ligand IMA5 was synthesized for dye-sensitized solar cells (DSSCs). The performance of these DSSCs has been enhanced via di-anchoring metal-free organic sensitizers, denoted IMA1-4, with structural motif A-π-D-π-A and incorporating phenyl-dibenzothiophene-phenyl (Ph-DBT-Ph) as the main building block but with different anchoring groups (A). These new organic sensitizers were well-characterized and used as efficient co-sensitizers. Their photophysical, electrochemical and photovoltaic properties were studied. Furthermore, molecular modeling studies using DFT calculations were used to investigate their suitability as effective sensitizers/co-sensitizers. The molecular orbital isodensity showed distinguishable delocalization of the intramolecular charge in the DBT moiety. The photovoltaic characterization showed that IMA3 had the best DSSC performance (η = 2.41%). In addition, IMA1-4 was co-sensitized in conjunction with the newly synthesized IMA5 complex to enhance light harvesting across expanded spectral regions and thus improve efficiency. The solar cells co-sensitized with IMA2, IMA3 and IMA4 exhibited improved efficiency (η) of 6.25, 6.19 and 5.83%, respectively, which outperformed the device employing IMA5 alone (η = 5.54%) owing to the improvement in the loading of IMA2, IMA3 and IMA4 in the presence of IMA5 on the surface of the TiO2 nanoparticles, and charge recombination was suppressed. This journal is

Full text of DOI:10.1039/d0ra03916k

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Efficiency enhancement of ruthenium-based
DSSCs employing A–p–D–p–A organic Co-
sensitizers†
Cite this: RSC Adv., 2020, 10, 27940
a
b
*
Islam M. Abdellah
and Ahmed El-Shafei
A new bipyridyl Ru(II) sensitizer incorporating triphenylamine and the 3,4-ethylenedioxythiophene (EDOT)
ancillary ligand IMA5 was synthesized for dye-sensitized solar cells (DSSCs). The performance of these
DSSCs has been enhanced via di-anchoring metal-free organic sensitizers, denoted IMA1–4, with
structural motif A–p–D–p–A and incorporating phenyl-dibenzothiophene-phenyl (Ph-DBT-Ph) as the
main building block but with different anchoring groups (A). These new organic sensitizers were well-
characterized and used as efficient co-sensitizers. Their photophysical, electrochemical and photovoltaic
properties were studied. Furthermore, molecular modeling studies using DFT calculations were used to
investigate their suitability as effective sensitizers/co-sensitizers. The molecular orbital isodensity showed
distinguishable delocalization of the intramolecular charge in the DBT moiety. The photovoltaic
characterization showed that IMA3 had the best DSSC performance (h ¼ 2.41%). In addition, IMA1–4 was
co-sensitized in conjunction with the newly synthesized IMA5 complex to enhance light harvesting
across expanded spectral regions and thus improve efficiency. The solar cells co-sensitized with IMA2,
IMA3 and IMA4 exhibited improved efficiency (h) of 6.25, 6.19 and 5.83%, respectively, which
outperformed the device employing IMA5 alone (h ¼ 5.54%) owing to the improvement in the loading of
IMA2, IMA3 and IMA4 in the presence of IMA5 on the surface of the TiO2 nanoparticles, and charge
recombination was suppressed.
Received 1st May 2020
Accepted 8th July 2020
DOI: 10.1039/d0ra03916k
rsc.li/rsc-advances
the electrochemical processes required for effective DSSCs. Thus,
systematic methodology is needed to address these issues and
1. Introduction
thus to design new sensitizers characterized by high-efficiency
photoconversion.16,17 In this regard, Ru(II)-based sensitizers have
been shown to be viable sensitizers for DSSCs owing to their
unusual metal-to-ligand charge transitions (MLCT), unique
excited photostability and photophysical properties. The excep-
tional light harvesting and durability characteristics of these
photosensitizers are attributed to the transition (MLCT) through
which the photoelectric charge moves to the TiO2 faster than the
electron recombination with the oxidized dye molecule, instead of
moving through the circuit.18–20 In addition, metal-free organic
sensitizers are the most favored candidates compared to Ru(II)-
based sensitizers because they have many advantages, such as
a exible model, cost-effective synthesis and superior molar
extinction coefficients accompanied with intramolecular charge
transfer (ICT) from an electron-rich donor to an anchoring unit
through a p-spacer unit upon light absorption.21–24 In addition to
the requirement to improve DSSC performance, co-sensitization
is one of the most promising approaches for improving effi-
ciency in DSSCs using a combination of organic sensitizers
(visible absorption) and Ru(II) (NIR-absorbing) complexes in order
to obtain a wide spectral response in the visible light region and
thus improve the absorption of light.1,25 Actually, co-sensitization
of DSSCs signicantly increases the performance of photovoltaics
Dye-sensitized solar cells (DSSCs) are one of the third-generation
of photovoltaic (PV) technology that represents clean and
sustainable energy used to generate electricity from abundant
sunlight to tackle the global energy crisis.1,2 As promising solar
cells, DSSCs have attracted interest from academics and industry
because of their low cost, stability, high efficiency and ease of
manufacture.3–6 DSSCs comprise several different components,
such as conductive glass, a mesoporous semiconductor lm,
electrolytes and sensitizers, so efficient tuning is crucial to real-
izing the highest efficiency for these major components.7–11 In
DSSCs, the molecular geometry of the sensitizer should be engi-
neered and designed to achieve broad UV absorption spectra,
harmonious thermodynamic properties, suitable molecular
orbital energy levels and excellent stability.12–15 In addition, the
thermodynamic and kinetic properties of the sensitizer are very
important for the initiation of light harvesting and for obtaining
aDepartment of Chemistry, Faculty of Science, Aswan University, Aswan, 81528, Egypt.
E-mail: islamabdellah2@aswu.edu.eg
bPolymer and Color Chemistry Program, North Carolina State University, Raleigh,
27606, USA. E-mail: Ahmed_El-Shafei@ncsu.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03916k
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Fig. 1 Graphical representation of organic sensitizers IMA1–4 with A–p–D–p–A structure and the IMA5 complex.
compared to a single sensitizer.26–28 This is attributed to prevent- when co-sensitized with D131 color dye,30 N3 dye when co-
ing the aggregation of the dye and minimizing charge recombi- sensitized with simple aniline-based D–A architecture enhanced
nation, increasing the efficient accumulation of both types of dyes the PCE by up to 7.02%,1 Ru sensitizer (SPS-01) co-sensitization
on the TiO2 surface owing to the difference in the molecular sizes with a metal-free dye containing thienyluorene (JD1) increased
of the dyes, thus facilitating the harvesting of a maximum number the PCE by 8.30% (ref. 31) and a JK2 and SQ01 co-sensitized DSSC
of incident photons by the cell.29 Different organic push–pull showed a 7.43% improvement in efficiency.32 Furthermore, triple
sensitizers have been used in DSSCs as effective co-sensitizers for co-sensitization of Y1 + TP2A + HSQ4 improved the PCE by
Ru(II) complexes compared to individual dye cells; for example, 7.48%.33
a black dye increased photocurrent efficiency (PCE) by up to 11%
Scheme 1 Synthesis of bi-anchoring metal-free organic photosensitizers. (i) p-Boronic acid benzaldehyde, K2CO3, Pd(PPh3)4, and DMF. (ii)
AcONH4 and AcOH-glacial.
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Scheme 2 (iii) Aliquat-336, K2CO3, Pd(PPh3)4, H2O, THF, 90 ꢂC. (iv) 4,40-Dimethyl-2,20-dipyridyl, Me3SiCl , 115 ꢂC, pressure tube. (v)a–c (a)
Dichloro-(p-cymene)-ruthenium(II) dimer, anhydrous DMF, 95 ꢂC, 5 h. (b) 2,20-Bipyridinyl-4, 40-dicarboxylic acid, 145 ꢂC, 5 h. (c) NH4SCN, 140 ꢂC,
5 h.
Herein, we report the synthesis and characterization of four rhodamine-3-acetic acid, cyanoacetic acid, 2-methyl quinoline-
new organic sensitizers with A–p–D–p–A architecture carrying 6-carboxylic acid and 5-oxo-1-phenyl-pyrazole-3-carboxylic acid
different acceptor units, viz. 1-phenyl-pyrazol-5-one-3-carboxylic were purchased from Sigma-Aldrich, Alfa Aesar and Ark Pharm.
acid IMA1, cyanoacetic acid IMA2, 2-methylquinoline-6- Furthermore, 2,20-bipyridinyl-4,40-dicarboxylic acid,34 N,N-
carboxylic acid IMA3, rhodamine-3-acetic acid IMA4 and a new diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline
ruthenium(II)-based complex (IMA5) incorporating a bipyridine (7)24 and 2-bromo-(3,4-ethylenedioxythiophene)-5-carbaldehyde
linked with two branches of a TPA-EDOT molecular motif as the (8)35 were synthesized as reported with the detailed proce-
electron donor, and 2,20-bipyridinyl-4,40-dicarboxylic acid was dures included in the ESI.† All solvents were purchased from
used as the main anchoring ligand, as shown in Fig. 1. The Fisher Scientic. 1H-NMR spectra were recorded utilizing
synthetic schemes and structures of the new dyes IMA1–5 are a Bruker AVANCE 500 MHZ using DMSO-d6 as the solvent and
depicted in Schemes 1 and 2. The dyes were used as sensitizers/co- tetramethylsilane for calibrating the chemical shi. The mass
sensitizers for the fabrication of DSSCs to evaluate their photo- spectra were recorded using a high-resolution Thermo Scientic
voltaic performance and the interfacial charge recombination Exactive Plus. The FTIR spectra were recorded for the pure solid
process. The target molecules and all compounds were well- using a Bruker ALPHA spectrophotometer. The UV-vis spectra
characterized using various spectral techniques, such as FT-IR, were recorded in 1 ꢀ 10ꢁ5 M solutions in an appropriate solvent
1H-NMR and high-resolution mass spectroscopy analysis. Their using a Varian Cary 3 UV-vis spectrophotometer. Cyclic vol-
optical bandgap (E0–0) and electronic energetics (GSOP and ESOP) tammetry (CV) was carried out for all sensitizers in an appro-
were measured experimentally using cyclic voltammetry and the priate anhydrous solution at a scan rate of 50 mV sꢁ1 with 0.1 M
orbital charge distributions over the dye molecules were deter- tetra-n-butylammonium hexauorophosphate (n-Bu)4N+(PF6)ꢁ
mined utilizing density functional theory (DFT).
as the supporting electrolyte at room temperature using a Vertex
electrochemical workstation. The photovoltaic parameters of
the DSSCs were measured in an Oriel SOL3A class AAA solar
simulator with an AM 1.5G spectral lter. A QEX10 measure-
ment system was used to run IPCE experiments. The electro-
chemical impedance spectra, including Nyquist and Bode
curves, were obtained using a Biologic SP-150 with a AAA solar
2. Experimental
2.1. Materials and methods
2,8-Dibromodibenzo[b,d]thiophene, p-boronic acid benzalde-
hyde, pd(dppf)2Cl2, K2CO3, Pd(PPh3)4, EDOT, POCl3,
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simulator. Molecular modeling calculations (DFT) were carried NMR (500 MHz, DMSO-d6) d: 9.85 (s, 2H), 7.72–7.65 (m, 2H),
out using the Gaussian 09 soware package and the calcula- 7.40–7.32 (m, 4H), 7.16–7.05 (m, 6H), 7.02–6.95 (m, 2H).
tions were performed remotely at the NC State University High
Performance Computing (HPC).
2.2.2. Synthesis of 4,40-((dibenzo[b,d]thiophene-2,8-diylbis(4,1-
phenylene))bis (methanylylidene))bis(5-oxo-1-phenyl-4,5-dihydro-
1H-pyrazole-3-carboxylic acid) (IMA1). A mixture of 4,40-(dibenzo
[b,d]thiophene-2,8-diyl)dibenzaldehyde (2) (0.393 g, 1 mmol) and 1-
phenyl-pyrazol-5-one-3-carboxylic acid (3) (0.4 g, 2 mmol) in 30 mL
of glacial acetic acid was reuxed in the presence of ammonium
2.2. Synthesis and characterization
The synthetic routes for the metal-free organic dyes with A–p–D– acetate (0.3 g, 3.9 mmol) for 6 hours under an argon atmosphere at
p–A architecture (IMA1–4) along with ruthenium complex IMA5 118 ꢂC. Upon completion of the reaction, the reaction mixture was
are presented in Schemes 1 and 2. The synthesis starts with the cooled to room temperature and poured into ice-cold water to
C–C Suzuki coupling reaction of 2,8-dibromodibenzo[b,d]thio- provide an orange precipitate. The precipitate was ltered and
phene (1) with two moles of p-boronic acid benzaldehyde in puried by column chromatography using silica gel and CHCl3-
potassium carbonate solution and tetrakis(triphenylphosphine)
: CH3OH (10 : 3) as the mobile phase to obtain a yellow-orange
palladium catalyst to afford 4,40-(dibenzo[b,d]thiophene-2,8-diyl) solid, which was crystallized from hexane. HRMS-ESI (m/z): [M +
dibenzaldehyde (2). The dialdehyde (2) then undergoes a Knoeve- H]+ calcd for C46H29N4O6S: 765.18133; found: 765.18394 (error, DM:
nagel condensation reaction with active methylene compounds, 3.419 ppm). FT-IR (cmꢁ1): 3374 (OH carboxylic), 3026 (]CH
such as rhodamine-3-acetic acid (3), cyanoacetic acid (4), 2-methyl alkene), 1706 (C]O), 1596 (C]C aromatic conjugation), 860–680
1
quinoline-6-carboxylic acid (5) and 1-phenyl-pyrazol-5-one-3- (aromatic CH bending). H NMR (500 MHz, DMSO-d6) d: 9.95 (s,
carboxylic acid (6), in the presence of ammonium acetate catalyst 2H), 8.12–8.07 (m, 2H), 8.03 (dd, J ¼ 1.8, 0.6 Hz, 2H), 7.99–7.94 (m,
to give the target organic sensitizers IMA1–4. Synthesis of the target 4H), 7.92–7.84 (m, 2H), 7.76–7.53 (m, 4H), 7.39–7.34 (m, 4H), 7.16–
Ru complex starts with the C–C Suzuki cross-coupling reaction of 7.02 (m, 8H).
N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline
2.2.3. Synthesis of 3,30-(dibenzo[b,d]thiophene-2,8-diylbis(4,1-
(7)24 and 2-bromo-(3,4-ethylenedioxythiophene)-5-carbaldehyde phenylene))bis(2-cyanoacrylic acid) (IMA2). A mixture of 4,40-
(8)35 to prepare the required aldehyde (9). The aldehyde (9) (dibenzo[b,d]thiophene-2,8-diyl)dibenzaldehyde (2) (0.393 g, 1
undergoes a condensation reaction with 4,40-dimethyl-2,20-dipyr- mmol) and cyanoacetic acid (4) (0.212 g, 2.5 mmol) in 30 mL of
idyl in the presence of Me3SiCl to afford the ancillary ligand (10). glacial acetic acid was reuxed in the presence of ammonium
Finally, the target Ru(II) complex IMA5 is synthesized via a one-pot acetate (0.3 g, 3.9 mmol) for 6 hours under an argon atmosphere at
three-step reaction protocol, wherein the precursor ancillary ligand 118 ꢂC. Upon completion of the reaction, the reaction mixture was
(10) is reacted with dichloro-(p-cymene)-ruthenium(II) dimer fol- cooled to room temperature and poured into ice-cold water to
lowed by 2,20-bipyridyl-4,40-dicarboxylic acid and ammonium provide a yellowish precipitate. The precipitate was ltered and
thiocyanate. All the new compounds and photosensitizers were puried by column chromatography using silica gel and CHCl3-
puried using column chromatography using an appropriate
: CH3OH (10 : 3) as the mobile phase to obtain a yellow color solid,
eluent and their synthesis was conrmed with various spectral which was crystallized from hexane. HRMS-ESI (m/z): [M ꢁ H]ꢁ
techniques, as described in the ESI (Fig. S4–S27).† More details on calcd for C32H17N2O4S: 525.09145; found: 525.09246 (error, DM:
the synthetic procedures for IMA1–5 are provided in the following 4.012 ppm). FT-IR (cmꢁ1): 3331 (OH carboxylic), 3033 (]CH
sections.
alkene), 2256 (CN), 1703 (C]O), 1590 (C]C aromatic conjuga-
2.2.1. Synthesis of 4,40-(dibenzo[b,d]thiophene-2,8-diyl) tion), 860–680 (aromatic CH bending). 1H NMR (500 MHz, DMSO-
dibenzaldehyde (2). A DMF solution of 2,8-dibromodibenzo d6) d: 8.92 (s, 2H), 8.85 (d, J ¼ 1.7 Hz, 2H), 8.46–8.39 (m, 4H), 8.36
[b,d]thiophene (1) (0.342 g, 0.001 mmol), p-boronic acid benz- (dd, J ¼ 8.8, 1.8 Hz, 2H), 7.89–7.86 (m, 2H), 7.79–7.63 (m, 4H), 7.56–
aldehyde (0.329 g, 0.00219 mmol) and potassium carbonate 7.53 (m, 2H).
solution (0.691 g, 0.00499 mmol) were added to a three-necked
ask. The solution was purged with argon for 30 min and then phenylene))bis(ethene-2,1-diyl))bis(quinoline-6-carboxylic
tetrakis(triphenylphosphine)palladium (0.034 g, 0.03 mmol) (IMA3).
mixture of 4,40-(dibenzo[b,d]thiophene-2,8-diyl)
2.2.4. Synthesis of 2,20-((dibenzo[b,d]thiophene-2,8-diylbis(4,1-
acid)
A
was added. The reaction mixture was stirred at 80 ꢂC overnight. dibenzaldehyde (2) (0.393 g, 1 mmol) and 2-methyl-quinoline-6-
The reaction was followed by TLC until reaction completion, carboxylic acid (5) (0.411 g, 2.2 mmol) in 30 mL of glacial acetic
then the reaction mixture was le to cool down and quenched acid was reuxed in the presence of ammonium acetate (0.3 g, 3.9
by adding 50 mL of water, followed by extraction with ethyl mmol) for 6 hours under an argon atmosphere at 118 ꢂC. Aer the
acetate (3 ꢀ 30 mL). The organic layer was dried using anhy- reaction completion, the reaction mixture was cooled to room
drous Mg2SO4 and the solvent was removed under vacuum. The temperature and poured into ice-cold water to afford a reddish
main product was puried by silica column chromatography precipitate. The precipitate was ltered and puried by column
with a mixture of hexane and ethyl acetate (3 : 1). The chromatography using silica gel and CHCl3 : CH3OH (10 : 3) as the
compound crystallized from ethanol and gave a pure white mobile phase to obtain a red solid, which was crystallized from
powder. HRMS-ESI (m/z): [M + H]+ calcd. for C26H17O2S: hexane. HRMS-ESI (m/z): [M]+ calcd for C48H30N2O4S: 730.66641;
393.09438; found: 393.09408 (error, DM: ꢁ0.767 ppm). FT-IR found: 730.66785 (error, DM: 3.031 ppm). FT-IR (cmꢁ1): 3396 (OH
(cmꢁ1): 3015 (]CH alkene), 1707 (C]O), 1583 (C]C carboxylic), 3070 (]CH alkene), 1702 (C]O), 1595 (C]C aromatic
aromatic conjugation), 860–680 (aromatic CH bending). 1H conjugation), 860–680 (aromatic CH bending). 1H NMR (500 MHz,
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DMSO-d6) d: 9.94 (s, 2H), 8.10–8.06 (m, 2H), 8.01 (d, J ¼ 1.8 Hz, 1H), deposited with the addition of 50 mL of ice and water. Finally,
7.96–7.93 (m, 4H), 7.89 (d, J ¼ 1.3 Hz, 1H), 7.72 (d, J ¼ 8.6 Hz, 2H), vacuum ltration was performed to supply the well-washed
7.39–7.32 (m, 7H), 7.14–7.07 (m, 7H), 7.05–7.03 (m, 4H).
antenna ligand. The antenna ligand was then dried overnight at
2.2.5. Synthesis of 2,20-((dibenzo[b,d]thiophene-2,8-diylbis(4,1- 50 C giving a 68% yield. The antenna ligand was recrystallized
phenylene))bis (methanylylidene))bis(4-oxo-2-thioxothiazolidin-3-yl- from acetone to form pure dark brown crystals. HRMS-ESI (m/z):
5-ylidene))diacetic acid (IMA4). A mixture of 4,40-(dibenzo[b,d] [M + H]+ calcd for C62H46N4O4S2: 975.30332; found: 975.30276
thiophene-2,8-diyl)dibenzaldehyde (2) (0.393 g, 1 mmol) and (error, DM: ꢁ0.578 ppm). FT-IR (cmꢁ1): 3033 (]CH), 1587 (C]C
ꢂ
1
rhodamine-3-acetic acid (6) (0.478 g, 2.5 mmol) in 30 mL of glacial aromatic conjugation), 860–680 (aromatic CH bending). H NMR
acetic acid was reuxed in the presence of ammonium acetate (500 MHz, DMSO-d6) d: 8.67–8.63 (m, 2H), 8.61–8.57 (m, 2H), 7.67–
(0.3 g, 3.9 mmol) for 18 hours under an argon atmosphere at 118 ꢂC. 7.59 (m, 4H), 7.36 (d, J ¼ 2.0 Hz, 2H), 7.34–7.27 (m, 12H), 7.10 (dd, J
Upon completion of the reaction, the reaction mixture was cooled to ¼ 2.7, 1.3 Hz, 2H), 7.07–6.99 (m, 14H), 4.43 (s, 8H).
room temperature and poured into ice-cold water to provide
2.2.8. Synthesis of ruthenium(II) complex (IMA5). The
a yellowish precipitate. The precipitate was ltered and puried by synthesis of IMA5 was carried out in a single-pot three-step
silica gel column chromatography using CHCl3 : CH3OH (10 : 3) as reaction. The reactions were carried out under argon gas in
the mobile phase to obtain dark yellow solid, which was crystallized a 100 mL ask connected with a condenser. The ask was
from hexane. HRMS-ESI (m/z): [M ꢁ H]ꢁ calcd for C36H22N2O6S5: charged with anhydrous DMF, dichloro-(p-cymene)-ruth-
737.00086; found: 737.00141 (error, DM: 0.745 ppm). FT-IR (cmꢁ1): enium(II) dimer (0.3 g, 0.5 mmol) and ancillary ligand (10)
3337 (OH carboxylic), 1649 (C]O), 1517 (C]C aromatic conjuga- (0.975 g, 1 mmol). The reaction mixture was stirred at 95 ꢂC for
tion), 860–680 (aromatic CH bending). 1H NMR (600 MHz, DMSO- 5 h. Then, 2,20-bipyridyl-4,40-dicarboxylic acid was added
d6) d: 9.06 (s, 2H), 8.63 (d, J ¼ 1.8 Hz, 2H), 8.54 (d, J ¼ 1.7 Hz, 2H), (0.244 g, 1 mmol) and the temperature was raised to 145 ꢂC and
8.05 (d, J ¼ 20.0 Hz, 2H), 7.87–7.79 (m, 4H), 7.76 (dd, J ¼ 8.6, 2.9 Hz, the reaction was allowed to run for 6 hours. Aer the 6 hours, an
1H), 7.72–7.65 (m, 4H), 7.62–7.51 (m, 1H), 4.75 (s, 4H).
excess of NH4NCS (0.5 g) was added to the reaction mixture and
2.2.6. Synthesis of 7-(4-(diphenylamino)phenyl)-2,3-dihy- then the reaction was allowed to run for an additional 4 hours at
drothieno[3,4-b][1,4]dioxine-5-carbaldehyde (9). In a three-necked 140 ꢂC. The product was cooled to 25 ꢂC and transferred to
ask
a mixture of N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2- a 250 mL round bottom ask and then the DMF was evaporated
dioxaborolan-2-yl)aniline (7) (0.408 g, 1.1 mmol), 2-bromo-(3,4- using a rotary evaporator. Ice was added to the ask and the
ethylenedioxythiophene)-5-carbaldehyde (8) (0.249 g, 1 mmol) and insoluble precipitate was ltered and washed with deionized
tetrakis(triphenylphosphine)palladium (0.034 g, 0.03 mmol) was H2O and ether. Upon drying, the dye was dissolved in CH3OH
dissolved in THF and degassed under an argon atmosphere for 15 with the addition of 2 mL of tetrabutylammonium hydroxide
minutes then a K2CO3 (2.5 mL, 2 M) solution was added. The (TBAOH) and then puried on a silica gel column. The main
reaction mixture was stirred at 80 ꢂC for 5–6 hours and followed by band (violet) was collected and acidied using 0.1 M HCl to
TLC until completion. Aer the reaction completion, it was le to reduce the pH to 2.0 and then allowed to precipitate for 48
cool down and quenched by adding (50 mL) of water then hours at low temperature. The precipitate was then ltered and
extracted with CH2Cl2 (3 ꢀ 40 mL). The organic layer was dried washed with plenty of deionized water to bring the pH to
using anhydrous Mg2SO4 and the organic solvent was removed neutral. The pure dye was then dried overnight and collected as
under vacuum. The crude product was puried by column chro- dark crystals (yield 74%). HRMS-ESI (m/z): [M + H]+ calcd for
matography on silica with CHCl3. The compound was crystallized
C76H55N8O8S4Ru: 1437.20637; found: 1437.20761 (error: DM,
from hexane and give a yellow crystal. HRMS-ESI (m/z): [M + H]+ 0.86184 ppm). FT-IR (cmꢁ1): 3388 (OH carboxylic), 3060 (]CH
calcd for C25H19NO3S: 414.11584; found: 414.11557 (error, DM: alkene), 2932 (CH for CH2) 2104 (SCN), 1718 (C]O), 1589 (C]C
ꢁ0.643 ppm). FT-IR (cmꢁ1): 3032 (]CH alkene), 2803 (CH for aromatic conjugation), 860–680 (aromatic CH bending). 1H
CH2) 1637 (C]O), 1519 (C]C aromatic conjugation), 860–680 NMR (500 MHz, DMSO-d6) d: 8.77 (S, 2H), 8.16–8.00 (m, 4H),
1
(aromatic CH bending). H NMR (500 MHz, DMSO-d6) d: 9.80 (s, 7.88–7.78 (m, 4H), 7.58 (d, J ¼ 7.5 Hz, 2H), 7.29–7.16 (m, 14H),
1H), 7.82–7.6 (m, 2H), 7.64–7.55 (m, 2H), 7.26 (t, J ¼ 7.5 Hz, 4H), 7.07 (d, J ¼ 1.5 Hz, 2H), 7.04–6.95 (m, 14H), 6.92 (dd, J ¼ 7.4,
7.11–6.90 (m, 4H), 7.21 (tt, J ¼ 7.4, 2.0 Hz, 2H), 4.37 (s, 4H).
2.2.7. Synthesis of 4,40-(([2,20-bipyridine]-4,40-diylbis(ethene-
2,1-diyl))bis(2,3-dihydrothieno[3,4-b][1,4]dioxine-7,5-diyl))bis(N,N-
diphenylaniline) (10). The ancillary ligand (10) was synthesized
under pressure in a glass tube containing 4,40-dimethyl-2,20-
1.5 Hz, 2H), 6.89 (dd, J ¼ 7.5, 1.4 Hz, 2H), 4.36 (s, 8H).
3. Results and discussion
3.1. Photophysical and electrochemical properties
bipyridine (0.184 g, 1 mmol), 7-(4-(diphenylamino)phenyl)-2,3- The UV-vis absorption/emission spectra of IMA1–4 were measured
dihydrothieno[3,4-b][1,4]dioxine-5-carbaldehyde (9) (0.8269 g, 2 in DMSO solution with a concentration of 1 ꢀ 10ꢁ5 M, as dis-
mmol), 1.52 mL of chlorotrimethylsilane (12 mmol), and 50 mL of played in Fig. 2a and b, and their characteristic spectral data are
anhydrous DMF. The tube was well closed by the cap and heated at tabulated in Table 1. The lower wavelength bands around 250–
100 ꢂC in an oil bath for 48 hours with continuous stirring. Over 300 nm in the absorption spectra of IMA1–4 can be assigned to p
the course of 48 hours, the color of the reaction mixture changed / p* transitions localized within the phenyl-DBT-phenyl (p–D–p)
from yellow to dark orange. At the end of the reaction, the pressure moieties. Moreover, the bathochromic shi bands in the region of
was released aer the tube was cooled and the solvent was 350–430 nm can be accredited to intramolecular charge transfer
removed using a rotary evaporator; a dark orange liquid was (ICT) from the donor (DBT) to the anchoring moiety via the
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Fig. 2 (a) UV-vis absorption spectra and (b) emission spectra of 1 ꢀ 10ꢁ5 M IMA1–4 in DMSO. (c) UV-vis absorption spectrum of 1 ꢀ 10ꢁ5
M
ancillary ligand (10) in CHCl3. (d) UV-vis absorption spectra of 1 ꢀ 10ꢁ5 M IMA5 in CHCl3.
conjugated phenyl segment, which is critical to absorbing lower capacity to harvest light increases for photosensitizers character-
energy light with reasonable molar absorptivity.36 Although the ized by higher molar extinction coefficients.37,38 On the other hand,
maximum
wavelengths
(ICT)
for
photosensitizers the UV-vis absorption/emission spectrum of IMA5 was measured
IMA1 (lmax ¼ 411 nm) and IMA4 (lmax ¼ 383 nm) are bath- in 1 ꢀ 10ꢁ5 M CHCl3 solution and its spectral behavior is depicted
ochromically shied compared to those of IMA2 (lmax ¼ 347 nm) in Fig. 2d and the corresponding data are outlined in Table 1.
and IMA3 (lmax ¼ 339 nm), the molar extinction coefficients Fig. 2d shows that IMA5 possesses three distinctive absorption
of 0.40 ꢀ 105 Mꢁ1 cmꢁ1 and 0.29 ꢀ 105 Mꢁ1 cmꢁ1 for IMA1 and bands. The band in the region of 290–320 nm is attributed to the
IMA4, respectively, are extremely low compared to those for p–p* electronic transition of the bipyridine ligand, while the peak
IMA2
(3max
¼
0.85
ꢀ
105
Mꢁ1
cmꢁ1
)
and in the 360–400 nm region can be ascribed to ligand-to-ligand
IMA3 (3max ¼ 0.95 ꢀ 105 Mꢁ1 cmꢁ1). It is well known that the charge transfer (LLCT) mixed with metal-to-ligand charge
Table 1 Photophysical and electrochemical data for the synthesized sensitizers
Oxd
Onset
Dye
lmax (nm)
3max (105 Mꢁ1 cmꢁ1
)
Stokes shi (nm) I (nm) Emax (nm) E0–0 (eV)
E
GSOP (eV) ESOP (eV) DGinjꢂ (eV)
IMA1 411 (ICT)
IMA2 347 (ICT)
IMA3 339 (ICT)
IMA4 383 (ICT)
0.401
0.848
0.953
0.291
167
173
164
175
110
482
433
401
458
557
578
520
503
558
615
2.57
2.86
3.09
2.70
2.22
0.28
0.24
0.20
0.21
0.31
ꢁ5.54
ꢁ5.50
ꢁ5.46
ꢁ5.47
ꢁ5.57
ꢁ2.97
ꢁ2.64
ꢁ2.37
ꢁ2.77
ꢁ3.35
1.23
1.56
1.83
1.43
0.85
IMA5 505 (d–p*) 0.624
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Fig. 3 Energy level diagram showing the different potentials of the DSSC components along with the GSOP and ESOP of the sensitizers IMA1–5.
transfer (MLCT) (pd–p*) and the broad peak at 480–525 nm cor- the oxidized dyes.39 The measurements were performed using
responding to the longest wavelength can be credited to metal-to- a Vertex electrochemical instrument with a three-electrode cell
ligand charge transfer (MLCT) (pd–p*), which corresponds to comprising a glassy carbon working electrode where the oxida-
electron transfer from the HOMO to LUMO energy levels. This is tion or reduction takes place, a platinum disc as the counter
conrmed by comparing the absorption spectrum of the ancillary electrode, and an Ag/AgCl reference electrode. The obtained
ligand (10), which is characterized by the presence of p–p* and voltammograms were used to calculate the ground state oxida-
(LLCT) peaks only, as shown in Fig. 2c, with that of the IMA5 tion potential (GSOP) from the rst jump in the voltammogram,
Oxd
Onset
complex, which is characterized by the presence of one more peak which represents the onset oxidation potential (E
), as shown
for MLCT. The IMA5 complex is characterized by a molar extinc- in the ESI (Fig. S1 and S2).† The onset potential values were
tion coefficient (3max) of 0.624 ꢀ 105 Mꢁ1 cmꢁ1). The higher calibrated by cyclic voltammetry measurement of ferrocene (Fc/
extinction coefficient of the IMA5 complex in the visible region is Fc+), as shown in the ESI (Fig. S3).† The resulting onset oxidation
attributed to the presence of the strong electron donor part, which potential was converted to NHE by applying the following equa-
comprises TPA-EDOT and is directly connected to the bipyridine to tion: GSOP/NHE ¼ oxidation onset ꢁ GSOP/Fc + 0.63. The
form a ligand that contains extended p conjugation, and the calculated GSOP/NHE was then converted to electron volts by
directionality of the excited state by perfect tuning of the ligand applying the following equation: GSOP/eV ¼ GSOP/NHE + 4.7. On
LUMO energy level with the donating groups. Moreover, the the other hand, the excited state oxidation potential (ESOP) was
emission spectra of the metal-free organic sensitizers displayed calculated from the GSOP value and the calculated energy
a single emission band in the range of 500–600 nm and the bandgap (E0–0) by applying the following equation: ESOP ¼
maximum emission wavelength (lemi) is in the order of IMA1 (578 [GSOP ꢁ E0ꢁ0]. The calculated data are tabulated in Table 1. From
nm) > IMA4 (558 nm) > IMA2 (520 nm) > IMA3 (503 nm), while the the results, it is obvious that all the dyes display GSOP energy
IMA5 complex displays a characteristic lemi in the longer wave- levels below the CB level of TiO2 (ꢁ4.2 eV)40 and far away from the
length region of 600–650 nm. Furthermore, the wavelength of the electrolyte potential (ꢁ5.2 eV),41 which enables the dye regener-
intersection (I) of the absorption and uorescence spectra gives the ation process. On the other hand, the ESOP levels are energeti-
optical bandgap (E0–0), which is calculated by converting the cally higher than that of the CB potential of TiO2 to permit
wavelength of the intersection (I) from nm to eV. The optical band electron injection from the excited dye molecules and decrease
gaps are in the following order: IMA3 (3.09 eV) > IMA2 (2.86 eV) > the recombination process.
IMA4 (2.70 eV) > IMA1(2.57 eV) > IMA5 (2.22 eV) and the Stokes
A comparison of the electronic energy levels of the dyes
shis are: IMA1 (167 nm), IMA2 (173 nm), IMA3 (164 nm), IMA4 IMA1–5 and the TiO2 and Iꢁ/I3 redox couple energy levels is
ꢁ
(175 nm), and IMA5 (110 nm).
summarized in Fig. 3. The energy diagram shows that all of the
Cyclic voltammetry (CV) of IMA1–5 is critical for exploring the dyes are thermodynamically favorable for electron injection and
electronic processes at the mesoporous TiO2/dye/electrolyte dye regeneration in the fabricated devices.
interface and showing the required energy levels for electron
injection to the conduction band (CB) of TiO2 and regeneration of
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Fig. 4 Optimized geometry and HOMO–LUMO molecular orbitals for sensitizers IMA1–5.
complex. All of these conditions make it easy for IMA1–4 to ll
the gaps le by the bulky IMA5 complex on the TiO2 surface. It
results in more dye packing on the TiO2 surface, which helps to
reduce dye aggregation and decrease the recombination process
between the injected electrons on the TiO2 semiconductor lm
and the electrolyte.42,43
3.2. Theoretical studies
Theoretical studies (quantum calculations) were performed on
sensitizers IMA1–5 utilizing DFT to understand their geomet-
rical electronic distribution and evaluate their photophysical
properties for use in DSSCs. All calculations were performed
through North Carolina State University's High-Performance
Computing utilizing GAUSSIAN 09 soware. The ground state
geometries were optimized using the B3LYP energy functional
and the DGTZVP basis set. The solvation effect was taken into
consideration utilizing the CPCM model in DMSO for dyes
IMA1–4 and CHCl3 for dye IMA-5. The resulting 3D optimized
structures of IMA1–5 with the isosurfaces of the HOMO–LUMO
frontier molecular orbitals are presented in Fig. 4. It was
obvious that all sensitizers achieved good orbital distribution
between the HOMO and LUMO isosurfaces. In the case of the
metal-free organic sensitizers IMA1–4, the HOMOs are accu-
mulated on the DBT moiety, which represents the donor part,
while the LUMOs are distributed on the acceptor parts, namely
the pyrazole carboxylic acid, cyanoacetic acid, quinoline
carboxylic acid and thiazole carboxylic acid moieties. On the
other hand, in the Ru(II) sensitizer IMA-5 the HOMO was located
on the donor (TPA-EDOT) moiety and the LUMO on the acceptor
part, which represents dibipyridine dicarboxylic acid.
3.3. Photovoltaic characterization of DSSCs
The DSSCs were fabricated utilizing dyes IMA1–5 with the
addition of chenodeoxycholic acid (CDCA) as a co-adsorbent.44
The DSSCs were fabricated on FTO glass, which was printed
with two layers of TiO2 and used as the working electrode
(photoanode). The prepared photoanode was immersed into
the dye/CDCA solution to allow the dye to anchor onto the
surface. The counter electrode was prepared by printing Pt paste
on FTO conductive glass. Both electrodes were sealed together
and iodolyte (redox couple) solution was injected into the device
interface; more details on the fabrication processes are
provided in the ESI.† The photovoltaic properties of the fabri-
cated DSSC devices sensitized with IMA5 and co-sensitizers
IMA1–4 on a TiO2 semiconductor electrode were analyzed
under standard AM 1.5 irradiation (100 mW cmꢁ2). The resul-
tant current–voltage (J–V) plots are presented in Fig. 5a and
b and the related data are summarized in Table 2. From the
results in Fig. 5a, IMA3 achieved the highest photocurrent
efficiency of h ¼ 2.41%, open circuit voltage (VOC ¼ 0.85 V),
short-circuit current (JSC ¼ 6.13 mA cmꢁ2) and ll factor
The optimized geometries for dyes IMA1–4 have maximum
˚
˚
˚
˚
lengths of 28 599 A, 17 760 A, 23 095 A, and 23 558 A, respec-
tively. The small dyes lengths and its planar structures can
make these dyes a strong co-sensitizer for the IMA5 octahedral
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Fig. 5 (a) J–V curves for the metal-free organic sensitizers. (b) J–V curves for the IMA5 complex alone and co-sensitized with IMA1-4 under 1.5
AM.
Table 2 Photovoltaic parameters of the sensitized/co-sensitized DSSCs for IMA1–5
Average values (PV parameters)
Sensitizer
(0.2 mM)
Co-sensitizer (0.2 mM)
No. of DSSCs
JSC (mA cmꢁ2
)
VOC (V)
FF (%)
h (%)
IMA1
IMA2
IMA3
IMA4
IMA5
__
__
__
__
4
4
4
4
3
3
3
3
3
1.41
4.05
6.13
0.53
0.59
0.58
0.53
0.61
0.62
0.66
0.65
0.63
73.07
71.49
67.76
65.79
64.54
61.21
60.58
61.71
64.85
0.54
1.74
2.41
0.88
5.54
5.44
6.25
6.19
5.83
2.51
__
14.07
14.25
15.67
15.44
14.36
IMA1
IMA2
IMA3
IMA4
(FF ¼ 67.76%) when compared to photocurrent efficiencies (h) harvesting in the 300–500 nm region owing to the addition of
of 0.54%, 1.74%, and 0.88% for IMA1, IMA2, and IMA4, metal-free organic co-sensitizers IMA1-4, which have a charac-
respectively. The photocurrent efficiencies of the organic dyes teristic high molar absorption coefficient. The improved VOC
IMA1-4 were found to be in the order of IMA3 > IMA2 > IMA4 > observed for the co-sensitized DSSC devices can be attributed to
IMA1, which is attributed to increasing electron injection from the lower rate of recombination between the injected electrons
the excited dye molecules to the TiO2 surface of the DSSCs. The in the TiO2 semiconductor conduction band and the redox
electron injection free energy (DGinjꢂ) was found to have the electrolyte (I3ꢁ/Iꢁ). Owing to their small size, the co-sensitizers
same trend as photocurrent efficiency and was calculated from provide better surface coverage27,45 by adsorbing into the pores
the difference between the CB of the TiO2 surface and ESOP, as and gaps in the TiO2, whereas the steric hindrance of the bulky
shown in Table 1.
ruthenium-based dye molecule prevents its adsorption. From
On the other hand, the relationship between the structures the results, the lower efficiency of IMA5 when co-sensitized with
of the co-sensitizers IMA1–4 and their performance with IMA1 (h ¼ 5.44%) and IMA4 (h ¼ 5.83%) compared to when co-
ruthenium dye IMA5 was studied by evaluating the photovoltaic sensitized with IMA2 (h ¼ 6.25%) and IMA3 (h ¼ 6.19%) can be
characterization of the co-sensitized DSSCs. The addition of attributed to the breaking of the continuous conjugation by the
different anchoring groups to the principal moiety of phenyl- pyrazole moiety in IMA1 and by the thiazole moiety in the case
DBT-phenyl was found to have a profound inuence on the of IMA4, which would prevent efficient electron injection into
photovoltaic properties of the co-sensitized DSSC devices, as the conduction band of TiO2.46 Among all the DSSCs, the DSSC
shown in Fig. 5b. The photovoltaic parameters of IMA5 alone device co-sensitized with IMA2 showed the highest Jsc, VOC, and
were h ¼ 5.54%, VOC ¼ 0.61 V, JSC ¼ 14.07 mA cmꢁ2, whereas PCE owing to the small size of IMA2, which includes a cyano-
aer employing co-sensitizers IMA1–4 the Jsc was enhanced to acetic acid moiety. These criteria increase the adsorption of the
14.25, 15.67, 15.44 and 14.36 mA cmꢁ2 and the VOC was molecule on the TiO2 surface and enhance its light-harvesting
enhanced to 0.62, 0.66, 0.65 and 0.63 V, respectively. The ability.
enhanced Jsc for IMA5 might be ascribed to the increased light
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Fig. 6 (a) IPCE curves for IMA1–4. (b) IPCE curves for IMA5 complex alone and co-sensitized with IMA1–4.
The IPCEs for the DSSCs sensitized with IMA1–5 and the by the pyrazole and thiazole moieties and the unfavorable
DSSCs sensitized with IMA5 and co-sensitized with IMA1–4 LUMO distribution.46
were measured and are plotted in Fig. 6a and b. The DSSCs
sensitized with the metal-free organic dyes IMA1–4 showed
3.4. Electrochemical impedance study of the DSSCs
IPCE spectra in the visible range (300–500 nm) with quantum
efficiencies of 4%, 27%, 33% and 12%, respectively, as shown in
Fig. 6a. The higher IPCE of IMA3 compared to the other dyes is
attributed to the high molar extinction coefficient of IMA3
(0.953 ꢀ 105 Mꢁ1 cmꢁ1). On the other hand, the DSSC devices
with IMA5 alone and co-sensitized with IMA1–4 show broad
coverage of IPCE spectra from the visible range to the near IR
range (300–700 nm) with quantum efficiencies of 40%, 54%,
47%, and 41%, respectively, compared to 42% for IMA5 alone,
as shown in Fig. 6b. The improved IPCEs of the DSSCs co-
sensitized with IMA2 and IMA3 compared to IMA5 alone is
attributed to the increased electron injection ability with co-
sensitizers incorporating cyanoacetic acid and quinoline
carboxylic acid moieties. While, the lower IPCEs of the DSSCs
co-sensitized with IMA1 and IMA4 is because of the low electron
injection efficiency owing to the disrupting of the conjugation
Electrochemical impedance spectroscopy was conducted to
explain the recombination rate and charge lifetime of the DSSCs
sensitized/co-sensitized with sensitizers IMA1–5. The Nyquist
and Bode plots for the devices with IMA1–4 are shown in Fig. 7a
and b and the corresponding data are summarized in Table 3.
The equivalent circuit tting was performed using (Bio-Logic)
soware utilizing the R1+ C2/R2 + C3/R3 tting model as shown
in Fig. 7a. In the EIS Nyquist plots, three semicircles were
observed, the rst and small semicircle in the higher frequency
region corresponds to the charge transfer process (R3) at the
counter electrode and electrolyte interfaces. The large or mid-
frequency region signies the recombination resistance (R2) at
the working electrode of the TiO2/dye/electrolyte interface.27,46
The third semicircle (low-frequency range) indicates the series
resistance (R1) of platinum and FTO glass. The obtained
recombination resistance values (R2) of the DSSCs sensitized
Table 3 Parameters obtained from applying the R1+ C2/R2 + C3/R3 fitting model on the impedance spectra of the sensitized/co-sensitized
DSSCsa
Sensitized DSSC devices
Co-sensitized DSSC devices
Parameters
IMA1
IMA2
IMA3
IMA4
IMA5
IMA5+IMA1
IMA5+IMA2
IMA5+IMA3
IMA5+IMA4
R1 (U cmꢁ2
C2 (mF cmꢁ2
n1
)
24.96
357
1
8.605
25
0.877
379.4
1.29
41.22
323
0.993
64.75
28
0.807
264.7
1.56
23.15
159
1
77.37
33
0.785
265.4
2.45
28.81
851
0.553
40.33
286
0.966
283.7
1.39
30.85
258
26.59
357
0.628
22.13
10.67
1
25
232
28.56
140
26.88
157
0.577
22.48
17.15
1
)
)
0.672
21.93
11.57
0.974
3.25
0.518
22.65
29.84
1
7.65
6.43
0.637
22.52
12.65
0.942
2.95
R2 (U cmꢁ2
)
C3 (mF cmꢁ2
n2
R3 (U cmꢁ2
seff (ms)
)
2.92
2.96
2.17
3.33
2.85
4.50
a
R1, R2, and R3 are the series resistance of Pt and TCO, the charge transfer resistance at the TiO2/dye/electrolyte interface, and the charge transfer
resistance at the Pt/electrolyte interface, respectively; C2 and C3 are the constant phase elements for the TiO2/dye/electrolyte and Pt/electrolyte
interface, respectively; n presents the degree of surface inhomogeneity; seff is the effective lifetime.
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Fig. 7 (a) Nyquist plots of DSSCs sensitized with IMA1–4 and equivalent circuit for R1+ C2/R2 + C3/R3 fitting model. (b) Nyquist plots of DSSCs
sensitized with IMA5 and co-sensitized with IMA1–4.
with IMA1–4 were 8.605, 64.75, 77.37, and 40.33 U, respectively. which indicates that dyes incorporating cyanoacetic acid
In fact, the higher the R2 value, the lower the charge recombi- anchoring groups are efficient co-sensitizers.
nation rate between the conduction band of the TiO2 semi-
Furthermore, the Bode phase plots were analyzed to deter-
conductor and the electrolyte. The R2 values of the DSSC devices mine the electron recombination lifetime in the CB of TiO2, as
sensitized with IMA1–4 decreased in the order of IMA3 > IMA2 > shown in Fig. 8a and b. The frequency peaks (f) obtained from
IMA4 > IMA1. The obtained R2 values were consistent with the the DSSCs sensitized with IMA1–4 in the Bode phase plots were
VOC values obtained from the J–V curves. Among all of the used to calculate the effective lifetime (sseff) of electrons injected
sensitizers, IMA3 achieved the highest R2 value which into the CB of TiO2 by utilizing the following relation:
ꢀ
ꢁ
enhancing the charge recombination resistance between the
semiconductor conduction band and the electrolyte compared
to the other sensitizers, which indicates that quinoline
carboxylic acid is an efficient anchoring group. On the other
hand, the EIS Nyquist plots of the DSSCs sensitized with IMA5
alone and co-sensitized with IMA1–4 showed recombination
resistance (R2) values in the order of IMA5+IMA2 > IMA5+IMA3
> IMA5+IMA4 > IMA5+IMA1 > IMA5. The R2 values followed the
same trend as the VOC values obtained from the J–V curves. All
co-sensitizers achieved higher recombination resistance (R2)
compared to IMA5 alone. Among all the co-sensitizers, IMA2
achieved the highest R2 value which enhancing the charge
recombination resistance compared to other co-sensitizers,
1
2pf
seff
¼
. The effective lifetimes, seff, for the devices
sensitized with IMA1–4 were calculated and found to decrease
in the following order: IMA3 (2.45 ms) > IMA2 (1.56 ms) > IMA4
(1.39 ms) > IMA1 (1.29 ms). The effective lifetimes, seff, for the
DSSCs sensitized with IMA5 alone and co-sensitized with IMA1–
4 showed the following order: IMA5+IMA2 > IMA5+IMA3 >
IMA5+IMA4 > IMA5+IMA1 > IMA5. Basically the longer electron
lifetime leads to a better suppression of back reactions between
the injected electrons and the electrolyte, which usually leads to
improvement of the VOC
The VOC values of the co-sensitized devices obviously follow
the same order as the seff values, clearly showing the great effect
47–49
.
Fig. 8 (a) Bode plots of DSSCs sensitized with IMA1–4. (b) Bode plots of DSSCs sensitized with IMA5 and co-sensitized with IMA1–4.
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of anchoring groups and co-sensitization on the electron
recombination processes between the electrolyte species and
electron transfer into the TiO2 semiconductor lm. Hence, the
decrease in the VOC for the IMA5 device can be explained by the
faster recombination relative to that of the IMA5+IMA2 co-
sensitized device, which can be attributed to the increase in
dye loading on the surface of the TiO2 by the adsorption of the
larger IMA5 complex followed by the adsorption of smaller
IMA1–4 molecules in such a way as to ll the gaps between
larger IMA5 molecules in the sensitization process, which helps
with the formation of a blocking layer covering the complete
TiO2 nanoparticle owing to increased dye loading.47,50
References
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4. Conclusions
In summary, a new series of organic sensitizers IMA1–4 with A–
p–D–p–A motif were synthesized and discussed along with the
Ru(II) complex IMA5 to be used as effective sensitizers/co-
sensitizers for DSSCs and their performance, photophysics,
electrochemistry and molecular modeling were compared. The
DSSCs fabricated with sensitizers IMA1–4 showed PCE perfor-
mance that decreased in the order of IMA3 > IMA2 > IMA4 >
IMA1, which depicts the anchoring nature (2-methylquinoline-6-
carboxylic acid, cyanoacetic acid, rhodamine-3-acetic acid, and 1-
phenyl-pyrazol-5-one-3-carboxylic acid). Among these sensitizers,
IMA3 showed the highest PCE, which can be ascribed to the
enhanced light harvesting coupled with better electron injection
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from 14.25 mA cmꢁ2 to 14.25, 15.67, 15.44 and 14.36 mA cmꢁ2
,
respectively when IMA1–4 were used as co-sensitizers. The cor-
responding enhancement in the JSC could be attributed to the
increased IPCE in the 300–600 nm range, possibly owing to the
complementary absorption properties of the co-sensitizers
(IMA1–4) with the IMA5 complex, thereby harvesting a larger
number of photons in this region, which in turn enhanced the JSC
and PCE. Moreover, improved VOC values were observed for the
IMA5 DSSC devices when co-sensitized with IMA1–4 compared to
the DSSC using the IMA5 dye alone, most likely owing to the
small size of the co-sensitizers, which may occupy pores and gaps
between the 3D ruthenium-based dye, which plays a key role in
suppressing charge recombination by acting as a pꢁhysical insu-
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Conflicts of interest
There are no conicts of interest to declare.
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