Simple diphenylamine based D-π-A type sensitizers/co-sensitizers for DSSCs: A comprehensive study on the impact of anchoring groups

Article abstract of DOI:10.1039/c9cp01032g

Herein, we report the design, synthesis and characterization of a new series of simple donor-π spacer-acceptor/anchor (D-π-A) type diphenylamine based metal-free organic dyes possessing three different anchoring groups, viz. 4-aminobenzoic acid (DTP), 2-(

Full text of DOI:10.1039/c9cp01032g

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Physical Chemistry Chemical Physics
Simple diphenylamine based D-π-A type sensitizers/co-sensitizersVfieow rArticle Online
DOI: 10.1039/C9CP01032G
DSSCs: A comprehensive study on the impact of anchoring groups
Rajalakshmi Kesavan a, Islam M. Abdellah b, Surya Prakash Singh c, Ahmed El-Shafei b*,
Airody Vasudeva Adhikari a*
a Organic Materials Laboratory, Department of Chemistry, National Institute of Technology
Karnataka, Surathkal, Mangalore 575 025, India.
(Corresponding Author: Airody Vasudeva Adhikari, avachem@gmail.com)
b Polymer and Color Chemistry Program, North Carolina State University, Raleigh, NC 27695, USA.
(Corresponding Author: Ahmed El-Shafei, ahmed_el-shafei@ncsu.edu)
c CSIR- Indian Institute of Chemical Technology, Tarnaka, Hyderabad, Telangana 500 007, INDIA.
Abstract
Herein, we report the design, synthesis and characterization of a new series of simple
Donor - π spacer - Acceptor/Anchor (D-π-A) type diphenylamine based metal-free organic
dyes carrying three different anchoring groups, viz. 4-aminobenzoic acid (DTP), 2-(4-
nitrophenyl) acetonitrile (DTN), and barbituric acid (DTB), connected with 2-(thiophene-2-
yl)-acetonitrile, as effective sensitizers and co-sensitizers in Dye Sensitized Solar Cells
(DSSCs). They were subjected to photophysical, electrochemical and theoretical studies. The
dyes exhibited characteristic λabs and λemi in the range of 445-485 and 545-570 nm,
respectively. Both optical and electrochemical band gaps were found to be in the range of 2.2
to 2.35 eV. The driving forces for injection (ΔGinj), recombination (ΔGrec) and regeneration
(ΔGreg) processes were evaluated to understand its feasibility. Finally, the DSSC devices were
fabricated employing new dyes as sensitizers as well as co-sensitizers along with Ru(II)
based N3 dye. Interestingly, DTP carrying 4-aminobenzoic acid as anchoring group displays
the best photoelectrochemical performance, viz. Photovoltaic Conversion Efficiency (PCE) =
4.4%, open circuit potential (VOC) = 0.577V, and short-circuit current density ( JSC) = 9.06
mAcm-2 with a broad Incident Photon Conversion Efficiency (IPCE) spectrum.
Co-
sensitization of the dyes brought about enhanced VOC values, compared to N3 dye alone.
Finally, different interface resistance values obtained from the Electrochemical impedance
spectroscopy (EIS) circuit fitting, were used to study the fundamental processes of energy
conversion.
1

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DOI: 10.1039/C9CP01032G
Introduction
The quest for clean and sustainable energy to tackle the global energy crisis has led to
attraction of academic communities and industries towards the field of DSSC.1–4 In DSSCs, a
plethora of photo-physical sequences occur in the sensitizer, photoactive semiconductor and
the electrolyte, thus an efficient turning is required for these major components to achieve the
highest efficiency.5–9 In fact, desirable processes like dye injection (injection from dye to the
conduction band (CB) of TiO2 (-0.5V vs. NHE) and dye regeneration (the reduction of
oxidized dye to the neutral state by accepting electron from the redox couple) contribute
effectively to the overall performance of the device.10 On the other hand, some unwanted
processes such as recombination of the injected electrons in CB of TiO2 by the oxidized dye
-
(recombination) or I3 in the solution (dark current) that take place during the electron cycle
bring about lowering of JSC and VOC values.10 Consequently, a systematic approach is needed
to tackle these issues and hence to design new sensitizers for improving the photoconversion
efficiency. 11,12
Further, sensitizer is a crucial component in DSSCs, which is chemically adsorbed
onto the semiconductor to harvest sunlight and then to produce excited electron-hole pair.9,13–
15 An ideal dye should have a highest occupied molecular orbital (HOMO) level higher than
CB of TiO2 for injection of electron, whereas a lowest unoccupied molecular orbital
-
(LUMO) level lower than the redox potential of I3 /I- (0.4 V vs. NHE) favouring easy dye
regeneration process.10,16,17 In this regard, metal-free dyes are the most preferring candidates
when compared to the Ru(II) based metal complexes, as they possess many advantages such
as design versatility, cost-effective synthesis and superior molar extinction coefficients. 10,16,18
Amongst the several approaches used for designing the effective sensitizers, D-π-A
architecture, wherein a rigid structure bearing electron donating and electron accepting units
are linked via a π-spacer is known to be one of the extensively explored simple designs to
improve the photovoltaic parameters of DSSCs. In these dyes, an intra-molecular charge
transfer (ICT) takes place conveniently from electron rich donor to anchoring unit through a
π-spacer unit upon irradiation.14,19
Against this background, in the present systematic study three new D-π-A architecture
dyes, viz. DTP, DTN and DTB as effective sensitizers/co-sensitizers for DSSCs have been
designed. The schematic representation is given in Figure 1. In the new structural design,
diphenylamine scaffold was used as a donor system owing to its electron-rich properties,
2

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Physical Chemistry Chemical Physics
while 2-(thiophene-2-yl)-acetonitrile was incorporated as a π-spacer unit mainlDyOId: 1u0e.103tVo9ie/Cwi9AtCsrtPic0le1O0n3l2inGe
good conjugation, high polarizability, thermal stability and an excellent charge transporting
capability.18,20–25 In addition, 4-aminobenzoic acid (DTP), 2-(4-nitrophenyl)acetonitrile
(DTN) and barbituric acid (DTB) were introduced as electron acceptor as well as anchoring
units.26–31 Particularly, anchoring groups are vital as they enable light harvesting,
intramolecular charge transfer, and interfacial charge transfer phenomena in the cell.15 Here,
4-aminobenzoic acid was selected as an anchoring unit because of its strong adsorbing
property to TiO2 through carboxylic functionality. Further, 2-(4-nitrophenyl)acetonitrile and
barbituric acid, containing no carboxylic acid group were chosen because of the fact that they
possess good electron accepting ability that facilitates an effective interaction with TiO2.
Furthermore, one of the major drawbacks of carboxylic acid group is its high recombination
rate of electrons which can be rectified to some extent by incorporating non-carboxylic acid
groups. Thus, a thorough study on the effect of selected anchoring groups on the electronic
processes involved in DSSCs is required.
The newly designed organic dyes have been synthesized from the simple
diphenylamine following multi-step synthetic route. The target molecules and their
intermediates were well-characterized using spectroscopic techniques. Their electronic
properties and optical band gap were calculated from absorption/emission studies. Further,
the ground state oxidation potential (GSOP) and excited state oxidation potential (ESOP)
values were calculated and driving forces of processes involved during energy conversion
(ΔGinj, ΔGrec, and ΔGreg) were estimated from cyclic voltammetric (CV) studies. Furthermore,
density functional theory (DFT) was used to map the ground/excited (HOMO/LUMO)
electron distribution, electrostatic potential (ESP) distribution and density of states (DOS)
using Turbomole 7.2V software. Finally, they were employed in the fabrication of DSSCs as
sensitizers to investigate their photovoltaic performance. Also, they were used as co-
sensitizers along with Ru(II) based N3 dye to study the effect of co-sensitization. The
structure of N3 dye is shown in Figure 1. Their photovoltaic parameters, electron impedance
data (RS, RCT, RPt) and anchoring strength were analysed in depth with respect to sensitization
as well as co-sensitization behaviour.
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DOI: 10.1039/C9CP01032G
Figure 1. Schematic representation of D-π-A architecture dyes and chemical structure of N3
dye
Results and discussions
Synthesis and characterization
Scheme 1 outlines the synthetic pathway of the newly designed dyes DTP, DTN and
DTB. The requisite intermediates from 1 to 5 were synthesized from well documented
procedures with slight modifications.20,24 Required 4-(hexyl(phenyl)amino)benzaldehyde was
obtained by the Vilsmeier-Hack reaction protocol. Further, (E)-2-(5-formylthiophene-2-yl)-
3-(4-(hexyl(phenyl)amino)phenyl)acrylonitrile was prepared by following Knovenagel
reaction. The final compounds DTP, DTN and DTB were obtained by subjecting the
penultimate compound to Schiff base and Knovenegel condensation reactions. The
synthesized dyes were purified using either recrystallization or column chromatography
technique. The FT-IR, NMR, Mass spectra and corresponding characterization data of the
dyes are given in electronic supplementary information (ESI) Figures S1- S14.
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DOI: 10.1039/C9CP01032G
O
S
a
b
c
H
N
H
N
N
N
N
1
2
4
d
3
O
OH
e
N
S
O
H
N
NO2
S
N
f
N
DTP
N
N
N
S
5
N
g
O
O
NH
DTN
DTB
O
NH
S
N
N
Scheme 1. Synthetic procedure for sensitizers: (a) 1-Bromohexane, NaH, DMF, RT,10 hours
(b) POCl3, DMF, RT, 12 hours (c) 2-(Thiophene-2-yl)-acetonitrile, NaOMe, CH3OH, RT, 6
hours (d) POCl3, DMF, RT, 15 hours (e) 4-Aminobenzoic acid, methanol, glacial acetic acid,
o
o
60 C, 3 hours (f) 2-(4-Nitrophenyl)acetonitrile, piperidine, ethanol, 60 C, 3 hours (g)
Barbituric, methanol, reflux, 10 hours
Theoretical studies
The electron delocalization in HOMO/LUMO levels, ESP maps and DOS of dyes
were evaluated using the Turbomole software of 7.2 software package.22,23,32 DFT
simulations were done via the Becke’s three-parameter hybrid functional and Lee-Yang-
Parr’s gradient-corrected correlation functional (B3LYP) using def-TZVPP basis set. Figure
2 portrays the electronic cloud delocalization in FMO levels. The 3-D vision of the molecules
shows the clear charge delocalization in ground and excited states, thereby indicating the
effectiveness of ICT. As supported from their HOMO energy levels, the electron density is
predominantly delocalized in the diphenylamine ring and the vinyl thiophene system of the
dyes. However, at their LUMO levels, the electron cloud is clearly shifted from the electron
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donor to the electron acceptor units (4-aminobenzoic acid, 2-(4-nitrophenyl) aceDtoOnI:i1t0r.1i0le39/aCn9dCP01032G
barbituric acid) to different extent because of their varied electron withdrawing nature.
Further, it can be seen from Figure 2 that, the electron cloud is slightly away from the donor
unit in the HOMO-1 level, whereas it is clearly concentrated on the donor and vinyl
thiophene units in the HOMO level. Interestingly, DTN shows a complete shifting of
electron density to the anchoring group not in the LUMO level but in the LUMO+1 level.
While in case of DTB the electron density is concentrated on the spacer unit and the
anchoring moiety. Thus, it can be concluded that, all the three dyes show a close proximity of
their LUMO levels to the anchoring group facilitating the electronic coupling between the
electron cloud at the LUMO level of the dye and the d orbitals of TiO2. The HOMO levels
obtained for dyes DTP (-5.3 eV), DTN (-5.2 eV) and DTB (-5.5 eV) are sufficiently lower
than the electrode potential of redox couple (-4.8 eV) facilitating the regeneration process.8
Likewise, obtained LUMO levels of DTP (-2.6 eV), DTN (-3.1 eV) and DTB (-3.0 eV)
clearly show that, electron injection is also facilitated as the values are higher than the CB of
TiO2 (-4.0 eV). 33The calculated band gap of dyes are in the order DTP (2.7 eV) > DTB (2.5
eV)> DTN (2.1 eV).
Figure 2. HOMO/LUMO levels of dyes DTP, DTN and DTB
Figure 3 depicts the ESP maps of DTP, DTN and DTB on the iso-surface. Generally,
ESP maps give the distribution of total charge of a molecule at a point in space around the
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molecule which produces net electrostatic effect.19,34,35 Further, ESP is associated with the
DOI: 10.1039/C9CP01032G
total charge distribution possessing dipole moments, partial charges, electronegativity and
site of chemical reactivity of a molecule. The varying electrostatic potential values at surface
are represented by different colours, and the potential increases in the order blue > green >
yellow> orange > red. 34 As shown in ESP maps of the dyes, electron density is concentrated
on both CN and the anchoring groups. In DTP and DTB, it can be seen that the electron
density is concentrated on the carbonyl group of anchoring unit. While in case of DTN a
clear electron density concentration of nitro group is observed. An obvious shift of the
electron density from electro positive donor to electro negative conjugation spacer and then
to anchoring group is noted. The ESP maps suggest that the electron distribution is
favourable for the electron transfer from the donor to the anchoring units.
The DOS plots of all the dyes are given in ESI (Figures S15-S17). From the figure, it
is clear that, as the energy increases the contribution of the s orbital has shifted to p-orbital.
Here, the p orbital contribution of O and S atoms facilitates binding of dye with TiO2. All the
three dyes show the similar pattern of DOS maps indicating that, all of them meet requisites
of a good sensitizer.
Figure 3. Molecular Electrostatic Potential (ESP) mapping on iso density surface (a) DTP (b)
DTN and (c) DTB
Photophysical studies
The UV-Vis absorption spectra of all the dyes were recorded in 10−5 M CHCl3
solution. Their corresponding spectral data are listed in Table 1 and the spectra are depicted
in Figure 4. The absorption bands obtained in the studies are quite comparable with reported
dyes bearing diphenyl amine core.24 Evidently, the spectra show two distinctive absorption
maxima. The lower maxima lie in the region of 340-380 nm, that corresponds to π-π*
electronic excitations within the diphenylamine moiety. Whereas the higher maxima is in the
range of 440-485 nm, which corresponds to promotion of electron from HOMO to LUMO
and a little promotion from HOMO-1 to LUMO.36 The obtained UV-Vis spectra further
confirms the ICT phenomenon, discussed under the section theoretical study (Figure 2).36
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The observed λabs redshift of DTB (483 nm) compared to that of DTP (466 nm) and DTN
DOI: 10.1039/C9CP01032G
(445 nm) is mainly due to the higher electron withdrawing nature of barbituric acid.9
To get a realistic picture of light harvesting ability of dyes they were adsorbed on
TiO2 and their UV-Vis spectrum was recorded in separate experiments. It is observed that
absorption peaks of adsorbed dyes are red shifted and broadened (Figure 3) because of the
interaction of anchoring group with the titania surface. Generally, when sensitizers anchor
onto the mesoporous TiO2 surface, the resulted deprotonation and aggregation (H and J-
aggregation) of dye molecules affect the optical absorption profile. Deprotonation and H-
aggregation always result in a blue shift in the absorption peak, while J-aggregation mainly
leads to the red shift in the absorption peak. 37–40 All the dyes show redshift, which can also
be explained by the fact that the adsorption can lead to more delocalisation of dye thereby
decreasing the energy of π* orbital.29,38 DTP (531nm) shows a greater redshift of ~ 65 nm
than DTN and DTB confirming the better ICT process. Further, all the absorption spectra
show a markedly broad profile, indicating their good light-harvesting ability. Moreover, DTP
displays more intense bands when compared to other two dyes mainly due to higher
anchoring strength of carboxylic group. Conclusively, DTP is a potential sensitizer from the
view of its absorption behaviour.
The emission spectra of the dyes were recorded and the corresponding spectra are
featured in Figure 4. All the dyes display characteristic λemi in the region of 545-570 nm. It
is seen that the λemi of DTB (546 nm) was blue shifted when compared to the DTP (566 nm)
and DTN (565 nm). The perceived blue shift is attributed to presence of strong electron
withdrawing barbituric acid in the dye.33 Further, their optical band-gap and Stokes shift
values were calculated from the intersection between normalized absorption and emission
spectra. Their optical band gaps are in the decreasing order: DTN (2.32 eV) > DTP (2.31
eV) > DTB (2.23 eV) and Stokes shifts are in the increasing order: DTB (61 nm) < DTP (105
nm) < DTN (119 nm).
In order to study DTP’s behaviour in acidic and basic media, its absorption spectra
were recorded in presence of little trifluoroacetic acid (TFA) and triethylamine (TEA),
separately and their corresponding spectra are depicted in Figure 4. Usually, the acid group
exists in equilibrium with deprotonated and normal form. Here, DTP (677 nm) shows a
bathochromic shift in presence of TFA. The reason is that the TFA can protonate the
carboxylate ion in the deprotonated form which can push the equilibrium towards more
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neutral form leading to a bathochromic shift of the dye. But, the addition of TEA results in
DOI: 10.1039/C9CP01032G
the blue shift of DTP (255 nm) which is due to the deprotonation of dye. Further, the
behaviour of DTN and DTB in solvents with different polarity is given in the ESI (Figures
S21- S22). From this study, it can be concluded that the charge transfer between donor and
acceptor is very much sensitive to the solvent condition.39
Figure 4. (a) and (b) Normalized absorption and emission spectra of DTP, DTN and DTB
recorded in CHCl3 (3X10-5M), respectively (c) Absorption spectra of DTP, DTN and DTB
adsorbed on TiO2 (d) UV-Vis spectra of DTP in acidic and basic medium
Table 1. Photophysical properties of DTP, DTN and DTB (*experimental and # theoretical)
Parameters
*λabs (nm)a
DTP
466
DTN
445
DTB
483
546
61
*λemi (nm)b
566
565
*Stokes shift (nm)
*Eo-o, optical (eV)c
*Eox(V)
105
119
2.31
0.64
-1.67
-1.17
-1.14
-0.24
-5.3
2.32
0.7
2.23
0.6
*Eox*(V)d
-1.66
-1.16
-1.2
-0.3
-5.2
-3.0
-1.63
-1.14
-1.1
-0.2
-5.5
-3.1
*ΔGinj (eV)
*ΔGrec(eV)
*ΔGreg(eV)
#EHOMO (eV)
#ELUMO (eV)
-2.6
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a Absorption and emission spectra was measured CHCl3 (3×10-5M) at room temperature. View Article Online
DOI: 10.1039/C9CP01032G
b Dyes were excited at their absorption maximum value.
c Optical band gap was calculated from the intersection between the absorption and emission
spectra.
*
ox
d
E
 E
 E
ox
The E* value was formulated by
.
00
Electrochemical studies
The cyclic voltammetric studies were performed to investigate the thermodynamics of
electron movement from the excited states of dyes to the CB of TiO2 and the dye regeneration
from the redox couple in the electrolyte to the oxidised dyes. The observed results are
furnished in Figure 5 and Table 1.
The measurements were performed at room temperature with a conventional three-
electrode configuration comprising of a platinum disc as passive electrode, a glassy carbon
(GC) as a working electrode, and a Ag/AgCl as a reference electrode. The potentials were
reported vs. ferrocene as a standard reference at a scan rate of 100 mV/s using 0.1M tetra n-
butyl ammonium hexafluorophosphate (n-Bu)4N+(PF6)- as a supporting electrolyte in
acetonitrile under Argon atmosphere. From the voltammograms it is observed that all the
dyes display two characteristic peaks, the first irreversible peak in the range 0.6-0.7 V is due
the removal of electron from the amine unit, and the spacer unit and the second quasi-
reversible peak, at about -1.6 to -1.7 V arises from the reduction of electron withdrawing
anchoring group.41,42 The obtained voltammograms of all the dyes are given in the ESI
E
(Figure S18). The experimental ox (HOMO) values of the dyes DTP, DTN and DTB are
E*
0.6 V, 0.64 V and 0.7 V, respectively. Their ox (LUMO) values as calculated from Eqn 1
are -1.67 V,-1.66 V and -1.63 V, respectively. Further, it is noted that the HOMO level of
-
dyes are sufficiently lower than the redox energy of I3 /I- (0.4 V vs. NHE), thereby favouring
the dye regeneration process. On the other hand, the calculated LUMO levels of dyes are
higher than the CB of TiO2 (-0.5V vs. NHE); this facilitates the electron injection
phenomenon.33,43–45
E
E*
From the studies, it is observed that DTB has attained the lowest ox and ox values
which is the result of electron withdrawing nature of the barbituric acid. The presence of
barbituric acid has lowered LUMO level of dye making it closer to CB of TiO2 and has
pushed the HOMO level close to electrolyte potential.4,5
E*ox  Eox  E00
. . . . . . . . . (1)
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Where, E(0–0) is the excited state energy of the dye, which is evaluated from theDiOnIt: e10r.s10e3c9t/iCo9nCP01032G
between the normalized absorption and emission spectra.
To study more precisely about the feasibility of dye injection, recombination and
ΔGinj ΔG
ΔG
reg were calculated using the
regeneration processes, the driving forces
,
rec and
Eqns 2-4. The calculated values are given in Table 1.
ΔGinj  E*ox  ECB
. . . . . . . . . (2)
ΔGrec  ECB  Eox
. . . . . . (3)
ΔGreg  EI/I  Eox . . . . . (4)
3
ECB
Where
(0.4 V vs. NHE).8 All the calculated ΔG values are negative indicating aforementioned
ΔG
ΔG
rec are very close
is the CB of TiO2 (-0.5V vs. NHE) and
E
is the redox potential of electrolyte
I3/I
processes are thermodynamically favourable. As the values of
to each other, the feasibility of losing energy through recombination process is high. The dye
ΔG
ΔG
and
inj
DTN with
process. Whereas DTB is susceptible to lose energy by recombination process because of its
ΔG
ΔG ΔG
inj = - 1.16 eV and
rec = - 1.2 eV shows the predominance of injection
higher
ΔG
rec (-1.17 eV,) and
inj (-1.14 eV) values. Similarly, DTP with
rec (-1.14 eV, )
ΔG
reg values of
and
DTP, DTN and DTB are -0.24, -0.3 and -0.2 eV, respectively. From the results, it is clear
ΔG
inj (-1.1 eV) values follows the same trend.33 Further, the calculated
that DTN has more negative value of
reg showing its spontaneity for dye regeneration.
Generally, the dye should have sufficient driving force for its regeneration in order to prevent
recombination between the photo-oxidized dye and injected electrons, thereby causing higher
JSC value in it.46
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DOI: 10.1039/C9CP01032G
E
Figure 5. Schematic representation of working principle of DSSC with the calculated ox and
E*
ox levels along with drifting energy values
Photovoltaic and electron impedance studies
The photoelectrochemical properties of the devices sensitized with DTP, DTN and
DTB were determined to establish the impact of their anchoring group on solar cell
performance. A detailed fabrication procedure is furnished in the enclosed ESI. In the
experiment, 0.3 mM dye solutions were prepared in 1:1:1 acetonitrile, tert. butanol and
dimethyl sulfoxide solvent; co-absorbent chenodeoxycholic acid (CDCA, 10 mM) was used
in the dye solution which acts as an anti-aggregation reagent. The dyes were adsorbed on
mesoporous TiO2 electrode in presence of 10 mM CDCA co-adsorbent and measurements
were made using a standard Oriel solar simulator (AM 1.5 G, 100 mW/cm2) attached to
Keithley 2400 source meter.23,47 The characteristic J-V curves of the photovoltaic devices are
given in Figure 6 and the photovoltaic parameters such as VOC, JSC, fill factor (FF) and η are
summarized in Table 2. The relation of different photovoltaic parameters are given in Eqn 5.
IVmax
JSCVOC FF
η 

. . . . . . . . . (5)
P A
P
in
in
where Pin is the power of incident light and A is the active surface area (0.18cm2).13 The
DSSC device with DTP showed maximum PCE (η) value of 4.4% (JSC=9.73mA/cm2, VOC
0.68V, and FF = 66.99%) when compared to devices with DTN (JSC=0.488mA/cm2, VOC
=
=
0.45V, and FF =72.35%) and DTB (JSC = 64mA/cm2, VOC =0.47V, and FF = 74.40%). It is
interesting to note that, the anchoring ability of 4-aminobenzoic acid is better than the
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majorly used cyano acetic acid reported earlier.24 Its higher JSC value can be attrDibOuI: t1e0.d103t9o/Ci9tCsP01032G
good anchoring property with TiO2; accordingly, the electron injection to the semiconductor
is facile when compared to the DTB and DTN. Normally, the lower adsorption of dye results
in poor light harvesting ability further leads to lower JSC value. JSC is related to IPCE
according to Eqn 6.
JSC  e IPCE ph.AM1.5G ()(d)
. . . . . . . . . (6)


where IPCE(λ) is the incident photon-to-current conversion efficiency at a fixed wavelength,
e is the unit charge, and φph.AM1.5G(λ) is the corresponding photon flux of the solar radiation
spectrum at a fixed wavelength.36 Experiments were carried out to obtain IPCE spectra of the
fabricated devices using a QEX10 spectral response measurement system. Figure 6 portrays
the IPCE spectra and from the spectra, it is clear that DTP shows a broad IPCE spectrum than
DTN and DTB. This can be attributed to the good adsorption of DTP on TiO2 through 4-
aminobenzoic acid in comparison with DTN and DTB.48
Figure 6. (a) Photocurrent density vs Voltage (b) IPCE characteristics for DTP, DTN and
DTB sensitized devices and their co-sensitized devices with N3 under illumination of
simulated solar light (AM 1.5, 100 mW/cm2 )
Further, EIS studies were carried out for the devices sensitized with new dyes to get a
better understanding of the interfacial charge-transfer processes.49,50 Generally, electron flow
in DSSC is coupled with electronic and ionic transports. The driving force for the
aforementioned transports is obtained due to the internal current generated in the device
because of the different processes taking place in the dark and illumination. 51 The equivalent
circuit in Figure 7 has been used to fit the EIS data and the obtained data is given in Table 2.
13

Physical Chemistry Chemical Physics
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For the sensitization of dyes, the capacitance Cµ and CPt in the equivalent circuDitOhI:a10v.1e03b9/eCe9nCP01032G
replaced with constant phase element (CPE) since electrodes are having a rough surface.52 In
the Nyquist plots shown in Figure 7, the higher frequency intercept on the real axis of semi-
circle corresponds to series ohmic resistance (RS) whereas the high-frequency region gives
the interface capacitance (CPt) and charge transport resistance (RPt) at the counter
electrode/electrolyte interface. Similarly, the low-frequency region attributes to chemical
capacitance (Cµ) and the charge recombination (RCT) resistance at the TiO2/dye /electrolyte
interface. 29,48,53
In the present study, the RS value of DTP (20.01 Ω) and DTN (20.27 Ω) were found
to be almost similar but it is quite high for DTB (28.39 Ω) due to its high solvent resistance.
Usually, larger the RPt value more difficult is the transfer of the injected electron from the
TiO2 back to the electrolyte. This indicates the suppression of back recombination in the cell
18
device.
Thus, the larger the RPt leads to enhancement of VOC. Here DTN (108.5Ω) and
DTB (325.1Ω) show very high RPt value, whereas DTP demonstrates (23.42Ω) a lower
value. Even though the trends of RPt values are not in accordance with the obtained VOC
ΔG
values, it is quite agreeing with the observed trend of
RCT values obtained for the tested dyes: DTN (628.1 Ω) > DTP (9.528 Ω) > DTB (6.4 Ω)
ΔGinj
ΔG
reg values. The descending order of
which is in accordance with calculated
and
rec of dyes. Although the dye DTN
shows a fairly good resistance, it has shown inferior cell performance because of its poor
anchoring property. Evidently, the dyes DTN and DTB give Nyquist plots with larger radii,
τ
which can be explained by the fast electron transport and long lifetime ( eff ) of electron in
the film.52 This happens if the Rr (dark reaction impedance) >> Rd (diffusion impedance),
where Nernst impedance comes into play. In contrast, if Rd >> Rr, Greischer impedance
predominates which can be seen in the case of DTP, where a small semi-circle is observed. 52
τ
An effective life time eff of electrons in the CB of TiO2 is calculated for all the dyes
τ
using Eqn 7 from Bode plots portrayed in Figure 7. eff depends on the density of charge
traps which further contributes to VOC value.41
1
τeff

. . . . . . . . . (7)
2 π f
Where, ƒ is the frequency of the corresponding peak in the Bode phase plot. The parameter,
τeff
is related to the injection of electron to CB of TiO2 and can be explained using two
14

Page 15 of 22
Physical Chemistry Chemical Physics
widely used theories, (i) Marcus-Greischer theory which states that rate constants (orVlieiwfeAr-ticle Online
DOI: 10.1039/C9CP01032G
times) proportional to the energetic overlap between the TiO2 (e-) and the electron acceptors.
(ii) the theory based on the trap-limited diffusion/recombination applied to explain electron
transport and recombination in DSSCs. 13,38,54,55 It is observed that, calculated eff values for
τ
the dyes is in the decreasing order of DTN (3.18 ms) > DTP (2.87 ms) > DTB (2.00 ms). In
principle, longer electron lifetime specifies the better suppression of back reactions between
the injected electrons and the electrolyte, which normally leads to improvement of the VOC
4
τ
value. In the present study, the obtained
and VOC values are not in agreement with the
eff
τeff
ΔGinj
theoretical data. The observed higher
can be explained with regard to the greater
energy, but the poor adsorption property of DTN has resulted in poor performance of the cell.
τ
DTP with eff (2.87 ms) brought about good performance of device due to its good anchoring
property.
Figure 7. Electrochemical Impedance Spectra (a) Nyquist plots (b) Bode plots for DTP,
DTN and DTB sensitized devices and their co-sensitized devices with N3 dye
Co-sensitization studies
DSSCs were fabricated using DTP, DTN and DTB as co-sensitizers along with well-
known N3 dye and the detailed procedure is explained in the ESI. The corresponding I-V
curves obtained in co-sensitization experiments are shown in Figure 6 and their parameters
are summarized in Table 2. The co-sensitized devices with DTP, i.e. N3+DTP (η =5.44%
JSC=14.25 mA/cm2, VOC = 0.62V, and FF = 61.21%) and DTB, i.e. N3+DTB (η =5.43%, JSC
= 13.13 mA/cm2, VOC=0.61V, and FF = 67.94%) showed slightly higher PCE than N3 dye
alone (η = 5.39%, JSC = 14.39mA/cm2, VOC = 0.60V, and FF = 61.98%) whereas with DTN
i.e., N3+DTN (η = 5.32%,JSC= 14.63 mA/cm2, VOC = 0.66V, and FF = 54.80%) has
displayed lower PCE. It is to be noted that, all the co-sensitized devices showed an enhanced
15

Physical Chemistry Chemical Physics
Page 16 of 22
VOC value, which is due to the incorporation of co-sensitizers in between buDlkOyI: 10N.1033V9ie/dCwy9ACertPic0le1O0n3l2inGe
-
molecules resulting in the minimization of N3 dye aggregation and prevention of I3 ions to
penetrate into the TiO2 surface. But, the IPCE spectrum of the N3 dye shows a broad
spectrum from 300 to 800 nm with a maximum value of 72.966% at 535 nm. Its value
dramatically decreases to 40% in the wavelength region of 350-400 nm due to the
-
competitive light absorption between I3 and N3 dye. This competitive absorption can be
reduced by a co-sensitization process which will result in a broad and intense IPCE spectrum.
Thus, the broad IPCE spectrum shown by N3+DTP and N3+DTB is a result of the reduction
-
of light absorption of I3 . 38 The dye DTN along with N3 displays an IPCE spectrum similar
to that of N3 dye alone which may be due to the improper adsorption of DTN in between
bulky N3 dye molecules because of the poor adsorbing property of DTN. Also, the low PCE
of N3+DTN (5.32%) can be explained by the decreased FF (54.80%) which may have arisen
due to the high RS value. 19,34,35
Figure 7 shows the Nyquist plots of DTP, DTN and DTB co-sensitized with N3 dye.
For all the co-sensitizers the radius of semicircles obtained is similar and is having two
semicircles, unlike the sensitized dyes.56 The dye DTB shows a higher RS (29.19 Ω) but
lower JSC (13.13 mAcm-2) value than N3 alone; however, still it shows a higher PCE value.
RCT observed for N3+DTP (46.89 Ω ) is greater than that of N3 dye whereas the RS value
(27.1 Ω ) is similar to that of N3 (27.1 Ω). The RCT trend shown for co-sensitized devices are
in the decreasing order: N3+DTN (348.71 Ω) > N3+DTP (32.07 Ω) > N3+DTB (25.14 Ω).
The RPt value for N3+DTP is quite higher (46.89 Ω) than that of N3 (42.63 Ω) alone showing
the efficient filling of the voids in between N3 molecules. Although N3+DTN shows less RPt
value (7.80 Ω), the VOC has been enhanced to 0.66V.
EIS Bode plots (Figure 7) for N3, N3+DTP, N3+DTN and N3+DTB exhibit two
characteristic peaks; one at a higher frequency corresponding to the charge transfer at the
Pt/electrolyte interface and another at a lower frequency conforming to the charge transfer at
the TiO2/dye/electrolyte interface. These two peaks are arising due to the difference in local
-
-
I3 concentration in the device. Under illumination, the I3 is formed in the device (in situ) due
-
to the dye regeneration at TiO2/electrolyte interface. But in the dark condition, I3 generates at
counter electrode and it penetrates to TiO2 film by diffusion.52,56 The characteristic peak
appeared at the lower frequency in the Bode plot is related to the charge recombination rate,
and its reciprocal is associated with the electron lifetime.29 The
values were calculated
τeff
16

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Physical Chemistry Chemical Physics
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using the Eqn 7 and they were found to be in the decreasing order, N3+DTN D(6OI.:4190.10m39s/)C9>CP01032G
N3+DTB (4.39 ms) > N3+DTP (3.18 ms).
Table 2. The photovoltaic performance and EIS parameters for DTP, DTN and DTB
sensitized devices and their co-sensitized devices with N3 dye
Sensitizer
JSC
VOC (V)
FF(%) Ƞ(%)
RS
RCT
RPt
τeff
(mAcm-2)
(Ω)
(Ω)
(Ω)
(ms)
DTP
DTN
9.73
0.488
0.64
0.68
0.45
0.47
0.60
0.62
0.66
0.61
66.99
72.35
74.40
61.98
61.21
54.80
67.94
4.41
0.16
0.22
5.39
5.44
5.32
5.43
2.87 20.01 9.528 23.42
3.18 20.27 628.1 108.5
2.00 28.39 6.482 325.1
7.98 27.21 22.74 42.63
DTB
N3
14.39
14.25
14.63
13.13
N3 +DTP
N3+DTN
N3+DTB
3.18
27.1 32.07 46.89
6.49 28.23 348.7 7.803
4.39 29.19 25.14 19.37
Anchoring studies of the dyes on TiO2
FTIR spectroscopy technique was used to gain an insight into the nature of anchoring
group and mode of its interaction with TiO2. For this study, FT-IR spectra of DTP, DTN and
DTB adsorbed on TiO2 were recorded and the acquired spectra of DTP is given in Figure 8
and the spectra of DTN and DTB is given in ESI (Figure S19 - S20).47 From the spectra, it is
clear that DTP shows good adsorption on TiO2 whereas DTN and DTB display feeble
adsorption spectra because of the poor adsorption property of nitro and barbituric acid to the
TiO2 surface respectively. Further, the feeble adsorption capacity of nitro and barbituric acid
results in the very low concentration of respective dye on the TiO2 surface. The Deacon and
Philips had formulated certain rules to deduce the type of adsorption of the carboxylic acid
group on the TiO2 surface.57 They correlated the difference (Δν) between symmetric (Δνs) and
asymmetric (Δνas) stretching frequencies of a carbonyl bond of carboxylic acid functionality
-
and corresponding co-ordination type (Δν=Δνas(COO-)-Δνs(COO ) ). Evidently, three scenarios
are possible; if (Δν) of the dye adsorbed onto TiO2 (Δνabs) is smaller than that of the free
molecule (Δνfree), i.e. (Δνabs) < (Δνfree), then bidentate (either bridging or chelating)
adsorption mode would be present; if (Δνabs) <<< (Δνfree), in all probability the chelating
17

Physical Chemistry Chemical Physics
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mode would be present; if (Δνabs) > (Δνfree), then dye molecule takes a monodentate binding
DOI: 10.1039/C9CP01032G
mode .47
From the spectra, it is clear that DTP shows good adsorption on TiO2 whereas DTN and
DTB display feeble adsorption spectra. The dye DTP shows prominent bands at 1602 and
2211 cm-1 corresponding to the >C=O and -C≡N stretching, respectively. The dye adsorbed
on TiO2 clearly depicts bands at 1654 and 1512 cm-1 for the asymmetric and symmetric
stretching modes of the >C=O group, specifying that the deprotonated carboxylic group is
involved in the adsorption on TiO2. Further, the appearance of asymmetric and symmetric
peaks reveal that, the dye is chemisorbed, thereby ruling out the possibility of physisorption.
The dye DTN shows a N=O stretching band at around 1503 cm-1 and C≡N stretching peak at
about 2200 cm-1. After adsorbing on the TiO2 surface, feeble peaks corresponding to Δνas and
Δνs of -NO2 group are observed at around 1440 and 1320 cm-1, respectively, which are almost
same as that of unadsorbed dye, probably due to the very weak interaction with TiO2
surface.28 Further, DTB demonstrates N-H stretching vibration at 3171 cm-1 and C≡N
stretching peak at 2211 cm-1, while the adsorbed dye displays a weak peak at 2211 cm-1 due
to C≡N stretching with no N-H, and the C=O stretching bands. In the FTIR spectra of all the
dyes, the appearance of O-H vibration peak may be due to the residual Ti(OH)2 present in the
26
powder. Thus, it can be concluded that, the DTP has chemisorbed strongly on the TiO2
surface by means of bidentate chelation or a bridging rather than an ester linkage through the
carboxylate unit.
Figure 8. FTIR spectrum of DTP adsorbed on TiO2 surface
18

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DOI: 10.1039/C9CP01032G
Conclusions
In summary, we synthesised three simple D-π-A architectured dyes (DTP, DTN and
DTB) as effective sensitizers/co-sensitizers in DSSCs. They comprise simple diphenylamine
as donor scaffold, 2-(thiophene-2-yl)-acetonitrile as a a π-spacer unit and three varying
anchoring units, i.e. 4-aminobenzoic acid, 2-(4-nitrophenyl) acetonitrile and barbituric acid.
Among them, DTP demonstrates the highest PCE value of 4.4% (JSC=9.73 mA/cm2,
VOC=0.68 V, and FF=66.99%), due to the strong anchoring tendency of the dye to the TiO2
surface, as confirmed by its FTIR and UV-Vis spectral studies. The observed high JSC value
(9.73 mA/cm2) of DTP can be ascribed to its broad IPCE spectrum. Further, the prominent
electronic charge separation from the donor to acceptor/anchoring group, brings about
efficient injection of excited electrons into CB of TiO2, contributing to high PCE value. In
case of DTN, the rate of injection is more than recombination rate, as revealed by its CV (
ΔG
ΔG
rec -1.2 eV values) and EIS studies. However, it possesses poor
inj = -1.16 eV and
adsorption to TiO2 surface which led to poor performance. Upon co-sensitization, all the dyes
displayed enhanced VOC values which can be explained on the basis of their ability to
suppress the recombination process. The obtained results throw light on the importance of
driving forces in the various processes taking place during energy conversion as well as the
resistances involved in interfacial charge transfer processes. Conclusively, anchoring strength
of the dye is a major efficiency determining factor in DSSCs.
Acknowledgements
The authors are grateful to the NITK, Surathkal, India, for providing laboratory facilities. The
authors are also indebted to the Department of Textile Engineering, Chemistry and Science at
NCSU, Rayleigh and CSIR- IICT, Hyderabad, India for providing fabrication facility and
synthetic facility.
Conflict of interest
There are no conflicts of interest to declare.
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