Structurally simple D–A-type organic sensitizers for dye-sensitized solar cells: effect of anchoring moieties on the cell performance

Article abstract of DOI:10.1007/s13738-017-1180-3

Abstract: In this work, we report synthesis and device fabrication studies of four metal-free D–A-type dyes (A1–A4) based on structurally simple N,N-dimethyl-4-vinyl aniline carrying four different acceptor/anchoring groups, as sensitizers for sensitizing photoanode (TiO₂). In the sensitizers, N,N-dimethylaniline ring acts as an electron donor, while barbituric acid, N,N-dimethyl barbituric acid, thiobarbituric acid and N,N-diethyl thiobarbituric acid function as electron acceptor/anchoring units. They were synthesized in good yield via Knoevenagel protocol in neutral condition without any catalyst. Further, they were subjected to structural, electrochemical and optical characterization in order to evaluate their structure, band gap and absorption/emission behavior. The studies reveal that all the four dyes have thermodynamic feasibility of electron injection as well as electron recombination; their optical band gaps were found to be in the range of 2.35–2.56?eV. High-quality crystals of A2 and A4 were grown by slow evaporation technique using its solution with 1:1 pet ether (60–80?°C)/ethyl acetate solvent mixture at room temperature. Their SC-XRD studies disclose that the crystals are in the triclinic system with space group P-1. Further, DFT studies were performed using Turbomole V7.1 software package to evaluate their optimized geometry and HOMO and LUMO levels. Finally, DSSC device fabricated with the dye A1 showed relatively good efficiency when compared to other dyes mainly due to the effective binding of barbituric acid on the surface of TiO₂ through NH or OH functional group. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-017-1180-3

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1180-3
ORIGINAL PAPER
Structurally simple D–A‑type organic sensitizers
for dye‑sensitized solar cells: efect of anchoring moieties
on the cell performance
Praveen Naik1 · Rui Su2 · Dickson D. Babu1 · Ahmed El‑Shafei2 ·
Airody Vasudeva Adhikari1
Received: 9 February 2017 / Accepted: 5 August 2017
© Iranian Chemical Society 2017
Abstract In this work, we report synthesis and device fab-
rication studies of four metal-free D–A-type dyes (A1–A4)
based on structurally simple N,N-dimethyl-4-vinyl aniline
carrying four diferent acceptor/anchoring groups, as sensi-
tizers for sensitizing photoanode (TiO2). In the sensitizers,
N,N-dimethylaniline ring acts as an electron donor, while
barbituric acid, N,N-dimethyl barbituric acid, thiobarbituric
acid and N,N-diethyl thiobarbituric acid function as electron
acceptor/anchoring units. They were synthesized in good
yield via Knoevenagel protocol in neutral condition without
any catalyst. Further, they were subjected to structural, elec-
trochemical and optical characterization in order to evaluate
their structure, band gap and absorption/emission behavior.
The studies reveal that all the four dyes have thermodynamic
feasibility of electron injection as well as electron recom-
bination; their optical band gaps were found to be in the
range of 2.35–2.56 eV. High-quality crystals of A2 and A4
were grown by slow evaporation technique using its solution
with 1:1 pet ether (60–80 °C)/ethyl acetate solvent mixture
at room temperature. Their SC-XRD studies disclose that
the crystals are in the triclinic system with space group P-1.
Further, DFT studies were performed using Turbomole V7.1
software package to evaluate their optimized geometry and
HOMO and LUMO levels. Finally, DSSC device fabricated
with the dye A1 showed relatively good efciency when
compared to other dyes mainly due to the efective binding
of barbituric acid on the surface of TiO2 through NH or OH
functional group.
Electronic supplementary material The online version
of this article (doi:10.1007/s13738-017-1180-3) contains
supplementary material, which is available to authorized users.
* Ahmed El-Shafei
ahmed_el-shafei@ncsu.edu
* Airody Vasudeva Adhikari
avachem@gmail.com
1
Organic Materials Laboratory, Department of Chemistry,
National Institute of Technology Karnataka,
Surathkal, Mangalore 575 025, India
2
Polymer and Color Chemistry Program, North Carolina State
University, Raleigh, NC 27695, USA
Vol.:(0123456789)
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J IRAN CHEM SOC
Graphical Abstract
and easy movement of electron from donor to the accep-
tor moieties in the sensitizer. Moreover, such molecules
exhibit an interesting optical property owing to intramo-
lecular charge transfer (ICT) [5]. Normally, in D–A-type
structures, the electron acceptor system acts as a good
anchoring group, which binds chemically on the surface
of the semiconductor TiO2. Several organic chromophores,
based on ammonia derivatives such as triphenylamine
[6–11], diphenylamine [11], substituted anilines [12, 13],
with “push–pull” structures have been reported as good
sensitizers with decent photon conversion efciency, when
used in DSSCs. Obviously, N,N-dimethyl aniline [14, 15]-
based dyes act as good donors because of their favora-
ble electronic structure, i.e., the amino group attached to
the phenyl ring plays an important role in the structural
chemistry due to the ability of its lone pair of electron to
interact with the π system of the benzene ring through p-
π conjugation [16]. Further, the two methyl substituents
on the nitrogen atom along with its lone pair enhance the
electron density on the phenyl ring of substituted aniline.
Also, its electron-donating capability is further enhanced
due to its planar structure. In most of the aforesaid reports,
cyanoacetic acid was used as an efective acceptor moi-
ety along with substituted anilines as donors. Recently,
many research groups have reported barbituric acid [17,
18] as an efcient acceptor/anchoring moiety when com-
bined with a donor system because of its strong electron
withdrawing nature and better anchoring capability on to
the surface of semiconductor (TiO2) through N–H or O–H
functionality, which facilitates enhanced charge injection.
Keywords n-type sensitizer · DSSC · Barbituric acid ·
Knoevenagel condensation · DFT and TDFT calculations
Introduction
The quest for neat and cost-efective renewable energy has
led to added research attention on organic photovoltaics. Over
the past two decades, DSSCs have emerged as one of the best
alternatives for silicon-based solar cells for harvesting solar
energy after the report of pioneering work of Gratzel and cow-
orkers in 1991 [1]. DSSC consists of mainly three compo-
nents, viz. photoactive anode (usually TiO2 semiconductor), a
sensitizer, and an electrolyte, out of which the sensitizer plays
a vital role in the photon conversion efciency (η). Further, the
sensitizers are broadly divided into two categories, i.e., metal-
based complexes and metal-free organic dyes. The latter class
of dyes possesses several advantageous over the metal-based
sensitizers such as design versatility, high molar extinction
coefcients, cost-efectiveness and relatively good efciency.
Therefore, a lot of eforts have been dedicated to the design
and development of new metal-free organic dyes as potential
sensitizers [2–4] for DSSC application.
A survey of the pertinent literature reveals that vari-
ous design strategies such as D–A, D–π–A, D–A–π–A and
D–D–π–A confgurations are being used for developing
new metal-free sensitizers with improved efciency [3].
Among them, D–A-type architecture has attracted much
attention mainly due to an efficient charge separation
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J IRAN CHEM SOC
Against this background, in the present work, we syn-
thesized four organic dyes derived from N,N-dimethyl-
4-vinyl aniline with D–A-type configuration (A1–A4),
wherein N,N-dimethylaniline group functions as an elec-
tron donor, while barbituric acid, N,N-dimethyl barbituric
acid, thiobarbituric acid and N,N-diethyl thiobarbituric acid
act as electron acceptor/anchoring units. In the device, on
photo-oxidation of these dyes, the electrons are expected
to be delocalized from the planar 4-vinyl phenyl ring to
electron-accepting barbituric acid conveniently. Then, the
accumulated charge on electron acceptor moiety would
be injected into the conduction band of TiO2 through NH
or OH binding [16]. This follows regeneration of the dye
by I−/I3− electrolyte system. Thus, it is anticipated that the
synthesized dyes would show enhanced photon conversion
efciency in DSSCs.
Experimental
Materials and methods
The starting materials 4-(N,N-dimethylamino)benzaldehyde
(S), barbituric acid, N,N-dimethyl barbituric acid, thiobar-
bituric acid and N,N-diethyl thiobarbituric acid were pro-
cured from Sigma-Aldrich and were used without purifying
further. All the solvents were dried prior to use. The reac-
1
tion progress was monitored by TLC technique. H NMR
and 13C NMR spectra were recorded using a Bruker Avance
400 MHz using DMSO-d6 as solvent and TMS as an internal
standard. SC-XRD study was made using Bruker APEX duo
II. UV–visible spectra of the dyes were recorded at room
temperature using Analytik Jena Specord S 600 spectro-
photometer, and FTIR spectra were run using Bruker Alpha
spectrophotometer. The fuorescence spectra were recorded
using Jasco FP6200 fuorescence spectrophotometer. The
cyclic voltammetric (CV) measurements were taken on an
Ivium Vertex electrochemical workstation in acetonitrile
by using 0.1 M (n-Bu)4N+(BF4)− as supporting electrolyte.
The CV experiments were conducted by using the three-
electrode system, consisting of glassy carbon as the work-
ing electrode, platinum as a counter electrode and Ag/AgCl
as a reference electrode, and data were recorded at a scan
rate of 100 mV/s. Mass spectra were recorded on Thermo
Scientifc-EXACTIVE (ESI–MS), whereas the elemental
analysis was carried out on a Flash EA1112 CHNSO ana-
lyzer (Thermo Electron Corporation). The DFT calculations
were performed using Turbomole, V7.1 software package.
The dyes were obtained from structurally simple and
inexpensive 4-(N,N-dimethylamino)benzaldehyde in
good yield following single-step Knoevenagel synthesis
protocol (Scheme 1). The 3-D structures of A2 and A4
were confrmed by their SC-XRD studies. Furthermore,
UV–visible absorption and photoluminescent emission
studies were conducted in order to investigate their opti-
cal properties. Also, the dyes were subjected to cyclic vol-
tammetry studies with the purpose of determining their
electrochemical band gap as well as HOMO and LUMO
levels. Density functional theory (DFT) and time-depend-
ent density functional theory (TD-DFT) calculations with
the def-TZVVP basis set were performed with the aim
of acquiring knowledge on their optimized geometry and
frontier molecular orbitals. Finally, A1–A4 were applied
as sensitizers toward the fabrication of DSSC to study
their photovoltaic performance.
General method for synthesis of A1–A4
The dyes were synthesized as per the procedures reported
[19–24] with small modifcations. In the method, 4-(N,N-
dimethylamino)benzaldehyde (S, 0.5 g, 3.355 mmol) was
dissolved in 15 mL of absolute ethanol and to this respective
barbituric acid derivative (4 mmol) dissolved in 10 mL of
absolute ethanol was added slowly with stirring. The reac-
tion mixture was further stirred at 60 °C for 10 h. The pre-
cipitated solid was fltered and washed with absolute etha-
nol. The dyes A1–A4 were crystallized from appropriate
solvents. Their characterization data are as follows:
5‑(4‑N,N‑Dimethylaminobenzylidene)pyrimi‑
dine‑2,4,6(1H,3H,5H)‑trione (A1) Orange-colored solid
crystallized from ethanol, yield 91%. 1H NMR (400 MHz
DMSO-d6, ppm): 11.07 (s, 1H), 10.94 (s, 1H) 8.44–8.22
(d, J = 8 Hz, 2H), 8.16 (s, 1H), 6.81–6.79 (d, J = 8 Hz,
2H), 3.13 (s, 6H). 13C NMR (400 MHz DMSO-d6, ppm):
165.16, 163.19, 155.93, 154.63, 150.77, 139.52, 120.45,
111.67, 110.03. FTIR (ATR, cm−1): 3442 (N–H stretch-
ing), 2945 (C–H stretching), 1678 (C=O stretching). TOF
Scheme 1 Synthetic route of the chromophores A1–A4: (i) barbi-
turic acid, ethanol, 60 °C, (ii) N,N-dimethyl barbituric acid, ethanol,
60 °C, (iii) thiobarbituric acid, ethanol, 60 °C, (iv) N,N-diethyl thio-
barbituric acid, ethanol, 60 °C
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J IRAN CHEM SOC
HRMS ES + Calculated for C13H13N3O3: 259.2523. Found:
260.1164 [M + H]+.
Crystal structure
Crystal structure of 5‑(4‑N,N‑dimethylamino‑benzylidene)‑
1,3‑diethyl‑2‑thioxo‑dihydro‑pyrimidine‑4,6‑dione (A2)
5‑(4‑N,N‑Dimethylaminobenzylidene)‑1,3‑dime‑
thyl‑pyrimidine‑2,4,6‑trione (A2) Orange solid crystal-
lized from dichloromethane, yield 90%. 1H NMR (400 MHz
DMSO-d6, ppm): 8.44–8.42 (d, J = 7.2 Hz, 2H), 8.24 (s,
1H), 6.83–6.81 (d, J = 8 Hz, 2H), 3.23 (s, 6H). 3.14 (s,
6H), 13C NMR (400 MHz DMSO-d6, ppm): 163.72, 161.66,
156.80, 154.73, 151.73, 139.58, 120.49, 111.68, 109.79,
28.98, 28.36, FTIR (ATR, cm−1): 3040.70 (C–H stretching),
1652 (C=O stretching). TOF HRMS ES + Calculated for
C15H17N3O3: 287.3041. Found: 288.1443 [M + H]+.
High-quality orange-colored single crystal of A2 was
obtained by slow evaporation of its solution with 1:1 pet
ether (60–80 °C)/ethyl acetate solvent mixture. The single
crystal was subjected to SC-XRD analysis, and obtained
crystal data are presented in Table 1. The SC-XRD studies
reveal that dye A2 crystallizes in triclinic space group P-1
with cell parameters, a = 8.7515(3) (Å), b = 8.9890(3) Å,
c = 9.0880(3) Å, V = 696.81(4) Å3, Z = 2. Figure 1a shows
the ORTEP diagram of the dye A2.
5‑(4‑N,N‑Dimethylaminobenzylidene)‑2‑thioxo‑dihy‑
dro‑pyrimidine‑4,6(1H,5H)‑dione (A3) Red solid
crystallized from ethanol, yield 85%. 1H NMR (400 MHz
DMSO-d6, ppm): 12.16 (s, 1H), 12.06 (s, 1H) 8. 49–8.47
(d, J = 6.8 Hz, 2H), 8.16 (s, 1H), 6.85–6.83 (d, J = 7.6 Hz,
2H), 3.17 (s, 6H). 13C NMR (400 MHz DMSO-d6, ppm):
177.98, 163.19, 155.93, 154.63, 150.77, 139.52, 120.45,
111.67, 110.03. FTIR (ATR, cm−1): 3499.50 (N–H stretch-
ing), 3049.93 (C–H stretching), 1684 (C=O stretching). TOF
HRMS ES + Calculated for C13H13N3O2S: 275.3179 Found:
276.0383 [M + H]+.
Crystal structure of 5‑(4‑N,N‑dimethyl‑
amino‑benzylidene)‑1,3‑diethyl‑2‑thioxo‑dihydro‑pyrimi‑
dine‑4,6‑dione (A4)
Superior-quality shining red-colored single crystal of dye
A4 was obtained by very slow evaporation of its solution
with 1:1 pet ether (60–80 °C)/ethyl acetate solvent mixture.
SC-XRD analysis was carried out, and obtained crystal
Table 1 Crystal and structure refnement data for dyes A2 and A4
5‑(4‑N,N‑Dimethylaminobenzylidene)‑1,3‑diethyl‑2‑thi‑
oxo‑dihydro‑pyrimidine‑4,6‑dione (A4) Red solid crys-
Compound
Formula
Formula weight
CCDC number
Temperature (K)
Crystal form
Color
Crystal system
Space group
a (Å)
A2
A4
C15H17N3O3
287.32
C17H21N3O2S
331.43
1
tallized from ether and ethyl acetate, yield 85%. HNMR
(400 MHz DMSO-d6, ppm): 8.48–8.46 (d, J = 8.8 Hz, 2H),
8.27 (s, 1H), 6.88–6.85 (d, J = 8.8 Hz, 2H), 4.46 (s, 4H),
3.19 (s, 6H), 1.22 (s, 6H), 13CNMR (400 MHz DMSO-d6,
ppm): 190.41, 159.30, 158.55, 155.54, 140.49, 112.08,
111.54, 109.27, 43.23, 12.77, FTIR (ATR, cm−1): 3055.08
(C–H stretching), 1682 (C=O stretching).TOF HRMS
ES + Calculated for C17H21N3O2S: 331.4215. Found:
332.1474 [M + H]+.
1,456,338
296 (2)
Block
Orange
Triclinic
P-1
8.7515 (3)
9.0880 (3)
81.822 (2)
81.299 (2)
83.835 (2)
696.81 (4)
2
1,490,875
296 (2)
Block
Red
Triclinic
P-1
7.9072 (2)
10.3716 (3)
88.3470 (10)
79.4360 (10)
86.0360 (10)
823.95 (4)
2
c (Å)
a (0)
b (0)
g (0)
Results and discussion
Volume (Å3)
Z
Synthesis and structure characterization
Density (g cm−3
)
1.369
0.097
1.336
0.210
µ (mm−1
)
The synthetic pathways of the four dyes are depicted in
Scheme 1. We have used Knoevenagel condensation pro-
tocol to obtain fnal compounds. The inexpensive starting
compound 4-(N,N-dimethyl amino)benzaldehyde (S) was
reacted with selected anchoring molecules, viz. barbituric
acid, 1,3-dimethylbarbituric acid, thiobarbituric acid and
1,3-diethylthiobarbituric acid to yield target molecules
A1–A4 in good yield. The melting point and spectral data
of dyes A1–A4 match with reported values [23, 24].
F (000)+
hmin, max
kmin, max
lmin, max
Refections collected
Rall, Robs
wR2_all, wR2_obs
GOOF
304
352
−9, 9
−10, 10
−11, 10
−11, 11
2736
0.0541, 0.0441
0.1350, 0.1207
0.674
−12, 12
−12, 12
3239
0.0440, 0.0393
0.1281, 0.1197
0.835
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J IRAN CHEM SOC
Fig. 1 a ORTEP diagram of dye A2, b ORTEP diagram of dye A4
data are presented in Table 1. The SC-XRD studies indi-
cate that dye A4 crystallizes in triclinic space group P-1
with cell parameters, a = 7.9072(2) (Å), b = 10.2460(3) Å,
c = 10.3716(3) Å, V = 823.95(4) Å3, Z = 2. Figure 1b
depicts the ORTEP diagram of the dye A4, from which it
is quite evident that the dye is almost planar, which favors
the fow of electron density from the donor segment to the
acceptor segment through the π-conjugation bridge and
thereby enhancing overall efciency of the sensitizers.
Fig. 3 Fluorescence emission spectra of dyes A1–A4 at 10−5 M con-
centration
clear that the dyes A1 and A2 exhibit single distinct absorp-
tion band, while dyes A3 and A4 display two major absorp-
tion bands. In case of compounds A1 and A2, the single peak
at around 470 nm is attributed to the intramolecular charge
transfer (ICT) from donor, i.e., 4-(N,N-dimethylamino)phe-
nylene group, to the corresponding acceptor unit, i.e., bar-
bituric acid or N,N-dimethyl barbituric acid. While the dyes
A3 and A4 show absorption bands in the shorter wavelength
(250 and 350 nm) corresponding to π–π* electronic excita-
tions localized within the 4-(N,N-dimethylamino)phenylene
group, whereas the intense band in the longer wavelength
region (496 and 500 nm) can be attributed to an intermo-
lecular charge-transfer (ICT) transition from the donor to the
acceptor segment [25]. The observed values of molar extinc-
tion coefcients in all the sensitizers ranging from 32,000 to
83,000 M−1 cm−1 endorse their fairly good light-harvesting
ability. Further, the calculated Stoke shift values for the sen-
sitizers A1–A4 were found to be fairly large, i.e., 41–79 nm,
which can be ascribed to the moderate charge transfer from
the donor to the acceptor unit.
Photophysical properties
The UV–visible absorption and photoluminescence emis-
sion spectra of dyes A1–A4 were recorded in chloroform
solution at a concentration of 10−5 M. Figures 2 and 3 depict
the UV–visible absorption and photoluminescence emission
spectra of A1–A4, respectively. The relevant spectral results
are presented in Table 2. From the absorption spectra, it is
Electrochemical measurements
Cyclic voltammetry (CV) experiments were carried out
in order to determine the ground-state oxidation potential
(GSOP) and excited-state oxidation potential (ESOP) values
of the synthesized sensitizers (A1–A4). These values ofer a
deeper insight into the thermodynamic driving force of elec-
tron injection from the ESOP of dye molecule into the con-
duction band edge of semiconductor TiO2 and dye regenera-
tion via replenishing the hole generated in the GSOP by the
redox electrolyte. The cyclic voltammetric traces of the dyes
A1–A4 were recorded using Ivium Vertex electrochemical
Fig. 2 UV–visible absorption spectra of dyes A1–A4 at 10−5 M con-
centration
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J IRAN CHEM SOC
Table 2 Optical and
Dye λabs (nm) λem (nm) Stokes shift ε (M−1 cm−1
)
Eox (V)
E0–0, optical HOMO (eV) LUMO
electrochemical characterization
data of dyes A1–A4
(nm)
(eV)
(eV)
A1 473
A2 467
A3 500
A4 498
514
546
561
566
41
79
61
68
32,000
45,500
59,000
83,000
1.25
0.78
1.12
0.89
2.54
2.57
2.35
2.36
−5.65
−5.18
−5.52
−5.29
−3.11
−2.67
−3.17
−2.93
λabs, Absorption maxima; λem, emission maxima; Eox, oxidation potential; E0–0, voltage of intersection point
between absorption and emission spectra
Fig. 5 Energy-level diagram of dyes A1–A4
providing ample driving force for efective dye regenera-
tion. In addition, ESOP of A1 (−2.86 eV), A2 (−3.00 eV),
A3 (−3.07 eV) and A4 (−3.23 eV) was found to be higher in
Fig. 4 CV traces of dyes A1–A4
energy than the conduction band edge of mesoporous TiO2
(−4.2 eV) [30], showing an efcient electron injection. Evi-
dently, electron injection free energy for dyes is in the order,
i.e., A4 > A2 > A3 > A1, while dye regeneration free energy
for them is in the same order, due to the fact that all the dyes
have similar electron-donating group as well as almost alike
HOMO delocalization [31].
workstation. A thin flm of the dye was obtained from the
dye solution using drop-cast method on a glassy carbon elec-
trode, which works as working electrode. Experiments were
performed using Pt electrode as a counter electrode and Ag/
AgCl as a reference electrode, immersed in an electrolyte
[0.1 M tetrabutylammonium tetrafuoroborate in acetonitrile]
at a scan rate of 100 mV/s. The obtained cyclic voltammo-
grams of A1–A4 are displayed in Fig. 6, and the correspond-
ing values are summarized in Table 2 (Fig. 4).
As per earlier reports, under optimized conditions, elec-
tron injection free energy greater than 150 meV should
result in efcient electron injection [32] and dye regenera-
tion free energy as low as 250 meV must lead to efcient
dye regeneration. All the synthesized dyes meet stringent
thermodynamic requirements, as shown in energy-level dia-
gram (Fig. 5). Further, it has been observed that negative
free energy for electron injection is the highest for A1. Also,
its dye regeneration free energy was found to be the lowest.
Accordingly, A1 showed the highest efciency among the
four dyes. In case of the dye A3, the negative free energy
for injection is almost same as that of A1, predicting that the
latter will be almost same efcient as the former. This is also
attributed to the observed lowest band gap for A3, whereas
the other two dyes A2 and A4 showed lower efciency as
predicted from their observed free energy values.
The obtained CV data were used to compute the oxida-
tion onset values (GSOP). The energy gap of the dye, E0−0
,
was calculated from its absorption and normalized emission
spectra. Further, E0−0 and GSOP values were employed to
calculate the excited-state oxidation potential (ESOP); the
values in volts (V) against NHE were converted to electron
volt (eV) according to the following equation [26–28]:
[
]
ESOP = (GSOP(V) + 4.7) − E0−0 eV
Figure 5 depicts the energy-level diagram, showing the
GSOP and ESOP values of A1–A4 dyes. It has been
observed that GSOP values of dyes A1 (−5.40 eV), A2
(−5.57 eV), A3 (−5.42 eV) and A4 (−5.59 eV) are lower
than those of I−∕I− redox system (−5.2 eV) [29], thus
3
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J IRAN CHEM SOC
Fig. 6 a Optimized structures and frontier molecular orbitals (HOMO and LUMO) of dyes A1–A4, b–e simulated absorption spectra of dyes
A1–A4, f–i simulated vibration spectra of dyes A1–A4
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J IRAN CHEM SOC
Fig. 6 (continued)
Theoretical calculation
excitations of dyes in the presence of time-dependent
perturbations. Typically, in TD-DFT, a time-dependent
Schrödinger equation is solved to obtain the excited-state
properties of the material. Here, an adiabatic approxima-
tion is made and following which temporal non-locality is
neglected. According to the aforementioned approximation,
at any point of time, the exchange–correlation (xc) func-
tional depends only on the instantaneous density [35].
As a result of aforementioned approximation, tradi-
tional approximations (time independent) can be applied
to the xc-functional derived for ground-state DFT, i.e., BP
(Becke–Perdew) and hybrid functionals (B3LYP) [34]. Typi-
cally, the precision of the results acquired using TD-DFT
profoundly depends on the functional and basis set employed
for the calculations. The simulated absorption spectrum of
A1 obtained at the B3LYP functional and def-TZVPP basis
set is depicted in Fig. 6b. Indeed, TD-DFT is known for its
inabilities in precisely predicting energies related to long-
range charge-transfer states due to the self-interaction error
in TD-DFT, which is owing to the electron transfer in the
extended charge-transfer state. The above-mentioned vari-
ance is not observed in the present study, as the molecules
under investigation are not large enough to display a long-
range intramolecular charge transfer. Figure 6b portrays
the simulated absorption spectrum of A1 obtained at the
B3LYP functional. It shows only one peak, corresponding
to its charge-transfer process. Similarly, A2 displays only
one peak as given in Fig. 6c. Simulated absorption spectra
of A3 and A4 are given in Fig. 6d, e, respectively, from
DFT and TD-DFT calculations were performed for all the
dyes using Turbomole V7.1 software in order to gain deeper
understanding of the electronic and optical properties of the
molecules. To begin with, the ground-state geometries were
optimized with semiempirical AM1 basis with MOPAC
in TmoleX. Further optimization of the aforementioned
geometries was performed using C1 point group symmetry
at DFT level via the Becke’s three-parameter hybrid func-
tional and Lee–Yang–Parr’s gradient-corrected correlation
functional (B3LYP) [33, 34]. Furthermore, in all the cal-
culations, default convergence criteria were maintained for
the optimizations. All calculations were performed using
def-TZVPP basis set. The optimized geometries and electron
density distributions in the HOMOs and LUMOs of the four
dyes are summarized in Fig. 6a. As evidenced from Fig. 6a,
in the HOMO level, the electron density is mainly located
on the 4-N,N-dimethylaniline segment of all the molecules,
whereas in the corresponding LUMOs, a clear shift of elec-
tron density toward acceptor group, i.e., barbituric acid, N,N-
dimethyl barbituric acid, thiobarbituric acid and N,N-diethyl
thiobarbituric acid, is observed. Thus, a close proximity of
the LUMO levels to the anchoring group would facilitate
considerable orbital overlap between the LUMO levels and
3d orbitals of titanium. This would ease efcient injection of
excited electrons into conduction band of TiO2.
Further, time-dependent density functional theory (TD-
DFT) studies were conducted to examine the electronic
1 3

J IRAN CHEM SOC
Table 3 Photovoltaic measurement data of dyes A1–A4
which it is evident that the molecules exhibit two distinct
bands, corresponding to π–π* and charge-transfer phenom-
enon. The simulated data are well in accordance with the
experimentally obtained spectra. These precise and reli-
able predictions made by TD-DFT studies indicate that the
functional and basis set chosen for TD-DFT calculations are
quite appropriate.
Dye
VOC (V)
JSC (mA cm−2
)
FF %
η %
A1
A2
A3
A4
0.501
0.460
0.476
0.472
0.70
0.23
0.70
0.35
67.0
60.4
67.1
64.5
0.244
0.065
0.223
0.108
Further, the stimulated vibration spectrum was obtained
using the B3LYP functional at the def-TZVPP basis set. Fig-
ure 6f, h depicts the simulated vibration spectra of A1 and
A3 obtained at the B3LYP functional. As observed from
the simulated spectra of A1 and A3, the molecules exhibit
a broad peak at around 3600 cm−1 that correspond to NH
functionality of the dyes. The obtained data match with the
experimental spectral data. Further, the simulated vibration
spectra of A2 and A4, as depicted in Fig. 6g, i, respectively,
clearly show the absence of NH functionality in the dyes.
Henceforth, the theoretically obtained UV–visible and vibra-
tion spectral data are in good agreement with the experimen-
tally acquired spectra.
VOC values mainly due to their heavy aggregation on the sur-
face of TiO2. Among the tested dyes, the dye A1 displayed
maximum efciency of 0.244% due to the strong binding
of barbituric acid on TiO2. While the dye A2 showed the
lowest efciency of 0.065% due to the non-availability of
free NH group to bind on the surface of mesoporous TiO2
efectively. The dye A3 exhibited an efciency 0.223% com-
parable to that of A1, owing to binding of sulfnyl as well
as NH group, whereas the dye A4 disclosed an efciency of
0.108%, which is attributed to non-availability of free NH
functionality. Compared to A1 and A3, the binding strength
of A2 and A4 on TiO2 surface is weak. The possible bind-
ing mode for A2 and A4 is monodentate coordinating mode
through a C=O or C=S group with Ti on the TiO2 surface,
while in A1 and A3, the binding mode is bidentate bridging
due to the tautomeric forms [38]. The weak binding strength
caused insufcient electron injection from the sensitizer to
the TiO2, leading to the decrease of Jsc and the overall ef-
ciency. The results obviously indicate that conversion ef-
ciency of all the dyes is not promising, even though they
display good photophysical and electrochemical properties.
It has been well established that one of the striking phe-
nomena, i.e., dye aggregation, signifcantly infuences its
electron injection behavior [39], causing adverse efect on
the overall performance of DSSCs. Evidently, the absence
of long-chain bulky groups in the dyes A1–A4 has favored
the excessive aggregation allowing the electrolyte to interact
directly with the TiO2 and hence instigating back current in
the cell [18, 40]. In conclusion, the absence of long hydro-
phobic chains results in the facilitated dye aggregation and
undesired charge recombination, leading to the decrease of
VOC values and the overall efciencies.
Photovoltaic characterization
In order to establish the structure–performance relationship
for synthesized dyes (A1–A4), DSSCs were fabricated as per
the protocol [36, 37] described in supplementary informa-
tion. In the experiment, dye solutions were prepared in a par-
ticular concentration and directly used for device fabrication
studies. The current–voltage (J–V) plots were obtained by
using Keithley 2400 source meter under illumination of AM
1.5G solar light from a solar simulator (SOL3A, Oriel). Fig-
ure 7 shows J–V curves of sensitizers A1–A4-based DSSCs
under AM 1.5G simulated sunlight with a light intensity of
100 mW/cm2. The observed photovoltaic parameters, viz.
open-circuit photovoltage (VOC), short-circuit photocurrent
density (JSC), fll factor (FF) and overall solar light to elec-
tricity conversion efciency (η), are summarized in Table 3.
From the results, it is clear that all the dyes show low JSC and
Conclusions
We have synthesized four structurally simple organic D–A-
type sensitizers (A1–A4), based on the donor N,N-dimethyl-
4-vinyl aniline carrying four diferent acceptor/anchoring
groups, viz. barbituric acid, N,N-dimethyl barbituric acid,
thiobarbituric acid and N,N-diethyl thiobarbituric acid, for
DSSC application. The synthesized dyes were subjected
to photophysical, electrochemical, theoretical and device
fabrication studies. The results reveal that the dyes show
Fig. 7 J–V characteristics of the DSSCs sensitized with dyes A1–A4
1 3

J IRAN CHEM SOC
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