Azafluorene Ornamented Thiazine Based Novel Fused Heterocyclic Organic Dyes for Competent Molecular Photovoltaics

Article abstract of DOI:10.1016/j.electacta.2017.06.151

Described herein is a series of azafluorene ornamented thiazine based novel fused heterocyclic organic dyes TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2. Nanocrystalline titania based DSSC performance along with photophysical/electrochemical behaviors of t

Full text of DOI:10.1016/j.electacta.2017.06.151

Accepted Manuscript
Title: Azafluorene Ornamented Thiazine Based Novel Fused
Heterocyclic Organic Dyes for Competent Molecular
Photovoltaics
Authors: Kesavan Stalindurai, Ayyanar Karuppasamy, Jia-De
Peng, Kuo-Chuan Ho, Chennan Ramalingan
PII:
S0013-4686(17)31389-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.06.151
EA 29789
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Electrochimica Acta
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Revised date:
Accepted date:
30-1-2017
15-5-2017
24-6-2017
Please cite this article as: Kesavan Stalindurai, Ayyanar Karuppasamy, Jia-De Peng,
Kuo-Chuan Ho, Chennan Ramalingan, Azafluorene Ornamented Thiazine Based
Novel Fused Heterocyclic Organic Dyes for Competent Molecular Photovoltaics,
Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.151
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Azafluorene Ornamented Thiazine Based Novel Fused Heterocyclic Organic
Dyes for Competent Molecular Photovoltaics
Kesavan Stalinduraia, Ayyanar Karuppasamya, Jia-De Pengb, Kuo-Chuan Hob,*, Chennan
Ramalingana,*
a Department of Chemistry, Kalasalingam University, Krishnankoil – 626 126, Tamilnadu, India
b Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
Corresponding author E-mail: ramalinganc@gmail.com (C. Ramalingan),
kcho@ntu.edu.tw (K.-C. Ho)
GRAPHICAL ABSTRACT
HIGHLIGHTS
 A series of azafluorene and thiazine based fused heterocyclic dyes has been synthesized.
 Photophysical / electrochemical behaviors of the dyes have been examined.
 Computational study - well separated distribution of electrons in donor and acceptor
motifs.
 The dye TBCPCA-2 carrying device exhibited the highest power conversion efficiency.
1

ABSTRACT
Described herein is a series of azafluorene ornamented thiazine based novel fused heterocyclic
organic dyes TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2. Nanocrystalline titania
based DSSC performance along with photophysical / electrochemical behaviors of the
aforementioned dyes have been examined. Utilizing computational approach, distribution of
electrons within the molecules has been explored. The devices possessing the novel fused dyes
TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 as sensitizers exhibited overall
power conversion efficiency range between 6.17±0.4 and 7.97±0.3%. The novel fused
heterocyclic organic dyes featuring di-tert-butylazafluorene structural motif such as TBCPCA-1
and TBCPCA-2 exhibit higher power conversion efficiency than the ones carrying sterically
congested tetra-tert-butyl-tri(azafluorene) scaffold viz., TBTCPCA-1 and TBTCPCA-2.
Amongst the devices fabricated with the utilization of novel heterocyclic dyes as sensitizers, the
device possessing the dye TBCPCA-2 as sensitizer displayed the highest power conversion
efficiency of 7.97±0.3% along with a Voc of 745±0.6 mV, a Jsc of 16.92±0.3 mA.cm-2 and a ff of
0.67±0.003.
Keywords: Azafluorene, Heterocycles, Organic dyes, Solar cells, Powerconversion efficiency
1. Introduction
In recent years, growing energy hassle and issue regarding global-warming make great
focal point of the scientists on the sources of renewable energy. To address these concerns,
inexorable research efforts have been committed to DSSCs [1-6], feature of an inter-connected
system of large band-gap nano semi-conducting materials, from materials syntheses, device-
physics and interface-engineering features. The efficiency of the DSSCs is fundamentally
2

affected by the sensitizer, the anode, the cathode, and the electrolyte. Of the factors that affect
significantly the device power conversion efficiency, the dyes participate a vital part for DSSCs
to achieve high PCE [7,8]. Conventionally, DSSCs with elevated PCEs were first accomplished
with Ru-polypyridyl based sensitizers [9-19], whose performances were soon after exceeded by
Zn-porphyrin based dyes [3,7]. Although the aforementioned ones are efficient sensitizers, they
have their own limitations. Specifically, a Ru-based dye has draw-backs of source paucity and
toxicity while Zn-porphyrin dye obtained in lower yield and requires awfully lethal
reactants/reagents. Thus, sensitizers with no metals, which possess D-π-A architecture, have also
fascinated attention due to molecular design flexibility, raw materials abundance, economic
suppleness, and bright colours [20-24]. It has recently been described that electrons (injected)
recombination with the dye (oxidized) / electrolyte rigorously reduces photo-current density [25].
Vitally, the ability of organic DSSCs is reliant on its structural and electronic distinctiveness [26-
33]. On comparing the semiconductor (CB) energy, the LUMO energy of the dyes has to be
superior for the thermodynamic possibility of charge injection whereas for the charge generation,
the dyes having strong and broader charge-transfer absorption are favorable [31]. Hence, several
structural modifications including incorporation of alkyl chains have been made to encumber the
reactions with recombination and participate to the raise in Jsc and Voc of the devices of DSSCs
[34-37]. In this context, DSSCs with various types of metal-free dyes as sensitizers have been
constructed and offered appreciable efficiencies [38-41]. This demonstrates the great potential of
organic dyes as efficient DSSC sensitizers. Although various types of organic dyes have been
documented as DSSC sensitizers, synthesis of novel organic dyes as efficient sensitizers is still
desired.
3

Figure 1
Metal free materials possessing azafluorene / thiazine based fused heterocycles have been
utilized as building blocks in a diverse range of electronic devices which include OLEDs and
DSSCs [42-60]. The first thiazine based fused heterocycle and cyanoacrylic acid structural motifs
possessing organic dye has been reported with PCE of 5.5% by Hagfeldt and Sun et al in 2007
[61] wherein N-butyl thiazine based fused heterocyclic structural motif play a role of donor and
acrylic acid scaffold serve as acceptor (A) (Figure 1). Besides, it is recognized that substituted
amine functionalities serve as competent electron donors in DSSCs and a diverse range of
organic dyes possessing amine functionalities as donors has been documented in the literature.
Since 2007, various types of amine functionalities on 6-position of thiazine based fused
heterocyclic unit (i.e., in A) have been introduced by several research groups around the globe
(A1-A7) and achieved power conversion efficiencies up to 7.1% [62-64]. We herein construct
four azafluorene ornamented thiazine based novel fused heterocyclic organic dyes wherein di-
tert-butylazafluorene
/
sterically conjested tetra-tert-butyltri(azafluorene) as donors,
alkylsubstituted fused thiazines as spacers, and acrylic acid as an acceptor as illustrated in Figure
2. We initially hypothesized that introduction of azafluorene / multi-azafluorenes would improve
the electron donating ability besides extending π-conjugation and, further incorporation of di /
multi tert-butyl bulky groups on azafluorene scaffold would restrain the intermolecular
aggregation, enhance the solubility and increases the light resistant capability of the dyes [50,65-
68]. The electron rich nitrogen and sulfur in fused thiazine structural motif would also facilitate
the donation of electrons in a competent manner. The non-planar butterfly conformation of the
thiazine ring of fused heterocyclic moiety would hamper aggregation of the dye and the
4

formation of intermolecular excimers besides the long chain alkyl group which would enhance
the solubility and control the aggregation [36,58,61]. Further, the π-delocalization of fused aryl
groups would extend over the complete thiazine possessing aromatic chromophore as two fused
aryl moieties in the same are set in an undersized angle (torsion) associated with sulfur and
nitrogen atoms. Eventually, incorporation of cyanoacrylic acid would competently
accept/withdraw the electrons and also graft on the TiO2 oxide layer.
Figure 2
2. Experimental Section
2.1. General
Standard procedures were utilized for distilling /drying the solvents. Reagent quality
reagents were purchased and used as such. Pre-silica coated aluminium plates (60, F254) were
utilized for TLC and the solvents specified were used as eluents. Column-Chromatography was
carried-out on Silica gel (spherical, 100-200 mesh) slurry packed in glass columns. The eluting
solvents employed for respective separations are provided in the individual experimental
procedures section. NMR spectra were acquired on a Bruker 400 MHz NMR spectrometer at 25
°C. Internal standard used for the NMR measurements was tetramethylsilane (TMS) while
deuterated chloroform was used as a solvent. Shimadzu IR Tracer-100 instrument in the range of
4000-400 cm-1 was utilized to record FT-IR spectra and samples with KBr pellet form were
utilized for recording the same. The IR spectral features are reported in wave number (cm-1).
VarioMICRO system was utilized for microanalyses. Cyclic voltametry experiments were
executed at ambient temperature using a three-electrode (conventional) configuration. The three
5

electrodes used are glassy carbon as a working electrode, Ag/AgNO3 as a reference electrode,
and a platinum wire as an auxiliary electrode. The E(1/2) figures were determined using 1/2(Ea +
p
Ecp) in which Ea is the anodic peak potential and Ec is the cathodic peak potential. Ferrocene
p
p
was used as a standard (internal) and the potentials were indicated with respect to the same. The
solvent and supporting electrolyte in all the experiments were dichloromethane and 0.1M
tetrabutylammonium perchlorate, respectively. JASCO V-650 UV-Vis spectrophotometer with
freshly prepared solutions was used for recording the absorption spectra. Guassian 09 program
[DFT, B3LYP; 6-311++G(d,p)] package was utilized for computational calculations.
2.2. Synthetic methodologies
The intermediates, diiodoazafluorene 2, N-tosyl diiodoazafluorene 3, di-tert-
butylazafluorene 4, N-tosyl-tetra-tert-butyl-tri(azafluorene) 5, tetra-tert-butyl-tri(azafluorene) 6,
N-alkyl fused thiazines (8 and 9), carbaldehydes of N-alkyl fused thiazines (10 and 11), and
bromocarbaldehydes of fused thiazines (12 and 13) were synthesized by adopting the literature
methods [69-71].
2.2.1. Synthesis of carbaldehyde 14
Bromocarbaldehyde of fused thiazine 12 (0.50 g, 1.38 mmol), di-tert-
butylazafluorene
4 (0.39 g, 1.38 mmol), K2CO3 (0.6 g, 4.34 mmol), copper-bronze (0.14
g, 2.16 mmol), and 18-crown-6 (0.06 g, 0.20 mmol) were charged in a double neck RB
flask (100 mL) equipped with reflux condenser under nitrogen atmosphere. Solvent
ortho-dichlorobenzene (ODCB) (20 mL) was then added using a syringe and stirred for
6

two minutes. The temperature was raised to reflux while stirring and it was continued for
48h. It was then filtered after cooling so as to eliminate the insoluble inorganic material.
The solid inorganic material was washed further with DCM (3x30 mL). The washed
solvent and the already collected filtrate were united and the same was washed with dilute
aq. NH3 followed by water. The organic-phase was dried using Mg2SO4, filtered and the
solvent was evaporated to furnish the crude sample. Column chromatographic separation
of the crude sample using hexane-dichloromethane mixture (4:6) eventually furnished the
1
pure title compound. Yield: 0.56 g (73%), m.p. 218-220°C. H NMR (400 MHz, CDCl3,
25°C, TMS): δ = 1.01 (t, J = 7.40Hz, 3H), 1.45 (s, 18H), 1.63-1.52 (m, 2H), 1.92-1.85 (m,
2H), 3.97 (t, J = 7.20Hz, 2H), 6.97 (d, J = 8.40Hz, 1H), 7.04 (d, J = 6.00 Hz, 1H), 7.34-
7.26 (m, 4H), 7.47-7.44 (m, 2H), 7.62 (s, 1H), 7.70-7.68 (m, 1H), 8.12 (d, J = 1.20Hz,
13
2H), 9.82 (s, 1H). C NMR (100 MHz, CDCl3. 25°C, TMS): δ = 13.84, 20.18, 29.74,
32.04, 34.77, 47.99, 109.12, 115.03, 116.29, 116.59, 123.30, 123.65, 124.50, 125.36,
125.69, 125.88, 128.65, 130.24, 131.31, 133.81, 139.25, 142.34, 142.94, 150.39, 190.00;
IR (KBr, cm-1): ν 424.3, 470.6, 505.4, 543.9, 572.9, 611.4, 651.9, 738.7, 801.1, 879.5,
920.1, 1029.9, 1101.4, 1143.8, 1197.8, 1236.4, 1257.6, 1294.2, 1325.1, 1363.7, 1410.0,
1471.7, 1579.7, 1602.9, 1693.5, 2860.4, 2955.0, 3055.2; elemental analysis calcd (%) for
C37H40N2OS: C 79.24, H 7.19, N 5.00, S 5.72; found: C 79.43, H 7.22, N 4.96, S 5.69.
2.2.2. Synthesis of carbaldehyde 15
A mixture of bromocarbaldehyde of fused thiazine 13 (0.54 g, 1.38 mmol), di-tert-
butylazafluorene 4 (0.39 g, 1.38 mmol), K2CO3 (0.60 g, 4.34 mmol), copper-bronze (0.14 g, 2.16
mmol), and 18-crown-6 (0.06 g, 0.20 mmol) in ODCB (20 mL) gave the title compound by
7

1
adopting the above method. Yield: 0.54 g (67%), m.p. 227-229°C. H NMR (400 MHz, CDCl3,
25°C, TMS): δ = 0.90 (t, J = 4.40Hz, 3H), 1.49-1.32 (m, 4H), 1.46 (s, 18H), 1.61-1.59 (m, 2H),
1.91-1.87 (m, 2H), 3.94 (t, J = 7.20Hz, 2H), 6.94 (d, J = 8.40Hz, 1H), 7.02 (d, J = 8.40Hz, 1H),
7.31-7.25 (m, 4H), 7.46-7.44 (m, 2H), 7.61 (s, 1H), 7.68 (d, J = 2.00Hz, 1H), 8.13 (d, J =
13
1.60Hz, 2H), 9.81 (s, 1H). C NMR (100 MHz, CDCl3, 25°C, TMS): δ = 14.05, 22.63, 26.60,
26.79, 31.45, 32.06, 34.77, 48.30, 109.13, 115.02, 116.30, 116.60, 123.31, 123.66, 124.46,
125.35, 125.67, 125.88, 128.63, 130.26, 131.29, 133.79, 139.25, 142.31, 142.94, 150.38, 190.02;
IR (KBr, cm-1): ν 424.3, 468.7, 543.9, 611.4, 651.9, 736.8, 810.1, 881.5, 920.1, 1029.9, 1101.4,
1143.8, 1197.8, 1255.7, 1294.2, 1327.0, 1363.7, 1409.9, 1471.7, 1579.7, 1602.9, 1693.5, 2860.4,
2956.9, 3055.2; elemental analysis calcd (%) for C39H44N2OS: C, 79.55; H, 7.53; N, 4.76; S,
5.45. Found: C, 79.81; H, 7.61; N, 4.73; S, 5.43.
2.2.3. Synthesis of carbaldehyde 16
A mixture of bromocarbaldehyde of fused thiazine 12 (0.25 g, 0.69 mmol), tetra-
tert-butyl-tri(azafluorene)
6 (0.50 g, 0.69 mmol), K2CO3 (0.30 g, 2.17 mmol), copper-
bronze (0.07 g, 1.08 mmol), and 18-crown-6 (0.03 g, 0.10 mmol) in ODCB (25 mL)
provided the title compound by using the above method. Yield: 0.44 g (64%), m.p. 318-
319°C. 1H NMR (400 MHz, CDCl3, 25°C, TMS): δ = 0.93 (t, J = 7.40Hz, 3H), 1.20-1.16
(m, 2H), 1.37 (s, 36H), 1.47-1.44 (m, 2H), 3.86 (t, J = 7.18Hz, 2H), 6.84 (d, J = 8.15Hz,
1H), 6.96 (d, J = 8.04Hz, 1H), 7.18 (s, 1H), 7.23 (d, J = 8.80Hz, 4H), 7.35-7.32 (m, 6H),
7.51-7.47 (m, 4H), 7.60 (s, 1H), 7.85 (d, J = 7.90Hz, 1H), 8.13-8.06 (m, 6H), 9.81 (s,
13
1H); C NMR (100 MHz, CDCl3, 25°C, TMS): δ = 13.83, 20.10, 29.79, 32.02, 34.74,
8

47.97, 109.10, 115.01, 116.21, 116.51, 123.32, 123.62, 123.65, 123.77, 124.52, 124.85,
125.32, 125.62, 125.77, 125.82, 128.62, 130.23, 131.33, 133.83, 139.23, 142.34, 142.94,
150.35, 190.04; IR (KBr, cm-1): ν 422.4, 468.7, 572.9, 611.4, 651.9, 738.7, 808.2, 877.6,
918.1, 1029.9, 1103.3, 1197.8, 1234.4, 1259.5, 1290.4, 1325.1, 1363.7, 1487.1, 1577.8,
1602.9, 1691.6, 2864.3, 2955.0, 3047.5; elemental analysis calcd (%) for C69H70N4OS: C
82.59, H 7.03, N 5.58, S 3.20; found: C 82.85, H 6.99, N 5.60, S 3.22.
2.2.4. Synthesis of carbaldehyde 17
A mixture of bromocarbaldehyde of fused thiazine 13 (0.27 g, 0.69 mmol), tetra-
tert-butyl-tri(azafluorene)
6 (0.5 g, 0.69 mmol), K2CO3 (0.30 g, 2.17 mmol), copper-
bronze (0.07 g, 1.08 mmol), and 18-crown-6 (0.03 g, 0.10 mmol) in ODCB (25 mL)
yielded the title compound by employing the above method. Yield: 0.48 g (68%), m.p.
332-334°C. 1H NMR (400 MHz, CDCl3, 25°C, TMS): δ = 0.89 (t, J = 4.00Hz, 3H), 1.24-
1.22 (m, 2H), 1.30-1.27 (m, 4H), 1.40 (br.s, 40H), 3.92 (t, J = 7.35Hz, 2H), 6.82 (d, J =
8.12Hz, 1H), 6.93 (d, J = 7.95Hz, 1H), 7.20-7.17 (m, 4H), 7.47-7.40 (m, 6H), 7.68 (s,
13
1H), 7.96-7.93 (m, 1H), 8.25-8.13 (m, 6H), 9.82 (s, 1H); C NMR (100 MHz, CDCl3,
25°C, TMS): δ = 14.04, 22.62, 26.66, 26.76, 31.41, 32.02, 34.74, 48.38, 109.10, 115.01,
116.31, 116.61, 123.32, 123.62, 123.65, 123.80, 124.42, 124.91, 125.32, 125.62, 125.82,
125.83, 128.62, 129.95, 130.23, 131.05, 131.23, 133.73, 139.23, 142.38, 142.84, 150.35,
190.09; IR (KBr, cm-1): ν 424.3, 468.7, 501.5, 547.8, 577.9, 611.4, 651.9, 738.7, 810.1,
879.5, 920.1, 1029.9, 1103.3, 1197.8, 1238.3, 1261.5, 1292.3, 1327.0, 1363.7, 1413.8,
1487.1, 1577.7, 1602.9, 1693.5, 2864.3, 2956.9, 3047.5; elemental analysis calcd (%) for
C71H74N4OS: C 82.68, H 7.23, N 5.43, S 3.11; found: C 82.86, H 7.26, N 5.40, S 3.13.
9

2.2.5. Synthesis of dye TBCPCA-1
A mixture of carbaldehyde 14 (0.22 g, 0.4 mmol) and cyanoacetic acid (0.04 g, 0.5
mmol) in the presence of ammoniam acetate (0.02 g, 0.22 mmol) in AcOH (15 mL) was
refluxed for 12h. The mixture was diluted with water after cooling and extracted with
DCM. The organic portion was separated, washed well with water, dried over Mg2SO4
and the solvent was removed. The crude thus obtained was subjected to column
chromatographic separation utilizing 10% methanol in ethyl acetate as eluting system to
eventually provide the title dye molecule. Yield: 0.20g, (79%), m.p. 165-166°C. 1H NMR
(400 MHz, CDCl3, 25°C, TMS): δ = 1.04 (t, J=7.20Hz, 3H), 1.33-1.25 (m, 2H), 1.46 (s,
18H), 1.55-1.51 (m, 2H), 3.96 (t, J=7.00Hz, 2H), 6.92 (d, J=8.80Hz, 1H), 7.03 (d,
J=8.75Hz, 1H), 7.31-7.26 (m, 5H), 7.46 (dd, J=8.40Hz, 1.60Hz, 2H), 7.96 (d, J=8.60Hz,
1H), 7.69(s, 1H), 8.12 (d, J=1.20Hz, 2H); 13C NMR (400 MHz, CDCl3, 25°C, TMS): δ =
13.83, 20.17, 29.74, 32.04, 34.77, 48.06, 98.18, 109.12, 115.19, 116.30, 116.58, 123.33,
123.69, 124.15, 124.85, 125.62, 125.69, 125.88, 130.66, 131.90, 134.06, 139.17, 141.68,
142.99, 149.79, 154.39, 167.90; IR (KBr, cm-1): ν 2221.8 (C≡N); elemental analysis calcd
(%) for C40H41N3O2S: C 76.52, H 6.58, N 6.69, S, 5.11; found: C 76.71, H 6.51, N 6.74, S
5.08.
2.2.6. Synthesis of dye TBCPCA-2
The title dye molecule was synthesized using carbaldehyde 15 (0.24 g, 0.4 mmol)
by employing the above method with no change in other reactants/reagents. Yield: 0.21g,
10

(81%), mp. 200-202°C. 1H NMR (400 MHz, CDCl3, 25°C, TMS): δ = 0.89 (t, J=5.20Hz,
3H), 1.29-1.23 (m, 2H), 1.36-1.33 (m, 4H), 1.45 (br.s, 24H), 3.92 (t, J=6.80Hz, 2H), 6.90
(d, J=8.80Hz, 1H), 7.00 (d, J=8.40Hz, 1H), 7.19-7.17 (m, 3H), 7.46-7.42 (m, 3H), 7.67 (s,
13
1H), 7.94 (d, J=8.40Hz, 1H), 8.09 (s, 1H), 8.12 (s, 2H); C NMR (100 MHz, CDCl3,
25°C, TMS): δ = 14.04, 22.74, 26.58, 31.43, 32.05, 34.77, 48.38, 98.16, 109.14, 115.17,
115.96, 116.31, 116.59, 123.36, 123.70, 123.99, 124.12, 124.85, 125.58, 125.65, 125.86,
127.75, 130.57, 130.66, 131.98, 134.07, 139.18, 141.61, 143.01, 149.81, 154.47, 167.97;
IR (KBr, cm-1): ν 2217.4 (C≡N); elemental analysis calcd (%) for C42H45N3O2S: C
76.91, H 6.92, N 6.41, S 4.89; found: C 76.69, H 6.98, N 6.47, S 4.84.
2.2.7. Synthesis of dye TBTCPCA-1
By utlilzing the above method, the title dye molecule was synthesized from the
reaction of carbaldehyde 16 (0.40 g, 0.4 mmol) with no alteration in other
1
reactants/reagents. Yield: 0.31g (73%), m.p. 260-261°C. H NMR (400 MHz, CDCl3,
25°C, TMS): δ = 0.93 (t, J=7.40Hz, 3H), 1.20-1.16 (m, 2H), 1.46 (s, 36H), 1.53-1.49 (m,
2H), 3.85 (t, J=6.80Hz, 2H), 6.83 (d, J=8.35Hz, 1H), 6.97 (d, J=8.35Hz, 1H), 7.17 (s,
1H), 7.25-7.22 (m, 4H), 7.37-7.33 (m, 6H), 7.51-7.49 (m, 4H), 7.61 (s, 1H), 7.88 (d,
13
J=8.80Hz, 1H), 8.06 (s, 4H), 8.12 (s, 2H); C NMR (100 MHz, CDCl3, 25°C, TMS): δ =
13.86, 20.18, 29.74, 32.06, 34.75, 48.06, 98.16, 109.09, 111.07, 115.36, 116.25, 119.32,
123.12, 123.60, 123.92, 125.51, 126.00, 130.95, 131.87, 132.83, 140.10, 140.38, 142.59,
142.89, 149.79, 154.39, 167.94; IR (KBr, cm-1): ν 2224.7 (C≡N); elemental analysis calcd
(%) for C72H71N5O2S: C 80.79, H 6.69, N 6.54, S 3.00; found: C 81.06, H 6.62, N 6.59, S
3.03.
11

2.2.8. Synthesis of dye TBTCPCA-2
Synthesis of the title molecule was achieved from the reaction of carbaldehyde 17
(0.41 g, 0.4 mmol) by using the above method with no modification in other
1
reactants/reagents. Yield: 0.30g (69%), m.p. 265-266°C. H NMR (400 MHz, CDCl3,
25°C, TMS): δ = 0.90 (t, J=5.10Hz, 3H), 1.29-1.23 (m, 2H), 1.36-1.32 (m, 4H), 1.45
(br.s, 38H), 3.92 (t, J=6.00Hz, 2H), 6.86 (d, J=8.40Hz, 1H), 6.96 (d, J=8.40Hz, 1H),
7.19-7.17 (m, 5H), 7.30-7.24 (m, 6H), 7.48-7.44 (m, 4H), 7.68 (s, 1H), 7.95 (d, J=8.00Hz,
13
1H), 8.09 (s, 4H), 8.17 (s, 2H); C NMR (100 MHz, CDCl3, 25°C, TMS): δ = 14.04,
22.62, 29.79, 31.41, 32.02, 34.74, 48.36, 98.17, 109.19, 115.15, 115.95, 116.36, 116.56,
123.33, 123.73, 123.93, 124.14, 124.84, 125.55, 125.65, 125.85, 127.77, 130.50, 130.60,
131.91, 134.04, 139.19, 141.61, 143.03, 149.89, 154.44, 167.92; IR (KBr, cm-1): ν 2220.5
(C≡N); elemental analysis calcd (%) for C74H75N5O2S: C 80.91, H 6.88, N 6.38, S 2.92;
found: C 80.67, H 6.94, N 6.33, S 2.88.
2.3. Fabrication of DSSC and characterization
A tin(IV) oxide conducting glass (fluorine doped; ~80% transmittance; 7 Ω sq-1) was
firstly cleaned utilizing a cleaner (neutral) and was then washed with water (deionized),
afterward with dimethyl ketone and then with 2,3-dimethylbutan-2-ol. To reach good mechanical
contact between the film of titanium(IV) oxide and the conducting glass, and also to isolate the
surface of the glass (conducting) from the electrolyte, the conducting exterior of the FTO was
treated with tetraisopropyl orthotitanate (1000 mg) in ethyleneglycol monomethyl ether (3000
12

mg) solution. By employing doctor blade technique, titanium(IV) oxide paste was coated on the
aforesaid glass. The dried titanium(IV) oxide film was steadily heated (450 °C) in O2 atm. and
accordingly sintered at the same temp. for 0.5 hour to coat each titanium(IV) oxide layer.
The titanium(IV) oxide photo-anodes of the DSSCs used in the tests were composed of a
15 μm thick transparent titanium(IV) oxide (13 nm) layer with a scattering titanium(IV) oxide
(300 nm) layer of 5 μm thickness. The titanium(IV) oxide film was immersed in a solution,
which contains dye (3 x 10-4 M), at ambient temperature for a day after the sintering process at
450°C followed by cooling (80°C). The solutions of N719-ruthenium complex (used as standard
/ reference) as well as all the new organic dyes were prepared in a mixture of solvents such as
acetonitrile and 2-methylpropan-2-ol (1:1; v/v). The photoanode, dye-sensitized titanium(IV)
oxide, thus equipped was kept on a conducting glass (sputtered; platinum) electrode, maintaining
the couple of electrodes which are divided by a Surlyn® film (25 μm thick). Then, the couple of
electrodes were sealed. The electrolyte - 0.1M LiI, 0.05 M I2, 0.6M DMPII, 0.5M TBP, and in
ACN and MPN solvent mixture (1:1 v/v).
In the counter electrode, a hole was made previously for the injection of electrolyte.
Utilizing a capillary, the injection of electrolyte was done and after which it was sealed. The
surface of the DSSCs was sheltered by a cover (0.16 cm2 - light illuminated area) and
subsequently it was illuminated by utilizing a solar simulator. Light-intensity (incident; 0.001 W
/ cm2) was standardized with the help of a standard Si-cell. The width (thickness) of the
titanium(IV) oxide film was evaluated using SEM. For recording UV-visible spectra, the dyes
were layered on the titanium(IV) oxide films. The baseline was corrected using a naked
13

titanium(IV) oxide coated FTO substrate. EIS were acquired from the potentiostat / galvanostat,
prepared with an FRA2 module [light-illumination (constant); 0.001W / cm2]. The range of
frequency examined was 10–65 kHz. The voltage (applied bias) was set at the Voc of the DSSC
between the conducting glass–titanium(IV) oxide–dye working electrode and the indium tin
oxide-platinum counter electrode, initiating from the short-circuit conditions; the equivalent AC
amplitude was 10 mV. The analysis of EI spectra was made by an equivalent-circuit model. The
curves of IPCE were acquired under short-circuit conditions. The source of light was a solar
simulator (PEC-L11, class-A quality; AM 1.5G) and the light was focused - via a
monochromator - on to the PV cell. The monochromator was added via visible-spectrum to
produce the IPCE (λ). The Jsc (mA /cm2) was measured with the help of a potentiostat-
galvanostat, and the IRF (incident-radiative-flux (W/ m2) was recorded with an OD (optical-
detector) along with a power-meter.
3. Results and Discussion
3.1. Syntheses of metal free dyes
The synthetic strategy for building of novel title metal-free heterocyclic dyes TBCPCA-
1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 is provided in Scheme 1. N-tosylated
intermediate 3 was synthesized from commercially available azafluorene 1 upon diiodination
using potassium iodide and potassium iodate in acetic acid followed by tosylation with the help
of p-toluenesulfonyl chloride in DMF. Besides, di-tert-butylated intermediate 4 was synthesized
from azafluorene 1 through dialkylation utilizing tert-butyl chloride in nitromethane in the
presence of zinc chloride. Ullmann type reaction between the intermediates 3 and 4 in the
presence of cuprous oxide in N,N-dimethylacetamide furnished intermediate 5, which upon
14

detosylation with potassium hydroxide in aqueous tetrahydrofuran and N,N-dimethylformamide
mixture gave intermediate 6. On the other hand, bromo and carbaldehyde possessing fused
thiazine intermediates 12 and 13 were synthesized from commercially available aryl fused
thiazine 7 by employing alkylation using alkyl halide in dimethylsulfoxide followed by
Vilsmeier-Haack and halogenation reactions. The Vilsmeier-Haack reaction was performed by
employing phosphorous oxy chloride and N,N-dimethyl formamide in chloroform while the
bromination reaction was carried out in acetic acid using bromine. The intermediates 12 and 13,
upon Ullmann type reaction with the previously synthesized intermediate 4 utilizing copper-
bronze and potassium carbonate in o-dichlorobenzene under PTC (18-crown-6) furnished the
carbaldehydes 14 and 15, respectively, which upon subjected to Knoevenagel type reaction with
cyanoacetic acid eventually provided the novel dyes TBCPCA-1 and TBCPCA-2, respectively.
Similarly, the intermediates 12 and 13, upon Ullmann type condensation with the already
synthesized intermediate 6 by employing copper-bronze and potassium carbonate in o-
dichlorobenzene under PTC (18-crown-6) furnished the intermediates 16 and 17, respectively
which are further converted in to novel metal free fused heterocyclic dyes TBTCPCA-1 and
TBTCPCA-2, respectively through Knoevenagel type condensation. The structures of novel
metal free fused heterocyclic dyes are established based on their physical and spectroscopic
methods.
Scheme 1
3.2. Photophysical properties
Table 1
15

The photo-physical properties of all the novel metal free heterocyclic dyes TBCPCA-1,
TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 are studied from the results of UV-Visible spectra
recorded in dichloromethane and anchored on TiO2 films. The spectra are furnished in Figure 3
and the subsequent data are provided Table 1. The absorption bands obtained from 250 nm to
375 nm are ascribed to localized π-π* electron transitions of the conjugated motif while the
absorption bands resulted from 380 nm to 600 nm are due to strong ICT between the fused
heterocyclic units and cycnoacrylic acid skeleton. Of the dyes, the one possessing di-tert-
butylazafluorene structural motif at the 6-position of fused thiazine unit (i.e., TBCPCA-1)
exhibited intramolecular charge transfer maximum at 456 nm, which is slightly higher than the
one with tetra-tert-butyl-tri-(azafluorene) substituted analogue TBTCPCA-1. Similar trend is
observed in the case of TBCPCA-2 when compared with TBTCPCA-2 though the difference in
magnitude is comparatively higher. Normally, it is apparent that increasing conjugation in a
molecule and / or assimilation of auxiliary electron releasing functional moieties provides the
absorption maximum with bathochromic shift. Accordingly, the absorption maxima of the dyes
possessing tetra-tert-butyl-tri-(azafluorene) structural motif such as TBTCPCA-1 and
TBTCPCA-2 are expected to be in the bathochromic region when compared to the absorption
maxima of the dyes having di-tert-butylazafluorene scaffold such as TBCPCA-1 and TBCPCA-
2. However, the obtained results are contrary to each other. The plausible rationale is the
sterically crammed tetra-tert-butyl-tri-(azafluorene) structural motif in the dyes TBTCPCA-1
and TBTCPCA-2 increases the inter- and intramolecular aromatic-aromatic interactions between
and with-in the dye molecules, respectively through edge-to-face (H-π) and / face-to-face
interactions on comparing the dyes TBCPCA-1 and TBCPCA-2, which possess less-sterically
16

congested di-tert-butylazafluorene scaffold. Further, enhancing the bulkiness of a molecule
generally decreases the dye-aggregation and consequently increases the molar extinction
coefficient. This concept is realistic in our case as well since the molar extinction coefficient of
the tetra-tert-butyl-tri-(azafluorene) scaffold containing dyes TBTCPCA-1 and TBTCPCA-2
are higher than that of the di-tert-butylazafluorene structural motif possessing dyes TBCPCA-1
and TBCPCA-2. Amongst the dyes synthesized, TBTCPCA-2 displayed the highest molar
extinction coefficient. It was observed that the absorption maxima of the dyes are comparatively
lower when they are adsorbed on TiO2 films on comparing with the absorption of the respective
dyes in dichloromethane solution. The absorption in the lower wavelength (hypsochromic shift)
in the case of the former than the latter could be attributed to deprotonation on TiO2 and / or H-
aggregation for those dyes [53,72,73].
Figure 3
3.3. Electrochemical properties
In addition to the light collecting yield of nanocrystalline titania (dye-adsorbed), it is
crucial that there are encouraging dye molecules energy off-sets with respect to the redox
electrolyte and nanocrystalline titania in order to construct efficient DSSCs. Consequently,
electrochemical activities of the novel heterocyclic dyes TBCPCA-1, TBCPCA-2, TBTCPCA-
1 and TBTCPCA-2 were recorded with the help of cyclic voltametry in dichloromethane
solution. The pictorial representations of the same are furnished in Figure 4 and the related data
are given in Table 1. The dyes HOMO-LUMO level energies were recognized on evaluating the
red-ox potentials of the heterocyclic dyes with oxidation potential of ferrocene. From the
17

aforementioned data, the thermodynamic viability for injection of electron to the CB of
nanocrystalline titania from the excited dye as well as the regeneration viability of the oxidized
dyes by electrolyte were estimated.
Figure 4
Picture (energy level) exhibiting the oxidation potentials (GS and ES) for the novel fused
heterocyclic dyes TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 is furnished in
Figure 5. These potentials in the ground state range between 1.20 and 1.26V Vs NHE [74].
Among the dyes, the oxidation potentials of di-tert-butylazafluorene donor possessing ones
TBCPCA-1 and TBCPCA-2 are relatively higher than that of the dyes containing tetra-tert-
butyl-tri-(azafluorene) analogues TBTCPCA-1 and TBTCPCA-2. Particularly, all the novel
fused heterocyclic dyes exerted oxidation potential adequately positive compared to the red-ox
-
potential of the electrolyte I-/I3 , which is reported as 0.4V (Vs NHE), and has been frequently
utilized in DSSCs. Thus, the oxidized fused heterocyclic dyes could ably be rejuvenated by the
-
electrolyte I-/I3 . At the same time, on comparing the first oxidation potential of the dyes
TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 with the conduction band edge of
nanocrystalline titania semiconductor, the former ones exhibit sufficiently negative than the latter
as the first oxidation potential of the former (i.e., dyes) lies between 1.19 and -1.35V while the
latter (i.e., semiconductor) reported to be -0.5V (Vs NHE). Specifically, the potential difference
between the fused heterocyclic dyes and the CB edge of nanocrystalline titania is greater than
0.69V and this difference in potential is ample to give the thermodynamic feasibility for the
electrons injection to the CB of nanocrystalline titania from the excited dyes [27]. The oxidation
18

potential results of ground and excited states of all the fused heterocyclic dyes imply that these
are estimated to serve as capable sensitizers in nanocrystalline titania based DSSCs.
Figure 5
3.4. Computational approaches
DFT calculations were performed to acquire further information regarding molecular
structure and electronic sharing of the FMO of the fused heterocyclic dyes TBCPCA-1,
TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 [75-77]. For the optimization of the chemical
structures of all the new heterocyclic dye molecules, DFT-B3LYP (Becke3 Lee-Yang-Parr)
method with 6-311++G(d,p) basis set using Gaussian 09 program package was used. The FMOs
of the heterocyclic dyes at the aforesaid level are provided in Figure 6.
Figure 6
The density of electrons in the HOMO level of the dyes TBTCPCA-1 and TBTCPCA-2
is mainly distributed on the fused thiazine scaffold whereas the electron density in the LUMO
level of the same is localized precisely on the cyanoacrylic acid segment. Similar trend is
observed in the cases of TBCPCA-1 and TBCPCA-2 wherein the distribution of electrons in the
homo level is on the fused thiazne structural motif while the the same in lumo level is on the
cyanoacrylic acid unit. As an overall observation, it is apparent that the excitation of electrons
from HOMO to LUMO by light induces facile shift of the electrons from fused thiazine
structural moiety to cyanoacrylic acid scaffold in the cases of TBTCPCA-1, TBTCPCA-2,
19

TBCPCA-1 and TBCPCA-2. Consequently, the aforementioned trend in the distribution of
electrons makes us to speculate that the electrons which are excited could adequately be injected
in-to the conduction band of nanocrystalline titania via the cyanoacrylic acid acceptor moiety. It
is well recognized that a charge transfer in a facile manner could avoid come into existence of
recombination of charge between dye molecules (oxidized) and photo-induced electrons at the
semiconductor/dye/electrolyte interface [78]. As can be seen in the Figure 6, the thiazine
structural motif is non-planar and puckered conformation with an equatorial disposition of alkyl
substituent; as a result, it restrains the aggregation because the aforementioned conformation
exposes the character of non-aromatic of the fused thiazine motif, which could decrease the π-π
stacking between the molecules of the dye [61].
3.5. Photovoltaic performance
Table 2
Fabrication of DSSCs was made using all the four heterocyclic dyes TBCPCA-1,
TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 as sensitizers on titania films in combination with
the liquid electrolytic system and the photovoltaic parameters of the fabricated cells were
acquired under irradiation at standard AM 1.5G. The noteworthy parameters of the DSSCs (ff,
Jsc, Voc, and η) are furnished in Table 2. The J-V uniqueness of the DSSCs (solar irradiation at
AM 1.5G), are given in Figure 7.
Figure 7
20

The dye-sensitized solar cell possessing TBCPCA-2 sensitizer exhibited the highest
photocurrent while the ones possessing TBCPCA-2 and TBTCPCA-2 sensitizers showed the
highest open-circuit voltage. On comparing the fused heterocyclic dyes containing di-tert-
butylazafluorene structural unit as an electron donor (TBCPCA-1 and TBCPCA-2), the ones
comprising tetra-tert-butyl-tri-(azafluorene) scaffold as the same (TBTCPCA-1 and
TBTCPCA-2) ended-up with lower photo-current density. The bulkier tetra-tert-butyl-tri-
(azafluorene) scaffold of the dyes TBTCPCA-1 and TBTCPCA-2 might probably have reduced
the dye loading on the nanocrystalline titania semiconductor surface and consequently light
harvesting tendency and thus culminated with the aforementioned lower photo-current density.
The low photocurrent density may occur due to poor contact between the nanocrystalline titania
semiconductor and the dye due to the presence of additional couple of bulky azafluorene
structural units. In addition, on comparing the dyes having di-tert-butylazafluorene scaffold as
donor (i.e., TBCPCA-1 and TBCPCA-2), the dyes possessing tetra-tert-butyl-tri-(azafluorene)
unit as the same (i.e., TBTCPCA-1 and TBTCPCA-2) exhibit larger dark current. This
increasing fashion of dark current from the former type of dyes to the latter ones implies that the
former ones are more capable to reduce electron leakage than the latter ones. Normally, this
could be understood with no doubt since on contrast to the molecules with larger in size which
-
build efficient barriers that hinders the approach of I3 towards the surface of nanocrystalline
titania, molecules with smaller in size are prone to form tightly packed monolayer as a
consequence of smaller amount of steric interruption.
21

IPCE appraised alongside the wavelength of irradiation for the dye sensitized solar cells
possessing the dyes TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 as sensitizers
are provided in Figure 8. All the aforementioned dyes demonstrated conversion efficiency of
photon-to-electron between 400 to 750 nm wavelength. Particularly, the dyes TBCPCA-1,
TBCPCA-2 and TBTCPCA-1 exerted high IPCE between 400 to 650 nm while the dye
TBTCPCA-2 displayed slightly lower IPCE compared to the others. Of all, the dye TBCPCA-2
showed the highest IPCE (80 to >95%). Further, all the dyes displayed power conversion
efficiencies between 6.17 and 7.97%. On comparing the power conversion efficiencies between
the dyes possessing di-tert-butylazafluorene structural motif as electron donor (i.e., TBCPCA-1
and TBCPCA-2) and the ones containing tetra-tert-butyl-tri-(azafluorene) scaffold as electron
donor (i.e., TBTCPCA-1 and TBTCPCA-2), the former category of dyes provided higher
efficiencies than the latter category of dyes. Among all the devices fabricated, the device with
the utilization of TBCPCA-2 as a sensitizer exhibited highest power conversion efficiency. This
highest efficiency initiated from the photo current density distinction when compared to others.
The higher IPCE and photo-current density values resulted for the DSSC with the TBCPCA-2 as
the sensitizer are trustworthy with the property of absorption [79-81]. The device with the
employment of the dye TBCPCA-2 as a sensitizer showed a ff of 0.67±0.003, a Voc of 745±0.6
mV, a Jsc of 16.92±0.3 mA cm-2, and PCE (η) of 7.97±0.3%, which is the highest one when
compared to the devices fabricated with the utilization of the other dyes TBCPCA-1,
TBTCPCA-1 and TBTCPCA-2 as sensitizers.
Figure 8
22

3.6. Electrochemical impedance spectroscopy
It is documented that as a consequence of electrons in comparatively huge quantity in the
nanocrystalline titania conduction band, high open-circuit voltage could be observed. The
solidity of electrons in the conduction band of nanocrystalline titania may be a result of enhanced
efficiency of charge collection in the equivalent dye-sensitized solar cell device. Therefore, EI
spectra were acquired (illumination - 100 mW.cm-2) to explore the relative charge-collection rate
in the DSSC devices with the dyes TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2.
As can be seen in Figure 9 (equivalent circuit 9a), reasonably large middle semicircles were
obtained in the Nyquist plot, measured under standard illumination conditions for the new metal
free heterocyclic dyes TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2. The resulted
semicircles suggest a huge electron transport resistance for their devices in the series and the
order is furnished herein: TBTCPCA-2 > TBTCPCA-1 > TBCPCA-1 > TBCPCA-2. The
small electron transport resistance for the dye TBCPCA-2 over TBCPCA-1, TBTCPCA-1 and
TBTCPCA-2 proposes enhanced efficiency of electron collection of the former compared to the
latter. A careful observation between the electron transport resistance and the power conversion
efficiency implies that the dye with low electron transport resistance showed higher power
conversion efficiency and hence there is a fine relationship between the same. Further, the
devices directed to superior photocurrent due to the enhanced efficiency of electron collection.
Therefore, the improved performance of the dye TBCPCA-2 over the other heterocyclic dyes
viz., TBCPCA-1, TBTCPCA-1 and TBTCPCA-2 may be credited to the superior electron-
gathering supported by the encouraging electron-releasing in the TBCPCA-2. Further, the RCt2 at
the interface of nanocrystalline titania-dye-electrolyte is lesser in the case of DSSC with the dye
TBCPCA-2 as sensitizer relative to the DSSCs with the dyes TBCPCA-1, TBTCPCA-1 and
23

TBTCPCA-2 as sensitizers (Table 2), implies an effective electron collection for the device with
the dye TBCPCA-2. This observation is in harmony with the superior values of short-circuit
current resulted for the device with the dye TBCPCA-2.
Figure 9
Additionally, DSSCs Bode-phase plots were measured under conditions of illumination
to judge the life time of electron within the semiconductor (τe). The pictorial representation of the
same is furnished in Figure 10. The frequency of the peaks resulted in the region of middle-
frequency can be used to judge the same. This provides a further suggestion of the rate of charge
recombination. The results of electron life time are reliable with the results of Voc. The definite
order of open-circuit voltage as well as electron life time for the novel heterocyclic dyes is
TBCPCA-2 ≈ TBTCPCA-2 > TBTCPCA-1 > TBCPCA-1. In order to determine the efficiency
of DSSCs, there are a couple of processes that can participate in an imperative role: (i)
aggregation of dye and (ii) injected electrons recombination process on nanocrytalline titania
semiconductor with the electrolyte and / or oxidized dye. Conversely, the aforementioned
processes are also controlled by the bonding between the dye and nanocrystalline titania, the
energy disparity between the conduction band of nanocrystalline titania and the dye’s excited
state and the dye rejuvenation by the redox electrolyte. It is recognized that the degree of the
move of CB commonly rely on the nanocrystalline titania semiconductor /dye interfacial-dipole
which, further is affected by the electronic blending between the nanocrystalline titania
semiconductor and the dye.
24

Figure 10
The resulted high Jsc of 16.92±0.3 mA.cm-2 for the DSSC with TBCPCA-2 as a
sensitizer could be owing to the finer electronic interaction of the dye with the nanocrystalline
titania semiconductor. Usually, electrochemical impedance spectrum of
a
FTO/TiO2/dye/electrolyte/Pt/ITO device exhibits three semicircles between 10 mHz to 65 kHz.
The RS is for the overall series resistance. The first semicircle is for the CT resistance at the
counter-electrode (RCt1) while the second and third ones are for the
-
semiconductor/dye/electrolyte interface (RCt2) and the Warburg diffusion method of I-/I3 in the
electrolyte (Zw), respectively. On the other hand, in our study, the predictable diffusion resistance
of the redox couple is in fact considerably overlapped by RCt2 because of a undersized length for
the diffusion of I- ion available with the 25 µm thin spacer utilized, and because of the
exploitation of low viscosity solvents in our electrolyte (viscosities of MPN and ACN are 1.60 cp
and 0.37 cp, respectively).
4. Conclusions
A series of azafluorene ornamented thiazine based novel fused heterocyclic organic dyes
TBCPCA-1, TBCPCA-2, TBTCPCA-1 and TBTCPCA-2 has been synthesized by adopting a
multi-step synthetic strategy. The dyes TBCPCA-1 and TBCPCA-2 possessing di-tert-
butylazafluorene structural motif as an electron donor showed comparatively higher power
conversion efficiencies over the dyes TBTCPCA-1 and TBTCPCA-2 containing sterically
congested tetra-tert-butyl-tri-(azafluorene) scaffold as an electron donor. Amongst the devices
fabricated with the utilization of novel dyes, the TBCPCA-2 carrying device exhibited the
25

highest power conversion efficiency of 7.97±0.3% along with the IPCE of 80 to >95%. In the
energy levels of HOMO-LUMO of all the novel heterocyclic dyes, well separated distribution of
electrons has been observed; imply ample release of excited electrons to the CB of the
semiconductor via carboxylic acid structural motif. Further, the elevated short-circuit current and
inferior CT resistance at the nanocrystalline titania/dye/electrolyte interface observed in
TBCPCA-2 possessing dye sensitized solar cell support the superior electronic interaction of the
nanocrystalline titania semiconductor with the dye and an effective electron collection for the
device, respectively. Attention towards the modification of donor and acceptor scaffolds so as to
improve the PCE further is under the way.
Acknowledgements
Financial assistance provided by the Department of Science and Technology (DST-SERB), New
Delhi, India under Fast-Track Scheme (SR/FT/CS-149/2011) and the Ministry of Science and
Technology of Taiwan (MOST-102-2221-E-002-186-MY3) are gratefully acknowledged.
26

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