Novel metal-free organic dyes possessing fused heterocyclic structural motifs for efficient molecular photovoltaics

Article abstract of DOI:10.1039/c6cp05722e

Reported herein are six novel metal free organic dyes such as PCA1-PCA3 and PCTA1-PCTA3 featuring fused heterocyclic structural motifs such as phenothiazine and alkylcarbazole. Photophysical/electrochemical properties and nanocrystalline TiO₂ based dye-sensitized solar cell performance of the same have been investigated. Electronic distribution within the molecules has been determined through a computational approach. The overall power conversion efficiencies of the devices with the utilization of dyes PCA1-PCA3 and PCTA1-PCTA3 as sensitizers ranges between 4.67 and 8.08%. The novel dyes possessing cyanoacrylic acid as electron acceptor, PCA1-PCA3, exhibit higher power conversion efficiency, short-circuit current, open-circuit voltage, and electron lifetime those with thioxothiazolidinylacetic acid, PCTA1-PCTA3, as the same. Of the devices fabricated by employing the new metal-free organic sensitizers, that with the dye PCA2 exerted a power conversion efficiency (PCE, η) of 8.08% with a short circuit current density (J_SC) of 16.45 mA cm^-2, an open circuit voltage (V_OC) of 735 mV and a fill factor (ff) of 0.68; this PCE is the highest amongst the devices fabricated.

Full text of DOI:10.1039/c6cp05722e

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Novel metal-free organic dyes possessing fused
heterocyclic structural motifs for efficient
molecular photovoltaics
Cite this:DOI: 10.1039/c6cp05722e
Ayyanar Karuppasamy,a Kesavan Stalindurai,a Jia-De Peng,b Kuo-Chuan Ho*b and
Chennan Ramalingan*a
Reported herein are six novel metal free organic dyes such as PCA1–PCA3 and PCTA1–PCTA3 featuring
fused heterocyclic structural motifs such as phenothiazine and alkylcarbazole. Photophysical/electrochemical
properties and nanocrystalline TiO2 based dye-sensitized solar cell performance of the same have been
investigated. Electronic distribution within the molecules has been determined through a computational
approach. The overall power conversion efficiencies of the devices with the utilization of dyes PCA1–PCA3
and PCTA1–PCTA3 as sensitizers ranges between 4.67 and 8.08%. The novel dyes possessing cyanoacrylic
acid as electron acceptor, PCA1–PCA3, exhibit higher power conversion efficiency, short-circuit current,
open-circuit voltage, and electron lifetime those with thioxothiazolidinylacetic acid, PCTA1–PCTA3, as the
same. Of the devices fabricated by employing the new metal-free organic sensitizers, that with the dye
PCA2 exerted a power conversion efficiency (PCE, Z) of 8.08% with a short circuit current density (JSC) of
16.45 mA cmÀ2, an open circuit voltage (VOC) of 735 mV and a fill factor (ff) of 0.68; this PCE is the highest
amongst the devices fabricated.
Received 18th August 2016,
Accepted 9th October 2016
DOI: 10.1039/c6cp05722e
www.rsc.org/pccp
extinction coefficients, and tunable absorption spectra and
energy levels.16–22 Principally, the performance of organic dye-
1 Introduction
Since the existing energy resources (fossil fuels) have been sensitized solar cells is dependent on its electronic and struc-
exhausted speedily during recent years, global consciousness is tural uniqueness. It is vital that the energy of LUMO level of the
rising in connection with the need to find alternative, renewable dyes is higher when compared with the energy of the conduction
energy resources, viz., solar cells, in order to suit the necessities band of the semiconductor for the thermodynamic possibility of
of today’s life. In this milieu, sizeable research endeavors have charge injection, whereas the dyes which possess broader and
been devoted to dye-sensitized solar cells (DSSCs) since the strong CT absorption are beneficial for charge generation.23
1
¨
discovery of ruthenium based cells by Gratzel et al. in 1991
Also, it has recently been reported that the recombination of
with the aim of eco-friendly and inexpensive conversion of solar injected electrons with the IÀ/I3À electrolyte/oxidized dye severely
energy to clean electricity. As a key module of DSSCs, sensitizers reduces the photocurrent density.24 To encumber the reactions
play a vital role in light harvesting and electricity conversion, with recombination and contribute to an increase in the short-
which significantly affect the power conversion efficiency of circuit current and open-circuit voltage of dye-sensitized solar
devices.2,3 Although the complexes of polypyridine–ruthenium cell devices, several structural alterations including incorpora-
and porphyrin–zinc are the hitherto best performing sensitizers tion of alkyl chains25–28 have been made.
in DSSCs,2,4–15 the former possess the apparent dilemma of
To date, a diverse range of novel metal-free organic dyes
heavy metal toxicity as well as resource insufficiency, while the possessing various structural motifs, including acyclic/cyclic
latter have the significant obstacles of poor synthetic yield as amines and other heterocyclic species have been developed as
well as the handling of highly toxic chemicals. Recently, organic sensitizers, and notable efficiencies in power conversion have
sensitizers have been studied intensively because of their unique been attained which demonstrate the great potential of metal-
features such as economic feasibility, unlimited resources, high free dyes as efficient sensitizers for DSSC applications.28–33
Organic sensitizers are generally composed of three structural
motifs: an electron donor; a linker with p-conjugation; and an
electron acceptor. This structural disposition is favorable for the
optimisation of dye-sensitized solar cells performance by modifying/
changing diverse parts in the molecules.34 Organic materials
a Department of Chemistry, Kalasalingam University, Krishnankoil – 626 126,
Tamilnadu, India. E-mail: ramalinganc@gmail.com
b Department of Chemical Engineering, National Taiwan University, Taipei 10617,
Taiwan. E-mail: kcho@ntu.edu.tw
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to donate electrons efficiently; (iv) the delocalization of p-electrons
could be spread over the complete phenothiazine chromophore, as
the two fused aryl scaffolds have been set in an undersized angle
(torsion) related to the sulfur and nitrogen atoms; (v) appropriate
alkyl functionalities could improve the water tolerance in the
electrolytes/improve the solubility and restrict aggregation;28
and (vi) cyanoacrylic acid/thioxothiazolidinylacetic acid could
act as acceptor/capable anchoring group.
2 Experimental
2.1 General
The reagents used herein are reagent grade and were utilized
as-received. All solvents were distilled/dried by employing
standard procedures before their utilization. Column chromato-
graphy was performed on silica gel (spherical, 100–200 mesh)
slurry packed in glass columns. The eluent systems used for
individual separations are furnished in the respective experi-
mental procedures. TLC was performed on pre-silica coated
aluminium plates (60, F254) and eluted with the solvents specified.
All FT-IR spectra were recorded on a Shimadzu IR Tracer-100 in the
range of 4000–400 cmÀ1 using KBr in pellet form, and the spectral
features are reported in terms of wavenumber (cmÀ1). All NMR
spectra were recorded on a NMR spectrometer (BRUKER 400 MHz)
at 25 1C with the use of tetramethylsilane (TMS) as an internal
standard and CDCl3 as solvent. Microanalyses were performed
using the varioMICRO system. Cyclic voltammetry experiments
were done at ambient temperature using a three-electrode (con-
ventional) configuration. The three electrodes used were Ag/AgNO3
as the reference electrode, glassy carbon as the working electrode,
and a platinum wire as the auxiliary electrode. The E(1/2) figures
were determined using 1/2(Eap + Epc), in which Eap is the anodic peak
potential and Ecp is the cathodic peak potential. The potentials were
indicated with respect to the internal standard, ferrocene. In all
the experiments, the supporting electrolyte and the solvent were
0.1 M tetrabutylammonium perchlorate and dichloromethane,
respectively. Absorption spectra were acquired on a JASCO V-650
UV-Vis spectrophotometer with the utilization of freshly prepared
solutions. Computational calculations were executed using the
Gaussian 09 program [DFT, B3LYP; 6-31G(d,p)] package.
Fig. 1 Structure of novel metal-free organic dyes PCA1–PCA3 and
PCTA1–PCTA3.
containing phenothiazine/carbazole structural components have
been used in a range of electronic devices, including DSSCs and
OLEDs.35–52 We herein report the synthesis, photo-physical,
computational, electrochemical, and photovoltaic characteristics
2.2 Synthetic procedures
of six novel organic dyes possessing alkylcarbazole, phenothia- The intermediates, 9-butyl-9H-carbazole (2a), 9-hexyl-9H-carbazole
zine and cyanoacrylic acid or thioxothiazolidinylacetic acid struc- (2b), 9-octyl-9H-carbazole (2c), 9-butyl-3-iodo-9H-carbazole (3a),
tural motifs, as given in Fig. 1.
9-hexyl-3-iodo-9H-carbazole (3b), 3-iodo-9-octyl-9H-carbazole (3c),
The design of the aforementioned novel organic dyes is based 3-bromo-9H-carbazole (4a), 3-bromo-9-ethyl-9H-carbazole (5a), and
on the following factors: (i) the butterfly (non-planar) conforma- 10H-phenothiazine-3-carbaldehyde (7) were synthesized by adopting
tion of phenothiazine could satisfactorily hinder aggregation of the literature methods.56–59
the molecule and the formation of intermolecular excimers;53,54
2.2.1 Synthesis of carbaldehyde 8a. To a solution of
(ii) the presence of a bulky alkyl carbazole moiety at the 10H-phenothiazine-3-carbaldehyde (0.50 g, 2.4 mmol) and
10-position of phenothiazine could also inhibit dye-aggregation, 3-bromo-9-ethyl-9H-carbazole (0.70 g, 2.5 mmol) in 1,2-dichloro-
increase the electron life-time of the dyes in the nanocrystalline benzene (DCB; 20 mL) under nitrogen atmosphere, were added
TiO2 conduction band, and improve charge separation at the copper powder (0.30 g, 4.7 mmol), powdered potassium carbonate
oxide-solution interface;28,55 (iii) the presence of electron-rich (0.90 g, 6.5 mmol) and 18-crown-6 (0.11 g, 0.4 mmol). The reaction
sulfur and nitrogen heteroatoms could cause the phenothiazine mixture was refluxed for 48 h. and filtered after attaining
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atmospheric temperature. It was then subjected to evaporation 13C NMR (400 MHz, CDCl3): d 189.82, 150.07, 143.34, 141.07,
using a rotary evaporator and the mixture was extracted with 139.91, 130.82, 130.80, 130.01, 127.54, 127.37, 127.11, 126.65,
dichloromethane after the addition of water. The DCM portion 126.57, 124.75, 123.50, 122.45, 122.35, 120.69, 119.87, 119.55,
was separated, dried on anhydrous sodium sulphate and con- 118.86, 116.78, 115.28, 110.93, 109.24, 43.49, 31.61, 29.06,
centrated using a rotary evaporator. The solid thus obtained 27.07, 22.62, 14.07; anal. calcd for C31H28N2OS: C, 78.12; H,
was subjected to column chromatography using 5–25% ethyl- 5.92; N, 5.88; S, 6.73. Found: C, 78.33; H, 5.89; N, 5.86; S, 6.70.
acetate in hexane as eluent to afford pure carbaldehyde 8a.
2.2.4 Synthesis of carbaldehyde 8d. A combination of
Yield: 0.51 g (51%), m.p. 140–142 1C. IR (KBr, cmÀ1): n 3061.0, carbaldehyde 7 (0.50 g, 2.4 mmol), 3-iodo-9-octyl-9H-carbazole
2960.7, 2922.7, 2818.0, 2723.7, 1685.8, 1460.1, 1375.3, 1313.4, (1.01 g, 2.5 mmol), Cu powder (0.30 g, 4.7 mmol), 18-crown-6
1244.1, 1197.8, 1124.5, 1072.4, 1039.6, 1004.9, 927.8, 894.9, (0.11 g, 0.4 mmol) and powdered K2CO3 (0.90 g, 6.5 mmol) in
817.8, 777.3, 744.5, 704.0, 653.9, 628.8, 578.6, 518.4, 462.7; 1,2-DCB (20 mL) under nitrogen atmosphere provided carb-
1H NMR (400 MHz, CDCl3): d 9.66 (s, 1H), 8.07–8.05 (m, 2H), aldehyde 8d by employing the above method. Yield: 0.71 g
7.65–7.39 (m, 5H), 7.28–7.20 (m, 2H), 6.97 (d, J = 7.20 Hz, 1H), (59%), m.p. 145–147 1C. IR (KBr, cmÀ1): n 3059.7, 2961.4,
6.83–6.75 (m, 2H), 6.22–6.17 (m, 2H), 4.45 (q, J = 7.07 Hz, 2H), 2922.2, 2848.9, 2725.4, 1772.4, 1681.9, 1595.1, 1572.0, 1409.9,
1.53 (t, J = 7.20 Hz, 3H); 13C NMR (400 MHz, CDCl3): d 189.65, 1367.5, 1313.5, 1249.9, 1203.6, 1078.2, 1045.4, 1018.4, 916.2,
1
150.04, 143.36, 140.62, 139.48, 130.92, 130.55, 129.91, 127.59, 815.9, 740.7, 636.5, 549.7, 507.3, 451.3, 428.2, 406.9; H NMR
127.35, 127.08, 126.68, 126.55, 124.90, 123.47, 122.53, 120.77, (400 MHz, CDCl3): d 9.67 (s, 1H), 8.09–8.05 (m, 2H), 7.63 (d, J =
119.94, 119.61, 118.91, 116.78, 115.29, 110.68, 109.00, 37.91, 8.40 Hz, 1H), 7.53–7.41 (m, 3H), 7.39 (d, J = 6.40 Hz, 1H), 7.26–
13.91; anal. calcd for C27H20N2OS: C, 77.12; H, 4.79; N, 6.66; 7.21 (m, 2H), 6.97 (d, J =7.20 Hz, 1H), 6.82–6.77 (m, 2H), 4.37
S, 7.62. Found: C, 77.44; H, 4.82; N, 6.62; S, 7.59.
(t, J = 7.40 Hz, 2H), 1.96–1.92 (m, 2H), 1.48–1.27 (m, 10H), 0.88
2.2.2 Synthesis of carbaldehyde 8b. A combination of (t, J = 7.20 Hz, 3H); 13C NMR (400 MHz, CDCl3): d 189.84,
carbaldehyde 7 (0.50 g, 2.4 mmol), 9-butyl-3-iodo-9H-carbazole 150.08, 143.34, 141.06, 131.91, 130.81, 130.78, 130.03, 127.53,
(0.87 g, 2.5 mmol), Cu powder (0.30 g, 4.7 mmol), 18-crown-6 127.38, 127.11, 126.64, 126.58, 124.74, 123.50, 122.45, 122.34,
(0.11 g, 0.4 mmol) and potassium carbonate (0.90 g, 6.5 mmol) in 120.69, 119.86, 119.55, 118.85, 116.78, 115.28, 110.93, 109.24,
1,2-DCB (20 mL) under nitrogen atmosphere afforded carb- 43.49, 31.67, 29.42, 29.25, 29.10, 27.42, 22.67, 14.15; anal. calcd
aldehyde 8b by adopting the above method. Yield: 0.80 g (74%), for C33H32N2OS: C, 78.53; H, 6.39; N, 5.55; S, 6.35. Found: C,
m.p. 136–138 1C. IR (KBr, cmÀ1): n 3053.3, 2953.0, 2924.1, 2854.7, 78.79; H, 6.41; N, 5.52; S, 6.33.
2727.4, 1683.9, 1595.1, 1573.9, 1552.7, 1492.9, 1465.9, 1373.3,
2.2.5 Synthesis of dye PCA1. To a solution of carbaldehyde
1317.4, 1284.6, 1249.9, 1199.7, 1157.3, 1128.4, 1076.3, 1043.5, 8a (200 mg, 0.48 mmol) in dry acetonitrile (10 mL), were added
1002.9, 933.6, 812.0, 783.1, 744.5, 633.5, 599.9, 509.2, 434.0, piperidine (10 mL, 0.10 mmol) and cyanoacetic acid (60 mg,
408.9; 1H NMR (400 MHz, CDCl3): d 9.60 (s, 1H), 8.01–7.98 0.71 mmol). The mixture was refluxed under nitrogen for 8 h.
(m, 2H), 7.56 (d, J = 8.40 Hz, 1H), 7.46–7.33 (m, 2H), 7.31 (d, J = After attaining ambient temperature, it was evaporated using a
1.60 Hz, 1H), 7.21–7.14 (m, 3H), 6.90 (d, J = 7.20 Hz, 1H), 6.75–6.71 rotary evaporator and the mixture was extracted with dichloro-
(m, 2H), 6.16–6.10 (m, 2H), 4.31 (t, J = 7.20 Hz, 2H), 1.90–1.85 methane after dilution with water. The DCM phase was separated,
(m, 2H), 1.45–1.40 (m, 2H), 0.95 (t, J = 7.40 Hz, 3H); 13C NMR dried using sodium sulphate, and evaporated. The crude thus
(400 MHz, CDCl3): d 189.73, 150.05, 143.35, 141.10, 139.94, 130.86, obtained was then subjected to column chromatography by
129.95, 127.53, 127.36, 127.08, 126.62, 126.55, 124.76, 123.47, employing 10–20% methanol in ethyl acetate as eluent to provide
122.45, 122.37, 120.68, 119.90, 119.55, 118.88, 116.76, 115.28, the target dye, PCA1. Yield: 183 mg (78%), m.p. 275–278 1C. IR
110.89, 109.21, 43.24, 31.22, 20.67, 13.92; anal. calcd for (KBr, cmÀ1): n 3057.2, 2966.5, 2877.8, 2821.9, 2223.9, 1678.1,
C29H24N2OS: C, 77.65; H, 5.39; N, 6.24; S, 7.15. Found: C, 77.88; 1564.3, 1489.1, 1465.9, 1327.0, 1302.0, 1257.6, 1209.4, 1089.8,
H, 5.42; N, 6.21; S, 7.11.
1020.3, 945.1, 883.4, 802.4, 746.5, 642.3, 594.1, 551.6, 461.0,
1
2.2.3 Synthesis of carbaldehyde 8c. A combination of carb- 424.3; H NMR (400 MHz, CDCl3): d 7.98–7.87 (m, 4H), 7.49–7.41
aldehyde 7 (0.50 g, 2.4 mmol), 9-hexyl-3-iodo-9H-carbazole (m, 3H), 7.33–7.25 (m, 3H), 6.90–6.81 (m, 1H), 6.73–6.70 (m, 2H),
(0.94 g, 2.5 mmol), Cu powder (0.30 g, 4.7 mmol), 18-crown-6 6.07 (bs s, 2H), 4.36 (br s, 2H), 1.35 (br s, 3H); anal. calcd for
(0.11 g, 0.4 mmol) and powdered K2CO3 (0.90 g, 6.5 mmol) in C30H21N3O2S: C, 73.90; H, 4.34; N, 8.62; S, 6.58. Found: C, 74.11; H,
1,2-DCB (20 mL) under nitrogen atmosphere provided carb- 4.36; N, 8.59; S, 6.55.
aldehyde 8c by employing the above method. Yield: 0.79 g
2.2.6 Synthesis of dye PCA2. A mixture of carbaldehyde 8b
(69%), m.p. 158–160 1C. IR (KBr, cmÀ1): n 3067.2, 2964.6, (215 mg, 0.48 mmol), cyanoacetic acid (60 mg, 0.71 mmol), and
2927.9, 2816.4, 2728.7, 1768.7, 1666.5, 1595.1, 1409.9, 1369.5, piperidine (10 mL, 0.10 mmol) in dry acetonitrile (10 mL) gave
1315.5, 1249.8, 1207.4, 1145.7, 1078.2, 1014.6, 923.9, 818.8, the target dye PCA2 by adopting the above method. Yield: 185
1
792.7, 744.5, 642.3, 626.9, 603.7, 547.8, 509.2, 455.2; H NMR mg (75%), m.p. 240–242 1C. IR (KBr, cmÀ1): n 3050.4, 2956.8,
(400 MHz, CDCl3): d 9.67 (s, 1H), 8.09–8.05 (m, 2H), 7.63 (d, J = 2870.1, 2218.1, 1676.1, 1560.4, 1487.1, 1464.0, 1319.3, 1300.0,
8.40 Hz, 1H), 7.53–7.41 (m, 3H), 7.39 (d, J = 6.80 Hz, 1H), 1280.7, 1255.7, 1205.5, 1166.9, 931.6, 906.5, 873.8, 806.3, 765.7,
7.26–7.20 (m, 2H), 6.97 (d, J = 7.20 Hz, 1H), 6.82–6.77 (m, 2H), 740.7, 661.6, 638.4, 594.0, 460.9, 420.5; 1H NMR (400 MHz,
6.23–6.17 (m, 2H), 4.37 (t, J =7.40 Hz, 2H), 1.96–1.93 (m, 2H), CDCl3): d 8.11–7.99 (m, 2H), 7.76–7.74 (m, 2H), 7.69–7.59
1.48–1.45 (m, 2H), 1.37–1.33 (m, 4H), 0.89 (t, J = 7.00 Hz, 3H); (m, 3H), 7.57–7.47 (m, 2H), 7.43–7.38 (m, 2H), 7.01–6.82 (m, 2H),
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6.22 (br s, 2H), 4.45 (br s, 2H), 2.01–1.98 (m, 2H), 1.58–1.54
2.2.10 Synthesis of dye PCTA3. The title compound, PCTA3
(m, 2H), 1.06 (br s, 3H); anal. calcd for C32H25N3O2S: was synthesized from the reaction between carbaldehyde 8d
C, 74.54; H, 4.89; N, 8.15; S, 6.22. Found: C, 74.77; H, 4.91; (303 mg, 0.60 mmol) and 2-(4-oxo-2-thioxothiazolidin-3-yl)acetic
N, 8.12; S, 6.19.
acid (140 mg, 0.73 mmol) in the presence of ammonium acetate
2.2.7 Synthesis of dye PCA3. A mixture of carbaldehyde 8d (25 mg, 0.32 mmol) in glacial acetic acid (20 mL) by adopting the
(242 mg, 0.48 mmol), cyanoacetic acid (60 mg, 0.71 mmol), and above method. Yield: 248 mg (61%), m.p. 250–252 1C. IR (KBr,
piperidine (10 mL, 0.10 mmol) in dry acetonitrile (10 mL) cmÀ1): n 3055.2, 2935.7, 2837.3, 1762.9, 1703.1, 1635.6, 1562.3,
provided the dye PCA3 by adopting the above method. Yield: 1464.0, 1386.8, 1309.7, 1228.7, 1190.1, 1111.0, 1051.2, 915.4,
189 mg (69%), m.p. 230–232 1C. IR (KBr, cmÀ1): n 3059.1, 898.8, 802.4, 742.6, 609.5, 555.5, 460.9; 1H NMR (400 MHz,
2918.3, 2850.8, 2220.1, 1689.6, 1562.3, 1489.1, 1465.9, 1429.3, DMSO-d6): d 8.29 (s, 1H), 8.20 (d, J = 7.60 Hz, 1H), 7.90 (d, J =
1327.0, 1296.2, 1255.7, 1209.4, 1174.6, 933.6, 914.3, 877.6, 8.00 Hz, 1H), 7.68–7.64 (m, 2H), 7.51–7.45 (m, 2H), 7.33 (s, 1H),
846.8, 767.7, 740.7, 633.5, 640.4, 597.9, 551.6, 453.3, 422.4; 7.22 (d, J = 6.00 Hz, 1H), 7.13–7.07 (m, 2H), 6.87 (br s, 2H), 6.18
1H NMR (400 MHz, CDCl3): d 8.00–7.89 (m, 3H), 7.56 (br s, (d, J = 8.80 Hz, 1H), 6.11 (br s, 1H), 4.63 (s, 2H), 4.46 (br s, 2H),
2H), 7.46–7.26 (m, 3H), 7.21–7.18 (m, 2H), 6.89 (br s, 1H), 1.82 (br s, 2H), 1.34–1.29 (m, 4H), 1.19 (br s, 6H), 0.81 (br s, 3H);
6.77–6.71 (m, 2H), 6.11 (br s, 2H), 4.31 (br s, 2H), 1.88 (br s, anal. calcd for C38H35N3O3S3: C, 67.33; H, 5.20; N, 6.20; S, 14.19.
2H), 1.40–131 (m, 4H), 1.19 (br s, 6H), 0.81 (br, 3H); anal. calcd Found: C, 67.54; H, 5.22; N, 6.22; S, 14.14.
for C36H33N3O2S: C, 75.63; H, 5.82; N, 7.35; S, 5.61. Found: C,
75.38; H, 5.84; N, 7.38; S, 5.63.
2.3 Dye-sensitized solar cell fabrication and characterization
2.2.8 Synthesis of dye PCTA1. To a solution of carb- A fluorine doped tin(IV) oxide conducting glass (B80% trans-
aldehyde 8b (270 mg, 0.60 mmol) in glacial acetic acid mittance; 7 O sqÀ1) was initially cleaned with the help of a
(20 mL), were added 2-(4-oxo-2-thioxothiazolidin-3-yl)acetic acid cleaner (neutral) and was then washed with water (deionized),
(140 mg, 0.73 mmol) and ammonium acetate (25 mg, 0.32 mmol). and subsequently with acetone as well as 2,3-dimethylbutan-2-ol.
The mixture was refluxed for 8 h and the solvent was removed To reach good mechanical contact between the film of titanium(IV)
using a rotary evaporator after attaining ambient temperature. oxide and the conducting glass, and also to isolate the surface
It was then extracted with DCM after diluting with water. The of the glass (conducting) from the electrolyte, the conducting
DCM phase was separated, dried on sodium sulphate, and exterior of the FTO was treated with tetraisopropyl orthotitanate
evaporated using a rotary evaporator. Column-chromatography (1000 mg) in ethyleneglycol monomethyl ether (3000 mg) solution.
separation of the crude thus obtained using 10–20% methanol By employing the doctor blade technique, titanium(IV) oxide paste
in ethyl acetate as eluent furnished the target dye, PCTA1. Yield: was coated on the aforesaid glass. The dried titanium(IV) oxide film
250 mg (67%), m.p. 290–293 1C. IR (KBr, cmÀ1): n 3035.7, 2924.1, was steadily heated (450 1C) in O2 atm and accordingly sintered
2861.2, 1703.1, 1564.3, 1491.0, 1465.9, 1435.0, 1388.8, 1311.6, at the same temperature for 0.5 h to coat each titanium(IV)
1247.9, 1188.2, 1107.1, 1051.2, 950.9, 802.4, 744.5, 709.8, 615.3, oxide layer.
1
559.4; H NMR (400 MHz, DMSO-d6): d 8.28 (s, 1H), 8.20 (d, J =
The titanium(IV) oxide photo-anodes of the dye-sensitized
6.80 Hz, 1H), 7.90 (d, J = 8.00 Hz, 1H), 7.67 (d, J = 7.60 Hz, 1H), solar cells used in the tests were composed of a 15 mm thick
7.60 (s, 1H), 7.51–7.45 (m, 2H), 7.31 (s, 1H), 7.23–7.19 (m, 1H), transparent titanium(IV) oxide (13 nm) layer with a scattering
7.12 (d, J = 8.00 Hz, 1H), 7.07 (br s, 1H), 6.87 (br s, 2H), 6.19 (d, J = titanium(IV) oxide (300 nm) layer of 5 mm thickness. The
8.00 Hz, 1H), 6.11 (br s, 1H), 4.51 (s, 2H), 4.47 (br s, 2H), 1.81 titanium(IV) oxide film was immersed in the solution, which
(br s, 2H), 1.39–1.31 (m, 2H), 0.92 (t, J = 7.00 Hz, 3H); anal. calcd contains dye (3 Â 10À4 M), at ambient temperature for a day
for C34H27N3O3S3: C, 65.68; H, 4.38; N, 6.76; S, 15.47. Found: C, after the sintering process at 450 1C followed by cooling (80 1C).
65.91; H, 4.40; N, 6.73; S, 15.42.
The solutions of N719–ruthenium complex (used as a standard/
2.2.9 Synthesis of dye PCTA2. The title compound, PCTA2 reference) as well as all the new metal free organic dyes were
was synthesized from the reaction between carbaldehyde 8c prepared in a mixture of solvents such as acetonitrile and
(285 mg, 0.60 mmol) and 2-(4-oxo-2-thioxothiazolidin-3-yl)acetic 2-methylpropan-2-ol (1 : 1; v/v). The photoanode, dye-sensitized
acid (140 mg, 0.73 mmol) in the presence of ammonium acetate titanium(IV) oxide, thus equipped was kept on a conducting glass
(25 mg, 0.32 mmol) in glacial acetic acid (20 mL) by employing (sputtered platinum) electrode, maintaining the electrode couple
the above method. Yield: 281 mg (72%), m.p. 280–282 1C. IR (KBr, which was divided by a Surlyns film (25 mm thick). Then, the
cmÀ1): n 3042.4, 2943.4, 2856.3, 1763.9, 1716.6, 1560.4, 1465.9, electrode couple was sealed. The electrolyte was 0.05 M I2, 0.1 M
1396.5, 1311.6, 1224.8, 1190.1, 1112.9, 1051.2, 952.8, 902.7, 798.5, LiI, 0.5 M TBP, 0.6 M DMPII, in an ACN and MPN solvent mixture
1
744.5, 605.7, 561.3, 511.1, 416.6; H NMR (400 MHz, DMSO-d6): (1: 1 v/v). In the counter electrode, a hole was made previously for
d 8.32 (s, 1H), 8.23 (d, J = 8.00 Hz, 1H), 7.92 (d, J = 7.60 Hz, 1H), the injection of electrolyte. Utilizing a capillary, the injection of
7.68 (br s, 2H), 7.53–7.47 (m, 2H), 7.36 (s, 1H), 7.25–7.22 (m, 1H), electrolyte was done, after which it was sealed. The dye loading
7.15 (d, J = 7.60 Hz, 1H), 7.09 (br s, 1H), 6.88 (br s, 2H), 6.21 measurements were carried out by desorbing the dye from the
(d, J = 8.80 Hz, 1H), 6.12 (br s, 1H), 4.70 (s, 2H), 4.48 (br s, 2H), practical TiO2 electrode with 0.1 M sodium hydroxide (NaOH) in
1.83 (br, s, 2H), 1.38 (br s, 2H), 1.28 (br s, 2H), 0.83 (br s, 3H); methanol (CH3OH) and then measuring the UV/Vis absorption
anal. calcd for C36H31N3O3S3: C, 66.54; H, 4.81; N, 6.47; S, 14.80. (UV-4802, Unico, USA) to calculate the dye loading by using a
Found: C, 66.37; H, 4.83; N, 6.45; S, 14.85.
pre-constructed calibration curve.
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The surface of the DSSCs was sheltered by a cover (0.16 cm2 – 3 Results and discussion
light illuminated area) and subsequently it was illuminated by
utilizing a solar simulator. Light-intensity (incident; 0.001 W cmÀ2
3.1 Synthesis of novel organic dyes
)
The synthetic approach for the construction of novel metal free
organic dyes PCA1–PCA3 and PCTA1–PCTA3 is furnished in
Scheme 1. 9-Alkyl-3-iodo-9H-carbazoles (3a–3c) were synthe-
sized from commercially available 9H-carbazole (1) by adopting
a two-step synthetic protocol; alkylation with alkyl halide in the
presence of potassium hydroxide at 80 1C followed by iodination
using potassium iodide and potassium iodate in acetic acid at
80 1C. 3-Bromo-9-ethyl-9H-carbazole (5a) was synthesized from
9H-carbazole (1) employing a two-step synthetic protocol viz.,
bromination using N-bromosuccinimide in N,N-dimethyl form-
amide at 0 1C to ambient temperature followed by ethylation
with ethyl bromide in the presence of potassium hydroxide at
80 1C. On the other hand, 3-formyl-10H-phenothiazine (7) was
synthesized from commercially available 10H-phenothiazine (6)
by formylation using 1,3,5,7-tetraazatricyclo[3.3.1.13,7]decane
in acetic acid under reflux conditions. The 3a–3c and 5a thus
synthesized, upon Ullmann type coupling with 10H-phenothiazine-
3-carbaldehyde (7) in the presence of copper powder and potassium
carbonate under phase transfer catalytic conditions in 1,2-dichloro-
benzene at reflux temperature, formed the corresponding carb-
aldehydes 8a–8d. The novel dyes PCA1–PCA3 and PCTA1–PCTA3
was standardized with the help of a standard Si-cell. The width
(thickness) of the titanium(IV) oxide film was evaluated using
SEM. To record UV-visible spectra, the dyes were layered on
the titanium(IV) oxide films. The baseline was corrected using a
naked titanium(IV) oxide coated FTO substrate. EIS were acquired
from the potentiostat/galvanostat, prepared with an FRA2
module (constant light illumination; 0.001 W cmÀ2). The range
of frequency examined was 10–65 kHz. The applied bias voltage
was set at the open-circuit voltage 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 the EI spectra was made by an equivalent-
circuit model. The IPCE curves 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 mono-
chromator onto the PV cell. The monochromator was added via
the visible-spectrum to produce the IPCE (l). The JSC (mA cmÀ2
)
was measured with the help of a potentiostat–galvanostat, and
the IRF (incident-radiative-flux; W mÀ2) was recorded with an OD
(optical-detector) along with a power-meter.
Scheme 1 Synthesis of novel organic dyes PCA1–PCA3 and PCTA1–PCTA3.
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were then synthesized by adopting the Knoevenagel type reaction probably the electron-accepting nature of the oxothioxothiazoli-
of corresponding heterocyclic aldehydes with cyanoacetic acid dine scaffold, which is higher compared to the cyanoacrylic acid
in the presence of piperidine in acetonitrile and 2-(4-oxo-2- moiety. Furthermore, among the dyes possessing a cyanoacrylic
thioxothiazolidin-3-yl)acetic acid in the presence of ammonium acid acceptor such as PCA1, PCA2, and PCA3, the one with a two
acetate in acetic acid, respectively, under reflux conditions.
carbon alkyl chain PCA1 exhibits slightly lower lmax compared to
the others, which possess comparatively longer alkyl chains (four
carbons and eight carbons). The probable reason is that the
increase in length of the alkyl chain imparts a reasonably strong
electron donating capability, or it may affect J-aggregation. In the
cases of dyes PCTA1–PCTA3 wherein thioxothiazolidinylacetic
acid acts as the acceptor moiety, there was no appreciable
change in the absorption maxima noticed. This implies that
increasing the length of the carbon chains (i.e., beyond four
carbons) probably inhibits the dye aggregation and enhances
the electron delocalization. Among the dyes with a cyanoacrylic
acid acceptor, PCA2 provides the highest molar extinction
coefficient. Similarly, among those with a thioxothiazolidinyl-
acetic acid acceptor, PCTA2 exhibits the maximum molar extinc-
tion coefficient. The enhanced extinction coefficient values favor
light harvesting and consequently photocurrent generation in
DSSCs. As can be seen from Table 1, the absorption peaks for
PCA1–PCA3 and PCTA1–PCTA3 on transparent TiO2 films are all
blue shifted to some extent compared to the absorption peaks in
dichloromethane, which could be ascribed to either H-aggregation
and/or deprotonation of the dyes on TiO2.47,60,61
3.2 Photophysical properties
The UV-visible spectra of novel organic dyes such as PCA1–PCA3
and PCTA1–PCTA3 in dichloromethane and anchored on TiO2
films are given in Fig. 2. The pertinent data for the same are
summarized in Table 1. Apart from the absorption bands
appearing below 400 nm, which are ascribed to the localized
p–p* electron transitions of the conjugated skeleton, absorption
bands occur between 400 and 600 nm corresponding to strong
intramolecular charge transfer between the heteroaromatic and
cyanoacrylic acid/thioxothiazolidinylacetic acid scaffolds. Of
the dyes, those with cyanoacrylic acid as the electron acceptor/
anchoring group absorbed at lower wavelengths (lmax = B20 to
30 nm) compared to those with thioxothiazolidinylacetic acid as
the electron acceptor/anchoring group. The rationale for this is
3.3 Electrochemical properties
In order to construct an efficient dye sensitized solar cell, it
is vital that there are positive energy off-sets of the molecules
of dyes in connection with the nanocrystalline TiO2 and
reduction–oxidation electrolytes, in addition to the light har-
vesting yield of a dye-adsorbed TiO2. Hence, the electrochemical
behavior of the novel organic dyes PCA1–PCA3 and PCTA1–PCTA3
was measured using cyclic voltammetry in dichloromethane
solution. The cyclic voltammograms and the related data are
furnished in Fig. 3 and Table 1, respectively.
The HOMO energy level and LUMO energy level of the dyes
were realized by comparing the dyes’ reduction–oxidation
potentials with ferrocene’s oxidation potential, recorded in
similar conditions. Having the aforementioned values in hand,
the regeneration viability of the oxidized dyes by electrolyte as
well as the thermodynamic feasibility for electron-injection into
the conduction band of nanocrystalline TiO2 were evaluated. As
can be seen from Fig. 4, the oxidation potentials of the dyes at
the ground state are in the range of 1.10–1.21 V vs. NHE. The
oxidation potential of the dyes PCTA1–PCTA3, which contain
a thioxothiazolidinylacetic acid acceptor/anchoring moiety, is
comparatively higher than that of those having a cyanoacrylic
acid acceptor/anchoring group (PCA1–PCA3). Most importantly,
the dyes PCA1–PCA3 and PCTA1–PCTA3 hold an oxidation
potential sufficiently positive compared to redox the potential
of the IÀ/I3À electrolyte, reported as 0.4 V (vs. NHE), commonly
employed in DSSCs. Hence, the oxidized species could be
regenerated efficiently by the IÀ/I3À electrolyte. Also, the oxida-
tion potential (first) of PCA1–PCA3 and PCTA1–PCTA3 is suffi-
ciently negative when compared to the conduction band edge
Fig. 2 Absorption spectra of the dyes PCA1–PCA3 and PCTA1–PCTA3
(a) in dichloromethane and (b) on a nanocrystalline TiO2 semiconductor.
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Table 1 Optical and electrochemical data of the dyes PCA1–PCA3 and PCTA1–PCTA3
HOMOb
(eV)
LUMOc
(eV)
E0–0
E0–0*
(V)
a
d
e
l
max, nm
FWHM
(nm)
lmax on
TiO2 (nm)
Eox (V)
Dye
(e/MÀ1 cmÀ1 103)
(DEp, mV)
(eV)
PCA1
PCA2
PCA3
PCTA1
PCTA2
PCTA3
471 (10.1)
478 (18.9)
480 (16.1)
501 (11.4)
500 (19.8)
500 (18.5)
112
109
104
113
109
111
440
442
441
460
462
461
0.418 (0.18)
0.437 (0.23)
0.444 (0.20)
0.327 (0.13)
0.363 (0.15)
0.348 (0.14)
5.218
5.237
5.244
5.127
5.163
5.148
3.069
3.043
3.146
3.084
3.176
3.163
2.149
2.194
2.098
2.043
1.987
1.985
À0.961
À0.987
À0.884
À0.946
À0.854
À0.867
a
Absorption spectra were measured in dichloromethane (DCM) solution. Redox potentials are reported with reference to the ferrocene internal
standard. Deduced from the oxidation potential using the formula HOMO = 4.8 + Eox
electrochemically deduced HOMO value. Calculated from the optical edge. Excited state oxidation potential versus NHE.
b
c
.
Obtained from the optical band gap and the
d
e
dyes is more than 0.35 V, it is sufficiently enough to furnish the
needed thermodynamic feasibility for electron injection to
the nanocrystalline TiO2 conduction band from the dye.34
The results of the potentials of the excited and ground states
of the dyes imply that the dyes are anticipated to be suitable as
capable sensitizers in TiO2-based DSSCs.
3.4 Theoretical approaches
To obtain further insight into the structures of the molecules
and the electronic sharing of the FMOs of PCA1–PCA3 and
PCTA1–PCTA3, density functional theory (DFT) calculations
were prosecuted.63 The frontier molecular orbitals of the dyes
at the B3LYP/6-31G(d,p) level are furnished in Fig. 5. In all the
dyes, the electron density is largely distributed on the pheno-
thiazine component/carbazole–phenothiazine components in
the HOMO state, whereas the electrons are localized mainly
on cyanoacrylic acid/thioxothiazolidinylacetic acid unit at the
LUMO level. Hence, excitation of electrons from HOMO to
LUMO by light induces a facile shift of the electrons from the
phenothiazine/carbazole structural motif to the cyanoacrylic
acid/thioxothiazolidinylacetic acid moiety. This distribution of
electrons causes us to speculate that the electrons which are
excited could be adequately injected to the nanocrystalline TiO2
conduction band through the acceptor moieties. As it is known,
a facile charge transfer could occur by recombination of charge
between the photo-injected electrons at the interface of semi-
conductor/dye/electrolyte and the oxidized dye molecules.64
Fig. 5 shows that the shape of the phenothiazine is non-planar
and thus restrains the aggregation as it exposes the non-aromatic
character of phenothiazine, which can reduce the p–p interaction
between the dye molecules.54 The alkyl carbazole units linked to
the nitrogen of phenothiazine increases the bulky nature of the
molecules; as a result the formation of eximers and aggregates is
averted.
Fig. 3 Cyclic voltamograms of the dyes PCA1–PCA3 and PCTA1–PCTA3.
3.5 Photovoltaic performance
Fig. 4 Energy level diagram displaying the ground and excited state
oxidation potentials observed for the dyes PCA1–PCA3 and PCTA1–
PCTA3.
The DSSCs were fabricated by the utilization of all the new dyes
as sensitizers on nanocrystalline TiO2 films in conjunction with
the liquid electrolytic system. The significant parameters of the
dye-sensitized solar cells, such as PCE (Z), fill-factor (ff), short-
of nanocrystalline TiO2, reported as À0.5 V (vs. NHE), as it is circuit current density (JSC) and open-circuit voltage (VOC), are
positioned in the range À0.85 to À0.99 V.62 Since the potential provided in Table 2. The J–V characteristics of the DSSCs, which
difference between the TiO2 conduction band edge and the were recorded under standard global AM 1.5G solar irradiation,
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Fig. 5 Electron distribution in the HOMOs and LUMOs for the dyes PCA1–PCA3 and PCTA1–PCTA3.
Table 2 Photovoltaic performance parameters of the DSSCs fabricated using the dyes PCA1–PCA3 and PCTA1–PCTA3
Electron
lifetime, te (ms)
Adsorbed dye
S. no.
Z (%)
VOC (mV)
JSC (mA cmÀ2
)
ff
fmax (Hz)
RCt2 (ohm)
(Â10À7 mol cmÀ2
)
PCA1
PCA2
PCA3
PCTA1
PCTA2
PCTA3
N719
7.47 Æ 0.2
8.08 Æ 0.3
7.09 Æ 0.2
4.67 Æ 0.2
5.26 Æ 0.2
4.91 Æ 0.2
9.02 Æ 0.1
735 Æ 0.5
735 Æ 0.6
723 Æ 0.5
656 Æ 0.4
642 Æ 0.5
642 Æ 0.4
745 Æ 0.4
15.06 Æ 0.2
16.45 Æ 0.3
14.43 Æ 0.2
10.01 Æ 0.2
11.45 Æ 0.2
11.26 Æ 0.2
17.01 Æ 0.2
0.68 Æ 0.002
0.68 Æ 0.003
0.68 Æ 0.002
0.71 Æ 0.003
0.72 Æ 0.002
0.68 Æ 0.003
0.71 Æ 0.002
7.01
7.01
10.42
15.52
18.92
18.92
5.75
22.69
22.69
15.27
10.25
8.41
8.41
27.68
15.76
15.23
16.21
19.42
18.12
18.71
13.98
1.45 Æ 0.03
1.43 Æ 0.02
1.38 Æ 0.03
1.28 Æ 0.03
1.26 Æ 0.02
1.20 Æ 0.02
1.36 Æ 0.03
are presented in Fig. 6b. Interestingly, the efficiencies for the acid-based dyes have a lower dark current than the cells with
DSSCs with cyanoacrylic acid-based dyes and thioxothiazoli- thioxothiazolidinylacetic acid-based dyes, and as a result of this
dinylacetic acid-based dyes range from 7.09 Æ 0.2 to 8.08 Æ 0.3% the cells with cyanoacrylic acid-based dyes lead to lower charge
and 4.67 Æ 0.2 to 5.26 Æ 0.2%, respectively, without obvious recombination and thus higher VOC values. In general, this could
differences with the change of their alkyl chains. In particular, be easily understood because, in contrast to larger moleculeÀs
the DSSC with PCA2 shows the highest efficiency of 8.08 Æ which create efficient barriers that block the approach of I3
0.3%, among all the synthesized dyes. Furthermore, the effi- towards the surface of nanocrystalline TiO2, smaller molecules
ciency of the cell with the PCTA2-based dye is the best among are prone to form firmly packed monolayers as a result of the
the thioxothiazolidinylacetic acid-based dyes (5.26 Æ 0.2%). smaller amount of steric hindrance. However, they show no
Table 2 also shows that the cells with cyanoacrylic acid-based obvious differences with the change of their alkyl chains.
dyes possess higher JSC and VOC than cells with thioxothiazoli-
IPCE values evaluated alongside the wavelength of irradiation
dinylacetic acid-based dyes, and as a result of this the cells with for the dye sensitized solar cells are furnished in Fig. 7. All the
cyanoacrylic acid-based dyes may achieve higher efficiency than dyes showed photon-to-electron conversion efficiency in the
the latter, despite the fact that all of the cells show similar region of 400–700 nm (wavelength). Specifically, the dyes
ff.54,65–70 Fig. 6a shows dark current measurements for the PCA1–PCA3 exhibited a very high IPCE in the region of
pertinent DSSCs. This measurement relates to the characteristic 400–600 nm, while the dyes PCTA1–PCTA3 displayed moderate
of DSSC which prevents electrons from being captured by the values in the region of 400–650 nm. Of all, the dye PCA2
electrolyte. The VOC value would be high if electrons find diffi- showed the highest IPCE (75 to 495%). In addition, the power
culty in recombination. The result shows that the cyanoacrylic conversion efficiencies of all the dyes range from 4.67–8.08%.
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Among them, the dyes PCA1–PCA3 provided much higher
efficiencies than the dyes PCTA1–PCTA3. The device with using
the dye PCA2 as a sensitizer displayed a short-circuit current
density (JSC) of 16.45 Æ 0.3 mA cmÀ2, an open-circuit voltage
(VOC) of 735 Æ 0.6 mV, a fill factor (ff) of 0.68 Æ 0.003, and a PCE
(Z) of 8.08%, which is the highest among the devices fabricated.
3.6 Electrochemical impedance spectroscopy
It is recognized that a high-open circuit voltage could be
observed as a result of electrons in fairly large amounts in the
nanocrystalline TiO2 conduction band. The compactness of
electrons in the nanocrystalline TiO2 conduction band may be
an outcome of the enhanced efficiency of charge-collection in the
analogous DSSC device. To explore the relative charge collection
rate in DSSC devices with PCA1–PCA3 and PCTA1–PCTA3 dyes,
electrochemical impedance spectra were measured under the
illumination of 100 mW cmÀ2. In the Nyquist plot (Fig. 8;
equivalent circuit 8a) measured at standard illumination condi-
tions for the new metal free dyes, relatively large intermediate
semicircles were observed. These signify a large electron trans-
port resistance for their devices in the series and the order is as
follows: PCTA1 4 PCTA3 4 PCTA2 4 PCA3 4 PCA1 4 PCA2.
The low electron transport resistance for the dye PCA2 compared
to PCA1, PCA2 and PCTA1–PCTA3 suggests the improved electron
collection efficiency of the former over the latter. Since the dye
possessing comparatively higher power conversion efficiency
showed low electron transport resistance, there is a good correla-
tion between the same. Furthermore, the devices guided enhanced
photocurrent due to the improved electron collection efficiency.
Fig. 6 J–V Characteristics of the DSSCs fabricated using the novel dyes
PCA1–PCA3 and PCTA1–PCTA3 under (a) dark (b) at 100 mW cmÀ2
(AM 1.5G).
Fig. 8 Nyquist plots observed for the DSSCs under illumination conditions;
(a) equivalent circuit.
Fig. 7 IPCE plots of the DSSCs fabricated using the novel dyes
PCA1–PCA3 and PCTA1–PCTA3.
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Consequently, the better performance of the dye PCA2 over normally exhibits three semicircles in the range between 10 mHz
the others such as PCA1, PCA2 and PCTA1–PCTA3 may be and 65 kHz. The ohmic serial resistance (RS) corresponds to the
ascribed to the enhanced electron collection supported by the overall series resistance. The first semicircle corresponds to the
encouraging electron injection in the former dye. Additionally, CT resistance at the counter-electrode (RCt1); at the same time,
as can be seen in Table 2, the CT resistance at the interface of the second one is for the semiconductor/dye/electrolyte interface
the nanocrystalline TiO2/dye/electrolyte (RCt2) is lower in the (RCt2). The Warburg diffusion method of IÀ/I3 in the electrolyte
À
case of dyes with cyanoacrylic acid acceptor (PCA1–PCA3) (Zw) is related to the third semicircle. Nevertheless, in this work
relative to the dyes with thioxothiazolidinylacetic acid acceptor the conventional diffusion resistance of the redox couple is
(PCTA1–PCTA2), implying effective electron collection for the actually significantly overlapped by RCt2 because of the short
cyanoacrylic acid dyes. This observation is in accord with the length for the diffusion of the IÀ ion with the thin spacer utilized
high short-circuit current values observed for the cyanoacrylic (25 mm thick), and due to the utilization of low viscosity solvents
acid dyes.
in our electrolyte (viscosities of ACN and MPN are 0.37 cp and
In addition, taking Bode phase plots (Fig. 9) into consideration, 1.60 cp, respectively).
the frequency of the peaks obtained in the middle-frequency
region can be utilized to judge the lifetime of an electron within
the semiconductor (te), providing an added indication of the
charge recombination rate. The electron lifetime data are compar-
able to the open circuit voltage data. The specific order of electron
lifetime as well as open-circuit voltage for the dyes PCA1–PCA3 and
PCTA1–PCTA3 is PCA1 E PCA2 4 PCA3 4 PCTA1 4 PCTA2 E
PCTA3. For the determination of the power conversion efficiency
of DSSCs, there are two processes that can play a critical role. One
is dye aggregation and the other is the injected electrons recombi-
nation process on a TiO2 semiconductor with the electrolyte and/or
oxidized dye. However, these processes are also controlled by the
difference in energy between the CB of nanocrystalline TiO2 and
the dye’s excited state, the bonding between the nanocrystalline
TiO2 and the dye, and the dye regeneration by the redox electrolyte.
It is known that the degree of the shift of conduction band
generally depends on the nanocrystalline TiO2 semiconductor/
dye interfacial dipole which, additionally is affected by the
electronic coupling between the nanocrystalline TiO2 semi-
conductor and the dye. The observed high JSC for the DSSC
with PCA2 could be due to the superior electronic interaction
between the dye and the nanocrystalline TiO2 semiconductor.
The EIS spectrum of a FTO/TiO2/dye/electrolyte/Pt/ITO device
4 Conclusions
Six novel metal free organic dyes possessing fused heterocyclic
structural units such as phenothiazine and carbazole as an
electron donor/spacer and cyanoacrylic acid/thioxothiazolidinyl-
acetic acid as an acceptor/anchoring moiety have been synthesized.
The dyes PCA1–PCA3 with a cyanoacrylic acid acceptor motif
exhibited comparatively higher power conversion efficiency
than the dyes PCTA1–PCTA3 with a thioxothiazolidinylacetic
acid acceptor motif. Of the devices fabricated with the new
dyes, that with PCA2 showed a PCE of 8.08%, which is the
highest among the dyes synthesized, and also an IPCE of 75%
to 495%. Well separated electron distribution in the energy
levels of the HOMOs and LUMOs of all the dyes was noted. Akin
to power conversion efficiency, the dyes PCA1–PCA3 exerted
higher values of short-circuit current, open-circuit voltage, and
electron lifetime than the dyes PCTA1–PCTA3. The observed
high short-circuit current and low CT resistance at the interface
of the semiconductor/dye/electrolyte in the device fabricated
with the PCA2 supported better electronic interaction between
the semiconductor and dye as well as effective electron collection
for the device. A research focus in connection with additional
structural alterations to attain further enhancement in the power
conversion efficiency is in progress.
Acknowledgements
Financial assistance provided by the Council of Scientific and
Industrial Research (CSIR), New Delhi, India [02(0099)/12/EMR
II] and the Ministry of Science and Technology of Taiwan
[MOST-102-2221-E-002-186-MY3] is gratefully acknowledged.
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