Molecular Design of Carbazole-based Dyes and the Influence of Alkyl Substituent on the Performance of Dye-Sensitized Solar Cells

Article abstract of DOI:10.1080/15421406.2015.1106895

Carbazole is an alternant polycyclic aromatic hydrocarbon consisting of three fused rings with a large, aromatic system, containing nitrogen atom showing extensive electron delocalization. In this work, carbazole applied as π-conjugated bridge to construct electron donor–π–electron acceptor (D–π–A) organic dyes, where barbutiric acid and thiazolidine-2,4-dione as electron acceptor. The effects of these three acceptors and length of alkyl on the performance of the DSSCs were investigated systematically along with their photophysical and photo electrochemical properties. These series of organic dyes include (B-CH, B-C B, DT-CB, DB-CB). Our investigation indicate among dyes containing butyl as same donor, DT-CB exhibited the maximum overall conversion efficiency of 1.44% and from dyes by deferent electron donor B-CH shows conversion efficiency of 1.47%.

Full text of DOI:10.1080/15421406.2015.1106895

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Molecular Design of Carbazole-based Dyes and the
Influence of Alkyl Substituent on the Performance
of Dye-Sensitized Solar Cells
A. S. Beni, M. Zarandi, A. R. Madram, Y. Bayat, A. N. Chermahini & R. Ghahary
To cite this article: A. S. Beni, M. Zarandi, A. R. Madram, Y. Bayat, A. N. Chermahini & R.
Ghahary (2016) Molecular Design of Carbazole-based Dyes and the Influence of Alkyl
Substituent on the Performance of Dye-Sensitized Solar Cells, Molecular Crystals and Liquid
Crystals, 629:1, 29-43, DOI: 10.1080/15421406.2015.1106895
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1106895
Published online: 16 Jun 2016.
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Date: 17 June 2016, At: 02:50

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Molecular design of carbazole-based dyes and the influence of
alkyl substituent on the performance of dye-sensitized solar
cells
a
a
b
b
c
d
A. S. Beni , M. Zarandi , A. R. Madram , Y. Bayat , A. N. Chermahini , and R. Ghahary
a
b
Department of Chemistry, Faculty of Science, Yasouj University, Yasouj, Iran; Department of Chemistry and
c
Chemical Engineering, Malek Ashtar University, Tehran, Iran; Department of Chemistry, Isfahan University of
Technology, Isfahan, Iran; Department of Physics, Sharif University, Tehran, Iran
d
ABSTRACT
KEYWORDS
Carbazole is an alternant polycyclic aromatic hydrocarbon consisting
of three fused rings with a large, aromatic system, containing nitro-
gen atom showing extensive electron delocalization. In this work, car-
bazole applied as π-conjugated bridge to construct electron donor–
π–electron acceptor (D–π–A) organic dyes, where barbutiric acid and
thiazolidine-2,4-dione as electron acceptor. The effects of these three
acceptors and length of alkyl on the performance of the DSSCs were
investigated systematically along with their photophysical and photo
electrochemical properties. These series of organic dyes include (B-CH,
B-C B, DT-CB, DB-CB). Our investigation indicate among dyes contain-
ing butyl as same donor, DT-CB exhibited the maximum overall conver-
sion efficiency of 1.44% and from dyes by deferent electron donor B-CH
shows conversion efficiency of 1.47%.
Dye-sensitized solar cells
(
DSSCs); barbutiric acid; alkyl;
carbazole
1
. Introduction
Today’s one of the most urgent in energy field is finding good resource replacement for tradi-
tional and commonly used source of energy. Therefore to accomplish this purpose application
of solar energy is highly desirable because it has so many advantages. The Sun is an obvious
source of clean and cheap energy. Photovoltaic technologies harness the power of sun. Among
known Photovoltaic technologies, dye-sensitized solar cells (DSSCs), developed by Grätzel
and co-workers, have attracted so many attention in two past decades due to their promising
potential to actualize solar-to-electricity conversion. This generation of solar cells overcomes
the problems such as requirement of high costs and highly energy consuming preparation
methods of old generation. Two basic actions in the dye-sensitized solar cell (DSSC) include:
the charge generation is done at the semiconductor-dye interface and the semiconductor and
the electrolyte do the charge transport. Therefore, one crucial component of solar cells is pho-
tosensitizer. Great academic efforts have been fuscous on synthesis of versatile dyes [1–12].
Metal complexes based on ruthenium and zinc have been extensively studied for DSSCs and
high efficiencies over 10%–11% [13,14] and 12% [15] have been obtained. Nevertheless, these
metals have some problems such as high cost, relatively low molar extinction coefficients and
CONTACT A. S. Beni
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
 Taylor & Francis Group, LLC
salimibeni@mail.yu.ac.ir
©

3
0
A. S. BENI ET AL.
Scheme . Chemical structure of novel dye B-CH, B-C B, DT-CB, and DB-CB
rarity. Organic dyes show higher molar extinction coefficients, and need only simple synthesis
and purification processes at lower cost, therefore tremendous investigation have been done
to their development in all over the world. Some of these photosensitizers include different
group such as coumarin [16–18], merocyanine [19], indoline [20,21], polyene [22], hemicya-
nine [23,24], fluorene [25–26], triphenylamine (TPA) [27–36], tetrahydroqui-noline [37,38],
carbazole [39,40], and heteroanthracene (phenothiazine (PTZ) and phenoxazine (POZ)) [41–
4
7] and so on. Generally, organic dyes in order to act as photosensitizer should contain an
electron donor—π bridge—electron acceptor structure to show facile intramolecular charge
transfer and efficient light harvesting ability. Therefore, a π bridge unit should have a good
conjugation across a donor and an acceptor for efficient light harvesting and intramolecular
charge transfer.
Carbazole is an alternant polycyclic aromatic hydrocarbon consisting of three fused rings
with a large, aromatic system, containing nitrogen atom showing extensive electron delocal-
ization. In this paper, four carbazole-based D–π–A dyes were synthesized and used as sensi-
tizers in DSCs. The four dyes were denoted as B-CH, B-C B, DT-CB, and DB-CB, as shown
in Scheme 1.
2
2
. Experimental details
.1. Materials and characterization
All chemicals were purchased from Merck and Sigma Aldrich. Solvents and other chemicals
are reagent grade and used without further purification. TLC was performed on TLC-Grade
silica gel-G/UV 254 nm plates (n-hexane, ethyl acetate). Melting points were measured on a
1
13
Barnstead Electrothermal (BI 9300) apparatus. H, CNMR spectra were recorded on a FT-
1
NMR Bruker Avanced ultra shield spectrometer (frequency line, 400. MHz for HNMR and
13
1
00 MHz for C NMR; solvents, CDCl and DMSO-d ). Mass spectra of the products were
3 6
measured with a HP (Agilent Technologies) 5937 Mass Selective Detector. Elemental analyses
C, H, N) were done with a Heraeus CHN-O-Rapid analyzer. FT-IR spectra were recorded in
the matrix of KBr with JASCO FT-IR-680 plus spectrometer.
(
2.2. Photophysical and electrochemical measurements
Cyclic voltammetry (CV) studies were performed by biologic SP 150 electrochemical analyzer
−1
with scan rate of 100 mVs in dimethylformamide containing 0.1 M Bu NPF as the sup-
4
6
porting electrolyte, platinum wire as counter and work electrodes and Ag/AgCl as reference
electrode. The UV–Vis absorbance spectra were measured on a Perkin Elmer (Lambda 25)

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
31
spectrophotometer. Current density–voltage measurements were performed using simulated
−
2
1
.5 AM sunlight with an output power of 100 mW.cm . Electrochemical impedance spec-
−1
5
troscopy (EIS) was measured in the frequency range of 10 –10 Hz and in the dark under
0.70V bias. Incident photon-to-current efficiencies (IPCE) were measured with mono-
−
16
2
−2
chromatic incident light of 1×10 photon/cm under 100 mW/cm with bias light in DC
mode (Jarrel Ash monochromator, using a 100 W halogen lamp) and a calibrated photodiode
(Thorlabs) was used to study the optical properties of layers.
2.3. Fabrication of DSSCs
The preparation of the photo electrode was performed by application of doctor-blade tech-
nique on a conducting glass (FTO, 15 ꢀ/sq, Dyesol). The paste containing TiO nanoparticles
2
with a diameter of 20 nm (PST-20T, Sharifsolar) was spread on FTO glass. The films were
dried at 120°C for 6 min and then sintered at 500°C for 30 min. After sintering the TiO films
2
−4
were immerged into 3×10 M dyes (DB-CH, DTB-CH, and DT-CH) in acetonitrile/tert-
butyl alcohol(1:1; v/v) for overnight. Afterward the dye-covered nanoparticles electrode and
Pt-counter electrode were assembled into a sandwich type cell and sealed with 30 μm-thick
spacer at 120°C for a few seconds. After the injection of the electrolyte solution, which con-
sists of 0.5 M LiI, 0.05 M I , and 0.1 M 4-tert-butylpyridine in acetonitrile into the interspace
2
between the photo anode and the counter electrode, the photo electrochemical property of
the DSSC was measured.
2
.4. Computational methods
The optimization of the geometries of B-CH, B-C B, DT-CB, and DB-CB have been done
by using the B3LYP method with the 6–31 G (d) basis set. Density functional theory (DFT)
calculations of dyes were performed by Gaussian 03 package.
2.5. Synthesis of dyes

... Synthesis of -Butyl-H-carbazole
To a stirred solution of carbazole (0.2 g, 1.1 mmol) in DMSO (3 mL), NaOH (0.4 g) was
added and then the corresponding n-butylbromide (0.137 g, 1.01 mmol) was added drop wise.
After complete addition, the reaction mixture was heated at 90°C overnight. The organic layer
was separated, washed with water, dried over Na SO and concentrated. Pure product was
2
4
obtained after silica gel column chromatography (normal hexane) as a white solid.
White powder, Yield: 83%, mp: 54–55°C
1
HNMR (300 MHz, CDCl ): 8.22 (m, 2H), 7.55 (m, 4H), 7.35 (m, 2H), 4.42 (t, J = 6.3 Hz,
3
2
H), 1.98 (m, 2H), 1.52 (m, 2H), 1.06 (t, J = 7.2 Hz, 3H) ppm.
1
−
FT-IR (cm ): 3043, 2953, 2928, 1918, 1625, 1591, 1482, 1463, 1348, 1325.

... Synthesis of -Hexyl-H-carbazole
To a stirred solution of carbazole (0.2 g, 1.1 mmol) in DMSO (3 mL), NaOH (0.4 g) was added.
Then the corresponding hexylbromide (0.335 g, 1.01 mmol) was added dropwise. After com-
plete addition, the reaction mixture was heated at 90°C overnight. The organic layer was sep-
arated, washed with water, dried over Na SO and concentrated. Pure product was obtained
2
4
after silica gel column chromatography (normal hexane) as a white solid.
White powder, yield: 85%, mp: 75–77°C

3
2
A. S. BENI ET AL.
1
HNMR (400 MHz, CDCl ) δ (ppm): 0.87 (t, J = 7.043 Hz, 3H), 1.26–1.43 (m, 6H), 1.87
3
(
=
m, 2H), 4.30 (t, J = 7.043 Hz, 2H), 7.22 (t, J = 7.825 Hz, 2H), 7.40–7.48 (m, 4H), 8.09 (d, J
7.825 Hz, 2H)
13
CNMR (100 MHz, CDCl ) δ (ppm): 14.6, 23.1, 27.5, 29.4, 32.1, 43.4, 109.2, 119.2, 120.9,
3
1
1
23.3, 126.1, and 140.9
−1
IR (cm ): 3051, 2955, 2923, 2869, 2856, 1628, 1594, 1487, 1453, 1376, 1327, 1241, 1217,
192, 1153, and 1128
... -Butyl-H-carbazole--carbaldehyde()
Phosphorylchloride (1.55 mL, 1.25 eq) was added drop wise to N, N-dimethylformamide
DMF, 0.88 mL, 1.15 eq) for 1 h at 0°C. The mixture was stirred and then N-(butyl) car-
(
bazole (0.25 g, 1 mmol) added over 1 h at room temperature. After standing for 5 hr at 90°C,
the mixture was poured into ice–water (30 mL), stirred 5 hr, and neutralized with sodium
hydroxide. The solution was extracted three times with ethyl acetate and dried with Na SO .
2
4
The solvent was removed under reduced pressure. The residue was purified by silica gel col-
umn chromatography (eluent: ethyl acetate/n-hexane = 1:9).
Cream powder, Yield: 55%, mp: 62–64°C
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 10.06 (s, 1H), 8.76 (s, 1H), 8.30 (d, J = 8 Hz, 1H),
6
7
1
.99 (m, 1H), 7.79 (d, J = 8 Hz, 1H), 7.70 (d, J = 8 Hz, 1H), 7.54 (m, 1H), 7.31 (t, J = 7.6 Hz,
H), 4.46 (t, J = 8 Hz, 2H), 1.76 (m, 2H),1.29 (m, 2H), and 0.87 (t, J = 8 Hz, 3H)
13
CNMR (DMSO-d , 100 MHz) δ (ppm): 191.8, 143.4, 140.8, 128.2, 126.4, 124, 122.1,
6
1
20.8, 120.40, 110, 109.8, 42.2, 30.6, 19.8, and 13.8
FT-IR (cm ): 2985, 1691, 1625, 1594, 1473, 1352, 1133, and 808
−1

... -Hexyl-H-carbazole--carbaldehyde))
Phosphorylchloride (1.55 mL, 1.25 eq) was added drop wise to DMF (0.88 mL, 1.15 eq) for
hr at 0°C. The mixture was stirred and then N-(hexyl) carbazole (0.25 g, 1 mmol) added
1
over 1 hr at room temperature. After standing for 5 hr at 90°C, the mixture was poured into
ice–water (30 mL), stirred 5 hr, and neutralized with sodium hydroxide. The solution was
extracted three times with ethyl acetate and dried with MgSO . The solvent was removed
4
under reduced pressure. The residue was purified by silica gel column chromatography (elu-
ent: ethyl acetate/n-hexane = 1:9).
Cream powder, Yield: 55%, mp: 70–74°C
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 10.06 (s, 1H), 8.77 (s, 1H), 8.30 (d, J = 8 Hz, 1H),
6
7
1
.79 (d, J = 8Hz, 1H), 7.70 (m, 1H), 7.55 (m, 1H), 7.31 (t, 8Hz, 1H), 4.46 (t, J = 8 Hz, 2H),
.77 (m, 2H), 1.25 (m, 6H), and 0.87 (t, J = 8 Hz, 3H)
13
CNMR (DMSO-d , 100 MHz) δ (ppm): 13.7, 22, 26, 28.4, 30.9, 42.5, 109.7, 110.2, 120,
6
1
1
20.7, 122.11, 122.2, 124, 126.6, 126.7, 128.2, 140.4, 142.6, and 192
IR (cm ): 3052, 2955, 2929, 2857, 2727, 1893, 1686, 1593, 1469, 1383, 1352, 1339, 1329,
240, 1177, 1135, 807, 765, 748, and 730.
−1

... Synthesis of butyl-carbazole-,-dicarbaldehyde ()
Phosphorylchloride (2.31 mL, 2.5 eq) was added drop wise to DMF (1.76 mL, 2.3 eq) for
hr at 0°C. The mixture was stirred and then N-(butyl) carbazole (0.22 g, 1 mmol) added
1
over 1 hr at room temperature. After standing for overnight at 90°C, the mixture was poured
into ice–water (30 mL), stirred 5 hr, and neutralized with sodium hydroxide. The solution
was extracted three times with ethyl acetate and dried with MgSO . The solvent was removed
4

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
under reduced pressure. The residue was purified by silica gel column chromatography (elu-
ent: ethyl acetate/n-hexane = 1:9).
Cream powder, Yield: 60%, mp: 145–150°C
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 10.9 (s, 2H), 8.87 (s, 2H), 8.77 (d, J = 8 Hz, 2H),
6
8
1
.08 (d, J = 8 Hz, 2H), 4.50 (t, J = 12 Hz, 2H), 1.75 (m, 2H), 1.30 (m, 2H) and 0.87 (t, J =
.11 Hz, 3H)
13
CNMR (DMSO-d , 100 MHz) δ (ppm): 191.9, 144.3, 129.3, 127.3, 124.5, 122.2, 110.7,
6
4
2.7, 30.6, 19.7 and 13.6
FT-IR (cm ): 2956, 2863, 2811, 2722, 1685, 1629, 1627, 1592, 1488, 1205 and 1125
−1
... Synthesis of -[-butyl--(,-dioxo-,-thiazolan--ylidenmethyl)-H--carbaz-
olyl]methylidene-,-thiazolane-,-dione) DT-CB)
Butyl-carbazole-3,6-dicarbaldehyde (0.31 g, 1 mmol) and thiazolidine-2,4-dione (0.23 g,
2
mmol) and ammonium acetate (0.15 g, 1.9 mmol) were dissolved in acetic acid (2 mL).
The reaction mixture was refluxed for 5 hr. The orange crystals thus obtained were filtered,
washed with excess water three times. The dye BD-CH obtained was purified by crystalliza-
tion from EtOAc/n-hexane.
Orange, Yield: 63%
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 12.50 (s, 2H), 8.76 (s, 2H), 8.40 (s, 2H), 7.89 (s,
6
2
(
H), 7.64 (d, J = 4 Hz,2H), 7.61 (d, J = 4 Hz, 2H), 4.36 (t, J = 6 Hz, 2H), 1.73 (m, 2H), 1.26
m, 2H) and 0.86(t, J = 8 Hz, 3H)
CNMR (DMSO-d , 100 MHz) δ (ppm): 168, 167.5, 141.3, 132.8, 128.3, 124.6, 123.4,
13
6
1
22.4, 119.8, 110.6, 42.5, 30.6, 19.7, and 13.6
Mass: m/z 478 (M+1), 477, 434, 406, and 376
−1
FT-IR (cm ): 3433, 3117, 2959, 2771, 1734, 1683, 1629, 1583, 1488, 1458, 1432, 1388, 1321,
1
4
295, 1276, 1251, and 1213
Anal. Calcd for C24H19N3O4S2 (477.08): C, 60.37; H, 4.01; N, 8.80 Found: C, 60.42; H,
.0 6; N, 8.90
... Synthesis of -[-butyl--(,,-trioxohexahydro--pyrimidinylidenmethyl)-H--
carbazolylmethylene]hexahydro-,,-pyrimidinetrione
DB-CB)
(
Butyl-carbazole-3,6-dicarbaldehyde (0.31 g, 1 mmol), and barbutiric acid (0.29 g, 2 mmol)
were dissolved in absolute ethanol (3 mL). The reaction mixture was refluxed for 5 hr. The
dark orange crystals thus obtained were filtered, washed with excess water three times. The
dye BTD-CB obtained was purified by crystallization from EtOAc/n-hexane.
Dark orange, Yield: 87%
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 11.35 (s, 2H), 11.25 (s, 2H), 9.25 (s, 2H), 8.46 (s,
6
2
H), 8.40 (d, J = 8, 2H), 7. 70 (d, J = 8 Hz, 2H), 4.40 (t, J = 6, 2H), 1.74 (m, 2H), 1.28 (m,
2
H), and 0.8 (t, J = 8, 3H)
13
CNMR (DMSO-d , 100 MHz) δ (ppm): 164.0, 162.5, 160.08, 155.3, 150.3, 143.6, 134.3,
6
1
28.3, 124.7, 122.3, 115.1, 109.1, 42.7, 30.6, 19.7, and 13.6
Mass: m/z 501 (M+1), 499, 487, 481, 453, and 431
−1
FT-IR (cm ): 3417, 3201, 3065, 2957, 2930, 2854, 1740, 1734, 1667, 1492, 1531, 1483, 1440,
402, 1377, 1309, 1241, and 1197
1
4
Anal. Calcd for C26H21N5O6 (499.47): C, 62.52; H, 4.24; N, 14.02 Found: C, 62.61; H,
.33; N, 15.51.

3
4
A. S. BENI ET AL.

... Synthesis of -(-Butyl-H-carbazol--ylmethylene)-pyrimidine-, , -trione
(
B-CB)
Butyl-carbazole-3-carbaldehyde (0.25 g, 1 mmol), and barbutiric acid (0.15 g, 1 mmol) were
dissolved in absolute ethanol (3 mL). The reaction mixture was refluxed for 5 h. The orange
crystals thus obtained were filtered, washed with excess water three times. The dye B-CB
obtained was purified by crystallization from EtOAcn-hexane.
Orange, Yield: 78%
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 11.33(s, 1H), 11.20(s, 1H), 9.30(s, 1H), 8.64(d, J
6
=
8, 2H), 8.53(s, 1H), 8.18 (d, J = 8, 2H), 7.71 (t, J = 10, 1H), 7.54 (m, 2H), 7.32 (t, J = 8, 1H),
.45 (t, J = 12, 2H), 1.76 (m, 2H), 1.30 (m, 2H), and 0.88 (t, J = 8, 3H)
CNMR (DMSO-d , 100 MHz) δ (ppm): 164. 2, 162.4, 156.9, 150.2, 143.1, 140.7, 133.3,
4
13
6
1
29.5, 126.7, 123.4, 122.4, 122.1, 120.3, 120.5, 114.0, 110.3, 109.3, 42.4, 30.6, 19.7, and 13.7
Mass: m/z 361.4 (M+1), 362, 361, 318, 261, and 204
−1
FT-IR (cm ): 3852, 3841, 3742, 3687, 3674, 3648, 3618, 3437, 3189, 3060, 2957, 2862, 1669,
539, 1740, and 1401
1
Anal. Calcd for C21H19N3O3 (361.4): C, 69.79; H, 5.30; N, 11.63 Found: C, 69.9; H, 5.57;
N, 11.65
... Synthesis of -(-Hexyl-H-carbazol--ylmethylene)-pyrimidine-, , -trione
(B-CH)
Hexyl-carbazole-3-carbaldehyde (0.27 g, 1 mmol), and bituric acid (0.15 g, 1 mmol) were dis-
solved in absolute ethanol (3 mL). The reaction mixture was refluxed for 5 hr. The orange crys-
tals thus obtained were filtered, washed with excess water three times. The dye B-CB obtained
was purified by crystallization from EtOAc/n-hexane.
Orange, Yield: 82%
1
HNMR (DMSO-d , 400 MHz) δ (ppm): 11.32 (s, 1H), 11.20 (s, 1H), 9.30 (s, 1H), 8.60 (d,
6
J = 8, 2H), 8.16 (s, 1H), 7.70 (d, J = 8, 2H), 7.54 (t, J = 10, 1H), 7.30 (m, 2H), 7.32 (t, J = 8,
1
=
H),4.45 (t, J = 6, 2H), 3.36 (m, 2H), 2.51 (m, 2H), 1.77 (m, 2H), 1.24 (m, 2H), and 0.78 (t, J
8, 3H)
13
CNMR (DMSO-d , 100 MHz): 164.2, 162.4, 156.9, 150.2, 143.1, 140.6, 133.4, 129.5, 126.7,
6
1
23.5, 122.4, 122.1, 120.6, 120.4, 114.0, 110.3, 109.2, 42.6, 30.9, 28.5, 26.0, 22.0, and 13.7
Mass: m/z 390 (M+1), 389, 318 and 204
−1
FT-IR (cm ): 3901, 3826, 3744, 3423, 3190, 3059, 2954, 2921, 2855, 1692, 1523, 1464, 1401,
and 1146
Anal. Calcd for C23H23N3O3 (389.17): C, 70.93; H, 5. 95; N, 10.79 Found: C, 70.99; H,
.07; N, 10.89
6
3
3
. Results and discussion
.1. Synthesis and characterization
The molecular structures and synthetic routes of the four organic dyes B-CH, B-C B, DT-CB,
and DB-CB are represented in Schemes 1 and 2, respectively. All of the dyes have been syn-
thesized by the stepwise synthetic protocol. Every dye contains three essential parts include
π donor, linker and π acceptor. Carbazole is an aromatic hydrocarbon chose as linker. In
order to investigate the effect of π donor and π acceptor on the dye performances, two donor
units include butyl and hexyl and barbutiric acid and thiazolidine-2,4-dione were applied as
acceptor. Firstly, hexyl and butyl moieties were introduced to carbazole by the reaction in

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
Scheme . Synthetic routes of DB-CH, DTB-CH, and DT-CH (a) -Bromohexyl, NaOH, DMSO, °C, overnight
(
(
b) POCl, DMF, °C, overnight (c) -Bromobutyl, NaOH, DMSO, °C, overnight (d) POCl, DMF, °C,  hr
e) Ethanol, °C,  hr (f) Acetic acid, Amonium acetate, °C,  hr.

36
A. S. BENI ET AL.
Figure . Absorption spectra of dyes B-CH, B-C B, DT-CB, and DB-CB in THF.
the presences of NaOH and DMSO as solvent. The role of alkyl unit in addition of donor,
decrease aggregation of dyes in TiO surface and fulfill better dye efficiency. In the next step
2
9
-alkyl-9H-carbazole undergoes Vilsmeier–Haack reaction for formylation. In order depicted
in Scheme 2, 9-butyl-9H-carbazole dose Vilsmeier–Haack reaction in one and two sides. Nev-
ertheless, 9-Hexyl-9H-carbazole was subjected to mono aldehyde. In final synthetic protocol
different precursors 1, 2, and 3 were used for preparation of dyes. In this step, active methy-
lene such as barbutiric acid as six-member ring and thiazolidine-2,4-dione as five-member
ring react by formylated precursors by Knoevenagel condensation. The overall yield for final
step was (63%–85%).
3
.2. UV–Vis absorption spectra
The ultraviolet–visible (UV–Vis) absorption spectra of the four dyes in diluted THF solution
−
4
(
3×10 M) are presented in Fig. 1, and the corresponding data are collated in Table 1. All
dyes except DB-CB showed one broad peak in region (350–540) nm. The DB-CB showed two
Figure . Absorption spectral of dyes B-CH, B-C B, DT-CB, and DB-CB on TiO film.


MOLECULAR CRYSTALS AND LIQUID CRYSTALS
37
peaks in 379 and 474 nm. It is notable absorption band is in the UV region (300–370 nm)
corresponding to the π–π/ electron transitions of the conjugated molecules; and the other is
in the visible region (400–550 nm) that can be assigned to an intramolecular charge transfer
(ICT). The three other dyes show peak in λmax of 427,426 and 387 for B-CH, B-CB and DT-CB
respectively. The λmax of B-CH and B-CB correspond to visible region but the λmax of DT-CB
correspond to UV region. These differences attributed to their structure, both of B-CH, B-CB
have one barbutiric acid as acceptor unit. DB-CB has two barbutiric acids as acceptor unit.
However, DT-CB contains two five member ring thiazolidine-2, 4-dione as acceptor.
In order to study the absorption of dyes in TiO surface, absorption spectral of dyes B-CH,
2
B-C B, DT-CB and DB-CB on TiO film is displayed in Fig. 2. All dyes showed one broad peak
2
in the range of 380–600 nm. The λmax for B-CH, B-CB, DT-CB, and DB-CB is 459, 439.5, 381,
and 453 nm, respectively. Roughly all dyes show red shift in the range of 6–32 nm with respect
to those in solutions. It is presumed that the red and blue shifts of the absorption spectra may
be contributed to the formation of aggregates on the TiO surface [14,28]. Therefore, B-CH
2
due to having longer alkyl group shows less aggregation. Other obvious phenomena in absorp-
tion of dyes in TiO surface related to threshold of spectral. Higher threshold in absorption
2
of dyes on TiO surface attributed to higher aggregations. By the way, DT-CB shows shorter
2
threshold and most absorption on TiO surface. These results indicate in addition of donor
2
unit, acceptor unite play important role in absorption and aggregation phenomenon because
DT-CB despite having lower alkyl unit shows less aggregation. Also it may be attributed to dif-
ferent function of anchoring groups. Their function may be differing in solution and anchored
to TiO molecules.
2
3
.3. Electrochemical properties
In order to evaluate the possibilities of electron transfer from the excited states of B-CH, B-C
B, DT-CB, and DB-CB to the conduction band of TiO and regeneration of the dyes, cyclic
2
voltammetry was applied to investigate the redox behavior (Fig. 3) and related information
are presented in Table 1.
The HOMO levels of dyes B-CH, B-C B, DB-CB, and DT-CB is 1.03, 1.09, 1.02, and 1.03 V
vs. NHE), respectively. All dyes have near HOMO levels, but all are satisfactory in due to be
(
Table . UV-Vis and electrochemical data.
Dye
B-CH
B-CB
DB-CB
DT-CB
a
a
UV(λmax/nm)
UV(λmax/nm)


− .
.

−.


−.


−.
Reduction(V vs.
b
NHE)
c
Oxidation(V vs. NHE)
.
.
.
.
.
.
.
.
d
E–
f
HOMO/LUMO (ev)
Band gap (ev)f
−./−.
−./−.
−./−.
−./−.
.
.
.
.
a
b
c
Maximum absorption on TiO film.
Maximum absorption in THF.
Oxidation of dyes measured by cyclic voltametery in . M BuNPF in THF solution as supporting electrolyte, Ag/AgCl as
reference electrode and Pt as counter electrode. Potentials measured vs Ag/AgCl was converted to normal hydrogen
electrode (NHE) by addition of +.V[]
d
Reduction was calculated by HOMO–E–.
e
f
E– transition energy measured at the onset of absorption spectra.
DFT/BLYP calculated values.

3
8
A. S. BENI ET AL.
Figure . CVs of the B-CH, B-C B, DT-CB, and DB-CB dye films in DMF containing . M BuNPF at a scan rate
−

of  mVs , vs Ag/AgCl.
more positive than the iodine/iodide redox potential value (0.4v) vs. NHE. These values indi-
cate all dyes fulfill efficient regeneration of dye cation after photo induced electron injection
in to TiO film. The LUMO levels of dyes B-CH, B-C B, DB-CB, and DT-CB is −1.56, −1.43,
2
−
1.34, and −1.72 V (vs. NHE), respectively. These values are more negative than TiO con-
2
duction band edge (−0.5 vs NHE). Therefore, ensure electron injection from the excited dye
in to the conduction band of TiO . E 0–0 transition energy measured at the onset of absorption
2
spectra. The difference in band gap originates from different structural units spatially accep-
tor unit. DT-CB has the most band gap, 2.75 and other dyes have near band gap. E0–0 is 2.59,
2.57, and 2.36 for B-CH, B-C B, and DB-CB, respectively. Potentials measured vs. Ag/AgCl
was converted to normal hydrogen electrode (NHE) by addition of +0.2V. The schematic
energy levels of B-CH, B-C B, DT-CB, and DB-CB based on absorption and electrochemical
data are shown in Fig. 4.
Figure . Schematic energy levels of B-CH, B-C B, DT-CB, and DB-CB based on absorption and electrochem-
ical data.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
39
Figure . The electron distribution of the HOMO and LUMO of B-CH, B-C B, DT-CB, and DB-CB.
3
.4. Theoretical calculation
To visualize and gain further insight into the results from cyclic voltammetry, density function
theory (DFT) calculations were carried out by the B3LYP/6-31G (d) levels. Electron distribu-
tions of HOMO and LUMO level of dyes is shown in Fig. 5. In HOMO level dyes don’t show
similar electron distribution. Electrons in CB-H are located in carbazole and first CH2 of hexyl
Figure . J–V characteristics of cells with different dyes as sensitizer.

4
0
A. S. BENI ET AL.
Table . Photovoltaic parameters for cells.
B-CH
B-CB
DB-CB
DT-CB
JSC (mA.cm 
VOC (V)
FF
−
)
.
.

.
.

.
.

.
.

η (%)
.
.
.
.
and upper side of barbutiric acid, for CB-B in addition of carbazole total ring of barbutiric acid
contain electron distribution. DB-CB is similar to CB-H but contain both of barbutiric rings.
Electron distribute all over carbazole and acceptor units in DT-CH. The electrons locate simi-
larly in LUMO level. Two dyes B-CH and B-CB have the same distribution over the some part
of carbazole and barbutiric acid. In addition, DB-CB and DT-CB have comparable electron
distribution in all over carbazole and two acceptor units.
3.5. Photovoltaic properties
Figure 6 shows the current density–voltage (J–V) curves of the cells with different dyes as sen-
sitizer under AM 1.5 light illumination. The photoelectrode chemical properties of these cells
are listed in Table 2; FF, VOC, and JSC are fill factor, open circuit photovoltage, and short-
circuit photocurrent, respectively. We obtained a solar energy to electricity conversion effi-
−
2
−2
ciency of 1.47% (Jsc = 3.87 mA cm , Voc = 0.55V, FF = 0.68), 1.26% (Jsc = 3.11 mA cm ,
Voc = 0.56V, FF = 0.72), 1.44% (Jsc = 4.48 mA cm ,Voc = 0.52V, FF = 0.61) and 0.33%
−2
−2
(
Jsc = 1.08 mA cm ,Voc = 0.47 mV, FF = 0.65) with DSCs-based B-CH, B-CB, DT-CB, and
DB-CB, respectively. For the DT-CB & B-CH dye, cells have better performance compared to
other dyes. A larger Voc and Jsc are a prerequisite for higher power conversion efficiencies
in DSCs. The increased Jsc value of DT-CB compared with other three dyes mainly derived
from its relatively better light harvesting ability reflecting in its better IPCE spectrum. The
increased energy to electricity conversion efficiency of B-CH compare to DT-CB is related to
higher Voc.
Incident photon-to-electron conversion efficiency (IPCE) spectra of the cells are shown in
Fig. 7. The IPCE curves of DSSCs based on B-CH, B-CB, DT-CB, and DB-CB exhibited broad
Figure . IPCE spectra of cells.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
41
Figure . (a) EIS spectra of cells, (b) equivalent circuit for (a).
absorption in the range of 400–650 nm, with maxima at 19.9, 16.8, 28.4, and 3.5%, respec-
tively. There is no consistent with the absorption spectra in Fig. 2 and ICPE of the DB-CH.
This malfunction may be attributed to two barbutiric acids in structure of DB-CB. Actually
one of barbutiric acids has different spatial orientation. Therefore it cannot anchor to TiO2
molecules. It withdraw the electron population on itself and decrees electron population on
other barbutiric acid anchor to TiO molecules. Therefore DB-CH cannot act properly as a
2
(
D–π–A) organic dye. Roughly other dyes are consistent with the absorption spectra in solu-
tion and ICPE. In comparison with dyes B-CH gives the higher IPCE value, which implies the
dye would show a relatively large photocurrent in DSSCs [49]. It is obvious that the DT-CB
dye as sensitizer in cells show higher IPCE which is in a good agreement with the results of
J-V characterization. The B-CH & DT-CB have higher IPCE in higher wavelengths.
To investigate the charge recombination in the DSSCs based on the four dyes, EIS spectra
dyes measured in the dark under −0.70V bias. Fig. 8 (a) shows the electrochemical impedance
spectroscopic (EIS) analysis of the cells and equivalent circuit is represented in Fig. 8 (b).
The larger semicircle at lower frequencies corresponded to the charge transfer processes at
the TiO /dye/electrolyte interface, while the smaller semicircle at higher frequencies corre-
2
sponded to the charge transfer processes at the Pt/electrolyte interface. The results of EIS data
fits listed in Table 3. R , τ and C correspond to the charge transfer process occurring at
ct
μ
the TiO /dye/electrolyte interface, electron life time and chemical capacitance, respectively.
2
CPE ct-T and CPE ct-P are parameters of constant phase elements charge transfer (CPE ct).
Important parameters form DSSCs, electron lifetime (τ), are extracted from the (C ) and R
μ
ct
using τ = C × R . Radius of the larger semicircle in the Nyquist plot decreased in the fol-
μ
ct
lowing order: DB-CB > B-CB > B-CH > DT-CB indicating that DT-CB and DT-CB have

4
2
A. S. BENI ET AL.
Table . EIS parameters obtained from modeling the EIS result.
Rct (ꢀ.cm)
CPE ct-T (−)
CPE ct-P
τ (ms)
Cμ ( μF.cm

−
)
B-CH
B-CB
DB-CB
DT-CB
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
more efficient electron transportation at the TiO /dye/electrolyte interface. In comparison of
2
smaller semicircle, DT-CB has bigger radius. This phenomenon shed out the little deficiency
of DT-CB in comparison to DT-CB. This dye has less efficient electron transportation at the
Pt/electrolyte interface. According to J-V curve results, the cell prepared with B-CH dye has
the largest efficiency. The EIS spectra analysis proves this claim and the R value is low, too.
ct
4
. Conclusion
Four metal-free organic dyes comprising a carbazole linker and butyl and hexyl as the com-
mon electron donor and novel anchoring group, active methylene such as barbutiric acid as
six member ring and thiazolidine-2,4-dione as five member ring were designed and synthe-
sized for investigation in dye-sensitized solar cells (DSSCs). Actually, effect of two electron
donor and electron acceptor on the efficiency of sensitizer was studied. Hexyl and butyl chose
for overcome the obstacle of aggregation and evaluate the effect of length of alkyl on this
phenomenon. Result indicates dye contain hexyl show better overall efficiency (η (%): 1.47).
In order to study anchoring group effect two dyes DT-CB and DB-CB by same donor unit
evaluated. DT-CB by thiazolidine-2,4-dione as five member ring has better performance and
overall of its efficiency was (η (%): 1.44). It is notable to understand the effect of number of
anchoring group DB-CB and B-CB can be compare. It is clear B-CB by the efficiency of (η
(%): 1.26) is more suitable for application in dye-sensitized solar cells.
Acknowledgment
We are grateful to the Yasouj University and industrial Sharif University.
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