Carbazole based A-π-D-π-A dyes with double electron acceptor for dye-sensitized solar cell

Article abstract of DOI:10.1016/j.orgel.2013.11.020

Three novel carbazole-based A-π-D-π-A-featured dyes (CSG1-CSG3) have been designed, synthesized for applications in dye-sensitized solar cells and fully characterized with NMR, MS, IR, UV-vis and electrochemical measurements. These dyes share the same donor (N-hexylcarbazole) and acceptor/anchoring group (cyanoacrylic acid), but differs in conjugated linkers incorporated, such as benzene, furan or thiophene, to configure the novel A-π-D-π-A framework for effective electron flow. The power conversion efficiencies were observed to be sensitive to the π-bridging linker moiety. The photovoltaic experiments showed that dye with a benzene linker exhibited a higher open-circuit voltage (0.699 V) compared to thiophene and furan linker. Among all dyes, CSG2 containing a thiophene linker exhibited the maximum overall conversion efficiency of 3.8% (J_SC = 8.90 mA cm^-2, V_OC = 584 mV, FF = 0.74) under standard global AM 1.5 G solar condition. Under similar fabrication conditions, champion dye N719 exhibited the maximum overall conversion efficiency of 6.4% (J_SC = 14.74 mA cm^{- 2}, V_OC = 606 mV, FF = 0.716).

Full text of DOI:10.1016/j.orgel.2013.11.020

Organic Electronics 15 (2014) 266–275
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Carbazole based A-p-D-p-A dyes with double electron acceptor
for dye-sensitized solar cell
K.S.V. Gupta a, Thogiti Suresh a, Surya Prakash Singh a, Ashraful Islam b, Liyuan Han b,
Malapaka Chandrasekharam a,
⇑
a Network of Institutes for Solar Energy, Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka,
Hyderabad 500607, India
b Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2013
Received in revised form 4 November 2013
Accepted 4 November 2013
Available online 22 November 2013
Three novel carbazole-based A-p-D-p-A-featured dyes (CSG1–CSG3) have been designed,
synthesized for applications in dye-sensitized solar cells and fully characterized with
NMR, MS, IR, UV–vis and electrochemical measurements. These dyes share the same donor
(N-hexylcarbazole) and acceptor/anchoring group (cyanoacrylic acid), but differs in conju-
gated linkers incorporated, such as benzene, furan or thiophene, to configure the novel
A-
p
-D-
p
-A framework for effective electron flow. The power conversion efficiencies were
-bridging linker moiety. The photovoltaic experiments
Keywords:
Dye sensitized solar cells
Double acceptor dye
A-p-D-p-A structure
Anchoring stability
observed to be sensitive to the
showed that dye with a benzene linker exhibited a higher open-circuit voltage (0.699 V)
p
compared to thiophene and furan linker. Among all dyes, CSG2 containing a thiophene lin-
ker exhibited the maximum overall conversion efficiency of 3.8% (JSC = 8.90 mA cmꢀ2
,
VOC = 584 mV, FF = 0.74) under standard global AM 1.5 G solar condition. Under similar fab-
rication conditions, champion dye N719 exhibited the maximum overall conversion effi-
ciency of 6.4% (JSC = 14.74 mA cmꢀ2, VOC = 606 mV, FF = 0.716).
Carbazole
Linker
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
using a simple co-adsorbent developed by our group [12]
under standard AM 1.5 solar light irradiation. Record effi-
Dye-sensitized solar cells (DSSCs), one of the most
promising alternatives to crystalline silicon-based photo-
voltaics for converting clean, inexhaustible sunlight to elec-
tricity have received significant research interest due to
their low fabrication cost and relatively high power conver-
sion efficiency (g), since the seminal work was reported in
1991 by O’Regan and Graetzel [1–10]. In the past 5 years,
such solar cells have witnessed tremendous progress. For
ciencies of 12.3% under AM 1.5 conditions have been
achieved by using Zn(II)-porphyrin dyes and cobalt poly
pyridyl redox electrolyte [13]. Recently, organic–inorganic
hybrid perovskites have attracted attention as light-
harvesting materials for mesoscopic solar cells [14–18].
Moreover, solar cells based on these hybrids exhibited a
highest power conversion efficiency of 15.0% under stan-
dard AM 1.5 conditions [18]. Although metal based dyes
have high efficiency and long-term stability, they have
encountered problems such as relatively low molar
extinction coefficient, limited resources, morphological
variations, challenging synthesis and tedious purification
which might retard future large scale power production
based on DSSCs [4].
example, a power conversion efficiency (
been reached with bipyridyl-based ruthenium sensitizer
[11] and particularly a certified record of 11.4% has also
g) of 11.9% has
g
been achieved very recently for terpyridine based dyes
On the other hand, metal-free organic dyes have been
developed for DSSCs. Their advantages are due to their
⇑
Corresponding author. Tel.: +91 040 2719 1710; fax: +91 40 2719
3186.
E-mail address: chandra@iict.res.in (M. Chandrasekharam).
1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.11.020

K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
267
high molar absorption coefficients, inexpensive syntheses
by using well-established synthetic methodologies and
they present optical, electronic, and electrochemical prop-
erties that can be modulated through appropriate molecu-
lar design [19–23]. Moreover, organic dyes normally
possess higher molar extinction coefficients than Ru dyes
(<20,000 Mꢀ1 cmꢀ1), which make them ideal for solid state
DSSCs, utilizing thinner TiO2 layers. In particular, encour-
aging efficiency up to 10.3% has been reported by Wang
et al. using metal free organic dyes [22]. Among metal-free
organic dyes, a common strategy in the design of highly
efficient sensitizers for DSSCs is linking the electron donor
(D) and acceptor (A) systems (D–A) through the
p-conju-
gated linkers with TiO2 surface anchoring groups (e.g., car-
boxylate) integrated onto the acceptor moiety [24].
Fig. 1. Chemical structures of organic dyes CSG1–CSG3.
Variation of the
p-conjugated linker seriously affects the
efficiency of the intramolecular charge transfer, and par-
tially determines the DSSC performance of the correspond-
ing cells [25]. However, the low conversion efficiency and
low stability are important issues still to be resolved
[26]. Another crucial factor is the binding strength of dye
on the TiO2 surface, which affects the conversion efficiency
and stability of DSSC device. Strong binding of the dye on
TiO2 not only improves adsorption but also induces effi-
cient charge injection. One of the drawbacks of most of
the organic dye molecules is the presence of single anchor-
ing functionality which can be a serious constraint for infe-
rior performance as compared to Ru(II) sensitizers, where
1–4 anchoring groups are present [27]. Thus, several di/
multi anchoring organic dyes have been synthesized for
use in DSSCs, and have demonstrated better cell perfor-
cyanoacrylic acid moiety acting as the anchoring group
and variation of -conjugated linkers. Finally, the three
new sensitizers have been applied successfully to the sen-
sitization of nanocrystalline TiO2-based solar cells and the
corresponding photovoltaic properties, electronic and opti-
cal properties are also presented.
p
2. Experimental
2.1. Materials and instruments
All reactions were carried out under nitrogen atmo-
sphere. Solvents were distilled from appropriate reagents.
All reagents were purchased from Sigma–Aldrich. 1H and
13C NMR spectra were recorded on Avance 300 or
500 MHz spectrometer. Chemical shifts were reported in
parts per million (d) downfield from tetramethylsilane
(TMS) as an internal standard in CDCl3. Low resolution
mass spectrometry was performed using LCQ iontrap mass
spectrometer (Thermo Fisher, Sanjose, CA, USA) equipped
with an ESI source. IR spectra were recorded on a Perkin–
Elmer 1800 series FTIR spectrometer and samples were
analyzed as thin films on KBr pellets.
mance than single D-p-A sensitizers with an improved
photoresponse, photocurrent and stability [28–33].
3,6-Functionalized carbazole is a nonplanar compound
which can improve the hole transporting ability of the
materials and prevent the formation of dye aggregates
[34]. Due to its unique optical, electrical, and chemical
properties, carbazole has been used widely as a functional
building block or substituent in the construction of organic
molecules for use as light-emitting and hole-transporting
layers in OLED devices [35–42], as host materials for elec-
trophosphorescent applications [43], and as active compo-
nents in solar cells [44,45]. Moreover, upon introduction of
a carbazole unit into the structure, the thermal stability
and glassy state durability of the organic molecules im-
proves significantly [46,47].
Furthermore, alkyl-functionalized carbazole dyes have
been designed and used in DSSCs to improve both perfor-
mance and stability. Kohjiro Hara et al. showed that the
presence of hexyl substituents increased the electron life-
time and consequently open circuit voltage [48–53]. We
and others reported that the substitution of alkyl chains
on dye peripheral can inhibit the recombination losses in
DSSC [48,54–57]. As mentioned above, the introduction
The optical spectra of dyes in solution were recorded
with a UV–vis spectrophotometer (Shimadzu) while their
emission measurements were performed using a Fluorolog
3, JY Horiba fluorescence spectrometer. Electrochemical
data were recorded using Autolab potentiostat/ Galvano-
stat PGSTAT30. The cyclic voltammogram curves were ob-
tained from a three electrode cell in 0.1 M Bu4NPF6 N,N-
dimethylformamide solution at a scan rate of 100 mV sꢀ1
,
Pt wire as a counter electrode and an Ag/AgCl reference
electrode and calibrated with ferrocene.
2.1.1. Synthesis of 3,6-dibromo-9 hexyl-9H carbazole(3)
To a solution of 3,6-dibromo-9H-carbazole (0.200 g,
0.615 mmol) in dry N,N-dimethylformamide was added
KOH (0.105 g, 1.845 mmol) and allowed to stir for
30 min. To this, 1-bromo hexane (0.962 mL, 0.799 mmol)
dissolved in dry N,N-dimethylformamide was added drop
wise and allowed to room temperature with stirring for
12 h. After complete consumption of the dibromocarbazole
(TLC), crushed ice was added and stirred well. The
of
p-linker moieties were expected to allow red-shift of
the spectrum, and broaden the spectral region of absorp-
tion. Based on the above consideration, and our experience
on metal free organic sensitizers and carbazole substituted
ruthenium [58–60], we have designed and synthesized
three new efficient organic carbazole-based double elec-
tron acceptor dyes (CSG1–CSG3 shown in Fig. 1) with

268
K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
precipitate formed filtered and dried. Then, the obtained
crude solid was subjected to silica gel chromatography
using a mixture of ethyl acetate and n-hexane as an eluent
to obtain pure product 3 (83%). 1H NMR (CDCl3, 300 MHz) d
8.13 (s, 2H), 7.55–7.52 (m, 2H), 7.25–7.27 (m, 2H), 4.24–
4.22 (t, 2H, J = 6.929 Hz), 1.79–1.84 (m, 2H), 1.24–1.35
(m, 6H), 0.873–0.87(m, 3H). 13C NMR (CDCl3, 300 MHz) d
139.2, 128.9, 123.3, 123.1, 111.8, 110.3, 43.2, 31.4, 28.7,
26.8, 22.4, 13.9. MS (ESI) m/z: 411 (M+2)+.
refluxed for 36 h under nitrogen atmosphere. After cooling
the mixture down to room temperature, filtered through
Celite, extracted with ethyl acetate. The extracts were
combined and dried with anhydrous Na2SO4, and then fil-
tered. The solvent was completely removed by rotary
evaporation. The solid residue was purified by column
chromatography using ethyl acetate-hexane as the eluent
to get 6 (40%). 1H NMR (CDCl3, 300 MHz) d 9.64 (s, 2H),
8.56–8.58 (m, 2H), 7.90–7.93 (m, 2H), 7.36–7.42 (m, 4H),
6.86–6.88 (m, 2H), 4.27 (t, 2H, J = 6.789 Hz), 1.83–1.89
(m, 2H), 1.27–1.34 (m, 6H), 0.86–0.90 (m, 3H). 13C NMR
(CDCl3, 300 MHz) d 176.7, 160.7, 151.5, 141.5, 123.8,
123.1, 120.7, 118, 109.4, 43.4, 31.8, 29.6, 26.8, 22.6, 14.1.
MS (ESI) m/z: 440 (M+1)+.
2.1.2. Synthesis of 4,40-(9-hexyl-9H-carbazole-3,6-
diyl)dibenzaldehyde (4)
4-Formylphenylboronic acid (0.92 g, 0.583 mmol), 3,6-
dibromo-9 hexyl-9H carbazole (0.100 g, 0.243 mmol) and
Pd(PPh3)4 (0.056 g, 20 mol%) in dimethoxyethane (5 mL)
and a 2 M Na2CO3 aqueous solution (2 mL) was degassed
with nitrogen. Then, the reaction mixture was refluxed
for 36 h under nitrogen atmosphere. After cooling the mix-
ture down to room temperature, filtered through Celite,
extracted with ethyl acetate. The extracts were combined
and dried with anhydrous Na2SO4, and then filtered. The
solvent was completely removed by rotary evaporation.
The solid residue was purified by column chromatography
using ethyl acetate-hexane as the eluent to get 4 (41%). 1H
NMR (CDCl3, 300 MHz) d 10.08 (s, 2H), 8.44–8.46 (m, 2H),
7.98–8.02 (m, 2H), 7.89–7.93 (m, 4H), 7.79–7.83 (m, 2H),
7.52–7.55 (m, 4H), 4.38 (t, 2H, J = 6.798 Hz), 1.89–1.96
(m, 2H), 1.26–1.30 (m, 6H), 0.85–0.91(m, 3H). 13C NMR
(CDCl3, 300 MHz) d 191.9, 147.9, 141, 134.5, 130.3, 127.5,
125.6, 123.5, 119.3, 109.5, 43.4, 31.5, 28.9, 26.9, 22.5,
13.9. MS (ESI) m/z: 460 (M+1)+.
2.1.5. Synthesis of 3,30-(4,40-(9-hexyl-9H-carbazole-3,6-
diyl)bis(4,1-phenylene))bis(2-cyanoacrylic acid) (CSG1)
Compound 4, 4,40-(9-hexyl-9H-carbazole-3,6-diyl)
dibenzaldehyde (0.100 g, 0.217 mmol), dissolved in acetic
acid was condensed with cyanoaceticacid (0.055 g,
0.651 mmol) in the presence of ammonium acetate
(5 mol%). Then, the reaction mixture was refluxed well
for 12 h under N2. After cooling to room temperature, the
reaction mixture was poured into a crushed ice and solid
obtained was washed thoroughly with water to remove ex-
cess of acetic acid and cyanoacetic acid. Finally, washed
with pentane, hexane and 3% ethyl acetate in hexane to af-
ford a fine yellow colored solid CSG1 (80%). 1H NMR (CDCl3,
300 MHz) d 8.51 (s, 2H),8.28 (s, 2H), 8.09–8.14 (d, 4H),
7.89–7.94 (m, 4H), 7.81–7.86 (d, 2H), 7.54–7.59 (m, 2H),
4.35–4.42 (m, 2H), 1.87–1.95 (m, 2H), 1.28–1.43 (m, 6H),
0.83–0.89 (m, 3H). 13C NMR (CDCl3 + DMSO, 300 MHz) d
161.6, 151.8, 143.5, 138.8, 129.5, 127.6, 124.9, 123.1,
121.2, 114.4, 108.1, 100.3, 40.7, 29.1, 26.6, 24.2, 20.1,
11.9. IR (KBr, cmꢀ1) 3437.15, 2926.63, 2222.89, 1697.04,
1571.09, 1482.98, 1425.39, 1271.44, 1186.64, 802.87,
705.08. MS (ESI-) m/z: 572 (Mꢀ1)+. MS (ESI-) m/z: 592
(Mꢀ1)+.
2.1.3. Synthesis of 5,50-(9-hexyl-9H-carbazole-3,6-
diyl)dithiophene-2-carbaldehyde (5)
A mixture of 5-formylthiophen-2-ylboronic acid (0.91 g,
0.583 mmol), 3,6-dibromo-9 hexyl-9H carbazole (0.100 g,
0.243 mmol) and Pd(PPh3)4 (0.056 g, 20 mol%) in dime-
thoxyethane (5 mL) and a 2 M Na2CO3 aqueous solution
(2 mL) was degassed with nitrogen. Then, the reaction
mixture was refluxed for 36 h under nitrogen atmosphere.
After cooling the mixture down to room temperature, fil-
tered through Celite, extracted with ethyl acetate. The ex-
tracts were combined and dried with anhydrous Na2SO4,
and then filtered. The solvent was completely removed
by rotary evaporation. The solid residue was purified by
column chromatography using ethyl acetate-hexane as
the eluent to get 5 (45%). 1H NMR (CDCl3, 300 MHz) d
9.89 (s, 2H), 8.40–8.42 (m, 2H),7.77–7.82 (m, 2H), 7.42–
7.48 (m, 4H), 6.86–6.88 (m, 2H), 4.31 (t, 2H, J = 6.78 Hz),
1.84–1.94 (m, 2H), 1.27–1.32 (m, 6H), 0.87–0.90 (m, 3H).
13C NMR (CDCl3, 300 MHz) d 182.5, 155.5, 141.2, 137.8,
124.8, 124.5, 122.9, 118.4, 109.5, 43.3, 31.3, 29.5, 26.7,
22.4, 13.9. MS (ESI) m/z: 472 (M+1)+.
2.1.6. Synthesis of 3,30-(5,50-(9-hexyl-9H-carbazole-3,6-
diyl)bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid) (CSG2)
Compound 5, 5,50-(9-hexyl-9H-carbazole-3,6-diyl)
dithiophene-2-carbaldehyde (0.100 g, 0.212 mmol), dis-
solved in acetic acid was condensed with cyanoacetic acid
(0.054 g, 0.636 mmol) in the presence of ammonium ace-
tate (5 mol%). Then, the reaction mixture was refluxed well
for 12 h under N2. After cooling to room temperature, the
reaction mixture was poured into a crushed ice and solid
obtained was washed thoroughly with water to remove ex-
cess of acetic acid and cyanoacetic acid. Finally, washed
with pentane, hexane and 3% ethyl acetate in hexane to af-
ford a fine red colored solid CSG2 (85%). 1H NMR (CDCl3,
300 MHz) d 8.69 (s, 2H), 8.36–8.39 (m, 2H), 7.84–7.93 (m,
4H), 7.69–7.71 (m, 2H), 7.58–7.62 (m, 2H), 4.38–4.44 (m,
2H), 1.82–1.88 (m, 2H), 1.23–1.36 (m, 6H), 0.83–0.88 (m,
3H). 13C NMR (CDCl3 + DMSO, 300 MHz) d 162, 153.1,
144.4, 139, 131.6, 122.2,121.8, 120.9, 116.9, 114.6,
2.1.4. Synthesis of 5,50-(9-hexyl-9H-carbazole-3,6-
diyl)difuran-2-carbaldehyde (6)
5-Formylfuran-2-ylboronic acid (0.82 g, 0.583 mmol),
3,6-dibromo-9 hexyl-9H carbazole (0.100 g, 0.243 mmol)
and Pd(PPh3)4 (0.056 g, 20 mol%) in dimethoxyethane
(5 mL) and a 2 M Na2CO3 aqueous solution (2 mL) was
degassed with nitrogen. Then, the reaction mixture was
93.7,40.93, 29.2, 27.3, 24.4, 20.2, 12.1. IR (KBr, cmꢀ1
)
3441.84, 2923.68, 2210.32, 1684.30, 1567.97, 1413.42,
1258.66, 1214.40, 1181.26, 1069.26, 786.74. MS (ESI-) m/
z: 604 (Mꢀ1)+.

K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
269
2.1.7. Synthesis of 3,30-(5,50-(9-hexyl-9H-carbazole-3,6-
2.4. Photovoltaic characterization
diyl)bis(furan-5,2-diyl))bis(2-cyanoacrylic acid)(CSG3)
Compound 6, 5,50-(9-hexyl-9H-carbazole-3,6-diyl)difu-
ran-2-carbaldehyde (0.100 g, 0.227 mmol), dissolved in
acetic acid was condensed with cyanoaceticacid (0.058 g,
0.681 mmol) in the presence of ammonium acetate
(5 mol%). Then, the reaction mixture was refluxed well
for 12 h under N2. After cooling to room temperature, the
reaction mixture was poured into a crushed ice and solid
obtained was washed thoroughly with water to remove ex-
cess of acetic acid and cyanoacetic acid. Finally, washed
with pentane, hexane and 3% ethyl acetate in hexane to af-
ford a fine red colored solid CSG3 (84%). 1H NMR (CDCl3,
300 MHz) d 8.71 (s, 2H), 8.05–8.11 (m, 4H), 7.78–7.83 (d,
2H), 7.60–7.64 (d, 2H), 7.29–7.32(d,2H), 4.42–4.49 (m,
2H), 1.76–1.84 (m, 2H), 1.23–1.32 (m, 6H), 0.76–0.84 (m,
3H). 13C NMR (DMSO, 300 MHz) d 164, 160.1, 146.9,
141.2, 137.4, 122.3, 120, 117.7, 116.7, 110.5, 108.3, 42.5,
30.7, 28.3, 25.9, 21.9, 13.7. IR (KBr, cmꢀ1)3434.65, 2925.3,
2213.17, 1688.43, 1589.92, 1528.92, 1460.66, 1417.66,
1263.42, 1234.49, 1191.4, 1037.3, 708.19. MS (ESI-) m/z:
572 (Mꢀ1)+.
The current–voltage characteristics were measured
using the previously reported method with a solar simula-
tor (AM-1.5, 100 mW cmꢀ2, WXS-155S-10: Wacom Denso
Co., Japan) [62]. Monochromatic incident photon-to-
current conversion efficiency (IPCE) for the solar cell,
plotted as
a function of excitation wavelength, was
recorded on a CEP-2000 system (Bunkoh-Keiki Co., Ltd.).
3. Results and discussion
3.1. Synthesis
The photosensitive dyes in this study are depicted in
Fig. 1. Synthetic routes to the as synthesized organic dyes
are illustrated in Scheme 1. Carbazole (1) was used as
starting material. The dibromocarbazole derivative 2 was
synthesized according to previously published procedure
in 90% yield [63]. This dibromocarbazole (2) was N-alkyl-
ated using n-hexylbromide and KOH as base in DMF med-
ium at room temperature for 12 h to give compound 3 in
83% yield. In the next step, Suzuki coupling reaction of 3
with (4-formylphenyl)-, (5-formylthiophen-2-yl)-, or
(5-formylfuran-2-yl)-boronic acid afforded corresponding
Knoevenagel precursors 4–6 in quantitative yields (40–
45%). These reactions were carried out in DME medium
by using Pd(PPh3)4 as catalyst under reflux conditions for
48 h. Finally, these aldehyde intermediates were converted
into corresponding dyes through Knoevenagel reaction by
treating with cyanoacetic acid using piperdine as base
affording 80%, 85% and 84% yields for CSG1, CSG2 and
CSG3 respectively. These target dyes were fully character-
ized by NMR spectroscopy and mass spectrometry. All the
compounds found to be consistent with the proposed
structures.
2.2. Preparation of TiO2 electrode
Nanocrystalline TiO2 photoelectrodes of about 20 lm
thickness (area: 0.25 cm2) were prepared using a variation
of a method reported by Graetzel and co-workers [61].
Fluorine-doped tin oxide coated (FTO) glass electrodes
(Nippon Sheet Glass Co., Japan) with a sheet resistance of
8–10 ohm mꢀ2 and an optical transmission of >80% in the
visible range were used. Anatase TiO2 colloids (particle size
ꢁ13 nm) were obtained from commercial sources (Ti-Nan-
oxide D/SP, Solaronix). The nanocrystalline TiO2 thin films
of approximately 20 lm thickness were deposited onto the
conducting glass by screen-printing. The film was then sin-
tered at 500 °C for 1 h. The film thickness was measured
with a Surfcom 1400A surface profiler (Tokyo Seimitsu
3.2. Photophysical properties
Co., Ltd.). The electrodes were impregnated with
a
50 mM titanium tetrachloride solution and sintered at
500 °C. The dye solutions (2 ꢂ 10ꢀ4 M) were prepared in
1:1 acetonitrile and tert-butyl alcohol solvents. The elec-
trodes were immersed in the dye solutions and then kept
at 25 °C for 20 h to adsorb the dye onto the TiO2 surface.
The UV–vis absorption spectra of CSG1, CSG2 and CSG3
in DMF solution are depicted in Fig. 2 and the correspond-
ing data are summarized in Table 1. The band located at
shorter wavelength is attributed to the
transition of the chromophore. The absorption band at
369–293 nm can be ascribed to localized aromatic p–
p*
p
p* electron
ꢀ
transitions. The absorption band at around 359–427 nm
can be ascribed to an intramolecular charge transfer (ICT)
between the carbazole donor and the cyanoacetic acceptor
[64], the absorption maximum wavelength (kmax) of CSG1,
CSG2 and CSG3 was at 362, 415, and 427 nm respectively,
and the corresponding molar extinction coefficients
2.3. Fabrication of dye-sensitized solar cell
Photovoltaic measurements were performed in a two-
electrode sandwich cell configuration. The dye-deposited
TiO2 film was used as the working electrode and a plati-
num-coated conducting glass as the counter electrode.
The two electrodes were separated by a surlyn spacer
were 3.43 ꢂ 104, 3.27 ꢂ 104 and 3.92 ꢂ 104 Mꢀ1 cmꢀ1
,
respectively (i.e., increasing in the order CSG2 < CSG1 <
CSG3). The kmax of CSG3 with the furan linker was
red-shifted 3 nm compared to that of CSG2 with the thio-
phene linker, while the kmax of CSG2 was red-shifted
38 nm compared to that of dye CSG1 with the benzene
linker. This is due to the better delocalization of
(40 lm thick) and sealed up by heating the polymer frame.
The electrolyte was composed of 0.6 M dimethylpropyl-
imidazolium iodide (DMPII), 0.05 M I2, 0.5 M TBP and
0.1 M LiI in acetonitrile and is introduced into the device
by vacuum filling method through the predrilled hole on
the counter electrode.
electrons over the
p-conjugated molecules when furan

270
K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
Scheme 1. Synthetic route for organic dyes CSG1–CSG3. Reagents and conditions: (i) NBS in DMF, toluene, 0 °C, 2 h; (ii) 1-bromohexane, KOH, DMF, RT,
12 h; (iii) (OH)2B-Ar-CHO, Na2CO3, DME, reflux, 48 h; (iv) cyanoaceticacid, CH3COONH4, CH3COOH, reflux, 6 h.
and thiophene were used as linkers in this A-
framework.
p-D-
p-A
intramolecular charge transfer transition from the donor
to acceptor within the A- -D- -A framework. Since the
p
p
The fluorescence emission spectra of all the dyes exhib-
ited strong luminescence maxima in the wavelength re-
gion 486–502 nm recorded in DMF medium (Fig. 3) upon
their excitation of absorption maximum value. To find
out their excited state behavior, the fluorescence emission
properties for CSG1-CSG3 were examined in solvents of
different polarity. The examples of solvatochromic behav-
ior of the dyes CSG1-CSG3 are illustrated in Fig. 4 and cor-
responding data presented in Table 2. The emission peak
wavelength decreases as the solvent polarity increases. A
clear negative solvatochromism observed for the dyes in
fluorescence further supports the presence of an
dyes are dipolar in nature, polar solvents are expected to
stabilize them in the ground and excited states equally.
However a significant blue shift observed for the emission
spectra recorded for polar solvents such as DMF and THF
when compared to that measured in CHCl3 solution is
interesting and indicate a less polar excited state [65].
The absorption spectra of CSG1, CSG2 and CSG3 on TiO2
films are shown in Fig. 5. The maximum absorption peaks
for CSG1, CSG2 and CSG3 on TiO2 films are at 391, 427, and
396 nm, respectively. Compared with the solution spectra,
the absorption bands of the CSG1 and CSG2 dyes on the
TiO2 films are red shifted by 29 and 12 nm, respectively.

K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
271
form dye aggregates on the semiconductor surface. This
phenomenon leads to higher photo-current densities
[64,71]. Based on this assumption, CSG2, showing the
smallest change of absorption spectrum on TiO2, seemed
to be more efficient in suppressing the formation of dye
aggregates on the TiO2 surface.
To explain the anchoring mode of CSG1–CSG3 on TiO2,
FTIR spectra of pristine CSG1–CSG3 powders and that of
dyes adsorbed on TiO2 have been measured (Fig. 6). The
IR spectra of the sensitized films display two characteris-
tic bands centered at ꢁ1592 and ꢁ1355 cmꢀ1, respec-
tively, are associated with the COOꢀ antisymmetric and
symmetric stretching vibrations of carboxylate groups
coordinated with surface titanium atoms. The FT-IR spec-
tra reveal that bands observed at ꢁ1685 cmꢀ1 assigned
to the free carboxylic acid groups of powdered CSG1–
CSG3 dyes completely disappeared indicating that
CSG1–CSG3 dyes are absorbed on the TiO2 surface with
both of the anchors [33].
40000
30000
20000
10000
0
a
CSG1
CSG2
CSG3
300
400
500
600
Wavelength (nm)
Fig. 2. UV–vis absorption spectra of three organic dyes in DMF.
Table 1
Optical and redox parameters of CSG1–CSG3.
a
b
c
d
e
Dye
kabs (nm)
/Mꢀ1 cmꢀ1
kem
EOX
E0–0
(V)
ELUMO
3.3. Electrochemical properties
(
e
)
(nm)
(V vs. SCE)
(V vs. SCE)
CSG1
CSG2
CSG3
362(34,300)
415(32,700)
427(39,200)
494
502
486
1.15
1.07
1.05
2.77
2.66
2.66
ꢀ1.62
ꢀ1.59
ꢀ1.61
To evaluate the thermodynamically allowed electron
transfer processes from the excited dye molecule to the
conduction band of TiO2, cyclic voltammetry (CV) mea-
surements were performed. The ground and excited state
oxidation potentials of the three dyes were measured
and the results summarized in Table 1.
a
b
c
Absorption spectra.
Emission spectra.
Eox is oxidation potential.
d
E0–0 is voltage of intersection point between absorption and emission
Fig. 7 shows a schematic energy diagram of three dyes
based on absorption and electrochemical data. The
ground-state oxidation potentials EOX of CSG1, CSG2 and
CSG3, corresponding to the highest occupied molecular
orbital (HOMO) level of dyes, range from 1.05 to 1.15 V
vs. SCE. These values are more positive than the Iꢀ3 =Iꢀ redox
potential (0.42 V vs. SCE) [72–75], indicating that the oxi-
dized dye formed after electron injection into the conduc-
tion band of TiO2 and thermodynamically accepted
electrons from Iꢀ ions from the electrolyte [76,77]. The ex-
cited-state oxidation potentials EꢃOX, which reflect the low-
est occupied molecular orbital (LUMO) level of the
sensitizers, play an important role in the electron injection
process. The excited-state oxidation potential EꢃOX was cal-
culated using: EꢃOX = EOX – E0ꢀ0 where E0ꢀ0 is the zeroth–
zeroth transition value obtained from the intersection of
the normalized lowest energy absorption peak and highest
energy fluorescence peak. The LUMO levels of these dyes
are in the range of ꢀ1.59 to ꢀ1.62 V vs. SCE, which are
more negative than the conduction band edge of TiO2
(ꢀ0.5 V vs. SCE) [78–80]. The driving force is sufficient
for efficient charge injection, provided that an energy gap
between dye LUMO and TiO2 conduction band (CB)
is >0.2 eV [81,82]. Thus, the electron injection process from
the excited dye molecule to the TiO2 conduction band and
the subsequent dye regeneration are energetically permit-
ted. Such electronic structures thus ensure a favorable exo-
thermic flow of charges throughout the photo-electric
conversion. Further, the LUMO level of CSG1 (ꢀ1.62 V vs.
NHE) is more negative than that of CSG2 and CSG3
(ꢀ1.59 and ꢀ1.61 V vs. NHE, respectively), indicating that
CSG1 dye has more efficient electron injection.
pectra.
e
ELUMO was calculated by Eox – E0–0
.
CSG1
CSG2
CSG3
1.00
0.75
0.50
0.25
0.00
400
500
600
700
Wavelength (nm)
Fig. 3. Emission spectra of the dye CSG1–CSG3 recorded in DMF.
This red shift can be explained by the formation of
J-aggregates of the dye on TiO2 [66]. Whereas the absorp-
tion band of the CSG3 on the TiO2 film is blue shifted by
31 nm, which is attributed to the formation of H-type
aggregates or deprotonation of the carboxylic acid units.
Similar phenomena have been observed for several other
organic dyes [67–70]. Results of previous research indi-
cated that the smaller kmax shifts associated with dyes from
solution vs. the solid state suggest a smaller tendency to

272
K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
CHCl3
CHCl
CHCl
THF
1.00
0.75
0.50
0.25
0.00
c
1.00
0.75
0.50
0.25
0.00
1.00
3
a
3
b
THF
THF
DMF
DMSO
DMF
DMF
DMSO
DMSO
0.75
0.50
0.25
0.00
420
480
540
600
450
525
600
675
450
525
600
675
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fig. 4. Emission spectra of the dye (a) CSG1 (b) CSG2 and (c) CSG3 recorded in different solvents.
Table 2
Emission data of the CSG1–CSG3 in different solvents.
Dye
kem (nm)
CHCl3
THF
DMF
DMSO
CSG1
CSG2
CSG3
544
540
527
520
502
495
494
502
486
524
525
512
CSG1
CSG2
CSG3
1.2
0.8
0.4
0.0
Fig. 6. FT-IR spectra of CSG1–CSG3 in KBr and adsorbed on TiO2 film.
400
500
600
700
Wavelength (nm)
Fig. 5. Absorption spectra of the three dyes absorbed onto TiO2 film
electrodes.
3.4. Photovoltaic performance in dye-sensitized solar cells
Fig. 8 shows the incident photon-to-current conversion
efficiency (IPCE) as a function of incident wavelength for
DSSCs based on CSG1, CSG2 and CSG3. The dye-coated
TiO2 film was used as the working electrode, platinized
FTO glass as the counter electrode, and 0.5 M 1,2-di-
methyl-3-propylimidazole iodide (DMPII), 0.05 M I2,
0.1 m LiI, and 0.5 M 4-tert butylpyridine (TBP) in acetoni-
trile as the redox electrolyte. In comparison to CSG1, the
onset wavelengths for CSG2 and CSG3 were shifted to long-
er wavelength. The onset wavelengths for CSG2 and CSG3
were around 700 nm while that with CSG1 was around
625 nm, suggesting the increased donor ability of CSG2
and CSG3. Excellent IPCE performance was observed from
350 to 550 nm, CSG1 with a maximum of 82% at 420–
490 nm, CSG2 with a maximum of 72% at 365–545 nm
and CSG3 with a maximum of 66% at 360–525 nm.
Fig. 7. The schematic energy levels of CSG1–CSG3 based on absorption
and electrochemical data.
On the other hand, the IPCE of CSG1 is higher than that
of CSG3 in the region of 330–510 nm, although the IPCE va-
lue of CSG1 is significantly lower than CSG3 in the remain-
ing visible region (see Fig. 8), which suggests that CSG1 has
a higher cell performance. Despite of highest absorption
wavelength, the decreased IPCE values observed for CSG3
can be attributed to the increase of dye aggregation on
TiO2 surface [21,83]. The sensitizer CSG2 has broader
action spectra compared to CSG1 which means that it
can generate greater conversion efficiency. In addition,

K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
273
100
75
50
25
0
10.0
CSG1
CSG2
CSG3
CSG1
CSG2
CSG3
7.5
5.0
2.5
0.0
0.0
0.2
0.4
0.6
0.8
300
400
500
600
700
Voltage (V)
Wavelength (nm)
Fig. 9. Photocurrent–voltage characteristics of the three dye-sensitized
solar cells under illumination of simulated solar light (AM 1.5,
100 mW cmꢀ2).
Fig. 8. Action spectra of monochromatic incident photo-to-current con-
version efficiency (IPCE) of the three dye-sensitized solar cells.
although the IPCE of CSG2 are lower than those of CSG1 in
the region 370–495 nm, the IPCE values of CSG2 are higher
and broader than those of CSG1 in the remaining region,
which indicates that the CSG2-sensitized TiO2 electrode
would generate the highest conversion yield among the
three dyes. In the whole region, the IPCE values of CSG2
are more or less similar in intensity and broader than those
of CSG1 and CSG3 in the remaining region, which indicates
that the CSG2-sensitized TiO2 electrode could generate the
highest conversion yield among the three sensitizers.
The photocurrent density–photovoltage (J–V) curves of
the DSSCs based on CSG1–CSG3 are shown in Fig. 9, and
key parameters including short-circuit current density
(JSC), open-circuit voltage (VOC), fill factor (FF) and overall
Table 3
DSSC performance parameters of CSG1–CSG3 and standard N719.
JSC (mA cmꢀ2
)
VOC (V)
FFc
gd (%)
a
b
Dye
CSG1
CSG2
CSG3
N719
6.36
8.90
7.61
0.699
0.584
0.579
0.606
0.757
0.740
0.732
0.716
3.4
3.8
3.2
6.4
14.74
a
b
c
JSC: short photocurrent density.
VOC: open-circuit photovoltage.
ff: fill factor.
d
g
: power conversion efficiency.
energy capture in a narrower region of the solar spectrum
than the CSG2 and CSG3 based devices.
power conversion efficiency (
For the three dyes, the CSG2-sensitized cell exhibited a
maximum
of 3.8% (JSC = 8.90 mA cmꢀ2, VOC = 584 mV,
FF = 0.74). Under the same conditions, the cells sensitized
with CSG1 and CSG3 gave JSC of 6.36 and 7.61 mA cmꢀ2
VOC of 699 and 579 mV and FF of 0.75 and 0.73, correspond-
ing to of 3.4% and 3.2%, respectively. Under similar fabri-
g) are summarized in Table 3.
Under illumination, VOC of the device is the potential
difference between the quasi-Fermi level of the electrons
in the conduction band (CB) of TiO2 film and redox poten-
tial of the electrolyte ðIꢀ=Iꢀ3 Þ. The pronounced rate of elec-
tron recombination between the injected electron onto the
CB of TiO2 and the Iꢀ3 or oxidized sensitizer could reduce
the VOC. In addition, the band-edge shift occurring due to
the interaction of the sensitizer with TiO2 will also alter
the VOC. However, a strong dipole present at the TiO2/sen-
sitizer interface will contribute to a more negative shift in
the Fermi level of the TiO2 conduction band. The highest
VOC observed for the devices fabricated using the sensitizer
CSG1 may be attributed to the stronger interaction be-
tween the TiO2 and the sensitizer while the lowest VOC
exhibited by CSG2 and CSG3 may be probably due to poor
interaction between the sensitizer and TiO2 [75].
g
,
g
cation conditions, champion dye N719 exhibited the
maximum overall conversion efficiency of 6.4%
(JSC = 14.74 mA cmꢀ2
,
VOC = 606 mV, FF = 0.716). Among
the three organic dyes, CSG2 sensitized cell has the highest
resulting from its relatively higher JSC, which can be
g
attributed to its broader absorption and enhanced molar
extinction coefficient in the visible region. The short-cir-
cuit current (JSC) is related to the molar extinction coeffi-
cient of the dye molecule, a higher molar extinction
coefficient showing good light-harvesting ability and yield-
ing a higher short-circuit current. The relative lower pla-
teau from CSG3 despite a high driving force for electron
injection might arise from dye aggregation behavior on
the TiO2 surface, decreasing electron injection yield. This
assumption is consistent with the absorption curve for a
CSG3 loaded TiO2 film, which showed the largest spectral
shift relative to that in solution. On the other hand, the de-
vice constructed from CSG1 resulted in the highest plateau.
This finding further supports aggregation behavior as a fac-
tor affecting IPCE plateaus. Nonetheless, CSG1 based device
did not give the highest current density, due to solar
4. Conclusions
Three metal-free organic dyes (CSG1–CSG3) comprising
a carbazole moiety and cyanoacrylic acid as the common
electron donorand anchoring group with different linker
moieties such as benzene, thiophene, or furan were de-
signed and synthesized for application in dye-sensitized
solar cells (DSSCs). The results based on photovoltaic
experiments show that dye with a benzene linker exhibit
a
higher open-circuit voltage (0.699 V) compared to

274
K.S.V. Gupta et al. / Organic Electronics 15 (2014) 266–275
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