Carbazole-based sensitizers for potential application to dye sensitized solar cells

Article abstract of DOI:10.1007/s12039-015-0794-1

Two push-pull molecules employing carbazole and alkyl thiophene (CAR-THIOHX) or carbazole and triphenylamine (CAR-TPA) as donor moieties, with the cyanoacrylic group as the acceptor, have been designed and synthesized by simple organic transformations. Photophysical and electrochemical studies revealed the potential of these two systems in dye sensitized solar cells (DSSC). Under standard irradiation conditions, CAR-TPA and CAR-THIOHX exhibited 2.12 and 1.83% of overall power conversion efficiencies respectively. The moderate photovoltaic efficiency of the sensitizers has been attributed to the poor light absorption of the sensitizers in the visible region. Density functional theory (DFT) calculations have shown a strong intramolecular charge transfer character, with the HOMOs of both the sensitizers exclusively localized on the corresponding donor moieties and LUMOs on the cyanoacrylic acid acceptor. On the other hand, the calculated high dihedral angle between the carbazole donor and the phenyl bridge for these sensitizers impedes the conjugation along the dyes backbone, and thus leads to less extended and intense absorption spectra in the visible region. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0794-1

J. Chem. Sci. Vol. 127, No. 3, March 2015, pp. 383–394. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-015-0794-1
Carbazole-based sensitizers for potential application to dye sensitized
solar cells
a
a
a
NARESH DUVVA , RAVI KUMAR KANAPARTHI , JAIPAL KANDHADI ,
b
b,c
b,∗
GABRIELE MAROTTA , PAOLO SALVATORI , FILIPPO DE ANGELIS and
LINGAMALLU GIRIBABU
Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,
a,∗
a
Hyderabad 500 007, India
b
Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze
Tecnologie Molecolari, Via Elce di Sotto 8, I-06123, Perugia, Italy
c
D3-Computation, Istituto Italiano di Tecnologia, Via Morego 30, I-16163 Genova, Italy
e-mail: giribabu@iict.res.in; filippo@thch.unipg.it
MS received 6 May 2014; revised 8 September 2014; accepted 26 September 2014
Abstract. Two push-pull molecules employing carbazole and alkyl thiophene (CAR-THIOHX) or carbazole
and triphenylamine (CAR-TPA) as donor moieties, with the cyanoacrylic group as the acceptor, have been
designed and synthesized by simple organic transformations. Photophysical and electrochemical studies
revealed the potential of these two systems in dye sensitized solar cells (DSSC). Under standard irradiation
conditions, CAR-TPA and CAR-THIOHX exhibited 2.12 and 1.83% of overall power conversion efficien-
cies respectively. The moderate photovoltaic efficiency of the sensitizers has been attributed to the poor light
absorption of the sensitizers in the visible region. Density functional theory (DFT) calculations have shown a
strong intramolecular charge transfer character, with the HOMOs of both the sensitizers exclusively localized
on the corresponding donor moieties and LUMOs on the cyanoacrylic acid acceptor. On the other hand, the
calculated high dihedral angle between the carbazole donor and the phenyl bridge for these sensitizers impedes
the conjugation along the dyes backbone, and thus leads to less extended and intense absorption spectra in the
visible region.
Keywords. Sensitizer; metal-free; push-pull; carbazole; DFT.
1
. Introduction
the molecular structure and can be synthesized easily
and rather inexpensively by following well-established
design protocols. Moreover, it is possible to tune the
absorption, fluorescence emission and electrochemical
properties in a desired manner through suitable molec-
ular design strategies for achieving better efficiency.
More interestingly, the high molar extinction coeffi-
In recent years, metal-free organic materials, because
of their unique optical and electronic properties, have
been used quite extensively in optoelectronic devices
which include solar cells.1 Dye sensitized solar cells’
–5
(DSSCs) research progressed immensely after the pio-
5
−1
−1
neering work of O’Regan and Grätzel on electrochem-
ical cell which exhibited high power conversion effi-
ciency using ruthenium(II) polypyridyl complexes.
Among the various ingredients of the DSSC, the dye
which sensitizes the TiO semiconductor, plays an
important role in achieving high efficiency and durabil-
ity of the cell. Although the ruthenium(II) based com-
plexes are very promising in harvesting solar energy,
they have certain drawbacks which turned the attention
of the researchers towards metal-free organic dyes, por-
cients (up to 10 M cm ) of these metal-free sen-
sitizers are particularly attractive. Among the various
molecular architectures, donor-π-acceptor (D-π-A) is
found to show promising conversion efficiencies. So
far, various electron donor moieties such as triph-
enylamine, carbazole, indoline, squaraines, etc., have
been employed in the D-π-A architecture for designing
6
2
8
,9
efficient sensitizers. Recently, a triphenylamine based
sensitizer has shown outstanding power conversion effi-
ciency of 10.3% by using Co(II/III) redox couple.10
Over the past decade, molecular sensitizers based
on triphenylamine, squaraine and indole have been
well studied and documented. Carbazole is a hetero-
cyclic aromatic system and its derivatives have been
7
phyrins and other molecular systems. The metal-free
organic dyes have the advantage of a large diversity in
∗
For correspondence
383

384
Naresh Duvva et al.
11
successfully employed in molecular electronics. How-
ever, carbazole based sensitizers have not progressed
much compared to the aforesaid donor systems.
COOH
COOH
NC
NC
Miyasaka and co-workers employed a carbazole
based polymer (poly(N-vinyl-carbazole)) for the first
time in dye-sensitized solar cell as a hole transport-
ing layer and the cell showed power efficiencies of 2.4
and 2.0% at 0.25 and 1 sun (AM 1.5) light irradia-
tion, respectively. Since then, many efforts have been
made to improve the efficiency and durability of the
carbazole based DSSCs by following molecular engi-
neering strategies. Koumura and co-workers reported
N
N
S
S
N
N
6
C H
13
6 13
C H
12
CAR-THIOHX
CAR-TPA
Scheme 1. Molecular structures of CAR-THIOHX and
a series of carbazole sensitizers for DSSCs and sys- CAR-TPA.
tematically investigated the effect of a π-conjugated
system of n-hexyl-substituted oligothiophenes on the density at N9 site can be enriched and hence the device
DSSC performances.1
3,14
A DSSC device fabricated performance. It is expected that the directional prop-
from one of the derivatives showed overall power con- erty of the excited dye further helps this new dye,
version efficiency of 7.7% under standard illumination CAR-TPA to have better electron flow from the triph-
conditions. Tan and co-workers reported a new sen- enylamine to carbazole. Due to the enhanced electron
sitizer based on carbazole in which triphenylamine is donating ability of the triphenylamine moieties, CAR-
used as auxiliary donor and the DSSC device showed TPA dye is expected to show better power conversion
efficiency as high as 7.03%.15 Recently, Han and co- efficiency compared to the CAR-THIOHX.
workers reported efficient carbazole based sensitizer
bearing flexible circle chain embracing π-spacer which
16
2. Experimental
has an efficiency of 9.2%. In most of the carbazole sen-
sitizers, cyanoacrylic acid group is commonly used as
an anchoring group. However, Harima and co-workers
reported new carbazole based sensitizers bearing pyri-
dine anchoring group and achieved the efficiency of
Triphenylamine, carbazole, bis(pinacolato)diborane, N-
bromosuccinamide (NBS), Pd(PPh ) Cl , pottasium
acetate (KOAc), pottasium tert-butoxide (t-BuOK),
cyanoacetic acid and ammonium acetate (NH OAc)
were procured from Sigma Aldrich and used as such.
All solvents were procured from BDH (India) and
3
2
2
17
4
1
.61%. By substituting proper functional groups at
C3, C6 and N9 positions of the carbazole scaffold,
one can tune the optoelectronic properties and thus
the device performance. Recently, C2 and C7 positions
have also been exploited in DSSCs to achieve excellent
power conversion efficiencies 6.8–7.2%.18
Here, we report synthesis, characterization and
device studies of two new carbazole derivatives, CAR-
THIOHX and CAR-TPA by exploiting alkyl sub-
stituted thiophene and triphenylamine at C3 and C6
positions as donor moieties (scheme 1). The CAR-
THIOHX consists of an anchoring group at N9 posi-
tion and two hexyl substituted thiophene moieties at C3
and C6 positions of carbazole. In CAR-TPA, tripheny-
lamine auxiliary donors have been covalently attached
at C3 and C6, keeping the same anchoring group at
N9 position. There are a few triphenylamine and indole
22
dried wherever it is necessary prior to use. Analyti-
cal reagent grade solvents were used for synthesis and
distilled laboratory grade solvents were used for chro-
matography. Milli-Q (Millipore) water was used for
synthesis and purification. Silica gel (ACME, 100–200
mesh) was used for column chromatography and thin-
layer chromatography (TLC) was performed by using
precoated silica gel on aluminum plates (Merck, 60-
F ). Either gravity or flash chromatography was used
254
for compound purification. Dual solvent system was
eluted gradiently for the separation of reaction mix-
tures and the major intense band was collected and
concentrated.
based molecular sensitizers in which carbazole is used
2.1 Synthesis
as auxiliary donors.1
9,20
Very recently, some of us have
explored the possibility of using the carbazole moiety as 2.1a 4-Bromo triphenyl amine (1): Compound 1 was
21
an antenna donor in phenothiazine based dyes. How- synthesized by following reported procedures with
2
3,24
ever, very few carbazole sensitizers consist of auxiliary slight modifications.
To a solution of tripheny-
13
donor moieties. We believe that by substituting triph- lamine (2.1 g, 8.16 mmol) in 40 mL CCl , NBS (1.6 g,
4
enylamine donor at C3 and C6 position, the electron 8.16 mmol) was added and the reaction mixture

Carbazole sensitizers with applications in DSSCs
385
refluxed for 4 h under nitrogen (N ) atmosphere. During 2.1d 4-(3,6-Dibromo-9H-carbazol-9-yl)benzaldehyde
2
the reaction solid was formed which, conformed as suc- (4): Compound 3 (1.60 g, 5.9 mmol) was dissolved in
◦
cinimide. After filter the reaction mixture filtrate was toluene (10 mL) and cooled to 0 C in an ice bath. Then
evaporated and recrystalized from ethanol. Yield 93%. a solution of NBS (2.60 g, 13.2 mmol) in DMF (15 mL)
1
H NMR (500 MH , CDCl , TMS) δ (PPM):7.31 (d, was added dropwise to the dissolved compound 3 and
Z
3
2
H, J = 8.4 Hz), 7.24 (t, 4H, J = 7.6 Hz), 7.07 (d, 4H, stirred for 30 min. The reaction mixture was poured
J = 8 Hz), 7.02 (t, 2H, J = 7.5 Hz), 6.93 (m, 2H). in ice-cold water and the obtained precipitate was
+
ESI-MS: m/z 323 [M ].
filtered, washed with cold methanol for several times,
and dried under reduced pressure. Yield ∼92%. H
1
2
.1b N-(4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2- NMR (500 MH , CDCl , TMS) δ(PPM): 7.3–7.53 (m,
Z 3
yl)phenyl)-N-phenylbenzenamine (2): Compound 2 4H, J = 7.54 Hz), 7.74–7.79 (dd, 2H, J = 8.3 Hz),
was synthesized according to the modified reported 8.07–8.3 (m, 4H, J = 8.4 Hz), 10.12 (s, 1H). ESI-MS:
2
5
+
procedures.
1
4-Bromo triphenylamine, 1 (0.32 g, m/z 431 [M +H].
mmol), bis(pinacolato)diborane (0.38 g, 1.5 mmol),
KOAc (0.4 g, 4 mmol) and Pd(PPh ) Cl (35 mg,
3
2
2
2
.1e 4-(3,6-Bis(4-(diphenylamino)phenyl)-9H-
0
.05 mmol) were dissolved in minimum amount of
carbazol-9-yl)benzaldehyde (5): Compound 5 was
synthesized by palladium catalyzed Suzuki coupling
between compound 4 and compound 2. In a typical
synthesis, compound 4 (0.22 g, 0.48 mmol), compound
degassed N,N-dimethyl formamide (DMF) in a 100 mL
round bottom (RB) flask. Subsequently, the reaction
mixture was subjected to reflux for 6 h under nitrogen
atmosphere and the reaction was monitored by TLC.
After completion of reaction, the solvent was evapo-
rated under vacuum and crude compound was extracted
with dichloromethane and concentrated using rotary
evaporator. The crude compound was purified by column
chromatography by employing silica gel as stationary
phase and ethyl acetete:hexane (2:98) as eluting solvent.
2
(0.53 g, 1.43 mmol), KOAc (0.43 g, 3.84 mmol) and
Pd(PPh ) Cl (0.05 mmol) were dissolved in DMF and
degassed for 30 min. Then the reaction mixture was
subjected to heat at boiling point of the solvent for 6 h
3
2
2
under N atmosphere and the reaction was monitored by
2
TLC. After completion of the reaction, the solvent DMF
was evaporated, extracted from dichloromethane and
water mixture. Then the finally obtained crude com-
pound was purified by column chromatography on
1
Yield ∼75%. H NMR (500 MH , CDCl , TMS) δ
Z
3
(
PPM): 1.35(s, 12H), 7.05,(t, 4H, J = 7.99 Hz), 7.12(d,
4
H, J = 7.99 Hz), 7.24–7.29 (t, 4H, J = 6.9 Hz), 7.68
silica as stationary phase and CHCl : Hexane (3:7)
+
3
(
d, 2H, J = 7.98 Hz). ESI-MS: m/z 371 [M ].
1
as eluting solvent. Yield ∼75%. H NMR (500 MH ,
Z
CDCl , TMS) δ (PPM): 7.00–7.08 (t, 4H, = 7.176),
3
2.1c 4-(9H-Carbazol-9-yl)benzaldehyde (3): Com-
7
.13–7.3 (m, 20H, J = 8.309 Hz), 7.53–7.69 (m, 8H,
pound 3 was synthesized as per the reported pro-
J = 8.687 Hz), 7.81–7.87 (d, 2H, J = 8.309 Hz),
.12–8.18 (d, 2H, J = 8.34 Hz), 8.35 (d, 2H, J = 1.32
Hz), 10.12(s, 1H). IR (KBr, cm ): 2952, 2854 (alkyl
), 1645(ν , ν ) 1451, 1230, 751. ESI-MS:
26
cedure.
In 50 mL RB flask, carbazole (2.0 g,
8
1
1.9 mmol) and t-BuOK (1.33 g, 11.9 mmol) were dis-
−1
◦
solved together in DMF (10 mL) and heated at 110 C
under stirring for 30 min. 4-Fluorobenzaldehyde (1.5 g,
ν
C−H
S
C=O
+
m/z757 [M ].
11.9 mmol) was taken in 5 mL of DMF and added
dropwise to the reaction mixture over 30 min. Then the
reaction mixture refluxed for another 36 h at the same 2.1f 4-(3,6-Bis(5-hexylthiophen-2-yl)-7,8-dihydro-
temperature under nitrogen atmosphere. After comple- carbazol-9-yl)benzaldehyde (6): Compound 6 was
tion of the reaction, the reaction mixture was cooled synthesized by Suzuki coupling reaction of compound
to room temperature and poured in excess amount of 4. In a typical synthesis compound 4 (0.3 g, 0.7 mmol)
ice-cold water (700 mL), from which solid precipitated. and 5-hexylthiophene-2-yl-2-boronic acid (0.3 g,
Then the solid was filtered under reduced pressure and 1.404 mmol) were dissolved in dimethoxyethane, DME
dried. Finally, the crude compound was purified by (12 mL) to which Na CO solution (2 M, 5 mL of),
2
3
column chromatography employing silica gel as sta- Pd(PPh ) (0.1 mmol) were added and reaction mix-
3
4
tionary phase and CHCl : Hexane (1:1) as mobile ture was degassed for 30 min. The reaction mixture
3
1
phase. Yield ∼60%. H NMR (500 MH , CDCl , TMS) was then refluxed for 18 h under N atmosphere. After
Z
3
2
δ(PPM): 7.29–7.36(t, 2H, J = 6.798 Hz), 7.4–7.52(m, completion of the reaction (monitored by TLC), excess
4
H, J = 8.30 Hz), 7.76–7.81(d, 2H, J = 9.06 Hz), water was added to the reaction mixture, extracted with
8
.09–8.17(t, 4H, J = 7.54), 10.11(s, 1H). ESI-MS: dichloromethane, organic layer separated and concen-
+
m/z 271 [M ].
trated on rotary evaporator. Crude solid compound

386
Naresh Duvva et al.
was purified by column chromatography using silica controlled X-WIN software. The elemental analysis
gel as stationary phase and ethyl acetate:hexane (1:10) was carried out on Elementar, Vario MICRO CUBE
1
as eluting solvent. Yield ∼70%. H NMR (500 MH , analyzer. Cyclic voltammetric (CV) measurements
Z
CDCl , TMS) δ(PPM): 0.76–0.88 (t, 6H, J = 6.798 were performed on a PC-controlled CH instruments
3
Hz), 1.12–1.4 (m, 12H, J = 3.022), 1.51–1.71 (m, 4H, (CHI 620C electrochemical analyzer). For all measure-
J = 7.554 Hz), 2.69–2.79 (t, 4H, J = 7.554 Hz), 6.67 ments, 1 mM solution of compounds in CH Cl sol-
2
2
(
(
d, 2H, J = 3.77 Hz), 7.05 (t, 2H, J = 3.77 Hz), 7.28 vent were used at scan rate of 100 mV/s in which 0.1 M
d, 2H, J = 8.30 Hz), 7.46–7.53 (dd, 2H, J = 1.51 tetrabutyl ammonium perchlorate (TBAP) is used as
Hz), 7.56–7.62 (d, 2H, J = 8.30 Hz), 7.91–8.00 (d, supporting electrolyte. The working electrode is glassy
2
(
H, J = 3.77 Hz), 8.15 (d, 2H, J = 1.51 Hz), 9.96 carbon, standard calomel electrode (SCE) is reference
−1
s, 1H). IR (KBr, cm ): 2897 (alkyl ν ), 1796 (ν , electrode and platinum wire served as auxiliary elec-
C−H
S
+
ν
C=O
) 1449, 1179, 732. ESI-MS: m/z 604 [M ].
trode. After a cyclic voltammogram (CV) had been
recorded, ferrocene was added, and a second voltam-
2
.1g CAR-TPA: Finally the desired compound, mogram was measured. The optical absorption spectra
CAR-TPA sensitizer was synthesized by following were recorded on a Shimadzu (Model UV-3600) spec-
27
Knoevenagel reation procedure. Compound 5 (0.76 g, trophotometer. Concentrations of solutions are ca. to be
−
6
1
mmol), cyanoacetic acid (0.43 g, 5 mmol) and ammo- 1 × 10 M in all solvents. Steady-state fluorescence
nium acetate (0.24 g, 3 mmol) were dissolved in acetic spectra were recorded on a Fluorolog-3 spectrofluo-
acid (10 mL) and refluxed for 14 h under nitrogen atmo- rometer (Spex model, Jobin Yvon) for solutions with
sphere. The reaction was monitored by TLC and at the optical density at the wavelength of excitation (λex
=
end of reaction, the reaction mixture poured in ice-cold 350 nm) ≈0.05. Fluorescence quantum yields (φ) were
water and extracted with chloroform. Collected organic estimated by integrating the fluorescence bands of
layers were evaporated and dried. Then the obtained the each individual compounds and comparing with
solid compound was purified by column chromatography standard quinine sulfate fluorescence emission (φ =
using silica gel, stationary phase and MeOH:CHCl (1:10) 0.577 in 0.1 M H SO ). Fluorescence lifetime measure-
2 4
3
as eluting solvent. Yield ∼76%. Elemental analysis of ments were carried on a picosecond time-correlated
C H N O (calculated mass % in parentheses): C, 84.40 single photon counting (TCSPC) setup (FluoroLog3-
58
40
4
2
1
(
84.44); H, 4.90 (4.89); N, 6.80 (6.79). H NMR (500 Triple Illuminator, IBH Horiba Jobin Yvon) employ-
MH , CDCl , TMS) δ(PPM): 7–7.1 (t, 4H, J = 7.176), ing a picosecond light emitting diode laser (NanoLED,
Z
3
7
.13–7.3 (m, 20H, J = 8.309 Hz), 7.4–7.69 (m, 8H,
J = 8.687 Hz), 7.81–7.87 (d, 2H, J = 8.309 Hz), were recorded by monitoring the fluorescence emission
.12–8.18 (d, 2H, J = 8.34 Hz), 8.35 (d, 2H, J = 1.32 maxima of the triads (λem = 430 nm). Photomulti-
Hz), 8.34 (s, 1H). IR (KBr, cm ): 3415 (ν ) 2926, plier tube (R928P, Hamamatsu) was employed as the
λ
ex = 405 nm) as excitation source. The decay curves
8
−1
O−H
2
1
854 (alkyl νC ), 2220(ν , ν ), 1728(ν , ν ), detector. The lamp profile was recorded by placing a
−H S C≡N S C=O
+
454, 1276, 747. ESI-MS: m/z 845 [M +Na].
scatterer (dilute solution of Ludox in water) in place
of the sample. The width of the instrument response
2
.1h CAR-THIOHX: This compound was synthe- function (IRF) was limited by the full width at half
sized as per the procedure adopted for preparing CAR- maxima (FWHM) of the excitation source, ∼625 ps
TPA. Elemental analysis of C H N O S (calculated at 405 nm. Decay curves were analyzed by non-linear
42
42
2
2 2
mass% in parentheses): C, 75.21 (75.19); H, 6.30 least-squares iteration procedure using IBH DAS6 (ver-
1
(
6.31); N, 4.20 (4.18). H NMR (500 MH , CDCl , sion 2.3) decay analysis software. The quality of the
Z 3
2
TMS) δ(PPM): 0.80–0.91(t, 6H, J = 6.798 Hz), 1.1– fits was judged by the χ values and distribution of the
1
2
7
.4(m, 8H), 1.54–1.69(m, 4H, J = 6.798), 2.67(s, 4H), residuals.
.85(s, 4H), 7.28–7.39(dd, 5H, J = 5.85 Hz), 7.46–
.83(m, 5H, J = 8.49), 8.06–8.26(dd, 4H, J = 8.68
2
.3 Cell fabrication
−
1
Hz), 8.35(s, 1H). IR (KBr, cm ): 3406(ν ) 2926,
O−H
2
1
852 (alkyl ν ), 2217(ν , ν ) 1624(ν , ν ), 1598, FTO glass (TEC-15, 2.2 mm thickness, Solaronix) was
514, 1362, 1227, 794. ESI-MS: m/z 670 [M ].
C−H
S
C≡N
S
C=O
+
used for transparent conducting electrodes. The sub-
strate was first cleaned in an ultrasonic bath using
a detergent solution, acetone and ethanol respectively
2.2 Instrumentation
(each step 15 min long). The FTO glass plates were
1H NMR spectra were recorded on a Brucker 300 immersed into a 40 mM aqueous TiCl solution at 70 C
◦
4
Avance NMR spectrometer (300 MHz) using PC for 30 min and washed with water and ethanol. Two

Carbazole sensitizers with applications in DSSCs
387
kind of TiO film were tested: a monolayer of opaque the conductor-like polarizable continuum model (C-
2
31
32
TiO paste (Dyesol, 18NR-AO), or a layer of trans- PCM) as implemented in GAUSSIAN 09. In order
2
parent TiO paste (Dyesol, 18NR-T) coated by a scat- to determine the frontier orbital energies and the cor-
2
tering layer (Dyesol, WER 2.0), spread on the FTO responding electronic distribution, we performed sin-
glass plates by doctor blade. The TiO layer films gle point calculations at the B3LYP/6-31G* level in
2
were treated in an ethanol chamber and dried for solution.
◦
5
0
min at 120 C. The TiO coated electrodes (active area
2
2
◦
.2 cm ) were gradually heated under air flow at 325 C
for 5 min, at 375 C for 5 min, at 450 C for 15 min,
and 500 C for 15 min. After the sintering process, the
3
. Results and Discussion
◦
◦
◦
The synthetic approach to these single donor (CAR-
THIOHX) and double donor (CAR-TPA) metal-free
organic sensitizers involves Miyaura borylation, Suzuki
and Knoevenagel reactions and the synthetic routes are
outlined in schemes 2 and 3. Initially, base catalyzed
C-N bond formation was performed between carbazole
and 4-fluorobenzaldehyde and the corresponding prod-
uct subjected to bromination. Then by following pal-
ladium(0) catalyzed Suzuki coupling reaction, either
donor TPA boronic acid or thiophene boronic acid have
been connected to carbazole at 2, 7 positions. Finally,
anchoring as well as electron acceptor group was intro-
duced by reacting either 4 or 5 with cyanoacetic acid via
Knoevenagel condensation leading to CAR-TPA and
CAR-THIOHX. Detailed reaction procedures have
been discussed in the experimental section of this paper.
These two single and double donor metal-free
organic sensitizers, CAR-THIOHX and CAR-TPA
were fully characterized by vary spectroscopic methods
and elemental analysis. Preliminary characterization of
both sensitizers was done by using elemental analy-
TiO film was again treated with 40 mM TiCl solu-
2
4
tion, then rinsed with water and ethanol. The elec-
◦
trodes were heated at 500 C for 30 min and after cool-
◦
ing (80 C) were immersed for 6 hours into sensitizing
bath. This consisted of EtOH:THF = 9:1 solutions of
dyes in 0.2 mM concentration, with 10.0 mM of 3a,7a-
dihydroxy-5b-cholic acid (CDCA) added. Counter elec-
trodes were prepared by coating with a drop of H PtCl
2
6
solution (2 mg of Pt in 1 mL of ethanol) a FTO plate
(
4
TEC 15/2.2 mm thickness, Solaronix) and heating at
◦
00 C for 15 min. The TiO sensitized photoanode
2
and Pt counter electrode were assembled into a sealed
sandwich-type cell by a hot-melt ionomer film (Surlyn,
25 μm thickness, Dyesol). The electrolyte solution, the
commercially available Iolitech ES-0004 HP, contain-
ing 1butyl-3methylimidiazolium iodide, iodine, guani-
diniumthiocyanate and tert-butylpyridine, in a mixture
of valeronitrile and acetonitrile, with LiI 0.1 M added
where required, was inserted by vacuum backfilling.
Then, the hole was sealed by using additional Surlyn
patch and a cover glass and finally a conductive Ag-
based paint was deposed at the electrical contacts.
Photovoltaic measurements were recorded by means
of AM 1.5 solar simulator equipped with a Xenon lamp
1
sis, ESI-MS, IR and H NMR spectroscopies. Elemen-
tal analysis results were consistent with the assigned
formulations. The mass spectra obtained by the ESI-
MS technique, the molecular ion peak was observed
+
+
(
LOT-ORIEL LS 0106). The power of incoming radia-
at m/z 845 [M +Na] and 670 [M ] corresponds to
the presence of CAR-TPA and CAR-THIOHX respec-
tively. Stretching frequencies in IR spectra at 1728
2
tion, set at 100 mW/cm , was checked by a piranome-
ter. J–V curves were obtained by applying an exter-
nal bias to the cell and measuring the generated pho-
tocurrent with a Keithley model 2400 digital source-
meter, under the control of dedicated LabTracer 2.0
software. A black shading mask was employed to avoid
the overestimation of the measured parameters.
−1
and 1624 cm corresponds to the presence of carbonyl
groups of CAR-TPA and CAR-THIOHX, respectively
(
see Supporting information).
UV–Vis electronic absorption spectra of CAR-TPA
and CAR-THIOHX have been recorded in various sol-
vents with different polarities and the spectra are illus-
trated in figure 1. As can be seen in the figure 1, CAR-
TPA and CAR-THIOHX exhibit prominent absorp-
2.4 Computational details
The ground state equilibrium geometries of the iso- tion peaks in the UV region, at about 300 nm, which
lated compounds were optimized in vacuum using the are assigned to π-π* transitions. At longer wavelengths
28
B3LYP exchange-correlation (xc) functional and the we observe other absorption signals, bands or shoul-
6
-31G* basis set.29 The UV-Vis absorption spectra der of the n-π* transitions, with low molar extinction
of the dyes in solution were simulated by TDDFT coefficients. These may be attributed to the intramolec-
calculations with the Coulomb-attenuating B3LYP ular charge transfer (ICT) transitions from the donor
CAM-B3LYP) method30 and a 6-31G* basis set. carbazole to the cyanoacrylic moieties.
33–35
The strong
(
Solvent (Ethanol, EtOH) effects were included by solvent dependency, solvatochromism further suggest

388
Naresh Duvva et al.
O
O
B
N
Br
Pd(pph ) Cl KoAC
NBS
CCl4
3 2
2
N
N
O
O
B B
O
O
2
1
H
O
H
O
H
O
H
N
DMF
NBS
+
DMF,Toluene
tBuoK
N
N
F
Br
Br
3
4
Pd(pph
3
)
2
Cl
2
(2)
KoAC
CN
H
O
COOH
NC
COOH
N
N
-
+
3 4
ACOH, CH COO NH
N
N
N
N
5
CAR-TPA
Scheme 2. Synthetic scheme of CAR-TPA.
H
O
CN
COOH
(
4)
NC COOH
N
Pd(pph3)4
-
+
ACOH, CH
3
COO NH
4
N
+
Na Co3
2
OH
DME,H O
B
2
S
S
S
OH
6
S
Hex
S
Hex
Hex
Hex
CAR-THIOHX
Scheme 3. Synthetic scheme of CAR-THIOHX.
the ICT character of the bands (table 1). As the polarity excited-state quenching.36–39 However, our experiments
of the medium decreases (in low polar solvents DCM using low concentrated dye solutions also reveal the
and toluene), the ICT band blue shifted (negative sol- same behaviour that ruled out the aggregation of the
vatochromism) and intensity increased to several fold dyes at those concentrations. The observed negative
presumably due to the aggregation tendency of both the solvatochromism could also be attributed to the possible
40–42
dyes. Dyes aggregation is known to be a major prob- deprotonation of the carboxylic acid group.
lem which reduces the overall conversion efficiency of
Fluorescence emission was also recorded in differ-
DSSC devices due to the reduced amount of absorbed ent solvents (toluene, THF, DCM, DMF and ethanol)
light (caused by filtering effects) and intermolecular and spectra in DCM are displayed in figure 2. The

Carbazole sensitizers with applications in DSSCs
389
2
1
1
.0
.5
.0
2
1
1
0
0
.0
.5
.0
.5
.0
Toluene
THF
DCM
DMF
Ethanol
Toluene
THF
DCM
DMF
Ethanol
(
a)
(b)
0.5
0.0
3
00
400
500
300
400
500
Wavelength (nm)
Wavelength (nm)
Figure 1. Absorption spectra of (a) CAR-THIOHX & (b) CAR-TPA in different solvents.
Table 1. Absorption and emission maxima of CAR-THIOX and CAR-TPA in different solvents.a
CAR-THIOX
λmax, nm, (log ε)
CAR-TPA
λmax, nm,(log ε)
CAR-THIOX
λem, nm, (φ)
CAR-TPA
a
a
b
λem, nm, (φ)b
Solvent
Toluene
THF
CH2Cl2
DMF
430 (3.97)
395 (3.90)
428 (3.83)
383 (sh)
401 (4.11)
388 (4.08)
403 (4.01)
338 (3.94)
325 (3.92)
411
408
411 (0.08)
413 (0.01)
415 (0.04)
416 (0.05)
408 (0.08)
407 (0.03)
410 (0.04)
412 (0.07)
Ethanol
390 (3.72)
a
Error limits: λmax, ± 1 nm, log ε, ± 10%, ε in M−1 cm . λem, ± 1 nm; φ, ± 10%.
−1
b
0
0
.00002
.00000
CAR-THIOHX
CAR-TPA
ex=350 nm
3
2
1
0
-0.00002
-0.00004
400
450
500
550
Wavelength (nm)
0
.0
0.5
1.0
1.5
2.0
Figure 2. Emission spectra CAR-THIOHX and CAR-
TPA in CH2Cl2 (λex = 350 nm).
Potential (V) vs. SCE
Figure 3. Cyclic voltammogram of CAR-TPA in CH2Cl2
wavelength of emission maxima and the fluorescence solvent.
quantum yields are given in table 1. These dyes exhib-
ited moderate fluorescence quantum yields and large fluorescence decay curves (λex 301 nm and λem at
Stokes shift. From the table 1, one can easily conclude 415 nm) and found to be 2.42 and 1.66 ns respectively
that the Stokes shift is due to the ICT from donor to (see Supporting information).
43
acceptor. It was also found that the fluorescence emis-
For efficient electron injection, LUMO or oxidation
sion maxima is red-shifted with increasing the solvent potential of a dye sensitizer should be above the con-
polarity (fluorescence solvatochromism) which further duction band edge of TiO and similarly for efficient
2
supports the excited state ICT nature (see Supporting dye regeneration, HOMO of a dye sensitizer should lie
information). Singlet state lifetimes of CAR-THIOHX below the energy level of redox system (for example
44
and CAR-TPA in CH Cl were estimated from the iodine/iodide). In order to evaluate thermodynamic
2
2

390
Naresh Duvva et al.
Table 2. Electrochemical data of CAR-THIOX and CAR-TPA in CH2Cl2.
E1/2, V vs. SCEa
b
∗c
Eox
Dye
Ox
Red
E0−0, eV
CAR-THIOX
CAR-TPA
1.90
0.85, 1.20,
.50, 1.91
−1.42
2.95
3.09
−1.05
−1.59
−1.10, −1.81
1
a
Error limits: E1/2 ± 0.03 V, 0.1 M TBAP; Glassy carbon working electrode, Stan-
dard calomel electrode is reference electrode, Pt electrode is auxillary electrode.
b
Calculated from the intersection point of absorption and emission spectra. Error
c
∗
limits, E0−0 = ± 0.05 eV. Calculated from the equation E = Eox− E0−0.
ox
feasibility of the electron transfer process in these
sensitizers and to assess the energy levels cylic voltam-
metric experiments were carried out. Cyclic voltam-
mogram of CAR-TPA obtained by this technique
is shown in figure 3 and the redox potentials for
both dyes are shown in table 2. Both the sensitizers
exhibit either reversible or quasi-reversible oxidation or
reduction processes. The oxidation of CAR-TPA has
four one-electron peaks. The first two reversible oxi-
dation peaks belong to triphenyl moiety. The third
irreversible oxidation at 1.51 V vs SCE belongs to
carbazole moiety,34 whereas irreversible oxidation of
CAR-THIOHX appeared at 1.90 V. The reduction of
CAR-TPA and CAR-THIOHX takes place at −1.10
and −1.42 V vs. SCE, respectively. The first reduc-
tion potential of CAR-THIOHX exhibits positive shift
compared to CAR-TPA, which might be due to the
more electron releasing hexyl groups than the triphenyl
groups. These measurements further suggest that the
Figure 4. Optimized geometries of CAR-THIOHX and
CAR-TPA sensitizers.
The isodensity plot of the HOMO and LUMO molec-
ular orbitals of both CAR-THIOHX and CAR-TPA
dyes, obtained at B3LYP/6-31G* level of theory, is
shown in figure 5, while their energy level are reported
in table 3. Both the dyes show a strong ‘push-pull’
character; the HOMO is localized on the main pri-
mary donor carbazole as well as on auxiliary donor
either thiophene or triphenylamine, while the LUMO
mainly localized on the acceptor cyanoacrylic acid
group. The HOMO localization in the outer molec-
∗
excited state oxidation potential (E ) is more negative
ox
45
than the TiO conduction band edge (−0.8 V vs SCE)
2
and E energy level is more positive than the redox
ox
46
potential of iodine/iodide system (0.2 V vs SCE) for
these dyes, allowing for working DSSC devices.
3.1 Theoretical investigation
In order to obtain insight into the effect of differing ular region should ensure the exposure of the oxi-
donor groups (on carbazole scaffold) on the electronic, dized dye, generated after electron injection into the
optical and geometrical properties of CAR-THIOHX semiconductor, to the electrolyte redox couple, there-
and CAR-TPA, these two dyes have been theoretically fore favoring rapid regeneration of the oxidized sen-
modeled, the optimized structures are shown in figure 4. sitizer. The LUMO distribution close to the anchoring
We note that both dyes have a similar structure. The car- group enhances the orbital overlap with the titanium
◦
bazole plane is 47 twisted with respect to the phenyl conduction band states, which is considered an essen-
34
47
ring bonded at the N9 position. The strong deviation tial requirement for efficient charge injection process.
from the planarity between the carbazole donor and Interestingly, these two dyes show only slight differ-
the phenyl-bridge and the cyanoacrylic acid group hin- ences in the LUMO energies (table 3), due to the fact
ders the conjugation along the dye backbone, thus lead- that we have the same anchoring group. On the other
ing to an increase of the band gap and a less extended hand, we see a sizeable difference in the HOMO energy
absorption spectra toward the visible region.
values, with the CAR-THIOHX dyes having a more

Carbazole sensitizers with applications in DSSCs
391
Figure 5. Isodensity plots of HOMO and LUMO involved in the excited
states of CAR-THIOHX and CAR-TPA sensitizers.
Table 3. Energies (in eV) of the HOMO and LUMO molecular orbitals and energy
gap (ꢀE) of CAR-THIOHX and CAR-TPA sensitizers.
Dye
HOMO (eV)
LUMO (eV)
ꢀE (eV)
CAR-THIOHX
CAR-TPA
−5.26
−4.95
−2.67
−2.66
2.59
2.29
by a series of additional transitions, constituting the
π-π* absorption band at about 300 nm. The main
absorption band, as anticipated, has a strong ICT char-
acter, and it is constituted by an HOMO-LUMO tran-
sition (>50%), (see Supplementary Information), lead-
ing to charge displacement from the carbazole donor
to the cyanoacrylic anchoring group. We observe a
slightly blue shifted absorption maxima (377 nm) for
the CAR-THIOHX dye compared to the CAR-TPA
(380 nm), with similar oscillator strengths. These val-
ues are in good agreement with the experimental data
in ethanol solution (see figure 1 and table 1), even if
in the experimental spectra the ICT absorption bands
are less intense and appear only as a shoulder of the
higher π-π* band in the UV region. The low extinc-
tion coefficient observed for the charge transfer transi-
tion can be related to the breaking of the conjugation
between the carbazole moiety and the phenyl bridge,
leading to a poor overlap between the HOMO and
Figure 6. Simulated absorption spectra (CAM-B3LYP/6-
1G* in EtOH) for both dyes.
3
stabilized HOMO level, leading to a higher band gap. LUMO frontiers orbitals and thus to a less transition
In figure 6, we report the calculated absorption spec- probability.
tra of the both investigated dyes in ethanol solution.
All the calculated electronic transitions energies, along tion bands in the visible region, with extinction coeffi-
The fact that we observe only the tails of the absorp-
−1
−1
with their oscillator strengths and molecular orbital cient in the order of 10000 M cm , is detrimental for
compositions of dyes are summarized in supplementary the photovoltaic performances of these dyes. With this
information.
low spectral extension (and therefore poor solar light
Both dyes show a similar absorption profile, with a harvesting) in the visible range one can expect only low
single, low-lying transition, followed, at higher energy, photocurrent values.

392
Naresh Duvva et al.
Figure 7. J-V curves of CAR-THIOX and CAR-TPA sensitized devices.
Table 4. Photovoltaic parameters for DSCs using an opeque TiO2 monolayer, measured under simulated AM 1.5. G, 1
SUN illumination
Dye
TiO2
Additives
Jsc (mA/cm2)
Voc (mV)
ff
η(%)
CAR-THIOHX-1
CAR-THIOHX-2
CAR-THIOHX-3
CAR-TPA -1
CAR-TPA -2
CAR-TPA -3
Opaque
Opaque
–
3.19
3.93
3.36
3.87
4.19
3.72
722
646
736
720
610
755
0.69
0.64
0.74
0.70
0.68
0.76
1.58
1.62
1.83
1.94
1.75
2.12
LiI 0.1M
–
–
LiI 0.1M
–
Trans+SL
Opaque
Opaque
Trans+SL
a
2
Error limits: JSC: ± 0.20 mA/cm , VOC = ± 30 mV, ff = ± 0.03.
3.2 Photovoltaic measurements
anyway, we noted that the less extended absorption
−
−
spectra and the competition with the I /I based elec-
3
Finally, the photovoltaic performances of CAR- trolyte solution for the light absorption, limit the
THIOHX and CAR-TPA have been evaluated in photocurrent production and thus the photovoltaic per-
devices employing the iodine/iodide redox system. formances of these dyes. In order to improve the
Ethanol:THF (9:1 v/v) solution has been utilized for photocurrent values, we tested the effect of the LiI
grafting the sensitizers on to nanocrystalline TiO sur- additive (0.1 M) in the redox electrolyte. It has been
2
face. Figure 7 illustrates photovoltaic response of the noted that Li ions are able to downshift the TiO con-
2
constructed DSSCs of both CAR-TPA and CAR- duction band, facilitating the charge injection process
THIOHX sensitizers which are investigated under var- and then improving the generated photocurrent.48,49
ious experimental conditions, while the PV parameters As expected, we obtain a gain in the photocur-
2
are reported in table 4. In all the cases, the Chenodeoxy- rent generation (3.93 mA/cm for CAR-THIOHX and
2
cholic acid, CDCA, was employed as a co-adsorbent 4.19 mA/cm for CAR-TPA), but unfortunately the
in the dye solution to minimize the aggregation of the overall efficiency does not improve due to the strong
dyes on the TiO surface. In the first test, an opaque reduction of the V in the order of 80–100 mV. Finally,
2
oc
TiO layer was used. The CAR-TPA sensitizer has we have tested a TiO scattering layer in combi-
2
2
shown better power conversion efficiencies compared nation with a transparent film, in order to improve
to the CAR-THIOHX sensitizer, due to the higher the efficiency. In these conditions we obtained the
photocurrents developed by the CAR-TPA dye, while best results, with power conversion efficiency of 2.12
similar values of Voc and FF are measured. This is and 1.83% for respectively CAR-TPA and CAR-
probably motivated by presence of the triphenylamine THIOHX dyes. In any case, the obtained improve-
auxiliary donor moiety, enhancing the charge trans- ments of the overall efficiency is quite low, in the
fer directionality toward the acceptor. In both cases, order of 10–15%, due to the still low Jsc values.

Carbazole sensitizers with applications in DSSCs
393
On the other hand, the Voc and FF values are
satisfactory, testifying to the good potential of this class
of sensitizers, once the light harvesting power will be
increased through a careful selection of suitable donor
and π-bridge moieties. The low efficiency of these sen-
sitizers is probably due to the poor absorption properties
in the visible region. If the absorption of these sen-
sitizers push towards visible region then one can expect
good efficiency. This can be achieved by introducing
4. Kim H N, Guo Z Q, Zhu W H, Yoon J and Tian H 2011
Chem. Soc. Rev. 40 79
. Zhang S, Yang X, Numata Y and Han L 2013 Energy
Environ. Sci. 6 1443
. O’Regan B and Gratzel M 1991 Nature 353 737
7. Giribabu L, Kanaparthi R K and Velkannan V 2012
5
6
Chem. Rec. 12 306
. Kanaparthi R K, Kandhadi J and Giribabu L 2012
Tetrahedron 68 8383
. Mishra A, Fischer M K R and Bauerle P 2009 Angew.
Chem. Int. Ed. 48 2474
8
9
more extended π-conjugated groups in its molecular 10. Hum J –H, Baranhoff E, Kessler F, Moehl T, Ahmad S,
Bessho T, Marchioro A, Ghadiri E, Moser J –E, Yi Ch,
Nazeeruddin M K and Gratzel M 2012 Nature Commun.
structure.
3
1
1
1. Wang H -Y, Liu F, Xie L -H, Tang C, Peng B, Huang W
4
. Conclusions
and Wei W 2011 J. Phys. Chem. C 115 6961
1
2. Ikeda N and Miyasaka T A 2005 Chem. Commun. 1886
3. Wang Z S, Koumura N, Cui Y, Takahashi M, Sekiguchi
H, Mori A, Kubo T, Furube A and Hara K 2008 Chem.
Mater. 20 3993
In conclusion, we have designed, synthesized and char-
acterized two new sensitizers based on carbazole scaf-
fold having one or two donors in its molecular struc-
ture. Photophysical and electrochemical properties have
revealed that these sensitizers are potential candidates
for DSSC applications. On the other hand, caused by
1
1
4. Koumura N, Wang Z -S, Mori S, Miyashita M, Suzuki
E and Hara K 2006 J. Am. Chem. Soc. 128 14256
5. Shen P, Tang Y, Jiang S, Chen H, Zheng X, Wang X,
Zhao B and Tan S 2011 Org. Electron. 12 125
1
the narrow absorption in visible region, the overall con- 16. Liu J, Yang X, Islam A, Numata Y, Zhang S, Salim N T,
Che H and Han L 2013 J. Mater. Chem. A 1 10889
7. Ooyama Y, Yamaguchi N, Imae I, Komaguchi K, Oshita
J and Harima Y 2013 Chem. Commun. 49 2548
8. Venkateswararao A, Justin Thomas K R, Lee C -P and
Ho K –C 2013 Tetrahedron Lett. 54 3985
version efficiency is found to be low due to the poor
1
1
photocurrent, J . Theoretical investigation has shown
SC
that the poor light absorption in the visible region
◦
depend on the geometry of the two dyes, having ∼50
twist between the carbazole donor plane and the phenyl 19. Yang C -H, Lin W -C, Wang T -L, Shieh Y -T, Chen W
-
J, Liao S -H and Sun Y -K 2011 Mater. Chem. Phys.
30 635
0. Baheti A, Singh P, Lee C -P, Thomas J K R and Ho K
C. 2011 J. Org. Chem. 76 4910
bridge, leading to the ‘breakout’ of the conjugation
along the dyes backbone and thus to less extended and
intense absorption spectra. Though only moderate pho-
1
2
-
tovoltaic efficiencies were obtained, these two-donor 21. Marotta G, Anil Reddy M, Singh S P, Islam A, Han L,
carbazole-based dyes can be considered as a promis-
ing class of sensitizers, underlining once again the key
importance of the selection of optimized π-bridge moi-
eties for organic dyes. Studies towards increasing the
De Angelis F, Pastore M and Chandrasekharam M 2013
ACS Appl. Mater. Interfaces 5 9635
2
2. Purification of Laboratory Chemicals 2003 W L
F Armango and Ch L L Chai (Eds.) (NewYork:
Butterworth Heinemann)
DSSC performance of the carbazole dyes are underway 23. Shi L, Zhu D, He Q, Li Y, Chen Y, Sun Y, Fu Y, Wen D,
in our laboratory.
Cao H and Cheng J 2012 J. Mate. Chem. 22 11629
4. Bacher E, Bayerl M, Rudati P, Reckefess N, Muller, C
D, Meerholz K and Nyuken O 2005 Macromolecules 38
2
1
640
Acknowledgements
2
2
5. Zhang L, Zhang S, Yin L, Ji Ch, Li K, Li Y and Wang Y
2013 New J. Chem. 37 632
The authors are thankful to the joint DST-EU project
6. Hu B, Zhuge F, Peng S, Chen X, Pan L, Yan Q and Li R
‘ESCORT’ (FP7-ENERGY-2010, contract no. 262910)
–W 2012 J. Mater. Chem. 22 520
for the financial support of this work. JK thanks CSIR 27. Wu W, Yang J, Hua J, Tang J, Zhang L, Long Y and Tian
H 2010 J. Mater. Chem. 20 1772
for senior research fellowship.
2
2
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