Carbazole-bridged double D-A dye for efficient dye-sensitized solar cell

Article abstract of DOI:10.1016/j.jphotochem.2014.09.007

A di-anchoring organic dye based on an acceptorπ spacerdonorsecondary donordonorπ spacer acceptor structural motif has been synthesized and used as a sensitizer for dye-sensitized solar cells (DSSCs). The new synthetic approach uses two π spacers, two acceptors and two different donors such as arylamino and carbazole units. The double donoracceptor dye CBZ-3 is linked by a carbazole unit as a secondary donor which increases the donoracceptor strength of the dye and proved to increase the η and J_sc values of the photovoltaic cell compared to the corresponding mono-anchoring system CBZ-2 and the monodonor dye CBZ-1. With the non-planar structure of the arylamino group and the alkyl chains which efficiently prevent the formation of the dye aggregates, the dye exhibited a high open-circuit photovoltage (V_oc) of 650 mV, a short-circuit photocurrent density (J_sc) of 7.20 mA cm^-2 and the solar energy-to-electricity conversion efficiency (η) of 3.23%.

Full text of DOI:10.1016/j.jphotochem.2014.09.007

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Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 1–10
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Carbazole-bridged double D–A dye for efficient dye-sensitized solar
cell
Songyos Pramjit, Utt Eiamprasert, Panida Surawatanawong, Pattida Lertturongchai,
*
Supavadee Kiatisevi
Center for Alternative Energy, Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Mahidol
University, Rama VI Road, Ratchathewi, Bangkok 10400, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 16 May 2014
Received in revised form 3 September 2014
Accepted 17 September 2014
Available online 22 September 2014
A di-anchoring organic dye based on an acceptor–
p spacer–donor–secondary donor–donor–p spacer–
acceptor structural motif has been synthesized and used as a sensitizer for dye-sensitized solar cells
(DSSCs). The new synthetic approach uses two spacers, two acceptors and two different donors such as
arylamino and carbazole units. The double donor–acceptor dye CBZ-3 is linked by a carbazole unit as a
secondary donor which increases the donor–acceptor strength of the dye and proved to increase the
p
h
and Jsc values of the photovoltaic cell compared to the corresponding mono-anchoring system CBZ-2 and
the monodonor dye CBZ-1. With the non-planar structure of the arylamino group and the alkyl chains
which efficiently prevent the formation of the dye aggregates, the dye exhibited a high open-circuit
photovoltage (Voc) of 650 mV, a short-circuit photocurrent density (Jsc) of 7.20 mA cmꢀ2 and the solar
energy-to-electricity conversion efficiency (h) of 3.23%.
ã 2014 Elsevier B.V. All rights reserved.
1. Introduction
region and the formation of aggregates on the semiconductor
surfaces were suggested to be the causes of the low efficiencies of
organic dyes [18].
The most common investigated class of organic sensitizers is
the D–p–A structure, where an electron donor (D), such as
Although solar power is one of the most popular renewable
energy resources, there are two major drawbacks that prevent it
from widespread use: the high manufacturing cost and the low
electrical energy efficiency conversion rate [1]. Among the devices
used to generate electricity in solar power, dye-sensitized solar
cells (DSSCs) become more attractive as the production costs are
low and the efficiency rates have been much improved [2]. DSSCs
using Ru(II) polypyridyl complexes as the sensitizers were
reported to reach the power conversion efficiencies of more than
11% under standard solar illumination [3–9]. However, the wide
application is still limited due to expensive resources and difficult
purification process of Ru complexes.
As alternatives to metal-based dyes, organic dyes have received
much attention recently because of their higher molar extinction
coefficients, lower production costs, and easier structural tuning in
comparison with metal-based dyes [10–12]. Although organic dyes
have shown impressive photovoltaic performance with efficiencies
of 5–9% [13–17], the conversion efficiencies still lag behind those of
the Ru complexes. The narrow absorption bands in the visible
arylamino derivative is linked to an electron acceptor (A), most
notably cyanoacrylic acid, aiming to facilitate vectorial charge
transfer upon light absorption, through a
p-conjugated unit (p) to
tune the spectral coverage and light harvesting efficiency [11].
Diverting from the conventional approach, Abbotto and co-
workers have employed the new chromophore design,
a
dibranched di-anchoring D– –A dye (A– –D– –A), to achieve
p
p
p
increased photocurrent (di-branched structure) and enhanced
stability (presence of di-anchoring moiety) when compared to the
corresponding monobranched mono-anchoring dye [19]. Since
this discovery, many new model classes of non-linear multi-
branched di-anchoring organic sensitizers have been proposed as
pictorially depicted in Fig. 1, for examples: (a) a symmetric double
D–
chain as known as A–
asymmetric double D–
with different electron donors which is linked by an alkyl group
(A– –A) [21]; (c) a symmetric tribranched A– –D–
–D–Alk–D0–
p0–D– –A architecture (where p0 is other than
), which implies
the use of two donor and two acceptor/anchoring fragments and
different spacers [16]; and (d) an asymmetric tribranched
p
–A structural architecture bridged by a nonconjugated alkyl
–D–Alk–D– –A (Alk = alkyl) [20]; (b) an
–A approach constituted by two side arms
p
p
p
p
p
p
p
p
* Corresponding author. Tel.: +66 22015131.
E-mail addresses: supavadee.mon@mahidol.ac.th, kleinejoy@gmail.com
(S. Kiatisevi).
p
http://dx.doi.org/10.1016/j.jphotochem.2014.09.007
1010-6030/ã 2014 Elsevier B.V. All rights reserved.

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S. Pramjit et al. / Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 1–10
A–
p
–D–p00–D–p0–A dye in which two different D–
p
–A side arms
of the secondary donor to the dye system and improvement in the
are linked through donor endcapping moieties by a conjugated
bridge [22]. The results revealed that the DSSCs based on most of
these dyes showed better performances than the corresponding
dye containing only one anchoring functionality owing to larger
structural variety to obtain panchromatic response, enhanced
efficiencies has been observed [17,26].
Here, we have modified the design by introducing two D–
side arms bridged by a different secondary donor. The new
approach based on the A– –A (where D is other than
–D–D0–D–
D0) implies the use of a secondary donor (D0) instead of a
spacer.
Our approach differs from the other reports in that the two D– –A
arms are connected either through saturated nonconjugated links,
which thus limits -structure extension, or through a spacer, in
which the use of extended -spacers results in rod-shaped
p–A
p
p
p
optical density as a result of extended
p
-conjugated framework
p
and multi-bonding to TiO2 nanoparticles [16].
Compared with dyes containing only a single donor, the
addition of a secondary donor is beneficial to improve the
photoelectric conversion efficiency of DSSCs due to an increase
in both donor–acceptor strength and molar absorptivity of the
materials, as well as a broader absorption spectrum of the dye.
Furthermore, the physical separation of the dye cation from the
TiO2 electrode surface will be increased, which efficiently retards
recombination between the photoelectron and the oxidized dyes
[23–25]. Various studies have been conducted by the introduction
p
p
p
molecular structures, which could increase aggregation between
dye molecules and result in electron-electrolyte recombination
[27]. To achieve high-performance organic dyes, we modified the
dye structure to contain a secondary donor and two anchoring
functionalities, which magnifies the extent of electron
delocalization and improves the ability to donate the electron of
the sensitizer. In order to get more insight into the impact on the
Fig. 1. Various designs of multibranched D–{–A dyes.

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3
mass spectra were recorded on HR-TOF-MS Micromass model
VQ-TOF2. The N719 dye was purchased from Solaronix Company.
Column chromatography was performed by using Merck silica gel
60 PF254 (Art 7734).
2.2. Synthetic procedures and characterizations
2.2.1. 9-Butylcarbazole (2)
A mixture of carbazole 1 (5 g, 30 mmol), potassium hydroxide
(10 g, 180 mmol) and DMF (50 ml) was stirred at room temperature
for 3 h. Then, a solution of 1-iodobutane (6.8 ml, 60 mmol) in DMF
(10 ml) was slowly added. After stirring at room temperature for
additional 10 h, the mixture was poured into water (700 ml), and a
white precipitate was formed immediately. After filtration, the
precipitate was purified by column chromatography (100% hexane)
Fig. 2. Molecular structures of CBZ-1–3.
to give 2 as a white solid (6.21 g, 93%). 1H NMR (300 MHz, CDCl3):
d
(ppm) = 8.14 (d,J = 7.6 Hz, 2H), 7.44–7.54 (m, 4H), 7.25–7.30 (m, 2H),
performance of the solar cells under these conditions, we designed
4.35 (t, J = 7.2 Hz, 2H), 1.86–1.96 (m, 2H), 1.39–1.52 (m, 2H), 1.01 (t,
a new dye, CBZ-3 that has D = phenylamino, A = cyanoacrylic acid,
J = 7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3):
d (ppm) = 140.41, 125.52,
D0 = carbazole, and
p
= phenyl, as shown in Fig. 2. The key
122.78, 120.31, 118.66, 108.63, 42.79, 31.10, 20.55, 13.87. HRMS
advantages of choosing carbazole as the secondary electron donor
are as follows: (i) the alkyl-substituents on electron-releasing
nitrogen heteroatoms of carbazole play important roles in tuning
of morphology and suppressing aggregation resulting in enhanced
long-term stability of the solar cell [15]; (ii) carbazole is a stable
and cheap starting material and, therefore, the synthesis of dyes
can be afforded and commercialized at a very low cost; (iii) with
good electron donor, large molar absorption coefficients resulting
from increasing light-harvesting units should give rise to enhance
(ESI): calcd for C16H17N [M + H]+ 224.1439, found 224.1432.
2.2.2. 3,6-dibromo-9-butylcarbazole (3)
At ambient temperature NBS (1.6 g, 9 mmol) was added to a
solution of 2 (1 g, 4.5 mmol) in CH2Cl2 (40 mL). The reaction
mixture was then stirred for overnight. After water was added, the
mixture was extracted with CH2Cl2. Crude solid obtained by
removing the solvent was purified by column chromatography to
give 3 as a colorless solid (3.24 g, 94%). 1H NMR (300 MHz, CDCl3):
d
the
The diphenylamino group is considered as both primary
electron donor and spacer that are represented by the
h
value of DSSCs.
(ppm) = 8.03 (d, J = 1.5 Hz, 2H), 7.53 (dd, J = 8.9, 1.5 Hz, 2H), 7.16 (d,
J = 8.9 Hz, 2H), 4.07 (t, J = 7.1 Hz, 2H), 1.72–1.80 (m, 2H),1.28–1.40 (m,
p
2H), 0.95 (t, J = 7.4 Hz, 3H). 13C NMR (75 MHz, CDCl3):
d (ppm) =
phenylamino unit and a phenyl group, respectively. The linking
of the carbazole primary electron donor and acceptor groups via
the amino and conducting phenyl unit in organic DSSC dyes has
some advantages in terms of the electron-releasing nitrogen
heteroatom and its nonplanar structure that therefore can likewise
impede the molecular aggregation. In addition, another consider-
able advantage of a small dye consisting of carbazole and units like
CBZ-3 is its simple preparation and convenient isolation compared
to larger dyes.
In this paper, CBZ-3 was synthesized, characterized for DSSCs,
and compared with the corresponding monoanchoring sensitizer,
CBZ-2 which was prepared to investigate the effect of the acceptor
unit number. The dye containing only one donor unit, CBZ-1, was
also studied. Our results strongly indicate that the double branched
acceptors and the nonplanar dye structure with a central carbazole
as a secondary donor and two arylamino side arms as primary
donors are important for highly efficient solar-cell performance of
DSSCs.
139.14, 128.85, 123.27, 123.08, 111.80, 110.26, 42.94, 30.88, 20.40,
13.76. HRMS (APCI): calcd for C16H15Br2N [M + H]+ 381.9629, found
381.9686.
2.2.3. 9-Butyl-3,6-diformylcarbazole (4)
5 mL of n-BuLi (6.3 mmol) was slowly added to a solution of 3
(0.7 g, 2.5 mmol) in THF (15 mL) at ꢀ78 ꢁC under nitrogen by using a
syringe. The mixture was stirred at this temperature for 2 h and
then 1 mL of DMF was slowly added. The reaction mixture was
allowed to warm to room temperature and hydrolyzed with 20 ml
water. The mixture was extracted with CH2Cl2 and the solvent was
evaporated off. The crude product was finally purified by column
chromatography on silica gel using CH2Cl2 as the eluent to afford 4
as a pale brown solid (0.49 g, 70%). 1H NMR (300 MHz, CDCl3):
d
(ppm) = 10.13 (s, 2H), 8.65 (s, 2H), 8.08 (d, J = 8.6 Hz, 2H), 7.55 (d,
J = 8.6 Hz, 2H), 4.38 (t, J = 7.2 Hz, 2H),1.88–1.92 (m, 2H),1.39–1.44 (m,
2H), 0.98 (t, J = 7.3 Hz, 3H). 13C NMR (75 MHz, CDCl3):
d (ppm) =
191.50, 144.74, 129.58, 127.81, 124.20, 123.11, 109.74, 43.54, 30.97,
20.43, 13.75. HRMS (ESI): calcd for C18H17NO2 [M + H]+ 280.1338,
found 280.1367.
2. Experimental section
2.1. Materials and measurements
2.2.4. 2,20-Dicyano-3,30-(9-butylcarbazol-3-yl)-diacrylic acid (CBZ-1)
A mixture of 4 (1.3 g, 4.6 mmol) and cyanoacetic acid (1.57 g,
18 mmol) was vacuum-dried. Acetonitrile (20 mL) and piperidine
(0.45 ml, 4.6 mmol) were added to the mixture. The solution was
refluxed for overnight. After the solution was cooled down to room
temperature, the organic layer was removed in vacuo. The pure
product was obtained by silica gel chromatography using 20%
methanol/ethylacetate as the eluent to afford CBZ-1 as a yellow
All reagents were obtained from commercial suppliers at the
highest purity and used without further purification. All reactions
were carried out under nitrogen atmosphere. THF was distilled
from sodium/benzophenone. DMF and dichloromethane were
distilled from CaH2 under N2 atmosphere. Extracts were dried with
anhydrous Na2SO4 and filtered before removal of the solvent by
evaporation. Nuclear magnetic resonance (NMR) spectra were
collected on a Bruker DPX-300 MHz instrument using tetrame-
thylsilane as internal standard. UV–vis spectra of the dyes in
CH2Cl2 solutions were recorded in a quartz cell with 1 cm path
length on a JASCO V-530 spectrophotometer. The high resolution
powder (57% yield). 1H NMR (300 MHz, DMSO-d6):
d (ppm) = 8.76
(d, J = 1.4 Hz, 2H), 8.44 (s, 2H), 8.26 (dd, J = 8.9, 1.4 Hz, 2H), 7.85 (d,
J = 8.9 Hz, 2H), 4.47 (t, J = 7.0 Hz, 2H),1.72–1.82 (m, 2H),1.23–1.35 (m,
2H), 0.88 (t, J = 7.3 Hz, 3H). 13C NMR (75 MHz, DMSO-d6):
d
(ppm) = 163.89, 154.69, 143.27, 128.76, 125.06, 123.71, 122.36,

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S. Pramjit et al. / Journal of Photochemistry and Photobiology A: Chemistry 296 (2014) 1–10
Scheme 1. Synthesis of CBZ-1. Reagents and conditions: (a) 1-iodobutane, KOH, DMF, rt; (b) NBS (2 equiv), rt; (c) n-BuLi (2 equiv), THF, ꢀ78 ꢁC then DMF; (d) cyanoacetic acid
(2 equiv), piperidine, CH3CN.
117.02, 111.06, 100.02, 42.80, 30.63, 19.64, 13.62. HRMS (ESI): calcd
for C24H19N3O4 [M]+413.1376, found 413.2590.
added and heated at 120 ꢁC for 2 h. The solution was then extracted
with CH2Cl2 and the organic phase was washed with NaHSO4 to
remove I2. The crude product was purified by column chromatog-
raphy (silica gel, 10% EtOAc/hexane) to give a red solid in 63% yield.
2.2.5. 9-butyl-3,6-diiodo-9H-carbazole (5)
A solution of compound 2 (200 mg, 0.9 mmol) and KI (193.1 mg,
1.2 mmol) in acetic acid (15 mL) was stirred at 120 ꢁC for 30 min.
After cooling to room temperature, KIO3 (287.3 mg, 1.3 mmol) was
1H NMR (300 MHz, CDCl3): (ppm) = 8.31 (d, J = 1.6 Hz, 2H), 7.72
d
(dd, J = 8.4, 1.6 Hz, 2H), 7.16 (dd, J = 8.4 Hz, 2H), 4.20 (t, J = 7.02 Hz,
2H), 1.74–1.84 (m, 2H), 1.24–1.40 (m, 2H), 0.87 (t, J = 7.32 Hz, 3H). 13
C
Scheme 2. Synthesis of CBZ-2–3. Reagents and conditions: (a) KI, AcOH, 120 ꢁC, 30 min, then KIO3 120 ꢁC, 2 h; (b) iodobenzene, Cu, tBuOK, DMF, 170 ꢁC, 24 h; (c) Cu, K2CO3
nitrobenzene, 210 ꢁC, 24 h; (d) NBS (2 equiv), rt; (e) n-BuLi (2 equiv), THF, ꢀ78 ꢁC, then DMF; (f) cyanoacetic acid (1 equiv), piperidine, CH3CN; (g) cyanoacetic acid (2 equiv),
piperidine, CH3CN.