Influence of triazole dendritic additives in electrolytes on dye-sensitized solar cell (DSSC) performance

Article abstract of DOI:10.1039/c1jm10334b

Novel triazole dendritic molecules bearing dimethyl isophthalate surface group are synthesized. The enhancement of triazole and carboxylate groups in the dendritic structures significantly alter the absorption, electrochemical and DSSCs behaviors. With de

Full text of DOI:10.1039/c1jm10334b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 7700
www.rsc.org/materials
PAPER
Influence of triazole dendritic additives in electrolytes on dye-sensitized solar
cell (DSSC) performance
a
a
a
Sebastian Raja, Chinnadurai Satheeshkumar, Perumal Rajakumar, Shanmugam Ganesanb
*
and Pichai Maruthamuthub
Received 21st January 2011, Accepted 22nd March 2011
DOI: 10.1039/c1jm10334b
Novel triazole dendritic molecules bearing dimethyl isophthalate surface group are synthesized. The
enhancement of triazole and carboxylate groups in the dendritic structures significantly alter the
absorption, electrochemical and DSSCs behaviors. With dendritic additive DSSCs cell exhibited higher
Voc values and power conversion efficiency (h) in IÀ/I3À redox couple under simulated AM 1.5 at
40 mW cmÀ2
.
Among several organic frameworks, dendrimer have been
suggested to be an efficient design criterion for new artificial light
harvesting and solar energy conversion devices.12–14
Introduction
Dye-sensitized solar cells (DSSCs) have been the subject of
intense research during the past years due to their high photo-
electric conversion efficiency, simple assembly technology and
potential economic advantages.1–4 A typical solar cell has three
components: (i) a sensitized photoanode, which is typically a dye
sensitized nanocrystalline TiO2 film on a transparent fluorine-
doped tin oxide (FTO) conducting glass; (ii) an electrolyte
solution containing IÀ/I3À as a redox couple; and (iii) a cathode,
which is a platinized (Pt) FTO conducting glass.
Dendrimers are perfect monodisperse macromolecules with
a regular and highly branched three-dimensional architecture,
which offer a variety of applications in optoelectronic and elec-
tronic devices.15–18
In this connection, 1,2,3-triazole offer various applications as
flurophores, chemosensors and charge transfer agents.19–21
Recently, we have reported the synthesis of various novel triazole
dendrimers with chalcone and glucose moiety at the periphery
through the ‘click’ approach22,23 and application of such den-
drimers as additive to enhance the performance of DSSC.24
The introduction of N-containing heterocycles at the
periphery in dendrimer as an additive with electrolytic solution
could improve the efficiency of DSSC and the presence of such
units at the surface would further increase the efficiencies of the
DSSCs due to the valence effect.25
Although there are many factors that determine the photo-
voltaic performance of the DSSCs, the role of the electrolyte in
enhancing performance is obviously a crucial one. Adding
certain compounds to the electrolytic solution can remarkably
improve solar cell performance. One of the most frequently used
additive is TBP.5 Gratzel and coworkers6 conducted a study on
À
an IÀ/I3 electrolyte with TBP as an additive in acetonitrile and
found that the addition of TBP drastically increase the Voc, the
fill factor and the solar energy conversion efficiency without
altering Jsc.
Inspired by the wide applications of 1,2,3-triazole unit in
dendrimers, we report herein the synthesis of dendritic molecules
1 and 2 (Fig. 1) and their optical and electrochemical behaviours.
Further, feasibility of enhancing solar cell performance with
Organic molecules play a significant role in photovoltaic and
light harvesting systems due to their excellent optical and elec-
trical properties.7–9 N-containing organic molecules are
commonly used an additive in electrolytic solution to improve
the performance of DSSC.10 N-containing heterocycles other
than TBP such as pyrazole, imidazole, 1,2,4-triazole, pyridine,
pyrimidine and pyrazine have been recently used as an additive in
DSSC.11
À
1,2,3-triazole dendrimer 1 and 2 as an additive using IÀ/I3
electrolyte has been also investigated.
Results and discussions
Synthesis
Dendritic molecules 1 and 2 were synthesized through ‘click’ and
O-alkylation methodologies. In order to synthesise such den-
drimers 1 and 2, dendron 4 and 7 were synthesized24 in good
yields and summarized in Schemes 1 and 2.
Hexakis(azidomethyl)benzene 826 and hexakis(bromomethyl)
benzene 927 were used as core units for the preparation of den-
drimers 1 and 2. Dendrimer 1 was obtained in a 84% yield by the
aDepartment of Organic Chemistry, University of Madras, Guindy
Campus, Chennai, 600 025, India. E-mail: perumalrajakumar@gmail.
com; Fax: +91 044 22300488; Tel: +91 44 2220 2810
bDepartment of Energy, University of Madras, Guindy Campus, Chennai,
600 025, India
7700 | J. Mater. Chem., 2011, 21, 7700–7704
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Scheme 1 (i) 1.25 equiv. propargyl bromide, K2CO3, DMF, 24 h, 60 ꢀC,
4 (91%). (ii) CuSO4$5H2O (5 mol %), NaAsc, (10 mol %), THF: H2O
(1 : 1), rt, 12 h, 1 (84%).
Scheme 2 (i) 2.1 equiv. NaN3, DMF, rt, 48 h (ii) 2.0 equiv. KOH,
ethanol, reflux, 3 h, 6 (75%) (iii) 4 (2.1 eq.), CuSO4$5H2O (5 mol %),
NaAsc, (10 mol %), THF: H2O (1 : 1), rt, 12 h, 7 (64%) (iv) K2CO3,
DMF, 60 ꢀC, 48 h, 2 (79%).
Fig. 1 Molecular structures of dendritic compound 1 and 2.
treatment of 1.0 equiv. of azide 8 with 6.2 equiv. of alkyne 4
under ‘click’ chemistry reaction conditions viz., 5 mol %
CuSO4$5H2O and 10 mol % sodium ascorbate in THF–H2O
(1 : 1) for 12 h at room temperature (Scheme 1).
1
The H NMR spectrum of 1 displayed three sharp singlets at
d 3.78, d 5.02 and d 5.99 for carboxylate methyl, N-methylene and
O-methylene protons in addition to twenty four aromatic
protons. In 13C NMR spectrum, compound 1 showed three peaks
at d 29.5, d 52.3, and d 62.0 for carboxylate methyl, N-methylene
and O-methylene carbons respectively. The ester carbonyl
carbon appeared at d 165.6 in addition to seven aromatic
carbons. The mass spectrum (MALDI-TOF) of 1 showed
a molecular ion peak at m/z 1897. The structure of 1 was further
confirmed from elemental analysis.
Fig. 2 UV-vis absorption spectra of dendrimers 1 and 2 in DMSO at
Reaction of 1.0 equiv. of bromide 9 with 6.2 equiv. of dendron
7 under O-alkylation using K2CO3 in DMF at 60 ꢀC for 48 h gave
the dendrimer 2 in 79% yield (Scheme 2).
room temperature.
Dendrimer 1 exhibited only one absorption band at 307 nm,
however, dendrimer 2, showed two absorption bands, a weak
band at 285 nm and strong band at 308 nm, respectively (Fig. 2).
As the number of triazole and carboxylate unit increases in the
dendrimer structure the absorption also increases, which indi-
cates that the amount of light absorbed by the dendritic antenna
increases from one generation to the next.28
In the 1H NMR spectrum, dendrimer 2 displayed seventy two-
proton singlet at d 3.86 for carboxylate methyl, athirty six-proton
singlet at d 5.13 for benzylic and N-methylene, a twenty four-
proton singlet at d 5.44 for O-methylene protons in addition to
aromatic protons. In 13C NMR spectrum, dendrimer 2 displayed
four signals at d 52.4, d 53.6, d 62.1 and d 64.0 for carboxylate
methyl, N-methylene and O-methylene carbons in addition to ten
aromatic carbons. The carbonyl carbon of carboxylate ester
appeared at d 165.8. The mass spectrum (MALDI-TOF) of 2
showed the molecular ion peak at m/z 4354. The structure of 2
was further confirmed from elemental analysis.
Table 1 Optical and electrochemical parameters were measured for the
compounds 1 and 2
S. No. Dendrimer (nm) UV-Visa Epc1a(V) Epc2a(V) Epc3a(V)
Absorption studies
1
2
1
2
307
285, 308 À0.96
À0.94
À1.06
À1.07
À1.23
À1.22
The UV-vis absorption spectra of compounds 1 and 2 in DMSO
are presented in Fig. 2. The UV-vis absorption maxima are listed
in Table 1.
a
Measured in DMSO (1 Â 10À5 M) solution.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7700–7704 | 7701

View Article Online
Electrochemical studies
The redox behavior of dendrimers 1 and 2 were studied by cyclic
voltammetry (CV) in DMSO (1 Â 10À5 M) at room temperature.
Both the compounds have an electrochemical response, and the
cyclic voltammogram is shown in Fig. 3, with redox potentials
are listed in Table 1. The redox potential of dendrimers 1 and 2
are almost similar, and a multi-reduction peak was observed in
the cyclic voltammogram.
Fig. 4 Structure of cis-dithiocyanato-N,N-bis(2,2-bipyridyl-4,4-dicar-
In Fig. 3, dendrimer 1 exhibited three irreversible reduction
waves at À0.94 V, À1.06 V and À1.23 V. Similarly, dendrimer 2
exhibited three irreversible reduction waves at À0.96 V, À1.07 V
and À1.22 V. The multi-reduction pattern of the dendritic
molecules may be due to the presence of the large numbers of
triazole isophthalate as the building and surface groups. The
presence of triazole units is responsible for the CV behaviour of
the dendrimers.
boxylic acid)-ruthenium(II)] (N3 dye).
drastically altered. The chemical reaction of charge-transfer
complexation of dendrimers in electrolyte solution is given below.
À
Dendrimer + I3 / Dendrimer I2 + IÀ
The photovoltaic performance of the dendrimers 1 and 2 are
summarized in Table 2.
Dye-sensitized solar cell studies
The DSSC study revealed that the dendrimers bearing triazole
and ester groups, contain more nitrogen and oxygen atoms, form
a charge transfer complex with I2 in the redox couple and result
in a decrease in the sublimation of iodine and also reflects in high
performance of DSSC. Moreover, the efficiency of the DSSC
increases with an increase in triazole and ester units into the
dendritic structures, which is usually referred to as the multiva-
lent effect25 in dendrimer chemistry. For example, dendritic
structure 2, bearing a large number of triazole and ester groups
shows a high DSSC performance (Fig. 5). Among the dendritic
structures 1 and 2, compound 2 showed a high Voc of 0.675 V and
a h value of 7.2% in DSSC. These performances are higher than
those reported earlier by us for dendrimer additives.24
In conclusion, photoelectrochemical and DSSC properties
increase with increasing dendrimer core length, leading to better
performance.
In an attempt to improve the solar cell performance, many studies
have examined the role of the electrolytic solution in enhancing
performance.29 In general, the amine derivatives drastically
increase the open-circuit voltage (Voc) due to the electron
donating nature of the nitrogen lone pair.30–32 In addition to this,
various nitrogen based organic molecules like aminopyridine,
pyrimidine, pyrrolidinopyridine, pyrrolidinoethylpyridine have
also been used as additives in DSSCs.30 Based on the above fact,
the dendrimers 1 and 2 are evaluated for DSSCs due to high
molecular weight and the presence of a large number of electron
donating atoms. The dendrimers 1 and 2 form charge transfer
complexes with iodine in the redox couple (IÀ/I3À) in photovoltaic
devices. The present investigation reports that the dendrimer
additive in the IÀ/I3À electrolyte solution influences ruthenium dye
(Fig. 4) sensitized solar cell performance and the Voc values are
Fig. 3 Cyclic voltammogram of dendrimer 1 and 2 in DMSO at room
temperature.
Fig. 5 Current–Voltage curve of dendrimer 1 and 2.
Table 2 The photovoltaic properties of the dendrimers with a KI and I2 redox couple under illumination of 40 mW cmÀ2 at AM 1.5
S. No
electrolyte system
Jsc (mA cmÀ2
)
Voc (mV)
fillfactor
efficiency h (%)
1
2
3
KI/I2
KI/I2/dendrimer 1
KI/I2/dendrimer 2
1.8
6.0
6.9
420
620
675
0.44
0.61
0.62
0.8
5.7
7.2
7702 | J. Mater. Chem., 2011, 21, 7700–7704
This journal is ª The Royal Society of Chemistry 2011

View Article Online
with 0.05 micron alumina powder and cleaned in 1 : 1 acetone–
ethanol mixture in an ultrasonic bath to remove impurities,
rinsed with water and then dried in air. Then the electrode was
cleaned by cycling between the potentials of À1.6 to +0.6 V
versus AgCl in 1 Â 10À5 M of tetrabutylammoniumhexa-
fluorophosphate (TBAPF6) in DMSO at a scan rate of 50 mV sÀ1
for approximately 30 min until reproducible scans were recorded.
All the electrochemical experiments were performed in a quies-
cent solution at room temperature (25 Æ 1 ꢀC).
Conclusions
Novel triazole based dendritic structures were synthesized
through the click and O-alkylation approaches. In addition,
optical, electrochemical and dye-sensitized solar cell (DSSC)
studies indicate better performance for higher generation den-
drimers. Optical, electrochemical and DSSC studies revealed that
the enhancement of triazole and ester moieties in the dendritic
structures significantly alter their behaviors. Furthermore, as the
number of triazole units increases in dendrimers of higher
generation the DSSC performance also increases.
Synthesis of compound 1
A mixture of azide (0.11 g, 0.26 mmol), alkyne (0.41 g, 1.67 mmol)
and CuSO4 (5 mol %)/sodium ascorbate (10 mol %) mixture in
aqueous THF solution (1 : 1) stirred for 12 h at room temperature.
The residue obtained after evaporation of the solvent was washed
thoroughly with water and dissolved in CHCl3 (150 mL). The
organic layer was separated, washed with brine (1 Â 150 mL),
dried (anhydrous Na2SO4) and evaporated to give the crude tri-
azole, which was purified by column chromatography (SiO2) using
the mixture of CHCl3 with MeOH (24 : 1) ratio as the eluent.
Experimental
Materials and methods
Chemicals and solvents (AR quality) were used as received
without any further purification. All melting points are uncor-
rected. 1H NMR and 13C NMR spectra were recorded on
BRUKER 300 MHz instruments. 1H NMR spectra were recorded
using deuterated chloroform (CDCl3) as solvent. Tetramethylsi-
lane (TMS) was used as an internal standard. Matric-assisted
laser-desorption ionization time-of-light (MALDI-TOF) mass
spectrometry was performed on a Voyager De-Pro mass spec-
trometer. Elemental analyses were carried out using a Perkin-
Elmer CHNS 2400 instrument. Column chromatography was
performed on silica gel (ACME, 100–200 mesh). Routine moni-
toring of the reaction was made using thin layer chromatography
developed on glass plates coated with silica gel-G (ACME) of
25 mm thickness and visualized with iodine. The UV-vis spectra
were recorded on a Shimadzu 260 spectrophotometer.
ꢀ
Dendrimer 1: Yield: 84%; Mp: 169–171 C; MALDI-TOF-MS:
m/z 1897 (M+); 1H NMR (300 MHz, CDCl3): d 3.78 (s, 36H); 5.02
(s, 12H); 5.99 (s, 12H) 7.33 (s, 6H); 7.84 (s, 12H); 8.07 (s, 6H). 13
C
NMR (75 MHz, CDCl3): d 29.5, 52.3, 62.0, 118.9, 119.8, 122.9,
123.2, 131.3, 142.6, 158.0, 165.6. Anal. calc. for C90H84N18O30: C,
56.96; H, 4.46; N, 13.29. Found: C, 56.87; H, 4.53; N, 13.37.
Synthesis of compound 2
A mixture of bromide (0.11 g, 0.17 mmol) and phenol (0.75 g,
1.07 mmol) in dry DMF at 60 ꢀC for 48 h. The reaction mixture
was then allowed to cool to room temperature and poured into
ice water. The resulting precipitate was filtered, washed thor-
oughly with water and dissolved in CHCl3 (150 mL). The organic
layer was separated, washed with brine (1 Â 150 mL), dried
(anhydrous Na2SO4) and evaporated to give the crude den-
drimer, which was purified by column chromatography (SiO2)
using CHCl3–MeOH (23 : 2) ratio as the eluent. Dendrimer 2:
Yield: 79%; Mp: 115–119 ꢀC; MALDI-TOF-MS: m/z 4354 (M+);
1H NMR (300 MHz, CDCl3): d 3.86 (s, 72H); 5.13 (s, 36H); 5.44
(s, 24H) 6.72 (s, 12H); 6.81 (s, 6H); 7.66 (s, 24H); 7.94 (s, 12H);
8.14 (s, 12H). 13C NMR (75 MHz, CDCl3): d 52.4, 53.6, 62.1,
64.0, 114.6, 119.9, 120.3, 123.4, 124.1, 131.8, 137.5, 143.3, 158.1,
159.2, 165.8. Anal. calc. for C216H198N36O66: C, 59.58; H, 4.58;
N, 11.58. Found: C, 59.49; H, 4.47; N, 11.67.
General procedure (A) for dye-sensitized solar cell studies
The TiO2 photoelectrode was prepared as described earlier.31 The
N3 dye was adsorbed on the TiO2 surface by soaking the TiO2
photoelectrode in an ethanol solution of the N3 dye (5 Â 10À5
M
concentration) for 24 h at room temperature. The photoelectrode
was washed dried and immediately used for the measurement of
solar cell performance. A sandwich type photoelectrochemical cell
was composed of a dye-coated TiO2 photoanode. Platinum coated
fluorinated tin oxide (FTO) conducting glass act as a counter
electrode. The electrolyte solution was injected into the space
between two electrodes. The electrolyte solution was composed of
3.2 Â 10À5 M of KI, 4.1 Â 10À6 M of I2, and the dendrimer of 6.4 Â
10À6 M additives in 10 mL DMF solvent. The solar to electric
energy conversion efficiency was measured under simulated solar
light at 40 mW cmÀ2. The photocurrent-photovoltage was
measured using a BAS 100A electrochemical analyzer. The
apparent cell area of TiO2 photoelectrode was 1 cm2 (1 cm  1 cm).
Acknowledgements
The authors acknowledge UGC, New Delhi, India for financial
support. We thank DST, New Delhi for NMR facilities under the
DST-FIST scheme to the Department of Organic Chemistry,
University of Madras, Chennai, India. SR thanks CSIR, New
Delhi for providing SRF.
General procedure (B) for electrochemical studies
Cyclic voltammetric measurements were performed in a conven-
tional three electrode system on CHI model 1100A series electro
chemical analyzer (CH Instrument, USA). Glassy carbon
electrode (GCE) was used as the working electrode with Pt foil
(large surface area) and a silver-silver chloride (Ag/AgCl) as the
counter and reference electrodes, respectively. Prior to each
electrochemical experiment, this GCE was mechanically polished
References
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 E. Palomaras, J. N. Chifford, S. A. Haque, T. Lutz and R. J. Durrent,
J. Am. Chem. Soc., 2003, 125, 475–482.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7700–7704 | 7703

View Article Online
3 M. Adachi, Y. Murta, J. Takao, M. Sakamoto and F. Wang, J. Am.
Chem. Soc., 2004, 126, 14943–14949.
17 J. Lu, P. F. Xia, P. K. Lo, Y. Tao and M. S. Wong, Chem. Mater.,
2006, 18, 6194–6203.
4 S. H. Lin, Y. C. Hsu, J. T. Lin, C. K. Lin and J. S. Yang, J. Org.
Chem., 2010, 75, 7877–7886.
18 N. Satoh, T. Nakashima and K. Yamamoto, J. Am. Chem. Soc., 2005,
127, 13030–13038.
5 T. Hoshokawa, T. Ikebe, R. Kikuchi and K. Eguchi, Electrochim.
Acta, 2006, 51, 5286–5294.
6 (a) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
€
E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am.
Chem. Soc., 1993, 115, 6382–6390; (b) C. J. Barbe, F. Arendse,
€
P. Comte, M. Jirousek, F. Lenzmann, V. Shklover and M. Gratzel,
J. Am. Ceram. Soc., 1997, 80, 3157–3171.
7 M. Morisue, S. Yamatsu, N. Haruta and Y. Kobuke, Chem.–Eur. J.,
2005, 11, 5563–5574.
8 P. Chen, J. H. Yum, F. De Angelis, E. Mosconi, S. Fantacci, S. Moon,
€
R. H. Baker, J. Ko, M. K. Nazeeruddin and M. Gratzel, Nano Lett.,
2009, 9, 2487–2492.
9 X. Zhang, Y. Cui, R. Katoh, N. Koumura and K. Hara, J. Phys.
Chem. C, 2010, 114, 18283–18290.
10 T. Nakashima, N. Satoh, K. Albrecht and K. Yamamoto, Chem.
Mater., 2008, 20, 2538–2543.
19 O. David, S. Maisonneuvesss and J. Xie, Tetrahedron Lett., 2007, 48,
6527–6530.
20 K. Varazo, F. Xie, D. Gulledge and Q. Wang, Tetrahedron Lett.,
2008, 49, 5293–5296.
21 P. D. Jarowski, Y. L. Wu, W. B. Schweizer and F. Diederich, Org.
Lett., 2008, 10, 3347–3350.
22 P. Rajakumar and S. Raja, Synth. Commun., 2009, 39, 3888–3897.
23 P. Rajakumar, R. Anandhan and V. Kalpana, Synlett, 2009, 9, 1417–1422.
24 P. Rajakumar, S. Raja, C. Satheeshkumar, S. Ganesan,
P. Maruthamuthu and S. A. Suthanthiraraj, New J. Chem., 2010,
34, 2247–2253.
25 Y. Li, Y. Y. Cheng and T. W. Xu, Curr. Drug Discovery Technol.,
2007, 4, 246–254.
26 Y. Song, E. K. Kohlmeir and T. J. Meade, J. Am. Chem. Soc., 2008,
130, 6662–6663.
ꢀ
ꢀ ꢀ
ꢀ
27 J. Zavada, M. Pankova, P. Holy and M. Tichy, Synthesis, 1994, 11,
ꢀ
11 H. Kusama, H. Orita and H. Sugihara, Langmuir, 2008, 24, 4411–
4419.
12 C. Devadoss, P. Bhrathi and J. S. Moore, J. Am. Chem. Soc., 1996,
118, 9635–9644.
1132–1132.
28 T. H. Tae-Hyuk kwon, M. K. Kim, J. Kwon, T. Y. Shin, S. J. Park,
C. L. Lee, J. J. Kim and J. I. Hong, Chem. Mater., 2007, 19, 3673–3680.
29 Y. Huana, G. Schlichthorl, A. J. Nozik, M. Gratzel and A. J. Frank,
J. Phys. Chem. B, 1997, 101, 2576–2582.
ꢀ
13 V. Adronw and J. M. Frechet, J. Chem. Soc. Chem. Commun., 2000,
18, 1701–1710.
14 T. S. Kang, K. H. Chen, J. S. Hona, S. H. Moon and K. J. Kim,
J. Electrochem. Soc., 2000, 147, 3049–3053.
15 D. L. Jiang, C. K. Choi, K. Honda, W. S. Li, T. Yuzawa and T. Aida,
J. Am. Chem. Soc., 2004, 126, 12084–12085.
30 G. Schlichthorl, S. Y. Huana, J. Spragne and A. J. Frank, J. Phys.
Chem. B, 1997, 101, 8141–8155.
31 S. Ganesan, B. Muthuraaman, J. Madhavan, P. Maruthamuthu and
S. A. Suthanthiraraj, Sol. Energy Mater. Sol. Cells, 2008, 92, 1718–
1722.
16 F. Loiseau, S. Campagna, A. Hameurlaine and W. Dehaen, J. Am.
Chem. Soc., 2005, 127, 11352–11363.
32 H. Kusama and H. Arakawa, Sol. Energy Mater. Sol. Cells, 2004, 81,
87–89.
7704 | J. Mater. Chem., 2011, 21, 7700–7704
This journal is ª The Royal Society of Chemistry 2011