Pyridine derivatives; New efficient additives in bromide/tribromide electrolyte for dye sensitized solar cells

Article abstract of DOI:10.1039/c5ra15894j

In this work, two new inexpensive pyridine derivatives, propyl isonicotinate (PIN) and isopropyl isonicotinate (IPIN), have been synthesized through a simple and low cost method and for the first time, they have been applied as effective additives in bromide/tribromide electrolyte in dye sensitized solar cells (DSSCs). Although the iodide/triiodide redox shuttle shows remarkable performance in DSSCs, but bromide/tribromide couple has a more positive redox potential in comparison to this couple. Therefore, it is good idea to design dyes whose HOMO and LUMO levels match with the redox potential of bromide/tribromide and the conduction band (CB) of TiO₂, respectively. We have synthesized 3-(4-carbazole-9-yl-phenyl)-2-cyano-acrylic acid (TC301) and 2-cyano-3-(4-(3,6-di-tert-butyl-9H-carbazole-9-yl)phenyl) acrylic acid (TC302) as two carbazole dyes and applied them with modified electrolyte in DSSCs. The influence of PIN and IPIN additives in bromide/tribromide redox electrolyte on the DSSC performances is investigated. In comparison to electrolyte without additive, adding 0.5 M of these additives to the electrolyte solution leads to an increase in the open circuit voltage (V_oc) and short current density (J_sc), consequently the energy conversion efficiency (η) improves. Electrochemical impedance spectroscopy show that the enhancement in V_oc is due to increasing electron density in the CB of TiO₂ so that a shift in the Fermi level (E_F) occurs. It leads to a suppression in electron recombination that has beneficial effect on the V_oc. Furthermore, cyclic voltammetry results reveal that PIN and IPIN has similar effect mechanisms to 4-tert-butylpyridine (TBP) additive in the DSSCs. Our findings show that TBP can be replaced with PIN and IPIN in the DSSCs.

Full text of DOI:10.1039/c5ra15894j

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PAPER
Pyridine derivatives; new efficient additives in
bromide/tribromide electrolyte for dye sensitized
solar cells
Cite this: RSC Adv., 2015, 5, 86191
Omid Bagheri, Hossein Dehghani* and Malihe Afrooz
In this work, two new inexpensive pyridine derivatives, propyl isonicotinate (PIN) and isopropyl isonicotinate
(IPIN), have been synthesized through a simple and low cost method and for the first time, they have been
applied as effective additives in bromide/tribromide electrolyte in dye sensitized solar cells (DSSCs).
Although the iodide/triiodide redox shuttle shows remarkable performance in DSSCs, but bromide/
tribromide couple has a more positive redox potential in comparison to this couple. Therefore, it is good
idea to design dyes whose HOMO and LUMO levels match with the redox potential of bromide/
2
tribromide and the conduction band (CB) of TiO , respectively. We have synthesized 3-(4-carbazole-9-
yl-phenyl)-2-cyano-acrylic acid (TC301) and 2-cyano-3-(4-(3,6-di-tert-butyl-9H-carbazole-9-yl)phenyl)
acrylic acid (TC302) as two carbazole dyes and applied them with modified electrolyte in DSSCs. The
influence of PIN and IPIN additives in bromide/tribromide redox electrolyte on the DSSC performances is
investigated. In comparison to electrolyte without additive, adding 0.5 M of these additives to the
electrolyte solution leads to an increase in the open circuit voltage (Voc) and short current density (Jsc),
consequently the energy conversion efficiency (h) improves. Electrochemical impedance spectroscopy
show that the enhancement in Voc is due to increasing electron density in the CB of TiO
2
so that a shift
Received 8th August 2015
Accepted 5th October 2015
in the Fermi level (E ) occurs. It leads to a suppression in electron recombination that has beneficial
F
effect on the Voc. Furthermore, cyclic voltammetry results reveal that PIN and IPIN has similar effect
mechanisms to 4-tert-butylpyridine (TBP) additive in the DSSCs. Our findings show that TBP can be
replaced with PIN and IPIN in the DSSCs.
DOI: 10.1039/c5ra15894j
www.rsc.org/advances
3
(
they have stable oxidized and reduced forms), lifetime and
1
. Introduction
4
reactivity of the excited state. Furthermore, photophysical
Dye-sensitized solar cells (DSSCs) have attracted lot of attention, properties such as absorption in the visible range made this
since the breakthrough work by O'Regan and Gratzel in 1991, class of compounds attractive for fundamental and practical
because of their potential of becoming a clean, unlimited, science studies. There are two most problematic components of
1
renewable energy source and low-cost photovoltaics. The a dye sensitized solar cell. One of them is ruthenium based dye.
device is based on an optically transparent mesoporous lm of Despite many advantages of ruthenium complexes as sensitizer,
semiconductor, usually titanium dioxide coated on a conduc- the idea of developing organic sensitizers originates from the
tive glass. The dye is adsorbed on the semiconductor substrate. need to nd cheaper and easier ways to synthesize dyes
The sensitizer harvests light in the visible range and injects compared to those based on the ruthenium. The main advan-
electrons in the semiconductor conduction band. The oxidized tages of organic dyes can be summarized as follows: very high
dye is then regenerated by electron donation from the electro- molar extinction coefficients, simple modelling and facile
2
lyte, which usually contains the iodide/triiodide redox couple.
synthesis, absorption energies that can be easily tuned and low
5,6
Not every dye can be a sensitizer; this depends on its physical cost. Another problem is associated to iodide/triiodide elec-
and chemical properties which need to satisfy certain require- trolyte and they are numerous: it is corrosive, volatile and has
ments in order to maximize electron injection when adsorbed high color. Probably the most critical drawback to iodide/
on the semiconductor. Ruthenium polypyridine complexes are triiodide is its redox potential that is signicantly more nega-
a class of compounds that have been widely studied in the past tive than thermodynamically necessary for cell operation.
three decades is due to their chemical stability, redox properties Consequently, the open circuit voltage (Voc) attainable from
a DSSC containing iodide/triiodide electrolyte is less than is
theoretically possible. Because the overall efficiency of a solar
Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O.
cell scales directly with V , this constitutes a signicant
oc
Box 87317-51167, Kashan, I. R. Iran. E-mail: dehghani@kashanu.ac.ir; Fax: +98-315-
7,8
limitation.
5
91-2397; Tel: +98-315-591-2386
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The bromide/tribromide couple has a more positive redox 24 h. The uorine doped tin oxide coated glass substrates (FTO)
potential (about 1.1 V vs. NHE) in comparison to the iodide/ (transmission >90% in the visible, sheet resistance 8 U per
9
triiodide couple (about 0.35 V vs. NHE). Thus, the electrolyte square, 2.3 mm thickness) and surlyn spacer (30 mm) were
containing the bromide/tribromide redox system can improve purchased from Dye sol (Australia). The detailed preparing
2
greatly the photovoltage of the DSSCs if used together with well- processes for TiO paste (P25, Degussa, Germany) were similar
30
designed dyes. The research results approved that the DSSCs to our previous article.
with bromide/tribromide electrolyte had higher Voc values than
that of the DSSCs with iodide/triiodide electrolyte for appointed
dyes.
2
.2. Instrumentation
10–15
The FT-IR spectrum was recorded by using a Magna 550 Nicolet
spectrometer using KBr pellets. H NMR spectrum was recorded
Liquid electrolytes typically consist of a redox couple and
additives dissolved in a liquid solvent. The photovoltaic prop-
1
on Bruker DPX-400 MHz NMR spectrometer. Cyclic voltamme-
try (CV) was used to study the electrochemical properties of the
electrolytes. The CV spectra were obtained with a Sama 500
potentiostat (Isfahan, Iran) using 0.1 M lithium perchlorate
trihydrate in acetonitrile as a supporting electrolyte, a glassy
carbon working electrode (2 mm diameter) and platinum wire
counter electrode, an Ag/AgCl/KCl (sat) as a reference electrode
were employed as electrodes. The electrochemical impedance
spectroscopy (EIS) measurements of the cells were achieved
under AM 1.5 G simulated light (Luzchem) using potantiostat/
galvanostat (PGSTAT 100, Autolab, Eco-Chemie), at an AC
amplitude of 5 mV within the frequency range from 0.01 Hz to
16
erties of DSSCs can be optimized by employing additives. The
redox couple potential, semiconductor surface state, shi of
the conduction band edge, recombination kinetics, as well as
the photovoltaic parameters of DSSCs can be improved by
17,18
adding a small amount of additives.
One class of the most
commonly used additives in liquid electrolytes are nitrogen-
containing heterocyclic compound. 4-tert-Butylpyridine (TBP)
is the most famous used additive in DSSCs. The Lewis base trait
of TBP is caused that it deprotonates the TiO surface, therefore
2
2
causing a negative shi of the CB of the TiO . The effect of TBP
has been extensively investigated in dye sensitized solar cell
over the past several years, and several mechanisms have been
100 kHz. The photocurrent–voltage (I–V) characteristics of the
19–22
put forward.
It has been proposed that the dramatic
cells were measured using a Keithley model 2400 digital source
meter (Keithley, USA). The obtained spectra from the EIS
measurements were tted by Z-view soware (v2.9c, Scribner
Associates Inc.).
increase of the Voc arising from the introduction of the TBP can
be attributed to either the suppression of the dark current at
TiO /electrolyte interface or the negative shi of the CB in the
2
TiO lm, or a combination of both. Despite wide use of TBP as
2
additive in dye sensitized solar cell, it's price is so much and
many investigations have been devoted to replace it with other
2
.3. Synthesis of additives and dyes
23–29
pyridine derivatives.
Although pyridine derivatives have
2.3.1. Additives
widely been used to modify iodide/triiodide electrolyte, but no
2.3.1.1. Propyl isonicotinate. Isonicotinic acid (20 g,
example of their application as additive in bromide/tribromide 0.16 mol) was reuxed with excess thionyl chloride (40 mL, 0.55
electrolyte have not been reported. So, in this work, two inex- mol) for 1 hour. Excess thionyl chloride was removed by evap-
pensive pyridine derivative (IPIN and PIN) were synthesized oration in vacuum to leave the acid chloride. The crude product
ꢀ
through a simple and low cost method with commercial and was washed with diethyl ether and dried in vacuum at 40 C.
available materials. Beside, effects of propyl isonicotinate and Isonicotinoyl chloride hydrochloride were yielded as white
31
isopropyl isonicotinate additives are investigated in the crystals (27.6 g, yield 96%). Propanol (16.5 mL, 0.22 mol) was
bromide/tribromide electrolyte on the DSSC performance. By added in to the isonicotinoyl chloride hydrochloride (4 g,
means of the electrochemical impedance spectroscopy and 0.022 mol) and reux for 2 hours. Aqueous sodium bicarbonate
cyclic voltammetry approach, we decided to study the perfor- solution (20 mL) was added, then the product extracted into
mance of carbazole dyes sensitized solar cells based on redox dichloromethane (3 ꢁ 15 mL), drying with MgSO , and then
4
couple bromide/tribromide that contain these new additives. evaporation to give propyl isonicotinate (2.52 g, yield 68%).
1
The adsorption of PIN and IPIN on the semiconductor surface
H NMR (400 MHz, CDCl ): d (ppm) 8.78 (2H, d, J ¼ 6 Hz),
3
suppresses the electron recombination from the TiO
2
to the 7.85 (2H, d, J ¼ 6 Hz), 4.31 (2H, t, J ¼ 4.8 Hz), 1.80 (2H, sext,
ꢂ1
tribromide in the electrolyte effectively and nally enhances J ¼ 7 Hz), 1.02 (3H, t, J ¼ 7.6 Hz). IR (KBr), n (cm ): 1729 (ester
V
oc, Jsc and h. To the best of our research, for a DSSC containing carbonyl group stretching), 1596 and 1405 (C]C stretching
bromide/tribromide electrolyte, PIN and IPIN are the rst bands of the pyridine ring), 1282 and 1121 (C–O bands in the
additives which have been replaced TBP. ester group), 2969 (C–H stretching band of the propyl group).
.3.1.2. Isopropyl isonicotinate. Isopropyl isonicotinate was
2
synthesized by similar method as described above by reacting
isonicotinoyl chloride hydrochloride (4 g, 0.022 mol) and iso-
2
. Experimental
2.1. Materials
propanol (16.8 mL, 0.22 mol), (2.38 g, yield 64%) product was
1
All solvents and reagents were purchased from Merck and obtained aer purication: H NMR (400 MHz, CDCl ): d (ppm)
3
Aldrich Companies. Propanol and isopropanol were distilled 8.64 (2H, d, J ¼ 4.4 Hz), 7.71 (2H, d, J ¼ 6 Hz), 5.14 (1H, m), 1.25
ꢂ1
under vacuum prior to the use. For preparation of dry acetoni- (6H, d, J ¼ 6.4 Hz); IR (KBr), n (cm ): 1725 (ester carbonyl group
ꢀ
trile in the electrolytes it was kept over 4 A molecular sieves for stretching), 1596 and 1407 (C]C stretching bands of the
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pyridine ring), 1283 and 1105 (C–O bands in the ester group), dichloromethane: methanol (v/v, 10 : 1) as eluent to afford the
1
2
1
982 (C–H stretching band of the isopropyl group), 1327 and dye TC301 as a yellow solid (123 mg, yield 70%). H NMR
351 (C–H bending bands of the isopropyl group).
(400 MHz, DMSO): d (ppm) 8.25 (2H, d, J ¼ 7.6 Hz), 8.18 (2H, d, J
.3.1.3. Allyl isonicotinate. Allyl isonicotinate was synthe- ¼ 7.2 Hz), 8.11 (1H, s), 7.79 (2H, d, J ¼ 7.2 Hz), 7.420–7.496
sized by similar method mentioned above by reacting iso- (4H, m), 7.30 (2H, t, J ¼ 6.8 Hz).
nicotinoyl chloride hydrochloride (4 g, 0.022 mol) and allyl To prepare 3,6-di-tert-butyl-9H-carbazole, a mixture of
alcohol (32.34 mL, 0.22 mol), (2.35 g, yield 64%) product was carbazole (5.00 g, 0.03 mol) and anhydrous AlCl (4.00 g, 0.03
(100 mL). At 0 C, a solution of
Cl (20 mL) was
2
1
1
3
1
ꢀ
3 2 2
obtained aer purication: H NMR (400 MHz, CDCl ): d (ppm) mol) were dissolved in CH Cl
8
5
.67 (2H, d, J ¼ 6 Hz), 7.70 (2H, d, J ¼ 6 Hz), 5.93 (1H, m), tert-butyl chloride (6.6 mL, 0.06 mol) in CH
2
2
.32 (1H, d, J ¼ 17.2 Hz), 5.22 (1H, d, J ¼ 10.4 Hz), 4.75 (2H, d, added slowly. The mixture was stirred for 24 h at room
ꢂ1
J ¼ 4.8 Hz); IR (KBr), n (cm ): 1648 (C]C stretching band of the temperature. Then, the mixture was poured into water, extrac-
allyl group), 1725 (ester carbonyl group stretching), 1596 and ted with CH Cl (3 ꢁ 50 mL), and dried with MgSO . Solvent was
2
2
4
1
409 (C]C stretching bands of the pyridine ring), 1281 and evaporated to afford the crude products, which were recrystal-
1
122 (C–O bands in the ester group). Synthetic pathway of three lized three times from petroleum ether to give white crystals
1
additives is presented in Fig. 1.
(4.25 g, yield 51%). H NMR (400 MHz, acetone): d (ppm) 8.19
2
.3.2. Dyes (TC301 and TC302). 4-(9H-Carbazole-9-yl) (2H, s), 7.46 (2H, d, J ¼ 7.2 Hz), 7.40 (2H, d, J ¼ 8.2 Hz), 1.43
32
benzaldehyde was synthesized according to the literature.
(18H, s).
A mixture of 4-uorobenzaldehyde (1.06 mL, 10 mmol), K CO
To synthesize 4-(3,6-di-tert-butyl-9H-carbazole-9-yl)benzalde-
2
3
11,32
(4.15 g, 30 mmol) and carbazole (1.67 g, 10 mmol) in DMF (15 hyde,
a mixture of 4-uorobenzaldehyde (1.06 mL,
mL) was reuxed for 12 h, cooled to room temperature and 10 mmol), K CO (4.15 g, 30 mmol), and 3,6-di-tert-butyl-9H-
2
3
poured into ice water. The resulting precipitate was ltered carbazole (2.79 g, 10 mmol) in DMF (15 mL) was reuxed for 12
and washed with water, methanol, ethanol, and n-hexane. h and cooled to room temperature. Aer poured into ice water,
A yellowish white solid was obtained aer drying under extracted with CH
vacuum (2.02 g, yield 75%). H NMR (400 MHz, CDCl ): d (ppm) with Na SO
3 2 4
2
Cl
and evaporated. Finally, the crude product was
0.13 (1H, s), 8.13–8.17 (4H, m), 7.81 (2H, d, J ¼ 8.4 Hz), puried by silica gel column chromatography with CH Cl
.52 (2H, d, J ¼ 7.6 Hz), 7.45 (2H, t, J ¼ 6.8 Hz), 7.35 (2H, t, J ¼ petroleum ether (v/v, 1 : 1) as eluent affording a pale yellow
2
(3 ꢁ 40 mL). The organic phase was dried
1
1
7
7
2
2
:
1
.2 Hz).
solid (1.58 g, yield 42%). H NMR (400 MHz, CDCl ): d (ppm)
3
To prepare 3-(4-(9H-carbazole-9-yl)phenyl)-2-cyanoacrylic 9.90 (1H, s), 7.95 (2H, s), 7.92 (2H, d, J ¼ 8 Hz), 7.79 (2H, d, J ¼ 8
11
acid (TC301), 4-(9H-carbazole-9-yl)benzaldehyde (136 mg, Hz), 7.278–7.309 (4H, m), 1.28 (18H, s).
.50 mmol) was dissolved in 10 mL of acetonitrile in a three- 2-Cyano-3-(4-(3,6-di-tert-butyl-9H-carbazole-9-yl)phenyl) acrylic
necked ask. Then cyanoacetic acid (85 mg, 1 mmol) and acid (TC302) was synthesized by similar method for TC301,
0
1
5
a few drops of piperidine (63.98 mg, 0.75 mmol) was added and but with 4-(3,6-di-tert-butyl-9H-carbazole-9-yl)benzaldehyde
heated under reux for 4 h at argon atmosphere. The reaction (192 mg, 0.50 mmol) instead of 4-(9H-carbazole-9-yl)
mixture was cooled to room temperature and the solvent was benzaldehyde. The dye TC302 was obtained as yellow solid
1
removed by rotary evaporation. The residue was extracted with (167 mg, yield 72%). H NMR (400 MHz, DMSO): d (ppm) 8.28
dichloromethane and 0.1 M HCl solution. Organic phase was (2H, s), 8.16 (2H, d, J ¼ 8.4 Hz), 8.10 (1H, s), 7.76 (2H, d, J ¼ 8
puried by silica gel column chromatography with Hz), 7.76 (2H, d, J ¼ 8 Hz), 7.46 (2H, d, J ¼ 6.8 Hz), 7.40 (2H, d,
J ¼ 8.4 Hz), 1.39 (18H, s). Synthetic pathway of two dyes is
observed in Fig. 2.
Fig. 2 (a) 4-Fluorobenzaldehyde, K
2 3
CO , DMF, reflux, 12 h (ref. 11 and
2
Fig. 1 (a) SOCl , reflux, 1 h (ref. 31) (b) isopropanol, reflux, 2 h (c) 32) (b) 2-cyanoacetic acid, piperidine, acetonitrile, reflux, 4 h (ref. 15)
propanol, reflux, 2 h (d) allyl alcohol, reflux, 2 h.
3 2 2
(c) t-BuCl, AlCl , CH Cl , Rt, 24 h (ref. 11).
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2
.3.3. 1-Butyl-2,3-dimethylimidazolium bromide (BDMIBr). from the dyes to the CB of TiO
2
. This is p-stacked aggregation of
1
-Butyl-2,3-dimethylimidazolium bromide (BDMIBr) was dye molecules on TiO surface. Dye aggregation is usually
2
synthesized according to ref. 33. Briey, 1-bromobutane (7 mL, considered a phenomenon that should be avoided although
.4 mmol) was slowly added to a stirred solution of 1,2-dime- a controlled aggregation could be considered benecial. One
thylimidazole (4 mL, 4.2 mmol) in ethyl acetate. The mixture was way to avoid this problem is incorporate long alkyl chains and
6
36
then reux. Finally, two phases were formed. The top phase was aromatic groups into the dye structure to prevent aggregation.
discarded. The bottom phase was recrystallized four times in
In our study, we have synthesized two carbazole dyes as
ethyl acetate: acetonitrile (v/v, 3 : 2). Residual solvents were simple and low cost dyes for applying in the DSSCs. To gain
removed under vacuum in the presence of P
(24 h). BDMIBr further insight into the electronic properties of the carbazole
2 5
O
was obtained as a white hygroscopic solid. The structure of the dyes, DFT (density functional theory) calculations were per-
1
resulting salt was conrmed by H NMR spectroscopy (400 MHz, formed at the B3LYP/6-31+G (d) level. The frontier orbital plots
CDCl
.724–1.798 (2H, m), 2.76 (3H, s), 3.99 (3H, s), 4.18 (2H, t, J ¼ 7.2 lowest unoccupied molecular orbital (LUMO) of the TC301 and
3
): d (ppm) 0.91 (3H, t, J ¼ 7.2 Hz), 1.301–1.375 (2H, m), of the highest occupied molecular orbitals (HOMO) and the
1
Hz), 7.50 (1H, d, J ¼ 2.1 Hz), 7.72 (1H, d, J ¼ 2.1 Hz).
TC302 dyes are shown in Table 1. In these dyes like other
D–p–A systems, HOMOs are delocalized over the p-conjugated
system in congurations centering on the donor side while the
LUMOs are mainly delocalized on the acceptor side. Absorp-
tion spectra of solutions of TC301 and TC302 in THF have
been shown in Fig. 3. The results showed that the absorption
maximum were at 340 nm for TC301 and 347 nm for TC302.
The absorption maxima of TC302 dye has a red-shied
compared to TC301 dye and the HOMO–LUMO energy gap of
TC301 was larger than that of TC302. Therefore, tert-butyl
groups facilitate absorption at long wavelengths. Furthermore,
in TPA based dyes it is conrmed that presence of tert-butyl
group as an electron donor group can improve the photovol-
2
.4. Cell fabrication
To prepare transparent working electrodes, FTO glass were rst
cleaned with deionized water, acetone and ethanol in an
ultrasonic bath. Aer cleaning the FTO, they were treated with
ꢀ
a 40 mM aqueous TiCl
4
solution at 70 C for 30 min. Then a TiO
2
paste was spread onto the FTO substrate with a glass rod using
Scotch tape as spacer (doctor blade deposition). In this study,
two layers of TiO particles were deposited onto FTO glass and
2
used as photoelectrode. The TiO
2
lms were then heated at 500
ꢀ
C for 30 minutes. Aerwards, the electrodes were immersed
aqueous as described previously and then were
into the TiCl
4
37
taic performance.
ꢀ
ꢀ
calcined at 500 C for 15 min. Aer the lm was cooled to 40 C,
the prepared thin lm electrodes were immersed into a 2 ꢁ
ꢂ4
10
M dye bath in THF solution and maintained in the dark for
3
.2. Photovoltaic performance and electrical impedance
8
h. Aer dye loading, the TiO lms were rinsed with THF and
2
analysis
assembled with platinum counter electrodes that were prepared
During the course of our study, four electrolytes were prepared
for DSSCs consisted of 0.05 M Br , 0.6 M BDMIBr with 0.5 M
2
different additives (TBP, IPIN, PIN and bare (electrolyte without
additive)). The photovoltaic parameters of the DSSCs employing
the TC301 and TC302 sensitizers and bromide/tribromide
electrolyte with different additives under simulated sunlight
by thermal decomposition of H
2
PtCl
6
solution (2 mg in 1 mL of
ꢀ
ethanol) and heating at 400 C for 15 min. To prepare sealed
cells, the two electrodes were sealed together with a hot-melt
polymer lm frame around the titanium dioxide area. Electro-
lyte was introduced via the hole drilled through the counter
electrode by vacuum lling.
3
. Results and discussion
Table 1 Frontier molecular orbitals of the HOMO and LUMO calcu-
lated with DFT at the B3LYP/6-31+G (d) level of TC301 and TC302 dyes
3.1. Spectral properties of dyes
Dye
HOMO
LUMO
In terms of molecular structure, organic dyes can be divided in
34
three major parts; donors, linkers and acceptors. Linkers are
usually p-conjugated system that link by bridges electron-
donating (D) and electron-accepting (A) groups; so-called
D–p–A sensitizers. Light absorption induces charge separation TC301
between the donor and the acceptor moieties of the chromo-
phore where the photo-absorption properties involve intermo-
35
lecular charge transfer (ICT) between A and D. One of the most
relevant features that researchers would like to improve in the
design of a new organic sensitizer is its absorption in the red
region of the visible spectra. This can be obtained by expanding
the p-conjugation of the linkers and increasing the electron-
donating and electron-accepting capability of donors and
acceptors. In a DSSC using organic dyes, there is a factor which
is generally thought inuences negatively the electron injection
TC302
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Fig. 4 Photocurrent voltage characteristic plots of the DSSCs based on
TC301 dye in the presence of various additives in bromide/tribromide
ꢂ2
electrolyte systems under 100 mW cm illumination (AM 1.5 G).
Fig. 3 Absorption spectra of solutions of the dyes in THF.
ꢂ2
at 100 mW cm were compared in Table 2. Interestingly, the
data for all of additives in the bromide/tribromide electrolyte
indicate an increase in open circuit voltage and the short circuit
current density compared to bare electrolyte. The photocurrent
density–voltage (J–V) curves are shown in Fig. 4 and 5. To
investigate the inuence of additive in bromide/tribromide
electrolyte on performance the cell, one of the dyes for
example, TC302 was selected. The values of the Jsc, Voc and ll
factor (FF) of the DSSCs based on the electrolyte without any
ꢂ2
additive were 3.33 mA cm , 0.80 V and 0.62, respectively,
yielding a conversion efficiency (h) of 1.65%. Overall, among the
various additives in the redox shuttles, the best energy conver- Fig. 5 Photocurrent voltage characteristic plots of the DSSCs based on
TC302 dye in the presence of various additives in bromide/tribromide
sion efficiency of 3.76%, achieved for IPIN, is the highest re-
ported for DSSCs constructed with our modied electrolyte
ꢂ2
electrolyte systems under 100 mW cm illumination (AM 1.5 G).
based on TC302 dye. In this additive, compared to TBP, we
observed even a 30% increase in Jsc, were explainable for the
obtained efficiency increase from 2.99% to 3.76%. This cell
yielded an improvement of an additional 26% in efficiency.
However, other electrolyte containing PIN exhibits less
enhancing in efficiency (11%) compared to TBP.
Furthermore, AIN was applied as an additive in the electro-
than TC301 dye. As mentioned in Section 3.1, this is attributed
lyte solution. Our results showed that it is not suitable additive
double bond of allyl group. This reaction leads to disappear of
electrolyte solution color and the concentration of bromine
decreases.
Based on the obtained photovoltaic results (Table 2), it is
clear that performance of dye TC302 as a sensitizer is better
to the presence of tert-butyl group in the structure of TC302 dye.
This is conrmed by absorption spectra solutions of TC301 and
TC302 in THF (Fig. 3). The results showed that tert-butyl groups
for bromide/tribromide electrolyte because bromine reacts with
facilitate absorption at long wavelengths and nally is caused
TC302 dyes in presence of various additives in bromide/tribromide a higher DSSC performance.
Table 2 Photovoltaic performance of DSSCs based on TC301 and
36,37
ꢂ2
electrolyte systems under AM 1.5 (100 mW cm ) illumination with an
The photoelectrochemical behavior of the DSSC based on
TC302 dye containing different additives can be followed
successfully using EIS spectroscopy. Fig. 6 shows the typical
Nyquist plot of our devices. The semicircle prole of the Nyquist
plot of a DSSC represents the charge transfer resistances at
different interfaces. We observe the rst semicircle corre-
sponding to the charge transfer impedance at the counter
electrode/electrolyte interface, the second semicircle is associ-
2
active area of 0.18 cm
ꢂ2
Dye/additive
Jsc [mA cm
]
Voc [V]
FF
h [%]
TC301/no additive
TC301/TBP
TC301/IPIN
TC301/PIN
TC302/no additive
TC302/TBP
3.08
3.74
4.93
4.47
3.33
4.37
5.69
5.29
0.79
1.06
0.95
0.92
0.80
1.00
0.94
0.91
0.65
0.67
0.70
0.68
0.62
0.68
0.70
0.69
1.58
2.68
3.30
2.81
1.65
2.99
3.76
3.35
2
ated with the charge transfer impedance of the TiO electrode
and the electrolyte. The equivalent circuit used for tting the
impedance spectra of the DSSC is shown in the inset of Fig. 6. In
TC302/IPIN
TC302/PIN
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the equivalent circuit, R
s
, RCE and RCT represent series resis- becomes difficult, which leads to a decrease in the photocur-
tance, charge transfer resistance at the counter electrode/ rent. Therefore, we expect to design two new pyridine-based
electrolyte interface and electron recombination resistance at additive to adjust the interaction between the N atom of pyri-
the TiO /electrolyte interface, respectively. C and CPE1 are dine ring and the Ti(IV) ions, making the negative shi of the
2
attributed to the chemical capacitance at the counter electrode/ conduction band edge of TiO
electrolyte interface and the TiO /electrolyte interface, respec- negative shi of the conduction band edge can increase
Table 3 was extracted by using an equivalent circuit to the open-circuit voltage and the short-circuit photocurrent. For
2
moderately. This moderate
2
38,39
tively.

t the Nyquist plot.
a DSSC, the photocurrent is determined by the following two
Electron lifetime in TiO
2
(s) is one of the main parameters, processes: (1) the photoexcited dye molecules inject electrons
which could be extracted from the highest frequency peak of the into the conduction band of the TiO . (2) The injected electrons
2
second arc (umax) in the Nyquist plot by using s ¼ 1/umax. It are extracted from the nanocrystalline to the outer electrical
40
shows the electron recombination in TiO lm.
circuit to form the photocurrent. It has been concluded that if
2
Adding additives in to the electrolyte lead to an increases in the negative shi of the conduction band edge be small enough,
41
the electron lifetime and charge transfer resistance at the TiO
2
/
the electron injection will not be hindered. Due to presence of
electrolyte interface. Hence, charge recombination is inhibited ester group as electron withdrawing group on the structure of
upon adding pyridine derivatives into electrolyte and thus an IPIN and PIN, the adsorption of these isonicotinate lead to
increase in Voc is obtained. Among these additives, TBP shows a small shi of the conduction band edge and do not hamper
the highest Voc in the DSSCs. In the molecule of TBP, the tert- the injection of the electrons. Thus, the process (2), i.e. the
butyl group as an electron donating group increases the charge charge extraction efficiency of the injected electrons, is decisive
density of the nitrogen atom of the pyridine ring, which to the photocurrent of the cell.
enhances the interaction between the nitrogen atoms and
TBP showed the largest resistance at Pt/electrolyte interface.
the Ti(IV) ions. It seems that for the pyridine based additives, This means that in device based on TBP, the reduction of
there is a stronger interaction so that the more negative shi of bromine is the slowest at the Pt counter electrode. This indi-
23
the conduction band edge there will be. The negative shi of cates that the catalytic performance of Pt can be deteriorated in
the conduction band edge is benecial to the increase of the Voc the presence of TBP. The increased RCE were also reason for the
However, the negative shi is too much, the electron injection lowered J in device based on TBP compared to IPIN and
.
sc
10,42
from the photoexcited dye into the conduction band of the TiO2 PIN.
3.3. Cyclic voltammetry analysis
CV is a powerful tool to analyze the reaction kinetics between
the bromide and tribromide ions in the electrolyte solution.
Cyclic voltammograms of bromide/tribromide electrolytes are
shown in Fig. 7.
Four peaks are observed for redox process in the electrolyte
in every curve, which are attributed to anodic peaks potential,
reactions (1) and (2) and cathodic peaks potential, reactions (3)
43
and (4).
Fig. 6 Nyquist plots from the EIS spectra for the DSSC based on
TC302 dye in the presence of various additives in the bromide/tri-
bromide electrolyte.
Table 3 Electrochemical parameters obtained by fitting the EIS of
DSSCs with different additives using an equivalent circuit for TC302
dye
ꢂ1
eff (s
ꢂ2
ꢂ2
Additive
k
)
RCT (ohm cm
)
RCE (ohm cm
)
s (ms)
TBP
PIN
IPIN
11.12
14.91
14.91
42.85
21.34
29.10
13.50
5.95
6.02
90.09
67.06
67.06
Fig. 7 Cyclic voltammograms for the bromide/tribromide electrolyte
without any additive (bare) and the electrolytes in the presence of TBP,
IPIN and PIN as additives.
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ꢂ
ꢂ
ꢂ
3
Br % Br3 + 2e
(1) nally enhance efficiency in comparison to electrolytes
contain TBP.
(2)
3ꢂ
2
ꢂ
2
3
Br % 3Br
+ 2e
ꢂ
3ꢂ
Br
2
+ 2e % 2Br
(3) Acknowledgements
ꢂ
ꢂ
ꢂ
Br3 + 2e % 3Br
(4) The authors gratefully acknowledge from the University of
Kashan for supporting this project by Grant No. 159183/24.
In comparison to bare electrolyte, it is observed that addi-
tion of additives into the electrolyte causes a shi of redox
reaction peaks to more negative or positive positions. For References
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