Julolidine-based organic dyes with neutral and anion anchoring groups for dye-sensitized solar cells

Article abstract of DOI:10.1166/jnn.2015.11536

Two simple organic dyes (J1 and J2) containing julolidine as the electron donor were synthesized. The simple structure of julolidine attached to the cyanoacetic acid group formed two compounds (anion and neutral forms of E-CCVJ), which showed two different efficiencies when applied to dyesensitized solar cells (DSSCs). Overall conversion efficiencies of 0.91% and 1.21% were obtained for DSSCs based on the cyanoacetic acid (J1) and cyanoacetate (J2) derived dyes, respectively. Compared to the cyanoacetic acid terminated dye, the current density, open circuit voltage and conversion efficiency of the solar cells based on cyanoacetate dye were increased by approximately 24%, 8% and 32%, respectively. The electrochemical impedance analysis showed that the better charge transfer of TiO₂ (e^-) and electron lifetime (τ_e) for J2 dye as compared with J1. The power conversion efficiency was found to be quite sensitive to small structural changes to the anchoring moiety.

Full text of DOI:10.1166/jnn.2015.11536

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 8813–8819, 2015
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Julolidine—Based Organic Dyes with Neutral and Anion
Anchoring Groups for Dye-Sensitized Solar Cells
Le Quoc Bao, Nguyen Thi Hai, Chi Hwan Lee, Suresh Thogiti∗, and Jae Hong Kim∗
Department of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongsangbuk-do, 712-749, Republic of Korea
Two simple organic dyes (J1 and J2) containing julolidine as the electron donor were synthesized.
The simple structure of julolidine attached to the cyanoacetic acid group formed two compounds
(anion and neutral forms of E-CCVJ), which showed two different efficiencies when applied to dye-
sensitized solar cells (DSSCs). Overall conversion efficiencies of 0.91% and 1.21% were obtained
for DSSCs based on the cyanoacetic acid (J1) and cyanoacetate (J2) derived dyes, respectively.
Compared to the cyanoacetic acid terminated dye, the current density, open circuit voltage and
conversion efficiency of the solar cells based on cyanoacetate dye were increased by approximately
24%, 8% and 32%, respectively. The electrochemical impedance analysis showed that the better
charge transfer of TiO2 (e−ꢀ and electron lifetime (ꢁeꢀ for J2 dye as compared with J1. The power
conversion efficiency was found to be quite sensitive to small structural changes to the anchoring
moiety.
Delivered by Publishing Technology to: Florida State University, College of Medicine
Keywords: Dye Sensitized Solar Cells, Julolidine Sensitizers, Nanocrystalline TiO2, UV-Vis
IP: 74.40.123.8 On: Mon, 19 Oct 2015 07:30:01
Absorption.
Copyright: American Scientific Publishers
1. INTRODUCTION
metals in dye synthesis has limited their commercial
applicability. Metal-free organic dyes as alternative sen-
sitizers have attracted considerable attention because of
their potential advantages, such as control of the struc-
ture of organic compounds, the use of inexpensive and
environmentally benign resources, high molar extinction
coefficient, and facile synthesis.13–16 To date, a myriad
organic dyes, such as triphenyl amine,17–19 coumarne,20–22
cyanine,23ꢀ24 perylene,25–27 indoline,28–31 fluorine,32–34
Since O’Regan and Grätzel introduced Dye-Sensitized
Solar Cells (DSSCs) in 1991, they have attracted con-
siderable interest in the scientific community.1–3 DSSCs
have also attracted the attention of chemists, physics, and
engineers around the world owing to their construction
and operation, their high incident-solar-light-to-electricity
conversion efficiency and low cost of production.4–6 To
increase the performance of DSSCs, it is very important
to create highly efficient sensitizers. The ideal sensitizer
for a DSSC should have a high molar extinction coeffi-
cient and absorb solar radiation strongly with absorption
bands preferably covering the broad ranges of wavelengths
across visible to near infrared regions. In addition, the sen-
sitizer should anchor strongly to the semiconductor surface
to achieve good electronic communication and inject elec-
trons efficiently into the conduction band of TiO2.
carbazole,35–37
phenothiazine,38–40
phenoxazine,41–43
tetrahydroquinoline,44–47 and ullazine,48–50 have been eval-
uated as sensitizers and the highest efficiency cells have
achieved comparable PCEs (>10%) to those with metal-
based dyes.51–54 For example, an impressively high cell
efficiency of 12.8% has also been achieved using a metal-
free sensitizer.15 On the other hand, a better fundamental
understanding is needed to design a new, efficient and
stable organic sensitizers for future devices. For instance,
the adsorption mode influences the electronics states of
the dye/TiO2 interface, such as the conduction band posi-
tion of the TiO2 surface, the photo-absorption spectra of
the dye itself55–57 and the electron injection efficiency.58
Conventionally, the deprotonated anchor is regarded as
being more stable than the protonated anchor.59 Therefore,
Recently, DSSC based on a Zn-porphyrin complex
were reported to have a power conversion efficiency
(PCE) of 13%7 and those with the ruthenium complex
dyes also reached 11%8–12 under standard AM 1.5G sun-
light. On the other hand, the use of rare and expensive
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 11
1533-4880/2015/15/8813/007
doi:10.1166/jnn.2015.11536
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Julolidine—Based Organic Dyes with Neutral and Anion Anchoring Groups for Dye-Sensitized Solar Cells
Bao et al.
investigation of the protonation/deprotonation of dye
carboxyl anchor is still important.
2.2.1. Synthesis of 9-Julolidinecarboxaldehyde (1)
Julolidine (0.5 g, 2.89 mmol), DMF (0.255 g, 3.49 mmol)
and POCl3 (0.535 g, 3.49 mmol) were dissolved in DCM
(15 mL) and the mixture was stirred at room temperature
for 4 h under an inert argon atmosphere. The solution’s
color turned green and the degree of advancement was fol-
lowed by TLC. The solution was treated with aq. NaOH
(2 M) and the crude product then was extracted with
Et2O. After two aqueous washings, the organic phase was
dried on MgSO4, filtered and concentrated under vacuum.
The product was then purified on column chromatogra-
phy using 40%–50% Et2O: Hexane was used as the elu-
ent to give 0.48 g (83.08%) (the reaction yield was up
to 92% when 1.5 g julolidine was used as starting mate-
rial) of a light yellow solid product.1H NMR (300 MHz,
CDCl3ꢂ:ꢃ = 9ꢄ597 (s, 1 H), 7.29 (s, 1 H), 3.308 (t, J =
5ꢄ7 Hz, 4 H), 2.787 (t, J = 6ꢄ2, 4 H), 2.002–1.92 (m, J =
6ꢄ3, 4 H).
9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) is a mem-
ber of a class of molecules commonly known as “molecu-
lar rotor.” This material was first introduced by Sawada in
1992 as one of a series of fluorophores with greater water
solubility to study the association phenomena in biologi-
cal systems.60 Several julolidine dyes have been used as
sensitizers in DSSCs because of their large ꢁ-conjugated
system and better electron donating property.61–65 In this
study, anion and neutral derivatives of cyanoacetic acid
were introduced to the simple julolidine based dyes for
anchoring in an attempt to achieve better anchoring of
the cyanoacetate anion and increase photovoltaic perfor-
mance through potential electrostatic interactions with the
semiconductor (TiO2ꢂ surface. Therefore, two new organic
dyes J1 (neutral) and J2 (anion), were designed and
applied to DSSCs and compared with standard N71912 dye
under similar fabrication and measurement conditions. To
explain the photovoltaic properties of the ionic dye sys-
tem in DSSCs, the UV-Vis spectra of the two compounds,
current density–voltage characteristics, impedance analy-
sis, and the incident photo-to-current conversion efficien-
cies were studied.
2.2.2. Synthesis of 9-(2-carboxy-2-cyanovinyl)
Julolidine (CCVJ)
9-Julolidinecarboxaldehyde (0.5 g, 2.49 mmol), cyano-
acetic acid (0.3 g, 3.53 mmol), and piperidine (0.3 g,
3.52 mmol) were dissolved in 15 mL AcCN and stirred
ꢀ
for 5 h at 60 C. The reaction progress was monitored by
TLC. Subsequently, water was added to the solution and
2. EXPERIMENTAL DETAILS
the crude product was extracted with CH Cl . After two
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2
2
2.1. Equipment and Measurements
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aqueous washings, the organic phase dried on Na2SO4, fil-
1
All H NMR spectra were recorded on a Varian Mercury
NMR 300 MHz spectrometer using CDCl3 and DMSO-
d6, which were both purchased from Alfa Aesar Company.
The chemical shifts were referenced to TMS. The absorp-
tion spectra were recorded on an Agilent 8453 UV-Vis
spectrophotometer. The active areas of the dye-absorbed
TiO2 films were estimated using a digital microscope cam-
era with image-analysis-software (Moticam 1000). The
photovoltaic I–V characteristics of the prepared DSSCs
Copyright: American Scientific Publishers
tered and concentrated under vacuum. The product was
then purified by column chromatography using 5% MeOH:
CH2Cl2 and 0.05 g (7.5%) of a solid product (CCVJ) was
obtained. Finally, the crude product was recrystallized with
MeOH to produce two compounds, which were called J1
(neutral CCVJ) and J2 (anion CCVJ), respectively. J1 and
1
J2 were tested using the H NMR method. J1—1H NMR
(300 MHz, DMSO): ꢃ = 7ꢄ849 (s, 1 H), 7.487 (s, 2 H),
3.337 (t, J = 5ꢄ7 Hz, 4 H), 2.690 (t, J = 6ꢄ0 Hz, 4 H),
1.907 (m, J = 5ꢄ7 Hz, 4 H). J2—1H NMR (300 MHz,
DMSO): ꢃ = 7ꢄ688 (s, 1 H), 7.326 (s, 2 H), 3.265 (t, J =
5ꢄ7 Hz, 4 H), 2.680 (t, J = 6ꢄ0 Hz, 4 H), 1.880 (m, J =
6ꢄ0 Hz, 4 H).
were measured under 1 sunlight intensity (100 mW cm−2
,
AM1.5), which was verified with an AIST-calibrated Si-
solar cell (PEC-L11, Peccell Technologies, Inc., City,
State, Country). The incident monochromatic photon-to-
current efficiencies (IPCEs) were plotted as a function of
the light wavelength using an IPCE measurement instru-
ment (PEC-S20, Peccell Technologies, Inc.).
2.3. Assembly and Characterization of DSSCs
The transparent conducting glass substrates were cleaned
sequentially with ethanol, DI water and acetone with ultra-
sonication. The TiO2 pastes were prepared using ethyl
cellulose (Aldrich), Lauric acid (Fluka, City, State, Coun-
try), and terpineol (Aldrich). The TiO2 particles used were
ca. 20–30 nm in diameter. The prepared TiO2 paste was
doctor-bladed onto pre-cleaned glass substrates, followed
2.2. Synthesis of Sensitizers
Julolidine was purchased from TCI and stored at low tem-
peratures. N ꢀN -dimethylformamide (DMF), cyanoacrylic
acid and piperidine were obtained from Sigma-Aldrich
and used as received. Phosphoryl chloride (POCl3ꢂ and
other solvents, such as dichloromethane (DCM), chloro-
form (CHCl3ꢂ and diethyl ether (Et2O), were purchased
from Alfa-Aesar and used as received. All reactions were
performed under an inert gas (N2ꢂ and the products were
stored in the dark.
ꢀ
by drying at 70 C for 30 min and 30 min calcination at
500 ꢀC. The scattering layer consisting of rutile TiO2 parti-
cles (250 nm in a size) was deposited on mesoporous TiO2
films. These layers were dipped into an aqueous solution
ꢀ
of TiCl4 (0.04 M) at 70 C for 30 min. The sensitizers
8814
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Bao et al.
Julolidine—Based Organic Dyes with Neutral and Anion Anchoring Groups for Dye-Sensitized Solar Cells
(J1 and J2) were dissolved in DMF (0.3 mM) at room
temperature and stirred for 24 h. The TiO2 layers were
immersed in the solutions for 24 h.
subjected to Vilsmeier–Haack formylation in the presence
of dichloromethane. The crude reaction mixture after work
up was separated by column chromatography to obtain
pure 9-julolidinecarboxaldehyde (1) in good yield. Kno-
evenagel condensation of intermediate 1 with cyanoacetic
acid in the presence of piperidine for 5 h at 60 ꢀC
yielded a crude dye, which was purified by recrystal-
lization using MeOH to give the sensitizers, J1 and J2.
9-Julolidinecarboxaldehyde (1) and the sensitizers (J1 and
The Pt counter electrodes were prepared by thermal
reduction of the filmsꢀ dip-coated in H2PtCl6 (7.0 mM)
in 2-propanol at 400 C for 20 min. The dye-adsorbed
TiO2 and Pt counter electrodes were sandwiched between
60 ꢅm-thick Surlyn (Dupont 1702), which was used as a
bonding agent and spacer. Through a pre-punched hole on
the Pt counter electrode, a liquid electrolyte (I−/I3− redox
couple) was then introduced and finally sealed. The active
area of the dye-adsorbed TiO2 films was estimated using
a digital microscope camera with image-analysis-software
(Moticam 1000).
1
J2) were characterized by HNMR spectroscopy.
3.2. Optical Properties
Figure 1 shows the UV-Visible absorption spectra of J1
and J2 dyes in DMF. From the spectra, at lower con-
centrations, the absorption peaks for J1 and J2 appeared
to be the same (at approximately 375 nm). By increas-
ing the concentration of both J1 and J2, J1 showed a red
shift in absorption. Further increasing the concentration
resulted in a clear absorption peak at 441 nm. In contrast,
the solution of J2 in DMF, showed no such change in
the absorption spectra with changing concentration. The
relative ratio of the peaks depends not only on the concen-
tration, but also on the solution temperature. By increasing
3. RESULTS AND DISCUSSION
3.1. Synthesis
Julolidine and cyanoacetic acid were chosen as the donor
and acceptor moieties for the two metal-free organic sen-
sitizers, J1 and J2. Although there are a few reports on
the role of julolidine as a metal-free organic sensitizer, it
was used to synthesize the simplest julolidine sensitizers
with a small change in their anchoring moiety for DSSC
applications.
2.0
1.8
(A)
The synthesis of two new organic sensitizers was
carried out according to Scheme 1. Julolidine was
0.0015 mM J1
0.003 mM J1
0.0045 mM J1
0.006 mM J1
0.009 mM J1
0.015 mM J1
0.03 mM J1
0.045 mM J1
0.06 mM J1
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1.4
HOOC
Copyright: American Scientific Publishers
CN
1.2
1.0
0.8
0.6
0.4
0.2
0.0
N
N
CCVJ
DMF
POCl3
CH2Cl2
RT, 4h
MeOH
300
350
400
450
500
550
600
Wavelength (nm)
H
OOC
(B)
CHO
N
0.0015 mM J2
0.003 mM J2
0.0045 mM J2
0.006 mM J2
0.009 mM J2
0.015 mM J2
0.03 mM J2
0.045 mM J2
0.06 mM J2
N
C
0.8
0.6
0.4
0.2
0.0
N
(1)
NeutralE-CCVJ(J1)
Piperidine
AcCN
CNCH2COOH
60 ºC, 5 h
OOC
HOOC
CN
CN
N
N
300
350
400
450
500
550
600
Wavelength (nm)
CCVJ
AnionicE-CCVJ(J2)
Figure 1. UV-Vis absorption spectra of J1 (A) and J2 (B) in DMF at
Scheme 1. Synthesis of the CCVJ sensitizers J1 and J2.
J. Nanosci. Nanotechnol. 15, 8813–8819, 2015
various concentrations.
8815

Julolidine—Based Organic Dyes with Neutral and Anion Anchoring Groups for Dye-Sensitized Solar Cells
Bao et al.
1.4
(A) 1.75
J1
J2
0.06 mM J1 RT
Intensity
0.06 mM J1 40 ºC
0.06 mM J1 60 ºC
0.06 mM J1 80 ºC
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Decrease
1.50
1.25
1.00
0.75
0.50
0.25
0.00
Intensity
Increase
300
350
400
450
500
550
600
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(B) 1.0
0.8
Figure 3. UV-Vis spectra of J1 and J2 attached on TiO2 film.
0.06 mM J2 RT
anion groups in the molecule because this functional group
may increase the likelihood of favorable electrostatic inter-
actions with the TiO2 surface. The interactions between
the carboxylate group and the surfaceTi4+ ions may lead
to increased delocalization of the ꢁ∗ orbital of the conju-
gated framework. The energy of the ꢁ∗ level is decreased
by this delocalization, which explains the red shift in the
absorption spectra. The blue-shift of the absorption spectra
of dye J1 on TiO2 compared to solution can be attributed
to deprotonation of the carboxylic acid group and/or the
formation of H aggregates on the TiO2 surface.46ꢀ66 From
0.06 mM J2 40 ºC
0.06 mM J2 60 ºC
0.06 mM J2 80 ºC
0.6
0.4
0.2
0.0
300
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350
400
450
500
550
the absorption studies, the julolidine-based sensitizer con-
IP: 74.40.123.8 On: Mon, 19 Oct 2015 07:30:01
Wavelength (nm)
taining a carboxylate anion as the anchoring group may
Copyright: American Scientific Publishers
Figure 2. Effect of temperature on UV-Vis spectra of J1 (0.06 mM) in
DMF: Upon increasing the temperature to 80 ꢀC (A) and Effect of tem-
perature on UV-Vis spectra of J2 (0.06 mM) in DMF: Upon increasing
the temperature to 80 ꢀC (B).
be a good sensitizer compared to other sensitizers with a
cyanoacetic acid anchoring group for DSSC applications.
3.3. Photovoltaic Properties
The photovoltaic properties of the solar cells constructed
from J1 and J2 of the E-CCVJ TiO2 electrode were mea-
sured under simulated AM 1.5 irradiation (100 mW cm−2ꢂ.
The open-circuit photovoltage (VOCꢂ, short-circuit pho-
tocurrent density (JSCꢂ, fill factor (FF), and solar-to-
electricity conversion efficiencies (ꢆꢂ were compared with
those of N719 and are listed in Table I. Figures 4 and 5
show the incident photo-to-current conversion efficiencies
(IPCE) and current density–voltage (J–V ꢂ characteristics
of the devices based on the two dyes, respectively. The
DSSC coated with J2 exhibited a much better IPCE of
approximately 50% at approximately 450 nm in the solar
response range. For the J1 based cell, the IPCE reached its
maximum of 40% at 450 nm. J2 produced a higher IPCE
value between 350 nm and 550 nm than the J1 dye. On the
the temperature of the solution of J1 in DMF, the absorp-
tion intensity began decreasing at 441 nm and increasing
at 375 nm (Fig. 2(A)). These concentration- and tempera-
ture dependent behaviors of the J1 sensitizer suggest a red
shift in the absorbance after forming aggregates. In con-
trast, no change in the absorption spectrum was observed
with increasing temperature of the J2 solution in DMF
(Fig. 2(B)).
The absorption spectra of the spin-coated thin films of
J1 and J2 were recorded to investigate the interactions
between the molecules in the solid state (Fig. 3). Gener-
ally, the absorption maxima of the organic dyes on the
TiO2 films would change because of the different effects of
the deprotonation in the adsorption process and the aggre-
gation state of dyes on TiO2 films. In our case, a red shift
between the organic sensitizer J2 on the TiO2 films and in
solution was observed, which might be due to the J-type
aggregation. In addition, the absorption spectrum of the J2
sensitizer anchored onto the TiO2 film showed a slightly
broad profile compared to the solution spectrum, which
was beneficial for light harvesting. The broadening and red
shift in the spectrum of J2 might be due to the presence of
Table I. Photovoltaic parameters of the DSSCs based on J1, J2 and
standard N719.
Sensitizer
JSC (mA/cm2)
VOC (V)
FF (%)
ꢆ (%)
N719
J1
J2
16ꢄ84
3ꢄ186
3ꢄ955
0.7299
0.4214
0.4575
61.62
67.76
67.08
7ꢄ575
0ꢄ91
1ꢄ21
8816
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Bao et al.
Julolidine—Based Organic Dyes with Neutral and Anion Anchoring Groups for Dye-Sensitized Solar Cells
60
16
J1
J2
J1
J2
14
12
10
8
45
30
15
0
6
4
2
0
300
400
500
600
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Wavelength (nm)
Potential (V)
Figure 4. IPCE curves of the DSSCs using J1 and J2.
Figure 6. J–V curves obtained with DSSCs based on J1 and J2 dyes
under dark conditions.
other hand, the IPCE values for J1 and J2 decreased sig-
nificantly after 500 nm, which was attributed to a decrease
in the light harvesting property. The J1 sensitized solar
cell showed a JSC of 3.18 mA−2, VOC of 0.421 V and FF
of 67.76%, corresponding to a ꢆ of 0.91%, whereas the
J2 sensitized cell yielded a JSC of 3.95 mA−2, a VOC of
0.457 V and a FF of 67.08%, corresponding to a ꢆ of
1.21%. Under similar test conditions, the N719 dye gave
a referenced ꢆ value of 7.57%.
provides an opportunity for organic dyes to exhibit high
energy conversion performance.
A dark current test, which is a technique for testing
the capability of charge recombination, was conducted
and the results are shown in Figure 6. The onset for
J1 was slightly lower than J2. This suggests that the
back-electron-transfer process corresponding to a reaction
between the conducting-band electrons in the TiO2 and I3−
in the electrolyte under dark conditions occurs more easily
The observed JSC values of the DSSCs based on J2 were
in the DSSCs based on neutral E-CCVJ (J1) after adsorb-
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higher than those based on J1 under one sun illumina-
ing on the TiO layer. Compared to J1, the J and VOC of
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2
SC
tion, which is in agreement with the improved IPCE spec-
Copyright: American Scientific Publishers
the solar cells based on J2 are increased about 24% and
8%, respectively. The higher JSC and VOC for the DSSCs of
J2 lead to a higher efficiency. Finally, an increase in ꢆ of
approximately 32% was obtained from J1 to J2. A likely
reason for this may be the better electrostatic interactions
of the anion form of E-CCVJ with the TiO2 surface, which
ultimately gives J2 better conversion efficiency than J1.
To further understand the FTO/TiO2/dye interface,
we performed electrochemical impedance spectroscopy
trum. This suggests that the cyanoacetate anion-anchoring
dye J2 with a broad and red-shifted spectrum could inject
more electrons effectively into the TiO2 conduction band.
On the other hand, the cyanoacetic acid anchoring dye J1
decreased the IPCE of the dye. Therefore, the cells based
on the dye with the cyanoacetic acid anchoring group
showed narrower IPCEs than those based on the cyanoac-
etate anion-anchoring dye, resulting in lower JSC values.
Therefore, the cyanoacetate anion as an anchoring moiety
–5.0
4.5
J1
J2
–4.5
–4.0
–3.5
–3.0
–2.5
–2.0
–1.5
–1.0
–0.5
0.0
J1
4.0
J2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
15
20
25
30
Potential (V)
Z' (Ohm)
Figure 5. Current density–voltage (J–V ꢂ characteristics of the DSSCs
from J1 and J2 under the illumination of simulated solar light (AM 1.5,
100 mW cm−2ꢂ.
Figure 7. Impedance spectra (Nyquist plots) based on J1 and J2 under
AM1.5 light conditions.
J. Nanosci. Nanotechnol. 15, 8813–8819, 2015
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Julolidine—Based Organic Dyes with Neutral and Anion Anchoring Groups for Dye-Sensitized Solar Cells
Bao et al.
Table II. Electrochemical parameters of DSSCs.
plot also indicated increasing resistance to recombination.
The lower RCT and longer ꢈe observed for cyanoacetate
anion dye J2 compared to cyanoacetic acid dye J1 indi-
cates a more effective suppression of the back reaction of
the injected electron with the I3− in the electrolyte, reflect-
ing in the improvement of the JSC and VOC, yielding a
substantially enhanced power conversion efficiency.
Sample
RS(ꢇ)
RCE(ꢇ)
RCT(ꢇ)
ꢈe
J1
J2
16.45
17.98
3ꢄ587
4ꢄ23
7.727
5.249
12.63
15.92
studies (EIS) since this technique has been a useful tool
to estimate charge transfer resistance and to know the
dye regeneration efficiency. Figure 7 shows EIS results
for DSSCs comprised of FTO/TiO2/J1 and FTO/TiO2/J2
electrodes under illuminated. The smaller and larger semi-
circles in the Nyquist plots were attributed to the charge
transfer at the counter electrode and the working elec-
trode interface, respectively. RS is the series resistance,
RCT is the electron transfer resistance between the TiO2
film and electrolyte and RCE is the resistance of elec-
tron transport in the counter electrode. The fitting results
are summarized in Table II. The Nyquist plots (Fig. 7)
showed the radius of the semicircle corresponding to the
working electrode to be higher for FTO/TiO2/J1 compared
to FTO/TiO2/J2. The charge transfer resistance (RCTꢂ for
FTO/TiO2/J1 and FTO/TiO2/J2 were 7.72 and 5.24 ꢇ,
respectively (Table II). The decrease in charge transfer
resistance for FTO/TiO2/J2 compared to FTO/TiO2/J1
can be attributed to the better performance, i.e., photo-
regeneration of FTO/TiO2/J2 is much efficient, a result
4. CONCLUSION
Sensitizers based on julolidine with the simplest structure
were synthesized and used on DSSCs to test their effi-
ciencies. The simple structure of julolidine attached to the
cyanoacrylic acid group formed two compounds (anion
and neutral forms of E-CCVJ), which gave two different
efficiencies when applied to DSSCs. The J2 dye with the
cyanoacetate anion as the anchoring group showed red-
shift in the absorption band on the TiO2 film compared
to J1 dye with a cyanoacetic acid as the anchoring group.
The simple structure of the donor-anchoring CCVJ showed
reasonable results (0.91% for J1 and 1.21% for J2). Sensi-
tizer J2 showed a 32% improvement in the overall power
conversion efficiency compared to J1, which could be due
to its better electrostatic interactions with the TiO2 sur-
face. The EIS analysis of the DSSCs shows that the charge
transfer resistance (RCTꢂ is lowered and electron lifetime
(ꢈeꢂ is increased for anion anchored dye; this is consistent
that agrees well with the cell efficiency shown in Figures 4
Delivered by Publishing Technology to: Florida State University, College of Medicine
with the results of the photocurrents and the power conver-
sion efficiencies. The results suggest that the cyanoacetate
and 5 (Table I).
The recombination rate caused by the backward charge
IP: 74.40.123.8 On: Mon, 19 Oct 2015 07:30:01
Copyright: American Scientific Publishers
acceptor produces DSSCs with better properties than those
of the cyanoacetic acid acceptor in the julolidine dyes.
This data suggests that sensitizers based on julolidine com-
pounds could show impressive results if eligible ꢁ-bridges
can be added. Research on studying this class of julolidine
metal-free organic dyes towards achieving higher solar to
electricity conversion efficiencies is currently underway.
transfer was estimated using the Bode-phase spectra for
the DSSCs, as shown in Figure 8. The electron lifetimes
were calculated and shown in Table II. The frequency of
the characteristic peak in the Bode phase plot increased in
the order of J2 < J1. A lower characteristic frequency in
the Bode phase plot means a slower charge recombination
rate and higher VOC. As the reciprocal of the characteristic
frequency is correlated with electron lifetime (ꢈeꢂ, electron
lifetime increased in the order of J1 < J2. This sequence
of the electron lifetime calculated from the Bode phase
Acknowledgment: This research was supported by
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Min-
istry of Education, Science and Technology (grant number:
2012014395).
–10
J1
J2
–8
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Received: 19 July 2014. Accepted: 22 December 2014.
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