Triazoloisoquinoline-based dual functional dyestuff for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.materresbull.2012.10.025

Triazoloisoquinoline contains electron-rich nitrogen and oxygen heteroatoms in a heterocyclic structure with high electron-donating ability. By utilizing this feature, two organic dyesutffs containing triazoloisoquinoline were synthesized and used in the fabrication of dye-sensitized solar cells (DSSCs), overcoming the deficiency of ruthenium dyestuff absorption in the blue part of the visible spectrum. This method also fills the blanks of ruthenium dyestuff sensitized TiO₂ film, and forms a compact insulating molecular layer due to the nature of small molecular organic dyestuffs. The incident photon-to-electron conversion efficiency of N719 at shorter wavelength regions is 35%. After addition of triazoloisoquinoline-based dyestuff for co-sensitization, the IPCE at 350-500 nm increased significantly. This can be attributed to the increased photocurrent of the cells, which improves the dye-sensitized photoelectric conversion efficiency from 4.49% to 5.15%, which the overall conversion efficiency increased by about 15%. As a consequence, this low molecular weight organic dyestuff is a promising candidate as co-adsorbent and co-sensitizer for highly efficient dye-sensitized solar cells.

Full text of DOI:10.1016/j.materresbull.2012.10.025

Materials Research Bulletin 48 (2013) 146–150
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Triazoloisoquinoline-based dual functional dyestuff for dye-sensitized solar cells
b
c,
Che-Lung Lee a, Wen-Hsi Lee a, Cheng-Hsien Yang b, , Hao-Hsun Yang , Jia-Yaw Chang
*
*
a Department of Electrical Engineering, Nation Cheng Kung University, No. 1, Daxue Rd., East Dist., Tainan City 70101, Taiwan, ROC
b ShiFeng Technology Co., Ltd. Rm. 410, Bldg. R2, No. 31, Gongye 2nd Rd., Annan District, Tainan, 70955, Taiwan, ROC
c Department of Chemical Engineering, Nation Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da’an Dist., Taipei City 106, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history:
Triazoloisoquinoline contains electron-rich nitrogen and oxygen heteroatoms in a heterocyclic structure
with high electron-donating ability. By utilizing this feature, two organic dyesutffs containing
triazoloisoquinoline were synthesized and used in the fabrication of dye-sensitized solar cells (DSSCs),
overcoming the deficiency of ruthenium dyestuff absorption in the blue part of the visible spectrum. This
method also fills the blanks of ruthenium dyestuff sensitized TiO2 film, and forms a compact insulating
molecular layer due to the nature of small molecular organic dyestuffs. The incident photon-to-electron
conversion efficiency of N719 at shorter wavelength regions is 35%. After addition of triazoloisoquino-
line-based dyestuff for co-sensitization, the IPCE at 350–500 nm increased significantly. This can be
attributed to the increased photocurrent of the cells, which improves the dye-sensitized photoelectric
conversion efficiency from 4.49% to 5.15%, which the overall conversion efficiency increased by about
15%. As a consequence, this low molecular weight organic dyestuff is a promising candidate as co-
adsorbent and co-sensitizer for highly efficient dye-sensitized solar cells.
Received 23 May 2012
Received in revised form 5 October 2012
Accepted 15 October 2012
Available online 23 October 2012
Keywords:
A. Thin films
B. Chemical synthesis
C. Nuclear magnetic resonance
D. Electrical properties
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
trimethyl-1-octyl-3H-indolium (SQ1) [6] By using the concept of two
complementary organic dyestuffs, they achieved higher efficiency-
Dye-sensitized solar cells (DSSCs) have attracted much scientific
attention in recent years because of their low cost, relatively high
photoelectric conversion efficiency (11%) and simple fabrication
process[1–4]. However,itisnecessarytofurtherimprovetheenergy
conversion efficiency of DSSCs for successful commercialization.
Many studies have focused on the broadening of the absorption
spectra of the cells, including tandem DSSCs, hybrid DSSCs and co-
sensitized DSSCs [5–8]. The cost of the multilayered photoelectrode
process is high for the fabrication of tandem and hybrid DSSCs.
However, co-sensitized DSSCs have attracted much attention
because of the easy dying process.
In the literature, most reports of co-sensitized DSSCs focus on the
extension of solar absorption spectra from the visible to the near-
infrared (NIR) region [6,9,10]. For example, Yum et al. fabricated asolar
cell based on co-sensitization of 3-{50-[N,N-bis(9,9-dimethylfluorene-
2-yl)phenyl]-2,20-bisthiophene-5-yl}-2-cyanoacrylic acid (JK2) and 5-
carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-indol-2-ylide-
ne)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-
7.36% than that seen with a single dye device. Using this concept and a
multilayered photoanode, Miao et al. obtained a high efficiency of
11.05% by choosing (tri(isothiocyanato)(2,20:60,200-terpyridyl-4,40,400-
tricarboxylic acid)ruthenium(II)) (BD) to overcome the deficient areas
in the NIR absorption spectra of cis-dithiocyanate-N,N0-bis-(4-
carboxylate-4-tetrabutyl ammoniumcarboxylate-2,20-bipyridine)r-
uthenium(II) (N719) [9] However, to the best of our knowledge,
few studies focus on the use of co-sensitization to broaden the light
absorption spectra of N719 in the blue part of the visible spectrum. In
this work we report the synthesis of the organic dyestuff triazoloi-
soquinoline 5, which can be used as a co-adsorbent to modify the
monolayer of a N719 dying-TiO2 photoanode and co-sensitized
dyestuff to absorb in the blue part of the visible spectrum. By
combining the triazoloisoquinoline 5and the N719 sensitizers, greater
higher efficiency is obtained than seen with the individual dye cells.
2. Experimental
2.1. General information and materials
Scheme 1 outlines how the dyestuff was synthesized. All
starting materials were purchased from Aldrich and TCI, and were
used without further purification. N719, titania pastes (Ti-
Nanoxide T series) and transparent conducting oxide (TCO,
* Corresponding authors.
E-mail addresses: chelung168@gmail.com (C.-L. Lee),
jasonyang@fusol-material.com (C.-H. Yang), jychang@mail.ntust.edu.tw
(J.-Y. Chang).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.10.025

C.-L. Lee et al. / Materials Research Bulletin 48 (2013) 146–150
147
Br
N
O
N
N
Br
triethylamine
Cl
N
N
NMP
10% HCl
.
NH2 HCl
O
1
2
O
O
O
O
B
B
N
O
O
N
B
potassium acetate
Pd(dppf)Cl2
DMSO
N
O
3
S
Br
CHO
N
S
N
CHO
potassium carbonate
Pd(pph3)4
N
THF
O
4
5
O
N
N
OH
S
4
4
N
piperidine
CHCl3
N
COOH
O
NC
S
S
N
O
N
COOH
S
N
S
piperidine
CHCl3
N
S
O
N
O
6
COOH
Scheme 1. Synthesis of organic dyestuffs 5 and 6.
F-doped SnO2, 10
V
squareꢀ1) were purchased from Solaronix SA.
DMF solution on an Agilent 8453 spectrophotometer. 1H NMR and
13C NMR spectra were measured in d6-DMSO solution on a Bruker
Avance-300 (300 MHz) NMR spectrometers with tetramethylsi-
lane (TMS) as the internal standard. The EI-Mass spectra were
recorded on a Bruker APEX II.
With regard to the electrolyte, 0.6 M propylmethylimidazolium
iodide ([PMI][I]) + 0.1 M lithium iodide (LiI) + 0.03 M iodine
(I2) + 0.5 M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile
(MPN) was used in this work. The UV–vis spectra were measured in

148
C.-L. Lee et al. / Materials Research Bulletin 48 (2013) 146–150
2.2. Synthesis of 2-(4-bromophenyl)-[1,2,4]triazolo[3,4-
2.6. Synthesis of 2-(5-(4-(3-oxo-[1,2,4]triazolo[3,4-a]isoquinolin-
2(3H)-yl)phenyl) thiophen-2-yl) methylene)-4-oxo-2-
thioxothiazolidin-3-yl)acetic acid (6)
a]isoquinolin-3(2H)-one (2)
A three-neck flask was charged with
a-chloroformyl p-
bromophenyl hydrazine hydrochloride (5.3 g, 18.5 mmol). This
was dissolved in 50 ml of N-methyl pyrrolidinone (NMP) with
stirring. Triethylamine (2.8 ml, 27.7 mmol) was then added
dropwise into the mixture at room temperature. The mixture
was heated at 80 8C for 16 h. Then, after adding 10% hydrochloric
acid, the mixture was filtered to obtain a crude product. This
crude product was purified by recrystallization technique
(EtOAc-Toluene) to get compound 2: yellow crystals; yield
In a three-neck bottle, compound 4 (1.2 g, 3.3 mmol), 2-(4-oxo-
2-thioxothiazolidin-3-yl) acetic acid (rhodanine-3-acetic acid)
(0.7 g, 3.8 mmol) and piperidine (0.05 g, 0.57 mmol) were dis-
solved in chloroform. The mixed solution was refluxed for 24 h
with rapid stirring. After cooling, the resulting solution was poured
into EtOAc-MeOH. The separated solid was filtered and thoroughly
washed with EtOAc and MeOH and dried: reddish-orange powder;
yield 0.98 g (95.4%); 1H NMR (300 MHz, d6-DMSO):
d 8.32 (d, 1H,
4.2 g (66.7%); 1H NMR (300 MHz, d6-DMSO):
d
8.28 (d, 1H,
J = 7.60 Hz), 8.22 (d, 2H, J = 8.75 Hz), 8.21 (s, 1H), 8.01–7.95 (m,
2H), 7.85–7.67 (m, 6H), 7.04 (d, 2H, J = 7.47 Hz), 4.66 (s, 2H); ESI-
MS m/z 543 (MꢀH+).
J = 7.65 Hz), 8.08 (d, 2H, J = 8.81 Hz), 7.83 (d, 1H, J = 7.65 Hz),
7.77–7.66 (m, 5H), 7.03 (d, 1H, J = 7.44 Hz).
3. Fabrication of photovoltaic devices
2.3. Synthesis of 2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl)-[1,2,4] triazolo [3,4-a]isoquinolin-3(2H)-one (3)
Eight micrometer nanocrystalline TiO2 photoelectrodes were
prepared from a titania paste (Ti-Nanoxide T series, Solaronix SA).
The paste was applied to a transparent conducting oxide by doctor-
blading techniques and annealed at 450 8C for 30 min in air. The
thickness of the TiO2 films was measured with an Alpha-Step 300
profiler. When the TiO2 electrodes cooled down to around 100 8C,
the electrodes were dipped in dye solutions, which included
0.5 mM N719 and different concentrations of dyestuffs 5 or 6 in
tert-butanol/acetonitrile (AN) (1:1 in volume) (device A: N719:
dyestuff 5 (1:0); device B: N719: dyestuff 5 (1:1); device C: N719:
dyestuff 5 (1:0.25); device D: N719: Dyestuff 5 (0:1)). The TiO2
electrodes were immersed in the dye solutions and then kept at
25 8C for more than 12 h to allow the dye to adsorb to the TiO2
surface, and rinsed with the same solvents. The dye-loaded TiO2
film as the working electrode and Pt-coated TCO (about 20 nm) as
the counter electrode were separated by a hot-melt Surlyn sheet
In a round-bottom flask, compound 2 (5.6 g, 16.3 mmol),
4,4,5,5-tetramethyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-1,3,2-dioxaborolane (4.4 g, 17.1 mmol) and potassium acetate
(4.8 g, 49.0 mmol) were dissolved in dimethyl sulfoxide (DMSO)
under nitrogen atmosphere. After adding a catalyst of dichloro-
[1,10-bis(diphenylphosphino)ferrocenyl]palladium(II)
(Pd(dppf)Cl2), the mixed solution was heated at 80 8C for 6 h with
vigorous stirring. It was then poured into EtOAc-water for
extraction. The organic phase was concentrated and adsorbed
on silica gel and purified by column chromatography using
hexane/EtOAc mixture (5:1) as the eluant: white powder; yield
5.32 g (84.2%); 1H NMR (300 MHz, d6-DMSO):
d 8.31 (d, 1H,
J = 7.53 Hz), 8.17 (d, 2H, J = 8.23 Hz), 7.84 (d, 3H, J = 8.57 Hz), 7.77–
7.66 (m, 3H), 7.02 (d, 1H, J = 7.44 Hz), 1.32 (s, 12H).
(60
mm) and sealed together by pressing them under heat. The
2.4. Synthesis of 5-(4-(3-oxo-[1,2,4]triazolo[3,4-a]isoquinolin-2(3H)-
electrolytes were introduced into the gap between the working
and the counter electrodes from two holes predrilled on the back of
the counter electrode. Finally, the two holes were sealed with a
Surlyn film covering a thin glass slide under heat.
yl)phenyl) thiophene- 2-carbaldehyde (4)
In a three-necked round-bottomed flask (25 mL) equipped
with a reflux condenser, compound 3 (1.71 g, 4.4 mmol), 5-
bromothiophene-2-carbaldehyde (1.0 g, 5.2 mmol), and 2 M
potassium carbonate solution were added to a suspension of
Pd(PPh3)4 (3.0 mol%) in tetrahydrofuran (30 mL) at ambient
temperature under nitrogen. The reaction mixture was heated to
80 8C with rapid stirring for 16 h. After cooling, the resulting
solution was poured into water. The separated solid was filtered
and thoroughly washed with water–acetone and dried: pale
yellow powder; yield 1.0 g (61.6%); 1H NMR (300 MHz, d6-DMSO):
4. Photovoltaic measurement
The current density–voltage (J–V) characteristics in the dark
and under illumination were measured with a Keithley 2400
sourcemeter. The photocurrent was measured in a nitrogen-filled
glove box under a solar simulator (Oriel 96000 150W) with AM
1.5G-filtered illumination (100 mW cmꢀ2). The spectra-mismatch
factor of the simulated solar irradiation was corrected using an
Schott visible-color glass-filtered (KG5 color filter) Si diode
(Hamamatsu S1133) [11]. The active area of the device was
0.25 cm2.
d
9.23 (s, 1H), 8.32 (d, 1H, J = 7.56 Hz), 8.24 (d, 2H, J = 8.53 Hz), 8.07
(d, 1H, J = 3.81 Hz), 8.01 (d, 2H, J = 8.60 Hz), 7.86–7.67 (m, 5H),
7.04 (d, 1H, J = 7.39 Hz).
2.5. Synthesis of 2-cyano-3-(5-(4-(3-oxo-[1,2,4]triazolo[3,4-
5. Results and discussion
a]isoquinolin-2(3H)-yl) -phenyl) thiophen-2-yl)acrylic acid (5)
Co-adsorbents and co-sensitizers have both been extensively
studied in DSSCs. However, to the best of our knowledge, few
reports discuss the relationship between the two. Triazoloisoqui-
noline contains electron-rich nitrogen and oxygen heteroatoms in
a heterocyclic structure with high electron-donating ability. This
study is the first report of using triazoloisoquinoline dyestuffs as a
co-adsorbent to modify the monolayer of a N719 dying-TiO2
photoanode and co-sensitizer to absorb the blue part of the visible
spectrum. The synthetic routes of organic dyestuffs 5 and 6 are
depicted in Scheme 1. They consist of treatment of triazoloiso-
quinolines substituted tetramethyl-dioxaborolane with 5-formyl-
2bromothiophene under conditions for Suzuki coupling, followed
In a three-neck bottle, compound 4 (1.2 g, 3.3 mmol), 2-
cyanoacetic acid (0.6 g, 6.5 mmol) and piperidine (0.08 g,
0.98 mmol) were dissolved in chloroform. The mixed solution
was refluxed for 16 h with rapid stirring. After cooling, the
resulting solution was poured into EtOAc-MeOH. The separat-
ed solid was filtered and thoroughly washed with EtOAc and
MeOH and dried: pale orange powder; yield 1.2 g (85.0%); 1H
NMR (300 MHz, d6-DMSO):
d 8.32 (d, 1H, J = 7.50 Hz), 8.23 (s,
1H), 8.20 (d, 2H, J = 3.20 Hz), 7.93 (d, 2H, J = 8.72 Hz), 7.89–
7.67 (m, 6H), 7.03 (d, 1H, J = 7.46 Hz); ESI-MS m/z 437
(MꢀH+).

C.-L. Lee et al. / Materials Research Bulletin 48 (2013) 146–150
149
Table 2
Photovoltaic parameters of the DSSCs with N719: dyestuff 5 sensitizers in different
1.0
0.8
0.6
0.4
0.2
0.0
molar ratios under AM 1.5G sunlight.a
Jsc (mA cmꢀ2
)
Voc (V)
FF
h (%)
5
6
Dyestuffs
molar ratio
Device A
Device B
N719
9.36
9.66
0.72
0.67
0.67
0.66
4.49
4.31
N719-Dyestuff 5
1:1
Device C
N719-Dyestuff 5
1:0.25
10.68
3.25
0.70
0.58
0.69
0.61
5.15
1.14
Device D
Dyestuff 5
a
Performances of DSSCs were measured with 0.25 cm2 working area.
on N719 and dyestuff 5 were designed. All the essential properties
of these cells are listed in Table 2, and the respective J–V curves are
shown in Fig. 2. Under the standard AM 1.5G irradiation, the
300
350
400
450
500
550
600
Wavelength (nm)
maximum efficiency (h) for the N719-sensitized solar cell-device
A with an active area of 0.25 cm2 was calculated to be 4.49%, with a
Fig. 1. Normalized UV–vis spectra of dyestuffs 5 and 6 in DMF.
Jsc of 9.36 mA cmꢀ2, a Voc of 0.72 V and a fill factor (FF) of 0.67.
However, device D, based on dyestuff 5, showed relatively low Jsc
-
3.25 mA cmꢀ2, Voc-0.58 V and FF-0.61, leading to a lower
h
value
by condensation with cyanoacrylic acid or rhodanine-3-acetic acid
in the presence of piperidine.
The absorption spectra of the dyestuffs in DMF solutions are
shown in Fig. 1, and the corresponding data are presented in Table
1. The absorption spectra show two major bands at ca. 250–
350 nm and at ca. 350–600 nm. UV absorption is attributed to
of 1.14%. It is reasonable that dyestuff 5 exhibits narrower
absorption in the blue part of the visible region (as shown in Fig. 1)
and hence could not capture enough solar radiation energy in
comparison with N719. In addition, the large
system of triazoloisoquinoline causes stronger intermolecular
interaction, leading to unfavorable -stacked dye aggregation
on TiO2 in single sensitizer dying device D. Generally, close
stacked dye aggregation leads to inefficient electron injection,
resulting in a low value due to the formation of excited triplet
p-conjugated
p
–
p
p
localized aromatic
p
–
p
* transitions. In the visible region, this may
p-
be attributed to intramolecular charge transfer (ICT) absorption,
because an efficient charge-separated excited state could be
produced between the triazoloisoquinoline and the cyanoacrylic/
rhodanine-3-acetic acid moieties [12]. As shown in Table 1, the ICT
h
states [16]. Because of these physical properties, dyestuff 5 is used
as the co-adsorbent and co-sensitizer for DSSCs. Interestingly,
device C sensitized by N719-dyestuff 5 with 1–0.25 molar ratio
produced the best results, and even better than those seen with
devices A and B, with Jsc-10.68 mA cmꢀ2, Voc-0.70 V and FF-0.69
absorption (lmax) appeared at 380 nm for 5, and 451 nm for 6. The
molar extinction coefficients in the visible region were
29,651 Mꢀ1 cmꢀ1 for 5, and 29,353 Mꢀ1 cmꢀ1 for 6, and all of
these are twice as high as those of the ruthenium complexes [13].
Compound 6 with rhodanine-3-acetic acid as the acceptor shows a
significant bathochromic shift in the S0–S1 absorption band
compared with that of compound 5. This red-shift may favor
light harvesting and hence photocurrent generation in DSSCs.
and an overall conversion efficiency
h of 5.15%. One can see from
Fig. 2 and Table 2 that the Voc of devices B and C were around 0.67–
0.70 V by using co-sensitizers, but the Jsc of these devices showed
excellent performance in comparison with device A, which was
based on single sensitizer-N719. This result suggests that the
quasi-Fermi level of the co-sensitizer dyed TiO2 layer moves
downward relative to the iodide/triiodide, which leads to a
decrease in Voc, but extends the absorption area of the visible
Therefore, a simple cell was fabricated (FTO/8
mm TiO2/MPN
electrolyte/Pt) for verification this concept. All the essential
properties of these DSSC dyestuffs are listed in Table 1. Under
the standard AM 1.5G irradiation, the maximum efficiency (h (%))
for compound 5 with an active cell area of 0.25 cm2 was calculated
to be 1.2%, while the cell based on compound 6 showed a relatively
low short-circuit current (Jsc) and open-circuit voltage (Voc),
12
10
leading to a lower
h value of 0.54%. This result indicates that
rhodanine-3-acetic acid is a poor anchor in comparison to
cyanoacrylic acid. These results agree with the experimental data
in Liang et al. and Tian et al. [14,15]. However, it is necessary to
consider the efficiency of co-sensitizer, and also compound 6 based
on rhodanine-3-acetic acid is excluded from the follow-up
experiments.
8
Device A
Device B
6
Device C
Device D
4
In order to clarify the role played by the triazoloisoquinoline
dyestuff in the DSSCs, different molar ratios of cosensitizers based
Table 1
2
0
Optical and DSSC performance parameters of dyestuffs 5 and 6.
Dyestuff
l
max/nma
Jsc (mA cmꢀ2
)
Voc (V) FF
h
(%)b
(
e
/Mꢀ1 cmꢀ1
)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5
6
277 (19435), 380 (29651) 2.95
314 (19115), 451 (29353) 1.96
0.56
0.50
0.68 1.12
0.55 0.54
Voltage (V)
a
Fig. 2. J–V curves of solar cells sensitized by N719 (device A), N719: dyestuff 5 (1:1)
(device B), N719: dyestuff 5 (1:0.25) (device C), and dyestuff 5 (device D) with
Absorption spectra were measured in DMF.
Performances of DSSCs were measured with 0.25 cm2 working area and the
b
8.0
mm single layer nanocrystalline TiO2 electrodes.
photoanodes were dyed with DMF solution.

150
C.-L. Lee et al. / Materials Research Bulletin 48 (2013) 146–150
100
90
80
70
60
50
40
30
20
10
0
shielded, and the incident photo-to-electron conversion efficiency
at shorter wavelengths can be enhanced, and this increases Jsc and
. On the other hand, device B shows a downward trend at 570–
800 nm in comparison with device A, and the maximum IPCE is ca.
82% at 420 nm. This indicates that dyestuff 5 has considerable
superiority at 1–1 molar ratio, with some N719 molecules having
lost their positions on TiO2 and being replaced by dyestuff 5. This
h
Device A
Device B
Device C
Device D
increases the opportunities for
p–p stacking of organic dyestuff 5,
and decreases the contribution of N719 to the incident photon-to-
electron conversion efficiency [20,21]. This is the reason why
device B has lower conversion efficiency than device C in Table 2.
6. Conclusions
This study investigated the role of triazoloisoquinoline dye-
stuffs as co-adsorbents and co-sensitizers with N719. The results
show that co-adsorption of N719 sensitizer with dyestuff 5 onto
nanocrystalline TiO2 films significantly increases the photocurrent
in 1–0.25 molar ratio, thus enhancing the total conversion
efficiency. The cell produced in this work achieved an energy
conversion efficiency as high as 5.15% at 100 mW cmꢀ2 and AM
1.5G. This improved conversion efficiency is attributed to the
insulating molecular layer, which was composed of small molecule
organic dyestuff 5 and N719, and the light harvesting effect at
shorter-wavelength regions. It is anticipated that the findings of
this work will contribute to the development of co-adsorbent and
co-sensitizer bi-functional organic dyestuffs in DSSCs.
400
500
600
700
800
Wavelength (nm)
Fig. 3. IPCE action spectra of solar cells sensitized by N719 (device A), N719:
dyestuff 5 (1:1) (device B), N719: dyestuff 5 (1:0.25) (device C), and dyestuff 5
(device D) with 8.0
mm single layer nanocrystalline TiO2 electrodes.
spectrum after being co-sensitized with dyestuff 5 and N719,
which enhances Jsc. This result is also in agreement with the
observation of the incident photon-to-electron conversion
efficiency (IPCE) action spectra of DSSCs presented in Fig. 3. On
the other hand, device D-sensitized with dyestuff 5 exhibits a
lower Voc-0.58 V, while devices B and C co-sensitized with
dyestuff 5 and N719 show only small changes in the Voc value,
to 0.67 V and 0.70 V, respectively. These values are very close to
that of device A based on N719, which is 0.72 V. This result is
attributed to the co-adsorbent effect of dyestuff 5, which is
applied successfully to prepare an insulating molecular layer with
N719 [17,18].
Acknowledgements
The authors thank the National Science Council of the Republic
of China for financially supporting this research under Contract No.
NSC 101-2113-M-011-002.
References
IPCE as a function of wavelength was measured to evaluate the
photo-response of the photoelectrodes in the whole spectra region.
As shown in Fig. 3, the maximum IPCE of dyestuff 5 based device D
was ca. 43% at 420 nm. This indicates that dyestuff 5 has good light
harvesting effects at shorter wavelength regions. Device A exhibits
the maximum IPCE of ca. 52% at 570 nm for the N719 sensitized
solar cell. The incident photon-to-electron conversion efficiency of
N719 at shorter wavelength regions (ca. 35% at 420 nm) is thus not
as good as at longer wavelength owns [19]. This is a niche for
organic dyestuffs, such as dyestuff 5, which can enhance
absorption at shorter wavelength regions. When dyestuff 5 was
added to the dye solution, the IPCE of devices B and C at 350–
500 nm increased significantly. The maximum IPCE increased from
35% to 65% when 0.25 molar ratio dyestuff 5 was added to the dye
solution, and to 83% with 1 molar ratio dyestuff 5. Upon dyestuff 5
co-adsorption, the maximum IPCE improved significantly, com-
pared to that seen with device A, which can be attributed to the
increased photocurrent of the DSSCs. The related experimental
data are in line with the results in Table 2.
It is well known that organic dyestuffs will compete with N719
for the adsorption on the TiO2 surface. As shown in Fig. 3 for
devices B, and C and Table 2, there is a balanced relationship
between the dyestuff 5 and N719 co-sensitizers. The IPCE spectra
of device C shows a similar response at 570–800 nm, but broadens
significantly at 350–570 nm in comparison with device A. This
shows that N719 and dyestuff 5 are in balance at 1–0.25 molar
ratio. In this concentration of co-sensitizers, N719 may occupy
positions on the TiO2 surface, as in device A, and dyestuff 5 then
anchors to the blank to create a perfect insulating molecular layer.
Because of this layer, the charge recombination process can be
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