Dyes for sensitized solar cells by using [2.2]paracyclophane as a bridging unit

Article abstract of DOI:10.1016/j.tetlet.2014.07.034

Organic dyes that consist of a [2.2]paracyclophane moiety between a triphenylamine donor group and a cyanoacrylic acid acceptor group have exhibited considerably high values of open-circuit voltage (V_oc) in the range of 0.69-0.74 V. In an exper

Full text of DOI:10.1016/j.tetlet.2014.07.034

Accepted Manuscript
Dyes for Sensitized Solar Cells by Using [2.2]Paracyclophane as a Bridging
Unit
Chiung Hui Huang, Yuan Jay Chang
PII:
S0040-4039(14)01187-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.07.034
TETL 44877
Reference:
Tetrahedron Letters
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Received Date:
Revised Date:
Accepted Date:
27 May 2014
4 July 2014
9 July 2014
Please cite this article as: Huang, C.H., Chang, Y.J., Dyes for Sensitized Solar Cells by Using [2.2]Paracyclophane
as a Bridging Unit, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.07.034
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Dyes for Sensitized Solar Cells by Using
[2.2]Paracyclophane as a Bridging Unit
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Chiung Hui Huang and Yuan Jay Chang∗

1
Tetrahedron Letters
jo u rn al h ome p ag e : w ww .e lse vie r .c om
Dyes for Sensitized Solar Cells by Using [2.2]Paracyclophane as a Bridging Unit
Chiung Hui Huang and Yuan Jay Chang∗
Department of Chemistry, Tung Hai University, Taichung 407, Taiwan
ARTICLE INFO
ABSTRACT
Article history:
Received
Organic dyes that consist of a [2.2]paracyclophane moiety between a triphenylamine donor
group and a cyanoacrylic acid acceptor group have exhibited considerably high values of open-
circuit voltage (Voc) in the range of 0.69–0.74 V. In an experiment that involved using an ion
liquid electrolyte (E2 electrolyte), the values of Voc were increased to 0.03–0.04 V because of a
decrease in the concentration of LiI. A typical device demonstrated a maximal incident photon-
to-current conversion efficiency (IPCE) of 60% in the region of 350–475 nm, a short-circuit
photocurrent density (Jsc) of 8.80 mA·cm-2, an open-circuit photovoltage (Voc) of 0.74 V, and a
fill factor (FF) of 0.65, corresponding to an overall conversion efficiency of 4.24% of CPG2b.
The photophysical properties were analyzed using a time-dependent density functional theory
(TDDFT) model with the M062X functional.
Received in revised form
Accepted
Available online
Keywords:
Dye-sensitized solar cells (DSSCs)
[2.2]Paracyclophane
Deoxycholic acid (DCA)
Nonconjugated spacer
2009 Elsevier Ltd. All rights reserved.
Recently, several renewable energy sources have been
developed because of the increased consumption of fossil fuels
and consequent environmental pollution. Solar energy is widely
recognized as a solution to the global energy crisis. Since their
discovery by O’Regan and Grätzel in 1991,1 dye-sensitized solar
cells (DSSCs) are expected to demonstrate promising potential as
an alternative renewable energy source. DSSCs offer the benefits
of low-cost processes and attractive features such as flexibility
and colorful transparency. Although ruthenium-based and zinc-
porphyrin-based sensitizers have achieved high power conversion
efficiencies of over 12%, their use has revealed the problems of
limited resources and elaborate purification.2 Metal-free organic
dyes have the advantages of environmental friendliness, high
structural flexibility, low cost, generally high molar extinction,
and easy preparation and purification. Thus far, metal-free
organic dyes are abundant and demonstrate an energy-to-
electricity conversion efficiency in the range of 7%–10%.3
photocurrent can became a drawback because of the breakdown
of the π-conjugation.
In designing organic dyes for DSSCs, we expanded the π-
chromophore of a [2.2]PCP bridge and introduced a long alkoxy
chain to improve not only the value of short-circuit current (Jsc)
but also the value of open-current voltage (Voc). In our previous
investigation, we discovered that using nonconjugated π-
chromophores for DSSCs is possible by incorporating a unit of
paracyclophane to interrupt the π-system.5 The substituents of
alkoxy chains could reduce not only intramolecule aggregation
but also the dark current by decreasing the rate of charge
recombination. The Voc value of these materials can be expected
to achieve a high standard because of a slow rate of charge
recombination. To enhance the efficiency of electron injection,
we attempted to increase coplanarity of the bridge unit of
[2.2]paracyclophane with an adjacent moiety by connecting an
ethenyl group with the donor and acceptor.
However, conjugated π-chromophores in organic structures
are regarded as indispensable in performing many functional
properties of optoelectronic devices, especially in DSSC
applications. In a previous study, we demonstrated that, as a
NC
COOH
S
N
bridge
unit,
the
nonconjugated
chromophore
of
CP3
[2.2]paracyclophane (PCP) reduced the rate of charge
recombination and led to a high value of Voc. This indicated that a
high energy barrier for charge recombination across a [2.2]PCP
bridge may improve the value of Voc.
R
NC
COOH
R
R
The [2.2]PCP moiety is well known because the migration of
electrons in an organic medium may proceed effectively through
a nonfully conjugated system.4 Electrons can travel across the
gap of parallel-aligned aromatic rings, thereby exhibiting a
detectable flow of current. However, the relatively low value of
N
R
CPG2a (R=H)
CPG2b (R=OC6H13
)
Figure 1. The structure of CP3, CPG2a and CPG2b.
———
∗ Corresponding author: Tel: +886 4 23590248 Ext 305; Fax: +886 4 23590426 (Y. J. Chang). E-mail address: jaychang@thu.edu.tw.

2
Tetrahedron Letters
Two novel organic dyes of the CPG2-series and its synthetic
The absorption spectra of dyes in a diluted THF solution (3 ×
10-5 M) are shown in Figure 2(a), exhibiting three major
absorption regions appearing at 242–245 nm, 306–308 nm, and
400–419 nm. The first two regions were composed of localized
aromatic π-π* transitions, and the final one was assigned to a
charge-transfer (CT) transition. The CT band of CPG2b is 19 nm
and showed more of a redshift because of a higher efficiency of
the inductive effect with the hexyloxy substituents. The influence
of the electron-donating substituent was shown by a comparison
between CPG2a and CPG2b using theoretical models; the latter
exhibited stronger oscillator strengths calculated by TDDFT, that
is, 1.92 and 2.80 (Table S1). These values are consistent with
their CT transition region; both CPG2a and CPG2b showed high
molar extinction coefficient; that is, ε = 6.54 × 104 M-1 cm-1 and
7.02 × 104 M-1 cm-1, respectively (Table 1).
procedure in this study are shown in Scheme 1. A Heck reaction
of pseudo-para-4,12-Diethenyl[2.2]paracyclophane 15 with 4-
Bromobenzaldehyde or 4-Bromo-2,5-dihexyloxy-benzaldehyde
yielded a dialdehyde compound of 2 in the range of 41%–72%.6
The conjugating length was elongated again by inserting a
triphenylamine (TPA) electron-donating unit through a Wittig
reaction TPA-phosphonium salt with 2 in DMF. This yielded a
mono-aldehyde compound of 3 in the range of 56%–66%.7
Knoevenagel condensation was used to complete the two final
products by building up the cyanoacrylic acid acceptor unit,
thereby providing a favorable yield at CPG2a and CPG2b for
73% and 70%, respectively. Two final products were crystallized
into deep red solids and confirmed according to their spectra
(electrochemical impedance spectrum; EIS).
The oxidation potentials (Eox) were measured using cyclic
voltammetry (CV) in THF, and the results are listed in Table 1.
The HOMO levels corresponded to the first oxidation potentials
(Eox) of CPG2a and CPG2b. The LUMO levels were estimated
according to the values of Eox and the zero-zero absorption
wavelength, whereas the latter was obtained at the onset of
absorption spectrum. The HOMO-LUMO energy gap of CPG2b
(2.53) was narrower than that of CPG2a (2.72). The estimated
LUMO levels of both dyes are sufficiently higher than the
conduction band level of TiO2 (ca. -0.5 V vs. NHE), whereas
their HOMO levels are sufficiently lower than that of the
electrolyte ion pair I－/I3－ (ca. 0.4 V vs. NHE).8,9 This ensures an
exothermic flow of charges through the photoelectronic
conversion process. The HOMO-LUMO energy levels of dyes
warrant an efficient charge injection into the photoanode and
charge regeneration from the Pt cathode.
R
O
(ii)
(i)
R
R
56~66%
41~72%
O
1
R
2-X
X=a, R=H
X=b, R=OC6H13
R
O
R
R
(ii)
CPG2a
CPG2b
N
70~73%
R
3-X
Scheme 1. Synthesis of CPG2a and CPG2b. (i): K2CO3, NBu4Br,
Pd(OAc)2, and NEt3. (ii): K2CO3, 18-crown-6, and DMF.
(iii): NH4OAc, CNCH2COOH, and AcOH.
Figure 2. (a) Absorption spectra of CPG2a and CPG2b in THF. (b) Oxidative voltammograms of CPG2-series dyes. (c) Computed
optimum structure and dihedral angles of the CPG2-series dyes. (d) Computed HOMO and LUMO orbitals of CPG2-series dyes.

3
The electronic nature of the dyes was examined using
theoretical models. Full geometrical optimizations were
performed using the M062X/6-31G(d) hybrid functional
implanted in Gaussian09.10 The amine donor and the
cyanoacrylate acceptor are attached to [2.2]PCP with the vinyl
group in a pseudopara orientation. The dihedral angles between
the linker and PCP are considerably wide at 101°–115°. The
charge distribution in the frontier molecular orbitals shows that
the electron distributions in the HOMOs and LUMOs are well
separated by the PCP moiety (ESI Fig. S8), indicating that the π-
conjugation is severely interrupted. The electrons migrate from
the donor moiety to the anchoring group forming a charge
separated state, as evidenced by the high intensity of the CT
absorption band. However, the lower degree of π-conjugation
may slow the rate of charge recombination and maintain a longer
lifetime for the charge-separated state.11
enhancing the Voc value, which increased the gap between the
conduction band of TiO2 and the electrolyte. The relative
quantum efficiency of the CPG2-series was higher than that of
CP3, and the result attributed to the higher efficiency of electron
injection to the conduction band of TiO2. The devices
manufactured using CPG2b and electrolyte E2 performed higher
when exhibiting a Jsc of 8.80 mA·cm-2, a Voc of 0.74 V, and an FF
of 0.65, which totaled an overall quantum efficiency of 4.24%.
The order of Jsc values CPG2b > CPG2a are nearly proportional
to their relative magnitudes of molar absorptivity. The lower Jsc
value (8.65 mA·cm-2) of CPG2a was mainly ascribed to its
narrow absorption bandwidth and location on the ultraviolet
region (Fig. 2a).
However, the purpose of introducing the PCP unit, as
mentioned, is to reduce the rate of charge recombination. The
long chains have also been shown to be capable of reducing the
dark current by decreasing the rate of charge recombination, thus
elevating the Voc of the solar cells. It may effectively improve the
film morphology that covers the surface of TiO2. 12 This effect is
represented by EIS Nyquist and Bode plots in Fig. 3c and Fig. 3d.
The order of the relative values of dark current was found to be
consistent with the magnitude of Voc; that is, CPG2b > CPG2a.
Further information on the lifetime also corresponds with this
trend and is supported byfitting the middle frequency in the Bode
phase plots through the expression τ = 1/(2πf). A longer electron
lifetime indicates a slower charge recombination, thus leading to
a higher value of Voc. The trend of relative quantum efficiency
follows the same order of Jsc and Voc values. The highest
performance of a DSSC device was found in the one
manufactured using CPG2b and electrolyte E2, which exhibited
a Jsc of 8.80 mA·cm-2, a Voc of 0.74 V, and an FF of 0.65. The
overall conversion efficiency (η) was estimated at 4.24%.
The parameters of the DSSC devices fabricated using the two
dyes were short-circuit current (Jsc), open-circuit photovoltage
(Voc), fill factor (FF), and solar-to-electrical photocurrent density
(η), measured under AM 1.5 solar light (100 mW·cm-2). These
parameters are summarized in Table 1, and the photocurrent-
voltage (J-V) plots are shown in Fig. 3a. Two types of electrolyte
were used to achieve the most favorable result: system E1 was
composed of LiI (0.5 M), I2 (0.05 M), and TBP (4-tert-
butylpyridine) (0.5 M) in MeCN, and system E2 was composed
of 3-dimethylimidazolium iodide (DMII)(1.0 M) and
guanidinium thiocyanate (0.1 M), in addition to LiI (0.05 M), I2
(0.03 M), and TBP (0.5 M) in a mixed solvent of MeCN and
valeronitrile (85:15, v/v). The electrolyte system from E1 to E2
was changed to help prevent leakage and increase the Voc value to
ca. 0.03–0.04 V. The lower concentration of LiI in E2 raised the
potential Fermi level of the conduction band of TiO2, thus
Figure 3. (a) J-V curves of the CPG2-series dyes. (b) IPCE plots of the dyes compared with CP3. (c) EIS Nyquist plots of the CPG2-
series dyes in the dark. (d) Bode plots of the CPG2-series dyes.

4
Tetrahedron Letters
Table 1. Photochemical, electrochemical parameters, and photovoltaic devices manufactured using E1 and E2 electrolytes.
b
c
d
dye
λabsanm
(ε/ M-1 cm-1)
400 (65400)
419 (70900)
377 (41700)
---
λ
abs nm
Eox
E0-0
Ered
(V)
Jsc (mA·cm-2)
E1/E2e
Voc (V)
E1/E2
FF
E1/E2
ηf (%)
E1/E2
τ g (ms)
E1/E2
(film)
386
414
---
(V)
0.60
0.55
0.81
1.10i
(V)
2.72
2.53
2.83
2.60 i
CPG2a
CPG2b
CP3h
-2.12
-1.98
-2.02
-1.50 i
8.65/ 8.02
9.28/ 8.80
7.88/ ---
0.69/ 0.72
0.70/ 0.74
0.69/ ---
0.74/ 0.75
0.66/ 0.69
0.65/ 0.65
0.70/ ---
0.62/ 0.60
3.95/ 3.97
4.20/ 4.24
3.83/ ---
7.26/ 7.90
17.30/22.28
19.94/26.04
17.9/---
---
N719
---
15.62/ 17.47
ε: absorption coefficient; Eox: oxidation potential; E0-0: 0-0 transition energy.
a Absorptions were measured in THF. b Oxidation potentials of dyes (10-3 M) in THF containing 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV·s-1 (vs. Fc+/ Fc).
c E0-0 was determined from the onset of the absorption in THF. d Ered was calculated by Eox − E0-0 e Electrolyte 1 (E1): LiI (0.5 M), I2 (0.05 M), and TBP (0.5 M)
.
in MeCN. Electrolyte 2 (E2): 3-dimethylimidazolium iodide (1.0 M), LiI (0.05 M), I2 (0.03 M), guanidinium thiocyanate (0.1 M), and TBP (0.5 M) in
MeCN:valeronitrile (85:15, v/v). f Performance of DSSCs was measured in a 0.25 cm2 working area on an FTO (8Ω/square) substrate under AM 1.5 conditions.
g Values in the blanks were obtained by fitting the middle-frequency in the Bode phase plots through the expression τ = 1/(2πf), where f is the frequency. h see ref.
5. i see ref. 13.
Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M.
Science 2011, 334, 629–633.
In summary, we successfully synthesized two novel DSSCs
materials containing a [2.2]paracyclophane as the bridge moiety
3. a) Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska,
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Acknowledgments
This work was supported by the National Science Council
(NSC 101-2113-M-035-001-MY2) and Academia Sinica in
Taiwan. The molecule weight is grateful for a Postdoctoral
Fellowship in Academia Sinica, Taiwan. The computations were
performed using the computer facilities at the Academia Sinica
Computing Center, Academia Sinica.
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1
The H and 13C NMR spectra of all compounds, absorption
spectra in THF solution, absorption spectra on TiO2 film,
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spectra were used. Supplementary data associated with this
article can be found in the online version, at doi:xxx.
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