Paracyclophane as a bridging unit in the design of organic dyes for sensitized solar cells

Article abstract of DOI:10.1039/c1cc16248a

Organic dyes consisting of a [2.2]paracyclophane unit along the main chromophore are examined for their application in sensitized solar cells. These materials exhibit considerably high values of open-circuit voltage (V _oc) ranging 0.69-0.72 V, and an overall efficiency up to 3.8%.

Full text of DOI:10.1039/c1cc16248a

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[2.2]Paracyclophane as a bridging unit in the design of organic dyes for
sensitized solar cellsw
Yuan Jay Chang,a Motonori Watanabe,b Po-Ting Choua and Tahsin J. Chow*b
Received 8th October 2011, Accepted 10th November 2011
DOI: 10.1039/c1cc16248a
Organic dyes consisting of a [2.2]paracyclophane unit along the
main chromophore are examined for their application in sensitized
solar cells. These materials exhibit considerably high values of
open-circuit voltage (Voc) ranging 0.69–0.72 V, and an overall
efficiency up to 3.8%.
linked by a triarylene bridge (B), performed the best in DSSCs
with a remarkable quantum efficiency around 7%.5 In the
bridging units of these compounds the adjacent phenylene
rings are not fully co-planar, but twisted ca. 351 with respect to
each other. Such a slight distortion does not interrupt much
the p-conjugation between the D and A, as evidenced by their
absorption spectra. The p-conjugation along the bridge does
contribute significantly to the values of short-circuit current
(Jsc) in DSSC. In order to increase the value of open-current
voltage (Voc), which is another key factor determining the
efficiency, it is necessary to reduce the rate of charge recombi-
nation after absorption of light. In the case of triphenylene,
the dihedral angles between adjacent phenyl rings twist to a
larger angle in the charge-separated state. The highly twisted
geometry along B therefore reduces the rate of charge recom-
bination between D and A, and extends the lifetime of the
charge-separated state. The flexible rotation of the phenyl ring
along the bridge is believed to have played a critical role in
promoting the quantum efficiency of DSSC.
Conjugated p-chromophores in organic structures are regarded
as an indispensable part in performing many functional pro-
perties, especially in the applications of opto-electronic devices.
The electrons can be delocalized over a long distance through
the overlapping of p-orbitals. With proper doping, organic
materials can become significantly conductive. However, it is
also well-known that the migration of electrons in an organic
medium may proceed effectively through a not fully conjugated
system.1 For example, the electrons may tunnel through s-bonds
or migrate across non-bonded connections at high rate. A typical
case can be observed in the development of organic field-effect
transistors, where an intermolecular p–p interaction across
stacked aromatic rings has played the key factor. Cyclophanes
are a special group of aromatic compounds which consist of two
or more phenyl units locked face-to-face by aliphatic chains.2
Electrons and holes can move across the gap of parallel-aligned
aromatic rings to display a detectable flow of current.3 In our
design of dye-sensitized solar cells (DSSCs), the higher energy
barrier for charge recombination across a cyclophane bridge
may be beneficial to the promotion of quantum efficiency. The
concept of inserting a non-conjugated bridge in organic dyes
has never been investigated before. In this report the effect of
using a [2.2]paracyclophane (PCP) in the structure of DSSC
materials is examined for the first time.4
In this report four compounds containing a PCP unit at the
centre of their structures are prepared, i.e., CP1–CP4 (Fig. 1).6
A triarylamine donor group and a cyanoacrylate acceptor
group are separated by the PCP and located on different sides.
The synthetic procedures of CP1–CP4 are depicted in Scheme 1
(ESIw S1). The pseudo-para dibromide 1 was prepared first
from PCP through a bromination reaction.7 The triarylamine
moieties were added onto 1 by a Stille coupling reaction
catalyzed by PdCl2(PPh3)2.8 The experimental details for the
preparation of all the derivatives of 2 are given in the ESI.w
Compound 2 was formylated with DMF to yield 3, in which
the carbaldehyde group was condensed with a cyanoacetic
acid by a Knoevenagel reaction in the presence of ammonium
acetate leading to CP2 in 72% yield.9 The conversion of 2 to 4
In the choice of dye structures, our previous experience
indicated that compounds consisting of an arylamine electron
donor (D) and a cyanoacrylate electron acceptor (A), both
a Department of Chemistry, National Taiwan University, No. 1,
Sec. 4, Roosevelt Rd., Da’an Dist., Taipei 10617, Taiwan.
E-mail: d93223006@ntu.edu.tw; Fax: +886-2-27884179;
Tel: +886-2-27898641
b Institute of Chemistry, Academia Sinica, No. 128, Sec. 2,
Academia Rd., Nankang Dist., Taipei 115, Taiwan.
E-mail: tjchow@chem.sinica.edu.tw; Fax: +886-2-27884179;
Tel: +886-2-27898552
w Electronic supplementary information (ESI) available: Experimental
data (CP1–CP4), the 1H and 13C NMR spectra, theoretical calcula-
tion, CV spectra, EIS spectra, and electron diffuse lifetime spectra. See
DOI: 10.1039/c1cc16248a
Fig. 1 Structures of compounds CP1–CP4.
c
726 Chem. Commun., 2012, 48, 726–728
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p–p* transitions. The presence of alkoxy substituents in CP4
induces a slight red-shift with respect to CP3. The presence of
the PCP in the main chromophore does impose a significant
effect on the CT transition.
The oxidation potentials (Eox), corresponding to the highest
occupied molecular orbital (HOMO) level of the dyes, were
measured by cyclic voltammetry. The ionization energy
obtained in THF solutions decreased gradually in the order
of CP1 (0.88 V) > CP2 (0.82 V) > CP3 (0.81 V) > CP4 (0.57 V)
(Table 1). A comparison between CP1 and CP2 is particularly
noteworthy, as it reveals an effect by shifting the position of
PCP. The low ionization energy of CP4 is caused mainly by
the resonance effect of alkoxy substituents. Suitable HOMO
and LUMO (lowest unoccupied molecular orbital) levels are
required to match the conduction band (À0.5 V vs. NHE) of
the TiO2 electrode and the redox potential (0.4 V vs. NHE) of
the iodine/iodide electrolyte. The LUMO levels of the dyes
were estimated by the values of Eox and the 0–0 band gaps at
the onset of absorption spectra. The energy levels of all dyes
match well with the requirement for the injection of electrons
to the conductive band of TiO2.
Scheme 1 Reagents and conditions of the synthesis: (i) PdCl2(PPh3)2,
N-(4-(tributylstannyl)phenyl)-N-phenylaniline, N-(4-(5-tributylstannyl-
thiophen-2-yl)phenyl)-N-phenylaniline, or N-(4-(5-tributylstannylthio-
phen-2-yl)phenyl)-N-(4-hexyloxyphenyl)-4-hexyloxyaniline, DMF, 90 1C.
(ii) n-BuLi, DMF, THF, À78 1C. (iii) (a) PdCl2(PPh3)2, (5-(1,3-dioxolan-
2-yl)thiophen-2-yl)tributylstannane, DMF, 90 1C; (b) AcOH : THF : H2O
(4 : 2 : 1), 60 1C. (iv) Cyanoacetic acid, NH4OAc, AcOH, 90–100 1C.
was accomplished by another Stille coupling reaction with
(5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane, followed
by hydrolysis. The final step for the production of CP1, CP3
and CP4 was another Knoevenagel condensation to fuse a
cyanoacrylic acid moiety as the electron acceptor. All struc-
tures were confirmed by their spectroscopic data.
The UV-Vis absorption spectra of organic dyes in THF
solution are depicted in Fig. 2 and the parameters are listed in
Table 1. The compounds with a thiophenyl group next to the
cyanoacrylate acceptor (A), i.e., CP1, CP3 and CP4, display
longer absorption wavelengths than that without, i.e., CP2.
The spectra of CP2, CP3 and CP4, in which a thiophenyl group is
linked directly to the triarylamine donor (D), exhibit two major
absorption bands at 295–305 nm and 364–383 nm. The two shorter
wavelength bands were assigned to localized p–p* transitions, and
the long wavelength one to a charge-transfer (CT) transition.
The intensity of CT transitions is higher than that of the
The electronic nature of the dyes was examined by theoret-
ical models. Full geometrical optimizations were performed by
using the M062X/6-31G(d) hybrid functional implanted in
Gaussian09.10 The triarylamine donor and the cyanoacrylate
acceptor are attached to PCP in a pseudo-para orientation. The
dihedral angles between the aryl groups and PCP are quite
large, i.e., 651–751. The charge distribution in the frontier
molecular orbitals shows that the electron distributions in the
HOMOs and LUMOs do not mix with each other across the
PCP moiety (ESIw Fig. S14), indicating that the p-conjugation
is heavily interrupted. Upon exposure to light, electrons migrate
from D to A forming a charge separated state, as evidenced
by the high intensity of the CT absorption band. However,
the lower degree of p-conjugation may slow down the rate of
charge recombination, and maintains a longer lifetime for the
charge separated state.11
The parameters of DSSCs fabricated with these dyes, i.e.,
short circuit current (Jsc), open-circuit photovoltage (Voc),
fill factor (ff), and solar-to-electrical photocurrent density (Z)
measured under AM 1.5 solar light (100 mW cmÀ2), are
summarized in Table 1, and the photocurrent–voltage
(J–V) and incident photon to current conversion efficiency
(IPCE) plots are shown in Fig. 3 and 4. Among all dyes, the
devices made with CP3 performed the best. The order of
Jsc values CP3 > CP4 > CP1 is roughly proportional to
Fig. 2 Absorption spectra of organic dyes in THF.
Table 1 Photochemical and electrochemical parameters of the dyes
labs (nm)/(e (MÀ1 cmÀ1)) Eox (V) (vs. NHE) E0–0 (eV) Ered (V) (vs. NHE) Jsc (mA cmÀ2
)
Voc (V)
ff
Zd (%) te (ms)
a
b
c
Dye
CP1
CP2
CP3
CP4
N719
373(26 600)
364(38 700)
377(41 700)
383(39 000)
—
0.88
0.82
0.81
0.57
—
2.85
2.97
2.83
2.81
—
À1.97
À2.15
À2.02
À2.24
—
6.68
4.37
7.88
7.12
0.71
0.70
0.69
0.715
0.755
0.71 3.11
0.70 2.13
0.70 3.83
0.65 3.32
0.59 6.68
20.4
19.1
17.9
21.5
22.6
15.0
e: absorption coefficient; Eox: oxidation potential; E0–0: 0–0 transition energy measured at the onset of absorption spectra; Jsc: short-
circuit photocurrent density; Voc: open-circuit photovoltage; ff: fill factor; Z: total power conversion efficiency. t: electron diffuse lifetime of
b
DSSCs. a Absorption in THF. Oxidation potential in THF (10À3 M) containing 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV sÀ1 (vs. NHE).
c
d
e
Ered calculated by Eox À E0–0
.
Performance of DSSC measured in a 0.25 cm2 working area on a FTO (8 O/square) substrate. Lifetime (t) of
injected electrons measured by transient photovoltage at open circuit in acetonitrile using LiI (0.5 M) as electrolyte.
c
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Chem. Commun., 2012, 48, 726–728 727

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will not jeopardize the light harvesting property of the materials.
The non-conjugated chromophore may have an advantage of
reducing the rate of charge recombination, and lead to a high
value of Voc. In the present study the drawback of our materials
is mainly due to their narrow bandwidth in the absorption
spectra, which yielded low values of Jsc with respect to N719.
The concept of using a non-conjugated spacer group may
broaden our vision in the future design of organic dyes.
Notes and references
Fig. 3 J–V curves of the DSSC devices made with the dyes. The plots
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In summary, we have successfully demonstrated that inserting
a [2.2]paracyclophane moiety in the main chain of organic dyes
c
728 Chem. Commun., 2012, 48, 726–728
This journal is The Royal Society of Chemistry 2012