Embedding an electron donor or acceptor into naphtho[2,1-b:3,4-b′] dithiophene based organic sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1039/c3cc44258f

An electron donor and acceptor, respectively, is embedded into naphtho[2,1-b:3,4-b′]dithiophene based organic sensitizers to tune their optoelectronic properties. The DSSC based on FNE52 containing an auxiliary electron acceptor displays a maximum power conversion efficiency of 8.2% and good long-term stability.

Full text of DOI:10.1039/c3cc44258f

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Embedding an electron donor or acceptor into
naphtho[2,1-b:3,4-b0]dithiophene based organic
sensitizers for dye-sensitized solar cells†
Cite this: Chem. Commun., 2013,
49, 7445
Received 6th June 2013,
Accepted 25th June 2013
Quanyou Feng, Xiaowei Jia, Gang Zhou* and Zhong-Sheng Wang*
DOI: 10.1039/c3cc44258f
www.rsc.org/chemcomm
An electron donor and acceptor, respectively, is embedded into
naphtho[2,1-b:3,4-b0]dithiophene based organic sensitizers to tune
their optoelectronic properties. The DSSC based on FNE52 containing
an auxiliary electron acceptor displays a maximum power conversion
efficiency of 8.2% and good long-term stability.
Dye-sensitized solar cells (DSSCs) have received considerable
1
¨
attention since the original report in 1991 by Gratzel’s group.
Up to now, DSSCs incorporating ruthenium–polypyridine based
sensitizers and a zinc–porphyrin based co-sensitizer have already
achieved power conversion efficiencies (Z) above 11%2–4 and
12%,5 respectively. In recent years, considerable efforts have also
been made to develop metal-free organic dyes, and impressive
efficiencies in the range of 8–10% have been achieved for the
DSSCs with a liquid electrolyte.6–12
Fig. 1 Chemical structures of sensitizers FNE50, FNE51, and FNE52.
Recently, naphtho[2,1-b:3,4-b0]dithiophene (NDT) and its deriva-
tives have been vigorously investigated in organic photovoltaics13–15
and thin film transistors16,17 due to their unique properties, property of the organic sensitizer can be well tuned by simple
such as coplanarity, highly ordered p-stacked structures, high incorporation of an auxiliary electron-donating group, such as
charge density and high hole carrier mobility. However, to the 3,4-ethylenedioxythiophene (EDOT), or an electron-withdrawing
best of our knowledge, NDT based organic sensitizers have so group, such as benzothiadiazole. Herein, we report the synthesis
far not been explored for applications in DSSCs. In this work, and characterization of three NDT based organic sensitizers (Fig. 1).
a NDT unit is incorporated into the spacer part of the organic It is found that the incorporation of an electron acceptor is superior
dyes (Fig. 1). There are several advantages for the incorporation to that of an electron donor with respect to broadening of the
of the NDT unit into the p-conjugated spacer of organic absorption spectrum and improvement of the DSSC performance.
sensitizers. Firstly, the large planar conjugated system can As a result, the Z value significantly increases from 5.2% for FNE50
delocalize the p-electrons, which facilitates the efficient charge based DSSC to 8.2% for FNE52 based DSSC with a liquid electrolyte.
transfer through a NDT based bridge. Secondly, two linear Most importantly, the DSSC based on FNE52 with a quasi-solid-
alkoxyl chains can be easily introduced at the 5- and 6-positions state electrolyte displays an Z of 7.1% which remains 98% of the
on the NDT unit, which not only increase the solubility of the initial value after continuous light soaking for 1000 h.
resulting organic dyes but also minimize the formation of
The synthetic route to sensitizers FNE50, FNE51, and
molecular aggregates of the resulting dyes and reduce the charge FNE52 is depicted in Scheme S1 (ESI†), which starts from
recombination in the DSSCs. Thirdly, the charge transfer 5,6-dioctyloxynaphtho[2,1-b:3,4-b0]dithiophene.14 Details of the
synthetic route towards the resulting sensitizers are provided in
Department of Chemistry & Laboratory of Advanced Materials, Fudan University,
the ESI.† All the target compounds were characterized using
1H NMR, 13C NMR spectroscopy, and mass-spectrometry, and
2205 Songhu Road, Shanghai 200438, P. R. China.
E-mail: zhougang@fudan.edu.cn, zs.wang@fudan.edu.cn; Fax: +86-21-5163-0345;
were found to be consistent with the proposed structures.
Tel: +86-21-5163-0345
The UV-vis absorption spectra of the resulting sensitizers in
† Electronic supplementary information (ESI) available: Synthesis and characteri-
zation details for the resulting sensitizers. See DOI: 10.1039/c3cc44258f
THF solutions (ca. 10ꢀ5 M) are shown in Fig. 2, and their
c
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ꢀ
that all the HOMO values are more positive than Iꢀ/I3 redox
couples (B0.4 V), indicating that the reduction of the oxidized
dyes with Iꢀ ions is thermodynamically feasible. Correspondingly,
from HOMO levels and the optical band gap derived from the
wavelength at 10% maximum absorption intensity for the dye-
loaded TiO2 film,7 the LUMO levels for sensitizers FNE50,
FNE51, and FNE52 are calculated to be ꢀ1.24, ꢀ1.16, and
ꢀ1.00 V, respectively, indicating enough driving force of the
electron injection from the excited states of all the sensitizers.
As shown in Fig. S2 (ESI†), it should be noted that the embedding
of EDOT into the spacer part lifts up the HOMO level of the FNE51
dye relative to that of the FNE50 dye. This can be attributed to
the contribution of the increased electron density, which results
in a more delocalized HOMO. Upon introducing benzothiadiazole
into the sensitizer’s bridge, both HOMO and LUMO levels are
positively shifted through a contributing inductive effect,19 but
Fig. 2 Absorption spectra of sensitizers FNE50, FNE51, and FNE52 in THF solutions.
The absorption spectra of dye-loaded TiO2 films are shown as the inset.
absorption data are summarized in Table S1 (ESI†). As shown in the degree of shift of the HOMO level is smaller than that of the
Fig. 2, FNE50 displays the maximum absorption wavelength LUMO level. Consequently, the band gap of FNE52 decreases
at 471 nm. Upon incorporation of an EDOT unit, an electron- compared with that of FNE50.
donating group, in the p-conjugated spacer of the FNE50 dye, a
The DSSCs based on the resulting sensitizers with coadsorption
significant bathochromic shift of 24 nm, as well as increased of deoxycholic acid were fabricated using a liquid electrolyte
absorption intensity can be observed for sensitizer FNE51 due containing 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide
to the extended p-conjugation. After introducing an electron (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M tert-butylpyridine
withdrawing group, benzothiadiazole, into the organic dye skeleton, (TBP) in acetonitrile. The solar-to-electricity conversion efficiencies
a further bathochromically shifted maximum (526 nm) along of DSSCs were evaluated by recording the photocurrent density–
with an increased absorption coefficient (4.03 ꢁ 104 Mꢀ1 cmꢀ1
)
voltage ( J–V) characteristics at 100 mW cmꢀ2 simulated
can be found for the maximum absorption band of sensitizer AM1.5G solar light. As shown in Fig. 3, the short-circuit photo-
FNE52. Compared with the absorption maxima of sensitizer current density ( Jsc), open-circuit photovoltage (Voc), and fill
FNE50, the degree of the bathochromic shift and the increased factor (FF) of the FNE50 based DSSC are 10.59 mA cmꢀ2
,
absorption intensity for sensitizer FNE52 are both larger than 0.726 V, and 0.68, respectively, yielding an Z of 5.2%. The
those for sensitizer FNE51. This is obviously due to the DSSCs based on FNE51 and FNE52 produce Z of 5.5%
strengthened intramolecular charge transfer interactions in ( Jsc = 13.84 mA cmꢀ2, Voc = 0.615 V, FF = 0.65) and 8.2%
FNE52 caused by the introduction of an extra acceptor group ( Jsc = 16.36 mA cmꢀ2, Voc = 0.694 V, FF = 0.72), respectively.
in addition to the conjugation extension. The UV-vis absorption Under the same conditions, the standard cell fabricated with
spectra of the dye-loaded TiO2 films (B2 mm) are shown in the the well-known N719 dye2 exhibits an efficiency of 8.5%
inset of Fig. 2, which display the same trend for the absorption ( Jsc = 16.02 mA cmꢀ2, Voc = 0.737 V, FF = 0.72).
maxima as those for the absorption spectra in solutions. More-
To understand the enhancement of the Jsc value after
over, as compared with the maximum absorption band in incorporation of an auxiliary electron donor or acceptor in
solution, FNE50 displays a hypsochromically shifted absorption the dye molecule, action spectra of the incident photon-to-
maxima (11 nm) for the TiO2 film. This phenomenon has been current conversion efficiencies (IPCE) as a function of incident
observed for most of the organic sensitizers and is due to the
deprotonation of the cyanoacrylic acid group. However, upon
adsorption to the nanocrystalline TiO2 surface, almost no
shift in the absorption maximum but a broadened spectrum
can be observed for sensitizers FNE51 and FNE52, respectively,
which is beneficial to light-harvesting and short-circuit current
enhancement.
Suitable highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels of the
sensitizer are required to match the redox potential of the
redox couple in the electrolyte and the conduction band edge
of the nanocrystalline TiO2 for efficient sensitizer regeneration
and electron injection.18 As revealed by the cyclic voltammetry
spectra of the dye-loaded TiO2 films (Fig. S1, ESI†), the HOMO
level corresponding to the first half-wave potential is deter-
mined to be 0.87, 0.79, and 0.92 V (vs. NHE, the same below) for
Fig. 3 J–V curves for the DSSCs based on FNE50, FNE51, and FNE52. IPCE spectra
sensitizers FNE50, FNE51, and FNE52, respectively. It is clear are shown as the inset.
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parameters of FNE52 based quasi-solid-state DSSCs show a
best performance of 7.1% ( Jsc = 15.84 mA cmꢀ2, Voc = 658 mV,
FF = 0.68). Moreover, as revealed in Fig. S4 (ESI†), the quasi-
solid-state DSSC based on FNE52 remains as 98% of the initial
overall efficiency value after 1000 h of visible-light soaking,
indicating that sensitizer FNE52 is sufficiently stable for appli-
cation in DSSCs.
In summary, an electron donor and an electron acceptor,
respectively, is embedded into NDT based organic sensitizers.
It is found that the incorporation of a benzothiadiazole unit is
superior to that of a EDOT unit in broadening the absorption
spectrum of the sensitizer. Consequently, FNE52 based DSSCs
with a liquid and a quasi-solid-state electrolyte display an Z of
8.2% and 7.1%, respectively, and the Z value of the latter quasi-
solid-state DSSC remains as 98% of the initial value after
continuous light soaking for 1000 h.
Fig. 4 Electron lifetime as a function of electron density at the open circuit for
DSSCs based on the resulted sensitizers.
wavelength are recorded. As shown in the inset of Fig. 3, the
FNE50 based DSSC displays a broad solar light response with a
maximum value above 80%. As the effective conjugation length of
the sensitizer molecules gets longer via embedding of EDOT or a
benzothiadiazole unit into the molecular skeleton, the IPCE
spectral response for the DSSCs based on FNE51 and FNE52,
respectively, gets broader compared with that for the FNE50 based
DSSC, which is in good agreement with their UV-vis absorption
results (Fig. 2) and beneficial to light-harvesting and photocurrent
generation. With such an extended IPCE response, the DSSCs
based on FNE51 and FNE52 provide a higher photogenerated
current, respectively, as compared with the FNE50 based DSSC.
In contrast to the behavior of the short-circuit photocurrent,
the FNE50-sensitized solar cell exhibits the highest Voc among
these cells based on NDT dyes. To investigate the reason for the
differences between the Voc values, electron lifetime and electron
density are measured using the intensity modulated photo-
voltage spectroscopy (IMVS) measurement and charge extraction
method.20,21 Fig. 4 shows the electron lifetime as a function
of electron density at the open circuit for the DSSCs based on
the resulting sensitizers. The lifetime increases in the order
FNE51 o FNE52 o FNE50, indicating the lowest charge
recombination rate in FNE50 based DSSC. The retarded charge
This work was financially supported by the National Basic
Research Program (2011CB933302) of China, the National Natural
Science Foundation of China (90922004 and 51273045), NCET-
12-0122, Shanghai Pujiang Project (11PJ1401700), STCSM
(12JC1401500), Shanghai Leading Academic Discipline Project
(B108), and Jiangsu Major Program (BY2010147).
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