High-Performance Organic Dyes with Electron-Deficient Quinoxalinoid Heterocycles for Dye-Sensitized Solar Cells under One Sun and Indoor Light

Article abstract of DOI:10.1002/cssc.201900505

A series of Y-shaped sensitizers incorporating quinoxaline or quinoxalinoid moieties were prepared and applied in dye-sensitized solar cells (DSSCs). By the introduction of quinoxalinoid functionalities, the absorption extinction coefficients could be enh

Full text of DOI:10.1002/cssc.201900505

Accepted Article
Title: High Performance Organic Dyes with Electron-Deficient
Quinoxalinoid Heterocycles for Dye-Sensitized Solar Cells under
One Sun and Indoor Light
Authors: Man Ling Jiang, Jun-Jie Wen, Zi-Ming Chen, Wen-Hsuan
Tsai, Tzu-Chau Lin, Tahsin J. Chow, and Yuan Jay Chang
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To be cited as: ChemSusChem 10.1002/cssc.201900505
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DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
Title: High Performance Organic Dyes with Electron-Deficient Quinoxalinoid
Heterocycles for Dye-Sensitized Solar Cells under One Sun and Indoor Light
Man Ling Jiang, Jun-Jie Wen, Zi-Ming Chen, Wen-Hsuan Tsai, Tzu-Chau Lin,* Tahsin J.
Chow,* and Yuan Jay Chang*
((Optional Dedication))
M. L. Jiang, J.-J. Wen, W.-H. Tsai, Prof. T. J. Chow, Prof. Y. J. Chang
Department of Chemistry, Tunghai University, No.1727, Sec.4, Taiwan Boulevard, Xitun
District, Taichung 40704, Taiwan
E-mail: jaychang@thu.edu.tw
Z.-M. Chen, Prof. T.-C. Lin
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan
Keywords: Dyes sensitized solar cells, co-sensitization, quinoxaline, high performance under
indoor light
A series of Y shaped D-A-π-A sensitizers incorporating quinoxaline or quinoxalinoid
moieties (MM-1~MM-6) were prepared and applied onto dye-sensitized solar cells (DSSCs).
By the introduction of quinoxalinoid functionalities, the absorption extinction coefficient can
be enhanced. The molecular structures were modified by introducing an extra acceptor group
between the donor and the π-bridge, and also by incorporating electron donating substituents
at various positions. Dyes MM-5 and MM-6 were found to perform remarkably good light-
harvesting efficiency under both sunlight and indoor light. The DSSCs made with mixtures of
MM-3 and MM-6 displayed efficiency 7.92% under one sun (AM 1.5G). While operated
under indoor light, the efficiency was boosted to 27.76, 28.74, and 30.45% under 600 & 1000
& 2500 lux illuminations, respectively. The high performance of MM-3 and MM-6 mixtures
can be ascribed partly to a better dye-coverage on the TiO2 surface.
1. Introduction
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Despite intermittent problems, solar energy is the most abundant renewable energy source and
generally recognized to be a promising alternative energy source. Solar energy can be
harvested and converted into electricity for practical applications by various technologies such
as silicon-based solar cells (SSCs), dye-sensitized solar cells (DSSCs), organic photovoltaic
(OPVs) and perovskite solar cells (PSCs). In the past two decades, dye-sensitized solar cells
(DSSCs) have received considerable attention due to their facile fabrication process,
lightweight, cost-effective device module, and suitability for indoor applications, compared
with silicon-based photovoltaic cells.1-4 Current researches have demonstrated even a new
trend of an integration system by using a single device to harvest the energy and to store it
simultaneously. These integrated solar cells combine the technologies of the new-generation
solar cells (SSCs, DSSCs, OPVs, and PSCs) and the storage devices (lithium-ion batteries
(LIBs) and supercapacitors (SCs)) together.5-6
One of the most crucial components in the development of DSSC is the design of high-
performance organic sensitizers. O’Regan and Grätzel have reported the deposition of
ruthenium-based sensitizers N3, N719, and black dye onto nanoporous TiO2, and achieved an
optimal power conversion efficiency over 11%.7-10 Another report using porphyrin and
indenoperylene dyes has achieved a maximal efficiency up to 12%. However, the production
of these dyes were hampered by their low yields, complicated synthetic procedures, and
difficulty of purification.11-12 Yano and Hanaya used an alkoxysilyl anchored dye (ADEKA-1)
as a co-sensitizer along with the metal-free organic dye LEG-4 to achieve a high performance
of 14.3 %.13 Current reports had also showed the efficiency of DSSCs can be optimized by
using a cobalt electrolyte and a Pt-free counter electrode.14-15
An enormous amount of metal-free organic dyes has been investigated and developed for
DSSCs. Organic dyes have the advantages of environmental friendliness, lower cost, higher
structural flexibility, and easier preparation, etc. Most of them are molecules of linear shape
that consists of an electron donor (D) and an electron acceptor (A) that are separated by a π-
2
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bridge to form a dipolar D-π-A system.
Manipulating the morphology of sensitizer on nanocrystalline TiO2 surface is expected to
enhance electron injection and therefore suppresses the rate of charge recombination.
Increasing the loading amount on TiO2 surface may lead to a more compact dye layer, which
can prevent the electrode from direct contact with the electrolyte, and therefore improves the
photocurrent (Jsc). A good compact layer is usually formed with controlled intermolecular
aggregation. In generally, there are two ways to control intermolecular aggregation: the first is
by co-absorbing sterically rigid molecules, such as deoxycholic acid derivatives (DCA or
CDCA) that can help the sensitizer align parallel to each other and stand vertical to the
surface of TiO2. The second way is by introducing sterically hindered alkyl chains to the
sensitizer molecules to enhance their coverage on TiO2 surface. A commonly used strategy of
structural design is to synthesize X- or Y-shaped rigid molecules attached with long alkyl
substituents.16-19
Recently, Zhu designed a new type organic sensitizer in ”D-A-π-A” system by inserting
an extra A group between the D group and the π-bridge.20-22 These molecules displayed better
photovoltaic performance by broadening the absorption spectra, therefore extending the light-
absorption range. A variety of auxiliary A groups have been reported previously, such as
benzothiadiazole,23-27 benzotriazole,28-30 diketopyrrolopyrrole,31-34 fluoro-substitented,35-36
isoindgo,37-38 quinoxaline,39-43 thioazole,44-48 and triazine49 to tailor the absorption energy
levels. In our earlier report, we have shown that by the introduction of quinoxaline and
quinoxalinoid functionalities, the absorption extinction coefficient may be doubled.50-51 In this
work, we continue these designs by incorporating quinoxalinoid groups in six Y-shaped D-A-
π-A systems (MM-1~MM-6), and apply them onto DSSCs (Figure 1). The synthetic
procedures are shown in Scheme 1, whilst all compounds were confirmed by spectroscopic
analysis.
The effect of inserting quinoxaline or quinoxalinoid into the dye structure is apparent. The
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dye with a phenothiazine moiety at the 5-position of quinoxaline (MM-5) displayed higher
photocurrent and PCE values under AM1.5 solar condition (100 mW∙cm-2) than those without
(MM-3 and MM-4). This observation is consistent with our previous reports, due to more
effective formation of an intramolecular charge transfer state.52-53 Although the bulky moiety
of 9,9-dihexyl-N,N-diphenyl-9H-fluoren-2-amine can increase the absorption intensity
through π-π* transitions, it also reduces the dye-loading amount on TiO2 surface. The
sensitizer structure was further modified by replacing the bulky moiety by a smaller phenyl
ring, and the resulted MM-6 displayed a high conversion efficiency of 6.96% with short-
circuit current (Jsc) 15.5 mA∙cm-2, open-circuit voltage (Voc) 701 mV and fill factor (ff) 0.64
under DCA treatment at AM1.5 solar condition.
The effect of panchromatic co-sensitizationIn is also evaluated, which may enhance the
performance of DSSCs by improving film morphology and dye-coverage.22, 54-59 We found
that upon co-sensitization of MM-3 and MM-6 the intensity of light absorption increased due
to an improvement of dye-coverage on TiO2 surface (Table S3). The best performance of the
device with co-sensitization of MM-3 and MM-6 displayed a short-circuit current (Jsc) 17.7
mA∙cm-2, an open-circuit voltage (Voc) 717 mV, a fill factor (ff) 0.63, corresponding to an
overall conversion efficiency 7.92% under AM1.5 solar condition.
In addition to sunlight, indoor light is also essential to our daily life. Indoor light is used to
brighten our offices, factories, hospitals, stores, laboratories, libraries, production halls, first
aid rooms, and showrooms, etc., where most people spend more than half of their lifetimes.
The intensity of indoor light, mostly generated by luminescent devices such as fluorescent and
LED lamps, rages typically in 300–1500 lux. It would be beneficial if indoor light can be
harvested to provide the electric power of certain indoor applications. In this regard, DSSC is
recognized to be a more efficient device than silicon cells, because it performs better in longer
long wavelength region.
It is encouraging that through optimizing the spectral match with light sources (TL84,
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D65 and CWF), many effective organic sensitizers have been developed. Grätzel and
Hagfeldt's group has demonstrated that by using XY1 and D35 dyes with a copper-based
electrolyte the PCE can be promoted to a remarkable level of 28.9% under 1000 lux of
fluorescent light.60 Wei and Yeh's group reported that an acetylene-based anthracene sensitizer
TY6 yielded 28.56% PCE under 6000 lux T5 fluorescent light.61 Recently, Lin and Wei used
pyrazine-incorporated panchromatic sensitizer MD7 that reached PCE values of 18.85% and
27.17%, respectively, under 300 lux and 6000 lux of T5 fluorescent light.62 By mixing our
dyes MM-3 and MM-6, we achieved PCE values of 27.76%, 28.74% and 30.45% under 600
lux, 1000 lux and 2500 lux, respectively, under TL84 fluorescent light.
<< Figure 1 >>
2. Results and discussion
2.1 Synthesis of Sensitizers
The synthesis of compounds MM1~MM-6 started from 2,7-dibromo-9,9-dihexyl-9H-fluorene
(1) (Scheme 1). The bromo group of 1 was first modified by condensing it with an arylamine
via a Buchwald-Hartwig reaction to produce compound 2. The central acetylene moiety was
constructed by coupling two units of 2 with acetylenedicarboxylic acid simultaneously
through a Sonogashira reaction to yield compounds 3 in 60% yield.50 Compound 3 was then
subjected to an oxidation with KMnO4 to afford the -diketone 4 in 90%. The reaction of
diketone 4 with various quinoxalinoid ortho-diamines produced the respective products of 5,
7 and 12.51 The length of π-bridge can be elongated by inserting an extra thiophene or
phenylene unit through Stille coupling reaction to produce the aldehydes 6, 8, 15 and 17 in
60%–76%. Finally, all the aldehydes were subjected to Knoevenagel condensation reactions
to yield the target compounds. More details of the synthesis were described in ESI†.
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<< Scheme 1 >>
2.2 Absorption spectra
The ultraviolet–visible absorption spectra of all dyes, measured in 3.0 × 10−5 M CH2Cl2
solutions, were shown in Figure 2. The MM-series dyes exhibited two broad, high-intensity
absorption peaks in the range of 359–484 nm. The short wavelength absorptions (265–383 nm;
3.78-8.87x104 M-1cm-1) are attributed to localized π–π* transitions, whereas the long
wavelength ones (435–484 nm; 3.57-8.62x104 M-1cm-1) are assigned to the characteristic
intramolecular charge transfer (ICT) transitions between the donor group and acceptor
cyanoacrylic acid moiety. Compared with phenylene π-spacers MM-1 and MM-3, an
extension of π-conjugation by the insertion of a thiophene π-linkage in MM-2 and MM-4 red-
shifts the absorption spectra by approximately 18–22 nm. The ICT bands of MM-5 and MM-
6 are quite similar to each other, however, in the π–π* transition region the absorption
intensity of MM-5 displays a higher intensity due to the presence of two substituents 9,9-
dihexyl-N,N-diphenyl-9H-fluoren-2-amine at the 2,3-positions of quinoxaline. Comparing
with the absorption spectra in solutions, the spectra of dyes in solid films on TiO2 exhibit
similar yet broader ICT bands due to a stronger interaction between the dyes and TiO2 surface.
The ICT bands of MM-1 and MM-2 exhibit slight red-shift as a result of intermolecular J-
aggregation. The blue-shifted ICT bands of MM-4 and MM-6 may be ascribed either to H-
aggregation or to the deprotonation of cyanoacrylic acid moiety.63-64 By measuring the offset
of absorption band, the zero-zero energy of all dyes can be estimated to be in the ranges of
2.07–2.54 eV.
<< Figure 2 >>
2.3 Cyclic voltammetry
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To examine the electro-stability and the HOMO and LUMO energies of dyes MM-1~MM-6,
cyclic voltammetry experiments were performed. All the MM-series dyes displayed nearly
reversible waves (Figure S26). Repeated scans (15 cycles) were performed on dyes MM-3
and MM-6, while no apparent difference was observed on the redox waves (Figure S27).
This result indicated that our dyes are electrochemically stable under an ambient condition.65-
67
The donor and acceptor moieties at the 5,8-position of quinoxaline (MM-5 and MM-6)
strongly influence their oxidation potentials. The HOMO energy levels of MM-5 and MM-6
are 0.19–0.22 eV higher than those of MM-1~MM-4. It leads to narrower HOMO–LUMO
energy gaps in a range of 0.14-0.47 eV (Figure 3). The introduction of fluorene side arms to
quinoxalinoid of MM-1~MM-4 shows virtually no effect to the HOMO levels, nevertheless
the HOMO–LUMO gaps of MM-1 and MM-2 are slightly smaller. This result is consistent
with the red-shift of ICT bands in the absorption spectra, and can be verifies by the difference
of dipole moments according to theoretical computations (Table S1 and Figure S24). The
narrower HOMO–LUMO energy gaps of MM-2 and MM-4, compared with those of MM-1
and MM-3, are apparently induced by the presence of a thiophene unit. The LUMO levels of
the dyes were determined by measuring the Eox and the zero–zero band gaps at the offset of
the absorption spectra (Figure 2a and Table 1). The estimated HOMO and LUMO levels are
illustrated in Figure 3. The variation of energy levels was analysed by using theoretical
computations as discussed in the following.
<< Figure 3 >>
2.4 Theoretical computations
The optimized structures of all compounds were obtained by time-dependent density
functional theory (TD-DFT) calculations using B3LYP/6-31G(d) basis set in Gaussian 16W.
Compounds containing phenylene groups, i.e. MM-1 and MM-2, displayed a similar degree
of co-planarity comparing with others that possessing thiophene groups (Figure S24). The
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electron density distributions in the frontier orbitals of all dyes are illustrated in Figure 4 and
Figure S25. The electron density in the HOMOs is mainly localized around the arylamine
donor moieties, while that of the LUMOs around the cyanoacrylic acid acceptor moieties. The
calculated HOMO/LUMO levels were −4.89/−2.71 eV for MM-1, −4.89/−2.79 eV for MM-2,
−4.88/−2.62 eV for MM-3, −4.92/−2.70 eV for MM-4, −4.83/−2.73 eV for MM-5, and
−4.83/−2.82 eV for MM-6. These results are consistent with the observed λmax values in the
absorption spectra, i.e., 569 nm, 590 nm, 549 nm, 558 nm, 590 nm, and 617 nm for MM-
1~MM-6, respectively.
Upon photoexcitation an electron is promoted from the HOMO to the LUMO and
therefore it induces a considerable amount of electron density redistribution. The donor-
acceptor chromophore across the 5,8-position of quinoxaline in MM-5 and MM-6 shows a
larger degree of electron density shift upon photoexcitation than those along the 2,3-position
of quinoxaline in MM-1~MM-4. Such a charge redistribution can be clearly verified by the
calculated dipole moments, i.e. MM-6 (9.4549 D) > MM-5 (9.4514 D) > MM-2 (7.6565 D) >
MM-1 (7.5003 D)> MM-4 (6.5937 D)> MM-3 (5.7328 D). Therefore, the ICT bands in the
dyes MM-5 and MM-6 exhibit a bathochromic shift comparing with that the dyes MM-
1~MM-4. The physical and theoretical parameters and the ICT transition dipole moments of
the MM-series dyes are summarized in Table S1.
<< Figure 4 >>
2.5 Photovoltaic performance of DSSCs
The DSSC devices were fabricated by using the designed dyes as photosensitizers, which
were absorbed on the TiO2 surface according to standard procedure. The electrolyte used in
-
the devices contained a I-/I3 solution (including LiI (0.5 M) and I2 (0.05 M)). Deoxycholic
acid (DCA) was used in the experiments of co-absorption, and N719 was served as the
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reference dye. The incident photocurrent conversion efficiency (IPCEs) and photocurrent–
voltage (J–V) curve of all dyes were measured either under AM 1.5 solar light (100 mW∙cm−2)
or under indoor light using a D65 (6500 K, average north USA sky daylight), a TL84 (4100 K,
European shop fluorescent), or a CWF (4150 K, cool white fluorescent, shop lighting) lamp.
The short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and solar-to-
electrical photocurrent density (η) are summarised in Table 1.
The J–V curve and IPCE of all devices are plotted in Figure 5. Compared with MM-1 and
MM-2, an apparent improvement of 76–80 mV in Voc was observed for dyes MM-3, MM-4,
and MM-5 that contain more long-chain alkyl substituents. Sensitizer MM-6 also exhibited a
higher Voc value of 663 mV without DCA co-absorbent, due to a better morphology and
higher loading amount (Table S3). Higher morphology quality leads to a better coverage of
−
TiO2 surface, and reduces the degree of contact between Li+ or I3 ions and the TiO2 surface.
A better surface blocking increases the energy gap between the Fermi levels of TiO2 and
electrolyte, therefore enhances the value of Voc.68-70 The dye morphology on the TiO2 surface
plays a key factor for reducing the charge recombination in DSSCs.71-73
<< Figure 5 >>
<< Figure 6 >>
<< Table 1 >>
In order to optimize the dye alignment on the TiO2 surface, the addition co-absorbent
DCA was used to prevent the dye molecules from lying down onto the TiO2 surface. An
imporved dye alignment can reduce the rate of charge recombination. Upon adding DCA (10
mM) as a co-absorbent, the Voc of DSSCs can increase in 1.66%–5.89% (Table 1). In the case
of MM-6 the Jsc decreased slightly by the addition of DCA due to the reduction of loading
amount, yet the Voc was enhanced significantly to 5.89%. This result indicated that the smaller
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size of MM-6 enabled better vertical alignment on the TiO2 surface (Table S3). Both FF and
Voc were improved and the power conversion efficiency increased by 3.88%. The device
performance of MM-6 displayed a short-circuit current (Jsc) 15.5 mA∙cm-2, an open-circuit
voltage (Voc) 701 mV, a fill factor (ff) 0.64, corresponding to an overall conversion efficiency
6.96% with DCA treatment under AM1.5 solar condition. However, the power conversion
efficiencies of MM-1~MM-5 reduced slightly in 0.01~0.26% due to the bulky substituents
leading to lower loading amount in the presence of DCA co-absorbent.
We further investigated the effect of co-deposite of MM-3 and MM-6 together in
different ratios in order to search for better spectral coverage and higher photocurrent. Dye
MM-3 was deposited on TiO2 first, followed by MM-6. The most appropriate deposition
times for MM-3 and MM-6 were 4 hr and 8 hr, respectively. The device exhibited even
higher Jsc and Voc values than the co-deposition of MM-6 and DCA, i.e., a Jsc value of 17.7
mA∙cm−2 with broad IPCE (Figure 5 (c)-(d)) and a high Voc of 717 mV.
Electrochemical impedance spectroscopy (EIS) was examined to further analyze the
photovoltaic properties of the DSSCs devices. Nyquist and Bode plots were drawn based on a
forward bias under dark condition (Figure 6). In the Nyquist plots the major semicircles that
observed for each sensitizer are associated with charge recombination resistance (Rrec) at the
TiO2/electrolyte/dye interface. A larger semicircle relates to Rrec at low frequency, indicating a
lower tendency of charge recombination and thus a smaller dark current.75 The radii of the
semicircles increase in the order of MM-1 (132 Ω) < MM-2 (144 Ω) < MM-3 (180 Ω) ≈
MM-4 (186 Ω) < MM-5 (196 Ω) ≈ MM-6 (194 Ω) < MM-6+DCA (244 Ω) < MM-6+MM-3
(255 Ω) < N719 (270 Ω). This trend is consistent with the Voc of the dyes, i.e., MM-1 (583
mV) < MM-2 (585 mV) < MM-3 (661 mV) ≈ MM-4 (661 mV) < MM-5 (663 mV) ≈ MM-6
(662 mV) < MM-6+DCA (701 mV) < MM-6+MM-3 (717 mV) < N719 (736 mV). The high
performance of co-deposition of MM-3 and MM-6 is confirmed by a larger arc of the major
semicircle (255 Ω) in Nyquist plot and a longer electron lifetime (46.2 ms). The optimal
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device power conversion efficiency of 7.92% under one sun AM1.5 condition is higher than
that using standard N719 (7.75%) dye under the same condition.
The electron diffusion lifetime (τe) of the device was related to the value of 1/2πf. A shift
to lower frequency (f) in a Bode plot corresponds to a longer electron lifetime. The electron
lifetime of the dyes increases in the order of MM-1 (17.0 ms) < MM-2 (18.7 ms) < MM-3
(35.3 ms) ≈ MM-4 (36.8 ms) < MM-5 (38.1 ms) ≈ MM-6 (38.8 ms) < MM-6+DCA (40.8 ms)
< MM-6+MM-3 (46.2 ms), where a higher τe indicates a lower charge recombination rate
(Fig. 6b & 6d). The larger charge recombination resistance and the longer electron diffusion
lifetime may be attributed to compact packing of the dyes.
The presence of 5 or 6 long alkyl/alkoxy chains in the structure of dyes MM-3, MM-4, and
MM-5 improves the morphology and coverage on TiO2 surface, thus results in higher values
of Rrec and τe than those of MM-1 and MM-2. Sensitizer MM-6 is an exception, i.e., it also
exhibits higher values of Rrec and τe even without the long-chain substituents. The high degree
of regular shape of MM-6 induces a more compact molecular packing, therefore blocks the
−
Li+ or I3 ions in electrolyte effectively from a direct contact with the electrode and reduces
the dark current.
<< Figure 7 >>
<< Table 2 >>
<< Figure 8 >>
2.6 DSSCs performance under indoor light condition
We have demonstrated that the solar cells made with the MM-series dyes showed promising
conversion efficiency under 1 sun illumination. We continued to explore the possibility of
using these dyes under indoor light illuminations in the intensity range of 600-2500 lux using
three kinds of lamps (D65, 6500 K; TL84, 4100 K; CWF, 4150 K). These MM-dyes display a
high degree of spectral overlap with the indoor light emission wavelength (Fig. S28).
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The indoor-light cell efficiency and parameters at 600, 1000 and 2500 lux are
summarized in Table 2. The current density–voltage (J–V) curve of the DSSCs under 2500 lux
TL84 fluorescent light is shown in Figure 7. The devices made with sensitizers MM-1~MM-4
showed cell efficiencies less than 13% even under a stronger illumination of 2500 lux (TL84),
due to the narrow absorption band widths that is similar to the situation under one sun. The
two sensitizers MM-5 and MM-6, which possesses the donor and acceptor at 5,8-positions of
quinoxaline, displayed efficiencies higher than 18% under 1000 lux (TL84). Sensitizer MM-6
showed the highest performance among all, i.e., 24.37% under 600 lux, 27.58% under 1000
lux, and 27.82% under 2500 lux. These efficiencies are comparable to those of the N719-
based cells under similar conditions, i.e., 17.21% (600 lux), 20.10% (1000 lux) and 24.63%
(2500 lux), respectively. The performance of MM-6 is only slightly lower than the record-
high TY6 (28.56% at 6000 lux) reported by Wei and Yeh.61 The performance of MM-6 in this
work with DCA co-absorbent showed improved performance of 25.42% (600 lux), 27.40%
(1000 lux) and 28.95% (2500 lux). Moreover, the co-sensitization of MM-3 and MM-6
exhibited remarkable performance of 27.76% (600 lux), 28.84% (1000 lux) and 30.45% (2500
lux). Such performance is highly competitive with the co-sensitization system of XY-1 and
D35 using Cu(II/I)(tmby) as the redox mediator (25.5% at 200 lux; 28.9% at 1000 lux), which
has been developed by Grätzel and Hagfeldt.60
We compared the efficiency of DSSCs made with the following three different
compositions under illumination of indoor light sources D65, CWF, and TL84: (1) MM-6
only; (2) MM-6 co-absorbed with DCA; and (3) MM-6 co-absorbed with MM-3. The results
are shown in Figure 8 and Table S4. The efficiency of D65 at 1000 lux is similar to that of one
sun, because the spectrum of D65 is a mimic of sunlight shining through a window. Compared
with the CWF condition, a better result is observed under TL84 illumination (Figure 8). The
co-sensitization of MM-3 and MM-6 performed the best after fine-tune the mole ratios to
achieve an optimal morphology on TiO2 surface. The best performance PCE was found to be
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higher than 30% (30.45 % at 2500 lux) under a TL84 fluorescent indoor light source.
A stability test on the devices were performed at 25 °C under TL84 indoor light and AM1.5
in an ambient condition. The device co-sensitized with MM-3 and MM-6 retained 81.42%
(600lux), 80.80% (1000lux), and 82.53% (2500lux) efficiency, respectively, after 168 h. The
plots of decay curves of PCEs are shown in Figure S29, and the list of parameters is provided
in Table S5.76 And stability test on device co-sensitized with MM-3 and MM-6 retained 95%
after 1hr under AM1.5 continuous illumination in Figure S30, this result indicated our
devices possesses good stability under an ambient condition.
3. Conclusion
We demonstrated the successful application of a series D-A-π-A sensitizers incorporating
quinoxaline or quinoxalinoid moieties (MM-1~MM-6) in high-performance DSSC under one
sun and indoor light illumination (D65, CWF and TL84) conditions. Six Y-shaped metal-free
organic dyes (MM-1~MM-6) were synthesized through stepwise modifications on the
quinoxalinoid moieties. Dyes with donor substituents at 5,8-positions of quinoxaline (MM-5
and MM-6) displayed higher photocurrent and PCE than those at 2,3-positions (MM-3 and
MM-4). The dye MM-6 displayed a remarkable performance for DSSC upon co-deposition
with DCA, even without the two large fluorene chromophores. It showed short-circuit current
(Jsc) 15.5 mA∙cm-2, open-circuit voltage (Voc) 701 mV, and fill factor (ff) 0.64, corresponding
to an overall conversion efficiency of 6.96% under AM1.5 solar condition. We also found that
a co-deposition of MM-3 and MM-6 together can improve the dye-coverage on TiO2 surface.
The best performance of the device co-deposited with MM-3 and MM-6 displayed short-
circuit current (Jsc) 17.7 mA∙cm-2, open-circuit voltage (Voc) 717 mV, and fill factor (ff) 0.63,
corresponding to an overall conversion efficiency of 7.92% under AM1.5 solar condition. The
most impressive result was obtained when MM-3 and MM-6 were used on indoor
applications. The devices co-sensitized with MM-3 and MM-6 achieved the highest
13
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10.1002/cssc.201900505
ChemSusChem
performance of PCE exceeding 30% (30.45 % at 2500 lux) under a TL84 fluorescent lamp.
4. Experimental Section
Characterization and reagents: All reactions and manipulations were performed under
a nitrogen atmosphere, and solvents were freshly distilled according to standard
1
13
1
procedures. The H and C NMR were recorded on a Bruker AVIII HD 400. The H and
13C NMR chemical shifts were reported on  scale downfield from Me4Si. The coupling
constants (J) were given in hertz. Absorption spectra were recorded on a SHIMADZU
UV-1800 spectrophotometer. Redox potentials were measured using cyclic voltammetry
on a CHI 620 analyser. All measurements were performed in CH2Cl2 solution containing
0.1 M tetrabutylammonium hexaflourophosphate as supporting electrolyte under ambient
conditions after purging the solution with N2 for 10 min. The conventional three-electrode
configuration was employed that consists of a glassy carbon working electrode, a platinum
counter electrode, and an Ag/Ag+ reference electrode calibrated with ferrocene/
ferrocenium (Fc/Fc+) as an internal reference. Mass spectra were recorded on a JEOL
JMS-700 double-focusing mass spectrometer. Analytical thin layer chromatography (TLC)
was performed on Silica gel 60 F 254 Merck. Column chromatography was performed
using the silica gel from Merck (Kieselgel Si 60; 40–63 μm). Solvent THF was distilled
with sodium benzophenone ketyl. Toluene and methylene chloride were distilled with
CaH2. All other solvents and reagents were reagent grade, purchased from Acros, Alfa,
Merck, Lancaster, TCI, Sigma-Aldrich, and Showa, separately, and used without further
purification.
Fabrication and Characterization of DSSCs: The FTO conducting glass (2.2 mm thick,
fluorine doped tin oxide over-layer, transmission ＞90% in the visible, sheet resistance 7
Ω/square), titania-oxide pastes of Ti-Nanoxide T/SP (adsorption layer) and Ti-Nanoxide
14
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R/SP (scattering layer) were purchased from Solaronix. A thin film of TiO2 (adsorption
layer~12μm; scattering layer: ~6μm) was coated on a 0.28 cm2 FTO glass substrate. It was
immersed in a methylene chloride solution containing 3 × 10-4 M dye sensitizers for 12 h,
then rinsed with anhydrous acetonitrile and dried. The other piece of FTO with sputtering
100 nm thick Pt was used as a counter electrode. The active area was controlled at a
dimension of 0.28 cm2 by adhering 60 μm thick polyester tape on the Pt electrode. The
photocathode was placed on top of the counter electrode and was tightly clipped together
to form a cell. Electrolyte was then injected into the seam between two electrodes. An
acetonitrile solution containing LiI (0.5 M), I2 (0.05 M) and 4-tert-butylpyridine (TBP)
(0.5 M) was used as the electrolyte. Devices made with commercial dye N719 under the
same condition (3 × 10−4 M, Solaronix S.A., Switzerland) were used as references. The
cell parameters were obtained under incident light that had an intensity of 100 mW∙cm−2
measured using a thermopile probe (Oriel 71964), which was generated by a 300 W solar
simulator (Oriel Sol3A Class AAA Solar Simulator 9043A, Newport), and passed through
an AM 1.5 filter (Oriel 74110). The light intensity was further calibrated using an Oriel
reference solar cell (Oriel 91150) and adjusted to 1.0 sun. The monochromatic quantum
efficiency was recorded using a monochromator (Oriel 74100) under a short circuit
condition. Electrochemical impedance spectra of DSSCs were recorded by an
Impedance/CIMPS/IVMS analyzer (Zahner Ennium).
Fabrication of dye-loading TiO2 on FTO glass: The dye-loading TiO2 on FTO glass (7
Ω/sq) was used as working electrode, which has 3 x 1 cm dimension (8 μm thick). The
film TiO2 (Ti-Nanoxide T/SP, SOLARONIX) was adhered on FTO glass by screen-
o
o
printing. It was pre-dried 1 h at 100 C, followed by heating at 500 C. It was then
immersed in a solution of the dye (3 x 10-4 M in anhydrous CH2Cl2) for 12 h, then rinsed
with anhydrous CH2Cl2 and dried.
15
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10.1002/cssc.201900505
ChemSusChem
Device performance under indoor light: The system of stabilized indoor light sources
(model CMS-PV101, designed by Industrial Technology Research Institute, Taiwan)
consisted of D65 (6500 K, Average North USA sky daylight), TL84 (4100 K, European shop
fluorescent), CWF (4150 K, cool white fluorescent, shop lighting), U30 (3000 K, shop
lighting) and A (2856 K, typical home lighting) lamps, which meet the standards of SEMI
PV80-0218 and CIE. All light sources are intensity adjustable in a range of 0 to 2500 lux,
with non-uniformity less than 2 % (within the area of 20 cm × 20 cm) and temporal instability
less than 2 %. A calibrated spectroradiometer (ISM-Lux, Isuzu Optics, Japan) was embed at
underlying system. The acquired illuminance value was attained by altering LED lamp lifting
platform to a moderate position and incessantly confirmed by spectroradiometer until it
reached firmly stable condition. After then, the DSSC device as put at the upper site of the
spectroradiometer and the J-V curves were measured via a computer-controlled digital source
meter (Keithley 2401, USA) under various dim light illumination, followed the requirements
of SEMI PV57-1214.
Quantum Chemistry Computations: Geometry optimization of the dyes were
accomplished by Gaussian G16W program (B3LYP/6-31G* hybrid functional). For excited
states, the time-dependent density functional theory (TDDFT) and the B3LYP functional were
used. The frontier orbital plots of the highest occupied and lowest unoccupied molecular
orbitals (hereafter abbreviated as HOMO and LUMO, respectively) were drawn using
GaussView 06.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
16
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10.1002/cssc.201900505
ChemSusChem
This work was financial supported by the Ministry of Science and Technology of Taiwan
(MOST 107-2113-M-008-010-, 107-2113-M-029-010-, and 107-2113-M-029-011-). Tunghai
University and National Central University are gratefully acknowledged.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
References
[1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev., 2010, 110, 6595.
[2] F. D. Rossi, T. Pontecorvo, T. M. Brown, Applied Energy, 2015, 156, 413.
[3] C.-L. Wang, P.-T. Lin, Y.-F. Wang, C.-W. Chang, B.-Z. Lin, H.-H. Kuo, C.-W. Hsu, S.-
H. Tu, C.-Y. Lin, J. Phys. Chem. C, 2015, 119, 24282.
[4] M.-C. Tsai, C.-Li Wang, C.-W. Chang, C.-W. Hsu, Y.-H. Hsiao, C.-L. Liu, C.-C. Wang,
S.-Y. Lin, C.-Y. Lin, J. Mater. Chem. A, 2018, 6, 1995.
[5] S. Yun, Y. Qin, A. R. Uhl, N. Vlachopoulos, M. Yin, D. Li, X. Han, A. Hagfeldt, Energy.
Environ. Sci., 2018, 11, 476.
[6] S. Yun, Y. Zhang, Q. Xu, J. Liu, Y. Qin, Nano Energy, 2019, 60, 600.
[7] Md. K. Nazeeruddin, A. Key, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N.
Vlachopoulous, M. Grätzel, J. Am. Chem. Soc., 1993, 115, 6382.
[8] M. K. Nazeeruddin, P. Péchy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P.
Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A.
Bignozzi, M. Grätzel, J. Am. Chem. Soc., 2001, 123, 1613.
[9] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, T. Sekiguchi, M. Grätzel,
Nature Materials, 2003, 2, 402.
[10] F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S.
M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc., 2008, 130, 10720.
17
This article is protected by copyright. All rights reserved.

10.1002/cssc.201900505
ChemSusChem
[11] A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G.
Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Grätzel, Science, 2011, 334, 629.
[12] Z. Yao, M. Zhang, H. Wu, L. Yang, R. Li, P. Wang, J. Am. Chem. Soc. 2015, 137, 3799.
[13] K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J.-i. Fujisawa, M. Hanaya, Chem. Commun.,
2015, 51, 15894.
[14] S. Yun, Y. Liu, T. Zhang, S. Ahmad, Nanoscale, 2015, 7, 11877.
[15] S. Yun, A. Hagfeldt, T. Ma, Adv. Mater., 2014, 26, 6210.
[16] X. Ren, S. Jiang, M. Cha, G. Zhou, Z.-S. Wang, Chem. Mater. 2012, 24, 3493.
[17] X. Lu, X. Jia, Z.-S. Wang, G. Zhou, J. Mater. Chem. A, 2013, 1, 9697.
[18] H. Li, Y. Hou, Y. Yang, R. Tang, J. Chen, H. Wang, H. Han, T. Peng, Q. Li, Z. Li, ACS
Appl. Mater. Interfaces 2013, 5, 12469.
[19] R. Y.-Y. Lin, F.-L. Wu, C.-H. Chang, H.-H. Chou, T.-M. Chuang, T.-C. Chu, C.-Y. Hsu,
P.-W. Chen, K.-C. Ho, Y.-H. Lo, J. T. Lin, J. Mater. Chem. A, 2014, 2, 3092.
[20] Y. Wu, W. Zhu, Chem. Soc. Rev., 2013, 42, 2039.
[21] W. Li, Y. Wu, Q. Zhang, H. Tian, W.-H. Zhu, ACS Appl. Mater. Interfaces 2012, 4,
1822.
[22] K. Pei, Y. Wu, H. Li, Z. Geng, H. Tian, W.-H. Zhu, ACS Appl. Mater. Interfaces 2015,
7, 5296.
[23] W. Zhu, Y. Wu, S. Wang, W. Li, X. Li, J. Chen, Z.-S. Wang, H. Tian, Adv. Funct.
Mater. 2011, 21, 756.
[24] S. Haid, M. Marszalek, A. Mishra, M. Wielopolski, J. Teuscher, J.-E. Moser, R.
Humphry-Baker, S. M. Zakeeruddin, M. Grätzel, P. Bäuerle, Adv. Funct. Mater. 2012,
22, 1291.
[25] Y. Wu, M. Marszalek, S. M. Zakeeruddin, Q. Zhang, H. Tian, M. Grätzel, W. Zhu,
Energy Environ. Sci., 2012, 5, 8261.
18
This article is protected by copyright. All rights reserved.

10.1002/cssc.201900505
ChemSusChem
[26] S. Chen, H. Jia, M. Zheng, K. Shen, H. Zheng, Phys. Chem. Chem. Phys., 2016, 18,
29555.
[27] D. Joly, M. Godfroy, L. Pellejà, Y. Kervella, P. Maldivi, S. Narbey, F. Oswald, E.
Palomares, R. Demadrille, J. Mater. Chem. A, 2017, 5, 6122.
[28] Y. Cui, Y. Wu, X. Lu, X. Zhang, G. Zhou, F. B. Miapeh, W. Zhu, Z.-S. Wang, Chem.
Mater. 2011, 23, 4394.
[29] J. Mao, F. Guo, W. Ying, W. Wu, J. Li, J. Hua, Chem. Asian J. 2012, 7, 982.
[30] S. Chaurasia, W.-I. Hung, H.-H. Chou, J. T. Lin, Org. Lett. 2014, 16, 3052.
[31] S. Qu, C. Qin, A. Islam, Y. Wu, W. Zhu, J. Hua, H. Tian, L. Han, Chem. Commun.,
2012, 48, 6972.
[32] T. W. Holcombe, J.-H. Yum, Y. Kim, K. Rakstysac, M. Grätzel, J. Mater. Chem. A,
2013, 1, 13978.
[33] X. Li, Z. Zheng, W. Jiang, W. Wu, Z. Wang, H. Tian, Chem. Commun., 2015, 51, 3590.
[34] P. Ganesan, A. Yella, T. W. Holcombe, P. Gao, R. Rajalingam, S. A. Al-Muhtaseb, M.
Grätzel, M. K. Nazeeruddin, ACS Sustainable Chem. Eng. 2015, 3, 2389.
[35] D.-Y. Chen, Y.-Y. Hsu, H.-C. Hsu, B.-S. Chen, Y.-T. Lee, H. Fu, M.-W. Chung, S.-H.
Liu, H.-C. Chen, Y. Chi, P.-T. Chou, Chem. Commun., 2010, 46, 5256-5258.
[36] Y. J. Chang, T. J. Chow, J. Mater. Chem., 2011, 21, 9523.
[37] W. Ying, F. Guo, J. Li, Q. Zhang, W. Wu, H. Tian, J. Hua, ACS Appl. Mater. Interfaces
2012, 4, 4215.
[38] S.-G. Li, K.-J. Jiang, J.-H. Huang, L.-M. Yang, Y.-L. Song, Chem. Commun., 2014, 50,
4309.
[39] K. Pei, Y. Wu, A. Islam, Q. Zhang, L. Han, H. Tian, W. Zhu, ACS Appl. Mater.
Interfaces 2013, 5, 4986.
19
This article is protected by copyright. All rights reserved.

10.1002/cssc.201900505
ChemSusChem
[40] J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi, P. Comte, K. Pei, T. W.
Holcombe, M. K. Nazeeruddin, J. Hua, S. M. Zakeeruddin, H. Tian, M. Grätzel, J. Am.
Chem. Soc. 2014, 136, 5722.
[41] L.-P. Zhang, K.-J. Jiang, G. Li, Q.-Q. Zhang, L.-M. Yang, J. Mater. Chem. A, 2014, 2,
14852.
[42] Y. Gao, X. Li, Y. Hu, Y. Fan, J. Yuan, N. Robertson, J. Hua, S. R. Marder, J. Mater.
Chem. A, 2016, 4, 12865.
[43] J.-S. Ni, T.-Y. Chiu, W.-S. Kao, H.-J. Chou, C.-C. Su, J. T. Lin, ACS Appl. Mater.
Interfaces 2016, 8, 23066.
[44] D. H. Lee, M. J. Lee, H. M. Song, B. J. Song, K. D. Seo, M. Pastore, C. Anselmi, S.
Fantacci, F. De Angelis, M. K. Nazeeruddin, M. Grätzel, H. K. Kim, Dyes Pigm., 2011,
91, 192.
[45] J. Kim, H. Choi, J. Lee, M. Kang, K. Song, S. O. Kang, J. Ko, J. Mater. Chem., 2008,
18, 5223.
[46] S . Kim, H. Lim, K. Kim, C. Kim, T. Y. Kang, M. J. Ko, N. G. Park, IEEE J. Sel. Top.
Quantum Electron., 2010, 16, 1627.
[47] S. Haid, M. Marszalek, A. Mishra, M. Wielopolski, J. Teuscher, J. Moser, R. Humphry-
Baker, S. M. Zakeeruddin, M. Grätzel, P. Bäuerle, Adv. Funct. Mater., 2012, 22, 1291.
[48] Y. Z. Wu, M. Marszalek, S. M. Zakeeruddin, Q. Zhang, H. Tian, M. Grätzel, W. H. Zhu,
Energy Environ. Sci., 2012, 5, 8261.
[49] J. Liu, K. Wang, F. Xu, Z. Tang, W. Zheng, J. Zhang, C. Li, T. Yu, X. You,
Tetrahedron Lett., 2011, 52, 6492.
[50] T.-C. Lin, F.-L. Guo, M.-H. Li, C.-Y. Liu, Chem. Asian J. 2013, 8, 2102.
[51] T.-C. Lin, Y.-H. Lee, C.-Y. Liu, B.-R. Huang, M.-Y. Tsai, Y.-J. Huang, J.-H. Lin, Y.-K.
Shen, C.-Y. Wu, Chem. Eur. J. 2013, 19, 749.
20
This article is protected by copyright. All rights reserved.

10.1002/cssc.201900505
ChemSusChem
[52] D. W. Chang, H. J. Lee, J. H. Kim, S. Y. Park, S.-M. Park, L. Dai, J.-B. Baek, Org.
Lett., 2011, 13, 3880.
[53] C. Y. Jung, C. J. Song, W. Yao, J. M. Park, I. H. Hyun, D. H. Seong, J. Y. Jaung, Dyes
and Pigments 2015, 121, 204.
[54] J.-J. Cid, J.-H. Yum, S.-R. Jang, M. K. Nazeeruddin, E. Marťnez-Ferrero, E. Palomares,
J. Ko, M. Grätzel, T. Torres, Angew. Chem. Int. Ed. 2007, 46, 8358.
[55] J.-H. Yum, S.-R. Jang, P. Walter, T. Geiger, F. Nüesch, S. Kim, J. Ko, M. Grätzel, M.
K. Nazeeruddin, Chem. Commun., 2007, 4680.
[56] S.-Q. Fan, C. Kim, B. Fang, K.-X. Liao, G.-J. Yang, C.-J. Li, J.-J. Kim, J. Ko, J. Phys.
Chem. C 2011, 115, 7747.
[57] J.-H. Yum, E. Baranoff, S. Wenger, Md. K. Nazeeruddin, M. Grätzel, Energy Environ.
Sci., 2011, 4, 842.
[58] S. K. Balasingam, M. Lee, M. G. Kang, Y. Jun, Chem. Commun., 2013, 49, 1471.
[59] L. Yu, K. Fan, T. Duan, X. Chen, R. Li, T. Peng, ACS Sustainable Chem. Eng. 2014, 2,
718.
[60] M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P. Liska, J. Hua, S. M.
Zakeeruddin, J.-E. Moser, M. Grätzel, A. Hagfeldt, Nature Photonics, 2017, 11, 372.
[61] Y. S. Tingare, N. S. Vinh, H.-H. Chou, Y.-C. Liu, Y.-S. Long, T.-C. Wu, T.-C. Wei, C.-
Y. Yeh, Adv. Energy Mater. 2017, 1700032.
[62] M. B. Desta, N. S. Vinh, C. P. Kumar, S. Chaurasia, W.-T. Wu, Jiann T. Lin, T.-C. Wei,
E. W.-G. Diau, J. Mater. Chem. A, 2018, 6, 13778.
[63] M. K. Nazeeruddin, P. Péchy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P.
Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A.
Bignozzi, M. Grätzel, J. Am. Chem. Soc., 2001, 123, 1613.
[64] T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A. Anderson, X. Ai, T. Lian, S.
Yanagida, Chem. Mater., 2004, 16, 1806.
21
This article is protected by copyright. All rights reserved.

10.1002/cssc.201900505
ChemSusChem
[65] S. Yun, H. Zhang, H. Pu, J. Chen, A. Hagfeldt, T. Ma, Adv. Energy Mater., 2013, 3,
1407.
[66] S. Yun, M. Wu, Y. Wang, J. Shi, X. Lin, A. Hagfeldt, T. Ma, ChemSusChem, 2013, 6,
411.
[67] S. Yun, H. Pu, J. Chen, A. Hagfeldt, T. Ma, ChemSusChem, 2014, 7, 442.
[68] G. Boschloo, L. Häggman, A. Hagfeldt, J. Phys. Chem. B 2006, 110, 13144.
[69] Z. Ning, Y. Fu, H. Tian, Energy Environ. Sci., 2010, 3, 1170.
[70] S. A. Haque, E. Palomares, B. M. Cho, A. N. M. Green, N. Hirita, D. R. Klug, J. R.
Durrant, J. Am. Chem. Soc. 2005, 127, 3456.
[71] R. Kern, R. Sastrawan, J. Ferber, R. Stangl, J. Luther, Electrochim. Acta 2002, 47, 4213.
[72] J. Bisquert, J. Phys. Chem. B, 2002, 106, 325.
[73] Q. Wang, J. Moser and M. Grätzel, J. Phys. Chem. B, 2005, 109, 14945.
[74] M. K. Nazeeruddin, F. D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito,
B. Takeru, M. Grätzel, J. Am. Chem. Soc., 2005, 127, 16835.
[75] Z. Ning, Q. Zhang, W. Wu, H. Pei, B. Liu, H. Tian, J. Org. Chem., 2008, 73, 3971.
[76] S. Yun, P. D. Lund, A. Hinsch, Energy Environ. Sci., 2015, 8, 3495.
22
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Figure 1. Organic sensitizers of MM-series.
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Scheme 1. Synthesis condition of organic dye sensitizer: (i) Pd2(dba)3, dppf, sodium tert-
butoxide, diphenylamine, toluene, 110 ℃; (ii) Acetylenedicarboxylic acid, dppb, DBU,
PdCl2(PPh3)2, DMSO (iii) KMnO4, NaHCO3, Aliquat 336, CH2Cl2/H2O; (iv) CH3COOH,
110 ℃; (v) (4-(1,3-dioxolan-2-yl)phenyl)tributylstannane/(5-(1,3-dioxolan-2-yl)thiophen-2-
o
yl)tributylstannane, PdCl2(PPh3)2, DMF, 100 C; then THF/HCl, rt.; (vi) CNCH2COOH,
piperidine, chloroform, reflux.; (vii) HBr, Br2, 60 oC; (viii) NaBH4, ethanol, rt.; (ix) (10-(4-
(hexyloxy)phenyl)-10H-phenothiazin-3-yl)boronic acid, Pd(PPh3)4, K2CO3, THF/toluene.
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Figure 2. (a) Absorption spectra of dyes MM-1~MM-6 in THF (3.0 × 10-4M); (b) Absorption
spectra of dyes MM-1~MM-6 on TiO2 thin film.
Figure 3. HOMO-LUMO energy levels of MM-series organic sensitizers.
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Figure 4. The frontier HOMO and LUMO orbitals of MM-series organic sensitizers
estimated by time-dependent DFT/B3LYP (6-31G* basis set).
Table 1 Photochemical and electrochemical parameters of the dyes.
b)
c)
Sensitizer
λmaxa) (nm)/
λmax
Eox
E0-0
Ered
Jsc (mA∙cm-2)
wo/w DCA
Voc (mV)
wo/w
FF(%)
wo/w DCA
PCEd) (%)
wo/w DCA
(ε (M-1cm-1))
(TiO2)
(V)
0.74
0.74
0.71
0.73
0.54
0.52
(eV)
(V)
DCA
MM-1e)
MM-2e)
MM-3e)
MM-4e)
MM-5e)
MM-6e)
435
(4.63x104)
457(4.20
x104)
440
466
425
436
470
474
2.31
2.21
2.54
2.43
2.10
2.07
-1.57
-1.47
-1.83
-1.70
-1.56
-1.55
7.75(±0.2)/
6.84(±0.1)
8.05(±0.3)/
7.51(±0.1)
7.93(±0.2)/
7.35(±0.2)
9.08(±0.1)/
8.51(±0.2)
14.0(±0.2)/
12.7(±0.3)
17.1(±0.3)/
15.5(±0.2)
16.5(±0.3)/
—
583(±4)/
613(±5)
585(±3)/
603(±2)
661(±2)/
680(±2)
661(±5)/
672(±3)
663(±2)/
681(±2)
662(±3)/
701(±2)
685(±2)/
—
61.3(±0.2)/
66.0(±0.3)
66.1(±0.1)/
63.5(±0.3)
67.4(±0.2)/
68.1(±0.3)
64.6(±0.2)/
66.2(±0.1)
63.1(±0.1)/
65.2(±0.1)
59.2(±0.2)/
64.1(±0.3)
64.5(±0.4)/
---
2.77(±0.10)/
2.76(±0.08)
3.11(±0.13)/
2.85(±0.08)
3.53(±0.11)/
3.40(±0.12)
3.88(±0.08)/
3.79(±0.10)
5.86(±0.11)/
5.64(±0.16)
6.70(±0.17)/
6.96(±0.14)
7.29(±0.20)/
---
427(8.06
x104)
445(8.62
x104)
470(3.65
x104)
484(3.57
x104)
MM-3+MM-6
f)
(2hr+10hr)
MM-3+MM-6
17.7(±0.2)/
717(±5)/
—
62.8(±0.2)/
---
7.92(±0.22)/
---
f)
—
(4hr+8hr)
MM-3+MM-6
15.2(±0.3)/
703(±4)/
—
65.2(±0.3)/
---
6.95(±0.27)/
---
f)
—
(6hr+6hr)
N719
—
—
1.10g)
2.60g)
-1.50g)
17.5(±0.2)/
736(±4)/
---
60.1(±0.1)/
---
7.75(±0.13)/
---
—
a)Absorption and Emission are in CH2Cl2; b)Oxidation potential in THF (10-3 M) containing 0.1
c)
d)
M (n-C4H9)4NPF6 with a scan rate 50 mV∙s-1 (vs. NHE); Ered calculated by Eox − E0-0;
Performance of DSSCs measured in a 0.28 cm2 working area on a FTO (8Ω/square) substrate.
e)
Electrolyte: LiI (0.5 M), I2 (0.05 M), and TBP (0.5 M) in MeCN; The TiO2 electrode was
dipped in methene chloride of sensitizer with 3 x 10-4 M for 12hr; The TiO2 electrode was
f)
dipped in methene chloride of MM-3 (3 x 10-4 M), rinsed with ACN, then dipped in methene
chloride of MM-6 (3 x 10-4 M); g)see reference 74.
26
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10.1002/cssc.201900505
ChemSusChem
Figure 5. (a) J-V plots and (b) IPCE of DSSCs devices of MM-1~6 without DCA co-
absorbent. (c) J-V plots and (d) IPCE of DSSCs devices of MM-6 by adding DCA co-
absorbent and cosensitization system with MM-3. The plots were measured under the light
intensity of 1.0 sun.
27
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10.1002/cssc.201900505
ChemSusChem
Figure 6. Impedance spectra of MM-series dyes in CH2Cl2 (a-b) Nyquist plots (c-d) Bode
phase plots at -0.73 V bias in the dark. The electron diffusion lifetime τe = 1/2πf.
0.35
0.30
0.25
0.20
N719
MM-5
MM-6
0.15
MM-6+DCA
MM-3+MM6
0.10
(2500 lux TL84)
0.05
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (V)
Figure 7. J-V plots and of MM-5, MM-6, MM6 with DCA and co-sensitization of MM-3
and MM-6 under TL84 illuminance (2500 lux).
28
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10.1002/cssc.201900505
ChemSusChem
Table 2. Photovoltaic parameters of the MM-series dyes measured using TL84 indoor light.
Sensitizer
Irradiance
(mW/cm2)
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
0.110
0.185
0.462
Illuminance (lux)
Jsc (μA∙cm-2)
Voc (mV)
FF(%)
PCE (%) a)
MM-1
MM-2
600
1000
2500
600
1000
2500
600
1000
2500
600
1000
2500
600
1000
2500
600
1000
2500
600
1000
2500
600
1000
2500
600
21.7(±1.3)
40.3(±3.5)
120(±7.2)
30.5(±2.1)
48.1(±4.5)
133(±6.9)
31.7(±1.9)
65.8(±2.5)
131(±5.5)
44.2(±1.7)
78.6(±1.5)
170(±6.1)
52.0(±2.2)
95.1(±3.5)
248(±8.7)
75.6(±3.5)
128(±3.7)
309(±7.9)
75.4(±2.2)
129(±4.1)
303(±4.3)
84.5(±2.8)
137(±3.8)
312(±4.7)
61.5(±2.8)
107(±3.2)
273(±4.5)
405(±4.5)
415(±4.3)
462(±3.7)
419(±3.5)
459(±4.8)
481(±3.5)
427(±2.1)
436(±3.9)
492(±3.1)
417(±5.1)
444(±3.9)
496(±4.6)
480(±4.5)
550(±2.8)
570(±3.7)
555(±4.5)
570(±4.8)
597(±3.3)
562(±4.3)
620(±3.2)
625(±2.7)
537(±3.1)
598(±2.5)
621(±3.6)
493(±2.5)
530(±4.8)
611(±3.5)
55.84(±0.72)
83.33(±2.27)
64.08(±0.78)
58.21(±0.27)
71.55(±1.53)
65.25(±0.88)
60.16(±0.31)
62.28(±0.55)
70.70(±1.12)
61.17(±0.37)
58.72(±0.19)
66.51(±0.46)
55.15(±0.33)
67.27(±0.89)
65.02(±1.13)
64.53(±0.57)
70.72(±1.22)
69.68(±0.85)
66.64(±0.55)
63.44(±0.24)
70.54(±0.88)
67.84(±0.18)
64.96(±0.25)
72.59(±1.02)
62.91(±0.27)
65.62(±1.55)
68.22(±1.53)
4.43(±0.41)
7.50(±0.97)
7.72(±0.59)
6.71(±0.61)
8.52(±1.08)
9.03(±0.67)
7.35(±0.57)
9.64(±0.56)
9.86(±0.64)
10.16(±0.67)
11.07(±0.35)
12.14 (±0.63)
12.38(±0.86)
18.99(±1.09)
19.89(±1.19)
24.37(±1.82)
27.58(±1.85)
27.82(±1.22)
25.42(±1.42)
27.40(±1.15)
28.95(±0.86)
27.76(±1.39)
28.74(±1.06)
30.45(±1.06)
17.21(±1.08)
20.10(±1.39)
24.60(±1.11)
MM-3
MM-4
MM-5
MM-6
MM-6+DCA(10mM)b)
MM-6+MM-3
(8hr+4hr) c)
N719
1000
2500
a)
V : open-circuit voltage; Jsc: short-circuit current; FF: fill factor; PCE: power-conversion
oc
efficiency. The data are based on three measurements. Experiments were conducted by using
TiO2 photoelectrodes, with approximately 12 mm thickness and 0.28 cm2 working area on FTO
(8Ω/square) substrate; b)The TiO2 soaked with dyes and DCA (10 mM); c) The TiO2 was dipped
in methene chloride of MM-3 (3 x 10-4 M) for 4hr, rinsed with ACN, then dipped in methene
chloride of MM-6 (3 x 10-4 M) for 8 hr.
29
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