Phenothiazine dye featuring encapsulated insulated molecular wire as auxiliary donor for high photovoltage of dye-sensitized solar cells by suppression of aggregation

Article abstract of DOI:10.1016/j.electacta.2019.02.013

Two efficient dye-sensitized solar cells have been fabricated by two novel D–D–π–A phenothiazine-based organic dyes (PH2 and PH3) with an encapsulated insulated molecular wire (EIMW) as an auxiliary donor. The cell sensitized by PH2 with EIMW as an auxiliary donor shows a much higher photovoltage (V_oc) relative to the reference dye PH1 without EIMW, because the former dye can inhibit dye aggregation and suppress the charge recombination effectively. The results show that the cell sensitized by PH2 with co-adsorption of chenodeoxycholic acid obtains a high power conversion efficiency, even higher than that of the cell based on N719. Thus, an effective way to increase the photovoltage and efficiency of the DSSCs has been developed by introducing the EIMW into the organic dyes due to the effective inhibition of dye aggregation and charge recombination.

Full text of DOI:10.1016/j.electacta.2019.02.013

Accepted Manuscript
Phenothiazine dye featuring encapsulated insulated molecular wire as auxiliary donor
for high photovoltage of dye-sensitized solar cells by suppression of aggregation
Tao Hua, Zu-Sheng Huang, Ke Cai, Lingyun Wang, Hao Tang, Herbert Meier, Derong
Cao
PII:
S0013-4686(19)30239-7
https://doi.org/10.1016/j.electacta.2019.02.013
EA 33604
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 10 December 2018
Revised Date: 3 February 2019
Accepted Date: 3 February 2019
Please cite this article as: T. Hua, Z.-S. Huang, K. Cai, L. Wang, H. Tang, H. Meier, D. Cao,
Phenothiazine dye featuring encapsulated insulated molecular wire as auxiliary donor for high
photovoltage of dye-sensitized solar cells by suppression of aggregation, Electrochimica Acta (2019),
doi: https://doi.org/10.1016/j.electacta.2019.02.013.
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ACCEPTED MANUSCRIPT
Phenothiazine Dye Featuring Encapsulated Insulated Molecular
Wire as Auxiliary Donor for High Photovoltage of Dye-Sensitized
Solar Cells by Suppression of Aggregation
Tao Hua a, Zu-Sheng Huangb, Ke Caia, Lingyun Wanga, Hao Tanga, Herbert Meier*a,c and Derong
Cao*a
a
State key Laboratory of Luminescent Materials and Devices, School of Chemistry and Chemical
Engineering, South China University of Technology, Guangzhou 510641, China
b School of Pharmaceutical Sciences, Wenzhou Medical University, Wenzhou 325035, China
c Institute of Organic Chemistry, University of Mainz, Mainz 55099, Germany
*Corresponding author. E-mail: drcao@scut.edu.cn (D. Cao), hmeier@uni-mainz.de (H. Meier)
ABSTRACT
Two efficient dye-sensitized solar cells have been fabricated by two novel D–D–π–A
phenothiazine-based organic dyes (PH2 and PH3) with an encapsulated insulated
molecular wire (EIMW) as an auxiliary donor. The cell sensitized by PH2 with
EIMW as an auxiliary donor shows a much higher photovoltage (Voc) relative to the
reference dye PH1 without EIMW, because the former dye can inhibit dye
aggregation and suppress the charge recombination effectively. The results show that
the cell sensitized by PH2 with co-adsorption of chenodeoxycholic acid obtains a
high power conversion efficiency, even higher than that of the cell based on N719.
Thus, an effective way to increase the photovoltage and efficiency of the DSSCs has
been developed by introducing the EIMW into the organic dyes due to the effective
inhibition of dye aggregation and charge recombination.

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Keywords: encapsulated insulated molecular wire; dye-sensitized solar cells;
auxiliary donor; suppression aggregation
1. Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable attention from
both academic and industrial research communities in the past 25 years, because of
their favorable ecological feature, economical characteristic and ease to tune various
structural characters [1-8]. Although a variety of photovoltaic materials have been
concerned, such as perovskites [9,10], organic small molecules, polymers [11-13] and
quantum dots solar cells. However, Dye-sensitized solar cells still play an important
role in striving towards the next generation solar technology due to their flexible
structure modification and superior performance. Up to now, power conversion
efficiencies (PCEs) of dye-sensitized solar cells have exceeded 13% [14,15]. The
enormous efficiency breakthrough of DSSCs has been greatly benefited from new
materials design and device engineering, including the use of donor–donor–π spacer–
acceptor (D–D–π–A) architectures [16-20], the larger plane structure [4,21], the
effectively π–spacer [22-24], co-adsorption [25,26] or co-sensitization [27-30],
amendment of the interfacial morphology of TiO2 [31-42], reform of the electrolyte
composition [43-47], and so on.
Phenothiazine dyes were one class of the most attractive metal-free dyes in the
past few years because the phenothiazine units possess strong electron-donating
ability and non-planar butterfly conformation [48-51]. Besides, phenothiazine unit can
be modified easily with a considerable efficiency simultaneously. Recent research

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found that introducing an auxiliary donor at the terminal of phenothiazine unit could
effectively improve the PCE of the dyes [52-57]. Zhu et al reported a series of
phenothiazine-based dyes by introducing thiophene and ethylenedioxythiophene, their
dimers and the mixtures of the both to adjust dye aggregation, suppress charge
recombination and improve dye regeneration. The results show that thiophene
analogues as auxiliary donors not only improve light adsorption property and adjust
energy levels, but also have an effect on dye aggregation and interfacial charge
recombination [58].
At present, a majority of organic sensitizers are provided of donor, π–spacer, and
acceptor (D–π–A configuration) and most of them possess rigid rod-shaped skeleton
[59-61]. However, the rigid rod-shaped skeletons of dyes are easily generated
intermolecular aggregation and charge recombination on the surface of TiO2 [62-66].
The two negative effects not only accelerate the process of self-quenching of
generated electron and cut down the electron injection into TiO2, but also reduce the
stability of these dyes. In addition, aggregation and charge recombination of organic
dye on the surface of TiO2 also restrict the improvement of the Jsc and Voc. At present,
the main strategies to restrain intermolecular aggregation and charge recombination of
the sensitizers are introducing alkyl chain [67-69], and bulky group [70-72] or
co-sensitization [73,74], and so on. But these tactics do not show obvious effect on
the two negative aspect sometimes, such as the triiodide can easily penetrate through
the alkyl chain or the interval to reach the surface of TiO2 [75].
Therefore, a more effective method is highly desired, which not only can prevent

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the redox couple approach of the surface of TiO2, but also can suppress the
intermolecular aggregation. For example, introducing a more appropriate group into
the ordinary dyes at an appropriate site could form a compact sensitizer layer at the
surface of the TiO2. Rotaxane has attracted considerable attention in the recent years
because of specific framework consisting of mechanically interlocked threads and
macrocycles, which has remarkable potential to evolve into molecular switches and
multifunctional rotaxane-based molecular systems in future [76-78]. Introducing the
rotaxane-like molecule into the DSSCs at an appropriate site, not only suppress the
dye aggregation and reduce the charge recombination, but also immobilizes the
conjugated backbone in a planar configuration [79]. Encapsulated insulated molecular
wire (EIMW) is a rotaxane-like configuration which may play a role as a barrier
between the two dye molecules. Pyrene is a polycylic aromatic hydrocarbon which
has extensive electron delocalization nature [80,81]. It could be exploited as a
sensitization unit by coupling with electron rich moieties [82]. Thus, introducing the
pyrene into the donor of PTZ may enhance the light absorption capacity, meanwhile,
introducing the EIMW unit into the system may suppress dye aggregation.
Based on the above considerations, two novel phenothiazine dyes (PH2 and PH3)
with EIMW as an auxiliary donor were designed and synthesized (Fig. 1). To the best
of our knowledge, EIMW unit is introduced as the auxiliary donor into the organic
dyes for the first time. PH1 without EIMW was prepared as a reference dye. The
photophysical, electrochemical and photovoltaic properties of the dyes and their
DSSCs were studied systematically.

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Fig. 1. Chemical structures of the sensitizers (PH1, PH2 and PH3).
2. Experiment
2.1 Materials and Reagents
The raw materials were purchased from J&K, Energy Chemical and Adamas in
analytical grade. Toluene, THF, dioxane and 1,2-dichloroethane were distilled by
sodium. Other solvents and regents were of an analytical grade and used without
further purification. The reactions were monitored by TCL until the reaction
completely. Chromatographic separations were carried out on silica gel (300-400
mesh).
2.2 Instruments and Characterization
1H and 13C NMR spectra were recorded on Bruker 400 MHz instrument in CDCl3,
DMSO-d6 and THF-d8. HRMS spectra were recorded on an Agilent Technologies
1290 Infinity mass spectrophotometer. Melting point was measured on SGW X-4B
microscopic melting point apparatus. Ultraviolet-Visible (UV-Vis) spectra of the dyes
in solution were recorded on a Shimadzu UV-2450 spectrophotometer.
Ultraviolet-Visible (UV-Vis) spectra of the dyes anchored on TiO2 film were recorded
on a UV-3010 spectrophotometer. The PL spectra were recorded on a Fluorolog III
photoluminescence spectrometer. Electrochemical redox potentials were performed by
cyclic voltammetry (CV) using a three electrode cell on an electrochemistry
workstation at a scan rate of 50 mV s-1. The dye adsorbed on TiO2 films were used as
a working electrode. A platinum wire was utilized as the counter electrode and
Ag/AgCl (0.01 M) as reference electrode. Tetrabutylammonium hexafluorophosphate

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(TBAPF6, 0.1 M in dry acetonitrile) was used as the supporting electrolyte. The
ferrocene/ferrocenium (Fc/Fc+) redox couple acts as an internal potential. The incident
monochromatic photon-to-current conversion efficiency (IPCE) spectra were achieved
on a Spectral products DK240 monochromator from the 300~800 nm region. The
current-voltage characteristics were recorded on a Keithley 2400 source meter under
simulated AM 1.5G (100 mW cm-2) illumination with a standard solar light simulator
with a mask. The electrochemical impedance spectra (EIS) were recorded on a Zahner
Zennium electrochemical workstation under dark conditions.
2.3 Fabrication of dye-sensitized solar cells
The fabrication of the cell was carried out according to the reported procedures
[83-85]. Fluorine-doped tin oxide (FTO) glasses were washed with water, ethanol and
acetone in an ultrasonic bath to remove impurity. TiO2 nanoparticles (10 nm) were
prepared by a hydrothermal treatment with a precursor solution containing 5 mL
Ti(OBu)4, 10 mL ethanol, 9 mL acetic acid and 25 mL deionized water. The TiO2
powder (0.5 g) was ground for 40 min in the mixture of 4.0 mL ethanol, 0.1 mL acetic
acid, 1.5 g terpineol and 0.25 g ethyl cellulose to achieve a homogeneous suspension.
Finally, the plup suspension was sonicated for 20 min in an ultrasonic bath to produce
a viscous white TiO2 sizing agent. The TiO2 photoanodes were prepared by the
screen-printing process on FTO glass, the thickness of the TiO2 photoanode films was
controlled about 16 ꢀm (12 ꢀm thick transparent layer and 4 ꢀm thick scattering
layer). The prepared films were annealed through a temperature programming. The
o
o
main program as follow: 325 C for 5 min, 375 C for 5 min, 450 oC for 15 min, and

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o
500 C for 15 min then cooling to room temperature. The TiO2 films were treated by
o
0.04 M TiCl4 aqueous solution for 30 min at 70 C to improve photovoltaic
performance. Then TiO2 films were rinsed with deionized ethanol and water, then
o
sintered again at 520 C for 30 min. After the films cooling to room temperature, the
films were immersed in a 3.0 × 10-4 M solution of the dyes for 24 h (The solution is a
mixture solvent of acetonitrile and tert-butanol (volume ratio of 1:1) at 25 °C [86].
For the co-adsorption, chenodeoxycholic acid was added into the dye solution at a
concentration of 1 mM. Afterward these films were washed with ethanol and dried
naturally. The dye-loaded TiO2/FTO glass films were assembled into a sandwiched
type together with a Pt/FTO counter electrode. The active area of the dye coated TiO2
film was 0.16 cm2.
Scheme 1. Synthesis routes of dyes PH1−3.
3. Results and Discussion
3.1 Synthesis
Compounds 4 and 12 were synthesized according to the references [87-89].
Compound 14 was conveniently obtained by Suzuki coupling reaction from
compound 13 and compound 6, then compound 14 was hydrolyzed with CF3COOH to
give the target product PH2. Compound 15 was synthesized by NBS with 14. PH3
was obtained through a similar hydrolysis reaction to that of PH2. All new
compounds were characterized by multiple spectroscopic tools (NMR, HRMS). The
synthesis route of the dyes PH1–3 is depicted in Scheme 1.
3.2 Spectroscopic studies
The UV-Vis absorption spectra of PH1, PH2 and PH3 were recorded in CH2Cl2 (Fig.

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2a), and the detailed data are summarized in Table 1. The three dyes have two major
broad absorption bands in the range of 300−650 nm. The high-energy bands located at
300−400 nm are due to localized aromatic π–π* electronic transition of the
chromophores; the low-energy bands located at 420−650 nm are ascribed to
intramolecular charge transfer (ICT) from the donor to the acceptor. The maximum
absorption wavelengths of PH1, PH2 and PH3 are 493, 506 and 508 nm, respectively.
In comparison with PH1, the λmax of PH2 displays a red-shift of 12 nm due to a more
coplanar configuration by EIMW unit. The rigid circle chain fixes the bithiophene in a
coplanar configuration, which increases the probability of the ICT transition from the
donor to the acceptor. This appearance can be verified in the later by the DFT
calculation. It is surprised that PH3 only has a 2 nm red-shift relative to PH2.
Interestingly, an additional shoulder peak appears in the UV-Vis absorption spectra of
the three dyes, which is according to the additional π–π* electron transitions due to
introducing the additional donor and the peaks became more obvious follow with the
planarity of the molecule improving. The molar extinction coefficients for PH1, PH2
and PH3 are 2.15×104, 2.39×104 and 2.89×104 M-1cm-1, respectively. PH2 and PH3
have higher molar extinction coefficients than PH1. When the dyes adsorbed on TiO2,
all of the absorption spectra are broadened. The maximum absorption of PH1 and
PH2 exhibit 27 and 14 nm blue-shift with respect to those in the solution, respectively.
The reason of this hypochromatic shift may be ascribed to deprotonation of the
carboxylic acid and intermolecular aggregation. PH2 shows a slight blue-shift
although it presents better planarity, which might be due to introducing the EIMW

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into the system. Interestingly, PH3 displays about 40 nm blue-shift. This phenomenon
maybe due to introducing the pyrene into the system, which let dye PH3 has a better
planarity, the circle chain could not suppress the aggregation completely.
To understand the origins of the additional absorption band in D–D–π-A dyes, we
determined the UV-Vis absorption spectra of the additional donor (bithiophene,
EIMW and pyrene), the results are shown in Figure S31. The bithiophene unit and
EIMW unit show a shoulder peak at 330 nm and the shoulder peak of the latter more
obvious. In addition, pyrene unit shows an absorption peak at 340 nm. While
above-mentioned building blocks connect with phenothiazine system, the absorption
peaks are red-shift to near 400 nm. Introducing the additional donor to phenothiazine,
the dye shows extra absorption peak due to additional electron transition, and the
intensity of the absorption is improved along with extending conjugation[58,90]. In
addition, the absorption spectra of dyes co-adsorbed CDCA in solution and the cells
with electrolyte solution were determined, as shown in Figure S32 and S33. When
CDCA was added, the cell sensitized by PH1 presented obvious red-shift in
comparison with the cell without CDCA. The results indicate a higher degree
aggregation of dye PH1 on the TiO2 films.
Table 1. Photophysical and electrochemical properties of the dyes
Fig. 2. The UV-Vis absorption spectra of these dyes (a) and the absorption spectra of
these dyes adsorbed on TiO2 films (b).
3.3 Electrochemical properties
To evaluate the thermodynamic possibility of electron injection and regeneration

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of the three dyes, cyclic voltammograms were recorded on CH3CN to determine the
oxidation potentials (Fig. 3). The oxidation potentials correspond to the HOMOs, and
the electrochemical data are summarized in Table 1. The potential was externally
calibrated by ferrocenium/ferrocene (Fc+/Fc) couple and then was calculated versus
NHE electrode (E1/2 (Fc+/Fc) = 0.53 V vs Ag/AgCl in our case, and 0.64 V vs NHE)
[91]. The HOMO level of PH1, PH2 and PH3 are 0.94, 0.95 and 1.14 V, respectively.
-
These values are more positive than the I-/I3 redox potential value (0.4 V vs. NHE),
indicating that the oxidized dyes can be efficiently regenerated by the electrolyte. The
E0-0 values are 2.15, 2.16 and 2.24 V, respectively, which are estimated from the
intersection of the normalized absorption and emission spectra. The LUMO levels of
PH1–PH3 calculated from HOMO – E0-0 are –1.21, –1.21 and –1.10 V, respectively.
The values are more negative than the CB level of the TiO2 (–0.5 V vs. NHE), which
implies that an efficient electron injection process from the excited dye into the
conduction band of TiO2 can occur for the dyes.
Fig. 3. Cyclic voltammograms (a) and Schematic energy diagram of these dyes (b).
Table 2. The computed energy levels and the spatial distribution of the frontier
molecular orbitals of the dyes
3.4 Theoretical calculations
Density function theory (DFT) calculations at the DFT-B3LYP/6-31G level with
Gaussian 09 suite of programs were utilized to investigate the electron-density
distributions of the three dyes as shown in Table 2. The HOMO levels of PH1–3 are
mainly delocalized throughout the bithiophene and phenothiazine rings, the LUMO

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orbitals are delocalized in cyanoacetic acid. It can be found that clear variation when
introducing the EIMW into the system. When introducing the pyrene group into the
conjugation structure, the LUMO orbitals no obvious changed, but the HOMO
orbitals delocalized throughout the pyrene group to the p-(hexyloxy)benzoyl group.
The electronic properties of the three dyes indicate that such well-separated orbital
distribution is conducive to the electron transfer from HOMO to LUMO and benefits
for inducing the electron injection from the LUMO into the conduction band of TiO2.
The optimized ground-state geometries of the three dyes are shown in Table 2.
For PH1, the dihedral angles between the two thiophenes, thiophene and
phenothiazine are 2.87 o and 18.55 o, respectively. For PH2 with EIMW unit linked to
bithiophene, the dihedral angles between the two thiophene units, thiophene and
phenothiazine are 2.41o and 14.7o, respectively. Obviously, PH2 possesses a better
coplanar configuration than PH1. This result indicates a better ICT effect for PH2
than PH1 and it is consistent with the absorption properties described above. However,
when pyrene group was introduced into the conjugation structure, the torsion angle of
the two thiophenes, thiophene and phenothiazine, pyrene and thiophene were 2.15 o,
14.86 o and 49.19 o, respectively.
3.5 Photovoltaic performance of the DSSCs
Fig. 4. exhibits the current-voltage characteristics of the DSSCs fabricated with the
three phenothiazine dyes based on an irradiance of simulated AM 1.5G sunlight (100
mW cm-2). The detail photovoltaic parameters of short-circuit photocurrent (Jsc),
open-circuit photovoltage (Voc), fill factor (FF) and overall conversion efficient (η)

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are shown in Table 3. The cell sensitized by PH1 exhibits the lowest η of 5.64 % with
a Jsc of 11.2 mA cm-2, a Voc of 730 mV and an FF of 0.69. PH2 exhibits an efficiency
of 7.08%, with a Jsc of 13.40 mA cm-2, a Voc of 760 mV, and a FF of 0.69. In
comparison to PH1, the higher Jsc of PH2 may be ascribed to the higher molar
extinction coefficient. When introducing pyrene into the system, PH3 exhibits a slight
decrease of Jsc compared to the other two dyes, which is attributed to the more serious
dye aggregation. The good planarity of pyrene might aggravate intermolecular
aggregation. In addition, compared with PH1, the Voc of PH2 was improved 30 mV,
which is ascribed to the EIMW unit in the architecture. The steric properties and size
of the circle chain in the sensitizers may have an obvious impact on the recombination
process at the photoelectrode interface in the cells [92]. In addition, the open-circuit
voltage (Voc) is similar for PH2 and PH3.
The photoresponse properties of these cells sensitized by PH1–3 also evaluated by
incident photon-to-current conversion efficiency (IPCE) spectra, as shown in Fig. 4a.
The three dyes exhibit a broad spectral response range from 300 to 700 nm, indicating
that all the three dyes can absorb the visible light convert it into photocurrent
efficiently. The IPCE value of PH2 is over 60% from 400 to 600 nm with a maximum
IPCE value of 70% at 550 nm, showing a higher IPCE value than the others. This
result in good accordance with the J–V (current–voltage) measurements. It is
surprised that PH3 show a good light harvesting ability in absorption spectra (Fig. 2),
but lower IPCE values compared to PH2. In comparison with PH2, the pyene unit
was introduced to PH3, so the lower IPCE values due to more serious intermecular

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aggregation, leading to lower charge injection efficiency [84].
Fig. 4. (a) IPCE spectra and (b) J−V curves of these cells based on dyes PH1-3.
Fig. 5. J − V characteristics of these DSSCs with 1 mM CDCA as a co-adsorbent.
An appropriate amount of chenodeoxycholic acid (CDCA) was added as a
co-adsorbent to see if the novel auxiliary donor reduces dye aggregation and charge
recombination. The J – V curves were measured and shown in Fig. 5, and the specific
data were listed in Table 3. The photovoltaic conversion efficiencies were improved
for all the three cells when CDCA was added as a co-adsorbent. The efficiency of the
cell based on PH1 was improved about 40% relative to that without CDCA. Such a
big improvement indicates that obvious aggregation occurred for PH1. However, only
about 11% and 20% improvements are for PH2 and PH3, respectively. The less
improvement with CDCA, for PH2 and PH3 in comparison with PH1 confirms that
the EIMW unit can inhibit dye aggregation to a certain extent.
Table 3. Photovoltaic performance parameters of the DSSCs based on PH1–3
3.6 Electrochemical impedance spectra ꢀEISꢁ
To more explicit the electron recombination in the DSSCs, electrochemical
impedance spectroscopy (EIS) was conducted in dark. The Nyquist plots for the
DSSCs based on the dyes are shown in Fig. 6 and the detail data shown in Table 4.
The first semicircle (Rce) is belonged to the resistance of charge transfer at the counter
electrode/electrolyte interface, while the second semicircle (Rrec) is belonged to the
resistance of charge transfer at the TiO2/dye/electrolyte interface. Charge transfer

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resistance of the counter electrode (Rce) and recombination resistance (Rrec) were
analyzed by Z-view software using an equivalent circuit. Similar Rce values were
obtained in the low frequency range (7.57Ω cm-2 for PH1, 9.54 Ω cm-2 for PH2,
10.91 Ω cm-2 for PH3 because of using the same counter electrode and electrolyte at
this experiment. The second semicircle (Rrec) values for PH1, PH2 and PH3 are 52.92,
68.29 and 108.4 Ω cm-2, respectively. The electron lifetime (τct) of DSSCs were
calculated by applying equations of τct = Rct × Cchem. The corresponding electron
lifetimes for PH1, PH2 and PH3 are 78, 91 and 156 ms, respectively. The electron
lifetime trend is in order of PH1 < PH2 < PH3, which clearly indicates that the
recombination of the injected electron with I-/I3 in the electrolyte can be effectively
-
suppressed by EIMW.
Fig. 6. Electrochemical impedance spectra (Nyquist plot) of the DSSCs measured in
the dark.
Table 4. Parameters obtained by fitting the impedance spectra of the DSSCs with
PH1–3
4. Conclusion
In summary, we designed and synthesized two novel phenothiazine dyes with
encapsulated insulated molecular wire as an auxiliary donor, and studied the
photoelectric properties of their dye-sensitized solar cells. Compared to PH1 without
EIMW, the rigid circle chain of EIMW of PH2 can inhibit the dye aggregation greatly,
resulting in much higher photovoltage of the cell. In addition, the circle chain plays a

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role as a compact layer on the surface of the TiO2, leading to suppress the charge
recombination effectively. The results indicate that it is an effective way to increase
the photovoltage and efficiency of the DSSCs by introducing the EIMW into the
organic dyes.
ASSOCIATED CONTENT
Supporting Information
Experimental details, 1H NMR, 13C NMR of all of the new compounds and HRMS of
PH1, PH2 and PH3, spectroscopic data.
ACKNOWLEDGMENTS
We are grateful to the National Key Research and Development Program of
China (2016YFA0602900), the National Natural Science Foundation of China
(21572069, 21772045), and the Natural Science Foundation of Guangdong Province,
China (2018B030311008).
NOTES
The authors declare no competing financial interest.
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Figure and Table Captions
Table 1. Photophysical and electrochemical properties of the dyes
Table 2. The computed energy levels and the spatial distribution of the frontier
molecular orbitals of the dyes
Table 3. Photovoltaic performance parameters of the DSSCs based on PH1–3
Table 4. Parameters obtained by fitting the impedance spectra of the DSSCs with
PH1–3
Scheme 1. Synthesis routes of dyes PH1–3.
Fig. 1. Chemical structures of the sensitizers (PH1, PH2 and PH3).
Fig. 2. The UV-Vis absorption spectra of the dyes in solution (a) and the absorption
spectra of these dyes adsorbed on TiO2 films (b).
Fig. 3. Cyclic voltammograms (a) and Schematic energy diagrams of these dyes (b).
Fig. 4. (a) IPCE spectra and (b) J−V curves of the DSSCs based on dyes PH1-3.
Fig. 5. J − V characteristics of the DSSCs with 1 mM CDCA as a co-adsorbent.
Fig. 6. Electrochemical impedance spectra (Nyquist plot) of the DSSCs measured in
the dark.

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Table 1. Photophysical and electrochemical properties of the dyes
HOMOc LUMOe
Dye
λmax/nma
ε/M-1 cm-1
λmax/nm b E0-0/eVd
PH1
PH2
PH3
493
506
508
21500
23900
28900
466
492
466
2.15
2.16
0.94
0.95
1.14
–1.21
–1.21
–1.10
2.24
a
b
Absorption maximum measured in CH2Cl2 (1
×
10-5 M). Absorption maximum peak of dye
c
adsorbed on TiO2. Oxidation potential measured in CH3CN containing 0.1 M (n-C4H9)4NPF6
with a scan rate of 50 mVs-1 (vs. a normal hydrogen electrode (NHE)). d Zero
–
zero band gap was
e
determined from the intersection of the normalized absorption and emission spectra. LUMO =
HOMO – E0-0.
Table 2. The computed energy levels and the spatial distribution of the frontier
molecular orbitals of the dyes
PH1
PH2
PH3
optimized
geometries
HOMO
LUMO

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Table 3. Photovoltaic performance parameters of the DSSCs based on PH1–3
Jsc (mA cm-2)
Voc (V)
0.73
0.76
0.77
0.74
0.78
0.79
0.68
FF
η (%)
5.64
7.08
6.51
7.92
7.88
7.85
7.61
Dye
11.20
0.69
0.69
0.67
0.67
0.69
0.69
0.68
PH1
13.40
PH2
12.68
PH3
PH1+CDCA
PH2+CDCA
PH3+CDCA
N719
16.08
14.79
14.48
16.41
The electrolyte solution is composed of 0.6 M 1-metyl-3-propylimidazoliumiodide (PMII), 0.05 M
LiI, 0.10 guanidiniumthiocyanate, 0.03 I2 and 0.5 tert-butylpridine in
M
M
M
acetonitrile/valeronitrile (85 : 15) was injected from a hole made on the counter electrode into the
space between the sandwiched cells. The active area of those dyes coated on TiO2 was 0.16 cm2.
The photoanodes were immersed in a 3.0 × 10-4 M solution of the dyes for 24 h (The dye solutions
were prepared in the mixture of CH3CN/tert-BuOH = 1:1).
Table 4. Parameters obtained by fitting the impedance spectra of the DSSCs with
PH1–3
Dye
PH1
PH2
PH3
Rce (Ω cm-2)
Rrec (Ω cm-2)
CPE (μF)
1477
τ (ms)
78
7.57
52.92
9.54
68.29
1336
91
10.91
108.4
1441
156

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Scheme 1.
Synthesis routes of dyes PH1–3. a.1-(Hexyloxy)-4-iodobenzene, Pd2(dba)3, t-BuOK,
P(t-Bu)3, toluene, 110 oC, 24 h, 78%; b. POCl3, DMF, 1,2-dichloroethane, 60 oC, 72%;
c. NBS, THF, rt, 8 h, 85.5%; d. NH4AC, CH3COOH, tert-butyl cyanoacetate, toluene,
o
130 C, 4 h, 93.9%; e. KOAC, Pd(dppf)2Cl2, bis(pinacolato)diboron, 1,4-dioxane, 95
oC, 18 h, 64%; f. 5-bromo-2,2'-bithiopheneꢃ Pd(PPh3)4, Na2CO3, CH3CH2OH/H2O,

ACCEPTED MANUSCRIPT
toluene, 18 h, 90 oC, 67.5%; g. CF3COOH, rt, 3h, 91%; h.
(2,6-dimethoxyphenyl)boronic acid, Pd2(dba)3, s-Phos, K3PO4, toluene, 105 oC, 18 h,
75%; i. BBr3, CH2Cl2, 0 oC, 2 h, 68%; j. Cs2CO3, DMSO, 5-bromo-1-pentene, 100 oC,
o
10 h, 88%; k. 2nd generation Grubbs catalyst, CH2Cl2, 40 C, 3 h, 59.6%; l. Pd/C,
o
o
CH2Cl2/CH3OH, H2, 5h, 35 C, quantitatively; m. NBS, THF, 0 C, 5min, 82%; n.
o
Compound 6, Pd(PPh3)4, Na2CO3, CH3CH2OH/H2O, toluene, 18 h, 90 C, 52%; o.
CF3COOH, rt,
4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane,
CH3CH2OH/H2O, toluene, 18 h, 90 oC, 62%; r. CF3COOH, rt, 3 h, 87%.
3
h, 90%; p. NBS, THF,
0
oC,
Pd(PPh3)4,
6
h, 52%; q.
Na2CO3,

ACCEPTED MANUSCRIPT
Fig. 1.
Fig. 2

ACCEPTED MANUSCRIPT
Fig. 3
Fig.4

ACCEPTED MANUSCRIPT
Fig. 5
Fig. 6