A co-sensitized approach to efficiently fill the absorption valley, avoid dye aggregation and reduce the charge recombination

Article abstract of DOI:10.1016/j.electacta.2016.08.072

Compared with the development of novel organic dyes, co-sensitization is a much more convenient way to achieve highly efficient dye-sensitized solar cells (DSSCs). Here, two simple donor-(π conjugated spacer)-acceptor dyes, in which hexyloxy-substituted phenyl as an electron donor, cyanoacrylic acid as the electron acceptor and different spacers thiophene (LD03) or furan (LD04) as the π-linkers, were designed and synthesized as co-sensitizers for N719-based DSSCs. The photophysical, electrochemical, photovoltaic performances and electrochemical impedance spectroscopy of these DSSCs were investigated in detail. When compared to the DSSCs fabricated from individual N719, the co-sensitized DSSCs (N719?+?LD03 and N719?+?LD04) showed significantly enhanced power conversion efficiencies (PCEs), short-circuit photocurrent density (J_sc) and open circuit voltage (V_oc) simultaneously, which owing to the co-sensitizers could effectively overcome the competitive light absorption by I₃^? in the electrolyte, avoid dye aggregation and reduce the charge recombination. Consequently, the co-sensitized DSSC (N719?+?LD04) gave the highest PCE of 8.955% (J_sc?=?17.628?mA?cm^?2,?V_oc?=?0.758?V and fill factor?=?0.670), exhibiting an improvement of 13.412% compared to the device sensitized with N719 alone (PCE?=?7.896%) under illumination (AM 1.5G, 100?mW?cm^?2).

Full text of DOI:10.1016/j.electacta.2016.08.072

Accepted Manuscript
Title: A co-sensitized approach to efficiently fill the absorption
valley, avoid dye aggregation and reduce the charge
recombination
Author: Junsheng Luo Zhongquan Wan Chunyang Jia Yan
Wang Xiaochun Wu
PII:
S0013-4686(16)31783-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.08.072
EA 27850
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
24-5-2016
20-7-2016
15-8-2016
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<AT>A co-sensitized approach to efficiently fill the absorption valley, avoid dye
aggregation and reduce the charge recombination
<AU>Junsheng Luoa, Zhongquan Wana, Chunyang Jiaa,*
##Email##cyjia@uestc.edu.cn##/Email##, Yan Wanga, Xiaochun Wub
<AU>
<AFF>aState Key Laboratory of Electronic Thin Films and Integrated Devices, School
of Microelectronics and Solid-State Electronics, University of Electronic Science and
Technology of China, Chengdu 610054, PR China
<AFF>bCollege of Chemistry and Materials Science, Sichuan Normal University,
Chengdu 610068, PR China
<PA>Tel.: +86 28 83201991; Fax: +86 28 83202569.
<ABS-Head><ABS-HEAD>Graphical abstract
<ABS-P>
<ABS-P><xps:span class="xps_Image">fx1</xps:span>
<ABS-HEAD>Highlights► Two simple co-sensitizers were synthesized for
N719-based DSSCs. ► The co-sensitizers could avoid dye aggregation and reduce
charge recombination. ► Both Jsc and Voc of the co-sensitized DSSCs were enhanced
relative to single N719. ► Co-sensitization of a DSSC with N719 and LD04 exhibited
the highest PCE of 8.955%.
<ABS-HEAD>Abstract
<ABS-P>Compared with the development of novel organic dyes, co-sensitization
is a much more convenient way to achieve highly efficient dye-sensitized solar cells
(DSSCs). Here, two simple donor-(π conjugated spacer)-acceptor dyes, in which
hexyloxy-substituted phenyl as an electron donor, cyanoacrylic acid as the electron
acceptor and different spacers thiophene (LD03) or furan (LD04) as the π-linkers, were
1

designed and synthesized as co-sensitizers for N719-based DSSCs. The photophysical,
electrochemical, photovoltaic performances and electrochemical impedance
spectroscopy of these DSSCs were investigated in detail. When compared to the
DSSCs fabricated from individual N719, the co-sensitized DSSCs (N719 + LD03 and
N719 + LD04) showed significantly enhanced power conversion efficiencies (PCEs),
short-circuit photocurrent density (Jsc) and open circuit voltage (Voc) simultaneously,
which owing to the co-sensitizers could effectively overcome the competitive light
-
absorption by I3 in the electrolyte, avoid dye aggregation and reduce the charge
recombination. Consequently, the co-sensitized DSSC (N719 + LD04) gave the highest
PCE of 8.955% (Jsc = 17.628 mA cm-2, Voc = 0.758 V and fill factor = 0.670), exhibiting
an improvement of 13.412% compared to the device sensitized with N719 alone (PCE
= 7.896%) under illumination (AM 1.5G, 100 mW cm-2).
<KWD>Keywords: Dye-sensitized solar cells; Co-sensitization; Absorption valley;
Dye aggregation; Charge recombination
<H1>1. Introduction
As a kind of prospective photovoltaic devices, dye-sensitized solar cells (DSSCs)
have been intensively investigated since DSSCs were reported for the first time by
Grätzel and co-workers in 1991 [1]. Generally, a conventional DSSC contains three
components: a dye-sensitized TiO2 photoanode, a redox electrolyte/hole transporter
and a counter electrode [2]. Upon irradiation, light is absorbed by sensitizer, which
creates a high-energy state and results in electron injection from the photoexcited
sensitizer into the conduction band (CB) of TiO2; the oxidized sensitizer is
2

subsequently backed to the ground state by electron transfer from redox couple. As a
key component of DSSCs, the sensitizer plays an important role in light harvesting,
electron injection as well as the device stability [3]. To date, many efforts have been
devoted to develop new and highly efficient sensitizers, which can be divided into two
general classes: metal-complex sensitizers and metal-free organic sensitizers [4-18].
For DSSCs using single sensitizer, the record high power conversion efficiencies
(PCEs) over 13% [15], 11.5% [16] and 12.5% [17], have been achieved for
zinc-porphyrin, ruthenium complex and metal-free organic sensitizers, respectively,
which were lower than that of perovskite solar cells. Perovskite solar cells, as a new
class of thin film solid state solar cells, have received widespread attention in recent
years [19-21]. The highest certified efficiency over 20% was recently reported for a
solar cell based on formamidinium lead iodide, which was fabricated by an
intramolecular exchange process [22]. However, there are two key issues for the
application of perovskite solar cell in industry, toxicity and device stability.
Meanwhile, the perovskite materials (typically CH3NH3PbI3 and CH3NH3PbI3-xClx) are
susceptible to the following factors: oxygen and moisture, UV light, solution process
(solvents, solutes, additives) and temperature [23]. Therefore, significant degradation
of organic-inorganic halide perovskite materials is typically observed under ambient
conditions. Compared with perovskite solar cells, the DSSCs have unique advantages,
such as low production cost, excellent stability, environmental friendly and easy of
fabrication [24-26]. However, it is necessary to further improve the PCEs to enable
successful commercialization. Many attempts, such as innovation of photoactive
3

materials and photo-anodes, utilization of new electrolytes and counter electrodes,
co-sensitization of two or multiple dyes, have been conducted to improve the
photovoltaic efficiencies of DSSCs [27-32].
Among various approaches, co-sensitization is an effective strategy to enhance
photovoltaic properties of DSSCs through combination of two or more dyes on
semiconductor film together. This strategy usually can improve the light harvesting
ability both in the absorption intensity and absorption breadth so as to increase
photocurrents of the devices. To date, series of co-sensitization systems have been
proposed and demonstrated improved photovoltaic performances with respect to
DSSCs based on individual sensitizers [33-38]. It is worth noting that the three
record-breaking DSSCs are related to the strategy of co-sensitization. In 2012, a record
efficiency (11.4%) was achieved for DSSCs based on a co-sensitized system of
metal-free dye and black dye by the work of Han and coworkers [33]. Subsequently, a
new record high PCE of 12.3% was reported with co-sensitization of a porphyrin dye
YD2-o-C8 and a metal-free dye Y123 [37]. In 2015, an impressive photovoltaic
performance was achieved 14.7% with collaborative sensitization by silyl-anchor and
carboxy-anchor dyes [39]. To the best of our knowledge, the PCE of 14.7% represented
the highest PCE value reported to date for DSSCs. Overall, these results indicate that
there is still room for improving photovoltaic performance by co-sensitization.
According to the previous report, the ideal candidate for a co-sensitizer should have
following characteristics: (i) it should have a large molar extinction coefficient (ε) in
the infrared region or around 400 nm to recover the dip in the incident
4

-
photon-to-current conversion efficiency (IPCE) spectra induced by I3 ; (ii) the structure
of the co-sensitizer should be suitable for avoiding competitive adsorption with main
sensitizer while effectively suppressing the aggregation of dyes on the TiO2 surface;
-
(iii) it should be able to reduce the recombination of electrons in the TiO2 film with I3
by the formation of a compacted molecule monolayer covering the bare TiO2 surface
[33].
Based on the above requirements, we reported the synthesis and characterization of
two simple donor-(π conjugated spacer)-acceptor type dyes containing
hexyloxy-substituted phenyl as electron-donating unit, thiophene or furan as π-spacer,
cyanoacrylic acid group as acceptor and anchor, coded as LD03 and LD04,
respectively, which were applied in DSSCs by co-sensitization with N719. The
chemical structure of two co-sensitizers is shown in Fig. 1. It was found that the
short-circuit photocurrent density (Jsc) and open circuit voltage (Voc) of the
co-sensitized DSSCs (N719 + LD03 and N719 + LD04) were significantly enhanced
relative to the DSSC sensitized with individual N719, resulting in impressively
improved photovoltaic performances. Consequently, the co-sensitized DSSC based on
LD04 with N719 gave the highest PCE of 8.955%, exhibiting an improvement of
13.412% compared to the device sensitized with N719 alone (PCE = 7.896%) under the
same conditions.
Fig. 1. is here
<H1>2. Experimental section
5

<H2>2.1. Materials and reagents
The starting materials 4-hexyloxyphenylboronic acid,
5-bromo-thiophene-2-carbaldehyde and 5-bromo-furan-2-carbaldehyde were
purchased from Aladdin. The TiO2 electrodes and electrolyte solution (DHS-Et23)
were achieved from Dalian Heptachroma Solar Tech, Co., Ltd., China. The solvents
(methanol and tetrahydrofuran) were dried by standard procedures. All other solvents
and chemicals obtained from commercial sources, and used as received without further
purification.
<H2>2.2. Characterization
NMR spectra were obtained with a Brücker AM 400 spectrometer (relative to
TMS). High resolution mass spectra (HRMS) were determined with a Waters ESI mass
spectrometer. The UV-vis absorption spectra of the dyes either in butanol-acetonitrile
(1/1, v/v) solution or on the adsorbed TiO2 films (individual sensitizer and
co-sensitizers) were measured by HITACHI (model U-2910) UV-vis
spectrophotometer. Emission spectra were performed using a HITACHI (model
F-4600) spectrophotometer. Cyclic voltammetry (CV) experiments were performed in
N, N-dimethylformamide (5 × 10-4 M) at a scan rate of 10 mV s-1 with a three electrode
configuration consisting of a Pt wire counter electrode, a Ag/AgCl (saturated KCl)
reference electrode and a Pt disk working electrode, using 0.1 M n-Bu4NPF6 as the
supporting electrolyte.
<H2>2.3. Fabrication of DSSCs
6

The commercial TiO2 electrodes were sintered at 500 °C for 30 min in an air
atmosphere prior to use. After cooling down to around 80 °C, the TiO2 electrodes were
immersed in a dye bath containing 0.2 mM LD03, LD04 and N719 in solution
butanol-acetonitrile (1/1, v/v), then kept at room temperature for 15 h in the dark.
Afterward, the dye-adsorbed TiO2 electrodes were rinsed with ethanol to get rid of the
excess dyes and dried in air flow. Typically, co-sensitized electrode was immersed in a
solution (0.2 mM N719 in butanol-acetonitrile (1/1, v/v)) for 15 h and then washed with
ethanol. It was further immersed in a solution (0.2 mM LD03 or 0.2 mM LD04 in
butanol-acetonitrile (1/1, v/v)) for 3 h and then washed with ethanol. The final process
for the fabrication of the DSSCs was performed by assembling the dye-adsorbed TiO2
electrodes and Pt counter electrodes with a 25 μm thick thermoplastic Surlyn frame. An
electrolyte solution was injected into interlayer between two electrodes through a hole
which was then sealed with thermoplastic Surlyn covers and a glass coverslip.
<H2>2.4. Photovoltaic characterization
The current-density voltage (J-V) curves of the DSSCs were measured using an
electrochemical workstation (CHI 660C, Shanghai Chenhua) under AM 1.5G
simulated solar light (100 mW cm-2) (CHF-XM-500W, Trusttech Co. Ltd., Beijing,
China). The incident light intensity (Pin) was calibrated with a standard Si solar cell.
The PCE of the DSSC is calculated from Jsc, Voc, fill factor (FF) and Pin according to the
following equation:
Voc JscFF
PCE % 
 
100 (1)
P
in
7

The spectra of monochromatic IPCE for solar cells were performed by using a
commercial setup (QTest Station 2000 IPCE Measurement System, CROWNTECH,
USA). The electrochemical impedance spectroscopy (EIS) of the DSSCs was recorded
under dark conditions using an electrochemical workstation (CHI 660C, Shanghai
Chenhua) over a frequency range from 10-1 Hz to 105 Hz with a bias potential of 0.75 V.
<H2>2.5. Synthesis
All reactions were carried out under nitrogen atmosphere. Suzuki coupling reaction of
compound 1 with compounds 2 and 3 afforded corresponding Knoevenagel precursor
compounds 4 and 5, respectively. This reaction was carried out in
methanol/tetrahydrofuran = 1:4 (v/v) medium by using Pd(PPh3)4 as catalyst under
reflux condition for 12 h. These aldehyde intermediates (compounds 4 and 5) were
converted into corresponding target dyes through Knoevenagel condensation reaction
by treating with cyanoacrylic acid using piperdine as catalytic for LD03 and LD04,
respectively. The synthetic routes of the dyes LD03 and LD04 are shown in Scheme 1.
Scheme 1 is here
<H3>2.5.1. 5-(4-Hexyloxy-phenyl)-thiophene-2-carbaldehyde (4)
A mixture of 4-hexyloxyphenylboronic acid (1) (210 mg, 0.95 mmol),
5-bromo-thiophene-2-carbaldehyde (2) (120 mg, 0.63 mmol), Pd(PPh3)4 (37 mg, 0.03
mmol), 1 M aqueous solution of K2CO3 (2 mL) and 20 mL methanol/tetrahydrofuran =
1:4 (v/v) were added in a 100 mL three-necked flask and degassed with nitrogen. Then,
the reaction mixture was stirred at 67 ℃ for 12 h under nitrogen atmosphere. After
cooling to room temperature, the reaction was quenched by water and the organic layer
8

extracted with CH2Cl2. The organic layer was dried with anhydrous MgSO4 and the
solvent was removed by rotary evaporation. The crude product was purified by silicon
gel column chromatography (eluent petroleum ether: dichloromethane = 1: 1) to give a
cream colored solid 150 mg. Yield: 55%. 1H NMR (400 MHz, CDCl3) δ: 9.89 (s, 1H),
7.73 (d, J = 3.2 Hz, 1H), 7.63 (d, J = 6.8 Hz, 2H), 7.32 (d, J = 3.2 Hz, 1H), 7.29 (s, 1H),
6.97 (d, J = 7.2 Hz, 2H), 4.03 (t, J = 5.2 Hz, 2H),1.86-1.80 (m, 2H), 1.52-1.47 (m, 2H),
1.38 (d, J = 2.4 Hz, 4H), 0.94 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ:
182.65, 160.37, 154.72, 141.47, 137.68, 127.81, 125.57, 122.91, 115.13, 68.26, 31.57,
29.15, 25.69, 22.60, 14.03 ppm.
<H3>2.5.2. 5-(4-Hexyloxy-phenyl)-furan-2-carbaldehyde (5)
4-Hexyloxyphenylboronic acid (1) (190 mg, 0.86 mmol),
5-bromo-furan-2-carbaldehyde (3) (100 mg, 0.57 mmol), Pd(PPh3)4 (33 mg, 0.03
mmol) in 20 mL methanol/ tetrahydrofuran = 1:4 (v/v) and a 1 M K2CO3 aqueous
solution (2 mL) was degassed with nitrogen. Then, the reaction mixture was stirred at
67 ℃ for 12 h under nitrogen atmosphere. After the reaction, water was added and the
mixture extracted with CH2Cl2. The organic layer was dried with anhydrous MgSO4
and then filtered. The solvent was removed by rotary evaporation. The crude product
was purified by silicon gel column chromatography (eluent petroleum ether:
dichloromethane = 1: 1) to give a white solid 136 mg. Yield: 58%. 1H NMR (400 MHz,
CDCl3) δ: 9.62 (s, 1H), 7.77 (d, J = 6.8 Hz, 2H), 7.32 (d, J = 2.8 Hz, 1H), 6.97 (d, J = 6.8
Hz, 2H), 6.73 (d, J = 2.8 Hz, 1H), 4.03 (t, J = 5.2 Hz, 2H), 1.82-1.78 (m, 2H), 1.50 (t, J
= 6.8 Hz, 2H), 1.38 (d, J = 2.8 Hz, 4H), 0.94 (t, J = 2.8 Hz, 3H); 13C NMR (100 MHz,
9

CDCl3) δ: 176.82, 160.53, 159.97, 151.62, 128.21, 126.97, 121.57, 114.96, 106.20,
68.22, 31.57, 29.52, 25.69, 22.59, 14.02 ppm; HRMS (ESI, m/z): [M + H]+ calcd for
(C17H20O3): 273.1412; found, 273.1370.
<H3>2.5.3. 2-Cyano-3-[5-(4-hexyloxy-phenyl)-thiophen-2-yl]-acrylic acid (LD03)
In a 100 mL three-necked flask, 5-(4-hexyloxy-phenyl)-thiophene-2-carbaldehyde
(4) (80 mg, 0.28 mmol), cyanoacrylic acid (35 mg, 0.42 mmol), CHCl3 (15 mL), a few
drops of piperidine were added and heated to refluxed under a nitrogen atmosphere for
12 h. Then, water (10 mL) was added to the reaction mixture. The mixture was acidified
with 2 M HCl (30 mL) and extracted with CHCl3. Organic phase was dried over
anhydrous MgSO4. The solvent was removed by rotary evaporation and the crude
product was purified by silicon gel column chromatography (eluent ethanol:
dichloromethane = 1: 8) to give yellow solid 68 mg. Yield: 68%. 1H NMR (400 MHz,
CDCl3) δ: 8.33 (s, 1H), 7.78 (d, J = 2.0 Hz, 1H), 7.66 (d, J = 5.6 Hz, 2H), 7.36 (d, J = 2.0
Hz, 1H), 6.97 (d, J = 7.2 Hz, 2H), 4.03 (t, J = 4.8 Hz, 2H), 2.00-1.97 (m, 2H), 1.52-1.50
(m, 2H), 1.33 (d, J = 5.2 Hz, 4H), 0.94 (t, J = 4.8 Hz, 3H); HRMS (ESI, m/z): [M + H]+
calcd for (C20H21NO3S): 356.1242; found, 356.1248.
<H3>2.5.4. 2-Cyano-3-[5-(4-hexyloxy-phenyl)-furan-2-yl]-acrylic acid (LD04)
5-(4-Hexyloxy-phenyl)-furan-2-carbaldehyde (5) (100 mg, 0.37 mmol), cyanoacrylic
acid (47 mg, 0.56 mmol) and a few drops of piperidine in CHCl3 (20 mL) was heated to
refluxed under a nitrogen atmosphere for 12 h. After that water (10 mL) was added to
the reaction mixture. The solution was acidified with 2 M HCl (30 mL) and extracted
with CHCl3. Organic phase was dried over anhydrous MgSO4 and then filtered. The
10

solvent was removed by rotary evaporation. The crude product was purified by silicon
gel column chromatography (eluent ethanol: dichloromethane = 1: 8) to give a yellow
solid 82 mg. Yield: 65%. 1H NMR (400 MHz, CDCl3) δ: 7.99 (s, 1H), 7.85 (d, J = 6.8
Hz, 2H), 7.38 (s, 1H), 7.01 (d, J = 6.4 Hz, 2H), 6.85 (s, 1H), 4.04 (t, J = 4.8 Hz, 2H),
1.83 (t, J = 5.6 Hz, 2H), 1.50 (t, J = 3.6 Hz, 2H), 1.38 (d, J = 2.0 Hz, 4H), 0.94 (t, J = 6.8
Hz, 3H).
<H1>3. Results and discussion
The co-sensitization of LD03 or LD04 with N719 on a TiO2 films was achieved via
a stepwise approach, which accomplished adsorption of two dyes on the TiO2
photoanodes in a consecutive manner. The J-V characteristics of the DSSCs based on
individual sensitizer and co-sensitizers have been investigated under illumination (AM
1.5G, 100 mW cm-2). The J-V curves are shown Fig. 2 and corresponding photovoltaic
parameters are summarized in Table 1 for devices made of LD03 dye alone, LD04
dye alone, N719 dye alone and the N719 + LD03, N719 + LD04 co-sensitized
systems. Devices sensitized with only LD03 and LD04 yielded a PCE of 1.813% (Jsc
= 3.905 mA cm-2, Voc = 0.652 V and FF = 0.712) and 2.570% (Jsc = 5.027 mA cm-2,
Voc = 0.710 V and FF = 0.720), respectively. At the same time, the N719 sensitized
DSSC gave a Jsc of 16.883 mA cm-2, a Voc of 0.714 V and a FF of 0.655, resulted in a
PCE of 7.896%, while the co-sensitized DSSCs N719 + LD03 and N719 + LD04
showed PCEs of 8.598% (Jsc = 17.673 mA cm-2, Voc = 0.736 V and FF = 0.661) and
8.955% (Jsc = 17.628 mA cm-2, Voc = 0.758 V and FF = 0.670), respectively.
Impressively, the co-sensitized DSSCs (N719 + LD03 and N719 + LD04) yielded
11

significant enhancement in PCEs of devices over the individually sensitized devices
with improvements in both Jsc and Voc. In order to present the enhanced performances
compared with other co-sensitized molecules, the table containing photovoltaic
parameters of previously reported single sensitizers and the co-sensitized systems has
been listed in part of supporting information (see Table S1). Meanwhile, the
corresponding chemical structures have also been listed in Fig. S1 to clearly
understand the correlation between molecular structures and the photovoltaic
performances.
Fig. 2 is here
Table 1 is here
To account for the enhanced Jsc upon co-sensitization, UV-vis absorption spectra
have been carried out to study the combined light-harvesting effect of the two
sensitizers. The absorption spectra of LD03, LD04 and N719 in butanol-acetonitrile
(1/1, v/v) are shown in Fig. 3a and the photophysical data are collected in Table 2. As
shown in Fig. 3a and Table 2, the dyes LD03 and LD04 showed strong absorptions near
392 nm (ε = 2.653 × 104 M-1 cm-1) and 421 nm (ε = 3.028 × 104 M-1 cm-1), respectively,
which were assigned to the localized π-π* transitions of the conjugated systems.
Meanwhile, the N719 appeared two broad visible bands at 530 nm (ε = 1.311 × 104
M-1 cm-1) and 385 nm (ε = 1.271 × 104 M-1 cm-1) as well as a UV absorption band at
310 nm (ε = 4.671 × 104 M-1 cm-1), the former two absorption bands were attributed to
the metal-to-ligand charge transfer (MLCT) transitions while the latter at UV region
was assigned to intraligand (π-π*) charge-transfer transitions [40]. It is obvious that
12

-
the ε values of LD03 and LD04 are much higher than those of N719 and I3 (2.5 × 104
M-1 cm-1) in the low wavelength region of ~340-430 nm [33, 41]. Therefore, the
co-sensitization of N719 with LD03 or LD04 can achieve enhanced Jsc in
-
DSSCs through offsetting the loss of light absorption by I3 . Also, the absorption
spectra of individual dye N719 and co-sensitizers (N719 + LD03 and N719 + LD04)
adsorbed onto the surface of TiO2 films were depicted and shown in Fig. 3b. It is
obvious that the maximal absorption peak for N719 locate at 535 nm. The absorption
peak is slightly red-shifted by 5 nm for N719 when anchored at the TiO2 surface as
compared to the solution spectrum. This result indicates that N719 formed
J-aggregation on TiO2 surface, which is not conducive to solar cells achieve better
efficiency. Upon co-sensitization with LD03 or LD04 on TiO2 film, the maximal
absorption peak of N719 shifted back to 530 nm, which indicate that the co-sensitizers
can prevent the aggregation of N719 and adjust the arrangement of N719 molecules
toward a more nearly parallel configuration on the surface of TiO2, resulting in a
better charge transfer [35]. Moreover, it is observed that the co-adsorbed TiO2 films
exhibit enhanced light absorption in the region of ∼380-500 nm when compared with
the individually sensitized N719 film, which ensure harvesting more sunlight, resulting
in impressively improved Jsc.
Fig. 3 is here
Furthermore, for the co-sensitized systems, the enhancement of Jsc values is also
understandable from the IPCE shown in Fig. 4. Amplification of IPCE spectra in the
different wavelengths are shown in Fig. 4b and Fig. 4c. It is obvious that the DSSC
13

containing individual N719 has a broad IPCE spectrum from 300-800 nm with a
maximum value of 72.966% at 535 nm; but a dramatical decrease of the IPCE to 40% in
the wavelength range of ~350-400 nm is due to the competitive light absorption
-
between I3 and N719. It is found that LD03 and LD04 co-sensitized with N719,
respectively, not only fill the valley, but also broaden the IPCE spectra (Fig. 4b and
Fig. 4c) and consequently achieve better photocurrents. Meanwhile, addition of LD03
restored the decrease and broadened the absorption of the IPCE spectrum more
efficient than LD04, which are in good agreement with the Jsc values: N719 + LD03
(17.673 mA cm-2) > N719 + LD04 (17.628 mA cm-2) > N719 (16.883 mA cm-2).
Fig. 4 is here
The dip in IPCE spectrum of N719 based DSSC near 400 nm was recovered to an
extent by the cocktail DSSCs with N719 and the dyes LD03, LD04, which required
that the new two co-sensitizers could easily inject electron from the dyes into the TiO2
film. In order to investigate the possibilities of electron transfer from the excited states
of dyes to the CB edge of the TiO2 electrode, and dyes regeneration from the redox
couple in the electrolyte to the oxidized state dyes, the redox behaviors were performed
by cyclic voltammetry (CV) in N, N-dimethylformamide. The CV curves are shown in
Fig. 5a and the corresponding data are collected in Table 2. The first oxidation
potentials (Eox) correspond to the highest occupied molecular orbitals (HOMO) levels.
As shown in Fig. 5a and Table 2, the HOMO levels of the dyes are 1.327 V for LD03
-
and 1.334 V for LD04 vs. NHE; these values are more positive than the I-/I3 redox
potential value (0.4 V vs. NHE), which facilitate electron transfer from I- to the dyes.
14

The zero-zero band gaps (E0-0) were 2.956 V for LD03 and 2.804 V for LD04,
estimated from the intersection of the normalized absorption and emission spectra.
The normalized absorption and emission spectra of LD03 and LD04 were recorded in
butanol-acetonitrile (1/1, v/v) solution, as shown in Fig. 5b and Fig. 5c, resprctively.
The lowest unoccupied molecular orbital (LUMO) were calculated from the EHOMO
E0-0. As a result, the LUMO levels of co-sensitizers LD03, LD04 are -1.629 and
-
-1.470 V, respectively, which are more negative than the CB edge of TiO2 (-0.5 V vs.
NHE), which indicate that the electron injection from excited co-sensitizer into the
CB of TiO2 is thermodynamically favorable. The energy diagram of the two
co-sensitizers is shown in Fig. 6.
Fig. 5 is here
Fig. 6 is here
Table 2 is here
The high photocurrents of DSSCs based on co-sensitization were manifested by
UV-vis absorption spectrum and IPCE response. In addition, the co-sensitized systems
have higher Voc than single N719 is worth being investigated. To elucidate the reasons
for the differences between the Voc values, the electrochemical impedance spectroscopy
(EIS) was measured to investigate the effects on charge recombination, transport and
collection under a bias of 0.75 V with a frequency range of 10-1 Hz to 105 Hz in the dark
[42, 43]. The equivalent circuit (Fig. 7a) has been used to fit the EIS of the DSSCs,
and the corresponding fitting data are summarized in Table 3. Rs represents the series
resistance. Cμ and Rct are the chemical capacitance and the charge recombination
15

resistance at the TiO2/dye/electrolyte interface, respectively. CPt and RPt are the
interface capacitance and charge transport resistance at the counter
electrode/electrolyte interface, respectively. Fig. 7b and 7c show the Nyquist and Bode
phase plots for DSSCs based on LD03, LD04, N719, N719 + LD03 and N719 +
LD04. As shown in Fig. 7b, two major semicircles for each DSSC were observed in the
EIS Nyquist plot. The small semicircle at a higher frequency is attributed to charge
transfer at Pt counter electrode/electrolyte interface (RPt). It is obvious that the DSSCs
based on single sensitizer and co-sensitized system have similar values of RPt
(3.99-10.74 Ω) although existence of slightly small discrepancies, which are consistent
with many literatures [44, 45]. On the other hand, the larger semicircle in the lower
frequency range is related to the resistance of electron transport at the
TiO2/dye/electrolyte interface, i.e., the charge recombination resistance between
−
electrons on the TiO2 CB and I3 species in the electrolyte (Rct). A smaller Rct value
means a faster charge recombination rate and a larger dark current, ultimately
achieving a smaller Voc. It can be seen from Table 3 the fitted values of the Rct were in
the order of LD03 (26.34 Ω) < LD04 (31.14 Ω) < N719 (33.70 Ω) < N719 + LD03
(36.86 Ω) < N719 + LD04 (67.57 Ω), which appears to be consistent with the
aforementioned variation trend of Voc values.
From the Nyquist plots, we can also achieve another important data RS. The RS is
equal to the value of high-frequency intercept on the real axis. The fitted values of Rs
for LD03, LD04, N719, N719 + LD03 and N719 + LD04 are 21.80, 22.10, 24.76,
31.49 and 25.33 Ω, respectively. It is evident that the Rs of DSSCs based on single
16

sensitizer and co-sensitized system showed considerable fluctuation, which are
consistent with many literatures [44, 46] and can be explained by the FF obtained in J-V
curves under standard global AM 1.5 illumination. As we know, the Rs has a significant
effect on FF, and a higher Rs seems to a lower FF. As shown in Table 1, it is obvious
that the DSSCs based on single co-sensitizers (LD03 and LD04) have higher FF, which
is in accordance with smaller similar Rs (21.80-22.10Ω). Meanwhile, the DSSCs based
on N719, N719 + LD03 and N719 + LD04 show the lower FF, which are closely linked
to the higher Rs (24.76-31.49 Ω).
Furthermore, the changing tendencies of Voc values can be manifested by electron
lifetime (τe), which is estimated from peak frequency (f) in the lower frequency region
of EIS Bode phase plots using τe = 1/(2πf). From Fig. 7c, it can be seen that for the
devices, the peak frequency decreased in the order of LD03 > LD04 > N719 > N719 +
LD03 > N719 + LD04, and the electron lifetimes were enhanced in reverse with the
calculated values of LD03 (5.871 ms) < LD04 (10.978 ms) < N719 (13.467 ms) <
N719 + LD03 (16.104 ms) < N719 + LD04 (20.528 ms). The longer electron lifetimes
for co-sensitized DSSCs are indicative of effective suppression of the back reaction of
-
the injected electron with the I3 in the electrolyte, resulting in the higher Voc.
Meanwhile, we can also understand the relation between the electron lifetime and Voc.
The Voc value of a DSSC depends on the difference between the quasi-Fermi level of
TiO2 and the redox potential of the electrolyte (Eredox) as it is illustrated by the
following equation：
17







ECB
q
kT
q
n
Eredox
q
Voc 

ln

(2)
NCB
where ECB is the CB edge of TiO2, q is the electron charge, k is the Boltzmann
constant, T is absolute temperature, n is the number of electrons in TiO2 and NCB is
the effective density of states. Eredox is constant for both DSSCs due to the same
-
electrolyte was used (I-/I3 ) in both cases. Therefore, Voc depends only on the first two
terms, i.e., ECB and electron concentration in TiO2. Since N719, LD03 and LD04 are
able to inject electrons from their excited states into TiO2, the electron density is
obviously increased in case of the co-sensitized DSSCs. Moreover, small molecule
LD03 and LD04 tend to fill up the spaces of large-molecule N719 layer, which would
-
greatly increase the overall surface molecular coverage, leading to impede the I3
percolating to the TiO2 surface and suppression of the back reaction, while electron
lifetime in CB of TiO2 is increased. The longer electron lifetime in the co-sensitized
DSSCs is beneficial for charge accumulation in CB of TiO2 under open circuit
voltage, thus increasing electron density and Fermi level of TiO2. Therefore, the Voc
value of a DSSC increases in accordance with its electron lifetime. Overall,
co-sensitization of N719 with LD03 or LD04 effectively blocks the interfacial charge
recombination loss and enhances the electron lifetime, which accounts well for the
higher Voc value.
In order to further illuminate the enhancement of recombination resistances for
the co-sensitized systems, we recorded the dark current-voltage characteristics. Dark
-
current can give a measure of the recombination of injected electrons with I3 in the
18

electrolyte in a DSSC. Dark J-V characteristics of the solar cells sensitized by single
N719 and the co-sensitized systems (LD03 + N719 and LD04 + N719) are presented
the Fig. 7d and reveal that dark currents are lower for the co-sensitized DSSCs,
indicating suppression of the charge recombination rate, ultimately achieved higher
recombination resistances.
Fig. 7 is here
Table 3 is here
Generally speaking, the two new co-sensitizers possess the following key
features: (i) hexyloxy-substituted phenyl as an electron-donating unit, thiophene or
furan as π-spacer and cyanoacrylic acid group as an acceptor and anchor, these results
being LD03 and LD04 with a strong absorption around 400 nm that could effectively
-
overcome the competitive light absorption by I3 ; (ii) hexyloxy group was added in
LD03 and LD04 to avoid dye aggregation; (iii) compared with Ru-complex dye N719,
the two new dyes have simple molecular structures and small volumes. It is
well-known that small molecules with alkyl chains tend to form densely packed
monolayer due to the weak steric hindrance [47]. Furthermore, the post-adsorption of
small molecules into the interstitial sites of large-molecule layer would greatly increase
-
the overall surface molecular coverage, thus impeding the I3 percolating to the TiO2
surface (see Fig. 8) [48]. For these reasons, dyes LD03 and LD04 are suitable as
co-sensitizers for the N719 to enhance photovoltaic performances of DSSCs.
Fig. 8 is here
<H1>4. Conclusions
19

In conclusion, two organic donor-(π conjugated spacer)-acceptor dyes were
synthesized as co-sensitizers for N719 based DSSCs, in which employ thiophene
(LD03) or furan (LD04) as the π-linkers in combination with the hexyloxy-substituted
phenyl electron-donating unit and cyanoacrylic acid acceptor. The absorption and IPCE
-
spectra reveal that the dyes can overcome the competitive light absorption by I3 and
lead to the improvement of Jsc. Meanwhile, the co-sensitizers can prevent the
aggregation of N719 and adjust the arrangement of N719 molecules toward a more
nearly parallel configuration on the surface of TiO2. Furthermore, the EIS data
indicate that the strategy of co-sensitization can reduce recombination of injected
-
electron in TiO2 with the I3 and enhance electron lifetime, resulting in higher Voc.
Consequently, with respect to the DSSC with N719 alone, PCEs were improved by
8.891% and 13.412% in N719 + LD03 and N719 + LD04 DSSCs, respectively, and the
highest PCE was achieved by a DSSC fabricated with N719 + LD04 (PCE = 8.955%,
Jsc = 17.628 mA cm-2, Voc = 0.758 V and FF = 0.670). We hope thar the present work
can provide a reference for the synthesis of simple, low-cost co-sensitizers and
achievement of more promising co-sensitized solar cells.
<ACK>Acknowledgements
We are grateful to the National Natural Science Foundation of China (Grant nos.
21572030, 21272033, 21402023) for financial support.
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23

</BIBL>
<Figure>Figure captions:
<Figure>Fig. 1. Chemical structure of the two new co-sensitizers.
<Figure>Fig. 2. (a) J-V curves for DSSCs sensitized with LD03 and LD04; (b) J-V
curves of DSSCs based on N719 and co-sensitized systems (N719 + LD03 and N719
+ LD04).
<Figure>Fig. 3. (a) UV-vis absorption spectra of LD03, LD04 and N719 in
butanol-acetonitrile (1/1, v/v) solution; (b) UV-vis spectra of the N719 and
co-sensitized systems anchored onto nanoporous TiO2 films.
<Figure>Fig. 4. (a) IPCE spectra for DSSCs sensitized with N719, co-sensitized with
N719 and LD03 or with N719 and LD04; (b) and (c) amplification of IPCE spectra in
different wavelengths.
<Figure>Fig. 5. (a) Cyclic voltammograms of the dyes in N, N-dimethylformamide
solution; (b) and (c) the normalized absorption and emission spectra of the LD03 and
LD04 in butanol-acetonitrile (1/1, v/v) solution, respectively.
<Figure>Fig. 6. Schematic energy levels of the two co-sensitizers based on absorption
and electrochemical data.
<Figure>Fig. 7. (a) Equivalent circuit for the DSSCs. (b) Nyquist plots and (c) Bode
phase plots for DSSCs based on the single sensitizer and co-sensitization measured in
the dark under 0.75 V bias. (d)J-V curves under dark condition for DSSCs sensitized by
N719 and co-sensitized systems.
24

<Figure>Fig. 8. The spatial arrangement of the two types of dyes adsorbed on TiO2
surface.
<Table>Table 1 Photovoltaic properties of LD03, LD04, N719 and the co-sensitized
systems.
Jsc /
Dye
Voc/V
FF
PCE/%
mA cm-2
3.905
0.652
0.710
0.714
0.736
0.758
0.712
0.720
0.655
0.661
0.670
1.813
2.570
7.896
8.598
8.955
LD03
LD04
5.027
16.883
17.673
17.628
N719
N719+LD03
N719+LD04
<Table>Table 2 Absorption and electrochemical data of dyes LD03 and LD04.
b
d
ε at λmax
/104 M-1
cm-1
EHOMO
V vs.
NHE
1.327
1.334
/
ELUMO /
a
Dye
λmaxa /nm
λex /nm
E0-0c/V
V vs.
NHE
392
421
2.653
460
463
2.956
2.804
-1.629
-1.470
LD03
LD04
3.028
a Absorption maximum (λmax) and emission spectra (λex) in 2 × 10-5 M
butanol-acetonitrile (1/1, v/v) solution.
b EHOMO values were measured in N, N-dimethylformamide with 0.1 M n-Bu4NPF6 as
supporting electrolyte (scanning rate: 10 mV s-1, working electrode: Pt disk, counter
electrode: Pt wire and reference electrode: Ag/AgCl), potentials measured vs.
Ag/AgCl were converted to NHE by addition of +0.2 V, namely EHOMO = Eox + 0.2.
25

c E0–0 values were calculated from the intersection of the normalized absorption and
emission spectra.
d ELUMO = EHOMO - E0-0.
<Table>Table 3 EIS parameters of the DSSCs.
Dye
LD03
RS (Ω)
21.80
22.10
24.76
31.49
25.33
Rct （Ω）
26.34
RPt（Ω）
6.84
τe/ms
5.871
31.14
6.56
10.978
13.467
16.104
20.528
LD04
33.70
3.99
N719
36.86
5.90
N719+LD03
N719+LD04
67.57
10.74
TDENDOFDOCTD
26