Co-sensitization of N719 with polyphenylenes from the Bergman cyclization of maleimide-based enediynes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ta01861g

The co-sensitization of N719 with conjugated polyphenylenes for dye-sensitized solar cells (DSSCs) was demonstrated. The conjugated polyphenylenes (6-BC and 7-BC), which contain two triphenylamine (TPA) groups as electron donors and carboxylic acids as ad

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Journal of Materials Chemistry A
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ARTICLE
Co-sensitization of N719 with polyphenylenes from
Bergman cyclization of maleimide-based enediynes
for dye-sensitized solar cells
Cite this: DOI: 10.1039/x0xx00000x
a
b
a
b
b
a
Luhua Dong , Zhiwei Zheng , Youfu Wang , Xing Li , Jianli Hua* , Aiguo Hu*
Received 00th January 2012,
Accepted 00th January 2012
The co-sensitization of N719 with conjugated polyphenylenes for dye-sensitized solar cells
DSSCs) was demonstrated. The conjugated polyphenylenes (6-BC and 7-BC) with two
DOI: 10.1039/x0xx00000x
(
triphenylamine (TPA) groups as electron donor and carboxylic acid as adsorption groups were
prepared from thermal-triggered Bergman cyclization polymerization of two maleimide-based
www.rsc.org/
enediynes
6 and 7, respectively. Both of the short-circuit photocurrent density (JSC) and the
open-circuit voltage (VOC) of the co-sensitized DSSCs were enhanced due to the increase of
the light harvesting and the retardation of the electron recombination and dye aggregation upon
N719 co-sensitized with the conjugated polyphenylenes. The co-sensitized (7-BC+N719
)
.
-2
DSSC showed a high efficiency of 9.68% (VOC of 750 mV, JSC of 18.18 mA cm and FF of
.71), exhibiting an improvement by 16% compared to the devices sensitized with N719 (PCE
of 8.34%) alone under the identical condition.
0
Co-sensitization of two or more dyes (having
Introduction
complementary absorption bands) is a profound strategy to
extend and increase the light harvesting ability and hence the
photocurrent. Research works focused on ruthenium complexes
Dye-sensitized solar cells (DSSCs), one of the most promising
candidates for the renewable energy, are under intense
investigations because of their easy fabrication process, low
price and high energy conversion efficiency . Among its
constituents, the sensitizer plays a critical role in light-
harvesting and the subsequent charge injection into the
conduction band of TiO2 and affects the power conversion
efficiency (PCE) directly. Although various kinds of sensitizers,
3
2-35
co-sensitized with metal-free organic dyes
organic small molecules
, hydrophobic
1
8,19
36-38
1, 2
or quantum dots
showed the
fact that the ideal candidate for a co-adsorbent should have the
following three characteristics. First, it should have a large
molar extinction coefficient in the infrared or around 400 nm
region to compensate for light-harvest loss by the electrolytes
-
-
3
-8
9-
(
I /I ). Second, the structure of the molecule should effectively
such as porphyrin compounds , and metal-free organic dyes
3
1
3
suppress the aggregation of the dye on the TiO surface. In
have been rapidly developed in recent years, Ru(II)-
2
1
4-17
addition, it should be able to reduce the recombination of the
polypyridyl complexes
(such as N749, N3, N719 etc.) are
-
electrons in the TiO2 film with I3 by forming a compact
regarded as classic sensitizers since their panchromatic
absorption. So far, DSSCs sensitizers based on ruthenium
complexes have an overall solar energy conversion efficiency
molecule monolayer covering the bare TiO surface.
2
Here we developed an effective method to optimize DSSC
1
8
based on N719 by co-sensitizing with
polyphenylenes prepared from maleimide-based enediynes
through thermal-triggered Bergman cyclization
a
conjugated
(
ƞ) exceeding 11% . The wide absorption ranges from the
visible to near-infrared (IR) regions endows DSSCs based on
ruthenium complexes high efficiency; however, their low molar
extinction coefficients limit the efficiency being further
enhanced. A major drawback limiting the advancement of
DSSCs is the uncontrolled charge recombination at the
3
9
polymerization . The polyphenylenes with carboxylic acid
adsorption groups absorbed on the unoccupied locus on TiO2
surface serves as an electrolyte-blocking shell. Both the Jsc and
1
9
the Voc of the co-sensitized DSSCs were significantly
enhanced relative to N719 sensitized alone DSSCs, by
improving electron injection more effectively and reducing the
electron recombination, resulting in a PCE of 9.68% for the
DSSC based on the co-adsorption system, which was raised by
TiO /dye/electrolyte interface regardless the photophysical
2
nature of the dyes. Actually, in most cases the electron in the
conduction band of TiO recombination with the electrolyte is
considered most detrimental to the PCE of DSSCs. To this end,
several strategies have been developed to disrupt the
undesirable interaction between the electrolyte and the TiO2
2
1
6% in comparison with that of N719 sensitized alone. Our
approach provides a new strategy for optimizing DSSCs.
electrode, such as passivating the surface states of TiO by co-
2
2
0-23
adsorbents
(CDCA, DPA etc.), adding a blocking/tunneling
2
4-26
layer at TiO surface
(Al O , ZnO, SiO etc.), incorporating Experimental
2
2
3
2
2
7-29
an alkyl chain in dye backbone
by polymerization
and post-surface passivation
3
0, 31
etc.
Materials
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ARTICLE
Journal Name
1
13
Triethylamine, N,N-dimethylformamide (DMF) were dried
H and C NMR spectra were recorded on Bruker AM-400
over calcium hydride and distilled before use. Other chemicals MHz using tetramethylsilane as an internal standard. High
were commercial grade and used as received. N-(4- resolution mass spectra (HR-MS) were obtained using a Waters
ethynylphenyl)-N-phenylbenzenamine
dibromofuran-2,5-dione ( ) were synthesized according to polymers were performed on a Mass spectrometer 4800 plus
literature procedures. The detailed synthesis of the precursors MALDI-TOF/TOF MS Analyzer (AB Sciex, USA) equipped
(
2
)
and
3,4- LCT Premier XE spectrometer. MALDI-TOF MS of the
3
(
compounds
1
~
5
) is presented in the supporting information.
with a Nd:YAG laser emitting at 355 nm, and operating at an
accelerating voltage of 20 kV in reflection mode. Electron
paramagnetic resonance (EPR) spectroscopy was performed
with a Bruker EMX-8/2.7. The spectrometer settings were:
sweep width, 40 G; time constant, 163.84 ms; conversion time,
Synthesis of enediyne compounds
tert-Butyl 2-(3,4-bis(2-(4-(diphenylamino)phenyl)ethynyl)-2,5-
dioxo-2H-pyrrol-1(5H)-yl)acetate (Enediyne 6)
Compound
mmol), CuI (47.6 mg, 0.25 mmol), and Pd(PPh ) Cl (131.6 mg,
0
40.96 ms; scan time, 83.89 s; 1024 points; modulation
4 (0.92 g, 2.5 mmol), compound 2 (1.38 g, 5.125
amplitude, 1.00 G; and microwave power, 2.0 mW. Differential
scanning calorimetry (DSC) was performed with a Pyris
Diamond thermal analysis work station equipped with a model
3
2
2
.1875 mmol) were mixed in a 50 mL Schlenk flask. Then
degassed DMF (5 mL) and triethylamine (1.5 mL) were added
successively under nitrogen atmosphere. The reaction mixture
was stirred at 50 ºC for 2 h. The resulting dark brown solution
was concentrated and partitioned with saturated NaCl solution
and dichloromethane (DCM), and the organic layer was dried
8
22e DSC module under a constant nitrogen flow. The
absorption spectra of sensitizer dyes in solution and adsorbed
on Titania films were measured with a Varian Cary 500
spectrophotometer. Cyclic voltammograms (CV) were obtained
on a Versastat II electrochemical workstation (Princeton
Applied Research) using a three-electrode cell with a glassy
carbon working electrode, a Pt wire auxiliary electrode, and a
saturated calomel reference electrode (SCE) in saturated KCl
solution. Ferrocene (Fc) was used as an internal reference and
0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6)
over anhydrous MgSO . After removal of the solvent under
4
vacuum, the crude product was purified through column
chromatography (200~300 mesh silica, Petroleum ether (PE) :
1
DCM = 1 : 1) to yield an atropurpureus solid (1.1 g, 59%). H
NMR (CDCl , 400 MHz, ppm): 7.43 (d, J = 8.8 Hz, 4H), 7.30
3
(
=
9
1
8
dd, J = 7.4 Hz, 7.5 Hz, 8H), 7.14 (d, J = 7.5 Hz, 8H),7.11 (t, J
7.4 Hz, 4H), 6.97 (d, J = 8.8 Hz, 4H), 4.23 (s, 2H), 1.46 (s,
was used as the supporting electrolyte in CHCl . The scan rate
was 100 mV s .
3
. -1
1
3
H). C NMR (CDCl , 100 MHz, ppm): 166.9; 165.9; 149.7;
3
46.5; 133.6; 129.5; 125.7; 124.4; 120.7; 113.3; 111.4; 82.9;
DSSC Fabrication and Photovoltaic Performance Measurements
1.4; 58.5; 53.4; 28.0. HR-MS (ESI): m/z calcd. for
+
C H N O (M) : 745.2941, found: 745.2942.
A double-layer TiO photoelectrode of 17 μm thick, composed
5
0
39
3
4
2
of a 12-μm-thick nanoporous layer and a 5-μm-thick scattering
2
tert-Butyl 4-(3,4-bis(2-(4-(diphenylamino)phenyl)ethynyl)-2,5-
layer (area: 0.25 cm ), was prepared by screen printing on a
dioxo-2H-pyrrol-1(5H)-yl)benzoate (Enediyne 7)
Compound
conducting glass substrate. Dye solutions of 6-BC
5 (0.43 g, 1 mmol), compound 2 (0.55 g, 2.05 compound 8 with a concentration of 3×10 M in CHCl , and
,
7-BC and
−
4
3
mmol), CuI (19.0 mg, 0.1 mmol), and Pd(PPh ) Cl (52.6 mg, mixed solutions of 6-BC+N719 (1:1) and 7-BC+N719 (1:1, 1:2,
3
2
2
0.075 mmol) were mixed in a 50 mL Schlenk flask. Then 1:3) in acetonitrile/n-butyl alcohol = 1:1 were used to uptake
degassed DMF (5 mL) and triethylamine (1.5 mL) were added the dye onto the TiO film. The TiO films were immersed in
2
2
successively under nitrogen atmosphere. The reaction mixture the dye solution for 16 h. Photovoltaic measurements were
was stirred at 50 ºC for 2 h. The resulting solution was performed in a sandwich-type solar cell in conjunction with an
concentrated and partitioned with saturated NaCl solution and electrolyte consisting of a solution of I (0.05 M), LiI (0.1 M),
2
DCM, and the organic layer was dried over anhydrous MgSO4. dimethylpropylimidazolium iodide (0.6 M), and 4-tert-butyl
After removal of the solvent under vacuum, the crude product pyridine (0.5 M) in acetonitrile. The dye-deposited TiO film
2
was purified through column chromatography (200~300 mesh and a platinum-coated conducting glass were separated by a
silica, PE : DCM = 1 : 2) to yield a dark purple solid (0.43 g, Surlyn spacer (40 μm thick) and sealed by heating the polymer
1
5
3%). H NMR (DMSO-d , 400 MHz, ppm): 8.02 (d, J = 8.5 frame. The photocurrent density−voltage (J−V) of the sealed
6
2
Hz, 2H), 7.55 (d, J = 8.5 Hz, 2H), 7.49 (d, J = 8.6 Hz, 4H), 7.40 solar cells was measured with a black metal mask of 0.25 cm ,
(
=
(
dd, J = 7.4 Hz, 8.2 Hz, 8H), 7.19 (t, J = 7.4 Hz, 4H), 7.16 (d, J illuminating the cell through the conducting glass from the
1
3
8.2 Hz, 8H), 6.88 (d, J = 8.6 Hz, 4H), 1.56 (s, 9H). C NMR anode side with a solar simulator (WXS-155S-10) at AM 1.5
−
2
CDCl , 100 MHz, ppm): 165.5; 164.3; 149.6; 145.7; 135.5; illumination (light intensities: 100 mW cm ). Monochromatic
3
1
1
33.7; 130.3; 130.0; 129.6; 126.4; 126.0; 125.7; 125.1; 119.4; IPCE was measured with monochromatic incident light of
1
6
−2
−2
11.4; 110.6; 99.5; 81.2; 81.0; 27.7. HR-MS (ESI): m/z calcd. 1×10 photons cm under 100 mW cm in director current
+
for C H N O (M) : 807.3097, found: 807.3065.
mode measurements, which were made on a CEP-2000 system
Bunkoh-Keiki Co. Ltd.)
5
5
41
3
4
(
Polymerization
Electrochemical Impedance Spectra (EIS) Measurements
The Bergman cyclization polymerizations were performed
under vacuum in a sealed glass tube in a heating bath set at the EIS experiments were carried out in the dark using a ZAHNER
exothermic peak temperature (DSC) of the corresponding ZENNIUM electrochemical workstation with frequency
enediyne compound for 8 h. The heating bath for
6 was ranging from 50 mHz to 100 kHz and a potential amplitude of 5
diphenyl ether to reach 260 ºC and for
7
was xylene to 150 ºC.
mV. The obtained impedance spectra were fitted with the
ZSimpWin software (v3.10) in terms of appropriate equivalent
circuits.
Characterizations
Results and discussion
2
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ARTICLE
Conjugated polyphenylenes can be facially prepared by respectively. The thermal-triggered Bergman cyclization
thermal-triggered Bergman cyclization of maleimide-based polymerization of two enediyne compounds was conducted in a
4
0
enediynes at elevated temperature . The conjugated sealed bottle under vacuum to eliminate any species that may
polyphenylenes (6-BC and 7-BC) with triphenylamine as the quench radicals. The enediyne compounds were heated in a
electron donor, carboxy acid as the acceptor/anchor and the heating bath set around their peak temperature for 8 hours.
maleimide-based enediyne as the bridge of electron donor and Atropurpureus solids were obtained in a quantitative yield.
acceptor was synthesized with a similar strategy and used as the
co-adsorbents with N719 in DSSCs. The general synthetic
routes to prepare the conjugated polyphenylenes are shown in
Scheme1, and detailed syntheses of the precursors and
characterization data are provided in the Supporting
Information. The enediyne compounds
6 and 7 with ester
groups were synthesized via Sonogashira coupling reaction. A
methylene bridge is presented between maleimide and electron
acceptor in enediyne
6, while in enediyne 7, a -bridge
(
phenylene group) is presented, with the expectation of more
effective intramolecular electron transport through the bridging
group. In one way, the two compounds with enediyne structure
Figure 1. a) MALDI-TOF mass spectrum of the oligomer part of 7-BC; b) EPR
undergo thermal-triggered Bergman cyclization and the two spectrum of the freshly prepared 7-BC in CHCl3.
adjacent carbonyl groups in the maleimide moiety lower the
energy barrier of Bergman cyclization, permitting the reaction
to take place at moderate temperature. Meanwhile, the tert-
butyl group was deprotected under the reaction temperature
The MALDI-TOF mass spectrum of the oligomer part of
-BC is shown in Fig. 1a. Several sets of peaks with an m/z
7
interval of 751.24 are observed, which is consistent with the
calculated m/z value of a monomer unit of enediyne with the
(
>150 ºC), forming carboxylic acid as the adsorption groups.
7
The tert-butyl ester protecting groups of compound were also
7
tert-butyl group being removed (751.26). The occurrence of the
Bergman cyclization was further confirmed with electron
removed by treating with trifluoroacetic acid to afford the
corresponding carboxylic acid functionalized enediyne
4
0
paramagnetic resonance (EPR) . Freshly prepared 7-BC was
dissolved in chloroform and subjected to EPR analysis. As
shown in Fig.1b, the intense peak at around 3519 G indicates
compound
8
, which was used as a control in DSSC.
4
1
the presence of carbon-centered radicals in the sample .
Moreover, we demonstrated the obvious diminution of the peak
N
-
1
O
O
at 2177 cm (Fig.S5) in FT-IR spectra corresponding to the
Br
O
N
a)
O
b)
N
stretching vibration absorption of C C, which further verified
≡
6
-BC
Br
O
O
O
O
the occurrence of Bergman cyclization polymerization.
4
6
The UV-Vis absorption spectra are shown in Fig. 2a. Both
the two polymers 6-BC and 7-BC with similar backbone
structure show a broad absorption range covering a large
proportion of the visible region and even to near-infrared (NIR)
region, which indicated the formation of conjugated systems
N
N
O
OH
O
N
)
O
42
N
N
d
after Bergman cyclization polymerization . The higher
8
N
O
O
absorption intensity of 7-BC is mainly due to the presence of
the phenyl group, which enhanced the conjugate degree and
conduced to intramolecular electron transfer. The intensive
Br
Br
O
O
O
O
c)
N
N
O
O
)
e
5
−1
-1
7
absorption at 307 nm (ε = 67 400 M cm , shown in Table 1)
was assigned to the localized π-π* transitions of the conjugated
7
-BC
−
1
-1
systems. The defined peak at 486 nm (ε = 19 840 M cm ) was
correspond to n–* transition in the maleimide moiety. In
addition, the cocktail solutions of 7-BC+N719 showed much
more intensive absorption in all the UV-Vis and NIR regions
relative to dye N719, while 6-BC+N719 (1:1) (molar ratio, the
N
N
N
N
O
O
OH
O
N
N
O
OH
O
O
same below) only shows negligible enhancement over N719
.
n
n
And with the decreased ratio of the 7-BC, the enhancement
effect of the 7-BC+N719 mixture system was decreased. As a
6
-BC
7-BC
comparison, the absorptions of enediyne monomer compound
8
were not that strong as well as that of its mixture solution with
Scheme 1. Synthesis of enediyne compounds and related Bergman cyclization. a) N719 (Fig. S8).
and c) Compound , Pd(PPh Cl , CuI, DMF, triethylamine, 50 ºC, 5h; d) The UV-vis absorption of N719, 6-BC, 7-BC, 6-
2
3
)
2
2
trifluoroacetic acid, 0 ºC, 1 h; b) Heating in a refluxing diphenyl ether bath; e) BC+N719(1:1), 7-BC+N719 on thin TiO film (~ 4 μm) was
2
Heating in a refluxing xylene bath.
shown in Fig. 2b. To get the most suitable mixture ratio to
optimize the optical absorption properties of the device, films
The thermal-triggered Bergman cyclization of the dipped with 7-BC mixed with N719 in different ratios (1:1, 1:2
enediyne compounds was analyzed by differential scanning and 1:3). The maximum absorption peaks of 7-BC+N719 in
calorimetry (DSC). Fig. S4a and S4b show exothermal peaks at different ratio on TiO films was consistent with respect to that
2
214 ºC for compound
6
and at 138 ºC for compound
7
,
in solution (530 nm), which excludes the H or J aggregation in
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3
the interaction with the surface of TiO2 . And with the Table 1 Photophysical and electrochemical properties of 6-BC and 7-BC
decreased ratio of the 7-BC, the absorption intensity of the 7-
BC+N719 mixture system decreased as well, despite of the
weak film absorption of 7-BC alone. In contrast, the absorption
intensity of 6-BC alone was relatively strong (Fig.S9), showing
a
a
ε
b
c
c
Co-
adsorbent
λ
max
-1
-
HOMO
E
0-0
LUMO
(
M cm
(nm)
1
(V/vs.NHE)
(V)
3.10
2.25
(V/vs.NHE)
)
3
05
30060
6360
67400
6
-BC
-BC
0.78
-2.32
good adsorption on TiO . With the intense adsorption
432
307
486
2
competition between 6-BC and N719, the absorption of 6-
BC+N719 (1:1) decreased sharply since the lower adsorbing
7
0.76
-1.49
19840
capacity of N719
.
a. Absorption peaks (λmax) and molar extinction coefficients (ε) in the
solution of CHCl ; b. HOMO was measured in CHCl and calibrated with
ferrocene with cyclic voltammetrys (CV) at a scan rate of 100 mVs ; c. E0–0
3
3
.
-1
stimated from the absorption thresholds in absorption spectra of dyes
adsorbed on TiO
HOMO.
2
film, LUMO estimated by subtracting E0–0 from the
Polyphenylene co-adsorbents and dye compound
-BC N719, compound N719 6-BC+N719 7-BC+N719
1:1, 1:2, 1:3) based devices are denoted as device 1~9,
8，6-BC,
7
(
,
8
+
,
,
respectively. Figure 4 shows the current−voltage (J−V)
characteristics and incident photon-to-current conversion
efficiency (IPCE) action spectra of these liquid electrolyte-
,
7-BC in based DSSCs. Their photovoltaic parameters are listed in Table
Figure 2. UV-Vis absorption spectra of sensitizers. a) in solution: 6-BC
CHCl (0.05 mmol/L) and N719, 6-BC+N719 (1:1) , 7-BC+N719 (1:1, 1:2, 1:3) in mix 2. All the devices based on the two polyphenylene co-
3
solution of acetonitrile and n-butyl alcohol (v/v=1:1)(the concentration of N719
constant be 0.05 mmol/L); b) N719 alone and co-sensitization system on thin
adsorbents (6-BC and 7-BC) and enediyne compound
8 showed
poor power conversion efficiency of 0.16% and 0.06%
respectively under AM1.5 simulated sun light. Although two
conjugated polyphenylene co-adsorbents showed broad light
absorption range, their performances in DSSCs are not as high
as expected as sensitizers. This is because that the HOMO
energy levels of 6-BC and 7-BC were 0.78 V and 0.76 V,
TiO
2
film (4 μm).
To estimate the HOMO and LUMO energy levels and thus
to evaluate the feasibilities of electron injection and dye
regeneration, the electrochemical and optical properties of the
organic dyes are of prime importance. The electrochemical
-
-
respectively, which are close to the energy level of I /I ,
3
properties of 6-BC and 7-BC in CHCl were investigated by
3
leading to hard regeneration of oxidation state dye. In addition,
in order to inject electrons effectively, the energies of excited
dyes states (LUMO level) should not only be higher than ECB of
TiO , but also have good orbit overlap with TiO conduction
cyclic voltammetry (CV) (Fig. 3). The measurement shows
oxidation potential of 0.78 V and 0.76 V (vs. NHE) for 6-BC
and 7-BC respectively, which correspond to the highest
occupied molecular orbital (HOMO) energy of the
polyphenylene compounds. These value are more positive than
2
2
15, 44, 45
band (higher than ECB about 0.3 eV
). Though the LUMO
level of 6-BC and 7-BC is higher than ECB of TiO , the
difference values are too high for an effective overlap with the
-
-
2
the redox couple I /I (~0.4 V vs. NHE), thus achieve favorable
3
kinetics for dye regeneration. On the other hand, the lowest
unoccupied molecular orbital (LUMO) was calculated to be -
ECB of TiO . So the process of electron injection is suppressed
2
and the performances of the DSSC devices based on the
2.32 V (vs. NHE) for 6-BC and -1.49 V (vs. NHE) for 7-BC
,
polyphenylenes alone were poor. For the co-adsorption systems,
devices showed more encouraging results. Devices based on 6-
BC+N719 (1:1) showed an efficiency of 7.53%, slightly lower
than the device based on N719 (ŋ = 8.34%) alone. It may be
caused by the decreasing of the absorption of N719, as there are
intense competitive adsorption between 6-BC and N719 (see
above). Devices based on 7-BC+N719 (1:1) and (1:3) showed
efficiency of 9.02% (VOC of 748 mV, JSC of 17.96 mA cm , FF
of 0.67) and 8.76% (VOC of 748 mV, JSC of 17.84 mA cm , FF
of 0.66), respectively, with both of VOC and JSC a slightly
both of which are higher than the conduction band of TiO
(∼−0.5 V vs NHE) and thus enables efficient charge injection
2
1
5
kinetics. The photophysical and electrochemical properties
parameters are tabulated in Table 1.
6
7
-BC
-BC
0.10
0.05
0.00
.
-2
.
-2
.
-2
increase over N719 (VOC of 731 mV, JSC of 17.33 mA cm , FF
of 0.66) device. Particularly, the device based on 7-BC+N719
(
1:2) showed the most optimal performance, with the power
conversion efficiency of 9.68% (V of 750 mV, JSC of 18.18
OC
.
-2
mA cm , FF of 0.71), which was raised by 16% in comparison
0
1
2
with that of N719 sensitized alone. Comparing with the
Potential (V)
efficiency of device based on enediyne compound
8 co-
Figure 3. cyclic voltammetry curves of 6-BC and 7-BC in a CHCl
Bu NPF
3
solution (0.1 M
sensitized with N719 (ŋ = 5.99%), it can be perceived that the
conjugated polyphenylenes generated by thermal-triggered
Bergman cyclization polymerization play an important role for
the performance improvement of DSSCs.
4
6
;
To investigate the improvement in the co-sensitization of
JSC, their IPCE spectra were carefully compared with individual
sensitization. The IPCE is associated with the product of light
harvesting efficiency (ƞ ), charge injection efficiency (ƞ ), and
lh
inj
4
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ARTICLE
4
6
charge collection efficiency (ƞcoll) . As shown in Fig. 4b, the electron recombination in DSSCs based on 6-BC+N719 (1:1),
co-sensitizer of 7-BC makes a preferable contribution to IPCE 7-BC+N719 (1:1, 1:2, 1:3) and N719 under -0.70 V bias
improvement in the whole light harvesting range of N719
,
applied voltage in the dark. The Nyquist plot, Bode plot, and
though the device based on 7-BC alone showed an IPCE band equivalent circuit are shown in Figure 5 a–c, respectively. In
in a narrow range in UV with low value (37%). The DSSC Figure 5c, R , R and Rrec represent series resistance and
S
CE
sensitized with N719 had a broad IPCE spectrum across the charge-transfer resistances at the counter electrode (CE) and
4
7
entire visible region extending up to 800 nm and exhibited dye/TiO /electrolyte interface, respectively . The series
2
IPCE value >70% at the range 470-570 nm. On this basis, 7- resistance (R ) and electrolyte reduction resistance (RCE)
S
BC+N719 systems improved the light harvesting efficiency corresponding to the first semicircle and the Rrec corresponds
effectively in full absorption range, especially, for device of 7- to the middle semicircle in Nyquist plot (Figure 5a). Generally,
BC+N719 (1:2) the value reached the highest at 76%, for well-operational DSSCs, the VOC performance is closely
indicating that photoinduced charge injection is quite efficient sensitive to the charge 4 transport processes at the
8-50
for this co-sensitization systems at short-circuit.
TiO /dye/electrolyte interface
. Obviously, with the same
2
electrode material and same electrolyte, five DSSCs show close
R and RCE value (see in Table 3). The strategy to restrain
S
Table 2 Photovoltaic parameters of DSSCs devices 1-9 measured under
simulated AM 1.5G solar light (100 mW.cm )
-
2
charge recombination will increase the radius of the second
semicircle (in Nyquist plots), moving the phase angle peak (in
V
OC
J
SC
PCE
(%)
Device
Dye
.
2
FF
5
0
(
mV)
436
483
479
731
561
709
748
750
748
(mAcm- )
0.23
0.51
0.55
17.33
5.09
17.07
17.96
18.18
17.84
Bode plots) to low frequency, and increasing the RREC value .
Apparently, from Fig.5a and 5b, the larger width of the second
semicircle of the 7-BC+N719 (1:1, 1:2, 1:3) cells and lower
frequency of phase angle peak indicate a larger charge-transfer
resistance (compared to the N719 cell) at the
1
2
3
4
5
6
7
8
9
8
0.55
0.65
0.59
0.65
0.41
0.62
0.67
0.71
0.66
0.06
0.16
0.16
8.34
1.19
7.53
9.02
9.68
8.76
6-BC
7-BC
N719
8+N719
TiO /dye/electrolyte interface. And the R of the 7-BC+N719
6-BC+N719(1:1)
7-BC+N719(1:1)
7-BC+N719(1:2)
7-BC+N719(1:3)
2
REC
in the ratios of 1:1-1:3 are 364.9 Ω, 418.9 Ω and 394.8 Ω,
respectively, higher than that of N719 (209.8 Ω) and 6-
BC+N719 (1:1) (184.2 Ω). The electron lifetime  can be
e
51
calculated from the equation of 1/2πfp , where f is the peak
p
It is observed that the photoelectric properties of all the
systems are different from that obtained from the UV-vis
frequency of lower frequency region in the Bode phase plots.
And the calculated electron lifetime decreased in the order of 7-
BC+N719 (1:2) (115.3 ms) > 7-BC+N719 (1:3) (106.1 ms) >
absorption on TiO films. Though the light harvesting ability of
2
all the 7-BC+N719 co-adsorption systems on film were slight 7-BC+N719 (1:1) (99.5 ms) > N719 (78.4 ms) > 6-
poor than N719 alone since the adsorption capacity of N719
was decreased, the performances of the corresponding devices
BC+N719(1:1) (53.9 ms), well in line with VOC together with
RREC. Two plausible mechanisms may account for the
significantly increased of Voc: 1) aggregation of N719
are better than N719. This is probably due to the adsorbed 7-
-
BC plays an important role on blocking I3 from approaching molecular was effectively inhibited due to the existence of the
TiO2 surface to reduce electron recombination, which is
corroborated with the results of dark current tests (Fig.S11), or
restraining N719 aggregation to promote electron injected
conjugated polyphenylene 7-BC
injections were improved; 2) electron recombination between
the injected electrons and the electrolyte was suppressed
, therefore the electron
effectively. And these benefits counteract the initial loss of light owning to the formation the electrolyte-blocking shell with 7-
harvesting. In addition, the results of the AM1.5 studies
paralleled with the IPCE results of 6-BC+N719 and 7-
BC+N719 suggesting that there are advantages for using
conjugated polyphenylenes with more effective intramolecular
electron transport.
BC on unoccupied locus, lengthening the electron lifetimes.
2
60
40
220
200
180
160
a)
6-BC+N719(1:1)
7-BC+N719(1:1)
7-BC+N719(1:2)
b)
6
7
+N719(1:1)
+N719(1:1)
2
220
7+N719(1:2)
7+N719(1:3)
N719
7-BC+N719(1:3)
200
180
160
140
120
100
N719
140
120
100
80
80
60
40
20
0
6
0
0
4
20
0
0
50
100 150 200 250 300 350 400 450
0
10
1
10
2
10
3
10
4
10
Z'/
Frequence (Hz)
Figure 4. a) J-V curves of DSSCs sensitized with N719, 6-BC+N719 (1:1) and 7-
BC+N719 (1:1, 1:2, 1:3) under AM 1.5G irradiation; b) IPCE spectra of the DSSCs
based on N719 and co-sensitization system.
Figure 5. Impedance spectra for DSSCs based on 6-BC+N719 (1:1), 7-BC+N719
1:1, 1:2, 1:3) and N719 measured at a bias of −0.70 V in the dark. a) Nyquist
plots; b) Bode phase plots; c) equivalent circuits. The lines of parts a) and b) show
(
The high photocurrent of DSSCs based on 7-BC+N719 is
obvious from its higher IPCE response than that of N719. In theoretical fits using the equivalent circuits in (c).
addition, the insight into its slight higher Voc (about 20 mV
higher than N719) is worth to be investigated. Electrochemical
impedance spectroscopy (EIS) was employed to study the
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Journal Name
Table 3 Parameters obtained by fitting the impedance spectra of the DSSCs
with sensitizers by using the equivalent circuit (Figure 5 c).
Acknowledgements
The authors acknowledge the financial support from National
Natural Science Foundation of China (91023008, 21474027,
Dye
R
S
[Ω]
R
CE [Ω]
30.9
24.5
14.6
36.6
39.4
R
rec [Ω]
e
τ [ms]
53.9
99.5
115.3
106.1
78.4
6
7
7
7
-BC+N719(1:1)
-BC+N719(1:1)
-BC+N719(1:2)
-BC+N719(1:3)
N719
7.2
8.2
7.7
8.6
7.0
184.2
364.9
418.9
394.8
209.6
21172073 and 91233207) and Shanghai Leading Academic
Discipline Project (B502). A.H. thanks the “Eastern Scholar
Professorship” support from Shanghai local government.
Notes and references
Shanghai Key Laboratory of Advanced Polymeric Materials, School of
The long-term stability test was performed for 7-
BC+N719(1:2) co-sensitization system, which showed that the
DSSC was stable after 1000 h under full sunlight intensity. The
a
Materials Science and Engineering, East China University of Science and
Technology, Shanghai, 200237, China.
values changed over time for VOC, J , FF and PCE are plotted
SC
b
in Figure
6
. VOC is slightly increased during the first 200 h, and
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
slightly decreased to 745 mV by the end of the test, only 5 mV East China University of Science and Technology, Shanghai 200237,
lower than the initial value (750 mV), indicating good
adsorption stability of the co-sensitization system. JSC exhibited
a slightly decreased trend during the period of 1000 h and
finally reached 16.84 mA cm , which may originate from the
slow dye degradation. The PCE of the device slowly degraded
to 9.13%, remaining at 95 % of the initial value after 1000 h of
China.
Electronic Supplementary Information (ESI) available: [detailed synthesis
of other compounds, NMR spectra of all new compounds, DSC, EPR,
UV-vis and dark current test]. See DOI: 10.1039/b000000x/
.
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Co-sensitization of N719 with polyphenylenes from Bergman
cyclization of maleimide-based enediynes for dye-sensitized
solar cells
a
b
a
b
b
a
Luhua Dong , Zhiwei Zheng , Youfu Wang , Xing Li , Jianli Hua* , Aiguo Hu*
N719 co-sensitized with a conjugated polyphenylene (7-BC) shows a high efficiency
.
-2
of 9.68% (VOC of 750 mV, JSC of 18.18 mA cm and FF of 0.71) in dye-sensitized
solar cell, exhibiting an improvement by 16% compared to the devices sensitized with
N719 (PCE of 8.34%) alone under the identical condition.