Efficient light-scattering functionalized TiO2 photoanodes modified with cyanobiphenyl-based benzimidazole for dye-sensitized solar cells with additive-free electrolytes

Article abstract of DOI:10.1039/c2jm32607h

Light-scattering functionalized cyanobiphenyl-based benzimidazole was synthesized and applied in the surface modification of dyed TiO₂ photoanodes for dye-sensitized solar cells (DSSCs). DSSCs based on the ionic liquid and all-solid-state electrolytes were fabricated and characterized, without the addition of additives in the electrolytes. Compared with electrolytes containing the additive N-butylbenzimidazole (NBB), surface modification of dyed TiO₂ photoanodes with cyanobiphenyl-based benzimidazole could enhance the values of the open-circuit photovoltage (V _oc), short-circuit photocurrent density (J_sc) and the overall PCE of the fabricated DSSCs, due to the suppressed charge recombination rate, the conductivity optimization of the electrolytes and enhanced light harvesting capability at the TiO₂ photoanode/electrolyte interface. Under the simulated air mass 1.5 solar spectrum illuminations at 100 mW cm ^-2, the fabricated devices achieved a cell efficiency of ～6.63% and ～5.82% for ionic liquid and all-solid-state electrolytes, respectively. These results provide an alternative strategy for high performance DSSCs with additive-free electrolytes.

Full text of DOI:10.1039/c2jm32607h

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PAPER
Efficient light-scattering functionalized TiO2 photoanodes modified with
cyanobiphenyl-based benzimidazole for dye-sensitized solar cells with
additive-free electrolytes†
a
b
a
a
a
a
Jie Zhao, Baoquan Sun, Lihua Qiu, Huizi Caocen, Qing Li, Xiaojian Chen and Feng Yan
a
*
Received 25th April 2012, Accepted 20th July 2012
DOI: 10.1039/c2jm32607h
Light-scattering functionalized cyanobiphenyl-based benzimidazole was synthesized and applied in the
surface modification of dyed TiO2 photoanodes for dye-sensitized solar cells (DSSCs). DSSCs based on
the ionic liquid and all-solid-state electrolytes were fabricated and characterized, without the addition
of additives in the electrolytes. Compared with electrolytes containing the additive N-
butylbenzimidazole (NBB), surface modification of dyed TiO2 photoanodes with cyanobiphenyl-based
benzimidazole could enhance the values of the open-circuit photovoltage (Voc), short-circuit
photocurrent density (Jsc) and the overall PCE of the fabricated DSSCs, due to the suppressed charge
recombination rate, the conductivity optimization of the electrolytes and enhanced light harvesting
capability at the TiO2 photoanode/electrolyte interface. Under the simulated air mass 1.5 solar
spectrum illuminations at 100 mW cmꢀ2, the fabricated devices achieved a cell efficiency of ꢁ6.63% and
ꢁ5.82% for ionic liquid and all-solid-state electrolytes, respectively. These results provide an alternative
strategy for high performance DSSCs with additive-free electrolytes.
restrain the interfacial electron–hole recombination. The addi-
tives added in the electrolytes have been considered as one of the
most important components, with which significant improve-
ment in the photovoltaic parameter of the DSSCs could be
achieved.4–6 The influence of electrolyte additives, including 4-
tert-butylpyridine (TBP),7 N-alkylbenzimidazole,8,9 benzimida-
zolyl functionalized ionic liquids,10 and other alkylpyridines or
N-heterocyclic compounds4 has been extensively studied. In
general, N-heterocycles could be effectively attached to the TiO2
surface via the N–Ti4+ interaction, and shift the conduction band
edge of TiO2 toward higher energies and retard the interfacial
electron recombination to increase the electron life time.4,7,11,12
Therefore, these additives could give significant improvement of
the DSSCs with the high open-circuit photovoltage (Voc), fill
factor (FF), and the high PCE.7 Actually, addition of the additive
generally decreases the conductivity of the electrolytes and the
diffusion coefficients of the redox mediator species containing
the redox couples (such as Iꢀ/I3ꢀ), resulting in a decrease of
short-circuit photocurrent density (Jsc).13,14 Therefore, optimi-
zation of the electrolytes with high conductivity and diffusion
coefficients of redox mediator species, however, without any
additives, is highly desirable for the DSSCs.
Introduction
Much attention has been paid to dye-sensitized solar cells
(DSSCs), owing to their advantages of simple assembly tech-
nology, high power conversion efficiency (PCE), and relatively
low manufacturing cost.1–3 Recently, a PCE over 12% (13.1% at
0.5 Sun) has been achieved by Yella and coworkers using a
cobalt(II/III)-based tris(bipyridyl)tetracyanoborate complex as
the redox mediator in combination with a custom synthesized
donor–p–bridge–acceptor zinc porphyrin dye YD2-o-C8 and
another organic cosensitizer Y123.3 In a DSSC device, photo-
excitation of the monolayer sensitizer coated on the TiO2 films
enables the injection of electrons into the conduction band of the
oxide. The electrons cross the films and are collected by a charge
collector into the external current circuit, and then return to the
device through the counter electrode. The complete separation of
the functions of light absorption, electrons and holes transfer
allows much more favorable materials selectivity and optimiza-
tion.3 However, the key point for efficiency enhancement is to
aJiangsu Key Laboratory of Advanced Functional Polymer Design and
Application, Department of Polymer Science and Engineering, College of
Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou 215123, P. R. China. E-mail: fyan@suda.edu.cn; Tel:
+86-512-65880973
bJiangsu Key Laboratory for Carbon-Based Functional Materials &
Devices, Institute of Functional Nano & Soft Materials (FUNSOM),
Soochow University, Suzhou 215123, P. R. China
In addition, surface passivation of TiO2 photoanodes by
carefully optimizing and controlling the thickness of an ultrathin
insulator layer, such as metal oxide,15,16 carbonates,17 organic
silane18 or poly(methylsiloxane)19 has been applied to improve
the performance in DSSCs.4,15–20 Recently, dendrons21 and
metal–organic frameworks (MOFs)22 coated on TiO2
† Electronic supplementary information (ESI) available: Additional
tables. See DOI: 10.1039/c2jm32607h
18380 | J. Mater. Chem., 2012, 22, 18380–18386
This journal is ª The Royal Society of Chemistry 2012

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photoanodes also effectively enhance the device performance of
DSSCs. Co-adsorbants, such as phosphonic acid and bis-(3,3-
dimethy-butyl)-phosphinic acid23 and donor–acceptor type
molecules,24 can also be applied for the surface modification of
TiO2 photoanodes to assist favourable packing of dye molecules
by avoiding aggregation.4 These surface passivation methods
could suppress the electron recombination.
N-Methyl-N-butylpyrrolidinium iodide (P1,4I) was synthesized
and purified as described in the literature.37
Synthesis of 1-bromo-6-(4-cyanobiphenyl-40-oxy)hexane (BrCH)
BrCH was synthesized by the reaction of 4-cyano-40-hydroxy-
biphenyl (3.90 g, 20 mmol) with a large excess of 1,6-dibromo-
hexane (12.20 g, 50 mmol) in dry acetone (100 mL) with
potassium carbonate (13.80 g, 100 mmol) as a base.38 1H NMR
(400 MHz, CDCCl3): 7.60–7.72 (m, 4H), 7.53 (d, 2H), 6.98 (d,
2H), 4.0 (t, 2H), 3.43 (t, 2H), 1.91 (m, 2H), 1.83 (m, 2H) 1.53 (m,
4H). Formula mass: 358.27. Microanalysis Calcd: C, 63.69%; H,
5.64%; N, 3.91%. Found: C, 63.61%; H, 5.72%; N, 3.80%.
Due to their unique properties, cyanobiphenyl based liquid
crystals (LCs) have been extensively studied and used as optical
materials, such as in non-linear materials,25,26 display and
photonic devices,27,28 chemical and biological sensors.29,30 The
special and orientational direction of molecules at the molecular
level and particular 3D network could provide effective path-
ways for the electroactive species to move freely.31,32 In addition,
they exhibit a wide transparency range with an optical absorp-
tion,23,24 and light-scattering properties.33–35 Recently, the cya-
nobiphenyl-functionalized liquid crystal-based electrolytes have
been used as the electrolytes with light-scattering properties for
DSSCs. Photoelectrodes with enhanced light-harvesting pro-
perties could improve the device efficiency.36 However, the extra
additives in the electrolytes are still indispensable even in these
cases. This triggered our interest in exploring the possibility to
fabricate the DSSCs with additive-free in electrolytes.
Synthesis of N-6-(4-cyanobiphen-yl-40-oxy)hexyl benzimidazole
(NCHB)
To a stirred solution containing benzimidazole (1.42 g, 12 mmol)
and BrCH (3.58 g, 10 mmol) in 20 mL of dry acetonitrile, ground
poweredꢂKOH (1.40 g, 25 mmol) was added.7 After stirring for
8 h at 0 C, the mixture was washed with acetonitrile and then
poured into water (100 mL), and finally extracted by dichloro-
ethane (50 ꢃ 3 mL). White powder, 2.94 g (yield: 74.3%). 1H
NMR (400 MHz, CDCCl3): 7.90 (s, 1H), 7.82 (d, 1H), 7.60–7.70
(dd, 4H), 7.52 (d, 2H), 7.41 (d, 1H), 7.24–7.34 (m, 2H), 6.96 (d,
2H), 4.20 (t, 2H), 3.97 (dd, 2H), 1.72–2.02 (m, 4H), 1.35–1.62 (m,
4H). Formula mass: 395.5. Microanalysis Calcd: C, 78.95%; H,
6.38%; N, 10.63%. Found: C, 78.83%; H, 6.47%; N, 10.58%.
Here, for the first time, cyanobiphenyl-based benzimidazole
was synthesized and applied for the surface modification of TiO2
photoanodes to increase the light-harvesting efficiency of pho-
toanodes. DSSCs based on the surface modified TiO2 photo-
anodes, using ionic liquid and all-solid-state electrolytes were
fabricated, without the addition of any other additives in the
electrolytes. Photovoltaic measurements show that cyanobi-
phenyl-based benzimidazole modified TiO2 photoanodes could
enhance the Voc, Jsc and the overall PCE values of the fabricated
DSSCs, due to the suppressive charge recombination rate, the
conductivity optimization of the electrolytes and enhanced light
harvest capability at the TiO2 photoanode/electrolyte interface.
Such an alternative strategy of surface modification of TiO2
photoanodes for the DSSCs with additive-free electrolytes could
be applied to the most DSSCs to achieve high device
performances.
Device fabrication
The fabrication of DSSCs was as reported previously.39,40 In
brief, the cleaned FTO glass was covered at two parallel edges
with an adhesive tape to control the thickness of mesoporous
TiO2 film. Two layers of TiO2 particles were deposited onto
cleaned FTO glass and used as photoelectrodes. A 10 mm thick
film of 20 nm sized TiO2 particles was deposited onto the FTO
glass electrode using the doctor-blade technique. The film was
ꢂ
dried at 125 C for 5 min. Then, a second 5 mm thick layer of
200 nm light-scattering anatase particles were coated on the top
of the first TiO2 layer. The resulting TiO2 films were annealed at
500 ꢂC for 15 min. After cooling to 80 ꢂC, the obtained TiO2
electrode was immersed in 0.3 mM solution of Z907 in acetoni-
trile and tert-butyl alcohol at room temperature for 24 h.
Afterward, the dye-sensitized TiO2 electrode was washed with
anhydrous ethanol and dried with nitrogen stream. To prepare
the Pt counter electrode, two drops of 5 mM H2PtCl6 in ethanol
was placed onto the cleaned FTO glass substrate, followed by
drying and annealing at 400 ꢂC for 15 min.
Experimental procedure
Materials
Benzimidazole,
1,6-dibromohexane,
4-cyano-40-hydroxy-
biphenyl, tert-butyl alcohol and succinonitrile were purchased
from Alfa Aesar and used as received. N-butylbenzimidazole
(NBB) and iodine (I2) were purchased from Fluka. TiCl4 and
H2PtCl6 were purchased from Aldrich. cis-RuLL0(SCN)2 (L ¼
4,40-dicarboxylic acid-2,20-bipyridine, L0 ¼ 4,40-dinonyl-2,20-
bipyridine) (Z907) was purchased from Solaronix SA (Switzer-
land) and used without any further purification. Fluorine-doped
tin oxide overlayer (FTO) glass electrodes (15 U per Sq), slurries
containing 20 nm-sized mesoporous and 200 nm-diameter light-
scattering TiO2 colloidal were purchased from Dalian Hepat
Chroma Solar Tech. Co., Ltd (China). 1,3-Dimethylimidazolium
iodide (DMII), 1-ethyl-3-methylimidazolium iodide (EMII), 1-
ethyl-3-methylimidazolium tetracyanoborate (EMITCB) and
guanidinium thiocyanate (GSCN) were purchased from Merck.
Before the device fabrication of Devices C, D and G, 50 mL of
0.3 M NCHB (or NBB) solution in acetonitrile/dichloroethane
(10 : 1, v : v) was filled into the porous dye-coated TiO2 photo-
anode. Then the solvent was evaporated under vacuum at 80 ꢂC.
DSSCs were fabricated by sandwiching the completely melted
electrolyte (80–100 ꢂC) between a dye-sensitized TiO2 electrode and
a pre-drilled Pt counter electrode by a 25 mm hot melt ring (Surlyn,
DuPont). The resulting cells were placed at 80 ꢂC in a vacuum to
remove air and guarantee optimum filling and fine electrical
contact. The produced devices were sealed with a Surlyn sheet and
a thin glass cover by heating. The employed electrolyte for Devices
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J. Mater. Chem., 2012, 22, 18380–18386 | 18381

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A,
C
and
D
contains DMII/EMII/EMITCB/I2/GSCN
(12 : 12 : 16 : 1.67 : 0.67), without NBB. While the reference elec-
trolyte used for Devices B contains DMII/EMII/EMITCB/I2/NBB/
GSCN (12 : 12 : 16 : 1.67 : 3.33 : 0.67), with NBB.9 The reference
all-solid-state electrolytes for Devices E and G are composed of
P
1,4I : I2 : succinonitrile (5 : 1 : 100),39 and with an additional
0.5 M NBB in the electrolyte for Device F.
Scheme 1 Synthetic route, chemical structure of N-6-(4-cyanobipheny-l-
40-oxy)hexyl benzimidazole (NCHB), and its thermal properties (melting
point (mp), and decomposition temperature(Td)).
Characterization and measurements
1H NMR spectra were recorded on a Varian 400 MHz spec-
trometer. The conductivity of electrolytes was characterized in an
ordinary cell composed of a Teflon tube and two identical
stainless steel electrodes (diameter of 1 cm) on a CHI660c elec-
trochemical workstation at room temperature. Fourier trans-
form infrared (FTIR) spectra of the synthesized compounds were
recorded on a Varian CP-3800 spectrometer in the range of 4000–
400 cmꢀ1. Absorption spectra of NCHB and the transmittance of
the dyed TiO2 films without and with NCHB (illuminated from
the FTO side) were observed with a UV Lambda 750 UV-vis/
NIR spectrometer in the range of 350–800 nm. While, the
reflectance of the dyed TiO2 films without and with NCHB
(illuminated from the NCHB side) were recorded on the same
spectrometer equipped with barium sulfate-coated integrating
sphere. Thermal analysis was carried out on Universal Analysis
2000 thermogravimetric analyzer (TGA). Samples were heated
from 50 to 500 ꢂC at a heating rate of 10 ꢂC minꢀ1 under a
nitrogen flow. Differential scanning calorimetry (DSC)
measurements were performed under a nitrogen atmosphere with
Fig. 1 FTIR spectrum of (a) benzimidazole, (b) 4-cyano-40-hydroxy-
biphenyl, (c) BrCH, and (d) NCHB.
3380 cmꢀ1 (Fig. 1b).42 Compared with Fig. 1a, the sharp band at
2230 cmꢀ1 (Fig. 1b–d) is indicative of the presence of the C^N
group.42 The relatively high melting point and decomposition
temperature (>330 ꢂC, Fig. S3, ESI†) of the compound indicate
that NCHB could offer a highꢂthermal stability, far beyond the
operating temperature (50–80 C) of the DSSCs.43
ꢀ1
ꢂ
a heating rate of 10 C min in a temperature range of ꢀ50–
250 ꢂC on a DSC-Q200. EIS measurements on a Tafel plots were
recorded on a CHI660c electrolchemical workstation at room
temperature in two-electrode mode of DSSCs. The EIS of devices
were tested using a CHI660c electrochemical workstation. Under
dark conditions, the bias voltage for the impedance measurement
was ꢀ0.70 V and the frequency ranged from 0.01–105 Hz. The
photocurrent density–voltage (J–V) curves of the assembled
DSSCs shielded by an aluminum foil mask with an aperture area
of ꢁ0.1 cm2 were measured with a digital source meter (Keithley,
model 2612) under simulated air mass (AM) 1.5 solar spectrum
Fig. 2 shows the photocurrent density–voltage (J–V) curves for
all the devices under simulated AM 1.5 solar irradiance at 100
mW cmꢀ2. As shown in Table 1, the photovoltaic parameters of
the DSSCs without (Devices A, C and D) and with (Devices B)
the additive, N-butylbenzimidazole (NBB) in the electrolytes,
respectively. Compared with Devices A and C, Device B with
NBB as the additive in the electrolyte as previously reported by
9
illumination at 100 mW cmꢀ2
. Incident photo-to-current
€
Gratzel shows a relatively higher cell efficiency (PCE ¼ 5.98%),
indicate that NBB could effectively improve the photovoltaic
parameter of the DSSCs. However, Device C which was treated
conversion efficiency (IPCE) plotted as a function of excitation
wavelength was recorded on a Keithley 2612 source meter under
the irradiation of a Xenon lamp with a monochromater (Oriel
CornerstoneTM 260 1/4). The photoelectrochemical parameters,
such as the fill factor (FF) and power conversion efficiency (PCE)
were calculated according to the previous reports.
Results and discussion
Scheme 1 shows the synthetic route, chemical structure and the
thermal properties of N-6-(4-cyanobiphenyl-40-oxy)hexyl benz-
imidazole (NCHB). The purity and chemical structure are
1
confirmed by H nuclear magnetic resonance (NMR) (Fig. S1
and S2, ESI†) and Fourier transform infrared spectroscopy
(FTIR). As shown in Fig. 1, a broad absorption peak at 2900
cmꢀ1 is assigned to the stretching vibrations of N–H of benz-
imidazole rings.41 The stretching vibration belonging to the O–H
group of 4-cyano-40-hydroxybiphenyl is observed at about
Fig. 2 J–V characteristics of Devices A–D measured under simulated
AM 1.5 solar spectrum irradiation at 100 mW cmꢀ2 (solid curves) and in
the dark (dash curves). Cell area is 0.1 cm2.
18382 | J. Mater. Chem., 2012, 22, 18380–18386
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Table 1 J–V characteristics of Devices A–D measured under simulated
AM 1.5 solar spectrum irradiation at 100 mW cmꢀ2. Cell area: 0.1 cm2
Table 2 The Jsc calculated from the IPCE spectrum and measured from
the J–V curves for Devices A, B and D
TiO2 surface
Devicea coating/additive Voc [V] Jsc [mA cmꢀ2
Device
]
FF
PCE [%]
A
B
D
A
B
C
D
—/—
0.567
0.611
0.598
0.643
14.21
14.79
14.31
15.36
0.663 5.34
0.662 5.98
0.666 5.70
0.671 6.63
Calculated Jsc (mA cmꢀ2
Measured Jsc (mA cmꢀ2
)
11.15
14.21
12.36
14.79
13.42
15.36
—/NBB
NBB/—
NCHB/—
)
a
The reference ionic liquid electrolyte for Device B containing: DMII/
EMII/EMITCB/I2/NBB/GSCN (12 : 12 : 16 : 1.67 : 3.33 : 0.67),9 and
without the additive NBB for Devices A, C and D. No surface
modification has been done for the reference Devices A and B, while
NBB and NCHB are used for surface modification for Devices C and
D, respectively.
elementary charge will lead to a calculated Jsc value, which are
consistent with the measured data, as shown in Table 2. It should
be noted that liquid-based DSSCs showed slower response times
than that of solid-state photovoltaics.4 The deviation of the
measured Jsc values was acceptable since the filling of trap states
should be responsible for the current response results because of
the large internal surface of the photoelectrodes.46 In addition,
low stead-state value of photo-induced current at large bias light
intensity (1 Sun) will slight decrease the IPCE values, due to the
ion transport limitation of the electrolyte.47
with NBB at the dyed TiO2 photoelectrode/electrolyte interface
does not achieve a comparative efficiency in comparison to
Device B, which might be due to the fast dissolution of NBB
from the photoelectrode/electrolyte interface into the electrolyte
to increase the electron recombination. On the other hand,
NCHB is insoluble in the ionic liquid electrolyte applied in this
work due to the anisometric molecules including van der Waals
interactions and p–p stacking.44 Moreover, the employed elec-
trolyte without NBB shows a higher conductivity of 9.06 ꢃ 10ꢀ4
S cmꢀ1 than that of NBB-based electrolyte (8.52 ꢃ 10ꢀ4 S cmꢀ1),
which also leads to a higher performance for Device D. There-
fore, addition of NCHB does not decrease the conductivity of the
electrolyte, and Device D with the modification of NCHB at the
dyed TiO2 electrode/electrolyte interface represents the best solar
cell performance (PCE ¼ 6.63%), which is much higher than that
of the reference Device B (PCE ¼ 5.98%), resulting in about 11%
efficiency improvement. Furthermore, NCHB treatment can
increase the light-harvesting efficiency of photoanodes as dis-
cussed below (Fig. 4). These results demonstrate the possibility of
fabricating high performance DSSCs based on additive-free
electrolytes.
As shown in Fig. 4, the light-transmitting and light-scattering
properties of the dyed TiO2 films were investigated. Compared
with the film control, it is observed that coating a scattering of
200 nm TiO2 particles on the control film can reduce the trans-
mittance and enhance the reflectance of TiO2 photoanodes.
However, a small amount of light leakage through the scattering
layer could be observed. Further addition of light scattering
layers can reflect more visible light, however, cracks of TiO2
photoanodes during sintering processes could not be avoi-
ded.48–50 Here, the treatment of TiO2 photoanodes with NCHB
could further decrease the transmittance and increase the
reflectance of the TiO2 photoanodes (Fig. 4). Moreover, the low
UV absorption of NCHB film (Fig. S4, ESI†) represents a wide
transparency range with an optical absorption. Therefore,
NCHB could effectively reflect the light of the dyed TiO2 film.
The high reflectance in the visible light region indicates the
multi-reflection of light in TiO2 film leading to an increase of
Fig. 3 represents the incident photo-to-current conversion
efficiency (IPCE) values of typical Devices A, B and D. The
broad feature appears covering the visible spectrum range from
450 to 650 nm. The maximum IPCE values at 530 nm are 69%,
75% and 81% for Devices A, B and D, respectively. As previously
reported,45 using a solar photon flux of standard AM 1.5 spec-
trum, a simple multiplication with the IPCE spectrum and the
Fig. 4 (a) The transmittance of dyed TiO2 films: control, control +
scattering layer, and control + scattering layer + NCHB; (b) the reflec-
tance of dyed TiO2 films: control, control + scattering layer, and
control + scattering layer + NCHB; (c) the transmittance of a DSSC
without and with NCHB, respectively; (d) schematic representation for
the sunlight scattering principle of a DSSC.
Fig. 3 The IPCE vs. wavelength profiles for typical Devices A, B and D,
respectively.
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Table 3 EIS results and calculated electron life time before recombi-
nation for Devices A, B and D at ꢀ0.70 V in the dark
Device
R1/U
Rct/U
R3/U
fmax
se/ms
A
B
D
0.97
1.39
1.21
36.82
39.51
50.96
9.14
9.58
8.95
25.82
24.13
21.66
6.17
6.60
7.35
light-harvesting efficiency of the photoelectrode. Therefore, the
transmittance of the DSSC treated with NCHB at the photo-
electrode/electrolyte interface is obviously lower than that
without NCHB modification (Fig. 4c), demonstrating the
sunlight harvest is enhanced by NCHB treatment, which results
in the remarkable enhancement in device performance, including
Jsc and IPCE as mentioned above (Table 1 and Fig. 3). A
hypothesized operational scheme of such a device is shown in
Fig. 4d.
Fig. 6 Time-course variation of the efficiencies for Devices A–D, during
accelerated aging tests under the successive one sun visible-light soaking
with UV cutoff filter at 50 ꢂC.
that the adsorption of the dyed TiO2 electrode with NCHB could
effectively reduce the electron recombination, leading to
enhanced electron transfer and further improved cell perfor-
mance. These results show good consistency with the previous
J–V values, demonstrating that the treatment of dyed TiO2
electrode with NCHB is an efficient method for the high
performance DSSCs without any additives in the electrolytes.
The long-term stability of the fabricated DSSCs was investi-
gated via an accelerating aging test at 50 ꢂC with successive one
sun visible-light soaking shown in Fig. 6. After an accelerating
aging test of 50 days, Devices A–D remained at 91%, 89%, 88%
and 91% of their initial efficiency, respectively. All the devices
retained almost over 90% of their initial efficiency in this period.
However, Device C shows a sharp reduction of the PCE during
the initial few days because of the dissolution of NBB from the
dyed TiO2 electrode into the electrolyte to leading to increased
electron recombination and resultant decreased cell perfor-
mance. It should be noted that the Device D treated with NCHB
at the photoanode/electrolyte interface still maintains almost
91% of its initial performance over 50 days, and also represents a
good stability.
Photovoltaic electrochemical impedance spectra (EIS) were
further used to investigate the interfacial charge transfer
processes of the DSSCs measured at ꢀ0.70 V in the dark,51–54 and
the resistance values of devices were summarized in Table 3.
Fig. 5a shows the typical Nyquist plots of EIS spectra of Devices
A, B and D. From low to high frequency, three semicircles are
observed in the Nyquist plots. R1, Rct, and R3 represent charge-
transfer resistance at the counter electrode, the resistance of
TiO2/FTO, TiO2/electrolyte interface, and the Nernst diffusion in
the electrolyte, respectively.54 The semicircle in the intermediate-
frequency region is larger, the electron recombination at the
TiO2/electrolyte interface is weaker.54,55 Thus, higher Rct renders
the suppressive electron recombination, which is favourable of
the performance enhancement. The low values of R1 represent a
favourable charge-transfer process for the reduction of I3ꢀ to Iꢀ
at the Pt counter electrode/electrolyte interface. It can be seen
that Device B shows higher R1 and R3 values than those of
Devices A and D, indicating that the addition of NBB in elec-
trolyte reduces the conductivity of the electrolyte, which is
undesirable for performance improvement.
Application of NCHB is further extended to all-solid-state
DSSCs based on N-methyl-N-butylpyrrolidinium iodide (P1,4I)-
doped succinonitrile plastic crystal electrolytes, as reported by
Fig. 5b shows the Bode phase plots for the DSSC measured
at ꢀ0.70 V in the dark. The frequency (fmax, the maximum
frequency of the frequency peak) and the electron life time before
recombination (se) (Table 3) were calculated according to the
following equation:56
Gratzel.39 The J–V curves for Devices E–G are shown in Fig. 7,
€
and the photovoltaic parameters are summarized in Table 4.
Under the same experimental conditions, Device G, in which the
1
1
se ¼
¼
umax 2pfmax
Compared with Devices A and B, the highest values of Rct
(50.96 U) and se (7.35 ms) were obtained for Device D, indicating
Fig. 7 J–V characteristics of solid-state Devices E, F and G measured
under simulated AM 1.5 solar spectrum irradiation at 100 mW cmꢀ2
(solid curves) and in the dark (dash curves). Cell area is 0.1 cm2.
Fig. 5 (a) Nyquist plots of EIS spectra and (b) Bode phase plots
measured for Devices A, B and D at ꢀ0.70 V in the dark.
18384 | J. Mater. Chem., 2012, 22, 18380–18386
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Table 4 J–V characteristics of Devices E–G measured under simulated
AM 1.5 solar spectrum irradiation at 100 mW cmꢀ2. Cell area: 0.1 cm2
TiO2 surface
Devicea coating/additive Voc [V] Jsc [mA cmꢀ2
]
FF
PCE [%]
E
F
G
—/—
—/NBB
NCHB/—
0.577
0.614
0.630
12.39
13.16
13.65
0.682 4.87
0.660 5.33
0.676 5.82
Fig. 9 (a) The IPCE vs. wavelength profiles for Devices E–G and (b)
Time-course variation of the efficiencies for Devices E–G, during accel-
erated aging tests under the successive one sun visible-light soaking with
UV cutoff filter at 50 ꢂC.
a
Solid-state electrolyte for Devices
E
and
G
containing:
P1,4I : I2 : succinonitrile (5 : 1 : 100),39 and with 0.5 M NBB for Device F.
Table 6 The Jsc calculated from the IPCE spectrum and measured from
the J–V curves for Devices E–G
Device
E
F
G
Calculated Jsc (mA cmꢀ2
Measured Jsc (mA cmꢀ2
)
10.28
12.39
11.91
13.16
13.74
13.65
)
Fig. 8 (a) Nyquist plots of EIS spectra and (b) Bode phase plots
measured for Devices E–G at ꢀ0.70 V in the dark.
the first ten days, the efficiency is moderately enhanced due to an
increase of the regeneration of dye Z907 and thus an increase of
Jsc.9 By the accelerating aging test, Devices E–G remained 93%,
90% and 95% of their initial efficiency, respectively, showing an
excellent long-time stability. These results further confirm the
conclusion that NCHB with light-scattering properties could
successfully enhance the device performance of the DSSCs by a
simple treatment at the photoelectrode/electrolyte interface.
Table 5 EIS results and calculated electron life time before recombi-
nation for Devices E–G at ꢀ0.70 V in the dark
Device
R1/U
Rct/U
R3/U
fmax
se/ms
E
F
G
0.94
1.66
1.12
36.85
43.28
56.39
8.88
12.23
8.21
27.17
23.91
21.39
5.86
6.66
7.44
Conclusions
dyed TiO2 photoanode is treated with NCHB, shows the highest
PCE of 5.82%, much higher than that of the reference Device E
(PCE ¼ 4.87%), leading to an efficiency enhancement of 19.5%.
In the Nyquist plots and Bode phase plots of all-solid-state
In conclusion, we demonstrate herein a simple strategy for the
fabrication of high performance DSSCs via covering modifica-
tion of TiO2 photoanode surface with light-scattering function-
alized cyanobiphenyl-based benzimidazole. Compared with the
electrolytes containing NBB, surface modification of dyed TiO2
photoanode with NCHB could enhance the Voc and Jsc values
and the overall PCE of the fabricated DSSCs because of the
suppression of charge recombination, the conductivity optimi-
zation of the electrolytes and the enhancement of light-harves-
ting efficiency of the TiO2 photoanodes. This alternative strategy
of surface modification could be applied to most DSSCs
including quasi- or solid-state ones to achieve high device
performances.
Devices E–G shown in Fig. 8 and Table 5, low R1 values also
ꢀ
represent an effective charge-transfer process relying on the I3
/
Iꢀ redox couple for oxidized dye regeneration. Compared with
Devices E and F, larger Rct (56.39 U) at the photoelectrode/
interface and longer se (7.44 ms) for Device G is also indicative of
a more effective suppression of charge recombination by NCHB
treatment. Furthermore, higher R3 value of Device F (12.23 U)
than those of Devices E and G indicates that the addition of NBB
in all-solid-state electrolyte reduces the conductivity of the elec-
trolyte, which is unfavourable for performance enhancement.
The solid-state electrolyte without NBB shows a higher ionic
conductivity of 3.87 ꢃ 10ꢀ4 S cmꢀ1 than that of NBB-based
electrolyte (3.52 ꢃ 10ꢀ4 S cmꢀ1), resulting in a higher perfor-
mance for Device G. These results further confirm the conclusion
that NCHB with light-scattering properties could successfully
enhance the device performance of the DSSCs by a simple
treatment at the photoelectrode/electrolyte interface.
Acknowledgements
This work was supported by Natural Science Foundation of
China (no. 21174102), The Natural Science Foundation of
Jiangsu Province (BK2011274), Research Fund for the Doctoral
Program of Higher Education (20103201110003), and the Project
Funded by the PAPD of Jiangsu Higher Education Institutions.
As shown in Fig. 9a, a broad feature appears covering the
visible spectrum range from 450 to 700 nm, and the maximum
IPCE values at 530 nm are 64%, 70% and 76% for Devices E–G,
respectively, which are consistent with the calculated data (Table
6). The long-term stability of Devices E–G was also investigated
and the characterization data were presented in Fig. 9b. During
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