Enhance the performance of dye-sensitized solar cells by co-sensitization of 2,6-bis(iminoalkyl)pyridine and N719

Article abstract of DOI:10.1039/c3ra44194f

Three organic dyes 2,6-bis(iminoalkyl)pyridines [2,6-(2,6-R ₂C₆H₂NCMe)₂]C₅H ₃N (R = methyl, ethyl, isopropyl) (named DM, DE and DP, respectively) were synthesized and assembled onto nanocry

Full text of DOI:10.1039/c3ra44194f

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PAPER
Enhance the performance of dye-sensitized solar cells by
co-sensitization of 2,6-bis(iminoalkyl)pyridine and
N719†
Cite this: RSC Adv., 2013, 3, 25908
a
a
Liguo Wei,ab Yulin Yang, Ruiqing Fan, Ping Wang,a Liang Li,a Jia Yu,a Bin Yanga
*
*
and Wenwu Caoac
Three organic dyes 2,6-bis(iminoalkyl)pyridines [2,6-(2,6-R2C6H2N]CMe)2]C5H3N (R ¼ methyl, ethyl,
isopropyl) (named DM, DE and DP, respectively) were synthesized and assembled onto nanocrystalline
TiO2 film to prepare 2,6-bis(iminoalkyl)pyridine/N719 co-sensitized photoelectrodes for dye-sensitized
solar cell (DSSCs) applications, and their photoelectrochemical performances were studied. In each of
the three composite electrodes, the aggregation of N719 was alleviated and its spectra response was
enhanced by the 2,6-bis(iminoalkyl)pyridines in region of 400 to 750 nm, and the total resistance of
DSSCs was decreased after co-sensitizing with 2,6-bis(iminoalkyl)pyridine, all of which facilitate to
improve efficiency of DSSCs. The optimized cell which was co-sensitized with DM gave a short circuit
current density of 16.57 mA cmꢀ2, an open circuit voltage of 0.72 V and a fill factor of 0.59
corresponding to an overall conversion efficiency of 7.00% under standard global AM1.5 solar
irradiation conditions, which is 28.91% higher than that for DSSCs only sensitized by N719 (5.43%). The
incident photo-to-current conversion efficiency, electrochemical impedance spectroscopy, surface
photovoltage spectroscopy and UV-visible adsorption spectra were used to analyze their
photoelectrochemical performances, and X-ray single-crystal structural analysis was used to analyze the
structures of 2,6-bis(iminoalkyl)pyridines.
Received 6th August 2013
Accepted 3rd October 2013
DOI: 10.1039/c3ra44194f
www.rsc.org/advances
complement absorption bands sensitized on semiconductor
lm together, extending the light harvesting ability so as to
1. Introduction
Dye-sensitized solar cells (DSSCs) are promising next-
generation alternatives to conventional silicon-based
photovoltaic devices because of their great advantages—
light weight, low cost and easy processing.1–4 Since O'Regan
increase photocurrents of the devices. Ehret et al. studied
the nanocrystalline TiO2 solar cell sensitized by various
dicarboxylated cyanines and found that the use of mixed
cyanine dyes could improve photoelectric conversion effi-
ciency.14 Zhang and coworkers examined the mechanism
of the co-sensitization with squarylium and N3 dyes by
¨
and Gratzel reported a high-efficiency (7.10%) DSSC based
on Ru-complex dye and nanocrystalline TiO2 mesoporous
lm in 1991,1 a lot of interest has been generated on this
topic.5–8 Dye as the most crucial element in DSSCs has
attracted considerable attention, and most research focuses
on synthesis of new dyes,9,10 low-cost natural dyes, and
co-sensitizing using multicolor dyes.11–13 The co-sensitiza-
tion strategy is proved to be an effective approach to enhance
device performance by combination of two or more dyes that
have larger molar extinction coefficients or dyes that have
time-resolved spectroscopy.15 Gratzel and coworkers combined
¨
bisthiophene dye (JK2) with squarylium cyanine dye (SQ1) as
co-sensitizers and obtained an overall conversion efficiency of
7.43%.16 Wang et al. reported that the enhanced performance
of chlorophyll-sensitized solar cells have been obtained by
co-sensitization of a-type Chl with b-, or c-type Chl.17 Phthalocy-
anine dye (TT1) co-sensitized with various organic dyes (JK2 or
D2) exhibited an enhanced device performance.18,19 Co-sensitiza-
tion of TiO2 lms with a black dye and organic dyes (D131 or Y1)
signicantly enhanced photocurrent and the device perfor-
mance attained h ¼ 11.0–11.4%.20,21 In 2011, the highest value
reported for the power conversion efficiency of DSSCs has
reached 12.3% through co-sensitization method,22 which is a
new milestone in this research eld, stimulating investigation of
the development of new sensitizer to promote device perfor-
mance by co-sensitization. Presently, metal-free organic dyes
aDepartment of Chemistry, Harbin Institute of Technology, Harbin 150001, P.R. China.
E-mail: ylyang@hit.edu.cn; fanruiqing@hit.edu.cn; Fax: +86-451-86418270
bModern Analysis and Research Center, Heilongjiang University of Science and
Technology, Harbin 150027, P.R. China
cMaterials Research Institute, The Pennsylvania State University, Pennsylvania 16802,
USA
† CCDC 952322 and 952323. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ra44194f
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have attracted much attention and were widely used for co- and dried with blower to complete the loading of N719. The
sensitization of DSSCs.23–27
co-sensitized electrodes were prepared by further immersing
In our previous work, metal–organic complexes were used to the electrode in saturated (ca. 0.3 mM) 2,6-bis(iminoalkyl)-
co-sensitize DSSCs and the performances were enhanced,13,28,29 pyridine in absolute ethanol solution for 2 h to complete the
while in this paper, three metal-free organic dyes 2,6-bis(imi- co-sensitizer loading and then dried with blower again. The
noalkyl)pyridines [2,6-(2,6-R2C6H2N]CMe)2]C5H3N (R
¼
electrolyte used in this work was 0.5 M LiI + 0.05 M I2 + 0.1 M
methyl, ethyl, isopropyl) (named DM, DE and DP, respectively), tert-butyl pyridine in a 1 : 1 (volume ratio) of acetonitrile–
whose chemical structure are shown in Fig. 1, were selected as propylene carbonate. The platinum counter electrode was
co-sensitizers to study the photoelectrochemical performances prepared by depositing H2PtCl6 paste onto the FTO glass
ꢁ
of dye-sensitized solar cells. Since DM, DE and DP are iso- substrates and then sintered at 450 C for 30 min. The cells
structural, but substitute groups on the 2- and 6-positions of were assembled by sandwiching the electrolyte between the
phenyl groups are different, the relationships between molecule dye sensitized photoanode and the counter electrode and
structure and photoelectrochemical performance have also assembly was held together using mini-binder clips.
been investigated. Through co-sensitization of these organic
dyes with N719 sensitizer, respectively, we have achieved a
higher efficiency than that for single N719 sensitized solar cells.
2.3. Characterization
2,6-bis(iminoalkyl)pyridines play an important role in matching All characterizations were carried out under ambient pressure
optimization with dyes in complexes for increasing photo- and temperature. Photocurrent–photovoltage (I–V) curves were
electrical conversion efficiencies.
recorded by Keithley model 2400 digital source meter using a
mask with an aperture area of 0.16 cmꢀ2. The irradiance of
AM1.5 global sunlight from a ltered 500 W xenon lamp light
source was set at 100 mW cmꢀ2 and was calibrated by a stan-
dard silicon solar cell (no. NIMMS1123, calibrated by National
Institute of Metrology, P. R. China). Based on I–V curve, the ll
factor (FF) is dened as: FF ¼ (Jmax ꢂ Vmax)/(Jsc ꢂ Voc) where Jmax
and Vmax are the photocurrent density and photovoltage for
maximum power output; Jsc and Voc are the short-circuit
photocurrent density and open-circuit photovoltage, respec-
tively, the overall energy conversion efficiency h is dened as: h
¼ (FF ꢂ Jsc ꢂ Voc)/Pin where Pin is the power of the incident
light, Jsc and Voc are also the short-circuit photocurrent density
and open-circuit photovoltage, respectively, and FF is the ll
factor. EIS were recorded by CHI660D Electrochemical Analyzer
(Chenhua, China), and the measurements were taken over a
frequency range of 0.1–100 kHz under standard global AM1.5
solar irradiation, or in the dark by applying a forward bias
equivalent to Voc. IPCE was measured on an EQE/IPCE spectral
response system (Newport). The SPS instrument was assembled
by Jilin University. Monochromatic light was obtained by
passing light from a 500 W xenon lamp through a double-prism
monochromator (SBP300, China), and the signals were
collected by a lock-in amplier (SR830, Stanford). UV-visible
absorption spectra of DM, DE and DP in ethanol solution were
recorded on a Germany Jena model SPECORD S600 UV-visible
spectrophotometer and the spectra of co-sensitized TiO2 lms
were recorded on a Japan Shimadzu model UV-2250 spectro-
photometer. The single-crystal X-ray diffraction data for DM, DE
and DP were collected on a Rigaku R-AXIS RAPID IP diffrac-
tometer equipped with graphite-monochromated Mo KR radi-
2. Experimental section
2.1. Synthesis of DM, DE and DP
The 2,6-diacetylpridine was prepared according to a published
procedure.30 DM, DE and DP were synthesized according to
modied published procedures in a good yield by condensation
of 2,6-diacetylpyridine with the corresponding aniline in
reuxing absolute methanol in the presence of a catalytic
amount of formic acid (Scheme 1).31,32
2.2. Fabrication of DSSCs
Dye-sensitized solar cells were fabricated using the following
procedure. The TiO2 paste was cast onto the FTO substrate
(90% transmittance in the visible, 15
U per square,
purchased from NSG, Japan) by the screen-printing method,
ꢁ
followed by drying at 100 C for 5 min and this process was
repeated six times, then followed by sintering at 500 ꢁC for
15 min in air to obtain a transparent TiO2 photoelectrode
with the thickness of ca. 10 mm. The mesoporous TiO2
photoelectrode was immersed in 0.3 mM [cis-bis(isothio-
cyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)-ruthenium(II)bis-
tetrabutylammonium] (known as N719, Solaronix Company,
Switzerland) solution in absolute ethanol solution for 14 h
and washed with ethanol to remove the unabsorbed N719
˚
ation (l ¼ 0.71073 A), operating at 293 ꢃ 2 K. The structures
were solved by direct methods and rened by full-matrix least-
squares based on F2 using the SHELXTL 5.1 soware package
(G M. Sheldrick, SHELXTL NT Crystal Structure Analysis
Package [CP], version 5.10; Bruker AXS, Analytical X-ray System:
Madison, WI, 1999). The hydrogen atoms residing on the
carbon atoms were located geometrically. All non-hydrogen
atoms were rened anisotropically.
Fig. 1 Molecular structures of DM, DE and DP.
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Scheme 1 Synthesis process of 2,6-bis(iminoalkyl)pyridines.
conjunction with the solvent model. The electronic distribu-
tions of the frontier molecular orbitals of the DM, DE and DP in
ethanol are shown in Fig. 3. The highest occupied molecular
orbital (HOMO) of DM, DE and DP contributes to the two
2,6-dialkyl-substituted phenyl rings (donor) and the lowest
unoccupied molecular orbital (LUMO) contributes to the pyridyl
rings (acceptor). The two imino functional groups with conju-
gated C]N bridge work as a p conjugated bridge, so DM, DE
and DP are donor–p-conjugated–acceptor (D–p–A) type metal
free organic dyes. The HOMO and LUMO energy level of DM,
estimated from DFT calculations are ꢀ5.61 and ꢀ2.93 eV,
respectively, and for DE and DP they are ꢀ5.64 and ꢀ2.89 eV,
ꢀ5.63 and ꢀ2.85 eV, respectively.
3. Results and discussion
3.1. Structure of 2,6-bis(iminoalkyl)pyridines
The main difference of DM, DE and DP is the substitute groups
on the 2- and 6-positions of phenyl groups, which are methyl,
ethyl and isopropyl, respectively, as shown in Fig. 1.
The X-ray structural analysis of the single-crystals reveals
that DM, DE and DP are isostructural. The dyes DM, DE and DP
possess the structures with an approximate Cs symmetry about
a plane bisecting the central pyridine ring. As shown in Fig. 2,
the pyridyl ring of DP as an example is coplanar with the two
imino functional groups. The two imino C]N bonds have
typical double-bond characteristics, with bond lengths of
˚
1.266(3) and 1.272(3) A, which are similar to that in DM and
To ensure effective electron injection from the LUMO of the
sensitizer into the conduction band of semiconductor (TiO2),
the LUMO level of the dye must be higher than the conduction
band edge of TiO2.35 It is observed that ELUMO for DM, DE and
DP is more positive than the conduction band of TiO2 (ꢀ4.40
eV).36 This provides a substantial driving force for electron
DE.33,34 The two 2,6-diisopropyl-substituted phenyl rings are
approximately perpendicular to the ethylidenepyridine ring,
with the dihedral angles being 87.059(79) and 72.949(88)ꢁ. The
dihedral angles are decreased with the increased size of
substituted alkyls. Although the complex includes three N
atoms, there are no obvious hydrogen-bond or p–p interactions
in the crystal structure.
3.2. Molecular orbitals and band structure calculations of
DM, DE and DP
Energy-level matching is crucial in selecting sensitizer. In order
to further understand the energy band structure in DM, DE and
DP, we performed ab initio calculations for the molecular
orbitals based on the time-dependent density functional theory
(TD-DFT) with 6-31G basis set. The geometric parameters
obtained from X-ray diffraction analyses were fully optimized in
Fig. 3 The electronic distribution of the frontier molecular orbitals of DM, DE
Fig. 2 The asymmetric unit of DP.
and DP computed in the ethanol using TDFT/MPWIK/6-31G.
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injection. The potential of the reduced DM, DE and DP (EHOMO
)
occurred for N719 adsorbed on TiO2, but co-sensitization with
is more negative than the redox potential of Iꢀ/I3 (ꢀ4.85 eV)37 DM, DE and DP prevents the aggregation of N719 and adjusts
and far below that of the conduction band of TiO2. This gives the arrangement of N719 molecules toward a more uniform
sufficient driving force for dye regeneration (Scheme 2). It orientation which is benecial for charge transfer. Moreover, the
shows that the energy levels of DM, DE and DP are appropriate adsorption was increased in the region of 350–750 nm, which
ꢀ
for the DSSCs system containing TiO2.
could render better light-harvesting in this region and further
enhance the performance of co-sensitized DSSCs.
3.3. UV-visible absorption spectra of 2,6-bis(iminoalkyl)-
pyridine in ethanol and co-sensitized TiO2 photoelectrodes
3.4. Surface photovoltage spectrum analysis of 2,6-
bis(iminoalkyl)pyridine co-sensitized photoelectrodes
The absorption spectra of DM, DE and DP in ethanol and
co-sensitized with N719 on TiO2 lms is shown in Fig. 4(a) and
(b), respectively. It can be seen from Fig. 4(a) that DM, DE and
DP all display a large absorption peak at about 279 nm and a
small absorption peak at about 335 nm, respectively. When
DM, DE and DP is mixed with N719, respectively, the
absorption spectra show that the absorption of N719 is
enhanced only in the region of 250–450 nm. The enhanced
band seems too narrow and too close to the UV region that it
does not work when they are used to co-sensitize TiO2 pho-
toelectrodes with N719. But when DM, DE and DP was used to
co-sensitize with N719 on TiO2 lms, respectively, it gives a
surprising result that the absorption of N719 was remarkably
enhanced in the UV-visible region as shown in Fig. 4(b).
Fig. 4(b) shows the absorption spectra of DM, DE and DP on
N719/TiO2 lms, respectively. The experimental results indi-
cate that aggregation of N719 occurred on TiO2 nanocrystal
surface. Aggregations of dye was reported as early as in 1936,38
and from then on it is accepted that the dyes have a strong
tendency to aggregate at the solid/liquid interface due to
strong attractive forces between molecules.39 Usually, dye
aggregates have three forms: blue-shied H-aggregates
(parallel orientation), red-shied J-aggregates (tilted orienta-
tion), and both red- and blue-shied herring-bone
aggregates.40–44 As shown in Fig. 4(b), the two maximum
absorption peaks shied from 383 to 367 nm and 525 to
559 nm, respectively, compared with in ethanol solution; aer
The surface photovoltage spectrum (SPS) method is commonly
applied to TiO2 for DSSCs, which is a well-established contact-
less technique for surface state distribution.45,46 Fig. 5 is the SPS
of photoanodes sensitized with single N719 sensitizer and co-
sensitized with 2,6-bis(iminoalkyl)pyridine. The curves in Fig. 5
indicate that co-sensitizing could enhance the photovoltage
signal between 300 and 750 nm. The photovoltage signals
increase with the order of DM/N719 > DE/N719 > DP/N719 >
N719, which indicates that the separate efficiency of electrons
and holes correlate strongly with DM, DE, DP. The results of
UV-visible absorption spectra and SPS demonstrate that the
composite TiO2 is benecial to enhance the spectra response
aer co-sensitization in DSSC photoelectrodes. The improve-
ment of composite photoelectrode co-sensitized by DM is
more evident than that by DE and DP. It is known that the
electron-donating ability of the alkyl group is in the order of 2,6-
dimethylphenyl > 2,6-diethylphenyl > 2,6-diisopropylphenyl and
the steric hindrance on the 2,6-dialkyl-substituted phenyl ring
is in the order of 2,6-dimethylphenyl < 2,6-diethylphenyl < 2,6-
diisopropylphenyl. Stronger electron-donating ability and lower
steric hindrance of the alkyl group on the 2,6-dialkyl-
substituted phenyl ring is more suitable for enhancing the
spectra response.
3.5. Photoelectrochemical performance of dye-sensitized
solar cell
The 2,6-bis(iminoalkyl)pyridine co-sensitized and single dye
sensitized photoanodes were assembled with counter elec-
trodes and electrolyte to prepare organic dye/N719 co-sensi-
tized DSSCs devices or single dye sensitized DSSCs. Fig. 6
presents the performances of the co-sensitized DSSCs in
terms of Jsc and Voc, and the results of all DSSCs are listed in
Table 1. Fig. 6 and Table 1 indicate that the performances of
cells co-sensitized with 2,6-bis(iminoalkyl)pyridine and N719
are improved in the order of DM/N719 > DE/N719 > DP/N719
> N719. As shown in Table 1, take the DM/N719/TiO2
composite photoelectrode as an example, the co-sensitized
solar cell based on a DM/N719/TiO2 electrode yields a
remarkably high photocurrent density ( Jsc) of 16.57 mA cmꢀ2
under standard global AM1.5 solar irradiation conditions
without an expense of lower open circuit voltage (Voc) but a
little increased from 0.71 V to 0.72 V. The cell shows a better
photo-electricity conversion efficiency of 7.00%, which is
higher than that for DSSCs using single N719 (5.43%) and
DM (0.69%). In this way the cell efficiency was improved by
more than 28.91% compared with that for single N719. This
co-sensitization
two maximum absorption peaks shied back to 375 and 547 nm,
respectively. This result indicates that herring-bone aggregates
with
2,6-bis(iminoalkyl)pyridine,
the
Scheme 2 Schematic energy diagram of HOMO and LUMO for dyes compared
to the energy levels calculated for TiO2.
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Fig. 4 UV-visible absorption spectra of dyes in ethanol solution and on TiO2 photoelectrodes (a) in ethanol solution and (b) on TiO2 photoelectrodes.
Meanwhile, the effect of DM, DE and DP soaking time on the
performances of co-sensitized DSSCs was also investigated. As
shown in Fig. 7, h and photocurrent density increased within
2 hours soaking time of DM, DE and DP, and do not change
too much aer soaking more than 2 hours; voltage for DM
co-sensitized DSSCs increases slightly with the increase of
soaking time and for DE co-sensitized DSSCs it rstly decreases
and then increases, while for DP co-sensitized DSSCs it is reversed,
but all the voltage value for DM, DE and DP co-sensitized DSSCs
are around 0.7 V; ll factor for DM, DE and DP co-sensitized
DSSCs keep stable with the increase of soaking time. The photo-
voltaic parameters shown in Fig. 7 indicate that all the parameters
could reach the maximum value when soaking time of DM, DE
and DP is 2 hours, and prolonging soaking time does not enhance
the performances of DSSCs too much. Therefore, 2 hours soaking
time for DM, DE and DP is appropriate for the DSSCs co-sensitized
with N719 whose soaking time is 14 h. This is also the reason
why 2 hours is xed as DM, DE and DP soaking time in all our
experiments.
Fig.
5
SPS of co-sensitized photoelectrodes and N719 sensitized
photoelectrode.
IPCE is dened as the number of electrons generated by light
in the external circuit divided by the number of incident
photons. The IPCE spectra of different devices are plotted as a
function of excitation wavelength and are presented in Fig. 8. It
is found that DM, DE and DP co-sensitized with N719, respec-
tively, could enhance the spectral response in the whole visible
region and consequently enhance the photocurrent perfor-
mance. Herein, co-sensitization of the nanostructured TiO2
electrodes could markedly increase the photocurrent density as
reported in Table 1. As can be seen from the spectra in Fig. 7,
the spectrum obviously exhibits an upward shi in the wave-
length range from 400 to 750 nm. It shows that co-sensitization
of 2,6-bis(iminoalkyl)pyridine with N719 has higher light-
harvesting. Compared with the results of UV-visible absorption
spectra and SPS, it is clear that the improvement of IPCE
performance is attributed to the enhanced spectral response,
better charge transfer and higher light-harvesting.
Fig. 6 J–V curves for DSSCs based on co-sensitized photoelectrodes and N719
sensitized photoelectrode.
Table 1 J–V performance of DSSCs based on different photoelectrodes
Photoelectrode
Jsc (mA cmꢀ2
)
Voc (V)
FF
h (%)
N719/TiO2
DM/TiO2
DE/TiO2
12.68
3.65
2.46
0.71
0.51
0.55
0.45
0.72
0.72
0.71
0.60
0.37
0.49
0.45
0.59
0.61
0.62
5.43
0.69
0.66
0.49
7.00
6.99
6.91
DP/TiO2
2.44
DM/N719/TiO2
DE/N719/TiO2
DP/N719/TiO2
16.57
15.83
15.64
3.6. Electrochemical impedance spectroscopy (EIS)
EIS is a useful technique for the analysis of electronic and ionic
result suggests that co-sensitization of TiO2 photoelectrode transport processes in an electrochemical device. It is a steady
with the 2,6-bis(iminoalkyl)pyridine and N719 is an effective state method to measure current response based on the appli-
way to improve the efficiency of DSSCs.
cation of an AC voltage at different frequencies.47–49 The EIS
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Fig. 7 Effect of DM, DE and DP soaking time on the performances of co-sensitized DSSCs (a) efficiency, (b) open circuit voltage, (c) short circuit current and (d) fill factor.
transfer at the TiO2/electrolyte interface, and redox charge
transfer at the counter electrode, respectively.
Under light illumination, the second semicircle radius in the
Nyquist plot decreased aer co-sensitizing with 2,6-bis(imi-
noalkyl)pyridine, and the values are in the order of DM/N719 <
DE/N719 < DP/N719 < N719, which indicates a decrease of the
electron transfer impedance and an increase of charge transfer
rate at this interface (Fig. 9(a)); while in the dark, this semicircle
radius in the Nyquist plot increased aer co-sensitizing with
2,6-bis(iminoalkyl)pyridine, and the values are in the order of
DM/N719 > DE/N719 > DP/N719 > N719, which indicates an
increase of electron recombination impedance and a reduction
of the interfacial charge recombination rate (Fig. 9(c)).
It was reported that under Voc conditions, the electrons
injected into the oxide semiconductor should be recombined at
Fig. 8 IPCE spectra of DSSCs based on single N719 sensitized and co-sensitized
photoelectrodes.
investigation of the DSSC device provides valuable informa- the oxide/dye/electrolyte interface because there is no current
tion for the understanding of photovoltaic parameters (Jsc, Voc
passing through the external circuit.50 Thus the lifetime of
,
FF, and h). Herein, we utilize EIS to analyze charge carrier electrons in the oxide lm (sr) can be estimated according to the
dynamics in the interfacial regions of solid–liquid layers and equation: sr ¼ 1/(2pfmax), where fmax is the maximum frequency
the effect of 2,6-bis(iminoalkyl)pyridine in composite photo- of the mid-frequency peak.51 As shown in Fig. 8(b), the fmax value
electrodes on the performance of DSSCs. Impedance spectra shis to lower frequency aer co-sensitizing with 2,6-bis(imi-
of DSSC devices with different 2,6-bis(iminoalkyl)pyridine/ noalkyl)pyridine, which for N719 sensitized DSSC is 21.5 Hz,
N719 co-sensitized photoanode were measured under stan- bigger than the value for co-sensitized DSSCs (17.4, 21.2 and
dard global AM1.5 solar irradiation or in the dark by applying 21.2 Hz for DM/N719, DE/N719 and DP/N719, respectively).
a forward bias equivalent to open circuit potential. As indi- Therefore, the electron lifetimes for DM/N719, DE/N719 and
cated in Fig. 9, a typical EIS spectrum exhibits three semi- DP/N719 are calculated to be 9.2, 7.5, 7.5 ms, respectively,
circles in the Nyquist plot or three characteristic frequency longer than 7.4 ms for single N719. The same trend also can be
peaks in a Bode phase angle presentation. In the order of seen in the dark bode phase plot (Fig. 9(d)). The longer electron
increasing frequency the features are attributed to the Nernst lifetime and higher value of recombination resistance for
diffusion in the electrolyte (not clearly visible), electron co-sensitized DSSCs indicates more effective suppression of the
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Fig. 9 Impedance spectra of DSSC devices based on different photoelectrodes measured under standard AM1.5 G solar irradiation (a and b) or in the dark (c and d).
Nyquist plots: (a) and (c); Bode phase plots: (b) and (d).
back reaction between photoelectrons in the conduction band composite solar cells. The symbols R and C describe a resistance
of TiO2 and the electrolyte.52 It is possibly due to the better and a capacitance, respectively, Rs is series resistance, R1 and R2
coverage of the 2,6-bis(iminoalkyl)pyridine/N719/TiO2 photo- are the charge transfer resistance, O, which depends on the
electrode, which inhibits the surface recombination of elec- parameters Yo,1 and B, accounts for a nite-length Warburg
trons at the electrode/electrolyte interface, this is conrmed by diffusion (Zw) and Q is the symbol for constant phase element,
Fig. 8(c). Lower steric hindrance make it easy to adhere on the CPE (its parameters are Yo,2 and n).
surface of TiO2 lms and lead to better coverage, as the steric
Table 2 lists parameters obtained by tting the impedance
hindrance is in the order of DM < DE < DP, the coverage of DM is spectra of composite solar cells measured under standard
better than DE and DP. On the other hand, as discussed above, AM1.5 G solar irradiation using the equivalent circuit in Fig. 10.
co-sensitizing with 2,6-bis(iminoalkyl)pyridine could prevent As shown in Table 2, the series resistance Rs does not obviously
the aggregation of N719 and adjust arrangement of the N719 change; the resistance R1 which is estimated from the high
molecules towards a more uniform orientation which is bene- frequency range semicircle in Nyquist plots decrease with the
cial for charge transfer and further prolongs electron lifetime. introduction of 2,6-bis(iminoalkyl)pyridine, and the resistance
On all accounts, the longer electron lifetime and better R2 which is estimated from the second semicircle of Nyquist
suppression of back electron recombination enable co-sensi- plots also decrease with the introduction of 2,6-bis(iminoalkyl)-
tized DSSCs to achieve a higher solar cell efficiency.
pyridine, and the total resistance of every co-sensitized DSSCs
In order to further understand the complex charge transfer with 2,6-bis(iminoalkyl)pyridine decrease compared with single
process in DSSCs, a physical model has been proposed,53,54 and N719 sensitized DSSCs, these are all consistent with the results
the equivalent circuit represented in Fig. 10, Rs[C1(R1O)](R2Q2), list in Fig. 9. The total resistance decrease is benecial for
is used to model this system, representing interfaces in electron transfer and further improving the performance of
DSSCs. This suggests that the high performances in DSSCs
sensitized with 2,6-bis(iminoalkyl)pyridine/N719 are also due to
decreases of internal cell resistance.
3.7. Relationship between structures and
photoelectrochemical properties
Photoelectrochemical performances indicate that DM, DE and
DP which are isostructural could prevent the aggregation of
Fig. 10 Equivalent circuit used to represent interfaces in DSSCs consisting of FTO
glass substrate/TiO2/dye/I3ꢀ/IꢀkPt/FTO glass substrate.
25914 | RSC Adv., 2013, 3, 25908–25916
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Table 2 Parameters obtained by fitting the impedance spectra of solar cells measured under standard AM1.5 G solar irradiation using the equivalent circuit
O
Q2
DSSC samples
Rs (U)
C1 (10ꢀ4 F)
R1 (U)
Yo,1 (10ꢀ1 S)
B (S1/2
)
R2 (U)
Yo,2 (10ꢀ5 S1/2
)
n
N719/TiO2
25.66
25.62
25.46
26.02
6.24
7.38
6.47
7.15
15.56
12.42
12.81
11.84
1.38
1.34
1.23
1.32
0.50
0.51
0.51
0.54
14.03
11.45
11.65
12.93
9.03
8.92
7.50
0.77
0.75
0.76
0.75
DM/N719/TiO2
DE/N719/TiO2
DP/N719/TiO2
10.30
N719 and adjust the arrangement of the N719 molecules containing only N719 (5.43%). Jsc is enhanced because the
toward a more uniform orientation, enhance spectra response aggregation of N719 is alleviated and its spectral response is
in the whole visible region, and further improve the photo- enhanced in the region of 400–750 nm by 2,6-bis(iminoalkyl)-
electrochemical performances. However, the substituent pyridines, and the total resistance of DSSCs is decreased
groups on the 2- and 6-positions of phenyl groups are aer co-sensitizing with 2,6-bis(iminoalkyl)pyridines which are
different, they are isopropyl, methyl and ethyl, respectively, good for charge transfer. It is worth noting that the newly
these make a little difference in their performances. It is very developed co-sensitized electrode is prepared only by a simple
interesting that the electron-donating ability of the alkyl group sequential adsorption process, which is inexpensive and easily
(2,6-dimethylphenyl > 2,6-diethylphenyl > 2,6-diisopropyl- handled, and thereby deserves to be explored for the application
phenyl) is in line with the overall energy conversion efficiency in DSSCs.
of co-sensitized DSSCs. Meanwhile, lower steric hindrance on
the 2,6-dialkyl-substituted phenyl ring (2,6-dimethylphenyl <
2,6-diethylphenyl < 2,6-diisopropylphenyl) leads to a higher
efficiency of co-sensitized DSSCs. It is well known that higher
electron-donating ability contributes better electron transfer
21371040), the National key Basic Research Program of China
and smaller steric hindrance making it easier to adsorb on
TiO2 lm and inhibit the surface recombination of electrons at
the electrode/electrolyte interface more efficiently. Both
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant nos 21071035, 21171044 and
(973 Program, no. 2013CB632900), and the key Natural Science
Foundation of the Heilongjiang Province, China (no. ZD201009,
ZD201101).
of them are responsible to enhance the performance of
co-sensitized DSSCs. Meanwhile, the different substitutes on
Notes and references
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25916 | RSC Adv., 2013, 3, 25908–25916
This journal is ª The Royal Society of Chemistry 2013