Gel electrolyte materials formed from a series of novel low molecular mass organogelators for stable quasi-solid-state dye-sensitized solar cells

Article abstract of DOI:10.1039/c4ta02895c

Electrolyte materials are the key components in dye-sensitized solar cells (DSCs) and are very crucial to the performance and long-term stability of DSCs. We developed a series of diamide derivatives as novel low molecular mass organogelators (LMOGs) for DSCs. These LMOGs contain different numbers (2, 6, 5 and 9) of methylene groups (-CH₂-) between the two amide carbonyl groups and exhibit distinctive self-assembly behaviors. The gel electrolytes prepared by these LMOGs possess high gel-to-solution transition temperatures (over 100 °C) and the stability of DSCs is largely enhanced. More importantly, the parity of the number of -CH₂- and their special molecular arrangements have a remarkable influence on the self-assembly of the gelators resulting in a significantly different morphology, and further influence the photovoltaic performances of DSCs. It is found that the LMOGs containing odd-numbered -CH₂- lead to a much better charge transport of the gel electrolytes, inducing a longer electron lifetime and higher incident photon-to-electron conversion efficiency compared with the LMOGs containing even-numbered -CH₂-. Finally, a superior quasi-solid-state DSC based on the gelator containing five -CH₂- is obtained, which exhibits a photoelectric conversion efficiency of 7.53% and excellent thermal and light-soaking stabilities during accelerated aging tests.

Full text of DOI:10.1039/c4ta02895c

Journal of
Materials Chemistry A
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PAPER
Gel electrolyte materials formed from a series of
novel low molecular mass organogelators for
stable quasi-solid-state dye-sensitized solar cells†
Cite this: J. Mater. Chem. A, 2014, 2,
5921
1
a
a
a
a
a
a
Li Tao,‡ Zhipeng Huo,‡* Yong Ding, Lu Wang, Jun Zhu, Changneng Zhang,
a
b
ac
b
Xu Pan, Mohammad K. Nazeeruddin,* Songyuan Dai* and Michael Gratzel
¨
Electrolyte materials are the key components in dye-sensitized solar cells (DSCs) and are very crucial to the
performance and long-term stability of DSCs. We developed a series of diamide derivatives as novel low
molecular mass organogelators (LMOGs) for DSCs. These LMOGs contain different numbers (2, 6, 5 and
9
2
) of methylene groups (–CH –) between the two amide carbonyl groups and exhibit distinctive self-
assembly behaviors. The gel electrolytes prepared by these LMOGs possess high gel-to-solution
ꢀ
transition temperatures (over 100 C) and the stability of DSCs is largely enhanced. More importantly, the
2
parity of the number of –CH – and their special molecular arrangements have a remarkable influence
on the self-assembly of the gelators resulting in a significantly different morphology, and further
influence the photovoltaic performances of DSCs. It is found that the LMOGs containing odd-numbered
–
CH
and higher incident photon-to-electron conversion efficiency compared with the LMOGs containing
even-numbered –CH –. Finally, a superior quasi-solid-state DSC based on the gelator containing five
CH – is obtained, which exhibits a photoelectric conversion efficiency of 7.53% and excellent thermal
and light-soaking stabilities during accelerated aging tests.
2
– lead to a much better charge transport of the gel electrolytes, inducing a longer electron lifetime
Received 9th June 2014
Accepted 25th July 2014
2
DOI: 10.1039/c4ta02895c
–
2
www.rsc.org/MaterialsA
enhance the efficiency of the DSC. One approach is to broaden
the photoresponse and maximize the light-harvesting efficiency.
Introduction
Over the last few decades, solar energy has emerged as the most For example, the hyperbranched titania architectures in the
2,3
practical and long lasting alternative to conventional fossil-fuel photoanode for DSCs have been investigated by Kuang et al.
based energy sources. To utilize solar energy for a variety of and a signicant enhancement in power conversion efficiency
4
applications one would need to convert it into electricity using (11.01%) was obtained. Another important way to improve the
photovoltaic cells. Among various kinds of photovoltaic devices, photovoltaic performance of DSCs is to develop novel electro-
dye-sensitized solar cells (DSCs) have been regarded as a prom- lytes. The best photoelectric conversion efficiency of DSCs based
5
ising candidate for next generation solar cells and aroused on the liquid electrolyte has achieved 13%. However, it is clear
intensive interest over the past decade owing to their low cost, that stability is a prerequisite for the application of any photo-
low energy consumption, environmentally friendly materials, voltaic technology and the extent of application is limited by the
6,7
simple fabrication process and comparatively high power level of stability that can ultimately be achieved. In order to
1
conversion efficiency. Many strategies have been reported to improve the stability of liquid electrolyte based DSCs, many
strategies have emerged, such as room-temperature molten ionic
salts (ILs), p-type semiconductors, and organic and inorganic
a
8–12
Key Laboratory of Novel Thin Film Solar Cells, Division of Solar Energy Materials and
hole conductors.
ILs possess many unique properties
Engineering, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031,
P. R. China. E-mail: zhipenghuo@163.com; Tel: +86 551 65592190
including low vapor pressure, broad electrochemical windows
and non-ammability. Recently, the efficiency of DSCs based on
the eutectic melt-based ionic liquid electrolytes reached over
b
Laboratory for Photonics and Interfaces, Institution of Chemical Sciences and
Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH-
13
1
015 Lausanne, Switzerland. E-mail: mdkhaja.nazeeruddin@ep.ch; Tel: +41 21 8%. Despite this, the uidity of ILs can still lead to leakage of
6
c
93 6124
DSCs. Therefore, the quasi-solid-state electrolytes based on low
molecular mass organogelators (LMOGs) have attracted much
attention in recent years. LMOGs can nely disperse as aniso-
tropic aggregates within the organic solvent resulting in a three-
dimensional structure to effectively gelate the liquid electrolyte to
form a quasi-solid-state electrolyte. The key factor in gel
Beijing Key Laboratory of Novel Thin Film Solar Cells, North China Electric Power
University, Beijing, 102206, P. R. China. E-mail: sydai@ipp.ac.cn; Tel: +86 10
6
†
1
‡
1772268
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ta02895c
Li Tao and Zhipeng Huo contributed equally to this research.
This journal is © The Royal Society of Chemistry 2014
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formation is the self-assembly process of LMOGs driven by non- dodecanamide (gelator D) were synthesized by reacting laur-
covalent interactions, including hydrogen bonding, p–p inter- oylchloride with different alkyl diamines in the mixture of water
actions, metal–ligand coordination, van der Waals forces, and ether. Aer the reaction, the solvent was removed under
14
hydrophobic effects and so on. More importantly, the gel elec- vacuum and the gelators were obtained as white powder used
22
trolyte based on LMOGs has good ionic conductivity and pore- with further recrystallization by ethanol. The products were
15
1

lling property, which are similar to the liquid electrolyte, and conrmed by proton nuclear magnetic resonance spectra ( H
the network of the gel can reduce the leakage of the liquid elec- NMR) (Bruker Avance III 500, Bruker, Switzerland) and
trolyte effectively to improve the long-term stability of DSCs. elemental analysis (VarioEL III, Elementar, Germany). Speci-
16
Many compounds, such as 12-hydroxystearic acid, cyclo- cally, the gelators A, B, C and D contain two, six, ve and nine
1
7
hexanecarboxylic acid-[4-(3-octadecylureido)phenyl]amide, 1- –CH – between the two amide carbonyl groups in didodeca-
2
18
ethyl-3-methylimidazolium thiocyanate,
and tetradodecy- noylamides of a,u-alkylidenediamines (n ¼ 2, 5, 6 and 9),
19
lammonium bromide, have been developed as LMOGs and respectively.
applied in DSCs, respectively. These literature reports studied the
inuences of the additive LMOGs in the liquid electrolyte on the Electrolyte preparation
gel to the solution transition temperature (Tgel) of the gel elec-
1
,2-Methyl-3-propylimidazoliumiodide (DMPII) was prepared as
trolyte and photovoltaic performances of the DSCs. In addition,
other researchers generally discussed the gelation capability of
23
reported previously. The liquid electrolyte for DSCs was
composed of 0.1 mol L iodine (I
anhydrous lithium iodide (LiI: 99%, Aldrich), 0.5 mol L N-
methylbenzimidazole (NMBI: 99%, Aldrich), and 1 mol L
DMPII in 3-methoxypropionitrile (MePN: 99%, Fluka). The gel
electrolyte was prepared by adding an 8 wt% (vs. liquid elec-
trolyte) gelator into the liquid electrolyte and heated to 120 C
under stirring until the gelator was dissolved. Aer cooling to
ꢁ1
ꢁ1
2
: 99%, Aldrich), 0.1 mol L
ꢁ
ꢁ
different LMOGs or demonstrated the difference in the I /I
ꢁ1
3
redox transport property between the liquid and gel electro-
ꢁ1
20,21
lytes.
However, little research on the relationship of the gel
morphology with the charge transport property of the gel elec-
trolyte and the kinetic processes of the electron transport/
recombination in the quasi-solid-state dye sensitized solar cells
ꢀ
(QS-DSCs). Different from other studies, in this paper, we
ꢀ
room temperature (25 C), the gel electrolytes A, B, C and D with
gelators A, B, C and D were respectively formed and used in
DSCs.
synthesized a series of didodecanoylamides of a,u-alkylidenedi-
amines as LMOGs to gelate the 3-methoxypropionitrile (MePN)
based liquid electrolytes. Depending on the even- or odd-
numbered –CH – in the molecular structure of LMOGs, these
2
Fabrication of dye-sensitized solar cells
LMOGs exhibit intriguingly changeable self-assembly behaviors
in electrolytes, which results in quite different microscopic
structures of the gel networks. We systematically studied how the
molecular structures of the gelators inuence the self-assembled
The colloidal TiO
2
mircospheres were prepared by hydrolysis of
titanium tetraisopropoxide as shown in the previous litera-
24
ture. Nanocrystalline electrodes about 11.3 mm thickness were
obtained by screen-printing TiO
2
paste on FTO glass (TEC-8,
3D network structures of the gels, and how these different
ꢀ
LOF). Aer sintering at 450 C for 30 min in air then cooling to
microstructures inuence the electrochemical properties of the
gel electrolytes, the photovoltaic performances and stabilities of
the QS-DSCs. By using the polarized optical light microscopy, it is
found that the self-assembly behavior of the LMOGs containing
ꢀ
about 120 C, the nanoporous TiO2 photoelectrode was
immersed in an ethanol solution of 0.5 mM cis-dithiocyanate-
N,N-bis-(4-carboxylate-4-tetrabutylammoniumcarboxylate-2,2-
bipyridine) ruthenium(II) (N719 dye) for 14 h. The platinized
odd-numbered –CH
the molecules of the gelators is quite different from those con-
taining even-numbered –CH –. And not surprisingly, these
different self-assembly modes inuence the morphologies of the
D network structures of the gel electrolytes. Furthermore, it is
2
– between the two amide carbonyl groups in
counter electrodes were obtained by spraying H
2
PtCl
6
solution
ꢀ
to FTO glass followed by heating at 410 C for 20 min. DSCs were
2
assembled by sealing the dyed nanoporous TiO
and the counter electrode with thermal adhesive lms (Surlyn
702, Dupont, USA). The liquid electrolyte was injected into the
2
photoelectrode
3
1
very important that the electrochemical characteristics and
photovoltaic performance of QS-DSCs were signicantly changed
with these different self-assembly behaviors. Therefore, it seems
reasonable and important for both fundamental research and
practical application of photovoltaic devices to investigate and
understand the relationships of the LMOGs molecular structures,
microscopic structures of gel electrolytes, photovoltaic perfor-
mances and stabilities of QS-DSCs.
internal space between two electrodes through the hole on the
counter electrode, which was later sealed by a cover glass and
thermal adhesive lms. The gel electrolytes were heated to
ꢀ
1
20 C under stirring until the gel transforms into a liquid
completely. Then, the electrolyte (hot solution) was rapidly
injected into the cell and the cell was sealed similar to the liquid
electrolyte. Aer cooling to room temperature, a uniform
motionless gel layer was formed in the cell.
Experimental section
Polarized optical light microscopy
Synthesis of LMOGs
For optical microscopic investigations, a piece of the gel (8 wt%
0
0
N,N -1,2-Ethanediylbis-dodecanamide (gelator A); N,N -1,6-hex- of LMOGs in MePN) was placed onto a glass slide and protected
0
ꢀ
anediylbis-dodecanamide (gelator B), N,N -1,5-pentanediylbis- with a cover slip. The sample was heated to 120 C at a rate of
dodecanamide (gelator C) and N,N -1,9-nonanediylbis- 10 C min and the micrographs were obtained during cooling
0
ꢀ
ꢁ1
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between crossed polarizers using a microscope (DM2500P, mV within the frequency range from 1 MHz to 10 mHz in the
Leica, Germany) equipped with a hot-stage (LTSE-420, Linkam, dark.
UK) and a camera (Micropublisher 5.0 RTV, Qimaging, Canada)
ꢀ
ꢁ1
at a rate of 1 C min
.
Characterization of incident photon-to-electron conversion
efficiency (IPCE)
Differential scanning calorimetry
The photocurrent action spectra were recorded using a QE/IPCE
The gel to solution transition temperature (Tgel) of the gel measurement kit consisting of a 300 W xenon lamp (69911,
electrolyte was determined by using a differential scanning Newport, USA), a 1/4 m monochromator (74125 Oriel Corner-
calorimeter (DSC-Q2000, TA, USA). Approximately 5–7 mg of stone 260, Newport, USA), a dual channel power meter (2931-C,
each sample was weighed and sealed in an aluminum pan and Newport, USA) and the calibrated UV silicon photodetector
ꢀ
ꢁ1
heated at a rate of 10 C min under nitrogen ow from 25 to (71675, Newport, USA).
1
ꢀ
35 C for DSC measurement.
Photovoltaic characterization and stability tests
Conductivity measurements
The photovoltaic performance of DSCs with the active area of
2
The ionic conductivities of the liquid and gel electrolytes were
determined by two electrode thin-layer cells with two platinum
0
2
.16 cm with a black mask was measured by using a Keithley
420 digital source meter (Keithley, USA), and controlled using
23
black electrodes and an ac impedance technique on an elec-
trochemical workstation (IM6e, Zahner, Germany) over a
frequency range of 50 MHz to 1000 kHz with a voltage ampli-
tude of 10 mV at 0 V bias.
Test point soware under a 450 W xenon lamp (Orial, USA) with
ꢁ
2
a lter (AM 1.5, 100 mW cm ). The incident light intensity was
calibrated with a standard crystalline silicon solar cell before
each experiment.
Hermetically sealed cells were used for long-term stability
ꢀ
Voltammetric measurements
tests. The cells were stored in the oven at 60 C for thermal
stress experiment. Furthermore, the successive one sun light
soaking experiment was also carried out. DSCs covered with a
UV cutoff lter (up to 394 nm) were irradiated with an open
Steady-state voltammograms were recorded on a electro-
chemical workstation (Autolab 320, Metrohm, Switzerland) at
ꢀ
2
5 C in two-electrode mode of the DSC equipped with a 5.0 mm
ꢁ2
circuit under AM1.5, (XQ3000, 100 mW cm , Shanghai B.R.
Science Instrument Co., Ltd, China) and the air temperature
platinum ultramicroelectrode (CHI107, CH Instruments Inc.,
USA) as the working electrode, a 1 mm radius platinum disk
electrode (CHI102, CH Instruments Inc., USA) as the counter
ꢀ
was set to 50 C during the light soaking experiment. J–V
2
5,26
measurements were carried out at room temperature aer
allowing these cells to cool down and equilibrate for 30 min.
electrode and the reference electrode.
The steady-state
ꢁ
1
current–voltage curves were obtained at a scan rate of 5 mV s
.
FT-IR spectrophotometry measurements
Results and discussion
To conrm the formation of the lithium bond between the Gelation study of the gel electrolytes
gelator and LiI, we prepared four samples: (a) the MePN solvent,
The molecular structure has an important inuence on the
(b) solution of LiI (0.1 M) dissolving in the MePN solvent, (c) the
molecular self-assembly and microscopic 3D network of the gel
gel based on MePN and gelator C (8 wt%) without LiI and (d) the
gel based on MePN and gelator C with LiI (the ratio of the molar
concentration of gelator C and LiI is 1.7 : 1). These samples
were put into the polyethylene cell for measurement and
30–32
electrolyte.
Fig. 1 shows the chemical structures of the
gelators bridged by a straight carbon chain varying in length by
, 5, 6 and 9 –CH – between the two amide carbonyl groups. As
can be seen, the zigzag arrangement of the carbon chain directs
2
2
ꢁ
1
analyzed in a range of 800 to 3200 cm with 16 scans at a
22,33
the amide carbonyl groups in the different direction.
The
ꢁ1
resolution of 2 cm . IR spectra for all the samples were
measured by using KBr pellets and acquired using a FT-IR
spectrometer (NICOLET8700, Thermo Fisher Scientic Inc.,
USA).
carbon chain of the even number (n ¼ 2 or 6) directs the two
amide carbonyl groups antiparallel (the opposite direction),
while the carbon chain of the odd number (n ¼ 5 or 9) is parallel
(the same direction). Consequently, a diamide molecule of an
even-numbered carbon chain is able to form two pairs of
hydrogen bonds with two other molecules in a plane. On the
IMVS/IMPS and electrochemical impedance measurements
The experimental setup for intensity-modulated photocurrent other hand, a diamide molecule of the odd-numbered carbon
and photovoltage spectroscopy (IMPS and IMVS) measurements chain forms four independent hydrogen bonds with four other
27–29
22,33
has been described elsewhere.
Intensity-modulated molecules not in a plane.
In addition, the dodecanoyl
measurements were carried out by using an electrochemical moieties of all these gelators favorably interact with each other
workstation (IM6e, Zahner, Germany) with light emitting within van der Waals forces. These different self-assemblies
diodes (l ¼ 610 nm) driven by export (Zahner, Germany). respectively result in ribbon and woven 3D networks of the gels
Electrochemical impedance spectroscopy (EIS) measurements containing even-numbered –CH – (gelators A and B) and odd-
2
were performed on an electrochemical workstation (Autolab numbered –CH
2
– (gelators C and D), which can be further
320, Metrohm, Switzerland), at a perturbation amplitude of 10 conrmed by polarized optical micrographs.
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Fig. 1 Chemical structures of didodecanoylamides of a,u-alkylide-
nediamines (n ¼ 2, 5, 6, and 9) (above) and self-assembly structures of
gelator B (n ¼ 6) (middle) and gelator D (n ¼ 9) (below).
The in situ gelation process of gelator B (n ¼ 6) and gelator
D (n ¼ 9) in the MePN solvent was investigated using polar-
ized optical light microscopy in the same area of each sample.
ꢀ
ꢀ
Firstly, the gels were heated up to 130 C at a rate of 10 C
min
ꢁ
1
to ensure complete dissolution of the gelators in
MePN. Subsequently, these hot solutions were cooled to 30 C
at a rate of 1 C min . The photographs of Fig. 2(a)–(d) were
taken during the cooling process at 110 C, 108 C, 103 C and 100 C, (f) 97 C, (g) 95 C, and (h) 30 C in the MePN solvent.
Fig. 2 Optical micrographs between crossed polarizers showing the
formation processes of the network structures of gel B (n ¼ 6) at (a)
ꢀ
ꢀ
ꢁ1
ꢀ
ꢀ
ꢀ
ꢀ
1
10 C, (b) 108 C, (c) 103 C, and (d) 30 C and gel D (n ¼ 9) at (e)
ꢀ
ꢀ
ꢀ
ꢀ ꢀ ꢀ ꢀ
ꢀ
3
0 C, respectively, which show the formation process of the
self-assembly gel network of gelator B. Fig. 2(e)–(h) were
ꢀ
ꢀ
ꢀ
ꢀ
obtained at 100 C, 97 C, 95 C and 30 C, respectively, and
show the formation process of self-assembled gel network of
gelator D. It is noteworthy that gelator B and gelator D show
the remarkable differences in the crosslinking modes, which
can be clearly seen from the circular areas of Fig. 2. As pre-
dicted, gelator B forms a ribbon network, on the other hand,
gelator D forms a woven network. These different character-
istics of the gel network could lead to different thermody-
ꢀ
chain. The T values of gel A and gel B are 112.2 C and
gel
ꢀ
1
(
15.4 C, respectively, which are higher than those of gel C
ꢀ
ꢀ
100.2 C) and gel D (106.8 C). It indicated that the gelators
containing even-numbered –CH – groups can construct a more
stable network structure than those containing odd-numbered
CH – groups. In addition, four more –CH – groups were
increased in the molecule of gelator B than that of gelator
A; therefore, the Tgel is also increased with the increase of the
length of the carbon chain. This is because the longer
carbon chains could increase the van der Waals forces
between the molecules of gelators. This phenomenon is also
observed in gelator C and gelator D. Furthermore, all the four
2
–
2
2
3
4,35
namic and electrochemical properties of gel electrolytes.
Since the woven network has a much smaller bril size than
that of the ribbon network, we presume that the ions can
move more easily in the woven network than in the ribbon
network.
ꢀ
gel electrolytes exhibited high Tgel (over 100 C) which ensures
The thermodynamic properties of gelators A–D in MePN
based electrolytes were investigated by using a differential
scanning calorimeter and the transition temperature (Tgel) from
the gel state to the liquid state were obtained. The photo of gel
electrolytes is shown in Fig. 3. As can be seen, the Tgel values
the gel state of the electrolytes at the operating temperature of
the solar cell.
Electrochemical characteristics of the gel electrolytes
2
depend on the parity of the number of –CH – groups between The Arrhenius equation was used to describe the conductivity–
the two amide carbonyl groups and the length of the carbon temperature behavior of the liquid and gel electrolytes. The
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Table 1 The values of activation energy of gel electrolytes A–D at
around the Tgel
ꢁ1
Activation energy (kJ mol
)
Electrolyte
Below Tgel
Above Tgel
Gel A (n ¼ 2)
Gel B (n ¼ 6)
Gel C (n ¼ 5)
Gel D (n ¼ 9)
16
18
12
14
11
10
11
10
when the temperature is below the Tgel, the activation energy
values of gel A and gel B are higher than those of gel C and gel D.
This is due to the completely different bril sizes and types of
the microstructures of the gels caused by different self-assem-
blies of gelators.
Fig. 3 Differential scanning calorimetric thermograms of the gel
electrolytes.
ꢁ
3ꢁ
The diffusion of redox species (I /I ) is associated with the
conductivity which provides information of the mobility of the
36
ions in the electrolyte. Moreover, it is well known that the
ꢁ
diffusion coefficient of I3 plays an important role in increasing
temperature dependence shows a classical Arrhenius behavior
obeying the eqn (1):
37
the short circuit photocurrent density (Jsc
dark reaction at the TiO photoelectrode/electrolyte interface in
(1) the QS-DSCs. Steady-state cyclic voltammetric analysis was
)
and reducing the
2
16,18
s ¼ A exp(ꢁE /kT)
A
carried out in order to understand the charge transport mech-
where s is the conductivity, A is the pre-exponential factor, E
A
is anism in the liquid and gel electrolyte system. Fig. 4(b) shows
the activation energy, and k is Boltzmann's constant. Fig. 4(a) the comparative steady-state voltammograms of the liquid
ꢀ
shows the ionic conductivity–temperature plot of liquid and gel electrolyte and gel electrolytes at 25 C. The apparent diffusion
ꢁ
ꢁ
electrolytes. Table 1 lists the values of the activation energy of coefficients (Dapp) of redox species (I and I3 ) were calculated
the gel electrolytes with different gelators. As can be seen in from the anodic and cathodic steady-state currents (I ) using
ss
38
Fig. 4(a) and Table 1, the activation energy values of the gel the following equation.
electrolytes are changed around the Tgel of gel electrolytes.
When the temperature is below Tgel, the gel electrolytes show
I
ss ¼ 4nrcFDapp
(2)
higher activation energy values than liquid electrolytes (10 kJ
ꢁ
1
where n is the electron number in the electrode reaction, F is the
Faraday constant, and c is the bulk concentration of electro-
active species, Dapp is the apparent diffusion coefficient, and r is
the radius of the Pt ultramicroelectrode. As shown in Fig. 4(b)
and Table 2, the Dapp values of I and I
lower than those of the liquid electrolyte. And the Dapp values of
mol ). However, when the temperature is above Tgel, the acti-
vation energy values of the gel electrolytes are almost equal to
that of the liquid electrolytes. This result suggested that the 3D
network of the gel could obstruct the physical diffusion of the
charges in the electrolytes, which leads to a lower ionic
ꢁ
ꢁ
3
of gel C are slightly
16
conductivity than that of the liquid electrolyte. Moreover,
Fig. 4 (a) Arrhenius plots for the conductivity of the liquid electrolyte and gel electrolytes A–D and (b) cyclic voltammograms of MePN-based
ꢀ
ꢁ1
liquid and gel electrolytes with a Pt ultramicroelectrode at 25 C (scan rate ¼ 5 mV s ).
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Table 2 The steady-state current and apparent diffusion coefficient of
iodide and triiodide in different electrolytes
The inuences of gelation on the kinetic processes of electron
transport and recombination
3ꢁ
7
ꢁ
ꢁ7
3ꢁ
ꢁ
I
ss (I
)
I
ss (I )
D
app (I
)
D
app (I )
Importantly, the amide carbonyl groups of gelators A–D can
ꢁ
ꢁ6
2
ꢁ1
ꢁ6
2
ꢁ1
+
40,41
Electrolyte
(10 A) (10 A) (10 cm s
)
(10 cm s
)
interact with Li .
the microstructures of the gels probably cause different Li
As a result, the different morphologies of
+
Liquid
1.45
0.30
0.23
5.98
1.39
0.98
4.15
2.76
3.76
0.78
0.59
2.55
1.73
4.20
0.98
0.69
2.92
1.94
adsorption behaviors on the surface of the mesoporous TiO
2
Gel A (n ¼ 2)

lm, therefore, the kinetic processes of the electron transport
Gel B (n ¼ 6)
Gel C (n ¼ 5) 0.99
and recombination can probably be changed in the devices
based on the different gel electrolytes, which should be dis-
cussed in detail. In the gel electrolytes, the interaction between
Gel D (n ¼ 9) 0.67
+
ꢁ
3ꢁ
the amide carbonyl groups of the LMOGs and Li can be
I and I of gel C and gel D are much higher than those of gel A
and gel B. Due to the distinctive self-assembly, the gelators
containing odd-numbered –CH – groups form woven 3D
networks and provide much better channels for charge diffu-
sion. Therefore, the redox ions can still diffuse smoothly in the
gel electrolytes based on woven 3D networks despite the high
investigated by IR spectra and the results are shown in Scheme
1
(A). It is accepted that the shi of the stretching vibration
2
ꢁ1
ꢁ1
absorption band (1680 cm to 1630 cm ) of the carbonyl
group (nC]O) in amide is considered to be an important infor-
41
mation to conrm the interaction mentioned above. Scheme
A(a) and A(b) show the IR spectra of the pure MePN solvent and
1
viscosity of the gel electrolytes. Moreover, the increased Dapp
ꢁ
solution of LiI dissolving in the MePN solvent, respectively, and
there is no change in these two samples. Scheme 1A(c) shows
the IR spectra of the MePN based gel without LiI, and the nC]O
can accelerate the supply of I
increase the Jsc of QS-DSCs. Therefore, due to the larger Dapp
3
to the Pt counter electrode and
3
9
,
the QS-DSCs based on gel electrolytes containing odd-
numbered –CH – groups probably have higher J than the QS-
ꢁ
1
is observed at 1648 cm . However, the nC]O in the spectra of
the MePN based gel with LiI (as shown in Scheme 1A(d)) shis
to the much lower frequency region and is observed at 1633
2
sc
DSCs based on gel electrolytes containing even-numbered
CH – groups.
From the above analysis we can conclude that the methylene
units between the two amide carbonyl groups in the gelator
molecule is the key in the self-assemble behavior. The special
molecular arrangement creates different morphologies, types
and sizes of the microscopic 3D network of the gel electrolyte.
The type and size of the gel microstructures can directly inu-
ence the charge transport properties of the gel electrolytes.
–
2
ꢁ
1
cm . This obvious shi reveals the interaction between the
+
+
C]O of the gelator molecule and Li . As is known that the Li
can adsorb on the surface of the mesoporous TiO lm and
2
2
intercalate into the TiO lattice as shown in Scheme 1B(a). The
+
interaction between the carbonyl groups and Li can results in a
+
decreased adsorption of Li on the surface of the mesoporous
2
TiO lm as shown in Scheme 1B(b), this phenomenon can
Scheme 1 (A) Infrared spectra of (a) MePN solvent, (b) solution of LiI dissolving in MePN solvent, (c) gel based on MePN and gelator C without LiI
+
and (d) gel based on MePN and gelator C with LiI; (B) adsorption of Li on the mesoporous TiO
being gelated.
2
film surface before and after the liquid electrolyte
15926 | J. Mater. Chem. A, 2014, 2, 15921–15930
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strongly affect the electron transport and recombination factors for the change of the open circuit potential. Generally,
42,43
kinetics, and surface state energy distribution in DSCs.
the increase in Voc could be caused by the negative shi of the
The photoinduced charge (Q) in the mesoporous TiO lms TiO conduction band edge. On the other hand, if the conduc-
2
2
4
0,44–46
was detected by IMPS/IMVS measurements.
dence of the open circuit potential (Voc) on ln Q is shown in decreased Voc
Fig. 5. For a given Q, the increase in Voc is attributed to an The gelation can also inuence the kinetic processes of
upward shi of the TiO conduction band edge. In contrast, the electron transport and recombination. Fig. 6 presents the
decrease in Voc is attributed to a downward shi of the TiO comparison of the electron transport time (s ) and the electron
conduction band edge. As shown in Fig. 5, the conduction band recombination lifetime (s ) calculated by s
¼ 1/2pfimps and s
The depen- tion band edge moves to the positive potential, it could lead to a
.
2
2
d
n
d
n
edge of the TiO lm in the QS-DSCs based on gel electrolytes ¼ 1/2pfimvs, where the fimps and fimvs are the frequencies of the
2
shis towards negative potentials compared with the liquid lowest imaginary component in the IMPS and IMVS plot,
+
36
electrolyte based DSC. Generally, adsorption of Li ions from respectively. In addition, the electron collection efficiency
the electrolyte to the surface of the mesoporous TiO lm (hcoll) is determined by the competition between the transport
increases the charge injection from the excited state dye due to of electrons through the TiO lm and the recombination of
the change in the conduction band edge to more positive photoinjected electrons with the redox electrolyte or oxidized
2
2
4
2,47
potentials.
In the gel electrolytes, the interaction between dye, which could be evaluated by hcoll ¼ 1 ꢁ s
d n
/s (ref. 48) as
+
the amide carbonyl groups and Li results in a decreased shown in Fig. 6(c). Due to the different inuences of the steric
+
adsorption of Li on the surface of the mesoporous TiO lm. hindrance effect of the gel microstructures and lengths of alkyl
The decreased Li adsorption causes the conduction band edge chains of the gelators on the Li absorption, the decreased
of the TiO lm in the QS-DSCs to shi upward. The ribbon 3D adsorption and intercalation of Li can reduce the amount of
2
+
+
+
2
networks of gel electrolytes based on the LMOGs containing trap states and shallow the surface state energy distribution on
39,49
even-numbered –CH
steric hindrance effects on the Li adsorption than the woven electron transportation should be accelerated, and the order
D networks of gel electrolytes based on the LMOGs containing of s is gel B < gel A < gel D < gel C as can be seen in Fig. 6(a).
odd-numbered –CH – groups (gelators C and D) due to bigger Furthermore, the process of electron recombination can be
bril sizes and more compact microstructures of the ribbon 3D reected by recombination current density (J ) which depends
2
– groups (gelator A and B) exhibit stronger the surface of mesoporous TiO
2
lms.
Consequently, the
+
49
3
d
2

r
networks. Therefore, the conduction band edges of the TiO2 on the rate constant for back electron transfer k, the concen-

lm in the DSCs with gelators C and D are more positive than tration of the oxidized half of the redox couple c and the
ox
43,50
those with gelators A and B. In addition, the steric hindrance electron density n, as shown in eqn (3):
effects are increased with the increase of the alkyl chain length
g
n
J
r
¼ kcox
(3)
of the gelator, and the stronger steric hindrance effects decrease
+
the adsorption amount of Li on the surface of the mesoporous
2 2
TiO lm. As a result, the conduction band edge of the TiO lm
in the DSCs with gelator B (n ¼ 6) is more negative than that in
the DSCs with gelator A (n ¼ 2). This result can also be obtained
between gelator D (n ¼ 9) and gelator C (n ¼ 5). Furthermore,
2
the TiO conduction band edge movement is one of the decisive
Fig. 6 Light intensity dependence of (a) electron transport time (s
electron recombination lifetime (s
Voc as a function of ln Q in the DSCs based on the liquid ciency (hcoll) in DSCs based on the liquid electrolyte and gel
d
), (b)
n
) and (c) electron collection effi-
Fig. 5
electrolyte and gel electrolytes.
electrolytes.
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where exponent g has typical values of 2–3 (ref. 51 and 52) but Table 3 Photovoltaic characteristics of DSCs based on the liquid
ꢁ2
42
electrolyte and gel electrolytes at AM 1.5 (100 mW cm )
can be larger. Compared with the liquid electrolyte, the
crosslinked 3D networks of gel electrolytes obstruct the diffu-
ꢁ2
Solar cell
V
oc (mV)
J
sc (mA cm
)
FF
h (%)
ꢁ
ꢁ
sion of I and I3 resulting in an increased cox in the TiO2/
electrolyte interface, and the J is increased and the electron Liquid
lifetime is shortened. As can be seen in Fig. 6(b), the s
r
714
682
662
701
690
15.87
14.09
12.64
15.30
14.84
0.70
0.70
0.69
0.70
0.70
7.93
6.69
5.75
7.53
7.20
of the Gel A (n ¼ 2)
Gel B (n ¼ 6)
n
liquid electrolyte based DSC is longer than that of the gel
electrolyte based DSCs. And owing to the more obstructed
ribbon 3D networks for charge diffusion than the woven 3D
networks and the increased steric hindrance effect with the long
Gel C (n ¼ 5)
Gel D (n ¼ 9)
length of the carbon chains, the s is decreased in the QS-DSCs
n
based on gelators C, D, A and B in sequence, which also indi-
cates that the electron recombination in the TiO
interface is accelerated with the decreased s
DSCs, and further lead to a decreased Voc
2
/electrolyte above (liquid > gel C > gel D > gel A > gel B). As shown in Table
in the gel based 3, the Jsc of the QS-DSCs based on gelators C and D is higher
than those of the QS-DSCs based on gelators A and B, which
n
.
results from the higher IPCE and Dapp. Fig. 7 also shows that
Photovoltaic performances and stabilities of quasi-solid-state
dye-sensitized solar cells fabricated based on LMOGs
the dark current increased aer the liquid electrolyte was
gelated, which results from the aggravated electron recombi-
nation in the TiO
2
photoelectrode/electrolyte interface by
Fig. 7 shows a typical photocurrent density–voltage curve at AM
ꢁ2
gelation. Moreover, the order of the dark currents for these
DSCs is gel B > gel A > gel D > gel C > liquid, which indicated
that the recombination is slower in the woven-shaped 3D
network based QS-DSCs than those in the ribbon 3D network
based QS-DSCs.
1.5 (100 mW cm ), the dark current–voltage curve and IPCE
spectra for the DSCs based on the liquid electrolyte and gel
electrolytes A–D. The photovoltaic performances of the short
circuit photocurrent density (Jsc), open circuit potential (Voc
)
and photoelectric conversion efficiency (h) are listed in Table 3.
2
The TiO conduction band edges of DSCs based on the gel
It is well known that the IPCE value is the ratio of the observed
electrolytes were shied to a more negative potential compared
with the DSC based on the liquid electrolyte, which would
contribute to the Voc. However, on the other hand, the Voc is also
53
photocurrent divided by the incident photon ux.
IPCE ¼ hlh
h
inj
h
coll
(4)
inuenced by the electron recombination in the TiO photo-
2
55,56
where hlh is the product of the light-harvesting efficiency of the electrode/electrolyte interface.
Therefore, it can be
dye, hinj is electron injection efficiency and hcoll is the electron concluded that the lower Voc of QS-DSCs is mainly caused by the
collection efficiency. The h , which is determined by the increased electron recombination. Finally, compared with the
lh
loading and extinction coefficient of the dye, is considered to DSCs based on the gels A, B and D, due to the faster diffusion of
be the same for all the DSCs in this article due to the equal ions, higher electron injection efficiency and lower electron
5
4
adsorption capacity of N719 on the TiO
according to the eqn (4), it can be conrmed that the change of taining ve –CH
2
electrode. Therefore, recombination, a superior QS-DSC based on the gelator con-
– groups (gel C) was obtained. Although this
2
IPCE is the same as the order of hinj and hcoll as concluded device has the slightly lower Jsc and Voc than those of the DSC
based on the liquid electrolyte, the h (7.53%) of this QS-DSC is
very close to that of the DSC based on the liquid electrolyte
(7.93%).
Importantly, although the photovoltaic performances of
QS-DSCs based on LMOGs are slightly lower than the corre-
sponding liquid based DSC, they exhibit excellent long-term
stability. As is shown in Fig. 8, the photoelectric conversion
efficiencies of all QS-DSCs could retain 85–93% of their initial
ꢀ
values aer 60 C thermal stress for 1000 h, but the liquid
electrolyte based DSC retained only 76%. Moreover, during
successive one sun light soaking for 1000 h, the photoelectric
conversion efficiency of the liquid electrolyte based DSC
retained 83% of its initial value, while the photoelectric
conversion efficiencies of QS-DSCs retained 95–97% of their
initial values, which indicates that the network of the gel
electrolyte could hinder the leakage of the liquid electrolyte
effectively. These accelerated aging results revealed that the
QS-DSCs are more stable than the liquid based DSC, which is
caused by the good intrinsic stability of the LMOGs and the
high Tgel of the gel electrolytes.
ꢁ2
Fig. 7 J–V curves at AM 1.5 (100 mW cm ) and dark conditions for
the DSCs based on the liquid electrolyte and gel electrolytes. The inset
shows the IPCE spectra for these DSCs.
15928 | J. Mater. Chem. A, 2014, 2, 15921–15930
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stability using the functional gel electrolytes by designing the
molecular structures of LMOGs at the molecular level.
Acknowledgements
This work was nancially supported by the National Basic
Research Program of China (Grant no. 2011CBA00700), the Sino
Swiss Science and Technology Cooperation Program (no.
IZLCZ2_139056), the External cooperation program of Chinese
academy of science (no. GJHZ1220), The National High Tech-
nology Research and Development Program of China (Grant no.
2011AA050510), the National Natural Science Foundation of
China (no. 21103197, 21173227 and 21273242) and the Program
of Hefei Center for Physical Science and Technology (no.
Fig. 8 (a) Normalized device efficiency variation with the DSCs based 2012FXZY006).
ꢀ
on the liquid electrolyte and gel electrolytes at 60 C and (b) under
ꢀ
successive one sun light soaking with a UV cutoff filter at 50 C for
1
000 h.
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