1-Alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids: Novel and low-viscosity ionic liquid electrolytes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c7ra12904a

Dye-sensitized Solar Cells (DSCs) based on ruthenium complex N719 as sensitizer have received much attention due to their affordability and high efficiency. However, their best performance is only achieved when using volatile organic solvents as electrolyte solutions, which are unstable under prolonged thermal stress. Thus, we developed a new series of 1-alkenyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids used as robust DSC electrolytes. These ionic liquids exhibit low viscosity, high conductivity, and thermal stability. The implementation of 1-but-3-enyl-3-methyl-imidazolium trifluoromethanesulfonate, [ButMIm]OTf, into DSCs gave the best photovoltaic performance. The results are fairly comparable to those reports for other popular ionic liquid electrolytes currently used in DSC field. An insightful discussion on the relationship between the structure of these new ionic liquids and the J-V characterization as well as electrochemical impedance measurement of DSCs will give more interesting information. The results are useful for large-scale outdoor application of DSCs.

Full text of DOI:10.1039/c7ra12904a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
1-Alkenyl-3-methylimidazolium
trifluoromethanesulfonate ionic liquids: novel and
low-viscosity ionic liquid electrolytes for dye-
sensitized solar cells†
Cite this: RSC Adv., 2018, 8, 13142
a
Trang Ngoc Nguyen,a Vinh Son Nguyen,a
*
Phuong Tuyet Nguyen,
Hai Truong Nguyen,b Dung Kim Thi Ngob and Phuong Hoang Tran
b
*
Dye-sensitized Solar Cells (DSCs) based on ruthenium complex N719 as sensitizer have received much
attention due to their affordability and high efficiency. However, their best performance is only achieved
when using volatile organic solvents as electrolyte solutions, which are unstable under prolonged
thermal stress. Thus, we developed
a
new series of 1-alkenyl-3-methylimidazolium
trifluoromethanesulfonate ionic liquids used as robust DSC electrolytes. These ionic liquids exhibit low
viscosity, high conductivity, and thermal stability. The implementation of 1-but-3-enyl-3-methyl-
imidazolium trifluoromethanesulfonate, [ButMIm]OTf, into DSCs gave the best photovoltaic
performance. The results are fairly comparable to those reports for other popular ionic liquid electrolytes
currently used in DSC field. An insightful discussion on the relationship between the structure of these
new ionic liquids and the J–V characterization as well as electrochemical impedance measurement of
DSCs will give more interesting information. The results are useful for large-scale outdoor application of
DSCs.
Received 29th November 2017
Accepted 31st March 2018
DOI: 10.1039/c7ra12904a
rsc.li/rsc-advances
performance of DSCs.5,6 Such so-called “non-robust” electro-
lytes also caused a dramatically fast thermal degradation of
Introduction
Ruthenium dyes – crucial components in DSCs, leading to a loss
of the cell performance.7,8 By replacing traditional electrolytes
with ionic liquid ones which are known as “robust electrolytes”,
thermal stability of the dyes were notably improved to guarantee
the passing of serious damn heat test.9,10 Hence, more and more
researches on the application of ionic liquids as an alternative
electrolytes in DSC devices due to their easy synthesis, unique
properties, thermal stability, highly ionic conductivity, non-
volatility, broad electrochemical potential window, relative
non-ammability, and low toxicity have been reported.11–17
The ionic liquids are entirely composed of cations (ammo-
nium, phosphonium, guanidinium, pyridinium, and sulfo-
nium) and anions (halides and complex anions such as BF4,
PF6, OTf, NTf2, etc.) with low melting point below 100 ꢀC.18–21 By
virtue of this property, ionic liquids have gained widespread
applications in DSCs led as solvents in liquid electrolytes and
organic salts in quasi-solid-state electrolytes.22–27 1-Hexyl-3-
methylimidazolium iodide was rstly used in DSCs by Papa-
georgiou's group with the aim of reducing the volatility of
Dye-sensitized solar cells (DSCs) have been developed as
promising photovoltaic devices since their rst invention by
Gratzel and O'Regan.1–3 The three basic components con-
structing a simple DSC are mesoporous metal oxide semi-
conductor layers deposited on one out of two conductive
transparent electrodes, a photosensitizer (or dye) thoroughly
loaded onto the mesoporous lm, and an electrolyte solution
lled up the space between the as-prepared electrode and the
other one to assemble a functional DSC. As one of most
common electrolytes, the solution of triiodide/iodide redox
couple in a high dielectric constant organic solvent is usually
employed in high performance DSCs.2 An efficiency of 12% has
been so far reported as the highest power conversion using this
kind of liquid electrolytes in DSC devices applied Ruthenium
dyes.4 However, high volatility and unavoidable leakage of
organic solvents exert a negative impact on the longevity and
aDepartment of Applied Inorganic Chemistry, Faculty of Chemistry, University of
Sciences, Viet Nam National University, Ho Chi Minh City 70000, Viet Nam. E-mail:
ngtuyetphuong@gmail.com
electrolyte.28
However,
the
traditional
1-alkyl-3-
methylimidazolium iodide is too viscous and the high concen-
tration of iodide preventing them from the practical application
of DSCs. Recently, the use of some binary ionic liquid electro-
lytes allows the reduction of electrolyte viscosity. Wang and
bDepartment of Organic Chemistry, Faculty of Chemistry, University of Sciences, Viet
Nam National University, Ho Chi Minh City 70000, Viet Nam. E-mail: thphuong@
hcmus.edu.vn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12904a
13142 | RSC Adv., 2018, 8, 13142–13147
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Table
1
The preparation of 1-alkenyl-3-methylimidazolium tri-
coworkers reported
a solvent-free electrolyte 1-methyl-3-
ethylimidazolium dicyanamide,29 1-ethyl-3-methylimidazolium
thiocyanate,30 and 1-ethyl-3-methylimidazolium selenocya-
nate,31 and 1-ethyl-3-methylimidazolium tricyanomethanide,32
1-ethyl-3-methylimidazolium tetracyanoborate33 blending with
standard iodide-based ionic liquids achieved a high conversion
efficiency of 6.5 to 8.3%. More latterly, Bidikoudi reported the
use of the electrolytes prepared by blending a low viscosity ionic
liquid 1-ethyl-3-methylimidazolium dicyanamide with methyl-
imidazolium iodide to enable the devices to attain the effi-
ciencies of 4.4 to 6.5%.34 Although these DSCs devices showed
the high conversions, the long-term stability was not good.
Therefore, the search for alternative electrolytes in DSCs devices
has been still studied extensively.
fluoromethanesulfonates under solvent-free conditions
Condition (ꢀC, min)
Yielda (%)
MWb
Entry
ILs
Step 1
Step 2
Dc
1
2
3
[AMIm]OTf
[ButMIm]OTf
[PentMIm]OTf
100, 20
100, 20
100, 20
100, 15
100, 15
100, 30
90
88
89
78
72
68
a
b
Isolated yield. Microwave heating: the reaction was performed in
closed vessels using a CEM Discover monomode oven with strict
c
control of pressure and temperature (power 10 W). Conventional
heating: the reaction was performed in a thermostat-controlled oil bath.
In this paper, we proceed with the synthesis of low-viscosity
1-alkenyl-3-methyimidazolium triuoromethanesulfonate ionic
liquids whose alkenyl chain consists of three to ve carbon
atoms. The synthesized ionic liquids were structurally identied
as well as further characterized for physical properties such as
viscosities, conductivities, and thermal stabilities. Then, these
low-viscosity ionic liquids were used to prepare electrolytes to
be implemented to the functional DSCs. The cells were nally
characterized by J–V curve and electrochemical impedance
measurement.
in the [PF6] anion by pentauoroethyl groups tremendously
decreases the viscosity of imidazolium ionic liquids from 548 to
74 cP.35 Consequently, we developed a new series of low-
viscosity imidazolium triate ionic liquids bearing different
alkenyl substituents. As expected, the replacement of halide
anions with triate anion reduced the viscosity of imidazolium
ionic liquids remarkably. The viscosity of synthesized ionic
liquids was given in Table 2. As reported by Chen and
coworkers, the viscosity values of a range of ILs having the same
cation but different anions increase in the order: [DCA] < [NTf2]
< [TfO] < [BF4] < [PF6] < [OAc].36 For example, the value of
viscosity for [BMIm][OTf] is 75 cP, whereas it is only 45 cP for
[BMIm][NTf2].36 In the current work, we found that the
replacement of alkyl substituents by alkenyl substituents led to
decrease the viscosity of ionic liquids. The viscosity of [ButMIm]
[OTf] is only 16.9 cP (Table 2). Generally, the viscosity of ionic
liquids increases with the length of alkyl side chain due to the
increase of the van der Waals interaction.36 However, there is
somewhat different in the viscosity of [AMIm][OTf]/33.7 cP >
[ButMim][OTf]/16.9 cP presumably due to a more exibility of
butenyl than allyl.
Result and discussion
The low-viscosity ionic liquids were easily prepared in high
yields via a two-step pathway procedure under solvent- and
catalyst-free condition assisted by microwave irradiation or
conventional heating. The alkylation of imidazole by alkyl
bromides followed by anion metathesis with lithium triate
resulted in the formation of three new ionic liquids, 1-allyl-3-
methylimidazolium triuoromethanesulfonate [AMIm]OTf, 1-
but-3-enyl-3-methyl-imidazolium
triuoromethanesulfonate
Thermal gravimetric analyses (TGA) of [AMIm]OTf, [But-
MIm]OTf, [PentMIm]OTf were conducted to test their thermal
stability and the results were presented in Fig. 1. The analysis
[ButMIm]OTf, and 1-pent-4-enyl-3-methylimidazolium tri-
uoromethanesulfonate [PentMIm]OTf (Scheme 1).
The results from Table 1 showed that the ionic liquids were
obtained in excellent yield (68–90%) with high purity.
Compared to conventional heating, microwave irradiation was
a better method to afford the ionic liquids in higher yields.
The fact that ionic liquids are more viscous than traditional
organic solvents can impede them from being used as electro-
lytes in DSCs. Fortunately, the viscosity of a given ionic liquid
can be greatly governed by the nature of its anion as reported by
Willner.35 For example, the replacement of three uorine atoms
shows
that
the
1-alkenyl-3-methylimidazolium
tri-
uoromethanesulfonateꢀ ionic ꢀliquids are stable at high
ꢀ
temperature (up to 180 C, 200 C, and 280 C for [AMIm]OTf,
[ButMIm]OTf, [PentMIm]OTf, respectively), which ensure these
ionic liquids suitable as the electrolyte in DSCs.
Fig. 2 shows current–voltage J–V characteristics of the cells
using ionic liquid electrolytes mixed with PMII in comparison
with the popular one 1-ethyl-3-methylimidazolium tetracyano-
borate (EMITCB) under AM 1.5G illumination. The respectively
photovoltaic parameters are involved in Table 3. In general, all
the J–V curves show similar shape in all three cases of
Table 2 Viscosity and conductivity of 1-alkenyl-3-methylimidazolium
trifluoromethanesulfonates
[AMIm]OTf [ButMIm]OTf [PentMIm]OTf
Viscosity (cPs)
33.7
3.96
16.9
12.13
94.4
0.68
Conductivity (mS cmꢁ1
)
Scheme
1
Synthesis
of
1-alkenyl-3-methylimidazolium
trifluoromethanesulfonates.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 13142–13147 | 13143

View Article Online
RSC Advances
Paper
Table 3 Photovoltaic performance of DSCs used the synthesized
ionic liquid electrolytes in comparison to the popular one EMITCB
under the light intensity of 1000 W mꢁ2. Bold values are selected for J–
V characteristics illustration in Fig. 2
Efficiency,
%
Ionic liquid
JSC, mA cmꢁ2
VOC, V
FF
[ButMlm]OTf
11.413
11.115
11.311
11.660
11.560
10.820
11.042
10.828
0.659
0.652
0.653
0.626
0.624
0.631
0.630
0.632
0.653
0.673
0.655
0.694
0.684
0.692
0.693
0.701
4.907
4.876
4.833
5.067
4.933
4.727
4.820
4.791
[AMIm]OTf
[PentMlm]OTf
which is about 4.8% compared to 4.9% and 5.0% with [ButMIm]
OTf and [AMIm]OTf, respectively. The viscosity and ionic
conductivity can be explained as the main factors responsible
for these differences. The results of photovoltaic performance
correspond reasonably with the viscosity and conductivity
values of our ionic liquids in which [ButMIm]OTf has the lowest
viscosity (16.9 cPs) and the highest conductivity (12.13
mS cmꢁ1), thus leading to the best voltage and current. Further
data summarized in Table S1 (ESI†) shows that DSCs with our
ionic liquids are well comparable to the results reported else-
where with different ionic liquids.
Fig. 1 TGA curves of [AMIm]OTf, [ButMIm]OTf, and [PentMIm]OTf.
Fig. 3 shows an analysis of the electrochemical impedance
spectroscopy (EIS) of the DSCs using the synthesized ionic
liquid electrolytes. The measurement was performed in the
dark under a forward bias of ꢁ0.60 V. As shown in Fig. 3; three
arcs can be observed in the Nyquist plots in the frequency range
of 0.01 Hz to 100 kHz. The rst arc at the high-frequency range
(103 to 105 Hz) is related to the charge transfer processes at
counter electrode|electrolyte interface, the second arc at the
middle frequency range (1–103 Hz) is related to carrier transport
resistance (RCT) in TiO2|dye|electrolyte interfaces, and the nal
one at the low-frequency range (2 ꢃ 10ꢁ2 to 1 Hz) is associated
with ionic diffusion in the electrolyte.
Fig. 2 Typical J–V curves of DSCs using ionic liquid electrolytes with
[PentMIm]OTf ( ); [ButMIm]OTf ( ); [AMIm]OTf ( ) and EMITCB (
)
under the light intensity of 1000 W mꢁ2. The electrolyte composition:
0.05 M I2, 0.1 M PMII, 0.6 M GuNCS, 0.5 M NBB, and either one of these
four ionic liquids.
As can be seen from Fig. 3, the charge transfer resistant RCT
are about 43, 35, 29 U cmꢁ2 for the case of using [ButMIm]OTf,
[PentMIm]OTf and [AMIm]OTf, respectively, indicating that the
back electron transfer at the interface between TiO2 photo
synthesized ionic liquid electrolytes. The open circuit voltage
(VOC) of DSC with [ButMIm]OTf is equivalent to EMITCB, and
higher than the ones with two other ionic liquids about 20–
30 mV while the short circuit current density (JSC) values are just
slightly different between the three ionic liquid electrolytes.
As can be seen in Table 3 there were no considerable
differences in the photovoltaic parameters among the cells
belonging to this electrolytes series. The photovoltaic perfor-
mance of DSC with the synthesized ionic liquids is comparable
to DSC with the popular EMITCB. Cells using [PentMIm]OTf
and [AMIm]OTf are almost the same in VOC (0.63 V in average)
which is slightly lower than cells using [ButMIm]OTf. However,
the ll factor (FF) values with these two ionic liquid electrolytes
are about 69–70% and higher than [ButMIm]OTf (ꢂ65–66%).
The cells with [PentMIm]OTf have the lowest in JSC values,
therefore the lowest in the overall efficiency energy conversion
Fig. 3 Nyquist plots showing impedance data of DSCs using tri-
fluoromethanesulfonate electrolytes. The data were obtained at an
applied potential of 0.60 V. The solid lines are model fit to the data.
13144 | RSC Adv., 2018, 8, 13142–13147
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
anodes and electrolytes increases within the order of the less back electron transfer between electrolyte mediators and
mentioned series ionic liquids. Therefore, DSC with [ButMIm] photo-anodes, as well as better electrolyte ionic diffusion,
Otf responds the high performance in both voltage and current. leading to the best photovoltaic performance with [ButMIm]OTf
Differently, RCT of cells using [PentMIm]OTf is observed to be electrolyte. These results are well-agreed with the low viscosity
higher than the ones with [AMIm]OTf while the JSC values show and good conductivity values of [ButMIm]OTf. Whereas the
a completely opposite tendency. This can be explained by the cases of [AMIm]OTf and [PentMIm]OTf are more complicated.
effects of other parameters as charge transport resistance Rt; The ionic liquid [AMIm]OTf owns lower viscosity and a bit
capacitance and electron life time of the DSCs which will be higher conductivity than [PentMIm]OTf, so its DSCs give better
discussed further below.
current and voltage. However, the RCT values of DSCs with
The third arc related to ionic electrolyte diffusion can be [AMIm]OTf are lower than [PentMIm]OTf as aforementioned.
evaluated by the shape of the arc and diffusion resistant value This can be explained by the contribution of other parameters
RD obtained from it. The shapes of the third arcs with [Pent- including Rt; capacitance and s to the current value. Fig. 4b and
MIm]OTf and [AMIm]OTf are similarly whereas the one with c show that DSCs with [AMIm]OTf have higher capacitance, and
[ButMIm]OTf is different in the slope of the le side upward of especially lower charge transport resistant Rt than [PentMIm]
the arc. This illustrates that with [ButMIm]OTf the electrolyte OTf. The difference between these values RCT; Rt; and capaci-
performs better ionic diffusion, and there is less electron tance might be due to their dissimilar structure. The short
transfer between the interaction of electrolyte and the two alkenyl chain might not somehow help to protect the interac-
electrodes. The values of RD were obtained to be 18, 22, 16 U tion between electrolytes and electrodes, therefore DSCs with
cmꢁ2 for [ButMIm]OTf, [PentMIm]OTf and [AMIm]OTf, [AMIm]OTf have low RCT, hence, less efficient electron life time
respectively.
than DSCs with [PentMIm]OTf. But the viscosity and conduc-
From the calculation and simulated data, the values of Rt, tivity of the ionic liquids have more impact to the cell perfor-
RCT and capacitance were extracted. We also obtained further mance, therefore it gives a nally higher overall energy
analysis on Bode plots. From the Bode plots, the electron life conversion in the case of [AMIm]OTf.
times (s) of these cells were estimated using eqn (1), where the
fmax value was read off the Bode plot at the phase angle peak
observed from the second peak.
Experimental section
General procedure for the preparation of 1-allyl-3-
methylimidazolium triuoromethanesulfonate ([AMIm]OTf)
under microwave irradiation
1
s ¼
(1)
2pfmax
In a 5 mL round-bottom ask, a mixture of 1-methylimidazole
(2 mmol, 0.1640 g) and allyl bromide (2 mmol, 0.2409 g) was
irradiated under microwave heating at 100 ꢀC for 20 min.
Then, LiOTf (2 mmol, 0.3120 g) was added and the resulting
mixtures were further irradiated at 100 ꢀC for 15 min. The
reaction mixture was cooled to ambient temperature, and then
diluted with 5 mL of acetonitrile. Aer ltration through cel-
ite, the solvent was removed. The crude products were washed
with diethyl ether and concentrated by rotary evaporator to
Fig. 4 shows linear curves of RCT; Rt; capacitance; as well as s
values when the applied potentials increase in all the three ionic
liquids, indicating that all the cells respond with the same
behaviour at different potentials. The data again conrm the
1
obtain [AMIm]OTf whose structure was conrmed by H, 13C
NMR spectroscopy, and HRMS (ESI).
DSCs fabrication and characterization
The dye-sensitized solar cells were constructed by two elec-
trodes prepared from glass substrates (Pilkington ꢁ8 U cmꢁ2
)
coated with F-doped SnO2 (FTO) as previously described in our
publications elsewhere.37,38 DSC electrolyte was a mixture of
0.05 M I2, 0.1 M PMII, 0.6 M GuNCS, 0.5 M NBB, and either one
of four ionic liquids including 1-ethyl-3-methylimidazolium
tetracyanoborate (EMITCB) and the three synthesized ionic
liquids [AMIm]OTf or [ButMIm]OTf, or [PentMIm]OTf. Each
experiment produced 8 to 10 DSCs.
Photovoltaic measurements of DSCs were performed
following the protocol as described37 and the DSCs were masked
with a 0.1444 cm2 active area of the anode electrode.
Electro impedance spectroscopy (EIS) was carried out using
an electrochemical interface workstation (Schlumberger SI-
Fig. 4 Charge electron transfer resistant, RCT (a); charge transport
resistant, Rt (b); capacitance (c) and electron life time, s (d) of the DSCs
using [ButMIm]OTf, [AMIm]OTf, [PentMIm]OTf calculated from
simulated data and the Bode plots. Data was collected at various
applied potentials.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 13142–13147 | 13145

View Article Online
RSC Advances
Paper
1286) and a HF frequency response analyzer (Schlumberger SI- 10 P. T. Nguyen, T. A. P. Phan, N. H. T. Ngo, T. V. Huynh and
1255). Different bias potentials from ꢁ0.3 V to open-circuit T. Lund, Solid State Ionics, 2018, 314, 98–102.
voltage value, synchronized with a modulated voltage of 11 C. T. Li, C. P. Lee, C. T. Lee, S. R. Li, S. S. Sun and K. C. Ho,
10 mV with a frequency range of 100 kHz to 10 MHz, were ChemSusChem, 2015, 8, 1244–1253.
applied in the dark. The data was analyzed by using Z-View 12 H. H. Lin, J. D. Peng, V. Suryanarayanan, D. Velayutham and
soware with the appropriate equivalent circuit.
K. C. Ho, J. Power Sources, 2016, 311, 167–174.
13 M. Ghavre, O. Byrne, L. Altes, P. K. Surolia, M. Spulak,
B. Quilty, K. R. Thampi and N. Gathergood, Green Chem.,
2014, 16, 2252–2265.
Conclusions
Three low-viscosity ionic liquids, based on 1-alkenyl-3- 14 W. S. Chi, H. Jeon, S. J. Kim, D. J. Kim and J. H. Kim,
methyimidazolium cations and triuoromethane (OTf-) Macromol. Res., 2013, 21, 315–320.
anion, where the alkenyl chain length was 3 to 5 carbon 15 Z. Wang, L. Wang, Y. Zhang, J. Guo, H. Li and F. Yan, RSC
atoms, were synthesized and implemented in DSC electro- Adv., 2017, 7, 13689–13695.
lytes. [ButMIm]OTf shows the lowest viscosity although its 16 Y.-F. Lin, C.-T. Li, C.-P. Lee, Y.-A. Leu, Y. Ezhumalai, R. Vittal,
structure does not have the shortest alkenyl chain. The solar
cells using [ButMIm]OTf in electrolytes also respond the best
M.-C. Chen, J.-J. Lin and K.-C. Ho, ACS Appl. Mater. Interfaces,
2016, 8, 15267–15278.
photovoltaic performance due to its best conductivity and 17 M. Gorlov and L. Kloo, Dalton Trans., 2008, 2655–2666.
low viscosity. The cells using [AMIm]OTf were obtained 18 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576.
better short circuit current values than the ones with [Pent- 19 M. A. Martins, C. P. Frizzo, A. Z. Tier, D. N. Moreira,
MIm]OTf. All the DSCs applied these three ionic liquids show
comparable performance to the present popular ones
N. Zanatta and H. G. Bonacorso, Chem. Rev., 2014, 114,
PR1–PR70.
demonstrating their high potential for the use in DSCs. For 20 K. Goossens, K. Lava, C. W. Bielawski and K. Binnemans,
further application of these ionic liquids in DSC devices, the Chem. Rev., 2016, 116, 4643–4807.
long term stability of DSCs with these ionic liquid electrolytes 21 P. C. Marr and A. C. Marr, Green Chem., 2016, 18, 105–128.
should be investigated.
22 J.-D. Decoppet, S. B. Khan, M. S. A. Al-Ghamdi, B. G. Alhogbi,
¨
A. M. Asiri, S. M. Zakeeruddin and M. Gratzel, Energy
Technol., 2017, 5, 321–326.
23 M. H. Khanmirzaei, S. Ramesh and K. Ramesh, Mater. Des.,
2015, 85, 833–837.
Conflicts of interest
There are no conicts to declare.
´
24 B. Gelinas and D. Rochefort, Electrochim. Acta, 2015, 162, 36–
44.
Acknowledgements
25 J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan and G. Luo,
This research is funded by Vietnam National Foundation for
Chem. Rev., 2015, 115, 2136–2173.
Science and Technology Development (NAFOSTED) under grant 26 G. M. A. Girard, M. Hilder, H. Zhu, D. Nucciarone,
number 104.03-2014.26.
K. Whitbread, S. Zavorine, M. Moser, M. Forsyth,
D. R. MacFarlane and P. C. Howlett, Phys. Chem. Chem.
Phys., 2015, 17, 8706–8713.
References
27 G. P. Lau, H. N. Tsao, C. Yi, S. M. Zakeeruddin, M. Gratzel
and P. J. Dyson, ChemSusChem, 2015, 8, 255–259.
¨
1 B. O'Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, 28 N. Papageorgiou, J. Electrochem. Soc., 1996, 143, 3099–3108.
¨
Chem. Rev., 2010, 110, 6595–6663.
29 P. Wang, S. M. Zakeeruddin, J.-E. Moser and M. Gratzel, J.
3 L. M. Peter, J. Phys. Chem. Lett., 2011, 2, 1861–1867.
Phys. Chem. B, 2003, 107, 13280–13285.
4 Q. Yu, Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang and P. Wang, 30 P. Wang, S. M. Zakeeruddin, R. Humphry-Baker and
¨
M. Gratzel, Chem. Mater., 2004, 16, 2694–2696.
ACS Nano, 2010, 4, 6032–6038.
5 A. G. Kontos, T. Stergiopoulos, V. Likodimos, D. Milliken, 31 P. Wang, S. M. Zakeeruddin, J.-E. Moser, R. Humphry-Baker
¨
H. Desilvesto, G. Tulloch and P. Falaras, J. Phys. Chem. C,
2013, 117, 8636–8646.
6 D. Bari, N. Wrachien, R. Tagliaferro, S. Penna, T. M. Brown,
A. Reale, A. Di Carlo, G. Meneghesso and A. Cester,
Microelectron. Reliab., 2011, 51, 1762–1766.
and M. Gratzel, J. Am. Chem. Soc., 2004, 126, 7164–7165.
32 P. Wang, B. Wenger, R. Humphry-Baker, J.-E. Moser,
J. Teuscher, W. Kantlehner, J. Mezger, E. V. Stoyanov,
¨
S. M. Zakeeruddin and M. Gratzel, J. Am. Chem. Soc., 2005,
127, 6850–6856.
7 P. Tuyet Nguyen, R. Degn, H. Thai Nguyen and T. Lund, Sol. 33 Y. Bai, Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang,
Energy Mater. Sol. Cells, 2009, 93, 1939–1945.
S. M. Zakeeruddin and M. Gratzel, Nat. Mater., 2008, 7,
626–630.
34 M. Bidikoudi, L. F. Zubeir and P. Falaras, J. Mater. Chem. A,
2014, 2, 15326–15336.
8 P. Tuyet Nguyen, A. Rand Andersen, E. Morten Skou and
T. Lund, Sol. Energy Mater. Sol. Cells, 2010, 94, 1582–1590.
9 T. Lund, P. T. Nguyen, H. M. Tran, P. Pechy,
¨
S. M. Zakeeruddin and M. Gratzel, Sol. Energy, 2014, 110, 35 N. V. Ignat'ev, U. Welz-Biermann, A. Kucheryna, G. Bissky
96–104.
and H. Willner, J. Fluorine Chem., 2005, 126, 1150–1159.
13146 | RSC Adv., 2018, 8, 13142–13147
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
36 G. Yu, D. Zhao, L. Wen, S. Yang and X. Chen, AIChE J., 2012,
58, 2885–2899.
T. Lund, S.-N. Le and P. T. Nguyen, Appl. Surf. Sci., 2017,
399, 515–522.
37 V. A. Tran, T. T. Truong, T. A. P. Phan, T. N. Nguyen, 38 T. Lund, P. T. Nguyen and T. Ruhland, J. Electroanal. Chem.,
T. V. Huynh, A. Agresti, S. Pescetelli, T. K. Le, A. Di Carlo,
2015, 758, 85–92.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 13142–13147 | 13147