Enhancements of Trifluoroacetic Acylated Five-membered Heterocyclic Compounds Using as Additives in Dye Sensitized Solar Cells

Article abstract of DOI:10.1002/jccs.201500440

In this study, five-membered heterocyclic compounds are trifluoroacetic acylated for the purpose of providing more long pairs to enhance electrolyte in dye sensitized solar cells (DSSCs). Four five-membered heterocyclic compounds will be trifluoroacetic acylated with trifluoroacetic anhydride by Friedel-Crafts acylation: furan, thiophene, pyrrole and N-methylpyrrole. The properties will be measured by cyclic voltage (CV), Fourier transform infrared spectroscopy (FTIR), solar simulator, and electrochemical impedance spectroscopy (EIS). We find out that furan and thiophene which we add in electrolyte as additives can increase short circuit current and photovoltaic efficiency, and furthermore, all the trifluoroacetic acylated heterocyclic compounds perform better photoelectric abilities than non-trifluoroacetic acylated one. The photovoltaic efficiency will be increased from 4.439% to 5.197% when 1wt% trifluoroacetic acylated thiophene is added in electrolyte as additives.

Full text of DOI:10.1002/jccs.201500440

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Enhancements of Trifluoroacetic Acylated Five-membered Heterocyclic Compounds
Using as Additives in Dye Sensitized Solar Cells
Chung-Chun Wang and Yeh-Fang Duann*
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan,
Republic of China
(Received: Oct. 30, 2015; Accepted: Mar. 7, 2016; Published Online: Apr. 23, 2016; DOI: 10.1002/jccs.201500440)
In this study, five-membered heterocyclic compounds are trifluoroacetic acylated for the purpose of pro-
viding more long pairs to enhance electrolyte in dye sensitized solar cells (DSSCs). Four five-membered
heterocyclic compounds will be trifluoroacetic acylated with trifluoroacetic anhydride by Friedel-Crafts
acylation: furan, thiophene, pyrrole and N-methylpyrrole. The properties will be measured by cyclic volt-
age (CV), Fourier transform infrared spectroscopy (FTIR), solar simulator, and electrochemical imped-
ance spectroscopy (EIS). We find out that furan and thiophene which we add in electrolyte as additives can
increase short circuit current and photovoltaic efficiency, and furthermore, all the trifluoroacetic acylated
heterocyclic compounds perform better photoelectric abilities than non-trifluoroacetic acylated one. The
photovoltaic efficiency will be increased from 4.439% to 5.197% when 1wt% trifluoroacetic acylated
thiophene is added in electrolyte as additives.
Keywords: Friedel-Crafts acylation; Heterocyclic; Trifluoroacetic anhydride; DSSCs.
INTRODUCTION
The generation of energy is one of the most important
the cation-p binding ability conversely. Most of the hetero-
cyclic elements are poor electron donors when it comes to
cation-p binding. Cation-heterocycle interaction is not al-
ways a cation-p interaction, in some cases it is more favor-
able for the cation to be bound directly to a lone pair, like
the case in pyridine-Na+ complexes,13 for example. The in-
teraction between the cation and the carbonyl oxygen is a
common example of a Lewis acid and base reaction. The
carbonyl oxygen (the Lewis base) donates a pair of elec-
trons to the cation (the Lewis acid). This interaction has the
very important effect of increasing the polarity of the car-
bon-oxygen double bond. The mechanism is shown in
Scheme 1.
and imperative problems nowadays. Solar energy1 which is
radiant light and heat continuously from the Sun using a
range of technologies such as solar heating and photovol-
taic seems like a feasible solution for this issue. Photovol-
taic cell which converts the energy of light directly into
electricity by the photovoltaic effect is one of the possible
devices that we used recently and widely.2-6 DSSCs devel-
oped by Grätzel and coworkers are high efficiency,7 low-
cost, and flexible alternative for photovoltaic cells. Re-
markably high quantum efficiency has been reported with
conversion efficiency up to 13%.8
Heterocycle is a cyclic compound that has atoms of at
least two different elements as members of its ring.9 Every
carbocyclic compound, regardless of structure and func-
tionality, may be converted into a heterocyclic compound
by replacing one or more of the ring carbon atoms with a
different element in theory. There are one or more than one
lone pairs on oxygen, nitrogen, and sulfur. These are the
most common heterocyclic elements that we usually used.10
Heterocycle is often activated towards cation-p binding11,12
when the lone pair on the heterocyclic element is incorpo-
rated into the aromatic system. When the lone pair does not
contribute to aromaticity, heterocyclic element will weaken
Friedel-Crafts acylation which is the acylation of aro-
matic rings using a strong Lewis acid catalyst with an acyl
chloride or acid anhydride has been reported since 1877.
The reaction conditions are similar to the Friedel-Crafts
alkylation.14 Due to the electron withdrawing effect of the
carbonyl group, the ketone product is always less reactive
than the original molecule. That’s why there is no carbo-
cation rearrangement15 occurring. There are lots of cata-
lysts suitable for Friedel-Crafts acylation such as alumi-
num chloride (AlCl3),16 zinc chloride (ZnCl2),17 iodide
(I2),17 ferric chloride (FeCl3),17 iron (Fe),17 and antimony
pentafluoride (SbF5).18 It is inconvenience that most of
* Corresponding author. Tel: +886-2-2771-2171#2562; Fax: +886-2-2731-7117; Email: f10421@mail.ntut.edu.tw
Supporting information for this article is available on the www under http://dx.doi.org/10.1002/jccs.201500440
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The iodide anion will be freer to redox in the electrolyte because both the cation-heterocycle interaction and the
lone pairs on the carbonyl oxygen are attracted by the lithium cation
Scheme 1
these catalysts are anhydrous. Minute amount of ferric
chloride hexahydrate (FeCl3×6H2O) has been reported as
catalyst for Friedel-Crafts acylation of pyrrole and substi-
tuted pyrrole,19 and production yield is excellent. It is also
appropriate for furan20,21 and thiophene,22,23 and further-
more it is not necessary anhydrous.
7.805 were extracted through simple distillation directly, and pure
chloroform would be recycled during distillation.
Trifluoroacetic acylated pyrrole and N-methylpyrrole were
prepared by the same steps but reacted in ice bath. Liquid phase
extraction would be used in the process of removing ferric chlo-
ride, and 1N sodium hydroxide (NaOH) solution was added in ad-
justing pH value to 10~11.25 The bottom layer solution of liquid
phase extraction was collected and crystallized for purification of
trifluoroacetic acylated pyrrole (TFAA_Pyrrole): FTIR: 3340
(N-H), 1660 cm-1 (C=O); 1HNMR (200 Hz, CDCl3, in ppm) d 6.41
(m, 1H, -H4), d 7.20~7.26 (m, 2H, -H3, -H5), d 9.60 (br, 1H, -N-H).
Trifluoroacetic acylated N-methylpyrrole (TFAA_N-Methyl-
In this study, five-membered heterocyclic compounds
will be trifluoroacetic acylated in order to combine cation-
heterocycle interaction and the lone pairs in carbonyl group
these two characteristics. Both of these major properties
are attracted by cations. The iodide anion will be freer to re-
dox in the electrolyte because heterocyclic compounds are
attracted by the lithium cation. We hope it will be helpful
for enhancing iodine/iodide electrolyte and increasing pho-
tovoltaic efficiency in DSSCs.
1
pyrrole): FTIR: 1670 cm-1 (C=O); HNMR (200 Hz, CDCl3, in
ppm) d 3.97 (s, 3H, -N-CH3), d 6.24 (q, 1H, -H4), d 7.02 (m, 1H,
-H3), d 7.02 (m, 1H, -H5) would be received by simple distillation
from the bottom layer solution of liquid phase extraction.
EXPERIMENTAL
Assembly of DSSCs: Fluorine doped tin oxide glasses
(FTO) were cleaned with methanol, acetone, and deionized water
(DI water) in ultrasonic bath (DELTA DC300H) for each 20 min-
utes and dried in a vacuum oven. Titanium dioxide26 (TiO2) work-
ing electrode anode film with a thickness of 16-20 mm was pre-
pared by spin coating on FTO in high temperature annealed at 450
°C for two hours. TiO2 paste was made from 9 g titanium oxide
powder (Degussa P25), 1 g polyethylene glycol (PEG, Mw =
6000), 2 ml Acetylacetone, 0.2 ml Triton-X 100, and 18 ml DI wa-
ter. The paste was ball milled for 60 minutes and ultrasonic for 24
hours. The TiO2 anodes was further sensitized by immersing into
a 0.5 mM anhydrous ethanol solution of N719 dye (bis(tetrabutyl-
ammonium)-cis-bis(thiocyanato)bis(2,2’-bipyridine-4,4’-dicarb
oxylate)-Ru(II)).27-29 The cell was fabricated by sandwiching re-
dox electrolyte between dye sensitized TiO2 working electrode
and platinum (Pt) counter electrode7,30 which was supported on
FTO glass. The redox electrolyte was composed of 0.5 M lithium
iodide (LiI), 0.05 M iodine (I2), 0.5 M 4-tert-butylpyridine31,32
(4-TBP), 5 ml acetonitrile, and 5 ml propylene carbonate (PC). 1
wt% of trifluoroacetic acylated five-membered heterocyclic com-
Trifluoroacetic acylation of five-membered heterocyclic
compounds: Ferric chloride hexahydrate (FeCl3×6H2O) had been
found as an effective catalyst for Friedel-Crafts acylation of
furan, thiophene, pyrrole, and N-methylpyrrole.19-23 In the pro-
cess of trifluoroacetic acylation, furan was reacted with the same
molar ratio trifluoroacetic anhydrate and minute amount of ferric
chloride hexahydrate, which was used as catalyst, at room tem-
perature for hours until trifluoroacetic anhydrate was exhausted.
Chloroform was used as solvent. Reaction termination was deter-
minate by thin layer chromatography24 (TLC). The trifluoroacetic
acylation of thiophene was similar to the previous process, but it
had to be heated under reflux temperature. Ferric chloride was re-
moved from solution by silica column filtration, and hexane
would be used as eluent. Hexane would be separated during rotary
evaporatoration. Trifluoroacetic acylated furan (TFAA_Furan):
FTIR: 1692 cm-1 (C=O); 1HNMR (200 Hz, CDCl3, in ppm) d 6.67
(q, 1H, -H4), d 7.46 (m, 1H, -H3), d 7.79 (q, 1H, -H5) and
trifluoroacetic acylated thiophene (TFAA_Thiophene): FTIR:
1690 cm-1 (C=O); 1HNMR (200 Hz, CDCl3, in ppm) d 7.172, d
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CHEMICAL SOCIETY
Trifluoroacetyl 5-membered Heterocycle in DSSCs
pounds would be added to electrolyte in the experimental group.
Electrochemical and photovoltaic characterizations:
The electrochemical behaviors were recorded on a conventional
electrochemical workstation CHI614D. It is including a Pt work-
ing electrode, a saturated calomel reference electrode (SCE), and
a platinum sheet counter electrode (CE). The cyclic voltammetry
(CV) curves were recorded from -1.0 to +0.6 V. Before the mea-
surement, the supporting electrolyte consisting of 0.115 mg
heterocyclic compound, 50 mM LiI, 10 mM I2, and 500 mM
LiClO4 in 10 ml acetonitrile was degassed using nitrogen for 10
minutes.33
more electronegative than carbon, so the oxygen atom pulls
electron density away from the carbon. When the carbon is
more positive, the bond length between oxygen and carbon
will be longer. The energy of the carbonyl will be de-
creased, and the wavelength will be increased.39,40 Figure 1
shows that the difference of the wavelengths of the car-
bonyl group between the acetylthiophene and the TFAA_
Thiophene in FTIR. The wavelengths of the carbonyl
group between the acetylthiophene and the TFAA_Thio-
phene are shifted from 1657 cm-1 to 1690 cm-1. As we
have expected, trifluoromethyl group pulls electron away
from the carbonyl carbon atom causing the oxygen much
more negative. Figure 2 shows that the redox peaks of
acetylthiophene and TFAA_Thiophene in CV. The details
are shown in Table 1. In Figure 2 and Table 1 we can obvi-
The photovoltaic testing of DSSCs could be measuted us-
ing simulated solar light as well as light from a Xenon arc lamp.
The incident light intensity was controlled at 1000 W/m2
(AM1.5).30 Electrochemical impedance was measured in a fre-
quency range of 10-1 Hz to 106 Hz and an AC amplitude of 5 mV at
room temperature under the same simulated solar light as photo-
voltaic testing.34,35
Other characterizations: Fourier transform infrared
spectrometry (FTIR) and Nuclear magnetic resonance (NMR)
would be used to identify the functional groups and structures.
FTIR spectra were recorded on Perkin-Elmer Spectrum 100 FTIR
spectrometer from 4000 cm-1 to 650 cm-1. NMR spectra were
measured by Varian Mercury-300 NMR spectrometer, and D-sol-
vent would be D-chloroform (1Hd = 7.26 ppm).
RESULTS AND DISCUSSION
Trifluoroacetic acylation
Five-membered heterocyclic compounds are useful
to enhance the short circuit current in DSSCs because of
providing extra lone pair electrons.36-38 Comparing with
methyl group on the carbonyl carbon, trifluoromethyl
group is more electronegative. It will make the carbonyl
carbon more positive and the carbonyl oxygen more nega-
tive. In the other words, the bond length between the car-
bonyl oxygen and the carbonyl carbon will be longer. That
is why we choose trifluoroacetic acylated five-membered
heterocyclic compounds which are acylated by Friedel-
Crafts acylation to offer a carbonyl group and extra lone
pair electrons in this experiment. According to Planck’s
Equation:
Fig. 1. The difference of the wavelength of the car-
bonyl group between acetylthiophene and
TFAA_Thiophene in FTIR.
E = hn = hc / l
A photon has an energy E, and this energy is propor-
tional to its frequency n where h is Planck’s constant, l is
the wavelength, and c is the speed of light. Oxygen is far
Fig. 2. The difference of redox peaks of acetylthio-
phene and TFAA_Thiophene in CV.
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centage38 that we add in won’t enhance current anymore.
The optimal concentration of heterocyclic compounds that
we add in electrolyte as additives will be 1wt% in this
study.
Table 1. The parameters of each redox peaks of acetylthiophene
and TFAA_Thiophene in CV
ERed1 (V) EOx1 (V) IRed1 (A) IOx1 (A)
I-/I3
-0.562
-0.571
-0.359 1.665E-5 2.651E-5
-0.389 2.383E-5 3.162E-5
-0.395 2.849E-5 3.448E-5
-
1%_Acetylthiophene
The appropriate scan rate and the optimal concentra-
tion of additives are 0.05 V/s and 1 wt% in both electro-
chemical and photovoltaic testing. Trifluoroacetic acylated
five-membered heterocyclic compounds are synthesized
by Friedel-Crafts acylation19-23 in the previous experi-
ments. Figure 5a and 5b show the difference between add-
ing five-membered heterocyclic compounds and trifluoro-
acetic acylated five-membered heterocyclic compounds in
CV. We can clearly observe the electrolytes which we add
pyrrole and N-methylpyrrole in as additives are very differ-
ent with the other five-membered heterocyclic compounds
1%_TFAA_Thiophene -0.508
ously find out that the current of reduction peak of I3 to I-
-
and the oxidation peak of I- to I3 in TFAA_Thiophene are
-
bigger than the current in acetylthiophene. Comparing the
FTIR and CV, trifluoroacetic acylation is better than
acetylation for providing extra lone pair electrons.1
Electrochemical behaviors
From the stacking CV curves added thiophene in
electrolyte at different scan rates, we can find out an out-
ward extension of all the peaks in Figure 3a. By plotting
in Figure 5a. The oxidation peaks at 2I3 -2e-®3I2 in pyrrole
-
peak current density corresponding to I3 «I- versus square
–
and N-methylpyrrole are obviously bigger than the others
three redox peaks. Electropolymerization43-46 of pyrrole
root of scan rate, linear relationship is observed in Figure
3b. This result indicates that the redox reaction is a diffu-
sion controlled mechanism on Pt counter electrode and also
in heterocyclic compound added electrolyte system.41 Ac-
cording to the book of Bard, A. J., for fast electron transfer
process with diffusion limited, the peak current is propor-
tional to the square root of the scan rate.42 In other words,
scan rate will not affect the electrochemical potential but
only proportional to current density. It looks like that add-
ing heterocyclic compounds in electrolyte is still a revers-
ible system. Because of the CV curve becoming unstable
under higher scan rate, it will be suggested to run with
medium scan rate.
Iodine/iodide based electrolytes can be optimized in
terms of efficiency and stability. Heterocyclic compounds
added in DSSCs are used for helping electrolyte to enhance
short circuit current density. Although the concentration is
proportional to current in a reversible Nernstian system,
but the original iodine/iodide based electrolyte is still sup-
planted by the heterocyclic additives. The optimal weight
percent of heterocyclic compounds that we shall add in
electrolyte will be determined by cyclic voltammetry. The
CV results of different weight percent thiophene additives
added in iodine/iodide based electrolyte from 0.1 wt% to
10 wt% are shown in Figure 4a and 4b. Concentration is not
proportional to current density obviously. That is because
more heterocyclic compounds that we add in, less iodine/
iodide based electrolyte will remain. Although hetero-
cyclic compounds are helpful for catching lithium ions in
iodine/iodide based electrolyte, but too much weight per-
Fig. 3. (a) The stacking CV curves of thiophene added
electrolyte at different scan rates. (b) The rela-
tionship between peak current density and squ-
are root of scan rates.
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Trifluoroacetyl 5-membered Heterocycle in DSSCs
Fig. 4. (a) The different weight percent of thiophene
we add in iodine/iodide based electrolyte from
0.1 wt% to 1 wt%. (b) The different weight per-
cent 1 wt% to 10 wt%.
Fig. 5. (a) The difference between adding five-mem-
bered heterocyclic compounds in CV. (b) The
difference between adding trifluoroacetic acy-
lated five-membered heterocyclic compounds.
and N-methylpyrrole are considered to be synthesized at
this working potential. It causes the concentration changed
and affects electrons transportation in electrolyte unfortu-
nately. In contrast, electropolymerization47 won’t occur
when five-membered heterocyclic compounds trifluoro-
acetic acylated, and it is helpful for interrupting electro-
polymerization occurring and enhancing the redox current
in CV in Figure 5b. According to atomic orbital,48 furan (O:
[He]2s22p4) and thiophene (S: [Ne]3s23p4) have two pairs
of lone pair. Pyrrole (N: [He]2s22p3) has only one pair of
lone lair, and N-methylpyrrole has no extra lone pair be-
cause of methyl group. So furan and thiophene will be
better than pyrrole and N-methylpyrrole for providing ex-
tra lone pairs attracted by cation, and TFAA_Furan and
TFAA_Thiophene are also better than TFAA_Pyrrole and
TFAA_N-Methylpyrrole. Furthermore, sulfur atom is big-
ger than oxygen atom in periodic table and therefore causes
the lone pair in the outermost electron shell of atom more
negative. That is the reason why thiophene added electro-
lyte is better than furan, and TFAA_Thiophene is also
better than TFAA_Furan in electrochemical behaviors.
These results and theories will be applicable for all the next
experiments in this study.
Photovoltaic and EIS performances
Figure 6a and 6b show the photovoltaic characteris-
tics of DSSCs, and the parameters reflecting DSSCs pro-
perties are summarized in Table 2. Figure 6a shows the
DSSCs added furan and thiophene in as additives get
higher efficiency (h) than pyrrole, N-methylpyrrole, and
the un-added one in photovoltaic testing. These results may
be attributed to the electropolymerizations44-46 of poly-
pyrrole (PPy) and polymethylpyrrole occurring during
photovoltaic testing. Although the DSSC added pyrrole in
as additive achieves higher short circuit current13 at begin-
ning, the polymerization occurring during photovoltaic
testing changes electrolyte concentration, and causes the
cell unstable.42 The DSSCs added trifluoroacetic acylated
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Table 2. The parameters of DSSCs added heterocyclic
compounds and trifluoroacetic acylated heterocyclic
compounds individually in photovoltaic testing
Voc (V) Jsc (A/cm2) FF h (%)
I-/I3
0.7113 1.159E-02 53.86 4.439
-
1%_Furna
0.7245 1.154E-02 55.00 4.597
0.7190 1.181E-02 56.88 4.830
0.6984 1.209E-02 51.27 4.331
0.7158 1.092E-02 56.12 4.387
0.7208 1.224E-02 55.01 4.852
0.7138 1.338E-02 54.43 5.197
0.7141 1.112E-02 56.33 4.472
1%_Thiophene
1%_Pyrrole
1%_N-Methylpyrrole
1%_TFAA_Furan
1%_TFAA_Thiophene
1%_TFAA_Pyrrole
1%_TFAA_N-Methylpyrrole 0.7137 1.203E-02 52.18 4.480
trochemical independence spectra.
Nyquist plots in Figure 7 illustrate the electrochemi-
cal impedance characteristics of five-membered hetero-
cyclic compounds and trifluoroacetic acylated five-mem-
bered heterocyclic compounds that we added in DSSCs as
additives. According to the Randles-type circuit49,50 that in-
serts in Figure 7, the intercept on the real axis represents the
series resistance (Rs) which is a reflection of total resis-
tance. Then the first semicircle (R1) from the left is related
with counter electrode which is Pt in this study, the second
one (R2) is associated with TiO2/dye/electrolyte interface
where the electrons transfer occurred, and the third one
(R3) is the diffusion rate of I-/I3 inner electrolyte.50 The de-
-
Fig. 6. (a) The I-V curve of DSSCs added heterocyclic
tail parameters are summarized in Table 3. It can be easily
found out that the most difference at Nyquist plots in elec-
trochemical independence spectra is the second semicircle
R2. For furan, thiophene, TFAA_Furan, and TFAA_Thio-
compounds. (b) The I-V curve of DSSCs added
trifluoroacetic acylated heterocyclic compounds.
-
five-membered heterocyclic compounds in as additives
achieve better efficiency than the five-membered hetero-
cyclic compounds and the un-added one in Figure 6b. For
TFAA_Furan and TFAA_Thiophene, trifluoroacetic acy-
lation provides a carbonyl group for an extra lone pair, and
it is helpful for increasing short circuit current in DSSCs.
For TFAA_Pyrrole and TFAA_N-methylpyrrole, trifluoro-
acetic acylation stops the electropolymerization47 occur-
ring, and also provides a carbonyl group for enhancing
short circuit current in photovoltaic testing. The optimal
phene that we add in I-/I3 electrolyte, the resistances of R2
are obviously decreased due to that the additives12,36 are
helpful for improving short circuit current in DSSCs. These
results can be compared with Table 2. For pyrrole and
N-methylpyrrole that we add in I-/I3 electrolyte, the re-
-
sistances of R2 are much bigger than the un-added one be-
cause of the electropolymerization44,46 which will affect the
concentration and increase the interface resistance between
working electrode and electrolyte occurring. It is particular
that adding pyrrole in electrolyte will also increase the re-
additive for I3 «I- redox reaction in this study is TFAA_
sistance R3 which is the diffusion rate of I-/I3 inner electro-
-
-
Thiophene which achieves the higher Jsc in photovoltaic
testing. There is a significant enhancement in DSSCs, and
the highest efficiency is 5.197% with 1 wt% TFAA_
Thiophene in comparison with 4.439% with un-added pure
iodine/iodide based electrolyte. The further relationship
between additives and resistance will be discussed in elec-
lyte. It is associated with electropolymerization of poly-
pyrrole44 which will change the concentration and disturb
electrons transfer in electrolyte. For TFAA_Pyrrole and
TFAA_N-methylpyrrole that we add in I-/I3 electrolyte, R2
-
are clearly smaller than pyrrole and N-methylpyrrole. It is
because that trifluoroacetic acylation will interrupt electro-
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Trifluoroacetyl 5-membered Heterocycle in DSSCs
reduces photovoltaic efficiency by reason of the resistance
between the working electrode and the electrolyte increas-
ing. Trifluoroacetic acylation which can provide more long
pairs and prevent electropolymerization occurring is help-
ful for five-membered heterocyclic compounds, further-
more, the current density and the short circuit current will
increase significantly in electrochemistry and photovol-
tage.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the supports by
the Material and Chemical Research Laboratories of Indus-
trial Technology Research Institute (ITRI).
Fig. 7. The electrochemical impedance characteristics
of each five-membered heterocyclic compounds
and trifluoroacetic acylated five-membered
heterocyclic compounds we added in DSSCs.
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