The influence of the push-pull effect and a π-conjugated system in conversion efficiency of bis-chalcone compounds in a dye sensitized solar cell

Article abstract of DOI:10.1016/j.molstruc.2017.04.059

Chalcone and its related compounds are known to be π-conjugated compounds, which can be potentially used in different electronic areas include dye sensitized solar cell (DSSC). A total of six bis-chalcone compounds (1–6) have been synthesized using a Clai

Full text of DOI:10.1016/j.molstruc.2017.04.059

Accepted Manuscript
The influence of the push-pull effect and a π-conjugated system in conversion
efficiency of bis-chalcone compounds in a dye sensitized solar cell
Kien Yung Teo, Mee Hing Tiong, Hung Yee Wee, Nornadia Jasin, Zhi-Qiang Liu,
Ming Yang Shiu, Jyun Yang Tang, Jenn-Kai Tsai, Rafizah Rahamathullah, Wan M.
Khairul, Meng Guan Tay
PII:
S0022-2860(17)30501-X
10.1016/j.molstruc.2017.04.059
MOLSTR 23682
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 9 December 2016
Revised Date: 16 April 2017
Accepted Date: 16 April 2017
Please cite this article as: K.Y. Teo, M.H. Tiong, H.Y. Wee, N. Jasin, Z.-Q. Liu, M.Y. Shiu, J.Y. Tang,
J.-K. Tsai, R. Rahamathullah, W.M. Khairul, M.G. Tay, The influence of the push-pull effect and a π-
conjugated system in conversion efficiency of bis-chalcone compounds in a dye sensitized solar cell,
Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.04.059.
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Graphical Abstract
OH
O
OMe
MeO
O
OH
OH
O
O
OH
Breaking π-conjugated effect
Push Pull effect

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The Influence of the Push-Pull Effect and a π-Conjugated System in Conversion Efficiency
of Bis-chalcone Compounds in a Dye Sensitized Solar Cell
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Kien Yung Teoa, Mee Hing Tionga, Hung Yee Weea, Nornadia Jasina, Zhi-Qiang Liub, Ming
Yang Shiuc, Jyun Yang Tangc, Jenn-Kai Tsaic, Rafizah Rahamathullahd, Wan M. Khairuld, Meng
Guan Taya,*
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aDepartment of Chemistry, Faculty of Resource Science and Technology, Universiti Malaysia
Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia.
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b State Key Lab of Crystal Materials, Shandong University, Jinan, 250100, China.
cDepartment of Electronic Engineering, National Formosa University, Yunlin County 63201,
Taiwan.
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dSchool of Fundamental Science, Universiti Malaysia Terengganu, 21030 Kuala Terengganu,
Terengganu, Malaysia.
*Corresponded author email address: mgtay@unimas.my
Abstract
Chalcone and its related compounds are known to be π-conjugated compounds, which can be
potentially used in different electronic areas include dye sensitized solar cell (DSSC). A total of
six bis-chalcone compounds (1-6) have been synthesized using a Claisen-Schmidt condensation
method under basic conditions. The compounds were used as the dye in DSSC to test their solar
conversion efficiency. In the process of solar cell fabrication, titanium(II) oxide (TiO2) coated
glass was used as the working electrode, whereas the mixture of iodine (I2), lithium iodide (LiI),
4-tertbutylpyridine (4-TBP) and 1,2-dimethyl-3-propylimidazolium (DMPII) in 3-

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methoxypropionitrile were used as the electrolyte. The DSSC was fabricated by immersing the
TiO2 glass into the respective bis-chalcone compound solution and dried into the oven at 45 °C
for 120 minutes. Lastly, the working electrode and counter electrode were sealed using surlyn of
60 µm thickness. The efficiency test was conducted under AM 1.5G illumination with the
incident light intensity of 100 mW/cm2. Among the six bis-chalcone derivatives, compound 2,
namely 1,4-bis-2-hydroxychalcone, was recorded with the highest efficiency (0.054%) compared
to the others (0.022-0.035%). The presence of a π-conjugated system and the push-pull effect in
the molecule were found to enhance the conversion efficiency of DSSC. Details of the results are
discussed in the present paper.
Keyword: Bis-chalcone, dye sensitized solar cell (DSSC), π-conjugation system, push-pull effect
1. Introduction
Solar energy has great potential to be used as a renewable energy source in the future. However,
in the current market, over 90% solar cells are made of silicon, which, despite giving high
conversion efficiency are expensive [1]. As a result, the organic type dye sensitized solar cell
(DSSC) was developed, which offers low cost [2], however, the conversion efficiency is still far
lower than the silicon type solar cell [3]. Despite DSSC giving low conversion efficiency, this
type of solar cell still attracts attention from different fields of researchers due to several
advantages [4]. In addition to the low cost, the organic dye can be coated onto the plastic type
substrates which can increase the flexibility of the cell [2]. Moreover, by understanding the

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structure-property relationship, chemists can structurally design the dyes in order to enhance the
sun light absorption properties [4].
Chalcone is a natural compound found in apple seeds [5]. The structure of chalcone consists of
two aromatic rings that linked by α,β-conjugated keto-enol group (Fig. 1).
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Fig. 1. Structure of chalcone
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Chalcone and its derivatives are widely used in non-linear optics (NLO), optical computing,
optical communication, optical limiting, photoinitiated polymerization, electrochemical sensing
and Langmuir films [6-8]. According to Kamath and co-workers, chalcones are used as NLO due
to their good optical power limiting properties and third-order nonlinearity. In general, chalcones
offer large nonlinear coefficients due to the fact that they consist of a conjugated double bond in
between two planar benzene rings [9]. The delocalization of π-electrons within these double
bond and benzene rings are the one that responsible for the third order nonlinear optical
responses. Furthermore, the effect can also be increased by enhancing the ability of electron
donating groups [10].
In fact, some researchers found that small molecules containing donor (D)-acceptor (A) groups
connected by π-conjugated system can pose high solar conversion efficiency [11-14]. For this
reason, Chambon and co-workers have synthesized a series of 2’-hydroxychalcones coordinated
to borondifluoride, a strong π-acceptor, in order to enhance the push-pull character by having
donor-acceptor (D-A) groups in the molecule. The group was able to achieve an efficiency
conversion of 1.13% when the boron-chalcones complexes were used as donor materials while

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[6,6]phenyl-C61-butyric acid methyl ester (PC61BM) as the electron accepting material in the
solar cells [15]. In addition, flavonoid, which is a natural pigment from the same family with
chalcone, and its’ application in DSSC has been reported [16].
Herein, we report the solar conversion efficiency of six bis-chalcone compounds (Fig.2). The
synthesis, photophysical properties [8], and the biological activities [17-21] of compounds 1-4
have already been reported, whereas compounds 5-6 are novel compounds. To our knowledge,
none of these compounds have been reported concerning their solar conversion efficiency in
DSSC.
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Fig. 2. Bis-chalcone compounds 1-6 that have been tested for conversion efficiency in DSSC
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The influence of the push-pull effect in conversion efficiency can be observed by comparing
compounds 3 and 4 to compounds 1 and 2. There are two methoxy groups present at the 2 and 5
positions at the central phenyl ring, which act as electron donor in compounds 3 and 4.
Meanwhile, the bis-chalcone analogues in compounds 5 and 6 are located at the meta-position in
order to reduce π-conjugated effect along the molecule.
2. Experimental

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All chemicals used for the synthesis of compounds 1-6 were purchased from Fluka, Aldrich and
Acros-organic. Unless otherwise stated, the chemicals and solvents were used without further
purification.
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Ethanol (1.00 L) was purified by mixing with 1.5 g of iodine and 3.5 g of magnesium turning in
a round bottom flask. The mixture was refluxed until its colour changed from dark brown to
colourless and then collected via distillation to obtain dry ethanol.
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2.1 Characterization
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The IR spectra of bis-chalcone compounds were recorded using a Thermo Scientific Fourier
transform infrared spectrometer (FTIR) model - NICOLET iS10 in KBr dish in the range of
4000-400 cm-1. The resolution used for obtaining the IR spectra was 4.000 with the scan’s
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number of 36 in a deuterated triglycine sulphate (DTGS) KBr detector. The H NMR spectra
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were recorded by using JEOL, 500 MHz FT-NMR spectrometer, and the chemical shifts were
reported relative to TMS internal standard and referenced via residual proton NMR resonances
of the deuterated solvent (CDCl3: δH 7.26; δC 77.7 ppm). The absorption spectra were recorded
with JASCO V-630 spectrophotometer. The elemental analysis of carbon, hydrogen, and
nitrogen were carried out by using CHN analyzer Thermo-flash EA 1112 Series. Melting points
were determined using open capillary in Stuart MP3 melting point apparatus.
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2.2 General procedures for the preparation of bischalcone
The procedures of preparing bis-chalcone compounds are similar to those reported in the
literature [8]. Compounds 1-4 have been reported in literature [8, 17-23], while compounds 5 and
6 were newly synthesized. The general procedures of bis-chalcone compounds 1-4 preparation

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are described as below, whereas the spectroscopic data of compounds 1-4 can be found in S.I or
our earlier publication [8].
Two equivalent molar amount of R-acetophenone [where R=H (1 & 3) and ortho-OH (2 & 4)]
was added into 100 mL beaker and was dissolved in 20 mL of ethanol. 10 mL of 10 % aqueous
KOH was poured gently into the beaker containing R-acetophenone and the mixture was stirred
for about 10 minutes. 1,4-terephthaldehyde (0.67 g, 0.0050 mol) and 2,5-dimethoxy-1,4-
terephthaldehyde (0.20 g, 0.0010 mol) were added into beaker (1 & 3) and (2 & 4), respectively.
The mixture was stirred vigorously under room temperature for 4 hours. After that, 3M HCl was
added until the solution was neutralized. The obtained precipitate was filtered, rinsed with
distilled water and ethanol. The washed precipitate was allowed to air dried. Finally, the
precipitate was purified via slow evaporation by using THF.
For compounds 5 and 6, precaution steps were taken as the precipitate easily agglomerate during
the neutralization process. Therefore, the neutralization process was carried out under cold
condition. The preparation procedures and spectroscopic data of compounds 5 and 6 are as
described below:
Acetophenone (0.360 g, 3.0 mmole for compound 5) and 2-hydroxyacetophenone (0.408 g, 3.0
mmole for compound 6) was dissolved in 20 mL of ethanol, respectively. 10 mL of 10% aqueous
KOH was added gently into the solution and the mixture was stirred for about 10 minutes. Iso-
terephthaldehyde (0.200 g, 1.5 mmol) was added into the mixture and stirred vigorously at room
temperature for 4 hours. After that, the reaction flask was immersed into ice bath and 3M HCl
was added until the solution was neutralized. The obtained precipitate was filtered, rinsed with
distilled water and ethanol. The washed precipitate was allowed to air dried. The precipitate was
purified using recrystallization via slow evaporation in tetrahydrofuran (THF).

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2.2.1 (2E,2’E)-3,3’-(1,3-phenylene)bis(1-phenylprop-2-en-1-one), 5
White solid. Yield: 0.530 g, 95%. M.P.: 136.4-137.3 °C. IR (KBr, cm-1): 1660 (vC=O), 1607
(vC=C), 1442 and 1420 (vC=C, aromatic ring). 1H-NMR (500 MHz, CDCl3) δ: 8.05 (d, 4H, J = 8
Hz, Ar-H), 7.89 (d, 2H, J = 16 Hz, CH=CHβ), 7.82 (s, 1H, Ar-H), 7.69 (d, 2H, J = 8 Hz, Ar-H),
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7.60 (d, 2H, J = 16 Hz, CHα=CH), 7.62 (t, 2H, 8Hz, Ar-H), 7.53 (m, 5H, Ar-H). C-NMR (125
MHz, CDCl3) δ: 190.5, 143.9, 138.1, 135.8, 133.1, 130.3, 129.7, 128.8, 128.7, 128.4 and 123.1.
UV-Vis (DCM) (λmax/nm): 261 and 307. Anal. Cal. for C24H18O2: C, 85.18; H, 5.36; Found: C,
85.62; H, 5.43.
2.2.2 (2E,2’E)-3,3’-(1,3-phenylene)bis[1-(2-hydroxyphenyl)prop-2-en-1-one), 6
Yellow solid. Yield: 0.161 g, 27%. M.P.: 178-180 °C. IR (KBr, cm-1): 3455 (vOH), 1639
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(vC=O), 1575 (vC=C), 1487 and 1441 (vC=C, aromatic ring). H-NMR (500 MHz, CDCl3) δ:
12.74 (s, 2H, OH), 7.96 (d, 2H, J = 6 Hz, Ar-H), 7.95 (d, 2H, J = 15 Hz, CHα=CH), 7.91 (s, 1H,
Ar-H), 7.74 (d, 2H, J = 5 Hz, Ar-H), 7.72 (d, 2H, J = 15 Hz, CH=CHβ), 7.53 (t, 3H, J = 8Hz, Ar-
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H), 7.05 (d, 2H, J = 8 Hz, Ar-H), 6.98 (t, 2H, J = 7 Hz, Ar-H). C-NMR (125 MHz, CDCl3) δ:
193.59, 163.74, 144.39, 136.75, 135.61, 130.56, 129.79, 128.91, 121.37, 120.01, 119.08
and118.82. UV-Vis (DCM) (λmax/nm): 260, 315 and 351. Anal. Cal. for C24H18O4: C, 77.82; H,
4.90; Found: C, 77.84; H, 4.95.
2.3 Dye sensitized solar cell (DSSC) fabrication
The fabrication method of DSSC followed that reported in the literature [24] with slight
modification. Fig. 3 shows a schematic assembly of DSSC, whereas the completed DSSC is

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shown in Fig.4. The preparation procedures of each individual component as described in the
following sub-sections 2.3.1-2.38.
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Fig. 3. Schematic structure of DSSC
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Fig. 4. Completed DSSC
2.3.1 Preparation of TiO2 paste
Titanium oxide (TiO2) P25 (5 g) was transferred into a mixture of 30 mL of tert-butylalcohol and
15 mL of deionized water in a beaker. It was stirred for overnight to homogenize the paste.
2.3.2 Preparation of TiO2 working electrode
FTO glass (1.5 cm×3.0 cm) was used in the fabrication. The glass was sonicated with acetone,
followed by methanol and deionized water. It was dried at 45 °C for about 10 minutes. Upon

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completion, it was treated under UV light for 30 minutes. The treated FTO glass was directly
used for the TiO2 coating by using the doctor blade technique. The TiO2 coating was started from
the thickness of 5 µm, followed by 10, 15, 20, 35, 40, 45 and lastly 50 µm. Each coat was kept
for 60 minutes interval gap. The excess TiO2 was scraped out using microscopic slide to create a
dimension of 0.5 cm×0.5 cm as a working electrode. The film was treated to 150 °C for 90
minutes and annealed to 450 °C for 30 minutes.
2.3.3 Preparation of standard dye cis-[bis(isothiocyanato)bis(2,2’-pyridinyl-4,4’-dicarboxylic
acid)ruthenium(II)](N3) solution
The standard dye namely N3 (0.0223 g, 0.03 mmol) was dissolved into a mixture of 50 mL
acetonitrile and 50 mL tert-butylalcohol in a beaker. Lights were turned off during the
preparation due to light sensitive nature of N3. The mixture was capped and sonicated for about
30 minutes. The beaker was covered with aluminium foil to avoid direct contact with light.
2.3.4 Preparation of bis-chalcone compounds as organic dye
Respective bis-chalcone compound (0.03 mmol) was dissolved into 20 mL of acetonitrile and 20
mL of tertbutylalcohol in a beaker. The mixture was capped and sonicated for 30 minutes.
2.3.5 Preparation of Pt counter electrode
The electron beams evaporation technique (Fig. 5) was used to coat the Pt counter electrode.
This technique involves the use of electricity to generate an electron beam in a high pressure
vacuum chamber of about 7.5×10−5 Torr (10−2 Pa) and control of magnetic field to deflect the
beams by 180° or 270° onto the evaporation material. The kinetic energy produced from the

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electricity drives the generated electron beams and upon contact with the evaporation material, it
is converted into thermal energy producing heat to melt or sublimate it. Once sufficient heat and
pressure were reached, the vapour evaporated upwards and formed a thin layer film on the
substrate that is attached onto the substrate dome.
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Fig. 5. Electron beams evaporation technique
2.3.6 Preparation of Electrolyte
The electrolyte consists of 0.05 mol iodine (I2), 0.5 mol lithium iodide (LiI), 0.5 mol 4-
tertbutylpyridine (4-TBP) and 1 mol 1,2-dimethyl-3-propylimidazolium (DMPII) in 15 mL 3-
methoxypropionitrile (MPN).

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2.3.7 Preparation of dye-sensitized working electrode
The TiO2 working electrode was immersed into the N3 dye in a dark room and put into the oven
at 45 °C for 120 minutes. Upon completion, the film was taken out and dipped into the
acetonitrile solution three times. It was allowed to air dried.
For the bis-chalcone organic dye, the prepared TiO2 working electrode was immersed into the
respective bis-chalcone compound and put into the oven at 45 °C overnight. Upon completion,
the film was take out and was allowed to dry in air.
2.3.8 Sealing of Dye-sensitized solar cell
The working electrode and counter electrode were sealed using the surlyn of 60 µm thickness
(surlyn 60 µm). Electrolyte was injected into the small drilled hole. The hole was sealed with
another piece of small glass using the surlyn 60 µm. Silver paste was applied on both of the
conductive sides.
2.4 Conversion efficiency measurement
The incident photon to current efficiency (IPCE) was measured under light illumination of air
mass 1.5 global (AM 1.5G) with the incident light intensity of 100 mW/cm2. The IPCE data were
recorded using Keithley 2400 source meter with illuminating monochromatic light on the solar
cells. The working dimension for the solar cell was set at 0.25 cm2. The J/V measurement graph
of solar circuit current density (Jsc, mA/cm2) against voltage (V) was plotted using the Origin8
Pro SR4 software. The power conversion efficiency of the solar cell can be calculated using the
equation as shown in Eq. 1 below:

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ꢀ
ꢁꢂ
ꢃ ꢄ ꢃ ꢆꢆ
ꢅꢂ
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Eq. 1
η
=
ꢇ
ꢈꢉ
where
Jsc = short circuit current (mA/cm2)
Voc = open circuit photovoltage (V)
FF = fill factor (%)
Pin = incident light intensity (100 mW/cm2)
η
= power conversion efficiency (%)
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2.5 Electrochemical setup
The electrochemical experiments were carried out via electron impedance spectroscopy (EIS)
which comprises three-electrode system; glassy carbon electrode (GCE) as working electrode,
platinum wire as counter electrode and Ag/AgCl as reference electrode. The system was
connected to AUTOLAB PGSTAT302 model potentiostat driven by General Purpose
Electrochemical System (GPES) software. These experiments were carried out in
dichloromethane solution containing 0.1 M tetrabutylammonium hexafluorophosphate
[NBu4]PF6 as supporting electrolyte and sample concentration of 1 × 10-3 M. All measurements
were performed at room temperature.
3. Results and discussion
3.1 Synthesis and spectroscopic characterization
The bis-chalcone compounds 1-6 were synthesized through Claisen-Schmidt condensation. The
reactions were carried out under basic conditions for 6-12 hours and neutralized using 3M HCl
upon the completion of the reaction. For compounds 5 and 6, precautionary steps were taken as
the precipitate easily agglomerate during the neutralization process. Therefore, neutralization
process was carried out under cold condition in order to avoid the agglomeration problem.

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The formation of bis-chalcone compounds can be confirmed from the presence of C=C-C=O
conjugation stretching band at 1600-1700 cm-1 in the IR spectra (See SI under Fig. S1-S6) [8].
Meanwhile, another strong stretching band appears at 1570 - 1600 cm-1 is attributed to the
aliphatic C=C bond in the structure. For bis-chalcone compounds 2, 4 and 6, a broad signal
presents at the region of 3300 – 3400 due to the presence of OH group at the ortho position of
wing phenyl rings.
1
Furthermore, in the H NMR spectra (see SI under Fig. S7-S12), a pair of doublets can be
observed at 8.05-7.80 ppm and 7.71-7.51 ppm with the coupling constant, J, values of 16 Hz.
The pair of doublets are attributed to the α- and β-protons located in the Cβ=Cα-C=O moiety,
confirming the trans conformation of the bis-chalcone compounds were obtained.
For the UV-Vis spectra (SI under Fig. S19-S24), all the bis-chalcone compounds show two
absorption bands namely 268–278 nm for the π→π* transition in the phenyl ring and 333-383
nm for the n→π* in the C=O moiety (Fig. 6). As well as these two absorption bands, compounds
3-4 also have another additional absorption band at 421 nm attributed to intra-molecule charge
transfer (ICT) due to the electron transfer from ethylenic ꢊ-orbitals to the carbonyl ꢊ-orbital [8].
2
379 nm
1.5
1
Abs
277 nm
0.5
0
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400
Wavelength [nm]
500
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Fig. 6 UV-Vis spectrum of compound 2
3.2 Application of bis-chalcone in solar cell efficiency
The standard dye, cis-[bis(isothiocyanato)bis(2,2’-pyridinyl-4,4’-dicarboxylicacid)ruthenium(II)]
complex (N3), was used in order to ensure the consistency of our solar cell fabrication as well as
the measurement. The test was conducted in triplicates and a power conversion efficiency of
4.57, 4.52 and 4.59% was recorded for trials 1, 2 and 3, respectively. The small standard
deviation (±0.04) indicating the fabrication technique and measurements were consistent.
Fig. 7 shows the J/V measurement of the bis-chalcone series which was divided into two
categories, with and without –OH substituted compounds. From the results obtained (Table 1),
compound 2 showed the best solar conversion efficiency of 0.054%, which is due to the good
absorption of the compound 2 dye in the TiO2 layer, where a yellow color can be clearly seen on
the surface. The result is also supported by the fill factor (FF) value which measured up to
51.604%. Fill factor is generally defined as the ideal of the device system work. Several factors
that can affect the FF value include the thickness of TiO2, the electrolyte, the resistance of the
cell and also the conversion of photocurrent absorbed by the dye [25]. Thus, the better absorption
of the dye sensitizer allows for more photons are being absorbed, which results in a greater
photocurrent that could be transported to the nanocrystalline TiO2 layer and therefore increase
the overall performance.
On the other hand, the effect of a substituent can be clearly observed. The structure of compound
2 with an –OH substituent on both ends of the molecule promotes the charge transfer within the
molecule and delivers the electron towards the TiO2 semiconductor layer leading to a solar
conversion efficiency of 0.054%. In comparison with compound 1, which does not contain –OH

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groups in the molecule, a much lower solar conversion efficiency with only 0.030% was
recorded.
0.25
0.20
0.15
0.10
0.05
0.00
0.20
0.15
0.10
0.05
0.00
Compound 2
Compound 4
Compound 6
Compound 1
Compound 3
Compound 5
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
Voltage (V)
Voltage (V)
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Fig. 7 J/V measurement of bis-chalcone compounds (without –OH substituent, compounds 1, 3
and 5, left); (with –OH substituent, compounds 2, 4 and 6, right)
Table 1: Summary of power conversion efficiency of bis-chalcone compounds
Compound Voc (V) Jsc (mA/cm2) Fill factor (%) Efficiency (%)
0.410
0.460
0.440
0.370
0.361
0.390
0.144
0.225
0.150
0.193
0.144
0.182
50.649
51.604
48.570
43.970
41.620
49.423
0.030
0.054
0.032
0.032
0.022
0.035
1
2
3
4
5
6
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Furthermore, the presence of
π-conjugation in the structure also plays a vital role in solar
conversion efficiency. This effect was observed when we changed the substitution position on
the central phenyl ring from para (compound 2) to meta-position (compound 6). Compound 6
showed 20% lower solar conversion efficiency (0.035%) compared to compound 2 (0.054%),
despite also having the –OH substituted functional group. According to He and coworkers, the π-

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conjugation effect in the molecule bonded by a phenyl meta-branching point is destroyed,
causing the localization of the electron-hole pair and electronic decoupling [26]. Hence, this
affects the overall performance of the dye-sensitizer to facilitate the transfer of the electron to the
semiconductor layer. Due to this reason, the conversion efficiency for compound 5 is the lowest
with a solar efficiency of only 0.022%.
Compounds 3 and 4 were synthesized with two methoxy groups attached at 2 and 5 positions of
the central phenyl ring at the para-branching point. The initial aim of introducing methoxy
groups to the central phenyl ring of compounds 3 and 4 was to induce push-pull effect along the
molecule. Interestingly, compound 3 is highly luminescent with the
λ emission centered at 505
nm and the of 0.57 in DCM solution [8]. Indeed, compound 3 showed slightly higher
Փ
conversion efficiency compared with compounds 1 and 5, and a higher Voc and Jsc value were
recorded. However, compound 4, which has both methoxy and hydroxyl substituent groups,
showed the least efficiency in comparison with compounds 2 and 6. This may due to the poor
solubility of compound 4 in the solvent system, where the crystal of compound 4 was found to
reform on top of the TiO2 surface. This caused an increase in the thickness of the dye sensitizer
layer and the direct contact with the counter electrode would results in an increase in cell
resistance and impacting the Voc and Jsc values.
3.5 Electrochemistry
The electrochemical behaviours of bis-chalcone compounds 2, 4 and 5 were investigated further
by using cyclic voltammetry analysis in order to determine the redox reaction and potential range
of different electrochemical processes. Cyclic voltammograms of 2, 4 and 5 at glassy carbon

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electrode were recorded in dichloromethane solution using 0.1 M [NBu4]PF6 as supporting
electrolyte in the potential range -3.0 V to 3.0 V as depicted in Fig. 8.
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Fig. 8 Cyclic Voltamograms of bis-chalcone 2, 4 and 5 in DCM/0.1M [NBu4]PF6 at 0.05 V s-1
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scan rate.
Fig. 8 shows the redox potential behaviour with the presence of oxidation and reduction peaks
for all three compounds. The anodic peak which corresponded to bis-chalcone oxidation
appeared at around 1.74 V to 2.05 V due to the consumption of adsorbed chalcone and/or its
oxidative product occurs on the electrode surface [27, 28]. For electrochemical reduction, cyclic
voltammogram of 2 reveals well defined quasi-reversible reduction in the range -0.5 V to -2.3 V.
The quasi-reversibility of the reduction process is confirmed with the corresponding peak
separation value [29] between the catholic peak potential (Epc= -1.58 V) and the two anodic peak
potential (Epa= -1.79 V) and (Epa= -1.23 V). The existence of this cyclic voltammetry (CV) wave

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may be ascribed to the reduction of C=O group [27,30] and showed a mixed adsorptive and
diffusive response for the system [31]. These findings were supported by previous literatures
describing on similar type of molecular frameworks where they focused on proton coupled
electron transfers leading to the reduction of the carbonyl bond to form the hydroxyl and
hydrogenation of the vinylic bond [27, 31]. In conclusion, the behaviour of compounds 2, 4 and
5, and the existence of redox potential subsequently gave ideal characteristics for potential of the
fabrication of DSSC.
4. Conclusion
Six bis-chalcone compounds were synthesized and their solar conversion efficiencies in DSSC
were measured. Compound 2 gave the highest conversion efficiency (0.054%) compared to the
others (0.022-0.035%). The higher efficiency of compounds 1 and 2 compared to compounds 5
and 6, respectively, indicates the importance of π-conjugation system in conversion efficiency. In
addition, the presence of push-pull effect in compound 3 also resulted a higher conversion
efficiency compared to compounds 1 and 5.
Acknowledgements
The authors wish to thank to Ministry of Higher Education, Malaysia, for financial support under
Fundamental Research Grant Scheme with the number of [FRGS/SG01(01)/1067/2013(13)]. In
addition, the authors also would like to thank Dr. Andrew Crawford for proofreading to this
article.

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Highlights
•
•
The influence of π-conjugated system and push-pull effect to DSSC’ efficiency
The synthesis and characterization of bis-chalcone compounds
•
Fabrication of DSSC and the conversion efficiency measurement