Synthesis and characterization of organic photo-sensitizers containing multi-acceptors for the application of dye-sensitized solar cells

Article abstract of DOI:10.1080/15421406.2010.497115

Organic photo-sensitizers containing multi-acceptors in a chromophore have been synthesized and characterized for the application of dye-sensitized solar cell (DSSC). In this study, we have used intramolecular push-pull system containing triphenylamine as the electron donor with different number of cyanoacetic acid moieties as electron acceptor/anchoring groups in a chromophore. The experimental results have revealed that when the induced electron acceptor increases, the larger amounts of dyes are adsorbed on the TiO₂ surface in DSSC, resulting in the increase of short circuit photocurrent density. Our results suggest that the organic dyes with multi-electron acceptors moieties are promising for getting higher solar-to-electricity conversion efficiencies in DSSC. Copyright Taylor & Francis Group, LLC.

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Synthesis and Characterization of
Organic Photo-Sensitizers Containing
Multi-Acceptors for the Application of
Dye-Sensitized Solar Cells
Hyo Jeong Jo a , Young Cheol Choi b , Jang-Hyun Ryu c , Jung Hyeon
Kang c , Nam Kyu Park c , Do Kyung Lee b & Jae Hong Kim a
a Department of Display and Chemical Engineering, Yeungnam
University, Gyeongsan, Gyeongsangbuk-do, Republic of Korea
b R&D Affairs Department, Gumi Electronics & Information
Technology Research Institute, Gumi, Gyeongsangbuk-do, Republic
of Korea
c Solar Cell Research Center, Korea Institute of Science and
Technology (KIST), Seoul, Republic of Korea
Available online: 14 Nov 2010
To cite this article: Hyo Jeong Jo, Young Cheol Choi, Jang-Hyun Ryu, Jung Hyeon Kang, Nam Kyu Park,
Do Kyung Lee & Jae Hong Kim (2010): Synthesis and Characterization of Organic Photo-Sensitizers
Containing Multi-Acceptors for the Application of Dye-Sensitized Solar Cells, Molecular Crystals and
Liquid Crystals, 532:1, 55/[471]-64/[480]
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Mol. Cryst. Liq. Cryst., Vol. 532: pp. 55=[471]–64=[480], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.497115
Synthesis and Characterization of Organic
Photo-Sensitizers Containing Multi-Acceptors for
the Application of Dye-Sensitized Solar Cells
HYO JEONG JO,1 YOUNG CHEOL CHOI,2
JANG-HYUN RYU,3 JUNG HYEON KANG,3
NAM KYU PARK,3 DO KYUNG LEE,2
AND JAE HONG KIM1
1Department of Display and Chemical Engineering, Yeungnam
University, Gyeongsan, Gyeongsangbuk-do, Republic of Korea
2R&D Affairs Department, Gumi Electronics & Information Technology
Research Institute, Gumi, Gyeongsangbuk-do, Republic of Korea
3Solar Cell Research Center, Korea Institute of Science and Technology
(KIST), Seoul, Republic of Korea
Organic photo-sensitizers containing multi-acceptors in a chromophore have been
synthesized and characterized for the application of dye-sensitized solar cell
(DSSC). In this study, we have used intramolecular push-pull system containing
triphenylamine as the electron donor with different number of cyanoacetic acid
moieties as electron acceptor=anchoring groups in a chromophore. The experimental
results have revealed that when the induced electron acceptor increases, the larger
amounts of dyes are adsorbed on the TiO2 surface in DSSC, resulting in the increase
of short circuit photocurrent density. Our results suggest that the organic dyes with
multi-electron acceptors moieties are promising for getting higher solar-to-
electricity conversion efficiencies in DSSC.
Keywords Dye-sensitized solar cells; intramolecular charge-transfer chromo-
phore; multi-acceptors; organic photo-sensitizers
1. Introduction
Since Gra¨tzel et al. reported the first efficient dye-sensitized solar cells (DSSCs) in
1991 [1], they have attracted much attention due to their relatively high power
conversion efficiency and potentially low cost production [2–10]. To date, high per-
formance and good stability of DSSC based on Ru-dyes as a photosensitizer had
Address correspondence to Prof. Jae Hong Kim, School of Display and Chemical Engin-
eering, Yeungnam University, 214-1 Dae-dong, Gyeongsan, Gyeongsangbuk-do 712-749,
Korea (ROK). Tel.: (þ82)53-810-2521; Fax: (þ82)53-810-4631; E-mail: Jaehkim@ynu.ac.kr
or Dr. Do Kyung Lee, R&D Affairs Department, Gumi Electronics & Information Technology
Research Institute, Bongsan-ri, Sandong-Myeon, Gumi, Gyeongsangbuk-do 730-853, Korea
(ROK). Tel.: (þ82)54-479-2111; Fax: (þ82)54-479-2050; E-mail: dklee@geri.re.kr
55=[471]

56=[472]
H. J. Jo et al.
been widely addressed in the literatures. The DSSC with Ru-bipyridyl complexes (N3
and N719), and the one with black ruthenium dye have achieved power conversion
efficiencies up to 11.2% and 10.4%, respectively [3,4].
However, the Ru-based dyes are facing the problem of manufacturing costs and
environmental issues. In order to obtain the cheaper photosensitizers for the DSSC,
the metal-free organic photosensitizers are strongly desired. The metal-free organic
dyes offer superior molar extinction coefficients, low cost, and diverse molecular
structures as compared to the conventional Ru-based dyes. Recently, novel photosen-
sitizers such as coumarin [5], merocyanine [6], cyanine [7], indoline [8], hemicyanine
[9], triphenylamine [10–13], dialkylaniline [14], bis(dimethylfluorenyl)-aminophenyl
[15,16], phenothiazine [17], tetrahydroquinoline [18], and carbazole [19] based dyes
have achieved the solar-to-electrical power conversion efficiencies up to 5–9%.
In general, organic dyes are designed to be of intramolecular donor-acceptor
structure that is called a push-pull architecture, which can induce the electron density
change from electron donor to electron acceptor by the light irradiation following
subsequent electron injection to the TiO2 via the anchoring group.
Recently, we have reported [20] the organic dyes with double electron acceptors
in a chromophore as a photosensitizer. It was indicated that the organic dyes with
double electron acceptors exhibited the high power conversion efficiency in the
DSSC due to the increase of electron extraction paths from electron donor as com-
pared with those of single electron acceptor type. Also, this earlier report has the
important implications for the needs of conducting studies on the organic dyes
containing multi-acceptors with a viewpoint of photovoltaic performance.
In this work, we have studied on the synthesis and characterization of the organic
dyes containing different number of electron acceptor moieties in a molecule. In parti-
cular, photovoltaic properties of the dyes in DSSC device are mainly discussed.
2. Experimental
2.1. Synthesis
Organic dyes based triphenylamine were synthesized by the well-known reactions
such as Vilsmeier-Haack formulation reaction and Knoevenagel reaction.
4-(Diphenylamino)-benzaldehyde, 4,40-(phenylazanediyl) dibenzaldehyde and 4,40,
400-(phenyl azanediyl)tribenzaldehyde were prepared by treating triphenylamine with
different equivalent of POCl3 in DMF, according to the procedures in literature [21].
The detailed synthetic procedures for the organic dyes were described as follows.
4-Formyltriphenylamine (1a). Triphenylamine (40 g, 163 mmol) was dissolved in
80 mL DMF. POCl3 (76 mL, 815 mmol) was added drop to drop to the mixture at
0ꢀC. As the temperature was increased to room temperature, the mixture was
changed to a clear bright red solution. The reaction mixture was heated to 45ꢀC
and then stirred for an additional 2 hr. The mixture was poured in an ice-bath and
then neutralized with sodium bicarbonate. Recrystallization of resulting
precipitates from ethanol afforded crystals of 4-Formyltriphenylamine. Pale yellow
solids were collected by filtering. (42.9 g, 96%). 1H NMR (300 MHz, CDCl3): d
9.79 (s, 1H), 7.30–7.36 (t, 4H), 7.67 (d, 2H), 7.15–7.18 (m, 6H), 7.00 (d, 2H).
40400-Diformyltriphenylamine (1b). Triphenylamine (4.900 g, 20 mmol) and
anhydrous DMF (40 mL) were added to a 250-mL round bottomed flask under

Organic Photo-Sensitizers Containing Multi-Acceptors
57=[473]
nitrogen. The mixture was cooled to 0ꢀC and phosphorus oxychloride (18.0 mL,
200 mmol) was added drop to drop. The mixture was heated to 85ꢀC for 10 hr and
the ensuing mixture was poured into an ice water. The mixture was neutralized to
pH 7 with aqueous 2 N NaOH solution and then the crude product was extracted
with dichloromethane. The product was purified by column chromatography
(silicagel, petroleum ether=ethyl acetate ¼ 4:1) to obtain pure product (4.81 g,
1
80%). H NMR (300 MHz, CDCl3): d 9.89 (s, 2H), 7.76–7.78 (d, 4H), 7.38–7.41 (t,
2H), 7.25–7.28 (t, 1H), 7.17–7.19 (m, 6H).
4,40,400-(Phenyl azanediyl)tribenzaldehyde (1c). Triphenylamine (4.900 g, 20 mmol)
and anhydrous DMF (40 mL) were added to a 250-mL round bottomed flask
under nitrogen. The mixture was cooled to 0ꢀC and phosphorus oxychloride
(18.0 mL, 300 mmol) was added drop by drop. The reacting mixture was stirred at
85ꢀC for 10 hr and the ensuing mixture was poured into ice water. It was
neutralized to pH 7 with aqueous 2 N NaOH solution, and the crude product
extracted with dichloromethane. The product was collected by filtration and
purified by column chromatography (silicagel, petroleum ether=ethyl acetate ¼ 4:1)
1
to obtain pure product (1.97 g, 30%). H NMR (300 MHz, DMSO-d6): d 9.85 (s,
3H), 7.83–7.7.81 (d, 6H), 7.20–7.17 (d, 6H).
2-Cyano-3-(4-diphenylamino-phenyl)-acrylic Acid (TPA1). 4-(Diphenylamino)
benzaldehyde (1 g, 3.7 mmol) and 2-cyanoacetic acid (0.47 g, 5.5 mmol) were added
to 15 mL of glacial acetic acid and refluxed for 3 hr in the presence of 0.42 g
(5.5 mmol) of ammonium acetate. After cooling to room temperature, the mixture
was poured into ice water. The precipitate was filtered, washed by distilled water,
dried under vacuum, and purified by column chromatography (ethyl acetate=
ethanol ¼ 9:1), resulting in yellow powder of TPA1 (0.23 g, 80%). 1H NMR
(300 MHz, DMSO-d6): d 8.13 (s, 1H), 7.90–7.93 (d, 2H), 7.40–7.43 (t, 4H),
7.20–7.27 (m, 6H), 6.84–6.87 (d, 2H).
2-Cyano-3-(4-(phenyl(4-vinylphenyl)amino)phenyl)acrylic
Acid
(TPA2). 4,40-
(phenylazanediyl)dibenzaldehyde (0.5 g, 1.7 mmol) and 2-cyanoacetic acid (0.31 g,
3.7 mmol) were added to 15 mL of glacial acetic acid and refluxed for 3 hr in the
presence of 0.28g(3.7 mmol) of ammonium acetate. After cooling to room
temperature, the mixture was poured into ice water. The precipitate was filtered and
washed by distilled water. The product was purified by column chromatography
1
(ethyl acetate=ethanol ¼ 8:2), resulting in orange powder of TPA2 (305mg, 65%). H
NMR (300 MHz, DMSO-d6): d8.23 (s, 2H), 8.01–8.04(d, 4H), 7.47–7.51(t, 2H),
7.32–7.36(t, 1H), 7.23–7.26(d, 2H), 7.16–7.18 (d, 4H).
3-(4-{[4-(2-Carboxy-2-cyano-vinyl)-phenyl]-phenyl-amino}-phenyl)-2-cyano-acrylic
Acid (TPA3). 4,40,400-(phenylazanediyl)tribenzaldehyde and (0.3 g, 0.3 mmol) and
2-cyanoacetic acid (0.09 g, 1.0 mmol) were added to 15 mL of glacial acetic acid
and refluxed for 3 hr in the presence of 0.08g(1 mmol) of ammonium acetate.
After cooling to room temperature, the mixture was poured into ice water. The
precipitate was filtered and washed by distilled water. The product was purified
by column chromatography (Ethyl acetate=ethanol ¼ 7:3), resulting in dark orange
powder of TPA3 (0.4 g, 85%). 1H NMR (300 MHz, DMSO-d6): d 7.91(s, 3H),
7.77–7.80 (d, 6H), 7.01–7.04 (d, 6H).

58=[474]
H. J. Jo et al.
2.2. Fabrication Method of Dye-Sensitized Solar Cell (DSSC)
The fabrication procedure of DSSC is as follows. The conducting glass substrate
(FTO; TEC8, Pilkington, 8 X=cm2, Thickness of 2.3 mm) was cleaned in ethanol
by ultrasonication. TiO2 pastes were prepared using ethyl cellulose (Aldrich), Lauric
acid (Fluka) and Terpineol (Aldrich). The size of TiO2 particles is ca. 20–30 nm. The
prepared TiO2 paste was coated on the pre-cleaned conducting glass substrate using
doctor-blade, and sintered at 500ꢀC for 30min. The thickness of the sintered TiO2 layer
was measured with Alpha-step IQ surface profiler (KLA Tencor). The TiO2 paste as
scattering layer was re-coated over the sintered layer using ca. 250 nm size of TiO2 par-
ticle and then sintered again at 500ꢀC for 30 min. The prepared TiO2 film was dipped in
0.04M of TiCl4 aqueous solution at 70ꢀC for 30 min. For dye adsorption, the annealed
TiO2 electrodes were immersed in dye solution (0.5 mM of dye in DMF; TPA series or
N3=N719) at 50ꢀC for 3 hr. Pt counter electrodes were prepared by thermal reduction
of thin film formed from 7 mM of H2PtCl6 in 2-propanol at 400ꢀC for 20min. The
dye-adsorbed TiO2 electrode and Pt counter electrode were assembled using
60mm-thick Surlyn (Dupont 1702) as a bonding agent. A liquid electrolyte was intro-
duced through a pre-punctured hole on the counter electrode. The electrolyte was com-
posed of 3-propyl-1-methyl-imidazolium iodide (PMII, 0.7 M), lithium iodide (LiI,
0.2 M), iodine (I2, 0.05 M), and t-butylpyridine (TBP, 0.5 M) in acetonitrile=valeroni-
trile (85:15). The active areas of the dye-adsorbed TiO2 films were estimated by a digital
microscope camera with an image-analysis-software (Moticam 1000).
2.3. Instrumental Analysis
1H NMR spectra were recorded on a Varian Mercury NMR 300 Hz spectrometer.
The redox properties of three dyes were examined using a cyclic voltammetry
(Model:CV-BAS-Epsilon) with 100 mV=s scan rate. The electrolyte solution was
0.10 M tetrabutylammonium hexafluorophosphate (TBAPF6) in freshly dried aceto-
nitrile. The Ag=AgCl and Pt wire (0.5 mm in diameter) electrodes were utilized as
reference and counter electrodes, respectively.
3. Results and Discussion
3.1. Synthesis and Optical Properties of Organic Dyes
The intramolecular electron donor-acceptor type of organic dyes, which are containing
triphenylamine as an electron donor and cyanoacetic acid as an acceptor, were synthe-
sized as shown in Scheme 1. The organic photosensitizers containing multi-electron
acceptor=anchoring groups, TPA2 and TPA3, were synthesized to investigate the
correlation between the number of acceptors and photon-to-current efficiency.
Figure 1 shows the UV-Vis absorption spectra of the synthesized dyes in DMSO
solution (a) and on the TiO2 film (b). It is observed that the molar extinction
coefficient of the dye increases with increasing the number of electron acceptor in
a chromophore. The absorption peaks of the organic dyes adsorbed on the TiO2 sur-
face are found to be slightly blue-shifted, compared with those of solution state. This
blue-shift of absorption peak may be caused by the formation of the H-aggregate of
organic dyes on the TiO2 surface. The optical and electrochemical properties of the
organic dyes are summarized in Table 1.

Organic Photo-Sensitizers Containing Multi-Acceptors
59=[475]
Scheme 1. Synthetic procedures and molecular Structures of organic dyes (a) DMF, POCl3,
1,2-dichloroethane, and (b) cyanoacetic acid, NH4OAc, CH3COOH.
3.2. Electrochemical Properties
Highly efficient DSSC requires that the LUMO level of dye molecules should be
more negative than the conduction band edge of TiO2 to ensure the effective electron
injection from the excited dye molecules to the conduction band of TiO2 film and
that the HOMO level of the dye should be more positive than the redox potential
of Iꢁ3 =Iꢁ to ensure the regeneration of dye molecules [22]. The HOMO levels of
the dye molecules have been estimated from the cyclic voltammetry curves, as shown
in Figure 2. The HOMO levels of TPA2 and TPA3 containing multi-acceptors in a
chromophore show similar energy level with ꢁ5.57 and ꢁ5.54 eV, respectively. Those
Figure 1. (a) Absorption spectra of TPA1, TPA2, and TPA3 dyes dissolved in DMSO, and (b)
absorption spectra of the as-synthesized dyes absorbed on TiO2 films, which were measured in
the diffuse reflectance mode.

60=[476]

Organic Photo-Sensitizers Containing Multi-Acceptors
61=[477]
Figure 2. Cyclic voltammograms of the organic dyes.
values are more negative to compare with that of single acceptor system in TPA1.
The zeroth–zeroth transition E0-0 values have been estimated from the cross-point
of the normalized UV absorption spectra; the LUMO levels of the dyes have been
calculated from the following equation: ELUMO ¼ EHOMO ꢁ E0-0; and the results
are also summarized in Table 1.
3.3. Photovoltaic Performance of the Dyes
The photovoltaic performance of organic dyes and the amount of dyes adsorbed on
the surface of TiO2 in DSSC are summarized in Table 2. For comparison, the
efficiency of DSSC device with N719 dye under the same conditions is also indicated
in Table 2, which is represented the power conversion efficiency of 8.3%. The corre-
sponding photocurrent–voltage curves are shown in Figure 3. The organic dyes con-
taining multi-acceptor, TPA2 and TPA3, show better photovoltaic properties in a
short circuit photocurrent density (Jsc) of 8.7 and 8.0 mAcmꢁ2, compared with that
of TPA1 in 7.1 mAcmꢁ2. The more dye adsorbed on the TiO2 surface, the better Jsc is
observed. In the case of TPA3 with three acceptor moieties in a chromophore, it
shows relatively low open circuit voltage (Voc) to compare with that of TPA2 in
Table 2. DSSC performance data of organic dyesa
Jsc=
Active
Layer
Amount=10ꢁ8
Dye
mAcmꢁ2 Voc=mV FF=% g=% area=cm2 thickn.=mm
mol cmꢁ2
TPA1
TPA2
TPA3
N719
7.1
8.7
8.0
15.6
732.7
740.3
630.0
780.0
70.62 3.7
69.95 4.5
65.73 3.3
68.45 8.3
0.421
0.433
0.393
0.464
7.46
7.35
7.41
6.20
223
815
554
2099
aWith scattering layer (250 nm, 4 mm) þ TiCl4 Treatment on FTO glass, Blocking layer, TiO2
paste : B32, Organic Dye 0.5 mM, PMII (0.7 M) þ LiI (0.2 M) þ I2 (0.05 M) þ TBP (0.5 M) in
ACN=VN ¼ 85:15, Pt electrode solution (7 mM), 60 mm surlyn, Dr.blade (2 T).

62=[478]
H. J. Jo et al.
Figure 3. Photocurrent density versus voltage curves of the organic dyes for DSSC.
the DSSC, which affects the efficiency of DSSC, significantly. It has been reported
that for photo-sensitizers containing protonated carboxylic groups, anchoring
groups are adsorbed the TiO2 surface and hence most of their protons transfer to
the TiO2 [3]. Therefore, the multi-anchoring TPA3 can supply a higher concentration
of the proton onto the TiO2 surface than that of the single-anchoring TPA1. The
high proton concentration gives rise to the larger positive charges on the TiO2 sur-
face, leading to the increasing recombination rate and the lowering Voc value in
DSSC. This is because that the increase in the recombination reaction contribute
to the decrease of the energy gap between the redox couple of iodine=tri-iodide
and the Fermi level, resulting from the lowering the Fermi level of TiO2. The electron
recombination dynamic experiment for the multi-anchoring TPA3 is needed for the
further information.
Figure 4. Spectra of monochromatic incident photon-to-current conversion efficiencies for
DSSC based on the organic dyes.

Organic Photo-Sensitizers Containing Multi-Acceptors
63=[479]
The incident photon-to-current conversion efficiencies (IPCEs) for the DSSCs
have been also detected with light from a 500-W Xe lamp that is focused through
a monochromator onto the photovoltaic cell and their results are shown in
Figure 4. The IPCE values are determined at 5 nm intervals according to Eq. (1) [23].
1240ðev ꢃ nmÞ JscðmA=cm2Þ
IPCE % ðkÞ ¼
ꢂ
ꢂ 100
ð1Þ
kðnmÞ
/ðmW=cm2Þ
The IPCE of organic dyes shows more than 70% in a spectral range from 400 nm
to 500 nm. The highest value 83% is detected in TPA2 at 460 nm, which is good
agreement with the result of Jsc value in Table 2.
4. Conclusions
We have designed and synthesized the organics dyes with multi-electron acceptor
moieties in a chromophore. Effects of the number of electron acceptors in a chromo-
phore and their photovoltaic properties in the DSSC device have been also demon-
strated in this work. Compared to other dyes, the DSSC based on TPA2 shows the
most efficient solar-to-electrical power conversion efficiency, which is the maximum
g value of 4.5% (Voc ¼ 740 mV, Jsc ¼ 8.7 mA=cmꢁ2, FF ¼ 0.68%) under simulated
AM 1.5 irradiation (100 mW=cm2). Also, the larger amount of protonated car-
boxylic groups in the multi-anchoring organic dye, TPA 3, led to the increase in
the recombination reaction, resulting in the decrease of the open-circuit voltage in
DSSC. A study on the electron recombination dynamics of TPA3 and the stability
of DSSC based on the organic dyes are now in progress.
Acknowledgments
This work was supported by National Research Foundation of Korea Grant funded
by the Korean Government (2009-0067142).
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