Organic dyes incorporating 9,10-dihydrophenanthrene moiety for dye-sensitized solar cells

Article abstract of DOI:10.1080/15421406.2020.1743940

A series of new dipolar organic dyes containing 9,10-dihydrophenanthrene has been synthesized as sensitizers for the application in dye-sensitized solar cells. These new materials containing less rigid π–linker as sensitizers exhibit good performance with conversion efficiencies ranging from 1.58percent to 6.21percent reaching 11 ～ 83percent of the ruthenium dye N719-based cell under the same condition.

Full text of DOI:10.1080/15421406.2020.1743940

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Organic dyes incorporating 9,10-
dihydrophenanthrene moiety for dye-sensitized
solar cells
Yung-Sheng Yen, Jui-Ling Hsu, Yung-Tse Cheng, Ying-Chan Hsu & Jiann T. Lin
To cite this article: Yung-Sheng Yen, Jui-Ling Hsu, Yung-Tse Cheng, Ying-Chan Hsu & Jiann T.
Lin (2020) Organic dyes incorporating 9,10-dihydrophenanthrene moiety for dye-sensitized solar
cells, Molecular Crystals and Liquid Crystals, 703:1, 32-38, DOI: 10.1080/15421406.2020.1743940
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2020, VOL. 703, NO. 1, 32–38
https://doi.org/10.1080/15421406.2020.1743940
Organic dyes incorporating 9,10-dihydrophenanthrene
moiety for dye-sensitized solar cells
Yung-Sheng Yena, Jui-Ling Hsua, Yung-Tse Chengb, Ying-Chan Hsub, and
Jiann T. Linb
b
aDepartment of Chemistry, Chung Yuang Christian University, Chung Li, Taiwan; Institute of Chemistry,
Academia Sinica, Nankang, Taipei, Taiwan
KEYWORDS
ABSTRACT
D-p-A; 9,10-
A series of new dipolar organic dyes containing 9,10-dihydrophenan-
threne has been synthesized as sensitizers for the application in dye-
sensitized solar cells. These new materials containing less rigid
p–linker as sensitizers exhibit good performance with conversion effi-
ciencies ranging from 1.58% to 6.21% reaching 11ꢀ 83% of the
ruthenium dye N719-based cell under the same condition.
dihydrophenanthrene; dye-
sensitized solar cells;
organic photosensitizers
Introduction
Dye-sensitized solar cells (DSSCs) have attracted intensive attention due to their rela-
tively high power conversion efficiency and low-cost converting of solar energy to clean
electricity [1]. The photosensitizers clearly play a crucial role in DSSCs. Photosensitizers
can be categorized into three classes: ruthenium (Ru)-based organometallic dyes, zinc-
porphyrins sensitizers and metal-free organic sensitizers composed of donor–p–bridge
acceptors (D–p–A). To date, the best cell efficiencies of the ruthenium complexes, Zn-
porphyrins and metal-free organic dyes have achieved 11.8% [2], 13.0% [3] and 13.0%
[4], respectively. However, the high cost and limited availability of ruthenium resources
as well as the complex synthetic procedures and purification difficulty for zinc-porphy-
rin dyes restrict their further practical application in DSSCs.
Recently, metal-free organic sensitizers have been intensively investigated owing to
their low cost, high molar extinction coefficients and good flexibility of molecular
design [5–7]. Generally, metal-free organic dyes are adopted as a representative donor-
p-bridge-acceptor (D-p-A) architecture, which promotes an efficient photoinduced
intramolecular charge transfer (ICT) properties [8–10].
We have been interested in the development of new dipolar organic dyes featuring
donor-to-accepter configuration bridged by different rigid spacers for solar cell applica-
tions [11]. Incorporating a rigid spacer into D-p-A structure, such as fluorene, facili-
tated effective intramolecular charge separation and led to high conversion efficiencies.
On the contrary, a more rigid spacer is expected to favor electron recombination after
electron injection from the dye to the conduction band of TiO2. We therefore decided
CONTACT Yung-Sheng Yen
ysyen@cycu.edu.tw
Department of Chemistry, Chung Yuan Christian University,
32023 Chung Li, Taiwan.
ß 2020 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
to develop sensitizers incorporating a less rigid congener of fluorene, 9,10-dihydrophe-
nanthrene in the spacer. Herein, we report a new series of dyes with a 9,10-dihydrophe-
nanthrene entity in the conjugated spacer. DSSCs using these dyes as the sensitizers will
be also discussed.
Experimental
Unless otherwise specified, all the reactions were performed under nitrogen atmosphere
using standard Schlenk techniques. All solvents and chemicals were commercially avail-
able and used without further purification (unless specially mentioned), and all reac-
tions dealing with air- or moisture-sensitive compounds were carried out under
nitrogen atmosphere using standard Schlenk techniques. Structural analysis was con-
1
ducted with H NMR and 13C NMR spectra were recorded on a Bruker 300-MHz or
400-MHz spectrometer. Absorption spectra were recorded on a Cary 50 probe UV-Vis
spectrophotometer. Mass spectra (FAB) were recorded on a VG70-250S mass spectrom-
eter. Elementary analyses were performed on a Perkin-Elmer 2400 CHN analyzer.
Cyclic voltammetric (CV) was used to investigate the electrochemical characteristics of
these dyes (Model: CHI-621B).
1
DP-1: H NMR (400 MHz, THF-d8): d 8.36 (s, 1H), 7.85 (d, J ¼ 4.0 Hz, 1H), 7.81 (d,
J ¼ 4.0 Hz, 1H), 7.73 ꢁ 7.65 (m, 2H), 7.61 (s, 1H), 7.58 (d, J ¼ 9.3 Hz, 1H), 7.27 ꢁ 7.23
(m, 4H), 7.11 ꢁ 7.09 (m, 4H), 7.03 ꢁ 6.95 (m, 4H), 2.79 ꢁ 2.76 (m, 2H), 2.52 ꢁ 2.48 (m,
2H). 13C NMR (THF-d8): d 154.61, 148.81, 148.71, 146.65, 140.19, 139.70, 138.61,
136.72, 132.12, 130.14, 129.13, 126.68, 125.85, 125.76, 125.47, 124.97, 124.72, 123.95,
123.54, 122.96, 68.72, 67.23, 67.01, 30.68, 29.85, 29.74, 25.36, 25.16, 24.96. MS (FAB):
m/z 524.16 (calcd [M]þ).
DP-2: 1H NMR (400 MHz, acetone-d6): d 8.03 (s, 1H), 7.85 (d, J ¼ 8.1 Hz, 1H),
7.79 ꢁ 7.72 (m, 5H), 7.35 ꢁ 7.31 (m, 4H), 7.13 ꢁ 7.07 (m, 6H), 6.97 ꢁ 6.94 (m, 2H), 4.84
(s, 2H), 2.94 ꢁ 2.92 (m, 2H), 2.82 ꢁ 2.80 (m, 2H). 13C NMR (THF-d8): 193.0, 167.4,
167.2, 153.6, 148.7, 139.6, 138.6, 137.7, 137.3, 136.4, 132.2, 130.4, 130.1, 129.9, 129.1,
126.5, 126.3, 125.7, 125.6, 125.5, 124.7, 123.9, 123.6, 122.9, 120.7, 45.4, 29.9, 29.8. MS
(FAB): m/z 630.11 (calcd [M]þ).
1
DP-3: H NMR (400 MHz, THF-d8): d 8.34 (s, 1H), 7.82 ꢁ 7.80 (m, 1H), 7.76 ꢁ 7.74
(m, 1H), 7.72 ꢁ 7.68 (m, 2H), 7.30 ꢁ 7.23 (m, 4H), 7.09 (d, J ¼ 8.0 Hz, 4H), 7.02 ꢁ 6.94
(m, 4H), 4.46 ꢁ 4.43 (m, 4H), 2.89 ꢁ 2.77 (m, 2H), 2.75 ꢁ 2.51 (m, 2H). 13C NMR
(THF-d8): d 164.35, 148.73, 141.08, 139.73, 139.24, 138.15, 135.99, 131.03, 130.13,
129.30, 127.06, 126.55, 125.72, 125.45, 124.29, 123.90, 123.60, 123.01, 110.70, 95.67,
29.83. MS (FAB): m/z 582.16 (calcd [M]þ).
1
DP-4: H NMR (400 MHz, acetone-d6): d 8.43 (s, 1H), 7.97 (d, J ¼ 4.1 Hz, 1H), 7.89
(d, J ¼ 8.4 Hz, 1H), 7.81 (d, J ¼ 8.4 Hz, 1H), 7.77 ꢁ 7.72 (m, 7H), 7.40 ꢁ 7.38 (m, 2H),
7.35 (dd, J ¼ 7.3, 1.3 Hz, 1H), 7.31 (dd, J ¼ 3.9, 1.5 Hz, 2H), 7.29 (dd, J ¼ 6.0, 1.2 Hz,
1H), 7.25 (d, J ¼ 1.2 Hz, 2H), 7.16 (dd, J ¼ 8.1, 2.1 Hz, 2H), 7.08 (dd, J ¼ 2.1, 8.4 Hz,
1H), 7.05 (d, J ¼ 2.1 Hz, 1H), 2.95 ꢁ 2.80 (m, 4H), 2.02 ꢁ 1.82 (m, 8H), 0.34 (t,
J ¼ 7.3 Hz, 12H). 13C NMR (THF-d8): d 164.13, 154.67, 152.08, 150.49, 149.13, 148.14,
146.67, 142.45, 140.22, 139.61, 138.54, 138.12, 136.77, 135.96, 132.06, 128.92, 127.82,
127.30, 126.70, 125.86, 125.72, 124.95, 124.69, 124.51, 123.53, 123.42, 122.68, 121.33,

34
Y.-S. YEN ET AL.
Figure 1. Structure of the dyes.
120.16, 119.99, 116.70, 99.70, 68.58, 56.99, 33.49, 29.95, 29.76, 9.00. MS (FAB): m/z
812.34 (calcd [M]þ).
Assembly of DSSCs
The photoanode used was the TiO2 thin film (15 lm of 20 nm particles as the absorbing
layer and 6 lm of 400 nm particles as the scattering layer) coated on fluorine-doped tin
oxide (FTO, 15X/square) glass substrate [12] with a dimension of 0.5 ꢂ 0.5 cm2. A plati-
nized FTO produced by thermopyrolysis of H2PtCl6 was used as the counter electrode.
The TiO2 thin film was dipped into the THF solution containing 3 ꢂ 10ꢁ4 M of dye
sensitizers for at least 12 h. After rinsing with CH3CN, the photoanode adhered with a
polyester tape of 30 lm in thickness and with a square aperture of 0.36 cm2 was placed
on the top of the counter electrode and the two were tightly clipped them together to
form a cell. Electrolyte was then injected into the void space of the cell, which was then
R
V
sealed with the Torr Seal cement (Varian, MA, USA). The electrolyte was composed
of 0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tert-butylpyridine dissolved
in acetonitrile.
Results and discussion
The structures of the new dyes are shown in Fig. 1. The new organic dyes were synthe-
sized by similar procedures, and only the synthetic route of DP-3 was illustrated in
Scheme 1. DP-1 was prepared according to published procedure for comparison pur-
poses [13]. Two was obtained from Pd(dba)2 catalyzed Buchwald-Hartwig cross-cou-
pling reactions of one with the diphenylamine. The Suzuki-Miyaura cross-coupling
reaction of
3
with 7-bromo-2,3-dihydrothieno[3,4-b][1,4]dioxine-5-carbaldehyde

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
Scheme 1. Synthesis routes for DP-3. (i) Pd(dba)2, diphenylamine, t-BuONa, DPPF, toluene (ii) (a) n-
BuLi, (b) B(OMe)3, (c) 2 N HCl (iii) 7-bromo-2,3-dihydrothieno[3,4-b][1, 4]dioxine-5-carbaldehyde,
Pd(PPh3)4, 2 M Na2CO3 (iv) cyanoacetic acid, NH4OAc, acetic acid.
afforded 4. Finally, Dp-3 was obtained by Knoevenagel condensation of four with cya-
noacetic acid in the acetic acid in the presence of ammonium acetate.
The absorption spectra of the dyes in THF solutions are shown in Fig. 2. All of the
dyes exhibited two prominent transition bands in THF, the one below 400 nm is attrib-
ꢃ
uted to the p ꢁ p transition of the conjugate molecules and the other ranging from
ꢃ
410 ꢁ 450 nm is attributed to the more delocalized p ꢁ p transition possibly with intra-
molecular charge transfer character. Rhodanine-3-acetic acid derivative DP-2 leads, as
expected, to the most pronounced bathochromic shift both in the absorption because of
the greater conjugation of the 4-oxo-2-thioxothiazolidine ring compared with the cyano-
acetic group. In consistence with Ko’s observation [14], DP-4 with the strongest elec-
tron donor, bis-diethylfluorenylamine, has the largest kabs value compared to that of
DP-1. When the dyes were absorbed on TiO2 surface(Fig. 2b), the absorption spectra of
these dyes are broadened and slightly blue-shifted to that of in solutions, indicating
deprotonation of the carboxylic acid [15,16] or H-aggregation of the dyes on TiO2 sur-
face [17,18].
Photovoltaic devices
The photovoltaic performance statistics under a solar condition (AM 1.5) illumination
are collected in Table 1. Fig. 3a shows the photocurrent-voltage (J-V) curves of the cells.
The incident photon-to-current conversion efficiencies (IPCE) of the dyes on TiO2 are
plotted in Fig. 3b. Except for DP-2, the incident photon-to-current conversion efficien-
cies (IPCE) of the dyes exceed 65% in the 400 to 500 nm region. The importance of
light-harvesting to the cell efficiency is also supported by the high conversion efficiency
of DSSC based on DP-4 in spite of the relatively shorter sR value. Comparison of the
devices with dyes DP-1 and DP-2 shows that the photovoltaic performances are signifi-
cantly declined upon replacing cyanoacetic acid with rhodanine-3-acetic acid, with the
power conversion efficiency of the solar cell based on cyanoacetic dye DP-1 being three
times higher than that of rhodanine dye DP-2. It was due to lack of conjugation
between the rhodanine ring and the carboxyl anchoring group for organic dyes [19].

36
Y.-S. YEN ET AL.
Figure 2. Absorption of the dyes (a) in the THF solution (b) absorbed on TiO2 film.
Table 1. Performance parameters of DSSCs constructed using the dyes.
Dye
VOC (V)
JSC (mA/cm2)
g (%)
FF
sR(ms)a
DP-1
DP-2
DP-3
DP-4
N719
0.65
0.54
0.67
0.68
0.72
12.69
4.63
11.92
14.24
15.64
5.29
1.58
5.33
6.21
7.43
0.64
0.64
0.67
0.65
0.66
9.1
7.7
6.4
4.1
8.9
Experiments were conducted using TiO2 photoelectrodes with approximately 18 lm thickness and 0.25 cm2 working
area on the FTO (15 X/sq.) substrates.
asR is lifetime of an electron measured by transient photovoltage for DSSCs constructed using the dyes.
Figure 3. (a) Incident photon-to-current conversion efficiency spectra s and (b) current densityꢁ volt-
age curve for the DSSCs.
The electrochemical impedance spectroscopy of the DSSCs was further used to ana-
lyze the resistance of the DSSCs. Fig. 4a shows the Nyquist plots of DSSCs with differ-
ent dyes under dark condition by applying a voltage of ꢁ0.50 V. The radius of the
larger semicircle decreases in the order DP-3 > DP-4 > DP-2 > DP-1, indicating
increases electron recombination rate in the order of DP-3 < DP-4 < DP-2 < DP-1. Its
means that the electron recombination from the conduction band to electrolyte occurs
more difficulty. The Bode phase plots are also shown in Fig. 4b. The difference in Bode
phase plots occurs in the lower frequency lying in the range of 1 ꢁ 102 Hz means a lon-
ger electron recombination lifetime if regeneration of the oxidized sensitizer is assumed

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
37
Figure 4. Electrochemical impedance spectra of DSSC for dyes measured in the dark: (a) Nyquist and
(b) Bode plots.
to be fast. This results approximately are in agreement with the observed shifted in the
VOC values.
Conclusions
We have synthesized dipolar compounds containing arylamine unit bridged by 9,10-
dihydrophenanthrene conjugated spacer. DSSCs using these sensitizers exhibited effi-
ciencies ranging from 11–82% under one sun (AM 1.5) illumination. We have also
studied that rhodanine-3-acetic acid is not a good anchoring group because of the bro-
ken conjugation between the acceptor and anchor from the intervening methylene
group. The bis-diethylfluorenyl amino-containing dye DP-4 has the best cell perform-
ance, and the conversion efficiency reaches 6.20%, which is approximately 83% of the
standard cell based on N719.
Funding
We acknowledge the support of the Ministry of Science and Technology of Taiwan.
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