Novel coumarin sensitizers based on 2-(thiophen-2-yl)thiazole π-bridge for dye-sensitized solar cells

Article abstract of DOI:10.1039/c5pp00216h

2-(Thiophen-2-yl)thiazole was introduced as a π-bridge into diethylamino coumarin and novel coumarin sensitizers were synthesized with cyanoacrylic acid or rhodanine acetic acid as electron acceptor. Their light-harvesting capabilities and photovoltaic performance were investigated and compared with those of a similar sensitizer bearing a phenylthiophene bridge. Replacement of benzene in the π-bridge with a thiazole ring contributes to the improvement of the light-harvesting capability and hence superior J_SC. 7-Diethylamino coumarin dye with a 2-(thiophen-2-yl)thiazole bridge and cyanoacrylic acid acceptor shows the most efficient photoelectricity conversion efficiency which has the maximum value of 4.78% (V_OC = 690 mV, J_SC = 9.79 mA cm^-2, and ff = 0.71) under simulated AM 1.5 G irradiation (100 mW cm^-2).

Full text of DOI:10.1039/c5pp00216h

View Article Online
View Journal
Photochemical &
Photobiological
Sciences
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Han, R. Kang, X.
Zu, Y. Cui and J. Gao, Photochem. Photobiol. Sci., 2015, DOI: 10.1039/C5PP00216H.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/pps

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 1 of 8
View Article Online
DOI: 10.1039/C5PP00216H
Journal Name
ARTICLE
Novel coumarin sensitizers based on 2ꢀ(thiophenꢀ2ꢀyl)thiazole πꢀ
bridge for dyeꢀsensitized solar cell
Received 00th January 20xx,
Accepted 00th January 20xx
Liang Han, Rui Kang, Xiaoyan Zu, Yanhong Cui and Jianrong Gao*
2ꢀ(Thiophenꢀ2ꢀyl)thiazole was introduced as a πꢀbridge into diethylamino coumarin and novel coumarin sensitizers were
synthesized with cyanoacrylic acid or rhodanine acetic acid as electron acceptor. Their lightꢀharvesting capabilities and
photovoltaic performance were investigated and compared with those of the similar sensitizer bearing phenylthiophene
bridge. Replacement of benzene in the πꢀbridge with thiazole cycle contributes to the improvement of lightꢀharvesting
capability and hence superior JSC. 7ꢀDiethylamino coumarin dye with 2ꢀ(thiophenꢀ2ꢀyl)thiazole bridge and cyanoacrylic acid
acceptor shows the most efficient photoelectricity conversion efficiency which has the maximum value of 4.78% (VOC = 690
mV, JSC = 9.79 mA/cm2, and ff = 0.71) under simulated AM 1.5 G irradiation (100 mW/cm2).
DOI: 10.1039/x0xx00000x
www.rsc.org/
rarity of ruthenium metal in the earth’s crust, tricky synthesis,
1.Introduction
and purification steps.
Therefore, metalꢀfree organic
sensitizing dyes have called great attention for their large
diversity in the molecular structure, simple preparation, and
purification process at lower cost.11 Typical metalꢀfree organic
sensitizers are based on a donorꢀπ bridgeꢀacceptor system (Dꢀπꢀ
A) to achieve effective charge separation and transfer. Many
electronꢀrich compounds have been found to be the desirable
donor for metalꢀfree organic sensitizers, such as triarylamine,12
coumarin,13 indoline,14,15 carbazole,16 and phenothiazine/
phenoxazine.17,18
Coumarin derivatives are characterized with good
photoresponse in the visible region, good longꢀterm stability,
and appropriate lowest unoccupied molecular orbital (LUMO)
levels matching with the conduction band of TiO2, which brings
coumarin compound a good candidate for the donor of the
sensitizer. Hara designed a series of coumarin dyes with an
optimal power conversion efficiency of 8.2% which ranks the
highest among the reported coumarin sensitizers.19 Since
introduction of additional donor or acceptor is a better choice to
optimize the absorption spectrum than a flexible extended πꢀ
bridge, Kim reported novel DꢀAꢀπꢀA coumarin dye containing
benzothiadiazole chromophore20 and Yu et al. synthesized
branched coumarin dyes based on a 2DꢀπꢀA system to prevent
πꢀaggregation and improve the photovoltaic performance.21
However, those coumarin sensitizers feature the coumarin
donors substituted by rigid nitrogenꢀcontaining cycle which are
prepared through redundant synthetic process.
The development and utilization of solar energy have been one
of the most important scientific and technological challenges in
recent years. The solar cell is a promising approach to convert
light into electrical energy through photovoltaic process. 1 The
commercially available solar cells are mainly composed of
inorganic silicon semiconductors which need silicon with great
purity and accordingly high cost. 2 Therefore, hybrid solar cells
characteristic of low material cost, easy manufacturing process
and flexibility in the scaling processes, appear to be a good
alternative to crystalline siliconꢀbased photovoltaics.3,4 In
various hybrid solar cells, dye sensitized solar cells (DSSC)
have attracted extensive interests and are on the verge of
commercialization.5
In DSSC, sensitizing dyes play
achievement of high efficiency and longꢀterm durability.
a critical role in the
A
sensitizer is a dye with a broad absorption spectrum and a high
molar extinction coefficient, which has strong interaction with
semiconductor surface through functionality. 6ꢀ8 A few Ru (II)
complexes have been reported to be the sensitizers with high
photoelectric conversion efficiency. The most famous Ru (II)
complexes are bis(tetrabutylammonium)ꢀcisꢀdi(thiocyanato)ꢀ
bis(4,4’ꢀdicarboxyꢀ2,2’ꢀbipyridine)ruthenium (II) (N719 dye)
and trithiocyanatoꢀ(4,4’,4’’ꢀtricarboxyꢀ2,2’:6’,2’’ꢀterpyridine)
ruthenium (II) (black dye), achieving a high photoelectric
conversion yield (η) of > 11% under AM 1.5 G irradiation.9,10
However, Ru (II) complexes suffer the disadvantages of the
In our previous work, we have reported several coumarin
sensitizers characteristic of easily available 7ꢀdiethylamine
coumarin donor. Compound ZXꢀ1 bearing the diethylamino
coumarin donor, thiophene πꢀbridge and cyanoacrylic acid
acceptor was synthesized and achieved the photoelectric
conversion efficiency 3.52%.22 Based on this, we modified the
πꢀconjugation system and selected benzene, biphenyl and
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 2 of 8
View Article Online
DOI: 10.1039/C5PP00216H
ARTICLE
Journal Name
phenylthiophene as πꢀbridge to synthesize WHBꢀseries FTO glass was first cleaned in a detergent solution using an
coumarin sensitizers.23 The photovoltaic test indicates that ultrasonic bath for 15 min, and then rinsed with water and
phenylthiophene bridge favors the photovoltage improvement ethanol. For the fabrication of photoelectrodes, two different
compared with thiophene bridge. However, introduction of the titanium nanoxide pastes were deposited by screenꢀprinting,
bulky phenyl group weakens the coplanarity of whole molecule resulting in the TiO2 electrodes (0.16 cm2, with light scattering
and hence the electron transfer, which is not good for the anatase particles). The total thickness of TiO2 film was 18 ꢁm.
photocurrent. Therefore, WHBꢀ5 with the diethylamino The TiO2 electrodes were gradually heated under an air flow at
coumarin donor, phenylthiophene πꢀbridge and cyanoacrylic 325 °C for 5 min, at 375 °C for 5 min, and at 450 °C for 15 min,
acid acceptor gives a litter better power conversion efficiency and finally, at 500 °C for 15 min. After being cooled to 80 °C,
3.62%. To improve the electron transfer and the power the TiO2 electrode was immersed into the dye solution in a
conversion efficiency, we further changed the phenyl unit with mixture of MeOH and CHCl3 (VMeOH: VCHCl3 = 1:10) and kept
an electronꢀwithdrawing thiazole. Herein, novel coumarin at room temperature for 24 h to assure complete dye uptake,
sensitizers with 2ꢀ(thiophenꢀ2ꢀyl)ꢀthiazole πꢀbridge (Fig. 1
,
which were then rinsed with MeOH to remove excess dye.
ZXYꢀa and ZXYꢀb) were synthesized and better efficiency Meanwhile, the counter electrodes were prepared by screenꢀ
4.78% was achieved with this molecular engineering.
printing a 50 nm Pt layers on the cleaned FTO plates. Open
cells were fabricated in air by clamping the different
photoelectrodes with platinized counter electrodes. The
electrolyte used here is composed of the CH3CN solution of 0.3
M 1ꢀmethylꢀ3ꢀpropylimidazolium iodide (MPII), 0.03 M I2,
0.07 M LiI, 0.1 M guanidinium thiocyanate and 0.4 M 4ꢀtertꢀ
butylpyridine (TBP). The active area of DSSCs was 0.16 cm2.
2.3 Photovoltaic characterization
Photovoltaic performance of the DSSCs was evaluated at AM
1.5 G illumination (100 mW/cm2; PeccellꢀL15, Peccell, Japan)
using a Keithley digital source meter (Keithley 2601, USA).
The incident light intensity was 100 mW/cm2 calibrated with a
standard Si solar cell (BSꢀ520, Japan). The action spectra of
Fig. 1 Structure of the coumainꢀbased dyes
monochromatic
incident
photonꢀtoꢀcurrent
conversion
efficiency (IPCE) for solar cell were performed by using a
commercial setup (PECꢀS20 IPCE Measurement System,
Peccell, Japan). Electrochemical impedance spectroscopy (EIS)
experiments were conducted using
a computerꢀcontrolled
potentiostat (ZeniumZahner, Germany). The measured
frequency ranged from 100 mHz to 1 MHz while the AC
amplitude was set to 10 mV. The bias of all EIS measurements
was set to the VOC of the corresponding dyes. The cyclic
voltammetry were recorded with
a computer controlled
electrochemical analyzer (IviumStat, Holland) at a constant
scan rate of 100 mV/s. Measurement were performed with a
three electrode cell in 0.1
M
Bu4NClO4 N,Nꢀ
dimethylformamide solution, Pt wire as a counter electrode and
an Ag/AgCl reference electrode and calibrated with ferrocene.
The HOMO energy versus vacuum (EHOMO) is obtained though
the equation: EHOMO= ꢀ [EOX + 4.7 eV].24
Scheme 1 Syntheses of ZXYꢀseries dyes
2.4 The detailed experimental procedures and characterization
data
2.4.1 Synthesis of 3ꢀacetylꢀ7ꢀdiethylamino coumarin (I)
2. Experimental sections
Piperidine (1 mL) was added in the solution of 4ꢀdiethylamino
salicylaldehyde (19.3 g, 100 mmol) and ethyl acetoacetate (18.5
g, 150 mmol) in ethanol (200 mL) and the reactant was refluxed
for 5 h. The solvent was removed and the residue was
recrystallized with ethanol to give yellow solid (21.3 g, 82.3%).
m.p. 150ꢀ153 °C [ Ref.23 150ꢀ152 °C]
2.1 Materials and instruments
Nuclear magnetic resonance spectra were recorded on Bruker
Avance III 500 MHz and chemical shifts were expressed in
ppm using TMS as an internal standard. Mass spectra were
measured using a Waters Xevo QꢀTof Mass Spectrometer.
Absorption spectra were measured with SHIMADZU (model
UV2550) UV–Vis spectrophotometer. Fluorescence spectra
2.4.2 Synthesis of 3ꢀbromoacetylꢀ7ꢀdiethylamino coumarin (II)
After the addition of hydrobromide (4.5 mL
，40.2 mmol) in
were
obtained
on
a
SHIMADZU
RFꢀ5301PC
the solution of compound (5.2 g, 20.1 mmol) in acetic acid
I
spectrofluorometer.
(100 mL), liquid bromine (1.1 mL, 20.1 mmol) was slowly
2.2 Fabrication of DSSCs
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 3 of 8
View Article Online
DOI: 10.1039/C5PP00216H
Journal Name
ARTICLE
added to the mixture. The reactant was stirred overnight and m.p. 249~250 ºC; 1H NMR (500 MHz, DMSOꢀ
then filtrated. The filter cake was washed with ethanol and 1H, COOH), 8.28 (s, 1H, thiazoleꢀH), 8.01 (dd,
d
J
6) δ: 13.49 (bs,
= 3.5, 1.0 Hz,
= 5.0, 1.1
= 9.0 Hz, 1H, coumarinꢀH),
= 9.0,
= 2.3 Hz, 1H, coumarinꢀH),
= 7.0 Hz, 4H, CH2CH3),
J = 7.0 Hz, 6H, CH2CH3); HRꢀESIꢀMS for
used directly in the next step without further purification.
1H, thiopheneꢀH), 7.97 (s, 1H, CH=C), 7.93 (dd, J
2.4.3 Synthesis of 7ꢀdiethylaminoꢀ3ꢀ(2ꢀ(thiophenꢀ2ꢀyl)thiazolꢀ4ꢀ Hz, 1H, thiopheneꢀH), 7.63 (d,
J
yl)coumarin (III
Thiopheneꢀ2ꢀcarbothioamide (1.5 g, 10.7 mmol) was added in 2.4 Hz, 1H, coumarinꢀH), 6.65 (d,
the solution of compound II (3.6 g, 10.7 mmol) in ethanol (100 4.73 (s, 2H, CH2COOH), 3.50 (q,
)
7.29 (dd, J = 5.0, 3.8 Hz, 1H, coumarinꢀH), 6.80 (dd, J
J
J
mL) and the reactant was refluxed for 2 h. After being filtrated, 1.17 (t,
the filter cake was recrystallized with ethanol to afford yellow C26H21N3O5S4: Found: 582.0294 [MꢀH]ꢀ; Calcd. 583.0364
1
solid (2.5 g, 61.2%). m.p. 169~172 ºC; H NMR (500 MHz,
CDCl3) δ: 8.74 (s, 1H, thiazoleꢀH), 8.38 (s, 1H, coumarinꢀH),
3. Results and Discussion
7.59 (d,
thiopheneꢀH), 7.46 (d,
(m, 1H, thiopheneꢀH), 7.13 (dd,
H), 7.05 (d, = 8.4 Hz, 1H, coumarinꢀ8ꢀH), 3.31 (q,
4H, CH2CH3), 1.14 (t, = 7.1 Hz, 6H, CH2CH3).
2.4.4 Synthesis of 5ꢀ(4ꢀ(7ꢀ(diethylamino)ꢀ2ꢀoxoꢀ2Hꢀchromenꢀ3ꢀ
yl)thiazolꢀ2ꢀyl)ꢀthiopheneꢀ2ꢀcarbaldehyde (IV
J
= 2.7 Hz, 1H, thiopheneꢀH), 7.54 (d,
= 4.1 Hz, 1H, coumarinꢀH), 7.42ꢀ7.39
= 5.0, 3.7 Hz, 1H, coumarinꢀ
= 7.0 Hz,
J = 8.5 Hz, 1H,
J
3.1 Synthesis
J
Diethylaminosalicylaldehyde reacted with ethyl acetoacetate to
afford 3ꢀacetylꢀ7ꢀdiethylaminocoumarin I which was brominated at
the acetyl group to produce the bromide II. The thiazole ring was
formed through Hantzsch synthesis between the bromide II and
thiopheneꢀ2ꢀcarbothioamide to give 7ꢀdiethylaminoꢀ3ꢀ(2ꢀ(thiophenꢀ
2ꢀyl)thiazolꢀ4ꢀyl)coumarin III. The aldehyde group was introduced
into the thiophene of compound III through Vilsmeier reaction and
then condensed with cyanoacetic acid or rhodanine acetic acid to
yield the target compounds ZXYꢀa and ZXYꢀb.
J
J
J
)
After the addition of POCl3 (5 mL), DMF (5 mL) solution was
stirred for 0.5 h and then was added into the solution of
compound III (2.5 g, 6.6 mmol) in DMF (30 mL). After stirred
for 8 h at 60 ºC, the mixture was poured into ice water (300 mL)
and adjusted to pH = 7 with 10% NaOH solution. After being
filtrated, the filter cake was washed with water and ethanol and
dried to afford yellow solid (1.22 g, 45.1%). m.p. 215~216 ºC;
1H NMR (500 MHz, CDCl3) δ: 10.19 (s, 1H, CHO), 8.44 (s, 1H,
3.2 Optical properties
The UV absorption of ZXYꢀa and ZXYꢀb in CH3OHꢀCHCl3
cosolvent were shown in Fig. 2(a) and the corresponding data were
listed in Table 1 with those of WHBꢀ5 for comparison. ZXYꢀa and
ZXYꢀb display two distinct maximum absorption peaks: one occurs
at about 400 nm and is caused by πꢀπ* transition; another peak
locates at 460ꢀ490 nm and may be attributed to intramolecular
charge transfer (ICT) from the diethylamino coumarin donor to the
acceptor bearing carboxyl group through 2ꢀ(thiophenꢀ2ꢀyl)thiazole
πꢀbridge. Due to the expansion of πꢀconjugation system, ZXYꢀb
with rhodanine acetic acid acceptor exhibits longer maximum
absorption wavelength and larger molar extinction coefficient than
ZXYꢀa with cyanoacrylic acid acceptor. The ICT absorption peak
of ZXYꢀa shifts bathochormically when compared to that of WHBꢀ5,
which indicates the replacement of the phenyl ring with the electronꢀ
withdrawing thiazole ring, favors the intramolecular charge transfer.
thiazoleꢀH), 7.68 (dd,
(dd, = 5.0, 1.0 Hz, 1H, thiopheneꢀH), 7.42 (d,
thiopheneꢀH), 7.15 (dd, = 5.0, 3.7 Hz, 1H, coumarinꢀH), 6.66
(dd, = 8.9, 2.5 Hz, 1H, coumarinꢀH), 6.55 (d, = 2.4 Hz, 1H,
= 7.1 Hz, 4H, CH2CH3), 1.25 (t, = 7.1
J
= 3.8, 1.0 Hz, 1H, coumarinꢀH), 7.53
J
J= 9.0 Hz, 1H,
J
J
J
coumarinꢀH), 3.47 (q,
Hz, 6H, CH2CH3)
J
J
2.4.5 Synthesis of 2ꢀcyanoꢀ3ꢀ(5ꢀ(4ꢀ(7ꢀ(diethylamino)ꢀ2ꢀoxoꢀ2Hꢀ
chromenꢀ3ꢀyl)ꢀthiazolꢀ2ꢀyl)thiophenꢀ2ꢀyl)acrylic acid (ZXY-a
)
Compound IV (0.2 g, 0.5 mmol) and cyanoacetic acid (0.085 g,
1.0 mmol) were dissolved in acetonitrile (5 mL) and then
piperidine (0.01 mL, 0.1 mmol) was added. The reactant was
refluxed for 6 h and cooled. After the filtration, the filter cake
was washed with acetonitrile (20 mL×3) and then recrystallized
with CHCl3 to give dark red solid (150 mg, 62.9%). m.p.
1
241~243 ºC; H NMR (500 MHz, DMSOꢀ
d
6) δ: 8.29 (s, 1H,
= 3.7, 1.0 Hz,
= 5.0, 0.9 Hz, 1H, thiopheneꢀH),
= 9.0 Hz, 1H, coumarinꢀH), 7.29 (dd, = 5.0, 3.8 Hz,
1H, coumarinꢀH), 6.81 (dd, = 9.0, 2.4 Hz, 1H, coumarinꢀH),
6.64 (d, = 2.3 Hz, 1H, coumarinꢀH), 3.50 (q, = 7.0 Hz, 4H,
CH2CH3), 1.17 (t, = 7.0 Hz, 6H, CH2CH3); HRꢀESIꢀMS for
CH=CCN), 8.27 (s, 1H, thiazoleꢀH), 7.98 (dd,
1H, thiopheneꢀH), 7.96 (dd,
7.62 (d,
J
J
J
J
J
J
J
J
C24H19N3O4S2: Found: 476.0749 [MꢀH]ꢀ; Calcd. 477.0817.
2.4.6 Synthesis of 2ꢀ(5ꢀ((5ꢀ(4ꢀ(7ꢀ(diethylamino)ꢀ2ꢀoxoꢀ2Hꢀ
chromenꢀ3ꢀyl)thiazolꢀ2ꢀyl)thiophenꢀ2ꢀyl)methylene)ꢀ4ꢀoxoꢀ2ꢀ
thioxthiazolidinꢀ3ꢀyl)acetic acid (ZXY-b
)
Compound IV (0.3 g, 0.73 mmol) and rhodanine acetic acid
(0.28 g, 1.46 mmol) were dissolved in acetonitrile (7 mL) and
then piperidine (0.01 mL, 0.1 mmol) was added. The reactant
was refluxed for 6 h and cooled. After the filtration, the filter
cake was washed with acetonitrile (20 mL × 3) and then
recrystallized with CHCl3 to give dark red solid (170 mg, 40 %).
Fig.2 UV spectra of coumarin sensitizing dyes
(a) in CH3OHꢀCHCl3 cosolvent(VCH3OH:VCHCl3=1:10);
(b) on sensitized TiO2 electrode (1.0 ꢁm in thickness)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 4 of 8
View Article Online
DOI: 10.1039/C5PP00216H
ARTICLE
Table 1 Optical properties of coumarin sensitizing dyes
Journal Name
level. As shown in Table 2, when thiazole ring was introduced to
replace the benzene ring, the HOMOꢀ1 and LUMO energy level
were lowered down while the LUMO energy level was reduced more
than the HOMOꢀ1 energy level. Since the transition from the
HOMOꢀ1 to LUMO plays the leading role after the photoexcitation
in these coumarin sensitizers (vide infra), the Egap between the
HOMOꢀ1 and LUMO is actually reduced by the introduction of the
thiazole ring. Therefore, the replacement of benzene by thiazole
benefits the molecular coplanarity and small energy gap, which leads
to longer maximum absorption wavelength of ZXYꢀa than that of
WHBꢀ5.
a
b
c
λmax
ε a
λmax
λmax
Compd
/ (nm)
/ (Mꢀ1·cmꢀ1)
/ (nm)
/ (nm)
ZXYꢀa
401, 460
45333, 23433
400
581
ZXYꢀb
WHBꢀ5
413, 489
438
50800, 34167
37300
405
432
607
500
a
maximum absorption wavelength λmax and molar extinction
coefficient at λmax of dyes measured in CH3OHꢀCHCl3 cosolvent (V
CH3OH: VCHCl3=1:10)；b maximum absorption wavelength λmax of
dyes on sensitized TiO2 electrodes; c maximum emission wavelength
measured in CH3OHꢀCHCl3 cosolvent (V CH3OH: VCHCl3=1:10)
Table 2 Energy level data of ZXYꢀa and WHBꢀ5 at the B3LYP/6ꢀ
31G(d) level
Generally, dye molecules are deprotonated on TiO2 film to form a
carboxylateꢀTiO2 unit which may facilitate the charge injection and
at the same time lead to the blue shift of the absorption peak.
Moreover, an occurrence of dye aggregation on TiO2 film induces
the broadening of the absorption spectrum and the blue shift (Hꢀ
aggregate) or red shift (Jꢀaggregate) of the absorption peak. Fig. 2(b)
listed the absorption spectra of ZXYꢀa and ZXYꢀb on TiO2 film and
the hypsochromical shift of the maximum absorption peak was
observed. On the other hand, the onset of the absorption spectra has
been pushed only about 50 nm further while the whole spectra
region appears not be broadened on TiO2 film when compared with
that in solution. Therefore, we speculate the hypsochromical shift of
the absorption peak may be mainly ascribed to the deprotonation of
the carboxylic acid which lowers the electronꢀwithdrawing ability of
the carboxylic acid and accordingly weakens the ICT.
EHOMOꢀ1
/ eV
EHOMO
eV
/
ELUMO
eV
/
Egap∗ /
eV
Molecule
ZXYꢀa
WHBꢀ5
∗Egap= ELUMO ꢀ EHOMOꢀ1
ꢀ6.17
ꢀ6.09
ꢀ5.40
ꢀ5.42
ꢀ2.93
ꢀ2.70
3.24
3.39
Fig. 4 listed frontier molecular orbitals of coumarin dyes. As can be
seen, the HOMOs of ZXYꢀsensitizers are mostly delocalized at the
coumarin donor and the LUMOs are mainly composed of the
electronꢀwithdrawing acceptor, while the HOMOꢀ1 of ZXYꢀ
sensitizers are delocalized over the whole molecules. Since the
overlap of the HOMOꢀ1 with LUMO is better than that of the
HOMO, the electron transition after photoexcitation from the
HOMOꢀ1 to LUMO in ZXYꢀsensitizers may be easier than that from
the HOMO to LUMO.
3.3 Computational studies
Moreover, the energy gap of ZXYꢀa and ZXYꢀb between the
HOMO and LUMO shown in Fig. 5 are 2.47 eV and 2.41 eV,
respectively, and the difference 0.06 eV is too small to meet the
difference 29 nm of the experimental absorption data. On the other
hand, the energy gap of ZXYꢀb between the HOMOꢀ1 and LUMO is
2.95 eV and smaller than that of ZXYꢀa 3.24 eV, which agrees with
the redꢀshifted maximum absorption of the former in Fig. 2.
Therefore, it also indicates that the electronic transition of ZXYꢀ
sensitizers after the photoexcitation is mainly caused by the
transition from the HOMOꢀ1 to LUMO.
To gain an insight into the relationship between the structure and the
photophysical properties, the groundꢀstate geometries of coumarin
dyes have been optimized by DFT with the Gaussian package, using
the hybrid B3LYP functional and the standard 6ꢀ31G(d) basis set.
The fully optimized geometrical structure of ZXYꢀa is shown in Fig.
3 and also listed that of WHBꢀ5 for comparison. In WHBꢀ5
molecule, large deviation from coplanarity between the coumarin
and benzene is observed. The presence of the benzene ring induces
large distortion and weakens the conjugation of the whole molecule.
With the replacement of benzene by thiazole of small size, ZXYꢀa
molecule exhibits improved planarity configuration, which favors
the intramolecular charge transfer and accordingly the light
absorptivity. On the other hand, the introduction of the electronꢀ
withdrawing thiazole may contribute to the adjustment of energy
Fig.3 Optimized geometries (side view) of ZXYꢀa and WHBꢀ5 at
the B3LYP/6ꢀ31G(d) level
Fig. 4 Frontier molecular orbitals (HOMOꢀ1, HOMO, LUMO, and
LUMO+1) of coumarin dyes
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 5 of 8
View Article Online
DOI: 10.1039/C5PP00216H
Journal Name
ARTICLE
injection into the conduction band of TiO2.25, 26 When compared with
WHBꢀ5, ZXYꢀa also displays superior IPCE spectrum, which
demonstrates again the replacement of benzene with thiazole favors
charge separation and collection.
Fig. 5 The energy level of the frontier molecular orbitals of
coumarin dyes at B3LYP/6ꢀ31G(d)
To further explain the electronic transition after the photoexcitation,
the TDDFT calculation is performed at the B3LYP/6ꢀ31G(d) level
based on the above optimized groundꢀstate geometries of coumarin
dyes and the related data are listed in Table 3. Two most important
electronic transitions are described for each compound: one is the
electronic transition with longest absorption wavelength and the
other electronic transition has the largest oscillator strength. The
absorption peak (408.91 nm) of ZXYꢀa with the maximum oscillator
strength corresponds to S0→S2 transition, and the electrons mainly
transfer from the HOMOꢀ1 to LUMO. The calculated UV
absorption of ZXYꢀb with the maximum oscillator strength centers
at 447.73 nm corresponding to S0→S3 transition, which is also
composed of the electron transfer from the HOMOꢀ1 to LUMO. The
results of TDDFT calculation accord with both the suggestion of the
frontier molecular orbitals and the experimental absorption data.
3.4 Photovoltaic performance
Fig. 6 IPCE curve for DSSCs based on coumarin dyes with 18.0 ꢁm
nanocrystalline TiO2 electrodes
The JꢀV curves of DSSC based on coumarin sensitizing dyes are
shown in Fig. 7 and the corresponding parameters JSC, VOC, ff and η
are listed in Table 4. With a better IPCE spectrum, ZXYꢀa has a
shortꢀcircuit photocurrent density of 9.79 mAꢂcmꢀ2, well above of
0.97 mAꢂcmꢀ2 of ZXYꢀb and 7.72 mAꢂcmꢀ2 of WHBꢀ5. In addition,
the openꢀcircuit photovoltage of ZXYꢀa is higher than that of ZXYꢀ
b due to the effective electron injection efficiency of the former.
Therefore, the optimal photovoltaic performance of 4.78% came
from DSSC based on ZXYꢀa, which exhibited a JSC value of 9.79
mA/cm2, VOC value of 0.69 V, and ff value of 0.71.
The IPCE action spectra of DSSCs based on coumarin dyes have
been measured under AM 1.5 G irradiation (100 mW/cm2) and are
shown in Fig. 6. ZXYꢀa shows a broad IPCE response from 350 to
550 nm with a maximum value of 73% in the plateau region. IPCE is
mainly influenced by the light harvesting ability and the overall
electron injection efficiency. As can be seen, when compared with
ZXYꢀa, ZXYꢀb displays a little broader absorption spectra on TiO2
film, which indicates ZXYꢀb has similar or even better light
harvesting ability than ZXYꢀa. However, Fig. 6 shows that ZXYꢀb
exhibits distinctly inferior IPCE action spectra to ZXYꢀa, which
indicates the weak electron injection efficiency of the former. In fact,
the presence of methylene group between the carboxylic acid and
rhodanine cycle of ZXYꢀb breaks the conjugation system of the
whole molecule, which weakens the overlap of the electron density
of the dyes with the semiconductor and deteriorates the electron
Fig. 7 Currentꢀpotential curve (JꢀV) for DSSCs based on coumarin
dyes
Table 3 Electronic transition data of coumarin dyes obtained at the TDDFT//B3LYP/6ꢀ31G(d) level
Molecule
Electronic transitions
S0→S1
λabs (nm)
571.79
408.91
580.60
447.73
f
Main conf. coefficient*
HOMO→LUMO
0.1415
1.2492
0.2390
1.2192
0.70386
0.63367
0.70259
0.68545
ZXYꢀa
S0→S2
HOMOꢀ1→LUMO
HOMO→LUMO
HOMOꢀ1→LUMO
S0→S1
ZXYꢀb
S0→S3
*Main conf. coefficient stands for the main composition and the corresponding coefficient of the electron transition after the molecule
absorbs the ultravioletꢀvisible light.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 6 of 8
View Article Online
DOI: 10.1039/C5PP00216H
Journal Name
ARTICLE
Table 4 Parameters for DSSCs based on coumarin sensitizing dyes:
openꢀcircuit photovoltage VOC, shortcircuit photocurrent density JSC
,
fill factor ff, solar energyꢀtoꢀelectricity conversion efficiency η.
VOC
(V)
JSC
Compd
ZXYꢀa
ff
η%
4.78
(mA·cmꢀ2)
0.69
0.50
0.66
9.79
0.96
7.72
0.71
0.69
0.71
ZXYꢀb
WHBꢀ5
0.33
3.62
Electrochemical impedance spectroscopy (EIS) measurement
provides the charge transfer resistance information of the TiO2ꢀdyeꢀ
electrolyte interface. The EIS measurement of ZXYꢀsensitizers is
evaluated in dark and the Nyquist and Bode spectra are shown in
Fig. 8. In the dark under forward bias, electrons are transported
ꢀ
through the mesoscopic TiO2 network and react with I3 and
Fig. 8 Electrochemical impedance spectra for DSSCs based on
coumarin sensitizing dyes (a) Nyquist plot; (b) Bode plot; (c)
equivalent circuits
simultaneously Iꢀ is oxidized to I3 at the counter electrode (CE).27
ꢀ
The reaction resistance of DSSCs with a constant phase element
(CPE) and resistance (R) is obtained through an equivalent circuit
shown in Fig. 8(c) using the software (ZSimpWin). In the
equivalent circuit, the chargeꢀtransfer resistances at the
photoanode/electrolyte interface and CE are denoted by Rct and Rce
cyanoacrylic acid acceptor. Large resistance leads to hard charge
recombination, which should contribute to the improvement of VOC
.
respectively, while Rs represents the overall series resistance. The In fact, ZXYꢀb sensitized DSSC shows lower VOC than ZXYꢀa
corresponding parameters Rs and Rct are listed in Table 5. Due to
the same electrolyte and electrode in the materials and surface area,
similar series resistance are observed in these DSSCs. Generally, the
semicircle in the frequency 1ꢀ1k Hz in the Nyquist diagram
corresponds to the electron transfer at the TiO2/electrolyte
interface.28 The larger the radius of the semicircle, the higher the
charge transfer resistance. Larger radius in the Nyquist diagram
relates to smaller frequency in the Bode diagram, which means
charge recombination is difficult to happen. Compared with ZXYꢀa
sensitized DSSC, ZXYꢀb sensitized DSSC displays bigger
semicircle in the Nyquist diagram and smaller frequency in the Bode
diagram, which means it has larger Rct than ZXYꢀa as shown in
Table 5. This result was similar to our previous work22 and the
resistance of the DSSC based on ZXYꢀb bearing rhodanine acetic
acid acceptor is larger than that of ZXYꢀa bearing
sensitized DSSC, which was caused by the inferior charge injection
efficiency of the former as suggested by its low JSC, since VOC is
mainly determined by both the electron injection efficiency and
charge recombination degree.
Table 5 Parameters obtained by fitting the impedance spectra of
DSSCs based on coumarin sensitizers using the equivalent circuit
Compd
Rs (ꢃ)
Rct (ꢃ)
ZXYꢀa
6.929
6.337
467.3
509.1
ZXYꢀb
Conclusions
Two novel coumarin sensitizers were synthesized with
diethylamino coumarin as the electron donor, 2ꢀ(thiophenꢀ2ꢀ
yl)thiazole as the πꢀbridge and cyanoacrylic acid or rhodanine
acetic acid as the electron acceptor. Compared with the similar
compound with phenylthiophene πꢀbridge, the replacement of
benzene by thiazole cycle favors intramolecular charge transfer
and helps the red shift of the maximum absorption. The JSC of
ZXYꢀa was improved to 9.79 mA/cm2 from 7.72 mA/cm2 of
WHBꢀ5 when shared with the same electron donor and
acceptor while just different in πꢀbridge. In these coumarin
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 6
Please do not adjust margins

Please do not adjust margins
Photochemical & Photobiological Sciences
Page 7 of 8
View Article Online
DOI: 10.1039/C5PP00216H
Journal Name
ARTICLE
14. B. Liu, B. Wang, R. Wang, L. Gao, S. H. Huo, Q. B. Liu, X. Y.
Li and W. H. Zhu, Influence of conjugated πꢀlinker in DꢀDꢀπꢀA
indoline dyes: towards longꢀterm stable and efficient dyeꢀ
sensitized solar cells with high photovoltage, J. Mater. Chem.
sensitizers, the photovoltaic performance was mainly
influenced by the electron injection and ZXYꢀb with rhodanine
acetic acid acceptor displayed poor JSC and VOC though with
better absorption spectrum and less charge recombination.
Therefore, with the replacement of benzene by thiazole, the
photoelectricity conversion efficiency was increased to 4.78%.
A
, 2014, 2, 804ꢀ812
15. W. H. Zhu, Y. Z. Wu, S. T. Wang, W. Q. Li, X. Li, J. Chen, Z.
S. Wang and H. Tian, Organic DꢀAꢀ π ꢀA solar cell sensitizers
with improved stability and spectral response, Adv. Funct.
Mater. 2011, 21, 756ꢀ763
16. L. Han, X. Y. Zu, Y. H. Cui, H. B. Wu, Q. Ye and J. R. Gao,
Acknowledgements
Novel DꢀAꢀπꢀA carbazole dyes containing benzothiadiazole
chromophores for dyeꢀsensitized solar cells, Org. Electron.
2014, 15, 1536ꢀ1544.
,
The authors gratefully acknowledge for funding: National
Natural Science Foundation of China (21406202); National
Natural Science Foundation of China (21176223); and Natural
Science Foundation of Zhejiang province (LY13B020012).
17. Y. Hua, S. Chang, J. He, C. S. Zhang, J. Z. Zhao, T. Chen, W.
Y. Wong, W. K. Wong and X. J. Zhu, Molecular engineering of
simple phenothiazineꢀbased dyes to modulate dye aggregation,
charge recombination, and dye regeneration in highly efficient
dyeꢀsensitized solar cells, Chem. Eur. J., 2014, 20, 6300ꢀ6308.
18. H. J. Tan, C. Y. Pan, G, Wang, Y. Y. Wu, Y. P. Zhang, Y. P.
Zou, G. P. Yu and M. Zhang, Phenoxazineꢀbased organic dyes
with different chromophores for dyeꢀsensitized solar cells, Org.
Electron., 2013, 14, 2795ꢀ2801.
19. Z. S. Wang, Y. Cui, Y. Danꢀoh, C. Kasada, A. Shinpo and K.
Hara, Thiopheneꢀfunctionalized coumarin dye for efficient dyeꢀ
sensitized solar cells:ꢄ electron lifetime improved by
coadsorption of deoxycholic acid, J. Phys. Chem. C, 2007, 111,
7224ꢀ7230.
20. K. D. Seo, I. T. Choi, Y. G. Park, S. Kang, J. Y. Lee, H. K.
Kim, Novel DꢀAꢀπꢀA coumarin dyes containing low bandꢀgap
chromophores for dyeꢀsensitised solar cells, Dyes Pigm., 2012,
94, 469ꢀ474.
21. B. Liu, R. Wang, W. Mi, X. Li and H. Yu, Novel branched
coumarin dyes for dyeꢀsensitized solar cells: significant
improvement in photovoltaic performance by simple structure
modification, J. Mater. Chem., 2012, 22, 15379ꢀ15387.
22. L. Han, X. Zhou, Q. Ye, Y. J. Li and J. R. Gao. Synthesis and
photoelectric properties of coumarin type sensitizing dyes,
Chin. J. Org. Chem., 2013, 33, 1000ꢀ1004.
23. L. Han, H. B. Wu, Y. H. Cui, X. Y. Zu, Q. Ye and J. R. Gao,
Synthesis and density functional theory study of novel
coumarinꢀtypedyes for dye sensitized solar cells, J. Photochem.
Photobio. A, 2014, 290, 54ꢀ62.
24. Y. Wang, Z. Xie, G. Gotesman, L. Wang, B. P. Bloom, T. Z.
Markus, D. Oron, R. Naaman and D. H. Waldeck.
Determination of the electronic energetics of CdTe nanoparticle
assemblies on Au electrodes by photoemission, electrochemical,
and photocurrent studies, J. Phys. Chem. C, 2012, 116, 17464ꢀ
17472.
25. B. Liu, W. Q. Li, B. Wang, X. Y. Li, Q. B. Liu, Y. Naruta and
W. H. Zhu, Influence of different anchoring groups in indoline
dyes for dyeꢀsensitized solar cells: Electron injection,
impedance and charge recombination, J. Power Sources, 2013,
234, 139ꢀ146.
26. J. Wiberg, T. Marinado, D. P. Hagberg, L. C. Sun, A. Hagfeldt
and B. Albinsson, J. Phys. Chem. C, 2009, 113, 3881ꢀ3886.
27. Q. Wang, J. E. Moser and M. Grätzel, Electrochemical
impedance spectroscopic analysis of dyeꢀsensitized solar cells,
J. Phys. Chem. B, 2005, 109, 14945ꢀ14953.
Notes and references
1. R. W. Miles, K. M. Hynes and I. Forbes, Photovoltaic solar
cells: An overview of stateꢀofꢀtheꢀart cell development and
environmental issues, Prog. Cryst. Growth Ch., 2005, 51, 1ꢀ42.
2. C. Strümpel, M. McCann, G, Beaucarne, V. Arkhipoc, A.
Slaoui, V. Švrček, C. del Canizo and I. Tobias, Modifying the
solar spectrum to enhance silicon solar cell efficiencyꢀAn
overview of available materials, Sol. Energy Mat. Sol. C., 2007,
91, 238ꢀ249.
3. M. Grätzel, Recent advances in sensitized mesoscopic solar
cells, Accounts Chem. Res., 2009, 42, 1788ꢀ1798.
4. Y. Wang, K. Liu, P. Mukherjee, D. A. Hines, P. Santra, H. Y.
Shen, P. Kamat and D. H. Waldeck, Driving charge separation
for hybrid solar cells: photoꢀinduced hole transfer in conjugated
copolymer and semiconductor nanoparticle assemblies, Phys.
Chem. Chem. Phys., 2014, 16, 5066ꢀ5070.
5. A. Hafeldt, G. Boschloo, L. C. Sun, L. Kloo and H. Pettersson,
Dyeꢀsensitized solar cells, Chem. Rev., 2010, 110, 6595ꢀ6663.
6. BꢀG. Kim, K. Chung and J. Kim, Molecular design principle of
allꢀorganic dyes for dyeꢀsensitized solar cells, Chem. Eur. J.
,
2013, 19, 5220ꢀ5230.
7.
Y. Z. Wu, W. H. Zhu, Organic sensitizers from DꢀπꢀA to DꢀAꢀ
πꢀA: effect of the internal electronꢀwithdrawing units on
molecular absorption, energy levels and photovoltaic
performances, Chem. Soc. Rev., 2013, 42, 2039ꢀ2058.
8. J. N. Clifford, E. NartinezꢀFerrero, A. Viterisi and E.
Palomares, Sensitizer molecular structureꢀdevice efficiency
relationship in dye sensitized solar cells, Chem. Soc. Rev., 2011,
40, 1635ꢀ1646.
9. H. Ozawa, Y. Okuyama, H. Arakawa, Dependence of the
efficiency improvement of blackꢀdyeꢀbased dyeꢀsensitized solar
cells on alkyl chain length of quaternary ammonium cations in
electrolyte solutions, Chemphyschem, 2014, 15, 1201ꢀ1206.
10. C. F. Chi, S. C. Su, I. P. Liu, C. W. Lai and Y. L. Lee. Charge
transfer and performance enhancement of dyeꢀsensitized solar
cells by utilization of a tandem structure, J. Phys. Chem. C
2014, 118, 17446ꢀ17451
,
11. A. Mishra, M. K. R. Fischer and P. Bäuerle, Metalꢀfree organic
dyes for dyeꢀsensitized solar cells: from structure: property
relationships to design rules, Angew. Chem. Int. Ed., 2009, 48,
2474ꢀ2499.
12. A. Baheti, K. R. J. Thomas, C. P. Lee, C. T. Li and K. C. Ho.
Organic dyes containing fluorenꢀ9ꢀylidene chromophores for
efficient dyeꢀsensitized solar cells, J. Mater. Chem. A, 2014, 2,
5766ꢀ5779
28. K. Pan, Y. Z. Dong, C. G. Tian, W. Zhou, G. H. Tian, B. F.
Zhao and H. G. Fu, TiO2ꢀB narrow nanobelt/TiO2 nanoparticle
composite photoelectrode for dyeꢀsensitized solar cells,
Eletrochimica Acta, 2009, 54, 7350ꢀ7356.
13. X. G. Liu, J. M. Cole, P. G. Waddel, T. C. Lin, J. Radia and Z.
Zeidler, Molecular origins of optoelectronic properties in
coumarin dyes: toward designer solar cell and laser
applications, J. Phys. Chem. A, 2012, 116, 727ꢀ737
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Photochemical & Photobiological Sciences
Page 8 of 8
View Article Online
DOI: 10.1039/C5PP00216H
Novel coumarin sensitizers based on
2ꢀ(thiophenꢀ2ꢀyl)thiazole πꢀbridge for dyeꢀsensitized solar cell
Liang Han, Rui Kang, Xiaoyan Zu, Yanhong Cui, Jianrong Gao ∗
(College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310032)
14
12
10
8
COOH
NC
S
N
O
O
N
S
ZXY-a
η
η
=4.78%
WHB-5
=3.62%
6
N
O
O
4
NC
COOH
S
2
0
0.0
0.2
0.4
0.6
0.8
Voltage / V
Diethylaminocoumarin dye with 2ꢀ(thiophenꢀ2ꢀyl)thiazole bridge and cyanoacrylic
acid acceptor was synthesized and showed the photoelectricity conversion efficiency
of 4.78% (VOC = 690 mV, JSC = 9.79 mA/cm2, and ff = 0.71) under simulated AM 1.5
irradiation (100 mW/cm2).
∗ Corresponding author. Phone: +86 0571 88320891; Email: gjrarticle@163.com