D-A-π-A featured sensitizers containing an auxiliary acceptor of benzoxadiazole: Molecular engineering and co-sensitization

Article abstract of DOI:10.1039/c5ta01371b

Two novel D-A-π-A organic sensitizers (WS-24 and WS-26) incorporating a benzoxadiazole (BOD) unit are synthesized for dye sensitized solar cells. An additional n-hexylthiophene unit is incorporated into WS-26 to decrease unfavourable dye aggregation, thus

Full text of DOI:10.1039/c5ta01371b

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DOI: 10.1039/C5TA01371B
Journal Name
RSCPublishing
ARTICLE
D–A–π–A featured sensitizers containing auxiliary
acceptor of benzoxadiazole: molecular engineering and
co-sensitization†
Cite this: DOI: 10.1039/x0xx00000x
Haibo Zhu,‡ Yongzhen Wu,‡ Jingchuan Liu, Weiwei Zhang, Wenjun Wu and Weiꢀ
Hong Zhu*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Two novel DꢀAꢀπꢀA organic sensitizers (WSꢀ24 and WSꢀ26
(BOD) unit are synthesized for dye sensitized solar cells. An additional
)
incorporating a benzoxadiazole
ꢀhexylthiophene unit is
www.rsc.org/
n
incorporated in WSꢀ26 to decrease unfavourable dye aggregation, thus suppressing charge
recombination and increasing the solubility. Incorporating an auxiliary acceptor of benzoxadiazole
(BOD) into πꢀbridge can red shift absorption bands, and sharply decreases the LUMO level. In
virtue of coꢀsensitization, the WSꢀ26 based DSSC device can achieve as high as 9.72% in
photovoltaic efficiency.
In our previous study, we have found that the absorption spectra
of DꢀAꢀπꢀA dyes can regularly shift to long wavelength when
1 Introduction
increasing the electronꢀwithdrawing ability of auxiliary acceptor.
Among the benzoheterocycle based auxiliary acceptors, BTD shows
higher electronꢀwithdrawing ability than that of BTZ and QN, thus
Dye sensitized solar cells (DSSCs) have attracted considerable
attention due to their low cost and easy fabrication since the initial
work by Grätzel’s group.1 The sensitizers play an important role and
can be divided into two categories: metal complexes and metalꢀfree
organic dyes.2 Up to now, the ruthenium–polypyridine and zinc–
porphyrin based sensitizers show the champion performances, with
power conversion efficiencies (PCE) around 11%~13%.3,4 During
the past decades, metalꢀfree organic dyes usually constructed with a
donorꢀπ bridgeꢀacceptor (DꢀπꢀA) configuration have been
intensively studied owing to their lower cost, easy purification and
more flexibility in molecular design.5,6 For further improving the
photovoltaic efficiency, it is necessary to extend and broaden the
absorption spectra in DꢀπꢀA organic dyes. In this respect, our group
has proposed a novel DꢀAꢀπꢀA concept for designing novel organic
sensitizers which incorporate an electronꢀwithdrawing group as
auxiliary acceptor into the π bridge.7 As demonstrated, the additional
acceptor not only decreases the HOMOꢀLUMO energy gap, but also
redistributes the charge density across the dye backbone, being
capable of facilitating the photoꢀinduced charge separation and
stabilizing the electronꢀrich donor (such as indoline unit). A series of
electronꢀwithdrawing building blocks, such as benzothiadiazole
resulting in
a broader lightꢀharvesting in BTDꢀbased dyes.
According to the knowledge from polymer science,13 upon
exchanging sulphur atom with an oxygen atom, the resulting
benzoxadiazole (BOD) can induce higher electronꢀwithdrawing
ability than BTD unit. In this regard, incorporation of BOD unit in
DꢀAꢀπꢀA based sensitizers should be preferable to the light
harvesting. Moreover, compared with BTD unit, the oxygen atom in
BOD is expected to have less interaction with I2 and/or I3− ions than
the sulphur atom, which is beneficial to retard the electron
recombination.14 However, the sensitizer containing BOD unit used
in DSSCs has rarely reported.15
O
N
N
COOH
CN
O
N
O
O
WS-24
COOH
CN
S
N
O
N
N
COOH
CN
O
S
(BTD),8
benzotriazole
(BTZ),9
quinoxaline
(QN),10
N
S1
diketopyrrolopyrrole (DPP)11 and other units12 have been exploited
as the auxiliary acceptor. For instance, a structural engineering on
the donor of quinoxaline based DꢀAꢀπꢀA dye (YA422) in
combination with a fine device optimization leads to a high
efficiency of 10.65%.10e A systematic study on the πꢀbridge of DPP
based DꢀAꢀπꢀA dyes greatly improves the efficiency up to 10%.11c
C6H13
WS-26
Fig. 1 Chemical structures of WS-24, WS-26, and co-sensitizer dye S1.
Herein we reported two BOD based DꢀAꢀπꢀA dyes WSꢀ24 and
WSꢀ26 (Fig. 1) bearing furan as the linker, cyanoacrylic acid as the
acceptor moiety and the indoline as the donor (Scheme S1 in ESI†).
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DOI: 10.1039/C5TA01371B
One more nꢀhexylthiophene unit was incorporated in WSꢀ26 to electrochemical workstation (Zahner IM6e). The spectra were
decrease unfavourable dye aggregation, thus suppressing charge scanned in a frequency range of 0.1 Hz ꢀ 100 kHz at room
recombination and increasing the solubility.16 Interestingly, WSꢀ26 temperature under a series of applied bias potential with a magnitude
based DSSCs exhibit a 38% increment in the power conversion of the alternative signal of 10 mV and characterized using ZꢀView
efficiency (PCE) with respect to that of WSꢀ24, which can be software (Solartron Analytical).
rationalized by the longer electron lifetime in the former device.
Furthermore, we found that dye WSꢀ26 is more compatible with a
known coꢀsensitizer S115 in a coꢀsensitization test.17 After coꢀ
sensitization with S1, the PCE of WSꢀ24 based device slightly
decreased from 6.21% to 5.87%, while the WSꢀ26 based device
increased from 8.61% to 9.72%.
2.3 Cell Fabrication
A doubleꢀlayer TiO2 photoelectrode (a 12 µm thick nanoporous layer
and a 4 mm thick scattering layer), was prepared by screen printing
on conducting glass substrate following the reported procedure.18
A
dyeꢀloaded TiO2 electrode was dipped in the solution of dye with 3 ×
10–4 M concentration in chloroform / methanol (v/v, 4/1) for 12 h,
rinsed with ethanol and dried. The coꢀsensitized cells were prepared
by immersing a TiO2 film in the solution of WSꢀ24 or WSꢀ26 for 12
h, and then were immersed into S1 with 3 × 10–4 M concentration in
a mixture of chloroform /methanol (v/v, 4/1) for 1 h. The dyeꢀ
deposited TiO2 film and a platinumꢀcoated conducting glass were
separated by a Surlyn spacer (40 µm thick) and sealed by heating the
polymer frame. In this work, 0.5 M 1ꢀbutylꢀ3ꢀmethylimidiazolium
iodide (BMII), 0.1 M 1,2ꢀdimethylꢀ3ꢀpropylimidazolium iodide
(DMPII), 0.1 M LiI, 0.05 M I2, 0.1 M guanidinium thiocyanate
(GuSCN) and 0.5 M tertꢀbutylpyridine (TBP) in 85/15 mixture of
acetonitrile and valeronitrile were used as the redox electrolyte.
2 Experimental
2.1 Characterization
1H and 13C NMR spectra were recorded on Bruker AM 400
spectrometer with tetramethylsilane (TMS) as internal standard.
HRMS were performed using
spectrometer. The UV−vis spectra were measured with a Varian
Cary 500 spectrophotometer. The cyclic voltammograms (CV) were
determined with
(Princeton Applied Research) using a threeꢀelectrode cell with a Pt
working electrode, Pt wire counter electrode, and regular calomel
a Waters LCT Premier XE
a Versastat II electrochemical workstation
reference electrode in
tetrabutylammonium hexafluorophosphate (TBAPF6) was used as
the supporting electrolyte in DMF.
a
saturated KCl solution, 0.1
M
2.4 Materials and Synthesis
19
Bromoꢀsubstituted
dibromobenzo[ ][1,2,5]oxadiazole
according to the established references. 4ꢀFormylfuranboronic
acid (c), 2ꢀ(4ꢀhexylthiophenꢀ2ꢀyl)ꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀ
indoline
(
(b)
a
)
and
4,7ꢀ
20
c
were synthesized
2.2 Cell device fabrication and characterizations
Photocurrent density−voltage (I−V) curves were obtained by
dioxaborolane (d) were purchased from J&K Scientific, Ltd., and
used as received without further purification. THF was preꢀdried
over 4 Å molecular sieves and distilled under argon atmosphere from
sodium benzophenone ketyl immediately prior to use. All other
solvents and chemicals used in this work were of reagent grade and
used without further purification.
illuminating the cell through the FTO substrate from the
photoanode side under standard AM 1.5 conditions with a solar
simulator (WXSꢀ155Sꢀ10). The photocurrent action spectra
were measured with a CEPꢀ2000 system (BunkohꢀKeiki Co.
Ltd.).
Electrochemical impedance spectroscopy (EIS) for DSSCs was
performed using
a twoꢀelectrode system under dark with
Scheme 1 Synthetic routes to WS-24 and WS-26. Reagents and conditions: (i) n-BuLi, -78 ºC, B(OCH3)3, THF; (ii) Pd(PPh3)4, K2CO3, THF; (iii) cyanoacetic acid,
piperidine, acetonitrile; (iv) NBS, dichloromethane (DCM), 0 ºC.
Synthesis of compound 1. To a solution of compound
4.87 mmol) in dry THF (30 mL) in a dried schlenk tube was to this solution. The reaction mixture was stirred at the same
added ꢀBuLi (2.4 mL, 5.76 mmol) in hexane dropwise at ꢀ78 temperature for 4 h, then gradually warmed up to room
°C, under argon in dark. After stirring for 2 h at this temperature and used for the next Suzuki coupling reaction
a (1.60 g, temperature, B(OCH3)3 (0.7 mL, 6.25 mmol) was added slowly
n
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5TA01371B
without purification. In a threeꢀneck roundꢀbottom flask was / MeOH = 20 / 1,
dissolved compound
V
/
V
) on silica gel to yield the product as
b
(1.60 g, 5.76 mmol) in THF (20 mL), a purple black powder 115 mg, yield 66.8%. 1H NMR (400 MHz,
aqueous solution of 2 M K2CO3 (10 mL) and Pd(PPh3)4 (150 DMSOꢀd6, ppm): δ 7.90ꢀ7.98 (m, 5 H), 7.48 (d, J = 3.6 Hz, 1 H),
mg, 0.13 mmol) were added. After the solution was heated at 7.41 (d, J = 3.6 Hz, 1 H), 7.25 (d, J = 8.4 Hz, 2 H), 7.21 (d, J = 8.4
80°C, the unpurified mixture prepared above was added slowly. Hz, 2 H), 6.92 (d, J = 8.5 Hz, 1 H), 4.95 (m, 1 H), 3.89 (m, 1 H),
The reaction mixture was stirred for an additional 8 h. After 2.30 (s, 3 H), 2.01 (m, 1 H), 1.65ꢀ1.87 (m, 4 H), 1.41 (m, 1 H). 13
C
cooling, water was added and the reaction mixture was NMR (100 MHz, DMSOꢀd6, ppm): δ 162.82, 151.02, 149.44,
extracted with CH2Cl2. The combined organic layer was 148.66, 148.40, 146.52, 138.94, 135.61, 133.91, 131.47, 129.82,
washed with brine, dried over anhydrous Na2SO4, and 128.71, 127.44, 125.89, 124.40, 124.04, 121.93, 120.17, 118.43,
evaporated under reduced pressure. The crude product was 114.22, 113.49, 113.49, 106.80, 68.44, 44.43, 34.87, 32.93, 23.89,
purified by column chromatography (CH2Cl2 / petroleum = 1 / 20.40. HRMS (m/z): [M + H]+ calcd. for (C32H25N4O4), 529.1876;
8,
V
/
V
) on silica gel to yield red powder 1.30 g, yield 59.9%. 1H found, 529.1874.
NMR (400 MHz, CDCl3, ppm):
δ
7.78 (s, 1 H), 7.72 (dd, J1 = 8.4
Synthesis of compound 3. Compound 3 was obtained as a red solid
by similar procedure to that for compound 2, but with compound c
instead of compound b, 240 mg, yield 67%. 1H NMR (400 MHz,
CDCl3, ppm): δ 7.94 (d, J = 1.1 Hz, 1 H), 7.85 (s, 1 H), 7.77 (dd, J1
= 8.4, J2 = 1.8 Hz, 1 H), 7.60 (d, J = 7.4 Hz, 1 H), 7.48 (d, J = 7.4
Hz, 1 H), 7.23 (d, J = 8.6 Hz, 2 H), 7.18 (d, J = 8.4 Hz, 2 H), 6.99 (s,
1 H), 6.98 (d, J = 8.7 Hz, 1 H), 4.87 (m, 1 H), 3.92 (m, 1 H), 2.68 (t,
J = 7.6 Hz, 2 H), 2.35 (s, 3 H), 2.16ꢀ2.03 (m, 1 H), 2.01ꢀ1.88 (m, 2
H), 1.86ꢀ1.74 (m, 1 H), 1.74ꢀ1.63 (m, 3 H), 1.61ꢀ1.56 (m, 1 H), 1.44ꢀ
1.30 (m, 6 H), 0.90 (t, J = 6.9 Hz, 3 H). 13C NMR (100 MHz, CDCl3,
ppm): δ 149.14, 148.29, 144.91, 139.80, 137.96, 135.70, 132.11,
Hz, J2 = 2.0 Hz, 1 H), 7.63 (d, J = 7.5 Hz, 1 H), 7.34 (d, J = 7.5 Hz,
1 H), 7.22 (d, J = 7.1 Hz, 2 H), 7.18 (d, J = 7.1 Hz, 2 H), 6.95 (d, J =
8.4 Hz, 1 H), 4.88 (m, 1 H), 3.91 (m, 1 H), 2.35 (s, 3 H,), 2.10 (m, 1
H), 1.94 (m, 2 H), 1.79 (m, 1 H), 1.68 (m, 1 H), 1.61 (m, 1 H). 13C
NMR (100 MHz, CDCl3, ppm): δ 150.14, 149.59, 148.91, 139.60,
135.82, 134.60, 132.37, 130.16, 129.91, 128.27, 125.41, 124.58,
124.18, 120.79, 107.38, 104.50, 69.43, 45.27, 35.32, 33.57, 24.40,
20.87. HRMS (m/z): [M + H]+ calcd. for (C24H21N3O79Br),
446.0868, found 446.0866; [M + H]+ calcd. for (C24H21N3O81Br),
448.0848, found 448.0847.
Synthesis of compound 2. To a 100 mL threeꢀneck roundꢀ 129.87, 129.58, 128.49, 128.07, 126.71, 125.59, 125.08, 124.57,
bottom flask was added compound (300 mg, 0.67 mmol), 120.93, 120.60, 107.47, 69.38, 45.34, 35.28, 33.64, 31.70, 30.66,
1
THF (30 mL), a aqueous solution of 2 M K2CO3 (10 mL), 4ꢀ 30.45, 29.04, 24.42, 22.65, 20.85, 14.13. HRMS–ESI (m/z): [M +
formylfuranboronic acid (380 mg, 2.70 mnol) and Pd(PPh3)4 H]+ calcd. for (C34H36N3OS), 534.2579, found: 534.2579.
(120 mg, 0.1 mmol) were added. The mixture was refluxed for
Synthesis of compound 4. To a 100 mL threeꢀneck roundꢀ
12 h under argon. After cooling, THF was removed under
reduced pressure. Water was added and the reaction mixture
was extracted with CH2Cl2. The combined organic layer was
washed with brine, dried over anhydrous Na2SO4, and
evaporated under reduced pressure. The crude product was
purified by column chromatography (CH2Cl2 / petroleum = 1 /
bottom flask was added compound (220 mg, 0.41 mmol),
3
CH2Cl2 (40 mL). The DCM (20 mL) solution of NBS (110 mg,
0.62 mmol) was added via a syringe at 0 °C. The mixture was
stirred for 6 h under this temperature. Water was added, the
organic layer was separated and removed under reduced
pressure. The crude product was purified by column
chromatography (CH2Cl2 / PE = 1 / 4) on silica gel to yield red
solid 227 mg, yield 90.1%. 1H NMR (400 MHz, CDCl3, ppm): δ
7.84 (s, 1 H), 7.74ꢀ7.79 (m, 2 H), 7.49 (d, J = 7.4 Hz, 1 H), 7.46 (d, J
= 7.4 Hz, 1 H), 7.22 (d, J = 8.6 Hz, 2 H), 7.18 (d, J = 8.5 Hz, 2 H),
6.96 (d, J = 8.4 Hz, 1 H), 4.87 (m, 1 H), 3.92 (m, 1 H), 2.62 (t, J =
7.6 Hz, 2 H), 2.35 (s, 3 H), 2.17ꢀ2.02 (m, 1 H), 2.00ꢀ1.89 (m, 2 H),
1.86ꢀ1.74 (m, 1 H), 1.73ꢀ1.62 (m, 3 H), 1.61ꢀ1.50 (m, 1 H), 1.44ꢀ
1.25 (m, 6 H), 0.90 (t, J = 6.9 Hz, 3 H). 13C NMR (100 MHz, CDCl3,
ppm): δ 149.28, 149.09, 148.01, 143.77, 139.71, 137.71, 135.74,
132.21, 129.89, 128.95, 128.92, 128.16, 126.59, 125.35, 124.90,
124.57, 120.67, 119.56, 110.41, 107.44, 69.40, 45.31, 35.29, 33.61,
31.64, 29.74, 29.70, 28.97, 24.41, 22.63, 20.86, 14.12. HRMS–ESI
1,
V/V) on silica gel to yield purple red powder 158 mg, yield
1
51.0%. H NMR (400 MHz, CDCl3, ppm):
δ
9.71 (s, 1 H), 8.08
(d, J = 7.5 Hz, 1 H), 7.89 (s, 1 H), 7.86 (dd, J1 = 8.4 Hz, J2 = 2.0 Hz,
1 H), 7.59 (m, 2 H,), 7.43 (d, J = 3.8 Hz, 1 H), 7.23 (d, J = 7.1 Hz, 2
H), 7.19 (d, J = 7.1 Hz, 2 H), 6.97 (d, J = 8.4 Hz, 1 H), 4.90 (m, 1
H), 3.93 (m, 1 H), 2.36 (s, 3 H), 2.11 (m, 1 H), 1.97 (m, 2 H), 1.79
(m, 1 H), 1.69 (m, 1 H), 1.60 (m, 1 H). 13C NMR (100 MHz, CDCl3,
ppm): δ 177.27, 154.04, 152.15, 149.87, 148.97, 147.14, 139.43,
135.91, 132.56, 129.94, 128.77, 128.35, 124.76, 124.73, 124.45,
120.94, 113.97, 113.65, 107.40, 69.49, 45.25, 35.36, 33.53, 24.38,
20.88. HRMS (m/z): [M + H]+ calcd. for (C29H24N3O3), 462.1818;
found, 462.1820.
Synthesis of dye WSꢀ24. To a 50 mL threeꢀneck roundꢀbottom (m/z): [M + H]+ calcd. for (C34H35N3OS79Br), 612.1684, found:
flask was added compound
(150 mg, 0.33 mmol), cyanoacetic 612.1686; [M + H]+ calcd. for (C34H35N3OS81Br), 614.1664, found:
2
acid (280 mg, 3.3 mmol), acetonitrile (20 mL), piperidine (0.5 614.1686.
mL). The mixture was refluxed for 7 h under argon. After
Synthesis of compound 5. Compound 5 was obtained as a red solid
cooling, the mixture was diluted with CH2Cl2 (50 mL). The
combined organic layer was washed with water and brine, dried
over Na2SO4, and evaporated under reduced pressure. The
crude product was purified by column chromatography (CH2Cl2
by similar procedure to that for compound 2, but with compound 4
(196 mg, 0.32 mmol) instead of compound 1, 134 mg, yield 66.7%.
1H NMR (400 MHz, CDCl3, ppm): δ 9.68 (s, 1 H), 8.00 (s, 1 H),
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Fig. 2 Absorption spectra of WS-24 and WS-26: (a) in chloroform/methanol
mixed solution (v/v = 4/1) and (b) anchored on a transparent TiO2 film.
7.89 (s, 1 H), 7.83 (d, J = 8.5 Hz, 1 H), 7.67 (d, J = 7.5 Hz, 1 H),
7.54 (d, J = 7.4 Hz, 1 H), 7.37 (d, J = 3.7 Hz, 1 H), 7.25 (d, J = 8.5
Hz, 2 H), 7.21 (d, J = 8.5 Hz, 2 H), 7.00 (d, J = 8.4 Hz, 1 H), 6.72 (d,
J = 8.4 Hz, 1 H), 4.91 (m, 1 H), 3.95 (m, 1 H), 2.93 (t, J = 7.7 Hz, 2
H), 2.38 (s, 3 H), 2.11ꢀ2.07 (m, 1 H), 2.09ꢀ1.90 (m, 2 H), 1.89ꢀ1.67
(m, 5 H), 1.53ꢀ1.44 (m, 2 H), 1.44ꢀ1.33 (m, 4 H), 0.93 (t, J = 6.9 Hz,
3 H). 13C NMR (100 MHz, CDCl3, ppm): δ 176.86, 154.48, 151.38,
149.43, 149.09, 148.12, 144.43, 139.64 138.81, 135.79, 132.31,
131.34, 129.90, 129.44, 128.31, 127.45, 125.92, 125.25, 124.80,
124.61, 120.75, 119.34, 109.24, 107.44, 69.43, 45.31, 35.31, 33.59,
31.67, 30.11, 29.92, 29.32, 24.40, 22.64, 20.86, 14.10. HRMS–ESI
(m/z): [M + H]+ calcd. for (C39H38N3O3S), 628.2634, found:
628.2634.
The absorption spectra of two sensitizers in chloroform/methanol
mixed solution (v/v = 4/1) and anchored on a transparent TiO2 film
are shown in Fig. 2, and the detailed parameters are summarized in
Table 1. As expected, both dyes show typical absorption spectra of
DꢀAꢀπꢀA dyes with characteristic three absorption bands in the UV
region and visible region, which is different from the normal DꢀπꢀA
dyes that always show two absorption bands.7b Although WSꢀ26
contains one more nꢀhexylthiophene unit in the πꢀconjugation, the
two dyes display almost the same maximum absorption wavelength
(551 nm vs. 552 nm) as well as threshold. The long wavelength
absorption band is apparently redꢀshifted compared to BTDꢀbased
dye WSꢀ2 (533 nm)7a under the same conditions, which should be
arisen from the stronger electronꢀwithdrawing properties of BOD
unit. The main difference in the absorption spectra of WSꢀ24 and
WSꢀ26 is the middle absorption band, where the latter shows a 50
nm red shift compared to the former. This phenomenon is consistent
with our previous results.16 The two sensitizers displayed the same
trend for the absorption spectra as those in solutions after anchored
Synthesis of dye WSꢀ26. Compound WSꢀ26 was obtained as a deep
red solid by similar procedure to that for compound WSꢀ24, but with
compound 5 (126 mg, 0.20 mmol) instead of compound 2,102 mg,
1
yield 73.4%. H NMR (400 MHz, DMSO, ppm): δ 7.96 (s, 1 H),
7.78 (s, 2 H), 7.72 (d, J = 8.4 Hz, 1 H), 7.59ꢀ7.69 (m, 2 H), 7.54 (d, J
= 3.7 Hz, 1 H), 7.12ꢀ7.26 (m, 4 H), 6.94 (d, J = 3.5 Hz, 1 H), 6.84 (d,
J = 8.5 Hz, 1 H), 4.83ꢀ4.93 (m, 1 H), 3.76ꢀ3.88 (m, 1 H), 2.81 (t, J =
7.3 Hz, 2 H), 2.28 (s, 3 H), 1.92ꢀ2.12 (m, 1 H), 1.67ꢀ1.87 (m, 3 H),
1.52ꢀ1.66 (m, 3 H), 1.15ꢀ1.47 (m, 7 H), 0.86 (t, J = 6.7 Hz, 3 H). 13C
NMR (100 MHz, DMSO, ppm): δ 163.84, 153.57, 148.31, 148.25,
147.38, 147.18, 143.86, 138.97, 137.96, 136.86, 135.42, 131.25,
130.38, 129.75, 128.11, 128.00, 127.67, 125.78, 125.72, 124.03,
123.99, 119.94, 117.75, 115.98, 111.34, 106.74, 96.75, 68.36, 44.41,
34.88, 32.92, 31.08, 29.11, 28.44, 23.91, 22.08, 20.39, 13.90.
HRMS–ESI (m/z): [M+] calcd. for (C42H38N4O4S), 694.2614, found:
694.2621.
onto nanocrystalline TiO2 film (Fig. 2b). They showed
a
hypsochromic shift of about 22 nm upon adsorption, which is
probably due to the deprotonation of the cyanoacrylic acid group.10
Table 1 Photophysical and electrochemical properties of sensitizers WS-24
and WS-26
a
b
d
Dyes
λmax
εa
λmax
HOMOc
E0-0
LUMOe
(nm)
(M-1 cm-1)
(nm)
528
(V)
(eV)
1.79
(V)
WS-24
551
398
315
552
446
328
26470
17950
24050
38534
23000
27640
1.01
0.99
-0.78
-0.84
3 Results and discussion
WS-26
530
1.83
Note: aAbsorption parameters were obtained in chloroform/methanol mixed
solution (v/v = 4/1). bAbsorption parameters were obtained on 3 μm
c
nanocrystalline TiO2 film. The HOMO was obtained in DMF with ferrocene
(0.69 V vs. NHE) as external reference. dE0-0 dvalues were estimated from the
wavelength at 10% maximum absorption intensity for the dye-loaded 3 μm
e
nanocrystalline TiO2 film. The LUMO was calculated according to LUMO =
HOMO – E0-0
.
Fig. 3 Cyclic voltammograms of WS-24 and WS-26 in DMF solution.
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5TA01371B
To further investigate the electronic properties, their cyclic
voltammetry was carried out in
DSSCs were fabricated according to the literature.15The
typical threeꢀelectrode photocurrent densityꢀvoltage (J–V) curves of DSSCs based on WSꢀ
a
electrochemical cell (Fig. 3 and Table 1). WSꢀ24 and WSꢀ26 exhibit 24 and WSꢀ26 are shown in Fig. 4a, and the detailed parameters are
similar HOMO levels due to their same donor, calculating to be 1.01 collected in Table 2. For single dye based devices, WSꢀ24 show a
V and 0.99 V (vs NHE), respectively. The driving forces for dye moderate efficiency of 6.21%, with shortꢀcircuit current density (Jsc)
regeneration of WSꢀ24 and WSꢀ26 are around 0.6 eV (0.4 V vs NHE of 13.35 mA cmꢀ2, openꢀcircuit voltage (Voc) of 671 mV and fill
ꢀ
for Iꢀ/I3 redox couple), ensuing the efficient regeneration of the factor (FF) of 0.693. Compared to previously reported BTD based
oxidized dyes by Iꢀ after electron injection.21 Correspondingly, dye WSꢀ2 (Voc = 661 mV), 16 the Voc of WSꢀ24 is improved in spite
calculated from HOMO levels and the optical band gap, the LUMO of smaller HOMOꢀLUMO energy gap. The low Jsc of WSꢀ24 should
levels of WSꢀ24 and WSꢀ26 are sharply deepened to ꢀ0.78 V and ꢀ be attributed to unfavourable dye aggregation. The incorporation of a
0.84 V, respectively. Here the driving forces for electron injection hexylthiophene in WSꢀ26 greatly decreases the dye aggregation and
from the excited dyes to the conduction band of TiO2 (ꢀ0.5 V vs improves the Jsc and Voc to 17.84 mA cmꢀ2 and 709 mV, respectively,
NHE) of WSꢀ24 and WSꢀ26 are 0.28 eV and 0.34 eV. These driving resulting in an enhanced efficiency of 8.61%. The improvement of
forces are theoretically enough for effective electron injection, Jsc can be rationalized from the measurement of incident photon to
assuming that an energy gap of 0.2 eV is indispensable for efficient electron conversion efficiency (IPCE) spectra as shown in Fig. 4b.
electron injection.22
Both WSꢀ24 and WSꢀ26 show a broad spectral response from 350 to
750 nm, while the plateau of WSꢀ26 is 20% higher than that of WSꢀ
24. These results suggest that a structural modification on the πꢀ
bridge of BOD based DꢀAꢀπꢀA dyes has significant influence on
their performance in DSSCs.
Previously, we have demonstrated an effective coꢀsensitizer S1
(Fig. 1) for improving the photovoltaic performance of DꢀAꢀπꢀA
dyes. On one hand, the intensive absorption of S1 in the blue region
of solar spectrum can compensate the spectra of DꢀAꢀπꢀA dyes, on
the other hand, S1 plays role as coꢀadsorbent that suppresses the
unfavourable dye aggregation on the surface of TiO2
nanocrystallines. We also fabricated devices of WSꢀ24 and WSꢀ26
coꢀsensitized with S1. Intriguingly, we found that coꢀsensitized dye
S1 has a positive effect on WSꢀ26 and a negative effect on WSꢀ24.
The coꢀsensitization of WSꢀ26 with S1 (WSꢀ26/S1) improves the
photovoltaic efficiency from 8.61% to 9.72%, with Jsc = 19.84 mA
cmꢀ2, Voc = 694 mV, FF = 0.706. The major improvement is the Jsc,
which can be manifested by the IPCE spectra (Fig. 4b). The increase
across the whole IPCE spectra and the blueꢀshift of the spectra
threshold indicated that S1 mainly decrease the unfavourable dye
aggregation of WSꢀ26. In comparison, the coꢀsensitization of WSꢀ24
with S1 (WSꢀ24/S1) gives an efficiency of 5.87%, which is almost
same as the single WSꢀ24 based device. Apparently, the different
effect of S1 coꢀsensitization on dye WSꢀ24 and WSꢀ26 should be
related to the πꢀbridge structures. The unchanged IPCE spectra of
WSꢀ24 before and after coꢀsensitization with S1 indicated that a
molecular matching is crucial to the coꢀsensitization strategy.23
The Voc values of WSꢀ26 based devices are always higher than
that of WSꢀ24 by ~35 mV whether or not in the presence of coꢀ
sensitizers. Since all the devices employ the same electrolyte, the
difference of Voc should be attributed to the shift of electron Fermi
level in TiO2, which is related to the conduction band position of
TiO2 and the charge recombination rate in device.24 The chemical
structure of organic dyes on TiO2 can affect both the energy level of
TiO2 and charge recombination resistance. To clarify the effect of πꢀ
bridge on Voc, electrochemical impedance spectroscopy (EIS)
measurements were performed. At a given voltage, WSꢀ24 and WSꢀ
26 exhibited similar chemical capacitance, ruling out the shift of the
conduction band (Fig. 5a). Thus the difference in Voc should be
attributed to different charge recombination rate in DSSCs, which is
characterized by the electron lifetimes (τe). As shown in Fig. 5b, at a
Fig. 4 (a) J-V curves and (b) IPCE spectra for DSSCs based on WS-24, WS-26
and with the co-sensitization of S1 under AM 1.5G simulated solar light (100
mW cm−2).
Table 2 Photovoltaic parameters of DSSCs based on WS-24, WS-26, and co-
sensitization with S1.
Dyes
Jsc / mA cm-2
13.35±0.20
13.25±0.18
17.84±0.24
19.84±0.38
Voc / mV
671±8
662±3
709±6
694±8
FF
η / %
WS-24
0.693±0.005
0.669±0.008
0.681±0.011
0.706±0.006
6.21±0.17
5.87±0.18
8.61±0.32
9.72±0.39
WS-24/S1
WS-26
WS-26/S1
Note: the composition of the electrolyte is: 0.5 M BMII, 0.1 M DMPII, 0.1 M
LiI, 0.05 M I2, 0.1 M GuSCN and 0.5 M TBP in 85/15 mixture of acetonitrile
and valeronitrile. The parameters were obtained from the averaged five
devices.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

Journal of Materials Chemistry A
Page 6 of 8
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5TA01371B
fix voltage, WSꢀ26 showed longer electron lifetime than that of WSꢀ This work was supported by NSFC for Creative Research Groups
24, suggesting that the incorporated hexylthiophene unit in the πꢀ (21421004) and Distinguished Young Scholars (21325625),
conjugation can reduce the contact between I3ꢀ in the electrolyte and NSFC/China, and the Oriental Scholarship.
the TiO2 electrode, and thus suppressing charge recombination.
Notes and references
Shanghai Key Laboratory of Functional Materials Chemistry,Key
Laboratory for Advanced Materials and Institute of Fine Chemicals,
Collaborative Innovation Center for Coal Based Energy (iꢀCCE), East
China University of Science and Technology, Shanghai 200237, P. R.
China; Eꢀmail: whzhu@ecust.edu.cn
‡These authors contributed equally to this work.
†Electronic Supplementary Information (ESI) available: NMR and Mass
spectra. See DOI: 10.1039/b000000x/
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In summary, we have synthesized two novel BODꢀbased DꢀAꢀπꢀA
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acceptor of BOD into πꢀbridge can further red shift absorption bands,
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developing high efficiency DSSCs, and the structure engineering as
well as device optimization is crucial to enhance the photovoltaic
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DOI: 10.1039/C5TA01371B
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DOI: 10.1039/C5TA01371B
Journal Name
RSCPublishing
ARTICLE
D–A–π–A featured sensitizers containing
auxiliary acceptor of benzoxadiazole:
molecular engineering and coꢀsensitization
Haibo Zhu, Yongzhen Wu, Jingchuan Liu, Weiwei
Zhang, Wenjun Wu and Wei-Hong Zhu*
Benzoxadiazoleꢀbased D–A–π–A organic sensitizers can red
shift absorption bands and predominantly deepen LUMO
level, realzing photovoltaic efficiency as high as 9.72% via
cosensitization process.
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 8