Organic dyes incorporating the benzo[1,2-b:4,5-b′]dithiophene moiety for efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/ol201858b

A new class of organic sensitizers incorporating a benzo[1,2-b:4,5- b′]dithiophene (BDT) unit as conjugated spacer has been synthesized and successfully used for dye-sensitized solar cells (DSSCs). The length of the π-conjugated spacers has a strong impac

Full text of DOI:10.1021/ol201858b

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5424–5427
Organic Dyes Incorporating the
Benzo[1,2-b:4,5-b0]dithiophene Moiety for
Efficient Dye-Sensitized Solar Cells
Xiaoli Hao, Mao Liang,* Xiaobing Cheng, Xiaoqing Pian, Zhe Sun, and Song Xue*
Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384,
People’s Republic of China
liangmao717@126.com; xuesong@ustc.edu.cn
Received July 11, 2011
ABSTRACT
A new class of organic sensitizers incorporating a benzo[1,2-b:4,5-b0]dithiophene (BDT) unit as conjugated spacer has been synthesized and
successfully used for dye-sensitized solar cells (DSSCs). The length of the π-conjugated spacers has a strong impact on electro-optical properties
of these dyes, leading to the conversion efficiencies ranging from 4.17 to 5.68% under AM 1.5 G irradiation. This result indicates that the BDT unit is
a promising candidate in organic sensitizers.
There has been considerable interest to develop cheap
and easily accessible renewable energy sources due to
increasing energy demands and concerns about global
warming. Compared to traditional silicon-based solar
cells, dye-sensitized solar cells (DSSCs) are viewed as
promising candidates for renewable clean energy sources
by virtue of their low manufacturing cost and impressive
photovoltaic performance.1 To achieve high solar power
conversion efficiency, great research efforts are focused on
designing and synthesizing new photosensitizers: Ru-based
complexes2 and organic sensitizers.3,4 The former holds
the record of validated efficiency of over 11%.2c Organic
sensitizers with robust availability, ease of structural tuning,
and generally high molar extinction coefficients, have
emerged as a competitive alternative to the Ru-based
counterpart.5 So far, the metal-free organic sensitizers have
gained promising solar energy-to-electricity conversion effi-
ciencies (η) comparable to Ru-based complexes.6
€
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r
10.1021/ol201858b
Published on Web 09/29/2011
2011 American Chemical Society

Scheme 1. Synthetic Route of Dyes
Figure 1. Structure of dyes.
BDT unit for constructing wide-spectral response organic
chromophores for DSSCs (XS32ꢀ34, Figure 1).
In order to investigate the structural modification of the
spacer group upon the photophysical, electrochemical,
and photocurrent action spectra and currentꢀvoltage
characteristics of the DSSCs, single and binary π-conju-
gated spacers were applied into these dyes, apart from the
blocks of a lipophilic alkoxy-substituted triphenylamine
electron donor (D) and a hydrophilic cyanoacrylic acid
electron acceptor (A). The synthetic route of XS32ꢀ34 is
shown in Scheme 1. The propoxy-substituted BDT moiety
was synthesized and purified according to the literature.8a
Treatment of the intermediate 1 with n-BuLi and DMF
afforded aldyhyde 2. Intermediate 3 was prepared from
bromonation reaction with bromine. Reaction of 3 with
4aꢀc via the palladium-catalyzed Stille coupling reaction
afforded aldyhydes 5aꢀc, which underwent Knoevenagel
condensation with cyanoacetic acid to form the target dyes.
These compounds are soluble in common organic solvents.
The optical and electrochemical properties of the three
dyes are summarized in Table 1. In CH2Cl2 solution
(Figure 2), the dyes display uninterrupted strong absorp-
tion between 350ꢀ550 nm, which is desirable for harvest-
ing more solar light. The dyes XS33 and XS34 featur-
ing a BDT-based binary spacer have a maximum molar
absorption coefficient (ε) of 39 ꢁ 103 Mꢀ1 cmꢀ1 at 483 and
502 nm, respectively. While the maximum molar absorption
coefficient of XS32 is reduced to 22 ꢁ 103 Mꢀ1 cmꢀ1 at 472
nm. This observation could be rationalized in terms of an
obviously electron-richer character of the binary spacer.
Upon adsorption onto TiO2, XS33ꢀ34 show similar hypso-
chromic effect compared with that measured in solution
[Figure S2a, Supporting Information (SI)], which could be
ascribed to a weaker electron-withdrawing capability of the
carboxylate titanium assembly than the carboxylic acid.10
Aside from electron donor (D) and acceptor (A) in a
typical DꢀπꢀA system, the conjugated bridging segment
(π) iswidely recognizedfor its significance for performance
control of DSSCs. Introduction of fused ring building
blocks,7,6b involving cyclopentadithiophene, thieno[3,4-b]-
thiophene, and dihexyl-substituted dithienosilole in the
π-spacer, has proved to be an effective strategy for enhan-
cing the light-harvesting capacity, leading to impressive
performances under standard global air mass 1.5 (AM 1.5 G)
illumination. Development of new fused ring building
blocks may lead to synthesis of interesting photosenzi-
tizers with tailor-made photophysical, electrochemical,
and other properties to fulfill those requirements for
high performance of DSSCs.
Recently, copolymers containing benzo[1,2-b:4,5-b0]-
dithiophene (BDT) as the donor unit have exhibited out-
standing photovoltaic properties for applications in poly-
mer bulk heterojunction (BHJ) solar cells.8 There are two
merits of choosing BDT as the π-conjugated spacer of organic
dyes for DSSCs: First, the large planar conjugated structure
favors light harvesting and thermal stability. Second, higher
oxidation potential of BDT relative to fused thiophene (e.g.,
thieno[3,4-b]thiophene) may result in high open-circuit
voltage.9 Thus, the BDT unit can be an attractive component
in organic sensitizers. However, organic dyes based on a BDT
unit have not been reported to our best knowledge. We
therefore set out to synthesize compounds consisting of a
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Org. Lett., Vol. 13, No. 20, 2011
5425

Table 1. Optical and Electrochemical Properties of the Three
Dyes
Table 2. DSSCs Performance Parameters of the Three Dyes
With N719 As a Reference
dye
λmax a/nm (ε/103 M-1 cm-1)
Eox b/V
Eox*c /V
dye
JSC/ mA cmꢀ2
VOC/mV
ff
η/%
τ/ms
XS32
XS33
XS34
297 (31), 355 (29), 472 (22)
302 (38), 391 (34), 483 (39)
300 (36), 385 (38), 502 (39)
0.93
0.76
0.82
ꢀ1.34
ꢀ1.42
ꢀ1.48
XS32a
XS32b
XS33a
XS33b
XS34a
XS34b
N719a
11.9
6.8
713
620
627
612
675
636
735
0.67
0.67
0.64
0.68
0.65
0.68
0.72
5.68
2.82
4.17
3.04
5.05
3.24
7.73
98
11
21
9
10.4
7.3
a Absorption spectra were measured in CH2Cl2 solutions. b First
oxidation potentials (vs NHE) in acetonitrile internally calibrated with
ferrocene (0.63 V vs NHE). c Eox* were estimated from Figure S4, SI.
11.5
7.5
26
18
14.6
a Dye bath concentration of 300 μM. b Dye bath concentration of 30 μM.
Figure 2. Absorption spectra of dyes in dichloromethane.
Figure 3. JꢀV curves of DSSCs based on the three dyes. Two
different dye bath concentrations (300 and 30 μM) were used for
prepartion of TiO2 electrode.
The three dyes exhibit two reversible oxidative waves in
the cyclic voltammogram (Figure S5, SI). The first oxida-
tion potentials of XS33ꢀ34 [0.76ꢀ0.82 V vs normal hydro-
gen electrode (NHE)] are more negative than that of XS32
(0.93 V vs NHE), which can be well understood on account
of the electron-richness character of a BDT-based binary
spacer in comparison with a single BDT unit. Theꢀy are all
more positive than the Nernst potential of Iꢀ/I3 redox
couple (0.4 V vs NHE), ensuring regeneration of the
oxidized dyes by Iꢀ after electron injection. The excited-
state redox potentials of the three dyes (ꢀ1.34 to ꢀ1.48 V
vs NHE) are more negative than the conduction band of
TiO2 (ECB, ꢀ0.5 V vs NHE),1b which provide sufficient
driving forces for electron injection. Thus, these oxidized
dyes can be regenerated from the reduced species in the
electrolyte to give an efficient charge separation.
Photovoltaic characteristics of the DSSCs based on the
three dyes as well as the standard N719 dye under AM 1.5 G
condition (100 mW cmꢀ2) are displayed in Table 2, and the
JꢀV curves plotted in Figure 3. The XS32 showed promising
power efficiency of 5.68% compared to the dyes XS33 and
XS34 featuring a binary conjugated spacer. The somewhat
higher efficiency observed for XS32 is mainly due to the
relatively larger short-circuit photocurrent density (JSC) and
open-circuit photovoltage (VOC). The best efficiency of XS32
reached ∼73% of the ruthenium dye N719-based standard
cell fabricated and measured under similar conditions.
The preliminary evaluation of the incident monochro-
matic photon-to-current conversion efficiency (IPCE) of
the cells based on XS32ꢀ34 is displayed in Figure S6, SI.
The XS32-sensitized cell showed absorption wavelength
ranges from 400 to 700 nm, with a maximum of 89% at
497 nm. Solar cells based on XS33 and XS34 show broad
IPCEs in accordance to the broad absorption spectrum
achieved by increasing the π-linker conjugation. Accord-
ing to the absorption measurements of XS32ꢀ34 on TiO2
transparent films (Figure S2b, SI), it is found that the
absorbance-to-photocurrent conversion efficiency (APCE)
values around the maximum are in the order of XS33 <
XS34 < XS32.
The VOC values are in the sequence of XS33 < XS34 <
XS32. When the binary conjugated spacer is displaced by
the single spacer, the VOC rises remarkably (e.g., spacer
from EDOT-DBT toDBT, VOC rises from 627 to 713 mV).
The respectable increase of VOC obtained by the delicate
change in molecular structure is intriguing. To scrutinize
the origin of the improvement in VOC, measurement of the
electrochemical impedance spectroscopy (EIS) is per-
formed. Typical EIS Nyquist and Bode phase plots mea-
sured in the dark under forward bias (ꢀ0.7 V) are shown in
Figure 4. The larger semicircle at lower frequencies repre-
sents the interfacial charge-transfer resistances (RCT) at the
TiO2/dye/electrolyte interface.12 The fitted RCT increases
in the order of XS33 (92Ω) <XS34 (145Ω) <XS32 (588Ω),
(11) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Zaban,
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Org. Lett., Vol. 13, No. 20, 2011

dyes promoted electron recombination from the FTO elec-
trode to the redox electrolyte because of complex formation
between dyes and I3ꢀ (or I2).14 We have performed similar
forward bias current density measurements by adding the
three dyes to the redox electrolyte (Figure S7, SI). It is
found that the recombination reaction as the oxidation
onset increases in the sequence of XS33 < XS34 < XS32,
which is consistent with the trend observed for VOC. It is
possible that XS33 and XS34 with more cꢀonjugated dye
structures increase the concentration of I3 at the TiO2
surface. These results give an explanation of the different
electron lifetimes yielded by DSSCs based on the three dyes.
To get further insight into the molecular structure and
electron distribution of dyes, the geometries of the dyes are
optimized by density functional theory (DFT) calculations
at the B3LYP/6-31G(d) level (Figures S11 and S12, SI).
The highest occupied and lowest unoccupied molecular
orbitals (HOMOs and LUMOs, respectively) in the dyes
largely populate on the triphenylaminemoietyand anchor-
ing carboxylic group, respectively. Thus, the HOMOꢀ
LUMO excitation induced by light irradiation could move
the electron distribution from the triphenylamine part to
the cyanoacrylic acid segment. Preliminary temperature-
dependent DFT (TDDFT) computations (Table S1, SI)
also highlight the electron-donating character of BDT toward
excitation. There are two electronic transitions (HOMO-
1fLUMO and HOMO-2fLUMO) contributing to the
uninterrupted strong absorption between 350 and 550 nm for
the dyes. Both the transitions have significant oscillator strength
(f) and extent of charge transfer from the BDT to the
2-cyanoacrylic acid, revealing the advantages of DBT units in
ameliorating the spectral response of the triphenylamine dyes.
In summary, we developed new benzo[1,2-b:4,5-b0]-
dithiophene-containing organic dyes with single or binary
π-conjugated spacers. These dyes exhibit intense and broad
electronic absorption. The fabricated DSSCs from XS32
exhibit high efficiencies reaching ∼73% of the standard cell
using N719 as the sensitizer. The results demonstrate that
BDT unit can produce high VOC in DSSCs and is a
promising candidate for the effective sensitizer. Further
applications of the BDT derivatives to the organic dyes are
now in progress, and we believe that improvement of effi-
ciency can be achieved by careful molecular design.
Figure 4. Nyquist plots and Bode phase plots (insert) for DSSCs
based on the three dyes measured in the dark under ꢀ0.7 V bias.
Top right corner is the equivalent circuit.11
which is consistent with the sequence of VOC values in the
devices. The larger RCT value means that the electron recom-
bination from the conduction band to the electrolyte occurs
more difficultly. Clearly, electron recombination in devices
based on XS33 and XS34 is faster than that of XS32.Byfitting
the EIS curves, another important parameter for DSSCs,
electron lifetime (τ), could be extracted from the chemical
capacitance (Cμ) and RCT using τ = Cμ ꢁ RCT. The fitted τ
increases in the order of XS33 (21 ms) < XS34 (26 ms) <
XS32 (98 ms), giving an explanation of the different VOC
yielded by DSSCs based on the three dyes.
Recently, several research groups have found that effec-
tive surface blocking is essential for long electron
lifetime.13 To study how surface blocking for the different
dyes influences the VOC, different dye bath concentrations
(300 and 30 μM) were used for the preparation of TiO2
electrode. As shown in Figure 3, the JSC increases with
elevated dye loading for all three dyes. Furthermore, when
increasing the dye load for XS32-based solar cells, the VOC
rises remarkably from 620 to 713 mV. In contrast, the
enhancements of VOC for XS33 and XS34-based solar cells
were not obvious. It suggests that the surface blocking
effect for the three dyes is on the order of XS32 > XS34 >
XS33. We also derived the surface coverages (Γ) of the
three dyes. It is found that the surface coverage of XS32
(6.92 ꢁ 10ꢀ7 mol cmꢀ2) is ∼1.7-fold higher than that of the
other dyes (4.01 ꢁ 10ꢀ7 and 4.23 ꢁ 10ꢀ7 mol cmꢀ2 for
XS33 and XS34, respectively). The increased dye loading
of XS32 relative to XS33 and XS34 significantly contributes
to the observed superior surface blocking. Apart from the
surface blocking, the interaction between dye and acceptor
species needs to be evaluated for all new candidate dyes. In a
study on the VOC of DSSCs using ruthenium phthalocyanine
dyes, O’Regan et al. claimed that the presence of sensitizing
Acknowledgment. We gratefully acknowledge the finan-
cial support from the National 863 Program (2009AA-
05Z421), the National Natural Science Foundation of China
(21003096), and the Tianjin Natural Science Foundation
(09JCZDJC24400).
Supporting Information Available. Synthetic procedures
and characterization for new compounds, absorption and
emission spectra, cyclic voltammogram, IPCEs action spec-
tra, plots of RCT, Cμ, and τ, frontier molecular orbitals of the
dyes and TDDFT calculation. This material is available free
of charge via the Internet at http://pubs.acs.org.
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