Rational molecular engineering of cyclopentadithiophene-bridged D-A-π-A sensitizers combining high photovoltaic efficiency with rapid dye adsorption

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Dye-sensitized solar cell (DSSC) is considered as a feasible route to the clean and renewable energy conversion technique. The commercial application requires further enhancements on photovoltaic efficiency and simplification on the device fabrication. For avoiding the unpreferable trade-off between photocurrent (JSC) and photovoltage (VOC), here we report the molecular engineering and comprehensive photovoltaic characterization of three cyclopentadithiophene-bridged D-A-π-A motif sensitizers with a change in donor group. We make a careful choice on the donor and conjugation bridge for synergistically increasing JSC and VOC. Comparing with the reference dye WS-2, the photovoltaic efficiency with the single component dye of WS-51 increases by 18%, among one of the rare examples in pure metal-free organic dyes exceeding 10% in combination with traditional iodine redox couples. Moreover, WS-51 exhibits several prominent merits on potentially scale-up industrial application: i) facile synthetic route to target molecule, ii) simple dipping procedure without requirement of co-sensitization, and iii) rapid dye adsorption capability.

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## OPEN

received: 08 February 2015

accepted: 21 May 2015

Published: 11 June 2015

# Rational molecular engineering of cyclopentadithiophene-bridged D-A-π-A sensitizers combining high photovoltaic efficiency with rapid dye adsorption

Qipeng Chai 1 , Wenqin Li 1,2 , Jingchuan Liu 1 , Zhiyuan Geng 3 , He Tian 1 & Wei-hong Zhu 1

Dye-sensitized solar cell (DSSC) is considered as a feasible route to the clean and renewable energy
conversion technique. The commercial application requires further enhancements on photovoltaic
efficiency and simplification on the device fabrication. For avoiding the unpreferable trade-off
between photocurrent ( _J_ SC) and photovoltage ( _V_ OC), here we report the molecular engineering and
comprehensive photovoltaic characterization of three cyclopentadithiophene-bridged D-A-π-A motif
sensitizers with a change in donor group. We make a careful choice on the donor and conjugation
bridge for synergistically increasing _J_ SC and _V_ OC. Comparing with the reference dye WS-2, the
photovoltaic efficiency with the single component dye of WS-51 increases by 18%, among one of the
rare examples in pure metal-free organic dyes exceeding 10% in combination with traditional iodine
redox couples. Moreover, WS-51 exhibits several prominent merits on potentially scale-up industrial
application: i) facile synthetic route to target molecule, ii) simple dipping procedure without
requirement of co-sensitization, and iii) rapid dye adsorption capability.

Dye-sensitized solar cells (DSSCs) have aroused great enthusiasm as an alternative photoelectric conversion device 1,2, with extensive efforts to pursuing high photovoltaic efficiency and low cost for their
commercial attractiveness 3–9 . In DSSCs, sunlight is harvested by dye sensitizers that are attached onto
the surface of a wide band gap semiconductor oxide (typically TiO2). Generally, organic sensitizers govern photon harvesting and electron conduction inside nanoporous TiO2 electrode, being one
of the decisive factors to high photon-conversion-efficiency (PCE). Organic sensitizers usually have a
donor-π -bridge-acceptor (D−π− A) feature, with a characteristic intramolecular charge transfer (ICT)
band that harvests sunlight for photon-to-electron conversion. However, the absorption bands for most
pure organic dyes based on D−π− A motif always fall in short wavelength region (less than 500 nm) 3,4 .
Moreover, due to the deprotonation effect, conventional D−π− A sensitizers bring forth large blue-shifts
when anchored onto TiO2 photoanodes, leading to a great loss in photocurrent generation.

To minimize the limitation, a concept of D− A−π− A motif was proposed for designing a new
generation of efficient and stable organic sensitizers, in which an auxiliary electron-withdrawing unit

1Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and
Institute of Fine Chemicals, Collaborative Innovation Center for Coal Based Energy (i-CCE), East China University
of Science and Technology, Shanghai 200237, P. R. China. 2 School of Urban Development and Environmental
Engineering, Shanghai Second Polytechnic University, Shanghai 201209, P. R. China. 3 Gansu Key Laboratory of
Polymer Materials, College of Chemistry and Chemical Engineering, Key Laboratory of Eco-environment-related
Polymer Materials, Ministry of Education, Northwest Normal University, Lanzhou, Gansu 730070, P. R. China.
[Correspondence and requests for materials should be addressed to W.H.Z. (email: whzhu@ecust.edu.cn)](mailto:whzhu@ecust.edu.cn)

Figure 1. Molecular structures of CPDT-bridged D-A-π-A sensitizers WS-37, WS-38 and WS-51 derived
from reference dye WS-2 .

was incorporated into the conjugated bridge to facilitate electron transfer from the donor to the acceptor 10–25 . As an electron deficient unit, the incorporated additional unit can efficiently modulate the
energy levels, absorption bands, and function as a great relief of the blue shifts when the dye molecules
anchored on TiO2 anode, which is beneficial to the photocurrent generation with extending the incident
photon-to-electron conversion efficiency (IPCE) onset 10 . For instance, our group systematically studied a
series of 2,1,3-benzothiadiazole (BTD)-based sensitizers based on D− A−π− A motif, showing high PCE
of 8.7% ( WS-2 ) and 9.0% ( WS-9 ) with the iodine redox couples 11,13 . However, the photovoltage in these
BTD based D− A−π− A sensitizers is always unsatisfactory due to the serious electron recombination.
The known WS-2 endowed a moderate photovoltage in the range of 580–650 mV 11,12, which critically
limits the photovoltaic efficiency to a great extent.

As well known, the presence of long alkyl chains on conjugated bridge could efficiently decrease
the intermolecular interactions and retard the electron recombination, thus resulting in an obvious
improvement of _V_ OC14,26. As an effective building block, 4 _H_ -cyclopenta[2,1- _b_ :3,4- _b_ ’]dithiophene (CPDT)

has been exploited for constructing the π− conjugated skeleton of organic sensitizers due to its excellent co-planarity and electron donating capability 27–29 . Moreover, the long alkyl chains can be feasibly
introduced at the bridging carbon atoms of CPDT. Herein, we further replace the thiophene moiety (in
the D− A−π− A motif of WS-2 ) with the enlarged segment dioctyl-substituted CPDT for the sake of
extending IPCE response and attenuating the interfacial interactions. The three CPDT-bridged D-A-π -A
sensitizers (Fig. 1) with a change in donor group were rationally designed. Comparing with the reference
dye WS-2, the CPDT-based corresponding dye WS-51 exhibits a panchromatic profile with a synergistic
enhancement in _J_ SC and _V_ OC, achieving an impressive 10.1% PCE without requirement of co-sensitization.
More interestingly, instead of normal long term adsorption (> 12 h), WS-51 shows a rapid adsorption
property and reaches a PCE over 9% after only initial 2 h dye adsorption. As a consequence, dye WS-51
exhibits several prominent merits, such as convenient synthesis, simple dipping procedure, no necessary
co-sensitization, and rapid dye adsorption, which are highly preferable for potentially industrial scale-up
application owing to its high efficiency and time-saving dye adsorption.

Results
Molecular design and synthesis. As an excellent co-planarity, the enlarged conjugation CPDT was
utilized instead of thiophene to effectuate the red-shift and enhancement in ICT absorption band for
extending light-harvesting. Moreover, the introduction of long alkyl chains would also attenuate the
interfacial recombination, for achieving both gains in _J_ SC and _V_ OC. Several classical donor groups were
incorporated in molecular skeleton to modulate the orbital levels for balancing the thermodynamic force
between electron injection and dye regeneration. As illustrated in Fig. 2, we utilized the facile starting material of 4,7-dibromobenzo[ _c_ ][1,2,5]thiadiazole to develop these sensitizers. Dioctyl-substituted
CPDT moiety was attached to the additional unit of BTD via Suzuki reaction and afforded a monosubstituted product 2 . Then the formylation gave aldehyde 3 under room temperature. Subsequently, the donor
groups and the framework 3 coupled together via Suzuki reaction to give the sensitizer precursors 4,

Figure 2. Synthetic route to WS-37, WS-38 and WS-51. Reaction conditions: i) Pd(PPh3)4, 2 M K2CO3
aqueous solution, THF, Ar, 90 ºC, ii) POCl3, DMF, 25 ºC, iii) Pd(PPh3)4, 2 M K2CO3 aqueous solution, THF,
Ar, 90 ºC, and iv) piperidine, acetonitrile, Ar, 90 ºC.

which were eventually converted to the target dyes ( WS-37, WS-38 and WS-51 ) by reflux for 8 h in the
presence of piperidine and acetonitrile. All the dye sensitizers are dark purple in solid state, and exhibit
purple red to dark purple in solutions with gradually enhanced electron donating capability in the donor
units.

Red-shift and molar extinction coefficient enhancement in absorption band. The absorption spectra of dyes ( WS-37, WS-38 and WS-51 ) in mixed solution (CHCl3:CH3OH, v/v = 4:1) and
on TiO2 films were preliminary studied (Fig. 3 and Table 1). In similar with WS-2, the three BTD
based D− A−π− A sensitizers exhibit three major electronic absorption bands: i) the π –π * electron
transitions in UV region (near 300 nm), ii) the ICT band in visible region (around 540 nm), and iii)
the additional absorption band or shoulder from subordinate orbital transition (near 460 nm, DFT calculation, Supplementary Table S1). The direct comparison between WS-2 and WS-51 highlights the
effect of CPDT conjugation bridge on the ICT absorption band as well as molar extinction coefficients.
Compared with WS-2 ( _λ_ max = 533 nm), the larger co-planarity of CPDT unit in the skeleton of WS-51
results in a red-shift by 18 nm. Moreover, WS-51 exhibits 1.6-fold higher molar extinction coefficient
( _ε_ = 43000 M −1 cm −1 ) than WS-2 ( _ε_ = 16700 M −1 cm −1, Table 1). The tendency is well consistent with the
calculated band gap and oscillator strength (Supplementary Table S1). Moreover, in spite of giving the
similar absorption profile (Fig. 3), the gradually enhanced electron donating capability can also red shift
the absorption band, from triphenylamine unit ( WS-37, _λ_ max = 536 nm), dialkoxy-substituted triphenylamine unit ( WS-38, _λ_ max = 546 nm) to indoline unit ( WS-51, _λ_ max = 551 nm). In this sense, the indoline
unit in WS-51 is stronger than the traditional donor unit of tripenylamine unit 11,16 . Additionally, the blue
shifts on TiO2 films for the BTD based D− A−π− A sensitizers are all around 20 nm (Table 1), much
less than that of conventional D−π− A motif, highlighting the additional electron-withdrawing effect of
BTD on the weakening influence by deprotonation.

Electrochemical data and DTF simulation. To further determine the energy level position of frontier orbitals, cyclic voltammetry was performed in CH2Cl2 with tetra- _n_ -butylammonium hexafluorophosphate (TBAHFP) as a supporting electrolyte. The formal oxidation potentials are corresponding to
the HOMO levels, appearing at 0.82 and 0.90 V ( _vs_ NHE) for WS-51 and WS-2, respectively. The 0.08 V

Figure 3. Absorption spectra of sensitizers WS-37, WS-38 and WS-51 and reference dye WS-2 in mixed
solution (CHCl3:CH3OH, v/v = 4:1, a ) and on 3 μ mTiO2 films ( b ).

Dye _λ_ max [nm] a _ε_ [M −1 cm −1 ] a _λ_ max on TiO2 [nm] b HOMO [ _V_ ] c _E_ 0-0 [e _V_ ] d LUMO [ _V_ ] d

WS-2 533 16700 515 0.90 1.90 –1.00

395 14100

321 21100

WS-37 536 37300 514 1.05 1.96 –0.91

457 16700

308 20900

WS-38 546 41000 529 0.85 1.89 –1.04

459 17200

308 22700

WS-51 551 43000 533 0.82 1.83 –1.01

460 27800

314 22700

Table 1. Absorption and electrochemical properties of sensitizers WS-37, WS-38 and WS-51 and
reference dye WS-2. a Absorption peaks ( _λ_ max) and molar extinction coefficients ( _ε_ ) in mixed solution
(CHCl3:CH3OH, v/v = 4:1). b Absorption peaks on TiO2 films. c HOMO were measured in CH2Cl2 with
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: Pt; reference
electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fc + ) as an external reference. Counter electrode:
Pt, and the scan rate is 100 mV/s. d _E_ 0-0 was estimated from the absorption thresholds from absorption
spectra of dyes adsorbed on the TiO2 film, LUMO is estimated by subtracting _E_ 0-0 from the HOMO.

Figure 4. Calculated frontier molecular orbitals (HOMO and LUMO) of sensitizers WS-2, WS-37,
WS-38 and WS-51.

upshift can be ascribed to the contribution of a higher electron-rich effect of CPDT relative to thiophene
moiety. Moreover, all the CPDT-based dyes exhibit two reversible oxidation waves (Supplementary Fig. S1).
The resulting formal oxidation potentials are found at 1.05, 0.85 and 0.82 V for WS-37, WS-38 and WS-51,
respectively. Here the uplifted HOMO levels are in accordance with the increasing electron donating
ability for each donor group. Derived from the absorption thresholds on TiO2 films, the resulting LUMO
levels for WS-2, WS-37, WS-38 and WS-51 are - 1.00, - 0.91, - 1.04 and - 1.01 V, respectively. Given
the chemical potential of iodine redox couples (0.4 V _vs_ NHE) and the conduction band position of
TiO2 (− 0.5 V), there exists sufficient thermodynamic force for the occurrence of dye regeneration and
electron injection 30,31 .

The electron distribution in the frontier orbitals for these sensitizers is presented in Fig. 4, and their
optimized geometrical configurations are shown in Supplementary Fig. S2. The HOMO electrons in all
these four sensitizers are delocalized throughout the entire framework, indicating that they can be treated
as highly conjugated configurations with excellent electron transfer channels for electron injection. The
LUMO electrons are predominantly distributed on the BTD-CPDT-anchor unit. Obviously, the good
overlap between the HOMO− LUMO orbital can facilitate the electron migration from donor to the
anchor unit, then to the conduction band of semiconductor TiO2. Besides, the small dihedral angles
between BTD and CPDT (Supplementary Fig. S2) emphasize the convenience in electron flow through
the entire skeleton.

Synergistic enhancement of _J_ SC and _V_ OC. Generally, the photovoltaic efficiency can be optimized
from the enhancement of _J_ SC and _V_ OC. However, the unpreferable trade-off between _J_ SC and _V_ OC is always
observed. For instance, the broad spectral response with high photocurrent has been easily achieved
in BTD-based sensitizers, while their _V_ OC is always limited to 670 mV 11,12 . In contrast, the high _V_ OC is
easy to realize in benzotriazole-based sensitizers but their photocurrent becomes discouraging to some
extent 14–15 . As well known, the quantum conversion yield in response region and the spectrum coverage range determine the generation of photocurrent density. Our motivation is to look for a sensitizer holding the capability of panchromatic coverage and particularly efficient electron conversion,
keeping the synergistic enhancement of _J_ SC and _V_ OC. Fig. 5a shows the IPCE curves as a function of
excitation wavelength for the developed sensitizers. Notably, the IPCE onsets are well consistent with
the corresponding electron donating ability, that is, indoline ( WS-51) - dimethoxy-substituted triphenylamine ( WS-38 ) > triphenylamine ( WS-37 ). Especially, all the CPDT-based devices have small notches
around 400 nm, which is attributed to the weak absorbance in this region. As a matter of fact, WS-51

Figure 5. IPCE action spectra ( a ), current− voltage characteristics ( b ), plots of cell capacitance ( _C_ μ, c ),
interface charge transfer resistance ( _R_ CT, d ) and calculated electron lifetime ( _τ_, e ) under a series potential
bias of DSSCs based on sensitizers WS-2, WS-37, WS-38 and WS-51 (dye bath solvent: CHCl3:C2H5OH,
v/v = 1:1, dipping time: 12 h)

exhibited the extensive plateau IPCE response, above 80% in the entire visible region of 400 – 700 nm.
Indeed, among the four dye sensitizers, the predominance from both the CPDT conjugation bridge and
indoline unit in WS-51 realized a broadest spectral response, thus presenting a high current density
(19.69 ± 0.09 mA cm −2 ) with respect to WS-2 (18.24 ± 0.24 mA cm −2 ). Here the integrals of IPCE curves
are in good agreement with the measured current density, showing less than 5.2% error. However, when
compared with WS-37 and WS-51, WS-38 displayed the worst IPCE response value with around 60%
plateau in the visible region, resulting in a low _J_ SC (12.32 mA cm −2 ).

Besides _J_ SC, the different donor parts in WS-37, WS-38 and WS-51 have little impacts on _V_ OC (690–
700 mV). Consequently, we shed the improvement of photovoltage ( _V_ OC) on the facile introduction of
hydrophobic long alkyl group on CPDT as anti-aggregation chain. The reference sensitizer WS-2 displayed a moderate photovoltage below 650 mV, which critically limited the photovoltaic efficiency to a
great extent. Interestingly, the _V_ OC was increased by around 50 mV when simply changing the conjugated
bridge from thiophene to CPDT unit. Compared with WS-2, the incorporation of dioctyl-substituted
CPDT in WS-51 can offer the possibility to reduce the intermolecular interactions or block molecular
aggregation. In fact, the negative effect upon the addition of a co-adsorbent CDCA (Table 2) is suggestive that the steric two octyl chains can efficiently suppress the formation of undesirable dye aggregates.
Considering the limitation to the relatively low _V_ OC in WS-2, the rational molecular strategy with the
dialkyl substituted CPDT conjugated bridge and indoline unit in WS-51 can guarantee the balance of _J_ SC
and _V_ OC, especially pursuing the goal of outstanding device efficiency. As a result, under standard AM
1.5 conditions, WS-51 -based solar cells realized an outstanding photovoltaic efficiency of 10.08 ± 0.05%
( _J_ SC = 19.69 mA cm −2, _V_ OC = 700 mV, _FF_ = 0.73). In this way, the synergistic uplift from both _J_ SC and _V_ OC
in WS-51 lead to an increase by 18.7% in photovoltaic efficiency with respect to WS-2 . Additionally, the
power conversion efficiency of WS-51 can remain above 90% of initial value under one-sun illumination
after 1000 h, indicative of good sensitizer stability.

Generally, the _V_ OC is defined as the potential difference between Femi level of TiO2 ( _E_ Fn) and the
chemical potential of redox mediators in the electrolyte. Due to the identical redox species in the test,
the _V_ OC is determined by the position of conduction band of TiO2 and the electron density in the TiO2
nanoparticles 32–35 . To identify the position of TiO2 conduction band, we fitted the cell capacitance ( _C_ μ)
responses under a series bias potential, determined from the typical electrochemical impedance spectroscopy (EIS). In these solar cells sensitized with these four dyes, the logarithm of _C_ μ was enhanced at
the almost identical slope, showing a linear increase with the given bias potential (Fig. 5c). Obviously,
the observed similar _C_ μ value at fixed potential is indicative of no essential influence on the conduction
band of TiO2. In other words, the values of _V_ OC is directly correlated with the electron density in TiO2.
Compared with WS-2, the congener WS-51 exhibited an obvious uplift of the charge transfer resistance

Dye _J_ SC / mA cm −2 _V_ OC / mV _FF_ _η_

WS-2 18.24 ± 0.24 649 ± 5 0.718 ± 0.003 8.49 ± 0.13

WS-37 16.23 ± 0.31 692 ± 4 0.716 ± 0.002 8.04 ± 0.12

WS-38 12.32 ± 0.25 699 ± 5 0.727 ± 0.004 6.27 ± 0.12

WS-51 19.69 ± 0.09 700 ± 4 0.731 ± 0.002 10.08 ± 0.05

WS-51 (5 mM
18.85 ± 0.12 687 ± 3 0.718 ± 0.001 9.29 ± 0.04
CDCA)

Table 2. Photovoltaic parameters of DSSCs measured under AM 1.5 conditions (dye bath solvent:
CHCl3:C2H5OH, v/v = 1:1, dipping time: 12 h).

( _R_ CT) under a series bias potential. In particular, the _R_ CT of WS-51 -based device was 6-fold as that for
WS-2 under 0.65 V (Fig. 5d). Obviously, WS-51 is more efficient for inhibition of the interfacial electron
recombination process 36–38 . Also, WS-37 and WS-38 displayed a comparable _R_ CT and calculated electron
lifetime ( _τ_, Fig. 5e) with WS-51 . Obviously, the different donor parts in WS-37, WS-38 and WS-51 have
a negligible effect on the interfacial interactions or _V_ OC. As a consequence, compared with WS-2, it is
the dioctyl-substituted CPDT bridge that plays a major role in the observed synergistic enhancement of
_J_ SC and _V_ OC, specifically resulting in a 18.7% increase in photovoltaic efficiency of WS-51 .

Rapid dye uptake. In our previous work, CH2Cl2 was chosen as dye bath solvent for WS-2 . In a
preliminary test, the photovoltaic performance of WS-2 is not satisfying due to the serious aggregation. Therefore, chenodeoxycholic acid (CDCA) was used as a co-adsorbent for the consequent optimization of the devices 11–13 . Herein, we adopted a binary solvent system (CHCl3:C2H5OH, v/v = 1:1) to
optimize the DSSC performance. Meanwhile, the dye uptake experiments were attentively performed to
gain insight into the origin of the remarkable difference on photovoltaic performances between the two
kinds of dye bath solvents. Fig. 6 depicted the dye adsorption profiles as a function of time for WS-2 and
WS-51 . In both cases, the dye uptake amounts were increased significantly in initial 2 h. Afterwards, the
uptrends slowed down gradually, and reached plateau at a certain time (> 8 h). The dye uptake amount
for WS-2 in CH2Cl2 was 2-fold of that in binary solvent system after complete adsorption. In sharp
contrast, WS-51 did not show any difference in either CH2Cl2 or the binary solvents (CHCl3:C2H5OH).
That is, the dye loading amount of WS-51 in the two kinds of dye bath solvents was fundamentally
equal at any dye soaking time. In particular, over 90% dye molecules of WS-51 were binding onto TiO2
nanoparticles after initial 2 h dye adsorption. Moreover, FTIR spectroscopic analyses confirmed the same
binding mode in the two above-mentioned dye bath solvents, along with the identical peak locations:
carboxylate asymmetric stretching vibration (− COO − _as_ ) around 1612 cm−1 and symmetric stretching

carboxylate asymmetric stretching vibration (− COO − _as_ ) around 1612 cm−1 and symmetric stretching

vibration (− COO − _s_ ) around 1400 cm−1 (Supplementary Fig. S3).

According to Fig. 6, it is obvious that the device performance may have a close relationship with the
dye uptake amount for a designated dye in a given dye bath solvent. In view of the sharp increase in dye
adsorption amount during the initial dipping time, we measured the DSSC performance with different
dye adsorption amount. When using CH2Cl2 as dye bath solvent, the photovoltaic efficiency around 7%
was obtained with 1 h adsorption of WS-2, and further improvement to 7.6% was reached after another
1 h soaking (Fig. 7a, Table 3). However, when the dipping time was delayed to 12 h, the _J_ SC was decreased
sharply from 17.67 to 12.43 mA cm −2, resulting in a low photovoltaic efficiency of 5.50%. The decrease in
photovoltaic performance implies that the serious aggregation for WS-2 would take place during the time
from 2 h to 12 h. That is why we have to make use of chenodeoxycholic acid (CDCA) as a co-adsorbent
for the performance optimization 11–13 . In contrast with WS-2, the photovoltaic efficiency of WS-51 was
enhanced with the increasing dipping time (Fig. 7b, Table 3). In fact, the overall performance point of
inflection was not observed, even without the requirement of co-sensitization, which might be attributed to the existing long dioctyl chain in CPDT unit. An impressive conversion efficiency of 9% was
obtained with only 2 h adsorption of WS-51, around 94% of that in 12 h common dipping condition.
As a consequence, the rapid dye uptake of WS-51 becomes very attractive, which may avoid the tedious
time-consuming process in development of large scale commercial DSSCs.

To further scrutinize the different _V_ OC of WS-51 under various dye-soaking time, the EIS analyses
were also performed. In initial 1 h, the dye coverage is not sufficient to some extent, resulting in a relatively low light harvesting efficiency and a moderate photocurrent density. Besides, the more unoccupied
sites on TiO2 nanoparticles may facilitate the recombination between the injected electrons and the redox
species, thus presenting a relatively low _V_ OC (637 mV). With subsequent 1 h dipping, no relative shifts
of conduction band in TiO2 were observed (Fig. 7c). Whereas, the charge transfer resistances displayed
significant enhancements (Fig. 7d) due to the consequent coverage of the unoccupied sites, which is more
efficient to retard the deleterious electron recombination. With a 12 h common dipping time, the devices
indicated a relative downward CB compared with those for 1 h and 2 h conditions. Meanwhile, the _R_ CT
values of 12 h under a series bias potential were also between those of 1 h and 2 h. As a consequence,
the combined effect explains the trend of the electron lifetime ( _τ_ 2h - _τ_ 12h - _τ_ 1h, Fig. 7e), which is well

Figure 6. Absorption profiles of WS-2 ( a ) and WS-51 ( b ) on TiO2 photoanodes under different dye bath
system.

consistent with the _V_ OC change tendency from 637, 705 to 683 mV (Table 3) with dipping time for 1, 2
and 12 h, respectively.

Discussion
The photovoltaic efficiency can be optimized from the enhancement of _J_ SC and _V_ OC. However, the unpreferable trade-off between _J_ SC and _V_ OC is always observed. Our main motivation is to look for a sensitizer
holding the capability of panchromatic coverage and particularly efficient electron conversion. In this
work, we make a rational molecular design on the component in the sensitizer framework for the sake
of keeping the synergistic enhancement of _J_ SC and _V_ OC39–41: i) A powerful electron donating group of

indoline pushes the photoexcitation electron vigorously through the molecular channel to the internal
TiO2 nanoparticles, ii) the auxiliary electron-withdrawing unit BTD extends the spectral response and
weakens the blue shift on TiO2 films, and iii) the enlarged π -conjugation bridge of CPDT with long alkyl
chain further extends the response wavelength, enhances the light harvesting capability, and blocking
aggregation for free co-sensitization.

We modulated the electron donating group and conjugation bridge in three cyclopentadithiophenebridged D-A-π -A motif sensitizer, for balancing dye regeneration and electron injection. The device
performance indicates that the donor groups have a determined influence on the photocurrent but
with little impact on photovoltage. The combination of a powerful electron donating unit indoline, a
strong electron-deficient moiety BTD and an enlarged π -conjugation CPDT brings forth a sensitizer
named WS-51 with several characteristics, such as a panchromatic absorption profile, no requirement
of co-sensitization, and rapid dye adsorption capability. WS-51 achieved an impressive device efficiency
up to 10.1%, which is very rare in single organic dye working with iodine electrolyte. Moreover, WS-51
presents a rapid dye adsorption, and the photovoltaic device efficiency in a 2 h dipping can reach 94%
that for common 12 h soaking, effectively avoiding the time-consuming method for industrial application
in the future. As demonstrated, WS-51 is a promising dye sensitizer in DSSCs, especially in consideration
of the commercialization of DSSCs owing to its facile synthesis, high efficiency and rapid dye adsorption.

Methods
General. All materials and detailed synthetic procedure are collected in Supplementary Information.
1H and 13C NMR and HRMS were recorded on Bruker 400 and Waters ESI mass spectroscopy, respectively,

Figure 7. Current− voltage characteristics of WS-2 ( a ), WS-51 ( b ), plots of cell capacitance ( _C_ μ, c ),
interface charge transfer resistance ( _R_ CT, d ) and calculated electron lifetime ( _τ_, e ) under a series potential
bias of DSSCs based on WS-51 under a series dye soaking time (1, 2 and 12 h) with dye bath solvent of
CH2Cl2.

Dye/dipping time _J_ SC/mA cm −2 _V_ OC/mV _FF_ _η_

WS-2 /1 h 16.09 ± 0.28 608 ± 3 0.703 ± 0.002 6.88 ± 0.11

WS-2 /2 h 17.67 ± 0.23 617 ± 4 0.693 ± 0.003 7.63 ± 0.09

WS-2 /12 h 12.43 ± 0.36 628 ± 6 0.704 ± 0.003 5.50 ± 0.17

WS-51 /1 h 17.89 ± 0.15 637 ± 6 0.688 ± 0.006 7.84 ± 0.16

WS-51 /2 h 19.01 ± 0.22 705 ± 3 0.673 ± 0.002 9.02 ± 0.03

WS-51 /12 h 19.85 ± 0.08 683 ± 6 0.704 ± 0.003 9.55 ± 0.07

Table 3. Photovoltaic parameters of WS-2 and WS-51 measured with a transitory dye adsorption
obtained from the averaged four devices (dye bath solvent of CH2Cl2 ).

for the characterization of intermediates and target molecules. The UV-vis absorption spectra were
obtained with CARY 100 spectroscopy, and infrared (IR) spectra were performed using Nicolet 380 FTIR
spectrometer with dye-loaded TiO2 powder. Cyclic voltammetry was measured with a three-electrode
system, a calomel electrode in saturated KCl solution as reference electrode, Pt as working electrode,
and a Pt wire as counter electrode, respectively. The redox couple of Fc/Fc + was measured as external
standard.

Device fabrication and photovoltaic characterization. The photoelectrode was fabricated by
repeating screen printing process with commercial colloidal paste (Dyesol 18NR-T) layer (12 μ m) and
scattering layer (4 μ m), respectively. Afterwards, the TiO2 films were heated gradually under an air flow
at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min. Prior to dye adsorption,
the TiO2 films were post treated by 0.04 M TiCl4 solution to increase the surface area and improve the
connectivity of the nanoparticles. Subsequently, the photoanodes sintered once again and cooled to room
temperature. Then they were immersed into a binary solvent system (CHCl3:C2H5OH = 1:1) or CH2Cl2
with sensitizers (3 × 10 −4 M), respectively. For the counter electrode, the H2PtCl6 in 2-propanol solution
presented a uniform distribution on FTO glass by spin coating method, and the cathode was heated
under 400 °C for deposition of platinum. Eventually, the two electrodes were sealed with thermoplastic
Surlyn, and an electrolyte solution was introduced through one hole in the counter electrode to finish
the sandwiches type-solar cells. The electrolyte is composed of 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, and

0.5 M TBP in acetonitrile. The photocurrent-voltage ( _I_ - _V_ ) curves were measured under AM1.5G simulated solar light by illuminating the cell through the FTO substrate from the photoanode side. The incident photon-to-charge carrier efficiencies (IPCEs) were obtained on a Newport-74125system (Newport
instruments). Electronic impedance spectra (EIS) measurements was performed with an impedance analyzer (Solartron Analytical, 1255B) using DSSC devices under 20 °C in the dark. The applied frequency
range was 10 −1 –10 5 Hz, and the magnitude of the sinusoidal perturbations was 5 mV. The bias potential
varied between 450 and 650 mV, or from 500 to 700 mV, with about 50 mV progressive increase, and the
spectra was characterized with Z-View software.

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Acknowledgements
This work was supported by NSFC for Creative Research Groups (21421004) and Distinguished Young
Scholars (21325625), NSFC/China, the Oriental Scholarship, National Major Scientific Technological
Special Project (2012YQ15008709), Science and Technology Commission of Shanghai Municipality (No.
14YF1410500 and 15XD1501400), Shanghai Young Teacher Supporting Foundation (No. ZZEGD14011),
and the Fundamental Research Funds for the Central Universities (WJ1416005).

Author Contributions
W.-H.Z., H.T. and Q.P.C. conceived the experiments and designed the study. Q.P.C. carried out the
synthesis. Q.P.C. and J.C.L. performed cell measurements and analyzed the data. W.-H.Z., Q.P.C. and
W.Q.L. wrote manuscript. Z.Y.G. did the quantum chemical calculations. All authors discussed results
and contributed to the interpretation of data.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Chai, Q. _et al._ Rational molecular engineering of cyclopentadithiophenebridged D-A-π-A sensitizers combining high photovoltaic efficiency with rapid dye adsorption. _Sci._
_Rep._ 5, 11330; doi: 10.1038/srep11330 (2015).

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