Conjugated polymer with rigid donor poly(para-divinylphenylamino) backbone and pendant cyanoacetic acid acceptor for dye sensitized solar cells

Article abstract of DOI:10.1002/pola.27334

A donor backbone [poly(para-divinylphenylamino)]-acceptor (cyanoacetic acid side group) type conjugated polymer (P2) has been synthesized and used as the active material for dye-sensitized solar cells. DFT calculation shows that the insertion of vinyl lin

Full text of DOI:10.1002/pola.27334

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Conjugated Polymer with Rigid Donor Poly(para-divinylphenylamino)
Backbone and Pendant Cyanoacetic Acid Acceptor for Dye Sensitized
Solar Cells
Zhen Fang,1 Danlu Wu,2 Shahar Keinan,1 Bin Liu3
1Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599
2Department of Chemistry, Tufts University, Medford, Massachusetts 02155
3Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4,
Singapore 117576
Correspondence to: Z. Fang (E-mail: Fangzh78@unc.edu)
Received 24 May 2014; accepted 12 July 2014; published online 6 August 2014
DOI: 10.1002/pola.27334
ABSTRACT: A donor backbone [poly(para-divinylphenylamino)]-
acceptor (cyanoacetic acid side group) type conjugated poly-
mer (P2) has been synthesized and used as the active material
for dye-sensitized solar cells. DFT calculation shows that the
insertion of vinyl link in the polymer backbone leads to a pla-
nar structure in P2 and changes the excited state significantly.
Photoelectrochemical cells based on the DSSC format were
fabricated using the polymers as sensitizers. The cell con-
structed using P2 exhibits a considerably high peak IPCE and
J-V response, with an overall power conversion efficiency of
C
V
3.67%.
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KEYWORDS: conjugated polymer; dye sensitized solar cell; struc-
ture-property relation; thin film
elements for high DSSC efficiency. In particular, CPs with
main-chain donor and side-chain acceptor structure exhibit
high efficiencies in DSSCs.16,17 Interest on tuning the donor/
acceptor pair and further questions on understanding the
structure-property relationship led us to design novel CPs,
while maintaining the D-A motif. Specifically, we previously
designed a poly(triphenylamines) (P1 in Chart 1) with cya-
noacetic acid side group and thiophenyl as the link, which
shows strong charge separation at the excited states and
favors electron injection to semiconductor, that is, TiO2.18
This design led to a high efficient polymer DSSCs with a PCE
of ꢀ3.4%. Nevertheless, the light absorption band remained
narrow due to the twisted phenyl-phenyl backbone struc-
ture. In this presentation, an improvement on the previous
design is tested, by inserting a vinyl link between two tri-
phenylamines (P2 in Chart 1). This stilbene derivative fea-
tures a planar structure and is more favorable for extending
conjugation. We describe here the synthesis and characteri-
zation of P2. A detailed computational model is presented
for the charge separation states, showing enhanced intramo-
lecular charge transfer (ICT) characters for P2. P2 was also
fabricated as the active materials in DSSC, with broader
external action efficiencies. Another consideration was that
polymer size significantly affects dye adsorption onto TiO2
INTRODUCTION Taking advantages of relatively low manufac-
turing cost and capability of large scale production on flexi-
ble substrates,1,2 dye-sensitized solar cells (DSSCs) have
been regarded as the most promising alternative to conven-
tional inorganic photovoltaic cells, for example, silicon,3 CIGS,
and CdTe.4 A typical DSSC is composed of a TiO2 photoanode
sensitized by a transition metal complex, such as Ru com-
plexes, accomplishing maximum power conversion efficien-
cies (PCE) up to ꢀ11%.5 A record efficiency of 12.3% was
recently achieved using a zinc porphyrin with Co(II/III) tris(bi-
pyridyl) as the electrolyte.6 Nevertheless, several critical dis-
advantages, for example, high cost materials (noble metals)
and environmental safety issue of heavy metals, prompt the
research of metal-free organic chromophores with donor-
acceptor (D-A) structures.7,8 Currently, DSSCs with pure
organic dyes have comparable PCEs (ꢀ10%).9–11
Within the large number of organic materials for photovol-
taic application, conjugated polymers (CPs) with D-A struc-
ture have been extensively developed as active materials in
bulk heterojunction solar cells.12–14 However, CP sensitizers
in DSSCs are much less discussed.15 Such CPs feature strong
light harvesting, excellent electron-hole dissociation, and
multiple anchoring functionalities, all of which are crucial
Additional Supporting Information may be found in the online version of this article.
C
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CHART 1 Structures of P1 and P2.
particles due to the difficulty for larger size polymers diffus-
ing through the nanoscale pores in TiO2 film. Thus, in the
present research, we carefully controlled the degree of poly-
merization (DP) to ensure effective polymer adsorption on
films and optimized PCE.
DSSC Fabrication and Characterization
The cells were fabricated as a sandwich with fluorine doped
tin(IV) oxide (FTO) conducting glass, nanocrystalline TiO2 as
the wide band-gap semiconductor with the dyes adsorbed,
I2/I3 electrolyte for charge regeneration and a Pt counter
2
electrode as a catalyst. The TiO2 paste, which was purchased
R
V
EXPERIMENTAL
from Solaronix was doctor-bladed onto a clean FTO glass
slide and sintered at 450 ꢁC for 30 min. The sintered elec-
trode was immersed into the polymer solution in DMF
[ꢀ0.2 mM, based on repeat units (RU)] for 24 h to allow for
dye adsorption. The counter electrode was prepared using
the reported method from drop-casting a solution of H2PtCl6
(2 mg in 1 mL ethanol) followed by heat treatment at 400
ꢁC for 15 min.19 Finally, an electrolyte solution containing
0.03 M I2, 0.05 M LiI, 0.1 M guanidinium thiocyanate, 1 M
Materials and Synthesis
All reagents were used as received unless otherwise speci-
fied. Toluene was distilled from sodium/benzophenone. Trie-
thylamine was refluxed with calcium hydride and distilled.
DMF was stirred with calcium hydride and distilled under
reduced pressure. 5-(4-(Bis(4-bromophenyl)amino)styryl)
thiophene-2-carbaldehyde was synthesized according to
previous report.18 N,N0-bis(4-bromophenyl)aniline, tributyl
(vinyl)tin, bis(triphenylphosphine)palladium chloride, palla-
dium acetate, tristolylphophine, cyanoacetic acid, and piperi-
dine were used as received from Sigma-Aldrich. Chloroform,
acetonitrile and other solvents were purchased from Fisher
Scientific. All reactions were carried out under nitrogen or
argon flow unless otherwise noted.
1,3-dimethyl imidazolium iodide, and 0.5
M
tert-
butylpyridine in a mixture of acetonitrile and valeronitrile
(volume ratio 5 85:15) was injected. The device was
assembled and sealed according to the literature.20 The
active area of the cell was ꢀ0.28 cm2.
The current-voltage characteristics of the cells were meas-
ured with a Keithley 2400 source meter under AM1.5 (100
mW/cm2) solar simulator. For IPCE measurements, the cells
were illuminated by monochromatic light from an Oriel Cor-
nerstone spectrometer, and the current response under short
circuit conditions was recorded at 10 nm intervals using a
Keithley 2400 source meter. The light intensity at each wave-
length was calibrated with an energy meter (S350, UDT
Instruments).
General Methods and Instrumentation
NMR spectra were obtained on a Bruker instrument operat-
ing at 400 MHz with chloroform-D and/or DMSO-D6 as sol-
vents. The molecular weight was measured by GPC and
calibrated by polystyrene standards with DMF as the solvent.
UV–vis absorption spectra were recorded on a Hewlett Pack-
ard 8453 spectrophotometer. Emission spectra were
recorded on an Edinburgh Instruments FLS920 emission
spectrometer, equipped with a Xenon light; the spectra are
corrected for instrument response. Excitation was at
400 nm, with inclusion of a 450 nm long-pass optical filter
before the detector. Cyclic voltammetry (CV) was carried out
on a computer-controlled CHI660D electrochemical worksta-
tion, where a glassy carbon electrode served as the working
electrode, a platinum electrode as the counter electrode, and
a Ag/AgNO3 as the reference electrode. Ferrocene (Fc) was
used as an external reference. A solution of tetrabutylammo-
nium hexafluorophosphate (0.1 M) in dry DMF was used
as the supporting electrolyte, and the scan rate was
Computational Methods
All molecular geometries were calculated by density func-
tional theory (DFT) with the B3LYP functional and the 6–
31G** basis set.21 Solvent environment effects were
described by using the polarizable continuum model with
the integral equation formalism variant for DMF. Tighter con-
vergence criteria and a more accurate numerical integration
grid were specified, to ensure finding the exact geometrical
minima. Frequencies were calculated and checked to make
sure that all frequencies were positive. Electronic spectra
were calculated using TD-DFT, based on the procedure
100 mV s21
.
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SCHEME 1 Synthesis of P2
previously outlined by Jacquemin et al.22 The geometry-
optimized structures were used in the TD-DFT calculations,
with the PBE0 functional,23 and the same basis-set and sol-
vent effects as in the geometry optimization. The adiabatic
approximation of time dependent DFT was used to solve for
60 singlet excited states.24 All calculations were done in
Gaussian 09, Revision C.01.25
in dark at 100 ꢁC for 5 h. A small amount of dichlorome-
thane was added to dilute the solution. After filtered, the
solution was added to diethyl ether to yield NP2 as a dark
red solid (0.092 g, 76%). Further purification was conducted
using dissolve-precipitate cycles several times, followed by a
size exclusion chromatography using THF as the eluent.
1H_NMR (400 MHz, CDCl3) d 9.83 (s, 1H), 7.64–6.97 (m,
31H). Mn 5 3500, Mw/Mn 5 1.5.
N,N0-Bis(4-vinylphenyl)benzenamine (1)
A mixture containing N,N0-bis(4-bromophenyl)aniline (0.5 g,
1.24 mmol), tributylvinyltin (2 mL, 6.70 mmol), 2,6-di-tert-
butylphenol (0.020 g, 0.10 mmol), Pd(PPh3)2Cl2 (0.035 g,
0.05 mmol), and toluene (10 mL) was bubbled with argon
for 20 min, and then stirred overnight at 100 ꢁC under
argon. After cooled to room temperature, the mixture was
diluted by addition of ether. KF (0.05 g) was added and
stirred for 4 h. The mixture was washed with water and
dried over anhydrous Na2SO4. After the solvent was distilled
out under reduced pressure, the crude was purified over
silica gel using hexane as the eluent to yield 1 as a yellow
oil (0.15 g, 41 %). 1H_NMR (400 MHz, CDCl3) d 7.28–7.21
(m, 6H), 7.09–7.00 (m, 7H), 6.64 (dd, 2H, J 5 9.6 Hz), 5.63
(d, 2H, J 5 17.5 Hz), 5.15 (d, 2H, J 5 10.8 Hz); 13C_NMR (100
MHz, CDCl3) d 147.19, 136.17, 132.10, 129.22, 127.06,
124.53, 124.34, 123.82, 123.58, 123.12, 122.89, 112.25;
Anal. Calcd C22H19N: C, 88.85; H, 6.44; N, 4.71; Found: C,
88.79; H, 6.56; N, 4.65.
P2
To a solution containing NP2 (0.056 g, 0.086 mmol, in RUs),
chloroform (10 mL), acetonitrile (1 mL), and DMF (1 mL),
cyanoacetic acid (0.020 g, 0.24 mmol), and piperidine
(0.5 mL) were added. The mixture was refluxed for 4 h
under nitrogen. After the solvent was distilled out under
reduced pressure, the residue was dissolved in DMF, filtered
and precipitated from acetone to yield P2 as an orange solid
(0.045 g, 73%). 1H_NMR (300 MHz, CDCl3) d 8.25 (br, 1H),
7.62–6.97 (m, 31H); Anal. Calcd (C48H33N3O2S)n: C, 80.53; H,
4.65; N, 5.87; Found: C, 80.29; H, 4.91; N, 5.90; Mn 5 3800,
Mw/Mn 5 1.5.
RESULTS AND DISCUSSION
The general synthetic route for P2 is illustrated in Scheme 1.
N,N’-Bis(4-vinylphenyl)benzenamine (1) was synthesized
from N,N0-bis(4-bromophenyl)aniline and tributyl(vinyl)tin
by Stille coupling reaction, with a yield of ꢀ41%. 5-(4-
(Bis(4-bromophenyl)amino)styryl)thiophene-2-carbaldehyde
was synthesized according to the literature,18 which was
condensed with monomer 1 to yield precursor polymer NP2
by Heck cross coupling reaction. Following the polymeriza-
tion reaction, NP2 was treated with a preparative size exclu-
sion chromatography to remove catalyst and oligomers. The
aldehyde group was then functionalized with cyanoacetic
NP2
To a solution containing 5-(4-(bis(4-bromophenyl)amino)s-
tyryl)thiophene-2-carbaldehyde (0.10 g, 0.18 mmol) and 1
(0.055 g, 0.18 mmol) in DMF (3 mL), previously bubbled
with argon for 20 min, palladium acetate (0.002 g, 0.0074
mmol), tris(o-tolyl)phosphine (0.011 g, 0.037 mmol), and
triethylamine (1 mL) were added. The mixture was stirred
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TABLE 1 Properties of Polymers
kabs (nm)a
e (104 cm21M21
)
kem (nm)a
/
b
Mn
PDI
fl
P1c
P2
3100
3800
1.70
1.50
372, 445
411, 490
3.6
3.9
615
715
0.021
0.0025
a
c
Estimated error in k max is 6 5 nm.
Determined using quinine sulfate in 0.1 M H2SO4 as the standard,
fl 5 0.546.27
The properties of P1 are in well agreement with previous work.18
b
/
acid via Knoevenagel condensation reaction in the presence
of piperidine as the catalyst. The condensation was moni-
tored by UV–vis absorption spectroscopy (see Fig. S1 in the
Supporting Information). The rising absorption band at
490 nm as a result of ICT characteristic indicates the appear-
ance of cyanoacetic acid; while the neutral precursor NP2
shows a single p-p* transition band at ꢀ420 nm correspond-
ing to the backbone and the thiophene side chains. A
detailed electronic state delocalization study will be dis-
cussed later in the calculation section.
lower excited state relative to P1, which is in agreement with
the shift in absorption spectra. Both P1 and P2 feature large
Stokes shifts of ꢀ200 nm. These large Stoke shifts suggest sig-
nificant energy loss upon excitation, which is likely due to the
ICT and/or the twisted polymer backbone (P1). The fluores-
cence quantum yields decrease significantly from P1 (2.1%)
to P2 (0.25%). Normally, this dramatic decrease is likely due
in part to narrowed energy gap resulting in increased nonra-
diative decay for P2.29
To better understand the structure-property relationship in
P1 and P2, we have calculated the frontier orbitals of the P1
and P2 RUs. Our previous calculation in a single P1 mono-
mer suggests a significantly efficient charge separation upon
excitation.18 However, in the previous calculation a possible
significant interaction from the approximate neighbor units
was omitted. In order to overcome this, we present here cal-
culations for two RUs, that is, ABAB type, instead of a single
monomer. These calculations were conducted on DFT opti-
mized geometry, as specified in the methods section. The
optimized molecular structures of both P1 and P2 are
shown in Figure 2. The connecting two triphenylamines in
P1 are twisted, while those in P2 are planar and transconfi-
guration, indicating an extended conjugation that is larger
than P1, accounting for the redshift (ꢀ40 nm) in the p-p*
transition band in UV–vis spectra. For each optimized geome-
try, we calculated the ground-state electronic transition using
TD-DFT (details can be found in the Methods section) and
determined the orbitals that contributed to these transitions.
The molecular weight and polydispersity index (PDI) was
determined by analytical GPC using polystyrene standards.
The number average molecular weight (Mn) of P2 is 3800,
with a PDI of 1.5 (Table 1). The DP, which is ꢀ5, was calcu-
lated from the formula weight of the RU and Mn. This DP
suggests the phenylene RU number of about 20, which is
similar to P1, and in agreement with the literature reported
optimal chain length for effective adsorption onto nanopo-
rous TiO2.26 The 1H_NMR spectrum of P2 after Knoevenagel
condensation does not show the protons of ACHO (ꢀ9.80
ppm) in NP2, consistent with complete deprotection.
The absorption and emission spectra of P2 were measured
in DMF solution. The spectra are shown in Figure 1, and a
tabulation of absorption and emission maxima, extinct coeffi-
cient (e) values, and fluorescence quantum yields (/fl) is pro-
vided in Table 1. The polymer concentration is calculated
based on the RU. For comparison, the absorption and emis-
sion spectra of P1 are also included. Both polymers feature
two-band absorption, where the long wavelength band corre-
sponds to an ICT from the electron-rich (triphenylamines-
vinyl-thiophene) segments to the electron-deficient (cyano-
acetic acid) unit, and the high energy band corresponds to
the p-p* transition of backbones. This character is typical for
donor-acceptor CPs and has been discussed.28 We note that
the p-p* transition band of P2 red-shifts by ꢀ40 nm relative
to that of P1, indicating an extended conjugation backbone
in P2. Meanwhile, a comparable redshift in the ICT band of
P2 relative to P1 is observed, which will be discussed in
details later in the calculation section. Another distinct
observation in the absorption spectra is that the extinct coef-
ficient of P2 (3.9 3 104 cm21M21 at 490 nm) increases by
10% relative to that of P1 (3.6 3 104 cm21M21 at 445 nm),
which suggests larger light harvesting capability for P2.
P2 exhibits weak red fluorescence with kmax 5 715 nm, while
P1 exhibits relatively strong fluorescence with kmax 5 615 nm.
The significant redshift of emission for P2, suggests a much
FIGURE 1 Normalized absorption and emission spectra of P1
and P2 in DMF solution, [RU] 5 10 mM.
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FIGURE 2 Optimized molecular structures of P1 and P2, two RUs are illustrated. Black: Carbon; Gray: Hydrogen; Blue: Nitrogen;
Red: Oxygen; Yellow: Sulfur. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
We show here, Figure 3, the contributing orbitals for the
strongest excitations from the high (3.22 eV for P1, 2.68 eV
for P2, HOMO to LUMO12, 14 transition) and low (1.89 eV
for P1, 1.72 eV for P2, HOMO to LUMO transition) energy
transition, see also Supporting Information. For both poly-
mers, the HOMO electrons are mainly localized at the triphe-
nylamines backbones; while small amount is delocalized at
the thiophene-vinyl-cyanoacetic acid in P2. A valuable note is
that both HOMOs are most localized at central three triphe-
nylamines, likely suggesting an effective conjugation length.30
The high energy p->p* transition in P2 is from orbitals
HOMO to LUMO12 and has a better overlap with stronger
oscillator strength (2.04) than that in P1, which corresponds
to HOMO to LUMO12 and LUMO14 with weaker oscillator
strength (1.49). The low energy transition is also a p->p*
transition, Figure 3, lower part. For both polymers, the
LUMO orbital is completely localized on the side groups, i.e.
vinyl-thiophene-vinyl-cyanoacetic acid. It demonstrates an
effective charge separation once the molecules are excited. It
is in agreement with the measured ICT band in absorption
spectra. The excited states of P2 are equally delocalized on
each side chains, suggesting a better charge separation than
those of P1. This pronounced intramolecular charge separa-
tion favors electron injection from polymer to an n-type
semiconductor, for example, TiO2.
Polymer DSSC device was fabricated as a sandwich of sensi-
tized TiO2 photoanode, electrolyte solution and Pt counter
FIGURE 3 Calculated frontier orbitals for P1 (top) and P2 (lower) RUs for the high and low energy transition. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 4 Photocurrent action spectra (a) and J-V characteristics (b) of two typical cells made from P1 and P2, selected from three
comparable devices each. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
electrode. The electrolyte solution contains 0.03 M I2, 0.05 M
LiI, 0.1 M guanidinium thiocyanate, 1 M 1,3-dimethyl imida-
zolium iodide, and 0.5 M tert-butylpyridine in a mixture of
acetonitrile and valeronitrile (volume ratio, 85:15). For each
polymer, three devices have been fabricated and tested to
affirm the reproducibility. The fabrication and test conditions
are described in details in the Supporting Information.
Herein, we show the typical performance as shown in Figure
4. Figure 4(a) illustrates the photocurrent action spectra
(IPCE) of the polymer sensitized solar cells obtained using
monochromatic illumination under short circuit conditions.
the delocalization at more acceptors, as shown in Figure 3,
both LUMO values are still positive compared to the TiO2 con-
duction band (ECB 5 23.9 eV),34 for charge injection into the
semiconductor.
Figure 4(a) shows the IPCE spectrum of P2 has broader
absorption relative to that of P1, which is consistent to that
seen in the polymers’ absorption spectra (Fig. 1). In particu-
lar, the DSSC cell fabricated with P1 has a peak external
quantum efficiency of about 40%, whereas the cells from P2
have a maximum IPCE value of 50%, indicating that at short
circuit condition P2 gives rise to a more efficient external
quantum yield than P1, which is in part due to the increased
extinct coefficient of P2. The current-voltage (J-V) character-
istics under AM1.5 illumination (100 mW cm22) of the poly-
mer sensitized DSSCs are shown in Figure 4(b). The
performances of the cells in terms of short-circuit current
density (Jsc), open-circuit voltage (Voc), fill factor (FF) and
overall PCE (gcell) are summarized in Table 3, along with the
maximum IPCE values. The photovoltaic parameters (Jsc, Voc,
The electrochemical properties of two polymers were charac-
terized using CV method in anhydrous and degassed DMF
solution (See Fig. S2 in the Supporting Information). The redox
values were listed in Table 2, where Eox is the oxidation poten-
tial of the ground state. The HOMO energy is estimated by the
equation, HOMO 5 2Eox versus Fc/Fc1 2 5.1 eV, where the Fc/
Fc1 redox couple is ꢀ25.1 eV versus vacuum in the Fermi
scale.31 This estimation has been established to determine the
energy levels of donor-acceptor polymers by CV.32 Assuming
the electron injection occurs after a relaxed excited state, the
LUMO energy is estimated as the oxidation potential of relaxed
state, Eoꢂx5 Eox 2 E0-0, where E0-0 5 1240/kem, estimated from
the emission maximum. The similar oxidation potential is in
agreement with their similar HOMO calculated localization at
triphenylamines backbones, and sufficiently more negative
than the redox potential of the iodine/iodide couple (24.95
eV),33 for the photo-oxidized polymers efficiently reduced by
iodide. Although the lower LUMO level of P2 is in part due to
FF, and gcell) of the cell made from P1 are 8.66 mA cm22
,
0.66 V, 0.58, and 3.30%. In contrast, the cell made from P2
has enhanced performance. Jsc, Voc, FF, and gcell of P2 are
8.82 mA cm22, 0.70 V, 0.59, and 3.67%, respectively.
In particular, the short-circuit current density is proportional
to the light harvesting efficiency (LHE) and electron injection
efficiency (Uinj). While the larger absorption coefficient of P2
contributes to an enhanced LHE, Uinj is proportional to the
driving force of injection, which is representative to the dif-
ference between the relaxed excited state energy level of
polymers (Eoꢂx) and the ECB of TiO2, assuming the electron
TABLE 2 Electrochemical Properties of Polymers
TABLE 3 Summary of Solar Cell Characteristics under AM1.5
E
ox/V vs
E0–0
(eV)b
HOMO
(eV)
LUMO
(eV)
Illumination
Compound
Fc1/Fca
P1
P2
0.32
0.36
2.02
1.73
25.42
25.46
23.40
23.73
Voc
Jsc
(mA/cm2)
FF
Maximum
IPCE (%)
gcell
(V)
(%)
(%)
P1
P2
0.66
0.70
8.66
8.82
58
59
42.0
50.6
3.30
3.67
a
Determined as the onset oxidation in CV spectra.
b
Estimated from emission maximum, 1240/kem
.
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rently, P2 has a weaker driving force than P1 due to the
more negative LUMO. However, the enhanced extinct coeffi-
cient of P2 relative to P1 is indicative of a larger LHE. Tak-
ing both factors together, there is a small increase in the
photocurrent, that is, from 8.66 to 8.82 mA/cm2. Voc repre-
sents the energy difference between the redox potential of
electrolyte and the quasi-Fermi level of TiO2, while the for-
mer remains constant and the latter one shifts due to many
factors, for example dipole dependence from different
dyes.35 The increased dipole moment of the RU of P2 rela-
tive to that of P1 due to introduction of vinyl links and the
planar backbones may produce an upshift of the quasi-Fermi
level and consequently enhance the Voc of the DSSC.
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10 W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F.
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11 B. -G. Kim, K. Chung, J. Kim, Chem. Eur. J. 2013, 19, 5220–
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CONCLUSIONS
12 J. Hou, H.-Y. Chen, S. Zhang, Y. Yang, Y. Wu, G. Li, J. Am.
Chem. Soc. 2009, 131, 15586–15587.
In conclusion, a donor (triphenylamines backbone)-acceptor
(cyanoacetic acid side group) type CP (P2) has been synthesized
and used as active materials for DSSCs. DFT calculation shows
that the HOMO electrons are mainly localized at the approxi-
mate three triphenylamine-vinyl and thiophene links, suggest-
ing the effective conjugation length. The twisted backbone in P1
becomes planar in P2. The significant charge transfer state fea-
tures a dominant low energy transition, and is localized at each
thiophene-vinyl-cyanoacetic acid side groups; while polymers
without vinyl in the backbone (P1) only alternatingly localize
the ICT at every side group. Significant red shift at both p-p*
transition band (high energy) and ICT band in P2 relative to P1
leads to an enhanced light harvesting capability.
13 W. Ma, C. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct.
Mater. 2005, 15, 1617–1622.
14 Y. Li, Acc. Chem. Res. 2012, 45, 723–733.
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16 D. W. Chang, S.-J. Ko, J. Y. Kim, S.-M. Park, H. J. Lee, L.
Dai, J.-B. Baek, Macromol. Rapid Commun. 2011, 32, 1809–1814.
17 H. Tan, C. Pan, G. Wang, Y. Wu, Y. Zhang, X. Chen, Y. Zou,
G. Yu, M. Zhang, RSC Adv. 2013, 3, 16612–16618.
18 W. Zhang, Z. Fang, M. Su, M. Saeys, B. Liu, Macromol.
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19 S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Gratzel, M. K.
Nazeeruddin, M. Gratzel, Thin Solid Films 2008, 516, 4613–4619.
Photoelectrochemical cells based on the DSSC format were
fabricated using the polymers as sensitizers. By carefully
controlling the molecular weight of the polymer, a consider-
able amount of P2 has been adsorbed on metal oxide. The
cell constructed using P2 exhibits a considerably higher
peak IPCE than that using P1. The DSSC fabricated with P2
20 S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska, R.
ꢀ
Charvet, P. Comte, M. K. Nazeeruddin, M. Pechy, M. Takata, H.
€
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21 (a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652; (b) C.
Lee, W. Yang, R. G. Parr, Phys. Rev. B Condens. Matter 1988,
37, 785–789.
exhibits enhanced performance, with Jsc of 8.82 mA cm22
,
22 (a) D. Jacquemin, E. A. Perpete, G. E. Scuseria, I. Ciofini, C.
Adamo, J. Chem. Theory Comput. 2008, 4, 123–135; (b) D.
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Chem. 2010, 110, 2121–2129.
Voc of 0.70 V, FF of 59%, and gcell of 3.67%. In general, the
design principle in this study shows that the insertion of
vinyl in the backbone results in planar conformation and
improved light harvesting. Future work may search for more
efficient donor-acceptor couples and study the intrinsic pho-
ton excitation, charge separation, and injection.
23 (a) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett.
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ACKNOWLEDGMENT
25 M. J. T. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.
P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F.
Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
The authors acknowledge support from the Department of
Chemical and Biomolecular Engineering, National University of
Singapore, and support from the UNC EFRC: Center for Solar
Fuels, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-SC0001011 supporting Z.
Fang and S. Keinan.
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