Influence of the auxiliary acceptor on the absorption response and photovoltaic performance of dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201402608

Three new dyes with a 2- (1,1-dicyanomethylene)rhodanine (IDR-I, -II, -III) electron acceptor as anchor were synthesized and applied to dye-sensitized solar cells. We varied the bridging molecule to fine tune the electronic and optical properties of the dyes. It was demonstrated that incorporation of auxiliary acceptors effectively increased the molar extinction coefficient and extended the absorption spectra to the near-infrared (NIR) region. Introduction of 2,1,3-benzothiadiazole (BTD) improved the performance by nearly 50%. The best performance of the dye-sensitized solar cells (DSSCs) based on IDR-II reached 8.53% (short-circuit current density (J_sc)=16.73 mAcm^-2, open-circuit voltage (V_oc)=0.71 V, fill factor (FF)= 71.26%) at AM 1.5 simulated sunlight. However, substitution of BTD with a group that featured the more strongly electron-withdrawing thiadiazolo [3,4- c]pyridine (PT) had a negative effect on the photovoltaic performance, in which IDR-III-based DSSCs showed the lowest efficiency of 4.02%. We speculate that the stronger auxiliary acceptor acts as an electron trap, which might result in fast combination or hamper the electron transfer from donor to acceptor. This inference was confirmed by electrical impedance analysis and theoretical computations. Theoretical analysis indicates that the LUMO of IDR-III is mainly localized at the central acceptor group owing to its strong electron-withdrawing character, which might in turn trap the electron or hamper the electron transfer from donor to acceptor, thereby finally decreasing the efficiency of electron injection into a TiO₂ semiconductor. This result inspired us to select moderated auxiliary acceptors to improve the performance in our further study.

Full text of DOI:10.1002/asia.201402608

FULL PAPER
DOI: 10.1002/asia.201402608
Influence of the Auxiliary Acceptor on the Absorption Response and
Photovoltaic Performance of Dye-Sensitized Solar Cells
ZhiFang Wu,[a] Xin Li,*[b] Jing Li,[a] JianLi Hua,*[a] Hans ꢀgren,[b] and He Tian[a]
Abstract: Three new dyes with a 2-
(1,1-dicyanomethylene)rhodanine
(Jsc)=16.73 mAcmꢀ2, open-circuit volt-
age (Voc)=0.71 V, fill factor (FF)=
71.26%) at AM 1.5 simulated sunlight.
However, substitution of BTD with
a group that featured the more strongly
hamper the electron transfer from
donor to acceptor. This inference was
confirmed by electrical impedance
analysis and theoretical computations.
Theoretical analysis indicates that the
LUMO of IDR-III is mainly localized
at the central acceptor group owing to
its strong electron-withdrawing charac-
ter, which might in turn trap the elec-
tron or hamper the electron transfer
from donor to acceptor, thereby finally
decreasing the efficiency of electron in-
jection into a TiO2 semiconductor. This
result inspired us to select moderated
auxiliary acceptors to improve the per-
formance in our further study.
(IDR-I, -II, -III) electron acceptor as
anchor were synthesized and applied to
dye-sensitized solar cells. We varied the
bridging molecule to fine tune the elec-
tronic and optical properties of the
dyes. It was demonstrated that incorpo-
ration of auxiliary acceptors effectively
increased the molar extinction coeffi-
cient and extended the absorption
spectra to the near-infrared (NIR)
region. Introduction of 2,1,3-benzothia-
diazole (BTD) improved the perfor-
mance by nearly 50%. The best perfor-
mance of the dye-sensitized solar cells
(DSSCs) based on IDR-II reached
8.53% (short-circuit current density
electron-withdrawing thiadiazoloACTHNUTRGNEUG[N 3,4-
c]pyridine (PT) had a negative effect
on the photovoltaic performance, in
which IDR-III-based DSSCs showed
the lowest efficiency of 4.02%. We
speculate that the stronger auxiliary ac-
ceptor acts as an electron trap, which
might result in fast combination or
Keywords: charge transfer · donor–
acceptor systems · dyes/pigments ·
electron trap · sensitizers
Introduction
a high absorption coefficient. In addition to that, organic
dyes have a versatile functional group for tuning electronic
and optical properties.[4] One successful strategy to improve
the performance of DSSCs is by varying the components
such as donor, bridge, and acceptor. Previously our group
reported a highly efficient dye that incorporates the auxili-
ary acceptor 2,1,3-benzothiadiazole (BTD).[5] The auxiliary
electron acceptor significantly increased the molar extinc-
tion coefficient as well as short-circuit current density (Jsc)
and redshifted the absorption, thereby resulting in an effi-
ciency of 9.02%. Meanwhile, substitution of the traditional
cyanoacetic acid with the stronger acceptor of 2-(1,1-dicya-
nomethylene)rhodanine (DCRD) displayed similar proper-
ties.[6]
Over the past decades, dye-sensitized solar cells (DSSCs)
have been extensively investigated as an alternative to tradi-
tional solar cells based on silicon.[1] To date, power-conver-
sion efficiencies (PCE) have reached more than 11% with
ruthenium-based dyes.[2] However, ruthenium-based dyes
have some disadvantages such as high cost, use of noble
metals, and difficulties in the synthesis and purification that
limit their wider application. Recently, the most efficient of
the DSSCs reached 13%, which was reported by Grꢀtzel
et al.[3] The main advantage of DSSCs that use metal-free or-
ganic dyes is that they have facile synthetic strategies and
Because dyes with good planarity are vulnerable to form
unfavorable p–p stacking, this aggregation might lead to
back-electron transfer, thereby limiting electron accumula-
tion in the TiO2 conduction band and giving low open-cir-
cuit voltage (Voc), whereas long-chain alkyls were widely
used to disrupt the aggregation and improve the solubility
of dye. Based on dye RD-II,[6] we substituted the traditional
[a] Z. Wu, Dr. J. Li, Prof. Dr. J. Hua, Prof. Dr. H. Tian
Key Laboratory for Advanced Materials
Institute of Fine Chemicals
East China University of Science and Technology
Shanghai 200237 (China)
Fax : (+86)-264252756
E-mail: jlhua@ecust.edu.cn
[b] Dr. X. Li, Prof. Dr. H. ꢁgren
Department of Theoretical Chemistry and Biology
School of Biotechnology
triphenylamine-branching chromophore with
a stronger
donor indoline derivative and incorporated n-hexylthio-
phene into the bridge to construct dye IRD-I.
ACHUTNGREN[NUG 1,2,5]ThiadiazoloACHTUNGTNER[NUGN 3,4-c]pyridine (PT) possesses a higher
KTH Royal Institute of Technology
Stockholm, 10691 (Sweden)
E-mail: lixin@theochem.kth.se
electron-withdrawing ability than BTD. PT-based polymers
have been widely used in applications such as bulk hetero-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402608.
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Xin Li, JianLi Hua et al.
junction (BHJ) solar cells.[4j] Heeger reported the highest
conversion efficiency of 6.7% for solution-processed small-
molecule solar cells by incorporating PT into the small mol-
ecule.[4o] Herein, based on the strategy that both a strong
donor and acceptor can shift the absorption band toward
the near-infrared (NIR) region and increase the molar ex-
tinction coefficient, which can ultimately improve the short-
circuit current density (Jsc),[6,7] we further introduced BDT
or PT into the bridge to construct dyes IDR-II and IDR-III,
respectively.
Scheme 1. Molecular structures of IDR-I–III.
Results and Discussion
Synthesis and Characterization
The molecular structures of
dyes IDR-I–III and the synthet-
ic routes to them are shown in
Schemes 1 and 2. All of the
dyes contain the same strong
donor (indoline) and acceptor
(DCRD). We varied the
p
bridge to tune the electronic
and optical properties of IDR-
I–III. 4,7-Dibromobenzothiadia-
zole (BTD) and 3-hexylthio-
phene are low cost and com-
mercially available. The critical
intermediate (7) was obtained
by a selective Suzuki coupling
reaction.[8] In the Suzuki cou-
pling reaction, preferential oxi-
dative coupling occurs at the
more
electron-deficient
4-
carbon position of pyridine de-
rivates, thus leading to the pyr-
idyl N atoms proximal to the
donor. It is important to high-
light the regiochemistry of 7, in
which the pyridal N atoms of
the PT acceptor unit are orient-
ed towards the 3-hexylthio-
phene moiety (proximal config-
uration). All the bromination
products were used for the next
step without further purifica-
Scheme 2. Synthetic route to dyes IDR-I–III. Reagents and conditions: a) NBS, CH2Cl2, and CH3COOH, 6 h.
b) 5-Formyl-2-thiopheneboronic acid, K2CO3, [Pd(dppf)Cl2], THF, H2O, and N2 atmosphere. c) NBS, CH2Cl2,
and CH3COOH, 6 h. d) 7-Bromo-1,2,3,3a,4,8b-hexahydro-4-(4-methylphenyl)cyclopent[b]indole, nBuLi, iso-
propyl borate, [Pd(dppf)Cl2], K2CO3, THF, H2O, and N2 atmosphere. e) SOCl2 (excess amount), reflux. f) 3-
Hexylthiophene, nBuLi, isopropyl borate, K2CO3, [Pd(dppf)Cl2], THF, H2O, and N2 atmosphere. g) NBS,
CH2Cl2, 6 h. h) 7-Bromo-1,2,3,3a,4,8b-hexahydro-4-(4-methylphenyl)cyclopent[b]indole, nBuLi, isopropyl
borate, K2CO3, [Pd(dppf)Cl2], THF, H2O, and N2 atmosphere. i) NBS, CH2Cl2, and CH3COOH, 6 h. j) 5-
Formyl-2-thiopheneboronic acid, K2CO3, [Pd(dppf)Cl2], THF, H2O, and N2 atmosphere. k) DCRD, EtOH,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
tion. The target products IDR- NaOH, reflux.
I–III were synthesized by
means of Knoevenagel conden-
Optoelectronic Properties
sation reaction in the presence of a catalytic amount of pi-
peridine in THF. All compounds were purified by means of
column chromatography or recrystallization and character-
The UV/Vis absorption spectra of the samples in CH2Cl2 are
shown in Figure 1, and the data are summarized in Table 1.
Generally, organic dyes show two main absorption bands:
intramolecular charge transfer (ICT) absorption and a p–p*
absorption band.[9] In this paper, however, dyes IDR-II and
IDR-III display three absorption bands. The absorption
bands in the UV region for the three dyes originated from
1
ized by standard H and 13C NMR spectroscopy and HRMS.
IDR-I–III are black powders, thus indicating great light-har-
vesting ability.
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Figure 1. UV/Vis absorption spectra of solutions of IDR-I–III in CH2Cl2.
Table 1. Optical and electrochemical properties of IDR-I–III.
Figure 2. UV/Vis absorption spectra of IDR-I–III adsorbed on TiO2
films.
[a]
[b]
[c]
[d]
eꢃ104
EHOMO
(vs NHE)
E0–0
ELUMO
ACHTUNGTRNE(NUNG vs NHE)
had no clear shift when adsorbed on film, which is possibly
because strong electron-withdrawing units weaken the large
Dye
lmax
[nm]
G
]
N
[eV]
blueshift in the ICT band caused by the deprotonation of
IDR-I
IDR-II
IDR-III
504
538
582
2.66
3.53
3.15
0.75
0.70
0.68
2.02
1.94
1.78
ꢀ1.37
ꢀ1.24
ꢀ1.10
[5]
ꢀ
the DCRD group ( CONH).
Cyclic voltammetry (CV) was carried out to determine
the redox properties of dyes IDR-I–III (Figure 3). The oxi-
dation potentials that correspond to the HOMO are deter-
mined exclusively by the donors. As shown in Table 1, the
three dyes exhibited the same HOMO level; they are more
positive than that of the electrolyte mediator, thus indicating
that the oxidized dye could be regenerated effectively by
Iꢀ1/I3ꢀ. E0–0 was estimated from the intersection of the UV/
Vis absorption and the emission spectra of the dyes; the
values were 2.02, 1.94, and 1.78 eV, respectively. The de-
crease in the bandgap is consistent with the increase in the
electron-withdrawing ability of the auxiliary acceptor and
the redshift in the absorption spectra. The LUMO was esti-
mated to be ꢀ1.37, ꢀ1.24, and ꢀ1.10 V versus the normal
hydrogen electrode (NHE), respectively, thus indicating
a sufficient driving force for electron injection from the oxi-
dized dye into the conduction band. It is worth noting that
all the three dyes are redox stable since their CV curves
almost overlap after repeated scanning.
[a] Absorption maximum in CH2Cl2 (1ꢃ10ꢀ5 m). [b] The ground-state oxi-
dation potential of dyes was measured in a dichloromethane with 0.1m
tetra-n-butylammonium hexafluorophosphate (TBAPF6) as electrolyte
(Pt working electrode, saturated calomel electrode (SCE) reference elec-
trode calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external ref-
erence, Pt counter electrode). [c] E0–0 was estimated from the intersection
of UV/Vis absorption and emission spectra of the dyes. [d] ELUMO was
calculated as EHOMOꢀE0–0
.
local p–p* transitions and their peaks were located at 383,
333, and 333 nm, respectively. IDR-I and IDR-II showed the
maximum absorption wavelength at 504 and 538 nm, respec-
tively. The corresponding molar extinction coefficients were
26657 and 35365mꢀ1 cmꢀ1, which could be ascribed to ICT.
Clearly, the incorporation of the auxiliary acceptor redshift-
ed the absorption spectra and increased the molar extinction
coefficient. This phenomenon is consistent with previous re-
ported results. We further substituted the BTD group with
a stronger acceptor (PT) to construct IDR-III. IDR-III
showed the maximum wavelength (lmax) at 582 nm as well as
a molar extinction coefficient of 31573mꢀ1 cmꢀ1 and realized
a broad spectral response. Compared to IDR-I, the redshifts
of the absorption band of IDR-II and IDR-III were 33 and
80 nm, respectively. The absorption spectra of IDR-II and
IDR-III extended to 750 and 800 nm, respectively, thus indi-
cating better light-harvesting ability. Therefore a high Jsc
value could be expected. The origin of an additional band
around 450 nm and higher absorption intensity for IDR-III
relative to IDR-II in this band will be discussed below.
The absorption spectra of IDR-I–III on transparent film
are shown in Figure 2; the maximum absorption peaks were
located at 527, 539, and 578 nm, respectively. Compared to
the absorption spectra of samples in solution, the redshift of
23 nm for IDR-I could be ascribed to the formation of J ag-
gregation.[10] However, the spectra of IDR-II and IDR-III
Figure 3. Cyclic voltammetry curves of solutions of IDR-I–III in CH2Cl2.
3
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Photovoltaic Properties
Organic dyes tend to aggregate when anchored to TiO2 film,
so a coadsorption additive was used to disrupt aggregation
and improve cell performance. Photocurrent density–voltage
(I–V) data under different chenodeoxycholic acid (CDCA)
concentrations are listed in Table 2. The devices were opti-
mized using 10ꢃ10ꢀ3 m CDCA as a coadsorption additive,
which was able to significantly improve the fill factor (FF).
Table 2. Photovoltaic performance of CDCA with different concentra-
tions coadsorbed DSSCs based on dyes IDR-I–III.
Dye
CDCA [m]
JSC [mAcmꢀ2
]
VOC [V]
FF
h [%]
Figure 5. IPCE curve of DSSCs based on dyes IDR-I–III.
IDR-II
IDR-II
IDR-II
IDR-I
5ꢃ10ꢀ3
10ꢃ10ꢀ3
20ꢃ10ꢀ3
10ꢃ10ꢀ3
10ꢃ10ꢀ3
15.54
16.73
15.25
11.21
11.01
0.67
0.71
0.68
0.62
0.54
64.89
71.26
64.77
72.25
67.13
6.83
8.53
6.74
5.00
4.02
(IPCE) to that of IDR-I. The IPCE of IDR-II reached 65%
from 450 to 650 nm, and nearly 45% at 700 nm. However,
substitution of BTD with a group (PT) that featured a stron-
ger electron-withdrawing ability had a negative effect on
performance. DSSCs based on IDR-III showed a Jsc of
11.01 mAcmꢀ2, a Voc of 0.54 V, and a FF of 67.13%, thus
giving the lowest power conversion efficiency (h) of 4.02%.
Interestingly, the previously reported dye that contained the
PT group also gave a lower efficiency, which was ascribed to
the low voltage of 0.43 V.[12] Because molecular planarity fa-
cilitates dye aggregation, thereby resulting in fast recombi-
nation, we performed a theoretical analysis to calculate the
dihedral angles between the auxiliary acceptor and the adja-
cent aromatic ring. As shown in Scheme 3 and Table 3, after
incorporating an auxiliary acceptor, the dihedral angle (a)
between the indoline donor and auxiliary acceptor increased
from 22.9 to 31.68 for IDR-II, which partially contributed to
decreased recombination. Relative to IDR-I, the increased
dihedral angle of IDR-II is consistent with an increased
IDR-III
The photocurrent density–voltage curves of the cells were
measured under an irradiance of 100 mWcmꢀ2 simulated
AM 1.5 sunlight. CDCA (10ꢃ10ꢀ3 m) was used as a coadsorp-
tion additive to construct the cells based on three dyes. As
shown in Figure 4 and Table 2, DSSCs based on dye IDR-I
displayed
a
short-circuit
photocurrent
(Jsc)
of
11.21 mAcmꢀ2, an open-circuit voltage (Voc) of 0.62 V, and
a fill factor of 72.25%, thereby resulting in a power conver-
sion efficiency (h) of 5.00%. After introducing BTD, a signif-
icant increase in Jsc was observed relative to dye IDR-I. The
device based on dye IDR-II showed a Jsc of 16.73 mAcmꢀ2,
a Voc of 0.71 V, and a FF of 71.26%, thus giving a power
conversion efficiency (h) of 8.53%. This improvement was
ascribed to an increase in molar extinction coefficient and
a redshift of the absorption band that extended beyond
700 nm. BTD also played a role in facilitating the electron
transfer from donor to acceptor, and then electrons were in-
jected into the conduction band.[7a,11] As shown in Figure 5,
over the whole simulated AM 1.5 spectra, IDR-II had a rela-
tively high incident photon-to-electron conversion efficiency
Scheme 3. Dihedral angles between auxiliary acceptors and the adjacent
aromatic ring.
Table 3. Optimized dihedral angles in the three sensitizer dyes IDR-I–
III.
Compound
a [8]
b [8]
IDR-I
IDR-II
IDR-III
22.9
31.6
30.9
–
0.2
0.2
Figure 4. I--V curves of devices based on dyes IDR-I–III.
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Xin Li, JianLi Hua et al.
Jsc.[13a] However, the dihedral angle a of IRD-III is almost
the same as that of IRD-II. Furthermore, the dihedral
angles b between the auxiliary acceptor and the adjacent
hexylthiophene in IRD-II and IRD-III are both 0.28. Thus,
compounds IRD-II and IRD-III show a very similar degree
of twisting, and we could therefore rule out the planariza-
tion effects of the pyridinothiadiazole unit upon fast recom-
bination. Herein we attribute the decreased efficiency to the
electron-withdrawing ability that is too strong, which results
in fast combination of electrons in the conduction band of
titanium dioxide with electron mediators.[13] We also infer
that the acceptor that is too strong behaves like an electron
trap, which hinders the electron transfer from donor to ac-
ceptor.[14] This speculation was confirmed by means of com-
putational analysis, which will be discussed below. A de-
creased energy bandgap (0.33 eV) between the HOMO of
the dye IDR-III and the electrolyte mediator (Iꢀ1/I3ꢀ),
which might fail to provide sufficient driving force for an
electron-transfer reaction, might possibly lower the dye-re-
generation efficiency. Both of these factors have a synergistic
impact upon lowering Jsc. Although IDR-III displayed
a better IPCE beyond 630 nm and a superior absorption
performance, this superiority was offset by a lower IPCE in
the region between 350 and 630 nm.
To gain insight into the lowest open-circuit voltage of
IDR-III, which was ascribed to fast electron recombination,
we performed electrochemical impedance spectroscopy. The
Bode and Nyquist plots are shown in Figures 6 and 7. The
radius of the large semicircle decreased in the order of
IDR-III<IDR-I<IDR-II, thus indicating that electron-re-
combination resistance actually increased in the order of
IDR-II>IDR-I>IDR-III, which was consistent with varia-
tion in the Voc value. In the Bode phase plot, the peak of the
middle frequency correlates with the electron lifetime, so
the low frequency corresponds to long electron lifetime,
which reflects slow recombination. The lifetime increased in
the order of IDR-II>IDR-I>IDR-III. This result was also
consistent with the increase in Voc. The electron-combination
resistance and electron lifetime could also explain the low
Figure 7. Bode-phase plots of impedance spectra that were all measured
by applying a bias of 0.65 V in the dark.
Jsc of IDR-I and IDR-III relative to IDR-II. We were in-
spired that we could select an auxiliary acceptor with mod-
erate electron-withdrawing ability to facilitate electron
transfer without resulting in fast recombination.
The electron lifetime (tn), which reflects the response-
time constant for recombination, is the product of recombi-
nation resistance and capacitance.[15] The fitted relationship
between the electron lifetime in DSSCs and the capacitance
is shown in Figure 8, which was obtained from Nyquist plots
Figure 8. Electron lifetime as a function of capacitance in IDR-I–III-
based DSSCs, which were obtained by EIS under various biases in the
dark.
measured under different biases in the dark. According to
Figure 8, the electron lifetime for IDR-II-based DSSCs was
the longest, whereas IDR-III-based DSSCs had the shortest
electron lifetime, which was one magnitude lower than
IDR-II, thus indicating a fast recombination rate in IDR-
III-based DSSCs.
Computational Analysis
We used theoretical computation to understand the differen-
ces in the absorption spectra and performance of the three
dyes. The frontier molecular orbitals of compounds IDR-I,
Figure 6. Nyquist-phase plots of impedance spectra that were all mea-
sured by applying a bias of 0.65 V in the dark.
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Xin Li, JianLi Hua et al.
Table 4. Computed contour plots of frontier molecular orbitals at the B3LYP/6-31G* level of theory.
IDR-I
IDR-II
IDR-III
HOMOꢀ1
HOMO
LUMO
LUMO+1
IDR-II, and IDR-III are presented in Table 4, in which one
can see that the LUMO is mostly localized at the peripheral
acceptor group of compound IDR-I. In compound IDR-II,
the LUMO receives contribution from both the central and
the peripheral acceptor groups, and the LUMO+1 is largely
localized on the central acceptor group. This is similar to
previously reported D–A–p–A-type sensitizer dyes for high-
performance solar cells.[16] On the contrary, the LUMO of
compound IDR-III shows considerable localization on the
central acceptor group, and the peripheral acceptor group
contributes much less to the LUMO than the other two
compounds. This may lead to inefficient electron injection
into the TiO2 semiconductor upon solar-light irradiation. We
further analyzed the fragmental contribution to the frontier
molecular orbitals of these sensitizer dyes from their D, A,
p, and A groups, as shown in the Supporting Information.
Here the contribution from the peripheral acceptor group to
LUMO follows the order: IDR-I>IDR-II>IDR-III. In
compound IDR-II, the peripheral acceptor group contrib-
utes more to the LUMO and the central acceptor group
contributes more to the LUMO+1. However, in compound
IDR-III this is reversed; the central acceptor group contrib-
utes more to the LUMO, whereas the peripheral acceptor
group contributes more to the LUMO+1. This agrees with
the inefficient electron injection from IDR-III and may thus
hamper the efficiency of the sensitizing process.
ceptor. The S2 states arise from combined electronic transi-
tions of HOMOꢀ1!LUMO and HOMO!LUMO+1;
however, the corresponding absorptions are at different
spectral regions, that is, approximately 350 nm for IDR-I
and approximately 440 nm for IDR-II and IDR-III, respec-
tively. As shown in Figure 9, the oscillator strengths of the
S2 states are unfortunately underestimated by the TD-DFT
calculations; nonetheless, the theoretical simulations provide
explanations for the different absorption spectra of these
sensitizer dyes. The absorption band around 450 nm, which
is present in IDR-II and IDR-III and absent in IDR-I, origi-
nates from electronic transitions involving the central ac-
ceptor group. This absorption band exhibits stronger intensi-
ty in IDR-III than that in IDR-II, which reflects the stron-
ger electron-withdrawing character of the central acceptor
group in IDR-III.
Our theoretical calculations thus provide interpretation
for several phenomena relevant to the performance of the
sensitizers. First, compound IDR-I does not have an absorp-
From time-dependent (TD) DFT calculations we obtained
the excitation wavelengths of compounds IDR-I, IDR-II,
and IDR-III in the visible spectral region, in which the
lowest singlet excitation energies are in excellent agreement
with the experimentally observed spectra. As shown in the
Supporting Information, all the S1 states receive a large con-
tribution from the HOMO!LUMO transition correspond-
ing to electron transfer from the donor to the terminal ac-
Figure 9. Simulated absorption spectra of IDR-I–III.
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Xin Li, JianLi Hua et al.
using a normal three-electrode cell with a Pt working electrode, a Pt-wire
counter electrode, and a regular calomel reference electrode in saturated
KCl solution. The current density versus voltage ACTHNUTRGNEUG(N J–V) characteristics of
tion band at around 450 nm, since the central acceptor
group is not present. Second, the central acceptor group of
IDR-III has stronger electron-withdrawing character than
that of IDR-II, which leads to a redshifted absorption maxi-
mum and an enhanced absorption band around 450 nm.
Third, the LUMO of IDR-III is largely localized at the cen-
tral acceptor group owing to its strong electron-withdrawing
character, which may in turn hamper the efficiency of elec-
tron injection into the TiO2 semiconductor. The frontier mo-
lecular orbitals and absorption spectrum of IDR-II show
similar characters to previously reported D–A–p–A sensitiz-
er dyes[16] and are thus expected to be beneficial for photo-
voltaic efficiency.
the DSSCs were measured by recording J–V curves using a Keithley 2400
source meter under the illumination of AM 1.5 G simulated solar light
(Newport-91160 equipped with a 300 W Xe lamp and an AM 1.5 G
filter). The incident light intensity was calibrated to 100 mWcmꢀ2 with
a standard silicon solar cell (Newport 91150 V). Action spectra of the in-
cident monochromatic photon-to-electron conversion efficiency (IPCE)
for the solar cells were obtained with a Newport-74125 system (Newport
Instruments). The intensity of the monochromatic light was measured
with a Si detector (Newport-71640).
Synthesis of 2-Bromo-3-hexylthiophene (1)
3-Hexylthiophene (2.00 g, 11.9 mmol) was dissolved in a mixed solution
of CH2Cl2 (20 mL) and CH3COOH (20 mL), then N-bromosuccinimide
(NBS; 2.22 g, 12.5 mmol, 1.05 equiv) was added. After the reaction was
complete, the mixture was neutralized with saturated NaHCO3, CH2Cl2
(100 mL) was added, and the organic layer was separated. The aqueous
solution was washed with CH2Cl2 twice. After removing the solvent, the
residue was used for the next step without further purification.
Conclusion
In summary, a different auxiliary acceptor was introduced
into the molecular design of organic sensitizers, and three
dyes were synthesized. Among them, IDR-II-based DSSCs
showed the highest overall conversion efficiency of 8.53%
with Voc =710 mV, Jsc =16.73 mAcmꢀ2, and a fill factor
(FF)=0.71 after a 10 mm chenodeoxycholic acid (CDCA)
Synthesis of 3’-Hexyl-[2,2’-bithiophene]-5-carbaldehyde (2)
In a 100 mL round-bottomed flask with a condenser, compound 1 (1.90 g,
7.70 mmol), 5-formyl-2-thiopheneboronic acid (1.00 g, 6.40 mmol), K2CO3
(1.38 g, 100 mmol), and a catalytic amount of [PdACTHNUTRGEN(UNG dppf)Cl2] (dppf=1,1’-
bis(diphenylphosphino)ferrocene) was dissolved in a solution of THF
(20 mL) and H2O (4 mL). The mixture was heated at reflux overnight
under an N2 atmosphere. After the reaction was complete, CH2Cl2
(20 mL) was added and the organic layer was separated. Then, the aque-
ous solution was washed with CH2Cl2 twice. After removing the solvent
by evaporation, the residue was purified by chromatography to afford
compound 2 (1.40 g, 79%). 1H NMR (400 MHz, CDCl3): d=9.79, (s,
1H),7.62 (d, J=3.9 Hz, 1H), 7.19 (d, J=5.1 Hz, 1H), 7.13 (d, J=3.9 Hz,
1H), 6.89 (d, J=5.2 Hz, 1H), 2.73 (dd, 2H), 1.50–1.52 (m, 6H), 1.31–1.21
(m, 2H), 0.80 ppm (t, J=6.6 Hz, 3H).
treatment
under
standard
illumination
conditions
(100 mWcmꢀ2 simulated AM 1.5 solar light). We illustrate
that an auxiliary acceptor with appropriate electron-with-
drawing ability increases the molar extinction coefficient,
extends the absorption spectra, and facilitates electron
transfer, which finally improve the Jsc. In contrast, too
strong an auxiliary acceptor facilitates the recombination
between electrons in the conduction band with an electro-
lyte mediator and traps electrons resulting in hampering of
the electron transfer from donor to acceptor. In our future
work, we will further explore the effect of the auxiliary ac-
ceptor on the photovoltaic performance.
Synthesis of 5’-Bromo-3’-hexyl-[2,2’-bithiophene]-5-carbaldehyde (3)
Compound 2 (1.10 g, 3.96 mmol) was dissolved in CH2Cl2 and cooled to
08C, then NBS (0.70 g, 3.96 mmol) was added into the solution and the
mixture was stirred for 6 h. After the reaction was complete, CH2Cl2
(20 mL) was added and the organic layer was separated. Then the aque-
ous solution was washed with CH2Cl2 twice. Removal of the solvent af-
forded the product, which was used for the next step without further pu-
rification.
Experiment Section
Synthesis of 3’-Hexyl-5’-{4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]in-dol-7-yl}-[2,2’-bithiophene]-5-carbaldehyde (4)
Materials
Optically transparent fluorine-doped SnO2 (FTO) conducting glass
Compound 3 (300 mg, 0.84 mmol), compound 5 (an excess amount), a cat-
alytic amount of [PdACHTNUGTRNE(UNG dppf)Cl2], and K2CO3 (1.38 g, 10 mmol) were dis-
(transmission >90% in the visible region, sheet resistance 15 Wsquareꢀ1
)
was obtained from the Geao Science and Educational Co. Ltd. of China
and cleaned by a standard procedure. Methoxypropionitrile (MPN) was
purchased from Aldrich. Tetra-n-butylammonium hexafluorophosphate
(TBAPF6), 4-tert-butylpyridine (4-TBP), and lithium iodide were bought
from Fluka, and iodine (99.999%) was purchased from Alfa Aesar. THF
was pre-dried over 4 ꢁ molecular sieves and distilled under an argon at-
mosphere from sodium benzophenone ketyl immediately prior to use. Di-
chloromethane was distilled under normal pressure and dried over calci-
um hydroxide. Compound 5 was synthesized according to reported litera-
ture.[5] All other chemicals were purchased from Aldrich and used as re-
ceived without further purification.
solved in a solution of THF (25 mL) and H2O (5 mL) in a 50 mL round-
bottomed flask. Then the mixture was heated at reflux overnight under
an N2 atmosphere. After the reaction was complete, CH2Cl2 (20 mL) was
added and the organic layer was separated, then the aqueous solution
was washed with CH2Cl2 twice. All of the organic solution was collected.
After removal of the solvent, the residue was purified by chromatogra-
phy to afford the desired compound
4
(250 mg, 56.5%). 1H NMR
(400 MHz, CDCl3): d=9.79 (s, 1H), 7.62 (d, J=3.8 Hz, 1H), 7.20 (s, 2H),
7.4–7.10 (m, 5H), 6.96 (s, 1H), 6.78 (d, J=8.3 Hz, 1H), 4.75 (t, J=
7.2 Hz, 1H), 3.77 (t, J=8.3 Hz, 1H), 2.76–2.73 (m, 2H), 2.27 (s, 3H),
2.00–1.95 (m, 1H), 1.84 (s, 2H), 1.67–1.59 (m, 4H), 1.40–1.30 (m, 3H),
1.28–1.22 (m, 4H), 0.86 ppm (t, J=6.8 Hz, 3H).
Instrumentation
Synthesis of 4,7-Dibromo-[1,2,5]thiadiazoloACHTNUTRGNEUNG[3,4-c]pyridine (6)
NMR spectra were obtained by using a Brꢄcker AM 400 spectrometer.
The absorption spectra of the dyes in solution were measured with
a Varian Cary 500 spectrophotometer. MS were recorded on an ESI mass
spectrometer. The cyclic voltammograms of dyes were obtained with
a Versastat II electrochemical workstation (Princeton applied research)
Compound 3, 4-diaminopyridine (4 g, 4.98 mmol), and an excess amount
of SOCl2 were added to a 250 mL three-necked round-bottomed flask
with a condenser, then the mixture was heated at reflux for 5 h and
cooled to room temperature. The excess amount of thionyl chloride was
7
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Xin Li, JianLi Hua et al.
evaporated and the residue was slowly neutralized with saturated
NaHCO3. Once the reaction was quenched, CH2Cl2 (150 mL) was added,
then the organic layer was separated. The aqueous solution was washed
twice with CH2Cl2 and dried with MgSO4. After removing the solvent,
the crude product was purified by recrystallization from methanol to
afford compound 6 (2.71 g, 73%). 1H NMR (400 MHz, CDCl3): d=
8.56 ppm (s, 1H).
N2 atmosphere. After the reaction was complete, CH2Cl2 (20 mL) was
added and the organic layer was removed by separation, and the aqueous
layer was washed with CH2Cl2 twice. All of the organic layer was collect-
ed. After removing the solvent, the crude product 10 was purified by
chromatography. 1H NMR (400 MHz, CDCl3): d=9.92 (s, 1H), 8.67 (s,
1H), 8.53 (s, 1H), 7.83 (s, 1H), 7.80–7.75, (m, 2H), 7.40 (d, 1H), 7.21 (q,
4H), 7.03 (d, 1H), 4.91–4.88 (m, 1H), 3.97–3.93 (m, 1H), 2.96–2.92 (m,
2H), 2.36 (s, 3H), 2.16–2.06 (m, 1H), 1.97 (d, 2H), 1.86–1.78 (m, 3H),
1.70–1.65 (m, 2H), 1.48 (m, 2H), 1.36–1.35 (m, 4H), 0.91 ppm (t, J=
6.8 Hz, 3H); HRMS: m/z: calcd for C38H37N4OS3: 661.2129; found:
661.2130 [M+H]+.
Synthesis of 7-Bromo-4-(4-hexylthiophen-2-yl)-[1,2,5]thiadiazolo
ACHTUNGTRENN[UNG 3,4-c]-
pyridine (7)
3-Hexylthiophene (0.48 mL) was dissolved in freshly distilled THF
(10 mL) and then cooled down to ꢀ788C for 10 min, then nBuLi
(1.60 mL, 4.02 mmol) was added dropwise to the mixture. After 30 min
of stirring at the same temperature, isopropyl borate (0.93 mL,
4.02 mmol) was slowly injected into the reaction solution. After stirring
for a further 2 h, the solution was warmed to room temperature and the
mixture was stirred overnight. The crude mixture was used without fur-
ther purification. The solution was mixed with compound 6 (1.00 g,
Synthesis of 2-{5-[-(3’-hexyl-5’-{4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indol-7-yl}-[2,2’-bithiophen]-5-yl)methylene]-4-
oxothiazolidin-2-ylidene}malononitrile (IDR-I).
Equimolar quantities of compound 4 (200 mg, 0.38 mmol) and DCRD
(62 mg, 0.38 mmol) in EtOH (20 mL) were stirred under basic conditions
(1 drop, 20% aqueous NaOH solution) and heated at reflux overnight.
After the reaction was complete, the solvent was removed and the mix-
ture was purified by chromatography to afford the desired product as
a black powder (120 mg, 47%). 1H NMR (400 MHz, CDCl3): d=8.90 (s,
1H), 7.71 (s, 1H), 7.53 (d, J=3.9 Hz, 1H), 7.46 (s, 1H), 7.31 (d, J=
8.2 Hz, 1H), 7.28–7.26 (m, 2H), 7.19 (s, 4H), 6.82 (s, 1H), 4.87 (s, 1H),
3.83 (s, 1H), 2.77 (s, 2H), 2.28 (s, 3H), 2.10–1.98 (m, 1H), 1.88–1.72 (m,
3H), 1.71–1.56 (m, 3H), 1.48–1.37 (m, 3H), 1.36-1.27 (m, 4H), 0.86 ppm
(t, J=6.8 Hz, 3H); 13C NMR ([D6]DMSO, 100 MHz): d=180.39, 178.35,
147.75, 147.72, 143.81, 142.00, 140.63, 139.88, 139.78, 133.86, 127.80,
126.70, 126.50, 125.41, 125.25, 123.67, 122.31, 121.18, 120.18, 118.36,
117.00, 68.75, 48.09, 45.95, 45.02, 35.27, 33.57, 31.14, 30.28, 29.62, 28.94,
24.46, 22.45, 20.87, 14.45, 14.32 ppm; HRMS: m/z: calcd for
C39H35N4OS3: 671.1973; found: 671.1974 [MꢀH]ꢀ.
4.02 mmol, 1.50 equiv),
a catalytic amount of [PdACHTUNTGNRUEGN(dppf)Cl2], K3PO4
(1.70 g, 3 equiv), H2O (2 mL), and then heated at reflux overnight under
an N2 atmosphere. After the reaction was complete, CH2Cl2 (20 mL) was
added. The organic layer was separated and the aqueous solution was
washed with CH2Cl2 twice. All of the organic solution was collected and
dried with Na2SO4. After removal of the solvent solvent by evaporation,
the crude product was purified by chromatography to afford the desired
product 7 (760 mg, 75.2%). 1H NMR (400 MHz, CDCl3): d=8.66 (s,
1H), 8.52 (s, 1H), 7.24 (s, 1H), 2.73–2.69 (m, 2H), 1.73–1.67 (m, 2H),
1.38–1.30 (m, 6H), 0.90 ppm (t, J=7.0 Hz, 3H).
Synthesis of 7-Bromo-4-(5-bromo-4-hexylthiophen-2-yl)-
[1,2,5]thiadiazoloACHTUNGTRENNUNG[3,4-c]pyridine (8)
Compound 7 (484 mg, 1.27 mmol) was dissolved in CH2Cl2 and cooled to
08C, then NBS (248 mg, 1.39 mmol) was added into the solution and the
mixture was stirred for 6 h. After the reaction was complete, CH2Cl2
(20 mL) was added and the organic layer was removed by separation.
Then the aqueous solution was washed with CH2Cl2 twice. Removal of
the solvent afforded product 8, which was used for the next step without
further purification.
Synthesis of 2-[5-({3’-hexyl-5’-(7-{4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indol-7-yl}benzo[c]ACTHUNTGRENNG[U 1,2,5]thiadiazol-4-yl)-[2,2’-
bithiophen]-5-yl}methylene)-4-oxothiazolidin-2-ylidene]malononitrile
(IDR-II).
Equimolar quantities of compound 5 and DCRD in EtOH (20 mL) were
stirred under basic conditions (1 drop, 20% aqueous NaOH solution)
and heated at reflux overnight. After the reaction was complete, the sol-
vent was removed and the mixture was purified by chromatography to
afford the desired product as a black powder. 1H NMR (400 MHz,
[D6]DMSO): d=8.01 (d, J=7.6 Hz, 1H), 7.96 (s, 1H), 7.81 (s, 1H), 7.72–
7.70 (m, 3H), 7.53 (d, J=4.0 Hz, 1H), 7.32 (d, J=3.9 Hz, 1H), 7.20 (q,
J=8.5 Hz, 1H), 6.91 (d, J=8.4 Hz, 1H), 4.86 (s, 1H), 3.84 (s, 1H), 2.81
(s, 2H), 2.29 (s, 3H), 2.13–1.98 (m, 1H), 1.90–1.72 (m, 3H), 1.72–1.56 (m,
3H), 1.50–1.36 (m, 3H), 1.36–1.27 (m, 4H), 0.86 ppm (t, 3H); 13C NMR
(100 MHz,[D6]DMSO): d=180.12, 178.20, 152.97, 151.86, 147.50, 140.71,
139.50, 139.43, 134.96, 130.79, 129.73, 128.71, 127.91, 126.42, 125.85,
125.20, 122.70, 119.48, 106.73, 68.28, 47.72, 44.55, 33.08, 31.00, 30.94,
29.84, 28.72, 23.96, 22.06, 20.37, 13.94 ppm; HRMS: m/z calcd for
C45H39N6OS4: 807.2068; found: 807.2068 [M+H]+.
Synthesis of 4-(5-Bromo-4-hexylthiophen-2-yl)-7-{4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indol-7-yl}-[1,2,5]thiadiazolo
ACHTUNGTREN[NUNG 3,4-c]pyridine (9)
Compound 8 (490 mg, 1.07 mmol), compound 5 (an excess amount), a cat-
alytic amount of [Pd(dppf)Cl2], K2CO3 (1.38 g, 10 mmol), and solution of
ACHTUNGTRENNUNG
THF (25 mL) and H2O (5 mL) were added to a 50 mL round-bottomed
flask. Then the mixture was heated at reflux overnight under an N2 at-
mosphere. After the reaction was complete, CH2Cl2 (20 mL) was added
and the organic layer was removed by separation, and then the aqueous
solution was washed with CH2Cl2 twice. All of the organic solution was
collected. After removing the solvent, the residue was purified by chro-
matography to afford desired compound 9 (420 mg, 62.6%). 1H NMR
(400 MHz, CDCl3): d=8.61 (s, 1H), 8.34 (s, 1H), 7.80 (s, 1H), 7.74 (d,
J=8.3 Hz, 1H), 7.25–7.17 (m, 4H), 7.02 (d, J=8.4 Hz, 1H), 6.82 (d, J=
8.4 Hz, 1H), 4.90–4.87 (m, 1H), 3.94 (t, J=8.3 Hz, 1H), 2.66 (t, J=
7.7 Hz, 2H), 2.36 (s, 3H), 2.10–2.06 (m, 2H), 1.94 (s, 2H), 1.84–1.76 (m,
1H), 1.72–1.66 (m, 3H), 1.37 (m, 6H), 0.91 ppm (t, J=6.9 Hz, 3H); 13C
NMR (CDCl3, 100 MHz): d=155.32, 147.88, 147.07, 143.41, 142.97,
140.35, 140.35, 138.83, 134.67, 130.97, 130.70, 128.82, 127.74, 126.02,
124.26, 123.14, 119.50, 114.15, 106.53, 68.29, 44.34, 34.22, 32.62, 30.62,
28.82, 28.68, 27.93, 23.40, 21.60, 19.82, 13.10 ppm; HRMS: m/z: calcd for
C33H34BrN4S2: 629.1408; found: 629.1403 [M+H]+.
Synthesis of 2-[5-({3’-Hexyl-5’-(7-{4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indol-7-yl}-[1,2,5]thiadiazoloACHTNUTRGNEUNG[3,4-c]pyridin-4-yl)-
[2,2’-bithiophen]-5-yl}methylene)-4-oxothiazolidin-2-
ylidene]malononitrile (IDR-III).
Equimolar quantities of compound 10 (196 mg, 0.297 mmol) and DCRD
(48.7 mg, 0.297 mmol) in EtOH (20 mL) were stirred under basic condi-
tions (1 drop, 20% aqueous NaOH solution) and heated at reflux over-
night. After the reaction was complete, the solvent was removed and the
mixture was purified by chromatography to afford the desired product as
a black powder. 1H NMR (400 MHz, CDCl3): d=8.64 (s, 1H), 8.49 (s,
1H), 7.81 (s, 1H), 7.74 (d, J=8.2 Hz, 1H), 7.71 (s, 1H), 7.32 (s, 2H),
7.23–7.17 (m, 4H), 7.03 (d, J=8.6 Hz, 1H), 4.89 (s, 1H), 3.95 (s, 1H),
2.93 (s, 2H), 2.35 (s, 3H), 2.16–2.04 (m, 1H), 2.02–1.90 (m, 3H), 1.83–
1.77 (m, 6H), 1.55–1.45 (m, 4H), 0.89 ppm (t, J=6.8 Hz, 3H); 13C NMR
(100 MHz, [D8]THF): d=179.08, 176.53, 154.16, 146.7, 146.20, 142.17,
139.92, 139.01, 138.87, 138.20, 137.83, 133.55, 132.50, 131.95, 130.18,
Synthesis of 3’-Hexyl-5’-(7-{4-(p-tolyl)-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]-indol-7-yl}-[1,2,5]thiadiazolo
[2,2’-bithiophene]-5-carbaldehyde (10)
ACHTUNGERTN[NUNG 3,4-c]pyridin-4-yl)-
Compound 9 (380 mg, 0.605 mmol), 5-formyl-2-thiopheneboronic acid
(122.7 mg, 0.786 mmol, 1.3 equiv), a catalytic amount of [Pd(PPh3)4], and
AHCTUNGTRENNUNG
K2CO3 (1.38 g) were dissolved in a mixed solvent of THF (20 mL) and
H2O (5 mL). Then the mixture was heated at reflux overnight under an
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Xin Li, JianLi Hua et al.
129.58, 127.70, 126.76, 125.19, 124.84, 123.32, 122.69, 119.03,
118.34,105.35, 113.99, 113.64, 67.26, 44.12, 43.51, 33.30, 31.62, 29.85, 28.56,
28.06, 27.46, 20.71, 18.07 11.69 ppm; HRMS: m/z: calcd for C44H36N7OS4:
806.1864; found: 806.1864 [MꢀH]ꢀ.
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Acknowledgements
This work was supported by NSFC (91233207, 2116110444, 21172073,
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Received: June 4, 2014
Revised: July 3, 2014
Published online: && &&, 0000
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ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ

FULL PAPER
Solar Cells
ZhiFang Wu, Xin Li,* Jing Li,
JianLi Hua,* Hans ꢀgren,
He Tian
&&&& — &&&&
Influence of the Auxiliary Acceptor on
the Absorption Response and Photo-
voltaic Performance of Dye-Sensitized
Solar Cells
Bring me sunshine: New sensitizers
with indoline as donor and a 2,1,3-ben-
efficiency of 8.53% for a 2-(1,1-dicya-
nomethylene)rhodanine-based dye-
sensitized solar cell (DSSC; see
figure). The results indicate that the
incorporation of BTD can improve the
DSSC performance.
zothiadiazole (BTD)/
ACHTUNGTREN[NUNG 1,2,5]thiadiazolo-
AHCTUNGTRENN[GUN 3,4-c]pyridine (PT) moiety as the aux-
iliary acceptor were synthesized. By
fine-tuning the auxiliary acceptor, we
obtained a photoelectric conversion
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Chem. Asian J. 2014, 00, 0 – 0
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!