Stable dyes containing double acceptors without COOH as anchors for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201204948

The electron acceptor 2-(1,1-dicyanomethylene) rhodanine is a promising alternative to cyanoacrylic acid as an anchoring group for organic dyes. For example, the RD-II-based dye-sensitized solar cell has an overall conversion efficiency of 7.11 % and long-term stability. Copyright

Full text of DOI:10.1002/anie.201204948

Angewandte
Chemie
DOI: 10.1002/anie.201204948
Organic Solar Cells
Stable Dyes Containing Double Acceptors without COOH as Anchors
for Highly Efficient Dye-Sensitized Solar Cells**
Jiangyi Mao, Nannan He, Zhijun Ning, Qiong Zhang, Fuling Guo, Long Chen, Wenjun Wu,
Jianli Hua,* and He Tian*
Dedication to Professor Klaus Mꢀllen on the occasion of his 65th birthday
Dye-sensitized solar cells (DSSCs) have attracted consider-
able attention. The sensitizer is normally regarded as the
essential component of DSSCs.[1] In recent years, metal-free
organic dyes have been the central topic of development of
DSSCs, and they usually possess the structure of donor–p-
bridge–acceptor (D-p-A).[2–8] Among donor groups, triphe-
nylamine and its derivatives have shown promise in the
development of DSSCs owing to their nonplanar structure
suppressing the dye aggregation.[9] Electron acceptors have
been mainly focused on carboxylic acids, cyanoacrylic acid,
and rhodanine-3 acetic acid moieties as they can bind to the
TiO2. In particular, the introduction of cyanoacrylic acid as
electron-accepting unit has witnessed the rapid development
of organic dyes as sensitizers. This structure features evident
advantages of both wide absorption spectra in the visible
region and good electron injection efficiency. Based on this
same electron acceptor, significant improvement in the
efficiency of organic dyes has been achieved by regulating
the donor and the conjugating unit. However, dyes with
COOH as anchor unit will dissociate on TiO2 surface under
long irradiation times.[10] Thus, it is pivotal to increase the
binding strength of the anchor unit. It was known that metal
can form strong chelating bond with heterocycle ligand.[11]
Recently, pyridine,[12] phosphinic acid,[13] and 8-hydroxylqui-
noline[10] have been explored as anchoring groups; never-
theless, the efficiencies of DSSCs based on these acceptors
still lag behind that of cyanoacrylic acid counterparts. The
bottleneck of this effort lies in either the low electron-
injection efficiency or the limited broadening of absorption
spectra.
electron injection efficiency owing to its following character-
istics: 1) The double electron acceptors of both rhodanine and
dicyanomethylene can guarantee strong electron-accepting
ability;[14] 2) the O and N atoms of rhodanine in DCRD can
chelate to titanium ions on the TiO2 surface and achieve
strong electron coupling between the excited-state energy
level of the dyes and the conduction band of TiO2; and 3) the
anchor unit is localized on the middle acceptor, which can
reduce the electron transfer distance from the donor to TiO2.
Sensitizers coded as RD-I and RD-II (Scheme 1) based on
Scheme 1. a) Molecular structures of dyes RD-I, RD-II, CA-I, and CA-II;
b) Molecular structures of RD-II and its tautomer.
this electron acceptor were synthesized, and reference dyes
CA-I and CA-II with cyanoacrylic acid were also synthesized
for comparison. For this peculiar electron-acceptor DCRD,
the O and N on the rhodanine unit can both form strong
chelating bonds with Ti cations, furnishing enhanced stability
of the dyes in comparison with COOH anchor. Both RD-I
and RD-II show broadened absorption spectra than CA-I and
CA-II, respectively. More importantly, these dyes based on
the new electron acceptor show comparable electron injection
efficiency to cyanoacrylic acid counterparts, and the overall
efficiencies of RD-I and RD-II are evidently higher than CA-
I and CA-II, respectively. To the best of our knowledge, this is
probably the first report of dyes with alternative electron
acceptor showing higher efficiency than cyanoacrylic acid
counterparts.
Herein, we report a novel electron acceptor 2-(1,1-
dicyanomethylene)rhodanine (DCRD), which can not only
bring wide absorption spectra, but also maintain excellent
[*] J. Mao, N. He, Dr. Z. Ning, Dr. Q. Zhang, F. Guo, L. Chen, Dr. W. Wu,
Prof. J. Hua, Prof. H. Tian
Key Laboratory for Advanced Materials, Institute of Fine Chemicals,
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (China)
E-mail: jlhua@ecust.edu.cn
tianhe@ecust.edu.cn
[**] This work was supported by NSFC/China (2116110444, 21172073,
and 21190033), the National Basic Research 973 Program
(2011CB808400), the Fundamental Research Funds for the Central
Universities (WJ0913001), and the Ph.D. Programs Foundation of
the Ministry of Education of China (20090074110004).
The UV/Vis absorption spectra of the four dyes in diluted
solutions of CH2Cl2 and on thin transparent TiO2 films (5 mm
in thickness) after 12 h adsorption in CH2Cl2 solutions are
shown in the Supporting Information, Figure S1, and their
spectral data are listed in Table 1. The absorption spectra of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204948.
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Table 1: Optical and electrochemical properties data and DSSC performance parameters of RD-I, RD-II, CA-I, and CA-II.
E8’*[e] [V]
(vs. NHE)
MoleculescmÀ2[f]
Jsc
Voc
[mV]
FF
h [%][g]
[a]
[b]
[d]
Dye
lmax [nm]
lmax
[nm]
E8’ox[c][V]
(vs.NHE)
E0-0
(e, 104 LmolÀ1 cmÀ1
)
[eV]
[mAcmÀ2
]
RD-I
RD-II
CA-I
484 (4.42)
519 (3.14)
440 (3.79)
477 (2.36)
473
477
423
458
0.82
0.70
0.89
0.74
2.03
1.88
2.20
1.97
À1.21
À1.18
À1.31
À1.23
1.16ꢀ1015
1.20ꢀ1015
1.23ꢀ1015
1.25ꢀ1015
9.31
13.94
8.23
742
746
789
804
0.72
0.68
0.72
0.70
5.00
7.11
4.69
6.39
CA-II
11.34
[a] Absorption maximum in CH2Cl2 solution (3ꢀ10À5 m). [b] Absorption maximum on 5 mm TiO2 transparent films. [c] The ground-state oxidation
potential of dyes were measured in a dichloromethane with 0.1m tetra-n-butylammonium hexafluorophosphate (TBAPF6) as electrolyte (Pt working
electrode, SCE reference electrode calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external reference, Pt counter-electrode. [d] E0-0 was estimated
from the absorption thresholds from UV/Vis absorption spectra of the dyes. [e] E8’* was calculated as E8’oxÀE0-0. [f] Adsorption amount per unit area of
TiO2 film was measured after 12 h in the dye solution. [g] The photocurrent–voltage characteristics were measured with coadsorbent (10 mm CDCA)
for 13 mm thick TiO2 film with liquid electrolyte (0.05m I2, 0.1m LiI, 0.1m DMPII, and 0.5m TBP) at full sunlight (AM1.5G, 100 MWcmÀ2).
the dyes display the maximum absorption wavelength (lmax
)
nism (Supporting Information, Figure S4). For the dye
powder, the characteristic multiple bands corresponding to
NH stretching vibration (nN-H) were observed at 3430–
3200 cmÀ1, as the amide group can bond to form dimers by
intermolecular hydrogen bonding (Supporting Information,
Figure S5). At the same time, the characteristic bands for the
at 484 nm for RD-I, 519 nm for RD-II, 440 nm for CA-I, and
477 nm for CA-II, and this band is ascribed to the intra-
molecular charge transfer from the donor to the acceptor. The
corresponding maximum molar extinction coefficients (e) of
the four dyes are 4.42 ꢀ 104, 3.14 ꢀ 104, 3.79 ꢀ 104, and 2.36 ꢀ
104 LmolÀ1 cmÀ1, respectively. The red-shifts of the absorption
band of RD-I and RD-II are approximately 40 nm in
comparison to CA-I and CA-II, respectively. Furthermore,
the e values of RD-I and RD-II are higher than those of CA-I
and CA-II. These results show that the rhodanine bearing the
dicyanomethylene group exhibits stronger acceptor ability in
the dye, thus leading to the red-shift of absorption maximum
and enhancement of the extinction coefficient.
cyano group (C N) were clearly observed at 2205 cmÀ1. The
ꢀ
=
À
frequency of carbonyl (C O) stretching vibration and N H
bending vibration (dN-H) of the rhodanine were also raised to
1705 and 1660 cmÀ1 owing to the strong electron-withdrawing
=
property of dicyanomethylene group, though the C O and N-
H absorption of amides usually occur at 1650 and 1570 cmÀ1,
respectively. When the dyes were adsorbed on TiO2 surface,
the FTIR peak for cyano group remained detected at the
same frequencies while the absorption for nN-H, dN-H, and the
After anchoring on TiO2 film, the lmax of RD-I, RD-II,
CA-I, and CA-II hypsochromically shifted to 473, 477, 423,
and 458 nm, respectively, which may be ascribed to deproto-
nation and some type aggregates that always result in blue-
shift of absorption peak. Noticeably, the absorption peak of
RD-II on TiO2 film shows a sharp shift in comparison to that
in the solution of CH2Cl2. Cyclic voltammetric data for RD-I,
RD-II, CA-I, and CA-II are listed in Table 1. The ground-
state oxidation potential (E8’ox) of RD-I, RD-II, CA-I, and
CA-II are 0.82, 0.70, 0.89, and 0.74 V, respectively, which are
sufficiently low compared to the redox potential of the iodide/
triiodide electrolyte couple, and hence dye regeneration is
facilitated. On the other hand, the excited-state oxidation
potentials (E8’*) of RD-I, RD-II, CA-I, and CA-II, derived
from ground-state oxidation potential and optical energy gap
(E8’oxÀE0-0), are À1.21, À1.18, À1.31, and À1.23 V, respec-
tively. The E8’* potential of the four dyes were more negative
than the conduction-band edge of TiO2 (À0.5 V vs. NHE),
ensuring an efficient electron-injection process from the
excited dye into the conduction band of TiO2. Notably, the
dyes RD-I and RD-II with DCRD double acceptors as the
anchoring group feature relatively higher E8’* potentials than
corresponding dyes CA-I and CA-II with cyanoacrylic acid,
demonstrating stronger electron-withdrawing capability of
the DCRD group.
=
C O stretching bands disappeared. Noticeably, two new
bands emerged at around 3305 and 1640 cmÀ1, which can be
attributed to the characteristic absorption of hydroxy groups.
The presence of O H vibration may be due to the residual
À
Ti(OH)2 present in the powder.[16] Based on the fact that the N
and O atoms of 8-hydroxylquinoline can chelate to TiO2,[10]
we speculated that the DCRD moiety could form its tautomer
(Scheme 1b) in CH2Cl2, in which carbonyl heterocycle with a-
hydrogen underwent equilibration with its enol tautomer.[17]
Furthermore, the FTIR spectra of RD-I and RD-II adsorbed
on TiO2 nanoparticles indicated the formation of coordinate
bonds between the O and N atoms of rhodanine in RD-I and
RD-II tautomer and the Lewis acid sites of the TiO2 surface
(Supporting Information, Figure S6). To confirm this infer-
ence, we have also synthesized the model dye RDA-I
(Supporting Information, Scheme S1) in which the hydrogen
atom in lactam was substituted by butyl group. The TiO2 film
was soaked in a 3 ꢀ 10À4 m RDA-I bath in CH2Cl2 solution for
12 h at room temperature; no evidence of adsorption of the
dye molecules RDA-I was observed. Consequently, it can be
believed this is a new anchoring mode for 2-(1,1-dicyano-
methylene) rhodanine moiety without COOH as the novel
electron-withdrawing group for DSSCs.
RD-I and RD-II tend to aggregate when anchored on
TiO2 films, which is unfavorable for photocurrent generation.
Co-adsorption of chenodeoxycholic acid (CDCA) with dye
molecules has been used to prevent aggregation and improve
solar cell performance. Figure 1a shows the action spectra of
incident photon-to-current conversion efficiency (IPCE) as
To our knowledge, the bidentate mode is the way that the
dyes with a COOH anchoring group anchor on the TiO2
surface.[15] Regarding the anchoring mode for RD-I or RD-II,
FTIR spectra of RD-I–II powders and the dyes adsorbed on
TiO2 have been measured to explore the adsorption mecha-
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electrode through the strong coordinate bonding. Interest-
ingly, the open circuit photo-voltage of CA-II is best among
RD-I–II and CA-I-based solar cells. To understand this,
electrochemical impedance spectroscopy (EIS) was employed
to study the electron recombination in DSSCs based on these
dyes under À0.70 V bias applied voltage in the dark. The
Nyquist plot and Bode plot are shown in the Supporting
Information, Figure S7.[18] From the EIS measurements, the
radius of the larger semicircle decreases in the order CA-II >
CA-I > RD-II > RD-I, indicating that the electron recombi-
nation resistance increases from RD-I, RD-II, and CA-I to
CA-II and is reflected in the improvements of Voc. This result
is in agreement with the observed shift in the Voc values shown
in Table 1.
The stability of sensitizers is an especially critical factor
for the practical application of DSSCs. The stability of the
dyes adsorbed on nanocrystalline TiO2 films was tested by
irradiating the sample with simulated solar light without
redox mediators (Supporting Information, Figure S8 and S9).
The absorbance of RD-II and CA-II remained at 95% and
92% of the initial value, respectively, after 30 min visible-light
illumination. The stability of RD-II is a little bit higher than
CA-II, indicating that DCRD is a promising acceptor to
enhance the stability of the organic sensitizers. To further
prove the excellent stability of the sensitizer, RD-II was used
for the ionic-liquid electrolyte DSSCs using 1-butyl-3-methyl
imidazolium, 1-methyl-3-trimethylsilyl imidazolium, iodine,
benzimidazole, and guanidine thiocyanate (BMII/MSII/I2/BI/
GNCS = 20:20:1.67:0.67:3.33) as redox electrolyte. Figure 2
shows the photovoltaic performance during a long-term
accelerated aging of a RD-II-sensitized solar cell under
AM1.5 irradiation. As shown in Figure 2, the photovoltaic
parameters of RD-II-based DSSCs with ionic-liquid electro-
lytes obtained performance of 5.15%. The Jsc, Voc, and FF
barely change during the 1000 h irradiation, indicating that
Figure 1. a) IPCE action spectra for DSSCs based on RD-I, RD-II, CA-I,
and CA-II; b) Current–voltage characteristics of DSSCs based on RD-I,
RD-II, CA-I, and CA-II co-adsorbed with 10 mm CDCA.
a function of incident wavelength for DSSCs. In good
agreement with absorption spectra, the IPCE action spectrum
for DSSCs based on RD-II is broader than those of RD-I,
CA-I, and CA-II. It is worth noting that the IPCE of RD-II
keeps a high plateau at visible region until 710 nm. The
electron injection efficiency (Finj) of RD-I, RD-II, CA-I, and
CA-II in lmax calculated to be 86.7, 91.2, 78.6, and 81.0,
respectively (see the Supporting Information). It was found
that the Finj values of dyes containing RD are a little higher
than the dyes with CA group, indicating that these dyes based
on the new electron acceptor show comparable electron
injection efficiency to cyanoacrylic acid counterparts. Fig-
ure 1b shows photocurrent-voltage (I–V) of DSSCs based on
RD-I, RD-II, CA-I, and CA-II co-adsorbed with 10 mm
CDCA in ethanol using 0.1m DMPII, 0.1m LiI, 0.05m I2,
and 0.5m TBP as redox electrolyte under simulated AM1.5G
irradiation (100 mWcmÀ2), and the adsorption amounts of
dyes on TiO2 for RD-I, RD-II, CA-I and CA-II, the short-
circuit photocurrent (Jsc), open-circuit photovoltage (Voc), fill
factor (FF), and overall conversion efficiency (h) are shown in
Table 1. The relatively high photovoltaic performances of
RD-I and RD-II are attributed to both the red-shift of
absorption band and enhancement of molar extinction
coefficient owing to the introduction of the strong electron-
withdrawing group of DCRD. Thus, dyes RD-I and RD-II can
inject electrons efficiently from DCRD to the CB of the TiO2
Figure 2. Variations of photovoltaic parameters (Jsc, Voc, FF, and h)
with aging time for the DSSCs device based on RD-II and ionic-liquid
electrolyte under visible light soaking at 708C.
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RD-II bond strongly to TiO2, and it is substantially stable
under working conditions.
In summary, we demonstrated for the first time that the
electron acceptor 2-(1,1-dicyanomethylene) rhodanine is
a promising alternative anchoring group to cyanoacrylic
acid for sensitizing dyes. The double acceptor units bring
a large bathochromic shift in absorption spectrum. The
formation of coordinate bonds between the O and N in the
middle acceptor rhodanine of RD-I and RD-II tautomers and
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as electron acceptor. Furthermore, these dyes show remark-
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may pave the way to develop novel high efficiency and stable
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Received: June 25, 2012
Published online: && &&, &&&&
Keywords: acceptors · anchoring groups ·
.
dye-sensitized solar cells · rhodanines
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Organic Solar Cells
The electron acceptor 2-(1,1-dicyano-
methylene) rhodanine is a promising
alternative to cyanoacrylic acid as an
anchoring group for organic dyes. For
example, the RD-II-based dye-sensitized
solar cell has an overall conversion
efficiency of 7.11% and long-term stabil-
ity.
J. Mao, N. He, Z. Ning, Q. Zhang, F. Guo,
L. Chen, W. Wu, J. Hua,*
H. Tian*
&&&& — &&&&
Stable Dyes Containing Double Acceptors
without COOH as Anchors for Highly
Efficient Dye-Sensitized Solar Cells
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