Engineering organic sensitizers for iodine-free dye-sensitized solar cells: Red-shifted current response concomitant with attenuated charge recombination

Article abstract of DOI:10.1021/ja203708k

With a new metal-free donor-acceptor photosensitizer featuring the 2,6-bis(thiophen-2-yl)-4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′] dithiophene-conjugated spacer and the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle, we present a highly efficient io

Full text of DOI:10.1021/ja203708k

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pubs.acs.org/JACS
Engineering Organic Sensitizers for Iodine-Free Dye-Sensitized Solar
Cells: Red-Shifted Current Response Concomitant with Attenuated
Charge Recombination
Yu Bai,†,‡ Jing Zhang,† Difei Zhou,†,‡ Yinghui Wang,† Min Zhang,† and Peng Wang*,†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
‡Graduate School, Chinese Academy of Sciences, Beijing 10039, China
S Supporting Information
b
ABSTRACT: With a new metal-free donorꢀacceptor
photosensitizer featuring the 2,6-bis(thiophen-2-yl)-4,4-di-
hexyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene-conjugated
spacer and the tris(1,10-phenanthroline)cobalt(II/III) re-
dox shuttle, we present a highly efficient iodine-free dye-
sensitized solar cell displaying a power conversion efficiency
of 9.4% measured at 100 mW cmꢀ2 simulated AM1.5
conditions.
ye-sensitized solar cells (DSCs) are presently under intense
academic and industrial investigations because of their
D
grand potential to serve as a low-cost alternative to existing
photovoltaic technologies.1 A state-of-the art DSC fabricated by
sensibly combining a panchromatic ruthenium dye,2 an iodine
electrolyte, and a thick mesoporous titania film has been notar-
ized with a record solar-to-electricity conversion efficiency of
11.1% thus far.3 Owing to a resource restriction of the noble
metal ruthenium, metal-free organic pushꢀpull dyes have been
actively pursued for the wide availability of their raw materials.4
In this context, impressive progress on iodine-free DSCs dis-
playing an excellent photovoltage has been attained very recently,
based upon the combination of high-absorptivity organic dyes
with either a solid organic hole-transporter5 or some iron-,6
cobalt-,7 and copper-containing8 redox shuttles, all of which
feature considerably downward-shifted Fermi levels with respect
to the common iodine electrolytes. However, these explorations
have not yet afforded power conversion efficiencies comparable
to those of champion DSCs based on organic dyes and iodine
electrolytes.4dꢀf In this work, we employ 2,6-bis(thiophen-2-yl)-
4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene as the π-
conjugated linker to synthesize a new dye, coded C229
(Figure 1A), which exhibits outstanding efficiencies of 9.4ꢀ
10.3% in DSCs based on the tris(1,10-phenanthroline)cobalt
electrolyte.
Figure 1. (A) Dye structures. (B) Photocurrent action spectra of C228
and C229 cells employing an iodine or cobalt electrolyte. (C) JꢀV
characteristics measured at 100 mW cmꢀ2 simulated AM1.5 conditions.
A (2.7+4.1)-μm-thick bilayer titania film was employed for cell fabrica-
tion. An antireflection film was adhered to a testing cell during
measurement. All cells were measured with a metal mask having an
aperture area of 0.158 cm2. The spectral distribution of our light resource
simulates the AM1.5 solar emission (ASTM G173-03). The validity of
our data is further confirmed by comparing the overlap integral of the
measured IPCE spectrum with the standard AM1.5 solar emission
spectrum and the experimental Jsc, showing <5% error.
In comparison with the C228 dye (Figure 1A) possessing the
2,7-bis(thiophen-2-yl)-9,9-dihexylfluorene segment, C229 takes
on an augmented maximum molar absorption coefficient at a
desirable longer wavelength (as shown in Supporting Informa-
tion (SI), Figure S1), owing to a better electron delocalizability of
its π-conjugated spacer.9 We further examined the photocurrent
action spectra (Figure 1B) of DSCs with C228 or C229 dye, in
conjunction with an iodine-free electrolyte composed of 0.3 M
tris(1,10-phenanthroline)cobalt(II)
di[bis(trifluoromethanesulfonyl)imide] (Co-phen), 75 mM ni-
trosonium tetrafluoroborate (NOBF4), 0.1 M lithium bis-
(trifluoromethanesulfonyl)imide (LiTFSI), and 0.5 M 4-tert-
butylpyridine (TBP) in acetonitrile. Apart from a comparable
Received: April 22, 2011
Published: July 07, 2011
r
2011 American Chemical Society
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Table 1. Photovoltaic Parametersa
Table 2. Parametersa Derived From Electrical Impedance
Analyses
electrolyteb d/μm Pin/mW cmꢀ2 Jsc/mA cmꢀ2 Voc/V FF η/%
dye/electrolyte Ec ꢀ EF,redox/eV nc/1016 cmꢀ3 U0k/1020 cmꢀ3 sꢀ1
C228 Dye
C228/cobalt
C229/cobalt
C228/iodine
C229/iodine
1.05 ( 0.003
1.02 ( 0.004
0.96 ( 0.002
0.93 ( 0.003
13 ( 3
92 ( 8
28 ( 5
4 ( 1
320 ( 30
34 ( 4
cobalt
iodine
2.7 + 4.1
2.7 + 4.1
100.00
100.00
7.60
7.78
0.83 0.74 4.7
0.76 0.74 4.4
9 ( 2
C229 Dye
2800 ( 90
a Ec, titania conduction band edge; EF,redox, electrolyte Fermi-level; nc,
free electron density of our cells at the 100 mW cmꢀ2 AM1.5 conditions;
U0k, effective rate constant of charge recombination at the titania/
electrolyte interface. The errors are estimated from measurements on
three cells.
cobalt
2.7 + 4.1
5.0 + 4.1
2.7 + 4.1
12.48
23.27
1.92
3.58
0.77 0.79 9.3
0.79 0.78 9.6
0.82 0.76 9.7
0.85 0.73 9.4
0.76 0.80 10.2
0.79 0.78 10.3
0.82 0.74 10.1
0.84 0.70 8.4
0.68 0.65 6.7
50.09
7.74
100.00
12.48
15.31
2.08
23.27
3.88
50.09
8.32
superiority of employing cyclopentadithiophene relative to fluor-
ene as the conjugated spacer in DꢀπꢀA dyes. This concept is
actually borrowed from the wonderful design of a historically
seminal conjugated DꢀA polymer.10 Note that, in our earlier
work,11 we proved the advantage of cyclopentadithiophene
relative to dithiophene in the DSC dye design, and this strategy
was also applied by Gr€atzel's group.7d In contrast to the cobalt
cells, the iodine congeners exhibit evidently attenuated Voc,
which can be primarily ascribed to a 0.23 V negative shift of
the electrolyte equilibrium potential (SI, Figure S3; iodine,
ꢀ0.30 V and cobalt, ꢀ0.07 V versus Fc+/Fc). Moreover, it is
noteworthy that the dye alteration in the iodine cells from C228
to C229 has caused a Voc attenuation of 0.08 V, sharply
contrasting a 0.02 V enhancement in the cobalt cells.
100.00
14.25
iodine
100.00
15.20
a The spectral distribution of our light resource simulates the AM1.5
solar emission (ASTM G173-03) with a mismatch of <5%. d, thickness
of titania film; Pin, incident light intensity. b Cobalt electrolyte: 0.3 M
Co-phen, 75 mM NOBF4, 0.1 M LiTFSI, and 0.5 M TBP in acetonitrile.
Iodine electrolyte: 0.3 M DMII, 75 mM I2, 0.1 M LiTFSI, and 0.5 M TBP
in acetonitrile.
summit of incident photon-to-collected electron conversion
efficiencies (IPCEs) of ∼88%, the photosensitizer transforma-
tion from C228 to C229 brings on ∼150 nm red-shifting of the
photocurrent onset wavelength.
The solar-to-electricity conversion efficiencies of these two
cobalt cells were also evaluated by recording the JꢀV character-
istics at 100 mW cmꢀ2 simulated AM1.5 conditions, and the
detailed photovoltaic parameters are collected in Table 1. As
Figure 1C presents, the short-circuit photocurrent density (Jsc),
open-circuit photovoltage (Voc), and fill factor (FF) of the C228
cell are 7.6 mA cmꢀ2, 0.83 V, and 0.74, respectively, generating a
power conversion efficiency (η) of 4.7%. In good accord with the
integrals of IPCEs over the standard AM1.5 solar emission
spectrum, the C229 dye evokes a notably improved Jsc = 15.31
mA cmꢀ2, which affords a 2 times higher η = 9.4%, with an
impressive Voc = 0.85 V. Three batches of 12 cells exhibit an
efficiency ranging from 9.3% to 9.6% (see SI, including Figure S2
and Table S1, for details). Note that other organic sensitizers
previously reported by our group all present lower efficiencies in
cells employing iodine-free electrolytes. The new record effi-
ciency of 9.4% achieved in full sunlight constitutes a break-
through, setting a new benchmark for iodine-free DSCs.7 With a
thicker bilayer titania film, impressive power conversion efficien-
cies of >10% (Table 1) under relatively weak light irradiation
have been achieved with C229.
To clarify the importance of combining iodine-free redox
couples with organic dyes, an iodine control electrolyte consist-
ing of 0.3 M 1,3-dimethylimidazolium iodide (DMII), 75 mM
iodine, 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile was further
formulated to fabricate the iodine control cells. As Figure 1B
depicts, the dye substitution of C228 with C229 affects the
photocurrent action spectra of the iodine cells in a similar pattern
as the cobalt counterparts. JꢀV characteristics of these iodine
cells measured under full sunlight are also included in Figure 1C
for comparison, with the detailed photovoltaic parameters listed
in Table 1. It is noted that, regardless of the electrolyte selection,
the C229 dye displays a better cell efficiency, proving the
The open-circuit photovoltage of a DSC under illumination is
regarded as the energy difference between the Fermi level of
titania (EF,n) and that of a redox electrolyte (EF,redox), as
described by Voc = (EF,n ꢀ EF,redox)/e, where e is the elementary
charge. The opposite Voc variation for the cobalt and iodine cells
in the case of sensitizer change from C228 to C229 must stem
from a redox-couple-correlated alternation of EF,n level, which
could be understood via EF,n = Ec + kBT ln(nc/Nc). Here, Ec is the
titania conduction band edge, kB the Boltzmann constant, T the
absolute temperature, nc the free carrier density, and Nc the
density of accessible states in the conduction band. The quantity
of Ec ꢀ EF,redox could be roughly estimated by analyzing the
chemical capacitance (Cμ) of titania films12 (see SI for details).
Numerical analysis of an electrical impedance spectroscopy using
the transmission line model13 can afford the Cμ at a given
potential bias (V). Fitting the Cμ data in Supporting Information
Figure S4A yields the Ec ꢀ EF,redox values, and the results are
collected in Table 2. Apparently, a downward Ec shift of 30 meV
arises in both electrolytes upon replacing C228 with C229.
On the basis of this analysis, we can estimate nc (Table 2) of
each cell at open circuit under full sunlight via nc = Nc exp-
{[eVoc ꢀ (Ec ꢀ EF,redox)]/kBT} (see SI for details). It is noticed
from Table 2 that, upon employing the cobalt electrolyte, the
C229 cell contains more charges than the C228 counterpart,
which could be ascribed to better light absorption and slow
charge recombination in the case of C229; however, the use of
the iodine electrolyte evokes an even lower nc level in the C229
cell than in the C228 congener. The validity of this deduction
could be further elaborated by comparing the effective charge
recombination rate constant, U0k, which can be obtained by
analyzing interfacial charge recombination resistances Rct (SI,
Figure S4B).14 In the cobalt cells, C229 prompts a U0k almost
1 order of magnitude smaller than that with C228 (Table 2).
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Figure 2. Optimized geometries of (A) C228ꢀI2 and (B) C229ꢀI2
complexes. The interaction energies between iodine and sulfur atoms are
also included. Iodine, purple; carbon, green; sulfur, yellow; nitrogen,
blue; oxygen, red; hydrogen, gray.
Figure 3. (A) Transient absorption spectra of 2.4-μm-thick dye-coated
titania films immersed in an inert electrolyte of 0.1 M LiTFSI and 0.5 M
TBP in acetonitrile. The spectra were recorded upon nanosecond laser
excitation at 500 nm. (B) Absorption decays of ∼11-μm-thick C228-
coated titania film in the inert (1), iodine (2), and cobalt (3) electrolytes.
The compositions of our iodine and cobalt electrolytes are described in
the text. Excitation wavelength and pulse fluence: 45 μJ cmꢀ2 at 552
(trace 1), 555 (trace 2), and 551 nm (trace 3). Probe wavelength,
782 nm. (C) Absorption decays of ∼11-μm-thick, C229-coated titania
films immersed in the inert (4), iodine (5), and cobalt (6) electrolytes.
Excitation wavelength and pulse fluence, 36 μJ cmꢀ2 at 683 (trace 4),
687 (trace 5), and 682 nm (trace 6); probe wavelength, 782 nm. The
smooth lines in panels B and C are stretched exponential fittings over
raw data obtained by averaging over 500 laser shots.
Upon replacing the cobalt electrolyte with the iodine one,
however, a >2 orders of magnitude larger U0k, i.e., faster charge
recombination, is observed for the C229 cell relative to the C228
counterpart (C228, 9 ꢁ 1020 cmꢀ3 sꢀ1; C229, 2800 ꢁ 1020 cmꢀ3
s
ꢀ1). These comparisons, coupled with the preceding discussion
on Ec ꢀ EF,redox, provide sound evidence for the aforementioned
redox-couple-related Voc variation from C228 to C229.
To gain insight into the redox-shuttle-correlated interfacial
charge recombination kinetics, we further resorted to quantum
calculation to evaluate the interaction between dye molecules
and electron acceptors in electrolytes, which may affect the
distribution profile of oxidized species near the titania nanocrys-
tal surface. It is known that electron-deficient halogen atoms such
as iodine can form complexes with other electron-rich atoms
through a non-covalent interaction, i.e., halogen bonding.15 The
adverse impact of possible halogen bonding on interfacial charge
recombination in DSCs has been proposed by several groups,16
but no very convincing spectral data have been reported so far.
The redox-couple-correlated variation of charge recombination
upon dye alternation presented in this paper has supplied a new
clue to understand the halogen-bonding issue in DSCs. Herein
we compare the interaction energies of dyeꢀI2 complexes for
C228 and C229. Our calculation results (Figure 2) show that,
with respect to C228, the C229 dye contains more interaction
sites (sulfur atoms), favoring the formation of dyeꢀI2 complexes.
Thereby, we may expect a much higher iodine concentration in
the vicinity of a C229 dye-coated titania interface, leading to
accelerated interfacial charge recombination.
that, with respect to their neutral counterparts, both oxidized
C228 and C229 dye molecules display strong absorption in the
near-infrared region. Because of the signal sensitivity of our
photomultiplier tube detector, a 782-nm probe light and a thick
titania film were used in the following kinetic measurements. The
excitation wavelengths were also carefully selected according to
an optical density of ∼0.2 of dye-coated titania films, ensuring a
similar electron distribution profile in the studied samples. In the
presence of an inert electrolyte, the slow absorption decays
(traces 1 and 4 in Figure 3B,C) can be attributed to the back
electron transfer from titania to oxidized dye molecules. Further-
more, obviously accelerated absorption decays (traces 2, 3, 5,
and 6) were observed upon using the practical cobalt or iodine
electrolyte, owing to the occurrence of dye regeneration by
either cobalt(II) or iodide ions. The average time (Æτæ) of these
charge-transfer processes was derived by fitting the traces to a
stretched exponential function (ΔA ꢀ A exp(ꢀ(t/τ)R), collected
in SI Table S2). The kinetic branch ratios of oxidized dye mole-
cules involving dual-channel charge-transfer reactions exceed
50 in all our cells, ensuring an efficient interfacial charge separa-
tion. A ∼50 mV negative movement of the formal redox
potential from C228 to C229 (SI, Figure S5) induces a general
deceleration of dye regeneration in both cobalt and iodine elec-
trolytes, which however exhibit a distinctive variation for the
two redox couples (iodine, ∼7-fold deceleration; cobalt, ∼2-fold
Although the variation of redox shuttle from iodine to cobalt
does not exert a noticeable influence on the IPCE summit of
DSCs based on the C228 or C229 dye, it is still valuable to
measure the dual-channel charge-transfer kinetics of oxidized dye
molecules with titania electrons and electron-donating species in
electrolytes.17 We first recorded the transient absorption spec-
trum (Figure 3A) of a C228 or C229 dye-coated titania film with
an intensified charge-coupled detector camera. It can be noticed
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deceleration). This valuable observation suggests that, in com-
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part should endow a large chance of uplifting the ground-state
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To summarize, we have demonstrated an iodine-free dye-
sensitized solar cell exhibiting an unprecedented power conver-
sion efficiency of 9.4% at 100 mW cmꢀ2 simulated AM1.5G
conditions, based on a new organic pushꢀpull dye C229 in
conjunction with the tris(1,10-phenanthroline)cobalt(II/III) re-
dox shuttle. With respect to its C228 counterpart, the C229 dye,
when combined with the cobalt electrolyte, not only evokes a
red-shifted photocurrent response but concomitantly prompts a
Voc improvement, in sharp contrast to a Voc decrease for the
iodine control cells. This is further rationalized in terms of the
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’ AUTHOR INFORMATION
Corresponding Author
peng.wang@ciac.jl.cn
’ ACKNOWLEDGMENT
The National 973 Program (No. 2007CB936702 and No.
2011CBA00702), the National Science Foundation of China
(No. 50973105), the CAS Knowledge Innovation Program
(No. KGCX2-YW-326), and the CAS Hundred Talent Program
are acknowledged for financial support. We are grateful to Dyesol
for supplying the scattering paste and to DuPont Packaging and
Industrial Polymers for supplying the Bynel film.
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