Zinc porphyrins with a pyridine-ring-anchoring group for dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201201136

Anchoring groups are extremely important in controlling the performance of dye-sensitized solar cells (DSCs). The design and characterization of sensitizers with new anchoring groups, in particular non-carboxylic acid groups, has become a recent focus of DSC research. Herein, new donor-p-acceptor zinc-porphyrin dyes with a pyridine ring as an anchoring group have been designed and synthesized for applications in DSCs. Photophysical and electrochemical investigations demonstrated that the pyridine ring worked effectively as an anchoring group for the porphyrin sensitizers. DSCs that were based on these new porphyrins showed an overall powerconversion efficiency of about 4.0% under full sunlight (AM 1.5G, 100 mWcm^-2).

Full text of DOI:10.1002/asia.201201136

FULL PAPER
DOI: 10.1002/asia.201201136
Zinc Porphyrins with a Pyridine-Ring-Anchoring Group for Dye-Sensitized
Solar Cells
Jianfeng Lu,[a] Xiaobao Xu,[a] Zhihong Li,[a] Kun Cao,[a] Jin Cui,[a] Yibo Zhang,[a]
Yan Shen,*[a] Yi Li,[b] Jun Zhu,[b] Songyuan Dai,*[b] Wei Chen,[a] Yibing Cheng,[a, c] and
Mingkui Wang*[a]
Abstract: Anchoring groups are ex-
tremely important in controlling the
performance of dye-sensitized solar
cells (DSCs). The design and character-
ization of sensitizers with new anchor-
ing groups, in particular non-carboxylic
acid groups, has become a recent focus
a pyridine ring as an anchoring group
have been designed and synthesized
for applications in DSCs. Photophysical
and electrochemical investigations
demonstrated that the pyridine ring
worked effectively as an anchoring
group for the porphyrin sensitizers.
DSCs that were based on these new
porphyrins showed an overall power-
conversion efficiency of about 4.0%
Keywords: anchoring
groups
·
under
full
sunlight
(AM 1.5G,
dyes · electron transfer · porphyri-
noids · sensitizers
of DSC research. Herein, new donor
100 mWcmꢀ2).
ꢀ
ꢀ
ꢀ
p acceptor zinc porphyrin dyes with
ꢀ ꢀ
ꢀ ꢀ
Introduction
with a donor p acceptor (D p A) structure that attained
overall power-conversion efficiencies of above 10%.[4] More
recently, Yella et al. reported a new benchmark in DSCs,
with an overall efficiency of 12.3%, by using a judiciously
engineered porphyrin sensitizer (YD2-o-C8) along with
Dye-sensitized solar cells (DSCs) have drawn growing inter-
est as low-cost alternatives to conventional inorganic photo-
voltaic devices.[1] Recently, remarkable improvements have
been made in the power-conversion efficiency of DSCs,
within the range 11–12.3%, which can largely be attributed
to the development of sensitizer- and hole-transporting ma-
terials.[2] Porphyrins are promising sensitizers for molecular
photovoltaics because of their intense absorption from the
visible to the near-IR regions, as well as their outstanding
photochemical and thermal stabilities.[3] Recently, Diau and
a co-sensitizer and a cobaltACHTNUGRTNEUNG
(II/III)-based redox electrolyte.[5]
This system generates a strikingly high photovoltage (close
to 1 V) and has an impressive photocurrent density, owing
to the molecular design of YD2-o-C8, which contains long
alkyl chains that are attaching onto the porphine core.
The goal of this study is to explore the application of por-
phyrins as light-harvesting materials with various anchoring
groups, instead of the conventional carboxylic acid groups
that bind to the TiO2 surface. Although carboxylic acids
show remarkable performance in DSCs, there is an ongoing
quest to find suitable alternatives due to a number of rea-
sons, including the positive shift (versus NHE) of the con-
duction-band potential, which originates from proton inter-
calation into the titania during the dye-dipping process,
thereby lowering the open-circuit voltage of a device. As an
example of effective molecular engineering, modification of
the anchoring groups can yield new and efficient sensitizers
in DSCs. In this case, 2-hydroxybenzonitrile, nitro groups,
phosphinic acid, and 8-hydroxylquinoline have been intro-
duced into organic dyes as new anchoring groups, thereby
affording large red-shifts and long-term stability.[6] The N-
substituted pyridinium ion has also been employed as an ac-
ceptor in sensitizers for applications in molecular photovol-
taics.[7] Very recently, Ooyama et al. reported interesting re-
sults on organic dyes that contained pyridine as an anchor-
ing group.[8] A coordinate bond was formed between the
pyridine N atom and TiO2 through a shared a pair of elec-
trons, rather than an ester linkage as with carboxylic groups,
ꢀ
co-workers reported zinc porphyrin dyes (named LD14)
[a] J. Lu, X. Xu, Z. Li, K. Cao, J. Cui, Y. Zhang, Prof. Y. Shen,
Prof. W. Chen, Prof. Y. Cheng, Prof. M. Wang
Michael Grꢀtzel Center for Mesoscopic Solar Cells
Wuhan National Laboratory for Optoelectronics Department
School of Optical and Electronic Information
Huazhong University of Science and Technology
1037 Luoyu Road, Wuhan 430074 (P. R. China)
Fax : (+86)27-87792225
E-mail: ciac_sheny@mail.hust.edu.cn
mingkui.wang@mail.hust.edu.cn
[b] Y. Li, Prof. J. Zhu, Prof. S. Dai
Institute of Plasma Physics
Chinese Academy of Sciences
(P. R. China)
E-mail: sydai@ipp.ac.cn
[c] Prof. Y. Cheng
Department of Materials Engineering
Monash University
Melbourne,Victoria, 3800 (Australia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201201136.
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Yan Shen, Songyuan Dai, Mingkui Wang et al.
which leads to efficient electron injection, owing to good
electronic communication.
Herein, pyridine is introduced as an anchoring group into
and THF), as well as in other common organic solvents. The
structures of all of these new porphyrins were verified by
1H NMR, 13C NMR, UV/Vis, fluorescence, and FTIR spec-
troscopy and MS (ESI-Q-TOF); the spectra are presented in
the Supporting Information (Figures S1–S9 and S14–S16).
The NMR spectra of these compounds are consistent with
their proposed structures, thus showing the expected fea-
tures with the correct integration ratios (see the Experimen-
tal Section in the Supporting Information). Accurate-mass
data analysis provided direct evidence for the structures of
dyes LW11, LW12, and LW13 and the experimental values
match the calculated values for the molecular weight of
each compound (see the Supporting Information, Figures S3,
S6, and S9). Figures S14–16 in the Supporting Information,
show the FTIR spectra of various sensitizers. For dye LW13,
the C=O stretching band of the carboxy group is observed
at 1685 cmꢀ1. The characteristic stretching bandings for the
C=N or C=C groups are clearly observed at about 1590,
1500, and 1455 cmꢀ1, which can be assigned to the pyridine
ring in the sensitizers.[8a]
ꢀ ꢀ
ꢀ ꢀ
ꢀ
a donor p acceptor (D p A)-structured zinc porphyrin
sensitizer, that is 5-(4-pyridine)ethynyl-15-(4-N,N-dimethyla-
mino-phenyl)ethynyl-10,20-bisACTHNUGRTNEUNG[2,6-di(dodecyloxy)phenyl]-
porphinato zinc(II) (LW11). Scheme 1 shows the molecular
Further confirmation of the formation of compounds
LW11, LW12, and LW13 was obtained from steady-state ab-
sorption and emission measurements (Figure 1 and the Sup-
porting Information, Figures S11–S13). The electronic ab-
Scheme 1. Molecular structures of the LW11, LW12, LW13, and LD14
sensitizers.
structures of our proposed porphyrins based on this design.
Three new porphyrins have been designed and synthesized
and their use as sensitizers in DSC has been evaluated. The
LD14 dye, reported by Diau and co-workers was also pre-
pared and used as a reference for comparison.[4]
Results and Discussion
In this work, three new porphyrin dyes with different an-
choring groups (LW11 and LW12 with pyridine rings, LW13
with carboxylic acid groups) were designed and synthesized
as analogues of dye LD14 to better understand the influence
of the anchoring group on the photovoltaic parameters of
their corresponding DSCs. The introduction of bulky ortho-
substituted alkoxylphenyl chains at the meso positions of
the porphine cores in these compounds significantly sup-
presses dye aggregation and interfacial charge recombinatio-
n.[4a,b,5] As shown in Scheme 1, dyes LW11 and LD14 contain
a common 4-ethynyl-N,N-dimethylaniline (electron-donat-
ing) group. Dye LW11 contains a pyridine moiety, which
acts as an electron-withdrawing group and has the same
function as the carboxylic acid anchoring group in dye
LD14. Dyes LW12 and LW13 have symmetric structures,
with two 4-ethynylpyridine groups and two 4-ethynylbenzoic
acids at the meso positions, respectively. The synthesis of
these porphyrin sensitizers is given in the Supporting Infor-
mation, Scheme S1–S3.[3a,9,10] Sensitizers LW11, LW12, and
LW13, as well as their corresponding precursor compounds
(6–9), are highly soluble in aromatic solvents (i.e., toluene
Figure 1. Absorption spectra (a) and fluorescence emission spectra (b) of
LW11, LW12, LW13, and LD14 in THF.
sorption spectra of these porphyrins show typical S (450–
460 nm) and Q bands (650–670 nm). Sensitizers LW11 and
LD14 show broadening of the S band and a red-shift
(11 nm) of the Q band compared to those of dyes LW12 and
LW13, which might be due to effective inner-charge transfer
ꢀ ꢀ
in the former two dyes with the donor p acceptor struc-
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Yan Shen, Songyuan Dai, Mingkui Wang et al.
Table 1. Absorption wavelength, fluorescence maxima,[a] and first porphyrin-ring
redox potentials of porphyrins in THF.
band (about 0.5 eV) of TiO2 ensured a thermody-
namic driving force for the charge-generation pro-
cess.
Dye
Absorption[a]
lmax [nm] (loge
[mꢀ1 cmꢀ1])
Emission[a]
Potentials and energy level
lmax [nm] Eox [V][b] (vs.
E0–0 [V][c] (vs.
EoxꢀE0–0
The photocurrent–voltage characteristics of the
LW11 (device A), LW12 (device B), LW13 (devi-
ce C), and LD14 (device D)-based DSCs, measured
under simulated full sunlight (AM 1.5), are shown
in Figure 2a. The photovoltaic parameters, that is,
the open-circuit voltage (Voc), fill factor (FF), short-
circuit current density (Jsc), and power conversion
efficiency (h) for devices A–D are listed in Table 2.
Device A (with dye LW11, which contains a pyridine
moiety as the anchoring group) exhibits Voc =
0.63 V, Jsc =8.2 mAcmꢀ2, and FF=0.77, thus giving
an overall power-conversion efficiency of 3.96%.
NHE)
NHE)
[V]
LW11
LW12
LW13
457
665
453
657
457
663
N
682, 742 0.734
667, 725 0.952
672, 732 0.866
1.845
1.873
1.859
1.840
ꢀ1.111
ꢀ0.921
ꢀ0.993
–
LD14[d] 459
667ACHTUNGTRENNUNG
684, 749
–
[a] Absorption and emission data were measured in THF at 258C. Excitation wave-
lengths: 457 nm (LW11), 453 nm (LW12), 457 nm (LW13). [b] The first porphyrin-ring
oxidation was performed at 258C with each porphyrin (0.5 mm) in THF/0.1m TBAP/
N2 with a GC working electrode, a Pt counter electrode, and a Ag/AgCl reference
electrode with organic solvent at a scan rate of 50 mVsꢀ1. [c] E0–0 was estimated from
the intersection wavelengths of the normalized UV/Vis absorption and fluorescence
spectra. [d] LD14 was synthesized according to reference [4a].
ꢀ ꢀ
Compared to previously reported donor p accept-
or-conjugated organic dyes with non-carboxylic
acids as electron-withdrawing anchoring groups,[6,8]
the porphyrin sensitizers presented herein show im-
proved photovoltaic performance, which could be
ture.[7] The measured peak molar extinction coefficient (e)
of the Q band for sensitizers LW11, LW12, and LW13 are
62.3ꢂ103, 38ꢂ103, and 48.7ꢂ103 mꢀ1 cmꢀ1, respectively; the
corresponding data are presented in Table 1. These new por-
phyrins present higher light-harvesting capacity than other
previously reported organic sensitizers that incorporated
pyridine derivatives as anchoring groups.[8] Significant aug-
mentation in the intensity of the Q band relative to that of
the S band was observed in dye LW11, as compared to the
LW12 dye, which might be attributed to the loss of symme-
try in this compound. This phenomenon could be caused by
attributed to their strong light-adsorption properties and
broadened absorption spectra. More recently, organic dyes
that contained 2-(1,1-dicyanomethylene)rhodanine moieties
with high photovoltaic performance (power-conversion effi-
ciency of about 7%) have been reported by Tian and co-
workers.[13] The incorporation of another pyridine moiety
into the donor moiety in the porphyrin structure (LW12) led
to a decrease in the Jsc and Voc values of the corresponding,
ꢀ
the splitting in the p p* levels and a decrease in the
HOMO–LUMO gap.[11] Thus, the absorption spectra of dye
LW11 is red-shifted by 15 nm compared to that of LW12,
which reveals that the light-harvesting ability of these high
ꢀ
molar extinction coefficient Zn porphyrin dyes can be en-
hanced by careful design of the electron-donating moietie-
s.[4a–b,12]
As shown in Figure 1b, the fluorescence emission spectra
of sensitizers LW11, LW12, and LW13 in THF show an emis-
sion centered at 682, 667, 672 nm, respectively. The excita-
tion transition energies (E0–0) were evaluated to be 1.845,
1.873, and 1.859 eV, respectively. We investigated the elec-
trochemical properties of these porphyrins by performing
cyclic voltammetry measurements (see the Supporting Infor-
mation, Figure S10). The oxidation and reduction potentials
of these porphyrins are listed in Table 1. The first oxidation
potentials of LW11, LW12, LW13, and LD14 were deter-
mined to be at +0.734, +0.952, and +0.866 V (vs. NHE), re-
spectively, which were 0.234, 0.452, and 0.366 V higher than
that of the iodide electron donor, thus offering enough of
a driving force for the dye-regeneration process (see the
Supporting Information, Figure S17). The excited-state
redox potentials (ED/D+) for these three sensitizers were cal-
culated to be ꢀ1.111, ꢀ0.922, and ꢀ0.993 V (Table 1 and the
Supporting Information, Figure S17), respectively. The nega-
tive offset of (ED/D+) relative to the edge of the conduction
Figure 2. a) I–V curves of LW11 (device A), LW12 (device B), LW13 (de-
vice C), and LD14 (device D)-based DSCs under simulated full sunlight
(AM 1.5G). b) Photocurrent action spectra of the corresponding devices;
the cells were measured by using a metal mask (area: 0.072 cmꢀ2).
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Table 2. Photovoltaic parameters of solar cells that were sensitized with
porphyrins LW11 (device A), LW12 (device B), LW13 (device C), and
LD14 (device D) under AM 1.5G light (100 mWcmꢀ2).
IPCE is related to light-harvesting efficiency, charge-injec-
tion efficiency, and charge-collection efficiency. As discussed
above, the adsorption of LW11 is much lower than that of
the other three compounds (Table 2). Thus, device A may
Device Dye loading[a]
[pmolcm2]
Jsc
Voc
[V]
FF
h
a[b]
[mAcmꢀ2
]
[%]
ꢀ ꢀ
benefit from the donor p acceptor structure of dye LW11.
A
B
C
D
6.6
24.9
39.9
34.9
8.2
2.84
9.83
17.38
0.63
0.54
0.63
0.73
0.77 3.96 0.29
0.74 1.24 0.35
0.74 4.59 0.23
0.71 9.01 0.27
Thus, good coupling between the donor and acceptor for the
inner-charge transfer of sensitizers increases the electron-in-
jection efficiency.
Nanosecond time-resolved laser experiments were per-
formed to elucidate the dynamics of interception of the dye
cation by iodide.[15] Figure 3 shows the transient adsorption
[a] The amount of adsorbed dye per unit area of TiO2 film was controlled
by the immersion time of the TiO2 (20 nm) electrode in the dye solution.
The measured sample area of dye-loading was 0.96 cm2 with a thickness
of 10 mm. The porosity of the film was about 50%. [b] The parameter
a (a ¼ T=T0) is a dimensionless quantity that represents the ratio of the
experimental temperature (T) to the characteristic temperature (T0) of
a specific material.
thus yielding an efficiency of 1.24%. Switching these two
pyridine rings with carboxylic acids caused the efficiency to
increase to 4.59% for LW13-sensitized solar cells. Dye
LW13, which contained a carboxylic acid anchoring group,
showed a higher Jsc value than those of DSCs that were
based on sensitizers LW11 and LW12 (with pyridine rings).
Changing one of the carboxylic acid groups with the 4-eth-
ynyl-N,N-dimethylaniline moiety as the electron donor pro-
duced dye LD14. An overall power-conversion efficiency of
9.01% was obtained for device D (with sensitizer LD14),
owing to a contribution from a high Jsc value. In this study,
a transparent TiO2 film (size: 20 nm, thickness: 7.5 mm) was
used, which explains the lower photocurrents and solar-to-
electric-power-conversion efficiencies compared to the
values reported by Diau and co-workers (PCEꢁ10%), in
which an optimized electrolyte composition and a photoa-
node structure were used.[4a,b] Compared to the other three
dyes, dye LW11 showed much lower dye loading onto the
TiO2 surface (Table 2). Dyes LW13 and LD14 bind to titani-
um ions on the surface of titania through the carboxylic acid
group, which binds much more strongly than the pyridine
group through a coordinate bond.[8,14] We also observed that
the incorporation of one pyridine group in dye LW11 de-
creased the mass of loaded dye comparing with that of dye
LW12, which had a symmetrical pyridine structure (Table 2).
However, the photovoltaic performance of the former DSC
was higher than that of the latter device (3.96% versus
1.24%). A small difference in dye loading was observed be-
tween dyes LW13 and LD14, thus indicating almost total
surface coverage by these dyes onto the TiO2 nanoparticle,
with a diameter of about 20 nm after 2 h (dye-dipping time).
The dye-loading was estimated to be about 49.8 pmolcmꢀ2
from the film thickness, area, and porosity in this experi-
ment.
Figure 3. Transient adsorption spectra of LW11 that was stained onto
TiO2: Observed kinetics in MeCN with no iodide added (MeCN) and in
the presence of redox electrolyte w08 under similar conditions.
spectra of LW11 that was stained onto TiO2 under different
atmospheres as an example. The transient optical signal at
l=580 nm shows the concentration of the oxidized state of
ꢀ
the Zn porphyrin sensitizers after ultrafast photoinduced
electron injection from the dye into the conduction band of
TiO2. In the absence of a redox shuttle, the drop in the ab-
sorbance signal reflects the dynamics of the recombination
of conduction-band electrons with the oxidized dye. The
decays were well-fitted by a single exponential with a half-
life of t=221, 288, and 588 ms, for LW11, LW12, and LW13,
respectively. In the presence of MeCN that contained redox
electrolyte, the decay of the oxidized dye was markedly ac-
celerated. The half-life was estimated to be t=6, 6, and 5 ms
for LW11, LW12, and LW13, respectively. These results indi-
cate that the sensitizers are quickly regenerated and that the
dye cations are efficiently intercepted by the redox media-
tor. The insertion of the pyridine ring into the Zn porphyr-
ins decreases the dye-regeneration process by a factor of
1.2. The yield of interception by iodide for these three por-
phyrin sensitizers can be evaluated to be 97, 98, and 99%
for LW11, LW12, and LW13, respectively.
Unraveling the details of the electron-recombination dy-
namics between the photoinjected electrons on the TiO2 sur-
face and the oxidized electrolyte in various devices was un-
dertaken by employing transient photovoltage-decay meas-
urements and charge-extraction measurements. Figure 4a
shows a plot of Voc versus the open-circuit charge density.[16]
The Voc value increased linearly with the logarithm of
charge density for all devices. This change in Voc value can
be explained qualitatively by a shift in the conduction-band
edge (Ec) with respect to the quasi-redox potential of the
The photocurrent action spectra of DSCs with sensitizers
LW11, LW12, LW13, and LD14 are shown in Figure 2b. The
plots of incident photon-to-collected-electron conversion ef-
ficiency (IPCE) show two characteristic bands within the
ranges 400–500 and 650–750 nm, with a broad plateau of
over 50% for devices A, C, and D. Device B showed much
lower IPCE values compared to the other devices. The
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shift (~20 mV) relative to that of device A was seen for
device D, in which LD14 was used. A plot of electron life-
time as a function of charge density for the LW dyes is
shown in Figure 4b. The charge density was adjusted by
varying the intensity of the bias light impinging on the cell.
The electron lifetime is longer for device A than for devices
B and C, thus indicating lower interfacial charge recombina-
tion in this device. Figure 4c shows plots of chemical capaci-
tance (Cm) versus Voc for various dyes. We observed a charac-
teristic exponential rise of Cm as a function of Voc. The chem-
ical capacitance is defined as the amount of electronic
charge that is necessary to increase the Fermi level of the
nanocrystalline oxide film by 1 eV. Chemical capacitance
can provide information about the shift of the conduction-
band edge of TiO2 and the distribution of trap states. At
a given Voc value, the Cm value is larger for device B than for
devices A and C, thus indicating a lower Fermi level of
device B compared to the other dyes, in which two 4-ethy-
nylpyridineone groups are included. Parameters a and T0
are used to describe the density of trap states (DOS).[18] The
a parameter (a ¼ T=T0) is a dimensionless quantity that ex-
presses the ratio of the experimental temperature (T) to the
characteristic temperature (T0) of a specific material. By
using an exponential function to fit the chemical-capacitance
curves (Figure 4c), values of aꢁ0.29, 0.35, 0.23, and 0.27
were obtained for devices A, B, C, and D, respectively. Com-
paratively larger values of a correspond to a steeper drop of
the DOS below the conduction-band edge. It has been re-
ported that photovoltage can be influenced by either the
shift of the TiO2 Fermi level with respect to the electrolyte
potential or to the differences in the recombination reaction
of electrons in TiO2 and the electrolyte.[18] The results in
Figure 4 indicate that the recombination process is the
major factor that affects the photovoltaic parameters of the
LW11- and LW12-based DSCs. A plausible explanation of
the serious recombination of device B could be the symmet-
rical structure of dye LW12.
Conclusions
In conclusion, new porphyrin dyes that contained pyridine
moieties as electron-withdrawing anchoring groups were de-
signed, synthesized, and successfully employed as sensitizers
Figure 4. Transient photovoltage- and photocurrent-decay measurements
and charge-extraction measurements of LW11 (device A), LW12 (devi-
ce B), LW13 (device C), and LD14 (device D): a) Plots of electron densi-
ty as a function of open-circuit cell voltage. b) Plots of electron-recombi-
nation lifetime as a function of cell electron density. c) Plots of chemical
capacitance as a function of open-circuit cell voltage.
ꢀ ꢀ
in DSCs. DSC devices that used the donor p acceptor-
structured porphyrin dye with pyridine as an anchoring
group achieved power-conversion efficiencies of about 4%;
thus, they out-performed other porphyrin sensitizers with
non-carboxylic acid anchoring group. The porphyrins that
contained pyridine (as an acceptor moiety) groups formed
coordinate bonds with the TiO2 surface, as evidenced by the
absorption of TiO2 films and transient photovoltage-decay
measurements. This study demonstrates that porphyrins with
pyridine moieties as anchoring groups show good push–pull
properties that benefits inner-charge transfer and electron
injection after dye excitation. Further studies, such as
strengthening the interactions between the dyes and TiO2 to
electrolytes. We observed that the conduction-band edge of
the TiO2 underwent an upward displacement in devices that
employed sensitizers with pyridine as the anchoring group
(LW11 and LW12) compared to those with carboxylic acid
anchors. These energy shifts principally influence the devi-
ces’ open-circuit voltage.[17] At an identical photoinduced
charge density (5ꢂ1017 cmꢀ3), the photovoltage of device C
(LW13) is approximately 60 mV lower than that of devices
A and B (LW11 and LW12, respectively). A further negative
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diodes (0.05 s square pulse-width, 100 ns rise and fall time) that were
controlled by a fast solid-state switch were used as the perturbation
source. The voltage dynamics were recorded on a PC-interfaced Keithley
2602A source meter with a 500 ms response time. The perturbation light
source was set to a suitably low level for the voltage-decay kinetics to be
mono-exponential. By varying the intensity of white-light bias, the re-
combination lifetime could be estimated over a range of open-circuit vol-
tages. The chemical capacitance of the TiO2/electrolyte interface and the
DOS at Voc were calculated according to Cm ¼ DQ=DV, where DV is the
peak of the photovoltage transient and DQ is the number of electrons in-
jected during the red-light flash. The latter parameter is obtained by inte-
grating a short-circuit transient photocurrent that is generated from an
identical red-light pulse. Before the LEDs switched to the next light in-
tensity, a charge-extraction routine was executed to measure the electron
density in the film. In the charge-extraction techniques, the LED illumi-
nation source was turned off within <1 ms, whilst, simultaneously, the cell
was switched from open circuit to short circuit. The resulting current, as
the cell returned to V=0 and J=0, was integrated to give a direct mea-
surement of the excess charge in the film at that Voc value.
increase the dye loading, thus augmenting the photocurrent,
are underway.
Experimental Section
Materials
All solvents and reagents, unless otherwise stated, were of puriss quality
and used as received. Standard Schlenk techniques were employed to
manipulate oxygen- and moisture-sensitive chemicals. The starting re-
agents and 4-ethynyl-N,N- dimethylaniline were purchased from Aldrich
and used as received. 4-Ethynylbenzoic acid was synthesized according to
a literature procedure.[10a] THF was dried with sodium sand and benzo-
phenone indicator, CH2Cl2, Et2O, and triethylamine (TEA) were dried
with calcium hydride before use. Reactions were performed under a dry
nitrogen atmosphere.
Device Fabrication
FTO glass plates (3 mm thickness, 7 W/square, Nippon Sheet Glass) were
cleaned in a detergent solution in an ultrasound bath for 15 min and then
rinsed with deionized water and EtOH for 15 min. The cleaned glass
plates were immersed into 40 mm TiCl4 (aq) at 708C for 30 min and then
washed with water and EtOH. A transparent layer of 20 nm TiO2 parti-
cles (thickness: 7.5 mm) was first printed on the FTO-conducting glass
electrode and then coated with a second layer of 400 nm light-scattering
anatase particles (thickness: 5 mm, WER2-O, Dyesol). The thickness of
the film was measured by using the Profile system (DEKTAK, VECCO,
Bruker). The details for the preparation of the TiO2 films have been de-
scribed elsewhere.[18] First, the TiO2 film was sintered at 5008C for
30 min and then cooled to about 808C in air. Then, the TiO2 film electro-
des were dipped into a 200 mm dye solution in a mixture of toluene and
EtOH (1:1 v/v) at RT for 5 h. After washing with EtOH and drying in
air, the sensitized titania electrodes were assembled with thermally plati-
nized conductive glass electrodes. The working and counter electrodes
were separated by using a hot melt ring (thickness: 45 mm, Surlyn,
DuPont) and sealed by heating. The internal space was filled with liquid
electrolyte by using a vacuum back-filling system. The electrolyte (W08)
for the devices was 0.1m LiI, 0.05m I2, 0.6m PMII, and 0.5m 4-tert-butyl-
pyridine in a mixture of valeronitrile and MeCN (15:85 v/v).
Acknowledgements
Financial support from the Director Fund of the WNLO, the NSFC
(21103578, 21161160445, 20903030, and 201173091), the 973 Program of
China (2011CBA00703 and 2013CB922102), the Natural Science Founda-
tion of Hubei Province (2011CDB0.4), the Talents Recruitment Program
and the Fundamental Research Funds for the Central Universities
(HUST: 2011TS021, 2011QN040, and 2012YQ027), and the Program of
New Century Excellent Talents in University of the CME (NCET-10-
0416) is gratefully acknowledged. The authors thank the Analytical and
Testing Center at the HUST and the Key Laboratory of Novel Thin-Film
Solar Cells at the IPP for performing the characterization of the sensitiz-
ers.
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Received: November 28, 2012
Published online: February 19, 2013
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