A desirable hole-conducting coadsorbent for highly efficient dye-sensitized solar cells through an organic redox cascade strategy

Article abstract of DOI:10.1002/chem.201100813

Under the sun! A new low-molecular-weight and multiple-functional coadsorbent (HC-A) for highly efficient dye-sensitized solar cells has been developed. The NKX2677/HC-A-sensitized solar cell exhibited remarkably enhanced conversion efficiency by a factor

Full text of DOI:10.1002/chem.201100813

COMMUNICATION
DOI: 10.1002/chem.201100813
A Desirable Hole-Conducting Coadsorbent for Highly Efficient
Dye-Sensitized Solar Cells through an Organic Redox Cascade Strategy
Bok Joo Song, Hae Min Song, In Taek Choi, Sang Kyun Kim, Kang Duk Seo,
Min Soo Kang, Myung Jun Lee, Dae Won Cho, Myung Jong Ju, and Hwan Kyu Kim*[a]
Over the past decade, dye-sensitized solar cells (DSSCs)[1]
based on nanocrystalline mesoporous metal oxides (typically
TiO2) have been intensively studied and developed as a
promising, low-cost alternative to conventional silicon pho-
tovoltaic devices. The components of such DSSCs are a dye
sensitizer, a TiO2 metal oxide coated on conductive glass, a
redox electrolyte couple, and a counter electrode. As illus-
trated in Scheme 1, the power-conversion efficiency of
DSSCs is strongly dependent upon the minimization of
charge recombination losses at the TiO2/dye/electrolyte in-
terface (pathways 3 and 4).[2] There are two recombination
pathways of importance in DSSCs, in which electrons photo-
injected into the TiO2 electrode can recombine with either
dye cations or with redox electrolytes. Moreover, such
charge recombination leads to losses in both the short-cir-
cuit photocurrent (Jsc) and the open-circuit photovoltage
(Voc), resulting in a decrease in overall energy conversion ef-
ficiency (h).
only the insertion of a metal oxide blocking layer,[10] but
also by energetic cascades for multistep hole conductors
(HC),[11] as well as superior molecular sensitizer dyes[12] and
the insertion of a barrier layer between the sensitizer dye
and the HC.[2] To date, however, the desired redox inter-
ꢀ
mediate mediators between the dye and Iꢀ/I3 redox electro-
To reduce the possible charge recombination pathways oc-
curring at the TiO2/dye/electrolyte interface, several kinds of
additives, such as decylphosophonic acid (DPA), dineohexyl
bis(3,3-dimethylbutyl)phosphinic acid (DINHOP), and che-
nodeoxycholic acid (CDCA) have been introduced to
adsorb onto the TiO2 surface.[3] Among them, for example,
cholic acid (CA) derivatives as coadsorbents in DSSCs,
based on a Ru–pyridyl complex,[4] courmarin,[5] porphyrin,[6]
phthalocyanine,[7] and naphthalocyanine dye,[8] have been in-
vestigated well. Deoxycholic acid (DCA) is often used as a
coadsorbent to break up organic and Ru–dye aggregates
and hence to significantly improve Voc and Jsc.[5,9] Moreover,
alternative approaches directed toward the minimization of
recombination losses have been extensively studied by not
[a] Dr. B. J. Song, H. M. Song, I. T. Choi, S. K. Kim, K. D. Seo,
M. S. Kang, M. J. Lee, Dr. D. W. Cho, Dr. M. J. Ju, Prof. H. K. Kim
Department of Advanced Materials Chemistry and
Center for Advanced Photovoltaic Materials (ITRC) and
Center for Integrated Materials and Devices for
Next Generation Photovoltaic System (WCU)
Korea University, 208 Seochang, Jochiwon
ChungNam 339-700 (Korea)
Scheme 1. a) Schematic diagram of the ideal charge-transfer processes
occurring at a dyed TiO2/electrolyte interface including organic HC, and
b) one of the charge-transfer processes proposed herein. Pathways: (1)
Electron injection from the dye or HC-A excited state into the conduc-
tion band of the TiO2, (2) dye regeneration by electron transfer from a
redox couple, (2*) regeneration of the dye and HC-A by electron trans-
fer from a redox couple, (3,4) charge recombination with dye cation and
Fax : (+82)41-867-5396
E-mail: hkk777@korea.ac.kr
ꢀ
I3 ions. Reaction pathway (5) corresponds to the hole-transfer from the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100813.
HOMO of the dye cation to the HOMO of the HC-A, and (5*) hole
transfer from the HOMO of the HC-A to redox electrolyte.
Chem. Eur. J. 2011, 17, 11115 – 11121
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11115

lyte have been intensively looked for to increase the open
circuit voltage of Voc and improve the device performance
through a cascade-type hole-hopping channel, but these
have not yet been developed (see Scheme 1a).[13] For this, by
minimizing the energy difference between the HOMO of
the sensitizing dye and the work function of the organic HC,
it is possible to develop DSSCs with higher energy conver-
sion efficiency.[14]
and 0.5m 4-tert-butylpyridine (tBP) using dehydrated
CH3CN as a solvent. The dye NKX2677 was prepared ac-
cording to the literature.[15]
The UV/Vis absorption spectra for the dye-adsorbed TiO2
films with different coadsorbents are compared in Figure 1.
The molar extinction coefficient (e) of HC-A was estimated
Among the metal-free organic dyes studied in DSSCs,
coumarin-based dyes[15] are somewhat promising sensitizers
for DSSCs based on TiO2 films, because of their good pho-
toresponse in the visible region, good long-term stability
under light-soaking conditions (one sun),[16] appropriate
lowest unoccupied molecular orbital (LUMO) levels match-
ing the conduction band of TiO2, and low cost. However,
p–p stacking of organic dye molecules usually occurs, be-
cause of the strong intermolecular interaction. The strong
p-electron interaction in oligothiophene, in particular,
allows easy formation of p stacks or lamellar structure.[17]
Although p–p stacking is advantageous to light-harvesting,
because of its broad feature in the UV/Vis absorption spec-
trum, p-stacked aggregates usually lead to inefficient elec-
tron injection and thus result in low power-conversion effi-
ciency.[18] Prohibition of p–p stacking by use of an additive
in the dye solution is a typical way to improve efficiency of
organic dye-sensitized solar cells suffering from the p–p
stacking problem. Coadsorption of the dye with additives
and structural modification of the dye molecules has proven
to be effective to dissociate p–p stacking or dye aggregation
and thus to improve solar-cell efficiency.[19]
In the present study, we have synthesized a carbazole
moiety[20] of 4-{3,6-bis[4-(2-ethylhexyloxy)phenyl]-9H-carba-
zol-9-yl}benzoic acid (bEHCBA: hole conductor HC-A) as
an alternative coadsorbent with a new function not seen in
DCA or CA derivatives to enhance the overall conversion
efficiency of DSSCs by reducing the possible charge-recom-
bination pathways occurring at the TiO2/dye/electrolyte in-
terface. Also, we have investigated the charge-transfer pro-
cess occurring in co-sensitized DSSCs including a processes
proposed here that proceeds via an organic hole conductor
(Scheme 1b).
Figure 1. UV/Vis absorption spectra for TiO2 films after immersion for
12 h in 0.3 mm NKX2677 dye (c), 0.3 mm NKX2677 dye + 40 mm
DCA (g), 0.3 mm NKX2677 dye + 10 mm HC-A (d), and 10 mm
HC-A alone (ꢀ··ꢀ··ꢀ). The bare TiO2 transparent film with the thickness
of 8 mm was used as a reference for the spectrum scan.
to be 44200mꢀ1 cmꢀ1 at 296 nm. NKX2677 exhibited an ab-
sorption maximum peak at 511 nm in ethanol (with
e=64300mꢀ1 cmꢀ1),[16] but the NKX2677-sensitized TiO2
film exhibited a blue-shifted maximum peak at 501 nm.
Such a blue shift was attributed to the solvent effect and de-
protonation of carboxylic acid upon adsorption.[21] The
NKX2677/DCA-sensitized TiO2 film showed a similar ab-
sorption peak position, but the absorbance at 501 nm de-
creased from 3.42 to 2.21, indicating that the amount of dye
on the TiO2 surface decreased by about 35%. The dye/HC-
A-sensitized TiO2 film showed an absorption peak at
around 511 nm and an additional sharp absorption peak
around 355 nm, corresponding to the HC-A molecule. This
red shift may be caused by the breakup of H-aggregation of
NKX2677.
According to Figure 1, the absorbance at maximum peak
of NKX2677 decreased from 3.42 to 0.73, indicating an ap-
proximate 78% drop in dye adsorption upon coadsorption
with HC-A. When the dye with DCA and HC-A coadsor-
bed on the TiO2 film, the deprotonation of the carboxylic
acid group in the dye induced less adsorption of NKX2677
onto the TiO2 surface, owing to the electrostatic repulsion of
negatively charged carboxylic groups.[9] Moreover, less dye
adsorption for HC-A than DCA coadsorption was mainly
attributed to the bigger molecular size of HC-A. Thus, The
HC-A with a Y-shaped bulky structure could be allowed to
be more efficient than DCA in decreasing dye adsorption
and preventing p–p aggregation between the dye molecules
in the pores of TiO2 films. The amount of the dyes adsorbed
on the TiO2 surface were estimated spectroscopically by de-
As depicted in Figure S2 in the Supporting Information,
HC-A with a low molecular weight and Y-shaped structure
can strongly absorb UV light, leading to the light-harvesting
effect in the shorter wavelength regions. It is anticipated to
increase the photocurrent of DSSCs. The introduction of the
bulky ethylhexyloxy group into a carbazole-based HC-A
was also achieved in order to enhance its solubility in organ-
ic solvents, as well as the potential of inserting into the bar-
rier layer to retard recombination losses.[2] Its oxidation
mid-potential was determined from cyclic voltammetry
(CV) in DMF containing 0.1m TBAACTHNUTRGEN(UNG PF6) to be 1.35 V
versus NHE. Details of chemical synthesis and characteriza-
tion of HC-A, cell fabrication, and device performance
measurements are described in the Supporting Information.
The redox liquid electrolyte contained 0.1m LiI, 0.05m I2,
0.6m 1,2-dimethyl-3-n-propylimidazolium iodide (DMPII),
11116
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Dye-Sensitized Solar Cells
COMMUNICATION
Table 1. Photovoltaic performance parameters of the DSSCs under the
sorbing the dyes with a 0.1m solution of NaOH in 50:50
(vol%) THF/H2O, and the surface concentrations were de-
termined to be 2.02ꢁ10ꢀ7, 1.31ꢁ10ꢀ7, and 4.1ꢁ10ꢀ8 molcmꢀ2
forNKX2677-, NKX2677/DCA-, NKX2677/HC-A-based
TiO2 films, respectively (see Figure S4 in the Supporting In-
formation).
The incident photon conversion efficiency (IPCE) spec-
trum and J–V curve of HC-A-based solar cell under stan-
dard test conditions (STC: AM 1.5 G, 100 mWcmꢀ2 irradia-
tion at 258C) are shown in Figure S5 in the Supporting In-
formation. The photovoltaic performance of HC-A-based
solar cell was obtained with a short circuit current (Jsc) of
0.885 mAcmꢀ2, an open circuit voltage (Voc) of 560 mV, a fill
factor (FF) of 73.76%, and an overall conversion efficiency
(h) of 0.37% under STC. The maximum IPCE of HC-A-
based solar cell was about 50% at 350 nm, indicating that
HC-A has the same effect as DCA, in addition, it has a
light-harvesting effect at short-wavelength regions.
STC (AM 1.5 G, 100 mWcmꢀ2 irradiation at 258C).
Relative ratio
of dye adsorbed [V]
Voc
Jsc
[mAcmꢀ2
FF
[%]
h
G
]
[%]
NKX2677
NKX2677/DCA
NKX2677/HC-A 0.39
HC-A
1.00
0.67
0.579 15.49
0.608 16.81
0.688 17.25
74.36 6.68
74.86 7.67
74.93 8.89
73.76 0.37
–
0.560
0.885
by dye molecule aggregation or close p-p stacking. Such ag-
gregate formation has been suggested to cause unwanted in-
termolecular energy transfer or nonradiative decay path-
ways, thus reducing the electron injection efficiency.[22] How-
ever, in both the case of DCA and HC-A coadsorption,
even though the amount of dye on the TiO2 surface was re-
markably reduced, the maximum IPCE was improved up to
80 and 85%, respectively. Taking the ꢁ85% transmittance
of the TCO glass into account, the IPCE value for the TiO2
film loaded with HC-A is 100%, even without an antire-
flecting coating film. Because the dye monomer can inject
electrons to TiO2 more efficiently than aggregate,[18] the
coabsorbed dye/DCA TiO2 film obtained from the exposure
of TiO2 film to the dye solution with 40 mm DCA may con-
tain a mixture of aggregate and dye monomer, judged from
the IPCE spectrum. On the other hand, the coadsorbed dye/
HC-A TiO2 film obtained from exposure of TiO2 film to the
dye solution with 10 mm HC-A is mainly composed of dye
monomer. Upon HC-A coadsorption, the maximum IPCE
was remarkably improved up to 65% at 360 nm than that of
the other two devices, maybe contributing to increase the
photocurrent of the DSSCs. Even with its lower dye adsorp-
tion, NKX2677/HC-A exhibited a 5% higher value than the
maximum IPCE of NKX2677/DCA. It implies that HC-A
can act as a more effective spacer than DCA among dye
molecules and thus suppresses the p–p stacking of the dye
molecules, which in turn retards the charge recombination
and hence improves the device performance significantly.
Thus HC-A has the same effect like DCA; in addition, it
has a light-harvesting effect at short wavelength as seen in
IPCE data. In other words, it has multiple-functioned effects
that are the prevention of the p–p stacking of organic dye
molecules like DCA and the light-harvesting effect at short-
er-wavelength regions. As indicated in Figure 2b and
Table 1, the photovoltaic performance of the NKX2677-sen-
sitized DSSC was lower than the others, which influenced
the overall conversion efficiency (h=Voc ꢁJsc ꢁFF). For the
NKX2677/DCA-sensitized DSSC, compared with NKX2677-
sensitized DSSC, Voc increased from 0.579 to 0.608 V, Jsc in-
creased from 15.49 to 16.81 mAcmꢀ2, and the overall con-
version efficiency increased from 6.68 to 7.67%.
Figure 2 shows the IPCE and J–V characteristics of the
DSSCs using different additives under the STC. Their pho-
tovoltaic performance is summarized in Table 1. The IPCE
of the NKX2677-sensitized solar cell was lower than that of
the others and was ascribed to charge recombination caused
As shown in Figure 2, Jsc increased in the same order of
integrated photocurrent from the IPCE spectrum. For the
NKX2677/HC-A-sensitized DSSC, compared to the
NKX2677-sensitized DSSC, Voc increased from 0.579 to
0.688 V, Jsc increased from 15.49 to 17.25 mAcmꢀ2, and the
overall conversion efficiency increased from 6.68 to 8.89%.
Figure 2. a) The IPCE spectra, b) J–V curves of the DSSCs, and dark-cur-
~
*
rent density of NKX2677 (&), NKX2677/DCA ( ), NKX2677/HC-A ( ),
and film thickness and active area of 16
tively.
(8+8) mm and 0.16 cm2, respec-
ACHTUNGTRENNUNG
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H. K. Kim et al.
It was observed that the coadsorption of either DCA or
HC-A improved Jsc and Voc, simultaneously, but coadsorp-
tion with HC-A enhanced Voc more significantly than with
DCA, even with the lower dye adsorption. Also, Voc in-
creased to 29 mV for DCA and 107 mV for HC-A coadsorp-
tion. These results indicate that HC-A is more efficient than
DCA for retarding charge recombination and hence im-
proves photovoltaic performance, that is, Jsc and Voc, signifi-
cantly.
To further clarify the effect of HC-A on the Voc, electro-
chemical impedance spectroscopy (EIS), which is a powerful
tool to elucidate the electronic and ionic transporting pro-
cesses in DSSCs, was measured under dark conditions. EIS
can provide valuable information for the understanding of
photovoltaic performance parameters (Jsc, Voc, FF, and h) in
DSSCs. The enhancement of Voc is usually associated with
the negative shift of conduction band edge or the suppres-
sion of charge recombination. The value of Voc is deter-
mined by the potential difference between the Fermi level
of TiO2 and the chemical potential of the redox species
(Ered) in the electrolyte, which can be described as Equa-
tion (1),[23] in which g is characteristic constant of TiO2 tail-
ing states, kB is the Boltzmann constant, T is temperature, e
is the elementary charge, and Ne is the effective density of
states at the TiO2 conduction band edge.
ꢀ
ꢁ
kBT
e
Ne
n
VOC ¼ Ered ꢀ ECE ꢀ g
ln
ð1Þ
Considering that Ered would not change strongly in DSSCs
fabricated under similar conditions, Voc is determined by the
potential of the conduction band edge (ECB) and the elec-
tron density (n) in TiO2. These two parameters are closely
related to the surface charge and charge recombination, re-
spectively. Possible factors influencing Voc for DSSCs involve
the TiO2 surface blocking, conduction band movement, and
electrolyte-dye interaction.
Typical EIS Nyquist plots and Bode phase plots measured
under dark conditions at a forward bias of ꢀ0.65 V for
DSSCs based on NKX2677, NKX2677/DCA and NKX2677/
HC-A are shown in Figure 3. The equivalent circuit present-
ed in Figure 3b was used to fit the experimental data of all
of the samples. Rs is the series resistance accounting for the
transport resistance of the TCO and the electrolyte. Cm and
Rct are the chemical capacitance and the charge recombina-
tion resistance at the dyed TiO2/electrolyte interface, respec-
tively. CPt and RPt are the interfacial capacitance and charge
transport resistance at the Pt/electrolyte interface, respec-
tively. The larger semicircle at lower frequencies represents
the interfacial charge transfer resistance (Rct) at the dyed
TiO2/electrolyte interface. The fitted Rct increased in
the order of NKX2677 (14 W) <NKX2677/DCA (33 W)
<NKX2677/HC-A (106 W), which were consistent with the
sequence of Voc values in the devices.
Figure 3. a) Nyquist plots and b) fitted Bode-phase plots of electrochemi-
cal impedance spectra for the DSSCs based on NKX2677 (&), NKX2677/
~
*
DCA ( ), and NKX2677/HC-A ( ); the inset is an equivalent circuit
(see text for explanation).
recombination in devices based on NKX2677 and
NKX2677/DCA is faster than that of NKX2677/HC-A. The
results could be cross-checked by measuring the J–V charac-
teristics of the DSSCs in the dark (Figure 2b). The onset po-
tentials at the dark current value of ꢀ1 mAcmꢀ2 for the
three different devices were in the order of NKX2677
(453 mV) <NKX2677/DCA (469 mV) <NKX2677/HC-A
(533 mV). Their sequence was in accordance with that of Rct
values. This dark current change indicates that the coadsorp-
tion of HC-A leads to the suppression of the charge recom-
ꢀ
bination between injected electron and I3 ions in the elec-
trolyte, favorable for Voc gain.
To further investigate the influences of the shift in the
TiO2 conduction band edge and charge recombination on
Voc, impedance spectra were also measured by varying the
applied potential at equal intervals in the vicinity of Voc. The
charge-transfer resistance at the dyed TiO2/electrolyte inter-
face (Rct), the film capacitance (Cm) and the electron lifetime
(t=CmRct) were derived from EIS by fitting the experimen-
tal data through an appropriate equivalent circuit.[24] The
fitted Cm and Rct values together with t are plotted in
The smaller Rct value means that the electron recombina-
tion from the conduction band to the electrolyte occurs
more easily, thus resulting in lower Voc. Clearly, the electron
11118
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Dye-Sensitized Solar Cells
COMMUNICATION
As shown in Figure 4a, at the given whole potential range
of Va, Cm for NKX2677/HC-A and NKX2677/DCA was not
changed significantly, but was increased compared to
NKX2677. It indicates a sequential negative shift of the con-
duction band edge: NKX2677/HC-A> NKX2677/DCA>
NKX2677. Generally, the negative shifts of ECB lead to im-
provement of Voc of DSSCs. As shown in Figure 4b, the in-
creased Voc was mainly ascribed to the major increase in Rct
for NKX2677/HC-A compared to NKX2677/DCA. At the
forward bias of ꢀ0.65 V, the device based on NKX2677/HC-
A showed a similar value of Cm (678 to 665 mF), but an in-
creased value of Rct (from 33 to 106 W), compared to that of
NKX2677/DCA, yielding a higher t of 72 ms. The fitted t in-
creased in the order of NKX2677 (6 ms) <NKX2677/DCA
(22 ms) <NKX2677/HC-A (72 ms). It gives an explanation
of the different values of Voc from three DSSCs. In the
range of potentials studied, the order of t was in good ac-
cordance with that of Voc. It implies that the charge recom-
bination rather than the position of CB governs Voc. From
these results, the use of HC-A in DSSCs decreases the
charge recombination between injected electrons and the
oxidized dye or the redox couple, and explains the large
open circuit voltage of Voc obtained for NKX2677/HC-A-
sensitized device.
Furthermore, to clarify the additional role of hole-con-
ducting coadsorbent as proposed in Scheme 1b, the detailed
kinetic studies should be systematically and thoroughly ach-
ieved. However, these kinetic studies are extremely compli-
cated and constitute another big research area to be pro-
gressively undertaken. These studies are out of the scope of
this study, in which we just demonstrated a new class of a
low-molecular-weight and multiple-functioned coadsorbents
for highly efficient dye-sensitized solar cells. Figure 5 shows
that the transient absorption spectra (TAS) for the
Figure 4. a) Chemical capacitance (Cm), b) interfacial charge transfer re-
sistance (Rct), and c) electron lifetime (t) fitted from impedance spectra
&
*
)
under a series of applied potentials. NKX2677 ( ), NKX2677/DCA (
~
and NKX2677/HC-A ( ).
Figure 4. This exponential rise with the increase of forward
bias is a behavior typical of Cm that is described by Equa-
tion (2),[25] in which e is elementary charge, kB is the Boltz-
mann constant, T is temperature (298 K in this study), a is a
constant related to the distribution of electronic states
below the conduction, Ered is the chemical redox potential of
the redox couples in the electrolyte.
ꢂ
ꢃ
e2
kBT
a
ð2Þ
Cm ¼
exp
ðEred þ eVa ꢀ ECE
Þ
Figure 5. Transient absorption spectra for a) the NKX2677-, b) the
NKX2677/DCA-, and c) the NKX2677/HC-A-based TiO2 films in the ab-
sence of redox electrolytes were investigated with a nanosecond pulse
laser at 504 nm, and d) and e) the HC-A-based TiO2 films in the absence
of redox electrolytes were investigated with a nanosecond pulse laser at
504 and 355 nm, respectively. * corresponds to the spectrum originating
from HC-A radical cation.
kBT
Considering that Ered would not change strongly in DSSCs
fabricated under similar conditions,[26] therefore, Cm is gov-
erned by the applied potential (Va) and ECB
.
Chem. Eur. J. 2011, 17, 11115 – 11121
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11119

H. K. Kim et al.
NKX2677ꢂDCA- and NKX2677/HC-A-based TiO2 films in
the absence of the redox electrolyte were investigated with
a nanosecond pulse laser of 504 nm. The absorption maxima
for NKX2677ꢂDCA-based TiO2 films were observed
around 700 nm, which can be assigned to NKX2677 radical
cation absorption according to an article reported by Dur-
rant and co-workers,[27] indicating that the electron-transfer
process efficiently occurred from the dye to TiO2 [Eq. (3)].
Two distinct transient absorption spectra for the NKX2677/
HC-A-based TiO2 film were observed around 700 and
830 nm. Though the electrochemical interactions between
the NKX2677 and HC-A are energetically unfavorable (see
Scheme 1b), this hole-transfer mechanism may strongly
depend on the energetic overlap between the oxidized dye
and HC-A.[28] The latter absorption could be assigned to
HC-A radical cation of the carbazole cation radical spe-
cies,[29] because it cannot absorb the 504 nm pulse laser di-
rectly. On the other hand, the transient absorption spectrum
originating from the organic HC-A radical cation around
830 nm could not be observed from the HC-A-based film as
depicted in Figure 5. Thus, HC-A radical cation should be
generated through the hole-transfer [Eq. (4) and see path-
way (5) in Scheme 1b] from NKX2677 radical cation, as
shown below [Eqs. (3)–(8)]:
version efficiency by a factor of 1.33, compared to the
NKX2677-based DSSC under STC. The photovoltaic perfor-
mance of its device exhibited a remarkably high Jsc of
17.25 mAcmꢀ2, Voc of 688 mV, FF of 74.93%, and a conver-
sion efficiency of 8.89% at 100 mWcmꢀ2, which is the best
result ever reported for a DSSC employing the organic dye
NKX2677. The detailed kinetics for a cascade-type hole-
hopping process proposed here is very complicated and con-
stitutes another big research area to be progressively under-
taken.
Experimental Section
Details of the chemical synthesis and characterization as well as the cell
fabrication methods, photochemical measurements, and laser flash pho-
tolysis are described in the Supporting Information. UV/Vis spectra were
recorded with a SHIMADZU (UV-2401 PC) instrument. The FT-IR and
NMR spectra were recorded with a JASCO-4200 spectrophotometer and
a BRUKER AC-500 instrument. The corrected fluorescence emission
spectra were measured on a VARIAN CARY Eclipse fluorescence spec-
trophotometer. The mass spectra were taken by a JEOL JMS-600W mass
spectrometer. Electrochemical impedance spectra (EIS) of DSSCs were
measured with an impedance analyzer (VersaSTAT 3, AMETEK) con-
nected to a potentiostat under dark conditions at room temperature. The
spectra were scanned in a frequency range of 0.1–105 Hz and ac ampli-
tude 10 mV at room temperature.
þ
ꢀ
*
NKX2677 ! NKX2677 þ TiO2ðe Þ
HC-A þ NKX2677þ ! NKX2677 þ HC-Aþ
TiO2ðeꢀÞ þ NKX2677þ ! NKX2677
TiO2ðeꢀÞ þ HC-Aþ ! HC-A
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
Acknowledgements
This research was supported by MKE (The Ministry of Knowledge Econ-
omy), Korea, under the ITRC support program supervised by the IITA
(Institute for Information Technology Advancement (IITA-2008-C1090-
0804-0013), WCU (The Ministry of Education and Science) program
(R31-2008-000-10035-0) and by the Converging Research Center Pro-
gram through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science, and Technology (Grant Number:
2011K000596).
NKX2677þ þ 3Iꢀ ! NKX2677 þ I3
2 HC-Aþ þ 3Iꢀ ! 2 HC-A þ I3
ꢀ
ꢀ
Transient absorption spectra for the NKX2677ꢂDCA-
and NKX2677/HC-A-sensitized TiO2 films in the presence
of a redox electrolyte and also the transient absorption
traces for the HC-A-based TiO2 films in the presence of
electrolyte were investigated with a nanosecond pulse laser
at 504 and 355 nm, respectively (see Figure S6 in the Sup-
porting Information). The TAS amplitude of the carbazole
cation radical species was somewhat reduced in the presence
of a redox electrolyte, while HC-A radical cation could not
be observed from the HC-A-based film. It implies that elec-
tron migration could occur from a redox electrolyte to the
carbazole cation radical species in Scheme 1b.
Keywords: dyes/pigments
· dye-sensitized solar cells ·
organic hole conductor · pi interactions · redox chemistry
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To conclude, we just demonstrated that HC-A has multi-
ple functions, such as the light-harvesting function as a
short-wavelength light absorption dye molecule to increase
Jsc, the prevention effect of the p–p stacking of organic dye
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Chem. Eur. J. 2011, 17, 11115 – 11121

Dye-Sensitized Solar Cells
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Received: March 17, 2011
Published online: August 23, 2011
Chem. Eur. J. 2011, 17, 11115 – 11121
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11121