J. Phys. Chem. B 2005, 109, 22513-22522
22513
A Strategy To Increase the Efficiency of the Dye-Sensitized TiO2 Solar Cells Operated by
Photoexcitation of Dye-to-TiO2 Charge-Transfer Bands
Eunju Lee Tae, Seung Hwan Lee, Jae Kwan Lee, Su San Yoo, Eun Ju Kang, and
Kyung Byung Yoon*
Center for Microcrystal Assembly, Department of Chemistry, and Program of Integrated Biotechnology,
Sogang UniVersity, Seoul 121-742, Korea
ReceiVed: July 8, 2005; In Final Form: September 21, 2005
Dye-sensitized nanoporous TiO2 solar cells (DSSCs) can be classified into two types, namely, Type-I and
Type-II. Type-I DSSCs are the DSSCs in which electrons are injected from the adsorbed dyes by photoexcitation
of the dyes followed by electron injection from the excited dyes to TiO2 (pathway A). Type-II DSSCs are the
DSSCs in which electrons are injected not only by pathway A but also by direct one-step electron injection
from the dyes to TiO2 by photoexcitation of the dye-to-TiO2 charge-transfer (DTCT) bands (pathway B). The
DSSCs employing catechol (Cat) or its derivatives as the sensitizers have been the typical examples of TypeII DSSCs. However, their solar energy-to-electricity conversion efficiencies (η) have never exceeded 0.7%,
and the external quantum efficiencies (EQE) at the absorption maximums of the DTCT bands have never
exceeded 10%. We found that the attachment of electron-donating compounds such as (pyridin-4-yl)vinyl
and (quinolin-4-yl)vinyl, respectively, to Cat (designated as Cat-v-P and Cat-v-Q, respectively) leads to 2and 2.7-fold increases, respectively, in η, driven by large increases in short circuit current (Jsc). The EQE
increased from 8.5 to 30% at 400 nm upon changing from Cat to Cat-v-P, at which only the DTCT band
absorbs. In the case of the Cat-v-Q-sensitized DSSC, even the η obtained by exciting only the DTCT band
was higher than 1%. Interestingly, the illumination of only the DTCT band resulted in the increase of fill
factor from 62.6% to 72.3%. This paper provides for the first time an insight into the strategy to increase the
η values of Type-II DSSCs.
Introduction
Dye-sensitized TiO2 solar cells (DSSCs) have been developed
as commercially compelling alternatives to the more expensive,
conventional p-n junction semiconductor solar cells.1-12 A great
number of dyes have been tested as sensitizers during the past
15 years in pursuit of high efficiency (η). Accordingly, the η
has reached ∼11% in the laboratory scale.1 The DSSCs can be
classified into two types, namely, Type I and Type II, depending
on the electron-injection pathway from the dye to the conduction
band (CB) of TiO2.
The first pathway (pathway A) is photoexcitation of the local
band of the adsorbed dye followed by electron injection from
the excited dye to the CB of TiO2 as illustrated in Scheme 1A.
This pathway can also be called the “two-step” electron injection
pathway. The dyes that bind to the surface of TiO2 using
carboxylic acid groups have been known to inject electrons to
TiO2 according to pathway A. These dyes are classified as
Type-I dyes, and a variety of Ru(II) complexes,1-5 coumarin
derivatives,6 metal-porphyrin complexes,7-9 and others10-12
having carboxylic acid units, belong to this type. The corresponding DSSCs sensitized by Type-I dyes are classified as
Type-I DSSCs.
Another pathway (pathway B) is direct, “one-step” electron
injection from the ground state of the dye to the CB of TiO2 by
photoinduced charge-transfer (CT) excitation of the dye-to-TiO2
CT (DTCT) bands as illustrated in Scheme 1B.13,14 The dyes
having enediol units have been known to bind to the surface of
* To whom correspondence should be addressed. E-mail: yoonkb@
sogang.ac.kr.
SCHEME 1: Two Different Types of Electron Injection
Pathways from an Adsorbed Dye to TiO2 in DSSCs
TiO2 through chelation of surface Ti(IV) ions with the enediol
groups, generally giving rise to very strong DTCT bands.15-26
Notably, catechol (Cat)13,15,16 and its derivatives such as
dopamine (Dop),16a,19 fluorone,20 numerous natural pigments
such as bromopyrogallol red (Bpg)21 (see Chart 1), and
anthocyanins having catechol moieties21-26 give strong DTCT
bands in the visible region upon binding to TiO2. The related
compounds such as salicylic acid,16a,16b ascorbic acid,17 and
dihydroxycyclobutenedione16a also give strong DTCT bands in
the visible region upon binding to TiO2. Various transition-metal
cyanides have also been known to form visible CT complexes
with surface Ti(IV) ions through one of the nitrile groups27-31
and hence give rise to strong DTCT bands. In close relation to
this, we recently revealed that polycyclic aromatic compounds
also form visible CT complexes with TiO2.32
Several reports have demonstrated that the photoexcitation
of DTCT bands indeed gives rise to very fast (<100 fs) direct
10.1021/jp0537411 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/02/2005
22514 J. Phys. Chem. B, Vol. 109, No. 47, 2005
CHART 1: Structures and Abbreviations of the Type-II
Dyes Tested in This Report
electron injection from the dyes to TiO215a,18,22,31b in compliance
with the Mulliken’s CT theory.33 Thus, the dyes binding to the
surface of TiO2 using enediol and nitrile units can inject
electrons to the CB of TiO2 by pathway B. In this respect, they
are classified as Type-II dyes and the corresponding DSSCs
sensitized by Type-II dyes are classified as Type-II DSSCs.
In fact, most of Type-II dyes adsorbed on TiO2 absorb both
local (arising from the dye itself) and DTCT bands at the
wavelengths (λ) longer than ∼320 nm. Knowing that most of
the transparent conducting oxide (TCO) glasses begin transmitting light at ∼320 nm, the photovoltaic currents generated from
Type-II DSSCs are inevitably generated by both pathways (A
and B). Indeed, Hupp,28,34 Lian,31 Thompson and Meyer,30
Persson,13 Kumar,16c Brunschwig,29 Ghosh,35 and their coworkers pointed out the role of pathway B in the dye-to-TiO2
electron injection and in the overall photovoltaic performance
of Type-II DSSCs. In contrast to the above, several earlier
reports18,21,22,23a,26b on Type-II DSSCs did not take pathway B
into account during the analyses of electron injection pathways
from the dyes to the CB of TiO2 despite the facts that the
corresponding DTCT bands were manifest in the visible region
with no overlap with the local bands of the Type-II dyes and
that the action (incident-photon-to-current, IPCE) spectra clearly
showed the production of photovoltaic currents from the regions
specific to DTCT bands.
Unlike Type-I dyes, Type-II dyes have not been rigorously
employed as the dyes for DSSCs, despite the facts that Type-II
dyes can inject electron to TiO2 by two pathways and the
electron-injection efficiency of pathway B, in principle, should
be unity. The important reason for the above phenomenon lies
on the fact that the overall η values of the Type-II DSSCs have
never exceeded 0.7%4,16c,22,24,25a,26,30 (under the standard condition of one sun, 100 mW cm-2 with AM 1.5 filter), unlike
Type-I dye-sensitized DSSCs (Type-I DSSCs) whose η values
can reach almost 11%. Knowing that both one- and two-step
electron injection pathways operate in Type-II DSSCs, the fact
that the overall η value is smaller than 0.7% indicates that the
η values arising purely from pathway B are much less than
0.7%. Consistent with this, the external quantum efficiencies
(EQEs) arising purely from pathway B have never exceeded
10% in any wavelength in the absence of externally applied
bias potentials. One of the key reasons for the EQEs of pathway
B being so low lies on the fact that the back-electron-transfer
(BET) rates from reduced TiO2 to the oxidized dye are faster
Tae et al.
for pathway B than for pathway A. Indeed, reports have shown
that large portions (>75%) of charge recombination occur within
a few picoseconds in pathway B.15a,18,21b,22,31b In this respect,
the development of the methods for increasing the true efficiency
operated by pathway B is not only a big challenge for its own
sake but also of great interest from the academic and practical
points of view.
Stemming from our experience gained in the area of achieving
long-lived charge separation between the donors and acceptors
encapsulated within zeolite cages and placed at the zeolitesolution interfaces,36 we have recently been interested in
developing high efficiency Type-II DSSCs. By use of Cat and
their novel derivatives as model Type-II dyes, we now report
that the Type-II DSSC sensitized by (quinolin-4-yl)vinylattached Cat (designated as Cat-v-Q, see Chart 1) gives rise
to a 3-fold increase in EQE and a more than 2-fold increase in
overall η with respect to those of Cat-sensitized DSSC, despite
the fact that most of the photovoltaic currents were produced
by pathway B. Thus, the overall η obtained by Cat-v-Q was
1.3%, breaking for the first time the 0.7% barrier. Furthermore,
after coadsorption of deoxycholic acid, the η value increased
to 1.6%, demonstrating for the first time the potential of TypeII DSSCs to be developed into commercially viable DSSCs.
This paper also compares the photovoltaic characteristics of the
related (pyridin-4-yl)vinyl-attached catechol (designated as Catv-P) and the conventional Type-II DSSCs such as Cat, Dop,
and Bpg, respectively (Chart 1).
Experimental Section
Materials. 3,4-Dihydroxybenzaldehyde (97%, Aldrich), 4-pyridinecarboxaldehyde (TCI), 4-quinolinecarboxaldehyde (97%,
Aldrich), tetrabromomethane (Aldrich, 99%), chloromethyl
methyl ether (MOMCl, tech. Aldrich), N,N-diisopropylethylamine (99.5%, Aldrich), triphenylphosphine (PPh3, 99%, Aldrich), triethyl phosphite (98%, Aldrich), potassium tert-butoxide
(1.0 M in tetrahydrofuran, Aldrich), titanium(IV) isopropoxide
(Ti(OiPr)4, 98+%, Acros), titanium tetrachloride (TiCl4, 99.9%,
Aldrich), 2-propanol (Junsei), acetic acid (glacial, 99.99+%,
Aldrich), hydroxy propyl cellulose (HPC, Mw ) 80 000,
Aldrich), 4-tert-butylpyridine (TBP, 99%, Aldrich), Triton
X-100 (Aldrich), acetylacetone (Aldrich, 99%), and Mucasol
(Merz) were purchased and used as received. Cat (99%,
Aldrich), Dop (98%, Aldrich), Bpg (TCI), and deoxycholic acid
(DCA, 99%, Aldrich) were purchased and used as received.
3-Fluorocatechol (99%, Aldrich), 4-chlorocatechol (98%, TCI),
4-nitrocatechol (97%, Aldrich), 4-methylcatechol (95%, Aldrich), 3-methoxycatechol (99%, Aldrich), and 3,5-di(tert-butyl)catechol (99%, Aldrich) were purchased and used as received.
1-Hexyl-3-methylimmidazolium iodide (HMI+I-) was prepared
by reacting 3-methylimmidazole with 1-hexyl iodide. CH2Cl2
(Junsei) and acetonitrile (Junsei) were distilled over CaH2, and
valeronitrile (Aldrich) was used as received. Tetrahydrofuran
(THF, Junsei) was distilled from Na and benzophenone. Column
chromatography was performed using a Merck silica gel 60.
F-doped tin-oxide glass (FTO glass, 8 Ω cm-1, thickness of
FTO layer ) 100 µm, thickness of glass support ) 3 mm) was
the product of Libby Owens Ford. Thermoplastic film (Surlyn)
with the thickness of 100 µm was the product of DuPont.
Preparation of Cat-v-P and Cat-v-Q. The two compounds
were prepared according to Scheme 2.
Preparation of 3,4-Dimethoxymethyloxybenzaldehyde (1).
3,4-Dihydroxybenzaldehyde (1.38 g, 10 mmol) was dissolved
Efficiency of the Dye-Sensitized TiO2 Solar Cells
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22515
SCHEME 2: Procedure for the Synthesis of Cat-v-P and Cat-v-Q
in CH2Cl2 (10 mL). N,N-Diisopropylethylamine (2.58 g, 20
mmol) was added into the above solution at 0 °C, and MOMCl
(1.93 g, 24 mmol) was then slowly added into the solution
keeping the temperature at 0 °C. After stirring for 30 min at 0
°C, the reaction mixture was diluted with CH2Cl2 and washed
with water and brine. The organic layer was dried and
concentrated in vacuo. Column chromatography over silica gel
(1:4 ethyl acetate-hexane) gave solid 3,4-dimethoxymethyloxybenzaldehyde (2.17 g, 96%): 1H NMR (500 MHz, CDCl3,
ppm) 9.87 (s, 1H), 7.69 (d, J ) 1.5 Hz, 1H), 7.52 (d, J ) 8 Hz,
1H), 7.29 (d, J ) 8.5 Hz, 1H), 5.35 (s, 2H), 5.29 (s, 2H), 3.54
(s, 3H), 3.53 (s, 3H); 13C NMR (500 MHz, CDCl3, ppm) 190.9,
152.7, 147.5, 131.1, 126.4, 115.9, 115.4, 95.5, 55.5, 56.4, 56.4.
High-resolution mass spectrometry (HRMS) m/z for (C11H14O4
+ H): calcd., 227.0919; obsd., 227.0925.
Preparation of 3,4-Dimethoxymethyloxybenzyl Alcohol
(2). 1 (1.13 g, 5 mmol) was dissolved in methanol (10 mL).
NaBH4 (229 mg, 6 mmol) was slowly added into the above
solution at 0 °C. After stirring for 3 h at 0 °C, the solvent was
evaporated. After dissolving the reaction mixture in CH2Cl2 (5
mL), the solution was washed with saturated NaHCO3 solution
and subsequently with brine. The organic layer was collected,
dried, and concentrated in vacuo. Column chromatography over
silica gel (ethyl acetate:hexane ) 1:1) gave 2 (1.12 g, 98%) as
a liquid: 1H NMR (500 MHz, CDCl3, ppm) 7.14 (d, J ) 2 Hz,
1H), 7.10 (d, J ) 8 Hz, 1H), 6.92 (dd, J ) 5 Hz, 1H), 5.21 (s,
2H), 5.19 (s, 2H), 4.55 (s, 2H), 3.49 (s, 3H), 3.48 (s, 3H); 13C
NMR (500 MHz, CDCl3, ppm) 147.3, 146.6, 135.7, 121.2,
116.9, 115.7, 95.5, 64.8, 56.2, 56.1. HRMS m/z for C11H16O5:
calcd., 228.0998; obsd., 227.0996.
Preparation of 3,4-Dimethoxymethyloxybenzylbromide
(3). 2 (1.14 g, 5 mmol) was dissolved in CH2Cl2 (5 mL). PPh3
(1.31 mg, 5 mmol) and CBr4 (1.69 mg, 5 mmol) were
sequentially added into the solution at 0 °C. After stirring at 0
°C for 24 h, the solvent was evaporated, and the reaction mixture
was dissolved in CH2Cl2. The solution was washed with water
and subsequently with brine. The organic layer was dried and
concentrated in vacuo. Silica gel chromatography (ethyl acetate:
hexane ) 1:2) gave 3 as a liquid (1.38 g, 95%): 1H NMR (500
MHz, CDCl3, ppm) 7.20 (d, J ) 2 Hz, 1H), 7.11 (d, J ) 8.5
Hz, 1H), 7.00 (dd, J ) 5 Hz, 1H), 5.25 (s, 2H), 5.23 (s, 2H),
4.47 (s, 2H), 3.54 (s, 3H), 3.50 (s, 3H); 13C NMR (500 MHz,
CDCl3, ppm) 147.5, 147.4, 132.1, 123.5, 117.5, 116.7, 95.6,
56.5, 56.4, 33.9. HRMS m/z for C11H15BrO4: calcd., 290.0154;
obsd., 290.0157.
Preparation of Diethyl 3,4-Dimethoxymethyloxybenzylphosphonate (4). 3 (1.46 g, 5 mmol) was dissolved in
P(OEt)3 (997 mg, 6 mmol), and the solution was refluxed for
24 h at 150 °C. The reaction mixture was diluted by adding
CH2Cl2 (5 mL), and the solution was washed with water and
subsequently with brine. The organic layer was dried and
concentrated in vacuo. Column chromatography over silica gel
(ethyl acetate) gave 4 (1.57 g, 90%) as a liquid: 1H NMR (500
MHz, CDCl3, ppm) 6.94 (s, 1H), 6.91 (d, J ) 8.5 Hz, 1H),
6.72 (d, J ) 8 Hz, 1H), 5.06 (s, 2H), 5.02 (s, 2H), 3.86 (m,
4H), 3.32 (s, 3H), 3.31 (s, 3H), 2.95(s, 1H), 2.87 (s, 1H), 1.09
(t, J ) 7 Hz, 6H); 13C NMR (500 MHz, CDCl3, ppm) 146.9,
146.0, 125.5, 123.5, 118.1, 116.7, 95.2, 61.7, 55.8, 33.3, 32.2,
16.2, 16.1, 16.0, 15.9. HRMS m/z for (C15H25O7P + H): calcd.,
349.1416; obsd., 349.1424.
Synthesis of MOM-Protected Cat-v-P and Cat-v-Q
[(MOM)2-Cat-v-P and (MOM)2-Cat-v-Q]. 4 (360 mg, 1
mmol) was dissolved in THF (2 mL). Potassium tert-butoxide
(KOtBu, 1.2 mmol) was added to the above solution at -78
°C. After stirring at -78 °C for 30 min, a THF solution (5 mL)
of 4-pyridinecarboxaldehyde (123 mg, 1 mmol) or 4-quinolinecarboxaldehyde (157 mg, 1 mmol) was slowly added into
the solution. After stirring at -78 °C for 24 h, the reaction was
quenched by adding saturated NH4Cl solution (2 mL). The
reaction mixture was then diluted with CH2Cl2 (5 mL), washed
with water, and subsequently with brine. The organic layer was
dried and concentrated in vacuo. Column chromatography over
silica gel (ethyl acetate:hexane ) 1:4) gave the products as a
liquid: (MOM)2-Cat-v-P (yield ) 58%) and (MOM)2-Catv-Q (yield ) 35%). 1H NMR revealed that the products are
(cis and trans) geometric isomers with cis:trans isomeric ratios
of 0.29:1 [(MOM)2-Cat-v-P] and 0.8:1 [(MOM)2-Cat-v-Q],
respectively. trans-(MOM)2-Cat-v-P: 1H NMR (300 MHz,
CDCl3, ppm) δ 8.56 (d, J ) 5 Hz, 2H), 7.38 (s, 1H), 7.34 (d,
J ) 5 Hz, 2H), 7.23 (d, J ) 17 Hz, 1H), 7.17 (br s, 2H), 6.89
(d, J ) 17 Hz, 1H), 5.30 (s, 2H), 5.27 (s, 2H), 3.56 (s, 3H),
3.53 (s, 3H). cis-(MOM)2-Cat-v-P: 1H NMR (300 MHz,
CDCl3, ppm) δ 8.48 (d, J ) 5 Hz, 2H), 7.19 (d, J ) 5 Hz, 2H),
7.04 (d, J ) 8 Hz, 1H), 7.01 (s, 1H), 6.85 (d, J ) 8 Hz, 1H),
6.69 (d, J ) 12 Hz, 1H), 6.43 (d, J ) 12 Hz, 1H), 5.22 (s, 2H),
5.05 (s, 2H), 3.51 (s, 3H), 3.40 (s, 3H). trans-(MOM)2-Catv-Q: 1H NMR (300 MHz, CDCl3, ppm) δ 8.58 (d, J ) 5 Hz,
1H), 8.21 (d, J ) 8 Hz, 1H), 8.13 (d, J ) 8 Hz, 1H), 7.437.75 (m, 5H), 7.20-7.30 (m, 2H), 6.86 (s, 1H), 5.32 (s, 2H),
5.29 (s, 2H), 3.58 (s, 3H), 3.55 (s, 3H). cis-(MOM)2-Cat-v-
22516 J. Phys. Chem. B, Vol. 109, No. 47, 2005
Q: 1H NMR (300 MHz, CDCl3, ppm) δ 8.80 (d, J ) 5 Hz,
1H), 8.03 (d, J ) 8 Hz, 1H), 7.43-7.75 (m, 4H), 7.20-7.30
(m, 2H), 6.95 (d, J ) 8 Hz, 1H), 6.81 (d, J ) 2 Hz, 1H), 6.72
(dd, J ) 2, 8 Hz, 1H), 5.14 (s, 2H), 4.80 (s, 2H), 3.45 (s, 3H),
3.20 (s, 3H).
Preparation of trans-Cat-v-P and trans-Cat-v-Q. The cis
and trans isomeric mixture of (MOM)2-Cat-v-P was dissolved
in THF (5 mL). HCl (1 N, 2 mL) was added into the above
solution, and the mixture was refluxed for 3 h at 70 °C. The
reaction mixture was diluted with saturated solution of NaHCO3
(5 mL), and the product (mixture of cis- and trans-Cat-v-P)
was extracted with CH2Cl2. The organic layer was dried over
MgSO4 and concentrated in vacuo. The cis:trans ratio of the
obtained Cat-v-P mixture was 0.25:1. Repeated recrystallization
in hexane-CH2Cl2-MeOH gave pure trans isomer (trans-Catv-P, yield ) 16.2%, for simplicity, Cat-v-P will represent the
trans isomer hereafter). Interestingly, in the case of Cat-v-Q,
the cis isomer disappeared during the deprotection process and
only trans isomer remained (yield ) 22.9%, for simplicity, Catv-Q will represent the trans isomer hereafter). Cat-v-P: 1H
NMR (300 MHz, CD3OD, ppm) δ 8.43 (d, J ) 6 Hz, 2H),
7.52 (d, J ) 6 Hz, 2H), 7.36 (d, J ) 16 Hz, 1H), 7.10 (d, J )
2 Hz, 1H), 6.97 (dd, J ) 2, 8 Hz, 1H), 6.92 (d, J ) 16 Hz,
1H), 6.79 (d, J ) 8 Hz, 1H); 13C NMR (500 MHz, CD3OD,
ppm) 150.0, 147.0, 146.8, 145.6, 134.6, 128.6, 122.1, 121.0,
120.2, 115.3, 113.3. HRMS m/z for (C13H11O2N + H); calcd.,
214.0868; obsd., 214.0868. cis-Cat-v-P: 1H NMR (300 MHz,
CD3OD, ppm) δ 8.37 (d, J ) 6 Hz, 2H), 7.29 (d, J ) 6 Hz,
2H), 6.74 (d, J ) 12 Hz, 1H), 6.68 (d, J ) 8 Hz, 1H), 6.68 (d,
J ) 2 Hz, 1H), 6.59 (dd, J ) 2, 8 Hz, 1H), 6.41 (d, J ) 12 Hz,
1H). Cat-v-Q: 1H NMR (300 MHz, CD3OD, ppm) 8.77 (d, J
) 5 Hz, 1H), 8.38 (d, J ) 8 Hz, 1H), 8.03 (d, J ) 8 Hz, 1H),
7.64-7.81 (m, 4H), 7.41 (d, J ) 16 Hz, 1H), 7.21 (d, J ) 2
Hz, 1H), 7.08 (dd, J ) 2, 8 Hz, 1H), 6.84 (d, J ) 8 Hz, 1H);
13C NMR (500 MHz, CD OD, ppm) 149.6, 148.0, 147.0, 145.6,
3
144.9, 136.4, 129.7, 129.0, 126.7, 126.6, 123.8, 120.4, 118.5,
116.3, 115.4, 113.5. HRMS m/z for (C17H14O2N + H); calcd.,
264.1025; obsd., 264.1031.
Preparation of TiO2 Gel and 1:1 TiO2-HPC Paste. TiO2
nanoparticles were prepared according to the following procedure. Distilled deionized water (160 mL) was introduced into a
500-mL round-bottom flask, and then glacial acetic acid (51
mL) was added to the water. The flask was placed in an ice
bath and stirred for a few minutes to allow the solution to cool.
2-Propanol (6 mL) was added to a dropping funnel followed
by Ti(OiPr)4 (24 mL). The Ti(OiPr)4 solution was slowly dripped
into the cooled acetic acid solution at a rate of approximately
one or two drops per second over the course of 30 min, while
the solution was vigorously stirred. After the dripping was over,
white precipitate was formed within the round-bottomed flask.
The heterogeneous solution was heated to 80 °C using a water
bath with vigorous stirring. The white precipitate disappeared
upon heating the solution. The bluish, gel-like solution was
further heated at the temperature for 3-4 h. The slightly milky
and bluish viscose colloidal solution was transferred into a
cylindrical Teflon container, which tightly fits within an
autoclave. The autoclave was placed in an oven, and the
temperature of the oven was increased to 230 °C. After 13 h,
the temperature of the oven was allowed to cool to room
temperature during the course of 24 h. The solution was
sonicated for 30 min and then concentrated by evaporating water
using a rotary evaporator until the TiO2 concentration became
∼12 wt %. The average particle size of the nanocrystalline TiO2,
which was determined using the Scherrer equation, was 19.6
nm. To make TiO2-HPC paste, 10 g of 12 wt % aqueous TiO2
Tae et al.
solution and 0.58 g of HPC were introduced into a small vial
and the mixture was magnetically stirred for 2 days prior to
use. The weight ratio of TiO2 and HPC in the above paste was
1:1.
Preparation of Dye-Coated TiO2 Film. A large FTO glass
plate was cut into smaller pieces with the size of 30 × 60 mm2.
One horizontal and three evenly spaced vertical line scratches
were made on the glass side of each small FTO glass plate using
a glass knife to be able to divide the FTO glass plate into eight
smaller pieces with the dimension of 15 × 15 mm2 after fixation
of TiO2 film on the FTO side. The line-scratched FTO glass
plates were washed by sonication in 3% mucasol solution, and
rinsed sequentially with distilled deionized water and ethanol.
In a plastic container, 2 M TiCl4 (2 mL) and distilled deionized
water (78 mL) were mixed. The washed line-scratched FTO
glass plates were placed within the TiCl4 solution, and the
container was placed in an oven whose temperature was set at
60 °C for 30 min. The line-scratched FTO glass plates coated
with TiO2 buffer layer [(TiO2)b/FTO/G-8] were washed with
distilled deionized water and ethanol. The washed (TiO2)b/FTO/
G-8 plates were dried under a stream of N2. TiO2 films were
deposited on (TiO2)b/FTO/G-8 plates by the doctor-blade method
using Scotch tape as the spacer. The thickness of the TiO2 film
was controlled by the number of Scotch tape layers. The
thickness of the TiO2 layer used in this report was ∼13 µm.
The air-dried TiO2 film-coated (TiO2)b/FTO/G-8 [TiO2/(TiO2)b/
FTO/G-8] plates were sintered at 450 °C for 1 h.
The sintered TiO2/(TiO2)b/FTO/G-8 plate was carefully
broken into 8 smaller pieces with the dimensions of 15 × 15
mm2. The areas of TiO2 layers on the sintered, small TiO2/
(TiO2)b/FTO/G [TiO2/(TiO2)b/FTO/G-1] plates were reduced to
∼8.0 × 3.0 mm2 by removing the extra TiO2 layers using a
sharp-edged knife. The position of the remaining area of TiO2
on the plate was 3.0 mm away from the top edge, 9.0 mm away
from the bottom edge, and 3.5 mm away from the left and right
edges, respectively. Independently, each dye was dissolved in
ethanol. The concentrations of dyes ranged from 0.25 to 1 mM.
The TiO2/(TiO2)b/FTO/G-1 with small TiO2 area [(TiO2)sa/
(TiO2)b/FTO/G-1] plates were immersed into each dye solution
for varying periods of time (15 min to 15 h) at room temperature.
The dye-coated (TiO2)sa/(TiO2)b/FTO/G-1 [Dye-(TiO2)sa/(TiO2)b/
FTO/G-1] plates were washed with copious amounts of ethanol.
Measurements of DTCT Bands. For the measurements of
the DTCT bands, each Dye-(TiO2)sa/(TiO2)b/FTO/G-1 was cut
into 9 × 15 mm2, while retaining the dye-coated small-area
TiO2 layer within the center of the above plate. The smaller
Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plate was placed vertically on
top of the bottom of a 10 × 10 × 40 mm3 rectangular cuvette.
After filling the cuvette with CH3CN, the spectrum of the
smaller Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plate was measured
with respect to CH3CN contained within the reference cuvette.
The genuine DTCT band was obtained by subtracting the above
spectrum with the one obtained from the colorless (TiO2)sa/
(TiO2)b/FTO/G-1 plate which was obtained by removing the
dye off the TiO2 by burning the Dye-(TiO2)sa/(TiO2)b/FTO/
G-1 on a stream of hot wind (500 °C) for 15 min. Since FTO
has strong absorption at λ < 380 nm, the spectrum of DyeTiO2/FTO/G was shown at the spectral region of 380 < λ e
800 nm.
Measurements of Adsorbed Amounts of Dyes on TiO2. A
Dye-(TiO2)sa/(TiO2)b/FTO/G-1 was immersed in 3 mL of
NaOH solution (2 M) for 15 h. This caused desorption of the
dye from TiO2 into the solution while keeping the TiO2 layer
intact. The supernatant solution was introduced into a cuvette,
Efficiency of the Dye-Sensitized TiO2 Solar Cells
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22517
and the spectrum of each dye in the basic solution was measured.
The quantification of each dye was made based on the λmax
and the molar extinction coefficient of each dye in the basic
solution as follows: Cat (λmax ) 315 nm and ) 10 000 M-1
cm-1), Dop (λmax ) 280 nm and ) 9 900 M-1 cm-1), Bpg
(λmax ) 265 nm and ) 14 900 M-1 cm-1), Cat-v-P (λmax )
290 nm and ) 20 300 M-1 cm-1), and Cat-v-Q (λmax ) 304
nm and ) 12 400 M-1 cm-1).
Fabrication of Sandwich-Type Cells. Small FTO glass
plates with the dimension of 15 × 15 mm2 [FTO/G-1] were
prepared. Onto each small FTO glass plate two holes with the
diameter of 0.75 mm were made from the FTO side to the glass
side using a small diamond-coated drill tip with the diameter
of 0.75 mm at the positions of (-4.5, 0 mm) and (4.5, 4.5 mm)
from the center of the plate, where the horizontal and vertical
edges are defined as the x and y axes, respectively. Independently, a 2-propanol solution of H2PtCl6 was prepared by
dissolving H2PtCl6 (20.5 mg, 0.05 mmol) into 2-propanol (10
mL). Two drops of the 5 mM H2PtCl6 solution was dropped
onto each FTO/G-1 plate having two holes [FTO/G-1H2]. After
evenly spreading the H2PtCl6 solution, 2-propanol was allowed
to evaporate in the atmosphere. The H2PtCl6-coated FTO/G1H2 plates were heated under a hot stream of air (380 °C) for
20 min. The Pt-coated FTO/G-1H2 (Pt/FTO/G-1H2) plates were
used as counter electrodes. Dye-(TiO2)sa/(TiO2)b/FTO/G-1
plates were placed with TiO2 side facing up. Onto each Dye(TiO2)sa/(TiO2)b/FTO/G-1 plate, four narrow Surlyn tapes (width
) 1.0 mm) were placed along the top, left, and right edges of
each plate and 5 mm away from the bottom edge of each plate
so that the area surrounded by Surlyn tapes can also enclose
the two holes of a Pt/FTO/G-1H2 plate when they are attached
together. Subsequently, a Pt/FTO/G-1H2 plate was gently placed
with the FTO side face down onto the Surlyn-placed Dye(TiO2)sa/(TiO2)b/FTO/G-1 plate in such a way that the two holes
of the Pt/FTO/G-1H2 plate fit within the area surrounded by
Surlyn tapes on the bottom plate. The sandwiched assembly
was placed on a hot plate whose temperature was maintained
at 130 °C and pressed for 7 min under the pressure of 1.2 N m.
After being cooled to room temperature, an electrolyte solution
was introduced into the void space created between the two
electrodes through one of the holes. The composition and
concentration of electrolyte solution were 0.6 M HMI+I-, 0.05
M I2, 0.1 M LiI, and 0.5 M TBP in the 1:1 mixed solution of
acetonitrile and valeronitrile. After filling up the void space with
the electrolyte solution, the holes were blocked with small glass
plates using Surlyn as the adhesive. The nonoverlapped areas
of Dye-sa-TiO2/Tb/FTO/G-1 and Pt/FTO/G-1H2 plates were
coated with indium metal.
Coadsorption of DCA onto Dye-Coated TiO2 Film. One
or two Dye-(TiO2)sa/(TiO2)b/FTO/G-1 plates were dipped into
an ethanol solution of DCA (50 mM, 3 mL) for 15 h. The plates
were then washed with ethanol.
Photovoltaic Measurements of DSSCs. The η values for
the DSSCs were measured under a standard condition of one
sun (100 mW cm-2, AM 1.5 filter). The IPCE spectrum or the
EQE at an excitation wavelength was determined from eq 1
IPCE (%) ) (1240/λex) × (Jsc/Iinc) × 100
(1)
where Jsc is the short-circuit photocurrent (A cm-2), Iinc is the
incident light intensity (W cm-2), and λex is the excitation
wavelength.
Instruments. 1H and 13C NMR spectra were recorded on a
Varian Inova 300- or 500-MHz NMR spectrometer. The
chemical shifts are reported with respect to that of tetrameth-
ylsilane in CDCl3 and CD3OD. High-resolution mass spectra
were obtained from a JEOL JMS 700 installed in the Korea
Institute of Basic Science, Kyungpook National University. SEM
images of TiO2 films were obtained from a field emission
scanning electron microscope (Hitachi S-4300) at an acceleration
voltage of 10-20 kV. A platinum/palladium alloy (in the ratio
of 8 to 2) was deposited with a thickness of about 15 nm on
top of the samples. X-ray diffraction patterns for the characterization of TiO2 nanocrystals were obtained from a Rigaku
D/MAX-1C with the monochromatic beam of Cu KR. The UVvis spectra of the samples were recorded on a Shimadzu UV3101PC. The diffuse reflectance UV-vis spectra of the solid
samples were obtained using an integrating sphere. Currentvoltage (J-V) curves were obtained from a Keithley 2400 source
meter under the light produced from an Oriel 191 solar simulator
equipped with an AM 1.5 filter. The light source was an Oriel
1000-W Xe lamp. The applied light intensity was 100 mW
cm-2. The intensity of the incident light was calibrated using a
KG5-filtered, monocrystalline Si reference solar cell, PVM 37
(ISO tracking number 1006), which was calibrated at the
National Renewable Energy Laboratory in Golden, Colorado,
USA. Each data in the Tables represents the average value
obtained from three samples. The IPCE spectra for the cells
were measured on an IPCE measuring system (PV Measurements), in which monochromatic light is generated using a
tungsten light source filtered by a grating monochromator and
order-sorting filters. The light was modulated with a mechanical
chopper (Digirad C-980). A broadband light is simultaneously
applied to the sample cell to bring the cell into the end-use
conditions. The current generated by the sample cell at a specific
voltage (generally V ) 0) is converted to a voltage by a
transimpedance amplifier and measured with a lock-in amplifier
(Stanford Research Systems SR810) synchronized to the mechanical chopper. The photon flux of the monochromatic light
was determined by measuring the signal produced by a
calibrated photodiode and comparing the signal to the photodiode’s known spectral response information. Cyclic voltammetry measurements were performed on a Bioanalytical Systems, Inc. CV-50W voltammetric analyzer using a standard
three-electrode cell with a platinium wire working electrode
(electrode area, 5.0 cm2), a platinum gauze counter electrode,
and an Ag/AgCl (sat. KCl) reference electrode in DMF
containing a 0.1 M (Bu)4NClO4 electrolyte. The sweep rate was
0.1 V s-1. Quantum mechanical calculations of the frontier
orbitals of (H2O)2(OH)2Ti-Cat, (H2O)2(OH)2Ti-Cat-v-P, and
(H2O)2(OH)2Ti-Cat-v-Q were done with jaguar v 5.5 suite
(Schrödinger, Portland, US).37 B3LYP38 flavor of density
functional theory (DFT) calculations were adopted with LACVP**
basis set, which uses Hay-Watt effective core potential and basis
set for Ti39 and 6-31G** basis sets for other atoms (H, C, O,
and N).
Results and Discussion
Photovoltaic Parameters of Type-II DSSCs Sensitized by
Cat, Dop, and Bpg. To compare the photovoltaic parameters
of Cat-v-P and Cat-v-Q with those of the typical dyes that
have been employed in Type-II DSSCs under the same
experimental condition, we prepared a series of Type-II DSSCs
sensitized by Cat, Dop, and Bpg, respectively. The UV-vis
spectrum of each dye in solution (the local band of each dye)
and the diffuse-reflectance UV-vis spectrum of bare TiO2 (the
local band of TiO2) are shown in each panel of Figure 1, together
with the UV-vis spectrum of the corresponding Dye-TiO2/
FTO/G (the composite spectrum of the local bands of each dye
22518 J. Phys. Chem. B, Vol. 109, No. 47, 2005
Tae et al.
TABLE 1: Photovoltaic Parameters of the Type-II DSSCs
Sensitized by the Dyes Shown in Chart 1
dye
Voc (mV)
Jsc (mA cm-2)
FF
η (%)
Aaa
Cat
Dop
Bpg
Cat-v-P
Cat-v-Q
N719
557
452
445
561
562
750
1.51
0.40
0.79
2.74
3.53
12.4
66.5
67.8
63.1
68.1
66.5
66.6
0.6
0.1
0.2
1.1
1.3
6.2
0.17
0.01
0.10
0.07
0.08
0.09
a
Figure 1. The UV-vis spectrum of a Type-II dye in acetonitrile (dotdashed line), the diffuse-reflectance UV-vis spectrum of bare TiO2
(dashed line), the UV-vis spectrum of the corresponding Dye-(TiO2)sa/
(TiO2)b/FTO/G-1 plate (solid line), and the corresponding IPCE curve
of the cell (square-dashed line) with Cat (A), Dop (B), and Bpg (C),
as the dye.
and TiO2 and the corresponding DTCT band). The corresponding IPCE curve was overlaid on the UV-vis spectrum of each
Dye-TiO2/FTO/G.
In the cases of Cat and Dop, the onsets of the local bands
were 300 and 332 nm, respectively, while the onset of TiO2
appeared at ∼390 nm. Therefore, the new absorption bands that
appear at 400 e λ < 700 nm represent only the DTCT bands.
This means that, in the above two cases, the corresponding local
bands do not overlap with the DTCT bands in the visible region.
Yet, the IPCE curves very closely follow the corresponding
traces of DTCT bands. This unambiguously demonstrates that,
in the visible region (λ g 400 nm), the photocurrents are
generated only by pathway B. Furthermore, the close matching
between the IPCE curve and DTCT band indicates that the
electron injection efficiency is the function of the absorption
probability. The EQEs at 400 nm were 8.5 and 4.2%, respectively, which are about maximum EQEs obtainable by only
pathway B in the cases of Cat- and Dop-sensitized DSSCs,
respectively. In both cases, the EQE rapidly increased at ∼390
nm at which the onset of the local band of TiO2 begins, due to
additional photocurrent generation by TiO2.
The photovoltaic parameters of the Cat- and Dop-sensitized
DSSCs are listed in Table 1. The open-circuit voltage (Voc) and
the Jsc of Cat-sensitized DSSC were significantly higher than
those of Dop-sensitized DSSC. The corresponding η values were
0.6 and 0.1%, respectively. Although both values are still
relatively very small compared to that of N719-sensitized Type-I
Adsorbed amount in µmol cm-2.
DSSC compared in the last entry of Table 1 (6.2%), the η value
of Cat-sensitized DSSC was significantly higher than that of
Dop-sensitized DSSC, consistent with the fact that the Voc and
Jsc of Cat-sensitized DSSC were significantly higher than those
of Dop-sensitized DSSC. The adsorbed amounts of Cat and Dop
onto the TiO2 films under our experimental conditions were
0.17 and 0.01 µmol cm-2, respectively. Thus, the adsorbed
amount of Dop was much less than that of Cat. This fact reveals
that the adsorbed amounts of catechol derivatives onto TiO2
vary depending on their nature, and this is likely to be one of
the reasons for the η of Dop-sensitized DSSC being much poorer
than that of Cat-sensitized DSSC. Interestingly, however, their
fill factors (FFs) were similar (∼67) in both cases.
In the case Bpg, the absorption tail of the local band extended
up to 700 nm, while the absorption maximum (λmax) in the
visible region was 485 nm (Figure 1). The Bpg-adsorbed TiO2
films showed an additional band with λmax at ∼560 nm
consistent with the report of Ghosh and the co-workers.21b This
new absorption band was assigned as the DTCT band by Ghosh
and the co-workers.21b Thus, in the case of Bpg, there is no
spectral region in which only DTCT band absorbs since both
local and DTCT bands strongly overlap in the visible region.
In any case, the IPCE curve closely followed the trace of the
spectrum of Bpg-adsorbed TiO2 film, indicating that even in
this case the photocurrent is generated not only by pathway A
but also by pathway B in the visible region. Therefore, the Bpgsensitized DSSC is a typical example in which both pathways
operate in the whole spectral region. In this respect, Bpgsensitized DSSC is distinguished from Cat- and Dop-sensitized
DSSCs.
The photovoltaic parameters of the Bpg-sensitized DSSC are
listed in Table 1 (entry 3). Its Voc was similar to that of Dopsensitized DSSC. However, its Jsc value was placed between
those of Dop- and Cat-sensitized DSSCs. The FF was slightly
smaller than those of Cat- and Dop-sensitized DSSCs. The
overall η lied between those of Cat- and Dop-sensitized DSSCs.
Interestingly, in the above three DSSCs, a correlation exists
between the adsorbed amount and η, with both increasing in
the order: Cat > Bpg > Dop.
Photovoltaic Parameters of Cat-v-P-Sensitized DSSC. In
the case of Cat-v-P, the λmax of the local band appeared at 343
nm with the molar extinction coefficient of 24 500 M-1 cm-1
in CH3CN as shown in Figure 2A. The onset of the spectrum
was 400 nm. The Cat-v-P-adsorbed TiO2 gave an orangecolored CT band with the λmax of ∼500 nm which is ∼50-60
nm red-shifted with respect to that of Cat, indicating that, as
expected, Cat-v-P is a better donor than Cat. In support of
this, the oxidation potential of Cat-v-P (0.91 V vs Ag/AgCl in
DMF, scan rate ) 0.1 V s-1) shifted by 150 mV to the negative
with respect to that of Cat (1.06 V vs Ag/AgCl in DMF).
The onset of the Cat-v-P-to-TiO2 DTCT band was 700 nm.
The IPCE curve of Cat-v-P-sensitized DSSC nearly superimposed with the DTCT band in the visible region as shown in
Figure 2A, indicating that the photocurrent generated in the
Efficiency of the Dye-Sensitized TiO2 Solar Cells
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22519
SCHEME 3: Proposed Mechanism Partially Responsible
for the Increase in Efficiency and EQE in DSSCs
Sensitized by Cat-v-P and Cat-v-Q
Figure 2. The UV-vis spectrum of a Type-II dye in (dot-dashed line),
the diffuse-reflectance UV-vis spectrum of bare TiO2 (dashed line),
the UV-vis spectrum of the corresponding Dye-(TiO2)sa/(TiO2)b/FTO/
G-1 plate (solid line), and the corresponding IPCE curve of the cell
(square-dashed line) with Cat-v-P (A) and Cat-v-Q (B) as the dye.
Figure 3. The calculated electron densities of the HOMO and LUMO
of (H2O)2(OH)2Ti-Cat (A), (H2O)2(OH)2Ti-Cat-v-P (B), and
(H2O)2(OH)2Ti-Cat-v-Q (C).
visible region arises purely by pathway B and that the efficiency
of electron injection by DTCT is linearly proportional to the
extinction coefficient of the DTCT band. The IPCE value at
400 nm was 30%, which corresponds to a more than 3-fold
increase with respect to that of Cat-sensitized DSSC. It is also
important to note that this IPCE value is the highest ever
observed in the Type-II DSSCs. This also demonstrates the
potential of Type-II DSSCs to be developed into commercially
viable DSSCs.
Although the Voc (561 mV) value was only slightly higher
than that of Cat-sensitized DSSC (557 mV), the obtained Jsc
(2.7 mA cm-2) was 1.8 times higher than that of Cat-sensitized
DSSC (1.5 mA cm-2) (Table 1). However, the FF was nearly
the same with that of Cat-sensitized DSSC. The overall η was
1.1, which was again 1.8-times higher than that of Cat-sensitized
DSSC. Since Voc and FF are nearly the same, it is concluded
that the increase in η is caused by the increase in Jsc. Most
interestingly, the adsorbed amount of Cat-v-P in the cell was
only 0.07 µmol cm-2, which is only ∼40% of that of Cat in
the Cat-sensitized DSSC. On the basis of the fact that η
increased with increasing the adsorbed amount of dye in the
cases of Cat, Dop, and Bpg, the above result indicates that the
electron injection efficiency from Cat-v-P to TiO2 is about 4.5
times higher than that of Cat to TiO2 per molecule. This means
that the attachment of v-P to Cat leads to a large (4.5-fold)
increase in electron injection efficiency.
As a possible means to account for the above phenomenon,
the highest-occupied molecular orbitals (HOMOs) and lowestunoccupied molecular orbitals (LUMOs) of Ti(IV)-coordinated
Cat, Cat-v-P, and Cat-v-Q were calculated based on density
functional theory, and the results are compared in Figure 3. For
this calculation, the Ti(IV) ion was chelated to a catechol or a
catechol moiety and coordinated to two H2O molecules and two
OH- groups as in the report of Prezhdo and co-workers.40 In
the case of Ti-Cat, while the electron density was delocalized
almost entirely over the Cat in the HOMO, the electron density
completely shifted to Ti(IV) and the surrounding H2O and OH
ligands consistent with the DTCT nature of the HOMO-LUMO
transition as shown in Figure 3A. This phenomenon is well
explained in the report of Prezhdo and co-workers.40
In the case of Ti-Cat-v-P, however, while the HOMO is
delocalized over the entire Cat-v-P, the LUMO is localized
largely on Ti and to a certain extent on Cat-v moiety as well,
showing that a charge shift occurs to a certain degree from the
P moiety to Ti-Cat-v moiety, as shown in Figure 3B. On this
basis, we propose that intramolecular consecutive charge shift
from P to Ti-Cat-v moiety plays an important role in
contributing to such a dramatic increase in the electron injection
efficiency from Cat-v-P to TiO2, as illustrated in Scheme 3,
where D1 and D2 stand for Cat-v and P, respectively. This
means that eventual photoinduced electron transfer occurs from
P to TiO2.
The above situation can better be described as the typical
acceptor (A)-primary donor (D1)-secondary donor (D2) triad
system where A ) TiO2, D1 ) Cat-v moiety, and D2 ) P, in
which eventual charge separation occurs between A and D2 via
charge shift between D1 and D2. Efficient charge separation has
also been observed between TiO2 and D2 upon DTCT excitation
of TiO2-D1-D2 triad systems where D1 ) Dop and D2 )
biological systems such as a DNA double helix, biotin, or
22520 J. Phys. Chem. B, Vol. 109, No. 47, 2005
Figure 4. Linear relationship between the oxidation potentials of Cat,
Cat-v-P, and Cat-v-Q and the absorption maximums of the corresponding DTCT bands.
avidin.41 In close relation to the above, the CT excitation of the
quantum efficiency of photoinduced electron transfer from D1
and A placed within zeolites or across zeolite-solution interfaces significantly increases upon attaching D2 next to D1 due
to consecutive intramolecular charge shift from D1 to A followed
by electron transfer from D2 to D1+.42 In any case, the calculated
HOMO and LUMO of Ti-Cat-v-P confirm the DTCT nature
of the ∼500-nm band.
In addition to the above explanation, we also propose that
the red-shift of the DTCT band caused by the increase in the
donor strength of the dye by attaching a more electron rich
substituent (v-P) to Cat leading to a substantial increase in the
absorption of visible light contributes to the increase in the
electron injection efficiency. Thus, we conclude that both the
consecutive charge shift leading to the retardation of back
electron-transfer rate discussed above and the red-shift of the
DTCT band as a result of increasing the donor strength of the
dye leading to the increase in the absorption of visible light are
responsible for the increase in η.
Photovoltaic Parameters of Cat-v-Q-Sensitized DSSC. In
the case of Cat-v-Q, the λmax of the local band appeared at
383 nm with the molar extinction coefficient of 21 600 M-1
cm-1 in CH3CN as shown in Figure 2B. The onset of the
spectrum was 435 nm. The Cat-v-Q-adsorbed TiO2 was purple,
and the λmax of the DTCT band was ∼580 nm, which is about
∼80 nm red-shifted with respect to that of Cat-v-P. The onset
of the Cat-v-Q-to-TiO2 DTCT band was 774 nm, which is also
red-shifted with respect to that of Cat-v-P by 74 nm. The above
red shifts indicate that, as expected, Cat-v-Q is a better donor
than Cat-v-P. In support of this, the oxidation potential of Catv-Q (0.83 V vs Ag/AgCl in DMF, scan rate ) 0.1 V s-1) shifted
by 80 mV to the negative with respect to that of Cat-v-P (0.91
V vs Ag/AgCl in DMF).
Interestingly, the plot of the λmax of the DTCT band with
respect to the oxidation potential of the dye for Cat, Cat-v-P,
and Cat-v-Q gave a linear relationship as shown in Figure 4.
This result is rather unexpected considering the fact that the
Mulliken’s CT theory holds for a weak donor-acceptor
interaction between a donor and an acceptor33 but not for the
case of Cat-binding TiO2 since there exists a strong interaction
(binding) between the dye and TiO2. Indeed, when we plotted
the λmax of the DTCT band with respect to the oxidation potential
of the dye for Cat, 3-fluorocatechol, 4-chlorocatechol, 4-nitrocatechol, 4-methylcatechol, 3-methoxycatechol, and 3,5-di(tertbutyl)catechol, such a linear relationship was not observed.
Although a systematic study is needed to elucidate the intriguing
phenomenon of the linear relationship, it indicates that Cat, Catv-P, and Cat-v-Q belong to a homologous series of dyes that
bind to TiO2 in a same manner.
The IPCE curve of Cat-v-Q-sensitized DSSC also nearly
superimposed with the corresponding trace of DTCT band in
Tae et al.
the region (435 e λ < 800 nm) in which only the DTCT band
absorbs, indicating that the photocurrent generated in the visible
region arises by pathway B and that the efficiency of electron
injection by DTCT simply follows the extinction coefficient of
DTCT band. The IPCE value at 440 nm was 25%, which is ∼3
times higher than that of Cat-sensitized DSSC.
The measured Voc (562 mV) of Cat-v-Q-sensitized DSSC
was again slightly higher than that of Cat-sensitized DSSC (557
mV) but was essentially the same with that of Cat-v-Psensitized DSSC (561 mV) (Table 1). This indicates that the
attachment of (pyridin -4-yl)vinyl or (quinolin-4-yl)vinyl does
not affect Voc. However, the obtained Jsc (3.53 mA cm-2) was
more than 2.3 times higher than that of Cat-sensitized DSSC
(1.51 mA cm-2). The FF was still the same with that of Catsensitized DSSC. The overall η was 1.3, which was ∼2.2 times
higher than that of Cat-sensitized DSSC. The fact that Voc and
FF remain nearly the same also suggests that the increase in η
is driven by the increase in Jsc. In the case of Cat-v-Q-sensitized
DSSC, the adsorbed amount of Cat-v-Q in the cell was 0.08
µmol cm-2, which is only 47% of that of Cat in the Catsensitized DSSC. The above result therefore indicates that the
electron injection efficiency from Cat-v-Q to TiO2 is about
4.7 times higher than that of Cat to TiO2 per molecule.
The calculated HOMO and LUMO of Ti(IV)-coordinated
Cat-v-Q also showed that a charge shift occurs from the Q
moiety to Ti-Cat-v moiety, as shown in Figure 3C. Therefore,
as in the case of Cat-v-P-sensitized DSSC, we propose that
the intramolecular consecutive charge shift illustrated in Scheme
3 giving rise to eventual charge separation between TiO2- and
Q+ arising from A-D1-D2 triad system, partially contributes
to the dramatic increase in the electron injection efficiency from
Cat-v-Q to TiO2, since by this way the lifetime of chargeseparated state is elongated, giving rise to a higher chance to
the electrolyte (I-) to reduce the oxidized dye. In addition, we
also propose that the large red-shift of the DTCT band caused
by the increase in the donor strength of the dye contributes to
the increase in the electron injection efficiency.
The above two sets of results (Cat-v-P and Cat-v-Q) clearly
demonstrate that the attachment of a second donor group (P or
Q) to Cat leads to a dramatic increase in electron injection
efficiency. Interestingly, the conjugating bridge, the vinyl group,
also behaves similarly as Cat.
Effect of Adsorbed Amounts of Cat, Cat-v-P, and Catv-Q on η. Interestingly, the effect of adsorbed amount of dye
on the photovoltaic parameters was dramatically different
depending on the type of dye. Thus, while the overall η
increased with the adsorbed amount and saturated at a certain
amount in the case of Cat-sensitized DSSC, the reverse behavior
was observed in the cases of DSSCs sensitized by Cat-v-P and
Cat-v-Q, respectively, as shown in Table 2. Although the
reason the above two sets of dyes show different behaviors is
not clear at this stage we propose that Cat-v-P and Cat-v-Q
have higher propensities to aggregate between the neighboring
dyes upon increasing the concentration of the dyes on the surface
and the aggregation is responsible for the decrease of the ηs.
This report thus demonstrates for the first time that the adsorbed
amount of Type-II dye sensitively affects the photovoltaic
parameters of the DSSCs and the relationship between the
adsorbed amount and the η is sensitively governed by the nature
of the dye.
Effect of Coadsorption of DCA on Photovoltaic Parameters of Type-II DSSCs. It has been well established that the
coadsorption of DCA leads to an increase in η to a certain
extent.6b,7,9 We also coadsorbed DCA onto Type-II-dye adsorbed
Efficiency of the Dye-Sensitized TiO2 Solar Cells
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22521
TABLE 2: Effect of Adsorbed Amount of Dye on the
Photovoltaic Parameters of DSSCs Sensitized by Cat,
Cat-v-P, and Cat-v-Q
dye
Cat
Cat-v-P
Cat-v-Q
a
Aaa
Voc (mV)
Jsc (mA cm-2)
FF
η (%)
0.05
0.10
0.18
0.25
0.31
0.05
0.09
0.17
0.09
0.15
0.21
552
573
552
568
549
579
538
520
557
526
507
0.59
1.36
1.69
1.55
1.69
2.59
2.05
1.55
3.19
2.46
1.71
65.8
64.3
65.4
63.2
61.2
70.2
66.0
61.5
70.1
65.9
61.6
0.2
0.5
0.6
0.6
0.6
1.0
0.7
0.5
1.3
0.9
0.5
Adsorbed amount in µmol cm-2.
TABLE 3: Effect of Coadsorbed DCA on the Photovoltaic
Parameters of DSSCs Sensitized by the Dyes Shown in
Chart 1a
Voc (mV) Jsc (mA‚cm-2)
dye
Cat
Dop
Bpg
Cat-v-P
Cat-v-Q
554 (557)
470 (452)
473 (445)
568 (561)
579 (562)
1.28 (1.51)
0.31 (0.40)
1.10 (0.79)
3.08 (2.74)
4.29 (3.53)
FF
η (%)
Aab
69.7 (66.5)
68.2 (67.8)
68.5 (63.1)
69.5 (68.1)
62.6 (66.5)
0.5 (0.6)
0.1 (0.1)
0.4 (0.2)
1.2 (1.1)
1.6 (1.3)
0.11 (0.17)
0.01 (0.01)
0.04 (0.10)
0.05 (0.07)
0.07 (0.08)
a
Numbers in the parentheses stand for the corresponding values
obtained in the absence of DCA (from Table 2) for easier comparison.
b
Adsorbed amount in µmol‚cm-2.
TiO2 films. As summarized in Table 3, Voc and Jsc, and FF
somewhat increased upon coadsorption of DCA except Cat- and
Dop-sensitized DSSCs. As a result, the overall η increased by
100% (Bpg), 9% (Cat-v-P), and 23% (Cat-v-Q), respectively.
Thus, in the case of Cat-v-Q-sensitized DSSC, the η increased
to 1.6%, which is again the highest value ever observed from
Type-II-dye-sensitized DSSCs. Considering that the η of N-719sensitized DSSC prepared under our experimental condition was
only 6.2%, which is about a half of that achieved by Grätzel’s
group1 we believe that the η of Cat-v-Q-sensitized DSSC can
be increased over 3% under an optimized condition. This result
demonstrates the potential of Type-II DSSCs to be developed
into commercially viable DSSCs.
While the DCA-induced increases in Voc were not high, that
is, only 6% (Bpg), 1% (Cat-v-P), and 3% (Cat-v-Q),
respectively, the increases in Jsc were substantial, that is, 39%
(Bpg), 12% (Cat-v-P), and 22% (Cat-v-Q), respectively. This
shows that the DCA-induced increases in η are actually driven
by the increase in Jsc, despite the fact that the adsorbed amounts
of the dyes decreased by 60% (Bpg), 29% (Cat-v-P), and 13%
(Cat-v-Q), respectively, upon adsorption of DCA. This suggests
that the suppression of back electron transfer from the injected
electrons residing on TiO2 to I3- plays an important role in
giving rise to the above increase in Jsc. The adsorption of DCA
did not affect much on the FF.
Minimum η Arising Purely from Pathway B. To obtain
the minimum η arising purely from pathway B, we placed a
400-nm (in the case of DSSC sensitized by Cat, Dop, and Catv-P) or 435-nm cutoff filter (in the case of Cat-Q) on top of
each DSSC during the J-V curve measurements to excite only
the corresponding DTCT bands (but not the local bands).
Consistent with the decrease in the excitation wavelength range,
the overall η decreased substantially, by up to 40%. The cell
parameters under the condition of partial illumination are
summarized in Table 4. Interestingly, while Voc values decreased
significantly (by ∼7-18%) upon sensitization of only the DTCT
bands in the cases of Cat- and Dop-sensitized DSSCs, only slight
reductions in Voc (by ∼2%) were observed from the DSSCs
sensitized by Cat-v-P and Cat-v-Q upon excitation of only
the corresponding DTCT band. In other words, when Cat-v-P
and Cat-v-Q were sensitizers, Voc remained nearly intact as
opposed to the cases where Cat and Dop were sensitizers. In
contrast to Voc, Jsc values decreased rather monotonically by
large amounts (30-41%) regardless of the type of dye upon
illuminating only the DTCT bands, consistent with the decrease
in the number of electrons injected from the dyes to TiO2 caused
by the reduction in the band of excitation wavelength.
We propose that the above sensitizer-dependent different
behavior of Voc and Jsc originates from the difference in the
back electron-transfer rate. Thus, in the cases of Cat and Dop,
most of the electrons injected from the dyes to TiO2 by pathway
B are transferred back to the dyes due to the faster back electron
transfer rates, and only small portions of electrons that were
trapped within deep trap sites, which are lower in energy state,
survive the back electron-transfer process, leading to the arrival
to the FTO glass. In contrast, as a result of slower back electrontransfer rates, even the electrons injected to the sites of TiO2
with higher energy states can manage to arrive in FTO glass.
Interestingly, while the FFs of Cat- and Dop-sensitized DSSCs
decreased by 6 and 15%, respectively, those of DSSCs sensitized
by Cat-v-P and Cat-v-Q increased by 3 and 15%, respectively,
upon excitation of only the DTCT bands. This phenomenon was
highly reproducible. In fact, FF is a function of Voc and is
supposed to increase with increasing Voc.43 Therefore, the above
phenomenon is quite unusual from the facts that the Voc values
remained nearly intact or even slightly decreased, and that Jsc
decreased. The reasons giving rise to the above unprecedented
phenomenon are not clear at this stage and the elucidation of
such reasons is the subject of future studies.
Overall, this report demonstrates that the attachment of a
second donor such as P or Q to Cat via a vinyl bridge leads to
a dramatic increase in η (by 2- and 2.7-fold, respectively),
revealing for the first time the potential of Type-II DSSCs to
be developed into commercially viable DSSCs. The EQEs at
the maximum λmax values of the DTCT bands were 30% (Catv-P) and 25% (Cat-v-Q), respectively, which correspond to
3.5- and 2.9-fold increases with respect to that of Cat-sensitized
DSSC. The above increases in η are in fact driven by the
increases in Jsc. We propose that both the consecutive charge
shift from the secondary donor to a primary donor leading to
the retardation of back electron-transfer rate and the red shift
of the DTCT band caused by the increase in the donor strength
TABLE 4: Photovoltaic Parameters of the DSSCs Sensitized by Cat, Dop, Cat-v-P, and Cat-v-Q under the Condition of Only
DTCT Excitationa
dye
λexb
Voc (mV)
∆c
Jsc (mA‚cm-2)
∆c
FF
∆c
η (%)
Cat
Dop
Cat-v-P
Cat-v-Q
g400
g400
g400
g435
517 (554)
385 (470)
555 (568)
568 (579)
-7
-18
-2
-2
0.80 (1.28)
0.21 (0.31)
2.17 (3.08)
2.52 (4.29)
-38
-32
-30
-41
65.2 (69.7)
57.5 (67.8)
71.3 (69.5)
72.3 (62.6)
-6
-15
3
15
0.3 (0.5)
0.1 (0.1)
0.9 (1.2)
1.0 (1.6)
a
Numbers in the parentheses stand for the corresponding values obtained under the condition of full-range illumination (from Table 3) for
comparison. b Excitation wavelength in nm.. c Difference in %.
22522 J. Phys. Chem. B, Vol. 109, No. 47, 2005
of the dye by attaching the secondary donor to the primary donor
which leads to the increase in the absorption of visible light
are responsible for the increase in η. The η obtainable by only
pathway B can be higher than 1%. This report also reveals an
intriguing phenomenon that, in the cases of Cat-v-P- and Catv-Q-sensitized (Type-II) DSSCs, the FF increases upon exciting
only the DTCT band despite the fact that the reduction in the
excitation range led to a significant decrease in Jsc (by 3041%).
Acknowledgment. We thank the Ministry of Science and
Technology, the Ministry of Commerce, Industry, and Energy,
and Sogang University for supporting this work through the
Creative Research Initiatives (CRI), the Integrated Biotechnology (IB), and the Internal Research Fund programs, respectively.
We also thank Professor Sungu Hwang at Milyang University
for DFT calculations shown in Figure 3.
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