Rhodanine dyes for dye-sensitized solar cells: Spectroscopy, energy levels and photovoltaic performance

Article abstract of DOI:10.1039/b812154k

Three new sensitizers for photoelectrochemical solar cells were synthesized consisting of a triphenylamine donor, a rhodanine-3-acetic acid acceptor and a polyene connection. The conjugation length was systematically increased, which resulted in two effec

Full text of DOI:10.1039/b812154k

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Rhodanine dyes for dye-sensitized solar cells : spectroscopy, energy levels
and photovoltaic performancew
a
b
c
a
Tannia Marinado, Daniel P. Hagberg, Maria Hedlund, Tomas Edvinsson,z
c
a
c
a
Erik M. J. Johansson, Gerrit Boschloo,y Hakan Rensmo, Tore Brinck,
˚
Licheng Sun* and Anders Hagfeldty*
b
a
Received 16th July 2008, Accepted 23rd September 2008
First published as an Advance Article on the web 7th November 2008
DOI: 10.1039/b812154k
Three new sensitizers for photoelectrochemical solar cells were synthesized consisting of a
triphenylamine donor, a rhodanine-3-acetic acid acceptor and a polyene connection. The conjugation
length was systematically increased, which resulted in two effects: first, it led to a red-shift of the
optical absorption of the dyes, resulting in an improved spectral overlap with the solar spectrum.
Secondly, the oxidation potential decreased systematically. The excited state levels were, however,
calculated to be nearly stationary. The experimental trends were in excellent agreement with density
functional theory (DFT) computations. The photovoltaic performance of this set of dyes as sensitizers
in mesoporous TiO solar cells was investigated using electrolytes containing the iodide/triiodide
2
redox couple. The dye with the best absorption characteristics showed the poorest solar cell efficiency,
due to losses by recombination of electrons in TiO
the electrolyte led to a strongly reduced photocurrent for all dyes due to a reduced electron injection
efficiency, caused by a 0.15 V negative shift of the TiO conduction band potential.
2
with triiodide. Addition of 4-tert butylpyridine to
2
1
. Introduction
Here we present a study on three sensitizers, with a general
structure donor–linker–acceptor, D–L–A, where the triphenyl-
amine moiety acts as an electron donor and rhodanine-3-acetic
acid moiety as an acceptor. The linker conjugation is system-
atically changed with methine and thiophene units to increase
the spectral response. The dyes will be referred to as L0, L1
and L2, respectively, according to the linker length, see Fig. 1.
At present there is a strong interest in developing metal free
sensitizers for substitution of conventional ruthenium
1
–5
complexes
in dye sensitized solar cells (DSCs). Organic
sensitizers have the advantage of high extinction coefficients
and can thus also meet the demand of good light harvesting
efficiency with thinner TiO
systems such as ionic liquids
demand thinner TiO films because of mass transport limita-
tions or insufficient pore filling. A great variety of organic
2
films. New less volatile redox
The rhodanine-3-acetic acid anchoring group has shown very
23–25
6
–9
10–12
and hole conductors
promising efficiencies for coumarin and indoline dyes
also recently for triphenylamine based dyes by Liang and Tian
and
2
27,28
et al.
1
3–19
20–22
sensitizers based on polyene- triphenylamine,
23–26
and indoline
coumarin,
moieties give respectable conversion efficiencies
Spectroscopic and electrochemical properties were charac-
terized experimentally and by density functional theory (DFT)
computations. Photoelectron spectroscopy (PES) was used for
of 5–9% with the traditional iodide/triiodide redox system.
However, still a better fundamental understanding of the ener-
getic, kinetic and geometric interplay between the oxide/dye/
electrolyte constituents is needed to be able to design new efficient
and stable organic sensitizers for future devices.
element specific information on the electronic and molecular
29–32
structure of the dye sensitized surface.
The behavior of the
dyes in functional solar cell devices was further investigated by
current–voltage (IV), incident-photon-to-current-efficiency
(
IPCE), charge extraction, voltage and current decay measure-
a
ments. For this series of dyes used in DSCs two main effects
were studied and discussed: (i) the dependence of the linker
conjugation length and (ii) the influence of the additive 4-tert-
butylpyridine (4-TBP) on the overall solar cell performance for
this series of dyes.
Center of Molecular Devices, Royal Institute of Technology, School
of Chemical Science and Engineering, Physical Chemistry,
100 44 Stockholm, Sweden. E-mail: anders.hagfeldt@fki.uu.se
b
c
Center of Molecular Devices, Royal Institute of Technology, School
of Chemical Science and Engineering, Organic Chemistry,
100 44 Stockholm, Sweden. E-mail: lichengs@kth.se
Department of Physics, Uppsala University, Box, 530, SE-751 21
Uppsala, Sweden
w Electronic supplementary information (ESI) available: Detailed
synthetic procedures; preparation and characterization methods of
dye-sensitized solar cells and additional experimental results; details of
quantum chemical calculations; photoelectron spectroscopy details
and additional results. See DOI: 10.1039/b812154k
2
. Experimental
2
.1 Synthesis
The dyes were synthesized according to well known reactions;
the detailed synthetic procedures are described in the ESI.w As
a representative, the synthesis of L1 is illustrated in Scheme 1.
Suzuki coupling of 4-(diphenylamino) bromobenzene and
z Current address: Uppsala University, Department of Materials
Chemistry, Box 538, 751 21 Uppsala, Sweden.
y Current address: Uppsala University, Department of Physical and
Analytical Chemistry, Box 259, 751 05 Uppsala, Sweden.
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few minutes and drying in an air flow. DSCs were further
prepared using standard methods given in the ESIw and
characterized using methods given there.
3. Results and discussion
3
.1 Spectroscopic and electrochemical characterization
The optical and electrochemical properties of the dyes in
solution and adsorbed onto TiO are summarized in
2
Table 1. A red-shift is observed with increasing the linker
conjugation length; the absorption maxima in acetonitrile
solution were observed at 459, 476 and 501 nm, for L0, L1
and L2, respectively. The corresponding molar extinction
coefficients (emax) were approximately 44 000, 38 000 and
ꢁ ꢁ1
1
3
9 000 M cm for L0, L1 and L2, respectively, at absorp-
tion maximum. The high extinction coefficients of these dyes
meet the requirements for solid state dye-sensitized solar cells,
1
0
2
where thinner TiO films are used to obtain efficient devices.
The ground state oxidation potential (ED/D+) of the dye is
an important parameter when evaluating the driving force for
regeneration of the oxidized dye via the redox system. The
oxidation potential must be sufficiently more positive than the
potential of the redox couple iodide/triiodide to obtain fast
regeneration kinetics. Sluggish regeneration kinetics are un-
Fig. 1 Molecular structure of the sensitizers used in this study: L0, L1
and L2. The number is related to the length of the conjugated linker
between the triphenylamine and the rhodanine-3-acetic acid moieties.
3
1,33,34
favorable for the overall solar cell efficiency.
The oxida-
tion potentials of the dyes in solution were measured by
differential pulse voltammetry (DVP) and showed a clear
dependence on the conjugation length: EL0/L0+ 4 EL1/L1+
4
EL2/L2+, see Table 1. Although the absolute values might
differ slightly when the dyes are adsorbed onto TiO2, the
internal trend is expected to be similar. As the redox potential
of the iodide/triiodide electrolyte is about 0.3 V vs. NHE, the
regeneration driving force can be calculated to 0.69, 0.85 and
Scheme 1 Synthetic route to dye L1.
0
.97 V, for L2, L1 and L0, respectively. This may be compared
to 0.70 V for the conventional N719 ruthenium complex
5
-formyl-2-thiophene-boronic acid under microwave irradia-
tion provided compound 1. Condensation of the aldehyde
2
(
oxidation potential of 1 V vs. NHE). The estimated excited
state potential, ED*/D+, calculated from ED/D+–E0ꢁ0, seems
to be sufficiently more negative than the TiO CB edge for all
moiety with rhodanine-3-acetic acid by the Knoevenagel
0
reaction with excess amount of piperidine yielded L1 as a
precipitate which was filtered and washed with acetonitrile and
2
three dyes and hence should in theory have enough driving
force for effective injection, as reported in Table 1.
PES measurements of the valence structure of sensitized
0
water. The carboxylate-salt L1 was dissolved in dichloro-
methane (DCM), washed with aq. HCl (1 M) and converted
to the corresponding carboxylic acid L1.
2
TiO reveals the HOMO energy levels of the adsorbed organic
dyes, see Fig. 2. The results support the trend observed in the
electrochemical measurements. When compared to PES data
2
.2 Dye-sensitized solar cells
3
0
Mesoporous TiO films (ca. 10 mm thick) were immersed in
from adsorbed N719, the peak positions of the HOMO
energy levels of L2, L1 and L0 are shifted towards higher
binding energy by approximately 0.2, 0.3 and 0.4 eV,
2
500 mM dye solutions in acetonitrile for 12–14 h in room
temperature, followed by immersion in pure acetonitrile for a
Table 1 Experimental spectral and electrochemical properties of the dyes
b
ꢁ1
ꢁ1
a
c
d
e
Dye
lmax/nm
e /M cm
l
em /nm
E
D/D+ /V vs. NHE
E
(oꢁo) /eV
ED*/D+ /V vs. NHE
i
ii
ii
ii
L0
L1
L2
459 , 438
44 000
38 000
39 000
607
668
726
1.27
1.15
0.98
2.38
2.19
2.08
ꢁ1.11
ꢁ1.04
ꢁ1.10
i
476 , 459
501 , 469
i
a
i
Absorption of dyes in acetonitrile solution , adsorbed onto TiO
ii
b
c
2
, and emission spectra in acetonitrile. Absorption coefficient in THF. The
ground state oxidation potential, ED/D+, of the dyes were measured with DPV under following conditions: working electrode; Pt, electrolyte;
+
0
6
.1 M tetrabutylammonium hexafluorophosphate, TBA(PF ) in acetonitrile. Potentials measured vs. Fc /Fc were converted to NHE by addition
4
8 d
e
of +0.63 V.
E
(oꢁo) is estimated from the intercept of the normalized absorption and emission spectra in acetonitrile.
ED*/D+ is estimated by
subtracting E(0ꢁ0) to ED/D+
.
1
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calculation overestimates the energy of the absorption max-
imum of L2 in solution by 0.36 eV, whereas the PBE1PBE
method does better with an overestimation of only 0.19 eV.
The results are not surprising since TDDFT methods are
known to underestimate the energies for charge-transfer
states, and the underestimation generally increases with the
distance of charge separation. The error can be partly reduced
by the inclusion of Hartree–Fock exchange, which explains the
slightly better performance of the PBE1PBE functional, as the
latter includes more Hartree–Fock exchange than B3LYP.
However, using the full Hartree–Fock exchange in the TDHF
method leads to energies of the absorption maximum that are
too low by 0.7–1.3 eV (data not shown here). Inclusion of
solvent at the B3LYP and PBE1PBE levels using the PCM
(
polar continuum model) method generally reduces the transi-
tion energy by 0.21–0.23 eV. The gas phase results are,
however, in better agreement with the experiments than the
PCM results, for both the B3LYP and PBE1PBE methods.
The reason for this is most likely a cancellation of errors; the
underestimation of transition energies in the DFT calculations
are partly compensated for by the higher transition energies in
gas phase compared to solution. The computed oscillator
strengths decrease slightly on going from L0 to the longer
L1, which is in qualitative agreement with the experiments.
The strong absorption calculated for L2 is not, however,
supported by the experiments.
Fig. 2 Photoelectron spectroscopy (PES) valence electronic structure
of TiO sensitized with L0, L1 and L2. The measurement was
performed with a photon energy of 100 eV. The energy scale was
calibrated using the Ti3p peak of the TiO substrate.
2
2
In Table 3, the experimental oxidation potentials are listed
together with the computed oxidation potentials and reorga-
nization energies. It can first be noted that the B3LYP and
PBE1PBE methods yield oxidation potentials for all three dyes
that are too positive by approximately 0.6 and 0.5 V, respec-
tively. The relative position of the oxidation potentials within
this group of dyes that are calculated are, however, in very
good agreement with experiment. This indicates that the
computations are able to reproduce differences in the oxida-
tion properties between the dyes. It is interesting that the
computed reorganization energy is almost identical for
the three dyes. It is noted that the polyene diphenyl aniline
cyanoacetic acid (D5), which showed high performance
respectively. While only one feature can be resolved in the
HOMO level measured for L0 and L1, the less symmetric peak
obtained in the measurements of L2 indicates that more than
one state may be involved in the HOMO level of L2.
3
.2 Quantum chemical calculations
In Table 2 we report experimental and computed absorption
maxima for the lowest energy electronic transitions. Details of
the quantum chemical calculations are given in the ESI.w
Starting with the B3LYP and PBE1PBE gas phase results,
we note that both methods provide a good prediction for lmax
of the shortest dye, L0; the calculated wavelengths are 466 and
1
4
in solar cells reported by our group has a very similar
calculated reorganization energy (0.97 eV).
4
40 nm, respectively, which compares well with the experi-
mental lmax of 459 nm for L0 in acetonitrile. However, the
computations are not able to correctly predict the magnitude
of the change in lmax going towards longer dyes. In going from
L0 to L2 the energy of the absorption maximum decreases by
Fig. 3 depicts the B3LYP computed LUMO orbitals of the
dyes. In all three dyes the strongest absorption band is
dominantly of HOMO–LUMO character, i.e. the expansion
coefficient for the HOMO–LUMO transition is higher than
0.65. The electron redistribution of the orbitals indicates a
charge separation upon electronic excitation. Both HOMO
0.23 eV experimentally, whereas the computational methods
predict a decrease of 0.53–0.54 eV. For example, our B3LYP
Table 2 Experimental and computed spectral properties of the lowest energy absorption band of the dyes
a
b
ꢁ1
ꢁ1
c
d
e
f
l
max /nm
e /M cm
Exp.
l
max /nm
f
B3LYP
l
max /nm
f
PBE1PBE
Dye
Exp.
B3LYP
PBE1PBE
i
i
i
i
L0
L1
L2
459
476
501
44 000
38 000
39 000
466, 510i
543, 605i
586, 653
0.89, 0.93i
0.88, 0.89i
1.22, 1.23
440, 476i
0.95, 0.99i
503, 553i
542, 596
0.94, 0.93i
1.35, 1.34
a
b
Absorption maxima of dyes in acetonitrile solution at room temperature. Absorption coefficient in THF. TDDFT-B3LYP/6-31+G (d)
c
d
computed vertical excitation wavelengths. (i) in acetonitrile using the PCM approach. TDDFT-B3LYP/6-31+G (d) computed oscillator
e
strengths. TDDFT-PBE1PBE/6-31+G (d) computed vertical excitation wavelengths. TDDFT-PBE1PBE /6-31+G (d) computed oscillator
f
strengths.
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Table 3 Experimental and computed electrochemical properties of the dyes
a
b
c
DED/D+
D/D+[V vs. NHE]
Exp.
DED/D+
D/D+[V vs. NHE]
B3LYP
DED/D+
ii
ii
ii
d
E
E
(ED/D+[V vs. NHE] )
PBE1PBE
l
o
/eV
(kcal mol )
ꢁ
1
Dye
i
0 (1.27)
ii
i
0 (0.73)
ii
i
0 (0.81)
ii
L0
L1
L2
0.96 (22.1)
0.95 (22.0)
0.94 (21.6)
i
ii
ii
i
ii
i
ii
ii
ꢁ0.12 (1.15)
ꢁ0.20 (0.53)
ꢁ0.19 (0.62)
i
i
i
ꢁ0.29 (0.98)
ꢁ0.33 (0.40)
ꢁ0.30 (0.51)
(
i) The relative difference in ground oxidation potential, DED/D+, between the dyes normalized to the value for L0 and (ii) the absolute ground
a
state oxidation potential, ED/D+ vs. NHE of (exp) Dyes measured under following conditions; Working electrode; Pt, Electrolyte; 0.1 M
+
6
tetrabutylammonium hexafluorophosphate, TBA(PF ) in acetonitrile. Potentials measured vs. Fc /Fc were converted to NHE by addition of
48 b
c
(B3LYP) Values computed using equation. 1. at the PCM-B3LYP/6-31+G(d) level. (PBE1PBE) Values computed at the
+0.63 V.
PCM-PBE1PBE/6-31+G(d) level. The electron-transfer reorganization energy computed using equation. 2 at the PCM-B3LYP/6-31+G(d) level.
d
Table 4 Solar cell characteristics of DSCs based on the L0, L1 and
L2 under AM 1.5G illumination (100 mW cm ). The electrolytes used
ꢁ2
were: (1) 0.5 M LiI and 0.05 M I
0
2
in acetonitrile; (2) same, but with
.5 M 4-tert-butylpyridine added
a
ꢁ2
Conditions
Z/%
Voc/V
J
SC/mA cm
ff
L0, electrolyte 1
L0, electrolyte 2
L1, electrolyte 1
L1, electrolyte 2
L2, electrolyte 1
L2, electrolyte 2
2.7
1.3
2.3
2.0
1.7
0.8
0.50
0.61
0.44
0.58
0.41
0.49
10.0
3.7
10.5
5.9
7.7
2.9
0.54
0.58
0.50
0.58
0.54
0.56
a
The TiO
2
film thickness was B10 mm. Electrodes were dyed during
2
1
5 h in 500 mM dye solutions in acetonitrile. The active area was 0.48 cm .
Fig. 3 The frontier molecular orbitals of the HOMO (bottom) and
LUMO (top) calculated with PCM-PBE1PBE/6-31+G(d) level. The
electron redistribution of the orbitals shows a clear charge separation
upon electronic excitation.
rise to a modest increase of VOC by about 0.1 V, a dramatic
decrease of the short-circuit current density (JSC) was
observed, resulting in lower overall solar cell efficiencies.
Spectra of the incident photon-to-current-conversion effi-
ciency (IPCE) for L0, L1 and L2-based solar cells with
electrolyte 1 (no 4-TBP) are shown in Fig. 4. In the range
and LUMO are partially located on the conjugated system
which supports the observed dependence of the oxidation
potential upon the linker conjugation length. The LUMO
orbital does not extend out to the carboxylate group that links
5
00–550 nm, IPCE maxima of up to 84%, 74% and 50% were
found for L0, L1 and L2, respectively. The effect of 4-TBP
addition to the electrolyte is pronounced in the IPCE spectra.
the dye to the TiO surface, indicating a less strong electronic
2
coupling between the dye LUMO and the acceptor states of
TiO2 in comparison to for example cyanoacetic acid as
anchoring group where the conjugation is extended out on
the carboxylic group. In this context, it is also interesting to
compare our results with the high efficiency indoline dye
reported by Hourichi et al. using the same rhodanine-3-acetic
2
3,25,26
acid anchoring group as in the present study.
Our
calculation of the indoline dye (not shown here) shows a lack
of electron density of the LUMO on the anchoring group,
which also suggests a less strong electronic coupling with TiO2
acceptor states.
3
.3 Solar cell performance
The photovoltaic performance of solar cells based on L0, L1
ꢁ
2
and L2 dyes under AM 1.5G illumination, 100 mW cm , is
summarized in Table 4. The efficiencies obtained with solar
2
cells with electrolyte 1 (0.5 M LiI and 0.05 M I in acetonitrile)
were Z = 2.7% for L0, Z = 2.3% for L1 and Z = 1.7% for L2,
showing that higher efficiencies were obtained for the dyes
with a shorter conjugation length. In order to improve
the open-circuit potential (VOC), 0.5 M 4-tertbutylpyridine
Fig. 4 Spectra of monochromatic incident photon-to-current con-
version efficiency (IPCE) for DSC based on L0 (K), L1 (’) and L2
(
4-TBP) was added to the electrolyte. Although this gave
(c), respectively. Electrolyte: 0.5 M LiI and 0.05 M I
2
in acetonitrile.
1
36 | Phys. Chem. Chem. Phys., 2009, 11, 133–141
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Dye orientation: The dye orientation on the surface was
further investigated by analyzing N1s (nitrogen) and S2p
core-level PES. For a randomly ordered molecular layer
(
dye powder on conducting glass), the PES N1s intensities
should show a 1 : 1 ratio of the peak originating from the
triphenylamine moiety (N1sTPA) and that from the rhodanine-
3
-acetic acid moiety (N1sR3A). As observed in Fig. 6, the N1s
signal from these samples can indeed be reasonably well curve
Fig. 5 Spectra of monochromatic incident photon-to-current con-
version efficiency (IPCE) for DSCs based on (a) L0 (b) L1 and (c) L2;
with (- - -) and without (—) 4-TBP in the electrolyte, (d) normalized
IPCE spectra for DSC based on L0.
Fig. 5a–c shows IPCE spectra of L0, L1 and L2 using
electrolyte with and without 4-TBP. Fig. 5d shows normalized
IPCE spectra of L0 based solar cells with the two electrolytes.
A pronounced blue shift of the IPCE spectrum is observed for
L0. Similar blue shifts are found for L1 and L2.
3
.4 Origin of differences in photovoltaic performance
of L0, L1 and L2
We will divide this section in two parts: First, we will discuss
the internal differences observed in electrolyte without 4-TBP,
and second we will discuss the effect of 4-TBP on the solar cell
performance.
3
.4.1 Performance of L0, L1 and L2 sensitizers in absence
of 4-TBP. Dye-load: Although the organic sensitizers differ
only slightly in chemical structure, there are significant differ-
ences in dye load, photocurrent generation, charge recombi-
nation kinetics and energy levels in the DSC. The dye load was
2
determined by desorbing the dye from the TiO film using an
alkaline solution and recording the UV-Vis spectrum. The L2
dye load was equal or higher than the L0 dye load. This was
unexpected as L2 is a larger molecule, and this could be an
indication of aggregation of L2. The coverage of dye on the
sensitized TiO samples can also be estimated by PES mea-
2
surement by a comparison between the intensity of a dye
specific core level peak and a substrate (TiO
2
) specific peak.
The coverage of dye on the sensitized TiO surface was found
2
to vary between the sensitizers. The relative intensity ratio of
the S2p from the double bonded sulfur in the anchor group
2
and Ti2p peak originating from the TiO is 0.66, 0.73 and 1 for
Fig. 6 N1s PES spectra of L0 (top figure), L1 (middle figure) and L2
L0, L1 and L2, respectively. This result is a further indication
that more L2 than L0 is adsorbed on the TiO2 surface. It
should be noted that PES is a surface sensitive techniques,
(
lower figure). Both spectra of multilayers (ML) and spectra of dye-
sensitized TiO2 are shown for the different dyes. The measurements
were performed with a photon energy of 1487 eV for the ML and both
probing only a few a
˚
¨
ngstroms into the material.
2
1487 and 758 eV for the dye-sensitized TiO .
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Table 5 Intensity ratios of N1s core level PES peaks for the dyes on
TiO
2
N1sR3A/N1sTPA
N1sR3A/N1sTPA
Dye
hn = 1487 eV
hn = 758 eV
L0
L1
L2
D5
0.43
0.41
0.32
0.37
0.33
0.28
0.23
0.23
fitted with two components having the same intensity. The
peak at lower binding energy can be assigned to N1s and
TPA
3
.
5
the peak at higher binding energy to N1sR3A
In dye-sensitized TiO electrodes the ratio between these
2
two peaks changed. An intensity ratio between the N1sR3A
and N1sTPA peaks of about 0.5 was found at a photon energy
of 1487 eV, which gives a strong evidence for a dominating
orientation of the molecule having the triphenylamine moiety
Fig. 7 UV-Vis absorption spectra converted to LHE(%) of L0 (K),
3
5
2
pointing out from the TiO surface. The surface sensitivity
L1 (’) and L2 (c) adsorbed onto TiO
electrolyte. Inset: UV-Vis absorption spectra of L0, L1 and L2,
respectively, adsorbed onto TiO in the presence of electrolyte.
2
in the presence of redox
depends on the kinetic energy of the emitted electrons and can
be improved by decreasing the photon energy. This explains
why the intensity ratio between N1sR3A and N1sTPA decrea-
sees further in the measurements performed at lower photon
energy of 758 eV, see Table 5. Comparing the N1s ratios in
more detail we observe that the intensity ratio decreases in the
order L0, L1, L2. This change is in accordance with the
decrease in the distance between the two nitrogen atoms in
the different molecules. Previous measurements of the D5
molecule, also containing two spectroscopically distinguish-
able nitrogen atoms at a distance similar to that of L1, show
2
The calculated LUMO levels for dyes L0, L1 and L2 are
similar, see Table 1, and located well above the TiO conduc-
2
tion band edge. Furthermore, the dyes have identical binding
groups and similar electronic structure, suggesting that elec-
tronic coupling to CB states will be similar. Therefore, no
significant differences in finj between the dyes are expected.
Regeneration of the oxidized dye: Photoinduced absorption
3
4,36
3
5
(
PIA) spectroscopy
were able to inject electrons into TiO
demonstrated clearly that the dyes
upon excitation, as
very similar ratios. The PES analysis of the dye-sensitized
samples therefore suggests that differences in molecular
orientation among the L0, L1, L2 series is small and the
preferred orientation is having the triphenylamine moiety
pointing out from the surface and is similar to that of the
D5 molecule. Additional measurements using the S2p signals
confirm the results (see ESI).w
2
spectral characteristic of the oxidized dye molecules were
observed (see Supporting Information). The spectral charac-
teristic of the oxidized dye disappeared from the PIA spectrum
when the redox electrolyte was added (see ESI).w This suggests
that the regeneration of the dye (reduction of the oxidized dye
by iodide) is efficient. As the oxidation potentials for the dyes
differ, rate constants for regeneration may differ, and, based
on driving force, are expected to be decrease in the order
L0 4 L1 4 L2. Ultrafast laser spectroscopy studies, to be
published elsewhere, are presently being performed on the
system to determine injection and recombination kinetics, and
their impact on the photocurrent generation. Different limita-
tions in the ultrafast kinetics can reduce the value of Zreg and
may explain in part the observed differences in IPCE.
Light-harvesting efficiency: L2-sensitized TiO
2
films absorb
more sunlight than L1 or L0-sensitized films. The light-
A
ꢁ
harvesting efficiency (LHE = (1 ꢁ 10 ), with A the absor-
bance of the film) was measured with dyes adsorbed on B3 mm
2
thick TiO films, and is plotted in Fig. 7. As expected, L2 has
the most red-shifted onset, followed by L1. Due to the
high extinction coefficients, the LHE is 100% in the range
4
00–500 nm for all dyes. The IPCE in the same range was,
however, significantly below 100%, see Fig. 4. IPCE can be
expressed as follows:
Charge collection efficiency: Losses of injected electrons
during transport in the porous TiO film are given by good
2
IPCE(l) = LHE(l)ꢂfinjꢂZregꢂZcc
(1)
approximation by the ratio of electron transport time and
electron lifetime. These characteristic time constants were
determined using photocurrent and photovoltage transients
following small modulations of light intensity as previously
where finj, Zreg and Zcc are the quantum yields of charge
injection, dye regeneration and charge collection efficiency,
respectively. For L0 an IPCE of 84% was obtained. As
reflection losses are estimated to be in the order of 15%, it
appears that finj, Zreg and Zcc are all nearly 100% for this dye.
We will now establish which factors limit the IPCE of
L1 and L2.
3
7–41
described. No significant differences in electron transport
times plotted against short-circuit charge density in the film
are found (see ESI),w which can be expected as the transport is
determined mostly by properties of the mesoporous TiO
2
3
9
electrode and not by the dye. Electron lifetimes measured
at the same light intensities, but under open-circuit conditions,
differ significantly between the dyes, see Fig. 8a. Electron
lifetimes under short-circuit conditions were estimated using
Injection efficiency: The injection efficiency is given by the
ratio of the rate constant for electron injection and the sum of
all deactivation rate constants for the excited dye molecule.
1
38 | Phys. Chem. Chem. Phys., 2009, 11, 133–141
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and L0 does not protect electrons in TiO
2
from recombination
with triiodide, but is in fact promoting this reaction. The
origin of the catalysis of triiodide reduction may be the
formation of
(
a complex between the dye and iodine
3
or triiodide), as was recently suggested by O’Regan et al.
4
Differences in binding constants of dye and iodine (or triiodide)
are possibly the origin of the differences in electron lifetimes
and dark currents.
3
.4.2 Influence of 4-TBP additive on the photovoltaic
performance. Position of the conduction band: The position of
the TiO conduction band (CB) depends on the surface charge,
2
+
which is affected by the electrolyte composition. Li ions
2
adsorb relatively strongly at the TiO /electrolyte interface
and cause thereby a positive shift of the conduction band
edge potential. Adsorption of negatively charged ions or Lewis
base molecules, such as 4-TBP, shifts the CB edge to more
negative potential. 4-TBP has successfully been used to
increase the efficiency of N719-sensitized TiO
2
solar cells as
it improves the open-circuit voltage, while the short-circuit
2
,37
Fig. 8 (a) Electron lifetime as function of open-circuit voltage. Time
constants were determined using time-resolved small modulation
techniques. Solar cells based on L0 (K), L1 (’) and L2 (c),
currents is hardly affected.
charge and open-circuit voltage for DSCs with and without
-TBP in the electrolyte, we applied a charge extraction
To study the relation between
4
respectively, with 0.5 M LiI, 0.05 M I
2
in acetonitrile. (b) Extracted
44
method. Results are presented in Fig. 9 for L1-sensitized
solar cells. For a given charge, a higher open-circuit voltage,
charge as a function of open-circuit potential in dye sensitized solar
cells based on L0 (K), L1 (’) and L2 (c), respectively, with
V
OC, is obtained when 4-TBP is present in the electrolyte. This
2
electrolyte (1) 0.5 M LiI, 0.05 M I in acetonitrile. (c) Dark current-
implies that the conduction band edge is shifted to more
negative potential, provided that distribution of traps is not
voltage characteristics of DSC based on L0 (K), L1 (’) and L2 (c),
respectively.
3
7
affected.
IPCE spectra are significantly lower and blue-shifted in the
presence of 4-TBP, see Fig. 5. To explain these observations,
we will again factorize the IPCE using eqn (1).
the extracted charge under short-circuit conditions and the
relation between charge and electron lifetime at open circuit
condition that were measured separately (see ESI).w Zcc was
estimated to be 0.9 for L2-sensitized solar cells and 1.0 for L1
and L0. In conclusion, the differences in IPCE as observed in
Fig. 4 can be attributed to differences electron collection and
possibly also to differences in dye regeneration efficiency.
Electron lifetimes in dye-sensitized solar cells depend sig-
nificantly on the type of sensitizing dye, as has been observed
Light-harvesting efficiency: The absorption spectrum of the
adsorbed dyes did not change in the presence of 4-TBP.
Furthermore, no significant differences in dye-desorption
was observed when sensitized films were exposed to redox
4
2,43
before in several studies.
reflects the recombination between electrons in the TiO
This is remarkable as the lifetime
and
2
triiodide in the electrolyte. The relatively large differences in
electron lifetime between the dyes used in this study are even
more surprising, considering the similarity of the dyes. In
Fig. 8a electron lifetimes of DSCs based on L0, L1 and L2
are shown as a function of open-circuit voltage. Lifetimes
decrease in the order L0 4 L1 4 L2. Adsorbed dye can affect
the position of the TiO conduction band edge. The relation
2
between charge and open-circuit voltage was therefore studied,
see Fig. 8b. No significant differences in photovoltage–charge
relation were found for the different dyes, indicating that the
dyes affect the surface in a similar way. Differences in electron
recombination are thus only due to the nature of the dye, and
not due to band edge shifts.
It appears that dye molecules can catalyze the reduction of
4
3
triiodide. To confirm this, dark current–voltage character-
istics were measured, see Fig. 8c. The dark current decreases in
the order L2, L1 and L0, which agrees well with the electron
lifetime results. The higher dye load of L2 in comparison to L1
Fig. 9 Extracted charge as a function of open circuit potential in
L1-sensitized solar cells with electrolyte 1 (K) 0.5 M LiI, 0.05 M I
2
in acetonitrile and 2 (&) 0.5 M LiI, 0.05 M I , 0.5 M 4-TBP in
2
acetonitrile.
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electrolyte with and without 4-TBP. The blue-shift of the
IPCE must originate from one of the other factors.
Regeneration of the oxidized dye: We do not expect that
conditions. In contrast to L2, high IPCE values are obtained
for D5, even in the presence of 4-TBP, and significantly higher
photovoltages are found. While the principle anchoring group
to the TiO2 is identical, i.e. carboxylic acid, computational
results show that in case of D5 the LUMO is extended to this
group, whereas it is not in case of the rhodanine-3-acetic acid
of L2 (see Fig. 3). This suggests that a combination of reduced
injection driving force (B0.2 V) and weaker coupling between
4
-TBP interferes significantly with the dye regeneration,
although fast spectroscopic studies are needed to confirm this.
Zreg is likely to be similar to the situation without 4-TBP in the
electrolyte.
Charge collection efficiency: In contrast to N719-sensitized
4
5–47
TiO
2
solar cells,
4-TBP did not increase the electron
the excited state level in L2 and TiO
injection kinetics and poorer injection efficiencies in cases
where the TiO CB levels are relatively close in energy to the
2
CB levels result in slower
lifetime in L0, L1 and L2-sensitized solar cells. The collection
efficiency is therefore not expected to be much affected. The
presumed blocking effect of 4-TBP, by binding to uncovered
2
excited state levels of the dye, i.e. when 4-TBP is present in the
TiO
2
surface will be less important for small dye molecules.
electrolyte.
Furthermore, if the main reaction pathway of recombination
is through a possible dye-iodine (-triiodide) complex, the
surface blocking will not have much effect.
4. Conclusions
Injection efficiency: The observed changes in the IPCE
spectra are most likely due to changes in the injection effi-
ciency upon addition of 4-TBP. As discussed above, addition
A series of three closely related dyes has been designed,
synthesized and investigated in dye-sensitized solar cells. The
dyes consist of an electron donor moiety, triphenylamine, and
an electron acceptor moiety, rhodanine-3-acetic acid, linked
together using methine and thiophene moieties. Increase of the
conjugation length resulted in a more red-shifted spectral
response and less positive oxidation potentials. The calculated
excited state levels were stationary. DFT calculations using
PCM solvation corrected B3LYP and PBE1PBE was in good
agreement with experimental electrochemical and optical data.
The photovoltaic performance of this set of dyes as sensitizers
of 4-TBP leads to a shift of the TiO
higher energy. This will decrease the density of acceptor states
in the TiO which are located at the same energy as the excited
2
conduction band edge to
2
dye levels. This will result in a decrease of the electron injection
rate constant and the injection efficiency. The observed blue
shift in the IPCE spectra suggests that the injection efficiency is
wavelength dependent and is illustrated in Fig. 10. If electron
injection occurs from hot (= not vibronically relaxed) excited
states a wavelength-dependent injection rates and injection
efficiency may be expected. The excited state resulting from
absorption of a red photon has less energy than that resulting
from a blue photon, and its energy may not be high enough for
successful electron injection.
2
in mesoporous TiO solar cells was investigated using electro-
lytes containing the iodide/triiodide redox couple. The dye
with the best absorption characteristics, L2, showed the
poorest solar cell efficiency, due to insufficient dye regenera-
tion and charge collection efficiencies. Addition of 4-tert-
butylpyridine to the electrolyte led to a strongly reduced
photocurrent for all dyes due to a reduced electron injection
It is interesting to compare the L2 sensitizer to the closely
1
4,35
related D5 dye
acceptor group, cyanoacetic acid. This causes a difference in
that differs in structure only by the
efficiency, caused by a 0.15 V negative shift of the TiO
conduction band potential.
2
excited state potential, ED*/D+: values of ꢁ1.04 V for L2 and
ꢁ
1.36 V for D5 vs. NHE are estimated under the same
Acknowledgements
This work was supported by the Swedish Energy Agency, the
Swedish Research Council, Knut och Alice Wallenberg Foun-
dation, the Carl Trygger Foundation, the Swedish Foundation
for Strategic Research (SSF) and BASF AG Ludwigshafen.
We also thank ECN for supplying the TiO
Industrial Research and Development Corporation for screen-
printing the TiO electrodes. Ms Rong Zhang in Dalian is
2
paste, and IVF
2
acknowledged for the help with MS and Haining Tian and
Xichuan Yang for helpful discussions of the dye synthesis.
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