Phenoxazine dyes for dye-sensitized solar cells: Relationship between molecular structure and electron lifetime

Article abstract of DOI:10.1002/chem.201003730

A series of metal-free organic dyes with a core phenoxazine chromophore have been synthesized and tested as sensitizers in dye-sensitized solar cells. Overall conversion efficiencies of 6.03-7.40 % were reached under standard AM 1.5G illumination at a light intensity of 100 mW cm^-2. A clear trend in electron lifetime could be seen; a dye with a furan-conjugated linker showed a shorter lifetime relative to dyes with the acceptor group directly attached to the phenoxazine. The addition of an extra donor unit, which bore insulating alkoxyl chains, in the 7-position of the phenoxazine could increase the lifetime even further and, together with additives in the electrolyte to raise the conduction band, an open circuit voltage of 800 mV could be achieved. From photoelectron spectroscopy and X-ray absorption spectroscopy of the dyes adsorbed on TiO₂ particles, it can be concluded that the excitation is mainly of cyano character (i.e., on average, the dye molecules are standing on, and pointing out, from the surface of TiO₂ particles). Copyright

Full text of DOI:10.1002/chem.201003730

FULL PAPER
DOI: 10.1002/chem.201003730
Phenoxazine Dyes for Dye-Sensitized Solar Cells: Relationship Between
Molecular Structure and Electron Lifetime
Karl Martin Karlsson,[a] Xiao Jiang,[b] Susanna K Eriksson,[c] Erik Gabrielsson,[a]
Hꢀkan Rensmo,[d] Anders Hagfeldt,[c] and Licheng Sun*[a]
Abstract: A series of metal-free organ-
ic dyes with a core phenoxazine chro-
mophore have been synthesized and
tested as sensitizers in dye-sensitized
solar cells. Overall conversion efficien-
cies of 6.03–7.40% were reached under
standard AM 1.5G illumination at a
light intensity of 100 mWcmꢀ2. A clear
trend in electron lifetime could be
seen; a dye with a furan-conjugated
linker showed a shorter lifetime rela-
tive to dyes with the acceptor group di-
rectly attached to the phenoxazine. The
addition of an extra donor unit, which
bore insulating alkoxyl chains, in the 7-
position of the phenoxazine could in-
crease the lifetime even further and,
together with additives in the electro-
lyte to raise the conduction band, an
open circuit voltage of 800 mV could
be achieved. From photoelectron spec-
troscopy and X-ray absorption spec-
troscopy of the dyes adsorbed on TiO2
particles, it can be concluded that the
excitation is mainly of cyano character
(i.e., on average, the dye molecules are
standing on, and pointing out, from the
surface of TiO2 particles).
Keywords: charge transfer · dyes/
pigments · energy conversion · sen-
sitizers · solar cells
Introduction
too high to be able to really compete with the conventional
energy sources in the market. Dye-sensitized solar cells
(DSCs) could be a very attractive choice to potentially
lower production costs. A typical DSC consists of a wide
bandgap semiconductor photoanode, an anchored molecular
sensitizer, a redox electrolyte, and a counter electrode.[2–6]
Among these components, the sensitizing dye plays a vital
role in the light-harvesting efficiency (LHE). The develop-
ment of new sensitizers for DSCs is very important to im-
prove of the overall efficiency of the cell through a better
understanding about the function of the cell and its limita-
tions. During the early years of DSC research the focus was
mainly on metal complexes as sensitizers. Lately more atten-
tion has been paid to metal-free organic dyes due to their
high molar-extinction coefficients, easy synthesis and struc-
tural modification, as well as lesser environmental concerns
compared to the heavy-metal-based complexes. So far,
metal-free organic dyes show lower performance than their
metal-based analogues because of lower LHE in the visible
and near-infrared (NIR) regions. However, due to heavy re-
search within this field, the metal-free organic sensitizers
have now gained promising solar energy-to-electricity con-
version efficiencies (h). With top efficiencies reaching
10%,[7,8] the distance to the best inorganic dyes—with effi-
ciencies of approximately 11%[9,10]—should not be unreach-
able anymore. Many different kinds of chromophores have
been examined in DSCs (such as porphyrin,[11,12] phthalocya-
Recently, concern for the usage of classical energy sources,
such as fossil fuels, has readily increased due to environmen-
tal and long-term shortage issues.[1] Research into alterna-
tive energy sources is currently receiving extensive attention
all around the world. The abundance of energy coming from
the sun makes solar cells a very attractive alternative for the
major sources used today. However, the production cost of
the currently used crystalline silicon-based solar cells is still
[a] K. M. Karlsson, E. Gabrielsson, Prof. L. Sun
Organic Chemistry
School of Chemical Science and Engineering
Royal Institute of Technology (KTH)
10044 Stockholm (Sweden)
Fax : +(46)8-761-2333
E-mail: lichengs@kth.se
[b] Dr. X. Jiang
Inorganic Chemistry
School of Chemical Science and Engineering
Royal Institute of Technology (KTH)
10044 Stockholm (Sweden)
[c] S. K. Eriksson, Prof. A. Hagfeldt
Department of Physical and Analytical Chemistry
Uppsala University
75105 Uppsala (Sweden)
[d] Dr. H. Rensmo
nine,[13]
indoline,[14,15]
perylene,[16]
triphenylamine
Molecular and Condensed Matter Physics
Department of Physics and Astronomy
Uppsala University, Box 516
75120 Uppsala (Sweden)
(TPA),[7,8,17–22] and hetero-anthracene)[23–27] and give efficien-
cies in the range of 5–11%. Recently, TPA-based chromo-
phores have gained great attention due to their potential as
electron donors. We have compared the effect of different
electron donors in DSCs and showed that sensitizers with a
Supporting information (experimental data for the synthesis of the
dyes; dye loadings; and the structure of D5) for this article is avail-
able on the WWW under http://dx.doi.org/10.1002/chem.201003730.
Chem. Eur. J. 2011, 17, 6415 – 6424
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phenothiazine (PTZ) electron-donating unit gave promising
efficiencies, with even better performance than the TPA-
based dyes.[24] This is because the PTZ unit displayed a
stronger electron-donating ability than the TPA unit (0.848
and 1.04 V vs. the normal hydrogen electrode (NHE), re-
spectively).[28] Phenoxazine (POZ) shows similar structural
and electrochemical properties to PTZ (0.880 V vs.
NHE),[28] which implies that POZ could also be incorporat-
ed as a dye sensitizer. Despite this, most of the previous
work has focused on PTZ as the donor.[23,26,29,30] In 2009, we
published two dyes based on POZ, TH301 and TH305,
which gave efficiencies up to 7.7%,[25] and the panchromatic
dye TH304 with absorption into the NIR region.[27] A com-
parison between POZ- and PTZ-based sensitizers showed
that the POZ-based dye TH301 gave an efficiency of 6.2%
and the structurally similar PTZ-based T2-1 showed an effi-
ciency of 5.5% under similar conditions.[25,26]
POZ dyes have been used in other applications, such as
laser dyes,[31] indicators,[32] biological stains,[33] anticancer
agents,[34] hole-transporting materials,[35] and in host–guest
systems.[36]
There are some general considerations to take into ac-
count for the design of a sensitizer for DSCs: 1) strong and
broad light absorption for efficient light harvesting, 2) ap-
propriate energy levels for efficient electron injection and
dye regeneration, and 3) easy and reliable synthesis with
cheap, nontoxic materials suitable for future large-scale pro-
duction.
An important feature of POZ is the easy structural modi-
fication on both sides of the core to tune the properties of
the dye. To further evaluate the role of the POZ unit in sen-
sitizers, a series of dyes were designed (Scheme 1) and syn-
thesized (Schemes 2 and 3). The absorption properties can
be modified by different approaches, which include extend-
ing the conjugation,[21] increasing the electron-donating abili-
ties of the donor,[19,37] or enhancing the electron-withdrawing
abilities of the acceptor.[26,27,38] By extending the conjugation
with a furan ring linker (MP08) the absorption spectrum can
be redshifted.[39,40] The introduction of an additional electron
donor in the 7-position on the POZ (MP03, MP05) will in-
crease the energy levels, mostly affecting the HOMO, which
will be lifted towards a more negative value to generate a
bathochromic shift. The influence of insulating alkyl chains
has been shown to be beneficial for the performance of the
cell due to decreased aggregation, increased solubility, and
surface protection that will depress the recombination of the
injected electron with the oxidized dye or the redox species
in the electrolyte. Therefore, a long hydrophobic chain was
introduced as substituent on the POZ nitrogen atom for
three of the dyes (MP03, MP08, MP13). By changing this
chain for
a hexyloxy-substituted benzene ring (MP05,
MP12), a structure that resembles the TPA structure is ob-
tained. This extra ring could increase the donating proper-
ties of the dye even further and, because it will not be co-
AHCTUNGTREGpNNNU lanar with the POZ core, it could also decrease dye aggre-
gation. 2-Cyanoacrylic acid was used as the acceptor and an-
choring group for all the dyes. The strong electron-donating
property of POZ will not only increase the energy of the
HOMO significantly, but also increase the energy of the
LUMO of the dye. This presents the possibility to increase
the conduction band of TiO2 with additives such as 4-tert-bu-
tylpyridine, to improve the voltage and total efficiency. The
LUMO lifting effect is more relevant if the donor is close to
the acceptor. Photoelectron spectroscopy (PES) and X-ray
absorption spectroscopy (XAS) can, because of the surface
sensitivity of these techniques, be used to study the binding
geometry of the dye to the semiconductor surface and also
to study the energy of the HOMO versus the semiconductor
and unoccupied levels in the dye. In this study the best per-
forming dye (MP05) was chosen to investigate this aspect
further.
To estimate the energy levels and spectral properties of
the dyes, density functional theory (DFT) and time-depen-
dent DFT (TD-DFT) calculations were carried out for all
the molecules prior to the synthesis.
Results and Discussion
Photophysical properties of the dyes: To better understand
the properties of the dyes, quantum chemical calculations
were performed. The optimized geometries of the HOMO,
Scheme 1. Structures of the dye sensitizers.
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Phenoxazine Dyes for Dye-Sensitized Solar Cells
FULL PAPER
Scheme 2. Synthesis of dyes MP03, MP13, and MP08.
Scheme 3. Synthesis of dyes MP05 and MP12.
LUMO, and HOMOꢀ1 (Figure 1) show the dihedral angle
between the aryl substituent on the POZ nitrogen atom and
the plane of the POZ unit to be 908 (MP05, MP12). Because
of this large dihedral angle, caused by steric effects, little
overlap between the two p systems is expected, thus, the
HOMO does not delocalize over the aryl substituent. A
much smaller dihedral angle of about 458 is found between
the POZ core and the substituent in the 7-position (MP03,
Chem. Eur. J. 2011, 17, 6415 – 6424
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L. Sun et al.
&
Figure 2. Calculated gas-phase absorption spectra of the dyes ( =MP03,
~
!
=MP05, =MP08, ꢁ=MP12, N=MP13).
The absorption spectra of the different dyes in CH2Cl2
were measured and are shown in Figure 3; the correspond-
ing experimental absorption data are listed in Table 2. As
can be seen from Scheme 1, the alkyl-chain substituent on
the nitrogen atom in MP03 is replaced by a 4-hexyloxyphen-
yl unit in MP05, with the intention to improve the light har-
vest in the visible region. However, this exchange does not
make a contribution to an improved absorption property.
The absorption spectrum of MP05 is slightly blueshifted
compared to MP03. The same trend can be found in the
comparison between dyes MP12 and MP13. Based on the
above DFT study, the adverse contribution of the aryl unit
is mainly due to its perpendicular relationship to the POZ
core. This gives a poor orbital overlap, which leads
Figure 1. Frontier orbitals of the dyes, generated with an iso value of
0.02.
MP05), hence, delocalization of the HOMO over the sub-
stituent p system is enabled. TD-DFT calculations (Table 1)
show that the first vertical transition for the dyes consists of
82–85% of a HOMO!LUMO excitation. One of the dyes,
MP08, also shows a strong (oscillator strength (f)=0.87) ab-
to inefficient conjugation. Because the side chain
located on the POZ nitrogen atom did not improve
the light harvest, the modification on the 7-position
Table 1. Calculated vertical transitions and energy levels for the different dyes.
Dye
Excited state 1
Excited state 2
Energy levels [V vs. NHE]
of POZ was made to further investigate the effect
of substitution on the absorption property of the
dyes. Compared with MP13, the extinction coeffi-
cient of MP03 is distinctly increased by the intro-
duction of a 7-(2,4-dibutoxyphenyl) unit onto the
POZ and the absorption spectrum of MP03 is shift-
ed to the longer wavelength region. Compared with
absorption oscillator absorption oscillator LUMO HOMO HOMOꢀ1
peak [nm] strength peak [nm] strength
MP03 504
MP05 513
MP08 523
MP12 486
MP13 473
0.41
0.43
0.49
0.35
0.33
332
331
366
326
326
0.26
0.36
0.87
0.31
0.16
ꢀ2.26
ꢀ2.36
ꢀ2.01
ꢀ2.27
ꢀ2.16
0.52
0.37
0.60
0.60
0.78
1.29
1.23
1.56
1.97
2.13
sorption peak around 350 nm. This is expected to give a
higher LHE at short wavelengths compared to the other
dyes. From the calculations, it is also found that dye MP08,
with its extended conjugated system, has an absorption max-
imum at the longest wavelength of the series. Due to the
small overlap between the POZ unit and N-aryl substituent
in MP05 and MP12 there is very little difference in the cal-
culated absorption spectra (Figure 2) compared to the relat-
ed alkyl-substituted chromophores (MP03 and MP13, re-
spectively). There is, however, a small redshift for these N-
aryl-substituted dyes. As discussed above, substitution at the
7-position does not suffer from inefficient overlap of the p
systems and, hence, results in redshifted absorption spectra
of MP03 and MP05 versus MP13 and MP12, respectively.
~
&
Figure 3. Absorption spectra of the dyes in CH2Cl2 ( =MP03, =MP05,
!
=MP08, ꢁ=MP12, N=MP13).
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Phenoxazine Dyes for Dye-Sensitized Solar Cells
FULL PAPER
Table 2. Absorption, emission, and electrochemical properties of the dyes in CH2Cl2.
Due to the electron-donating properties, the intro-
duction of a 2,4-dibutoxyphenyl substituent at the
7-position of the POZ (MP03, MP05) lifts both the
HOMO and LUMO energy levels of the dyes rela-
tive to unsubstituted MP13 and MP12. The shift is
larger for the HOMO, which results in the redshift-
ed absorption illustrated in Figure 3, due to the
smaller energy-level gap. As seen from Table 2, the
excited-state oxidation potentials (LUMO levels) of
these dyes are more negative than the conduction-
band edge (CB) of TiO2 (approximately ꢀ0.5 V vs.
NHE), which means that the electrons could be ef-
ficiently injected into the TiO2 conduction band
from the excited dyes and simultaneously oxidized
dyes are formed. The large difference between the
TiO2 CB and the LUMO of the dye suggests that a
high concentration of additive (e.g. 4-tert-butylpyri-
dine) in the electrolyte could be used to increase
the CB to yield a higher open-circuit photovoltage (Voc). On
the other hand, the ground-state oxidation potentials
(HOMO levels) are more positive than the iodide/tri-iodide
redox potential (0.4 V vs. NHE), which indicates that the
oxidized dyes could be efficiently regenerated by the elec-
trolyte.
[a]
[a]
[b]
[c]
[d]
Dye
Absmax
[nm]
Absmax on
e
Emmax
[nm]
E(S+/S)
[V vs.
NHE]
E(0–0)
[eV]
E(S+/S*)
[V vs.
NHE]
TiO2 film[a] [104 mꢀ1 cmꢀ1
]
[nm]
MP03 506
MP05 500
MP08 518
MP12 486
MP13 491
456
455
448
451
448
2.22
2.03
1.92
1.98
1.88
627
625
626
601
607
0.962
0.967
0.918
1.133
1.090
2.17
2.19
2.16
2.27
2.25
ꢀ1.208
ꢀ1.223
ꢀ1.242
ꢀ1.137
ꢀ1.160
[a] Absorption (Abs) and emission (Em) data of dyes in CH2Cl2. [b] The ground-state
oxidation potential of the dyes was measured under the following conditions: Pt work-
ing electrode and Pt counter electrode; electrolyte, tetrabutylammonium hexafluoro-
phosphate (0.05m) in CH2Cl2. Potentials measured vs. Fc+/Fc were converted to po-
tentials vs. NHE by the addition of +0.63 V. [c] 0–0 transition energy, E(0–0), estimated
from the intercept of the normalized absorption and emission spectra in ethanol.
[d] Estimated LUMO energies (E
estimated highest occupied molecular orbital (HOMO) energies obtained from the
ground-state oxidation potential and the 0–0 transition energy, E(0–0)
ACHTUNGTREN(NUNG LUMO) vs. NHE) from the difference between the
.
MP12, MP05 also shows a redshifted absorption spectrum
and enhanced extinction coefficient due to the introduction
of the 2,4-dibutoxyphenyl unit. Although there is a dihedral
angle between the POZ core and the 2,4-dibutoxyphenyl
substituent, shown in Figure 1, the orbital overlap between
the two moieties is still favorable for the absorption proper-
ties of the dyes. The effect of the addition of an electron-do-
nating substituent in the 7-position of POZ is consistent
with earlier results.[25] In addition, it was found that MP08,
with a furan ring linker, shows the most redshifted and
broadest absorption spectrum of all of the dyes, due to its
efficient conjugation system. All dyes show relatively low
extinction coefficients with values around 20000mꢀ1 cmꢀ1.
When adsorbed onto the TiO2 film, the absorption spectra
(Figure 4, <Table 2) for all dyes are blueshifted compared
to those in solution (Figure 3, <Table 2). This could be an
effect of dye aggregation[41] and/or deprotonation of the car-
boxylic acid groups upon adsorption to the TiO2 surface,
which decreases the strength of the electron acceptor.[42,43]
Figure 5 shows the incident photon-to-current conversion
efficiency (IPCE) as a function of light excitation wave-
length. The onsets of the IPCE spectra increase in the order
MP12<MP13<MP05<MP03<MP08, which is in good
agreement with the absorption spectra for all of the dyes.
Electrochemical study: The electrochemical properties of
the sensitizers are shown in Table 2. The oxidation poten-
tials of these dyes were measured by cyclic voltammetry.
~
!
&
Figure 5. IPCE for POZ-based DSCs ( =MP03, =MP05, =MP08, ꢁ
=MP12, N=MP13).
Photovoltaic performance: The photovoltaic performance
characteristics of DSCs based on the POZ sensitizers, under
standard AM 1.5G illumination (100 mWcmꢀ2) are summar-
ized in Table 3 and the current–voltage characteristics are
shown in Figure 6. One can see from Scheme 1 that MP13 is
the basic structure, and three different kinds of structural
modification on the different positions of POZ core have
been selected to examine the substituent effects on the pho-
tovoltaic performance of DSCs. First, a hexyloxy-substituted
phenyl unit was introduced to replace the long alkyl chain
~
&
Figure 4. Absorption spectra of the dyes on TiO2 film ( =MP03,
=
!
MP05, =MP08, ꢁ=MP12, N=MP13).
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L. Sun et al.
Table 3. Photovoltaic performance of DSCs based on the different dyes.
Dye
Jsc [mAcmꢀ2
]
Voc [V]
Fill Factor
h [%]
MP03
MP05
MP08
MP12
MP13
14.38
13.09
14.22
10.67
12.25
0.78
0.80
0.70
0.77
0.76
0.64
0.70
0.60
0.74
0.72
7.17
7.40
6.03
6.07
6.70
Figure 7. Electron lifetime (Jsc) as a function of open-circuit voltage (Voc)
~
!
&
for DSCs ( =MP03, =MP05, =MP08, +=MP12, N=MP13).
delocalized across the whole donor at the HOMO level.
This increases the conjugation and strengthens the electron-
donating ability of the donor in MP03. It was observed that
the presence of a 2,4-dibutoxyphenyl unit in MP03 shifts the
absorption maximum to the longer wavelength region and
broadens the spectrum (Figure 3). At the same time, the ex-
tinction coefficient was also enhanced. As a result, the pho-
tocurrent of DSCs based on MP03 is increased greatly due
to the advantageous absorption properties compared to
MP13. The same trend can also be found for MP05 com-
pared to MP12.
Figure 6. Current-voltage characteristics of DSCs based on dyes MP03–
MP13 under 100 mWcmꢀ2 AM 1.5G illumination ( =MP03, =MP05,
&
*
~
!
^
=MP08, =MP12, =MP13).
located on the POZ nitrogen atom (MP12). It was expected
that the electron reservoir of the benzene ring would con-
tribute to better conjugation for MP12 and, hence, yield en-
hanced photovoltaic behavior. The photocurrent however,
decreased significantly with an almost unchanged photovolt-
age. This is mainly due to the disadvantageous absorption
properties of MP12 relative to MP13. According to the DFT
study (Figure 1), the phenyl substituent is almost perpendic-
ular to the POZ core, which leads to poor orbital overlap.
As a result the benzene ring cannot be of benefit to the con-
jugated system. It was confirmed that the introduction of
phenyl unit not only blueshifted the absorption spectrum,
but also lowered the extinction coefficient (discussed
above). In addition, the presence of the relatively large hex-
yloxy-phenyl unit decreases the dye loading on the TiO2 sur-
face (see the Supporting Information, Table S1). As a result,
the photocurrent of MP12 dropped greatly due to the ineffi-
cient light harvest (Table 3). The same effect is observed on
comparison of MP05 with MP03.
Because the hexyloxy-phenyl unit on the nitrogen atom
was not in favor of high LHE, a 7-(2,4-dibutoxyphenyl) unit
was introduced onto the POZ moiety (MP03, Scheme 1).
The photovoltage of MP03-based DSCs is slightly higher
than those based on MP13 (Table 3). This is probably due to
the enhanced surface protection caused by the presence of a
large substituent at the end of molecule, which inhibits the
recombination of electrons in TiO2 with oxidized species in
the electrolyte to a certain extent. This can be confirmed by
consideration of electron lifetime as a function of Voc of the
DSC (Figure 7). According to the DFT study (Figure 1), al-
though there is a slight dihedral angle, MP03 still shows a
relatively planar structure. Therefore, the electron can be
The effect of a linker unit on the photovoltaic perfor-
mance of DSCs was also examined. It was found that MP08
shows a higher photocurrent than MP13 due to the extended
conjugation system, which produces a broader, redshifted
absorption spectrum and a higher extinction coefficient.
However, the photovoltage of MP08-based DSCs is much
lower than all the other dyes. As illustrated in Figure 7,
MP08 shows quite a short electron lifetime at a certain volt-
age, which is mainly responsible for the lower Voc. It is
known that a short electron lifetime usually results from
pronounced electron recombination with the electrolyte. It
has been observed in previous studies that iodine has the
tendency to form iodine–electron donor complexes with
atoms that contain lone-pair electrons, such as oxygen, nitro-
gen, and sulfur. Therefore, it is possible for MP08 to form
dye–iodine complexes due to the presence of the furan ring.
This would cause an increased concentration of tri-iodide
near the TiO2 surface and subsequently result in an en-
hanced electron recombination compared to the dyes with-
out a furan ring.[44–46] This problem can be avoided by the
use of alternative redox systems, without iodine. It could
also be an effect of the longer distance between the insulat-
ing alkyl chains and the TiO2 surface, which leads to a less
protected surface and hence a shorter lifetime. The extend-
ed p conjugation can also lead to an increased intermolecu-
lar p–p interaction, which can explain the higher dye load-
ing for MP08 (see the Supporting Information, Table S1)
compared to the other dyes.[41] This extended conjugation
can also enhance the charge recombination and result in a
lower Voc.[47] These results are consistent with those pub-
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Phenoxazine Dyes for Dye-Sensitized Solar Cells
FULL PAPER
lished for PTZ-based dyes.[23,26] The fairly low currents mea-
sured for all of the dyes are probably a result of absorption
at short wavelengths, which affects the LHE of the solar
cell. For better LHE, the absorption properties of the dyes
need to be improved; a broader, redshifted spectrum and
also an increase in extinction coefficients are desirable. With
Jsc =13.09 mAcmꢀ2 and Voc =0.80 V, together with a fill
factor of 0.70, MP05 is the best dye in the series, with a
solar energy-to-electricity conversion efficiency of 7.4%. As
a comparison, previously reported dye TH305 showed 7.3%
efficiency under the same working conditions.[48]
closer to the TiO2 surface than the POZ group. The ratios
are similar to those of adsorbed dye D5 (see the Supporting
Information for the structure of D5)[49,50] and, therefore, in-
dicate that, on average, the molecules are aligned on the
surface with the POZ group pointing out, as illustrated in
Figure 9.
PES and XAS: The performance of the DSC largely de-
pends on the electronic and molecular surface structure.
PES techniques are well-suited to study such characteris-
tics.[49–51] PES and XAS were, therefore, specifically used in
the present study to investigate the surface structure of the
most efficient interface, MP05/TiO2.
Figure 9. Illustration of MP05 standing on the TiO2 surface. Nitrogen
atoms are highlighted in blue.
Figure 8 shows the N1s spectra of MP05, measured with
758 and 535 eV photon energy. Each asymmetric spectrum
can be fitted with two components that originate from the
nitrogen atom in the cyano (CN) group (398.8 eV) and the
nitrogen atom in the POZ group (400.0 eV), respectively.[49]
When the intensities from the different peaks are compared,
the ratios between the intensity of the peaks that correspond
to the CN and POZ nitrogen atoms are found to be 0.31
and 0.46 at photon energies of 535 and 758 eV, respectively.
Because the surface sensitivity is strongly dependent on the
kinetic energy of the emitted electrons, the spectrum mea-
sured at 535 eV is more surface sensitive than the spectrum
measured at 758 eV.[49,51] This indicates that the CN group is
In Figure 10 (left), the valence electronic spectrum for
MP05 on TiO2 is shown. The contribution from the TiO2 va-
lence electronic structure is mainly located at binding ener-
gies higher than 3 eV and the spectral features observed
below 3 eV are totally dominated by contributions from the
dye.[50] In this outermost valence structure region, a peak
positioned at 1.4 eV binding energy is observed, which origi-
nates from the HOMO-level of the dye adsorbed to TiO2.
The HOMO peak measured for D5 in our earlier report is
positioned at 1.6 eV and the value for N3 is about the same
as for MP05.[50,51] Although the theoretical modelling dis-
Figure 8. N1s spectra measured with different photon energies. The small-
er feature is from the CN group, the larger from the POZ group. When
PES is used, the mean free path of the emitted electrons depends on the
kinetic energy of the electrons. Therefore, measurements with lower
photon energy will be more surface sensitive compared to measurements
with higher photon energy.
Figure 10. Left: Part of the valence spectrum of MP05/TiO2. The HOMO
level is found at 1.4 eV binding energy. Right: N1s-XAS spectrum, in
which a large resonance is seen at 399.9 eV photon energy.
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L. Sun et al.
cussed above suggests a single HOMO at an energy 0.86 eV
from the HOMOꢀ1 energy level, a closer look at the PES
measurements shows a broad and asymmetric feature. One
possible explanation is that there are surface contributions
from dye molecules with different surface structures, for ex-
ample, the angle between the aryl substituent on the POZ
and the plane of the POZ differ between the molecules.
Figure 10 (right) shows the N1s XAS spectrum of MP05
on TiO2. Two sharp resonances can be seen at 398.7 and
399.9 eV. These resonances originate from the CN unit and
the structures at higher energy are from the POZ unit.[50]
The XAS spectra can be used to shed some light on the
nature of the excitation; this is of importance in UV-Vis ab-
sorption. There are large differences in the electronic relax-
ation that determines the valence- and core-excitation ener-
gies. When an electron is emitted, a core-hole potential is
created and the relaxation of the occupied orbitals will,
therefore, be different. When positioning the XAS spectrum
by subtracting the energy of the corresponding core level
this contribution partly cancels out. By using this procedure,
the energy position of the valence- and core-excitations will
mainly differ in how the excited electrons couple to the va-
lence- or core-hole, respectively. The two unoccupied states
with CN character were found 1.5 and 2.7 eV, respectively,
from the HOMO peak. The UV-Vis absorption shows a
maximum peak at 2.72 eV, which is in agreement with the
XAS data. One can compare data from other dyes, in this
case D5, to see relative differences. The unoccupied state at
the CN moiety in D5 is located 2.8 eV from the HOMO
level and the corresponding absorption peak in the UV-Vis
spectrum is found at 2.6 eV. These data are measured on dry
films (both D5 and MP05) but the electrochemistry is per-
formed in solution, in which other effects have to be consid-
ered. However, the measurements support the theoretical
conclusion that the absorption transition at 2.72 eV in MP05
is a charge-transfer excitation with the accepting orbital
partly located at the CN moiety.
Experimental Section
Synthesis: The dyes were synthesized as shown in Schemes 2 and 3, from
a commercially available phenoxazine precursor. The N-alkylation to
give 1 was performed in acetone with 1-bromo-octane, NaOH, and palmi-
tyl trimethyl ammonium bromide as a phase-transfer catalyst. For the 4-
hexyloxyphenyl-substituted phenoxazines, MP12 and MP05, the initial ar-
ylation to give 7 followed a procedure reported by Buchwald and co-
workers.[52,53] Formylation of 1 and 7 was accomplished by the Vilsmey-
er–Haack reaction. Higher yields of 4 and 8 were obtained by heating
the reaction mixture to reflux in CHCl3 overnight, compared to heating
in pure dimethylformamide (DMF).[25] Bromination of 1, 4, and 8 with N-
bromosuccinimide (NBS) provided the coupling reaction precursors 2, 5,
and 9 in good yield, however, for the unformylated phenoxazine 2 a mix-
ture of mono- and dibrominated products was formed. It was possible to
separate the products by chromatography, but the yield of the desired
compound 2 was lower than desired. The Suzuki reactions[54] of 2, 5, and
9 were carried out with unprotected 5-formyl-2-furanboronic acid and
2,4-dibutoxyphenylboronic acid by using palladium bis(diphenylphosphi-
no)ferrocene (PdACHTNURGTNEUNG(dppf)Cl2) as a catalyst. The final step in the synthesis
for each dye was Knoevenagel condensation of the respective aldehydes
(3, 4, 6, 8, or 10) with 2-cyanoacetic acid, in the presence of piperidine.
All of the dyes and intermediate compounds were characterized by
NMR spectroscopy. The dyes were also characterized by HRMS. Full ex-
perimental data is provided in the Supporting Information.
General procedure for the preparation of solar cells: Fluorine-doped tin
oxide (FTO) glass plates (Pilkington-TEC15) were cleaned sequentially
with detergent solution, water, and ethanol in an ultrasonic bath over-
night. The conducting glass substrates were immersed into aqueous TiCl4
solution (40 mm) at 708C for 30 min and washed with water and ethanol.
The screen-printing procedure was repeated (layers ꢁ4 mm) with TiO2
paste (ꢁ18 nm colloidal particles, Dyesol LTD.) to obtain a transparent
nanocrystalline film (thickness ꢁ12 mm; area=0.25 cm2). A scattering
layer (ꢁ3 mm, PST-400C, JGC Catalysts and Chemicals LTD) was depos-
ited and a final thickness of 15.7ꢂ0.5 mm was attained. The TiO2 electro-
des were gradually heated in an oven (Nabertherm Controller P320) fol-
lowing
a temperature gradient program through four levels: 1808C
(10 min), 3208C (10 min), 3908C (10 min), and 5008C (60 min). After sin-
tering, the electrodes once again passed a post-TiCl4 treatment, as de-
scribed above. A final sintering, at 5008C for 30 min, was performed.
When the temperature had decreased to 708C, the electrodes were im-
mersed into a solution of the dye (0.2 mm) and coadsorbent, chenodeoxy-
cholic acid, (6 mm) in EtOH (99.5%). The electrodes were left in solu-
tion for 18–19 h, in the dark, at room temperature. After adsorption of
the dyes, the electrode was rinsed with EtOH. The electrodes were as-
sembled with a platinized counter electrode by using a hot-melt Surlyn
film. The redox electrolyte, which consisted of I2 (99.9%, 0.04m), LiI
(99.9 %, 0.06m), 4-tert-butylpyridine (99%, 0.4m), and 3-hexyl-1,2-dime-
thylimidazolium iodide (98%, 0.6m) in acetonitrile, was introduced
through a hole drilled in the back of the counter electrode. Finally, the
hole was sealed with the Surlyn film.
Conclusion
A series of five metal-free organic dyes, based on the phe-
noxazine structure, have been synthesized. By using stan-
dard conditions, the solar cells based on these sensitizers
with liquid electrolyte gave overall conversion efficiencies of
6.03–7.40%. The difference in performance could be ex-
plained by the different absorption properties and electron
lifetimes for the cells. The power-conversion efficiency in-
creased with increasing electron lifetime, which yielded
higher Voc values. From data obtained by PES, it can be con-
cluded that the excitation in MP05 (studied with UV-Vis ab-
sorption) is of mainly CN character. Additionally, the dye
molecules are, on average, standing on, and pointing out,
from the TiO2 surface.
Photocurrent density-voltage (J-V) measurements: The prepared solar
cells were characterized by current-voltage characteristics and incident
photon-to-current conversion efficiency (IPCE). Current-voltage charac-
teristics were carried out with an AM 1.5G solar simulator, 300 W xenon
(ozone free). The incident light intensity was 1000 Wmꢀ2 calibrated with
a standard Si solar cell. For the J-V curves, the solar cells were evaluated
by using a black mask on the cell surface to avoid diffusive light (cell
area=0.25 cm2, aperture area=0.49 cm2).[55] IPCE measurements were
carried out with a computerized set-up, which consisted of a xenon arc
lamp (300 W Cermax, ILC Technology), followed by a 1/8 m monochro-
mator (CVI Digikrom CM 110). The data collection was performed with
a Keithley 2400 source/meter and a Newport 1830-C power meter with
818-UV detector head.
Photophysical measurements: The UV-Vis absorption spectra of the dye-
loaded transparent film and the dye solution (1ꢁ10ꢀ5 m in CH2Cl2) were
recorded on a Lambda 750 spectrophotometer by using a normal quartz
sample cell (1 cm path length). The fluorescence spectra of the dye solu-
6422
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6415 – 6424

Phenoxazine Dyes for Dye-Sensitized Solar Cells
FULL PAPER
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the same concentration as the UV-Vis measurement.
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ACHTUNGTNER(NUNG Fc/
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We acknowledge the Swedish Research Council, Swedish Energy Agency
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