Efficient solar cells sensitized by porphyrins with an extended conjugation framework and a carbazole donor: From molecular design to cosensitization

Article abstract of DOI:10.1002/anie.201406190

Porphyrin dyes containing the carbazole electron donor have been designed and optimized by wrapping the porphyrin framework, introducing an additional ethynylene bridge to extend the wavelength range of light absorption, and further suppression of the dye aggregation by introducing additional alkoxy chains. Application of a cosensitization approach results in improved current density (J_sc) and open-circuit voltage (V_oc) values, thus achieving the highest cell efficiency of 10.45 %. This work provides an effective combined strategy of molecular design and cosensitization for developing efficient dye-sensitized solar cells (DSSCs). In addition, carbazole has been demonstrated to be a promising donor for porphyrin sensitizers. All wrapped up: Porphyrin dyes containing the carbazole electron donor have been designed and optimized by wrapping the porphyrin framework, using an ethynylene bridge, and aggregation suppression by introducing additional alkoxy chains. Using a cosensitizer, the highest cell efficiency of 10.45 % is achieved. This work provides an effective combined strategy of molecular design and cosensitization for developing efficient DSSCs.

Full text of DOI:10.1002/anie.201406190

Angewandte
Chemie
DOI: 10.1002/anie.201406190
Solar Cells
Efficient Solar Cells Sensitized by Porphyrins with an Extended
Conjugation Framework and a Carbazole Donor: From Molecular
Design to Cosensitization**
Yueqiang Wang, Bin Chen, Wenjun Wu, Xin Li, Weihong Zhu, He Tian, and Yongshu Xie*
Abstract: Porphyrin dyes containing the carbazole electron
donor have been designed and optimized by wrapping the
porphyrin framework, introducing an additional ethynylene
bridge to extend the wavelength range of light absorption, and
further suppression of the dye aggregation by introducing
additional alkoxy chains. Application of a cosensitization
approach results in improved current density (Jsc) and open-
circuit voltage (Voc) values, thus achieving the highest cell
efficiency of 10.45%. This work provides an effective com-
bined strategy of molecular design and cosensitization for
developing efficient dye-sensitized solar cells (DSSCs). In
addition, carbazole has been demonstrated to be a promising
donor for porphyrin sensitizers.
diazole bridge between the ethynyl and benzoic acid moieties,
and using a cobalt-based electrolyte.[7]
In the design of efficient dyes, one of the most commonly
encountered problems is the relatively weak absorption of
light in the near-IR region.[8] To overcome this problem, an
additional ethynylene bridge may be introduced to expand
the p-conjugated framework and enhance the absorption.
However, one adverse effect of this approach is to induce
severe dye aggregation, which may lower the cell efficiency.
Accordingly, in most cases, the sensitizers incorporating an
additional ethynylene bridge show decreased cell efficien-
cies.[9,10] These observations have been a serious obstacle for
designing efficient sensitizers, which was also encountered in
our previous work. For example, porphyrin derivative Q1 has
an additional ethynylene bridge compared to Q2 (Scheme 1).
Thus the onset wavelength of its absorption was extended
from l = 655 nm to 695 nm, but the cell efficiency was
decreased from 5.51% to 2.22%.[11] For porphyrin sensitizers
containing a carbazole donor,[11,12] the highest efficiency is
5.74%,[12f] which is slightly higher than that for Q2, but much
lower than the value of 12.5% reported for the non-porphyrin
carbazole-containing dye ADEKA-1.[13] This large difference
prompted us to develop efficient porphyrin sensitizers with
a carbazole donor through molecular design with focus on
extending the p-conjugated framework in combination with
multiple anti-aggregation approaches. In addition, as por-
phyrins usually absorb weakly around l = 500 nm, we further
utilize a cosensitization approach[14] to enhance the efficien-
cies by improving the absorption in this wavelength region.
Based on these considerations and the molecular structure
of Q2, four long alkoxy chains were introduced at the
ortho positions of the meso-phenyl substituents to wrap the
porphyrin framework[6] and suppress dye aggregation. Por-
phyrin sensitizer XW1 (Scheme 1) demonstrated a cell effi-
ciency of 7.13%, significantly higher than that of Q2. An
ethynylene bridge was then inserted between the carbazole
donor and the porphyrin framework, forming molecule XW2,
to extend the absorption wavelength range, but the cell
efficiency decreased to 6.84%. To suppress the adverse dye
aggregation associated with the ethynylene bridge, methoxy
or hexyloxy groups were introduced onto the phenylene
group adjacent to the donor. Cell efficiencies improved, with
values of 7.32% and 7.94% achieved for XW3 and XW4,
respectively. Cosensitizer C1 (see Figure 1, inset, for molec-
ular structure) was next employed, whose absorption peak
lies at approximately l = 500 nm. Thus, the cosensitization of
XW4 and C1 affords the highest efficiency of 10.45%. These
results provide a molecular design strategy involving an
extension of the p-conjugated framework together with
D
ye-sensitized solar cells (DSSCs) have been widely inves-
tigated because of their low production cost, ease of
fabrication, and relatively high solar energy conversion
efficiencies.[1] In recent years, significant efforts have been
devoted to improving cell efficiencies.[2] In this respect, many
efficient sensitizers with donor–p-acceptor frameworks have
been developed.[3] Among them, porphyrins have received
increasing attention because of their strongly absorbing Soret
bands (l = 400–450 nm) and moderately intense Q bands (l =
550–650 nm) which cover the visible to the near-IR region of
the electromagnetic spectrum.[4] Additionally, their chemical
structures can be systematically modified to understand the
effect of structural properties on cell efficiencies.[2c,5] Until
recently, highly efficient porphyrin sensitizers have typically
been functionalized with two bis(ortho-alkoxy)-wrapped
meso-phenyl groups for reducing dye aggregation, and an
ethynyl benzoic acid group as the acceptor, a strategy which
was pioneered and developed by Diau and Yeh et al.[5,6] This
year, the highest DSSC efficiencies of 13.0% and 12.75%
have been reported by Grꢀtzel and co-workers for porphyrin
dyes with octyloxy-wrapped structures, utilizing a benzothia-
[*] Y. Q. Wang, B. Chen, Dr. W. J. Wu, Dr. X. Li, Prof. Dr. W. H. Zhu,
Prof. Dr. H. Tian, Prof. Dr. Y. S. Xie
Key Laboratory for Advanced Materials
and Institute of Fine Chemicals
East China University of Science and Technology
Shanghai 200237 (P. R. China)
E-mail: yshxie@ecust.edu.cn
[**] This work was supported by NSFC/China (21072060, 91227201),
National Basic Research 973 Program (2013CB733700), the
Oriental Scholarship (NCET-11-0638), and the Fundamental
Research Funds for the Central Universities (WK1013002).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406190.
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imately l = 500 nm, which compensates well the
poor absorption of the porphyrin dyes in this
region and indicates that C1 may be a good
cosensitizer for the porphyrin dyes. When XW1–
XW4 were adsorbed onto the TiO2 films, the
spectra were significantly broadened (see Fig-
ure S43), which is favorable for light harvesting.
Cyclic voltammetry (Figure S44) was mea-
sured to evaluate the possibility of electron trans-
fer from the excited dye molecules to the con-
duction band of TiO2, and the corresponding data
are summarized in Table S2. For porphyrins
XW1–XW4, similar HOMO levels of 0.87, 0.85,
0.82, and 0.83 V, respectively, were calculated. The
LUMO levels of XW1–XW4 were estimated to be
À1.09, À1.02, À1.04, and À1.03 V, respectively.
The fact that the HOMO levels are comparable
can be attributed to the similar electron-donating
ability of the donors in the four dyes. Compared
with XW1, the additional ethynylene bridges in
XW2–XW4 caused a decrease in the HOMO–
LUMO energy gaps, resulting in a red shift of the
absorption bands, and relatively positive LUMO
levels. The LUMO levels of all these dyes lie
above the conduction band of TiO2 (À0.5 V versus
NHE), and the HOMO levels lie well below the
iodide/triiodide redox potential value (0.4 V
versus NHE; see Figure S45). These results indi-
cate that the electron injection and dye regener-
Scheme 1. Molecular structures of XW1–XW4 and previously reported molecules Q1
and Q2.
multiple suppression of dye aggregation. The combination of
the optimized dye with a cosensitization approach is effective
for developing efficient DSSCs.
The carbazole donor moiety was attached at the meso-
position of the porphyrin unit by Suzuki or Sonogashira cross-
coupling reactions (see the Supporting Information for
synthetic details). The acceptor moiety was then introduced
by a Sonogashira cross-coupling reaction as the correspond-
ing acid ester. Finally, the target sensitizers were obtained
through hydrolysis of the esters. The compounds were
characterized using 1H NMR and 13C NMR spectroscopy
and HRMS (Figures S1–S42 in the Supporting Information).
The UV/Vis absorption spectra of the dyes in THF are
shown in Figure 1 and the corresponding data are listed in
Table S1 (Supporting Information). The absorption spectra of
the porphyrin dyes have features which are typical of
porphyrins, with an intense Soret band in the range of l =
400–500 nm and less intense Q bands in the range l = 550–
700 nm. The high molar absorption coefficients fulfil the
requirement for a DSSC sensitizer. Compared with XW1, the
additional ethynylene bridges in derivatives XW2–XW4 were
observed to induce an obvious red shift of both the Soret and
the Q bands. The additional alkoxy groups in XW3 and XW4
induced a slight red shift of approximately 4 nm relative to
XW2. All of the porphyrin dyes demonstrate very weak
absorption between l = 480 and 550 nm. In contrast, cosensi-
tizer C1 demonstrates a broad absorption peak at approx-
Figure 1. UV/Vis and near-IR absorption spectra of dyes XW1–XW4
and cosensitizer C1 in THF. Inset: molecular structure of C1.
ation processes are thermodynamically feasible for each of
these dyes.[15]
Density functional theory (DFT) calculations were
employed to understand the electron-density distributions
of the frontier orbitals of the dyes. The corresponding
molecular orbital profiles are shown in Figure S46. As
expected, the electron densities of the HOMO orbitals of
the porphyrin dyes are mainly delocalized between the donor
and the porphyrin framework, whereas the LUMO orbitals
are predominantly delocalized over the anchoring group and
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the porphyrin framework. Thus, the electron transfer from the
HOMO to the LUMO can easily result in electron redistrib-
ution from the donor to the anchoring moiety. Therefore, the
electron injection from the LUMO to the conduction band of
TiO2 is theoretically feasible. Consequently, we fabricated
DSSCs using these dyes as the sensitizers.
Table 1: Photovoltaic parameters of the DSSCs under AM1.5 G illumi-
nation (power=100 mWcmÀ2).
Dyes
J
sc [mAcmÀ2
]
Voc [mV]
FF
PCE [%]
Q1[11]
5.43
570
680
716
680
694
702
780
746
697
705
736
806
0.72
0.71
0.66
0.64
0.68
0.70
0.65
0.71
0.70
0.70
0.71
0.68
2.22
5.51
7.13
6.84
7.32
7.94
5.67
9.24
8.96
9.05
10.45
9.63
Q2[11]
11.30
14.99
15.73
15.60
16.22
11.21
17.53
18.22
18.42
20.15
17.65
XW1
XW2
XW3
XW4
The current-density–voltage (J–V) curves (Figure 2a) of
the DSSCs based on the individual porphyrin dyes were
measured
under
simulated
AM1.5 G
irradiation
(100 mWcmÀ2), and the photovoltaic parameters are sum-
C1
XW1 + C1[a]
XW2 + C1[a]
XW3 + C1[a]
XW4 + C1[a]
N719
[a] The cosensitization was carried out using an optimized stepwise
approach: the TiO2 electrode was immersed in a solution of the
porphyrin (0.2 mm) in a mixture of toluene and ethanol (1:4 v/v) for
10 hours and then immersed in a solution of C1 (0.3 mm) in a mixture of
chloroform and ethanol (3:7 v/v) for 1.5 hours. The cosensitized films
were then assembled into DSSC devices. FF=fill factor.
and XW4, respectively. The corresponding PCE values were
successively improved to 7.32% and 7.94% for XW3 and
XW4, respectively. These values are higher than that of XW1,
indicating that efficient sensitizers can be designed by
combining an additional ethynylene bridge with extra
alkoxy chains. By this approach, the adverse dye aggregation
effect which accompanies the extension of the p-conjugated
framework can be suppressed, and thus the cell efficiency may
be improved.
Based on the efficiency data, we continued to investigate
the effect of the molecular structural changes on the cell
current characteristics to gain more information for further
improvement of cell efficiencies. Figure 2b shows the incident
photon-to-current conversion efficiency (IPCE) as a function
of wavelength. All the porphyrin dyes show a broad peak
around l = 450 nm and two well-separated peaks between l =
550 and 700 nm, corresponding to the Soret and Q bands,
respectively. Compared with XW1, the ethynylene bridge and
the alkoxy groups in XW2–XW4 red shifted the onset
wavelengths of photocurrent response from l = 700 nm to
l = 750–820 nm, resulting in the current density increase from
14.99 to 15.60–16.22 mAcmÀ2 (Table 1).
The IPCE curves for XW1–XW4 exhibit low values
around l = 500 nm, with the lowest value lying between 33%
and 44%, which may be as a result of the weak absorption of
the porphyrin dyes around l = 500 nm. These observations
may be unfavorable for improving the cell efficiencies. In
contrast, C1 shows an intense absorption band in this spectral
region (Figure 2b), indicating that C1 may be an ideal
cosensitizer to compensate the absorption of the porphyrin
dyes in this spectral region. Thus, more DSSCs were
fabricated by employing the cosensitization approach. The
photovoltaic behavior and corresponding parameters are
shown in Figure 3 and Table 1. As expected, the lower
IPCE values at approximately l = 500 nm increased signifi-
cantly, with the values remaining higher than 58% in this
region (Figure 3b). As a result, the cosensitized devices show
Figure 2. a) Current-density–voltage (J–V) curves and b) IPCE action
spectra for the DSSCs based on XW1–XW4 and C1.
marized in Table 1. The solar-to-electric power conversion
efficiencies (PCE) for dyes XW1–XW4 lie in the range of
6.84–7.94%, which are significantly higher than the values of
2.22% and 5.51% measured for Q1 and Q2, respectively.
These results may be attributed to the effective suppression of
dye aggregation by the dodecoxy chains at the ortho positions
of the meso-phenyl substituents. As a result, XW1 exhibits
a PCE of 7.13%. Upon introduction of an additional
ethynylene bridge, the PCE falls to 6.84% for XW2, despite
the fact its onset of absorption occurs at l = 698 nm, at much
longer wavelength than for XW1 (l = 664 nm). The decreased
efficiency measured for XW2 may be related to the more
severe dye aggregation caused by the extended p-conjugated
framework.[9] To further suppress the dye aggregation and
thus improve the cell efficiency, two additional methoxy and
hexyloxy chains were introduced to afford derivatives XW3
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Information) shows that the range of Cm values lie in the order
of XW2 < XW1 < XW3 ꢀ XW4, indicating a sequential pos-
itive shift of the conduction band edge.[7a,16b,17] At a given
voltage bias, the fitted RCT and t values lie in the order of
XW2 < XW3 < XW4 < XW1 (Figures S47b,c), showing the
same order of decreasing charge recombination rate and
increasing electron lifetime.[17] The cosensitized DSSCs also
demonstrated the same trend (Figure S48), which is consistent
with the Voc values measured for the cosensitized DSSCs. As
compared with the other three dyes, the most negative TiO2
conduction band edge of XW2 tends to increase the
Voc value.[7a,16b,17] On the contrary, the shortest electron life-
time tends to decrease the Voc value. As a result of the
contradictory effects, derivative XW2 demonstrates the
lowest Voc value of 680 mV, indicating that the Voc value is
predominantly determined by the charge recombination
process, rather than the position of the TiO2 conduction band.
In addition to high cell efficiencies, the photostability of
sensitizers is an important parameter for the practical
application of DSSCs.[18] To investigate photostabilities, the
dyes adsorbed on nanocrystalline TiO2 films were illuminated
with simulated solar light without redox mediators for
30 minutes (Figure S49). The absorption of all the dyes was
found to be only slightly decreased relative to the initial value,
indicating that all four porphyrin dyes have satisfactory
photostability.
Our results clearly show that the wrapping at the
ortho positions of the meso-substituents is effective for
improving the Voc and Jsc values, as well as the cell efficiencies.
Thus, the cells based on XW1–XW4 showed significantly
improved efficiencies (6.84–7.94%) compared with those of
Q1 and Q2 (2.22% and 5.51%). Compared with XW1, the
ethynylene bridge in XW2 is favorable to induce an extension
of the absorption spectral range to a longer wavelength.
Consequently, the Jsc values are improved from 14.99 to
15.73 mAcmÀ2, but the Voc values decrease from 716 to
680 mV. This effect is a result of the enhanced charge
recombination associated with the more severe dye aggrega-
tion in XW2 caused by its larger conjugated framework, thus
resulting in a decrease of the cell efficiency from 7.13% to
6.84%. The methoxy and hexyloxy groups in XW3 and XW4
are favorable for suppressing the adverse dye-aggregation
effect, giving rise to improved Voc values of 694 and 702 mV,
respectively. Compound XW4 demonstrated the highest cell
efficiency, with a value of 7.94%. Further application of the
cosensitizer C1 with XW1–XW4 significantly improved the
Jsc values to 17.53–20.15 mAcmÀ2 and slightly improved the
Voc values to 705–746 mV. Finally, the cell efficiencies were
improved to 8.96–10.45%, showing the best performance for
carbazole-based porphyrin-sensitized solar cells.
Figure 3. a) Current-density–voltage (J–V) curves and b) IPCE action
spectra for the cosensitized DSSCs.
enhanced current density values (Jsc) compared with the
corresponding individual porphyrin-sensitized devices. The
onset wavelengths of photocurrent response for the cosensi-
tized cells containing XW2–XW4 vary in the range of l = 750–
820 nm, which is significant longer than that of l = 720 nm
observed for XW1 because of the presence of the additional
ethynylene bridge. In agreement with these observations, the
Jsc values for the cosensitized cells of XW2–XW4 lie in the
range of 18.22–20.15 mAcmÀ2, larger than the corresponding
value for XW1 (17.53 mAcmÀ2).
For most cosensitization systems,[16] the open-circuit
voltage (Voc) is intermediate in value between those of the
individual single-dye-sensitized cells. The relatively high
Voc value of 780 mV observed for the cells of C1 resulted in
enhanced Voc values of 697–746 mV for the cosensitized
DSSCs compared to those of the corresponding individual
porphyrin-sensitized devices (Voc values in the range 680–
716 mV). In addition, high PCE values of 8.96–10.45% were
achieved for the cosensitization of XW1–XW4 with C1. It is
clear that this cosensitization approach results in enhanced Jsc,
Voc, and efficiencies for cells employing the porphyrin dyes.
To investigate the effect of the TiO2 conduction band shift
and the electron transport and charge recombination pro-
cesses on Voc, electrochemical impedance spectroscopy (EIS)
measurements were performed as a function of potential bias.
By fitting the EIS spectra, the chemical capacitance (Cm),
interfacial charge-transfer resistance (RCT), and the electron
lifetime (t) can be determined.[17] Figure S47a (Supporting
In summary, efficient porphyrin sensitizers have been
developed by using a carbazole electron donor, and molecular
design strategies have been systematically employed to
optimize the dye structures by wrapping the porphyrin
framework with four long alkoxy chains, introducing an
ethynylene bridge, and further suppression of dye aggregation
by employing additional alkoxy chains. Finally, a non-por-
phyrin dye C1, with a high Voc value and strong absorption at
approximately l = 500 nm, was used as the cosensitizer. This
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[8] a) K. Lim, M. J. Ju, J. Song, I. T. Choi, K. Do, H. Choi, K. Song,
cosensitization approach significantly enhanced the Jsc value
and slightly enhanced the Voc. Thus, the highest cell efficiency
of 10.45% was achieved. This value is higher than that
obtained for the ruthenium dye N719[19] (Table 1), and
significantly higher than the best efficiency of 5.74% pre-
viously reported for porphyrin dyes containing a carbazole
donor.[12f] In conclusion, this work provides an effective
combined strategy of molecular design and cosensitization for
developing efficient DSSCs. We have also demonstrated for
the first time that carbazole is a promising donor moiety for
developing efficient porphyrin sensitizers. Considering the
structural characteristics and the energy-level distribution, it
is anticipated that cell efficiencies may be further improved
by using a benzothiadiazole-bridged acceptor group as well as
the Co3+/Co2+ redox electrolyte,[7] which is under investiga-
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Published online: August 11, 2014
Keywords: dyes/pigments · energy conversion · porphyrins ·
.
sensitizers · solar cells
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