N -annulated perylene as an efficient electron donor for porphyrin-based dyes: Enhanced light-harvesting ability and high-efficiency Co(II/III)-based dye-sensitized solar cells

Article abstract of DOI:10.1021/ja409291g

Porphyrin-based dyes recently have become good candidates for dye-sensitized solar cells (DSCs). However, the bottleneck is how to further improve their light-harvesting ability. In this work, N-annulated perylene (NP) was used to functionalize the Zn-porphyrin, and four push-pull -type NP-substituted and fused porphyrin dyes with intense absorption in the visible and even in the near-infrared (NIR) region were synthesized. Co(II/III)-based DSC device characterizations revealed that dyes WW-5 and WW-6, in which an ethynylene spacer is incorporated between the NP and porphyrin core, showed pantochromatic photon-to-current conversion efficiency action spectra in the visible and NIR region, with a further red-shift of about 90 and 60 nm, respectively, compared to the benchmark molecule YD2-o-C8. As a result, the short-circuit current density was largely increased, and the devices displayed power conversion efficiencies as high as 10.3% and 10.5%, respectively, which is comparable to that of the YD2-o-C8 cell (η = 10.5%) under the same conditions. On the other hand, the dye WW-3 in which the NP unit is directly attached to the porphyrin core showed a moderate power conversion efficiency (η = 5.6%) due to the inefficient π-conjugation, and the NP-fused dye WW-4 exhibited even poorer performance due to its low-lying LUMO energy level and nondisjointed HOMO/LUMO profile. Our detailed physical measurements (optical and electrochemical), density functional theory calculations, and photovoltaic characterizations disclosed that the energy level alignment, the molecular orbital profile, and dye aggregation all played very important roles on the interface electron transfer and charge recombination kinetics.

Full text of DOI:10.1021/ja409291g

Article
pubs.acs.org/JACS
N‑Annulated Perylene as An Efficient Electron Donor for Porphyrin-
Based Dyes: Enhanced Light-Harvesting Ability and High-Efficiency
Co(II/III)-Based Dye-Sensitized Solar Cells
Jie Luo,† Mingfei Xu,‡ Renzhi Li,‡ Kuo-Wei Huang,§ Changyun Jiang,† Qingbiao Qi,∥ Wangdong Zeng,∥
Jie Zhang,† Chunyan Chi,∥ Peng Wang,*,‡ and Jishan Wu*,†,∥
†Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore, 117602
‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P.R. China
§Division of Physical and Life Sciences and Engineering and KAUST Catalysis Center, King Abdullah University of Science and
Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
∥Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
S
* Supporting Information
ABSTRACT: Porphyrin-based dyes recently have become good candidates for dye-sensitized solar cells (DSCs). However, the
bottleneck is how to further improve their light-harvesting ability. In this work, N-annulated perylene (NP) was used to
functionalize the Zn-porphyrin, and four “push−pull”-type NP-substituted and fused porphyrin dyes with intense absorption in
the visible and even in the near-infrared (NIR) region were synthesized. Co(II/III)-based DSC device characterizations revealed
that dyes WW-5 and WW-6, in which an ethynylene spacer is incorporated between the NP and porphyrin core, showed
pantochromatic photon-to-current conversion efficiency action spectra in the visible and NIR region, with a further red-shift of
about 90 and 60 nm, respectively, compared to the benchmark molecule YD2-o-C8. As a result, the short-circuit current density
was largely increased, and the devices displayed power conversion efficiencies as high as 10.3% and 10.5%, respectively, which is
comparable to that of the YD2-o-C8 cell (η = 10.5%) under the same conditions. On the other hand, the dye WW-3 in which the
NP unit is directly attached to the porphyrin core showed a moderate power conversion efficiency (η = 5.6%) due to the
inefficient π-conjugation, and the NP-fused dye WW-4 exhibited even poorer performance due to its low-lying LUMO energy
level and nondisjointed HOMO/LUMO profile. Our detailed physical measurements (optical and electrochemical), density
functional theory calculations, and photovoltaic characterizations disclosed that the energy level alignment, the molecular orbital
profile, and dye aggregation all played very important roles on the interface electron transfer and charge recombination kinetics.
strategy, researchers have focused on the functionalization of
the β position3 or meso position4 of the porphyrin core to
generate dyes with appropriate energy levels and strong light-
harvesting capabilities in the visible range. Recent development
of a zinc porphyrin sensitizer (YD2-o-C8, Figure 1) co-
sensitized with an organic dye (Y123) using a cobalt-based
electrolyte has attained a power conversion efficiency (η) of
1. INTRODUCTION
Dye-sensitized solar cells (DSCs) have received considerable
attention due to their high power-conversion efficiencies, ease
of fabrication, and low production costs.1 Among the various
synthetic dyes, porphyrins have been considered to be
promising candidates for DSCs because of their superior
light-harvesting ability in the visible region and easy chemical
tuning of their physical properties.2 The key to achieving highly
efficient DSCs lies in the design and synthesis of stable organic
dyes with appropriate push−pull structure. On the basis of this
Received: September 8, 2013
© XXXX American Chemical Society
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Figure 1. Molecular structures of YD2-o-C8 and a series of perylene-functionalized porphyrin dyes WW-1−WW-6.
12.3%, which is superior to those based on Ru complexes,
setting a new milestone in this area.5 Further improvement of
the device performance will strongly rely on the development
of new dyes with even stronger light-harvesting abilities both in
the visible (∼400−700) and the near-infrared (NIR) region
(∼700−1400 nm). The DSCs based on YD2-o-C8 showed a
panchromatic photocurrent response in the whole visible
region with the lowest-energy onset at 725 nm,5 indicating
that there is yet plenty of space to further widen the light-
harvesting region. One can expect that attachment or fusion of
a π-extended chromophore to the porphyrin can further
improve its light-harvesting properties. This has been attempted
by using cyclic aromatics-substituted porphyrins,6 porphyrin
dimers,7 and fused porpyrins.8 Although these dyes showed
broad-range light absorption even extended to the NIR region,
the device performances are usually not as good as expected,
mainly due to dye aggregation and inappropriate energy level
alignment. Co-sensitizing has been demonstrated to be an
efficient way to improve light-harvesting properties but are
usually accompanied by a simultaneous voltage loss.9 Mean-
while, dye absorption structures play a key role in the device
performance and should be considered carefully.10 Therefore, it
is very challenging to design a suitable dye for high-efficiency
DSCs. On the one hand, we need organic dyes with strong
absorption in the visible and NIR region. On the other hand,
the energy level alignment, dye aggregation, surface coverage,
charge transfer and recombination kinetics, and excited lifetime
of the dyes, etc. all need to be carefully considered.
Among various π-extended polycyclic aromatic hydro-
carbons, perylene and its derivatives have been extensively
studied due to their excellent photophysical properties (e.g.,
high extinction coefficient and high fluorescence quantum
yield), charge transport properties, as well as outstanding
chemical, thermal, and photochemical stability.11 Furthermore,
the availability of their active sites (peri- or bay positions) leaves
room for further chemical modifications. In fact, perylene-based
dyes have been successfully used for both organic photo-
voltaics12 and DSCs.13 Therefore, one can expect that
functionalization of porphyrin core with a perylene moiety
will provide new dyes with largely enhanced light-harvesting
ability. Our groups recently reported the synthesis of two
perylene anhydride fused porphyrin dyes (WW-1 and WW-2,
Figure 1) and their application in DSCs.14 The devices showed
broad incident photon-to-current conversion efficiency (IPCE)
action spectra covering the entire visible spectral region and
even extending up to 1000 nm. However, only low power
conversion efficiencies have been obtained mainly due to their
mismatched energy levels, strong dye aggregations and short
excited state lifetime. In particular, the low-lying LUMO energy
level (−3.92 eV) of both compounds made the electron
injection from the photoexcited dyes to the conduction band of
the TiO2 unfavorable. To further modify the structure, we
decided to use a new perylene building block, the N-annulated
perylene (NP), in which the nitrogen atom is annulated at the
bay positions.15 The unique structural character and the
electron-rich nature of NP make regioselective functionalization
at the peri-positions and fusion of this moiety onto other
electron-rich units (e.g., NP itself, Zn-porphyrin, BODIPY, etc.)
feasible.16 Moreover, flexible alkyl chains or bulky groups can
be readily introduced to the amine site, which can significantly
improve its solubility and suppress dye aggregation. In this
work, four NP-substituted and fused Zn-porphyrin dyes WW-
3−WW-6 (Figure 1) were designed and used in DSCs. The
molecular design is based on the following considerations: (1)
NP shows strong absorption in visible range (λabsmax = 430 nm)
with high fluorescence quantum yield.17 At the same time, it is a
good electron donor, thus a “push−pull” structure is generated
in these new dyes; (2) like YD2-o-C8, the 4-ethynylbenzoic
acid was chosen as the acceptor due to its rigid structure and
efficient π-conjugation between the donor and acceptor
moieties; (3) bulky 3,5-di-tert-butylbenzyl, 2,4,6-trimethylphen-
yl (mesityl) as well as o-alkoxy-substituted phenyl groups were
chosen to surmount the solubility problem and to reduce dye
aggregation.18 At the same time, they may also retard the
interfacial electron back-injection from the conduction band of
the nanocrystalline TiO2 film toward the oxidized redox
mediator; (4) for WW-3 and WW-4, the NP unit is directly
linked or fused to the Zn-porphyrin core, which allows a good
comparison of their physical properties as well as device
performance; (5) for WW-5 and WW-6, the introduction of an
ethynylene spacer between the NP unit and the porphyrin core
leads to an efficient π-conjugation and red-shift of the
absorption spectrum compared to WW-3, thus improving
their light-harvesting properties. The physical properties
(optical, electrochemical) of these dyes were investigated by
various experimental methods assisted by time-dependent
density functional theory (TD DFT) calculations. The DSC
devices were fabricated and characterized by using standard
liquid cells with Co(II/III) as redox electrolyte. Some of these
dyes showed largely enhanced light-harvesting abilities (e.g.,
B
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WW-5 and WW-6), and as a result, high power conversion
efficiencies comparable to that of YD2-o-C8 under the same
conditions were achieved. Our research also gave a clear
structure−property device performance relationship, which is of
importance for future design.
and high-resolution mass spectrometry (see Supporting
Information [SI]).
Optical Properties. The absorption spectra of WW-3−
WW-6 together with YD2-o-C8 in dichloromethane (DCM)
are shown in Figure 2, and the data are collected in Table 1.
2. RESULTS AND DISCUSSION
Synthesis. The synthetic route towards N-annulated
perylene-substituted and fused porphyrin dyes WW-3−WW-6
is shown in Scheme 1. For WW-3 and WW-4, the synthesis
Scheme 1. Synthesis of N-Annulated Perylene-Substituted
and Fused Porphyrin Dyes WW-3−WW-6: (a) Pd(PPh3)4,
Cs2CO3; (b) NBS, CHCl3; (c) 4-ethynylbenzoic acid,
Pd(PPh3)4, CuI, TEA; (d) FeBr3, 1,2-dichloroethane; (e)
Pd(PPh3)4, CuI, TEA
Figure 2. UV−vis−NIR absorption spectra of YD2-o-C8 and WW-3−
WW-6 in DCM (10−5 M).
The absorption spectrum of WW-3 basically is the super-
imposition of the absorption of the Zn-porphyrin and the NP
chromophores, showing an intensive absorption band at 438
nm (ε = 194,700 M−1 cm−1) (p-band of NP plus the Soret band
of porphyrin) and two weak split Q bands at 566 nm (ε =
12,500 M−1 cm−1) and 617 nm (ε = 16,700 M−1 cm−1),
indicating that the NP and the Zn-porphyrin units are weakly
coupled. In contrast, the maximum absorption for the NP-fused
porphyrin dye WW-4 was much more red-shifted to 792 nm (ε
= 75,400 M−1 cm−1) due to the efficient π-extension after
fusion. Two intense absorption bands at 444 nm (ε = 55,200
M−1 cm−1) and 540 nm (ε = 69,900 M−1 cm−1) were also
observed. The ethynylene-linked dye WW-5 showed a very
different absorption spectrum from WW-3. It not only
possesses a strong B (or Soret) band with a peak at 441 nm
(ε = 158,500 M−1 cm−1) and two shoulders at 466 nm (ε =
115,300 M−1 cm−1) and 502 nm (ε = 61,500 M−1 cm−1), but
also an intense and broad Q-band at 679 nm (ε = 75,400
M−1 cm−1). The long alkoxy-substituted porphyrin dye WW-6
exhibits a very similar absorption spectrum to that of WW-5,
but with a slightly blue-shift of its Q bands (672 nm, ε = 76,900
M−1 cm−1). The big difference between WW-5/WW-6 and
WW-3 indicated that the incorporation of an ethynylene group
between the porphyrin and NP units would help to extend the
π-conjugation and decrease the HOMO−LUMO gap, thus
giving a longer-wavelength absorption spectrum. Compared to
YD2-o-C8, both WW-5 and WW-6 gave more intense and red-
shifted Q-bands. The optical energy gaps (Egopt) of WW-3−
WW-6 were estimated from the absorption onset as 1.94, 1.33,
1.71, and 1.74 eV, respectively.
commenced with the preparation of the key precursor 3, which
was synthesized by Suzuki coupling of porphyrin boronic ester
119 and monobrominated N-annulated perylene 2,16b followed
by bromination with N-bromosuccinimide (NBS). Treatment
of compound 3 through Sonogashira−Hagihara coupling
reaction with 4-ethynylbenzoic acid afforded the NP-substitued
Zn-porphyrin WW-3 in 70% yield. FeBr3-promoted intra-
molecular oxidative cyclodehydrogenation of 3 followed by
similar Sonogashira−Hagihara coupling reaction successfully
gave the NP-fused Zn-porphyrin WW-4 in 23% yield for two
steps. It should be noted that FeBr3 is the key reagent to obtain
the fused intermediate 4. Other synthetic methods such as
using Sc(OTf)3/DDQ16,20 gave complex mixtures, while using
21
FeCl3 afforded the cyclized product but with the bromine
Electrochemical Properties. Electrochemical properties of
WW-3 to WW-6 were investigated by cyclic voltammetry (CV)
and differential pulse voltommetry (DPV) in deoxygenated
DCM solution containing 0.1 M tetra-n-butylammonium
hexafluorophosphate (TBAPF6) as supporting electrolyte
(Figure 3 and Figure S1 in SI). The potential was externally
calibrated by ferrocenium/ferrocene (Fc+/Fc) couple and then
was calculated versus NHE electrode (E1/2 (Fc+/Fc) = 0.53 V
vs Ag/AgCl in our case, and 0.64 V vs NHE). Multiple quasi-
atom replaced by chlorine, which is unreactive for the
subsequent Sonogashira−Hagihara coupling reaction. Synthesis
of WW-5 and WW-6 was fulfilled through stepwise
Sonogashira−Hagihara coupling reaction of their correspond-
ing dibrominated porphyrin 5a or 5b first with ethynyl-
substituted N-annulated perylene 6 and then with 4-
ethynylbenzoic acid. The structures and purity of all the
intermediates and the final compounds were identified by NMR
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Table 1. Summary of Optical and Electrochemical Properties of NP-Functionalized Porphyrin Dyes WW-3−WW-6
ε (M−1 cm−1
)
Egopt (eV)
E1/2 (V)
E1/2 (V)
HOMO (eV)
LUMO (eV)
EgEC (eV)
ox
red
dye
λabs (nm)
WW-3
438
566
617
444
540
792
441
466
502
679
441
467
503
672
194700
12500
16700
55200
69900
75400
158500
115300
61500
75400
153800
116900
75800
76900
1.94
0.85
1.08
1.49
0.83
1.11
−1.12
−1.51
−4.96
−3.09
1.87
WW-4
WW-5
1.33
1.71
−0.54
−1.03
−4.76
−4.86
−3.60
−3.27
1.16
1.59
0.74
0.97
1.58
1.78
0.84
1.06
1.56
1.82
−0.96
−1.31
WW-6
1.74
−1.22
−1.59
−4.90
−3.15
1.75
measured under similar conditions has a HOMO of −4.92 eV
and LUMO of −3.00 eV, with an EgEC of 1.92 eV.5
TD DFT Calculations. In order to gain better under-
standing of the molecular geometries, molecular orbitals, optical
properties, and electronic properties of the porphyrin dyes
WW-3−WW-6, TD DFT calculations at the B3LYP/6-31G**
level of theory were performed. Figure 4 shows their optimized
molecular structures and frontier molecular orbital profiles
together with the calculated energy levels of HOMOs and
LUMOs. The HOMO of WW-3 is mainly localized on the NP
moiety, while its LUMO is homogeneously distributed through
the porphyrin core and the acid binding group. Such an orbital
partition feature is due to the nearly orthogonal arrangement of
Figure 3. Cyclic voltammograms of WW-3−WW-6 in dry DCM with
TBAPF6 as supporting electrolyte, AgCl/Ag as reference electrode, Au
disk as working electrode, Pt wire as counter electrode, and scan rate at
50 mV/s.
reversible or irreversible redox waves were observed for all four
ox
compounds (see the list of half wave potentials E1/2 (anodic
red
scan) and E1/2 (cathodic scan) in Table 1, vs NHE). The
HOMO and LUMO energy levels were estimated from the
onset of the first oxidation and reduction wave respectively.22
The measured HOMO energy level of the Co(II)-bpy cation is
−4.63 eV and the theoretical LUMO energy level of the
isolated TiO2 nanoparticle is −3.44 eV.23 The low-lying LUMO
energy level of WW-4 indicates that its driving force for
electron injection may not be sufficient. The electrochemical
energy gaps (EgEC) were calculated accordingly to be 1.87, 1.16,
1.59, and 1.75 eV, which are in agreement with the optical
energy gap values.
Thus, fusion of a NP unit to the porphyrin core (in WW-4)
significantly lowers the LUMO and lifts up the HOMO
compared to the NP-substituted porphyrin WW-3, leading to a
narrowed energy gap. Putting additional ethynylene linker
between the NP and porphyrin units (in WW-5 and WW-6)
also results in a decrease of LUMO and slight up-lift of
HOMO, accompanied with a diminished energy gap.
Replacement of the mesityl groups in WW-5 by 2,6-
didodecoxyphenyl units in WW-6 leads to a slight up-lift of
the LUMO and an slight increase of the energy gap (by 0.13
eV), which is in consistence with the observed slight blue-shift
in the absorption spectrum. For comparison, dye YD2-o-C8
Figure 4. Calculated HOMO and LUMO profiles and energy levels of
WW-3−WW-6 (B3LYP/6-31G**).
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the NP and porphyin unit which does not allow sufficient π-
orbital overlap for extended π-conjugation, in agreement with
the superimposition character of the absorption spectrum. For
the fused dye WW-4, both the HOMO and LUMO are
delocalized throughout the highly conjugated backbone. The
HOMO and LUMO are not well segregated, and the electrons
are less distributed on the acid binding moiety in LUMO,
implying that it may be difficult for the photoexcited electrons
to inject into the conduction band of TiO2 via the carboxylic
group adsorbed on the TiO2 surface. For both WW-5 and WW-
6, HOMO and LUMO showed a disjointed character, with the
HOMO delocalized along both the perylene moiety and
porphyrin core, and the LUMO delocalized through the
porphyrin core and the acid binding group. Hence, the
introduction of ethynylene group between the porphyrin core
and NP unit in WW-5 and WW-6 is believed to give a better
overlap between donor and acceptor moieties, thus providing a
fast charge transfer transition. At the same time, the LUMO
electron distributions of WW-5 and WW-6 share a pattern
similar to that ofWW-3; therefore, electron injection from the
carboxylic group into the conduction band of TiO2 will not be
affected significantly. The didodecoxyphenyl groups in WW-6
are nearly perpendicular to the porphyin plane and envelop the
porphyrin core, which may help to suppress the dye
aggregation and also slightly lift up the HOMO and LUMO
as demonstrated by Lin and Yeh et al.5,18d The calculations also
predict that WW-3−WW-6 show the longest-wavelength
absorption maximum at 569.9, 739.6, 715.8, and 684.1 nm,
respectively, (Figures S2−S5 in SI), which follow the same
trend with the experimental observations.
Figure 5. (a) Current−voltage characteristics of the WW-3−WW-6-
sensitzed solar cells and the YD2-o-C8 cell and (b) the corresponding
IPCE action spectra.
Photovoltaic Properties. The newly synthesized dyes
were adsorbed onto a bilayer (4.2 + 5.0 μm) titania film24 by
using an optimized binary solvent system of THF and ethanol
(volume ratio 1:4) to serve as a working electrode for
photovoltaic characterizations. Our cobalt electrolyte is
composed of 0.25 M tris(2,2′-bipyridine) cobalt(II) di[bis-
(trifluoromethanesulfonyl)imide], 0.05 M tris(2,2′-bipyridine)-
cobalt(III) tris[bis(trifluoromethanesulfonyl)imide], 0.5 M 4-
tert-butylpyridine (TBP), and 0.1 M lithium bis-
(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile.
Figure 5 shows the photocurrent density−voltage (J−V)
characteristics and the corresponding IPCE action spectra for
WW-3−WW-6 cells measured under standard photovoltaic
reporting conditions (global AM1.5 sunlight with irradiance of
100 mW cm−2 at 298 K), with YD2-o-C8 as the reference cell.
The photovoltaic parameters under various irradiances (G) are
listed in Table 2. The Jsccal values calculated from IPCE are also
shown for comparison with the measured JSC to confirm the
accuracy of the energy conversion efficiency.
When WW-3 was tested for the photovoltaic performance, it
only showed a moderate power conversion efficiency of 5.6%,
which can be mainly attributed to the low short-circuit current
(Jsc) value (9.81 mA cm−1). Dye WW-3 showed IPCE values
exceeding 35% from 390 nm up to 666 nm, with a big gap at
the middle, which is consistent with its absorption spectrum.
The highest IPCE value can reach 73% at 458 nm, indicating an
efficient electron injection from the photoexcited dyes to the
conduction band of the TiO2. However, the moderate light-
harvesting ability limits the Jsc value and eventually the
efficiency. Dye WW-4 showed even worse device performance
under the same condition. A very weak photocurrent response
was observed, although its absorption spectrum is extended to
the NIR region up to 920 nm. Both the Jsc and open-circuit
voltage (Voc) gave very disappointing results (3.00 mA cm−1
and 0.500 V, respectively). The relatively low efficiency
presumably ascribed to its low-lying LUMO energy level
(−3.60 eV) and the nondisjointed character of its HOMO/
LUMO profiles, which make the electron injection from the
excited dyes to the conduction edge TiO2 much less efficient.
Fortunately, dyes WW-5 and WW-6, possessing an ethynylene
bridge between the porphyrin core and the perylene moiety,
exhibited much better performances which are comparable to
that of the benchmark molecule YD2-o-C8. Indeed, when WW-
5 was employed, an impressive broad IPCE action spectrum
covering the pantochromatic visible region and part of the NIR
region was achieved. The IPCE values are high (>65% from
410 to 760 nm) and comparable to that of YD2-o-C8. The
most distinct character is that the onset of IPCE action
spectrum was further red-shifted from 725 nm for YD2-o-C8
cell to 815 nm for WW-5 cell; as a result, a high Jsc of 18.4 mA
cm−2 was achieved for WW-5-sensitized solar cells, in
comparison to a Jsc of 16.33 mA cm−2 for the YD2-o-C8 cell.
The Voc value of WW-5 cell (0.766 V), however, is lower than
that of the latter (0.868 V), and its fill factor (FF) is also slightly
lower; as a result, the overall power conversion efficiency of the
WW-5 cell reached 10.3%, which is very close to that of YD2-o-
C8 cell (η = 10.5%) at the same testing condition. The WW-6-
based cell showed a similar pantochromatic IPCE action
spectrum with the onset (785 nm) blue-shifted compared to
that of WW-5 cell, which is consisent with their absorption
spectra. Accordingly, the Jsc value decreased to 17.69 mA cm−2.
The Voc (0.809 V), however, was significantly improved, and
the power conversion efficiency was as high as 10.5%, which is
the same for the YD2-o-C8 cell. Under a lower irradiation
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Table 2. Photovoltaic Parameters of Cells Measured at Various Irradiances (G) of Simulated AM1.5 Sunlight
dye
G [mW cm−2
]
Jsc [mA cm−2
cal
]
Jsc [mA cm−2
]
Voc [V]
FF
η [%]
YD2-o-C8
12.58
23
2.03
3.72
8.09
16.17
9.40
4.52
2.37
4.34
9.44
18.87
2.31
4.22
9.17
18.34
2.03
3.79
8.31
16.33
9.81
3.00
2.39
4.44
9.71
18.43
2.32
4.31
9.42
17.69
0.781
0.810
0.842
0.868
0.744
0.500
0.698
0.719
0.746
0.766
0.730
0.755
0.786
0.809
0.791
0.786
0.773
0.743
0.767
0.299
0.779
0.780
0.766
0.733
0.777
0.776
0.762
0.735
10.0
10.5
10.8
10.5
5.6
50
100
100
100
12.58
23
WW-3
WW-4
WW-5
0.3
10.3
10.8
11.1
10.3
10.5
11.0
11.3
10.5
50
100
12.58
23
WW-6
50
100
condition at G = 50 mW cm−2, the η values of the WW-5 (η =
11.1%) and WW-6 cells (η = 11.3) were even higher than that
of YD2-o-C8 cell (η = 10.8%) (Table 2).
It is widely recognized that for a fixed redox electrolyte in
DSCs, the rise or fall of Voc is determined by the titania electron
quasi-Fermi-level (EF,n), which intrinsically stems from a
change of titania conduction band edge (Ec) and/or a variation
of titania electron density.25 At a given flux of photocarrier
generation, the titania electron density is determined by the
interfacial recombination rate of titania electrons with electron-
accepting species in electrolytes and/or dye cations. Thereby
charge extraction and transient photovoltage decay measure-
ments were further carried out to understand the electronic
origins of the aforesaid Voc fluctuation. As shown in Figure 7a,
the cells made with YD2-o-C8, WW-5, and WW-6 feature a
similar extracted charge (Q) at the same potential bias Voc,
In order to understand the performance differences from a
chemistry perspective, a dye-loading density study of porphyrin
dyes on TiO2 film was conducted. WW-3−WW-6 gave dye-
loading densities of 1.69 × 10−8, 1.84 × 10−8, 1.81 × 10−8, and
1.62 × 10−8 cm−2 μm−1, respectively. In comparison, YD2-o-C8
gave a relative lower density of 1.38 × 10−8 cm−2 μm−1,
indicating a higher power conversion ability of this dye. We also
conducted electronic absorption spectra of 4-μm-thick, trans-
parent dye-grafted titania films immersed in the Co-bpy
electrolyte (Figure S6 in SI). The results indicate that the
IPCE difference is not induced by the light absorption abilities.
To further understand the dye structure−device performance
relationship, we conducted further physical measurements on
the WW-5 and the WW-6 cells with the YD2-o-C8 cell as a
reference. For a short-circuit DSC device, it is known that the
Jsc is measured at the condition of a considerable low electron
density in the nanocrystalline titania film, and the charge
recombination flux is significantly reduced. Thereby, the
measured Jsc is roughly proportional to the photocarrier
generation flux. The validity of this analysis motivated us to
compare the dye structure-correlated Voc variation at a given Jsc
by measuring J−V curves under various light intensities and
plotting Voc as a function of Jsc (Figure 6). It is noted that at a
given Jsc, the YD2-o-C8 cell exhibits a higher Voc than WW-6,
while WW-5 gives the lowest Voc value.
Figure 7. (a) Comparison of the charges extracted from the dye-
grafted titania films at a certain open-circuit photovoltage. (b)
Comparisons of electron lifetime at a given open-circuit photovoltage.
Figure 6. Open-circuit photovoltage plotted as a function of short-
circuit photocurrent density.
F
dx.doi.org/10.1021/ja409291g | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
suggesting a fixed conduction-band edge of titania with respect
to the electrolyte Fermi-level. As Figure 7b presents, the YD2-
o-C8 cell displays longer charge recombination lifetime (τn)
than WW-6 and WW-5 cells at a given charge Q, accounting for
the aforementioned more superior photovoltage at a given Jsc
for YD2-o-C8 than for WW-6, followed by that for WW-5. The
observed higher Voc for WW-6 cell as compared to WW-5 cell
could be ascribed to the efficient suppression of dye
aggregation as well as the retardance of charge recombination
between the TiO2 film and the Co(II/III) redox electrolyte. In
addition, the slight rise of the HOMO of WW-5 in comparison
to that of WW-6 may be another reason, which could diminish
the dye regeneration. Compared to YD2-o-C8, WW-6 has a
very similar HOMO energy level but with a lower-lying LUMO,
which could make the electron injection to the TiO2 film at a
slower rate.
harvesting abilities, molecular orbital profiles, energy level
alignment, and dye aggregation, all of which are important
factors we must carefully consider. Further work to improve
device performance by using new dyes with even better light-
harvesting abilities and optimized energy levels is underway in
our laboratories.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures and characterization data of all new
compounds. Details for all physical characterizations and
theoretical calculations. Photovoltaic test procedure and
mechanism studies. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
chmwuj@nus.edu.sg or wuj@imre.a-star.edu.sg
peng.wang@ciac.jl.cn
■
The short-circuit currents of the YD2-o-C8, WW-5 and
WW-6 cell increase linearly with light intensity up to about 1
sun, and this is illustrated in Figure 8. This linearity shows that
the photocurrent is not limited by diffusion of the cobalt
electrolyte within the nanocrystalline film.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.W. acknowledges financial support from the IMRE Core
Funding (IMRE/13-1C0205) and MOE Tier 2 grant
(MOE2011-T2-2-130). P.W. thanks the National 973 Program
(No. 2011CBA00702) and the National Science Foundation of
China (No. 51203150). K.-W.H. acknowledges financial
support from KAUST.
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