Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Article abstract of DOI:10.1039/c5cp05658f

Porphyrins have been demonstrated as one of the most efficient sensitizers in dye-sensitized solar cells (DSSC). Herein, we investigated a series of porphyrin sensitizers functionalized with various π-spacers, such as phenyl for LD14, thiophene for LW4, thiophene-phenyl for LW5, and 2,1,3-benzothiadiazole (BTD)-phenyl for LW24. Photo-physical investigation by means of time-resolved fluorescence and nanosecond transient absorption spectroscopy revealed an accelerated inner charge transfer in porphyrins containing the BTD-phenyl π-spacer. Implementation of an auxiliary electron-deficient BTD unit to the porphyrin spacer also results in a broad light-harvesting ability extending up to 840 nm, contributing to an enhanced charge transfer character from the porphyrin ring to the anchoring group. When utilized as a sensitizer in DSSCs, the LW24 device achieved a power conversion efficiency of 9.2%, higher than those based on LD14 or LW5 porphyrins (PCE 9.0% or 8.2%, respectively) but lower than that of the LW4 device (PCE 9.5%). Measurements of transient photovoltage decays demonstrate that the LW24 device features the up-shifted potential band edge of the conduction band of TiO₂, but involves serious charge recombination in the dye/TiO₂ interface. The findings provide insights into the molecular structure and the charge-transfer characteristics for designing efficient porphyrin sensitizers for DSSC applications.

Full text of DOI:10.1039/c5cp05658f

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ARTICLE
Novel porphyrins-preparation, characterization, and
application in solar energy conversion
Cite this: DOI: 10.1039/x0xx00000x
‡
a
‡a
a
b
b
Jianfeng Lu, Hao Li, Shuangshuang Liu, YuꢀCheng Chang , HuiꢀPing Wu , Yibing
a,c
b
a
Cheng , Eric WeiꢀGuang Diau * and Mingkui Wang *
Received 00th January 2012,
Accepted 00th January 2012
Porphyrins have been demonstrated as one of most efficient sensitizers in dyeꢀsensitized solar
cells (DSSC). Herein, we investigated a series of porphyrin sensitizers functionalized with
various πꢀspacers, such as phenyl for LD14, thiophene for LW4, thiopheneꢀphenyl for LW5,
and 2,1,3ꢀbenzothiadiazole (BTD)ꢀphenyl for LW24. Photoꢀphysical investigation by means of
timeꢀresolved fluorescence and nanosecond transient absorption spectroscopy revealed an
accelerated inner charge transfer in porphyrin containing the BTDꢀphenyl πꢀspacer.
Implementing of an auxiliary electronꢀdeficient BTD unit to the porphyrin spacer also results
in a broad lightꢀharvesting ability extended up to 840 nm, contributing from an enhanced
charge transfer character from porphyrin ring to the anchoring group. When utilized as a
sensitizer in DSSC, the LW24 device achieved the power conversion efficiency of 9.2%,
higher than those based on LD14 or LW5 porphyrins (PCE 9.0% or 8.2%, respectively) but
lower than the LW4 device (PCE 9.5%). Measurements of transient photovoltage decays
demonstrate that the LW24 device features the upꢀshifted potential band edge of the
conduction band of TiO , but involves serious charge recombination in the dye/TiO interface.
DOI: 10.1039/x0xx00000x
www.rsc.org/
2
2
The findings provide insight between the molecular structure and the chargeꢀtransfer
characteristics for designing of efficient porphyrin sensitizers for DSSC applications.
2
a
Introduction
broaden and red shift their absorption bands. Detailed investigation
has showed that porphyrin’s optoelectronic property could be
efficiently modulated by adjusting the electronic unit between
Emulation of photosynthesis and efficient and sustainable utilization
of solar energy have become one of the greatest scientific challenges
6
porphyrin core and anchor. This particular unit is termed as the
st
in the 21 century. Porphyrins and their derivatives play vital role in
conjugating ‘spacer’ in this study as discussed below. The progress
of porphyrin applications in DSSC over the past few years has
evidenced the large contribution from a rational design of spacer
groups, including the DSSC PCE record of 13.0% from porphyrin
SM315, which incorporates a new spacer by implementing an
auxiliary electronꢀdeficient unit of benzothiadiazole (BTD) into
photosynthesis due to their superior lightꢀharvesting ability in the
visible spectrum, easy tuning photoꢀ and electrochemistry through
functionalization of the periphery (meso and βꢀpositions) as well as
variation of the metal centre. In addition to this, these properties
lead to fruitful otptoelectronic application, such as molecular
1
2
electronics, solar cells, hydrogen evolution, and so on. Synthetic
3c
phenyl spacer. However, it is worthy to note the DSSC PCE
porphyrin are widely incorporated in prototype dyeꢀsensitized solar
cells (DSSC), in which specifically rational designed porphyrin
sensitizers have afforded top power conversion efficiency (PCE),
dramatically decreased to 2.5% if the spacer was changed from
BTDꢀphenyl to BTD. Furthermore, Palomares et al. reported that
7
only a tiny change of single atom in spacer could play big influence
on porphyrins property. They found that the DSSC efficiency can be
largely improved from 2.55% to 10.41% by changing BTDꢀfuran to
3
outperforming the standard Ruꢀpolypyridyl dyes. Most of highly
efficient porphyrin sensitizers have a feature of donorꢀacceptor
4
conjunction. However, typical DꢀA porphyrin sensitizers has
8
BTDꢀthiophene. These results expose the crucial role of spacer’s
intense but sharp light absorption in the B (450 nm) and Q (550ꢀ650
nm) bands, bereft of absorption around 500ꢀ600 nm and beyond 750
nm. In pursuit of panchromatic porphyrin sensitizers, extensive
attention has been paid on modification of the donor and acceptor
structure for DꢀA porphyrins, resulting in variations in optical and
structure in porphyrin sensitizers. Therefore, a modification at the
spacer is expected to induce significant effect on the porphyrin
sensitizers’ optical, redox, and their photovoltaic properties,
suggesting a feasible choice rather than on the donor to modulate the
3cꢀ3e, 7ꢀ8
optoelectronic properties of porphyrin sensitizers.
Following
5
photovoltaic property. Notably, elongation of π conjugation and
loss of symmetry in porphyrin molecules have been recognized to
this strategy, a bunch of porphyrin sensitizers were developed for
3d,3e
DSSC, showing over 10% PCE.
It is also worth to note that a
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same group could have distinct different effect on porphyrin’s Result and Discussion
spectral and photovoltaic properties when placed at different
The molecular structures of porphyrin LD14, LW4, LW5, and LW24
are presented in Scheme 1. Rigid anchor structure and long alkoxyl
chain are designed to reduce the recombination reaction induced by
molecule aggregation. As shown in Scheme 1, the only difference
for the four porphyrins structures is the spacer moiety, aiming to
make a clear comparison of spacers for porphyrins. The LD14 with
phenyl and LW4 with thiophene are supposed to study the influence
of spacer’s electronꢀdonating ability on optoelectronic properties of
porphyrins and their photovoltaic devices. While the LW5 and
LW24 dyes are designed to determinate whether an electronꢀ
deficient or rich aromatic as a spacer extender is more suitable for
DSSC.
positions, such as donor or acceptor moieties (i.e., two different
mesoꢀpositions). For example, Officer and Gordon demonstrated that
the acceptor moiety, which act as a bridge between the porphyrin
and the oxide semiconductor surface, have stronger interaction with
9
the macrocycle than the donor moiety in such DꢀA type porphyrins.
Wang et al reported that the DSSC performance could be improved
by
50%
when
placing
electronꢀdeficient
unit
2,3ꢀ
diphenylquinoxaline (DPQ) at the acceptor moiety rather than at
10
donor moiety. On the other hand, there are also extensive work on
arylꢀethynyl substituted tetrapyrroles, showing variations in optical
and redox properties with variations in the aryl group attached to an
ethynyl. For example, by attaching additional potent auxochromes to
the donor mesoꢀpositions, Lin et al also reported highly efficient
The synthesis of LW24 is summarized in Scheme 2. Unlike the
synthesis approach for SM315 or LCVCꢀseries porphyrins as
1
1
porphyrin sensitizers covering NearꢀIR region. We notice that few
studies have been done on relationship between device property and
porphyrin structure through analysis of the photoꢀinduced charge
transfer via spacers.
3c,7ꢀ8
reported by Grätzel and Palomares,
we introduced preꢀ
synthesized acceptors to porphyrin at the last step, which has been
1
4
demonstrated to be an efficient and high purity ensured method.
Firstly, the 4,7ꢀdibromobenzo[c][1,2,5] thiadiazole (compound 1)
was coupled with (4ꢀmethoxyphenyl) boronic acid via Pdꢀcatalyzed
Suzuki coupling reaction. Then, treating the compound 2 with
trimethylsilylacetylene, Pdꢀcatalyzed Sonogashira coupling reaction
was carried out to obtain compound 3. Later on, compound 4 was
achieved in the K CO /MeOH alkalescence environment. Finally, a
Due to the importance of spacer in porphyrins, we have
tested various spacers in DꢀA structured sensitizers.
Consequently, we found that by changing spacer unit from
phenyl to thiophene, the devices PCEs could be improved from
1
2
9
.0% to 9.5%. Therein, thiophene is an electronꢀrich unit
2
3
while BTD is electronꢀdeficient unit. A question still opens
about the effect of conjugating spacer units on photoꢀinduced
charge transfer in porphyrins. In this article, we report our
finding on the conjugating spacer group to the terminal benzoic
group that is the determinant in the variations in optical, redox
of porphyrins. Four DꢀA type porphyrin sensitizers
standard Sonogashira crossꢀcoupling reaction was applied to
synthesize the DꢀA structured Por-2, which was further treated with
sodium hydroxide and diluted hydrochloric acid to get final target
molecule.
S
S
N
N
N
N
a)
b)
Br
Br
Br
COOCH3
1
3
differentiated in the conjugating spacers, i.e. phenyl (LD14),
thiophene (LW4), thiopheneꢀphenyl (LW5) or BTDꢀphenyl
LW24), were investigated. The selected spacer units possess
1
2
S
S
N
N
N
N
c)
(
Si
COOCH3
COOCH3
electronꢀdeficient or rich properties, thus providing significant
information to rational design highly efficient porphyrin
sensitizers. We found that with a solely aromatic spacer in these
porphyrins, the resulted DSSC presents an improved
performance due to an increasing of electronꢀdonating ability
from phenyl to thiophene. The electron deficient aromatic BTD
shows more beneficial than electronꢀrich thiophene as a spacer
extending unit.
3
4
C12H25
C12H25
C12H25
C12H25
O
O
O
O
S
N
N
N
N
N
N
N
d)
Zn
COOCH3
N
Zn
Br
N
N
N
N
O
O
O
O
C12H25
C12H25
C12H25
C12H25
Por-1
Por-2
C12H25
C12H25
O
O
S
N
N
C12H25
C12H25
C12H25
C12H25
N
N
N
O
O
O
O
e)
COOH
N
Zn
COOH
N
N
N
N
N
N
N
N
N
S
Zn
COOH
N
Zn
N
O
O
LW24
C12H25
C12H25
LW5
LD14
O
O
O
O
O
O
O
O
C12H25
C12H25
C12H25
C12H25
C12H25
C12H25
C12H25
C12H25
Scheme 2. Synthesis of the LW24 porphyrins: a) 1, (4ꢀmethoxyphenyl)
boronic acid, Pd(PPh , THF, 2 M K CO , reflux, overnight; b) 2, TMSA,
Pd(OAc) , PPh , CuI, THF/TEA, 45 °C, 24 h; c) 2, K CO , MeOH, room
temperature, 6 h; d) Por-1, 4, Pd(PPh /CuI, THF/TEA, 45 °C, 18 h; e) i)
Por-2, 20% NaOH(aq.), THF, 40 °C, 2 h; ii) diluted HCl.
3
)
4
2
3
S
2
3
2
3
N
N
N
N
N
N
S
COOH
N
N
N
N
3 4
)
N
Zn
N
Zn
COOH
LW4
O
O
O
O
LW24
C12H25
C12H25
C12H25
C12H25
Scheme 1. Molecular structure of the porphyrins LW4, LW5, LW24
and LD14.
2
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DOI: 10.1039/C5CP05658F
300
200
100
300
200
100
300
200
100
300
200
100
0
Fig. 2 a) The Q band absorption spectra of LW4 and LD14; b) The Q band
LD14
absorption spectra of LW5 and LW24; c) The steadyꢀstate emission of LD14
ꢀ6
LW4, LW5, and LW24. All measurements were done with 4×10 mol/L
THF solution at 25 °C.
LW4
The fluorescent emission maxima of LD14, LW4, LW5 and LW24
dyes in THF solution are found at 684, 687, 691, and 710 nm,
showing mirror image to their Q absorption bands as depicted in Fig.
LW5
2
c. The increased Stokes shift of the LW24 (20 nm for LW24)
compared with the other porphyrins (17, 16, and 17 nm for LD14,
LW4, and LW5) indicates a strong dipole moment in the excited
LW24
15
states of this porphyrin. The bathochromic shift as well as broader
PL spectra is an indication of intense charge transfer character within
LW24 porphyrins’ excited state. This will be discussed below.
400
500
600
700
Wavelength (nm)
I-/I -
TiO2
V (vs. NHE)
-1.5
LD14
LW4
LW5
LW24
3
Fig. 1 The absorption spectra of LD14, LW4, LW5, and LW24. All
-
-
3.0
3.5
ꢀ6
measurements were done in 4×10 mol/L in THF solution at 25 °C.
LUMO
-
3.36
-1.0
-3.38 -3.38
-
3.45
The absorption spectra of these porphyrin dyes are shown in Fig. 1
and corresponding data are tabulated in Table 1. The resulted dyes
exhibit two typical porphyrin B and Q absorption band at 450ꢀ570
and 650ꢀ750 nm. The former band involves the transition from the
ground state to the second excited state (S →S ), while the latter
CB
-
4.0
-0.5
0
1
.84
1.83 1.82
1.78
-
4.5
0
2
3c
-4.9
stems from the weaker transition to the first excited state (S →S ).
As depicted in Fig. 2a, the Q band of the LW4 is slightly redꢀshift by
nm comparing to the LD14, causing from the spacer substitution
0
1
-
-
5.0
5.5
0.5
1.0
HOMO
-
5.18
-
5.22 -5.21
4
-5.23
from phenyl to thiophene. For the thiopheneꢀphenyl conjugated LW5,
the Qꢀband redꢀshifts about 3 nm compared to that of LW4. Indeed,
elongation of π conjugation and loss of symmetry in porphyrin
eV (vs. Vacuum)
Fig. 3 Energy diagram of the sensitizers LD14, LW4, LW5, LW24, redox
couple, and the conduction band of TiO . The HOMO and LUMO levels
2a
molecule result in broadened and red shifted absorption bands.
2
were obtained from the ground state redox potentials and the optical energies.
Thus, we can ascribe the redꢀshift of LW4 (compared to LD14) to
the latter reason, while the LW5 (compared with LD14 or LW4) to
the former. When replacing thiophene (an electronꢀrich unit) with
BTD (an electronꢀdeficient aromatic unit), the Q band dramatic redꢀ
shifts by 15 nm from 674 nm for porphyrin LW5 to 689 nm for
porphyrin LW24, as presented in Fig. 2b. Otherwise, the Q band
maximum molar extinction coefficient of LW24 increases 30% to
And the difference of the potential for the NHE (Uredox) versus an electron in
16
vacuum (EF,redox) can be given as: EF,redox [eV] = ꢀ4.5ꢀ eUredox [V].
Table 1. Absorption, fluorescence, and first porphyrinꢀring redox potential of
various porphyrins (LD14, LW4, LW5 and LW24) in THF at 25 °C.
[e]
PL
[c]
E
ox /V
[
d]
4
ꢀ1
ꢀ1
4
ꢀ1
ꢀ1
Absorption
E0ꢀ0 /V EoxꢀEoꢀo
8
.8×10 M cm comparing to LW5 (6.6×10 M cm ), indicating a
[b]
[a]
1
2e
Dye
Emission lifetime
λmax /nm
λ
/nm
(vs.
NHE)
promising application for thinꢀfilm DSSCs. Since the conjugation
length of LW5 and LW24 are very similar, the above results suggest
that the porphyrin dye’s absorption spectrum greatly dependant on
the intrinsic property of the spacer unit. It also means that a versatile
synthesis platform for panchromatic porphyrin could be expected by
adjusting the aromatics in the spacer part.
max
3
(vs. NHE) /V
ꢀ1
ꢀ1
(
10 /M cm )
τ /ns
[f]
459(253.8)
LD14
684
687
691
710
1.69
1.54
1.66
0.89
0.72
0.71
0.68
0.73
1.84
1.83
1.82
1.78
ꢀ1.12
6
67(64.1)
62(234.1)
71(56.4)
4
6
LW4
LW5
ꢀ1.12
ꢀ1.14
ꢀ1.05
100
466(283.1)
674 (65.9)
464(177.6)
LD14
LW4
LD14
LW4
LW5
9
0
4 nm
15 nm
b)
c)
1.0
a)
LW5
LW24
80
LW24
LW24
6
89(87.8)
70
o
[
a] Absorption and emission data were measured in THF at 25 C; [b]
60
50
Excitation wavelength/nm: LD14, 459; LW4, 462; LW5, 466; LW24, 464; [c]
0.5
4
0
First porphyrin ring oxidation; Electrochemical measurements were
30
o
performed at 25 C with each porphyrin (0.5 mM) in THF/0.1 M TBAP/N
2
,
20
GC working and Pt counter electrodes, Ag/AgCl reference electrode, scan
1
0
ꢀ1
rate=50 mV s ; [d] Estimated from the intersection wavelengths of the
normalized UVꢀvis absorption and the fluorescence spectra; [e] The
0
0.0
850
6
00
640
680
720
600
640
680
720
550
600
650
700
750
800
Wavelength (nm)
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fluorescence time were measured under a laser excitation of 445 nm; [f] Fig 4. Timeꢀresolved photoluminescence decay traces of dyeꢀgrafted
LD14 was synthesized according to the literature ref. 13.
mesoporous titania film and dye in THF solvent. Excitation wavelength: 445
nm, detected at: LD14, 684 nm; LW4, 687 nm; LW5, 691 nm; LW24, 710
nm. The traces with scatter are raw data; the solid curves are the results fitted
according to the equation.
The HOMOs calculated from their first oxidation potentials (Eox)
are determined to be ꢀ5.22, ꢀ5.21, ꢀ5.18 and ꢀ5.23 eV, while the
LUMOs are ꢀ3.38, ꢀ3.38, ꢀ3.36 and ꢀ3.45 eV, if we transform the
1
6
cyclic voltammetry (CV) to energy level in vacuum. The HOMO
and LUMO values of the LD14 and LW4 porphyrins are identical,
while difference in LW5 and LW24 is clearly indicated. It appears
that the electronꢀdeficient aromatic unit of BTD shifts the LUMO
level negatively, while the electronꢀrich of thiophene unit mainly
plays influence on the HOMO level. The distribution for HOMO and
LUMO energy levels in such DꢀA structured porphyrin molecules
are largely localized on the donor moiety and the acceptor moiety,
Nanosecond transient absorption spectrum were performed to
scrutinize the dynamics of the recombination of electrons injected in
+
the conduction band of TiO (e ) with the oxidized dye (S ) and that
2
cb
21
of the dye regeneration reaction with iodide. Fig. 5 shows the
transients at 550 nm for with the samples on titania in the presence
and absence of the redox complex. The pump pulses at 460 nm are
attenuated with neutral density filters before excitation of the sample
ꢀ2
with a fluence less than 40 ꢁJ cm . The decays have been fitted by a
17
respectively. In this case, the variation of energy levels could be
explained by assuming the BTD unit serving as auxiliary acceptor in
LW24, while thiophene serving as auxiliary donor in LW5. As
21
stretchedꢀexponential function eq(1) as follows:
−
(t /τ
)β
A(t) = Ane
eq(1)
depicted in Fig. 3, the conduction band of the TiO locates at ꢀ4.0 eV,
2
1
6
while the redox potential of the electrolyte is ꢀ4.9 eV. Thus these where τ is the characteristic lifetime, β being the stretch parameter
porphyrins are sufficient to ascertain the injection and regeneration (0<β<1, β=1 corresponding to a monoexponential decay). The eq(1)
reaction from a thermodynamic point of view (Fig. 2). Densityꢀ takes account of the exponential distribution of energy states in the
functional theory (DFT) calculations were also performed on the conduction band, the presence of trap states, and the different dye
2
1b
LD14 and LW4 dyes to gain insight into the electronic structures of distributions onto the semiconductor surface.
In the devices
their frontier molecular orbitals. As shown in Fig. S5, the HOMOs containing solely inert electrolytes without redox couple, the
are localized on the donor (dimethylamine) and macrocycle moieties; electron recombination reaction is only on the interactions between
in contrast, the LUMOs are highly visible on the macrocycle and the the dye and the electrons located in the conduction band and the trap
acceptor moieties (πꢀspacer and carboxyl acid).
states of TiO . Thus, the extracted lifetimes of LD14, LW4, LW5
2
and LW24 was estimated to be 56, 38, 8.8 and 11.4 ms as listed in
Table 2. In the presence of redox couple, the oxidized dye molecules
are not only reduced by the recombination process, but also the
regeneration process via the electrolyte as well. In this case, the
acquired lifetimes of LD14, LW4, LW5 and LW24 are 31, 35, 42
and 52 ꢁs. To investigate the nonꢀexponential decay analysis
precisely, the average time constants and averaged rate are
Utilizing timeꢀcorrelated single photon counting technique, we
measured the fluorescence decay lifetime of LD14, LW4, LW5 and
LW24 dyes, getting insight into the electron injection process from
18
the photoexcited porphyrin to the TiO conduction band. Fig. 4
2
presents the fluorescence decay for the porphyrin dyes in THF and
on nanocrystalline TiO films in the presence of electrolytes used in
2
the photovoltaic experiments. In THF solution, the lifetime of the
LD14, LW4, LW5 and LW24 dyes is 1.69, 1.54, 1.66 and 0.89 ns (in
Table 1). The LW24 porphyrin shows the fastest decay among the
tested samples, being nearly half of the other three porphyrins. It
confirms a strong electronꢀcoupling of the donor and acceptor in the
2
1aꢀb
calculated according to the literature.
1
0
0
0
0
.0
.8
.6
.4
.2
LD14/solvent
LD14/W08
1
9
s
s
excited state. When those porphyrins adsorbed onto the TiO2
nanoparticles surface, a strong quenching of emission was observed.
The lifetime of the excited singlet state of LD14, LW4, LW5 and
LW24 and LD14 dyes in the adsorbed state is estimated to about 150
ps. This confirms a rapid electron injection from the excited state of
τ
β
=56 m
=0.51
τ
β
=31
=0.52
ꢁ
0.0
0.2
-
1.0
1E-4
1E-3
0.01
0.1
1E-7
1E-6
1E-5
1E-5
1E-4
1E-4
LW4/W08
1E-3
LW4/solvent
0.8
0.6
0.4
0.2
0.0
s
s
τ
β
=38 m
=0.52
τ
β
=35
=0.47
ꢁ
2
0
sensitizers into the TiO conduction band.
2
-0.2
1
0
0
.01E-5
1E-4
1E-3
0.01
LW5/solvent
0.1
1E-7
1E-6
1E-4
1E-3
1
LW5/ACN
.8
.6
LD14-THF
LD14-TiO2
LW5-THF
LW5-TiO
LW24-THF
LW24-TiO
2
y=y +A exp(-x/
τ)
0.4
0.2
y=y +A exp(-x/
τ)
LW4-THF
LW4-TiO2
0
1
0
1
2
s
s
τ
=8.8 m
=0.34
τ=42
ꢁ
0.0
LD14
LW5
β
0.2
β
=0.25
-
1.21E-5
τ=1.69 ns
τ=1.66 ns
1E-4
1E-3
0.01
LW24/solvent
0.1
1E-6
1E-5
1E-3
0.01
1.0
LW24/W08
LW4
0.8
0.6
LW24
τ=0.89 ns
τ=1.54 ns
0.4
0.2
0.0
s
s
τ
β
=11.4 m
=0.46
τ
β
=51
=0.25
ꢁ
-0.2
1E-4
1E-3
0.01
0.1 1E-7
1E-6
1E-5
1E-4
1E-3
0.01
Time (s)
Time (s)
0
1
2
3
4
50
1
2
3
4
5
Time delay (ns)
4
| J. Name., 2012, 00, 1-3
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Journal Name
Physical Chemistry Chemical Physics
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ARTICLE
DOI: 10.1039/C5CP05658F
Fig. 5 Transient absorption spectra (TAS) of LD14, LW4, LW5, and LW24
The driving force of the regeneration reaction can be informed
porphyrins on mesoporous TiO , with or without redox couple (iodine based from the difference of the electrolyte’s redox potential and the dye’s
2
electrolyte, coded W08). The traces with scatter are raw data; the solid curves HOMO level. As illustrated in Fig. 3, it is evaluated to be similar
are the results fitted according to the equation. Excitation wavelength: 460 driving force values of 0.32, 0.31, 0.28, and 0.33 eV for the LD14,
nm; detection wavelength: 550 nm.
LW4, LW5, and LW24, respectively. Therefore, we can conclude
that the variation in φ is not induced by the thermodynamic
driving force. Generally, regeneration of the oxidized dye in
reg
Table 2. Parameters of the stretched exponential function used in the fit of
22
the transient absorption signals for various porphyrins (LW4, LW5, LW24
homogeneous medium can be reasonably well described by the
and LD14) on TiO
electrolyte. Excitation wavelength is 460 nm and the detective wavelength is
50 nm. τ and β are the parameters acquired from eq(1), τobs is the averaged
2
electrode in the presence and absence of iodineꢀbased
21c
classic theory of electron transfer developed by Marcus. Within
this context, all the investigated porphyrins are designed with the
same donor and porphinate group. We may ascribe the different rate
of electron transfer between donor and acceptor to the variation of
the conjugation length and intrinsic property of the spacer. As
presented in Table 2, it is likely that an identical φreg could be found
for these dyes possessing with similar conjugation length as
evidenced in LW5 and LW24, or LD14 and LW4. Furthermore, the
result infers that conjugation length plays critical influence onto the
regeneration reaction in porphyrin based DSSC systems, rather than
5
1
time constant, krec and kreg are the rate constant of recombination and
regeneration process, respectively. φreg is the quantum yield of the dye
regeneration.
ꢀ
ꢀ
ACN
Β
I /I
3
φ
reg
τ
1
τ
(ꢁs
)
1
τ
(ꢁs
)
obs
k
s
rec
ꢀ1
k
reg
ꢀ1 ꢀ1
dye
τ
obs
(m
β
(M s )
(%)
(ms)
s)
LD1
4
LW
4
LW
5
LW
0.5
1
0.5
2
0.3
4
0.5
2
0.4
7
0.2
5
0.2
5
5
5
6
8
108
71
31
35
42
51
70
79
9.3
4.7×10
99.9 their intrinsic properties of those units. Thus, the regeneration is
quite similar for the four samples regardless of electronꢀdeficient or
5
3
14.1
43.5
37.0
4.2×10
99.9
rich unit inserted between the porphine and benzoic acid.
10
08
12
24
4
20
18
16
14
100
80
60
40
20
0
8
.8
23
2.9×10
95.6
95.5
11. 0.4
4
2
7
2.4×10
2
4
4
6
12
LD14-devices
LW4-devices
LW5-devices
LW24-devices
In this configuration, the rate constant (k ) for recombination
10
8
rec
process calculated from the inverse of the recombination average
LD14 device
LW4 device
LW5 device
LW24 device
6
ꢀ
1
time constants are 9.3, 14.1, 43.5 and 37.0 s for LD14, LW4, LW5
and LW24, respectively. Thus, the regeneration rate constant, kreg,
can be obtained by using eq(2) with the previously calculated value
krec for the recombination:
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
400
450
500
550
600
650
700
750
800
850
Voltage (V)
Wavelength (nm)
Fig. 6 a) J–V curves of and the DSSC devices based on LD14, LW4, LW5,
and LW24 porphyrins measured under simulated AM 1.5G full sunlight; b)
IPCE spectra of LD14, LW4, LW5, and LW24 porphyrins devices. The cells
kobs − k
rec
kreg
=
eq(2)
−
[
I ]
ꢀ2
ꢀ
were measured with a mask (area= 0.09 cm ).
where the [I ] refers to the concentration of the reduced species of
5
5
the redox pair. The k calculated from eq(2) are 4.7×10 , 4.2×10 ,
By using an optimized binary solvent of toluene and ethanol
.9×10 and 2.4×10 s . The obviously slower oxidized dye (volume ratio 1:1), the porphyrins were sensitized onto a bilayer (7.5
reg
2
4
ꢀ1
2
regeneration rate of LW5 and LW24 could be explained by the + 5.0 ꢁm) titania film to serve as a working electrode. We evaluated
longer conjugation length of molecules. Considering the different these porphyrins in DSSC devices in combination with iodineꢀbased
results from PL decay and TAS, it suggested different charge electrolyte. The currentꢀvoltage (JꢀV) characteristics and the
transfer processes from porphyrin sensitizers to semiconductor or to corresponding action spectra of incident photons to electrons
reduction species in the presence of electrolyte. Table 2 gives the conversion efficiency (IPCE) of the devices are shown in Fig. 6a and
obtained rate constants for the cells sensitized with LD14, LW4, 6b. The photovoltaic parameters, i.e., PCE, short circuit photocurrent
LW5 and LW24. Finally, the quantum yield of the dye regeneration density (J ), the open circuit photovoltage (VOC), and fill factor
SC
2
1
(
φreg) for the complete device is determined from eq(3):
(FF) extracted from Fig. 6a are summarized in Table 3. The LD14
devices show a PCE 9.01% with a VOC 734 mV, while LW4 devices
exhibit 9.53% with a superior VOC of 751 mV. The better
−
k [I ]
reg
ϕ
(reg) =
eq(3)
kobs
performance of the latter is mainly benefited from the higher VOC.
However, for LW5, the J of the corresponding devices decrease
SC
As showed in Table 2, the oxidized dye regeneration efficiencies
ꢀ2
nearly 2 mA cm , in spite of an enhanced lightꢀharvesting ability
are 99.9%, 99.9%, 95.6% and 95.5% for LD14, LW4, LW5 and
LW24 devices, respectively. It is worth to note that the porphyrins
(
or BTDꢀphenyl) exhibit slightly lower value of regeneration
efficiency than that having shorter ones (phenyl or thiophene).
around 725ꢀ750 nm. For the LW24 featured with BTD extended
spacer, the IPCE tail of the corresponded devices significantly
broadens to nearly 840 nm. Thus, the LW24 DSSC devices achieve a
LW5 and LW24) featuring with longer π spacer (thiopheneꢀphenyl
ꢀ2
JSC of 17.14 mA cm . LW24 devices not only show higher PCE than
LW5 devices, but also superior than LD14. The IPCE intensity is a
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

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Journal Name
function of charge collection, electron injection in the TiO , and dye group can accelerate conductionꢀband electron recapture by the
2
2
2
3b,3c,7
regeneration efficency. As discussed above, the oxidized dye sensitizer.
Thus, we ascribe the lower VOC of the LW24 in
regeneration efficiencies of LW5 or LW24 are lower than LD14 or comparison with LW4 devices to a serious recombination reaction.
LW4, which may give a clue for the lower IPCE values in the former
LD14 devices
LW4 devices
LW5 devices
LW24 devices
LD14 devices
LW4 devices
LW5 devices
LW24 devices
two kinds devices.
80
a)
b)
100
Table 3. Photovoltaic parameters (with standard deviations) of DSSC devices
6
4
2
0
using LD14, LW4, LW5, and LW24 dyes in combination with iodineꢀbased
ꢀ2
electrolyte under an irradiation at 100 mW cm . Five independent cells were
0
measured to obtain the average values. The cells were measured using a mask
10
ꢀ2
(
area= 0.09 cm ).
0
0
.55
0.60
0.65
0.70
0.75
0.2
0.4
0.6
0.8
1.0
ꢀ
2
Device
LD14
LW4
J
SC (mA cm )
17.38±0.10
17.65±0.18
15.41±0.12
17.14±0.20
V
OC (V)
FF
PCE(%)
9.01±0.06
9.53±0.07
8.16±0.10
9.21±0.12
19
-3
Voltage (V)
n (10 cm )
t
0.734±0.006
0.751±0.008
0.719±0.004
0.737±0.012
0.710±0.002
0.720±0.003
0.735±0.005
0.723±0.012
Fig. 7 Transient photovoltage decay and charge extraction measurements of
LD14, LW4, LW5, and LW24 based DSSC devices. a) Comparisons of
chemical capacitance at a certain open–circuit photovoltage; b) Comparisons
of electron lifetime at a given electron density.
LW5
LW24
Conclusions
The VOC is defined as the energy difference between the quasiꢀ
Fermi level (E ) of the TiO under illumination and the Nernst
potential of the redox electrolyte (EF,redox). Since all the devices
In summary, we have shown a systematic investigation of the
influence from varied πꢀspacers in porphyrin sensitizers on the
F
2
2
2
photoꢀinduced charge transfer processes and photovoltaic
performance. The results reveal the electronꢀdeficient unit BTD
inserted between porphyrin core and the benzoic acid anchoring
group could lead to a broadened lightꢀharvesting spectral region
here utilize similar electrolytes, TiO photoanode and fabrication
processes, the variations of VOC can be rationalized explained based
2
on the changes of electronic E , which depends on the electron
F
2
3
density in the TiO2. In this case, we performed transient photoꢀ
voltage decay and charge extraction measurements to understand the
as well as an upward shift of the conduction band edge of TiO2.
However, the electronꢀrich unit thiophene at the same site
physical origin of the V difference between the four porphyrins
OC
2
4
shows less significant impact. These results could be explained
by the difference of electron coupling in the excited state as
well as internal charge transfer within the molecule. Porphyrins
with solely aromatic spacer shows prolonged electron lifetime
and faster dyeꢀregeneration reaction in comparison with the
spacer extended porphyrins. The compromised effect brought
about by the extension of the conjugation length results in the
overall conversion efficiency of 9.2% for LW24, while LW4
with shorter spacer presented the highest efficiency of 9.5%.
We believe that this structure–property relationship will shed
based DSSC. Fig. 7a presents the capacitance (C ) at different
ꢁ
voltages for various DSSC. The C follows the expression of C ∝
ꢁ
ꢁ
25
exp(qV/mkT), where k is the Boltzmann constant, T is the absolute
temperature, and m is related to the shape of the distribution of the
density of states. Similar slopes for the plots in Fig. 7a indicate the
same trap state distribution in the TiO for various devices. At the
2
identical C , the openꢀcircuit voltage of the LW24 device is about
ꢁ
1
0ꢀ20 mV higher than the other devices (Fig.7a). Whilst the LD14
devices show a slight 5 mV lower than LW4 and LW5 devices, and
5 mV lower than LW24. Sensitizers adsorbed on the semiconductor
1
light on
a better molecular design and synthesis of
surface can modify its electronic properties, such as band bending,
on account of different dipole moments (∆φ∝Nꢁcosθ, where ∆φ is
the electrostatic potential drop, N being the number of adsorbed
molecules per surface area, ꢁ being the molecular dipole moment, θ
being the tilt angle between the axes of the dipole moment and the
panchromatic porphyrin sensitizers to further improve the
photovoltaic performance for a DSSC.
Acknowledgements
JL, HL, SL, YC, and MW thank the Director Fund of WNLO,
the 973 Program of China (2014CB643506 and
surface normal of the TiO particles). Neglecting the amount and tilt
2
2
7
angle of the four porphyrins adsorbed on TiO2, it is likely that the
2
013CB922104) and NSFC (21173091) for financial support;
higher TiO band bending edge in LW24 devices could be explained
2
they also thank the Analytical and Testing Centre at HUST for
performing characterization of various samples. YCC, HPW
and EWGD received support from National Chiao Tung
University (NCTU), Ministry of Science and Technology
by the bigger dipole moment of LW24 porphyrin. Fig. 7b presents
the electron lifetime (τ) as a function of electron density, which
further provides the information about recombination between the
3
injected electrons in the TiO and the oxidized form of electrolytes.
2
(
MOST) of Taiwan and Ministry of Education (MOE) of
All devices showed similar shape of the curves in Fig. 7b, which
Taiwan. The authors also thank Prof. Jie Xu for the DFT
calculation results. JL thanks NCTU (Hsinchu, Taiwan) and
WNLO (Wuhan, China) for supporting of his visit to NCTU.
may be ascribed to the efficient insulation of the TiO surface by the
2
2
8
same four dodecyloxyꢀchains. At a given electron density, the
electron lifetime of LW4 device is longer than the other three
porphyrins, while the LW24 device is the shortest one. The BTD
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

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View Article Online
ARTICLE
DOI: 10.1039/C5CP05658F
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DOI: 10.1039/C5CP05658F
Novel porphyrins − preparation, characterization, and application in solar energy conversion
By: J. Lu, H. Li, S. Liu, Y. Chang, H. Wu, Y. Cheng, E. Diau, and M. Wang
Accelerated inner charge transfer in porphyrin promotes broad light-harvesting ability up to 840
nm and a conversion efficiency of 9.2%.