Enhanced performance of quasi-solid-state dye-sensitized solar cells by branching the linear substituent in sensitizers based on thieno[3,4-c]pyrrole-4, 6-dione

Article abstract of DOI:10.1002/asia.201200720

Thieno[3,4-c]pyrrole-4,6-dione-based organic sensitizers with triphenylamine (FNE38 and FNE40) or julolidine (FNE39 and FNE41) as electron-donating unit have been designed and synthesized. A linear hexyl group or a branched alkyl chain, the 2-ethylhexyl group, is incorporated into molecular skeleton of the dyes to minimize intermolecular interactions. The absorption, electrochemical, and photovoltaic properties for these sensitizers were then systematically investigated. It is found that the sensitizers have similar photophysical and electrochemical properties, such as absorption spectra and energy levels, owing to their close chemical structures. However, the quasi-solid-state dye-sensitized solar cells (DSSCs) based on the two types of sensitizers exhibit very different performance parameters. Upon the incorporation of the short ethyl group on the hexyl moiety, enhancements in both open-circuit voltage (V_oc) and short-circuit current (J _sc) are achieved for the quasi-solid-state DSSCs. The V_oc gains originating from the suppression of charge recombination were quantitatively investigated and are in good agreement with the experimentally observed V_oc enhancements. Therefore, an enhanced solar energy conversion efficiency (I·) of 6.16 %, constituting an increase by 23 %, is achieved under standard AM 1.5 sunlight without the use of coadsorbant agents for the quasi-solid-state DSSC based on sensitizer FNE40, which bears the branched alkyl group, in comparison with that based on FNE38 carrying the linear alkyl group. This work presents a design concept for considering the crucial importance of the branched alkyl substituent in novel metal-free organic sensitizers. Copyright

Full text of DOI:10.1002/asia.201200720

FULL PAPER
DOI: 10.1002/asia.201200720
Enhanced Performance of Quasi-Solid-State Dye-Sensitized Solar Cells by
Branching the Linear Substituent in Sensitizers Based on
ThienoACTHNUGTRNEUNG[3,4-c]pyrrole-4,6-dione
Quanyou Feng, Weiyi Zhang, Gang Zhou,* and Zhong-Sheng Wang*[a]
Abstract:
Thieno
[3,4-c]pyrrole-4,6-
quasi-solid-state dye-sensitized solar
cells (DSSCs) based on the two types
of sensitizers exhibit very different per-
formance parameters. Upon the incor-
poration of the short ethyl group on
the hexyl moiety, enhancements in
both open-circuit voltage (Voc) and
short-circuit current (Jsc) are achieved
for the quasi-solid-state DSSCs. The
Voc gains originating from the suppres-
sion of charge recombination were
quantitatively investigated and are in
good agreement with the experimental-
ly observed Voc enhancements. There-
fore, an enhanced solar energy conver-
sion efficiency (h) of 6.16%, constitut-
ing an increase by 23%, is achieved
under standard AM 1.5 sunlight with-
out the use of coadsorbant agents for
the quasi-solid-state DSSC based on
sensitizer FNE40, which bears the
branched alkyl group, in comparison
with that based on FNE38 carrying the
linear alkyl group. This work presents
a design concept for considering the
crucial importance of the branched
alkyl substituent in novel metal-free or-
ganic sensitizers.
dione-based organic sensitizers with tri-
phenylamine (FNE38 and FNE40) or
julolidine (FNE39 and FNE41) as elec-
tron-donating unit have been designed
and synthesized. A linear hexyl group
or a branched alkyl chain, the 2-ethyl-
hexyl group, is incorporated into mo-
lecular skeleton of the dyes to mini-
mize intermolecular interactions. The
absorption, electrochemical, and photo-
voltaic properties for these sensitizers
were then systematically investigated.
It is found that the sensitizers have
similar photophysical and electrochem-
ical properties, such as absorption spec-
tra and energy levels, owing to their
close chemical structures. However, the
Keywords: branched alkyl chain ·
donor–acceptor systems · dye-sensi-
tized solar cells · quasi-solid-state ·
sensitizers
Introduction
phyrin dye designated YD2-o-C8 with another organic dye
using a cobalt-based electrolyte.[4]
Dye-sensitized solar cells (DSSCs) have recently been ex-
tensively studied as a promising candidate for low-cost,
lightweight, and scalable solar cells.[1] Undoubtedly, to ach-
ieve high photon-to-current conversion efficiency, one of the
key components in DSSCs is the sensitizer, which absorbs
light, injects electrons into the conduction band of nano-
structured titania, and then is regenerated by the redox elec-
trolyte.[2] Up to now, such cells, in which mostly ruthenium
complexes are used as sensitizers, have achieved power con-
version efficiencies above 11% under standard global air
mass 1.5 solar (AM1.5) conditions.[3] However, the practical
application of ruthenium-based dyes has become a critical
problem owing to the limited availability and high cost of
ruthenium metal and the extensive purification steps. Inspir-
ingly, DSSC devices with an efficiency of 12.3% have re-
cently been achieved through cosensitization of a zinc por-
As a promising and cost-effective alternative to dyes con-
sisting of metal complexes, metal-free organic dyes have at-
tracted considerable attention for practical applications
owing to their lower cost of production and better flexibility
in terms of molecular tailoring.[2a,5] Generally, organic sensi-
tizers used for efficient DSSC devices are composed of
three units, consisting of a donor, an acceptor, and a connect-
ing p-conjugation bridge (D-p-A). In the last decade, many
kinds of organic dyes with such configuration have been ex-
plored for DSSCs, including sensitizers based on thiophene
derivatives,[6] furan,[7] selenophene,[8] and pyrrole,[9] and im-
pressive efficiencies over 8% have been achieved in liquid
electrolyte systems.[6e,h,10] To further extend the absorption
capability and tune the energy level, a few organic sensitiz-
ers in which electron-withdrawing or electron-deficient
groups were incorporated, such as benzothiadiazole,[11] ben-
zotriazole,[12] thienopyrazine,[13] quinoxaline,[14] diketopyrro-
lopyrrole,[15] bithiazole,[16] and quinacridone,[17] have been
also utilized in DSSCs.
[a] Q. Feng, W. Zhang, Prof. Dr. G. Zhou, Prof. Dr. Z.-S. Wang
Lab of Advanced Materials, Department of Chemistry
Fudan University
In comparison to the DSSCs with liquid electrolytes,
quasi-solid-state DSSCs[18] have shown greatly improved
long-term stability as there is less or no possibility for leak-
age of the electrolyte. Therefore, in view of promising com-
mercial applications, the study on quasi-solid-state DSSCs
2205 Songhu Road, Shanghai, 200438 (P. R. China)
E-mail: zhougang@fudan.edu.cn
zs.wang@fudan.edu.cn
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Gang Zhou, Zhong-Sheng Wang et al.
has been an active area of research in this field. Recently,
we have integrated thieno[3,4-c]pyrrole-4,6-dione (TPD)
Results and Discussion
AHCTUNGTRENNUNG
Synthesis and Structural Characterization
into organic sensitizers for quasi-solid-state DSSCs[19] as the
TPD unit has been employed to construct low band gap co-
polymers for bulk heterojunction solar cells.[20] In general,
the rod-like configuration always induces undesirable p–p
stacking and intermolecular aggregation, which may lead to
self-quenching of excited states and hence inefficient elec-
tron injection. Therefore, the introduction of a hexyl chain
on the pyrrole ring was highly beneficial to prevent the in-
termolecular interaction, and an improved DSSC perfor-
mance was obtained.[19] However, the performance of the re-
sulting dye-based DSSCs was still limited since a linear
hexyl chain substituted on the p-conjugation unit was insuf-
ficient to suppress charge recombination and to improve the
device performance according to our previous study.[6e]
Thus, other alkyl groups should be introduced into the mo-
lecular skeleton. Meanwhile, simple incorporation of alkyl
groups on the thiophene unit may induce a molecular twist
of the spacer skeleton, which always leads to a hypsochromic
shift of the absorption band and hence reduces the light har-
vesting efficiency. Therefore, in this work, by adding the ex-
tremely short ethyl group to the existing hexyl chain on the
TPD moiety, we investigated the effect of branched alkyl
chains on photovoltaic properties and charge recombination
dynamics. Quasi-solid-state DSSCs based on sensitizers
FNE40 and FNE41 bearing a branched 2-ethylhexyl chain
(Figure 1) yielded a higher open-circuit photovoltage (Voc)
than those based on FNE38 and FNE39, which carry the
The synthetic approach to obtain sensitizers FNE38–FNE41
starting from thienoACTHNUTRGNEUNG
[3,4-c]furan-1,3-dione (1)[21] is depicted
in Scheme 1. First, a hexyl or 2-ethylhexyl group was intro-
duced by refluxing 1 with the corresponding primary amine
Scheme 1. Synthetic route for sensitizers FNE38–FNE41.
in acetic acid. After bromination by N-bromosuccinimide
(NBS), a Suzuki coupling[22] with thiophene-2-boronic acid
produced spacers 4 that would bridge the donor and anchor
groups. Next, under refluxing with a Vilsmeier reagent,[23]
the corresponding aldehyde-substituted derivatives 5 were
synthesized. After another bromination by NBS, an electron
donor, triarylamine or julolidine, was attached through Stille
coupling,[24] thus providing the precursors 7. In the last step,
the obtained precursors were converted into the correspond-
ing sensitizers FNE38–FNE41 by Knoevenagel condensa-
tion[25] with cyanoacetic acid under refluxing acetonitrile in
the presence of piperidine. All target sensitizers were char-
Figure 1. Chemical structures of sensitizers FNE38–FNE41.
linear hexyl chain, owing to lower interfacial charge recom-
bination losses. The effect of the introduced alkyl groups on
the Voc gain was systematically investigated by performing
controlled intensity modulated photovoltage spectroscopy
(IMVS) measurements, which are in good agreement with
the experimental observation. The quasi-solid-state DSSC
based on FNE40 produced an enhanced Voc in combination
with a higher short-circuit current (Jsc) and therefore results
in a conversion efficiency (6.16%) that is 23% higher than
that of the FNE38-based quasi-solid-state DSSC.
1
acterized by H NMR and 13C NMR spectroscopy, and mass
spectrometry, and were found to be consistent with the pro-
posed structures.
Absorption Properties
The electronic absorption spectra of the dyes in THF solu-
tions are shown in Figure 2a, and the corresponding photo-
physical data are summarized in Table 1. In the electronic
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Gang Zhou, Zhong-Sheng Wang et al.
ethylhexyl groups on the TPD units suppressed the dye ag-
gregation in THF and obstructed intermolecular interac-
tions. Upon replacing the triphenylamine unit in FNE38 and
FNE40 with a stronger electron-donating group, julolidine
(FNE39 and FNE41), a significant bathochromic shift of
25 nm and 19 nm, respectively, was observed for the absorb-
ance maximum. Apparently, this shift is caused by the in-
crease in the electron-donating ability and the expansion of
the p conjugation.
FNE38–FNE41 adsorbed on transparent TiO2 films (Fig-
ure 2b) have absorbance maxima at 435, 439, 437, and
448 nm, respectively. The hypsochromic shift of 50, 71, 50,
and 58 nm, respectively, in comparison to the maxima in
THF may originate from the deprotonation of the carboxyl-
ic acid, which would weaken the ICT interaction. Such hyp-
sochromic shifts in the absorption spectra of organic dyes
adsorbed on TiO2 films have also been observed in other
metal-free organic dyes.[6d,27] Moreover, compared with the
TiO2 films loaded with FNE38 and FNE39, the TiO2 films
loaded with FNE40 and FNE41 have an absorbance de-
creased by 20% and 26%, respectively. This is probably due
to the incorporation of the additional ethyl group, which in-
creases the molecular size of the dye and therefore reduces
the amounts of sensitizers absorbed on the TiO2 films. The
dye loading amount of FNE38 and FNE40 is 2.4ꢁ10ꢀ8 and
2.3ꢁ10ꢀ8 molcmꢀ2 mmꢀ1, respectively. When the donor tri-
phenylamine is replaced by julolidine, dye loading is in-
creased to 2.7ꢁ10ꢀ8 and 2.5ꢁ10ꢀ8 molcmꢀ2 mmꢀ1 for FNE39
and FNE41, respectively.
Figure 2. Absorption spectra of FNE38–FNE41 in THF solution (a) and
on transparent titania films of 3.5 mm thickness (b).
absorption spectra, all the dyes exhibit two distinct absorp-
tion bands. The absorption band in the high-energy region
(380–430 nm) corresponds to the p–p* electron transition of
the conjugated backbone, and the one in the low-energy
region (450–600 nm) can be assigned to an intramolecular
charge transfer (ICT) from the electron-donating unit to the
electron-withdrawing group.[6d,26] As shown in Figure 2a, the
absorption band of FNE38 has a maximum at 485 nm. Upon
replacement of the hexyl chain in FNE38 with a branched 2-
ethylhexyl chain in FNE40, a similar absorption spectrum
with a maximum at 487 nm was obtained, as expected owing
to the nearly identical chemical structures of these two sen-
sitizers. The absorption spectra of FNE38 and FNE40 were
moreover recorded at different concentrations to study the
intermolecular interactions. No obvious shift in the maxi-
mum was observed when the concentration was decreased
from 10ꢀ4 m to 10ꢀ6 m, thus indicating that both hexyl and 2-
Electrochemical Properties
To obtain and understand the molecular orbital energy
levels, cyclic voltammetry (CV) measurements were carried
out to determine the oxidation potential of the dyes on TiO2
with tetrabutylammonium hexafluorophosphate in dry ace-
tonitrile as supporting electrolyte. The cyclic voltammo-
grams are shown in Figure 3 and the relevant CV data are
collected in Table 1. The first half-wave potential corre-
sponding to the HOMO level was determined to be 1.13,
0.71, 1.13, and 0.72 V (vs. NHE) for sensitizers FNE38,
FNE39, FNE40, and FNE41, respectively. Correspondingly,
as calculated from the HOMO level and the optical band
gap[5] derived from the wavelength at 10% maximum ab-
sorption intensity for the dye-loaded TiO2 film, the LUMO
levels for sensitizers FNE38, FNE39, FNE40, and FNE41
Table 1. Photophysical and electrochemical properties of sensitizers FNE38–FNE41.
Dye
Absorption
HOMO [V][b]
E0ꢀ0 [eV]
LUMO [V]
[a]
lmax [nm][a]
e [Mꢀ1 cmꢀ1
]
lmax
on TiO2 [nm]
FNE38
FNE39
FNE40
FNE41
485
510
487
506
3.2ꢁ104
1.7ꢁ104
3.0ꢁ104
1.4ꢁ104
435
439
437
448
1.13
0.71
1.13
0.72
2.08
1.79
2.07
1.75
ꢀ0.95
ꢀ1.08
ꢀ0.94
ꢀ1.03
[a] Absorption peaks (lmax) and molar extinction coefficients (e) were measured in THF solutions (ca. 10ꢀ5 m). [b] The potentials (vs. NHE) were calibrat-
ed with ferrocene.
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Gang Zhou, Zhong-Sheng Wang et al.
The thienoACTHNUTRGNEUG[N 3,4-c]pyrrole-4,6-dione unit and the two neigh-
boring thiophene rings lie almost in the same plane, which
indicates that no obvious steric hindrance exists between the
thienoACTHNURTGNEU[GN 3,4-c]pyrrole-4,6-dione moiety and the neighboring
thiophene unit, and the introduction of the linear or
branched alkyl chain on the pyrrolidinedione unit can
hardly twist the conformation of the conjugated spacer in
the sensitizers.
Photovoltaic Performance of DSSCs
Quasi-solid-state DSSCs based on the four sensitizers were
fabricated with a polymer gel electrolyte.[18a] To achieve the
best DSSC performance, the content of tert-butylpyridine
(TBP) was first optimized for the quasi-solid-state DSSC
based on FNE40 since it provided the best performance in
our initial measurements. The performance parameters for
the quasi-solid-state DSSCs are summarized in Table 2.
Figure 3. Cyclic voltammograms of the resulting dye-loaded TiO2 films.
were calculated to be ꢀ0.95, ꢀ1.08, ꢀ0.94, and ꢀ1.03 V, re-
spectively, thereby indicating the dyes have enough driving
force for electron injection from their excited states. There-
fore, it can be concluded that the hexyl and 2-ethylhexyl
substituents on the pyrrolidine-2,5-dione moiety have no dis-
tinguishable influence on both HOMO and LUMO energy
levels.
Table 2. Photovoltaic performance for FNE40-based quasi-solid-state
DSSCs with electrolytes containing different concentrations of TBP and
I2.
Ciodine [M]
CTBP [M]
Voc [mV]
Jsc [mAcmꢀ2
]
FF
h [%]
0
584
601
613
623
608
584
566
14.08
13.03
11.49
9.60
14.68
14.08
13.68
0.68
0.68
0.71
0.71
0.69
0.68
0.68
5.56
5.33
4.98
4.23
6.16
5.56
5.30
Theoretical Approach
0.05
0.15
0.3
0.1
To gain insight into the geometrical and electronic proper-
ties of the studied sensitizers, density functional calculations
were conducted using the Gaussian 03 program package at
the B3LYP/6-31G(d) level.[28] It can be clearly seen from
Figure 4 that for all dyes the HOMO orbitals are mainly
composed of the triphenylamine or julolidine moiety, with
contributions from the neighboring thiophene extending to
the TPD moiety, while the LUMO delocalized over the
acrylic acid unit through p-conjugation, thus allowing signifi-
cant charge separation within the dye and hence efficient
electron injection upon photoexcitation. Furthermore, upon
replacing the hexyl chain with the 2-ethylhexyl chain, a simi-
lar coplanarity can be found in the spacer of FNE40 and
FNE41 in comparison with FNE38 and FEN39, respectively.
0.05
0.1
0.3
0
When the concentration of TBP was increased, the Voc value
increased slightly from 584 to 623 mV, while the Jsc value
dramatically decreased from 14.08 to 9.60 mAcmꢀ2, in
agreement with the reported phenomenon that the presence
of TBP improves Voc but decreases Jsc. This is mainly attrib-
uted to the negative shift of the conduction band of TiO2
caused by the adsorption of TBP on the TiO2 surface, which
lowers the driving force for the electron injection but in-
creases the energy level difference between the Fermi level
of TiO2 and the redox potential of the electrolyte under
light. As a result, although TBP has proved to be an effi-
cient additive to improve the DSSC performance,[29] the
highest conversion efficiency h for the FNE40-based quasi-
solid-state DSSC in this work was achieved with a polymer
gel electrolyte without TBP.
To further investigate the relationship between the addi-
tive concentration and the performance of the quasi-solid-
state DSSCs, the content of another additive, I2, in the elec-
trolyte was also optimized. As shown in Table 2, a remark-
able decrease was observed for both Voc and Jsc values with
increasing the I2 concentration. This result stems from the
fact that an increased I2 content in the electrolyte increases
the probability for electron recombination between the in-
jected electrons and the electron acceptors in the electrolyt-
e.[6e] Combining the above two effects of TBP and I2 concen-
Figure 4. Calculated frontier molecular orbitals and experimental energy
level diagram of FNE38–FNE41.
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Gang Zhou, Zhong-Sheng Wang et al.
trations, it was found that the best performance for FNE40-
based quasi-solid-state DSSC was obtained with 0.05m I2
but without TBP in the electrolyte.
group in comparison to those based on sensitizers with the
linear hexyl group.
As p–p stacked dye aggregates are inefficient for electron
injection and thus photocurrent generation,[30] DCA is usual-
ly used as a coadsorbate to break up the dye aggregates for
higher photocurrent generation.[29d,30b] Therefore, the effect
of the coadsorption of DCA on the DSSC performance is
attributed to the aggregation degree of the organic dyes ad-
sorbed on the TiO2 surface. To investigate the intermolecu-
lar interactions of the four dyes, the effect of DCA coadsor-
bate on the DSSC performance was investigated. As shown
in Table 3, upon the coadsorption of DCA, Jsc increased
from 12.92 and 9.42 mAcmꢀ2 to 13.54 and 9.76 mAcmꢀ2 for
the quasi-solid-state DSSCs based on FNE38 and FNE39,
respectively, thus indicating that the hexyl groups in FNE38
and FNE39 are still insufficient to completely suppress the
intermolecular interactions among the dye molecules anch-
ored on the TiO2 surface. By contrast, coadsorption of DCA
reduced the photocurrent for the quasi-solid-state DSSCs
based on FNE40 and FEN41 owing to the lower amount of
dye adsorbed. This indicates that the branched alkyl chain
can inhibit the intermolecular interactions more effectively
than the linear alkyl chain. Therefore, the devices based on
FNE40 and FEN41 containing a branched alkyl chain pro-
duced a higher photocurrent than those based on their ana-
logues with a linear alkyl chain.
The differences in Voc for the quasi-solid-state DSSCs
based on the dyes with the linear or branched chain was in-
vestigated through charge extraction and IMVS measure-
ments. For a DSSC with a given electrolyte system, the gen-
eration of Voc depends not only on the position of the con-
duction band of TiO2 but also on the charge recombination
rate in DSSCs. To understand why the DSSC based on the
sensitizer with the 2-ethylhexyl group gives a higher Voc
compared to that based on the sensitizer with the hexyl
group, the relative position of the conduction band in the
DSSCs based on the different dyes was scrutinized. Accord-
ing to the method developed by Frank and coworkers,[31] the
movement of the conduction band edge contributes to the
change of Voc at constant photoinduced charge density (Q),
which was measured by using the charge extraction tech-
nique[32] under illumination with an LED light (532 nm). A
plot of Voc versus the charge density at open circuit for the
quasi-solid-state DSSCs is shown in Figure 6. It can be
found that the Voc increases linearly with the logarithm of Q
for all DSSCs. The four curves for the corresponding devices
have almost the same slope (150 mV) and overlap with each
other. The fact that Voc is almost the same for all DSSCs at
a fixed Q indicates the identical conduction band position
for each case. Therefore, it can be concluded that there is no
influence on the conduction band edge when the alkyl
group is changed from the linear hexyl to the branched 2-
ethylhexyl chain.
The photovoltaic performance of the quasi-solid-state
DSSCs using 5% PVDF-HFP (w/w), 0.1m LiI, 0.05m I2, and
0.6m DMPImI in MPN as redox electrolyte (see the Experi-
mental Section) were evaluated by recording the photocur-
rent–voltage (J–V) curve at 100 mWcmꢀ2 simulated AM1.5
conditions (Figure 5); the detailed photovoltaic parameters
are listed in Table 3. The Jsc, Voc, and fill factor (FF) values
of the FNE38-sensitized cell are 12.92 mAcmꢀ2, 567 mV,
and 0.68, respectively, generating a h of 5.01%. It can be
seen from Table 3 that the quasi-solid-state DSSC incorpo-
rating FNE40 results in both enhanced Voc and Jsc values;
hence, an increase in h by 23% to 6.16% as compared to
the FNE38-sensitized solar cell was achieved. For the juloni-
dine-based dyes, upon incorporation of the branched 2-eth-
ylhexyl chain, the h increased from 2.77% for the FNE39-
based quasi-solid-state DSSC to 3.41% for the FNE41-
based quasi-solid-state DSSC. The quasi-solid-state DSSCs
based on FNE40 and FNE41 yielded a higher Jsc than the
DSSCs based on FNE38 and FNE39, respectively. Mean-
while, the Voc increased from 567 and 470 mV for quasi-
solid-state DSSCs based on FNE38 and FNE39, respectively,
to 608 and 505 mV for quasi-solid-state DSSCs based on
FNE40 and FEN41, respectively. Consequently, the overall
conversion efficiency was improved for the quasi-solid-state
DSSCs based on sensitizers with the branched 2-ethylhexyl
Figure 5. J–V curves for quasi-solid-state DSSCs based on FNE38–
FNE41.
Table 3. Photovoltaic performance parameters of quasi-solid-state
DSSCs based on FNE38–FNE41 with or without DCA.
Sensitizer
Voc [mV]
Jsc [mAcmꢀ2
]
FF
h [%]
FNE38
FNE38+DCA
FNE39
FNE39+DCA
FNE40
FNE40+DCA
FNE41
567
574
470
488
608
594
505
469
12.92
13.54
9.42
0.68
0.66
0.63
0.63
0.69
0.68
0.63
0.66
5.01
5.16
2.77
2.99
6.16
5.71
3.41
3.15
9.76
Therefore, the improvement of Voc from FNE38 to
FNE40 or FNE39 to FNE41, respectively, should be attrib-
uted to the inhibition of the interfacial charge recombina-
tion, which is often referred to as electron lifetime. Figure 7
14.68
14.27
10.77
10.19
FNE41+DCA
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Gang Zhou, Zhong-Sheng Wang et al.
Figure 8. Charge recombination current as a function of charge density at
open circuit for quasi-solid-state DSSCs based on sensitizers FNE38–
FNE41.
Figure 6. Voc as a function of charge density at open circuit for quasi-
solid-state DSSCs based on sensitizers FNE38–FNE41.
and a fixed light intensity, Jr equals the charge-injection cur-
rent density Jinj. Neither of them can be directly measured,
but they can be estimated from the measurement of Jsc. At
short circuit, Jr is taken as Jsc in value under the same light
intensity because of the negligible charge recombination at
short circuit.[34] At a given charge density, the charge recom-
bination current density is 3.6 times larger for the quasi-
solid-state DSSC based on FNE38 than that based on
FNE40, and 3.4 times larger for the quasi-solid-state DSSC
based on FNE39 than that based on FNE41, thereby indicat-
ing that the recombination rate constant is significantly re-
duced when the linear hexyl chain is replaced with the
branched 2-ethylhexyl group. The retarded charge recombi-
nation rate will reduce electron loss at open circuit. When
more charges accumulate in TiO2, the Fermi level moves
upward and Voc increases. For a DSSC, the electron density
(n) rises exponentially with Voc, or Voc increases with the log-
arithm of electron density, as shown in Equation (2):[35]
Figure 7. Electron lifetime as a function of charge density at open circuit
for quasi-solid-state DSSCs based on sensitizers FNE38–FNE41.
shows the electron lifetime as a function of Q for DSSCs
using the four dyes as sensitizers. The electron lifetime was
calculated by Equation (1):[33]
Voc ¼ mclnðnÞ
ð2Þ
where mc is 150 mV derived from Figure 6. When the ethyl
group is added to the hexyl group, Voc solely originating
from the suppression of charge recombination varies as the
electron density (n) or charge density (Q) changes, thus fol-
lowing Equation (3):[35–36]
1
ð1Þ
t ¼
2pfmin
where fmin is the frequency at the top of the semicircle in
IMVS. As illustrated in Figure 7, the quasi-solid-state
DSSCs based on FNE40 and FNE41 have a 4.3 and 6.5
times longer electron lifetime, respectively, at a certain
charge density in comparison to those based on FNE38 and
FNE40, respectively, which is attributed to the attenuated
charge recombination at open circuit. The data indicate that
the presence of the branched 2-ethylhexyl chain suppresses
the charge recombination between electrons in the TiO2
film and electron acceptors in the electrolyte more efficient-
ly than the linear hexyl chain.
n2
DVoc ¼ mcln ¼ mcln
n1
Q2
Q1
ð3Þ
At 10 Wmꢀ2 LED light (532 nm), the extracted charge
densities are 58.8 and 75.6 mCcmꢀ2 for the FNE38- and
FNE40-based DSSCs, respectively. According to the Q–Voc
plots, the Voc gain from the FNE38-based DSSC to the
FNE40-based DSSC arising solely from the increased
charge density is calculated to be 38 mV, explaining why the
FNE40-based DSSC produces an about 41 mV higher Voc
than the FNE38-based DSSC. Correspondingly, the Voc gain
from the FNE39-based DSSC to the FNE41-based DSSC
arising solely from the increased charge density is calculated
Furthermore, to investigate the effect of alkyl chains on
charge recombination rate, the charge recombination cur-
rent density (Jr) at open circuit was plotted against the
charge density (Q), as shown in Figure 8. At open circuit
Chem. Asian J. 2013, 8, 168 – 177
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Gang Zhou, Zhong-Sheng Wang et al.
to be 36 mV, which is consistent with our experimental ob-
servation (35 mV).
thienoACTHNGUTERN[UNG 3,4-c]pyrrole-4,6-dione moiety. It is found that the
structure of the alkyl group has no obvious effect on the ab-
sorption spectra and energy levels. However, the quasi-
solid-state DSSCs based on the sensitizers with the branched
alkyl substituent have a significantly improved photovoltaic
performance owing to reduced intermolecular interactions
and retarded charge recombination rate. As a result, the
conversion efficiency (h) of the quasi-solid-state DSSC
based on FNE40 is, with a value of 6.16%, significantly
higher (23%) compared with that based on FNE38 under
standard AM 1.5 sunlight without the use of a coadsorbant
agent. Our ongoing work will focus on obtaining sensitizers
that have even broader absorption bands to harvest more in-
cident photons while reducing electron loss from charge re-
combination.
Figure 9 shows the incident photon-to-electron conversion
efficiency (IPCE) spectra of the quasi-solid-state DSSCs
based on the resulting sensitizers. The IPCE of the quasi-
solid-state DSSC based on FNE40 exhibits a plateau be-
tween 440–660 nm, with an IPCE value over 80%, while
Experimental Section
General Information
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured on
a Varian Mercury Plus-400 spectrometer. The spectra were recorded in
CDCl3 or [D6]DMSO. The chemical shifts are expressed in ppm down-
field from tetramethylsilane (TMS). UV/Vis absorption spectra were re-
corded on a Shimadzu UV-2550PC spectrophotometer. Elemental analy-
ses were performed on an Elemental Vario EL3 elemental analyzer.
Electrochemical redox potentials were obtained by cyclic voltammetry
(CV) measurements, which were performed on a CH electrochemical
workstation (CHI604D) using a typical three-electrode electrochemical
cell in a solution of tetrabutylammonium hexafluorophosphate (0.1m) in
anhydrous acetonitrile at a scan rate of 50 mVsꢀ1 at room temperature
under nitrogen. Dye-adsorbed TiO2 film (6 mm thickness, 0.25 cm2) on
conductive glass was used as the working electrode, a Pt wire as the
counter electrode, and an Ag/Ag+ electrode as the reference in anhy-
drous acetonitrile. The potential of the reference electrode was calibrated
with ferrocene, and all potentials given are against the normal hydrogen
electrode (NHE).
Figure 9. IPCE action spectra for quasi-solid-state DSSCs based on
FNE38–FNE41.
that based on FNE38 has a similar plateau with a slightly
decreased maximum and a narrower shape. Although the
FNE40-loaded TiO2 film exhibits a lower absorption intensi-
ty compared with the FNE38-loaded TiO2 film, the relative-
ly higher IPCE value could result from the reduced charge
recombination rate in the FNE40-based quasi-solid-state
DSSC in comparison to that based on FNE38. Similarly,
upon the replacement of the triphenylamine group with ju-
lolidine, the quasi-solid-state DSSC based on FNE41 shows
a slightly higher IPCE value as compared to that based on
FNE39. However, the quasi-solid-state DSSC based on
either FNE39 or FNE41 results in much lower IPCE values
with a maximum below 60%. Since IPCE is a product of
the electron injection efficiency, light-harvesting efficiency,
and charge collection efficiency,[29a] the relative low IPCE
values are probably due to the low electron injection effi-
ciency and/or charge collection efficiency. As the driving
force for the regeneration of the excited FNE39 or FNE41
molecules is insufficient, as revealed by the electrochemical
study, the electron injection efficiency or the charge collec-
tion efficiency is possibly dramatically reduced, thus lower-
ing the IPCE significantly under similar adsorption condi-
tions.
Materials and Reagents
All chemicals and reagents were purchased from commercial sources and
used as received. Tetrahydrofuran (THF) was distilled from sodium/ben-
zophenone. Chloroform (CHCl3) was distilled from CaH2. All reactions
and manipulations were carried out under N2 atmosphere using Schlenk
techniques. The syntheses of 5-(2-ethylhexyl)-1,3-di(thiophen-2-yl)-
thieno
ACHTUNGTRENNUNG
ported elsewhere. 9-(Tributylstannyl)-1,2,3,5,6,7-hexahydropyridoACHTUNGTRENNUNG
ij]quinoline was synthesized in a similar way as N,N-diphenyl-4-(tributyl-
stannyl)aniline.[38]
Syntheses5-(5-(2-Ethylhexyl)-4,6-dioxo-3-(thiophen-2-yl)-5,6-dihydro-
thieno
ACHTUNGTREN[NUNG 3,4-c]pyrrol-1-yl)thiophene-2-carbaldehyde
A solution of 5-(2-ethylhexyl)-1,3-di(thiophen-2-yl)-thienoACTHNUGRTNE[NUG 3,4-c]pyrrole-
4,6-dione (390 mg, 0.91 mmol) in dry CHCl3 (10 mL) was added at 08C
to a cold Vilsmeier reagent, which was prepared with 2 mL of POCl3 in
DMF (2 mL). The mixture was stirred at 908C for 20 h and then
quenched with 10% aqueous solution of NaOAc (30 mL) at 08C. After
neutralization with 25% NaOH solution to pH 7, the mixture was ex-
tracted thrice with CH2Cl2. The combined organic layer was washed with
H2O and brine, dried over Na2SO4, and evaporated under reduced pres-
sure. The crude product was purified by column chromatography
(CH2Cl2/petroleum ether (PE) 1:1) on silica gel to obtain the desired
product (346 mg, 83%) as a yellow solid. 1H NMR (400 MHz, CDCl3):
d=9.92 (s, 1H), 8.17 (d, J=4.0 Hz, 1H), 8.05 (d, J=4.0 Hz, 1H), 7.74 (d,
J=4.0 Hz, 1H), 7.48 (d, J=4.4 Hz, 1H), 7.14 (t, J=4.4 Hz, 1H), 3.56 (d,
Conclusions
In summary, a series of sensitizers based on thienoACTHNUTRGENUG[N 3,4-c]pyr-
role-4,6-dione containing triphenylamine or julolidine as
electron donor have been designed and synthesized. A
linear or branched alkyl group was introduced into the
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Gang Zhou, Zhong-Sheng Wang et al.
J=7.2 Hz, 2H), 1.87–1.80 (m, 1H), 1.37–1.29 (m, 8H), 0.95–0.88 ppm (m,
FNE40
A mixture of 5-(3-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-5-(2-ethyl-
6H); 13C NMR (100 MHz, CDCl3): d=182.81, 162.77, 144.65, 141.03,
138.76, 136.91, 134.13, 132.19, 131.71, 130.91, 130.43, 129.76, 128.88, 42.85,
38.50, 30.80, 28.80, 24.113, 23.26, 14.33, 10.68 ppm; elemental analysis
calcd (%) for C23H23NO3S3: C 60.36, H 5.07, N 3.06; found: C 61.24, H
5.07, N 2.89.
hexyl)-4,6-dioxo-5,6-dihydro-thieno[3,4-c]pyrrol-1-yl)thiophene-2-carbal-
ACHTUNGTRENNUNG
dehyde (132 mg, 0.19 mmol) and cyanoacetic acid (48 mg, 0.57 mmol) in
CHCl3 (20 mL) was refluxed in the presence of piperidine (0.13 mL) for
12 h under a N2 atmosphere. After cooling, the mixture was diluted with
CH2Cl2, washed with water and brine, dried over Na2SO4, and evaporated
under reduced pressure. The crude product was purified by column chro-
matography (CH2Cl2/MeOH 10:1) on silica gel to yield the desired prod-
5-(3-(5-Bromothiophen-2-yl)-5-(2-ethylhexyl)-4,6-dioxo-5,6-dihydro-
thienoACHTUNGTRENNUNG[3,4-c]pyrrol-1-yl)thiophene-2-carbaldehyde
1
N-bromosuccinimide (654 mg, 3.67 mmol) was added in portions to a solu-
uct as a brown powder (103 mg, 71%). H NMR (400 MHz, [D6]DMSO):
tion of 5-(5-(2-ethylhexyl)-4,6-dioxo-3-(thiophen-2-yl)-5,6-dihydro-thieno-
ACHTUNGTRENNUNG[3,4-c]pyrrol-1-yl)thiophene-2-carbaldehyde (561 mg, 1.23 mmol) in
d=8.10 (s, 1H), 8.07 (d, J=4.0 Hz, 1H), 8.00 (d, J=4.0 Hz, 1H), 7.73 (d,
J=4.0 Hz, 1H), 7.59 (d, J=8.4 Hz, 2H), 7.47 (d, J=4.0 Hz, 1H), 7.33 (t,
J=7.8 Hz, 4H), 7.14–7.06 (m, 6H), 6.92 (d, J=8.4 Hz, 2H), 3.49 (d, J=
CH2Cl2 (30 mL) and glacial acid (30 mL) at room temperature. The reac-
tion mixture was stirred at room temperature for 12 h and quenched with
water (10 mL). Subsequently, the mixture was extracted thrice with
CH2Cl2. The combined organic layer was washed with H2O and brine,
dried over Na2SO4, and evaporated under reduced pressure. The crude
product was purified by column chromatography (CH2Cl2/PE 1:1) on
silica gel to obtain the desired product (521 mg, 79%) as a yellow solid.
1H NMR (400 MHz, CDCl3): d=9.84 (s, 1H), 8.03 (d, J=4.0 Hz, 1H),
7.63 (d, J=4.0 Hz, 1H), 7.54 (d, J=4.0 Hz, 1H), 6.94 (d, J=4.0 Hz, 1H),
3.46 (d, J=7.2 Hz, 2H), 1.80–1.77 (m, 1H), 1.38–1.28 (m, 8H), 0.94–
0.89 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=182.49, 162.28,
162.17, 144.76, 140.42, 136.77, 136.61, 134.01, 133.52, 131.40, 131.33,
130.53, 130.50, 117.86, 42.74, 38.49, 30.76, 28.74, 24.08, 23.28, 14.38,
10.65 ppm; elemental analysis calcd (%) for C23H22BrNO3S3: C 51.49, H
4.13, N 2.61; found: C 51.55, H 4.23, N 2.70.
7.2 Hz, 2H), 1.74–1.70ACTHNUTRGENUG(N m, 1H), 1.31–1.26 (m, 8H), 0.92–0.84 ppm (m,
6H); 13C NMR (100 MHz, CDCl3): d=166.76, 166.46, 152.39, 152.01,
151.76, 151.45, 151.28, 150.70, 150.66, 150.44, 149.94, 149.70, 149.53,
147.26, 146.60, 146.25, 131.13, 129.58, 129.03, 125.19, 125.14, 125.11,
124.48, 123.70, 123.36, 122.11, 121.76, 68.38, 41.47, 38.93, 32.17, 30.57,
23.95, 22.94, 14.37 ppm; HRMS (ESI, m/z): [M-H]ꢀ calcd for
C44H36N3O4S3, 766.1868; found 766.1879; elemental analysis calcd (%) for
C44H37N3O4S3: C 68.81, H 4.86, N 5.47; found: C 69.08, H 5.55, N 5.29.
FNE41
Obtained in a similar way as FNE40 (yield 59%). 1H NMR (400 MHz,
[D6]DMSO): d=8.04 (s, 1H), 8.03 (d, J=4.0 Hz, 1H), 7.93 (d, J=4.0 Hz,
1H), 7.67 (d, J=4.0 Hz, 1H), 7.25 (d, J=4.0 Hz, 1H), 7.03 (s, 2H), 3.41
(d, J=7.2 Hz, 2H), 3.17–3.14 (m, 4H), 2.68 (t, J=6.4 Hz, 4H), 1.87–1.82
(m, 4H), 1.62–1.59 (m, 1H), 1.28–1.21 (m, 8H), 0.87–0.82 ppm (m, 6H);
13C NMR (100 MHz, CDCl3): d=165.95, 163.34, 156.71, 154.90, 151.41,
148.08.15, 146.55, 141.53, 139.73, 139.44, 139.18, 132.81, 131.10, 130.43,
130.33, 126.74, 125.00, 124.16, 121.80, 119.26, 68.38, 50.13, 38.93, 32.16,
31.66, 30.41, 29.93, 24.96, 22.93, 21.99, 14.39 ppm; HRMS (ESI, m/z):
[MꢀH]ꢀ calcd for C38H36N3O4S3, 694.1868; found 694.1872; elemental
analysis calcd (%) for C38H37N3O4S3: C 65.58, H 5.36, N 6.04; found: C
64.82, H 6.25, N 5.15.
5-(3-(5-(4-(Diphenylamino)phenyl)thiophen-2-yl)-5-(2-ethylhexyl)-4,6-
dioxo-5,6-dihydro-thieno
ACHTUNGTRENNUNG[3,4-c]pyrrol-1-yl)thiophene-2-carbaldehyde
A mixture of 5-(3-(5-bromothiophen-2-yl)-5-(2-ethylhexyl)-4,6-dioxo-5,6-
dihydro-thieno[3,4-c]pyrrol-1-yl)thiophene-2-carbaldehyde (158 mg,
0.29 mmol), N,N-diphenyl-4-(tributylstannyl)aniline (620 mg, 1.16 mmol),
[Pd(PPh3)4] (100 mg, 0.09 mmol), and DMF (15 mL) was stirred at 1008C
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
for 12 h under N2 atmosphere. After cooling, the solvent was removed
and the residue was subjected to flash column chromatography (CH2Cl2/
PE 1:1) on silica gel to yield the desired product as a red solid (132 mg,
65%). 1H NMR (400 MHz, CDCl3): d=9.89 (s, 1H), 8.15 (d, J=4.0 Hz,
1H), 8.03 (d, J=4.0 Hz, 1H), 7.71 (d, J=4.0 Hz, 1H), 7.47 (d, J=8.5 Hz,
2H), 7.29 (t, J=7.8 Hz, 4H), 7.19 (d, J=4.0 Hz, 1H), 7.13 (d, J=7.6 Hz,
4H), 7.10–7.04 (m, 4H), 3.55 (d, J=7.2 Hz, 2H), 1.85–1.81 (m, 1H),
1.34–1.29 (m, 8H), 0.94–0.84 ppm (m, 6H); 13C NMR (100 MHz, CDCl3):
d=182.56, 162.63, 162.56, 148.96, 148.55, 147.28, 144.39, 141.14, 138.72,
136.81, 133.23, 132.44, 131.79, 130.26, 130.12, 129.69, 128.10, 126.98,
126.51, 125.25, 123.92, 123.50, 122.74, 42.77, 38.53, 30.83, 29.97, 28.82,
23.33, 14.41, 10.71 ppm; elemental analysis calcd (%) for C41H36N2O3S3:
C 70.25, H 5.18, N 4.00; found: C 71.19, H 5.95, N 3.16.
Fabrication of Dye Sensitized Solar Cells and Measurements
Fluorine-doped SnO2 glass (FTO, 15 Wsqꢀ1, Nippon Sheet Glass) sub-
strates were cleaned in a detergent solution by using an ultrasonic bath,
washed with acetone and water, and then dried using a flow of N2. Nano-
crystalline TiO2 films (10 mm) consisting of a 5 mm thick transparent layer
(~20 nm nanoparticles) and a 5 mm thick scattering layer (~100 nm parti-
cles) were prepared using a screen printing technique, followed by sinter-
ing at 5258C under an air flow.[39] After cooling, the TiO2 films were im-
pregnated in a 0.05m aqueous TiCl4 solution for 30 min at 708C and then
rinsed with deionized water. The TiCl4-treated TiO2 films were annealed
at 4508C for 30 min and then cooled to 1008C before immersion into
a dye solution (0.3 mm in chloroform/ethanol 7:3), with or without deoxy-
cholic acid (DCA, 20 mm), for 16 h to allow the dye to adsorb to the
TiO2 surface. After the adsorption step, the electrodes were rinsed with
chloroform and acetonitrile. The resulting photoelectrode and Pt counter
electrodes were assembled into a sealed sandwich solar cell with a ther-
moplastic frame (Surlyn, 30 mm thick). A quasi-solid-state gel electrolyte
was prepared by mixing 5 wt% poly(vinylidenefluoride-co-hexafluoro-
propylene) (PVDF-HFP)[40] with a redox solution containing a 0.1m LiI,
0.05m I2, and 0.6m 1,2-dimethyl-3-propylimidazolium iodide (DMPImI)
in 3-methoxypropionitrile (MPN) under heating at 1008C until all solids
were dissolved. After introducing the hot gel solution into the internal
space of the cell from the two holes predrilled on the back of the counter
electrode, a uniform polymer gel layer was formed between the working
and the counter electrodes, and then the holes were sealed with a Surlyn
film covered with a thin glass slide under heat.
5-(3-(5-(1,2,3,5,6,7-HexahydropyridoACTHNUTRGNE[UNG 3,2,1-ij]quinolin-9-yl)thiophen-2-yl)-
5-(2-ethylhexyl)-4,6-dioxo-5,6-dihydro-thieno
2-carbaldehyde
ACHTUNGTRNE[NUNG 3,4-c]pyrrol-1-yl)thiophene-
A mixture of 9-(tributylstannyl)-1,2,3,5,6,7-hexahydropyrido
noline (560 mg), 5-(3-(5-bromothiophen-2-yl)-5-(2-ethylhexyl)-4,6-dioxo-
5,6-dihydro-thieno[3,4-c]pyrrol-1-yl)thiophene-2-carbaldehyde (70 mg,
0.13 mmol), [Pd(PPh3)4] (100 mg, 0.09 mmol) in 10 mL DMF was heated
ACHTUNGTREN[NGNU 3,2,1-ij]qui-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
at 1008C overnight under N2 atmosphere. The solvent was removed and
the residue was subjected to flash column chromatography on silica gel
(CH2Cl2/PE 1:1) to give the target compound as a deep red solid (79 mg,
97%). 1H NMR (400 MHz, CDCl3): d=9.92 (s, 1H), 8.19 (d, J=4.0 Hz,
1H), 8.02 (d, J=4.0 Hz, 1H), 7.75 (d, J=4.0 Hz, 1H), 7.12 (d, J=4.0 Hz,
1H), 7.11 (s, 2H), 3.58 (d, J=7.2 Hz, 2H), 3.21 (t, J=5.6 Hz, 4H), 2.78
(t, J=6.4 Hz, 4H), 2.02–1.96 (m, 4H), 1.88–1.82 (m, 1H), 1.38–1.27 (m,
8H), 0.94–0.88 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=182.80,
182.73, 163.03, 162.91, 151.22, 144.19, 143.76, 141.55, 139.79, 137.00,
132.73, 132.64, 131.90, 130.07, 128.25, 125.00, 121.64, 120.25, 50.12, 42.84,
38.47, 30.80, 29.93, 28.82, 27.90, 24.10, 23.30, 21.97, 14.35, 10.69 ppm; ele-
mental analysis calcd (%) for C35H36N2O3S3: C 66.85, H 5.77, N 4.45;
found: C 67.53, H 6.57, N 3.57.
The current density–voltage (J–V) characteristics of the DSSCs were
measured by recording J–V curves using a Keithley 2400 source meter
under the illumination of AM1.5G simulated solar light coming from
a solar simulator (Oriel-91193 equipped with a 1000 W Xe lamp and an
AM1.5 filter). The DSSCs were fully covered with a black mask with an
aperture area of 0.2304 cm2 during measurements. The incident light in-
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Gang Zhou, Zhong-Sheng Wang et al.
tensity was calibrated with a standard silicon solar cell (Newport 91150).
The electron lifetimes were measured with intensity modulated photo-
voltage spectroscopy (IMVS),[35] whereas charge densities at open circuit
were measured using the charge extraction technique.[32] IMVS analysis
and charge extraction were carried out on an electrochemical worksta-
tion (Zahner XPOT, Germany), which includes a green light-emitting
diode (LED, 532 nm) and a corresponding control system. The intensity-
modulated spectra were measured at room temperature with a light in-
tensity ranging from 5 to 45 Wmꢀ2, in a modulation frequency ranging
from 0.1 Hz to 10 kHz, and with a modulation amplitude less than 5% of
the light intensity. Three identical devices were tested in each case, with
a standard deviation less than 5%. Action spectra of the monochromatic
incident photon-to-electron conversion efficiency (IPCE) for the solar
cells were obtained with an Oriel-74125 system (Oriel Instruments). The
intensity of monochromatic light was measured with a Si detector (Oriel-
71640).
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Acknowledgements
This work was financially supported by the National Basic Research Pro-
gram (2011CB933302) of China, the National Natural Science Founda-
tion of China (90922004, 50903020, and 20971025), Shanghai Pujiang
Project (11J1401700), Shanghai Leading Academic Discipline Project
(B108), and Jiangsu Major Program (BY2010147).
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