Rigid triarylamine-based D-A-π-A structural organic sensitizers for solar cells: The significant enhancement of open-circuit photovoltage with a long alkyl group

Article abstract of DOI:10.1039/c3ra43057j

Five new organic D-A-π-A sensitizers DIA6-DIA10 containing benzothiadiazole (BTD) or benzotriazole (BTZ) as auxiliary electron withdrawing units have been synthesized. Their photophysical, electrochemical and photovoltaic properties have been studied. Consequently, although all of these dyes have long alkyl chains grafted on a π-bridge, DSSCs with dyes containing a BTZ unit displayed higher open circuit voltages (757-829 mV) than those containing a BTD unit (682-712 mV). This may be because the nitrogen-based heterocyclic group of BTZ can lift the conduction band edge and reduce the charge recombination that arises from the absence of the sulfur atom. Benefiting from this and the introduction of alkyl chains on the nitrogen atom, DSSCs with one dye containing BTZ and thieno[3,2-b]thiophene (TT) as the π-spacer (DIA7) showed the highest open circuit voltage (V_oc = 829 mV). The addition of one more thiophene unit to DIA6 broadens the absorption spectra, leading the DIA8 device to produce the largest photocurrent. Among these sensitizers, the best photovoltaic performance was obtained by the DIA8 device, with a short circuit photocurrent density (J_sc) of 13.45 mA cm ^-2, an open-circuit photovoltage (V_oc) of 757 mV, and a fill factor (FF) of 70%, corresponding to an overall conversion efficiency (η) of 7.15% which reached 93% of the reference dye N719-based cell under standard global AM 1.5 solar light conditions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43057j

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Entry for the Table of Contents
Xiaohao Hu, Shengyun Cai, Guojian Tian, Xin Li, Jianhua Su and Jing Li……...…… Page 1– Page 8
, Rigid triarylamineꢀbased DꢀAꢀπꢀA Structural Organic Sensitizers for Solar Cells: Significant Enhancement of
OpenꢀCircuit Photovoltage with Long Alkyl Group
Five new organic DꢀAꢀπꢀA sensitizers containing benzothiadiazole or benzotriazole as auxiliary electron withdrawing units
were synthesized and successfully applied in dyeꢀsensitized solar cells. It was found that the DSSCs based on these dyes
could achieve high openꢀcircuit voltage performance and high overall conversion efficiency.

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Rigid triarylamine-based D-A-π-A Structural Organic Sensitizers for
Solar Cells: Significant Enhancement of Open-Circuit Photovoltage with
Long Alkyl Group
Xiaohao Hu,a Shengyun Cai,a Guojian Tian,a Xin Li,a Jianhua Sua and Jing Lia*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Five new organic D-A-π-A sensitizers DIA6-DIA10 containing benzothiadiazole (BTD) or benzotriazole
(BTZ) as auxiliary electron withdrawing units have been synthesized. Their photophysical,
electrochemical and photovoltaic properties have been studied. Consequently, although all of these dyes
10 have long alkyl chains grafted on π-bridge, DSSCs with dyes containing BTZ unit displayed higher open
circuit voltages (757–829 mV) than those containing BTD unit (682–712 mV). This may be because that
the nitrogen-based heterocyclic group of BTZ can lift the conduction band edge and reduce charge
recombination arise from the absence of sulfur atom. Benefited from it and the introduction of alkyl
chains on the nitrogen atom, DSSCs with one dye containing BTZ and thieno[3,2-b]thiophene (TT) as
15 the π-spacer (DIA7) showed the highest open circuit voltage (Voc = 829 mV). The addition of one more
thiophene unit to DIA6 broadens the absorption spectra, leading DIA8 device to produce the largest
photocurrent. Among these sensitizers, the best photovoltaic performance was obtained by DIA8 device:
a short circuit photocurrent density (Jsc) of 13.45 mA cm-2, an open-circuit photovoltage (Voc) of 757 mV,
and a fill factor (FF) of 70%, corresponding to an overall conversion efficiency (η) of 7.15% which
20 reached 93% of the reference dye N719-based cell under standard global AM 1.5 solar light conditions.
planar11 triphenylamine donors were reported, the efficiency are
Introduction
much better than simple triarylamine based dyes. In our previous
study, we have synthetized a rigid amine electron donor with a
50 two-locking structure which was proved to be helpful to increase
the ε of the corresponding dyes as well as the lifetime of electrons
injected into TiO2 conduction band.12 The Jsc of these dyes based
DSSCs can indeed display a good performance as a result of good
light-harvesting ability. However, the relatively low Voc became a
55 drawback, due to dye aggregation and high rate of charge
recombination. Herein, we re-examine our concept by
introducing long alkyl chains on the π-spacer. Based
Dye-sensitized solar cells (DSSCs) have been considered as a
promising, sustainable and environmental friendly energy device
to meet the demands of mankind in the future.1-2 Up to now, a
25 high photo-to-electricity conversion yield (η) exceeding 12.0%
under standard AM 1.5 sunlight irradiation has been obtained by
DSSCs based on Ru-complex dyes.3 Compared with metal
complexes, the metal-free sensitizers have also attracted
considerable scientific attention due to their low cost, facile
30 structural modifications, easier preparation and purification.4
Generally, donor-π-acceptor (D-π-A) structure is the most
common characters of organic dyes.5 Recently, Tian and co-
workers have put forward a so called ‘‘D-A-π-A’’ structure,
which introduces an auxiliary acceptor between the donor and π
35 bridge to further optimize the HOMO and LUMO levels and
broaden the absorption spectra.6
As is well-known, short circuit photocurrent density (Jsc) and
open circuit voltage (Voc) have become the primary concern in the
design of dyes, because they directly influence the overall
40 conversion efficiency. Dyes with high molar extinction
coefficients (ε) and broad absorption spectra are advantageous to
the light harvesting of DSSCs, thus beneficial to get high Jsc
values.7 Besides, high Voc values can be obtained by preventing
-
the intermolecular π-π stacking among sensitizers and keeping I3
45 ions away from the TiO2 electrode surface in the DSSCs.8
Recently a series of sensitizers bearing semi-rigid9 or rigid10
Scheme 1 Molecular structures of dyes (DIA6-DIA10).
60
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Scheme 2 Synthetic route to dyes DIA6-DIA10.
on that, we have designed and synthesized five new dyes DIA6- 35 further investigate the DSSCs performance, the stability of dyes
5
DIA10 (Scheme 1) with the D–A–π–A configuration using
benzothiadiazole (BTD) or benzotriazole (BTZ) as auxiliary
electron withdrawing units. All of these dyes were designed
based on the following considerations: 1) the attached alkyl
and devices was also measured.
Results and Discussion
Synthesis and Characterization
10 chains could keep sufficient solubility in common organic
solvents. More importantly, the introduced alkyl chains can
The synthetic route to the five new dyes DIA6-DIA10 is depicted
effectively inhibit the intermolecular aggregation functioned as 40 in Scheme 2. The starting materials 1 was synthesized according
DCA coadsorbents and charge recombination for the purpose of
reducing voltage losses;13 2) like pyrimidine, and benzimidazole
15 which have been tested as additives in the electrolyte for the sake
of increasing Voc, the nitrogen-containing heterocyclic group of
to our previous work.12 The intermediates 2, 3, B1, B2 and D1-
D4 were constructed by common Suzuki coupling, formylation
and bromination reaction (detailed synthesis procedures were
described in ESI). Among them, the synthesis of B1 involved the
BTZ can be expected to improve Voc.14 Compared with BTZ, 45 protection and deprotection of formyl group. The key π-spacer
BTD is a stronger electron-withdrawing unit which can improve
the distribution of donor electrons and broaden absorption spectra
20 into the NIR region.15 Here we employ BTD and alkyl grafted
thiophene as π-spacer, which can be expected to obtain desirable
moieties B3, B4 were synthesized according to previous report.17a
The first final product DIA6 were obtained by the direct
condensation of 3 with cyanoacetic acid in the presence of a
catalytic amount of piperidine in THF. The other final products
photo response ability and suppressed dark current from TiO2 to 50 DIA7-DIA10 were obtained firstly through Suzuki coupling
the electrolyte. As BTZ unit has some properties different from
the BTD unit while replacing sulfur atom by nitrogen one, such
25 as more electron-rich, thus the effects posed by the two units on
the DSSCs performances could be compared with; 3) it's worth
reaction between bromo-substituted materials
1
and
corresponding aldehyde precursor, followed by treatment with
cyanoacetic acid in the presence of piperidine in THF. The
purification of intermediates was performed by flash column
mentioning that besides general thiophene or n-hexyl-thiophene 55 chromatography and subsequent HPLC. All the intermediates and
units, thieno[3,2-b]thiophene (TT) unit was also incorporated in
the dyes due to its better π-conjugation and smaller geometric
30 relaxation energy upon oxidation;16 4) different numbers of
thiophene or n-hexyl-thiophene units will influence DSSCs
target dyes were characterized by standard spectroscopic methods
including 1H NMR, 13C NMR and HRMS.
Optical and electrochemical properties
The UV−Vis absorption spectra of DIA6-DIA10 in THF and on
60 TiO2 films are shown in Fig. 1 and the corresponding data are
summarized in Table 1. All of these compounds exhibit one
performances. Above all, these dyes based DSSCs are expected
to have broad response spectra and suppressed charge
recombination, thus leading to high performance. In order to
2
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major broad absorption band in the visible light region which is
(DIA8 or DIA10) is observed when introducing one more
attributed to the intramolecular charge transfer (ICT) between the 20 thiophene unit into DIA6 or DIA9. To be mentioned, DIA9 with
donor and the acceptor. The absorption maxima in ICT band for
DIA6-DIA10 were located at 431, 444, 465, 478 and 510 nm,
respectively. The corresponding molar extinction coefficients are
3.8×104, 5.6×104, 4.6 ×104, 2.9×104 and 3.1×104 M-1 cm-1. The
BTD unit exhibits the maximum absorption peak at 478 nm that
is red shifted by 47 nm as compared with DIA6 containing BTZ
unit which has the same length of chain. The red-shift may arise
from the stronger electron-withdrawing ability of BTD unit, thus
5
order of these values DIA7 > DIA8 > DIA6 > DIA10 > DIA9 are 25 resulting in stronger ICT effect on the dye molecule.16d
perfectly consistent with that of the corresponding oscillator
When adsorbed on TiO2, the dyes have mild changes (5-13nm)
with respect to that in solution. Two points could account for this
phenomenon: 1) the alkyl chain plays an important role, reducing
intermolecular aggregation when sensitizers are absorbed onto
30 the TiO2 surface; 2) the incorporation of BTZ and BTD units is
beneficial to countervail the deprotonation effect, which
contributes to the observed small shift. 17
6
DIA6
DIA7
(a)
DIA8
DIA9
DIA10
5
4
3
2
1
0
The electrochemical behaviors of the dyes were measured by
cyclic voltammetry (CV), and the results are given in the ESI. As
35 shown in Table 1, the first oxidation potentials, taken as the
HOMO levels of DIA6-DIA10, are calculated to be 1.19, 1.25,
1.14, 1.15 and 1.10 V (vs. NHE, same below), respectively,
-
which are more positive than I3 /I- redox couples (0.4 V),
indicating that the oxidized dyes formed after electron injection
40 into the conduction band of TiO2 could thermodynamically
accept electrons from I- ions. In particular, the lowest HOMO
potential-energy level was found in DIA7, which implies that the
device made with it may show the most efficient charge
regeneration.18 The LUMO levels of the sensitizers were
45 estimated by the values of Eox and the 0–0 band gaps (E0-0),
whilst the latter values were calculated by the onset of absorption
spectra in Fig. 1(b). Consequently, all the LUMO values of these
dyes (-1.09, -0.98, -0.96, -0.93 and -0.83V, respectively) are more
negative than the conduction band (Ecb) of TiO2 (–0.5 V),
50 indicating the electron injection from the LUMO orbital of these
sensitizers to the conduction band of TiO2 were energetically
permitted.
400
500
600
700
Wavelength/nm
(b)
DIA6
DIA7
DIA8
DIA9
DIA10
3
2
1
0
Computational analysis
400
500
600
700
To gain a better understanding of the geometrical configuration
55 and electronic distribution of the frontier as well as other close-
lying orbitals, density functional theory (DFT) calculations were
carried out. In the calculations, the long alkyl chains in these
compounds were replaced by ethyl groups to save computational
time while preserving their steric effect. The hybrid B3LYP
60 functional and the Ahlrichs split valence SVP basis set was
Wavelength/nm
10
Fig. 1 (a) UV-Vis absorption spectra of DIA6-DIA10 in THF (1×10-5 M);
(b) absorption spectra of DIA6-DIA10 adsorbed on TiO2 film.
strengths (f) as determined by TDDFT (2.09, 2.07, 1.84, 1.35 and
1.28, respectively). By comparison, it is found that the absorption
15 maximum of ICT for DIA7 is more red-shifted than that of
DIA6, in that TT could enhance the electron delocalization to
more extent than thiophene. Owing to the expansion of the
conjugation systems, the bathochromic shift by 34 or 32 nm
Table 1 Optical and electrochemical of DIA6-DIA10.
e
Dye
DIA6
DIA7
DIA8
DIA9
DIA10
f a
λ
maxb/nm( ε/104 M-1 cm-1)
431 (3.8)
444 (5.6)
λ
maxc/nm on TiO2
Adsorbed amount d (mol cm-2)
8.6 × 10-8
E0-0
E
HOMOf(V)
1.19
1.25
1.14
1.15
ELUMOg(V)
-1.09
-0.98
-0.96
-0.93
-0.83
1.84
2.09
2.07
1.28
1.35
426
432
452
484
512
2.28
2.23
2.10
2.08
1.93
7.6 × 10-8
465 (4.6)
478 (2.9)
510 (3.1)
7.5 × 10-8
5.5 × 10-8
4.8 × 10-8
1.10
a
b
Oscillator strengths at ICT bands calculated by DFT/B3LYP; Recorded in THF; Absorption maximum on TiO2 transparent films; Dye absorbed
c
d
e
amounts on 8 ꢀm TiO2 films when THF was used as the solvent for dye loading; E0-0 was estimated from the intersection of absorption and emission
spectra; f HOMOs were internally calibrated with ferrocene (0.59 V versua NHE) measured in CH3CN containing 0.1 M (n-C4H9)4NPF6 with a scan rate of
65 100 mV s-1 (working electrode: Pt; reference electrode: SCE); Counter electrode: Pt wire; The LUMO was calculated with the expression of ELUMO
=
g
EHOMO – E0-0
.
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650, and 740 nm, respectively, which are broadened significantly
with respect to the absorption spectra on sensitized films. This
40 phenomenon could be ascribed to the scattering effect of the large
particles in the film and the reflective effect of the Pt counter
electrode. The IPCE values of these dyes reached a maximum of
85.1% at 440 nm, 88.2% at 490 nm, 86.2% at 500 nm, 72.6% at
490 nm and 63.2% at 520 nm respectively. The highest IPCE
45 value was achieved by DIA7 device, which may be a result of the
high molar extinction coefficients. Although the IPCE value for
DIA8 device was almost the same as that for DIA6 and DIA7
devices, DIA8 exhibited better performance with broader
(a)
DIA6
DIA7
80
DIA8
DIA9
DIA10
Fig. 2 Dihedral angles between the neighbouring units.
60
employed to optimize molecular geometries of the dyes and to
compute their frontier molecular orbitals, using the ORCA
program package.19-21 Subsequent time-denpendent DFT
(TDDFT) calculations were performed to gain information of the
basis set implemented in Gaussian 09 program package.22
40
20
0
5
As is depicted in Fig. 2, in the ground state, the geometry of
300
400
500
600
700
10 the dyes is not planar. For DIA6-DIA8, only one major twist was
observed in the molecular structure, which is located between the
donor and neighbouring π-spacer and the corresponding dihedral
angles are 25.4o, 26.0o and 16.4o, respectively. However, for
DIA9 and DIA10, the molecules are mainly twisted at two
15 positions: One is located between the donor and neighbouring π-
spacer and the corresponding dihedral angles are 32.0o and 51.0o;
the other is located between the acceptor and the neighbouring π-
spacer and the corresponding dihedral angles are 31.2o and 31.8o.
It is obvious that the structures of DIA9 and DIA10 are more
20 twisted than the other dyes. The reason can be explained by that
the presence of alkyl chain on the thienyl unit may cause steric
hindrance, resulting in larger dihedral angles and twisted
conformation. This can be further used to explain that the loading
amounts of DIA9 (5.5 × 10-8 mol cm-2) and DIA10 (4.8 × 10-8
25 mol cm-2) on the TiO2 surface were lower than that of DIA6-
DAI7 (8.6 × 10-8, 7.6 × 10-8 and 7.5 × 10-8 mol cm-2,
respectively). Because a more twisted conformation could make
the molecule occupy a larger surface area than a molecule of
linear shape. 23
Wavelength/nm
DIA6
(b)
15
10
5
DIA7
DIA8
DIA9
DIA10
0
-5
0.0
0.2
0.4
0.6
0.8
Votage / V
50
Fig. 3 The performances of DSSCs sensitized by DIA6-DIA10: (a) IPCE
spectra for DSSCs based on DIA6-DIA10 with dye baths in THF; (b) J-V
curves for DSSCs based on DIA6-DIA10 with dye baths in THF upon
AM 1.5 solar light irradiation and under dark condition.
55
30 DSSCs performance
spectra region, indicating that DIA8 sensitized TiO2 electrode
would generate a higher conversion yield compared to these dyes.
The maximum IPCE for DIA9 and DIA10 devices were lower
than that of the others, which may be attributed to the combined
To investigate the light harvesting ability of DIA6-DIA10,
DSSCs based on dye-sensitized TiO2 (12 ꢀm) films and the
electrolyte in the experimental section were fabricated and the
performances were measured. Fig. 3(a) shows the action spectra
35 of the incident photon-to-current conversion efficiency (IPCE)
for DSSCs with these dyes when THF was used as the solvent for
dye loading. The IPCE onsets for DIA6-DIA10 are 620, 630, 680,
60 impacts of the electron injection yield from the excited dye to the
CB of the TiO2 (related with the LUMO energy level), the
absorption capacity and the amount of dye adsorbed on the TiO2
film.24
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The current density-voltage (J-V) curves of the DSSCs based
DIA8 device, which shows a maximum Jsc of 13.45 mA cm-2, a
Voc of 757 mV, a FF of 70% and an η of 7.15%. The high η was
on these dyes under standard global AM 1.5 solar light conditions
(100 mW cm-2) are displayed in Fig. 3(b). The detailed 55 mainly attributed to the large Jsc which may be derived from good
parameters, i.e., short-circuit photocurrent (Jsc), open-circuit
photovoltage (Voc), fill factor (FF), and power conversion
efficiency (η) are summarized in Table 2. When THF was used as
the solvent for dye loading, the DSSCs based on DIA6-DIA10
exhibited inferior conversion efficiency of 5.65%, 6.23%, 7.15%,
5.20% and 5.82%. The Jsc of DSSCs with dyes based on BTZ
IPCE performance. Besides, the replacement of sulfur element by
nitrogen element may cause the Voc of DIA8 device higher than
that of DIA9 and DIA10 devices.
5
Electrochemical impedance spectroscopy
60 Electrochemical impedance spectroscopy (EIS) was performed to
further elucidate the photovoltaic properties of these dyes. The
Nyquist plot and Bode plot are shown in Fig. 4 (a) and 4 (b),
respectively. The Nyquist plots for DIA6-DIA10 sensitized cells
were measured under a forward bias of -0.70V in the dark with a
10 unit decreased in the order of DIA8 > DIA7 > DIA6, which is
consistent with that of their corresponding IPCE performance
mentioned above. Compared with DIA9, the Jsc of DIA10 device
is much higher, despite lower maximum IPCE values, indicating
the broader absorption spectra is beneficial to obtain better light-
15 harvesting ability and higher light-to-electricity conversion
efficiency.
250
DIA6
DIA7
(a)
DIA8
DIA9
DIA10
200
150
100
50
Table 2 The performances of DSSCs based on DIA6-DIA10 in THF.
Dye
DIA6a
DIA7a
DIA8a
DIA9a
DIA10a
N719b
Jsc/mA cm-2
9.54 0.26
10.56 0.28
13.45 0.36
9.71 0.33
11.82 0.47
17.79 0.29
Voc/mV
808 16
829 10
757 21
712 14
682 12
718 10
FF (%)
73 1
71 1
70 2
75 1
72 1
60 2
η (%)
5.65 0.18
6.23 0.20
7.15 0.23
5.20 0.33
5.82 0.19
7.64 0.27
a
The electrolytes was composed of 0.1 M lithium iodide, 0.6 M 1,2-
dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M I2, 0.5 M 4-
20 tertbutylpyridine (4-TBP) in acetonitrile (AN) as a liquid electrolyte;
0
b
0
100
200
300
Z//ohm
400
500
600
The electrolytes was composed of 0.1 M DMPII, 0.05 M lithium iodide,
0.03 M I2, 0.5 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M
guanidinium thiocyanate, and 0.5 M 4-TBP in AN : valeronitrile (85:15,
v/v) as a liquid electrolyte. Average of four samples.
65
60
40
20
0
(b)
DIA6
DIA7
DIA8
DIA9
DIA10
25
The generation of Voc is related to the charge recombination
rate and electron injection efficiency in DSSCs. Among all of the
dyes, DIA6–DIA8 based DSSCs (with the BTZ group) showed
higher Voc than DIA9 and DIA10 based ones (with BTD). There
30 are two possible reasons: 1) compared with BTD-based
sensitizers, the BTZ unit contains no sulfur element which is
attractive to iodine in the electrolyte. Therefore, the BTZ-based
sensitizers can decrease the iodine concentration at the vicinity of
TiO2 and reduce charge recombination;25 2) the nitrogen-based
35 heterocyclic group of BTZ may lift the conduction band edge of
TiO2 and improve Voc.6c Obviously, DIA7 based DSSCs exhibits
the highest open-circuit voltage (829 mV). Two reasons could
take account for this phenomenon: 1) since the DIA7 dark-
current onset potential shifted to a larger value than that of other
40 dyes (as shown in Fig. 3(b)), suggesting that DIA7 can inhibit
100m
1
10
100
1k
10k
100k
Frequence/Hz
-
charge recombination between injected electrons and I3 ions in
the electrolyte more effectively; 2) DIA7 owned lowest HOMO
potential-energy level which leads to sufficient driving force for
dye regeneration. It is noteworthy, DIA7 based DSSCs exhibited
45 higher values of Jsc, Voc and η than DIA6, suggesting that as an
alternative to thiophene, the TT moiety can be advantageously
incorporated into the dye sensitizer as a π-spacer and improve the
cell efficiency. On the other hand, introducing an extra π-spacer
between the donor and auxiliary acceptor seems to increase the
50 Jsc of DSSCs due to expanded light response region (for example,
DIA6 and DIA8). Such a regular phenomenon may be helpful to
design new D-A-π-A dyes. The best performance is found in
Fig. 4 Impedance spectra of DSSCs based on DIA6-DIA10 with dye
baths in THF measure at bias -0.70 V in the dark: (a) Nyquist plots; (b)
70 bode phase plots; (c) equivalent circuit of DSSCs based on DIA6-DIA10.
The lines of (a) and (b) show theoretical fits using the equivalent circuits
(c).
frequency range of 0.1 Hz to 100 kHz. Since the semicircle for
Nernst diffusion within the electrolyte was overlapped by the
75 intermediate-frequency large semicircle, only two semicircles
were observed in the Nyquist plots. The smaller semicircle at
high frequency was assigned to the redox charge-transfer
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response at the Pt/electrolyte interface, while the larger one at the
intermediate frequency represented the charge transfer resistance
To further evaluate the long term stability of DIA7，DIA8-
based DSSC, a less volatile ionic liquid electrolyte (containing
at the TiO2/dye/electrolyte interface (Rrec). It is related to the 55 0.6 M 1-propyl-3-methylimidazolium iodide, 0.15 M iodine, 0.1
charge recombination rate, e.g., a larger Rrec indicates a slower
charge recombination and therefore a smaller dark current (shown
in Fig. 3(b)). The radius of the biggest semicircle Rrec values
decreased in the order of DIA7 > DIA6 > DIA8 > DIA9 >
DIA10. The detailed data analyzed by software (ZSimpWin)
using an equivalent circuit shown in Fig. 4(c) can further
M
guanidiniumthiocyanate and 0.5
M
NMBI in 3-
5
methoxypropionitrile) was employed. Table 4 and Fig. 5 shows
Table 4 Photovoltaic performance of DSSCs based on DIA7 and DIA8
with solvent-free ionic liquid electrolyte.
Dye
DIA7
DIA8
Jsc/mA cm-2
8.99 0.11
10.85 0.08
Voc/mV
693 10
674 11
FF (%)
62 1
62 1
η (%)
3.88 0.12
4.54 0.16
10 demonstrate the results mentioned above. By fitting the EIS
spectra to an electrochemical model26, several important
parameters such as RS (the series resistance), Rrec and RCE (charge
transfer resistance at the Pt/electrolyte interface) can be obtained.
Among these parameters, RS and RCE are associated with the
15 smaller semicircle. As listed in Table 3, RS and RCE of these dyes
based DSSCs show almost the same value because of the same
electrolyte and electrode the devices used. The Rrec of the DIA6-
60 a Average of three samples.
the variations of photovoltaic parameters during a long-term
accelerated aging on DIA7 and DIA8 based solar cells, keeping
constant performance during 500 h aging test under one sun at 60
65 oC. Upon a continuous 500 h of light soaking, the efficiencies of
the DSSCs based on DIA7 and DIA8 showed a maximum change
of 5.9% and 3.6%. These results suggested that the two dyes are
promising candidates for practical application.
Table 3 Parameters obtained by fitting the impedance spectra of the
DSSCs with sensitizers DIA6-DIA10, using the equivalent circuit.
Dye
RS (ꢁcm-2)
13.9
15.8
13.7
15.0
Rrec (ꢁcm-2)
272.6
543.9
113.6
34.4
28.4
RCE (ꢁcm-2)
τe (ms)
136
152
113
86
DIA6
DIA7
DIA8
DIA9
DIA10
5.6
12.7
6.0
7.3
7.0
1.0
DIA7
0.8
0.6
0.4
13.7
82
12
8
20
DIA10 dyes are 272.6, 543.9, 113.6, 34.4 and 28.4 ꢁcm-2,
respectively. The result appears to be consistent with the increase
of Voc in the DSSCs with dyes. The smaller Rrec value means the
electron recombination from the TiO2 to electron acceptors in the
4
1.0
0.8
0.6
0.4
25 electrolyte occurs more easily, thus resulting in lower Voc. The
electron lifetime measured by the relation τe = 1/ (2πf) (f is the
peak frequency) decreased in the order of DIA7 (152 ms) > DIA6
(136 ms) > DIA8 (113 ms) > DIA9 (86 ms) > DIA10 (82 ms),
while the peak frequency in Bode plot enhanced in the reverse.
30 All of the data mentioned above could well support the order of
the Voc of DSSCs based on these dyes. It is reasonable that DIA7
device has a high Voc value due to the long electron lifetime and
slow charge recombination rate.
6
4
2
0
0
100
200
300
400
500
Time (h)
1.0
0.8
0.6
0.4
DIA8
Stability of Dyes and Devices
16
12
8
35 Photo-stability is one of the most important parameters for cell
operation for durable application. According to the research by
Katoh and coworkers,27 we measured the absorption spectra of
dye-loaded TiO2 film before and after 30 min of AM 1.5
simulated solar light irradiation, and observed the variation in the
40 absorption curves. The details of DIA6-DIA10 are showed in
Fig. S3. It could be observed that the variations were huge before
and after light irradiation without the use of UV cutoff filter.
However the situation appeared entirely different when the UV
cutoff filter at 400 nm was used, suggesting that these dyes were
45 easily degraded by ultraviolent light. As shown in Fig.S3 (b),
since no distinct absorption peak shifts were observed, sensitizers
DIA6-DIA10 were basically stable. From the comparison of
DIA6 and DIA7, the introduction of TT instead of thiophene lead
to less decrease in absorbance, implying that DIA7 had a higher
50 photo-stability. On the other hand, extending the chain length of
the oligothiophene moiety seems to be beneficial to stabilize the
oxidized dyes upon light irradiation, such as DIA6 and DIA8.
1.0
0.8
0.6
0.4
8
6
4
2
0
100
200
300
400
500
Time (h)
70
Fig. 5 Variations of photovoltaic parameters (Jsc, Voc, FF, and η) with
aging time for the DSSC devices based on DIA7-DIA8 and ionic liquid
electrolyte under visible-light soaking.
Conclusions
75 In summary, five rigid triarylamine based D-A-π-A structural
dyes, with BTD or BTZ as auxiliary electron withdrawing units,
6
| Journal Name, [year], [vol], 00–00
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RSC Advances
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DOI: 10.1039/C3RA43057J
were successfully synthesized and characterized. The result
shows that the Voc values of DIA6-DIA8 with a BTZ moiety are
higher than those of DIA9 and DIA10 with a BTD group. TT-
(ZAHNERElektrik GmbH & CoKG, Kronach, Germany). The
spectra were scanned in a frequency range from 0.1 Hz to 100
kHz at room temperature with applied potential set at open
containing DIA7 based DSSCs exhibits the highest Voc of 829 60 circuit. The alternate potential amplitude was set at 10 mV.
5
mV due to its lowest HOMO potential-energy level, the long
electron lifetime and slow charge recombination rate. The
additional thiophene or n-hexyl-thiophene in DIA8 or DIA10
leads to a significant redshift in the UV-Vis properties and
The DSSCs were evaluated by recording the J–V curves with a
Keithley 2400 source meter under the illumination of an AM 1.5
solar simulator equipped with a 300 W xenon lamp (model no.
91160, Oriel). The power of the simulated light was calibrated to
photocurrent response by extending the conjugated system. 65 100 mW cm-2 using a Newport Oriel PV reference cell system
10 However, owing to the strong steric hindrance caused by n-hexyl-
thiophene unit, a remarkable molecular twist exists in the spacer
of DIA9 and DIA10, which decreases the dye-loading amount as
well as IPCE values. Because of the broad absorption spectrum
(model 91150V). A mask of 0.25 cm-2 was used to cover around
the cell active area. The voltage step and delay time of
photocurrent were 10 mV and 40 ms, respectively. The
photocurrent action spectra were measured with an IPCE test
and efficient electron injection into the conduction band of TiO2, 70 system consisting of a model SR830 DSP Lock-In Amplifier and
15 DIA8 with the large Jsc, exhibits the best performance overall
efficiency of 7.15% among these dyes (Jsc of 13.45 mA cm-2, a
Voc of 757 mV, and a FF of 70%). Based on the above findings,
the insertion of an extra π-spacer between the donor and auxiliary
acceptor and avoiding the long alkyl chain tightly close to the
20 acceptor are good designs for the D-A-π-A structured dyes.
a model SR540 Optical Chopper (Stanford Research Corporation,
USA), a 7IL/PX150 xenon lamp and power supply, and a
7ISW301 spectrometer. Impedance spectra were recorded using a
CHI-660c electrochemical station.
75 Fabrication of DSSCs
The pretreatment of transparent conducting oxide (FCO) glass
and fabrication of solar cells were performed according to the
published procedures.28 Excepted that the coated TiO2 film was
consisted of a 8 ꢀm thin TiO2 film and a 4 ꢀm scattering layer
80 (Ti-Nanoxide 300 ). DSSCs were fabricated using DIA6-DIA10
as sensitizers with an effective working area of 0.25 cm2, dye
adsorbed TiO2 films pasted onto FCO glass as the working
electrode.
Experimental
Materials
Optically transparent FTO conducting glass (fluorine doped
SnO2, transmission >90% in the visible, sheet resistance 15ꢁ per
25 square) was obtained from the Geao Science and Educational Co.
Ltd. of China and cleaned by a standard procedure. Titania pastes
of Ti-Nanoxide T/SP (13 nm) and Ti-Nanoxide 300 were also
Synthesis:
purchased
from
there.
Tetra-n-butylammonium
85
(E)-2-cyano-3-(5-(7-(8,8-diethyl-8H-indolo[3,2,1-
de]acridin-3-yl)-2-octyl-2H-benzo[d][1,2,3]triazol-4-
yl)thiophen-2-yl)acrylic acid (DIA6)
hexafluorophosphate (TBAPF6), 4-tert-butylpyridine (TBP), 2.4
30 M BuLi solution in hexane and the reaction catalyst Pd(PPh3)4
were obtained from J&K. Lithium iodide, 1,2-dimethyl-3-
propylimidazolium iodide (DMPII) were from Fluka and iodine
(99.999% purity) was from Alfa Aesar. Acetonitrile with HPLC
purity was purchased from SK Chemicals. THF was pre-dried
35 over sodium and distilled under argon atmosphere from sodium
benzophenone ketyl immediately prior to use. All other chemicals
were used as received without further purification. The starting
materials 4,7-dibromo-2-octyl-2H-benzo[d][1,2,3]triazole, 4,7-
dibromobenzo[c][1,2,5]thiadiazole and 3-bromo-8,8-diethyl-8H-
40 indolo[3,2,1-de]acridine were commercially available.
A mixture of compound 3 (651 mg, 1mmol), 2-cyanoacetic acid
(320 mg, 4 mmol) and 3 drops of piperidine in 20 ml THF was
90 refluxed for 4 h under N2 atmosphere. After cooling to r.t., the
solvent was removed by rotary evaporation. The residue was
purified by column chromatography using silica gel and firstly
using pure dichloromethane as the eluent to exclude the
impurities and then using THF and methanol mixture (1 : 1) as
95 the eluent to obtain the pure product as light red solid (503 mg,
73%). 1H NMR (400 MHz, DMSO-d6)
, δ: 9.05 (s, 1H), 8.52 (d, J
= 9.1 Hz, 1H), 8.44 (d, J = 10.4 Hz, 1H), 8.31 (d, J = 8.3 Hz, 1H),
8.22 (m, 2H), 8.12 (d, J = 7.5 Hz, 1H), 8.00 (m, 2H), 7.88 (s, 1H),
7.66 (d, J = 8.1 Hz, 1H), 7.52 (d, J = 7.5 Hz, 1H), 7.45 (q, J = 7.4
100 Hz, 2H), 7.27 (t, J = 7.6 Hz, 1H), 4.93 (t, J = 6.9 Hz, 2H), 2.12
(m, 6H), 1.37 (m, 4H), 1.21 (m, 6H), 0.78 (t, J = 6.5 Hz, 3H),
0.42 (t, J = 7.2 Hz, 6H). 13C NMR (126 MHz, THF-d8), δ: 149.45,
144.89, 142.91, 141.94, 137.83, 137.63, 137.31, 137.24, 136.35,
130.86, 129.99, 129.12, 128.20, 127.91, 127.75, 127.50, 125.86,
105 125.42, 124.34, 124.11, 123.19, 122.92, 121.65, 121.33, 121.00,
119.42, 119.28, 117.90, 114.53, 113.78, 56.72, 46.29, 38.13,
31.48, 28.82, 28.63, 26.28, 22.35, 14.19, 9.76. HRMS (m/z): [M +
H]+ calcd for C45H44N5O2S, 718.3216; found, 718.3213;
elemental analysis calcd (%) for C45H43N5O2S : C 75.28, H 6.04,
110 N 9.76; found: C 75.11, H 5.74, N 9.65.
Instruments and Characterization
NMR spectra were obtained on a Brucker AM 400 spectrometer
and HRMS were performed using a Waters LCT Premier XE
spectrometer. Elemental analysis was performed on a German
45 elementar vario EL III instrument. The absorption spectra of
sensitizers in solution and adsorbed on TiO2 films were measured
with
a Varian Cary 500 spectrophotometer. The cyclic
voltammograms of dyes were determined with a Versastat II
electrochemical workstation (Princeton applied research) using a
50 normal three-electrode cell with a Pt working electrode, a Pt wire
counter electrode, and a regular calomel reference electrode in
saturated KCl solution, 0.1
M
tetrabutylammonium
hexaflourophosphate (TBAPF6) was used as the supporting
electrolyte in cyanoacetic acid. The electrochemical impedance
55 spectroscopy (EIS) measurements of all the DSSCs were
(E)-2-cyano-3-(5-(7-(8,8-diethyl-8H-indolo[3,2,1-
de]acridin-3-yl)-2-octyl-2H-benzo[d][1,2,3]triazol-4-
yl)thieno[3,2-b]thiophen-2-yl)acrylic acid (DIA7)
performed using
a
Zahner IM6e Impedance Analyzer
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |
7

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DOI: 10.1039/C3RA43057J
Compound DIA7 was synthesized according to the same
de]acridin-3-yl)-4-hexylthiophen-2-
1
procedure as that of DIA6, as red solid in 74% yield. H NMR 60 yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-hexylthiophen-2-yl)acrylic
(400 MHz, DMSO-d6), δ: 9.05 (s, 1H), 8.57 (s, 1H), 8.50 (d, J =
8.9 Hz, 1H), 8.42 (d, J = 9.5 Hz, 1H), 8.29 (d, J = 9.7 Hz, 2H),
8.12 (m, 2H), 7.98 (m, 2H), 7.66 (d, J = 7.6 Hz, 1H), 7.52 (d, J =
7.5 Hz, 1H), 7.44 (m, 2H), 7.26 (t, J = 7.4 Hz, 1H), 4.93 (t, J =
acid (DIA10)
Compound DIA10 was synthesized according to the same
procedure as that of DIA6, as blackish red solid in 76% yield. 1H
NMR (400 MHz, DMSO-d6), δ: 8.43 (d, J = 8.7 Hz, 1H), 8.38 (s,
5
6.8 Hz, 2H), 2.11 (m, 6H), 1.36 (m, 4H), 1.21 (m, 6H), 0.78 (t, J 65 1H), 8.28 (s, 1H), 8.24 (d, J = 8.4 Hz, 1H), 8.20 (d, J = 8.4 Hz,
= 6.7 Hz, 3H), 0.41 (t, J = 7.2 Hz, 6H). 13C NMR (126 MHz,
1H), 8.15 (s, 2H), 8.10 (m, 2H), 7.66 (m, 2H), 7.50 (d, J = 7.6 Hz,
1H), 7.45 (m, 1H), 7.41 (t, J = 5.4 Hz, 1H), 7.26 (t, J = 7.6 Hz,
1H), 2.78 (m, 4H), 2.12 (ddd, J = 31.5, 13.8, 7.3 Hz, 4H), 1.65
(m, 4H), 1.27 (m, 12H), 0.86 (t, J = 6.9 Hz, 3H), 0.75 (t, J = 6.9
THF-d8), δ: 146.08, 145.09, 142.99, 141.92, 139.32, 139.10,
10 137.92, 137.73, 137.39, 130.80, 130.09, 129.22, 128.35, 128.04,
127.64, 125.96, 125.63, 125.53, 124.44, 124.26, 124.10, 123.32,
123.05, 122.40, 121.42, 121.10, 119.90, 118.88, 118.02, 114.66, 70 Hz, 3H), 0.40 (t, J = 7.2 Hz, 6H). 13C NMR (126 MHz, THF-d8),
113.92, 56.84, 46.39, 38.27, 31.62, 29.78, 28.99, 28.75, 26.42,
22.49, 14.33, 9.91. HRMS (m/z): [M
H]+ calcd for
δ: 164.24, 153.93, 151.80, 151.61, 145.25, 140.84, 139.31,
137.82, 137.55, 136.77, 135.97, 131.24, 130.88, 129.96, 129.27,
128.20, 127.80, 127.68, 127.65, 126.73, 126.49, 125.77, 125.35,
124.73, 124.06, 123.14, 122.97, 122.94, 121.13, 121.01, 117.97,
75 117.57, 114.37, 113.76, 46.27, 38.18, 31.34, 31.27, 30.79, 30.56,
29.07, 28.84, 28.67, 28.59, 22.43, 22.39, 14.22, 14.17, 9.74.
HRMS (m/z): [M + H]+ calcd for C53H53N4O2S3, 873.3331; found,
873.3334; elemental analysis calcd (%) for C53H52N4O2S3 : C
72.90, H 6.00, N 6.42; found: C 72.76, H 5.99, N 6.27.
+
15 C47H44N5O2S2, 774.2936; found, 774.2933; elemental analysis
calcd (%) for C47H43N5O2S2 : C 72.93, H 5.60, N 9.05; found: C
72.65, H 5.39, N 8.93.
(E)-2-cyano-3-(5-(7-(5-(8,8-diethyl-8H-indolo[3,2,1-
de]acridin-3-yl)thiophen-2-yl)-2-octyl-2H-
20 benzo[d][1,2,3]triazol-4-yl)thiophen-2-yl)acrylic acid (DIA8)
Compound DIA8 was synthesized according to the same
1
procedure as that of DIA6, as dark red solid in 72% yield. H
NMR (400 MHz, DMSO-d6), δ: 8.68 (d, J = 1.8 Hz, 1H), 8.40
80 Acknowledgments
(d, J = 8.9 Hz, 1H), 8.20 (m, 4H), 8.15 (d, J = 7.6 Hz, 1H), 7.93
25 (d, J = 10.5 Hz, 1H), 7.89 (d, J = 7.7 Hz, 1H), 7.85 (m, 2H), 7.78
(d, J = 3.8 Hz, 1H), 7.65 (d, J = 7.7 Hz, 1H), 7.51 (d, J = 7.5 Hz,
1H), 7.46 (d, J = 8.1 Hz, 1H), 7.42 (d, J = 7.6 Hz, 1H), 7.26 (t, J
= 7.5 Hz, 1H), 4.92 (t, J = 6.9 Hz, 2H), 2.11 (m, 6H), 1.28 (m,
10H), 0.78 (t, J = 6.8 Hz, 3H), 0.40 (t, J = 7.3 Hz, 6H). 13C NMR
30 (126 MHz, THF-d8), δ: 145.36, 144.17, 141.55, 141.35, 137.75,
137.67, 137.57, 137.51, 137.01, 135.73, 129.98, 129.27, 128.24,
127.92, 127.83, 126.67, 126.17, 125.39, 124.70, 124.69, 124.59,
124.25, 124.13, 124.11, 123.23, 123.08, 122.48, 121.78, 121.10,
118.22, 118.12, 114.44, 114.19, 56.78, 46.27, 38.10, 31.49,
35 29.56, 28.85, 28.65, 26.29, 22.37, 14.21, 9.78. HRMS (m/z): [M +
H]+ calcd for C49H46N5O2S2, 800.3093; found, 800.3094;
elemental analysis calcd (%) for C49H45N5O2S2 : C 73.56, H 5.67,
N 8.75; found: C 73.40, H 5.38, N 8.59.
This work was supported by NSFC/China (91233207), the
Fundamental Research Funds for the Central Universities
(WJ1114050) and Scientific Committee of Shanghai
(12ZR1407100).
85
Notes and references
a Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science & Technology, 130 Meilong
Road, Shanghai, 200237, PR China. Fax: +86-21-64252758; Tel: +86-
90 21-64252758; E-mail: jli@ecust.edu.cn
† Electronic Supplementary Information (ESI) available: (1) Frontier
molecular orbital contours of DIA6-DIA10, obtained from single point
calculations at the CAM-B3LYP/SVP level of theory; (2) Cyclic
voltammograms of dyes attached on 8 ꢀm TiO2 films; (3) Absorption
95 curves of dyes DIA6-DIA10 upon light irradiation of AM 1.5 solar light
(30 min) with and without UV cutoff filter at 400 nm; (4) Synthetic
procedure of intermediate 2, 3, B1, B2 and D1-D4; (5) Fig. S4-S8 1H and
13CNMR spectrum of DIA6-DIA10. See DOI: 10.1039/b000000x/
(E)-2-cyano-3-(5-(7-(8,8-diethyl-8H-indolo[3,2,1-
40 de]acridin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-
100
105
110
115
120
1
(a) B. O’Regan and M. Grätzel, Nature, 1991, 353, 737; (b) M.
Grätzel, Nature, 2000, 35, 3523; (c) A. Hagfeldt, G. Boschloo, L. C.
Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010, 110, 6595; (d) A.
Mishra, M. K. R. Fischer and P. Bäuerle, Angew. Chem., Int. Ed.,
2009, 48, 2474.
hexylthiophen-2-yl)acrylic acid (DIA9)
Compound DIA9 was synthesized according to the same
1
procedure as that of DIA6, as dark red solid in 74% yield. H
NMR (400 MHz, DMSO-d6), δ: 8.96 (s, 1H), 8.52 (d, J = 8.8 Hz,
45 1H), 8.36 (d, J = 7.5 Hz, 1H), 8.30 (d, J = 6.9 Hz, 3H), 8.24 (s,
1H), 8.12 (dd, J = 7.3, 3.8 Hz, 2H), 7.67 (d, J = 8.0 Hz, 1H), 7.52
(d, J = 7.3 Hz, 1H), 7.45 (m, 2H), 7.28 (t, J = 7.4 Hz, 1H), 2.86 (t,
J = 7.1 Hz, 2H), 2.12 (m, 4H), 1.67 (m, 2H), 1.34 (m, 6H), 0.88
(t, J = 6.6 Hz, 3H), 0.42 (t, J = 7.1 Hz, 6H). 13C NMR (126 MHz,
50 THF-d8), δ: 164.53, 153.51, 152.58, 152.20, 143.98, 137.78,
137.59, 137.37, 133.60, 131.76, 130.03, 129.67, 129.23, 128.24,
128.05, 127.93, 127.69, 125.68, 125.41, 124.17, 123.79, 123.26,
122.92, 121.91, 121.25, 118.53, 117.99, 114.50, 113.56, 46.27,
38.11, 31.30, 30.93, 28.75, 28.56, 22.38, 14.23, 9.75. HRMS
55 (m/z): [M + H]+ calcd for C43H39N4O2S2, 707.2514; found,
707.2511; elemental analysis calcd (%) for C43H38N4O2S2 : C
73.06, H 5.42, N 7.93; found: C 72.89, H 5.32, N 7.76.
2
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DOI: 10.1039/C3RA43057J
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