Spirally configured cis-stilbene/fluorene hybrids as bipolar, organic sensitizers for solar cell applications

Article abstract of DOI:10.1039/c2cc17079e

Hybrids based on a dibenzosuberene core bearing a spiro-fluorene junction at the C-5 position and with amino donor and β-thiophenyl-α- cyanoacrylic acid acceptor groups at C-3 and C-7, respectively, serve as new organic sensitizer materials for solar cell applications. Solar cell devices based on these materials show a conversion efficiency (η) of up to 6.1% (V_oc = 697 mV, J_sc = 12.2 mA cm^-2, FF = 0.72) under AM 1.5 G conditions. The best IPCE values exceed 75% within the 450-550 nm absorption range.

Full text of DOI:10.1039/c2cc17079e

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COMMUNICATION
Spirally configured cis-stilbene/fluorene hybrids as bipolar, organic
sensitizers for solar cell applicationsw
Wei-Shan Chao,a Ken-Hsien Liao,b Chien-Tien Chen,*b Wei-Kai Huang,c Chi-Ming Lanc
and Eric Wei-Guang Diau*c
Received 15th November 2011, Accepted 21st March 2012
DOI: 10.1039/c2cc17079e
Hybrids based on a dibenzosuberene core bearing a spiro-fluorene
junction at the C-5 position and with amino donor and b-thiophenyl-
a-cyanoacrylic acid acceptor groups at C-3 and C-7, respectively,
serve as new organic sensitizer materials for solar cell applications.
Solar cell devices based on these materials show a conversion
at the C-3 and C-7 positions, respectively. Recently, notable
power conversion efficiencies (Z) of DSSC employing trans
b-styryl (3.1–5.9%),8 b-thiophenylstyryl (4.1–7.0%),9 stilbenyl
(4.6–9.1%),10a,b and fluorenylvinylene (5.6%)10c blocks as
p-spacers for bipolar dyes have been reported. To date, the
corresponding cis-stilbene variants have never been explored.
In principle, the rigid spirally configured, cis-stilbene frame-
work would impose a better p-conjugation. Therefore, the
bipolar, DBE/spiro-fluorene hybrid assembly might have the
advantages of increasing light absorptions for superior light
harvesting, decreasing back electron transfer for bꢀetter charge
separation, and preventing approaching of I3 into the
dye/TiO2 interface for retarded charge recombination.
efficiency (g) of up to 6.1% (Voc = 697 mV, Jsc = 12.2 mA cmꢀ2
,
FF = 0.72) under AM 1.5 G conditions. The best IPCE values
exceed 75% within the 450–550 nm absorption range.
The interest on dye-sensitised solar cells (DSSCs) began with
the breakthrough made by Gratzel and co-workers1 who have
¨
documented efficiencies of 11% with Ru/bi- or terpyridine-
based sensitizers.2,3 Conversely, DSSCs made of porphyrin
and phthalocyanine dyes gradually catch up the leads.4 Metal-
free dyes bearing perylene, coumarin, or indoline core have
also been used for DSSCs.5 Some devices made of organic dyes
employing an EDOT–dithienosilole linked spacer have reached
a level of efficiency comparable to those of Ru dyes.6 The
structural design of an organic dye is often based on the
donor–(p bridge)–acceptor (D–p–A) motif due to facile intra-
molecular charge transfer with evident bathochromic shift of
the light absorption profiles.
The target dyes 1–5 were obtained by a synthetic protocol
illustrated in Scheme 1.11 Hartwig coupling reaction was used
to append five different arylamine groups at the C-3 position
of the 3,7-dibromo-STIF.7c The resulting intermediates were
subjected to Suzuki coupling reactions (72–80% yields) by
treatment with 5-formylthiophene–boronic acid.11 Subsequent
condensation of the aldehyde intermediates with cyanoacetic
acid in catalytic piperidine (10 mol%) provided the desired
dyes 1–5 in 72–78% yields.11
The stacked absorption spectra of D-STIF–TCA dyes 1–5
and the unconstrained 20 bearing a cis-stilbene core in THF
solution and TiO2 film are displayed in Fig. 1 and the
corresponding data are shown in Table 1. In all cases except
4 (D = Fl2N), two strong absorption bands with lmax centered
at 377 ꢁ 3 nm and 455 ꢁ 3 nm, respectively, were observed.
The former band can be attributed to p–p* transitions, and the
latter one to photoinduced, intramolecular charge transfer
(PICT) from a given arylamine donor to a-cyanoacrylic acid
In recent years, we have explored the utility of the dibenzo-
suberene (DBE) unit as a configurationally confined cis-stilbene
(STI) template when spirally linked to fluorene at the C-5
position. The resulting hybrid (STIF) serves as a new type of
optoelectronic template in organic light-emitting diode appli-
cations with great success.7 We sought to extend its utility
towards DSSC applications by appending amino donor (D)
and b-thiophenyl-a-cyanoacrylic acid (TCA) acceptor groups
a Department of Chemistry, National Taiwan Normal University,
#88, Sec. 4, Ding-jou Road, Taipei 11677, Taiwan
b Department of Chemistry, National Tsing Hua University,
#101, Sec. 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan.
E-mail: ctchen@mx.nthu.edu.tw; Fax: +886-3-5711082;
Tel: +886-3-5739240
c Department of Applied Chemistry and Institute of Molecular Science,
National Chiao Tung University, 1001 University Rd., Hsinchu 30010,
Taiwan. E-mail: diau@mail.nctu.edu.tw; Fax: +886-3-5723764;
Tel: +886-3-5131524
w Electronic supplementary information (ESI) available: Experimental
procedures, characterization and NMR spectra for all compounds.
CCDC 868581 and 871304. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2cc17079e
Scheme 1 The design and synthesis of D-STIF–TCA dyes.
c
4884 Chem. Commun., 2012, 48, 4884–4886
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Fig. 2 The calculated frontier molecular orbitals of Ph2N-, Fl2N-,
and IMS-STIF–TCA dyes.
ꢀ
potential of the Iꢀ/I3 redox pair (+0.35 V vs. NHE),
providing favourable conditions for hole transfer.
In addition, theoretical calculations on the frontier molecular
orbitals of these dyes by Density Functional Theory (DFT) at
the B3LYP/6-31+G(d) level support facile PICT from the
D-STIF conjugated blocks to the thiophene-CA terminals (Fig. 2).
Fig. 3 shows J–V curves of the devices made of dyes 1–5, 20,
and N719; the corresponding photovoltaic properties are
summarized in Table 2. Under AM 1.5 G illumination condi-
tions, the device efficiencies for the D-STIF–TCA dyes are in
the range of 5.40–6.12%. The short-circuit current densities
(JSC) range from 11.2 to 12.8 mA cmꢀ2 (cf. 8.25 mA cmꢀ2 for
20 and 14.4 mA cmꢀ2 for N719) and the open circuit voltages
Fig. 1 Stacked absorption spectra of 1–5 in (a) THF and (b) TiO2 film.
through conjugated STIF and thiophene linker. The PICT
band is shifted to 492 nm in the case of 4 presumably due to
enhanced conjugation. In marked contrast, 20 exhibits a blue
shift by 46 nm for the PICT absorption band with a drama-
tically reduced e value by 57% (Fig. 1a). Moreover, the
absorption spectrum of 20 on film grows up in the shorter
wavelength region and is much broader than that in solution,
which indicates an H-type aggregation of 20 on film.
The oxidation potentials (Eox) of D-STIF–TCA dyes were
obtained by cyclic voltammetry measurements. In all cases except
4 (Eox = 0.36 eV), the oxidation potentials which are attributed
to the oxidation of different arylamine moieties at C3 (Table 1)
fall in the range of 0.52–0.63 eV. Their relative trend towards
oxidation follows the order of 4 (D = Fl2N) > 5 E 1 E 3 > 2
(D = NPh2), indicating the strongest electron-donating nature
of the Fl2N group.5d The excited-state potentials [E(S+/S*)] of
the dyes 1–5 (ꢀ1.78 to ꢀ2.02 V vs. NHE) were deduced from
respective Eox and zero–zero excitation energy (E0–0) as calcu-
lated by the longest absorption band edge. Their [E(S+/S*)]
are 1.2–1.5 V higher than the conduction band energy of the
TiO2 working electrode (ꢀ0.57 V vs. NHE), which secure electron
injection from the photo-excited dyes into the TiO2 electrode.
Conversely, their E(S+/S) are 0.13–0.23 V lower than the chemical
Fig. 3 Plots of photocurrent density vs. voltage for DSSCs based on
D-STIF–TCA dyes 1–5, 20, and N719 under AM 1.5 G simulated solar
light (100 mW cmꢀ2).
Table 1 Absorption, fluorescence and electrochemical properties of D-STIF–TCA and 20 dyes
Dye
Abs., lmax (e ꢂ 10ꢀ3)a/nm
Em., lmaxb/nm
E(S+/S)c/V
E(S+/S*)c/V
E0–0d/eV
MeN-STIF–TCA (1)
PhN-STIF–TCA (2)
NpN-STIF–TCA (3)
Fl2N-STIF–TCA (4)
IMS-STIF–TCA (5)
PhN-STB–TCA (20)
375 (20.0), 452 (17.8)/442 (0.88)
379 (20.1), 452 (17.1)/461 (0.64)
380 (24.1), 457 (19.4)/462 (0.68)
374 (39.7), 492 (16.8)/476 (0.55)
369 (25.2), 456 (23.1)/461 (0.65)
406 (10.9)/374-435 (0.56)
622
607
640
615
621
467/576
+0.54
+0.63
+0.55
+0.36
+0.52
+0.51
ꢀ1.79
ꢀ1.78
ꢀ1.82
ꢀ2.02
ꢀ1.83
ꢀ1.87
2.33
2.41
2.37
2.38
2.35
2.38
a
b
c
Measured in THF and on TiO2 film. Measured in THF. Electrochemical data of D-STIF–TCA and 20 dyes measured in DMF containing 0.1 M TBA
(PF6). Measurements were conducted by using glassy carbon as a working electrode and a Pt counter electrode with a scan rate of 100 mV sꢀ1. Potentials are
quoted with reference to the internal ferrocene standard (E1/2 = 423 mV vs. Ag/AgCl). d The band-gap, E0–0, was derived from the observed optical edge.
c
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Table 2 Photovoltaic properties of the DSSCs made of D-STIF–TCA, 20
and N719 dyesa
nature of the IMS group in dye 5. Apparently, IPCE spectra
show that smaller JSC values of our dyes compared to that of
N719 are due to their poorer light harvesting ability in the
550–750 nm regions.
When compared with the device efficiency for 20, the signi-
ficant improvement of the device performances by 58–62% for
the spirally configured 2–4 strongly supports our molecular design
concept of introducing a confined cis-stilbene core to reduce dye
aggregation with improved conjugation and light harvesting.12
Dye
VOC/mV JSC/mA cmꢀ2 FF
Z (%) DL/nmol cmꢀ2
1
2
3
4
5
677
705
702
697
680
11.2
12.8
12.3
12.2
11.7
14.4
8.3
0.71 5.40
185.7
213.8
204.5
217.5
255.9
158.3
284.1
0.66 5.96
0.70 6.04
0.72 6.12
0.73 5.83
0.70 7.83
0.67 3.77
N719 777
20
678
a
Measured under the standard AM 1.5 G illumination (100 mW cmꢀ2);
Notes and references
the active area is 0.16 cm2 and the thickness of TiO2 films is 16 mm.
1 M. K. Nazeeruddin, A. Kay, L. Rodicio, B. R. Humphry,
E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am.
¨
Chem. Soc., 1993, 115, 6382.
¨
2 (a) M. Gratzel, J. Photochem. Photobiol., A, 2004, 164, 3;
¨
3 (a) M. K. Nazeeruddin, F. DeAngelis, S. Fantacci, A. Selloni,
¨
(b) B. O’Reagen and M. Gratzel, Nature, 1991, 353, 737.
G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Gratzel, J. Am.
¨
Chem. Soc., 2005, 127, 16835; (b) Y. Chiba, A. Islam,
Y. Watanabe, R. Komiya, N. Koide and L. Han, Jpn. J. Appl.
Phys., Part 1, 2006, 45, L638.
4 (a) H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009,
´ ´
42, 1809; (b) M. V. Martınez-Dıaz, G. de la Torrea and T. Torres,
Chem. Commun., 2010, 46, 7090; (c) M. G. Walter, A. B. Rudine
and C. C. Wamser, J. Porphyrins Phthalocyanines, 2010, 14, 759.
5 (a) S. Kim, J. K. Lee, S. O. Kang, J. Ko, J. H. Yum, A. Fantacci,
F. D. Angelis, D. Di Censo, M. K. Nazeeruddin and M. Grazel,
¨
J. Am. Chem. Soc., 2006, 128, 16701; (b) M. Xu, S. Wenger, H. Bala,
D. Shi, R. Li, Y. Zhou, S. M. Zakeeruddin, M. Gratzel and P. Wang,
¨
J. Phys. Chem. C, 2009, 113, 2966; (c) H. Qin, S. Wenger, M. Xu,
F. Gao, X. Jing, P. Wang, S. M. Zakeeruddin and M. Gratzel, J. Am.
¨
Chem. Soc., 2008, 130, 9202; (d) Y. Bai, J. Zhang, D. Zhou, Y. Wang,
M. Zhang and P. Wang, J. Am. Chem. Soc., 2011, 133, 11442;
(e) J. Liu, F. Wang, F. Fabregat-Santiago, S. G. Miralles, X. Jing,
J. Bisquert and P. Wang, J. Phys. Chem. C, 2011, 115, 14425.
6 (a) Y. I. Kim and B. A. Gregg, J. Phys. Chem., 1994, 98, 2412;
(b) S. Erten-Ela, D. Yilmaz, B. Icli, Y. Dede, S. Icli and E. U. Akkaya,
Org. Lett., 2008, 10, 3299; (c) Y. Shibano, T. Umeyama, Y. Matano
and H. Imahori, Org. Lett., 2007, 9, 1971; (d) M. Miyashita,
K. Sunahara, T. Nishikawa, Y. Uemura, N. Koumura, K. Hara,
A. Mori, T. Abe, E. Suzuki and S. Mori, J. Am. Chem. Soc., 2008,
130, 17874; (e) W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang,
F. Wang, C. Pan and P. Wang, Chem. Mater., 2010, 22, 1915.
7 (a) C.-T. Chen and I.-C. Chou, J. Am. Chem. Soc., 2000, 122, 7662;
(b) C.-T. Chen, Y. Wei, J.-S. Lin, M. V. R. K. Moturu,
W.-S. Chao, Y.-T. Tao and C.-H. Chien, J. Am. Chem. Soc.,
2006, 128, 10992; (c) Y. Wei and C.-T. Chen, J. Am. Chem. Soc.,
2007, 129, 7478; (d) Y. Wei, S. Samori, S. Tojo, M. Fujitsuka,
J. S. Lin, C.-T. Chen and T. Majima, J. Am. Chem. Soc., 2009,
131, 6698; (e) C.-T. Chen, J.-S. Lin, M. Murthy, Y.-T. Tao and
C.-H. Chiang, Chem. Commun., 2005, 3980.
Fig. 4 The incident photon-to-current conversion efficiency stacked
spectra for D-STIF–TCA, 20, and N719 based DSSCs.
(VOC) range from 677 to 705 mV (cf. 678 mV for 20 and
777 mV for N719) and the fill factor (FF) falls in the range of
0.66–0.73 (cf. 0.67 for 20 and 0.70 for N719). Notably, all the
device performances of dyes 1–5 are 43–63% better than that of
20 (Z = 3.77%) and reached 75–78% of the N719-based DSSC
(Z = 7.83%) fabricated and measured under similar conditions.
The stacked spectra of incident photon-to-current conversion
efficiency (IPCE) for devices made of D-STIF–TCA, 20 and N719
dyes are shown in Fig. 4. The band onsets of their IPCE plots fall
in the range of 670–690 nm. Notably, their IPCE performances
exceed 66% from 420 to 540 nm in all cases (cf. 53–67% for 20
and 66–75% for N719) except 1 (D = NMePh; 63–66%) and
exhibit the highest values (73–79%) around 490 nm (cf. 66% for
1 and 20 at 490 nm). In comparison with 2, the uniformly lower
IPCE profiles from 350 to 575 nm for 1 and from 350 to 640 nm
for 20, respectively, are responsible for their relatively smaller JSC
and poorer overall photovoltaic performances.
8 (a) A. Abbotto, N. Manfredi, C. Marinzi, F. De Angelis,
E. Mosconi, J.-H. Yum, Z. Xianxi, M. K. Nazeeruddin and
M. Gratzel, Energy Environ. Sci., 2009, 2, 1094; (b) S. Kolemen,
¨
O. A. Bozdemir, Y. Cakmak, G. Barin, S. Erten-Ela, M. Marszalek,
J.-H. Yum, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Gratzel and
¨
E. U. Akkaya, Chem. Sci., 2011, 2, 949.
In all cases except 1 and 5, the IPCE values are uniformly
higher than those of N719 by up to 10–20% from 350 to 490 nm.
Because of the limit of light absorption, the IPCE curves of
devices for 1–5 start to drop above 570 nm. The change of
donor group from common NAr2 to IMS group as in 5 led to
slight erosion of its IPCE even though there exists a stronger
absorption band at 455 nm as compared to those for 1–3
(Fig. 1). The amount of dye-loading (DL) of the 5/TiO2 film
was much larger than those of the others, indicating that the
degree of dye aggregation might play a role to reduce the JSC
of the device for 5 as compared to those of the devices for 2–4.
This interpretation is consistent with the rigidity and planar
9 (a) G. Li, K.-J. Jiang, Y.-F. Li, S.-L. Li and L.-M. Yang, J. Phys.
Chem. C, 2008, 112, 11591; (b) J. Heo, J.-W. Oh, H.-I. Ahn,
S.-B. Lee, S.-E. Cho, M.-R. Kim, J.-K. Lee and N. Kim, Synth.
Met., 2010, 160, 2143.
10 (a) S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park,
M.-H. Lee, W. Lee, J. Park, K. Kim, N.-G. Park and C. Kim,
Chem. Commun., 2007, 4887; (b) The power conversion efficiencies
vary from 4.6 to 5.4% based on our own device fabrication and
property measurement; (c) H. Zhou, P. Xue, Y. Zhang, X. Zhao,
J. Jia, X. Zhang, X. Liu and R. Lu, Tetrahedron, 2011, 67, 8477.
11 See ESIw for details.
12 An X-ray crystal structure (with a SQEEZE refinement) of Ph2N-
STIF–FCA indicates slight co-planarity in the confined cis-stilbene
core in view of the dihedral angle of 221 between CQC and
flanking phenyl groups11
.
c
This journal is The Royal Society of Chemistry 2012
4886 Chem. Commun., 2012, 48, 4884–4886