Synthesis of dendritic triphenylamine derivatives for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.09.015

Three new donor-π-Acceptor (D-π-A) metal-free organic dyes, in which dendritic triphenylamine acts as donor, and furan, thiophene and benzene as π-linkers, were synthesized, characterized and used for the application of dye sensitized solar cell (DSSC). Among the fabricated DSSCs, the device based on the furan as π-linker exhibited a short circuit current density of 14.56 mA cm^-2, an open circuit photo voltage of 0.71 V and a fill factor of 0.59, implying a power conversion efficiency (η) of 6.10%. The devices sensitized by dyes with thiophene and benzene as π-linker possessed relative lower η, 5.92% and 4.97%, respectively.

Full text of DOI:10.1016/j.dyepig.2012.09.015

Dyes and Pigments 96 (2013) 407e413
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of dendritic triphenylamine derivatives for dye-sensitized solar cells
,
,
,
a
a
a
a,
Junhui Jia a 1, Yuan Zhang b 1, Pengchong Xue a , Peng Zhang , Xin Zhao , Baijun Liu , Ran Lu
*
*
a State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
b College of Chemistry, Beijing Normal University, Beijing 100875, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new donor-
donor, and furan, thiophene and benzene as
application of dye sensitized solar cell (DSSC). Among the fabricated DSSCs, the device based on the furan
as
-linker exhibited a short circuit current density of 14.56 mA cmꢀ2, an open circuit photo voltage of
0.71 V and a fill factor of 0.59, implying a power conversion efficiency ( ) of 6.10%. The devices sensitized
-linker possessed relative lower , 5.92% and 4.97%,
p
-acceptor (D-
p
-A) metal-free organic dyes, in which dendritic triphenylamine acts as
Received 14 June 2012
Received in revised form
8 August 2012
Accepted 21 September 2012
Available online 29 September 2012
p
-linkers, were synthesized, characterized and used for the
p
h
by dyes with thiophene and benzene as
respectively.
p
h
Keywords:
DSSC
Ó 2012 Elsevier Ltd. All rights reserved.
Dendritic triphenylamine
p-Linker
Furan
Thiophene
Benzene
1. Introduction
configuration. Recently, some efficient metal-free organic dyes with
triphenylamine as electron donor have been synthesized [8]. This
year, we reported one dye with Y-shaped conformation bearing
triphenylamino-vinyl as arms (TT Scheme 1). The photo-to-
electrical conversion efficiency of the DSSC sensitized with
branched TT reached 5.12% [8c]. As reported so far, furan-based
[12], thiophene-based [13] heterocycles or phenylenevinylene
Dye-sensitized solar cells (DSSCs) have been an intensive
research field due to their capability to convert solar light into
electricity with low production costs and environmental pollution
[1] since Grätzel and co-workers reported it in 1991 [2]. At present,
Ru complex photosensitizers have overall solar energy conversion
efficiency (h) approaching more than 12% in small area [3], but the
unit [14] were introduced as p-spacer to expect the red-shift and
limited resources and elaborated purification of Ru complexes
make the wide application become difficult. In recent years, metal-
free organic sensitizers have emerged as competitive alternatives
because of relatively low cost, ease of structural tuning, high molar
extinction coefficients, control of absorption wavelength and more
environmentally friendly properties [4]. Many efficient metal-free
organic dyes based on coumarin [5], indoline [6], hemicyanine
[7], triphenylamine [8], fluorine [9], phenothiazine [10], and tet-
rahydroquinoline units [11] had been developed and their DSSCs
showed good performances. However, to design efficient metal-
free organic dyes is still a challenge.
broadening of the absorption spectra. In this paper, we had
designed and synthesized three new dendritic TT-based organic
sensitizers (TT1-3) containing a furan, a thiophene and a benzene
moiety as
p-spacer, respectively. Finally, the sensitizers had been
applied successfully to sensitization of nanocrystalline TiO2-based
solar cells and the corresponding electronic, optical and photovol-
taic properties were also presented. It was found that the devices
based on the furan, thiophene and benzene as
power conversion efficiencies ( ) of 6.10, 5.92 and 4.97%, respec-
tively. The result suggests that the introduction of furan and thio-
phene as -spacer increases the efficiency of DSSC.
p-linker exhibited
h
p
Triphenylamine has been widely used in opto- and electro-
active materials for its good electron donating and transporting
capability, as well as its special propeller starburst molecular
2. Experimental section
2.1. Material and methods
* Corresponding authors.
E-mail addresses: xuepengchong@jlu.edu.cn (P. Xue), luran@mail.jlu.edu.cn
UVevis absorption spectra were determined on a Shimadzu UV-
1601PC spectrophotometer. Photoluminescence (PL) spectra were
carried out on a Shimadzu RF-5301 luminescence spectrometer. IR
(R. Lu).
1
The first two authors contributed equally to this work.
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.015

408
J. Jia et al. / Dyes and Pigments 96 (2013) 407e413
Scheme 1. Molecular structures of TT1, TT2 and TT3.
spectra were measured using a Germany Bruker Vertex 80v FT-IR
spectrometer by incorporating samples in KBr disks. Mass spectra
were performed on Agilent 1100 MS series and AXIMA CFR MALDI/
TOF (matrix assisted laser desorption ionization/time-of-flight) MS
(COMPACT). C, H, and N analyses were taken on a Vario EL cube
elemental analyzer. Cyclic voltammetry was carried out with a CHI
604B electrochemical working station at room temperature at
a scan rate of 50 mV sꢀ1. Tetrahydrofuran (THF) was distilled over
sodium and benzophenone. DMF were distilled from phosphorous
pentoxide. CH2Cl2 was distilled by hydrogenated calcium and other
chemicals were used as received without further purification. All
reactions were monitored using TLC plates. All chromatographic
separations were carried out on silica gel (300e400 mesh). FTO
electrodes were tightly held and heated at 120 ꢁC for 25 s in seal
machine (DHS-ES2) to seal the two electrodes. A drop of an elec-
trolyte was put on the hole in the back of the counter electrode. It
was introduced into the cell via vacuum backfilling. Finally, the hole
was sealed using Surlyn film and a cover glass (0.13e0.17 mm
thickness). The edge of the FTO outside of the cell was roughened
with sandpaper. An electrolyte solution used was 0.6 M PMII
(1-propyl-3-methylimidazolium iodide), 0.1 M LiI, 0.05 M I2, 0.5 M
TBP (4-tert-butylpyridine) and 0.05 mM GUSCN (guanidine thio-
cyanate) in dry acetonitrile.
2.3. Synthesis
glasses (2.2 mm thickness, 15
U
cmꢀ2, Nippon Sheet Glass) pasted
m and Pt electrodes
4-(Bis(4-(4-(diphenylamino)styryl)phenyl)amino)benzalde-
hyde (1) was synthesized according to the reported procedure [8c].
with nanocrystalline-TiO2 in thickness of 10
m
were bought from Dalian HeptaChroma Solar Tech Co., Ltd.
2.3.1. 4-(4-(Diphenylamino)styryl)-N-(4-(4-(diphenylamino)styryl)
phenyl)-N-(4-vinylphenyl)aniline(2)
2.2. Preparation of solar cells
Methyltriphenylphosphonium iodide (1.25 g, 3.10 mmol),
compound 1 (1.00 g, 1.23 mmol) and t-BuOK (0.36 g, 3.13 mmol)
were dissolved in 25 mL dry THF at 0 ꢁC, and the solution was
stirred at room temperature for 3 h. After removing the solid by
filtration, the solvent was removed and the residue was purified by
column chromatography (petroleum ether/CH2Cl2 ¼ 3/1, v/v) to
give yellow solid (0.68 g, 69% in yield). mp 180e182 ꢁC; [15] 1H
The dye-anchored TiO2 film as a working electrode was
prepared by following procedure. The area of the nanocrystalline-
TiO2 film coated on FTO glasses was 0.24 cm2 (0.55 cm of diameter).
The wet film was dried in air at 100 ꢁC for 15 min followed by
another 15 min at 150 ꢁC. Then, the film was calcined at 350 ꢁC for
10 min. After that, the film was treated at 450 ꢁC for 30 min under
oxygen atmosphere. When the temperature decreased to 100 ꢁC,
the electrodes were immersed in a dry CH2Cl2 solution of the dye
(0.5 mM) and kept at room temperature for 12 h. A sandwich cell
was prepared by using the dye-anchored TiO2 film as a working
electrode and Pt electrode as a counter one, which was assembled
with a hot-melt-ionomer film of Surlyn polymer gasket. The
NMR (500 MHz, CDCl3, ppm):
d
7.37 (d, 8H), 7.31 (d, J ¼ 8.5 Hz, 2H),
7.27 (s, 2H), 7.24 (s, 2H), 7.11 (d, J ¼ 7.5 Hz, 10H), 7.07 (d, J ¼ 8.5 Hz,
6H), 7.03 (m, 10H), 6.95 (s, 4H), 6.67 (m, 1H), 5.65 (d, J ¼ 18.0 Hz,
1H), 5.17 (d, J ¼ 11.0 Hz, 1H). MS (MALDI-TOF): m/z 810.0 [M þ Hþ]
(Calcd for C60H47N3: 809.38). Anal. Calcd (%) for C60H47N3: C, 88.96;
H, 5.85; N, 5.19, Found: C 88.84, H 5.73, N 5.24.

J. Jia et al. / Dyes and Pigments 96 (2013) 407e413
409
2.3.2. 5-(4-(Bis(4-(4-(diphenylamino)styryl)phenyl)amino)styryl)
furan-2-carbaldehyde(3)
3027, 3421; 1H NMR (500 MHz, DMSO-d6, ppm):
d
7.79 (s, 1H), 7.53
(d, J ¼ 8.5 Hz, 4H), 7.50 (d, J ¼ 8.5 Hz, 6H), 7.32 (t, J ¼ 8.0 Hz, 8H), 7.24
(d, J ¼ 7.0 Hz, 2H), 7.20 (s, 1H), 7.11 (s, 4H), 7.04 (m, 18H), 6.95 (d,
J ¼ 8.5 Hz, 4H), 6.76 (d, J ¼ 3.5 Hz,1H). 13C NMR (125 MHz, DMSO-d6,
To a solution of compound 2 (0.40 g, 0.50 mmol), compound 5-
bromofuran-2-carbaldehyde (0.08 g, 0.45 mmol) and Bu4NBr
(0.58 g, 1.80 mmol) in dry DMF (50 mL) anhydrous K2CO3 (0.25 g,
1.80 mmol) and Pd(OAc)2 (5.0 mg, 0.02 mmol) were added. The
mixture was stirred under an N2 atmosphere at 110 ꢁC for 24 h.
After cooling to room temperature, the mixture was poured into
water (600 mL) and extracted with CH2Cl2. The combined organic
phases were washed with brine, and dried with anhydrous Na2SO4.
After the solvent was removed, the residue was purified by column
chromatography (silica gel, petroleum ether/CH2Cl2 ¼ 2/1, v/v) to
give 0.32 g orange-red solid (78% in yield). mp 66.0e68.0 ꢁC. IR
(KBr, cmꢀ1): 958, 1024, 1174, 1281, 1394, 1489, 1510, 1595, 1668,
ppm)
d 155.72, 147.57, 147.23, 145.93, 142.64, 133.21, 131.66, 129.28,
128.52, 127.31, 126.31, 124.95, 124.48, 123.63, 123.01, 115.94, 115.63,
115.22, 112.47, 112.20, 103.59. MS (MALDI-TOF): m/z 971.2 [M þ Hþ]
(Calcd for C68H50N4O3: 970.39). Anal. Calcd (%) for C68H50N4O3. C,
84.10; H, 5.19; N, 5.77, Found: C, 84.19; H, 5.10; N, 5.85.
2.3.6. (E)-3-(5-((E)-4-(Bis(4-((E)-4-(diphenylamino)styryl)phenyl)
amino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (TT2)
The synthetic procedure for TT2 was similar to that of TT1. The
crude product was purified by column chromatography (silica gel)
using methanol/dichloromethane (v/v ¼ 1/10) as the eluent to give
red solid (74% inyield). mp 230.0e232.0 ꢁC. IR (KBr, cmꢀ1): 961,1055,
1282, 1363, 1491, 1506, 2215, 2353, 3028, 3423; 1H NMR (500 MHz,
1716, 2955, 3022, 3431; 1H NMR (500 MHz, DMSO-d6, ppm):
d 9.55
(s, 1H), 7.61 (s, 1H), 7.59 (d, J ¼ 4.0 Hz, 2H), 7.55 (d, J ¼ 9.0 Hz, 4H),
7.51 (d, J ¼ 8.5 Hz, 4H), 7.33 (t, J ¼ 8.5 Hz, 10H), 7.29 (s, 1H), 7.15 (s,
1H), 7.11 (d, J ¼ 2.0 Hz, 4H), 7.09 (s, 1H), 7.07 (d, J ¼ 3.0 Hz, 4H), 7.06
(t, J ¼ 2.0 Hz, 6H), 7.04 (s, 4H), 7.02 (s, 1H), 6.96 (d, J ¼ 8.5 Hz, 4H),
6.83 (d, J ¼ 3.5 Hz,1H). MS (MALDI-TOF): m/z 904.1 [M þ Hþ] (Calcd
for C65H49N3O2: 903.38). Anal. Calcd (%) for C65H49N3O2, C, 86.35; H,
5.46; N, 4.65, Found: C, 86.29; H, 5.52; N, 4.57.
DMSO-d6, ppm):
d 7.98 (s, 1H), 7.56 (m, 4H), 7.50 (m, 6H), 7.37 (d,
J ¼ 8.5 Hz,1H), 7.32 (m, 9H), 7.24 (d, J ¼ 3.5 Hz,1H), 7.10 (m, 8H), 7.04
(m, 12H), 7.00 (s, 1H), 6.96 (t, J ¼ 7.5 Hz, 6H). 13C NMR (125 MHz,
CDCl3, ppm)
d 163.42, 147.88, 147.46, 147.01, 146.99, 146.36, 140.20,
136.09, 136.06, 132.85, 132.10, 130.91, 130.27, 130.18, 130.05, 128.44,
127.95, 127.92, 127.20, 127.04, 126.76, 125.17, 124.52, 124.28, 123.66,
123.23, 120.32, 120.01, 110.37. MS (MALDI-TOF): m/z 987.7 [M þ Hþ]
(Calcd for C68H50N4O2S: 986.37). Anal. Calcd (%) for C68H50N4O2S C,
82.73; H, 5.10; N, 5.68, Found: C, 82.83; H, 5.06; N, 5.73.
2.3.3. 5-(4-(Bis(4-(4-(diphenylamino)styryl)phenyl)amino)styryl)
thiophene-2-carbaldehyde (4)
The synthetic procedure for 4 was similar to that of 3, the rough
product was purified by column chromatography (silica gel,
petroleum ether/CH2Cl2 ¼ 3/2, v/v) to give orange solid (75% in
yield). mp 32.0e34.0 ꢁC. IR (KBr, cmꢀ1): 958, 1045, 1178, 1219, 1282,
1317, 1448, 1491, 1506, 1595, 1653, 1697, 1732, 3029, 3441; 1H NMR
2.3.7. (E)-3-(4-((E)-4-(Bis(4-((E)-4-(diphenylamino)styryl)phenyl)
amino)styryl)phenyl)-2-cyanoacrylic acid (TT3)
The synthetic procedure for TT3 was similar to that of TT1. The
crude product was purified by column chromatography (silica gel)
using methanol/dichloromethane (v/v ¼ 1/10) as the eluent to give
red solid (64% in yield). mp 168.0e170.0 ꢁC. IR (KBr, cmꢀ1): 963,
1178, 1284, 1363, 1493, 1510, 1589, 1634, 2218, 2923, 3024, 3427; 1H
(500 MHz, DMSO-d6, ppm):
d
9.84 (s, 1H), 7.92 (d, J ¼ 4.0 Hz, 1H),
7.54 (m, 3H), 7.48 (m, 6H), 7.39 (t, J ¼ 8.5 Hz, 1H), 7.32 (m, 10H), 7.21
(m, 2H), 7.11 (d, J ¼ 7.0 Hz, 1H), 7.07 (m, 6H), 7.02 (m, 12H), 6.97 (s,
1H), 6.94 (t, J ¼ 5.5 Hz, 4H), 5.74 (s, 1H). MS (MALDI-TOF): m/z 920.4
[M þ Hþ] (Calcd for C65H49N3OS: 919.36). Anal. Calcd (%) for
C65H49N3OS, C, 84.84; H, 5.37; N, 4.57, Found: C, 84.89; H, 5.47; N,
4.68.
NMR (500 MHz, DMSO-d6, ppm):
d
¼ 8.01 (s, 1H), 7.87 (d, J ¼ 8.5 Hz,
2H), 7.57 (d, J ¼ 8.5 Hz, 4H), 7.49 (d, J ¼ 8.5 Hz, 4H), 7.31 (t, J ¼ 8.0 Hz,
10H), 7.12 (t, J ¼ 9.0 Hz, 8H), 7.07 (d, J ¼ 7.5 Hz, 4H), 7.02 (m, 14H),
6.95 (d, J ¼ 8.5 Hz, 4H). 13C NMR (125 MHz, CDCl3, ppm)
d 147.94,
2.3.4. 4-(4-(Bis(4-(4-(diphenylamino)styryl)phenyl)amino)styryl)
benzaldehyde (5)
147.45, 147.36, 146.66, 145.61, 144.37, 139.26, 138.89, 133.37, 132.83,
132.11, 129.66, 128.66, 128.32, 127.61, 127.27, 126.78, 125.38, 124.82,
124.04, 123.35, 98.87. MS (MALDI-TOF): m/z 981.7 [M þ Hþ] (Calcd
for C70H52N4O2: 980.41). Anal. Calcd (%) for C70H52N4O2 C, 82.69; H,
5.34; N, 5.71, Found: C, 82.75; H, 5.25; N, 5.82.
The synthetic procedure for 5 was similar to that of 3, the rough
product was purified by column chromatography (silica gel,
petroleum ether/CH2Cl2 ¼ 2/1, v/v) to give yellow solid (75% in
yield). mp 80.0e82.0 ꢁC. IR (KBr, cmꢀ1): 964, 1028, 1076, 1163, 1175,
1219, 1277, 1317, 1419, 1456, 1491, 1508, 1588, 1693, 1734, 3028,
3. Results and discussion
3426; 1H NMR (500 MHz, DMSO-d6, ppm):
d 9.97 (s, 1H), 7.89 (d,
J ¼ 8.0 Hz, 2H), 7.78 (d, J ¼ 8.0 Hz, 2H), 7.57 (d, J ¼ 8.5 Hz, 2H), 7.48
(m, 6H), 7.45 (s, 1H), 7.42 (s, 1H), 7.32 (m, 9H), 7.24 (s, 1H), 7.21 (s,
1H), 7.12 (d, J ¼ 7.5 Hz, 1H), 7.08 (t, J ¼ 3.0 Hz, 6H), 7.06 (s, 2H), 7.03
(d, J ¼ 8.5 Hz, 10H), 7.00 (t, J ¼ 3.0 Hz, 3H), 6.95 (d, J ¼ 8.5 Hz, 3H).
MS (MALDI-TOF): m/z 914.2 [M þ Hþ] (Calcd for C67H51N3O: 913.4).
Anal. Calcd (%) for C67H51N3O, C, 88.03; H, 5.62; N, 4.60, Found: C,
87.94; H, 5.54; N, 4.67.
3.1. Synthesis of TT1, TT2, TT3
The synthetic routes for triphenylamine-based dyes TT1, TT2,
and TT3 are sketched in Scheme 2. Compound 1 and 2 were
synthesized according to the reported procedures [8c,15]. The Heck
reaction between compounds 1 and 5-bromofuran-2-carbaldehyde
catalyzed by Pd(OAc)2 at 110 ꢁC afforded compound 3 in a yield of
78%. Then, compound 3 could be easily transformed into compound
TT1 via Knoevenagel reaction in a yield of 77%. Similarly, TT2 and
TT3 were obtained in the yields of 74% and 64%, respectively. All the
intermediates and the target products were purified by column
chromatography and the new compounds were characterized with
FTIR, NMR, elemental analysis and MALDI/TOF mass spectroscopy.
2.3.5. (E)-3-(5-((E)-4-(Bis(4-((E)-4-(diphenylamino)styryl)phenyl)
amino)styryl)furan-2-yl)-2-cyanoacrylic acid (TT1)
A mixture of compound 3 (0.10 g, 0.11 mmol), cyanoacetic acid
(23.50 mg, 27.5 mmol) and ammonium acetate (20 mg) in glacial
acetic acid (10 mL) was stirred at 120 ꢁC for 12 h. After it was cooled
to room temperature, the resulting precipitate was collected by
filtration and washed with water. The crude product was purified by
column chromatography (silica gel) using methanol/dichloro-
methane (v/v ¼ 1/10) as the eluent. 75 mg of dark-red solid was
obtained (77% in yield). mp 200.0e202.0 ꢁC; IR (KBr, cmꢀ1): 962,
1026,1176,1207,1227,1284,1315,1396,1417,1507,1593, 2218, 2352,
3.2. Optical properties
Absorption spectra of three dyes in dilute solutions of THF
(2 ꢂ 10ꢀ6 M) are shown in Fig. 1 and their absorption data are listed
in Table 1. In the UVeVis spectra, the dyes exhibit three major

410
J. Jia et al. / Dyes and Pigments 96 (2013) 407e413
Scheme 2. Synthesis of triphenylamine-based dyes TT1, TT2 and TT3.
prominent bands, appearing at 290e310 nm, 370e420 nm and
450e525 nm, respectively. The absorption band of 290e310 nm can
TiO2 films (2 mm in thickness) are shown in Fig. 1b. The maximal
absorption peaks for TT1, TT2 and TT3 on TiO2 films appeared at
461, 447 and 424 nm, respectively, appearing similar order to that
in solution for maximal peaks of dyes. Moreover, the absorption
bands in films exhibited blue shift about 30 nm compared with
those in solutions, which might originate from the aggregation of
the dyes on the surface of TiO2 nanoparticles. Such a phenomenon
has been also found in other organic dyes [[4a,10a,17]].
be ascribed to a localized aromatic
ylamine [16] and the band around 370e420 nm is due to the
delocalized * transition. The third bands with longer wave-
p-p* transition of typical triar-
p-p
length can be attributed to the intramolecular charge transfer (ICT)
between the triphenylamine donor and the acceptor. The dyes from
TT1 to TT3 show strong visible bands with maxima at 496
(0.43 ꢂ 104 Mꢀ1 cmꢀ1), 492 (0.69 ꢂ 104 Mꢀ1 cmꢀ1) and 451 nm
(0.71 ꢂ 104 Mꢀ1 cmꢀ1), which are red-shifted and expanded,
compared with TT dye [8c] which contains only one vinyl group as
the spacer. This result indicates that furan and thiophene rings are
3.3. Electrochemical properties
To estimate the levels of the dyes and elucidate the effect of p-
better
p-spacer than benzene ring to increase the extent of
spacer on the energy level, cyclic voltammetry was employed using
a three-electrode cell and an electrochemistry work station (CHI
604). The working electrode was a glass carbon disc, the auxiliary
conjugation and ICT. Moreover, the normalized UVevis absorption
spectra of triphenylamine-based dyes deposited on transparent
Fig. 1. UVevisible absorption spectra of TT1-3 dyes (a) in THF (conc. ¼ 2.0 ꢂ 10ꢀ6 M) and (b) adsorbed on TiO2 films.

J. Jia et al. / Dyes and Pigments 96 (2013) 407e413
411
Table 1
Absorption and electrochemical properties of the TT1-3 dyes.
a
Dye
labs (nm) (
3
ꢂ 10ꢀ4 Mꢀ1 cmꢀ1
)
Eox (V) vs NHEb
E0-0 (ev)c
ELUMO (V) vs NHEd
ꢀ1.44
Egap (eV)e
HOMOf (ev)
LUMOf (ev)
TT1
298 (0.57)
0.69
2.13
0.94
ꢀ4.66
ꢀ2.52
ꢀ2.66
ꢀ2.55
408 (11.81)
496 (0.43)
304 (0.58)
392 (10.71)
492 (0.69)
303 (0.56)
393 (10.67)
451 (0.71)
TT2
TT3
0.70
0.70
2.11
2.14
ꢀ1.41
ꢀ1.44
0.91
0.94
ꢀ4.69
ꢀ4.64
a
Measured in THF (2 ꢂ 10ꢀ6 M),
3
(ꢂ10ꢀ4 Mꢀ1 cmꢀ1).
b
c
d
e
f
The ground station potential (first oxidation peak) of the dyes was measured in CH2Cl2 with ferrocene/ferrocenium (Fc/Fcþ) as an internal reference.
E0-0 was determined from the edge of the absorption spectrum.
ELUMO ¼ EoxꢀE0-0
.
Egap is the energy gap between the ELUMO of the dye and the conduction band level of TiO2 (ꢀ0.5 eV vs NHE).
Energy levels of HOMO and LUMO vs vacuum were given after geometrical optimization.
electrode was a Pt wire, and Ag/Agþ was used as reference elec-
trode. Tetrabutylammonium tetrafluoroborate (TBABF4, 0.1 M) was
used as the supporting electrolyte in dry CH2Cl2 and the ferroce-
nium/ferrocene (Fc/Fcþ) redox couple was used as an internal
potential reference [18].
excited state oxidation potentials of three triphenylamine-based
dyes are more negative than the potential of the TiO2 conduction
band edge (ꢀ0.5 V vs NHE), providing sufficient thermodynamic
driving force (0.91e0.94 V, Table 1) for electron injection from the
excited dyes to TiO2 electrode.
Their data are summarized in Table 1. The first oxidation
potentials (Eox), corresponding to the HOMO levels, for TT1, TT2 and
TT3 are 0.69 V, 0.70 V and 0.70 V (vs NHE, Table 1), respectively,
3.4. Molecular orbital calculations
indicating that the p-spacer has slight influence on energy level of
To gain an insight into the geometrical and electronic structures
of the dyes, density functional theory (DFT) calculations for the
three triphenylamine-based sensitizers were performed at B3LYP/
6-31 level with the Gaussian 03W program package. In the opti-
mized structures of three dyes, the electron cloud of the highest
occupied frontier molecular orbitals are delocalized over the tri-
phenylamine entities, while the lowest unoccupied molecular
dyes. This result also suggests that HOMO energy levels are more
positive than the iodine/iodide redox potential value (0.4 V vs NHE).
Therefore, sufficient driving forces are large enough to efficiently
regenerate dye through the recapture of the injected electrons from
I
ꢀ to the dye cation radical. In addition, the excited-state oxidation
potential of the dye plays an important role in the electron-
injection process from the excited state dye to TiO2 electrode, and
it can be evaluated from the equation: ELUMO ¼ EoxꢀE0-0, in which
Eox is the first oxidation potentials and E0-0 is the 0-0 excitation
energy. We gained the E0-0 values of TT1, TT2 and TT3 from the edge
of absorption spectra of the dyes in THF, which are 2.13 eV, 2.14 eV
and 2.11 eV, respectively. As a result, ELUMO values for TT1, TT2 and
TT3 were ꢀ1.44 V, ꢀ1.41 V and ꢀ1.44 V vs NHE, respectively. So, the
orbitals are mainly localized over the
p-spacer and cyanoacrylic
acid groups. It suggests that HOMO / LUMO excitation moves the
electron density distribution from triphenylamine donor moiety to
cyanoacrylic acid acceptor moiety via different
p-spacers (see
Fig. 2). The effective intramolecular charge transfer (ICT) process
indicates that the electron injection from the excited dyes to TiO2
CB is feasible.
Fig. 2. The frontier orbital plots of the HOMO and LUMO of TT1, TT2 and TT3.

412
J. Jia et al. / Dyes and Pigments 96 (2013) 407e413
Table 2
Photovoltaic parameters of the DSSCs sensitized with TT1, TT2 and TT3 in full
sunlight (AM 1.5 G, 100 mW cmꢀ2).
Dyes
Dye loada
(mol cmꢀ2
Voc (V)
Jsc
ff
h (%)
)
(mA
cmꢀ2
)
TT1
TT2
TT3
1.41 ꢂ 10ꢀ7
1.21 ꢂ 10ꢀ7
8.76 ꢂ 10ꢀ8
0.71
0.70
0.65
14.56
13.65
12.14
0.59
0.62
0.63
6.10
5.92
4.97
a
Amount of the dyes absorbed on TiO2 film.
3.5. Fabrication of DSSCs and photovoltaic properties of DSSCs
The electrochemical cell used for photovoltaic measurements
consisted of a dye-adsorbed TiO2 electrode (the thickness of TiO2
film is 10 mm), a counter electrode and an organic electrolyte. The
dye-adsorbed TiO2 electrode and Pt-counter electrode were
assembled into a sealed sandwich type cell. The photovoltaic
performances of TiO2 films sensitized by TT1, TT2 and TT3 with
a liquid electrolyte under standard global AM 1.5 solar light
condition (100 mW cmꢀ2) are summarized in Table 2 and the
photocurrentevoltage curves of the devices were shown in Fig. 3
The TT1-sensitized cell gave a short circuit photocurrent density
(Jsc) of 14.56 mA cmꢀ2, an open circuit voltage (Voc) of 0.71 V and
a fill factor of 0.59, corresponding to an overall conversion effi-
ciency of 6.10% (see Table 2). Under the same condition, the TT2-
and TT3-sensitized cell gave a Jsc of 13.65 and 12.14 mA cmꢀ2, Voc of
0.70 and 0.65 V, and a fill factor (ff) of 0.62 and 0.63, corresponding
to an overall conversion efficiency of 5.92% and 4.97%, respectively.
It is evident that TT1 possesses the highest Voc, the device of TT3
has the lowest Voc. To further scrutinize the difference in Voc value,
the measurement of dark currentevoltage characteristics was
performed. From the dark JeV curves (Fig. 3), the onsets of the dark
current of the three DSSCs were derived as TT1 (683 mV) > TT2
(658 mV) > TT3 (646 mV), suggesting good accordance with that of
the Voc values, indicating that the increasing of Voc was associated
with the decreased tendency of the electron recombination from
TiO2 to the Iꢀ/Iꢀ3 redox couple in the electrolyte [19]. This result is
rational if considering the loading amounts of three dyes in TiO2
film. It is well known, the larger loading amount of dyes on the
surface of TiO2 nanoparticles can suppress the electron
Fig. 4. Incident photo-to-current conversion efficiency of devices sensitized with TT1,
TT2 and TT3.
recombination between TiO2 to Iꢀ3 because of less exposed surface
of TiO2. The amount of dyes adsorbed on TiO2 films was estimated
by desorbing the dye with NaOH solution (Table 2) [8c]. The result
shows that TT1 has largest loading amount on TiO2 films, but lowest
absorbing amount could be observed for TT3, giving the same
sequence as that of Voc
.
It is also clear that TT1 has highest short current, and the Isc
based on TT2 is larger than that of TT1. To understand the difference
between Isc of the devices sensitized by three dyes, their IPCE
(monochromatic incident photon-to-current conversion efficiency)
responses were examined. As shown in Fig. 4, the low-energy
onsets of the IPCE spectra correlate well with E0-0 of the indi-
vidual dyes, meaning that the LUMO energy levels of dyes are high
enough to allow the electron of dyes at excited state to inject into
the conductive band of TiO2 nanoparticles. Moreover, solar cells
based on TT1 and TT2 showed similar IPCE, but the spectral
response of TT1 were broader than that of TT2, and the IPCE of TT1
exceeded 70% in the spectral range of 424e501 nm, which reached
a maximum (73.6%) at 475 nm. The IPCE of TT2 reached a maximum
(70.3%) at 477 nm. Because of the similar spectral response ranges
of TT1 and TT2 in IPCE, it is possible reason that the different
electron injection yields of dyes lead the change of their Isc [20].
Solar cell based on TT3 has lowest IPCE, implying poor electron
injection efficiency or serious charge recombination in a solar cell
based on TT3. Moreover, its spectral respond is relative narrow. As
a consequence, the lowest short current could be observed in TT3
devices.
4. Conclusion
In summary, three new triphenylamine-based organic dyes
(TT1-3) containing benzene, thiophene and furan moieties as
linkers for dye-sensitized solar cells had been synthesized. These
dyes were successfully adsorbed on TiO2 films, and efficient dye
sensitized solar cells had subsequently been fabricated. It was
found that the introduction of furan and thiophene
p-linkers to
tune the HOMO-LUMO level increased the spectral response range
and their solar cells showed higher IPCE than that of dye with
benzene as p-linker. As a consequence, the solar cells based on dye
with furan and thiophene linker exhibit 6.10 and 5.92% conversion
efficiency, respectively. The device sensitized by dye with benzene
Fig. 3. Current densityevoltage curves under full sunlight (real) (AM 1.5 G,
100 mW cmꢀ2) and in the dark (dashed) of devices sensitized with TT1, TT2 and TT3
using a volatile electrolyte.
as
p-linker possessed relative lower h, 4.97%. This work indicates

J. Jia et al. / Dyes and Pigments 96 (2013) 407e413
413
(e) Xu W, Peng B, Chen J, Liang M, Cai F. J Phys Chem C 2008;112:874e80;
that furan and thiophene rings in our dyes are excellent linker
relative to benzene ring.
(f) Wu WJ, Yang JB, Hua JL, Tang J, Zhang L, Long YT, et al. Efficient and stable dye-
sensitized solar cells based on phenothiazine sensitizers with thiophene units.
J Mater Chem 2010;3:1772e9;
(g) Tian HN, Pan JX, Chen RK, Liu M, Zhang QY, Hagfeldt A, et al. Adv Funct Mater
2008;18:3461e8.
Acknowledgments
[9] Kim S, Lee JK, Kang SO, Ko J, Yum JH, Fantacci S, et al. Molecular engineering of
organic sensitizers for solar cell applications. J Am Chem Soc 2006;128:
16701e7.
[10] (a) Tian HN, Yang XC, Chen RK, Pan YZ, Li L, Hagfeldt A, et al. Phenothiazine
derivatives for efficient organic dye-sensitized solar cells. Chem Commun
2007:3741e3;
This work was supported by the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education
Ministry; The National Natural Science Foundation of China
(21103067,
20874034
and
51073068);
973
Program
(b) Teng C, Yang XC, Yang C, Tian HN, Li SF, Wang XN, et al. Influence of triple
(2009CB939701); Open Project of State Key Laboratory of Supra-
molecular Structure and Materials (SKLSSM201102).
bonds as
p-spacer units in metal-free organic dyes for dye-sensitized solar
cells. J Phys Chem C 2010;114:11305e13.
[11] Chen RQ, Yang XC, Tian HN, Sun LC. Tetrahydroquinoline dyes with different
spacers for organic dye-sensitized solar cells. J Photochem Photobiol A 2007;
189:295e300.
Appendix A. Supplementary material
[12] (a) Walker B, Tamayo AB, Dang X-D, Zalar P, Seo JH, Garcia A, et al. Nanoscale
phase separation and high photovoltaic efficiency in solution-processed,
small-molecule bulk heterojunction solar cells. Adv Funct Mater 2009;19:
3063e9;
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2012.09.015.
(b) Yamamoto T, Zhou ZH, Kanbara T, Shimura M, Kizu K, Maruyama T, et al.
References
p-conjugated donor-acceptor copolymers constituted of p-excessive and p-
deficient arylene units. Optical and electrochemical properties in relation to
CT structure of the polymer. J Am Chem Soc 1996;118:10389e99;
(c) Araki Y, Wada T, Yoshikawa O, Sagawa T, Yoshikawa S, Imahor H.
Synthesis, photophysical and photovoltaic properties of porphyrin-furan and
-thiophene alternating copolymers. J Phys Chem C 2009;113:10798e806;
(d) Lin JT, Chen PC, Yen YS, Hsu YC, Chou HH, Yeh MCP. Organic dyes con-
taining furan moiety for high-performance dye-sensitized solar cells. Org Lett
2009;11:97e100.
[1] Grätzel M. Dye-sensitized solar cells. J Photochem Photobiol C 2003;4:145e53.
[2] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye
sensitized colloidal titanium dioxide films. Nature 1991;353:737e40.
[3] Yum JH, Jung I, Baik C, Ko J, Nazeeruddin MK, Grätzel M. High efficient donor-
acceptor ruthenium complex for dye-sensitized solar cell applications. Energy
Environ Sci 2009;2:100e2.
[4] (a) Hara K, Wang ZS, Sato T, Furube A, Katoh R, Sugihara H, et al. Oligothio-
phene-containing coumarin dyes for efficient dye-sensitized solar cells. J Phys
Chem B 2005;109:15476e82;
[13] (a) Katoh R, Furube A, Mori S, Miyashita M, Sunahara K, Koumuraa N, et al.
Highly stable sensitizer dyes for dye-sensitized solar cells: role of the oligo-
thiophene moiety. Energy Environ Sci 2009;2:542e6;
(b) Hara K, Sato T, Katoh R, Furube A, Ohga Y, Shinpo A, et al. Molecular design
of coumarin dyes for efficient dye-sensitized solar cells. J Phys Chem B 2003;
107:597e606;
(c) Hagberg DP, Edvinsson T, Marinado T, Boschloo G, Hagfeldt A, Sun L.
A novel organic chromophore for dye-sensitized nanostructured solar cells.
Chem Commun 2006:2245e7.
(b) Zhu WH, Wu YZ, Wang ST, Li X, Chen J, Wang Z-S, et al. Organic D-A-p-A
solar cell sensitizers with improved stability and spectral response. Adv Funct
Mater 2011;21:756e63.
[14] Hwang SY, Lee JH, Park C, Lee H, Kim C, Park C, et al. A highly efficient organic
sensitizer for dye-sensitized solar cells. Chem Commun 2007:4887e9.
[15] Li KP, Qu JL, Xu B, Zhou YH, Liu LJ, Peng P, et al. Synthesis and photovoltaic
properties of novel solution-processable triphenylamine-based dendrimers
with sulfonyldibenzene cores. New J Chem 2009;33:2120e7.
[16] (a) Ning ZJ, Zhang Q, Wu WJ, Pei HC, Liu B, Tian H. Starburst triarylamine
based dyes for efficient dye-sensitized solar cells. J Org Chem 2008;73:3791e
7;
[5] (a) Hara K, Sayama K, Ohga Y, Shinpo A, Suga S, Arakawa H. A coumarin-
derivative dye sensitized nanocrystalline TiO2 solar cell having a high solar-
energy conversion efficiency up to 5.6%. Chem Commun 2001:569e70;
(b) Wang ZS, Cui Y, Dan-oh Y, Kasada C, Shinpo A, Hara K. Molecular design of
coumarin dyes for stable and efficient organic dye-sensitized solar cells. J Phys
Chem C 2008;112:17011e7.
(b) Ning Z, Fu Y, Tian H. Improvement of dye-sensitized solar cells: what we
know and what we need to know. Energy Environ Sci 2010;3:1170e81.
[17] Fischer MKR, Wenger S, Wang MK, Mishra A, Zakeeruddin SM, Grätzel M, et al.
[6] Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of dye-sensitized
solar cells based on metal-free indoline dyes. J Am Chem Soc 2004;126:
12218e9.
D-p-A sensitizers for dye-sensitized solar cells: linear vs branched oligo-
thiophenes. Chem Mater 2010;22:1836e45.
[18] Chen H, Huang H, Huang X, Clifford JN, Forneli A, Palomares E, et al. High
molar extinction coefficient branchlike organic dyes containing di(p-tolyl)
phenylamine donor for dye-sensitized solar cells applications. J Phys Chem
C 2010;114:3280e6.
[7] Yao QH, Meng FS, Li FY, Tian H, Huang CH. Photoelectric conversion properties
of four novel carboxylated hemicyanine dyes on TiO2 electrode. J Mater Chem
2003;13:1048e53.
[8] (a) Zhang G, Bala H, Cheng Y, Shi D, Lv X, Yu Q, et al. High efficiency and stable
dye-sensitized solar cells with an organic chromophore featuring a binary
p-
conjugated spacer. Chem Commun 2009:2198e200;
[19] Ning ZJ, Zhang Q, Pei HC, Luan JF, Lu CG, Cui YP, et al. Photovoltage
improvement for dye-sensitized solar cells via cone-shaped structural design.
J Phys Chem C 2009;113:10307e13.
[20] Wang ZS, Koumura N, Cui Y, Takahashi M, Sekiguchi H, Mori A, et al. Hex-
ylthiophene-functionalized carbazole dyes for efficient molecular photovol-
taics: tuning of solar-cell performance by structural modification. Chem Mater
2008;20:3993e4003.
(b) Yum J, Hagberg DP, Moon S, karlsson KM, Marinado T, Sun L, et al. Angew
Chem Int Ed 2009;48:1576e80;
(c) Jia JH, Cao KY, Xue PC, Zhang Y, Zhou HP, Lu R. Y-shaped dyes based on triphe-
nylamine for efficient dye-sensitized solar cells. Tetrahedron 2012;68:3626e32;
(d) Zhou HP, Xue PC, Zhang Y, Zhao X, Jia JH, Zhang XF, et al. Fluorenylvinylenes
bridged triphenylamine-based dyes with enhanced performance in dye-sensitized
solar cells. Tetrahedron 2011;67:8477e83;