Unsymmetrical squaraine sensitizers containing auxiliary arylamine donor for NIR-harvesting on dye-sensitized solar cell

Article abstract of DOI:10.1016/j.dyepig.2014.03.008

Developing sensitizers capable of absorbing light intensely in the red to near-IR region is important to improve the performance of dye-sensitized solar cells. In this article, three new squaraine sensitizers containing diarylamines with varied alkyl or alkoxyl chain substituents as auxiliary donors were synthesized and utilized. The effects of different donor tail chains, i.e., methyl, methoxyl and hexyloxy groups on the photophysical, electrochemical and photovoltaic properties were investigated. Results indicated the alkoxyl chains brought about a more broadened photoresponse than the alkyl chain and the long hydrocarbon chain was superior to short chains in suppression of electron recombination. Therefore, with a bis(4-(hexyloxy) phenyl) amine substituent, a squaraine sensitized solar cell presented expansion of the incident photon-to-current conversion from the red to near infrared wavelength region up to 800 nm and hindered electron recombination, resulting in an efficiency of 3.39% under AM 1.5 irradiation.

Full text of DOI:10.1016/j.dyepig.2014.03.008

Dyes and Pigments 106 (2014) 128e135
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Unsymmetrical squaraine sensitizers containing auxiliary arylamine
donor for NIR-harvesting on dye-sensitized solar cell
*
Linfeng Liu, Jiangzhao Chen, Zhiliang Ku, Xiong Li, Hongwei Han
Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan
430074, Hubei, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Developing sensitizers capable of absorbing light intensely in the red to near-IR region is important to
improve the performance of dye-sensitized solar cells. In this article, three new squaraine sensitizers
containing diarylamines with varied alkyl or alkoxyl chain substituents as auxiliary donors were syn-
thesized and utilized. The effects of different donor tail chains, i.e., methyl, methoxyl and hexyloxy
groups on the photophysical, electrochemical and photovoltaic properties were investigated. Results
indicated the alkoxyl chains brought about a more broadened photoresponse than the alkyl chain and
the long hydrocarbon chain was superior to short chains in suppression of electron recombination.
Therefore, with a bis(4-(hexyloxy) phenyl) amine substituent, a squaraine sensitized solar cell presented
expansion of the incident photon-to-current conversion from the red to near infrared wavelength region
up to 800 nm and hindered electron recombination, resulting in an efficiency of 3.39% under AM 1.5
irradiation.
Received 23 October 2013
Received in revised form
1 March 2014
Accepted 4 March 2014
Available online 14 March 2014
Keywords:
Squaraine dye
Near infrared absorption
Dye-sensitized
Solar cell
Auxiliary donor
Arylamine
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
light-absorber toward TiO2 surface. However, the photoresponse
wavelength is still too short for utilizing the solar energy in NIR
Dye-sensitized solar cells (DSSCs) have attracted considerable
attention as potential photovoltaic devices because of their high
power conversion efficiency, ease of fabrication and relatively low
cost [1e3]. Up to now, various types of metal-free organic dyes have
been developed with their advantages such as low cost of pro-
duction, high molar absorption coefficient, and tunable molecular
design [4e9]. However, most of these organic dyes barely absorb
sunlight beyond 600 nm. Therefore, for the improvement on the
efficiency of DSSCs, it is important to develop sensitizers capable of
absorbing light intensely in the red to near-IR (NIR) region, which
accounts for about 50% of solar energy [10]. In this regard, squaraine
dyes are one of the most promising candidates owing to their
extremely high absorptivity in NIR region and flexible synthesis
procedure [11e19].
region, which requires an extended p-conjugated structure to
bathochromically shift the absorption band. On the other hand, the
planar molecular structure of SQ01 is likely to cause intermolecular
close pep aggregation, leading to self-quenching and reduction of
electron injection into semiconductor film. In addition, the indo-
lium donor ability is relatively weak, and better performance may
be achieved by introducing an additional donor moiety into indo-
lium. Therefore, the optimization of chemical and physical prop-
erties of squaraine dyes through molecular modification is essential
for further increasing the incident photon conversion efficiency
(IPCE) response at long wavelengths.
The 4,40-disubstituted triphenylamine unit is well known not
only for its strong electron-donating ability, but also its contribu-
tion to suppressing charge recombination and dye aggregation
owing to its nonplanar structure and substituent [21]. Addition of a
nonplanar triphenylamine unit to the tail of SQ01 molecule is
supposed to prevent the aggregation of the dye molecules and in-
crease the distance between the oxidizing hole and TiO2 surface,
and thus enhance the efficiency. Herein, we report three new
squaraine dyes by simply introducing three types of alkyl or alkoxyl
substituted diarylamines to the indolium donor of sensitizer SQ01
to obtained WH-SQ01 with methyl groups, WH-SQ02 with
methoxyl groups and WH-SQ03 with hexyloxy groups, the
In 2007, Yum et al. developed a new unsymmetrical squaraine
dye (SQ01) [20] bearing carboxylic groups directly attached to the
indole unit as potential alternative NIR sensitizer which achieved
high monochromatic external quantum efficiencies at around
650 nm, originating from the unidirectional flow of charge from the
* Corresponding author. Tel./fax: þ86 27 8779 3027.
E-mail address: hongwei.han@mail.hust.edu.cn (H. Han).
http://dx.doi.org/10.1016/j.dyepig.2014.03.008
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
129
structures of which have been presented in Fig. 1. In order to
highlight the role of substituted diphenylamine unit, we also syn-
thesized a reported sensitizer with no added diphenylamine unit in
donor (SQ01) for comparison. Efforts have also been made to
investigate the influence of the alkyl or alkoxyl substituted diaryl-
amines donor by UVevis absorption spectroscopy, cyclic voltam-
metry (CV), electrochemical impedance spectroscopy (EIS), and
photovoltaic measurements in terms of structure-property
correlation.
diethyl ether three times. Compound 1 was obtained as a brown
solid (33.5 g, 85%). 1HNMR (CDCl3)
(ppm): 7.78e7.64 (m, 3H, ArH),
4.79 (q, 2H, 7.5 Hz, eCH2e), 3.14 (s, 3H, eCH3),1.7 (s, 6H, eCH3),1.61
(t, 3H, 8.0 Hz, eCH3).
d
2.3.2. General synthesis of 2
A Schlenk flask was charged with 1 (0.197 g, 0.8 mmol), either
di-p-tolylamine or bis(4-methoxyphenyl)amine or bis(4-(hex-
yloxy)phenyl)amine (0.6 mmol), sodium tert-butoxide (0.192 g,
2 mmol), Pd(dba)2 (6 mg, 0.01 mmol), tri-tert-butylphosphine
(4.22 mg, 0.02 mmol), and toluene (10 mL). After refluxing for 15 h,
the mixture was cooled to room temperature and then poured into
water. The organic layer was extracted by CH2Cl2 and dried over
anhydrous MgSO4. After removal of the solvent the products were
used without further purification for next step.
2. Experimental
2.1. Materials
All reagents and solvents were obtained from commercial
sources and used without further purification unless otherwise
noted. Toluene was distilled from CaH2 under a nitrogen atmo-
sphere. 5-Bromo-2,3,3-trimethylindolenine [22] and bis(4-(hex-
yloxy)phenyl)amine [23] was prepared by following the reported
procedures.
2.3.3. General synthesis of 3
Compound 2 (0.5 mmol), 3, 4-diethoxy-3-cyclobutene-1,2-
dione (0.085 g, 0.5 mmol) and triethylamine (1.5 mL) were dis-
solved in ethanol (10 mL) and heated under reflux for 30 min. The
solvent was removed from the green solution and the crude
product was purified by column chromatography (silica gel) with
ethyl acetate as eluent. The following compounds were obtained in
this way:
2.2. Characterization
The 1H NMR (13C NMR) spectra were recorded at 400 MHz
(100 MHz) on a Bruker-AV400 spectrometers. The chemical shifts
were recorded in parts per million (ppm) with TMS as the internal
reference. ESI mass spectra were determined using Finnigan LCQ
Advantage mass spectrometer. FTIR spectra (as KBr pellets) were
obtained using Bruker Vertex 70. Elemental analyses were per-
formed with Thermo Quest Flash EA1112. UVevis spectra were
obtained using a PerKinElmer Lambda 950 spectrophotometer. The
cyclic voltammetry measurements were conducted in a three-
electrode system in an argon purged electrolyte solution on PAR-
STAT 2273 Electrochemical Workstation. Pt wire served as the CE,
and Agþ/Ag served as the reference electrode. The electrolyte
contained 1 mM dye and 0.1 M TBAP in DMF. The potential of the
reference electrode was adjusted by recording the cyclic voltam-
mogram for 0.01 M ferrocene in DMF containing 0.1 M TBAP.
2.3.3.1. 3-((5-(di-p-tolylamino)-1-ethyl-3,3-dimethylindolin-2-
ylidene)methyl)-4-ethoxycyclobut-3-ene-1,2-dione (3a). Orange red
solid (0.202 g, 80%). UVevis (EtOH) lmax: 451 nm. FTIR (KBr, cmꢀ1
)
nmax: 3466, 2962, 2922, 2855, 1770 (nC¼O), 1709 (nC¼O), 1608, 1547,
1419, 1311, 1301, 1220, 1124, 1042, 917, 870. 1HNMR (CDCl3)
d
(ppm):
7.08 (m, 5H, ArH), 6.96 (m, 5H, ArH), 6.74 (d, 1H, 8.1 Hz, ArH), 5.39
(s, 1H, eCH¼), 4.91 (q, 2H, 7.2 Hz, eNCH2e), 3.89 (q, 2H, 7.5 Hz, e
OCH2e), 2.33 (s, 6H, eCH3), 1.58 (s, 6H, eCH3), 1.54 (t, 3H, 8.1 Hz, e
CH3), 1.34 (t, 3H, 8.0 Hz, eCH3). 13CNMR (CDCl3)
d (ppm): 192.8;
187.0; 186.8; 173.3; 168.0; 145.6; 144.1; 142.3; 137.2; 132.0; 129.9;
123.7; 123.5; 118.5; 108.6; 80.6; 69.8; 48.2; 37.8; 26.8; 20.8; 15.9;
11.5. MS (ESI): m/z ¼ 506.2 [M]þ, calculated 506.3.
2.3.3.2. 3-((5-(bis(4-methoxyphenyl)amino)-1-ethyl-3,3-
dimethylindolin-2-ylidene)methyl)-4-ethoxycyclobut-3-ene-1,2-
2.3. Synthesis
dione (3b). Orange red solid (0.228 g, 85%). UVevis (EtOH) lmax
459 nm. FTIR (KBr, cmꢀ1
nmax: 3534, 2970, 2929, 2830, 1770 (nC¼O),
1707 (nC¼O), 1606, 1544, 1421, 1318, 1298, 1219, 1123 1035, 917, 870.
1HNMR (CDCl3)
(ppm): 7.01 (d, 4H, 8.8 Hz, ArH), 6.95 (d, 1H,
2.1 Hz, ArH), 6.85 (d, 1H, 8.4 Hz, ArH), 6.81 (d, 4H, 9.0 Hz, ArH), 6.70
(d, 1H, 8.6 Hz, ArH), 5.38 (s, 1H, eCH ¼ ), 4.89 (q, 2H, 7.0 Hz, e
:
2.3.1. 5-bromo-1-ethyl-2,3,3-trimethyl-3H-indolium iodide (1)
5-Bromo-2,3,3-trimethylindolenine (11.9 g, 50 mmol) and
iodoethane (9.36 g, 60 mmol) were dissolved in acetonitrile
(100 mL) and heated under reflux under nitrogen for 40 h. The
solvent was evaporated and the crude product was washed with
)
d
Fig. 1. Molecular structures of sensitizers.

130
L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
NCH2e), 3.87 (q, 2H, 7.6 Hz, eOCH2e), 3.80(s, 6H, eOCH3), 1.56 (s,
6H, eCH3), 1.53 (t, 3H, 8.1 Hz, eCH3), 1.35 (t, 3H, 8.5 Hz, eCH3).
1505,1355,1276,1199,1170,1091,1072,1048, 970, 937, 917, 826, 789,
772, 691, 452. 1HNMR (CDCl3)
(ppm): 8.10 (d,1H, 8.1 Hz, ArH), 8.03
d
13CNMR (CDCl3)
d
(ppm): 192.9; 186.9; 186.7; 173.2; 168.0; 155.5;
(s, 1H, ArH), 7.05 (d, 4H, 8.7 Hz, ArH), 7.03 (d, 1H, 8.1 Hz, ArH), 6.90e
6.82 (m, 6H, ArH), 6.09 (s,1H, eCH¼), 5.96 (s,1H, eCH¼), 4.14 (q, 2H,
8.1 Hz, eNCH2e), 3.94 (t, 2H, 7.4 Hz, eNCH2e), 3.81 (s, 6H, eOCH3),
1.72 (s, 6H, eCH3), 1.65 (s, 6H, eCH3), 1.45e1.21 (m, 15H), 0.89 (t, 3H,
144.9; 142.3; 141.4; 136.4; 125.6; 121.6; 116.8; 114.7; 108.5; 80.4;
69.7; 55.5; 48.2; 37.8; 26.8; 15.9; 14.6; 11.5. MS (ESI): m/z ¼ 538.2
[M]þ, calculated 538.3.
6.9 Hz, eCH3). 13CNMR (CDCl3)
d (ppm): 180.4; 174.2; 171.2; 170.3;
2.3.3.3. 3-((5-(bis(4-(hexyloxy)phenyl)amino)-1-ethyl-3,3-
dimethylindolin-2-ylidene)methyl)-4-ethoxycyclobut-3-ene-1,2-
167.1; 155.9; 147.1; 146.9; 144.0; 141.8; 135.0; 131.0; 124.0; 123.9;
120.4; 115.5; 110.4; 108.0; 87.5; 87.2; 55.6; 50.1; 48.2; 43.5; 39.1;
31.7; 29.3; 29.1; 27.2; 27.0; 26.5; 22.9; 22.5; 14.0; 12.4. MS (ESI): m/
z ¼ 807.5 [M]þ, calculated 807.4. Anal. Calcd for: C51H57N3O6: C,
75.81; H, 7.11; N, 5.20. Found: C, 75.72; H, 6.98; N, 5.12.
dione (3c). Orange red solid (0.258 g, 76%). UVevis (EtOH) lmax
459 nm. FTIR (KBr, cmꢀ1
nmax: 3436, 2962, 2929, 2870, 1774 (nC¼O),
1712 (nC¼O), 1606, 1552, 1505, 1309, 1222, 1124, 1045, 924, 877.
1HNMR (CDCl3)
(ppm): 6.99 (d, 4H, 9.0 Hz, ArH), 6.96 (s,1H, 2.0 Hz,
:
)
d
ArH), 6.85 (d, 1H, 8.5 Hz, ArH), 6.81 (d, 4H, 8.8 Hz, ArH), 6.71 (d, 1H,
8.3 Hz, ArH), 5.37 (s, 1H, eCH ¼ ), 4.91 (q, 2H, 7.0 Hz, eNCH2e), 3.93
(t, 4H, 7.5 Hz, eOCH2e), 3.85 (q, 2H, 7.3 Hz, -OCH2-), 1.79 (m, 4H, e
CH2e),1.57 (s, 6H, eCH3),1.56e1.42 (m, 6H),1.39e1.29 (m,12H), 0.91
2.3.8(E)-2-((E)-(5-(bis(4-(hexyloxy)phenyl)amino)-1-ethyl-3,3-
dimethylindolin-2-ylidene)methyl)-4-((5-carboxy-3,3-dimethyl-1-
octyl-3H-indolium-2-yl)methylene)-3-oxocyclobut-1-enolate (WH-
SQ03)
(t, 6H, 9.2 Hz, eCH3). 13CNMR (CDCl3)
d
(ppm): 192.8; 186.9; 186.6;
Dark blue powder (0.058 g, 30.6%).Mp:155 ꢁC (decomp). FTIR
173.1; 167.9; 155.0; 145.0; 142.2; 141.3; 136.2; 125.5; 121.4; 116.6;
115.2; 108.4; 80.3; 69.6; 50.8; 48.2; 31.6; 29.7; 26.7; 25.8; 22.6; 15.8;
14.0; 11.4. MS (ESI): m/z ¼ 678.4 [M]þ, calculated 678.4.
(KBr, cmꢀ1
)
nmax: 2926, 2856, 1701 (nCOOH), 1601 (nC¼O), 1567 (nC¼O),
1505, 1355, 1277, 1201, 1170, 1091, 1072, 1049, 970, 937, 918, 825,
775, 456. 1HNMR (CDCl3)
(ppm): 8.08 (d, 1H, 8.6 Hz, ArH), 7.98 (s,
d
1H, ArH), 7.03e6.98 (m, 5H, ArH), 6.90 (d, 1H, 8.6 Hz, ArH), 6.86e
6.79 (m, 6H, ArH), 6.06 (s, 1H, eCH¼), 5.94 (s, 1H, eCH¼), 4.13 (q,
2H, 8.5 Hz, eNCH2e), 3.95e3.85 (m, 6H), 1.82e1.68 (m, 10H), 1.50e
2.3.4. General synthesis of 4
3 (0.4 mmol) were dissolved in ethanol (10 mL). Under reflux
sodium hydroxide (0.04 g, 1.0 mmol) in H2O (1 mL) was added and
heated for 5 min. The mixture was concentrated and cold ethanol
(15 mL) was added. The yellow product precipitated and was
washed with ethanol three times. After removal of the solvent the
products were used without further purification for next step.
1.20 (m, 33H), 0.91 (m, 9H, eCH3). 13CNMR (DMSO-d6)
d (ppm):
181.6; 181.0; 174.5; 170.4; 167.6; 165.9; 155.4; 146.9; 146.7; 143.8;
141.3; 140.6; 135.4; 130.7; 126.3; 124.9; 123.4; 120.7; 115.8; 115.2;
112.2; 109.3; 87.2; 87.1; 68.0; 49.9; 47.9; 43.0; 31.6; 31.4; 29.2; 29.1;
29.0; 27.2; 26.7; 26.6; 26.3; 25.6; 22.5; 22.5; 14.3; 12.5. MS (ESI): m/
z ¼ 947.6 [M]þ, calculated 947.6. Anal. Calcd for: C61H77N3O6: C,
77.26; H, 8.18; N, 4.43. Found: C, 77.43; H, 7.94; N, 4.46.
2.3.5. General synthesis of WH-SQ01, WH-SQ02 and WH-SQ03
4 (0.2 mmol) and 5-carboxy-2,3,3-trimethyl-1-octyl-3H-indo-
lium iodide (0.19 mmol) were dissolved in a mixture of benzene
(10 mL ) and n-butanol (10 mL ). The mixture was refluxed with a
DeaneStark apparatus for 18 h. The solvent was partly removed
until the product precipitated. The crude product was filtered off
and purified bycolumn chromatography (CH2Cl2: methanol ¼ 30:1).
2.4. Device fabrication
FTO glass plates with high transparency in the visible range
purchased from CSG Holding Co. LTD and cleaned in a detergent
solution using an ultrasonic bath for 15 min, and then rinsed with
water and ethanol. Photoelectrode consisted of a TiO2 film with a
triple-layer structure. A compact blocking layer of dense TiO2 was
deposited onto a cleaned Fluorine doped Tin Oxide (FTO) glass
substrate by immersing the FTO glass plates into a 40 mM aqueous
2.3.64-((5-Carboxy-3,3-dimethyl-1-octyl-3H-indolium-2-yl)
methylene)-2-((E)-(5-(dip-tolylamino)-1-ethyl-3,3-
dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (WH-
SQ01)
TiCl4 solution at 70 ꢁC for 30 min. Successive depositions of a 12
thick transparent layer (PST-18NR, JGC Catalysts and Chemicals Ltd.,
Japan) and a 4 m thick light-scattering layer (PST-400C, JGC Cat-
mm
Dark blue powder (0.069 g, 44.6%).Mp:199 ꢁC (decomp). FTIR
(KBr, cmꢀ1
)
nmax: 2929, 2859, 1695 (nCOOH), 1597 (nC¼O), 1571 (nC¼O),
1504,1356,1272,1202,1168,1092,1071,1049, 971, 936, 917, 831, 788,
772, 693, 448. 1HNMR (CDCl3)
(ppm): 8.10 (d,1H, 8.8 Hz, ArH), 8.03
m
alysts and Chemicals Ltd., Japan) of nanocrystalline TiO2 were
printed layer by layer and then sintered at 500 ꢁC for 15 min. After
that, The TiO2 “triple-layer” film was treated with 40 mM TiCl4
solution, and then sintered at 500 ꢁC for 30 min. After cooling down
to 80 ꢁC, the photoelectrode was sensitized with a 0.1 mM sensi-
tizer (with 5 mM chenodeoxycholic acid (CDCA)) ethanol solution.
The counter electrode was Pt catalyst on FTO glass deposited with a
drop of H2PtCl6 solution (0.02 M 2-propanol solution) and then
heated treatment at 400 ꢁC for 15 min. The cell was encapsulated by
d
(s,1H, ArH), 7.11 (d,1H, 2.0 Hz, ArH), 7.09 (d, 4H, 8.3 Hz, ArH), 6.97 (d,
4H, 8.3 Hz, ArH), 6.95 (d, 1H, 7.2 Hz, ArH), 6.90 (d, 1H, 6.0 Hz, ArH),
6.88 (s, 1H, ArH), 6.08 (s, 1H, eCH¼), 5.96 (s, 1H, eCH¼), 4.14 (q, 2H,
8.5 Hz, eNCH2e), 3.92 (t, 2H, 7.3 Hz, eNCH2e), 2.42 (s, 6H, eCH3),
1.79 (s, 6H, eCH3),1.71 (s, 6H, eCH3),1.43 (t, 3H, 7.3 Hz, eCH3),1.38e
1.20 (m, 12H, eCH2e), 0.89 (t, 3H, 7.0 Hz, eCH3). 13CNMR (CDCl3)
d
(ppm): 180.9; 174.8; 171.3; 170.2; 167.2; 147.1; 146.1; 145.1; 143.6;
135.8; 132.7; 131.0; 160.0; 129.7; 124.2; 124.0; 123.8; 122.4; 117.3;
110.4; 108.1; 87.6; 87.2; 53.2; 53.2; 50.0; 48.1; 43.5; 39.1; 31.7; 29.3;
29.1; 27.2; 27.0; 26.9; 26.5; 25.5; 20.8; 14.0; 12.3. MS (ESI): m/
z ¼ 775.5 [M]þ, calculated 775.4. Anal. Calcd for: C51H57N3O4: C,
78.93; H, 7.40; N, 5.41. Found: C, 78.66; H, 7.63; N, 5.38.
a 45 mm thick hot-melt film (Surlyn, Solaronix). After sealing, the
electrolyte solution consisting of 0.6 M DMPII, 0.05 M LiI, 0.05 M I2,
and 0.05 M TBP in ACN was injected through the hole in the CE, and
then the hole was sealed with thermoplastic Surlyn film and a glass
coverslip.
2.3.72-((E)-(5-(bis(4-methoxyphenyl)amino)-1-ethyl-3,3-
dimethylindolin-2-ylidene)methyl)-4-((5-carboxy-3,3-dimethyl-1-
octyl-3H-indolium-2-yl)methylene)-3-oxocyclobut-1-enolate (WH-
SQ02)
2.5. Photovoltaic measurements
Current-voltage (IeV) characteristics were measured with a
Keithley 2400 source/meter and a Newport solar simulator (model
91160) giving light with AM 1.5 G spectral distribution, which was
calibrated using a certified reference solar cell (Fraunhofer ISE) to
Dark blue powder (0.085 g, 52.6%).Mp:179 ꢁC (decomp). FTIR
(KBr, cmꢀ1
) nmax: 2926, 2855, 1701 (nCOOH), 1599 (nC¼O), 1573 (nC¼O),

L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
131
an intensity 100 mW cmꢀ2. A black mask with a circular aperture
slightly smaller (0.159 cm2) than the active area of the square solar
cell (0.25 cm2) was applied on top of the cell. The incident photon
conversion efficiency (IPCE) was measured using a 150 W xenon
lamp (Oriel) fitted with a monochromator (Cornerstone 260) as a
monochromatic light source. The illumination spot size was chosen
to be slightly smaller than the active area of the DSC test cells. IPCE
photocurrents were recorded under short-circuit conditions using a
Keithley 2400 source meter. The monochromatic photon flux was
quantified by means of a calibrated silicon photodiode. Electro-
chemical impedance spectroscopy (EIS) of the symmetric cell was
carried on PARSTAT 2273 Electrochemical Workstation in the fre-
quency range 0.1e105 Hz with 10 mV AC amplitude. The electrolyte
used was the same as that used for the fully functional DSCs. The
Most methods for extending
p
-conjugation in squaraine dyes, and
thereby achieving panchromatic absorption require many lengthy
procedures or repeated Suzuki cross-coupling reactions, yet our
synthetic strategy required only a few steps.
3.2. Optical properties
The adsorption spectra of SQ01, WH-SQ01, WH-SQ02 and WH-
SQ03 in ethanol solution are showed in Fig. 2 and the corre-
sponding data are summarized in Table 1. As expected, all the new
dyes presented show better absorption in the 550e720 nm range.
Compared to SQ01, which shows intense narrow absorption band at
620 nm, WH-SQ01 exhibits a greatly broadened and red shifted
band with a peak at 652 nm (ε ¼ 86000 dm3 molꢀ1 cmꢀ1), as a
distance between two electrodes was 45
mm and the active area
consequence of the extended
pesystem of the dye molecule WH-
was 0.5 cm2.
SQ01 owing to auxiliary dip-tolylamino group. Therefore, WH-
SQ01 can efficiently absorb the photons with much lower energy
than the reference dye SQ01. Instead of methyl, WH-SQ03 pos-
sesses hexyloxy-disubstituted diphenylamine with stronger
electron-push effect than WH-SQ01. Thus the absorption
3. Results and discussion
3.1. Synthesis
maximum
of
WH-SQ03
is
located
at
660
nm
The synthetic routes to the sensitizers WH-SQ01, WH-SQ02 and
WH-SQ03 started from 5-bromo-2,3,3-trimethylindolenine and the
corresponding substituted diphenylamine and the route is depicted
in Scheme 1. The compounds 2 were obtained by HartwigeBuch-
wald coupling with the corresponding substituted diphenylamine
with 5-bromo-1-ethyl-2,3,3-trimethyl-3H-indolium iodide. A Key
step in the route is the selective condensation reaction of squaric
acid diethyl ester with indolium derivatives 2 allowing for an un-
symmetrical combination with the carboxy-indolium synthon
yielding the target compounds. The obtained dyes are dark blue
solids and freely dissolve in common organic solvents. The struc-
ture and purity of the sensitizers were confirmed by NMR, ESI-MS.
(ε ¼ 114000 dm3 molꢀ1 cmꢀ1), 8 nm red-shifted than that of WH-
SQ01. The additional absorption of WH-SQ03 at shorter wave-
lengths is significant compared to that of SQ01, which shows
negligible absorption in the 440e520 nm range, while the molar
absorption coefficient of WH-SQ03 varies from 3500 to
7000 dm3 molꢀ1 cmꢀ1 in this region. With a methyloxy disubsti-
tuted diphenylamine group linking to the donor part of SQ01, WH-
SQ02 (lmax ¼ 658, ε ¼ 122000 dm3 molꢀ1 cmꢀ1) behaves in a
similar way to WH-SQ03. Fig. 3 shows the UV/vis spectra of 0.1 mM
dyes solution in ethanol along with 5 mM CDCA absorbed onto
4
mm TiO2 films. The absorption spectrum of dyes on the TiO2
electrode is broadened. This should be caused by the formation of
Scheme 1. Synthetic route of sensitizers.

132
L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
Fig. 2. UVevis spectra of sensitizers in EtOH.
Fig. 3. UVevis spectra of the sensitizers on TiO2 films.
the J-aggregates of sensitizers on the TiO2 surface and/or the
interaction between the sensitizers and TiO2 [24]. Analyzing the
absorption properties carefully, it can be concluded that WH-SQ03
exhibits broader and additional absorption in the visible region and
extends the absorption onset to the longer wavelength, which
should favor the light harvesting of the devices and thus increase
the photocurrent response region, resulting in enhanced short-
circuit photocurrent density (JSC).
band level of TiO2 at ꢀ0.5 V vs. NHE, ensuring favorable electron
injection from the excited sensitizer to TiO2 electrode.
3.4. Theoretical approach
In order to evaluate the geometrical configuration and electron
distributions of the resulted sensitizers, density functional calcu-
lations were conducted using the Gaussian 03 program package at
the B3LYP/6-31G(d) level [25]. As depicted in Fig. 5, the calculation
reveals that the HOMO-1 is mainly contributed to the squaraine
core and the HOMO is distributed along the donor part through the
3.3. Electrochemical properties
To evaluate the feasibility of electron transfer from the highest
occupied molecular orbital (HOMO) of the dye to the conduction
band edge of the nanocrystalline TiO2, the oxidation potentials of
the sensitizers were determined by cyclic voltammetry (CV, see
Fig. 4). All the new dyes exhibit one reversible oxidative wave at
positive potential, which were attributed to the oxidation of the
diphenylamine-substituted indole group. The first half-wave po-
tentials (E1/2) for WH-SQ01, WH-SQ02 and WH-SQ03, which are
taken as the HOMO levels of the dyes, were measured to be 0.78,
0.71 and 0.72 V versus NHE, respectively. Obviously, the higher
HOMO of these dyes compared with SQ01 (0.83 V) derives from the
strong electron-push effect of auxiliary disubstituted diphenyl-
amine group. All the HOMO values are more positive than Iꢀ/I3-
redox couples (w0.4 V vs. NHE), indicating that the reduction of the
oxidized dyes with Iꢀ ions is thermodynamically feasible. For the
band gap estimated from the observed optical edge, WH-SQ01,
WH-SQ02 and WH-SQ03 also showed lower E0e0 (1.72, 1.70 and
1.69 V) than that of SQ01 (1.85 V) which is consistent with their
absorption peaks in the UVevis spectrum. The LUMO energy levels
p
-conjugated system, while the LUMO is distributed over the
conjugated backbone connecting to squaraine core and the
LUMOþ1 is localized within the carboxylic group. These electron
distributions allow sufficient charge separation within the dye.
Upon photoexcitation, the electrons could successively transfer
from the auxiliary diphenylamine donor to the acceptor unit, then
to the TiO2 film.
3.5. Photovoltaic performance of DSSCs
The photovoltaic characteristics (J-V curves) of the solar cells
containing these sensitizers were presented in Fig. 6 and their
calculated from subtraction between HOMO and E0e0
,
were ꢀ0.94, ꢀ0.99 and ꢀ0.98 V for WH-SQ01, WH-SQ02 and WH-
SQ03, respectively, which were more negative than the conduction
Table 1
Absorbance and electrochemical properties of the sensitizers.
lmax/nm ε/dm3
Mꢀ1 cmꢀ1
Eox/V E0e0a/V ELUMOb/V Amounts/
10ꢀ7 mol cmꢀ2
WH-SQ01 652
WH-SQ02 658
WH-SQ03 660
86000
122000
114000
158500
0.78 1.72
0.71 1.70
0.72 1.69
0.83 1.85
ꢀ0.94
ꢀ0.99
ꢀ0.98
ꢀ1.02
0.65
0.66
0.67
0.48
SQ01
620
a
The band gap, E0e0 was derived from the observed optical edge.
ELUMO was calculated from Eox ꢀ E0e0
b
.
Fig. 4. Cyclic voltammogram curves of sensitizers in solution of DMF.

L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
133
Table 2
Photovoltaic performance data of sensitizers. Illumination: 100 mW cmꢀ2 simulated
AM 1.5 G solar light.
Voc/mV
Jsc/mA cmꢀ2
FF
h/%
Rrec/U
WH-SQ01
WH-SQ02
WH-SQ03
SQ01
532.1
531.8
545.1
556.7
8.15
8.91
8.95
7.99
0.72
0.69
0.69
0.73
3.14
3.29
3.39
3.23
115
115
235
150
WH-SQ03 are almost the same. Therefore, it is obvious that the
enhanced JSC in dyes with auxiliary disubstituted diphenylamine
group is not originated from their adsorbed amounts [26]. In order
to understand the difference of JSC, the action spectra of the inci-
dent photo-to-current conversion efficiencies (IPCE) as a function
of incident wavelength for the DSSCs based on the synthesized dyes
are recorded and presented in Fig. 7. It could be found that WH-
SQ01 based DSSC displays a maximum IPCE of 46% at 656 nm and
converts photons with lower energy (up to 750 nm) compared to
SQ01 (710 nm), while WH-SQ02 and WH-SQ03 convert slightly
more lower energy than WH-SQ01, which is in consistent with the
UV/vis spectra. The additional IPCE of dyes at shorter wavelengths
within 440e520 nm are in the order of WH-SQ01 < WH-
SQ02 < WH-SQ03, which is in accordance with the tendency of the
UV/vis spectra. All of these new dyes with auxiliary disubstituted
diphenylamine groups exhibit higher JSC than the reference dye
SQ01 (7.99 mA cmꢀ2). For another important performance
parameter, VOC increases by w13 mV for WH-SQ03 relative to WH-
SQ02 and WH-SQ01-based cells, which indicates that the long
hydrocarbon chain in WH-SQ03 is superior to the short hydrocar-
bon chain in WH-SQ02 and WH-SQ01 in enhancing VOC. This
enhancement could be attributed to that the suppression of the
charge recombination, originating from blocked recapture of the
photoinjected electrons by triiodide ions and repressed aggregation
of the conjugation backbone, which is induced by the long hydro-
carbon chain substituted to the diphenylamine unit.
Fig. 5. Frontier orbital of the sensitizers optimized at the B3LYP/6-31G(d) level.
parameters were summarized in Table 2. Actually, dozens of de-
vices have been fabricated and tested, and they all show similar
tendency and the given data are representative. Therefore it can be
assured that the ascribed differences in JSC and VOC are superior to
experimental error. Under standard global AM 1.5 solar conditions,
the DSSC based on WH-SQ01 produced a short-circuit photocurrent
(JSC) of 8.15 mA cmꢀ2, an open-circuit voltage (VOC) of 532.1 mV, and
a fill factor (FF) of 0.72, corresponding to an overall conversion
efficiency (h) of 3.14%. Under the same condition, upon the incor-
poration of methoxy or hexyloxy groups on the diphenylamine
donor unit, WH-SQ02 and WH-SQ03 sensitized DSSC showed JSC of
8.91 and 8.95 mA cmꢀ2, VOC of 531.8 and 545.1 mV, and FF of the
same 0.69, corresponding to
h of 3.29 and 3.39, respectively. It can
be found that JSC increased by w0.8 mA cmꢀ2 for WH-SQ02 and
WH-SQ03 with alkoxy chains in comparison to WH-SQ01 with
methyl groups. To clarify the influence of the substituent groups of
the dyes on the JSC value, we have measured the amount of dyes
adsorbed on the TiO2 film. It could be found in Table 1 that the
adsorbed amounts of 0.65 ꢂ 10ꢀ7 mol cmꢀ2 for WH-SQ01,
0.66 ꢂ 10ꢀ7 mol cmꢀ2 for WH-SQ02, and 0.67 ꢂ 10ꢀ7 mol cmꢀ2 for
To further investigate the charge recombination, electro-
chemical impedance spectroscopy (EIS) was then performed. The
Nyquist plots are presented in Fig. 8(a), which were measured in
the dark under a forward bias of ꢀ0.5 V. The large semicircle at
middle frequencies is ascribed to the electron recombination
resistance and chemical capacitance at the dyed-TiO2/electrolyte.
By fitting the Nyquist plots to an electrochemical model, the Rrec
Fig. 7. Spectra of monochromatic incident photon-to-current conversion efficiency
Fig. 6. JeV curves of DSSCs sensitized with SQ01, WH-SQ01, WH-SQ02, and WH-SQ03.
(IPCE) for DSSCs based on these sensitizers.

134
L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
Fig. 8. Electrochemical impedance spectroscopy (EIS) for DSSCs base on the sensitizers. (A) Nyquist plots, (B) Bode phase plots in the dark under a forward bias of ꢀ0.5 V.
[2] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells.
values of WH-SQ01 and WH-SQ02, were calculated to be the same
Chem Rev 2010;110:6595e663.
[3] Bisquert J. Dilemmas of dye-sensitized solar cells. ChemPhysChem 2011;12:
1633e6.
[4] Mishra A, Fischer MKR, Bäuerle P. Metal-free organic dyes for dye-sensitized
solar cells: from structure: property relationships to design rules. Angew
Chem Int Ed Engl 2009;48:2474e99.
[5] Hwang S, 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;46:
4887e9.
115
U, smaller than that of WH-SQ03 (235 U). The higher electron
recombination resistance observed from WH-SQ03 indicated more
effective repression of the charge recombination, which could be
reflected in the improvements of VOC, resulting in enhanced device
efficiency. The Bode phase plots shown in Fig. 8(b) support the
differences in the Rrec. The middle-frequency peaks of the DSSCs
based on WH-SQ03 shifts to lower frequency relative to that of
WH-SQ01 and WH-SQ02, indicating longer electron lifetime for
WH-SQ03 sensitized TiO2 electrode. These results explain the in-
crease in WH-SQ03, and demonstrate that the charge recombina-
tion can be suppressed by the long hydrocarbon chain substituted
to the diphenylamine unit.
[6] Bai Y, Zhang J, Zhou D, Wang Y, Zhang M, Wang P. Engineering organic sen-
sitizers for iodine-free dye-sensitized solar cells: red-shifted current response
concomitant with attenuated charge recombination.
J Am Chem Soc
2011;133:11442e5.
[7] Qin C, Peng W, Zhang K, Islam A, Han L. A novel organic sensitizer combined
with a cobalt complex redox shuttle for dye-sensitized solar cells. Org Lett
2012;14:2532e5.
[8] Ito S, Miura H, Uchida S, Takata M, Sumioka K, Liska P, et al. High-conversion-
efficiency organic dye-sensitized solar cells with a novel indoline dye. Chem
Commun 2008;41:5194e6.
4. Conclusions
[9] Yen Y-S, Chou H-H, Chen Y-C, Hsu C-Y, Lin JT. Recent developments in
molecule-based organic materials for dye-sensitized solar cells. J Mater Chem
2012;22:8734e47.
In summary, three unsymmetrical squaraine dyes containing an
auxiliary arylamine donor have been synthesized and utilized as
NIR sensitizers for DSSC application. Their photophysical, electro-
chemical and photovoltaic properties have been systematically
investigated. Compared to the reference dye SQ01, the introduction
of auxiliary arylamine donor brings bathochromical shift of the
absorption band and the long alkoxy chain suppresses charge
recombination. A solar cell based on WH-SQ03 showed the best
light to electricity conversion efficiency of 3.39%. These results
reveal that the structures of the donor parts of squaraine signifi-
cantly affect the absorption character and charge recombination,
and then the photovoltaic performance of DSSCs based on them.
Further improvement on DSSCs with full spectrum absorption
could be expected with a co-sensitizer containing highlight ab-
sorption in the wavelength region of 400 nme600 nm.
[10] Qin C, Wong W-Y, Han L. Squaraine dyes for dye-sensitized solar cells: recent
advances and future challenges. Chem Asian J 2013;8(8):1706e19.
[11] Burke A, Schmidt-Mende L, Ito S, Grätzel M. A novel blue dye for near-IR ‘dye-
sensitised’ solar cell applications. Chem Commun 2007;3:234e6.
[12] Geiger T, Kuster S, Yum J-H, Moon S-J, Nazeeruddin MK, Grätzel M, et al.
Molecular design of unsymmetrical squaraine dyes for high efficiency con-
version of low Energy photons into electrons using TiO2 nanocrystalline films.
Adv Funct Mater 2009;19:2720e7.
[13] Kim S, Mor GK, Paulose M, Varghese OK, Baik C, Grimes CA. Molecular design
of near-IR harvesting unsymmetrical squaraine dyes. Langmuir 2010;26:
13486e92.
[14] Li J-Y, Chen C-Y, Lee C-P, Chen S-C, Lin T-H, Tsai H-H, et al. Unsymmetrical
squaraines incorporating the thiophene unit for panchromatic dye-sensitized
solar cells. Org Lett 2010;12:5454e7.
[15] Choi H, Kim J-J, Song K, Ko J, Nazeeruddin MK, Gratzel M. Molecular engi-
neering of panchromatic unsymmetrical squaraines for dye-sensitized solar
cell applications. J Mater Chem 2010;20:3280e6.
[16] Shi Y, Hill RBM, Yum J-H, Dualeh A, Barlow S, Grätzel M, et al. A high-effi-
ciency panchromatic squaraine sensitizer for dye-sensitized solar cells. Angew
Chem 2011;123:6749e51.
[17] Fu C, Lei L, Sun K, Xia P, Yuan H, Xiao D, et al. VIS harvesting unsymmetrical
squaraine dye for dye-sensitized solar cells. Renew Energy 2012;38:163e8.
[18] Chang C-H, Chen Y-C, Hsu C-Y, Chou H-H, Lin JT. Squaraine-arylamine sen-
sitizers for highly efficient p-type dye-sensitized solar cells. Org Lett 2012;14:
4726e9.
Acknowledgements
The authors acknowledge the financial support from the Min-
istry of Science and Technology of China (863, No.
SS2013AA50303), the National Natural Science Foundation of China
(Grant No. 61106056), the Fundamental Research Funds for the
Central Universities (HUSTNY022), and Scientific Research Foun-
dation for Returned Scholars, Ministry of Education of China.
[19] Delcamp JH, Shi Y, Yum J-H, Sajoto T, Dell’Orto E, Barlow S, et al. The role of
p
bridges in high-efficiency DSCs based on unsymmetrical squaraines. Chem.-
Eur. J 2013;19:1819e27.
[20] Yum J-H, Walter P, Huber S, Rentsch D, Geiger T, Nüesch F, et al. Efficient far
red sensitization of nanocrystalline TiO2 films by an unsymmetrical squaraine
dye. J Am Chem Soc 2007;129:10320e1.
[21] Liang M, Chen J. Arylamine organic dyes for dye-sensitized solar cells. Chem
Soc Rev 2013;42:3453e88.
References
[22] Jin L-M, Li Y, Ma J, Li Q. Synthesis of novel thermally reversible photochromic
axially chiral spirooxazines. Org Lett 2010;12:3552e5.
[1] O’Regan B, Gratzel M. A low-cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO2 films. Nature 1991;353:737e40.

L. Liu et al. / Dyes and Pigments 106 (2014) 128e135
135
[23] Zhao H, Tanjutco C, Thayumanavan S. Design and synthesis of stable triaryl-
amines for hole-transport applications. Tetrahedron Lett 2001;42:4421e4.
[24] Paek S, Choi H, Kim C, Cho N, So S, Song K, et al. Efficient and stable
panchromatic squaraine dyes for dye-sensitized solar cells. Chem Commun
2011;47:2874e6.
[25] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 09, revision A.02. Wallingford, CT: Gaussian, Inc.; 2009.
[26] Yu Q-Y, Liao J-Y, Zhou S-M, Shen Y, Liu J-M, Kuang D-B, et al. Effect of hy-
drocarbon chain length of disubstituted triphenyl-amine-based organic dyes
on dye-sensitized solar cells. J Phys Chem C 2011;115:22002e8.