Unsymmetrical squaraines with new linkage manner for high-performance solution-processed small-molecule organic photovoltaic cells

Article abstract of DOI:10.1039/c5ra24186c

Squaraines are promising donor materials because of their strong and broad absorption band in the visible and near infrared regions which is suitable for application in organic photovoltaic (OPV) cells. Two unsymmetrical squaraines (USQs), namely BIBISQ and TIBISQ, with two electron-donating aryls directly linked to the electron-withdrawing squaric acid core (Y-manner) can act as high performance donor materials for solution-processed bulk-heterojunction (BHJ) OPV cells. Both USQs show ideal low bandgaps (1.47 eV for BIBISQ and 1.39 eV for TIBISQ) with an intense and broad absorption band in the range of 500-900 nm, and relatively low HOMO levels of ～-5.10 eV. The BHJ-OPV device based on both of them simultaneously showed excellent short current density (J_sc) (over 13 mA cm^-2), open circuit voltage (V_oc) (0.84 V), fill factor (FF) (0.49) and power conversion efficiency (PCE) of over 5% under a blend ratio of USQs:PC₇₁BM = 1:3. These results indicate that the two USQs are quite promising candidates for small molecular (SM) OPV cells and the Y-manner should be a quite successful linking method for USQs.

Full text of DOI:10.1039/c5ra24186c

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DOI: 10.1039/C5RA24186C
Unsymmetrical squaraines with new linked manner for
high-performance solution-processed small-molecule organic
photovoltaic cells†
a
a,b
a
a
a
a
Lin Yang,‡ Daobin Yang,‡ Yao Chen, Qian Luo, Mangang Zhang, Yan Huang,*
a
b
b
Zhiyun Lu,* Hisahiro Sasabe,* and Junji Kido
a
Key Laboratory of Green Chemistry and Technology (Ministry of Education),
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China. E-mail:
huangyan@scu.edu.cn, luzhiyun@scu.edu.cn
b
Department of Organic Device Engineering, Yamagata University, 4-3-16 Jonan,
Yonezawa, Yamagata 992-8510, Japan. Email: h-sasabe@yz.yamagata-u.ac.jp.
‡
The first two authors contributed equally to this work.

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Abstract
Squaraines have been promising donor materials because of their strong and broad
absorption band in the visible and near infrared regions which is suitable for
application in organic photovoltaic (OPV) cells. Two unsymmetrical squaraines
(USQs), namely BIBISQ and TIBISQ, with two electron-donating aryls directly
linked to the electron-withdrawing squaric acid core (Y-manner) could act as high
performance donor materials for solution-processed bulk-heterojunction OPV cells.
Both of two USQs show ideal low bandgaps (1.47 eV for BIBISQ and 1.39 eV for
TIBISQ) with an intense and broad absorption band in the range of 500-900 nm, and
relatively low HOMO levels of ~ -5.10 eV. The BHJ-OPV based on both of them
-2
simultaneously show excellent J (over 13 mA cm ), V (0.84V), FF (0.49) and
sc
oc
PCEs of over 5% under the blend ratio of USQs:PC BM=1:3. These results are
71
indicating that the two USQs are quite promising candidates for small molecular (SM)
OPV and the Y-manner should be quite perspective linked method for USQs.
Keywords squaraine, organic photovoltaic cell, linked manner, increased donor
content

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1
. Introduction
Owing to its nanometer-scaled phase separation in the donor-acceptor (D-A) blending
system thus more efficient D/A interfacial contact, bulk-heterojunction (BHJ) organic
photovoltaic (OPV) device has been considered as a promising technique for
1
, 2
renewable energy source applications.
In recent decade, small molecular (SM)
photovoltaic donor materials have attracted considerable attention for preparing
BHJ-OPVs due to their unique advantages over their polymer counterparts, such as
well-defined molecular structure and molecular weight, facile synthesis and high
3
purity without batch to batch variations. Therefore, many kinds of small molecular
photovoltaic donor materials have been developed, including oligothiophenes, dyes,
4
-8
fused acenes, triphenylamine-based molecules and so on. Up to now, the power
conversion efficiency (PCE) of SMOPVs have been achieved as high as >10% with a
high donor content (D:A=1:0.8) when employed an oligothiophene derivative
9
(DRCN5T) as donor material.
Recently, squaraines (SQs) have emerged as promising donor materials in OPVs
due to their strong and broad absorption band in the visible and near infrared
1
0-16
regions.
As far, the record power conversion efficiency (PCE) of squaraines based
1
7
BHJ-OPVs is 5.50% with a rather low donor content (D/A ratio is 1:6) and the PCE
1
8
of tandem devices based on squaraines is up to 8.3%.
In recent years, our group has designed a series of USQs in which the D and D’
segments are connected to the A subunit directly and via a methylidene bridge,
respectively (X-manner in Scheme 1), and the PCE of these USQs based BHJ-OPV

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devices is improved from 1.54% to 4.29%, however, the optimized D/A ratios fall into
1
9-22
the region of 1:8~1:5 with a low donor content.
According to the literature
reported, the photophysical and chemical properties of squaraines are correlated
2
3, 24
highly with their connected manners,
which are closely related with their OPV
performance. Therefore, very recently, our group has firstly designed some USQs
with their two electron-donating aryls linked directly to the electron-withdrawing
2
5
squaric acid core in Y-manner (Scheme 1). The OPV device based on them rendered
-2
a short current density (J ) of up to 12.03 mA cm and a PCE of 4.35% with a D/A
sc
ratio of 1:5. These preliminary results indicated that D and D’ linked to A in Y-manner
should be quite promising linked manner for USQs donor materials. However, the
PCE of them is unsatisfactory due to their relatively low open circuit voltage (Voc,
0
.78 V) and fill factor (FF, 0.45). It is noteworthy that one of their donor segments is
pyrrole derivative, whose electron-donating ability is much better than phenyl groups,
which leads to a relatively high HOMO energy level and then low Voc.
Scheme 1. Two linking manners of photovoltaic USQs derivatives.
Therefore, from these previous results, two novel USQs with Y-manner linked have
16, 26
been designed and synthesized. The phenyl groups, such as N,N-dibutylaniline
or
4
-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole which are the promising
20, 27
groups in OPV donor materials were used to be
D
subunit;
5
-(8,9,10,10a-tetrahydrobenzo[e]cyclopenta[b]indol-7(7aH)-yl)benzene-1,3-diol, was

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25
DOI: 10.1039/C5RA24186C
chosen to act as D’ subunit. The molecular structures of two novel USQs (BIBISQ
and TIBISQ) are shown in Scheme 2. Both of two USQs show ideal low bandgaps of
1
.47 eV for BIBISQ and 1.39 eV for TIBISQ with an intense and broad absorption
2
8
band in the range of 500-900 nm, and relatively low HOMO levels of ~ -5.10 eV.
The BHJ-SMOPV based on both of them simultaneously show enhanced V (0.84V)
oc
-2
and FF (0.49). Meanwhile, the excellent J of over 13 mA cm is achieved due to the
sc
relatively high content of USQs (USQs:PC BM=1:3). Ultimately, the PCEs of
71
BHJ-SMOPV based on USQs are over 5%. Especially, the BHJ-SMOPV based on the
-2
TIBISQ shows a high J of up to 13.50 mA cm and an excellent PCE of 5.49%,
sc
2
9
17, 30, 31
which could compare to the highest Jsc and PCE
among all the reported
squaraine based BHJ-OPV cells. These results indicate that Y-manner is indeed a
quite promising linking way for the construction of high performance OPV USQs.
2
. Materials and methods
2
.1. Instruments and characterization
1
13
H and C NMR spectra were recorded on a Bruker Avance AV ΙΙ-400 MHz
instrument with tetramethylsilane as internal standard. High resolution mass spectra
were measured on a Shimadzu LCMS-IT-TOF. The purity of the two USQs were
measured by EZChrom Elite for Hitachi high performance liquid chromatography
-
6
-1
(
DAD and RI detector). Absorption spectra of two USQs in 5×10 mol L
chloroform solution and thin film states were measured with a Perkin Elmer Lamdba
50 UV-Vis scanning spectrophotometer.
9

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2
.2. Electrochemistry measurement
-1
Cyclic voltammetry was performed in 0.10 mol L tetrabutylammonium
-4
-1
perchlorate/anhydrous dichloromethane (2.5×10 mol L ) with a LK 2010
electrochemical work station, using a three-electrode cell with a Pt disk working
-1
electrode, a Pt wire counter electrode and a Ag/AgNO (0.1 mol L in acetonitrile)
3
reference electrode. Solutions have been pre-degassed by argon bubbling for 30 min
prior to each experiment. At the end of each measurement, ferrocenium/ferrocene
redox couple was added as an internal potential reference.
2
.3. Preparation of organic photovoltaic cells
Photovoltaic devices were fabricated on indium-tin oxide (ITO) coated-glass
-1
substrate (sheet resistance = 15 Ω sq ) with a layered structure of ITO/ MoO (8 nm)/
3
USQs:PC BM (60 nm)/ BCP (6 nm)/ Al (100 nm). The patterned ITO-coated glass
7
1
substrates were cleaned through sequential sonication in detergent, deionized water,
acetone and isopropanol for 20 min each. The cleaned substrates were dried in an
oven at 65 °C for 12 h before using. The substrate was treated by UV-ozone for 30
min, then immediately transferred into a high vacuum chamber for deposition of 8 nm
-4
-1
MoO at pressure of less than 1×10 Pa with a rate of 0.2 Å s . Subsequently, the
3
photoactive layer (thickness: 60±5 nm) was fabricated by spin-casting a blend of the
-1
USQs and PC BM in chloroform solution with total concentration of 20 mg mL
71
(
3500 rpm, 45 s) under a N -filling glove box at 25 °C. Finally, the substrates were
2
transferred back to the high-vacuum chamber, where BCP (6 nm) and Al (100 nm)
-4
were deposited as the top electrode at pressures of less than 2×10 Pa with a rate of

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-1
-1
DOI: 12 0.1039/C5RA24186C
0
.20 Å s and 2.0~3.0 Å s , respectively. The active area of OPV cells is 9 mm . To
obtain the average data related to device performance, several batches of devices (4
cells per batch) for each set of conditions were fabricated and tested. Current
density-voltage (J-V) and external quantum efficiency (EQE) characterization of
organic photovoltaic cells were performed on a CEP-2000 integrated system
manufactured by Bunkoukeiki Co. The integration of EQE data over a AM 1.5G solar
spectrum yielded calculated J values with an experimental variation of less than 5%
sc
-2
was relative to the J measured under 100 mW cm simulated AM 1.5G light
sc
illumination. Hole-only devices were fabricated with the structures of ITO/ MoO (8
3
nm)/ USQs:PC BM (60 nm)/ MoO (8 nm)/ Al (100 nm).
7
1
3
Samples for atomic force microscopy (AFM) measurements were prepared by
spin-casting from USQs:PC BM=1:3 in chloroform solution on glass substrates.
71
2
.4. Synthesis
The synthetic routes of intermediates and target molecules USQs are outlined in
Scheme 2. Compounds 1-9 were prepared according to the procedures described in
2
5, 27, 32, 33
the literatures.
n-Butanol and toluene were distilled from sodium freshly
prior to use. All the other chemicals, reagents, and solvents were used as received
from the suppliers.
2
.4.1 N,N-dibutylaniline (1)
A mixture of aniline (5 mL, 52.63 mmol), n-C H Br (17 mL, 157.89 mmol) and
4
8
Na CO (22.37 g, 210.53 mmol) in 25 mL of 9:1 v/v dimethylformamide
2
3
(DMF)/N-methylpyrrolidinone (NMP) was heated at 120 °C for 12 h. The reaction

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mixture was then cooled to room temperature and filtered to remove the insoluble
material. The precipitate was washed with ethyl acetate, and the combined filtrate was
evaporated to dryness. Water was the added, and the aqueous phase was extracted
with ethyl acetate. After silica gel column chromatography (hexane), a pale yellow oil
(9.18 g, 85%) was obtained.
2
.4.2 4-(p-Tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (3)
A mixture of compound 2 (0.80 g, 5.02 mmol), 1-bromo-4-methylbenzene (0.86 g,
.02 mmol), NaOBu-t [sodium tert-butoxide] (0.72 g, 7.53 mmol), Pd(OAc)2
5
[
palladium (II) acetate] (34 mg, 3%), and P(t-Bu) HBF [tri(tert-butyl)phosphine
3 4
tetrafluoroborate] (73 mg, 5%) in anhydrous toluene (50 mL) was refluxed under Ar
for 6 h. After cooled down, the reaction mixture was filtered, and the filtrate was
concentrated in vacuo. The resulting crude product was purified by silica gel column
1
chromatography (hexane) to give 3 (0.86 g, 70%) as colorless oil. HNMR (400 MHz,
CDCl , ppm) δ 7.20 (d, J = 8.4 Hz, 2H, ArH), 7.15 (d, J = 8.4 Hz, 2H, ArH), 7.12 (d, J
3
=
7.2 Hz, 1H, ArH), 7.05 (t, J = 8.0 Hz, 1H, ArH), 6.93 (d, J = 8.0 Hz, 1H, ArH), 6.72
t, J = 7.2 Hz, 1H, ArH), 4.77 (t, J = 8.8 Hz, 1H, CH), 3.84 (t, J = 8.8 Hz, 1H, CH),
.33 (s, 3H, CH ), 2.08-1.99 (m, 1H, CH ), 1.95-1.84 (m, 2H, CH ), 1.82-1.75 (m, 1H,
(
2
3
2
2
CH ), 1.70-1.60 (m, 1H, CH ), 1.59-1.47 (m, 1H, CH ).
2
2
2
2
.4.3 3-Chloro-4-(4-(dibutylamino)phenyl)cyclobut-3-ene-1,2-dione (5)
The mixture of squaryl chloride 4 (1.00 g, 6.67 mmol), N,N-dibutylaniline 1 (1.37 g,

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6
.67 mmol) in dried toluene (50 mL) was refluxing for 5 h, then the reaction mixture
was cooled and evaporated under vacuum. The residue was purified by silica gel
column chromatography (hexane/dichloromethane=2:1) to give 5 (0.71 g, 30%) as
1
yellow solid. HNMR (400 MHz, CDCl , ppm) δ 8.15 (d, J = 9.2 Hz, 2H, ArH), 6.75
3
(
d, J = 9.2 Hz, 2H, ArH), 3.44 (t, J = 7.6 Hz, 4H, CH ), 1.69-1.61 (m, 4H, CH ),
2 2
1
.46-1.37 (m, 4H, CH ), 1.03 (t, J = 7.2 Hz, 6H, CH ).
2 3
2
.4.4 3-Chloro-4-(4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)cyclob
ut-3-ene-1,2-dione (6)
The mixture of squaryl chloride 4 (0.40 g, 2.65 mmol), 3 (0.60 g, 2.41 mmol) in
dried toluene (30 mL) was refluxing for 5 h, then the reaction mixture was cooled and
evaporated under vacuum. The residue was purified by silica gel column
chromatography (hexane/dichloromethane=2:1) to give 6 (0.22 g, 25%) as orange
1
solid. HNMR (400 MHz, CDCl , ppm) δ 8.01 (d, J = 8.4 Hz, 1H, ArH), 7.96 (s, 1H,
3
ArH), 7.26-7.16 (m, 4H, ArH), 6.74 (d, J = 8.8 Hz, 1H, ArH), 4.99 (t, J = 8.4 Hz, 1H,
CH), 3.89 (t, J = 8.4 Hz, 1H, CH), 2.38 (s, 3H, CH ), 2.14-2.01 (m, 1H, CH ),
3
2
1
.92-1.85 (m, 2H, CH ), 1.76-1.66 (m, 2H, CH ), 1.56-1.44 (m, 1H, CH ).
2 2 2
2
.4.5 3-N,N-(dibutylamino)phenyl-4-hydroxy-3-cyclobutene-1,2-dione (7)
(0.71 g 2.22 mmol) was dissolved in a mixture of acetic acid (30 mL),
5
concentrated hydrochloric acid (4 mL) and water (10 mL). This mixture was refluxed
for 2 h, and cooled to room temperature. Water (200 mL) was added dropwise into the

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mixture, then the yellow precipitate 7 (0.59 g, 88%) was obtained by filtration,
1
washed with ether and dried. HNMR (400 MHz, DMSO-d , ppm) δ 7.86 (s, 2H,
6
ArH), 6.81 (s, 2H, ArH), 3.38 (s, 4H, CH ), 1.49 (s, 4H, CH ), 1.34-1.23 (m, 4H, CH ),
2
2
2
0
.92 (t, J = 6.8 Hz, 6H, CH3).
2
.4.6 3-Hydroxy-4-(4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)cyclo
but-3-ene-1,2-dione (8)
(0.22 g, 0.60 mmol) was dissolved in a mixture of acetic acid (15 mL),
6
concentrated hydrochloric acid (2 mL) and water (5 mL). This mixture was refluxed
for 6 h, and cooled to room temperature. Water (100 mL) was added dropwise into the
mixture, then the brown precipitate 8 (0.20 g, 90%) was obtained by filtration, washed
1
with ether and dried. HNMR (400 MHz, DMSO-d , ppm) δ 7.74 (s, 1H, ArH), 7.72
6
(
d, J = 8.8 Hz, 1H, ArH), 7.26 (d, J = 8.4 Hz, 2H, ArH), 7.23 (d, J = 8.4 Hz, 2H, ArH),
.85 (d, J = 8.4 Hz, 1H, ArH), 4.99 (t, J = 6.4 Hz, 1H, CH), 3.87 (t, J = 8.4 Hz, 1H,
CH), 2.30 (s, 3H, CH ), 2.11-2.02 (m, 1H, CH ), 1.80-1.60 (m, 4H, CH ), 1.43-1.33
6
3
2
2
(m, 2H, CH2).
2
1
.4.7 4-(4-(Dibutyliminio)cyclohexa-2,5-dien-1-ylidene)-2-(2,6-dihydroxy-4-(8,9,10,
0a-tetrahydrobenzo[e]cyclopenta[b]indol-7(7aH)-yl)phenyl)-3-oxocyclobut-1-enolat
e (BIBISQ)
A mixture of compound 9 (0.17 g, 0.53 mmol) and 7 (0.16 g, 0.53 mmol) in butanol
(4 mL) and toluene (12 mL) was refluxed at 140ºC under Ar for 12 h. Then the

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reaction mixture was concentrated in vacuo, and the crude product was purified by
column chromatography (dichloromethane) and followed by recrystallization from
dichloromethane/methanol to give green shiny crystals of BIBISQ (0.25 g, 78%).
1
Purity: 100% (HPLC, eluent: THF/CH OH=1:9). m.p. 219-221ºC. HNMR (400 MHz,
3
CDCl , ppm) δ 12.84 (s, 2H, OH), 8.15 (d, J = 8.8 Hz, 2H, ArH), 7.87 (d, J = 8.0 Hz,
3
1
8
2
H, ArH), 7.83 (d, J = 8.4 Hz, 1H, ArH), 7.79 (d, J = 9.2 Hz, 1H, ArH), 7.71 (d, J =
.8 Hz, 1H, ArH), 7.53 (t, J = 7.2 Hz, 1H, ArH), 7.44 (t, J = 7.2 Hz, 1H, ArH), 7.09 (s,
H, ArH), 6.40 (s, 2H, ArH), 4.98-4.93 (m, 1H, CH), 4.39-4.34 (m, 1H, CH), 3.44 (t, J
=
7.6 Hz, 4H, CH ), 2.39-2.25 (m, 2H, CH ), 2.12-1.99 (m, 2H, CH ), 1.79-1.67 (m,
2 2 2
4
H, CH ), 1.58-1.49 (m, 2H, CH ), 1.44-1.35 (m, 4H, CH ), 0.99 (t, J = 7.2 Hz, 6H,
2 2 2
13
CH ). CNMR (100 MHz, CDCl , ppm) δ 182.8, 178.1, 166.5, 164.6, 155.2, 152.6,
3
3
1
40.1, 132.5, 130.9, 130.7, 130.1, 128.8, 128.6, 126.9, 124.5, 123.3, 117.8, 115.2,
+
112.3, 108.0, 97.0, 69.7, 51.2, 44.7, 35.1, 33.3, 29.6, 24.8, 20.2, 13.9. HRMS (ESI)
+
m/z: [M+H] calcd. for C H N O , 601.3061; found, 601.3062. Anal. Calcd for
3
9
41
2
4
C H N O : C, 77.97; H, 6.71; N, 4.66. Found: C, 78.11; H, 6.94; N, 5.12.
39
40
2
4
2
.4.8 2-(2,6-Dihydroxy-4-(8,9,10,10a-tetrahydrobenzo[e]cyclopenta[b]indol-7(7aH)
-yl)phenyl)-4-(4-(p-tolyl)-1,3,3a,8b-tetrahydrocyclopenta[b]indol-4-ium-7(2H)-yliden
e)-3-oxocyclobut-1-enolate (TIBISQ)
A mixture of compound 9 (0.18 g, 0.58 mmol) and 8 (0.20 g, 0.58 mmol) in butanol
(4 mL) and toluene (12 mL) was refluxed at 140ºC under Ar for 12 h. Then the
reaction mixture was concentrated in vacuo, and the crude product was purified by

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column chromatography (dichloromethane) and followed by recrystallization from
dichloromethane/methanol to give green shiny crystals of TIBISQ (0.27 g, 73%).
1
Purity: 99.0% (HPLC, eluent: THF/CH OH=1:9). m.p. 248-249ºC. HNMR (400
3
MHz, CDCl , ppm) δ 12.69 (s, 2H, OH), 8.02 (d, J = 8.4 Hz, 1H, ArH), 7.97 (s, 1H,
3
ArH), 7.84 (d, J = 8.4 Hz, 1H, ArH), 7.79 (d, J = 8.4 Hz, 1H, ArH), 7.75 (d, J = 8.8
Hz, 1H, ArH), 7.69 (d, J = 8.8 Hz, 1H, ArH), 7.52 (t, J = 7.2 Hz, 1H, ArH), 7.40 (t, J
=
7.2 Hz, 1H, ArH), 7.26 (d, J = 8.0 Hz, 2H, ArH), 7.20 (d, J = 8.4 Hz, 2H, ArH), 6.75
(
(
(
d, J = 8.0 Hz, 1H, ArH), 6.38 (s, 2H, ArH), 4.98 (s, 1H, CH), 4.91 (s, 1H, CH), 4.32
s, 1H, CH), 3.85 (s, 1H, CH), 2.39 (s, 3H, CH ), 2.36-2.19 (m, 2H, CH ), 2.12-1.84
3
2
1
3
m, 5H, CH ), 1.77-1.66 (m, 3H, CH ), 1.57-1.45 (m, 2H, CH ). CNMR (100 MHz,
2
2
2
CDCl , ppm) δ 182.9, 177.6, 165.8, 164.5, 155.1, 154.5, 140.1, 136.9, 136.8, 135.7,
3
1
33.1, 130.9, 130.8, 130.3, 130.1, 128.8, 128.6, 126.9, 126.2, 124.5, 123.3, 123.2,
20.6, 115.2, 108.1, 108.0, 97.0, 70.9, 69.7, 44.7, 44.4, 35.5, 35.1, 33.3, 32.8, 24.8,
1
+
+
2
4.0, 21.1. HRMS (ESI) m/z: [M+H] calcd. for C H N O , 645.2748; found,
43 37 2 4
6
45.2748. Anal. Calcd for C H N O : C, 80.10; H, 5.63; N, 4.34. Found: C, 80.52;
43 36 2 4
H, 5.74; N, 4.82.
3
. Results and discussion
.1 Synthesis and characterization
The synthetic routes of intermediates and target molecules USQs are outlined in
Scheme 2. Compounds 1-9 were prepared according to the procedures described in
3
2
5, 27, 32, 33
the literatures.
The two objective molecules BIBISQ and TIBISQ were

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prepared by condensation of 9 with 7 and 8, respectively. The molecular structures of
1
13
the USQs were characterized by H NMR, C NMR, HR-ESIMS and element
analysis. The purity of both two USQs is conformed to be >99.0% by HPLC analysis.
Scheme 2. Synthetic routes of two USQs. (a) Na CO (4.0 equ.), dimethylformamide
2
3
(
DMF)/N-methylpyrrolidinone (NMP)=9:1 (v/v), 120 °C , 12 h, 85%; (b) NaOBu-t
1.5 equ.), Pd(OAc) (3% equ.), P(t-Bu) HBF (5% equ.), anhydrous toluene, reflux
(
2
3
4
under Ar, 6 h, 70%; (c) anhydrous toluene, reflux under Ar, 5 h, 30% for 5 and 25%
for 6; (d) acetic acid/water=3:1 (v/v), concentrated HCl, reflux, 2-6 h, 88% for 7 and
9
0% for 8; (e) n-butanol/toluene=1:3 (v/v), 140ºC under Ar, 12 h, 78% for BIBISQ
and 73% for TIBISQ.
3
.2 Optical properties
The UV-Vis absorption spectra of BIBISQ and TIBISQ in dilute chloroform
solution and thin solid films (spin-casted from chloroform solution) are shown in Fig.

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1
, and the relative data are summarized in Table 1. In solution, analogous to most of
1
9-22, 25
squaraines,
both the two compounds show intense absorption (molar extinction
5
-1
-1
coefficient > 10 M cm ) in Vis-NIR regions. The λ of BIBISQ and TIBISQ are
max
6
91 nm and 713 nm, respectively, with full width at half maxima (FWHM) of 53 nm
or 61 nm. In thin film state, both the two compounds display drastically red-shifted
and broadened absorption bands (λmax: 748 nm for BIBISQ, 760 nm for TIBISQ;
FWHM: 184 nm for BIBISQ, 217 nm for TIBISQ). This should be attributed to their
intense intermolecular interactions in condensed state. The intense and wide
absorption band of the two compounds in 500-900 nm should be beneficial to the
harvesting of sunlight hence the enhancement of J of their OPV devices. Through
sc
the absorption spectra of their solid film samples, the optical bandgap of BIBISQ and
TIBISQ is estimated to be 1.47 eV and 1.39 eV, respectively, which is an ideal value
2
8
for photovoltaic donor materials.
Table 1. Optical and electrochemical properties of the two USQs.
a
b
abs
opt
g
c
d
λ
abs (nm)
λ
FWHM (nm)
E
E
ox
HOMO
LUMO
Compound
(
log ɛ)
(nm)
Solution
Film
184
(eV)
(V)
(eV)
(eV)
BIBISQ
TIBISQ
691 (5.32)
713 (5.50)
748
760
53
61
1.47
1.39
0.32
0.28
-5.12
-5.08
-3.65
-3.69
217
a
-6
b
c
d
Measured in dilute chloroform solution (5.00×10 M); measured in thin film state;
HOMO= (-4.80- Eox) eV;
LUMO= E
opt
g
+HOMO.

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Fig.1. Absorption spectra of two USQs in solution (a) and thin films (b).
3
.3 Electrochemical properties
Cyclic voltammetry experiments were carried out on BIBISQ and TIBISQ in
dichloromethane solution with tetrabutylammonium perchlorate as the supporting
electrolyte, and the results are shown in Table 1 and Fig. 2. The HOMO level of
BIBISQ and TIBISQ is estimated to be –5.12 eV and –5.08 eV respectively by
+
comparing the oxidation wave peak of the compounds with Fc/Fc redox couple
1
5
whose energy level is 4.80 eV below vacuum. And the corresponding LUMO energy
level of BIBISQ and TIBISQ is calculated to be –3.65 eV and –3.69 eV, respectively
3
4
through their HOMO level and optical bandgap data. From these results, we can
roughly guess the V of BHJ-OPVs based on these two USQs and PC BM may be in
oc
71
3
5
0
.7-0.9 V.
Fig. 2. Cyclic voltammograms of the two USQs (a) and the energy levels of the
components for OPV devices (b).
3
.4 Photovoltaic characteristics
To evaluate their photovoltaic performance, BHJ-OPV cells with BIBISQ or
TIBISQ as electron donor material and [6,6]-phenyl-C butyric acid methyl ester
7
1
(
PC BM) as electron acceptor material have been fabricated with different D/A
71

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blending ratios. The device structure is ITO/MoO (8 nm)/USQs:PC BM (60 nm)/
3
71
BCP (6 nm)/ Al (100 nm). The corresponding data and curves are shown in Table 2,
Fig. 3 and 4. The results indicate that similar trend in photovoltaic performance of
OPV devices based on both BIBISQ and TIBISQ could be observed when their D/A
ratio varies from 1:1 to 1:8. Taking TIBISQ as an example, when the blend ratio is
changed from 1:1 to 1:8, the V and FF of the corresponding BHJ-OPV devices are
oc
nearly unaltered, while the J is found to show distinct variation, which matches with
sc
the corresponding EQE spectra of the devices. As shown in Fig 3b, in the EQE
spectra of TIBISQ-devices, three major bands could be identified, with their peaks
approximately located at 380 nm, 480 nm and 760 nm, respectively. The first two
high-energy bands should stem from the absorption of PC BM; and the third
7
1
low-energy band at ~760 nm should arise from the absorption of TIBISQ. With the
increasing composition of TIBISQ, the intensity of the third band peaked at ~760 nm
increases gradually, which is consistent with the corresponding absorption spectrum
of D/A blend film with similar D/A mixing ratio (shown in Fig. 3c). Accordingly, with
increasing D/A ratio from 1:8 to 1:3, the J of TIBISQ-device increases gradually
sc
-2
from 10.92 to 13.02 mA cm . However, in the case of TIBISQ-device with D/A ratio
of 1:1, the intensity of the third band in its EQE spectrum decreases rather than
increased drastically, despite the fact that the 1:1 blend film displays the strongest
absorption at this band, as shown in Fig. 3c. This may ascribed to the drastically
different morphology of the blend films between 1:1 and other D/A ratios (vide Fig.
5
). Similarly, for BIBISQ-devices, the optimized D/A ratio is also found to be 1:3.

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Therefore, taking advantage of the more effective absorption in the NIR region due to
the increased content of USQ in the 1:3 optoelectronic film, the resulting devices
-2
could both show impressive J of ~13 mA cm , which is much higher than that of the
sc
15, 25
reported USQ-based BHJ-SMOPVs.
Additionally, according to the experimental
and calculation results (Fig. 6), the hole mobility of the as-prepared 1:3 D/A blending
-5
-5
2
-1 -1
films is relatively high (3.25 × 10 and 6.21 × 10 cm V s for BIBISQ- and
TIBISQ-based film in sequence), which would also contribute to the high J and
sc
3
6
FF, hence high PCE of 4.97%~5.08% of the devices.
Table 2. Photovoltaic performance of USQs-OPV devices.
2
b
Active layer (w/w)
Voc (V) J (mA/cm ) FF
PCE (%)
sc
TIBISQ:PC BM=1:1
0.83
0.83
0.84
0.84
0.83
0.83
0.84
0.85
0.86
0.83
9.95
13.02
11.15
10.92
13.50
8.23
0.45 3.72 (3.65)
0.47 5.08 (5.03)
0.50 4.68 (4.57)
0.48 4.40 (4.32)
0.49 5.49 (5.39)
0.43 2.94 (2.81)
0.46 4.97 (4.92)
0.49 4.57 (4.46)
0.47 4.21 (4.13)
0.49 5.34 (5.24)
7
1
TIBISQ:PC BM=1:3
7
1
TIBISQ:PC BM=1:5
7
1
TIBISQ:PC BM=1:8
7
1
a
TIBISQ:PC BM=1:3
7
1
BIBISQ:PC BM=1:1
7
1
BIBISQ:PC BM=1:3
12.87
10.97
10.41
13.13
7
1
BIBISQ:PC BM=1:5
7
1
BIBISQ:PC BM=1:8
7
1
a
BIBISQ:PC BM=1:3
7
1
a
b
Thermal annealed at 80 °C for 10 min; the average PCEs are provided in parentheses.

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Fig. 3. The J-V characteristics (a), EQE characteristics (b) of the TIBISQ devices; and
absorption spectra (c) of the photoactive layers with different D/A ratios.
Fig. 4. The J-V characteristics (a), EQE characteristics (b) of the OPV devices and
absorption spectrum (c) of the blend films with the different blend ratios of
BIBISQ:PC BM.
7
1
Fig. 5. Tapping-mode AFM height images (5 × 5 μm) of TIBISQ:PC BM blend films
7
1
with different ratio (a, 1:1; b, 1:3).

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Fig. 6. Current density-voltage characteristics of hole-only single-carrier devices
using USQs:PC BM=1:3 blend films as active layer
71
Upon thermal annealing (80 °C, 10 min), the device performance of both BIBISQ
and TIBISQ is observed to be enhanced, with PCE of 5.34% and 5.49%, respectively.
The enhanced PCEs should be attributed not only to the improved J (for BIBISQ,
sc
-2
-2
from 12.87 to 13.13 mA cm ; for TIBISQ, from 13.02 to 13.50 mA cm ) which is
consistent with their EQE measurement results (shown in Fig. 3b and Fig. 4b), but
also to the enhanced FF (for BIBISQ, from 0.46 to 0.49; for TIBISQ, from 0.47 to
-2
0
.49). Moreover, the impressive J (13.50 mA cm ) and excellent PCE (5.49%) of
sc
2
9
17, 30, 31
TIBISQ-based BHJ-OPV could compare to the highest Jsc and PCE
among
all the reported squaraine based BHJ-OPV cells. These results indicate that the
Y-mannered molecular backbone is indeed quite promising for the construction of
perspective OPV USQs with increased donor content in photoactive layer.
It is worthwhile to mention that the optimized D/A composite ratio in these devices
based on two USQs is 1:3. Generally, a larger content of donor material in the blend
film is propitious to the harvesting of sunlight hence the enhancement of PCE of the
3
7
device, therefore, the typical D/A ratio in highly efficient OPV devices lies in the

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9
DOI: 10.1039/C5RA24186C
range of 1:1~1:2. However, although many D/A composite ratio has been employed
in the BHJ OPV devices based on squaraines, the record PCE of 5.50% for BHJ-OPV
using symmetrical squaraines (SSQs, D-A-D-type) could be achieved with a rather
1
7
low D/A ratio (1:6) according to Wei’s report. Similarly, for our reported USQs
1
9-22
donor materials, the optimized D/A ratios fall into the region of 1:8~1:5.
The low
content of donor material in photoactive layer may hamper the photovoltaic
performance of these squaraine materials. Therefore, the present investigation shows
the much enhanced content of USQ donor materials in active layers (the optimized
D/A ratios of BHJ-OPVs based on BIBISQ and TIBISQ are both 1:3) is beneficial to
absorb more solar light in the NIR region, which contributes to the impressive J over
sc
-2
1
3 mA cm and PCE over 5% of BHJ-OPVs, which would open a more confident
way to improve OPV performances of squaraine materials by molecular tailoring.
4
. Conclusion
In conclusion, two novel USQs bearing Y-manner molecular platform have been
developed. Both of them show relatively low bandgap of < 1.5 eV, intense and wide
absorption band in 500-900 nm, and relatively low HOMO energy level of ~ –5.10 eV
as well. BHJ-OPV devices based on them could show relatively high V of 0.84 V,
oc
-2
rather impressive J of over 13 mA cm and high FF of 0.49, consequently high PCE
sc
of over 5.0% even at a relatively high donor content (the optimized D/A ratio is 1:3).
Consequently, Y-manner is indeed a quite promising linking way for the construction
of high performance OPV USQs in terms of increased donor content.

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Acknowledgments
We acknowledge the financial support for this work by the National Natural
Science Foundation of China (project No. 21190031, 21372168 and 21432005),
Fujian Key Laboratory of Polymer Materials (Fujian Normal University)
(FJKL-POLY 201502) and China Scholarship Council. We are grateful to the
Comprehensive Training Platform of Specialized Laboratory, College of Chemistry,
Sichuan University for providing NMR and HR-MS data for the intermediates and
objective compounds.
Notes and references
a
Key Laboratory of Green Chemistry and Technology (Ministry of Education),
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China. E-mail:
huangyan@scu.edu.cn, luzhiyun@scu.edu.cn
b
Department of Organic Device Engineering, Yamagata University, 4-3-16 Jonan,
Yonezawa, Yamagata 992-8510, Japan. Email: h-sasabe@yz.yamagata-u.ac.jp.
1
13
†
Electronic Supplementary Information (ESI) available: The H NMR, C NMR
and HRMS spectra of BIBISQ and TIBISQ
The first two authors contributed equally to this work.
.
‡
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Two unsymmetrical squaraines were employed as donors for high performance
-2
BHJ-OPVs with Jsc >13 mA cm and PCE > 5%.

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3
8x18mm (300 x 300 DPI)