Two different donor subunits substituted unsymmetrical squaraines for solution-processed small molecule organic solar cells

Article abstract of DOI:10.1016/j.orgel.2016.02.009

Two unsymmetrical squaraines (USQs) with different donor (D) subunits as photovoltaic materials, namely USQ-11 and USQ-12, were designed and synthesized to investigate the effect of different D subunits on the optoelectronic properties of USQs for the first time. The two USQs compounds were characterized for optical, electrochemical, quantum chemical and optoelectronic properties. By changing the two different D subunits attached to the squaric acid core from 2,3,3-trimethylindolenine to 2-methylbenzothiazole, the HOMO energy levels could be tuned with a stepping of 0.07 eV, and quite different solid state aggregations (H-or J-aggregation) were observed in the thin film by UV-Vis absorption spectra, which were attributed to their distinct steric effects and dipole moments. Solution-processed bulk-heterojunction small molecule organic solar cells fabricated with the USQ-11/PC₇₁BM (1:5, wt%) exhibited extremely higher PCE (4.27%) than that of the USQ-12/PC₇₁BM (2.78%). The much enhanced PCE should be attributed to the simultaneously improved V_oc, J_sc and FF.

Full text of DOI:10.1016/j.orgel.2016.02.009

Organic Electronics 32 (2016) 179e186
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Two different donor subunits substituted unsymmetrical squaraines
for solution-processed small molecule organic solar cells
,
b
,
,
,
, **
Daobin Yang a 1, Yan Jiao a 1, Yan Huang a *, Taojun Zhuang b, Lin Yang a, Zhiyun Lu a
,
Xuemei Pu a, Hisahiro Sasabe b ***, Junji Kido b
,
, ****
a Key Laboratory of Green Chemistry and Technology (Ministry of Education), College of Chemistry, Sichuan University, Chengdu 610064, PR China
b Department of Organic Device Engineering, Research Center for Organic Electronics, Yamagata University, Yonezawa 992-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two unsymmetrical squaraines (USQs) with different donor (D) subunits as photovoltaic materials,
namely USQ-11 and USQ-12, were designed and synthesized to investigate the effect of different D
subunits on the optoelectronic properties of USQs for the first time. The two USQs compounds were
characterized for optical, electrochemical, quantum chemical and optoelectronic properties. By changing
the two different D subunits attached to the squaric acid core from 2,3,3-trimethylindolenine to 2-
methylbenzothiazole, the HOMO energy levels could be tuned with a stepping of 0.07 eV, and quite
different solid state aggregations (H- or J-aggregation) were observed in the thin film by UV-Vis ab-
sorption spectra, which were attributed to their distinct steric effects and dipole moments. Solution-
processed bulk-heterojunction small molecule organic solar cells fabricated with the USQ-11/PC71BM
(1:5, wt%) exhibited extremely higher PCE (4.27%) than that of the USQ-12/PC71BM (2.78%). The much
enhanced PCE should be attributed to the simultaneously improved Voc, Jsc and FF.
Received 6 January 2016
Received in revised form
4 February 2016
Accepted 9 February 2016
Available online xxx
Keywords:
Unsymmetrical squaraines
Donor subunits
H- and J-aggregation
Solution-processed
Small molecule organic solar cells
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
years, since they possess extremely high molar extinction co-
efficients, intense and broad absorption in Vis-NIR spectral regions
Solution-processed bulk-heterojunction (BHJ) organic solar cells
(OSCs) based on small molecules (SM) have shown great potential
as an alternative to more conventional polymer-based OSCs,
because of their well-defined molecular structure, high purity and
batch to batch reproducibility [1,2]. So far, the power conversion
efficiency (PCE) of small molecule based OSCs already exceeded 10%
[3], however further improved PCE is still needed for commercial
application, thus much research effort has been devoted to the
molecular tailoring of photovoltaic materials, so that the correla-
tions between molecular structure and photovoltaic properties
could be revealed.
[4e9]. Generally, they can be divided into symmetrical squaraines
(SQs) and unsymmetrical squaraines (USQs). In comparison with
SQs, USQs bearing a D-A-D0 rather than D-A-D molecular skeleton
should be more promising due to their much higher structural
tunability. At the beginning, the PCEs of the USQs-based BHJ-OSCs
were unstatisfactory (0.16e2.05%), which should be ascribed to
their low Voc (0.24e0.69 V), Jsc (1.40e9.05 mA cmꢀ2) and FF
(0.27e0.45) [10e13]. Recently, a series of novel USQs materials for
BHJ-SMOSCs have been developed by our group, in which 1,1,2-
trimethyl-1H-benzo[e]indole was used as D subunit, and 2,6-
dihydroxyphenyl groups were used as D0 segments [14e17].
Through delicate molecular design mainly at the end-capping
group of the D0 segments, such as diisobutylamino, diarylamino,
carbazyl, indoline and tetrahydroquinoline groups, the PCEs of
these USQs-based BHJ-SMOSCs increased from 1.54% to 4.29%,
which were attributed to their high Voc (0.75e1.12 V), Jsc
(5.40e11.03 mA cmꢀ2) and FF (0.36e0.48). So far, the PCEs of
solution-processed USQs-based BHJ-SMOSCs have reached 6.00%
[18,19], which were higher than that of the classic SQs materials
(PCE ¼ 5.50%) [20]. These encouraging results indicated that
indoline unit could acts as a quite promising end-capping unit to
Squaraines have drawn more and more attention in recent
* Corresponding author.
** Corresponding author.
*** Corresponding author.
**** Corresponding author.
E-mail addresses: huangyan@scu.edu.cn (Y. Huang), luzhiyun@scu.edu.cn
(Z. Lu), h-sasabe@yz.yamagata-u.ac.jp (H. Sasabe), kid@yz.yamagata-u.ac.jp
(J. Kido).
1
The first two authors contributed equally to this work.
http://dx.doi.org/10.1016/j.orgel.2016.02.009
1566-1199/© 2016 Elsevier B.V. All rights reserved.

180
D. Yang et al. / Organic Electronics 32 (2016) 179e186
construct high performance photovoltaic materials.
2.2. Fabrication of organic solar cells and characterization
So far, our group focused on the molecular tailoring of D0 seg-
ments in USQs, especially for the end-capping units [14e17].
However, another side (D subunit) is also extremely important for
Bulk-heterojunction small molecule organic solar cells were
fabricated with indium-tin- oxide (ITO) coated glass as substrate.
USQs
materials.
2,3,3-Trimethylindolenine
and
2-
The sheet resistance of ITO is 15 U
sqꢀ1. Patterned ITO-coated glass
methylbenzothiazole as a promising donor subunits widely used
in symmetrical squaraines with excellent hole mobilities
[10,21e23]. Hence, 2,3,3-trimethylindolenine and 2-methylbenzo-
substrates were sequentially cleaned using detergent, deionized
water, acetone, and isopropanol in an ultrasonic bath for 30 min
each. The cleaned substrates were dried in an oven at 65 ꢁC for 12 h
before using. The substrates were treated by UV-ozone for 20 min,
then immediately transferred into a high vacuum chamber for
deposition of 8 nm MoO3 at pressures of less than 1 ꢂ 10ꢀ4 Pa with
a rate of 0.2 Å sꢀ1. Subsequently, photoactive layers (thickness:
50 nm) were fabricated by spin-coating a blend of the USQs and
PC71BM in chloroform with total concentration of 20 mg mLꢀ1
under a N2-filling glovebox at 35 ꢁC, then the blend films were
thermal annealed at 80 ꢁC for 10 min. Finally, the substrates were
transferred back to the high-vacuum chamber, where BCP (6 nm)
and Al (100 nm) were deposited as the top electrode at pressures of
less than 8 ꢂ 10ꢀ5 Pa with a rate of 0.20 Å sꢀ1 and 2 ꢂ 10ꢀ4 Pa with a
rate of 1.5e4.0 Å sꢀ1, respectively, resulting in a final SMOSCs with
the structure of ITO/MoO3(8 nm)/USQs:PC71BM(50 nm)/BCP(6 nm)/
Al(100 nm). The active area of OSC cell is 9 mm2. Current density-
voltage (J-V) and external quantum efficiency (EQE) characteriza-
tions of organic solar cells were performed on a CEP-2000 inte-
grated system manufactured by Bunkoukeiki Co. The integration of
EQE data over an AM 1.5G solar spectrum yielded calculated Jsc
values with an experimental variation of less than 6% relative to the
Jsc measured under 100 mW cmꢀ2 simulated AM 1.5G light illu-
mination. Hole-only devices were fabricated with the structure of
ITO/MoO3(8 nm)/USQ-11(47 nm) or USQ-12(60 nm) or USQs:
PC71BM(50 nm)/MoO3(8 nm)/Al(100 nm).
thiazole were used as
D
subunits, 5-(1,3,3a,8b-tetrahy-
drocyclopenta[b]-indolyl)benzene-1,3-diol group acted as D0
segment [17], two novel USQs materials, namely USQ-11 and USQ-
12 (shown in Fig. 1), were designed and synthesized to investigate
the effect of different D subunits on the electronic and steric effect
of USQs for the first time. Their optical, electrochemical, quantum
chemical, hole mobility, morphology and photovoltaic properties
were investigated further below.
2. Experimental section
2.1. Instruments and characterization
1H NMR and 13C NMR spectra were measured using a Bruker
Avance AV II-400 MHz spectrometer, and the chemical shifts were
recorded in units of ppm with TMS as the internal standard. High
resolution mass spectra was obtained from a Shimadzu LCMS-IT-
TOF. Thermogravimetry analysis (TGA) was performed on
a
Perkin-Elmer TGA Q500 instrument in an atmosphere of N2 at a
heating rate of 10 ꢁC minꢀ1. The purity of USQs compounds were
measured by EZChrom Elite for Hitachi high performance liquid
chromatography (DAD and RI detector). Electronic absorption
spectra of both solution and thin-films of the USQs were recorded
using a Perkin Elmer Lamdba 950 UV-Vis scanning spectropho-
tometer. The ground-state geometries and electronic structures of
the USQs were calculated with Gaussian 09 software, using density
functional theory (DFT) based on B3LYP/6-31G(d) and B3LYP/6-
311G(d, p) levels. The solution samples were prepared in chloro-
form solution at a concentration of 3.00 ꢂ 10ꢀ6 mol Lꢀ1, while the
thin film samples were obtained by spin-coating from chloroform
solution (4 mg mLꢀ1, 1500 rpm/45 s) on quartz substrates. Cyclic
voltammetry measurements were carried out in 2.5 ꢂ 10ꢀ4 mol Lꢀ1
anhydrous dichloromethane (DCM) with tetrabutyl ammonium
perchlorate (Bu4NClO4) under an argon atmosphere at a scan rate of
50 mV sꢀ1 using a LK 2005A electrochemical workstation. The CV
system was constructed using a Pt disk, a Pt wire, and a Ag/AgNO3
(0.1 mol Lꢀ1 in acetonitrile) electrode as the working electrode,
counter electrode and reference electrode, respectively, and the
potential of the Ag/AgNO3 reference electrode was internally cali-
brated using the ferrocene/ferrocenium redox couple (Fc/Fcþ),
which has a known reduction potential of ꢀ4.80 eV relative to
vacuum level. The morphologies of the active layers were analyzed
by atomic force microscopy (AFM) in tapping mode in air under
room temperature using Bruker instrument, the active layers were
fabricated on ITO/MoO3 (8 nm) substrates.
2.3. Synthesis
Compound 3a and 3,4-diethoxy-cyclobut-3-ene-1,2-dione were
prepared according to the procedures described in the literature
[14,17]. The synthetic details of intermediates 1a, 2a, 1b, 2b, 1c and
2c were shown in Support Information. n-Butanol and toluene were
distilled from sodium freshly prior to use. All other chemicals were
obtained from commercial sources and used as-received without
further purification.
2.3.1. 4-((1-Butyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-
2-(2,6-dihydroxy-4-(1,3,3a,8b-tetrahydrocyclopenta[b]indol-4(2H)-
yl)phenyl)-3-oxocyclobut-1-enolate (USQ-11)
A mixture of compound 3a (596 mg, 2.22 mmol) and 1c (622 mg,
2.00 mmol) in n-butanol (50 mL) and toluene (50 mL) were added
into a round bottom flask. The mixture was refluxed with a Dean-
Stark apparatus for 36 h. After reaction was cooled down, the sol-
vents were removed under reduced pressure. This crude product
was purified by silica gel chromatography using dichloromethane/
methanol (50:1, v/v) as the eluent to afford green solid. The solid
was recrystallized from a 1:6 vol ratio of dichloromethane and
methanol mixture to afford green crystals USQ-11 (817 mg, 73%),
Fig. 1. Chemical structure of the USQs compounds.

D. Yang et al. / Organic Electronics 32 (2016) 179e186
181
mp. 214e215 ꢁC. 1H NMR (400 MHz, CDCl3, ppm)
d
:12.33 (2H, s,
are summarized in Table 1. In dilute solution, USQ-11 and USQ-12
exhibited very similar absorption profiles with the wavelength of
maxima absorption (lmax) of ~667 nm and full width half maxima
(FWHM) of ~1000 cmꢀ1, with a considerably high molar extinction
coefficient of > 105 Mꢀ1 cmꢀ1. In comparison with the absorption in
solution, the absorption bands of the thin films of the two USQs
compounds were significantly broadened (FWHM: from 900 to
3910 cmꢀ1 for USQ-11, from 1080 to 5060 cmꢀ1 for USQ-12).
However, quite different shapes of absorption bands could be
observed in thin film, the absorption maximum of the USQ-11 thin
film was red-shifted by 31 nm from the monomer peak in solution;
while 47 nm blue-shift could be observed in the USQ-12 thin film.
These drastic spectral changes are attributed to squaraines aggre-
gations. As we know, squaraines showed pronounced aggregation
features as they are known to form H- and J-aggregation in solid
films [24,25]. The H-aggregation displayed a blue-shift in the ab-
sorption maximum, which contributed to only a moderate increase
in the Jsc; while the J-aggregation exhibited a red-shift in the ab-
sorption maximum that resulted in a significantly increase in the Jsc
[11,26e28]. Thus, for USQ-11, the absorption peak exhibited a
bathochromic shift from 667 nm to 698 nm, which is probably due
to J-aggregation; while for USQ-12, the absorption peak showed a
hypsochromic shift from 668 nm to 621 nm, which is attributed to
H-aggregation. The branched two methyl groups of indolenine
eOH), 7.43 (1H, d, J ¼ 7.2 Hz, ArH), 7.38 (2H, d, J ¼ 8.4 Hz, ArH), 7.29
(1H, t, J ¼ 7.2 Hz, ArH), 7.18e7.10 (3H, m, ArH), 6.95 (1H, t, J ¼ 7.2 Hz,
ArH), 6.34 (2H, s, ArH), 5.92 (1H, s, ]CHe), 4.70 (1H, t, J ¼ 7.2 Hz,
eNCHe), 4.10 (2H, t, J ¼ 7.6 Hz, eNCH2e), 3.91 (1H, t, J ¼ 7.2 Hz, ]
CHe), 2.10e2.01 (2H, m, eCH2e), 2.00e1.91 (2H, m, eCH2e),
1.86e1.78 (2H, m, eCH2e), 1.77 (6H, s, eCH3), 1.70e1.62 (1H, m,
eCH2e), 1.53e1.43 (2H, m, eCH2e), 1.42e1.35 (1H, m, eCH2e), 1.03
(3H, t, J ¼ 7.6 Hz, -CH3); 13C NMR (100 MHz, CDCl3, ppm)
d: 173.7,
170.3, 162.9, 152.6, 144.0, 142.6, 141.6, 136.9, 128.2, 127.4, 125.5,
124.8, 122.5, 122.3, 113.6, 110.7, 104.7, 96.7, 87.7, 68.6, 50.3, 45.5,
44.2, 34.6, 33.8, 29.4, 26.6, 24.3, 20.3, 13.8; HR-MS (ESI): m/z [MþH]
561.2753, calcd.: 561.2756; purity: 99.2% (HPLC, eluent: THF/
CH3CN ¼ 1/9); elemental anal. calcd for C36H36N2O4: C 77.12, H 6.47,
N 5.00; found, C 76.71, H 6.70, N 4.91.
2.3.2. 4-((3-Butylbenzo[d]thiazol-3-ium-2-yl)methylene)-2-(2,6-
dihydroxy-4-(1,3,3a,8b-tetra- hydrocyclopenta[b]indol-4(2H)-yl)
phenyl)-3-oxocyclobut-1-enolate (USQ-12)
USQ-12 was obtained from the reaction of compound 3a
(297 mg, 1.11 mmol) and 2c (300 mg, 1.00 mmol) according to the
procedure described for the synthesis of USQ-11. The solid was
recrystallized from a 1:4 vol ratio of dichloromethane and meth-
anol mixture to afford green solid (270 mg, 49%), mp. 240e241 ꢁC.
1H NMR (400 MHz, CDCl3, ppm)
d
: 7.73 (1H, d, J ¼ 7.6 Hz, ArH), 7.47
hindered the p-p stacking of USQ-11, thereby enhancing J-aggre-
(1H, t, J ¼ 8.0 Hz, ArH), 7.38e7.31 (3H, m, ArH), 7.16 (2H, t, J ¼ 7.2 Hz,
ArH), 6.92 (1H, t, J ¼ 7.6 Hz, ArH), 6.26 (2H, d, J ¼ 3.2 Hz, ArH), 6.06
(1H, s, ]CHe), 4.66 (1H, t, J ¼ 8.0 Hz, eNCHe), 4.29 (2H, t,
J ¼ 7.6 Hz, eNCH2e), 3.90 (1H, t, J ¼ 8.0 Hz, ]CHe), 2.09e1.99 (2H,
m, eCH2e), 1.98e1.91 (2H, m, eCH2e),1.89e1.81 (2H, m, eCH2e),
1.70e1.62 (1H, m, eCH2e), 1.53e1.43 (2H, m, eCH2e), 1.42e1.36
(1H, m, eCH2e), 1.05 (3H, t, J ¼ 7.6 Hz, eCH3); 13C NMR (100 MHz,
gation; on the contrary, the benzothiazole segment showed a
planar structure, which is very beneficial for formation H-aggre-
gation [29]. Therefore, the totally different solid state aggregations
were attributed to their distinct steric effects. Determined by the
onset position of the absorption spectra of the USQs in the thin
films, optical band-gaps were calculated to be 1.53 and 1.52 eV,
respectively, for USQ-11 and USQ-12.
CDCl3, ppm) d: 182.1, 180.9, 169.8, 164.6, 164.3, 161.6, 161.3, 150.7,
144.5, 140.3, 136.5, 128.9, 128.1, 127.3, 126.1, 124.8, 122.7, 121.6, 113.1,
112.7, 103.3, 96.7, 96.5, 88.2, 68.4, 47.3, 45.4, 34.7, 34.0, 29.9, 24.3,
20.2, 13.7; HR-MS (ESI): m/z [MþH] 551.2005, calcd.: 551.2005;
purity: 99.6% (HPLC, eluent: THF/CH3CN ¼ 1/9); elemental anal.
calcd for C33H30N2O4S: C 71.98, H 5.49, N 5.09, S 5.82; found, C
71.46, H 5.58, N 4.91, S 5.56.
3.3. Electrochemistry properties
To estimate the energy level of the HOMO of these USQs com-
pounds, their electrochemical properties were investigated by cy-
clic voltammetry. As shown in Fig. 3 and Table 2, during anodic
scan, quasireversible oxidation processes could be observed in the
two USQs compounds, and their Eonset ox values were determined
to be 0.30 and 0.23 V relative to Fc/Fcþ for USQ-11 and USQ-12,
respectively. Accordingly, the HOMO energy levels of USQ-11 and
USQ-12 were calculated to be ꢀ5.10 and ꢀ5.03 eV, respectively, by
comparison with the Fc/Fcþ redox couple whose energy level
is ꢀ4.80 eV in vacuum [30]. In comparison with the USQ-11, the
USQ-12 possessed 0.07 eV higher HOMO energy level, which may
be attributed to its stronger electron-donating capability of 2-
methylbenzothiazole than that of 2,3,3-trimethylindolenine
group. Therefore, higher Voc could be expected when USQ-11 was
used as electron-donors materials to fabricate OSCs [31]. Moreover,
the LUMO energy levels of USQ-11 and USQ-12 were calculated to
be ꢀ3.57 and ꢀ3.51 eV, respectively, which are deduced from their
HOMO levels and corresponding optical band-gaps [15].
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes to the USQs compounds are illustrated in
Scheme 1. Intermediate 1b and 2b were synthesized similarly
starting with Knoevenagel condensation reaction of compounds 1a
and 2a, respectively, with the 3,4-diethoxy-cyclobut-3-ene-1,2-
dione. Then intermediate 1c and 2c were obtained by hydrolysis
of compounds 1b and 2b, respectively. Next, they were further
condensed with compound 3a to afford the unsymmetrical squar-
aines USQ-11 and USQ-12. Both of the two objective USQs com-
pounds displayed good solubility in common organic solvents, such
as chloroform (>10 mg mLꢀ1
)
and 1,2-dichlorobenzene
(>15 mg mLꢀ1). Moreover, high quality thin films of the USQs
compounds could be obtained through spin-coating from solution,
suggesting that they are very suitable for solution-processing. As
shown in Fig. S1 (shown in SI), both of the two USQs compounds
exhibited excellent thermal stability over 270 ꢁC under N2 atmo-
sphere (vide. Table 1).
3.4. DFT calculation
To gain further insights into the effect of different D subunits on
the electronic properties of these USQs compounds, quantum
chemical DFT calculations were performed. As shown in Fig. 4, both
of the two USQs shown similar electronic structures, their HOMOs
are delocalized on the whole molecular skeleton, while their
LUMOs exhibited a few difference, since the indoline groups
contributed much to the HOMO but little to the LUMO. As shown in
Table 2, the HOMO energy levels of USQ-11 and USQ-12 were
calculated to be ꢀ5.19 and ꢀ5.12 eV, respectively, and the LUMO
3.2. Optical properties
The UV-vis absorption spectra of the USQs compounds in
chloroform solution and thin films are shown in Fig. 2, and the data

182
D. Yang et al. / Organic Electronics 32 (2016) 179e186
Scheme 1. Synthetic routes to the USQs compounds.
Table 1
Optical properties of the USQs compounds.
Egopt
Compound
Absorption labs max (nm)
FWHM (cmꢀ1
)
Td
Solution (ε105 Mꢀ1 cmꢀ1
)
Film
Solution
Film
(eV)
(ꢁC)
USQ-11
USQ-12
667 (2.41)
668 (1.91)
698, 648
672, 621
900
1080
3910
5060
1.53
1.52
271
283
Fig. 2. Absorption spectra of the USQs compounds in solution and thin films.
energy levels of USQ-11 and USQ-12 were calculated to be ꢀ2.93
and ꢀ2.87 eV, respectively, which well reproduce their corre-
sponding experimental values, indicating that these computational
results are reliable.
why USQ-12 mostly shown H- aggregation behavior [29], a geom-
etry preferred in the presence of strong dipole-dipole coupling,
which can be endow the compound with a more enhanced higher
hole mobility [22,32,33].
Moreover, as shown in Fig. 5, although the dipole moment of
both compounds pointed in the same direction, their magnitude
was quite different. USQ-11 (dipole moment 3.24 D) exhibited in
fact a sizably less pronounced charge transfer character than that of
USQ-12 (dipole moment 5.23 D). Such difference might explain
3.5. Hole mobility and morphology
The hole mobility of the two pristine USQs as well as the USQs/
PC71BM (1:5, wt%) blend film samples were evaluated by the space

D. Yang et al. / Organic Electronics 32 (2016) 179e186
183
charge limited current method [34], and the J-V characteristics of
the devices are shown in Fig. 6. The hole mobility of thꢀe1USQ-11
neat film was calculated to be 4.33 ꢂ 10ꢀ5 cm2 Vꢀ1
s , which
was much lower than that of the USQ-12 (8.61 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1),
which was ascribed to its much higher dipole moment. However,
the hole mobility of the USQ-11/PC71BM blend film was calculated
to be 2.03 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1, which is nearly 4 times higher than
that of the USQ-12/PC71BM (5.10 ꢂ 10ꢀ6 cm2 Vꢀ1
mobility in blend films is beneficial for Jsc.
s
ꢀ1). The higher
The morphologies of the blend films were investigated by AFM.
As shown in Fig. 7, both of the two USQs/PC71BM blend films
exhibited similar root mean square (RMS) of 1.32e1.45 nm, how-
ever, the USQ-12/PC71BM blend films displayed quite larger phase
separation than that of the USQ-11/PC71BM, which could be the
result of limited mixing with PC71BM. Therefore, the low hole
mobility of the USQ-12/PC71BM should be ascribed to the poor
miscibility.
3.6. Organic solar cells
To evaluate the photovoltaic performance of the two USQs
compounds, devices with structure of ITO/MoO3 (8 nm)/USQs
PC71BM (50 nm)/BCP (6 nm)/Al (100 nm) have been fabricated. The
photovoltaic properties of the OSCs devices are listed in Table 3, the
corresponding current density-voltage (J-V) curves, and external
quantum efficiency (EQE) plots are displayed in Figs. 8 and 9,
respectively. Firstly, the effect of different blend ratios of USQs/
PC71BM on photovoltaic properties were investigated. For the USQ-
11 compound, when the USQ-11/PC71BM blend ratios increased
from 1:1 to 1:3, the Jsc and FF of the devices improved significantly
from 6.41 to 10.21 mA cmꢀ2 and 0.40 to 0.47, respectively, while no
distinct changes can be observed for the Voc of the devices; when
Fig. 3. Cyclic voltammogram of the USQs compounds.
Table 2
Cyclic voltammetry and DFT calculated data for the USQs compounds.
HOMOa
(eV)
LUMOb
(eV)
HOMOc
(eV)
LUMOc
(eV)
mg
c
onest
Compound
E
ox
(D)
(V)
USQ-11
USQ-12
0.30
0.23
ꢀ5.10
ꢀ5.03
ꢀ3.57
ꢀ3.51
ꢀ5.19
ꢀ5.12
ꢀ2.93
ꢀ2.87
3.24
5.23
a
HOMO values derived from cyclic voltammetry measurements.
LUMO ¼ Egopt þ HOMO.
b
c
Data derived from DFT calculation.
Fig. 4. The electron density distributions of USQ-11 (left) and USQ-12 (right).
Fig. 5. The dipole moments of USQ-11 (left) and USQ-12 (right).

184
D. Yang et al. / Organic Electronics 32 (2016) 179e186
Fig. 6. J-V characteristic of the hole-only devices using the neat film of USQs (a), and USQs/PC71BM (1:5, wt%) blend films (b) as active layer.
1:3, the Jsc and FF of the devices enhanced from 7.15 to
9.34 mA cmꢀ2 and 0.41 to 0.45, respectively, and with a further
increased blend ratios from 1:3 to 1:7, the Jsc and FF of the devices
gradually decreased from 9.34 to 8.44 mA cmꢀ2 and 0.45 to 0.41,
respectively, the PCE dropped to 2.53%. Thus, the highest PCE
(3.03%) was obtained when the USQ-12/PC71BM blend ratio was
1:3, with Voc of 0.72 V, Jsc of 9.34 mA cmꢀ2 and FF of 0.45.
To investigate the effect of different D subunits on the photo-
voltaic properties of the two USQs compounds, the data of devices
fabricated in USQs/PC71BM blend ratio of 1:5 was used for discus-
sion. The Voc of USQ-11-based device was 0.86 V, 0.14 V higher than
that of the USQ-12. The results are not very consistent with the
HOMO energy levels of the USQ-11 as just 0.07 eV lower than that
of the USQ-12. This indicated that there are large voltage losses in
the USQ-12-based devices. Theoretically, the Voc of OSCs is limited
by the difference between the HOMO of the donor and the LUMO of
the acceptor [31]. However, charge transfer states, disorder induced
changes in the hole and electron quasi-Fermi levels, and electronic
coupling between the donor's ground state have been shown to
cause losses in the Voc [35e38]. In addition to the much enhanced
Voc, the USQ-11-based device also exhibited quite higher Jsc
(10.34 mA cmꢀ2) than that of the USQ-12 (8.97 mA cmꢀ2). In
principle, Jsc correlates highly with the light-harvesting capability,
carrier mobility, and the morphology of the active layer [39e41]. To
elucidate the origin of the enhanced Jsc in USQ-11-based device, the
EQE plots of the two devices were recorded. As shown in Fig. 9, the
USQ-11-based device exhibited a broader spectral response and
higher EQE values than that of USQ-12. Moreover, the hole mobility
of USQ-11/PC71BM blend film displayed nearly 4 times higher than
that of the USQ-12/PC71BM blend film. Thus, compared with the
USQ-12-based device, the enhanced Jsc of USQ-11-device should be
ascribed to the higher hole mobility of the blend film, better phase
separation as well as the broader absorption band. Therefore, 2,3,3-
trimethylindolenine subunit substituted USQ-11 displayed much
higher photovoltaic performance (PCE ¼ 4.27%) than that of 2-
methylbenzothiazole subunit substituted USQ-12 (PCE ¼ 2.78%).
Fig. 7. AFM phase images and height images of the USQs/PC71BM (1:5, wt%) blend
films, USQ-11/PC71BM (a,c) and USQ-12/PC71BM (b,d).
the blend ratios increased further from 1:3 to 1:7, there are not
distinct changes for the Voc, Jsc and FF of the devices. Thus, the best
blend ratio of USQ-11/PC71BM-based device was 1:5, with Voc of
0.86 V, Jsc of 10.34 mA cmꢀ2, FF of 0.48 and PCE of 4.27%. While for
the USQ-12/PC71BM system, the blend ratios increased from 1:1 to
Table 3
Photovoltaic performances of SMOSCs based on USQs: PC71BM.
a
a
Donor
Ratio
(D:A)
Voc
Jsc
(mA cmꢀ2
FFa
PCEa
(%)
(V)
)
4. Conclusion
USQ-11
1:1
1:3
1:5
1:7
1:1
1:3
1:5
1:7
0.86 (0.86)
0.85 (0.85)
0.86 (0.86)
0.86 (0.86)
0.73 (0.73)
0.72 (0.72)
0.72 (0.72)
0.73 (0.73)
6.41 (6.36)
10.21 (10.00)
10.34 (10.26)
10.33 (10.06)
7.15 (6.85)
9.34 (9.12)
8.97 (8.91)
8.44 (8.43)
0.40 (0.40)
0.47 (0.47)
0.48 (0.48)
0.47 (0.47)
0.41 (0.41)
0.45 (0.45)
0.43 (0.43)
0.41 (0.40)
2.20 (2.19)
4.08 (4.00)
4.27 (4.24)
4.17 (4.07)
2.14 (2.05)
3.03 (2.95)
2.78 (2.76)
2.53 (2.46)
In conclusion, the effect of D subunits on the optoelectronic
properties of small molecules have been evaluated by comparing
the two USQs compounds. By changing the different D subunits
attached to the squaric acid core from 2,3,3-trimethylindolenine to
2-methylbenzothiazole, two novel USQs compounds, namely USQ-
11 and USQ-12, were designed and synthesized. Both of them dis-
played similar absorption bands in their solutions, however,
USQ-12
a
The first values are the best data obtained, while the values in parentheses are
average values from 4 devices.

D. Yang et al. / Organic Electronics 32 (2016) 179e186
185
Fig. 8. J-V curves characteristics of SMOSCs devices with different blend ratios.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.02.009.
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China Scholarship Council and National Natural Science Foundation
of China (No. 51573108, 21190031, 21372168, 21432005). We are
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