A new small molecule with indolone chromophore as the electron accepting unit for efficient organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.08.027

A new small molecule (TIBDT) with indolone chromophore as the electron acceptor unit and benzodithiophene as electron donor unit was synthesized and first applied in organic solar cells (OSCs). TIBDT was characterized by NMR, TGA, UV-Vis absorption spectroscopy and cyclic voltammetry measurements. Results show that TIBDT possesses excellent thermal stability, appropriate absorption spectra, low-lying highest occupied molecular orbital (HOMO) level and high hole mobility. The OSCs based on TIBDT: PC₇₁BM (1:1, w/w) showed a power conversion efficiency (PCE) up to 3.94% with an open circuit voltage (V_oc) of 0.89 V, short circuit current (J_sc) of 7.36 mA cm^-2, fill factor (FF) of 60.2% under the illumination of AM 1.5 G, 100 mW cm^-2 without solvent additives and thermal annealing treatment. These preliminary investigations show that indolone chromophore can probably be an excellent electron accepting unit for constructing high performance optoelectronic materials.

Full text of DOI:10.1016/j.dyepig.2014.08.027

Dyes and Pigments 113 (2015) 458e464
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A new small molecule with indolone chromophore as the electron
accepting unit for efficient organic solar cells
,
,
b
,
, *
Ling Fan a, Ruili Cui a, Lihui Jiang a *, Yingping Zou a *, Yongfang Li c, Dong Qian a
a College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
b Institute of Super-Microstructure and Ultrafast Process in Advanced Materials, Central South University, Changsha 410083, China
c Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new small molecule (TIBDT) with indolone chromophore as the electron acceptor unit and benzodi-
thiophene as electron donor unit was synthesized and first applied in organic solar cells (OSCs). TIBDT
was characterized by NMR, TGA, UVeVis absorption spectroscopy and cyclic voltammetry measure-
ments. Results show that TIBDT possesses excellent thermal stability, appropriate absorption spectra,
low-lying highest occupied molecular orbital (HOMO) level and high hole mobility. The OSCs based on
TIBDT: PC71BM (1:1, w/w) showed a power conversion efficiency (PCE) up to 3.94% with an open circuit
voltage (Voc) of 0.89 V, short circuit current (Jsc) of 7.36 mA cmꢀ2, fill factor (FF) of 60.2% under the
illumination of AM 1.5 G, 100 mW cmꢀ2 without solvent additives and thermal annealing treatment.
These preliminary investigations show that indolone chromophore can probably be an excellent electron
accepting unit for constructing high performance optoelectronic materials.
Received 2 July 2014
Received in revised form
18 August 2014
Accepted 27 August 2014
Available online 7 September 2014
Keywords:
Indolone chromophore
Benzodithiophene
Organic solar cells
High open circuit voltage
High fill factor
© 2014 Elsevier Ltd. All rights reserved.
High performance
1. Introduction
molecular structure is one of the most successful structure to
satisfy these requirements [7]. For instance, the small molecule p-
Bulk heterojunction (BHJ) organic solar cells (OSCs) have
drawn broad attentions over the past decade because of their
advantages in lightweight, flexibility and simple manufacturing
process [1]. In the past few years, we have witnessed a significant
progress of BHJ OSCs with a PCE up to 9% [2] for single-layer BHJ
and 10% [3] for tandem solar cells. Though higher PCEs have been
obtained in polymer solar cells (PSCs), the imperfections of broad
molecular weight distribution, tedious purification and difficult
reproducibility restrict their wide applications [4]. Meanwhile,
small molecule photovoltaic materials are drawing increasing
attentions due to their certain molecular structure, definite mo-
lecular weight, high purity and excellent repeatability [5]. Lately,
considerable efforts on small molecule donor materials are dedi-
cated to design new molecular structures. To further increase the
efficiency of small molecule OSCs (SM-OSCs), the new donor
materials should exhibit: 1) good solubility for solution process-
ing; 2) a broad absorption to harvesting photons; 3) a relatively
low HOMO energy level to enhance the Voc and 4) high hole
mobility [6] Up to date, acceptoredonoreacceptor (AeDeA) small
DTS(FBTTh2)2 [8] and p-SIDT-(FBTTh2)2 [9], both of them exhibi-
ted a broad absorption, a low-lying HOMO energy level, a mod-
erate mobility and a fairly high PCE.
Recently, the isoindigo unit has attracted great interest as an
acceptor building block for conjugated polymers due to its large
coplanar and strong electron-withdrawing character. To date,
conjugated polymers based on isoindigo unit have led to PCEs over
7% [10]. However, the twist of isoindigo unit could reduce
donoreacceptor interactions and adversely affect the inter-chain
stacking [11]. Thus, the expanded isoindigo with two indolone
unit was introduced into organic photovoltaics. This structure could
expand the conjugated core effectively, and avoid the steric
repulsion between the protons on the phenyl rings and the
carbonyl oxygens of the oxindoles successfully. Up to now, PCE of
6.4% [11] has been obtained in PSC with expanded isoindigo as
acceptor unit. Nevertheless, to our knowledge, small molecules
based on the indolone unit for OSCs have rarely been reported.
Simultaneously, benzodithiophene (BDT) and BDT derivatives as
the core of donor materials are widely used in organic photovol-
taics, mostly due to its superior extended conjugation and planar
structure, which probably can enhance the FF and Jsc. Furthermore,
BDT-containing molecules usually have a low HOMO level [12],
which can provide a relatively high Voc.
* Corresponding authors. College of Chemistry and Chemical Engineering, Central
South University, Changsha 410083, China.
E-mail addresses: jianglh@csu.edu.cn (L. Jiang), yingpingzou@csu.edu.cn,
yingpingzou@mail.csu.edu.cn (Y. Zou), qiandong6@vip.sina.com (D. Qian).
http://dx.doi.org/10.1016/j.dyepig.2014.08.027
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

L. Fan et al. / Dyes and Pigments 113 (2015) 458e464
459
In this work, we designed and synthesized a new molecule
TIBDT based on BDT unit as electron donor core, indolone as the
electron acceptor and terminated with hexyl-substituted bithio-
phene. This small molecule exhibits broad absorption in visible
range, a low-lying HOMO energy level and a high mobility. All of the
OSC devices based on TIBDT blend with PC71BM exhibited a high FF
of above 50% and Voc nearly of 0.9 V. Especially when the weight
ratio of TIBDT and PC71BM was 1:1, a highest PCE of 3.94% was
achieved with a Voc of 0.89 V, Jsc of 7.36 mA cmꢀ2 and a FF of 60.2%
(see Scheme 1).
system. XRD patterns were obtained using BraggeBrentano Ge-
ometry ( e2 ) with Cu K radiation as an X-ray source in the
reflection mode at 45 kV and 300 mA. The morphologies of the
TIBDT/PC71BM blend films were investigated by a SPI 3800 N
atomic force microscope (AFM).
q
q
a
2.3. Fabrication and characterization of organic solar cells
The SM-OSC devices were fabricated on cleaned and UV/ozone
treated patterned ITO glasses. PEDOT:PSS (poly(3,4-ethylene diox-
ythiophene): poly(styrene sulfonate)) (Baytron PVP Al 4083, Ger-
many) was filtered through a 0.45
mm poly(tetrafluoroethylene)
2. Experimental
(PTFE) filter and spin coated at 3000 rpm for 40 s on the ITO sub-
strate as the first layer and was dried at 150 ꢁC for 15 min under
ambient atmosphere. The active layers were dissolved in chloro-
form (CHCl3) for half an hour, and then spin-cast at 2000 rpm for
60 s above the PEDOT:PSS layer. Films were allowed to dry for
30 min under inert atmosphere at the room temperature. Cathodes
were deposited by sequential thermal evaporation of 20 nm of
calcium followed by 80 nm of aluminum under a shadow mask
with a base pressure of ca. 10ꢀ5 Pa. The active area of the devices is
4 mm2. Device characterization was carried out under AM 1.5 G
irradiation with the intensity of 100 mW cmꢀ2 (Oriel 67005,
500 W), calibrated by a standard silicon cell. JeV curves were
recorded with a Keithley 236 digital source meter.
2.1. Materials
Palladium(0)tetrakis(triphenylphosphine)
(Pd(PPh3)4),
5-
formylthiophen-2-ylboronic acid (3) and 6-bromoindolin-2-one
(5) were obtained from Acros Organics, and they were used as
received. Other starting materials and reagents were purchased
commercially as analytically pure and used without further puri-
fication unless stated otherwise.
2.2. Characterization
All 1H NMR and 13C NMR spectra were recorded using an
AVANCE III 500 MHz spectrometer in Chloroform-d solution at
298 K with tetramethylsilane (TMS) as internal reference (0 ppm).
MALDI-TOF was performed on a Bruker Autoex III instrument.
Elemental analyses were performed on a Flash EA112 instrument.
UVeVis absorption spectra were recorded on a SHIMADZU UV-
2700 spectrophotometer. The film on quartz used for UVeVis
measurements were prepared from spin-coating chloroform solu-
tion. Thermogravimetric analysis (TGA) measurements were con-
ducted on a PerkineElmer TGA-7 with a heating rate of 10 K minꢀ1
under inert atmosphere. The electrochemical cyclic voltammetry
(CV) curve was recorded with a computer controlled Zahner IM6e
electrochemical workstation with platinum electrode (1.0 cm2), a
platinum wire and Ag/Agþ (0.1 M) as the working electrode,
counter electrode and reference electrode respectively in an
anhydrous and argon-saturated solution of 0.1 M tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution
with a scanning rate of 50 mV sꢀ1. X-ray diffraction (XRD) was used
to investigate the crystallinity of spin-coated TIBDT thin films
prepared on silicon substrates, XRD measurements of the polymer
thin film were carried out with a 2 kW Rigaku X-ray diffraction
2.4. Synthesis
Reagents and conditions:(a) DMF, NBS, rt, 24 h; (b) 5-
formylthiophen-2-ylboronic acid, potassium carbonate, Pd(PPh3)4,
90 ꢁC, 14 h; (c) 6-bromoindolin-2-one, piperidine, methanol, 70 ꢁC,
24 h; (d) DMF (40 mL), potassium carbonate, 2-ethylhexyl bromide,
100 ꢁC, 24 h; (e) toluene, Pd(PPh3)4, 110 ꢁC, 24 h.
2.4.1. 2-bromo-5-hexylthiophene (1)
A solution of 2-hexylthiophene (6.72 g, 40 mmol) in CHCl3
(100 mL) was placed in a 250 mL three-necked flask at 0 ꢁC, and
then DMF (40 mL) with NBS (7.12 g, 40 mmol) mixture was added
dropwisely. The reaction mixture was warmed up to room tem-
perature and stirred for overnight absent from light, after this the
mixture was poured into water, extracted by CH2Cl2, organic phase
was washed several times with sodium hydrogen carbonate
aqueous solution, and dried by a rotary evaporator to give a crude
product. Further purification was carried out by column chroma-
tography on the silica gel using petroleum ether as the eluent to
S
S
S
S
S
S
S
N
O
N
O
S
Scheme 1. Structure of TIBDT.

460
L. Fan et al. / Dyes and Pigments 113 (2015) 458e464
obtain a colorless liquid (9.1 g, yield 92%). 1H NMR (500 MHz, CDCl3)
2.4.5. Synthesis of TIBDT
Using 50 mL one-necked flask, compound 4 (0.145 g,
d
6.84 (d, J ¼ 3.6 Hz, 1 H), 6.53 (d, J ¼ 3.5 Hz, 1 H), 2.74 (t, J ¼ 7.6 Hz,
a
2 H), 1.68e1.58 (m, 2 H), 1.42e1.22 (m, 6 H), 0.99e0.82 (m, 3H). MS
(m/z) for C10H15BrS: calcd. 246.01; found, 246.01 (Mþ). Elemental
Anal. calcd for C10H15BrS (%):C, 48.59; H, 6.12; Br, 32.32; S, 12.97.
Found (%): C, 48.62; H, 6.13; S, 13.01.
0.25 mmol) and 5 (0.0905 g, 0.1 mmol) was dissolved in toluene
(10 mL), the solution was flushed with N2 for 20 min, and then Pd
(PPh3)4 (10 mg) was added into the solution, the reaction mixture
was flushed with N2 for another 25 min. The reaction mixture was
heated to 110 ꢁC gradually and stirred for 24 h at this temperature
under N2 atmosphere. After cooled to room temperature, the
mixture was poured into methanol, stirred for 10 min and then
filtered to give a crude product. Crude product was further purified
by column chromatography on silica gel using petroleum ether/
dichloromethane (2:1, v/v) as the eluent. The product was then
recrystallized from chloroform and dried under vacuum at 40 ꢁC
overnight to obtain a red solid (80 mg, yield 50%). 1H NMR
2.4.2. 50-hexyl-2,20-bithiophene-5-carbaldehyde (2)
Using a 250 mL three-necked flask, 2-bromo-5-hexylthiophene
(1) (2.47 g, 10 mmol) and 5-formylthiophen-2-ylboronic acid
(1.716 g, 11 mmol) were dissolved in toluene (120 mL), then ethanol
(40 mL) and potassium carbonate (2.76 g, 20 mmol) was added, the
solution was flushed with N2 for 20 min, and Pd(PPh3)4 (0.45 g) was
added into the solution. The mixture was flushed with N2 for
another 25 min. The reaction mixture was heated to 90 ꢁC gradu-
ally, and stirred for 14 h at this temperature under N2 atmosphere.
After cooled to room temperature, the mixture was poured into
water, extracted with ethyl acetate. The organic phase was washed
with water, saturated sodium chloride solution and dried with
anhydrous magnesium sulfate. After filtration, the organic phase
was concentrated under reduced pressure at 40 ꢁC. Crude product
was further purified by column chromatography on silica gel using
petroleum ether/dichloromethane (1:1, v/v) as the eluent to obtain
a pale yellow solid (1.53 g, yield 55%). 1H NMR (500 MHz, CDCl3)
(500 MHz, CDCl3)
d
7.66 (s, 2H), 7.35 (d, J ¼ 11.8 Hz, 4H), 7.30 (s, 2H),
7.24 (d, J ¼ 7.8 Hz, 2H), 7.12 (dd, J ¼ 18.5, 6.0 Hz, 4H), 7.01e6.91 (m,
6H), 6.66 (d, J ¼ 3.5 Hz, 2H), 3.72 (d, J ¼ 6.9 Hz, 4H), 2.94 (d,
J ¼ 6.1 Hz, 4H), 2.75 (s, 4H), 1.97e1.71 (m, 6H), 1.65 (d, J ¼ 15.4,
7.6 Hz, 4H), 1.58e1.21 (m, 40H), 1.09e0.83 (m, 32H). 13C NMR
(125 MHz, CDCl3)
d 166.84, 147.20, 146.13, 145.61, 144.24, 141.77,
138.38, 137.26, 136.03, 134.61, 133.39, 129.94, 127.73, 125.88, 124.51,
124.28, 124.19, 124.18, 123.02, 119.24, 118.58, 105.46, 44.04, 41.43,
37.99, 34.46, 32.60, 31.53, 30.87, 30.27, 29.36, 29.13, 28.59, 27.20,
25.73, 24.22, 23.14, 22.63, 14.16, 11.01. MS (m/z) for C96H116N2O2S8:
calcd. 1584.68; found, 1584.71 (Mþ). Elemental Anal. calcd for
d
9.61 (d, J ¼ 7.7 Hz, 1H), 7.54e7.48 (m, 1H), 7.24 (d, J ¼ 3.9 Hz, 1H),
6.72 (d, J ¼ 3.6 Hz, 1H), 6.43 (dd, J ¼ 15.5, 7.7 Hz, 1H), 2.81 (t,
J ¼ 7.6 Hz, 2H), 1.75e1.63 (m, 2H), 1.44e1.23 (m, 6H), 0.94e0.83 (m,
3H). MS (m/z) for C15H18OS2: calcd. 278.08; found, 278.08 (Mþ).
Elemental Anal. calcd for C15H18OS2 (%): C, 64.71; H, 6.52; O, 5.75; S,
23.03. Found (%): C, 64.75; H, 6.47; S, 23.06.
C96H116N2O2S8 (%): C, 72.68; H, 7.37; N, 1.77; O, 2.02; S, 16.17. Found
(%): C, 72.73; H, 7.34; N, 1.75.
3. Results and discussion
3.1. Synthesis and characterization
2.4.3. (Z)-6-bromo-3-((50-hexyl-2,20-bithiophen-5-yl)methylene)
indolin-2-one (3)
The general synthetic route for TIBDT is outlined in Scheme 2.
The BDT unit was synthesized according to the previous literature
[13]. TIBDT was synthesized by Stille-coupling reaction between
compound 4 and 5 as a red powder. The molecular structure of
TIBDT was verified by 1H NMR and 13CNMR. The NMR spectra were
shown in the supporting information (Figs. S1 and S2). The corre-
sponding proton positions and numbers are in accordance with the
chemical structures of TIBDT. TIBDT is readily soluble in common
organic solvents such as chloroform, toluene, chlorobenzene and o-
dichlorobenzene.
In a 50 mL flask, 50-hexyl-2,20-bithiophene-5-carbaldehyde (2)
(0.556 g, 2 mmol), 6-bromoindolin-2-one (0.53 g, 2.5 mmol),
methanol (20 mL), piperidine (0.2 mL) were mixed and refluxed at
70 ꢁC under N2 for 24 h. The reaction mixture was cooled to room
temperature and filtered, the precipitate was washed with meth-
anol three times and dried under vacuum at 40 ꢁC overnight to give
a red solid and the product was used directly in the next step
without further purification.
2.4.4. (Z)-6-bromo-1-(2-ethylhexyl)-3-((50-hexyl-2,20-bithiophen-
5-yl)methylene)indolin-2-one (4)
3.2. Thermal stability
Compound 3 (0.94 g, 2 mmol), DMF (40 mL), potassium car-
bonate (2.76 g, 20 mmol) was mixed under N2 and stirred for half an
hour, then 2-ethylhexyl bromide (1.93 g, 10 mmol) was added. The
reaction mixture was heated to 100 ꢁC carefully in oil bath and
stirred for 24 h under N2 atmosphere at this temperature. After
cooled to room temperature, the reaction mixture was poured into
water, extracted with CH2Cl2. The organic phase was washed with
water, saturated sodium chloride solution and dried with anhy-
drous magnesium sulfate. After filtration, the organic phase was
concentrated under reduced pressure at 40 ꢁC. Crude product was
further purified by column chromatography on silica gel using
petroleum ether/dichloromethane (4:1, v/v) as the eluent, then
recrystallized from ethanol to obtain the target product (4) as a red
The thermal property of TIBDT was investigated by thermog-
ravimetric analysis (TGA) under nitrogen atmosphere. The results
(shown in Fig. 1) reveal that TIBDT has excellent thermal stability
with thermal decomposition temperatures (5% weight loss) over
350 ꢁC, which indicates that TIBDT is thermally stable enough for
device fabrication.
3.3. Optical and electrochemical properties
The photophysical characteristics of TIBDT were investigated by
UltravioleteVisible (UVeVis) absorption spectra. As shown in Fig. 2,
there are two dominating absorption bands between 300 nm and
600 nm for the solution, one from 300 nm to 375 nm with a peak at
325 nm, the other from 375 nm to 600 nm with a peak at 514 nm.
solid (0.35 g, yield 30%). 1H NMR (500 MHz, CDCl3)
d 7.69e7.61 (m,
2H), 7.37 (d, J ¼ 8.1 Hz, 1H), 7.25 (d, J ¼ 3.6 Hz, 1H), 7.19 (ddd,
J ¼ 10.9, 5.2, 2.8 Hz, 2H), 7.00 (dd, J ¼ 9.2, 5.0 Hz, 1H), 6.76 (t,
J ¼ 5.1 Hz, 1H), 3.78e3.63 (m, 2H), 2.84 (dd, J ¼ 14.7, 7.2 Hz, 2H),
2.10e1.81 (m, 5H),1.50e1.20 (m, 12H), 1.02e0.83 (m, 10H). MS (m/z)
for C31H38BrNOS2: calcd. 583.16; found, 583.16 (Mþ). Elemental
Anal. calcd for C31H38BrNOS2 (%):C, 63.68; H, 6.55; Br,13.67; N, 2.40;
O, 2.74; S, 10.97. Found (%): C, 63.71; H, 6.53; N, 2.38.
The first transition around 325 nm might be assigned to
pep*
transition and the second band in the 514 nm region can be
attributed to an intramolecular charge transfer (ICT) between the
BDT donor block and the electron acceptor indolone group [14].
Compared to the solution absorption, the absorption peak in thin
film state is red-shifted about 20 nme535 nm, indicating that

L. Fan et al. / Dyes and Pigments 113 (2015) 458e464
461
Scheme 2. Synthesis route to TIBDT. (a) DMF, NBS, rt, 24 h; (b) 5-formylthiophen-2-ylboronic acid, potassium carbonate, Pd(PPh3)4, 90 ꢁC, 14 h; (c) 6-bromoindolin-2-one,
piperidine, methanol, 70 ꢁC, 24h; (d) DMF (40 mL),potassium carbonate, 2-ethylhexyl bromide, 100ꢁC, 24 h; (e) toluene, Pd(PPh3)4, 110 ꢁC, 24 h.
TIBDT has larger intermolecular interactions in solid state than that
in solution. The optical bandgap ðEgoptÞ deducing from TIBDT film
absorption edge (ledge) is 1.90 eV according to Egopt ¼ 1240/ledge. The
Film
Solution
1.0
optical parameters of TIBDT were listed in Table 1.
Electrochemical property of TIBDT was evaluated by cyclic vol-
tammetry (CV) in a 0.1 mol Lꢀ1 solution of Bu4NPF6 in acetonitrile
under argon at room temperature, with a scanning rate of
100 mV sꢀ1 (shown in Fig. 3). The HOMO level of TIBDT could be
estimated according to the onset oxidation potential, the LUMO
(lowest unoccupied molecular orbital) energy level could be
calculated by Egopt and HOMO level. Assuming the absolute energy
level of ferrocene/ferrocenium (Fc/Fþc ) to be ꢀ4.8 eV below vacuum
[15], the relative potential of Fc/Fþc is 0.09 V against Ag/Agþ. The
onset oxidation potential of TIBDT was 0.49 V vs. Ag/Agþ, thus the
0.8
0.6
0.4
0.2
0.0
TIBDT
100
300 350 400 450 500 550 600 650 700
Wavelength (nm)
90
80
70
60
50
40
Fig. 2. UVeVis absorption spectra of TIBDT in chloroform solution and in thin film.
Table 1
Optical and electrochemical properties of TIBDT.
Small
molecule
Absorption spectra
Cyclic voltammetry
Solutiona
Filmb
p-doping
n-doping
c
lmax (nm)
lmax
ledge
Egopt
Eox/HOMOd
LUMOd
(eV)
(eV)
(nm)
(nm)
(V)/(eV)
TIBDT
514
535
650
1.90
0.49/ꢀ5.20
ꢀ3.30
0
100
200
300
400
500
a
Measured in chloroform solution.
Cast from chloroform solution.
o
b
c
Temperature ( C)
Bandgap estimated from the onset wavelength of the optical absorption.
Fig. 1. TGA curve of TIBDT with a heating rate of 10 K minꢀ1
.
HOMO ¼ ꢀe(Eox þ 4.71) (eV); LUMO ¼ (HOMOþEgopt) (eV).
d

462
L. Fan et al. / Dyes and Pigments 113 (2015) 458e464
-26
TIBDT
TIBDT
-27
-28
-29
-30
-31
-32
-33
0.04 mA
-34
0.0
0.5
Potential
1.0
vs Ag/Ag
1.5
0
100
200
300
400
500
600
700
800
+
)
(
V
0.5
0.5
(V/d) (V/cm)
Fig. 3. Cyclic voltammogram of TIBDT film on platinum electrode in 0.1 mol Lꢀ1
Bu4NPF6, CH3CN solution.
Fig. 5. ln(Jd3/V2) vs (V/d)0.5 plot of the TIBDT/PC71BM blend for the hole mobility by
SCLC method.
HOMO energy level of TIBDT could be calculated according to the
following equations: HOMO ¼ ꢀe(Eoxþ 4.71) (eV) [16]. We can get
LUMO levels according to the equation: LUMO ¼ Egopt þ HOMO (eV)
[17]. The corresponding HOMO energy level and LUMO energy level
of TIBDT was ꢀ5.20 eV and ꢀ3.30 eV, respectively. All electro-
chemical parameters of TIBDT were listed in Table 1. Results show
that TIBDT has a deep HOMO level, implying that this small
molecule is relatively stable against oxidization. Meanwhile, a low-
lying HOMO level is beneficial to obtaining high Voc [18].
a short and close
thus confirms the crystalline nature. And this compact
stacking and ordered structure of TIBDT are probably beneficial for
charge transportation, then leading to an increase in FF [19].
pep distance of 3.62 Å. The diffraction pattern
pep
4
TIBDT/PC71BM=2:1
(a)
TIBDT/PC71BM=1:1
2
TIBDT/PC71BM=1:2
TIBDT/PC71BM=1:3
3.4. X-ray diffraction (XRD)
0
As well known, solid state packing can affect the photovoltaic
performance of conjugated materials, thus XRD measurement was
used to investigate the crystalline structure of TIBDT film. Fig. 4
shows the XRD curve of this small molecule cast from chloro-
form. The XRD pattern shows two resolved peaks. The d-spacing
-2
-4
-6
-8
values can be calculated using Bragg0s equation, 2dsin
q
¼ n
l. The
first peak is positioned at 2
q
¼ 5.45ꢁ corresponding to an ordered,
self-organized lamellar structure with an interlayer distance of
16.21Å, which is formed by parallel stacks of main chains that are
separated by the alkyl side-chains. The diffraction peak at larger
angles is much broader and less intense, its position indicates that it
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
is a second order reflection. The peak at 2
q
¼ 24.40ꢁ corresponds to
70
TIBDT:PC71BM=2:1
TIBDT:PC71BM=1:1
TIBDT:PC71BM=1:2
TIBDT:PC71BM=1:3
(b)
TIBDT
60
50
40
30
20
10
0
300
400
500
600
700
Wavelength(nm)
10
20
30
40
50
2 Theta (Degree)
Fig. 6. (a) JeV curves of OSCs based on TIBDT:PC71BM (2:1,1:1,1:2 and 1:3, w/w), under
illumination of AM 1.5G, 100 mW cmꢀ2 (b) EQE spectra of the OSCs based on
TIBDT:PC71BM (2:1,1:1,1:2 and 1:3, w/w).
Fig. 4. XRD spectrum of TIBDT film.

L. Fan et al. / Dyes and Pigments 113 (2015) 458e464
463
Table 2
typical current densityevoltage (JeV) curves based on TIBDT:
PC71BM blend with different weight ratios under the illumination
of AM 1.5G, 100 mW cmꢀ2, the corresponding parameters of these
devices are summarized in Table 2. Results show that all the devices
possess promising Voc (0.88e0.91 V), which are originated from the
low-lying HOMO level of TIBDT. Obviously, the best performance
was obtained at a D/A weight ratio of 1:1, the device possesses a PCE
of 3.94% with a Voc of 0.89 V, a Jsc of 7.36 mA cmꢀ2 and a FF of 60.2%.
When decreasing D/A weight ratio to 1:2 and 1:3 or increasing to
2:1, slightly lower Jsc and FF were obtained, which resulted in a
decreased PCE of 2.43%, 1.32% and 3.25%, respectively. To verify this
result, external quantum efficiency (EQE) (as shown in Fig. 6b) was
characterized. All EQE curves revealed a broad wavelength range
between 300 nm and 700 nm, and exhibited a maximum EQE value
of 50% at 546 nm with a D/A weight ratio of 1:1. These results were
consistent with Jsc. Furthermore, the Jsc calculated by integrating
the EQE curve under AM 1.5G reference spectrum matches well
with the Jscvalue obtained from the JeV measurements within an
experimental error due to the spectral mismatch of the solar
simulator [21].
Photovoltaic parameters of the OSCs based on TIBDT:PC71BM blends with different
D/A weight ratios, under the illumination of AM 1.5 G, 100 mW cmꢀ2
.
Active layer
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCE (%)
TIBDT:PC71BM ¼ 2:1
TIBDT:PC71BM ¼ 1:1
TIBDT:PC71BM ¼ 1:2
TIBDT:PC71BM ¼ 1:3
0.91
0.89
0.88
0.90
6.79
7.36
5.01
2.72
52.52
60.23
55.02
53.73
3.25
3.94
2.43
1.32
3.5. Hole mobility
We measured the hole mobility of TIBDT with the SCLC model
using a device structure of ITO/PEDOT:PSS/TIBDT:PC71BM (1:1, w/
w)/Au [20]. The results are plotted as ln(Jd3/V2) vs (V/d)0.5, as shown
in Fig. 5. Here, J means current density, d is the thickness of the
device, and V ¼ VapplꢀVbi, where Vappl is the applied potential and
Vbi is the built-in potential. SCLC model can be described by the
"
#
qffiffiffiffiffiffiffiffiffiffi
VꢀVbi
d
ðVꢀVbi
Þ
equation:JSCLC
¼
98 εoεrmo
2 exp 0:89g
, and the hole
d3
mobility of TIBDT can be calculated to be 3.3*10ꢀ2 cm2 Vꢀ1 sꢀ1. This
reasonable hole mobility is helpful to obtaining high PCE.
3.7. Morphology
3.6. Photovoltaic property
In order to characterize the surface morphology of the active
laver, the active layer surface were measured by tapping-mode
atomic force microscopy (AFM), which is kept in close proximity
(or contact) to the sample by a controller. As shown in Fig. 6, the
root mean square roughness (RMS) of these films were 103.2, 2.78,
8.6, 3.77 nm for the D/A weight ratio of 2:1, 1:1, 1:2, 1:3, respec-
tively. The difference of RMS indicates that the morphology
In order to estimate the photovoltaic potential of TIBDT as donor
material, devices were fabricated with
TIBDT:PC71BM/Ca/Al configuration. Devices with different donor/
acceptor (D/A) weight ratios (2:1, 1:1, 1:2 and 1:3) were fabricated
to explore the optimal device performance. Fig. 6a shows the
a
ITO/PEDOT:PSS/
Fig. 7. AFM surface morphology (a), (b), (c) and (d) of the TIBDT: PC71BM blend films cast from chloroform solutions with different ratios (2:1, w/w), (1:1, w/w), (1:2, w/w) and (1:3,
w/w), respectively.

464
L. Fan et al. / Dyes and Pigments 113 (2015) 458e464
changed in the mixed films by the increase of the PC71BM. On the
other hand, the active layer with the D/A weight ratio of 1:1
(Fig. 7(b)) shows the smoothest surface and best phase separation,
which can improve the exciton diffusion, thus leading to a higher
value of Jsc and the highest FF [22]. Fill factor is not only due to the
film quality, but also the unbalanced charge mobility and molecular
packing of the blend layer. Obviously, the film with donor:acceptor
of 2:1 (w/w) showed the largest domains of phase separation and
RMS, which led to its lowest FF. Device with donor: acceptor of 1:3
(w/w) showed comparable RMS and slightly different phase sepa-
ration, compared to 1:2 (w/w), which led to slightly different FF.
Generally, for the active layers with donor/acceptor weight ratios of
2:1, 1:2 and 1:3 (Fig. 7(a), (c) and (d) respectively), the films un-
dergo significant phase separation, forming large plate-shaped
crystallites during blend film deposition, which is not beneficial
for exciton diffusion and charge generation, [23] thus resulting in
reduced FF, so that lower PCEs of the OSCs were obtained. Obvi-
ously, the better phase separation combined with higher EQE can
account for the better photovoltaic properties for the OSCs based on
TIBDT:PC71BM with a weight ratio of 1:1.
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4. Conclusions
In summary, a new BDT-based small molecule with indolone
chromophore as the electron accepting unit-TIBDT, was firstly
synthesized and applied in OSCs. TIBDT possesses excellent solu-
bility and thermal stability, appropriate absorption spectra, mod-
erate hole mobility and low-lying HOMO level. The BHJ OSCs device
based on TIBDT:PC71BM (1:1, w/w) as the active layer afforded a PCE
up to 3.94% with a Voc of 0.89 V, Jsc of 7.36 mA cmꢀ2, FF of 60.2%
without solvent additives or thermal annealing treatment. With the
rational structure modifications from TIBDT, we can firmly expect
higher PCE after further device optimization. These investigations
(especially outstanding Voc and high FF) show that indolone chro-
mophore can probably be an excellent electron accepting unit for
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Acknowledgments
This work was supported by NSFC (Nos. 51173206,
21161160443), National High Technology Research and Develop-
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Appendix A. Supplementary data
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