Novel A-D-A type small molecules with β-alkynylated BODIPY flanks for bulk heterojunction solar cells

Article abstract of DOI:10.1016/j.orgel.2017.06.054

A serial of novel A-D-A type small molecules with BODIPY linked through alkynyl with various electron donor units such as fluorene, carbazole, benzodithiophene and phenothiazine, namely F-BDP, C-BDP, B-BDP and P-BDP, respectively, were designed and synthe

Full text of DOI:10.1016/j.orgel.2017.06.054

Accepted Manuscript
Novel A-D-A type small molecules with β-alkynylated BODIPY flanks for bulk
heterojunction solar cells
Junxu Liao, Yongjun Xu, Hongbin Zhao, Qiao Zong, Yutang Fang
PII:
S1566-1199(17)30311-7
10.1016/j.orgel.2017.06.054
ORGELE 4179
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 11 April 2017
Revised Date: 19 June 2017
Accepted Date: 20 June 2017
Please cite this article as: J. Liao, Y. Xu, H. Zhao, Q. Zong, Y. Fang, Novel A-D-A type small molecules
with β-alkynylated BODIPY flanks for bulk heterojunction solar cells, Organic Electronics (2017), doi:
10.1016/j.orgel.2017.06.054.
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ACCEPTED MANUSCRIPT
Four A-D-A type BODIPY derivatives with ethinyl bridges were designed and
synthesized for small molecule organic solar cells (SM-OSCs). All these molecules
show appealing photovoltaic performance. Among them, B-BDP/PC BM based OSC
7
1
exhibit not only good photoelectric characteristic but also superior charge transport
property and favorable nanoscale morphology relative to the other counterparts and
2
thus pronounce to a higher PCE of 4.65% with a J of 11.84 mA cm and a FF of
sc
5
3.8%, which is one of the best values in BODIPY based OSCs.

ACCEPTED MANUSCRIPT
Novel A-D-A type small molecules with β-alkynylated BODIPY flanks for bulk
heterojunction solar cells
a, b
a, ∗
a, ∗
a
b
Junxu Liao , Yongjun Xu , Hongbin Zhao , Qiao Zong , and Yutang Fang
a
School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan
23808, People’s Republic of China
5
b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641,
People’s Republic of China
Abstract:
A serial of novel A-D-A type small molecules with BODIPY linked through alkynyl with various
electron donor units such as fluorene, carbazole, benzodithiophene and phenothiazine, namely
F-BDP, C-BDP, B-BDP and P-BDP, respectively, were designed and synthesized. Introducing
the alkynyl bridge leads to extending the molecular absorption spectrum to the range of 320-700
5
-1
-1
nm with high molar extinction coefficients (10 cm M ) and strong fluorescence quenching. The
molecules showed relatively low HOMO ranging from −5.02 to −5.24 eV as estimated from cyclic
voltammetry measurements. Interestingly, B-BDP with BDT as donor exhibits more obviously
red-shifted absorption in the solid state compared to F-BDP, C-BDP and P-BDP. Furthermore,
the solution-processed bulk-heterojunction organic solar cell based on B-BDP/PC BM present
7
1
superior charge transport property and more favorable nanoscale morphology, resulting in a
2
significant higher J of 11.84 mA cm and FF than the other counterparts, thus achieved a higher
sc
∗
∗
Corresponding author. Tel.: +86-139-2583-6155; fax: +86-769-2286-2605; e-mail: hnllxyj@126.com.
Corresponding author. Tel.: +86-158-1831-8389; fax: +86-769-2286-2601; e-mail: zhaohbhanlf@163.com.

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PCE of 4.65%, which is one of the best values among the ever reported BODIPY based organic
solar cells.
Keywords: BODIPY derivatives, organic solar cells, acceptor-donor-acceptor, optoelectronic
properties, DFT calculations, photovoltaic performances
1
. Introduction
Solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) are promising to solve
the growing energy crisis and environmental deterioration worldwide. In the past decades,
BHJ-OSCs have attracted enormous attention due to the advantages of low-cost, light-weight, and
flexibility [1-5]. Depending on the molecular weight, the organic semiconductors used in OSCs
can be either polymers or π-conjugated small molecules (SMs), both of which have achieved
remarkable power conversion efficiencies (PCE) of over 10% in single-junction devices [6-11]. In
comparison with polymer counterparts, SMs present many additional merits such as well-defined
molecular structures, low molecular weight, simple purification and superior batch-to-batch
reproducibility, and have the potential to produce low-cost and large-scale OSCs for commercial
applications in the near future [12-13], and there is a wide scope to enhance the PCE of
BHJ-OSCs based on SMs.
In the small molecule based organic solar cells (SM-OSCs), for achieving high efficiency
photovoltaic properties, the donor materials should be tailored to combine the following
characteristics: (i) relatively low optical absorption band gaps and high absorption coefficients to
guarantee efficient photo-response [14-16], (ii) suitable energy level offsets with a fullerene
acceptor to ensure efficient charge separation [17-20], (iii) extended π-electron delocalization and
backbone planarity to lead to high charge mobility [21-22]. A common method to develop

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efficient photovoltaic materials with high light harvesting ability is construction of narrow band
gap molecules by using donor-acceptor (D-A) structure [2, 23-24]. Another strategy to promote
molecular light harvesting ability in OSCs is introducing the dye building blocks such as
rhodanine [25-28], DPP [29-32], porphyrin [33-36], and so on [37].
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) have proved to be highly versatile dyes
owing to their remarkable photophysical properties including strong ground-state absorption,
outstanding redox activity, intense fluorescent emission, high photoluminescence quantum yield
and excellent chemical stability [38-39]. Moreover, BODIPY can be easily tailored by chemical
transformation and their photophysical properties can be readily tuned by structural modifications
at the periphery and at the boron atom of the BODIPY skeleton [40-43], which provide additional
opportunities to meet the different requirements for diverse applications. Roncali et al. reported
the first examples of BODIPY donors involving two styryl units along with PC BM as the
61
electron acceptor for solution-processed BHJ-OSCs [44]. Subsequently, other groups reported
some BHJ-OSCs based on BODIPY SMs and polymers [45-49]. In 2016, our group reported a
serial of D-A-D type BODIPY derivatives with various electron donors at 2,6-positions for
BHJ-OSCs, and the best device exhibits 2.15% PCE [50]. Sharma et al. have demonstrated the
high efficiency OSCs with alkynyl carbazole modified BODIPY derivatives as the donor materials,
which marked one of the significant advances of this dye family's applications [51]. Very recently,
Bulut et al. reported a BODIPY dye, BET, with triazatruxene groups linked at 2,6-positions of
BODIPY core through alkynyl bridge. An encouraging PCE of 5.8%, with a high FF of 65%, was
achieved in BHJ-OSCs [52]. However, although BODIPY derivatives showed great potential as
donor materials for OSCs, to date, the best PCE based on BODIPY were in 4.7%-6% range

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[44-52], which are moderate values in SM-OSCs. This is mainly due to the fact that the
photovoltaic materials based on BODIPY derivatives have not been fully developed, leading to the
high-performance devices are still rare. Thus, exploring novel excellent BODIPY based
photovoltaic materials is challengeable and is urgently needed.
In BODIPY based SM-OSCs, the device performance is often limited by the inappropriate
molecular structure and the unfavorable morphology. To overcome these shortcomings and exploit
novel high-efficiency materials, in this work, we design and synthesize four A-D-A type small
molecules with BODIPY linked through alkynyl with various electron donor units such as
fluorene, carbazole, benzodithiophene and phenothiazine, namely F-BDP, C-BDP, B-BDP and
P-BDP, respectively (Chart 1). The molecular design rationale is as follow: First, introducing
ethinyl can effectively reduce the dihedral angle between the eletron donor and BODIPY core,
consequently achieve a coplanar structure. The coplanar molecular geometry is expected to
contribute to greater crystallinity, improved charge transport, and a lower band gap of the
semiconductor. Second, BODIPY intrinsiclly possess strong electronegativity and intense
absorption. Considering that fluorene [53], carbazole [53], benzodithiophene [54] and
phenothiazine [55] are excellent electron-rich building blocks in the organic photoelectric material,
the designed D-A-D molecules are supposed to induce intramolecular charge transfer and lead to
broad and strong absorption throughout the visible and even NIR region. Third, compared with
fluorene and carbazole, S containing benzodithiophene and phenothiazine have the ability to form
additional intermolecular non-covalent SꢀꢀꢀS band to strengthen the intermolecular interaction,
which induce positive effects on electronic properties, charge transport, film morphology, and
photovoltaic properties. With this comparative series, we are able to investigate these effects. The

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thermal stability, physical properties, and electrochemical behavior of these SMs are
systematically studied. The BHJ-OSC devices using the new BODIPY derivatives as electron
donors combined with [6,6]-phenyl-C -butyric acid methyl ester (PC BM) as electron acceptor
7
1
71
are fabricated and their photovoltaic performances are investigated to insight their potential
photovoltaic applications. Significant effect of central electron donating units on these
opto-electronic properties was observed in these SMs. The best photovoltaic performance with a
2
PCE of 4.65% and a short circuit current density of 11.84 mA cm were obtained in the B-BDP
based OSCs.
2
. Experimental section
2
.1 Materials
4-hydroxybenzaldehyde, 2,7-dibromo-9H-fluorene, 3,6-dibromo-9H-carbazole, pyrrole, iodine
chloride, 1-bromooctane, trimethylsilylacetylene, bis(triphenylphosphine)palladium(II)dichloride,
copper(I) iodide, indium(III) chloride, 4,4,4',4',5,5,5',5'-octamethyl-2,2'-di(1,3,2-dioxaborolane),
boron trifluoride ether complex and 4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile
(TCQ) were purchased from commercial sources. 4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene
(11) and 10-octyl-10H-phenothiazine (15) were prepared as described in the literature [56-57].
Pyrrole was distilled from CaH before use. THF and toluene were dried and distilled over sodium
2
and benzophenone. Dichloromethane and methanol were dried under 4A molecular sieves.
Column chromatography was performed on silica (300-400 mesh).
2
.2 Characterization and measurement
1
13
H NMR and C NMR spectra were recorded on a Bruker Avance spectrometer (600 and 151

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MHz, respectively) with tetramethylsilane (TMS, 0 ppm) as the internal standard using CDCl as
3
solvent. Matrix-assisted laser desorption/ionization time of flight mass spectra (MALDI-TOF-MS)
were recorded on a Bruker ultraflex-II spectrometer using α-cyano-4-hydroxyl-cinnamic acid
(α-CHCA) matrix. High resolution mass spectra (HRMS) were recorded on an Agilent 6530
Accurate Mass Q-TOF LC-MS instrument. Thermogravimetric analysis (TGA) was carried out
o
-1
o
using a Netzsch TG 209 analyzer at a heating rate of 10 C min up to 600 C. Differential
scanning calorimetry (DSC) was recorded on a NETZSCH DSC 200F3 instrument. UV-vis
absorption spectra were collected on an SHIMADZU UV-2550 UV-vis spectrophotometer and
emission spectra were recorded by using a Hitachi F-4500 instrument. Emission spectra were fully
corrected by reference to a standard lamp. Cyclic voltammetric (CV) was carried out on a
CHI660E electrochemical workstation utilizing the three electrode configuration consisting of a
gold electrode (working electrode), platinum wire (auxillary electrode) and Ag/AgCl electrode
(
reference electrode). The experiments were carried out in CH Cl solution using 0.1 M
2 2
tetrabutylammonium hexafluorophosphate (n-Bu NPF ) as supporting electrolyte. The solution
4
6
was deoxygenated at room temperature under nitrogen protection. The transmission electron
microscope (TEM) was carried out on a FEI Tecnai T20 instrument. X-ray diffraction (XRD)
experiments were performed on a Rigaku D/max-2500 X-ray diffractometer. The theoretical study
was performed on the 6-31G** basis set in Gaussian09 using the density functional theory (DFT),
as approximated by the B3LYP.
2
.3 Fabrication and characterization of the photovoltaic cells
Solar cell devices with a typical configuration of indium tin oxide (ITO)/Poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/SMs:PC BM/Ca/Al
was
7
1

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fabricated as follows: The ITO glass was pre-cleaned with deionized water, acetone and
isopropanol in turn for 15 min. The organic residues were further removed by treating with
UV-ozone for 1 h. Then the ITO glasswas modified by spin-coating PEDOT: PSS (30 nm) on it.
o
After the ITO glasses were dried in oven at 150 C for 15 min, the active layer was spin-coated on
the ITO/PEDOT:PSS using a blend solution of SMs:PC BM (30 mg/mL in o-dichlorobenzene,
7
1
variants with donor/acceptor weight ratio, and different contents of DIO, respectively). Ca (20 nm)
and Al (80 nm) electrode was then subsequently thermally evaporated on the active layer under
-4
2
the vacuum of 1×10 Torr. The active area of the device was 0.1 cm , and the thicknesses of the
active films were recorded by a profilometer (Dektak XT, Bruker). The devices were characterized
-2
under the illumination of simulated AM 1.5 G, 100 mW cm using a solar simulator. The current
density-voltage (J-V) measurement of the devices was conducted on a computer controlled
Keithley 2400 Source Measure Unit. The external quantum efficiency (EQE) measurements were
performed in air using a source meter, silicon photodiode, and a computer-controlled light
source-monochromator-lock-in system with a scan increment of 20 nm per point.
2
2
.4 Synthesis
.4.1 Synthesis of compound 4
To a stirred solution of compound 3 (1.0 g, 2.5 mmol) in methanol (20 mL) and CH Cl (20 mL)
2
2
in a 100 mL three-neck flask were charged ICl (0.49 g, 3.0 mmol) under argon atmosphere for 1 h.
The progress of the reaction was monitored by TLC using hexane:CH Cl as eluent. After
2
2
completion of the reaction, the mixture was poured into water and extracted with CH Cl . The
2
2
organic layers were separated, dried over Mg SO , and filtered. Solvents were removed under
2
4

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reduced pressure and the crude residue was further purified by column chromatography
1
(
hexane:CH Cl 20:1, v/v) to afford the compound 4 as a red solid (0.78 g, yield: 60%). H NMR
2
2
(
400 MHz, CDCl ): δ 7.97 (s, 1H), 7.82 (s, 1H), 7.53 (s, 1H), 7.51 (s, 1H), 7.04 (d, J = 8.8 Hz,
3
4
1
1
2
5
H), 6.62–6.58 (m, 1H), 4.06 (t, J = 6.5 Hz, 2H), 1.84 (dd, J = 14.7, 6.8 Hz, 2H), 1.53–1.29 (m,
1
3
0H), 0.90 (t, J = 6.9 Hz, 3H). C NMR (101 MHz, CDCl ): δ 162.17, 146.82, 145.96, 145.05,
3
35.94, 135.73, 135.01, 132.93, 132.56, 125.66, 119.34, 114.78, 68.47, 31.86, 29.37, 29.29, 29.17,
+
6.07, 22.72, 14.17. MALDI-TOF-MS, m/z: calcd for C H BF IN O [M] : 522.115; found
2
3
26
2
2
22.098.
2
.4.2 Synthesis of compound 5
According to the literature [58], to a mixtrue of 2,7-dibromo-9H-fluorene (3.24 g, 10 mmol),
bromooctane (4 mL, 23 mmol) and tetrabutylammonium bromide (TBAB, catalytic amount ) in
DMSO (100 mL) was added sodium hydroxide solution (25 mL, 50 wt%). The resulting mixture
was stirred at room temperature for 8 h. The progress of the reaction was monitored by TLC using
hexane:CH Cl as the eluent. After completion of the reaction, the reaction mixture was poured
2
2
into water and extracted with CH Cl . The organic layers were separated, dried over anhydrous
2
2
Mg SO , and filtered. Solvents were removed under reduced pressure and the crude residue was
2
4
further purified by column chromatography (hexane as eluent) to afford the compound 5 as a
1
white solid (4.95 g, yield: 91%). H NMR (400 MHz, CDCl ): δ 7.51 (d, J = 8.5 Hz, 2H),
3
7
.47–7.41 (m, 4H), 1.97–1.85 (m, 4H), 1.29–0.97 (m, 20H), 0.83 (t, J = 7.1 Hz, 6H), 0.58 (s, 4H).
1
3
C NMR (101 MHz, CDCl ): δ 152.44, 139.19, 130.14, 126.16, 121.46, 121.12, 55.64,
3
4
0.25 ,31.76, 29.86, 29.18, 29.16, 23.62, 22.60, 14.09.

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2
.4.3 Synthesis of compound 6
According to the modified literature [57], to the mixture of compound 5 (3.0 g, 5.5 mmol), Pd
PPh ) Cl (0.53 g, 0.55 mmol) and CuI (0.217 g, 1.1 mmol) in Et N (50 mL) was added
(
3
2
2
3
o
trimethylsilylacetylene (3.1 mL, 22 mmol). The resulting suspension was stirred at 80 C for 24 h
under argon atmosphere. The progress of the reaction was monitored by TLC using
hexane:CH Cl as the eluent. After cooled to room temperature, the reaction mixture was filtried
2
2
and the filtrate was evaporated to dryness. The solid obtained was dissolved in CH Cl , washed
2
2
with brine, dried with anhydrous Mg SO and filtered. Solvents were removed under reduced
2
4
pressure and the crude residue was further purified by column chromatography (hexane as eluent)
1
to afford the compound 6 as a white solid (2.44 g, yield: 76%). H NMR (400 MHz, CDCl ): δ
3
7
2
1
2
.59 (d, J = 7.8 Hz, 2H), 7.45 (d, J = 7.9 Hz, 2H), 7.41 (s, 2H), 1.96–1.89 (m, 4H), 1.24–1.00 (m,
1
3
0H), 0.82 (t, J = 7.1 Hz, 6H), 0.51 (s, 4H), 0.28 (s, 18H). C NMR (101 MHz, CDCl ): δ 150.94,
3
40.87, 131.24, 126.22, 121.73, 119.86, 106.09, 94.26, 55.24, 40.36, 31.81, 29.99, 29.26, 23.60,
2.62, 14.12, 0.07.
2
.4.4 Synthesis of compound 7
To a stirred solution of compound 6 (2.33 g, 4 mmol) in THF (20 mL) was added
tetrabutylammonium fluoride (2 mL, 1 mol/L) dropwise. The resulting mixture was further stirred
at room temperture for 30 min. After completion of the reaction, the reaction mixture was poured
into water and extracted with ethyl acetate. The organic layers were separated, dried over
anhydrous Mg SO , and filtered. Solvents were removed under reduced pressure and the crude
2
4
residue was further purified by column chromatography (hexane as eluent) to afford the compound

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1
7
7
0
1
as a white solid (1.68 g, yield: 96%). H NMR (400 MHz, CDCl ): δ 7.63 (d, J = 7.8 Hz, 2H),
3
.48 (dd, J = 7.8, 1.3 Hz, 2H), 7.46 (s, 2H), 3.15 (s, 2H), 1.96–1.90 (m, 4H), 1.22–1.02 (m, 20H),
1
3
.82 (t, J = 7.1 Hz, 6H), 0.60–0.53 (m, 4H). C NMR (101 MHz, CDCl ): δ 151.07, 141.00,
3
31.26, 126.56, 120.85, 119.98, 84.54, 55.23, 40.21, 31.77, 29.94, 29.21, 23.66, 22.59, 14.07.
+
MALDI-TOF-MS, m/z: calcd for C H [M-H] : 437.329; found 437.235.
3
3
42
2
.4.5 Synthesis of compound 8
Compound 8 was synthesized according to the same procedure as for preparing compound 5
using 2,7-dibromo-9H-carbazole as reactant. The crude product was purified on silica column
1
using hexane as eluent to give the pure compound 8 as a white solid in 92% yield. H NMR (400
MHz, CDCl ): δ 7.87 (d, J = 8.3 Hz, 2H), 7.52 (s, 2H), 7.33 (d, J = 8.3 Hz, 2H), 4.17 (t, J = 7.3 Hz,
3
1
3
2
H), 1.92–1.76 (m, 2H), 1.38–1.21 (m, 10H), 0.87 (t, J = 6.7 Hz, 3H). C NMR (101 MHz,
CDCl ): δ 141.33, 122.51, 121.47, 121.26, 119.69, 111.99, 43.57, 31.80, 29.31, 29.19, 28.79,
3
2
7.19, 22.65, 14.12.
2
.4.6 Synthesis of compound 9
Compound 9 was synthesized according to the same procedure as for preparing compound 6
using compound 8 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 9 as a white solid in 72% yield. H NMR (400 MHz, CDCl ): δ 7.96
3
(d, J = 8.0 Hz, 2H), 7.50 (s, 2H), 7.34 (d, J = 8.0 Hz, 2H), 4.26–4.21 (m, 2H), 1.85 (dd, J = 14.1,
1
3
7
.2 Hz, 2H), 1.40–1.27 (m, 10H), 0.88 (d, J = 6.5 Hz, 3H), 0.31–0.29 (m, 18H). C NMR (101
MHz, CDCl ): δ 140.55, 123.27, 122.67, 120.37, 120.32, 112.42, 106.45, 93.85, 43.20, 31.84,
3
2
9.36, 29.21, 28.94, 27.21, 22.64, 14.12.

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2
.4.7 Synthesis of compound 10
Compound 10 was synthesized according to the same procedure as for preparing compound 7
using compound 9 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 10 as a white solid in 95% yield. H NMR (400 MHz, CDCl ): δ 8.00
3
(d, J = 8.0 Hz, 2H), 7.55 (s, 2H), 7.38–7.35 (m, 2H), 4.24 (t, J = 7.4 Hz, 2H), 3.17 (s, 2H), 1.84
1
3
(
dd, J = 14.7, 7.3 Hz, 2H), 1.39–1.24 (m, 10H), 0.87 (t, J = 6.9 Hz, 3H). C NMR (101 MHz,
CDCl ): δ 140.52, 123.23, 122.83, 120.55, 119.34, 112.76, 84.89, 76.95, 43.25, 31.81, 29.36,
3
+
2
9.20, 28.92, 27.25, 22.64, 14.12. MALDI-TOF-MS, m/z: calcd for C H N [M] : 327.199;
2
4
25
found 327.233.
2
.4.8 Synthesis of compound 12
To a stirred solution of compound 11 (3.0 g, 6.72 mmol) in CH Cl (40 mL) was added liquid
2
2
o
bromine (2.26 g, 14.12 mmol) dropwise at 0 C. The resulting mixture was stirred at room
temperature for further 7 h. After completion of the reaction, the reaction mixture was poured into
water and extracted with ethyl acetate. The organic layers were separated, dried over anhydrous
Mg SO , and filtered. Solvents were removed under reduced pressure and the crude residue was
2
4
further purified by column chromatography (hexane as eluent) to afford the compound 12 as a
1
white solid (3.74 g, yield: 92%). H NMR (400 MHz, CDCl ): δ 7.42 (d, J = 5.5 Hz, 2H), 4.15(t, J
3
1
3
=
6.5 Hz, 4H), 2.04–1.89 (m, 4H), 1.46–1.22 (m, 20H), 0.90 (t, J = 6.7 Hz, 6H). C NMR (101
MHz, CDCl ): δ 142.73, 131.16, 130.90, 123.14, 114.91, 74.17, 31.84, 30.45, 29.38, 29.28, 25.99,
3
2
2.69, 14.15.
2
.4.9 Synthesis of compound 13

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Compound 13 was synthesized according to the same procedure as for preparing compound 6
using compound 12 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 13 as yellow oil in 80% yield. H NMR (400 MHz, CDCl ): δ 7.57 (s,
3
2
6
1
H), 4.21 (t, J = 6.5 Hz, 4H), 1.92–1.78 (m, 4H), 1.33 (d, J = 15.4 Hz, 20H), 0.90 (t, J = 6.3 Hz,
1
3
H), 0.28 (s, 18H). C NMR (101 MHz, CDCl ): δ 143.88, 131.80, 130.28, 126.00, 122.89,
3
01.67, 97.81, 74.17, 31.85, 30.50, 29.41, 29.29, 26.00, 22.71, 14.18.
.4.10 Synthesis of compound 14
2
Compound 14 was synthesized according to the same procedure as for preparing compound 7
using compound 13 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 14 as yellow oil in 96% yield. H NMR (400 MHz, CDCl ): δ 7.63 (s,
3
2
0
8
H), 4.23 (t, J = 6.5 Hz, 4H), 3.48 (s, 2H), 1.88–1.82 (m, 4H), 1.35 (dd, J = 14.4, 10.3 Hz, 20H),
13
.91 (d, J = 6.5 Hz, 6H). C NMR (101 MHz, CDCl ): δ 143.99, 131.72, 130.26, 126.63, 121.93,
3
3.50, 77.29, 74.26, 31.86, 30.48, 29.41, 29.30, 26.02, 22.71, 14.17. MALDI-TOF-MS, m/z: calcd
+
for C H O S [M] : 494.231; found 494.207.
30
38
2 2
2
.4.11 Synthesis of compound 15
Compound 15 was synthesized according to the same procedure as for preparing compound 5
using 10H-phenothiazine as reactant. The crude product was purified on silica column using
1
hexane as eluent to give the compound 15 as colorless oil in 92% yield. H NMR (400 MHz,
CDCl ): δ 7.16–7.12 (m, 2H), 7.11 (s, 2H), 6.89 (dd, J = 8.1, 6.9 Hz, 2H), 6.84 (d, J = 7.7 Hz, 2H),
3
1
3
3
.85–3.77 (m, 2H), 1.83–1.73 (m, 2H), 1.36–1.15 (m, 10H), 0.86 (t, J = 5.9 Hz, 3H). C NMR
(
101 MHz, CDCl ): δ 145.38, 127.48, 127.23, 124.93, 122.36, 115.44, 47.46, 31.84, 29.31, 27.05,
3

ACCEPTED MANUSCRIPT
2
6.96, 22.73, 14.22.
2
.4.12 Synthesis of compound 16
Compound 16 was synthesized according to the same procedure as for preparing compound 12
using compound 15 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 16 as yellow oil in 93% yield. H NMR (400 MHz, CDCl ): δ
3
7
1
1
.26–7.18 (m, 4H), 6.66 (d, J = 8.5 Hz, 2H), 3.73 (t, J = 7.1 Hz, 2H), 1.80–1.67 (m, 2H),
1
3
.43–1.16 (m, 10H), 0.86 (t, J = 6.8 Hz, 3H). C NMR (101 MHz, CDCl ): δ 144.14, 130.13,
3
29.71, 126.43, 116.67, 114.75, 47.61, 31.76, 29.23, 29.18, 26.83, 26.65, 22.67, 14.17.
.4.13 Synthesis of compound 17
2
Compound 17 was synthesized according to the same procedure as for preparing compound 6
using compound 16 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 17 as yellow oil in 70% yield. H NMR (400 MHz, CDCl ): δ 7.22
3
(dd, J = 8.4, 1.9 Hz, 2H), 7.18 (d, J = 1.9 Hz, 2H), 6.71 (d, J = 8.5 Hz, 2H), 3.81–3.75 (m, 2H),
1
3
1
.79–1.69 (m, 2H), 1.39–1.22 (m, 10H), 0.87 (t, J = 6.8 Hz, 3H), 0.23 (s, 18H). C NMR (101
MHz, CDCl ): δ 144.73, 131.19, 130.59, 123.97, 117.21, 114.97, 104.33, 94.03, 47.66, 31.74,
3
2
9.20, 29.15, 26.76, 26.62, 22.64, 14.12.
2
.4.14 Synthesis of compound 18
Compound 18 was synthesized according to the same procedure as for preparing compound 7
using compound 17 as reactant. The crude product was purified on silica column using hexane as
1
eluent to give the compound 18 as yellow oil in 94% yield. H NMR (400 MHz, CDCl ): δ
3
7
.28–7.24 (m, 2H), 7.20 (d, J = 1.7 Hz, 2H), 6.75 (d, J = 8.4 Hz, 2H), 3.82–3.77 (m, 2H), 3.04 (s,

ACCEPTED MANUSCRIPT
1
3
2
H), 1.79–1.72 (m, 2H), 1.26 (dd, J = 11.8, 5.5 Hz, 10H), 0.86 (t, J = 6.7 Hz, 3H). C NMR (101
MHz, CDCl ): δ 145.04, 131.45, 130.75, 124.10, 116.27, 115.11, 82.90, 77.14, 47.68, 31.73, 29.19,
3
+
2
9.15, 26.80, 26.66, 22.63, 14.11. MALDI-TOF-MS, m/z: calcd for C H NS [M] : 359.171;
24
25
found 359.186.
2
.4.15 Synthesis of compound F-BDP
According to the modified procedure [57], compound 4 (131 mg, 0.25 mmol), Pd(PPh ) Cl (7
3
2
2
mg, 0.01 mmol) and CuI (2.0 mg, 0.01 mmol) were dissolved in dry toluene (10 mL) and Et N (10
3
mL) under nitrogen. Degassed compound 7 (44 mg, 0.1 mmol) in toluene (5 mL) solution was
added. After purging with nitrogen for 15 min, the mixture was stirred at room temperature for 12
h under nitrogen. After completion of the reaction, the reaction mixture was poured into water and
extracted with ethyl acetate. The organic layers were separated, dried over anhydrous Mg SO ,
2
4
and filtered. Solvents were removed under reduced pressure and the crude residue was further
purified by column chromatography (ethyl acetate/hexane 1:5, v/v) to give the desired F-BDP as
1
black purple solid (88 mg, yield: 72%). H NMR (400 MHz, CDCl ): δ 8.06 (s, 2H), 7.98 (s, 2H),
3
7
8
1
.64 (d, J = 7.6 Hz, 2H), 7.57 (s, 2H), 7.55 (s, 2H), 7.48–7.43 (m, 4H), 7.10 (s, 2H), 7.05 (d, J =
.6 Hz, 6H), 6.62–6.58 (m, 2H), 4.06 (t, J = 6.5 Hz, 4H), 1.97–1.93 (m, 4H), 1.89–1.82 (m, 4H),
1
3
.37–1.03 (m, 40H), 0.90 (t, J = 6.8 Hz, 6H), 0.82 (d, J = 6.8 Hz, 6H), 0.59 (s, 4H). C NMR (101
MHz, CDCl ): δ 162.08, 151.32,151.14, 147.54, 144.78, 140.69, 135.48, 134.17, 132.58, 132.37,
3
1
5
1
31.58, 130.55, 126.95, 125.81, 121.77, 120.23, 120.01, 119.07, 114.71, 92.85, 82.73, 68.45,
5.23, 40.33, 31.84, 31.79, 30.01, 29.99, 29.35, 29.26, 29.25, 29.16, 26.05, 23.73, 22.65, 22.69,
+
4.13, 14.09. MALDI-TOF-MS, m/z: calcd for C H B F N O [M] : 1226.734; found 1226.352.
7
9
92
2
4
4
2
+
HRMS, m/z: calcd for C H B F N O [M] : 1226.7343; found 1226.7340.
7
9
92
2
4
4
2

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2
.4.16 Synthesis of compound C-BDP
Compound C-BDP was synthesized according to the same procedure as for preparing
compound F-BDP using compound 4 and compound 10 as reactants. The crude product was
purified on silica column using hexane as eluent to give the desired compound C-BDP as black
1
purple solid in 68% yield. H NMR (400 MHz, CDCl ): δ 8.08 (s, 2H), 8.02–7.97 (m, 4H), 7.58 (d,
3
J = 1.9 Hz, 2H), 7.57–7.55 (m, 2H), 7.52 (s, 2H), 7.34 (dd, J = 8.1, 1.2 Hz, 2H), 7.12 (s, 2H),
7
.08–7.04 (m, 6H), 6.60 (dd, J = 4.3, 1.8 Hz, 2H), 4.26 (t, J = 7.3 Hz, 2H), 4.07 (t, J = 6.5 Hz, 4H),
1
3
1
.86 (dd, J = 14.2, 7.2 Hz, 6H), 1.41–1.24 (m, 30H), 0.92–0.85 (m, 9H). C NMR (101 MHz,
CDCl ): δ 162.08, 147.53, 144.88, 144.72, 140.72, 135.48, 134.19, 132.59, 132.34, 131.59, 125.83,
3
1
2
22.72, 122.58 120.52, 120.30, 119.15, 114.71, 114.20, 111.85, 93.22, 82.29, 68.45, 43.30, 31.84,
9.39, 29.36, 29.27, 29.21, 29.16, 28.97, 27.31, 26.06, 22.69, 22.63, 14.14, 14.10.
+
MALDI-TOF-MS, m/z: calcd for C H B F N O [M] : 1115.604; found 1115.271. HRMS, m/z:
7
0
75
2
4
5
2
+
calcd for C H B F N O [M] : 1115.6043; found 1115.6035.
7
0
75
2
4
5
2
2
.4.17 Synthesis of compound B-BDP
Compound B-BDP was synthesized according to the same procedure as for preparing
compound F-BDP using compound 4 and compound 14 as reactants. The crude product was
purified on silica column using hexane as eluent to give the desired compound B-BDP as black
1
purple solid in 58% yield. H NMR (400 MHz, CDCl ): δ 8.04 (s, 2H), 8.00 (s, 2H), 7.56 (d, J =
3
7
4
1
.1 Hz, 6H), 7.07 (dd, J = 10.8, 7.9 Hz, 8H), 6.61 (d, J = 2.5 Hz, 2H), 4.24 (t, J = 6.5 Hz, 4H),
.07 (t, J = 6.5 Hz, 4H), 1.85 (dd, J = 13.3, 6.6 Hz, 8H), 1.33 (d, J = 18.3 Hz, 40H), 0.92–0.88 (m,
13
2H). C NMR (101 MHz, CDCl ): δ 162.19, 147.87, 145.62, 145.36, 144.36, 143.83, 135.68,
3

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1
8
2
1
34.09, 132.83, 132.61, 131.94, 131.54, 130.34, 125.70, 125.11, 122.89, 119.38, 114.77, 88.67,
5.46, 74.17, 68.47, 53.43, 31.85, 31.83, 30.52, 29.71, 29.41, 29.35, 29.26, 29.15, 26.04, 26.03,
+
2.68, 14.13. MALDI-TOF-MS, m/z: calcd for C H B F N O S [M] : 1282.637; found
7
6
88
2
4
4
4 2
+
282.170. HRMS, m/z: calcd for C H B F N O S [M] : 1282.6369; found 1282.6358.
7
6
88
2
4
4
4 2
2
.4.18 Synthesis of compound P-BDP
Compound P-BDP was synthesized according to the same procedure as for preparing
compound F-BDP using compound 4 and compound 18 as reactants. The crude product was
purified on silica column using hexane as eluent to give the desired compound P-BDP as black
1
purple solid in 61% yield. H NMR (400 MHz, CDCl ): δ 7.99 (s, 2H), 7.95 (s, 2H), 7.55 (s, 2H),
3
7
.53 (s, 2H), 7.23 (dd, J = 8.4, 1.9 Hz, 2H), 7.18 (d, J = 1.9 Hz, 2H), 7.04 (d, J = 8.6 Hz, 8H), 6.76
(d, J = 8.5 Hz, 2H), 6.58 (dd, J = 4.2, 1.7 Hz, 2H), 4.06 (t, J = 6.5 Hz, 4H), 3.81 (t, J = 7.2 Hz,
1
3
2
H), 1.88–1.75 (m, 6H), 1.42–1.23 (m, 30H), 0.89 (dd, J = 14.0, 6.9 Hz, 9H). C NMR (101 MHz,
CDCl ): δ 162.06, 147.48, 144.82, 144.60, 135.42, 134.14, 132.55, 132.24, 131.46, 130.72, 129.97,
3
1
3
25.82, 124.11, 118.93, 117.23, 115.16, 114.70, 91.12, 82.33, 77.74 , 77.09, 76.78, 68.44, 47.74,
1.83, 31.73, 29.34, 29.26, 29.18, 9.15, 26.82, 26.72, 26.04, 22.68, 22.62, 14.11, 14.09.
+
MALDI-TOF-MS, m/z: calcd for C H B F N O S [M] : 1147.576; found 1147.256. HRMS, m/z:
7
0
75
2
4
5
2
+
calcd for C H B F N O S [M] : 1147.5764; found 1147.5760.
7
0
75
2
4
5
2
3
. Results and discussion
3
.1 Synthesis and characterization
The synthetic routes for the novel A-D-A type β- alkynylated BODIPY derivatives, F-BDP,
C-BDP, B-BDP and P-BDP are depicted in Scheme 1. For the construction of these molecules,
we utilized the Pd-catalyzed Sonogashira reaction to connect two BODIPY units covalently to the

ACCEPTED MANUSCRIPT
central electron-donating unit with alkynyl bridges, where β-iodinated BODIPY 4 and bisalkynyl
substituted donor moiety should be prerequisite for the synthesis. Firstly, iodination of BODIPY 3
with 1.3 eqiv of ICl in the mixture solvent of methanol and methylene chloride readily afford the
β-iodinated BODIPY 4 with a moderate yield. Secondly, treatment of bisbrominated alkyl donors
5
, 8, 12, 16, with trimethylsilylacetylene in the presence of catalytic amount of Pd(PPh ) Cl and
3 2 2
CuI in Et N, following by charging with tetrabutylammonium fluoride smoothly give the desired
3
bisalkynyl substituted donors 7, 10, 14, 18. With these key intermediates in hand, finally, the target
molecules F-BDP, C-BDP, B-BDP and P-BDP, were achieved by the Pd-catalyzed Sonogashira
coupling of β-iodinated BODIPY 4 with the corresponding electron donors. These newly
synthesized target molecules were sufficiently soluble in THF, CHCl , CH Cl , o-dichlorobenzene
3
2
2
to permit their characterization in solution as well as their application in BHJ-OSCs devices. The
structure assignments of these novel molecules were confirmed by NMR spectroscopy,
MALDI-TOF-MS and HRMS.
3
.2 Thermal properties
The thermal properties of four SMs are evaluated with TGA and DSC. The recorded TGA and
DSC curves are depicted in Fig. 1 and Fig. S1 (ESI), respectively. The corresponding data are
summarized in Table 1. As displayed, all of four SMs shown high thermal stability with the
o
o
o
decomposition temperatures (T , at 5% weight loss) observed at 314 C, 300 C, 295 C and 301
d
o
C for F-BDP, C-BDP, B-BDP and P-BDP, respectively, which is good enough for the OSCs
application. It indicates that the different electron donors have significant effect on their thermal
properties. Fluorene-based F-BDP has higher thermal stability than carbazole- and phenothiazine-
based C-BDP and P-BDP, and BDT based B-BDP exhibits the relatively lower T value. In DSC,
d
o
o
o
the dyes were heated from 25 to 300 C and then again cooled to 25 C with a rate of 10 C/min
o
under nitrogen flow. As shown in Fig S1, F-BDP exhibits an endothermic peak at 187 C, which
indicates the melting temperature of F-BDP. Nevertheless, no exothermic peak is observed during

ACCEPTED MANUSCRIPT
the cooling process in DSC curve. Similarly, C-BDP, B-BDP and P-BDP display endothermic
o
o
o
peaks at 191 C, 184 C and 169 C, respectively. The results show that all of these SMs have high
thermal stability and good enough for the application in optoelectronic devices.
3
.3 Absorption spectra
The UV-vis absorption spectra of reference BODIPY 3, F-BDP, C-BDP, B-BDP and P-BDP
are shown in Fig. 2, and their corresponding optical data are summarized in Table 1. As shown,
both SMs exhibited the similar absorption spectra covering from 300 to 650 nm with two obvious
absorption peaks, in which the high-lying region from 300 to 450 nm originate from the localized
π–π* transition of the conjugated backbones, whereas the low-lying region from 470 to 650 nm is
assigned to the S –S transition along with the ICT interaction between central donor moiety and
0
1
terminal BODIPY acceptor moieties. In comparison with the reference BODIPY 3, SMs all
showed obviously broad and red-shifted (~ 50 nm) absorption spectra. It indicated that the
conjugate length was effectively extended by flanking two BIDOPY units on the central donor
unit though acetylene bridges, leading to an intense ICT, and therefore, resulting a narrow band
gap. The optical band gaps calculated from the absorption onset wavelength (λonset) were 1.89 eV
for F-BDP, 1.87 eV for C-BDP, 1.84 eV for B-BDP and 1.79 for P-BDP, according to the
opt
formula of Eg = 1240/λonset. The molecular extinction coefficients (ε) of BODIPY 3, F-BDP,
4
4
4
4
4
4
C-BDP, B-BDP and P-BDP are 5.9 × 10 , 4.8 × 10 , 8.2 × 10 , 8.4 × 10 , 3.7 × 10 and 4.7 × 10
-
1
-1
cm M , respectively (Table 1). Such high molecular extinction coefficients enable these SMs for
good light-harvesting ability. As displayed, F-BDP and C-BDP exhibit higher ε value than the
typical BODIPY, is mainly attributed to the intense ICT between the strong electron-donating
fluorene and carbazole segment and the electron-accepting BODIPY dye units. On the other hand,

ACCEPTED MANUSCRIPT
B-BDP and P-BDP display relatively lower ε value than that of F-BDP and C-BDP, suggesting
that the central electron donating motif can easily tune the molecular extinction coefficients.
In comparison with the absorption spectrum in solution, broader absorption bands with
significant bathochromic shifts were observed in thin film (Fig. 3) for these SMs. This could be
attributed to the molecular aggregation and interchain interactions in the solid state, which
resulting an additional increasement of conjugation length. It is worth noting that B-BDP exhibits
a much more obviously bathochromic shifts than other SMs. It is due to the fact that B-BDP has
the ability to form short non-covalent SꢀꢀꢀS contact with adjacent molecules [59], leading to better
intermolecular interactions and further extended the conjugation length. Broad and intense
absorption properties of these SMs make them good candidates for the light-harvesting and
photovoltaic applications.
3
.4 Emission spectra
The photoluminescence (PL) of F-BDP, C-BDP, B-BDP and P-BDP in CH Cl solution (1×
2
2
-7
1
0 mol/L) were investigated at room temperature. Rhodamine B (= 0.71) was used as standard in
methanol to estimate the fluorescence quantum yield. All the SMs were excited by the λmax of their
absorption spectra and the normalized emission spectra are illustrated in Fig. 4, and the emission
band maxima, Stokes shift and quantum yields are listed in Table 1, respectively. Unlike to the
ever reported that the BDOIPY derivatives are usually with small Stokes shifts, these SMs exhibit
obvious Stokes shifts (55-89 nm), resulting in maximum emission peak at 632, 643, 624 and 603
nm, respectively (Table 1 and Fig. 4). This can be explained on the basis of intramolecular energy
transfer from the BODIPY π-π* excited state to the lower lying singlet excited state of the donor
moieties and/or to the existence of new non-radiative pathways from the BODIPY π-π* excited

ACCEPTED MANUSCRIPT
state to the ground state. In general, the strong donor-acceptor interaction and coplanar molecular
configuration will decrease the excitation energy of the excited state and will shift the
fluorescence emission band to the long wavelength region. Accordingly, as shown in Fig. 4,
F-BDP and C-BDP, incorporating donor units with strong electron-donating capacity, display
more significant Stokes shifts. On the other hand, F-BDP, C-BDP, B-BDP and P-BDP all give
low fluorescent quantum yields (0.02-0.05). These could be attributed to the increased internal
conversion according to the energy gap law that states the non-radiative deactivation probability
of S -S increases as the energy gap of S -S decreases in an extended conjugating system. PL
0
1
0
1
quenching provides the direct evidence for excition dissociation, and efficient PL quenching is
necessary to obtain efficient organic solar cells. The experimental observations indicate that these
SMs are well meet requirements of material for OSCs.
3
.5 Electrochemical properties
To investigate the electrochemical behaviors of F-BDP, C-BDP, B-BDP and P-BDP, their
electronic states were studied by cyclic voltammetry (CV) in CH Cl solution containing 0.1M
2
2
-1
Bu NPF at a scan rate of 100 mV s , and the cyclic voltammograms were calibrated with
4
6
+
ferrocene/ferrocenium (Fc/Fc ) as an external reference. The CV curves were depicted in Fig. 5
and the results are summarized in Table 2. As shown in Fig. 5, F-BDP, C-BDP, B-BDP and
P-BDP present differential first onset oxidation potential as 0.96, 0.90, 0.83 and 0.74 V,
ox
respectively, indicating the central donor have significant effects on the E onset. The reduction
ox
opt
potentials (E ) calculated from E
− Eg are approximately matched with the experimental
onset
red
values obtained from reduction waves. The HOMO and LUMO levels were estimated using the
equation:

ACCEPTED MANUSCRIPT
ox
onset
opt
EHOMO = [– (E
– 0.52) – 4.8] eV, ELUMO = EHOMO + Eg eV
where 0.52 V is the value for ferrocene vs Ag/AgCl and 4.8 eV is the energy level of Fc/Fc+
relative to the vacuum energy level. The calculated HOMO energy levels of these SMs are
obtained as -5.24 eV for F-BDP, -5.18 eV for C-BDP, -5.11 eV for B-BDP and -5.02 eV for
P-BDP, respectively, as list in Table 2. On the contrary, the LUMO values are observed with
minor difference for the SMs, suggesting that the reduction of the BODIPY moiety within the
molecule is relatively immune to the core donor moiety. Their low-lying HOMO energy levels
suggest that the OSCs based on these materials as donors and PCBM as acceptor can obtain high
V .
oc
3
.6 Theoretical calculations
To better understand the variation in photophysical and electrochemical properties between
F-BDP, C-BDP, B-BDP and P-BDP, the density functional theory (DFT) theoretical calculation
was conducted on the electronic structures of them to observe how they impact these properties.
The optimized ground-state geometries and electronic distribution in HOMO-LUMO levels were
presented in Fig. 6. All the calculations were carried out with the Gaussian 09 program suite by
using the B3LYP method and 6-31 G* basis set. In the optimized ground-state, we observed that
o
the dihedral angle between the central donor motif and flanked BODIPY are calculated to be 0.7
o
o
o
for F-BDP, 11.6 for C-BDP, 11.0 for B-BDP and 19.1 for P-BDP, respectively. In comprision
to those similar molecules wihtout ethinyl bridges reported in the previous literature [50], the
ethinyl linked SMs show much more planar molecular geometry. For example, BDP3, with two
fluorene groups directly substituted at the 2,6-positions of BODIPY, display dihedral angle of
o
2
5.7 between the fluorene group and the central BODIPY core. In contrast, F-BDP, with ethinyl

ACCEPTED MANUSCRIPT
bridges between the fluorene group and BODIPY present in this paper, exhibit significant smaller
o
value of 0.7 . Similarly result was observed that C-BDP also show smaller dihedral angles than
o
o
BDP4 (11.6 vs 22.8 ). These comparative results demonstrated that the ethinyl bridge play an
important role in optimization of molecular configuration. A near-planar molecular configuration
in D-A conjugate can efficiently arouse intense ICT, resulting in decrease of band gap and red
shifted absorption, which was clearly observed in absorption spectra. Furthermore, planar
molecular structure facilitate for hole mobility and energy transfer, which are considered as key
factors for high-performance photovoltaic devices. The efficient energy transfer from BODIPY
units to the donor unit in the excited state has been verified by the PL spectra with large Stokes
shift and low fluorescence quantum yield. From the electron distributions of F-BDP and C-BDP,
it can be seen that π- electrons in the HOMOs are delocalized over the entire molecule backbone,
offering effective orbital interactions among the stacked π-systems, while their LUMOs are
mainly localized on the BODIPY moieties, indicating that the HOMO-LUMO transition that is the
dominant contributing configuration to the S state in F-BDP and C-BDP should be ascribed to a
1
mixture of π–π* and ICT characters. Moreover, the feature of widely spread HOMOs enable to
lead to lower HOMO level (-5.3 eV), as observed in Fig. 6. However, for B-BDP and P-BDP, the
HOMOs are centered in donor moiety with some antibonding character between the donor unit
and the acceptor unit, and the LUMOs are located in the two BODIPY moieties, indicating that the
HOMO-LUMO transition in B-BDP and P-BDP should mainly be ascribed to the ICT. Fig. 6 also
presented the HOMO-LUMO energy levels and energy differences of these SMs. The calculated
values of HOMO levels are -5.32 eV for F-BDP, -5.31 eV for C-BDP, -5.16 eV for B-BDP, and
-5.05 eV for P-BDP, respectively, while the values of LUMO levels are -3.13 eV for F-BDP,

ACCEPTED MANUSCRIPT
-3.13 eV for C-BDP, -3.17 eV for B-BDP, and -3.07 eV for P-BDP, respectively, which are
approximately in accordance with the trends of the experimental observation in CV. In additional,
cal
opt
the trend of the predicted Eg values is in consistent with the estimated Eg calculated from the
ground-state absorption. The relatively narrow band gap and low-lying HOMO energy levels
make them excellent donor materials for OSCs.
3
.7 Photovoltaic properties
The solution processed bulk hetero-junction organic solar cells were fabricated with a structure
of ITO/PEDOT:PSS/active layer/Ca/Al, in which F-BDP, C-BDP, B-BDP or P-BDP was used as
the donor material and PC BM as the acceptor material. With careful optimization of the
7
1
processing parameters such as the ratio of donor to acceptor (D:A, w/w), the thickness of active
layer, the volume of additives and the annealing temperature (Figs. S2–S6, Tables S1–S5,
Supporting Information), their best performances were obtained with a 1:1 ratio of donor to
PC BM acceptor and the thicknesses of 100, 80, 80 and 90 nm, associate with thermal annealing
71
o
at 120, 100, 80 and 100 C for 10 min obtained from an CB solution at a total solid concentration
-1
of 25 mg mL under a spin-coating rate of 2000 rpm, for F-BDP, C-BDP, B-BDP and P-BDP,
respectively, in the presence of 1% 1,8-diiodooctane (DIO) additive. Fig. 7 shows the current
density–voltage (J–V) characteristics of F-BDP, C-BDP, B-BDP and P-BDP based BHJ-OSCs
-2
under illumination of AM 1.5G 100 mW m and their corresponding data are listed in Table 3. It
is further found that B-BDP based device exhibit the highest PCE of 4.65% with a J of 11.84 mA
sc
-
2
cm , a V of 0.73 V and a FF of 53.8% in the optimized conditions, which are good values
oc
among the BODIPY derivatives based SM-OSCs. Comparative studies show that the B-BDP and
P-BDP (3.33%) based devices have higher PCE values than the F-BDP (2.60%) and C-BDP

ACCEPTED MANUSCRIPT
(3.02%) based devices (Fig. 7, Table 3). This is due to the fact that B-BDP and P-BDP based
devices present the higher Jsc and FF values, originated from their superior photon-electron
conversion and carrier transmission properties in the OSCs. It should be note that P-BDP
displayed a higher PCE than the ever reported phenothiazine based OSCs [60-61], though the
samples are rather scarce. To our pleasant surprise, the BHJ-OSCs based on these β-alkynylated
BODIPYs exhibit significant enhancements in device parameters in comparison with the
BHJ-OSCs based on β position directly substituted analogues reported in previous works [50],
implying that the alkynyl play a important role in the improvement of photovoltaic performances,
especially for the J through efficient strengthening of light-harvesting, exciton diffusion, exciton
sc
dissociation, charge transport, and/or collection process in cells via adjustment of the molecular
coplanar. On the other hand, F-BDP, C-BDP and B-BDP based cells show the larger V values
oc
than P-BDP based cell, which is in agreement with their deeper lying HOMO levels. To be note
that these BHJ-OSCs all exhibit decent FF (~50%) compared to other BODIPY derivatives
analogues. This may be caused by the appropriate length of alkyl chain attachment on the
backbone ensure the appropriate crystalline and enable to form good microstructure between the
donor and PC BM.
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1
To further illustrate the differences of photovoltaic performance between these F-BDP, C-BDP,
B-BDP and P-BDP/PC BM based OSCs devices, external quantum efficiencies (EQEs) of these
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1
devices under optimization condition were measured. The recorded EQE curves are shown in Fig.
. As displayed, the EQE profiles are similar with their absorption spectra in film. A broad
8
response to the sunlight in 300-800 nm region with different EQEs values are observed for four
SMs:PC BM based OSCs. Higher EQE values of 50% and 40% are exhibited in the B-BDP and
71

ACCEPTED MANUSCRIPT
P-BDP based devices, respectively. The calculated J values are 6.46, 8.01, 11.27 and 9.22 mA
sc
-
2
cm for F-BDP, C-BDP, B-BDP and P-BDP/PC BM blends respectively given by integrated
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1
EQE curves, which are in close agreement with the J values obtained from the J-V measurements.
sc
It is further found that B-BDP and P-BDP/PC BM blends based devices exhibit stronger
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1
absorption in the 600-700 nm regions than F-BDP and C-BDP/PC BM blends based ones,
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1
especially cell based on B-BDP spread the absorption to 800 nm, which caused the higher EQE
values. This is due to the fact that although the sunlight energy in the longer wavelength region is
weak, however, the electronic transitions are capable of transformation with sufficient efficiency
of photons to electrons, which can be theoretical supported by the electronic distribution
characteristic in HOMO-LUMO levels (Fig. 6).
3
.8 Hole mobility
In addition to the optical absorption and energy levels, the charge carrier mobility of donor
material is another important factor to ensure efficient charge transport toward the electrodes and
also to suppress the competing charge recombination processes. In order to get some ideas about
the influence of the chemical structures on their charge transporting properties, we evaluated and
compared the hole mobilities (ꢀ ) of F-BDP, C-BDP, B-BDP and P-BDP in the same BHJ blends
h
1
/2
using a space charge-limited current (SCLC) technique. Fig. 9 showed the J −V curves of the
vertical hole-only devices with structures of ITO/PEDOT:PSS/SMs:PC BM/Au. The hole
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1
1
/2
mobilities were determined by fitting the dark J −V characteristics by adopting the Mott−Gurney
2
3
equation [62]: J = (9/8)ε ε ꢀ (V /L ), in which J is current density, L is film thickness of active
0
r h
layer (80 nm), ꢀ is hole mobility, ε is relative dielectric constant of the transport medium, ε is
h
r
0
-12
-1
permittivity of free space (8.85 × 10 F m ), V is internal voltage in the device and V = V -Va
appl

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-
Vbi, where Vappl is applied voltage, V is voltage drop, and V is the built-in voltage due to the
a
bi
relative work function difference of the two electrodes. As compiled in Table 3, the hole-only
-5
2
-1 -1
-4
2
-1 -1
mobilities of these SMs are calculated to be 9.61 × 10 cm V s , 1.34 × 10 cm V s , 4.53 ×
-4
2
-1 -1
-4
2
-1 -1
1
0
cm V
s
and 3.11 × 10 cm V
s
in the hole-only SMs/PC BM based devices,
71
respectively. It clearly demonstrated that B-BDP and P-BDP based devices present higher
hole-only mobilities than F-BDP and C-BDP based counterparts, which are in agreement with the
FF values obtained from the J-V measurements.
3
.9 Film morphology
T
he morphologies of the SMs/PC BM blend films under the optimized processing conditions
71
were recorded with transmission electron microscopy (TEM) and shown in Fig. 10. The observed
dark phases are assigned to the PC BM domains because of its relatively higher electron
71
scattering density. As can be seen, F-BDP/PC BM and C-BDP/PC BM blend films exhibit large
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1
71
black phases with a width around 70-30 nm, On the contrary, the images from B-BDP/PC BM
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1
and P-BDP/PC BM blend films present improved morphology with a continuous acicular fibrous
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1
structure and a decreased size of around 16 nm. It implying that the continuously interpenetrated
networks are formed in B-BDP and P-BDP based blend films. In general, good morphology
would lead to high exciton dissociation and charge separation efficiency and thus enhance device
performance. These results are in good agreement with the hole-mobility and photovoltaic
observations.
3
.10 XRD
To further understand the aggregation behavior of the molecule in film, as well as reveal its

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influence on the photoelectric properties and photovoltaic performance, the XRD pattern of
B-BDP, with the best PCE, was recorded in the pure film and in the blend film with PC BM
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1
under the optimized conditions. As shown in Fig. 11, a sharp peak was observed in the small angle
o
regions with 2θ = 5 , corresponding to the interchain d-spacing of 17.66 Å. It indicates that
o
B-BDP pack close in thin film. Moreover, a broad peak in the wider angle with 2θ = 20-25 was
also observed, which is attributed to the interlayer π-π staking of the main chain. To get some idea
of the change in crytallinity of the blended active layer, we have further recorded the XRD pattern
of B-BDP/PC BM blend film under the optimized device conditions. As can be seen, the intensity
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1
o
of the diffraction peak at 2θ = 5 was significant increased under the DIO and annealing
treatments as compared with the B-BDP film, implying the enhancement in the crystalline nature
of the blend. Meanwhile, the π-π staking characteristic peak was obviously decreased. It can be
explained as a certain phase separation between the B-BDP and PC BM inhibits the
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1
over-aggregation of the donors, which are demonstrated in the TEM observations. The
crystallinity of the active layer is one of key factor, which affect the charge transport and light
harvesting efficiency of the device, thereby impact the device performance of the OSCs. These
results are in agreement with the relatively high hole mobility and EQE, which are reasonable for
the high FF and J in OSC.
sc
4
. Conclusions
In summary, four linear A-D-A type small molecules with BODIPY terminal linked through
alkynyl with various electron donating units, F-BDP, C-BDP, B-BDP and P-BDP were designed
and synthesized. The molecules show good thermal stability, strong spectral coverage with
moderate HOMO energy levels. The influence of alkynyl bridges and the central electron donor

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on optical, electrochemical and photovoltaic properties was systematically investigated. As
anticipated, the alkynyl group effectively makes electron donor and receptor cell of these
molecules in coplanar configurations, resulting in broad absorption, efficient exciton dissociation
and good aggregation morphology. All these SMs/PC BM based OSCs displayed moderate
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1
photovoltaic performance with PCE ranging from 2.60% to 4.65%. Among them, B-BDP
exhibited superior intermolecular interaction in film, leading to enhanced light absorption, as well
as increased hole mobility and promoted surface morphology. Consequently, an encouraging PCE
-2
value up to 4.65% with a J of 11.84 mA cm , a V of 0.73 V and a FF of 53.8% was obtained
sc
oc
for the B-BDP/PC BM based cell, which is high level in BODIPY based OSCs. This work
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1
demonstrates that BODIPY derivatives remain a type of promising photovoltaic material as long
as elaborately tailored.
Acknowledgements
We greatly acknowledge the financial support from Nature Science Foundation of Guangdong
Province (2016A030313131, 2016A030313128, 2014A030313630), China Postdoctoral Science
Foundation (2017M612656), Young Innovative Talents Program of Guangdong Colleges and
Universities (2016KQNCX164) and National Nature Science Foundation of China (51476036,
2
1342003).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
Reference