Novel D-A-D type dyes based on BODIPY platform for solution processed organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2016.01.020

Four novel symmetrical donor-acceptor-donor (D-A-D) type boron dipyrromethene (BODIPY) based dyes, namely BDP1-4, featuring an electron-withdrawing BODIPY motif as the central core, and several types of electron-donating segments, including thiophene, bit

Full text of DOI:10.1016/j.dyepig.2016.01.020

Accepted Manuscript
Novel D-A-D type dyes based on BODIPY platform for solution processed organic
solar cells
Junxu Liao, Hongbin Zhao, Yongjun Xu, Zhuodi Cai, Zaixi Peng, Wentao Zhang,
Weinan Zhou, Bohong Li, Qiao Zong, Xiaoxi Yang
PII:
S0143-7208(16)00031-0
10.1016/j.dyepig.2016.01.020
DYPI 5073
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 November 2015
Revised Date: 18 January 2016
Accepted Date: 23 January 2016
Please cite this article as: Liao J, Zhao H, Xu Y, Cai Z, Peng Z, Zhang W, Zhou W, Li B, Zong Q, Yang
X, Novel D-A-D type dyes based on BODIPY platform for solution processed organic solar cells, Dyes
and Pigments (2016), doi: 10.1016/j.dyepig.2016.01.020.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Novel D-A-D type dyes based on BODIPY platform for solution processed
organic solar cells
Junxu Liao a, Hongbin Zhao b, ,Yongjun Xu b, , Zhuodi Cai c, Zaixi Peng b, Wentao Zhang b,
Weinan Zhou b, Bohong Li b, Qiao Zong b, Xiaoxi Yang a, b
a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641,
People's Republic of China
b College of Energy and Chemical Engineering, Dongguan University of Technology, Dongguan 523808, People's
Republic of China.
c Chemical Industry and Engineering Society of Dongguan, Dongguan, 523808
Abstract
Four novel symmetrical donor-acceptor-donor (D-A-D) type boron dipyrromethene (BODIPY)
based dyes, namely BDP1-4, featuring an electron-withdrawing BODIPY motif as the central core,
and several types of electron-donating segments, including thiophene, bithiophene, fluorene and
carbazole, flanked at its 2,6-positions as terminal groups were synthesized and characterized. All
of these BDPs exhibit panchromatic absorption covering the wavelength range 300-900 nm with
high extinction coefficients (0.78 × 105-1.22 × 105 cm-1 M-1) and relatively lower HOMO energy
levels from -5.17 to -5.48 eV. Furthermore, BDP1-4 show good thermal stabilities, excellent
film-forming abilities and low fluorescent quantum yields. BDP2 flanking with bithiophene units
demonstrates superior charge transport property and favorable nanoscale morphology relative to
BDP1, BDP3 and BDP4. The power conversion efficiency (PCE) values of the organic solar cells
Corresponding author. Tel.: +86-769-2286-1233; fax: +86-769-2286-1233; e-mail: zhaohbhanlf@163.com.
Corresponding author. Tel.: +86-769-2286-2605; fax: +86-769-2286-2605; e-mail: hnllxyj@126.com.
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(OSCs) based on the BDPs/PC61BM (1:0.5, w/w) are 1.49% for BDP1, 2.15% for BDP2, 1.06%
for BDP3 and 1.56% for BDP4, respectively.
Keywords: BODIPY dyes, organic solar cells, donor-acceptor-donor, optoelectronic properties,
DFT calculations, photovoltaic performances
Introduction
Organic solar cells (OSCs) are booming as one of the promising technologies for mitigation of
the energy crisis because of their potential low-cost fabrication, lightweight, color-tunable feature,
and mechanical flexibility [1-4]. Solution processed small molecule organic solar cells have
attracted more and more attention benefited from their definite molecular weight, easy purification,
and well-reproducible device performances [5-10]. For the traditional fullerene based organic
solar cell, the visible and near-infrared solar photons are normally absorbed by the blended donor
material because the fullerene acceptor, typically, PC61BM or PC71BM absorbs the solar photons
at the near-ultraviolet and visible range. In the solar spectrum ca. 50% of solar photons is located
in the wavelength of 600−1000 nm and the highest photon flux of the solar spectrum is distributed
around 600−800 nm [11]. Therefore, designing narrow-band gap (< 1.6 eV, or with onset
absorption beyond 800 nm) small-molecule donors are required in the field of small-molecule
organic solar cells.
Boron dipyrromethene (BODIPY) dyes have attracted considerable interest in recent years
owing to a unique combination of facile synthesis, stability, high absorption coefficient, and high
photoluminescence efficiency [12-16], along with the spectroscopic and photophysical properties
of BODIPYs can be fine-tuned by attachment of ancillary residues at the appropriate positions of
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the BODIPY core [17-18]. The BODIPY derivatives commonly possess solution-state absorption
spectra with absorption maxima exceeding 500 nm, even up to 1000 nm, by a suitable
modification [17-21]. In addition, BODIPY inherently bear deep HOMO energy levels (< 5.2 eV)
[17]. Therefore, the relatively high extinction coefficient of the BODIPY core combined with deep
HOMO energy levels and propensity to π-stack in solid-state, make BODIPY-based materials
attractive candidates for organic solar cells [17]. However, to our knowledge, there are only
several reports on organic solar cells based on BODIPY to date. Roncali et al. reported the first
examples of BODIPY donors involving two styryl units along with PC61BM as the electron
acceptor for solution-processed bulk-heterojunction (BHJ) OSCs [22-23]. Subsequently, other
groups reported BHJ OSCs based on BODIPY small molecules and polymers [16, 24-34], and the
best PCE based on BODIPY small molecules was only 5.05% [35]. Therefore, the development of
novel BODIPY based small molecule materials with versatile structures and further obtaining
insight into their potential photovoltaic application has aroused our interest.
Inspired by the strategy for constructing low-band gap materials with conjugated donor–
acceptor (D–A) system [2], herein, the highly flexible BODIPY platform was selected for the
development of D–A structural small molecular-based panchromatic donors. In this contribution,
we present four novel D-A-D type BODIPY dyes, BDP1-4, consisting of an electron-withdrawing
BODIPY as central acceptor, and different electron-donating aromatic groups, including thiophene,
bithiophene, fluorine and carbazole, symmetrically flanked at 2,6-positions of BODIPY nucleus as
terminal conjugated donors (Scheme 1). We expect that these chromophores would significant
broadening of absorption and improving light-harvesting ability in the whole sunlight region. The
photophysical properties and electrochemical behaviors of these dyes were studied systematically
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to reveal the relationships between the structures and the photoelectric characteristics.
Time-dependent density functional theory (TD-DFT) calculations were performed to further
understanding their structural and electronic features. Furthermore, the photovoltaic performances
of these dyes were investigated in detail to insight their potential OSCs applications.
2. Experimental section
2.1. Materials
2,2'-((4-(octyloxy)phenyl)methylene)bis(1H-pyrrole) (3), 5,5-difluoro-10-(4-(octyloxy)phenyl)-
5H-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ium-5-uide (4) and 2,8-dibromo-5,5-difluoro-10-
(4-(octyloxy)phenyl)-5H-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ium-5-uide
(5)
were
prepared following the reported procedures [34, 36-37]. Compound 6, 7, 8 and 9 were synthesized
according to the literatures [38-40]. THF and toluene were dried and distilled over sodium and
benzophenone. DMF, dichloromethane, ethyl acetate were dried under 4A molecular sieves.
Pyrrole was distilled from CaH2 before use. All other reagents and chemicals were purchased from
commercial sources (Aldrich, TCI, J&K) and used without further purification. Column
chromatography was performed on silica (300–400 mesh).
2.2. Measurements and characterization
All 1H and 13C NMR spectra were recorded on a Bruker Avance spectrometer (600 and 151
MHz, respectively) and a Bruker Avance spectrometer (400 and 101 MHz, respectively) with
tetramethylsilane (TMS) as the internal standard. Matrix-assisted laser desorption/ionization time
of flight mass spectra (MALDI-TOF-MS) were recorded on Bruker ultraflex-II spectrometer,
matrix-assisted laser desorption ionization mass spectrometry using an α-cyano-4-hydroxy-
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cinnamic acid (α-CHCA) matrix. High resolution mass spectra (HRMS) were recorded on an
Agilent 6530 Accurate Mass Q-TOF LC-MS instrument. Absorption spectra were collected on an
SHIMADZU UV-2550 spectrophotometer and emission spectra were recorded by using a Hitachi
F-4500 instrument. Excitation and 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 (auxiliary electrode) and Ag/AgCl electrode (reference electrode). The experiment
was performed in dry dichloromethane solution using 0.1 M n-Bu4NPF6 as supporting electrolyte.
The solution was deoxygenated at room temperature under nitrogen protection.
Thermogravimetric analyses (TGA) were conducted under a dry nitrogen gas flow at a heating
o
rate of 20 C min-1 on a NETZSCH TG 209F3 instrument. Differential scanning calorimetric
(DSC) were recorded on a NETZSCH DSC 200F3 instrument. Topographic images of the films
were obtained on a Veeco MultiMode atomic force microscopy (AFM) in the tapping mode using
an etched silicon cantilever at a nominal load of ~2 nN, and the scanning rate for a 5 ꢀm × 5 ꢀm
image size was 1.0 Hz. Quantum chemical calculations for complexes BDP1-4 were conducted at
the DFT level of theory, in conjunction with the B3LYP functional [41] and the 6-31G* basis set
[42], as implemented in the Gaussian 09 program package [43].
2.3. Device fabrication and characterization
OSC devices were fabricated using indium tin oxide (ITO) glass as an anode, Ca/Al as a
cathode, and a blend film of the BDPs/PC61BM as a photosensitive layer. After a 30 nm buffer
layer of poly(3,4-ethylenedioxy-thiophene) and polystyrene sulfonic acid (PEDOT:PSS) was
spin-coated onto the precleaned ITO substrate, the photosensitive layer was subsequently prepared
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by spin coating a solution of the BDPs/PC61BM (1:0.5, w/w) in CHCl3 on the PEDOT:PSS layer
with a concentration of 25 mg mL-1, and dried under nitrogen at room temperature in a
nitrogen-filled glove-box. Ca (10 nm) and Al (100 nm) were successively deposited on the
photosensitive layer in vacuum and used as top electrodes. The current density-voltage (J-V)
characteristics of the devices were carried out on a computer controlled Keithley 236 source
measurement system under simulated 100 mW cm-2 (AM 1.5 G) irradiation from a Newport solar
simulator. Light intensity was calibrated with a standard silicon solar cell. The active area was 0.1
cm2 for each cell. The thicknesses of the spun-cast films were recorded by a profilometer
(Alpha-Step 200, Tencor Instruments). The external quantum efficiency (EQE) was measured by a
Stanford Research Systems model SR830 DSP lock-in amplifier coupled with WDG3
monochromator and a 150W xenon lamp.
2.4. Synthesis of target molecules
2.4.1. Synthesis of BDP1
Compound 5 (277 mg, 0.5 mmol) was dissolved in toluene (30 mL), followed by addition of
tributyl(thiophen-2-yl)stannane 6 (448 mg, 1.2 mmol), Pd(PPh3)4 (23 mg, 0.02 mmol). The
o
resulting mixture was refluxed at 110 C under argon atmosphere for 24 h. Then the reaction
mixture was brought to room temperature, washed with brine, and extracted by ethyl acetate.
Organic layers were combined, dried over anhydrous Na2SO4, filtered, and evaporated to dryness.
Purification was performed by column chromatography on silica gel using petroleum ether/ethyl
acetate (8:1, v/v) as eluent, from which the desired product BDP1 was obtained as a dark blue
solid (132 mg, yield 47%). mp: 156-158 oC. 1H NMR (400 MHz, CDCl3) δ 8.17 (s, 2H), 7.62 (d, J
= 8.3 Hz, 2H), 7.23 (d, J = 4.9 Hz, 2H), 7.18 (d, J = 2.8 Hz, 2H), 7.11 (d, J = 8.3 Hz, 2H),
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7.07–7.02 (m, 2H), 7.01 (s, 2H), 4.10 (t, J = 6.3 Hz, 2H), 1.98–1.82 (m, 1H), 1.64–1.19 (m, 12H),
0.98–0.74 (m, 3H). 13C NMR (101 MHz, CDCl3) δ 162.02, 146.57, 141.00, 136.02, 132.46,
127.96, 127.28, 126.27, 125.87, 124.76, 124.31, 123.29, 114.83, 68.48, 31.78, 31.57, 29.30, 26.10,
25.77, 22.71, 14.16. MALDI-TOF-MS, m/z: calcd for C31H31BF2N2OS2 [M]+: 560.194; found
560.198. HRMS, m/z: calcd for C31H31BF2N2OS2 [M]+: 560.1939; found 560.1937.
2.4.2. Synthesis of BDP2
Compound BDP2 was synthesized according to the same procedure as for preparing BDP1
using [2,2’-bithiophen]-5-yltributylstannane 7 as reactant. The crude product was purified on
silica column using petroleum ether/ethyl acetate (8:1, v/v) as eluent to give the pure compound
o
1
BDP2 as dark blue solid in 52% yield. mp: 172-173 C. H NMR (400 MHz, CDCl3) δ 8.18 (s,
2H), 7.62 (d, J = 8.2 Hz, 2H), 7.23 (d, J = 5.0 Hz, 2H), 7.17 (d, J = 3.0 Hz, 2H), 7.11 (t, J = 7.2 Hz,
6H), 7.06–7.00 (m, 2H), 6.99 (s, 2H), 4.11 (t, J = 6.3 Hz, 2H), 1.87 (d, J = 14.0Hz, 2H), 1.68–1.16
(m, 12H), 0.98–0.75 (m, 3H). 13C NMR (101 MHz, CDCl3) δ 162.06, 146.21, 140.92, 137.20,
136.26, 135.58, 134.76, 132.51, 127.95, 127.80, 125.85, 124.56, 124.47, 124.07, 124.00, 123.72,
114.86, 68.52, 31.95, 29.75, 29.41, 29.31, 26.04, 22.82, 14.11. MALDI-TOF-MS, m/z: calcd for
C39H35BF2N2OS4 [M]+: 724.169; found 724.232. HRMS, m/z: calcd for C39H35BF2N2OS4 [M]+:
724.1693; found 724.1690.
2.4.3. Synthesis of BDP3
To the solution of compound 5 (277 mg, 0.5 mmol) and 2-(9,9-dimethyl-9H-fluoren-2-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane 8 (387 mg, 1.2 mmol) in toluene (20 mL) was added
Pd(PPh3)4 (23 mg, 0.02 mmol) and sodium carbonate solution (10 mL, 2 M in water), the mixture
o
was stirred at 90 C under argon atmosphere for 24 h. Then the reaction mixture was brought to
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room temperature, washed with brine, and extracted by ethyl acetate. Organic layers were
combined, dried over anhydrous Na2SO4, filtered and evaporated to dryness. The residue was
purified by column chromatography on silica gel using petroleum ether/ethyl acetate (6:1, v/v) as
eluent, afforded the desired product BDP3 as dark blue solid (205 mg, yield 41%). mp: 181-182
oC. 1H NMR (400 MHz, CDCl3) δ 8.37 (s, 2H), 7.71 (dd, J = 14.4, 8.0 Hz, 6H), 7.60 (s, 2H), 7.54
(d, J = 7.7 Hz, 2H), 7.45 (d, J = 6.4 Hz, 2H), 7.35 (dd, J = 11.5, 6.1 Hz, 4H), 7.20 (s, 2H), 7.16 (d,
J = 8.2 Hz, 2H), 4.13 (t, J = 6.1 Hz, 2H), 1.89 (dd, J = 13.7, 6.7 Hz, 2H), 1.64–1.15 (m, 12H),
1.02–0.75 (m, 15H). 13C NMR (101 MHz, CDCl3) δ 161.89, 154.51, 153.83, 146.26, 141.55,
138.74, 135.87, 134.57, 132.64, 131.82, 127.40, 127.11, 126.25, 125.06, 124.57, 122.65, 120.57,
120.03, 119.65, 114.81, 77.37, 76.74, 68.43, 46.99, 31.79, 29.74, 29.40, 29.29, 29.27, 27.29, 26.14,
22.68, 14.15, 1.04. MALDI-TOF-MS, m/z: calcd for C53H51BF2N2O [M]+: 780.406, found
780.485. HRMS, m/z: calcd for C53H51BF2N2O [M]+: 780.4063, found 780.4059.
2.4.4. Synthesis of BDP4
Compound BDP4 was synthesized according to the same procedure as for preparing BDP3
using 9-octyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole 9 as reactant. The
crude product was purified on silica column using petroleum ether/ethyl acetate (10:1, v/v) as
o
1
eluent to give the pure compound BDP4 as dark blue solid in 59% yield. mp: 126-127 C. H
NMR (400 MHz, CDCl3) δ 8.46 (s, 2H), 8.34 (s, 2H), 8.20 (d, J = 7.6 Hz, 2H), 7.79 (d, J = 8.4 Hz,
2H), 7.74 (d, J = 8.4 Hz, 2H), 7.56 (t, J = 7.5 Hz, 2H), 7.48 (d, J = 8.8 Hz, 4H), 7.37–7.30 (m, 4H),
7.23 (d, J = 8.5 Hz, 2H), 4.38 (t, J = 7.0 Hz, 4H), 4.21 (t, J = 6.4 Hz, 2H), 2.03–1.88 (m, 6H),
1.70–1.22 (m, 30H), 1.04–0.85 (m, 9H). 13C NMR (101 MHz, CDCl3) δ 161.74, 145.30, 141.26,
140.97, 139.97, 135.93, 135.04, 132.48, 126.46, 125.98, 124.15, 123.85, 123.55, 123.40, 122.82,
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120.51, 119.05, 117.22, 114.70, 109.07, 108.92, 68.49, 43.29, 31.81, 29.73, 29.41, 29.39, 29.29,
29.28, 29.19, 29.02, 27.34, 26.14, 22.71, 22.62, 14.14, 14.07. MALDI-TOF-MS, m/z: calcd for
C63H73BF2N4O [M]+: 950.585; found 950.769. HRMS, m/z: calcd for C63H73BF2N4O [M]+:
950.5845; found 950.5834.
3. Results and discussion
3.1. Synthesis and thermal property
The synthetic routes for the four novel D-A-D type BODIPY dyes are depicted in Scheme 1.
For the construction of these dyes, we utilized the Suzuki and Stille cross-coupling reaction to
connect two electron-donating moieties covalently to the 2,6-positions of BODIPY nucleus, where
a bisbromated BODIPY 5 should be prerequisite for the synthesis. Thus, our synthetic approach
started with the preparation of 5 from commercially available 4-hydroxybenzaldehyde 1 through a
four-step sequence involving alkylation-condensation-chelation-bisbromination reactions. In
particular, alkylation of 1 with 1-bromooctane readily gave 2 in quantitatively. Condensation of 2
with excess pyrrole using InCl3 as catalyst efficiently afforded the desired dipyrromethane 3. Then,
chelating reaction was performed between 3 and BF3·Et2O in the presence of chloranil to give the
corresponding BODIPY 4. Subsequently, bisbromination of 4 with NBS in a mixture of DMF and
CH2Cl2 solution efficiently provided the key intermediate 5. Finally, the classical Stille and Suzuki
cross-coupling reactions were performed to execute the conversion from 5 to the target BODIPY
dyes BDP1-4. These BDPs exhibit good solubility in common organic solvents such as ethyl
acetate, dichloromethane and THF. All these intermediates and the final BODIPY dyes were
readily available, and the success in the synthesis of these products was fully characterized by a
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range of spectroscopies including 1H NMR, 13C NMR, MALDI-TOF-MS and HRMS.
TGA and DSC curves provide useful information about the thermostability of these four dyes.
The TGA curves of BDP1-4 are depicted in Fig.1, and the corresponding data are summarized in
Table 1. The decomposition temperature (Td) level of 221 oC for BDP1, 305 oC for BDP2, 328 oC
o
for BDP3 and 301 C for BDP4 are observed at a 5% weight loss under nitrogen protection,
respectively. In DSC, the dyes were heated from 25 to 300 oC and then again cooled to 25 oC with
a rate of 10oC/min under nitrogen flow. The chromatograms are shown in Fig S1-Fig S4 (ESI).
BDP1 exhibits an endothermic peak at 157 oC, which indicates the melting temperature of BDP1.
o
This melting temperature is approximately consistent with the value (156-158 C) obtained by
melting point meter. Nevertheless, no exothermic peak is observed during the cooling process in
o
o
o
DSC curve. Similarly, BDP2-4 display endothermic peaks at 172 C, 180 C and 127 C,
respectively. The results show that all of these BDPs have high thermal stability and good enough
for the application in optoelectronic devices.
3.2. Electronic absorption
The normalized absorption spectra of these BODIPY dyes BDP1-4 in dichloromethane solution
are depicted in Fig. 2, and the corresponding optical data are summarized in Table 1. As shown in
Fig. 2, the absorption spectrum of the reference molecule 4 is characterized by a strong S0→S1
(π-π*) transition at 509 nm and a weaker broad band around 420 nm arising from the S0→S2 (π-π*)
transition. BDP1, BDP2, BDP3, and BDP4 show broad absorption with three distinct bands in the
absorption spectra. The first one at the shorter wavelength region at around 280-350 nm is
assigned to localized π-π* transition of the electron-rich donors, such as thiophene, bithiophene,
fluorene and carbazole moieties. The second one region at around 350-500 nm is attributed to the
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S0→S2 (π-π*) transition of BODIPY core. The third one at the longer wavelength region at around
500-800 nm is ascribed to a mixture of S0→S1 (π-π*) transition and the intramolecular charge
transfer (ICT) transition between electron-rich donors and an electron- deficient acceptor.
Compared with reference 4, the maximum absorption wavelength λmax is red-shifed by 112 nm for
BDP1, 169 nm for BDP2, 115 nm for BDP3, and 146 nm for BDP4, which can be assigned to the
extended π-conjugation of the backbone. BDP2 and BDP4 display more bathochromic shifts than
BDP1 and BDP3 can be explained as bithiophene and carbazole segments inherently possess
stronger electron-donating ability, resulting in more intensive ICT transitions. Fig. 2 also reveals
that the absorption spectra of BDPs lie in the broad region range of 300–800 nm from the visible
to a portion of the NIR region of the solar spectrum, which is a desirable property for generating
higher photocurrents in solar cells. The optical band gaps (Egopt, Table 1) of the four BDPs
estimated from the UV-vis onset absorptions are 1.71, 1.59, 1.77, and 1.65 eV for BDP1, BDP2,
BDP3 and BDP4, respectively. Noteworthy, all these dyes show high extinction coefficients range
from 7.8 × 104 to 1.2 × 105 cm-1 M-1 in solution, which ensured their excellent light harvesting
ability. In comparison with the absorption spectrum in CH2Cl2 solution, broader absorption bands
with obvious bathochromic shifts and higher coefficients were observed in the solid state (Fig. 3).
This could be attributed to the molecular stacking in solid state, which suggests an increased
conjugation length in the solid state due to the π-π stacking of the nearly planar conformations of
BODIPY. Consequently, these BDPs exhibit panchromatic absorption covering the wavelength
from 300 nm to 900 nm with extinction high coefficients. Furthermore, all these dyes exhibit
perfect solubility and film-forming ability. The desirable combination of attractive photophysical
characteristics, good solubility, favorable film-forming ability and functional group tolerance
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indicate that these dyes are promising applying in solution processed organic solar cells devices.
3.3. Photoluminescence
The photoluminescence of complexes BDP1-4 in different solvents were investigated at room
temperature. Rhodamine B (= 0.71) was used as standard in methanol to estimate the fluorescence
quantum yield [44].Their normalized emission spectra in CH2Cl2 solution at a concentration of 1×
10-7 mol/L are illustrated in Fig. 4. The emission band maxima and quantum yields are listed in
Table 1. Excitation of these compounds at their respective three absorption bands at room
temperature resulted in the same near-infrared emissive BODIPY dyes with emission maxima of
671, 748, 668 and 720 nm, respectively. This can be explained on the basis of a photo-induced
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 state to the ground state [45]. The results also imply that there is a thorough intermolecular
energy transfer of the excitons from the BODIPY core to the donor unit. These BODIPY dyes with
donor units at 2,6-positions (BDP1-4) present red-shifted emissions compared to the BODIPY 4,
which indicate that the substituents significantly affect the emission and bring forth an effective
intramolecular energy transfer for the excitons in the D-A units. The significant Stokes shifts of
these dyes are range from 1263 to 1567 cm-1. Taking these features into account, we can assign the
observed emission to the ICT emission. Solvent dependence study was performed in solvents with
various polarities, such as methanol, dichloromethane, acetone and ethyl acetate (Fig. S5, ESI).
The minor solvatochromic affect were observed for BDP1-4 indicate that these emissions were not
caused by solvent effect and/or aggregation effect. Additionally, the excitation spectra of BDP1-4
are essentially in agreement with their absorption spectra. These results further confirm the ICT
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emission nature of these dyes.
On the other hand, BDP1-4 give low fluorescent quantum yields (Table 1), which could be
attributed to the increased internal conversion according to the energy gap law that states the
non-radiative deactivation probability of S0-S1 increases as the energy gap of S0-S1 decreases in a
highly extended conjugating system. In general, photoluminescence (PL) quenching provides the
direct evidence for exciton dissociation, and thus efficient PL quenching is necessary to obtain
efficient organic solar cells.
3.4. Electrochemical properties
In order to investigate the electrochemical behaviors of BDP1-4, the electronic states of these
dyes were studied through cyclic voltammetry in CH2Cl2 solution at a scan rate of 100 mVs-1
using Bu4NPF6 as supporting electrolyte. Potentials were standardized with ferrocene/ferrocenium
(Fc/Fc+) couple as internal reference vs Ag/AgCl. The cyclic voltammograms of BDP1-4 are
illustrated in Fig. 5, and their electrochemical properties are summarized in Table 2. As shown in
Fig. 5, dyes BDP1-4 present onset oxidation potential as 1.15, 0.91, 1.20 and 0.89 V, respectively.
It indicates that the aromatic substituents have significant effects on the Eoxonset. The reduction
potentials (Ered) calculated from Eox
- Eg are approximately matched with the experimental
onset
values obtained from reduction waves. The corresponding ionization potential (can be considered
as the highest occupied molecular orbital, HOMO, energy level) of BDP1-4 are calculated (-5.43,
-5.19, -5.48 and -5.17 eV, respectively) from the equation EHOMO = [-(Eox
- 0.52) - 4.8] eV,
onset
where 0.52 V is the value for ferrocene versus Ag/AgCl and 4.8 eV is the energy level of Fc/Fc+
relative to the vacuum energy level. These dyes are actually located at very low energy levels,
which is attractive property in bulk-heterojunction (BHJ) solar cell device. The important factor
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for this result is that the strong electron donating group would cause an intensive ICT process. The
reduction potential versus NHE (Ered), which corresponds to the electron affinity (can be
considered as the lowest unoccupied molecular orbital, LUMO, versus NHE) can be obtained
from Eox
and Eg. The results are shown in Table 2. It should be noted that HOMO/LUMO
onset
levels of these dyes are favorable match with the n-type semiconductor such as PCBM, which
suggests that they are capable of OSCs applications.
3.5. Theoretical calculations
For a better understanding of the structural and electronic features of these novel dyes, the
TD-DFT theoretical calculations were further performed, and the optimized structures and the
electronic distribution in HOMO and LUMO levels are presented in Fig. 6 and Fig. 7. All
calculations were carried out with the Gaussian 09 program suite by using the B3LYP method and
6-31 G* basis set. From the optimized ground-state geometries of the dyes in Fig. 6, we can see
that the dihedral angle between the donor units and the BODIPY framework are computed to be
12.2o for BDP1, 3.5o for BDP2, 25.7o for BDP3, and 22.8o for BDP4, respectively. The optimized
ground-state geometries indicate that all these dyes exhibit relatively coplanar molecule structures.
Generally, the high degree of molecular coplanarity facilitate the effect ICT transition in D-A
structural molecule, resulting in bathochromic shifted absorption, which is consistent with the
UV-vis absorption spectra observed in solution.
The electron distributions of BDP1–4 indicate that π-electrons in the HOMO 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. Therefore, the
HOMO–LUMO transition that is the dominant contributing configuration to the S1 state in
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BDP1–4 should be ascribed to a mixture of π-π* and ICT characters, which is consistent with their
absorption assignments. On the other hand, the calculated results clearly demonstrate that strong
electron-donating substituent with more coplanar geometry, such as dithiophene in BDP2,
increases the energy level of HOMO more than that of LUMO, thus the HOMO–LUMO gap
decreases, causing a bathochromic shift of the π-π*/ICT band. This trend follows that observed
from the UV-vis absorption measurement. Fig. 7 presents the HOMO–LUMO energy differences
cal
(energy band gaps, calculated Egcal) of BDP1–4. We can see that the trend of the predicted Eg
opt
values (1.09–1.43 eV) is in consistent with the estimated Eg (calculated from the ground-state
absorption, 1.59–1.77 eV) in Table 1.
3.6. Photovoltaic performance
BHJ OSCs were fabricated by solution processed with a general device structure of
ITO/PEDOT:PSS/active layer/Ca/Al to investigate the photovoltaic properties. The device
fabrication processes are described in detail in the Experimental section. The active layer
consisted of the donor of BDPs and the acceptor of PC61BM. In this contribution, optimal
fabrication conditions were obtained from a CHCl3 solution with a typical concentration of 25
mg/mL at room temperature. As we known, the performances of OSCs could be strongly affected
by the processing parameters, including the BDPs/PC61BM ratio (w/w), thicknesses and annealing
temperatures of the photoactive layer. In order to understand these parameters influence on the
photovoltaic properties, the BDP1:PC61BM based devices at different donor-acceptor ratios, as
well as thicknesses of the photoactive layer and the annealing temperatures were systematically
investigated, and the corresponding data are summarized in Table S1 (ESI). The best performance
was achieved with a donor-acceptor ratio of 1:0.5 and the active layer thickness of 110 nm
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obtained from a CHCl3 solution at a total solid concentration of 25 mg/mL under a spin-coating
rate of 1500 rpm, followed by annealing at 100 oC for 10 min.
At these optimized conditions, Fig. 8 shows the current density-voltage (J-V) characteristics of
these devices using BDPs as donor and PC61BM as acceptor, and the corresponding data are listed
in Table 3. As a result, a maximal PCE of 1.49% with a Voc of 0.64 V, a Jsc of 6.02 mA cm2 and FF
of 39% was obtained in the BDP1 based device. As we expected, the BDP2 based device
demonstrated the highest PCE of 2.15% with a Voc of 0.78 V, a Jsc of 6.77 mA cm2 and FF of 41%.
The PCE of 1.06% and 1.56% were obtained in the BDP3 and BDP4 based devices, respectively.
It is obvious that the better performance of the BDP1, BDP2 and BDP4 based devices relative to
BDP3 based device is mainly contributed by the significantly higher Jsc values, which probably
cause by their broader absorption in visible and near-infrared range.
The external quantum efficiency (EQE) curves of the devices under the optimized conditions
were also investigated. As shown in Fig. 9, the EQE curves for these BDPs based devices exhibit
very broad wavelength range from 300 nm to nearly 900 nm, which match their absorption
assignments. The maximum EQE of 35% at 692 nm is observed in the BDP2 based device.
According to the EQE curves and the solar irradiation spectra, the integral current density of
values of OSCs are calculated to be 5.96, 6.63, 3.65 and 5.21 mA cm-2 for BDP1, BDP2, BDP3
and BDP4, respectively, which are in consistent with the values obtained from J-V measurements.
3.7. Hole mobility of the BDPs films
.
Generally, a high hole mobility is beneficial for efficient hole transport within an active layer
without large photocurrent loss caused by recombination with opposite charges. In order to get
some ideas about the influence of the chemical structures on their charge transporting properties,
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hole mobilities (ꢀh) of the hole-only devices were measured by the space-charge limited-current
(SCLC) method [46] with a device structure of ITO/PEDOT:PSS/active layer/Au. The SCLC is
estimated by the Mott Gurney equation [47]: J = 9ε0εrꢀhV2/8L3, in which J is current density, L is
film thickness of active layer (85 nm), ꢀh is hole mobility, εr is relative dielectric constant of the
transport medium, ε0 is permittivity of free space (8.85 × 10-12 F m-1), V is internal voltage in the
device and V = Vappl –Va –Vbi, where Vappl is applied voltage, Va is voltage drop, and Vbi is the
built-in voltage due to the relative work function difference of the two electrodes. As depicted in
Fig. 10, the hole-only mobilities of these BDPs are calculated to be 8.92 × 10-5 cm2 V-1 s-1, 6.35 ×
10-4 cm2 V-1 s-1, 3.11× 10-5 cm2 V-1 s-1 and 6.02 × 10-5 cm2 V-1 s-1 in the hole-only BDPs/PC61BM
based devices, respectively (Table 3). It is obvious that BDP2 presents relatively higher hole
mobility than BDP1, BDP3 and BDP4, which is also responsible for the high FF of BDP2 and the
better performance of the devices using this molecule. It firmly verifies that rigid molecular
structure facilitates the hole mobility.
3.8. Morphologies of the active layers
.
Atomic force microscopy (AFM) can provide useful information about D-A blend films, such as
their miscibility, distributions, and interpenetrating net structures. To understand the differences in
device performance when using different BDPs as donors, the morphology of blend film under the
optimized condition (D/A weight ratio of 1:0.5) was tested using AFM. Fig. 11 shows the AFM
height and phase images of the blend films of the four BDPs and PC61BM. The film surface are
relatively flat with root-mean-square (RMS) roughness of 0.496 nm for BDP1/PC61BM, 0.563 nm
for BDP2/PC61BM, 0.436 nm for BDP3/PC61BM and 0.581 nm for BDP4/PC61BM, respectively,
and relatively small phase domains are observed in the height images. This indicates that these
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BDPs have good miscibility with PC61BM. The phase images in Fig.11 demonstrate that the
donor/acceptor interpenetrating network of the blend films of BDP2 and BDP4 are better than
those of BDP1/PC61BM and BDP3/PC61BM. In general, good surface morphology would lead to
higher exciton dissociation efficiency and thus improve device performance. The good
photovoltaic performance of the BDP2/PC61BM active layer should be also related to the suitable
morphology of the active layer.
4. Conclusions
In summary, a series of novel symmetrical D-A-D structured BODIPY-based dyes, BDP1-4,
featuring an electron-withdrawing BODIPY motif as the central core, and several types of
electron-donating segments, including thiophene, bithiophene, fluorene and carbazole, flanked at
its 2,6-positions as terminal groups were successfully synthesized and characterized. These BDPs
are soluble in common organic solvents, possess panchromatic absorption covering the
wavelength range 300−900 nm, and have relatively lower HOMO energy levels of −5.17 ∼ −5.48
eV. The BDPs exhibit excellent thermal stability and good miscibility with PC61BM. BDP2 show
not only good photoelectric characteristic but also superior charge transport property and
favorable nanoscale morphology relative to BDP1, BDP3 and BDP4 and thus pronounce to the
higher PCE value of 2.15% under the illumination of AM 1.5G, 100 mW cm-2 with a Voc of 0.78 V,
a Jsc of 6.77 mA cm2 and FF of 41%. The results indicate that these series of BODIPY based dyes
are potential good candidates for the efficient organic solar cell materials. Further engineering of
the molecular structure and optimization of the morphology, and variation of the donor moieties,
are currently underway in our laboratory in the quest to boost device efficiencies.
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Acknowledgements
We greatly acknowledge the financial support from the Nature Science Foundation of
Guangdong Province (2014A030313630), the National Nature Science Foundation of China
(51476036, 21342003), Project for High-level Personnel of University in Guangdong Province
([2013]246), the Production-Study Research Combinational Project of Guangdong Province and
Ministry of Education (2012B091100296), the University Science and Technology Innovation
Key Project of Guangdong Province (cxzd1148) and the University and Scientific Research
Institution Key Project of Dongguan City (2012108101005).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
References
[1] Li G, Zhu R, Yang Y. Polymer solar cells. Nat Photonics 2012; 6:153-61.
[2] Cheng YJ, Yang SH, Hsu CS. Synthesis of conjugated polymers for organic solar cell
applications. Chem Rev 2009;109:5868-923.
[3] Li Y. Molecular design of photovoltaic materials for polymer solar cells: toward suitable
electronic energy levels and broad absorption. Acc Chem Res 2012;45:723-33.
[4] Dou L, You J, Hong Z, Xu Z, Li G, Street RA, Yang Y. 25th Anniversary article: a decade
of organic/polymeric photovoltaic research. Adv Mater 2013;25:6642–71.
[5] Lin YZ, Li YF, Zhan XW. Small molecule semiconductors for high-efficiency organic
photovoltaics. Chem Soc Rev 2012;41:4245-72.
[6] You JB, Dou LT, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. A polymer tandem solar
19

ACCEPTED MANUSCRIPT
cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446-55.
[7] Liu YS, Chen CC, Hong ZR, Gao J, Yang YM, Zhou HP, et al. Solution-processed
small-molecule solar cells: breaking the 10% power conversion efficiency. Sci Rep
2013;3:3356-63.
[8] You JB, Chen CC, Hong ZR, Yoshimura K, Ohya K, Xu R, et al. 10.2% power conversion
efficiency polymer tandem solar cells consisting of two identical sub-cells. Adv Mater
2013;25:3973-8.
[9] Zhang Q, Kan B, Liu F, Long G, Wan X, Chen X, et al. Small-molecule solar cells with
efficiency over 9%. Nat Photonics 2014;9:35-41.
[10] Kan B, Zhang Q, Li M, Wan X, Ni W, Long G, Wang Y, et al. Solution-processed organic
solar cells based on dialkylthiol-substituted benzodithiophene unit with efficiency near 10%,
J Am Chem Soc 2014;136:15529-32.
[11] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells. Chem Rev
2010;110:6595-663.
[12] Marsico F, Turshatov A, Weber K, Wurm FR. A metathesis route for BODIPY labeled
polyolefins. Org Lett 2013;15:3844-7.
[13] Ziessel R, Ulrich G, Haefele A, Harriman A. An artificial light-harvesting array constructed
from multiple bodipy dyes. J Am Chem Soc 2013;135:11330-44.
[14] Kamkaew A, Lim SH, Lee HB, Kiew LV, Chung LY, Burgess K. BODIPY dyes in
photodynamic therapy. Chem Soc Rev 2013;42:77-88.
[15] Kim BS, Ma B, Donuru VR, Liu H, Fréchet JMJ. Bodipy-backboned polymers as electron
donor in bulk heterojunction solar cells. Chem Commun 2010;46:4148-50.
20

ACCEPTED MANUSCRIPT
[16] Bura T, Leclerc N, Fall S, Leveque P, Heiser T, Retailleau P, et al. High-performance
solution-processed solar cells and ambipolar behavior in organic field-effect transistors with
thienyl-BODIPY scaffoldings. J Am Chem Soc 2012;134:17404-7.
[17] Bessette A, Hanan GS. Design, synthesis and photophysical studies of dipyrromethene-based
materials: insights into their applications in organic photovoltaic devices. Chem Soc Rev.
2014:43:3342-405.
[18] Loudet A, Burgess K. BODIPY dyes and their derivatives:ꢀ syntheses and spectroscopic
properties. Chem Rev 2007;107:4891-932.
[19] Donuru VR, Vegnesa GK, Velayudham S, Green S, Liu H. Synthesis and optical properties
of red and deep-red emissive polymeric and copolymeric BODIPY dyes. Chem Mater
2009;21:2130-8.
[20] Meng G, Velayudham S, Smith A, Luck R, Liu H. Color tuning of polyfluorene emission
with BODIPY monomers. Macromolecules 2009;42:1995-2001.
[21] Wang Y, Chen J, Zhen Y, Jiang H, Yu G, Liu Y, et al. Synthesis and properties of novel
near-infrared dye based on BODIPY and diketopyrrolopyrrole units. Mater Lett
2015;139:130-3.
[22] Rousseau T, Cravino A, Bura T, Ulrich G, Ziessel R, Roncali J. BODIPY derivatives as
donor materials for bulk heterojunction solar cells. Chem Commun 2009;1:673-5.
[23] Rousseau T, Cravino A, Bura T, Ulrich G, Ziessel R, Roncali J. Multi-donor molecular bulk
heterojunction solar cells: improving conversion efficiency by synergistic dye combinations.
J Mater Chem 2009;19:2298-300.
[24] Rousseau T, Cravino A, Ripaud E, Leriche P, Rihn S, De Nicola A, et al. A tailored hybrid
21

ACCEPTED MANUSCRIPT
BODIPY-oligothiophene donor for molecular bulk heterojunction solar cells with improved
performances. Chem Commun 2010;46:5082-4.
[25] Lin HY, Huang WC, Chen YC, Chou HH, Hsu CY, Lin JT, et al. BODIPY dyes with
β-conjugation and their applications for high-efficiency inverted small molecule solar cells.
Chem Commun 2012;48:8913-5.
[26] Liu W, Tang A, Chen J, Wu Y, Zhan C, Yao J. Photocurrent enhancement of BODIPY-
based solution-processed small-molecule solar cells by dimerization via the meso position.
ACS Appl Mater Interfaces 2014;6:22496-505.
[27] Xiao L, Wang H, Gao K, Li L, Liu C, Peng X, et al. A-D-A type small molecules based on
boron dipyrromethene for solution-processed organic solar cells. Chem Asian J
2015;10:1513-8.
[28] Zhang X, Zhang Y, Chen L, Xiao Y. Star-shaped carbazole-based BODIPY derivatives with
improved hole transportation and near-infrared absorption for small-molecule organic solar
cells with high open-circuit voltages. RSC Adv 2015;5:32283-9.
[29] Shimizu S, Iino T, Saeki A, Seki S, Kobayashi N. Rational molecular design towards
Vis/NIR absorption and fluorescence by using pyrrolopyrrole aza-BODIPY and its highly
conjugated structures for organic photovoltaics. Chem Eur J 2015;21:2893-904.
[30] Mirloup A, Leclerc N, Rihn S, Bura T, Bechara R, Hébraud A, et al. A deep-purple-grey
thiophene–benzothiadiazole–thiophene BODIPY dye for solution-processed solar cells. New
J Chem 2014;38:3644-53.
[31] Kim YJ, Lee GB, Jeon CW, Kim Y-H, Chung DS, Park CE. A push–pull organic
semiconductor with efficient intramolecular charge transfer for solution-processed small
22

ACCEPTED MANUSCRIPT
molecule solar cells. RSC Adv 2015;5:3435-42.
[32] Chen JJ, Conron SM, Erwin P, Dimitriou M, McAlahney K, Thompson ME. High-efficiency
BODIPY-based organic photovoltaics. ACS Appl Mater Interfaces 2015;7:662-9.
[33] Cortizo-Lacalle1 D, Howells CT, Pandey UK, Cameron1 J, Findlay1 NJ, Inigo1 AR, et al.
Solution processable diketopyrrolopyrrole (DPP) cored small molecules with BODIPY end
groups as novel donors for organic solar cells. Beilstein J Org Chem 2014;10:2683-95.
[34] Liao J, Xu Y, Zhao H, Wang Y, Zhang W, Peng F, et al. Synthesis, optical, electrochemical
properties and photovoltaic performance of novel panchromatic meso- thiophene BODIPY
dyes with a variety of electron-donating groups at 3,5-positions. RSC Adv 2015;5:86453-62.
[35] Jadhav T, Misra R, Biswasb S, Sharma GD. Bulk heterojunction organic solar cells based on
carbazole–BODIPY conjugate small molecules as donors with high open circuit voltage.
Phys Chem Chem Phys 2015;17:26580-8.
[36] Liao J, Wang Y, Xu Y, Zhao H, Xiao X, Yang X. Synthesis, optical and electrochemical
properties of novel meso-triphenylamine-BODIPY dyes with aromatic moieties at 3,5-
positions. Tetrahedron 2015;71:5078-84.
[37] Zhao H, Liao J, Ning J, Xie Y, Cao Y, Chen L, et al. Efficient synthesis of novel
bis(dipyrromethanes) with versatile linkers via indium(III) chloride-catalyzed condensation
of pyrrole and dialdehydes. Adv Synth Catal 2010;352:3083-8.
[38] Duan ZF, Yang ZG, Liu DJ, Cai L, Hoshino D, Morita T, et al. Facile synthesis and
characterization of a novel thiophene-fused polycyclic aromatics based on pyrene. Chin
Chem Lett 2011;22:819-22.
[39] Liu W, Pink M, Lee, D. Conjugated polymer sensors built on π-extended borasiloxane cages.
23

ACCEPTED MANUSCRIPT
J Am Chem Soc 2009;131:8703-7.
[40] Li X-C, Liu Y, Liu MS, Jen AK-Y. Synthesis, properties, and application of new luminescent
polymers with both hole and electron injection abilities for light-emitting devices. Chem
Mater 1999;11:1568-75.
[41] Axel DB. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys
1993;98:5648-52.
[42] Gill PMW, Johnson BG, Pople JA, Frisch MJ. The performance of the Becke-Lee-Yang-Parr
(B-LYP) density-functional theory with various basis sets. Chem Phys Lett
1992;197:499-505.
[43] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian
09, revision B. 01. Wallingford CT: Gaussian, Inc; 2010.
[44] Karstens T, Kobs K. Rhodamine B and rhodamine 101 as reference substances for
fluorescence quantum yield measurements. J Phys Chem 1980;84:1871-2.
[45] Zhao H, Wang B, Liao J, Wang H, Tan G. Synthesis, spectral, electrochemical properties,
and photovoltaic performance of structurally constrained BODIPY dyes with 4,4’-dimethyl-
triphenylamine at the 2,6-positions. Tetrahydron Lett 2013;54:6019-22.
[46] Malliaras GG, Salem JR, Brock PJ, Scott C. Electrical characteristics and efficiency of
single-layer organic light-emitting diodes. Phys Rev B 1998;58:13411-4.
[47] Fan Q, Li M, Yang P, Liu Y, Xiao M, Wang X, et al. Acceptor-donor-acceptor small
molecules containing benzo[1,2-b:4,5-b']dithiophene and rhodanine units for solution
processed organic solar cells. Dyes Pigments 2015;116:13-9.
24

ACCEPTED MANUSCRIPT
Scheme 1. The synthetic route of BDP1-4.
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Fig. 1. TGA curves of BDP1-4 at a heating rate of 20 oC min-1 under nitrogen atmosphere.
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Fig. 2. Normalized absorption spectra of BODIPY 4 and dyes BDP1-4 in CH2Cl2 solution.
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Fig. 3. Normalized absorption spectra of dyes BDP1-4 in quartz film.
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