Efficient polymer solar cells processed by environmentally friendly halogen-free solvents

Article abstract of DOI:10.1039/c6ra04835h

The use of environmentally friendly halogen-free organic solvents for the fabrication of polymer solar cells will be of great importance for future practical applications. In this work, a new alternative conjugated polymer with 3,4-bis(octyloxy)-phenyl su

Full text of DOI:10.1039/c6ra04835h

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Efficient polymer solar cells processed by
environmentally friendly halogen-free solvents†
Cite this: RSC Adv., 2016, 6, 39074
Zhe Zhang, Xuejuan Zhang, Jicheng Zhang, Xue Gong, Yahui Liu, Heng Lu,
Cuihong Li* and Zhishan Bo*
The use of environmentally friendly halogen-free organic solvents for the fabrication of polymer solar cells
will be of great importance for future practical applications. In this work, a new alternative conjugated
polymer with 3,4-bis(octyloxy)-phenyl substituted benzo[1,2-b:4,5-b]dithiophene as the donor unit and
benzo[c][1,2,5]thiadiazole as the acceptor unit was synthesized and used as the donor material for
polymer solar cells. This polymer showed good solubility in halogen-free solvents such as toluene, o-
xylene and so on. The blend film morphology, charge mobility and photovoltaic performance were
investigated in halogen-free solvents. The photovoltaic devices fabricated from o-xylene with N-methyl-
Received 23rd February 2016
Accepted 12th April 2016
2-pyrrolidone as additive provided the best power conversion efficiency of 4.57%, comparable to that
fabricated from halogenated solvents such as 1,2-dichlorobenzene/1,8-diiodooctane with a power
conversion efficiency of 4.33%. Our results demonstrate that halogen-free solvents are promising for the
fabrication of high efficiency polymer solar cells.
DOI: 10.1039/c6ra04835h
www.rsc.org/advances
P3HT : ICBA blend by halogen-free solvents and achieved
a comparable PCE that is similar to the devices prepared from
Introduction
Bulk heterojunction (BHJ) organic solar cells (OSCs) have DCB. Hou et al. modied conjugated polymers and fullerene
experienced rapid progress in the past decade and a power derivatives by triethylene glycol monoether functional group
conversion efficiency (PCE) higher than 9% has been achieved and a PCE of 5.23% was obtained by processing the active layer
1
–11
for both single junction and tandem OSCs.
advantage of OSCs is that they can be fabricated by low cost inert phosphonate moiety to the side chain of conventional
solution processing technology.
The main with halogen-free solvents. Wang et al. attached a polar and
12–18
The most frequently used photovoltaic polymers and a PCE of 2.11% was achieved by
solvents for the fabrication of OSCs are halogenated organic using anisole as the processing solvent. It is still challenging to
solvents such as chlorobenzene (CB) and o-dichlorobenzene fabricate PSCs using halogen-free organic solvents to achieve
(
DCB) that possess high boiling points, adequate viscosity, and efficiency as high as that fabricated using halogenated solvents.
superior solubility for nonpolar conjugated oligomers/polymers Nevertheless, the use of environmentally friendly halogen-free
and fullerene derivatives. Unfortunately, these halogenated organic solvents for the fabrication of PSCs will be of great
solvents are detrimental to human health and the environment, importance for future practical applications.
thus their use should be carefully considered from the view-
points of large-scale roll-to-roll production and practical appli- and so on are biodegradable that produce little negative effect
Non-halogenated organic solvents, such as toluene, o-xylene
19–21
cation.
Increasing attention has been paid to the use of on environment and human health. In comparison with halo-
halogen-free organic solvents, such as toluene, o-xylene, 1,2,4- genated organic solvents, halogen-free organic solvents are
trimethylbenzene, 1,2-dimethylnaphalene, anisole and so on, to usually poor solvents for fullerene derivatives such as PC71BM.
22–34
replace halogenated ones for fabricating PSCs devices.
For When halogen-free organic solvents were used as processing
example, Jen et al. used toluene solvents to fabricate devices solvent, during the drying of active layer fullerene derivatives
based on PIDII-DFBT : PC BM blends and PCE of 7.26% has will rst precipitate to form large aggregates, leading to unfa-
7
1
35–37
been achieved. Li et al. fabricated the devices based vorable morphology and low efficiency.
It is possible to
adjust the solubility of PC BM in halogen-free organic solvents
7
1
by using the additive. N-Methyl-2-pyrrolidone (NMP), which has
Beijing Key Laboratory of Energy Conversion and Storage Materials, College of
Chemistry, Beijing Normal University, Beijing 100875, China. E-mail: licuihong@
bnu.edu.cn; zsbo@bnu.edu.cn
ꢀ
a boiling point of 202 C and is a good solvent for PC71BM (48
ꢁ1
mg mL ), can be used as additive for halogen-free organic
38
solvents to tune the morphology of active layer. In addition,
†
Electronic supplementary information (ESI) available: Experimental part
1
including TGA curves, DSC curves, XRD curves, SCLC curves, and H NMR and NMP is minimally ammable, of low toxicity and environmen-
13
1
C NMR spectra of monomers and H NMR spectra of polymers. See DOI:
tally friendly. Encouraged by these ideas, we designed and
1
0.1039/c6ra04835h
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ꢁ1
synthesized bis(alkoxy)phenyl substituted benzo[1,2-b:4,5-b] 43 kg mol
and 3.0, respectively. The related data are
dithiophene (BDT) based polymers (PBBDTBT), since BDT summarized in Table 1. PBBDTBT was fully soluble in common
based polymers have been proved to be excellent donor mate- organic solvents such as dichloromethane (DCM), chloroform
5
,11,39,40
rials.
The chemical structure of PBBDTBT is shown in (CF), o-xylene and DCB at room temperature, and could be fully
ꢀ
Scheme 1. As expected, PBBDTBT showed a good solubility in dissolved in toluene at about 50 C. Thermogravimetric analysis
halogen-free organic solvents such as toluene and o-xylene. (TGA) and differential scanning calorimetry (DSC) experiments
Aer the addition of NMP as additive, the solubility of PBBDTBT were carried out under nitrogen atmosphere at a heating rate of
in the solvent mixture decreased, leading to proper phase 10 C min to investigate thermal properties of PBBDTBT. As
separation for the blend lms and enhanced photovoltaic shown in Fig. S3 and S4,† the TGA curve demonstrated that
performance for the devices. The initial test revealed that the PBBDTBT is of good thermal stability with a 5% decomposition
photovoltaic performance of PBBDTBT : PC BM based devices temperature up to 333 C. DSC measurement showed no
fabricated with o-xylene/NMP as the processing solvent obvious glass transition in the range of 60 to 250 C, reecting
produced a PCE of 4.57%, higher than the PCE of 4.33% for PBBDTBT could be amorphous. X-ray diffraction (XRD) experi-
ꢀ
ꢁ1
ꢀ
7
1
ꢀ
devices fabricated from halogenated solvents (DCB/DIO).
ments were carried out to investigate the packing properties of
PBBDTBT in the solid state. As shown in Fig. S5,† PBBDTBT
ꢀ
presents two diffraction peaks, the rst peak at 3.67 in the
Results and discussion
Material synthesis and characterization
small angle region could be ascribed to the distance between
polymer chains separated by exible side chains and the second
ꢀ
broad peak centered at 22.55 in the wide angle region could be
Synthetic routes for the monomer and polymer are shown in
ascribed to the p–p stacking distance of polymer backbones.
According to the Bragg equation, these distances were calcu-
41
42
43,44
Scheme 1. Compound 1,
2
and 5
were synthesized
according to the literature procedure. The treatment of
compound 2 with n-BuLi by lithium halogen exchange reaction
afforded lithium intermediate, which was quenched with
ꢀ
lated to be 24.09 and 3.91 A, respectively. Such a broad
diffraction peak in the wide angle region indicated that this
polymer is amorphous.
0
commercially available benzo[1,2-b:4,5-b ]dithiophene-4,8-
dione to afford the intermediate diol. The diol was converted
to compound 3 in a yield of 85% by the reaction with SnCl2- Optical and electrochemical properties
$
2
2H O in dilute hydrochloride acid. Abstraction of the two
UV-vis absorption characteristics of PBBDTBT were investigated
active hydrogen atoms of compound 3 by n-BuLi was followed
by reaction with trimethyltin chloride to furnish monomer 4 in
a yield of 87%. Stille polycondensation of monomer 4 and
compound 5 afforded copolymer PBBDTBT as a black-blue
in different solvents such as toluene, o-xylene and DCB (1 ꢂ
ꢁ5
10
M) at room temperature and as lms. As shown in Fig. 1, in
toluene, o-xylene and DCB solutions PBBDTBT exhibited similar
absorption spectra with a weak absorption peak at 419 nm and
an intense absorption peak at 574 nm and a should peak at
solid. The number average molecular weights (M ) and poly-
n
dispersity (PDI) of PBBDTBT were determined by gel perme-
ation chromatography (GPC) using THF as an eluent and
narrowly distributed polystyrenes as calibration standards to be
ꢁ1
about 612 nm, with a molar extinction coefficient of 50.34 L g
ꢁ1
cm in o-xylene. The rst peak at 419 nm could be ascribed to
the p–p* transition and the second peak and the shoulder
could be attributed to the intramolecular charge transfer
interaction (ICT) between BDT unit and benzo-2,1,3-thiadiazole
7
unit. In going from solution to lm, PBBDTBT displayed
a broader spectrum with two red-shied peaks at 433 and 590
nm and a shoulder peak at 625 nm, indicating the formation of
ordered stacking in some extent as lms. The absorption data
are also summarized in Table 1. The optical band gap (Eg,opt) of
PBBDTBT was evaluated from the lm absorption onset to be
1.76 eV. Upon blending with PC71BM, lms spin coated from
toluene and o-xylene solutions revealed similar absorption
properties; whereas lms spin coated from DCB solutions
exhibited a slightly broad absorption peak with the shoulder
peak at 630 nm became more intense, indicating that polymer
chains could form more ordered packing when spin coated
from DCB solutions. The absorption valley of polymers in the
short wavelength region was compensated by the absorption of
PC71BM.
Electrochemical properties of PBBDTBT were investigated by
cyclic voltammetry (CV) with a standard three-electrode elec-
trochemical cell. As shown in Fig. 2, the oxidation onset of
PBBDTBT lm was determined to be 0.72 V and the HOMO level
Scheme 1 Synthetic routes for PBBDTBT.
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Table 1 Physical, electronic, and optical properties of PBBDTBT
a
d
ꢀ
b
Polymer
T
( C)
l (nm) solution
l (nm) lm
E
g,opt (eV)
HOMO (eV)
LUMO (eV)
PBBDTBT
333
419, 574
433, 590, 627
1.76
ꢁ5.43
ꢁ3.67
a
b
T
d
is the 5% weight-loss temperature under N
2
atmosphere. Calculated from the onset absorption of PBBDTBT, Eg,opt ¼ 1240/lonset
.
Fig. 2 Cyclic voltammogram of PBBDTBT as films on a platinum
ꢁ
1
4 6
electrode in 0.1 mol L Bu NPF acetonitrile solution at a scan rate of
ꢁ
1
1
00 mV s .
fabricated from halogen-free solvents such toluene and o-xylene.
Devices fabricated from DCB solutions were also tested for
a comparison. All devices with varied D/A ratios exhibited in
Table S1,† and the best photovoltaic performance in a donor/
acceptor weight ratio of 1 : 1.5. J–V curves of optimized devices
measured under simulated solar illumination (AM1.5G, 100 mW
ꢁ2
cm ) are shown in Fig. 3a and the device parameters are
summarized in Table 2. For plain devices fabricated with toluene
and o-xylene as the processing solvents, inferior PCEs of 0.73%
and 1.35% were achieved; whereas a PCE of 3.22% was obtained
using DCB as the processing solvent. The different photovoltaic
performance might be due to the inferior solubility of PC71BM in
toluene or o-xylene to DCB. The boiling point and evaporation
speed of processing solvent have a signicant inuence on the
morphology of blend lms. Active layers spin-coated from
toluene and o-xylene solutions exhibited a severe phase separa-
30
tion with larger spherical grains. The unfavorable lm
morphology is responsible for the poor photovoltaic performance
of devices fabricated with toluene or o-xylene as the processing
Fig. 1 UV-vis absorption spectra of (a) PBBDTBT in diluted toluene, o-
xylene and DCB solutions and as films; (b) PBBDTBT : PC71BM (1 : 1.5
by weight) blending films spin coated from toluene, o-xylene and DCB
solutions; (c) molar extinction coefficient of PBBDTBT in diluted o- solvent. To tune the morphology of blend lms different addi-
xylene.
tives were utilized. When 3% NMP (by volume) was used as
additive in the toluene solution, an improved PCE of 2.10% with
ꢁ2
V
oc of 0.71 V, Jsc of 5.80 mA cm and FF of 0.51 was achieved.
of PBBDTBT was calculated to be ꢁ5.43 eV using the equation: Similarly, higher PCE of 4.57% with V of 0.73 V, Jsc of 8.94 mA
oc
ꢁ2
E
¼ ꢁe(E + 4.71) (eV). The LUMO level was calculated to cm and FF of 0.70 was obtained for devices fabricated from o-
HOMO
ox
be ꢁ3.67 eV according to the equation ELUMO ¼ EHOMO + Eg,opt
.
xylene with 4% NMP (by volume) as additive. The increased Jsc
and FF contributed to the overall enhanced photovoltaic perfor-
mance. In contrast, when DCB containing 3% DIO (by volume)
was used as the processing solvent, a PCE of 4.33% was achieved.
Notably, optimized devices fabricated from o-xylene/NMP solu-
tions showed superior performance to that fabricated from DCB/
DIO solutions, demonstrating that halogen-free solvent o-xylene
is promising for future commercial application.
The results are also summarized in Table 2.
Photovoltaic properties
A
conventional device structure of ITO/PEDOT : PSS/
PBBDTBT : PC71BM/LiF/Al was adopted to investigate the
photovoltaic performance of PBBDTBT : PC71BM based PSCs
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Table 2 Summary of the photovoltaic properties of the solar cells
Hole mobility
ꢁ2
a
2
ꢁ1 ꢁ1
Solvent
Toluene
o-Xylene
DCB
Solvent additive
No
V
oc (V)
J
sc (mA cm
)
FF
PCE (%) best/ave
d (nm)
(cm V
s )
ꢁ6
ꢁ5
ꢁ6
ꢁ5
ꢁ5
ꢁ5
0.77
0.71
0.78
0.73
0.76
0.71
2.77
5.80
3.86
8.94
6.62
8.97
0.34
0.51
0.45
0.70
0.64
0.68
0.73/0.61
2.10/1.96
1.35/1.24
4.57/4.50
3.22/3.11
4.33/4.25
194
171
128
138
124
137
4.15 ꢂ 10
1.05 ꢂ 10
8.73 ꢂ 10
1.48 ꢂ 10
1.18 ꢂ 10
1.52 ꢂ 10
3% NMP
No
4% NMP
No
3% DIO
a
The average PCEs were based on ten devices.
in Table 2. Devices were fabricated from different solvents with
a typical structure of ITO/PEDOT : PSS/PBBDTBT : PC71BM
(1 : 1.5 w/w)/Au. Electron mobilities of blend lms were
measured and the results are summarized in Table S2.† Devices
were fabricated from different solvents with a typical structure
of FTO/PBBDTBT : PC71BM (1 : 1.5 w/w)/Al. Dark J–V experi-
ments were carried out and the hole mobility of PBBDTBT was
2
3
calculated from J ¼ 93
o r
3 mV /8L . The curves are shown in Fig. S6
and S7.† PBBDTBT : PC71BM based plain devices fabricated
from toluene, o-xylene and DCB exhibited hole mobilities of
ꢁ
6
ꢁ6
ꢁ5
2
ꢁ1 ꢁ1
4
.15 ꢂ 10 , 8.73 ꢂ 10 , and 1.18 ꢂ 10
cm
V
s ,
ꢁ5
respectively. Hole mobilities were improved to 1.05 ꢂ 10 , 1.48
ꢁ
5
ꢁ5
2
ꢁ1 ꢁ1
ꢂ 10 , 1.52 ꢂ 10 cm V
s
for devices fabricated from
toluene containing 3% NMP, o-xylene containing 4% NMP, and
DCB containing 3% DIO, respectively. The improvement of hole
mobilities, which matched better the electron mobility, could
be ascribed to the optimization of lm morphology aer using
solvent additive. More balanced charge mobility is benecial for
the charge transport in the devices, leading to higher Jsc and FF.
The blend lm morphology
The morphology of the active layer has an obvious inuence on
the photovoltaic performance of devices. Therefore, atomic
force microscopy (AFM) was used to investigate the morphology
of blend lms spin coated from different solvents. As shown in
Fig. 4, the blend lms spin coated from toluene and o-xylene
solutions exhibited a relatively rough surface topography with
Fig.
3 (a) The J–V characteristics of devices based on
ꢁ2
PBBDTBT : PC71BM under the illumination of AM1.5G (100 mW cm );
(
b) EQE curves of solar cells fabricated under different conditions.
a
large spherical aggregates and the root-mean-square (R ) values
are 16.6 and 12.0 nm, respectively. Such high roughness values
can be attributed to low solubility of PC71BM in toluene and o-
xylene. Interestingly, aer NMP was used as the additive in
toluene and o-xylene, the blend lms showed distinct differ-
ences in morphologies from the system without NMP. Both
External quantum efficiencies (EQEs) were measured to
examine the accuracy of Jsc from the J–V measurements. As
shown in Fig. 3b, EQE spectra of the devices processed by
toluene, o-xylene, and DCB are in the range of 350 to 700 nm.
The results demonstrated that devices fabricated with additives
have higher EQE values than that fabricated without using
additives, indicating the use of additive is favorable to form
appropriate morphology for charge separation and transport.
The Jsc calculated from the integration of EQE curves agreed
well with that obtained from J–V measurement, verifying the
accuracy of the PCE obtained from J–V measurements.

lms became smooth, and the roughness values were decreased
to be 9.29 and 3.53 nm, respectively. The solubility of conju-
gated polymers and PC71BM has a signicant inuence on the
morphology of blend lms. PBBDTBT exhibited a good solu-
bility in toluene, o-xylene and DCB; whereas the solubility of
PC71BM in toluene and o-xylene is poorer than that in DCB.
When toluene or o-xylene is used as the processing solvent,
PC BM will rstly precipitate from the solution to form large
7
1
Hole and electron mobilities
aggregates during drying of the active layer. Aer adding NMP,
Hole mobilities of blend lms were measured with space charge the solubility of polymer in blend solutions will decrease,
limited current (SCLC) method and the results are summarized leading to the formation of polymer networks, which can
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mixture was stirred for 5 h at the room temperature. The
mixture was poured into ice water and extracted by dichloro-
methane (DCM) three times. The combined organic phase was
dried over magnesium sulfate and evaporated to afford the
crude product. Further purication was carried out on a silica
gel column, eluting with pure petroleum ether. White crystals
1
were obtained (1.92 g, 65%). H NMR (400 MHz, CDCl
3
): d 7.58
(
(
d, J ¼ 5.6 Hz, 1H), d 7.50 (s, 2H), d 7.09 (d, J ¼ 5.6 Hz, 1H), d 7.00
13
d, J ¼ 8.6 Hz, 1H), d 3.97–3.92 (m, 4H), d 1.89–0.99 (m, 30H).
C
NMR (100 MHz, CDCl ): d 149.11, 148.99, 138.23, 136.13,
3
1
3
2
31.92, 130.21, 127.11, 123.18, 121.86, 114.91, 113.67, 69.28,
1.87, 31.84, 29.45, 29.42, 29.33, 29.29, 26.13, 26.05, 22.71,
2.69, 14.13. Anal. calcd for C54
H
78
O
4
S
2
: C 75.83; H 9.19. Found:
C 75.87; H 9.52.
0
4
,8-Bis(1,2-bis(octyloxy)-4-phenyl)-benzo[1,2-b:4,5-b ]-
Fig. 4 AFM height images of PBBDTBT : PC71BM (1 : 1.5, w/w) blend
films spin-coated from different solvents.
dithiophene-2,6-bis(trimethyltin) (4)
To a solution of compound 3 (0.5 g, 0.585 mmol) in THF (40 mL)
ꢀ
at ꢁ78 C was slowly added 2.4 M n-butyllithium solution in n-
hexane (0.61 mL). Aer the addition, the solution was warmed
slowly to room temperature and kept for 1 h. Aer cooling down
prevent the formation of large spherical PC71BM aggregates
during the drying of blend lms. As for blend lms spin-coated
from DCB solutions, the morphology became rougher and the
ꢀ
to the ꢁ78 C, 1.0 M trimethyltin chloride solution in THF (1.29
mL) was added. The mixture was stirred overnight at the room
temperature and quenched with water. The mixture was
extracted with diethyl ether three times and dried by over
a
R value increased from 1.65 to 2.29 nm aer using DIO as
additive. Obviously, the addition of a small amount of polar
solvent as the additive to non-halogenated solvents such as
toluene and o-xylene can signicantly promote micro-phase
separation for the blend lms and have a profound effect on
the photovoltaic performance.
MgSO , the solvent was removed and the residue was recrys-
4
1
tallized from hexane to afford the compound 4 (0.62 g, 90%). H
3
NMR (400 MHz, CDCl ): d 8.04 (s, 1H), d 7.70–7.68 (m, 2H),
d 7.04 (d, J ¼ 8.04 Hz, 1H), d 4.00–3.93 (m, 4H), d 1.84–1.01 (m,
1
3
3
1
1
2
3
0H). d 0.39–0.25 (m, 9H). C NMR (100 MHz, CDCl ): d 148.93,
Conclusions
48.78, 142.47, 141.78, 136.99, 132.53, 131.00, 128.64, 121.80,
15.08, 113.51, 69.23, 69.18, 31.87, 31.84, 29.46, 29.42, 29.33,
9.32, 29.26, 26.15, 26.07, 22.72, 22.69, 14.14, 14.12. Anal. calcd
3,4-Bis(octyloxy)-phenyl substituted benzo[1,2-b:4,5-b]dithio-
phene (BDT) based polymer (PBBDTBT) was designed and
synthesized. PBBDTBT showed a good solubility in halogen-free
solvents such as toluene, o-xylene and so on. The inuence of
non-halogenated solvent on the morphology, charge mobility
for C H O S Sn : C 61.02; H 8.02. Found: C 61.07; H 7.86.
6
0
94
4
2
2
PBBDTBT
and photovoltaic performance was thoroughly investigated. 4 (120 mg, 0.1016 mmol) and 5 (72.62 mg, 0.1016 mmol) were
PBBDTBT : PC71BM based PSCs processed from non- dissolved in toluene (5 mL) and DMF (1 mL) in a 50 mL double-
halogenated solvent o-xylene/NMP produced the best PCE of neck round-bottom ask. The reaction container was carefully
ꢁ
2
4
.57% with a Voc of 0.73 V, a Jsc of 8.94 mA cm and an FF of degassed before and aer the addition of Pd(PPh ) (5.0 mg,
3 4
0.70, higher than the devices fabricated using halogenated 4.32 mmol). The mixture was stirred and reuxed under nitrogen
solvents such as DCB/DIO with a PCE of 4.33%. Our results have atmosphere for 24 h. Then the mixture was cooled to room
demonstrated that non-halogenated solvents are promising for temperature and poured into methanol (150 mL), the precipi-
the fabrication of high efficiency PSCs.
tates was subjected to Soxhlet extraction with methanol, hexane
and DCM. The polymer was recovered as a solid from the DCM
fraction by natural sedimentation from methanol. Polymers
were obtained as black-blue solid (106.5 mg, 73%). GPC (PS
Experimental part
0
4,8-Bis(1,2-bis(octyloxy)-4-phenyl)-benzo[1,2-b:4,5-b ]-
ꢁ1
ꢁ1
standards): M
.0.
n
¼ 43 kg mol , M
w
¼ 130 kg mol and PDI ¼
dithiophene (3)
3
Under protection of nitrogen, 2.4 M n-butyllithium (n-BuLi)
solution in n-hexane (3.05 mL) was slowly added to 4-bromo-1,2-
Acknowledgements
ꢀ
bis(octyloxy)benzene (3 g, 7.25 mmol) in THF (50 mL) at ꢁ78 C,
ꢀ
0
and the mixture was kept at ꢁ78 C for 1 h. Benzo[1,2-b:4,5-b ] Financial support from the NSF of China (51003006,
dithiophene-4,8-dione (0.76 g, 3.45 mmol) was added, and the 21161160443), the 973 Programs (2011CB935702), Beijing
mixture was stirred overnight at the room temperature. Then Natural Science Foundation (2132042), Program for Changjiang
2 2
SnCl $2H O (1.8 g) in 20% HCl (25 mL) was added and the Scholars and Innovative Research Team in University and the
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Fundamental Research Funds for the Central Universities are 22 K. S. Chen, H. L. Yip, C. W. Schlenker, D. S. Ginger and
gratefully acknowledged.
A. K. Y. Jen, Org. Electron., 2012, 13, 2870.
2
2
3 B. R. A ¨ı ch, S. Beaupr ´e , M. Leclerc and Y. Tao, Org. Electron.,
2014, 15, 543.
4 P. T. Tsai, C. Y. Tsai, C. M. Wang, Y. F. Chang, H. F. Meng,
Z. K. Chen, H. W. Lin, H. W. Zan, S. F. Horng, Y. C. Lai
and P. C. Yu, Org. Electron., 2014, 15, 893.
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