A facile method to synthesize [A'(D'AD)2]-based push-pull small molecules for organic photovoltaics

Article abstract of DOI:10.1039/c5ra06660c

An efficient route to synthesize, for the first time, a series of small molecules based on the [A'(D'AD)₂] architecture was developed using selective direct heteroarylation of the C-H bond with a Pd(AcO)₂/Bu₄NBr simple catalytic system. The C-H arylation of the unsymmetrical compound 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)benzo[c][1,2,5] thiadiazole (5) showed that the (C5) in the ethyldioxythiophene moiety is more reactive towards C-H arylation than its counterpart in the thiophene unit. The new small molecules bear two different acceptors and/or donors, which led to decrease the electron band gap and improve the strength of the push-pull system as well as the amount of intramolecular charge transfer from donor to acceptor units. Lactams and imide containing acceptors are used as second electron withdrawing units due to their well conjugated structures, strong π-π interactions and high electron affinity as well as their potential as electron withdrawing units in photovoltaic conjugated materials. All small molecules showed broad absorption spectra with optical band gaps, which were estimated to be in the range of 1.72-1.29 eV. From cyclic voltammetry, the highest occupied molecular orbital (HOMO) energy level could be tuned by changing the second acceptor, suggesting high open circuit voltage (V_oc). The EHID(EDBTT)₂:PC₇₁BM-based solar cells reached a maximum PCE of 3.24%.

Full text of DOI:10.1039/c5ra06660c

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Graphical Abstract
Different Acceptors
Different Donors
2
0
O
O
O
O
V = 0.83 V
S
S
oc
N
N
N
N
-2
-2
Jsc = 8.13 mA/cm
A
S
S
-
-
-
4
6
8
FF = 0.48
Push-pull system
S
S
PCE = 3.24%
S
N
EHID(EDBTT)2
N
O
O
N
S
O
O
-10
O
O
N
A
N
-
0.2 0.0 0.2 0.4 0.6 0.8 1.0
S
-2
DTDPP
EHID
HTPD
Current density (mA/cm )
TEXT
Synthesis of promising new constituted low bandgap small molecules based on the push-pull (Donor-
Acceptor) system and obtained power conversion efficiency up to 3.24%.

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RSC advances
Paper
A Facile method to synthesize [A`(D`AD) ]-based push-pull small
2
molecules for organic photovoltaics
Received 00th January 20xx,
Accepted 00th January 20xx
Mohamed Shaker, Jong-Hoon Lee, Cuc Kim Trinh, Wonbin Kim, Kwanghee Lee, and Jae-Suk Lee*
DOI: 10.1039/x0xx00000x
An efficient route to synthesize, for the first time, a series of small molecules based on the [A`(D`AD)
2
] architecture was
developed using selective direct heteroarylation of the C-H bond with a Pd(AcO) /Bu NBr simple catalytic system. The C-H
2
4
www.rsc.org/
arylation of the unsymmetrical compound 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2yl)benzo[c][1,2,5]
thiadiazole (5), showed that, the (C5) in the ethyldioxythiophene moiety is more reactive towards C-H arylation than its
counterpart in the thiophene unit. The new small molecules bear two different acceptors and/or donors which led to
decrease the electron band gap and improve the strength of push-pull system as well as the amount of intramolecular
charge transfer from donor to acceptor units. Lactams and imide containing acceptors are used as second electron
withdrawing units due to their well-conjugated structure, strong π-π interaction and the lactam and high electron affinity
as well as their potential as electron withdrawing units in photovoltaics conjugated materials. All small molecules showed
broad absorption spectra with the optical band gaps which were estimated to be in the range of 1.72–1.29 eV. From the
Cyclic voltammetry, the high occupied molecular orbital (HOMO) energy level could be tuned with the changing of the
2
second acceptor, suggesting high open circuit voltage (Voc). The EHID(EDBTT) :PC71BM-based solar cells reached a
maximum PCE of 3.24%.
electron donors and blended with PCBM, and showed in a
1
0,11
Introduction
power conversion efficiency of in the range of 0.2 0.8%.
~
Recently, a series of conjugated donor-acceptor (push-pull)
chemical structures were developed. That helps to broad the
absorption window, increase the efficiency of intramolecular
Recently, much focus has been on the solution processing of
organic solar cells development. The organic small molecules
have unique advantages, such as, low cost, flexibility,
1
2a,b
1
-4
charge transfer (ICT)
enhance the net short circuit current (Jsc
applications. Among these structures, a series of conjugated
along the backbone structure, and
lightweight, and large-area device fabrication. Moreover,
small molecules offer potential advantages over the
conjugated polymer, such as, their defined molecular
structure, high purity, easy purification, easy mass-scale
production and good batch-to-batch reproducibility. The
previous advantages encouraged many scientists to dear their
challenges to design the perfect small molecule chemical
structure to improve the optical and electrical properties, as
1
2c-15
)
in device
(
D-A-D) and (A-D-A) small molecules based on 2,1,3-
6-19
1
20
benzothiadiazole,
Thieno[3,4-c]pyrrole-4,6-dione,
25-27
2
1-24
diketopyrrolo-pyrrole,
and isoindigo
as electron
accepting units were integrated into two low-energy gap
oligothiophenes. These materials showed broad absorption
bands. Insertion of second donor into (D-A-D) small molecule
5
-8
well as the power conversion efficiency
that recently
2
8
2
structure offered a variety of architectures, such as [A(D-D`) ]
achieved up to 10%. The the first bulk heterojunction solar cell
9
was realized in 1995. This design improved the charge
2
9-31
and [D`(D-A-D)
2
]
building blocks.
In parallel, most of organic small molecules are synthesized
3
extraction and enhanced both of light absorption and the
photocurrent density. Ten years later, the first solution-
processed small molecules were introduced as donor material
in the molecular bulk heterojunction (BHJ). That materials
based on the oligothiophenes which investigated as the novel
2,33
via Stille and/or Suzuki cross-coupling reactions.
Despite
the high selectivity of these synthesis methodologies, they
have some disadvantages, such as, contamination by
organometallic intermediates. Recently, we succeeded in the
3
4
synthesis of isoindigo based small molecules via Stille and
Suzuki cross-coupling methodologies, which needed
organometallic monomer derivatives that contaminated
several byproducts in the net product. To avoid the difficulty
that faced during the synthesis of some stannyl and/or boronic
esters, catalyzed C-H direct heteroarylation was preferred.
In this work, Pd(OAc)
2
catalyzed C-H direct arylation
35-37
methodology
was used to synthesize a new series of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
Journal Name
4
1
[
A`(D`AD)
dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2yl)benzo[c]
1,2,5]thiadiazole (EDBTT) (5) was the key structure of this dione (HTPD) (11) were synthesized by literature procedures.
2
] conistituted small molecules. Compound 4-(2,3- N,N’-bis(2-ethylhexyl)-isoindigo (9), 2-tributylstannyl-3,4-ethylene-
4
2
dioxythiophene (4), 1,3-dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-
4
3
[
class of materials (Scheme 1), which contain ethylene
dioxythiophene (EDOT) and thiophene as electron rich cores
which offered strong (ICT) with acceptors and extended the
Characterization
3
6
1
13
low energy optical transitions.
In addition, 2,1,3-
H and C NMR spectra were measured with a Varian spectrometer
benzothiadiazole (BT) afforded high electron affinity, which (400 and/or 300MHz). The UV/Vis absorption spectra were
increased across the backbone structure through the obtained with a Varian Cary UV/Vis/NIR-5000 spectrophotometer
introduction of another electron accepting core such as 2,5- on the pure samples. Differential scanning calorimetry (DSC) was
bis(2-ethylhexyl)-3,6-di(thien-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
performed with a TA instrument (DSC-TA Q-20) under nitrogen at a
N,N’-bis(2-ethylhexyl)isoindigo heating rate of 10 °C/min. Cyclic voltammetry (CV) measurements
EHID) (9) and\or 5-hexylthieno[3,4-c]pyrrole-4,6-dione (HTPD) were performed with B-class solar simulator:
11) between the two arms of (EDBTT) to obtain our targets Potentiostate/Galvanostate (SP-150 OMA company); the supporting
(
2H,5H)-dione (DTDPP) (7),
(
a
(
A`(EDBTT) (Scheme 1) and showed the success of CH-arylation electrolyte
2
was
tetrabutylammonium-hexafluorophosphate
methodology in the synthesis of much complicated small (Bu NPF ) in acetonitrile (0.1 M) and the scan rate was 50 mV/s. A
6
4
molecules in minimal steps.
three-electrode cell was used; a Pt wire and silver/silver chloride
All of the previous acceptors have high electron affinities [Ag in KCl (0.1 M)] were used as the counter and reference
that help to low-lying the HOMO levels and maintain high electrodes, respectively. The HOMO and LUMO energy levels (EHOMO
oxidation potentials. In addition, the attached aliphatic chains and ELUMO) of the materials were determined from the following
to these acceptor moieties induced solubility of the obtained relationships: EHOMO = –Eox – 4.71 and ELUMO = –Ered – 4.71 (in eV),
2
small molecules in the most of organic solvents, which helps where Eox and Ered are the onset oxidation and reduction potentials,
the solution processable solar cells.
respectively, of the material vs. the Ag/AgCl reference electrode.
The small molecules thin films for electrochemical measurements
were drop-coated from a materials/chloroform solution on ITO
glass slides, 10 mg/mL. XRD experiments were performed with a
Bruker D8 advanced model diffractometer and with Cu-Kα radiation
O
O
S
S
N
N
(n-Bu)
3
Sn
2
S
S
(n-Bu)
3
Sn
4
N
N O
O
N
N
S
Br
Br
Br
S
S
Pd(PPh
3
)
o
2
Cl
2
/ Toluene
S
Pd(PPh
3
)
2
Cl
2
/ Toluene
1
o
(EDBTT)
5
1
10 C,12-24hr
3
110 C,12-24hr
(
λ = 1.54 Å) at a generator voltage of 40 kV and a current of 40 mA.
O
N
Br
S
O
N
Fabrication and characterization of photovoltaic devices
N
S
Br
S
O
O
S
O
S
N
O
N
O
N
N
S
N
S
The bulk heterojunction organic photovoltaic devices were
7
O
S
S
S
fabricated with
a
structure of ITO/PEDOT:PSS/organic
DTDPP(EDBTT)
8)
2
(
O
Br
material:PC BM/Ca/Al. The ITO substrates were sequentially
71
N
O
N
O
O
S
S
N
O
N
O
N
S
cleaned by ultrasonication using a detergent, deionized water,
acetone and isopropyl alcohol. Surface treatment was performed by
exposing the ITO to UV/ozone treatment. PEDOT:PSS (Clevios P VP
AI4083) solution was spin coated at 5000 rpm for 20 s on an ITO
substrate, serving as an anode. The coated substrate was dried at
150 ⁰C for 10 min in air and was transferred into a glove box filled
with N . The small molecule:PC BM (1:1, w/w), dissolved in
N
Br
O
N
S
5
C-H arylation
9
O
N
(
EDBTT)
S
S
EHID(EDBTT)
2
(10)
N
S
O
O
N
O
O
O
S
O
S
S
N
O
O N S
N
Br
S
Br
N
1
1
2
71
S
S
chlorobenzene with a concentration of 20 mg/mL, was spin cast on
the PEDOT:PSS layer and dried at 100 ⁰C for 5 min except for (first
small molecule). The cathode, Ca (10 nm) and Al (100 nm), was
HTPD(EDBTT)
(12)
2
o
2 4
C-H arylation = Pd(AcO) / AcOK / (Bu) NBr / DMF / 80 C, 24hr
-
6
deposited in a vacuum chamber (10 Torr). The active area was
Scheme
1 Synthetic route of DTDPP(EDBTT)2, EHID(EDBTT)2 and
2
4
.64 mm . The current–voltage (J–V) characteristics were measured
HTPD(EDBTT)2 via C-H arylation reaction.
under air mass (AM) 1.5 G illumination with light-source operation
2
at 100 mW/cm . The incident photon-to-current efficiency (IPCE)
Experimental
measurements were performed using a QE-IPCE 3000 spectral
response/QE/IPCE measurement system (Titan Electro-Optics Co.
Ltd.).
Materials
3
,4-ethylenedioxythiophene (Sigma-Aldrich, 97%), thiophene-3,4-
dicarboxylic acid (Frontier Scientific), 2,1,3-benzothiadiazole (Sigma-
Aldrich, 98%), 2-(tributylstannyl)- thiophene (Sigma-Aldrich, 97%) Synthesis of monomers and small molecules
are used as received. The materials 4,7-dibromo-2,1,3-
Synthesis
of
4-bromo-7-(thiophen-2-yl)-2,1,3-
3
8,39
benzothiadiazole (1),
2,5-bis(2-ethylhexyl)-3,6-di(5-bromothien-
benzothiadiazole (3). 4,7-Dibromo-2,1,3-benzothiadiazole (1.00 g,
4
0
2
-yl)pyrrolo[3,4-c]- pyrrole-1,4-(2H,5H)-dione (7), 6,6’-dibromo-
3
.40 mmol) and tributyl(thiophen-2-yl)-stannane (1.39 g, 3.70
This journal is © The Royal Society of Chemistry 20xx
2
| J. Name., 2012, 00, 1-3
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ARTICLE
mmol) were dissolved in anhydrous Toluene (50 ml) and
Synthesis of DTDPP(EDBTT)
2
. Compound 5 (0.186 g, 0.520
dichlorobis(triphenylphosphine)-palladium(II) (0.120 g, 0.170 mmol) mmol),
2,5-bis(2-octyldodecyl)-3,6-di(5-bromothien-2-yl)pyrrolo-
was added at room temperature and degassed with argon for 20 [3,4-c]pyrrole-1,4-(2H,5H)-dione (7) ( 0.143 g, 0.21 mmol), to afford
1
min. The mixture was refluxed at 110 °C for 24 hr under nitrogen 8 ( 0.148 g, 60% yield), as a deep violet solid. H NMR (400 MHz,
3
atmosphere. After have been allowed to cool to room temperature, CDCl ), δ (ppm): 9.17 (d, J = 3.30 Hz, 2H, 2xCH, DPP-thiophene),
the reaction mixture was extracted with EtOAc and the organic 8.40 (d, J = 5.1 Hz, 2 H, 2xCH, benzothiadiazole), 8.06 (s, 2 H, 2xCH,
layers were separated, washed thoroughly with water, and finally benzothiadiazole), 7.83(d, 2H, 2xCH, thiophene), 7.36 (d, 2H, 2xCH,
dried with anhydrous Na
vacuum and the crude product was purified by neutralized silica gel J = 4.30 Hz, 2H, 2xCH, thiophene), 4.91-4.82 (m, 8H, 4xCH
flash column chromatography (hexane) to generate a yellow solid 3.05 (m, 4 H, 2xCH -, hexyl-C1), 1.68 (m, 2H, 2xCH-, hexyl-C2), 1.19-
), δ (ppm): 7.99 1.33 (m, 16H, 2x(CH , hexyl-C3-C5 & 2xCH , ethyl-), 0.740-0.790
4.03 Hz, 1H, CH, thiophene), 7.75 (d, 1H, CH, (m, 12H, 4xCH -, ethylhexyl-); MALDI TOF [M+1] found: 1237.65
: C, 60.17; H, 4.56; N,
.03 Hz, 1H, CH, thiophene), 7.12 (dd, 1H, CH, thiophene); C NMR 6.79; O, 7.76; S, 20.73 found: C, 60.11; H, 4.50; N, 6.78; O, 7.69; S,
100 MHz, CDCl3), δ (ppm): 153.68, 151.90, 138.41, 128.08, 127.99, 20.67.
2
SO
4
. The solvent was evaporated under DPP-thiophene), 7.12 (d, J = 4.66, Hz 2H, 2xCH, thiophene), 6.89(d,
2
, EDOT),
2
1
(
0.670 g, 66% yield) of 3. H NMR (400 MHz, CDCl
d,
3
2
)
3
2
+
(
J
=
3
56 6 6 8
benzothiadiazole), 7.60 (d, 1H, CH, benzothiadiazole), 7.38 (d, J = (calcd: 1237.66); Anal. Calcd for C62H N O S
1
3
4
(
+
1
27.23, 127.00, 125.76, 112.28; MALDI TOF [M+1] found: 297.17
calcd: 297.19); Anal. Calcd for C10 BrN : C, 40.41; H, 1.70; Br,
6.89; N, 9.43; S, 21.58; Found: C, 40.39; H, 1.70; Br, 26.77; N, 9.49;
Synthesis of EHID(EDBTT)
mmol), reacted with 6,6`-dibromo-N,N`-bis(2-ethylhexyl)-isoindigo
(0.152 g, 0.230 mmol), to afford 10 (0.101 g, 57% yield), as a dark
2
. Compound 5 (0.186 g, 0.520
(
H
5
2 2
S
2
9
S, 21.51.
Synthesis
thiophen-2-yl)-2,1,3-benzothiadiazole (5)(EDBTT). A mixture of 3 2xCH, thiophene ), 8.10 (s, 2H, 2xCH, benzothiadiazole), 7.86 (s, 2H,
3.00 g, 10.0 mmol), 2-tributylstannyl-3,4-ethylenedioxythiophene 2xCH, benzothiadiazole), 7,43 (dd, 2H, 2xCH, thiophene) 7.31 (d, J =
4) (4.50 g, 11.0 mmol) and Pd(PPh Cl (0.350 g, 0.500 mmol) was 4.00 Hz, 2H, 2xCH, thiophene), 7.26 (dd, J = 7.55 Hz, J = 2.30 Hz,
1
blue solid. H NMR (400 MHz, CDCl
3
), δ (ppm): 9.10 (d, J = 8.40 Hz,
of
4-(3,4-ethylenedioxythiophene-7`-yl)-7- 1H, indole), 9.02 (d, J = 8.40 Hz, 1H, indole), 8.41 (d, J = 4.60 Hz, 2H,
(
(
(
3
)
2
2
1
2
dissolved in THF (50 mL) and degassed with nitrogen for 20 min. 2H, 2xCH, indole), 6.83 (d, J = 1.87 Hz, 2H, 2xCH, indole), 4.49-4.45
The reaction mixture was stirred and heated under reflux at 73 °C (m, 8H, 4xCH , EDOT), 3.66 (m, 4 H, 2xCH -, hexyl-C1), 1.84(m, 2H,
for 16 h. After having been allowed to cool to room temperature, 2xCH-, hexyl-C2), 1.35-1,32 (m, 16H, 2x(CH , hexyl-C3-C5 & 2xCH
the reaction mixture was extracted with ethyl acetate and the ethyl-), 0.920-0.890 (m, 12H, 4xCH -, ethylhexyl-); MALDI TOF
organic layers were separated, washed thoroughly with water, and [M+1] found: 1199.56 (calcd: 1199.57); Anal. Calcd for
finally dried with anhydrous Na SO . The solvent was evaporated : C, 64.08; H, 4.87; N, 7.01; O, 8.00; S, 16.04; found: C,
2
2
2
)
3
2
,
3
+
2
4
64 58 6 6 6
C H N O S
under vacuum and the crude product was purified by silica gel flash 64.06; H, 4.87; N, 6.99; O, 8.01; S, 15.91.
column chromatography (n-hexane/ EtOAc 10:1) to generate 5
Synthesis of HTPD(EDBTT)
mmol), reacted with 1,3-dibromo-5-hexyl-5H-thieno[3,4-c]pyrrole-
,6-dione 11 (0.093 g, 0.230 mmol), to afford 12 (0.128 g, 37%
2
. Compound 5 (0.186 g, 0.520
1
3
(1.25 g, 34% yield) as an orange red solid. H NMR (400 MHz, CDCl ),
δ (ppm): 8.33 (d, J = 4.00 Hz, 1H, CH, thiophene), 8.07 (s, 1H, CH,
benzothiadiazole), 7.86 (s, 1H, CH, benzothiadiazole), 7.41 (d, J =
4
1
yield), as a dark brown solid. H NMR (400 MHz, CDCl
.45 (d, J = 4.05 Hz, 2 H, 2xCH, thiophene ), 8.14 (s, 2 H, 2xCH,
benzothiadiazole), 7.91 (s, 2 H, 2xCH, benzothiadiazole), 7.68 (d, J =
3
), δ (ppm):
4
.00 Hz, 1H, CH, thiophene), 7.19 (dd, 1H, CH, thiophene), 6.57 (s,
H, CH, EDOT-thiophene), 4.38 (t, J = 12.00 Hz, 2H, CH , EDOT), 4.30
, EDOT); C NMR (100 MHz, CDCl3), δ
ppm): 152.46, 152.28, 141.60, 140.46, 139.56, 127.90, 127.12,
8
1
2
1
3
(t, J = 12.00 Hz, 2H, CH
2
4
.00 Hz, 2H, 2xCH, thiophene), 7.45 (dd, 2H, 2xCH, thiophene), 4.53-
.51 (m, 8H, 4xCH , EDOT), 3.66 (m, 2 H, CH -, hexyl-C1), 1.55 (m, 2
-, hexyl-C2), 1.31 (m, 6 H, 3xCH , hexyl-C3-C5), 0.870 (t, 3H,
-, hexyl-C6); MALDI TOF [M+1] found: 950.07 (calcd: 950.20);
Anal. Calcd for C44 : C, 55.62; H, 3.29; N, 7.37; O, 10.10; S,
3.62; found: C, 55.98; H, 3.29; N, 7.37; O, 10.13; S, 23.55.
(
4
2
2
+
1
3
2
8
26.11, 113.44, 102.39, 64.99, 64.28; MALDI TOF [M+1] found:
H, CH
CH
2
2
58.44 (calcd: 358.46); Anal. Calcd for C16 : C, 53.61; H,
H
10
N
2
O
2
S
3
+
3
.81; N, 7.81; O, 8.93; S, 26.84; found: C, 53.60; H, 2.80; N, 7.79; O,
.90; S, 26.87.
31 5 6 7
H N O S
2
General Procedure for Thermal direct Pd catalyzed CH-
arylation Cross-Coupling. Under dry condition, a mixture of 5
Results and discussion
(0.360 g, 1.00 mmol), desired dibromo-derivative (7, 9 and/or 11)
(0.480 mmol), potassium acetate (0.590 g, 6.00 mmol),
Synthesis and characterization
tetrabutylammonium bromide (TBAB) (0.640 g, 2.00 mmol) and
palladium acetate (0.040 g, 0.200 mmol) in dry DMF (15 ml) was
degassed with nitrogen for 20 min then heated under reflux for 24
As shown in Scheme 1, 4,7-dibromo-2,1,3-benzothiadiazole (1)
was coupled with 2-(tributylstannyl)thiophene (
coupling conditions to afford 4-bromo-7-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazole ( ) in a good yield. Compound
was then reacted with 2-tributylstannyl-3,4-ethylenedioxy-
thiophene ( ) to obtain co-monomer . To optimize the
2) under Stille
̊
h at 80 C. The reaction mixture was cooled to room temperature
3
3
followed by adding a mixture of CH
2
Cl
2
(50 ml) and water (25 ml).
4
SO ,
The organic layer was separated and dried with anhydrous Na
2
4
5
then filtered and concentrated under reduced pressure. The crud
product was purified by flash chromatography on silica gel eluted
by n-hexane/EtOAc (10:3) to give the corresponding product
derivative.
synthesis of new small molecules using direct C-H arylation,
several reaction conditions were utilized as shown in (Table S1,
ESI†). The optimal reaction condition was achieved by using
Pd(OAc)
2
as a catalyst, terabutylammonium bromide (TBAB)
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and KOAc as
a base. The temperature, solvent, and All of the new materials structures were thermally stable and
concentration were held constant for this work, based on were confirmed with NMR spectroscopy, FT-IR and elemental
1
3a,b
general procedures was reported in the literature.
analysis. The FT-IR spectra provided an evidence for the
)(O-Tol) catalyst was not sufficient for presence of the lactam and imide groups by showing two
this reaction, even combined with TBAB and/or tris(2- peaks assigned to free carbonyl (-C=O) stretching vibration in
3
3
Furthermore, Pd(OAc
2
-1
†
methoxyphenyl)phosphine (L2), (Table S1. ESI†). By using one the range of 1682–1800 cm (Fig. S1, ESI ).
step, highly selective, Pd-catalyzed direct arylation coupling
reaction, compound
5 coupled with a series of electron Thermal stability
withdrawing moieties, 7, 9 and/or 11, to afford DTDPP(EDBTT)
2
The DSC curve exhibited clear melting peaks and phase
transitions of crystallization of the A`(D`AD)2 based small
(
8
), EHID(EDBTT)
Scheme 1).
The selectivity resulted from the difference in reactivity
2 2
(10) and HTPD(EDBTT) (12), respectively,
(
molecules, DTDPP(EDBTT)
2
, EHID(EDBTT)
Fig. 1b). The melting (T ), crystallization (T
) temperatures are summarized in (Table S2,
2
and HTPD(EDBTT)
2
(
m
c
)
and
between (C5) in the EDOT moiety and its counterpart in the
thiophene unit. The (C5) in the EDOT moiety is more active due
to the effect of the cyclic ether oxygen atom, which acts as a
directing group. This cyclic ether oxygen attracted the Pd
catalyst by coordination bond and makes the catalyst much
nearby position of (C5) in the EDOT moiety, see the proposed
reacꢀon mechanism pathway (Scheme S1, ESI†). This simple
catalytic system decreases the obtained by-products and
avoids the use of organometallic reagents, as well as helps to
decomposition (T
d
ESI†). As known, the molecular weights, intermolecular
interactions and the alkyl chain affected the thermal
properties. The melting point of DTDPP(EDBTT)
from 157 to 174 ⁰C in case of EHID(EDBTT) the due to the high
planar structure and rigidity of isoindigo unit. Furthermore,
DTDPP(EDBTT) showed slightly phase transition signal
2
increased
2
2
observed at around 101⁰C supposed to be a result of twisted
structure for the present molecule, due to the steric hindrance
44
3
3
obtain a reasonable product yield.
at the linkage between EDOT and benzothiadiazole unit.
2 m
The HTPD(EDBTT) showed low T at 85 ⁰C due to the
attachment of the hexyl- chain to the small size TPD unit. The
thermal stability of all the small molecules was observed by
TGA at a heating rate of 100 °C/min under nitrogen (Fig. 1a).
When the threshold for the thermal decomposition was
defined at a mass loss of 5%, all the materials possess good
thermal stability with decomposition temperatures above 350
(
a)
100
8
6
4
2
0
0
0
0
⁰
C.
HTPD(EDBTT)2
DTDPP(EDBTT)2
EHID(EDBTT)2
X-ray diffraction analysis
To the study the crystalline structure and the planarity of the
obtained materials 8, 10 and 12, X-ray diffraction (XRD)
measurements were obtained (Fig. S2, ESI†). New small
molecules exhibited two important diffraction peaks, the first
1
00
200
300
Temperature
400
500
(b)
HTPD(EDBTT)2
one at approximately 2θ = 5.79° (DTDPP(EDBTT) ), 6.64°
2
2 2
(EHID(EDBTT) ) and 5.45° (HTPD(EDBTT) ), which corresponds
to a d-spacing of 15.4, 14.0 and 17.1 Å, respectively, These
spacing referred to the interchain spacing between the planes
of the main conjugated backbone of these small molecule
separated by the corresponding ethylhexyl- and/or hexyl-
chains. The high intensities observed in the diffraction peaks of
5
5
5
0
100
150
150
150
200
200
200
250
300
EHID(EDBTT)2
250
0
0
100
100
300
2 2
DTDPP(EDBTT) and EHID(EDBTT) proved a better ordering in
DTDPP(EDBTT)2
solid state, which would led to good film morphology for BHJ
solar cells. The second peak assigned at 2θ = 21.46°
250
300
2
(DTDPP(EDBTT) ) and 21.30° (HTPD(EDBTT)2) with a d-spacing
Temperature
of 4.1 and 4.3 Å, respectively, demonstrated the π-π stacking
between the small molecule backbones and implied the long
range ordering and highly organized assembled structure in
solid state. Fig. S3 (ESI†) showed the proposed packed
Fig. 1 (a) TGA thermograms of DTDPP(EDBTT)
HTPD(EDBTT)
DTDPP(EDBTT)
heating rate of 10 °C/min.
2
, EHID(EDBTT)
2
and
2
and (b) Differential scanning calorimetry (DSC) curves of
, EHID(EDBTT) and HTPD(EDBTT) under nitrogen at a
2 2
structures in solid stat for DTDPP(EDBTT) , EHID(EDBTT) and
2
2
2
HTPD(EDBTT)
2
.
4
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ARTICLE
Table 1 Optical and electrochemical properties of all of the materials in this study.
UV/Vis absorption
Cyclic voltammetry
a
Materials
Solution
Flim
elc
op
E
(eV)
HOMO
E
(eV)
LUMO
Eg
λ
max
λ
max
λ
onset
Eg
c
b
(eV)
(
nm)
(nm)
836
641
524
(nm)
(eV)
1.29
1.51
1.72
d
d
d
DTDPP(EDBTT)2
(630)(704)
586
962
822
721
-5.49
-5.52
-5.55
c
-3.80(-4.20)
-3.89(-4.01)
-3.87(-3.83)
1.69
1.63
1.68
EHID(EDBTT)
HTPD(EDBTT)
2
2
496
a
b
op
elc
Measured in CHCl
3
. Estimated using the onset of the UV−vis spectrum (Eg = 1240/λonset). Calculated from Eg = ELUMO − EHOMO
.
d
Calculated by the equation E LUMO = E HOMO + E g,opt
Optical properties
EHID(EDBTT)
2
showed λmax values of 641 nm. The lowest
λmax value was 524 nm in the case of HTPD(EDBTT)
was expected to have the lowest ICT. In contrast,
DTDPP(EDBTT) showed two λmax peaks in solution at 630 and
04 nm, which combined into one peak at 836 nm in the film.
All of small molecules have two different high-energy
2
, which
The optical properties were investigated using UV-visible
absorption spectroscopy (Fig. 2), and the data are summarized
in Table 1. Considering that, the EDBTT unit is fixed, the
changes in maximum absorption depends on the active
2
7
conjugated length induced by the introduced of second absorption bands in the range of 300 to 450 nm which have
acceptors. In general, absorption in range of 423–468 nm been attributed to a more localized π–π* transition due to the
ascribed to ICT between the EDOT and thiophene donors with presence of two different electron accepting units along the
backbone structure.
the acceptors. In film, In case of DTDPP, small molecule
8
In particular, when the donor is highly electropositive, the
polarizability of the acceptor unit becomes a critical factor to
exhibited a broad absorption with onset absorption (λonset) at
62 nm. The DTDPP is considered as (D-A-D) unit due to the
presence of two thiophene units around the DPP core, which
extended the effective conjugation length
and donor and strong electron acceptor units along the conjugated
bathochromically shifted the wide absorption band associated materials backbone decreases the electron band gap and improves
9
1
2a
determine the amount of ICT. So that, using strong electron
3
4
with the DPP core and thiophene rings.
the strength of push-pull system as well as the performance of
organic solar cells by controlling the photo-induced ICT from donor
1.2
1.0
0.8
0.6
0.4
0.2
0.0
12b
to acceptor units. Confirming the previous deduction, all the new
(
a)
DTDPP (EDBTT)2
EHID (EDBTT)2
2
synthesized A`(D`AD) based small molecules showed more narrow
HTPD (EDBTT)2
optical band gaps than their counterparts of small molecules with
(D-A-D) building block which have same number of building units
21-24
and also based on the same electron withdrawing units (DTDPP
,
25-27,34a
20
ID
and TPD ), see Table 1.
Electrochemical properties
Cyclic voltammetry (CV) measurements were obtained for all
materials (Fig. 3a). The HOMO and low unoccupied molecular
orbitals (LUMO) energy levels were derived from the onset of
oxidation (Eox) and reduction potentials (Ered) of the CV curves
of the drop-cast films, using a platinum electrode in 0.1 M of
400
600
800
1000
1200
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(
b)
DTDPP(EDBTT)2
EHID(EDBTT)2
HTPD(EDBTT)2
4 6
Bu NPF acetonitrile solution at a scan rate of 50 mV/s versus a
Ag/AgCl reference electrode. According to the equations
HOMO = -(Eox + 4.71) eV and LUMO = -(Ered + 4.71) eV. The
interface barrier present between the small molecule film and
the electrode surface made the estimated electronic E
g
values
are quite higher than the optical E estimated in thin films. All
g
of the materials have low-lying (LUMO) because of the
increased electron defficiency by the addition of one more
2 2 2
acceptor. DTDPP(EDBTT) , EHID(EDBTT) and HTPD(EDBTT)
showed LUMO energy levels at -3.80, -3.89 and -3.87 eV,
respectively (Fig. 3b).
4
00
600
800
1000
1200
Wavelength (nm)
3
Fig. 2 UV-vis spectra of small molecules (a) in CHCl solution and (b) in
films.
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(
a) Reduction scan
Oxidation scan
2
(
a)
0
2
-
DTDPP(EDBTT)2
EHID(EDBTT)2
HTPD(EDBTT)2
-4
-
6
8
-
2.0
-1.5
-1.0
-0.5
0.00.0
0.5
1.0
1.5+
2.0
+
HTPD(EDBTT)2
Potential(V vs Ag/Ag )
Potential(V vs Ag/Ag )
-
EHID(EDBTT)2
(
b)
DTDPP(EDBTT)
2
-
3.80
-10
-3.89
-3.87
-
0.2
0.0
0.2
0.4
0.6 0.8 1.0
-
-
-
-
4
5
6
7
Voltage (V)
E HOMO
E LUMO
5
0
(
b)
-
5.49
-5.52
40
-5.55
30
20
10
0
HTPD(EDBTT)2
EHID(EDBTT)2
DTDPP(EDBTT)2
Fig. 3 (a) Cyclic voltammograms of all small molecules in 0.1 M solution of
TBAPF in acetonitrile at room temperature and (b) HOMO and LUMO
energy levels diagram.
6
3
00
400
500
600
700
800
DTDPP(EDBTT)
.80 eV) among the materials under study, but also showed
the highest-lying HOMO energy level at -5.49 eV because the
2
not only exhibited highest-lying LUMO (-
Wavelength (nm)
3
Fig.
4 (a) Current density-voltage (J-V) characteristics of the small
molecules:PC71BM solar cells and (b) Corresponding external quantum
presence of four thiophene units which increased the electron efficiency (EQE) spectra measured under illumination of monochromatic
donating properties as well as increase the HOMO energy light.
2 2
level. EHID(EDBTT) and HTPD(EDBTT) showed low-lying
HOMO energy levels at -5.52 and -5.55 eV, respectively (Fig.
elc
All the device parameters are summarized in Table 2. The
photovoltaic performances were significantly affected by the
changing of the lactam and/or imide containing acceptor core
3
b). The EHOMO, ELUMO and the electrochemical band gaps (Eg )
are summarized in Table 1.
in
the
small
molecules
structures
and
the
Photovoltaic properties
EHID(EDBTT) :PC BM based solar cells reached a maximum
71
2
To estimate the photovoltaic performances of the new
synthesized small molecules, BHJ solar cells, conventional
devices structure ITO/PEDOT:PSS/organic material:PC71BM
2
PCE of 3.24%, with a V of 0.83 V, J of 8.13 mA/cm , and fill
oc
sc
factor (FF) of 0.48. The Voc values of each optimized device
decreased gradually from 0.88 to 0.75 V as the electron
withdrawing ability of the accepting units TPD, DPP and ID
/Ca/Al were fabricated. The optimized results were obtained
by varying the small molecule:PC71BM weight ratios, active
decreased. As confirmed by the CV results, The HTPD(EDBTT)2
3
4
layer thicknesses, and annealing temperature. The optimized
active layer thickness was around 80 nm for all the small
molecule:PC71BM blend layers.
has the HOMO level of -5.65 eV and exhibited a high Voc value
of 0.88 V. Thus EHID(EDBTT) and DTDPP(EDBTT) showed V
2
2
oc
values of 0.83 and 0.75 V, respectively.
Unlike the Voc trend, EHID(EDBTT) exhibited a high Jsc
2
2
value of 8.13 mA/cm , while HTPD(EDBTT) showed low Jsc
Table 2 Photovoltaic Parameters of the Solar Cells
2
2
value of 3.51 mA/cm , due to its narrow light absorption
Materials:PC71BM
1:1)
V
(V)
oc
J
sc
FF
PCE
(%)
(
mA/cm
window as well as low ICT. The J value of DTDPP(EDBTT) was
sc
2
2
2
.87 mA/cm (Fig. 4a). To examine the accuracy of the J from
6
sc
DTDPP(EDBTT)
EHID(EDBTT)
HTPD(EDBTT)
2
0.75
0.83
0.88
6.87
8.13
3.51
0.49
0.48
0.26
2.52
3.24
0.80
the J-V measurements, the corresponding external quantum
efficiencies of the fabricated solar cells were measured under
the illumination of monochromatic light (Fig. 4b). All the Jsc
results calculated from integrating the EQE with an AM 1.5G
2
2
reference spectrum matched with the J obtained from the J-V
sc
measurements. As shown in Fig. 4b, the EHID(EDBTT) showed
2
6
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much higher EQE values up to 45% in the broad wavelength New photovoltaic devices and a number of additives will be
range of 400 750 nm, compared to those of HTPD(EDBTT) and further studied to improve the FF as well as the PCE.
. Moreover, in spite of the broad absorption of
, it showed low external quantum efficiencies
−
2
DTDPP(EDBTT)
DTDPP(EDBTT)
2
2
Acknowledgements
around 34% (Fig.4b) reflected some leakage in the number of
charge carriers.
This work was supported by the Global Frontier R&D Program
(
2013-073298) in the Center for Hybrid Interface Materials
HIM) funded by the Ministry of Science, ICT & Future Planning
Morphology analysis
(
In order to imply the effect of the active layer morphology on and the GIST-Caltech Research Collaboration Project through a
the device performance, the morphologies of all the small grant provided by GIST in 2014.
molecule:PC BM blend films were studied by atomic force
7
1
microscopy (AFM), as shown in Fig. 5).
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