Synthesis and characterization of donor-acceptor semiconducting polymers containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4: H -dithieno[3,2- b:2',3'- d] pyrrole for organic solar cells

Article abstract of DOI:10.1039/d0nj02616f

In this study, new donor-acceptor (D-A) conjugated polymers based on 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EPDP) and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (BTBP) or 4,7-

Full text of DOI:10.1039/d0nj02616f

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C. D. Tran and H. Tran Nguyen, New J. Chem., 2020, DOI: 10.1039/D0NJ02616F.
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Synthesis and Characterization of Donor-Acceptor Semiconducting
Polymers Containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-
dithieno[3,2-b:2',3'-d]pyrrole for Organic Solar Cells
Huyen Le Thi Mai1, Nhung Thanh Thi Truong1,2, Thiet Quoc Nguyen3, Bao Kim Doan1, Dat Hung
Tran1, Le-Thu T.Nguyen2, Lee Woosung4, Jae Woong Jung5, Mai Ha Hoang6, Ha Phuong Ky Huynh2,
Chau Duc Tran*2, Ha Tran Nguyen*1,2
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1National Key Laboratory of Polymer and Composite Materials, Ho Chi Minh City University of
Technology, Vietnam National University–Ho Chi Minh City (VNU–HCM), 268 Ly Thuong Kiet, District
10, Ho Chi Minh City, Vietnam.
2Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT), Vietnam
National University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam
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Institute of Applied Materials Science, Vietnam Academy of Science and Technology, 01 TL29, District
12, Ho Chi Minh City, Vietnam
4 Smart textile R&D group, Korea Institute of Industrial Technology, Ansan, (KITECH)
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Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea
6 Institute of Chemistry, Vietnam Academy of Science and Technology, Hanoi, Vietnam
* Corresponding author. Email address: nguyentranha@hcmut.edu.vn
Abstract
In this study, new donor-acceptor (D-A) conjugated polymers based on 4-(4-((2-
ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EPDP) and 3,6-bis(5-bromothiophen-2-yl)-2,5-
bis(2-ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (BTBP) or 4,7-bis(5-bromothiophen-2-
yl)benzo[c][1,2,5]thiadiazole (BT) have been synthesized successfully via direct(hetero)arylation
polymerization using Pd(OAc)2 and PCy3.HBF4 as catalyst system. The obtained D-A conjugated
1
polymers have been characterized via H NMR, GPC, FTIR, DSC, XRD, PL and UV-Vis methods.
Afterward, the D-A conjugated polymers have been applied for the fabrication of organic solar cells based
on bulk-heterojunction (BHJ) structures.
Key words: Donor-acceptor (D-A) conjugated polymers, Direct Arylation Polymerization, Organic Solar
Cells, Bulk heterojunction structure, Dithieno [3,2-b:2’,3’-d]pyrrole
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1. Introduction
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Today, organic solar cells (OSCs) have emerged as potential candidates as renewable energy
resources because of their tremendous advantages.1-7 The power conversion efficiencies (PCEs) of OSCs
have enhanced up to 18% where conjugated copolymers as donor moieties combined with fullerene/non-
fullerene derivatives as acceptors were applied in bulk-heterojunction structures.8-17 Among novel
conjugated polymers, the type of alternating donor–acceptor (D-A) conjugated copolymers containing
both electron donor and electron acceptor segments has exhibited extremely narrow bandgaps, red-shifts
in absorption wavelength and excellent thermal stability. However, one of the main challenges in organic
photovoltaics is the loss of energy in photon absorption process and the optimal balance between the
PCEs and open-circuit voltage (Voc) which influences the gap between HOMO and LUMO levels in
donor and acceptor materials.18 Hence, selecting appropriate donor and acceptor units in D-A materials
plays a key role in handling these issues. Recently, conjugated polymers based on dithieno[3,2-b:2,3-
d]pyrroles (DTPs) have showed favorable results, including reduced band gaps, high carrier mobilities,
and the stability in the oxidized state owing to the electron-rich N atom in their structure.19,20
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Among strong electron–withdrawing units, benzothiadiazole (BT) and diketopyrrolopyrrole (DPP)
have been used efficiently in D-A conjugated polymers for OSC and organic field-effect transistor
(OFET) devices because of their planar and electron withdrawing structures.1,4,6 In 2013, Dou et. al
reported the charge transport properties of a low band gap conjugated polymer named PBDTT-Se-DPP,
which exhibited a PCE of 7.2% (Eg= 1.38 eV; JSC = 16.8 mA/cm-2; VOC= 0.69 V).21 In 2014, Ashraf et. al
reported the polymer PDPP2TT-T with PCE up to 8% (Eg= 1.39 eV; JSC= 23.5 mA/cm-2; VOC = 0.57 V).22
Following this achievement, PDPP2TT-TT has been synthesized and explored with PCE reaching to 9.4%
(Eg= 1.35 eV; JSC = 20.1 mA/cm-2; VOC= 0.67 V) in 2015.23 On the other hand, BT unit is also a favorable
electron-drawing with good π-π stacking, strong intermolecular interactions and simple synthesis
procedure.24 In 2019, Chen et. al reported the polymers PE2: BTA3 and PE1: BTA3 with PCE of 5.83%
(JSC= 8.06 mA/cm-2; VOC = 1.26 V) and 8.43 % (JSC= 11.95 mA/cm-2; VOC = 1.11 V), respectively.25 The
photovoltaic polymer PDPP2TzDTP based on DTP and BT was studied by Li et al. with a PCE of 5.6%
(Eg= 1.67 eV; JSC = 14.9 mA/cm-2; VOC = 0.69 V) in 2015 [10]. Geng et al. also reported the conjugated
polymer PDTP-DT2BT with a PCE of 3.12% (Eg= 1.59 eV; JSC = 11.43 mA/cm-2; VOC = 0.31 V, PCE =
3.12 %).26 Nagarjuna et. al reported OSCs based on PTB7-Th polymer in combination with PC70BM and
modified PC70BM (CN-PC70BM), with PCEs of 5.4% and 8.2%, respectively.27 Wan et. al also reported
PTB7-Th:CN-PC70BM based OSCs with the structure of ITO/PEDOT:PSS/PTB7-Th: PC61BM (1:1.5,
w/w)/C60 –N (16 nm)/Al, giving a PCE of 10.8% in the presence of 1.5% DIO + 1.5% NMP additives,
whereas without additives only a PCE of 8.1% was obtained.28 This result suggested that the modification
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of acceptor moieties of PCBM and binary additives have an important role for incorporating donor and
acceptor units for charge transport properties. On the other hand, conjugated polymers based on DTP
moieties exhibit HOMO levels closed to that of PC60BM acceptor units as well as good stability in the
oxidized state, which could be more promising for OSCs in comparison with PTB7-Th copolymer
systems. However, there have not been any reports related to the synthesis and characterization of
conjugated polymers based on DPP and 4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole with 4-
(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole.
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Therefore, in this research, the alternating D-A conjugated polymers poly[4-(4-((2-
ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole-alt-3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-
ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4(2H)-dione] (P1) and Poly[4-(4-((2-ethylhexyl)oxy)phenyl)-4H-
dithieno[3,2-b:2',3'-d]pyrrole-alt-4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole] (P2) have
been designed and synthesized via direct (hetero)arylation polymerization using palladium(II) acetate
(Pd(OAc)2) and tricyclohexylphosphine tetrafluoroborate (PCy3.HBF4) as catalyst and ligand,
1
respectively. The obtained novel conjugated polymers have been characterized via H NMR, FT–IR,
XRD, UV-Vis and photoluminescence (PL) spectroscopies as well as thermal analysis including
differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Additionally, the organic
solar cells based on these conjugated polymers have been fabricated and their PCEs have been evaluated.
2. Experimental Section
2.1. Materials
2-ethylhexyl bromide (95%), palladium(II) acetate (Pd(OAc)2, 98%), tricyclohexylphosphine
tetrafluoroborate
(PCy3·HBF4,
97%),
3,3′dibromo-2,2′bithiophene,
benzamide,
N,N′-
dimethylethylenediamine (DMEDA, 85%) and pivalic acid (PivOH, 99%), tri(tert-butyl)phosphine (P(t-
Bu)3 were purchased from Sigma-Aldrich/Kantochem and used as received. 4-Iodine phenol (99%),
potassium carbonate (K2CO3, 99%), sodium tert-butoxide (NaOt-Bu, 98%), magnesium sulfate (MgSO4,
98%), and copper iodine (CuI, 99.9%) were purchased from Acros/Merck. Chloroform (CHCl3, 99.5%),
dichloromethane (CH2Cl2, 99.5%), toluene (99.5%), acetonitrile (CH3CN, 99.8%) and dimethylacetamide
(DMAc, 99%) were purchased from Fisher/Acros/Sigma as well as dried using molecular sieves under
N2. Acetone (99.6%), n-heptane (99%), methanol (99.8%) and ethyl acetate (99%) were purchased from
Fisher/Acros and used as received.
2.2. Measurements
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1H NMR spectra were recorded in deuterated chloroform (CDCl3) with tetramethylsilane (TMS) as
an internal reference, on a Bruker Avance 500 MHz. Fourier transform infrared (FT-IR) spectra, collected
as the average of 264 scans with a resolution of 4 cm−1, were recorded from a KBr disk on the FTIR
Bruker Tensor 27. Size exclusion chromatography (SEC) measurements were performed on a Polymer
PL-GPC 50 gel permeation chromatography (GPC) system equipped with an RI detector, with chloroform
(CHCl3) and tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL min−1. Molecular weights and
molecular weight distributions were calculated with reference to polystyrene standards. Elemental
analyses were performed by the Dumas combustion method, using a Costech ECS 4010 Elemental
Analyzer
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UV–visible absorption spectra of all polymers in solution and polymer thin films were recorded on
a Shimadzu UV-2450 spectrometer over the wavelength range 300–900nm. Fluorescence spectra (PL)
were measured on a Horiba IHR 325 spectrometer.
Differential scanning calorimetry (DSC) measurements were carried out with a DSC 204 F1—
NETZSCH instruments under nitrogen flow (heating rate 10 °C min−1). Thermogravimetric analysis
(TGA) measurements were performed under nitrogen flow using a STA 409 PC Instruments with a
heating rate of 10°C min−1 from ambient temperature to 1000°C.
Wide-angle powder XRD patterns were recorded at room temperature on a Bruker AXS D8
Advance diffractometer using Cu-K훼 radiation (k = 0.15406 nm), at a scanning rate of 0.05°s−1. The data
were analyzed using DIFRAC plus Evaluation Package (EVA) software. The d-spacing was calculated
from peak positions using Cu-K훼 radiation and Bragg’s law. Atomic force microscopy (AFM) images
were obtained using a Bruker Dimension 3100 atomic force microscope. Elemental analyses were
performed by the Dumas combustion method, using a Costech ECS 4010 elemental analyzer.
Electrochemical measurements were performed on an AUTOLAB machine (software NOVA 1.11)
using an Au disc working electrode and a Pt wire counter electrode. Solutions consisted of 0.1 M
TBAPF6 in CH3CN and were purged with argon for 20 min prior to data collection. All potentials are
referenced to a Ag/Ag+ reference electrode (0.1 M AgNO3/0.1 M TBAPF6 in CH3CN; 0.320 V vs. SCE)
and internally standardized with ferrocene (vs. Ag/Ag+). EHOMO values were determined in reference to
ferrocene. The polymer film was coated on electrode washed with CH3CN and placed in a cell with a
fresh electrolyte solution for electrochemical characterization.
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The current density–voltage (J–V) characteristic curves were recorded in a computer-controlled
Keithley 2400 source unit. Solar cell performance was measured using Air Mass 1.5 Global (AM 1.5G)
under a simulated solar irradiance.
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2.3. Synthesis of 1-((2-ethylhexyl)oxy)-4-iodobenzene (compound 3) (M1)
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4-Iodine phenol (1000 mg, 4.55 mmol) (compound 1), 2-ethylhexyl bromide (1757 mg, 9.09 mmol)
(compound 2), K2CO3 (1258 mg, 9.10 mmol) were dissolved in acetonitrile (10 ml) in a 100 ml flask.
Then, the mixture was stirred and heated at 82 °C for 48 h under nitrogen atmosphere. After finishing and
cooling down to room temperature, the compound was extracted in CHCl3, washed with distilled water,
dried over anhydrous K2CO3, filtered and vacuum-evaporated to remove the solvent. The crude product
was purified by silica column chromatography using n-hexane as the eluent and dried under vacuum at 50
°C in 24 h in order to obtain the isolated product as a light yellow oil (2.09 g, Rf = 0.7, yield: 97%).
1H NMR (500 MHz, CDCl3), δ (ppm): 7.52 (d, 2H), 6.68 (d, 2H), 3.80 (m, 2H), 1.7 (m, 1H), 1.33-
1.44 (m, 8H), 0.91 (m, 6H). Analysis calculated for C14H21IO: C, 50.61; H, 6.37; I, 38.20; O, 4.82. Found:
C, 49.21; H, 6.15; I, 38.3; O, 6.34.
2.4. Synthesis of 4H‐dithieno[3,2‐b:2′,3′‐d]pyrrole (compound 6) (M2)
The synthesis of the compound 4H‐dithieno[3,2‐b:2′,3′‐d]pyrrole (M2) was carried out by the
previously reported procedure.29 A flame-dried Schlenk two-neck round-bottom flask with a magnetic stir
bar is charged with toluene and H2O. The solvent was degassed through freeze-pump-thaw cycle for 3
times. Then DMEDA (423.12 mg; 4.8 mmol) and CuI (228.54 mg; 1.2 mmol) were added to the flask
under N2 atmosphere, followed by stirring for several minutes. Benzamide (872.21 mg; 7.2 mmol), 3,3’-
Br2-2,2’-bithiophene (1944.3 mg; 6 mmol) and K2CO3 (2487.78 mg; 18 mmol) were added and the
mixture was degassed via freeze-pump-thaw cycle for three times. In the following stage, the reaction was
heated at 110 °C for 48 h. Afterward, the reaction was cooled to room temperature, quenched with H2O
and extracted with dichloromethane. The organic layer was washed with NaCl solution and distilled water
then dried over K2CO3. The mixture was filtered and concentrated via rotary evaporation. The crude
product was purified by silica gel chromatography using ethyl acetate/n-hexane = 1:15 (v/v) and dried
under vacuum at 50 °C in 24 h to obtain the pure compound as a white crystalline solid (630 mg, Rf=
0.17, yield: 58.57%).
1H NMR (500 MHz, CDCl3), δ (ppm): 8.31 (s, 1H), 7.02 (d, 2H), 7.14 (d, 2H). Analysis calculated
for C8H5NS2: C, 53.60; H, 2.81; N, 7.81; S, 35.77. Found: C, 54.10; H, 2.67; N, 7.85; S, 35.38.
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2.5. Synthesis of 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (compound 7)
(M3)
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In a two-neck flask connected with a Schlenk line, Pd(OAc)2 (2.51 mg; 0.023 mmol), P(t-Bu)3
(4.52 mg; 0.058 mmol) and toluene (5 ml) were added and stirred at the room temperature for 10 min. In
the next step, (M1) (139 mg; 0.846 mmol), (M2) (50 mg; 0.564 mmol) and NaOt-Bu (80.44 mg; 1.692
mmol) were added. The mixture reaction was bubbled with nitrogen for 30 min and then stirred at 110 °C
for 24 h. After cooling to ambient temperature, the mixture was extracted with dichloromethane, washed
with NaCl solution and distilled water as well as dried over K2CO3, filtered, concentrated via rotary
evaporation. The crude product was purified by column chromatography (silica gel) using ethyl acetate/n-
hexane = 1:200 (v/v) as eluent and dried under vacuum at 50 °C in 24 h in order to obtain the main
product as a green solid (24.2 mg, Rf= 0.45, yield: 22.6%).
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1H NMR (500 MHz, CDCl3), δ (ppm): 7.47 (d, 2H), 7.14 (d, 2H), 7.08 (d, 2H), 7.04 (d,2H), 3.89
(d,2H), 1.78 (m, 1H), 1.48-1.34 (m, 8H), 0.96 (m, 6H). Analysis calculated for C22H25NOS2: C, 68.89; H,
6.57; N, 3.65; O, 4.17; S, 16.72. Found: C, 69.2; H, 6.62; N, 3,71; O, 4.07; S, 16.40.
2.6 Synthesis of conjugated polymer Poly[4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-
d]pyrrole-alt-3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4(2H)-
dione] (compound 9) (P1)
In a glove box, 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (M3) (50 mg
0.1304
mmol)
and
3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-2,5-dihydropyrrolo[3,4-
c]pyrrole-1,4-dione (88.98 mg 0.1304 mmol) were dissolved in 3 ml of DMAc. Subsequently, Pd(OAc)2
(1.46 mg 0.007 mmol), PCy3.HBF4 (4.8 mg 0.01 mmol), PivOH (13.31 mg 0.1304 mmol) and K2CO3
(54.07 mg 0.391 mmol) were added to the monomers’ solution. Before being removed from the glove
box, the vial was sealed with a rubber cap. Then, the vial was heated in a 100 °C in oil bath for 24 h.
After being cooled to room temperature, the reaction mixture was diluted with 30 mL of chloroform. The
mixture was precipitated in methanol and purified via Soxhlet extraction with n-heptane, acetone and
chloroform in order to remove low molecular weight fractions. The resulting polymer was isolated by
filtration, washed with acetone and finally dried under reduced pressure at 50 °C for 24 h. A dark blue
solid was obtained with a yield of 69%.
1H NMR (500 MHz, CDCl3), δ (ppm): 8.50 – 9.25 (m, 2H), 6.61 – 7.61 (m, 8H), 4.00 (d, 4H), 0.7 –
1.95 (m, 45H). GPC: Mn = 12.700 g/mol. Đ (Mw/Mn) = 1.87. Analysis calculated for [C52H63O3N3S4]14
found: C, 67.81; H, 8.01; N, 4.94; S, 13.76.
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2.7 Synthesis of conjugated polymer of poly[4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-
d]pyrrole-alt-4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole] (compound 11) (P2)
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In a glove box, 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (50 mg 0.1304
mmol) and 4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole (59.75 mg 0.1304 mmol) were
dissolved in 3 ml of DMAc. Then, Pd(OAc)2 (1.46 mg 0.007 mmol), PCy3.HBF4 (4.80 mg 0.013 mmol),
PivOH (13.31 mg 0.1304 mmol) and K2CO3 (54.07 mg 0.3912 mmol) were added to the monomer
solution. Before being removed from the glove box, the vial was sealed with a rubber cap. Then, the vial
was heated in a 100 °C oil bath for 24 h. After being cooled to room temperature, the reaction mixture
was diluted with 30 mL of chloroform. The mixture was precipitated in methanol and purified via Soxhlet
extraction with n-heptane, acetone and chloroform in order to remove low molecular weights fraction.
The resulting polymer was isolated by filtration, washed with acetone and finally dried under reduced
pressure at 50 °C for 24 h. A dark purple solid was obtained with yield of 65%.
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1H NMR (500 MHz, CDCl3), δ (ppm): 8.07 – 7.53 (m, 6H), 7.09 (m, 4H), 3.96 (s, 2H), 2.69 (s,
2H), 2.81 (s, 2H), 0.8 – 1.98 (m, 37H). GPC: Mn = 14.800 g/mol. Đ (Mw/Mn) = 1.65. Analysis calculated
for [C48H55ON3S5]17 found: C, 66.80; H, 6.37; N, 5.01; S, 17.82.
2.8 Solid state film preparation
Solid state polymer thin films were prepared by spin-coating from polymer solutions in chloroform
with a concentration of 3 mg/ml and then dried at 70 °C for 60 min.
2.9 Fabrication and characterization of bulk-heterojunction polymer solar cell structure
First, the ITO substrates (10 Ω/sq) were cleaned by ultrasonication with deionized water, acetone
and isopropyl alcohol respectively. The devices were fabricated with a bulk-heterojunction (BHJ)
structure with an configuration of ITO/PEDOT:PSS/conjugated polymer:PC61BM/Ca/Al. The film was
annealed at 200 °C for 1 h, and then transferred into a N2-filled glove box to spin-cast the active layer.
The solution containing a mixture of polymer:PC61BM (weight ratio is 1:1.5) in chlorobenzene solvent
with 1.0 vol% of 1,8-diiodooctane (DIO). This was then followed by thermal annealing at 100 °C for 5
min. Then, Ca and Al were successively deposited by thermal evaporation in a vacuum less than 1 x 10-6
Torr with deposition rate of 0.3 nm s-1.
2.10 Fabrication and characterization of OFETs
The polymers P1 and were applied to fabricate the OFETs on oxidized silicon wafer to determine
the charge mobilities of semiconducting polymers. 80 nm thick films of the P1 and P2 conjugated
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polymers were spin-coated on silicon wafer substrates under argon environment, followed by film drying
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and annealing at 150 C for 15 min. Finally, OFET devices were completed by evaporating gold source
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and drain contacts through a shadow mask. The electrical properties of OFETs devices Keithley 4200
semiconductor parameter analyzer equipment was used to characterize the electrical properties of OFETs
devices.
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3. Results and Discussion
3.1 Synthesis and characterization of monomers and conjugated polymers
The synthetic route for preparation of 1-((2-ethylhexyl)oxy)-4-iodobenzene (Compound 1, M1),
4H‐dithieno[3,2‐b:2′,3′‐d]pyrrole (M2) and 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-
d]pyrrole (M3) is presented in Scheme 1. M1 was synthesized by nucleophilic substitution that following
to the procedure of previous literature.30 M2 was synthesized by Ullmann intramolecular cyclization
reaction following the previous report.29 Then, monomer M3 was formed from M1 and M2 via Buchwald-
Hartwig cross coupling reaction where Pd(OAc)2, P(t-But)3, NaOt-Bu were used as catalyst and ligand,
1
respectively. In the H NMR spectrum of monomer M3 (see Fig. S1, support information), the peaks at
7.04 ppm and 7.47 ppm are assigned for protons of “α” and “β” of thiophene ring, while those at 7.08
ppm and 7.14 ppm correspond to the protons of benzene ring linked with thiophene via C-N linker. The
peaks at 3.89 ppm and peaks from 0.8 ppm to 1.8 ppm are attributed to the alkyl chains of monomer M3.
In addition, the integration of the peaks indicates that 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-
b:2',3'-d]pyrrole (M3) was synthesized successfully.
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Scheme 1. Synthesis of monomers and D-A polymers P1 and P2.
The direct arylation polycondensation was carried out by Fagnou catalyst system where Pd(OAc)2
was used as catalyst and PCy3.HBF4 as ligand. The reaction was conducted in the DMAc solvent at 100
°C. At the early stage of the reaction, the color of the reaction was light yellow, and then changed to dark
pink after 1 h and turned to dark blue after 24 h for the synthesis of P1. In the case of P2, the color of the
reaction changed to deep orange after 1 h and turned to dark purple after 24 h. Polymers P1 and P2 were
dissolved in CHCl3 and filtrated via a celite layer to eliminate the Pd catalyst, and then the polymers were
obtained by precipitation in cold n-heptane. The yields of polymerizations fluctuated between 60 and
70%. Moreover, the D-A copolymers exhibited good solubility in common organic solvents such as
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CHCl3, THF, dichlorobenzene and toluene. The gel permeation chromatography (GPC) results showed
that the number average molecular weights (Mn) of P1 and P2 were 12700 g mol-1 and 14800 g mol-1 with
polydispersity indexes (Đ) of 1.87 and 1.65, respectively. However, the GPC traces of the polymers
exhibited the unsymmetrical shape containing a shoulder in the range of low retention time, which were
attributed to the high molecular weight fraction in the polymer mixture (Figure 1). It should be noted that
the D-A polymers were polymerized according to the step-growth polycondensation. Therefore, couplings
of oligomer/polymer segments appeared to form extremely high molecular weight fractions compared to
their average molecular weights.
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Figure 1. GPC curves of D-A conjugated polymers P1 and P2.
The vibrational properties of polymers P1 and P2 were investigated by FTIR spectroscopy (see Fig.
S2). The FTIR spectra of P1 and P2 exhibited the bands from 2853 cm-1 to 2922 cm-1 attributed to the C-
H stretching mode of hexyl group and C-H in aromatic structure. The band from 1585 cm-1 to 1492 cm-1 is
ascribed to the aromatic C=C stretching and C-H deformation vibrations.31,32 The broad band between
1403 cm−1 and 1409 cm−1 is presented for the C–N stretching of pyrrole moieties. In addition, the
characteristic band at 1658 cm-1 indicates the existence of the ketone C=O group of N-benzoyl dithieno
[3,2-b:2′,3′-d] pyrrole moiety in the structure of polymer P1. The band from 577 to 824 cm-1 is ascribed to
the C-S stretching vibrations of thiophene structures.
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The structures of D-A polymers P1 and P2 were also investigated by 1H NMR spectroscopy. In the
1H NMR spectrum of P1 (Figure 2A), the peaks from 0.80 to 1.90 ppm are attributed to the alkyl side
chain of P1. The peak at 4.0 ppm corresponds to the methylene protons of alkyl side chain (peak “d”,
Figure 2A). The multi-peaks from 6.5 ppm to 7.5 ppm (peaks “a”, “b”, “f” and “g”) correspond to the
protons in thiophene, 4H‐dithieno[3,2‐b:2′,3′‐d]pyrrole and pyrrole aromatic structures. In addition, the
broad peak at 8.98 ppm (peak “c”) is assigned as a characteristic peak of thiophene units. In the 1H NMR
spectrum of P2 (Figure 2B), the peaks from 0.80 ppm to 1.90 ppm are also ascribed to the alkyl side
chains. The peaks at 2.69 ppm and 8.07 ppm correspond to the hexyl methylene protons, which is well-
known for the 3-hexylthiophene unit. The peak at 4.0 ppm (peak “a”) is assigned to the methylene
protons of the branched alkyl chain of M3 moieties. The peaks from 6.90 ppm to 8.07 ppm (peak “b”,
“c”, “e”, “f”, and “g”) are characteristic for the protons in thiophene, 4H‐dithieno[3,2‐b:2′,3′‐d]pyrrole
and benzo[c][1,2,5]thiadiazole. Based on the GPC, FTIR and 1H NMR results, it can be demonstrated that
D-A polymers P1 and P2 were obtained through direct heteroarylation polymerization. These polymers
were also characterized via 13C NMR spectroscopy. However, the results did not support clearly the
determination of formation of polymers.
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Figure 2. 1HNMR spectra of P1 (a) and P2 (b) in CDCl3.
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3.2 Optical and electrochemical properties of conjugated polymers
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The absorption properties of polymers P1 and P2 were investigated through UV-Vis absorption
spectroscopy, which was performed in dilute CHCl3 solution (ca. 10−6 mol L–1) and in solid state films.
Polymer P2 showed an absorption maximum at 550 nm, whereas polymer P1 showed a strong red shift
absorption maximum at 730 nm, which is 180 nm higher than for P2. This observation can be explained
by that normally the DPP unit has an absorption maximum above 650 nm. Thus, incorporation of DPP
moieties in the D-A polymer structure led to a strong red shift in absorption. In addition, the main reason
causing the absorption red shift of P1 is the strong intermolecular π – π stacking interactions between
conjugated main chains, which enhances rigidity in its structure. Figure 3(b) presents the optical
absorption spectra based on the solution-cast thin films of P1 and P2. The polymer P1 and P2 thin films
exhibited absorption maxima at 800 nm and 680 nm, respectively. This phenomenon indicates again the
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strong intermolecular 휋–휋 stacking interactions of P1 in comparison with P2. Moreover, the onset values
opt
of P1 and P2 were determined to be about 959 nm and 800 nm, corresponding to optical band gaps (Eg
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of 1.31 eV and 1.55 eV, respectively. Furthermore, the molar extinction coefficients () of P1 and P2
were 1.2 x 10-4 and 1.5 x 10-4, respectively. The parameters of optical properties of D-A polymers are
summarized in Table 1.
Figure 3. Absorption spectra of polymers P1 and P2 in CHCl3 (a) and in solid state films (b).
To investigate the photoluminescence (PL) of P1 and P2, the PL spectra of P1 and P2 were
measured in different solvents as shown in Figure 4. In CHCl3 and THF (ca. 10−3 mol L–1), P1 displayed
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an emission peak at 712 nm upon excitation at 450 nm, whereas P2 exhibited an emission peak at 735 nm
upon excitation at 450 nm. The fluorescence quantum yields (F) of D-A polymers in dilute CHCl3 were
analyzed in comparison to 9,10-diphenylanthracene as a standard (F = 0.9). The F of P1 and P2 showed
the same value of 0.43, likely indicating the strong 휋 –휋 stacking effect in both polymer structures.33 The
PL data are presented in Table 1.
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Figure 4. Fluorescence spectra of polymers P1 and P2 in CHCl3 (concentrations of 10-3).
Table 1. UV–vis absorption and fluorescence emission maximum wavelength, and the fluorescence
quantum yields (F) of polymer P1 and P2.
D-A Polymers
Solvent (CHCl3)
Film
UV (λmax
(nm)
Optical
bandgap
(Eopt) (eV)c
1.31
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extinction
coefficient (ε)
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PL
(nm)
712
φF
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(nm)
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P2
730
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0.43
790
550
735
670
1.55
1.5 x 104
The LUMO energy levels of the D-A polymers were determined from the optical band gaps
which were calculated from onset absorption, while the HOMO energy levels are possibly estimated from
cyclic voltammetry. Measurements of cyclic voltammetry for polymer P1 and P2 were calibrated against
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ferrocene, as an internal standard, of which ionization potential value is -4.8 eV for ferrocene/ferrocenium
(Fc/Fc+) redox system. The cyclic voltammetry curves and energy level diagrams of the D-A polymers
are presented in Figure 5. The oxidation onset potentials of P1 and P2 were 0.8 and 1.1 eV, corresponding
to the HOMO energy levels of -5.2 eV and -5.5 eV, respectively. The LUMO energy levels of P1 and P2
calculated from the values of the optical band gaps and HOMO energy levels were -3.89 and -3.95 eV,
respectively. It is very clear that the LUMO levels of P1 and P2 are close to that of PCBM acceptor,
suggesting that the polymers are useful for preparation of photovoltaic cells.
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Figure 5. Cyclic voltammograms and energy diagrams of polymer P1 and P2.
3.3. Thermal and solid state properties of conjugated polymers
The thermal properties of polymers P1 and P2 were characterized by TGA and DSC. TGA
analysis was performed under nitrogen flow to determine the stability of these D-A polymers in the range
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from room temperature to 950 C. The polymer P2 exhibited a good thermal stability with a
decomposition temperature (5% weight loss) above 350 oC, whereas polymer P1 showed a lower thermal
stability with a decomposition temperature (5% weight loss) around 250 oC (see Figure 6(a)). The second
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heating run DSC diagrams in range from room temperature to 300 oC of the D-A polymers are presented
in Figure 6(b). No melting transition in this temperature range was detected for both P1 and P2.
Normally, the main chain of conjugated polymers is very stiff due to their strong intermolecular 휋–휋
stacking interaction, which results in high melting temperatures above 200 °C.34-36
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Figure 6. The TGA (a) and DSC (b) diagrams of polymers P1 and P2.
The molecular ordering of these D-A conjugated polymers in the solid state was characterized
through powder XRD measurements (Figure 7). Polymer P1 exhibited several distinctive crystalline
diffractions at 2θ = 18o (4.9 Å), 28o (3.2 Å) and 40o (2.3 Å), which closely correspond to the conjugated
main chain distance, 휋–휋 stacking spacing and the distance between highly stacked hetero-aromatic rings,
repectively,37-38 while the broad halo centered at 23° is likely attributed to side chain disorder. On the
other hand, for polymer P2, the XRD pattern showed two broad diffractions at ca. 23 and 39o and
diffractions associated to strong stacking orientation seem absent (or maybe overlaid by the amorphous
scattering halo). This suggests that P2 more weakly aggregates than P1.
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Figure 7. XRD patterns of polymer P1 (a) and P2 (b).
The microscopic and nanoscopic morphologies of thin films of D-A polymers were checked by
AFM in tapping contact mode. The drop-casting technique was applied to prepare thin films onto the
wafer substrates from a good solvent. Figure 8 shows the AFM images of polymers P1 and P2 after
solvent annealing at 150 oC. The results revealed that the surface of polymer P1 was smoother than that of
P2. The roughness value of P1 was determined 0.49 whereas P2 exhibited a roughness value of 2.11.
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Figure 8. AFM images of D-A polymer P1 (a, 2D dimension) (b, 3D dimension) and polymer P2
(c, 2D dimension) (d, 3D dimension).
The polymers P1 and were further applied to fabricate the OFETs on oxidized silicon wafer to
determine the charge mobilities of semiconducting polymers.
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Figure 9. Drain current (ID) versus drain voltage (VDS) characteristics at different gate voltage for OFET of
(A) P2 and (B) P1.
Thin film transistors based on P1 and P2 conjugated polymers exhibited typical p-channel output
characteristics due to plot of drain current ID vs. drain voltage at different gate voltage (VG) (Figure 9).
The mobilities of OFETs devices have been calculated according to the linear regime theory at low drain
voltage following equation (1)39,40
IDS = (푊)µhCi(VG – VT – VD/2)VD (1)
퐿
Where µh is the field-effect mobility of holes, W is the channel width, L is the channel length, the
channel length and width of the transistor are 10 m and 700 m respectively. The Ci is the capacitance
per unit area of the gate dielectric layer (SiO2, 300 nm. Ci = 11 nF/cm2). VG, VT and VD are gate, threshold
and drain voltages, respectively. The thin film transistors based on P1 and P2 conjugated polymers
exhibited hole mobilities of 1.98 x 10-2 cm2/Vs and 9.54 x 10-3 cm2/Vs with an on/off ratio about 105 and
103, respectively. This result suggested that the synthesized conjugated polymers are promising for
organic solar cell application.
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Figure 10. AFM height images (5 x 5 m) of the blended films: (a) P1:PC61BM (1:1.5 w/w) and (b)
P2:PC61BM (1:1.5 w/w).
As shown in Figure 10, the AFM images of the P2:PC61BM blended film showed a rough
surface, with an RMS of 2.18 nm, which inhibited charge migration and exciton diffusion. In contrast, the
AFM image of the P1 and PC61 BM blended film shows much finer domains, with an RMS of 0.62 nm,
which resulted in a much higher fill factor.
3.4 Organic solar cell device fabrication
Bulk heterojunction (BHJ) polymer solar cells were fabricated following the conventional
structure ITO/PEDOT:PSS/conjugated polymer:PC61BM/Ca/Al.41 The Jsc, Voc, fill factor (FF) and PCE
values of the OSCs based on a BHJ structure of polymer P1 and polymer P2 are presented in Table 2. The
OSCs based on P1 and P2 exhibited the same Voc of 0.79 V. This suggested that the Voc of OSCs is not
completely reflected by the HOMO level of conjugated polymers, since the Voc of cells much depends on
the interaction of the electrode and activated layers as well as the parameters of cell fabrication process.
The bilayer structure solar cells based on P1 and P2 showed an average PCE of 4.85% and 3.99%,
respectively. The current density–voltage (J–V) curves of the polymer solar cell devices recorded under
AM 1.5 illumination with an intensity of 100mW.cm-2 are plotted in Figure 11A. The EQE spectra of
these devices exhibited a pronounced contribution to photocurrent generation in the wavelength range of
400-850 nm (Figure 11B). This implies that the transport of generated charges from the P1(P2)/PC61BM
interface to the anode layer via the PC61BM electron channels was quite effective, which contributes to
the relatively good device performance. On the other hand, the device fabricated from P1 reveals better
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EQE values than from P2. This might be attributed to either less effective contact interface between P2
and PC61BM or to less efficient charge transport in polymer P2. It should be noted that the PCE depends
on not only the bandgap but also on the charge-carrier mobility. Therefore, the photovoltaic parameters of
the BHJ solar cells fabricated from polymer P1 with PC61BM acceptor compound revealed that the PCE
of P1 (4.85%) was much better than that of polymer P2 (3.99%) as a result of the lower bandgap energy
level and higher ordering structure of P1 giving rise to more efficient charge transport.
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Table 2. Photovoltaic parameters of OSCs based on polymer P1 and polymer P2 using PC61BM as
acceptor counterpart in optimized ratios under the illumination of AM 1.5G, 100 mW.cm-2.
PCE (Avg.)
D-A Polymer/Fullerene
Voc (V)
Jsc (mA/cm2)
FF (%)
%
P1/PC61BM
P2/PC61BM
0.79
0.79
12.82
13.24
47.98
38.24
4.85
3.99
Figure 11. Current–voltage characteristics of the bulk heterojunction polymeric solar cells prepared from
polymers P1 and P2 as donor polymer, and PC61BM fullerene acceptor (A) and EQE spectra of solar cells
based on P1 and P2 (B).
4. Conclusion
In conclusion, newly designed D-A conjugated polymers based on 4-(4-((2-
ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EPDP) and 3,6-bis(5-bromothiophen-2-yl)-2,5-
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bis(2-ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (BTBP) or 4,7-bis(5-bromothiophen-2-
yl)benzo[c][1,2,5]thiadiazole (BT) units were synthesized successfully in 65-69% yields through
direct(hetero)arylation polymerization. The D-A polymers P1 and P2 were characterized, exhibiting low
bandgap energy levels of 1.31 eV and 1.55 eV, respectively. Both polymers P1 and P2 were used to
fabricate BHJ organic solar cells in combination with PC61BM with PCEs of 4.85% and 3.99%,
respectively. This work demonstrated the potential of new D-A conjugated polymers for achieve efficient
organic photovoltaic devices.
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Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
This research was fully supported by the Vietnam National Foundation for Science and Technology
Development (NAFOSTED) under grant number “104.02-2017.339”
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Synthesis and Characterization of Donor-Acceptor Semiconducting
Polymers Containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-
b:2',3'-d]pyrrole for Organic Solar Cells
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Huyen Le Thi Mai1, Nhung Thanh Thi Truong1, Thiet Quoc Nguyen4, Bao Kim Doan1, Dat Hung Tran1,
Le-Thu T.Nguyen2, Lee Woosung3, Jae Woong Jung5, Mai Ha Hoang6, Ha Phuong Ky Huynh1, Chau Duc
Tran*2, Ha Tran Nguyen*1,2
The
novel
donor–acceptor
conjugated
polymers
and
containing
4-(4-((2-
ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole
2,5-bis(2-ethylhexyl)-3,6-
di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione were successfully synthesized via
direct arylation polymerization. The obtained donor–acceptor conjugated polymers have been
applied for the organic solar cells which exhibited the PCE of 4.85%.