Step-by-step improvement in photovoltaic properties of fluorinated quinoxaline-based low-band-gap polymers

Article abstract of DOI:10.1016/j.orgel.2017.04.025

A series of fluorinated quinoxaline-based polymers with typical donor-π-acceptor configurations has been synthesized by Stille coupling reaction. The electron-donating dialkoxy-substituted benzodithiophene (BDT) is connected to the electron-withdrawing 2,

Full text of DOI:10.1016/j.orgel.2017.04.025

Accepted Manuscript
Step-by-step improvement in photovoltaic properties of fluorinated quinoxaline-based
low-band-gap polymers
Sella Kurnia Putri, Yun Hwan Kim, Dong Ryeol Whang, Min Seok Lee, Joo Hyun Kim,
Dong Wook Chang
PII:
S1566-1199(17)30183-0
10.1016/j.orgel.2017.04.025
ORGELE 4063
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 14 February 2017
Revised Date: 1 April 2017
Accepted Date: 24 April 2017
Please cite this article as: S.K. Putri, Y.H. Kim, D.R. Whang, M.S. Lee, J.H. Kim, D.W. Chang, Step-by-
step improvement in photovoltaic properties of fluorinated quinoxaline-based low-band-gap polymers,
Organic Electronics (2017), doi: 10.1016/j.orgel.2017.04.025.
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ACCEPTED MANUSCRIPT
GA
S2

ACCEPTED MANUSCRIPT
Step-by-Step Improvement in Photovoltaic Properties of Fluorinated Quinoxaline-Based Low-
Band-Gap Polymers
Sella Kurnia Putri,a† Yun Hwan Kim,b† Dong Ryeol Whang,c Min Seok Lee,a Joo Hyun Kim,*, b and
Dong Wook Chang*, a
aDepartment of Industrial Chemistry, Pukyong National University, 48547 Pusan, Republic of
Korea
bDepartment of Polymer Engineering, Pukyong National University, 48547 Pusan, Republic of
Korea
cLinz Institute for Organic Solar Cells (LIOS)/Institute of Physical Chemistry, Johannes Kepler
University Linz, 4040 Linz, Austria
Corresponding Authors
D. W. Chang: dwchang@pknu.ac.kr
J. H. Kim: jkim@pknu.ac.kr
†Author Contributions
S. K. Putri and Y. H. Kim equally contributed to this research.
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Abstract
A series of fluorinated quinoxaline-based polymers with typical donor-π-acceptor
configurations has been synthesized by Stille coupling reaction. The electron-donating
dialkoxy-substituted benzodithiophene (BDT) is connected to the electron-withdrawing 2,3-
diphenylquinoxaline (DPQ) acceptor through a thiophene bridge. To investigate the effect
of strong electron-withdrawing moieties, fluorine atoms have been systematically
incorporated at various positions on the DPQ units such as the 6,7-positions, the para
positions of the phenyl substituent on the 2,3-positions, and both locations to afford PBDT-
QxF PBDT-FQx, and PBDT-FQxF, respectively. Because of significant contributions
-
,
from the fluorine atoms, these polymers display quite differentiated optical and
electrochemical properties in comparison with those of their non-fluorinated counterpart,
PBDT-Qx. In addition, a gradual improvement in power conversion efficiencies (PCEs) is
observed in the order of PBDT-Qx
inverted-type polymer solar
,
PBDT-QxF
,
PBDT-FQx, and PBDT-FQxF, when
with configuration of
cells (PSCs)
a
ITO/ZnO/polymers:PC71BM/MoO3/Al were fabricated. In particular, PBDT-FQxF, with
four fluorine atoms on the 6,7-positions and the para-positions of the phenyl substituent on
2,3-positions of DPQ, exhibits the highest PCE of 6.60% with an open-circuit voltage (Voc
)
of 0.91 V, a short-circuit current density (Jsc) of 10.15 mA/cm-2, and a fill factor (FF) of
71.5%.
Keywords: Low band gap polymer; benzothiadiazole; 2,3-diphenylquinoxaline; fluorine; polymer
solar cells
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1. Introduction
Owing to their unique advantages such as lightweight, easy processability, and superior
flexibility, polymer solar cells (PSCs) have attracted great interest during recent decades.1-3 Upon
formation of bulk heterojunction (BHJ) structures between conjugated polymeric electron donors
and electron acceptors (mostly fullerene-based materials) in an active layer of PSCs, the charge
separation and transportation of photo-generated excitons can be greatly facilitated, which enables
efficient conversion from solar light to electrical energy. Unceasing and innovative efforts,
including material developments and device optimizations can greatly improve the power
conversion efficiency (PCE) of PSCs to more than 10 %.4-7
So far, many low-band-gap conjugated polymers have been intensively developed for use as
photo-active components in PSCs.8-9 They usually possess alternating electron-donating (D) and
electron-withdrawing (A) building blocks along their conjugated backbones, leading to a significant
reduction in the band gap through efficient formation of intramolecular charge transfer (ICT) states.
In particular, the quinoxaline moiety has recently attracted great attentions as a major electron-
accepting building block in low-band-gap polymers, because of its unique advantages such as high
electron affinity, simple preparation, and structural versatility.10-16
Recently, the incorporation of an electron-withdrawing fluorine atom, especially onto electron-
accepting units of conjugated polymer structures, has come to be regarded one of the more
promising approaches for enhancing photovoltaic performances of PSCs.17 The high electron
affinity and small size of the fluorine atom can efficiently modulate the energy levels by lowering
both the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital
(HOMO) energy level and minimizing the undesirable steric hindrance of conjugated polymers,
respectively.17 Therefore, a significant improvement in PCEs to more than 8% has been obtained
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from fluorinated quinoxaline-based PSCs.18-19 Among various quinoxaline derivatives, 2,3-
diphenylquinoxaline (DPQ) is usually considered as an essential motif for introducing fluorine
atoms, because of its facile preparation and structural diversity.14 For example, Chen et al.
demonstrated a high PCE of up to 8.0% from PSCs with a 6,7-difluorinated DPQ-based low-band-
gap copolymer.20 Similar promising results have been observed from other polymers with mono- or
difluorinated DPQ units on its 6,7-positions.21-25 Apart from the 6,7-positions of DPQ, it has been
also reported that the introduction of fluorine atoms on the para-position of phenyl ring on 2,3-
positions of the DPQ moiety can significantly improve the photovoltaic properties of PSCs.26-27
Although, these results clearly underline the usefulness of fluorine atoms on diverse positions of
DPQ moieties for photovoltaic applications, more studies are required to better understand the
effects of fluorine substituents on the various characteristics of DPQ-based conjugated polymers.
Herein, we synthesized a series of DPQ-based low-band-gap polymers with a typical D-π-A
configuration, in which an electron-donating dialkoxy-substituted benzodithiophene (BDT) was
linked to electron-accepting DPQ derivatives through a thiophene linker. For systematic
investigations, fluorine atoms were incorporated onto the 6,7-positions of DPQ, the para-positions
of the phenyl substituents on the 2,3-positions of DPQ and both locations on the DPQ unit in a non-
fluorinated standard polymer, PBDT-Qx, to afford PBDT-QxF, PBDT-FQx, and PBDT-FQxF,
respectively (Figure 1). This simple but methodical approach enabled us to well comprehend the
positional and populational influence of fluorine atoms on various aspects of DPQ-based low-band-
gap polymers, including optical and electrochemical properties together with photovoltaic
characteristics.
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Figure 1. Chemical structures of PBDT-Qx, PBDT-QxF, PBDT-FQx, and PBDT-FQxF.
2. Experimental Section
2.1. Materials and instruments
4,7-Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(1),
5,6-difluoro-4,7-di(thiophen-2-yl)-
benzo[c][1,2,5]thiadiazole (2), and 2,6-bis(trimethyltin)-4,8-bis(2-octyldodecyloxy)benzo[1,2-b:4,5-
b′]dithiophene (11) were synthesized according to the previous literatures.28-30 Benzil and 4,4′-
difluorobenzil were purchased from Aldrich and TCI, respectively. All other chemicals and solvents
were obtained from Aldrich Chemical Co., Inc. 1H, 13C, and 19F NMR spectra were measured with a
JEOL JNM ECP-400 spectrometer. UV-visible spectra were recorded on a JASCO V-530 UV-Vis
spectrophotometer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF)
spectroscopy was conducted by using a Bruker Ultraflex spectrometer. Gel permeation
chromatography (GPC) was measured on an Agilent 1200 series instrument with THF as the eluent.
Cyclic voltammetry (CV) measurements were carried out by using a VersaSTAT3 potentiostat
(Princeton Applied Research) with tetrabutylammonium hexafluorophosphate (0.1 M, Bu4NPF6) as
the electrolyte in acetonitrile. For CV measurements, a glassy carbon electrode coated with
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polymers and a platinum wire were used as the working and counter electrode, respectively. A silver
wire was used as a pseudo-reference electrode with a ferrocene/ferrocenium external standard. The
thickness of the films was measured with an Alpha-Step IQ surface profiler (KLA-Tencor Co.), and
the atomic force microscopy (AFM) topography images were taken by using a Bruker (NanoScope
V) microscope operated in tapping mode.
2.2. Fabrication and analysis of photovoltaic devices
[6,6]-Phenyl C71 butyric acid methyl ester (PC71BM, catalog no. nano-cPCBM-SF) was obtained
from Nano-C, Inc. To fabricate inverted type PSCs with indium tin oxide (ITO)/ZnO/active layer
(polymer:PC71BM)/MoO3/Ag, a 25-nm-thick ZnO film was initially deposited on an ITO surface by
using a sol-gel process. The partially crystalline ZnO film was prepared by thermal curing of
predeposited ZnO precursors at 200 °C for 10 min. The solution of ZnO precursors was prepared by
the dissolution of zinc acetate dehydrate (0.164g) and ethanolamine (0.05 ml) in methoxyethanol
(1mL) and stirring the mixture for 30 min prior to film deposition. The active layer with a thickness
of 80 nm was fabricated using a chloroform solution of polymeric donor and PC71BM acceptor by
spin-coating at 600 rpm for 60 s. Prior to spin-coating, the blended solution was filtered through a
0.2 µm polytetrafluoroethylene membrane filter. Finally, a 20-nm-thick MoO3 layer and 100-nm-
thick Ag layer were consecutively deposited by thermal evaporation at 2 × 10-6 Torr through a
shadow mask with a device area of 0.13 cm2. The J-V characteristics of device were analyzed by
using a KEITHLEY Model 2400 source-measure unit under AM 1.5G illumination at 100 mW/cm2
from a 150 W Xe lamp. The conditions of solar simulation were calibrated before the measurements
by using a Si reference cell with a KG5 filter certified by the National Institute of Advanced
Industrial Science and Technology.
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Scheme 1. Synthesis of PBDT-Qx, PBDT-QxF, PBDT-FQx and PBDT-FQxF. (i) Zinc, acetic
acid, 80 °C, 12 h; (ii) α-diketone, acetic acid, reflux, overnight; (iii) N-bromosuccinimide (NBS),
RT, overnight; (iv) Pd(Ph3)4, toluene, 90 °C, 48 h.
2.3. Syntheses.
2.3.1. Synthesis of DPQ-based compounds 3, 4, 5 and 6
A mixture of the appropriate thiophene-attached benzothiadiazoles (1 or 2, 2.5 mmol) and zinc
powder (20 equiv., 50 mmol) in acetic acid (60 mL) was stirred at 80 °C for 12 h. Upon completion
of the reaction, the mixture was filtered to remove zinc powder and the filtrate was carefully
collected. After addition of α-diketone (benzil or 4,4′-difluorobenzil, 2.5 mmol) to the filtrate, the
solution was heated to reflux overnight. The solution was cooled to room temperature, and the
mixture was poured into water and extracted with chloroform. The organic layers were separated,
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dried over magnesium sulfate, and filtered. Solvents were removed under reduced pressure, and the
crude residue was further purified by column chromatography with chloroform/ethanol (1/4, v/v).
2.3.2. 2,3-Diphenyl-5,8-di(thiophen-2-yl)quinoxaline (3)
4,7-Di(thiophene-2-yl)benzo[c]-[1,2,5]thiadiazole (1) and benzil were used as reactants. Yield: 62%
(light green solid). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.17 (s, 2H), 7.89 (d, 2H, J = 3.76 Hz),
13
7.75 (m, 4H), 7.53 (d, 2H, J = 5.12 Hz), 7.42-7.38 (m, 6H), 7.20 (dd, 2H, J = 3.76, 5.12 Hz). C
NMR (100 MHz, CDCl3): δ (ppm) = 154.3, 141.4, 141.3, 139.9, 133.9, 133.1, 131.7, 131.5, 130.9,
129.7, 129.3, 129.1. MALDI-TOF MS: m/z calcd, 446.586; found, 446.127 [M+].
2.3.3. 2,3-Di(4-fluorophenyl)-5,8-di(thiophen-2-yl)quinoxaline (4)
4,7-Di(thiophene-2-yl)benzo[c]-[1,2,5]thiadiazole (1) and 4,4′-difluorobenzil were used as
reactants. Yield: 58% (light green solid). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.15 (s, 2H), 7.84
(d, 2H, J = 3.76 Hz), 7.72 (dd, 4H, J = 5.36, 8.60 Hz), 7.52 (d, 2H, J = 5.12 Hz), 7.18 (dd, 2H, J =
5.12, 3.76 Hz), 7.09 (t, 4H, J = 8.60 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) = 167.1, 165.2,
154.0, 153.0, 141.2, 139.8, 135.0, 131.6, 129.9, 129.3, 129.1, 118.2, 118.1, 110.0. 19F NMR (376
MHz, CDCl3): δ (ppm) = -111.51. MALDI-TOF MS: m/z calcd, 482.567; found, 483.121 [M+].
2.3.4. 6,7-Difluoro-2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (5)
5,6-Difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (2) and benzil were used as reactants.
Yield: 68% (yellow solid). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.05 (d, 2H, J = 3.76 Hz), 7.72
13
(m, 4H), 7.65 (d, 2H, J = 5.12 Hz), 7.41-7.37 (m, 6H), 7.22 (dd, 2H, J = 3.76, 5,12 Hz). C NMR
(100 MHz, CDCl3): δ (ppm) = 152.2, 138.8, 135.6, 131.6, 131.5, 131.4, 131.2, 130.8, 129.9, 129.0,
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127.3 19F NMR (376 MHz, CDCl3): δ (ppm) = -128.68. Mass, m/z calcd, 482.07; found, 481.85
[M+].
2.3.5. 6,7-Difluoro-2,3-bis(4-fluorophenyl)-5,8-di(thiophen-2-yl)quinoxaline (6)
5,6-Difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (2) and 4,4′-difluorobenzil were used
1
as reactants. Yield: 65% (orange solid). H NMR (400 MHz, CDCl3): δ (ppm) = 8.00 (d, 2H, J =
3.76 Hz), 7.68 (dd, 4H, J = 5.36, 8.84 Hz), 7.65 (d, 2H, J = 5.12Hz), 7.23 (dd, 2H, J = 3.76, 5.12
13
Hz), 7.08 (t, 4H, J = 8.84 Hz). C NMR (125 MHz, CDCl3): δ (ppm) = 164.3, 162.6, 150.2, 134.7,
134.0, 132.4, 132.3, 130.9, 130.5, 130.1, 126.7, 117.9, 115.7, 115.5. 19F NMR (376 MHz, CDCl3): δ
(ppm) = -111.09, -128.18. Mass, m/z calcd, 518.55; found, 517.85 [M+].
2.3.6. Synthesis of dibromo-DPQ compounds 7, 8, 9, 10
A mixture of the appropriate DPQ compound (3, 4, 5, 6; 1.0 mmol)) and N-bromosuccinimide
(NBS; 2.2 mmol) in THF (30 mL) was stirred at room temperature for 12 h. The reaction mixture
was poured into water and extracted with ethyl acetate. The organic layers were separated, dried
over magnesium sulfate and filtered. Solvents were removed under reduced pressure, and the crude
residue was further purified by recrystallization from ethyl acetate/ethanol (1/4, v/v).
2.3.7. 5,8-Di(5-bromothiophen-2-yl)-2,3-diphenylquinoxaline (7)
1
3 was used as the starting material. Yield: 72% (orange solid). H NMR (400 MHz, CDCl3): δ
(ppm) = 8.12 (s, 2H), 7.71 (m, 4H), 7.59 (d, 2H, J = 2.68 Hz), 7.44-7.40 (m, 6H), 7.14 (d, 2H, J =
13
2.68 Hz). C NMR (100 MHz, CDCl3): δ (ppm) = 154.8, 153.9, 142.2, 140.8, 139.2, 133.2, 133.1,
131.9, 130.9, 128.3, 128.2, 119.8. MALDI-TOF MS: m/z calcd, 604.378; found, 603.993 [M+].
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2.3.8. 5,8-Di(5-bromothiophen-2-yl)-2,3-bis(4-fluorophenyl)quinoxaline (8)
1
4 was used as the starting material. Yield: 65% (light red solid). H NMR (400 MHz, CDCl3): δ
(ppm) = 8.09 (s, 2H), 7.68 (dd, 4H, J = 5.36, 8.88 Hz), 7.55 (d, 2H, J = 4.32 Hz), 7.13-7.09 (m, 6H).
13C NMR (100 MHz, CDCl3): δ (ppm) = 167.0, 165.3, 153.5, 142.0, 139.2, 136.7, 135.0, 133.2,
131.8, 128.5, 119.9, 118.3, 118.2. 19F NMR (376 MHz, CDCl3): δ (ppm) = -111.02. MALDI-TOF
MS: m/z calcd, 640.359; found, 639.992 [M+].
2.3.9. 5,8-Di(5-bromothiophen-2-yl)-6,7-difluoro-2,3-diphenylquinoxaline (9)
1
5 was used as the starting material. Yield: 75% (orange solid). H NMR (400 MHz, CDCl3): δ
(ppm) = 7.76 (d, 2H, J = 4.28 Hz), 7.65 (m, 4H), 7.42-7.37 (m, 6H), 7.13 (d, 2H, J = 4.28 Hz). 13C
NMR (100 MHz, CDCl3): δ (ppm) = 152.6, 138.3, 135.1, 133.1, 131.8, 131.7, 131.6, 131.2, 130.1,
130.0, 129.1, 119.7. 19F NMR (376 MHz, CDCl3): δ (ppm) = -128.36. Mass, m/z calcd, 640.36;
found, 639.55 [M+].
2.3.10. 5,8-Di(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(4-fluorophenyl)quinoxaline (10)
1
6 was used as the starting material. Yield: 70% (orange solid). H NMR (400 MHz, CDCl3): δ
(ppm) = 7.78 (d, 2H, J = 4.04 Hz), 7.64 (dd, 4H, J = 5.36, 8.84 Hz), 7.17 (d, 2H, J = 4.04 Hz), 7.11
(t, 4H, J = 8.60 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) = 164.4, 162.8, 150.7, 134.2, 133.5,
132.4, 132.2, 131.2, 131.1, 129.5, 119.0, 115.8, 115.7. 19F NMR (376 MHz, CDCl3): δ (ppm) = -
110.59, -127.90. Mass, m/z calcd, 676.34; found, 675.60 [M+].
2.3.11. Synthesis of DPQ-based polymers by palladium-catalyzed Stille reaction
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In a Schlenk flask, BDT monomer (0.25 mmol), dibrominated DPQ-based monomer (0.25 mmol)
and Pd(PPh3)4 (3 mol%) were dissolved in dry toluene (10 ml). After N2 had been bubbled through
for 15 min, the reaction mixture was heated to 90 °C and vigorously stirred for 2 d under a N2
atmosphere and then 2-trimethylstannylthiophene and 2-bromothiophene a were consecutively
added s end-capping agents with an interval of 3 h. Once polymerization was complete, the mixture
was cooled to room temperature and poured into methanol. The precipitated black solids were
collected and further purified by Soxhlet extraction with methanol, acetone, hexane, and
chloroform. The polymers in the chloroform fraction were recovered by precipitation into methanol
again. Finally, the polymer was dried in a vacuum oven at 50 °C.
2.3.12. PBDT-Qx
1
11 and 7 were used as reactants. Yield: 83% (deep purple solid). H NMR (400 MHz, CDCl3): δ
(ppm) = 8.02-7.72 (br, 8H), 7.65-7.36 (br, 6H), 7.24-7.15 (br, 2H), 7.10-6.95 (br. 2H), 4.27-3.92 (br,
4H), 2.01-1.83 (br, 2H), 1.81-1.22 (br, 64H), 1.20-1.01 (br, 12H). Molecular weight by GPC:
number-average molecular weight (Mn) = 49.22 KDa, polydispersity index (PDI) = 2.64. Elemental
analysis: calcd (%) for C78H98N2O2S4: C 76.55, H 8.07, N 2.29, S 10.48; found: C 76.08, H 8.15, N
2.31, S 10.57.
2.3.13. PBDT-QxF
1
11 and 8 were used as reactants. Yield: 80% (deep purple solid). H NMR (400 MHz, CDCl3): δ
(ppm) = 8.12-7.65 (br, 10H), 7.48-7.32 (br, 2H), 7.23-7.01 (br, 4H), 4.27-3.95 (br, 4H), 2.01-1.83
(br, 2H), 1.81-1.22 (br, 64H), 1.20-1.01 (br, 12H). Molecular weight by GPC: Mn = 38.30 KDa,
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PDI = 3.01. Elemental analysis: calcd (%) for C78H96F2N2O2S4: C 74.36, H 7.68, N 2.22, S 10.18;
found: C 74.16, H 7.84, N 2.16, S 10.23.
2.3.14. PBDT-FQx
1 and 9 were used as reactants. Yield: 85% (deep purple solid). 1H NMR (400 MHz, CDCl3): δ
(ppm) = 8.21-7.85 (br, 4H), 7.82-7.41 (br, 8H), 7.21-7.01 (br, 4H), 4.27-3.95 (br, 4H), 2.01-1.83 (br,
2H), 1.81-1.22 (br, 64H), 1.20-1.01 (br, 12H). Molecular weight by GPC: Mn = 42.88 KDa, PDI =
2.53. Elemental analysis: calcd (%) for C78H96F2N2O2S4: C 74.36, H 7.68, N 2.22, S 10.18; found: C
74.11, H 7.81, N 2.24, S 10.22.
2.3.15. PBDT-FQxF
11 and 10 were used as reactants. Yield: 78% (deep purple solid). 1H NMR (400 MHz, CDCl3): δ
(ppm) = 8.22-7.61 (br, 6H), 7.49-7.31 (br, 2H), 7.22-7.05 (br, 6H), 4.27-3.95 (br, 4H), 2.01-1.83 (br,
2H), 1.81-1.22 (br, 64H), 1.20-1.01 (br, 12H). Molecular weight by GPC: Mn = 37.09 KDa, PDI =
3.24. Elemental analysis: calcd (%) for C78H94F4N2O2S4: C 72.30, H 7.31, N 2.16, S 9.90; found: C
71.91, H 7.50, N 2.24, S 10.23.
3. Results and discussion,
3.1. Synthesis and thermal properties
The detailed synthetic routes of all monomers and polymers are outlined in Scheme 1. Firstly, 4,7-
Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(1),
5,6-difluoro-4,7-di(thiophen-2-yl)-
benzo[c][1,2,5]thiadiazole (2), were reduced with Zn powder to give ortho-diamine intermediates,
which were consecutively condensed with benzil or 4,4′-difluorobenzil to produce the related DPQ
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compounds of 3, 4, 5, and 6. The combination of two benzothiadiazoles and two benzils can
generate four kinds of thiophene-containing DPQ derivatives with different positions and numbers
of fluorine atoms in their chemical structures. Bromination of 3, 4, 5, and 6 with NBS then readily
afford dibrominated DPQ monomers of 7, 8, 9, and 10, respectively. In addition, the electron-
donating dialkoxy-substituted BDT monomer (11) was prepared to construct polymers with D-π-A
structures. In particular, the relatively long 2-octyldodecyloxy chains were incorporated into BDT
monomer to avoid solubility problems presumably caused by the DPQ-based comonomers (7, 8, 9,
and 10) without alkyl or alkoxy chains. Finally, polymerization of 11 and the relevant DPQ co-
monomers (7, 8, 9, and 10) under Stille coupling condition successfully yielded four different
polymers, PBDT-Qx, PBDT-QxF, PBDT-FQx, and PBDT-FQxF, respectively. The chemical
structures of all monomers and polymers were clearly confirmed by various analytical techniques,
and these polymers display good solubility in common organic solvents such as chloroform, THF,
and toluene. GPC measurements with THF as the eluent exhibit the relatively high number-average
molecular weights of 49.22, 38.30, 42.88, and 37.09 kDa for PBDT-Qx, PBDT-QxF, PBDT-FQx,
and PBDT-FQxF, respectively, with corresponding polydispersity index (PDI) values of 2.64, 3.01,
2.53, and 3.24. Thermogravimetic analysis (TGA) of all polymers at a heating rate of 10 °C/min
under a N2 atmosphere were carried out, and the results are shown in Figure 2. All polymers showed
good thermal stability with a similar high onset of decomposition temperature of around 320 °C at 5
wt % loss (Td5%). However, no discernible thermal transitions of polymers were detected during the
cooling and heating processes of differential scanning calorimetry (DSC) measurements in the
temperature range from 25 to 250 °C (Figure S1 in the electronic supporting information (ESI)), due
to the amorphous nature of the polymers.
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Figure 2. TGA thermograms of PBDT-Qx, PBDT-QxF, PBDT-FQx and PBDT-FQxF at a
heating rate of 10 °C/min under N2.
3.2. Optical and electrochemical properties
As shown in Figure 3a, the UV-Vis spectra of the polymer films exhibit two broad absorption bands
in the shorter wavelength region (350-470 nm) and longer wavelength region (500-750 nm),
respectively. The absorption band at shorter and longer wavelength region correspond to the π-π*
transition of the polymer and to the intermolecular charge transfer (ICT), respectively, which is
characteristic of conjugated polymers comprising donor-π-acceptor architecture. The positions of
absorption bands of the polymer films are more red-shifted than those of the solutions (Figure S2)
owing to increased intermolecular interactions in the solid state. As summarized in Table 1, the
absorption maxima of the PBDT-Qx, PBDT-QxF, PBDT-FQx, and PBDT-FQxF film at ICT
region are 624, 610, 611 and 611 nm, respectively. The optical band gaps of PBDT-Qx, PBDT-
QxF, PBDT-FQx, and PBDT-FQxF figured out from the absorption edges are 1.67, 1.71, 1.73, and
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1.73 eV, respectively. The absorption maximum in the longer-wavelength region and the optical
band gap are blue-shifted upon an increase in the number of fluorine atoms on the DPQ unit. The
absorption spectra of the polymer thin films containing fluorine atoms show shoulders in the longer-
wavelength region, which is not observed in the spectrum of PBDT-Qx. These shoulders are
attributed to intermolecular or intramolecular aggregation through F-F and F-H interactions.31-34
Herein, the blue-shifted absorption maximum is not seemed to be attributed from stronger ICT. One
possible reason is that fluorine atoms increase the band gap by reducing the HOMO levels of the
polymers. The changes in the absorption spectra and optical band gaps clearly demonstrate the
effects of fluorine atoms on the optical and electronic properties of the polymers. Similar results
34-38
have been reported for other fluorine-substituted conjugated materials.31-32,
The molar
extinction coefficients (ε) at the absorption maximum wavelength of the polymer solutions with
fluorine atoms (Figure S2) are higher than that of PBDT-Qx owing to stronger ICT in fluorine-
substituted polymers.
Figure 3. (a) UV-visible spectra of polymer films on glass substrate (film spectra are offset for
clarity) and (b) cyclic voltammograms of polymers.
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Table 1. Summary of optical and electrochemical properties of the polymers.
ꢀ
ꢁꢂꢃꢁ(nm)a
HOMO (eV)d
LUMO (eV)e
ꢋꢌꢍꢉ
ꢉꢅꢊ
ꢀ
(nm)c
ꢄꢃꢇꢅꢆꢆꢈ(eV)b
741
1.67
724
1.71
717
1.73
718
1.73
PBDT-Qx
PBDT-QxF
PBDT-FQx
PBDT-FQxF
430, 624
431, 610
425, 611
426, 611
- 5.29
- 5.35
- 5.36
- 3.62
- 3.64
- 3.63
- 3.74
- 5.47
b
c
aAbsorption edge of the film, Estimated from the ꢀꢁꢂꢃꢁ , Maximum wavelength of the film,
dEstimated from the oxidation onset potential, eCalculated from the optical band gap and the HOMO
energy level.
As shown in Figure 3b, cyclic voltammetry (CVs) measurements of the polymers show
irreversible oxidation processes. The HOMO energy levels of the polymers were estimated from the
oxidation onset potential of the CV measurements. The HOMO energy levels of PBDT-Qx, PBDT-
QxF, PBDT-FQx, and PBDT-FQxF are -5.29, -5.35, -5.36 and -5.47 eV, respectively. With an
increase in the number of substituted fluorine atoms, the HOMO levels become deeper due to the
strong electronegativity of fluorine. The LUMO energy level of PBDT-Qx, PBDT-QxF, PBDT-
FQx, and PBDT-FQxF were determined from the HOMO energy levels and the optical band gaps
as -3.62, -3.64, -3.63, and -3.74 eV, respectively. Energy level diagrams of the polymers, PC71BM,
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and other materials in the device are illustrated in Figure 4. In this device configuration, facile
exciton dissociation from the polymer and PC71BM as well as an energetically favored charge
transfer process, can efficiently take place.
Figure 4. Energy level diagrams of polymers and materials in this research.
3.3. Theoretical calculations
The density functional theory (DFT) at the B3LP/6-31G** level was carried out with the Gaussian
09 program to explore the electronic structures of the DPQ-based low-band-gap polymers. For
simple computational calculation, the 2-octyldodecyl side chains on the BDT unit and the long
polymer backbones were replaced with methyl groups and two repeating units, respectively. To
obtain optimized molecular geometries, conformational effects were carefully considered, because
there are two possible conformers with respect to the quinoxaline unit and adjacent thiophene
linkers. One is a syn-conformer, i.e., the nitrogen atoms in quinoxaline and the sulfur atom in
thiophene face on the same direction, and the other is an anti-conformer, i.e., the nitrogen atoms in
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quinoxaline and the sulfur atom in thiophene face opposite directions. The calculated potential
energy differences between syn- and anti-conformers (Esyn-Eanti) for PBDT-Qx, PBDT-QxF,
PBDT-FQx, and PBDT-FQxF are - 0.09, - 0.11, - 0.18, and - 0.24 eV, respectively (Figure S3 in
the ESI). Therefore, the syn-conformation is more favored for all polymers, which agrees well with
previous results.15 The frontier molecular orbitals of the two repeating units with theoretical
HOMO/LUMO energy levels are shown in Figure 5. Overall, the calculated HOMO/LUMO energy
levels exhibit similar trends to those observed in the UV-vis and CV experiments. In Particular, the
HOMO energy levels are significantly lowered, when fluorine atoms are incorporated on the 6,7-
positions of the DPQ unit. In addition, the electrons are delocalized along the chain direction at the
HOMO level, while they are mostly localized in the electron-withdrawing DPQ unit at the LUMO
level.
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Figure 5. Frontier molecular orbitals of two-repeating unit models with HOMO/LUMO energy
levels calculated at the B3LYP/6-31G** level for (a) PBDT-Qx, (b) PBDT-QxF, (c) PBDT-FQx,
and (d) PBDT-FQxF.
3.4. Photovoltaic properties
In order to investigate the photovoltaic properties of the polymers, we fabricated and tested inverted
type polymer solar cells (PSCs) with a structure of ITO/ZnO (40 nm)/polymer:PC71BM (80
nm)/MoO3 (10 nm)/Ag (100 nm). PSCs based on PBDT-Qx and PBDT-QxF with different blend
ratios of polymers and PC71BM from 3:1 to 3:9 (w/w) have been fabricated and tested to optimize
the device fabrication conditions (see Table S1). In accordance with the preliminary results, we
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fabricated PSCs based on a blend ratio of 3:3 (polymer: PC71BM) with 1,8-diiodooctane (DIO) as a
processing additive. The current density-voltage (J-V) curves at the optimized blend ratio under AM
1.5G simulated illumination are shown in Figure 6a, and the photovoltaic parameters are
summarized in Table 2. In previous results, the role of fluorine atoms in conjugated polymers for
39
photovoltaic applications were mainly focused on enhancing open-circuit voltage (Voc).3,
However, concurrent increases in the short-circuit current (Jsc) and fill factor (FF) have also been
frequently observed.21, 33, 35, 40-41
Similarly, the PCEs of all polymers used in this study gradually improved with a change in the
position or an increase the number of fluorine atoms, due to increasing positive contributions for all
parameters including Voc, Jsc, and FF. The devices based on the polymers with fluorine substituents,
PBDT-QxF, PBDT-FQx, and PBDT-FQxF showed PCEs of 4.52%, 5.56%, and 6.60%,
respectively, which are significantly higher than that of the non-fluorinated PBDT-Qx (3.14%). The
Voc values of the devices fabricated with PBDT-QxF, PBDT-FQx, and PBDT-FQxF are 0.78,
0.82, and 0.91 V, respectively, which show 27.9%, 34.4%, and 49.2% increase relative to the device
based on PBDT-Qx (0.61 V). These gradual enhancements in Voc are attributed to the deeper
HOMO energy levels of the polymers, cause by incorporated fluorine atoms at different positions
and with different numbers in their chemical structures. The Jsc data of the devices based on PBDT-
QxF, PBDT-FQx, and PBDT-FQxF are -8.76, -9.62, and -10.15 mA/cm2, respectively, which also
indicate 8.4%, 19.1%, and 25.6% increases compared with that of the PBDT-Qx based device (-
8.08 mA/cm2). The change of the Jsc data of the devices under 1.0 sun illumination showed very
good agreement with the incident photon-to-current efficiency (IPCE; Figure 6b) curves. The IPCE
data at 400-700 nm of the devices based on the polymers of PBDT-QxF, PBDT-FQx, and PBDT-
FQxF are higher than that of the device fabricated with PBDT-Qx. The series resistance (Rs) data
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were obtained from the inverse slope at high current regime of J-V curves in the dark (inset of
Figure 6a). As shown in Table 2, the Rs data showed smaller values upon an increase in the number
of fluorine atoms and agreed well with the Jsc and FF values.
Figure 6. (a) Current density vs. voltage curves of PSCs under 1.0 sun condition (inset: under the
dark condition) and (b) IPCE spectra of PSCs based on PBDT-Qx (square), PBDT-QxF (circle),
PBDT-FQx (triangle) and PBDT-FQxF (inverted triangle).
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Table 2. The best photovoltaic parameters of the PSCs. The averages for the photovoltaic
parameters of each device are given in parentheses.
Jsc (mA/cm2)
8.08
Voc (V)
0.61
FF (%)
PCE (%)
Rs (Ω
cm2)a
63.7
3.14
4.02
3.60
2.59
2.27
PBDT-Qx
PBDT-QxF
PBDT-FQx
PBDT-FQxF
(7.87 ± 0.19)
8.76
(0.61 ± 0.00)
0.78
(63.1 ± 0.5) (3.05 ± 0.07)
66.1 4.52
(66.0 ± 0.1) (4.42 ± 0.06)
70.5 5.56
(70.2 ± 0.3) (5.46 ± 0.08)
71.5 6.60
(71.6 ± 0.4) (6.49 ± 0.07)
(8.57 ± 0.11)
9.62
(0.78 ± 0.00)
0.82
(9.49 ± 0.13)
10.15
(0.82 ± 0.00)
0.91
(9.97 ± 0.09)
(0.91 ± 0.00)
aSeries resistance estimated from the corresponding best device.
Electron- and hole- only devices with a structure of ITO/PEDOT:PSS (35 nm)/ polymer:PC71BM
(ca. 90 nm)/Au (50 nm) and ITO/Al (50 nm)/polymer:PC71BM (ca. 90 nm)/Al (100 nm),
respectively, have been fabricated and tested to investigate the charge transporting properties. As
shown in Figure 7a&b, the J-V curves showed a characteristic of space charge limited current
(SCLC) and can be expressed by the Mott-Gurney law;42
9
ꢄꢔ
ꢕ
ꢎ = ꢏꢐꢑꢒꢓ
8
where J is the current density,
µ is the charge mobility, E is the electric field, ε0εr is the permittivity
the active layer, and L is the thickness of the active layer. The electron mobility of the device with
PBDT-Qx, PBDT-QxF, PBDT-FQx, and PBDT-FQxF are 2.5 × 10-3, 2.9 × 10-3, 2.6 × 10-3, and
2.4 × 10-3 cm2V-1s-1, respectively. Notably, the electron mobility of the polymers with or without
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fluorine atoms are almost identical. The hole mobility of the devices with PBDT-Qx, PBDT-QxF,
PBDT-FQx, and PBDT-FQxF are 6.95 × 10-4, 8.67 × 10-4 s-1, 9.83 × 10-4 and 9.86 × 10-4 cm2V-1s-
1, respectively. These results indicate an increased hole mobility upon fluorination. The ratios of
hole and electron mobility of the polymers with fluorine atoms are higher than that of the device
based on PBDT-Qx, and the highest value for PBDT-FQxF is displayed. SCLC results indicate
why the devices based on fluorine-substituted polymers showed better Jsc and the FF values.
Figure 7. Current density vs. voltage curves of (a) electron- and (b) hole-only devices (inset: current
density vs. voltage - built-in voltage (Vbi) curves with fitted lines) based on PBDT-Qx (square),
PBDT-QxF (circle), PBDT-FQx (triangle) and PBDT-FQxF (inverted triangle).
Figure 8 shows the surface morphology of the active layers of the devices. The images were
obtained by tapping-mode atomic force microscopy (AFM) measurements. The root-mean-squares
(RMSs) of the surface of the active layers based on PBDT-QxF, PBDT-FQx, and PBDT-FQxF are
2.04, 1.35, 1.67 nm, respectively, which values are almost identical to that of the surface of PBDT-
Qx (1.66 nm). The surface morphology and the RMS data hardly affect the performances of the
photovoltaic devices.
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Figure 8. AFM images of the active layers based on (a) PBDT-Qx, (b) PBDT-QxF, (c) PBDT-
FQx, and (d) PBDT-FQxF.
4. Conclusions
A series of fluorinated DPQ-based polymers, in which the electron-donating BDT moiety was
connected with the electron-withdrawing DPQ unit through a thiophene bridge, have been
synthesized by using Stille coupling reactions. Owing to the existence of two long 2-
octyldodecyloxy chains on BDT, all polymers exhibited good solubility in common organic
solvents. To elucidate the positional and populational effects of strong electron-withdrawing
fluorine atoms on the various properties of DPQ-based polymers, fluorine atoms were
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systematically incorporated into diverse positions on the DPQ unit in the polymers. Relative to the
non-fluorinated counterpart of PBDT-Qx, the fluorinated DPQ-based polymers denoted as PBDT-
QxF, PBDT-FQx, and PBDT-FQxF contained fluorine atoms on the 6,7-positions of DPQ, the
para-positions of the phenyl substituents on 2,3-positions of DPQ and both locations of the DPQ
unit, respectively. Optical and electrochemical investigation indicated the significant effects of
fluorine atoms on the electronic properties of the polymers, such as band gaps and HOMO-LUMO
energy levels. Furthermore, the gradual but noticeable improvements in the PCEs of these polymers
in inverted-type PSCs were observed by increasing the number or changing the position of fluorine
substituents on the DPQ unit of the polymers. Although, the PCE of non-fluorinated PBDT-Qx was
limited to 3.14 %, enhanced PCEs of 4.52%, 5.56% and 6.60% are obtained from PBDT-QxF,
PBDT-FQx, and PBDT-FQxF, respectively. This step-by-step enhancement in the PCEs of
fluorinated DPQ-based polymers can be attributed to the positive contributions of the fluorine
atoms, such as a gradual increase in Voc and concomitant improvement in Jsc and FF through
lowering of the HOMO energy levels and more balanced electron-hole mobility with smaller series
resistance, respectively. Therefore, this study can provide valuable insights into the development of
fluorinated conjugated polymers by careful control of structure-property relationships for various
applications like photovoltaic cells and field-effect transistors.
Acknowledgements
This research work was supported by Basic Science Researches (2013R1A1A2007716,
201603930154) through the National Research Foundation (NRF) of Korea funded by the
Ministry of Education, Science and Technology (MEST).
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