New benzodithiophene- and benzooxadiazole/benzothiadiazole-based donor–acceptor π-conjugated polymers for organic photovoltaics

Article abstract of DOI:10.1002/pola.28145

A new set of push-pull type 2D-conjugated polymers (P1–P4) were designed and synthesized where A1, A2 (oxygen analogues) and A3, A4 (sulfur analogues) are electron deficient units used as co-monomers. On introduction of new repeating units into the polymer backbone, significant changes were observed in optoelectronic properties. Furthermore, the heteroatom exchange in new repeating units has also brought notable changes in photophysical properties, in particular P1 and P2 (oxygen analogues) showed bathochromic shift in UV-vis absorption spectra and deeper HOMO energy levels than P3, P4 (sulfur analogues). Interestingly P1, P3 absorption spectra shows a vibronic shoulder (659, 652 nm) peak in lower energy region, and this might originated from non-covalent interactions between the electron rich and electron deficient units. In addition, the systematic investigation of these polymers with additive and solvent treatment, yielded in enhanced power conversion efficiency of 4.29% for P3-based devices in bulk heterojunction organic solar cells.

Full text of DOI:10.1002/pola.28145

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New Benzodithiophene- and Benzooxadiazole/Benzothiadiazole-Based
Donor–Acceptor p-Conjugated Polymers for Organic Photovoltaics
Kakaraparthi Kranthiraja,1 Kumarasamy Gunasekar,1 Sang Ho Park,1 In-Nam Kang,2
Jin Yong Lee,3 Myungkwan Song,4 Sung-Ho Jin1
1Department of Chemistry Education, Graduate Department of Chemical Materials, Institute for Plastic Information and Energy
Materials, Pusan National University Busan 609-735, Republic of Korea
2Department of Chemistry, The Catholic University of Korea, Bucheon, Republic of Korea
3Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea
4Surface Technology Division, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea
Correspondence to: S.-H. Jin (E-mail: shjin@pusan.ac.kr)
Received 14 February 2016; accepted 18 April 2016; published online 00 Month 2016
DOI: 10.1002/pola.28145
ABSTRACT: A new set of push-pull type 2D-conjugated poly-
mers (P1–P4) were designed and synthesized where A1, A2
(oxygen analogues) and A3, A4 (sulfur analogues) are electron
deficient units used as co-monomers. On introduction of new
repeating units into the polymer backbone, significant changes
were observed in optoelectronic properties. Furthermore, the
heteroatom exchange in new repeating units has also brought
notable changes in photophysical properties, in particular P1
and P2 (oxygen analogues) showed bathochromic shift in UV-
vis absorption spectra and deeper HOMO energy levels than
P3, P4 (sulfur analogues). Interestingly P1, P3 absorption spec-
tra shows a vibronic shoulder (659, 652 nm) peak in lower
energy region, and this might originated from non-covalent
interactions between the electron rich and electron deficient
units. In addition, the systematic investigation of these poly-
mers with additive and solvent treatment, yielded in enhanced
power conversion efficiency of 4.29% for P3-based devices
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in bulk heterojunction organic solar cells. 2016 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 00,
000–000
KEYWORDS: conjugated polymers; electron deficient units; het-
eroatom; organic solar cells; power conversion efficiency;
structure-property relations; UV-vis spectroscopy
The optoelectronic properties of the conjugated polymers
were significantly influenced by insertion of conjugated
heterocyclic units such as 1,3-di(2-bromothien-5-yl)25-(2-
ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione, benzothiadiazole
(BT), and so forth.3 Thus, the choice of suitable (A) unit in
D–A polymers can effect on optoelectronic properties of
resulting polymers in many ways such as, it can reduce
the polymer band gap, affect the lowest unoccupied molec-
ular orbital (LUMO) energy level and it can also make
polymer backbone more planar by locking via non-
covalent interactions between heteroatoms of D and A
units and these interactions can significantly affect the
bulk properties of resulting conjugated polymers.4–8 In
addition, it has been observed that variations in optoelec-
tronic and photovoltaic properties of conjugated polymers
by simple exchange of heteroatom either in electron rich
or electron deficient units of D–A polymers, which could
able to show its clear impact on the photovoltaic
properties.9
INTRODUCTION Development of new p-conjugated polymers
and continuous device optimization could able to achieve high
power conversion efficiency (PCE) in bulk heterojunction
polymer solar cells (BHJ PSCs).1 To further improve the PCE,
a systematic design strategy is required, in particular by fine
tuning of optoelectronic properties of conjugated polymers
such as appropriate energy levels, reduced band gap, planar
polymer backbone etc. Synthesis of an alternating donor–
acceptor (D–A) copolymer by connecting an electron rich (D)
and an electron deficient (A) unit is the best strategic way to
get desired properties for a conjugated polymer used in
PSCs.2 In this regard, several kinds of D and A units have
been designed and synthesized for potential PSCs applications
so far. However, the accessibility of effective electron deficient
moieties has been limited for PSC application when compared
to the immense development of donor blocks in D–A conju-
gated polymers. Therefore, it is equally essential to design
and synthesis of new electron deficient units for efficient PSCs
via fine tuning of optoelectronic properties to get high PCEs.3
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic route for monomers. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
Among the few effective electron deficient units, BT has
been widely investigated unit in many conjugated copoly-
mers. However, various kinds of modifications have been
made on BT, and able to get high PCEs.10 In this work, we
have designed and synthesized new ethylenedialkoxy (EDA)
substituted BT as electron deficient units (A1, A2, A3, A4) to
improve the solubility without disrupting the other optoelec-
tronic properties of resulting polymers as shown in Scheme
1 and 2. These new monomers A1, A2 (oxygen analogues),
A3, A4 (sulfur analogues) were used in synthesis of D–A
polymers P1–P4 with good solution processability. We found
significant changes in optoelectronic, thermal, and photovol-
taic properties of new polymers on doping the new electron
deficient units as repeating units. In particular, P1, P3 show
vibronic shoulder at 650–660 nm which was not observed in
P2, P4. Additionally P1, P2 (oxygen analogues) show batho-
chromic shift in absorption spectra and also deep highest
occupied molecular orbital (HOMO) levels compared to the
P3, P4 (sulfur analogues), and effect was further sustained
on PSCs and polymer field effect transistor (PFETs)
characteristics.
for the 1H and 13C NMR, respectively. The UV-vis absorption
spectra were recorded on a JASCO V-570 spectrophotometer
at room temperature. Thermal gravimetric analysis (TGA)
was carried out on a Mettler Toledo TGA/SDTA 851e, under
nitrogen (N2) atmosphere at heating rate of 10 8C min21
.
Number-average molecular weight (Mn), weight average
molecular weight (Mw), and polydispersity index (PDI) were
determined against polystyrene as standard by GPC using
photoluminescence (PL) gel 5 lm MLXED-C column on an
Agilent 1100 series liquid chromatography system with tet-
rahydrofuran (THF) as an eluent. Cyclic voltammetry (CV)
studies were carried out in a 0.1 M solution of tetrabutylam-
monium perchlorate in anhydrous acetonitrile at a scan rate
of 100 mV s21 using CHI 600C potentiostat (CH Instru-
ments). A three electrode cell with platinum electrode as the
working electrode, Ag/AgCl as the reference electrode and a
platinum (Pt) wire as the counter electrode were used.
PFETs Fabrication and Characterization
The charge characteristic properties of P1–P4 were exam-
ined by bottom contact geometry PFETs. Thin films were
spin casted from a 5 mg mL21 P1–P4 in chloroform under a
N2 atmosphere onto n-octyltrichlorosilane, n-doped silicon
wafer with a 300-nm thickness of SiO2 dielectric layer. The
PFETs were then annealed for 10 min at 140 8C in a glove
box under N2 atmosphere. The field effect mobility was cal-
culated in the saturation regime using the equation IDS5
(mWCi/2L)(VG–VT)2, where IDS is the drain-source current, l
is the field effect mobility, W is the channel width (120 mm),
L is the channel length (12 mm), Ci is the capacitance per
EXPERIMENTAL
Materials and Instruments
All chemicals and reagents were purchased from Sigma-
Aldrich Chemical Co., Alfa Aesar and used as received. 1H
and 13C NMR spectra were recorded on a Varian Mercury
Plus 300 MHz spectrometer in CDCl3 using tetramethylsilane
as the internal reference. The chemical shifts were reported
in ppm relative to the singlet of CDCl3 at 7.26 and 77 ppm
2
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SCHEME 2 Synthetic route for polymers. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
À
Á
unit area of the gate dielectric layer, and VG is the gate
voltage.
J 5 ð9=8Þere0l V2=L3
where er is the dielectric constant (er 5 3), e0 is permittivity
of free space, L the thickness of the active layer, l is the
charge mobility, and V is the voltage drop across the device.
Fabrication and Characterization of the BHJ PSCs
The ITO substrates that have been used for fabrication were
cleaned ultrasonically with detergent, water, acetone, and
isopropyl alcohol. PEDOT:PSS (Clevios P, AI 4083) with
40 nm thick layer of was coated on the electrodes by spin
coating a diluted soluiton of PEDOT:PSS with isopropyl alco-
hol with the dilution ratio being 1:2 and baked at 150 8C for
10 min in oven. Active layer of P1–P4:PC71BM was spin
coated using a mixture of polymer:PC71BM was dissolved in
chlorobenzene (CB) and further mixed with 1–3 vol % 1,8
diiodooctane (DIO) and then washed with methanol (MeOH).
LiF (0.7 nm) and Al cathode (100 nm) were deposited
deposited via evaporation on the active layer. The BHJ PSCs
performances were measured under simulated AM 1.5G illu-
mination (100 mW cm22). The irradiance of the sunlight
simulating illumination was calibrated using a standard Si
photodiode detector fitted with a KG5 filter. The J–V curves
were measured using a Keithley 2400 source measurement
unit. The space charge limited current (SCLC) mobilities l
were extracted using the standard trap free SCLC transport
equation:
Synthesis of Monomers
Synthesis of [6,7]-Dihydro-[1,4]-Dioxino-[2,3-F[2,1,3]-
Benzooxadiazole (3)
Compound 2 (6.0 g, 26.5 mmol), sodium azide (8.6 g, 131.9
mmol), n-Bu4NBr (1.7 g, 1.7 mmol) were added to anhy-
drous toluene (50 mL) reflux for 24 h. And then triphenyl
phosphine (8.2 g, 10.5 mmol) was added very slowly to reac-
tion mixture and continued heating for 24 h (total 48 h).
Cool the reaction mixture to room temperature and evapo-
rated the excess of toluene. Crude was purified by flash
chromatography on short bed of silica gel with chloroform.
The filtrate was concentrated and recrystallized from ethanol
to get orange needle like structures of compound 3 (3.0 g,
63%).
1H NMR (CDCl3, 300 MHZ): 7.47 (s, 2H), 4.75 (s, 4H); 13C
NMR (CDCl3, 75 MHZ): d (ppm) 149.73, 146.96, 97.74, 64.56.
HRMS: Calculated for C8H6N2O3 178.0378; found: 178.0378.
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Synthesis of 4,9-Dibromo-6,7-Dihydro-[1,4] Dioxino
[20,30:4,5] Benzo[1,2,-C][1,2,5]Oxadiazole (A1)
1H NMR (CDCl3, 300 MHZ): d (ppm) 4.54 (s, 4H); 13C NMR
(CDCl3, 75 MHZ): d (ppm) 148.89, 147.16, 97.17, 65.16.
HRMS: Calculated for C8H4Br2N2O2S 351.8340; found:
351.8340.
Compound 3 (1.8 g, 10.0 mmol) was dissolved in methylene
chloride (100 mL), to this acetic acid (15 mL) bromine
(1.6 mL, 30.0 mmol) were added. The resulting mixture was
stirred for 3 days under dark, then the reaction mixture was
poured into sodium hydroxide solution and aqueous phase
was extracted with methylene chloride (2 3 100 mL) and
dried over anhydrous MgSO4. The crude was purified by col-
umn chromatography on silica with hexane: methylene chlo-
ride (6:4, v/v) to get light yellow powder compound A1
(2.2 g, 64%).
Synthesis of 4,9-Di(thiophene-2-Yl)26,7-Dihydro-
[1,4]Dioxino[2,3,4,5]Benzo[1,2-C][1,2,5]Thiazole (6)
A3 (1.0 g, 2.8 mmol), 2-tributylstannyl thiophene (2.7 mL,
8.0 mmol), and Pd(PPh3)4 (0.3 g, 0.3 mmol) were dissolved
in anhydrous toluene and purged with N2 for 20 min then
reflux overnight (12 h). Crude was purified by column chro-
matography on silica with hexane: methylene chloride (8:2,
v/v) to furnish orange solid (80%).
1H NMR (CDCl3, 300 MHZ): d (ppm) 4.52 (s, 4H); 13C NMR
(CDCl3, 75 MHZ): d (ppm); 146.95, 146.45, 90.63, 65.25.
HRMS: Calculated for C8H4Br2N2O3 335.8589; found:
335.8591.
1H NMR (CDCl3, 300 MHZ): d (ppm) 8.45 (d, 2H), 7.51 (d,
2H), 7.22 (d, 2H), 4.61 (s, 4H). 13C NMR (CDCl3, 75 MHZ): d
(ppm) 149.43, 143.76, 134.25, 130.45, 127.18, 110.82, 64.27,
29.67. HRMS: Calculated C16H10N2O2S3 357.9904; found:
357.9904.
Synthesis of 4,9-Di(thiophene-2-yl)26,7-dihydro-
[1,4]dioxino[2,3,4,5]benzo[1,2-C][1,2,5]oxadiazole (5)
Compound A1 (1.0 g, 2.9 mmol), 2-tributyl stannyl thiophene
(2.3 mL, 7.4 mmol), and Pd (PPh3)4 (0.3 g, 0.3 mmol) were
added to anhydrous toluene (20 mL) and purged with N2 for
20 min and then refluxed the mixture for overnight (12 h).
Evaporate the excess of toluene and extracted with methyl-
ene chloride and dried over anhydrous MgSO4. Crude was
purified by column chromatography on silica with hexane:
methylene chloride (8:2, v/v) to furnish orange solid A1
(70%).
Synthesis of 4,9-Di(5-bromothiphen-2-Yl)26,7-Dihydro-
[1,4]Dioxino[20,30:4,5] Benzo[1,2-C] [1,2,5]Thiadiazole (A4)
Synthesized same procedure as followed for A2 as red solid
(86%). 1H NMR (CDCl3, 300 MHZ): d (ppm) 8.33–8.35 (d,
2H), 7.17–7.18 (d, 2H), 4.62 (s, 4H). HRMS: Calculated for
C16H8Br2N2O2S3 516.8094; found: 515.8094.
General Polymerization Procedure
The general procedure for the synthesis of polymer P1 is as
follows: To a microwave tube BDTT (0.3 g, 0.26 mmol), and
A1 (0.089 g, 0.26 mmol), Pd2 (dba)3 (5.4 mg, 2 mol %), (o-
tol)3 P (16 mg, 16 mol %) were dissolved in anhydrous CB
(3 mL). The reaction mixture was purged with N2 for 15
min. Tube was heated in the reactor at 120 8C for 1 h for P1
and P3;120 8C for 10 min for P2 and P4.
1H NMR (CDCl3, 300 MHZ): d (ppm) 8.44–8.46 (m, 2H),
7.47–7.50 (m, 2H), 7.20–7.24 (m, 2H), 4.64 (s, 4H); 13C NMR
(CDCl3, 75 MHZ): d (ppm); 145.73, 143.55, 133.11, 130.57,
127.76, 105.55, 64.44, 28.48. HRMS: Calculated for
C16H10N2O3S2 342.0133; found: 342.0136.
Synthesis of 4,9-Di(5-bromothiphen-2-Yl)26,7-Dihydro-
[1,4]Dioxino[20,30:4,5] Benzo[1,2-C] [1,2,5]Oxadiazole (A2)
Compound 5 (0.6 g, 1.7 mmol) dissolved in THF (50 mL),
NBS (0.6 g, 3.5 mmol) in THF (10 mL) was added drop wise
to the mixture at 0 8C and then slowly warm to room tem-
perature and stirred for 6 h and reaction mixture was
poured into water (200 mL) extracted with chloroform and
dried over anhydrous MgSO4. Crude was purified by column
chromatography on silica gel with hexane: methylene chlo-
ride (9:1, v/v) to furnish red solid (74%).
After cooling to room temperature, the reaction mixture was
poured into MeOH to obtain precipitate, the resulted precipi-
tate was purified by soxhlet method using MeOH, hexane,
acetone, and chloroform. The chloroform fraction was evapo-
rated to get polymer. Mn and PDI of P1–P4 are listed in Sup-
porting Information Table S1.
Poly[4,8-bis(2-hexyldecyl-2-thiophenyl)-benzo[1,2-b:4,5-b0]-
dithiophene-alt-4,9-dibromo-6,7-dihydro-[1,4] dioxino [20,30:4,5]
1
benzo[1,2,-c][1,2,5]oxadiazole)] (P1, 60%). H NMR (CDCl3, 300
1H NMR (CDCl3, 300 MHZ): d (ppm) 8.23–8.24 (m, 2H),
7.17–7.18 (m, 2H), 4.65 (s, 4H). HRMS: Calculated for
C16H8Br2N2O3S2 499.8343; found: 499.8323.
MHZ): d (ppm) 8.8 (br, 2H), 7.92 (br, 4H), 4.24 (br, 4H), 3.01
(br, 4H), 1.32 (br, 50 H), (0.86, br 12H). Anal. Calcd for
C58H78N2O3S4: C, 71.12; H, 8.03; N, 2.86; found: C, 70.88; H,
7.89; N, 2.66.
Synthesis of 4,9-Dibromo-6,7-Dihydro-[1,4] Dioxino
[20,30:4,5] Benzo[1,2,-C][1,2,5]Thiadiazole (A3)
Poly[4,8-bis(2-hexyldecyl-2-thiophenyl)-benzo[1,2-b:4,5-b0]-
dithiophene-alt-4,9-di(5-bromothiphen-2-yl)26,7-dihydro-[1,4]
dioxino[20,30:4,5]benzo[1,2-c] [1,2,5]oxadiazole (P2, 73%). 1H
NMR (CDCl3, 300 MHZ): d (ppm) 8.8 (br, 10H), 4.62 (br, 4H),
2.98 (br, 4H), 1.35 (br, 50H), 0.87 (br, 12H). Anal. Calcd for
C66H82N2O3S6: C, 69.31; H, 7.23; N, 2.45; found: C, 68.28, H,
7.18, N, 2.55.
Compound 4 (1.0 g, 6.0 mmol) was dissolved in 50 mL of
chloroform cool to 0 8C and then add SOBr2 (3.2 mL, 42.0
mmol) then pyridine (2 mL) then heat the reaction mixture
at 70 8C for overnight (12 h), crude was purified by column
chromatography on silica with hexane: methylene chloride
(8:2, v/v) to furnish yellow solid (38%).
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FIGURE 1 UV-vis absorption spectra of (a) P1–P4 solution, (b) P1–P4 film, (c) P1, P3, P5, and P6 solution, (d) P1, P3, P5, and P6
film. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
1
Poly[4,8-bis(2-hexyldecyl-2-thiophenyl)-benzo[1,2-b:4,5-b0]-
dithiophene-alt-4,9-dibromo-6,7-dihydro-[1,4] dioxino [20,30:4,5]
benzo[1,2,-c][1,2,5]thiadiazole)] (P3, 73%). 1H NMR (CDCl3,
300 MHZ): d (ppm) 9.6 (br, 2H), 8.9 (br, 4H), 4.43 (br, 4H),
3.00 (br, 4H), 1.33 (br, 50H), 0.88, (br, 12H). Anal. Calcd for
C58H78N2O2S5: C, 69.97; H, 7.90; N, 2.81; found: C, 68.88; H,
7.80, N, 2.72.
confirmed by H NMR and 13C NMR and HRMS spectroscopy.
The intermediates 1, 2, 4, A3, A4 were synthesized and con-
firmed according to literature.11,12
Thermal Properties
The thermal stability of polymers was investigated via TGA
at scanning rate of 10 8C min21. All the polymers possess
good thermal stability to fabricate PSCs, which is revealed by
its thermal decomposition temperatures of 387–391 8C as
shown in Supporting Information Figure S1. It is worth to
mention that new electron deficient units doped polymer
have better thermal stability than alkoxy substituted BT con-
taining polymers.13
Poly[4,8-bis(2-hexyldecyl-2-thiophenyl)-benzo[1,2-b:4,5-b0]-
dithiophene-alt-4,9-di(5-bromothiphen-2-yl)26,7-dihydro-
[1,4]dioxino[20,30:4,5] benzo[1,2-c] [1,2,5]thiadiazole (P4,
64%). 1H NMR (CDCl3, 300 MHZ) d (ppm) 8.76-8.07 (br,
10H), 4.56 (br, 4H), 2.92 (br, 4H), 1.30 (br, 50H), 0.86 (br,
12H). Anal. Calcd for C66H82N2O2S7: C, 68.35; H, 7.13; N,
2.42; found: C, 68.23, H, 7.06, N, 2.34.
Optical Properties
The UV-vis absorption spectra of P1–P4 in chloroform solu-
tion and film state are shown in Figure 1(a,b). P1–P4 shows
p 2 p* transition and intramolecular charge transfer (ICT)
bands, which are originated from the interactions between
polymer backbone, and charge transfer between electron
rich and deficient units respectively. The ICT bands of P1, P2
(oxygen analogues) are red shifted than P3, P4 (sulfur ana-
logues) which could be attributed to the more electro nega-
tivity of oxygen than that of sulfur in their electron deficient
repeating unit of backbone, which led to enhanced ICT
resulting in more reduced optical band gaps for P1, P2
RESULTS AND DISCUSSION
Synthesis and Characterization
6, 7-Dinitro-2,3-dihydor[b][1, 4] dioxine was synthesized
using previously published procedure,11,12 and then followed
by ring closing with NaN3 and triphenylphosphine resulted
the compound 1 and then followed by bromination results
electron deficient unit A1, which on coupling with 2-
tributylstannyl thiophene and then brominating the second
position of thiophene spacer results the electron deficient
unit A2. All new electron deficient monomers were further
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FIGURE 2 Temperature (a, b) and concentration (c, d) dependent UV-vis absorption spectra of P1 and P3. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
(1.66, 1.64 eV) than P3, P4 (1.68, 1.65 eV). The P1–P4
showed red shifted absorption spectra in film state relative
that of solution state. Further P2, P4 showed reduced band
gaps than P1, P3 due to the extended backbone conjugation
with thiophene p-spacer. Interestingly P1, P3 shows addi-
tional shoulder peak at (659, 652 nm) in their absorption
spectra which was not observed in P2, P4 and we assumed
that the vibronic shoulder would be originated from non-
covalent interactions between sulfur and oxygen atoms in
benzodithiophene (BDT) and EDA bridge in electron rich and
deficient units of P1, P3.14
gradual increase in temperature (25–100 8C) gradual dip in
the intensity of shoulder peak was observed, which infer
that both the polymers P1 and P3 have strong aggregation
properties at room temperature which might be due to non-
covalent interactions, and this aggregation is gradually
reduced at higher temperature. Thus, the effective conjuga-
tion length will decrease because of more twisting or copla-
narity of the repeating units at elevated temperatures.
However, the P1, P3 polymers still have significant absorp-
tion shoulder at elevated temperature indicating the pres-
ence of polymer aggregates.15–17 Furthermore, we also
measured the concentration dependent absorption spectra
for P1 and P3, as shown in Figure 2(c,d). The intensity of
shoulder peak increases when concentration of polymer (P1,
P3) increases indicates that the existence of strong interac-
tions between the sulfur atom of BDT unit and oxygen atom
of EDA bridge.
To better understand this phenomenon, we have additionally
synthesized polymers P5, P6 without EDA bridge. The absorp-
tion spectra of P5, P6 do not have any additional shoulder
peaks in their absorption spectra as shown in Figure 1(c,d),
whereas in P1, P3 solution and film state spectra show very
strong shoulder peaks. This would partially explain the origin
of vibronic shoulder in P1, P3 due to interactions between
oxygen atom of EDA bridge and sulfur atom in BDT.
Altogether from the above results it is clear that the origin
of vibronic shoulder in P1 and P3 is due to the non-covalent
interactions between the sulfur atoms of BDT and oxygen
atoms in EDA bridge of new electron deficient units. Thus,
the newly synthesized electron deficient units not only
improve the solubility but also induce the non-covlent inter-
actions between the electron rich and deficient units which
will help in new avenues such as attaining additional
To further understand the reason for origin of vibronic
shoulder, temperature dependent absorption spectra were
measured for P1 and P3 in dilute CB as shown in Figure
2(a,b). The vibronic shoulder peak at 650–660 nm was very
sharp and pronounced at room temperature, whereas on
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TABLE 1 Optical and Electrochemical Properties of P1–P4
Polymer
Solution
Film
Film
Egopt
HOMO
LUMO
kmax (nm)a
338, 592, 643
358, 583
kmax (nm)b
344, 605, 659
366, 597
kmax (nm)c
746
(eV)d
1.66
1.64
1.68
1.65
(eV)
(eV)
P1
P2
P3
P4
25.40
25.29
25.36
25.21
23.74
23.64
23.68
23.56
756
346, 582, 643
358, 577
347, 603, 652
363, 597
738
751
a,bAbsorption maxima from UV-vis absorption spectra in chloroform
solution and film state.
cThe onset of the film absorption edge.
dEstimated from the onset of the absorption in thin films (Egopt 5 1240/
onset).
k
planarity, reduced bandgap, and p 2 p stacking and so forth,
which might be beneficial for improving the hole mobility
and photovoltaic properties.14
BT-based polymers (PBDT-TBT) with zig-zag polymer confor-
mation due to high torsion angle of 568.13
PFETs Characteristics
The charge transport properties of new electron deficient
units containing polymers (P1–P4) were investigated by fab-
ricated polymer thin film transistor devices. Here, PFETs
were fabricated in bottom contact geometry device on a sili-
con wafer with channel lengths (12 mm) and width (120
mm) under N2 atmosphere. All devices were fabricated with
polymer thin films as active layer, which were annealed at
140 8C. The transfer characteristics of the devices were
shown in Figure 4, and the corresponding data are summar-
ized in Supporting Information Table S2. P1–P4 delivered
prominent field effect hole mobilities, indicating that they
possess typical p-type behavior. Among the four polymers P2
and P4 showed higher hole mobility than P1 and P3 owing
to their extended conjugation in P2 and P4 via introduction
of p-spacers. The mobilities of P1 and P3 are higher than
that of P5 and P6 indicates enhanced planarity of P1 and P3
than P5, P6, this enhanced planarity of P1 and P3 which
might be attained via non-covalent interactions of the heter-
oatoms of electron rich and electron deficient units.
Electrochemical Properties
CV measurements were carried out to measure the HOMO
and LUMO of the P1–P4. The corresponding cyclic voltammo-
grams are shown in Supporting Information Figure S2 and
the respective values are listed in Table 1. The calculated
HOMO energy levels are 25.40, 25.29, 25.36, and 25.21 eV.
The LUMO energy levels (23.74, 23.64, 23.68, and 23.56
eV) were calculated from Egopt and corresponding HOMO
energy levels. The HOMO energy levels of P1 and P2 showed
deeper HOMO levels than P3, P4 because of high electro neg-
ativity of oxygen in P1and P2. The HOMO energy levels of
P1, P3 are deeper than P2, P4 as introduction of thiophene
spacer in P2, P4 increases the electron rich part in backbone.
Expectedly, all the donor polymers showed the LUMO value
greater than 0.3 eV of LUMO level of PC71BM. The HOMO
and LUMO levels of the new polymers clearly indicating their
good air stability and efficient exciton dissociation at donor/
acceptor interfaces, respectively.
Photovoltaic Properties
Computational Study
The photovoltaic performance of new electron deficient unit
doped copolymers P1–P4 were used in making conventional
BHJ PSC devices, with an architecture of ITO/PEDOT:PSS/
P1–P4:PC71BM/LiF/Al, and device performances were inves-
tigated under the illumination of AM 1.5G (100 mW cm22).
It is known that careful choice of suitable processing solvent
and blend ratio are important to get nanoscale morphology
for BHJ PSC photoactive layer to obtain best device perform-
ance. The solvent for solution processing and concentration
of blend (polymer to PC71BM) were optimized as CB and 1:2
and for P1 is (1:1.5). Initially the optimized blends of pris-
tine devices of P1–P4 exhibited moderate PCEs with maxi-
mum of 2.08% for P2. The best photovoltaic parameters
obtained for optimized blends of pristine devices are shown
in Table 2, and Figure 5 shows the current density-voltage
(J–V) curves for champion devices P1–P4. Among the four
polymers, P1-, P2-based devices shown high open circuit vol-
tages (Voc) (0.87, 0.64 V) than P3, P4 (0.84, 0.56 V) because
of deeper HOMO levels. Even though P1 and P3 possess
higher Voc values, the PCEs are moderate due to poor short-
To gain deep insight into the optoelectronic properties of
new electron deficient unit’s doped new conjugated poly-
mers (P1–P4), density functional theory (DFT) calculations
were carried out using a suite of Gaussian 09 programs.
Becke’s three parameterized Lee-Yang-Parr exchange func-
tional (B3LYP) and 6-31G* basis sets were used.18 The DFT
calculated HOMOs and LUMOs are shown in Figure 3. The
HOMOs and LUMOs of P1–P4 are 24.88, 24.83, 24.79,
24.75 eV and 22.23, 22.43, 22.17, 22.37 eV respectively
which are well concurred with the experimental values.
Interestingly the torsional angles between the electron rich
and new electron deficient units (3.17, 7.518) shows P1, P3
are highly planar compared to the P2, P4 (19.67, 21.658).
The above results also partially explaining that the possibil-
ity of enhanced non-covalent interactions between the sulfur
atom of BDT and oxygen atom of EDA bridge in electron
deficient units in P1, P3. Here, it is noteworthy to mention
that our new electron deficient unit doped conjugated poly-
mers are more planar than conventional alkoxy substituted
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FIGURE 3 DFT calculated HOMO and LUMO of P1–P4 repeating units. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
circuit current density (Jsc) and fill factor (FF), which can be
explained from their poor nanoscale morphology.
To further optimize the photovoltaic properties of P1–P4-
based devices, we have used polar solvent treatment for
active layers before deposition of Al electrode.19,20 Here, we
used several polar solvents and found that MeOH is the best
suited for our polymers and showed enhanced PCEs in Table
3. Even though the effect of solvent treatment on active
layers was positive toward all the four polymers, interest-
ingly P3-based devices showed significant improvement in
photovoltaic properties on treatment with various polar sol-
vents such as ethanol (EtOH), acetone, and MeOH. The PCEs
of P3-based devices showed good improvement after solvent
treatment with maximum PCEs of 3.49 (EtOH), 2.89 (ace-
tone), 4.29% (MeOH), respectively, the corresponding J–V,
external quantum efficiency (EQE) of P3-based PSCs are
shown in Figure 6(a,b). This enhanced PCEs attributed to the
improved Jsc and FF after solvent treatment. This improved
Jsc and FF might originated from nanoscale morphology
Active layer morphology can be improved via various ways
such as addition of high boiling additives, polar solvent treat-
ment, thermal annealing. As an initial attempt to improve
the photovoltaic properties of P1–P4-based devices, a small
amount of external additive DIO was added to the active
layers. On addition of DIO to the active layers, the photovol-
taic properties of P1–P4-based devices were slightly
improved. Among the four polymers, optimized P2-based
BHJ PSCs showed maximum PCE of 2.43% after addition of
1% DIO to the active layer. This improved PCE could be
attributed to the enhanced Jsc and FF. Rest of the polymers
also showed little improvement in PCE after addition of DIO
to the active layer, the corresponding J–V curves are shown
in Figure 5(a,b).
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FIGURE 4 Transfer characteristics of P1–P6. [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
attained after solvent treatment in P3-based active layers
which was further studied with electrical impedance spec-
troscopy (EIS) and atomic force microscopy (AFM) studies
(vide infra).19,20
EQE curves of P1–P4-based devices without and with DIO
are shown in Figure 5(c,d); all the devices showed photo
response from 300 to 700 nm, and the EQE spectra of P1–
P4 are similar to their absorption spectra’s. Although all
TABLE 2 Photovoltaic Properties of Optimized BHJ PSCs of P1–P4 without and with DIO
Polymer
DIO (%)
Polymer:PC71BM
Thickness (nm)
Jsc (mA cm22
)
Voc (V)
FF (%)
PCE (%)
P1
P2
P3
P4
P1
P2
P3
P4
0
0
0
0
1
1
1
1
1:1.5
1:2
85–90
75–80
85–95
80–85
85–90
75–80
85–95
80–85
2.90
5.77
5.85
6.09
3.65
6.76
7.29
6.94
0.87
0.67
0.78
0.56
0.83
0.64
0.76
0.52
45.99
53.61
40.64
38.13
45.09
55.76
42.89
42.47
1.17
2.08
1.86
1.30
1.39
2.43
2.37
1.53
1:2
1:2
1:1.5
1:2
1:2
1:2
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FIGURE 5 J–V, EQE characteristics of P1–P4-based PSCs without and with DIO. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
pristine devices showed a weak spectral response in visible
region, after addition of additive to the active layers, little
improvement of EQE response was observed in all the PSCs.
Further improvement in spectral response was observed
after polar solvent treatment to the active layers, especially
for P3 up to 65–70% at 350–450 nm and 50% at 500–
700 nm as shown in Figure 6(b), which was well correlated
with its higher Jsc. The integrated Jsc values were obtained
from the integration of the EQE spectra are well concurred
well with Jsc obtained from the J–V measurements.
impedance results well correlates with device data as
reported in Table 4 for P3-based devices treated with sol-
vents like MeOH, EtOH, and acetone. The Nyquist plot of P3-
based devices clearly depicts the significant reduction in
bulk resistance of all the solvent treated devices, especially
for MeOH, which can be inferred from the diameter of semi-
circle on the real axis (Re Z) of the EIS curve. Thus, among
all the solvents the MeOH showed considerably reduced
interfacial resistance for P3-based devices.
PL Quenching Studies
PL quenching studies [Fig. 6(d)] were performed to the
active layer blends of P3 to understand the effect of solvent
treatment, here we measured PL of P3: PC71BM, P3: PC71BM
(DIO), P3: PC71BM (DIO/MeOH). Emission intensity was
decreased gradually for P3: PC71BM (DIO/MeOH) blends
Impedance Spectroscopy Studies
To understand this improved PCEs after solvent treatment,
we probed EIS to the active layers of P3-based devices with-
out and with various solvent treatment, and their corre-
sponding Nyquist plots are shown in Figure 6(c). The
TABLE 3 Photovoltaic Properties of BHJ PSCs of P1–P4 with MeOH Treatment
Polymer
Polymer: PC71BM
Thickness (nm)
Jsc (mA cm22
)
Voc (V)
FF (%)
PCE (%)
P1
P2
P3
P4
1:1.5
1:2
90
95
90
85
3.83
7.24
9.99
7.90
0.84
0.64
0.78
0.53
47.48
57.80
54.92
42.95
1.53
2.69
4.29
1.81
1:2
1:2
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FIGURE 6 J–V, EQE, Nyquist plots, and PL quenching of BHJ PSCs of P3. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
compared to the P3: PC71BM (DIO) and P3: PC71BM on addi-
tion of additive and solvent treatment emission intensity was
decreased, and the emission intensity was further decreased
when tested with P3: PC71BM (DIO), as well as solvent
treated active layers. From this study, it is clear that addition
of small amount of DIO and further solvent treatment
improves the exciton dissociation at donor acceptor inter-
face, which is well correlated with the improved PCEs of P3-
based devices on addition of DIO and solvent treatment.
Morphology
Morphology of the active layers of P1–P4 was investigated
via tapping mode AFM. To understand the differences in the
pristine, additive and solvent treated active layer morphol-
ogy of P1–P4:PC71BM-based blends were analyzed by AFM
and the corresponding images are shown in Supporting
Information Figure S3, as pristine active layers concerned
P2:PC71BM showed better morphology than rest of the
blends which was well correlated with its maximum PCE for
pristine blends. The poor photovoltaic parameters of pristine
P1-based devices were due to unfavorable morphology
occurred in the active layer due to improper mixing of poly-
mer and PC71BM. Furthermor, the active layer morphologies
were improved slightly with proper mixing of polymer and
PC71BM on addition of DIO, and this could the reason for
slight improvement in the PCEs of P1–-P4-based devices,
whereas polar solvent treatment has shown very good effect
on all the active layers, and attain smooth morphologies
Hole Mobility
The hole mobility of the P1–P4:PC71BM blend films meas-
ured via the SCLC method. The calculated SCLC hole mobility
of P1–P4: PC71BM are 1.31 3 1025, 1.17 3 1024, 9.83 3
1025, 3.22 3 1025 cm2 V21
s
21. The obtained SCLC hole
mobility characteristics are shown in Supporting Information
Figure S4. The obtained SCLC mobilities are well correlated
with the photovoltaic properties.
TABLE 4 Optimized Photovoltaic Properties of BHJ PSCs of P3 with Various Solvent Treatments
Polymer:
Polymer
PC71BM
Solvent
Jsc (mA cm22
)
Voc (V)
FF (%)
PCE (%)
P3
P3
P3
1:2
1:2
1:2
MeOH
EtOH
9.99
8.29
7.56
0.78
0.76
0.77
54.92
55.01
49.85
4.29
3.49
2.89
Acetone
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3 Y. He, W. Hong, Y. Li, J. Mater. Chem. C 2014, 2, 8651–8661.
compared to pristine and additive-based active layers, espe-
cially P3-based active layer showed very good phase separa-
tion after MeOH treatment with reduced root mean square
roughness, this favorable nanoscale morphology of P3-based
devices would be the reason for improved Jsc and FF which
results in maximum PCE of 4.29%.
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CONCLUSIONS
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Polym. Chem. 2012, 3, 99–104.
In summary, we have synthesized new electron deficient
units and were successfully used as repeating units in new
polymers, all the new polymers showed good optoelectronic
properties, in particular P1 and P3 showed strong vibronic
shoulders due to non-covalent interactions between sulfur
atom of BDT and oxygen in EDA bridge induces better pla-
narity of polymer backbone by non-covalent locks, thus
results enhanced PFET hole mobility for P1, P3 than P5,
P6. We also found notable changes in photophysical proper-
ties on exchanging the heteroatom in new electron deficient
unit and which are reflected on their optoelectronic and
photovoltaic properties. Furthermore, P1–P4 were used in
BHJ PSCs as donor materials and they delivered a maxi-
mum PCE of 4.29% for P3-based PSCs on MeOH treatment,
which was further studied by EIS, the reduced interfacial
resistance of MeOH treated active layers was well con-
curred with the enhanced FF and Jsc of the P3-based PSCs.
Therefore, from the above observations, we believe that the
new electron deficient unit doped polymers exhibit good
optoelectronic properties and reasonable PCEs which dem-
onstrate that new electron deficient units can be considered
as potential candidates for the PSCs. Further improvement
in PCE of PSCs via structural modification and device opti-
mization is underway.
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ACKNOWLEDGMENTS
This work was supported by the New & Renewable Energy
Core Technology Program of the Korea Institute of Energy Tech-
nology Evaluation and Planning (KETEP), granted financial
resource from the Ministry of Trade, Industry & Energy (No.
20143030011560) and the National Research Foundation
(NRF) (2011-0028320) by the Ministry of Science, ICT & Future
Planning, Republic of Korea.
19 K. Kranthiraja, K. Gunasekar, W. Cho, M. Song, Y. G. Park,
J. Y. Lee, Y. Shin, I. N. Kang, A. Kim, H. Kim, B. S. Kim, S. H.
Jin, Macromolecules 2014, 47, 7060–7069.
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