Synthesis and photovoltaic performance of novel thiophenyl-methylene-9H- fluorene-based low bandgap polymers

Article abstract of DOI:10.1016/j.polymer.2013.07.001

Three novel donor-acceptor polymers have been synthesized with Suzuki cross-coupling between thiophenyl-methylene-9H-fluorene donating unit with thiazolo[5,4-d]thiazole (PFTTTz), benzothiadiazole (PFTODTBT) or [1,2,3]triazolo[4,5-g]quinoxaline (PFTDTBTzQ)

Full text of DOI:10.1016/j.polymer.2013.07.001

Polymer 54 (2013) 4930e4939
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and photovoltaic performance of novel thiophenyl-
methylene-9H-fluorene-based low bandgap polymers
,
a
Jiefeng Hai a, Baofeng Zhao b, Fujun Zhang c, Chuan-Xiang Sheng d , Liangming Yin ,
*
,
a,
*
Yang Li a, Enwei Zhu a, Linyi Bian a, Hongbin Wu b , Weihua Tang
*
a Key Laboratory of Soft Chemistry and Functional Materials (Ministry of Education of China), Nanjing University of Science and Technology,
Nanjing 210094, PR China
b Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices,
South China University of Technology, Guangzhou 510640, PR China
c Key Laboratory of Luminescence and Optical Information (Ministry of Education of China), Beijing Jiaotong University, Beijing 100044, PR China
d School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel donor-acceptor polymers have been synthesized with Suzuki cross-coupling between thi-
ophenyl-methylene-9H-fluorene donating unit with thiazolo[5,4-d]thiazole (PFTTTz), benzothiadiazole
(PFTODTBT) or [1,2,3]triazolo[4,5-g]quinoxaline (PFTDTBTzQ) accepting unit. The polymer PFTTTz-THF
prepared from tetrahydrofuran (THF) presented higher molecular weight and narrower polydiversity
than that from toluene (PFTTTz-toluene), leading to higher photovoltaic performance in BHJ devices of
these polymers blended with PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester). These good solution
processable polymers exhibit a bandgap of 1.66e1.93 eV and a low highest occupied molecular orbital
(HOMO) energy level w ꢀ5.32 eV. Polymers PFTTTz and PFTODTBT displayed strong absorption in the
range of 300e650 nm, while PFTDTBTzQ showed a further 100 nm extended absorption band. Overall
efficiencies over 1.5% are achieved for BHJ devices fabricated from blends of PFTTTz with PC71BM
as active layer. A maximum power conversion efficiency of 2.21% is obtained by the use of interlayer poly
[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(30-( N, N-dimethylamino)propyl)-2,7-fluorene)] (PFN) and Ca
between active layer and Al anode.
Received 20 March 2013
Received in revised form
29 June 2013
Accepted 2 July 2013
Available online 9 July 2013
Keywords:
Low bandgap polymers
Polymer solar cells
Thiophenyl-methylene-9H-fluorene
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Some design criteria for an ideal donor polymer materials can be
summarized as follows: (1) the donor polymer should have high
In the past decade, polymer solar cells (PSCs) adopting bulk
heterojunction (BHJ) device structure have attracted considerable
attention owing to their easy tunability of polymers’ chemical
properties and potential in fabricating lightweight, low-cost, and
flexible large area devices [1e5]. Tremendous progress in the
research of PSCs has been made in recent years, whose power
conversion efficiencies (PCEs) have been improved from 1% to over
9% and even a highest record of 10.6% under the collective efforts of
both device physicists and material chemists [6e8]. However, there
are still some key issues needed to be solved before its commercial
application, such as dependable encapsulation technology to
secure long-life working devices, and robust large-area processing
for device making [2e4,9].
extinction coefficient and optimal optical bandgap (Eg); (2) the
donor polymer should have good solubility and appropriate
compatibility with fullerene derivatives; (3) the molecular energy
levels of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) of the donor polymer
should well match with those of acceptors; (4) the donor polymer
should have high hole mobility [10]. To meet the these criteria, a
large library of donor polymers designed by alternating electron-
rich (D) and electron-deficient (A) units along the conjugated
backbone have been developed as DeA alternating copolymers for
highly efficient PSCs [6e8,11e22].
Recently, 9-alkylidene-9H-fluorene has been explored as a
donating unit in the synthesis of D-A alternating narrow bandgap
polymers for PSCs, with a highest PCE reaching 6.2% [15,23,24].
Featuring a sp2-hybridized carbon at the 9-position, 9-alkylidene-
9H-fluorene presents a more planar conformation than 9,9-dialkyl-
substituted fluorene, which facilitates the formation of close
* Corresponding authors. Fax: þ86 25 84317311.
E-mail addresses: cxsheng@mail.njust.edu.cn (C.-X. Sheng), hbwu@scut.edu.cn
(H. Wu), whtang@mail.njust.edu.cn (W. Tang).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.07.001

J. Hai et al. / Polymer 54 (2013) 4930e4939
4931
packing of polymer backbones in solid state to enhance charge
carrier transport [25]. In order to improve the isotropic hole
transport along polymer backbone and extend absorption spec-
trum of the as-designed polymers, a novel conjugated donating
unit, thiophenyl-methylene-9H-fluorene, was designed by intro-
ducing a thiophene unit into 9-alkylidene-9H-fluorene. New D-A
polymers were further prepared by alternating the thiophenyl-
methylene-9H-fluorene donating unit with accepting unit
including thiazolo[5,4-d]thiazole (TTz) [26e31], 40,70-di-2-thienyl-
50,60-dialkoxy-20,10,30- benzothiadiazole (ODTBT) [15,32e36] and
[1e3]triazolo[4,5-g]quinoxaline (DTBTzQ) [37e39]. These novel
copolymers possesses two distinctive characteristics in molecular
design: i) the first time to use thiophenyl-methylene-9H-fluorene
as electron donating unit for D-A copolymer construction for PSCs;
ii) thiophenyl-methylene-9H-fluorene increases conjugate length
comparing with 9-alkylidene-9H-fluorene, which has higher elec-
tron density. The thiophenyl-methylene-9H-fluorene unit en-
PEDOT/PSS from a solvent of o-dichlorobenzene (o-DCB) solution
(concentration, 10 mg/mL) at 1000 rpm for 2 min. Thirdly, a 5 nm
interlayer material PFN was spin-coated on the top of active layer
through its methanol solution in the presence of small amount of
acetic acid or Ca layer was deposited under high vacuum. Finally, Al
layer was subsequently evaporated through a shadow mask to
define the active area of the devices (w2 ꢂ 8 mm2) and form a top
anode to complete the photovoltaic device fabrication. The cur-
rentevoltage (JeV) characteristics and PCE were measured with a
Keithley 2400 sourcemeter under 1 sun, AM 1.5G spectrum from a
solar simulator (Oriel model 91192) at room temperature in a ni-
trogen filled glove-box. Solar simulator illumination intensity was
determined using a monocrystal silicon reference cell (Hamamatsu
S1133, with KG-5 visible color filter) calibrated by the National
Renewable Energy Laboratory (NREL).
2.3. Materials
hances the coplanarity of the polymer backbone and extent the
conjugation.
p-
All starting materials and reagents were purchased from Sigmae
Aldrich Chemical Co. or Acros and used without further purification.
THF was dried over sodium/benzophenone and freshly distilled
before use. Chloroform and dichloromethane were distilled from
CaH2 under nitrogen. 2-Dodecylthiophene [40], (40,70-di-2-thienyl-
50,60-didodecyloxy)-20,10,30-benzothiadazole(ODTBT)[36]2-dodecyl-
4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-5,6-dinitro-2H-benzo[d] [1e
3]triaz-ole [37], and 1,2-bis(3,4-bis(dodecyloxy)phenyl)ethane-1,2-
dione [41,42] were prepared according to the known literature
procedures.
In this paper, the synthesis, characterization and photovoltaic
properties of thiophenyl-methylene-9H-fluorene based narrow
bandgap polymers are reported. The thermal, optical and electro-
chemical properties of the resulting polymers were systematically
studied. Importantly, the promising potential of blends of PFTTTz
with PC71BM as the active layer for photovoltaic cells was
demonstrated, with a best PCE of 2.21% obtained in the initial de-
vice measurement.
2. Experimental section
2.4. Synthesis
2.1. Characterization
2.4.1. 5-Dodecylthiophene-2-carbaldehyde (1) [43]
1H NMR and 13C NMR spectra were characterized on Bruker
AVANCE 500-MHz (Bio-Spin Corporation, Europe) spectrometer
with chloroform-d as solvent and tetramethylsilane (TMS) as in-
ternal standard. Gel permeation chromatography (GPC) analysis
were measured with on a Waters 717-2410 instrument with poly-
styrenes as the standard and THF as an eluant (flow rate
1.0 mL/min). The thermogravimetric analysis (TGA) analysis was
conducted with a TA instrument TGA/SDTA851e under nitrogen
atmosphere at a heating rate of 20 ꢁC/min. Differential scanning
calorimetry (DSC) analysis was performed on a DSC instrument
DSC823 under N2 with a heating rate from 0 ꢁC to 280 ꢁC. Ultra-
violetevisible (UVevis) absorption spectra were recorded on a UVe
vis instrument Evolution 220. The electrochemical cyclic voltam-
To a stirred solution of 5-dodecylthiophene [40] (4.00 g,
15.84 mmol) in anhydrous DMF (3.47 g, 47.53 mmol), POCl3 (7.29 g,
47.53 mmol) was added slowly at 0 ꢁC and stirred for 20 min at this
temperature. The mixture was warmed to 75 ꢁC and refluxed for
2 h, and then it was poured into ice water and neutralized by NaOH
to neutrality. After extraction with CHCl3, the combined organic
layers were washed with water and brine, dried with MgSO4,
filtered, and concentrated via rotary evaporation. The residue was
purified by column chromatography on silica gel using petroleum
ether/ethyl acetate (4:1, v/v) to give the pure product as a pale
yellow solid (3.26 g, 73%). 1H NMR (500 MHz, CDCl3):
d (ppm) 9.81
(s, 1H), 7.60 (d, J ¼ 3.7 Hz, 1H), 6.90 (d, J ¼ 3.7 Hz, 1H), 2.86 (t,
J ¼ 7.6 Hz, 2H), 1.74e1.67 (m, 2H), 1.31e1.23 (m, 18H), 0.88 (t,
J ¼ 7.0 Hz, 3H).
metry was conducted in 0.1
M tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) acetonitrile solution at a scan rate of
50 mV/s on electrochemistry workstation (CHI660D, Chenhua
Shanghai) with the polymer film on Pt plate, Pt slice, and saturated
calomel electrode (SCE) as working electrode, counter electrode
and reference electrode, respectively.
2.4.2. 2,7-Dibromo-fluorene (2) [44]
To a solution of fluorene (14.7 g, 88.44 mmol) in dry chloroform
(150 mL) at 0 ꢁC was added with a solution of Br2 in chloroform
(80 mL). The reaction was stirred overnight at room temperature in
darkness. After quenched with saturated aqueous Na2S2O3, the
reaction mixture was separated and the organic phase was washed
with water, dried with MgSO4 and concentrated in vacuo. The crude
product was recrystallized in ethanol to give the title product as a
2.2. Photovoltaic device fabrication and characterization
The BHJ solar cells were prepared with a device structure of ITO/
PEDOT:PSS(40 nm)/polymer:PC61BM (1:1, 1:2, 1:3, w/w) or PC71BM
(1:2, w/w)/PFN or Ca/Al (80 nm). The ITO glass substrates were
sonicated sequentially in deionized water, acetone, deionized water
and isopropanol. Immediately prior to device fabrication, the sub-
strates were treated by oxygen plasma for 4 min. Firstly, poly(3,4-
ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS,
H.C. Starck) thin film (40 nm) was spin-coated and then baked at
150 ꢁC for 15 min. Secondly, the PFTTTz:PC71BM (1:2 weight ratio)
or PFTDTBTzQ:PC61BM (1:1, 1:2,1:3 weight ratio) active blend layer,
with a nominal thickness of w80 nm, was spin-coated on top of the
white solid (26.3 g, 92%). 1H NMR (500 MHz, CDCl3):
d (ppm) 7.68 (s,
2H), 7.61 (d, J ¼ 8.1 Hz, 2H), 7.52 (d, J ¼ 8.1 Hz, 2H), 3.87 (s, 2H).
2.4.3. 2,7-Dibromo-9-((5-dodecylthiophen-2-yl)methylene)-9H-
fluorene (3) [45]
2,7-Dibromo-fluorene (2) (0.648 g, 2.00 mmol), potassium
tert-butoxide (0.247 g, 2.20 mmol) were dissolved in ethanol
(100 mL). The mixture was refluxed for 1 h before added with
5-dodecylthiophene-2-carbaldehyde (1) (0.617 g, 2.00 mmol). The
mixture was refluxed for another 2 h. The reaction was cooled to

4932
J. Hai et al. / Polymer 54 (2013) 4930e4939
room temperature and water (100 mL) was added. The crude
product was extracted with dichloromethane and the combined
organic layers were washed with water and brine, dried with
MgSO4, filtered, and concentrated via rotary evaporation. The res-
idue was purified by column chromatography on silica gel using
petroleum ether to give the title compound as a yellow solid (1.05 g,
6.25e6.23 (m, 1H), 3.85 (d, J ¼ 5.7 Hz, 2H), 1.82e1.77 (m, 1H), 1.27e
1.50 (m, 32H), 0.93 (t, J ¼ 7.0 Hz, 6H).
2.4.7. 3-((2-Octyldodecyl)oxy)thiophene-2-carbaldehyde (6) [26]
To the stirred solution of anhydrous DMF (40 mL) at 0 ꢁC under
nitrogen atmosphere was added with POCl3 (2.42 g, 15.76 mmol)
over 10 min. After addition, the reaction mixture was stirred at that
temperature for 3 h. Then 3-((2-octyldodecyl)oxy)thiophene (5)
(5.00 g, 13.13 mmol) was added dropwise at 0 ꢁC. The reaction
mixture was heated to 70 ꢁC for 12 h. The reaction was cooled to
room temperature and poured it into ice water and then slowly
neutralize with 10% NaOH solution. The crude product was
extracted with dichloromethane and the combined organic layers
were washed with water and brine, dried with MgSO4, filtered, and
concentrated via rotary evaporation. After column chromatography
on silica gel using petroleum ether/ethyl acetate (8:1), the title
product was obtained as a colorless liquid (4.03 g, 75%). 1H NMR
90%). 1H NMR (500 MHz, CDCl3):
d
(ppm) 8.50 (d, J ¼ 1.6 Hz, 1H),
7.84 (d, J ¼ 1.6 Hz,1H), 7.60e7.53 (m, 3H), 7.50e7.45 (m, 2H), 7.32 (d,
J ¼ 3.6 Hz, 1H), 6.86 (d, J ¼ 3.6 Hz, 1H), 2.91 (t, J ¼ 7.6 Hz, 2H), 1.78e
1.71 (m, 2H), 1.46e1.39 (m, 2H), 1.37e1.20 (m, 16H), 0.88 (t,
J ¼ 6.9 Hz, 3H). 13C NMR (CDCl3, 125 MHz):
d (ppm) 150.77, 141.83,
138.85, 137.74, 136.44, 135.96, 131.82, 131.29, 130.74, 127.29, 125.35,
123.32, 122.45, 121.25, 121.07, 32.22, 31.79, 30.68, 29.96, 29.67,
29.45, 23.00, 14.44.
2.4.4. 2,20-(9-((5-dodecylthiophen-2-yl)methylene)-9H-fluorene-
2,7-diyl)bis(4,4,5,5- tetramethyl-1,3,2-dioxaborolane) (M1) [15]
A mixture of (9-((5-dodecylthiophen-2-yl)methylene)-9H-flu-
orene (3)) (1.00 g, 1.71 mmol), bis-pinacoldiboron (1.04 g,
4.09 mmol), PdCl2(dppf)2 (62 mg, 0.08 mmol), and potassium ac-
etate (1.67 g, 17.05 mmol) in MDF (50 mL) was stirred for 20 h at
100 ꢁC. The reaction mixture was cooled to room temperature and
water (50 mL) was added. The crude product was extracted with
diethyl ether and the combined organic layers were washed with
water and brine, dried with MgSO4, filtered and concentrated via
rotary evaporation. The residue was purified with column chro-
matography on silica gel using petroleum ether/ethyl acetate (30:1)
to give a yellow oil; after recrystallization from ethanol to give the
desired product as a pale yellow solid (1.16 g, 70%). 1H NMR
(500 MHz, CDCl3):
6.83 (d, J ¼ 5.4 Hz, 1H), 4.03 (d, J ¼ 5.6 Hz, 2H), 1.83e1.77 (m, 2H),
1.48e1.19 (m, 32H), 0.88 (t, J ¼ 7.0 Hz, 6H).
d
(ppm) 10.00 (s, 1H), 7.61 (d, J ¼ 5.4 Hz, 1H),
2.4.8. 2,5-Bis(3-((2-octyldodecyl)oxy)thiophen-2-yl)thiazolo[5,4-d]
thiazole (7) [26]
Dithiooxamide (220 mg, 1.84 mmol), phenol (1.5 g), and 3-((2-
octyldodecyl)oxy)thiophene-2-carbaldehyde (6) (1.50 g, 3.67
mmol) were combined in a round-bottom flask and refluxed at
temperature 160 ꢁC for 3 h. The crude product was purified by
column chromatography on silica gel using petroleum ether/chlo-
roform (3:2) to give pure product as brown yellow solid (800 mg,
(500 MHz, CDCl3):
d
(ppm) 8.97 (s, 1H), 8.20 (s, 1H), 7.83e7.78 (m,
24%). 1H NMR (500 MHz, CDCl3):
d
(ppm) 7.32 (d, J ¼ 5.5 Hz, 2H),
3H), 7.77e7.73 (m, 2H),7.36 (d, J ¼ 3.4 Hz, 1H), 6.83 (d, J ¼ 3.4 Hz,
1H), 2.89 (t, J ¼ 7.7 Hz, 2H), 1.79 (m, 2H), 1.39 (s, 12H), 1.35 (s, 12H),
1.33e1.23 (m, 18H), 0.88 (t, J ¼ 7.0 Hz, 3H). 13C NMR (CDCl3,
6.89 (d, J ¼ 5.5 Hz, 2H), 4.14 (d, J ¼ 4.8 Hz, 4H), 1.88e1.82 (m, 2H),
1.64e1.57 (m, 4H), 1.55e1.47 (m, 4H), 1.40e1.20 (m, 56H), 0.86 (t,
J ¼ 7.0 Hz, 12H).
125 MHz):
d (ppm) 149.46, 143.54, 141.26, 140.33, 137.22, 136.25,
134.99, 134.55, 133.48, 131.37, 128.11, 127.83, 126.59, 125.09, 120.68,
119.81, 119.63, 84.08, 83.92, 32.25, 31.93, 30.78, 30.00, 29.72, 29.56,
25.22, 23.03, 14.48.
2.4.9. 2,5-Bis(5-bromo-3-((2-octyldodecyl)oxy)thiophen-2-yl)
thiazolo[5,4-d]thiazole (TTz-Br) [26]
To a solution of 2,5-bis(3-((2-octyldodecyl)oxy)thiophen-2-yl)
thiazolo[5,4-d] thiazole (7) (0.80 g, 0.89 mmol) in CHCl3 (16 mL)
and acetic acid (8 mL) solution, NBS (348 mg,1.96 mmol) solution in
CHCl3/AcOH (10/5 mL) was added dropwise and stirred at 0 ꢁC for
2 h. Then the reaction solution was stirred at room temperature
overnight. The mixture was washed with water and brine, dried
with MgSO4, filtered, and concentrated via rotary evaporation. After
column chromatography on silica gel using petroleum ether/chlo-
roform (5:1), the pure product was obtained as a yellow solid
2.4.5. 3-Methoxythiophene (4) [46]
To a stirred solution of sodium methanolate (3.18 g, 58.88 mmol)
in methanol (50 mL) were added with N-methyl pyrrolidone
(50 mL) and 3-bromothiophene (8 g, 49.07 mmol). The mixture was
heated to 110 ꢁC and then CuI (934 mg, 4.91 mmol) was added in
one portion and kept reflux overnight. The reaction was cooled to
room temperature and water (100 mL) was added. The crude
product was extracted with dichloromethane and the combined
organic layers were washed with water and saturated aqueous
NH4Cl, dried with MgSO4, filtered, and concentrated via rotary
evaporation. The residue was further distilled under reduced
pressure to afford the title compound as a colorless liquid (4.95 g,
(800 mg, 85%). 1H NMR (500 MHz, CDCl3):
d (ppm) 6.90 (s, 2H), 4.09
(d, J ¼ 4.9 Hz, 4H), 1.88e1.78 (m, 2H), 1.62e1.44 (m, 8H), 1.38e1.18
(m, 56H), 0.86 (t, J ¼ 7.0 Hz, 12H).
2.4.10. 2-Dodecyl-4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-2H-
benzo[d] [1e3]triazole- 5,6-diamine (9) [37]
88%). 1H NMR (500 MHz, CDCl3):
d (ppm) 7.19e7.16 (m, 1H), 6.76-
6.74 (m, 1H), 6.26-6.24 (m, 1H), 3.81 (s, 3H).
2-Dodecyl-4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-5,6-dinitro-
2H-benzo[d] [1e3]tri-azole (8) (3.48 g, 4.54 mmol), iron powder
(5.07 g, 90.85 mmol) and acetic acid (100 mL) were added into a
round bottom flask purged with nitrogen. The reaction was stirred
at 70 ꢁC for 4 h, after which was allowed to cool to room temper-
ature, poured into water (200 mL) and extracted using dichloro-
methane. The combined organic layers were washed with water
and brine, dried with MgSO4, filtered, concentrated via rotary
evaporation and purified by column chromatography on silica gel
using petroleum ether/ethyl acetate to give the pure product as a
2.4.6. 3-((2-Octyldodecyl)oxy)thiophene (5) [46]
A mixture of 3-methoxythiophene (4) (5.26 g, 46.07 mmol),
2-octyldodecan-1-ol (27.51 g, 92.15 mmol), potassium bisulfate
(1.25 g, 9.21 mmol) were dissolved in toluene (50 mL) and heated at
115 ꢁC for 20 h under N2. The reaction mixture was cooled to room
temperature and water (50 mL) was added. The crude product was
extracted with dichloromethane and the combined organic layers
were washed with water and brine, dried with MgSO4, filtered, and
concentrated via rotary evaporation. The residue was purified by
column chromatography on silica gel using petroleum ether to give
the pure product as a colorless liquid (8.70 g, 50%). 1H NMR
brown oil (2.64 g, 82%). 1H NMR (500 MHz, CDCl3):
d (ppm) 7.25
(s, 2H), 7.05 (s, 2H), 4.58 (t, J ¼ 7.4 Hz, 2H), 3.54e2.86 (br, 4H), 2.63
(d, J ¼ 6.8 Hz, 4H), 2.07e2.00 (m, 2H), 1.66e1.58 (m, 2H), 1.41e1.20
(m, 34H), 0.94e0.86 (m, 15H).
(500 MHz, CDCl3):
d (ppm) 7.20e7.16 (m, 1H), 6.80e6.77 (m, 1H),

J. Hai et al. / Polymer 54 (2013) 4930e4939
4933
2.4.11. 6,7-Bis(3,4-bis(dodecyloxy)phenyl)-2-dodecyl-4,9-bis(4-(2-
ethylhexyl) thiophen-2-yl)-2H- [1e3]triazolo[4,5-g]quinoxaline (11)
[37]
2.4.14. PFTTTz-toluene
Toluene was used for the synthesis of PFTTTz with Suzuki cross-
coupling reaction. Typically, mixture of TTz-Br (259 mg,
a
2-Dodecyl-4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-2H-benzo[d]
[1e3]triazole-5,6-diamine (9) (844 mg, 1.20 mmol) and 1,2-bis(3,4-
bis(dodecyloxy)phenyl)ethane-1,2-dione (10) [42] (1.36 g,
1.43 mmol) was dissolved in acetic acid (50 mL). The reaction flask
was purged with nitrogen and heated at 100 ꢁC for 24 h. The re-
action mixture was cooled to room temperature and water
(100 mL) was added. The crude product was extracted with
dichloromethane and the organic layer was washed thoroughly
with water and then diluted sodium bicarbonate solution. The
combined organic layers were washed with water and brine, dried
with MgSO4, filtered and concentrated via rotary evaporation. The
residue was purified with column chromatography on silica gel
using petroleum ether/ethyl acetate to give the title compound as a
0.381 mmol), M1 (400 mg, 0.381 mmol), toluene (15 mL) and
aqueous Na2CO3 (7 mL, 2 mol/L) was carefully degassed before and
after Pd(PPh3)4 (16 mg, 0.02 mmol) was added. The mixture was
refluxed with vigorous stirring for 65 h under at 85 ꢁC under ni-
trogen. By following the same work-up procedure as for PFTTTz-
THF, the title polymer PFTTTz-toluene was obtained as a purple
black polymer (246 mg, 49%). 1H NMR (500 MHz, CDCl3):
d (ppm)
8.85e7.89 (br, 2H), 7.88e7.31 (br, 5H), 7.16e6.70 (br, 4H), 4.50e3.88
(br, 4H), 3.09e2.84 (br, 2H), 2.12e0.75 (m, 101H).
2.4.15. PFTODTBT
Standard Suzuki cross-coupling reaction was employed for the
synthesis of PFTODTBT. Typically, ODTBT-Br (335 mg, 0.492 mmol),
M1 (407 mg, 0.492 mmol), and Pd(PPh3)4 (16 mg, 0.02 mmol) were
dissolved in a mixture of toluene (12 mL) and aqueous Na2CO3
(5 mL, 2 mol/L). The mixture was refluxed with vigorous stirring for
48 h under an argon atmosphere. After cooling to room tempera-
ture, the mixture was poured into methanol. The precipitated
material was collected and was Soxhlet-extracted in order with
methanol, acetone and then with chloroform. The chloroform so-
lution was concentrated to a small volume to precipitate the
polymer out of methanol. Finally, the purple black polymer
PFODTBT was collected by filtration and dried under vacuum at
purple black liquid (1.85 g, 95%). 1H NMR (500 MHz, CDCl3):
d (ppm)
8.77 (d, J ¼ 1.2 Hz, 2H), 7.58 (d, J ¼ 2.0, 2H), 7.30 (dd, J ¼ 2.0, 8.3 Hz,
2H), 7.14 (s, 2H), 6.84 (d, J ¼ 8.5 Hz, 2H), 4.96 (d, J ¼ 7.2 Hz, 2H),
4.06e3.97 (m, 8H), 2.71 (d, J ¼ 5.1 Hz, 2H), 2.35-2.29 (m, 2H), 1.88e
1.78 (m, 8H), 1.73e1.68 (m, 2H), 1.53-1.19 (m, 106H), 0.98-0.81 (m,
27H). 13C NMR (125 MHz, CDCl3):
d (ppm) 151.09, 150.11, 148.61,
142.20, 141.24, 135.38, 133.54, 133.27, 131.33, 126.40, 123.86, 118.84,
116.01, 112.57, 77.25, 69.08, 57.54, 40.40, 34.48, 32.59, 31.92, 29.67,
29.46, 29.38, 29.11, 29.012, 26.73, 26.11, 26.06, 25.70, 23.10, 22.68,
14.20, 14.09, 10.97.
40 ꢁC overnight (324 mg, 60%). 1H NMR (500 MHz, CDCl3):
d (ppm)
2.4.12. 6,7-Bis(3,4-bis(dodecyloxy)phenyl)-4,9-bis(5-bromo-4-(2-
ethylhexyl)thiophen-2-yl)-2-dodecyl-2H- [1e3]triazolo[4,5-g]
quinoxaline (DTBTzQ-Br)
8.85-8.21 (m, 4H), 7.88e7.32 (m, 8H), 6.93e6.84 (br, 1H), 4.33e3.99
(br, 4H), 3.03e2.82 (m, 2H), 2.10e1.89 (br, 4H), 1.87e1.71 (br, 2H),
1.52e1.00 (m, 54H), 0.93e0.77 (br, 9H).
A
solution of 6,7-bis(3,4-bis(dodecyloxy)phenyl)-2-dodecyl-
4,9-bis(4- (2-ethylhexyl)thiophen-2-yl)-2H- [1e3]triazolo[4,5-g]
quinoxaline (11) (1850 mg, 1.14 mmol) in CHCl3 was cooled to 0 ꢁC.
Then NBS (458 mg, 2.57 mmol) was added in small portions over
30 min and then the mixture was stirred overnight. The combined
organic layers were washed with water and brine, dried with
MgSO4, filtered, and concentrated via rotary evaporation. After
column chromatography on silica gel using petroleum ether/ethyl
acetate, the title compound was obtained as a black purple solid
2.4.16. PFTDTBTzQ
By following similar procedure as for PFODTBT, PFDTBTzQ was
prepared by the cross-coupling between DTBTzQ-Br (510 mg,
0.287 mmol) and M1 (196 mg, 0.287 mmol) under the catalysis of
Pd(PPh3)4 (16 mg, 0.02 mmol) in a mixture solution of toluene
(20 mL) and 20% aqueous tetraethylammonium hydroxide (5 mL).
The mixture was refluxed with vigorous stirring for 48 h under an N2
atmosphere. After cooling to room temperature, the mixture was
poured into methanol. The precipitated material was collected and
was Soxhlet-extracted in order with methanol, acetone and then
with chloroform. The chloroform solution was concentrated to a
small volume to precipitate the polymer out of methanol. Finally, the
purple black polymer PFDTBTzQ was collected by filtration and
dried under vacuum at 40 ꢁC overnight (483 mg, 82%). 1H NMR
(960 mg, 47%). 1H NMR (500 MHz, CDCl3):
d (ppm) 8.66
(d, J ¼ 3.7 Hz, 2H), 7.76 (s, 2H), 7.07 (dd, J ¼ 2.0, 8.4 Hz, 2H), 6.77
(d, J ¼ 8.4 Hz, 2H), 4.95e4.90 (m, 2H), 4.18 (t, J ¼ 6.6 Hz, 4H), 4.03
(t, J ¼ 6.5 Hz, 4H), 2.64 (d, J ¼ 7.0 Hz, 4H), 2.33e2.26 (m, 2H), 1.92e
1.82 (m, 8H), 1.80e1.76 (m, 2H), 1.46e1.19 (m, 106H), 0.98e0.84
(m, 27H). 13C NMR (125 MHz, CDCl3):
d (ppm) 151.67, 150.43,
149.23, 141.77, 140.48, 135.27, 133.00, 131.20, 124.41, 118.23, 117.30,
115.67, 112.32, 69.57, 69.28, 57.70, 40.22, 33.84, 32.79, 32.22, 29.98,
29.69, 29.41, 29.09, 26.98, 26.55, 26.38, 25.95, 23.38, 22.94, 14.39,
11.19.
(500 MHz, CDCl3): d (ppm) 8.98e8.87 (m, 2H), 8.63 (s, 1H), 7.98
(s, 1H), 7.88e7.81 (m, 2H), 7.76 (s, 1H), 7.74e7.69 (m, 2H), 7.66e7.59
(m, 2H), 7.34e7.28 (m, 2H), 6.82e6.71 (m, 2H), 6.63 (s, 1H), 5.01 (br,
2H), 3.95 (br, 4H), 3.84 (br, 4H), 2.91-2.78 (m, 4H), 2.72 (br, 2H), 2.40
(br, 2H), 1.77 (br, 8H), 1.53e1.02 (m, 128H), 0.93e0.79 (m, 30H). 13C
2.4.13. PFTTTz-THF
NMR (125 MHz, CDCl3): d (ppm) 151.14, 150.18, 149.34, 144.50,
TTz-Br (259 mg, 0.381 mmol), M1 (400 mg, 0.381 mmol), and
Pd(PPh3)4 (16 mg, 0.02 mmol) were dissolved in a mixture of THF
(15 mL) and aqueous Na2CO3 (7 mL, 2 mol/L). The mixture was
refluxed with vigorous stirring for 65 h under at 85 ꢁC under N2.
After cooling to room temperature, the mixture was poured into
methanol. The precipitated material was collected and was
Soxhlet-extracted sequentially with methanol, acetone and then
with chloroform. The chloroform solution was concentrated to a
small volume to precipitate out the polymer out of methanol.
Finally, the purple black polymer PFTTTz-THF was collected by
filtration and dried under vacuum at 40 ꢁC overnight (327 mg, 65%).
144.22, 142.28, 140.63, 139.78, 137.81, 136.78, 136.374, 135.03, 134.03,
133.80, 133.37, 131.38, 130.86, 129.88, 129.38, 125.57, 124.82, 123.96,
121.10, 119.50, 118.58, 115.61, 112.47, 69.11, 57.60, 40.73, 32.71, 31.92,
31.50, 30.35, 29.67, 29.36, 26.82, 26.04, 25.807, 23.14, 22.68, 14.10.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes for donating monomer M1 and accepting
monomers TTz-Br and DTBTzQ-Br are outlined in Scheme 1. The
thiophenyl-methylene-9H-fluorene based donating unit was
constructed by a condensation between 5-dodecylthiophene-2-
carbaldehyde (1) and 2,7-dibromo-fluorene (2) with a yield of 90%.
1H NMR (500 MHz, CDCl3):
d (ppm) 9.08e8.06 (br, 2H), 8.05e7.33
(br, 5H), 7.19e6.58 (br, 4H), 4.45e3.81 (m, 4H), 3.14e2.82 (br, 2H),
2.04e0.77 (m, 101H).

4934
J. Hai et al. / Polymer 54 (2013) 4930e4939
Scheme 1. Synthetic route for monomers.
Scheme 2. Synthetic route for the target copolymers PFTTTz-THF, PFTTTz-toluene, PFTODTBT and PFTDTBTzQ.

J. Hai et al. / Polymer 54 (2013) 4930e4939
4935
A further boration of 2,7-dibromo-(9-((5-dodecylthiophen-2-yl)
methylene)-9H-fluorene (3)) afforded the desired thiophenyl-
methylene-9H-fluorene based donating monomer M1 in 70% yield.
The thiazolo[5,4-d]thiazole based accepting monomer TTz-Br was
prepared with a five-step protocol starting from 3-bromothiophene.
The diamine 9 which was the reduction of 8 underwent a conden-
sation with 1,2-bis(3,4-bis(dodecyloxy)phenyl)ethane-1,2-dione (10)
in acetic acid to afford the key intermediate compound 11 in 95%
yield. Bromination of 11 with N-bromosuccinimide in a solution of
chloroform afforded DTBTz-Br with a good yield.
As depicted in Scheme 2, three polymers PFTTTz, PFTODTBT
and PFTDTBTzQ were prepared using Suzuki cross-coupling reac-
tion with a yield of 49e82%. For the synthesis of PFTTTz, THF and
toluene were explored for the reaction, and the resulted polymers
are named as PFTTTz-THF and PFTTTz-toluene, respectively. The
as-synthesized four polymers exhibit excellent solubility in com-
mon organic solvents, including tetrahydrofuran, chloroform,
o-dichlorobenzene, toluene, etc. Molecular weight of the polymers
and polydispersity (PDI) was measured by gel permeation chro-
matography (GPC) method using polystyrene as standard and
chloroform as eluant (Table 1). The as-prepared polymers PFTTTz-
Fig. 1. TGA curve of copolymers at the heating rate of 20 ꢁC/min under nitrogen
atmosphere.
THF, PFTTTz-toluene, PFTODTBT and PFTDTBTzQ possess
a
number-average molecular weight (Mn) of 11,190, 9456, 6393 and
14,230, respectively, and a corresponding polydispersity index
(PDI) of 1.22, 1.40, 1.66 and 1.69. A close look at the Mn and PDI data
of PFTTTz-THF and PFTTTz-toluene would find that THF is a better
solvent than toluene for the synthesis of PFTTTz, indicated by the
higher Mn and narrower PDI.
The thermal property of the polymer was investigated with TGA
(Fig. 1). The onset temperature (Td) with 5% weight loss of PFTTTz-
THF, PFTTTz-toluene, PFTODTBT are 352, 315 and 317 ꢁC, respec-
tively. These high Td values indicate the good thermal stability of
the polymers, which are beneficial for their application in opto-
electronic devices. PFTDTBTzQ, however, exhibits a relatively low Td
(270 ꢁC). Interestingly, no obvious phase transition process could be
observed in the range from 0 to 300 ꢁC for all the resulting polymers
even in several cycles of DSC testing.
35 nm red-shifted than that of its solution. PFTDTBTzQ film
showed absorption band in the range of 550e750 nm with a lmax at
669 nm, which was 19 nm red-shifted than that of its solution. The
red-shifted absorption of polymers film indicated that strong
intermolecular interaction and aggregation exist in the solid-state
of these polymers, which is probably related to the increased
extent of
pep stacking of the backbones and increased polariz-
ability of the solid-state. PFTODTBT and PFTDTBTzQ exhibited two
evident absorption bands, the first absorption band at the shorter
wavelength was attributed to the pep* transition of the conjugated
main chain and the second band at the longer wavelength was
owed to the intramolecular charge transfer (ICT) interaction be-
tween the 9-((5-dodecylthiophen-2-yl)methylene)-9H-fluorene
donating unit and TTz or DTBTzQ accepting unit.
The optical bandgaps (Eg) of these copolymers were determined
from the UVꢀvis absorption onsets in the solid state according to
the empirical equation: Eg ¼ 1240/lonset eV. The Eg of PFTTTz-THF,
PFTTTz-toluene, PFTODTBT and PFTDTBTzQ was determined to be
1.93, 1.96, 1.92, and 1.66 eV, respectively. From the Eg data of
PFTTTz-THF, PFTTTz-toluene, PFTODTBT and PFTDTBTzQ, one
could conclude that the electron-withdrawing ability of accepting
DTBTzQ is stronger than those of ODTBT and TTz units. PFTTTz
exhibited a lower Eg than structure analog PCDTTz (2.07 eV) [47].
The Eg of PFTODTBT is smaller than benzothiadiazole based poly-
fluorene (2.04 eV), but larger than 9-alkylidene-9H-fluorene based
PADFTBT (1.84 eV) [15]. It showed that 9-((5-dodecylthiophen-2-
yl)methylene)-9H-fluorene unit could effectively adjust the opti-
cal property of the resulting polymers.
3.2. Photophysical properties
Fig. 2 shows the UVevis absorption spectra of the polymers in
dilute (10ꢀ5 M) chloroform solutions and solid films, with the ab-
sorption maximum (lmax) data summarized in Table 1. PFTTTz-THF
film showed absorption band in the range of 350e650 nm with a
lmax at 581 nm, which was 12 nm red-shifted than its solution.
PFTTTz-toluene film showed absorption band in the range of 350e
650 nm with a lmax at 577 nm, which was 7 nm red-shifted than its
solution. The red-shifted lmax of PFTTTz-THF than PFTTTz-toluene
film can be explained with longer conjugation length of PFTTTz-
THF due to its larger Mn. PFTODTBT film showed absorption band
in the range of 450e650 nm with a lmax at 548 nm, which was
Table 1
Molecular weights, thermal property, bandgap and energy levels of the polymers.
PDIa
Td (ꢁC)b
lmax (nm)c
lmax (nm)d
lonset (nm)d
Eg (eV)e
HOMO (eV)
LUMO (eV)f
a
Polymers
Mw
PFTTTz-THF
PFTTTz-toluene
PFODTBT
13,657
13,238
10,628
24,031
1.22
1.40
1.66
1.69
352
315
317
270
569
570
513
650
581
577
548
669
640
634
646
748
1.93
1.96
1.92
1.66
ꢀ5.24
ꢀ5.23
ꢀ5.43
ꢀ5.32
ꢀ3.31
ꢀ3.27
ꢀ3.51
ꢀ3.66
PFTDTBTzQ
a
Mw and PDI of the polymers determined by GPC using polystyrene standards in THF.
5% weight-loss temperatures under N2 and heat from 50 to 800 ꢁC at a rate of 20 ꢁC/min.
UVevis absorption in dilute CHCl3 solution.
b
c
d
e
f
UVevis absorption in thin films.
Optical bandgap calculated as Eg ¼ 1240/lonset (eV).
LUMO ¼ HOMO þ Egopt (eV).

4936
J. Hai et al. / Polymer 54 (2013) 4930e4939
Fig. 2. Absorption spectra of four polymers in chloroform solution and solid films.
3.3. Electrochemical properties
PFTDTBTzQ are calculated to be ꢀ5.24, ꢀ5.23, ꢀ5.43 and ꢀ5.32 eV,
respectively (Table 1). The deep HOMO levels of these polymers
should be beneficial to their chemical stability in ambient conditions.
In addition, the deep HOMO levels of copolymers are desirable for
higher Voc of the PSCs with copolymers as donor materials [1]. The
LUMO level of PFTTTz-THF, PFTTTz-toluene, PFTODTBT and
PFTDTBTzQ were thus calculated to be ꢀ3.31, 3.27, ꢀ3.51, ꢀ3.66 eV,
The highest occupied molecular orbital (HOMO) energy levels
of the conjugated polymers were determined by electrochemical
cyclic voltammetry (CV). Fig. 3 shows the CV traces of polymers film
on Pt electrode in 0.1 M Bu4NPF6 acetonitrile solution at a scan rate
of 50 mV/s. The potential of ferrocene 0.40 V versus SCE is used as
internal standard. On the basis of 4.8 eV below vacuum for the
energy level of Fc/Fcþ, the HOMO level of the polymers is calculated
from the onset oxidation potentials (Eooxnset), while the LUMO levels
are calculated using HOMO and optical Egopt according to the
following equations:
which were calculated from the Egopt and HOMO energy levels of the
onset
polymers, respecteively. The difference E
of PFTTTz-THF and
ox
PFTTTz-toluene is attributed to the difference Mn of them.
À
Á
onset
ox
HOMO ¼ ꢀe E
þ 4:4 ðeVÞ;
LUMO ¼ HOMO þ EgoptðeVÞ
The onset potential for oxidation (Eooxnset) was observed to be 0.84,
0.83, 1.03, and 0.92 eV for PFTTTz-THF, PFTTTz-toluene, PFTODTBT
and PFTDTBTzQ, respectively. Accordingly, the corresponding
HOMO energy level of PFTTTz-THF, PFTTTz-toluene, PFTODTBT and
Fig. 4. The HOMO and LUMO energy levels of the copolymers.
0.0
-0.4
-0.8
PFTDTBTzQ:PC BM 1:1
61
-1.2
PFTDTBTzQ:PC BM 1:2
61
PFTDTBTzQ:PC BM 1:3
61
0.0
0.2
0.4
0.6
0.8
Voltage(V)
Fig. 5. Current density versus voltage (JeV) characteristics of devices using different
PFTDTBTzQ:PC61BM weight ratios as active layer under AM 1.5 G irradiation with an
Fig. 3. Cyclic voltammograms of polymers films on platinum electrode in 0.1
Bu4NPF6 acetonitrile solutions at a scan rate of 50 mV/s.
M
intensity of 100 mW cmꢀ2
.

J. Hai et al. / Polymer 54 (2013) 4930e4939
4937
Table 2
Table 3
Photovoltaic properties of polymer solar cells based PFTDTBTzQ:PC61BM blends at
Photovoltaic properties of polymer solar cells based PFTDTBTzQ:PC71BM (1:2)
different ratios.
blends.
Cells
Polymer/PC61BM
D/A
Voc [V]
Jsc [mA/cm2]
FF [%]
PCE [%]
Cells
Polymer/PC71BM
D/A
Voc [V]
Jsc [mA/cm2]
FF [%]
PCE [%]
A
B
C
PFTDTBTzQ
PFTDTBTzQ
PFTDTBTzQ
1:1
1:2
1.3
0.79
0.76
0.74
1.59
1.73
1.54
39.8
43.3
46.2
0.50
0.57
0.53
A
B
PFTDTBTzQa
PFTDTBTzQb
1:2
1:2
0.79
0.81
1.59
1.81
39.8
40.7
0.50
0.60
a
No annealing.
Annealing at 90 ꢁC.
b
The HOMOeLUMO energy diagrams of the polymers and
PC71BM are shown in Fig. 4. It is obvious that the HOMO energy
levels of the four polymers are almost identical due to the presence
of the same polyfluorene backbone. However, their LUMO energy
levels are strongly dependent upon the accepting unit. Among the
three accepting units used, the [1e3]triazolo[4,5-g]quinoxaline
based PFTDTBTzQ has the lowest lying LUMO level, due to the
strongest electron-deficiency of this aromatic moiety. Therefore,
PFTDTBTzQ has the narrowest Eg of 1.66 eV and thus much wider
absorption band. The relatively low HOMO levels of polymer do-
nors not only endows polymer with good chemical stability in
ambient conditions but also higher open circuit voltage (Voc) for
OPV devices, as the Voc is proportional to the difference between
the LUMO level of fullerene acceptor and the HOMO level of the
donor [1]. Importantly, the frontier energy levels of the as-prepared
polymers are appropriately aligned with those of PC71BM.
Table 4
Photovoltaic properties of polymer solar cells based PFTODTBT:PC71BM (1:2)
blends.
Cells
Polymer/PC71BM
D/A
Voc [V]
Jsc [mA/cm2]
FF [%]
PCE [%]
A
B
PFTODTBT
PFTODTBT
1:1
1:2
0.76
0.67
5.17
4.54
33.1
31.8
1.30
0.97
of 1:2 showed the best device performance. The PFTDTBTzQ/
PC61BM (1:2, w/w) device gave an open-circuit voltage (Voc) of
0.76 V, a short circuit current (Jsc) of 1.73 mA cmꢀ2, a fill factor (FF)
of 43.3%, and resulted in a power conversion efficiency (PCE) of
0.57% (Fig. 5 and Table 2).
PC71BM was further used to optimize the device performance
because PC71BM had electronic properties similar to those of
PC61BM but possessed a much stronger absorption in the visible
region [33], which could improve the absorption of the donor
material. The current density-voltage (JeV) curves of devices based
on the PFTDTBTzQ/PC71BM blend before and after thermal
annealing are compared in Fig. 6. The polymer/PC71BM (1:2, w/w)
after thermal annealing at 90 ꢁC showed improved device perfor-
mance. The PFTDTBTzQ/PC71BM device gave a Voc of 0.81 V, a Jsc of
1.81 mA cmꢀ2, a FF of 40.7%, and resulted in a PCE of 0.60% (Fig. 6
and Table 3).
Different PFTODTBT/PC71BM weight ratios, such as 1:1, 1:2 have
been investigated to optimize the photovoltaic properties. The
typical device structure is ITO/PEDOT:PSS/polymer:PC71BM/PFN/Al.
The corresponding photovoltaic performances of devices with
different ratios are summarized in Table 4. The polymer/PC71BM
weight ratio of 1:2 showed the best device performance. The
PFTODTBT/PC71BM (1:2, w/w) device gave a Voc of 0.76 V, a Jsc of
5.17 mA cmꢀ2, a FF of 33.1%, and resulted in a PCE of 1.30% (Fig. 7
and Table 4).
3.4. Photovoltaic properties
The photovoltaic property of PFTDTBTzQ was evaluated in BHJ
device by blending PFTDTBTzQ with PC61BM acceptor as active
layer. The photoactive layers were prepared by spin-coating of the
blend solution of PFTDTBTzQ:PC61BM dissolved in o-DCB, in which
PFTDTBTzQ shows good solubility, on ITO/PEDOT:PSS patterned
substrates (40 nm). The thickness of all the blends is around 80 nm.
The active area of the cells is 0.16 cm2. The typical device structure
is ITO/PEDOT:PSS/polymer:PC61BM/PFN/Al. All devices were pre-
pared under an inert nitrogen atmosphere and characterized in air
without encapsulation, where JeV measurements were performed
under simulated AM 1.5 G illumination (100 mW/cm2). Different
PFTDTBTzQ/PC61BM weight ratios, such as 1:1, 1:2, and 1:3 have
been investigated to optimize the photovoltaic properties. The
corresponding photovoltaic performances of devices with different
ratios are summarized in Table 3. The polymer/PC61BM weight ratio
1
0
0
-2
-4
-1
-2
annealing
PFTODTBT:PC BM 1:1
71
no annealing
PFTODTBT:PC BM 1:2
71
-3
-6
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Voltage (V)
Fig. 6. Current density versus voltage (JeV) characteristics of devices based on
PFTDTBTzQ:PC71BM as active layer under AM 1.5 G irradiation with an intensity of
Fig. 7. Current density versus voltage (JeV) characteristics of devices based on
PFTODTBT:PC71BM as active layer under AM 1.5 G irradiation with an intensity of
100 mW cmꢀ2
.
100 mW cmꢀ2
.

4938
J. Hai et al. / Polymer 54 (2013) 4930e4939
Fig. 8. (a) Current density versus voltage (JeV) characteristics of devices using PFTTTz-toluene (device A, B and C) or PFTTTz-THF (device D, E and F):PC71BM (1:2) as active layer
under AM 1.5 G irradiation with an intensity of 49.1 mW cmꢀ2. (b) JeV characteristics of the same devices in the dark.
ITO/PEDOT:PSS/PFTTTz:PC71BM/PFN(5 nm)/Al for device B and E;
Table 5
and ITO/PEDOT:PSS/PFTTTz:PC71BM/PFN(5 nm)/Ca(5 nm)/Al for
device C and F. Typical current density-voltage (JeV) curves for
devices with this blend composition are presented in Fig. 8a for two
polymers.
Photovoltaic properties of polymer solar cells based PFTTTz-toluene or PFTTTz-
THF:PC71BM (1:2) blends.
Cells Polymer/PC71BM Cathode
Voc [V] Jsc [mA/cm2] FF [%] PCE [%]
A
B
C
D
E
F
PFTTTz-toluene
PFTTTz-toluene
PFTTTz-toluene
PFTTTz-THF
PFTTTz-THF
PFTTTz-THF
Al
PFN/Al
PFN/Ca/Al 0.62
Al
PFN/Al
PFN/Ca/Al 0.70
0.70
0.62
1.93
2.17
2.29
2.35
2.44
2.48
54.4
57.7
61.4
57.7
55.6
62.4
1.50
1.58
1.77
1.66
1.77
2.21
Table 5 summarizes the photovoltaic performance of BHJ device
of PFTTTz under the same device fabrication condition. Obvious
impact of cathode interfacial layer was observed on the device per-
formance. The devices of PFTTTz-toluene achieved an increased PCE
from 1.50% (device A), 1.58% (device B) to 1.77% (device C) after
inserting the cathode interfacial layer Ca or PFN. On the other hand,
the devices with high Mn PFTTTz-THF achieved an increased PCE
from 1.66% (device D), 1.77% (device E) to 2.21% (device F) after
inserting the cathode interfacial layer Ca or PFN. Comparing device A,
B, C, D, E, and F, we could conclude that a PFN/Ca/Al trilayer cathode
is better than a PFN/Al bilayer cathode and an Al single layer cathode.
Cathode interfacial layer PFN could establish better interfacial con-
tacts by decreasing the series resistance, resulting in enhanced
electron collection at the cathode and decreasing the possibility of
holeeelectron recombinations in the active layer [48]. A bilayer
cathode or a trilayer cathode could increase the device configura-
tions. The best performing device F in the preliminary test exhibited
a maximum PCE value of 2.21%, with a Voc of 0.70 V, a Jsc of 2.48 mA/
cm2, and a FF of 0.62. Efficient charge collection in the case of
PFTTTz-THF:PC71BM (device F) would be consistent with the slightly
larger dark current for this blend than for any of the others (Fig. 8b).
The performance of PFTTTz-THF:PC71BM (device D, E, F) is relatively
better than that of PFTTTz-toluene:PC71BM (device A, B, C). It obvi-
ously deduced that higher Mn of polymer has positive effect on de-
vice performance through increasing Voc, Jsc and FF.
0.60
0.64
The photovoltaic properties of PFTTTz-THF and PFTTTz-toluene
were evaluated in BHJ device by blending PFTTTz-THF or PFTTTz-
toluene with PC71BM acceptor as active layer. PC71BM was chosen
due to its similar electronic properties to PCBM but a much stronger
absorption in the visible region. The photoactive layers were pre-
pared by spin-coating of the blend solution of polymer:PC71BM
(1:2, w/w) dissolved in o-DCB, in which PFTTTz-THF (device D, E
and F) or PFTTTz-toluene (device A, B and C) shows good solubility,
on ITO/PEDOT:PSS patterned substrates (40 nm). The thickness of
all the blends is around 80 nm. The active area of the cells is
0.16 cm2. All devices were prepared under an inert nitrogen at-
mosphere and characterized in air without encapsulation, where Je
V measurements were performed in the dark and under simulated
AM 1.5 G illumination (49.1 mW/cm2). The effect of insertion of
PFN or Ca interfacial layer on device performance was studied.
And thus three device structures (Fig. 8 and Table 5) were
explored, i.e. ITO/PEDOT:PSS/PFTTTz:PC71BM/Al for device A and D;
Fig. 9. AFM image of the surface morphology of (a) PFTTTz-toulene:PC71BM (1:2) thin film and (b) PFTTTz-THF:PC71BM (1:2) thin film.

J. Hai et al. / Polymer 54 (2013) 4930e4939
4939
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A new electron-donating unit, thiophenyl-methylene-9H-fluo-
rene, was developed and used for the design of D-A polymer for PSC
application for the first time. By conjugated thiophene with flour-
ene unit, thiophenyl-methylene-9H-fluorene shows more planar
conformation and electron donating ability. By alternating thio-
phenyl-methylene-9H-fluorene with thiazolo[5,4-d]thiazole (TTz),
benzothiadiazole (ODTBT), and [1e3]triazolo[4,5-g]quinoxaline
(DTBTzQ) the narrow bandgap copolymers, PFTTTz-THF, PFTTTz-
toluene, PFTODTBTand PFTDTBTzQ exhibited and ideal bandgap of
1.66e1.96 eV. Primary device tests on these copolymers delivered a
best PCE of 2.21%. Our research results suggest that thiophenyl-
methylene-9H-fluorene unit can be a choice of new donor to
construct D-A polymer for fine-tuning photophysical, electro-
chemical and photovoltaic properties of the resulted polymers.
Acknowledgments
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We gratefully acknowledged the financial support from the
National Natural Science Foundation of China (Grant No. 21074055,
61006014 and 51225301), Program for New Century Excellent
Talents in University (NCET-12-0633), Doctoral Fund of Ministry of
Education of China (No. 20103219120008), and the Fundamental
Research Funds for the Central Universities (30920130111006 and
30920130111008).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.07.001.
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