Low band-gap D-A conjugated copolymers based on anthradithiophene and diketopyrrolopyrrole for polymer solar cells and field-effect transistors

Article abstract of DOI:10.1002/pola.27163

Two conjugated copolymers PADT-DPP and PADT-FDPP based on anthradithiophene and diketopyrrolopyrrole, with thiophene and furan as the π-conjugated bridge, respectively, were successfully synthesized and characterized. The number-averaged molecular weights of the two polymers are 38.7 and 30.2 kg/mol, respectively. Polymers PADT-DPP and PADT-FDPP exhibit broad absorption bands and their optical band gaps are 1.44 and 1.50 eV, respectively. The highest occupied molecular orbital energy level of PADT-DPP is located at -5.03 eV while that of PADT-FDPP is at -5.16 eV. In field-effect transistors, PADT-DPP and PADT-FDPP displayed hole mobilities of 4.7 × 10^-3 and 2.7 × 10^-3 cm²/(V s), respectively. In polymer solar cells, PADT-DPP and PADT-FDPP showed power conversion efficiency (PCE) of 3.44% and 0.29%, respectively. Atomic force microscopy revealed that the poor efficiency of PADT-FDPP should be related to the large two-phase separation in its active layer. If 1,8-diiodooctane (DIO) was used as the solvent additive, the PCE of PADT-DPP remained almost unchanged due to very limited morphology variation. However, the addition of DIO could remarkably elevate the PCE of PADT-FDPP to 2.62% because of the greatly improved morphology. Our results suggest that the anthradithiophene as an electron-donating polycyclic system is useful to construct new D-A alternating copolymers for efficient polymer solar cells.

Full text of DOI:10.1002/pola.27163

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Low Band-Gap D–A Conjugated Copolymers Based on Anthradithiophene
and Diketopyrrolopyrrole for Polymer Solar Cells and Field-Effect
Transistors
Jun Huang, Yongxiang Zhu, Lianjie Zhang, Ping Cai, Xiaofeng Xu, Junwu Chen, Yong Cao
Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China
University of Technology, Guangzhou 510640, People’s Republic of China
Correspondence to: L. Zhang (E-mail: lianjiezhang@scut.edu.cn) or J. Chen (E-mail: psjwchen@scut.edu.cn)
Received 27 January 2014; accepted 11 March 2014; published online 25 March 2014
DOI: 10.1002/pola.27163
ABSTRACT: Two conjugated copolymers PADT-DPP and PADT-
FDPP based on anthradithiophene and diketopyrrolopyrrole,
with thiophene and furan as the p-conjugated bridge, respec-
tively, were successfully synthesized and characterized. The
number-averaged molecular weights of the two polymers are
38.7 and 30.2 kg/mol, respectively. Polymers PADT-DPP and
PADT-FDPP exhibit broad absorption bands and their optical
band gaps are 1.44 and 1.50 eV, respectively. The highest occu-
pied molecular orbital energy level of PADT-DPP is located at
25.03 eV while that of PADT-FDPP is at 25.16 eV. In field-effect
transistors, PADT-DPP and PADT-FDPP displayed hole mobili-
ties of 4.7 3 1023 and 2.7 3 1023 cm2/(V s), respectively. In
polymer solar cells, PADT-DPP and PADT-FDPP showed power
conversion efficiency (PCE) of 3.44% and 0.29%, respectively.
Atomic force microscopy revealed that the poor efficiency of
PADT-FDPP should be related to the large two-phase separa-
tion in its active layer. If 1,8-diiodooctane (DIO) was used as
the solvent additive, the PCE of PADT-DPP remained almost
unchanged due to very limited morphology variation. How-
ever, the addition of DIO could remarkably elevate the PCE of
PADT-FDPP to 2.62% because of the greatly improved mor-
phology. Our results suggest that the anthradithiophene as an
electron-donating polycyclic system is useful to construct new
D–A alternating copolymers for efficient polymer solar cells.
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Chem. 2014, 52, 1652–1661
KEYWORDS: anthradithiophene; charge transport; conjugated
polymers; diketopyrrolopyrrole; macrocycles; polymer solar
cells
Planar polycyclic aromatics are favored to be as D-type units
because of their good planarity and extended p-conjugated
length, which are beneficial for p–p stacking between poly-
mer main chain, charge separation, and transport as well as
tunable energy levels. One of the famous tricyclic aromatic
rings is benzodithiophene (BDT) unit, which has been
attracted a great deal of attention. BHJ PSCs with BDT-based
copolymers as the polymer donors have shown high power
conversion efficiencies (PCEs) over 8%.15–23 In addition to
the tricyclic ring system, the larger p-conjugated tetracyclic
aromatics are also of great interest to researchers. For
instance, Peng’s group24 synthesized the copolymer based on
naphthodithiophene (NDT), a tetracyclic donor unit, and
achieved high efficiency of 6.92%.
INTRODUCTION The development of low band-gap conju-
gated polymer donor plays an important role in the realiza-
tion of bulk heterojunction (BHJ) polymer solar cells (PSCs)
with high efficiency as an economically viable source of
renewable energy1–4 and organic field-effect transistor
(OFET) with high mobility to apply in display technology.5,6
Combination of an electron-donating (D) unit with an
electron-accepting (A) heterocycle along the polymer back-
bone has been proven to be an effective strategy to exploit
new low band-gap conjugated polymer.7,8 Generally, the high-
est occupied molecular orbital (HOMO) energy level of a D–A
copolymer is mainly determined by D-type moiety and its
lowest unoccupied molecular orbital (LUMO) energy level is
dominantly attributed to A-type moiety.9–11 As a conse-
quence, alternating hybridization of a strong D-type unit and
a strong A-type unit could give rise to narrow band gap D–A
copolymer to match solar spectrum and then could achieve
high current. Moreover, the D–A conjugated structure is also
benefit to achieve a high mobility.5,12–14
With the success example of NDT, the property of new poly-
cyclic aromatic ring is greatly encouraged to be studied. Sev-
eral groups focus on the opto-electronic performance based
on pentacyclic aromatic ring, among which the anthradithio-
phene (ADT) unit has been investigated as a building block
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SCHEME 1 Synthesis routes for polymers PADT-DPP and PADT-FDPP.
of small molecular semiconducting materials for OFETs25–29
and photovoltaic cells.25,30–32 Katz group28 first reported the
field-effect mobilities of ADT-based small molecules, which
were in the range of 0.01–0.02 cm2/(V s). Recently, the hole
mobility of ADT-based small molecule was achieved as high
as 5.4 cm2/(V s) by Anthony and Jurchescu29 through the
replacement of a trialkylsilyl substituent with a trialkyl-
germyl group. Malliaras’s group25 modified the structure of
triethylsilyl ADT-based donor for solar cells and the resulting
PCE was only 1%. It is of interest to introduce ADT as a
building block for novel conjugated polymers. In a report by
Bao’s group,30 two ADT-based copolymers comprising cyclo-
pentadithiophene and dialkylfluorene were synthesized. The
two polymers showed OFET hole mobility as high as 0.01
cm2/(V s), however, the highest PCE in BHJ solar cells was
merely 0.94%. Furthermore, Bao’s group31 reported a conju-
gated copolymer based on 5,11-diethynyl-ADT and diketopyr-
rolopyrrole. The polymer exhibited enhanced thin-film order
and high OFET hole mobility of 0.1 cm2/(V s) was achieved.
Recently, Choi’s group32 reported a group of ADT-based
copolymers. The alternating copolymer based on ADT and
thieno[3,4-b]thiophene (TT) displayed a not high PCE of
1.5% in a solar cell, obviously lower than those achieved
with terpolymers based on ADT, BDT, and TT. So far, it is a
challenging work to realize an efficient solar cell with ADT-
based alternating copolymer.
as A-unit (Scheme 1). The diketopyrrolopyrrole core is
widely considered as a strong acceptor, which has exhib-
ited great potential in polymer materials of high effi-
ciency8 or high OFET mobility.5 The two ADT-based
polymers, named as PADT-DPP and PADT-FDPP, comprised
thiophene and furan as the p-conjugated bridges, respec-
tively, so as to study the spacer effect of heterocycles in
the conjugated polymers. PADT-DPP and PADT-FDPP
exhibit broad absorption bands with absorption edges
more than 825 nm. The two polymers possessed OFET
hole mobilities of 1023 cm2/(V s). When measured by
space-charge-limited current (SCLC) method, the two poly-
mers exhibited hole mobilities at a level of 1024 cm2/(V s),
suggesting fairly good hole transport property. With the poly-
mers as donors in BHJ solar cells, PCEs up to 3.46% were
achieved.
EXPERIMENTAL
Materials
All reagents solvents, unless otherwise specified, were
obtained from Aldrich, Acros, and TCI Chemical and were
used as received. Anhydrous tetrahydrofuran was distilled
over sodium/benzophenone under N2 prior to use. All manip-
ulations involving air-sensitive reagents were performed
under an atmosphere of dry argon. 3,6-Bis(5-bromothien-2-
yl)-2,5-bis(2-octyldodecanyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione (DPP) and 3,6-bis(5-bromofuran-2-yl)-2,5-bis(2-octyl-
dodecanyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (FDPP) were
prepared similarly according to literatures.33,34
In this work, two new ADT-based conjugated copolymers
with a D–A alternating skeleton were designed, in which
the ADT was used as the D-unit and diketopyrrolopyrrole
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Instrumentations
278 ꢀC, then n-BuLi (2.5 M in THF, 1.1 mmol) was added
dropwise into the solution. Keeping the mixture stirring at
278 ꢀC for 1 h, then warmed to room temperature and
stirred for another 1 h. A Me3SnCl (1 M in THF, 1.1 mmol)
solution was added in one portion at 278 ꢀC. The mixture
was kept at 278 ꢀC for 1 h and then stirred for overnight.
Subsequently, 10 mL water was added, and extracted with
CH2Cl2. Then, the organic layer was dried over MgSO4 and
concentrated by vacuum. The crude product was recrystal-
lized by ethanol two times and finally afforded red-needle
crystals (353 mg, 78%).
1H NMR spectra were recorded on a Bruker AV 300 spectrom-
eter with tetramethylsilane (TMS) as the internal reference.
Molecular weights of the polymers were obtained on a Waters
GPC 2410 using a calibration curve of polystyrene standards,
with tetrahydrofuran as the eluent. Elemental analyses were
performed on a Vario EL elemental analysis instrument (Ele-
mentar). UV–vis absorption spectra were recorded on an HP
8453 spectrophotometer. Cyclic voltammetry was performed
on a CHI660A electrochemical work station with platinum
electrodes at a scan rate of 50 mV/s against an Ag/Ag1 refer-
ence electrode with a nitrogen-saturated solution of 0.1 M tet-
1H NMR (CDCl3, 300 MHz), d (ppm): 8.78 (d, 4H), 7.50 (s,
2H), 4.24 (t, 4H), 2.12 (t, 4H), 1.71 (t, 4H), 1.71–1.28 (m,
32H), 0.88 (t, 6H), 0.48 (s, 18H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 146.82, 144.87, 144.72, 141.44, 141.27, 140.02,
139.94, 131.82, 123.08, 122.66, 122.32, 115.38, 114.20,
31.95, 30.78, 29.75, 29.70, 29.40, 26.30, 22.72, 14.15, 28.44.
rabutylammonium
hexafluorophosphate
(Bu4NPF6)
in
acetonitrile. Potentials were referenced to the ferrocenium/
ferrocene couple using ferrocene as an internal standard. The
deposition of a copolymer on the electrode was done by the
evaporation of a dilute THF solution. Tapping-mode atomic
force microscopy (AFM) images were obtained using a Nano
Scope NS3A system (Digital Instrument) to observe the sur-
face morphology of active layers of polymer/PC71BM blends.
Polymerization
Both of the two polymers were performed by palladium(0)-
catalyzed Stille polycondensation reactions with equivalently
molar ratio of a bis(trimethylstannyl)-substituted monomer
to the dibromo monomer under argon protection. The purifi-
cation of the polymers was conducted in air. The preparation
of PADT-DPP and PADT-FDPP was performed according to
the same procedures as follows.
1,7-Dithia-dicyclopenta[b,i]anthracene-5,11-dione (1)
1,4-Cyclohexanedione (0.84 g, 7.5 mmol) was added to a
solution of 2,3-thiophene dicarbaldehyde (2.1 g, 15 mmol) in
ethanol (100 mL). Fifteen percentage of potassium hydroxide
solution (6 mL) was added dropwise and then precipitate
was formed. After stirred for 4 h at room temperature, the
light yellow precipitate was obtained by filtration and
washed several times by ethanol in 99% yield, without fur-
ther purification for next step.
Polymer PADT-DPP
Compound 3 (200 mg, 0.229 mmol) and 3,6-bis(5-bromothien-
2-yl)-2,5-bis(2-hexyldecanyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione (DPP) (207 mg) were dissolved in a mixture of 12 mL
chlorobenzene and 2 mL DMF in a flask under argon. The
solution was flushed by argon for 20 min, then Pd(PPh3)4
(5%) was added. After 20 min flushed by argon, the reaction
mixture was heated to 120 ꢀC slowly. The solution was
stirred for 24 h under argon atmosphere. Then, the solution
was cooled down to room temperature and poured into 500
mL methanol. The precipitated solid was placed in a Soxhlet
thimble, and extracted consecutively with methanol, ethyl
acetate, and chlorobenzene. The chlorobenzene fraction was
concentrated and poured into methanol. The title polymer
was obtained as dark green solid and the yield was 70%.
High temperature gel permeation chromatography (GPC)
(1,2,4-trichlorobenzene, 140 ꢀC): Mn 38.7 kg/mol; Mw/Mn
3.14 (Table 1). ELEM. ANAL. calcd. for (C80H112N2O4S4)n: C,
71.44; H, 8.26; N, 2.06. Found: C, 74.25; H, 8.72; N, 2.16. The
solubility of the polymer in deuterated chloroform or deuter-
ated o-dichlorobenzene was too low to obtain reliable NMR
data.
5,11-Bis(octyloxy)anthra[2,3-b:6,7-b0]dithiophene (2)
Compound 1 (0.5 g, 1.56 mmol), sodium hyposulfite (2.71 g,
15.6 mmol), and tetrabutylammonium bromide (0.08 g, 0.25
mmol) were put into a 50 mL flask. Under the protection of
nitrogen, 20 mL CH2Cl2 and 5 mL H2O were added by syringe.
After stirred for 15 min, potassium hydroxide (1.74 g, 31.2
mmol) in 3 mL water was added dropwise. Keeping stirring
for 5 min, 1-bromooctane (3.9 g, 15.6 mmol) was added in
one portion. The mixture was stirred at room temperature for
5 h, and zinc powder (0.51 g, 7.8 mmol) was added carefully.
ꢀ
Then, the reaction solution was refluxed at 50 C for 3 days.
The reactant was extracted by 150 mL CH2Cl2 and dried with
MgSO4. After removing the solvent in vacuum, purification
was performed via silica gel column chromatography, using
CH2Cl2:petroleum ether 5 1:4 as the eluent. Red solid (0.39 g)
of compound 2 was obtained in 38% yield.
1H NMR (CDCl3, 300 MHz), d (ppm): 8.81 (s, 4H), 7.49 (d,
2H), 7.42 (d, 2H), 4.26 (t, 4H), 2.15 (t, 4H), 1.72 (t, 4H),
1.54–1.22 (m, 32H), 0.88 (t, 6H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 138.80, 137.70, 129.02, 128.91, 125.19, 123.75,
123.01, 116.61, 115.29, 63.09, 32.80, 31.90, 31.79, 30.76,
29.59, 29.38, 29.26, 26.27, 25.72, 22.70, 14.13.
Polymer PADT-FDPP
This polymer was synthesized as the same procedure of
PADT-DPP, using 3,6-bis(5-bromofuran-2-yl)-2,5-bis(2-hexylde-
canyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (FDPP) as the
dibromo monomer and the yield ꢀwas 85%. High temperature
GPC (1,2,4-trichlorobenzene, 140 C): Mn 30.2 kg/mol; Mw/Mn
4.12 (Table 1). ELEM. ANAL. calcd. for (C80H112N2O6S2)n: C,
75.70; H, 8.56; N, 2.26. Found: C, 76.14; H, 8.95; N, 2.22.
5,11-Bis(octyloxy)-2,8-bis(trimethyltin)anthra[2,3-b:6,7-
b0]dithiophene (3)
Compound 2 (300 mg, 0.46 mmol) was dissolved into 40 mL
fresh THF under the nitrogen. The mixture was cooled to
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TABLE 1 Polymerization Yields, Molecular Weights, and
Decomposition Temperatures of PADT-DPP and PADT-FDPP
a
Yield
(%)
Mn
Td
(ꢀC)b
a
Polymer
(kg/mol)
Mw/Mn
PADT-DPP
70
85
38.7
30.2
3.14
4.12
319
298
PADT-FDPP
a
Estimated by GPC at 150 ꢀC with 1,2,4-trichlorobenzene as the eluent.
Temperature for 5% weight loss measured by TGA at a heating rate of
b
20 ꢀC/min under nitrogen.
The solubility of the polymer in deuterated chloroform or
deuterated o-dichlorobenzene was too low to obtain reliable
NMR data.
FIGURE 2 UV absorption spectra of PADT-DPP and PADT-FDPP
in chlorobenzene.
Fabrication and Charaterization of Solar Cells
controlled atmosphere of nitrogen dry box (Vacuum Atmos-
phere) containing less than 10 ppm oxygen and moisture.
The power conversion efficiencies of the resulting polymer
solar cells were measured under 1 sun, AM 1.5G (air mass
1.5 global) spectrum from a solar simulator (Oriel model
91192) 100 mW/cm2. The current density2voltage (J2V)
characteristics were recorded with a Keithley 2410 source
unit. The external quantum efficiencies of the conventional
solar cells were measured with a commercial photo modula-
tion spectroscopic setup, including a xenon lamp, an optical
chopper, a monochromator, and a lock-in amplifier operated
by a PC computer, and a calibrated Si photodiode was used
as a standard.
Patterned indium tin oxide (ITO)-coated glass with a sheet
resistance of 15220 ohm/square was cleaned by a surfac-
tant scrub and then underwent a wet-cleaning process inside
an ultrasonic bath, beginning with deionized water, followed
by acetone and 2-propanol. After oxygen plasma cleaning for
5 min, a 40-nm-thick poly(3,4-ethylenedioxythiophene):po-
ly(styrenesulfonate) (PEDOT:PSS) (Bayer Baytron 4083)
anode buffer layer was spin-cast onto the ITO substrate and
then dried by baking in a vacuum oven at 80 ꢀC overnight.
The active layer based on ADT-based polymer: PC71BM 5
1:2, with a thickness of 85 nm, was then deposited on top of
the PEDOT:PSS layer, by casting from a chlorobenzene solu-
ꢀ
tion, and drying at 70 C for 10 min. A DMF solution of PC-
EP wasꢀspin-coated on the top of the active layer and dried
at 120 C for 10 min to form a thin cathode interlayer of 10
nm. The thickness of the PEDOT:PSS and the active layer
was verified by a surface profilometer (Tencor, Alpha-500).
Determination of the thickness of the interlayer followed a
previously published paper.10 Finally, a 100 nm aluminum
layer was evaporated with a shadow mask. The overlapping
area between the cathode and the anode defined a pixel size
of 0.16 cm2. Except for the deposition of the PEDOT:PSS
layers, all the fabrication processes were performed inside a
FET Fabrication and Characterization
To measure the hole mobilities of the polymers, FETs were
fabricated in a top contact geometry using silver as the
source and drain electrode. Highly n-doped silicon and ther-
mally grown silicon dioxide (300 nm) were used as the back
gate and gate dielectric, respectively. Octyltrichlorosilane
(OTS) was then used for surface modification of the gate
dielectric layer. The copolymers films (60 nm) were spin-
coated on OTS treated substrates from chlorobenzene solꢀu-
tion (18 mg/mL) at 2000 rpm, and then annealed at 100 C
FIGURE 1 TGA curves of PADT-DPP and PADT-FDPP with a
FIGURE 3 UV absorption spectra of PADT-DPP and PADT-FDPP
heating rate of 20 ꢀC/min under nitrogen.
films.
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TABLE 2 Absorption Maxima, Optical Band Gaps, and Electrochemical Properties of the PADT-DPP and PADT-FDPP
HOMOd (eV)
LUMOe (eV)
a
b
c
Polymer
kmax-sol (nm)
kmax-film (nm)
Eg (eV)
Eox (V)
PADT-DPP
775
745
767
751
1.44
1.50
0.23
0.36
25.03
25.16
23.59
23.66
PADT-FDPP
a
d
Chlorobenzene solutions.
Calculated according to HOMO 5 2e(Eox14.8).
Calculated from the HOMO and optical band gap.
b
c
e
Thin solid film casted from chlorobenzene.
Optical band gap estimated with the absorption edge of thin solid
film.
for 10 min. Then silver film (50 nm) was deposited under
vacuum as the source and drain electrodes. The width to
length ratio (W/L) of the FET devices is 100/1. The OFETs
characterizations were performed in the atmosphere using a
probe station and a semiconductor parameter analyzer (Agi-
lent 4155C). Then field-effect mobility was calculated from
the standard equation for saturation region in metal-dioxide-
semiconductor field effect transistors: IDS 5 (W/2L)lCi(VG 2
VT)2, where IDS is the drain-source current, l is the field-
effect mobility, W and L are the channel width and length, Ci
is the capacitance per unit area of the dielectric layer (Ci 5
11 nF/cm2), VG is the gate voltage, and VT is the threshold
voltage.
exhibit high hole mobility up to 5.4 cm2/(V s).29 Then the
dione compound 1 was reacted with sodium hyposulfite, fol-
lowed by alkylation of 1-bromooctane to afford the dioctoxy
compound 2. It is worth noting that the subsequent use of
zinc dust as the second reductant can result in a modified
yield of 38%. In the last step, the monomer 3 could be read-
ily afforded, and the pure powder of monomer 3 was
obtained in a good yield of 78%, after twice recrystallized
from ethanol.
The DPP and FDPP monomers were prepared according to
those in literatures.33,34 Two 2-hexyl dodecanyl side chains
of a branched structure were attached on the diketopyrrolo-
pyrrole unit so as to impart the target polymers with enough
solubility. The alternating copolymers PAD-DPP and PADT-
FDPP were performed by palladium(0)-catalyzed Stille poly-
condensation reactions (Scheme 1) and their corresponding
polymerization yields are summarized in Table 1, with the
values of 70% and 85%, respectively.
SCLC Measurement
Hole-only devices were fabricated to measure the hole
mobility using SCLC method with a device configuration of
ITO/PEDOT:PSS/Copolymer:PC71BM/MoO3/Al. The mobility
was determined by fitting the dark current to the model of a
single carrier SCLC, described by the equation:
Two polymers can be easily dissolved in warm chloroben-
zene (CB) and o-dichlorobenzene. Their molecular weights
were performed by GPC at 150 ꢀC with 1,2,4-trichloroben-
zene as the eluent (Table 1). The Mn values of PADT-DPP and
PADT-FDPP are 38.7 and 30.2 kg/mol, with polydispersity
index (Mw/Mn) of 3.14 and 4.12, respectively. Thermal stabil-
ity was investigated by thermogravimetric analysis (TGA), as
shown in Figure 1. PADT-DPP showed decomposition tem-
peratures at 319 ꢀC while PADT-FDPP has a slightly lower
9
V2
h d3
J5 e0erl
;
8
where J is the current density, lh is the mobility under zero
field, E0 is the permittivity of free space, Er is the material
relative permittivity, d is the active layer thickness, and V is
the effective voltage. The effective voltage can be obtained
by subtracting the built-in voltage (Vbi) and the voltage drop
(Vs) from the substrate’s series resistance from the applied
voltage (Vappl), V 5 Vappl 2 Vbi 2 Vs. The hole-mobility can
be calculated from the slope of the J1/22V curves.
RESULTS AND DISCUSSION
Synthesis of Monomers and Polymers
As depicted in Scheme 1, the bis(trimethylstannyl)anthradi-
thiophene monomer 3 could be easily synthesized by three
steps that were reported by Choi and coworkers.32 We modi-
fied some of the procedures, from which higher yields could
be achieved. For the step one, 1,4-cyclohexanedione and 2,3-
thiophene dialdehyde were underwent a cyclization reaction
for a longer time of 4 h under the basic condition (15% KOH
solution), which formed the dione compound 1 with a per-
fect yield of 99%. It should be noted that there were two
isomers of compound 1 mixed together and it was difficult
to separate them from each other.25,28 However, in previous
reports, ADT-based small molecules of isomer nature could
FIGURE 4 Cyclic voltammograms of PADT-DPP and PADT-
FDPP films.
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FIGURE 5 The FET output characteristics of (a) PADT-DPP and (c) PADT-FDPP and transfer characteristics of (b) PADT-DPP and
(d) PADT-FDPP.
decomposition temperature at 298 ꢀC. No peaks associated
with crystallization/melting transitions of the polymers were
detected by differential scanning calorimetry (DSC) analyses.
absorption peaks for the polymer films do not show obvious
shifts. However, the films of the both polymers exhibit remark-
ably enhanced absorptions of shoulder peaks at 694 nm for
PADT-DPP and 677 nm for PADT-FDPP. Overall, for the solution
or the film, PADT-DPP with thiophene spacers can display a
broader absorption of the second band than that of PADT-FDPP
with furan spacers. The optical band gaps (Eg) for PADT-DPP
and PADT-FDPP were calculated from the onset wavelengths of
the film absorption spectra, which are 1.44 and 1.5 eV, respec-
tively (Table 2).
UV–Vis Absorption and Energy Level
The UV–vis absorption spectra of PADT-DPP and PADT-
FDPP in CB solutions are shown in Figure 2. Both the poly-
mers display two absorption bands, similar to many D–A
conjugated copolymers. The peaks for the first band are
located at 385 and 392 nm for PADT-DPP and PADT-FDPP,
respectively. The second band at the long-wavelength range,
with the absorptions peaks at 775 for PADT-DPP and 745
nm for PADT-FDPP, should be attributed to intramolecular
charge transfer (ICT) of the D–A conjugated polymers.24
The extent of ICT transition is an indirect parameter to
assess the electron-donating ability of a donor unit.35 The
ratio of the maximum absorbance of the first band to the
second band in PADT-DPP is 0.64, which is higher than that
in PADT-FDPP with a value of 0.45. The results indicate
that furan as spacer could enhance the electron-donating
ability of the ADT unit in D–A copolymer, which in accord-
ance with the electron-donating features on benzodifuran
unit.36
The HOMO levels of the copolymers (Table 2) were
obtained from the onsets of the oxidation potentials during
the cyclic voltammetry (CV) measurement. The correspond-
ing CV spectra are shown in Figure 4. Polymers PADT-DPP
and PADT-FDPP showed reversible oxidations during the CV
scans. The HOMO level of PADT-DPP is at 25.03 eV, while
that of PADT-FDPP have a slightly lower HOMO level of
25.16 eV. The results indicate that the HOMO levels of the
ADT-based polymers are mainly determined by the ADT
unit and the thiophene spacers result in a higher HOMO
level if compared with the furan spacers. The LUMO levels
of the ADT-based copolymers are given from the corre-
sponding optical band gaps and HOMO levels. The calcu-
lated LUMO level for PADT-DPP is at 23.59 eV, which is
slightly higher than that of PADT-FDPP (23.66 eV).
The absorption spectra of polymer films are shown in Figure 3.
In comparison to the absorptions in the CB solutions, the
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TABLE 3 OFET Hole Mobilities and Space-Charge-Limited Cur-
rent Mobilities of PADT-DPP and PADT-FDPP
Polymer
lFET [cm2/(V s)]
lSCLC [cm2/(V s)]
PADT-DPP
4.7 3 1023
2.7 3 1023
4.98 3 1024
4.40 3 1024
PADT-FDPP
Field-Effect Transistor and SCLC Measurement
Several ADT-based small molecules have exhibited high hole
mobilities in OFETs.27,29 To measure OFET hole mobilities of
PADT-DPP and PADT-FDPP, we fabricated OFET devices with
a top contact configuration, where PADT-DPP or PADT-FDPP
film (60 nm) was spin-coated on an octyltrichlorosilane
(OTS)-treatedꢀ SiO2 gate insulator. The polymer films were
dried at 100 C for 10 min and then silver film (50 nm) was
deposited under vacuum as the source and drain electrodes.
The width to length ratio (W/L) of the FET devices is 100/1.
The main reason for using silver as the electrode metal is
due to its great potential in printable electronics: solution-
processable silver nanoparticle ink can be sintered at a low
temperature of 100 ꢀC or less.37 Moreover, silver is a much
cheaper metal than gold, a widely used electrode metal for
OFET devices. In a recent report, an excellent polymer hole
mobility of 1.92 cm2/(V s) was achieved in OFET devices
with silver electrodes.38
FIGURE 6 SCLC curves of the hole-only devices based on pris-
tine films of PADT-DPP and PADT-FDPP.
cm2/(V s) for PADT-FDPP, indicating that the two polymers
are fairly good for the charge transport in polymer solar
cells.
Photovoltaic Performance
Bilayer cathode consisting of a hydrophilic conjugated poly-
mer and a high work-function metal electrode has shown
great potential to improve the device performances, where
open-circuit voltage (Voc), short-circuit current density (Jsc),
and fill factor (FF) could be increased simultaneously in
some cases.10,39 2,7-Carbazole-1,4-phenylene-based alternat-
ing copolymer (PCP-EP) with phosphonate end groups as
pendants has exhibited good modification effects as cathode
interlayer in light-emitting diodes and solar cells.35,40 Here,
we also selected the PCP-EP/Al as the bilayer cathode for
the constructions of solar cells with a device configuration of
ITO/PEDOT:PSS (40 nm)/(polymer:PC71BM 5 1:2) (80 nm)/
PCP-EP (10 nm)/Al (100 nm). The blend ratio of 1:2 for the
active layer could show the best efficiency for the two D–A
polymers PADT-DPP and PADT-FDPP.
The output and transfer characteristics of the OFETs are
shown in Figure 5. At different gate voltages (VG), the drain
current (ID) of the devices could reach saturation along with
the drain voltage (VD) [Fig. 5(a,c)]. From the slopes of the
curves of (ID)1/2 versus VG [Fig. 5(b,d)], the calculated hole
mobilities (lFET) are 4.7 3 1023 and 2.7 3 1023 cm2/(V s)
for PADT-DPP and PADT-FDPP, respectively (Table 3), and the
corresponding on/off current ratios both are 1 3 103. The
somewhat low on/off current ratios of the OFET devices may
be related to the silver source and drain electrodes, which
may result in contact resistance due to its lower work func-
tion than that of gold. The lFET values of PADT-DPP and
PADT-FDPP also suggest that the thiophene spacers can sup-
ply a higher mobility in the resulting ADT-based polymer if
compared with the furan spacers.
ADT-based small molecules have exhibited high hole mobility
up to 5.4 cm2/(V s)29 because of the forming of ordered
film. Relatively, the much lower lFET values for PADT-DPP
and PADT-FDPP in this work imply that the films of PADT-
DPP and PADT-FDPP are of poor order. Fine tuning of the
side chains of the ADT-based polymers would be useful to
improve their hole mobilities.
OFET performance largely relies on the lateral mobility of a
semiconducting film. However, carrier transport in polymer
solar cells is more related to the transportation in vertical
direction, which could be obtained by SCLC. Hole-only devi-
ces were fabricated to measure the SCLC hole mobility
(lSCLC) for the pristine polymer films (Fig. 6). The resulting
data are listed in Table 3. PADT-DPP shows a lSCLC of 4.98 3
1024 cm2/(V s), slightly higher than that of 4.40 3 1024
FIGURE 7 J–V characteristics of polymer solar cells as meas-
ured under illumination of AM 1.5G simulated solar light at
100 mW/cm2.
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TABLE 4 Photovoltaic Parameters of Polymer Solar Cells Based on PADT-DPP and PADT-FDPP as Measured Under Irradiation of
AM 1.5G at 100 mW/cm2
Polymer
DIO
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEa (%)
PADT-DPP
No
3%
No
3%
0.64
0.64
0.57
0.63
8.73
9.74
1.44
7.61
61.6
55.5
35.3
54.6
3.44 (3.33)
3.46 (3.37)
0.29 (0.26)
2.62 (2.56)
PADT-FDPP
a
Data in the parentheses are the averaged values based on over five devices.
The measurements of the photovoltaic performances of the
ADT-based copolymers as donors in the BHJ PSCs were per-
formed under illumination of AM 1.5G simulated solar light
at 100 mW/cm2. The current density–voltage (J–V) charac-
teristics of the BHJ PSCs are shown in Figure 7. Their solar
cell parameters are listed in Table 4. The device based on
polymer PADT-DPP exhibited a not high open-circuit voltage
(Voc) of 0.64 V, which was generally agreement with its
HOMO level. The measured short-circuit current (Jsc) and fill
factor (FF) of the device were 8.73 mA/cm2 and 61.6%,
respectively. The resulting PCE of 3.44% was achieved. The
average PCE for PADT-DPP was 3.33%. The device based on
PADT-FDPP showed a lower Voc of 0.57 V, a very bad Jsc of
1.44 mA/cm2, and a low FF of 35.3%, giving a very poor
PCE of 0.29%. The first furan-based D–A copolymer in the
34
ꢀ
report by Frechet et al. also suffered from a low PCE of
0.86% (PC71BM as the acceptor), but a solvent additive could
improve its efficiency by more than fivefold. In this work, we
used 1,8-diiodooctane (DIO) as the additive and the corre-
sponding results were also listed in the Table 4. As expected,
the performance of PADT-FDPP-based device can be
improved greatly, with an elevated Voc of 0.63 V, a larger Jsc
of 7.61 mA/cm2, and a higher FF of 54.6%, so the resulting
PCE (2.62%) is improved by ninefold. Nevertheless, the pho-
tovoltaic performance of PADT-DPP was practically not
changed in the presence of the DIO additive because of
unchanged Voc, slightly increased Jsc, and slightly decreased
FF. We tried 1-chloronaphthalene (CN) as the solvent
FIGURE 8 Tapping-mode AFM images of (a) PADT-DPP, (b) PADT-DPP13% DIO, (c) PADT-FDPP, and (d) PADT-FDPP13% DIO
films.
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cal band gaps of 1.44 and 1.50 eV, respectively. Their HOMO
levels are between 25.03 and 25.16 eV. PADT-DPP and PADT-
FDPP displayed OFET hole mobilities of 4.7 3 1023 and 2.7 3
1023 cm2/(V s), respectively, while their SCLC hole mobilities
were 4.98 3 1024 and 4.40 3 1024 cm2/(V s), respectively.
PADT-DPP showed obviously higher PCE of 3.44% than the
0.29% for PADT-FDPP. The poor efficiency of PADT-FDPP
should be related to the island-shaped morphology of its active
layer. DIO as the solvent additive did not contribute to the PCE
of PADT-DPP due to very limited morphology changing. How-
ever, the addition of DIO could remarkably elevate the PCE
PADT-FDPP to 2.62% because of the obviously improved two-
phase separation. These efficiencies are higher than those of
the previously reported anthradithiophene-based conjugated
polymers, suggesting the anthradithiophene as an electron-
donating polycyclic system is useful to construct new D–A
alternating copolymers for efficient polymer solar cells.
FIGURE 9 EQE curves of the polymer solar cells.
additive. Unfortunately, lower photovoltaic performances
than those with DIO were found.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support of
National Natural Science Foundation of China (Nos.
21225418 and 51173048), National Basic Research Program
of China (973 program Nos. 2013CB834705 and
2014CB643505), Doctoral Fund of Ministry of Education of
China (20120172110008), and GDUPS (2013).
Further insight into the microstructure of the active layer, it is
essential to explore the morphology of blend film. AFM was
applied to reveal the morphology of active layer surface. The
AFM topographic images of the blend film are shown in Figure 8.
For PADT-DPP-based blend films, the morphology fabricated
without DIO [Fig. 8(a)] is very close to that with 3% DIO [Fig.
8(b)], which is in good agreement with the corresponding photo-
voltaic performance with DIO as the solvent additive. As shown
in Figure 8(c), the domain size of the blend film based on PADT-
FDPP and PC71BM is very large, which is detrimental to charge
separation and transport. With 3% DIO as the solvent additive,
the former island shape morphology of PADT-FDPP disappears
[Fig. 8(d)], and a pattern of fairly continuous phase separation
emerges, from which the efficiency can be greatly improved. The
morphology study indicates that the polymer containing furan
spacer needs the assistance of a solvent additive to deposit the
blend film, so as to form a better interpenetrating network
toward good charge separation and transport.
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