Effects of including electron-withdrawing atoms on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene-based donor-acceptor polymers: towards an acceptor design for efficient polymer solar cells

Article abstract of DOI:10.1039/C7RA01049D

Three new D-A polymers PIDTT-DTBO, PIDTT-DTBT and PIDTT-DTFBT, using indacenodithieno[3,2-b]thiophene (IDTT) as the electron-rich unit and benzoxadiazole (BO), benzodiathiazole (BT) or difluorobenzothiadiazole (FBT) as the electron-deficient unit, were synthesized via a Pd-catalyzed Stille polymerization. The included electron-withdrawing atoms of the acceptor portion were varied between O, S, and F for tailoring the optical and electrochemical properties and the geometry of structures. Their effects on the film topography, photovoltaic and hole-transporting properties of the polymers were thoroughly investigated via a range of techniques. As expected, the stronger electron-withdrawing BO unit affords red-shifted absorption, low-lying HOMO and LUMO levels for the polymer PIDTT-DTBO. However, it depicts lower hole mobility and a less efficient charge collection in the active layer compared to the polymer PIDTT-DTBT. In addition, degradation of the solubility is observed in the fluorinated polymer PIDTT-DTFBT. As a result, a BHJ PSC (ITO/PEDOT:PSS/polymer:PC₇₁BM/interlayer/Al) fabricated with PIDTT-DTBT attains the best power conversion efficiency (PCE) of 4.91%. These results thus demonstrate the potential effects of electronegative atoms on IDTT-based polymers and the structure-function correlations of such electron-donor materials for efficient PSCs.

Full text of DOI:10.1039/C7RA01049D

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PAPER
Effects of including electron-withdrawing atoms
on the physical and photovoltaic properties of
indacenodithieno[3,2-b]thiophene-based donor–
acceptor polymers: towards an acceptor design for
efficient polymer solar cells
Cite this: RSC Adv., 2017, 7, 20440
a
b
c
c
c
*
* *
Xiaofeng Xu, Jiangman Sun, Junwu Chen and Yong Cao
Ping Cai,
Three new D–A polymers PIDTT-DTBO, PIDTT-DTBT and PIDTT-DTFBT, using indacenodithieno[3,2-b]
thiophene (IDTT) as the electron-rich unit and benzoxadiazole (BO), benzodiathiazole (BT) or
difluorobenzothiadiazole (FBT) as the electron-deficient unit, were synthesized via a Pd-catalyzed Stille
polymerization. The included electron-withdrawing atoms of the acceptor portion were varied between
O, S, and F for tailoring the optical and electrochemical properties and the geometry of structures. Their
effects on the film topography, photovoltaic and hole-transporting properties of the polymers were
thoroughly investigated via a range of techniques. As expected, the stronger electron-withdrawing BO
unit affords red-shifted absorption, low-lying HOMO and LUMO levels for the polymer PIDTT-DTBO.
However, it depicts lower hole mobility and a less efficient charge collection in the active layer
compared to the polymer PIDTT-DTBT. In addition, degradation of the solubility is observed in the
fluorinated polymer PIDTT-DTFBT. As a result, a BHJ PSC (ITO/PEDOT:PSS/polymer:PC71BM/interlayer/
Al) fabricated with PIDTT-DTBT attains the best power conversion efficiency (PCE) of 4.91%. These
results thus demonstrate the potential effects of electronegative atoms on IDTT-based polymers and the
structure–function correlations of such electron-donor materials for efficient PSCs.
Received 24th January 2017
Accepted 31st March 2017
DOI: 10.1039/c7ra01049d
rsc.li/rsc-advances
band-gap materials, conjugated polymers based on alternating
electron-donating (D) and electron-withdrawing (A) moieties
Introduction
can offer us an opportunity for tuning the D–A strength and
nature of p-electron delocalization, in order to match the
requirements of band gaps and energy levels (HOMO and
LUMO) for ideal electron-donor materials.19–21 Up to now,
a strategy toward using weak donor and strong acceptor units
for polymer skeleton construction is especially effective to
improve device performance.22 The weak donor moiety would
anchor a low-lying HOMO level, meanwhile, strong acceptor
moiety would optimize the value of a favorable LUMO level and
narrow band-gap.23–25
Recently, some electron-rich arenes with high level of fusion
have been reported.26 Their rigid backbones with horizontally or
vertically extended p-conjugation were formed by fastening or
fusing adjacent rings. Especially, a class of ladder-type frame-
works with forced planarization structures has been regarded as
appealing electron-donating moieties.27–38 First, the appropriate
planarization of aromatic rings can easily afford a red-shied
absorption spectrum, and also positively impact the intermo-
lecular interactions between conjugated main chains. In addi-
tion, the relatively higher level of coplanar backbones are
conducive to intermolecular charge carrier hopping and
suppress the interannular rotation, which is an effective way to
As a novel renewable energy source, polymer solar cells (PSCs)
have attracted growing attention from academic and industry
communities as a potential solar energy harvesting medium by
virtue of their intriguing lightweight, exible and large-area
features at low manufacturing costs.1,2 During the past
decade, bulk-heterojunction polymer solar cells (BHJ PSCs),
using a solution-processed active layer composed of an electron-
donor and an electron-acceptor, sandwiched between an ITO
anode and a low work-function metal cathode, have been
intensively investigated.3,4 To date, signicant progress has
been made in polymer/fullerene based BHJ PSCs with varying
degrees of success.5–18 Since the development of BHJ PSCs
largely relies on the rational design and synthesis of narrow
aSchool of Materials Science and Engineering, Guangxi Key Laboratory of Information
Materials, Guilin University of Electronic Technology, Guilin 541004, P. R. China.
E-mail: caiping@guet.edu.cn
bDepartment of Chemistry and Chemical Engineering, Chalmers University of
¨
Technology, SE-412 96, Goteborg, Sweden. E-mail: xixu@chalmers.se
cInstitute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of
Luminescent Materials & Devices, South China University of Technology, Guangzhou
510640, P. R. China. E-mail: psjwchen@scut.edu.cn
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improve the intrinsic charge mobility. Recently, we reported (BT) as the building block due to its easy synthesis, strong
a class of ladder-type D–A polymers composed of a indacenodi- electron-withdrawing property and excellent stability.
thieno[3,2-b]thiophene-based arene (IDTT; weak donor) as an Compared with BT unit, benzoxadiazole (BO) unit would have
electron-rich unit and diphenylquinoxaline (Qx; strong enhanced electron-withdrawing ability due to the stronger
acceptor) as an electron-decient unit with the best power electronegativity of oxygen than that of sulfur.50 Another effec-
conversion efficiency of 6.8% for BHJ PSCs.39,40 Generally, tive method is covalently addition of electronegative groups on
indacenodithiophene (IDT)-based polymers demonstrate low- the BT ring.23,51 Since the most electronegative element is uo-
lying HOMO levels, efficient solar energy harvest and high rine, two uorine atoms are fastened on 5, 6 position of BT core
hole mobility.41–44 In order to further extend linear p-conjuga- to form the diuorobenzothiadiazole (FBT) unit. Moreover, the
tion of IDT unit, the central phenyl ring was covalently fastened electron-withdrawing (BO, BT, FBT) and electron-donating
with two thieno[3,2-b]thiophenes (TT) to form the IDTT skel- (IDTT) moieties are coupled via bis-thiophene linkages to
eton, which can afford an enhanced planarization via form D–p–A conjugation, which is oen employed to improve
increasing the pentacyclic rings to heptacyclic rings. Moreover, the planarity of conjugated backbones. On the basis of the
four branched and aliphatic side chains can afford sufficient above consideration, three new conjugated polymers PIDTT-
solubility for post-processing and device fabrication. The DTBO, PIDTT-DTBT and PIDTT-DTFBT were successfully
appealing optoelectronic characteristics of IDTT make it prepared (Scheme 2). The solubility, thermal stability, UV-vis
promising building blocks for D–A conjugated polymers.45–47 absorption, electrochemical properties and dipole moments
Very recently, there are evidences indicating that IDTT unit is were systematically investigated to understand the structure–
also a promising building block to synthesize small molecular property correlation. BHJ PSCs using PC71BM as the electron
acceptors used for non-fullerene PSCs.48,49 Open questions acceptor were fabricated for evaluation. The hole mobility and
remain regarding further structural modications for IDTT lm morphology of the blend layers were also discussed.
based polymers, tailoring the deep HOMO level without sacri-
cing much of narrow band-gap, and its relationship to the
trade-off between obtained Voc, Jsc and FF for more efficient
device performance.
In an effort to provide more insight to the effect of electron-
withdrawing moieties on IDTT-based polymers, in this study we
incorporate three different electron acceptors (BO, BT and FBT)
and chlorobenzene were distilled over sodium/benzophenone
for comparison (Scheme 1). First, we choose benzodiathiazole
Experimental section
Materials
All reagents and solvents were obtained from Aldrich, Alfa
Aesar, and TCI Chemical Co. Anhydrous tetrahydrofuran(THF)
under N2 prior to use. All manipulations involving air-
Scheme 1 Monomer and polymer synthesis.
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Scheme 2 Polymer structures.
sensitive reagents were performed under an atmosphere of our previous work. Monomer 4,7-bis(5-(trimethylstannyl)thio-
argon.
phen-2-yl)benzo[c][1,2,5]thiadiazole (M3) and 5,6-diuoro-4,7-
bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(M4) were prepared according to the literatures.7,52 The detailed
synthetic processes of 4,7-bis(5-(trimethylstannyl)thiophen-2-yl)
benzo[c][1,2,5]oxadiazole (M2) are as follows.
Characterization
1H and 13C NMR spectra were recorded on a Bruker AV 300
spectrometer with tetramethylsilane (TMS) as the internal
reference. Elemental analyses were performed on a Vario EL
elemental analysis instrument (Elementar Co.). The molecular
4,7-Di(thiophen-2-yl)benzo[c][1,2,5]oxadiazole (DTBO)
weight of polymers were obtained on a Polymer Laboratories PL 4,7-Dibromobenzo[c][1,2,5]oxadiazole (1.39 g, 5.0 mmol), 2-
220 Chromatograph using linear polystyrene as references, with tributylstannylthiophene (4.66 g, 12.5 mmol) and Pd(PPh3)2Cl2
ꢀ
150 C 1,2,4-trichlorobenzene as the eluent. UV-vis absorption (0.18 g, 0.25 mmol) were dissolved in anhydrous THF (50 mL).
spectra were recorded on a Shimadzu UV 2450 spectropho- The reaction mixture was heated to 75 ꢀC for 12 hours under an
tometer. Thermogravimetric analysis (TGA) was carried out on argon atmosphere. Aer cooling to room temperature, the
a PerkinElmer Diamond TGA/DTA, at a heating rate of 10 ꢀC organic layer was extracted with dichloromethane (100 mL),
minꢁ1 from 50 ꢀC to 650 ꢀC under a nitrogen atmosphere. washed successively with water and then dried over anhydrous
Differential scanning calorimeter (DSC) measurement was MgSO4. The residue was puried by column chromatography
carried ouꢁt 1on a PerkinElmer Diamond DSC, at a heating rate of (dichloromethane/hexane ¼ 1 : 5 as the eluent) and recrystal-
10 ꢀC min from 25 ꢀC to 200 ꢀC under a nitrogen atmosphere. lized from THF\ethanol to give the nal compound as a red
1
Cyclic voltammetry was carried out on a CHI660A electro- solid (1.18 g, yield: 83%). H NMR (300 MHz, CDCl3), d (ppm):
chemical workstation with platinum electrodes at a scan rate of 8.11 (d, J ¼ 4.8 Hz, 2H), 7.62 (s, 2H), 7.44 (d, J ¼ 6.0 Hz, 2H), 7.20
50 mV sꢁ1 against the Ag/Ag+ reference electrode with an argon- (m, 2H). 13C NMR (75 MHz, CDCl3), d (ppm): 147.9, 137.9, 128.7,
saturated solution of 0.1
M tetrabutylammonium hexa- 128.6, 127.0, 126.4, 122.1. Anal. calcd (%) for C14H8N2OS2: C,
uorophosphate (Bu4NPF6) in acetonitrile (CH3CN). The depo- 59.13; H, 2.84; N, 9.85; S, 22.55. Found: C, 58.14; H, 2.30; N, 9.60;
sition of a polymer on the electrode was done by the evaporation S, 21.99.
of its dilute chloroform solution. Tapping mode atomic force
microscopy (AFM) images were obtained using a NanoScope
NS3A system (Digital Instrument).
4,7-Bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]
oxadiazole (M2)
To a solution of 2,2,6,6-tetramethylpiperidine (TMP) (0.64 g, 4.5
mmol) in anhydrous THF (50 mL) was added a 2.2 M solution of
Synthesis
The synthetic routes of monomers and polymers are shown in n-BuLi in hexane (2.05 mL, 4.5 mmol) at ꢁ78 ꢀC under an argon
Schemes 1 and 2. Monomer M1 was synthesized according to atmosphere. The reaction mixture was warmed to room
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ꢀ
temperature, stirred for another 30 min and cooled to ꢁ78 C scrub and then underwent a wet-cleaning process inside an
again. A solution of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]oxadia- ultrasonic bath, beginning with deionized water followed
zole (0.43 g, 1.5 mmol) in anhydrous THF (10 mL) was added by acetone and isopropanol. Aer oxygen plasma cleaning
dropwise. The solution turned deep purple immediately and was for 5 min, a 40 nm thick poly(3,4-ethylenedioxythiophene):
ꢀ
stirred at ꢁ78 C for 1 hour. Then Me3SnCl (1.20 g, 6.0 mmol) poly(styrenesulfonate) (PEDOT:PSS) (Bayer Baytron 4083)
was added in one portion. The reaction mixture was allowed to anode buffer layer was spin-cast onto the ITO substrate and
ꢀ
warm to room temperature and stirred overnight. The resulting then dried by baking in a vacuum oven at 80 C for overnight.
mixture was poured into water, extracted with diethyl ether (100 The active layer, with a thickness in the range of 80–90 nm, was
mL) and dried over anhydrous MgSO4. Aer the solvent was then deposited on top of the PEDOT:PSS layer by spin-coating
removed, the residue was recrystallized from THF\ethanol to from a chlorobenzene or dichlorobenzene solution. The inter-
1
give the nal compound as a red solid (0.72 g, yield: 79%). H layer solution in methanol was spin-coated on the top of the
NMR (300 MHz, CDCl3), d (ppm): 8.17 (d, J ¼ 3.6 Hz, 2H), 7.62 (s, active layer to form a thin interlayer of 3–5 nm. The thickness of
2H), 7.28 (d, J ¼ 3.6 Hz, 2H), 0.43 (s, 18H). 13C NMR (75 MHz, the PEDOT:PSS and active layer was veried by a surface pro-
CDCl3), d (ppm): 147.9, 143.5, 141.0, 136.6, 129.4, 126.4, 121.7, lometer (Tencor, Alpha-500). At last, a 100 nm aluminium layer
ꢁ8.1. Anal. calcd (%) for C20H24N2OS2Sn2: C, 39.38; H, 3.97; N, was thermally evaporated with a shadow mask at a base pres-
4.59; S, 10.51. Found: C, 39.72; H, 3.47; N, 4.74; S, 10.47.
sure of 3 ꢂ 10ꢁ4 Pa. The overlapping area between the cathode
and anode dened a pixel size of 0.15 cm2. The thickness of the
evaporated cathodes was monitored by
a quartz crystal
General procedure for polymerization
thickness/ratio monitor (model: STM-100/MF, Sycon). Except
the deposition of the PEDOT:PSS layers, all the fabrication
processes were carried out inside a controlled atmosphere of
nitrogen drybox (Vacuum Atmosphere Co.) containing less than
10 ppm oxygen and moisture. The power conversion efficiencies
of the resulting polymer solar cells were measured under 1 sun,
AM 1.5 G (air mass 1.5 global) spectrum from a solar simulator
(Oriel model 91192) (100 mW cmꢁ2). The current density–
voltage (J–V) characteristics were recorded with a Keithley 236
source unit. The external quantum efficiencies of PSCs were
measured with a commercial photomodulation spectroscopic
setup (DSR 100UV-B) including a xenon lamp, an optical
chopper, a monochromator, and a lock-in amplier operated by
a PC computer (Zolix Instruments), and a calibrated Si photo-
diode was used as a standard.
The three polymers were synthesized by the palladium(0)-
catalyzed Stille coupling reaction with a dibromide monomer
and a bis(trimethylstannyl)-substituted monomer under an
argon atmosphere.
0.25 mmol of dibromide monomer, 0.25 mmol of
bis(trimethylstannyl)-substituted monomer, tris(dibenzylide-
neacetone)dipalladium(0) (Pd2(dba)3) (7 mg) and tri(o-tolyl)
phosphine (P(o-Tol)3) (9 mg) were dissolved in anhydrous
chlorobenzene (15 mL). The mixture was reuxed with vigorous
stirring for 72 h under an argon atmosphere. Aer the mixture
was cooled to room temperature, it was poured into 250 mL of
methanol. The precipitated material was dissolved and ltrated
through a funnel and precipitated again. The polymer was
washed in a Soxhlet extractor with acetone (12 h), hexane (12 h),
and chloroform (12 h). The chloroform fraction was concen-
trated and poured into methanol (250 mL). Finally, the precip-
itate was collected and dried in vacuum to afford the title Hole mobility measurement
polymer as a dark ber.
The SCLC mobility was determined by tting the dark current to
the model of a single carrier SCLC, which is described by the
equation:
Polymer PIDTT-DTBO. 273 mg. Yield: 74%. GPC: Mn ¼ 33.5
kDa; Mw/Mn ¼ 2.2. 1H NMR (CDCl3, 300 MHz, d): 8.04 (br, 2H),
7.49 (br, 6H), 7.21 (br, 10H), 6.86 (br, 8H), 3.79 (br, 8H), 1.74 (br,
4H), 1.41–1.26 (m, 32H), 0.86 (br, 24H). Anal. calcd (%) for
(C90H94N2O5S6)n: C, 73.23; H, 6.42; N, 1.90; S, 13.03. Found: C,
74.14; H, 7.22; N, 1.81, S, 13.49.
Polymer PIDTT-DTBT. 265 mg. Yield: 71%. GPC: Mn ¼ 30.3
kDa; Mw/Mn ¼ 2.4. 1H NMR (CDCl3, 300 MHz, d): 8.05 (br, 2H),
7.85 (br, 2H), 7.48 (br, 4H), 7.24 (br, 10H), 6.84 (br, 8H), 3.79 (br,
8H), 1.74 (br, 4H), 1.41–1.26 (m, 32H), 0.86 (br, 24H). Anal. calcd
(%) for (C90H94N2O4S7)n: C, 72.44; H, 6.35; N, 1.88; S, 15.04.
Found: C, 73.15; H, 6.97; N, 1.73; S, 15.63.
9
V2
d3
J ¼ 303rmh
8
where J is the current, mh is the charge mobility at zero eld, 30 is
the free-space permittivity, 3r is the relative permittivity of the
material, d is the thickness of the active layer, and V is the
effective voltage. V ¼ Vappl ꢁ Vbi ꢁ Vs, Vappl is the applied
potential, Vbi is the built-in potential and Vs is the voltage drop
due to the substrate series resistance. The hole mobility can be
calculated from the slope of the J1/2–V curve.
Polymer PIDTT-DTFBT. 195 mg. Yield: 51%. GPC: Mn ¼ 8.3
kDa; Mw/Mn ¼ 3.6. Anal. calcd (%) for (C90H92F2N2O4S7)n: C,
70.74; H, 6.07; N, 1.83; S, 14.69. Found: C, 71.45; H, 6.89; N, 1.74;
S, 15.11.
Results and discussion
Design and synthesis
The synthetic routes for monomers and polymers are shown in
Scheme 1. Initial reactions started from respective bromination
PSC fabrication and characterization
Patterned indium tin oxide (ITO) coated glass with a sheet of benzo[c][1,2,5]oxadiazole and benzo[c][1,2,5]thiadiazole in
resistance of 15–20 ohm per square was cleaned by a surfactant the presence of excess bromide. Two obtained dibromo
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compounds 4,7-dibromobenzo[c][1,2,5]oxadiazole and 4,7-
dibromobenzo[c][1,2,5]thiadiazole were anked by two thio-
phene rings via Pd-catalyzed Stille coupling reaction, respec-
tively. Finally, respective reactions of 4,7-di(thiophen-2-yl)benzo
[c][1,2,5]oxadiazole (DTBO) and 4,7-di(thiophen-2-yl)benzo[c]
[1,2,5]thiadiazole (DTBT) with freshly made 2,2,6,6-tetrame-
thylpiperidyl lithium (LTMP) and then trimethylchloro-
stannane, afforded monomer M2 and M3 in good yield. Here,
LTMP is used as a non-nucleophilic base due to its strong steric
hindrance coming from the tetramethylpiperidide ring. Mono-
mer M1 was synthesized according to our previous work.39
Monomer M4 was prepared for comparison according to the
literature.52 The chemical structure of all the compounds were
conrmed using 1H NMR, 13C NMR and elemental analysis. (see
Experiment section for details).
The preparations of three polymers PIDTT-DTBO, PIDTT-
DTBT and PIDTT-DTFBT were accomplished via Pd2(dba)3-
catalyzed Stille coupling reaction. Aer polymerization, the
crude polymers were puried by ltration and continuously
extracting with acetone, hexane and chloroform in the Soxhlet
extractor. The chemical structure of polymer PIDTT-DTBO,
PIDTT-DTBT and PIDTT-DTFBT (Scheme 2) were conrmed by
1H NMR and elemental analysis. Polymer PIDTT-DTBO and
PIDTT-DTBT were readily soluble in tetrahydrofuran (THF),
chloroform, toluene and chlorobenzene (CB). However, aer
polymerization and precipitation, the crude polymer PIDTT-
DTFBT was only partially soluble in warm CB and dichloro-
benzene (DCB). Its poor solubility may be ascribed to enhanced
intermolecular non-covalent interactions of the uorinated
polymer backbones. No clear 1H NMR spectrum of PIDTT-
DTFBT was observed presumably due to strong aggregation of
polymer main chains in solution. We failed to obtain a thick
and smooth lm from the solution of polymer PIDTT-DTFBT,
thus hampering investigation of its absorption spectrum, elec-
trochemical properties and device fabrication. The molecular
weight of three polymers were evaluated by high temperature
Fig. 1 TGA and DSC plot of polymers with a heating rate of 10 ꢀC
minꢁ1 under a nitrogen atmosphere.
ꢀ
gel permeation chromatography (GPC) at 150 C with 1,2,4-tri-
chlorobenzene as eluent. The number-average molecular weight
(Mn) of polymer PIDTT-DTBO and PIDTT-DTBT was 33.5 and
30.3 kDa with polydispersity index (PDI) of 2.2 and 2.4,
respectively. For polymer PIDTT-DTFBT, a rather lower Mn of 8.3
kDa and board PDI up to 3.6 was obtained. It is noted that once
the limit of solubility was reached in hot chlorobenzene, the
was observed from two cooling and heating cycles between
ꢀ
25 C and 230 ꢀC.
Absorption spectra
length of polymer skeletons would stop increasing and precip- In order to determine the effects of electron-withdrawing atoms
itate may appear during polymerization.
on the optoelectronic properties of IDTT-based polymers, UV-
vis absorption spectra were obtained for each polymer in solu-
tion and lm (Fig. 2, Table 1). The absorption spectra of poly-
mer PIDTT-DTBO and PIDTT-DTBT show two distinct
Thermal properties
Thermal stability of polymers was investigated using thermog- absorption bands in the wavelength range of 350–450 and 500–
ravimetric analysis (TGA), as shown in Fig. 1(a). The TGA curves 700 nm, corresponding to the p–p* transition and intra-
exhibit onset temperatures with 5% weight loss (Td) of polymer molecular charge transfer (ICT) between the donor and acceptor
PIDTT-DTBO, PIDTT-DTBT and PIDTT-DTFBT are between 380 units. The absorption maximum (lmax) of polymer PIDTT-DTBO
ꢀ
and 430 C, indicating these polymers have sufficient stability and PIDTT-DTBT in dilute chloroform solutions are at 610 and
for optoelectronic applications. In addition, DSC measurement 585 nm, respectively. In thin lms, the absorption lmax of
was carried out to investigate the phase transitions and crys- polymer PIDTT-DTBO and PIDTT-DTBT are red-shied to 622
tallization properties of the polymers. As shown in Fig. 1(b), and 607 nm, owing to closer p–p stacking of polymer main
neither melting point (Tm) nor crystallization temperature (Tc) chains in the solid state. The absorption edges of the lm
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transfer between the IDTT donor and BO acceptor units. Since
the oxygen atom has relatively stronger electronegativity, the
stronger D–A interactions within polymer PIDTT-DTBO would
be originated from more electron-withdrawing ability of BO unit
than that of BT unit. Generally, enhancement of light harvest is
benecial to higher Jsc achievement of PSCs. As shown in
Fig. 2(b), we also investigate the effects of thermal annealing on
the absorption spectra in solid lms. For each polymer, no
spectral red-shis are recorded aer thermal annealing at
100 ꢀC for 10 min. The result appear to suggest that the polymer
backbones can attain desirable p–p stacking in the solid state
without any thermal treatment. In addition, as compared with
the previously reported polymer PIDTT-Qx incorporating IDTT
and Qx units, very similar absorption spectra are observed in the
neat polymer lms.39
Electrochemical properties
Fig. 3 shows the cyclic voltammograms of polymer lms on Pt
disk electrode in a 0.1 M Bu4NPF6 acetonitrile solution. The
highest occupied molecular orbital (HOMO), lowest unoccupied
molecular orbital (LUMO) energy levels and electrochemical
band-gaps of polymers were obtained from the onsets of
oxidation and reduction potentials with an Ag/Ag+ electrode as
reference. HOMO level of ferrocene (ꢁ4.80 eV) was used as the
internal standard. The onset oxidation potentials (4ox) of poly-
mer PIDTT-DTBO and PIDTT-DTBT are 0.87, 0.82 V vs. Ag/Ag+
electrode, respectively. The onset reduction potentials (4red) are
ꢁ0.82 and ꢁ0.80 V vs. Ag/Ag+ electrode, respectively. The HOMO
and LUMO energy levels were calculated according to the
equations, HOMO ¼ ꢁe(Eox + 4.49) (eV), LUMO ¼ ꢁe(Ered + 4.49)
(eV). The results of electrochemical measurements are
summarized in Table 1. The HOMO level of polymer PIDTT-
DTBO is around ꢁ5.36 eV, which is 0.05 eV lower than that of
polymer PIDTT-DTBT (ꢁ5.31 eV). The HOMO shi would be
ascribed to stronger electronegativity of oxygen than sulfur
atom in acceptor part. Since the Voc of BHJ PSCs is positively
correlated to the energy difference between HOMO level of
electron donor and LUMO level of electron acceptor, the low-
lying HOMO level of polymer PIDTT-DTBO is the precondition
for a high Voc achievement. Aer the isolated acceptors are
coupled by thiophene units on both sides, a little higher HOMO
energy (up-lying HOMO level) are recorded compared to the
Fig. 2 (a) Normalized UV-vis-NIR absorption spectra of polymers in
chloroform. (b) Normalized UV-vis-NIR absorption spectra of polymer
films before and after annealing at 100 ꢀC for 10 min.
spectra are located at 732 and 721 nm, respectively. Compared
to the absorption spectrum of PIDTT-DTBT, polymer PIDTT-
DTBO exhibits a bathochromic shi (10 nm) in the long wave-
length regime, implying stronger intramolecular charge
Table 1 Optical and electrochemical properties of polymers
UV-vis absorption spectra
Cyclic voltammetry
Solution
p-Doping
Film
n-Doping
lonset
4oxd/HOMOe
(V)/(eV)
4red/LUMOf
(V)/(eV)
Eogpt (eV)c
Eegcg (eV)
a
b
Polymer
labs (nm)
labs (nm)
(nm)
PIDTT-DTBO
PIDTT-DTBT
610
585
622
607
732
721
1.69
1.72
0.87/ꢁ5.36
0.82/ꢁ5.31
ꢁ0.80/ꢁ3.69
ꢁ0.82/ꢁ3.67
1.67
1.64
a
b
c
d
Absorption peak in chloroform solution. Absorption peak in lm. Optical band gap. Onset voltage of oxidation process during cyclic
voltammetry with Ag/Ag+ reference electrode. Calculated according to HOMO ¼ ꢁe(Eox + 4.49) (eV). LUMO ¼ ꢁe(Ered + 4.49) (eV).
e
f
g
Electrochemical band gap.
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1.72 eV, respectively, which match well with their electro-
chemical band-gaps within the experimental error.
Recently, theoretical calculations were used to study the
effect of dipole moments on performance of conjugated mole-
cules in PSCs.53,54 Here, the additional dipole moment of 4,7-
di(thiophen-2-yl)benzooxadiazole (DTBO) unit was performed
with hybrid DFT B3LYP/6-31g*. In comparison with 4,7-
di(thiophen-2-yl)benzothiadiazole (DTBT) (0.13 Debye),55 DTBO
unit exhibits a codirectional but higher dipole moment of 2.32
Debye, which would be ascribed to the inclusion of more elec-
tronegative oxygen atom. Compared to DTBO and DTBT units,
5,6-diuoro-4,7-di(thiophen-2-yl)benzothiadiazole (DTFBT) has
an opposite dipole moment of 1.64 Debye due to the 5,6-
diuorinated BT core.56 The natural differences of dipole
moment would affect the molecular conformations on acceptor
portion during thin lm fabrication. When using a polar solvent
like chlorobenzene, the conformation with a higher dipole
moment may appear dominantly, thus affecting intermolecular
interactions in the solid lm. As a result, it is noted that the
replacement of BT by FBT unit can decrease the nonplanar
conformation and promote the p–p stacking of polymer main
chains,52 which can also explain the noticeable degradation of
solubility for our polymer PIDTT-DTFBT.
Fig. 3 Cyclic voltammogram of the polymer film on a platinum
electrode measured in a 0.1 M Bu4NPF6 acetonitrile solution at a scan
rate of 100 mV sꢁ1
.
direct D–A structure of our previous work.30 It is ascribed to the
increased planarity and enhanced p-delocalization brought by
the thiophene rings, which is common in many D–p–A conju-
gated polymers. The LUMO levels of polymer PIDTT-DTBO and
PIDTT-DTBT are ꢁ3.69 and ꢁ3.67 eV, respectively. Similarly, the
larger electron-withdrawing effect from oxygen results in a little
stable LUMO level of BO compared to BT. According to the
previous report, the HOMO and LUMO levels of PIDTT-Qx are
ꢁ5.39 and ꢁ3.73 eV respectively, which are relatively higher
than those of PIDTT-DTBO and PIDTT-DTBT.39 As shown from
the Fig. 4, the energy difference between the LUMO levels of two
polymers and that of PC71BM (ꢁ4.2 eV) are large enough for
Photovoltaic properties
To investigate the photovoltaic properties of polymer PIDTT-
DTBO and PIDTT-DTBT, bulk-heterojunction polymer solar
cells (BHJ PSCs) with the device architecture of ITO/PEDOT:PSS
(40 nm)/polymer:PC71BM (85 nm)/PFN interlayer (3 nm)/Al (100
nm) were fabricated. An alcohol-soluble polymer PFN was used
as an interfacial modier between the active layer and Al
cathode, which can intensely promote the interfacial contact
and electron extraction between the active layer and metallic
cathode.57 Polymer and PC71BM was dissolved in o-dichloro-
benzene for spin-coating. The measurements of photovoltaic
performances were carried out under illumination of AM 1.5 G
simulated solar light at 100 mW cmꢁ2. Different weight ratios of
polymer : PC71BM were carefully optimized. The corresponding
PSC parameters (short-circuit current density Jsc, open circuit
voltage Voc, and ll factor FF) are summarized in Table 2, and
the J–V curves are shown in Fig. 5(a) and (b). From the device
based on polymer PIDTT-DTBO with an optimum D/A ratio
(1 : 4), a PCE of 4.37% with Voc ¼ 0.88 V, Jsc ¼ 9.97 mA cmꢁ2 and
FF ¼ 49.84% was obtained, while the device based on polymer
PIDTT-DTBT shows a higher PCE of 4.82%, with Voc ¼ 0.86 V, Jsc
¼ 10.05 mA cmꢁ2, and FF ¼ 55.80%. The slightly increased Voc
of PIDTT-DTBO-base device is in accordance with its low-lying
HOMO level compared to polymer PIDTT-DTBO. In addition,
thermal annealing was employed to improve the photovoltaic
performances of the PSCs. Aer annealing at 100 ꢀC for 10 min,
the Voc value of each polymer doesn't alter and little changes of
FF are observed. The Jsc of PIDTT-DTBO-based devices increase
from 9.97 to 10.50 mA cmꢁ2, its PCE thus increase from 4.37 to
4.60%. Compared to PIDTT-DTBT, the PIDTT-DTBO-based
device displays a little larger Jsc value, which is in accordance
efficient exciton dissociation. According to the equation, Egec
¼
e(Eox ꢁ Ered) (eV), the electrochemical band-gaps of polymer
PIDTT-DTBO and PIDTT-DTBT are calculated to be 1.67 and
1.64 eV respectively, which are similar with that of PIDTT-Qx
(1.66 eV).39 The coupling of bis-thiophenes to acceptor unit
shows minimal impact of the band-gaps. The optical band-gaps
of polymer PIDTT-DTBO and PIDTT-DTBT estimated from the
corresponding absorption edges of solid lms are 1.69 and
Fig. 4 HOMO and LUMO energy levels of the polymers and PC71BM. with its 10 nm bathochromic absorption. For polymer PIDTT-
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Table 2 Photovoltaic performances of the PSCs based on polymer:PC71BM under the illumination of AM 1.5, 100 mW cmꢁ2
Polymer
Ratioa
Annealing temperature
Voc (V)
Jsc (mA cmꢁ2
)
FF (%)
PCEc (%)
PIDTT-DTBO
1 : 1
1 : 2
1 : 3
1 : 4
1 : 4
1 : 1
1 : 2
1 : 3
1 : 4
1 : 4
r.t.b
r.t.
r.t.
r.t.
0.88
0.88
0.88
0.88
0.88
0.84
0.86
0.86
0.86
0.86
5.02
8.24
9.19
9.97
10.50
4.15
6.93
8.87
10.05
10.20
32.34
37.48
43.40
49.84
49.74
30.15
34.89
51.04
55.80
55.99
1.35 ꢃ 0.07 (1.43)
2.64 ꢃ 0.08 (2.72)
3.42 ꢃ 0.08 (3.51)
4.24 ꢃ 0.09 (4.37)
4.47 ꢃ 0.10 (4.60)
0.93 ꢃ 0.10 (1.05)
1.95 ꢃ 0.10 (2.08)
3.78 ꢃ 0.08 (3.89)
4.70 ꢃ 0.10 (4.82)
4.79 ꢃ 0.09 (4.91)
100 ꢀ
r.t.
C
C
PIDTT-DTBT
r.t.
r.t.
r.t.
100 ꢀ
a
b
c
Polymer : PC71BM weight ratio. Room temperature without annealing. The values in the parentheses are from the best devices, and the
statistics are from twelve devices.
Fig. 5 (a, b) Current density–voltage (J–V) characteristics of PSCs before and after annealing at 100 ꢀC for 10 min (c, d) EQE curves of PSCs
before and after annealing at 100 ꢀC for 10 min.
DTBT, the measured Jsc exhibits relatively limited increase, the 500 nm is attributable to the absorption of PC71BM in the
corresponding PCE displays a little increase from 4.88 to 4.91%. visible region. Compared to the devices without annealing, both
To evaluate the photoresponse of PSCs, external quantum devices aer annealing can demonstrate a higher EQE in the
efficiencies (EQE) before and aer annealing at 100 ꢀC for long wavelength region (550 to 750 nm). It's coincident with the
10 min were measured. As shown in Fig. 5(c) and (d), the EQE J–V results, indicating the solar absorption and conversion
curves generally correspond with UV-vis absorption spectra of becomes more efficiently in the relative strong region (500 to
polymer PIDTT-DTBO and PIDTT-DTBT in the range of 400 to 750 nm) of solar irradiance. The devices based on polymer
750 nm. The enhanced quantum efficiency in the region of 400– PIDTT-DTBT respond to a higher photo conversion efficiency
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over the whole visible region, implying more efficient charge
collection and less charge recombination at the junction
between polymer PIDTT-DTBT and PC71BM.
In order to determine the effects of thermal annealing on lm
topography, the surface morphology of polymer/PC71BM blend
lms before and aer post-thermal treatment were studied via
atom force microscopy (AFM). As shown in Fig. 6(a–c), without
any thermal annealing, the blend lms of polymer PIDTT-DTBO
and PIDTT-DTBT exhibit smooth morphology with low root
mean-square roughness (Rq) of 0.32 and 0.37 nm, respectively.
Generally, a smooth lm with uniform nanoscale network is
desirable in PSCs. Any large domain and aggregate would hinder
exciton dissociation, since its size may exceed the average diffu-
sion lengths of exciton. As shown in Fig. 6(b–d), aer thermal
ꢀ
annealing at 100 C for 10 min, the Rq is raised to 0.42 nm for
polymer PIDTT-DTBO. Some tiny visible domains appears in its
blend lm, suggesting the polymer affords better intermixing
with PC71BM, which corresponds to the obvious improvement
(5%) of device efficiency. And the post-thermal treatment slightly
increase the Rq to 0.40 nm for polymer PIDTT-DTBT, which may
attribute to the slightly stronger aggregation and thus leading to
the improved device performance.
Fig. 7 The J1/2–V characteristics of polymer PIDTT-DTBO and PIDTT-
DTBT-based hole-only devices.
(SCLC) method. The structure of hole-only device is ITO/
PEDOT:PSS/polymer:PC71BM (85 nm)/MoO3/Al. Aer thermal
annealing at 100 ꢀC for 10 min, the calculated mobility of
polymer PIDTT-DTBO-based device is 9.2 ꢂ 10ꢁ5 cm2 Vꢁ1 sꢁ1
,
while a higher hole mobility of 1.1 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1 is
recorded for polymer PIDTT-DTBT-based device. The better
hole-transporting property of polymer PIDTT-DTBT would
Hole mobility
As shown in Fig. 7, the hole mobility of polymer/PC71BM blend
lms were investigated using the space charge limited current
Fig. 6 Tapping mode AFM topography images (3 ꢂ 3 mm2) of polymer\PC71BM blend films, (a) PIDTT-DTBO\PC71BM before annealing. (b)
PIDTT-DTBO\PC71BM after annealing at 100 ꢀC for 10 min (c) PIDTT-DTBT\PC71BM before annealing. (d) PIDTT-DTBT\PC71BM after annealing at
100 ꢀC for 10 min.
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benet more balanced hole and electron transport in the poly- Electronic Technology (UF15011Y), and the EU projects
mer/PC71BM blend layer. Since the FF value of PSC is positively SUNFLOWER (FP7-ICT-2011-7, Grant number: 287594).
correlated to the balance of hole and electron transport, the
calculated mobility are consistent with the device results, from
which polymer PIDTT-DTBT-based devices show higher FF in
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