Synthesis and electronic energy-level regulation of imide-fused poly(thienylene vinylene) derivatives

Article abstract of DOI:10.1002/pola.26923

A series of novel poly(thienylene vinylene) derivatives (PTVs), P20-P24, with imide substituents were designed and synthesized by palladium-catalyzed Stille coupling polymerization, wherein the imide substituent density was decreased gradually, which allowed us to explicitly study the effect of electron-deficient substituent on the optical, electrochemical, and photovoltaic properties of the PTVs. All of the four polymers showed broad absorption bands with optical bandgaps between1.66 and 1.78 eV. By reducing density of electron-deficient imide group, the LUMO energy levels of the polymers could be tuned gradually from -3.75 to -3.43 eV, with HOMO levels upshifted from -5.64 to -5.16 eV. Bulk heterojunction solar cells with the polymers as donor and PC₇₁BM as acceptor demonstrated very different excitons dissociation behavior. With decreasing the imide-fused unit density, the open-circuit voltage (V_OC) values in the devices decreased from 0.78 to 0.62 V, whereas the short-circuit currents (J_SC) increased from 0 to 2.26 mA cm ^-2 and then decreased to 1.01 mA cm^-2. By adjusting the electron-withdrawing imide substituent density, power conversion efficiency of the PTVs-based solar cells can be increased to four times, reached 0.86%. To the best of our knowledge, this is the first systematic study of the relationship between molecular energy level and photovoltaic properties of PTVs. Copyright

Full text of DOI:10.1002/pola.26923

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Synthesis and Electronic Energy-Level Regulation of Imide-Fused
Poly(thienylene vinylene) Derivatives
Xiaochen Wang,1,2,3* Chen Gao,1* Kai Wang,1,3 Xi Fan,3 Haiqiao Wang,1 Xiaoyu Li,2
Zhi-Guo Zhang,3 Yongfang Li3
1State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
2Key Laboratory of Carbon Fiber and Functional Polymers of Ministry of Education, Beijing University of Chemical Technology,
Beijing 100029, China
3CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Correspondence to: H. Wang (E-mail: wanghaiqiao@mail.buct.edu.cn) or X. Li (E-mail: lixy@mail.buct.edu.cn) or
Y. Li (E-mail: liyf@iccas.ac.cn) or Z. G. Zhang (E-mail: zgzhangwhu@iccas.ac.cn)
Received 26 March 2013; accepted 20 August 2013; published online 16 September 2013
DOI: 10.1002/pola.26923
ABSTRACT: A series of novel poly(thienylene vinylene) deriva-
tives (PTVs), P20-P24, with imide substituents were designed
and synthesized by palladium-catalyzed Stille coupling poly-
merization, wherein the imide substituent density was
decreased gradually, which allowed us to explicitly study the
effect of electron-deficient substituent on the optical, electro-
chemical, and photovoltaic properties of the PTVs. All of the
four polymers showed broad absorption bands with optical
bandgaps between1.66 and 1.78 eV. By reducing density of
electron-deficient imide group, the LUMO energy levels of the
polymers could be tuned gradually from 23.75 to 23.43 eV,
with HOMO levels upshifted from 25.64 to 25.16 eV. Bulk het-
erojunction solar cells with the polymers as donor and
PC71BM as acceptor demonstrated very different excitons dis-
sociation behavior. With decreasing the imide-fused unit den-
sity, the open-circuit voltage (VOC
) values in the devices
decreased from 0.78 to 0.62 V, whereas the short-circuit cur-
rents (JSC
)
increased from
0
to 2.26 mA cm22 and then
By adjusting the electron-
decreased to 1.01 mA cm22
.
withdrawing imide substituent density, power conversion effi-
ciency of the PTVs-based solar cells can be increased to four
times, reached 0.86%. To the best of our knowledge, this is
the first systematic study of the relationship between molecu-
C
V
lar energy level and photovoltaic properties of PTVs.
2013
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2013, 51, 4975–4982
KEYWORDS: conjugated polymers; electrochemistry; electronic
energy level; polymer solar cells; poly(thienylene; vinylene)
(PTVs); synthesis; thieno[3,4-c]pyrrole-4,6-dione (TPD)
we reported a new poly(thienylene vinylene) derivative,
POTBTV,22 with electron-deficient benzothiadiazole unit
inserted into the backbone. PCE of solar cell based on
POTBTV reached 1.53%. These results indicate that the pho-
tovoltaic performance of the PTV derivatives could be greatly
improved by connecting electron-withdrawing substituents
to either the main chain or side chains of PTVs molecules.
However, to achieve efficient PTVs, it is necessary to further
study the effect of the electron-deficient substituent on the
optical, electrochemical, and photovoltaic properties.
INTRODUCTION In recent years, low-bandgap-conjugated
polymers have drawn much attention due to their better
match to the solar energy spectrum for the application in
polymer solar cells (PSCs).1–7 Poly(thienylene vinylene)
derivatives (PTVs) is an important class of low-bandgap
polymers.8–15 PTVs have already proven to have broad
absorption spectra (bandgap ca. 1.5–1.8 eV) combining with
high hole mobility (up to 0.2 cm2 V21 s21),16 which makes
them attractive candidates for photovoltaic applications.
However, power conversion efficiency (PCE) of bulk hetero-
junction (BHJ) PSCs based on PTVs as donor were rather
low (0.2–0.3%).17–20 Huo et al. synthesized poly(3-carboxy-
lated thienylenevinylene) (P3CTV)21 with an electron-
withdrawing carboxylate substituent, which possesses lower
HOMO energy level and enhanced fluorescence. The PCE of
the PSCs based on P3CTV as donor reached 2.01%. Recently,
Herein, three monomers, trans-1,2-bis(tributylstannyl)ethy-
lene, 2,5-dibromo thiophene, and imide-fused thiophene (N-
alkylthieno[3,4-c]pyrrole-4,6-dione, TPD) dibromide were
copolymerized using the Pd-catalyzed Stille reaction to pro-
duce a series of PTVs, P20-P24, with molar feed ratios of
TPD to pristine thiophene unit ranging from 2:0 to 2:4, as
*Xiaochen Wang and Chen Gao contributed equally to this work.
C
V
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on a Shimadzu spectrometer model UV-3150. Absorption
spectra measurements of the polymer solutions were carried
out in chloroform (analytical reagent) at 25 ꢀC. Absorption
spectra measurements of the polymer films were carried out
on the quartz plates with the polymer films spin-coated from
the polymer solutions in chloroform (analytical reagent) at 25
ꢀC. The electrochemical cyclic voltammetry was conducted on
a Zahner IM6e electrochemical workstation with a glassy car-
bon, Pt wire, and Ag/Ag1 electrode as the working electrode,
counter electrode, and reference electrode, respectively, in a
0.1-mol L21 tetrabutylammonium hexafluorophosphate
(Bu4NPF6) acetonitrile solution. Polymer thin films were
formed by drop-casting polymer solutions in chloroform on
the working electrode and then dried in air.
SCHEME 1 Molecular Structure of P20-P24.
shown in Scheme 1, to study the effect of the imide-fused
unit density on the physicochemical and photovoltaic proper-
ties of the polymers. Imide was chosen as the electron-
withdrawing substituent because the polymers based on
imide-fused thiophene, TPD, have demonstrated excellent
photovoltaic properties as electron donors for OPVs.23–26
Note here that the symmetric structures of TPD and pristine
thiophene make these polymers regioisomer-free, in contrast
to 3-alkyl or 3-carboxylated thienylenevinylene polymer,
which made it more convenient to evaluate the effect of the
electron-deficient substituent density.
Fabrication of Photovoltaic Devices
PSCs were fabricated with ITO glass as a positive electrode,
Ca/Al as a negative electrode, and the blend film of the poly-
mer:PCBM between them as a photosensitive layer. The ITO
glass was precleaned and modified by a thin layer of
PEDOT:PSS, which was spin-cast from a PEDOT:PSS aqueous
solution (Clevious P VP AI 4083 H. C. Stark, Germany) on the
ITO substrate, and the thickness of the PEDOT:PSS layer was
about 60 nm. The photosensitive layer was prepared by spin-
coating a blend solution of the polymer and PC71BM in o-
dichlorobenzene (DCB) on the ITO/PEDOT:PSS electrode.
Then, the Ca/Al cathode was deposited on the polymer layer
by vacuum evaporation under 3 3 1024 Pa. The thickness of
the photosensitive layer was about 80 nm, measured on an
Ambios Tech XP-2 profilometer. The effective area of one cell
was about 4 mm2. The current–voltage (J vs. V) measurement
of the devices was conducted on a computer controlled Keith-
ley 236 source measure unit. A xenon lamp with an AM 1.5 fil-
ter was used as the white-light source, and the optical power
at the sample was 100 mW cm22. Atomic force microscopy
(AFM) images of the thin films were obtained on a Nanoscope
V AFM (Digital Instruments) operating in tapping mode.
The UV–vis absorption spectra of these polymers do not
have a strong dependence on the electron-deficient substitu-
ent density. As can be expected, the highest occupied molec-
ular orbital (HOMO) energy levels upshifted gradually (from
25.64 to 25.16 eV) with the decrease of the electron-
deficient substituent density. As a result, photovoltaic per-
formance of the PSCs based on the blend of the polymers as
donor and PC71BM as acceptor changed regularly. More
importantly, the effect of electron-deficient substituent den-
sity was studied systematically to better understand the rela-
tionship between the molecular energy levels and
photovoltaic properties of the PTVs.
EXPERIMENTAL
Materials
Synthesis of the Monomer and Polymers
2-Octyldodecylamine (4)27 and thieno[3,4-c]furan-1,3-dione
(5)23 were synthesized according to the literatures. Other
reagents and solvents were commercial grade and used as
received without further purification. All reactions were per-
formed under nitrogen atmosphere.
5-(2-Octyldodecyl)thieno[3,4-c]pyrrole-4,6-dione (6)
Thieno[3,4-c]furan-1,3-dione (5) (7.7 g, 50 mmol) and 2-
octyldodecylamine (4) (16 g, 52.5 mmol) were dissolved in tolu-
ene (300 mL) and the mixture was refluxed for 24 h. The reac-
tion mixture was cooled down and the solvent was removed
under reduced pressure. The residue solid was dissolved in thi-
onyl chloride (250 mL) and the mixture was refluxed for 3 h.
After the removal of the volatiles, the remnant was added to
water (300 mL) and extracted with dichloromethane (3 3 100
mL). The extracts were combined and washed with water and
brine then dried over anhydrous sodium sulfate. After filtration,
the solvent was removed under reduced pressure. The residue
crude product was purified by column chromatography on silica,
eluting with petroleum ether/dichloromethane (from 2:0 to
2:1), to give a white solid (20 g, 92%).
Measurements and Characterization
The molecular weights of the polymers were measured by
gel permeation chromatography (GPC) method. The GPC
measurements were performed on Waters 515-2410 with
polystyrenes as reference standard and tetrahydrofuran
(THF) as an eluent. All new compounds were characterized
by nuclear magnetic resonance spectra (NMR). The NMR was
recorded on a Bruker AV 600 spectrometer at room temper-
ature. Chemical shifts of 1H NMR and 13C NMR were
reported in ppm. Splitting patterns were designated as s
(singlet), t (triplet), d (doublet), m (multiplet), and br
(broaden). Elemental analyses were performed on a Flash
EA 1112 analyzer. UV–vis absorption spectra were recorded
1H NMR (600 MHz, CDCl3): d (ppm) 7.80 (s, 2H, Ar-H), 3.51 (d,
2H, CH2), 1.85 (m, 1H, CH), 1.34 (m, 2H, CH2), 1.24 (m, 30H,
CH2), 0.88 (t, 6H, CH3). 13C NMR (600 MHz, CDCl3): d (ppm)
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SCHEME 3 Synthetic route of polymers P20-P24.
(br, 2H), 2.45 (br, 1H), 1.34 (br, 32H), 0.91 (br, 6H). ELEM.
ANAL. Found: C: 73.18, H: 9.44, N: 3.05.
P21
The feed ratio of TPD:7:8 was 2:1:3. Yield: 37%. 1H NMR
(600 MHz, THF-d8): d (ppm) 8.80–5.80 (br, 18 H), 2.49 (s,
10 H), 2.45 (s, 5 H), 1.32 (br, 160 H), 0.92 (br, 30 H). ELEM.
ANAL. Found: C: 70.98, H: 7.79, N: 2.18.
SCHEME 2 Synthetic route of monomer TPD.
162.90, 136.63, 125.43, 42.73, 36.85, 31.89, 31.87, 31.43,
29.95, 29.61, 29.58, 29.53, 29.32, 29.27, 26.24, 22.66, 14.09.
P22
The feed ratio of TPD:7:8 was 1:1:2. Yield: 71%. 1H NMR
(600 MHz, THF-d8): d (ppm) 9.00–5.40 (br, 29 H), 2.49 (s,
10 H), 2.45 (s, 5 H), 1.32 (br, 160 H), 0.91 (br, 30H). ELEM.
ANAL. Found: C: 69.45, H: 6.61, N: 1.56.
13-Dibromo-5-(2-octyldodecyl)thieno[3,4-c]pyrrole-4,6-
dione
5-(2-Octyldodecyl)thieno[3,4-c]pyrrole-4,6-dione 6 (4.34 g,
10 mmol) was dissolved in a mixture of sulfuric acid (20 mL)
and trifluoroacetic acid (60 mL). NBS (5.37 g, 30 mmol) was
added to the solution and the reaction mixture was stirred at
room temperature overnight. The mixture was diluted with
water (200 mL) and then extracted with dichloromethane.
The organic phase was dried over anhydrous magnesium sul-
fate and the solvent was evaporated under reduced pressure.
The crude product was purified by column chromatography
using petroleum ether/dichloromethane (from 1:0 to 1:1) to
afford the title product as a white solid (4.9 g, 83%).
P24
The feed ratio of TPD:7:8 was 1:2:3. Yield: 33%. 1H NMR
(600 MHz, THF-d8): d (ppm) 7.80–5.80 (br, 48 H), 2.49 (s,
10 H), 2.45 (s, 5 H), 1.32 (br, 160 H), 0.92 (br, 30 H). ELEM.
ANAL. Found: C: 68.14, H: 5.65, N: 1.05.
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthesis routes of monomers and corresponding
polymers are outlined in Schemes 2 and 3, respectively.
2-Octyldodecylamine (4) was synthesized from 2-octyl-1-
dodecanol as shown in Scheme 2. Polymers P20–24 were
synthesized by palladium-catalyzed Stille coupling polymer-
1H NMR (600 MHz, CDCl3): d (ppm) 3.48 (d, 2H, CH2), 1.82
(m, 1H, CH2), 1.34 (m, 2H, CH2), 1.25 (m, 30H, CH2), 0.88 (t,
6H, CH3). 13C NMR (600 MHz, CDCl3): d (ppm) 160.61,
134.73, 112.86, 43.09, 36.84, 31.90, 31.88, 31.45, 29.89,
29.62, 29.58, 29.53, 29.33, 29.27, 26.27, 22.66, 14.10.
1
ization and confirmed by H NMR spectroscopy and elemen-
tal analysis. Based on the H NMR spectroscopy (Fig. 1) and
1
elemental analysis, the composition of the polymers were in
well agreement with the expected results (as shown in
Table 1), indicating that the synthetic approach adopted here
General Procedures of Polymerization
The synthesis of the polymers were carried out using
palladium-catalyzed Stille coupling between monomer 1,3-
dibromo-5-(2-octyldodecyl)thieno[3,4-c]pyrrole-4,6-dione (TPD),
2,5-dibromothiophene (7), and (E)-1,2-bis(tributylstannyl)-
ethane (8), as shown in Scheme 2. Under the protection of
nitrogen, monomers were dissolved in chlorobenzene. The
solution was flushed with nitrogen for 10 min, and then
Pd2(dba)3 (2.5% in mol), (o-tol)3P (20% in mol) was added.
After being purged for 15 min, the mixture was heated at
ꢀ
140 C for 48 h. After cooled to room temperature, the reac-
tion mixture was added dropwise to 100 mL methanol, fil-
tered through a Soxhlet thimble, and then subjected to
Soxhlet extraction with ethanol, hexane, and chloroform. The
polymer recovered from chloroform solution was purified by
preparative column chromatography. Then, the product was
dried under vacuum for 1 day to yield the titled polymers.
P20
The feed ratio of TPD:7:8 was 1:0:1. Yield: 72%. 1H NMR
(600 MHz, THF-d8): d (ppm) 9.05–6.00 (br, 2H), 2.49
FIGURE 1 1H NMR spectra of polymer solutions in THF-d8.
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TABLE 1 Polymerization Results and Characterization of the Copolymers
Feed Ratio
(TPD:7)
Copolymer Composition
(m:n)a
Polymer
Mn (K)
Mw (K)
PDI
P20
P21
P22
P24
2:0
2:1
2:2
2:4
2:0
32.2
20.0
29.6
15.1
100.0
63.4
85.7
49.3
3.1
3.2
2.9
3.3
2:0.8
2:1.9
2:3.8
a
Composition of the polymers were determined by elemental analyses.
enables the design and adjustment of the electron-deficient
substituent density in copolymer expediently according to
the demand. All the polymers can be readily dissolved in
common solvents, such as chloroform, toluene, THF and
chlorobenzene, and processed to form smooth and pinhole-
free films upon spin-coating. Feed ratio, copolymer composi-
tion, number average molecular weight (Mn), weight average
molecular weight (Mw), and polydispersity index (PDI) of the
polymers are listed in Table 1.
Ag/Ag1 electrode, respectively. The energy levels of HOMO
and LUMO of the polymers were calculated according to the
equations:
HOMO 52IP 52eðEox 14:71ÞðeV Þ;
LUMO 52EA 52eðEred 14:71ÞðeV Þ:
(1)
(2)
From the value of Eox and Ered of the polymers, the HOMO
and the LUMO were calculated and listed in Table 2. It had
been well recognized that the LUMO energy level of the
donor polymer must be positioned above that of the
acceptor PCBM by a reasonable value (e.g., 0.3 eV)6 to over-
come the binding energy of the excitons for efficient charge
separation.1,4,6 By reducing density of electron-deficient
Optical Properties
Figure 2 shows absorption spectra of the polymers in chloro-
form solution (1 mg/100 mL) and as spin-coated films on
quartz substrates. Table 2 summarizes the optical data,
including the absorption peak wavelengths (kabs), absorption
edge wavelengths (konset), and the optical band gap (Egopt).
All of the polymers showed broad absorption bands covering
the visible region in both chloroform solutions and solid
films (Fig. 2). In chloroform solutions, absorption intensity of
the polymers increased gradually from P20 to P24. The
absorption peak wavelength and edge wavelength were red-
shifted from P20 to P22, indicating the enhancement of
intramolecular charge transfer (ICT) effect. Extraordinarily,
P24 showed an absorption edge wavelength between P21
and P22 and an observable blue-shift of its absorption peak
in both solution and film states compared to P22. This phe-
nomenon may be due to the relative low molecular weight
and low electron-deficient imide group density in P24, which
can limit the ICT effect. In the film state, the absorptions
peaks of all the polymers were red-shifted by 9–18 nm com-
pared to those in solution. These bathochromic shifts were
on account of the intermolecular association and p–p stack-
ing of these polymer chains in the solid states.
Electrochemical Properties
Cyclic voltammetry had been employed and considered as an
effective tool in measuring the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels of conjugated oligomers and
polymers.28,29
Cyclic voltammograms of the polymer films are shown in
Figure 3. The onset oxidation potential (Eox)/onset reduction
potential (Ered) of P20, P21, P22, and P24 were 0.93/
20.96, 0.72/21.08, 0.60/21.16, and 0.45/21.28 V versus
FIGURE 2 Absorption spectra of the polymers in (a) chloroform
solution (concentration of 1 mg/100 mL) and (b) solid film on a
quartz plate.
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TABLE 2 Optical and Electrochemical Properties of the Polymers
opt
ec
kabs in CHCl3
kabs in films
konset in films
Eg
HOMO
(eV)
LUMO
(eV)
Eg
Polymer
(nm)
(nm)
(nm)
(eV)
(eV)
P20
P21
P22
P24
578, 628
588
587, 636
598
696
725
745
740
1.78
1.71
1.66
1.68
25.64
25.43
25.31
25.16
23.78
23.63
23.55
23.43
1.86
1.80
1.76
1.73
592
606
564
582
imido group, the HOMO and LUMO energy levels of the poly-
mers were upshifted gradually from P20 to P24, as shown
in Figure 4. The differences of LUMO energy levels between
the polymers and PC71BM (ca. 23.91 eV)30 ranged from 0.16
to 0.48 eV; thus, excitons dissociation efficiencies in their
corresponding BHJ devices could be very different.
With the imide substitution increasing to a half of thiophene
units in P22, the photovoltaic performance of the PTV deriv-
ative was improved dramatically, as shown in Figure 6 and
Table 3. All the Voc, Jsc, and FF of the PSC based on P22 as
donor and PC71BM as acceptor improved significantly in
comparison with those of the devices with P24 as donor.
PCE of the PSC based on P22 reached 0.86%, which is about
four times of that of P24. The strong electron-withdrawing
substitution of imide could be the reason for the improved
photovoltaic performance, like that for P3CTV.21 With further
increase of imide substitution on the thiophene units in the
polymers, P21 showed poorer photovoltaic performance
with a PCE of 0.12% and a lower short-circuit current of
0.39 mA cm22. Because LUMO energy level downshifted
Photovoltaic Properties
To investigate the influence of electron-deficient substituent
density on photovoltaic properties of the PTV derivatives,
bulk heterojunction PSC devices with a configuration of ITO/
PEDOT:PSS/polymer:PC71BM/Ca/Al were fabricated. Active
layers of the polymer:PC71BM (1:2, w/w) were spin-coated
from DCB solutions. The current density-potential character-
istic of PSCs based on the blends of polymer:PC71BM under
illumination of AM1.5G, 100 mW cm22 are shown in Figure
5. The open-circuit voltage (Voc), short-circuit current (Jsc),
fill factor (FF), thickness and mobility of active layers, maxi-
mum PCE (PCEmax), and average PCE (PCEave) were summar-
ized in Table 3.
The PSCs based on the imide-containing PTVs possessed
higher Voc than that of devices based on alkyl-substituted
PTVs.8,21 This can be attributed to the lower HOMO levels of
the polymers due to the electron-withdrawing imide sub-
stituents. The PCE (0.23%) of the PSCs based on P24 was
close to that of typical PTVs,17–21 indicating that one-third
imide substitution of the thiophene units in P24 influences
the photovoltaic performance of the polymers very little.
FIGURE 4 Energy level diagrams for the polymers and
PC71BM.
FIGURE 3 Cyclic voltammograms of the polymer films on
FIGURE 5 Current density-voltage characteristics of PSCs
glassy carbon electrode in 0.1 mol L21 Bu4NPF6, CH3CN solu-
based on polymer:PC71BM blends under illumination of
tion with a scan rate of 100 mV s21
.
AM1.5G, 100 mW cm22
.
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TABLE 3 Photovoltaic Properties of the PSCs Based on the Polymers as Donor and PC71BM as Acceptor Under the Illumination of
AM1.5G, 100 mW cm22
Polymer
Voc (V)
Jsc (mA cm22
)
FF
PCEmax/PCEave (%)
Thickness (nm)
Mobility (cm2 V21 s21
)
P20
P21
P22
P24
2
2
2
2/2
106
117
109
101
5.1 3 1025
3.9 3 1024
5.8 3 1024
1.5 3 1024
0.78
0.75
0.62
0.39
2.26
1.01
0.40
0.51
0.36
0.12/0.11
0.86/0.82
0.23/0.21
gradually from P22 to P21 to P20 with the increase of the
imide substitution, the difference between the LUMO energy
levels of the polymer donor and PC71BM acceptor decreases
from P22 to P21 to P20. For P21, the difference decreased
to 0.28 eV, which is not large enough to overcome the bind-
ing energy of the excitons for efficient charge separation, so
that the Jsc and PCE of the corresponding solar cell
decreased remarkably. For P20 with imide group fused to
every thiophene ring in the polymer, photovoltaic response
could not be observed due to too small difference between
LUMO energy levels of P20 and PC71BM. In the other hand,
the HOMO energy levels of the polymers also decreased with
the increase of the imide substitution from P24 to P20;
therefore, the Voc of the PSCs with the polymer as donor
increased accordingly.
|HOMO(PTVs)-LUMO(PCBM)| and Voc of corresponding solar
cells increased. It seems that, for PTVs donor materials, the
term of empirical factor (0.3) in eq 3 should not be a con-
stant but a function of HOMO(PTVs). There might be some
additional energetic loss existed in the PTVs:PCBM system.
PCBM
LUMO
Voc5ð1=eÞðjEDonor
HOMOj2jE
jÞ20:3 V
(3)
To further study the performance of the PSCs, the morphol-
ogy and hole mobilities of the photosensitive layers were
measured. Figure 8 shows the AFM images of the active
layer surfaces. The average surface roughness (Ra) values of
the AFM topographic images were all less than 1 nm for the
blends of PTVs:PC71BM, indicating smooth surface of the
active layers. The P20:PC71BM film showed clear phase sepa-
ration with
a large domain size over 100 nm. The
According to the following eq 3 proposed by Brabec et al.,6
the calculated Voc should be 1.22, 1.10, and 0.95 V for P21,
P22, and P24, respectively, which are 0.33–0.44 V higher
than the experimental values. In order to reveal the reason
for the deviation of the Voc from the theoretical value, we
studied the properties of the previously reported PTVs pho-
tovoltaic materials and summarized the relationship between
HOMO and Voc in Figure 7. Simple linear fit line of the col-
lected data was also shown in Figure 7. In the case of low
|HOMO(PTVs)-LUMO(PCBM)| (high HOMO), the Voc of PTVs-
based solar cells was coincided with eq 3. With the increase
of |HOMO(PTVs)-LUMO(PCBM)|, the Voc of PTVs-based solar
cells deviated from the line of eq 3. In another word, with
the decrease of HOMO level of PTVs, the difference between
P24:PC71BM film gave a featureless phase separation image.
They were not good for exciton transportation and dissocia-
tion on the interface of polymer and PC71BM. For the
P21:PC71BM and P22:PC71BM blend films, the clear patterns
of polymer/PCBM domains showed good interpenetrating
networks which were beneficial to the exciton dissociation
and charge carriers transport.
The hole mobilities in the photosensitive layers were meas-
ured by the space charge-limited current (SCLC) method
using devices with structure of ITO/PEDOT:PSS/
polymer:PC71BM/Au. For unipolar transport in a trap-free
FIGURE 7 Open-circuit voltage (Voc) of different PTVs-based
bulk-heterojunction solar cells plotted versus the difference
between HOMO of the PTVs and LUMO of PC71BM. The
straight solid line represents a linear fit; dash line represents
the eq 3.
FIGURE 6 Short-circuit current of the solar cells based on the
polymers with variable LUMO levels.
4980
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POLYMER SCIENCE
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ARTICLE
FIGURE 8 Tapping mode AFM height images (2 3 2 lm2) of the polymer:PC71BM blend films.
semiconductor with an ohmic injecting contact, the SCLC can
be approximated by the Mott-Gurney equation31
substituent, via a palladium-catalyzed Stille coupling method.
The content of the imide substituent on thiophene units of
the PTVs was found to have a great impact on electrochemi-
cal and photovoltaic properties of the PTV derivatives. All of
the four polymers show broad absorption bands covering
the visible region with optical bandgaps of 1.78, 1.71, 1.66,
and 1.68, respectively. By increasing content of imide substit-
uent from P24 to P20, the LUMO and HOMO energy levels of
the polymers downshifted gradually from 23.43 to 23.75
eV and from 25.16 to 25.64 eV, respectively. P24 with one-
third imide substitution on thiophene units show a similar
photovoltaic performance with hexyl-substituted PTV. P22
with a half imide substitution on the thiophene units pos-
sesses a suitable LUMO and HOMO energy levels, and dis-
played an improved photovoltaic performance with PCE
increased four times to 0.86%. But further increase of the
content of imide substituent in P21 and P20 results in too
low LUMO energy level and decreased photovoltaic per-
formance. This work provide a facile electronic energy-level
regulation method, which allows ones to design photovol-
taic polymers for better understanding the relationship
between the molecular structure and photovoltaic proper-
ties of PTVs.
:
!
rffiffiffi
9
V
V2
L3
J ffi ere0l0exp 0:891c
(4)
8
L
where J is the current density, er is the dielectric constant of
the polymer, e0 is the free-space permittivity (8.85 3 10212
F m21), l0 is the charge mobility at zero field, c is a con-
stant, L is the thickness of the blend film layer, V 5 Vappl
2
Vbi, Vappl is the applied potential, and Vbi is the built-in
potential which results from the difference in the work func-
tion of the anode and the cathode (in this device structure,
Vbi 5 0.2 V). Figure 9 displayed ln(JL3/V2) versus (V/L)0.5
curve for the measurement of the hole mobility of the
copolymers by the SCLC method. As summarized in Table 3,
the hole mobilities of the polymers calculated using eq 4 are
5.1 3 1025, 3.9 3 1024, 5.8 3 1024, and 1.5 3 1024
cm2V21 21
for P20, P21, P22, and P24, respectively. Obvi-
s
ously, the photovoltaic performance of the polymers is not
determined by their hole mobilities, but mainly influenced
by the LUMO energy levels and the electron-withdrawing
substitution.
ACKNOWLEDGMENTS
CONCLUSIONS
This work was supported by Beijing Natural Science Founda-
tion (2122047), the Ministry of Science and Technology of
China (No. 2011AA050523 and 2010DFB63530), NSFC
(No.50933003 and 21021091) and the Chinese Academy of Sci-
ences (No. KGCX2-YW-39919-1 and GJHZ1124).
To investigate the influence of electron-deficient substituent
content on the photovoltaic properties of the PTV derivatives,
we designed and synthesized
a series of novel PTVs,
P20–P24, with various content of electron-deficient imide
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