Replacing Alkyl with Oligo(ethylene glycol) as Side Chains of Conjugated Polymers for Close π-π Stacking

Article abstract of DOI:10.1021/acs.macromol.5b00702

We synthesize and systematically study a series of conjugated polymers with oligo(ethylene glycol) (OEG) or alkyl chain as the side chain and poly[2,7-fluorene-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] as the polymer backbone. Replacing alkyl chain with OEG chain can decrease the π-π stacking distance of polymer backbone in thin film from 0.44 to 0.41 nm because OEG chain is more flexible than alkyl chain. As the result, the conjugated polymer with OEG side chain exhibits higher hole mobility, red-shifted absorption spectrum in thin film and smaller bandgap than those of the conjugated polymer with alkyl side chain. With the increase of the length of OEG side chain, the resulting conjugated polymers exhibit unchanged π-π stacking distance and decreased hole mobility. Moreover, owing to the large polarity of OEG chain, OEG side chain makes the conjugated polymer suitable for polymer solar cell (PSC) devices processed with polar nonhalogenated solvent, methoxybenzene. A power conversion efficiency of 4.04% is demonstrated with the resulting PSC devices. This work provides the new insight into the effect of OEG side chain on conjugated polymer, which can be used in the molecular design of novel conjugated polymer materials with excellent optoelectronic device performance. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.macromol.5b00702

Article
pubs.acs.org/Macromolecules
Replacing Alkyl with Oligo(ethylene glycol) as Side Chains of
Conjugated Polymers for Close π−π Stacking
Bin Meng,†,‡ Haiyang Song,† Xingxing Chen,†,‡ Zhiyuan Xie,† Jun Liu,*,† and Lixiang Wang*,†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China
‡University of Chinese Academy of Sciences, Beijing 100039, P. R. China
S
* Supporting Information
ABSTRACT: We synthesize and systematically study a series
of conjugated polymers with oligo(ethylene glycol) (OEG) or
alkyl chain as the side chain and poly[2,7-fluorene-alt-5,5-(4,7-
di-2-thienyl-2,1,3-benzothiadiazole)] as the polymer backbone.
Replacing alkyl chain with OEG chain can decrease the π−π
stacking distance of polymer backbone in thin film from 0.44
to 0.41 nm because OEG chain is more flexible than alkyl
chain. As the result, the conjugated polymer with OEG side
chain exhibits higher hole mobility, red-shifted absorption
spectrum in thin film and smaller bandgap than those of the
conjugated polymer with alkyl side chain. With the increase of
the length of OEG side chain, the resulting conjugated
polymers exhibit unchanged π−π stacking distance and
decreased hole mobility. Moreover, owing to the large polarity of OEG chain, OEG side chain makes the conjugated polymer
suitable for polymer solar cell (PSC) devices processed with polar nonhalogenated solvent, methoxybenzene. A power
conversion efficiency of 4.04% is demonstrated with the resulting PSC devices. This work provides the new insight into the effect
of OEG side chain on conjugated polymer, which can be used in the molecular design of novel conjugated polymer materials
with excellent optoelectronic device performance.
Scheme 1. Schematic Illustration of the Superior Flexibility of
OEG Chain Than Alkyl Chain
INTRODUCTION
■
Conjugated polymers have been widely used in optoelectronic
devices, such as organic light-emitting diodes (OLEDs), organic
field-effect transistors (OFETs), and polymer solar cells (PSCs),
with the great advantages of low cost, flexibility, and solution
processing.1 For solubility of conjugated polymers, a rigid
conjugated polymer backbone is always equipped with an alkyl
side chain.2 Oligo(ethylene glycol) (OEG) chain is well-known
for its hydrophilicity and is always used to endow molecules/
polymers solubility in water or polar organic solvents.3 Thus,
OEG chain has been used as side chain of conjugated polymers
for efficient OLED, OFET, and PSC devices processed with
polar nonhalogenated solvents.4 As optoelectronic properties of
conjugated polymers are affected by the side chain,5 the effect of
OEG side chain on properties of conjugated polymers is less
investigated and poorly understood. Researchers may doubt
whether hydrophilic OEG side chain would disturb the π−π
stacking of hydrophobic polymer backbone in conjugated
polymers. This manuscript aims to provide an insight into the
effect of OEG side chain on conjugated polymers.
the energy barrier of the rotation of O−CH2 in OEG chain (E =
0.08 eV) is smaller than that of CH2−CH2 in alkyl chain (E =
0.11 eV),6 making OEG chain more flexible than alkyl chain. For
conjugated polymers, alkyl side chain impedes the close π−π
We note that OEG chain is more flexible than alkyl chain. As
shown in Scheme 1, the two hydrogen atom in CH2 unit act as
steric hindrance for the rotation of CH2−CH2 unit in alkyl chain,
while the two lone electron pair in oxygen atom result in no steric
hindrance for the rotation of O−CH2 in OEG chain.6 Therefore,
Received: April 5, 2015
Revised: June 10, 2015
© XXXX American Chemical Society
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DOI: 10.1021/acs.macromol.5b00702
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a
Scheme 2. Chemical Structures and Synthetic Routes of the Four Polymers
a
Reagents and conditions: (i) p-tolylsulfate chloride, NaOH (aq), THF, room temperature; (ii) 2,7-dibrmofluorene, NaOH (aq, 50 wt %), (n-
C4H9)4NBr, toluene, reflux; (iii) 4,4,5,5-tetramethyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane, Pd(dppf)Cl2, CH3COOK,
DMF, 90 °C; (iv) Pd(PPh3)4, K2CO3 (aq, 2 M), Aliquat 336, toluene, 100 °C.
Table 1. Molecular Weights, Thermal Properties, π−π Stacking Distance, and Hole Mobility of the Four Polymers
polymer
Mn
PDI
Td (°C)
Tcc (°C)
Tm (°C)
dπ−π (nm)
μh (cm2 V−1 s−1
)
PFDTOBT
59600
55000
78760
50100
2.86
2.30
2.99
2.04
310
315
320
322
−
118
123
−
−
220
148
−
0.44
0.41
0.41
0.41
1.48 × 10−6
1.10 × 10−5
2.61 × 10−6
5.07 × 10−7
PFDTOBT-O2
PFDTOBT-O3
PFDTOBT-O4
stacking of polymer backbone because the stacking distance of
alkyl chain (4.1 Å) is larger than that of ideal π−π stacking
distance (ca. 3.4 Å).7 Therefore, compared with alkyl side chain,
the more flexible OEG side chain can favor the π−π stacking of
polymer backbone of conjugated polymers. As optical and
electronic properties of conjugated polymers are affected by the
solid state organization of polymer backbone,8 the close π−π
stacking favored by OEG side chain can affect the optical and
electronic properties of conjugated polymers.
Motivated by the above data and analysis, in order to disclose
the effect of OEG side chain on conjugated polymer, we
synthesize and systematically study a series of conjugated
polymers with OEG of different length or alkyl chain as the
side chain and poly[2,7-fluorene-alt-5,5-(4,7-di-2-thienyl-2,1,3-
benzothiadiazole)] (PFDTOBT-m) as the conjugated polymer
backbone. We find that replacing alkyl chain with OEG chain can
decrease the π−π stacking distance of polymer backbone from
0.44 to 0.41 nm. As the result, the polymers with OEG side chain
have higher hole mobility and smaller bandgap than those of the
polymer with alkyl side chain. Moreover, owing to the large
polarity of OEG chain, OEG side chain makes the conjugated
polymer suitable for PSC devices processed with polar
nonhalogenated solvent. This work provides the new insight
into the effect of OEG side chain on conjugated polymer, which
can be used in the molecular design of novel conjugated polymer
materials with excellent optoelectronic device performance.
Figure 1. GI-XRD patterns of the four polymers.
RESULTS AND DISCUSSION
■
Figure 2. DSC second heating curves of the four polymers.
Molecular Design. Scheme 2 shows the chemical structures
of the four conjugated polymers with the same polymer
backbone and different side chains. PFDTOBT-m is selected
as the conjugated polymer backbone9 because each fluorene unit
can be functionalized with two side chains with “sticking-out”
configuration. Since the “sticking-out” side chains can prevent
the close stacking of the conjugated polymer backbone, the
flexibility of the side chains may obviously affect the π−π stacking
distance of the polymer backbone.10 Thus, PFDTOBT-m is very
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Figure 3. J1/2 vs (Vappl − Vbi) curves of the hole-only devices of the four
polymers. (device structure: ITO/PEDOT:PSS/polymer/Au).
Figure 5. (a) Cyclic voltammogram of the four polymers in thin film. (b)
HOMO and LUMO energy levels of the four polymers and PC61BM.
further study the effect of the length of OEG side chain, we
design PFDTOBT-O3 and PFDTOBT-O4 with tri(ethylene
glycol) side chain and tetra(ethylene glycol) side chain,
respectively.
Synthesis. The synthetic routes for the four polymers are
depicted in Scheme 2. Tosylation of corresponding hydroxyl
compound and subsequent alkylation with 2,7-dibromofluorene
readily gave 2a−c. The key monomers 3a−c was prepared by
Miyaura borylation reaction of 2a−c and bis(pinacolato)-
diboron. Finally, we synthesized the four polymers using
Pd(0)-catalyzed Suzuki polycondensation with the correspond-
ing diboronic ester monomer and the dibromo monomer. All
these polymers exhibit good solubility in common chlorinated
solvents, e.g. chloroform, CB, and in some nonhalogenated
solvents, e.g., xylene, methoxybenzene (MOB). The chemical
Figure 4. UV−vis absorption spectra of the four polymers in CB
solution (a) and in thin film (b).
1
suitable for investigation of the effect of side chains on π−π
stacking of polymer backbone. To ensure good solubility and
high molecular weight of all the polymers, we use two hexyl unit
on the 2,1,3-benzothiadiazole unit.11 As shown in Scheme 2,
PFDTOBT with nonpolar octyl chains on the fluorene unit is a
conventional conjugated polymer and is used here as the
reference.12 To investigate the effect of OEG side chain, we
devise PFDTOBT-O2 with di(ethylene glycol) side chains of
similar length with that of the octyl chains in PFDTOBT. To
structures of the polymers are confirmed by H NMR and
elemental analysis. Their molecular weights are estimated by gel
permeation chromatography (GPC) using tetrahydrofuran
(THF) as the eluent and monodisperse polystyrene as the
standards. As listed in Table 1, the four polymers all have
number-average molecular weight (Mn) higher than 50 000 and
polydispersities (PDI) around 2.5.
Stacking in the Solid State. To investigate the solid state
organization of polymer chains, grazing incidence X-ray
Table 2. Photophysical and Electrochemical Properties of the Polymers
sol
film
film
ox
a
red
a
λmax
λmax
λonset
Egopt
(eV)
Eonset
Eonset
HOMO
(eV)
LUMO
(eV)
Egec
(eV)
polymer
(nm)
ξ (×104 L·mol−1·cm−1
)
(nm)
(nm)
(V)
(V)
PFDTOBT
517
515
517
515
5.64
5.25
5.39
4.68
523
547
547
549
606
616
615
613
2.05
2.01
2.02
2.02
0.76
0.60
0.56
0.52
−1.77
−1.70
−1.66
−1.60
−5.56
−5.40
−5.36
−5.32
−3.03
−3.10
−3.14
−3.20
2.53
2.30
2.22
2.12
PFDTOBT-O2
PFDTOBT-O3
PFDTOBT-O4
a
Onset potential vs Fc/Fc+.
C
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Figure 6. J−V curves (a) and EQE curves (b) of the PSC devices based on the polymers processed with CB as the solvent. J−V curves (c) and EQE
curves (d) of the PSC devices based on the polymers processed with MOB as the solvent.
diffraction (GI-XRD) of spin-coated film of the four polymers
was performed. As shown in Figure 1, all the four polymers show
weak diffraction peaks due to the lamellar stacking and moderate
diffraction peaks assigned to the π−π stacking of the polymer
backbone. The diffraction peak of PFDTOBT with alkyl side
chain is at 2θ = 20.3°, corresponding to the π−π stacking distance
250 °C. In contrast, the DSC curve of PFDTOBT-O2 shows an
exothermal peak at 118 °C with an area of 6.93 J·g−1 and an
endothermal peak at 220 °C with an area of 11.77 J·g−1, which is
attributed to the cold crystallization and melting of PFDTOBT-
O2, respectively. The comparison of the DSC curves of
PFDTOBT and PFDTOBT-O2 suggests that OEG side chain
make the resulting polymer more crystalline in solid state. This is
consistent with the closer π−π stacking of conjugated polymer
backbone of PFDTOBT-O2 than that of PFDTOBT. The cold
crystallization peak (Tcc = 123 °C, 1.71 J·g−1) and melting peak
(Tm = 148 °C, 1.62 J·g−1) in DSC curves become weak for
PFDTOBT-O3. The two peaks even diminish for PFDTOBT-
O4, indicating that longer OEG side chain makes the resulting
polymer less crystalline.
Hole Mobility. The hole mobility of the polymers was
estimated using space charge limited current (SCLC) method
according to the current density1/2 − voltage (J1/2 − V) curves of
the hole-only devices (device structure: ITO (indium tin oxide)/
PEDOT:PSS (poly(3,4-ethylenedioxythiophene) doped with
poly(styrenesulfonate))/polymer/Au) (Figure 3). As listed in
Table 1, the hole mobility of PFDTOBT-O2 (μh = 1.10 × 10−5
cm2 V−1 s−1) is much higher than that of PFDTOBT (μh = 1.48 ×
10−6 cm2 V−1 s−1), which is attributed to the aforementioned
close π−π stacking of PFDTOBT-O2 with OEG side chain.
Comparison of the hole mobility of PFDTOBT-O2,
PFDTOBT-O3 and PFDTOBT-O4 implies that the hole
mobility decreases with the increase of the OEG side chain
length. Since side chains are inert and charge carriers transport
on conjugated polymer backbone, the decreased hole mobility
with increased OEG side chain length is attributed to the
increased content of inert component and decreased content of
conjugated polymer backbone in the polymers. The hole
mobility data are consistent with the less ordered solid state
(dπ−π) of 0.44 nm (Table 1).10 In comparison, all the three
o
polymers having OEG side chains show 2θ = 21.7
,
corresponding to the dπ−π of 0.41 nm (Table 1). Compared
with alkyl side chain, OEG side chain favors more compact π−π
stacking of the conjugated polymer backbone because OEG side
chain is more flexible with smaller steric hindrance for the π−π
stacking. Researchers always doubt whether the hydrophilicity of
OEG side chain disturbs π−π stacking of hydrophobic
conjugated polymer backbone. Comparison of the XRD patterns
of PFDTOBT-O2 and PFDTOBT unambiguously verifies that
OEG side chain favors ordered organization of polymer
backbone in solid state and that the hydrophilicity of OEG
side chain cannot play a dominant role. Moreover, the same dπ−π
of PFDTOBT-O2, PFDTOBT-O3, and PFDTOBT-O4
suggests that the length of OEG side chain does not affect the
π−π stacking distance of conjugated polymer backbone. Because
optical and electronic properties of conjugated polymers are
affected by organization of polymer chains in solid state, the close
π−π stacking with OEG side chains is expected to change the
properties of the polymers in thin film.
Thermal Properties. According to thermogravimetric
analysis (TGA), all the four polymers have good thermal stability
with thermal degradation temperature (Td) higher than 310 °C
(Table 1). Then differential scanning calorimetry (DSC) was
employed to investigate the thermodynamic behaviors of the
four polymers. The second heating curves of these polymers are
shown in Figure 2. There is no obvious exothermal and
endothermal peak for PFDTOBT in the temperature range 50−
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Figure 8. J−V curves (a) and EQE curves (b) of the PSC devices based
on PFDTOBT-O2 processed with MOB and 5 vol % DIO. The inset is
the TEM image of the film of PFDTOBT-O2:PC61BM processed with
MOB and 5 vol % DIO.
Egopt of PFDTOBT-O2, PFDTOBT-O3, and PFDTOBT-O4 is
2.01 eV (Table 2).
Electrochemical Properties. Cyclic voltammetry (CV) of
the thin film was carried out to investigate the electrochemical
properties and estimate the LUMO/HOMO energy levels of the
four polymers. As shown in Figure 5a, all four polymers show
both oxidation and reduction waves assigned to the conjugated
polymer backbone. According to the onset potential of the
oxidation waves and the reduction waves, we estimate the
HOMO and LUMO energy levels of the four polymers. The
results are listed in Table 2 and compared in Figure 5b.
Compared with PFDTOBT, PFDTOBT-O2 with an OEG side
chain of similar length shows a much decreased LUMO level and
an increased HOMO level with smaller bandgap. Considering
their XRD and UV/vis absorption spectra, we attribute the
change of LUMO/HOMO mainly to the enhanced coplanarity
of the polymer backbone in PFDTOBT-O2 because of the
compact π−π stacking with flexible OEG side chain. Similar
energy level change with varied π−π stacking distance been
reported previously in conjugated polymers with different alkyl
chains.5 For PFDTOBT-O2, PFDTOBT-O3, and PFDTOBT-
O4, with the increase of the OEG side chain length, the HOMO
level increases and the LUMO level decreases, leading to smaller
bandgap. This is inconsistence with their similar absorption
spectra in thin film. The reason for the inconsistency is under
further investigation. As shown in Figure 5b, despite of the
difference, the LUMO levels of all the four polymers are all high
enough to ensure electron transfer to the typical electron
acceptor, phenyl-C61-butyric acid methyl ester (PC61BM),
indicating that all these four polymer can be used as effective
donor polymers for PSCs.14 On the other hand, the different
HOMO levels of the four polymers are expected to influence the
open-circuit voltage (VOC) of the resulting PSC devices.14
Figure 7. TEM images of the film of PFDTOBT (a, b), PFDTOBT-O2
(c, d), PFDTOBT-O3 (e, f), and PFDTOBT-O4 (g, h) blended with
PC61BM processed with CB (a, c, e, g) and MOB (b, d, f, h).
organization of polymer chains with increasing OEG side chain
length as revealed in DSC curves and XRD patterns.
Absorption Spectra. Figure 4 shows the UV−vis absorption
spectra in CB solution and in film of the four polymers. In CB
solution, the four polymers exhibit identical absorption spectra
with two absorption peaks at 390 and 516 nm, which are ascribed
to the π−π* transition and the intramolecular charge transfer of
the polymer backbone, respectively.12 The same absorption
spectra of the four polymers with different side chains in solution
imply that the OEG side chains do not affect the electronic
structure of the conjugated polymer backbone in isolated state.
In thin film, the long-wavelength absorption peak of PFDTOBT,
PFDTOBT-O2, PFDTOBT-O3, and PFDTOBT-O4 is red-
shifted to 523, 547, 547, and 549 nm, respectively. The redshift of
the absorption peak from in solution to in film is due to the
interaction of conjugated polymer backbone in solid state.13 The
absorption peak redshift of the three polymers with OEG side
chains (Δλ = 31−33 nm) is much larger than that (Δλ = 7 nm) of
PFDTOBT with alkyl side chain. This indicates the stronger
interaction of conjugated polymer backbone owing to the smaller
π−π stacking distance of the three polymers with OEG side
chains compared to PFDTOBT with alkyl side chain. On the
basis of the onset wavelength of absorption spectra, the optical
bandgap (Egopt) of PFDTOBT is estimated to be 2.05 eV and the
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Table 3. Characteristics of the PSC Devices of the Polymers with CB or MOB as the Processing Solvent
a
a
a
a
polymer
processing solvent
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
PFDTOBT
CB
0.99 0.01
0.94 0.01
0.90 0.01
0.82 0.02
1.04 0.01
1.03 0.01
0.96 0.01
0.87 0.02
1.02 0.01
6.20 0.11
6.40 0.06
6.61 0.10
4.40 0.09
4.69 0.11
6.79 0.05
5.36 0.09
5.02 0.12
7.48 0.06
0.36 0.01
0.42 0.01
0.40 0.01
0.34 0.02
0.41 0.01
0.47 0.01
0.45 0.02
0.40 0.01
0.52 0.01
2.28 0.07
2.58 0.04
2.35 0.07
1.20 0.10
2.00 0.05
3.29 0.03
2.30 0.05
1.72 0.11
4.00 0.04
PFDTOBT-O2
PFDTOBT-O3
PFDTOBT-O4
PFDTOBT
CB
CB
CB
MOB
MOB
MOB
MOB
MOB + DIO
PFDTOBT-O2
PFDTOBT-O3
PFDTOBT-O4
PFDTOBT-O2
a
The data shown are the average values obtained from 6 devices with standard deviation.
PSC Devices. To evaluate and compare the photovoltaic
PFDTOBT-O3, and PFDTOBT-O4 with OEG side chains,
more uniform and clear phase separation morphology is
observed when processed by MOB compared to processed
with CB (Figure 7c−h). The effect of processing solvent on the
morphology of polymer:PC61BM blend of the four polymers is in
accordance with the corresponding PSC device performance. For
the blend of PFDTOBT-O2:PC61BM processed with MOB,
which gives the best PSC device performance (Figure 7d), clear
phase separation of donor polymer (relatively white region) and
acceptor material (relative dark region) can be observed.
However, the phase separation domain size is much larger than
ideal (10−20 nm) for high photovoltaic performance.16 The
AFM images are shown in Figure S2. For PFDTOBT with alkyl
side chains, more uniform surface (root-mean-square (rms)
roughness: 0.24 vs 0.32 nm) is observed when processed with CB
compared to processed with MOB (Figure S2 a, e). In
comparison, for PFDTOBT-O2 with OEG side chains, the film
surface is more smooth when processed with MOB than that
processed with CB.
Device Improvement. Although the device based on
PFDTOBT-O2:PC61BM processed with MOB exhibits the
best photovoltaic performance, it still suffers from large phase
separation domain size, which limits further device performance
enhancement. To improve the morphology of the blend, we use
1,8-diiodooctane (DIO) as a solvent additive.17 We test the
volume ratio (DIO/MOB) of 1 vol %, 3 vol %, 5 vol %, 7 vol %
and 9 vol %, and find that 5 vol % is the optimized one (Figure S3
and Table S1, Supporting Information). As shown in the inset of
Figure 8b, after adding 5 vol % DIO to MOB, the phase
separation of PFDTOBT-O2:PC61BM blend is more obvious
and the phase separation domain size decreases to about 50 nm,
which is beneficial for good photovoltaic performance. The
resulting PSC device based on PFDTOBT-O2 processed with
MOB and 5 vol % DIO shows the PCE of 4.04% with the VOC of
1.03 V, JSC of 7.54 mA/cm2, and FF of 0.52 (Figure 8a and Table
3). The increased JSC is confirmed by the EQE curve, which
shows the average EQE value of about 55% (Figure 8b). The
increase of JSC and FF with DIO additive is due to the improved
phase separation of the PFDTOBT-O2:PC61BM active layer
(the inset of Figure 8b). Considering the poor overlap of the
absorption spectrum of PFDTOBT-O2 with solar spectra, we
believe that a PCE of exceeding 4% is an exciting result for device
of PFDTOBT-O2:PC61BM. Indeed, this PCE value is among the
highest reported for PSC devices of conjugated polymers with
the same polymer backbone, poly(fluorene-alt-co-(4,7-di-2-
thienyl-2,1,3-benzothiadiazole)).18 Our results indicate that
OEG side chain is a successful strategy to make donor polymers
suitable for processing with nonhalogenated solvent.
properties of the four polymers, PSC devices were fabricated
using the structure of ITO/PEDOT:PSS/polymer:PC61BM/
LiF/Al. The polymer:PC61BM ratio was fixed to be 1:4. Both
halogenated solvent CB and nonhalogenated solvent MOB were
used for the spin-coating of the active layers for comparison.15
Parts a and c of Figure 6 show the J−V curves of the PSC devices
of the four polymers processed with CB and MOB, respectively.
The control device of PFDTOBT processed with CB exhibits a
PCE of 2.35% with VOC = 1.00 V, JSC = 6.31 mA/cm2, and FF =
0.37. In comparison, the PSC device based on PFDTOBT-O2
processed with CB shows a higher PCE of 2.62% with an
obviously increased JSC and FF, which may attribute to its higher
hole mobility and better active layer morphology (vide infra).
The comparison of the two devices suggests that OEG side chain
has no negative effect on photovoltaic performance of the
polymers.
It is found that PFDTOBT-O2, PFDTOBT-O3, and
PFDTOBT-O4 with OEG side chains exhibit VOC lower than
that of PFDTOBT with alkyl side chains. Increase of the OEG
side chain length leads to decrease of VOC. The relationship of the
VOC with the chemical structure of the four polymers is
consistent with the CV results and their HOMO levels (Figure
5b).14 The device of PFDTOBT with MOB as the processing
solvent exhibits inferior photovoltaic performance compared
with the counterpart with CB as the solvent. In contrast, the
devices of PFDTOBT-O2 and PFDTOBT-O4 processed with
MOB show superior photovoltaic performance than the
corresponding devices processed with CB. These results suggest
that OEG side chains make conjugated polymers suitable for
processing with nonhalogenated solvent MOB. Among all these
device, the device of PFDTOBT-O2 processed with MOB
exhibits the best photovoltaic performance with the VOC of 1.03
V, JSC of 6.84 mA/cm2, and FF of 0.47, corresponding to the PCE
of 3.32%. The external quantum efficiency (EQE) curves of the
devices are shown in Figure 6, parts b and d. The integral JSC of
the EQE curves of the devices are all in errors within 5% with the
value from the J−V measurement. We note that the devices
processed with MOB as the solvent exhibits higher VOC than that
of the corresponding device processed with CB. The reason is
not clear yet and need further investigation.
Morphology. The effect of chemical structure and
processing solvent on the morphology of the active layer in the
PSC devices was investigated by transmission electron
microscopy (TEM) and atomic force microscopy (AFM). The
TEM results are shown in Figure 7. For PFDTOBT with alkyl
side chains, more uniform phase separation morphology is
observed when processed by CB compared to processed with
MOB (Figure 7a,b). In contrast, for PFDTOBT-O2,
F
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CONCLUSIONS
■
In summary, we have synthesized four conjugated polymers with
oligo(ethylene glycol) (OEG) or alkyl chain as the side chain and
investigated the effect of OEG side chain on the properties of
conjugated polymers. Replacing alkyl chain with OEG chain can
decrease the π−π stacking distance of polymer backbone in thin
film from 0.44 to 0.41 nm despite of the length of OEG chain,
because OEG chain is more flexible than alkyl chain. Compared
with PFDTOBT with alkyl chain, PFDTOBT-O2 with OEG
chain of similar length exhibits higher crystallinity, higher charge
carrier mobility and narrower bandgap. As the length of OEG
chain increasing, PFDTOBT-O3 and PFDTOBT-O4 exhibit
decreased hole mobility due to the increased content of inert
component in the polymers. The large polarity of OEG side
chain makes the conjugated polymer suitable for PSC devices
fabricated with polar nonhalogenated solvent. According to the
new insight on conjugated polymers with OEG side chain,
conjugated polymers with high charge carrier mobility by using
OEG side chains with proper length may be anticipated.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis procedure of the materials, characterization, fabrication
of PSC devices and hole-only devices, absorption spectra of the
materials in MOB solution, AFM topography images, J−V and
EQE curves, and characteristics of the PSC devices. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.macromol.5b00702.
(13) Wang, X. C.; Jiang, P.; Chen, Y.; Luo, H.; Zhang, Z. G.; Wang, H.
Q.; Li, X. Y.; Yu, G.; Li, Y. F. Macromolecules 2013, 46, 4805.
(14) (a) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58. (b) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153.
(c) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107,
1324.
AUTHOR INFORMATION
Corresponding Authors
*(J.L.) E-mail: liujun@ciac.ac.cn.
*(L.W.) E-mail: lixiang@ciac.ac.cn.
■
Notes
(15) (a) Meng, B.; Fu, Y. Y.; Xie, Z. Y.; Liu, J.; Wang, L. X. Polym. Chem.
2015, 6, 805. (b) Duan, C. H.; Cai, W. Z.; Hsu, B. B. Y.; Zhong, C. M.;
Zhang, K.; Liu, C. C.; Hu, Z. C.; Huang, F.; Bazan, G. C.; Heeger, A. J.;
Cao, Y. Energy Environ. Sci. 2013, 6, 3022. (c) Chueh, C.-C.; Yao, K.; Yip,
H.-L.; Chang, C.-Y.; Xu, Y.-X.; Chen, K.-S.; Li, C.-Z.; Liu, P.; Huang, F.;
Chen, Y. W.; Chen, W.-C.; Jen, A. K.-Y. Energy Environ. Sci. 2013, 6,
3241. (d) Guo, X.; Zhang, M. J.; Cui, C. H.; Hou, J. H.; Li, Y. F. ACS
Appl. Mater. Interfaces 2014, 6, 8190.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the 973 Project (No.
2014CB643504, 2015CB655001), the Nature Science Founda-
tion of China (No. 51373165), the Strategic Priority Research
Program of Chinese Academy of Sciences (No. XDB12010200),
and the “Thousand Talents Program” of China.
(16) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc.
Rev. 2011, 40, 1185.
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2011, 133, 20661.
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Sweelssen, J.; Koetse, M. M. Appl. Phys. Lett. 2007, 90, 143506.
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DOI: 10.1021/acs.macromol.5b00702
Macromolecules XXXX, XXX, XXX−XXX