Dithienosilole–phenylquinoxaline-based copolymers with A-D-A-D and A-D structures for polymer solar cells

Article abstract of DOI:10.1002/pola.28904

Two copolymers having D-A-D-A (P1) and D-A (P2) structures with quinoxaline acceptor unit and dithienosilole donor unit were synthesized and their optical and electrochemical (both experimental and theoretical) properties were investigated. The optical pr

Full text of DOI:10.1002/pola.28904

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Dithienosilole–Phenylquinoxaline-Based Copolymers with A-D-A-D and
A-D Structures for Polymer Solar Cells
1
1,2
1
1
3,4
M. L. Keshtov, A. R. Khokhlov, S. A. Kuklin, A. Yu Nikolaev, E. N. Koukaras,
5
Ganesh. D. Sharma
1
Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova St., 28, Moscow 119991, Russian
Federation
2
Lomonosov Moscow State University, Faculty of Physics, 1-2 Leninskiye Gory, Moscow 119991, Russian Federation
3
Nanotechnology and Advanced Materials Laboratory, Department of Chemical Engineering, University of Patras, Patras, GR
2
6500, Greece
4
Molecular Engineering Laboratory, Department of Physics, University of Patras, Patras, GR 26500, Greece
Department of Physics, The LNM Institute of Information Technology, Jamdoli, Jaipur, Rajasthan, India
5
Correspondence to: M. L. Keshtov (E-mail: keshtov@ineos.ac.ru) or G. D. Sharma (E-mail: gdsharma273@gmail.com)
Received 22 September 2017; accepted 26 October 2017; published online 00 Month 2017
DOI: 10.1002/pola.28904
ABSTRACT: Two copolymers having D-A-D-A (P1) and D-A (P2)
structures with quinoxaline acceptor unit and dithienosilole
donor unit were synthesized and their optical and electrochem-
ical (both experimental and theoretical) properties were investi-
gated. The optical properties showed that these copolymers P1
and P2 exhibit optical bandgaps of 1.54 and 1.62 eV, respec-
tively, with broader absorption profiles extending up to
with the PC71BM as acceptor for the fabrication of solution
processed bulk heterojunction PSCs. The optimized P1:PC71BM
and P2:PC71BM active layers treated with solvent vapor treat-
ment showed overall power conversion efficiency (PCE) of
7.16% and 6.57%, respectively. The higher PCE of P1-based
device as compared to P2 might be attributed to higher crystal-
linity of P1 and good hole mobility resulting more balanced
charge transport. V
2017 Wiley Periodicals, Inc. J. Polym. Sci.,
C
800 nm and 770 nm, respectively. The electrochemical investi-
Part A: Polym. Chem. 2017, 00, 000–000
gation of these two copolymers indicates that they exhibit suit-
able highest occupied molecular orbital and lowest unoccupied
molecular orbital energy levels for efficient exciton dissociation
and high open circuit voltage in the resultant polymer solar
cells (PSCs). These copolymers were used as donors along
KEYWORDS: bulk heterojunction solar cells; D-A copolymers;
polymer solar cells; power conversion efficiency; solvent vapor
annealing
INTRODUCTION Organic solar cells (OSCs) based on bulk
heterojunction (BHJ) active layer are promising alternative
for inorganic solar cells due to their low cost of fabrication,
The basic requirement for high performance conjugated
copolymers to get high PCE of resulting PSCs involves the
appropriate energy levels, narrow bandgap along with the
high molar extinction coefficient and broader absorption pro-
file, as well as high charge carrier mobility. In search of high
performance conjugated copolymers, the strategy of using
donor (D) and acceptor (A) conjugated copolymers with
alternating electron rich and electron deficient units have
1
light weight, flexibility, and short payback time. The best
performance is achieved using a bulk heterojunction where
an electron donor (usually a conjugated polymer) and an
acceptor (usually a fullerene) are blended to obtain a large
2
interfacial area for charge carrier generation. With the
5
been proven the effective way. Using this method, fine tun-
development of new low bandgap conjugated copolymers,
control of active layer morphology and interface engineering,
the small scale polymer solar cells (PSCs) based on single
ing of absorption profile, energy levels, and charge carrier
mobility can be realized by means of selecting different elec-
6
tron donor and acceptor units.
BHJ active layer now showed power conversion efficiency
3
(
PCE) reaching 10%–12% using fullerene and non fullerene
Among the various electron acceptor units used, quinoxaline
(Qx) is a strong electron acceptor unit because of the elec-
acceptors. Recently, Zhang and coworkers have reported an
excellent PCE of 13.1% for PSCs based on non-fullerene
2
tron withdrawing properties of the two sp -hybridized nitro-
4
7
acceptors.
gen atoms in the Qx units. After the optimization of both
Additional Supporting Information may be found in the online version of this article.
2017 Wiley Periodicals, Inc.
V
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SCHEME 1 Synthetic routes of monomer M1.
molecular design of copolymers and device engineering, an
impressive over all PCE of 11. 34% have been achieved for
higher crystalline nature of P1 and hole mobility in
P1:PC BM relative to P2.
7
1
3
g
Qx-based D-A polymer donors. In Qx building block, vari-
ous substitutions can be introduced at various positions to
EXPERIMENTAL
7
e
tailor the resulting copolymers/small molecules. Moreover,
the phenyl ring in Qx so called phenylquinoxaline (PhQx)
can easily be connected through the oxidative coupling reac-
tion to form the fused aromatic structures, posses higher
planarity and better intermolecular packing due to their
fused structure and extended conjugation length which
enhances the backbone planarity and p-p stacking of the cor-
The synthesis of intermediate compounds and monomers
are described in Supporting Information.
Synthesis of Copolymers P1
The polymerization was performed by a Stille coupling reac-
tion. In a 50-mL flask, monomers M1 (0.8250 g, 0.5 mmol)
and M2 (0.3722 g, 0.5 mmol) were dissolved in 15 mL of
dry toluene, and the solution was flushed with argon for 15
8
responding copolymers. Therefore, PhQx was thought to be
promising electron acceptor unit for the design of D-A
9
min, then 27 mg of Pd(PPh ) was added into the solution.
copolymers. On the other hand, dithienosilole (DTS) donor
3 4
The mixture was again flushed with argon for 20 min. The
reaction mixture was heated to reflux for 48 h. The reaction
mixture was cooled to room temperature and added drop-
wise to 400 mL methanol. The precipitate was collected and
further purified by Soxhlet extraction with methanol, hexane,
and chloroform (CF) in sequence. The CF fraction was con-
centrated and added drop-wise into methanol. Finally, the
precipitates were collected and dried under vacuum over-
night to get polymer P1 as a black solid (0.69 g, yield 72%).
has been used as donor unit along with different acceptor
units to design D-A copolymers and the PSCs based on these
copolymers have shown overall PCE in the range of 5%–
1
0
7.3%.
Taking the above considerations into account, herein, we
designed and synthesized two D-A copolymers with A-D
(P2) and A-D-A-D (P1) structures on the basis of Qx and
DTS as electron acceptor (A) and electron donor (D) units.
We have used these two copolymers as electron donor along
with PC BM as electron acceptor for the fabrication of BHJ
1
H NMR (400 MHz, CDCl
AOAAr), 2.24–2.07 (br, 8H, CH
1.82–0.70 (br, 138H,CH ,CH ). ELEM. ANAL. for (C120H160
): d (ppm) 9.71–7.08 (br, 14H,
7
1
3
PSCs and investigated the difference of the photovoltaic per-
formances of PSCs with the A-D and A-D-A-D structured
copolymers. Using the A-D-A-D copolymer showed higher
PCE than that for A-D copolymers, may be due to the
increase in the conjugation length for P1 as compared to P2.
The PSCs processed with the optimized P1:PC BM and
ArH), 4.72 (br, 4H, CH
2
2
AAr),
3
2
N S O Si ) . Calc: C, 75.58; H, 8.46; N, 2.94; S, 6.73. Found:
4
4
4
2 n
C, 75.01; H, 8.11; N, 2.54; S, 6.53.
Synthesis of Copolymers P2
7
1
P2:PC BM with solvent vapor annealing (SVA) treatment
showed overall PCE of 7.16% and 6.57%, respectively. The
larger PCE of P1-based devices has been attributed to the
Copolymer P2 as synthesized by following the same proce-
dure for P1 but using M3 instead of M1. Monomers M2
(0.3722 g, 0.5 mmol) and M3 (0.34830 g, 0.5 mmol) were
7
1
2
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SCHEME 2 Synthetic routes of copolymers P1 and P2.
used as starting materials in the polymerization. Finally, P2
(octyloxy)phenyl)quinoxaline (M3) using catalyst Pd(Ph P)4 in
3
was obtained as a black solid (0.32 g, yield 68%).
37% isolated yield.
1
H NMR (400 MHz, CDCl ): d (ppm) 9.56–7.01 (br, 14H,
All compounds were purified by silica gel column chroma-
tography, and their structures and purity were fully charac-
3
ArH), 3.43 (br, 4H, CH AAr), 2.07–2.0 (br, 8H, CH AAr),
1
O Si ) . Calc: C, 75.58; H, 8.46; N, 2.94; S, 6.73. Found: C,
2
2
1
.72–0.63 (br, 134H, CH ,CH ). ELEM. ANAL. for (C H N S
terized by H NMR spectroscopy and elemental analysis. In
3
2
60 80 2 2
1
particular, the H NMR spectrum of the monomer M1 (Sup-
2
1 n
75.21; H, 8.25; N, 2.44; S, 6.31.
porting Information Fig. S3) in aromatic region contains two
doublets at d 8.08 and 8.01 ppm, three singlet state d 7.87,
Device Fabrication and Characterization
7.52, and 7.37 ppm, and four multiple signals at d 7.27, 7.20,
The indium tin oxide-coated glass substrates were sequen-
tially cleaned in ultrasonicator using detergent, deionized
water, acetone, and isopropanol for 10 min for each. The
cleaned substrates were dried in an oven at 50 8C for over-
night. The PSCs were fabricated with the device structure ITO/
PEDOT:PSS/P1 or P2:PC BM/Al, where ITO/PEDOT:PSS act as
6
1
7
.97, and 6.93 ppm with related intensity ratio of
:1:1:1:1:2:2:1:1. We suggest that a single resonance at d
.87 ppm is attributed to H(s) protons of the central silole
fragment; two double signals at d 8.08 and 8.01 ppm are
assigned to H(a) and H(a ) protons of Qx rings. Two proton
sets, H(b), H(c), H(d), H(e) and H(b ), H(c ), H(d ), H(e ) give
the spectral patterns which are typical for 1,3-disubstituted
benzene derivatives. Thus, two broadened singlets at d 7.52
0
0
0
0
0
71
anode and Al act as cathode. First of all, a 40-nm thick hole
transport layer of PEDOT:PSS was spin cast on the top of pre-
cleaned ITO substrates and then annealed at 120 8C for 10
min. The P1:PC BM or P2:PC BM active layers with different
0
and 7.37 ppm correspond to isolated H(e) and H(e ) protons
of the nonequivalent alkoxyphenyl rings; two broadened
71
71
weight ratios were coated from CF solution of concentration 16
doublets at d 6.97 and 6.93 ppm correspond to H(b) and
21
0
0
mg mL . For the solvent vapor annealing the active layers
were exposed to THF vapors for 60 s. Finally, a 60-nm thick
aluminum electrode was deposited on the top of active layer
H(b ). Multilpets at d 7.27 and 7.20 ppm are from H(c), H(c )
0
and H(d), H(d ) protons, respectively. In the aliphatic region
1
of H NMR spectrum, there are signals from alkyl groups.
2
5
by thermal evaporation under a pressure less than 10 Torr.
The current–voltage (J-V) characteristics of the devices under
illumination were measured with computer controlled Keithley
electrometer. A xenon arc lamp-based solar simulator was used
as light source. The incident photon to current conversion effi-
ciency was measured illuminating the device through a mono-
chromator and resultant current was measured with Keithley
electrometer.
Two triplet signals at d 3.94 and 3.85 ppm are related to
protons of methylene groups directly connected to oxygen
atoms of two nonequivalent octylalkoxy groups. Other ali-
phatic multiplets are not exactly assigned but the ratio of
integrated intensity values agrees well with the suggested
structure of compound M1.
The alternating D-A copolymers P1 and P2 were synthesized
by Stille cross-coupling polymerization using tetrakis(triphe-
nylphosphine)palladium as the catalyst in toluene solution
RESULTS AND DISCUSSION
0
between the Œ1 or M3 and 5,5 -bis(trimethyltin)di-
0
0
Synthesis and Characterization
The synthetic routes of monomers (M1 and M3) are summa-
thieno[3,2-b:2 ,3 -d]silole (M2) with yields of 72%–68%
(Scheme 2). After polymerization, the polymer products
were isolated by first pouring the reaction mixtures into
methanol to precipitate the crude copolymers. Crude copoly-
mers were Soxhlet-extracted with methanol, acetone, hexane,
and CF in sequence. The resultant copolymers were obtained
by re-precipitation of their concentrated CF solutions from
methanol. The structures of the obtained copolymers were
0
0
rized in Scheme 1. 4,4 -Bis(2-ethylhexyl)-5,5 -bis(trimethyltin)di-
0
0
10(a)
thieno[3,2-b:2 ,3 -d]silole (M2)
octyloxy)phenyl)quinoxaline (M3) were prepared according
and 5,8-dibromo-2,3-bis(3-
1
1
(
to literature methods. Key monomer M1 was prepared through
0
the Stille coupling reaction between 5,5 -bis(trimethyltin)-
0
0
dithieno[3,2-b:2 ,3 -d]silole (M2) and 5,8-dibromo-2,3-bis(3-
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TABLE 1 Molecular Weights and Thermal Properties of
Polymers P1 and P2
a
b
Polymer
Yield (%)
M
n
(kDa)
M
w
(kDa)
PDI
d
T (8C)
P1
P2
72
68
15.3
14.1
38.3
43.7
2.50
3.1
336
336
a
Determined by GPC in CF on polystyrene standards.
b
Decomposition temperature, determined by TGA in nitrogen based on
% weight loss.
5
1
confirmed by H and 13C NMR spectroscopy (Supporting
Information Figs. S4 and S5) and elemental analysis. In par-
ticular, for copolymers P1, the characteristic peaks at 8.22–
6.97 ppm can be assigned to the resonance of protons on
the thiophene ring, benzene rings, and PhQx rings. There is
peak at d 3.92 ppm which is attributed to methylene group
attached to the phenoxy group. The peaks in the region of
2
2.08 ppm are assigned to the CH group attached to the
directly to the silicon atom of the DTS groups in the polymer
chains. All integral ratios of peak areas between the aromatic
and aliphatic signals agree which the corresponding molecu-
lar structures of copolymers. The two copolymers exhibited
good solubility in common organic solvents such as toluene,
CF, and o-dichlorobenzene. The molecular weight (M ) of the
n
copolymers were determined by gel permeation chromatog-
raphy (GPC) against polystyrene standards in a CF eluent
and were found to be in the range of 15.3–14.1 kDa with
polydispersity index of 2.5–3.1 (Table 1).
The thermal properties of the copolymers were investigated
by thermogravimetric analysis (TGA) (Fig. 1). Both materials
exhibit good thermal stability with 5% weight-loss tempera-
ture (T ) at 336 8C, indicating that high thermal stability of
d
its molecules is adequate for application in OSCs.
FIGURE 2 Normalized absorption spectra of (a) P1 and (b) P2
in CF solution and thin film cast from the CF. [Color figure can
be viewed at wileyonlinelibrary.com]
Optical Properties
The UV-vis spectra of copolymers in both CHCI3 solutions
and thin films are shown in Figure 2 and data listed in Table
2
. Two main absorption bands are notable for both copoly-
wavelength (300–500 nm) is attributed to the p-p transitions
of the main chain, with the second band at the longer wave-
length (550–750 nm) owing to the intermolecular charge
transfer (ICT) interaction between donor and acceptor units
mer films, the first absorption band at the shorter
1
2
in copolymers backbones. The copolymer P1 and P2 films
show a kmax at 624 and 625 nm, respectively, which is 12
and 19 nm redshifted than their solutions. This is a clear
indication of a higher structural organization in thin film.
The red-shifted absorption of the polymer films indicates
that a strong intermolecular interaction and aggregation
exist in the solid-state of these copolymers, which is proba-
bly related to the increased extent of the p-p stacking of the
backbones and an increased polarizability in the solid state.
The optical band gaps are calculated from the absorption
onsets of the solid films and are 1.54 and 1.63 eV for P1
and P2, respectively. The low value of optical bandgap for
P1 as compared to that of P2 may be attributed to the addi-
tional A and D units in P1. Moreover, the vibronic shoulder
in P1 is more pronounced as compared to the P2 and may
FIGURE 1 TGA curves of P1 and P2 at a heating rate of 10 8C
21
min . [Color figure can be viewed at wileyonlinelibrary.com]
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TABLE 2 Photophysical and Electrochemical Properties of P1 and P2
sol
max
film
max
op
g
ele
g
Polymers
k
(nm)
k
(nm)
E
(eV)
E
HOMO (eV)
ELUMO (eV)
E
(eV)
P1
P2
361, 612
361, 592
379, 636
378, 620
1.54
1.62
5.15
5.34
3.52
3.67
1.63
1.67
be ascribed to the stronger molecular aggregation due to the
presence to two donors and acceptor units in P1. These
results indicate that using A-D-A-D based copolymers is a
promising strategy to reduce the optical bandgap and
increasing the light harvesting efficiency of the active layer
in PSCs.
materials. The Ered values were observed to be 20.88 and
onset
20.73 V for P1 and P2, respectively. The high-lying value of
the P1 as compared to P2 may be due to the presence of
the donors and acceptor, leading to the stronger intramolec-
ular charge transfer in the polymer backbone as compared
_
to the P1. The LUMO levels were thus calculated to be 3.52,
_
and 3.67 eV for P1, and P2, respectively, which are much
Electrochemical Properties
higher than that of PC71BM (24.2 eV) to guarantee an ener-
The electrochemical properties of these copolymers were
investigated to elucidate the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
getically favorable electron transfer. The electrochemical
ec
band gaps (E ) are of 1.63 and 1.67 eV for P1 and P2,
g
respectively, and the observed trends agree well with optical
bandgaps the copolymers.
(
LUMO) energy levels. The CV characteristics of these poly-
mers are illustrated in Figure 3 and the relevant data are
summarized in Table 2. From CV measurements, both clear
oxidation peaks and reduction peaks were observed for
Theoretical Calculations
We have studied the corresponding oligomer structures theo-
retically by means of density functional theory (DFT)
and time-dependent DFT (TD-DFT). The computations
account for solvent effects with CF as solvent. For both the
1
3–17
copolymers P1 and P2. HOMO and LUMO energy levels of
ox
the copolymers were calculated from the oxidation ðE
Þ
onset
red
onset
and reduction ðE
Þ onsets, respectively, while the poten-
16
ground state and excited state computations, the B3LYP
tials are determined using ferrocene (Fc) as standard accord-
ing to the following equations:
1
7
and M06 functionals were used. To reduce the computa-
tional cost, we have introduced ethyl groups in place of all
alkyl groups. The initial structural optimizations were per-
ꢀ
ꢁ
ox
1=2
Fc=Fc1=
HOMO52 E
2E
14:8 eV
onset
18
formed using the Turbomole package. All of the reported
ꢀ
ꢁ
red
onset
1=2
Fc=Fc1
results are from computations that were performed using
LUMO52 E
2E
14:8 eV
19
the Gaussian package. Technical details on the computa-
tional methods and methodology used are provided in the
Supporting Information.
E
elec5HOMO2LUMO
g
1
=2
The potential of ferrocene E
50:40 eV versus SCE is
The main body of the structures that consists of DTS and Qx
moieties are somewhat planar with dihedral angles between
these moieties in the range of ꢀ208 – 288, depending on the
functional and the presence of solvent. The higher dihedral
angle values were computed using the M06 functional, that
better account for interactions that result in twisting of the
structure. The syndiotactic configuration is energetically
slightly more favorable (by less than 0.2 eV) compared with
Fc=Fc1
ox
used as an internal standard. The E
observed to be 0.75 and 0.94 V for P1 and P2, respectively.
Accordingly, the HOMO energy levels were calculated to be
5.15 and 25.34 eV for P1 and P2, respectively. The low-
lying HOMO energy levels of copolymers were desired for
achieving a higher Voc in the BHJ PSCs and made these
copolymers promising candidates for use as polymer donor
values were
onset
2
4 6
FIGURE 3 Cyclic voltammograms of copolymers P1 and P2 in a acetonitrile solution of 0.1 M Bu NPF with a scan rate of 100
21
mV s
.
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wavelength of the first excitation and of the excitations with
the largest oscillator strengths.
The HOMO–LUMO (HL) gap computed using the hybrid
B3LYP functional is underestimated by ꢀ 0.45 eV compared
with the value computed using the meta-hybrid M06 func-
tional. For optical gaps, only the difference is less at ꢀ0.15
eV. In Supporting Information Table S1, we also list contribu-
tions to the first allowed excitations that are larger than 4%.
The first excitation clearly exhibits a single-configuration
character. In Figure 4, we provide the isosurfaces (iso-
value 5 0.02) of the frontier orbitals for the monomer, dimer,
trimer, tetramer, and hexamer structures. The HOMO and
LUMO extend over the main body; however, the LUMO exhib-
its an increased contribution from the pyrazine of the Qx
moiety as well as from the sulfur atoms of the DTS moiety.
In Figure 5, we show the UV/Visual absorbance spectra for
all of the oligomers computed at the TD-DFT/M06 level of
theory, both accounting for solvent effects for CF and in gas
phase (for the monomer and dimer). Due to the small size of
the monomer structure, quantum confinement effects come
into play leading to larger energy gaps. As we move on to
oligomers with longer chain sizes their absorbance spectra
exhibit a very notable redshift in the long wavelength region
2
0–24
due to relaxation of quantum confinement
the corresponding experimental spectra.
approaching
FIGURE 4 Frontier orbitals of the (a) monomer, (b) dimer, (c)
trimer, (d) tetramer, and (e) hexamer structures. [Color figure
can be viewed at wileyonlinelibrary.com]
the isotactic isomer; however, they exhibit a significantly dif-
ferent dipole moment, with the isotactic isomer having over
three times higher value.
The energy levels of the frontier orbitals as well as the opti-
cal gaps were computed using the PBE, M06, and B3LYP
functionals. In Supporting Information Table S1, we provide
the frontier orbitals’ energy levels, the optical gap, the main
contributions to the first excitation, as well as the
FIGURE 5 Theoretical UV/Vis absorption spectrum evolution
with increasing oligomer chain size (calculated using the M06
functional).
6
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FIGURE 6 Theoretical energy gap versus oligomer chain size
calculated using the M06 functional).
(
The dependence of the energy gaps (optical and HOMO–
LUMO) with respect to the chain size, n, is shown in Figure 6.
After a chain size of n 5 4, there is only a marginal decrease
in the energy gaps. The data points have been fitted to the
c
function f ðnÞ5a1b=n . For the optical gap, we obtain the
values a51:55 6 0:12 eV, b51:10 6 0:12 eV, c51:08 6 0:27,
and for the HOMO–LUMO gap, we obtain a52:17 6
0
:06 eV, b51:23 6 0:06 eV, c51:20 6 0:15. Thus, the energy
gaps at the polymer limit are 1.55 eV and 2.17 eV for the
optical gap and HOMO–LUMO gap, respectively.
Photovoltaic Properties
With the aim to investigate the photovoltaic properties of
these copolymers as donor materials in BHJ PSCs, we have
fabricated the devices with structure ITO/P1 or P2:
PC BM/Al. The active layers made of copolymer:PC BM
were spin coated from CF solutions. It is well known that
the weight ratio of donor and acceptor used in active layer
plays an important role in the photovoltaic performance of
OSCs. To examine this dependence and optimize the PCEs,
we have fabricated the photovoltaic devices using donor/
acceptor weight ratios of 1:0.5, 1:1, 1:1.5, 1:2, and 1:2.5. We
found that the active layers with 1:2 weight ratio showed
the best photovoltaic response (as shown in Supporting
Information Tables S2 and S3). In addition, SVA from THF
FIGURE 7 (a) Current–voltage (J-V) characteristics under stimu-
22
lated AM1.5 G illumination at intensity of 100 mW cm
and
(b) IPCE spectra of PSCs based on optimized P1:PC71BM and
P2:PC71BM active layers. [Color figure can be viewed at
wileyonlinelibrary.com]
7
1
71
stimulated AM 1.5 G irradiation with an intensity of 100
22
mW cm are shown in Figure 7(a). The photovoltaic param-
eters of the optimized devices are complied in Table 3.
It can be seen from the Figure 7(a) and Table 3 that the Jsc
and FF are higher for the PSC based on P1:PC BM active
71
layer whereas the Voc is larger for P2:PC BM-based device.
71
The higher value of Voc for P2 may be related to deeper
HOMO energy level of P2 as compared to P1. The higher
PCE value of the device based on P1 is mainly attributed to
(
for 60 s) was applied in fabrication of BHJ PSCs to optimize
2
4
the morphology of the active layer. The J-V characteristics
of the fabricated devices with optimal conditions under
TABLE 3 Photovoltaic Parameters of PSCs with Optimized P1:PC71BM and P2:PC71BM Blended Films Processed with and Without
Solvent Vapor Annealing
22
22 a
)
Active Layers
J
sc (mA cm
)
V
oc (V)
FF
PCE (%)
J
sc (mA cm
b
b
c
P1:PC71BM (1:2)
P2:PC71BM (1:2)
P1:PC71BM (1:2)
P2:PC71BM (1:2)
8.64
7.3
0.86
0.94
0.82
0.89
0.48
0.45
0.65
0.61
3.57
3.09
7.16
6.57
8.58
7.23
13.44
12.05
13.38
11.98
c
a
c
Estimated from the IPCE spectra.
As cast from CF.
SVA.
b
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investigate the copolymer crystallinity and obtain informa-
tion on the difference in the FF and Jsc of the devices based
on P1 and P2, we performed the X-ray diffraction (XRD)
measurements. Figure 8 shows the XRD patterns of P1 and
P2 films cast from CF. The film of P1 showed pronounced
diffraction peak at 2h 5 5.06 corresponds to the d-spacing
of ꢀ15.42 A. In contrast, the P2 showed a weak diffraction
peak at same position as that for P1. Furthermore, the both
P1 and P2 exhibited the strong (010) diffraction peaks at
2
h 5 21.86, corresponding to the p-p stacking with d-spacing
FIGURE 8 X-ray diffraction patterns of as cast P1:PC71BM and
P2:PC71BM films. [Color figure can be viewed at wileyonlineli-
brary.com]
of ꢀ4.06 A. Although the position of the (100) and (010) dif-
fraction peaks is almost same but the intensity of these peaks
is higher for P1 as compared to P2, indicating that higher
crystalline nature of P1 as compared to P2, which is benefi-
cial for improved charge carrier mobility. These results
showed that the D-A-D-A (P1) instead of D-A (P2) is benefi-
cial for ordered molecular packing, and also confirmed from
the absorption spectra of copolymers in thin film (Fig. 1).
FF and J . The J value may be related to the low band gap
sc
sc
and slightly broad absorption profile of P1 relative to P2,
resulting in a higher PCE value for P1 (3.57%) than P2
(3.09%) using CF solvent and P1 (7.16%) and P2 (6.57%)
for SVA-treated active layers. The incident photon to current
conversion efficiency [as shown in Fig. 7(b)] is higher for the
devices based on P1:PC BM than that for P2:PC BM over
We have examined the surface topographies of the
P1:PC BM and P2:PC BM films that underwent SVA treat-
ment to identify differences in the active layer morphologies.
This was performed by atomic force microscopy (AFM) and
the corresponding images are provided in Figure 9. The
7
1
71
the full wavelength range, and well matched with the corre-
sponding absorption of the copolymers in longer wavelength
region. The IPCE band in the shorter wavelength region cor-
responds to the absorption spectra of PC BM, indicates that
71
71
7
1
P2:PC BM-based film exhibited a relatively smoother sur-
the both copolymers and PC BM contribute the photocur-
71
7
1
face with a root mean square (RMS) surface roughness value
rent generation in the PSCs. The integrated values of Jsc
of about 0.84 nm as compared to the P1:PC BM with a
deduced from the IPCE spectra of devices are shown in Table
71
RMS value of 1.12 nm. The improved surface roughness also
indicates that in the case of P1:PC71BM, the exciton dissocia-
tion and charge transport in relatively more effective than
that in P2:PC71BM and resulting higher PCE.
3
and agree well with the Jsc values obtained from the J-V
characteristics under illumination. It can be seen from the
IPCE spectra of the PSCs based on SVA-treated active layer
may be attributed to strong molecular aggregation due to
the penetrating of solvent vapor into the active layer during
solvent annealing process. The SVA treatment also changes
the morphology of the active layer, which leads to the
increase in the absorption characteristics of the active layers.
Generally, it was observed that the SVA leads to a redshift in
the absorption spectra of the ICT band of the polymer.
The hole mobility of the active layers cast from CF and with
SVA treatment were also compared. This was performed by
measuring the J-V characteristic in dark of the hole only devi-
ces with structure ITO/PEDOT:PSS/copolymer:PC BM Au
7
1
(
Fig. 10) and fitting these characteristics with space charge
2
5
limited current model. The hole mobility in P1:PC BM
(4.78 3 10
7
1
2
5
24
2
21
The polymer crystallinity is of high importance for the
charge transport in the active layer of the OSCs. To
and 2.19 3 10
cm Vs
for CF cast and
SVA, respectively) is higher than that for P2:PC BM (3.42 3
7
1
FIGURE 9 AFM images (3 mm 3 3 lm) of P1:PC71BM and P2:PC71BM films processed with SVA treatment. [Color figure can be
viewed at wileyonlinelibrary.com]
8
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ARTICLE
is the voltage at which the photocurrent is zero and V
the external applied voltage.
is
appl
2
6
The Veff determines the electric field in the bulk region and
thereby affects the carrier transport and photocurrent
extraction. At low V , the J varies linearly with Veff and
eff
ph
then tends to saturate as the Veff further increases. At high
Veff values mobile charge carriers quickly move toward the
corresponding electrode with minimum recombination. At
full saturation, we assume that all the excitons generated
after the absorption of light by the active layer are dissoci-
ated into free charge carriers and consequently collected by
the respective electrodes. The Jph saturate at lower Veff for
the device processed with solvent additive as compared to
the device processed without solvent additive, indicating that
the low Veff is needed for the exciton dissociation for former
FIGURE 10 Current–voltage (J-V) characteristics of hole-only
device. Therefore, at high V , the saturation photocurrent
density (Jphsat) is limited by the total number of absorbed
eff
devices. [Color figure can be viewed at wileyonlinelibrary.com]
photons and independent of bias voltage. We have estimated
the maximum generation rate of free charge carriers (Gmax
)
2
5
24
2
21
10
and 1.03 3 10 cm Vs for CF cast and SVA, respec-
according to Jphsat 5 qGmaxL, where q is the electronic charge
and L is the thickness of the active layer. The values of
Gmax for the devices based on P1:PC BM (as cast), P1:
tively). The higher hole mobility for either CF cast or SVA-
treated P1:PC71BM active layer indicates more balanced
charge transport in P1:PC BM active layers than that for
2
7
71
7
1
PC BM (SA), and P2:PC BM (as cast) and P2:PC BM (SVA)
7
1
71
71
P2:PC BM counterparts, leading to improvement in J and
7
1
sc
21
23
21
23
21
23
are 7.42 3 10 cm , 9.11 3 10 cm , 6.68 3 10 cm
,
FF. The blend films with SVA treatment exhibited higher hole
and electron mobilities and more balanced charge transport
than the blend films cast from CF, which is beneficial for the
higher Jsc and FF. These results are consistent with variation
in photovoltaic performance of the corresponding devices.
2
1
23
and 8.25 3 10 cm , respectively. Gmax is related to the
number of photons absorbed by the active layer and such
changes in the Gmax are consistent with the absorption spec-
tra of corresponding P1 and P2 layers. The charge collection
probability P has been estimated according to Pc 5 Jsc/
Jphsat and are 0.75, 0.86, 0.72, and 0.81 for P1:PC BM (as
c
2
8
To assess the effect of the solvent additive on the exciton
generation, dissociation, and charge transport properties, we
71
cast), P1:PC BM (SVA), P2:PC BM (as cast), and
7
1
71
have measured the photocurrent density (J ) versus the
71 c
P2:PC BM (SVA), respectively. The higher value of P for the
ph
effective voltage (V ) of the devices and the results are
shown in Figure 11. Jph is defined as Jph 5 J – J , where J
L D D
and JL are the current density in dark and under illumina-
devices with SVA treated active layer than the corresponding
devices based on as cast active layers is attributed to the
better phase separation, increased hole mobility and
improved balance in charge transport, and enhancement in
light harvesting efficiency.
eff
o
tion, respectively. Veff is defined as Veff 5 V – Vappl, where Vo
CONCLUSIONS
In conclusion, we have synthesized two copolymers with D-
A-D-A (P1) and D-A (P2) structures based on Qx and DTS as
electron acceptor (A) and electron donating (D) units,
respectively, for their applications as donors along with
PC71BM as acceptor for solution processed BHJ PSCs. Their
optical and electrochemical properties were characterized.
These copolymers possess deep HOMO energy levels, benefi-
cial for high open circuit voltage. The optimized (after the
optimization of weight ratios and solvent vapor treatment of
active layers) PSCs based on P1:PC BM and P2:PC BM
7
1
71
showed PCE of 7.16% and 6.57%, respectively. The higher
PCE of P1-based device as compared to P2 might be attrib-
uted to higher crystallinity that of P1 and hole mobility
resulting more balanced charge transport. Moreover, the sur-
face roughness in P1:PC BM is relatively more rough as
FIGURE 11 Variation in Jph with Veff for the devices based on
active layers processed under different conditions. [Color figure
can be viewed at wileyonlinelibrary.com]
7
1
compared to P2:PC BM, might be resulting more effective
7
1
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exciton dissociation and charge transport in later than for-
mer active layer.
(f) D. Gedefaw, M. Prosa, M. Bolognasi, M. Seri, M, R.
Andersson, Adv. Energy Mater. 2017, 1700575; (g) S. Xu, L.
Feng, J. Yuan, V. Cirmrova, G. Chen, Z. G. Zhang, Y. Li, H.
Peng, Y. Zhou, Org. Electron. 2017, 50, 7.
8
(a) Y. Zhang, J. Zou, H. L. Yip, K. S. Chen, D. F. Zeigler, Y.
ACKNOWLEDGMENTS
Sun, A. K. Y. Jen, Chem. Mater. 2011, 23, 2289; (b) Y. J. Cheng,
C. H. Chen, T. Y. Lin, C. S. Hsu, Chem. Asian J. 2012, 7, 818; (c)
X. Zhang, J. W. Shim, S. P. Tiwari, Q. Zhang, J. E. Norton, P.
T. Wu, S. Barlow, S. A. Jenekhe, B. Kippelen, J. L. Bredasa, S.
R. Marder, J. Mater. Chem. 2011, 21, 4971; (d) Y. Lee, Y. M.
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This work was supported by the Russian Science Foundation
(
grant number 14-13-01444 [to M.L.K., A.Y.N., and S.A.K.]).
G.D.S. is thankful to Director LNM Institute of Information Tech-
nology for financial support to visit Moscow for research work
and discussions.
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