Evaluating the influence of heteroatoms on the electronic properties of aryl[3,4-c]pyrroledione based copolymers

Article abstract of DOI:10.1016/j.polymer.2016.12.013

A donor-acceptor-type conjugated copolymer (PBDT-PPD) composed of benzodithiophene (BDT) and pyrrolopyrroledione (PPD) was synthesized using the Stille cross-coupling reaction. Using both experimental and theoretical data, the optical, electrochemical, and photovoltaic properties of PBDT-PPD were compared with those of its sulfur analog, PBDT-TPD, which is composed of BDT and thienopyrroledione (TPD). The optical bandgaps of the polymers were determined to be 1.86 and 2.20 eV, respectively. While both materials possessed similar highest occupied molecular orbital (HOMO) levels, the lowest unoccupied molecular orbital (LUMO) level for PBDT-PPD was raised relative to that of PBDT-TPD. Devices incorporating PBDT-PPD had a higher open-circuit voltage and fill factor, yet drastically lower short-circuit current density (J_sc) than PBDT-TPD leading to a lower power conversion efficiency (PCE). The lack of significant intramolecular charge transfer (ICT) combined with the high LUMO of PBDT-PPD resulted in poor spectral overlap with the solar spectrum, lowering J_sc. Additionally, there was poor electron injection into PCBM, which also reduced the PCE.

Full text of DOI:10.1016/j.polymer.2016.12.013

Polymer 109 (2017) 85e92
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Evaluating the influence of heteroatoms on the electronic properties
of aryl[3,4-c]pyrroledione based copolymers
,
Benjamin J. Hale a, Moneim Elshobaki b c, Ryan Gebhardt b, David Wheeler d, Jon Stoffer a,
d
b
,
, *
Aimee Tomlinson , Sumit Chaudhary e, Malika Jeffries-EL a
ꢀ
a Department of Chemistry, Iowa State University, Ames, IA 50011, United States
b Department of Materials & Science Engineering, Iowa State University, Ames, IA 50011, United States
c Department of Physics, Mansoura University, Mansoura 35516, Egypt
d Department of Chemistry & Biochemistry, University of North Georgia, Dahlonega, GA 30597, United States
e Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A donor-acceptor-type conjugated copolymer (PBDT-PPD) composed of benzodithiophene (BDT) and
pyrrolopyrroledione (PPD) was synthesized using the Stille cross-coupling reaction. Using both experi-
mental and theoretical data, the optical, electrochemical, and photovoltaic properties of PBDT-PPD were
compared with those of its sulfur analog, PBDT-TPD, which is composed of BDT and thienopyrroledione
(TPD). The optical bandgaps of the polymers were determined to be 1.86 and 2.20 eV, respectively. While
both materials possessed similar highest occupied molecular orbital (HOMO) levels, the lowest unoc-
cupied molecular orbital (LUMO) level for PBDT-PPD was raised relative to that of PBDT-TPD. Devices
incorporating PBDT-PPD had a higher open-circuit voltage and fill factor, yet drastically lower short-
circuit current density (Jsc) than PBDT-TPD leading to a lower power conversion efficiency (PCE). The
lack of significant intramolecular charge transfer (ICT) combined with the high LUMO of PBDT-PPD
resulted in poor spectral overlap with the solar spectrum, lowering Jsc. Additionally, there was poor
electron injection into PCBM, which also reduced the PCE.
Received 13 September 2016
Received in revised form
29 November 2016
Accepted 3 December 2016
Available online 6 December 2016
Keywords:
Conjugated polymer
Organic semiconductors
Donor-acceptor
Copolymers
© 2016 Published by Elsevier Ltd.
1. Introduction
broad optical absorption bands, deep HOMO energy levels, high
charge carrier mobilities, and LUMO levels with appropriate
Since the discovery of the first semiconducting polymer nearly
40 years ago, research involving conjugated polymers has been on
the rise. In particular, studies involving bulk hetero-junction (BHJ)
organic photovoltaic solar cells (OPVs), has increased exponentially
due to their potential low cost production, and use in lightweight,
flexible devices [1e9]. Although improvements in materials and
fabrication techniques have led to dramatic increases in OPV per-
formance, as determined by the power conversion efficiency (PCE),
a better understanding of structure-property relationships is still
desired [10,11]. The synthesis of conjugated polymers, comprised of
alignment to [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)
[8,15]. In addition, these materials can be further tuned by varying
the heteroatoms within the arenes. Indeed, a dramatic impact on
the physical, optical, and electronic properties of a material can be
achieved through heteroatom substitution [8,16]. For example,
large changes in optical absorption and solubility of many materials
have been observed upon replacing of thiophene with the iso-
electronic furan or selenophene [16e19]. While substitution
within a group (e.g. the group 16 chalcogens) can at often times
have predictable effects, the impact of substitution between groups
is often less straightforward [16,20,21].
alternating
p-electron rich and p-electron deficient arylene units,
allows for the selective tuning of optical and electronic properties
of the material [12e14]. This “donor-acceptor” strategy has given
rise to a variety of materials with desirable properties, such as
The thiophene containing 5-octylthieno[3,4-c]pyrrole-4,6-
dione (TPD) unit has been used as an
p-electron deficient moiety
in a variety of high efficiency donor-acceptor copolymers [22e24].
When polymerized with the
p-electron rich benzodithiophene
(BDT), BHJ OPV performance as high as 5.5% for 1.0 cm2 devices
(PBDT-TPD) has been reported [22]. When the thiophene was
* Corresponding author.
E-mail address: malikaj@bu.edu (M. Jeffries-EL).
http://dx.doi.org/10.1016/j.polymer.2016.12.013
0032-3861/© 2016 Published by Elsevier Ltd.

86
B.J. Hale et al. / Polymer 109 (2017) 85e92
replaced with furan (FPD), a widening of the optical bandgap was
observed, whereas switching with selenophene (SePD) resulted in
a reduction of the optical bandgap, relative to TPD [25]. OPVs
fabricated from the SePD based polymer showed a greatly reduced
short-circuit current density (Jsc), which resulted in a very low PCE
of 0.26% [26]. Although an improvement in performance has not
been seen with FPD or SePD, exploration outside of the group 16
elements has yet to be researched extensively.
A seldom studied alternative to TPD is the nitrogen analog
pyrrolo[3,4-c]pyrroledione (PPD), first reported in 1996 [27]. While,
the additional alkyl chain on the nitrogen atom of PPD can poten-
tially increase solubility, the impact of replacing sulfur with nitro-
gen is not well understood. Recently, PPD was used in a series of
donor-acceptor copolymers with varying results, and there was
no direct structural comparison to known high performing TPD
based materials [28]. Here, a PPD based copolymer was synthe-
sized, characterized and compared to the structurally analogous
PBDT-TPD. In addition to the physical methods, we also evaluated
both polymers through density functional theory.
thin films on glass substrates, are shown in Figs. 1 and 2, respec-
tively. PBDT-TPD had a broad absorption from 350 to 650 nm in
dilute solution with two peaks of nearly equal intensity at 552 and
627 nm, with a third slightly smaller peak at ~590 nm. The in-
tensities of the broad low energy transition suggests intramolecular
charge-transfer (ICT) interaction between the electron-rich BDT
and the electron-deficient TPD moieties [29]. In solution PBDT-PPD
displayed a lmax of 526 nm with a narrower and significantly blue
shifted absorption range of 350e550 nm, relative to PBDT-TPD.
Strong vibronic coupling can be seen in PBDT-PPD and PBDT-TPD,
suggesting the formation of aggregates in both solutions.
As a thin film, PBDT-TPD showed very little change in absorp-
tion when compared to its solution spectra, with a lmax of 624 nm.
A slight decrease in the intensity of the higher energy maximum
and an increase in the intensity of vibronic coupling was also seen,
with the vibronic coupling indicating highly ordered thin films [30].
Interestingly, the thin film of PBDT-PPD, had a lmax of 533 nm, with
a reduction in intensity, and a significant narrowing of the ab-
sorption range by a decrease of the
p-
p* transitions of the conju-
gated main chain. A comparison of the absorption profiles of the
two polymers shows PBDT-TPD has a stronger absorption across
nearly all wavelengths, relative to their respective lmax, and a
significantly broader absorption of 350e675 nm, versus the
350e550 nm range of PBDT-PPD. The optical bandgaps were
determined from the absorption onsets of the polymer films. The
measured optical bandgaps for PBDT-TPD and PBDT-PPD were
1.86 eV and 2.20 eV, respectively. The narrow absorption range of
PBDT-PPD and the wide bandgap suggest there is little, if any, ICT
occurring between the BDT and PPD moieties [31e33]. The optical
data is summarized in Table 2.
Cyclic voltammetry was used to investigate the redox behavior
and to estimate the HOMO energy levels of the polymers. The
HOMO and LUMO energy levels were calculated from the oxidation
onset using the adjusted energy level of ferrocene/ferrocenium (Fc/
Fcþ) as ꢁ4.7 eV vs vacuum and are summarized in Table 2. Both
polymers exhibited reversible reduction and irreversible oxidation
peaks (Supporting Information). The HOMO energy level for both
PBDT-TPD and PBDT-PPD were found to be ꢁ5.50 eV, while the
LUMO energy levels were found to be ꢁ3.54 eV and ꢁ3.10 eV, for
PBDT-TPD and PBDT-PPD respectively. The electrochemical
bandgaps of 1.96 eV for PBDT-TPD and 2.40 eV for PBDT-PPD are in
agreement with the optical bandgaps [34]. While both materials
had the same HOMO level, the significantly higher LUMO level and
narrowing of the optical absorption of PBDT-PPD reinforce the
suspicion that there is little intramolecular charge transfer along
the polymer backbone. To further investigate this possibility,
Density functional theory (DFT) calculations were performed.
2. Results and discussion
2.1. Synthesis and characterization of monomer and polymers
The PPD monomer 6 was prepared according the synthetic route
as illustrated in Scheme 1 [27]. Diethyl pyrrole-3,4-dicarboxlyate
was formed by condensation of diethyl fumarate and p-toluene-
sulfonylmethyl isocyanide followed by saponification to the dicar-
boxylic acid, 2. Compound
2 was then converted to the
corresponding anhydride by treatment with N,N0-dicyclohex-
ylcarbodiimide, which was ring opened with n-octylamine, and
closed with thionyl chloride to give 4. The unalkylated 4 was then
brominated using NBS in the dark. Compound 5 was then alkylated,
in a fashion similar to its structural isomer diketopyrrolopyrrole, in
DMF with potassium carbonate, 1-bromooctane, and 18-crown-6 to
give the final PPD monomer, 6, in moderate yield.
Alternating copolymers were synthesized by Stille cross-
coupling of the diarylhalide monomers (PPD or TPD) and the dis-
tannyl BDT monomer in anhydrous toluene, as shown in Scheme 2.
The molecular weight data for PBDT-TPD and PBDT-PPD were
determined by size exclusion chromatography in chloroform
against polystyrene standards. Both materials had reasonable
number averaged molecular weights (Mn) of 24.9 and 19.8 kDa for
PBDT-TPD and PBDT-PPD, respectively. The TPD based polymer,
PBDT-TPD, had poor solubility in organic solvents at room tem-
perature, but was readily dissolved in chlorobenzene and 1,2-
dichlorobenzene when heated, as reported by Leclerc et al. [22]
The PPD based polymer, PBDT-PPD, had greatly improved solubi-
lity and was readily dissolved in chloroform, chlorobenzene, and
1,2-dichlorobenzene at room temperature. The increased solubility
of PBDT-PPD is likely due to the lower degree of polymerization
(DPn) and the additional solubilizing alkyl chain on the nitrogen.
The thermal stabilities of the polymers were evaluated using
TGA under air (Supporting Information). PBDT-TPD and PBDT-PPD
demonstrated good thermal stability when heated, with a 5%
weight loss at 333 and 337 ꢀC, respectively. Differential scanning
calorimetry (Supporting Information) revealed no phase transitions
for PBDT-PPD below 250 ꢀC. The molecular weights and thermal
properties of PBDT-TPD and PBDT-PPD are summarized in Table 1.
2.3. Computational studies
DFT was used to evaluate the differences in the performance
between PBDT-TPD and PBDT-PPD. We began with the B3LYP
(Becke, three-parameter, Lee-Yang-Parr) [35] hybrid functional
which has been shown to produce comparable geometries to
Moller-Plesset second-order perturbation theory (a higher level of
theory) [36] at a fraction of the computational cost [37]. Although,
the improved performance of OPVs based PBDT-TPD can be
attributed to the smaller band gap and a lower lying LUMO level
relative to PBDT-PPD, other factors may be involved. Upon
completion of the DFT calculations, the frontier molecular orbitals
(FMOs) and electrostatic potential maps were generated (Fig. 3). For
PBDT-TPD, the terminal TPD ring appears to be lacking electron
density in the HOMO, whereas its LUMO is rich in electron density.
The opposite trend occurs for the terminal BDT ring within PBDT-
TPD, in which the HOMO is rich in electron density and the LUMO is
2.2. Optical and electrochemical properties
The optical properties of the polymers were investigated using
UV-Vis absorption spectroscopy. The normalized absorption
spectra of the polymers, both as dilute chloroform solutions and

B.J. Hale et al. / Polymer 109 (2017) 85e92
87
Scheme 1. Synthesis of PPD monomer 6.
Scheme 2. Synthesis of donor-acceptor copolymers PBDT-TPD and PBDT-PPD.
Table 1
Molecular weight and thermal properties of the synthesized polymers.
Yielda
(%)
Mn
Mw
Ðc
DPn
Td5%d
(ꢀC)
b
b
Polymer
(kDa)
(kDa)
PBDT-TPD
PBDT-PPD
82
73
24.9
19.8
44.8
33.6
1.8
1.7
35.1
24.6
333.7
337.8
a
Isolated yield.
b
Determined by GPC vs polystyrene standards in chloroform.
c
Dispersity: Mw/Mn.
d
Temperature at 5% weight loss with a heating rate of 20 ꢀC minꢁ1 under air.
Fig. 2. Normalized UV-Vis spectra of PBDT-TPD and PBDT-PPD thin films.
rich in the LUMOþ1. Collectively, these findings indicate that there
is donor-acceptor behavior within PBDT-TPD.
However this was not the case for PBDT-PPD as the both PPD
rings have electron density in the HOMO and the LUMO. Further-
more, there is complete lack of electron density on the PPD moiety
in the HOMO-1 and an absence of electron density in the LUMOþ1
of the terminal PPD unit. Thus donor-acceptor behavior alone was
not enough to explain the observed difference. We then turned our
attention to the geometry of the dimers. This data indicated that
the PBDT-TPD system was nearly planar with dihedral angles
ranging from 177 to 180ꢀ. On the other hand, the PBDT-PPD was
non-planar with dihedrals angles ranging from 146 to 151ꢀ. This
Fig. 1. Normalized UV-Vis spectra of PBDT-TPD and PBDT-PPD in CHCl3.
lacking in electron density. Additionally, both TPD rings show a lack
of electron density in the HOMO-1, whereas these rings are electron

88
B.J. Hale et al. / Polymer 109 (2017) 85e92
twist can be seen in the electrostatic potential map and also
adversely effects the distribution of electron density seen in the
FMOs. The lack of planarity is also responsible for raising the HOMO
and the LUMO and increasing the band gap. Collectively, these
factors result in the poor performance of PBDT-PPD.
A comparison between the experimental electrochemical re-
sults and the theoretical data is shown in Table 3. An absolute
difference of 0.35 eV and 0.22 eV was found in the HOMO and band
gap respectively, indicating there is good agreement between the
two data sets. In order evaluate the potential for charge transfer
within these copolymers the reorganization energy for both the
hole and the electron was computed and shown in Table 4. This
energy was generated for both the individual subunits as well as
the comonomer in each case. In the case of PBDT-TPD, the BDT
subunit had a much lower reorganization energy for electron than
for the hole indicating it takes less energy for this ring to accept a
negative charge than it does for a positive charge. The opposite is
true for TPD in which the hole reorganization energy is lower than
that of the electron. Putting the two subunits together and making
a comonomer shows that the material behaves more favorably as
an acceptor than a donor. For the PBDT-PPD copolymer, the PPD is
more accepting than donating in nature as indicated by the reor-
ganization energy. Like PBDT-TPD, PBDT-PPD is an also electron-
accepting material. Overall, all of these reorganization energies
are quite high and so while trends can be suggested based on their
magnitude it is doubtful either of these copolymers would be able
to charge-transfer at a reasonable rate.
2.4. Photovoltaic properties
Photovoltaic devices were fabricated with the structure ITO/
PEDOT:PSS/donor:PC71BM/Ca/Al, where ITO is indium tin oxide and
PEDOT:PSS
is
poly(3,4-ethylenedioxythiophene)-poly(-
styrenesulfonate). Characteristic JeV curves are shown in Fig. 4 and
the resulting data is summarized in Table 5. Solutions were cast
from a 25 mg mLꢁ1 total blend concentration in chlorobenzene
using a ratio of 1:2 polymer:PC71BM w/w. Spin-rates ranging from
1000 rpm to 1400 rpm were examined. The best devices of PBDT-
TPD gave a PCE of 2.5%, with an open-circuit voltage (VOC) of 0.68 V,
a short-circuit current density (JSC) of 7.98 mA cmꢁ2, and a fill factor
(FF) of 46%, while the best devices of PBDT-PPD gave a significantly
lower PCE of 0.63%, VOC of 0.87 V, JSC 1.3 mA cmꢁ2, and a FF of 57%.
In an effort to further improve the poor performance of PBDT-
PPD, devices were fabricated with the use of small quantities of the
high boiling solvent additives 1-chloronaphthalene (CN) and 1,8-
diiodooctane (DIO) at 5% v/v. In general, devices made from solu-
tion with additives had higher PCE in comparison to the ones
without additives. Devices fabricated using CN as an additive gave a
maximum PCE of 0.83%, an average PCE of 0.72%, VOC of 0.66 V, JSC of
2.5 mA cmꢁ2, but a greatly reduced FF of 44%. The devices using DIO
gave a significantly higher maximum PCE of 1.34% and an average of
1.23%, VOC of 0.76 V, JSC of 3.9 mA cmꢁ2, but also had a low FF of 42%.
Fig. 3. DFT calculated frontier orbital and electrostatic potential maps.
While improvement was observed using solvent additives, the
overall photocurrent remained very low.
The surface roughness and phase distribution of the polymers
were studied by atomic force microscopy (AFM) (Fig. 4). The AFM
roughness maps of PBDT-TPD:PC70BM and PBDT-PPD:PC70BM neat
thin-films show large domain sizes with root-mean square surface
Table 2
Optical and electrochemical properties of the synthesized polymers.
soln
max
film
max
Polymer
l
(nm)a
l
(nm)
Eogpt (eV)b
EHOMO (eV)c
ELUMO (eV)d
EEgCe
(eV)
PBDT-TPD
PBDT-PPD
552, 627
526
624
533
1.86
2.20
ꢁ5.50
ꢁ5.50
ꢁ3.54
ꢁ3.10
1.96
2.40
a
Measured in chloroform.
Measured from the optical onset.
b
c
d
e
ox
HOMO ¼ ꢁ(E
þ 4.7)eV.
onset
red
LUMO ¼ ꢁ(E
ꢁ 4.7)eV.
onset
EEgC ¼ LUMO ꢁ HOMO.

B.J. Hale et al. / Polymer 109 (2017) 85e92
89
Table 4
roughness (RMS) values of 3.99 and 5.17 nm, respectively. Whereas
Reorganization energies (in eV) for the subunits and their monomers.
the AFM height images reveal smooth topography for both poly-
mers with root-mean square (RMS) surface roughness values less
than 1.30 nm. The JSC of the PBDT-PPD device increased from
1.3 mA cmꢁ2 to 3.9 and 2.5 mA cmꢁ2 upon using as solvent addi-
tives of DIO and CN, respectively. This enhancement in JSC is a result
of the reduction in the domain size within the morphology of PBDT-
PPD-based thin-films with solvent additives as seen in AFM images
(Figs. 5 and 6). The films with additives have small grain sizes with
slight variation in the surface roughness (RMSDIO ¼ 5.59 nm, and
RMSCN ¼ 4.25 nm). This morphological change is beneficial for
suppressing the charge recombination and better charge transport
and dissociation is achieved.
BDT
PPD
TPD
PBDT-TPD
PBDT-PPD
a
lh
0.838
0.337
0.399
0.478
0.400
0.476
0.762
0.414
0.798
0.445
b
le
a
Hole reorganization: lh
Electron reorganization: le
.
b
.
The hole mobilities of PBDT-TPD and PBDT-PPD were examined
using the space-charge limited current (SCLC) method with a hole
only device structure of ITO/PEDOT:PSS/polymer:PC71BM/MoOx/Al.
The mobilities were calculated according to the equation:
ꢀ
.
ꢁ
9
8
JSCLC
¼
εoεrmh V2 L3
;
(1)
where εo is the permittivity of vacuum, εr is the permittivity of the
material, mh is the carrier mobility, V is the effective voltage, and L is
the thickness of the active layer. The hole mobilities were deter-
mined to be 4.36 ꢂ 10ꢁ6 for PBDT-TPD and 1.31 ꢂ 10ꢁ5 cm2 Vꢁ1 sꢁ1
for PBDT-PPD. The hole-only current-voltage characteristics are
shown in Fig. 7.
Since the PBDT-PPD blend has a higher mobility and more
favorable morphology than the PBDT-TPD blend, one of the most
likely causes for the low photocurrent of PBDT-PPD is the ab-
sorption profile of the material having poor overlap with the solar
spectrum [30]. Additionally, the high LUMO of PBDT-PPD may have
too high of an offset with that of PC71BM, leading to recombination
in the donor due to the poor rate of electron injection into the
acceptor [31].
Fig. 4. JeV curves of PBDT-TPD and PBDT-PPD photovoltaic devices.
Table 5
Summary of the characteristics of photovoltaic devices.
Polymera
Additiveb
JSC
VOC
(V)
FF
(%)
PCEave
(%)
PCEmax
(%)
e
(mA cmꢁ2
)
PBDT-TPD
PBDT-PPD
e
7.8
1.3
3.9
2.5
0.69
0.87
0.76
0.66
45
52
42
44
2.44
0.59
1.23
0.72
2.51
0.63
1.34
0.83
e
DIOc
CNd
a
Fabricated at a 1:1.5 weight ratio of polymer:PC71BM with a total solution
concentration of 25 mg mLꢁ1
.
b
5% v/v.
1,8-diiodooctane.
1-chloronaphthalene.
Average of six devices.
c
3. Conclusions
d
e
A novel conjugated polymer, PBDT-PPD was synthesized and
compared to the well-known sulfur analog PBDT-TPD. Both poly-
mers were used in OPVs and it was found that PBDT-PPD per-
formed worse than PBDT-TPD. Experimental and theoretical
studies on the optoelectronic properties of these polymers
demonstrated that PBDT-PPD had a lower electron affinity and
wider optical bandgap than PBDT-TPD. Furthermore, the ICT was
weaker in PBDT-PPD than in PBDT-TPD, and neither material was a
particularly good donor polymer. Collectively, these results suggest
that replacing sulfur with nitrogen in pyrrole dione monomers
could be a good strategy for designing efficient OPV materials,
however a different electron-donating group should be explored.
Research is ongoing in our group to improve upon these results.
4. Experimental
4.1. Materials
Air- and moisture-sensitive reactions were performed using
standard Schlenk techniques. Solvents used for palladium-
catalyzed reactions were deoxygenated prior to use by sparging
with argon for 30 min. The preparation of compounds 6 and 8 are
described in the Supporting Information. (4,8-bis((2-ethylhexyl)
oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethyl-
stannane) (BDT) [38] was prepared according to literature pro-
cedures. Thiophene-3,4-dicarboxylic acid was purchased from
Oakwood Chemicals and recrystallized from water before use. All
other chemical reagents were purchased commercially and used
without further purification unless otherwise noted.
Table 3
Comparison between theoretical and experimental values.
HOMO (eV)
DFT Expt
LUMO (eV)
Diff DFT Expt
Bandgap (eV)
Diff DFT Expt Diff
4.2. Characterization
PBDT-TPD ꢁ5.21 ꢁ5.50 0.29 ꢁ3.09 ꢁ3.54 0.45 1.82 1.86 0.04
PBDT-PPD ꢁ5.15 ꢁ5.50 0.35 ꢁ2.38 ꢁ3.10 0.72 2.42 2.20 0.22
Nuclear magnetic resonance (NMR) spectra were collected on

90
B.J. Hale et al. / Polymer 109 (2017) 85e92
Fig. 7. The current-voltage characteristics of PBDT-TPD:PC71BM and PBDT-PPD:PC71BM
photovoltaic devices in dark under ambient conditions.
Fig. 5. AFM height (left) and phase (right) images at 5
(aeb), PBDT-PPD:PC71BM (ced) thin-films.
m
m ꢂ 5
mm of PBDT-TPD:PC71BM
measurements were carried out using an e-DAQ e-corder 410
potentiostat with a scanning rate of 100 mV sꢁ1. The polymer films
were dropcast from 1e2 mg mLꢁ1 solutions in chlorobenzene onto
a platinum working electrode. Ag/Agþ and Pt wire were used as the
reference and auxiliary electrodes, respectively. The reported
values were referenced to Fc/Fcþ (ꢁ4.8 versus vacuum). All elec-
trochemical experiments were performed in deoxygenated aceto-
nitrile
under
an
argon
atmosphere
using
0.1
M
tetrabutylammonium hexafluorophosphate as electrolyte. Absorp-
tion spectra were obtained on a photodiode-array Agilent 8453 UV-
visible spectrophotometer using polymer solutions in CHCl3 and
thin films. The films were cast by spin coating 25 ꢂ 25 ꢂ 1 mm glass
slides using solutions of polymer (2.5e5.0 mg mLꢁ1) in CHCl3/o-
dichlorobenzene at a spin rate of 1200 rpm on a Headway Research,
Inc. PWM32 spin-coater. A Veeco Digital Instruments atomic force
microscope was used to capture the surface roughness and phase of
PBDTTPD- and PBDTPPD-based thins films. The tapping-mode AFM
was carried out using TESPA tip with scan rate of 0.6
scan size of 5 m.
m ꢂ 5
m
m sꢁ1 and
m
m
4.3. Computational modeling
To elucidate the difference in performance between PBDT-TPD
and PBDT-PPD we performed theoretical calculations using density
functional theory (DFT). The geometries of model oligomers (n ¼ 1,
2, 3, and 4) for both copolymers were optimized at the B3LYP/6-
31G* level in which the long side chains were truncated to
methyl groups in order to save computational expense. The first ten
excited states were determined through a time dependent density
functional theory treatment using the same level of theory as the
optimization. The HOMO, LUMO, and optical band gaps were pro-
duced by fitting the set of oligomers with the Kuhn expression:
[39,40].
Fig. 6. AFM height (left) and phase (right) images at
PPD:PC71BM with 5% DIO (aeb) and 5% CN (ced) solvent additives.
5
m
m
ꢂ
5 mm of PBDT-
Varian VXR-300, Varian MR-400, or Bruker Advance III-600 spec-
trometers. 1H NMR spectra were internally referenced to the re-
sidual solvent peak. In all spectra, chemical shifts are given in ppm
(
d
) relative to the solvent. Gel permeation chromatography (GPC)
measurements were performed on a Shimadzu Prominence GPC
with two 10 m AM Gel columns connected in series (guard,
m
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
10,000 Å, 1000 Å) in chloroform at 40 ꢀC relative to polystyrene
standards. Thermogravimetric analysis (TGA) were performed over
an interval of 30e850 ꢀC at a heating rate of 20 ꢀC minꢁ1 under
ambient atmosphere. Differential scanning calorimetryꢁ(1DSC) was
performed using a first scan heating rate of 15 ꢀC min to erase
thermal history and a second scan to measure transitions between
k0
E ¼ E0 1 þ 2 cos
k0
p
(2)
N þ 1
where E0 is the transition energy of a formal double bond, N is the
number of double bonds in the oligomer (thought to be identical
oscillators), and k0 =k0 is an adjustable parameter (indicative of the
strength of coupling between the oscillators). In addition, the
0
and 330 ꢀC under nitrogen. Cyclic voltammetry (CV)

B.J. Hale et al. / Polymer 109 (2017) 85e92
91
reorganization energy, which is a measure of charge mobility for
both the hole (lh) and electron (le), was calculated using: [41e44].
0.36 mmol) were dissolved in toluene (10 mL) and sparged with
argon for 30 min. Tris(dibenzylideneacetone)dipalladium(0)
(6.6 mg, 2 mol%) and tri(o-tolyl)phosphine (9.8 mg, 9 mol%) were
added and the reaction refluxed for 48 h. The polymer was end-
capped by refluxing with trimethyl(phenyl)tin (50 mg) for 4 h,
followed by refluxing with iodobenzene (0.1 mL) overnight. After
cooling to ambient temperature, the mixture was precipitated into
methanol and filtered through a Soxhlet thimble. The polymer was
washed with methanol (4 h), acetone (4 h), hexanes (12 h), and
extracted with chloroform. The chloroform fraction was then
concentrated and the polymer run through a short silica gel plug.
The resulting fraction was then concentrated (~5 mL) and precipi-
tated into methanol, filtered, and dried in vacuo to give the ex-
pected polymer as a dark orange solid (210.8 mg, 73%). Mn:
À
Á
À
Á
l
¼ E0ꢃ ꢁ E0 þ Eꢃ ꢁ E
(3)
where E0 and E are the energies of the neutral and charged
optimized geometries and the E0* and E* 0 are the energies of neutral
geometry with charge and the charged geometry set to neutral.
4.4. Fabrication of photovoltaic devices
All devices were produced via a solution-based, spin-casting
fabrication process. All polymers were mixed with PC70BM (1-
material) (mixed 1:1.5 with a total solution concentration of
25 mg mLꢁ1) dissolved in chlorobenzene (Sigma Aldrich) and
stirred overnight at 115 ꢀC at 800 rpm. ITO (sheet resistance:
20.1 kDa, PDI: 1.8; 1H NMR (600 MHz, CDCl3)
d 8.34 (s, 2H), 4.63 (s,
2H), 4.40 (s, 4H), 3.64 (s, 2H), 2.05e0.68 (m, 60H).
5e15
U
ꢁ1) coated glass slides (Delta Technologies) were cleaned by
consecutive 10 min sonication in (i) Alconox detergent (dissolved in
deionized water), (ii) deionized water, and then (iii) isopropanol.
The slides were then dried with N2 and cleaned with air plasma for
Funding
This work was supported by the 3M Foundation and Iowa State
University (ISU) and partially supported by DMR-1410088.
10 min. Filtered (0.45 mm) PEDOT:PSS (Clevios P™ 4083) was spin-
coated onto the prepared substrates (5000 rpm/60 s) and then
annealed at 120 ꢀC for 20 min. After cooling, the substrates were
transferred to an argon-filled glovebox. The polymer:PC70BM so-
Acknowledgements
lutions were filtered with 0.2 mm pore filter, and simultaneously
We wish to thank Steve Veysey and the ISU Chemical Instru-
mentation Facility for training and assistance with the thermal
analysis. We also thank Dr. Kamel Harrata and the ISU Mass Spec-
troscopy Laboratory for analysis. The OPVs were fabricated at the
ISU Microelectronics Research Center.
dropped onto the PEDOT:PSS-coated substrates and spin-cast at
1400 rpm for 60 s. The films were dried under petri-dish for 8 h. For
the active layers with solvent additives, 5% (v/v) of either CN or DIO
was added to the stock solution, and then deposited at the afore-
mentioned casting conditions. Finally, Ca (20 nm) and Al (100 nm)
were thermally evaporated through
a
shadow mask
Appendix A. Supplementary data
(area ¼ 0.1256 cmꢁ2) under vacuum of 10ꢁ6 mbar to complete the
devices. Current-voltage (J-V) data were generated by illuminating
the devices using an ELH Quartzline halogen lamp at 1 sun. The
solar simulator was calibrated using a crystalline silicon photodiode
with a KG-5 filter. The hole only devices were prepared following
the same procedure, except calcium was replaced with molybde-
num suboxide. The hole mobility was extracted from the SCLC
measurement using a Keithley 2400 SourceMeter in the dark under
ambient conditions.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2016.12.013.
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