5,10-dihydroindolo[3,2-b]indole-based copolymers with alternating donor and acceptor moieties for organic photovoltaics

Article abstract of DOI:10.1021/ma301987p

A series of new donor-acceptor π-conjugated copolymers incorporating 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron donating unit have been designed, synthesized, and explored in bulk heterojunction solar cells with diketopyrrolopyrrole and thienop

Full text of DOI:10.1021/ma301987p

Article
pubs.acs.org/Macromolecules
5
,10-Dihydroindolo[3,2‑b]indole-Based Copolymers with Alternating
Donor and Acceptor Moieties for Organic Photovoltaics
Zbyslaw R. Owczarczyk,* Wade A. Braunecker, Andres Garcia, Ross Larsen, Alexandre M. Nardes,
Nikos Kopidakis, David S. Ginley, and Dana C. Olson
National Center for Photovoltaics, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401,
United States
*
S Supporting Information
ABSTRACT: A series of new donor−acceptor π-conjugated copolymers
incorporating 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron
donating unit have been designed, synthesized, and explored in bulk
heterojunction solar cells with diketopyrrolopyrrole and thienopyrrole-
dione as the electron accepting units. A significant effect of the size and
shape of the pendant alkyl substituents attached to the DINI unit on the
optical and electronic properties of the copolymers is described. Our
study reveals a good correlation between the theoretical calculations
performed on the selected materials and the experimental HOMO,
LUMO, absorption spectra, and band gap energies of the corresponding
copolymers. The band gaps of the conjugated copolymers can be tailored
over 0.4 eV by the electron-withdrawing nature of the different acceptor
units to provide better overlap with the solar spectrum, and the energy
levels can be tuned ∼0.2 eV depending on the alkyl substituents
employed. For the polymers in this study, a nonoptimized power conversion efficiency as high as 3% was observed.
INTRODUCTION
fullerene acceptors and relatively deep highest occupied
molecular orbital (HOMO) of the polymer is desirable to
maximize the open circuit voltage (VOC) in OPV devices.
■
Organic photovoltaic (OPV) devices based on π-delocalized
conjugated polymers have attracted considerable interest due to
their potential for solution processable, lightweight, flexible, and
economically viable solar cells.
BHJ) has proven particularly successful for polymer solar cells,
8,9
Conjugated polymers with aromatic backbones comprising
alternating units of electron-rich (D) and electron-deficient
moieties (A) provide a unique method for fine-tuning polymer
band gaps to better harvest an expanded range of the solar
spectrum. Additionally, the electron donating/withdrawing
strength of the individual components can be adjusted to
further fine-tune the HOMO and LUMO values of the polymer
to permit efficient photoinduced charge transfer to the fullerene
1
−3
The bulk heterojunction
(
wherein the active layer is comprised of an interpenetrating
network of a narrow band gap conjugated polymer blended
with a fullerene derivative. The morphology of the active layer,
particularly phase separation of the polymer/fullerene blend,
plays a very important role in determining charge generation
and transport properties that are important to the function of
acceptor while maintaining a relatively high V , thereby
OC
8,9
1
,4,5
maximizing power conversion efficiency (PCE).
Conse-
the solar cell.
It is critical that a bicontinuous network be
quently, BHJ solar cells containing D−A conjugated copolymer
formed with large interfacial areas and appropriate domain sizes
to facilitate exciton dissociation and transport of separated
charges to the corresponding electrodes in the device.
Additionally, polymer/fullerene intercalation is also determined
by the size and density of the solubilizing alkyl chains along the
structures exhibit some of the highest PCEs for OPV devices to
10,11
date.
It is well established that polymer absorbers containing a
long, planar, π-conjugated backbone can offer broad overlap of
their absorption with the solar spectrum and often demonstrate
6
polymer backbone. Thus, systematic efforts are required to
12
high charge-carrier mobility due to π−π stacking interactions.
find those alkyl substituents on a polymer absorber that best
However, materials containing unsubstituted multiple fused
aromatic rings are notoriously insoluble in common organic
solvents used for the fabrication of OPV cells. 5,10-
Dihydroindolo[3,2-b]indole (DINI) represents a highly π-
facilitate the formation of optimal nanoscale phase-separated
4
−7
morphologies.
Electronic properties of the organic semiconductors present
in the active layer are also critically important to influencing
device performance. Ideally, the absorption of the conjugated
polymer should have broad overlap with the solar spectrum to
maximize photon collection. Additionally, suitable energy levels
of the polymer absorber are required to match those of the
Received: September 20, 2012
Revised: January 22, 2013
Published: February 5, 2013
This article not subject to U.S. Copyright.
Published 2013 by the American Chemical
Society
1350
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Article
a
Scheme 1. Synthesis of 5,10-Dihydroindolo[3,2-b]indole Comonomers
a
Reagents and conditions: (i) ethynyltrimethylsilane, toluene, Et N, cat. PdCl (PPh ) −CuI, rt, 12 h; (ii) KOH−MeOH, rt, 0.5 h, iodonitrobenzene,
3
2
3 2
toluene, Et N, cat. PdCl (PPh ) −CuI, rt, 12 h; (iii) KMnO , AcOH, H O, cat. TBABr, CH Cl , refux, 6 h; (iv) Zn, AcOH, HCl, 1 h at 40 °C, then 3
3
2
3
2
4
2
2
2
h at 80 °C; (v) alkyl halide or tosylate, NaH, DMF, rt, 8 h; (vi) Br , Py/CHCl , rt, 3 h; (vii) 2.1 equiv of n-BuLi, THF, −70 °C, 2 h, then 2-
2
3
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, −70 °C to rt, 4 h.
conjugated, electron-rich, and planar aromatic polycyclic fused
structure (Scheme 1) that, when copolymerized with an
electron-deficient unit (A) in alternating fashion, could lead
to strong π−π interactions among polymer chains and
potentially enable broad spectral absorption ideal for OPV
applications. The two unsubstituted nitrogen atoms on the
opposite sides of the aromatic skeleton of DINI offer a unique
opportunity to easily tailor the solubility and alkyl chain density
of the polymer. Despite these promising features, to the best of
our knowledge, DINI-containing copolymers have never been
investigated for OPV applications.
options for tailoring the alkyl groups along the copolymer
backbone. The varied electron withdrawing strengths of DPP
and TPD also allow the absolute energy levels of the DINI
copolymers to be tuned. Additionally, time-dependent density
functional theory computations suggest that D−A copolymers
containing DINI with DPP or TPD offer many desirable
characteristics that make these polymers attractive candidate
materials, including a high extinction coefficient, a broad
absorption which overlaps well with the solar spectral spectrum,
a relatively low band gap, appropriate energy levels, and a broad
delocalization of the HOMO along the polymer backbone. We
demonstrate the effect of the side chains on packing in the solid
state and correlate stronger coupling of the polymer chains with
a further red-shift of the absorption spectrum as well as with
improved efficiency for free carrier generation in bulk
heterojunctions. Finally, we show that improved free carrier
generation leads to higher short-circuit current density in OPV
devices under simulated sunlight.
In this work, DINI is copolymerized with the electron
accepting moieties diketopyrrolopyrrole (DPP) and thienopyr-
1
3
14
roledione (TPD). Recently, DPP and TDP based
copolymers have emerged as very attractive materials for both
thin-film transistors and solar cell devices. The compact planar
structure and strong electron-withdrawing ability of DPP and
TPD moieties could potentially benefit electron delocalization
along copolymer chains via the push−pull effect and promote
interchain π−π stacking. Such interactions should improve
charge carrier mobility and significantly affect the self-assembly
process, which can be further manipulated by changing the size
EXPERIMENTAL SECTION
Reagents and Instrumentation. All reagents and chemicals were
purchased from commercial sources (Aldrich, Acros, Strem, Fluka)
and used without further purification unless stated otherwise. All
■
6
of alkyl chains on the electron-withdrawing units. Unlike
reactions were performed under dry N . Solvents were dried when
2
benzothiadiazole, a robust electron withdrawing unit that is
widely used in OPV applications, DPP and TPD can be
functionalized on the nitrogen atoms with alkyl chains. This not
only improves the polymer’s solubility but also provides further
necessary or purified using Mbraun Solvent Purifier. Column
chromatography was performed with Fluka Silica Gel 60 (220−400
1
mesh). All small molecules were characterized by H NMR (400
MHz) and 13C NMR (100 MHz) on a Varian Unity Inova. Chemical
1
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shifts in the NMR spectra were reported in ppm relative to the singlet
Device Fabrication. Pattern glass/ITO substrates (Thin Films
Devices, Inc.) for photovoltaic measurements were cleaned prior to
device fabrication by sonication in acetone and isopropanol followed
by a O plasma treatment at 800 mTorr O pressure and 150 W power
at 7.26 ppm for CDCl . UV−vis absorption measurements were
3
performed using a Hewlett-Packard 8453 UV−vis spectrophotometer.
Gel permeation chromatography (GPC) measurements were
performed on a PL-Gel 300 × 7.5 mm (5 μm) mixed D column
using Agilent 1200 Series GPC-SEC Analysis System.
2
2
for 5 min. Devices with the standard architecture ITO/PEDOT:PSS/
B H J / C a / A l , w h e r e P E D O T : P S S i s p o l y ( 3 , 4 -
ethylenedioxythiophene):poly(styrenesulfonate), were fabricated by
spin-coating PEDOT:PSS (Clevios P VP Al 4083) followed by spin-
coating of the corresponding polymer:PC BM solution and
Polymer Molecular Weight Determination. Polymer samples
were dissolved in HPLC grade chloroform (∼1 mg/mL), stirred, and
heated at 50 °C for several hours, stirred overnight at rt, and then
filtered through a 0.45 μm PVDF filter. Size exclusion chromatography
was then performed on a PL-Gel 300 × 7.5 mm (5 μm) mixed D
column using an Agilent 1200 series autosampler, inline degasser, and
refractometer. The column and detector temperatures were 35 °C.
HPLC grade chloroform was used as eluent (1 mL/min). Linear
polystyrene standards were used for calibration. The same general
procedure was performed for larger scale preparatory GPC work. 4.5
mL of a ∼3 mg/mL polymer solution in HPLC grade chloroform was
injected on two PL-Gel 300 × 25 mm (10 μm) mixed D columns
connected in series. An Agilent 1200 series autosampler, inline
degasser, and diode array detector were employed. The column and
detector temperatures were 25 °C. HPLC grade chloroform was used
as eluent (10 mL/min).
6
1
completed by thermal deposition of 20 nm of Ca and 100 nm of Al
2
electrodes with an area of 11 mm via a shadow mask at a base
−
8
pressure of ∼3 × 10 Torr. The blend ratio of polymer to PC BM
6
1
was altered from 1:1 to 1:4.
Polymer BHJ films with ratio 1:2 P1−P8:PC BM were deposited
61
from 8 mg/mL polymer concentration in chlorobenzene (CB) or
CHCl3 solutions containing various amount (%) by volume 1,8-
diiodooctane cosolvent. All BHJ films were fabricated by spin-coating
filtered 70 °C solutions with 0.45 μm PTFE filters. Current−voltage
2
measurements of PV devices under a 100 mW/cm illumination
intensity (supplied by a tungsten halogen lamp and monitored with
Hamamatsu Si photodiodes equipped with KG5 filters) were carried
out with a Keithley 236 source-measuring unit in a N atmosphere.
2
Cyclic Voltammetry. All voltammograms were recorded at 25 °C
Theoretical Methods. We used a combination of ground-state
density functional theory (DFT) and time-dependent density
functional theory (TDDFT) to predict the properties of hydrogen-
terminated oligomers (with n = 1, 2, 3, 4) for the polymers reported in
this work. We performed all calculations using the default settings in
with a CH Instruments Model 600D potentiostat. Measurements were
−1
carried out under nitrogen at a scanning rate of 0.1 V s using a
platinum wire as the working electrode and a platinum wire as the
+
counter electrode. Potentials were measured vs Ag/Ag (and calibrated
+
18
vs Fc/Fc ) using 0.01 M AgClO and a 0.1 M Bu NBF salt bridge to
the Gaussian 09 electronic structure package, revision B.01, and we
4
4
4
+
minimize contamination of the analyte with Ag ions. Polymer films
were drop-cast onto a platinum wire working electrode from a 1 mg/
mL chloroform solution and dried under a stream of nitrogen prior to
measurement in a 0.1 M Bu NBF −acetonitrile solution.
Time-Resolved Microwave Conductivity. TRMC is a pump−
probe technique that has been extensively used to measure the
photoconductance of a film without the need for charge collection at
electrical contacts.
have been presented elsewhere.
a microwave cavity at the end of an X-band waveguide operating at ca.
GHz, and is photoexcited through a grid with a laser pulse with a 5
optimized the geometry of each oligomer in vacuum using the Becke-
style three-parameter density functional with the Lee−Yang−Parr
correlation function (B3LYP) with the 6-31G(d) basis set;
subsequently, we added diffuse functions to the basis (6-31+G(d))
for calculation of the orbital energies and optical absorption spectra of
the optimized structures, as described below. We found that adding
diffuse functions had little effect on the predicted absorption spectra;
the main change was a systematic shift of the molecular orbital
energies down by ∼200−300 meV.
4
4
1
5,16
The details of the experimental methodology
16,17
In brief, the sample is mounted in
To estimate optical absorption spectra, we calculated the lowest 12
excited states with TDDFT. In all cases, the alkyl groups substituents
were replaced by methyl groups because we have found that there is a
negligible effect on the HOMO and LUMO levels and the absorption
spectrum caused by replacing a methyl group with an ethyl or a small
branched alkyl group, such as isopropyl. For visualization purposes, we
broadened the predicted absorption spectra by convolving the discrete
gas-phase spectrum with a Gaussian having full width at half-maximum
of 0.15 eV and transforming the resulting spectra from functions of
9
ns laser pulse from an OPO pumped by a Nd:YAG laser. The relative
change of the microwave power, P, in the cavity, caused by the
photoinduced generation of free electrons and holes, is related to the
transient photoconductance, ΔG, by ΔP/P = −KΔG, where the
calibration factor K is experimentally determined individually for each
sample. Taking into account that the electrons and holes are generated
16
in pairs, the photoconductance can be expressed as
2
energy to functions of wavelength with the appropriate hc/λ Jacobian
ΔG = βq F I (ϕ μ)
∑
A 0
(1)
factor. The 0.15 eV broadening was determined empirically by
comparing the widths of typical spectra reported in the literature for
donor−acceptor type copolymers; no attempt was made to tune this
broadening to better match the measured spectra. For each oligomer,
e
where q is the elementary charge, β = 2.2 is the geometric factor for
e
the X-band waveguide used, I is the incident photon flux, F is the
0
A
fraction of light absorbed at the excitation wavelength, φ is the
the energy gap, E , is defined as the lowest energy excitation; note that
g
quantum efficiency of free carrier generation per photon absiorbed,
this energy corresponds to the reddest peak maximum in an
absorption spectrum rather than the absorption onset which often is
reported from experiment. We also report the energy of an electron in
the excited state as the optical LUMO (O-LUMO), which is the
calculated HOMO value plus the energy gap; physically, the O-LUMO
value seems most relevant for predicting whether exciton dissociation
may occur by injection of an electron into an electron accepting
species.
1
6
and ∑μ is the sum of the mobilities of electrons and holes. Equation
1
allows one to calculate the φ∑μ product:
ΔG
ϕ
∑
μ =
βq F I
A 0
(2)
e
At high absorbed photon flux φ∑μ decreases with I due to higher
0
1
6,17
order processes that limits the charge-carrier generation yield, φ.
Images of molecular geometries and molecular orbitals were
We use eq 3 (below) to fit the light intensity dependence of φ∑μ and
generated with Jmol, an open-source Java viewer for chemical
1
9
extrapolate to the low intensity range where φ∑μ does not depend on
structures in 3D. We display HOMO and LUMO contour levels
encompassing 80% of the total electron density for each orbital.
Synthesis of Comonomers. 1,2-Bis(2-nitrophenyl)ethyne (3). 2-
Iodonitrobenzene (24.9 g, 0.10 mol) was dissolved in dry toluene (220
mL) and triethylamine (25 mL), the solution was purged with
nitrogen, and PdCl (Ph P) (230 mg, 0.4 mmol) and CuI (115 mg,
16,17
I (the linear response regime).
0
A
ϕ ∑ μ = 1
+
BI0FA + CI0FA
(3)
2
3
2
where A, B, and C are fitting coefficients. At the low intensity range,
φ∑μ = A.
0.6 mmol) were added, followed by (trimethylsilyl)acetylene (15.5
mL, 0.11 mol). The mixture was stirred for 12 h, hexane (80 mL) was
1
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added, and the suspension was filtered through a Celite plug. The
resulting eluate was concentrated, the oily residue was dissolved in
methanol (100 mL), and a solution of KOH (7.0 g) in methanol (60
mL) was slowly added while maintaining the temperature below 25
Hz). 13C NMR (100 MHz, CDCl ): δ 140.92, 126.21, 121.34, 117.78,
3
114.52, 109.90, 49.57, 40.34, 30.86, 28.75, 24.23, 23.04, 13.98, 10.92.
5,10-Bis(2-hexyldecyl)-5,10-dihydroindolo[3,2-b]indole (8). This
compound was synthesized according to the general procedure
23
°C. The mixture was stirred for 0.5 h, concentrated to ca. 100 mL,
described above using 2-hexyldecyl p-toluenesulfonate in place of
poured into water (500 mL), and the product was extracted with ethyl
acetate. The extract was dried with sodium sulfate and filtered through
a short plug of silica prior to evaporation. The resulting crude (2) was
dissolved in toluene (200 mL) containing triethylamine (25 mL) and
an alkyl halide. Compound 8 was isolated in 84% yield as a pale yellow
1
solid. H NMR (400 MHz, CDCl
): δ 7.81 (d, 2H, J = 8.0 Hz), 7.42
3
(d, 2H, J = 8.4 Hz), 7.26 (t, 2H), 7.14 (t, 2H), 4.43 (d, 4H, J = 7.4
1
3
Hz), 2.23 (m, 2H), 1.17−1.36 (m, 48H), 0.84 (m, 12H). C NMR
2-iodonitrobenzene (23.0 g), the solution was purged with nitrogen,
(100 MHz, CDCl ): δ 140.98, 126.17, 121.31, 117.76, 114.57, 109.91,
3
and PdCl (Ph P) (230 mg, 0.4 mmol) and CuI (115 mg, 0.6 mmol)
49.94, 38.87, 31.85, 31.77, 29.95, 29.63, 29.49, 29.24, 26.53, 26.50,
22.65, 22.58, 14.10, 14.05.
5,10-Di(heptadecan-9-yl)-5,10-dihydroindolo[3,2-b]indole (9).
2
3
2
were added. The mixture was stirred for 12 h at rt, and the resulting
yellow precipitate was isolated by filtration and crystallized from
toluene to give 13.67 g (51% yield) of light-yellow solid; mp 187−189
This compound was synthesized according to a general procedure
2
0
1
1
using 9-heptadecane p-toluenesulfonate in 90% yield. H NMR (400
°
7
=
C dec (lit. 188−189 °C). H NMR (400 MHz, CDCl ): δ 7.50−
3
MHz, CDCl ): δ 7.92 (d, 2H, J = 7.8 Hz), 7.42−7.68 (m, 2H), 7.08−
.56 (m, 2H), 7.63−7.69 (m, 2H), 7.83 (d, J = 7.8 Hz, 2H), 8.14 (d, J
3
8.3 Hz, 2H). 13C NMR (100 MHz, CDCl ): δ 91.95, 118.07, 124.83,
7
1
.33 (m, 4H), 4.50−4.95 (m, 2H), 2.32 (m, 4H), 2.00 (m, 4H), 0.94−
.30 (m, 48H), 0.81 (t, 12H, J = 7.2 Hz). Multiples and broad protons
3
129.54, 133.11, 135.26, 149.48.
13
are due to a phenomenon of atropisomerism. C NMR (100 MHz,
1
,2-Bis(2-nitrophenyl)ethane-1,2-dione (4). Compound 3 (19.94
CDCl ): δ 142.03. 125.06, 121.25, 119.84, 117.58, 113.77, 112.64,
g, 74 mmol) was added to a mixture of potassium permanganate (35.2
g, 222 mmol), water (600 mL), TBAB (0.48 g, 1 mmol), methylene
chloride (800 mL), and acetic acid (40 mL). The mixture was
vigorously stirred at reflux for 7 h, cooled, and carefully decolorized
3
1
2
09.87, 59.98, 56.02, 35.14, 34.41, 31.74, 29.34, 29.31, 29.13, 26.94,
2.56, 14.04 (multiple carbon peaks are due to the phenomenon of
2
4
atropisomerism).
2
,7-Dibromo-5,10-bis(n-octyl)-5,10-dihydroindolo[3,2-b]indole
with NaHSO . The organic and aqueous were separated, and the
3
(
10). General Procedure. This compound was synthesized according
yellow organic was dried with Na SO and filtered through Celite plug.
2
4
22
to a modified literature procedure. To a solution of the crude 6 (4.30
After removal of solvent the resulting yellow crystalline solid was
washed with methanol to give 18.90 g (85% yield) of the title
g, 10 mmol) in a mixture of pyridine (8 mL) and chloroform (10 mL)
2
1
1
was added Br (1.1 mL, 21.5 mmol) in chloroform (10 mL) over a
2
compound; mp 206−207 °C (lit. 205−206 °C). H NMR (400
period of 30 min. After 2 h, the mixture was poured into 1 N HCl (200
mL) and ice, the product was extracted twice with ethyl acetate, and
the combined organic layers were washed twice with water and dried
over Na SO prior to evaporation. The crude product was crystallized
MHz, CDCl ): δ 7.70 (dd, J = 7.6 Hz, J = 1.2 Hz, 2H), 7.73−7.79
3
1
2
(
m, 2H), 7.86−7.91 (m, 2H), 8.29 (dd, J = 8.3 Hz, J = 0.5 Hz, 2H).
1
2
21
5
,10-Dihydroindolo[3,2-b]indole (5). To a stirred mixture
2
4
containing acetic acid (360 mL) and Zn powder (45.8 g, 700
mmol) was added 12 N HCl (3 mL) followed by 4 (21.00 g, 70
mmol), which was added in portions over 0.5 h while maintaining
temperature below 40 °C. The mixture was stirred at 80 °C for 1 h, an
additional amount of Zn (9.81 g, 200 mmol) was added, and stirring
was continued for another 2 h, before the reaction mixture was cooled
to rt. The solids containing unreacted Zn and the product were
isolated by filtration and washed twice with methanol, and the product
was eluted with hot DMF (70−80 °C, 3 × 50 mL). The cold eluent
was poured slowly to stirred cold water (1200 mL), precipitated white
solid was isolated by filtration, washed with water, several times with
from methanol−ethyl acetate to yield 3.59 g (61%) of the title
1
compound as a yellow solid. H NMR (400 MHz, CDCl ): δ 7.62 (d,
3
2
H, J = 8.4 Hz), 7.56 (d, 2H, J = 1.6 Hz), 7.26 (dd, 2H, J = 8.4 Hz, J
1 2
=
1.6 Hz), 4.34 (t, 4H, J = 7.2 Hz), 1.90 (m, 4H), 1.17−1.41 (m,
2
1
3
0H), 0.85 (t, 6H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl ): δ
3
41.09, 125.77, 121.37, 118.60, 115.34, 113.11, 112.73, 45.41, 31.74,
1.10, 29.28, 29.09, 27.03, 22.56, 14.03.
2
,7-Dibromo-5,10-bis(2-ethylhexyl)-5,10-dihydroindolo[3,2-b]-
1
indole (11). Yield 61%; a light orange solid. H NMR (400 MHz,
CDCl ): δ 7.60 (d, 2H, J = 8.5 Hz), 7.52 (d, 2H, J = 1.5 Hz), 7.24 (dd,
3
2
H, J = 8.5 Hz, J = 1.5 Hz), 4.18 (m, 4H), 2.06 (m, 2H), 1.16−1.42
1
2
methanol, and dried initially in air and then under vacuum to yield
13
(
(
m, 16H), 0.88 (t, 6H, J = 7.4 Hz), 0.81 (t, 6H, J = 7.2 Hz). C NMR
100 MHz, CDCl ): δ 141.39, 125.86, 121.15, 118.66, 115.18, 112.96,
1
1
3.06 g (91%) of the title compound. H NMR (400 MHz, acetone-
3
d6): δ 7.53 (d, J = 8.1 Hz, 2 H), 7.78 (d, J = 7.8 Hz, 2H), 7.53 (d, J =
.1 Hz, 2 H), 7.16−7.20 (m, 2 H), 7.06−7.10 (m, 2 H).
,10-Di(n-octyl)-5,10-dihydroindolo[3,2-b]indole (6). General
Procedure. This compound was synthesized according to modified
1
2
12.83, 49.72, 38.53, 31.84, 31.72, 31.58, 31.60, 29.87, 29.56, 29.44,
9.24, 26.39, 22.64, 22.58, 14.10, 14.04.
8
5
2
,7-Dibromo-5,10-bis(2-hexyldecyl)-5,10-dihydroindolo[3,2-b]-
indole (12). This compound was synthesized according to a general
procedure, except that after work-up, the crude material was extracted
from aqueous mixture with ethyl acetate, the organic layer was dried
over sodium sulfate prior to evaporation of the solvent, and the crude
product was purified on a short silica gel plug using hexane as an
22
literature procedure. NaH (2.64 g, 55 mmol, 50% in mineral oil) was
stirred with hexane (25 mL) under nitrogen for a few minutes before
stirring was turned off to allow NaH to settle at the bottom of the
flask. After 30 min, the hexane solution was removed under a slight
nitrogen pressure, using cannula tipped with a soft filter paper. The
activated NaH was mixed with dry DMF (45 mL). Subsequently, 5
1
eluent to give a light orange oil in 80% yield. H NMR (400 MHz,
CDCl ): δ 7.60 (d, 2H, J = 8.4 Hz), 7.52 (d, 2H, J = 1.2 Hz), 7.23 (dd,
3
(
4.08 g, 20 mmol) was added to the mixture in portions, followed by
2
1
1
3
1
H, J = 8.4 Hz, J = 1.2 Hz), 4.20 (d, 4H, J = 7.0 Hz), 2.12 (m, 2H),
1 2
n-octyl bromide (7.8 mL, 45 mmol), while maintaining temperature of
the reaction mixture below 35 °C. The mixture was then stirred for 10
h at rt before it was poured slowly into cold water (600 mL). The
precipitated solid material was isolated by filtration, washed with water
13
.16−1.40 (m, 48H), 0.86 (m, 12H). C NMR (100 MHz, CDCl ): δ
3
41.18, 125.80, 121.21, 118.62, 115.19, 112.98, 112.79, 50.21, 39.01,
1.93, 31.80, 30.01, 29.80, 29.29, 26.60, 26.42, 22.70, 22.65, 14.20,
4.09.
1
and methanol, and dried under vacuum to yield 8.10 g (94%). H
2
,7-Dibromo-5,10-bis(heptadecan-9-yl)-5,10-dihydroindolo[3,2-
1
NMR (400 MHz, CDCl ): δ 7.82 (d, 2H, J = 7.8 Hz), 7.44 (d, 2H, J =
b]indole (13). Yield 81%; a light orange solid. H NMR (400 MHz,
3
8
4
.1 Hz), 7.30 (t, 2H), 7.16 (t, 2H), 4.35 (t, 4H, J = 7.2 Hz), 1.91 (m,
H), 1.18−1.44 (m, 20H), 0.86 (t, 6H, J = 7.2 Hz). C NMR (100
CDCl ): δ 7.72 (d, 2H, J = 8.2 Hz), 7.62 (bs, 2H), 7.26 (bs, 2H),
3
13
4.27−4.83 (m, 2H), 2.22 (m, 4H), 1.87 (m, 4H), 0.90−1.28 (m, 48H),
MHz, CDCl ): δ 140.95, 126.17, 121.32, 117.78, 114.56, 109.91,
0.80 (t, 12H, J = 7.2 Hz) (multiple and broad protons are due to a
3
2
4
4
9.56, 45.40, 31.74, 31.12, 29.26, 29.02, 27.00, 22.53, 14.02.
,10-Bis(2-ethylhexyl)-5,10-dihydroindolo[3,2-b]indole (7). Yield:
phenomenon of atropisomerism).
13
5
C NMR (100 MHz, CDCl ): δ 142.74, 125.02, 121.16, 120.83,
3
1
96%. H NMR (400 MHz, CDCl ): δ 7.84 (d, 2H, J = 7.8 Hz), 7.45
115.41, 112.94, 112.41, 56.55, 34.95, 31.73, 29.24, 29.07, 26.76, 22.56,
14.04 (multiple carbon peaks are due to the phenomenon of
3
(
2
d, 2H, J = 8.2 Hz), 7.29 (t, 2H), 7.17 (t, 2H), 4.36 (m, 4H), 2.19 (m,
H), 1.20−1.45(m, 16H), 0.92 (t, 6H, J = 7.2 Hz), 0.85 (t, 6H, J = 7.2
2
4
atropisomerism).
1
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Macromolecules
Article
5
,10-Bis(n-octyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
-yl)-5,10-dihydroindolo[3,2-b]indole (14). General Procedure. This
compound was synthesized according to modified literature
(9% mol, 8.0 mg), and dibenzo-18-crown-6 (5% mol, 3.6 mg) were
dissolved in 10 mL of toluene, followed by addition of 1.0 mL of
K PO (2.5 M). The mixture was purged with nitrogen for 10 min and
2
3
4
25
procedure. To a solution of compound 10 (2.35 g, 4.0 mmol) in
THF (40 mL) was added dropwise 3.60 mL (9.0 mmol) of n-
butyllithium (2.5 M in hexane) at −70 °C. The mixture was stirred at
then vigorously stirred at 105 °C for 60 h, before bromobenzene (6
μL, 0.045 mmol) was injected to the reaction mixture. Three hours
later, phenylboronic acid (8.5 mg, 0.06 mmol) was added, and the
reaction mixture was stirred for an additional 5 h to complete the end-
capping process. A palladium scavenger complexing ligand (N,N-
−
78 °C for 2 h, and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane (2.65 mL, 13,0 mmol) was added rapidly to the suspension of
the bis-lithium salt in THF. After an additional 1 h at −78 °C, the
resulting mixture was warmed up to rt and stirred for 6 h, before it was
poured into cold water (300 mL), extracted with ethyl acetate, and
dried over sodium sulfate. The solvent was removed under reduced
pressure, and the residue was purified by crystallization form
2
8
diethylphenylazothioformamide) was then stirred with the reaction
mixture for 30 min before the polymer was precipitated into 200 mL
of MeOH−H O (15−1) mixture, filtered off, washed with methanol,
2
water, and acetone, and subsequently was purified via Soxhlet
extraction with acetone and chloroform. The chloroform fraction
was concentrated under reduced pressure, the material was
reprecipitated into methanol (150 mL), isolated by filtration, washed
with methanol, and vacuum-dried overnight to yield the corresponding
polymer in 50−80% yield.
methanol/ethyl acetate to yield the title product as light yellow
crystals (1.73 g, 63%). H NMR (400 MHz, CDCl ): δ 7.95 (s, 2H),
.84 (d, 2H, J = 7.8 Hz), 7.62 (d, 2H, J = 7.9 Hz), 4.54 (t, 4H, J = 7.2
Hz), 1.98 (m, 2H), 1.40 (s, 24H), 1.14−1.50 (m, 20H). C NMR
100 MHz, CDCl ): δ 140.48, 127.17, 124.03, 121.80, 117.21, 116.36,
1
3
7
13
(
Copolymers 6−8. Comonomer 18 (88.0 mg, 0.21 mmol), DINI
comonomer (0.21 mmol), Pd (dba) −CHCl complex (2 mol %), and
3
1
16.24, 83.58, 45.18, 31.80. 30.38, 29.39, 29.14, 22.58, 14.06.
,10-Bis(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)-5,10-dihydroindolo[3,2-b]indole (15). Yield: 57%; light
2
3
3
5
tri(o-tolyl)phosphine (8 mol %) were placed in a flask, purged with
three nitrogen/vacuum cycles, and subsequently dissolved in 5 mL of
dry chlorobenzene, from which oxygen was removed by purging with
nitrogen for 1 h. The mixture was stirred for 36 h at 110 °C, after
which 20 μL of 2-bromothiophene was injected as a capping agent.
The reaction was stirred for 2 h at 110 °C before 20 μL of 2-
1
yellow crystals (1.57 g, 57%). H NMR (400 MHz, CDCl ): δ 7.93
bs, 2H), 7.83 (d, 2H, J = 7.8 Hz), 7.61 (d, 2H, J = 7.8 Hz), 4.41 (m,
3
(
4
1
1
2
H), 2.19 (m, 2H), 1.39 (bs, 24H), 1.19−1.44 (m, 16H), 0.87 (m,
2H). 13C NMR (100 MHz, CDCl ): δ 141.02, 127.36, 123.86,
3
21.48, 117.35, 116.67, 116.18, 83.53, 49.35. 40.28, 30.66, 28.65,
4.94, 24.89, 24.25, 23.00, 14.04, 10.98.
(
tributyltin)thiophene was injected to complete the end-capping. After
an additional 2 h of stirring, a complexing ligand (N,N-
2
8
5
,10-Bis(2-hexyldecyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
diethylphenylazothioformamide) was stirred with the polymer to
remove any residual catalyst before being cooled to rt and precipitated
into methanol (100 mL). The precipitate was purified via Soxhlet
extraction overnight with methanol, for 2 h with acetone, and finally
was collected with chloroform. The chloroform solution was then
concentrated by evaporation; the material was reprecipitated into
methanol (150 mL), isolated by filtration, washed with methanol, and
vacuum-dried overnight to yield the corresponding polymer in 55−
olan-2-yl)-5,10-dihydroindolo[3,2-b]indole (16). This material was
synthesized according to the general procedure in 42% yield as an oil,
after purification on silica gel using a mixture of ethyl acetate and
hexane (1:7) as an eluent (R 0.25). H NMR (400 MHz, CDCl ): δ
.92 (s, 2H), 7.82 (d, 2H, J = 8.0 Hz), 7.59 (d, 2H, J = 8.0 Hz), 4.40
1
f
3
7
(
d, 4H, J = 7.4 Hz), 2.25 (m, 2H), 1.17−1.38 (m, 48H), 0.84 (m,
1
1
2
2H). 13C NMR (100 MHz, CDCl ): δ 141.04, 127.35, 123.88,
3
17.31, 116.67, 116.22, 83.46, 49.58, 38.74, 31.85, 31.76, 31.57, 29.89,
9.59, 29.49, 29.24, 26.48, 26.42, 24.88, 22.64, 22.55, 14.09, 14.05.
7
5% yield.
1
Copolymer P2. H NMR (400 MHz, CDCl ): δ 9.0−9.2 (m, 2H),
3
5
,10-Di(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
7
.0−7.8 (m, 8H), 3.7−4.5 (m, 8H), 0.6−2.3 (m, 60H).
borolan-2-yl)-5,10-dihydroindolo[3,2-b]indole (17). Yield 68%; a
1
Copolymer P3. H NMR (400 MHz, CDCl ): δ 7.5−8.0 (m, 10H),
1
3
white solid. H NMR (400 MHz, CDCl ): δ 8.02 (s, 2H), 7.95 (d, 2H,
3
4
2
.2−4.5 (m, 8H), 2.1−2.3 (m, 4H), 1.0−1.4 (m, 48H), 0.7−0.9 (m,
J = 8.0 Hz), 7.64 (d, 2H, J = 8.0 Hz), 4.63−5.00 (m, 2H), 2.33 (m,
4H).
4
=
H), 2.03 (m, 4H), 1.41 (s, 24H), 0.97−1.34 (m, 48H), 0.82 (t, 12H, J
1
Copolymer P4. H NMR (400 MHz, CDCl ): δ 9.1 (bs, 2H), 7.4−
7.0 Hz). 1 C NMR (100 MHz, CDCl ): δ 142.03, 126.31, 123.71,
3
3
3
8
.0 (m, 8H), 4.1−4.9 (m, 6H), 1.9−2.4 (m, 10H), 0.7−1.6 (m, 96H).
1
21.31, 119.37, 116, 70, 115.35, 83.47, 55.84, 35.07, 31.73, 29.45, 29.
2, 29.26, 29.13, 26.67, 24.86, 24.52, 22.54, 14.02 (multiple carbon
peaks are due to the phenomenon of atropisomerism).
1
Copolymer P5. H NMR (400 MHz, CDCl ): δ 9.1 (bs, 2H), 7.4−
3
3
8
1
.0 (m, 8H), 4.2−4.9 (m, 6H), 1.8−2.5 (m, 12H), 0.7−0.9 (m, 18H),
24
.0−1.6 (m, 68H).
1
,3-Bis(5-(trimethylstannyl)thiophen-2-yl)-5-(2-ethylhexyl)-
1
Copolymer P7. H NMR (400 MHz, CDCl ): δ 6.7−8.4 (m, H10),
3
thieno[3,4-c]pyrrole-4,6-dione (18). The brominated precursor of 18,
4
7
.2−4.5 (m, 4H), 3.5−3.7 (m, 2H), 1.8−2.1(m, 3H), 0.5−1.7(m,
1
4
,3-bis(5-bromothiophen-2-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-
26
4H).
,6-dione, was synthesized according to a literature procedure.
1
Copolymer P8. H NMR (400 MHz, CDCl ): δ 7.2−8.2 (m, 10H),
3
Compound 18 was synthesized in the following manner according to a
modified literature procedure. The precursor (1.00 g, 1.70 mmol)
was dissolved in 20 mL of dry THF and cooled to −78 °C under N . A
4
7
.4−4.9 (m, 2H), 3.4−3.6 (m, 2H), 1.7−2.5 (m, 9H), 0.6−1.6 (m,
27
4H).
2
n-butyllithium solution (2.5 M, 1.50 mL, 3.75 mmol) was added
dropwise, and the reaction mixture was stirred for 1 h at −78 °C.
Trimethyltin chloride solution (1 M, 4.3 mL, 4.30 mmol) was added
dropwise at −78 °C; the reaction was warmed to rt and stirred
overnight. The mixture was quenched with 50 mL of water and
extracted twice with 100 mL of ethyl acetate. The combined organic
RESULTS AND DISCUSSION
■
Material Synthesis. The synthesis of DINI, as described in
the Experimental Section, was prepared using modified
29
literature procedure and is illustrated in Scheme 1. Following
the Sonogashira Pd-catalyzed cross-coupling of o-iodonitro-
benzene (1) with ethynyltrimethylsilane to yield trimethyl((2-
nitrophenyl)ethynyl)silane (2), which was treated with KOH to
remove trimethylsilyl group, the resulting acetylene intermedi-
ate was reacted with another equivalent of 1 to yield 1,2-bis(2-
nitrophenyl)ethyne (3). After the oxidation of 3 to the dione 4,
ring closure was accomplished by reductive condensation to
give 5,10-dihydroindolo[3,2-b]indole (5). Subsequent alkyla-
tion reactions were performed in DMF using a moderate excess
of NaH. Derivatives of 3,6-bis(5-bromo-thiophen-2-yl)-2,5-di-
alkylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) were prepared
phase was dried over MgSO , and solvent was evaporated. The residue
4
was precipitated from isopropanol to yield 0.38 g (30%) of the title
1
compound as an orange solid. H NMR (400 MHz, CDCl ): δ 8.06 (s,
3
2
0
1
2
H), 7.16 (s, 2H), 3.54 (d, 2H), 1.84 (m, 1H), 1.40−1.20 (m, 8H),
.95 (m, 6H), 0.44 (s, 18H). 13C NMR (100 MHz, CDCl ): δ 163.72,
3
44.29, 138.54, 137.11, 136.95, 131.36, 128.30, 43.17, 38.85, 31.25,
9.26, 24.55, 23.71, 14.73, 11.13, −7.49.
Typical Proceedure for Polymer Synthesis. Poly[[5,10-bis(alkyl)-
,10-dihydroindolo[3,2-b]indole-2,7-diyl-alt-[2,5-bis(2-ethylhexyl)-
,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]] (P2−
5
2
P5). DPP comonomer (0.20 mmol), DINI comonomer (0.21
mmol), Pd (dba) −CHCl complex (1.5% mol, 3.0 mg), SPHOS
2
3
3
1
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a
Scheme 2. Synthesis of DPP−5,10-Dihydroindolo[3,2-b]indole Copolymers
a
Reagents and conditions: (i) cat. Pd (dba) -o-Tol P or SPHOS, K PO (aq), toluene, 105 °C, 60 h, then end-capping with phenylboronic acid and
2
3
3
3
4
bromobenzene.
1
3
according to previously reported procedure. The stannylated
derivative of thienopyrroledione (TPD) depicted in Scheme 2
P3−P5, solubility was dramatically improved and M
n
approaching 50 kg mol− was achieved (Table 1).
1
27
was synthesized from its brominated precursor. As depicted
in Scheme 2, copolymers of DPP and DINI (P1−P5) were
synthesized via a Suzuki Pd-catalyzed cross-coupling polymer-
ization protocol in toluene at 105 °C using an aqueous solution
of K PO as the hydroxyl anion source. During the course of
our study, it was found that a more basic aqueous solution of
K PO was superior to K CO in that it produced the desired
copolymer in higher yield. End-capping of the polymers was
performed using phenylboronic acid and 2-bromobenzene to
terminate residual active functionalities, which has been shown
to improve performance in photovoltaic devices. A palladium
scavenger was added to the reaction and stirred for 30 min
prior to work-up to facilitate catalyst removal before polymer
precipitation into methanol. The crude material was then
purified via Soxhlet extraction with MeOH followed by acetone
to remove oligomeric materials.
Table 1. Number-Average Molecular Weight (M ),
n
Polydispersity Index (PDI), and Optical and
Electrochemical Properties of Polymers
3
4
b
Egopt
d
e
Mn
(kDa)
λmax/λ0.1max
EHOMO
(eV)
polymer
PDI
(nm)
(eV)
3
4
2
3
P2
P3
P4
P5
P6
P7
P8
5.20
15.1
20.3
49.0
1.7
1.9
3.0
2.6
−
744/805
752/800
696/752
692/753
1.54
1.55
1.65
1.65
1.99
1.97
2.04
−5.0
−5.1
−5.2
−5.2
−5.1
−5.2
−5.3
30
31
a
c
−
10.0
530/625
1.4
532/630
520/607
15.2
1.4
a
b
Not sufficiently soluble for SEC measurements at RT. Measured in
chloroform solution; λ0 = wavelength at which absorption is 0.1 its
.1max
c
d
maximum value. Measured in dichlorobenzene solution. Calculated
from solution λ0.1max. EHOMO estimated from E1/2 measured vs Ag/Ag+
e
As bulky branched substituents on the polymer can have a
negative effect on carrier mobility if π−π stacking between the
+
in acetonitrile and calibrated against Fc/Fc (measured as 0.09 V vs
+
32
Ag/Ag ); E1 = (Ep,a + E )/2, where E and Ep,c are the peak+
/2
p,c
p,a
polymer chains is disrupted, we first attempted to synthesize a
DPP−DINI copolymer using as little alkyl chain density as
possible. P1 was synthesized with a linear n-octyl substituent on
the DINI unit and 2-ethylhexyl on DPP. However, P1 proved
quite insoluble in common organic solvents such as chloroform,
toluene, chlorobenzene, and DMF, even at temperatures above
potentials of the oxidation and reduction waves, respectively; Fc/Fc
34
energy level used in HOMO calculations was −4.80 eV.
While our theoretical calculations (vide infra) indicated that
these D−A copolymers containing DPP and DINI should have
quite low energy gaps, they also predicted that the HOMO
values would not be very deep. Thus, D−A copolymers of the
electron withdrawing TPD were also synthesized with DINI, as
copolymers of TPD are known for their relatively deep HOMO
1
00 °C. The introduction of a branched 2-ethylhexyl alkyl
group onto DINI resulted in noticeable improvement in
solubility of P2 in common organic solvents; however, the
highest number-average molecular weight (M ) that could be
attained for P2 under these conditions was only 5.2 kg mol .
We thus resorted to much larger branched alkyl derivatives.
When primary 2-hexyldecyl and secondary 9-heptadecanyl alkyl
substituents were employed on DINI to synthesize copolymers
3
3
n
values and consequently large values of VOC in OPV devices.
Copolymers containing TPD in BHJ solar cells have also
−1
33
recently delivered PCE in excess of 6%. Unfortunately, we
were not successful in synthesizing TPD and DINI copolymers
under the basic conditions required for Suzuki coupling, as the
1
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a
Scheme 3. Synthesis of TPD−5,10-Dihydroindolo[3,2-b]indole Copolymers
a
Reagents and conditions: (i) cat. Pd (dba) /o-Tol P, chlorobenzene, 105 °C, 60 h, then end-capping with 2-(tributylstannyl)thiophene and 2-
2
3
3
bromothiophene.
imide functionality in the TPD unit is base sensitive. All
attempts to directly stannylate the TPD unit for a Stille
coupling polymerization failed; however, we were successful in
stannylating the 1,3-bis(thiophenyl)-TPD compound in
Scheme 3. Thus, copolymers containing TPD and DINI were
synthesized by a Pd-catalyzed Stille cross-coupling copoly-
merization at 110 °C over 36 h in chlorobenzene, followed by
similar end-capping and purification procedures described
above. P6, having 2-ethylhexyl groups on both the TPD and
DINI units, was not sufficiently soluble for molecular weight
measurement at room temperature on our GPC. However, the
introduction of 2-hexyldecyl and 9-heptadecanyl groups onto
DINI gave the soluble polymers P7 and P8. Note that the
stannylated TPD compound does not appear to be particularly
Table 2. Theoretical Energy Values (eV) for DPP−DINI and
TPD−DINI Oligomers
polymer
n
HOMO
LUMO
optical LUMO
Egopt
DPP−DINI
1
2
3
4
1
2
3
4
−4.93
−4.78
−4.75
−4.74
−5.05
−4.96
−4.94
−4.93
−2.72
−2.82
−2.85
−2.86
−2.44
−2.54
−2.64
−2.64
−2.90
−3.01
−3.06
−3.08
−2.72
−2.86
−2.93
−2.96
2.03
1.77
1.69
1.66
2.33
2.10
2.01
1.97
TPD−DINI
stable at high temperatures; thus, values of M higher than
interactions. However, the different alkyl substituents can in
principle dramatically affect such interactions, which in turn can
influence absorption and the efficiency of free carrier generation
in the copolymer/fullerene blend. As illustrated at the top of
Figure 2, when branching occurs at the 1-position of the
secondary alkyl substituent (as is the case for the 9-
heptadecanyl substituents in P4, P5, and P8), steric interactions
will twist the side chains out of planarity with the polymer
backbone, effectively reducing interchain interactions between
otherwise planar backbone structures. On the other hand, when
the branching occurs in the 2-position (as in the 2-hexyldecyl
and 2-ethylhexyl substituents in P2, P3, P6, and P7), the alkyl
substituent can readily rotate to one side of the backbone or the
other, which may lead to improved solid-state packing.
Optical Properties of Polymers. The normalized
absorption spectra of the polymers displayed in Figure 2
were measured at rt in solutions of chloroform. As a first
observation, polymers with the DPP acceptor are substantially
red-shifted relative to those containing the TPD acceptor unit,
the λmax by as much as 220 nm (compare P3 with P6 in Table
1). This is consistent with theoretically calculated spectra, as
shown in Figure 1. Note that the computed spectra are for gas
phase structures, so any effects of aggregation or other
intermolecular interactions are not taken into account.
n
about 8 kg mol− for P7 and P8 could not be directly achieved.
1
−1
The values of 10 and 15 kg mol reported in Table 1 were
acquired only after purification of the polymer samples with a
preparatory GPC.
Theoretical Calculations. HOMO and LUMO energy
levels were calculated for oligomers n = 1−4 (where n = 1 is a
D−A pair), together with absorption spectra, for copolymers
based on DPP−DINI and TPD−DINI. Most of our discussion
will be confined to the n = 4 case, which is approaching the
polymer limit as evidenced by the convergence of the values in
Table 2. As shown in Table 2, the HOMO values for the TPD
containing oligomer are predicted to be 190 meV deeper than
that containing DPP. Meanwhile, the energy gap for the DPP
oligomer is predicted to be 310 meV more narrow than that of
TPD. Although some small discrepancies between computed
and measured values are observed, both of these results are
qualitatively in good agreement with measured values (see
Figure 1 and compare Tables 1 and 2).
The geometric structures of DPP−DINI and TPD−DINI
tetramers are illustrated in Figure S1 of the Supporting
Information. As seen in this figure, all oligomers are quite
planar, with dihedral angles between the alternating units being
less than 5°. Such planarity should facilitate π−π stacking
1
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Altering the alkyl chain substitution on the indoloindole unit
can dramatically affect the absorption of the polymers. For
DPP-containing polymers with structurally analogous 2-ethyl-
hexyl and 2-hexyldecyl substitution on the DINI unit (P2 and
P3, respectively), values of λ max are similar, as are values of
λ0.1max. Additionally, the spectra for these two polymers each
display a second local maxima at 684 nm. However, the λmax
and λ0.1max of polymer P4, with 9-heptadecanyl substitution, is
blue-shifted by ∼50 nm and only displays one maximum at 696
nm. P5, also with 9-heptadecanyl substitution on DINI but with
a linear n-octyl instead of a branched 2-ethylhexyl chain on the
DPP unit, displays a similar ∼50 nm blue-shift.
The dramatic difference in the solution absorption spectra of
these polymers might be attributed to the relative propensity of
11
the polymers to aggregate in solution. While our gas phase
calculations predict virtually no difference in the absorption
spectra for these different polymers, three-dimensional
modeling (Figure 2) does suggest that steric hindrance in the
9
-heptadecanyl chain of P4 and P5 with hydrogen atoms in the
DINI unit should force the alkyl chains out of plane with the
rigid aromatic backbone. However, when the alkyl chain
branching occurs in the 2-position, as in P2 and P3, the alkyl
chains can more easily rotate to one side of the backbone or the
other, which could ultimately allow better molecular packing in
the solid state and more pronounced aggregation in solution.
Indeed, when solutions of P2 and P3 are diluted by a factor of
1
0 and then 100, the relative intensity of the local maximum
Figure 1. (top) Computed absorption spectra for oligomers of DPP−
DINI (solid lines) and TPD−DINI (dashed lines). Spectra were
broadened artificially as described in the Experimental Section.
around 750 nm as compared to the one at 684 nm
systematically decreases, an observation consistent with
aggregation being the origin of the observed red-shift. The
same trend is observed for the TPD containing polymers,
although the relative shifts are much less pronounced and only
one maximum is observed for all three polymers.
(bottom) Normalized absorption spectra of polymers P2−P8 in
chloroform solution (P6 in o-dichlorobenzene).
Figure 2. Molecular image representing the lowest energy geometry of a DPP−DINI monomer where the alkyl chain branching occurs in the 1-
position (top), as in P4 and P5, and in the 2-position (bottom), as in P2 and P3. The carbon atoms associated with the alkyl chains in the images on
the left are highlighted for clarity.
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Figure 3. Optical band gaps and energy levels determined for DPP−DINI and TPD−DINI polymers from solution measurements of λ
electrochemical estimates of HOMO values.
and
.1max
0
The λmax values of the film absorbance spectra of the
polymers only red-shift ∼10 nm relative to the respective
solution spectra (see Figure S2). Interestingly, polymer P7
develops a shoulder in its solid state absorbance spectrum,
which is incidentally consistent with it ability to more easily
pack than P8.
2-ethylhexyl, 2-hexyldecyl, and 9-heptadecanyl substitution.
While P6 was sufficiently soluble in hot dichlorobenzene to
make a rough film for a voltammogram, no devices were made
for this polymer. However, VOC values in devices of P7 and P8
were measured as 757 and 797 mV, consistent with the relative
HOMO values of the two polymers, as well as the trend
observed for the DPP polymers. Figure 3 illustrates the
experimental energy levels for the seven polymers determined
from electrochemical film measurements of HOMO values
Electrochemical Characterization of Polymers. Cyclic
voltammetry was utilized to evaluate the HOMO energy levels
of polymers P2−P8. A full summary of the data is reported in
Table S1 and Figures S3−S10 and includes peak potentials of
the oxidation and reduction waves, the ratio of reverse to
forward peak current, and E1/2 values. Peak-to-peak separations
varied between 135 and 190 meV, and the ratio of reverse to
forward current varied between 0.16 and 0.77. EHOMO was
combined with optical gaps determined from λ0.1max
.
Time-Resolved Microwave Conductivity Analysis. To
determine the effect of polymer structure on the photophysical
properties of bulk heterojunctions of our polymers with
PC BM, we carried out time-resolved microwave conductivity
61
+
estimated from E1/2 values measured vs Ag/Ag and calibrated
(TRMC) measurements on films deposited onto quartz
substrates under the same conditions as the devices reported
in the Experimental Section. The purpose of these measure-
ments is to evaluate the free carrier generation and decay
dynamics in a blend film and correlate it to the microstructure
of the bulk heterojunction.
+
against the ferrocene/ferrocenium couple (Fc/Fc measured as
+
+
0
.09 V vs Ag/Ag ). The Fc/Fc energy level used in HOMO
34
calculations was assumed to be −4.80 eV. Thus, E
=
HOMO
+
−
(E + 4.71) eV, where E is reported vs Ag/Ag . Polymer
1/2 1/2
films were drop cast onto a platinum electrode from solutions
with uniform concentrations (1 mg/mL), as it is shown in
Figure S10 that higher concentrations (10 mg/mL) could give
erroneous results.
A few interesting observations can be made about these
voltammograms. The measured HOMO values for these DPP
polymers are quite dependent on the alkyl group substitution at
the DINI unit. In polymers P2, P3, and P4, with 2-ethylhexyl,
The comparison between films of P3:PC BM and
6
1
P4:PCB M is shown in Figure 4, where the product of the
61
yield for free carrier generation with the sum of mobilities of
electron and holes (eq 2) is plotted against the photon flux of
the excitation pulse absorbed by the sample (see Experimental
Section for details). The decrease of φ∑μ with increasing
absorbed photon flux has been attributed to non linear
1
6,17
2-hexyldecyl, and 9-heptadecanyl substitution, respectively, the
exciton−carrier interactions at high light intensities.
We
HOMO values are measured as −5.0, −5.1, and −5.2 eV. This
is consistent with the trend observed in the open-circuit
voltages of devices, with values of VOC measured as 562, 605,
and 680 mV for P2, P3, and P4, respectively. When the alkyl
chain on the DPP unit is changed from a branched 2-ethylhexyl
to a linear n-octyl chain (compare results for P4 and P5), no
difference is observed in the measured HOMO values.
use eq 3 (Experimental Section) to fit the intensity dependence
of φ∑μ and extrapolate to the low intensity range where φ∑μ
does not depend on light intensity (the linear response regime).
Figure 4 shows that the low-intensity limit of φ∑μ for the
P3:PCBM sample is an order of magnitude higher than that of
P4:PC BM. We attribute this to aggregation of the polymer
61
chains in the case of P3, as discussed above, that is expected to
The same trend in HOMO values with alkyl group
substitution is seen in TPD polymers P6, P7, and P8, with
HOMO values of −5.1, −5.2, and −5.3 eV, corresponding with
improve delocalization of charge that has been proposed to
35−37
enhance free carrier generation.
On the contrary, the steric
interactions of the side chain and the backbone of P4 cause
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Figure 5. J−V curves of polymer:PC BM solar cell devices.
61
Table 3. Device Characteristics of Photovoltaic Solar Cells
Based on Polymers P2−P8 with PC BM
6
1
Figure 4. The product of the yield for free carrier generation and the
sum of the mobilities of electrons and holes versus absorbed photon
flux for blends of P3:PC BM and P4:PC BM, measured by TRMC.
d
D:A
ratio
VOC
JSC
FF
PCE
2
polymer
solvent
(mV)
(mA/cm )
(%)
(%)
61
61
a
P2
P3
P4
P7
P8
CHCl3
1:1
1:2
1:2
1:2
1:2
568
608
673
757
797
10.5
9.7
47
53
48
44
59
2.8
The solid lines represent fits to the data using eq 3 (see Experimental
b
CB
3.0
Section). The inset shows normalized photoconductance transients at
12
2
CB
6.8
2.1
an absorbed photon flux of 7 × 10 photons/(cm pulse).
c
CB
1.6
0.52
0.98
c
CB
2.2
them to twist out of the plane of the backbone limiting
aggregation and causing the blue-shift of the absorption
spectrum discussed earlier. We propose that this same
structural feature limits the efficiency of free carrier generation
in P4:PC BM, as the polymer chains are pushed further apart
a
b
Chloroform with 1% v/v diiodooctane. Chlorobenzene solution
c
with 10% v/v diiodooctane. Chlorobenzene solutions with 4% v/v
diiodoocatne. Approximate PCE.
d
61
samples. Comparison of TPD- and DPP-based devices
fabricated under comparable processing conditions demon-
strates that devices containing TPD-based copolymers generate
noticeably higher VOC than those based on the DPP acceptor
unit. This observation is consistent with theoretical calculations
that predict the HOMO of the n = 4 oligomer of TPD-DINI to
be 190 meV deeper than that of DPP-DINI. However, despite
their higher VOC, devices of P7 and P8 suffered from reduced
in this case, and delocalization of the hole is limited. The decay
times of the photoconductance transients shown in the inset of
Figure 4 also show a slightly longer decay for P3:PC BM,
61
consistent with previous work on the dependence of photo-
3
5,37
carrier lifetimes on aggregation.
We note, however, that
unlike these previous systems, no ordering is observed in films
of P3 and P4 as no diffraction of X-rays was observed from
these films (data not shown) despite the prediction of a planar
backbone by DFT calculations. Finally, as shown in Figure 4,
the LUMO energies of P3 and P4 are very similar; therefore, a
change in charge generation yield (φ) due to changes in the
driving force for free carrier generation can be ignored.
J , FF, and PCE relative to those of P2−P4. The decreased
SC
performance of P7 and P8 devices might be partially attributed
to the hypsochromic shift of the absorption spectra and
consequently reduced photon harvest.
The TRMC results of Figure 4 provide some insight into the
improved performance of devices of P3:PC61BM. TRMC
probes the primary step in photocurrent generation in an OPV
device, namely the creation (under very low electric field) of
free carriers following photoexitation of the bulk hetero-
junction. While we cannot exclude differences in ∑μ of the two
samples, the similarity of the two systems (same conjugated
system, same PC BM loading) indicate comparable contribu-
Devices of DPP-based polymers, P2 and P3, which contain a
primary alkyl substituent on the DINI unit, produced
noticeably higher PCE and JSC than that of P4. The decreased
performance in devices of P4 might be attributed to the steric
interactions between the secondary alkyl chain on the nitrogen
atom and the aromatic backbone, as discussed earlier.
CONCLUSIONS
6
1
■
tion of the electron mobility to ∑μ. We therefore propose that
In summary, we have designed and synthesized a series of
conjugated copolymers comprised of a new electron-rich DINI
comonomer moiety and electron-deficeint DPP or TPD
moieties, from which BHJ solar cells could be fabricated with
relatively efficient preliminary device performance. A significant
effect of the size and shape of the pendant alkyl substituents
attached to the DINI unit was observed on the optical and
electronic properties of the copolymers. Theoretical calcu-
lations suggested that branching from the 2-position of the alkyl
substituent on DINI (as opposed to the 1-position) should
allow for better interchain interactions. These predictions were
consistent with the red-shifted absorbance, the higher peak
photoconductance, slower photoconductance decay (as deter-
mined by time-resolved microwave conductivity), and better
free carrier generation in P3:PC BM is more efficient due to
6
1
the proximity of polymer chains in this system, giving the
higher J observed in OPV devices (see below).
sc
Photovoltaic Characteristics. To investigate and compare
photovoltaic properties of the new absorbers, BHJ devices with
a configuration of ITO/PEDOT:PSS/polymer:PC BM/Ca/Al
6
1
were fabricated. Figure 5 shows J−V curves of the devices
2
under illumination with simulated AM1.5 at 100 mW/cm . The
photovoltaic parameters are summarized in Table 3. The active
layer blends for all copolymers investigated were spin-coated
from either chloroform or chlorobenzene (CB) solutions
depending on solubility. The weight ratios of the polymer/
PC BM in the active layer were also optimized for all polymer
61
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Macromolecules
Article
overall device performance observed for polymers with 2-
hexyldecyl substituents. The results provide future guidance for
the appropriate selection of polymer side chains in OPV device
fabrication.
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P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
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ASSOCIATED CONTENT
■
*
S
Supporting Information
Cyclic voltammograms for all polymers, absorption spectra of
polymers, geometric structures of some polymers, and H
1
NMR spectra for comonomers and copolymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
E-mail: Zbyslaw.Owczarczyk@nrel.gov.
*
Notes
(19) http://www.jmol.org/.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
K.; Suh, H. Bull. Korean Chem. Soc. 2006, 27 (7), 1043−1047.
This work was supported by the U.S. Department of Energy
under Contract DE-AC36-08-GO28308 with the National
Renewable Energy Laboratory through the DOE SETP
program.
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