Design of (X-DADAD)n type copolymers for efficient bulk heterojunction organic solar cells

Article abstract of DOI:10.1021/ma5023956

We show that extended TBTBT structure (T = thiophene, B = benzothiadiazole) can be used as an electron-deficient building block for designing conjugated polymers with deeply lying HOMO energy levels and narrow band gaps. The first carbazole-TBTBT copolymer P2 demonstrated power conversion efficiencies exceeding 6% in bulk heterojunction solar cells in combination with advanced operational stability, unlike conventional donor polymers such as PTB7, PBDTTT-CF, etc.

Full text of DOI:10.1021/ma5023956

Article
pubs.acs.org/Macromolecules
Design of (X-DADAD)n Type Copolymers for Efficient Bulk
Heterojunction Organic Solar Cells
Alexander V. Akkuratov,† Diana K. Susarova,† Oleg V. Kozlov,‡,∥ Alexander V. Chernyak,†
Yuriy L. Moskvin,§ Lubov A. Frolova,† Maxim S. Pshenichnikov,‡ and Pavel A. Troshin*,†
†Institute for Problems of Chemical Physics, Russian Academy of Sciences, Semenov Prospect 1, Chernogolovka 142432, Russian
Federation
‡Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
§Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences (Branch), Semenov Prospect 1/10, Chernogolovka,
Moscow Region 142432, Russian Federation
∥Faculty of Physics & International Laser Center, Lomonosov Moscow State University, Leninskie Gory 1, 119991, Moscow, Russia
S
* Supporting Information
ABSTRACT: We show that extended TBTBT structure (T = thiophene, B = benzothiadiazole) can be used as an electron-
deficient building block for designing conjugated polymers with deeply lying HOMO energy levels and narrow band gaps. The
first carbazole−TBTBT copolymer P2 demonstrated power conversion efficiencies exceeding 6% in bulk heterojunction solar
cells in combination with advanced operational stability, unlike conventional donor polymers such as PTB7, PBDTTT-CF, etc.
which is substantially longer as compared to the P3HT/PCBM-
based devices.14 Nonetheless, optoelectronic properties of
PCDTBT are not optimized with respect to PCBM used as
electron acceptor counterpart because of a 0.55 eV LUMO−
LUMO offset (Figure 1a). As a consequence, efficiency of
organic solar cells based on this material combination can
INTRODUCTION
■
The efficiency of organic fullerene/polymer bulk heterojunc-
tion solar cells has been increased dramatically during the past
few years mainly due to design and implementation of novel
electron donor materials with advanced optoelectronic proper-
ties.1 The vast majority of promising conjugated polymers were
built using so-called “push−pull” architecture by combining
different electron-deficient heterocycles such as quinoxaline,2
benzothiadiazole,3 1,4-diketopyrrolopyrrole,4 thieno[3,4-b]-
thiophene,5 thieno[3,4-c]pyrrole-4,6-dione,6 and isoindigo7
with electron-rich thiophene units or thiophene-based hetero-
cycles such as cyclopentadithiophene,8 dithienobenzene,9 or
dithienosilole10 in alternating copolymer structure. The recent
progress in the field is reviewed in a number of publications.11
As long as the efficiency of organic solar cells was substantially
improved, there appeared major concerns regarding their
stability.12
Figure 1. Frontier energy levels of PCDTBT/PCBM (a) and P2/
PCBM (b) binary systems.
Among hundreds of investigated polymers, the carbazole−
TBT copolymer PCDTBT13 has demonstrated an outstanding
stability. In particular, the lifetime of PCBM/PCDTBT solar
cells was estimated to be in the range of at least 6−8 years,
Received: November 26, 2014
Revised: March 9, 2015
© XXXX American Chemical Society
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Figure 2. Schematic illustration of the approach to design of novel conjugated polymers pursued in this work.
hardly exceed 7%.15 The reported reproducible experimental
solar cell efficiencies for PCDTBT/[70]PCBM blends fall in
the range of 5.6−6.6%.16 The energy diagram for the
PCDTBT−PCBM system (Figure 1) suggests that optoelec-
tronic properties of the polymer can be improved by lowering
its LUMO energy without affecting significantly the HOMO
position.
A number of previous attempts to modify the chemical
structure of PCDTBT in order to lower its LUMO energy and/
or reduce its band gap were unsuccessful and resulted in
inferior solar cell performances (see examples in Figure S1 and
Table S1, Supporting Information).
or toluene as eluents (flow rate 0.5 mL/min). The column was
calibrated using a series of commercial polystyrene standards obtained
from Fluka (THF as eluent) or using custom-made F8BT standards
with PDI < 1.5 (toluene used as eluent).
Synthesis of 1. Compound 1 was prepared from 4,7-dibromo-
2,1,3-benzothiadiazole (1.0 g, 3.40 mmol), 2-(tributylstannyl)-
thiophene (1.28 g, 3.43 mmol), and tetrakis(triphenylphosphine)-
palladium(0) (0.026 g, 0.022 mmol) using a procedure reported
previously.20
Synthesis of 2 and 3. Compound 2 was synthesized using 1 (2.97
g, 10 mmol), 2,5-bis(tributylstannyl)thiophene (3.31 g, 5 mmol), and
Pd(PPh3)4 (0.023 g, 0.02 mmol). Compound 3 was obtained by
bromination of 2 with NBS (N-bromosuccinimide) in warm 1,2-
dichlorobenzene (55 °C for 60 h). Detailed procedures and spectral
characteristics were reported previously.21
The approach pursued in this work is outlined schematically
in Figure 2.
Synthesis of 5. Compound 3 (1.0 g, 1.5 mmol) and (3-(2-
ethylhexyl)thiophen-2-yl)boronic acid (1.2 g, 5.0 mmol) were placed
under argon in a two-necked flask equipped with a reflux condenser.
Afterward, toluene (50 mL), 2 M aqueous solution of K2CO3 (2 mL),
Aliquat 336 (1 drop, ca. 80 mg), and tetrakis(triphenylphosphine)-
palladium(0) (10 mg) were added in the listed here sequence. The
obtained reaction mixture was stirred vigorously at 90−100 °C. The
course of the reaction was monitored with HPLC. The synthesis was
terminated when starting compound 3 disappeared and the amount of
monofunctionalization product was below 5%. The mixture was
poured into water and extracted with chloroform. Organic layer was
then dried over anhydrous MgSO4. Finally, the solvent was removed at
the rotary evaporator, producing a viscous oily residue. The crude
product was dissolved in 40 mL of toluene and filtered through a
syringe filter (PTFE, 0.45 μm). The solution was processed further
using a preparative Shodex GPC column (20 mm × 300 mm) and
toluene as eluent which resulted in isolation of pure precursor
compound. The latter was brominated using stoichiometric amount of
NBS in 1,2-dichlorobenzene at RT to afford the title compound 5 in
60−67% overall yield.
The PCDTBT repeating unit comprises in its molecular
framework the electron-deficient benzothiadiazole unit (accept-
or “A”), two adjacent electron donor thiophene rings (donor
“D”), and carbazole unit “X” which is a considerably weaker
electron donor compared to the thiophene.17 Therefore,
PCDTBT represents an example of the (-X-DAD)n family of
conjugated polymers. We proposed to introduce additional
alternating A and D elements in the polymer repeating unit
thus opening a pathway toward (-X-DADAD)n copolymers
which are expected to have advanced optoelectronic properties
compared to conventional (-X-DAD)n structures, particularly,
lower LUMO energies and narrower band gaps.
It should be noted that alternated donor−acceptor systems
are extensively utilized in the design of small molecular electron
donor materials for organic solar cells. A number of high
efficiency DADAD, DADADAD, and even DADADADAD
systems have been explored recently.18 Surprisingly, a potential
of this highly promising approach was not utilized in the
development of electron donor polymers for bulk hetero-
junction solar cells. In the present work we report the
application of this concept for designing a novel carbazole−
thiophene−benzothiadiazole copolymer, demonstrating high
photovoltaic performance in combination with an improved
stability.
5: 1H NMR (CDCl3, 600 MHz): δ (ppm) 8.07 (s, 2H), 7.96 (d, J =
3.8 Hz, 2H), 7.81 (d, J = 7.7 Hz, 2H), 7.72 (d, J = 7.7 Hz, 2H), 7.06
(d, J = 3.8 Hz, 2H), 6.85 (s, 2H); 2.68 (d, J = 7.2 Hz, 4H), 1.63 (m,
2H), 1.25−1.34 (m, 16H), 0.85 (m, 12H). 13C NMR (CDCl3, 126
MHz): δ (ppm) 152.44, 152.39, 140.47, 139.91, 139.35, 133.33,
132.65, 130.35, 128.44, 127.87, 127.73, 127.21, 125.52, 125.42, 125.11,
110.76, 40.34, 33.64, 32.69, 28.80, 25.84, 23.13, 14.18, 10.84.
Synthesis of P1. Monomers 3 (195 mg, 0.297 mmol) and 4 (200
mg, 0.297 mmol) were introduced into a 50 mL round-bottom three-
necked flask equipped with a thermometer and a reflux condenser.
Toluene (35 mL), 2 M aqueous solution of K2CO3 (0.6 mL), Aliquat
336 (1 drop, ca. 80 mg), and tetrakis(triphenylphosphine)-
palladium(0) (10 mg) were added in the listed here sequence. The
reaction mixture was degassed, immersed into an oil bath, and heated
at reflux. The molecular weight characteristics of the formed product
were controlled every 30 min. The reaction was stopped when the
polymer started to precipitate from the reaction mixture. The 4,4,5,5-
tetramethyl-2-phenyl-1,3,2-dioxaborolane (4.0 mg, 0.019 mmol) was
added, and the reaction mixture was heated for an additional 25 min.
Afterward, an excess of bromobenzene (300 mg, 1.9 mmol) was
introduced, and the mixture was stirred at reflux for another 25 min.
Then the mixture was cooled down to room temperature, and 50 mL
EXPERIMENTAL SECTION
■
General. All solvents and reagents were purchased from Sigma-
Aldrich or Acros Organics and used as received or purified according
to standard procedures. [70]PCBM and PCDTBT (Mw ∼ 200 000 g/
mol, PDI = 5−6) were synthesized and purified using previously
reported procedures.19,15 PTB-7, PBDTTT-CF, and PDTSTPD were
products of 1-Material Inc. (Quebec, Canada). Poly(3-hexylthio-
phene) of EE grade was purchased from Rieke Metals Co. AFM
images were obtained using NTEGRA PRIMA instrument (NT-MDT,
Russia).
Molecular weight characteristics of conjugated polymers were
obtained using Shimadzu LC20 instrument equipped with a
Phenomenex Luna Phenogel 5 μm column (0.78 × 30 cm, 5−500
kDa). The measurements were performed using freshly distilled THF
B
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of toluene was added. The resulting organic layer containing finely
dispersed precipitate of P1 was washed three times with deionized
water (50 mL) and poured to 60 mL of 2-propanol. The precipitated
polymer flakes were filtered into the cellulose thimble and processed
using Soxhlet extraction with hexanes (12 h), acetone (12 h),
dichloromethane (12 h), chloroform (8 h), chlorobenzene (12 h), and
1,2-dichlorobenzene (12 h). A considerable part of the polymer
remained undissolved in the thimble. The 1,2-dichlorobenzene extract
was concentrated in vacuum to 45 mL and poured in methanol (90
mL). The obtained solid was collected by filtration and dried under
vacuum. The polymer P1 was obtained as dark green, almost black,
flakes with a yield of 106 mg (40%). Mw = 29 100 g/mol, Mw/Mn =
3.3. UV−vis: λmax = 369, 407, and 582 nm; λedge = 726 nm.
Synthesis of P2. Monomers 4 (657.6 mg, 1.0 mmol) and 5 (1063
mg, 1.0 mmol) were introduced into a 50 mL round-bottom three-
necked flask equipped with a thermometer and a reflux condenser.
Toluene (25 mL), 2 M aqueous solution of K2CO3 (2 mL), Aliquat
336 (1 drop, ca. 80 mg), and tetrakis(triphenylphosphine)-
palladium(0) (10 mg) were added in the listed here sequence. The
reaction mixture was degassed, immersed into an oil bath, and heated
at reflux for 3−6 h. The molecular weight characteristics of the formed
product were controlled every 30 min. The reaction was intentionally
terminated when the weight-average molecular weight Mw reached ca.
150 000 g/mol. The 4,4,5,5-tetramethyl-2-phenyl-1,3,2-dioxaborolane
(4.0 mg, 0.019 mmol) was added, and the reaction mixture was heated
for additional 25 min. Afterward, an excess of bromobenzene (300 mg,
1.9 mmol) was introduced, and the mixture was stirred at reflux for
another 25 min. Then the mixture was cooled down to room
temperature, and the polymer was extracted with 500 mL of toluene;
the resulting solution was washed three times with deionized water
(250 mL), dried, and concentrated under vacuum (rotary evaporator)
to 40 mL. Addition of 150 mL of methanol precipitated the crude
polymer. Subsequent purification was achieved using several additional
dissolving/precipitation cycles. Finally, the precipitated polymer flakes
were filtered into the cellulose thimble and processed using Soxhlet
extraction with hexanes (12 h), acetone (12 h), dichloromethane (12
h), chloroform (8 h), and chlorobenzene (12 h). A very minor amount
of the polymer remained undissolved as a residue in the thimble. The
chlorobenzene extract was concentrated under vacuum to the volume
of ca. 20 mL, diluted with 20 mL of 1,2-dichlorobenzene, and
precipitated in methanol. The obtained dark green (almost black) solid
was collected by filtration and dried under vacuum. The total yield of
the purified polymer P2 varied between 70 and 85% depending on the
applied number of dissolving/precipitation cycles.
atmosphere at 90 °C for 10 min. The top electrode comprising Ca (20
nm) and Ag (100 nm) was deposited by thermal evaporation at the
pressure below 4 × 10−6 mbar in a vacuum chamber integrated inside a
MBraum glovebox.
The current−voltage (I−V) characteristics of the devices were
obtained in dark and under the simulated 100 mW/cm2 AM1.5 solar
irradiation provided by a KHS Steuernagel solar simulator integrated
in MBraun glovebox. The intensity of the illumination was checked
every time before the measurements using a calibrated silicon diode
with a known spectral response. The I−V curves were recorded in
inert atmosphere using Keithley 2400 source-measurement unit. The
external quantum efficiency spectra (EQE) were measured in normal
air atmosphere without applying any special encapsulation or
protection to the photovoltaic devices using a specially designed
setup (LOMO instruments, Russia).
Ultrafast PIA Spectroscopy. PIA spectroscopy was performed at
a visible-pump, IR-probe setup based on the Spectra-Physics
Hurricane system (∼120 fs, 800 nm, 1 kHz repetition rate) and two
optical parametrical amplifiers (Light Conversion TOPAS) operating
in the visible (400−800 nm) and IR (1.2−2.5 μm) regions (for more
details, refer to the Supporting Information). The wavelengths of the
excitation pulses were chosen near blend absorption maxima (at 560
and 630 nm for PCDTBT and P2, respectively). The wavelength of
the probe pulse was set near the maximum of the high-frequency
polaron peak at 1.2 μm (see Figure S8, Supporting Information). The
PIA transients were recorded with parallel or perpendicular polar-
izations with respect to the excitation, from which the isotropic
(population) PIA signals were calculated. Films of blends of P2 and
PCDTBT (Sigma-Aldrich) polymers with [70]PCBM (Solenne BV)
were spin-coated (1000 rpm, 2 min) from respective solutions on
microscope cover-glass substrates.
RESULTS AND DISCUSSION
■
Synthesis and Investigation of PCDTBTBT (P1).
Considering the concept discussed above and outlined in
Figure 2, a novel polymer PCDTBTBT (P1) can be considered
as a promising target. The synthesis of TBTBT building block
was performed following the previously reported procedures
applied for preparation of similar compounds.23 Suzuki-type
cross-coupling between the 2-thiopheneboronic acid and 4,7-
dibromo-2,1,3-benzothiadiazole produced 1 with a reasonable
yield (Scheme 1).
Cyclic Voltammetry Measurements. The cyclic voltammetry
measurements were performed for thin films (150−250 nm thick) of
polymers P1, P2, and PCDTBT deposited on a glassy carbon disc
electrode (working electrode, d = 5 mm, BAS Inc.) by drop-casting
from a 1-chloronaphthalene−1,2-dichlorobenzene (1:1 v/v) mixture in
the case of P1 and a chlorobenzene−chloroform (1:4 v/v) mixture in
the case of P2. The measurements were performed in a three-electrode
electrochemical cell using 0.1 M solution of Bu4NPF6 in acetonitrile as
supporting electrolyte, platinum wire as a counter electrode, and a
silver wire immersed in 0.01 M solution of AgNO3 in 0.1 M TBAP
(CH3CN) as a reference Ag/Ag+ electrode (BAS Inc.). Ferrocene was
used as internal reference. The electrolyte solution was purged with
argon before the measurements. The voltammograms were recorded
using an ELINS P-30SM instrument at room temperature with a
potential sweep rate of 50 mV/s.
Stille cross-coupling between 1 and 2,5-bis(tributylstannyl)-
thiophene afforded the target product 2. Dibromination of
Scheme 1. Synthesis of P1
Fabrication and Characterization of Photovoltaic Devices.
The conjugated polymer P2 (7 mg) and [70]PCBM (14 mg) were
dissolved together in 1 mL of 1,2-dichlorobenzene while stirring at
room temperature for 48 h. 1,8-Diiodooctane (DIO) was added to the
blend solution to achieve 0.63% volume concentration. The prepared
solution was filtered through the PTFE 0.45 μm syringe filter and
subjected to spin-coating at 900−1100 rpm for 150 s on the top of the
annealed PEDOT:PSS (Clevios HTL) films deposited on the
patterned ITO electrodes (see ref 22 for a general description of the
substrate preparation procedure). The obtained films were transferred
immediately inside glovebox and thermally annealed in an argon
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TBTBT in 1,2-dichlorobenzene led to the key monomer 3.
Suzuki−Miyaura polycondensation of 3 with carbazole-based
boronic component 4 produced the title polymer P1. The
optoelectronic characteristics of P1 are summarized in Table 1.
did not allow us to obtain thin films of reasonable quality
required for fabrication of organic solar cells.
Synthesis and Optoelectronic Properties of
PCDTTBTBTT (P2). In order to overcome the problem of
solubility, we have synthesized the conjugated polymer P2
bearing additional thiophene rings with solubilizing 2-ethyl-
hexyl substituents (Scheme 2). The key monomer 5 can be
synthesized from 3 using Suzuki or Stille cross-coupling
reactions with corresponding 3-alkylthiophene-based boronic
ester (acid) or stannane followed by dibromination of the
product using NBS.
The target polymer P2 obtained in high molecular weights
(Mw = 201 000, Mn = 49 700 g/mol, Figure S2) demonstrated
reasonably good solubility in organic solvents, which allowed us
to perform detailed investigation of optoelectronic and
photovoltaic properties of this material. Figure 3 shows that
P2 has significantly reduced band gap compared to PCDTBT.
At the same time, both polymers demonstrate very similar
oxidation potentials which imply that they have similar HOMO
energies (Table 1).24 Consequently, the narrow band gap of
1.65 eV originates mainly from the lower LUMO energy of P2
as compared to PCDTBT (Figure 1b). Thus, the proposed
chemical design based on the use of TBTBT building block
provided the material with desired electronic properties as it
follows from the energy diagrams for PCDTBT/PCBM and
P2/PCBM systems shown in Figure 1.
Table 1. Optoelectronic Properties of Polymers P1, P2, and
PCDTBT
a
polymer
P1
λmax (sol), nm HOMO, eV LUMO, eV Eg(opt), eV
584
610
553
−5.53
−5.44
−5.50
−3.83
−3.79
−3.60
1.70
1.65
1.90
P2
PCDTBT
a
HOMO energies were estimated from onsets of the oxidation
potentials using Fermi energy of −5.1 eV for the Fc+/Fc redox couple.
The absorption spectrum and cyclic voltammogram of P1 are
shown in Figure 3. It is seen from these data that P1 has
Comparing the properties of P1 and P2, one can also notice
that introduction of two additional alkylthiophene rings
increases the LUMO and HOMO energies of the polymer
and reduces its band gap by 0.05 eV. These changes are
expectable since bithiophene units in P2 induce stronger
electron donation effect on the neighboring benzothiadiazole
moieties as compared to the thiophenes in P1.
The polymer P2 also demonstrated reasonably good charge
transport properties. The SCLC mobility determined for P2 in
hole-only devices was ∼2.8 × 10−4 cm2 V−1 s−1, which is
comparable to the mobility estimated for PCDTBT (∼1.2 ×
10−4 cm2 V−1 s−1) and is somewhat lower than the mobility
determined for crystalline films of regioregular P3HT (5.9 ×
10−4 cm2 V−1 s−1, see Figure S3, Supporting Information).
Photovoltaic Properties of P2. Photovoltaic properties of
P2 were investigated in conventional bulk heterojunction solar
cells which had the following architecture: glass/ITO/
PEDOT:PSS/P2-PCBM blend/Ca/Ag. The P2/PCBM blends
were processed from pure 1,2-dichlorobenzene (DCB) and
from binary solvents based on DCB−1,8-diiodooactane (DIO)
mixtures. Volume concentration of DIO was varied from 0.12
to 6.0%, P2:PCBM ratios from 1:1 to 1:4, film thickness from
40 to 200 nm, and different annealing regimes were applied.
Such optimization revealed optimal P2:PCBM blend compo-
sition (1:2 w/w), DIO concentration (0.63%), film thickness
(70−80 nm), annealing temperature (90 °C), and time (10
min). The obtained results are given in Table 2.
The organic solar cells based on the P2/[70]PCBM blends
yielded highly reproducible efficiencies of 5.9−6.1% with the
power conversion efficiency of the best device as high as 6.4%
(Figure 4 and Figure S4 in Supporting Information).
The revealed optical and electronic properties of the P2/
[70]PCBM system strongly suggest that it has much higher
theoretical potential than the reference PCDTBT/[70]PCBM
blends. According to the model introduced by Scharber et al.,15
it should be possible to reach 9−10% in optimized solar cells
based on P2/[70]PCBM composites (Figure 5). It is very likely
Figure 3. Cyclic voltammograms (a) and absorption spectra (b) of
thin films of P1, P2, and PCDTBT.
significantly lower LUMO energy and narrower band gap
compared to PCDTBT. This finding confirmed our initial
hypothesis that replacement of TBT unit with extended
TBTBT block should improve optoelectronic properties of
the polymer.
Indeed, the solar cells based on the P1/PCBM blends could
potentially deliver higher open circuit voltages (due to lower
HOMO energy of P1), higher current densities (because of the
narrower band gap of P1), and higher power conversion
efficiencies compared to the PCDTBT/PCBM system.
Unfortunately, polymer P1 showed very low solubility in
organic solvents due to the presence of a large planar TBTBT
unit in its molecular framework. Therefore, low solubility of P1
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Scheme 2. Synthesis of P2
a
Table 2. Photovoltaic Properties of P2 and PCDTBT Blended with PCBM
system
additive
no
JSC, mA/cm2
VOC, mV
FF, %
54
η, %
5.3
P2/[70]PCBM (1:2)
13.3
730
b
P2/[70]PCBM (1:2)
0.63% DIO
no
13.3 (13.6 )
757 (775)
58 (62)
60
6.0 (6.4)
5.9
PCDTBT/[70]PCBM (1:3)
12.1
815
a
b
The presented values are statistically reproducible results obtained from at least 25 devices. Parameters of the best device are given in parentheses.
Figure 5. Theoretical potential of solar cells based on PCDTBT/
[70]PCBM and P2/[70]PCBM blends according to ref 15b.
Reproduced with permission from ref 15b. Copyright 2009 Royal
Society of Chemistry.
Processing these films from 1,2-dichlorobenzene comprising
0.63% (v/v) of DIO as additive led to a considerably improved
morphology. The size of the grains in these films was about
15−30 nm, which is considered to be close to the optimal
morphology required for efficient operation of fullerene/
polymer solar cells. The observed evolution of the film surface
structure induced by DIO additive correlates also with the
improvement in the solar cell performance (Table 2).
Figure 4. J−V (a) and IPCE (b) characteristics of the best solar cells
based on P2/[70]PCBM blend.
It is also notable that thermal annealing of the P2/
[70]PCBM composites led to decrease in the film roughness
and average grain size. This observation suggests good
compatibility between [70]PCBM and polymer P2, which
seem to undergo better intermixing at elevated temperatures.
Ultrafast Photoinduced Spectroscopy for P2/
[70]PCBM Blends. To evaluate loss channels of initial charge
generation, early time exciton and charge dynamics were
studied by ultrafast visible-pump−IR-probe photoinduced
that further improvement in the solar cell efficiency can be
achieved via thorough optimization of the material itself
(polymer P2), composite morphology, and the device
architecture similarly to the case of PCDTBT.15,18
Atomic Force Microscopy for Composites of P2 with
[70]PCBM. The AFM images (Figure 6) of the P2/[70]PCBM
composite films cast from pure 1,2-dichlorobenzene revealed
poorly organized (almost featureless) structure (Figure 6).
E
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Figure 6. AFM height images for the P2/[70]PCBM blends processed without (a, c, e) and with 0.63% of DIO additive (b, d, f). As-cast films (a, b)
were annealed at 90 °C for 10 min (c, d) and then additionally heated at 150 °C for 5 min (e, f).
absorption (PIA) spectroscopy.25 Briefly, the blends were
excited by the visible pump pulse close to the absorption
maxima (560 and 630 nm for PCDTBT and P2, respectively),
and IR (1200 nm) probe pulse was used to monitor the
dynamics of photogenerated species (for details, see Supporting
Information). Figure 7 shows transient PIA signals for the
blends based on P2 and PCDTBT as a reference, with
[70]PCBM.
Since polaron and exciton absorption spectra overlap at the
probe wavelength (Figure S8, Supporting Information), the
PIA dynamics consist of both polaronic and excitonic
responses. However, the two responses are readily separated
by their time signatures. A fast buildup of the signal near the
zero pump−probe delay is due to ultrafast (∼100 fs)
photogeneration of excitons in the polymer phase. Short-time
decay at ∼0.2 ps scale is assigned to absorption of polymer
excitons and their subsequent dissociation onto separated
charges due to electron transfer to [70]PCBM.26 The
contribution of the transient exciton absorption decreases
with the increase of [70]PCBM concentration because of
decreased share of photons absorbed by the polymer (Figure
S12, Supporting Information). All these prove that the reduced
to ∼0.35 eV LUMO−LUMO offset (Figure 1) does not
present any obstacle for efficient dissociation of the polymer
excitons.
At longer time scales, the PIA signals increase with
characteristic times of tens of picoseconds. This contribution
increases with the increase of [70]PCBM concentration.
Therefore, these dynamics are assigned to dissociation of
[70]PCBM excitons delayed by exciton diffusion in [70]PCBM
domains, via the hole transfer process.27 This assignment was
verified by substituting [70]PCBM with [60]PCBM which
lower absorption led to a substantially diminished amplitude of
the signal growth (Figure S10 in Supporting Information).
Hence, at short delays (∼1−5 ps) the PIA signals are
governed by the charges produced at polymer/[70]PCBM
interfaces. They mainly originate from the polymer excitons but
also with a small contribution from the [70]PCBM excitons
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PCDTBT.14 On the contrary, such reference polymers as
P3HT, PTB7, and MDMO-PPV underwent substantial
degradation under the specified conditions (change in the
optical density of the films by more than 10% was observed).
The operation stability of organic solar cells was investigated
under very similar conditions (metal halide lamps, temperature
80−85 °C). The device architecture was ITO/PEDOT:PSS-
(Clevious PH)/[60]PCBM-polymer blend/Mg/Al. It is seen
from Figure 8 that solar cells based on P2 and PCDTBT
Figure 7. Representative PIA transients for P2/[70]PCBM (a) and
PCDTBT/[70]PCBM (b) BHJ blends. Symbols show the exper-
imental data points while the solid lines show best fits (see Supporting
Information). All transients are normalized by the number of absorbed
photons. For the transients with other [70]PCBM concentrations,
refer to Figure S9 of the Supporting Information.
that appear to be generated next to the interface. At long delays
(>10 ps) the charges are generated from the diffusion-delayed
[70]PCBM excitons. Importantly, not all of these excitons will
be able to reach the interface due to their finite lifetime of ∼600
ps.28 Whether the [70]PCBM exciton dies inside the
[70]PCBM domain or is harvested at the interface depends
on the [70]PCBM domain size, i.e., the bulk heterojunction
morphology.
For both polymers, the PIA signal at shot times (for instance,
2 ps) decreases with increasing of [70]PCBM content due to
decreased share of polymer absorption (Figure S12, Supporting
Information). However, for the PCDTBT-based blends, this
decrease is compensated at longer times by efficient harvesting
of the [70]PCBM excitons which share in the total blend
absorption increases. This indicates close-to-optimal phase
separation of the polymer and [70]PCBM constituencies, with
[70]PCBM domain sizes less than the exciton diffusion length
of ∼10 nm.29 In contrast, for the P2-based blends the amount
of separated charges for the 1:3 blend is lower as compared to
the 1:1 blend for all delays. This points at the existence of large-
sized [70]PCBM clusters from which the [70]PCBM excitons
cannot dissociate, thereby suggesting possibilities for further
morphology optimization. This corroborates the results
obtained from AFM measurements (Figure 6).
Operational Stability of P2 in Devices. It was mentioned
above that photostability of conjugated polymers and long-term
operation stability of solar cells based on their blends with
fullerene derivatives are primary important issues which have to
be considered in the design of novel materials for organic
photovoltaics. We investigated photostability of thin polymer
films under continuous illumination with the light coming from
luminescent lamps inside argon glovebox (light power ∼60
mW/cm2; temperature of 75−85 °C; O2 and H2O concen-
trations below 1 ppm). Polymers P2 and PCDTBT did not
show any noticeable photobleaching within 6000 h. This result
agrees well with the previous reports on photostability of
Figure 8. Operation stability of organic solar cells based on the blends
of different polymers with [60]PCBM (a) and molecular structures of
the reference polymers used in the study (b).
demonstrate much superior stability compared to the similar
devices comprising reference polymer materials such as PTB7,
PBDTTT-CF, and PDTSTPD.
The analysis and discussion of the degradation processes
occurring in these devices lie outside the scope of the present
work and will be reported elsewhere. The obtained results
strongly suggest that combinations of carbazole, thiophene, and
benzothiadiazole units in P2 and PCDTBT are responsible for
advanced stability of these materials. From this perspective,
polymer P2 reported herein is considered as a promising
platform for construction of organic solar cells with improved
long-term stability, particularly in inverted device architectures.
CONCLUSION
■
In conclusion, we have shown that alternating copolymers of
novel (-X-DADAD)n family demonstrate superior optical and
electronic properties compared to the conventional (-X-DAD)n
G
DOI: 10.1021/ma5023956
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this family (P2) comprising TBTBT as a key building block
routinely demonstrated power conversion efficiencies exceeding
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ASSOCIATED CONTENT
* Supporting Information
■
S
GPC profile and NMR spectra for P2; SCLC data for P3HT,
PCDTBT, and P2; detailed description of ultrafast PIA
spectroscopy. This material is available free of charge via the
Internet at http://pubs.acs.org.
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J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009,
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Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133,
8416−8419. (c) Fang, G. J.; Liu, Y.; Fu, B.; Meng, B.; Zhang, Z.; Xie;
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AUTHOR INFORMATION
Corresponding Author
*E-mail troshin2003@inbox.ru; Tel +7-496-522-1418 (P.A.T.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(17) Banerji, N.; Gagnon, E.; Morgantini, P.-Y.; Valouch, S.; A.
Mohebbi, R.; Seo, J.-H.; Leclerc, M.; Heeger, A. J. J. Phys. Chem. C
2012, 116, 11456.
We thank Mrs. E. Levchenkova and Mrs. Z. Dzhivanova for
participation in the solar cell stability studies. Dr. D. V. Novikov
is acknowledged for support with the electrochemical measure-
ments. The major part of this work was funded by Lanxess
Germany. Investigation of some properties of P1 and P2 was
also supported by Russian President Science Foundation (MK-
5260.2014.3) and Russian Foundation for Basic Research (14-
03-31681-mol-a). O.V.K. acknowledges “Aurora - Towards
Modern and Innovative Higher Education” programme and
Russian Foundation for Basic Research, research project No.
14-02-31632, for financial support.
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