A selenophene analogue of PCDTBT: Selective fine-tuning of LUMO to lower of the bandgap for efficient polymer solar cells

Article abstract of DOI:10.1021/ma302133h

In an attempt to further improve the performance of the PCDTBT-based polymer solar cells (PSCs), we have synthesized a selenophene analogue of PCDTBT, namely, PCDSeBT, in which diselenienylbenzothiadiazole (DSeBT) monomer alternately flanks with a 2,7-carbazole unit. The intrinsic properties of PCDSeBT are not only characterized by UV-vis absorption, cyclic voltammetry (CV), and organic field-effect transistors (OFETs) but also the surface morphology, mobilities of space charge-limited current (SCLC) model, and polymer solar cells (PSCs) in its bulk-heterojunction (BHJ) active layer with [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) are evaluated in detail. It is found that PCDSeBT simultaneously has a low-lying highest occupied molecular orbital (HOMO) energy level at -5.4 eV and a low bandgap of 1.70 eV as required by the ideal polymers for BHJ PSCs. The high current of 11.7 mA/cm² is obtained for PCDSeBT-based PSCs, to our knowledge, which is among the highest short-circuit current density (J _SC) values obtained from a BHJ device consisting of PCDTBT derivatives and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM). The high J_SC value, along with a moderate fill factor (FF) of 45% and a high open-circuit voltage (V_OC) of 0.79 V, yields a power conversion efficiency (PCE) of 4.12%, which is about 37% increase in PCE from a PCDTBT-based reference device. On the basis of our results, one can be concluded that the DSeBT placement for construction of donor (D)-acceptor (A) polymers is an easy and effective way to realize both the higher J_SC and V _OC values in PSCs, as a consequence of the selective lower-lying lowest unoccupied molecular orbital (LUMO) with the HOMO being almost unchanged, together with the effective broadening on the absorption band.

Full text of DOI:10.1021/ma302133h

Article
pubs.acs.org/Macromolecules
A Selenophene Analogue of PCDTBT: Selective Fine-Tuning of LUMO
to Lower of the Bandgap for Efficient Polymer Solar Cells
Boram Kim,§ Hye Rim Yeom,§ Myoung Hee Yun, Jin Young Kim,* and Changduk Yang*
Interdisciplinary School of Green Energy and KIER-UNIST Advanced Center for Energy, Low Dimensional Carbon Materials
Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea
S
* Supporting Information
ABSTRACT: In an attempt to further improve the performance of
the PCDTBT-based polymer solar cells (PSCs), we have synthesized
a selenophene analogue of PCDTBT, namely, PCDSeBT, in which
diselenienylbenzothiadiazole (DSeBT) monomer alternately flanks
with a 2,7-carbazole unit. The intrinsic properties of PCDSeBT are
not only characterized by UV−vis absorption, cyclic voltammetry
(CV), and organic field-effect transistors (OFETs) but also the
surface morphology, mobilities of space charge-limited current
(SCLC) model, and polymer solar cells (PSCs) in its bulk-
heterojunction (BHJ) active layer with [6,6]-phenyl C71-butyric
acid methyl ester (PC71BM) are evaluated in detail. It is found that
PCDSeBT simultaneously has a low-lying highest occupied
molecular orbital (HOMO) energy level at −5.4 eV and a low bandgap of 1.70 eV as required by the ideal polymers for
BHJ PSCs. The high current of 11.7 mA/cm2 is obtained for PCDSeBT-based PSCs, to our knowledge, which is among the
highest short-circuit current density (JSC) values obtained from a BHJ device consisting of PCDTBT derivatives and [6,6]-phenyl
C61-butyric acid methyl ester (PCBM). The high JSC value, along with a moderate fill factor (FF) of 45% and a high open-circuit
voltage (VOC) of 0.79 V, yields a power conversion efficiency (PCE) of 4.12%, which is about 37% increase in PCE from a
PCDTBT-based reference device. On the basis of our results, one can be concluded that the DSeBT placement for construction
of donor (D)-acceptor (A) polymers is an easy and effective way to realize both the higher JSC and VOC values in PSCs, as a
consequence of the selective lower-lying lowest unoccupied molecular orbital (LUMO) with the HOMO being almost
unchanged, together with the effective broadening on the absorption band.
ple for a low HOMO (−5.45 eV) as required by the ‘ideal’ π-
conjugated polymers, which led to a high VOC value over 0.8 V
and outstanding stability against oxidation at high temper-
atures.5 The initial PCE reached 3.6%6 in a typical BHJ device
and has been subsequently improved to exceeding 6% by BHJ
layer thickness optimization and nanomorphology control
using mixed solvent mixtures.7 However, the bandgap of
PCDTBT is still relatively larger than the proposed 1.5−1.7 eV
of the ideal polymers for PCEs exceeding 10%. Recently, to
further lower the bandgap of PCDTBT without sacrificing its
high VOC, Hsu et al.8 and our group9 independently reported
PCDTBT-based ladder-type polymers with forced planarity of
which device performance showed impressive PCEs of up to
4.59% with reduced bandgap (ca. 0.2 eV) in comparison with
that of PCDTBT. However, the multiple synthetic steps for the
ladder-type PCDTBTs have restricted the realization for their
usage in BHJ devices.
INTRODUCTION
■
With deep understanding of efficient photoinduced electron
transfer within an interpenetrating network by simple blending
π-conjugated polymers (electron donor) with fullerene
derivatives (electron acceptor),1 bulk-heterojunction (BHJ)
polymer solar cells (PSCs) based on poly(3-hexylthiophene)
(P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM)
have been extensively investigated, leading to great progress in
power conversion efficiencies (PCEs) of 4−5%.2 Yet, the PCE
of P3HT-based PSCs is limited by its relatively large bandgap
(∼1.9 eV) with major loss of the solar radiation as well as low
open circuit-voltage (VOC ≅ 0.6 V) due to the relatively small
energy difference between the highest occupied molecular
orbital (HOMO) of P3HT and the lowest unoccupied
molecular orbital (LUMO) of PCBM. By taking into
consideration using low bandgap polymers to harvest more
solar photons, researchers have devoted much effort recently to
reducing the bandgap of polymers. This approach has provided
a remarkable short-circuit current density (JSC) of 16 mA/cm2,3
whereas, in general, the VOC has suffered4 because of a higher
than the HOMO optimal energy level in order to pursue a
lower bandgap. In the PSCs community, poly(2,7-carbazole-alt-
dithienylbenzothiadiazole) (PCDTBT) is one excellent exam-
Herein, we present an easily obtainable selenophene
analogue of PCDTBT, namely, poly(2,7-carbazole-alt-disele-
nienylbenzothiadiazole) (PCDSeBT) by replacing thiophenes
Received: October 11, 2012
Published: October 31, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of PCDSeBT
a
Reagents and conditions: all reactions used anhydrous solvent and Ar conditions; (i) KOH, DMSO, room temperature, 6 h; (ii) 2.05 equiv of n-
BuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF, −78 °C; (iii) Pd(PPh3)4, toluene/DMF (4:1), 100 °C; (iv) NBS, DMF, room
temperature, overnight; (v) Pd2(dba)3, P(o-Tol)3, K3PO4, toluene/deionized H2O (4:1), 90 °C, 3 days.
Figure 1. UV−vis absorption spectra of PCDSeBT in chloroform and the thin film on the quartz (a). Inset: images of PCDSeBT and PCDTBT in
chloroform solutions. Cyclic voltammogram of PCDSeBT thin film (b). Energy-level diagrams of PCDSeBT and PCDTBT (c). Graphical
representation of a methyl-substituted CDSeBT dimer (d).
with selenophene rings in the polymer backbone. Such a
structural modification would be expected the lower-lying
LUMO with the HOMO being largely unaffected due to the
individual effects of the smaller ionization potential (IP) and
almost the same electronegativity of selenium relative to sulfur
atom.10 In addition, interchain-charge transfer should be
facilitated by intermolecular Se···Se contacts, most likely
leading to a widen absorption band and enhanced mobility
that are desirable for high photocurrents.11 Our preliminary
investigation on the photovoltaic properties of PCDSeBT in
typical BHJ devices with [6,6]-phenyl C71-butyric acid methyl
ester (PC71BM) shows a highly respectrable PCE of 4.12% on
account of the high JSC and VOC.
RESULTS AND DISCUSSION
■
The synthetic routes to the monomers (M1 and M2) and
PCDSeBT are shown in Scheme 1. Compounds 1, 2, 3, and 4
were synthesized according to the previously reported
procedures.5a,12 The diboronic ester monomer M1 was easily
synthesized from precursor 3, by dilithiation at −78 °C,
followed by quenching with 2-iso-propoxy-4,4,5,5-tetrametyl-
1,3,2-dioxaborolane in anhydrous THF in a yield of 79%. As the
key building block selenium-containing heterocyclic intermedi-
ate di(2′-selenienyl)-2,1,3-benzothiadiazole (DSeBT) 612 was
prepared by Pd-catalyzed Stille coupling reaction of dibromo-
benzothiadiazole 4 and monostannylselenophene 5, which was
then dibrominated by N-bromosuccinimide (NBS) to yield
comonomer M2 (2 steps, 52% overall yield). The target
poly(2,7-carbazole-alt-diselenienylbenzothiadiazole)
(PCDSeBT) was obtained by Suzuki polymerization of M1 and
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M2 in degassed toluene/water with K3PO4 as an organic base,
Pd2(dba)3 as a catalyst, and P(o-tol)3 as the corresponding
ligand at 90 °C for 3 days. The crude polymer was purified by
reprecipitation and Soxhlet extraction with methanol, acetone,
hexane, and chlorobenzene. Details of synthesis of monomers,
polymerization, and their characterizations are given in the
Experimental Section. After purification, gel-permeation
chromatography (GPC) analysis against PS standard exhibits
a number-averaged molecular mass (Mn) of 83 000 g/mol with
a polydispersity (PDI) of 2.51. The polymer is readily soluble
in common solvents such as chloroform, toluene, THF, and
chlorobenzene.
compared to variations of the donor bandgap. On the basis of
the rules, the estimated PCE from the Egopt and the LUMO
value of PCDSeBT is 7.8%. This theoretical value is much
higher than that of PCDTBT (6.5%), indicating that a highly
promising material for PSCs considering the frontier orbital
energies.
Upon utilizing a model dimeric system containing methyl
groups, the optimized molecular geometry of PCDSeBT and its
frontier molecular orbitals of the calculated HOMO and
LUMO isosurfaces were computed by density functional theory
(DFT, B3LYP/6-31G) (Figure 1d). The HOMO of PCDSeBT
is well-distributed along the conjugated chains; its LUMO is
mainly centralized on BT accepting core as shaped by orbital
lobes, being predicted a p-type organic semiconductor.
In order to corroborate the effect of the heteroatom
substitution on charge transporting properties of the polymers,
top-contact organic field-effect transistors (OFETs) were
fabricated using solution processed PCDTBT and PCDSeBT,
respectively, as the semiconducting layer on the octadecyltri-
chlorosilane (OTS) self-assembled monolayer (SAM)-modified
Si/SiO2 substrates (see the detailed devices fabrication in the
Experimental Section). The OFET mobilities are calculated in
the saturation regime using the following equation: Ids = (W/
2L) × μ × Ci(Vgs − Vth)2, where W and L are the channel width
and length, respectively, Ci is the capacitance per unit area of
the insulation layer. Linear plot of Ids1/2 vs Vgs deduced from the
Ids vs Vgs measurement. These devices show characteristics of a
typical p-channel transistor with good drain-current Modu-
lation, well-defined linear, and saturation regions (Figure 2).
The summary of the performances is provided in Table 1.
The OFET device using as-cast PCDTBT films without
thermal annealing exhibits a hole mobility of 2.2 × 10−4 cm2/
V·s with an on/off current ratio (Ion/Ioff) of 4.44 × 103. For
PCDTBT thin films after annealing at evaluated temperatures,
As shown in Figure 1a, the UV−vis absorption spectra of
PCDSeBT both in chloroform solution and solid state exhibit
two distinct absorption bands, as typically observed for donor
(D)-acceptor (A) low bandgap materials. The absorption
maxima (λmax) of the film are located at 403 and 589 nm. The
shorter absorption band is due to π−π* transitions of the
polymer backbone,13 while the lower-energy peak is attributed
to intramolecular charge transfer (ICT) in the electron-rich and
electron-deficient units.14 It is worth noting that the maxima
absorption values of PCDSeBT in the film are slightly
broadened without any obvious red-shift when compared to
the solution. This verifies that only a small reorganization via
π−π stacking between the polymer chains is within view during
the film formation because of the interchain aggregates in
solution.15 This observation is in contrast to many conjugated
polymers, in which there is a red-shift that occurs in going from
the solution to the solid state due to the aggregation of the
polymer chains in the solid state,3,16 even the PCDTBT film is
red-shifted by 30 nm vs solution.5a Besides, the deep blue color
of PCDSeBT solution is observed unlike the red color of
PCDTBT in solution (see the digital photograph in Figure 1a).
Notably, although the absorption profile of PCDSeBT is
similar to that of the thiophene analogue, PCDTBT, the
absorption onset of PCDSeBT is significantly red-shifted by
∼70 nm, corresponding to a lower optical bandgap (Egopt
=
1.70 eV) than PCDTBT (Egopt = 1.85 eV). This apparently
demonstrates that, in agreement with earlier studies and
experimental studies,17 the replacement of thiophene with
selenophene results in a reduction of the optical bandgap due
to the increase of quinoid character in the polymer backbone.18
In a closer look of the film absorptions, it is also found that the
full width at half-maximum (fwhm) of PCDSeBT (155 nm) is
wider than that of PDCTBT (140 nm), as a matter of a
characteristic of strong intermolecular Se···Se interactions,
which can potentially further increase JSC value. The electro-
chemical properties of PCDSeBT were investigated by cyclic
voltammetry (CV). As shown in Figure 1b, PCDSeBT exhibits
clear reversible n-doping/dedoping processes in the negative
potential range, whereas quasi-reversible p-doping/dedoping
behaviors in the positive potential range are observed. The
HOMO and LUMO energy levels are estimated to be −5.40 eV
and −3.70 eV, respectively, using the ferrocene-ferrocenium
(Fc/Fc+) redox couple (4.8 eV below the vacuum level). Both
PCDTBT and PCDSeBT have nearly identical HOMO energy
levels, but PCDSeBT exhibits 0.10 eV lower-lying LUMO level
relative to PCDTBT (LUMO = −3.60 eV, see also Supporting
Information),5a verifying our presumption aforementioned
about the effect of selenium on the selective fine-tuning of
LUMO level. The material-design rules described by Scharber
et al.19 suggested that the PCEs of a BHJ solar cell should be
much more sensitive to changes of the donor LUMO level
Figure 2. Transfer characteristics of (a) PCDTBT- and (b)
PCDSeBT-based OFET devices at different annealing temperatures.
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Table 1. OFET performance of PCDTBT and PCDSeBT
a
polymer thin films at different annealing temperatures.
b
polymer
condition mobility (μh) (cm2/V·s)
Ion/Ioff
Vth[V]
PCDTBT
as-cast
120 °C
150 °C
as-cast
120 °C
150 °C
2.2 × 10−4
4.9 × 10−4
3.5 × 10−4
5.1 × 10−4
7.8 × 10−4
1.0 × 10−3
4.4 × 103
7.9 × 103
7.9 × 103
4.1 × 104
3.4 × 104
4.0 × 104
−21
−22
−34
−19
−9
PCDSeBT
−13
a
All device characteristics were obtained from the devices (L = 50 μm
b
and W = 1.5 mm) based on OTS-treated Si/SiO2 substrates. The on/
off current ratio (Ion/Ioff) were extracted from the linear-regime
transfer characteristics.
the mobilities are not markedly improved, implying that the
PCDTBT thin films remain nearly amorphous for both as-cast
and annealing at high temperature. On the other hand, it is
found that the unannealed thin films of PCDSeBT exhibit a
slightly higher mobility of μh = 5.1 × 10−4cm2/V·s, compared
with that of pristine PCDTBT. Interestingly, in contrast, as the
PCDSeBT thin films are annealed at a temperature of 120 and
150 °C, the mobilities are systematically improved up to 1.0 ×
10−3 cm2/V·s (Figure 2b), which is about 1 order of magnitude
higher than that of PCDTBT, possibly due to increased
interpolymer chain contact induced by Se···Se interactions with
the thermal annealing process.
To gain a better understanding of the influence of the
thermal annealing, the crystallinity and molecular organization
of PCDTBT and PCDSeBT thin films were investigated by X-
ray diffraction (XRD), both before and after annealing at 150
°C. Both the as-cast and annealed PCDTBT thin films fail to
reveal any defined scattering patterns (Figure 3a), indicating
the macroscopically disordered, amorphous structure. In the
case of PCDSeBT, the as-cast thin films show very weak
diffractions (Figure 3b). Upon thermal annealing, the primary
peak at 4.42°, corresponding to a d-spacing of 19.99 Å,
progressively intensifies and the secondary broad peak at
∼21.1° (d = 4.21 Å) that is probably related to π−π stacking
becomes visible (Figure 3b). On the basis of these XRD results,
we can conclude that the oriented microstructure of the as-cast
PCDSeBT thin films is kinetically limited but the polymer
chains reorganize to ordered lamellar packing structures upon
annealing. This observation is a plausible explanation for the
sudden rise in mobility of PCDSeBT as a function of the
thermal annealing.
Figure 3. X-ray diffraction patterns of PCDTBT(a) and PCDSeBT(b)
thin films before (i) and after annealed at 150 °C (ii).
Photovoltaic effects of PCDSeBT were investigated in BHJ
PSCs with the device structures of the standard configuration
glass/ITO/PEDOT:PSS/PCDSeBT:PC71BM/Al. The
PCDSeBT:PC71BM weight ratio was optimized from 1:1, 1:2,
1:3, to 1:4. The current-density−voltage (J−V) characteristics
of the devices under the illumination of simulated AM 1.5G
conditions (100 mW/cm2) are shown in Figure 4a and the
parameters are summarized in Table 2. Without extensive
optimization, the initial device fabricated from the blend
solutions with a PCDSeBT:PC71BM weight ratio of 1:1 shows
a PCE of 0.73% with a JSC of 2.98 mA/cm2, a VOC of 0.71 V, a
fill factor (FF) of 35%. As increasing the weight fraction of
PC71BM, the PCEs are gradually increased. The device
containing the 1:4 w/w blend as the photoactive layer after
thermal annealing at 150 °C delivers at superior performance
with the JSC of 11.7 mA/cm2, VOC of 0.79 V, and FF of 45%,
improving the PCE to 4.12%, an approximately 37% enhance-
Figure 4. J−V characteristics of PSCs based on PCDSeBT:PC71BM
under illumination of AM 1.5 G, 100 mW/cm2 (a). Incident photon-
to-current efficiency (IPCE) spectrum of PCDSeBT:PC71BM (1:4 w/
w)-based device (b).
ment in PCE, when compared to the reference cell based on
PCDTBT:PC71BM for the sake of comparison (see SI).20 It is
worth noting that the JSC, to the best of our knowledge, is
among the highest JSCs reported to date for PCSs based on
PCDTBT analogues,5,8,21 in a good agreement with the
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Table 2. Photovoltaic Properties of PSCs Based on
PCDSeBT
PCDSeBT:PC71BM
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
(1:1)
(1:2)
(1:3)
(1:4)
2.98
4.66
9.97
11.7
10.3
0.71
0.76
0.79
0.79
0.64
0.35
0.42
0.46
0.45
0.42
0.73
1.50
3.64
4.12
2.76
a
(1:4)
a
Before thermal annealing.
Figure 6. Measured J−V characteristics by the space charge limited
current (SCLC) method with PCDSeBT:PC71BM (1:4 w/w) films
under dark conditions before and after thermal annealing for hole-only
device (a) and electron-only device (b).
extended absorption of sunlight through the low bandgap and
broaden absorption band of PCDSeBT. In order to ensure the
accuracy of the measurements, we investigated the incident
photon-to-current efficiency (IPCE) spectrum of the optimized
device (1:4 w/w), exhibiting over 65% at the energetically
lower peak, i.e. at the λmax of PCDSeBT (Figure 4b). The JSC
value calculated by integrating the IPCE data with an AM 1.5G
reference spectrum is rather consistent with that obtained from
J−V plots within 10% error.
cm2/V·s) obtained with the thermally annealed film is one
order higher than that without post annealing (μhole = 4.02 ×
10−6 cm2/V·s), whereas in the both cases, the electron
mobilities (μelectron) are similar (1.24 × 10−3 and 5.58 × 10−3
cm2/V·s) for as-cast and annealed films, respectively, yielding a
smaller difference between the mobility ratio for the annealed
film. The higher mobilities and more balanced charge transport
should definitely favor the charge transportation in the blend
film and lead to high solar cell performance, supporting the
beneficial effect of the thermal annealing on
PCDSeBT:PC71BM cells.
Atomic force microscopy (AFM) was performed to gain
insight into the surface morphology of the optimized 1:4 w/w
blend films (Figure 5). The films before and after thermal
CONCLUSION
■
In conclusion, diselenienylbenzothiadiazole (DSeBT) units
have successfully been incorporated into a D−A polymer
backbone by Suzuki polymerization with 2,7-carbazole
comonomer, affording a poly(2,7-carbazole-alt-diselenienylben-
zothiadiazole) (PCDSeBT); This design motif is that
replacement of thiophenes to selenophene units in PCDTBT
that acts as a superior p-type photoactive material for PSCs,
induces a lower bandgap (1.70 eV) than that of PCDTBT (1.85
eV), while the ideal HOMO energy level (−5.40 eV) is still
maintained. Another pleasant surprise comes from PCDSeBT
with a relative wider absorption band observed, as a result of
strong intermolecular Se···Se interactions. The lower bandgap
with the broaden absorption band significantly improves a JSC
of 11.7 mA/cm2, which is among the highest JSC values for
PSCs based on PCDTBT derivatives, while the low HOMO
energy level maintains the high VOC value. The corresponding
PCE is 4.12%, about 37% higher than that obtained from a
PCDTBT-based reference device. This work provides a feasible
strategy for selective fine-tuning of LUMO levels, which
Figure 5. AFM images (6 μm × 6 μm) of PCDSeBT:PC71BM (1:4 w/
w) cells before (a) and after (b) thermal annealing at 150 °C,
respectively.
annealing have smooth surface topographies and very similar
features, where the formation of voids, typically attributed to
incompatibility between donor and acceptor components, is
not observed. However, the rms roughness is somewhat
reduced to from 0.47 to 0.43 nm with thermal treatment,
being favorable for charge separation and transport. This can
reasonably explain the enhanced efficiency via the postanneal-
ing effect. Note that, in stark contrast to PCDTBT:PC71BM
system in which thermal annealing reduced the photovoltaic
parameters (FF, JSC, and VOC),22 the performance of all
PCDSeBT-based devices for this study is considerably
improved as a result of thermal treatment. We can conclude
that the formation of crystalline nanoscale domains would be
slightly induced in PCDSeBT:PC71BM with a thermal
treatment in consequence of the ordered crystalline nature of
the PCDSeBT on annealing as supported by the XRD results
above as well as the large quinoidal character in the polymer
backbone as the inclusion of selenophene.23 The AFM of the
annealed film gives an evidence for slightly nanofibril-like
organization (see Figure 5b).
achieves a high JSC value, while nearly remaining a high VOC
.
Therefore, further exploration of PCDSeBT-based PSCs is still
warranted for high-performance PCEs by means of controlling
the film morphology and improving electric contacts.
EXPERIMENTAL SECTION
■
Materials and Characterization. Compounds 1, 2, 3, and 4 were
synthesized according to the previously reported procedures.1,2 All
starting materials were purchased either from Aldrich or Acros and
used without further purification. THF was distilled over sodium/
1
benzophenone. H NMR spectra were recorded on a Varian VNRS
600 MHz (Varian USA) spectrophotometer using CDCl3 as solvent
and tetramethylsilane (TMS) as the internal standard. UV−vis spectra
were taken on the Cary 5000 (Varian USA) spectrometer. Number-
average (Mn) and weight-average (Mw) molecular weights, and
polydispersity index (PDI) of the polymer products were determined
by gel permeation chromatography (GPC) with Agilent 1200 HPLC
Chemstation using a series of mono disperse polystyrene as standards
The hole and electron mobilities of PCDSeBT:PC71BM (1:4
w/w) blends were measured using the space-charge-limited
current (SCLC) method and the J−V characteristics were
plotted in Figure 6. The hole mobility (μhole = 4.68 × 10−5
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in THF (HPLC grade) at 308 K. Cyclic voltammetry (CV)
measurement was performed on AMETEK VersaSTAT 3 with a
three-electrode cell in a nitrogen bubbled 0.1 M tetra-n-butylammo-
nium hexafluorophosphate (n-Bu4NPF6) solution in acetonitrile at a
scan rate of 50 mV/s at room temperature. A used as the Ag/Ag+ (0.1
M of AgNO3 in acetonitrile) reference electrode, platinum counter
electrode and polymer coated platinum working electrode, respec-
tively. The Ag/Ag+ reference electrode was calibrated using a
ferrocene/ferrocenium redox couple as an internal standard, whose
oxidation potential is set at −4.8 eV with respect to zero vacuum level.
The HOMO energy levels were obtained from the equation HOMO =
apparatus with methanol, acetone, hexane, and chlorobenzene. The
chlorobenzene fraction (150−200 mL) was reduced to 30−40 mL
under reduced pressure precipitated into methanol (500 mL), filtered
through 0.45 μm nylon filter and finally dried overnight under the
vacuum. The dark violet polymer was obtained (yield %, Mn = 8.3 ×
104 g/mol, Mw = 2.0 × 105 g/mol, PDI = 2.51). 1H NMR (600 MHz,
CDCl3). δ (ppm): 8.26 (br, 2H), 8.14 (br, 2H), 7.92 (br, 2H), 7.71
(br, 2H), 7.52 (br, 2H), 7.39 (br, 2H), 4.63 (br, 1H), 2.35 (br, 2H),
2.04 (br, 2H), 1.53 (br, 8H), 1.25−1.14 (br, 16H), 0.78 (br, 6H).
Anal. Calcd: C, 64.73; H, 6.19; N, 5.27; S, 4.02; Se, 19.79. Found: C,
64.37; H, 6.07; N, 5.22; S, 3.90.
Fabrication of Photovoltaic Cells. Polymer solar cell devices
were fabricated according to the following procedure: First, the ITO
coated glass substrate was cleaned with detergent, then ultrasonicated
in distilled water, acetone and isopropyl alcohol, and then dried
overnight in an oven at the 100 °C. Poly(3, 4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Bay-
tron PH) was spin-cast at 5000 rpm for 40s. The substrate was then
dried for 10 min at the 140 °C in the air. Subsequently, it is moved
into a glovebox for spin-coating the active layer. A mixed solution of
PCDSeBT:PC71BM in o-dichlorobenzene (DCB) was then spin
coated at 700 rpm for 60s on top of the PEDOT:PSS layer to obtain a
BHJ film. Those samples were brought into a vacuum system (about
10−7 Torr), and an Al electrode (100 nm) was deposited on top of the
BHJ layer. Typical devices were thermally annealed in a Petri dish.
Thermal annealing was carried out by directly placing the completed
devices on a digitally controlled hot plate at 150 °C, in a glovebox
filled with nitrogen gas and after annealing. The devices were put on a
metal plate and cooled to room temperature. Measurements were
carried out with the solar cells inside the glovebox by using a high
quality optical fiber to guide the light from the solar simulator
equipped with a Keithley 2635 A source. The solar cell devices were
illuminated at an intensity of 100 mW/cm2. For more accurate
information the IPCE measurements were carried out with QEX7. The
device structure of the hole and electron only devices are ITO/
P E D O T : P S S / P C D S e B T : P C 7 1 B M / A u a n d F T O /
PCDSeBT:PC71BM/Al, respectively. The space-charge-limited current
onset
onset
−(Eox
− E(ferrocene)
+ 4.8) eV. The LUMO levels of polymers
onset
were obtained from the equation LUMO = −(Eredonset − E(ferrocene)
+ 4.8) eV.
Synthesis of 2,7-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxa-
borolan-2′-yl)-N-9″-heptadecanylcarbazole (M1). To a solution
of compound 3 (1.5 g, 2.67 mmol) in anhydrous THF (50 mL) at the
−78 °C, 1.6 M n-BuLi in hexane (4 mL) was added dropwise, the
mixture being stirred at this temperature under Ar flow for 1 h, and 2-
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.31 mL, 60.41
mmol) then being added. After 1h at −78 °C, the mixture was warmed
to room temperature and stirred overnight. The mixture was poured
into water, extracted with diethyl ether and dried over MgSO4. The
solvent was removed under reduced pressure, and the residue was
purified by recrystallization in methanol/acetone (ca. 10:1) to give a
1
title compound M1 as a white crystal (1.37 g, Yield 79%). H NMR
(600 MHz, CDCl3), δ (ppm): 8.14 (t, J = 7.2 Hz, 2H), 8.02 (s, 1H),
7.88 (s, 1H), 7.66 (br, 2H), 4.69 (m, 1H), 2.33 (m, 2H), 1.94 (m, 2H),
1.39 (br, 23H), 1.18 (br, 23H), 1.12 (br, 2H), 0.97 (m, 6H).
Synthesis of Tributyl(2-selenophenyl)stannane (5). To a
solution of selenophene (5.0 g, 38.17 mmol) in anhydrous THF (100
mL) at the −78 °C, 1.6 M n-BuLi in hexane (28.62 mL) was added
dropwise, the mixture being stirred at this temperature under Ar flow
for 1 h, and tributylchlorostannane (11.05 mL, 41.0 mmol) then being
added. The mixture was stirred at the −78 °C for 1.5 h and poured
into saturated aqueous NaHCO3. The organic phase was separated
and washed with saturated aqueous brine and then dried over MgSO4.
The solvent was removed under reduced pressure, and the residue was
purified by neutral alumina column chromatography (eluent hexane)
(SCLC) mobilities were estimated the Mott-Gurney square law JSCLC
=
1
9/8 × εrε0 × μ(V2/L3), where εr is the dielectric constant of the
material, ε0 is the permittivity of free space, L is the distance between
the cathode and anode, which is equivalent to the film thickness, and V
is the applied voltage.
to give a title compound 5 as colorless oil (12.43 g, Yield 77.5%). H
NMR (600 MHz, CDCl3), δ (ppm): 8.36 (d, J = 4.8 Hz, 1H), 7.50 (m,
2H), 1.55 (m, 6H), 1.35 (m, 6H), 1.09 (m, 6H), 0.90 (t, 9H).
Synthesis of 4,7-Di(2′-selenienyl)-2,1,3-benzothiadiazole
(6). To a solution of 4,7-dibromo-2,1,3-benzothiadiazole (2.0 g, 6.0
mmol) and compound 5 (6.29 g, 14.9 mmol) in anhydrous toluene
(40 mL) and anhydrous DMF (10 mL), Pd(PPh3)4 (0.4 g, 0.34 mmol)
was added. The mixture was refluxed for 3 days. The solvent was
removed under reduced pressure, and the residue was purified by silica
gel column chromatography (eluent CH2Cl2/hexane, 1:1) to give a
OFET Device Preparation and Measurement. Highly doped n+-
Si wafers were used as substrates, and a layer of 200 nm of silicon
dioxide (SiO2:grown by thermal oxidation) was used as the gate
dielectric layer. Au (60 nm) was successively evaporated with shadow
mask to obtain source and drain electrodes. The interdigitated
structure of the source-drain contacts determined a channel length of
50 μm and a channel width of 2950 μm. Substrates were cleaned by
acetone, isopropanol and dried at 100 °C oven for 20 min, and treated
with octadecyltrichlorosilane (OTS) at room temperature for over 12
h to treat the surface. Organic semiconductor layers (60 nm) were
deposited by spin-coating at 1500 rpm. All fabrication processes were
carried out in a glovebox filled with N2. Electrical characterization was
performed using a Keithley semiconductor parametric analyzer
(Keithley 4200-SCS) under N2 atmosphere. The electron mobility
(μ) was determined using the following equation in the saturation
regime; Ids = (WCi/2L) × μ × (Vgs − VTh)2, where Ci is the
capacitance per unit area of the SiO2 dielectric (Ci = 15 nF/cm2) and
VTh is the threshold voltage.
1
title compound 6 as a red solid (2.10 g, yield 78.4%). H NMR (600
MHz, CDCl3) δ (ppm): 8.19 (m, 4H), 7.91 (s, 2H), 7.45 (t, J = 3 Hz,
2H).
Synthesis of 4,7-Bis(5′-bromo-2′-selenienyl)-2,1,3-benzo-
thiadiazole (M2). To a solution of compound 6 (2.0 g, 5.07
mmol) in an anhydrous DMF (35 mL), N-bromosuccinimide (2.17 g,
12.17 mmol) was added, the mixture was stirred at room temperature
under Ar flow for 1day and poured into water. The red precipitate was
filtered off and washed with NaHSO4 solution. Recrystallization from
DMF twice gave a title compound M2 as dark red solid (1.72 g, yield
51.6%). 1H NMR (600 MHz, CDCl3), δ (ppm): 7.87 (s, 2H), 7.79 (d,
J = 4.8 Hz, 2H), 7.37 (d, J = 4.8 Hz, 2H).
Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2- se-
lenienyl-2,1,3-benzothiadiazole)] (PCDSeBT). In a flame-dried
flask, M1 (0.168 g, 0.30 mmol), M2 (0.20 g, 0.30 mmol), tri(o-
tolyl)phosphine (7 mg, 0.024 mmol), K3PO4 (0.32 g, 1.52 mmol), and
Pd2(dba)3 were dissolved in anhydrous toluene (8 mL) and deionized
H2O (2 mL). The mixture was stirred at the 90 °C under Ar flow for 3
days (end-capped with phenylboronic acid and bromobenzene). The
solution was precipitated in a mixture of methanol and ammonia (4:1
v/v, 300 mL). The precipitate was filtered off and washed on Soxhlet
ASSOCIATED CONTENT
■
S
* Supporting Information
Cyclic voltammetry (CV) measurements and CV of PCDTBT
and J−V characteristic of the PCDTBT:PC71BM-based cell.
This material is available free of charge via the Internet at
http://pubs.acs.org.
8663
dx.doi.org/10.1021/ma302133h | Macromolecules 2012, 45, 8658−8664

Macromolecules
Article
(18) (a) Chattopadhyaya, M.; Sen, S.; Alam, M. M.; Chakrabarti, S. J.
Chem. Phys. 2012, 136, 094904. (b) Patra, A.; Wijsboom, Y. H.; Leitus,
G.; Bendikov, M. Chem. Mater. 2011, 23, 896. (c) Zade, S. S.;
Zamoshchik, N.; Bendikov, M. Acc. Chem. Res. 2010, 44, 14.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (J.Y.K.) jykim@unist.ac.kr; (C.Y.) yang@unist.ac.kr.
Fax: +82-52-217-2909. Telephone: +82-52-217-2920.
(19) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
̈
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789.
(20) For our BHJ devices based on PCDTBT:PC71BM, PCE = 3.0%,
VOC = 0.81 V, JSC = 7.4 mA/cm2, and FF = 42%.
Author Contributions
§These authors contributed equally.
(21) Li, J.; Grimsdale, A. C. Chem. Soc. Rev. 2010, 39, 2399.
(22) (a) Chu, T.-Y.; Alem, S.; Tsang, S.-W.; Tse, S.-C.; Wakim, S.;
Lu, J.; Dennler, G.; Waller, D.; Gaudiana, R.; Tao, Y. Appl. Phys. Lett.
2011, 98, 253301. (b) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.;
Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J.
Nat. Photon. 2009, 3, 297.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2010-0002494) and the National Research Foundation of
Korea Grant funded by the Korean Government (MEST)
(2010-0019408), (2010-0026163), (2010-0026916), and
(NRF-2009-C1AAA001-0093020).
(23) Saadeh, H. A.; Lu, L.; He, F.; Bullock, J. E.; Wang, W.; Carsten,
B.; Yu, L. ACS Macro Lett. 2012, 1, 361.
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dx.doi.org/10.1021/ma302133h | Macromolecules 2012, 45, 8658−8664