Sustainable synthetic approach to π-conjugated arylacetylenic semiconductors for bulk heterojunction solar cells

Article abstract of DOI:10.1039/c3ra41048j

We report here the first study illustrating the attraction of fabricating solution-processed small molecule field-effect transistors and bulk heterojunction solar cells with semiconductors prepared using new sustainable synthetic procedures which minimize

Full text of DOI:10.1039/c3ra41048j

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Sustainable synthetic approach to p-conjugated
arylacetylenic semiconductors for bulk heterojunction
Cite this: RSC Advances, 2013, 3,
9288
solar cells3
Elena Bartollini,a Mirko Seri,b Sara Tortorella,a Antonio Facchetti,*cd Tobin J. Marks,*c
Assunta Marrocchi*a and Luigi Vaccaro*a
Received 2nd March 2013,
Accepted 26th March 2013
We report here the first study illustrating the attraction of fabricating solution-processed small molecule
field-effect transistors and bulk heterojunction solar cells with semiconductors prepared using new
sustainable synthetic procedures which minimize waste per kg of product (E-factor minimization).
DOI: 10.1039/c3ra41048j
www.rsc.org/advances
of waste generated is therefore highly desirable.3 Waste
production should be reduced by using minimal amounts of
Introduction
Bulk heterojunction solar cells (BHJ) composed of conjugated
polymer–fullerene blends currently display power conversion
efficiencies (PCEs) in the range of 6–9%.1 However, PCE values
and processing conditions are strongly affected by polymer
batch variations in solubility, molecular weight, polydispersity
and purity. These issues are absent with solution-processed
small molecule (SM) BHJ solar cells, where the high degree of
molecular precision circumvents the statistical variability of
polymers. A record PCE of 7% under AM 1.5G irradiation (100
mW cm22) has been recently achieved by Nguyen, Bazan and
co-workers2 for SM BHJ solar cells, thus demonstrating that
such devices can be competitive with their polymeric counter-
parts.
In parallel, by combining the advantages of solar cells
fabricated from small donor molecules with new eco-friendly
synthetic procedures, new opportunities for the production of
green energy from green materials may emerge. The synthetic
methodologies currently available for organic semiconductors
are demanding from both raw materials/chemical costs and
environmental points of view, and these aspects may seriously
affect their large-scale production. An effective waste-mini-
mized synthetic approach that focuses on reducing the volume
reagents and by using easily recoverable/recyclable catalytic
systems in order to simplify costly purification procedures.
Furthermore, the use of green alternative reaction media such
as water and solvent-free conditions, as well as new solid
catalytic systems, could lead to improved material perfor-
mance and stability. For many years we have been developing
synthetic protocols employing eco-compatible reaction proto-
cols, including the use of water,4 solvent-free conditions
6
(SolFC),5 and supported organocatalysts,
proving that
significant improvements in both chemical efficiency and
process sustainability can be obtained.
In this regard, and in continuation of our efforts7 to
understand the interrelationships between molecular struc-
ture and organic photovoltaic (OPV) cell performance, the
present work is devoted to the sustainable synthesis, char-
acterization, and implementation in OFETs/BHJ OPVs of a
series of asymmetrically substituted donor arylacetylenes 1–4
(Fig. 1).
In our design strategy, compounds 1–4 comprise an
anthracene core linked to two ethynylene–phenylene units at
both 9,10-positions which are peripherally functionalized with
electron-releasing and electron-accepting species at the extre-
mities, thus leading to a so-called push–pull architecture.
It is well established that as a consequence of a favorable
push–pull effect of donor and acceptor species, efficient
aLaboratory of Green Synthetic Organic Chemistry, Dipartimento di Chimica,
`
Universita degli Studi di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
E-mail: assunta@unipg.it; luigi@unipg.it
bConsiglio Nazionale delle Ricerche (CNR)-Istituto per la Sintesi Organica e la
`
Fotoreattivita (ISOF), via P. Gobetti 101, 40129 Bologna, Italy
cDepartment of Chemistry, the Materials Research Center, and the Argonne-
Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan
Road, Evanston, IL 62208, USA. E-mail: a-facchetti@northwestern.edu;
t-marks@northwestern.edu
dPolyera Corporation, 8045 Lamon Avenue, Skokie, IL 60077, USA
Electronic supplementary information (ESI) available: OFET/OPV fabrication
3
details, synthetic procedures/spectroscopic data/E-factor calculation, synthesis
optimization screening, electrochemical/UV-Vis/OFET data, XRD analysis of
donors, AFM images of blends and neat molecules. See DOI: 10.1039/c3ra41048j
Fig. 1 Chemical structure of compounds 1–4.
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intramolecular charge transfer may occur upon excitation,
lowering the band gap, which is desirable for OPV light
absorption at longer wavelengths.8 It is also known9 that such
band gaps can be simply controlled by adjusting the electron
donating strength of the donor and the electron withdrawing
strength of the acceptor units. In designing compounds 1–4 we
also bear in mind that anthracene is a promising candidate for
use in the development of organic semiconductor devices as it
may combine large carrier mobilities, greater solubility, and
improved stability10 as compared to pentacene, which has
become a benchmark in the field. Moreover, the anthracene
nucleus is a good chromophore unit. Finally, the n-hexyloxy
chains at the 3- and 4-positions of the terminal phenyl rings
are introduced to enhance the semiconductor solubility as well
as to promote their self-organization in the solid state. Indeed,
n-hexyloxy chains along the main molecular axis (i.e. at the
4-positions) may also promote enhanced p-stacking, thus
improving charge transport properties.11
Scheme 1 Synthesis of compound 1: i, trimethylsilylacetylene (1.5 eq),
PdEncatTPP30 (2 mol%), CuI (2 mol%), diisopropylamine (1.5 eq), 2-methyl-
THF, 18 h, 30 uC, 99%; ii, 10% TBAF/SiO2, 3 h, 30 uC, 96%; iii, 9-iodo-10-
bromoanthracene (0.9 eq), PdEncat30 (3 mol%), CuI (1 mol%), diisopropyla-
mine (27 eq), cyclopentyl methyl ether, 18 h, 55 uC, 90%; iv, 3,4-bis(hexylox-
y)ethynylbenzene
6 (0.9 eq), PdEncatTPP30 (6 mol%), CuI (4 mol%),
diisopropylamine (45 eq), cyclopentyl methyl ether, 20 h, 50 uC, 70%; v,
trimethylsilylacetylene (1.5 eq), PdEncatTPP30 (2 mol%), CuI (2 mol%),
diisopropylamine (1.5 eq), 2-methyl-THF, 3 h, 30 uC, 99%; vi, 10% TBAF/SiO2,
3 h, 30 uC, 96%.
The optoelectronic properties of these new compounds
have been studied by UV–Vis absorption and cyclic voltam-
metry. The thin film microstructure and morphology has been
investigated using X-ray diffractometry (XRD) and atomic force
microscopy (AFM), and is discussed in relation to molecular
structure and device response.
used in this case (1 mol%) essentially no alkyne homocoupling
by-product was observed.14 The resulting monocoupling
product 8 was then subjected to additional cross-coupling
with 3,4-bis(hexyloxy)ethynylbenzene 9 to provide the target
Results and discussion
compound 1 in yields up to 70% (Scheme 1, Table S4, ESI ).
3
Note that the previously reported synthesis15 of 3,4-bis(hex-
It is known that a low Environmental factor (E-factor, kg waste
per kg product) is one of the most essential contributions to
the overall efficiency of a synthetic process.12 The synthesis of
donors 1–4 was therefore carried out by means of unconven-
tional Pd-catalyzed Sonogashira coupling reactions to reduce
the E-factor of these processes, as exemplified in Scheme 1 for
compound 1. In all cases, various environmentally-sustainable
reaction media and different recoverable heterogeneous
palladium sources were considered and the obtained data
yloxy)ethynylbenzene 9 has in turn been revisited (Scheme 1,
Tables S5 and S6, ESI ), and it was found that the optimal
3
conditions for the Sonogashira cross-coupling involve
PdEncatTPP30 as the palladium source (2 mol %), 2% mol
of CuI co-catalyst, and 2-methyl-THF as reaction media. The
de-silylation reaction was optimized by using 10% TBAF/SiO2.
All of the new compounds were characterized by conventional
chemical and analytical methods (ESI ).
3
Note here the importance of using green reaction media
such as those selected in our protocols (i.e., 2-methyl-THF and
cyclopentyl methyl ether); organic solvents accounting for
more than 80% of the waste production of a process. Further,
carrying out the chemical reactions by using a recoverable
heterogeneous catalysts simplified the purification steps,
avoiding in most cases the need for chromatography, and
prevent metal contamination of the semiconductor which is
known to negatively affect performance. This is particularly
evident in comparing our optimized protocols and the
conventional ones previously reported,15,16 when available
are reported in Tables S1–S6, ESI.
The synthesis of compound 1 begins with Sonogashira
coupling between 1-nitro-2-hexyloxy-4-iodobenzene and
trimethylsilylacetylene. The most efficient result was accom-
plished in 2-methyl-THF, a reaction medium included in the
list of green solvents,13 and by using 2% mol of PdEncatTPP30
in the presence of 2% mol of CuI and 1.5 equiv. of base.
The subsequent desilylation reaction was optimized by
using TBAF/SiO2, allowing the desired acetylenic compound 7
to be isolated as a pure product in quantitative yield
3
5
(Scheme 1, Table S2, entry 11, ESI ). Note that the desilylation
3
(see ESI ).
3
step could be carried out by using only 10 mol % of TBAF/SiO2,
instead of (over)-stoichiometric amounts of TBAF that are
routinely employed. Next, the selective cross-coupling reaction
between 9-bromo-10-iodoanthracene and the alkyne 7 in
cyclopentyl methyl ether, included in the list of green
solvents,13 proceeded in 90% yield when PdEncat30 (3%
Calculation of the E-factors12 confirm that the present
approach is very efficient for waste reduction. In our case, the
E-factors range from y101 to y102 and are significantly
smaller than those for conventional protocols previously
reported,13,16 where an average of y5 6 103 was obtained. A
reduction of waste by 94.4–99.1% has been achieved for the
Sonogashira cross-couplings as well as the alkyne desilylation
mol) was used (Scheme 1, Table S3, entry 19, ESI ).
3
Importantly, due to the very low amount of CuI co-catalyst
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Table 1 Electrochemical and optical properties, and MO energy summary of compounds 1–4
lmax solution [nm]b
lmax film [nm]
a
a
Donor
Eox (V)
Ered (V)
EHOMO (eV)
ELUMO (eV)
1
2
3
4
0.82
0.76
21.41, 21.62
21.45, 21.61
—
25.62
25.56
25.64
25.47
23.13
23.11
23.28
23.23
278, 317, 455
318, 455,471
278, 472, 494c
460, 483
374, 396, 435
327, 470, 504
488, 529
0.84
0.63d, 0.81
21.64
440, 528
a
b
c
d
Onset values determined vs. Fc/Fc+ in CH2Cl2 solution, data not available for donor 3. CHCl3 solution. Data from ref. 16. E1/2 potential.
reactions (when applicable) by adopting the newly reported
105, after thermal annealing at 80 uC for 2 h. By further
increasing the annealing temperature up to 120 uC, no FET
responses are observed, likely due to drastic morphological
changes (vide infra).
procedures (see also ESI ).
3
The redox properties of compounds 1–4 were investigated
by cyclic voltammetry (CV, see Fig. S1, ESI ), and the results are
3
summarized in Table 1. The HOMO and LUMO energies of
semiconductors 1–4 were estimated from optical absorption
and CV data17 using the standard approximation that the Fc/
Fc+ HOMO level is 24.8 eV. These data are compiled in Table 1
and show that the HOMO energies of compounds 1–4 are
within the required range to ensure an acceptable Voc in a
PCBM BHJ device (vide infra). Note also that the LUMO energy
for compound 3 lies below those of anthracene derivatives 1, 2
and 4, reflecting the stronger electron-withdrawing character
of the ethynylene phenylene fragment 4-substituted by the
nitro group.
In order to better understand the effects of the various
substituents on the electrical performance of the present
arylacetylene derivatives, film microstructure and film mor-
phology were studied by X-ray diffraction (XRD), and tapping
mode atomic force microscopy (AFM). Fig. S4, ESI and Fig. 2B
3
show XRD data using standard h–2h scan for spin-cast 1–4
films on bare Si/SiO2 substrates. All the semiconductor films
are crystalline and exhibit multi-order X-ray reflections.
The diffraction data indicated that 1 films exhibit a single
low intensity reflection at 2h
= 3.95u (Fig. S4A, ESI ),
3
corresponding to a d-spacing of 26.8 Å. The estimated
molecular or long axis tilt angle w with respect to the substrate
normal, defined by the d-spacing and computed molecular
Optical absorption data for compounds 1–4 in chloroform
and as thin films, are summarized in Table 1 and Table S7,
ESI. The solid-state spectrum of 1 was blue-shifted relative to
lengths (Table S9, ESI ), is y 36.4u, suggesting predominant
3
3
that in solution (Fig. S2, ESI, Table 1). This result could be
edge-on molecular orientation on the substrate surface, with a
modest inclination.
We also investigated how film microstructure evolves as a
function of annealing at increasing temperatures (40–120 uC).
3
explained by H-aggregation of the molecules.18 In contrast, the
absorption peaks of the solid-state spectra of compounds 2–4
are y50 nm red-shifted relative to the solution spectra,16
which can be attributed to greater structural organization in
the solid state and/or formation of J-aggregates.
Fig. S4A, ESI shows that on going from 60 to 120 uC, an
3
extension of visible reflections at lower diffraction angles is
observed for arylacetylene 1. Moreover, the major peak shifts
towards smaller diffraction angles when the annealing
temperature is increased from 80 to 120 uC, whereas its
intensity decreases somewhat with the increase in tempera-
Bottom-gate top-contact OFETs were fabricated by spin
coating semiconductor solutions in CHCl3 (5 mg mL21 7d
on
)
doped Si(gate)/SiO2 (dielectric layer, 300 nm thick) substrates.
Vacuum-deposited Au source and drain electrodes completed
the FET structure. Device fabrication and characterization
ture. Further, the 2 film XRD scan (Fig. S4B, ESI ) exhibits a
3
details are given in the ESI. Typical p-type organic semi-
3
major reflection at 2h = 6.71u whereas for 3 films reflections at
conductor characteristics are observed for the compounds 1, 3
2h = 4.67u are observed (Fig. S4C, ESI ), corresponding to a
3
and 4, whereas semiconductor
Representative transfer plots for compounds 1, 3, and 4 are
shown in Fig. 2A, and Fig. S3, ESI. Table S8, ESI summarizes
2 was not FET active.
d-spacing of 27.6
diffraction features at 2h = 3.2u, 13.43u, 16.79u and 2h = 9.9u,
Å and 18.9 Å, respectively. Multiple
3
3
the FET performance response for optimal devices, including
the charge carrier mobility (m), on-off current ratio (Ion/Ioff),
and threshold voltage (VT). Semiconductors 1 and 3 exhibit
hole mobilities up to y4.5 6 1025 cm2 V21 s21 and 5.5 6
1025 cm2 V21 21, respectively, and Ion/Ioff approaching 3 6
s
104 and 3 6 101, respectively, when measured in vacuum. Film
thermal annealing over the range 40–120 uC was found to have
a
generally detrimental effect both on 1 and 3 FET
performance in terms of mobility, whereas the Ion/Ioff for 1 is
slightly enhanced.
Semiconductor 4 exhibits the best FET performance, with a
hole mobility up to 3.4 6 1022 cm2 V21 s21 and Ion/Ioff of 1 6
Fig. 2 (A) Typical transfer plot for a spin-coated 4 OFET annealed at 80 uC. (B)
XRD scans for films of semiconductor 4 spin-coated from chloroform onto bare
Si/SiO2 and annealed at 80 uC.
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14u, respectively, are also observed. The estimated molecular
tilt angles of these molecular structures (w y 34.0u and w y
46.8u, respectively) were similar to those observed for 1 (Table
Table 2 Bulk heterojunction photovoltaic response properties of compounds 1–
4.
Active layer (wt/wt) Ta (uC) Voc (V) Jsc (mA cm22
) FF (%) PCE (%)
a
S3, ESI ). From the XRD data in Fig. S4B and S4C, ESI it is also
3
3
1/PCBM (1 : 2)b
1/PCBM (1 : 2)
1/PCBM (1 : 1)b
1/PCBM (1 : 1)b
1/PCBM (2 : 1)
1/PCBM (2 : 1)
3/PCBM (1 : 2)
3/PCBM (1 : 2)
3/PCBM (1 : 1)
3/PCBM (1 : 1)
3/PCBM (2 : 1)
3/PCBM (2 : 1)
4/PCBM (1 : 2)
4/PCBM (1 : 2)
4/PCBM (1 : 1)
4/PCBM (1 : 1)
4/PCBM (2 : 1)
4/PCBM (2 : 1)
—
50
—
50
—
50
—
50
—
50
—
50
—
50
—
50
—
50
0.78
0.82
0.90
0.92
0.78
0.64
0.82
0.86
0.52
0.59
0.67
0.51
0.65
0.63
0.60
0.61
0.62
0.50
1.19
0.42
1.17
1.05
1.19
0.55
0.34
0.46
0.25
0.31
0.26
0.20
0.74
2.27
4.45
4.07
3.50
1.72
22
24
37
39
22
18
24
26
31
32
30
34
26
46
48
51
43
26
0.2
0.1
0.4
0.4
0.2
0.1
0.1
0.1
0.04
0.04
0.05
0.03
0.1
0.7
1.25
1.2
clear that in the case of 2 and 3, respectively, on increasing the
annealing temperature, thin film crystallinity generally
increases, presumably due to a more ordered p–p stacking of
the phenyl–ethyne–anthracene–ethyne–phenyl core. Next, the
2h diffraction angles basically remain unchanged regardless of
the investigated annealing temperature. The XRD plot shown
in Fig. 2B confirms the high crystallinity of 4 films, which
correlates well with the optimal OFET performance (Table S2,
ESI ). A primary reflection is observed at 2h = 2.21 correspond-
3
ing to a d-spacing of y40 A. Additionally, multiple Bragg
reflections at 2h = 4.46u, 6.62u, 9.02u, 11.31u, 13.55u, 15.77u and
18.05u (inset Fig. 2B) are also observed. Note that the
experimental d-spacing of semiconductor
4
(y40 Å) is
0.9
0.2
significantly larger than the semiempirical AM1-calculated
molecular length of this molecule (y27.64 A). This finding
correlates well with the typical19 antiparallel self-assembling of
a
b
Ta = annealing T/uC. Spin speed 4000 rpm, 1 min.
push–pull systems (Fig. S5, ESI ).
3
The AFM images of the 1-derived films on bare SiO2
instrumentation described previously.21 Device fabrication
and characterization details are given in the ESI.
Representative OPV current density–voltage (J–V) plots under
illumination for the present devices based on chromophores 1,
substrates (SI Fig. S6A, ESI ) indicate that the morphology
3
3
consists of plate-like grains, which increase in size when
annealed up to 60 uC. On the other hand, films of compound 2
(Fig. S6B, ESI ) exhibit a sponge-like surface morphology, the
3
3 and 4 are shown in Fig. S11, ESI and in Fig. 3A.
3
channel width increasing after annealing at 40–60 uC. Thermal
annealing over the range 80–120 uC results in extensive film
degradation for compound 1, whereas it induces a progressive
formation of large clusters in 2-derived films (Fig. S7 and S8,
The low photocurrents (JSC) of 1 and 3/PCBM blended films,
accompanied by poor FFs and, in the case of 3, small VOC
values, afford modest PCEs up to y0.4% for the compound 1.
For cells based on 4/PCBM (1 : 1) a maximum JSC value of 4.45
mA cm22 was achieved. Combined with good VOC values, and a
fill factor (FF) of y 50%, these data yield PCEs up to y1.3%.
The optimization of the D : A blend ratio for 4/C61-PCBM
based device, demonstrates that 1 : 1 is the best composition;
in fact, an increase or decrease of the PCBM content leads to
degradation in device performance. Representative AFM
images of the resulting morphologies are shown in Fig. S12,
ESI ). In the case of 3, the AFM images (Fig. S6C, ESI ) reveal
3
3
large crystal-like features protruding from the thin films. Upon
thermal annealing, decreased film continuity is observed with
increasing temperature, although the grain sizes increase,
which explains the depressed mobility.20 After thermal
annealing at 120 uC, well-isolated features are clearly visible
(Fig. S9, ESI ). Similar to the case of 3, the surface morphology
3
of as-spun 4 films reveals large features protruding from the
ESI.
3
films (Fig. S5D, ESI ). AFM characterization of the 4-derived
3
The 4/PCBM (1 : 1) device annealed at 50 uC exhibits a
slightly decreased PCE. Despite the slight improvement of the
FF value – over 50% – the drop in JSC (from 4.45 to 4.07 mA
films after thermal annealing (40–120 uC) is reported in Fig.
S10, ESI. Indeed, optimum annealing conditions for such
3
semiconductor are 80 uC/2 h, as evidenced by the structured
grains in Fig. S10C, ESI. This is consistent with the unique
3
XRD pattern observed as well as the FET data (vide supra).
BHJ OPV cells were fabricated by spin-coating from CHCl3,
under an ambient atmosphere, a blend of 1, 3 or 4 as the
donor and PCBM as acceptor onto cleaned ITO-coated glass
anodes modified by first spin coating on a PEDOT:PSS layer as
a hole extraction/electron blocking layer. After drying, the cells
were then completed by sequential vacuum deposition of LiF
and Al as the cathode. The solar cell device performance was
optimized by varying both the active layer 1, 3 or 4/PCBM (wt/
wt) blend composition, film thickness and annealing tem-
peratures. Table 2 summarizes the results of this screening
process. All devices were characterized under standard
AM1.5G 1 Sun test conditions using the NREL calibrated
Fig. 3 (A) J–V characteristics under illumination of optimized 4/PCBM based
OPVs as a function of D : A (w/w) composition. (B) EQE spectra of BHJ donor 4/
PCBM 1 : 2 wt/wt ratio (black line), 1 : 1 wt/wt ratio (red line), and 2 : 1 wt/wt
ratio (blue line).
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cm2) limits the efficiency. The improved hole conductivity
(higher FF), probably reflecting the good charge carrier
lenes 1, 3 and 4 are p-type semiconductors with a hole carrier
mobilities up to # 4 6 1022 cm2 V21 21. Organic BHJ solar
s
mobility (mh # 4 6 1022 cm2 V21
s
21), can be attributed to
cells using PCBM as electron acceptor were also fabricated
with these new extended arylacetylenes. Power conversion
efficiencies range from y0.1% to y1.3%. A direct correlation
is identified here between the nature/position of electron-
withdrawing groups substituted on the peripheral phenylene
fragment and the BHJ device VOC. Furthermore, a particular
crystal packing arrangement for arylacetylene 4 (antiparallel
self-assembling) consistently yields the best performing OFETs
and OPVs. All of the obtained results argue that film
morphology/microstructure is a major factor in determining
OFET/OPV characteristics.
Although the investigated arylacetylene performances are
not sufficient for practical applications, to the best of our
knowledge, this is the first example of combining the
advantages of SM-based organic optoelectronic TFT/OPV
devices and new sustainable synthetic procedures with
minimal waste production per kg of the desired material.
This approach may open the route to photovoltaics as a truly
viable technology for cost-competitive applications. Further
investigation along this line is currently in progress.
4 self-assembling by thermal treatment. However, a different
phase-separation, as suggested by the major change in the
thin-film morphology (see representative AFM images in Fig.
S13, ESI ), may influence the interfacial areas between the BHJ
3
components, thus limiting the exciton separation efficiency
and the resulting photocurrent.
A number of arguments have been presented in the
literature to describe the impact of material parameters on
device VOC, one of the most widely accepted involving the
relation between the donor HOMO and acceptor LUMO.22
However, there is also an observed and important relationship
between VOC and film morphology23 which may relate to the
observed impact of the substitution pattern on the VOC of the
solar cells. For instance, when a 1 : 1 wt/wt ratio with respect
to PCBM is used, donor 1 shows a higher VOC than derivatives
containing the electron-withdrawing group at the 4-position of
the peripheral ethynylene phenylene fragment. The magnitude
of this difference (y0.3 V) is greater than would be estimated
from the differences in HOMO energies (on the order of 0.15
eV). Contributing to this phenomenon may be the very
different morphology of 3- and 4- vs. 1-based films (Fig. S12,
ESI ).
3
Experimental
The optical absorption spectra of the 1-, 3–4/PCBM blend
films were also examined (Fig. S14, ESI ). The enhanced long
General
3
wavelength absorption by the 4/PCBM blends correlates with
higher device efficiencies. Spectral response data for the most
efficient of the present OPVs are shown in Fig. 3B. Plots for
devices using non-optimal PCBM content are included for
comparison. For 4/PCBM 1 : 2 wt/wt films the EQE maximum
is lower (y28% at 550 nm) than that obtained for devices
based on 1 : 1 wt/wt (y35% at 440 nm) and 2 : 1 wt/wt blends
(y31% at 440 nm). Additionally, the 1 : 1 and 2 : 1 spectral
responses exhibit a broader profiles, which may account for
the appreciably high Jsc response of the 1 : 1 and 2 : 1 vs. 1 : 2
device (4.45 and 3.50 vs. 2.27 mA cm22, respectively).
Melting points were determined on a Bu¨chi melting point
apparatus and are uncorrected. Chromatography was per-
¨
formed on Riedel de Haen silica gel (230–400 mesh ASTM).
NMR spectra were recorded on a Varian Associates VXR-400
multinuclear instrument (internal Me4Si). UV-vis spectra were
recorded on a Cary Model 1 UV-vis spectrophotometer.
Commercially available reagents were purchased from Sigma-
Aldrich Co. and utilized without further purification, unless
otherwise noted. PCBM was purchased from American Dye
Source, Inc. (ADS) and was further purified by several cycles of
sonication in toluene followed by filtration, and then sonica-
tion in pentane, followed by centrifugation. Petroleum ether as
the 40–60 uC boiling fraction was used. Electrochemistry was
performed on
a C3 Cell Stand Electrochemical Station
Conclusions
equipped with BAS Epsilon software (Bioanalytical Systems,
Inc., Lafayette, IN) in an electrolyte solution of 0.1 M
An effective waste-minimized approach to the synthesis of a
new family of soluble extended p-conjugated arylacetylenic
optoelectronic materials with a push–pull architecture is
reported. Since the organic solvents account for more than
80% of the synthetic waste production, green reaction media
were used. Furthermore, the use of recoverable heterogeneous
catalysts simplify the process purification steps, avoiding in
most cases the need for chromatography. This approach leads
to the definition of synthetic methodologies featuring a very
good E-factor for each semiconductor. By adopting these newly
defined synthetic protocols a 94.4–99.1% waste reduction is
demonstrated for the Sonogashira cross-couplings as well as
for the alkyne desilylation reactions (when applicable). Field
effect transistor measurements demonstrate that arylacety-
2
tetrabutylammonium hexafluorophosphate (Bu4N+PF6
) in
dry dichloromethane. Platinum wire electrode was used as
working electrode, platinum wire was used as counter
electrodes, and a silver wire was used as the pseudo-reference
electrode. A ferrocene/ferrocenium redox couple was used as
an internal standard and the potential values obtained in
reference to the silver electrode were converted to the vacuum
scale.
[4-(Hexyloxy)-3-nitrophenyl]ethynyltrimethylsilane 6
[4-(Hexyloxy)-3-nitro]iodobenzene15 5 (0.174 g, 0.5 mmol),
diisopropylamine (106 ml, 0.75 mmol), trimethylsilylacetylene
(106.3 mL, 0.75 mmol), PdEnCatTPP30 (0.025 g, 0.01 mmol),
CuI (0.0019 g, 0.01 mmol), and degassed 2-methyl-THF (0.5
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ml) were placed in a screw-capped vial equipped with a
magnetic stirrer. Next, the mixture was stirred at 30 uC for 18
h, then CH2Cl2 was added (1 ml), and the catalyst recovered by
filtration. The mixture was subsequently diluted with CH2Cl2
(2 ml), washed with water (3 ml 6 2), dried over Na2SO4,
filtered and evaporated to dryness to afford pure 6 (99% yield,
uC. After 18 h, the crude mixture was diluted with CHCl3 (2
ml), the catalyst was filtered off, and the solvent removed
under vacuum to give 9-bromo-10-[4-hexyloxy-(3-nitropheny-
l)ethynyl]anthracene 8. This latter was purified by re-crystal-
lization from ethyl acetate (2 6 25 ml) to give the pure
compound as yellow needles (0.176 g, 90% yield). 1H NMR
(CDCl3, d): 8.67 (m, 3H), 8.53 (m, 3H), 7.63 (m, 4H), 6.96 (m,
1H), 4.05(m, 2H), 1.83 (m, 4H) 1.42 (m, 4H), 0.87 (m, 3H); 13C
NMR (CDCl3, d): 153.5, 142.4, 140.9, 134.5, 134.4, 134.2, 130.9,
130.1, 126.3, 124.6, 118.4, 113.9, 114.9, 95.6, 87.7, 70.5, 31.5,
29.3, 25.6, 22.6, 14. An. Calcd. For: C28H24BrNO3: C, 66.94; H,
4.82; N, 2.79%. Found: C, 66.89; H, 4.83; N, 2.77%. E-factor =
[0.106 g (4-ethynyl-1-(hexyloxy)-2-nitrobenzene) + 0.149 g (9-
iodo-,10-bromoanthracene) + 0.57 g (cyclopentyl methyl ether)
+ 0.00074 g (CuI) + 1.063 g (diisopropylamine) + 0.029 g
(PdEnCat30) + 2.98 g (CHCl3) + 45.10 g (ethyl acetate) 2 0.176 g
(product 6 yield)]/0.176 g = 283.0.
1
yellow oil). H NMR (CDCl3, d): 7.93 (d, J = 2.4 Hz, 1H), 7.61
(dd, J = 2.4, 8.9 Hz, 1H), 6.95 (d, J = 8.9 Hz, 1H), 4.09 (m, 2H),
1.90 (m, 4H), 1.43(m, 4H), 0.89 (m, 3H). 13C NMR (CDCl3, d):
153.6, 140.5, 137.24, 129.14, 117.30, 113.5, 99.9, 94.3, 70.6,
31.5, 29.4, 25.6, 22.6, 14.0, 20.3. E-factor = [0.174 g (4-
(hexyloxy)-3-nitro-iodobenzene) + 0.076 g (diisopropylamine) +
0.408 g (2-methyl-THF) + 0.1474 g (trimethylsilylacetylene) +
0.0019 g (CuI) + 0.025 (PdEnCatTPP30) + 3.97 g (CH2Cl2) + 6.0 g
(H2O) 2 0.158 g (product 6 yield)]/0.158 g = 67.37. Not
accounting for drying agent.
4-Ethynyl-1-(hexyloxy)-2-nitrobenzene 7
9-{[3,4-Bis(hexyloxy)phenyl]ethynyl}-10-{[4-(hexyloxy)-3-
nitrophenyl]ethynylanthracene 1
In a screw capped vial equipped with a magnetic stirrer [4-
(hexyloxy)-3-nitrophenyl]ethynyltrimethylsilane 6 (0.159 g, 0.5
mmol) and SiO2-supported TBAF (0.0625 g, 0.05 mmol; 1.25
mmol g21) were added, and the resulting mixture was left
under vigorous stirring at 30 uC. After 3 h, ethyl acetate (1 ml)
was added. The catalyst was then filtered off and washed with
ethyl acetate (1.5 ml). The solvent was removed under vacuum
to give compound 9 as a yellow oil (0.118 g, 96% yield). 1H
NMR (CDCl3, d): 7.51 (m, 1H), 7.56 (m, 1H), 6.97 (m, 1H), 4.07
(m, 2H), 2.35 (s, 1H), 1.47 (m, 2H), 1.33 (m, 6H), 0.88 (m, 3H).
13C NMR (CDCl3, d): 153.7, 140.8, 138.1, 130.2, 114.8, 113.8,
79.2, 78.2, 70.5, 31.5, 29.4, 25.6, 22.6, 14.0. E-factor = [0.159 g
([4-(hexyloxy)-3-nitrophenyl]ethynyltrimethylsilane) + 0.0625 g
(TBAF/SiO2) + 2.25 g (ethyl acetate) 2 0.118 g (product 6
yield)]/0.118 g = 19.94.
In a screw-capped vial equipped with a magnetic stirrer 3,4-
bis(hexyloxy)ethynylbenzene 9 (0.130 g, 0.43 mmol), degassed
cyclopentyl methyl ether (CPME, 0.66 ml), bromo derivative 8
(0.196 g, 0.39 mmol), diisopropylamine (2.126 g, 21.06 mmol),
CuI (0.0029 g, 0.015 mmol), and PdEnCatTPP30 (0.082 g, 0.033
mmol) were consecutively added and the resulting mixture was
left under vigorous stirring at 50 uC. After 20 h, the crude
mixture was diluted with CHCl3 (2 ml), the catalyst was filtered
off, and the solvent removed under vacuum to give 9-bromo-
10-[4-hexyloxy-(3-nitrophenyl)ethynyl]anthracene 8. This latter
was purified by re-crystallization from ethyl acetate (2 6 25
ml) to give the pure compound as orange needles (0.197 g,
70% yield); m.p. 121–122u C. 1H NMR (CDCl3, d): 8.70 (m, 3H),
8.57 (m, 3H), 7.63 (m, 6H), 6.97 (m, 2H), 4.07(m, 6H), 1.85 (m,
6H), 1.50 (m, 6H), 1.34 (m, 6H), 0.92 (m, 9 H). 13C NMR (CDCl3,
d): 14.05, 22.54, 22.64, 25.47, 25.75, 28.83, 29.21, 29.31, 31.46,
31.63, 69.16, 69.48, 69.84, 84.97, 87.12, 99.59, 103.33, 113.29,
114.31, 115.38, 116.69, 119.29, 125.37, 126.59, 126.89, 127.31,
128.29, 131.75, 131.97, 136.725, 148.95, 150.28, 153.07. An.
Calcd. For: C48H53NO5: C, 79.64; H, 7.38; N, 1.93%. Found: C,
79.52; H, 7.37; N, 1.90%. E-factor = [0.130 g (3,4-bis(hexylox-
y)ethynylbenzene) + 0.196 g (8) + 0.57 g (cyclopentyl methyl
ether) + 0.0029 g (CuI) + 2.126 g (diisopropylamine) + 0.082 g
(PdEnCatTPP30) + 2.98 g (CHCl3) + 45.10 g (ethyl acetate) 2
0.197 g (product 6 yield)]/0.197 g = 258.8.
3,4-Bis(hexyloxy)ethynylbenzene 915
The desilylation reaction procedure described above for
preparing 7 is that used for preparing compound 9. Yield:
96% (yellow oil). 1H NMR (CDCl3, d): 0.85 (m, 6H), 1.34 (m,
12H), 1.75 (m, 4H), 2.93 (s, 1H), 3.92 (m, 4H), 6.73 (d, 1H, J =
8.2 Hz), 6.94 (m, 1H) 7.00 (dd, 1H, J = 1.9, 8.2 Hz). E-factor: see
ESI.
3
[3,4-Bis(hexyloxy)phenyl]ethynyl trimethylsilane 11
This compound was prepared by coupling 1,2-bis(hexyloxy)-4-
iodobenzene 1015 with trimethylsilylacetylene (1 : 1.5 molar
ratio), following the procedure described for 6. Yield: 99%
(yellow oil). 1H NMR (CDCl3, d): 0.19 (s, 9H), 0.85 (m, 6H), 1.34
(m, 12 H), 1.75 (m, 4H), 3.92 (m, 4H), 6.71 (d, 1H, J = 8.3 Hz),
6.91 (d, 1H, J = 1.9 Hz), 6.98 (dd, 1H, J = 1.9, 8.3 Hz). E-factor:
Device fabrication and measurements
OFET devices. Prime-grade n-doped silicon wafers having
300 nm thermally grown oxide (Process Specialties Inc.) were
used as device substrates. Before film deposition, the
substrates were cleaned by ultrasonic treatment with ethanol
and then in an oxygen plasma cleaner for 5 min under
vacuum. Films of compounds 1–4 were spin-coated from 0.5%
(w/v) CHCl3 solutions and, if annealed, heated under N2 at
various temperatures from 40 to 120 uC for 1 h, unless
otherwise noted. Spin-coated films were 30–40 nm thick as
determined by profilometry (Tencor P10). For FET device
fabrication, top contact gold (500 Å) were deposited by
evaporating gold (pressure , 1025 Torr); channel dimensions
see ESI.
3
9-Bromo-10-[4-hexyloxy-(3-nitrophenyl)ethynyl]anthracene 8
In a screw-capped vial equipped with a magnetic stirrer
4-ethynyl-1-(hexyloxy)-2-nitrobenzene 7 (0.106 g, 0.43 mmol),
degassed cyclopentyl methyl ether (CPME, 0.66 ml), 9-iodo-10-
bromoanthracene (0.149 g, 0.39 mmol), diisopropylamine
(1.063 g, 10.53 mmol), CuI (0.00074 g, 0.0039 mmol), and
PdEnCat30 (0.029 g, 0.011 mmol) were consecutively added
and the resulting mixture was left under vigorous stirring at 55
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were 25/50/100 mm (L) by 1.0/2.0/5.0 mm (W). The capacitance
of the insulator is 1 6 1028 F cm22 for 300 nm SiO2. TFT
device measurements were carried out at 21–23 uC in a
customized high-vacuum probe station (1 6 1026 Torr) or in
air. Coaxial and/or triaxial shielding was incorporated into
Signaton probes to minimize noise levels. TFT characteriza-
tion was performed with a Keithley 6430 subfemtoamperom-
eter (drain) and Keithley 2400 (gate) source meter, operated by
a locally written Labview program and GPIB communication.
OFET response parameters were extracted from I–V data using
the standard field-effect transistor equation.24 Mobilities (m)
were calculated in the saturation regime by the standard
relationship: msat = (2IDSL)/[WCi(VG 2 VT)2], where IDS is the
source–drain saturation current, Ci is the gate dielectric
capacitance (per area), VG is the gate voltage, and VT is the
silicon cantilevers in the tapping mode, using WinSPM
Software. Optical absorption spectra were acquired on a
Shimadzu 1601 spectrophotometer.
Acknowledgements
This research is supported as part of the ANSER Center, an
Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, and Office of Basic
Energy Sciences under Award Number DE-SC0001059. S.T.
thanks the REU program of the National Science Foundation
for support under award CHE-0755206. The authors acknowl-
edge the American Chemical Society for coordinating the IREU
exchange program. We acknowledge the Northwestern
University MRSEC for access to materials characterization
facilities (NSF grant DMR-1121262). We gratefully acknowl-
threshold voltage. The latter can be estimated as the intercept
1/2
of the linear section of the plot of VG vs. (IDS
)
OPV devices. These devices were fabricated by spin-coating a
blend of 1, 3 and 4 + PCBM (D : A wt : wt ratio, D = 1, 2; A = 1,
2), sandwiched between a transparent anode and a cathode.
The anode consisted of glass substrates pre-coated with
indium-tin-oxide (ITO, Delta Technologies; RS = 8–12 V).
Before device fabrication, the ITO-coated (150 nm) glass
substrates were cleaned by ultrasonic treatment with deio-
nized water + detergent, deionized water, isopropyl alcohol,
methanol, and acetone sequentially, and finally in a UV-ozone
cleaner for 30 min under ambient atmosphere. A thin layer of
PEDOT/PSS (Baytron P) was then spin-coated at 4000 rpm for 1
min (unless otherwise noted) on the anode to modify the ITO
surface. After baking at 150 uC for 15 min, the active layer was
deposited by spin-coating at 2000 rpm for 1 min (unless
otherwise noted), the D/A blends under air using a solution
concentration of 16 mg ml21. Dry chloroform was used as
solvent. Contact areas were cleaned with dry toluene and a
cotton swab. The cathode consisted of LiF (y0.6 nm, Acros,
99.98%) capped with Al (150 nm, Sigma-Aldrich, 99.999%).
The deposition rates used were 0.1 Å s21 for LiF and y2 Å s21
for Al, with a chamber pressure of 1.1 6 1026 Torr. Device
characterization was performed at 298 K using a Class A
Spectra-Nova Technologies solar cell analyzer having a xenon
lamp that simulates AM1.5G light from 400–1100 nm. The
instrument was calibrated with a monocrystalline Si diode
fitted with a KG3 filter to bring spectral mismatch to unity.
The calibration standard was calibrated by the National
Renewable Energy Laboratory (NREL). Four-point contacts
were made to the substrate with Ag paste and copper alligator
clips. Individual devices were isolated by a mask during testing
to avoid current collection from adjacent devices and edge
effects. Power conversion efficiencies were calculated from the
following equation: PCE = (JscVocFF)/P0, where Jsc (mA cm22) is
the short circuit current, Voc (V) the open circuit voltage, FF the
fill factor, and P0 the power of incident light source (mW
cm22).
`
edge the Ministero dell’Istruzione, dell’Universita e della
Ricerca (MIUR) within the projects PRIN and ‘‘Firb-Futuro in
Ricerca’’ and the Universita degli Studi di Perugia for financial
`
support.
References
1 (a) http://www.polyera.com/newsflash/polyera-achieves-
world-record-organic-solar-cell-performance (accessed
February 2013); (b) A. Marrocchi, D. Lanari, A. Facchetti
and L. Vaccaro, Energy Environ. Sci., 2012, 5, 8457; (c)
H. Xin, X. Guo, G. Ren, M. D. Watson and S. A. Jenekhe,
Adv. Energy Mater., 2012, 2, 575; (d) P. Sonar, S. P. Singh, E.
L. Williams, Y. Li, M. S. Soh and A. Dodabalapur, J. Mater.
Chem., 2012, 22, 4425; (e) A. Facchetti, Chem. Mater., 2011,
23, 733; (f) J. Li, H. S. Tan, Z.-H. Chen, W. P. Goh, H.
K. Wong, K. H. Ong, W. Liu, C. M. Li and B. S. Ong,
Macromolecules, 2011, 44, 690; (g) H. Zhou, L. Yang, A.
C. Stuart, S. C. Price, S. Liu and W. You, Angew. Chem., Int.
`
Ed., 2011, 50, 2995; (h) S. H. Park, A. Roy, S. Beaupre,
S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee
and A. J. Heeger, Nat. Photonics, 2009, 3, 297; (i) H.-Y. Chen,
J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu
and G. Li, Nat. Photonics, 2009, 3, 649; (j) H. Xin, X. Guo, F.
S. Kim, G. Ren, M. D. Watson and S. A. Jenekhe, J. Mater.
Chem., 2009, 19, 5303.
2 T. S. van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan,
Adv. Mater., 2012, 24, 3646.
3 (a) A. Facchetti, L. Vaccaro and A. Marrocchi, Angew. Chem.,
Int. Ed., 2012, 51, 3520; (b) P. Berrouard, A. Najari, A. Pron,
D. Gendron, P.-O. Morin, J.-R. Pouliot, J. Veilleux and
M. Leclerc, Angew. Chem., Int. Ed., 2012, 51, 2068; (c) J. Li,
Q. Bao, C. M. Li, W. Zhang, C. Gong, M.-B. Chan-Park,
J. Qin and B. S. Ong, Chem. Mater., 2010, 22, 5747.
4 (a) T. Angelini, D. Lanari, R. Maggi, F. Pizzo, G. Sartori and
L. Vaccaro, Adv. Synth. Catal., 2012, 354, 908; (b) D. Lanari,
O. Piermatti, F. Pizzo and L. Vaccaro, Synthesis, 2012, 44, 2181; (c)
S. Calogero, D. Lanari, M. Orru`, O. Piermatti, F. Pizzo and
L. Vaccaro, J. Catal., 2011, 282, 112; (d) S. Bonollo, D. Lanari,
F. Pizzo and L. Vaccaro, Org. Lett., 2011, 13, 2150; (e) S. Bonollo,
D. Lanari and L. Vaccaro, Eur. J. Org. Chem., 2011, 2587; (f)
Thin-film characterization
Thin films were analyzed by wide angle X-ray film diffracto-
metry (WAXRD) on a Rigaku ATX-G instrument using standard
h–2h techniques with Cu-Ka1 radiation. Atomic force micro-
scopy was performed on a Jeol 5200 SPM instrument with
9294 | RSC Adv., 2013, 3, 9288–9295
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
`
S. Bonollo, F. Fringuelli, F. Pizzo and L. Vaccaro, Synlett, 2007,
2683.
12 (a) R. A. Sheldon, Chem. Commun., 2008, 3352; (b) J. Aug’e,
Green Chem., 2008, 10, 225; (c) R. A. Sheldon, Green Chem.,
2007, 9, 1273; (d) R. A. Sheldon, Chem. Ind., London, U.K.,
1997, 12.
5 (a) D. Lanari, R. Ballini, A. Palmieri, F. Pizzo and L. Vaccaro,
Eur. J. Org. Chem., 2011, 2874; (b) F. Fringuelli, D. Lanari,
F. Pizzo and L. Vaccaro, Green Chem., 2010, 12, 1301; (c)
A. Zvagulis, S. Bonollo, D. Lanari, F. Pizzo and L. Vaccaro,
Adv. Synth. Catal., 2010, 352, 2489–2496; (d) R. Ballini,
L. Barboni, L. Castrica, F. Fringuelli, D. Lanari, F. Pizzo and
L. Vaccaro, Adv. Synth. Catal., 2008, 350, 1218; (e)
F. Fringuelli, R. Girotti, F. Pizzo and L. Vaccaro, Org. Lett.,
2006, 8, 2487; (f) D. Amantini, F. Fringuelli, O. Piermatti,
F. Pizzo, E. Zunino and L. Vaccaro, J. Org. Chem., 2005, 70,
6526.
´
´
13 R. K. Henderson, C. Jimenez-Gonzalez, D. J. C. Constable,
S. R. Alston, G. G. A. Inglis, G. Fisher, J. Sherwood, S.
P. Binks and A. D. Curzons, Green Chem., 2011, 13, 854.
`
14 (a) R. Chinchilla and C. Carmen Najera, Chem. Soc. Rev.,
2011, 40, 5084; (b) J. Sedelmeier, S. V. Ley, H. Lange and I.
R. Baxendale, Chem.–Eur. J., 2009, 4412; (c) Y.-Y. Kuang and
F.-E. Chen, Helv. Chim. Acta, 2009, 92, 897; (d) R. Chinchilla
`
and C. Najera, Chem. Rev., 2007, 107, 874.
15 L. Valentini, D. Bagnis, A. Marrocchi, M. Seri, A. Taticchi
and J. M. Kenny, Chem. Mater., 2008, 20, 32.
6 (a) S. Bonollo, D. Lanari, J. M. Longo and L. Vaccaro, Green
Chem., 2012, 14, 164; (b) S. Bonollo, D. Lanari, T. Angelini,
F. Pizzo, A. Marrocchi and L. Vaccaro, J. Catal., 2012, 285,
216.
16 (a) A. Marrocchi, A. Spalletti, R. Flamini, I. S. Ciorba,
M. Seri, F. Elisei and A. Taticchi, J. Photochem. Photobiol., A,
2010, 211, 162; (b) R. Flamini, I. Tomasi, A. Marrocchi,
B. Carlotti and A. Spalletti, J. Photochem. Photobiol., A, 2011,
223, 140.
17 C.-C. Wu, J. C. Sturm, R. A. Register, J. Tian, E. P. Dana and
M. E. Thompson, IEEE Trans. Electron Devices, 1997, 44,
1269.
18 (a) G. Pera, A. Villares, M. C. Lopez, P. Cea, D. P. Lydon and
P. J. Low, Chem. Mater., 2010, 22, 2041; (b) A. Villares,
G. Pera, S. Martin, R. J. Nichols, D. P. Lydon, L. Applegarth,
A. Beeby, P. J. Low and P. Cea, Chem. Mater., 2010, 22, 2041.
19 (a) N. Sakai, A. L. Sisson, S. Bhosale, A. Fu¨rstenberg,
N. Banerji, E. Vauthey and S. Matile, Org. Biomol. Chem.,
2007, 5, 2560.
20 (a) M. Mas-Montoya, R. P. Ortiz, D. Curiel, A. Espinosa,
A. Magali, A. Facchetti and T. J. Marks, J. Mater. Chem. C,
2013, 1, 1959; (b) R. P. Ortiz, A. Facchetti and T. J. Marks,
Chem. Rev., 2010, 110, 205.
21 M. D. Irwin, B. Bucholz, A. W. Hains, R. P. H. Chang and T.
J. Marks, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 2783.
22 C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz,
M. T. Rispens, L. Sanchez and J. C. Hummelen, Adv. Funct. Mater.,
2001, 11, 374.
23 (a) M. C. Scharber, D. Mu¨hlbacher, M. Koppe, P. Denk,
C. Waldauf, A. J. Heeger and C. J. Brabec, Adv. Mater., 2006,
18, 789; (b) D. M. Stevens, J. C. Speros, M. A. Hillmyer and
C. D. Frisbie, J. Phys. Chem. C, 2011, 115, 20806.
24 S. M. Sze, Physics of Semiconductor Devices, 2nd ed., John
Wiley & Sons, Taipei, Taiwan, 1981.
7 (a) A. Marrocchi, I. Tomasi and L. Vaccaro, Isr. J. Chem.,
2012, 52, 41; (b) M. Seri, A. Marrocchi, D. Bagnis, R. Ponce,
A. Taticchi, T. J. Marks and A. Facchetti, Adv. Mater., 2011,
23, 3827; (c) P. Wolfer, M. L. Santarelli, L. Vaccaro, L. Yu, T.
D. Anthopulos, P. Smith, N. Stingelin and A. Marrocchi,
Org. Electron., 2011, 12, 1886; (d) F. Silvestri, A. Marrocchi,
M. Seri, C. Kim, A. Facchetti, A. Taticchi and T. J. Marks, J.
Am. Chem. Soc., 2010, 132, 6108; (e) L. Valentini,
F. Mengoni, A. Taticchi, A. Marrocchi, S. Landi, L. Minuti
and J. M. Kenny, New J. Chem., 2006, 30, 939; (f)
L. Valentini, F. Mengoni, A. Taticchi, A. Marrocchi and J.
M. Kenny, J. Mater. Chem., 2006, 16, 1622.
8 (a) B. Carlotti, R. Flamini, A. Spalletti, A. Marrocchi and
F. Elisei, ChemPhysChem, 2012, 13, 724; (b) P. Sonar, S.
P. Singh, Y. Li., Z. Ooi, T. Ha, I. Wong, M. S. Soh and
A. Dodabalapur, Energy Environ. Sci., 2011, 4, 2288.
9 L. Minuti, A. Taticchi, A. Marrocchi, S. Landi and E. Gacs-
Baitz, Tetrahedron Lett., 2005, 46, 5735.
10 (a) J. E. Anthony, Chem. Rev., 2006, 106, 5028; (b) H. Meng,
F. Sun, A. B. Goldfinger, G. D. Jaycox, Z. Li, J. Marshall and
G. S. Blackman, J. Am. Chem. Soc., 2005, 127, 2406.
11 (a) A. Sung, M. Ling, M. L. Tang, Z. Bao and J. Locklin,
Chem. Mater., 2007, 19, 2342; (b) F. Wurthner and
R. Schmidt, ChemPhysChem, 2006, 7, 793; (c) J. Locklin,
M. E. Roberts, S. C. B. Mannsfeld and Z. Bao, Polym. Rev.,
2006, 46, 79.
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