Fluoro-functionalization of vinylene units in a polyarylenevinylene for polymer solar cells

Article abstract of DOI:10.1039/c2ta00177b

A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)- benzothiadiazole blocks with difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and characterized as donor materials in bulk heterojunction (BHJ) s

Full text of DOI:10.1039/c2ta00177b

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Materials Chemistry A
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Fluoro-functionalization of vinylene units in a
polyarylenevinylene for polymer solar cells†
Cite this: DOI: 10.1039/c2ta00177b
a
a
b
b
Antonio Cardone, Carmela Martinelli, Maria Losurdo, Elena Dilonardo,
b
c
c
d
e
Giovanni Bruno, Guido Scavia, Silvia Destri, Pinalysa Cosma, Luigi Salamandra,
Andrea Reale, Aldo Di Carlo, Aranzazu Aguirre, Bego n~ a Mili a´ n-Medina,
Johannes Gierschner and Gianluca M. Farinola*
e
e
f
f
f
d
A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)-benzothiadiazole blocks with
difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and
characterized as donor materials in bulk heterojunction (BHJ) solar cells with PCBM as the acceptor. The
solar cells with the fluorinated polymer show better photovoltaic performances than those recorded
with the non-fluorinated material. Comparative spectroscopic and computational studies, together with
morphological, electrical and optical characterization of thin films, have been carried out to shed light on
the reasons for the improvement of performances as induced by the double bond fluorination. Our
study introduces the fluorinated double bond as a new conjugated unit in donor polymers for BHJ solar cells.
Received 3rd September 2012
Accepted 12th October 2012
DOI: 10.1039/c2ta00177b
www.rsc.org/materialsA
derivative, for example [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM)), are one of the most effective device congura-
Introduction
1,2
Fast progress has been recorded in the last few years in the
tions in organic photovoltaics. The advantages of this approach
development of polymer solar cells (PSCs) as an alternative to
traditional silicon-based photovoltaics. Although the efficiency extended by roll-to-roll fabrication of plastic solar cells.
for fast production of large area devices have been recently
1
3
and lifetime of organic-based devices still remain lower than
The main factors that still limit the BHJ PSC performances
those of silicon solar cells, inexpensive and fast processing as and prevent their wide commercialization are the not optimized
well as the possibility to fabricate coloured and semitransparent efficiency and the low chemical stability of the organic active
4
photovoltaic modules on exible platforms make the new layer. Strong research efforts are devoted to design and
technology suitable for many applications, and continuous
improvements are expected in the near future. The BHJ solar
cells based on a p-conjugated polymer donor blended with an
synthesize donor polymers that are able to overcome the current
limitations. An electron donor material must have the following
characteristics: (a) light absorption matching as much as
electron acceptor (most commonly
a
soluble fullerene possible the solar spectrum for effective harvesting of the solar
photons; (b) high extinction coefficient; (c) proper alignment of
a
Istituto di Chimica dei Composti OrganoMetallici (ICCOM) di Bari, Consiglio the HOMO and LUMO energy levels with respect to the
Nazionale delle Ricerche (CNR), Via Orabona 4, 70125 Bari, Italy. E-mail:
5
acceptor; (d) good exciton and positive charge (holes) transport
cardone@ba.iccom.cnr.it; mrcr01ch@uniba.it
properties; and (e) thermal and chemical stability to optimize
the device lifetime.
b
Istituto per le Metodologie Inorganiche e dei Plasmi (IMIP) di Bari, Consiglio
Nazionale delle Ricerche (CNR), Via Orabona 4, 70125 Bari, Italy. E-mail: maria.
Hereby, the morphology of the BHJ active layer plays an
important role favouring exciton transport and dissociation as
losurdo@ba.imip.cnr.it; dlln01ch@uniba.it; giovanni.bruno@ba.imip.cnr.it
c
Istituto per lo Studio delle Macromolecole (ISMAC), Consiglio Nazionale delle
Ricerche (CNR), Via Bassini, 15, 20133 Milano, Italy. E-mail: scavia@ismac.cnr.it; well as the transport of separated charges to the respective
destri@ismac.cnr.it
6
7
electrodes, thus increasing the open circuit voltage (V ).
oc
d
Dipartimento di Chimica, Universit a` degli Studi di Bari Aldo Moro, Via Orabona, 4,
An important difference of organic semiconductors
compared to inorganic ones is the generally poor charge-carrier
mobility. However, organic semiconductors have a relatively
7
0125 Bari, Italy. E-mail: p.cosma@chimica.uniba.it; gianlucamaria.farinola@
uniba.it
e
Electronic Engineering Department, University of Rome Tor Vergata, Via del
5
ꢀ1
strong absorption coefficient (usually $ 10 cm ), which
Politecnico, 1, 00133 Rome, Italy. E-mail: luigi.salamandra@uniroma2.it; reale@
uniroma2.it; aldo.dicarlo@uniroma2.it
partially balances the low mobility, giving effective light
f
8
Madrid Institute for Advanced Studies, IMDEA Nanoscience, Calle Faraday 9, Ciudad absorption also in active lms thinner than 100 nm. The effi-
Universitaria de Cantoblanco, 28049 Madrid, Spain. E-mail: johannes.gierschner@
ciency of BHJ solar cells is also limited by the short exciton
diffusion length (i.e. the distance over which excitons travel
before undergoing deactivation), which is about 3–10 nm in
imdea.org; begona.milian@imdea.org; aranzazu.aguirre@imdea.org
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2ta00177b
1
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8
most organic semiconductors. Excitons formed at a distance polymers in BHJ solar cells, and opens the way to entirely new
from the donor–acceptor interfaces longer than the exciton families of materials for polymeric photovoltaics.
diffusion length have a lower probability of being harvested. As
a consequence, the active volume of this type of solar cells is
limited to a very thin region close to the donor–acceptor inter-
Results and discussion
Synthesis and characterization
faces; thus the increase of the absorption coefficient of the
polymeric donor material matching the solar emission spec-
trum is a key target.
The monomers M1 (ref. 16) and M2 (ref. 15a) were synthesized
following the procedures reported in the literature. The mono-
mer M3 was synthesized as outlined in Scheme 1.
Based on these considerations, an ideal donor polymer to be
blended with PCBM in BHJ conguration must exhibit a low
band gap (between 1.2 and 1.9 eV), with the HOMO energy level
between ꢀ5.2 and ꢀ5.8 eV and the LUMO energy level near
1,2-Bis(octyloxy)benzene 1 was obtained by a standard
alkylation of catechol with bromooctane in acetone at reux.
Nitration of compound 1 affords 1,2-bis(octyloxy)-4,5-dinitro-
benzene 2 in high yield. Reduction of compound 2 was per-
ꢀ
3.8/ꢀ4.0 eV to ensure an efficient charge separation, maxi-
9
mizing the Voc
.
17
2
formed with Pd/C and H in ethanol at 2 atm. The resulting
Several low band gap polymers have been reported in the
1,2-bis(octyloxy)-4,5-diaminobenzene (unstable) was immedi-
recent literature; these mainly include homopolymers, donor–
ately treated with thionyl chloride and triethyl amine in
1d–g
acceptor polymers and quinoid polymers.
A record average
18
dichloromethane to yield benzothiadiazole 3 that was, in turn,
efficiency of 7.4% has been achieved in a single layer congu-
ration using copolymers based on benzodithiophene (PTB)
brominated with bromine in acetic acid and sodium acetate to
give dibromobenzothiadiazole 4 in high yield. Compound 4
was reacted with 2-tributylstannylthiophene by the Stille cross-
2 3 2
coupling, in reuxing toluene using PdCl (PPh ) as the cata-
lyst, obtaining compound 5 as a highly uorescent yellow solid.
Finally, the treatment of compound 5 with N-iodosuccinimide
19
derivatives blended with phenyl-C -butyric acid methyl ester
7
1
1
0
(
8
PC71BM) and the present record laboratory efficiency of over
11
.6% has been measured with a tandem device conguration.
The introduction of double bonds in the conjugated back-
bone of semiconducting polymers has been proposed as a
(NIS) in chloroform and acetic acid at room temperature gave
structural modication to reduce the band gap as in poly-
the monomer M3 as an orange solid in 98% yield.
2
a,b,12
(
(
phenylenevinylene)s (PPVs)
and poly(thienylenevinylene)s
0
0
0
0
0
0
0
Poly[5,5-(5 ,6 -di-octyloxy-4 ,7 -di-2-thienyl-2 ,1 ,3 -benzothia-
13
PTVs). The vinylene units increase the effective conjugation
0
0
diazole)-alt-1,2-vinylene] PDTBTV and poly[5,5-(5 ,6 -di-octy-
length by inducing planarization of the polymeric backbone,
thus decreasing the band gap and extending the absorption in
the low energy spectral region. In fact, the effect of double
bonds in donor polymers for BHJ solar cells has been investi-
gated especially as a bridge between donor and acceptor units in
0
0
0
0
0
loxy-4 ,7 -di-2-thienyl-2 ,1 ,3 -benzothiadiazole)-alt-1,2-diuor-
ovinylene] PDTBTFV were synthesized via the Stille cross-
coupling reaction between M3 and M1 or M2, respectively
(
3 4
Scheme 2). Polymerizations were performed with Pd(PPh )
as the catalyst, and CuI in a stoichiometric amount at room
temperature in a mixture of DMF–THF as the solvent, for 1
week. End capping with iodobenzene removes the tributyltin
terminal groups. The polymers were puried by washing the
crude products in a Soxhlet apparatus with hexane and
methanol. A nal extraction with chloroform yields the pure
polymers as dark-violet (PDTBTV) and dark-blue (PDTBTFV)
powders in good yields (70% and 75%, respectively). Both the
polymers are soluble in chlorinated solvents such as chloro-
form, chlorobenzene and ortho-dichlorobenzene (ODCB). The
average molecular weights were determined by conventional
size exclusion chromatography (SEC), with polystyrene narrow
standards calibration and chlorobenzene as the mobile phase,
and the results are shown in Table 1 together with optical and
electrochemical data which are discussed below.
14
donor–acceptor type low band gap polymers.
We have recently reported general synthetic methods for
arylenevinylene oligomers and polymers containing uorinated
15
double bonds as conjugation units. The spectroscopic and
1
5h–j
optoelectronic properties
of the resulting materials have
15g
been studied, also with the support of theoretical calculations,
in comparison with those of the non-uorinated counterparts.
The results have shown that the uorination of the double bond
can tune optoelectronic properties and solid state organization
and it can improve the material lifetime in devices, in compar-
15j
ison with the corresponding non-uorinated materials.
Starting from these results, here we extend our investigation
to the impact of the uorinated double bond on the photovol-
taic performances of donor arylenevinylene polymers in BHJ
solar cells. We have synthesized two soluble conjugated poly-
mers alternating the double bonds (either uorinated or non-
Solar cells characterization

uorinated) with bis-thienyl-bis-alkoxybenzothiadiazole units.
The results of the synthesis, spectroscopic and computational Polymer BHJ solar cells were fabricated using PDTBTV or
studies, electro-optical and morphological characterizations are PDTBTFV as the donor material and PCBM as the acceptor.
reported in this article, emphasizing the impact of the uori- Measurements were performed on nonoptimized devices.
nation of double bonds on the spectroscopic, solid state and
A screening of the donor/acceptor (polymer/PCBM) weight
photovoltaic properties of the resulting donor polymer in BHJ ratio was preliminarily carried out, and the results are
solar cells with PCBM as the acceptor. Our study demonstrates summarized in Table 2.
that the uorinated vinylene unit can represent a suitable
PDTBTV shows only limited variations in the photovoltaic
structural motif to increase the performances of donor parameters with the D/A ratio, always maintaining quite
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Scheme 1 Synthetic route for the monomer M3.
low Voc and power conversion efficiency (PCE). The best signicantly higher Voc and PCE values (0.73–0.83 V and
performances were measured with a 2 : 7 (D : A) weight ratio. 0.83–1.24%, respectively) compared to PDTBTV (Voc 0.43–0.56 V
PDTBTFV shows better photovoltaic performances with and PCE 0.33–0.53%, respectively) for all the measured weight
Scheme 2 Synthetic route for the polymers PDTBTV and PDTBTFV.
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Table 1 Molecular weights, spectroscopic and electrochemical data for PDTBTV and PDTBTFV polymers
PDTBTV
PDTBTFV
DEFluor
a
Molecular weight
/M
M
w
36 900
43 700
M
n
w
4.67
4.46
abs
Absorption maximum lmax
Solution: 569 nm
Film: 633 nm
Solution: 669 nm
Film: 746 nm
ꢀ5.01 eV
ꢀ3.03 eV
1.98 eV
Solution: 548 nm
Film: 594 nm
Solution: 648 nm
0.08 eV
0.13 eV
0.06 eV
0.07 eV
ꢀ0.19 eV
ꢀ0.11 eV
0.08 eV
0.10 eV
ꢀ0.16 eV
ꢀ0.09 eV
0.15 eV
0.06 eV
em b
Emission maximum lmax
c
Film: (714 nm) , 768 nm
d
HOMO (solid state)
ꢀ5.20 eV
ꢀ3.14 eV
2.06 eV
d
LUMO (solid state)
EC
g
Electrochemical bandgap E
opte
g
Optical bandgap E
1.69 eV
1.79 eV
f
HOMOCALC (in vacuo)
ꢀ4.82 eV
ꢀ2.99 eV
4.92 eV
ꢀ4.98 eV
ꢀ3.08 eV
5.07 eV
f
LUMOCALC (in vacuo)
f
Ionization potential IPCALC
f
Electron affinity EACALC
3.12 eV
3.18 eV
a
b
c
d
Determined by size exclusion chromatography. Emission spectra are shown in the ESI.† Shoulder. Highest (lowest) (un)occupied molecular
orbitals HOMO (LUMO) were estimated via Koopmans theorem from the experimental electrochemical oxidation and reduction potentials (Eox, Ered
)
e
f
reported against vacuum, under the premise that ferrocene has an Eox at ꢀ4.80 eV vs. vacuum. From the onset of absorption. From DFT oligomer
calculations (m ¼ 4), IP and EA are calculated in solvent (dichloromethane).
Table 2 Photovoltaic parameters of BHJ cells based on a blend of PDTBTV or
PDTBTFV/PCBM: open circuit voltage VOC, short circuit current ISC, ll factor FF,
and power conversion efficiency h
Polymer/PCBM
weight ratio
V
oc
I
sc
ꢀ2
Polymer
[V]
[mA cm
]
FF [%]
h [%]
PDTBTV
2 : 7
0.55
0.56
0.43
0.73
0.75
0.83
3.33
2.42
3.15
2.94
3.55
4.32
28.83
24.50
33.51
38.57
37.97
34.56
0.53
0.33
0.46
0.83
1.05
1.24
3
1
: 7
: 1
PDTBTFV
2 : 7
3
1
: 7
: 1
ratios. For PDTBTFV the most efficient devices were obtained by
mixing the donor and acceptor in a 1 : 1 weight ratio.
Fig. 1 J–V curves (solid symbols) and power voltage characteristics (open
symbols) of the polymeric solar cells obtained from the blend of polymer PDTBTV
Thermal annealing of the active layer in the temperature with PCBM at different weight ratios.
ꢁ
range 70–150 C does not signicantly affect the photovoltaic
performances of both polymers, in agreement with both the
lack of any transition in differential scanning calorimetry (DSC)
of donor polymer and of the almost negligible modication of
the blend morphology with the temperature.
J–V curves of the solar cells and power voltage characteristics
of the solar cells based on PDTBTV and PDTBTFV are shown in
Fig. 1 and 2, respectively.
Polymer properties in solution
MOLECULAR GEOMETRY. The calculated structure for model
oligomers of both polymers in the gas phase shows an anti-
arrangement of the thiophene rings against the thiadiazole ring
ꢀ
1
(
Fig. 3), stabilized against the syn-form by 10.2 kJ mol (0.106
eV), due to (non-bonding) S–O interactions similar to the case of
20
PEDOT.
The thiadiazole ring drives the conformation somewhat out
of co-planarity; however, packing forces are expected to over-
Fig. 2 J–V curves (solid symbols) and power voltage characteristics (open
symbols) of the polymeric solar cells obtained from the blend of polymer
come this barrier to effectively planarize the backbone in the PDTBTFV with PCBM at different weight ratios.
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This is fully supported by the comparison with the lm
absorption spectra, also given in Fig. 4, which resemble the
position of the aggregate shoulder in the solution spectra. From
the PLE spectra the absorption maxima of the dissolved chains
can be determined to be 548 nm (2.26 eV) for PDTBTFV and 569
nm (2.18 eV) for PDTBTV, resulting in a uorination-induced
blue-shi of 0.07 eV. Such blue-shis were observed before and
ascribed to the specic interplay of the mesomeric (+M) and
inductive (ꢀI) character of the uorine atom, which can each
dominate the effect depending on the nature of the back-
21,22
23
bone
and substituent position.
The shi agrees very well with the one calculated by TD-DFT
(Fig. 5), although the absolute values are underestimated both
by the B3LYP and CAM-B3LYP functional. This is due to the
inherent limitation of the TD-DFT calculations with standard
functionals, overestimating long-range interactions. Although
CAM-B3LYP clearly gives better results, the difference with
experiment (ꢀ0.2 eV) is still signicant.
Fig. 3 Chemical structure of PDTBTV and PDTBTFV, geometry of oligomers with
m ¼ 1, 4.
The extinction coefficient was estimated from absorption
measurements on solutions with dened concentration,
solid state. The alkoxy groups point almost perpendicular to the
plane of the conjugated backbone.
considering the molar mass of the repeating units (MPDTBTV
¼
OPTICAL SPECTRA IN SOLUTION. The absorption spectrum of
PDTBTFV in solution shows a broad, unstructured main
absorption band at around 550 nm, whereas the spectrum of
PDTBTV is somewhat red-shied (Fig. 4). The long wavelength
side shows a pronounced shoulder centered at 660 nm for
PDTBTFV and 690 nm for PDTBTV.
ꢀ
1
ꢀ1
580 g mol , MPDTBTFV ¼ 616 g mol ), which gives 3PDTBTV ¼ 2.1
5 ꢀ1 5 ꢀ1 ꢀ1
ꢀ1
ꢂ 10 M cm and 3PDTBTFV ¼ 3.4 ꢂ 10 M cm , respec-
tively. In any case, the unknown degree of aggregation makes an
exact determination difficult; from the quantum-chemical
In order to check whether the shoulder is an intramolecular
property (owing to an electronic state with small oscillator
strength) or an intermolecular property through aggregates, we
recorded a PLE spectrum, which allows for photo-selection if
the quantum yield of the single chain is considerably higher
than that of the aggregates. Indeed, the PLE spectra in Fig. 4 do
not show the shoulder, and thus they represent the absorption
features of the dissolved chain. Therefore, the shoulder is
assigned to some (minor) aggregation, which is still present
despite the low concentrations, longer dissolution time and use
of a good solvent (ODCB).
Fig. 5 TD-DFT calculated chain length dependence of the absorption maxima
for PDTBTFV (blue) and PDTBTV (red) oligomers (mDTBTFV, mDTBTV, with m ¼
number of repetition units), employing the CAM-/B3LYP functionals as a function
of 1/m, which relates to the number of double bonds N along the conjugated
24
path as m ¼ 7N. Experimental polymer values in solution are shown as squares;
2
5
1/2
solid lines are Kuhn fits for m $ 2 according to E ¼ a{1 + bcos(p/(N + 1))}
.
Fig. 4 Normalized absorbance (solid) and PL excitation (dashed) spectra of
PDTBTFV (red) and PDTBTV (blue) in ODCB solutions; thin film absorption spectra
Inset: TD-DFT (CAM-B3LYP) calculated absorption spectra of 4DTBTFV (blue) and
4DTBTV (red) oligomers, depicting the first 20 electronic states (broadened by a
Lorentzian, half-width 0.5 eV).
(
thickness ¼ 15 nm) are shown for comparison (dotted line).
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Fig. 6 MO topologies of the PDTBTFV oligomers (m ¼ 1, 4).
calculation (Fig. 5), PDTBTFV might have just a slightly higher uoro-functionalization of the double bond is an overall stabili-
extinction coefficient (ca. 3%). zation of both HOMO and LUMO energy levels; similar values for
According to the TD-DFT calculation the main absorption the frontier MOs as well as for oxidation potentials and electron
band is ascribed to the symmetry allowed S ) S transition, affinities were obtained from the DFT calculations, see Table 1.
1
0
opt
mainly described by a HOMO / LUMO excitation but showing
The optical band gap of the lms, Eg , extracted from the
some minor contributions of higher excitations. While the onset of the absorption, was found at 1.79 eV for PDTBTFV and
HOMO is rather delocalized over the molecular backbone 1.69 eV for PDTBTV, i.e. signicantly smaller than the electro-
(Fig. 6), the LUMO is strongly localized in the benzothiadiazole chemical gaps thus demonstrating that electronic relaxation is
unit due to its acceptor properties; these effects however depend considerable in these polymers. The absorption maximum,
strongly on the specic electronic properties of the donor and which is dominated by the HOMO–LUMO transition, was found
26
acceptor fragments.
at 2.06 eV for PDTBTFV and 1.98 eV for PDTBTV (see ellipso-
The admixing of higher excitation states is not observed for metric characterization below), respectively.
4
,28
the monomer (m ¼ 1). This change in the CI (conguration
In agreement with previous studies
on conjugated poly-
interaction) description is the obvious reason why the chain mers bearing uorine atoms on aromatic rings, a rst positive
length dependence of the main absorption cannot be tted with effect of uoro-functionalization on the photovoltaic perfor-
a 2-parameter Kuhn equation; the latter works both for PDTBTV mances of the BHJ organic solar cells can be attributed to the
and PDTBTFV onlyfor m $ 2 (Fig. 5). This result shows once more lowering of the HOMO energy level of the donor material,
that simple conclusions of polymer properties from monomer
calculations, as frequently done, are not always possible.
E
HOMO(D), resulting in a better matching with the LUMO energy
27
level of the acceptor, ELUMO(A), which leads to an enhancement
of the VOC via
29
Thin lm properties
V
OC ¼ EHOMO(D) ꢀ ELUMO(A) ꢀ 0.3
ELECTROCHEMICAL CHARACTERIZATION. The HOMO and LUMO
energy levels in thin lms of the two polymers were estimated by with ELUMO(PCBM) ¼ 3.7 eV, and the HOMO levels of the poly-
cyclic voltammetry(CV) andthe resultsaresummarizedinTable 1. mers in Table 1. This gives a VOC ¼ 1.2 V for PDTBTFV, while for
The CV measurements were carried out using 0.1 M tetrabuty- PDTBTV VOC ¼ 1.0 V, which already leads to an increase of the
lammoniumhexauorophosphate (TBAPF
6
) in dry acetonitrile. PCE by 20%. Furthermore, two previous investigations stressed
28b,c
Cyclic voltammograms of the PDTBTFV and PDTBTV thin lms the inuence of uorination on hole mobility.
Finally, as
are shown in Fig. S10 of the ESI.† Both polymers present two discussed below, we nd a signicant enhancement of thin lm
sequential irreversible oxidation peaks in the range from +1.0 light absorption in PDTBTFV compared to PDTBTV, which we
to +2.0 V and a reversible reduction process with peaks between held responsible for an additional improvement in device
ꢀ
1.0 and ꢀ1.5 V. In particular, anodic peaks relative to PDTBTV performance.
are at +1.27 V and +1.64 V, while those of PDTBTFV are at +1.67 V
ATOMIC FORCE MICROSCOPY CHARACTERIZATION. The AFM
and +1.86 V. Cathodic peaks are at ꢀ1.33 V, ꢀ1.23 V and ꢀ1.06 V characterization was performed on thin lms of the two poly-
forPDTBTVandat ꢀ1.21 V andꢀ1.13 V for PDTBTFV, respectively. mers and of their blends with PCBM, deposited in the same
TheestimatedHOMOenergylevels(basedonKoopmanstheorem) conditions used for photovoltaic device fabrication.
is ꢀ5.20 eV for PDTBTFV and ꢀ5.01 eV for PDTBTV, while the
For pure PDTBTFV the morphology consists of a distribution
LUMO energy level is ꢀ3.14 eV for PDTBTFV and ꢀ3.03 eV for of interconnected clusters of polymer grains (Fig. 7a and b). In
PDTBTV. The electrochemical energy gap E , calculated as the the blend with PCBM the roughness decreases from Zav ¼ 5.6
g
difference between the HOMO and LUMO energies, is 2.06 eV for nm, Rms ¼ 1.5 nm for the pure polymer to Zav ¼ 2.3 nm,
PDTBTFVand1.98eVforPDTBTV, respectively. Hencetheeffectof
R
ms ¼ 0.6 nm due to the homogeneous distribution of the
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Fig. 8 (a) Height and (b) phase images of a pure PDTBTV layer. (c–e) Height and
Fig. 7 (a) Height and (b) phase images of a pure PDTBTFV film. (c–e) Height and
(d–f) phase images of a PDTBTV/PCBM blend.
(
d–f) phase images of a PDTBTFV/PCBM blend.
PCBM spherical structures onto the previous layer (see arrows measure lms also directly in the device conguration. Spectra
in Fig. 7e and f). The corresponding pure PDTBTV layer is less of the complex refractive index, i.e., N ¼ n + ik, (where n is the
organized into clusters and more amorphous (Fig. 8a and b) in real part of the refractive index and k is the extinction coeffi-
comparison with PDTBTFV. Also in this case a partial lling cient) are simultaneously measured in a very broad spectral
process of the original PDTBTV voids by the PCBM spheres is range from 0.75 eV to 6.5 eV. The absorption coefficient a,
evidenced by a decrease of roughness from the pure polymer which is related to the extinction coefficient through a ¼ 4pk/l
(
Zav ¼ 6.0 nm, R ¼ 1.5 nm) to the corresponding blend with (l; wavelength of the light), is also determined.
ms
PCBM (Z ¼ 1.8 nm, R ¼ 0.5 nm).
Furthermore, it allows for discerning possible optical anisot-
av
ms
Fig. 8e–f show PCBM distributed onto PDTBTV. It is worth ropy of the polymer lms by measuring spectra at various angles
noting that these short range morphologies obtained to deter- of incidence of light and also by rotating in-plane the sample.
mine the intimate distribution of the two components in the
Fig. 9a shows the experimental spectra of the pseu-
blend layer are compatible with a previous study on the long doextinction coefficient (measured for the whole system
range (5 micron) morphology dependence on the layer thick-
30
ness. Morphology doesn’t seem to signicantly change with
annealing for both the uorinated and non-uorinated poly-
mers (results not shown here).
ELLIPSOMETRIC CHARACTERIZATION OF THE THIN FILMS. It is well
known that for thin lms of conjugated polymers deposited
onto substrates the macroscopic optical response is not only
dependent on the intrinsic electronic structure of the chain but
also on inter-molecular interactions determined by their rela-
tive orientation, position and aggregation, as well as on the
interactions with the substrate. A versatile technique to address
both optical properties and morphology of thin lms is reec-
Fig. 9 (a) Experimental ellipsometric spectra of the pseudoextinction coefficient,
for the non-fluorinated, PDTBTV (red), and fluorinated, PDTBTFV (blue), films of
similar thickness of approximately 15 nm, and (b) the derived optical spectra of
the extinction coefficient, k, and, in the inset, of the absorption coefficient, a. The
31
tion variable-angle spectroscopic ellipsometry. The advantage
of this technique for the analysis of the lms is mainly that it
does not require transparent substrates; it can thus be used to main absorption bands for both films are indicated.
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substrate + lm) in the photon energy range of 0.75–6.5 eV for

lms with an approximate thickness of 15 nm. Here, for conve-
ꢁ
nience only the spectra acquired at an angle of incidence of 70
are shown; however, varying the angle of incidence and in-plane
rotation did not show signicant variation (see Fig. S11 in the
ESI†), thus indicating that the lms (with the morphology shown
here) are isotropic. Five main bands can be identied which
correspond to different optical transitions. Therefore, the optical
response of the two polymers has been modelled by a dispersion
32,33
relationincluding anensemble of veoscillators.
The derived
spectra of the extinction coefficient, k, and of the absorption
coefficient, a, for both the non-uorinated (PDTBTV) and uo-
rinated (PDTBTFV) polymers lms are reported in Fig. 9b.
The pattern of the observed optical transitions are similar to
Fig. 10 Ellipsometric spectra of the extinction coefficient and, in the inset, of the
absorption coefficient for the non-fluorinated PDTBTV and fluorinated PDTBTFV
polymers blended with PCBM (polymer/PCBM ratio 1 : 1 for both blends). For
comparison, the spectrum of PCBM is also shown.
33,34
those reported for other conjugated polymers,
where the
prominent bands below 4 eV are described by p–p* type singlet
transitions, involving both localized and delocalized states,
oen with a complex conguration interaction contribution;
the high energy transition V originates from states associated
with p–p* transitions localized in the benzene rings.
show that the blend lms can be approximated as isotropic
homogeneous materials. The optical response of the blend has
The main absorptions are at 2.05 eV and 2.25 eV for PDTBTV
and PDTBTFV, respectively. The blue shi upon uorination of
the vinylene unit was also observed in structurally similar
39,40
also been modelled by ve oscillators.
As can be seen in
Fig. 10, the spectrum of the blend lms is a superimposition of
41
the spectra of each component. In the spectral region above 3
15b,f
compounds
and is consistent with the shis of the polymer
42
eV, the electronic transition originates mainly from PCBM,
chains in solution and with the theoretical calculations (vide
supra). In any case, the blue shi (0.2 eV) is larger than in
solution (0.1 eV), thus pointing to somewhat improved packing
whereas below 3 eV, where PCBM has no characteristic bands,
the transition originates from the respective conjugated polymer
43
with similar spectral positions as in the pristine materials.
25
of the chains in PDTBTFV. Further indication for different
packing arises from the extinction coefficient k (Fig. 9) which is
higher for PDTBTFV compared to PDTBTV all over the investi-
gated spectrum.
As in the pristine samples, the absorption coefficient of the
PDTBTFV-PCBM blend still remains higher than that of the
PDTBTV-PCBM blend in the region of the main optical transi-
tion of the polymer (see Fig. 10).
Thus, the increased light absorption of the uorinated
polymer blended with PCBM might be a main reason for the
enhanced photovoltaic performance of PDTBTFV compared to
PDTBTV, since higher light absorption implies larger exciton
formation rates, so that a higher number of free charge carriers
can be generated. Such an increase of light absorption in BHJ
solar cells also allows for the use of thinner lms to better
match the diffusion length of excitons in conjugated polymers.
In particular, for the application in BHJ devices, it can be
useful to compare k values of our polymers with some reference
materials, e.g. P3HT; in fact PDTBTV shows a k value of 0.9 at its
30
maximum, which is comparable to that of P3HT, whereas
PDTBTFV has a maximum k value of 1.25. Considering the
limited increase in the absorption coefficient of PDTBTFV with
respect to PDTBTV estimated from solution measurements as
well as in the TD-DFT results (i.e., an increase of 3%, see Fig. 5),
the remarkable enhancement of k cannot be attributed to e.g.
increased D–A intramolecular charge transfer interaction in the
polymer backbone, as in previously reported donor–acceptor
Experimental
35–37
copolymers.
Instead, it must be due to interchain effects of Materials
both polymers in the lm. In fact, double bond uorination can
All chemicals were purchased from Aldrich, Alfa Aesar or Acros
and used without further purication. SOCl2 was freshly
distilled immediately prior to use. THF was distilled from
sodium and benzophenone under a nitrogen atmosphere.
be responsible for the increased intermolecular interactions
38
between almost coplanar adjacent polymer chains which,
depending on the nature of the chains and interactions, can
result in the observed increased absorption coefficient. Addi-
tionally, while the PDTBTV lm shows a sharper absorption
edge, the PDTBTFV lm even at lower energies has a higher
absorption coefficient (see the inset of Fig. 9), e.g. at an energy of
CH
2
Cl
2
and CHCl
3
were distilled from P
2
O
5
under a nitrogen
˚
atmosphere. DMF and Et
3
N were distilled from 4 A molecular
sieves under a nitrogen atmosphere. All solvents were freshly
distilled immediately prior to use.
1
2
.5 eV it is approximately one order of magnitude higher, being
3
ꢀ1
4
ꢀ1
.5 ꢂ 10 cm for PDTBTV and 2.7 ꢂ 10 cm for PDTBTFV.
General methods
The ellipsometric analysis has also been carried out on the
polymer blend used for the BHJ solar cells. Similarly to the pure Column chromatography was performed using silica gel 60,
polymer lms, ellipsometric measurements performed at 40–63 mm from Merck. Merck Silica gel 60 F254 aluminium
various angles of incidence as well as the AFM morphologies sheets were used for analytical TLC. Petroleum ether (boiling
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ꢁ
range 40–60 C) was used in the chromatography. FT-IR spectra pressure, the residue was diluted with diethyl ether (50 mL) and
were measured on a Perkin-Elmer 1710 spectrophotometer using stirred for 30 min with an aqueous KF (40%) solution (20 mL).
1
13
dry KBr pellets. H and C NMR spectra were recorded in CDCl3 The organic layer was separated from the aqueous medium and
on a Bruker AM 500 spectrometer at 500 MHz and 125 MHz, dried over anhydrous Na SO . Aer evaporation of the solvent at
2
4
respectively. The residual CHCl
3
signal at d ¼ 7.24 ppm and the reduced pressure, the crude product was puried by column
CDCl
3
signal at d ¼ 77.0 ppm were used as the standards for chromatography using petroleum ether–dichloromethane (1/1)
1
13
H NMR and C NMR spectra, respectively. MS spectra were as the eluent, to yield the pure product 5 as a yellow-red solid
ꢁ
1
recorded on
a
Shimadzu GCMS-QP 5000 spectrometer. (1.80 g, yield 90%). M
p
¼ 75–76 C. H NMR (500 MHz, CDCl
3
,
Elemental analyses were done by a Carlo Erba CHNS-O EA1108- d): 8.48 (dd, J ¼ 1.2, 5.2, 2H), 7.51 (dd, J ¼ 4.0, 5.2, 2H), 7.24 (dd,
Elemental Analyzer. Melting points were determined on a J ¼ 1.2, 4.0, 2H), 4.11 (t, J ¼ 7.2, 4H), 1.93 (quint, J ¼ 7.2, 4H),
Gallenkamp capillary melting point apparatus. UV-vis spectra 1.45 (quint, J ¼ 7.2, 4H), 1.40–1.24 (m, 16H), 0.90 (t, J ¼ 7.2, 6H).
1
3
were recorded on a Shimadzu UV-2401 PC spectrophotometer.
C NMR (125 MHz, CDCl , d): 151.90, 150.94, 134.07, 130.51,
3
Molecular weights were determined by SEC with a Hewlett- 127.26, 126.70, 117.57, 74.32, 31.79, 30.31, 29.48, 29.25, 25.93,
+
Packard HP 1050 liquid chromatograph instrument equipped 22.64, 14.09. EIMS (m/z (%)): 556 (M , 35), 447 (28), 332 (100),
with a Plgel 5 m Mixed-D300 ꢂ 7.5 mm column, using chloro- 303 (12). FT-IR (KBr): n ¼ 2926 (s), 2856 (m), 1463 (m), 1416 (m),
ꢀ
1
benzene (CB) as the eluent and polystyrene standard 1379 (m), 1279 (s), 1027 (m), 695 (s) cm . Anal. calcd for
calibration.
C
7
30
H
40
N
2
O
2
S
3
: C, 64.71; H, 7.24; S, 17.27. Found C, 64.75; H,
.22; S, 17.30%.
SYNTHESIS OF 4,7-DI-(5 -IODOTHIOPHEN-2 -YL)-5,6-DI-OCTY-
LOXY-2,1,3-BENZOTHIADIAZOLE (M3). A solution of 5 (1.80 g, 3.2
0
0
Synthetic procedures
SYNTHESIS OF 5,6-DI-OCTYLOXY-2,1,3-BENZOTHIADIAZOLE (3). mmol) in dry CHCl
3
(30 mL) and acetic acid (15 mL), prepared
ꢁ
The reaction was carried out following the procedure reported under a nitrogen atmosphere, was cooled at 0 C, then NIS (1.89
18
in the literature and the crude material was puried by g, 8.4 mmol) was added, and the resulting mixture was warmed
column chromatography using petroleum ether–ethyl acetate at room temperature and stirred overnight in the dark. The
(
96/4) as the eluent, followed by crystallization from ethanol, to reaction mixture was neutralized by aqueous NaOH (10%) and
yield the pure product 3 as a white solid (yield 85%). M
¼ 88–89 extracted by CHCl . The organic layer was dried over anhydrous
, d): 7.13 (s, 2H), 4.09 (t, J ¼ 6.3, Na SO and the solvent was removed under reduced pressure.
H), 1.91 (quint, J ¼ 6.3, 4H), 1.52 (quint, J ¼ 7.5, 4H), 1.42–1.24 The crude residue was puried by column chromatography
p
3
ꢁ
1
C. H NMR (500 MHz, CDCl
3
2
4
4
1
3
(
1
m, 16H), 0.89 (t, J ¼ 7.0, 6H). C NMR (125 MHz, CDCl
3
, d): using petroleum ether–dichloromethane (85/15) as the eluent,
54.10, 151.36, 98.35, 69.10, 31.78, 29.30, 29.24, 28.71, 25.99, followed by crystallization from MeOH, to yield the pure
+
2
2.66, 14.10. EIMS (m/z (%)): 392 (M , 18), 280 (16), 168 (100). product M3 as an orange solid (2.56 g, yield 98%). M
p
¼ 79–80
, d): 8.24 (d, J ¼ 4.0, 2H), 7.36 (d,
S: J ¼ 4.0, 2H), 4.12 (t, J ¼ 7.2, 4H), 1.93 (quint, J ¼ 7.2, 4H), 1.46
ꢁ
1
FT-IR (KBr): n ¼ 2918 (s), 2848 (m), 1499 (m), 1466 (m), 1311 (s),
C. H NMR (500 MHz, CDCl
3
ꢀ
1
1
36 2 2
217 (m), 1195 (m), 852 (m) cm . Anal. calcd for C22H N O
1
3
C, 67.30; H, 9.24; S, 8.17. Found C, 67.36; H, 9.20; S, 8.20%.
(quint, J ¼ 7.2, 4H), 1.40–1.24 (m, 16H), 0.90 (t, J ¼ 7.2, 6H).
C
SYNTHESIS OF 4,7-DIBROMO-5,6-DI-OCTYLOXY-2,1,3-BENZO- NMR (125 MHz, CDCl
, d): 151.62, 150.33, 140.17, 136.73,
3
THIADIAZOLE (4). To a mixture of 3 (4.0 g, 10.2 mmol) and 132.03, 117.06, 77.93, 74.54, 31.81, 30.25, 29.44, 29.27, 25.92,
sodium acetate (0.38 g, 4.6 mmol) in acetic acid (50 mL), 22.68, 14.13. FT-IR (KBr): n ¼ 2916 (s), 2849 (m), 1462 (w), 1416
ꢀ1
bromine (1.6 mL, 31 mmol) was slowly added, and the resulting (s), 1377 (m), 1285 (s), 1023 (m), 954 (w), 796 (m) cm . Anal.
38 2 2 2 3
mixture was stirred for 48 h at room temperature. The precipi- calcd for C30H I N O S : C, 44.56; H, 4.74; S, 11.90. Found C,
tate was ltered and washed with methanol. A recrystallization 44.58; H, 4.75; S, 11.98%.
from methanol yielded the pure product 4 as a white solid (5.1 g,
ꢁ
1
yield 91%). M
p
¼ 45–46 C. H NMR (500 MHz, CDCl
3
, d): 4.16 (t,
Synthesis of polymers
General procedure
J ¼ 6.8, 4H), 1.88 (quint, J ¼ 6.8, 4H), 1.53 (quint, J ¼ 7.5, 4H),
1
3
1
.44–1.24 (m, 16H), 0.89 (t, J ¼ 7.0, 6H). C NMR (125 MHz,
CDCl , d): 154.47, 150.34, 106.25, 75.13, 31.81, 30.24, 29.37, Poly[5,5-(5 ,6 -di-octyloxy-4 ,7 -di-2-thienyl-2 ,1 ,3 -benzothiadia-
9.25, 25.96, 22.64, 14.09. EIMS (m/z (%)): 552 (M + 2, 2), 550 zole)-alt-1,2-diuorovinylene] (PDTBTFV): M3 (2.0 g, 2.47 mmol)
0
0
0
0
0
0
0
3
2
+
(
(
(
M , 4), 548 (M ꢀ 2, 2), 440 (3), 438 (6), 436 (3), 328 (50), 326 and (E)-(1,2-diuoro-1,2-ethenediyl)bis[tributylstannane] M2
100), 324 (50). FT-IR (KBr): n ¼ 2921 (s), 2850 (m), 1469 (w), 1431 (1.59 g, 2.47 mmol) were dissolved under a nitrogen atmosphere
ꢀ
1
w), 1385 (w), 1285 (s), 1058 (w), 1028 (w), 944 (w) cm . Anal. in a mixture of dry THF (20 mL) and dry DMF (20 mL). Then
calcd for C22
8.00; H, 6.20; S, 5.85%.
5,6-DI-OCTYLOXY-4,7-DI-2 -THIENYL-2,1,3- ture for 6 days. Iodobenzene (0.52 g, 2.47 mmol) was added for
BENZOTHIADIAZOLE (5). To a solution of 4 (2.0 g, 3.6 mmol) and end-capping and the mixture stirred for another day. The
-tributylstannylthiophene (3.36 g, 9.0 mmol) in dry toluene reaction mixture was poured into methanol (300 mL) and the
40 mL), PdCl (PPh ) (50 mg, 0.07 mmol) was added and the crude polymer was allowed to precipitate. The crude polymer
H34Br
2
N
2
O
2
S: C, 48.01; H, 6.23; S, 5.83. Found C, Pd(PPh
3 4
) (0.14 g, 0.12 mmol) and CuI (0.5 g, 2.47 mmol) were
4
added and the resulting mixture was stirred at room tempera-
0
SYNTHESIS
OF
2
(
2
3 2
resulting mixture was reuxed for 24 h under a nitrogen was ltered and puried by Soxhlet extraction using hexane (24
atmosphere. Aer removal of the solvent under reduced h), methanol (24 h) and chloroform (12 h). The chloroform
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fraction was concentrated at about 30 mL and precipitated in Computational details
methanol (200 mL). Filtration and drying in vacuum yielded
Ground state geometries of the oligomers (in vacuo) with one to
four repetition units (m) were calculated by density functional
theory (DFT), without imposing symmetry. All calculations were
done employing the standard B3LYP functional and 6-311G*
ꢁ
PDTBTFV as a dark blue solid (1.13 g, yield 75%). M > 350 C.
p
1
H NMR (500 MHz, CDCl , d): 8.80–8.20 (bs, 2H), 7.24–6.80 (bs,
3
2
1
1
6
H), 4.40–4.00 (bs, 4H), 2.30–1.90 (bs, 4H), 1.80–1.15 (bs, 20H),
.14–0.70 (m, 6H). FT-IR (KBr): n ¼ 2921 (s), 1260 (w), 1097 (m),
44
basis set as dened in the Gaussian09 program package.
ꢀ
1
40 2 2 2 3
022 (m), 800 (m) cm . Anal. calcd for C32H F N O S : C,
Adiabatic electron affinities and ionization potentials were
calculated by optimization of the radical anions and cations,
taking into account the solvent (dichloromethane) with the
polarizable continuum model (PCM) as implemented in
Gaussian09. Time-dependent (TD) calculations based on the
DFT geometries were done by the TD-DFT methods (in vacuo),
employing both the B3LYP functional as well as the coulomb-
attenuating method (CAM-B3LYP) to correctly account for long-
range interactions. Polymer extrapolations were done using the
2.10; H, 6.51; S, 15.54. Found C, 62.31; H, 6.33; S, 15.36%.
0
0
0
0
0
0
0
Poly[5,5-(5 ,6 -di-octyloxy-4 ,7 -di-2-thienyl-2 ,1 ,3 -benzothia-
diazole)-alt-1,2-vinylene] (PDTBTV) was synthesized according
to the general procedure starting from the monomers M3 and
M1. Aer purication the polymer PDTBTV was obtained as a
ꢁ
1
dark violet solid (yield 70%). M
CDCl , d): 8.80–8.00 (bs, 2H), 7.40–6.60 (bs, 2H), 4.40–3.80 (bs,
H), 2.40–1.80 (bs, 4H), 1.80–1.10 (bs, 20H), 1.10–0.60 (bs, 6H).
p
> 350 C. H NMR (500 MHz,
3
4
FT-IR (KBr): n ¼ 2917 (s), 1278 (s), 1221 (m), 1170 (m) 1019 (s),
25
Kuhn approach as described elsewhere.
ꢀ
1
9
28 (m.) 797 (s) cm . Anal. calcd for C32
42 2 2 3
H N O S : C, 65.94; H,
7.26; S, 16.50. Found C, 65.60; H, 7.02; S, 16.80%.
Cyclic voltammetry
Cyclic voltammetry (CV) measurements were carried out under
inert nitrogen atmosphere with an AUTOLAB potentiostat
Device fabrication
Bulk heterojunction solar cells were fabricated in a nitrogen PGSTAT 10 using a classical three-electrode cell. The CV
protected atmosphere following a conventional architecture, by measurements were carried out in CH CN solutions of TBAPF
3
6
ꢀ
1
spin coating a thin layer of poly(3,4-ethylenedioxythiphene)- (0.10 M) at a scan rate of 100 mV s . An indium-tin oxide (ITO)
poly(styrenesulfonate) (PEDOT-PSS Clevios P, VP AI4083) on an electrode coated with a thin lm of the relevant polymer was used
ꢀ
1
+
ITO/glass substrate (Kintec ꢃ10 U cm ), previously patterned as the working electrode. A Pt wire and an Ag/Ag electrode were
with wet-etching in hydrobromic acid and cleaned in an ultra- utilized as the counter electrode and pseudo-reference electrode,
sonic bath with acetone and isopropyl alcohol (10 min each respectively. All measurements were calibrated against ferro-
45
step). The active layer was deposited by spin coating a blend of cene, the ionization potential of which is ꢀ4.80 eV. The evalu-
polymer and PC61BM dissolved in ortho-dichlorobenzene ation of the HOMO and LUMO levels of the polymers was carried
ODCB) (13 mg mL ). Finally, an aluminum layer (100 nm) was out by measuring the onset potential of the redox processes.
ꢀ
1
(
evaporated on top as a metal cathode.
Devices were characterized in air under a solar simulator Atomic force microscopy
ꢀ
2
KHS Solar Constant 1200 AM 1.5 Class B (100 mV cm ) with a
Morphological characterization has been carried out with
commercial AFM NT-MDT in non-contact force modulation
mode. Both height and phase images have been contemporarily
acquired. The typical parameters used are 50–60% of the orig-
inal modulation amplitude as a reference signal at the canti-
lever resonance.
parameter analyzer (Agilent E5291A). For the measurements, a 5
2
ꢂ 5 mm (as the active area of the device) shadow mask was
used to illuminate one cell at a time. The irradiation level was
veried at quote and position where the solar cell was placed by
means of a calibrate Pyranometer (Skye model SKS 1110).
Optical measurements
Spectroscopic ellipsometry
The concentrations in ortho-dichlorobenzene (ODCB; Sigma The optical response of lms was investigated by spectroscopic
1
5i,j,46
Aldrich) solution were 4.957 mM for PDTBTV and 2.042 mM for ellipsometry (SE).
Measurements were carried out at
ꢁ
PDTBTFV. Spin coated lms of 15 nm thickness were deposited various angles of incidence in the range of 55–75 to investigate
on quartz substrates.
anisotropy of optical transitions using a phase-modulated
The absorption spectra of solutions (standard quartz cells) spectroscopic ellipsometer (UVISEL, JobinYvon) in the energy
and lms were recorded in a Cary 50 CONC (Varian) spec- photon range of 0.75–6.5 eV with a resolution of 0.05 eV.
trometer. Photoluminescence (PL)/-excitation (PLE) spectra
Ellipsometry is a technique that measures the change in
were recorded in a Fuoromax-4 (Horiba JobinYvon), equipped polarization of light as a function of incident angle and wave-
with a Xe high pressure lamp and a red-sensitive photo- length. The measured parameters are J and D, which are related
multiplier tube (Hamamatsu R928). PL spectra were recorded to the reection coefficient, r, of the sample for p and s polarized
aer excitation of samples at 600 nm (PDTBTV) and 550 nm light, and ultimately to the sample optical properties by
ꢀ
ꢀ
(
7
PDTBTFV). PLE were recorded at a uorescence wavelength of
50 nm for both polymers (10 averaged scans). PL and PLE
ꢀ
ꢀ
r
p
iD
r ¼
¼ tanJ$e
jr
s
j
spectra were corrected for the characteristics of the detection
and excitation parts, respectively.
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"
#
2
2
Cajal Research fellow of the MINECO. A.A. is grateful for an
Amarout grant of the EC. Dr Antonio Cardone acknowledges the
ð1 ꢀ rÞ
ð1 þ rÞ
2
2
2
N ¼ ðn þ ikÞ ¼ sinf 1 þ tan f

nancial support from CNR by “Progetto EFOR”.
where N is the complex refractive index, and f is the angle of
incidence.
Notes and references
Because of its isotropy, the spectra of polymer lms were
modelled with a simple two layer model, i.e., substrate/lm/
surface roughness to extract the optical constants, namely the
refractive index, n, and the extinction coefficient, k, and
absorption coefficient, a, of the lms. The lm was described by
an ensemble of Lorentzian oscillators, as reported in detail in
the text. Table S1† reports the ellipsometric t parameters with
the corresponding errors, for the oscillators associated with the
main optical absorption bands.
1 (a) B. C. Thompson and J. M. J. Fr ´e chet, Angew. Chem., Int.
Ed., 2008, 47, 58; (b) S. Gnes, H. Neugebauer and
N. S. Saricici, Chem. Rev., 2007, 107, 1324; (c) A. Facchetti,
Chem. Mater., 2011, 23, 733; (d) H. Zhou, L. Yang and
W. You, Macromolecules, 2012, 45, 607; (e) J. Kirkpatrick,
C. B. Nielsen, W. Zhang, H. Bronstein, R. S. Ashraf,
M. Heeney and I. McCulloch, Adv. Energy Mater., 2012, 2,
260; (f) C. B. Nielsen, R. S. Ashraf, B. C. Schroeder,
P. D’Angelo, S. E. Watkins, K. Song, T. D. Anthopoulos and
I. McCulloch, Chem. Commun., 2012, 48, 5832; (g) Z. Fei,
R. S. Ashraf, Z. Huang, J. Smith, R. J. Kline, P. D’Angelo,
T. D. Anthopoulos, J. R. Durrant, I. McCulloch and
M. Heeney, Chem. Commun., 2012, 48, 2955.
Conclusions
In summary, we have reported the synthesis and properties of
PDTBTFV as the rst arylenevinylene polymer for BHJ with

uorinated double bonds. Its features and photovoltaic
2 (a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789; (b) J. J. M. Halls, C. A. Walsh,
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performances have been studied in comparison with those of its
non-uorinated counterpart, PDTBTV, in order to evaluate the
impact of the double bond uorination on properties of donor
polymeric semiconductors in BHJ polymer solar cells.
Combined spectroscopic and computational investigations,
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performances of the two polymers in blends with PCBM in BHJ
solar cells reveal a favourable effect of uorinated double bonds
which remarkably enhance the solar cells performance. This
result is attributed to a synergetic effect of double bond uori-
nation which (i) lowers the frontier MOs leading to an increase
of the VOC and (ii) increases the molar extinction coefficient by a
combined intra- and interchain effect. This latter might addi-
tionally allow reduction of BHJ lm thickness, and thus
enhances charge carrier generation at the interface and easy
collection of the carriers at the electrodes against their tendency
to recombination.
Experiments are in progress in order to improve the PCE
values of BHJ based on the uorinated polymer PDTBTFV as
donor materials via different approaches: introduction of the
LiF interlayer between the aluminium cathode and the organic
blend; use of processing additives in the active layer thin lm
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dell’Universit `a e della Ricerca (MIUR), Universit `a degli Studi di
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