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Cite this: Chem. Commun., 2013,
49, 4322
Received 31st October 2012,
Accepted 27th December 2012
DOI: 10.1039/c2cc37909k
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Orthogonal self-assembly and selective solvent vapour
annealing: simplified processing of a photovoltaic
blend†‡
Giovanna De Luca,ab Andrea Liscio,c Glauco Battagliarin,d Long Chen,d
` Scolaro,e Klaus Mu
Luigi Monsu
¨ llen,d Paolo Samorı`*f and Vincenzo Palermo*c
www.rsc.org/chemcomm
Selective solvent vapour annealing is used on a photovoltaic blend
to enhance the interaction between the electron acceptor and the
electron donor, simplifying thin films post-processing for photovoltaic applications. A remarkable improvement in the interfacial
charge transfer in the bulk hetero-junction is attained, as measured
by Kelvin Probe Force Microscopy.
A solution-based approach to the fabrication of organic electronic
devices is a key step toward high efficiency and low cost when
aiming at large-scale production.1 Organic semiconductors acting
as active materials in these devices are often chosen among
conjugated discotic molecules because of their extraordinary
opto-electronic features.2 The inherent complexity of organic
semiconductor thin films poses a major challenge to the control
of processing–structure–function relationships and to fine tuning
of device properties.3 For instance, a high degree of order at
various length scales needs to be reached to attain high performance of the deposited functional architectures.4 To this end,
both solution processability of the organic materials and solutioncasting protocols have to be optimized to control the interactions
between the active components, ideally down to the molecular
level. In this framework, supramolecular self-assembly is one of
the most diffused methods to drive the formation of new
a
` di
Dipartimento di Scienze del Farmaco e Prodotti per la Salute, Universita
Messina, V.le Annunziata, 98168 Messina, Italy
b
Istituto per i Materiali Compositi e Biomedici, Consiglio Nazionale delle Ricerche,
P.le Tecchio 80, 80125 Napoli, Italy
c
`, Consiglio Nazionale delle
Istituto per la Sintesi Organica e la Fotoreattivita
Ricerche, via Gobetti 101, 40129 Bologna, Italy.
E-mail: vincenzo.palermo@isof.cnr.it
d
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany
e
` di Messina,
Dipartimento di Scienze Chimiche, and C.I.R.C.M.S.B., Universita
V.le Stagno d’Alcontres 31, 98166 Vill. S. Agata, Messina, Italy
f
ISIS & icFRC, Universite´ de Strasbourg & CNRS, 8 alle´e Gaspard Monge,
67000 Strasbourg, France. E-mail: samori@unistra.fr
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed
issue.
‡ Electronic supplementary information (ESI) available: UV/vis spectra, AFM data
analysis and experimental details. See DOI: 10.1039/c2cc37909k
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Chem. Commun., 2013, 49, 4322--4324
functional structures.5 Orthogonal non-covalent interactions were
exploited to lead to high structural complexity of organic electronic
devices both at the molecular level,6 and at the processing
and post-processing stages.7 Here, orthogonal self-assembly is
employed to drive the interactions between complementary
species in ultra-thin films of a photovoltaic blend. This approach
allows coupling of the simplicity of an all-in-one deposition step
with the tailored modification of individual blend components, to
optimize the degree of order of the deposited materials. A selective
solvent-vapour annealing (SVA) post-treatment relying on the use
of orthogonal solvents is exploited to attain the targeted reorganization of the active components of the blend. Clearly, the choice of
the solvent to be employed in the annealing has to be made by
considering carefully the difference in the chemical nature of the
molecules under investigation. Morphological and electronic
properties of the films can be studied with a nanoscale resolution
by Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM),8 respectively. In particular, KPFM allows quantitative mapping of the surface potential of nano-objects with a lateral
and potential resolution below 30 nm and 10 mV, respectively. It
thus enables real-time exploration of the photovoltaic effect, as
demonstrated on continuous photovoltaic films with thicknesses of 50–200 nm,9 as well as on isolated acceptor–donor
nano-structures.10a,b Two semi-conducting systems, a perylene
bis(dicarboximide) derivative and a five-ring pentacene analogue
with four symmetrically fused thiophene rings (Fig. 1a), were
mixed in a 1 : 1 ratio for use as an electron acceptor–donor blend.
The chosen perylene acceptor (PEG–PDI)11 bears oligomeric PEG
side chains conferring to this molecule an amphiphilic character,
while the donor counterpart (DTBDT-C6)12 is extremely apolar. In
this way the casting process is simpler, since both functional
components can be processed in the same solution. Chloroform
(CHCl3) has been identified as suitable solvent both for the acceptor
and the donor. In fact, if blended in CHCl3 at a relatively high
dilution (B10 5 M), these two species can be still regarded as
isolated based on spectroscopic evidence (see ESI,‡ Fig. S1).
A phase separation on the nanoscale, needed to maximize
exciton splitting,13 is then desirable during or after blend
casting. To improve the morphology a selective SVA, which
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Fig. 1 (a) Chemical structures of a PEG–PDI acceptor, a perylene
bis(dicarboximide) derivative (top), and a DTBDT-C6 donor, a five-ring-fused
thiophene-based pentacene analogue (bottom). (b–e) AFM topography images
of PEG–PDI (b, c) and DTBDT-C6 (d, e) thin films before (b, d) and after (c, e) SVA in
methanol. Z-range: 20 nm.
affects only one component of the blend by exploiting the
different polarity of the two species, may also be carried out
on the deposited materials. For selective SVA we used methanol
(MeOH) as solvent: due to its high polarity, it interacts substantially only with the more polar component (vide infra).
When a PEG–PDI solution is spun on silicon (SiOx), the
molecules organize in patches of hundreds of nm size having
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quite jagged borders (Fig. 1b), seemingly as a result of the
fracture of a monolayered structure during solvent evaporation.
The patches display a uniform height of 2.0 0.4 nm as well as
a uniform root-mean square roughness (RRMS) amounting to
0.2 nm estimated on 1 mm2 images. Both values agree with
those observed on layered self-assembled architectures of PDI
derivatives bearing dove-tail substituents at the N atoms.14 The
morphology of PEG–PDI thin films is modified drastically upon
SVA in MeOH, with the formation of a very regular multilayered structure with well-defined steps (Fig. 1c). The thickness of the first layer is similar to that observed before SVA
(1.8 0.2 nm), while the layers above are thicker (2.9 0.2 nm)
(Fig. S2, ESI‡). This evidence suggests a Stranski–Krastanov
growth mechanism for the self-assembled structures, with
bottom layers and overlayers characterized by a different azimuthal orientation with respect to the basal plane. The
observed change in the degree of molecular organization can
be ascribed to the condensation of solvent molecules on the
film surface during the SVA. The adsorbed MeOH will strongly
interact with the oligo-PEG chains of the PDI, largely increasing
the mobility of the acceptor molecules on the surface. Then, the
patched monolayer, which formed during spin-coating under
kinetic control due to fast solvent evaporation, can turn into the
more stable multilayered morphology obtained after annealing.
When a CHCl3 solution of DTBDT-C6 is spin-coated on SiOx
substrates the molecules organize into elongated nano-objects
generally displaying heights between 10–20 nm, widths around
200 nm and lengths up to 2 mm with a surface coverage of 30 10% (Fig. 1d). Their sharp edges suggest that they have crystalline nature and, as easily anticipated, the SVA treatment in
MeOH does not affect markedly the morphology of the selfassembled nano-crystals of the apolar DTBDT-C6 molecules.
Having studied the behaviour of single components, we
moved to spin-coating equimolar acceptor–donor blends onto SiOx
Fig. 2 AFM topography (a, e) and the corresponding KPFM (b, c, f, g) images of thin films of equimolar donor–acceptor blends, before (a–c) and after (e–g) SVA in
methanol (DTBDT crystals are marked with white lines). The KPFM measurements were carried out in the dark (b, f) or when illuminated with white light (c, g). (d, h)
Line profiles measured across the arbitrary lines drawn in the corresponding KPFM images. Z-range: (a, e) 20 nm; (b, c) 100 mV; (f, g) 200 mV.
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Chem. Commun., 2013, 49, 4322--4324
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substrates. Indeed, the morphology observed in AFM topographical
images of the latter samples (Fig. 2a) may be described as the
simple superposition of the two components: elongated nanocrystals of the apolar DTBDT-C6 are surrounded by a patched, thin
layer of the polar PEG–PDI (e.g. no. 4 and 2, respectively, in Fig. 2a).
Crystal sizes, film thickness, as well as roughness values are similar
to those measured on the isolated materials, indicating that the
increase in concentration of the two blend components, due to
solvent evaporation at the spin-coating step, does not foster their
interaction. Rather, the two species will tend to phase-segregate,
without affecting each other’s behaviour during their self-assembly
processes. This is further confirmed by subjecting the blend thin
films to SVA in MeOH, which causes noticeable changes only in the
organization of the most polar component (Fig. 2e): PEG–PDI
monolayers reorganize into regular, multi-layered structures overlapping the pre-existing DTBDT-C6 nano-crystals. The charge transfer improvement at the donor–acceptor interface has been
monitored using KPFM. The blend under investigation can be
described as a vertical bulk hetero-junction. The bulk sensitivity of
KPFM15 allows mapping of the surface potential variation induced
by illumination directly at the 2D region where acceptor and donor
materials are in contact, up to a depth of about 100 nm. In general,
the measured potential is given by the vertical average of the
potential at the interface and the charge density redistribution
induced inside the material. Measurements were performed both
on samples illuminated with a white light and under dark, gaining
new insight into the photovoltaic activity of the supramolecular
architectures. Representative AFM and KPFM measurements are
shown in Fig. 2, and the photo-induced variation of the surface
potential (SPV) is measured before and after SVA in methanol. An
increase in potential contrast is achieved upon illumination, due to
exciton splitting and charge generation at the acceptor–donor
interface.10a The largest SPV (ca. 70 mV) corresponds to zones
where the two components are physically in contact (no. 3 in Fig. 2c
and d), i.e. where the vertical hetero-junction is formed. Accordingly, the SPV is smaller where the two components are not
properly in contact, or physically separated (e.g. no. 2 and 4 in
Fig. 2c and d). In these cases, the exciton splitting is prevented, and
the total charge does not vary in the measurement timescale (ms)
as the excitons lifetime lies on the ns scale. Besides the abovedescribed changes in the morphology of the blend thin films, the
SVA causes also a large increase in the SPV values measured at a
vertical hetero-junction (e.g. no. 3 in Fig. 2f–h). Here the maximum
SPV amounts to 170 mV, with an increase in ca. 140% with respect
to the pristine blend. In all the cases, the achieved SPVs weakly
depend on the height of the donor assemblies, suggesting that the
Debye length is larger than 20 nm. This evidence hints to an
improvement in the interfacial contact between donor and acceptor
phases, as well as the reorganization toward a higher degree of
order for the PEG–PDI phase, and the possible reduction in the
defect density. It is thus evident that mastering the balance of
intermolecular interactions through a careful design of the active
species may both simplify the fabrication of photovoltaic devices,
and lead to improved performance. In our hands, orthogonal
self-assembly was successfully exploited to modify at the nanoscale
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Chem. Commun., 2013, 49, 4322--4324
Communication
the arrangement of just one of the two phases of a complex
photovoltaic blend, optimizing the degree of order for the acceptor,
without affecting the ordered structures already formed spontaneously by the donor. A noticeable improvement at the interface in
the bulk hetero-junction, and/or a reduction in the defect density,
was attained due to better contact between the two phases and/or
increased order. Thermal vacuum annealing is surely effective
in improving material performance, but the use of vacuum is not
cost-effective for large scale, low cost production of materials. Thus
alternative, simpler and more tunable techniques are technologically interesting for surface treatments in organic photovoltaic
cells production. Plastic and food industries already use large
chambers for vapor treatment and annealing, confirming the
viability of this technique for large scale, industrial production.
This work was supported by the following projects:
ERC-SUPRAFUNCTION (GA-257305), EC Marie-Curie ITN
SUPERIOR (PITN-GA-2009-238177), the ESF-EUROCORES-EuroGRAPHENE-GOSPEL, and the International Center for Frontier
Research in Chemistry (icFRC).
Notes and references
1 E. Menard, M. A. Meitl, Y. Sun, J.-U. Park, D. J.-L. Shir, Y.-S. Nam,
S. Jeon and J. A. Rogers, Chem. Rev., 2007, 107, 1117.
2 (a) M. Mas-Torrent and C. Rovira, Chem. Soc. Rev., 2008, 37, 827;
(b) S. Sergeyev, W. Pisula and Y. H. Geerts, Chem. Soc. Rev., 2007,
36, 1902; (c) T. Weil, T. Vosch, J. Hofkens, K. Peneva and K. Mullen,
Angew. Chem., Int. Ed., 2010, 49, 9068.
3 S. S. Lee and Y.-L. Loo, Annu. Rev. Chem. Biomol. Eng., 2010, 1, 59.
4 V. Palermo and P. Samorı`, Angew. Chem., Int. Ed., 2007, 46, 4428.
5 (a) A. P. H. J. Schenning and E. W. Meijer, Chem. Commun., 2005, 3245;
(b) M. Van der Auweraer and F. C. De Schryver, Nat. Mater., 2004, 3, 507.
6 S.-L. Li, T. Xiao, C. Lin and L. Wang, Chem. Soc. Rev., 2012, 41, 5950.
´, J. Phys. Chem. C,
7 (a) C. W. Rochester, S. A. Mauger and A. J. Moule
2012, 116, 7287; (b) E. Treossi, A. Liscio, X. Feng, V. Palermo,
¨llen and P. Samorı`, Appl. Phys. A, 2009, 95, 15;
K. Mu
(c) A. A. Zakhidov, J.-K. Lee, H. H. Fong, J. A. DeFranco,
M. Chatzichristidi, P. G. Taylor, C. K. Ober and G. G. Malliaras,
Adv. Mater., 2008, 20, 3481.
8 (a) M. Nonnenmacher, M. P. Oboyle and H. K. Wickramasinghe,
Appl. Phys. Lett., 1991, 58, 2921; (b) A. Liscio, V. Palermo and
P. Samorı`, Acc. Chem. Res., 2010, 43, 541.
9 (a) H. Hoppe, T. Glatzel, M. Niggemann, A. Hinsch, M. C. Lux-Steiner
and N. S. Sariciftci, Nano Lett., 2005, 5, 269; (b) M. Chiesa, L. Burgi,
J.-S. Kim, R. Shikler, R. H. Friend and H. Sirringhaus, Nano Lett.,
2005, 5, 559; (c) V. Palermo, G. Ridolfi, A. M. Talarico, L. Favaretto,
G. Barbarella, N. Camaioni and P. Samorı`, Adv. Funct. Mater., 2007,
17, 472; (d) D. C. Coffey and D. S. Ginger, Nat. Mater., 2006, 5, 735.
¨llen and
10 (a) A. Liscio, G. De Luca, F. Nolde, V. Palermo, K. Mu
P. Samorı`, J. Am. Chem. Soc., 2007, 130, 780; (b) V. Palermo, M. B.
J. Otten, A. Liscio, E. Schwartz, P. A. J. de Witte, M. A. Castriciano,
M. M. Wienk, F. Nolde, G. De Luca, J. J. L. M. Cornelissen, R. A. J.
¨llen, A. E. Rowan, R. J. M. Nolte and P. Samorı`, J. Am.
Janssen, K. Mu
Chem. Soc., 2008, 130, 14605.
¨llen and
11 M. R. Hansen, T. Schnitzler, W. Pisula, R. Graf, K. Mu
H. W. Spiess, Angew. Chem., Int. Ed., 2009, 48, 4621.
12 P. Gao, D. Beckmann, H. N. Tsao, X. Feng, V. Enkelmann,
¨llen, Adv. Mater., 2009, 21, 213.
M. Baumgarten, W. Pisula and K. Mu
13 S. R. Scully and M. D. McGehee, J. Appl. Phys., 2006, 100, 034907.
14 (a) G. De Luca, A. Liscio, M. Melucci, T. Schnitzler, W. Pisula,
¨llen and P. Samorı`,
C. G. Clark, L. M. Scolaro, V. Palermo, K. Mu
J. Mater. Chem., 2010, 20, 71; (b) G. De Luca, A. Liscio, F. Nolde,
¨llen and P. Samorı`, Soft Matter,
L. M. Scolaro, V. Palermo, K. Mu
2008, 4, 2064.
¨llen, M. Fahlman,
15 A. Liscio, V. Palermo, O. Fenwick, S. Braun, K. Mu
F. Cacialli and P. Samorı`, Small, 2011, 7, 634.
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