chains bridge the boundary and provide efficient charge transport pathways
between crystalline grains in the fiber direction (see discussion on the concept of
tie molecules in Sect. 4.1.2). By contrast, fiber-to-fiber grain boundaries only allow
for intergrain chains with sharp bends and twists, which hamper charge transport.
Charge transport along the fiber axis is around one order of magnitude higher than
in the perpendicular direction.
Spherulite Growth
The order of P3HT thin films can be further realized via low nucleation densities
obtained by controlled homogeneous nucleation and self-seeding. Conventional
deposition techniques such as spin-coating usually result in a random orientation of
the crystalline lamellae due to the high and uncontrolled nucleation. Spherulitic
structures of classical, semicrystalline polymers such as polyethylene with low
nucleation density are typically generated from the melt. However, melt-annealing
of conjugated polymers is problematic due to the high melting temperatures and
potential thermal degradation. Although spherulites for P3AT have not been
observed after spin-coating and melt-annealing, Lu et al. and our group reported
the formation of P3AT spherulites by solvent vapor annealing [20, 109–111].
Lu et al. reported on the transition of form I to form II in poly(3-butylthiophene)s
associated with spherulite formation when thin films were treated with CS 2 vapor
[110, 111]. A precise control of nucleation density and, hence, spherulite size for
P3HT was achieved by us by introducing an approach based on the controlled
swelling and subsequent deswelling in CS 2 vapor of well-defined pressure
[20, 109]. Nucleation densities much smaller than those obtained after typical
spin-casting conditions were achieved, resulting in spherulitic structures of 10–
100 μm in diameter (see Fig. 21a). Thereby, the lamellar crystals composed of
highly ordered, edge-on oriented chains exhibited common in-plane alignment over
several tens of micrometers. Starting from a pre-cast film, swelling in solvent vapor
atmosphere of sufficient pressure results in a complete dissolution of P3HT crystals
present in the film after spin-coating. In this solution-like state, isolated chains
adopt a flexible coil configuration that is reflected in a single broad peak at 465 nm
in the absorption spectrum (illustrated in Fig. 21b). Recrystallization is induced by a
controlled deswelling of the film to form a supersaturated solution, and can be
followed in-situ by polarized optical microscopy (POM) and UV–vis absorption.
Nucleation density is adjusted by controlling the degree of supercooling or by selfseeding, the latter approach relying on a small number of nuclei that remain in the
solvent-swollen film and act as seeds in subsequent recrystallization. Following this
methodology, nucleation densities over several orders of magnitude can be realized
independently of crystal growth conditions.
By decreasing the nucleation density to such an extent that a single transistor can
be placed in an ordered crystalline domain, charge transport measurements in the
b-c-plane could be performed. For bottom gate, bottom contact transistors, average
mobilities of μ II ¼ 0.07 cm
2 /V s and μ ⊥ ¼ 0.20 cm
2 /V s parallel (II) and perpendicular (⊥) to the spherulite growth direction were found, meaning that charge
Morphology of P3HT in Thin Films
69
between crystalline grains in the fiber direction (see discussion on the concept of
tie molecules in Sect. 4.1.2). By contrast, fiber-to-fiber grain boundaries only allow
for intergrain chains with sharp bends and twists, which hamper charge transport.
Charge transport along the fiber axis is around one order of magnitude higher than
in the perpendicular direction.
Spherulite Growth
The order of P3HT thin films can be further realized via low nucleation densities
obtained by controlled homogeneous nucleation and self-seeding. Conventional
deposition techniques such as spin-coating usually result in a random orientation of
the crystalline lamellae due to the high and uncontrolled nucleation. Spherulitic
structures of classical, semicrystalline polymers such as polyethylene with low
nucleation density are typically generated from the melt. However, melt-annealing
of conjugated polymers is problematic due to the high melting temperatures and
potential thermal degradation. Although spherulites for P3AT have not been
observed after spin-coating and melt-annealing, Lu et al. and our group reported
the formation of P3AT spherulites by solvent vapor annealing [20, 109–111].
Lu et al. reported on the transition of form I to form II in poly(3-butylthiophene)s
associated with spherulite formation when thin films were treated with CS 2 vapor
[110, 111]. A precise control of nucleation density and, hence, spherulite size for
P3HT was achieved by us by introducing an approach based on the controlled
swelling and subsequent deswelling in CS 2 vapor of well-defined pressure
[20, 109]. Nucleation densities much smaller than those obtained after typical
spin-casting conditions were achieved, resulting in spherulitic structures of 10–
100 μm in diameter (see Fig. 21a). Thereby, the lamellar crystals composed of
highly ordered, edge-on oriented chains exhibited common in-plane alignment over
several tens of micrometers. Starting from a pre-cast film, swelling in solvent vapor
atmosphere of sufficient pressure results in a complete dissolution of P3HT crystals
present in the film after spin-coating. In this solution-like state, isolated chains
adopt a flexible coil configuration that is reflected in a single broad peak at 465 nm
in the absorption spectrum (illustrated in Fig. 21b). Recrystallization is induced by a
controlled deswelling of the film to form a supersaturated solution, and can be
followed in-situ by polarized optical microscopy (POM) and UV–vis absorption.
Nucleation density is adjusted by controlling the degree of supercooling or by selfseeding, the latter approach relying on a small number of nuclei that remain in the
solvent-swollen film and act as seeds in subsequent recrystallization. Following this
methodology, nucleation densities over several orders of magnitude can be realized
independently of crystal growth conditions.
By decreasing the nucleation density to such an extent that a single transistor can
be placed in an ordered crystalline domain, charge transport measurements in the
b-c-plane could be performed. For bottom gate, bottom contact transistors, average
mobilities of μ II ¼ 0.07 cm
2 /V s and μ ⊥ ¼ 0.20 cm
2 /V s parallel (II) and perpendicular (⊥) to the spherulite growth direction were found, meaning that charge
Morphology of P3HT in Thin Films
69