4.2 Inducing Order and Orientation in P3HT Thin Films
Due to the structural complexity of semicrystalline polymers such as P3HT, charge
transport bottlenecks are difficult to determine. As opposed to small molecules,
crystalline grains in conjugated polymers are usually too small to allow device
fabrication within a single grain or across a single, isolated grain boundary.
Therefore, typical macroscopic transport measurements based on field-effect transistors, for instance, average over a great number of randomly oriented crystals and
grain boundaries. It remains extremely challenging to measure the impact of the
structural anisotropy within a crystalline grain as well as the influence of grain
boundaries of different orientation on charge transport. The main approach for
overcoming this problem is to induce high in-plane orientation in thin polymer
films. These highly anisotropic layers can serve as a tool for the study of optical and
electronic properties along different structural and crystallographic directions. In
addition, the introduction of controlled defects enables the probing of the impact of
specific morphological features.
There are different approaches to the synthesis of highly ordered, anisotropic
films. For homogeneous layers, anisotropy is mainly induced by controlling nucleation and growth processes and by methods based on shearing forces such as
mechanical rubbing. In patterned thin films, confinement-induced orientation is
exploited to obtain structural anisotropy. In the following section, different ideas on
achieving structural anisotropy for homogeneous and patterned polymer layers and
its impact on charge transport is discussed.
4.2.1 Order in Homogeneous Thin Films
Control of Nucleation and Growth
Epitaxial Crystallization
Epitaxial crystallization on orienting surfaces as one elegant method to induce order
is thoroughly described by Brinkmann et al. [45]. We briefly highlight the main
findings regarding the charge transport anisotropy of such films. Following the
approach of directional epitaxial crystallization (DEC) adjusted by Brinkmann and
Wittmann [107] and De Rosa et al. [108], Jimison et al. produced highly anisotropic
layers of P3HT with fiber-like morphology after directional solidification on the
crystallizable solvent 1,3,5-trichlorobenzene (TCB) (see Fig. 20) [19]. Thereby, the
polymer backbone (c P3HT ) orients parallel to the long axis of the TCB needles
(c TCB ), with the majority of chains adopting a face-on orientation (Fig. 20d).
Transistor measurements in the a-c-plane of these anisotropic layers revealed a
low charge transport barrier at grain boundaries parallel to the fiber axis compared
with fiber-to-fiber grain boundaries. This result is explained by the relative grain
orientation. Grain boundaries along the fiber separate crystallites in which all the
chains orient along the fiber axis. In this geometry, it is likely that straight intergrain
Morphology of P3HT in Thin Films
67
Due to the structural complexity of semicrystalline polymers such as P3HT, charge
transport bottlenecks are difficult to determine. As opposed to small molecules,
crystalline grains in conjugated polymers are usually too small to allow device
fabrication within a single grain or across a single, isolated grain boundary.
Therefore, typical macroscopic transport measurements based on field-effect transistors, for instance, average over a great number of randomly oriented crystals and
grain boundaries. It remains extremely challenging to measure the impact of the
structural anisotropy within a crystalline grain as well as the influence of grain
boundaries of different orientation on charge transport. The main approach for
overcoming this problem is to induce high in-plane orientation in thin polymer
films. These highly anisotropic layers can serve as a tool for the study of optical and
electronic properties along different structural and crystallographic directions. In
addition, the introduction of controlled defects enables the probing of the impact of
specific morphological features.
There are different approaches to the synthesis of highly ordered, anisotropic
films. For homogeneous layers, anisotropy is mainly induced by controlling nucleation and growth processes and by methods based on shearing forces such as
mechanical rubbing. In patterned thin films, confinement-induced orientation is
exploited to obtain structural anisotropy. In the following section, different ideas on
achieving structural anisotropy for homogeneous and patterned polymer layers and
its impact on charge transport is discussed.
4.2.1 Order in Homogeneous Thin Films
Control of Nucleation and Growth
Epitaxial Crystallization
Epitaxial crystallization on orienting surfaces as one elegant method to induce order
is thoroughly described by Brinkmann et al. [45]. We briefly highlight the main
findings regarding the charge transport anisotropy of such films. Following the
approach of directional epitaxial crystallization (DEC) adjusted by Brinkmann and
Wittmann [107] and De Rosa et al. [108], Jimison et al. produced highly anisotropic
layers of P3HT with fiber-like morphology after directional solidification on the
crystallizable solvent 1,3,5-trichlorobenzene (TCB) (see Fig. 20) [19]. Thereby, the
polymer backbone (c P3HT ) orients parallel to the long axis of the TCB needles
(c TCB ), with the majority of chains adopting a face-on orientation (Fig. 20d).
Transistor measurements in the a-c-plane of these anisotropic layers revealed a
low charge transport barrier at grain boundaries parallel to the fiber axis compared
with fiber-to-fiber grain boundaries. This result is explained by the relative grain
orientation. Grain boundaries along the fiber separate crystallites in which all the
chains orient along the fiber axis. In this geometry, it is likely that straight intergrain
Morphology of P3HT in Thin Films
67