Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2 Regioregular Poly(3-hexylthiophene): Functional Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.2 Energy Levels from Electrochemical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.3 Optical Properties of P3HT Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3 Crystallization from Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4 Thin Film Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.1 Preparation and Characterization of P3HT Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2 Inducing Order and Orientation in P3HT Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
1 Introduction
Semiconducting polymers have attracted a great deal of attention in the last decades
because of their remarkable optoelectronic properties combined with low-cost
solution processability [1–4]. One of the most prominent semiconducting polymers
is poly(3-hexylthiophene) (P3HT), whose optoelectronic properties and device
performance have been investigated extensively in numerous studies
[1, 5–7]. Most of the fascinating properties of semiconducting polymers such as
P3HT derive from their strong tendency to crystallize, because strong
intermolecular interactions in well-ordered crystalline domains offer efficient
charge transport pathways through the polymer layer on a macroscopic scale
[8]. Polymer crystallization of flexible chains such as polyethylene (PE) has been
described by numerous theories [9, 10]. As a result of different processing
conditions, there is a great diversity of morphologies for these polymers, which
have been widely explored and reviewed [11] during the last 50 years. Crystalline
lamellae based on folded chains are unique for polymer crystals. Lamellae can
further aggregate and form semicrystalline spherulites under static conditions,
whereas an external field provokes anisotropic structures such as polymer fibrils
[12, 13]. Precise control of crystallization conditions can allow the growth of single
crystals [14, 15] formed of folded-chain lamellae, which are not only used for
investigations of the crystallographic structure, but also for exploring crystallization kinetics. From an engineering point of view, the precise morphology is of great
relevance because the mechanical properties strongly depend on the microstructure.
Crystallization of conjugated polymers like P3HT is more complex due to the
rigidity of the polymer backbone and the presence of side chains, which follow
distinct crystallization kinetics [16]. A precise crystallization mechanism for
conjugated polymers has not yet been fully established. The great advantage of a
conjugated polymer backbone, however, is that it allows for functional properties
with a characteristic absorption in the low-energy part of the spectrum and highly
anisotropic charge transport. Hence, the exact morphology affects not only the
mechanical but also the functional properties of the polymer layer and is therefore
40
K. Tremel and S. Ludwigs
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2 Regioregular Poly(3-hexylthiophene): Functional Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.2 Energy Levels from Electrochemical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.3 Optical Properties of P3HT Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3 Crystallization from Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4 Thin Film Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.1 Preparation and Characterization of P3HT Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2 Inducing Order and Orientation in P3HT Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
1 Introduction
Semiconducting polymers have attracted a great deal of attention in the last decades
because of their remarkable optoelectronic properties combined with low-cost
solution processability [1–4]. One of the most prominent semiconducting polymers
is poly(3-hexylthiophene) (P3HT), whose optoelectronic properties and device
performance have been investigated extensively in numerous studies
[1, 5–7]. Most of the fascinating properties of semiconducting polymers such as
P3HT derive from their strong tendency to crystallize, because strong
intermolecular interactions in well-ordered crystalline domains offer efficient
charge transport pathways through the polymer layer on a macroscopic scale
[8]. Polymer crystallization of flexible chains such as polyethylene (PE) has been
described by numerous theories [9, 10]. As a result of different processing
conditions, there is a great diversity of morphologies for these polymers, which
have been widely explored and reviewed [11] during the last 50 years. Crystalline
lamellae based on folded chains are unique for polymer crystals. Lamellae can
further aggregate and form semicrystalline spherulites under static conditions,
whereas an external field provokes anisotropic structures such as polymer fibrils
[12, 13]. Precise control of crystallization conditions can allow the growth of single
crystals [14, 15] formed of folded-chain lamellae, which are not only used for
investigations of the crystallographic structure, but also for exploring crystallization kinetics. From an engineering point of view, the precise morphology is of great
relevance because the mechanical properties strongly depend on the microstructure.
Crystallization of conjugated polymers like P3HT is more complex due to the
rigidity of the polymer backbone and the presence of side chains, which follow
distinct crystallization kinetics [16]. A precise crystallization mechanism for
conjugated polymers has not yet been fully established. The great advantage of a
conjugated polymer backbone, however, is that it allows for functional properties
with a characteristic absorption in the low-energy part of the spectrum and highly
anisotropic charge transport. Hence, the exact morphology affects not only the
mechanical but also the functional properties of the polymer layer and is therefore
40
K. Tremel and S. Ludwigs