crucial for device performance. For instance, charge transport is highly dependent
on the order within the polymer layer at multiple length scales: On a local scale,
charge transfer is affected by the extent of π-conjugation along the backbone as well
as intermolecular chain contacts. On a macroscopic scale, the interconnectivity of
crystalline grains plays a crucial role in the charge carrier mobility. Therefore,
improving device performance requires precise control of morphology, and new
methods to manipulate and control the polymer microstructure are continuously
pursued. From a research point of view, gaining a detailed understanding of the
correlation between morphology and functional properties is challenging and has
been the focus of various publications [1, 17–20].
This chapter gives an overview of recent reports on the control of morphology of
regioregular P3HT in thin films. Starting from a short introductory section about
synthesis, energy levels, and optical properties in solution, we then discuss the
preparation of one-dimensional (1D) fibers by controlled crystallization from
solution. We then focus on P3HT thin films obtained by solution processing from
good solvents, a procedure that is highly relevant for large-scale applications. The
impact of molecular parameters and processing conditions on the semicrystalline
morphology of P3HT on substrates is discussed in this context. The last part of the
chapter deals with the manufacture of oriented structures of P3HT with long range
order, with the clear perspective of understanding structure–function relations.
2 Regioregular Poly(3-hexylthiophene): Functional
Properties
2.1 Materials
One of the first syntheses of unsubstituted polythiophenes with high conductivity
after doping with iodine was reported in 1980 [21, 22]. However, due to the lack of
processability of these polymers, alkyl chains were attached to the conjugated
backbone to make the polymer soluble in common organic solvents and to allow
deposition from solution. Because 3-alkylthiophene is an asymmetric monomer,
there are three different regioisomers that vary in the relative orientation of the two
thiophene rings when coupled between the 2- and 5-position: 2-2
0 or head-to-head
coupling (HH), 2-5
0 or head-to-tail coupling (HT), and 5-5
0 or tail-to-tail coupling
(TT).
As a result, polymers based on 3-alkylthiophene exhibit a different
regiochemistry, which is illustrated in Fig. 1 for the example of P3HT.
Polythiophenes containing a mixture of the different couplings are referred to as
regioirregular or regiorandom. For polymers with irregular substituent distribution,
the thiophene units twist away from planarity as a result of unfavorable HH
coupling, which results in a drastic decrease in the conjugation length. In contrast,
regioregular polythiophenes, which contain exclusively HT couplings, can adopt a
Morphology of P3HT in Thin Films
41
on the order within the polymer layer at multiple length scales: On a local scale,
charge transfer is affected by the extent of π-conjugation along the backbone as well
as intermolecular chain contacts. On a macroscopic scale, the interconnectivity of
crystalline grains plays a crucial role in the charge carrier mobility. Therefore,
improving device performance requires precise control of morphology, and new
methods to manipulate and control the polymer microstructure are continuously
pursued. From a research point of view, gaining a detailed understanding of the
correlation between morphology and functional properties is challenging and has
been the focus of various publications [1, 17–20].
This chapter gives an overview of recent reports on the control of morphology of
regioregular P3HT in thin films. Starting from a short introductory section about
synthesis, energy levels, and optical properties in solution, we then discuss the
preparation of one-dimensional (1D) fibers by controlled crystallization from
solution. We then focus on P3HT thin films obtained by solution processing from
good solvents, a procedure that is highly relevant for large-scale applications. The
impact of molecular parameters and processing conditions on the semicrystalline
morphology of P3HT on substrates is discussed in this context. The last part of the
chapter deals with the manufacture of oriented structures of P3HT with long range
order, with the clear perspective of understanding structure–function relations.
2 Regioregular Poly(3-hexylthiophene): Functional
Properties
2.1 Materials
One of the first syntheses of unsubstituted polythiophenes with high conductivity
after doping with iodine was reported in 1980 [21, 22]. However, due to the lack of
processability of these polymers, alkyl chains were attached to the conjugated
backbone to make the polymer soluble in common organic solvents and to allow
deposition from solution. Because 3-alkylthiophene is an asymmetric monomer,
there are three different regioisomers that vary in the relative orientation of the two
thiophene rings when coupled between the 2- and 5-position: 2-2
0 or head-to-head
coupling (HH), 2-5
0 or head-to-tail coupling (HT), and 5-5
0 or tail-to-tail coupling
(TT).
As a result, polymers based on 3-alkylthiophene exhibit a different
regiochemistry, which is illustrated in Fig. 1 for the example of P3HT.
Polythiophenes containing a mixture of the different couplings are referred to as
regioirregular or regiorandom. For polymers with irregular substituent distribution,
the thiophene units twist away from planarity as a result of unfavorable HH
coupling, which results in a drastic decrease in the conjugation length. In contrast,
regioregular polythiophenes, which contain exclusively HT couplings, can adopt a
Morphology of P3HT in Thin Films
41