3 Coarse-Grained Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
3.1 Structure-Based Coarse-Graining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
3.2 Soft Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
4 Rate-Based Description of Charge Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
4.1 Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
4.2 Reorganization Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
4.3 Electronic Coupling Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
4.4 Site Energies . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . 156
4.5 Charge Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
4.6 Autocorrelation of Electronic Couplings and Site Energies . . . . . . . . . . . . . . . . . . . . . . . . . 167
5 First-Principles-Based Calculations for Large Models of Polymer . . . . . . . . . . . . . . . . . . . . . . . 169
5.1 The Charge Localization–Length Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
5.2 Strategies for Large-Scale Electronic-Structure Calculations of Polymer Models . 171
5.3 Results from the Computation of the Wavefunction for Large-Scale Polymer
Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
6 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
1 Introduction
Thiophene-based conjugated polymers have accompanied, if not originated, the
interest in conductive polymer materials and their application in organic field-effect
transistors (OFETs) and organic photovoltaic (OPV) devices [1]. The most studied
representative of this class of materials is poly(3-hexyl-thiophene) (P3HT) with its
regioregular (head-to-tail) isomer (see Fig. 1), as first synthesized by Rick
McCullough in 1992 [2]. Polythiophenes, however, were already an intensely
studied class of conjugated polymers, a rudimentary description of the compound
being published as early as 1883 [3]. The first polymerization reactions with high
yield and small concentrations of synthesis impurities were reported in 1980
[4, 5]. These compounds were essentially not processable due to the strong interaction of the conjugated backbones. In 1986, Elsenbaumer reported the synthesis of
easily processable poly(alkyl-thiophenes) (PATs) [6]. Solution-processed into thin
films, these materials could exhibit reasonable conductivities limited, however, by
the disorder that results from a regiorandom attachment of the side chains to the
thiophene monomers. It was the synthesis of regioregular (rr) P3ATs [2] that
eventually paved the way for applications in devices such as OFETs and OPV
cells. Here, we recapitulate key experimental results relevant to polymorphism,
formation of self-assembled nanostructures, and charge transport in rr-P3HT. An
extended overview is provided in the rest of the contributions of this volume,
various books, and monographs (e.g., [7])
Like many conjugated polymers, P3HT is a polymorph, i.e., forms different
crystal structures depending on processing conditions. The most frequently
observed are so-called forms I and II [8], which differ by the side chain conformation and interdigitation, inclination of conjugated backbones with respect to the
stacking direction, and the shift of successive (along the π-stacking direction)
polymer chains [9]. Form I, which is observed after annealing, is the structure
140
C. Poelking et al.
3.1 Structure-Based Coarse-Graining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
3.2 Soft Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
4 Rate-Based Description of Charge Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
4.1 Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
4.2 Reorganization Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
4.3 Electronic Coupling Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
4.4 Site Energies . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . 156
4.5 Charge Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
4.6 Autocorrelation of Electronic Couplings and Site Energies . . . . . . . . . . . . . . . . . . . . . . . . . 167
5 First-Principles-Based Calculations for Large Models of Polymer . . . . . . . . . . . . . . . . . . . . . . . 169
5.1 The Charge Localization–Length Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
5.2 Strategies for Large-Scale Electronic-Structure Calculations of Polymer Models . 171
5.3 Results from the Computation of the Wavefunction for Large-Scale Polymer
Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
6 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
1 Introduction
Thiophene-based conjugated polymers have accompanied, if not originated, the
interest in conductive polymer materials and their application in organic field-effect
transistors (OFETs) and organic photovoltaic (OPV) devices [1]. The most studied
representative of this class of materials is poly(3-hexyl-thiophene) (P3HT) with its
regioregular (head-to-tail) isomer (see Fig. 1), as first synthesized by Rick
McCullough in 1992 [2]. Polythiophenes, however, were already an intensely
studied class of conjugated polymers, a rudimentary description of the compound
being published as early as 1883 [3]. The first polymerization reactions with high
yield and small concentrations of synthesis impurities were reported in 1980
[4, 5]. These compounds were essentially not processable due to the strong interaction of the conjugated backbones. In 1986, Elsenbaumer reported the synthesis of
easily processable poly(alkyl-thiophenes) (PATs) [6]. Solution-processed into thin
films, these materials could exhibit reasonable conductivities limited, however, by
the disorder that results from a regiorandom attachment of the side chains to the
thiophene monomers. It was the synthesis of regioregular (rr) P3ATs [2] that
eventually paved the way for applications in devices such as OFETs and OPV
cells. Here, we recapitulate key experimental results relevant to polymorphism,
formation of self-assembled nanostructures, and charge transport in rr-P3HT. An
extended overview is provided in the rest of the contributions of this volume,
various books, and monographs (e.g., [7])
Like many conjugated polymers, P3HT is a polymorph, i.e., forms different
crystal structures depending on processing conditions. The most frequently
observed are so-called forms I and II [8], which differ by the side chain conformation and interdigitation, inclination of conjugated backbones with respect to the
stacking direction, and the shift of successive (along the π-stacking direction)
polymer chains [9]. Form I, which is observed after annealing, is the structure
140
C. Poelking et al.