In order to understand molecular ordering and electronic processes in bulk
heterojunction devices, blends of P3HT oligomers with fullerene and PCBM have
been simulated at different levels of resolution. Atomistic simulations showed that
bulk oligothiophenes (five chains of 20 repeat units each) tend to cluster better than
oligothiophene/fullerene systems [48]. Prototypical model interfaces have been
used to evaluate energetic profiles for electrons and holes [49]. Coarse-grained
simulations could observe the onset of phase separation [45], distributions of
domain sizes, and interface-to-volume ratios [50] in P3HT/PCBM mixtures. Nevertheless, the field of coarse-grained modeling of conjugated polymers is still in its
infancy. Complications reside in the large persistence length and anisotropic
nonbonded interactions that promote π–π stacking. We will discuss various strategies for developing coarse-grained models of conjugated polymers in Sect. 3.
3 Coarse-Grained Models
Apart from the local molecular packing discussed so far, mesoscopic ordering of
conjugated polymers is equally important for the functionality of organic semiconducting devices. In a bulk heterojunction solar cell, for example, domain sizes of
the donor and acceptor mesophases have considerable impact on cell efficiency. To
predict and analyze such effects, modeling strategies that target the morphology on
length scales reaching several hundreds of nanometers, the typical thickness of the
active layer in a polymer-based solar cell, are required. With the currently available
computational power, such system sizes can only be addressed on a coarse-grained
level.
The idea of coarse-graining relies on the separation of time and length scales.
For many polymeric systems, especially polymer melts, the chemical details,
although strongly affecting material behavior on the microscopic level, become
less important on the mesoscale, where simplified representations of polymer
architecture and interactions can be used. In conjugated polymers, however, the
mesoscale features of the morphology couple across many length scales: π–π
stacking for instance promotes the formation of lamellae, which in turn selfassemble into supralamellar structures. Predicting this hierarchical self-assembly
is the main target (and challenge) of coarse-grained modeling of conjugated
polymers.
Coarse-graining strategies can be subdivided into bottom-up and top-down
approaches [51, 52]. In bottom-up coarse-graining, the model is constructed to
reproduce physical quantities known from a more detailed description of the system
[53]. In other words, a fine-grained representation of the system is projected onto a
representation with fewer degrees of freedom. This projection is not unique, and
various techniques have been suggested, including structure-based coarse-graining
[53, 54], force-matching [55, 56], and relative entropy frameworks [57]. Top-down
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C. Poelking et al.
heterojunction devices, blends of P3HT oligomers with fullerene and PCBM have
been simulated at different levels of resolution. Atomistic simulations showed that
bulk oligothiophenes (five chains of 20 repeat units each) tend to cluster better than
oligothiophene/fullerene systems [48]. Prototypical model interfaces have been
used to evaluate energetic profiles for electrons and holes [49]. Coarse-grained
simulations could observe the onset of phase separation [45], distributions of
domain sizes, and interface-to-volume ratios [50] in P3HT/PCBM mixtures. Nevertheless, the field of coarse-grained modeling of conjugated polymers is still in its
infancy. Complications reside in the large persistence length and anisotropic
nonbonded interactions that promote π–π stacking. We will discuss various strategies for developing coarse-grained models of conjugated polymers in Sect. 3.
3 Coarse-Grained Models
Apart from the local molecular packing discussed so far, mesoscopic ordering of
conjugated polymers is equally important for the functionality of organic semiconducting devices. In a bulk heterojunction solar cell, for example, domain sizes of
the donor and acceptor mesophases have considerable impact on cell efficiency. To
predict and analyze such effects, modeling strategies that target the morphology on
length scales reaching several hundreds of nanometers, the typical thickness of the
active layer in a polymer-based solar cell, are required. With the currently available
computational power, such system sizes can only be addressed on a coarse-grained
level.
The idea of coarse-graining relies on the separation of time and length scales.
For many polymeric systems, especially polymer melts, the chemical details,
although strongly affecting material behavior on the microscopic level, become
less important on the mesoscale, where simplified representations of polymer
architecture and interactions can be used. In conjugated polymers, however, the
mesoscale features of the morphology couple across many length scales: π–π
stacking for instance promotes the formation of lamellae, which in turn selfassemble into supralamellar structures. Predicting this hierarchical self-assembly
is the main target (and challenge) of coarse-grained modeling of conjugated
polymers.
Coarse-graining strategies can be subdivided into bottom-up and top-down
approaches [51, 52]. In bottom-up coarse-graining, the model is constructed to
reproduce physical quantities known from a more detailed description of the system
[53]. In other words, a fine-grained representation of the system is projected onto a
representation with fewer degrees of freedom. This projection is not unique, and
various techniques have been suggested, including structure-based coarse-graining
[53, 54], force-matching [55, 56], and relative entropy frameworks [57]. Top-down
146
C. Poelking et al.