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T. Fujita
isolated molecule or a molecular crystal has now become a routine task. However,
an organic solar cell consists of a combination of donor and acceptor materials; the
packing structures in the vicinity of the D/A interface can be significantly disordered
and may differ from those in the bulk molecular crystal. Localized (e.g., molecular orbital (MO)) and delocalized (e.g., Bloch orbital) electronic states can coexist
in such disordered molecular aggregates. In addition, materials of different crystallinity, ranging from planar heterojunctions to bulk heterojunctions, have been used
in organic solar cells. Unified descriptions of localized and delocalized electronic
states are necessary to explore organic materials of different crystallinity.
Predicting the electronic states of disordered molecular aggregates requires
large-scale electronic structure calculations. However, the computation time rapidly
increases with increasing system size; thus, such calculations are difficult to perform
for disordered molecular aggregates, including D/A interfaces. We have developed
large-scale methods that are suitable for large molecular assemblies [38, 39, 42–45].
In particular, one recent development [38] has enabled an excited-state calculation for
a system containing over 2,000 atoms [44, 45]. In this chapter, we present our recent
efforts to explore the excited states of organic semiconductors on the basis of the
large-scale calculations. After briefly discussing the effects of intermolecular interactions and molecular aggregation on the electronic states, we present applications
of our methods to molecular clusters and a D/A interface.
The remainder of this chapter is organized as follows: In Sect. 7.2, we discuss the
effects of molecular assemblies on electronic states and introduce the polarization
and delocalization effects. In Sect. 7.3, we overview our ab initio approaches based on
the many-body Green’s function method and the fragment molecular orbital method.
In Sect. 7.4, we present numerical results for electronic states of pentacene (PEN)
clusters and illustrate the roles of polarization and delocalization effects. In Sect. 7.5,
we investigate the interfacial CT states in a D/A interface and discuss the role of
induced polarization in the e–h pair separation. In Sect. 7.6, we summarize this
chapter and briefly discuss future challenges for theoretical approaches.
7.2 Effects of Molecular Aggregation on Electronic States:
Polarization and Delocalization
In this section, we describe the key concepts regarding the electronic states of organic
materials. Because organic materials consist of molecules bound by relatively weak
non-covalent interactions, an organic molecule in the condensed phase exhibits electronic properties similar to those of an isolated molecule. Therefore, we consider the
electronic states of organic aggregates by exploring how intermolecular interactions
and molecular aggregations influence molecular electronic states. Here, we introduce the polarization and delocalization effects. The polarization effect indicates
the change in electronic states between a molecule in the gas phase and that in the
condensed phase, caused by the interaction between the considered molecule and
T. Fujita
isolated molecule or a molecular crystal has now become a routine task. However,
an organic solar cell consists of a combination of donor and acceptor materials; the
packing structures in the vicinity of the D/A interface can be significantly disordered
and may differ from those in the bulk molecular crystal. Localized (e.g., molecular orbital (MO)) and delocalized (e.g., Bloch orbital) electronic states can coexist
in such disordered molecular aggregates. In addition, materials of different crystallinity, ranging from planar heterojunctions to bulk heterojunctions, have been used
in organic solar cells. Unified descriptions of localized and delocalized electronic
states are necessary to explore organic materials of different crystallinity.
Predicting the electronic states of disordered molecular aggregates requires
large-scale electronic structure calculations. However, the computation time rapidly
increases with increasing system size; thus, such calculations are difficult to perform
for disordered molecular aggregates, including D/A interfaces. We have developed
large-scale methods that are suitable for large molecular assemblies [38, 39, 42–45].
In particular, one recent development [38] has enabled an excited-state calculation for
a system containing over 2,000 atoms [44, 45]. In this chapter, we present our recent
efforts to explore the excited states of organic semiconductors on the basis of the
large-scale calculations. After briefly discussing the effects of intermolecular interactions and molecular aggregation on the electronic states, we present applications
of our methods to molecular clusters and a D/A interface.
The remainder of this chapter is organized as follows: In Sect. 7.2, we discuss the
effects of molecular assemblies on electronic states and introduce the polarization
and delocalization effects. In Sect. 7.3, we overview our ab initio approaches based on
the many-body Green’s function method and the fragment molecular orbital method.
In Sect. 7.4, we present numerical results for electronic states of pentacene (PEN)
clusters and illustrate the roles of polarization and delocalization effects. In Sect. 7.5,
we investigate the interfacial CT states in a D/A interface and discuss the role of
induced polarization in the e–h pair separation. In Sect. 7.6, we summarize this
chapter and briefly discuss future challenges for theoretical approaches.
7.2 Effects of Molecular Aggregation on Electronic States:
Polarization and Delocalization
In this section, we describe the key concepts regarding the electronic states of organic
materials. Because organic materials consist of molecules bound by relatively weak
non-covalent interactions, an organic molecule in the condensed phase exhibits electronic properties similar to those of an isolated molecule. Therefore, we consider the
electronic states of organic aggregates by exploring how intermolecular interactions
and molecular aggregations influence molecular electronic states. Here, we introduce the polarization and delocalization effects. The polarization effect indicates
the change in electronic states between a molecule in the gas phase and that in the
condensed phase, caused by the interaction between the considered molecule and