7 First-Principles Investigations of Electronically …
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state, in the sense that it consists of a delocalized electron and a delocalized hole, the
efficient charge separation would take place as a result of the reduction of exciton
binding energy. However, as we have seen, a WM exciton state can be formed when
e–h interactions are sufficiently smaller than transfer integrals. This is often not the
case, particularly for organic bulk heterojunction solar cells, in which the transfer
integrals would be suppressed by the inherent structural disorder. Characterizing and
engineering the interfacial CT states are key for designing efficient and low-energyloss organic solar cells.
7.3 Electronic Structure Calculations for Optoelectronic
Properties
In this section, we present an overview of our theoretical developments for largescale electronic state calculations, aiming toward applications for organic materials.
Accurate estimations of e–h interactions are important for predicting the energy levels
of excited states. To this end, we have employed the many-body Green’s function
theory within the GW approximation. The effects of molecular aggregation on the
electronic states can be treated in combination with the fragment molecular orbital
method, a large-scale method suitable for organic materials.
First, the many-body Green’s function methods are briefly summarized. Next,
the advantages of the Green’s function methods will be discussed in comparison
to standard density functional theories, with benchmark numerical results for the
exciton binding energy of an isolated PEN molecule. Finally, the fragment molecular
orbital method is presented as to how the polarization and delocalization effects can
be incorporated.
7.3.1 Many-Body Green’s Function Method Within GW
Approximation
Here, we briefly summarize the many-body Green’s function methods. In general,
quantitative determinations of the energy levels of electronic states require a highly
accurate method, which goes beyond the standard density functional theory (DFT).
However, a highly accurate method requires a considerable computational time,
particularly for large molecular systems, and is not practical for organic materials
containing a large number of organic molecules or polymers. The many-body Green’s
function methods within the GW approximation can offer practical schemes for
calculating electronic states with reasonable accuracy. Here, we briefly describe the
GW and GW/Bethe-Salpeter equation (GW/BSE) methods. More comprehensive
descriptions on this approach can be found elsewhere [3, 11, 84].
165
state, in the sense that it consists of a delocalized electron and a delocalized hole, the
efficient charge separation would take place as a result of the reduction of exciton
binding energy. However, as we have seen, a WM exciton state can be formed when
e–h interactions are sufficiently smaller than transfer integrals. This is often not the
case, particularly for organic bulk heterojunction solar cells, in which the transfer
integrals would be suppressed by the inherent structural disorder. Characterizing and
engineering the interfacial CT states are key for designing efficient and low-energyloss organic solar cells.
7.3 Electronic Structure Calculations for Optoelectronic
Properties
In this section, we present an overview of our theoretical developments for largescale electronic state calculations, aiming toward applications for organic materials.
Accurate estimations of e–h interactions are important for predicting the energy levels
of excited states. To this end, we have employed the many-body Green’s function
theory within the GW approximation. The effects of molecular aggregation on the
electronic states can be treated in combination with the fragment molecular orbital
method, a large-scale method suitable for organic materials.
First, the many-body Green’s function methods are briefly summarized. Next,
the advantages of the Green’s function methods will be discussed in comparison
to standard density functional theories, with benchmark numerical results for the
exciton binding energy of an isolated PEN molecule. Finally, the fragment molecular
orbital method is presented as to how the polarization and delocalization effects can
be incorporated.
7.3.1 Many-Body Green’s Function Method Within GW
Approximation
Here, we briefly summarize the many-body Green’s function methods. In general,
quantitative determinations of the energy levels of electronic states require a highly
accurate method, which goes beyond the standard density functional theory (DFT).
However, a highly accurate method requires a considerable computational time,
particularly for large molecular systems, and is not practical for organic materials
containing a large number of organic molecules or polymers. The many-body Green’s
function methods within the GW approximation can offer practical schemes for
calculating electronic states with reasonable accuracy. Here, we briefly describe the
GW and GW/Bethe-Salpeter equation (GW/BSE) methods. More comprehensive
descriptions on this approach can be found elsewhere [3, 11, 84].