14
M. Hiramoto
Fig. 1.12 a Face-on and b edge-on orientations of π −π stacking. Molecular planes are indicated
in the figure. π −π stackings are formed perpendicular to the molecular planes
1.2.2 Photovoltage
1.2.2.1 HOMO–LUMO Gap Tuning
The maximum V oc of organic solar cells consisting of a combination of D/A organic
semiconductors is determined by the energetic difference between the LUMO of
the A molecule and the HOMO of the D molecule (V LUMO-HOMO ) (Fig. 1.13a) [33].
In other words, to obtain high V oc , a combination of D/A organic semiconductors
having a larger V LUMO-HOMO difference is necessary (Fig. 1.13). By using the newly
synthesized D molecules with deeper HOMO levels (Fig. 1.13b) [33] and the A
molecules having shallower LUMO levels (Fig. 1.13c) [34], cell efficiency can be
increased by the increase in V oc . HOMO–LUMO gap tuning is discussed in detail
in Chap. 5.
Fig. 1.13 a V LUMO-HOMO difference of donor and acceptor molecules. b V LUMO-HOMO difference
using a donor molecule having a deeper HOMO level. c V LUMO-HOMO difference using an acceptor
molecule having a shallower LUMO level