Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 2

semiconductors to absorb light and convert it to electrical energy. With the
inorganic solar cells technology running into cost bottlenecks for large area
applications, the simple and cheap fabrication process of OSCs provides a huge
potential for large area applications. Also, OSCs have the unique properties of
flexibility and lightweight that may also result in new applications such as portable
solar panels. The properties of OSCs are very interesting for understanding organic
devices. Their basic principles are described in this chapter.
1.2 Overview of Organic Semiconductors
Organic semiconductors are carbon-based materials possessing semiconductor
characteristics. Atoms within an organic semiconductor molecule are bonded by
conjugated p-bonds, while molecules are bonded to each other by weak van der
Waal's force, as opposed to the giant covalent structure exhibited by inorganic
semiconductors. The bonding structure gives organic semiconductors its unique
flexibility, light weight, and low sublimation point which allow easy processing.
From the macroscopic point of view, the band structure of organic semicon-
ductors can be treated similarly as inorganic semiconductors. The valence band is
normally filled with electrons and conduction band is normally free of electrons.
In organic semiconductors, the Highest Occupied Molecular Orbital (HOMO) and
the Lowest Unoccupied Molecular Orbital (LUMO) are analogs to the valence
band and conduction band respectively. The HOMO and LUMO of organic
semiconductors represent the hybridization between bonding and antibonding of
the conjugated p-electrons [ 3 , 4 ].
Organic semiconductors are composed of organic molecules which are formed
by a p-conjugated system. Carbon atoms are sp 2 hybridized and the sp 2 bonds form
three strong r-bonds with neighboring atoms [ 4 ]. The remaining p-orbitals of the C
atoms form a delocalized cloud of electrons through the formation of weaker
p-bonds. This bond structure forms a quasi-one-dimensional structure for the
conjugated organic semiconductors. The p-bond system can have different bond-
ing configurations according to the electron wavefunction overlap of neighboring
atoms. For example, in Fig. 1.1 , we can see two different states of the p-bonds,
with the bonding and antibonding states corresponding to different energy levels.
The HOMO and LUMO of organic semiconductors refer to energy bands that
correspond to different hybridization states of the p-bonds which will result in
Fig. 1.1 Illustration of
bonding-antibonding
interactions between the
HOMO/LUMO levels of an
organic semiconductor