Page 256 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 259

12.2 Operating Principles

The basic physical processes that give rise to the photovoltaic effect in an OPV [24–26] are
different to those which underpin the operation of conventional inorganic PVs due to a
fundamental difference between the structure of crystalline inorganic semiconductors and
molecular semiconductors: In the former, such as silicon, all atoms form strong covalent
bonds with their neighbors, and are part of a continuous periodic lattice that extends in
three dimensions [3]. Strong coupling of a very large number of equivalent atomic orbitals
on adjacent atoms gives rise to the formation of bands of closely spaced electron states
which, in a semiconductor at ambient temperature, are almost completely filled or com-
pletely empty corresponding to the valence band and conduction band respectively. The
high density of closely spaced electron states in a band allows charge carriers to move easi-
ly through the crystal lattice under the influence of an applied electric field. Charge carriers
can be vacancies in the valence band (i.e., holes) or electrons in the conduction band. While
it is often useful to consider charge carriers in semiconductors as particles, in crystalline
semiconductors they can also be described as waves, which extend over several crystal lat-
tice sites. The delocalized nature of charge carriers in crystalline inorganic semiconductors
helps to ensure that the coulombic force of attraction between electrons and holes is very
small. Additionally, the small background population of free electrons and holes that al-
ways exists in semiconductors at ambient temperature, also helps to screen the interaction
between charge carriers by polarization of the crystal lattice in the space between them,
a property that is embodied in the relative permittivity, or dielectric constant (ε r ), of the
material. due to the combined effect of the delocalization of charge carriers and the large
dielectric constant, the binding energy between a photo-generated electron and hole in a
crystalline inorganic semiconductor is typically much smaller than the thermal energy of
the charge carriers at room temperature: ∼25 meV. Consequently, absorption of a photon
of light with sufficient energy to excite an electron from the valence band to the conduction
band gives rise directly to the production of a free electron in the conduction band and free
hole in the valance band. Since these charges are not coulombically bound to one another
they can easily be extracted to the external circuit using a moderate electric field, such as
that which exists across the interface between n and p doped semiconductors. For this rea-
son, conventional PVs are based on a p-n junction [3], in which the same semiconductor
doped p and n-type forms the junction across which photo-generated charge carriers are
separated. Across a p-n junction there is no discontinuity (step) in the energy of the con-
duction and valence band edges across the interface between p and n-type regions.
In contrast, photon absorption in organic semiconductors does not result in the direct
formation of a free electron and hole, because the coulombic energy of attraction between
them is large: 0.2–1 eV [24–26]. This large binding energy is a consequence of the elec-
tronic structure of organic semiconductors in the solid state, which is dominated by that
of the constituent molecules. Coupling between the highest occupied molecular orbital
(HOmO) and lowest unoccupied molecular orbital (lumO) of adjacent molecules is weak
because the molecules are bound together only by weak van der Waals-type interactions.