Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 3

different energy levels of an organic semiconductor. When an electron is excited
from the HOMO to the LUMO of an organic semiconductor, the molecule itself is
excited into a higher energy state, as opposed to the actual excitation of a free electron
from the valence band to the conduction band in inorganic semiconductors.
The carrier transport mechanism in organic semiconductors is also different from
that of inorganic semiconductors. In organic semiconductors, thermally activated
'hopping' of carriers occurs to overcome the energy barriers within the disordered
conjugated polymer structure, thus allowing carrier transport within the semicon-
ductor. [ 5 ]. This is highly different for charge transport in inorganic semiconductors,
which can be described by movement offree carriers in the valence or conduction band.
The hopping transport mechanism gives organic semiconductors a rather low mobility
when compared to their inorganic counterparts. Up to*1.5 9 10 -3 m 2 V -1 s -1 hole
mobility is achieved for small molecule organic semiconductors [ 6 , 7 ], while silicon
has a mobility of up to *4.5 9 10 -2 m 2 V -1 s -1 [ 8 ]. On the other hand, electron
mobility for some small molecule materials reaches *1 9 10 -5 m 2 V -1 s -1 [ 9 , 10 ],
while silicon has a much higher electron mobility of 0.1 m 2 V -1 s -1 .Insolarcell
applications, the commonly studied P3HT:PCBM blend has hole and electron
mobilities of the order of *10 -7 -10 -8 m 2 V -1 s -1 in the mixed blend film [ 11 ]. The
low mobility, when compared to inorganic semiconductors, is a major disadvantage for
organic semiconductors and consequently, different devices were proposed to over-
come this weakness.

1.3 Operational Principles of OSCs

1.3.1 Exciton Generation

Upon the absorption of a photon, an electron in the organic semiconductor is excited
from the HOMO to the LUMO. This is analogous to exciting an electron from the
valence band to the conduction band of inorganic semiconductors. However, due to
the low dielectric constant and localized electron and hole wavefunctions in organic
semiconductors [ 12 , 13 ], strong Coulombic attraction exists between the electron-
hole pair. The resulting bound electron-hole pair is called an exciton, with a binding
energy of 0.1-1.4 eV [ 13 ], as opposed to a much lower binding energy of a few meV
in inorganic semiconductors. Hence, it is relatively higher possibility to generate free
charge carriers after absorption of photons in an inorganic semiconductor as the
electron-hole pairs easily dissociate by absorbing thermal energy; while strongly
bound excitons are generated in organic semiconductors.
The absorption coefficient of organic materials is commonly high at
*10 5 cm -1 [ 14 ]. Hence, although the thickness of the active layer of OSCs are
limited by electrical conduction, a few hundreds of nanometers of the active layer
is thick enough to absorb an adequate amount of light and show significant solar
cell characteristics [ 15 ]. To further improve absorption, particularly in the