Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 5

Fig. 1.3 Band alignment of
donor and acceptor materials
for a heterojunction
electron from the exciton to LUMO B is an energetically favorable process.
An electron is hence transferred from the exciton to HOMO B , while a hole remains in
HOMO A . As a result of this charge transfer process, materials A and B are called a
donor and acceptor respectively. The competing process of luminescence, which
involves the radiative recombination of excitons, occurs at a timescale of *1ns
[ 23 ]. In contrast, the charge transfer process occurs at a much shorter timescale of
*45 fs [ 24 ], allowing efficient exciton dissociation at the heterojunction. After
dissociation, electron-hole pairs form a charge pair called a geminate pair, which are
charges still Coulombically bound and have to be separated by an internal field.
The distance that excitons can diffuse before recombination is called the exciton
diffusion length. Common exciton diffusion lengths in organic semiconductors are very
short, at a few tens of nanometers [ 7 , 20 ]. Excitons generated at a distance from the
heterojunction longer than this length will recombine before reaching the hetero-
junction, resulting in lower exciton dissociation efficiency. Hence, the active layers
have to be kept thin in order to ensure that phase separation between the donor and
acceptor is within the exciton dissociation length. However, a thin active layer results
in a serious tradeoff of low absorption efficiency. Therefore, it is important to have a
large interface area for exciton dissociation and adequate phase separation to ensure
efficient exciton dissociation. To achieve this, innovative device architectures such as
the bulk heterojunction and nanostructured active layers have been suggested.
1.3.3 Carrier Transport
The geminate pairs generated after exciton dissociation have to travel to electrodes
for collection within their lifetimes. The main driving forces for the transport of
holes to the anode and electrons to the cathode are drift and diffusion currents [ 25 ].
The drift current corresponds to carrier movement along the potential gradient
within the solar cell. This potential gradient is mainly determined by the choice of