Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 104

Fig. 5.1 Photovoltaic
conversion events in polymer
solar cells with a bilayered
structure of hole-transporting
and electron-transporting
materials: (1) exciton
generation by photon
absorption, (2) exciton
diffusion into a donor-
acceptor interface, (3) charge
transfer at the interface, (4)
charge dissociation into free
carriers, and (5) charge
transport to each electrode

the figure, the photon absorption first produces singlet excitons that are electron-
hole pairs tightly bound by the Coulomb attraction. This exciton generation results
from an electronic transition from the ground state to an excited state due to the
photon absorption, which typically occurs on a time scale of femtosecond
(*10 -15 s). In contrast, the photon absorption in silicon-based solar cells produces
not excitons but freely mobile charge carriers directly at room temperature. This is
the most critical difference between them, which results from the lower dielectric
constant and larger effective mass (lower charge carrier mobility) in organic
semiconductors than in inorganic semiconductors [ 22 ]. A critical distance r C at
which the thermal energy of a charge carrier is equal to the Coulomb attractive
potential energy [ 23 ] would be as long as 14-19 nm in organic materials at room
temperature because dielectric constant is small (e = 3 - 4), while it would be
only 5 nm in crystalline silicon with a high dielectric constant (e = 11.9). Fur-
thermore, excitons are typically localized in organic materials as Frenkel excitons
or charge transfer (CT) excitons, while they are delocalized in crystalline silicon as
Wannier excitons with a radius much larger than the lattice spacing [ 23 ]. Conse-
quently, excitons generated in organic materials cannot be dissociated into free
carriers at room temperature. Instead, they can migrate randomly in films before
deactivating to the ground state (exciton diffusion). As a result, some excitons can
reach a donor-acceptor interface where they can be dissociated into free carriers.
The diffusion time is limited by the lifetime of excitons (typically \1 ns) and is
dependent on the phase-separated structures in donor-acceptor blend films. In a
finely mixed donor-acceptor blend, no exciton diffusion is required to generate free
carriers. In a large phase-separated blend such as RR-P3HT:PCBM annealed films,
the exciton diffusion is observed on a time scale of tens picoseconds (*10 -11 s) as
described in Sect. 5.6.1 . At the donor-acceptor interface, excitons can be separated
into the electron on the acceptor material (radical anion) and the hole on the donor
material (radical cation or polaron) if the energy gap at the interface is enough to
break the Coulomb attraction. The charge separation is promptly completed in the
order of *10 -14 s[ 19 , 24 ]. The electron and hole pair generated at the interface is