Basic Research Needs for Solar Energy Utilization - Office of ...

Figure 30 Schematic diagram (right) of an excitonic solar cell comprising a donor (orange) and an
acceptor (green) species. An energy-level diagram (left) with excitation into the donor shows the optical
and electronic bandgaps of the donor and acceptor and the band offset relative to the exciton binding
energy (EX).
transport mechanisms, which take a variety of different forms, depending on the quasi-particle
that is being transported. Excitons are charge-neutral and are transported by diffusion, while
charged carriers are transported by diffusion and/or drift in the built-in electric fields resulting
from the electrical contacts. The degree of structural order of the material is important in
determining transport properties. In the disordered limit (which includes most polymers), carrier
transport occurs predominantly via hopping between spatially localized states, the energy
distribution of which determines the intrinsic mobility. In the ordered limit of single crystals,
weak scattering among delocalized states produces band-like transport. Strong charge lattice
interactions lead to polaron states, which exist in both disordered and ordered structures.
Understanding the role of deep energy traps in highly disordered materials is vital for controlling
both charge transport and exciton lifetimes. These traps must be identified and characterized
individually in crystalline samples; this knowledge can be used for optimization of the extrinsic
mobility in device structures. A key component of this work is close interaction between
mobility measurements, transport spectroscopies, and theory, including electronic structure and
transport theories.
Charge Separation and Recombination at the Interface between Nanostructures
Although singlet excitons are the predominant photogenerated species, intersystem crossing to
the triplet state yields species that are longer-lived and lower in energy. Their role in enhancing
light-emitting devices has been a key success story, but little is known about their impact in
organic photovoltaics. Whether they are essential or must be avoided at all costs are questions
that need addressing. Although the exciton dissociation process is of fundamental importance to
extracting energy from absorbed photons in OPV structures, it is also essential to inhibit the
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