materials and fabrication approaches to solar cells. An overall concept that needs to be
maintained through all of this work will be scalability. The techniques that are being developed
need to able to be both effective and sufficiently rapid to allow low-cost fabrication of large-area,
defect-free, PV cell modules.
Approaches to Controlling Light Absorption and Scattering
To achieve high cell conversion efficiency, it is necessary to efficiently capture photons with
energies ranging from the visible into the near-infrared (IR). While the intrinsic absorption cross-
section of the active materials is important in this regard, advances in cell fabrication are needed
to allow the construction of photonic structures that tailor photon energies, and concentrate or
confine the optical energy into the active material. A key issue in the construction of active
layers for PV cells is that often the active materials need to be thin (ca. 100 nm) because of the
relatively short exciton diffusion distance. Consequently, strategies are needed that concentrate
or capture incident light in order to increase the interaction time or length with the active
material (see Figure 62). Structures that combine optical concentration (e.g., integrated lenses or
parabolic reflectors) with waveguides could lead to dramatic increases in light-harvesting
efficiency. Nonlinear processes such as photon up-conversion or down-conversion could lead to
an increase in the quantum efficiency for photon-to-exciton generation. Optical concentrators
could substantially increase the probability for nonlinear conversion processes.
Figure 62 Light-trapping by concentrator/waveguide structure increases
light absorption efficiency.New methods for the deposition of organic, inorganic, and hybrid structures on rigid and
flexibles substrates must be developed. These involve both wet and vapor deposition techniques
that are easily scalable and that will allow the proper stacking of active structures on both rigid
and plastic substrates. Various strategies can be employed using composite and hybrid structures
that are tailored, for instance, for self-assembly. Novel approaches to control the PV architecture
include, for example, the use of block co-polymers, blends, crystal engineering, as well as using
templated nanowires and quantum dots. Methods of controlling light absorption and scattering
phenomena through the use of photonic band structures, plasmonic structures and spectral
splitting need to be discovered. With this level of control, cell efficiencies have the potential of
being enhanced by adopting methods to localize the optical energy directly at or near the exciton
dissociation zone.