carrier injection between the individual cells has received little attention. Use of colloidal metals
to fulfill this function has proven to be surprisingly successful; the amount of metal required is
minimal, and beneficial field-enhancement effects improve the absorption properties of the
surrounding medium. Although the basic effects are understood, significant improvements in
device performance depend upon a far more detailed understanding of the properties of these and
related species when they are embedded in organic structures.
The contact electrodes continue to play a significant role in controlling device performance. The
deposition of metals as one contact and the use of ITO as the transparent conducting electrode
are limiting factors. New strategies are needed to develop transparent conductors and substrates
that take into account their “end use” in organic-based solar energy conversion devices. This
problem can be addressed by focusing on the interface with the existing transparent oxides or by
developing new molecule-based conductors that provide better compatibility.
SCIENTIFIC CHALLENGES
Key scientific challenges that must be overcome include optimizing the target molecular,
polymeric, and nanocrystalline structures to produce systems that will provide extraction of
energy over the full range of the solar spectrum, including the challenging near-infrared region.
There is also a need to understand the relationship between electronic structure and excited state
properties of the constituent species of these new systems and to understand how this
relationship is affected through their interactions with each other. The development and
understanding of structures that can exhibit efficient frequency up-conversion and down-
conversion for advanced, third-generation device structures will also benefit from such a
fundamental understanding. Factors that control the processes that occur at the interfaces
between dissimilar phases pose an additional challenge that must also be addressed. This
includes the interface within the components of the active photoconversion medium, at the
interface between stacked junctions of a tandem design and at the interface with the external
contacts. The challenge is to understand how to manipulate and enhance the transport of excitons
to the interface, where they can be dissociated and the resulting charge carriers transported away,
while inhibiting any recombination processes.
POTENTIAL IMPACT
The potential impact of success in the development of new organic photovoltaic systems and
device structures will be high-performance, light-weight, conformable, photovoltaic solar arrays
that contain sustainable and nontoxic species.
REFERENCES
C.J. Brabec, “Organic Photovoltaics: Technology and Market,” Sol. Energy Mater. Sol. Cells 83 ,
273–292 (2004).