MAXIMUM ENERGY FROM SOLAR PHOTONS AT LOW COST:
DESIGNED PLASTIC PHOTOVOLTAIC STRUCTURES
“Plastic” solar cells made from molecular, polymeric, or nanoparticle-based structures could
provide flexible, inexpensive, conformal, low-cost solar electricity systems. At present, the
efficiencies of such systems are too low (<5%, generally 2% or less) to be useful; performance
needs to be improved by a factor of 5–10. New materials chemistry, new device designs, and
fundamental understanding of the factors that limit the performance of these systems are needed.
EXECUTIVE SUMMARY
Organic photovoltaic (OPV) devices are in a
comparatively early stage of development. Since the
early proof-of-principle work on organic
photovoltaics in the mid-1980s, total solar to
electrical energy conversion efficiencies have been
pushed to ca. 5%. A vigorous period of research and
development is needed to refine structures,
processing, and cell fabrication techniques to
increase the efficiency of OPVs five- to ten-fold
with respect to current values.
Since the early developments in this field,
substantial improvements have occurred in synthesis
methods for fabrication of the molecular, polymeric,
and nanocrystalline building blocks used to make
organic photovoltaic converters. Significant
advances also have been made in the techniques
used to characterize organic systems and the
fundamental processes involved in operation of
organic photovoltaics (e.g., scanning probe
microscopy, time-resolved optical methods, etc.).
The development of high-efficiency organic solar
cells could lead to a revolution in solar electrical power generation — the promise of an
inexpensive “solar paint” that could be deployed over large areas in a convenient and
conformable format.
The research agenda to achieve these goals requires the design and development of robust
molecular systems with structural, optical, and electronic properties optimized for photovoltaic
energy conversion. This activity will necessarily include a close coupling of molecular and
device theory, directed organic synthesis and purification, photophysical characterization, and
processing. Organic solar cells that are based on the photogeneration of excitons (bound
electron-hole pairs) or excited states, rather than the direct formation of charge carriers,
necessitate a detailed understanding of the fundamental process of charge separation and
recombination at nanostructured interfaces between organic and hybrid interfaces. Parallel
research efforts will involve design of cell architectures to optimize the interactions of light with
Figure 27 Construction of a typical organic
photovoltaic device. The active layer (red)
can be a bilayer of vapor-deposited
molecules, or a bulk heterojunction
containing a polymer and C 60 or an inorganic
quantum structure.