Properties of Organic and Hybrid Photovoltaic Structures. Many basic science questions
need to be addressed to better understand the fundamental steps involved in energy conversion in
organic and hybrid systems. First, the morphology of the solid-state PV film needs to be defined,
then researchers need to develop an understanding of the relationship between the morphology
and the structure of the building blocks, the nature of the substrate, and the deposition method
used to fabricate the film. Physical methods are needed to elucidate the thermodynamics and
dynamics of the fundamental steps of light absorption and exciton generation, charge carrier
generation, and charge carrier diffusion to the collector electrodes. To obtain cells that are
durable enough for long-term use, photostability and photochemical degradation pathways need
to be improved.
Achieving the long-term objective of an organic PV solar cell with a power conversion
efficiency that approaches 50% will require solar cells that can extract more of the photon energy
available within the solar spectrum. This can be achieved through one of two methods:
(1) A device architecture designed to match individual solar photons, such as the
tandem solar cell (see Figure 8);(2) Up-conversion or down-conversion of the solar photons to match an existing
single-junction PV design (see Figure 9).While the first method requires specific design characteristics of the PV structures to have
absorption profiles that match the solar spectrum, combined with balanced charge-carrier
transport properties, the second requires control of the material to efficiently shift the photon
frequencies.
An intrinsic feature of the organic solar cell is the diversity of interfaces; either between
dissimilar organic materials acting as the photoconversion layer; with the contact electrodes,
where transparency and good ohmic contact are essential; or between multiple layers in a tandem
design, where carrier annihilation must be facilitated. These features provide additional
challenges that can be met through molecular design and an understanding of electronic
interactions at an interface.
Photoelectrochemistry
Photoelectrochemical (PEC) systems provide the best-known wet chemical method of converting
sunlight into electrical energy or chemical fuels. PEC systems developed during the mid-1970s
and 1980s for capture and conversion of solar energy into electricity and fuels are based on a
semiconductor electrode in contact with an electrolyte solution (Memming 2001; Bard et al.
2002; Nozik and Memming 1996; Nozik 1978; Grätzel 2001). The solid-liquid configuration
offers the following four advantages: (1) the junction required for efficient charge separation of
photogenerated electrons and holes is very easily formed by simply immersing the
semiconductor in an appropriate electrolyte solution; (2) the liquid electrolyte offers the
capability of a readily conformable and strain-free junction; (3) a third electrode can be added to
PEC cells to provide in-situ chemical storage for 24-hr/day power; and (4) the conversion of