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

process of charge carrier recombination. Transport of the resulting charged carriers away from
the interface so as not to form bound interface states is an equally important process to control,
but both these processes are poorly understood. The role of interfacial energy off-sets, dipole
layers, exciton binding energy, and spin states must be understood, and their relationships to the
electronic structure of the interface are important issues to address.
Semiconductor quantum dots, metal nanoparticles, and carbon nanotubes are all examples of
species that can be incorporated in an organic host to promote exciton dissociation and/or
additional charge carrier generation and transport. Similarly, photoexcited semiconductor
nanoparticles can undergo charge transfer upon contact with metal nanoclusters (such as gold
and silver). Such charge redistribution can influence the energetics of the composite by shifting
the Fermi level. A better understanding of the mediating role of metal nanoclusters, including
their size and shape dependence on the storage and transport of electrons, is needed to design the
next generation of hybrid systems. Metal nanoparticles have potential as components of the
interconnecting junction in a tandem solar cell, where they act as recombination centers, but they
can also dramatically influence the optical properties of the surrounding medium.
Third-generation OPV
To achieve a device with efficiency that approaches 50% will
require the development of organic species and device
architectures that can extract more energy from the solar
spectrum than can a single-junction device. The two basic
methods for achieving this goal are the development of
efficient structures for up- and down-conversion of solar
photons to match an existing, single-junction device; or the
construction of multiple, stacked single-junction devices that
are optimized for specific wavelengths of light within the solar
spectrum (Figure 31).
To achieve these objectives, research into the relationship
between the excited-state properties of organic molecules and
their structure is needed. For photons absorbed above the
optical bandgap, such a strategy can lead to systems with the
ability to either down-convert the initial excited excitons into
multiple ground-state excitons and ultimately into multiple
charge carriers, or systems that can facilitate the up-conversion
of sub-optical bandgap solar photons into excitons.
The progress made in the direction of stacked (tandem) solar
cells will be facilitated by the development of new deposition
procedures that can be adapted to provide the required
structures. Such issues as layer thickness and the creation of
multiple interfaces are nontrivial aspects of the problem that
are far from optimized and require attention. Furthermore, the
need for materials that can act as interconnectors for balanced
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Figure 31 Schematic of a
multi-layered, tandem organic
solar cell with three stacked
solar cells designed to absorb
different solar photons that
enter through the substrate.
Balanced charge transport in
each cell is indicated with
efficient recombination at the
two interconnectors.