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

important new experimental and computational approaches, can have a major impact on how we
address these vital cross-cutting issues.
Research Issues
Research needs associated with the interface science of photo-driven systems can be divided into
the following broad categories: (1) fabrication of controlled interfaces and thin films,
(2) development and application of new experimental probes of interface structure and dynamics,
and (3) development of new analytical and computational theory capable of elucidating the
relationship between interface structure and relevant interface processes, such as excitation,
charge separation or recombination, and reaction.
With regard to synthesis of high-quality interfaces, research needs include both the development
of highly controlled model research systems and the rational improvement of the array of
different interfaces that are currently impacting progress in solar energy conversion. Underlying
topics of particular importance include the control of interface composition and structure, both
on the atomic- and nano-length scales. The ability to control the nature and density of defects
and active catalytic sites is also of crucial importance. A broad study area with major potential
for both fundamental science and solar energy conversion is the control of hard-soft interfaces,
such as those that arise at the junction between inorganic materials and both conventional
organic materials and biological systems. Control of semiconductor heterointerfaces is also
crucial to facilitate advances in PV conversion, particularly for high-efficiency multi-junction
cells. The control of electrical transport properties at interfaces — a crucial defining factor in
preparing high-quality contacts — represents another research need.
Scientists have made great strides in using theoretical techniques to describe the molecular and
electronic structures of molecules, solids, and solid surfaces for systems exceeding 1,000 atoms.
However, problems related to solar energy conversion impose particular demands on theory.
Describing the potential experienced by carriers at material interfaces is an outstanding issue in
semiconductor device physics that will be even more important in solar energy conversion
because of the central role of interfaces and the variety of interfaces that need to be investigated.
The molecular structure and the charge transfer that occurs when molecules or metals are
adsorbed on semiconductor electrodes determine the interface potential profile experienced by
electrons traversing the interface. By developing an atomic-scale understanding of the
relationship between the interface structure and electronic potential, scientists can optimize the
properties of interfaces in terms of the carrier transport, carrier separation, and carrier
recombination. However, at present, there are no broadly applicable methods for calculating the
excited electronic structures of interfaces that could guide the design of electronic properties of
interfaces. Moreover, the coupling of molecules to semiconductor continua presents significant
challenges for describing the nonadiabatic dynamics leading to charge injection or photocatalytic
reactions at semiconductor surfaces. To be able to describe the excited states of extended
systems such as regular, as well as more realistic, defective interfaces, we will need to develop
new techniques and to extend emerging techniques, such as time-dependent density functional
theory.
84