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

Figure 19 Transparent conductive electrodes represent one of the major
challenges for PV devices. The problem involves both the bulk properties of
the electrode material and its interface with the PV media. The figure shows
the progress achieved in developing a new transparent conductive electrode
material based on carbon nanotubes. (Source: Wu 2004)
While PV devices typically comprise thin films with important interfacial components, some
schemes are inherently interfacial in character. This is the case in dye-sensitized devices, in
which photoexcitation and exciton breaking occur at the interface between two distinct media
(O’Regan and Grätzel 1991). It is clear that the efficiency and reliability of such devices is
determined, to a large degree, by physical and electronic structure at the interface, by the
dynamics of charge separation, and by the desired and side oxidation/reduction reactions at the
interface. A further important example of a solar energy conversion scheme that is inherently
dominated by interface characteristics is the photocatalysis process for solar fuel production
(Hermann 2005). In this process, photo-driven heterogeneous catalysis is controlled by the
detailed interface structure and composition and, more specifically, by the characteristics and
lifetime of the active catalytic site.
The above discussion demonstrates the central importance of interfaces in solar energy
conversion and the critical role that interfaces play in defining the performance of real devices.
The motivation for cross-cutting research in the interface science of photo-driven systems
reflects this importance. The need for this research is further supported by the major
opportunities for scientific advances and the commonality of issues that underlie these diverse
technologies. From a theoretical perspective, we are concerned with the relation between charge
transport and energy level structure and the physical and chemical nature of the interface. From
an experimental perspective, we need tools that are capable of probing buried interfaces in great
detail to elucidate the structure, not only of the ideal interface, but also of defects and active
catalytic sites. We also need interfacial probes that can follow the evolution of interfaces, not
only on the time scale of hours and days, but down to the femtosecond time scale on which the
fundamental processes of electronic motion, energy flow, and nuclear displacement in chemical
reactions take place. The challenges, both experimental and theoretical, are significant. As we
indicate below, however, research advances in the broader scientific community, including
83