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

NEW EXPERIMENTAL AND THEORETICAL TOOLS TO ENABLE
TRANSFORMATIONAL RESEARCH
Solar energy conversion systems involve many components to achieve the functions of light
capture, conversion, and storage. Experimental tools and theoretical capabilities that can
capture the behavior of these systems, which span many decades in space, time, and structure,
do not yet exist. Development of such tools would allow experimentalists to directly probe the
behavior of molecules, materials, structures and devices, and could enable the theoretical
prediction of optimally performing structures without having to first make the systems in the
laboratory.
EXPERIMENTAL TOOLS: REAL-TIME LOCAL PROBES FOR ATOMISTIC
STRUCTURE AND FUNCTION
Overview
Efficient conversion of solar energy to electricity and chemical fuels requires complex interplay
between multiple functional components and processes occurring in differing length and time
scales. Consider, for example, a Grätzel cell where photoexcited redox reactions on
nanostructured titania (TiO2) are used to generate electricity. The operation of the Graetzel cell
involves the efficient photon absorption by organic dye molecules, separation of an electron and
a hole at the molecule-TiO2 interface, electron transport through TiO2 grain boundaries, energy
relaxation and charge trapping, solution phase electrochemistry, and the mass transport through
the electrolyte solution. Essentially all known solar energy conversion processes involve
similarly complex physical and chemical processes intertwined with each other, and the
efficiencies and fidelity of solar energy conversion depend critically on the atomistic detail of the
molecule and material systems involved.
The design and optimization of an effective solar energy conversion system requires
experimental tools for investigating these complex, multi-scale processes and their interplay at
the system-wide level. Despite the spectacular expansion of experimental tools that has occurred
over the last several decades, none of the existing techniques allows a detailed atomistic
investigation of these complex processes in real time, pointing to the need for new,
transformative experimental tools in solar energy research.
Research Needs
In principle, an ideal experimental tool should be able to monitor physical and chemical
processes on the full range of length and time scales involving electronic, molecular, nanoscale,
and macroscopic degrees of freedom. This is a daunting challenge, and experimental tools with
the potential to address this complex multi-scale problem are only beginning to emerge
(see Figure 51). Electron microscopy and X-ray/neutron diffraction techniques have enabled the
detailed interrogation of bulk, interfacial, and nanoscale structures with atomic resolution, and
can be used for structural investigation of various components in solar energy conversion
systems.
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