Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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<strong>for</strong> the detailed understanding and optimization <strong>of</strong> these systems is a set <strong>of</strong> complementary<br />
advances in experimental techniques <strong>for</strong> structural and functional characterization from the<br />
atomic to the macroscopic in time and space, with continual interplay between experiment and<br />
theory.<br />
<strong>Research</strong> <strong>Needs</strong><br />
New theoretical, modeling, and computational tools<br />
are required to meet the challenges <strong>of</strong> solar energy<br />
research. Currently, highly accurate quantum<br />
mechanical schemes, based on density functional<br />
theory (see Figure 53), are well established to<br />
describe ground state structures <strong>of</strong> systems consisting<br />
<strong>of</strong> up to a few hundreds <strong>of</strong> atoms. In order to<br />
successfully describe the processes that are relevant<br />
to solar energy conversion, the capability <strong>of</strong> these<br />
approaches will need to be enhanced to deal with<br />
thousands <strong>of</strong> atoms: this will require the practical<br />
implementation <strong>of</strong> novel linear scaling<br />
methodologies. In addition, methods <strong>for</strong> excited-state<br />
potential energy surfaces will have to be developed<br />
and tested. Alternative approaches to deal with<br />
excited-state properties are based on time-dependent<br />
density functional theory, on many-body perturbation<br />
theory, and on quasi-particle equations, but a<br />
consensus on their accuracy is not broadly available<br />
yet, nor have these approaches been applied to<br />
systems with the complexity <strong>of</strong> the nanoscale components <strong>of</strong> solar energy conversion devices.<br />
Better schemes <strong>for</strong> excited states also will be useful to accurately predict band-gaps and band<br />
gap line-ups in a variety <strong>of</strong> solar energy systems.<br />
<strong>Solar</strong> energy conversion processes, such as the processes that lead to photosynthesis, are<br />
characterized by activated catalytic processes, which cannot be simulated on the short time scale<br />
<strong>of</strong> molecular dynamics simulations. In this case, approaches like first-principles molecular<br />
dynamics, which use a potential energy surface generated from ground-state density functional<br />
theory, need to be supplemented by approaches <strong>for</strong> finding chemical reaction pathways both at<br />
zero and at finite temperature. These approaches should allow us to characterize the reaction<br />
intermediates and transition states in chemical and photochemical reactions in processes like<br />
water-splitting, which is essential to solar hydrogen production by hydrolysis. Ab initio quantum<br />
mechanical methods will need to be extended to deal with up to tens <strong>of</strong> thousands <strong>of</strong> atoms, by<br />
means <strong>of</strong> parameterized empirical or semi-empirical approaches. To understand the complex<br />
organization and assembly <strong>of</strong> biological light harvesting systems that are made <strong>of</strong> non-covalently<br />
bonded molecular subunits, classical <strong>for</strong>ce fields are required: these will need improved<br />
<strong>for</strong>mulations <strong>for</strong> dispersion <strong>for</strong>ces. Finally, charge and energy transfer, trapping, and<br />
recombination/relaxation processes are crucial in all energy conversion devices from<br />
photovoltaic, to photoelectrochemical, to natural (biological) systems. Modeling these processes<br />
158<br />
Figure 53 Density functional theory<br />
calculations give the optimized geometry<br />
<strong>of</strong> a (Ph2PO2)6Mn4O4 cubane complex (a<br />
quasi-cubane Mn cluster) that is relevant<br />
to photosynthesis.