for the detailed understanding and optimization of these systems is a set of complementary
advances in experimental techniques for structural and functional characterization from the
atomic to the macroscopic in time and space, with continual interplay between experiment and
theory.
Research Needs
New theoretical, modeling, and computational tools
are required to meet the challenges of solar energy
research. Currently, highly accurate quantum
mechanical schemes, based on density functional
theory (see Figure 53), are well established to
describe ground state structures of systems consisting
of up to a few hundreds of atoms. In order to
successfully describe the processes that are relevant
to solar energy conversion, the capability of these
approaches will need to be enhanced to deal with
thousands of atoms: this will require the practical
implementation of novel linear scaling
methodologies. In addition, methods for excited-state
potential energy surfaces will have to be developed
and tested. Alternative approaches to deal with
excited-state properties are based on time-dependent
density functional theory, on many-body perturbation
theory, and on quasi-particle equations, but a
consensus on their accuracy is not broadly available
yet, nor have these approaches been applied to
systems with the complexity of the nanoscale components of solar energy conversion devices.
Better schemes for excited states also will be useful to accurately predict band-gaps and band
gap line-ups in a variety of solar energy systems.
Solar energy conversion processes, such as the processes that lead to photosynthesis, are
characterized by activated catalytic processes, which cannot be simulated on the short time scale
of molecular dynamics simulations. In this case, approaches like first-principles molecular
dynamics, which use a potential energy surface generated from ground-state density functional
theory, need to be supplemented by approaches for finding chemical reaction pathways both at
zero and at finite temperature. These approaches should allow us to characterize the reaction
intermediates and transition states in chemical and photochemical reactions in processes like
water-splitting, which is essential to solar hydrogen production by hydrolysis. Ab initio quantum
mechanical methods will need to be extended to deal with up to tens of thousands of atoms, by
means of parameterized empirical or semi-empirical approaches. To understand the complex
organization and assembly of biological light harvesting systems that are made of non-covalently
bonded molecular subunits, classical force fields are required: these will need improved
formulations for dispersion forces. Finally, charge and energy transfer, trapping, and
recombination/relaxation processes are crucial in all energy conversion devices from
photovoltaic, to photoelectrochemical, to natural (biological) systems. Modeling these processes
Figure 53 Density functional theory
calculations give the optimized geometry
of a (Ph 2 PO 2 ) 6 Mn 4 O 4 cubane complex (a
quasi-cubane Mn cluster) that is relevant
to photosynthesis.