Basic Research Needs for Solar Energy Utilization

A shortcoming of this approach is that the discovery process relies on accident or serendipity, or
targeting in a limited domain, and ultimate success requires a long research and development
process. A more desirable approach (Franceschetti and Zunger 1999) is one that emphasizes
design of materials with targeted properties as an integral part of the discovery process (see
Figure 55). New approaches to discovery-by-design can be based on several observations:

• Current research-oriented advanced materials synthesis and processing
  methods can produce a wide variety of both equilibrium and nonequilibrium
  atomic configurations — almost at will.


• The choice of atomic configuration in a material controls many of its physical
  properties.


• There are often too many possible atomic configurations for direct and
  explicit prediction of properties.

Thus, the challenge underlying these
observations is to identify an atomic
configuration (structure) with a given,
useful target property, out of an
astronomical number of possibilities^
(Franceschetti and Zunger 1999).
Progress in both theoretical and
experimental methods is needed. For
photovoltaics and photoelectrodes, the
materials properties that need to be
identified and optimized include
semiconductor band structure, band gap,
band edge energies, carrier mobilities,
electron affinity, work function,
oscillator strength and selection rules
(direct vs indirect band gap), phonon
spectrum, electron-phonon scattering parameters, lattice constants, atomic order-disorder
behavior, and defect structure. The specific properties required will depend upon the specific
type of device being considered.

Thermoelectrics

Comprehensive Theoretical Guidance on Thermal and Electronic Transport in Complex
Structures. Over the past decade, progress has been made in the theory of thermoelectricity,
noticeably the work of quantum size effects on the electronic power factor^ (Hicks and
Dresselhaus 1993), interface effects on the thermal conductivity^ (Chen 2001; Chen et al. 2003),
and the use of density functional theory for the electron and phonon band structures^ (Singh
2001). However, existing theoretical approaches lack predictive power. For bulk materials, the
challenges lie in predicting the structures of materials, and their electronic and phononic band
structures and transport properties, and in understanding the impact of defects in the materials on

Atomic Configuration

Electronic Structure

Eg, m*, fij,Tc

Target

Properties

Search

Atomic Configuration Algorithm

Electronic Structure

Eg, m*, fij,Tc

Target

Properties

Search

Algorithm

Figure 55 Materials by design. In one manifestation,

the process begins with a set of target properties.

A simulation tool is combined with a search algorithm

process to find a test atomic configuration, then

calculates the values of the properties and adjusts the

configuration if necessary. The loop is repeated until the

calculated properties match the target input.