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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A shortcoming <strong>of</strong> this approach is that the discovery process relies on accident or serendipity, or<br />
targeting in a limited domain, and ultimate success requires a long research and development<br />
process. A more desirable approach (Franceschetti and Zunger 1999) is one that emphasizes<br />
design <strong>of</strong> materials with targeted properties as an integral part <strong>of</strong> the discovery process (see<br />
Figure 55). New approaches to discovery-by-design can be based on several observations:<br />
• Current research-oriented advanced materials synthesis and processing<br />
methods can produce a wide variety <strong>of</strong> both equilibrium and nonequilibrium<br />
atomic configurations — almost at will.<br />
• The choice <strong>of</strong> atomic configuration in a material controls many <strong>of</strong> its physical<br />
properties.<br />
• There are <strong>of</strong>ten too many possible atomic configurations <strong>for</strong> direct and<br />
explicit prediction <strong>of</strong> properties.<br />
Thus, the challenge underlying these<br />
observations is to identify an atomic<br />
configuration (structure) with a given,<br />
useful target property, out <strong>of</strong> an<br />
astronomical number <strong>of</strong> possibilities<br />
(Franceschetti and Zunger 1999).<br />
Progress in both theoretical and<br />
experimental methods is needed. For<br />
photovoltaics and photoelectrodes, the<br />
materials properties that need to be<br />
identified and optimized include<br />
semiconductor band structure, band gap,<br />
band edge energies, carrier mobilities,<br />
electron affinity, work function,<br />
oscillator strength and selection rules<br />
(direct vs indirect band gap), phonon<br />
spectrum, electron-phonon scattering parameters, lattice constants, atomic order-disorder<br />
behavior, and defect structure. The specific properties required will depend upon the specific<br />
type <strong>of</strong> device being considered.<br />
Thermoelectrics<br />
Comprehensive Theoretical Guidance on Thermal and Electronic Transport in Complex<br />
Structures. Over the past decade, progress has been made in the theory <strong>of</strong> thermoelectricity,<br />
noticeably the work <strong>of</strong> quantum size effects on the electronic power factor (Hicks and<br />
Dresselhaus 1993), interface effects on the thermal conductivity (Chen 2001; Chen et al. 2003),<br />
and the use <strong>of</strong> density functional theory <strong>for</strong> the electron and phonon band structures (Singh<br />
2001). However, existing theoretical approaches lack predictive power. For bulk materials, the<br />
challenges lie in predicting the structures <strong>of</strong> materials, and their electronic and phononic band<br />
structures and transport properties, and in understanding the impact <strong>of</strong> defects in the materials on<br />
162<br />
Atomic Configuration<br />
Electronic Structure<br />
E g , m*, f fij, ij, T c<br />
Search<br />
Algorithm<br />
Target<br />
Properties<br />
Figure 55 Materials by design. In one manifestation,<br />
the process begins with a set <strong>of</strong> target properties.<br />
A simulation tool is combined with a search algorithm<br />
process to find a test atomic configuration, then<br />
calculates the values <strong>of</strong> the properties and adjusts the<br />
configuration if necessary. The loop is repeated until the<br />
calculated properties match the target input.