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

for electron and phonon band structures (Sing 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 transport properties. For nanostructured
materials, a crucial issue is the role of interfaces on electron and phonon transport. Although the
ultimate goal should be set at predictive tools, modeling should help in pointing directions for
materials synthesis and structural engineering. Insights gained through combined theoretical and
experimental studies on fundamental thermoelectric transport processes are invaluable in the
search of high-ZT materials.
New High-performance Bulk Materials. Several new bulk materials that demonstrate ZT>1
have been identified over the last 10 years. Diverse classes of potential materials need to be
developed so they may serve as sources for novel high-ZT compounds. Mechanisms for
decoupling electron transport from phonon transport in such materials through modification need
to be identified. Research opportunities along these directions need to be systematically pursued.
Nanoengineered Materials. Nanoscale engineering may be a revolutionary approach to
achieving high performance bulk thermoelectric materials. Recent results in bulk materials
(based on AgPbSbTe, called LAST) have shown ZT>2 in a bulk thermoelectric material (Hsu et
al. 2004). An intriguing finding is that this material exhibits a nanoscale substructure. Given the
former successes for high ZT in nanomaterials (quantum dots and superlattice materials), the
nanostructure observed in the LAST material may be essential for achieving a ZT>2. Therefore,
one approach to nanoscale engineering is to synthesize hybrid or composite materials that have
nanoscale thermoelectric materials inserted into the matrix of the parent thermoelectric material.
Developing synthetic processes to nanoscale substructures is an important undertaking.
Nanoscale thermoelectric materials that can independently reduce phonon transport without
deteriorating electronic transport have been implemented in Bi2Te3/Sb2Te3 superlattices
(Ventakatasubramanian et al. 2001) offering ZT~2.4 at 300K and quantum dot PbTe/PbTeSe
superlattices (Harman et al. 2002) offering ZT~2 at 550K. Most of the enhancements have been
attributed to lattice thermal conductivity reduction in nanoscale dimensions. It is anticipated that
further reduction is possible with a comprehensive understanding of phonon transport in lowdimensional
systems. There is also potential for significant ZT enhancement through quantumconfinement
effects (Hicks and Dresselhaus 1993).
Thermophotovoltaics
Significant progress has been made in TPV cells (Coutts et al. 2003; Aicher et al. 2004). The
efficiency of TPV systems depends critically on spectral control so that only useful photons
reach the PV cells. Ideally, spectral control should be done at the emitter side, although filters
standing alone or deposited on PV cells are also being developed. However, emitter temperatures
exceeding 1000°C impose great challenges on the stability of the materials and structures used in
a TPV system, especially for those components that provide spectral control. Photonic crystals
(Fleming et al. 2002), plasmonics, phonon-polaritons, coherent thermal emission (Greffet et al.
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