Needs of Direct-gap, Thin-film Photovoltaic Technologies. A second strategy for reducing
costs is to use thin-film materials that have a very high absorptivity for solar photons. Such
materials are called direct-band-gap semiconductors; Si is an indirect-band-gap semiconductor
and absorbs relatively weakly, thus requiring a relatively large thickness of Si in the PV cell to
absorb all the incident sunlight. Substantial research efforts have produced direct-band-gap CdTe
and CuInSe 2 solar cells with efficiencies approaching 20%. Industrial efforts to manufacture
cells made of these materials in high volumes are beginning to demonstrate success. However,
this process has been slower than expected because much of the basic science of these solar cells
is not understood. These polycrystalline solar cells are affected by many things, including the
grain structure obtained for growth on foreign substrates, the effects of intentional and
unintentional impurities on doping and performance (e.g., injection of sodium affects the
performance of CuInSe 2 cells), and the nature of the active junction and ohmic contacts formed
by poorly understood processes. A basic understanding of these issues would facilitate the
technology transfer to large-scale production, enabling a revolutionary growth of the PV
industry.
Needs of Concentrator Cell Technology. A third strategy would reduce costs by using
inexpensive optics to concentrate the light on small-area solar cells. Four recent
achievements/developments provide a foundation and momentum:
(1) An efficiency of 37.9% has been obtained, with possible pathways to higher
efficiencies.(2) Lattice mismatched III-V solar cells with performance approaching the
radiative limit have been demonstrated, implying that such cells may reach
efficiencies in the 40–50% range.(3) A 1-kW multi-junction concentrator system is now supplying electricity to the
grid, paving the way for larger prototypes and manufacturing; and(4) In 2004, installations with sizes >100 kW increased to 20 MW, implying that
a market appropriate for concentrators may be emerging.Taken together, these recent developments imply that concentrator cell technology may be
poised for rapid growth. Key to this growth is the integration of multiple materials for fabrication
of higher-efficiency solar cells.
Need for Revolution to Create New Technologies
In addition to investing in basic research to support a slope change in the present evolutionary
path of existing PV technologies — including crystalline Si, thin-film approaches, and multiple-
junction tandem cells — an aggressive, high-risk research program must be developed for as-yet-
unknown or nascent approaches to solar energy conversion. Such a research effort must target
the development of inherently high-efficiency and low-cost conversion concepts to rapidly