Figure 20 A grand challenge of
photovoltaics: How to bridge the gap
between existing photovoltaic devices and
the efficiency limits?Revolutionary Photovoltaic Devices: 50% Efficient Solar Cells
To enable solar electricity from photovoltaics to be competitive with, or cheaper than, present
fossil fuel electricity costs likely requires devices that operate above the existing performance
limit of energy conversion efficiency of 32% calculated for single-junction cells. At present, the
best single-junction solar cells have efficiencies of 20–25%. New concepts, structures, and
methods of capturing the energy from sunlight without thermalization of carriers are required to
break through this barrier and enable solar cells having efficiencies of greater than 50%.
EXECUTIVE SUMMARY
Mature energy conversion technologies typically operate close to their maximum thermodynamic
efficiency. For solar energy conversion, this efficiency is between 66% and 87%, depending on
the concentration and the spectrum. A grand challenge for photovoltaics is the development of
high-efficiency, low-cost photovoltaic structures that can reach these ultimate thermodynamic
efficiency limits. Existing photovoltaic devices, which are based primarily on single-junction
silicon, have made dramatic improvements over the 50 years of their development, and these
solar cells now achieve about three-quarters of the Shockley-Queisser efficiency limit of ~32%.
Discovering new technologies, processes, and materials that allow photovoltaic devices to
substantially exceed this efficiency while maintaining low cost are critical research goals for
photovoltaics.
The viability of achieving these goals has been
dramatically increased in the last few years due to the
combination of theoretical and material advances,
particularly improved understanding of materials and
their interaction with growth and defects; and through
new approaches, materials, and concepts relying on
phenomena allowed by low-dimensional structures.
The latter include approaches such as multiple
junctions (tandems), optical spectrum shifting,
multiple electron/exciton generation, multiple energy
level solar cells, and hot carrier solar cells.
Substantial scientific challenges exist in each of these
approaches, relating to understanding, modeling, and
controlling the basic physical mechanisms, as well as
to incorporating these physical phenomena into high-
performance solar cells (see Figure 20). The
development of solar cells based on such principles
would revolutionize photovoltaics by allowing high-efficiency, cost-effective solar cells, and
further, contribute directly to fundamental scientific advances. Moreover, since many solar
energy utilization technologies depend on the understanding and control of these physical
phenomena, advances in such high-efficiency photovoltaic devices contribute directly toward
enhanced understanding that underpins other solar conversion technologies, including organic
and photochemical conversion as well as biologically based solar conversion systems.