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