  The Shockley-Queisser limit describes the maximum solar energy conversion
efficiency achievable for a particular material and is the standard by which
new photovoltaic technologies are compared. This limit is based on the
principle of detailed balance, which equates the photon flux into a device to
the particle flux (photons or electrons) out of that device. Nanostructured
solar cells represent a new class of photovoltaic devices, and questions have
been raised about whether or not they can exceed the Shockley-Queisser limit.
Here we show that single-junction nanostructured solar cells have a theoretical
maximum efficiency of 42% under AM 1.5 solar illumination. While this exceeds
the efficiency of a non- concentrating planar device, it does not exceed the
Shockley-Queisser limit for a planar device with optical concentration. We
conclude that nanostructured solar cells offer an important route towards
higher efficiency photovoltaic devices through a built-in optical
concentration.
