  The ideal solar cell conversion efficiency limit known as the
Shockley-Queisser (SQ) limit, which is based on a detailed balance between
absorption and radiation, has long been a target for solar cell researchers.
While the theory for this limit uses several assumptions, the requirements in
real devices have not been discussed fully. Given the current situation in
which research-level cell efficiencies are approaching the SQ limit, a
quantitative argument with regard to these requirements is worthwhile in terms
of understanding of the remaining loss mechanisms in current devices and the
device characteristics of solar cells that are operating outside the detailed
balance conditions. Here we examine two basic assumptions: (1) that the
photo-generated carriers lose their kinetic energy via phonon emission in a
moment (fast thermalization), and (2) that the photo-generated carriers are
extracted into carrier reservoirs in a moment (fast extraction). Using a model
that accounts for the carrier relaxation and extraction dynamics, we
reformulate the nonequilibrium theory for solar cells in a manner that covers
both the equilibrium and nonequilibrium regimes. Using a simple planar solar
cell as an example, we address the parameter regime in terms of the carrier
extraction time and then consider where the conventional SQ theory applies and
what could happen outside the applicable range.
