  An isolated single-junction solar cell's temperature in outer space depends
only on its radiation exchange with its environment. We consider a cell with
zero emissivity below its bandgap and unity above it -- an idealization so far
considered to define the upper limit of photovoltaic power conversion
efficiency. For this case, we show that the detailed-balance and the energy
conservation laws that govern the cell's temperature and potential, cannot be
mutually solved. However, the cell's temperature and potential can be
determined if a finite amount of sub-bandgap emissivity is included, the exact
amount of which is found by minimizing the process's entropy generation.
Finally, we generalize this result to a photovoltaic system in contact with
some environment, hence for terrestrial conditions. Therefore, a universal
thermodynamic formulation of the photovoltaic effect emerges. Unlike former
attempts to thermodynamically justify the PV effect, our formalism applies to a
work producing system.
