The interesting case of a single-junction solar cell in outer space

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.