  We study the radiative decay, or fluorescence, of excitons in organic solar
cells as a function of its geometrical parameters. Contrary to their
non-radiative counterpart, fluorescence losses strongly depend on the
environment. By properly tuning the thicknesses of the buffer layers between
the active regions of the cell and the electrodes, the exciton lifetime and,
hence, the exciton diffusion length can be increased. The importance of this
phenomenon depends on the radiative quantum efficiency, which is the fraction
of the exciton decay that is intrinsically due to fluorescence. Besides this
effect, interferences within the cell control the efficiency of sunlight
injection into the active layers. An optimal cell design must rely on the
consideration of these two aspects. By properly managing fluorescence losses,
one can significantly improve the cell performance. To demonstrate this fact,
we use realistic material parameters inspired from literature data and obtain
an increase of power conversion efficiency from 11.3% to 12.7%. Conversely, not
to take into account the strong dependence of fluorescence on the environment
may lead to a sub-optimal cell design and a degradation of cell performance.
The presence of radiative losses, however small, significantly changes the
optimal thicknesses. We illustrate this latter situation with experimental
material data.
