  Because of the temporal incoherence of sunlight, solar cells efficiency
should depend on the degree of coherence of the incident light. However,
numerical computation methods, which are used to optimize these devices,
fundamentally consider fully coherent light. Hereafter, we show that the
incoherent efficiency of solar cells can be easily analytically calculated. The
incoherent efficiency is simply derived from the coherent one thanks to a
convolution product with a function characterizing the incoherent light. Our
approach is neither heuristic nor empiric but is deduced from first-principle,
i.e. Maxwell's equations. Usually, in order to reproduce the incoherent
behavior, statistical methods requiring a high number of numerical simulations
are used. With our method, such approaches are not required. Our results are
compared with those from previous works and good agreement is found.
