The role of the carrier’s recombination velocity S_i within the depletion Layer of p-n junction solar cell and the external bias voltage V_a across the junction in determining the current density “J” through the cell is revealed. The unsteady carrier diffusion equation is solved under illumination conditions considering a source spectral function G(λ). The efficiency of the device as a function of S_i , V_a , G(λ) is obtained. Computations considering a silicon solar cell are given as an illustrative example.