Analytic treatments of a one-dimensional model p-n homojunction or heterojunction cell exhibiting superposition are consolidated, extended and illustrated. The model cell consists of two quasi-neutral regions of finite width and a junction region which is treated as two back-to-back exhaustion layers. Analytic solutions are obtained for the carrier concentration and flux profiles (current densities), quasi-Fermi levels and carrier collection probabilities and efficiencies, in the dark and under illumination, as a function of distance across the cell. For the most part, it is assumed that the light intensity falls exponentially with depth, but we briefly examine also the case of isotropically forward-scattered light. Surface and bulk recombination are included, but recombination in the junction region and all current transport mechanisms other than injection/diffusion are assumed to be absent. The commonly used quasi-equilibrium assumption for carrier concentrations in the junction is derived and shown to be a reasonable approximation. The criteria for maximum photocurrent from the model cell are listed, and their applicability to cells of more complex architecture and behavior is indicated.