A very simple analytic analysis of p+in+ solar cell structures is undertaken based in the quasi-neutrality condition (n = p), which validity is verified. The only differential equation to be solved in the model is the classical continuity one. Non linearities, appear only in the boundary conditions. This model can be applied to p+nn + or n+pp+ cells under very high injection conditions. High carrier concentrations are required to support the current flow even in short-circuit conditions, so enhancing the recombination which, on the other hand, is reduced by high injection lifetime increase. Under-linearity between the short-circuit current and the photon flux is also deduced at very high irradiance. The short-circuit current under bifacial illumination is higher than the sum of those currents under front and back illumination alternatively, so leading to inherently better bifacial cells. In the Voc vs Jsc curve values of m in the logarithmic slope mkT/q range from values slightly below one to two. The value m=1 is not to be expected even if the base recombination is negligible due to Dember potential effects.