For a detailed understanding of solar cell operation and optimization it is necessary to know how the main performance parameters (open circuit voltage, short circuit current and fill factor) depend on material and structural parameters. In this work, we give analytic formulas for the case of solar cells consisting of a single layer absorber, derived from the drift-diffusion model under some simplifying assumptions. We provide insight into the dependency of these parameters on mobility, light intensity, contact workfunctions and recombination coefficients, and we derive simple formulas for limiting cases.