A 1-D drift-diffusion modeling for impurity photovoltaics is presented. The model is based on the self-consistent solution of Poisson's equation and carrier continuity equations incorporating generation and recombination mechanisms including the intermediate states. The model is applied to a prototypical solar cell device, where strong space charge effects and reduced conversion efficiency are identified for the case of lightly doped absorption regions. A doping compensation scheme is proposed to mitigate the space charge effects, with optimal doping corresponding to one-half the concentration of the intermediate states. The compensated doping device design provides calculated conversion efficiencies of approximately 40%, which is similar to the maximum expected values from prior 0-D models. The carrier transport between intermediate levels is shown to be noncritical for achieving the efficiency limit predicted by 0-D models. The qualitative behavior of the model is compared to existing experimental data on quantum dot solar cells.