A drift-diffusion model for spin-charge transport in spin-polarized p-n junctions is developed and solved numerically for a realistic set of material parameters based on GaAs. It is demonstrated that spin polarization can be injected through the depletion layer by both minority and majority carriers, making all semiconductor devices such as spin-polarized solar cells and bipolar transistors feasible. Spin-polarized p-n junctions allow for spin-polarized current generation, spin amplification, voltage control of spin polarization, and a significant extension of spin diffusion range.