[en] The main parts of a photovoltaic cell and their function are described. Photovoltaic cells are then classified in respect to their operation. The operation of typical cells is analyzed with the goal to obtain current-voltage characteristics in a self-consistent physical model. This is achieved by connecting the emitter diffusion current with the voltage drop in the junction by a doubly acting boundary condition, the electron density at the emitter-junction interface. The consequently obtained characteristics have near the open circuit voltage (Boltzmann range), the form of the commonly used shifted diode characteristic, however, with parameters in substantially improved agreement with the experiment. Outside the Boltzmann range, integration of transport and Poisson equation yields the shape of the characteristics. This theory is then extended to include photovoltaic cells with dominant interface recombination. Such interface recombination causes mostly a lowering of the open circuit voltage. The agreement between theory and experiment is surprisingly good for CdS/Cu₂S solar cells. (author)