Using the admittance analysis method, the design of a tandem of two cells stacked one on the top of the other and connected in series is modelled. The i-layer of the top cell is assumed to be made of a-Si:H and that of the bottom cell of a-SiGe:H, and the condition of current matching at maximum power point is applied to determine the tandem's optimal design. The effect of defects as recombination centres is also incorporated in the calculation of the voltage-current dependencies.