Using the admittance analysis method, the optimal design of a single junction a-Si : H solar cell is suggested and its photovoltaic parameters are calculated. The technique is then extended to design a tandem structure of two cells stacked one on the top of the other and connected in series. The top cell is considered of a-Si:H and bottom of a-SiGe:H and the condition of current matching is applied to determine the tandem＇s optimal design. The efficiency of the single-junction cell with the optimal design is predicted to be 13.1% and that of the tandem cell with the perfect current matching is 20.8%. The results of our calculations are discussed in the light of the recent experimental results.