Amorphous Si (a-Si) is potentially an attractive material for thin-film solar cells. However, its potential for high-efficiency devices to date has not been realized. In this paper, we examine the design considerations which can be used to develop high-efficiency devices from a-Si. The design analysis takes full advantage of the flexibility in bandgap and other material properties offered by a-Si. Several structures, such as light-trapped cells, graded layer cells, and tandem junction a-Si/a-Si cells are analyzed in detail for collection efficiency, Voc, and fill factors. It is shown that using these innovative structures, realistic conversion efficiencies exceeding 10 percent can be realized in a-Si in spite of the very low diffusion lengths (∼0.1 µm). Technological considerations for achieving such structures are also discussed.