The conversion efficiencies of amorphous silicon (a-Si) solar cells were found to decrease with an increase of the cell size. The cause of this effect was investigated, and a part of the cause was indicated by a distributed constant model. In order to enlarge the size of a solar cell with high efficiency, we have developed an integrated structure for the a-Si solar cell module. The best conversion efficiency of this module with a size of 10×10 cm2 was 3.4% in sunlight.