We have achieved a stabilized conversion efficiency of 8.9% in a single-junction a-Si solar cell and 10.6% in a double-junction a-Si/a-SiGe solar cell for a size of 1 cm(2), which are the world＇s highest values achieved so far for this size and structure. We have been investigating the improvement of stability in a-SiGe film with regard to the bottom cell i-layer, and the control of E-opt in a-SiGe film in order to confirm the tandem cell design. On the other hand, uniformity of +/- 1% has been obtained in conversion efficiency for many small cells fabricated in a size of 30 cm x 40 cm, evaluated by using a-Si single-junction structure. As a result, we have achieved the stabilized high-effective area conversion efficiency of 8.64% in a 30 cm x 40 cm a-Si/a-Si tandem submodule. The combination of the above techniques and further optimization can be expected to achieve a stabilized conversion efficiency of more than 10% for a 30 cm x 40 cm double-junction a-Si/a-SiGe submodule.