Research and development of our thin film Si solar cells are reviewed. By combining a poly-Si cell with an a-Si cell both of which are fabricated by plasma chemical vapor deposition (CVD) at low temperatures, a stabilized efficiency of 12% has been achieved for a-Si : H/poly-Si/poly-Si cell structure. The stabilized efficiency of 11.3% has been achieved for 10 segments of a-Si/poly-Si stacked cell. The segments are monolithically interconnected in series and have an area of 25 cm².