Key issues for the efficiency improvement of silicon basis stacked solar cells

Aiming for the development of high efficiency with low cost next generation solar cells, a series of systematic investigations has been made on a-Si//poly(μc)-Si thin film stacked solar cells. The basic concept of multi-band gap stacked solar cell and its historical background on the selection of material combinations are briefly introduced. Secondly, some key issues for efficiency improvement on the a-Si top cell are demonstrated. Then, a series of the experimental approaches on the double heterostructure a-Si top cell along with the issues are presented and obtained optimum design parameters on the a-Si top cell are reported. Thirdly, current topics on the bottom cell technologies are classified with active material thickness and their present status are overviewed. Then, some recent technologies on the microcrystalline (μc) basis bottom cells are reviewed. Finally a comparison is made on the realistic achievable theoretical efficiency obtained from computer simulation with some of the top data and remaining problems are discussed