Recent progresses in amorphous and microcrystalline silicon materials are reviewed. Photo-induced degradation is controlled not completely but strictly in amorphous silicon deposited at high growth rate of 2nm/sec and stable solar cell showing an efficiency of 8.2% is demonstrated. Mechanism of the growth of microcrystalline silicon is started to be understood using surface-reaction diagnostic techniques, which enables us to fabricate high efficiency microcrystalline silicon solar cells at low temperature as low as 140C.