For amorphous and microcrystalline silicon solar cells, key issues to determine their device performance are overviewed in tenus of material quality as well as interface nature. For amorphous silicon, the photodegradation is well correlated to the SiH2 density in the film, and it is demonstrated that the improvement of the degradation is attained by eliminating the contribution of higher silane radicals to the film growth. For microcrystalline silicon, electronic properties are much more sensitive to the impurities. The passivation of the impurity is essential for better performance. A possible way is to employ the low temperature processing for the intrinsic layer (T < 180 degrees C). It is also found that the low temperature processing is effective to avoid the damage at the p/i interface due to the defect creation accompanying the impurity (B) diffusion. Finally it is also demonstrated that the deterioration of the device performance at higher deposition rate can be avoided under higher gaseous pressure in the plasma CVD process. The origin of the deterioration is discussed in terms of the structural and electronic properties. (c) 2005 Elsevier B.V. All rights reserved.