In this Book, back reflectors for hydrogenated amorphous silicon and hydrogenated nanocrystalline silicon based solar cells have been systematically studied. 1) By using the optical scattering theory and PVOPTICS simulation program, it is found that to characterize the texture of the BR, not only texture height, but also the texture angle is needed. It has been deduced that the light trapping scheme with ideal rough BR should have an effective light path enhancement factor of n(n+1)2 in the absorption media, rather than 4n2, the generally quoted value in references. 2) This work studied optical and textural properties of Al and Ag deposited at various condition. On the basis of the studies, a stacked configuration of ZnO/Ag/Al BR has been proposed. It shows a higher performance than ZnO/Ag and ZnO/Al structure. High quality ZnO films and ZnO/Ag interface are studied. No obvious inter-diffusion and chemical reaction at the interface of Ag with ZnO are observed. 3) Using optimized BR with ZnO/Ag/Al/SS structure, a-Si:H/a-SiGe:H/nc-Si:H triple junction solar cells have been fabricated, with a high efficiency up to 12.5%.