Silver nanoparticle (Ag NP) enhanced back reflector (BR) has been used for effective light trapping in thin-film silicon solar cells. In this letter, we demonstrate experimentally that the photocurrent density of hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) based solar cells are enhanced by as much as 3.0 mA/cm(2) or 19.5% with medium sized (320 nm) Ag NPs as compared to the specular BR. In addition, it is demonstrated that the optimized Ag NP BR is also effective in multi-junction solar cells. It is believed that it is the appropriated surface morphology of the Ag NPs that leads to the improved light-trapping. It is found that the average haze of the Ag NP enhanced BR can be improved to as much as > 70% throughout the wavelength range 600-900 nm. The atomic-force-microscopy measurement shows that the optimized root mean square roughness of Ag NP BR is about 35.2 nm. (C) 2015 Elsevier B.V. All rights reserved.