In this paper, the effect of gold nanoparticles on n-i-p a-Si:H solar cells with different intrinsic layer (i-layer) thicknesses has been studied. 100 nm and 500 nm i-layer based n-i-p a-Si:H solar cells were fabricated and colloidal gold (Au) nanoparticles dispersed in water-based solution were spin-coated on the top surface of the solar cells. The Au nanoparticles are of spherical shape and have 100 urn diameter. Electrical and quantum efficiency measurements were carried out and the results show an increase in short-circuit current density (J(sc)), efficiency and external quantum efficiency (EQE) with the incorporation of the nanoparticles on both cells. J(sc) increases from 5.91 mA/cm(2) to 6.5 mA/cm(2) (similar to 10% relative increase) and efficiency increases from 3.38% to 3.97% (similar to 17.5% relative increase) for the 100 nm i-layer solar cell after plasmonic enhancement whereas J(sc) increases from 9.34 mA/cm(2) to 10.1 mA/cm(2) (similar to 7.5% relative increase) and efficiency increases from 4.27% to 4.99% (46.9% relative increase) for the 500 nm i-layer cell. The results show that plasmonic enhancement is more effective in 100 nm than 500 nm i-layer thickness for a-Si:H solar cells. Moreover, the results are discussed in terms of light absorption and electron hole pair generation. (C) 2015 Elsevier Ltd. All rights reserved.