One sided c-Si/a-Si:H heterojunction solar cells (Ag/Al/c-Si(n)/a-Si:H(i)/a-Si:H(p)/ITO/Ag) with open circuit voltage (V-oc) as high as 711 mV were fabricated on double side polished n- type c-Si wafer (thickness 275-325 lam, resistivity of 5-10 Omega cm) by RF-PECVD technique in multi-chamber system. The cells were fabricated with hydrogen plasma treatment of a-Si:H(i) layer of different thicknesses prior to the deposition of top a-Si:H(p) layer. The thickness, optical band gap and microstructure of different a-Si:H layers in the cells wen estimated using Spectroscopic ellipsometry. It was observed that hydrogen plasma treatment of a-Si:H (i) layer not only etched out the significant portion of the layer, but also improved the microstructure of the films anc passivated the interface defects without deteriorating the electronic transport properties. The V-oc, of 711 mV is higher than the values reported in literature so far for single sided solar cells. This improvement in V-oc is due to reduction of interface recombination sites, passivation of surface dangling bonds and decrease of series resistance with 2 min H-2 plasma treatment on a-Si:H(i) layer before deposition of a-Si:H(p) layer, which also improved the short circuit current density (J(sc)) and fill factor (FF). The results suggest hydrogen plasma treatment of a-Si:H(i) layer as an alternative approach to achieve surface passivation and improve open circuit voltage of c-Si/a-Si:H heterojunction cells.