Direct deposition of 10-15-mum-thick silicon layers onto foreign substrates yields a polycrystalline Si material with an average grain size in the range 1-5mum. The strong electrical activation of such large density and small grains is detrimental to solar cell performances. Here we investigate the passivation of such defects by ECR plasma hydrogenation. The electron beam induced current (EBIC) technique is used to reveal the electrical activity of the defects and to extract the minority carrier diffusion length on hydrogenated fine-grained polycrystalline silicon thin films. It is shown that the minority carrier diffusion length is mainly ruled by the grain size (1-2 mum) in the as-grown polycrystalline silicon layer. Hydrogen plasma treatments are performed. taking into account the deep defect passivation and the doping rate control. The diffusion length is then improved up to 10 mum, the same order of magnitude than the probed silicon layer thickness and larger than the mean grain size,