Hydrogen plasma treatment of polycrystalline silicon is performed to eliminate the detrimental effects of grain boundaries and grain itself on solar cell performances. Cast polycrystalline silicon solar cell reached a 13.9% photovoltaic conversion efficiency after hydrogenation. Efficiency improvement ratios after hydrogenation have various values, ranging from 3 to 76% on the same grain size samples. It is shown that grain boundary hydrogenation effects are small and crystalline grain performance itself is greatly improved by the plasma treatment. The grain hydrogenation effect may be attributed to the vanishing of the unintentionally introduced carbon impurity defect.