Hydrogenated microcrystalline silicon (mu c-Si:H) solar cells show decreasing open-circuit voltage with increasing crystalline volume fraction (F(c)). Therefore, most research groups focus on mu c-Si:H films produced close to the transition with amorphous silicon. Three cells with varying F(c) and large-grain fraction, F(lg) have been studied experimentally and by modeling. The large grained fluorinated mu c-Si:H cell appears to be an exception to the above rule, and modeling is used to understand why low and high F(c) mu c-Si:H and mu c-Si:F:H are used as the second intrinsic-layer in hypothetical double junction cells. Modeling suggests possible efficiencies of 11.3%, 8.4% and 13%, respectively. Three methods of double junction cell thickness optimization are discussed. (C) 2010 Elsevier B.V. All rights reserved.