Hydrogenated microcrystalline silicon germanium (mu c-Si1-xGex:H), with the advantage of its narrower variable band gap and higher absorption coefficient over the conventional hydrogenated microcrystalline silicon (mu c-Si:H), has been implemented as the bottom sub-cell absorber of the triple junction solar cells. By replacing mu c-Si:H i-layer with mu c-Si0.91Ge0.09:H i-layer in the triple junction solar cell, the bottom sub-cell thickness (D-bottom) could be reduced by almost a half, meanwhile a higher efficiency was attained. As a result, an initial efficiency of 12.02% in an a-Si:H/a-Si0.6Ge0.4:H/mu c-Si0.91Ge0.09:H triple junction structure with a total cell thickness as small as 1800 nm was achieved. It is demonstrated that the triple junction solar cell incorporating mu c-Si1-xGex:H bottom sub-cell with high efficiency and a relatively low thickness has a high potential for cost-effective photovoltaic applications. (C) 2013 Elsevier B.V. All rights reserved.