We report a study of a-Si:H/a-SiGe:H dual-bandgap p-i-n type tandem solar cells and Modules. Refined bandgap grading in the a-SiGe:H bottom cell results in higher Jsc (higher red response) without loss in FF or Voc. IMprovements in the tunnel junction consisting of doped widegap a-SiC:H and μC-Si lead to higher FF and lower optical loss. Undoped widegap a-SiC:H is used as interface layers in the top junction. Initial conversion efficiencies of 11.4% and 10.5% have been obtained for small-area cells and one-square-foot modules, respectively. The double junction cells and modules exhibit good stability and saturation of light-induced degradation, showing only ∼15% decrease in conversion efficiency after more than 1000 hours of simulated AM1.5 illumination. The stabilized efficiencies are 9.6% and 9.1%, respectively, for small area cell and large area Module. We also present some findings from the computer simulations of multijunction solar cell performance using a simple lump-circuit device Model.