This work first theoretically optimize the amorphous (a-Si:H) and the microcrystalline (μc-Si:H) devices characteristics, and then perform studies for micromorph tandem solar cells. The studies calculated by AMPS-1D show that the TCO work function has obvious influence on the open-circuit voltage. Moreover, the power conversion efficiency of an a-Si:H cell is optimized by changing the absorber's layer thickness and mobility gap. Furthermore, a critical doping concentration of μc-Si:H films limiting the barrier height of a grain boundary (GB) is observed. After optimizing sub cell mutually, we combine the individual junctions to construct a micromorph cell which has an efficiency of 9.24%.