Quadruple junction solar cells and modules are presented, which consist of hydrogenated amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) in the a-Si:H/a-Si:H/μc-Si:H/μc-Si:H configuration. The highest measured conversion efficiency of a mini-module with an aperture area of 61.44cm2 was 13.4- before and 12.0- after more than 1000 h of light soaking, respectively. In this paper, we discuss the advantages of the quadruple junction design over the common tandem design, which is ascribed mainly to the fact that the total absorber thickness can be increased while electronic properties and stability are maintained or even improved. The role of the μc-SiOx:H intermediate reflector is highlighted and an optimization of the doping concentration in this layer is presented. Furthermore, the advantage of the high maximum power voltage for the monolithic cell interconnection laser design of modules is shown.