This work illustrates the research activity carried out at ENEA for the achievement of high efficiency large area a-Si modules by a low cost process involving a simple single chamber PECVD reactor. Device optimization leading to small area (1 cm2) single and tandem a-Si junctions exceeding 10% efficiency is described. Details of fabrication process, such as back-reflector, microcrystalline layers, interface cleaning process, adjustment of i-layer absorption and device thermal annealing will be given. In addition, the results of laser scribing interconnection technique for module realization are also described. Back scribing and modified beam power distribution were effective for the improvement of module efficiency. A pin/pin tandem module with initial efficiency of 9.1%, on active area of 750 cm2, has been realized. This is the best efficiency ever reported by a European laboratory for a-Si large area tandem modules.