The conversion efficiency of a solar cell can substantially be increased by improved material properties and associated designs. In this book, it has been shown that how the parametric optimization can lead to successful design of amorphous silicon thin film solar cells by using a software called AMPS-1D (Analysis of Microelectronic and Photonic Structures) prior to fabrication. Solar cells of single junction based on hydrogenated amorphous (a-Si:H) have been analyzed by using AMPS-1D simulator. The efficiency of single junction a-Si:H can be achieved as high as over 19% after parametric optimization in the simulation. Therefore, the numerically designed and optimized a-SiC:H/a-SiC:H-buffer/a-Si:H/a-Si:H solar cells have been fabricated by using PECVD (plasma enhanced chemical vapor deposition) where the best initial conversion efficiency of 10.02% has been achieved (Voc= 0.88 V, Jsc= 15.57 mA/cm2 and FF= 0.73) as one of the highest recorded efficiency to date.