Single junction solar cells have a theoretical efficiency limit of 33.1% due to spectral mismatch. To overcome this limitation, two or more single junction cells with different bandgaps can be coupled together to achieve higher energy conversion efficiency by optimum spectral utilization. In this work, thin film silicon-based alloys are used to obtain a high potential and a current matched 2-junction solar cell is fabricated. The top cell is a-Si which has high bandgap of 1.8eV and the bottom cell is a-SiGe:H which has a tuneable bandgap between 1.4-1.6eV. This thesis will focus in optimizing the window layer of the top a-Si solar cell and the effects of Hydrogen Plasma Treatment (HPT) at the i/p and p/TCO interface. This top cell is fabricated in a PECVD cluster tool in n-i-p substrate configuration on ASAHI glass substrate. The use of PECVD allows for a better control of the layer properties by changing the gas flow rates and the deposition environment. Initially a reference cell is fabricated which had an open circuit voltage (Voc) of 804 mV, a fill factor (FF) of 0.63, a short circuit current density (Jsc) of 12.8 mAcm-2 and an efficiency of 6.56 %. To improve the performance of the solar cell the the effects of HPT on the material properties of the i-layer and the p-layer are studied at varying power, pressure and duration of exposure. The next step is to study the effects of the precursor gas flow rates for the deposition of p-nc-SiOx layer and optimize the thickness of the individual components of the window layer including the TCO. The optimization process involved trading off certain properties in favour of a high Voc and high FF. The final cell had an open circuit voltage (Voc) of 863V, a fill factor (FF) of 0.655, a short circuit current density (Jsc) of 12.5 mAcm-2 and an efficiency of 7.06 %. Thus, by only optimizing the non-active window layer an open circuit voltage gain of 60mV is achieved with an improved FF. This single junction cell is then fabricated on top of a-SiGe:H solar cell. With some iteration in the absorber layer properties of both the top and bottom cells, the final tandem cell had a Voc of 1395 mV, a FF of 0.69, the current limiting Jsc of the top cell at 8.34mAcm-2 and an overall efficiency of 7.99%. To further improve this tandem cell, the tunnel recombination junction can be further optimised.