Abstract: Si based tandem solar cells speak to a contrasting option to conventional compound III-V multi-junction cells as a promising approach to accomplish high efficiencies. In this study, we compare between an experimental model of a novel 4-terminal tandem cell design with a Back-Junction Back-Contacted (BJBC) c-Si and a theoretical model of a new structure of tandem solar cell based on c-Si using MATLAB code. The result obtained shows that we can arrived to the same efficiency η = 28% with the both of model but with less cost and problems of physics coherence in our model.