The ambient temperature and the defects density in the Hetero-junction with Intrinsic Thin layers solar cells (HIT) strongly influence their performances. In the first part, we presents the band diagram on the front/back simulated solar cell based on a-Si: H / c-Si (p)/a-Si:h. In second part, we modele the following layers structure: ZnO/a-Si:H(n)/a- Si:H(i)/c-Si(p)/a-Si:H(p)/Ag where we study the effect of the ambient temperature and the defects density in the gap of the crystalline silicon layer on the performance of the heterojunction solar cell with intrinsic layer (HIT).