Utilizing the two-dimensional (2D) nano-bands with graphene-like atom arrangement in the structure of the solar cells is of significant importance for the next generation of solar cells. In the present research, germanene (2D structure consisting of germanium atoms) was placed in ITO/germanene (1, 2, 3)/ \({\hbox {MoS}}_{2}\) (n)/a-SiGe: H (i)/c-Si (P)/Au heterojunction solar cell structures once as semiconductor layers with Al (germanene1), P (germanene2), and In (germanene3) dopant, separately. Then, the free-standing germanene was used as front contact in a structure consisting of germanene/ \({\hbox {MoS}}_{2}\) (n)/a-SiGe: H (i)/c-Si (P)/Au of the heterojunction cell. The impacts of different radiant intensities at 300 K temperature by the AM1.5 spectrum radiation were investigated using the AFORS-HET simulation tool. The highest efficiency was obtained in the presence of the germanene2 layer, which was 18.64%, 17.78%, and 19.56%, respectively, in 1 sun, 0.1 sun, and 100 sun radiant intensities. By applying the free-standing germanene in the structure of the proposed cell, the efficiency in radiant intensities of 1 sun, 0.1 sun, and 50 sun were 26.98%, 25.87%, and 27.99%, respectively. The results suggest that this 2D structure can improve the cell’s output parameters, especially the efficiency, positively affecting the solar cell function due to its monoatomic thickness. Therefore, germanene can be an emerging competitor to other 2D structures used in the structure of solar cells.
