Utilizing the two-dimensional (2D) nano-bands with graphene-like atom arrangement in the structure of the solar cells is of significant importance to present the next generation of solar cells. The present study used germanene (a 2D structure made of germanium atoms) as p-type and n-type semiconductor layers in structure ITO/germanene (1, 2, 3)/MoS2 (n)/a-SiGe: H (i)/c-Si (P)/Au of the heterojunction cell. In this structure, germanene (1, 2, 3) is the p-type germanene semiconductor layer doped with AL, the n-type germanene semiconductor layer doped with P, and the p-type germanene semiconductor layer doped with In, represented by germanene1, germanene2, and germanene3 respectively, and were used separately in the solar cell structure. Also, the free-standing germanene was used as front contact in structure germanene/MoS2 (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 AFORS-HET simulation tool. According to the results, in 1 Sun radiant intensity, by employing germanene1, germanene2, and germanene3 layers separately in the cell structure, the highest efficiency was achieved in the presence of the germanene2, which was 18.64. The highest efficiency in 0.1 Sun and 100 Sun radiant intensities was 17.78 and 19.56, respectively, both obtained in the presence of the germanene2 layer. 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 positive effects of the germanene layer, both as semiconductor layers and free-standing germanene (front contact), were observable in all investigated radiant intensities.