A semiconductor structure is described, which includes a semiconductor substrate (10) of one conductivity type, having a front surface (12) and a back surface (14). A first amorphous semiconductor layer (16) is applied on the front surface (12); and second (22) and third (32) amorphous semiconductor layers are disposed on portions of the back surface (14) of the substrate (10). The second and third layers are each compositionally graded through their depth, from substantially intrinsic at the interface with the substrate (10), to substantially conductive at their opposite surfaces. In some instances, the first semiconductor layer (16) is also compositionally graded, while in other instances, it is intrinsic in character. The semiconductor structures can function as solar cells; and modules which include a number of such cells represent another embodiment of the invention. Methods for making a photovoltaic device are also described..