Nanostructured thin films for multiband-gap silicon triple junction solar cells

By implementing nanostructure in multiband-gap proto-Si/proto-SiGe/nc-Si:H triple junction n–i–p solar cells, a considerable improvement in performance has been achieved. The unalloyed active layers in the top and bottom cell of these triple junction cells are deposited by Hot-Wire CVD. A significant current enhancement is obtained by using textured Ag/ZnO back contacts instead of plain stainless steel. We studied the correlation between the integrated current density in the long-wavelength range (650–1000 nm) with the back reflector surface roughness and clarified that the rms roughness from 2D AFM images correlates well with the long-wavelength response of the cell when weighted with a Power Spectral Density function. For single junction 2-μm thick nc-Si:H n–i–p cells we improved the Jsc from 15.2 mA/cm2 for plain stainless steel to 23.4 mA/cm2 using rough back reflector. We introduced profiling of the H2 dilution during growth of the nc-Si:H layer to prevent a transition to amorphous growth. The efficiency for a single junction n–i–p cell reached 8.5%, the highest reported value for HWCVD cells of this kind. Moreover, these cells show to be totally stable under light-soaking tests. Combining the above techniques, an efficiency of 10.9% has been obtained for triple junction cells (Jsc = 8.35 mA/cm2, Voc = 1.98 V, FF = 0.661). By using effective light trapping techniques and three different band-gap materials, the required thickness could be kept small (∼ 2.5 μm total).