Nanostructured thin films for multibandgap silicon triple junction solar cells

A considerable improvement in performance has been achieved for multibandgap proto-Si/proto-SiGe/nc-Si:H triple junction n–i–p solar cells in which hot-wire chemical vapor deposition (HWCVD) is used to obtain the absorber layers of the bottom and the top cell. To achieve this, optimized Ag/ZnO layers are prepared in house with respect to rms roughness and morphology. Apart from optimizing the light scattering from this back reflector, we found that it is important to control the morphology of surfaces to avoid cavities and shunting paths. A further drawback of such rough metallic surfaces is the parasitic absorption due to surface particle plasmons. Nevertheless, optimized back reflectors lead to an enhancement of the photocurrent of as much as 50%. The stable efficiency for a single junction n–i–p cell with optimized back reflector reached 8.6%, which is the highest reported value for n–i–p cells with HWCVD nc-Si:H i-layer. The triple junction cells of 2.5-μm thickness, using silicon germanium (1.5 eV) in the middle cell, have an efficiency of 11%. These cells are stable within 3.5% relative. By keeping all component cells very thin, these triple cells are almost insensitive to light-induced defects. The difference between the best single junction and triple junction n–i–p cells obtained so far in our laboratory and the reported best cells with plasma enhanced chemical vapor deposition (PECVD) i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers.