Thin film nanocrystalline silicon and nanostructured interfaces for multibandgap triple junction solar cells

We present the main research aspects encountered during the development of thin film silicon-based single and multijunction solar cells using hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. First of all, a significant current enhancement is obtained by using textured Ag/ZnO back contacts instead of plain stainless steel. The textured interface acts as a diffuse scattering mirror, due to which the optical path in the cell is lengthened. We studied the root-mean-square (RMS) roughness by 2D atomic force microscopy (AFM) and found that the morphology of the surfaces needs to be controlled 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 second large step forward is the use of multijunction (tandem) cells, in which at least one of the cells contains nanocrystalline silicon. This material behaves optically like 1.1 eV material and, used in the bottom cell, it forms a perfect combination with the 1.7 eV amorphous material in the top cell. Using silicon germanium (1.5 eV) in the middle cell, our triple junction cells have reached nearly 11% efficiency and they are stable within 3% relative. The difference between the best single junction and triple junction n-i-p cells obtained so far in our lab 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.