Interfaces in amorphous/crystalline silicon heterojunction solar cells

The passivation of the amorphous/crystalline silicon heterojunction (SHJ) and the hole transport across the same are the dominating topics in this thesis. The different chapters focus on: The influence of hydrogen on SHJ passivation and an alternative passivation process for surfaces with higher surface free energy, the application of anti-reflection nanostructures and the current generation in these structures, the application of a novel liquid precursor for SHJ passivation, the investigation of its conversion process and the differences between the resulting layer and PECV deposited a-Si:H layers, the transport across the SHJ and its dependence on the valence band offset and the future research topics for silicon heterojunction solar cells. Chapter 4 comprised an investigation of a two step process for the fabrication of (i)a-Si:H passivation layers on non-ideal c-Si surfaces. The more common approach of directly depositing the best possible passivation layer in a single step is sensitive to surface properties, such as the higher surface free energy of the (100) surface, and results in higher defect densities at Si-(100), or crystallographically undefined surfaces, than on Si-(111). Using a two-step approach allows to deposit structurally worse a-Si:H layers first. These layers are less likely to form epitaxial regions on surfaces with higher free surface energies. In a second step the layer is then exposed to a hydrogen plasma treatment, which allows to introduce additional hydrogen into the layer and reduce the structural disorder. The two step approach allows to form an epitaxy free and hydrogen rich interface at the SHJ. Chapter 5 describes the application of the aforementioned method to passivate nanotextured silicon surfaces with (i)a-Si:H. The technological question in this project was to enable epitaxy free (i)a-Si:H growth on the crystallographically undefined nanotexture and then apply a hydrogen plasma, which improves the structural quality of the layer, but does not induce crystallization. Additionally the nanotextured solar cells featured a low quantum efficiency at the shorter wavelengths of the visible spectrum. It was deduced from simulations that this low quantum efficiency is based on parasitic absorption in crystalline silicon and a follow-up experiment with nanotextures of varied height enabled to show that this parasitic absorption happens inside the nanotexture. The work on liquid silicon precursors for SHJ passivation is another application for the hydrogen plasma developed in chapter 4. In contrast to the previous application, the structural quality of the as-deposited a-Si:H layers was acceptable, as shown by photo electron spectroscopy measurements of the valence band. Amorphous silicon layers prepared from liquid precursors feature low hydrogen densities. Also, these layers tend to be macroscopically porous. Both issues can be solved by a HPT. In addition to achieving a well passivated SHJ with a liquid silicon precursor, the conversion of the liquid silicon precursor was investigated and the differences of these layers to PECV deposited material were discussed. Using minority carrier lifetime spectroscopy and XPS the conversion from polysilane to a-Si:H was found to happen at about 350°C. Chapter 7 focuses on the charge transfer across the SHJ. A parameter set for a-SiOx:H layers with stoichiometry from a-Si:H to a-SiO2 was developed. These layers feature higher band gaps and therefore higher valence band offsets to the crystalline silicon. Hence they were incorporated as passivation layers at the p/n-junction of SHJ solar cells to investigate the relation between Δ Ev at and the hole transport across the SHJ. The combination of experimental results and device simulation allows to deduce a significant influence of tunnel hopping in the hole transport across the SHJ, especially in the passivation layer. Finally, chapter 8 discusses future topics for research on the silicon heterojunction solar cells, connects them with results from this thesis and discusses results on tungsten oxide hole conduction layers and tandem cells consisting of perovskite top cells and silicon heterojunction bottom cells.