Ultrathin Parasitic SiO_z Layer Formation at Annealed Wet-Chemical NiO_x/Si Interfaces in Perovskite/Si Tandem Solar Cells

NiOx is a promising hole-transport layer for perovskite/Si tandem solar cells due to its high work function and long-term bulk stability. Besides that, it should also be inert to reactions with materials in its proximity. NiOx will be interfaced to Si using a TOPCon bottom cell and the question about the chemical stability of this interface arises. In this contribution, we investigate the effect of air-annealing between 200 degrees C and 600 degrees C on the microstructural, electronical, chemical and electrical properties of the wet-chemical NiOx/Si interface with X-ray photoelectron spectroscopy (XPS), electron-energy loss spectroscopy in a scanning transmission electron microscope (STEM-EELS) and micro transfer length measurements (mu-TLM). A mixed bulk phase consisting of NiOxHy after annealing at 200 degrees C is found, which is converted to pure NiOx above 300 degrees C. The Fermi energy shifts correspondingly towards the NiOx valence band, indicating suitable p-selective properties. A nm-thin parasitic SiOz layer at the NiOx/Si junction is confirmed after an annealing step as low as 300 degrees C, reaching a thickness of 3-4 nm after annealing at 500 degrees C. This interfacial layer acts as a barrier for electrical current transport, greatly increases the contact resistivity and may hinder tandem integration.