Thin film formation of wet chemically deposited NiO x and parasitic oxides at NiO x /Si interfaces during high-temperature annealing of selective contacts in perovskite/Si tandem solar cells

Wet chemically deposited NiO x is a promising hole-transport material in perovskite/Si tandem solar cells due to its high work function, chemical stability, and low cost. However, it requires subsequent annealing that may lead to the formation of SiO z at the Si interface to the bottom cell. These layers of nm thickness exhibit non-stoichiometric chemical composition and are detrimental for charge transport and passivation. In this contribution, we investigate the atomic and electronic structure as well as chemical properties of NiO x layers on Si after wet chemical deposition and annealing in air between 200 °C and 600 °C. For this purpose, photoelectron spectroscopy, X-ray diffraction, differential thermal analysis/thermogravimetric analysis coupled with Fourier transform infrared spectroscopy, time-of-flight secondary ion mass spectrometry, and electron-energy loss spectroscopy in a scanning transmission electron microscope are applied. After deposition and annealing at 200 °C, a mixed bulk phase consisting of NiO x H y is found which transforms to pure NiO x above 300 °C. A parasitic SiO z layer is observed at the NiO x /Si interface after this process, with a thickness of 3–4 nm after annealing at 500 °C. The impact of this interfacial layer for electrical current transport will be discussed concerning optimized performance and integrability in perovskite/silicon tandems.