Oxidation states in perovskite layers formed using various deposition techniques

Organic–inorganic halide perovskites have rapidly become emerging materials for photovoltaic applications, but a major factor causing instability in perovskite solar cells could be the elemental shift due to oxygen. The present work demonstrates the oxidation states of perovskite layers formed by (i) one-step deposition, (ii) solvent-to-solvent extraction, (iii) hot casting, and (iv) mixed-cation solution processing. X-ray photoelectron spectroscopy is used to investigate the compositional changes due to oxygen, and the spectra confirm the diffusion of oxygen in the perovskite layer. Mixed-cation perovskites show the most promising results among the four deposition techniques. The best photovoltaic performance is achieved by an SnO2-based mixed-cation perovskite solar cell with a power conversion efficiency of 18.75%. A power conversion efficiency of 16.74% is obtained by a TiO2-based mixed-cation perovskite solar cell when the optimum thickness of the TiO2 blocking layer is 60–65 nm.