XPS visualization of soft and hard Lewis base passivation of defects in MAPbI3 perovskite

Abstract

A new method for visualization of uncoordinated lead (Pb²⁺) in solution processed polycrystalline halide perovskite films is presented. The method is based on the measurements of XPS spectra with high energy resolution, which show a narrowing of the low-energy part of the Pb 4f-core level spectra due to passivation of halide vacancies with soft and hard Lewis base doping and reduction of contribution of uncoordinated lead (Pb²⁺) at binding energy of ∼137.8 eV. Based on XPS Pb 4f-spectra measurements the influence of size of hard Lewis base metal iodides on the passivation of surface defects in MAPbI₃ perovskite is discussed. It is found that at low concentrations (1–5 %) CaI₂, BaI₂, AgI with metal ionic radius ≥1.0 Å the metal atoms substitute the coordinated Pb²⁺ atoms whereas at higher concentrations they are located at grain boundaries and passivate the uncoordinated lead sites. For hard Lewis base metal iodides with small ionic radii (0.7–0.8 Å) (MnI₂, FeI₂, CuI) the situation is quite different and metals occupy the interstitial positions keeping the uncoordinated Pb²⁺ ions to be unchanged. The use of appropriate charge-transport layers and soft Lewis base molecular modifiers can improve the crystallization of perovskite films by reducing the number of defects formed, which is also reflected in the XPS Pb 4f_7/2 spectra.