Correlation between Trap State Properties and Ion Migration in Metal Halide Perovskites

Abstract

Organic-inorganic perovskites have attracted the interest of the research community for their use as absorber materials in photovoltaic devices and, more generally, as semiconductors with exceptional electronic properties. While the efficiencies of these solar cells have seen a strong rise in recent years, the understanding of physical pro- cesses of this material is still limited and devices face stability issues. Many dynamic phenomena, like current-voltage hysteresis, have been attributed to mobile ions and their interaction with charge carriers. Additionally, charge carrier trap states are known to have a strong impact on carrier dynamics. While crystal defects have been identified as an origin of these phenomena, the specific defect type resulting in mobile ions, trap states, or both is still under investigation. Since both ion migration and charge carrier trapping are influenced by common material modifications, a correlation between them seems possible and could provide information on their origin, as well as their role during device operation. In this work, the effects of defect modifications on trap states and ion migration properties were investigated via electrical measurements on solar cells with the pro- totypical CH3NH3PbI3 (MAPbI3 ) as active layer. The energetic distribution of charge carrier trap states is studied using thermally stimulated currents and thermal admittance spectroscopy, which allow the evaluation of energetic depth and density of electronic trap states. Temperature-dependent impedance spectroscopy and analysis of the frequency-dependent conductivity are used to extract activation energies for the movement of ionic charges in the material. Both techniques are used to study MAPbI3 solar cells and show at least one deep trap state and one mobile ionic species. The variation of stochiometry and addition of rubidium as an extrinsic modifier are used to target point defects in the material, which reveals the addition of at least one deep trap state and another mobile ionic species for rubidium incorporation. Measurements across the orthorhombic-to-tetragonal phase transition of MAPbI3 for perovskite cells with surface passivation indicate an intimate connection of the polarization current and the film surface. The persistence of the measured electronic trap states upon surface modification implies that the detected defects originate from the bulk of the perovskite film.