Alessandro Senocrate
The present work deals with the defect chemistry and charge transport properties in halide perovskites, and in particular in the archetypal methylammonium lead iodide. These materials are extensively researched due to their very promising application as light-harvesters in solar cells and in other optoelectronic devices. Notwithstanding the numerous studies dealing with these materials (especially with their optical and electronic properties, and with device application), a significant portion of the underlying physics and chemistry is still poorly understood. Indeed, the physico-chemical features behind their exceptional photo-electrochemical properties are still largely unknown. Moreover, these materials suffer from severe degradation processes presently impeding their practical application. In addition, the charge transport in these materials is not purely electronic, but rather shows a significant ionic portion due to mobile ionic defects. The nature of such ion conduction, alongside its effect on the photo-electrochemical properties and on the materials stability, has never been systematically investigated. The study of these aspects is the aim of this thesis, where:
At first, we study the charge transport properties of methylammonium lead iodide, with particular attention to the ionic contribution, and we perform a defect chemical study of the compound. We show how the ionic conductivity, in equilibrium conditions, can be unambiguously assigned to mobile iodine vacancies, with an electronic contribution due to electron holes or conduction electron depending on the iodine activity.
Secondly, we investigate the light effect on the charge transport previously characterized. Alongside an expected increase of the electronic contribution, we observe a striking enhancement of ionic conductivity in methylammonium lead iodide upon illumination. This remarkable observation is of fundamental relevance for both photovoltaics and solid state ionics fields. Here we also discuss a mechanism for such photo-enhanced ion conduction that relies on electron-ion interaction.
Subsequently, we analyze methylammonium lead iodide and other hybrid halide perovskites under oxygen exposure, both in the dark and under illumination. We show that light strongly affects the kinetics of oxygen interaction, so much so that under illumination methylammonium lead iodide completely degrades, while it is metastable in the dark. The oxygen, when incorporated in a sufficient amount in the lattice, also acts as acceptor dopant, greatly varying the ionic and electronic conductivities.
We then investigate stability of several halide perovskites with respect to temperature, oxygen, and illumination through thermodynamic considerations. Here we show how many of these processes are expected to be extremely severe for some of the compounds, underlining an important -and intrinsic- bottleneck for the application of these materials.
At last, we investigate the short-range ion dynamics in methylammonium lead iodide, since these are linked to stability and electronic transport properties. Here, we show that methylammonium dynamics conforms well to a fast bi-axial rotation becoming isotropic in the cubic phase. In the inorganic lattice, a strong nuclear coupling between Pb and I is present, alongside highly active iodine dynamics.
EPFL2018
