Crystal domain size control in organometal halide perovskite materials and its effect on ion and defect migration as well as its optoelectronic properties for photovoltaic application

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Perovskite materials based on organometal halides, such as methylammonium lead iodide (MAPI) found a lot of attention within the solar energy community in the very recent time. Their development in reported device efficiency is unprecedented, starting with an efficiency of around 3% in 2012 of more than 21% recently. Their general elemental abundancy and potential viability for low cost solution processing make them a very promising candidate for 3rd generation photovoltaics, but also light emitting devices. Despite this quick development, there are still many fundamental question remaining. We have shown that surprisingly high crystalline semiconductor qualities are achievable, however, the effect of grain boundaries and their overall effect on stability, trap states, disorder, ion and defect state migration has not completely understood. Object of this proposal is the control of crystalline domain sizes in a wide range within the perovskite films, and to investigate the subsequent effects on its optoelectronic properties. The first part of this project focuses on the crystal size control of organometal halide crystallites in thin films and their related mechanism for crystal growth. We will establish a method how to control the crystallisation kinetics and the crystal grain sizes on a very broad range using controlled atmospheres of selected solvent vapour. With this method we directly influence the crystallisation process, nucleation density and coalescence without having to alter precursor compositions. The aim is to provide a precise control of crystallite sizes on a broad range from nanometres to several tens of micrometers. Spectroscopic and microscopic methods track the respective results and kinetics. In-situ structural characterisation using X-ray scattering will be a second part of this project and will give precise information on the kinetics of crystallisation. The third part of this project focuses on the electro-optical characterisation and creates the ultimate relation to grain sizes. Specifically, this project will employ temperature dependent current transients, which allow an understanding for ion migration and hysteresis, as well as optical techniques such as absorption and photoluminescence spectroscopy as well as wide-field photoluminescence microscopy. Additional methods such as electro-absorption and photoelectron-spectroscopy, which are and will be established in my research group, are available as well. Combining these methods we can address the influence of grain boundaries on ion and defect migration, trap states and disorder as well as solar cell device performance, i.e. hysteresis, stability and charge recombination. This will provide a deep fundamental understanding on the thin film and material properties, and create respective design and processing rules for organometal halide perovskite semiconductors for their application in solar cells and related devices.