Fabian Panzer - Understanding the optical properties of halide perovskites and using them to understand their film formation

Understanding the optical properties of halide perovskites is crucially important to access the full potential of this new and exciting class of semiconductors in the future. In the first part of my talk, I will thus address the topic of understanding double peak photoluminescence (PL) of halide perovskites, as it has been repeatedly reported for single crystals and thin films, but no consensus about its origin has emerged yet. By different optical spectroscopic approaches and quantitative models, we demonstrate that the additional PL peak results from an extensive self-absorption effect, whose impact is intensified by strong internal reflections [1]. This knowledge allowed us to investigate in detail the temperature induced tetragonal to orthorhombic phase transition of the model perovskite methylammonium lead iodide (MAPbI) by means of PL spectroscopy. In the temperature range of the phase transition, measured PL spectra consist of multiple features, which could be assigned to different optical effects from the two crystal phases. This also allowed to quantify the fraction of tetragonal inclusions below the phase transition temperature and to estimate their size to be about 7–15 nm down to 120 K [2].

In the second part of my talk, I will outline how the optical properties of halide perovskites can be used to also understand their film formation process. To monitor absorption and PL during processing with a sufficient time resolution, we developed a detection system, with which it is possible to measure pairs of PL and absorption with a framerate of up to 15 Hz [3]. With that, we investigated the solution-based two-step processing of MAPbI in situ during spin coating. We find that the film formation takes place in five consecutive steps, including the formation of a MAPbI capping layer via an interface crystallization and the occurrence of an intense dissolution–recrystallization process. Consideration of confinement and self-absorption effects in the PL spectra, together with consideration of the corresponding absorption allows quantifying the growth rates of the initial interface crystallization to be 11 nm s-1 and the main dissolution–recrystallization process to be 445 nm s−1, emphasizing the importance of the latter to the overall processing [4].