Perovskite Heterostructure Investigations using Vacuum Evaporation and X-ray diffraction - PHIVE-X

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Perovskite photovoltaics have developed rapidly in recent years, reaching photovoltaic efficiencies well above 20% - close to the thermodynamic (Shockley-Queisser) limit. Furthermore, hybrid perovskites can be directly grown in 2D layered configuration by clever choice of the organic cation, thus creating a prestructured layer stack. For these types of perovskites, stability is increased immensely, even if only a thin layer of 2D perovskite is used as a barrier on top of a thicker “3D” thin-film. In this project, we plan to address the astonishing physical properties of these materials and the influence of dimensionality using vacuum evaporation. Though 2D-perovskites have been demonstrated from solution in a self-layering manner, 2D-layering using highly precise vacuum evaporation techniques has not been shown. Within PHIVE-X, we will use the variability in crystal structure and electric properties of the perovskites to follow two main paths: Realization of vacuum-deposited, self-structured 2D perovskites and manufacturing of highly-precise thin-film stacks of alternating perovskites with different stoichiometry – forced 2D perovskites. The latter is exclusively viable using the unique film control of vacuum deposition. We thus widen the possible material choices and open up a completely new field of perovskite research. By precisely controlling stoichiometry during growth, variations in the materials alter the band gap and refractive index in the ultra-thin films. In particular, we intend to exploit the opportunities of this method in a comprehensive manner, like tuning the band gap by almost 0.8 eV via interchanging iodide and bromine, as well as methyl ammonium and formamidinium. Both paths will be applied to device concepts: Self-structured and forced 2D perovskites will be implemented in solar cells as well as light-emitting devices and investigated towards their performance and stability. With forced 2D perovskites, we will also effectively form double heterostructures and superlattices. If correctly tuned, carrier and light confinement create a range of opportunities for optoelectronic applications like perovskite lasers. The novel structures created with this technique will be extensively studied with regard to the structural and electronic properties, combining the extensive experience of the Dresden and Tübingen groups.