Building a Rational Understanding of Novel 2D Hybrid Halide Perovskites in Bulk and in Films

Hoffman, Justin Michael

doi:https://doi.org/10.21985/n2-c7kv-c491

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Building a Rational Understanding of Novel 2D Hybrid Halide Perovskites in Bulk and in Films

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Two-dimensional (2D) hybrid halide perovskites have been the response to their exciting but woefully unstable 3D counterparts. These 2D perovskites have been shown to have respectable stabilities as photovoltaic absorbers, yet they lag behind the 3D perovskites in terms of efficiency. With the need to catch up to the efficiencies of the 3D devices, a call to understand the fundamental properties of 2D perovskites has been the natural result. In particular, changing the spacing cation leads to exciting changes in the structure, but there are no rational rules available to predict how a new spacer will affect the structure and thus the optical properties of the resultant structure. Additionally, when these compounds are made into films for photovoltaic devices, orientation of the layers with respect to the substrate becomes important, along with the composition of the film itself which is actually a composite of many phases. Here, I present three new spacers: propylammonium, isobutylammonium, and isoamylammonium, similar to the previously reported butylammonium cation with stabilizes a homologous series of Ruddlesden-Popper type 2D perovskites. The propylammonium cations lead to a new complex “step-like” homologous series with 2D structural dimensionality and 1D electronic dimensionality. The isoamylammonium cation leads to the first reported modulated 2D perovskite structure. I have also used in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) to analyze the film formation during spin coating. This had revealed that, when targeted layer thickness is low, an orientation mechanism prevails, in which a 3D-like phase first grows at the air-liquid interface and serves as a template for highly oriented 2D phases to grow beneath it. With higher layer thickness, an unoriented solvate phase (MA)2Pb3I8·2DMF grows in instead and slowly transforms to unoriented perovskite. These issues can be resolved with hot casting, in which heat is applying during spin coating remove excess solvent, or by using excess ammonium iodide in the precursor solution to favor the orientation mechanism. These results show how fundamental understanding of the effects of the spacer and mechanism of film formation can be used to improve the quality of materials and films for photovoltaic devices.

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