Directing the Self-Assembly of Conjugated Organic Ammonium Cations in Low-Dimensional Perovskites by Halide Substitution

At present, two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) are drawing significant interest because of their potential use in different optoelectronic applications, that is, photovoltaics and photodetectors. Here, we report on a series of 2D layered HOIPs (Bit-C3)₂PbX₄ (with X = Cl, Br, and I) containing a 2,2′-bithiophene chromophore functionalized with a propylammonium tethering chain as a model molecule. The optical properties, crystal structure, and phase behavior of the 2D layered HOIPs are studied in depth. The crystal structures with the chemical formula (Bit-C3)₂PbX₄ (with X = Cl, and Br) are successfully obtained. Contrastingly, different crystal structures with an inorganic framework containing face- and corner-sharing octahedra were identified for the iodide-based HOIP. The phase diversity and thermal stability of the (Bit-C3)₂PbX₄ (with X = Cl, Br, and I) thin films were investigated via in situ measurements. Here, the presence of lower-dimensional hybrids with reduced electronic dimensionality within the iodide-based thin film is demonstrated. Additionally, we show that the 2D hybrid thermal stability is dependent on the type of lead­(II)­halide framework employed. We suggest that, via halide substitution from iodide to bromide and chloride, the molecular degrees of freedom of the Bit-C3 ammonium cations are reduced by spatial confinement of a smaller inorganic framework, therefore, limiting the formation of lower-dimensional hybrids besides the targeted 2D layered HOIP. This study illustrates the importance of efficiently utilizing the space supplied by the inorganic framework in which the organic ammonium cations can reside within a 2D layered HOIP. This, in turn, dictates how the organic ammonium cations arrange themselves within the organic layer and influences the adopted crystal structure of the hybrid.