Because of their promising properties as semiconducting materials for optoelectronic applications, like solar cells, three-dimensional (3D) hybrid perovskites have been in the spotlight during the last years. This is due to two main factors: low temperature and precursor solution based synthesis and impressive certified photovoltaic efficiency larger than 20%. In light of this, 3D hybrid perovskites have been claimed as the new big thing in photovoltaics^[1]. However, in spite of their photovoltaics performances, hybrid 3D perovskites still suffer from significant material stability issues, which result in the degradation of real devices^[2].

Recently, two-dimensional (2D) hybrid perovskites got the attention of the scientific community, as they show largely improved stability against water and moisture, still retaining positive photovoltaic efficiencies^[3]. Together, 2D hybrid perovskites allow for much wider flexibility in their chemical composition. In fact, while in the 3D case the size of the cation is subject to the spatial constraint imposed by the octahedral corner-shared network, in 2D perovskites this is no more the case. As result, a huge library of organic cations becomes available. With these materials, we are hence expanding the field of possibilities of hybrid halide perovskites, but a clear connection is needed, at this point, linking the choice of the organic component and its effect on the opto-electronic properties of the material itself.

We report here density functional theory (DFT) calculations on alkyl-ammonium lead iodide perovskites, to study the influence of the alkyl chain length on the electronic structure and optical properties of the material. The reciprocal role of spin-orbit coupling and electronic correlation has been adressed, highlighting important differences with respect to the 3D case^[4]. We predict a significant change in the electronic structure (opening of the band gap and higher effective masses, in particular) using long against short chains. Indeed, if the inorganic layers fix the electronic properties, the length of the organic cation is shown to have an indirect effect. In the case of long, dodecyl chains, an opening of the electronic band gap occurs, due to the influence of the supramolecular packing of the organic spacers on the structure organization of the octahedra network. In this materials, in fact, the organic chains adopt a polyethylene-like packing, causing angle distortions in the inorganic framework and leading to the observed electronic band gap opening. These theoretical results are in agreement with experimental data and demonstrate that organic saturated chains can modify the electronic properties of layered halide perosvkite seminconductors^[5].