Organic-Inorganic Halide Perovskites (OIHPs) represent the big breakthrough of the last decade in low-cost photovoltaics (PV): [1] bulk OIHPs (with general stoichiometry ABX₃, A=organic cation; B=Pb²⁺, Sn²⁺, Ge²⁺; X = halides) are indeed characterized by unique features in solar-to-energy conversion as consequence of a perfect combination of physical and chemical factors.[2,3] Nevertheless, the presence of residual, non-trivial, issues yet prevents their exploitation in solar device mass production. In particular, the hydrophilic organic moiety present in the A-site of OIHPs (methylammonium, CH₃NH₃⁺, and also formamidinium,⁺HC(NH₂)₂) and Pb in the B-site of the most representative compound of the class, i.e. CH₃NH₃PbX₃, represent serious limitations for the applicability in devices. 

An ideal solution for overcoming this latter issue is replacing Pb with other, more environmental friendly, cations.[4]

On the other hand, plausible solution for the former is the replacement, total or partial, of methylammonium cations with other organic, hydrophobic, aromatic or aliphatic, long chain cations. Such cations (mainly, buthylammonium and phenethylammonium) are indeed able to stabilize the final compound against heat and moisture, a stabilization which is accompanied by a reduction in the dimensionality of the final systems. These so-called mixed 2D/3D Ruddlesden-Popper (RPPs) and Dion-Jacobson (DJ) perovskites have been recently suggested as a viable alternative to 3D bulk OIHPs for their photochemical stability coupled with high-performance optoelectronic properties.

A large number of experimental papers has appeared in literature witnessing the interest of the community towards this class of materials.[5] At the same time ab-initio studies focusing on the role of many-body effects are still very limited and the results in this sense are not yet conclusive.

In the present contribution, after a short introduction about the general properties of the 3D bulk organic-inorganic halide perovskites, we will discuss the optoelectronic properties of some Pb-free 3D bulk systems and of dimensionally confined Ruddlesden-Popper perovskites, as a result of the combination of Density Functional and Many-Body Perturbation Theory analysis. With a keen eye on the A-site and B-site cation replacement, we will discuss the excitonic features in 2D and in mixed 2D/3D halide, hybrid and full-inorganic, perovskites in Pb-based,[6] Pb-less,[7] and Pb-free [8] systems. Finally, in the framework of Pb-free systems, we will show some results of the most recent analysis of interfaces formed by Sn/Ge-alloyed OIHPs and GeI₂/GeO₂ in terms of structural, electronic, and optical features, focusing on pros and cons of such passivating mechanism.