Metal halide perovskites are outstanding materials for photovoltaics due to their excellent optoelectronic properties, such as direct band gaps, large absorption cross sections and long lifetimes and diffusion paths of the photo-generated charge carriers. Lead-based perovskites dominate the field by providing efficiencies of 26% in solar cell devices, but an increasing interest is arising in the development of tin-halide perovskites (THP) to replace toxic lead and reduce the band gap of the perovskite. Despite large efforts, the efficiencies of THPs are still sensibly lower than the lead-counterpart, mostly due to the native p-doping and the easy oxidation of tin(II) to tin(IV), compromising not only efficiency but also the long-term stability of the material.[1] Understanding the origin of such limitations and elaborating effective strategies to improve efficiency and stability are essential for an advancement in the field.  

In this presentation we provide a theoretical perspective of the fundamental properties of THPs by focusing on the microscopic origin of the p-doping, the role of chemical composition, as well as the potential strategies which can be adopted to increase efficiency and stability. Our analysis starts from the discussion of the defect chemistry of THPs by keeping a comparison with lead. The defect-related origin of the heavy p-doping and the parallel existence of non-radiative recombination channels, as well as the potential defect-activated degradation processes at the surface will be discussed.[2-3] The role of electron-phonon coupling in tuning the localization of the hole charge carriers (free carrier vs polaron) vs the doping density is analyzed.[4] Hence, the discussion moves to elucidate the mechanisms of action of some additives commonly used in the synthesis of THPs, such as SnF2 and EDAI2, in order to illustrate how they can mitigate p-doping and improve crystallization. In-silico designed de-doping strategies based on the incorporation of trivalent metal ions coupled to small halide-alloying will be presented.[5] Finally, we will discuss the physical and chemical properties of the interface between the THP and some SAM hole transport materials, e.g. MEO-nPACZ, by illustrating the effects of the SAM and perovskite chemical composition on the electronic alignment at the interface.