Metal halide perovskites are emerging materials for optoelectronics due to their excellent optoelectronic properties, such as direct and tunable band gaps, large absorption cross section, long lifetimes and diffusion paths of the charge carriers, coupled to an elevated defects tolerance.[1] The band gaps and the exciton binding energies of these materials can be tuned by varying the chemical composition of the inorganics or by reducing dimensionality by introducing large cations. Interestingly, reduction in dimensionality leads to the emergence of novel optical features, such as the sub-gap broad emission (BE) in 2D perovskites, whose origin is hotly debated.[2] Contrasting hypotheses assign BEs to the recombination of intrinsic self-trapped excitons (STEs) or to emission from native defects.[3-4] 

In this presentation the defect chemistry and photophysics of PEA2MX4 (PEA=phenethylammonium; M=Pb, Sn; X=I, Br, Cl) <100> 2D perovskites is discussed by a computational perspective, in order to provide a microscopic picture of the defects processes taking place in these quantum confined materials. The trends in the defects chemistry moving from 3D to 2D and the effects of the metal by replacing lead with tin will be discussed. Hence, by comparing predictions with experiments, the diverse hypothesis about the origin of the BE in this class of materials will be analyzed. DFT results show that sensibly lower defect densities are expected in 2D perovskites compared to 3D analogues, but a deepening of defect charge transitions in the band gap is reported due to QC. Despite the low calculated defect densities, emission from halide vacancies is compatible with the experimentally observed sub-gap features in the relatively large set of compounds considered in the study, suggesting that the density of these optically active centers is modulated by the crystallization kinetics. On the other hand, the simulation of STEs indicates that the self-trapping of holes and electrons is a feasible process only in the wide band gap Br- and Cl-based 2D perovskites, even though the process is thermodynamically hindered by the shallow nature of the transition.[5] 

Our work provides useful insights into the intrinsic and extrinsic mechanisms generating BEs in 2D perovskites and it demonstrates that a control of the crystallization process, e.g. by tuning PEA-halide stoichiometry in the precursors, is key to selectively control the optical features of these materials.