titre de la dissertation: « First Principle Simulations of Low-dimensional Hybrid Organic-Inorganic Perovskites for Optoelectronic Applications ».

Promoteur: Monsieur David BELJONNE et Filippo DE ANGELIS (CNR-SCITEC)

Résumé de la dissertation: Lead halide perovskite semiconductors have attracted a lot of interest in the field of materials science namely for optoelectronic applications. These crystalline systems, composed of at least three different entities in a general stoichiometry ABX₃, with, typically, A = methylammonium/formamidinium, B = Pb²⁺ or Sn²⁺ and X = Cl^–, Br^– or I^–, indeed show remarkable electrical and photophysical properties : strong optical absorption in the visible range, tuneable bandgap, decent charge carrier mobility, reduced charge recombination, high tolerance to defects, etc. As such, lead halide perovskite semiconductors have quickly become attractive compared to well-established competing technologies and are being studied for a wide range of applications including photovoltaic devices, light-emission displays, photodetectors, lasers, etc. In less than a decade, single-junction lead halide perovskite solar cells have now reached 25.7% certified best performing conversion efficiency. Sadly, three-dimensional perovskites are suffering from a lack of stability. This is currently the main challenge ahead. Among the different approaches to improve the stability, one involves reducing the dimensionality of the active material.By introducing A-site organic cations that are too large to fit inside the cavities formed by the inorganic octahedral network made of B and X atoms, the mixed organic-inorganic system spontaneously organizes onto two-dimensional nano-sheets separated by the A-site cations. This seems to prevent halide migrations, one of the causes of device degradation under operation, and improves the stability, but at the cost of poorer transport properties. Indeed, the physical barrier to halide migration induced by the use of inert organic cations usually based on saturated alkyl chains, is also acting as an insulator layer, preventing the transfer of charges and electronic excitations from one inorganic layer to another and, in fine, transport through the material to the electrodes. In order to keep the improved stability and try to enhance the charge transport properties, we decided to theoretically study, thanks to Density Functional Theory (DFT), novel two-dimensional hybrid lead-halide perovskites with electronically (in)active A-site spacers cations.First, we studied well-known and experimentally studied two-dimensional perovskites, composed of saturated alkyl chains of different lengths and in different phases, in order (i) to set-up our computational protocol; and (ii) to assess the possible indirect role induced by these inert spacers (namely on the geometric structure of the inorganic layout). Our calculations show that the inert organic cations exert some lattice distortion in the octahedral network, which in turn lead to changes in electronic structure. These effects vary with the length of the alkyl chains with long chains leading to stronger perturbations. We then moved to the modelling of low-dimensional perovskites that incorporate innovative electronically active cations, following two different strategies. The first one involves the synthesis and characterization of a one-dimensional perovskite-like octahedral network (made of face-shared instead of corner-shared octahedra), where a pyrene:tetracyanoquinodimethane Charge-Transfer Complex (CTC) is used in place of the organic linker. Experimental and theoretical investigations have been performed to assess charge transport in these mixed systems. An increased photocurrent is measured that is assigned to an interesting interaction between the organic and the inorganic components, likely prompting transient localization of the holes on the lead halide octahedral network. The second approach deals with three theoretically designed two-dimensional perovskites where we introduced different electronically active cations (with conjugated molecules/aromatic rings), paying special attention to the influence of the inorganic surface termination. These calculations highlight again the importance of the distance between the inorganic layers due to the length and the encumbrance of the organic cation, and the associated importance of octahedral distortion. Also, we assessed how the electronic structure depends on the organic-inorganic interface, showing that a <110> termination could be interesting for a better hybridization between the electronic states of the organic and the inorganic components. The choice of the organic cation is also key in order to achieve energy level alignment between the band edges of the inorganic layout and the frontier molecular orbitals of the organic cations.