Combined theoretical and experimental investigations of new perovskite compounds for solar cells applications

In this thesis, we aimed to contribute to a better understanding of the surfaces and interfaces involving perovskite compounds and governing the performances of PSCs, by using density-functional theory (DFT)-based computational protocols, with a comparison with experimental data, when available. More precisely, we first performed DFT calculations on TiO₂ anatase bulk and its (101) surface, and on the adsorption of four small organic ligands on the latter, aiming to select the best candidate for surface engineering of PSC. The benchmark calculations performed with different DFT models on bulk anatase indicated that hybrid functionals can both well reproduce the experimental lattice parameters with a small overestimation of the bandgap. For the adsorption calculations, a bidentate binding mode was found to be preferred for all ligands, in line with the vibrational frequencies investigation carried out which showed a general good agreement with available experimental data. From the properties computed, including density of states (DOS), dipole moments, and the work function obtained by experiments, it is derived that 4-Chlorobenzoate (CBA) and 4- Nitrobenzoate (NBA) are suitable candidates to improve the performance of PSC. We then simulated the heterointerface between the CH₃NH₃PbI₃ (MAPI) perovskite and TiO₂, in which the CBA ligand has been used to act as a bifunctional linker. Our calculation indicated that CBA can efficiently link the perovskite and the oxide moieties, ensuring stability of the interface through Ti-O and Pb-Cl interactions. Furthermore, CBA ligand can induce a favorable effect to improve band alignment and thus electron transfer from MAPI to TiO₂. Last, we reported preliminary calculations performed both on bulk α-CH(NH₂)₂PbI₃ (α-FAPI) as well as on two of its low-index surfaces, considering different terminations with both stoichiometric and non-stoichiometric slab models, which mainly revealed the structure-dependent properties. Overall, the obtained DFT results were in line with the experimental data. The DFT-based computational approach proposed in this thesis therefore allowed to better understand the basic mechanisms of PSCs, especially on the electron transfer from perovskite to TiO₂ (electron transfer layer). Moreover, general guidelines for the screening of molecules to be adopted as ligand to improve the performance of PSC could be drawn from the obtained results. The relationship between structure and perovskite properties could be established based on the DFT modeling carried out. Overall, the here proposed mixed theoretical/experimental approach could be extended to other semiconductor heterostructures in a wide variety of optoelectronic applications, where it could contribute to a better understanding of the working principles of these devices to improve their performances. (author)