Computational Modeling of Organic Cations and Crystal Defects in Hybrid Perovskites: Bulk, Surfaces, and Interfaces

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The photoconversion efficiency record of silicon-perovskite solar cells recently has reached 31.3%, owing to the novel hybrid perovskites ΆPbX3 that are used as high-energy (1.6-2.0 eV) photon absorber layers with suitable band gaps and high absorption coefficients of more than 10^4 cm-1. They form Wannier excitons of hundreds of Å diameter and tens of meV binding energy, and exhibit extraordinary charge carrier transport properties (ns lifetimes and μm diffusion lengths). ΆPbX3 is a new organic-inorganic material rich in van der Waals bonding. It consists of an ultrasoft 3D net of corner-shared inorganic PbX6 octahedral units, where X = I, Br, Cl, or a mixture (ΆPbIxBryCl3‒x‒y), and Ά stands for a light polar organic cation, typically methylammonium, formamidinium, or a mixture with Cs (MAxFAyCs1‒x‒yPbX3). The organic cations are a “glue” stabilizing the perovskite lattice by not only ionic bonding but also hydrogen-, π-anion-, etc. bonding inside the X12 dodecahedra that is absent in all-inorganic perovskites. The lead-halide inorganic skeleton defines the optoelectronic properties: The lowest energy electronic transitions mostly involve occupied X np and Pb 6s as well as unoccupied Pb 6p orbitals, whereas the Ά-cation orbitals do not contribute to the band edges. These properties are affected by the X composition, PbX6 tiltings and rotations, and bulk and surface defects. The main drawback of the hybrid perovskites is their instability: Not only the intrinsic instability associated with crystal phase transitions into photoinactive δ-phases but also the material degradation that is triggered by light, heat, oxygen, moisture, etc. The main aim of this thesis is to understand the stability of hybrid perovskites from different perspectives considering (i) the effects of the organic cations that have potential to fit in the structure and bond stronger with the lead-halide skeleton than methylammoniun and formamidinium, (ii) the structures of the photoinactive δ-phases, mechanisms of their formation, and the effects of the organic and inorganic cations, (iii) the structures of the point defects, mechanisms of halogen defect migration, and effects of the organic and inorganic cations, and (iv) the structures of the perovskite surfaces and their effects on the stabilization and interface engineering with the charge transport layers.


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