Interfacial Effects in Hybrid Perovskite Solar Cells : Insights from First Principles Studies

Development of sustainable energy sources is among the most urgent needs of modern society. Solar energy technologies, and in particular, hybrid perovskite solar cells (HPSCs), have demonstrated hard-to-beat performances with large capacities for improvement. However, maintaining their efficiencies, over 25% as of this date, under operating conditions, remains the first and foremost obstacle to their becoming a competitive commercial technology. In spite of massive endeavors in the field of perovskite solar technologies, their limited tolerance for moisture, extended periods of light exposure and high temperature have yet to be addressed before they can reach their Shockley-Queisser limit. It is however well established that both interlayer proximities (interfaces of the light absorbing layer (LAL) with the electron and the hole transporting layers, ETL and HTL), and intralayer proximities (organic-inorganic interfaces in 2D hybrid perovskites (2DHPs)) have paramount impact on determining the power conversion efficiency (PCE) as well as the stability of solar cells. In addition to well-known interlayer phenomena such as energy-level misalignment and formation of trap states that subsequently interfere with transport properties, intralayer organic-inorganic interfaces remain an interesting point of discussion, given recent blast-off in the investigation and the application of 2DHPs. Studies have revealed that incorporation of longer or bulkier cationic ligands into 3DHPs will provoke dimensionality reduction and formation of either 2D/3D or pure 2D phases comprising successive organic-inorganic layers that offer higher moisture and thermal stabilities. Crystal structures and optoelectronic properties of these systems are evidently governed to a great extent, by the intralayer effects otherwise known as "cross-linking of the conducting framework by the spacing layer" defined mainly by the nature of the cationic ligands, hence offering possibilities of compositional and structural tailoring towards desirable optoelectronic properties. In this thesis, we provide a comprehensive understanding of structural and optoelectronic properties of 2D perovskites based on a series of organic spacers and attempt to address cross-linking effects of organic-inorganic interfaces from first principles calculations also validated by experiments. On the same note, from another point of view, we explore a novel passivation strategy to alleviate deteriorating interfacial effects at the surface of formammidinium lead iodide as one of the most prominent light harvesting materials. Unfavorable interfacial trap states and structural degradations occur predominantly due to presence of surface intrinsic defects in the perovskite film and the aim is to find suitable strate- gies to diminish these deteriorating effects. By employing density functional theory (DFT), we unravel at the atomic level, the underlying mechanism and the role of crownether surface modulation that occurs through complexation of the under-coordinated surface ions. Eventually, we study the LAL/ETL interface as a potential defect accumulation and carrier recombination center. We computationally obtain the energetically most stable interfaces of MAPbI3/TiO2 at DFT level and probe the impact of different surface properties, including terminations, presence of vacancies as well as ion substitution, on the energy-level misalignment and mitigation of trap states that improve the PCE in MAPbI3 PSCs.