Dimensional Engineering in Perovskite Solar Cells and Its Impacts on Device Stability

The organic-inorganic hybrid perovskites are a class of semiconductors that combined the advantages of low-cost solution-processable fabricating and excellent photoelectric properties such as tunable bandgap, large absorption coefficient, and long carrier diffusion length. Researchers have already found many successful applications of perovskite materials in photoelectronic fields, such as photovoltaics, light emitting diodes (LEDs), photodetectors, radiation detectors, etc. For perovskite solar cells (PSCs), the power conversion efficiencies (PCEs) have dramatically increased from 3.8% in 2009 to 24.2% in 2019. Compared to the commercialized silicon solar cells, which come with a guaranteed power output of at least 80% for 25 years, the best lifetime of perovskite solar cells reported is about one year with a PCE of 12%. Therefore, the stability issues still prevent perovskite solar cells from competing with the established photovoltaic technologies. In this dissertation, we mainly focus on the stability enhancements for perovskite solar cells by using dimensional engineering. In Chapter 2, the operation mechanism in 2D layered perovskite solar cells was investigated for developing stable and high-performance layered perovskite devices. The real morphology of hot-cast layered perovskite films was examined by a variety of characterization techniques, which are found to be a mixture of layered and three dimensional (3D)-like phases with phase separations at multiple scales in both vertical and lateral directions. This phase separation is explained based on a top-crust peeling-off test. The carrier transport properties in the layered perovskite solar cells was further studied. Based on such morphology, the working mechanism of the layered perovskite solar cells is proposed. The impact of morphology on efficiency and stability of the hot-cast layered perovskite solar cells are further discussed to provide guidelines for the future investigation. In Chapter 3, the ion migration stability in the Ruddlesden-Popper type 2D layered perovskites was investigated, the mechanism of suppressed ion migration in low-dimensional perovskites was proposed to explain their enhanced stability compared to the conventional three-dimensional perovskites. In Chapter 4, a method to fabricate high efficiency and thermally stable perovskite solar cells by constructing 2D/3D stacked structures was developed which take advantage of the high efficiency of 3D perovskites as well as high stability of 2D perovskites. The mechanism of enhanced efficiency and stability in such 2D/3D stacking structures was also investigated