Perovskite solar cells (PSCs) are gaining increasing importance and attention in the last decade. Even though high conversion efficiencies have been reached, one of the major bottlenecks for the commercialization of PSCs is their stability. Issues at the interfaces in the multilayered PSC architecture are suspected to be the significant contributor in causing low stability. This doctoral thesis focusses on the analysis of various interfaces present in PSCs and traces methods to improve them. The investigation of individual interfaces is performed by developing suitable sub-cells, i.e., comprising only the particular interface of interest. Various optical and structural characterization methods are used to determine layer and interface properties. After this step, complete devices are manufactured from the sub-cells, and their final performance is investigated to conclude the role and optimization of the interfaces. The thesis accounts for the analysis of the interfacial quality of different in-room ambient processed n-i-p perovskite solar cells configurations, being based on hole-transport-layer (HTL)/Au or HTL-free/carbon-graphite (CG) electrodes. The findings show that developing suitable sub-cells allows to investigate the quality of the various interfaces of PSCs individually. Results from the sub-cell analysis are compared with the performance of complete devices to verify their significance. Thereby, methods to improve the interfaces can be found to achieve higher device efficiencies without compromising its stability.