Since CH3NH3PbI3 based perovskites were discovered as viable active materials for the next generation photovoltaic devices, their instability in different environmental conditions has been a constant challenge. In pursuit of a better understanding of the degradation mechanisms, perovskite solar cells have been fabricated and investigated by scientists in order to find correlations between the solar cell characteristics/performance and the interface variation. In this thesis, the perovskite reactivity to humidity is studied by exposing samples to D2O environment for different durations. The degradation process of CH3NH3PbI3 perovskite is examined in-situ by using time-of-flight secondary ion mass spectrometry (ToF-SIMS). 3D images are constructed through the layer-by-layer spatially resolved elemental distribution analysis and the D2O moisture penetration through the sample. The intermediate products of interaction with moisture are analyzed by ToF-SIMS and X-ray photoelectron spectroscopy (XPS).We also investigated the electrical operation-induced degradation on CH3NH3PbI3 perovskite solar cells. Upon exposure to electrical current, the structure and composition were examined by combining depth-resolved imaging with ToF-SIMS, XPS and field-emission scanning electron microscopy (FE-SEM). The results show that the interface of the perovskite and the meso-porous TiO2 intermix into each other during the initial operations of solar cell. This intermixing turns the efficiency upward and improves the power conversion efficiency (PCE) up to ~50%. Both depth profiles and SEM images proved that operating devices undergo irreversible changes in thickness, which results in a dramatic performance loss eventually. In addition to studying the degradation process of the perovskite, a new formation method was developed to achieve complete conversion of PbI2 to CH3NH3I3 on FTO/Compact TiO2 substrate by employing a quaternary ammonium salt as an additive in the PbI2 solution. This complete conversion improves perovskite solar cell efficiency up to ~45 % compared to devices made without additive (from 11% to 16% in PCE).