Zhaojie Zhang (Venkataraman group): Employing a 2D perovskite capping layer on the 3D perovskite is an effective strategy to improve the performance and stability of perovskite solar cells. However, when exposed to light, small A-site cations in 3D perovskite exchange with the bulky cations in the 2D layer and degrades of the 2D/3D interface. Therefore, to achieve long-term stability in perovskite solar cells, it is important to understand the nature of photogenerated charge carriers that cause cation migrations at the 2D/3D interface. In our recent work, we fabricated 2D/3D perovskite stacks on glass, ITO, ITO/PTAA, ITO/PTAA/CuI and ITO/SnO2. A combination of grazing incidence X-ray diffraction, steady-state and time-resolved photoluminescence studies reveals the link between the light-induced degradation and the photogenerated charge carrier dynamics. Upon illumination, the stability of the 2D layers follows this trend: ITO/PTAA/CuI≈ITO>ITO/PTAA>glass>ITO/SnO2 (from stable to unstable). This trend suggests that extracting holes efficiently from the 3D layer can improve the stability of the 2D layer. We also found that 2D/3D stacks degrade faster when illuminated from the 2D side instead of the 3D side. Our studies suggest that to achieve a stable 2D/3D interface, hole accumulation in the 3D layer should be avoided and the exciton density in the 2D layer should be reduced.