In comparison to the undistinguishable (by eye at least) layers composing the perovskite solar cells, the III-V layers are shiny, it is feasible to distinguish the GaAs substrate and the GaInP layer (see Figure )). In this image, the white stripes indicated by the red arrows seem to be an oxide layer on the TCS between the Ge and the GaInP. This oxide layer is also observed by SEM (see Figure ) and its thickness is estimated to be 15-20 µm. Another hypothesis on the origin of these white stripes is the breaking of the thin GaInP layer and the resulting rough surface reflects the light in a way that it appears white. A direct proof of the success of the TCS preparation is highly valued for refining polishing parameters on a sample. With an optical microscope, it can also be seen that the geometry of the sample is no longer a parallelepiped (see Figure ), a significant amount of material was removed towards the right of the sample giving access to the buried GaAs|GaInP interface. Such a TCS interface can also be characterized using SEM (see Figure )). From each side of the interface on the TCS, EDX measurements have been performed and reveal a stoichiometry close to 1:1 for the GaAs and close to 1.2:1 (Ga:P) and 0.5:1 (In:P) for the top layer (see Figure )). A non-negligible amount of carbon was also detected at the surface of the sample. This is mostly due to the polishing procedure in air and the use of diamond lapping films. Additional SEM pictures and EDX mapping of the Ge|GaInP and GaAs|GaInP interfaces are provided in Figures and.