Figure 5: Optical and surface analysis before/after selective dissolution of the substrate.

After the recycling process using a polar aprotic solvent, (a) the optical transmittance is preserved and (b) the X-ray diffraction patterns of the cleaned mp-TiO₂/FTO substrate exhibit peaks corresponding to anatase TiO₂ and FTO without MALI or PbI₂ (^ with black: FTO, # with green: anatase TiO₂, * with red: MALI). Results of secondary ion mass spectroscopy performed on (c) as-prepared mp-TiO₂/FTO, (d) MALI/mp-TiO₂/FTO and (e) cleaned mp-TiO₂/FTO. The background schematics of the spectra represent the device structure; thus, the initial values of the spectra at 0 s of sputtering time represent each ion concentration at the surface of each substrate. The results for the cleaned mp-TiO₂/FTO substrate reveal the presence of residual Pb²⁺ and I^- ions on the cleaned substrate. X-ray photoemission spectra of (f, g) Pb 4f and (h, i) I 3d on the substrate before and after selective dissolution, respectively. The binding energies of Pb 4f and I 3d correspond to Pb and I in the perovskite layer and in the form of residual PbI₂ on the mp-TiO₂/FTO and cleaned mp-TiO₂/FTO substrates, respectively.